Nawel Ayadi

Alexandre Lapeyre

Consumer purchase intentions for green products: Mediating role of WTP and moderating effects of framing

Keywords: willingness to pay, framing, internal reference price, financial perceived risk, ecological perceived benefits, green product

This research examines the role of consumers' willingness to pay (WTP) in the purchase decision process for a green product and investigates the moderating effects of two types of framing: price and ecological message claim. Using an experimental design, 262 participants were assigned randomly to one of nine conditions (3 price  3 ecological message frames).

The results reveal a mediating effect of WTP on the relationship between consumers' perceptions and purchase intentions. Furthermore, a price framed in aggregate terms reinforces the positive effect of WTP on purchase intention. The mere presence of an ecological message claim also enhances the relation between WTP and purchase intentions.

This research proposes ways marketers can promote their green products. The results encourage the use of ecological messages and frames of prices in an aggregate form to enhance consumers' purchase intentions through WTP.

Introduction

Price formulations are fundamental for marketers, so many researchers have focused on the concept of willingness to pay (WTP), which reflects the maximum amount of money a consumer will agree to give up for a certain quantity of a product or service [START_REF] Acquisti | Do Interruptions Pay Off? Effects of Interruptive Ads on Consumers' Willingness to Pay[END_REF][START_REF] Kohli | A Reservation-Price Model for Optimal Pricing of Multiattribute Products in Conjoint Analysis[END_REF][START_REF] Koschate-Fischer | Willingness to Pay for Cause-Related Marketing: The Impact of Donation Amount and Moderating Effects[END_REF][START_REF] Krishna | Effect of Dealing Patterns on Consumer Perceptions of Deal Frequency and Willingness to Pay[END_REF]. According to [START_REF] Miller | How Should Consumers' Willingness to Pay Be Measured? An Empirical Comparison of State-of-the-art Approaches[END_REF], measuring and determining consumers' WTP is critical for formulating competitive strategies, developing new products, and implementing pricing tactics. Prior WTP literature mainly examines ways to measure WTP (e.g., [START_REF] Miller | How Should Consumers' Willingness to Pay Be Measured? An Empirical Comparison of State-of-the-art Approaches[END_REF][START_REF] Wertenbroch | Measuring Consumer Willingness-to-Pay at the Point of Purchase[END_REF], consumer profiles (e.g., demographics, attitudes, values) that lead to distinct WTP levels (e.g., [START_REF] Laroche | Targeting Consumers Who Are Willing to Pay More for Environmentally Friendly Products[END_REF], and the specific relationships of WTP with consumer knowledge [START_REF] Bechwati | Willingness To Pay for Professional Services[END_REF], perceived value [START_REF] Dodds | Effects of Price, Brand and Store Information on Buyers' Product Evaluations[END_REF], or product design [START_REF] Kristensen | How Valuable is A Well-Crafted Design and Name Brand? Recognition and Willingness to Pay[END_REF]. Yet this stream of research remains underexplored, in the sense that the ways consumers analyze information and how these processes affect their judgment is fundamental for marketers. Therefore, this article explores the determinants of and influences on WTP to enhance understanding of consumers' purchase decision-making processes.

Consumer preferences depend on various individual perceptions; the ultimate consumption decision results from a trade-off between consumers' perceptions of the benefits and costs [START_REF] Luce | Tradeoff Difficulty: Determinants and Consequences for Consumer Decisions[END_REF]. Their WTP thus appears to result from a trade-off between certain benefits and costs; in turn, it influences purchase intentions. This effective conceptualization of WTP can help marketers overcome consumer-established barriers related to price. For example, for a green, or environmentally friendly, product, consumers' trade-off is specific to the risks and benefits they perceive in relation to its ecological features. Because such products tend to be more expensive than conventional offerings (Consumer Reports 2007), WTP likely has a central role in the purchase decision process. Furthermore, in response to consumers' increasing ecological consciousness [START_REF] Poll | Water Pollution Americans' Top Green Concern[END_REF][START_REF] Hartmann | Consumer Attitude and Purchase Intention Toward Green Energy Brands: The Roles of Psychological Benefits and Environmental Concern[END_REF][START_REF] Husted | An Exploratory Study of Environmental Attitudes and the Willingness To Pay for Environmental Certification in Mexico[END_REF][START_REF] Laroche | Targeting Consumers Who Are Willing to Pay More for Environmentally Friendly Products[END_REF][START_REF] Royne | The Public Health Implications of Consumers' Environmental Concern and Their Willingness to Pay for an Eco-Friendly Product[END_REF][START_REF] Thøgersen | Chinese Consumers' Adoption of a Green Innovation-The Case of Organic Food[END_REF], more firms offer environmentally friendly products, which requires them to find a way to make their green offers sufficiently attractive, despite any additional costs. Green products are intriguing in this context, in that despite consumers' stated agreement that they would pay more for such products, relatively few people buy them in reality [START_REF] Luchs | The Sustainability Liability: Potential Negative Effects of Ethicality on Product Preference[END_REF]).

To reconcile these phenomena, we propose examining the extent to which unique presentations of the same information might exert various effects on consumers' perceptions [START_REF] Tversky | The Framing of Decisions and the Psychology of Choice[END_REF]. For example, different frames applied to the same price and message claims might shape consumers' WTP [START_REF] Gamliel | The Effect of Framing on Willingness to Buy Private Brands[END_REF]. Consumers are sensitive to the contextual information contained in price offers, so price frames likely influence both the perceived costs of the transaction and consumers' final purchase decisions. [START_REF] Gourville | The Effects of Monetary Magnitude and Level of Aggregation on the Temporal Framing of Price[END_REF] calls for more research into the behavioural mechanisms that underlie this influence. In addition, environmentally oriented advertising might improve product evaluations, but marketers still know little about how to convey green claims effectively [START_REF] Grimmer | Green Marketing Messages and Consumers' Purchase Intentions: Promoting Personal Versus Environmental Benefits[END_REF][START_REF] Hartmann | Green Advertising Revisited -Conditioning Virtual Nature Experiences[END_REF][START_REF] Kong | When Does Green Advertising Work? The Moderating Role of Product Type[END_REF].

Such advertising even could be counterproductive if the claim appears misleading. Therefore, we investigate both price and ecological message framing strategies applied to green products to determine their effects on consumers' WTP and purchase intentions, as well as detail their usefulness for marketers. Our findings indicate a mediating effect of WTP on the relationship between consumers' perceptions of financial risk or ecological benefits and their purchase intentions. Price framed in aggregate terms and the mere presence of an ecological message claim can reinforce the positive effect of WTP on purchase intentions for a green product.

The rest of this article is organized as follows: In the next section, we review prior literature related to consumers' WTP and develop our hypotheses. After introducing the research methodology, we present the empirical analyses and results. The last section offers theoretical and managerial contributions, limitations, and further research directions.

Literature review and hypotheses

Consumer WTP

As we already defined it, WTP is the maximum amount of money that a consumer is willing to pay for a certain quantity of a product or service [START_REF] Acquisti | Do Interruptions Pay Off? Effects of Interruptive Ads on Consumers' Willingness to Pay[END_REF][START_REF] Kohli | A Reservation-Price Model for Optimal Pricing of Multiattribute Products in Conjoint Analysis[END_REF]. Knowledge of WTP helps companies understand consumers' price perceptions and meet their expectations, which depend on the proportion and importance of available attributes (e.g., degradability or recyclability of green products). For green products, consumers face a dilemma though, because they generally perceive the price as higher than that charged for conventional products. [START_REF] Husted | An Exploratory Study of Environmental Attitudes and the Willingness To Pay for Environmental Certification in Mexico[END_REF] argue that environmentally oriented firms suffer from high cost structures, which directly affect their final offer prices. [START_REF] Pickett-Baker | Pro-Environmental Products: Marketing Influence on Consumer Purchase Decision[END_REF] also note that in some cases, the initial price of newly introduced, low energy consumption products is quite high. The Green Gauge Global research project specifies that 6 in 10 consumers believe environmentally friendly product alternatives are too expensive (GFK 2012). As they trade off the benefits and risks of green products in the process of setting their WTP, consumers likely assess both positive and negative aspects.

We investigate three factors as potential determinants: ecological perceived benefits (EPB), financial perceived risk (FPR), and internal reference prices (IRP).

Determinants

Consumers use reference prices as comparison standards to assess the value of a product or service. Their evaluations are not in absolute terms but rather are relative to a reference point [START_REF] Garbarino | Interrelationships and Distinct Effects of Internal Reference Prices on Perceived Expensiveness and Demand[END_REF], which might be internal (memorized) or external (displayed in the marketplace) [START_REF] Dickinson | Conceptual Model of Reference Price Formation for Consumer Services[END_REF][START_REF] Monroe | Buyers' Subjective Perceptions of Price[END_REF][START_REF] Putler | Incorporating Reference Price Effects into a Theory of[END_REF]. As Dholakia and Simonson (2005, 206) explain, 'consumers' perceptions of attribute values and product preferences are determined largely by the reference points they use, rather than by absolute values alone…. price perceptions are strongly influenced by reference prices, such as the prices of products encountered previously or externally available store prices.' Thus, though external sources such as price advertisements might influence a consumer's IRP, it is fundamentally the result of a memory process that relies on prior experiences with similar products or brands. For our study context, we conceptualize IRP as the expected price that consumers perceive as fair for an equivalent good that has no green attributes [START_REF] Dickinson | Conceptual Model of Reference Price Formation for Consumer Services[END_REF][START_REF] Garbarino | Interrelationships and Distinct Effects of Internal Reference Prices on Perceived Expensiveness and Demand[END_REF]. If consumers establish a higher IRP, they are willing to pay a higher price. Despite this close relation between IRP and WTP [START_REF] Chandrashekaran | The Implication of Individual Differences in Reference Price Utilization for Designing Effective Price Communications[END_REF], [START_REF] Bearden | Examining Alternative Operational Measures of Internal Reference Prices[END_REF] demonstrate empirically that the concepts are distinct.

In addition to product costs, reference points, and IRP in particular [START_REF] Chandrashekaran | The Implication of Individual Differences in Reference Price Utilization for Designing Effective Price Communications[END_REF], can shape consumers perceptions of financial risk (FPR), which reflect the possible monetary costs of a transaction. A high IRP, indicating perceptions of high quality and added value for a product [START_REF] Grewal | The Effect of Store Name, Brand Name and Price Discounts on Consumers' Evaluations and Purchase Intentions[END_REF], should expand the margin at which a price appears acceptable and reduce FPR. Accordingly, H1: A higher IRP leads consumers to indicate a (a) higher WTP and (b) lower FPR.

Yet perceived risk generally has a negative impact on consumer perceptions and purchase intentions [START_REF] Alhakami | A Psychological Study of the Inverse Relationship Between Perceived Risk and Perceived Benefit[END_REF][START_REF] Dowling | Perceived Risk: The Concept and Its Measurement[END_REF][START_REF] Nordgren | Unpacking Perceived Control in Risk Perception: The Mediating Role of Anticipated Regret[END_REF]. If consumers anticipate poor outcomes, their expected value for the product declines, as does their WTP. Thus, higher FPR should lead to lower WTP. For green products, these perceptions may concentrate on ecological benefits. Accordingly, we define EPB in relation to consumers' beliefs about the efficacy of a claim that the offering will have a minimal negative impact on the environment. Consumers likely need to search for information to discern the ecological advantages of a new green product. According to [START_REF] Hirunyawipada | Consumer Innovativeness and Perceived Risk: Implications for High Technology Product Adoption[END_REF] though, they are less likely to engage in a search for information about new products if the potential losses of its adoption are highly salient. Thus, 'a high perceived financial risk might dissuade consumers from acquiring any further information about new products' (Hirunyawipada and Paswan 2006, 188). If FRP discourage consumers from pursuing the potential ecological benefits of a new product, logically, consumers' assessment of their FRP must occur prior to their consideration of EPB. That is, consumers need information about green attributes to establish a trade-off, but if FPR discourages them from acquiring information [START_REF] Hirunyawipada | Consumer Innovativeness and Perceived Risk: Implications for High Technology Product Adoption[END_REF] Modern consumers, with their increasing interest in environmental issues, appear far more aware of the ecological benefits of certain products than prior generations. This growing awareness has granted consumers a means to recognize key green product attributes.

Perceptions of ecological features should enhance the perceived value of green products [START_REF] Grewal | The Effect of Store Name, Brand Name and Price Discounts on Consumers' Evaluations and Purchase Intentions[END_REF][START_REF] Zeithaml | Consumer Perceptions of Price, Quality and Value: A Means-end Model and Synthesis of Evidence[END_REF]) and increase consumers' WTP to acquire such products. Therefore, H3: Higher EPB leads consumers to express higher WTP.

Mediating role of consumer WTP

Purchase intentions result from a trade-off of the advantages and disadvantages of a product or service, as perceived by consumers [START_REF] Dodds | The Effect of Brand and Price Information on Subjective Product Evaluations[END_REF][START_REF] Luce | Tradeoff Difficulty: Determinants and Consequences for Consumer Decisions[END_REF]. A positive purchase intention thus depends on IRP, FPR, and EPB. Considering the particularities of green products (i.e., additional costs, ecological features), WTP should have a more pronounced role in related purchase decision processes, because it represents the last assessment step, before the formation of purchase intention. That is, we have postulated that a consumer's WTP mainly depends on her or his IRP, FPR and EPB; here, we argue that these antecedents affect purchase intentions through WTP: H4: The effects of (a) IRP, (b) FPR, and (c) EPB on consumers' purchase intentions are mediated by WTP.

Moderating effects of framing strategies

The notion of framing derives from prospect theory [START_REF] Kahneman | Prospect Theory: An Analysis of Decisions Under Risk[END_REF]) and refers to 'the decision-maker's conception of the acts, outcomes, and contingencies associated with a particular choice' (Tversky and Kahneman 1981, 453). A simple change in the presentation of the same decision problem can lead to different individual preferences. Kühberger (1998, 24) explains that 'in decision theory, the term frame can be used in a "strict" and in a "loose" sense. The strict definition relates to the wording of formally identical problems, i.e., to a semantic manipulation of prospects whereby the exact same situation is simply redescribed.' For this study, we opt for strict framing, 1 to be able to explain differences in perceptions more fully. Vast literature on judgment and decision making explicates the ways in which semantic manipulation influence the framing of organizational issues; prior research generally asks participants to 'choose between two alternatives (i.e., a sure thing and a risky option) that are semantically manipulated to alter decision-makers' valuation of the (objectively) same outcomes' (Stapel and Koomen 1998, 133). Another classical manner to frame identical problems is to use different price formulations. We consider these two distinct types of framing: price and semantic manipulation of the ecological message.

Moderating effects of price framing

Price frames can affect consumers' monetary assessments (e.g., [START_REF] Gendall | Message Framing Effects on Price Discounting[END_REF], WTP, and purchase intentions. When prices appear in an aggregated format, the whole price is exhibited, whereas if they appear in a disaggregated format, the same objective price appears in segments, depending on a time schedule of payments (price displayed at a certain number of months or days). Disaggregated prices should be more effective in economic and psychological terms. As economic research has shown, the time value of money leads to varying consumer behaviours in response to prices [START_REF] Frederick | Time Discounting and Time Preference: A Critical Review[END_REF][START_REF] Kovacs | Identifying Individual Discount Rates and Valuing Public Open Space with Stated-Preference Models[END_REF]. Disaggregated prices also help reduce the perceived costs of a transaction [START_REF] Gourville | The Effects of Monetary Magnitude and Level of Aggregation on the Temporal Framing of Price[END_REF]. Therefore, a disaggregated or per day pricing strategy should help consumers encode price information and perhaps reduce purchase anxiety. Finally, consumers are more willing to accept prices framed as 'pennies-a-day' rather than in aggregate terms [START_REF] Sinha | Consumers' Perceptions of Promotional Framing of Price[END_REF], likely because they have difficulty computing prices [START_REF] Thomas | Penny Wise and Pound Foolish: the Left-Digit Effect in Price Cognition[END_REF], so consumers process that information only partially.

Accordingly, we hypothesize: H5: Price framing moderates the influence of consumers' WTP on purchase intentions.

Moderating effects of ecological message framing

Green products often feature specific communications to inform consumers about their environmental contributions. Such advertising claims should influence consumers' perceptions and help convince them to buy the green product, as well as moderate the effects of WTP on purchase intentions. First, consumers like to be informed and prefer to make product decisions that are consistent with their desires, such as a goal of reducing environmental damage [START_REF] Davis | Strategies for Environmental Advertising[END_REF]). If they recognize the benefits of a green product for preserving the environment, consumers might agree to its purchase more easily, especially in the presence of a concrete claim rather than in its absence. Second, the message focus (positive or negative) constitutes a specific type of ad framing. Previous studies note the effects of message framing on consumer responses to advertisements (e.g., [START_REF] Grewal | The Moderating Effects of Message Framing and Source Credibility on the Price-Perceived Risk Relationship[END_REF][START_REF] Martin | The Interaction of Message Framing and Felt Involvement in the Context of Cell Phone Commercials[END_REF] and prices [START_REF] Gamliel | Message Framing of Products Causes a Preference Shift in Consumers' Choices[END_REF] in various consumption domains. On average, a positive frame highlights the potential favourable outcomes and is more effective than a negative frame that emphasizes possible adverse consequences [START_REF] Arora | Price Bundling and Framing Strategies for Complementary Products[END_REF], because it generates more favourable associations and encoding [START_REF] Biswas | Consumer Choices Under Product Option Framing: Loss Aversion Principles or Sensitivity to Price Differentials?[END_REF]. However, [START_REF] Martin | The Interaction of Message Framing and Felt Involvement in the Context of Cell Phone Commercials[END_REF] note that negative frames are more persuasive and accepted in certain conditions. For this study, we create negative and positive frames with a semantic manipulation of the ecological claim that varies the terminology used to present the ecological message. The positive frame stresses the ecological contributions of the green product; the negative frame highlights the prevention of negative outcomes, though the added value for the environment is objectively identical. We predict:

H6: The framing of the ecological message claim moderates the influence of consumers' WTP on their purchase intentions.

Methodology

Research design

The quasi-experimental design featured advertisements for a green laptop with 3 (temporal price framing: euros-a-day price, euros-a-month price, and aggregate price)  3 (ecological message framing: no message, positive ecological message, and negative ecological message), full-factorial, between-subjects factors. The prices were manipulated only with respect to the framing criterion, so the offer maintained the same final price (1049€) across all conditions. Three modalities in our design reflected this consistent economic value: (1) aggregated price: 1049€, (2) disaggregated price per month: 49.95€ a month (over 21 months), and (3) disaggregated price per day: 1€ a day (over 2 years, 10 months and 14 days).

In all the experimental conditions, the offer was a green-oriented one (product name and characteristics). The framing featured an ecological message in the stimuli, and the message manipulation reflected the percentage of recycled materials in the product. Thus the positive framing indicated that a high percentage of materials were recycled ('80% of materials will be recycled with EcoLaptop'); the negative framing noted that 'only 20% of materials will not be recycled.'

Stimuli

The product had to be expensive enough to induce FPR but not prohibitively expensive for the respondents. The selected green laptop used a fictitious product name (EcoLaptop) and represented a relatively new offer for consumers. The brand name was not revealed, to avoid evoking any established attitudes. All stimuli ads appeared with a white background, the same product name, and the same picture of the laptop, to ensure attention to the focal manipulations (price and ecological message frames) (see Appendix 1).

Respondents' perceptions of the stimulus were controlled on the basis of a previous pretest, conducted with 28 participants whose profiles were similar to those of the respondents in the main study.

Participants

To ensure that we could compare the effects of the nine experimental conditions accurately, we needed a homogeneous population. The sample consisted of 262 graduate and undergraduate students from a French university who received an e-mail invitation to access a dedicated website. 2 We chose a student sample primarily to exploit its homogeneity [START_REF] Peterson | On the Use of College Students in Social Science Research: Insights from a Second-Order Meta-Analysis[END_REF]) and thereby increase internal validity, which Winer (1999, 349) calls 'a necessary condition for any experimental study. External validity is not of much concern in experimental work if the researcher cannot adequately show that the results found from an experiment are truly due to the manipulation(s).' In our sample, 51% of the participants were women, and the mean age was 22.11 years. Each participant was randomly assigned to one of nine conditions (3 price  3 ecological message frames) (28-31 subjects per condition).

Procedure

The online procedure prevented participants from turning back to pages they already had read or moving on to pages if they had not answer questions. They began by viewing an advertisement for the green laptop with the fictitious name. Depending on the experimental condition to which they had been assigned, this advertisement included different green message claims. The price was hidden. Participants declared their IRP and WTP for the EcoLaptop. Next, they viewed the same advertisement, except that it contained the 2 According to the Centre for Retail Research, the French market for green products in 2009 grew to €10.6 billion, or 2.4% of total sales. In addition, French consumers pay 51% more for green products (except food) on average, though their costs have increased only 29%. Sales of green products are predicted to double during the next five years (http://www.retailresearch.org). manipulated price of the EcoLaptop. Finally, they declared their intention to purchase this product and provided their perceptions of financial risk and ecological benefits.

Variable measures

Using translation and back-translation procedures, we developed French versions of existing English-language scales. For the IRP measure, we used three items from [START_REF] Vaidyanathan | The Role of Brand Familiarity in Internal Reference Price Formation: An Accessibility-Diagnosticity Perspective[END_REF]. The measure of FPR relied on Stone and Gronhaug's (1993) three-item, seven-point Likert subscale. We developed a new measure of EPB for this study; it consisted of a threeitem, seven-point Likert scale. We measured participants' WTP using a direct valuation method [START_REF] Miller | How Should Consumers' Willingness to Pay Be Measured? An Empirical Comparison of State-of-the-art Approaches[END_REF], with an open-ended question. Specifically, respondents indicated the price that they would be willing to pay for a EcoLaptop. Finally, purchase intentions were assessed with a dichotomous scale, to collect respondents' choices to purchase or not purchase the EcoLaptop (see Appendix 2).

Factor analysis and scale reliability

A factor analysis with Varimax rotation of the IRP, FPR and EPB items generated three factors with eigenvalues greater than 1. All items loaded above .7 on their intended factors, and no item loaded higher than .28 on a different factor [START_REF] Hair | Multivariate Data Analysis[END_REF]). The extracted factors fit the three expected variables. The Cronbach's alphas for IRP, FPR and EPB were .82, .81 and .78, respectively.

Results

Preliminary analyses

To assess the ecological message framing manipulation, we examined its influence on EPB.

Participants reported different ratings according to the three conditions-no message, negative ecological message, or positive ecological message. In an analysis of variance, these manipulations received support (F = 11.78, p < .001; Mabsence = -.36, SD = .94; Mnegative = -.01, SD = .95; Mpositive = .37, SD = .98). In the Tamhane T2 post-hoc comparison test, the mean differences were significant for the no message condition and both messages (p < .05 for negative, p < .001 for positive). As expected, the respondents perceived ecological benefits as higher in the positive condition than in the negative one (p < .05).

A comparison of the undergraduate and postgraduate student respondents included in the sample also ruled out the existence of an answer bias, because the mean value of WTP did not differ across these two groups (t = .33, p = .74; Mundergrad = .03, SD = .99; Mpostgrad = -.01, SD = 1.00). The two groups also were similar in the other responses we measured: IRP (t = -1.26, p = .21; Mundergrad = -.11, SD = .92; Mpostgrad = .05, SD = 1.03), FPR (t = .14, p = .89; Mundergrad = .01, SD = .95; Mpostgrad = -.01, SD = 1.02), EPB (t = 1.42, p = .16; Mundergrad = .12, SD = .96; Mpostgrad = -.06, SD = 1.02), and purchase intention (Pearson's  2 = .17, p = .68).

Direct influences

We relied on linear regressions to test H1-H3. Not only did IRP have a negative influence on FPR (β = -.18, t = -2.91, p < .01) but FPR also exerted a negative effect on EPB (β = -.26, t = -4.39, p < .001), in support of H1b and H2b. In another linear regression, with IRP, FPR, EPB, and their interactions as the independent variables and WTP as the dependent variable (Table 1), both IRP and EPB significantly influenced WTP (β = .67, t = 14.81, p < .001; β = .12, t = 2.59, p < .05, respectively). Specifically, higher levels of IRP and EPB led to higher WTP, in support of H1a and H3. Furthermore, WTP was significantly predicted by FPR (β = -.13, t = -2.87, p < .01), such that higher levels of FPR decreased WTP, in support of H2a. As we expected, when IRP was the independent variable, it had the greatest influence on WTP. The interaction terms of IRP and FPR and of IRP and EPB were not significant, whereas the interaction between FPR and EPB was a significant predictor of WTP (β = -.09, t = -2.26, p < .05). Higher ecological benefits intuitively are associated with greater financial perceived risk, because consumers are unsure of the quality of the newly introduced green product. In this case, the interaction term reduces WTP.

[Insert Table 1 about here]

WTP as a mediator

To assess the mediating role of a consumer's WTP, we used the PROCESS macro developed for SPSS [START_REF] Hayes | Introduction to Mediation, Moderation, and Conditional Process Analysis: A Regression-Based Approach[END_REF]. One constraint of this macro is that it can specify only a single independent variable. Therefore, we executed it separately for each independent variable in our model (IRP, FPR and EPB), leaving the others as covariates.

[Insert Table 2 about here] First, for the mediation of WTP between IRP and purchase intention, we determined a significant influence of IRP on WTP (a = .632), as we detail in Table 1. With a binary logistic regression model, we also examined the direct effect of WTP on purchase intentions (b = .908). As expected, WTP significantly predicted purchase intentions, such that greater WTP increased purchase intentions. In addition, IRP indirectly influenced purchase intentions through its effect on WTP. The bias-corrected bootstrap confidence interval for the indirect effect (ab = .574), based on 1,000 bootstrap samples, excluded zero (.161-1.066). Finally, the direct effect of IRP on purchase intention was not significant (c' = .211, p = .350), so we had no evidence that IRP influenced purchase intentions independent of its effect on WTP, suggesting an indirect-only mediation [START_REF] Zhao | Reconsidering Baron and Kenny: Myths and Truths about Mediation Analysis[END_REF].

Second, in the mediation analysis we conducted to test if FPR indirectly influenced consumers' purchase intention through its effect on WTP, we found a significant influence of FPR on WTP (a = -.138). The direct effect of WTP on purchase intentions, as we noted previously, revealed a significant influence. Furthermore, IRP indirectly influenced purchase intentions (ab = -.125, bootstrap confidence interval: -.316 to -.030), and its direct effect was also significant (c' = -1.065, p = .000). Thus we found a less central role of WTP in this case (complementary mediation).

Third, regarding the mediating role of WTP between EPB and purchase intentions, we determined that EPB significantly influenced WTP (a = .110). The confidence interval of the indirect effect (ab = .100) was entirely above zero (.007-.297). The direct influence of EBP on purchase intention was not significant (c' = -.084, p = .654). Thus we found indirect-only mediation. Overall, these results offered consistent support for all the elements of H4.

Price framing as a moderator

In H5, we predicted that the relationship between WTP and purchase intention changes with price framing. Therefore, we created three groups, reflecting the three experimental manipulations: euros-a-day, euros-a-month, and aggregate price. With hierarchical logistic regression analyses, we tested for the null hypothesis that the relationship between WTP and purchase intention would not vary across price frame groups. The findings in Table 3 (Models 1 and 2, N = 262) reveal that the chi-square statistic changed significantly (Δχ² = 17.381, p < .001) when we included the two product terms that reflected the interaction between WTP and the price frame dummy variables (euros-a-day and euros-a-month) in Model 1. That is, we affirmed a moderating effect of price framing. In Model 2, each additional unit of WTP increased purchase intentions in the aggregate price condition (β = 4.471). The interaction terms had negative influences on purchase intentions though (β = -3.761 and -3.392). Thus, the relationship between WTP and purchase intentions did not grow stronger when the price was disaggregated.

In supplementary analyses, we also compared responses for the per day and per month conditions (Table 3, Models 3 and 4, N = 173), as well as those obtained in the per month and aggregated price conditions (Table 3, Models 5 and 6, N = 177). The WTP-purchase intention relationship was not significantly different in the first comparison (Δχ² = .742, p = .389), but it differed in the second (Δχ² = 12.454, p = .000, β = -3.392).

[Insert Table 3 about here]

Ecological message framing as a moderator

We predicted that ecological message framing would moderate the relationship between WTP and purchase intention. To test H6, we created three groups, corresponding to the experimental conditions (no message, positive message, negative message), and applied hierarchical logistic regression analyses. The findings in Table 4 (Models 1 and 2, N = 260) reject the null hypothesis that the variation in χ² is attributable to hazard (Δχ² = 9.623, p = .008), in support of a moderating effect of ecological message framing.

In complementary analyses, we also tested whether the WTP-purchase intention relationship would be enhanced in the presence, rather than the absence, of ecological message claims. The results of the comparisons of no message with both positive (Table 4, Models 3 and 4, N = 172) and negative (Table 4, Models 5 and 6, N = 176) messages were consistent with this view (Δχ² = 5.963, p = .015; Δχ² = 7.058, p = .008, respectively).

Comparisons of the positive with the negative message condition (Table 4, Models 7 and 8, N = 172) also revealed that the WTP-purchase intention link was not stronger in the positive message frame (Δχ² = .040, p = .841).

[Insert Table 4 about here]

General discussion

This study has examined the determinants and consequences of consumer WTP for a green product, a relatively underresearched area. By considering the effects of both price and ecological message frames, we extend knowledge of framing effects on consumers' perceptions, within the WTP process, and thus produce several theoretical and practical insights.

First, most WTP studies explore measurement methods or examine variability across different types of products (Hamzaoui-Essoussi and Linton 2010). We go further by exploring a mediating role of WTP within the consumer decision-making process for a green product.

We specify that WTP depends on IRP and the trade-off between FPR and EPB. As expected, all three elements are significant predictors of WTP; in particular, the variables associated with price (IRP and FPR) have substantial effects. The findings thus affirm prior studies that focus on the price-product evaluation relationship to explain perceived value [START_REF] Beneke | The Influence of Perceived Product Quality, Relative Price and Risk on Customer Value and Willingness to Buy: A Study of Private Label Merchandise[END_REF] or WTP [START_REF] Dodds | Effects of Price, Brand and Store Information on Buyers' Product Evaluations[END_REF]. They also reinforce the positioning of IRP as a close antecedent of WTP [START_REF] Chandrashekaran | The Implication of Individual Differences in Reference Price Utilization for Designing Effective Price Communications[END_REF]. Furthermore, because EPB exerts the lowest influence on WTP, it appears that green product attributes, even if perceived as important, cannot compensate for high perceived costs [START_REF] Carrigan | The Myth of the Ethical Consumer-Do Ethics Matter in Purchase Behaviour?[END_REF].

Second, we confirm the crucial role of WTP for marketing [START_REF] Acquisti | Do Interruptions Pay Off? Effects of Interruptive Ads on Consumers' Willingness to Pay[END_REF]: It imposes a full mediating effect on the consumer purchase decision process (i.e., among IRP, EPB, and purchase intentions), which suggests that consumers rely on price, in the form of WTP, to make purchase decisions about a new green product. This evidence can help marketers determine demand for a new offer, such as a green laptop, and derive ways to shape buying decisions. Our finding also confirms the explanatory power of WTP as an indicator of demand, which reflects the trade-off consumers make. That is, identifying WTP offers an effective approach to determine the price a company should charge for its products [START_REF] Acquisti | Do Interruptions Pay Off? Effects of Interruptive Ads on Consumers' Willingness to Pay[END_REF] and to maximize the fit between firm profitability goals and consumer preferences and expectations. The managerial impact of WTP should be substantial, because 'it is one of the strongest indicators of brand loyalty and may be the most reasonable summary measure of overall brand equity' [START_REF] Li | Luxury Fashion Brand Consumers in China: Perceived value, Fashion Lifestyle, and Willingness to Pay[END_REF]Kambele 2012, 1517).

Third, marketers stand to gain from matching consumers' WTP with the right price, as well as framing the offer appropriately. Price framing significantly moderates the relationship between WTP and purchase intention. Perhaps less intuitively, the intensity of this effect decreases when the price is disaggregated. This finding confirms suggestions that a per day price framing might be ineffective in some conditions (e.g., [START_REF] Gourville | The Effect of Implicit Versus Explicit Comparisons on Temporal Pricing Claims[END_REF][START_REF] Gourville | The Effects of Monetary Magnitude and Level of Aggregation on the Temporal Framing of Price[END_REF]. We proffer two possible explanations. On the one hand, consumers likely use aggregate prices to derive their WTP for a product such as the EcoLaptop. If they are exposed to an offer in euros-a-day or euros-a-month form, they must perform mental calculations to match their WTP with the advertised price. This situation implies a strong disturbance in their judgment and purchase decision processes; they likely need time to analyze the prices and recover reference points, especially for a new green product. On the other hand, consumers could have been disturbed by a per day payment indication, particularly for a laptop-a product for which the price usually appears in aggregate form. This outcome is relevant, irrespective of whether the focal product is a green or a traditional version.

Fourth, we find a moderating effect of ecological messages on the WTP-purchase intention relationship: This effect is always stronger in the presence of an ecological message claim rather than in its absence. Thus, message claims appear highly relevant in advertising for green products [START_REF] Davis | Strategies for Environmental Advertising[END_REF][START_REF] Hartmann | Green Advertising Revisited -Conditioning Virtual Nature Experiences[END_REF][START_REF] Kang | Consumers' Willingness to Pay for Green Initiatives of the Hotel Industry[END_REF]). Yet the influence of WTP on purchase intention is not higher in the positive, compared with the negative, ecological message condition; rather, it appears slightly stronger in the negative condition, though this difference is not statistically significant. This finding challenges previous research (e.g., [START_REF] Biswas | Consumer Choices Under Product Option Framing: Loss Aversion Principles or Sensitivity to Price Differentials?[END_REF] that implies positive attribute framing consistently leads to more favourable evaluations. Other studies instead find no difference across framing conditions or indicate some benefits of negative framing [START_REF] Donovan | Positively Versus Negatively Framed Product Attributes: The Influence of Involvement[END_REF]. According to [START_REF] Maheswaran | The Influence of Message Framing and Issue Involvement[END_REF], in high-involvement conditions, respondents process messages in more detail and assign disproportionate weight to negatively framed information, such that the negative frame becomes more persuasive. This explanation is convincing for our study, which refers to a product category that is well known by young consumers. Alternatively, the respondents could have judged the ecological message claim as exaggerated and excessive in the positive condition, such that it seemed less trustworthy than the negative message. Consumers remain distrustful of many environmental claims [START_REF] Auger | What Will Consumers Pay for Social Product Features?[END_REF][START_REF] Grimmer | Company Environmental Performance and Consumer Purchase Intentions[END_REF]. [START_REF] Becker-Olsen | The Impact of Perceived Corporate Social Responsibility on Consumer Behavior[END_REF] and [START_REF] Kang | Consumers' Willingness to Pay for Green Initiatives of the Hotel Industry[END_REF] highlight the possibility that customers' WTP for green initiatives could turn negative if they perceive a profit motive or company interest other than an ecological one.

More developments should improve understanding of this result; at this point, managers must consider the use of green claims carefully [START_REF] Schlegelmilch | The Perils and Opportunities of Communicating Corporate Ethics[END_REF].

Limitations and directions for further research

The results of this study should be considered exploratory, in a stream of research (WTP for green products) in which empirical work is sporadic and insufficient. It also suffers from some limitations; additional empirical investigations are required to confirm the WTP formation process and purchase intentions toward green products. For example, the conceptual framework introduces only one type of perceived risk (financial), though perceived product performance also could be a major barrier to green products' selection.

New theoretical developments related to framing strategies could improve our understanding of the reasons consumers process prices thoroughly-or choose not to process them.

From a methodological perspective, the use of a student sample has been criticized [START_REF] Lynch | Theory and External Validity[END_REF]), though Winer (1999) argues that external validity is less of much concern when researchers can show that experimental results are truly due to the manipulations. The choice of a homogeneous sample improves the internal validity of the results and strengthens the theory test (Bergmann and Grahn 1997;[START_REF] Calder | Designing Research for Application[END_REF], which is why student samples continue to be used widely to test marketing theories [START_REF] Hirunyawipada | Consumer Innovativeness and Perceived Risk: Implications for High Technology Product Adoption[END_REF]. A student sample also is appropriate in specific situations, such as when students represent an important market segment-as is true for laptops. We predict that the respondents in our study would be very interested in an offer for an EcoLaptop. Students generally are interested in both technology and the environment, so our main results likely apply to other consumers with similar profiles. In contrast, consumers less involved in technology may attribute less weight to FPR in their trade-off, which may increase their WTP.

Among consumers less concerned about the environment, the EPB might take less weight in the trade-off, though our data already indicate a smaller influence of EPB on WTP. In this case, our conclusion seemingly would remain valid across samples.

We also relied on unobservable cognitive measures, which are classic tools. The measure of WTP still may be subject to hypothetical and other biases [START_REF] Morrison | Testing the Effectiveness of Certainty Scales, Cheap Talk, and Dissonance-Minimization in Reducing Hypothetical Bias in Contingent Valuation Studies[END_REF], which could be attenuated with Vickrey auctions or a demand-revealing method.

Finally, self-generated validity theory stipulates that question order could influence respondents' judgments. [START_REF] Podsakoff | Common Method Biases in Behavioral Research: A Critical Review of the Literature and Recommended Remedies[END_REF] suggest counterbalancing the question order; for this study, we only placed the measures of the dependent variables before those of the independent variables. Although this ordering represents a limitation, it also offers the best alternative, in that any other approach would have induced a primary disadvantage: 'it may disrupt the logical flow and make it impossible to use the funneling procedure (progressing logically from general to specific questions) often recommended in the survey research literature' (Podsakoff et al. 2003, 888).

  , they are unlikely to spend more for green products, about which they still know little. Using this reasoning, such that FPR represents an antecedent of EPB, we propose: H2: Higher FPR leads consumers to indicate lower (a) WTP and (b) EPB.

Figure 1

 1 Figure 1 contains the conceptual model, in which we summarize the predicted

  price of the EcoLaptop (when it is not promoted).IRP2 the most likely price you would find the EcoLaptop. purchasing of the EcoLaptop for use at home would be a bad way to spend my money.FPR2. If I bought the EcoLaptop for myself for use at home, I would be concerned that the financial investment I would make would not be wise. FPR3. If I bought the EcoLaptop for myself for use at home, I would be concerned that I really would not get my money'advantage of the EcoLaptop is that it is ecological. EPB2. The EcoLaptop is interesting because it allows for energy savings. EPB3. Thanks to the EcoLaptop, I can protect the environment. Willingness to pay* WTP. What price you would be willing to pay for the EcoLaptop? Purchase Intention*** PI. I will buy the EcoLaptop. * Open-ended item. ** Items assessed on seven-point Likert-type subscales, with 1 = 'strongly agree' and 7 = 'strongly disagree.' *** Dichotomous item.

[START_REF] Kühberger | The Influence of Framing on Risky Decisions: A Meta-Analysis[END_REF] argues that a loose framing goes beyond semantic manipulation and may result from other contextual features of the decision situation.


Céline Paeye

Thérèse Collins

Patrick Cavanagh

Evidence for trans-saccadic fusion

Introduction

In 1983, several studies 1-3 reported the failure of trans-saccadic fusion -the integration of pre-and post-saccadic percepts across saccades. We re-investigated this issue and addressed two potentially problematic aspects of the stimuli used in these previous studies:

• We used simpler objects where spatial alignment was not critical • We reduced the contrast of the post-saccadic stimulus to minimize the possibility that it might mask the pre-saccadic percept. In addition, we compared trans-saccadic perceptual integration with within-fixation integration 4 (i.e. the two successive stimuli were presented across a saccade or within a single fixation).

Results

Main Expt.: Integration response analysis

Conclusion

With the contrast of the post-saccadic stimulus set to each participant's threshold in a subsequent test, integration reports were more frequent (58.75% of the trials on average) than succession (35.33% of the trials) (Chi²(1) test, p<0.001).

Three embedded, independent staircases were used to converge on the contrast that maximized the frequency of reporting integration. The resulting threshold contrasts were generally very low. Contact: celine.paeye@gmail.com Irrespective of whether a saccade occurred between the successive presentation of two simple stimuli, participants most often integrated the two stimuli into a new, composite percept, provided that the second stimulus did not mask the first. This can explain why previous studies failed to find trans-saccadic fusion.

In the saccade condition, because participants often reported seeing integration rather than a succession of percepts, we believe that this integration was a perceptual phenomenon.

In addition, the systematic directional biases are compatible with remapping. Presumably, remapping could provide a prediction of the vertical line that could be integrated with the horizontal lines to give rise to the fused percept. An efference copy with a gain less than 1 predicts a directional bias. This short-lived fusion occurs in spatiotopic coordinates, suggesting that the prediction provided by remapping might help the visual system cope with the shifts in retinal coordinates produced by each saccade and contribute to our experience of visual stability.

  Second stimulus: 6-17 ms, median: 12 ms Critically, participants had 4 answer options: -A vertical line on the left side of the horizontal ones -A vertical line on the right side of the horizontal ones -Horizontal lines only, that succeeded the vertical one -No horizontal lines • Main experiment: One third of the participants (N=6) performed two experimental conditions: saccade & fixation. The actual position of the first stimulus (a vertical line) varied between the left and right extremities of the second one (3 horizontal lines): • Pre-test: Determination, via an adaptive staircase procedure, of best individual postsaccadic stimulus contrasts for reporting a perceptual integration in a subsequent test (N=17). Pre-test: best post-saccadic stimulus contrast Question: What did you see at the end of your eye movement? Integration of pre-and postsaccadic percepts Hypotheses With the contrast of the second stimulus adjusted individually, perceptual responses might depend on the: -Experimental condition -Actual position of the vertical line wrt the horizontal ones -Saccadic landing position.

  Succession of percepts (the 1 st was not visible any more) pre-test & main expt.) Fixation condition (main expt.) Response distributions in the saccade and fixation conditions were not statistically different (Chi²(2)=5.454, p=0.065). Integration responses did not depend on the position of the first vertical line: first vertical line wrt the center of the second stimulus (deg) • Neither in the saccade condition: (Chi²(4)=1.004, p=0.909)• nor in the fixation condition:(Chi²(4)=0.036, p=0.999) function of the saccade direction (or side of stimuli presentation in the fixation condition) were systematic in the saccade condition (Chi²(1)=99.613, p<0,001), but not in the fixation condition (Chi²(1)=0.021, p=0.885).

  1-Bridgeman, B., & Mayer, M. (1983). Failure to integrate visual information from successive fixations. Bulletin of the Psychonomic Society, 21, 285-286. 2-Irwin, D.E., Yantis, S., & Jonides, J. (1983). Evidence against visual integration across saccadic eye movements. Perception & Psychophysics, 34, 49-57. 3-O'Regan, J.K., & Lévy-Schoen, A. (1983). Integrating visual information from successive fixations: Does trans-saccadic fusion exist? Vision Research, 23, 765 4-Di Lollo, V. (1980). Temporal integration in visual memory. Journal of Experimental Psychology: General, 109(1), 75-97. This work was supported by the ERC Position (PC) Pdf file of this poster:


Karen Tatarian

Amit Marine Chamoux

Kumar Pandey

Mohamed Chetouani

Amit Kumar Pandey
email: amit.pandey@hansonrobotics.com

Conversational Role Coordinator for Groups in Human-Robot Interaction

Keywords: Social Intelligence, Proxemics, Footing, Situational Assessment

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Introduction

Among research on human-robot-interaction there is increasing interest in how robots should act in order to be perceived as socially intelligent. Social intelligence can be comprised of many factors, such as how the robot acts, speaks, and gestures. However, one element of robot social intelligence that has so far received little attention is proxemics. In human-human interaction, proxemics is defined as the study of the human use of space and its effects on the behavior, communication and social interactions [START_REF] Hall | A system for the notation of proxemic behavior1[END_REF]. A related concept is footing, which is used to describe the set-up of a conversation and the role of the participants in it [START_REF] Goffman | Behavior in public places: notes on the social organization of gatherings. Social theory[END_REF]. For example, participants can be either a speaker, listener, or bystander (not actively participating).

In human robot interaction, a social robot is not an isolated agent but rather an active participant in social interactions and conversations. As such, it needs to be able to identify the different roles of participants. In addition, it should be able to recognize its own role and adapt accordingly, whether as a speaker, listener, or bystander.

Accordingly, this paper presents a footing framework that coordinates the conversational roles for multi-party human robot interaction. The system takes into account the position, orientation, gaze, and speech of the participants and combines them with the governing theories of proxemics for situational assessment. The final output is the status of all participants around the robot in addition to the status of the robot itself. A study was conducted to better understand whether footing knowledge and status coordination for multi party interactions increase group affiliation and perceived social intelligence of a social robot.

Related Work

For a robot to be perceived as a socially-intelligent agent, it must be able to hold a successful social interaction, adapt to the social environment, and exhibit appropriate multi-modal behavior. For this reason, the robot is required to have the skills to understand and reason about the environment, not just from the perspective of the physical locations of the objects, but also from that of the 'mental' and 'physical' states of the human participants in the interaction [START_REF] Pandey | Romeo2 project: Humanoid robot assistant and companion for everyday life: I. situation assessment for social intelligence[END_REF].

There are three levels of situation awareness. The first level is perceiving the state of the elements found in the environment. The second level is building a goal the user wants to achieve based on the comprehension of the information inferred from the first level. Finally the third level is projecting on the future based on the perception and comprehension of the current situation [START_REF] Endsley | Toward a theory of situation awareness in dynamic systems[END_REF]. Based on this model, a generalized perception system has been proposed aiming to achieve an effective and more natural multi-modal human robot interaction [START_REF] Pandey | Romeo2 project: Humanoid robot assistant and companion for everyday life: I. situation assessment for social intelligence[END_REF].

In addition, proxemics and footing have been an increasingly studied phenomena in human-agent interactions, where the agent can be either virtual or physical. For instance, for virtual agents, the interest of better understanding footing using nonverbal signals is highlighted in [START_REF] Lee | Modeling Side Participants and Bystanders: The Importance of Being a Laugh Track[END_REF] and [START_REF] Pejsa | Who, me? how virtual agents can shape conversational footing in virtual reality[END_REF]. In [START_REF] Lee | Modeling Side Participants and Bystanders: The Importance of Being a Laugh Track[END_REF], human interactions were studied to model how footing and the different roles of a multi-party conversation affect the behavior of the participants.

Background

This section introduces a few definitions needed to better understand the footing framework introduced in Section 4 and represented in Figure 1. In addition, the interpersonal distances, as introduced by Hall [4], suggest that the limit of the personal distance, where interaction with friends takes place, is around 1.2 meters and the limit of the social distance, which is the space for formal interactions, is around 3.6 meters. However, any distance beyond that is considered a public space. Transactional Segment: In [START_REF] Shi | Spatial formation model for initiating conversation[END_REF], the transactional segment is defined as the half circle around the forward direction of the person with a radius of 2 meter. This identifies an object in that segment as the person's implicit attentional target. F-formation: When Kendon in [START_REF] Kendon | Conducting Interaction: Patterns of Behavior in Focused Encounters[END_REF] studied organizational patterns of social encounters, he defined the term F-formation, which is when two or more people adjust their spatial and orientation relationship in order to have equal, direct, and exclusive access. Furthermore, Schegloff [START_REF] Schegloff | Body torque[END_REF] also linked proxemics to the intentions of the participants. Front Zone:In estimating the participation status of a person based on observations of human interaction, [START_REF] Shi | Spatial formation model for initiating conversation[END_REF] defined the front zone as the area across an angle of 120 degree from the front of the person. Moreover, it was concluded that there is a perceived obligation to participate in a conversation when people are in each other's front zones. Gaze Zone: When two people fall in each other's gaze zones thus having their gazes meet, there is now an obligation for the participation in a conversation [START_REF] Shi | Spatial formation model for initiating conversation[END_REF].

Footing Framework

The inputs needed for the framework are the distance between the robot and each participant, the orientation of each participant with respect to the robot, the angle between the robot and each participant, and the attention of each participant around the robot (whether they are looking straight at robot or to the sides or up or down). First, the transactional segment, defined for this framework, is a half circle in front of the robot with a radius of 1.6 meters. The radius length was adjusted (from the one defined in Section 3) based on the data collected from human interactions with the robot Pepper and in the intention of it being within the inner circle of the social distance defined by [START_REF] Hall | The Hidden Dimension[END_REF]. In addition, it is important to have the transactional segment within the field view of the robot. Thus, any participant within the transactional segment is considered to have the intentions to interact (whether as addresser/addressee or bystander) and any participant outside this segment has the role of an overhearer. Second when an f-formation is detected based on the position and orientation of the participants, they are directly given the status of addresser/addressee. In addition, if there are any participants not part of the f-formation detected but still within the transactional segment, they are given the bystander status. Third, for this framework, the front zone refers to the 120 degree fan-shaped area with a 1.6 meters in front of the robot. Any participant in this zone can either be an addresser/addressee or a bystander depending on the gaze zone priority, but the rest of the participants outside this area now get assigned the bystander status. Fourth, the gaze zone here is a 30 degree fan-shaped area with a 1.6 meter distance. The participants who fall in this area get the priority of being assigned the active status, which is also referred to addresser/addressee, and all other participants within the transactional segment get the bystander status. Finally, a few rules are added to the framework. First, if at least one participant is assigned the active status, the robot gets an active status as well. Second, if the direction of orientation of the participants is facing each other and not the robot, the robot is now assigned bystander status and must act accordingly. Third, if one participant's direction of orientation is facing the robot while the others do not and speech is detected, the participant facing the robot gains priority of being an addresser/addressee.

Evaluation Experiment

A pilot study of 16 participants (average age of 27) was conducted as a preliminary evaluation of the proposed framework for group interactions. The main purpose is to answer the following research question "Does proper footing knowledge and status coordination for multi party interactions increase group affiliation and perceived social intelligence of a social robot?". In this study, a group of two participants plus a robot (forming a triadic interaction) are engaged in a trivia game with the robot that lasted on average 15 minutes. Once the interaction was over, the participants were asked to answer an ALMERE questionnaire. The ALMERE questionnaire was designed to assess the acceptance of assistive social agent technology [START_REF] Heerink | Assessing acceptance of assistive social agent technology by older adults: the almere model[END_REF]. The questionnaire includes the following constructs: anxiety (ANX), attitude towards technology (ATT), perceived adaptability (PAD), perceived enjoyment (PENJ), perceived sociability (PS), and social presence (SP). In addition, the positioning and orientation of the participants were also measured to analyze how the dynamics of the group interaction differs when the robot has the footing framework suggested. However, for this paper, the focus is on the results from the ALMERE questionnaire.

Study Design & Hypothesis

This study employs a 2×3 independent groups design. A behavior with the footing status framework (Condition1) and a control condition with out-of-the-box basic awareness (Condition 2) were manipulated as between groups independent variables. The participants were assigned randomly to the conditions. In addition, there are three situations that occur throughout the interaction: situation with two addressees, situation with one addressee and one bystander, and situation one bystander and one addressee. The situations are manipulated as within group independent variables. The order of the three independent variables in each condition are random to counterbalance. Perceived intelligence and sociability of the robot are measured as dependent variables. We hypothesize that the robot with the footing framework for conversational roles coordinator will score higher on the ALMERE survey specially for the perceived sociability, social presence and perceived adaptability constructs.

Set-Up & Experiment

Each interaction required a group of two participants at a time to play a trivia game. The trivia game consists of general knowledge questions to which the players have to answer by either 'True' or 'False'. It is made up of three levels not based on difficulty but rather on whether it is one player or two players in order to simulate the three situations mention in subsection 5.1. The first level is for two players (two addresses) and the second and third levels are for one player where the participants have to switch turns among each other (one addressee and one bystander). During levels two and three, the non-playing participant is invited to observe the other participant play and was given the freedom to move around as he/she wants. At the beginning, the robot would greet the players and at the end of the game say goodbye and thank them for playing. Moreover, the participants were divided randomly among condition 1 and condition 2 of the experiment. In condition 1, the robot uses the suggested framework in section 4 and adjusts it's gaze patterns accordingly to conform to the roles the participants have chosen based on their position and orientation. In condition 2, Pepper uses its basic awareness API, which is already embedded in it. In basic awareness, the robot studies its own surroundings and once a human is detected, the robot keeps tracking him/her. Basic awareness allows the robot to focus on only one human until another is detected.

Results

After the trivia game was over, the participants were asked to fill the ALMERE survey. For the overall evaluation score, shown in Figure 4, a repeated measures ANOVA was conducted. A significant main effect was found (p =0.0394) with the condition of the footing framework scoring higher on perceived sociability, social presence, and perceived adaptability. Therefore, the hypothesis is supported. In addition, the participants were asked to rate an extra set of questions from Extra Questions Did you feel attended to by the robot? Was the robot giving you enough attention? Did you feel that the robot was looking at you? Do you think the robot considered you as an important player? Table 1. Extra questions for evaluation Fig. 5. Result of extra questions 1 (strongly disagree) to 5(strongly agree). The questions are found in Table 1 and Figure 5 shows the evaluation of the result. The robot with the footing framework condition scored much higher than the robot with basic awareness. This shows that the participants who interacted with the robot, which was able to adjust its gaze patterns to their conversational roles, felt more attended to by the robot.

Discussion & Conclusion

In the results, we see that the robot for both conditions performed similarly in context of social presence and perceived enjoyment. These results are not surprising since it was the same robot with the same design and features. However, the robot with the footing framework scored higher in the perceived sociability section and showing that the robot with this condition had a more socially intelligent behavior. The robot with the footing framework also scored much higher on perceived adaptability indicating that the robot did adapt and conform to the change in the conversational roles throughout the interaction. Third, when looking at the attitude construct, once again the robot with the footing framework performed better with users rating a more positive feeling and attitude about the application of this robot. Finally, the users also rated that they were less anxious when interacting with the robot with the footing framework. Moreover, to further gather qualitative insight, the participants were asked to answer four extra questions shown in Table 1. The ANOVA results, in Figure 5, indicate a significant effect (p = 0.045) and the Footing framework scored higher. This gives an indication that in this condition, the users felt more attended to. This work is still in its early stages and with the pilot study results supported the preliminary hypothesis. The next steps include looking at the quantitative measures to better understand how the dynamics of the group interaction with this footing framework differs from one without. It is also interesting to see the effect of having additional social cues in the gaze mechanism, such as turntaking and floor-holding, and/or proactive adjustment of the robot's position in the conversation. The suggested framework still needs to be optimized for it to be more adaptive and increase the quality of interaction further.

Acknowledgment

This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 765955.

Fig. 1 .

 1 Fig. 1. Representation of the definitions of Transactional Segment, Front Zone, and Gaze Zone respectively.

Fig. 2 .

 2 Fig. 2. Overall system

Fig. 3 .

 3 Fig. 3. Framework for Footing in Human-Robot Interaction

Fig. 4 .

 4 Fig. 4. Overall evaluation of ALMERE questionnaire


Munirah D Albaqami

Asma Alothman

Ayman Nafady
email: anafady@ksu.edu.sa

Aqeel Shymaa S Medany

Ahmed Shah

Mazhar Umair Aftab

Hussain Ibupoto

Arfana Begum Mallah

Aneela Tahira

Matteo Tonezzer
email: matteo.tonezzer@cnr.it

Aqeel Ahmed

Mazhar Hussain Ibupoto

Brigitte Vigolo

Zafar Hussain Ibupoto

PhD Zafar Hussian Ibupoto

Utilization of polyvinyl amine hydrolysis product in enhancing the catalytic properties of Co3O4 nanowires: Toward potentiometric glucose bio-sensing application

Keywords: Hydrothermal method, Nucleation and Growth, Nanostructure, Oxidation

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Glucose is a fundamental need of living objects. The physiological glucose concentrations are in the range of 79.2-110 mg dl -1 at the fasting conditions in the human blood. The abnormal glucose concentration is an indicator of several metabolic problems and is considered as main causing agent for the different diseases including diabetes, obesity and cardiovascular. Therefore, the monitoring of glucose in human body is highly important 1 in order to prevent the life taking diseases. Several efforts have been put forwarded in this direction for the quantification of glucose with excellent sensitivity, efficiency, and swift response from the vitro and vivo conditions. The glucose biosensors have been developed and used for the detection of glucose including optical devices [START_REF] Yang | Optical property and adsorption isotherm models of glucose sensitive membrane based on prism SPR sensor[END_REF] , colorimetric [START_REF] Pang | Colorimetric detection of glucose based on ficin with peroxidase-like activity[END_REF] , fluorimetry [START_REF] Liu | Fluorometric and colorimetric sensor array for discrimination of glucose using enzymatic-triggered dual-signal system consisting of Au@Ag nanoparticles and carbon nanodots[END_REF] , amperometry [START_REF] Guascito | Amperometric non-enzymatic bimetallic glucose sensor based on platinum tellurium microtubes modified electrode[END_REF] , and potentiometric methods [START_REF] Ibupoto | Effect of Urea on the Morphology of Co3O4 Nanostructures and Their Application for Potentiometric Glucose Biosensor[END_REF] since several decades.

Among them, the electrochemical methods are versatile for the detection of glucose due to their numerous advantages such as high sensitivity, fast in response, stability, reproducibility and simplicity. Generally, glucose biosensors have been fabricated by using enzymatic approach [START_REF] Adeel | Recent advances of electrochemical and optical enzyme-free glucose sensors operating at physiological conditions[END_REF][START_REF] Wilson | Glucose oxidase: an ideal enzyme[END_REF] since decades. The use of amperometry mode of electrochemical methods at high current bias produces the interference effect from easily oxidizing agents such as ascorbic acid and uric acid during the glucose sensing. Therefore, the selectivity and destruction of biological cells are the major issues during amperometric methods. In contrast to amperometric glucose biosensors, the potentiometric measurement have high superiority because it does not use any current bias so the issue of selectivity and the destruction of biological samples is prevented during the analysis.

Today, an advancement in the field of nanostructured materials made it possible to manipulate the materials close to the atomic level with outstanding physical and chemical properties. Therefore taking the advantage of high surface to volume ratio of nanostructured materials which can easily and densely host the glucose oxidase enzyme and generate a strong output potential when the glucose oxidation reaction could take. To add this, various precious metal and metal oxide nanostructures with extent sensing performance have be used for the development of glucose sensors [START_REF] Soomro | Highly sensitive non-enzymatic glucose sensing using gold nanocages as efficient electrode material[END_REF][START_REF] Mei | Electrochemical non-enzymatic glucose sensor based on hierarchical 3D Co3O4/Ni heterostructure electrode for pushing sensitivity boundary to a new limit[END_REF][START_REF] Sheng | A highly sensitive non-enzymatic glucose sensor based on PtxCo1-x/C nanostructured composites[END_REF][START_REF] Poyraz | One-step synthesis and characterization of poly (o-toluidine) nanofiber/metal nanoparticle composite networks as nonenzymatic glucose sensors[END_REF][START_REF] Li | Electrodeposition of nickel oxide and platinum nanoparticles on electrochemically reduced graphene oxide film as a nonenzymatic glucose sensor[END_REF][START_REF] Hoa | A highly sensitive enzyme-free glucose sensor based on Co3O4 nanoflowers and 3D graphene oxide hydrogel fabricated via hydrothermal synthesis[END_REF][START_REF] Chowdhury | Binderless Solution Processed Zn Doped Co3O4 Film on FTO for Rapid and Selective Non-enzymatic Glucose Detection[END_REF][START_REF] Xu | Phase separation induced rhizobia-like Ni nanoparticles and TiO2 nanowires composite arrays for enzyme-free glucose sensor[END_REF][START_REF] Baghayeri | Amperometric glucose biosensor based on immobilization of glucose oxidase on a magnetic glassy carbon electrode modified with a novel magnetic[END_REF][START_REF] Ni | Enzyme-free glucose sensor based on heteroatom-enriched activated carbon (HAC) decorated with hedgehog-like NiO nanostructures[END_REF][START_REF] Wu | Nickel plasma modification of graphene for high-performance non-enzymatic glucose sensing[END_REF][START_REF] Ghosh | Tuning the visible photoluminescence in Al doped ZnO thin film and its application in label-free glucose detection[END_REF][START_REF] Wang | MoS2 as connector inspired high electrocatalytic performance of NiCo2O4 nanoplates towards glucose[END_REF][START_REF] Liu | NiCo2O4 nanoneedle-decorated electrospun carbon nanofiber nanohybrids for sensitive non-enzymatic glucose sensors[END_REF][START_REF] Liu | Biochemical properties of a new thermo-and solvent-stable xylanase recovered using three phase partitioning from the extract of Bacillus oceanisediminis strain SJ3[END_REF] . Cobalt oxide (Co3O4)is one of the notable transition metal oxide which exhibits plenty of favorable offers for the development of biosensors such as good biocompatibility, and excellent stability, low cost and can be produced from earth abundant materials [START_REF] Ding | Fluorescent sensing of sulfide ions based on papain-directed gold nanoclusters[END_REF] . These potential properties of Co3O4 have been capitalized for the wide range of electrochemical applications such as electrocatalysis, supercapacitors, batteries and electrochemical sensing devices [START_REF] Dalkiran | Amperometric xanthine biosensors based on chitosan-Co3O4-multiwall carbon nanotube modified glassy carbon electrode[END_REF][START_REF] Haldorai | An enzyme-free electrochemical sensor based on reduced graphene oxide/Co3O4 nanospindle composite for sensitive detection of nitrite[END_REF][START_REF] Zhou | Perforated Co3O4 nanoneedles assembled in chrysanthemum-like Co3O4 structures for ultra-high sensitive hydrazine chemical sensor[END_REF][START_REF] Li | 2D ultrathin Co3O4 nanosheet array deposited on 3D carbon foam for enhanced ethanol gas sensing application[END_REF][START_REF] Kim | Solution-processed n-ZnO nanorod/p-Co3O4 nanoplate heterojunction light-emitting diode[END_REF][START_REF] Qing | Facile synthesis of Co3O4 nanoflowers grown on Ni foam with superior electrochemical performance[END_REF] . The point of care for using Co3O4 based material for the potentiometric glucose biosensor is that it should have high catalytic activity, surface area and can produce a high output potential signal, therefore these properties of Co3O4 should be improved before to use it for glucose bio-sensing applications.

Keeping these challenges in mind, we have used a concept of hydrolyzing the polyvinyl amine as it could produce number of long chain of hydrocarbons, amide and hydroxide ions which further may play a vital role on the control of morphology and catalytic properties of Co3O4 nanostructures. The use of hydrolyzed products of polyamineand their contributing role on the well defined nanostructures ise not reported elsewhere.

In this study, we have used the reactive species produced from the overnight hydrolysis of polyvinyl amine for the growth of controlled Co3O4 nanostructures via hydrothermal method. These Co3O4 nanostructures were physically studied by SEM, EDS and XRD techniques. Then, the glucose oxidase was physically immobilized onto nanowires through electrostatic attraction for the development of potentiometric glucose biosensor. The proposed glucose biosensor has shown the linear response for the glucose concentrations from 0.0005 mM to 6 mM with a limit of detection of 0.0001mM. The fabricated glucose biosensor is stable, reproducible, selective and sensitive, therefore it can be used as an alternative sensing device for the analysis of glucose form real samples.

Materials and Methods

Materials

Cobalt chloride hexahydrate, urea, polyvinyl amine, glucose, uric acid, ascorbic acid, oxidase, 25% glutaraldehyde and saline phosphate buffer of pH 7.3. These chemicals were purchased from Sigma-Aldrich Karachi Pakistan. All the experiments were performed in the prepared phosphate buffer solution of pH 7.3 in deionized water.

Preparation of modified nanostructures of Co3O4 using polyvinyl amine via hydrothermal method

The nanostructures of Co3O4 were prepared by three steps methodology described as under: First, we have hydrolyzed the 0.01M polyvinyl amine in deionized water for overnight as its hydrolysis kinetics is slow. The aqueous solution could contain both the hydrocarbon chains, amide and hydroxide ions. The presence of hydrocarbon chain could accelerate the length of one dimensional Co3O4 nanostructures whereas the concentration of amide and hydroxide ions is enhancing the nucleation rate. The obtained nanostructures of Co3O4 have high surface area, excellent catalytic sites, thus produced an enhanced output potential during potentiometric measurements. The Co3O4 nanostructures were grown the gold coated glass substrate using hydrothermal method and the fabrication of gold coated glass electrode is described in our published work [START_REF] Ibupoto | Development of a disposable potentiometric antibody immobilized ZnO nanotubes based sensor for the detection of C-reactive protein[END_REF] . Secondly, the 0.1M concentrations of cobalt chloride hexahydrate and urea precursors were made in the hydrolyzed solution of polyvinyl amine. Then cleaned gold coated glass substrate was vertically hanged in the beaker. Then precursor's solution was sealed with aluminum sheet and hydrothermally treated at 95 o C for 5 hours in electric oven. Afterwards, the cobalt hydroxide nanostructures were washed several times with the deionized water and ethanol in order to wash out the unreacted amount of hydrocarbon and amide groups from the surface of materials. Then, the nanostructured fabricate substrate was dried in open air for overnight at room temperature. In the third step, we combusted the cobalt hydroxide nanostructures in the muffle furnace for 4 hours.

Finally, we obtained a dark product of Co3O4 and it was used for the structural and functional characterization. The powder of nanostructured material was also obtained by these steps. The pristine Co3O4 sample was prepared without the addition of hydrolyzed products of polyvinyl amine by the similar method. The synthesis process is further described by scheme 1.

Scheme 1:

Illustrates the synthesis process for the production of Co3O4 nanowires in the presence of overnight hydrolyzed polyvinyl amine solution

Characterization and bio-sensing application

The crystalline structure and morphology were studied by the powder X-ray diffraction and scanning electron microscopy techniques. For the structural studies, the Co3O4 material was scratched form the gold coated substrate. The composition of material was analyzed by the energy dispersive spectroscopy equipped with SEM. Scanning electron microscopy was carried out at 3kV with a ZEISS Gemini SEM 500 equipped with field emission gun. XRD was carried out under the conditions using the source of CuKα radiation (λ = 1.54050 Å), current of 45 mA, and an accelerating voltage of 45 kV.

The fabrication of glucose biosensor based on glucose oxidase functionalized Co3O4 nanostructures was as followed:

Prior to the fabrication of glucose biosensor, we dissolved a 10 mg of glucose oxidase in 0.01M phosphate saline solution of pH 7.3 and 100 µL of 5% glutaraldehyde as a cross linker for the prevention of self-enzyme reaction. Then the nanostructures of Co3O4 were dispersed in the enzyme solution for 10 mints and dried for 30 min. Then, glucose oxidase was immobilized onto Co3O4 nanostructures and used as working electrode for the potentiometric determination of glucose in phosphate buffer solution. The potentiometric measurements were performed with two electrode cell set up. The reference electrode was silver-silver chloride (Ag/AgCl) filled with 3M KCl solution. A 0.1M stock solution of glucose was prepared in 0.01M phosphate buffer solution of pH 7.3. The fresh solutions of glucose were prepared in the phosphate buffer solution using dilution method and used for the functional characterization. All the potentiometric measurements were performed at room temperature. All the potentiometric measurements were performed with pH meter from METTLER TOLEDO and dynamic response and sequential experiment was done on the potentiotat supplied from Netherland. The charge transport during potentiometric response was evaluated in 0.5mM glucose solution prepared in phosphate buffer solution of pH 7. The length of platelets may be in the range of few microns and size of self-assembled Co3O4 could be less than 150 nm. However, the overnight hydrolysis of polyvinyl amine has changed the morphology of Co3O4 from platelets to nanowires with a proper orientation as shown in Figure 2cd. The length of polyvinyl assisted Co3O4 could be 5-7 microns and the diameter of nanowires might be around 100 nm. Also, the hydrolyzed polyvinyl amine has shown Co3O4 nanostructures with high surface to volume ratio which is highly important for the loading of glucose oxidase enzyme and resulting an improved output potential signal during the potentiometric analysis. From SEM analysis, it is obvious that the polymer hydrolysis has played a very critical role in tuning the morphology of Co3O4 through the presence of large amount of hydrocarbon chains, and amide groups during the growth process. The growth kinetics of Co3O4 is well governed by the presence of these groups which strengthened the idea of controlling the morphology of metal oxides by using the overnight hydrolysis of polyvinyl amine. The chemical composition of both pristine and polyvinyl amine assisted Co3O4 samples was also investigated by EDS as shown in Figure 3a-b.

Both materials were containing the Co and O as the main elements.

3.2.Potentiometric analysis of glucose using glucose oxidase immobilized polyvinyl amine assisted Co3O4 nanostructures

The glucose oxidase was immobilized on the polyvinyl amine assisted Co3O4 nanowires and they were tested as a working electrode for the potentiometric determination of glucose in 0.01M saline phosphate buffer solution. The working electrode was soaked in saline phosphate buffer solution of pH 7.3 for overnight in order to quantify the stable output potential signal. The glucose oxidase was also immobilized on the pristine Co3O4 for the comparative study for understanding the role of Co3O4 nanowires obtained from the overnight hydrolysis of polyvinyl amine. Figure 4a-b shows the output potential response of pristine and polyvinyl amine assisted Co3O4 and it can be seen that the well-defined morphology with high surface to volume ratio is very important for the development of potentiometric glucose biosensors. The performance of pristine Co3O4 sample is poor in terms of analytical parameters such as regression coefficient, linear range and limit of detection as shown in Figure 4a. However, the performance of polyvinyl amine assisted Co3O4 is highly improved in terms of analytical properties including, regression coefficient of 0.99, wide linear range of 0.0005 mM to 6 mM, and low limit of detection of 0.0001mM. The improved performance of polyvinyl amine assisted Co3O4 could attributed to heavy loading of glucose oxidase, fast electron communication due to proper orientation of nanowires and also a highly biocompatible surface. Importantly, the Nernstian slope of 42 mVdec -1 is also obtained.

The bio-sensing mechanism on the glucose oxidase immobilized Co3O4 nanowires is mainly controlled by the interaction of glucose oxidase with glucose substrate which was further strengthened by the nanowires of Co3O4 as a co-catalyst. The co-catalytic properties of Co3O4 have supported the main glucose oxidase reaction towards the glucose oxidation. The most important reactions involved during the potentiometric measurements are as described. The reaction of glucose on the immobilized glucose oxidase onto the surface of Co3O4 nanowires resulted into the formation of unstable glucono-lactone species, which further gives rise to gluconic acid. Then, gluconic acid interacts with water and produces the gluconate and hydronium ions in the electrochemical cell as shown under [START_REF] Ibupoto | The synthesis of CuO nanoleaves, structural characterization, and their glucose sensing application[END_REF] :

H2O + O2 + β-D-glucose GOD → d-gluconolactone+H2O2 (1) d-glucono lactone spontaneously → gluconate -+ H + (2)
It is the most probable the concentration of hydronium and gluconate ions moving on the surface of nanowires of Co3O4 which produces the output potential during the measurement. Various biosensing parameters were also studied on the glucose oxide immobilized Co3O4 nanowires such as repeatability, reproducibility, dynamic response time, selectivity, and effect of pH and temperature. The repeatability of glucose oxidase immobilized Co3O4 nanowires was investigated via the use of same electrode with span of 12 hour each time as shown in Figure 5. It is clear that the developed electrode has a high potential to maintain the linear range and limit of detection, thus it can be considered as an alternative analytical protocol for the determination of glucose. The reproducibility is another performance evaluation indicator for the bio-sensing device and for this purpose inter-electrode signal was noticed by using ten independent working electrodes fabricated at the same conditions as shown in Figure 6. The inter-electrode study was performed in 1 mM glucose solution. The potentiometric response of each electrode was found less than 3% of relative standard deviation which reveals an outstanding performance of Co3O4 nanowires during the potentiometric analysis of glucose. The storage life of the proposed device was studied for 4 weeks and the performance of Co3O4 nanowires was found acceptable and significant as given in Table 1. The improved performance of biosensor is assigned to high isoelectric point of Co3O4 which strongly bound to the low isoelectric bio-membrane of glucose oxidase. The advantage of using potentiometric strategy is that it prevents the chances of interference form the easily oxidizing species like uric acid and ascorbic acid due to use of zero biasing current during the analysis. The potentiometric response of selectivity is shown in the Figure 7. It can be visualized that the biosensor signal is remained unaltered during the addition of common interfering agents like urea, uric acid, dopamine, cholesterol and ascorbic acid. The same concentration of glucose and interfering species was used during the selectivity study. This confirms that the biosensor is highly selective towards the quantification of glucose. The selectivity of glucose biosensor is only depending on two aspects such as enzyme-glucose reaction and selective signal to the glucose substrate. The enzyme based reactions are highly selective due to specific interaction with biomolecules and the glucose oxidase is known as the best example of enzyme only selective towards glucose. The potentiometric response is highly dependent on the pH of analyte solution and the most consideration during the development of biosensor is that it should be tested under the physiological pH values of human body, therefore we have studied the effect of pH. The pH study was carried out in 1mM glucose concentration. On other hand, the stability of Co3O4 should be verified as the metal oxides do not seem stable at certain pH values. The pH of analyte solution was adjusted using 0.1M HCl and NaOH solutions. The range of pH values between 3 and 13 was selected as shown in Figure 8a.

This study indicates that the Co3O4 nanowires has shown optimum output potential signal around pH 7 which confirms that material has high capability to work under physiological pH values. The signal is low at very high and a low pH values which reveals that Co3O4 material is unstable in highly alkaline and acidic pH conditions 33 . Also, the glucose oxidase enzyme deactivates at the low pH and high pH values, therefore the proposed configuration is most suitable for the quantification of glucose at normal pH values. Furthermore, the environmental conditions like temperature has also significant effect on the output potential signal. For this reason, we also studied the effect of temperature on the response of glucose biosensor as shown in Figure 8b. The activity of biosensor was enhanced with the increase in the temperature due to frequent collision of glucose molecules with immobilized glucose oxidase on the Co3O4 nanowires. The output potential of biosensor was found higher at 40 o C and further increase in the temperature drastically lowered the performance of biosensor due to denaturation of enzyme as shown in Figure 8b.

However, for the ease of measurement, we carried out all the measurements at room temperature.

After the use of biosensor, it was stored at 4 o C in refrigerator in order to prevent the effect of environmental conditions. The dynamic response of time to glucose biosensor was measured in 1mM glucose solution as shown in Figure 9.

The biosensor exhibited response time less than 1 s and revealing high sensitivity towards the detection of glucose. Moreover, the multi-step potentiometric technique was used to record the calibration plot and it was found that with successive addition of glucose the output signal was stable and increasing rapidly as shown in Figure 10. This study confirms that the proposed fabrication technology is accurate and has a high potential as an alternative tool to monitor the glucose levels from real samples. The analytical application of proposed glucose oxidase immobilized Co3O4 nanowires was also performed using standard addition method. This study is further illustrating the important aspect of proposed method about its practical suitability as shown in Table 3.

The comparative analysis of presented glucose biosensor was carried out in terms of different biosensor parameters such as linear range, sensitivity, response time, regression coefficient, limit of detection as given in Table 4. The proposed configuration has better or comparable to some of the recently reported glucose biosensors. Particularly a fast response time, low limit of detection and excellent regression coefficient value of 0.99 are indicating the superior performance of proposed study.

Conclusions

In summary, we have studied the influence of overnight hydrolysis of polyvinyl amine on the morphology and catalytic performance of Co3O4 nanostructures. The material synthesis was carried out by the hydrothermal method. The Co3O4 nanomaterial exhibits nanowire like morphology with high surface to volume ratio, which enabled the high loading of glucose oxidase via electrostatic attraction. The potentiometric response of glucose oxidase immobilized biosensor was probed using 0.01M saline phosphate buffer solution of pH 7.3. The proposed glucose biosensor has demonstrated a wide linear range of 0.0005 to 6 mM, with limit of detection of 0.0001mM. Also, the repeatability, reproducibility, dynamic response time, multistep potentiometric response, effect of pH and temperature were investigated along with storage life time and selectivity. Significantly, the proposed configuration has a high potential to monitor the glucose in real samples with outstanding analytical accuracy. The developed methodology for the preparation of Co3O4 with tuned morphology and enhanced catalytic features could be used as alternative method for the fabrication of a wide range of nanostructured materials for various electrochemical-based applications such as batteries, supercapacitors, water splitting and other biomedical applications.

Scheme 2 :

 2 Scheme 2: Sensing mechanism of glucose ontto cobalt oxide nanowires coated with glucose oxidase

Figure 11

 11 Figure 11 represents the typical Nyquist cruves estimated from EIS data for pure Co3O4 and polyvinyl assisted Co3O4 nanowires immobilized with glucose oxidase and without enzyme in phosphate buffer solution of pH 7.3. It is obvious from Figure 11 that charge transport is more favourable in the polyvinyl assisted Co3O4 nanowires compare to pure pure Co3O4 indicating the role of polyvinylamine in tuning the electrical communciation withing the material. However, afterthe immubilziation of gluxose oxidase, it has produced the barrier for the charge trnsfer due to the insulling proerpties of glutaradehyde and gluccosseoxidase itself[35, 36]. The measured charge transfer values for the pure Co3O4 and polyvinylamine assisted Co3O4 nanowires with and without glucose oxidase immoblization are enclosed in Table2. It can be seen that polyvinylamine assisted nanowires with glucose oxidase immobilization has rvealed better charge transport properties compare to the pure Co3O4.

Figure 1 .

 1 Figure 1. Powder XRD of (a) pristine Co3O4, (b) polyvinyl amine assisted Co3O4 nanowires

Figure 2 .Figure 3 .

 23 Figure 2. Typical SEM images (a,b) pristine Co3O4 (b) polyvinyl assisted Co3O4 nanowires Figure 3. EDS spectra (a) pristine Co3O4 (b) polyvinyl assisted Co3O4 nanowires

Figure 4 . 3 Figure 5 . 3 Figure 6 . 3 Figure 7 .Figure 8 . 3 Figure 9 . 3 Figure 10 .

 4353637839310 Figure 4. Calibration (a) pristine Co3O4 (b) polyvinyl assisted Co3O4 nanowires in saline phosphate buffer solution of pH 7.3 Figure 5. Repeatability calibration of polyvinyl assisted Co3O4 nanowires in saline phosphate buffer solution of pH 7.3 Figure 6. Reproducibility study of polyvinyl assisted Co3O4 nanowires in 1 mM glucose in saline phosphate buffer solution of pH 7.3 Figure 7. Interference study of polyvinyl assisted Co3O4 nanowires in saline phosphate buffer solution of pH 7.3 using different interfering agents like uric acid, dopamine, ascorbic acid, fructose, urea and glucose Figure 8. (a) Effect of pH in 1.0 mM glucose, (b) Effect of temperature in 1.0 mM glucose prepared in saline phosphate buffer solution of pH 7.3 Figure 9. Dynamic response time of polyvinyl assisted Co3O4 nanowires in 1.0 mM glucose in saline phosphate buffer solution of pH 7.3 Figure 10. Sequential addition of glucose concentration from 0.005mM to 5 mM prepared in saline phosphate buffer solution of pH 7.3 and the successive potentiometric response of polyvinyl assisted Co3O4 nanowires

Figure 11 :

 11 Figure 11: EIS analysis of pristine Co3O4 and polyvinyl assisted Co3O4 nanowires with and without immobilization of glucose oxidase in 0.5mM glucose prepared in phosphate buffer solution of pH 7.3.

Figure 1 Figure 2 Figure 3 22 Figure 4 Figure 5 Figure 6

 12322456 Figure 1

Figure 7 Figure 8

 78 Figure 7

Figure 9 25 Figure 10 Figure 11

 251011 Figure 9

Table 1 :

 1 Life time study of glucose biosensor based on glucose oxidase immobilized polyvinyl assisted Co3O4 nanowires

	No of weeks	Linear range (mM)	Slope (mVdec -1 )	Limit of detection
				(mM)
	1	0.0005-6	42	0.0001
	2	0.0006-6	44	0.0002
	3	0.0003-6	43	0.0003
	4	0.0005-6	44	0.0002

Table 2 :

 2 The calculated charge transfer values of Co3O4 nanostructures with and without enzyme immobilization

		Charge transfer resistance
	Sample	(Rct)
	Pristine Co3O4	4.3 x 10 3 Ohms
	Pristine + Glucose Enzyme	8.1 x 10 3 Ohms
	Sample Co3O4	1.7 x 10 3 Ohms
	Sample Co3O4 + Glucose Enzyme	2.2 x 10 3 Ohms

Table 3 :

 3 Practicality of glucose oxidase immobilized polyvinyl assisted Co3O4 nanowires

	Experiment no:	Proposed method	Commercial	% recovery of
			glucometer	proposed method
	1	5.2±0.02	5.15±0.01	100.97±0.05
	2	5.1±0.04	5.08±0.001	100.39±0.06
	3	5.3±0.03	5.23±0.01	101.33±0.04

Table 4 :

 4 Comparative analysis obtained results of presented glucose biosensor with reported works GOx: glucose oxidase; GO: graphene oxide; AZO: aluminum-doped zinc oxide; ZnO: zinc oxide; NWs: nanowires; Ag: silver, NPs:nanoparticles, BSA: bovine serum albumin; MIP: molecularly imprinted polymer; MAA: methacrylic acid; NRs: nanorods.

	Materials	Linear	Sensitivity Regression	Response	LOD	Reference
		range		coefficient	time
	Nafion-	2 mM to	15.44	0.997	26 s	1.89 mM 37
	GOx/GO/AZO/Ag	10 mM	mV/mM
	BSA-Nafion-	0.5-	35	-	1-4 s	-	38
	GOx/ZnO	1000	mV/decade
	NWs/Ag	µM
	GOx/ZnO	1 µM to	2.51	0.980	-	-	39
	NRs/Ag	10mM	mV/decade
	AgNPs-GOx/Ag-	0.1-3	8.62	-	-	10 µM	40
	ISE	mM	mV/decade
	MIP-based on	0.02-7	43.7	0.980	-	-	41
	MAA	mM	mV/mM

Acknowledgment

We extend our sincere appreciation to the Researchers Supporting Project (RSP-2022/79) at King Saud University, Riyadh, Saudi Arabia for partial funding of this work. We would like to thank the platform "Microscopies, Microprobes and Metallography (3M)" (Institut Jean Lamour, IJL, Nancy, France) for access to SEM facilities.

Data Availability Statement

The datasets generated analysed during the current study are available from the corresponding author on reasonable request

Conflict of interest statement