Nadjet Belblidia

Marcelo Dias De Amorim

Jeremie Leguay

Vania Conan

Jon Crowcroft

Serge Fdida

Jérémie Leguay

Contact Surround in Opportunistic Networks

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first.last@lip6.fr first.last@fr.thalesgroup.com first.last@cl.cam.ac.uk

Abstract-Is the temporal dimension alone sufficient to characterize contacts in opportunistic networks? Several studies analyze the temporal aspect of contacts with significant results concerning contact and inter-contact distributions. Nevertheless, only the temporal dimension does not give a complete overview of contact characterization. In this paper, we propose the surround indicator as a metric to exhibit the contact's surrounding environment in opportunistic networks. We evaluate the surround indicator on two existing datasets and show that contacts have too heterogeneous and too unstable surrounds to be considered only in terms of duration. Besides a large variability of the surrounding environment within the duration of a single contact, it is frequent to observe contacts of identical duration that exhibit differences in their surrounds of more than a hundred times.

I. CONTEXT AND PROBLEM DEFINITION

Important advances in the area of opportunistic networks have been achieved these latest years, including the proposal of routing and forwarding algorithms [START_REF] Balasubramanian | DTN routing as a resource allocation problem[END_REF], [START_REF] Leguay | Evaluating mobility pattern space routing for DTNs[END_REF], [START_REF] Lindgren | Probabilistic routing in intermittently connected networks[END_REF], [START_REF] Spyropoulos | Spray and wait: an efficient routing scheme for intermittently connected mobile networks[END_REF], [START_REF] Yuan | Predict and relay: an efficient routing in disruption-tolerant networks[END_REF], the investigation of social relationships [START_REF] Boldrini | Contentplace: social-aware data dissemination in opportunistic networks[END_REF], [START_REF] Daly | Social network analysis for routing in disconnected delay-tolerant manets[END_REF], [START_REF] Hui | Bubble rap: social-based forwarding in delay tolerant networks[END_REF], [START_REF] Sarafijanovic-Djukic | Island Hopping: Efficient Mobility-Assisted Forwarding in Partitioned Networks[END_REF], and the conception of applications to enable content sharing among users on the move [START_REF] Jung | Bluetorrent: Cooperative content sharing for bluetooth users[END_REF], [START_REF] Lebrun | Bluetooth content distribution stations on public transit[END_REF], [START_REF] Mcnamara | Media sharing based on colocation prediction in urban transport[END_REF]. The common point in all these contexts is that they rely on a thorough understanding of the properties governing contact patterns among users. This is a crucial step, as contacts in opportunistic networks may be characterized by a limited capacity due to, among others, user mobility, signal range, obstacles, and surrounding interferences. Providing fine characterization of contact capacity is however a difficult task, as it inherently depends on the physical conditions of the network and on the traffic demand.

A contact has two dimensions: temporal and spatial. The temporal dimension concerns the contact and intercontact distributions and has been the subject of several studies [START_REF] Chaintreau | Impact of human mobility on opportunistic forwarding algorithms[END_REF], [START_REF] Conan | Characterizing pairwise intercontact patterns in delay tolerant networks[END_REF], [START_REF] Karagiannis | Power law and exponential decay of inter contact times between mobile devices[END_REF], [START_REF] Tournoux | The accordion phenomenon: Analysis, characterization, and impact on DTN routing[END_REF], [START_REF] Galati | Human mobility in shopping mall environments[END_REF]. The spatial dimension of contacts is related to the contact surrounding environment. Although the surrounding environment has a direct impact on the contact capacity, it has been surprisingly underexplored in the literature. The few works that investigate this issue restrain themselves to the forwarding path characterization [START_REF] Chaintreau | The diameter of opportunistic mobile networks[END_REF].

In this paper, we provide elements to the intuition that contacts should not be considered only in terms of duration. that additional information providing simple indication on the contact surrounding conditions can bring a complementary knowledge for a better understanding of contact capacity.

We propose the surround indicator as a metric to exhibit the spatial dimension of contacts. The surround indicator gives explicit information about nearby contacts that are potential sources of interference with the contact under consideration. The surround indicator is computed from a directed graph representation of the network. Note that, although the proposed approach is merely indicative, such an estimation of the quality of a contact can be useful in a number of research problems. For example, to design more appropriate routing algorithms, to identify popular nodes, and to understand the network evolution through graph-related metrics, to cite a few. In this paper, we focus on both the characterization of opportunistic networks based on our metric and on the dynamics of the topology derived from this characterization.

We use the surround indicator to characterize the spatial dimension of contacts of two datasets collected in realworld experiments. First, we investigate the distribution of the surround indicator on a per-contact basis. Second, we examine the dynamics of the surrounding environment within the duration of a contact. Our idea here is to evaluate the common assumption that the topology remains static while two nodes meet. Our analyses reveal that:

• Opportunistic networking is much more dynamic than it is traditionally handled as. Besides the intermittent characteristic of connectivity, nodes also observe huge surround instability. • Most of the time, we observe a high variability of the surround indicator within the same contact. • In many cases, the surround indicator is expressed by extreme values, indicating zones that are inherently prone to strong interference. This observation has fundamental practical impact, as for example in routing protocols based on the notion of node centrality (which might be in fact unfeasible in many situations).

These results highlight the importance of the spatial dimension of contacts in opportunistic networks.

The remainder of the paper is structured as follows. In Section II, we present our proposal for the surround indicator and discuss the practical utilization of this metric. We investigate then in Sections III and IV the surround and dynamics of contacts of two datasets collected in real-world experiments. Finally, we conclude the paper in Section V and give some directions for future work.

II. EXHIBITING THE SPATIAL DIMENSION OF CONTACTS

Principles. We refer to the situation represented in Fig. 1(a) to explain how the surround indicator is computed. We suppose that all contacts are bidirectional. In one hand, If node x tries to send data to node y, the co-located contacts that may interfere with y reception are the ingoing contacts of y and also all their outgoing contacts, because of the omnidirectionality of wireless links. These contacts are represented with arrows in Fig 2(a). Similarly, if y tries to send data to x, the contacts that may interfere with the x reception are the ingoing contacts of x and all their outgoing contacts (cf., Fig. 2(b)). We can see through this example that the surround indicator reflects the receiver surround. Moreover, the value corresponding to each direction of contact can be very different if the surrounding environment of the involved nodes are heterogeneous. In the following, we give a formal definition of the surround indicator.

Formal definition. Let V be the set of nodes in the network and xy be the directional contact between x and y during the interval [t 1 , t 2 ], with x, y ∈ V. We refer to it as the target contact. N + x (t) and N - x (t) are respectively the sets of out-neighbors and in-neighbors of node x at time t. Loosely speaking, we define the surround indicator S xy (t) of the target contact xy at time t ∈ [t 1 , t 2 ] as the number of outgoing contacts of all y's in-neighbors except x :

S xy (t) = ∀z∈N - y (t)-{x} |N + z (t)|, ∀t ∈ [t 1 , t 2 ]. (1)
Note that the surround indicator points out how many sources of interference (i.e., co-located contacts) might have an influence on the reception quality of the target contact. In the worse case, where the destination is the neighbor of all nodes and all the destination neighbors have outgoing link to all nodes (in a complete graph for example),

S xy (t) = (N -1)(N -2) with N = |V |.
Interest. The surround indicator can be useful in a number of situations. A straightforward case is routing. For example, a possible policy would be to ignore small contacts with large surround indicators when alternative paths exist. A second case concerns bundle size; it could be useful for example to tune the size of a bundle in function of the surround indicators. The bundle is expected to find throughout the network. As a contact with a large surround indicator is much more prone to competitive flows, it could be better to try to transfer small messages to fairly share the bandwidth between co-located contacts. A third application is the detection of link popularity (as a complement to the notion of node popularity) based on the link's surround.

Dynamics. The surround indicator is likely to vary during the contact interval [t 1 , t 2 ], as some neighbors appear while others leave during the contact. We propose to decompose the contact into as many necessary sub-contacts with a stable surround indicator. Each sub-contact represents a different temporal view of a surround. We define the expansion as the number of sub-intervals associated to the same original contact. This representation allows not only characterizing neighborhood density through the surround indicator, but also to have a clue on the network dynamics. Indeed, a high value of the expansion suggests frequent changes of the surround indicator value and reflects contact surround instability.

The creation of sub-intervals is illustrated in Fig. 3. The duration of contact xy is divided into as many sub-intervals as the number of changes in the neighborhood. Thus, in the example of Fig. 3, the contact surround evolution generates the division of the duration [0, 15] into six sub-intervals as follows :

. . .

[x, y, 0, 15] . . . →                  [x, y, 0, 1] ; [0] [x, y, 1, 3] ; [1] [x, y, 3, 5] ; [2] [x, y, 5, 10] ; [3] [x, y, 10, 14] ; [2] [x, y, 14, 15] ; [1]
Therefore, the expansion is equal to six in this example. By investigating the dynamics within a contact, we somehow indirectly estimate the evolution of the contact capacity. Again, it is important to make it clear that our proposal is not a computation of the capacity, but an indication of the conditions under which a contact operates. We show in the following that this captures well the conditions under which a contact takes place.

Note that there are other possibilities to characterize a surrounding environment of a contact. An alternative would be to simply consider the degree of a node. We do not use this representation because it does not take into account the increase in the interference probability with the increase of the number of outgoing links. Another possibility would be to consider a contact as indivisible and use the average surround indicator value computed during the entire contact duration. However, we would not be able to have an estimation of the surround stability with this solution. Yet other alternatives exist and all of them have pros and cons. As discussing the particularities of each proposal is not the goal of this paper, we leave such a comparative study for future work.

III. THE SURROUND INDICATOR IN REAL SCENARIOS

We use the surround indicator to characterize the spatial dimension of contacts issue from the Infocom [START_REF] Chaintreau | Impact of human mobility on opportunistic forwarding algorithms[END_REF] and Roller-Net [START_REF] Tournoux | The accordion phenomenon: Analysis, characterization, and impact on DTN routing[END_REF] datasets. Both datasets have been generated through contact logs between Intel iMote nodes (equipped with a Bluetooth interface). Each iMote performs regular scans and registers the MAC addresses of the responding devices around. In the Infocom dataset, iMotes were distributed to 41 students participating in the four-day Infocom student workshop in 2005. The RollerNet dataset has been collected during a rollerblading tour in Paris. iMotes were distributed to 62 participants and the total duration of the tour was about three hours. These datasets are publicly available to the community through the Crawdad repository [START_REF]CRAWDAD Website[END_REF].

First, we wonder whether the two situations shown in Fig. 1 can really happen in a real setup. Fig. 4 shows the surround indicator associated to four contacts arbitrarily chosen from the Infocom dataset. We observe in Fig. 4(a) three distinct surrounding environments for contacts having the same duration. The first contact has a stable surround indicator around very low values (between 1 and 3). This situation represents a contact happening in a sparse zone. In contrast, the second contact has a stable surround indicator around large values (around 50) characterizing a much denser zone. Note that the two situations were observed in the same network. Furthermore, we could also observe in the same dataset contacts that are extremely dynamic through time, as in the third plot of Fig. 4(a). We observe this dynamics more clearly in longest contacts, as in the example shown in Fig. 4. In the remainder of this section, we will study the proportion of each situation in both the Infocom and Rollernet datasets.

We study the distribution of the surround indicator by considering the sub-intervals generated by changes in the neighborhood. Fig. 5 shows the cumulative distribution function of sub-contacts according to the surround indicator. For both datasets, the percentage of contacts with a null surround indicator does not exceed 0.55% for RollerNet and 0.59% for Infocom. Therefore, it is very rare that a contact happens in an isolated environment. Moreover, in extreme cases, the surround indicator reaches 212 in RollerNet and 378 in Infocom. Even if the surround indicator is just an indication of potential parallel communications, such a large number of surrounding contacts clearly shows that some zones are likely to have a limited number of working contacts, if any, due to mutual interference generated from exchanged control messages.

Let us focus now on the repartition of contact durations according to the surround indicator. Fig. 6 shows that, for both datasets, contact durations decrease when the surround indicator increases. We can explain this result by the surround evolution probability. Indeed, a contact has a small probability to observe a change of its surround in a sparse zone. As a consequence, this contact has less probability to be decomposed. On the other hand, in a dense environment, a contact has a larger probability to see one of its surrounding neighbors move during the contact time. Although it is not a surprising result, it has a major impact. By isolating a specific class of small contacts characterized by large surround indicator, we are able to compare contacts without any particular information about the medium characteristics. For example, a straightforward policy for a routing protocol would be to ignore the smallest contacts with large surround indicator when alternative paths exist.

Finally, we investigate the cumulative distribution of the contact duration according to the surround indicator, as shown in Fig. 7. Although the two curves show similar results around a surround indicator of 50, they behave quite differently for other values. Initially, the duration of the contact with a surround indicator equal to zero represents 18% of the total contact duration in Infocom dataset when it represents only 2% in RollerNet. Consider a simple routing protocol that would ignore contacts whose surround indicator is larger than say 25. In this case, links would be idle for more than 55% of the time in the RollerNet case while for only 35% of the time in Infocom. This result suggests that communication strategies must consider the specificities of each environment.

IV. DYNAMICS WITHIN CONTACTS

We investigate now the expansion distribution to have a clue on the network dynamics. Recall that a large expansion corresponds to frequent changes of the surround indicator and reflects contact surround instability. Conversely, a small expansion corresponds to a stable environment. Fig. 8 shows the distribution of the contacts according to the expansion. In RollerNet, about 50% of the contacts have an expansion larger than 12. Similarly, in Infocom, about 50% of the contacts have an expansion larger than 16. Furthermore, although these values are relative as the datasets have different durations, the expansion reaches extreme values in both cases. In RollerNet, the most instable contact surround has an expansion equal to 173, while it reaches 1,436 in Infocom.

It is important however to check whether the surround instability causing a high expansion reflects a minor change of the contact environment or if it is due to a significant neighborhood modification. Minor changes can be, for example, the consequence of a node that joins and leaves the contact neighborhood periodically causing a high expansion without real change in the surround. We compute, for each initial contact, the interquartile range of the surround indicator. The interquartile range is equal to the difference between the third quartile (75%) and the first quartile (25%) of the values taken by the surround indicator during the contact duration. It represents the dispersion of the surround indicator values. The interquartile range expresses the variability of the surround indicator associated to the same initial contact. Fig. 9 shows the distribution of the average interquartile range according to the expansion. In both datasets, when the expansion is below 60, the average interquartile range grows with the increase of the expansion value. In other words, the instability expressed by the expansion is due to noticeable changes in the contact environment during the interval [0, 60]. Note that 99% of contacts in RollerNet and 85% of contacts in Infocom have an expansion value less than 60.

It is also interesting to observe the extreme cases. Indeed, the most unstable contact surround in RollerNet (expansion equals to 173), which has an interquartile range of 19, shows the same average interquartile range as the contacts with an expansion of 26. Also, the most unstable contact surround in Infocom (expansion equals to 1,436) has an interquartile range of 25, which is lower than the average interquartile range of contacts with an expansion value of 53. In these cases, instability is due to minor but frequent changes in the contact surround.

Finally, we focus on the contact duration distribution according to the expansion values. Fig. 10 shows the durations of contacts having a particular expansion. The darker the point in the plot, the most representative the duration. In RollerNet, when we focus on the most representative values, we found that the expansion value increases with the contact duration. The accordion phenomenon that characterizes the RollerNet dataset explains in part this result [START_REF] Tournoux | The accordion phenomenon: Analysis, characterization, and impact on DTN routing[END_REF]. Indeed, such a phenomenon indicates a periodic evolution of contact surrounds. When the contact surround radically changes during the characteristic period, the longer the contact, the more varying the surround (in particular when the periodic time interval is much smaller than the contact average time). The same observation is not straightforward in the case of Infocom (cf., Fig. 10(b)), where the contact duration has a different distribution showing long contacts with few surround changes. This observation can be also explained by the dataset's specificities. The Infocom experimentation records student connectivity during the four days of a conference. The students were attending to the same presentations during the day. In this context, we can understand the contact surround stability. Moreover, we note some isolated contacts (with an expansion equal to 1), which last more than 5 hours. These contacts most likely represent roommates during night. This suggests that environment properties can have a real impact on contact characterization.

V. CONCLUSION AND FUTURE WORK

The main contribution of this paper is the introduction of the surround indicator as a metric to exhibit the spatial dimension of contacts in opportunistic networks. The surround indicator provides explicit information of the environment in which contact takes place. We use two different datasets with specific contact environment to study the density and the expansion of the contact surround. Our analyses show that the surrounding environment of a contact is more dynamic than indicated by solely contact and intercontact times. This observation highlights the use of the proposed surround indicator as a useful yet simple metric to point out the evolution of a contact surrounding environment. Future work includes investigating a dissemination strategy based on the surround indicator so that the next-hop selection mechanism depend on both its surrounding environment and the traffic demand in the network. The objective is to use central nodes to disseminate data when the traffic demand is low and prefer edge nodes when it is high. To this end, we intend to study the surround indicator predictability and its computation on the fly.

Fig. 1

 1 illustrates two completely different situations where contacts last for the same duration. If we consider only the temporal dimension, both contacts are then of course identical. Nevertheless, although the contacts have the same duration, the one in Fig.1(a) is clearly much more prone to interference as it happens in a denser zone. We argue x y (a) Contact in a dense zone.x y (b) Contact in a sparse zone.

Fig. 1 .

 1 Fig. 1. The same duration. Two situations.

  Destination node in a sparse zone.

Fig. 2 .

 2 Fig. 2. Two directions. Different surrounds.

Fig. 3 .

 3 Fig. 3. Dissecting a contact according to the surround indicator. In this example, the original contact xy is decomposed into six sub-intervals.

  Highly unstable contact surround.

Fig. 4 .

 4 Fig. 4. The surround indicator values associated to contacts arbitrarily chosen from Infocom dataset.

Fig. 5 .

 5 Fig. 5. Repartition of sub-contacts according to the surround indicator.

Fig. 6 .Fig. 7 .

 67 Fig. 6. Duration of contacts according to the surround indicator.

Fig. 8 .

 8 Fig. 8. Repartition of contacts according to the expansion.

Fig. 9 .

 9 Fig. 9. Repartition of the surround indicator interquartile range according to the expansion.

Fig. 10 .

 10 Fig. 10. Repartition of contact durations according to the expansion.

Author manuscript, published in "IEEE PIMRC 2010 Mobile and Wireless Networks (PIMRC 2010 Track 3), Istanbul, Turkey : France

ACKNOWLEDGMENT

This work is partially supported by the ANR project Crowd under contract ANR-08-VERS-006 and by the European Commission in the framework of the FP7 Network of Excellence NEWCOM++ under contract 216715.


Renata Bilewicz
email: bilewicz@chem.uw.edu.pl

Olga Swiech
email: oswiech@chem.uw.edu.pl

Kazimierz Chmurski
email: chmurkaz@chem.uw.edu.pl

Molecular Interactions of β-Cyclodextrins with Monolayers Containing Adamantane and Anthraquinone Guest Groups

Keywords: cyclodextrin, anthraquinone, adamantane, self assembled monolayer, association constant cyclodextrin, adamantane, anthraquinone, self assembled monolayer, association constant .manuscriptcentral.com/

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Introduction

Cyclodextrins, cyclic organic compounds obtained by enzymatic transformation of starch belong to one of the most intensively investigated classes of ''host'' molecules in supramolecular chemistry. The β-cyclodextrin (β-CD) is one of the most abundant natural oligomers and corresponds to the association of seven glucose units. The hydrophobic cavity and hydrophilic exterior makes the molecule an appropriate host for various guest molecules bound via non covalent bonds to form inclusion complexes [1,2]. This inclusion ability of cyclodextrins has attracted considerable attention due to applications in drug delivery systems, sensing devices and for the construction of molecular machines, designed to perform tailored mechanical tasks [3][4][5][6]. Adamantane, an apolar cage hydrocarbon, is one of guest molecules forming strong inclusion complexes with β-CD, with equilibrium constant above 10 4 M -1 [7]. Complexation between β-CD and adamantane and its derivatives has been exploited for molecular linking, gene delivery and sensor applications [8,9].

Anthraquinone derivatives are the largest group of naturally occurring quinones.

Redox cycling of anthraquinone is supposed to play a important role in the activation of many anthraquinone-based drugs under aerobic conditions [10]. The inclusion complex between anthraquinone and β-CD in aqueous solution was reported by Jiang et al [11]. To our knowledge there are only few report on the complexes of amino derivatives of anthraquinone with β-CD and weaker interactions due to the presence of amino group were shown [12,13].

In the present contribution, the complexation of β-cyclodextrin with 1-aminoanthraquinone (AAQ) in solution and with surface immobilized N- (1-anthraquinone) lipoamide (AQ-Lip), were investigated. (Fig. 1a) Since the anthraquinone group is electroactive, voltammetry can be used to follow the complexation reactions. The electrochemical behavior of a non-electroactive guest: N-(1-adamantane) lipoamide (AD-Lip) self-assembled in a monolayer on the gold electrode was also studied. (Fig. 1b). The monolayer covered electrode was exposed to the solution of electroactive derivative of β-CD: mono-6-deoxy-6-thioureido-(1-anthraquinone)-per-O-methyl-β-cyclodextrin (AQ-β-CD) and changes in the voltammograms are discussed in terms of the interaction between the β-CD and the adamantane moiety.

Experimental

Chemicals

All compounds used in this work for the syntheses were purchased from Aldrich and Fluka. to -1° C and kept under argon atmosphere. 1.1 equiv (4.11 mM 521 mg) of oxalil chloride was added followed by addition of 1ml of DMF. The reaction mixture was stirred for 3 hours. 1-aminoanthraquinone (3.8 mM, 899 mg) in 50 ml of dichloromethane was dropped into this mixture and the reaction was continued overnight, while the grey-green precipitate was formed. The latter was filtered and the mixture was evaporated to dryness by means of rotary evaporator. The title compound was isolated by column chromatography on silica gel with dichloromethane as eluent; Rf =0.14. Yield was 342mg (0.83 mM) as orange solid, 22.3%.

N-(1-anthraquinone) lipoamide (AQ-Lip

MS ES+: AcONa m/z 434.1 [M+Na].

N-(1-adamantane) lipoamide (AD-Lip

). 2.5 g of dicyclohexylocarbodiimide (DCC) was dissolved in DMF (8mL). 2.2 g (9.8 mM) of lipoic acid was added to this solution under magnetic stirring. Immediately, white precipitate was formed. The reaction mixture was diluted with 50 mL of acetonitrile. To this suspension 1 equiv (1.86g) of 1-amino adamantane hydrochloride was added followed by addition of 1mL of triethylamine and the reaction was continued overnight. The precipitate, N,N'-dicyclohexylurea was filtered off and all solvents were removed under reduced pressure. The solid residue was analysed by TLC on silica gel with 5% MeOH in dichloromethane as eluent, new compound was detected; Rf=0.16. This product was purified by column chromatography on silica gel with 4:1 v/v chloroform: acetone system as eluent; Rf=0. 15 were purged with purified nitrogen for 30 min and all experiments were performed at room temperature. Milli-Q ultra-pure water (resistivity 18.2 MΩ/cm) was used.

Preparation of the modified gold electrodes

The gold electrode was polished to mirror finish with 0.05 µm alumina powder and electrochemically cleaned by cycling in the range of potentials from -0.2V to 1.6V in 0.5M H 2 SO 4 solution until the typical cyclic voltammogram of a clean gold surface was obtained [14]. Modification of the gold electrodes was carried out by self-assembly from oxygen-free 0.1mM solutions of AQ-Lip and AD-Lip in DMF for 25 minutes. Next, the electrodes were immersed in 0.1mM solution of hexanethiol in DMF for 24 hours. The modified electrode was then washed with Milli-Q ultra-pure water.

Results and discussion

β-cyclodextrin complex formation with 1-aminoanthraquinone in solution

In phosphate buffer with 40 percent DMF (pH 9.1), the decrease of anodic and cathodic peaks of AAQ were observed upon addition of β-CD to the AAQ solution due to the smaller diffusion coefficient of the β-CD complex formed compared to the diffusion coefficient of free guest. Dependence of reduction peak current of AAQ on the ratio of β-CD to AAQ concentrations is shown in Fig. 2.

The formation constant of 1:1 cyclodextrin complex was calculated using Osa equation [15]. The ratio of association constants of the reduced and oxidized forms of AAQ are described by equation [3]:

] / ) ( [ 2 1 ' ' RT E E F S S c F e K K - - = ,
where, K S1 and K S2 are the association constants of the oxidized and reduced forms, respectively, and E F and E C are the formal potentials of free, and complexed forms, respectively. While the peak-to-peak separation increased upon addition of β-cyclodextrin, the formal potential did not change. In a 1:1 complex this indicated similar binding strength of β-cyclodextrin with the oxidized and reduced forms of AAQ.

The heterogeneous standard rate constant was calculated from equation [16]:

F o r P e e r R e v i e w O n l y ( ) ( ) 2 1 2 1 2 ox s red ox nFvD RT k D D π α         = Ψ
Where Ψ is a function fixed from product of electron number(n) and difference between anodic and cathodic peaks potential (E ox -E red ). The dependence Ψ = n(E ox -E red ) is tabulated. D ox and D red are diffusion coefficients of anodic and cathodic processes, α is a transfer coefficient, k s is heterogeneous rate constants, v, R, T, F, π are their usual meanings

The rate constant of the AAQ electrode process decreases upon addition of βcyclodextrin. The values of standard rate constants are 2.5•10 -3 cm/s and 0.5•10 -3 cm/s for AAQ and AAQ:β-CD system, respectively.

The complexation of AAQ by β-CD was confirmed using UV-Vis spectrometry. The addition of β-CD to the solution of AAQ resulted in the increase of AAQ absorbance β-cyclodextrin complex formation with AQ-Lip immobilized in a mixed monolayer at gold electrode Two-component monolayers containing AQ-Lip and hexanethiol showed a pair of reversible redox peaks; its anodic and cathodic peak potential where, respectively, -0.637V and -0.654V at 0.05 V/s scan rate. The cathodic and anodic peaks were almost symmetric and the formal potential is -0.645V (Fig. 3). The dependence of peak current, i p on scan rate, v is linear and i p is related to surface concentration of the electroactive component of the monolayer, Γ according to equation [17]:

T R A F n i p ⋅ ⋅ Γ ⋅ ⋅ ⋅ ⋅ = 4 2 2 ν
The surface concentration and molecular area of AQ-Lip modified electrodes calculated based on this equation were 7.73±0.39•10 -11 mol/cm 2 and 217±13 Å 2 .

Electrochemical desorption experiments were performed in 0.1M NaOH aqueous solution and the surface concentration of the thiolated molecules ( both components of the monolayer) was found to be 3.50± 0.17•10 -10 mol/cm 2 . The ratio of surface concentrations can The apparent rate constant, app k was obtained using equation [18]:

) exp( t k Q k i app app - = ,
where Q is the charge associated with converting the redox centers from one oxidation state to another.

The plot of ln(i) vs. time is linear. The experimental Tafel plot was fitted to theoretical line of the Butler-Volmer equations for low overpotentials region [19]:

      - - = T k e k k B ET ox 4 2 exp 0 η λ       + - = T k e k k B ET red 4 2 exp 0 η λ
where k ET is electron transfer rate at zero overpotential, k ox and k red are the apparent rate constants for anodic and cathodic processes, λ is reorganization energy, e 0 and k B are static dielectric and Boltzmann constants, and η is the applied overpotential .

The dependencies of ln k app vs η for mixed AQ-Lip -hexanethiol monolayer in the presence and absence of β-CD are shown in Fig. 4. The value of standard rate constant was found to be: 44.1±1.7 s -1 without β-CD, while in solutions containing 0.1mM β-CD it decreased to 31.2±0.8 s -1 In the presence of larger amounts of DMF, the rate constants decreased probably reflecting the interaction with the solvent and a more complicated mechanism. Practically, lack of differences upon addition of β-CD to solutions containing DMF may reflect weaker affinity of the β-CD cavity to AQ-Lip in solutions containing DMF.

In case of adamantane -β-cyclodextrin complexes both the host and the guest are nonelectroactive and a different method should be used for monitoring the complexation reaction. Our approach was to "decorate" β-CD with a side -group which is electroactive.

Therefore, β-cyclodextrin with an anthraquinone side -group was synthesized and its electrochemical properties were studied in solution (Fig. 5). The voltammogram showed a cathodic peak at -0.617V. The plot of the reduction peak current vs. square root of scan rate is shown in Fig. 6. Positive deviations from linearity at larger scan rates, can be explained by the contribution of adsorption of AQ-β-CD molecules on the electrode surface.

Since AD-Lip is nonelectroactive, the surface concentration of modified electrodes was calculated from electrochemical desorption of mixed AD-Lip -hexanethiol monolayer in 0.1 M NaOH. The surface concentration of the thiolated molecules in the mixed monolayer was 4.55 ± 0.9•10 -10 mol/cm 2 .

In phosphate buffer solution containing 25 percent DMF (pH 8.9), the interaction between mixed AD-Lip -hexanethiol monolayer modified electrode and AQ-β-CD was easily detected. Figure 2. Dependence of reduction peak current of 1-aminoanthraquinone on the ratio of concentrations of cyclodextrin to 1-aminoanthraquinone. Figure 3. Cyclic voltammogram of mixed AQ-Lip-hexanethiol modified gold electrode performed in phosphate buffer with 40% addition of DMF. Scan rate 50 mV/s. Figure 4. Tafel plot for mixed AQ-Lip -hexanethiol monolayer in the presence and absence of β-CD recorded in 0.1 M phosphate buffer without DMF, pH 9.1. Figure 5. Cyclic voltammogram of AQ-β-CD in phosphate buffer solution with 25% DMF. Scan rate 50 mV/s. Figure 6. The dependence of cathodic peak current on scan rate for AQ-β-CD solution. Supporting electrolyte: 0.1M phosphate buffer + 25% DMF, pH 8.9. Figure 7. Cyclic voltammograms recorded using a (a) gold electrode and electrode modified by: (b) mixture of N-(1-adamantane) lipoamide and hexanethiol, (c) hexanethiol, in phosphate buffer containing 25% DMF, pH 8.9. All electrodes were kept in 0.1 mM solution of mono-6-deoxy-6-thioureido-(1-anthraquinone)-per-Omethyl-β-cyclodextrin for two hours. • Abstract. Complexing abilities of β-cyclodextrin towards anthraquinone derivatives in solution and immobilized on gold surfaces were studied by voltammetry. The association constant of β-cyclodextrin with 1-aminoanthraquinone in solution was found to be 1.03±0.05•10 3 M -1 hence smaller than with anthraquinone. Capping the surface immobilized N-(1-anthraquinone) lipoamide with β-cyclodextrin lead to the decrease of heterogeneous electron transfer rate constant due to the change of the immediate environment around the electroactive group. To detect interactions of β-cyclodextrin with a nonelectroactive guest, N-(1-adamantane) lipoamide, the cyclodextrin was modified by attachment of an anthraquinone group as the electroactive marker. The appearance of the voltammetric peak corresponding to reduction of the anthraquinone side-group indicated binding of β-cyclodextrin to the N-(1-adamantane) lipoamide self-assembled in a monolayer on the gold electrode.

• keywords . cyclodextrin, adamantane, anthraquinone, self assembled monolayer, association constant

Introduction

Cyclodextrins, cyclic organic compounds obtained by enzymatic transformation of starch belong to one of the most intensively investigated classes of ''host'' molecules in supramolecular chemistry. The β-cyclodextrin (β-CD) is one of the most abundant natural oligomers and corresponds to the association of seven glucose units. The hydrophobic cavity and hydrophilic exterior makes the molecule an appropriate host for various guest molecules bound via non covalent bonds to form inclusion complexes [1,2]. This inclusion ability of cyclodextrins has attracted considerable attention due to applications in drug delivery systems, sensing devices and for the construction of molecular machines, designed to perform tailored mechanical tasks [3][4][5][6]. Adamantane, an apolar cage hydrocarbon, is one of guest molecules forming strong inclusion complexes with β-CD, with equilibrium constant above 10 4 M -1 [7]. Complexation between β-CD and adamantane and its derivatives has been exploited for molecular linking, gene delivery and sensor applications [8,9].

Anthraquinone derivatives are the largest group of naturally occurring quinones.

Redox cycling of anthraquinone is supposed to play a important role in the activation of many anthraquinone-based drugs under aerobic conditions [10]. The inclusion complex between anthraquinone and β-CD in aqueous solution was reported by Jiang et al [11]. To our knowledge there are only few report on the complexes of amino derivatives of anthraquinone with β-CD and weaker interactions due to the presence of amino group were shown [12,13].

In the present contribution, the complexation of β-cyclodextrin with 1-aminoanthraquinone (AAQ) in solution and with surface immobilized N-(1-anthraquinone) lipoamide (AQ-Lip), were investigated. (Fig. 1a) Since the anthraquinone group is electroactive, voltammetry can be used to follow the complexation reactions. The electrochemical behavior of a non-electroactive guest: N-(1-adamantane) lipoamide (AD-Lip) self-assembled in a monolayer on the gold electrode was also studied. (Fig. 1b). The monolayer covered electrode was exposed to the solution of electroactive derivative of β-CD: mono-6-deoxy-6-thioureido-(1-anthraquinone)-per-O-methyl-β-cyclodextrin (AQ-β-CD) and changes in the voltammograms are discussed in terms of the interaction between the β-CD and the adamantane moiety.

Experimental

Chemicals

All compounds used in this work for the syntheses were purchased from Aldrich and Fluka. to -1° C and kept under argon atmosphere. 1.1 equiv (4.11 mM 521 mg) of oxalil chloride was added followed by addition of 1ml of DMF. The reaction mixture was stirred for 3 hours. 1-aminoanthraquinone (3.8 mM, 899 mg) in 50 ml of dichloromethane was dropped into this mixture and the reaction was continued overnight, while the grey-green precipitate was formed. The latter was filtered and the mixture was evaporated to dryness by means of rotary evaporator. The title compound was isolated by column chromatography on silica gel with dichloromethane as eluent; Rf =0.14. Yield was 342mg (0.83 mM) as orange solid, 22.3%.

N-(1-anthraquinone) lipoamide (AQ-Lip

MS ES+: AcONa m/z 434.1 [M+Na].

N-(1-adamantane) lipoamide (AD-Lip). 2.5 g of dicyclohexylocarbodiimide (DCC) was

dissolved in DMF (8mL). 2.2 g (9.8 mM) of lipoic acid was added to this solution under magnetic stirring. Immediately, white precipitate was formed. The reaction mixture was diluted with 50 mL of acetonitrile. To this suspension 1 equiv (1.86g) of 1-amino adamantane hydrochloride was added followed by addition of 1mL of triethylamine and the reaction was continued overnight. The precipitate, N,N'-dicyclohexylurea was filtered off and all solvents were removed under reduced pressure. The solid residue was analysed by TLC on silica gel with 5% MeOH in dichloromethane as eluent, new compound was detected; Rf=0.16. This product was purified by column chromatography on silica gel with 4:1 v/v chloroform: acetone system as eluent; Rf=0.15. Yield was 1.6g (4.7 mM) as yellow solid, 48%. MS ES+ m/z 362.1 [M+Na]. were purged with purified nitrogen for 30 min and all experiments were performed at room temperature. Milli-Q ultra-pure water (resistivity 18.2 MΩ/cm) was used.

Mono(6-deoxy-6-thioureido(1-antraquinono))-per(2,3,6-O-metylo)-β β β β-cyclodextrin (AQ-

β β β β-CD).

Preparation of the modified gold electrodes

The gold electrode was polished to mirror finish with 0.05 µm alumina powder and electrochemically cleaned by cycling in the range of potentials from -0.2V to 1.6V in 0.5M H 2 SO 4 solution until the typical cyclic voltammogram of a clean gold surface was obtained [14]. Modification of the gold electrodes was carried out by self-assembly from oxygen-free 0.1mM solutions of AQ-Lip and AD-Lip in DMF for 25 minutes. Next, the electrodes were immersed in 0.1mM solution of hexanethiol in DMF for 24 hours. The modified electrode was then washed with Milli-Q ultra-pure water.

Results and discussion

β-cyclodextrin complex formation with 1-aminoanthraquinone in solution

In phosphate buffer with 40 percent DMF (pH 9.1), the decrease of anodic and cathodic peaks of AAQ were observed upon addition of β-CD to the AAQ solution due to the smaller diffusion coefficient of the β-CD complex formed compared to the diffusion coefficient of free guest. Dependence of reduction peak current of AAQ on the ratio of β-CD to AAQ concentrations is shown in Fig. 2.

The formation constant of 1:1 cyclodextrin complex was calculated using Osa equation [15]. The ratio of association constants of the reduced and oxidized forms of AAQ are described by equation [3]

: ] / ) ( [ 2 1 ' ' RT E E F S S c F e K K - - = ,
where, K S1 and K S2 are the association constants of the oxidized and reduced forms, respectively, and E F and E C are the formal potentials of free, and complexed forms, respectively. While the peak-to-peak separation increased upon addition of β-cyclodextrin, the formal potential did not change. In a 1:1 complex this indicated similar binding strength of β-cyclodextrin with the oxidized and reduced forms of AAQ.

The heterogeneous standard rate constant was calculated from equation [16]:

F o r P e e r R e v i e w O n l y ( ) ( ) 2 1 2 1 2 ox s red ox nFvD RT k D D π α         = Ψ
Where Ψ is a function fixed from product of electron number(n) and difference between anodic and cathodic peaks potential (E ox -E red ). The dependence Ψ = n(E ox -E red ) is tabulated. D ox and D red are diffusion coefficients of anodic and cathodic processes, α is a transfer coefficient, k s is heterogeneous rate constants, v, R, T, F, π are their usual meanings

The rate constant of the AAQ electrode process decreases upon addition of βcyclodextrin. The values of standard rate constants are 2.5•10 -3 cm/s and 0.5•10 -3 cm/s for AAQ and AAQ:β-CD system, respectively.

The complexation of AAQ by β-CD was confirmed using UV-Vis spectrometry. The addition of β-CD to the solution of AAQ resulted in the increase of AAQ absorbance β-cyclodextrin complex formation with AQ-Lip immobilized in a mixed monolayer at gold electrode Two-component monolayers containing AQ-Lip and hexanethiol showed a pair of reversible redox peaks; its anodic and cathodic peak potential where, respectively, -0.637V and -0.654V at 0.05 V/s scan rate. The cathodic and anodic peaks were almost symmetric and the formal potential is -0.645V (Fig. 3). The dependence of peak current, i p on scan rate, v is linear and i p is related to surface concentration of the electroactive component of the monolayer, Γ according to equation [17]:

T R A F n i p ⋅ ⋅ Γ ⋅ ⋅ ⋅ ⋅ = 4 2 2 ν
The surface concentration and molecular area of AQ-Lip modified electrodes calculated based on this equation were 7.73±0.39•10 -11 mol/cm 2 and 217±13 Å 2 .

Electrochemical desorption experiments were performed in 0.1M NaOH aqueous solution and the surface concentration of the thiolated molecules ( both components of the monolayer) was found to be 3.50± 0.17•10 -10 mol/cm 2 . The ratio of surface concentrations can The apparent rate constant, app k was obtained using equation [18]:

) exp( t k Q k i app app - = ,
where Q is the charge associated with converting the redox centers from one oxidation state to another.

The plot of ln(i) vs. time is linear. The experimental Tafel plot was fitted to theoretical line of the Butler-Volmer equations for low overpotentials region [19]:

      - - = T k e k k B ET ox 4 2 exp 0 η λ       + - = T k e k k B ET red 4 2 exp 0 η λ
where k ET is electron transfer rate at zero overpotential, k ox and k red are the apparent rate constants for anodic and cathodic processes, λ is reorganization energy, e 0 and k B are static dielectric and Boltzmann constants, and η is the applied overpotential .

The dependencies of ln k app vs η for mixed AQ-Lip -hexanethiol monolayer in the presence and absence of β-CD are shown in Fig. 4. The value of standard rate constant was found to be: 44.1±1.7 s -1 without β-CD, while in solutions containing 0.1mM β-CD it decreased to 31.2±0.8 s -1 In the presence of larger amounts of DMF, the rate constants decreased probably reflecting the interaction with the solvent and a more complicated mechanism. Practically, lack of differences upon addition of β-CD to solutions containing DMF may reflect weaker affinity of the β-CD cavity to AQ-Lip in solutions containing DMF.

In case of adamantane -β-cyclodextrin complexes both the host and the guest are nonelectroactive and a different method should be used for monitoring the complexation reaction. Our approach was to "decorate" β-CD with a side -group which is electroactive. Therefore, β-cyclodextrin with an anthraquinone side -group was synthesized and its electrochemical properties were studied in solution (Fig. 5). The voltammogram showed a cathodic peak at -0.617V. The plot of the reduction peak current vs. square root of scan rate is shown in Fig. 6. Positive deviations from linearity at larger scan rates, can be explained by the contribution of adsorption of AQ-β-CD molecules on the electrode surface.

Since AD-Lip is nonelectroactive, the surface concentration of modified electrodes was calculated from electrochemical desorption of mixed AD-Lip -hexanethiol monolayer in 0.1 M NaOH. The surface concentration of the thiolated molecules in the mixed monolayer was 4.55 ± 0.9•10 -10 mol/cm 2 .

In phosphate buffer solution containing 25 percent DMF (pH 8.9), the interaction between mixed AD-Lip -hexanethiol monolayer modified electrode and AQ-β-CD was easily detected. Figure 2. Dependence of reduction peak current of 1-aminoanthraquinone on the ratio of concentrations of cyclodextrin to 1-aminoanthraquinone. Figure 3. Cyclic voltammogram of mixed AQ-Lip-hexanethiol modified gold electrode performed in phosphate buffer with 40% addition of DMF. Scan rate: 50 mV/s. Figure 4. Tafel plot for mixed AQ-Lip -hexanethiol monolayer in the presence and absence of β-CD recorded in 0.1 M phosphate buffer without DMF, pH 9.1. Figure 5. Cyclic voltammogram of AQ-β-CD in phosphate buffer solution with 25% DMF. Scan rate 50 mV/s. Figure 6. The dependence of cathodic peak current on scan rate for AQ-β-CD solution. Supporting electrolyte: 0.1M phosphate buffer + 25% DMF, pH 8.9. Figure 7. Cyclic voltammograms recorded using (a) gold electrode and electrode modified by: (b) mixture of N-(1-adamantane) lipoamide and hexanethiol, (c) hexanethiol, in phosphate buffer containing 25% DMF, pH 8.9. All electrodes were kept in 0.1 mM solution of mono-6-deoxy-6-thioureido-(1-anthraquinone)-per-Omethyl-β-cyclodextrin for two hours.

  ). 770 mg (3.73 mM) lipoic acid was mixed with 30ml of dichloromethane in the reaction vessel. The solution was protected from light, cooled

  . Yield was 1.6g (4.7 mM) as yellow solid, 48%. MS ES+ m/z 362.1 [M+Na]. ). 257 mg (0.246 mM) of mono(6-amino-6-deoxy)-per(2,3,6-O-metylo) β-cyclodextrin was dissolved in dry pyridine (5 mL) and 1 equiv of 1-isothiocynatoanthraquinone dissolved in the same solvent was added to this solution at room temperature. After 16 hours, pyridine was evaporated using a rotary evaporator. Remaining traces of pyridine were removed by coevaporation with toluene. Solid residue was dissolved in dichlorometane and purified by

  is the observed diffusion coefficient, and D f and D c are diffusion coefficients of free guest and inclusion complex, respectively. K s is formation constant and [L] is the concentration of the ligand. D obs and D f can be calculated from the experiments. The value of K s can be obtained from the slope of the linear plot of D obs vs. (D f -D obs )/[L]. The formation constant was 1.03±0.05•10 3 M -1 .

  on these two measurements. For the two-component monolayer AQ-Lip:hexanethiol was 1:4.

  Fig.7 -curve a shows the CV curves of bare gold electrode. Curve b was recorded using the electrode covered with mixed AD-Lip -hexanethiol modified gold electrode and curve c shows, for comparison, the behavior of the AQ-β-CD system in singlecomponent hexanethiol monolayer. Interaction between bare gold electrode and AQ-β-CD leads to the appearance of a cathodic peak, at -0.777V. The hexanethiol modified gold electrode exposed to AQ-β-CD showed a cathodic peak at potentials -0.628V. Finally, the two-component monolayer containing both hexanethiol and Ad-Lip immersed in the AQ-β-CD solution leads to the appearance of two peaks. Thus, AQ-β-CD can affect AD-Lip monolayer in two ways. Firstly, the molecule can be incorporated between other molecules of the monolayer and interact with the electrode surface. This results in the formation of a peak at potential ca. -0.628V. In addition, the molecule interacts directly with the AD-Lip component of the monolayer giving the other peak at ca. -0.735V. The peak at -0.735V remains when the electrode is replaced to a clean supporting electrolyte solution and proves the specific interaction of the cyclodextrin with the AD-Lip component of the monolayer.

  Interaction of β-cyclodextrin with 1-aminoanthraquinone in the solution and with surface immobilized N-(1-anthraquinone) lipoamide slows down the rate of anthraquinone group reduction. In case of solution resident complex, the association constant can be easily evaluated based on the decrease of diffusion coefficient of the electroactive guest due to complexation. The association constant with 1-aminoanthraquinone was 1.03±0.05•10 3 M -1 hence smaller than that of anthraquinone equal to 2.86•10 3 M -1 [20]. The decrease of the electron transfer rate constants of the electroactive anthraquinone moiety upon complexation can be ascribed to the change of its immediate environment caused by the hydrophobicity of the β-CD cavity. Surface immobilized non-electroactive guest N-(1-adamantane) lipoamide was also found to bind β-CD from the solution. The monolayer covered electrode was exposed to the solution of electroactive derivative of β-CD: mono-6-deoxy-6-thioureido-(1-anthraquinone)per-O-methyl-β-cyclodextrin (AQ-β-CD) and then transferred to a pure supporting electrolyte solution. The cyclodextrin was modified by attachment of the anthraquinone group as the electroactive marker. The appearance of the voltammetric peak corresponding to the reduction of the anthraquinone side-group indicated binding of β-cyclodextrin to the N-(1-adamantane) lipoamide monolayer, since it remained upon replacing the modified electrode to the solution of pure supporting electrolyte.

Figure 1 .

 1 Figure 1. Structures of AQ-Lip(a) and AD-Lip(b).Figure2. Dependence of reduction peak current of 1-aminoanthraquinone on the ratio of concentrations of cyclodextrin to 1-aminoanthraquinone. Figure3. Cyclic voltammogram of mixed AQ-Lip-hexanethiol modified gold electrode performed in phosphate buffer with 40% addition of DMF. Scan rate 50 mV/s. Figure4. Tafel plot for mixed AQ-Lip -hexanethiol monolayer in the presence and absence of β-CD recorded in 0.1 M phosphate buffer without DMF, pH 9.1. Figure5. Cyclic voltammogram of AQ-β-CD in phosphate buffer solution with 25% DMF. Scan rate 50 mV/s. Figure6. The dependence of cathodic peak current on scan rate for AQ-β-CD solution. Supporting electrolyte: 0.1M phosphate buffer + 25% DMF, pH 8.9. Figure7. Cyclic voltammograms recorded using a (a) gold electrode and electrode modified by: (b) mixture of N-(1-adamantane) lipoamide and hexanethiol, (c) hexanethiol, in phosphate buffer containing 25% DMF, pH 8.9. All electrodes were kept in 0.1 mM solution of mono-6-deoxy-6-thioureido-(1-anthraquinone)-per-Omethyl-β-cyclodextrin for two hours.

  Molecular Interactions of β-Cyclodextrins with Monolayers Containing Adamantane and Anthraquinone Guest Groups Olga Swiech, Kazimierz Chmurski, Renata Bilewicz* Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland Olga Swiech, Faculty of Chemistry, University of Warsaw, ul. Pasteura 1, 02-093 Warsaw, Poland. Tel./Fax: +48 22 8220211/+48228225996, E-mail: oswiech@chem.uw.edu.pl Kazimierz Chmurski Faculty of Chemistry, University of Warsaw, ul. Pasteura 1, 02-093 Warsaw, Poland. Tel./Fax: +48 22 8220211/+48228225996, E-mail: chmurkaz@chem.uw.edu.pl, Renata Bilewicz, Faculty of Chemistry, University of Warsaw, ul. Pasteura 1, 02-093 Warsaw, Poland. Tel./Fax: +48 22 8220211/+48228225996, E-mail: bilewicz@chem.uw.edu.pl, Corresponding Author.

  ). 770 mg (3.73 mM) lipoic acid was mixed with 30ml of dichloromethane in the reaction vessel. The solution was protected from light, cooled

  is the observed diffusion coefficient, and D f and D c are diffusion coefficients of free guest and inclusion complex, respectively. K s is formation constant and [L] is the concentration of the ligand. D obs and D f can be calculated from the experiments. The value of K s can be obtained from the slope of the linear plot of D obs vs. (D f -D obs )/[L]. The formation constant was 1.03±0.05•10 3 M -1 .

  on these two measurements. For the two-component monolayer AQ-Lip:hexanethiol was 1:4.

Fig. 7 -

 7 curve a shows the CV curves of bare gold electrode. Curve b was recorded using the electrode covered with mixed AD-Lip -hexanethiol modified gold electrode and curve c shows, for comparison, the behavior of the AQ-β-CD system in singlecomponent hexanethiol monolayer. Interaction between bare gold electrode and AQ-β-CD leads to the appearance of a cathodic peak, at -0.777V. The hexanethiol modified gold electrode exposed to AQ-β-CD showed a cathodic peak at potentials -0.628V. Finally, the two-component monolayer containing both hexanethiol and Ad-Lip immersed in the AQ-β-CD solution leads to the appearance of two peaks. Thus, AQ-β-CD can affect AD-Lip monolayer in two ways. Firstly, the molecule can be incorporated between other molecules of the monolayer and interact with the electrode surface. This results in the formation of a peak at potential ca. -0.628V. In addition, the molecule interacts directly with the AD-Lip component of the monolayer giving the other peak at ca. -0.735V.The peak at -0.735V remains when the electrode is replaced to a clean supporting electrolyte solution and proves the specific interaction of the cyclodextrin with the AD-Lip component of the monolayer.

  Interaction of β-cyclodextrin with 1-aminoanthraquinone in the solution and with surface immobilized N-(1-anthraquinone) lipoamide slows down the rate of anthraquinone group reduction. In case of solution resident complex, the association constant can be easily evaluated based on the decrease of diffusion coefficient of the electroactive guest due to complexation. The association constant with 1-aminoanthraquinone was 1.03±0.05•10 3 M -1 hence smaller than that of anthraquinone equal to 2.86•10 3 M -1 [20]. The decrease of the electron transfer rate constants of the electroactive anthraquinone moiety upon complexation can be ascribed to the change of its immediate environment caused by the hydrophobicity of the β-CD cavity. Surface immobilized non-electroactive guest N-(1-adamantane) lipoamide was also found to bind β-CD from the solution. The monolayer covered electrode was exposed to the solution of electroactive derivative of β-CD: mono-6-deoxy-6-thioureido-(1-anthraquinone)per-O-methyl-β-cyclodextrin (AQ-β-CD) and then transferred to a pure supporting electrolyte solution. The cyclodextrin was modified by attachment of the anthraquinone group as the electroactive marker. The appearance of the voltammetric peak corresponding to the reduction of the anthraquinone side-group indicated binding of β-cyclodextrin to the N-(1-adamantane) lipoamide monolayer, since it remained upon replacing the modified electrode to the solution of pure supporting electrolyte.

Figure 1 .

 1 Figure 1. Structures of AQ-Lip(a) and AD-Lip(b).Figure2. Dependence of reduction peak current of 1-aminoanthraquinone on the ratio of concentrations of cyclodextrin to 1-aminoanthraquinone. Figure3. Cyclic voltammogram of mixed AQ-Lip-hexanethiol modified gold electrode performed in phosphate buffer with 40% addition of DMF. Scan rate: 50 mV/s. Figure4. Tafel plot for mixed AQ-Lip -hexanethiol monolayer in the presence and absence of β-CD recorded in 0.1 M phosphate buffer without DMF, pH 9.1. Figure5. Cyclic voltammogram of AQ-β-CD in phosphate buffer solution with 25% DMF. Scan rate 50 mV/s. Figure6. The dependence of cathodic peak current on scan rate for AQ-β-CD solution. Supporting electrolyte: 0.1M phosphate buffer + 25% DMF, pH 8.9. Figure7. Cyclic voltammograms recorded using (a) gold electrode and electrode modified by: (b) mixture of N-(1-adamantane) lipoamide and hexanethiol, (c) hexanethiol, in phosphate buffer containing 25% DMF, pH 8.9. All electrodes were kept in 0.1 mM solution of mono-6-deoxy-6-thioureido-(1-anthraquinone)-per-Omethyl-β-cyclodextrin for two hours.

Figure 1 .Figure 2 .Figure 3 .Figure 4 .Figure 6 .Figure 7 .

 123467 Figure 1. Structures of AQ-Lip (a), AD-Lip (b) and AQ-β-CD (c) 187x102mm (600 x 600 DPI)