domain: sdv info

text:
Juliette Gaillard

Bruno Veyrand²

Marielle Thomas

Xavier Dauchy

Virginie Boiteux

Philippe Marchand²

Bruno Le Bizec²

Damien Banas
email: damien.banas@univ-lorraine.fr

Cyril Feidt

Tissue Uptake, Distribution, and Elimination of Perfluoroalkyl Substances in Juvenile Perch through Perfluorooctane Sulfonamidoethanol Based Phosphate Diester Dietary Exposure

Perfluorooctane sulfonamidoethanol based phosphate diester (SAmPAP) is a potential perfluorooctanesulfonate (PFOS) precursor. To examine whether SAmPAP exposure would result in fish contamination by perfluoroalkyl and polyfluoroalkyl substances (PFASs), juvenile Eurasian perch were dietarily exposed to this compound (dosed group) or exposed to the same tank water but fed control feed (control group). SAmPAP and metabolites were monitored in the muscle, liver, and serum during the 45-day exposure phase and 35-day depuration phase. SAmPAP was only detected in the dosed group and the absorption efficiency (0.04-2.25%) was very low, possibly related to its low bioavailability in the gastrointestinal tract, steric constraints in crossing biological membranes, and clearing by enterohepatic circulation. Although SAmPAP was biotransformed and eliminated at a slow rate (t1/2 > 18 days), its biomagnification factor was low. The observed metabolites in fish were N-ethyl perfluorooctane sulfonamidoacetic acid, perfluorooctane sulfonamidoacetic acid, perfluorooctane sulfonamide, and PFOS. Considering that SAmPAP was the only source of PFASs in the tanks, the occurrence of metabolites indicates that SAmPAP could be biotransformed in fish and contribute to PFOS bioaccumulation. However, levels of metabolites were not significantly different in the dosed and control groups, indicating that metabolite excretion followed by re-exposure to these metabolites from water was the main uptake route.

INTRODUCTION

Perfluorooctanesulfonate (PFOS) is one of the most abundant per-and polyfluoroalkyl substances (PFASs) detected in wildlife and humans worldwide [START_REF] Calafat | Polyfluoroalkyl Chemicals in the U.S. Population: Data from the National Health and Nutrition Examination Survey (NHANES) 2003-2004 and Comparisons with NHANES 1999-2000[END_REF][START_REF] Giesy | Global Distribution of Perfluorooctane Sulfonate in Wildlife[END_REF][START_REF] Kannan | Perfluorooctanesulfonate and Related Fluorochemicals in Human Blood from Several Countries[END_REF] and has been associated with endocrinedisrupting and developmental effects in both wildlife and humans [START_REF] Du | Chronic effects of waterborne PFOS exposure on growth, survival and hepatotoxicity in zebrafish: A partial life-cycle test[END_REF][START_REF] Lopez-Espinosa | Thyroid Function and Perfluoroalkyl Acids in Children Living Near a Chemical Plant[END_REF][START_REF] Washino | Correlations between prenatal exposure to perfluorinated chemicals and reduced fetal growth[END_REF] . Dietary uptake of PFOS is suggested to be an important route to the total exposure for the general adult human population 7.8 and it has been shown that freshwater fish is a significant determinant of internal exposure to PFOS for specificpopulations, such as anglers [START_REF] Denys | Is the fresh water fish consumption a significant determinant of the internal exposure to perfluoroalkylated substances (PFAS)?[END_REF][START_REF] Yamada | Dietary exposure to perfluoroalkyl acids of specific French adult subpopulations: High seafood consumers, high freshwater fish consumers and pregnant women[END_REF][START_REF] Zhou | Highly Elevated Serum Concentrations of Perfluoroalkyl Substances in Fishery Employees from Tangxun Lake, China[END_REF] . PFOS concentrations are indeed particularly high in freshwater species, such as common carp or Eurasian perch 12.13 . A primary research interest is to determine to what extent the body burden of PFOS in aquatic organisms is due to the degradation of its higher molecular weight derivatives, referred to as PFOS precursors (PreFOS) [START_REF] Brandsma | Dietary exposure of rainbow trout to 8:2 and 10:2 fluorotelomer alcohols and perfluorooctanesulfonamide: Uptake, transformation and elimination[END_REF][START_REF] Chen | In Vivo and in Vitro Isomer-Specific Biotransformation of Perfluorooctane Sulfonamide in Common Carp (Cyprinus carpio)[END_REF][START_REF] Lee | Dietary Bioaccumulation of Perfluorophosphonates and Perfluorophosphinates in Juvenile Rainbow Trout: Evidence of Metabolism of Perfluorophosphinates[END_REF][START_REF] Martin | PFOS or PreFOS? Are perfluorooctane sulfonate precursors (PreFOS) important determinants of human and environmental perfluorooctane sulfonate (PFOS) exposure?[END_REF][START_REF] Peng | Isomer-Specific Accumulation of Perfluorooctanesulfonate from (N-Ethyl perfluorooctanesulfonamido)ethanol-based Phosphate Diester in Japanese Medaka (Oryzias latipes)[END_REF] . It has been estimated that PreFOS may be among the largest potential historical reservoirs of PFOS, given production volumes [START_REF] Paul | A First Global Production, Emission, And Environmental Inventory For Perfluorooctane Sulfonate[END_REF] . Perfluorooctane sulfonamidoethanol based phosphate diester (SAmPAP) is a typical PreFOS that was introduced in 1974 for use in foodcontact paper and packaging 17.20 and constituted one of 3M's major fluorochemical output before it was phased out in the United States, although it is still produced in some places, including China [START_REF]Fluorochemical Use, Ditribution and Release Overview[END_REF] . SAmPAP and potential SAmPAP transformation products have been recently detected in aquatic sediments to which SAmPAP would be expected to partition preferentially [START_REF] Ahrens | Partitioning Behavior of Per-and Polyfluoroalkyl Compounds between Pore Water and Sediment in Two Sediment Cores from Tokyo Bay, Japan[END_REF][START_REF] Benskin | Observation of a Novel PFOS-Precursor, the Perfluorooctane Sulfonamido Ethanol-Based Phosphate (SAmPAP) Diester, in Marine Sediments[END_REF][START_REF] Gebbink | Perfluoroalkyl Acids (PFAAs) and Selected Precursors in the Baltic Sea Environment: Do Precursors Play a Role in Food Web Accumulation of PFAAs?[END_REF][START_REF] Higgins | Quantitative Determination of Perfluorochemicals in Sediments and Domestic Sludge[END_REF] . While SAmPAP was predicted to be recalcitrant to both aqueous hydrolysis and microbial degradation 26.27 , biotransformation of SAmPAP through aqueous exposure in fish has been demonstrated [START_REF] Peng | Isomer-Specific Accumulation of Perfluorooctanesulfonate from (N-Ethyl perfluorooctanesulfonamido)ethanol-based Phosphate Diester in Japanese Medaka (Oryzias latipes)[END_REF] . However, little is known about SAmPAP biotransformation by aquatic organisms through dietary exposure and its potential to contribute to the bioaccumulation of PFOS. In the present study, fish were dietarily exposed to SAmPAP and levels of metabolites, including PFOS, were monitored in fish tissues in order to ascertain PFOS accumulation from SAmPAP exposure.

MATERIALS AND METHODS

Chemicals:

A list of analytical standards used in this study is provided in the Supporting Information (SI). Technicalgrade N-ethyl perfluorooctane sulfonamidoethanol (NEtFOSE)-based phosphate (SAmPAP, CAS 2965-52-8) originated from Defu and was provided by Holly Lee (Department of Chemistry, University of Toronto). The product was purified by Atlanchim (Saint-Herblain, France) in order to achieve a purity close to 99%. Potential impurities in the preparation were monitored by LC-MS using the full scan acquisition mode (m/z 60-800). No impurity was detected.

Food Preparation:

Two batches of commercial fish feed (Le Gouessant, Sturgeon Grower coul 5, 47% protein, 13% fat, 1.9% fiber, 11% ash) were prepared, one for the dosing of SAmPAP and one for the control feed. SAmPAP-spiked fish feed (target concentration 5 μg/g) was prepared using methanol as a vector followed by complete evaporation of the solvent (further details are presented in the SA). Spiked feed samples were taken every day of the exposure phase during preparation of meals for the dosed fish and pooled for analysis. The SAmPAP concentration in the dosed feed (n = 6) was 1631 ± 95 ng/g food. Large discrepancies between the target concentration (5000 ng/g food) and the actual concentration (1631 ng/g food) in the dosed feed may be due to solubility problems and sorption onto the labware walls. When preparing daily rations, subsamples (≈1 g) of each of the experimental feeds were collected. The control and dosed feed collected daily were pooled in separate jars. Both pooled samples were kept at -20 °C. These pooled samples of dosed and control feeds were analyzed, using the same method performed on fish, in order to measure feed contamination.

Fish Care and Sampling:

The present study was approved by the local Ethical Committee for Animal Experimentation (no. 02664.01). Juvenile perch (∼60 g) were purchased from a farm raising fish in a closed system (Asialor, Dieuze, France) and allowed to acclimate for a month prior to chemical exposure. Two weeks prior to chemical exposure, 90 fish were distributed in five glass tanks (280 L) in an aerated, recirculated system (circular pump and biological filter). During the experimentation, the light/dark period was 16 h/8 h. The water temperature was maintained at 22 °C, the pH at 8, and the dissolved oxygen at 7 mg/L [START_REF] Lazartigues | Accumulation and half-lives of 13 pesticides in muscle tissue of freshwater fishes through food exposure[END_REF] . Several steps were taken to control the fish environment, including a daily renewal of one-third of the water with a fresh water supply, daily draining of uneaten food and feces, and continuous filtration over an active carbon unit (Norit, 3-6 mm). Among the five tanks, three tanks were designated for the SAmPAP bioaccumulation test and two tanks were designated for the blank control (Figure 1). Each of the three test tanks were divided into two connected compartments, separated by a screen through which water but no feed pellets could pass. During the exposure phase (45 days), fish in the first compartment of the test tanks (dosed group), received the SAmPAP dosed feed and fish in the second compartment (control group) received the control feed. The purpose of including fish that were not dietarily exposed to SAmPAP in the same tank as the SAmPAP dosed fish was to determine the extent of uptake from water that may occur. In addition, to evaluate water contamination, five water samples (one from each tank) were collected at the start and at the end of the exposure phase. Water samples were stored at -20 °C until further analysis. During the depuration phase (35 days), all fish were fed control feed. The daily rate of feeding was equal to 1.5% of the mean fish weight. To maintain the same feeding rate, the amount of feed distributed was recalculated once a week using the weight of the fish sampled most recently. Different sampling schedules were applied to fish from (i) the dosed groups in the test tanks, (ii) the control group in the test tanks, and (iii) the blank control tanks (Figure 1). Fish sampling always occurred before feeding. For a given time point, three fish from the dosed group (one from the first compartment of each test tank); three fish from the control groups (one from the second compartment of each test tank), and two fish (one from each blank control tank) were collected. Each fish was anaesthetized by tricaine methanesulfonate. Whole blood was collected through puncture with syringes in the caudal vein. Fish were subsequently euthanized by a lethal overdose of tricaine methanesulfonate. Each fish was weighed and dissected to collect the liver and muscle tissue without skin. Livers and muscle of each fish were weighted. All samples were stored at -20 °C until further analysis.

2.4. Chemical Analysis: 2.4.1. Water Samples. Water samples were extracted and analyzed using previously developed methods 29.30 . Three sample treatments were applied depending on the considered compound. Detailed extraction methods, chromatographic gradients, and instrumental conditions are provided in the SI. The analytical methods were validated according to SANCO 11945/2013 guidelines, except for analytes analyzed by direct injection [perfluorooctane sulfonamide (FOSA), perfluorooctane sulfonamidoacetic acid (FOSAA), Nethyl perfluorooctane sulfonamidoacetic acid (NEtFOSAA), and SAmPAP]. Mean recoveries for Nmethyl perfluorooctane sulfonamide (NMeFOSA), N-ethyl perfluorooctane sulfonamide (NEtFOSA), and NEtFOSE in water samples ranged from 85 to 103% at 20 ng/L (quantification limit). Mean recoveries (n = 5) for PFHpS and PFOS in water samples ranged from 81% to 95% at 4 ng/L (quantification limit). The quantification of the four PFASs analyzed by direct injection was based on a standard addition calibration. Detection limits were 1 ng/L for FOSA and NEtFOSAA and 5 ng/L FOSAA and SAmPAP. The reliability of the results was ensured by within-run and intrasample controls systematically made for each sample batch. The within-run controls consist of calibration check standards inserted throughout the sample batch. Intrasample controls consist of spiking the analytes in the actual samples in order to verify the accuracy of the analytical method.

2.4.2. Food and Fish Samples. For food and fish sample analysis, 5 g (±500 mg) (for muscle or liver) or 1mL (for serum) was placed into a polypropylene centrifuger tube (50 mL), in which a mixture of labeled precursors was added (internal standards including 13C4-PFOS). For each sample as well as the blank, solid-liquid extraction was performed using sequentially 10 mL of ultrapure water, 200 μL of formic acid, and 15 mL of acetonitrile. The samples were then agitated via vortex for about 30 s. A mixture composed of MgSO4:NaCl (6:1.5, w/w) was added (7.5 g) and directly followed by vortex agitation in order to avoid agglomeration. After centrifugation at 2500g for 5 min, the supernatant layer was transferred into another 50 mL centrifuge tube, where 2.07 g of the mixture MgSO4:C18:EnviCarb (1.8:0.18:0.09, w/w/w) was added, followed by agitation via vortex for 30 s. After centrifugation at 2500g for 5 min, 8 mL of the purified extract was evaporated under nitrogen to 500 μL. The extract was then filtered on a 0.2 μm filter and transferred into 1.5 mL microtubes. The final volume was adjusted to 500 μL using acetonitrile. Before analysis, 50 μL of 13C8-PFOS (used as external standard) and 150 μL of the previous extract were transferred into a polypropylene vial. Seven standard solutions in methanol containing different amounts of target analytes (0, 0.5, 1, 2, 5, 10, 20 ng) and a fixed amount of internal standard (5 ng) were prepared to obtain calibration curves for each compound. Blank samples containing only extraction solvent and internal standard were included in all batches of analysis. Recoveries were in-between 30% and 50%, depending on the analytes and the matrices. The recovery was systematically corrected by the internal standard. Analyses were performed by liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) using an HPLC pump with a binary gradient system (Agilent Technologies, Santa Clara, CA) coupled to a 6410 triple quadrupole instrument (Agilent Technologies, Santa Clara, CA) or gas chromatography coupled to tandem mass spectrometry using positive chemical ionization (GC-PCI-MS/MS) using an Agilent 7000 instrument (Agilent Technologies, Santa Clara, CA) for volatile compounds (NMeFOSE and NEtFOSE). Two diagnostic signals (qualifier and quantifier SRM transitions, respectively) were monitored for each target compound.

Data Analysis

All values reported are means (±standard errors) unless stated otherwise. All statistical analyses were performed using R version 3.0.2 31 .

2.5.1. Physical Indices. Fish weight (FW in g) was best predicted by the exponential growth model, FW = a exp(kGt), where a is a constant, kG is the growth rate [in g/(g day)], and t is the time (in days). Differences between groups or experimental phases (i.e., exposure and depuration phases) were compared using the parametric method of grouped linear regression with covariance analysis. Liver somatic index (LSI in %) was calculated as LSI = 100 × (LW/FW), where LW is the liver weight (in g). The parametric one-way analysis of variance (ANOVA) was used to assess the differences in LSI among the different groups.

2.5.2. Dietary Uptake and Depuration of SAmPAP in Fish. Depuration rate constants (kT) were calculated by fitting tissue concentrations during the depuration phase to the first-order elimination equation, ln(CF) = a + kTt, where CF is the chemical concentration in fish (ng/g wet weight fish), a is a constant, t is time (days), and kT is the overall depuration rate constant (i.e., not growth corrected) 32.33 . To correct kT for growth dilution, the growth corrected elimination rate constant kTg was calculated as kTg = kT -kG, where kT is the overall depuration constant, and kG is the growth rate. Depuration halflives (t1/2 or t1/2G) were calculated as ln(2/kT) or ln(2/kTg). Absorption efficiencies (α or αG) were determined for SAmPAP by using iterative nonlinear regression to fit data in the exposure phase to the integrated form of the kinetic rate equation for constant dietary exposure: CF = (αFCD/kT)[1exp(-kTt)] or CF = (αgFCD/kTg)[1exp(-kTgt)], where F is the feeding rate (1.5% FW) and CD is the chemical concentration in diet (ng/g food). Biomagnification factors (BMF and BMFg) were calculated for SAmPAP using the kinetic equation method (i.e., BMF = αF/kT or BMFg = αGF/ kTg). 2.5.3. Uptake and Depuration of SAmPAP Metabolites in Fish. The uptake and depuration of SAmPAP metabolites in fish can be expressed using a typical bioaccumulation mass balance model 32.33 . In addition to the distinction between uptake from the water and from the diet, a distinction was made between metabolite uptake resulting from in vivo transformation of precursor compounds, denoted "PrePF-based uptake", and metabolite uptake resulting from direct exposure, denoted "PFbased uptake" where CF,PF is the concentration of the considered metabolite in fish (ng/g wet weight fish), CW,PF is the concentration of the metabolite in water (ng/mL water), CW,PrePF is the concentration of a precursor of the metabolite in water (ng/mL water), CD,SAmPAP is the concentration of SAmPAP in diet (ng/g food), kR,PF and kR,PrePF [mL water/(g ww fish d)] are the rate constants for uptake from the water of the metabolite or one of its precursors, kD,SAmPAP [g food/(g ww fish d)] is the rate constant for dietary uptake of SAmPAP, and Fbiotransf (unitless), the biotransformation factor of precursor compounds to metabolites. 2.5.4. Tissue Distribution. Liver-to-serum (LSRs), liver-to-muscle (LMRs), and serum-to-muscle (SMRs) ratios were calculated on each sampling day to evaluate partitioning of SAmPAP and metabolites in these compartments [START_REF] Nania | Monitoring of perfluorinated compounds in edible fish from the Mediterranean Sea[END_REF][START_REF] Peng | Tissue Distribution and Maternal Transfer of Poly-and Perfluorinated Compounds in Chinese Sturgeon (Acipenser sinensis): Implications for Reproductive Risk[END_REF][START_REF] Shi | Tissue distribution of perfluorinated compounds in farmed freshwater fish and human exposure by consumption[END_REF] . The burden of PFASs in the muscle, liver, and serum was calculated by multiplying tissue concentration by the mass or volume of the considered tissue. Muscle weight was calculated as 0.42 × FW, liver weight was measured at the time of sampling, and serum volume was calculated as 0.018 × FW 37.38 .

RESULTS AND DISCUSSION

3.1. Fish Health. The fish growth rates (kG) were 0.62 g/(g d), with no significant differences observed between the test and blank control tanks or between the uptake and depuration phases (Table S1, SI). Variability in size distributions among groups was reflected by a relatively low coefficient of determination (r2 = 0.4) value for growth rate. Mortality only occurred in the test tanks, with lower mortality in the dosed groups (4% mortality rate) compared to the control groups (33% mortality rate). The fact that mortality rates were higher in the test tanks compared to the blank control tanks contrasts a number of studies recording no mortality while having higher levels of PFSAs 14.18.39.40 . Mortality may have been induced from stress resulting from isolation of the control group from the larger dosed group, considering that juvenile perch is a gregarious fish. The mean LSI was 2.16%, which is consistent with previous studies [START_REF] Mairesse | Appearance and technological characteristics in wild and reared Eurasian perch, Perca fluviatilis (L.)[END_REF] . No significant differences between the test and blank control tanks and no increase over time were observed, both of which suggest the absence of liver enlargement.

3.2. SAmPAP Dietary Bioaccumulation. 3.2.1. Background Concentrations. SAmPAP was not detected in either the control feed, samples collected from the test tank 1 h before dosing (day 0), or samples collected from the blank control tanks during the whole course of the experiment. SAmPAP background concentrations were therefore very low (below limits of detection).

3.2.2. Relative Importance of Uptake from Diet and Water in the Bioaccumulation of SAmPAP. In the dosed group, SAmPAP was detected in the muscle, liver, and serum samples. In the control group, SAmPAP was detected in the liver and serum samples at low levels and was not detected in the muscle samples (Table 1 At the end of the exposure phase, serum and liver concentrations were significantly different between the dosed and control groups, and concentrations measured in the control group were always <8% of corresponding concentrations in the dosed group. SAmPAP uptake from water was therefore considered a negligible uptake route in comparison to dietary uptake. Assimilation efficiency, depuration rate constant, and biomagnification factors for SAmPAP were therefore calculated using the concentration data from the dosed group.

3.2.3. SAmPAP Uptake. During the exposure phase, fish from the dosed group received ≈24 ng of SAmPAP/g of fish, although some fish may have been feeding above or below the feeding rate (1.5%), as evidenced by the low coefficient of determination (r2 = 0.4) for the exponential growth model. SAmPAP was detected in the serum and liver samples of the dosed group within 1 day of exposure and in the muscle within 3 days of exposure (Figure 2). SAmPAP concentrations increased in the muscle, liver, and serum (p < 0.05) throughout the exposure phase. The absorption efficiencies observed for SAmPAP varied depending on the considered tissue (α = 0.04-2.25%, Table 2). SAmPAP absorption efficiencies were low when compared to other PFASs, such as FOSA (18%) or PFOS (120%), in rainbow trout14. 40 The reduced uptake of SAmPAP (molecular weight = 1204 amu, Table S1, SI) is consistent with the typical 500 amu molecular weight cutoff for oral absorption [START_REF] Lipinski | Experimental and computational approaches to estimate solubility and permeability in drug discovery and development settings1[END_REF] . This low absorption efficiency can indeed be the result of the reduced bioavailability in the gastrointestinal tract and/or steric or electrostatic effects limiting diffusive mass transfer through the epithelium of the gastrointestinal tract [START_REF] Arnot | Development and Evaluation of a Database of Dietary Bioaccumulation Test Data for Organic Chemicals in Fish[END_REF] . The first pass effect is unlikely given the absence of SAmPAP metabolites in liver on the first day of exposure. Higher concentrations in serum and liver compared to muscle support the importance of SAmPAP enterohepatic circulation in perch, as suggested for PFOS, together with intrinsic partition of these chemicals to the liver 40.43 . However, it is unlikely that SAmPAP is reabsorbed across the intestinal mucosa following excretion via the bile, given the low absorption efficiency of SAmPAP. a Estimated from the lower bounds 95% confidence interval of the slope.

The half-lives reported in the present study are much higher than those observed for SAmPAP in a bioconcentration study (i.e., 2.4-2.7 days in medaka) [START_REF] Peng | Isomer-Specific Accumulation of Perfluorooctanesulfonate from (N-Ethyl perfluorooctanesulfonamido)ethanol-based Phosphate Diester in Japanese Medaka (Oryzias latipes)[END_REF] . Differences between half-lives obtained from bioconcentration (i.e., chemicals enter the systemic circulation directly via the gills) and the present dietary accumulation study, together with higher concentrations in serum and liver compared to muscle, support the importance of SAmPAP enterohepatic circulation in perch, which is indeed suggestive of a prolonged retention of SAmPAP and longer half-life [START_REF] Martin | Dietary accumulation of perfluorinated acids in juvenile rainbow trout (Oncorhynchus mykiss)[END_REF] .

3.2.5. SAmPAP Biomagnification. The biomagnification factor (BMF) was calculated using the kinetic approach. The BMF was not significantly greater than 1, indicating that SAmPAP dietary exposure will not result in biomagnification in perch. If we examine the equation for biomagnification (BMF = αF/kT) and set the absorption efficiency to 1%, the depuration rate kT must be 0.000 15 (i.e., half-life of 4621 days) to obtain a biomagnification factor of 1. The low BMF of SAmPAP was therefore strongly associated with its low absorption efficiency.

Intermediate and Terminal

Metabolites Bioaccumulation. 3.3.1. Background Concentrations. PFOS was detected in fish serum from the blank control tanks, but there was no significant increase of PFOS concentrations over time, and serum concentrations were <13% and <7% of corresponding concentrations in the fish from the test tanks on day 45 and 80, respectively. Similarly, PFOS was detected in the predose fish (day 0) collected from the test tanks. PFOS background concentrations were low (0.1 ng/g muscle, 0.8 ng/g liver, and 1 ng/mL serum) but above the limits of detection in fish. These PFOS levels could be attributed to the presence of PFOS in the fish from the hatchery or to ambient levels in water and feed during the experiment, although PFOS was not detected in either of the two matrices. Assuming a bioconcentration factor of 4300, PFOS concentrations in the serum of the predose and control fish (1 ng/mL) could be the result of a water concentration below the quantification limit (i.e., water concentration of 0.25 ng/L when the quantification limit in water is 4 ng/L). Other potential SAmPAP metabolites were not detected in the predose fish collected from each test tanks or in the blank control tanks during the course of the experiment. These results suggest that the trends regarding PFOS and other PFASs observed in the test tanks were the result of SAmPAP transformation in the tanks and not attributable to background concentrations.

3.3.2.

Relative Importance of Uptake from Diet vs Water in the Bioaccumulation of Metabolites. Detection frequencies and concentrations of PFASs in the dosed and control fish from the test tanks can be found in Table 1. Four potential SAmPAP metabolites, including NEtFOSAA, FOSAA, FOSA, and PFOS, were detected. Occurrence of these compounds in the dosed and control fish from the test tanks provides evidence that SAmPAP is degraded in fish. No significant differences among NEtFOSAA, FOSA, PFOS, or ΣSAmPAP metabolite concentrations in either muscle, liver, or serum were observed between the dosed and control groups at the end of the exposure phase or at the end of the depuration phase while FOSAA was only detected in the dosed group (Table 1). Considering that SAmPAP was the only source of PFASs in the tanks, the occurrence of metabolites indicates that SAmPAP could be biotransformed in fish including fish gut. However, the similarity of concentrations between the dosed and control groups indicates that fish in the test tanks were exposed to NEtFOSAA, FOSA, and PFOS through water, which was not the expected pathway. The fact that uptakewater≫uptakediet indicates that SAmPAP ingestion and in vivo biotransformation did not directly result in the accumulation of metabolites but resulted in the excretion of metabolites that were subsequently absorbed from water. This could be the result of (i) low uptake rate of SAmPAP from diet (i.e., kD,SAmPAP in eq 1) compared to uptake rates from water (i.e., kR,PF or kR,PrePF) and/or (ii) minor biotransformation contributing to tissue accumulation (i.e., Fbiotransf,SAmPAP in eq 1) of SAmPAP to metabolites in fish. An illustrative calculation was made with PFOS to evaluate the plausibility of these assumptions. In the present study, kD,SAmPAP ≈αF ≈ 0.01 × 0.015 ≈1.5 × 10-4/day, while it has been reported that kR,PFOS≈100/day.39

The ratio kD/kR≈1.5 × 10-4/100 ≈1.5 × 10-6 of the rate constants for dietary uptake and respiration includes differences in chemical uptake efficiencies from the water and diet. Assuming the production of 2 mol of PFOS from 1 mol of SAmPAP and using this kD/kR ratio of approximately 1.5 × 10-6, the uptake rates by the two routes become equal when CD,SAmPAP/(2 × CW,PFOS) is approximately 1/(2 × 1.5 × 10-6) ≈3 × 105, e.g., if CW,PFOS ≈ 1 ng/L and CD,SAmPAP ≈ 1500 ng/g, which was close to the actual SAmPAP concentration in the dosed feed (i.e., 1631 ng/g). This illustrative calculation emphasizes that, in addition to the low uptake rate of SAmPAP from diet, minor biotransformation of SAmPAP to PFOS could explain that uptakewater ≫ uptakediet. These results are consistent with the minor biotransformation of SAmPAP to PFOS in the rat after a single oral dose and confirm the slow rate of SAmPAP metabolism is fish tissues, as previously mentioned [START_REF] Seacat | Diester-Pharmacokinetic Study in Rats[END_REF] . Surprisingly, SAmPAP metabolites were not detected in the water, except for NEtFOSAA, which was quantified in a sample collected at the end of the exposure in a single test tank at a concentration of 43 ng/L. Applying bioconcentration factors from previous studies and measured tissue concentrations to estimate water concentrations in the experimental tanks, as previously undertaken with PFOS background concentrations in predose fish, it appears that water concentrations were likely below the method detection limits of PFASs in water 15.39 . It should be noted that the increase in metabolite concentrations in the water from the test tanks during the exposure phase was limited by several processes, including water renewal and irreversible sorption on the tank filtration system and walls. Further observations on the transformation pathway, bioaccumulation, and elimination of SAmPAP metabolites were carried out from data obtained from the dosed group, as fish from the control group were only sampled at the end of the exposure phase and at the end of the depuration phase (Figure 1).

3.3.3. Metabolites Uptake from Water and Transformation Pathways of SAmPAP. Significant trends of greater concentrations of all metabolites except FOSAA as a function of exposure duration were observed in the muscle, liver and serum (Figure 2). This is consistent with a gradual transformation of SAmPAP to the more stable terminal product PFOS in the test tanks. The last compound to appear was PFOS at day 28, indicating that multiple transformations steps are necessary for its production. Proposed biotransformation pathways for SAmPAP to PFOS in fish are presented in the SI (Figure S1). In the present study, NEtFOSE was not detected in the test tanks, which could be the result of the partial volatility of this compound during sampling or of a relatively high detection limit. The lack of detection of NEtFOSE in the dosed fish may also be the result of a relatively short half-life in comparison to other PFASs 18.46 . Considering the metabolites detected in the test tanks and that FOSE was not analyzed, the following transformation pathways were suggested in accordance with previous studies: (i) SAmPAP → NEtFOSE →NEtFOSAA, (ii) SAmPAP→NEtFOSE→FOSE→FOSAA →FOSA→PFOS, and (iii) SAmPAP→NEtFOSE→FOSE→ FOSA → PFOS 18.47.48 . The formation of NEtFOSA from SAmPAP was however not observed, although this metabolite was detected in Japanese medaka [START_REF] Peng | Isomer-Specific Accumulation of Perfluorooctanesulfonate from (N-Ethyl perfluorooctanesulfonamido)ethanol-based Phosphate Diester in Japanese Medaka (Oryzias latipes)[END_REF] .

3.3.4. Metabolites Depuration in Fish. During the depuration phase, there was no significant decrease in the concentration of SAmPAP metabolites, although fish were fed the control feed as previously reported [START_REF] Peng | Isomer-Specific Accumulation of Perfluorooctanesulfonate from (N-Ethyl perfluorooctanesulfonamido)ethanol-based Phosphate Diester in Japanese Medaka (Oryzias latipes)[END_REF] . The low depuration of PFOS in the present study is in contradiction with previous studies and most likely resulted from continuous biotransformation of precursors during the depuration phase rather than slow elimination of PFOS [START_REF] Martin | Dietary accumulation of perfluorinated acids in juvenile rainbow trout (Oncorhynchus mykiss)[END_REF] . For NEtFOSAA only, slopes of ln(tissue concentrations) vs time were negative, although not statistically different from 0 (Figure 2). Differences in depuration kinetics among metabolites indicate that NEtFOSAA was generally depurated faster from fish than FOSA or PFOS. The reason for the lower elimination of FOSA and PFOS could be due to (i) continuing uptake from the water, (ii) in vivo biotransformation of precursors, and/or (iii) low elimination rates. Molar concentrations ratios ofΣPFOS precursors (i.e., sum of SAmPAP, NEtFOSAA, FOSAA, and FOSA) to PFOS were investigated as an indicator of degradation of precursors (Figure S2, SI). In the muscle and liver tissues, ΣPreFOS/PFOS were generally above 1, indicating higher concentrations of PreFOS compared to PFOS. In serum, the ratio was generally around 1, indicating similar concentrations between PFOS and precursors. For the three tissues, there was a significant increase in the ratio values during the exposure phase, while a significant decrease was observed during the depuration phase. These results reflect the from SAmPAP dosed feed to control feed and highlight the progressive transformation a PreFOS-dominated bioaccumulation to PFOS-dominated bioaccumulation.

Tissue Distribution of SAmPAP and Metabolites.

The mean concentrations of SAmPAP, NEtFOSAA, FOSAA, FOSA, and PFOS were higher in the liver and serum compared to muscle tissue (Table 3).

These results are consistent with (i) the demonstrated affinity of PFOS for fish serum albumin during extracellular transport [START_REF] Jones | Binding of perfluorinated fatty acids to serum proteins[END_REF] and (ii) the affinity of NEtFOSE, NEtFOSA, and PFOS for rat liver intracellular fatty acid-binding proteins during intracellular transport50. This pattern is indeed consistent with the resemblance of PFOS and PreFOS to fatty acids50-52. Besides these general considerations, ratio values differed among PFASs, indicating that tissue distribution is also compound specific, suggesting differences in partitioning and toxicokinetics within the fish. The highest LMR and SMR were measured for SAmPAP, indicating very low muscle concentrations. SAmPAP was equally distributed to serum and liver with LSRs close to 1. NEtFOSAA was primarily distributed to serum compared to liver, which might be explained by its strong affinity to albumin. FOSA was the only PFASs primarily distributed to liver, which might be explained by its strong affinity for lipids, unlike other acidic analytes53. PFOS concentrations were higher in the liver and serum, and reported LMRs are consistent with other studies in freshwater fish 54.55 . PFOS was primarily distributed to serum compared to liver, which is consistent with previous studies36.39. When considering the weight of the tissue sampled, muscle tissue (42% of fish body weight) comprised the majority of the burden for FOSA (Table 3). Liver-to-muscle (LMR), liver-to-serum (LSR), and serum-to-muscle (SMR) ratios (mean ± standard error) were calculated using concentration data from the dosed group. FOSAA was only detected in the liver and ratios could therefore not be calculated. bDistribution ratios were calculated by dividing tissue concentrations. cThe burden of PFASs in the muscle, liver, and serum were calculated by multiplying tissue concentration by the mass or volume of the considered tissue. Proportional distributions in fish body were calculated as the relative importance of tissue burden relative to the sum of the three tissue burdens.

NEtFOSAA and PFOS were equally distributed between muscle and serum. SAmPAP was mainly distributed to liver and serum. Linear regression was used to test for temporal trends in the evolution of LSRs, LMRs, or SMRs during the exposure and depuration phases. When considering the evolution of distribution ratios during the experiment, SAmPAP ratios remained constant (see Figure S1 in the SI). LMRs and LSRs of NEtFOSAA and PFOS significantly decreased during the exposure phase and increased during the elimination phase. FOSA LSRs significantly decreased during the exposure phase as well. An increase of LMRs and LSRs of PFOS during the depuration phase has already been reported in rainbow trout and could be attributable to longer elimination half-lives in the liver [START_REF] Falk | Tissue specific uptake and elimination of perfluoroalkyl acids (PFAAs) in adult rainbow trout (Oncorhynchus mykiss) after dietary exposure[END_REF] . There was no significant difference of distribution ratios of NEtFOSAA, FOSA, and PFOS between the dosed and control groups from the test tanks at the end of the exposure and depuration periods.

Environmental Implications.

Examination of wildlife monitoring data demonstrates the signification of bioaccumulation potential for PFOS, with high trophic level predators often reported with higher PFOS concentrations 57.58 . However, laboratory-derived BMF values obtained by exposing juvenile rainbow trout to dietary PFOS were less than 1. Similarly, there appear to be some foodweb-specific differences in the biomagnification of PFOS in the environment. Martin et al. [START_REF] Martin | Perfluoroalkyl Contaminants in a Food Web from Lake Ontario[END_REF] and Asher et al. [START_REF] Asher | Enantiospecific Perfluorooctane Sulfonate (PFOS) Analysis Reveals Evidence for the Source Contribution of PFOS-Precursors to the Lake Ontario Foodweb[END_REF] measured the highest mean concentrations of PFOSA and PFOS in a benthic macroinvertebrate and slimy sculpin, occupying low trophic levels of Lake Ontario. Similarly, Gebbink et al. [START_REF] Gebbink | Perfluoroalkyl Acids (PFAAs) and Selected Precursors in the Baltic Sea Environment: Do Precursors Play a Role in Food Web Accumulation of PFAAs?[END_REF] found the highest concentrations of precursor compounds, mainly NEtFOSAA and FOSA, in zooplankton from the Baltic Sea. The occurrence of PreFOS in the environment (water or sediment) may indeed lead to the contamination of low trophic levels, while it appears higher trophic levels are mostly contaminated by PFOS. In the present study, perch were dietarily exposed to SAmPAP in a recirculated system. SAmPAP ingestion and in vivo biotransformation did not directly contribute to the body burden of PFOS in Eurasian perch. In vivo biotransformation of SAmPAP rather contributed to the metabolite (including PFOS) burden measured in water, which could be subsequently bioaccumulated in fish through reexposure to the dissolved metabolites.

Figure 1 .

 1 Figure 1. Experimental design.

  dCF,PF/dt = (kR,PFCW, PF + 𝛴 𝑃𝑟𝑒𝑃𝐹 kR,PrePFFbiotransfCW,PrePF) + (kD,SAmPAPFbiotransf,SAmPAPCD,SAmPAP) -kTCF,PF (equation 1)

Figure 2 .

 2 Figure 2. Evolution of concentrations of SAmPAP and potential metabolites in the muscle (A), liver (B), and serum (C) of Eurasian perch in the test tanks (dosed and control groups) and in the blank control tanks.

Table 1 .

 1 ). Detection Frequencies and Concentrations (mean ± standard error) of SAmPAP and Metabolites in Muscle (ng/g wet weight), Liver (ng/g wet weight), and Serum (ng/mL) of Eurasian Perch at the End of the Exposure Phase and at the End of the Depuration Phase a Differences between the dosed and control groups from the test tanks were evaluated by the unpaired Student's t-test (*p < 0.1).

		Muscle	Muscle	Liver	Liver	Serum	Serum
		Dosed group	Control group	Dosed group	Control group	Dosed group	Control group
		Detection Frequencies (%) during the Whole Course of the Experiment
	SAmPAP	71	Nd	93	83	86	17
	NEtFOSAA	79	100	88	100	83	100
	FOSAA	Nd	Nd	19	Nd	Nd	Nd
	FOSA	79	100	76	100	71	100
	PFOS	100	100	100	100	100	100
		Concentrations (ng/g ww or ng/mL) at the End of the Exposure Phase
	SAmPAP	0.16 ± 0.08	0	5.99 ± 1.84*	0.44 ± 0.29*	9.37 ± 2.25*	0.67 ± 0.67*
	NEtFOSAA	1.16 ± 0.40	0.93 ± 0.15	2.23 ± 0.33	1.82 ± 0.24	13.77 ± 3.02	11.61 ± 1.68
	FOSAA	0	0	0	0	0	0
	FOSA	1.31 ± 0.29	0.84 ± 0.30	1.94 ± 0.38	2.06 ± 0.50	2.29 ± 0.49	1.6 ± 0.31
	PFOS	0.47 ± 0.12	0.53 ± 0.13	2.32 ± 0.73	3.11 ± 0.81	16.12 ± 3.71	16.13 ± 3.27
	𝜮 SAmPAP metabolites	2.94 ± 0.19	2.30 ± 0.15	6.50 ± 0.94	7.00 ± 0.88	32.18 ± 4.21	29.34 ± 3.39
		Concentrations (ng/g ww or ng/mL) at the End of the Depuration Phase
	SAmPAP	0.16 ± 0.03*	0	6.28 ± 1.85*	0.12 ± 0.06*	4.31 ± 1.25*	0*
	NEtFOSAA	0.74 ± 0.25	0.85 ± 0.28	2.29 ± 0.67	1.63 ± 0.49	6.22 ± 2.15	9.79 ± 3.31
	FOSAA	0	0	0.04 ± 0.04	0	0	0
	FOSA	1.92 ± 0.86	1.36 ± 0.30	5.37 ± 1.73	4.06 ± 0.81	2.49 ± 0.81	3.26 ± 1.08
	PFOS	1.34 ± 0.09	1.57 ± 0.22	11.60 ± 2.08	9.79 ± 0.83	39.23 ± 8.27	50.85 ± 8.48
	𝜮 SAmPAP metabolites	4.01 ± 0.99	3.77 ± 0.57	19.30 ± 2.21	15.48 ± 1.15	47.94 ± 9.67	63.90 ± 10.87

a

Table 2 .

 2 slow compared to that of other PFASs14.18.40 . In the absence of significant SAmPAP depuration, SAmPAP depuration rate constants and half-lives were estimated using the lower bounds 95% confidence interval of the slopes obtained (Table2)[START_REF] Benskin | Biodegradation of N-Ethyl Perfluorooctane Sulfonamido Ethanol (EtFOSE) and EtFOSE-Based Phosphate Diester (SAmPAP Diester) in Marine Sediments[END_REF] . Depuration Rate Constant (kT or kTg), Depuration Half-Life (t1/2 or t1/2g), Assimilation Efficiency (α or αg), Biomagnification Factor (BMF or BMFg), and Estimated Time To Achieve Steady-State (tss) for SAmPAP Using Gross Concentration Data or Growth-Corrected Concentration Data

	3.2.4. SAmPAP Depuration. During the depuration phase, the concentration of SAmPAP decreased in
	the dosed group, but slopes of ln(concentrations or growth corrected concentrations) vs time (i.e., kT or

kTg) were not statistically different from zero. Growth-corrected elimination rate constants (kTg = kT -kG) were indeed negative values (i.e., increase in SAmPAP growthcorrection concentrations during the depuration phase) except in the serum, as previously observed for other extremely high KOW chemicals (log KOW > 8).

44

Loss by fecal egestion, biotranformation, or respiratory ventilation of SAmPAP were therefore very

Table 3 .

 3 Tissue Distribution of SAmPAP and Metabolites a

			distribution ratios b		proportional distribution in the three tissues (%) c
		LMR	LSR	SMR	muscle	liver	serum
	SAmPAP	35.66 ± 3.21	0.88 ± 0.09	39.37 ± 2.73	19.72 ± 2.19	40.84 ± 3.13	39.44 ± 2.75
	NEtFOSAA	2.92 ± 0.16	0.27 ± 0.02	11.50 ± 0.58	55.25 ± 3.16	13.96 ± 3.27	30.79 ± 2.40
	FOSA	3.25 ± 0.28	1.71 ± 0.27	2.34 ± 0.26	80.55 ± 1.05	11.68 ± 0.84	7.77 ± 0.78
	PFOS	7.22 ± 0.40	0.23 ± 0.01	32.44 ± 1.27	37.47 ± 0.88	12.35 ± 0.46	50.18 ± 0.97
	a

ACKNOWLEDGMENTS

We gratefully acknowledge Holly Lee and Scott Mabury (Department of Chemistry, University of Toronto), for donating the technical-grade SAmPAP and for insightful input in the planning of this study, and Alain Iuretig and Yannick Ledore (UR Animal et Fonctionnalités des Produits Animaux, Universite de Lorraine/INRA), for their assistance during the fish experiment. The present study is supported by the Agence de l'Eau Rhin-Meuse, the Conseil Général de Moselle, the Région Lorraine and the Zone Atelier Moselle.

Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/abs/10.1021/acs.est.6b05598


Yinan Fan

Adrien Girard

Michael Waals

Caroline Salzemann

Alexa Courty

Ag@Pt Core-Shell Nanoparticles for Plasmonic Catalysis

Keywords: synthesis, seed-mediated growth, core-shell nanoparticles, plasmonic, catalysis

Bimetallic nanocatalysts (NCs) composed of two metal elements are a promising way to achieve high catalytic performance, based on so-called synergistic effects. In this context, we have designed bimetallic core-shell nanoparticles (NPs) combining a plasmonic metal Ag as core and a catalytic metal Pt as shell. Ag@Pt core-shell NPs were synthesized via a seedmediated growth process allowing fine control of the Ag core size and Pt shell thickness. In a first step, Ag seeds (8-13nm) with a narrow size distribution (<10%) were synthesized via chemical reduction by tuning the reaction time, heating ramp and temperature. In the second step, we tuned the Pt shell thickness (from 1 to 6 atomic layers) by adjusting the ratio of Pt precursor to silver seed concentrations. The Pt shell covered all the particles, mostly heterogeneously. The robustness and versatility of the synthesis method was tested and successfully applied to different systems (Ag/Au core and Pd/Pt shell). Subsequently, we studied the impact of size and shell thickness on Ag@Pt optical properties by ultraviolet-visible absorption spectroscopy (UV-Vis) and by discrete dipole approximation (DDA) calculations.

Finally, Ag@Pt core shell NPs were demonstrated to be more catalytically active than Ag and Pt NPs of similar sizes through a model reaction of reduction of 4-NP into 4-AP in the presence of NaBH4. These results highlight the synergistic catalytic effect of Ag and Pt species which may be due to the electronic structure of Ag@Pt core-shell NPs, but may also arise from the LSPR excitation of the Ag core. This shows the potential of these particles for applications in plasmonic catalysis

Introduction

Noble metallic nanoparticles (NPs) such as gold, silver and copper have been long recognized in both the catalysis and plasmonic communities. [START_REF] Che | The Influence of Particle Size on the Catalytic Properties of Supported Metals[END_REF][START_REF] Haruta | Size-and Support-Dependency in the Catalysis of Gold[END_REF][START_REF] Bhosale | Silver Nanoparticles: Synthesis, Characterization and Their Application as a Sustainable Catalyst for Organic Transformations[END_REF][START_REF] Dong | Nanosilver as a New Generation of Silver Catalysts in Organic Transformations for Efficient Synthesis of Fine Chemicals[END_REF] In particular, it has been shown that light excitation of plasmonic metallic NPs can induce photochemical reactions. [START_REF] Christopher | Visible-Light-Enhanced Catalytic Oxidation Reactions on Plasmonic Silver Nanostructures[END_REF][START_REF] Mukherjee | Hot Electrons Do the Impossible: Plasmon-Induced Dissociation of H 2 on Au[END_REF] Various mechanisms are known to be competitively or cooperatively at work during plasmonic catalysis, which are closely related to the excitation of localized surface plasmon resonances (LSPR) -collective oscillations of electrons. The main mechanisms include local amplifications of the LSPR-induced electric field that can modify some properties of neighboring molecules (absorption, Raman diffusion, emission...) but also promote chemical transformations, generation of hot electrons and local heating. [START_REF] Mukherjee | Hot Electrons Do the Impossible: Plasmon-Induced Dissociation of H 2 on Au[END_REF][START_REF] Gargiulo | From Optical to Chemical Hot Spots in Plasmonics[END_REF][START_REF] Liu | Chiral Plasmonic Nanocrystals for Generation of Hot Electrons: Toward Polarization-Sensitive Photochemistry[END_REF] Compared to conventional thermal-driven catalysis, plasmonic catalysis can significantly reduce the temperature required to achieve the desired catalytic activity and offers thus mild reaction conditions improving the stability and the selectivity of the target products [START_REF] Gellé | Applications of Plasmon-Enhanced Nanocatalysis to Organic Transformations[END_REF][START_REF] Li | Recent Advances in Plasmonic Nanostructures for Enhanced Photocatalysis and Electrocatalysis[END_REF] . In addition, plasmonic catalysis is also capable of exceeding the equilibrium limit of thermocatalytic reactions [START_REF] Mao | Beyond the Thermal Equilibrium Limit of Ammonia Synthesis with Dual Temperature Zone Catalyst Powered by Solar Light[END_REF] .

After significant successes in the production of single-component nanomaterials with controlled structure (size and morphology) [START_REF] Favier | Palladium Nanoparticles in Polyols: Synthesis, Catalytic Couplings, and Hydrogenations[END_REF][START_REF] Heuer-Jungemann | The Role of Ligands in the Chemical Synthesis and Applications of Inorganic Nanoparticles[END_REF][START_REF] Jin | Atomically Precise Colloidal Metal Nanoclusters and Nanoparticles: Fundamentals and Opportunities[END_REF][START_REF] Liu | Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles[END_REF] , research interest has turned to multicomponent nanomaterials, which are defined as a hybrid structure consisting of two or more nanoscale domains with distinct differences in chemical compositions or physical characteristics [START_REF] Liu | Core-Shell Ag-Pt Nanoparticles: A Versatile Platform for the Synthesis of Heterogeneous Nanostructures towards Catalyzing Electrochemical Reactions[END_REF][START_REF] Xu | Pentagram-Shaped Ag@Pt Core-Shell Nanostructures as High-Performance Catalysts for Formaldehyde Detection[END_REF][START_REF] Alayoglu | Ru-Pt Core-Shell Nanoparticles for Preferential Oxidation of Carbon Monoxide in Hydrogen[END_REF][START_REF] Ferrando | From Theory to Applications of Alloy Clusters and Nanoparticles[END_REF] . The appeal of these nanostructures lies in their possible synergistic effect due to the cooperation/interaction between different components or domains at the nanoscale, which often leads to improved performance or properties for a given application [START_REF] Mao | Beyond the Thermal Equilibrium Limit of Ammonia Synthesis with Dual Temperature Zone Catalyst Powered by Solar Light[END_REF] .

A number of strategies based on wet chemistry methods have been proposed and proven applicable for the synthesis of multicomponent nanostructures with desired properties, among which seed-mediated growth, galvanic replacement and ion exchange methods are capable of producing nanomaterials with heterogeneous properties [START_REF]Bimetallic Nanostructures: Shape-Controlled Synthesis for Catalysis, Plasmonics, and Sensing Applications[END_REF][START_REF] Han | Facile Direct Seed-Mediated Growth of AuPt Bimetallic Shell on the Surface of Pd Nanocubes and Application for Direct H2O2 Synthesis[END_REF][START_REF] Bhol | Modern Chemical Routes for the Controlled Synthesis of Anisotropic Bimetallic Nanostructures and Their Application in Catalysis[END_REF][START_REF] Rodríguez | Bimetallic Ag-Au Nanoparticles Inside Mesoporous Titania Thin Films: Synthesis by Photoreduction and Galvanic Replacement, and Catalytic Activity[END_REF][START_REF] Da Silva | Galvanic Replacement Reaction: Recent Developments for Engineering Metal Nanostructures towards Catalytic Applications[END_REF] .

Traditional metal catalysts such as platinum, rhodium and palladium are widely used in many thermal heterogeneous catalysis processes due to their high reactivity, in particular in the production and storage of hydrogen, CO oxidation, organic compound synthesis and may other. applications [START_REF] Moumen | Microwave Assisted Synthesis of Palladium Nanoparticles in an Aqueous Emulsion of Copolymer: Application to Catalysis[END_REF][START_REF] Liu | Getting Insights into the Temperature-Specific Active Sites on Platinum Nanoparticles for CO Oxidation: A Combined in Situ Spectroscopic and Ab Initio Density Functional Theory Study[END_REF] Compared to thermal catalysis, plasmonic catalysis can offer opportunities for reducing the energy input and increasing the selectivity and catalytic activity. The judicious association of traditional Pt metal with lower-cost plasmonic metals such as Ag in the form of a core-shell Ag@Pt NPs thus offers the possibility to retain of combining the plasmonic properties of Ag with the high catalytic performance of the Pt shell. These NPs therefore have great potential for plasmonic catalysis.

In this article, we report the synthesis of Ag@Pt core-shell NPs with low structural dispersion, controlled core size and shell thickness. The influence on the Pt shell growth of reaction time, heating ramp, temperature, [Pt shell] / [Ag seed] concentration ratio and seed size have all been thoroughly investigated. We show that this synthesis is versatile and robust and can thus be extended to different plasmonic core and catalytic shell materials. The core-shell NPs were fully characterized by scanning and transmission electron microscopy (SEM, TEM, STEM), Electron Energy Loss (EELS), and Energy-dispersive X-ray (EDS) spectroscopies. The plasmonic properties of Ag@Pt NPs were studied in relation to their structural parameters by UV-visible spectroscopy coupled with discrete dipole approximation (DDA) calculations.

Finally, the catalytic activity of the Ag@Pt NPs was tested through a model reaction of reduction of 4-NP into 4-AP in the presence of NaBH4.

Materials and methods

Chemical

Commercial reagents were used without further purification, and stored in a glovebox in order to avoid any traces of moisture or air contamination. Silver nitrate dioctyl ether, oleic Acid (90%), chloroform (≥ 99%), Pt(acac)2, Pd(acac)2 and gold (III) chloride trihydrate were purchased from Sigma-Aldrich. Oleylamine (80-90%) was bought from ACROS Organics.

Synthesis of Ag nanoparticle seeds

The synthesis of Ag NPs (Figure SI.1) was based on the reduction of silver nitrate (AgNO3) in oleylamine (OLA), which served as a solvent, reducing agent and stabilization agent. 1 mmol of AgNO3 was dissolved in 5ml of OLA and held at 30℃ for 35 minutes to ensure its complete dissolution. To avoid light degradation of the AgNO3 precursor, the reaction flask was covered with aluminum foil. The solution turned light yellow, indicating the formation of the Agoleylamine complex [START_REF] Lee | Versatile and Robust Synthesis Process for the Fine Control of the Chemical Composition and Core-Crystallinity of Spherical Core-Shell Au@Ag Nanoparticles[END_REF] . The solution was then heated up to a target temperature (120°C, 140°C or 240℃) using a set ramping rate (180, 220 or 450℃/h) in N2 atmosphere. At 120°C, the color of the solution turned dark brown, indicating the formation of Ag(0). Below this temperature the (OLA)n -Ag complex + was stable and no reduction occurred. After reaching the reaction temperature, the reaction solution was kept at this temperature for a variable reaction time (treaction) of between 30 and 120 minutes with strong stirring, before cooling to room temperature. The resulting solution was then washed with acetone or ethanol to remove oleylamine and excess by-products. The solution was centrifuged at 5500 rpm for 5 min and the precipitate was dispersed in chloroform (CHCl3). To stabilize the final Ag NPs, 20µL of oleic acid (OA) was then added. In some cases, additional washing processes were performed to further remove excess OLA or to reduce the size distribution via a size selective precipitation process [START_REF] Andrieux-Ledier | Synthesis of Silver Nanoparticles Using Different Silver Phosphine Precursors: Formation Mechanism and Size Control[END_REF] .

Synthesis of Au nanoparticle seeds

Au NPs were synthesized by reduction of gold (III) chloride trihydrate ( HAuCl4•3 H2O ) in OLA at 100℃ 27 . 0.025 mmol of gold (III) chloride trihydrate was dissolved in 5 ml of OLA at 30℃ for 30min, then the solution was heated to 100℃ with a ramping rate of 220℃/h. The solution was kept at this temperature for 20 min. The product was washed with acetone, followed by centrifugation at 5500 rpm for 5 min and dispersion in CHCl3.

Synthesis of core-shell NPs

The formation of core@shell NPs was achieved by a seeding process. In a test tube, 0.105 µmol L -1 of Ag (or Au) NP seeds dispersed in chloroform were dried by nitrogen flow and redispersed in 1.7 ml of dioctyl ether. 300µl of a Pt(acac)2 (or Pd(acac)2) solution in OLA (7mM) was added to the seed solution and the mixture was maintained at a target temperature for a given reaction time in a dry bath. Different temperatures (150°C, 200°C and 250°C) and reaction times (60 min, 90 min and 120 min) were used. After reaction, the mixture was cooled to room temperature. The bimetallic nanoparticles formed were then extracted by washing with ethanol (ethanol/reaction solution volume ratio of 1:2) and centrifugation at 5500 rpm for 5 min. The supernatant was removed and the collected precipitate was redispersed in 1ml of chloroform before adding 20µl of OA.

The shell thickness was controlled by varying the ratio R= [shell precursor]/ [metal seeds]. For Ag@Pt NPs, an initial minimum shell precursor concentration of 7 mmol/L was calculated as necessary to deposit at least one atomic layer of Pt on the surface of the larger 13.3nm Ag seeds at a concentration of 1.05*10 -4 mmol L -1 . This corresponds to an R value fixed at R=6.7*10 4 . For all syntheses, the volume of added Pt(acac)2 solution in OLA was kept constant while the concentration was adjusted from 7 mmol/L to 110 mmol/L, corresponding to a ratio variation by a factor of 16. Similarly, for Ag@Pd and Au@Pt NPs, different Pd(acac)2 or Pt(acac)2 concentrations were also used to get different Pt or Pd shell thicknesses.

Structural characterizations and composition

TEM images were obtained with a JEOL JEM1011 100 kV. The samples for TEM imaging were prepared by putting droplets of a dilute solution on a carbon-coated copper grid.

EDS was carried out in a JEOL-JSM-5510LV SEM operating at 20KV. Each sample was washed twice to remove more of the excess of ligands which have a large negative impact on the result of EDS (and EELS). The samples with high concentration were deposited on a conductive adhesive (carbon tape usually used to fix to the solid substrate) instead of the usual silicon substrate because of the close position of Si and Pt peaks in the EDS measurements. It should be noted that for a variation in composition between different positions on the sample of less than 3%, it is reasonable to assume that the sample is homogeneous. STEM images were taken using a high-angle annular dark field (HAADF) detector in a Nion Ultrastem 200 Cs-corrected microscope operating at 100kV with a probe size of about 0.07 nm. The probe convergence and HAADF detector minimum collection semi-angles were 35 and 75mrad respectively, meaning that HAADF images were dominated by Z-contrast, although some diffraction contrast is possible. EELS Elemental maps were obtained using electron energy-loss spectroscopy in the spectrum-image (SI) collection mode using the Ag M-and Pt M-edges.

UV-Visible spectroscopies. UV-visible absorption spectra in the range 300nm to 700nm were acquired from all samples with a VARIAN Cary 5000 UV-Vis-NIR spectrometer. QS UV-Vis cells with a length l=1mm were used to measure the NPs' optical response. The concentration of NP seeds in chloroform solution is determined using the Beer's Law formula: A310nm=l ε CM (1) where, A310nm is the UV-visible absorption value of the solution at 310 nm, ε is the molar extinction factor of the metal M and is equal respectively to 2973 and 1200 cm -1 LMol -1 for M=Ag and Pt; and CM is the total atomic concentration after synthesis. The NP concentration can be deduced by: 𝐶

!"# (𝑚𝑜𝑙 𝐿 $% ) = & ! '()* , "# -×/ $%&'()*,! , - 01 . /%)* 0 2 - ×!3 (2)
Discrete-Dipole Approximation (DDA). The absorption spectra are compared to those calculated using the discrete dipole approximation (DDA) method, which is a flexible and powerful method widely used to simulate the scattering and absorption by nanometer-sized objects of different geometries and dielectric properties. Its principle and accuracy are well described in previous articles [START_REF] Lee | Versatile and Robust Synthesis Process for the Fine Control of the Chemical Composition and Core-Crystallinity of Spherical Core-Shell Au@Ag Nanoparticles[END_REF][START_REF] Wei | Collective Surface Plasmon Resonances in Two-Dimensional Assemblies of Au and Ag Nanocrystals: Experiments and Discrete Dipole Approximation Simulation[END_REF][START_REF] Draine | Discrete-Dipole Approximation For Scattering Calculations[END_REF] . Here, the extinction spectra of Ag and Ag@Pt NPs are calculated using the free software DDSCAT 7.3 written by Draine and Flatau 31 as well as the dielectric functions of bulk Ag and Pt taken from Palik's handbook [START_REF]Handbook of Optical Constants of Solids[END_REF] . The refractive index of the surrounding medium is fixed at the value n = 1.44, which corresponds to that of the dispersion solvent, i.e. chloroform.

Results and discussion

Synthesis of Ag@Pt core-shell nanoparticles

The physicochemical properties of silver and platinum, including their reduction potentials are very different. [START_REF] Gilroy | Bimetallic Nanocrystals: Syntheses, Properties, and Applications[END_REF] Hence if both precursors coexist initially in the chemical media they can be successively reduced respective to their redox potential to form core-shell structures. [START_REF] Hou | One-Pot Synthesis of Noble Metal Nanoparticles with a Core-Shell Construction[END_REF][START_REF] Calvo | Thermodynamics of Nanoalloys[END_REF] However, considering that platinum has higher redox potential, higher cohesive energy and higher surface free energy than silver, a catalytic platinum core and a plasmonic silver shell is expected. [START_REF] Ferrando | From Theory to Applications of Alloy Clusters and Nanoparticles[END_REF][START_REF] Calvo | Thermodynamics of Nanoalloys[END_REF] In this context, to achieve the formation of the desired plasmonic-core@catalyticshell bimetallic structures and prevent any alloying, it is necessary to carry out a two-step successive reduction of the metals to selectively deposit the platinum at the silver surface. [START_REF] Han | Facile Direct Seed-Mediated Growth of AuPt Bimetallic Shell on the Surface of Pd Nanocubes and Application for Direct H2O2 Synthesis[END_REF][START_REF] Gilroy | Bimetallic Nanocrystals: Syntheses, Properties, and Applications[END_REF] For this purpose, we have developed a versatile and robust synthesis process in two-steps: i) formation of silver NP seeds with a low structural dispersion (size, shape, crystallinity) ii)

growth of the platinum shell on these seeds by heterogeneous reduction and nucleation at their surface. From this synthesis process, both core size and shell thickness can be controlled independently to obtain different Ag-coresize@Pt-shellthickness combinations (Figure 1). Since the ramping rate is fixed and reduction is observed as early at 120°C, we can reasonably assume that reduction and nucleation occur in a similar way for all samples.

Diffusion-controlled growth in the Ostwald ripening process is known to be influenced by temperature. [START_REF] Prasad | Digestive Ripening of Thiolated Gold Nanoparticles: The Effect of Alkyl Chain Length[END_REF] This is due to changes in interfacial energy, growth rate coefficients, and solubility with temperature. Therefore, as the diffusion and growth rate coefficients increase with increasing temperature, larger NP sizes are obtained at higher reaction temperatures. This is consistent with our observations (Figure 2) as well with those of Prasad et. al. As we can see, this peak shifts from 410 nm to 409 nm and to 406 nm, when the size of the NPs increases from 8.4nm, to 10.5nm and then to 13.3nm. This observation may seem surprising because it is in disagreement with the effect of the size of the NPs according to the Mie theory [START_REF] Horvath | Gustav Mie and the Scattering and Absorption of Light by Particles: Historic Developments and Basics[END_REF] . However, similar results were already reported [START_REF] Peng | Reversing the Size-Dependence of Surface Plasmon Resonances[END_REF] . In fact, a blue-shift then redshift switch is observed when increasing the size of the nanoparticles. The absorption band red shifts when the Ag NP size increases from 12.5 nm to 20 nm, but it blue shifts when the Ag NP size increases from 2 nm to 12.5 nm, in chloroform. The position of the LSPR peak in small NPs results from a balance between two counteracting quantum size effects, namely the effects induced by the electron spill-out yielding a red shift with decreasing size and a reduced screening of d surface electrons yielding a blue shift. The inversion behaviour of LSPR at the turning point around 12nm has been reported previously and was interpreted by a decrease in conductivity in the outer metallic layer compared to the conductivity of the inner Ag atoms due to chemical interactions, in particular with the presence of a capping agent. [START_REF] Peng | Reversing the Size-Dependence of Surface Plasmon Resonances[END_REF] However, this anomalous behaviour has been revisited in the light of more recent works based on correlated STEM-EELS and optical experiments, that pushed the accessible size range below 10nm. In addition to the known LSPR red-shift effect due to refractive index, the theoretical and experimental results showed that embedding NPs in an environment also alters the relative weight of the two-counteracting size-dependent LSPR shifts, yielding similar trend: a blueshift and red-shift upon decreasing the size of silica embedded Ag NPs. [START_REF] Campos | Plasmonic Quantum Size Effects in Silver Nanoparticles Are Dominated by Interfaces and Local Environments[END_REF] In chemically-stabilized NPs as in our case, the situation is seemingly different due to the electronic structure of the metal core being altered by the localized bonds with ligand and the liquid environment as compared to a solid silica matrix, and it is not surprising that contradictory trends have appeared in the literature over the years due to the sensitivity of this detailed balance upon solvent, surface sate of the NPs, nature of metal-ligand interaction, nature of interface (gas, solid, liquid)

and surface coverage of capping ligands. [START_REF] Campos | Plasmonic Quantum Size Effects in Silver Nanoparticles Are Dominated by Interfaces and Local Environments[END_REF] Our result indicates that in our conditions the balance is slightly tilted toward a larger contribution of the electronic spill-out yielding a redshift of the LSPR upon decreasing the NPs size below 12 nm, which may be supported by a decreased conductivity of the outer metallic layer. The characterization of the core-shell structure of the bimetallic NPs is not obvious using conventional TEM because of the lack of contrast between Ag core and Pt shell. To circumvent this problem and have no ambiguity on the structure, HAADF and EELS measurements were performed on different bimetallic samples, the EELS being used to make elemental maps (Figures 3 a andb). The Ag@Pt core-shell structure was confirmed with the silver core indicated in yellow and the platinum shell in blue (Figure 3b). The platinum shell appeared inhomogeneously distributed and discontinuous across the Ag NP surface. Furthermore, we observed that a thin atomic layer of silver was deposited on the platinum shell of some NCs in a discontinuous way (see arrow on Figure 3b). This atomic outer yellow layer can be explained based on a galvanic replacement mechanism, which consists in the oxidation of the silver core

Synthesis of

(having the lower redox potential) by the Pt ions (having the higher redox potential) [START_REF] Cobley | Engineering the Properties of Metal Nanostructures via Galvanic Replacement Reactions[END_REF] , in competition with the direct reduction of Pt ions by OLA and subsequent reduction of silver ions by OLA. The galvanic process is not predominant in our synthesis method, probably due to a lowering of the redox potential of platinum following its complexation with OLA (formation of Pt(OLA)4(Acac)2). [START_REF] Mourdikoudis | Oleylamine in Nanoparticle Synthesis[END_REF][START_REF] Yin | Effects of Metal Ion-Ligand Interactions on the Synthesis of Metal Nanostructures with Controlled Morphology[END_REF] The reduction rate of the precursor thus becomes higher than the rate of the galvanic reaction and Ag@Pt NPs are formed by the deposition of Pt(0) on silver NPs before Pt 2+ can react with Ag seeds. Figure 7 shows typical TEM images of bimetallic core-shell NPs obtained from an Ag core size of 8.4 nm (Figure 4a) with different Pt shell thicknesses. The Pt content has been determined by both TEM and EDS (Table SI.9.I) and a corresponding average shell thickness has been estimated. For the minimum Pt concentration corresponding to a ratio of 1R, Ag@Pt NPs keep the spherical shape of the original Ag seeds (Figure 4a). The average diameter of these NPs is 9.0 nm from which we can deduce a thickness of Pt shell of 0.4 nm (about1 Pt atomic layer). Multiplying R by 2, the Pt shell of Ag@Pt NPs starts to become inhomogeneous (Figure 4b), meanwhile the Pt shell thickness slightly increases to 0.85 nm (around 3 Pt layers).

When Stranski-Krstanov (SK): the atoms are first assembled in Frank-van der Merwe mode, but growth then switches to Volmer-Weber mode. [START_REF] Xia | Seed-Mediated Growth of Colloidal Metal Nanocrystals[END_REF] The small lattice mismatch between Ag and Pt (4.1%) and their identical crystalline structures should favour FM growth at least at the beginning or for low Pt content. Conversely, their strong difference in bulk cohesive energy and surface energy disfavours the FM mode to the benefit of the VW mode. As the deposition of Pt was carried out at relatively high temperature, 200°C, it is expected that Pt diffusion should be promoted. At low Pt concentration, considering the high temperature and the low surface energy of silver, Pt atoms should diffuse on the Ag surface relatively easily. However for higher Pt content, as soon as the Ag surface is covered by a Pt shell, this serves as new nucleation centre where Pt atoms stick to form heterogeneous growth due to Pt's higher dissociation energy compared to Ag, limiting the diffusion [START_REF] Xia | Seed-Mediated Growth of Colloidal Metal Nanocrystals[END_REF] . We can then assume that a mixed growth mode combining FM and VW modes (SK mode) is the most likely at higher Pt content, explaining the bumpy morphology. In the STEM-HAADF images, the bumpy surface is confirmed for high platinum contents (Figure 3a).

Figure 4. Representative TEM images of Ag8.4nm@Pt NPs synthesized for different [Pt precursor]/ [Ag seed] ratio equal to (a) 1R, (b) 2R, (c) 4R, (d) 8R

Using the same synthesis process as previously described, Ag@Pt core-shell NPs with tunable Pt thickness have been obtained from the 10.5nm (Figure 2b) and 13.3nm (Figure 2c) Ag NPs.

The TEM images and the corresponding size dispersion are reported in the supporting information (Figures SI.

Figure 5. Evolution of the Pt shell thickness of Ag@Pt NPs for different [Pt precursor]/ [Ag seeds] ratio and Ag core size

The Pt atomic percentage increases with the increase in [Pt precursor]/ [Ag seeds] ratio, regardless of the core size (Figure 5). However, for a fixed ratio, the atomic percentage of Pt is always larger for the smaller core size. This is not surprising. Indeed, as we always use the same concentration of Ag seeds whatever the core size, for smaller Ag NPs, the total NP surface is lower compared to larger sizes. Hence, for a given [Pt precursor]/ [Ag seeds] ratio, the ratio of platinum to the available Ag surface increases when the core size decreases. This explains the thicker shell of platinum on smaller seeds. In addition, as discussed previously, the platinum initially deposits on the silver surface and can diffuse relatively easily. As soon as the silver surface is covered by platinum, further growth occurs by heterogeneous nucleation of platinum on the formed platinum shell. The diffusion is less efficient due to the strong cohesion energy and more bumpy shells are then obtained. Figures 6a andb show representative STEM-HAADF images of Ag10.5nm @Pt4 atomic layers coreshell NPs. The Pt shell seems to grow directly on the surface of Ag NPs, since non clear discontinuity was observed between the lattices. High magnifications in Figures 6a1 and6a2 shows the lattice fringes of the Ag core and Pt shell respectively. The two lattice fringes with d-spacings of 0.21nm and 0.24 nm can be attributed respectively to (200) and (111) crystalline planes of Ag (Figure 6a1). The d-spacing of 0.23 nm is attributed to the (111) crystalline plane of Pt (Figure 6a2) [START_REF] Xu | Pentagram-Shaped Ag@Pt Core-Shell Nanostructures as High-Performance Catalysts for Formaldehyde Detection[END_REF][START_REF] Zhao | Biochemical Synthesis of Ag/AgCl Nanoparticles for Visible-Light-Driven Photocatalytic Removal of Colored Dyes[END_REF] . The corresponding EELS maps show the distribution of Ag and Pt in the nanoparticle (Figure 9c-d). They confirm the formation of core-shell NPs with an Ag core (yellow) and a more or less continuous Pt shell (blue).

Moreover, we observe some hollow structures in Figure 6d, constituting about 3%-4% of the total probed particles and many Ag@Pt NPs are coated with a very thin Ag layer as observed previously for Ag8.4nm@Pt4atomiclayer NPs. The outer thin Ag layer can be formed through galvanic replacement (GR) arising from the oxidation of the metal core ((lower redox potential) by the metallic salt (higher redox potential) [START_REF] Cobley | Engineering the Properties of Metal Nanostructures via Galvanic Replacement Reactions[END_REF] . The hollow NPs can thus be the result of the formation and coalescence of voids due to the Kirkendall effect (KE) coupled with GR [START_REF] Chee | Direct Observation of the Nanoscale Kirkendall Effect during Galvanic Replacement Reactions[END_REF] .

Stability of Ag@Pt core-shell NPs. In the targeted purpose of using the core-shell nanoparticles for catalytic application, we must ensure that they do not change over time. The combination of the low surface energy of silver and the high dissociation energy of platinum indeed makes the Ag@Pt systems thermodynamically unfavourable. An evolution toward nanoalloys such as AgPt@Ag or in a best case, Ag@PtAg cannot be excluded. A study of their stability was thus performed. The NP solutions were always stored in a glove box under nitrogen atmosphere.

We have studied their structural evolution with time. Two samples were tested: (i) a fresh sample (deposition of a colloidal solution synthesized 2 days before and left in the glove box) and (ii) an old sample (deposition of a colloidal solution synthesized 11 days before and left in the glove box). Both samples show a few hollow structures (Figure SI.12). The percentage of hollow structures increases very slightly from 3-4% to 4-5% with aging. This increase is not significant. These hollow structures are thus formed during the synthesis process and are stable in time. The GR and KE reaction between Ag seeds and Pt 2+ ions appear thus to weakly interfere with the growth of Pt on Ag seeds, resulting in a small amount of Ag/Pt alloy nanoparticles with hollow interiors instead of core-shell structures. 16 In the case of core-shell structures, we can also note that the outer thin Ag shell thickness does not increase with time, confirming its formation during the synthesis.

Optical properties of Ag@Pt NPs.

Plasmonic catalysis applications rely on matching the light wavelength to the LSPR. We have characterized the optical properties of the synthesized Ag@Pt NPs as a function of core size and shell thickness. Ag NPs are known to have a well-defined Localized Surface Plasmon Resonance (LSPR) band in the visible range. Figure 7a shows a series of experimental spectra collected from monometallic Ag8.4nmNPs and bimetallic core-shell Ag8.4nm@Pt NPs with different Pt thickness. Ag8.4nm NPs exhibit an LSPR band at around 410nm as expected [START_REF] Chapus | Tunable SERS Platforms from Small Nanoparticle 3D Superlattices: A Comparison between Gold, Silver, and Copper[END_REF] . The spectra were recorded from colloidal solutions at room temperature with a fixed concentration of 2*10 -8 mol L -1 . Moreover, the intensity of the Ag plasmon band decreased with the increase in the surface coverage by Pt atoms. The LSPR band maximum also shifts progressively to the blue (Table SI.13). For a Pt thickness of 1.6 nm (around 6 atomic layers) the LSPR is completely damped and not visible in the UV-Vis spectra. It should be noted that the Pt shell should therefore not exceed a thickness of about 1 nm (i.e. 4 atomic layers) in order to not overdamp the LSPR of the Ag core, which is necessary for plasmonic catalysis applications.

NPs with various shell thicknesses as indicated in both panels

To gain deeper insights into the relationship between the SPR bands and the shell thickness, DDA calculations were carried out to simulate the absorption spectra of Ag8.4nm and Ag8.4nm @Pt NPs. In these calculations, the refractive index of the surrounding medium is set to 1.4

to simulate the chloroform used as the solvent. The dielectric functions of bulk Ag and Pt were taken from Palik's handbook. [START_REF]Handbook of Optical Constants of Solids[END_REF] The calculated spectra obtained for different Pt shell thicknesses are plotted in figure 7b and are consistent with the general trend observed in the experimental spectra. They show also a blue shift and damping of the SPR band when Pt shell thickness increases, in good agreement with experimental absorbance spectra (Table SI.13).

We can note a broadening of the SPR band in the experimental spectra compared to the calculated spectra, which can be attributed to the size distribution of the NPs which is not taken into account in the calculations. These results confirm the core-shell structure of the NPs.

Extension to other core and shell materials

The synthesis method of Ag@Pt core-shell NPs that we developed in this work is easily transposable to other bimetallic systems and yields core-shell NPs while keeping a narrow size dispersion and homogeneous shape. We have tested the robustness of our synthesis by replacing i) the Pt shell by a Pd shell and ii) the Ag core by an Au core. Synthesis of Au@Pt NPs. For the synthesis of the core-shell Au@Pt NPs, we used the Au10.4nm

Synthesis of

NPs as seeds and the Pt(acac)2 precursor. Different [Pt precursor] / [Au seed] ratios (R,2R,4R and 8R) were tested. The reaction was performed at 200℃ for 1 h and followed by cooling to room temperature. The solution was precipitated out using ethanol (ethanol /reaction solution volume ratio of 1:2) and centrifuged at 5500 rpm for 5 min. The precipitate was then redispersed in chloroform.

For a [Pt precursor] / [Au seed] ratio equal to 1R, Au@Pt NPs with 0.3nm Pt shell are obtained, and their shape remains quasi-spherical as for the initial Au seeds (Figure SI.15 and Table SI.15). As previously observed for others core-shell, when increasing R to 2R and then 8R, the Au @Pt NPs become bumpy, the Pt shell thickness increases progressively from 0.6 nm to 1.2 nm according to the TEM images. The Pt shell thickness is deduced as previously from the composition determined by EDS. The efficiency of Pt overgrowth on Au is thus similar to that on Ag, which can be explained by a similar lattice mismatch for both metals with Pt (around 4.1 %). [START_REF] Zaleska-Medynska | Metal-Based Bimetallic Nanoparticles: The Effect of the Structure on the Optical, Catalytic and Photocatalytic Properties[END_REF]

Catalytic activity of Ag@Pt NPs

The catalytic activity of Ag@Pt core-shell NPs was studied through a model reaction of reduction of 4-nitrophenol (4-NP) in 4-aminophenol (4-AP) by sodium borohydride (NaBH4) in aqueous phase. This reaction is indeed reported as a model reaction for investigating the catalytic properties of nanoparticles made of silver, gold, palladium etc. [START_REF] Kuroda | Reduction of 4-Nitrophenol to 4-Aminophenol over Au Nanoparticles Deposited on PMMA[END_REF][START_REF] Noh | Synthesis and Catalytic Evaluation of Dendrimer-Templated and Reverse Microemulsion Pd and Pt Nanoparticles in the Reduction of 4-Nitrophenol: The Effect of Size and Synthetic Methodologies[END_REF][START_REF] Wang | Facile Synthesis of Well-Dispersed Pd-Graphene Nanohybrids and Their Catalytic Properties in 4-Nitrophenol Reduction[END_REF] The reaction This model reaction takes place in the aqueous phase, whereas all the NPs synthesized in this work are dispersed in an organic solvent such as chloroform. These NPs are therefore coated with hydrophobic ligands such as OLA associated to a very small amount of OA (see SI.16).

Thus, to use these NPs, it is necessary to transfer them into water. For this purpose, the organic ligands were replaced by hydrophilic ligands. We chose a shorter charged ligand, Mercaptosuccinic acid (MSA), to allow transfer of the NPs into water and to promote adsorption of reagents onto the free surface of the NPs for catalysis. MSA has a mercaptan function (HS) which gives it a stronger affinity for metallic NPs than OLA, which promotes ligand exchange. [START_REF] Dabera | Retarding Oxidation of Copper Nanoparticles without Electrical Isolation and the Size Dependence of Work Function[END_REF][START_REF] Yang | A Versatile AuNP Synthetic Platform for Decoupled Control of Size and Surface Composition[END_REF] The ligand exchange procedure used was adapted from the literature and is described in the supporting information. [START_REF] Salgueiriño-Maceira | Water-Based Ferrofluids from Fe x Pt 1-x Nanoparticles Synthesized in Organic Media[END_REF] It works for all the NPs synthesised in this work.

EDS analysis was performed to confirm the ligand exchange (see Table SI.16.1).

The catalytic activity of Ag@Pt NPs was compared with that of the monometallic NPs (Ag and Pt). The synthesis process of Pt NPs adapted from that used for Ag NPs and described in details in supporting information, allowed us to obtain NPs with a mean diameter of 9.7 ± 0.5 nm. Ag and Ag@Pt NPs with a core size of 8.4 nm close to that of Pt were thus selected and the thickness of the Pt shell was set at 1.1 nm (around 4 Pt atomic layers). This Pt shell thickness is the limit for observing Ag LSPR. The concentration of the NP solution was calculated from UV-Vis spectra (see experimental section) and then adjusted to have the same total surface area regardless of the NPs. The temperature was fixed at 20℃. 8). From the slop of the lines, we deduced the apparent rate constants (Kapp) (see table SI.17) of the reaction for Ag, A@Pt and Pt nanocatalysts. Monometallic Pt NPs showed a 24% increase in Kapp compared to that of Ag NPs while the Kapp for Ag@Pt1.1nm remains larger with a 50% increase compared to Pt NPs.

These results highlight the synergistic catalytic effect of Ag and Pt species, which may be due to several effects.

First, this effect may stem from the electronic structure of Ag@Pt core-shell NPs. DFT calculations have shown that in Ag@Pt core-shell NPs, electrons accumulate in the Pt region, and Pt atomic population become more negatively charged compared to Ag core subsequent to Pt coating, indicating a charge transfer from Ag to Pt layers. [START_REF] Ma | Core-Shell Ag@Pt Nanoparticles Supported on Sepiolite Nanofibers for the Catalytic Reduction of Nitrophenols in Water: Enhanced Catalytic Performance and DFT Study[END_REF] This is associated with a strong improve catalytic reaction by several process namely enhanced EM fields (plasmonic antenna), plasmonic heating effect and hot electron generation. [START_REF] Baffou | Nanoplasmonics for Chemistry[END_REF][START_REF] Xin | Copper-Based Plasmonic Catalysis: Recent Advances and Future Perspectives[END_REF] The latter two are related to the LSPR excitation and the former to the increase of photon flux at the NP due to the focusing of the electromagnetic field by the plasmonic antenna. Each of the effect show potential to contribute to the catalytic reaction of 4-NP reduction. The local photo-induced heating around an individual NP in a diluted solution under irradiation can be estimated [START_REF] Baffou | Nanoplasmonics for Chemistry[END_REF] and is very low (around 3 mK) (see details in SI. 19). This calculation is strictly valid for non-thermodynamically interacting plasmonic NPs, as thermodynamic coupling could produce much larger temperature variation. [START_REF] Baffou | Photoinduced Heating of Nanoparticle Arrays[END_REF] Here the concentration in our solution is so low that we can safely assume that the NPs can be considered as non-thermodynamically interacting. Therefore, we cannot attribute the increase in reactivity observed with Ag@Pt NPs to local heating due to Ag absorption. The role of hot electrons can be also excluded as the Pt shell thickness is too high [START_REF] Zhang | Plasmon-Induced Interfacial Hot-Electron Transfer Directly Probed by Raman Spectroscopy[END_REF] . Only antenna effect can be considered in our case, as it was demonstrated that enhanced EM fields can indeed promote the adsorptions of reactants on the NPs surface according to the literature [START_REF] Xin | Copper-Based Plasmonic Catalysis: Recent Advances and Future Perspectives[END_REF][START_REF] Mori | Enhancement of the Photoinduced Oxidation Activity of a Ruthenium(II) Complex Anchored on Silica-Coated Silver Nanoparticles by Localized Surface Plasmon Resonance[END_REF] and thus the charge transfer mechanism for the reduction of 4-NP into 4-AP. Ag@Pt NPs appears thus as promising candidate for catalysis owing to their intrinsic catalytic activity and to the LSPR of Ag core. Further experiments have to be carried out for the maximum utilization of these core-shell NPs with tunable composition and size.

Conclusion

Ag@Pt core-shell NPs were successfully synthesized with controlled size, size dispersion and chemical composition through a versatile approach using Ag seeds with selected sizes. Seed size control was obtained by vaying the reaction temperature and using a selected ramping rate.

The final Pt thickness of the core-shell NPs was shown to depend on the [shell precursor]/ [metal seeds] concentration ratio for an optimum reaction temperature of 200°C and a reaction time of 1 hour. The core-shell structure was confirmed by EELS maps and UV-visible spectroscopy coupled with DDA calculations. Finally, this protocol was extended to other plasmonic and catalytic bimetallic systems. Bimetallic core-shell Ag@Pd and Au@Pt NPs with controlled composition were synthesized without strong modification of the synthesis process.

The shell growth was still controlled by [metallic precursor]/[seed] concentration ratio. The plasmonic properties of the Ag8.4nm@Pt NPs were studied as a function of the Pt shell thickness by UV-vis absorption spectroscopy and DDA calculations. We obtained a critical Pt shell thickness of 1.1 nm (4 atomic layers) above which the Ag core LSPR is completely damped.

Figure 1 :

 1 Figure 1: Synthesis of plasmonic-core@catalytic-shell nanoparticles in a two-step seeding process a) Ag core synthesis and b) platinum shell growth. Core size and platinum shell thickness are controlled independently. T is the temperature and xR= x[Pt(acac)2]/ [Ag seeds] ratio

Figure 2 shows

 2 Figure 2 shows TEM images of silver NPs obtained after 60 minutes of reaction at different heating temperatures: 120℃, 140℃ and 240℃ with a fast-ramping rate of 450°C/h. The ramping times necessary to reach the target temperatures were 11 min., 16 min. and 36 min. respectively. This means that the total time was 71, 76 and 96 min. (Figure SI.5.I). As previously, the TEM images show polycrystalline spherical NPs. The size distributions (Figure SI.5.II) highlight an increase in the average NP size from 8.4 ± 0.7 nm to 10.5 ± 0.9 nm and 13.3 ± 1.3 nm for an increase in reaction temperature from 120°C to 140°C and 240°C respectively, with a size dispersion of less than 9% in all cases. As seen previously, the low polydispersity obtained after one hour of reaction can be explained by the high ramping rate which induces burst nucleation associated with probable digestive ripening during the first hour

Figure 2 .

 2 Figure 2. TEM images of Ag NPs obtained by fast ramping (450°C/h) after one hour of reaction at different reaction temperatures, (a) 120℃, (b) 140℃, (c) 240℃ and (d) the corresponding UV-visible spectra.

Figure 2 .

 2 Figure 2.d shows the different absorption spectra obtained for the three different NP sizes. It is worth noting that the maximum absorption peak position of Ag NPs is dependent on their size.

  Ag@Pt NPs. The Pt shell was grown at the surface of the Ag NPs seeds by adding 300µL of a solution of Pt(acac)2 salt solubilized in OLA in a single step to 1.7mL of a solution of silver NPs (9x10 -8 mol L -1 ) dispersed in diocthylether. The mixed solution was then kept at a fixed temperature for one hour. Reduction of the Pt(acac)2 salt was performed in the presence of the spherical Ag NPs and nucleation occurred directly on the silver surface, thereby inducing the formation of a platinum shell around the silver core. It was observed that a minimal temperature of 200°C is necessary to induce the reduction of platinum (Figure SI.6); for lower temperatures no platinum is detected. At 200°C, the proportions of platinum and silver determined by EDS are constant (less than 3% variation) over the whole sample, which confirms the formation of bimetallic Ag/Pt NPs, excluding the co-existence of two distinct populations of pure Ag and pure Pt. A higher temperature or a longer reaction time have no influence on the composition of the NPs (Figure SI.7). The corresponding absorption spectra clearly show a strong damping and broadening of the plasmon peak for bimetallic NPs compared to pure Ag, indicating that the silver environment has changed.

Figure 3 .

 3 Figure 3. (a) Typical HAADF-STEM dark field image of AgPt bimetallic NPs (b) corresponding EELS mapping with Ag core (yellow) and Pt shell (blue) confirming the formation of the core-shell structure.

  the [Pt precursor]/ [Ag seed] ratio is increased to 4R, 8R and 16R, the Pt shell thickness increases to 1.1 nm (about 4 Pt layers), 1.6 nm (about 6 Pt layers) and 2.0 nm (about 8 Pt layers), respectively (Figure 4c-d). By increasing the platinum content from 2R (Figure 4b) to 8R (Figure 4d), the NCs' surfaces become more and more irregular. At the highest content corresponding to 16R (Figure SI.9), most of the NCs have a very bumpy surface. The evolution from spherical NPs toward irregular morphology with the increase in the [Pt precursor]/ [Ag seeds] ratio can be explained by the different deposition/diffusion processes during the reactions at different Pt content. From a general point of view, the shell formation occurs by Pt atom deposition on sites that have the highest surface energy followed by diffusion to a site with a lower surface energy. In the literature three well-known growth modes for the shell have been described in the past (i) Frank-van der Merwe (FM): the atomic growth occurs layer by layer; (ii) Volmer-Weber (VW): the atoms form 3-D islands on the seed surface; (iii)

Figure 6 .

 6 Figure 6. Electron microscopy characterization of Ag@Pt core shell NPs with 10.5nm core and 4 Pt layers. (a), (a1), (a2) and (b) Typical HAADF-STEM dark field images of NPs, (c) and (d) corresponding EELS mapping (in blue Pt and in yellow Ag).

Figure 7 :

 7 Figure 7: (a) Experimental and (b) DDA calculated absorption spectra of core-shell Ag@Pt

  kinetic is easily followed by UV-vis spectroscopy during the colour changes associated with the conversion of 4-NP to 4-AP. In Figure SIX, we can observe a typical example of the trend of the absorption bands related to the consumption of 4-NP and the formation of 4-AP during the chemical reduction of the nitro functional group. In the absence of the catalysts, no reaction was observed at reasonable times.

Figure 8 .

 8 Figure 8. Plots of ln(C/C0) versus reaction time for the conversion of 4-NP with the use of Ag@Pt, Ag and Pt NPs as nanocatalysts.

(

  Ag)s-(Pt)d hybridization at the origin of the charge redistribution between Ag cores and Pt shells. The increased electron density in the Pt layer makes more electrons available for the NP reduction at the surface and may be responsible of the improved catalytic performance of Ag@Pt compared to pure Ag and Pt NPs. Secondly, the LSPRs of Ag NPs are excited during the recording of the in-situ UV-visible spectra performed in this study (see Figure SI.18). We did not observe the NP LSPR since concentration are extremely low, and the absorbance is dominated by 4-NP in the same spectral range at 400nm. In plasmonic catalysis, light can

  [START_REF] Prasad | Digestive Ripening of Thiolated Gold Nanoparticles: The Effect of Alkyl Chain Length[END_REF][START_REF] Shimpi | Digestive Ripening: A Fine Chemical Machining Process on the Nanoscale[END_REF]

  10. I and SI.10. II). The evolution of Pt shell thickness for different [Pt

precursor]/ [Ag seed] ratio and Ag core size is summarized in Table

SI.11.

  Ag@Pd NPs. Ag NPs 13.3 nm in diameter coated by oleylamine are selected as seeds and Pd(acac)2 was used as the source of Pd. Different [Pd precursor]/ [Ag seed] ratios (1R or 2R) were prepared by changing the Pd(acac)2 concentrations to get different Pd shell thicknesses as realized previously for Ag@Pt NPs. The Ag@Pd NPs were synthesized by heating the solution to 200℃ for 1h. The final solution was washed in ethanol followed by centrifugation during 5 min at 5500 rpm and the precipitated particles were then redispersed in chloroform. Typical TEM images of Ag@Pd NPs obtained for the two different [Pd precursor]/ [Ag seed] ratios (1R and 2R) are shown in Figure SI.14. The NPs present quasi-spherical shapes with a narrow size distribution for both ratios. The average Pd shell thicknesses deduced from EDS measurement is in agreement with TEM calculations (see Table SI.14). It increases from 0.3 to 0.5 nm when increasing the [Pd precursor]/ [Ag seeds] ratio from 1R to 2R. A series of spectra collected for monometallic Ag13.3nm NPs and bimetallic core-shell Ag13.3nm@Pd NPs with different Pd thickness is presented in Figure SI.14. Ag13.3nm NPs exhibit an SPR band at around 406nm. Moreover, the intensity of the Ag plasmon band decreases with the increase in the surface coverage by Pd atoms. The SPR band maximum also shifts progressively to the blue. This confirms the core-shell structure of the NPs.27

We have also demonstrated that the Ag@Pt1.1nm NPs were catalytically active in the conversion of 4-NP to 4-AP in the presence of NaBH4. In addition, these NPs present a higher activity than monometallic Ag and Pt NPs with similar sizes. These results therefore highlight the synergistic catalytic effect of Ag and Pt species which may be due to the electronic structure of Ag@Pt core-shell NPs, but which may also benefit from the LSPR excitation of the Ag core.

Further work is therefore underway to study the plasmonic catalytic activity of these core-shell NPs as a function of their composition and size. The high quality of the samples and the high degree of control of the core and shell parameters will be of great interest for these future studies, and should help to disentangle the different mechanisms involved in plasmonic catalysis.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge.

Additional figures illustrating the synthesis of AgNPs and Ag@Pt NPs with the evolution of the reaction conditions (ramping rates and temperatures), the characterization of the NPs (TEM images, size distributions and UV-vis spectra), the detail of the Pt thickness calculation from the TEM images, the ligand exchange process and characterization and finally additional data concerning the catalytic performance of Ag@Pt Pps for the reduction of 4NP into 4AP.

AUTHOR INFORMATION

Corresponding Author

*Pr Alexa Courty: alexa.courty@sorbonne-universite.fr; * Dr Caroline Salzmann: caroline.salzemann@sorbonne-universite.fr ;

Author Contribution

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT

The authors thank the Chinese Scholarship Council for the thesis grant. AC, CS and AG thank Sorbonne University and the CNRS for facilities. MW thanks the EU ESTEEM3 grant agreement (No 823717) and the French METSA network (FR CNRS 3507).

EELS mapping and in-situ UV-visible spectra for monitoring the catalytic activity of Ag@Pt NPs via the model reaction of reduction of 4-NP into 4-AP