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Redox Photocatalysis with Water-soluble Core-shell CdSe-ZnS Quantum Dots Timothée Chauviré, Jean-Marie Mouesca, Didier Gasparutto, Jean-Luc Ravanat, Colette Lebrun, Marina Gromova, Pierre-Henri Jouneau, Jérôme Chauvin, Serge Gambarelli, and Vincent Maurel J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b04396 • Publication Date (Web): 18 Jun 2015 Downloaded from http://pubs.acs.org on July 9, 2015

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The Journal of Physical Chemistry

Redox Photocatalysis With Water-Soluble Core-Shell CdSeZnS Quantum Dots Timothée Chauviré1,2, Jean-Marie Mouesca1,2, Didier Gasparutto1,2, Jean-Luc Ravanat1,2, Colette Lebrun1,2, Marina Gromova1,2, Pierre-Henri Jouneau3,4, Jérôme Chauvin5, Serge Gambarelli1,2, Vincent Maurel1,2* 1) Univ. Grenoble Alpes, INAC, SCIB, F-38000 Grenoble, France 2) CEA, INAC, SCIB, F-38054 Grenoble, France. 3) Univ. Grenoble Alpes, INAC, SP2M, F-38000 Grenoble, France 4) CEA, INAC, SP2M, F-38054 Grenoble, France. 5) Univ. Grenoble Alpes, DCM - UMR 5250, BP 53, F-38041 Grenoble CEDEX 9, France

ABSTRACT: The use of CdSe-ZnS type I quantum dots as redox photocatalysts with visible light in water is investigated by monitoring the oxidation reaction of 8-oxo-2’-deoxyguanosine and the reduction of nitrophenylalanin derivatives. The detection of reaction intermediates and the identification and quantitation of reaction products establish that both reactions are photocatalyzed simultaneously by CdSe-ZnS type I quantum dots. The photocatalyzed reactions are observed without the need of sacrificial reactants and with high turnover numbers (>400). These results demonstrate that CdSe-ZnS type I quantum dots can work efficiently as redox photocatalysts in water solution.

nanoparticles by avoiding interaction of the photoexcited charge with the medium surrounding the nanoparticle20,25.

Introduction Colloidal semiconductor nanostructures including quantum dots (QD’s) are promising candidates for applications in photocatalysis 1,2 because of their intense and tunable absorption in the visible range 3-5, low photobleaching6 and flexible functionalization chemistry by organic ligands 7. In the last few years many systems based on QD’s alone8, or coupled with inorganic catalysts9-12 or with enzymatic catalysts13,14 were reported as efficient photocatalysts for water reduction reaction. QD’s were also reported as efficient redox photocatalysts for the oxidation15,16 or the reduction 17-19 of organic substrates, without the need of additional catalysts. Up to now two types of QD’s were mainly studied for applications in photocatalysis. i/ Core only QD’s are made of only one type of semiconductor. They are studied in photocatalysis with the aim to give to substrates the easiest access to both photoexcited hole and electron15-19. ii/ Core-shell type II QD’s are made of two types of semiconductors (see scheme 1) whose energy levels promote the step of charge separation of the electron-hole pair by localizing photoexcited electron and hole in different parts of the nanoparticle20: one in the core, the other in the shell. Such nanostructures help the transfer of the charge localized in the shell to the substrates21,22 and can be used to build efficient photocatalytic systems11,23,24. Core-shell type I QD’s are made of two types of semiconductors (see scheme 1) whose energy levels promote the localization of photoexcited electron and hole in the QD’s core. They were initially designed to improve the photoluminescence of

Scheme 1: Energy levels and localization of photoinduced charge carriers in Core-Shell QD’s. “VB” and “CB” stand for valence band and conduction band energy levels, “e” and “h” in circles indicate the most stable levels of the photoinduced excited electron and hole respectively. Nevertheless few reports demonstrate that the charge transfer from photoexcited core-shell type I QD’s or nanorods to various acceptors26-29 and donors30 can be efficient. This efficiency can be explained by the decreasing of back electron transfer rates thanks to the large gap shell of type I core-shell QD’s. This effect was evidenced by Lian and co-workers, who described electron transfer from photoexcited CdSe-ZnS QD’s to

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anthraquinone and showed that the ZnS shell impedes the back electron transfer from anthraquinone radical anion to the positively charged QD’s (noted QD[h]+) and enhances dramatically the lifetimes of these species28,31. This positive effect of the ZnS shell makes CdSe-ZnS type I QD’s promising candidates for applications in photocatalysis. Up to now, to the best of our knowledge, no quantitative study of redox photocatalysis with type I QD’s was reported in the Literature. In the present paper the redox photocatalytic activity of water soluble CdSe-ZnS type I QD’s was investigated. The redox photocatalytic activity of CdSe-ZnS QD’s was tested by using substrates whose one-electron redox potentials in water are in principle well adapted for electron and hole transfer from photoexcited QD’s (see below). The electron and hole transfers between photoexcited CdSe-ZnS QD’s and the substrates were investigated by EPR spectroscopy, by study of the quenching of QD’s photoluminescence and by nanosecond flash photolysis. The decay of substrates was monitored by HPLC analysis and the products of reaction were identified by HPLC, mass spectrometry and coupled HPLC-MS/MS techniques (see Scheme 2).

Scheme 2. Principle of the study. Overview of the photocatalytic process reported here and of the different techniques used to explore excited states, intermediates and reaction products. Study of electron and hole transfers between substrates and quantum dots Identification of free radicals produced from the substrates by EPR spectroscopy EPR spectroscopy experiments were performed for samples containing CdSe-ZnS quantum dots and the substrate under study with in situ visible-light irradiation. These EPR experiments were performed at room temperature for NO2PhAlaMe (Figure 1A) and for the parent nitrophenylalanine (noted NO2PhAla) (Figure S4) as electron acceptors. In both cases a rather strong and well-resolved signal centered in the g~2 (~3360 G) region indicates clearly the formation of free radicals.

Chart 1: Chemical structures of Nitrophenylalaninemethylester (NO2PhAlaMe), Methylviologen (MV2+) and 8-oxo-2’deoxyguanosine (8oxodG) Results The water soluble CdSe-ZnS QD’s under study are covered by histidine ligands (see Experimental section) and were characterized by UV-visible, fluorescence (see ESI Figure S1a), TEM microscopy (see ESI Figure S1b) and NMR spectroscopy (see ESI Figure S2). Histidine ligands were chosen because they were reported as providing good water solubility, while not binding very strongly to the ZnS surface since they can be easily exchanged by stronger ligands32,33. Nitrophenylalanine methyl ester (NO2PhAlaMe, E0 = -0.46 V/NHE at pH=5.6, see ESI section S3) and methylviologen34 (noted MV2+, E0 = -0.45 V/NHE at pH=7 35) were tested as electron acceptors. 8-oxo2’-deoxyguanosine (8oxodG) was tested as an electron donor (E0 = 0.74V/NHE at pH=7 36).

Figure 1 : EPR spectra of NO2PhAlaMe- anion radical and 8oxodG+ cation radical obtained by charge transfer with photoexcited QD’s. Experimental EPR spectra (in black) are obtained during irradiation of QD’s (10 µM) in water solutions with A/ NO2PhAlaMe (10mM) at room temperature or with B/ 8oxodG (10 mM) at 170 K. (see experimental section for EPR settings). Simulated EPR spectra (in red) are computed by Easyspin Matlab toolbox37 (see parameters in SI Table S4 for the Frame A and in the main text for the Frame B).

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To the best of our knowledge EPR spectra of NO2PhAlaMeand NO2PhAla- have never been reported in the literature. In order to support the identification of these anion radicals, the hyperfine coupling constants obtained by numerical fitting of the experimental spectra were compared with DFT calculations (see ESI sections S4 and S5). The good agreement between experimental and calculated hyperfine coupling constants confirms that the irradiation of CdSe-ZnS quantum dots in the presence of electron acceptors NO2PhAlaMe and NO2PhAla produces the corresponding anion radicals NO2PhAlaMe- and NO2PhAla-. To complete the EPR study of electron acceptors, EPR experiments were performed at room temperature by in situ irradiation of an aqueous sample containing the electron acceptor MV2+ and CdSe-ZnS quantum dots (see Figure S5). A very intense EPR spectrum could be observed and was attributed to the MV+ free radical by comparison of the numerical simulation with hyperfine coupling constants reported in the literature for this radical38

Table 1 : Stern-Volmer analysis of QD photoluminescence quenching by the three different quencher NO2PhAlaMe, MV2+ and 8oxodG. [a] K is the Stern-Vollmer constant ob tained by linear fitting of the Stern-Vollmer plot with 1 . [b] kq is the bimolecular quenching constant calculated in the hypothesis of a dynamic quenching, then K = kq.t0. t0 is the average photoluminescence lifetime of QD’s taken here as t0 = 20 ns as a higher limit of typical values for CdSe-ZnS QD’s (see ref41).

Quencher

NO2PhAlaMe

MV2+

8oxodG

K (L.mol-1)[a]

160 ± 20

30100 ± 500

540 ± 30

kq (L.mol-1.s-1)[b]

8.0.109

1.5.1012

2.7.1010

In the case of 8oxodG, the quenching efficiency at low concentrations of quencher is slightly higher than for NO2PhAlaMe. The Stern-Vollmer plot is linear only at low concentrations with a downward curvature at higher concentrations. From the low concentrations part one can compute KSV = 540 L. mol -1 and kq = 2.7.1010 L.mol -1.s-1. Such a quenching rate close to 1010 L.mol -1.s-1 pleads again for a dynamic quenching.

In the case of the electron donor 8oxodG, we observed no signal by in situ irradiation of an aqueous solution containing 8oxodG and CdSe-ZnS quantum dots at room temperature. However, by in situ irradiation at 170 K, an EPR spectrum (See Fig 1B) was observed and exhibited only one broad line. This is not surprising since the radical 8oxodG+ is known to be very reactive and its detection by EPR was reported only in frozen solution39. The observed EPR spectrum was fitted by a 6.8 G wide lorentzian line at g = 2.0045 by analogy with the parameters reported by Sevilla et al.39,40 for the 8oxodG+ radical (linewidth = 5.3 G and g = 2.0048). The simulated and experimental spectra do not match perfectly and the spectral parameters used for the simulation are slightly different from those reported by Sevilla. These differences can be due to the fact that our experiments were performed at pH~8, when Sevilla et al. worked at slightly acidic pH (the pKa of 8oxodG+ /8oxodG(-H) couple is 6.6 36). So we most probably observed a spectrum due to a mixture of 8oxodG+ and 8oxodG(-H) while spectral parameters reported by Sevilla et al. correspond to pure 8oxodG+. Quenching of CdSe-ZnS quantum dots photoluminescence A quenching of the quantum dots photoluminescence was observed by adding each of the three substrates MV2+, NO2PhAlaMe and 8oxodG (see Figure 2, frames A, B and C, resp.). However the concentrations necessary to observe this quenching and the shape of Stern-Vollmer plots are different from one substrate to the other. In the case of NO2PhAlaMe, the Stern-Vollmer plot is approximately linear and its slope KSV = 160 L. mol -1 is low. The bimolecular quenching constant can be estimated to be kq = 8.109 L.mol -1.s-1 (see Table 1) by considering a photoluminescence lifetime of 20 ns as a higher limit of typical values for CdSe-ZnS QD’s (see for instance reference 41). The linear shape of the Stern-Vollmer plot and the fact that kq is lower than 1010 L.mol -1.s-1 plead for a dynamic quenching of QD’s photoluminescence42.

Figure 2. Quenching of QD’s photoluminescence by electron acceptors (NO2PhAlaMe and MV2+) and electron donor (8oxodG). Increasing quantities of MV2+ (frame A), NO2PhAlaMe (frame B) and 8oxodG (frame C) were added to 2µM QD’s aqueous solutions containing histidine (1mM) (See experimental section). Inserts: experimental Stern-Vollmer

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plots I0/I=f([quencher]) (black plots) and numerical linear fitting (red plots).

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Actually at higher Laser power, long-lived bleaching signals increased strongly (data not shown). At the Laser power used here (0.04 mJ/flash) each QD receives in average one photon per flash. It would be useful to work at even lower Laser power. Such a study was not possible due to the weakness of the signals and the limited sensitivity of our experimental setup.

In the case of MV2+, the quenching is much more efficient. The Stern-Vollmer plot is linear and its slope KSV = 3.0.104 L. mol -1 is high. It would correspond to a very high bimolecular quenching constant kq = 1.5.1012 L.mol -1.s-1 above the diffusion limit. These results are typical of a static quenching due to the formation of complexes between QD’s and MV2+. They are comparable to results reported by Impellizzeri et al26,41 for the quenching of CdSe-ZnS quantum dots by MV2+ in THF, which was attributed to static quenching with even higher Stern-Vollmer constants (KSV ~ 105 L. mol -1).

In conclusion for LFP experiments, samples containing QD’s alone and QD’s in the presence of 8oxodG exhibited an intense fast decaying bleaching that can be unambiguously attributed to QD[e-h]. Moreover these LFP experiments exhibited also a very weak bleaching signal at longer time scale, so it can be concluded that no efficient production of photoinduced QD[e-] was evidenced by these experiments.

Nanosecond Laser flash photolysis The nanosecond Laser flash photolysis (LFP) experiments reported in Figure 3 aimed to detect photoinduced transient species derived for the QD. In these experiments the absorbance is monitored at the wavelength of the QD’s first exciton peak (λ=510nm) in order to detect a possible bleaching and the influence of the donor 8oxodG and the acceptor MV2+ on this bleaching. A fast decaying bleaching signal was observed in the first 200 ns following the Laser flash for CdSe-ZnS QD’s alone and for CdSe-ZnS QD’s with 8oxodG (Frames A and B). The observation of such a bleaching in this time range for CdSe-ZnS QD’s was already reported by Lian et al.28 In agreement with these authors, this first signal was attributed to the decay of the QD’s exciton (noted QD[e-h] hereafter). The lack of influence of the addition of 8oxodG on this fast decaying bleaching suggests that 8oxodG does not bind to the QD’s surface. This is compatible with the dynamic quenching of QD’s photoluminescence reported above for 8oxodG. When MV2+ as an acceptor is added to a sample containing QD’s and 8oxodG, no more bleaching signal was observed (see Figure 3 frame C). This is consistent with the very efficient static quenching of photoluminescence of QD’s by MV2+. Some complementary LFP experiments monitoring the + MV free radical were performed (see Figure S7). From these experiments a quantum yield of ~1.7 % could be estimated for the photoinduced electron transfer from QD’s to MV2+.

Figure 3 : Transient absorption spectra measured at the first exciton peak (λ λ = 510 nm) for QD’s alone (frame A), QD’s + 8oxodG (frame B), QD’s + 8oxodG + MV2+ (frame C). Laser excitation conditions: wavelength = 355 nm, power = 0.04 mJ/pulse, flash duration = 5 ns. Samples A) 0.36 µM of QD’s , B) 0.36 µM of QD’s + 1 mM of 8oxodG and C) 0.36 µM of QD’s + 1 mM of 8oxodG + 1 mM of MV2+.

A weak bleaching signal (~0.1 to 0.2 mOD) seems to be present in the µs time range for both samples “QD’s alone” and “QD’s with 8oxodG” (see Frames A and B), and to be slightly more intense in the presence of 8oxodG. In the case of the sample containing QD’s and 8oxodG this long lived bleaching could be due to negatively charged quantum dots, noted QD[e] produced by electron transfer from 8oxodG to photoexcited QD’s (ε(bleaching QD[e-], 510 nm) = ε(QD, 510 nm)/2 ~ 80000 mol.L-1.cm-1, see reference43). These very weak bleaching signals contrast strongly with the intense and long lived (100 µs range) bleaching signal due to QD[e-] reported for instance by Sharma et al. for CdSe QD’s in the presence of phenylenediamine as an electron donor44. Moreover the weak bleaching signals observed here could also be due to a biphotonic excitation of a small fraction of QD’s in the sample.

Photocatalytic activity of CdSe-ZnS quantum dots for the oxidation of 8oxodG and the reduction of NO2PhAla-Me. Quantitative study of the decay of substrates In order to study quantitatively the photocatalytic activity of CdSe-ZnS QD’s towards 8oxodG and NO2PhAlaMe, aqueous samples containing QD’s and the substrate(s) under inert at-

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mosphere were irradiated by visible light using a 150 W halogen lamp equipped with a 420 nm high-pass filter (see Experimental Section). The decay of the substrates 8oxodG and/or NO2PhAlaMe was monitored by HPLC with UV-visible absorption detection (see Figure S9 for UV-vis spectrum and for HPLC chromatograms of these substrates).

The curves plotted in green and red in the Figure 4A show the variations of the 8oxodG concentration of a solution containing i/ QD’s as photocatalysts, ii/ 8oxodG as an electron donor and iii/ NO2PhAlaMe (green plot) or MV2+ (red plot) as an electron acceptor. In these experiments 8oxodG and the electron acceptor were introduced in great molar excess compared with quantum dots. With MV2+ as an electron acceptor, a 25 % decay of 8oxodG is observed during the first hour of irradiation. Then the reaction slows down and a ~60 % decay of the initial stock of 8oxodG is observed after 3 hours. A similar trend was observed when using NO2PhAlaMe as an electron acceptor, but the initial decay of 8oxodG is faster (~50% after one hour) and a decay of 80 % is observed after 3 hours. The variations of the NO2PhAlaMe concentration could be monitored by HPLC during the same experiment and are plotted in Figure 4B (red curve). It decreases by 60% after 3 hours of irradiation. The corresponding decrease of the concentration of NO2PhAlaMe (1.8 .10-3 mol.L-1) is more than twice higher than the decrease of the 8oxodG concentration (0.8.10-3 mol.L1 ) in the same experiment. Remarkably the kinectics of the NO2PhAlaMe concentration decay in this experiment in presence of 8oxodG is very close to the decay observed in the absence of any electron donor (black curve same panel 4B).

The black curve plotted in the Figure 4A shows the variations of the 8oxodG concentration during visible light irradiation of a sample containing i/ QD’s as photocatalysts, ii/ 8oxodG as an electron donor. 8oxodG was introduced in great molar excess (500 eq.) compared with QD’s. In the absence of any added electron acceptor, the 8oxodG concentration does not change notably during 3 hours of irradiation. The black curve plotted in the Figure 4B show the variations of the NO2PhAlaMe concentration during visible light irradiation of a sample containing i/ QD’s as photocatalysts, ii/ NO2PhAlaMe as an electron acceptor in great molar excess (1500 eq.) compared with QD’s. In the absence of any added electron donor, the NO2PhAlaMe concentration decreases markedly: a decay of 80 % is observed after 3 hours.

Figure 4 : HPLC monitoring of concentrations of 8oxodG and NO2PhAlaMe during the reactions photocatalyzed by CdSe-ZnS QD’s. Panel A: monitoring of [8oxodG]. Reactions are performed with 2.0 µM of QD’s in argon atmosphere and 1mM of 8oxodG without any additional electron acceptor (black squares), and with added electron acceptors NO2PhAlaMe (3 mM, red circles) or MV2+ (1 mM, green triangles). Panel B: monitoring of [NO2PhAlaMe]. Reactions are performed with 2.0 µM of QD’s and 3 mM of NO2PhAlaMe without any additional electron donor (black squares), and with added 8oxodG as electron donor (1 mM, red circles, same experiment as red curve in panel A). Panel C: HPLC chromatograms for the photoreaction performed with 2.0 µM of QD’s ,1 mM of 8oxodG and 1 mM of MV2+ (corresponding to panel A, green triangles). Panel D: HPLC chromatograms for the photoreaction performed with 2.0 µM of QD’s ,1 mM of 8oxodG and 3 mM of NO2PhAlaMe (corresponding to panels A and B, red circles).

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These experiments establish clearly that QD’s can photooxidize 8oxodG only in the presence of an electron acceptor, while QD’s can photo-reduce NO2PhAlaMe even in the absence of any electron donor. These results suggest that photoexcited electron and holes in CdSe-ZnS QD’s have different reactivities towards substrates in solution (see Discussion). A rather low overall quantum yield of 6.10-4 (see Discussion) was measured at 450 nm by ferrioxalate actinometry for the photocatalytic decay of 8oxodG in the presence of QD’s and NO2PhAlaMe (see ESI Figure S10). Moreover a lower limit of 400 is obtained for the Turn Over Number (TON), by considering the ratio of the consumed 8oxodG substrate over the quantity of CdSe-ZnS QD’s introduced in the reaction monitored in Figure 4.

Chart 2: Structure of observed reaction products: spiroiminodihydantoin (Sp), guanidinohydantoin (Gh) and Aminophenylalaninemethylester (NH2PhAlaMe)

Discussion Identification of the reaction products The two main products of oxidation of 8oxodG are spiroiminodihydantoin (Sp) and guanidinohydantoin (Gh) (See Chart 2). Both of them were identified by HPLC-MS/MS analysis. The HPLC-MS/MS chromatograms shown in Figure S11 were obtained by analyzing the sample already described on Figure 4A and C containing CdSe-ZnS QD’s, 8oxodG and MV2+ after 3 hours of irradiation. These chromatograms were recorded by monitoring the 300 – 184 and the 274 – 158 transitions for the detection of spiroiminodihydantoin (Sp) and guanidinohydantoin (Gh), respectively.45-48 These chromatograms are very similar to those of standard calibrated samples of Sp and Gh recorded in the same conditions. The comparison of the chromatograms of reaction products and standard samples provides an estimate of the reaction products concentrations: 3.2.10-4 mol.L-1 for Sp and 3.6.10-4 mol.L-1 for Gh. Remarkably, the measured decay of the concentration of 8oxodG in this experiment is 6.8 10-4 mol.L-1 so the measured quantities of Sp and Gh after irradiation account for the totality of the decay of 8oxodG. The products of reduction of NO2PhAlaMe formed by visible light irradiation of a sample containing CdSe-ZnS QD’s, 8oxodG and NO2PhAlaMe were investigated by recording the UV-visible absorption spectra (Figure S12) and mass spectra (Figure S13) of the fractions isolated after preparative HPLC of a sample irradiated for 3 hours (same sample as the one described in Figure 4). One fraction exhibited a mass spectrum with a parent ion a m/z = 195 (positive mode), which was attributed to the amino compound NH2PhAlaMe analogous to the initial nitro compound NO2PhAlaMe (See Chart 2). This fraction has the same UV-visible absorption spectrum as the peak observed at 4.8 minutes retention time in the analytical HPLC monitoring of the reaction (see Figure 4D). The identification of NH2PhAlaMe was confirmed by synthetizing this compound and checking that MS and MS/MS spectra obtained were the same for the product isolated by HPLC and for the reference compound (see Figure S13 part I and II). No other product of NO2PhAlaMe reduction could be detected.

Charge transfer from photoexcited CdSe-ZnS QD’s to NO2PhAlaMe, MV2+ and 8oxodG The transfer of one electron or one hole from photoexcited CdSe-ZnS QD’s to NO2PhAlaMe, MV2+ or 8oxodG was evidenced by the combination of the EPR spectroscopy, quenching of photoluminescence and flash photolysis experiments reported above. EPR spectroscopy experiments unambiguously identified the free radicals corresponding to electron transfer from QD’s to electron acceptors in the case of NO2PhAlaMe, MV2+ and to hole transfer from QD’s to the electron donor in the case of 8oxodG. The interaction of substrates with the photoexcited QD’s was confirmed by monitoring of QD’s photoluminescence quenching by these substrates. At last flash photolysis experiments on a system composed of QD’s, 8oxodG and MV2+, showed absorption signals corresponding to MV+ and provided an estimate of the quantum yield (~1.7%) for the formation of MV+. From these experiments one can conclude that one electron oxidation reactions up to the oxidation potential of 8oxodG are accessible to CdSe-ZnS QD’s (0.74V/NHE see, ref36) and that one electron reduction reactions up to the reduction potential of NO2PhAlaMe (NO2PhAlaMe, E0 = -0.46 V/NHE at pH=5.6) are accessible to CdSe-ZnS QD’s. CdSe-ZnS QD’s electron and hole reactivities towards electron acceptors and electron donors The flash photolysis experiments and the HPLC monitoring of substrates and reaction products during photoirradiation reported here suggest that the hole and the electron in photoexcited CdSe-ZnS QD’s have very different reactivities. Of course we cannot establish a general mechanism, since we studied only one electron donor and two electron acceptors. However we will use our results to suggest a mechanism that explain our observations and that may have a broader range of application. Three different cases were observed. Case A: QD + acceptor without donor. The electron transfer from excited QD’s to NO2PhAlaMe is efficient even in the

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absence of 8oxodG as an electron donor. This is evidenced by the decays of NO2PhAlaMe monitored by HPLC during photoirradiation experiments (See Figure 4B) and by the observation of EPR spectra of MV+ and NO2PhAlaMe- at room temperature without electron donor.

reacts with the electron acceptor in a first step leading to Aand QD[h+]. The hole of QD[h+] has probably a long life time. So in spite of its low reactivity, QD[h+] has enough time to react with the electron donor in a second step, producing D+ free radical and regenerating non-charged QD.

Case B: QD + donor without acceptor. In the absence of an electron acceptor, the hole transfer from excited QD’s to 8oxodG seems to be either not efficient or annihilated by a fast recombination of 8oxodG+ with the electron remaining on QD[e-]. This is evidenced by the absence of decay of 8oxodG observed in photoirradiation experiments without electron acceptor (See Figure 4A black trace) and by the weakness of the long lived (at µs time scale) bleaching observed by flash photolysis of QD’s in the presence of 8oxodG (See Figure 3 trace B). If the hole transfer from excited QD’s to 8oxodG had a quantum yield of 1% it would result in a bleaching intensity of ~0.45 mOD due to QD[e-], that would be easily detected by Laser Flash Photolysis (ε(bleaching QD[e-], 510 nm) = 80000 mol.L-1.cm-1). However the hole transfer in the absence of electron acceptor is not completely negligible as evidenced by the observation of 8oxodG+ and/or 8-OHdG(-H) free radicals by EPR at low temperature.

This interpretation is consistent with the experimental results reported by several groups about the effect of the ZnS shell on the electron transfer and the hole transfer from photoexcited CdSe-ZnS QD’s to substrates 28,30. First, according to Lian et al. 28, the electron wave function of the exciton QD[e-h] delocalizes not only in the CdSe core but also in the ZnS shell and can react more easily with substrates at the surface, than the hole whose wave function is less delocalized in the ZnS shell. Second, according to the same authors the ZnS shell prevents the recombination between the hole QD[h+] and A- after the step of electron transfer. Both effects support the mechanism proposed here for the reaction of CdSe-ZnS QD’s with both a donor and an acceptor.

Case C: QD + donor + acceptor. In the presence of an electron acceptor, not only the electron transfer to donor but also the hole transfer to 8oxodG is efficient. This is evidenced by the decays of 8oxodG observed in photoirradiation experiments with NO2PhAla-Me or MV2+ as an electron acceptor (See Figure 4A red and green traces, resp.)

These mechanisms are consistent with our experimental results, however they account only for one electron oxidations and reductions, while we observe final products corresponding to 2 electrons oxidation of 8oxodG and 6 electrons reduction of NO2PhAlaMe. We can speculate that similar steps repeat themselves several times and that QD’s transfer charges one by one to the substrates. Photocatalyzed reactions: 2 electrons photo-oxidation of 8oxodG and up to 6 electrons photo-reduction of NO2PhAlaMe From the study of reaction products and the monitoring of the 8oxodG decay reported above, it can be concluded that, in the presence of an electron donor, CdSe-ZnS quantum dots are able to photocatalyze the 2 electrons oxidation of 8oxodG into Spiroiminodihydantoïne (Sp) and Guanidinohydantoïne (Gh). These oxidation products are well documented as oxidation products of guanine and 8oxodG in DNA 46,49-51 but to the best of our knowledge this is the first time that they are obtained by using a photochemical reaction with QD’s instead of using organic photosensitizers or mineral oxidants. The measured inferior limit of turnover (400) clearly establishes that QD’s act as photocatalysts for this reaction.

Scheme 2 – Mechanisms for photoinduced charge transfer between excited QD’s and substrates. Case A: QD + acceptor, case B: QD + donor and case C: QD + donor+ acceptor.

The mechanisms corresponding to these three cases are pictured in the scheme 2. In the simple mechanisms corresponding to cases A and B, we propose that the CdSe-ZnS QD’s exciton, QD[e-h] reacts well with electron acceptors but poorly with electron donors. Then the case C can be rationalized by the following mechanism: in the presence of both electron donor and acceptor, the more reactive electron of QD[e-h]

The overall quantum yield measured for the photocatalyzed oxidation of 8oxodG (at λ=450 nm) is 6.10-4 and is far below the quantum yield of photocatalyzed reactions studied, for instance, for cleaning applications of TiO2 photocatalysis (typically in the range 1-10% 52). Two factors can explain this low value: 1/ according to the experiments of quenching of photoluminescence it is probable that 8oxodG does not adsorb efficiently to the surface of CdSe-ZnS QD’s, 2/ the QD’s under study have a very thick (6 mono-layers) ZnS shell. According to Lian et al.28 the positive effect of the ZnS shell, which limits the charge recombination between the hole QD[h+] and A- is beneficial at least up to 3 ZnS monolayers. The low quantum yield observed here suggests that 6 ZnS



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monolayers probably slow down too much the step of charge transfer between QD’s and substrates. Further studies are underway to investigate thinner ZnS shells and optimize this parameter. From the investigation of reaction products and the monitoring of the NO2PhAlaMe decay reported above it can be concluded that CdSe-ZnS quantum dots are able to perform a 6 electrons reduction of nitroaromatic compounds. We could not detect other reduction products (nitroso or hydroxyamino compounds for instance). To the best of our knowledge, such photoreductions of nitroaromatic compounds into the corresponding aromatic amine compounds were reported for CdS QD’s17, but not for CdSe QD’s or CdSe-ZnS QD’s. Conclusion The results reported here demonstrate that water-soluble type I CdSe-ZnS quantum dots are able to photocatalyze quantitatively the 2 electrons oxidation of 8oxodG with a turn-over number higher than 400. This is the first example of this photoreaction induced by QD’s. Our study also demonstrates that CdSe-ZnS QD’s are able to photocatalyze the 6 electrons reduction of nitrophenylalanin methyl ester, which was not reported to date for QD’s with a CdSe core. Even if the quantum yield reported here (6.10-4) is modest, the reported results confirm that ZnS shell can play a beneficial role and pave the way for the use of CdSe-ZnS QD’s as redox photocatalysts in water and using visible light.

Experimental section Chemicals: (S)-(+)-4-nitrophenyl-alanine methyl ester hydrochloride (NO2PhAla-Me), 4-nitro-D-phenylalanine hydrate (NO2PhAla), methylviologen-dichloride hydrate (MV2+), Dhistidine, Chloroform, and Methanol were purchased from Sigma-Aldrich without any further purification. 8-oxo-2’deoxyguanosine (8oxodG) was purchased from Berry & Associates. Quantum dots ligands exchange procedure: Core-Shell CdSeZnS QD’s coated with octadecylamine ligands were purchased from NN-labs. Ligand exchange of the quantum dots was realized by following a procedure reported by Zylstra et al.53 with slight modification. Briefly 300 µL of a solution with 0.1 M histidine and 0.25 M NaOH in methanol:H2O 3:1 is added to 300 µL of core-shell CdSe-ZnS QD’s coated with octadecylamine ligands in CHCl3. After mixing an immediate phase transfer of QD’s from organic phase to aqueous phase is observed. Then 200 µL of a 0.1 M histidine and 0.25 M NaOH solution in water is added to the aqueous phase. This aqueous solution is washed 4 times with CHCl3. Then 2 mL of a 0.1 M histidine and 0.25 M NaOH solution in water were added to the Quantum Dots. The Quantum Dots were concentrated and cleaned of excess of histidine via 5 successive filtrations with a 30 kDalton centrifugal filter (Millipore Amicon). The ob-

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tained water soluble core-shell CdSe-ZnS QD’s were dispersed in deionized water. EPR experiments: EPR spectra recorded at ambient temperature (EPR spectra of free radicals derived from NO2PhAla-Me and NO2PhAla) were obtained with a Varian E102 EPR spectrometer at X-band frequency. Deaerated solution of QD’s was put in a quartz suprasil flat cell (optical path 0.3 mm). Irradiation was achieved with a Luzchem XE300BF Lamp (300 W ozone free Xenon lamp) with a 455 nm high pass UV-Vis filter (Edmund Optic) during the detection of EPR signal. EPR spectra recorded at low temperature (EPR spectra of 8oxodG derived radical) were obtained with a Bruker EMX spectrometer operating at X-band frequency operating with an ER-4116 dual mode cavity and an Oxford Instruments ESR-900 flow cryostat. Measurements of g-value is obtained by comparison with DPPH EPR signal in the same conditions. Irradiation was achieved during acquisition with a Schott KL1500 halogen lamp without filter. Typical settings at room temperature (Varian spectrometer): 9.421 GHz frequency, 10 mW power, 0.2 G modulation amplitude; at 170 K (Bruker Spectrometer): 9.655 GHz frequency, 0.2 mW power and 5 G modulation amplitude.

DFT Calculations: DFT (Density Functional Theory) calculations have been performed with the code ADF 2012 (Amsterdam Density Functional) developed by E. J. Baerends54. Triple-zeta basis set quality has been used throughout all calculations with no frozen core option. Constrained geometry optimizations have been obtained at the GGA level with the VBP exchange-correlation potential (VBP for VWN + BP: Vosko, Wilk & Nusair55 + gradient corrective terms by Becke56 for the exchange and Perdew57 for the correlation). The constrained geometrical feature is the H1-C-C1-C2 dihedral angle (caution: this dihedral angle is shifted by 90° compared to the one represented in Figure S5 where H1 is referred to the normal of the aromatic plane). The hyperfine coupling constants have been computed with the ESR key using the B3LYP exchangecorrelation potential58,59 mixing 20% of Hartree-Fock exchange within the exchange-correlation potential. Fluorescence spectroscopy: A Perkin Elmer LS 55 spectrometer was used for all quenching of photoluminescence experiments. Temperature was set to 25 °C with a Julabo F 25 system. Absorbance of the quantum dots solution was fixed by dilution to 0.3 at the excitation wavelength λ = 440 nm. QD’s samples studied by fluorescence spectroscopy were obtained by diluting stock solutions with aqueous solution of histidine (1mM) and were agitated 2 hours before the starting photoluminescence quenching experiments in order to stabilize their fluorescence. Laser flash photolysis: Transient absorption was acquired using a LP920K system from Edinburgh Instruments. Excitation was carried out from the third-harmonic (355 nm) of a Brilliant-Quantel Nd:YAG Laser (pulse width 5 ns, repetition rate 6 Hz, energy 0.04 mJ/pulse). A Xe900 pulsed Xenon Lamp is used as probe source. The photons were dispersed using monochromator, transcripted by a R928 (Hamamatsu) photomultiplicator and recorded on a TDS3012C (Tektronix) oscilloscope. The samples were degassed using Argon prior to measurements.


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Photocatalysis experiments monitored by HPLC: Samples containing QD’s and substrates were prepared in an argon atmosphere. Each sample (volume = 300 µL) was then irradiated with a Schott KL1500 halogen lamp equipped with a 420 nm high pass UV-Vis filter (Edmund Optics) to eliminate UV radiation and a hot-filter (Schott) to remove IR. At each time of irradiation, QD’s were separated by centrifugation and 12 µL of solution was diluted with 12 µL of deionized water. 20 µL of the solution was then analyzed by HPLC.An Agilent Technologies 1200 series apparatus was used for HPLC analysis. A XTerra-RP18 Column was used with an elution grade of acetonitrile from 0% to 10% in H2O in 30 minutes and from 10% to 40% in 15 minutes. Products were monitored by multiabsorbance detection (DAD) at 230 nm and 260 nm. Mass Spectrometry and HPLC-MS/MS spectrometry: The mass spectra were recorded on a LXQ type THERMO SCIENTIFIC spectrometer, equipped with an electrospray ionization source and a linear-trap detector. Solutions were injected in the spectrometer at 10 µL.min-1 flow rate. Ionization voltage and capillary temperature were set at about 4 kV and 250 °C respectively.The data were acquired in positive mode with an injection time of 5-200 ms. The LXQ calibration (m/z 50-2000) was achieved according to the standard calibration procedure from the manufacturer (mixture of caffeine, MRFA and Ultramark 1621 ).The HPLC-MS/MS analyses were performed with a triple quadrupole mass spectrometer TSQ Quantum Ultra from Thermo Scientific equipped with a Thermo Accela Pump, an Accela autosampler, and an Accela photodiode array detector as described previously49

SUPPORTING INFORMATION The following results and analyses are provided in supporting information: DFT calculations of hyperfine couplings; ferrioxalate actinometry experiments, complementary experiments (by EPR, LFP, HPLC, MS and MS/MS), characterization of QD’s (by UVvisible absorption, fluorescence, TEM and NMR) and cyclic voltammetry analysis of NO2PhAlaMe. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

Funding Sources TC, SG and VM thank the French Environment and Energy Management Agency (ADEME) and CEA for co-funding the TC’s PhD thesis grant N° 3735. VM and SG thank the CEA “DSM-Energie” program for financial support.

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