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The role of metal cations in plasmon-catalyzed oxidation: a case study of p-aminothiophenol dimerization Zhiyang Zhang, Virginia Merk, Anja Hermanns, Wolfgang E.S. Unger, and Janina Kneipp ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02700 • Publication Date (Web): 09 Oct 2017 Downloaded from http://pubs.acs.org on October 9, 2017

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The role of metal cations in plasmon-catalyzed oxidation: a case study of p-aminothiophenol dimerization

Zhiyang Zhanga,b, Virginia Merka, Anja Hermannsb, Wolfgang E. S. Ungerb, and Janina Kneippa,b* a

Humboldt-Universität zu Berlin, Department of Chemistry and School of Analytical Sciences

Adlershof (SALSA), Brook-Taylor-Str. 2, 12489 Berlin, Germany b

BAM Federal Institute for Materials Research and Testing, Richard-Willstätter-Str. 11, 12489

Berlin. Germany

* To whom correspondence should be addressed: E-mail: [email protected]

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ABSTRACT The mechanism of the plasmon-catalyzed reaction of p-aminothiophenol (PATP) to 4,4’dimercaptoazobenzene (DMAB) on the surface of metal nanoparticles has been discussed using data from surface-enhance Raman scattering of DMAB. Oxides and hydroxides formed in a plasmon catalyzed process were proposed to play a central role in the reaction. Here, we report DMAB formation on gold nanoparticles occurring in the presence of the metal cations Ag+, Au3+, Pt4+ and Hg2+. The experiments are carried out under conditions, where formation of gold oxide or hydroxide from the nanoparticles can be excluded, and at high pH, where the formation of the corresponding oxidic species from the metal ions is favored. Based on our results, we conclude that under these conditions, the selective oxidation of PATP to DMAB takes place via formation of a metal oxide from the ionic species in a plasmon-catalyzed process. By evidencing the necessity of the presence of the metal cations, the reported results underpin the importance of metal oxides in the reaction.

Key words: metal ions, plasmonic catalysis, p-aminothiophenol, 4,4’-dimercaptoazobenzene, surface enhanced Raman scattering

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INTRODUCTION Plasmonic catalysis has attracted a lot of attention in heterogeneous catalysis due to its high throughput and selectivity.1-6 In these plasmon-mediated processes, light energy is converted into chemical energy. Many plasmon-catalyzed redox reactions have been selectively achieved on metal nanoparticles.2-3,

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Amongst them, the dimerization of p-aminothiophenol (PATP) to

4,4’-dimercaptoazobenzene (DMAB) has been widely used as a model reaction to study plasmon-catalyzed oxidation reactions on gold and silver nanoparticles.11-13, 15-16 Similarly, as a counterpart, the dimerization of p-nitrothiophenol (PNTP) to DMAB is often discussed as a typical plasmon-catalyzed reduction reaction.17 After the first experimental report on the formation of DMAB from PATP,18 the mechanism of this reaction has been discussed under many different conditions, including variation of pH, the presence of O2, potential, laser intensity, laser wavelength, temperature, molecule coverage, and metal nanoparticle species.15,

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Although all these parameters have an effect on the reaction, it is not entirely clear how they exert their influence. Most recently, it has been found that the plasmon-mediated activation of oxygen molecules adsorbed on nanoparticles is a crucial step for the oxidation process.16 In addition, density functional theory calculations reveal that the formation of metal oxides or hydroxides on nanoparticle surface can be key active intermediates for the plasmon-catalyzed oxidation.16 The experimental data presented in the work here support this discussion. PATP is one of the most frequently used molecules to study enhancement effects in surfaceenhanced Raman scattering (SERS).26-27 In addition to the strong electromagnetic enhancement that can underlie SERS,28 the Raman signature of PATP was found to experience a high chemical enhancement due to a photo-driven charge transfer process.26-27 In the last years, strong evidence from experiments and theory was provided that the SERS spectrum of PATP indicates 3


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the formation of DMAB, produced in a photo-induced dimerization of PATP on metal nanoparticles that function as SERS substrate.19, 23, 29 In our work, we use SERS to observe the dimerization of PATP on metal nanoparticles and to discuss a potential mechanism for the formation of DMAB based on surface-species mediated dimerization of PATP on gold nanoparticles. As will be shown, under basic conditions, Ag+, Au3+, Pt4+ and Hg2+ ions are a prerequisite for DMAB formation. Before the background of the recent discussion on the role of metal oxide and hydroxide species in the plasmon-assisted oxidation of PATP,16, 30 we provide experimental evidence that these ions can provide the metal species for such oxide and hydroxide formation, and support their role as important mediators in the dimerization reaction.

RESULTS AND DISCUSSION Ag+, Au3+, Pt4+ and Hg2+ ions induce the dimerization of PATP to DMAB. In this work, we discuss the dimerization of PATP on gold nanostructures. The gold nanostructures in solution in sub-nanomolar concentration with the PATP molecules, at surface coverage of ~38% display typical absorbance spectra that also indicate the formation of gold nanoaggregates (Figure S1). Different metal ions in the form of salts (cf. Experimental section in the Supporting Information) were added to the aqueous nanoparticle solutions in µM concentration. Figure 1A shows the SERS spectra before and after the addition of Ag+, Au3+, Pt4+ and Hg2+ ions at pH 9, excited at 633 nm. With addition of the ions, the bands typical of DMAB at 1143, 1392 and 1436 cm-1

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appear (Figure 1A). The spectra shown in Figure 2 further confirm the assignment of the typical DMAB bands to the dimer molecule. When the pH was decreased to pH 1.0 by adding HNO3 (Figure 2A), the bands did not change, confirming their assignment to DMAB.32 In contrast, after addition of NaBH4, a dramatic decrease of the three bands suggests that present DMAB was 4


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reduced to PATP

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, providing additional evidence that the spectra in Figure 1A are spectra of

DMAB (Figure 2B). It is worth noting that the DMAB signals do not disappear completely in the SERS spectrum after the addition of NaBH4 (Figure 2B). The possible reason is that some of the DMAB formed in the nanoparticles aggregates did not fully react with NaBH4. To exclude the possibility of the new bands originating from a potential complex of the amino group of PATP with the respective metal ions rather than from DMAB, we added Na2S. It has a high affinity to Ag+ (solubility product constant Ksp (Ag2S) =6 × 10-50), and would therefore disrupt a potential complex. The bands’ intensity did not change (Figure 2C), also confirming the presence of DMAB. In the control SERS spectrum of PATP with gold nanoparticles (Figure 2E), without addition of any further species only the main characteristic bands of PATP are found. Further confirmation that the three bands are signals of DMAB comes from a spectrum of the DMAB molecule when it is produced in a plasmon-catalyzed reduction of PNTP (Figure S2). The addition of Ag+, Au3+, Pt4+ and Hg2+ ions also caused the relative intensity at 390 cm-1, assigned to S-C-C and N-C-C 33 or C-C 26 deformation to decrease and that of two bands at 407 and 440 cm-1 to increase (Figure 1A, four top spectra). The latter indicates a more upright orientation of the PATP during the interaction of the amino group with the metal.34 The decrease at 390 cm-1 is in accord with the reaction to DMAB, which leads to a changed interaction with the gold nanostructure.35 We attribute the bands at 407 and 440 cm-1 to the metal-complexing interaction of PATP, probably forming a nitrene precursor of DMAB.35 With the addition of Na2S, they disappear both at pH 9 (Figure 2C) and at pH 2 (Figure S3A), which we interpret as disruption of such a precursor by Na2S. Interaction of AgNO3 with aminobenzene also leads to an increase at 409 cm-1 in a normal Raman experiment (Figure S3B) supporting assignment to a metal–nitrogen vibration. 34 5


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The SERS experiments were conducted at two different excitation wavelengths. Figure S4 indicates DMAB formation also for an excitation wavelength of 785 nm. With addition of Pb2+, Bi3+, Pd2+, Cu2+, Cd2+, Cr3+, Co2+, Zn2+, Fe3+ ions, no change in the SERS spectra of PATP on the gold nanoparticles is observed (Figure S5). The band intensity ratios at 1143, 1392, 1436 cm-1 (I1143, I1392, I1436) assigned to DMAB and at 1080 cm-1 (I1080) to both PATP and DMAB spectra were determined (Figure 1B). They are highest in the presence of Ag+, Au3+, Pt4+ and Hg2+ ions. For the example of silver ions, Figure 1C shows that the signal ratios increase with increased ion concentration. Considering no alterations in interaction of the PATP molecules with the gold nanostructures’ surface, the higher relative signals indicate a higher amount of DMAB. In principle, based on such an excellent sensitivity, SERS signals from DMAB could be used for Ag+ detection in environmental or biological samples as well. In our experiments, we detect Ag+ concentrations down to 0.1 µM (3σ rule), which is lower than the EPA rule (0.46 µM) for drinking water.36 The ratiometric analysis of the SERS spectra similar to signals from other SERS sensors,37-39 allows quantification in spite of the fluctuation of the absolute SERS signals. As the dimerization of PATP is an oxidation, Ag+, Au3+, Pt4+ and Hg2+ ions could act as electron acceptors with relatively high redox potential. In accord with this, the characteristic spectra of DMAB also appear in the presence of other strong oxidants, such as KMnO4 and NaClO (Figure S6). Nevertheless, as discussed below, influence of laser intensity as well as of pH32, 40-41 suggest further functions of the metal cations. Since the metal ion-induced dimerization is particularly efficient in the presence of the silver ions (cf. Figure 1B), and since the role of silver nanostructures in DMAB formation, also from other benzene thiols,42 has been discussed in quite

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some detail in the recent past,30, 42 we will focus in the following on the interpretation of further experimental evidence in the experiments with Ag+.

Ag+ induced formation of DMAB depends on laser intensity. Figures 3A and 3B show SERS spectra obtained with 633 nm excitation during Ag+ induced DMAB formation at a laser intensity of 5 kW/cm2 and 500 kW/cm2, respectively, and for different duration of irradiation at 633 nm. Comparing the relative intensities of the lines characteristic of DMAB, development of the signals is observed within a few seconds for both intensities (Figure 3A and Figure 3B). Signal does not evolve much further in the case of the lower intensity of 5 kW/cm2 (Figure 3A). In contrast, when the intensity of the laser is increased by two orders of magnitude, an ongoing rise of the DMAB signals is observed until 30-45 s of exposure (Figure 3B), and higher signal ratios are reached, as illustrated for the example of the DMAB band at 1143 cm-1 in Figure 3C. In the absence of the Ag+ ions, no DMAB formation was observed (Figure S7). We also confirmed these findings using gold nanoparticles that were immobilized on glass surfaces by 3aminopropyltriethoxysilane.43 Also there, DMAB is only formed in the presence of Ag+ ions (not shown).Using other excitation (and irradiation) wavelengths, the signal ratios differ, in agreement with ref.16 Higher ratios are obtained for the 532 nm and lowest for 785 nm (Figure S8A and S8C). Nevertheless, the temporal evolution of the maximum signal intensities is very similar (compare Figure S8B and S8D with Figure 3C). Even though such a semi-quantitative comparison is difficult due to variations in chemical enhancement, detector sensitivities, changes in re-absorption of the SERS light, as well as other factors that influence relative intensities at varied excitation wavelength, we find that the presence of the metal ions is crucial in all experiments. The laser intensity dependence of the catalytic activity indicates that a photo7


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catalytic reaction takes place. According to the theory of plasmonic catalysis,1-6 the generation of hot electron/hole pairs and heating due to the decay of the plasmons can greatly influence the redox reaction. Ag+ induced formation of DMAB is favored at high pH. Figure 3D shows the relative intensity of the band of DMAB at 1143 cm-1 for four pH values, the corresponding spectra are displayed in Figure S9A. A significant increase in the intensity is found at higher pH values, in accord with previous work.22, 32, 44-46 Since changes in pH could change the plasmonic properties of the gold nanoparticle solutions, which in turn could influence the efficiency of DMAB formation, we also employed immobilized gold nanoparticles in these experiments. Also there, absence of the typical DMAB signals was found for low pH, and high intensities at 1143, 1392, and 1436 cm-1 were observed for high pH (Figure S9B). The pH dependency of DMAB formation can have several reasons. First, in accord with the redox potential of PATP (pK1/2=5.3)47 being reported to decrease with increasing pH,48-49 oxidation of PATP is facilitated. Second, especially under basic conditions, the formation of different silver species may play an important role.15-16, 21-23, 30 Therefore, we further assessed the properties of the nanoparticles in the reaction.

Formation of metal oxide on gold nanoparticles. Figure 4 displays transmission electron micrographs of the gold nanoparticles before (Figure 4A) and after the incubation of PATPfunctionalized gold nanoparticles with Ag+ ions (Figure 4B and 4C). In the samples where dimerization reaction was observed by SERS, the surfaces of the gold nanoparticles appear rough, and small nanoparticles were found (Figure 4B and 4C). The EDX spectra of these samples indicate that these nanoparticles contain silver (Figure 4B and 4D). As these newly formed, 8


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smaller nanoparticles show a much lower density in comparison with the gold nanoparticles (Figure 4B and 4C), they may indicate formation of silver species other than Ag0. Figure 4E displays a high-resolution XPS spectrum of the Ag-3d signal in such a sample. The chemical shift as well as the asymmetry of the signal indicate that the major contribution to the XPS intensity originates from Ag0 at 368.2 eV (Ag3d5/2)

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(Figure 4E, pink trace) Nevertheless,

using a minor contribution at the binding energy of Ag+ in Ag2O at 367.7 eV (Ag3d5/2).52 (Figure 4E, green trace), a fit of reasonable quality is yielded for a contribution of the Ag2O signal of up to 10%). We attribute the high ratio of elemental Ag to the further photoreduction of Ag(I) by citrate.53 The presence of oxygen is evidenced by the O-1s signal around 532 eV (Figure 4F) that was assigned to AgOH, as well as to major contribution from oxygen species in the citrate.16, 54-55 Under basic conditions, Ag+ could react with OH– and produce Ag2O particles that were reported to serve as excellent photocatalyst.56 Based on the results of DFT calculations, the oxidation of PATP by Ag2O or AgOH would be energetically highly favourable.16 The production of such silver species as surface metal oxides (or of corresponding gold species) on nanoparticles was proposed recently to occur by activation of 3O2 in a surface plasmon assisted process that employs both electron transfer from silver / gold nanoparticles and local temperature increase.16 High pH would support the stabilization of Ag2O. Based on our results obtained at low laser intensities (Figure 3A), the DMAB formation on gold or silver nanoparticles could be attributed to the presence of Ag or Au oxide / hydroxide that is independent on the plasmon activation mechanism, e.g., by aging oxidation or incomplete reduction, or ion release57, the latter under neutral condition specifically occurring in the case of silver rather than on gold. 57-59 In accord with this, with the excitation wavelengths (633 nm, but also 785 nm and 532 nm) and low intensities, DMAB production is absent on gold nanoparticles and present on silver 9


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nanoparticles (Figure S10). On gold, with addition of Au3+ (Figure 1 and Figure S4) or Ag+ (Figure 1 and Figure S8), the DMAB production increases as well, further emphasizing that the metal ions on the gold nanoparticles play an important role. As reported above, we observe no formation of DMAB when we do not add metal ions (Figure 1A), neither, if support by the SPR is absent or insufficient, or when the metal oxide cannot be stabilized (Figure 3). In contrast, when Ag+, Au3+, Pt4+ and Hg2+ ions are added (Figure 1A) and SPR is assisting, the dimer product forms (Figure 3), and XPS data allow to infer on small amounts of Ag2O (Figure 4). Assuming activation of oxygen on the gold nanoparticles’ surface according to ref.16, this would imply that activated 2O2-, adsorbed to the gold nanoparticle surface, reacts directly with the added metal ions, forming the respective metal oxide at the surface of the gold nanoparticles. Based on our results and using the description for generation of the metal oxide from refs.16,

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, dimerization of PATP can then take place

according to Scheme 1, comprising the following steps: Ag+ + OH– → AgOH 2AgOH → Ag2O + H2O 2PATP + 2Ag2O → DMAB + 4Ag + 2H2O 4Ag + O2 → 2Ag2O The SERS data obtained with the gold and silver ions are in accord with the predicted activity of the corresponding oxides /hydroxides16. Also in the case of platinum oxide,62 strong rationale is provided for its role in catalyzing the dimerization.22 We therefore conclude that our results strongly support the proposed role of the oxide species and their formation from activated oxygen.16 10


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CONCLUSIONS Here, we have observed the important role of Ag+, Au3+, Pt4+ and Hg2+ cations in the plasmoncatalyzed oxidation of PATP on gold nanoparticles. As our data indicate, the metal cations induced dimerization of PATP is greatly influenced by laser intensity and pH. When high laser intensity is applied, the catalytic activity increases, indicating that a photo-catalytic reaction due to the effect of localized surface plasmon is involved. Under high pH conditions, where formation of the corresponding metal oxidic species from the added metal ions is favored, the dimerization reaction is also observed under conditions, where the formation of gold oxide or hydroxide from the nanoparticles directly can be excluded. Based on our results, we conclude that the selective oxidation of PATP to DMAB takes place via formation of a metal oxide from the ionic species in a plasmon-catalyzed process. In general, these results may have implications in the design and characterization of other reactions relying on effects of metal ions in nanoparticle catalyzed reactions.63

EXPERIMENTAL SECTION Materials. The following chemicals (all purchased from Sigma-Aldrich) were used in the experiments: HAuCl4·(H2O)3, H2PtCl6·(H2O)6, AgNO3, HgCl2, Pb(NO3)2, Bi(NO3)2,

PdCl2,

CuSO4, Cd(NO3)2, CrCl3, CoCl2, ZnCl2, FeCl3, KMnO4, HClO, Na2S, NaBH4, HNO3, NaOH, HClO, trisodium citrate, para-aminothiophenol, para-nitrothiophenol. Synthesis of gold nanoparticles. Gold nanoparticles were synthesized by citrate reduction of HAuCl4 according to Ref.64

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Sample preparation using gold nanoparticles in solution. PATP solution was added to the gold nanoparticles yielding a nanoparticle-to-molecule ratio of ~1:6000. The solution was kept at room temperature for 15 min. The PATP molecules coverage on gold nanoparticles is estimated to be around 38.4%. Then, the pH value was adjusted by 0.2 M NaOH or 1 M HNO3, respectively. The dimerization reaction was conducted at room temperature. Different metal ions with concentration range from 0.1 µM to 10 µM were added into 500 µL of the obtained PATPfunctionalized gold nanoparticles, depending on the metal ions that were applied. To verify the band assignments in the SERS spectra of DMAB as described in the manuscript text and Figure 2, samples of gold nanoparticles (0.044 nM) with PATP (0.25 µM) at pH 9 were prepared, and after incubation with 1 µM Ag+ 0.1 M HNO3, 4 mM NaBH4, and 4 mM Na2S, respectively were added.

Sample preparation using immobilized gold nanoparticles. The immobilized gold nanoparticles were prepared by 3-aminopropyltriethoxysilane as previously reported.43 The experiment was performed by immersion of the nanostructured surfaces in 100 µM PATP (2 mL) for 1 hour. After washing with deionized water, the immobilized nanostructure was kept in 10 µM Ag+ solution (pH 9 or pH 2) for 30 min at room temperature.

Raman experiments. Raman spectra of the final samples were measured under a Raman microscope (LabRamHR, Horiba, Jobin-Yvon, France) with a 60x water immersion objective using and excitation wavelength of 633 nm, 532 nm, and 785 nm, respectively. The timedependent spectra after irradiation with laser light of different intensities and wavelengths were obtained with 1 s acquisition time for each, using the same laser intensity for irradiation and 12


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excitation of the Raman scattering. Average spectra were calculated from ten 1s-measurements each. SERS mapping of DMAB on immobilized nanostructures was obtained by raster scanning with 1 s acquisition time over an area of 100 x 100 µm2 with a distance of 10 µm between each point in a rectangular grid.

XPS experiments. 10 µM Ag+ was added to 500 µL of the obtained PATP-functionalized gold nanoparticles at pH 9. After incubation for 10 min, the solution was centrifuged 3 times at 4000 rpm for 15 min to remove the Ag+ ions in the suspension. The supernatant was kept for a control experiment. The final sediment was then re-suspended in 50 µL buffer at pH 9. Insulating teflon tape was fixed on an Si wafer using adhesive tape. A 2 µL drop was applied onto the teflon tape. After the first drop had fully evaporated the second drop was applied at the same position. Two drops were applied in total. XPS measurements were carried out with an AXIS Ultra DLD photoelectron spectrometer (Kratos Analytical, Manchester, UK). XPS spectra were recorded using monochrome aluminium Kα radiation for excitation, at a pressure of approximately 5 × 10-9 mbar. The electron emission angle relative to the surface normal was 0° and the source-to-analyzer angle was 60°. The binding energy scale of the instrument was calibrated using ISO 15472 binding energy data.65 The instrument was set to the hybrid lens and the slot mode providing an approximately 300 × 700 µm² analysis area. The charge neutralizer was used. The acquired XPS spectra were analyzed using UNIFIT 2018. The binding energy scale was corrected for charging using an electron binding energy of 292.48 eV for the C1s (CF2) signal from the teflon tape.66 For the curve fitting of the high resolution Ag3d spectrum, a Gaussian/Lorentzian sum function peak shape model (G/L=0.2) was used in combination with a Tougaard background. For the curve 13


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fitting of the high resolution Ag3d spectrum a Gaussian/Lorentzian sum function peak shape model (G/L=0.2 constrained) with an asymmetry of -0.1163 (non-constrained) for Ag0 and 0 for Ag+ (constrained). A constrained FWHM of 1.12 eV was selected for both components. A Tougaard background was applied (B = 123.568, C = 43.463, C’ = -0.188, D = 1176.92, T0 = 0).

ASSOCIATED CONTENT Supporting Information UV-Vis spectra of the gold nanostructures for different experimental conditions, SERS spectra supporting the findings on metal ion induced DMAB formation, specifically details on pH dependence, control experiments at varied excitation wavelengths, and verification of spectral assignments to DMAB are provided. The information is available free of charge via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank Sören Selve (ZELMI, TU Berlin) for support regarding transmission electron microscopy, Dr. Jörg Radnik (BAM) for help with the XPS experiments, and Dr. Merwe 14


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Buurman and Prof. Dr. Ulrich Panne (BAM) for fruitful discussion. Financial support of the research by DFG GSC 1013 SALSA (fellowship to Z.Z.), ERC grant no 259432 (J.K.) and DFG FOR 2177 InCheM (Z.Z., J.K.) is gratefully acknowledged. REFERENCES (1) Linic, S.; Christopher, P.; Xin, H.; Marimuthu, A. Acc. Chem. Res. 2013, 46, 1890-1899. (2) Aslam, U.; Chavez, S.; Linic, S. Nat. Nanotechnol. 2017. Doi:10.1038/nnano.2017.131 (3) Christopher, P.; Xin, H.; Linic, S. Nat. Chem. 2011, 3, 467-472. (4) Sun, M.; Xu, H. Small 2012, 8, 2777-2786. (5) Zhang, Z.; Fang, Y.; Wang, W.; Chen, L.; Sun, M. Adv. Sci. 2016, 3, 1500215. (6) Zhang, Z.; Xu, P.; Yang, X.; Liang, W.; Sun, M. J. Photochem. Photobiol. c Chem. 2016, 27, 100-112. (7) Mukherjee, S.; Libisch, F.; Large, N.; Neumann, O.; Brown, L. V.; Cheng, J.; Lassiter, J. B.; Carter, E. A.; Nordlander, P.; Halas, N. J. Nano Lett. 2013, 13, 240-247. (8) Kim, C.; Suh, B. L.; Yun, H.; Kim, J.; Lee, H. ACS Catal. 2017, 7, 2294-2302. (9) Li, C.; Wang, P.; Tian, Y.; Xu, X.; Hou, H.; Wang, M.; Qi, G.; Jin, Y. ACS Catal. 2017, 7, 5391–5398 (10) Xie, W.; Schlücker, S. Nat. Commun. 2015, 6, 7570. (11) da Silva, A. G.; Rodrigues, T. S.; Correia, V. G.; Alves, T. V.; Alves, R. S.; Ando, R. A.; Ornellas, F. R.; Wang, J.; Andrade, L. H.; Camargo, P. H. Angew. Chem. 2016, 55, 7111-7115. (12) Wang, J.; Ando, R. A.; Camargo, P. H. Angew. Chem. 2015, 127, 7013-7016. (13) Yang, H.; He, L. Q.; Hu, Y. W.; Lu, X.; Li, G. R.; Liu, B.; Ren, B.; Tong, Y.; Fang, P. P. Angew. Chem. 2015, 127, 11624-11628. (14) Cui, L.; Wang, P.; Li, Y.; Sun, M. Sci. Rep. 2016, 6, 20458. (15) Wang, J.; Ando, R. A.; Camargo, P. H. ACS Catal. 2014, 4, 3815-3819. (16) Huang, Y. F.; Zhang, M.; Zhao, L. B.; Feng, J. M.; Wu, D. Y.; Ren, B.; Tian, Z. Q. Angew. Chem. 2014, 53, 2353-2357. (17) Sun, M.; Zhang, Z.; Zheng, H.; Xu, H. Sci. Rep. 2012, 2, 647. (18) Fang, Y.; Li, Y.; Xu, H.; Sun, M. Langmuir 2010, 26, 7737-7746. (19) Choi, H.-K.; Shon, H. K.; Yu, H.; Lee, T. G.; Kim, Z. H. J. Phys. Chem. Lett. 2013, 4, 10791086. (20) Xu, J.-F.; Luo, S.-Y.; Liu, G.-K. Spectrochim. Acta A Mol. Biomol. Spectrosc. 2015, 143, 35-39. (21) Xu, P.; Kang, L.; Mack, N. H.; Schanze, K. S.; Han, X.; Wang, H.-L. Sci. Rep. 2013, 3, 2997 (22) Sun, M.; Huang, Y.; Xia, L.; Chen, X.; Xu, H. J. Phys. Chem. C 2011, 115, 9629-9636. (23) Huang, Y.-F.; Zhu, H.-P.; Liu, G.-K.; Wu, D.-Y.; Ren, B.; Tian, Z.-Q. J. Am. Chem. Soc. 2010, 132, 9244-9246. (24) Canpean, V.; Astilean, S. Spectrochim. Acta A Mol. Biomol. Spectrosc. 2012, 96, 862-867. (25) Xu, J.-F.; Liu, G.-K. Spectrochim. Acta A Mol. Biomol. Spectrosc. 2015, 138, 873-877. (26) Osawa, M.; Matsuda, N.; Yoshii, K.; Uchida, I. J. Phys. Chem. 1994, 98, 12702-12707. (27) Park, W.-H.; Kim, Z. H. Nano Lett. 2010, 10, 4040-4048. 15


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Figures and scheme

Figure 1 (A) SERS spectra (averages of 10 spectra) of gold nanoparticles (0.044 nM) with PATP (0.25 µM) at pH 9 before and after incubation with Ag+, Au3+, Pt4+ and Hg2+ ions. Red labels indicate bands typical of DMAB. (B) Signal intensity ratios (I1143/I1080, I1392/I1080, I1436/I1080) of the intensity at 1143, 1392, 1436 cm-1(I1143, I1392, I1436) assigned to DMAB and the intensity at 1080 cm-1 (I1080) in the corresponding SERS spectra after incubation with different ions. (C) Signal ratios after incubation with Ag+ at different concentration. (λexcitation = 633 nm; excitation intensity: 500 kW/cm2; accumulation time per spectrum: 1s)

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Figure 2 SERS spectra of gold nanoparticles with PATP (0.25 µM) at pH 9 after incubation with 1 µM Ag+ with further addition of (A) HNO3, (B) NaBH4, (C) Na2S. (D) control spectrum of DMAB, forming upon addition of Ag+, (E) control spectrum of PATP. (λexcitation = 633 nm; excitation intensity: 500 kW/cm2; accumulation time per spectrum: 1s). Scale bars: 500 cps. For data at pH 2, please see Figure S3A.

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Figure 3 Laser intensity and pH effects on Ag+ induced dimerization of PATP. SERS spectra of gold nanoparticles with PATP in the presence of Ag+ with a laser intensity of (A) 5 kW/cm2 and (B) 500 kW/cm2at pH 9 after different irradiation times (0, 5, 15, 30 and 60 s). Spectra were acquired for 1s after the indicated duration of irradiation. (C) Signal intensity ratio I1143/I1080 in the SERS spectra as a function of irradiation duration for the two laser intensities. (D) Signal intensity ratio I1143/I1080 in the SERS spectra in the absence and presence of Ag+ at pH 2, pH 7, pH 9, and pH 11, respectively (λexcitation = 633 nm; laser intensity 500 kW/cm2, acquisition time 1s).

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Figure 4 TEM images of gold nanoparticles functionalized with PATP (A) before and (B) after incubation with Ag+ at pH 9. (C) Enlargement of the area indicated in (B). (D) EDX spectra from the areas marked in (D). (E) High resolution Ag3d XPS spectrum of a sample as shown in (B). The Ag3d signal is fitted with two doublets, the dominant one representing Ag0, the minor one representing Ag+ in Ag2O. (F) High-resolution O1s XPS spectrum of a sample as shown in (B). The vertical solid line indicates the binding energy of the oxygen species in AgOH.52 The vertical dashed line indicates the binding energy of a NaKL1L23 Auger transition at 536.5 eV which coincides with the O1s photoelectron line.55

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Scheme 1 Scheme for the proposed mechanism of Ag+ induced dimerization of PATP.

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