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Catalysis by Metal Nanoparticles in a Plug-in Optofluidic Platform: Redox Reactions of p-Nitrobenzenethiol and p-Aminothiophenol Zhiyang Zhang, Ulrich Gernert, Renata Gerhardt, Eva-Maria Höhn, Detlev Belder, and Janina Kneipp ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00101 • Publication Date (Web): 07 Feb 2018 Downloaded from http://pubs.acs.org on February 7, 2018

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ACS Catalysis

Catalysis by Metal Nanoparticles in a Plug-in Optofluidic Platform: Redox Reactions of p-Nitrobenzenethiol and p-Aminothiophenol Zhiyang Zhang1,2, Ulrich Gernert3, Renata F. Gerhardt4, Eva-Maria Höhn4, Detlev Belder4, and Janina Kneipp1,2* 1

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

Sciences Adlershof, Brook-Taylor-Str. 2, 12489 Berlin, Germany 2

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

12489 Berlin, Germany 3

Technical University Berlin, ZELMI, Strasse des 17. Juni 135, D-10623 Berlin, Germany

4

Institute of Analytical Chemistry, Department of Chemistry and Mineralogy, Leipzig

University, Linnéstraße 3, 04103 Leipzig, Germany E-mail: [email protected]

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Abstract: The spectroscopic characterization by surface-enhanced Raman scattering (SERS) has shown great potential in studies of heterogeneous catalysis. We describe a plug-in multifunctional optofluidic platform that can be tailored to serve both as variable catalyst material and for sensitive optical characterization of the respective reactions using SERS in microfluidic systems. The platform enables the characterization of reactions under controlled gas atmosphere and does not present with limitations due to nanoparticle adsorption nor memory effects. Spectra of the gold-catalyzed reduction of para-nitrothiophenol by sodium borohydride using the plug-in probe provide evidence that the borohydride is the direct source of hydrogen on the gold surface, and that a radical anion is formed as intermediate. The in situ monitoring of the photo-induced dimerization of para-aminothiophenol indicates that the activation of oxygen is essential for the plasmon-catalyzed oxidation on gold nanoparticles and strongly supports the central role of metal oxide species.

Keywords: heterogeneous catalysis, gaseous reactants, radicals, reusable, optofluidics, surface-enhanced Raman scattering (SERS), microfluidics


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INTRODUCTION Determining catalytic properties and understanding the underlying kinetics and mechanisms of a catalyzed reaction helps to utilize heterogeneous catalysis in new applications, and to design new catalyst materials. Progress in our understanding of heterogeneous catalysis requires us to study the reacting species in situ, at the surface of the catalyst,1-5 and must provide access to fast reactions, with high sensitivity. At the same time, in spite of the importance of the nanoscopic properties of the catalyst and individual catalytic sites, probing the catalyzed reaction at the microscopic or macroscopic level will help to assess the efficiency of a catalyst material. Yielding locally high sensitivity through monitoring molecules that interact directly with a surface of a catalyst material, surface-enhanced Raman scattering (SERS) was shown to be a promising tool to monitor heterogeneous catalysis6-13 Specifically, bifunctional, composite nanomaterials that show both catalytic activity and provide high plasmonic enhancement in SERS, such as Au-Pd,9 gold-SiO2-gold,8 Pt–SiO2– Au,12 or ‘mix-and-match’ nanostructured surfaces,7 enable the direct observation of nanoparticle catalysis. SERS can monitor reactions that are conducted in droplets or tubes containing the bifunctional nanostructures,7-8, 10-12, 14 including reactions occurring in droplets inside microfluidic chips.15-16 Apart from the simple mixing of colloidal metal nanostructures and reacting species in a microfluidic channel that was reported so far, more potential approaches exist to monitor reactions by SERS in optofluidic approaches. As was shown, the immobilization of nanoparticles that act as SERS substrate on the channel walls can provide stable SERS enhancement due to the robust plasmonic structures.17-21 Nevertheless, re-utilization of immobilized SERS active nanostructures on-chip is restricted to some specific cases,18 and the adsorbed molecules on the surfaces of the nanoparticles are usually difficult to be cleaned up. Although, in contrast, SERS substrates in droplets on-chip are always fresh, and time-scales 3 Environment ACS Paragon Plus

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and homogeneity of mixing enable fast probing,16 reaction monitoring faces several difficulties, specifically when a defined gas atmosphere has to be utilized in the reaction. The application of SERS-based optofluidics to reactions involving molecules in the gas phase has not been addressed so far, yet specifically some of the most frequently studied catalytic reactions do in fact need investigations under defined gas atmosphere in order to delineate their respective mechanism. As a prominent example, the reduction of para-nitrothiophenol (PNTP) to para-aminothiophenol (PATP) can take place by H2,9 and potential formation of H2 was recently also suggested as one way through which reduction by sodium borohydride can occur.16 Similarly, the presence of oxygen can be crucial for many reaction paths, e.g., when formation of metal oxides is required for catalysis to proceed.14, 22-23 Here, we report new details on the mechanism of two important catalyzed reactions under conditions of controlled gas atmosphere in a reusable optofluidic chip, (i) the catalytic reduction of PNTP by NaBH4, and (ii) the plasmon-supported dimerization of PATP to 4, 4′dimercaptoazobenzene (DMAB) in situ. Using a new kind of optofluidic platform that includes multifunctional gold nanostructures, our data give further insight into the role of NaBH4 and of an anion radical in the reduction of PNTP. With the multifunctional plug-in SERS probe, we further demonstrate the important role of oxygen in the plasmon-catalyzed dimerization of PATP that was reported for silver nanoparticles24 and also on gold nanoparticles. Specifically, supporting theoretical suggestions22 and our recent experimental findings14 that metal oxide species are crucial for DMAB formation, we conclude that the important role of 3O2 must lie in the formation of such a species. By integrating a ‘plug-in’ SERS probe as optical component, we enhance the sensing performance and simplify the design of the microfluidic system. As we discuss here, the presented approach enables SERS monitoring of heterogeneous catalysis in both the liquid and gaseous phase.


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RESULTS AND DISCUSSION As shown in Figure 1, a ‘plug-in’ SERS probe is generated from a glass rod (Figure 1A), onto which gold nanoparticles of ~30 nm in diameter were immobilized using aminopropyltriethoxysilane (APTES) using a procedure reported previously.25-26 The gold nanoparticles were prepared following a standard protocol.27 Alternatively, gold nanoparticles can also be immobilized on a slab of polydimethylsiloxane (PDMS) using silane chemistry and many ‘plug-in’ SERS probes can be obtained by punching out the slab (Figure 1B). The optical probes are inserted into a PDMS-glass microfluidic chip with appropriate pre-punched holes on the PDMS side (Figure 1C). For the assembly of the chip, the PDMSglass chip was firstly fabricated, and the holes were punched before sealing the chip. In the case of the PDMS probes, these can also be inserted through a pipette tip, as shown in Supporting Information Figure S1. Figure 1 A and B also show the nanoscopic structure of the surfaces of the different ‘plug-in’ probes, obtained by scanning electron microscopy and atomic force microscopy, respectively. On both, the glass rod (Figure 1A) and the PDMS (Figure 1B), the gold nanoparticles are uniformly distributed, with slight aggregation that leads to an appearance of a broad plasmon resonance visible in the absorbance spectra around 600 nm (Supporting Information Figure S2A and S2B), which is of particular benefit when the experiments are conducted using an excitation wavelength of 633 nm as was the case here. SERS spectra were obtained from large areas on the microscopic surfaces of the plug-in probes at high reproducibility (Supporting Information Figure S2C and S2D), evidenced also by the maps of the enhancement factors that were obtained at the microscopic scale, and that indicate uniform enhancement factors of around 106 (Supporting Information Figure S2E and S2F). We have found such enhancement factors before for other SERS substrates that employ immobilized gold nanoparticles for monitoring of catalytic reactions in droplets.7 5 Environment ACS Paragon Plus

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The electromagnetic enhancement in SERS is determined by both the excitation and the Raman field enhancement factor, each contributing quadratically to the SERS signal.28 Therefore, at high field enhancement factors, a high SERS signal can be obtained even at relatively low excitation fields. Low excitation laser intensities keep local field-dependent plasmonic catalysis, leading to wanted or unwanted reactions, at a low level. Reduction of PNTP. To avoid the influence of plasmon-catalyzed side reactions in the nanoparticle-catalyzed reduction of PNTP to PATP by sodium borohydride discussed here,7-8, 16

we first obtained spectra of PNTP with the plug-in probe at different laser intensities

(Figure 2) The first spectrum of Figure 2A excited at an intensity of 300 kW/cm2 clearly shows the characteristic bands at 1145 cm-1, 1392 cm-1, and at 1436 cm-1 of the 4, 4′dimercaptoazobenzene (DMAB)29-30 that forms as a result of plasmon-supported catalysis,31-38 and are, e.g., driven by plasmon-exciton coupling of varying degree.39-44 As illustrated by Figure 2B, the bands of the DMAB side product evolve over time, and are absent when excitation intensities of one or two magnitudes lower are applied for the SERS experiment, even under 10 min irradiation (Supporting Information Figure S3). Therefore, in the following experiments, 3 kW/cm2 was applied as the excitation laser intensity. Under this laser intensity, no plasmon-catalyzed reaction of PATP 14 took place either (Figure S6A). We find that the 30 nm gold nanoparticles, that were immobilized on the plug-in probes, not only exhibit high SERS enhancement but also possess excellent catalytic properties, as was illustrated by the reduction of PNTP by sodium borohydride (Figure S4). The particular size of the gold nanoparticles provides high SERS enhancement and is still not too large for sufficient catalytic activity.8 Following the idea of creating versatile surfaces with those nanoparticles that are suited best for the respective SERS and/or catalysis experiment (mixand-match principle),7, 26 we used here these 30 nm gold nanoparticles both as SERS substrate and as catalyst. At a low excitation intensity of 3 kW/cm2, this optofluidic plug-in SERS 6 Environment ACS Paragon Plus

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ACS Catalysis

platform enables the direct observation of a catalytic reaction at high sensitivity due to high enhancement and without interference from plasmon-catalyzed side reactions. The reduction of PNTP that resided on the immobilized gold nanoparticles was started by the flow of sodium borohydride solution through the microfluidic channel. Figure 3A displays SERS spectra recorded from the plug-in probe during the reaction at selected time points. At time zero (t=0 s), the spectrum (Figure 3A, bottom trace) shows four major bands of PNTP at 330 cm-1 (assigned to metal-molecule surface complex45), at 1083 cm-1 (νC-S)31, 46, 1346 cm-1 (νNO2)31, 46, and 1577 cm-1 (νC-C)31, 46. With increasing reaction time, the intensity of νNO2 at 1346 cm-1 gradually decays and is accompanied by the appearance of the typical bands of PATP at 390 cm-1 and 1595 cm-1 (cf. Figure 3A bottom to top). After ~30 s, the characteristic bands of PNTP completely disappear, and only the signals of PATP remain, indicating that all PNTP was reduced to PATP. As expected from the results shown in Figure 2, no formation of DMAB was observed during the monitoring time. As can be seen from the spectra in Figure 3A and also from the development of the intensities in the color-coded plot of the spectral region from 1300 cm-1 to 1370 cm-1 (Figure 3B), the band assigned to the stretching vibration of the nitro group (νNO2), shows a second component, red-shifted in frequency from 1346 cm-1 to 1334 cm-1 during the reaction, with the high-frequency component at 1346 cm-1 losing intensity quite quickly, as can also be seen from the intensity ratio of both bands (Figure 3C). Recently, in the case of photocatalytic reductions, electrochemical potential-dependent experiments and theoretical calculations indicated that such a low-frequency band could be assigned to the anion radical of nitrobenzenethiol (NBT•−, termed PNTP•− according to the nomenclature used here) or its conjugate acid produced from the single-electron reduction of PNTP as intermediate.47 In principle, also a change in coverage and orientation of PNTP could contribute to such changes in the SERS spectra.48 Nevertheless, also here, we assign the low-frequency band component 7 Environment ACS Paragon Plus

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at 1334 cm-1 to the intermediate of the anion radical PNTP•−. The essence of the reduction of PNTP is similar, in spite of the absence of a photocatalytic step here, providing a rationale for the appearance of the radical intermediate. Accordingly, the band at 1346 cm-1 is assigned to the original νNO2 of PNTP. Due to the high spectral resolution in our experiment compared to the spectra reported previously in ref.47, the vibration of the anion radical is observed as separate band or shoulder rather than a down-shift of a band representing both PNTP and PNTP•−. To obtain the kinetics of the reaction, the intensities of a band at 1577 cm-1 (characteristic band of PNTP) and the PATP band at 1595 cm-1 were used for quantification. The logarithm of the ratio of the relative intensities at different time points in the reaction is plotted in Figure 3D. As sodium borohydride was used in excess, it can be assumed that the reaction follows pseudo-first-order kinetics, and the reaction rate constant k can be determined by t = ln

[ ] [ ]

( ⁄ ) ) (⁄ )

= ln(

Here, [PNTP] is the relative concentration of PNTP, and I1577 and I1595 are the intensities of the bands at 1577 cm-1 and 1595 cm-1, respectively. We determine a rate constant for the reaction of 0.061 ± 0.004 s-1 (Figure 3D) which is similar to that of the platinum-catalyzed reaction (k = 0.011 s-1)7. The reactants are all transformed into product within 30 s (Figure 3D). In comparison, the band ratio of the NO2 stretching vibration of the PNTP and the intermediate PNTP•− radical anion decreases to a minimum within 17 s (Figure 3C). From 17 s to 30 s, the ratio remains almost constant (Figure 3C), and no further band shift nor change in the band shape happens (Figure 3A). This indicates that the transformation of PNTP to the intermediate PNTP•− is a faster step than the further conversion of the intermediate PNTP•− to PATP.


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ACS Catalysis

As was proposed recently,16 the platinum-catalyzed reduction of PNTP involves adsorbed hydrogen atoms that come from H2 gas, which was suggested to be generated from NaBH4. when pH is lower than 13.16 Since the pH used here is around 10 and the H2 generation from NaBH4 hydrolysis is possible, it is necessary to study the effect of H2 in our system. Using the plug-in SERS probe in the microfluidic system, we can test such a potential role of H2 directly in an H2 atmosphere on-chip, and did this here for the gold-catalyzed reduction of PNTP. Figure S5A and S5B show the SERS spectra and the ratio of the band intensities at 1577 cm-1 and 1595 cm-1 (I1577/I1595) with the H2 flow in the microfluidic channel over a time range of 5 min. The data indicate that the first-order reaction discussed above does not take place in the presence of H2 on the gold nanoparticles used here when NaBH4 is absent. This suggests that the 30 nm gold nanoparticles have less catalytic activity for the activation of H2, compared with, e.g., highly active Pt, Pd, or smaller gold nanoparticles.8, 9, 12 Based on all these result and assuming a Langmuir-Hinshelwood model16, 49-50, we conclude that the gold-catalyzed reduction of PNTP is determined by the borohydride, not the presence of H2, and followed by the formation of an anion radical, as shown in Scheme 1. The surface adsorbed atomic hydrogen (Au-H) formed from NaBH4 reacts with adsorbed PNTP to form the anion radical (PNTP•−) or its conjugate acid via a single electron reduction47. Subsequently, as the most plausible candidates,47 the two electron reduction product of PNTP (the conjugate acid of the dianion of PNTP), could be formed. Then, according to the wellstudied case of the reduction of nitrobenzene,51-53 as two further intermediates the nitroso compound and the hydroxylamine compound 51-53 (bottom of Scheme 1) can be formed before generating the final product PATP. Dimerization of PATP under controlled gas atmosphere. The versatility of the multifunctional plug-in SERS microfluidic platform in monitoring reactions involving a defined gas atmosphere shall be demonstrated by a second example. In the photo-induced 9 Environment ACS Paragon Plus

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dimerization of PATP on metal nanoparticles, considered a model reaction of plasmonic catalysis29-30, 54, the activation of 3O2 on metal nanoparticles was identified as a key step for the dimerization to occur.22, 24 Using the plug-in SERS platform, we investigate the photoinduced dimerization of PATP under stable and controllable gas atmosphere. Figure 4A shows the SERS spectra of the plug-in SERS probe functionalized with PATP during irradiation for 30 s at an excitation intensity of 30 kW/cm2, sufficient to induce dimerization, with the flow of air, containing O2, in the channel. The characteristic bands of DMAB gradually increase in intensity with increasing irradiation time when the air flow passes the plug-in SERS probe (Figure 4A), leading to high DMAB signals after a few seconds (Figure 4B, blue symbols). No DMAB formation is found in air flow under lower laser intensity of 3 kW/cm2 (Figure S6A), in accord with previous observations.14 As a control, under N2 flow the SERS spectra are almost unchanged during the monitoring time, and no significant amount of DMAB is formed (Figure 4B, black symbols and Figure S6B). The band ratio (I1143/I1080), chosen to represent the relative amount of DMAB compared to PATP illustrates this as well (Figure 4B). This result supports the necessity of oxygen in photo-induced dimerization of PATP, through the mechanism of 3O2 activation.22, 24 It was proposed that metal oxides are important in the dimerization reaction, and that they are generated through the plasmoncatalyzed activation of oxygen on the surface of the gold nanoparticles.22 Recently, we could prove this central role of the metal oxides by adding different metal ions to gold nanoparticles when excitation intensities were low.14 The absence of the reaction product in the absence of oxygen (Figure 4) suggests that no metal oxide or hydroxide species could be formed, and that the reaction cannot take place. Therefore, the results shown here (Figure 4A and 4B) further support the proposed role of such an oxide species and its formation from activated oxygen.22 As discussed previously,14 a small amount of DMAB, formed also in the absence of oxygen on the gold nanoparticles (compare Figure 4B) as well as on silver nanoparticles,55 could be 10 Environment ACS Paragon Plus

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ACS Catalysis

attributed to the presence of gold and of silver oxide / hydroxide, respectively, that is independent of the plasmon activation mechanism, e.g., by oxidation or incomplete reduction, or ion release,56 especially in the case of silver, the latter can become important.56-58 We therefore conclude that our results strongly support the proposed role of the oxide species and their formation from activated oxygen (Figure 4C).22 After the formation of DMAB, we tested the reactivity of DMAB on gold nanoparticles replacing the oxygen containing air with an H2 flow (Figure 4B, yellow symbols). As shown in Figure 4B and Figure S6C, the relative intensity of the DMAB bands did not change even after a duration of 300 s of H2 flow. This indicates that the DMAB was not reduced to PATP under the present conditions on the gold nanoparticles. This is different from the reduction on silver nanoparticles in the presence of H2 or H2O reported previously24, probably due to a more efficient action of the hot electron reduction.59 CONCLUSIONS To summarize, we developed an optofluidic platform to study heterogeneous catalytic reactions that involve both aqueous and gaseous reactants. Its central part is a plug-in SERS probe comprising immobilized nanostructures that can be chosen according to the demands regarding the catalyst and the plasmonic properties. The plug-in design allows the easy integration into existing Raman microscopic systems, so that this approach can be easily adapted. Most importantly, the chip can be re-used, and multiple, different plug-in probes can be applied. As an example, SERS-based on-line measurement of pH 60-61 can be combined with the observation of reaction product formation on different catalyst substrates in one chip, following the mix-and-match principle.7, 26 This would allow to study, e.g., the size dependency of the catalytic activity of gold nanoparticles of varying size, or the activity of different nanoparticle materials. Off-chip, e.g., the combination of gold and Pt nanoparticles


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was already successfully applied.7 Finally, compared to an application of flowing nanoparticles in a microfluidic system, only a small amount of catalyst material is consumed, and memory effects from the adhesion of colloidal nanoparticles on channel walls are avoided. Using bifunctional, catalytically active gold nanoparticles in the plug-in probe, here we studied the gold-catalyzed reduction of PNTP by sodium borohydride. The data provide evidence that the borohydride is the direct source of hydrogen on the gold surface, and that a radical anion as intermediate is formed subsequently in a single electron reduction process. Furthermore, on-chip characterization of another reaction, the photo-induced dimerization of PATP on the plug-in probe indicates that the activation of 3O2 is an essential step for the plasmon-catalyzed oxidation on the gold nanoparticles, providing further strong support for a central role of metal oxide species14, 22 in this reaction. The results indicate that the plug-in optofluidic platform will greatly facilitate other future studies of heterogeneous catalytic reactions. In general, in accord with an important goal of optofluidics to improve optical detection without increasing complexity of the microfluidic systems62, they strengthen current efforts16-18 to make SERS part of a universal analytic approach for microfluidics as well as other optical detection tools that rely on the optical properties of nanomaterials, including LSPR, surface-enhanced fluorescence, and surfaceenhanced infrared spectroscopy. EXPERIMENTAL SECTION Synthesis of gold nanoparticles Gold nanoparticles were synthesized by citrate reduction of tetrachloroauric(III) acid according to Ref.27


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ACS Catalysis

Preparation using immobilized gold nanoparticles on glass rod and PDMS block The gold nanoparticles were immobilized on the whole surface of a glass rod (2 mm diameter, length variable and corresponding to thickness of microfluidic chip) by 3aminopropyltriethoxysilane as previously reported for glass surfaces.26 Subsequently, all surfaces, except one of the flat ends were cleaned from gold nanoparticles. Alternatively, gold nanoparticles were also immobilized on the surface of a polydimethylsiloxane (PDMS) slab by 3-aminopropyltriethoxysilane after activation with O2 plasma according to a method modified from that reported in ref.63. Then, PDMS rods (2 mm diameter) with gold nanoparticles at one end were obtained by punching the PDMS block The gold-modified glass rods and PDMS rods were characterized by scanning electron microscopy and atomic force microscopy, respectively. The PDMS rod can also be inserted into a holder as shown in Figure S1. Fabrication of the Plug-in SERS Microfluidics Platform Silicon wafers were purchased at Active Business Company GmbH (Brunnthal, München, Germany).MicroChem SU-8 2050 polymer and developer (mr-Dev 600) were purchased from micro resist technology (Berlin, Germany). Polydimethylsiloxane (PDMS) was obtained from Biesterfeld Spezialchemie GmbH (Hamburg, Germany). A silicon wafer was baked out for 5 minutes at 200°C and treated for 5 minutes in an oxygen plasma (Femto, Electronic Diener). Approximately 3 ml of photoresist reagent was applied onto the wafer. Then the wafer was put into a spincoater. First it was rotated for 5 seconds at 500 rpm with an acceleration of 300 rpm/sec, then with the same acceleration it was increased to 1900 rpm for 30 s. This gave a thickness of about 50-60 µm, which corresponds to the height of the channels of the chip. For pre-hardening, the wafer was incubated on a hotplate at 65°C for 3 minutes and after that at 95°C for 7 minutes. Subsequently, the photomask was


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placed on top of the SU8 (SU-8 is a commonly used epoxy-based negative photoresist); and photomask and wafer were illuminated by a Hg lamp for 14 seconds and again incubated on a hotplate at 65°C for 2 minutes and at 95°C for 7 minutes. After some seconds of cooling, the wafer was gently washed in a bath with developer agent and rinsed with isopropanol. To prepare a PDMS mold, 10 parts by weight of silicon pre-polymer were thoroughly mixed with 1 part by weight of curing agent. The mixture was degassed under vacuum and poured onto the wafer comprising the SU8 structures. After the PDMS had hardened, it was gently peeled off the wafer and cut so that it met the outer dimensions of the chip. After punching holes into the PDMS layer for inlets (1.5 mm diameter), outlets (1.5 mm diameter) and access of glass rod (2 mm diameter) on the measurement channels, the PDMS was bonded to the glass slide that made the bottom of the chip as follows: A glass slide was baked out for 5 minutes at 200°C and treated for 10 minutes via oxygen plasma. The PDMS layer was treated for 30 seconds with oxygen plasma. For bonding, both layers were pressed together. The glass rod (2 mm diameter) functionalized with SERS probes is carefully inserted into the pre-punched access. Then, the inlet and the outlet are connected with the pump and with the container with tubing. The flow rate of 10 mM NaBH4 (pH around 10) during the reaction monitoring was set to 40 µL/ min. The gas flow was controlled with a balloon. Modification of SERS probes on immobilized gold nanoparticles The modification with para-nitrothiophenol (PNTP) was achieved by immersing the nanostructured surfaces in 100 µM PNTP (2 mL) for 24 hours. After washing with deionized water, the plug-in probe was inserted into the PDMS chip. Raman experiments Raman spectra were measured under a Raman microscope (LabRamHR, Horiba, Jobin-Yvon, France). For the measurements on chip, a 50x long working distance objective under 14 Environment ACS Paragon Plus

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ACS Catalysis

excitation wavelength of 633 nm was applied. To assess the presence of the photo-reduction of PNTP as side reaction, spectra were measured each second for a time range of 3-10 minutes under three different laser intensities (3 kW/cm2, 30 kW/cm2, 300 kW/cm2). For the estimation of SERS enhancement factor, a 60x water immersion objective under excitation wavelength of 633 nm was applied. For monitoring the gold-catalyzed reduction of PNTP by NaBH4, the laser intensity of 3 kW/cm2. For the photo-induced dimerization of PATP, the laser power was chosen to be 3 and 30 kW/cm2. All spectra were obtained with 1 s acquisition time. Estimation of SERS enhancement factor The SERS enhancement factors were determined using 5×10-6 crystal violet as the analyte with excitation of 633 nm (laser intensity: 5 kW/cm2, acquisition time: 1 s). Spectra were acquired at a step size of 10 mm (diameter of the probed spot: 1.5 mm). Then, the intensity of Raman band at 1622 cm-1 was chosen for the calculation. Details of the procedure are given in ref..64 ASSOCIATED CONTENT Supporting Information UV-Vis spectra of the gold nanosctructures for different experimental conditions, SERS spectra supporting the findings on-chip, specifically details on SERS enhancement factor, catalytic property of gold nanoparticles, control of photoreaction. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank Sören Selve (ZELMI, TU Berlin) for support regarding transmission electron microscopy, Christian Heck (BAM / HU / Uni Potsdam) for help with the AFM experiment, and Dr. Virginia Merk (HU) and Dr. Jéremy Bell (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., D.B.) is gratefully acknowledged. REFERENCES (1) Tao, F. F.; Salmeron, M., Science 2011, 331, 171-174. (2) Hansen, T. W.; Wagner, J. B.; Hansen, P. L.; Dahl, S.; Topsøe, H.; Jacobsen, C. J., science 2001, 294, 1508-1510. (3) Zhang, C.; Grass, M. E.; McDaniel, A. H.; DeCaluwe, S. C.; El Gabaly, F.; Liu, Z.; McCarty, K. F.; Farrow, R. L.; Linne, M. A.; Hussain, Z., Nat. Mater. 2010, 9, 944-949. (4) Tao, F.; Grass, M. E.; Zhang, Y.; Butcher, D. R.; Renzas, J. R.; Liu, Z.; Chung, J. Y.; Mun, B. S.; Salmeron, M.; Somorjai, G. A., Science 2008, 322, 932-934. (5) Wachs, I. E.; Roberts, C. A., Chem. Soc. Rev. 2010, 39, 5002-5017. (6) Heck, K. N.; Janesko, B. G.; Scuseria, G. E.; Halas, N. J.; Wong, M. S., J. Am. Chem. Soc. 2008, 130, 16592-16600. (7) Joseph, V.; Engelbrekt, C.; Zhang, J.; Gernert, U.; Ulstrup, J.; Kneipp, J., Angew. Chem. 2012, 51, 7592-7596. (8) Xie, W.; Walkenfort, B.; Schlücker, S., J. Am. Chem. Soc. 2012, 135, 1657-1660. (9) Huang, J.; Zhu, Y.; Lin, M.; Wang, Q.; Zhao, L.; Yang, Y.; Yao, K. X.; Han, Y., J. Am. Chem. Soc. 2013, 135, 8552-8561. (10) Li, J.; Liu, J.; Yang, Y.; Qin, D., J. Am. Chem. Soc. 2015, 137, 7039-7042. 16 Environment ACS Paragon Plus

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Figures and Scheme

Figure 1 Procedure for the fabrication of plug-in multifunctional SERS probes based on (A) glass and (B) PDMS. (C) Corresponding SEM and AFM image, revealing the 3aminopropyltriethoxysilane (APTES) immobilized gold nanostructures. (D) Schematic of the whole plug-in optofluidic platform for monitoring of nanoparticle-catalyzed reactions in a microfluidic platform.


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Figure 2 Optimization of the excitation laser intensity to avoid the photoinduced dimerization of PNTP. (A) SERS spectra of PNTP on a plug-in SERS probe with gold nanoparticles after irradiation of 120 s under different laser intensities. The characteristic Raman bands appearing at higher laser intensity at 1145, 1392 and 1436 cm-1 are assigned to the unwanted side product DMAB. (B)Time-dependent band ratio I1145/I1085 of the intensity at 1145 cm-1(I1145) assigned to DMAB and the intensity at 1085 cm-1 (I1085) (present in both PNTP and DMAB) in the corresponding SERS spectra under different laser intensity. (λexcitation 633 nm, acquisition time 1 s)


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Figure 3 (A) SERS spectra recorded during the catalytic reduction of PNTP on immobilized gold nanoparticles in aqueous NaBH4 solution at different times. (B) Color-coded intensity map of SERS spectra in range from 1300 to 1370 cm-1 for the catalytic reduction in the first 5 seconds. (C) The band ratios of the intensities at 1346 cm-1 (assigned to PNTP) and 1334 cm-1 (assigned to the intermediate PNTP•−). (D) Determination of the rate constant for the reduction of PNTP using relative concentration of PNTP and PATP. Quantification is achieved by comparing the intensities of their characteristic bands at 1577 and 1595 cm-1, respectively. (λexcitation 633 nm, intensity 3 kW/cm2, acquisition time1 s)


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Scheme 1 Proposed mechanism for the gold-catalyzed reduction of PNTP in the presence of NaBH4. The data discussed here indicate BH4- as source of the surface-adsorbed hydrogen, as well as the presence of the radical anion formed as intermediate in a one-electron reduction (first row). Further plausible steps would be formation of the two electron reduction product of PNTP (the conjugate acid of the dianion of PNTP),33 and of a nitroso and a hydroxylamine species 36-38 (bottom row).


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Figure 4 Influence of H2 gas on the plasmon-catalyzed dimerization of PATP on gold nanoparticles. (A) SERS spectra of the plug-in probe functionalized with PATP with irradiation of different times under a flow of air, containing oxygen. (B) Band ratio I1143/I1080 of the intensity at 1143 cm-1 (I1143) assigned to DMAB and the intensity at 1080 cm-1 (I1080) (representing PATP here) in the corresponding SERS spectra under conditions of varied gas flow (N2 flow, air flow and subsequent H2 flow). (C) Scheme for the plasmon-catalyzed dimerization of PATP and the reactivity of DMAB in the presence of H2. (λexcitation 633 nm, intensity 30 kW/cm2, acquisition time 1 s)


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