Surface-Enhanced Raman Spectroscopy Assisted by Radical Capturer for Tracking of Plasmon-Driven Redox Reaction

The deep understanding about the photocatalytic reaction induced by the surface plasmon resonance (SPR) effect is desirable but remains a considerable challenge due to the ultrafast relaxation of hole-electron exciton from SPR process and a lack of an efficient monitoring system. Here, using the p-aminothiophenol (PATP) oxidation SPR-catalyzed by Ag nanoparticle as a model reaction, a radical-capturer-assisted surface-enhanced Raman spectroscopy (SERS) has been used as an in-situ tracking technique to explore the primary active species determining the reaction path. Hole is revealed to be directly responsible for the oxidation of PATP to p, p′-dimercaptoazobenzene (4, 4′-DMAB) and O2 functions as an electron capturer to form isolated hole. The oxidation degree of PATP can be further enhanced through a joint utilization of electron capturers of AgNO3 and atmospheric O2, producing p-nitrothiophenol (PNTP) within 10 s due to the improved hole-electron separation efficiency.


Result and Discussion
Assembled Ag nanoparticle layer spin-coated from 50 nm of Ag NPs on a glass slide is used as plasmon-active substrate both for SERS analysis and catalysis reaction, which shows a wide plasmon resonance absorption band centred at 450 nm (Figures S1-S4). For assembled Ag nanoparticle layer adsorbed with PATP (Ag-PATP), Ag NPs and PATP are premixed before spin-coating. As shown in Fig. 1a, the SERS signal is collected under the irradiation of a 532 nm laser within 10 s. Compared with the Raman signal of PATP on the glass slide, three new peaks (blue line) assigned to the bend vibration of CH (β (CH)) at 1145 cm −1 and the stretching vibration of N = N (ν (N = N)) at 1390 and 1437 cm −1 are observed, implying PATP is oxidized to 4, 4′ -DMAB as driven by the SPR effect of Ag nanoparticle layer [28][29][30][31][32][33][34] . All the three strongly enhanced peaks of DMAB represent symmetric a g vibrational modes, strongly indicating the formation of DMAB from PATP through the N = N bond. These peaks are assigned to the a g12 , a g16 , and a g17 symmetric vibrational modes, respectively 35,36 . However, it is found that when ammonium oxalate (AO), a commonly used hole capturer is present, the reaction from PATP to 4, 4′ -DMAB is completely quenched since no signal from 4, 4′ -DMAB is observed. This result indicates the capturer-assisted strategy actually allows SERS to in-situ explore the reaction process in spite of the ultrafast relaxation process of hot carriers. Since both hole and its secondary radical • OH are strong oxidants, it still cannot be distinguished that PATP is oxidized by hole or • OH. Therefore, t-butanol (TBA) as the capturer for • OH is further adopted to understand its influence on the reaction 37 . However, strong peaks attributed to 4, 4′ -DMAB is still observed with preserved intensity, suggesting the reaction from PATP to 4, 4′ -DMAB is not altered by • OH (Fig. 1b). Therefore, it is undoubted that the oxidation of PATP is directly related to the hole decayed from the surface plasmon resonance of Ag nanoparticle layer. To get the original spectrum of PATP, it also can be detected on the Ag substrate by the irradiation of 785 nm on lower laser powers such as 0.50 μ W and 0.25 mW ( Figure S5).
The above results seem to be inconsistent with the current reports, where the hot electron together with oxygen is generally considered to be responsible for the plasmon-driven oxidation reaction 38,39 . To reveal the real role of oxygen and electron during the oxidation of PATP, the reaction was further carried out in N 2 atmosphere (Fig. 2a), which is significantly prohibited, suggesting O 2 indeed contributes to the oxidation of PATP. It is highly possible that O 2 may be reduced by hot electron to • O 2 − with strong oxidation capacity, which further cause the oxidation of PATP. To check this conjecture, a typical • O 2 − capturer, p-benzoquinone (BQ), is further applied in the SERS analysis. However, the synthesis conducted in the atmospheric environment in the presence of BQ does not cause any variation of 4, 4′ -DMAB signals (Fig. 2b), excluding the possible effect of • O 2 − on the oxidation of PATP. Intriguingly, when AgNO 3 is adopted as an electron capturer in N 2 atmosphere, the oxidation of PATP to 4, 4′ -DMAB occurs even in the absence of oxygen. A similar experiment was conducted in the water solution [40][41][42][43]  Electron spin resonance (ESR) is then further adopted to analyse the function of AgNO 3 in the oxidation of PATP by using 5, 5-dimethyl-1-pyrroline N-oxide (DMPO) as the indicator of • O 2 − . It is obvious that the intense signal appears on Ag nanoparticle layer under the laser irradiation, but the presence of AgNO 3 causes the decreased peak intensity of DMPO-• O 2 − (Fig. 2c), which thus proves two facts as follows. First, O 2 adsorbed on the surface of Ag nanoparticle layer can actually be reduced by SPR-derived electron. However, the presence of superoxide radical has no effect on the oxidation of PATP; Second, hot electrons can be indeed captured by AgNO 3 according to the retarded formation of • O 2 − . Based on these two facts, the oxidation of PATP in the absence of O 2 should be attributed to the improved concentration of holes due to the capture of electron by AgNO 3 (eqs 1-3, SI).
Generally, the SPR-derived hole-electron exciton from Ag nanoparticle layer resides in the Fermi energy level, which is harder to be separated than that formed from semiconductor [44][45][46][47][48][49] . Since the hole has been revealed as the exclusively active species for the oxidation of PATP instead of O-containing oxidants, the retarded oxidation from PATP to 4, 4′ -DMAB in the absence of O 2 should be attributed to the inefficient separation of hole from electron. As such, molecular O 2 should actually function as an electron capturer, which consumes electrons and produce enough holes to initiate the oxidation of PATP (eq 4, SI). Moreover, it is noted the plasmonic absorption of Ag nanoparticle layer in the presence of AgNO 3 (Ag-AgNO 3 ) is enhanced under UV-light irradiation (500 W Xe light, Fig. 2d), implying a possible reduction of AgNO 3 to Ag during the SERS analysis. To understand the influence of enhanced plasmon resonance intensity on the reaction, the SERS of PATP on the pre-irradiated Ag-AgNO 3 assembled nanoparticle layer has been further investigated in N 2 atmosphere. The signal intensity attributed to 4, 4′ -DMAB seems too low to be detected ( Figure S7), thus excluding the contribution from improved plasmon resonance intensity to the conversion efficiency.
Furthermore, it is found that when both of AgNO 3 and atmospheric O 2 are present, PATP is unprecedentedly transformed into a mixture of PNTP and 4, 4′ -DMAB within 10 s as characterized by the appearance of ν (NO 2 ) peak at ca. 1330 cm −1 (Fig. 3a), demonstrating the oxidation efficiency can be enhanced by improving the separation degree of hole-electron exciton. Both of AgNO 3 and O 2 should be involved in the oxidation of 4, 4′ -DMAB to PNTP since no PNTP can be produced when either of them is absent. The influence of AgNO 3 density (ρ Ag ) on the conversion efficiency was further investigated. It is found from Fig. 3a that 4, 4′ -DMAB is the dominant product at a laser power of 0.5 mW and ρ Ag of 2.3 * 10 −6 g/cm 2 , as evidenced by the strong peaks at 1440, 1380 and 1140 cm −1 from 4, 4′ -DMAB and a weak peak at 1330 cm −1 due to the ν (NO 2 ) of PNTP. The peaks of 4, 4′ -DMAB decreases when the laser power and ρ Ag increase to 2.5 mW and 2.3* 10 −5 g/cm 2 , along with an increasing intensity of ν (NO 2 ) peak. The intensity ratio between peaks at 1330 and 1390 cm −1 (I 1330 ν (NO 2 )/I 1390 ν (N = N)) was plotted to more clearly demonstrate the co-effect of AgNO 3 and laser power (Fig. 3a, red line). The ratio of I 1330 ν (NO 2 )/I 1140 β (CH) was also plotted and used as a reference (Fig. 3b, black line). The I 1330 ν (NO 2 )/I 1390 ν (N = N) and I 1330 ν (NO 2 )/I 1140 β (CH) obtained at 2.5 mW are almost doubled compared with those formed at 0.5 mW when ρ Ag is 2.3* 10 −6 g/cm 2 , which are further improved for ca. 30% and 80% when ρ Ag is increased to 2.3* 10 −5 g/cm 2 . A higher ρ Ag leads to the decreased intensity of both 4, 4′ -DMAB and PNTP (not shown), implying the shielding of Ag nanoparticle layer by overmuch addition of AgNO 3 . The actual composition of PNTP should be higher as valued from the peak intensity ratio between the a 1 and a g modes since the intensities of the a g modes of 4, 4′ -DMAB are significantly stronger than those of the a 1 modes of PNTP 50 , where a small amount of 4, 4′ -DMAB may already produce observable signal in SERS spectra. What's more, to investigate the role of laser power and exposure time 51 , Ag-AgNO 3 substrate was taken to detect PATP under 0.5 and 0.25 mW with different exposure times as Figure S8. The laser is both used for the light source of the SPR reaction and SERS analysis. The decreasing of the laser power decreases the reaction efficiency and so does the SERS sensitivity. We further investigated the reaction under 0.25 mW irradiation, where the Raman intensity is decreased without obvious variation of the reaction efficiency. The variation of the irradiation time to 5 s or 3 min does not obviously cause the change of the reaction process. However, when the exposure time extends to 10 min, the sample seems to be destroyed by the strong laser power, weakening the signal of the PNTP and DMAB on the substrate.
As a further check for the feasibility of radical-capture strategy to the in-situ SERS analysis of other SPR catalytic reactions, the SPR-catalysed reduction of PNTP, another typical reaction model has been further adopted here 31 . The results shown in Fig. 4a indicate the reduction of PNTP is retarded when AgNO 3 is present, accordant with the commonly-accepted understanding about the function of electrons in the SPR-catalysed reduction reaction 3,4 . On the contrary, the reduction can be promoted by conducting the reaction in N 2 atmosphere (Fig. 4b), which should be attributed to the eliminated consumption of electron by molecular O 2 . Even more, a higher  reduction degree of PNTP is achieved when AO is adopted to improve the electron concentration through consuming more holes (Fig. 4b).

Conclusions
In summary, we have explored the mechanism of SPR-catalysed reaction by a capturer-assisted SERS strategy using the oxidation of PATP on Ag nanoparticle layer as the model reaction. The adoption of AO and AgNO 3 as the capturers for hole and electron effectively leads to the separation of SPR-derived hot holes and electrons. The hot hole is directly responsible for the oxidation of PATP to 4, 4′ -DMAB. The oxidation of PATP is prohibited in N 2 atmosphere but occurs when AgNO 3 is further present. O 2 plays the role as an electron capturer in promoting the separation of hole-electron. The oxidation of PATP to PNTP has been unprecedentedly achieved in the atmospheric environment when the reaction is assisted by AgNO 3 . This study provides a novel way to deeply understand the mechanism of plasmon-related photocatalysis and photochemical reactions, which is expected to substantially push the development of SPR-induced green synthesis forward through rational and scientific design.

Method
Experimental Section. Preparation of Ag Nanoparticles with Diameter of ca. 50 nm. The preparation of Ag nanoparticles is adopted from a previously reported method 52  Characterization. Scanning electron microscopy (SEM) analysis was performed using a TESCAN nova III scanning electron microscope. Transmission electron microscopy (TEM) analysis was performed using a JEOL 2100 LaB6 TEM, at a 200 kV accelerating voltage. Raman spectra were recorded on a Renishaw inVia-Reflex Raman microprobe system equipped with Peltier charge-coupled device (CCD) detectors and a Leica microscope. Spectra were collected from the nanoparticle layer with an accumulation time of 10 s. Lasers with wavelength of 532 nm and 785 nm were used as the excitation light source, and a 50× objective with a numerical aperture (NA) of 0.75 was used to get the laser spot diameter of ~1 μ m. The electron spin resonance (ESR) technique (with DMPO) was used to detect the radical species over the catalyst on a Bruker EMX-8/2.7 spectrometer. DMPO was added to the suspension system before testing, and then the system was irradiated by visible light using a Xenon lamp. Electron spin resonance (ESR) technique is a very powerful and sensitive method for the characterization of the electronic structures of materials with unpaired electrons. By investigating the resonance line can obtain the information about status of the unpaired electrons in radical and its surrounding environmental, thereby obtaining information about the structure and chemical bonding of the substance, in order to identify the different types of free radicals and their levels.