Electrochemical Tip-Enhanced Raman Spectroscopy with Improved

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Electrochemical tip-enhanced Raman spectroscopy with improved sensitivity enabled by a water immersion objective Sheng-Chao Huang, Jiu-Zheng Ye, Xiao-Ru Shen, Qing-Qing Zhao, Zhicong Zeng, Mao-Hua Li, De-Yin Wu, Xiang Wang, and Bin Ren Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b01701 • Publication Date (Web): 30 Jul 2019 Downloaded from pubs.acs.org on July 31, 2019

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Analytical Chemistry

Electrochemical tip-enhanced Raman spectroscopy with improved sensitivity enabled by a water immersion objective Sheng-Chao Huang,§ Jiu-Zheng Ye,§ Xiao-Ru Shen, Qing-Qing Zhao, Zhi-Cong Zeng, Mao-Hua Li, De-Yin Wu, Xiang Wang,* Bin Ren* State Key Laboratory of Physical Chemistry of Solid Surface, The MOE Key Laboratory of Spectrochemical Analysis and Instrumentation, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China ABSTRACT: Electrochemical tip-enhanced Raman spectroscopy (EC-TERS) appears as a promising in situ nanospectroscopic tool for characterization and understanding of the electrochemical interfacial processes at the nanometer scale and molecular level. However, the wide application of EC-TERS is hampered by its low sensitivity as a result of the optical path distortion due to the refractive index mismatch of the multilayer media (air, glass, and electrolyte). Here we propose a new side-illumination EC-TERS setup by coupling a water immersion objective with a high numerical aperture (NA) to a STM scanning head customized with a large open space and a compact spectroelectrochemical cell. It not only effectively eliminates the optical distortion, but also increases the sensitivity remarkably, which allows the sensitive monitoring of the electrochemical redox processes of anthraquinone molecules. More importantly, EC-TERS is able to independently control the tip position and laser illumination position. By utilizing this feature, we reveal that the irreversible reduction reaction of anthraquinone observed in EC-TERS is induced by the synergistic effect of the negative potential and laser illumination rather than the localized surface plasmon (LSP). The highly improved sensitivity and the flexibility to control the tip and laser illumination position on the nanometer scale endows EC-TERS an important tool for the fundamental understanding of the photo- or plasmon electrochemistry and the interfacial structure-activity relationship of important electrochemical systems.

A comprehensive understanding of the electrochemical processes on the nanometer and molecular level is the key to establish a reliable structure-activity relationship and optimize the performance of the electrochemical interface for various electrochemical systems1, 2, including electrocatalysis, batteries, and photoelectrochemistry. Therefore, it requires characterization techniques that can work in the electrochemical environment with a high sensitivity and high spatial resolution. In this regard, electrochemical tip-enhanced Raman spectroscopy (ECTERS), which integrates nanometer spatial resolution3, single-molecule sensitivity4, 5, rich vibrational information of TERS6, 7 and flexible control of the energy of the electrode materials of electrochemistry, shows unique advantages for the molecular level and nanoscale analysis of the electrochemical interface processes. However, an EC-TERS setup involves a complex combination of electrochemical scanning probe microscope (SPM) (sensitive to vibration interference and has a limited open space) and a Raman system (needs to be sensitive enough to probe small amounts of surface species). How to couple a Raman optical system into the limited open space of SPM to achieve a high detection sensitivity while minimizing the interferences among different

instruments is the key issue in developing the EC-TERS setup. This is particularly important when the signal of the molecules is weak. In 2015, our group developed an EC-TERS setup with a horizontal illumination configuration.6 The laser beam was focused and the EC-TERS spectrum was collected by an air objective with a long working distance. The horizontal optical configuration enabled a well control of electrolyte thickness to less than 1 mm to reduce the optical distortion caused by the mismatch of refractive index of air, glass window, and electrolyte. However, the optical distortion still could not be completely eliminated. In addition, the low effective numerical aperture (NA) of ca. 0.3 limited the detection sensitivity of the setup. At almost the same time, Van Duyne group reported the AFM-based EC-TERS setup using the bottom illumination configuration.4, 5 The Raman optical path was working in a transmission mode through the optically transparent ITO and on the opposite side of the SPM tip. The spatial independence of SPM and Raman systems guaranteed an adequate open space to accommodate a high NA oil objective. Additionally, as the laser was focused from the bottom side into the sample surface without passing through the thick electrolyte layer, the optical distortion

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could be effectively avoided. However, this transmission EC-TERS configuration could not be applied to nontransparent electrodes such as metal single crystal electrodes that are of great significances in electrochemistry. Thereafter, both Domke group7, 8 and Van Duyne group9-11 developed side-illumination EC-TERS setup using air objectives. It was found that the detection sensitivity of the side illumination mode was only onetenth of that of the bottom illumination mode.9 Even for resonant molecules, it took 120 s to obtain EC-TERS spectra with reasonable good signal to noise ratio.9, 10 The overall performance of side-illumination EC-TERS is yet to be improved, on considering that it can be applied to both transparent and non-transparent electrode materials. In this paper, we introduce a new side-illumination ECTERS setup based on the water immersion objective with a short working distance and high NA, which not only minimizes the effect of the mismatch of refractive index of different media, but also increases the effective NA. We redesign the scanning head of scanning probe microscope (STM), the sample holder, and EC-TERS cell to offer a large open space to accommodate the water immersion objective with a high NA and short working distance. The setup shows a good performance in both STM imaging and a significantly improved TERS sensitivity. The high sensitivity allows us to systematically study the redox processes of a nonresonant molecule, thiolacetylterminated-phenylene ethynylene-substituted anthraquinone (2-AQ), on Au(111) surface. The ability to independently control the position of tips in EC-TERS setup allows us to distinguish the role of laser illumination and the tip induced localized surface plasmon (LSP) on the redox reaction, which cannot be easily achieved with other techniques. EXPERIMENTAL SECTION Electrochemical Cyclic Voltammetry Characterization In the cyclic voltammetry (CV) experiment, we used a Au(111) single crystal disc (MaTeck, Germany) with a diameter of 8 mm as the working electrode. The Au single crystal electrode was electrochemically polished and annealed before use to ensure the high facet quality. After that, it was immersed in 0.025 mM 2-AQ tetrahydrofuran solution for one hour. Then the electrode was rinsed by a large amount of ethanol to remove the physically adsorbed molecules on the surface, and dried with pure nitrogen gas. Before CV characterization, argon was purged into the electrochemical cell for 20 min to remove oxygen. The CV experiment was performed on CHI660D (CH Instruments, China). EC-TERS Characterization In the EC-TERS experiments, we used the Au(111) single crystal fabricated by Clavilier method12 as the working electrode. A gold bead with a Au(111) surface on the top was fixed on a Au rod, and the Au rod was fixed on a gold

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foil (Figure S1). The Au(111) single crystal surface was electrochemically polished and annealed before use. We followed our previous work to prepare 4-PBT ((4 ′ (pyridin-4-yl)biphenyl-4-yl)me-thanethiol) adsorbed Au(111) surface.13 In brief, a clean Au(111) single crystal was immersed in 0.4 mM 4-PBT ethanol solution for 1 hour. Then the electrode was rinsed by a large amount of ethanol to remove the physically adsorbed molecules on the surface, and dried with pure nitrogen gas. In the ECTERS characterization of 2-AQ, the procedure for preparing the monolayer 2-AQ assembled Au(111) electrode was the same as that in the CV characterization. A platinum wire was adopted as the counter electrode and a silver wire was used as the quasi reference electrode. The electrolyte was 0.2 M sodium dihydrogen phosphate/disodium hydrogen phosphate buffer solution with pH=6.75. The Ag tips used in EC-TERS experiments were prepared by using the electrochemical etching method,14, 15 and the tips were insulated with the thermosetting polyethylene.16 The EC-TERS experiments were performed on a homebuilt EC-TERS system that consists of a Raman optical head6, 17, modified scanning tunneling microscope (Veeco Nanoscope E, USA) and a home-designed EC-TERS cell. A 632.8 nm laser (CVI Melles Griot, USA) was introduced into the Raman optical head through the single-mode fiber. The laser beam was then focused onto the tip apex through the microscope objective to excite the TERS signal. The signals were collected by the same objective and passed through a 45° and a 0° 632.8 nm edge filter to remove the Rayleigh scattering light. The TERS signal was focused into the multimode optical fiber and coupled into the Shamrock 303i spectrometer (Andor, UK) equipped with a 600 g/mm grating and idus CCD (1024×256 pixels) detector. The Raman optical head was equipped with a white light imaging system to guide the accurate coupling of the laser focus with the tip. All experiments were carried out at room temperature. All chemicals used were of analytical reagent grade and the solutions were prepared using Milli-Q water. RESULTS AND DISCUSSION Novel Side Illumination EC-TERS Setup Based on a Water Immersion Objective In our previous EC-TERS setup, we used an air microscope objective. Therefore, the laser and TERS signals had to pass through air (n=1), glass window (n=1.46), electrolyte solution (n=1.33). The obvious difference of the refractive indices of the three layers causes severe distortion in the optical path.18, 19 If we adopt a water immersion microscope objective for the new ECTERS setup, the air in the gap between the objective and glass window is replaced by a drop of water (Figure 1). The difference in the refractive indices of the three layers, water (1.33), thin glass window (1.46), and electrolyte (1.33), becomes much smaller, thus the optical distortion could be largely eliminated.


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Analytical Chemistry However, the working distance of a water immersion objective with large NA (NA=0.8, WD=3.5mm) is usually much shorter than that of the air objective used in the conventional side illumination TERS. Therefore, it requires a large open space in the SPM head, which is not possible for a commercial SPM scanning head. We customized the STM scanner of a Veeco SPM, by cutting an angle on the sample holder and scanning head (Figure 1a) to provide a sufficient space to accommodate the water immersion objective. Accordingly, we redesigned the EC-TERS cell shown in Figure 1b to match the optical structure and SPM scanning head. The EC-TERS cell consists of a poly(chlorotrifluoro-ethylene) (Kel-F) body with a hole in the bottom for installation of the homemade Au(111) working electrode and a thin glass window attached to the side of the cell with an angle of 70° to the horizontal direction. The water immersion objective is tilted with an angle of 20° to the horizontal direction to maximally use the open space of the modified SPM system, see Figure 1c. In this case, although a small portion of the light is still blocked by the sample (S2 part), the effective collection area (S1) can account for 80% of the total area of the objective aperture and the

effective NA can reach 0.64, which is much higher than the 0.30 for the previous EC-TERS setup.6 Thus, a high excitation and collection efficiency can be achieved. To ensure an efficient coupling of the laser spot and the tip apex during EC-TERS experiments, the objective is mounted on a piezoelectric stage (NanoMax-TS, Thorlabs) to precisely drive the three-dimensional movement of the objective and the laser spot. One may argue that the movement of the objective may lead to a deviation of the objective axis from the optical axis of the laser beam, which may block part of the laser beam and decrease the excitation and collection efficiencies. However, even though the objective is driven to move to the full range of the stage (20 μm), the deviation is still much smaller than the beam diameter (about 8 mm). Therefore, it has negligible effect (20/8000=0.25%) on the laser focus and signal collection. More importantly, the coupling adjustment with the slow and precise piezoelectric control can largely overcome the mechanical interference as a result of the manual adjustment method (Figure S2 and S3)

Figure 1 (a) 3D drawing of the EC-TERS setup and (b) Schematic illustration of the EC-TERS setup. 1-laser beam; 2- STM scanner head; 3-sample holder; 4-water immersion objective; 5-Au single crystal (as working electrode); 6-cell body; 7insulated tip; 8-Ag quasi reference electrode; 9-Pt counter electrode. A bipotentiostat in the EC-STM setup is used to control the potential of the Au(111) electrode and the tip independently. (c) Scheme demonstrating the effective NA in our configuration. A part of light is blocked by the crystal surface. S1: the part of light that can be focused on the tip apex. S2: the part of light that is blocked by the substrate. We then characterized the performance of the new ECTERS setup. Benefitted from the less optical distortion by using the water immersion objective, we could obtain a clear microscopic image of the tip apex and its refection image on a Au(111) substrate (Figure 2a). The good imaging quality is conducive to guide the adjustment of coupling of the laser spot and the tip. To directly compare the performance of current setup with the previous setup, we use nonresonant 4-PBT molecules20 assembled on Au(111) as a model sample. It is clear from the STM image (Figure 2b) that the molecules form a compact selfassembled monolayer on the Au surface, similar to our previous study.6, 17 The appearance of a clear Au atomic terrace indicates that the designed EC-TERS setup retains the good STM imaging performance. Figure 2c shows the TERS spectra of the monolayer 4-PBT obtained from the old and new EC-TERS setups Note that the acquisition

condition in our experiments (with an integral time of 1s and laser power of 0.4 mW) is relatively moderate compared with that of reported works7, 9-11. To evaluate the sensitivity of the two setups, we estimate the S/N ratios (normalized to 1 mW power) of the spectra by using the 1605 cm-1 peak of 4-PBT. The S/N ratios are 162.9 for the new EC-TERS setup and 27.7 for the old setup. It amounts to about 5.9 times improvement in sensitivity. The improvement of the sensitivity is benefited from the elimination of the optical path distortion and the improvement of the effective NA. The effective NA (0.64) of the water immersion objective in the new EC-TERS is about 2.1 times of that of the air objective used in the old EC-TERS setup (~0.3). The improvement of the collection efficiency as a result of the increase of NA could be about 4.4 times, considering that the collection efficiency is proportional to the square of NA. The additional 1.3 fold



improvement in the sensitivity of the new setup may be contributed by the minimization of the optical distortion and better focus.

Figure 2 (a) Microscopic image of the tip above a Au(111) single crystal surface in the EC-TERS system. (b) ECSTM image of a 4-PBT-adsorbed Au(111) surface in 0.1 M NaClO4. The potential of the Au(111) was -200 mV. The bias voltage was 100 mV, and the tunneling current was 800 pA. (c) TERS spectra of 4-PBT adsorbed on a Au(111) surface obtained on our first-generation EC-TERS setup (red curve) and the new EC-TERS setup (black curve). The laser power was 0.4 mW and 0.9 mW in the new and the first-generation EC-TERS setup, respectively. The acquisition time was 1s. Inset: the molecular structure of 4-PBT. Study of the Electrochemical Redox Reaction of Anthraquinone In our previous EC-TERS study, we have already studied the adsorption behavior of 4-PBT on Au(111) surface.6 It would be interesting if we can further demonstrate the capability of EC-TERS on studying the redox behavior of the molecules important to electrochemistry. Among all the redox species of great interest to electrochemists, anthraquinone and its derivatives have attracted great interests. They are not only important electron mediators in the photosynthesis process,21, 22 but also important molecular wires in single molecule electronics.23, 24 We chose 2-AQ molecule as a model molecule and in-situ characterized the electrochemical behavior of monolayer 2-AQ molecules adsorbed on the Au(111) surface on the newly developed EC-TERS setup.

Figure 3 Cyclic voltammograms (CV) of 2-AQ adsorbed on Au(111). (a) CV in 0.2 M PB buffer solution (containing 0.2 M Na2HPO4 / NaH2PO4, pH=6.75). (b) Dependence of the anodic peak current (red curve) and cathodic peak current (black curve) on the scan rates.

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Figure 3(a) shows the CV curve of the 2-AQ molecules adsorbed on the Au(111) electrode at different scan rates. The CV exhibits a pair of redox peaks of 2-AQ at about 0.73 V and -0.64 V, which is similar to that of the reported CV of monolayer 2-AQ adsorbed Au electrode25. The peak potential separation (ΔEp) of the redox reaction was about 0.09V and increased with the increase of the scan rate, indicating the reaction was quasi-reversible.26,27 There is no obvious shift of the anodic peak with potential but an obvious shift of the cathodic peak to the negative potential, indicating the anodic process occurs much faster than the cathodic process.28 The latter involves the diffusion of proton in the electrolyte to the surface right after the reduction of the 2-AQ molecules. The linear dependence of the anode and cathode peak currents on the scan rate shown in Figure 3b agrees well with the surface reaction. To monitor the electrochemical redox reaction, ECTERS experiment was performed in the potential window covering the redox current peak. Figure 4a shows the ECTERS spectra of 2-AQ molecules with a high signal to noise ratio obtained on the new EC-TERS setup. When the potential of the Au(111) electrode was moved from -0.5 V to -0.9 V, a remarkable decrease in the intensity of 1671cm-1 peak was observed. This peak is attributed to the stretching vibration of C=O bond (Table S1). The 1671 cm-1 peak intensity-potential plot and charge-potential plot are shown in Figure 4b. The intensity of 1671 cm-1 peak decreases and the charge corresponding to the quantity of the electrochemically reduced 2-AQ molecules increases with the negative shift of the potential. Meanwhile, a shoulder peak at 1625cm-1 (assigned to C=C vibration of the anthracene ring in hydroquinone, see detailed assignment in Table S1) increases from -0.5 V to -0.9 V (Figure 4c), indicating the formation of the hydroquinone derivative (2-AQH2). In addition, the stretching vibration of the C≡C bond at 2215 cm-1 red shifts to 2210 cm-1 (Figure 4d). Our DFT calculation of 2-AQ (Spectrum 2 in Figure 4e) and the reduced form of thiol-terminated-phenylene ethynylene-substituted hydroquinone(2-AQH2) (Spectrum 1 in Figure 4e) agrees well with the EC-TERS spectra. The hydrogenation of C=O results in the formation of a large conjugated anthracene (three benzene rings) structure and an increased electron delocalization of the C≡C. As a result, the C≡C bond weakens and the C≡C vibration frequency redshifts. However, when the potential was gradually returned to 0.1 V, the intensity of 1671 cm-1 peak could only be partially recovered (Figure 4b), and the 1625 cm-1 peak remained to be observed (Figure 4c). Furthermore, the C≡C vibration peak does not return to its initial frequency (Figure 4d). These indicate that a part of 2-AQ molecules still remained in the reduction state when the potential was returned to -0.1 V. About 64%


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Analytical Chemistry

Figure 4. (a) Potential-dependent TERS spectra of 2-AQ molecules on Au(111) in 0.2 M PB buffer solution (pH=6.75). The acquisition time for each spectrum was 1s. The laser power was 1 mW. (b) Plots of the integrated peak intensity (1671 cm-1) and charge derived from CV against potential. (c) A plot of the integrated peak intensity (1625 cm-1) against potential. (d) A plot of the peak position (C ≡ C) against potential. The error bars in (b-d) show the deviation in the peak intensity or frequency in the fitting processes. (e) Calculated Raman spectra of 2-AQ molecule in the reduction state (Spectrum 1) and oxidation state (Spectrum 2). Inset: the molecular structures in the reduction state and oxidation state. (f) TERS spectrum of 2-AQ molecule obtained on a new site in the laser spot at -0.1 V after the cycle of potential (Spectrum 3) and the spectrum obtained at a new site out of the previous illuminated laser spot (Spectrum 4). The detailed schematic illustration of the experimental procedures is shown in Figure S15. molecules could not be reversibly oxidized estimated from the ratio of the peak area of 1625 cm-1 at -0.1 V to that at – 0.9 V (Figure S14). Additionally, the intensity of 1625 cm-1 peak at -0.1 V did not change further in the following cycles (see the intensity-cycle plot in Figure S8), indicating the irreversible reaction has been stabilized after the first cycle. Similarly, we also found the irreversible reduction of 2-AQ on the Au nanoparticles surface in the EC-SERS study (Figure S7). Such an irreversible redox reaction observed with EC-TERS study is contradictory to the quasi-reversible behavior observed with CV (Figure 3). On considering the experimental difference in EC-TERS and CV, the photoelectrochemistry as a result of laser illumination and/or the tip-induced localized surface plasmon (LSP) may account for the irreversible process. Interestingly, in EC-TERS, the laser was focused onto the Au(111) surface over an area much larger than that can be influenced by the tip LSP. Therefore, after performing the above potential dependent EC-TERS experiments (P1 in Figure S15, fixed laser spot position and fixed tip position), we moved the tip to a new position within the laser spot while keeping the potential at -0.1 V (P2, in Figure S15c). The molecules in the new position has experienced the laser illumination and negative potential of -0.9 V for reduction and -0.1 V for oxidation, but without the influence of the tip LSP at 0.9 V. We could still observe the 1625 cm-1 peak at the new position (Spectrum 3 in Figure 4f and Table S3), indicating that tip LSP is not the key factor influencing the reaction. We then moved the tip and the laser spot

out of the area that had experienced the laser illumination and negative potential of -0.9 V (P3 in Figure S15c) while keeping the potential at 0.1 V. Surprisingly, we did not observed the 1625 cm-1 band at the new positions (Spectrum 4 in Figure 4f and Table S3). This result indicates that the laser illumination is the key factor that led to the irreversible redox process of 2-AQ at the negative potential. Therefore, in such a complicated system with the presence of potential control, laser illumination and tip LSP, we can still scrutinize that the irreversible reaction was triggered by synergistic effect of negative potential and laser, not the tip LSP. In the literature, EC-SERS have been widely used to study the plasmon-enhanced chemical reactions and it is difficult for SERS to isolate the impact of laser illumination and LSP induced by laser as they always are co-localized. However, in EC-TERS, the LSP is only excited in the area (around 10 nm in diameter) below TERS tip, whereas the laser illuminates over a diffraction limited spot of around 1000 nm in diameter. With the spatial resolution of ECTERS, we can easily investigate the chemical fingerprint information of surface species on different sites inside the laser spot, thereby distinguishing the impact of laser illumination and LSP. This is a unique advantage of ECTERS. The irreversible reduction of 2-AQ is highly reproducible in both EC-TERS and EC-SERS. Up to now, it is not straightforward to understand why a part of hydroquinone molecules cannot recover to anthraquinone molecules with the positive movement of



potential. We speculate that a portion of 2-AQ molecules might be reduced drastically to some new species other than hydroquinones under the negative potential and laser illumination, which led to the irreversible reaction. On considering the molecular structure of 2-AQ, we analyzed the two most probable structures as the products of the irreversible reaction. One of the possibilities is that C ≡C was reduced to C=C at negative potentials. However, we did not observe the C=C band predicted to appear at 1644 cm-1 by DFT calculation (Figure S10) in the EC-TERS results. It indicates that the reduction of C ≡ C might not account for the irreversibility. We further consider the possibility of formation of 9,10-dihydroxy-9,10-dihydro-anthracenes or anthrones, which has been demonstrated to exist in the literature when some strong reducing agents like NaBH4 was used (Figure S11).29, 30 However, the 1625 cm-1 peak is expected to disappear as a result of the breaking of the anthracene ring according to the DFT calculation (Figure S12). Therefore, the detailed mechanism of this irreversible process remains open. The signature of the irreversible reaction can also be observed after the electrode has experienced several cycles of the redox reaction in the absence of laser illumination, indicating that the laser illumination could accelerate the irreversible reaction. On considering the importance of anthraquinones and its derivatives in electrochemistry and photo-electrochemistry, and that such an irreversible reaction may affect the performance of the redox couple in the photosynthesis or other electrochemical applications, it is of great significance to further explore the mechanism of the irreversible process.

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the fundamental study of the photo- or plasmon electrochemistry. The improved sensitivity of EC-TERS setup opens the door of studying the non-resonant small molecules at a short time scale, which may further extend the application of EC-TERS for the in situ nanoscale characterization and exploration of the interfacial structure-activity relationship of some important electrochemical systems, including energy storage and (photo)electrocatalysis in the future.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. EC-TERS setup, piezoelectric stage to eliminate the mechanical shock, method for estimating the signal to noise ratio of the spectra, spectra of 2-AQ on the old and new EC-TERS setups, EC-SERS of 2-AQ, multiple cycles CV of 2-AQ adsorbed on Au(111), variation of peak intensities at 1625 cm-1 and 1671 cm-1 with cycles, DFT calculation, analysis of the ratio of 2-AQH2 to 2AQ, the control experiment demonstrating the effect of LSPR and laser illumination (PDF)

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected]

Present Addresses ‡ College

of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu 730000, P.R. China. (Z.-C.Z.).

Author Contributions CONCLUSION

§These

authors contributed equally. (S.-C.H. and J.-Z.Y.)

We have developed a new EC-TERS setup based on a water immersion objective with a short working distance and high numerical aperture by customizing the STM scanning head with a large open space and designing a special spectroelectrochemical cell. Benefitted from the elimination of optical distortion and the increase of the effective NA, we significantly improved the sensitivity of the new EC-TERS setup by five times compared with the previous EC-TERS setup. We studied the electrochemical redox reaction of anthraquinone molecules on the new EC-TERS setup, and clear spectral features of the redox couple were successfully observed. We found that about 64% of the hydroquinone molecules as the reductive product of 2-AQ could not be reversibly oxidized although it is considered as a quasi-reversible electrochemical redox couple. We demonstrated that it was the synergistic effect of negative potential and laser illumination rather than the tip LSP that led to such an irreversible reaction. Such a conclusion may not be drawn without the capability of EC-TERS in independently controlling the tip position and laser illumination position, which shows unique advantages of EC-TERS in

The authors declare no competing financial interest.

Notes

ACKNOWLEDGMENT The authors acknowledge the financial supports from MOST of China (2016YFA0200601), NSFC (21633005, 21790354, 21503181, 21711530704), Natural Science Foundation of Fujian Province (2016J05046), China Postdoctoral Science Foundation (2017M622062) and Fundamental Research Funds for the Central Universities (lzujbky-2019-63).

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