Flexible SERS substrate In-situ monitoring of SPR-induced P-aminothiophenol dimerization

Surface-enhanced Raman spectroscopy (SERS) is an efficient technique for in-situ molecular investigation that is highly sensitive and has the ability to provide extensive fingerprint information. Additionally, the surface plasmons on the SERS substrate can also simultaneously induce the catalytic reaction. P-aminothiophenol (PATP) is a crucial model molecule utilized in SERS catalytic reactions. Through dimerization, PATP induced by the plasmon effect can produce 4,4’-dimercaptoazobenzene (DMAB). Here, a flexible SERS substrate with a monolayer of Au nanoparticles on polydimethylsiloxane (PDMS) is prepared and used for monitoring the dimerization of PATP. In the experiment, the SERS substrate is inverted, the side with Au nanoparticles monolayer is kept in contact with the PATP solution directly, and the laser is irradiated from the backside of the flexible substrate. The effects of Ag+ and oxygen concentrations on the reaction are investigated. This work paves the way for SERS methodology in in-situ monitoring of other liquid-phase catalytic reactions.


Introduction
Raman scattering is an inelastic scattering process through which molecular fingerprint information can be obtained, but the intensity is very low.Surface-enhanced Raman spectroscopy (SERS) technology is primarily on account of the electromagnetic enhancement arising from the surface plasmon resonance effect in the regions of metallic nanostructures normally less than 10 nm, and the Raman scattering intensity can be amplified by several orders of magnitude [1].SERS is a rapid, non-destructive, and ultrasensitive analytical technique that is capable of providing rich, sharp, and fingerprint-like vibrational signatures of target molecules up to even a single molecule level [2].Therefore, SERS is widely used in molecular detection and material characterization.Besides, surface plasmon resonance (SPR) on the SERS substrate can promote the catalytic process of some molecules, and one important model molecule in the SERS catalytic reaction is p-aminothiophenol (PATP).
According to previous research, PATP dimerization is relevant to many conditions, such as the presence and absence of oxygen, metal species, and pH [2].Here, Au nanoparticles (NPs) monolayer fabricated by oil-water interfacial self-assembly is transferred to a polydimethylsiloxane (PDMS) substrate, and the fabricated flexible and transparent SERS substrate is used to monitor the dimerization of PATP on Au NPs and to study the potential influences of oxygen and Ag + concentrations on PATP dimerization.This work provides the possibility for SERS technology in in-situ monitoring of catalytic reactions in the liquid phase.

Self-Assembly of Au NPs
Au NPs were synthesized by referring to a previously reported method [3].Citrate-stabilized Au NPs with closely packed monolayers were prepared by an oil-water interfacial self-assembly strategy.Briefly, 5 mL of freshly prepared Au nanoparticles solution was transferred into a beaker with ultrasonic treatment for 5 min, followed by 5 mL of hexane added above it to form a hexane-water interface.Then 2 mL of ethanol was slowly injected into the Au NP solution by using a syringe with a low rate (150 ȝL/min).The Au nanoparticles gradually rose to the water/hexane interface and self-assembled into a closely packed Au nanoparticles monolayer.After removing almost all of the hexane, the assembled monolayer Au film was transferred onto a patch of PDMS substrate by a lifting method.

Characterization
Scanning electron microscopy (SEM) images were taken by using JSM-7800F, JEOL.All SERS spectra were obtained by using a confocal Raman spectrometer (LabRAM HR Evolution, Horiba Jobin Yvon) with a 632.8 nm He-Ne laser line, and the diameter of the laser spot is about 1 ȝm.

Raman experiments
PATP solution with a suitable concentration was prepared in the centrifuge tube, and sodium hydroxide was added to adjust the pH value to 9. Different concentrations of silver nitrate were added into PATP solutions to obtain different concentrations of Ag + .O2 or N2 flowed into PATP solutions to make the presence or absence of oxygen.The SERS substrate was put inverted, and the side with Au-NPs monolayer was kept in contact with the PATP solution directly, and the laser was irradiated from the backside of the SERS substrate.

SERS Monitoring of SPR-induced PATP Dimerization at Different Conditions
The entire experiment was tested in an alkaline environment (pH=9).The concentration of Ag + and oxygen were used as variables to investigate their potential influences on PATP dimerization.

In Air Surroundings with Different Ag + Concentrations
Figure 2 illustrates how long the time is taken when the characteristic SERS peaks of 4,4'dimercaptoazobenzene (DMAB) appeared under different Ag + concentrations.As Figure 2(a) and Figure 2(b) demonstrate, the characteristic SERS peaks of DMAB at 1143, 1392, and 1436 cm -1 , could be clearly observed at 40 min and 8 min, with Ag + concentrations of 0 ȝM and 5 ȝM, respectively.However, in Figure 2(c), when the concentration of Ag + rose to 10 ȝM, only 30 s was taken and the characteristic SERS peaks of DMAB could be clearly observed.It is obvious that the higher the Ag + concentrations were, the faster the PATP dimerization occurred, confirming that Ag + could promote this reaction under alkaline conditions.The reason speculated was as follows.During the reaction, Ag + preferred to react with activated oxygen to form silver oxide as Ag + had a much lower activation energy.Silver oxide as a photocatalyst had an excellent performance [2], and PATP was more easily converted to DMAB with the existence of silver oxide.Therefore, adding Ag + to the PATP solution could effectively reduce the activation energy and promote the reaction kinetics.

Different concentrations of oxygen without Ag +
According to the prior work, the presence of oxygen is necessary for the specific PATP dimerization [2].Herein, to control the concentration of oxygen, O2 and N2 flowed into the PATP solution, respectively.In Figure 3(a), PATP dimerization occurred after 4 min under O2 conditions without Ag + .However, in Figure 3(b), it took 40 min to observe the characteristic SERS peaks of DMAB under N2 surroundings without Ag + .It was evident that the PATP dimerization reaction could be facilitated by the existence of oxygen.Here, the speculated reaction mechanism was as follows.During the reaction, the laser activated the oxygen on the Au NP surface, causing a PATP dimerization reaction to occur.Equation (1) provides a description of the reaction principle [4]: where ‫ݒ݄‬ represents the activation energy required for the reaction.However, as Figure 3(b) illustrates, PATP dimerization still occurred without the existence of oxygen, but with a much longer time.In this case, the dimerization of PATP was accomplished through SPRinduced electron transfer.Figure 3(c) illustrates the mechanism.Pairs of hot holes and hot electrons were excited when laser irradiated on the nanostructure of Au and Ag.These electrons moved away from the hot holes below the Fermi level and went to the bands above it.If there were acceptable electron acceptors close by, electron transfer would happen.The transfer of the holes was similar to the electrons, and the hole transfer happened if the HOMO level of the reactant molecule matched the level of holes [2].The plasma-driven electrons and holes transfer process could be understood as electron-hole transfer between PATP and the flexible SERS substrate directly when oxygen was not present.

Ag + Without Oxygen
As mentioned above, Ag + could be oxidized to silver oxide in air or O2 surroundings to facilitate PATP dimerization reaction.However, as shown in Figure 4, the reaction still occurred with the existence of Ag + in N2 surroundings.The reason speculated was as follows.Ag + could form the hydroxide in an alkaline environment, and the formed silver hydroxide could be dehydrated to generate silver oxide which can facilitate the PATP dimerization reaction [5].In other words, Ag + had another way to form silver oxide in alkaline conditions.

Conclusions
This work utilized a flexible SERS substrate to achieve in-situ monitoring of PATP dimerization.The effects of oxygen and Ag + concentration on this reaction were investigated.The results illustrated that, without the existence of oxygen, the primary mechanism for this reaction was electron-hole transfer between PATP and the flexible SERS substrate directly; with the presence of oxygen, the dominant mechanism for this reaction was oxidation.As for Ag + , it had two pathways to generate silver oxide under alkaline conditions.One is in the presence of oxygen, where silver oxide was directly formed from Ag + ; the other is in an N2 environment, where Ag + first generated silver hydroxide, and then the formed silver hydroxide converted into silver oxide.Thus, Ag + can promote PATP dimerization reaction kinetics in both O2 and N2 surroundings under alkaline conditions.This work provides the possibility for applying SERS technology in in-situ monitoring of other liquid-phase catalytic reactions.

Figure 1 (
a) shows the optical image of the self-assembly of the Au NPs monolayer at the hexane-water interface, and Figure1(b)is the optical image of the Au nanoparticles monolayer transferred to a PDMS substrate.Figure1(c) illustrates the SEM image of Au nanoparticles monolayer on PDMS.As shown in Figure1(c), Au nanoparticles with a diameter of 80 nm were successfully prepared and they were uniformly distributed on the PDMS substrate.Thus, it could be concluded that flexible SERS substrates with Au nanoparticles self-assembly monolayer were successfully fabricated.

Figure 1 .
Figure 1.(a) Optical image of self-assembly of Au NPs monolayer at hexane-water interface.(b) Optical image of Au nanoparticles monolayer transferred onto PDMS.(c) SEM image of Au nanoparticles monolayer on PDMS.

Figure 2 .
Figure 2. SERS spectra of PATP dimerization in air surroundings with different Ag + concentrations.(a) 0 ȝM; (b) 5 ȝM; (c) 10 ȝM.It is obvious that the higher the Ag + concentrations were, the faster the PATP dimerization occurred, confirming that Ag + could promote this reaction under alkaline conditions.The reason speculated was as follows.During the reaction, Ag + preferred to react with activated oxygen to form silver oxide as Ag + had a much lower activation energy.Silver oxide as a photocatalyst had an excellent performance[2], and PATP was more easily converted to DMAB with the existence of silver oxide.Therefore, adding Ag + to the PATP solution could effectively reduce the activation energy and promote the reaction kinetics.

Figure 3 .
Figure 3. SERS spectra of PATP dimerization under (a) O2 condition without Ag + , and (b) N2 condition without Ag + .(c) Schematic diagram of the PATP dimerization reaction mechanism without the existence of oxygen.