Effect of Intermolecular Distance on Surface-Plasmon-Assisted

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Effect of Intermolecular Distance on Surface-Plasmon-Assisted Catalysis shiwei wu, yu liu, caiqing ma, Jing Wang, Yao Zhang, Peng Song, and Lixin Xia Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00700 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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Effect of Intermolecular Distance on SurfacePlasmon-Assisted Catalysis Shiwei Wua, Yu Liua, Caiqing Maa, Jing Wanga, Yao Zhanga, Peng Songb,*, Lixin Xiaa,*

a College of Chemistry, Liaoning University, Shenyang 110036, China b College of Physical, Liaoning University, Shenyang 110036, China

----------------------* Corresponding authors. Tel.: +86 24 62202258; fax: +86 24 62202304. E-mail: [email protected] (L.X. Xia) [email protected] (P. Song)

ACS Paragon Plus Environment

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Abstract: 4-Aminothiophenol (PATP) and 4-aminophenyl disulfide (APDS) in contact with silver will form H2N-C6H4-S-Ag (PATP-Ag), and under the conditions of surface-enhanced Raman spectroscopy (SERS) a coupling reaction will generate 4,4-dimercaptoazobenzene (DMAB). DMAB is strongly Raman-active, showing strong peaks at ν≈1140, 1390, and 1432 cm-1, and is widely used in surface-plasmon-assisted catalysis. Using APDS, PATP, pnitrothiophenol (PNTP), and p-nitrodiphenyl disulfide (NPDS) as probe molecules, Raman spectroscopy and imaging techniques have been used to study the effect of intermolecular distance on surface-plasmon-assisted catalysis. Theoretically, PATP-Ag formed from APDS will be bound at proximal Ag atoms on the Ag surface due to S-S bond cleavage. The results show that APDS is more prone to surface-plasmon-assisted catalytic coupling due to the smaller distance between surface PATP-Ag moieties than those derived from PATP. Therefore, APDS shows higher reaction efficiency, better Raman activity, and better Raman imaging than PATP. Analogous experiments with PNTP and NPDS gave similar results. Thus, this technique has great application prospects in the fields of surface chemistry and materials chemistry.


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Introduction Surface-plasmon-assisted catalysis can play an important role in studies of reaction mechanisms1-8. In 2009, the surface catalytic coupling reaction from PATP to DMAB was firstly confirmed by theoretical study and further proved in the experiment in 20109,10. In 2011, Sun et al. confirmed both theoretically and experimentally that DMAB is generated by the coupling of two PATP molecules with the aid of surface plasmon resonance under SERS conditions11,12. And the structure of the DMAB molecule is trans12. This has been further verified by numerous other studies-13-18. Moreover, numerous reports have been concerned with the effects of various parameters on the surface-plasmon-assisted catalytic coupling reaction, such as atmosphere19-21, pH22, substrate, and other conditions23-25. However, there have not been many studies on the effect of intermolecular distance on surface-plasmon-assisted catalysis. PATP-Ag is generated when either APDS or PATP come into contact with silver26-28. However, with PATP, the PATP-Ag moieties will be randomly and individually dispersed on the Ag surface, whereas when derived from the S-S cleavage of APDS, the PATP-Ag moieties can be expected to be bound in a pairwise manner on the Ag surface with a fixed distance. Thus, DMAB generation from APDS will be more efficient than that from PATP under SERS conditions. Based on the above principle, the effect of intermolecular distance on surfaceplasmon-assisted catalysis can be understood by comparing the efficiencies of DMAB generation from APDS and PATP. In this paper, we mainly focus on this reaction. Furthermore, the surface plasmon-assisted catalytic reactions of PNTP and NPDS are very similar to those of PATP and APDS. PNTP-Ag will be formed when either PNTP or NPDS come into contact with the surface of a silver foil, at random sites with PNTP and at adjacent sites with NPDS.


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Materials and methods PATP, APDS, PNTP, NPDS, silver foil (99.998%), and absolute ethanol were purchased from commercial suppliers. An ethanolic solution of the organic compounds was dropped onto an area of silver foil of 0.3 × 0.3 mm2. After drying at room temperature, Raman spectra were acquired using a Renishaw inVia spectrometer. A 532 nm laser was used as the excitation light source, the power was 0.5% (~0.25 mW), and the Raman spectral range was 894.33−1687.17 cm−1 (center 1300 cm−1). The ethanol solution of PATP and APDS was separately mixed with the silver sol in a volume ratio of 1:1 and shaken for 5 minutes. UV-visible absorption spectra of PATP and APDS before and after mixing were measured. Absorption spectra were recorded on an Optizen 2120UV spectrophotometer.

Results and discussion The Raman spectra of APDS (a) and PATP (b) in the mid-frequency region (1000–1700 cm1

) are shown in Figure 1. These spectra are seen to be very similar. It can be seen from the

chemical structure of PATP and APDS that if two PATP molecules lose H+, APDS will be generated by establishing an S-S bond, hence it is not surprising that their Raman peaks are almost the same ，especially between 1000−1700 cm-1. In our experimental spectrum, APDS has an S-S vibration peak at 464 cm-1, and PATP has an S-H vibration peak at 2546 cm-1, indicating that the purity of our experimental drugs can be guaranteed29. Figure 1(c) shows the Raman spectrum of DMAB, which features three distinct strong peaks at ν≈1140, 1390, and 1432 cm-1. The Raman peak at ν≈1140 cm-1 corresponds to the C-N vibrational mode, and those at ν≈1390 and 1432 cm-1 can be ascribed to N=N vibrational modes10,30.


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Intensity / a.u.

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Figure 1. Raman spectra of (a) APDS, (b) PATP, and (c) DMAB.

The experimental evidence for the plasma-assisted coupling reaction from APDS to DMAB has been provided28. These three strong peaks were once considered to be the b2 enhancement mode of PATP, but finally proved to be the characteristic Raman peaks of DMAB following PATP coupling30. These three peaks are now widely used as an indicator of surface-plasmonassisted catalysis (DMAB formation) due to their very high Raman intensity and good discrimination30-34.


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Wavelength (nm) Figure 2. UV/Vis spectra of PATP and APDS before and after contact with silver.

Figure 2 shows the UV/Vis spectra of PATP and APDS before and after contact with silver. A new absorption peak appears at 975 nm, irrespective of whether PATP or APDS is added to the silver foil. This shows that when PATP and APDS come into contact with silver, a new material is generated. Since the newly generated material has the same UV absorption peak at 975 nm, we assume it to be the same species, PATP-Ag26,27. Raman mapping spectroscopy is an ideal technique to study the distribution of matter33-39. To investigate the effect of PATP-Ag spacing on the coupling reaction with the aid of plasmids, different concentrations of PATP and APDS were subjected to Raman mapping spectroscopy testing under the same experimental conditions and the results were compared. It can be seen from Figure 1(c) that the band at 1432 cm-1 is the strongest in the Raman spectrum of DMAB. This peak corresponds to an N=N vibration and is diagnostic of the formation of DMAB. Therefore, we selected the Raman peak at 1432 cm-1 as the main target for mapping and silver foil acts as a SERS substrate. An aliquot (5 µL) of a 10-4 M ethanolic solution of APDS was


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added, dropwise, to a silver foil surface and air-dried. The Raman spectra and surface mapping image are shown in Figure 3.

Figure 3. Raman spectra and mapping image of APDS at a concentration of 10-4 M and a volume of 5 µL and in the order of black, violet, blue, cyan, green, yellow, orange, and red, the color represents the intensity of the 1432 cm-1 peak from 0 to 400. (a) Mapping image of the peak position at 1432 cm-1 obtained using Wire 4.0 software and different colors indicate the distribution of 1432 cm-1 peak intensity on the surface of silver foil. (b) Corresponding Raman spectra in the color area. (c) 3D representation of Raman spectral mapping on the


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surface of silver foil (the Y-axis(Data set) corresponds to the Raman spectral data number, the upright axis indicates the Raman intensity).

Figure 3(a) is a mapping image of the peak at 1432 cm-1 generated using Wire 4.0 software. As can be seen, the whole area of the image is dominated by red, with only a few areas of orange/yellow/green/cyan. This shows that for APDS, when the laser impinges on the surface of the silver foil, a plasmon is generated. With the aid of this plasmon, most APDS undergoes a photocatalytic coupling reaction and is converted into DMAB. Figure 3(b) shows the Raman spectra for different colors of the mapping image. Figure 3(b1) shows the Raman spectrum of the blue region, which shows considerable noise and the baseline is not very smooth. Nevertheless, Raman peaks can be discerned at 1140, 1390, and 1432 cm-1, albeit with very low intensities, with that at 1432 cm-1 having a relative intensity of < 150. Figure 3(b2) shows the Raman spectrum of the green area. Compared with that of the blue area, this spectrum is much enhanced, the noise is reduced, the baseline is smoother, and the Raman peaks at 1140, 1390, and 1432 cm-1 can be more clearly discerned. Figure 3(b3) shows the Raman spectrum of the red region, which represents the bulk of the entire region. The spectrum in Figure 3(b3) is very smooth, with no baseline noise. The Raman peaks at 1140, 1390, and 1432 cm-1, characteristic of DMAB, are very clear, and there are no impurity peaks. Figure 3(c) shows the 3D distribution of Raman spectra of all scanning points on the surface of the silver foil. Viewing this 3D model as a topographic map, we can see that its plain area is very broad and flat, and almost no extraneous peaks appear. The characteristic peaks appear as a continuous range, especially the Raman peaks at 1140, 1390, and 1432 cm-1, which are very


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clear. The red coloration indicates that the relative intensity of these peaks is much higher than 400. It is clear from Figure 3 that when the laser impinges on the surface of the silver foil, plasmon-assisted catalysis occurs, and APDS is converted into DMAB. From the surface distribution of the 1432 cm-1 peak (Figure 3(a)), it is evident that DMAB was generated on the entire surface of the silver foil, and its concentration was very high in most areas (the intensity of the Raman peak is proportional to the amount of material). From the Raman spectrum (Figure 3(b)), it can be seen that the characteristic peaks of DMAB are very prominent; especially in the red region, the DMAB concentration is very high. In Figure 3(c), the Raman spectrum distribution of DMAB can be observed more clearly, verifying the above analysis. To investigate the effect of intermolecular distance on the surface-plasmon-assisted catalysis, we performed Raman spectroscopy and mapping experiments at higher concentrations of PATP.


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Figure 4. Raman spectra and mapping image of PATP at a concentration of 10-3 M and a volume of 5 µL and in the order of black, violet, blue, cyan, green, yellow, orange, and red, the color represents the intensity of the 1432 cm-1 peak from 0 to 400. (a) Mapping image of the peak position at 1432 cm-1 obtained using Wire 4.0 software and different colors indicate the distribution of 1432 cm-1 peak intensity on the surface of silver foil. (b) Corresponding Raman spectra in the color area. (b) Corresponding Raman spectra in the color area. (c) 3D representation of the Raman spectral mapping on the surface of silver foil (the Y-axis(Data set) corresponds to the Raman spectral data number, (the upright axis indicates the Raman intensity).


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Figure 4 shows Raman spectra and the mapping image of PATP at a concentration of 10-3 M and a volume of 5 µL. Figure 4(a) shows a mapping image of the peak position at 1432 cm-1 obtained using Wire 4.0 software. The entire area is covered by dark hues, mainly black, without any prominent bright colors. Figure 4(b1) shows the Raman spectrum corresponding to the black region. It is seen to be featureless, and no obvious Raman peak can be discerned. Figure 4(b2) shows the Raman spectrum in the purple region, in which the peaks become apparent. Despite the baseline noise, weak Raman peaks are discernible at 1140, 1390, and 1432 cm-1. Figure 4(b3) shows the Raman spectrum corresponding to the blue region. The baseline noise is reduced, and the peaks at 1140, 1390, and 1432 cm-1 are somewhat enhanced, but their intensity is still not high. Figure 4(c) shows the 3D distribution of Raman spectra at all scanning points on the surface of the silver foil. The mapping image base is irregular and mainly black, there are few peaks, and the peak intensity is not high, not exceeding a relative height of 150. Analysis of Figure 4 shows that only a small amount of PATP underwent plasmon-assisted catalysis to generate DMAB when the laser impinged on the surface of the silver foil bearing 5 µL of a 10-3 M ethanolic PATP solution, even though the concentration of PATP was ten times that of APDS (five times as much PATP-Ag on the silver surface). In order to further validate our theory, we continued to increase the amount of PATP.


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Figure 5. Raman spectra and mapping image of PATP at a concentration of 10-3 M and a volume of 10 µL and in the order of black, violet, blue, cyan, green, yellow, orange, and red, the color represents the intensity of the 1432 cm-1 peak from 0 to 400. (a) Mapping image of the peak position at 1432 cm-1 obtained using Wire 4.0 software and different colors indicate the distribution of 1432 cm-1 peak intensity on the surface of silver foil. (b) Corresponding Raman spectra in the color area. (b) Corresponding Raman spectra in the color area. (c) 3D representation of Raman spectral mapping on the surface of silver foil (the Y-axis(Data set) corresponds to the Raman spectral data number, the upright axis indicates the Raman intensity).

Figure 5 shows a mapping image from Raman scans obtained after dropping 10 µL of a 10-3 M ethanolic solution of PATP onto a thin piece of silver foil and allowing it to dry naturally. Comparing Figure 5(a) with Figure 4(a), it can be seen that the black areas are reduced, the blue


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and purple areas are more extensive, and there are some regions of cyan, green, and red. The black area accounts for about 1/3 of the total image area; the blue, purple, and cyan areas are in roughly the same proportions, and the green and red areas account for only small proportions. This shows that as the concentration of PATP is increased, the Raman peak intensity at 1432 cm1

on the surface of the silver foil also increases, that is, more PATP is converted into DMAB. Figure 5(b1, 2, 3, and 4) show the Raman spectra corresponding to the black, blue, cyan,

and red regions, respectively. As can be seen, with the color change the baseline becomes smoother, the characteristic peaks become more obvious, and the peak at 1432 cm-1 slowly intensifies. Figure 5(b4) shows the Raman spectrum of the red region, which differs greatly from Figure 3(b3). The baseline in Figure 3(b3) is very smooth and the Raman spectrum is very clean. In contrast, the baseline in Figure 5(b4) shows more noise. For the sake of comparison, we applied a maximum peak strength of 400. It can be seen from the data that the highest peak intensity in Figure 5(b4) is about 500, while for the same region in Figure 3(b3), the red region spectrum, the peak intensity is close to 6,000. Thus, despite relating to the same red region, the peak intensity in Figure 3(b2) is much greater than that in Figure 5(b4). Figure 5(c) shows the 3D distribution of Raman spectra, in which the base noise is seen to be evenly distributed, the peaks are obvious, the main color is blue-green, and some individual peaks appear red. The number of peaks was increased, and the relative maximum peak height was about 400. As can be seen from the analysis of Figure 5, as the number of PATP molecules is increased, the distance between bound PATP-Ag moieties decreases, and the probability of their coupling increases, resulting in more DMAB.


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Although the production of DMAB was increased, there is still a significant drawback compared with Figure 3. To obtain Raman spectra and a mapping similar to those in Figure 3, we increased the amount of PATP to 40 times that of APDS.

Figure 6. Raman spectra and mapping image of PATP at a concentration of 10-3 M and a volume of 20 µL and in the order of black, violet, blue, cyan, green, yellow, orange, and red, the color represents the intensity of the 1432 cm-1 peak from 0 to 400. (a) Mapping image of the peak position at 1432 cm-1 obtained using Wire 4.0 software and different colors indicate the distribution of 1432 cm-1 peak intensity on the surface of silver foil. (b) Corresponding Raman spectra in the color area. (c) 3D representation of Raman spectral mapping on the surface of silver foil (the Y-axis(Data set) corresponds to the Raman spectral data number, the upright axis indicates the Raman intensity).


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Figure 6 shows the Raman spectra and the resulting mapping obtained after dropping 20 µL of a 10-3 M solution of PATP in ethanol onto a silver foil followed by natural drying. As can be seen in Figure 6(a), the red area occupies most of the image. The proportions of orange, yellow, green, and blue are similar, accounting for about half of the total area. There is only a small purple area, and no black area. This shows that as the concentration of PATP is increased, the intensity of the Raman peak at 1432 cm-1 of the surface of the silver foil also increases, and PATP is converted into DMAB at almost every position thereon. The corresponding Raman spectra of the blue, green, and red regions are shown in Figure 6(b1, 2, and 3, respectively). The Raman spectra of the blue and green regions are similar, with maximum intensities of about 150 and 300. Figure 6 (b3) shows the Raman spectrum in the red region. The peak intensity is close to 1000, which is clearly less than Figure 3(b3), and larger than Figure 5(b3). Figure 6(c) shows the 3D profile of the corresponding Raman spectrum, in which it can be seen that the substrate is evenly distributed, the peaks are sharp, and the colors include red, orange, yellowish-green, cyan, and violet, arranged in order. The peak height is uniform, with a relative maximum of about 1000. As can be seen from the analysis of Figure 6, as the number of PATP molecules is increased, the distance between bound PATP-Ag moieties further decreases, and the probability of their coupling increases significantly, with DMAB generation almost everywhere on the silver foil. Through the above experimental data analyses, we can conclude that the distance between molecules has a great impact on the surface-plasmon-assisted catalytic reaction efficiency, and the new species PATP-Ag can be formed after contact of PATP or APDS with silver foil. APDS


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is adsorbed on the silver foil surface in pairs of PATP-Ag due to cleavage of the S-S bond, and is uniformly distributed at low concentrations. Thus, although APDS was applied at a lower concentration (10-4 M), it showed a higher reaction efficiency relative to PATP due to the pairwise binding of PATP-Ag, resulting in more DMAB. This was manifested in a higher Raman peak intensity at 1432 cm-1. Thus, under SERS conditions, surface-plasmon-catalyzed coupling reactions are less likely to occur for PATP due to the longer intermolecular distances. APDS interacts with silver to produce two adjacent PATP-Ag moieties and therefore has higher coupling reaction efficiency than PATP.

Figure 7. Raman spectra and mapping images of PATP and APDS at volumes of 5 µL and concentrations of 10-4 M and 10-3 M, respectively, and in the order of black, violet, blue, cyan, green, yellow, orange, and red, the color represents the intensity of the 1432 cm-1 peak from 0 to 400. (a1) Mapping image of the PATP with peak position at 1432 cm-1 obtained using Wire 4.0 software and different colors indicate the distribution of


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1432 cm-1 peak intensity on the surface of silver foil. (b1) 3D representation of Raman spectral mapping on the surface of silver foil of PATP (the Y-axis(Data set) corresponds to the Raman spectral data number, the upright axis indicates the Raman intensity). (a2) Mapping image of the APDS with peak position at 1432 cm-1 obtained using Wire 4.0 software and different colors indicate the distribution of 1432 cm-1 peak intensity on the surface of silver foil. (b2) 3D representation of Raman spectral mapping on the surface of silver foil of APDS (the Y-axis(Data set) corresponds to the Raman spectral data number, the upright axis indicates the Raman intensity).

Figure 7(a1) shows the Raman mapping following the deposition of 5 µL of PATP solution at a concentration of 10-4 M. As can be seen in Figure 7(a1), the image is dominated by the black area, with only one blue spot at the brightest point. From Figure 7(b1), it can be seen that the Raman peak at ν=1432 cm-1 has a low maximum intensity of 150. Indeed, the spectrum obtained from the surface of the silver foil is dominated by an impurity peak. This shows that for 10-4 M PATP, SERS conditions generate only a trace of DMAB, and no surface-plasmon-assisted catalysis occurs in most of the regions measured. It is worth reiterating that in obtaining Figure 7 and Figure 3, the experimental conditions and the substrate concentrations in terms of the number of molecules were exactly the same. APDS (Figure 3) generates two adjacent PATP-Ag moieties upon contact with silver, and so its reaction efficiency is much higher than that of randomly distributed PATP-Ag obtained from dilute, discrete PATP. Comparing Figure 7(a2, b2) with Figure 3, it is evident that when the reactant concentration is increased to 10-3 M, the DMAB formed by APDS covers the entire surface of the silver foil due to the close proximity of PATP-Ag moieties after APDS cleavage. The Raman spectrum of the resulting DMAB is very clear in Figure 7(b2), and the intensity of the Raman peak is very strong. The relative intensity of the Raman peak at ν=1432 cm-1 is greater than 400. This


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experimental result further validates that the distance between the molecules plays an important role in surface-plasmon-assisted catalysis in very low-concentration systems. In subsequent experiments, we further lowered the APDS concentration to 10-5 M.

Figure 8. Raman spectra and mapping images of APDS at volumes of 5 µL and 20 µL at a concentration of 105 M and in the order of black, violet, blue, cyan, green, yellow, orange, and red, the color represents the intensity of the 1432 cm-1 peak from 0 to 400. (a1) Mapping image of the APDS at a volume of 5 µL with peak position at 1432 cm-1 obtained using Wire 4.0 software and different colors indicate the distribution of 1432 cm-1 peak intensity on the surface of silver foil. (b1) 3D representation of Raman spectral mapping of a1 on the surface of silver foil (the Y-axis(Data set) corresponds to the Raman spectral data number, the upright axis indicates the Raman intensity). (a2) Mapping image of the APDS at a volume of 20 µL with peak position at 1432 cm-1 obtained using Wire 4.0 software and different colors indicate the distribution of 1432 cm-1 peak intensity on the surface of silver foil. (b2) 3D representation of Raman spectral mapping of a2 on the surface of silver foil (the Y-axis(Data set) corresponds to the Raman spectral data number, the upright axis indicates the Raman intensity).

Figure 8(a1, a2) show mappings of the Raman intensity at 1432 cm-1 following deposition of 5 µL and 20 µL, respectively, of 10-5 M APDS in ethanol on silver foil. Figure 8(b1, b2) show


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the corresponding 3D maps of the Raman spectra, scanning all points on the surface of the silver foil. As can be seen from Figure 8, when the concentration of APDS was further reduced to 10-5 M, deposition of 5 µL of this solution onto the surface of the silver foil still generated some DMAB. However, due to the low concentration of APDS, significant DMAB was not generated in all regions. When the volume of 10-5 M APDS solution was increased to 20 µL, DMAB was seen to be generated over most of the surface of the silver foil, and the relative intensity of the Raman peak at 1432 cm-1 exceeded 400. This reiterates that APDS has a higher coupling efficiency due to its ability to split into pairs of PATP-Ag moieties on the silver foil surface. We performed further experiments to verify our hypothesis that intermolecular distance plays an important role in surface-plasmon-assisted catalysis at very low concentrations. A large number of experimental and theoretical studies have shown that under SERS conditions, catalyzed coupling reactions of PNTPs through the catalysis of electron-hole pairs generated by surface plasmons generate DMAB39-43. So, we chose PNTP and NPDS, which are very similar to PATP and APDS, and are also susceptible to surface plasmon catalysis under similar conditions with similar experimental parameters.


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DMAB from NPDS

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Raman shift /cm

Figure 9. Raman spectra of (a) NPDS, (b) PNTP, (c) DMAB from NPDS, and (d) DMAB from PNTP.

Figure 9 shows the Raman spectra of PNTP, NPDS, and DMAB generated therefrom, which are seen to be very similar. Subsequently, we performed SERS experiments on silver foil surfaces, using the same methods and parameters as above, with 10-4 M NPDS and 10-3 M PNTP in ethanol.


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Figure 10. Raman spectra and mapping images of DMAB generated from PNTP and NPDS on a silver foil surface and in the order of black, violet, blue, cyan, green, yellow, orange, and red, the color represents the intensity of the 1432 cm-1 peak from 0 to 400. (a1, b1, c1) Mapping images of DMAB generated from 5 µL, 10 µL, and 20 µL, respectively, of 10-3 M PNTP, with the peak position at 1432 cm-1 on the surface of silver foil obtained using Wire 4.0 software. (a2, b2, c2) 3D representations of Raman spectral mappings of a1, b1, and c1, respectively (the Y-axis(Data set) corresponds to the Raman spectral data number, the upright axis indicates the Raman intensity). (d1) Mapping image of DMAB generated from 0.5 µL of 10-4 M NPDS, with the peak position at 1432 cm-1 obtained using Wire 4.0 software and different colors indicate the distribution of


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1432 cm-1 peak intensity on the surface of silver foil. (d2) 3D representation of Raman spectral mapping of d1 on the surface of silver foil (the Y-axis(Data set) corresponds to the Raman spectral data number, the upright axis indicates the Raman intensity).

Figure 10 shows the mappings and spectra of DMAB generated from NPDS and PNTP assisted by surface plasmons. Figure 10(a1, a2, a3) show the 1432 cm-1 Raman peak intensity distributions when 5, 10, and 20 µL of 10-3 M ethanolic PNTP were dropped onto the surface of the silver foil. Figure 10(a4) shows the corresponding results when 0.5 µL of a 10-4 M NPDS solution was dropped onto the surface of silver foil. Figure 10(b1–b4) show the corresponding 3D distributions of the Raman spectra. As can be seen from Figure 10, similar to the results of the APDS and PATP mappings, a lower concentration of NPDS is more efficient for generating DMAB under SERS conditions due to the shorter distance between two PNTP-Ag moieties formed upon dissociative deposition. With discrete PNTP molecules, due to the greater distance between PNTP-Ag moieties, even at higher concentrations, the efficiency of DMAB generation under SERS conditions is far lower than that with NPDS. Conclusion Intermolecular distance plays an important role in surface-plasmon-assisted catalysis. Its effect can be readily understood through Raman spectroscopy and mapping, which is thus of reference value for the study of surface-plasmon-assisted coupling reaction mechanisms. Due to cleavage of the S-S bond, the PATP-Ag formed from APDS is bound to the surface of silver foil in a pairwise manner at short distances (approximately one molecular bond length). Conversely, PATP-Ag formed from PATP is bound individually at much greater distances. In our experiments, the amount of PATP-Ag produced by PATP was 20 times that produced by APDS. However, due to the influence of intermolecular distance, APDS showed higher


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efficiency in the generation of DMAB. Thus, Raman imaging and spectral quality with APDS were far superior to those with PATP. Under the same experimental conditions, corresponding experiments with NPDS and PNTP gave similar results. Therefore, under SERS conditions for surface-plasmon-assisted catalysis, the distance between bound molecules plays an important role in the efficiency of the coupling reaction, especially at low substrate concentrations. Therefore, APDS and NPDS are more suitable for surface-plasmon-assisted catalysis than PATP or PNTP, offering higher Raman imaging efficiency, more accurate imaging, and more accurate positioning. Our research may greatly expand the application value of APDS and NPDS in the fields of surface chemistry and materials chemistry, and broadens the potential scope of surfaceplasmon-assisted catalytic reactions.

Acknowledgements This work was supported by the National Natural Science Foundation of China (21671089 and 21271095), the Shenyang Natural Science Foundation of China (F16-103-4-00), the Scientific Research Fund of Liaoning Province (LT2017010, 20170540409), the Innovative Talent Support Program of Liaoning Province (Grant No. LR2017062), the Liaoning Provincial Department of Education Project (Grant No. LFW201710), the Natural Science Foundation of Liaoning Province (Grant No. 201602345). We thank International Science Editing ( http://www.internationalscienceediting.com ) for editing this manuscript.


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Table of Contents Graphic

Effect of intermolecular distance on surface-plasmon-assisted catalysis.


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Effect of Intermolecular Distance on Surface-Plasmon-Assisted

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