Laser Power Threshold of Chemical Transformation on Highly Uniform

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Laser Power Threshold of Chemical Transformation on Highly Uniform Plasmonic and Catalytic Nanosurface Nobuyuki Takeyasu,*,† Ryusuke Kagawa,† Kohei Sakata,† and Takashi Kaneta† †

Graduate School of Natural Science and Technology, Okayama University, 3-1-1, Tsushima-naka, Kita-ku, Okayama 700-8530, Japan ABSTRACT: Noble metallic nanosurface exhibits both plasmonic and catalytic functions. Surface-enhanced Raman scattering of para-aminothiophenol (p-ATP) was measured on a highly uniform two-dimensional silver nanoparticle array at different intensities of an excitation laser (532 nm) ranging from 4 to 4000 W/mm2. It was observed that p-ATP was chemically transformed to 4,4′-dimercaptoazobenzene (DMAB) with the laser intensities of ≥40 W/mm2 during the Raman measurement. At 4 W/mm2, the Raman peaks of DMAB disappeared, which indicates that the laser intensity was insufficient for the chemical transformation although it was sufficient for the Raman measurement. The highly uniform silver nanoparticle array allowed quantitative analysis on the Raman peak intensity. The threshold of the chemical transformation from p-ATP to DMAB was estimated to be ∼18.8 W/mm2 on the silver nanoparticle array whose enhancement factor is ∼104.

1. INTRODUCTION Plasmonics has been intensively studied, which has revealed fundamentals on interaction between metals and light in nanoscales and which has enabled the development of new applications.1,2 Metallic nanostructures, gold and silver in most cases, allow light to confine at metallic nanosurface through surface plasmons. The confinement leads to large enhancement of electromagnetic (EM) field, and then, it is applicable to sophisticated functional optics, devices, and chemical/biomolecule sensing with spectroscopic methods.3−5 Surfaceenhanced Raman scattering/spectroscopy (SERS) is a powerful spectroscopic method that enables a highly sensitive molecular sensing technique benefiting from the enhanced EM fields. It was reported that anomalously intense Raman signals from pyridine were observed at the surface of silver electrodes,6 and the enhancement factor was reported to be ∼106.7,8 SERS research has been boosted with the remarkable development of nanofabrication and characterization techniques, leading to the sensitivity sufficient for the detection of ultratrace levels of target molecules down to a single molecule.9−12 One of the popular molecules used for SERS measurement is para-aminothiophenol (p-ATP).13−21 Thiol group (−SH) in pATP binds directly to the surface of gold/silver nanostructures, which results in accurate evaluation of the enhancement factor (EF) for a variety of geometries of the nanostructures. However, it has been observed that the SERS spectrum of pATP is different from one with the normal Raman measurement, where three peaks were additionally observed at 1142, 1389, and 1435 cm−1 only in the SERS measurement. The additionally observed peaks are a1g bands of 4,4′-dimercaptoazobenzene (DMAB) that is transformed chemically from pATP during the SERS measurement.22−25 Huang et al. have measured SERS spectrum of p-ATP under different chemical potentials, which revealed that the variety of the spectrum is due to transformation from p-ATP to DMAB.22 Sun et al. © XXXX American Chemical Society

showed that pH of the solution is also related to the transformation where lower pH avoids the transformation to DMAB.23 These reports have shown that such chemical transformation is induced by the laser illumination during SERS measurement. The environmental effects, such as pH, temperature, and electrochemical potential, on the SERS spectrum of p-ATP have been studied by Kim et al. for investigation of the Raman peaks observed only in SERS measurements although the origin of those peaks can be explained as b2 modes of p-ATP.26−29 Gold and silver work also as catalytically active materials and, then, promote chemical reactions.30−33 Reducing the size of the catalytic materials down to nanoparticles remarkably enhances the efficacy as catalysts.34 Therefore, gold/silver nanostructures inherently possess both plasmonic and catalytic functions. Recently, the reaction mechanisms of p-ATP have been intensively studied on noble metallic nanostructures.35−38 It is reported that oxygen molecules35,36 and nitrite38 are activated by surface plasmon, which induces the chemical transformation of p-ATP. Liu et al. observed SERS spectrum of p-ATP with randomly dispersed gold nanoparticles on a substrate under the different excitation power conditions.18 Raman peaks of DMAB were observed more than 0.08 W, or 20 W/mm2, at 633 nm of the excitation. It is known that silver exhibits more enhancement of EM field because the optical loss is lowest among the noble metals in the visible region, which may lead to the lower intensity threshold for the chemical transformation. In this article, we report investigation of Raman spectra of p-ATP on a silver nanosurface under illumination of a 532 nm laser at different laser intensities. We monitored the Raman peaks from p-ATP in ethanol solution through SERS measurement on the Received: February 21, 2016 Revised: May 13, 2016

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DOI: 10.1021/acs.jpcc.6b01756 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Extinction spectra of (a) AgNP solution and (b) 2D AgNP array whose plasmon resonance was tuned to Raman measurement with 532 nm of excitation laser. Inset shows SEM images of 2D AgNP array. The scale bar indicates 200 nm (50 nm for magnified image).

Figure 2. Raman measurement of p-ATP in ethanol solution. (a) Illustration of Raman measurement for p-ATP solution. (b) Raman spectra of 1 mM p-ATP with (red) and 1 M p-ATP without (blue) AgNP array. The excitation laser powers were 6 and 0.7 mW for with and without AgNP array, respectively.

emulsion, and then the change of the color of the solution was observed. A glass slide was immersed in the solution, and a proper amount of n-hexane was added to cover the water surface, which provided an oil/water interface. After the color of the aqueous phase disappeared, a 2D AgNP array structure was formed at the o/w interface. The 2D AgNP array was transferred to a glass substrate, and it was used as a SERS substrate. The SERS substrate was washed with deionized water and ethanol alternatively and was dried before SERS measurement. p-ATP in ethanol solution (1 mM) was prepared and measured with a microscopic Raman spectrometer (JASCO, NRS-5100). The excitation laser with the wavelength of 532 nm was focused onto the surface of the AgNP substrate with the objective lens (20×, NA = 0.45). The Raman scattering from the sample was collected also by the same objective lens. The exposure time was 10 s. The power of the excitation laser is variable from 6 μW to 6 mW by changing the optical density filter (O.D. = 0, 0.3, 0.6, 1, 1.3, 2, and 3).

two-dimensional silver nanoparticle (2D AgNP) array structure whose plasmon resonance has been tuned to Raman measurement with a 532 nm excitation. SERS spectrum of p-ATP was measured with the laser intensity ranging from 4 to 4000 W/ mm2. Drastic change of SERS spectrum was observed in the range of the laser intensity because of the chemical transformation from p-ATP to DMAB. Furthermore, the Raman spectra were quantitatively analyzed to estimate the threshold laser intensity for the chemical transformation on the 2D AgNP array structure.

2. EXPERIMENTAL SECTION n-Dodecylamine (C12H25NH2) was purchased from Tokyo Chemical Industry. n-Hexane was purchased from Kanto Chemical. Silver nitrate (AgNO3), sodium tetrachloroaurate(III) dihydrate (NaAuCl4·2H2O), trisodium citrate dihydrate (C6H5Na3O7·2H2O), sodium tetrahydroborate (NaBH4), and toluene were purchased from Wako Pure Chemical Industries. Deionized water was used in all experiments. All glass slides were cleaned by Piranha solution (concentrated H2SO4:30% H2O2 = 4:1) and were rinsed with deionized water. A 2D AgNP array structure was prepared by self-assembly of AgNPs with oil-in-water (o/w) emulsion (unpublished results). Citratecapped AgNPs were synthesized by a method in ref 39. We prepared 20 mM of n-dodecylamine in n-hexane solution (solution A). Solution A (0.05 mL) and water (3 mL) were mixed and emulsified with ultrasonic treatment for 1 min. One milliliter of the AgNPs solution (solution B) was added to the

3. RESULTS AND DISCUSSION Figure 1a shows an extinction spectrum of AgNP aqueous solution. The extinction peak is observed at 406 nm. The peak position depends on the particle size of the AgNPs. The inset of Figure 1b shows an image of the AgNP array, which was used as a SERS substrate, observed with a scanning electron microscope (SEM). It was observed that the size of AgNP was 40 ± 10 nm, which is consistent with the extinction peak of AgNP.40 B

DOI: 10.1021/acs.jpcc.6b01756 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 3. SERS spectra of p-ATP with different excitation laser intensities, 4 (purple), 40 (green), 400 (blue), and 4 kW/mm2 (red). The spectra were measured with (a) ascending and (b) descending laser intensities.

The AgNP array was more than several square centimeters and was uniform over the whole structure. Figure 1b shows the extinction spectrum of the AgNP substrate. The large red-shift and broadening of the extinction peak are observed, which indicate that the interparticle gap of the AgNPs is close enough for the interference between localized surface plasmon resonances. From the extinction spectrum, the AgNP substrate is applicable to the SERS measurement with 532 nm of the excitation laser. Figure 2a shows an illustration of Raman measurement of pATP in ethanol solution. We measured Raman spectra of pATP with and without AgNP array. We prepared 1 mM and 1 M of p-ATP in ethanol solution, and it was sandwiched by a cover glass and a glass (or AgNP array) substrate with a spacer. The Raman spectra of p-ATP were measured with and without the AgNP array, which are shown in Figure 2b. Altogether, 10 Raman peaks were clearly observed at 922, 1008, 1080, 1142, 1191, 1305, 1389, 1435, 1470, and 1574 cm−1 with the AgNP array, and the spectrum is different from one without the AgNP array. Among them, the strong three peaks observed at 1142, 1389, and 1435 cm−1 represent the existence of DMAB transformed from p-ATP during the SERS measurement.22,24,41 We estimate the enhancement factor (EF) in our SERS measurement. Figure 2b shows the Raman spectrum of 1 M of p-ATP in ethanol solution measured with normal Raman spectroscopy at 0.7 mW of the excitation laser power. The peak of νCS was observed at 1090 cm−1, although it was at 1080 cm−1 with AgNP substrate. The peak shift may be due to the effect of the adsorption of p-ATP onto the silver surface. We calculate EF from the comparison between these two peaks shown in Figure 2b. EF is obtained from the following equation:

EF =

ISERS/Nsuf INRS/Nvol

(1)

where ISERS is SERS signal intensity, Nsuf is the number of molecules at the surface for SERS, INRS is normal Raman intensity, and Nvol is the number of molecules in the focusing spot. ISERS/INRS is 22.5 ± 3.2. The volume of the focusing spot is calculated to be 4.98 μm3 from Rayleigh length (3.06 μm) and the beam waist (1.44 μm). Nvol is estimated to be 4.98 × 10−15 mol. In the estimation of Nsuf, we refer that the number of adsorbing p-ATP molecules on the silver surface is 5/nm2,41,42 since the concentration of 1 mM used in the experiment is sufficient to cover the entire surface of the silver structures with the molecules. Nsuf is, at most, estimated to be 5 × 1018 /m2 × π(0.72 × 10−6 m)2 = 8.1 × 106 = 1.3 × 10−17 mol. Then, EF is estimated to be 1.0 ± 0.14 × 104 from eq 1. The value of the EF is lower compared to other reports, which is due to the overestimation of the number of molecules at the silver surface. The EF in our previous report was ∼105 with toluene sample where 20 nm silver nanoparticles were used for 2D AgNP array (unpublished results). The illumination of the excitation laser induced the chemical transformation from p-ATP to DMAB on the AgNP array at 6 mW during the Raman measurements. The total amount of DMAB should depend on the amount of the exposed photons, which is (the laser intensity) × (the exposure time). We measured SERS spectra of the same sample on the AgNP array at different excitation laser powers for quantitative discussion on the production of DMAB. A series of SERS measurements were performed at the same position on the AgNP array with the laser power of 6 μW, 0.06, 0.6, and 6 mW (4, 40, 400, and 4000 W/mm2) where the laser powers were increased from 6 C

DOI: 10.1021/acs.jpcc.6b01756 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 4. Lattice-mapping measurement (10 × 10) of Raman spectra of p-ATP on 2D AgNP array. SERS measurements were performed over 0.5 × 0.5 mm at regular intervals of 50 μm. The horizontal axis of SERS spectra is the Raman shift from 1000 to 1500 cm−1. The excitation laser power was 0.6 mW.

μW to 6 mW. The SERS spectra measured with ascending laser power are shown in Figure 3a. The SERS spectrum of p-ATP with the laser intensity of 4 W/mm2 was different from that measured with 4 kW/mm2. The peaks of DMAB disappeared, and two strong peaks were observed at 1080 and 1595 cm−1. The peak observed at 1080 cm−1 is assigned to C−S stretching (νCS). Additionally, weak peaks were observed at 1180 and 1490 cm−1. The SERS spectrum is similar to the normal Raman spectrum of p-ATP, as shown in Figure 2b. The Raman signals of p-ATP are certainly enhanced at 4 W/mm2, or 6 μW. The enhanced Raman signals indicate that p-ATP molecules are adsorbed at the silver surface. However, the Raman peaks of DMAB were not observed in our experimental condition. The chemical reaction is sensitive to the environmental conditions. The laser intensity was insufficient for the transformation but sufficient for SERS measurement in our experimental condition although Raman signals were deteriorated. Raman peaks of DMAB newly appeared when the laser intensity increased 10 times, which is 40 W/mm2. The weak peaks at 1191, 1364, and 1574 cm−1 were observed as shoulders besides 1180, 1386, and 1595 cm−1, respectively. These shoulders also grew similarly to the other newly generated peaks when the laser intensity was further increased. The peak intensities at 1142, 1386, and 1426 cm−1 become almost comparable to the peaks at 1080 cm−1 at 400 W/mm2. When the laser intensity was 4 kW/mm2, the three peaks became larger than the peak at 1080 cm−1. The peaks at 1180, 1490, and 1595 cm−1, which had been observed with 4 W/mm2,

disappeared. The appearing/disappearing of the Raman peaks during the measurement indicates the transformation from pATP to DMAB. From the measurements, the peaks at 1080, 1180, 1490, and 1595 cm−1 are originated from p-ATP, and the other peaks are from DMAB. Similarly, a series of SERS measurements were performed at another position on the AgNP array with descending laser powers from 6 mW to 6 μW. The results are shown in Figure 3b. The spectrum with 6 mW was similar to the previously observed SERS spectrum with the same laser power even at the different measurement position. The reproducibility of the SERS spectrum is sufficiently high, which means that the fabricated AgNP array is highly uniform. No change was found on the shape of the four SERS spectra although the all peak intensities decreased according to the excitation laser powers. The result indicates that the spectral shape is not a function of the laser power but of the irreversible chemical transformation from p-ATP to DMAB. It was observed that the intensities of Raman peaks at 1080, 1140, 1387, and 1430 cm−1 were almost comparable at 0.6 mW or 400 W/mm2, although they originated from p-ATP and DMAB. For quantitative discussion, we examined the reproducibility of these Raman signal intensities over a wide area of the 2D AgNP array. We performed lattice-mapping measurement of SERS spectra of p-ATP at 10 × 10 positions on the AgNP substrate over 0.5 × 0.5 mm2, which is shown in Figure 4. The horizontal and vertical axes are the Raman shift ranging from 1000 to 1500 cm−1 and the Raman intensity in D

DOI: 10.1021/acs.jpcc.6b01756 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 5. Relationships between laser power and signal intensity with 0.06 (○), 0.3 (□), 0.6 (*), 1.5 (×), and 3 mW (+) observed at (a) 1142 and (b) 1426 cm−1. The intensities I1142 and I1426 are normalized by the peak intensity at 1080 cm−1, I1080.

threshold of the chemical transformation was achieved with the activation of the oxygen by the enhanced EM field,35,36 the inherent catalytic property of silver, and plasmonic heating. The AgNP array promotes the chemical transformation where the AgNP substrate works not only as plasmonic surface but also as catalytic surface.

arbitrary units, respectively. Circles marked in the optical microscope image indicate the measurement positions on the surface of the AgNP array. It was difficult to identify Raman signals in 10 spectra among them, which was maybe because of lack of silver nanostructures. However, the Raman peaks were observed in most of the measurement points, and the patterns of the observed SERS spectra were similar. The result allows further quantitative discussion on the amount of DMAB produced by the excitation laser with the intensity of Raman peaks. DMAB is identified by the Raman peaks observed at 1142, 1386, and 1426 cm−1, and these peaks grow as the laser power increases (Figure 3a). It was difficult to find the peaks at 6 μW because the peak intensities are less than the noise level. We were concerned that the Raman signals might be saturated at 6 mW. The Raman spectrum was measured at 0.06, 0.3, 0.6, 1.5, and 3 mW similarly to the ascending case in Figure 3a, and the relationship between the peak intensity and the laser power was analyzed, regarding the two peaks at 1142 and 1426 cm−1. The peak intensities were normalized by the peak intensity at 1080 cm−1 (I1080), which is assigned to νCS, because I1080 appears to have minimum intensity change by the transformation among all Raman peaks. Figure 5 shows the relationship between the Raman intensity ratios (I1142/I1080 and I1426 /I1080) and the laser power. Both ratios increase as the excitation laser power increases. It was found that the ratio was linearly proportional to the logarithm of the laser power although the slopes were different from each other. The slope is reflected by the Raman cross section of each mode. The fitting lines of I1142/I1080 and I1426/I1080 are across the horizontal axes at 28.4 and 27.5 μW, respectively. The values represent the threshold of the laser power for the chemical transformation from p-ATP to DMAB with the exposure time of 10 s. The values should be the same but sufficiently close. The averaged value is considered to be the threshold, which is ∼28.0 μW, or ∼18.8 W/mm2. In Figure 3a, the peaks of DMAB were observed at more than 60 μW, which is consistent with these results. The threshold is close to ref 18. It is also reported that 2.0 × 102 mW/cm2 is sufficient for the transformation on silver surface with 633 nm.38 The threshold is lower than our result, and the difference may be due to the environmental condition. In the normal Raman spectrum of p-ATP (Figure 2b), A1g modes of DMAB were not observed although the concentration was higher by a thousand times and the excitation laser power was 0.7 mW, or 467 W/mm2, which is larger than the threshold with AgNP array by more than 25 times. From these results, it is obvious that the threshold of the chemical transformation falls down on the AgNP array. The lower

4. CONCLUSION SERS measurements of p-ATP were performed on the highly uniform AgNP substrate at the different excitation laser intensities. No Raman peak of DMAB was observed at 4 W/ mm2 with 10 s, which was similar to the Raman spectrum of pATP measured with the normal Raman spectroscopy. The result indicates that the laser intensity is sufficient for Raman measurement but insufficient for chemical reaction from p-ATP to DMAB. The SERS peaks of DMAB were observed at 40, 400, and 4000 W/mm2. The comparison of Raman peak intensities of p-ATP and DMAB at the same laser intensity indicated high reproducibility, which means that our SERS substrate was sufficiently reliable for quantitative analysis. The peak intensities were normalized by the peak intensity at 1080 cm−1, and it was found that the threshold of the chemical reaction was roughly 28.0 μW, or 18.8 W/mm2, in the exposure time of 10 s. The EF was estimated to be 1.0 ± 0.14 × 104. No chemical transformation was observed without an AgNP substrate even with 0.7 mW of the excitation laser, which is larger than the threshold of the transformation. The AgNP substrate under illumination of light exhibits promotion of the chemical reaction, where the AgNP substrate works as both plasmonic and catalytic surface.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-(0)86-2517845. Author Contributions

The manuscript was written by N. T. The experiment was performed by R. K. and K. S. All authors discussed the experimental results. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by The Yakumo Foundation for Environmental Science. The authors gratefully thank Prof. J. Kano for maintaining the Raman microscope system. The E

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authors gratefully thank Division of Instrumental Analysis, Department of Instrumental Analysis & Cryogenics, Advanced Science Research Center, Okayama University, for the SEM measurements.

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DOI: 10.1021/acs.jpcc.6b01756 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.6b01756 J. Phys. Chem. C XXXX, XXX, XXX−XXX