Letter pubs.acs.org/JPCL
b2 Peaks in SERS Spectra of 4‑Aminobenzenethiol: A Photochemical Artifact or a Real Chemical Enhancement? Han-Kyu Choi,† Hyun Kyong Shon,‡ Hyunung Yu,‡ Tae Geol Lee,*,‡ and Zee Hwan Kim*,† †
Department of Chemistry, Korea University, Seoul 136-713, Korea Center for Nano-Bio Convergence, Korea Research Institute of Standards and Science, Daejeon 305-340, Korea
‡
S Supporting Information *
ABSTRACT: Strong b2 peaks (1142, 1391, 1438, and 1583 cm−1) in the SERS spectra of 4-aminobenzenethiol (ABT) have been regarded by many as a textbook example of chemically enhanced SERS signals. However, this interpretation is in serious doubt after the recent claim that they arise from 4,4′-dimercaptoazobenzenes (DMAB) photogenerated during the acquisition of SERS, not the genuine chemically enhanced signals of ABT. Subsequent attempts to prove or disprove this claim have failed to provide any decisive verdict. Here we present spectroscopic and mass spectrometric evidence that further support the photogeneration of DMABs from ABTs on an Ag surface. Furthermore, we show that the amount of the DMAB is sufficient to explain the b2 intensities of ABT.
SECTION: Spectroscopy, Photochemistry, and Excited States
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peaks even when they replaced the NH2 group in ABTs with a N(CH2CH3)2 group, which would completely block the formation of NN bonds, and hence no b2 peaks should arise. The following four questions need to be answered to completely settle down the ongoing dispute. (1) Do the DMABs really form via the photoreaction of ABTs? To fully address this question, one needs not only the SERS but also other complementary measurements that can cross-check the production of DMABs. (2) If we accept that DMABs are generated from ABT, are they responsible for the b2 peaks? In other words, do we have a sufficient number of DMABs? (3) How do we explain the dependences of b2 intensities on pH, temperature, and the potentials of metals? (4) What is the reaction mechanism for the DMAB formation? The aim of this Letter is to answer the first two questions. We find that the SERS peaks of DMAB/Ag match not only the major b2 peaks of ABT/Ag but also other minor peaks of ABT/ Ag that were previously unassignable to any vibrational modes of ABT. We also observe that the high-frequency part of the SERS spectra (2400−3200 cm−1, containing S−H and C−H stretching modes) is also the same for the DMAB/Ag and the laser-exposed ABT/Ag samples. The infrared reflection (IR) spectroscopy and mass spectrometry (MS) on the ABT/Ag sample show an extensive depletion of ABTs on surface and the production of a variety of photoproducts, some fraction of which can be assigned to the DMAB. We find that the surface
ince the early days of surface-enhanced Raman scattering (SERS),1 4-aminobenzenethiol (ABT) has served as a standard molecule for assessing the electromagnetic (EM)2 and chemical enhancement (CE)3−5 mechanism of SERS. In particular, the SERS spectra of ABT on Ag show peaks at 1142, 1391, 1438, and 1583 cm−1 (commonly denoted as 9b, 3, 19b, and 8b peaks, respectively), intensities of which sensitively depend on the potentials of metallic surfaces.6 This, together with much other evidence, has lead researchers7−17 to conclude that the peaks are the symmetry-forbidden b2 vibrational modes of ABT that derive intensities via vibronic charge-transfer (CT) resonance.3,4,18,19 Quite recently, however, Wu et al.,20 Fang et al.,21 and Huang et al.22 claimed that these peaks are the EM-enhanced peaks of 4,4′-dimercaptoazobenzenes (DMABs), which are created by the photo-oxidative coupling of two ABTs.23 Their claim (hereafter we call the azo-hypothesis) is based on the striking similarity between the spectra of ABT and DMAB, electronic structure calculation, and irreversible changes in the mass spectra (MS) of ABT on Ag surface (ABT/Ag) upon light irradiation. Similarity of the spectra was later confirmed by a few other researchers.24−26 The azo-hypothesis, although very compelling, is still hotly debated in the SERS community:19,24−48 the similarities of a few peaks in the spectra of ABT and DMAB might be a pure coincidence; the DMAB may be generated from ABT, yet there is no solid proof that they are really responsible for the observed b2 peaks. Furthermore, the hypothesis is seemingly irreconcilable with the existing body of data on ABT. For example, Kim and coworkers30 observed nearly identical b2 © 2013 American Chemical Society
Received: February 7, 2013 Accepted: March 15, 2013 Published: March 15, 2013 1079
dx.doi.org/10.1021/jz4002828 | J. Phys. Chem. Lett. 2013, 4, 1079−1086
The Journal of Physical Chemistry Letters
Letter
Figure 1. (a) Local photoactivation and SERS mapping of the ABT/Ag. (b) SERS spectra averaged over the exposed (red) and pristine (blue) regions of the same sample shown in inset images. The triangles in panel b point to the previously unassigned peaks of ABT (see the main text). The inset images in panel b are the SERS images of activated ABT/Ag monitoring 7a and 9b peaks of SERS spectra. The area inside the dashed square is the exposed region, whereas the outer area is the pristine region. (c) SERS spectrum of a monolayer of pure DMABs on Ag. In panels b and c, parts of spectra (thin solid lines) are enlarged by a factor of five to better show the details of spectra. (d) Time-trace (red) of 19b-peak of ABT/Ag and the fit to a rise-and-decay kinetics model (black, time constants for the rise (τrise) and decay (τdecay) are also shown). (e) 19b-intensity of DMAB/Ag and a fit to single exponential decay. (d) Similar to panel a but showing the influence of temporarily blocking (laser off) and unlocking (laser on) the laser on the time-evolution of the b2-peak.
time-resolved SERS measurement on a fixed laser spot (Figure 1d), we also find that the b2 intensities rapidly increase and saturate in
