Charge-Transfer-Induced Enantiomer Selective Discrimination of

Article pubs.acs.org/JPCC

Charge-Transfer-Induced Enantiomer Selective Discrimination of Chiral Alcohols by SERS Yue Wang,† Zhi Yu,‡ Xiaoxia Han,† Hongyang Su,† Wei Ji,§ Qian Cong,‡ Bing Zhao,*,† and Yukihiro Ozaki*,§ †

State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, P. R. China Key Laboratory of Bionic Engineering, Ministry of Education, Jilin University, Changchun 130022, P. R. China § Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo 669-1337, Japan ‡

S Supporting Information *

ABSTRACT: In our previous study, we proposed a label-free enantioselective discrimination methodology for chiral alcohols based on surface-enhanced Raman scattering (SERS) spectroscopy. This method does not use either chiral reagents or circularly polarized light (chiral light). In the present study, a series of SERS experiments were carried out with an achiral SERS probe molecule, p-aminobenzenethiol (PATP), as the chiral selector molecule to develop this method further and to explore the possible mechanism of enantioselective discrimination. The relative intensities of the nontotally symmetric (b2 mode) bands in the SERS spectra of PATP change depending on the presence of different enantiomeric environments, which is a manifestation of charge transfer (CT) processes. From laser excitation wavelength-dependent and concentration-dependent SERS experiments, we have further verified that this CT-induced chiral discrimination magnifies the differences between two chiral alcohol enantiomers interacting with PATP by SERS technology. We propose that the directions of CT from Ag nanoparticles to PATP molecules are different in different enantiomeric conditions, which results in differences in the CT transitions and significantly different SERS spectra. Therefore, this work allows discrimination between two enantiomers and breaks the traditional notion that chiral discrimination requires other chiral entities as chiral selectors or the involvement of chiral light in the system. We envision that this approach will be of great significance in the field of chiral separation and chiral catalysis.

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INTRODUCTION Chiral discrimination is an intriguing fundamental characteristic of living organisms as well as in various matters of chemical interest such as catalysis and synthesis. This biological feature has drawn intense attention in the fields of biochemistry and pharmaceuticals because of the inherent chirality of biological systems.1,2 Over past decades, a variety of techniques have been used to study enantiomeric discrimination, such as capillary electrophoresis (CE),3 nuclear magnetic resonance (NMR) spectroscopy,4 fluorescence spectroscopy,5 and vibrational optical activity (VOA).6,7 Most of these approaches require either the synthesis of a specialized chiral entity as a chiral selector8−10 or the use of circularly polarized light (chiral light).11,12 Despite much progress being achieved in the field of chiral discrimination, a more convenient and efficient strategy to differentiate between the enantiomers of chiral molecules is highly desirable. In particular, an approach for enantiomeric discrimination exploiting achiral selectors is necessary. In such a process, the complicated synthetic steps and time-consuming fabrication of chiral selectors can be eliminated; furthermore, the use of complicated chiral separation apparatus would be avoided. © XXXX American Chemical Society

The conventional and accepted method of chiral recognition is the measurement of Raman optical activity (ROA). In this method, circularly polarized light is used to yield the requisite information about the chirality of the system under study.13,14 Nevertheless, an unavoidable problem with ROA is that the signal intensity is weak. In fact, it is 3−5 orders of magnitude smaller than that of ordinary Raman scattering.13−15 To overcome the problem of weak ROA signal intensity, one must spend abundant time to acquire a distinct signal. Moreover, some research groups even combined surfaceenhanced Raman scattering (SERS) with ROA to enlarge the spectral difference.15,16 SERS spectroscopy retains the rich chemical and structural information afforded by Raman spectroscopy,17,18 which provides direct information about adsorbed molecules and their surrounding interactions. Due to the tremendous Raman signal enhancement derived from the electromagnetic (EM)19−21 and charge transfer (CT) mechanisms,22−25 SERS allows single-molecule detection sensitivity Received: November 5, 2016 Revised: December 5, 2016 Published: December 5, 2016 A

DOI: 10.1021/acs.jpcc.6b11128 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C and molecular specificity.17,26 It has emerged as a powerful technique for exploration of the structure, orientation, and adsorption of molecules and, furthermore, for quantitative and qualitative analysis of the trace molecules.18,27−32 The EM effect is long-range, arising from the localized surface plasmon resonance of the metal substrate, and is the dominant contributor to a SERS signal.19,20 On the other hand, CT enhancement is a short-range effect and a resonance-like process. It is dependent on the energy differences between the Fermi level of the metal substrate and the molecular orbitals of the adsorbate. Considering the tremendous signal enhancement due to the simultaneous contribution of the two mechanisms, SERS spectroscopy is extremely sensitive to slight changes in molecular structure, which may originate from changes in the molecular environment, such as temperature, pH, laser energy, and intermolecular interactions.33−35 Modulation of the minimum interfacial free energy by the adsorbed probe molecule in different environments results in changes in molecular electronic structure36 and, consequently, changes in the SERS spectrum in surface and interface systems. Therefore, differences in the SERS spectrum are expected for a SERS probe molecule placed in the different chiral environments. If the enantiomers of a chiral molecule interact with an achiral adsorbate molecule, corresponding changes in the SERS signal of the adsorbate should be seen, even if the changes are only subtle. In this case, chiral discrimination is achieved by SERS alone, with no chiral reagents or circularly polarized light used. In a previous paper, we reported a label-free chiral discrimination method for the distinction of some chiral alcohols.37 This conceptual approach challenged the traditional notion that chiral discrimination requires other chiral entities, as chiral selectors, or chiral light to be included in the system. In the present study, we have explored the mechanism involved in this label-free chiral discrimination using SERS, and a CT-induced enantioselective discrimination mechanism was proposed. p-Aminobenzenethiol (PATP), one of the most popular molecules in the study of the CT mechanism,38−41 was selected as the SERS probe molecule. A propensity rule can be empirically stated where the selective enhancement of the nontotally symmetric (b2) modes in SERS spectra of molecules with C2v symmetry, like PATP, can be considered to be an indicator for the participation of a CT process.38,43 The experimental evidence presented here, from the excitation wavelength-dependent and concentration-dependent SERS experiments, allowed interpretation of the origin of the labelfree enantiomeric discrimination, and we found that CT transitions play a key role in the discrimination process. This implies that there are significant differences between the interactions of R and S enantiomers with the probe molecule, resulting in differences in orientation, which affects the CT process. A DFT theoretical calculation was conducted to simulate the structure of this chiral discrimination system. It is manifested that the strict mirror symmetry between the two optimized structures of enantiomeric complexes constituted of chiral molecules and PATP decreases compared to that of a single pair of enantiomers. The calculation results support our supposition that differences existing between the structures of the complexes in two chiral environments are the origin of the different SERS spectral patterns. However, at the present stage of research in this area, it is difficult to completely clarify the origin of this label-free discrimination phenomenon. Our work has verified a general and simple approach for enantiomeric

discrimination that does not require a chiral entity. Importantly, this label-free method using SERS can tremendously enhance the small differences between two enantiomers observed in the traditional ROA technique, and achieve their discrimination. This study will, therefore, be of great significance due to both the exploration of a new application field for SERS and the great potential for label-free chiral recognition.

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EXPERIMENTAL SECTION Materials. p-Aminothiophenol (PATP, 97%), 1,1,1-trifluoro-2-propanol (TFIP, 97%), (R)-1,1,1-trifluoro-2-propanol (R-TFIP, 97%), 2-butanol (99.5%), (S)-2-butanol (99%), and (R)-2-butanol (99%) were purchased from Sigma-Aldrich Co. Ltd. and used without further purification. All other chemicals were of analytical grade and obtained from Beijing Chemical Reagent Factory. Distilled and deionized water from a Milli-Qplus system with a resistivity greater than 18 MΩ·cm was used throughout the study. Impurity checks were carried out for all chiral reagents and confirmed that the impurities do not affect the SERS experimental results. (For details, see Figure S1 in the Supporting Information.) Preparation of Ag Nanoparticles Substrate. Ag nanocolloids were synthesized by the method of Lee and Meisel to prepare Ag nanoparticles (NPs) substrates.44 Briefly, for a 100 mL solution, 18 mg of silver nitrate was added and brought to the boiling point. A 1% sodium citrate solution of 2 mL volume was used as the reductant. The solution was kept at 85 °C for 45 min. This produced a greenish yellow sol that contained Ag NPs with an average diameter of ∼70 nm. The substrates were prepared via a self-assembly method. The self-assembly was performed as follows. A positively charged, polyelectrostatic reagent (poly(diallyldimethylammonium chloride)) was adhered onto a hydroxylated glass slide surface to enable adsorption of the negatively charged Ag NPs. A layer of Ag NPs was then assembled on the glass surface by electrostatic interaction, completing the substrates formation. PATP molecules were then adsorbed onto the assembled Ag NPs through the formation of Ag−S bonds. After exhaustive rinsing with ethanol and drying with nitrogen gas flow, the substrates with adsorbed PATP were immersed in the same volume of chiral molecules in both their racemic and enantiomerically pure forms for in-solution SERS measurements. SERS Experiments. A Horiba LabRAM HR-800 Raman spectrophotometer equipped with a 514 nm Ar ion laser, and a Horiba-Jobin-Yvon LabRAM ARAMIS spectrometer equipped with a 633 nm He−Ne laser and a 785 nm exciting laser were used for measurement of all the Raman and SERS spectra. The efficient laser powers reaching the samples for these three lasers were 0.01 mW (514 nm), 3 mW (633 nm), and 15 mW (785 nm). The laser was focused on the surface of the sample through a 50× magnification long-distance objective lens with a 1 μm diameter spot size. SERS spectra were obtained with an acquisition time of 5 s using a holographic grating of 1800 grooves/mm on an interchangeable magnetic mount. The Raman band of a silicon wafer at 520.7 cm−1 was used to calibrate the spectrometer. Ultraviolet Photoelectron Spectroscopy (UPS) Experiments. Ultraviolet photoelectron spectroscopy (UPS) measurements were taken using a VG Scienta R3000 spectrometer with a He I (21.21 eV) radiation source (Scienta VUV5kpackage) under ultrahigh vacuum (10−11 mbar). B


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The Journal of Physical Chemistry C DFT Computational Details. In this study, a direct theoretical evidence of differences in the structures of the chiral discrimination complexes constituted of PATP molecules and TFIP enantiomers is provided. Theoretical calculation was obtained by the DFT method of m06-2X functional,45,46 which is a high-nonlocality functional with double the amount of nonlocal exchange. The triple split valence basis set of 6-31++G (d, p) was adapted with the dispersion correction D3. All calculations were carried out with the aid of the Gaussian 09 program.47

PATP SERS signal intensity. However, no evident change was observed in the SERS spectrum of the complex when immersed in R-TFIP. This appearing nontotally symmetric vibrations, which are forbidden in nonresonance conventional Raman spectra, is expected to explain the electronic interactions between molecular π-electron systems and metal surfaces.42 It indicated that a CT resonance contribution is involved in this chiral discrimination system. These significant changes in the SERS spectrum of PATP result from differences in the orientation of the two enantiomers with respect to the Ag-PATP complex; in particular, the hydrogen-bonding interaction of the hydroxyl group of TFIP with the amine group of PATP plays a key role in determining the orientation of the two enantiomeric complexes.37 When the Ag-PATP complex is placed in two different chiral environments (S and R enantiomers), the two chiral enantiomers have different effects on PATP, generating different CT energy states of the Ag-PATP complex. These different CT energy states can induce different CT processes between the adsorbed PATP and the Ag substrate, thus allowing chiral discrimination, as suggested in our previous work. In the current study, this chiral discrimination originates from the intrinsic difference between the two chiral enantiomers, and the CT contribution enlarges this difference in the SERS spectra. Another chiral alcohol, 2-butanol, was introduced into the Ag-PATP system instead of TFIP to demonstrate the general nature of the effect chiral surroundings have on the CT transition between the Ag NPs and PATP. As observed for TFIP, similar changes were expected in the intensities of the SERS spectra of the Ag-PATP complex soaked in 2-butanol and its enantiomers. For comparison, all SERS spectra of PATP were normalized to the intensity of the a1 mode at 1075 cm−1, assigned to the ν(C−S) mode, since it is one of the most intense SERS bands of PATP and one of the least susceptible to the surroundings of the molecule (Figure S2A). The spectra of the Ag-PATP complex immersed in (S)-2-butanol showed a decreased intensity in the b2 modes at 1139, 1398, 1442, and 1576 cm−1 compared to that immersed in the R enantiomer. In the case of the racemic mixture of 2-butanol, a change in the relative intensity was also observed due to the presence of S enantiomers. It is accepted that the selective enhancement of b2 modes in the SERS spectra of molecules with C2v symmetry can be used empirically as a propensity rule to recognize the participation of a CT process contributed by the Herzberg−

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RESULTS & DISCUSSION Figure 1 shows the fabrication process for the chiral discrimination system reported in this study. The Ag-PATP

Figure 1. Schematic of the fabrication process of the chiral discrimination system.

complex was fabricated on a glass substrate by the method described in the Supporting Information. The 633 nm excited SERS spectrum of the Ag-PATP complex (Figure 2A) shows the intense SERS features of PATP, dominated by the a1-type bands at 1075 and 1588 cm−1 and the b2-type bands at 1139, 1398, 1442, and 1576 cm−1. The band assignments of the SERS spectrum of PATP are summarized in Table S1, as reported previously.39,41 Upon exposure of the Ag-PATP complex to TFIP and R-TFIP, marked changes in both the spectral shape and intensity of the SERS signals were observed between the SERS spectra of PATP+R-TFIP and PATP+TFIP (Figure 2B). Notably, with the complex immersed in TFIP, the intensity of the b2-type bands at 1139, 1398, 1442, and 1576 cm−1 decreased dramatically, along with an overall decrease in

Figure 2. SERS spectra of (A) the Ag-PATP complex and (B) the Ag-PATP complex upon exposure to racemic TFIP and R-TFIP, respectively, with a 633 nm laser excitation. C


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Figure 3. (A) Normalized SERS spectra and (B) the degree of CT (ρCT) of the Ag-PATP complex after immersion in racemic TFIP and R-TFIP with excitation wavelengths of 514 (a, d), 633 (b, e), and 785 nm (c, f). All the SERS spectra were normalized to the band at 1075 cm−1.

Teller effect.23,38,48 In this case, the decrease in relative intensities indicates that an unfavorable CT state exists in the system containing S enantiomers, thus inhibiting the influence of the Herzberg−Teller contribution. We calculated the degree of CT (ρCT), as proposed by Lombardi et al.,23,49 to quantitatively estimate the influence of the chiral environment on the CT transition in the Ag-PATP complex. Figure S2B shows that the selective enhancement of the b2 modes decreased with the increase in the content of S enantiomer, further indicating that the S enantiomer selectively inhibits CT processes in the Ag-PATP complex. It can be inferred that the slight differences in orientation of the two enantiomers interacting with the Ag-PATP complex cause different electron density distributions in the two chiral systems. This further affects the CT process from the Ag substrate to the PATP molecules, generating distinct CT states between the two enantiomers. Furthermore, the SERS spectra of the Ag-PATP complex showed remarkable differences between the R and S enantiomers, which tremendously enlarge the differences caused by molecular chirality by about 2 orders of magnitude over the intensities of ROA spectra (Figure S3). To verify the CT transitions in our chiral discrimination process, excitation wavelength-dependent SERS experiments were conducted for the Ag-PATP complex immersed in racemic TFIP and R-TFIP with excitations at 514, 633, and 785 nm, as illustrated in Figure 3A. When racemic TFIP was involved in the system, both a dramatic reduction in the intensity of the b2 modes of PATP and a weakening of the global SERS signal intensity occurred with the use of 514 and 633 nm laser excitation wavelengths. Figure 3B shows the degree of CT of PATP in this chiral discrimination system. For the 514 and 633 nm excitations, the ρCT values for the b2 bands at 1139, 1398, 1432, and 1576 cm−1 were great than 50% when R-TFIP was present, indicating the key effect of CT

contribution on the SERS signal of PATP with interaction with R-TFIP. While interacting with racemic TFIP, the intensity of the b2 bands decreased to a large extent, manifesting that the CT contribution decreased. On the other hand, no obvious CT enhancement was observed with RTFIP involved in the system when the 785 nm excitation was used, and the ρCT values for the aforementioned b2 bands were in a range of 12.5−25%. When introducing racemic TFIP, only a small change occurred in the intensity of the b2 bands under 785 nm excitation. The ρCT values were compared at the three different laser excitations (Figure 4), and with exception of the 785 nm excitation, the CT contribution obviously decreased upon the introduction of racemic TFIP to the system. This implies that the S enantiomer inhibits the Herzberg−Teller

Figure 4. A comparison of the decrease in ρCT values for the bands at 1139, 1398, 1432, and 1576 cm−1 upon exposure to racemic TFIP under different excitation wavelengths. D


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shift occurred in the frequency of the a1 mode at 1075 cm−1 in the SERS spectra upon Ag-PATP changing from immersion in racemic TFIP to immersion in R-TFIP, as shown in Figure 7.

effect and further restrains the CT between the Ag surface and the adsorbed PATP molecules. Figure 5 illustrates the mechanism of this CT-induced chiral discrimination with regards to the corresponding energy levels

Figure 5. Energy level diagram of the Ag-PATP complex assembly at the energy of the 633 nm laser excitation.

Figure 7. Zoom of the 1000−1530 cm−1 spectral region of the SERS spectra of PATP after immersion in racemic TFIP and R-TFIP with excitation wavelengths of 514 (a), 633 (b), and 785 nm (c). The full SERS spectra of PATP+TFIP/R-TFIP at a 633 nm excitation wavelength are shown in Figure 2A.

of Ag and PATP. The Fermi level for Ag is 4.4 eV below the vacuum level, and the ionization potential of PATP is 6.36 eV, which is the energy of the highest occupied molecular orbital (HOMO), as shown in Figure 6A. The lowest unfilled molecular orbital (LUMO) corresponds to the 300 nm transition in the UV−vis spectrum of PATP (Figure 6B),38,50 equivalent to 2.23 eV. The energy of a 633 nm laser (ca. 1.96 eV) can promote an electron from the Fermi level of Ag to an energy level (2.44 eV) just below the LUMO level of the adsorbed PATP and close to the CT resonance region; therefore, a CT resonance can occur under the laser excitation of 633 nm. When PATP interacts with TFIP by intermolecular hydrogen bonding, the intrinsic differences between the two S and R enantiomers result in different bonding geometries for the structure of the Ag-PATP complex. This is evidenced by the observation that, at all three laser excitations, an obvious red

Moreover, the bandwidth of the band at 1075 cm−1, assigned to ν(C−S), is narrower when the complex interacts with TFIP, which indicates a more homogeneous orientation of PATP with respect to the surface of the Ag NPs. This further influences the CT transition between the Ag NPs and the adsorbed PATP. Therefore, it can be inferred that the chiral environment plays a role in the CT process. In the case of the 514 and 633 nm excitations, the laser energy was sufficient to cause a CT transition for Ag-PATP. Given that the S enantiomer environment inhibits the CT process in the system, it is reasonable that the SERS intensity of b2 modes of PATP decreased significantly in the presence of racemic TFIP

Figure 6. (A) UPS spectrum of PATP adsorbed on an assembled Ag NPs substrate. (B) UV−vis absorbance spectrum of PATP in ethanol. E


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Figure 8. (A) SERS spectra of the Ag-PATP complexes in (a) TFIP and TFIP diluted by CCl4 to a TFIP concentration of (b) 5, (c) 1, (d) 0.1, (e) 10−2, (f) 10−3, and (a) 0 M. All spectra were measured with a 633 nm excitation wavelength and normalized to the band at 1075 cm−1. (B) Plot of the degree of charge transfer (ρCT) for PATP, determined from the characteristic b2 band at 1142 cm−1, versus the concentration of racemic TFIP.

ment the spectroscopic data. In these chiral discrimination complexes, there is a strong propensity for the dipolar quinoidlike resonance structure of PATP NH2+ϕS−,50 to be an important contributor to the stability of the system. The S atom of PATP attached to the surface of the Ag nanoparticle possesses electronegativity, whereas the electropositive NH2+ end of the molecule interacts with the hydrogen atom in the hydroxyl group of the TFIP, which causes the positive center of the hydrogen bonding complex to shift correspondingly. It is conceivable that the energy and the dipole of the complex formed between a single R- or S-TFIP enantiomer and PATP are almost identical. However, differences in the dipolar direction of the two enantiomeric systems arise when the relative proportion of either TFIP enantiomer in the environment around PATP is increased, despite the energy of the two complexes system remaining more or less the same. We used trimers of the two enantiomeric TFIP molecules and a PATP molecule to develop the equilibrium geometry of the PATPTFIP/R-TFIP complex and simplistically simulate the chiral discrimination system, shown in Figure S4. The steric hindrance and the orientation of hydrogen bonding of the chiral complex was considered, and it was found that only TFIP molecules that hydrogen bond with PATP are orientated away from PATP and the steric hindrance is minimized when the trimmers are arranged in a linear distribution. According to the structure optimized by theoretical calculations, it is clear that the R and S enantiomeric complexes are not exactly mirror symmetrical, especially for the spatial positions of the three oxygen atoms of TFIP in the two enantiomeric trimers, as shown in Figure S4 (the frontal view (A) and side view (B)). It indicates that the strict mirror symmetry between the R and S enantiomeric complexes decreases after they hydrogen bond with a PATP molecule. In addition, it is evident that the electronegative centers in the R and S enantiomeric multimers are different, resulting in a difference in the charge distribution in the enantiomeric trimerPATP complexes. Therefore, either the degree or the direction of the CT transition from the Fermi level of Ag NPs to the LUMO level of PATP is different between the two enantiomeric complex systems, and is responsible for their significantly different SERS patterns of PATP.

compared to R-TFIP when using the laser excitations of 514 and 633 nm. At 785 nm, the laser excitation is insufficient to cause electron transfer in the system, and there was no noticeable CT contribution for the Ag-PATP complex with the introduction of R-TFIP at this laser excitation. In our chiral discrimination process, CT contributes to the amplification of very slight differences in the intermolecular interactions between the two Ag-PATP-TFIP/R-TFIP complexes, leading to remarkable differences in the relative intensities of their respective SERS spectra. The origin of the differences in our system is a key issue that must be understood. Given that there is negligible difference in the hydrogen bonding between single S/R enantiomers and PATP is negligible and there exist many hydrogen-bonded multimers in the enantiomeric alcohols, we propose that the multimers enlarge the difference in hydrogen bonding and are crucial to the chiral discrimination. When these S or R multimers interact with adsorbed PATP, the aforementioned differences in intermolecular interactions are observed. A reasonable explanation is that the different enantiomeric multimers engender differentiated effects on the electronic structure of PATP, resulting in different CT transitions. To investigate this hypothesis, we carried out concentration-dependent SERS experiments for the Ag-PATP complex immersed in different concentrations of R-TFIP. A racemic TFIP solution was diluted to several different concentrations using a small nonpolar molecule, CCl4. The SERS spectrum of PATP changed with the concentration of TFIP, as shown in Figure 8A. When TFIP was diluted to 10−2 M, the spectrum became stable and no obvious changes were observed upon further dilution. Differences in chirality could not be distinguished when the concentration of chiral enantiomer decreased below a critical concentration, suggesting that there were not sufficient molecules to form effective multimers to interact with PATP. From calculating the degree of CT as a function of TFIP concentration, as shown in Figure 8B, we determined that chiral discrimination was not possible when the concentration of TFIP was lower than 10−3 M. For further evaluation of the hypothesis that differences between the two enantiomeric multimers are responsible for the differences in CT observed in the two systems, a theoretical calculation of the optimized structures of the chiral discrimination complexes constituted of PATP and the enantiomeric TFIP would be particularly helpful to compleF


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CONCLUSION The mechanism of label-free, enantiomeric discrimination of alcohols by SERS spectroscopy has been systematically investigated by using PATP, an achiral molecule, as a chiral selector. We found that the intensity of the b2 modes in the SERS spectrum of PATP decreased after the introduction of racemic TFIP to the system; no obvious changes were observed when the sample was immersed in R-TFIP. To further explore this intriguing discrimination phenomenon, excitation wavelength-dependent SERS experiments were conducted, and it was found that different CT transitions occurred in the enantioselective process, causing the dramatically different SERS spectra. Concentration-dependent SERS experiments indicated that a certain number of enantiomeric molecular clusters exist in the chiral alcohols, resulting in the differentiated CT transitions of the Ag-PATP complex. The interaction of the Ag-PATP complex with these R and S chiral alcohol multimers induces different CT directions, leading to a different CT process and distinctly different SERS patterns. Overall, this work illustrates the mechanism of enantiomeric discrimination by an achiral selector in the absence of any chiral auxiliaries and deepens our understanding of this process. Moreover, this process is generally applicable for the chiral discrimination of alcohols, affording new possibilities for its use with other chiral molecules, and new insights into label-free chiral discrimination with applications in chemical and industrial fields.

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China, the 111 project (B06009), and the Development Program of the Science and Technology of Jilin Province (20110338, 20130305005GX).

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(1) Hao, H. P.; Wang, G. J.; Sun, J. G. Enantioselective Pharmacokinetics of Ibuprofen and Involved Mechanisms. Drug Metab. Rev. 2005, 37, 215−234. (2) Brown, J. M.; Davies, S. G. Chemical Asymmetric Aynthesis. Nature 1989, 342, 631−636. (3) Amini, A. Recent Developments in Chiral Capillary Electrophoresis and Applications of This Technique to Pharmaceutical and Biomedical Analysis. Electrophoresis 2001, 22, 3107−3130. (4) Shundo, A.; Labuta, J.; Hill, J. P.; Ishihara, S.; Ariga, K. Nuclear Magnetic Resonance Signaling of Molecular Chiral Information Using an Achiral Reagent. J. Am. Chem. Soc. 2009, 131, 9494−9495. (5) James, T. D.; Sandanayake, K. R. A. S.; Shinkai, S. Chiral Discrimination of Monosaccharides Using a Fluorescent Molecular Sensor. Nature 1995, 374, 345−347. (6) Stephens, P. J.; Devlin, F. J.; Pan, J. J. The Determination of the Absolute Configurations of Chiral Molecules Using Vibrational Circular Dichroism (VCD) Spectroscopy. Chirality 2008, 20, 643− 663. (7) Barron, L. D.; Hecht, L.; McColl, I. H.; Blanch, E. W. Raman Optical Activity Comes of Age. Mol. Phys. 2004, 102, 731−744. (8) Maier, N. M.; Franco, P.; Lindner, W. Separation of Enantiomers: Needs, Challenges, Perspectives. J. Chromatogr. A 2001, 906, 3−33. (9) Noyori, R. Chiral Metal Complexes as Discriminating Molecular Catalysts. Science 1990, 248, 1194−1199. (10) Lee, S. J.; Lin, W. A Chiral Molecular Square with MetalloCorners for Enantioselective Sensing. J. Am. Chem. Soc. 2002, 124, 4554−4555. (11) Tkachenko, G.; Brasselet, E. Optofluidic Sorting of Material Chirality by Chiral Light. Nat. Commun. 2014, 5, 3577. (12) Cameron, R. P.; Barnett, S. M.; Yao, A. M. Discriminatory Optical Force for Chiral Molecules. New J. Phys. 2014, 16, 013020. (13) Nafie, L. A. Infrared and Raman Vibrational Optical Activity: Theoretical and Experimental Aspects. Annu. Rev. Phys. Chem. 1997, 48, 357−386. (14) He, Y.; Wang, B.; Dukor, R. K.; Nafie, L. A. Determination of Absolute Configuration of Chiral Molecules Using Vibrational Optical Activity: A Review. Appl. Spectrosc. 2011, 65, 699−723. (15) Abdali, S. Observation of SERS Effect in Raman Optical Activity, a New Tool for Chiral Vibrational Spectroscopy. J. Raman Spectrosc. 2006, 37, 1341−1345. (16) pour, S. O.; Rocks, L.; Faulds, K.; Graham, D.; Parchaňský, V.; Bouř, P.; Blanch, E. W. Through-Space Transfer of Chiral Information Mediated by a Plasmonic Nanomaterial. Nat. Chem. 2015, 7, 591−596. (17) Ozaki, Y., Kneipp, K., Aroca, R., Eds. Frontiers of SurfaceEnhanced Raman Scattering: Single Nanoparticles and Single Cells; Wiley: Hoboken, NJ, 2014. (18) Kneipp, K., Moskovits, M., Kneipp, H., Eds. Surface-Enhanced Raman Scattering: Physics and Applications; Springer-Verlag: Berlin, 2006. (19) Camden, J. P.; Dieringer, J. A.; Zhao, J.; Van Duyne, R. P. Controlled Plasmonic Nanostructures for Surface-Enhanced Spectroscopy and Sensing. Acc. Chem. Res. 2008, 41, 1653−1661. (20) Moskovits, M. Surface-Enhanced Spectroscopy. Rev. Mod. Phys. 1985, 57, 783−826. (21) Rycenga, M.; Cobley, C. M.; Zeng, J.; Li, W. Y.; Moran, C. H.; Zhang, Q.; Qin, D.; Xia, Y. N. Controlling the Synthesis and Assembly of Silver Nanostructures for Plasmonic Applications. Chem. Rev. 2011, 111, 3669−3712. (22) Otto, A. The Chemical (Electronic) Contribution to SurfaceEnhanced Raman Scattering. J. Raman Spectrosc. 2005, 36, 497−509. (23) Lombardi, J. R.; Birke, R. L. A Unified View of SurfaceEnhanced Raman Scattering. Acc. Chem. Res. 2009, 42, 734−742.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b11128. The assignment of the SERS spectra of PATP; normal Raman and SERS spectra of racemic TFIP, R-TFIP, (S)2-butanol, and (R)-2-butanol; SERS spectra of Ag-PATP complex immersed in racemic 2-butanol, (R)-2-butanol, and (S)-2-butanol and their corresponding degree of CT value; the ROA-like data processing algorithm of the normalized SERS spectra of the Ag-PATP complex in (R)-2-butanol and (S)-2-butanol; the optimized structure of the PATP interacted with the trimer of the R enantiomer and S enantiomer of TFIP by DFT calculation; and some supporting explanations and instructions (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: +86 431 8519 3421. Tel: +86 431 8516 8473 (B.Z.). *E-mail: [email protected]. Fax: +81 79 565 9077. Tel: +81 79 565 8349 (Y.O.). ORCID

Bing Zhao: 0000-0002-9559-589X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank Prof. Fei Li and Prof. Zhonghan Hu from Jilin University for their support. This Research was Supported by the National Natural Science Foundation (Grant Nos. 21273091, 21221063, 21327803, 21411140235) of P. R. G


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