Chiral Recognition and Enantioselective Photoelectrochemical

May 7, 2014 - Oxidation toward Amino Acids on Single-Crystalline ZnO ... The photoelectrochemical (PEC) approach is quite popular for analysis owing t...
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Chiral Recognition and Enantioselective Photoelectrochemical Oxidation toward Amino Acids on Single-Crystalline ZnO Cheng Chen, Huijie Shi, and Guohua Zhao* Department of Chemistry, Key Laboratory of Yangtze River Water Environment, Ministry of Education, Tongji University, Shanghai 200092, People’s Republic of China S Supporting Information *

ABSTRACT: A novel chiral photoanode was fabricated by in situ constructing surface molecular imprinting (MI) sites on vertically aligned single-crystalline (SC) ZnO, employing the Lor D-amino acid enantiomer as templates. The photoelectrochemical (PEC) experiments showed that the photoanode exhibited chiral recognition and enantioselective PEC oxidation ability to the template enantiomer, compared with the other one. The photocurrent response of L-Phe on the LSC photoanode was 4.8 times the value of D-Phe. A similar result on the D-SC photoanode could also be observed. Moreover, it was found that the recognition factor obtained on the SC photoanode was 2.7-fold that of the polycrystalline counterpart. It was presumed that the enhanced PEC enantioselectivity may be attributed to the high-quality imprinting expression on the rigid surface of SC ZnO, on which the stereoselective adsorption ability was approximately 1.7 times that of the polycrystalline ZnO. The favorable photocatalytic activity of the one-dimensional SC photoanode further amplified the PEC chiral recognition ability by about 37%. Finally, the kinetics of PEC oxidation of the two enantiomers in racemic solution was investigated, and the rate constant on the proposed photoanode to the template enantiomer was above 1.75-fold that to the other enantiomer.

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important theoretical and actual meanings in the field of life, environment, and materials. The key to chemical identification of a chiral substance lies in the construction of chiral materials and corresponding recognition methods of molecular information. High-performance liquid chromatography and gas chromatography are among the most successful methods applied for the recognition and separation of chiral enantiomers,7,8 which are currently confined by the category of commercial chiral columns. The chiral enantiomer detection through an electrochemical method was realized by means of fabricating chiral molecules on the electrode surface.9−11 The photoelectrochemical (PEC) approach is quite popular for analysis owing to its briefness, speediness, accuracy, and ease of online detection.12 The photocatalytic (PC) and PEC oxidation method adopting semiconductor materials such as TiO2 and ZnO as catalysts have been widely studied and used for energy conversion and degradation of environmental pollutants.13 The PEC method for oxidizing substrate is fast, efficient, and degradation exhaustive, based on the strong oxidation ability of hydroxyl radicals that form on the electrode surface. But, that is also the reason why the technique is limited

INTRODUCTION Chirality is an essential property of nature. In nature, many biomacromolecules are chiral, such as amino acids, proteins, polysaccharides, and enzymes, as well as a large number of artificial molecules such as drugs1 and pesticides2 that often exist as a racemic mixture. In a nonchiral environment, the physicochemical properties of chiral enantiomers are almost the same, and the two enantiomers can be viewed as one substance. However, in a chiral environment, the enantiomers tend to exhibit diverse, even completely adverse characteristics, particularly when interacting with chiral compounds, and should be treated as two different chemicals.3,4 Among the countless chiral substances, amino acids are the most common and basic enantiomers. In a biological system, it exhibits enantioseletivity in the transformation, production, metabolism, and oxidation−reduction of amino acids. For instance, an Lamino acid is a kind of fundamental element to constitute protein, while a D-amino acid scarcely participates in the composition of protein, even leading to serious side effects on the living system.5 Enantioselectivity is not limited to the biological effects of chiral materials like teratogenesis, carcinogenicity, mutagenicity, and endocrine disruption.6 In fact, enantioselectivity prevails among lives and natural evolution. Therefore, the research for chiral recognition and development of a new method for selective chemical transformation of chiral enantiomers in a controlled way has very © 2014 American Chemical Society

Received: March 21, 2014 Revised: May 4, 2014 Published: May 7, 2014 12041

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Thus, the SC molecularly imprinting (MI, SC) ZnO PEC electrode exhibiting chiral molecular recognition ability toward L-/D-Phe was fabricated successfully. PEC approaches and chemisorption were introduced to investigate the molecular recognition and PEC oxidation ability toward the enantiomers in single and racemic systems. The factors affecting chiral PEC oxidation ability of the (MI, SC) ZnO electrode were also discussed, compared with that of the polycrystalline molecularly imprinting (MI, PC) ZnO electrode. Eventually, in the racemic system, the kinetics of selective catalytic oxidation toward phenylalanine enantiomers on the (MI, SC) ZnO electrode was also investigated. Our work proposed the fundamental research and fresh ideas for the fabrication of PEC electrodes with remarkable enantioselective capacity and stereoselective PEC oxidation of enantiomers.

to achieve selective PEC oxidation of a specific substrate in the hybrid system. As a consequence, there is significant technical difficulty to make selective catalytic degradation of contaminants come true. Of late years, multitudinous scientists have been trying to conquer the lack of selectivity in PEC oxidation technology via catalyst surface modification or combining with the molecular imprinting technique (MIT).14,15 Through addition of target material as the template molecule in the process of preparing molecularly imprinted materials, the active sites possessing selective recognition ability for the target molecule would be acquired.16 The integration of the MIT and the PEC approach could remarkably improve the capacity of selective oxidation of PEC. In addition to the employment of traditional organic polymers as an imprinting membrane, more and more people turn to fabricate molecularly imprinted sites on the surface of inorganic photocatalysts directly. Luo et al. constructed molecularly imprinted TiO2/WO3 nanocomposites with selective PC activity using 2-nitrophenol and 4-nitrophenol as template molecules, of which the PC selectivity toward target molecules becomes more than two times that of nonimprinted TiO2.17 But, for one thing, the common inorganic semiconductor photocatalysts such as TiO2 and ZnO often exist as disordered nanoparticles and nanofilms, of which the surface is usually irregular, fluctuant in three dimensions. For the other, TiO2 photocatalyst is often composed of mixed crystals of anatase and rutile. Hence, reproducibility of imprinting expression is affected to a certain extent by constructing the recognition sites on the surface directly. Moreover, enantiomers have the identical molecular weight and size and just differ in spatial structure. Therefore, how to distinguish the two enantiomers efficiently by a molecular imprinting based PEC method is still challenging. We speculate that, if the above two drawbacks of inorganic photocatalysts could be solved, the enantioselectivity of the PEC technique would be greatly enhanced. Our previous work indicated that in situ building molecular imprinting sites on the surface of one-dimensional singlecrystalline ZnO (SC ZnO) could bring about selective PEC oxidation of small organic molecules with disparate structures.18 It has two principal advantages: on one side, avoiding some disadvantages of traditional molecularly imprinted polymer, such as the low light absorption, poor stability, and easy degradation by light; on the other side, other than the mixedphase structure of TiO2, SC ZnO may make for the expression and rebuilding of the imprinting sites that are liable to access. All the above sufficiently guaranteed the recognition function of the imprinting sites and quick performance of the selective PEC reaction. As a result, we consider that enantioselectivity of PEC oxidation will be obtained if chiral molecular imprinting sites are fabricated directly on the surface of the one-dimensional SC ZnO. At the same time, the low defect rate of one-dimensional SC ZnO nanorods can well reduce the recombination sites of the photogenerated electron−hole and swiftly drive away the photogenerated electrons, which is beneficial for the separation of the photogenerated electron−hole. Therefore, the catalytic efficiency and the selective oxidizing ability of the PEC materials can be further improved. There are few reports about molecular recognition and selective oxidation of chiral enantiomers on the surface of PEC catalysts. In our work, the crystalline-seed-assisted hydrothermal method was adopted to construct molecularly imprinted sites on the surface of SC ZnO nanorods with phenylalanine enantiomers (L-/D-Phe) as template molecules.

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EXPERIMENTAL SECTION Materials and Apparatus. Titanium plate (purity of 99.99%) was clipped to 1.5 × 4.0 cm2 and served as the substrate electrode after anodic oxidation. L-Phenylalanine, Dphenylalanine, Zn(CH 3 COO) 2 ·2H 2 O, Zn(NO 3 ) 2 , and (CH2)6N4 were of analytical grade and used without further purification. Unless stated, all the solutions were prepared with deionized water. Field emission scanning electron microscopy (FE-SEM, Hitachi S-4800, Japan), high-resolution transmission electron microscopy (HR-TEM, JEOL, Japan), X-ray diffraction (XRD, Bruker D8, Germany), and Raman spectroscopy (Raman, Rainshaw invia) were undertaken to characterize the morphology and crystal structure of the as-prepared electrodes. The construction and rebinding process of molecularly imprinted sites on the electrode surface were studied through Fourier transform IR (FT-IR) characterization (Thermonico Let, Nexus, USA). Circular dichroism (CD) spectra were used for illustrating the enentioselective adsorption capacity of the imprinted electrode for the target. Fabrication of the Electrodes. First, the TiO2 NTs substrate was prepared by anodic oxidation19 on which the crystalline-seed-assisted hydrothermal method was adopted to construct SC ZnO according to the literature20 with certain modification. A 5 mM Zn(CH3COO)2 ethanol solution was spin-coated on TiO2 NTs several times, and then, a dense seed layer can be attained via 350 °C heat treatment. The onedimensional SC ZnO electrode was fabricated by putting the TiO2 NTs into a Teflon-lined stainless steel autoclave with aqueous solution containing 0.02 M Zn(NO3)2 and 0.02 M (CH2)6N4 and kept at 90 °C for 5 h. The L-Phe imprinted SC ZnO electrode (L-(SC, MI) ZnO) was prepared via addition of 1.00 mM template molecule L-Phe into the aqueous solution. The template molecules were removed by thermal treating at 500 °C for 0.5 h. For comparison, polycrystalline ZnO electrode (PC ZnO) was prepared via sol−gel method.21,22 The details were as follows: a certain amount of monoethanolamine was gradually dropped into the ethanol solution while stirring until 1 g of zinc acetate dehydrate (Zn(CH3COO)2·2H2O) in the solution was completely dissolved. The resultant solution was uniformly spin-coated on the TiO2 NTs substrate and then kept at 80 °C for 5 min. By controlling the coating cycles to be six, the final PC ZnO loading was kept the same as that of SC ZnO to be 0.003 g. The films were finally heated at 600 °C for 30 min in air. Similarly, the L-Phe imprinted polycrystalline ZnO electrode (L-(PC, MI) ZnO) was constructed via addition of 0.23 mM template molecular L-Phe in the spin-coating solution 12042

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Scheme 1. Schematic Illustration of the Fabrication of L-/D-(MI, SC) ZnO electrodes

Table 1. Variation of PEC Oxidation Current Density of SC ZnO, PC ZnO, (MI, MC) ZnO, and (MI, PC) ZnO Electrodes upon Addition of 1 × 10−8 M L- or D-Phe in 0.1 M Na2SO4 Solution electrode

PC ZnO

Iblank‑L (mA/cm2) IL (mA/cm2) Iblank‑D (mA/cm2) ID (mA/cm2) ΔIL (μA/cm2) ΔID (μA/cm2) SPEC

0.118 0.122 0.109 0.113 4.3 4.2 1.06

L-(MI,

PC) ZnO

D-(MI,

0.113 0.119 0.114 0.117 5.6 3.0 1.87

PC) ZnO

0.111 0.113 0.109 0.114 2.6 4.5 1.75

to ensure the ratio of template and Zn2+ coincidence with that for the L-(SC, MI) ZnO. Heat treatment at 500 °C was eventually performed to make the template L-Phe decomposed. The D-(SC, MI) ZnO and D-(PC, MI) ZnO electrodes were prepared with similar procedures, of which the template should be replaced to D-Phe. PEC Measurements. The PEC current of each electrode to diverse chiral enantiomers were measured by a CHI660C electrochemical workstation (CH Instrument, USA) with a conventional three-electrode system: the prepared electrodes with the illumination area of 13 × 15 mm2 was used as working electrode, a platinum foil was the counter electrode, and a saturated calomel electrode (SCE) was the reference electrode. A 200 W LA-410UV lamp was used as the UV irradiation source. The amperometric i-t method was employed to monitor the PEC current at a constant potential of 0.6 V (vs SCE) in 0.1 M Na2SO4 solution, and the working electrode was kept 3 cm away from the lamp center. Adsorption Experiments. The prepared electrodes were dipped in 50 mL of 2 × 10−8 M L-and D-Phe and rac-Phe solution separately for adsorption, of which the working area was controlled to be 1.0 × 1.0 cm2. After the solution was stirred for 30 min, the electrode was taken out and washed to eliminate the nonspecific organic substance and then heat treated at 500 °C for the Phe enantiomers adsorbed on the imprinted sites to remove. The whole process was repeated for

SC ZnO 0.490 0.523 0.503 0.537 33 34 1.03

L-(MI,

SC) ZnO

0.538 0.597 0.565 0.578 59 13 4.77

D-(MI,

SC) ZnO

0.533 0.544 0.494 0.543 11 49 4.81

three times, and the adsorbed solution was diluted with water to 100 mL, of which the concentration variation was tested respectively by the proposed PEC method that would be described in detail later.

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RESULTS AND DISCUSSION Fabrication and PEC Chiral Recognition of L- and D(MI, SC) ZnO Electrodes. As illustrated in Scheme 1, L- and D(MI, SC) ZnO electrodes were fabricated through a crystalseed-assisted hydrothermal method employing Phe enantiomers as templates. The templates were removed by heat treatment at 500 °C for half an hour. For comparison, L- and D(MI, PC) ZnO electrodes were also prepared by sol−gel method. The PEC chiral recognition ability of THE L-(MI, SC) ZnO electrode was investigated by monitoring the photocurrent response in single-component enantiomer and interferent coexisting solution. As shown in Table 1, the photocurrent increment (59 μA/cm2) induced by 1 × 10−8 M L-Phe on the L(MI, SC) ZnO electrode was much higher than that of D-Phe (13 μA/cm2) with the same concentration. However, the photocurrent response (33 μA/cm2) of L-Phe on SC ZnO was nearly the same compared with that of D-Phe (34 μA/cm2). It indicated that the introduction of the MIT can evidently increase the enantioselectivity in the PEC process. Further12043

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more, the photocurrent density (59 μA/cm2) of L-Phe on L(MI, SC) ZnO turned out to be more than 10 times of the value (5.6 μA/cm2) on L-(MI, PC) ZnO, which suggested that much more superior PEC recognition ability could be achieved on SC ZnO. Similar results can be obtained in similar experiments conducted on D-(MI, SC) ZnO and D-(MI, PC) ZnO electrodes. In order to further clarify the enantioselectivity of different electrodes, a recognition factor (SPEC) was defined as the ratio of photocurrent increment caused by the target molecule L-Phe and the nontarget molecule D-Phe, which was expressed as SPEC: SPEC =

ΔIL /Iblank‐L , ΔID/Iblank‐D

for

(L‐MI, SC) ZnO

D-Phe. When Sinterfere = 0, it signified that the additional interference enantiomer had no impact on the photocurrent response of the chiral target. The closer the value of Sinterfere approached zero, the better chiral recognition ability of the molecularly imprinted electrodes was embodied. As shown in Figure 1, the red and blue columns expressed the chiral recognition status (Sinterfere) of L-(MI, SC) ZnO and L-

and

(L‐MI, PC) ZnO electrodes SPEC =

ΔID/Iblank‐D , ΔIL /Iblank‐L

for

(D‐MI, SC) ZnO

and

(D‐MI, PC) ZnO electrodes

ΔIL/Iblank‑L and ΔID/Iblank‑D were the photocurrent enhancement rates on the electrode before and after addition of 1 × 10−8 M L-Phe and D-Phe enantiomer, respectively. If L-Phe was served as the target molecule, then D-Phe was the nontarget molecule, and vice versa. As a result, if SPEC was bigger than 1, the electrodes exhibited preferential PEC oxidation to the target molecule, that is, the template molecule instead of the other enantiomer. The higher the factor SPEC was above 1, the better enantioselectivity of the electrode in PEC oxidation was shown. As illustrated in Table 1, the SPEC values of SC ZnO (1.028) and PC ZnO (1.059) were approximately equal to 1, demonstrating that the nonimprinted electrodes were not able to distinguish the two enantiomers L-Phe and D-Phe. However, on the L-Phe imprinted electrode, the greater SPEC of L-(MI, SC) ZnO electrode (4.766) than SC ZnO electrode (1.028) can be acquired by the objective enantiomer L-Phe rather than D-Phe. In other words, the imprinted electrodes can preferably identify the goal compound, which confirmed that the L-Phe imprinted sites were successfully constructed on the SC ZnO and PC ZnO electrodes. It is worth noting that SPEC of L-(MI, SC) ZnO (4.766) was 2.6 times that of the L-(MI, PC) ZnO electrode (1.87), suggesting that the SC structure was more favorable to the clear impression of footprint cavities on ZnO and made the affinity and PEC recognition capacity of electrodes for the target molecule vastly promoted. The same outcomes were observed on the D-(MI, SC) ZnO and D-(MI, PC) ZnO electrodes. Furthermore, the photocurrent alteration on the L-(MI, SC) ZnO and L-(MI, PC) ZnO electrodes along with the addition of a different concentration of target L-Phe and diverse concentration proportions of D-Phe were tested. The photocurrent increase rate of adding D-Phe following the target L-Phe was employed to evaluate the interference effect of nontarget enantiomer to the PEC recognition ability on molecularly imprinted electrodes, which was represented by Sinterfere: Sinterfere =

Figure 1. Variation of photocurrent response on L-(MI, SC) ZnO and L-(MI, PC) ZnO electrodes induced by different concentrations of LPhe after addition of different concentration proportions of D-Phe.

(MI, PC) ZnO electrodes, respectively. As the concentration of −11 L-Phe was kept invariant (such as 4 × 10 M), the Sinterfere of L-(MI, SC) ZnO and L-(MI, PC) ZnO electrodes would rise with the increasing concentration of D-Phe. As the concentration of L-Phe was 4 × 10−9 M, the growth of the red bar was the slowest, indicating the best PEC recognition capacity of L(MI, SC) ZnO could be attained at the concentration. When the amount of L-Phe and the ratio of D-Phe remained constant, the Sinterfere of L-(MI, PC) ZnO was larger than that of L-(MI, SC) ZnO; hence, the taller blue collumn than the adjacent red one could be noticed. That was, the augment of D-Phe concentration in the electrolyte would make the photocurrent enlarged on both imprinted electrodes, but the photocurrent produced by D-Phe on the L-(PC, MI) ZnO electrode was more evident than that on L-(SC, MI) ZnO. It suggested more promising anti-interference ability of the SC structure L-(MI, SC) ZnO electrode. Although the photocurrent on L-(MI, SC) ZnO electrodes would be more or less enhanced by the nontarget substance D-Phe, the Sinterfere preserved no more than 30% until 50-fold concentration of D-Phe in the coexisting system. Therefore, the contribution caused by D-Phe to the photocurrent was vividly inferior to the target molecule L-Phe on the L-(SC, MI) ZnO electrode. While the concentration of L-Phe was setting as 4 μM, the Sinterfere of L-(SC, MI) ZnO in 100-fold of D-Phe mixed solution was still kept below 31%, but the value of L-(PC, MI) ZnO was as high as 156.3% (the data was not given in Figure 1).

ΔIL+D − ΔIL × 100% ΔIL

ΔIL+D was the photocurrent produced collectively by L-Phe and D-Phe, and ΔIL was that caused by L-Phe. It meant that ΔIL+D − ΔIL was the photocurrent triggered just by nontarget 12044

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After template removal, nearly all the characteristic peaks of LPhe disappeared, indicating that heat treatment at 500 °C could dislodge template L-Phe drastically. The studies of FT-IR characterization proved that, with the hydrothermal method, the SC ZnO electrodes with distinct imprinted sites for the template L-Phe can be acquired. For D-(MI, SC) ZnO and (MI, PC) ZnO electrodes, similar results have also been obtained (not shown). The SEM images displayed that both the morphology of L(MI, SC) ZnO (Figure 3a) and D-(MI, SC) ZnO (Figure 3d) were vertically aligned nanorods. The HR-TEM images and the corresponding selected area electron diffraction (SAED) patterns (Figure 3b, c, e, f) revealed that the ZnO nanorods were well-controlled six-prism structures grown in [0002] direction with SC structure.29,30 The addition of molecular template did not cause any influence on the morphology of SC ZnO. The XRD and Raman spectra (in Figure 4a, b) also revealed that the SC structure remained unchanged during the process of template molecule addition and removal. Therefore, it was presumed that the molecular imprinting sites were formed on the surface of SC ZnO, rather than in the catalyst lattice spacing. Actually, there are always abundant hydroxyls on the surface of the metal oxide.31 During the molecular imprinting preparation, the oriented polar groups, for example, carboxyl and amino groups on the template molecules (L-/DPhe) would interact with the surface hydroxyls, forming hydrogen bonding. Then, the recognition sites complementary to the templates in size, stereostructure, and functional groups distribution would be formed on the ZnO surface after template removal. Similar discussions can also be found concerning molecular imprinting site formation on other surface of inorganic oxide such as TiO2 and SiO2.32,33 The electrode characterization results gave clear evidence for the feasibility of the hydrothermal method to prepare (MI, SC) ZnO with footprints. It was considered that the excellent PEC chiral recognition ability of the (MI, SC) ZnO electrode was mainly ascribed to its preferential adsorption of the objective enantiomer. Therefore, the adsorptive property of the (MI, SC) ZnO electrode was further studied.

It is believed that the enhanced PEC chiral recognition ability for identifying enantiomers can be attributed to the successful construction of molecularly imprinted cavities on the rigid surface of SC ZnO electrode. Therefore, the fabrication and the crystal structure of the proposed electrodes were investigated and discussed in detail. The fabrication of footprints on the L-(MI, SC) ZnO electrode could be confirmed by the FT-IR characterization (in Figure 2). The wide peaks at 3400 cm−1 assigned to the O−H

Figure 2. FT-IR spectra of L-Phe, SC ZnO, and L-(MI, SC) ZnO before and after template removal.

vibration could be found in the spectra indicating the existence of surface hydroxyls on ZnO photoanodes23 (enlarged figure on the left of Figure 2). The characteristic absorption peaks at 1620, 1720, and 2600−3100 cm−1 assigned to CO vibrations of carboxyl group, carboxyl anions, and NH stretching vibration24,25 separately could be observed in the FT-IR spectra of L-Phe, which also included peaks at 1400−1600 cm−1 assigned to the framework vibration of benzene rings.25 From the spectra of SC ZnO, the peak at 495 cm−1 contained a dominating contribution of Zn−O−Zn vibration.26−28 Compared to SC ZnO, apart from the peaks at 495 cm−1, new peaks dedicated by vibrations of carboxyl, carboxyl anions, N−H, and benzene rings could be found in the spectra of the L-(MI, SC) ZnO with templates. It confirmed that the templates L-Phe were successfully embedded on the surface of SC ZnO NRs.

Figure 3. (a, d) SEM and (b, e) TEM images and (c, f) SAED patterns of L-(MI, SC) ZnO and D-(MI, SC) ZnO. 12045

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At the same time, quantitative determination of Phe enantiomers adsorbed on the molecularly imprinted electrodes was also performed, and the enantioseletive adsorption capacity was evaluated by the factor Sads: At Sads = A nont At and Anont are the adsorbed amount of the target and nontarget enantiomer, respectively. The Sads of L-(MI, SC) ZnO was calculated to be 2.273 (Figure 6), which was approximately

Figure 6. Sads of L- and D-(MI, SC) ZnO electrodes toward L-/D-Phe in racemic Phe solution.

Figure 4. (a) XRD patterns and (b) Raman spectra of PC ZnO, L-(MI, PC) ZnO, SC ZnO, and L-(MI, SC) ZnO.

The CD spectra of L-Phe, D-Phe, and racemic Phe solution before and after adsorption by L- and D-(MI, SC) ZnO electrodes are given in Figure 5. D-Phe had a negative CD

1.7-fold that of L-(MI, PC) ZnO (1.356). Likewise, the Sads of D-(MI, SC) ZnO was 1.8 times that of D-(MI, PC) ZnO. The results suggested that the molecular imprinting expression on SC electrodes was more efficient than that on polycrystalline imprinted electrodes. Meanwhile, the chiral adsorption ability of the present photoanode was comparable with that obtained on a kind of TiO2 nanothin films with imprinted (R)- and (S)enantiomers of propranolol, of which the highest value of selective factor for the target enantiomer was 2.61.35 It was even better than the adsorptive enantioselectivity of platinum matrix using L- or D-DOPA as templates where chiral selectivity ratio can reach 1.52.11 However, the Sads was found to be distinctly smaller than the SPEC obtained from the PEC test, which recommended that there existed some other elements leading to enhanced PEC enantioselectivity on SC imprinted electrode, except for the excellent adsorptive property. We put forward that the extra element was likely to be the preeminent photocatalytic activity of SC ZnO. As we can see in Table1, the photocurrent enhancement on the SC ZnO electrode (33 μA/cm2) was apparently higher than that (4.3 μA/cm2) on PC ZnO induced by the same amount of L-Phe, suggesting the advanced PEC oxidation ability of SC ZnO. Unlike L-(MI, PC) ZnO (5.6 μA/ cm2), the more intensive photocurrent response can also be attained on L-(MI, SC) ZnO (59 μA/cm2) by L-Phe. We surmise the L-Phe adsorbed on the imprinted cavities of the SC ZnO surface could be oxidized in a faster way resulting from the excellent one-dimensional SC structure, which could promote the separation of photoelectrons from vacancies. Furthermore, the faster oxidation for the target enantiomer would free up unoccupied imprinted sites, which was beneficial for the newly chiral template to spread and be adsorbed for further oxidation. As a consequence, through the favorable photocatalysis property, the stronger amplification function to enantioselectivety could be achieved on the SC phoanode.

Figure 5. CD spectra of L-Phe, D-Phe, and racemic Phe solution before and after adsorption by L- and D-(MI, SC) ZnO electrodes.

adsorption peak around 217 nm while L-Phe had a positive one around 214 nm. The weak negative peak in the CD spectra of racemic Phe solution (1:1 L-/D-Phe) was ascribed to the small difference of specific rotation between the two enantiomers. After adsorption by the L-(SC, MI) ZnO electrode in the racemic solution, the peak intensity of the CD spectra descended obviously, which meant that the peak moved toward the direction of D-Phe peaks. It proved that the L-Phe was enantioselectively adsorbed by the L-(MI, SC) ZnO electrode and the nontarget enantiomer D-Phe was left in the solution.34 For the D-(MI, SC) ZnO electrode, the CD spectra showed an evident shift toward the direction of L-Phe as well, which demonstrated that the D-(MI, SC) ZnO electrode was capable to adsorb the corresponding target D-Phe. 12046

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ZnO electrodes can be used to determinate L-Phe or D-Phe in racemic Phe solution in the range of 4 × 10−10 ∼ 4 × 10−8 M without considering the interference caused by the other coexisted enantiomer. If the concentration of L- and D-Phe was out of the linear range, the solution should be diluted to the range of 4 × 10−10 ∼ 4 × 10−8 M. The kinetics of PEC oxidation of 1 × 10−8 M racemic Phe solution on the D-(MI, SC) ZnO electrode under UV light irradiation were illustrated in Figure 7. Compared with L-Phe,

In summary, the principal reasons accounting for the attractive PEC enantioselective identification capability of (MI, SC) ZnO electrodes could be divided into two aspects: on one hand, the rigid surface of SC ZnO could facilitate the high-quality expression and steric stability of the imprinted cavities on the electrode surface, and the advantage of few defects can avoid the interference of nonspecific adsorption brought by the nontarget enantiomer. Conversely, the imprinted sites fabricated on the PC ZnO nanofilm may distort easily after template removal owing to the volatile configuration and numerous deficiencies, giving rise to the weaker binding affinity between template molecules and imprinted sites. On the other hand, the structure of onedimensional vertically aligned nanorods and low defects of SC are in favor of the efficient separation of the photogenerated electron and hole,36 leading to enhanced photocatalytic ability. When the template molecules have access to the imprinted sites, the photogenerated holes were quickly transferred to the electrode surface to oxidize template enantiomer, and the photogenerated electron can be rapidly driven away through the external circuit profiting from the one-dimensional originated structure, making a magnifying improvement on the PEC enantioselectivity. Elevated Enantioselective PEC Oxidation of (MI, SC) ZnO Electrodes. On account of the excellent PEC enantioselectivity, the L-(MI, SC) ZnO electrode was further applied for the PEC oxidation of L-Phe, D-Phe, and racemic Phe solution, and the relevant reaction kinetics were studied as well. The concentration variations of L-Phe and D-Phe in the solution along with response time were analyzed by the as-prepared photoanodes. During our experiments, it was observed that the photocurrent response detected by L-(MI, SC) ZnO electrodes would increase with the rising of L-Phe enantiomer concentration. As a result, the L-(MI, SC) ZnO electrode may be used for the PEC determination of L-Phe. The results showed that the photocurrent variation on L-(MI, SC) ZnO were proportional to the logarithm value of L-Phe concentration in the range from 4.0 × 10−12 to 4.2 × 10−6 M. The linear equation was ΔIL/Iblank = 42.43 + 3.39log C with a correlation coefficient of 0.9976, of which the limit of detection was estimated to be 4 × 10−12 M. In order to further make clear the accuracy of the PEC detection, the L-(MI, SC) ZnO electrode was adopted to analyze 1 × 10−7, 1 × 10−8, and 1 × 10−9 M spiked L-Phe solutions, and the average recoveries were 98.37%, 100.30%, and 103.17%, respectively. Analogously, the experiments convinced that ΔID/Iblank on D-(MI, SC) ZnO was also in linear relation with the logarithm value of D-Phe concentration in the range of 4.4 × 10−10 to 1.0 × 10−5 M. The linear equation was ΔID/Iblank = 30.84 + 2.61log C with a correlation coefficient of 0.9981. The limit of detection was estimated to be 4 × 10−12 M. The average recoveries for detection of 1 × 10−7, 1 × 10−8, and 1 × 10−9 M spiked D-Phe solutions were 102.14%, 98.45%, and 108.20%, respectively. It confirmed that the fabricated L- and D-(MI, SC) ZnO electrodes could be applied to monitor chiral enantiomers with favorable accuracy and sensitivity. More importantly, in the investigation of the enantioselectivity of (MI, SC) ZnO electrodes, it was found that the best PEC recognition capacity of L-(MI, SC) ZnO was obtained when the concentration ratio of D/L was smaller than 10. The same result was also achieved on the D-(MI, SC) ZnO electrode. Therefore, it was believed that the L- and D-(MI, SC)

Figure 7. Kinetics of PEC oxidation for L-Phe and D-Phe in racemic solution on the D-(MI, SC) ZnO electrode under UV light irradiation.

the D-(MI, SC) ZnO electrode exhibited superior PEC catalytic efficiency to oxidize the target enantiomer D-Phe. The faster DPhe enantiomer oxidation on D-(MI, SC) ZnO electrode could be attributed to the fact that D-Phe was more apt to be adsorbed and rebound in the molecular imprinting sites where they could be oxidized preferentially. Successive mass transport of D-Phe to the electrode surface and PEC oxidation brought about accelerated reaction rate of D-Phe on the D-(MI, SC) ZnO electrode compared with L-Phe. As shown in Figure 7, the PEC oxidation of L-Phe and D-Phe on the D-(MI, SC) ZnO electrodes followed a pseudo-first-order kinetics.37,38 The rate constant (k) for D-Phe on the D-(MI, SC) ZnO electrode was calculated to be 0.28 h−1, which was 1.75 times that of L-Phe (0.16 h−1) on the same electrode. In the racemic Phe solution, the higher PEC oxidation efficiency to the target molecule L-Phe other than nontarget DPhe was also achieved on the L-(MI, SC) ZnO electrode, which was consistent with that obtained on D-(MI,SC) ZnO. The rate constant value (k) for L-Phe oxidation in the racemic solution was about 1.52-fold of that for D-Phe. Therefore, we can confidently propose that fascinating enantioselective PEC oxidation toward the target molecule, that is, the template enantiomer, can be achieved by construction of molecular imprinting sites of chiral molecules on SC ZnO electrodes.

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CONCLUSION In this work, highly ordered and vertically aligned L-/D-(MI, SC) ZnO electrodes were constructed by a crystal-seed-assisted hydrothermal method employing Phe enantiomers as template molecules. For comparison, disordered polycrystalline L-/D(MI, SC) ZnO electrodes were also fabricated through the sol− gel method. It was found that, compared to (MI, PC) ZnO, much more promising PEC enantioselective recognition ability was achieved on the (MI, SC) ZnO electrode toward the target enantiomer. It can be attributed to the high-quality expression and steric stability of the molecular imprinting cavities resulting from the rigid surface of SC ZnO. Meanwhile, the one12047

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dimensional vertically aligned nanostructure are nondefective and in favor of efficient separation of the photogenerated electron and hole, leading to enhanced PEC oxidation ability that further amplified the PEC enantioselectivity. Therefore, the present work may provide a widely applicable method for fabricating chiral photoanode capable of identifying other chiral amino acids.

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ASSOCIATED CONTENT

S Supporting Information *

SEM and TEM images and SAED patterns of L-(MI, PC) ZnO and D-(MI, PC) ZnO; variation of PEC oxidation current density on D-(MI, SC) ZnO and D-(MI, PC) ZnO electrodes with addition of different concentration of D-Phe and different concentration proportions of L-Phe in 0.1 M Na2SO4 solution; photocurrent variation of prepared electrodes in 0.1 M Na2SO4 solution before and after the addition of 1 × 10−8 M L-Phe; linear calibration between photocurrent increase rate and logarithm value of L-Phe and D-Phe concentration respective on L-(MI, SC) ZnO and D-(MI, SC) ZnO electrodes; recovery for detecting various concentration of L- and D-Phe on L-(MI, SC) ZnO and D-(MI, SC) ZnO electrodes, respectively; the adsorption data of L- and D-Phe on prepared electrodes in single-component enantiomer and racemic solution. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-21-65981180; fax: +86-21-65982287; e-mail: g. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundations of China (nos. 21277099 and 21307091). REFERENCES

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