Development of Triphenylamine Functional Dye for Selective

Article pubs.acs.org/ac

Development of Triphenylamine Functional Dye for Selective Photoelectrochemical Sensing of Cysteine Shuo Wu,*,† Honglei Song,‡ Jie Song,† Cheng He,‡ Jun Ni,† Yanqiu Zhao,† and Xiuyun Wang† †

School of Chemistry, Dalian University of Technology, Dalian 116023, PR China Key Laboratory of Fine Chemistry, Dalian University of Technology, Dalian 116023, PR China

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S Supporting Information *

ABSTRACT: A novel triphenylamine-based organic dye, TTA, with an acrylic group is designed to graft TiO2 nanoparticles for sensitive and selective photoelectrochemical sensing. The synthesized TTA possesses a high molar absorption coefficient, leading to an enhanced photoelectron emission ability of the electron donor. The carboxyl group of TTA acts as not only an electron acceptor but also a linker to connect TTA to TiO2 nanoparticles. Under irradiation, TTA shows fast intramolecular charge transfer from triphenylamine to carboxyl group via the πbridge of thiophene moiety, thus producing a sensitive photocurrent response. Meanwhile, the acrylic moiety provides an active site for the Michael addition reaction, which would destroy the π-bridge and decrease the photocurrent response. Thus, a selective photoelectrochemical sensing strategy is proposed for detection of small biomolecules. Using cysteine as a model analyte, this sensing strategy shows a detectable range from 1 to 200 μM, without the interference from natural amino acids and various biological reducing reagents. This work offers a new photoelectrochemical route to highly selective and sensitive detection of biologically important small molecules. ince the first photoelectrochemical (PEC) sensor was reported by Willner et al. in 2001,1 PEC sensing has been recognized as an efficient detection technique for different fields, including the monitoring of trace amount of analytes in complex biological contexts.2−10 This technique is of elegant merits of high sensitivity, low background, and easy minimization. Given that some small biomolecules, such as biothiol, NADH, and glucose, are involved in many important psychological processes and closely related to many serious diseases,11−13 a variety of PEC sensing methods have been developed to detect these biologically important species.14−17 Their sensing principles mainly depend on the small biomolecule-induced oxidization or biocatalytic events, which lead to the change of electrical signal. However, because of the coexistence of electron donors in the complex biological systems and the poor stability of the biocatalysts, some PEC sensors cannot exclude the interference from other reducing reagents or suffer from the poor stability and unsatisfied reproducibility. Nowadays, it is still highly desirable but challenging to develop long-term stable, sensitive, and selective PEC sensing system for the detection of small biomolecules, which requires an ingenious design of the signaling strategy. Generally, a PEC sensing system is composed of two parts, a proper PEC transducer and an electron/hole donor.18 Among different PEC transducers, organic dye and TiO2 nanocomposites-based transducers have been demonstrated to be promising candidates for PEC sensing. For example, a dopamine-coordinated photoactive TiO2 nanoporous film has been successfully used for sensitive detection of NADH at the

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© 2014 American Chemical Society

irradiation wavelength up to 580 nm.14 The integration of porphyrin with TiO2 nanoparticle has led to a rapid and valid PEC approach to the determination of biothiol at a relatively low applied potential.15 The good performance can be attributed to the synergistic action of the TiO2 nanoparticle and the organic dye. In particular, the TiO2 nanoparticles possess large surface area, suitable energy distribution, and good photochemical stability, and therefore, they can efficiently transfer photoelectron from their conduction band to the underlying electrode, yielding a high photocurrent.14,15 On the other hand, the high absorption coefficient, ultrafast electron injection from dye to the nanoparticle, slow chargerecombination kinetics, and good chemical stability of the dye are also important factors for obtaining sensitive photocurrent response.15,16 Moreover, the carboxyl, phosphoryl, and sulfonic groups for anchoring the dye to TiO2 nanoparticles provide the intimate coupling of the excited states wave function of the dye with the conduction band manifold of TiO2.19 Thus, the design of novel organic dyes with these groups will promote the development of innovative PEC systems for highly efficient detection of small biomolecules. Recently, some organic dyes with high absorption coefficient and specific reactivity to biologically important small molecules have been reported.20−22 The high absorption coefficient results from the intramolecular charge transfer from electron Received: February 28, 2014 Accepted: May 19, 2014 Published: May 19, 2014 5922

dx.doi.org/10.1021/ac500790u | Anal. Chem. 2014, 86, 5922−5928

Analytical Chemistry

Article

Scheme 1. Schematic Mechanism of the Operating PEC System for Cys

Scheme 2. Synthesis Procedure of TTA

donor to electron acceptor through π-conjugated bridge (D−π−A ICT).23 A novel organic dye, 3−5-(4-diphenylamino-phenyl)-thiophene-2-yl-acrylic acid (TTA, Scheme 1), is designed for quick D−π−A ICT. The diphenylamino-phenyl moiety (triphenylamine/TPA) can act as an electron donor because of its electron-rich property, intrinsic high hole conducting capability, and nonplanar structure for avoiding dye aggregation,24−26 while the carboxyl group acts as both an electron acceptor and an anchoring group to achieve ultrafast electron injection to TiO2 nanoparticle. More importantly, the presence of acrylic moiety provides not only a π-conjugated bridge for ICT but also an active site for selective reaction with biomolecule.27 Interestingly, the reaction leads to the change of ICT efficiency, allowing for selective PEC sensing. To demonstrate the PEC sensing strategy, this work used cysteine (Cys) as a model molecule to react with the acrylic moiety of TTA. The reaction destroyed the π-conjugated system of TTA and thus reduced photoelectron transfer/ injection efficiency and photocurrent response (Scheme 1). Furthermore, the proposed detection method could effectively exclude the interferences from other kinds of amino acids, biothiols, and the biological relevant reducing species because of the selectivity of acrylic reaction and the steric hindrance. The success of this work may inspire more rational designs of PEC dyes for the selective detection of biologically important small molecules.

(China). Other reagents were all of analytical grade and used without further purification. Phosphate buffered solution (PBS, 0.1 M) was prepared by mixing the stock solutions of NaH2PO4 and Na2HPO4. Doubly distilled water was used in all experiments. Urine samples were donated by healthy volunteers. Apparatus. FT-IR spectra in KBr were collected on a Nicolet Avatar 360 FT-IR spectrometer. UV−vis spectra were recorded with a Lambda35 UV−vis spectrophotometer. Fluorescence spectra were recorded on a FS920 luminescence spectrometer (Edinburgh Instruments). 1H NMR was taken on a Varian Inova-400 spectrometer with TMS as an internal standard and CDCl3 or d6-DMSO as solvent. Mass spectra were measured using the HP1100LC/MSD electrospray ionization mass spectrometry (Agilent, ESI-MS). A 500-W Xe lamp equipped with a monochromator was used as the irradiation source. The PEC measurements were carried out on a CHI 660 C electrochemical working station (CH Instruments Co., China). A three-electrode system was employed with either a TTA-TiO2/FTO or a glassy carbon electrode (GCE) as the working electrode, a saturated calomel electrode (SCE) as the reference electrode, and a platinum wire as the counter electrode. Synthesis of TTA. The TTA was synthesized according to a three-step reaction as described in Scheme 2. Initially, TPA (29.4 g, 120 mmol) and NBS (21.4 g, 120 mmol) were dissolved in CCl4 (500 mL) in a closed vessel and refluxed for 4 h. The mixture was purified by filtration and recrystallization to yield a middle product N,N-diphenyl-4-bromoaniline (a), which showed the 1H NMR δ (CDCl3, 400 MHz, ppm): 7.32−7.22 (m, 6H), 7.06−7.02 (m, 6H) and 6.94−6.91 (m, 2H). Then, middle product a (200 mg, 0.617 mmol), 5-formylthiophene-2yl-2-boronic acid (192 mg, 1.23 mmol), K2CO3 (426 mg, 3.10 mmol), and Pd(dppf)Cl2 (51 mg, 0.06 mmol) were dissolved in a mixture of toluene and methanol (4 mL, 1:1 v/v) in a closed vessel. The mixture was heated at 70 °C for 40 min under microwave condition. Concentration by rotary evaporation and purification by column chromatography over silica gel

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EXPERIMENTAL SECTION Materials and Reagents. TPA, N-bromosuccinimide (NBS), pyridine, piperidine, 1,1′-bis(diphenylphosphino)ferrocenedichloropalladium(II) (Pd(dppf)Cl2), malonic acid, 5-formylthiophen-2-yl-boronic acid, and tetrabutyl ammonium bromide were purchased from JK Chemical (China). Cys, glutathione (GSH), homocysteine (Hcy), 16 kinds of essential amino acids, ascorbic acid (AA), uric acid (UA), and adrenaline (AD) were obtained from Sigma. TiO2 nanoparticles (25 nm), fluorine-doped tin oxide electrode (FTO) and terpineol were purchased from Yingkou Opvtech New Energy Co. Ltd. 5923



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Figure 1. (A) Normalized UV−vis (a) and fluorescent spectrum (b) of 10 μM of TTA in CH2Cl2, and (B) cyclic voltammogram of 1.0 mM of TTA in CH2Cl2, containing 0.1 M tetrabutyl ammonium bromide as supporting electrolyte and 0.1 mM of ferrocene as internal standard.

(CH2Cl2/EtOAc 30:1) yielded another middle product, 5-(4Diphenylamino-phenyl)-thiophene-2-carbaldehyde (b), which showed the 1H NMR δ (d6-DMSO, 400 MHz, ppm): 9.86 (s,1H), 8.01−8.00 (d, 1H), 7.70−7.68 (m, 2H), 7.62−7.61 (d, 1H), 7.38−7.34 (m, 4H), 7.15−7.09 (m, 6H), and 6.97−6.95 (d, 2H). At last, b (0.853 g, 2.4 mmol), malonic acid (0.305 g, 2.9 mmol), and piperidine (24.7 mg, 0.29 mmol) were dissolved in pyridine (15 mL) and refluxed for 24 h. The pyridine was removed by rotary evaporation and the reaction mixture was poured into water and washed with HCl. The precipitate was filtered and washed with hexane for three times and purified by silica gel column chromatography (CH2Cl2). The resulted earthy yellow pure compound was TTA with a molecular ion peak at 397.1142 (Supporting Information Figure S-1) and 1H NMR δ (400 MHz, d6-DMSO): 12.33 (s, 1H), 7.73−7.69 (d, 1H), 7.60−7.58 (d, 2H), 7.49−7.48 (d, 1H), 7.43−7.42 (d, 1H), 7.36−7.32 (m, 4H), 7.12−1.06 (m, 6H), 6.98−6.96 (d, 2H), and 6.14−6.10 (d, 1H) (Supporting Information Figure S-2). Preparation of TTA-TiO2/FTO. FTO electrodes (0.5 cm × 2 cm) were first cleaned with 1 mol L−1 NaOH and 30% H2O2, and then dipped into a cold TiCl4 solution. After immersion for 2 h, they were heated in air at 70 °C for 2 h to form a homogeneous and stable TiO2 nanoseed layer on the surface. The TiO2 seeds covered FTO electrodes were coated with 4 μL of TiO2 gel (a mixture containing 15 mg of 25 nm TiO2 nanoparticles, 4.26 mL of ethyl cellulose, and 81 mg of terpineol), dried at room temperature, and calcinated at 450 °C for 2 h. The obtained electrode was named as TiO2/FTO. Then, the TiO2/FTO was immersed into a CH2Cl2 solution containing 50 μM TTA for 4 h. After thoroughly washing with CH2Cl2, the obtained PEC sensor was dried under vacuum and named as TTA-TiO2/FTO. Detection of Cys. The performance of the TTA-TiO2/ FTO was tested by its photocurrent−time response in 0.1 M PBS containing 2.0 mM of AA at −0.1 V (versus SCE) under irradiation of 410 nm light. For Cys detection, the original photocurrent signal (Icontrol) of TTA-TiO2/FTO was first measured at the above-mentioned condition. Then the electrode was rinsed with water and incubated in PBS which contained desired concentration of Cys for 10 min. After incubation, the residual photocurrent signal (Iexp) was recorded at the same condition as the original one. The photocurrent decrease factor (PCDF) caused by Cys was calculated as follows:

PCDF(%) = (Icontrol − Iexp)/Icontrol

For real sample analysis, the urine samples donated by healthy volunteers were first filtrated by Millipore filter (0.2 μm) and twice diluted by PBS to make their Cys concentration suitable for detection. Recovery test was performed by adding aliquot amount of standard Cys into the diluted urine sample. The detection procedure was the same as the standard procedure. The corresponding data were listed in Supporting Information Table S1.

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RESULTS AND DISCUSSION Design and Synthesis of TTA. To fabricate a sensitive and selective dye-TiO2 based transducer for PEC sensing, ideally, the organic dye should fulfill the following essential characteristics: (1) a high molar absorption coefficient to enable efficient visible light harvesting and photoelectron emission, (2) appropriate steric properties to suppress dye aggregation, (3) more negative lowest unoccupied molecular orbital (LUMO) level than the conduction band of TiO2 to provide sufficient driving force (at least 0.2 eV) for electron injection, (4) suitable anchoring groups to ascertain fast photoelectron injection into the conduction band of TiO2 nanoparticles, and (5) specific active site for selectively recognizing target molecule. Thus, this work designed a TPA-based dye, TTA, with a typical D−π−A character (Scheme 1). In this case, the electron push−pull structure could induce the ICT from subunit A to D through the π-bridge to help harvest visible light. The TPA group was chosen as the electron donor due to its electron-rich property, high molar absorption coefficient, intrinsic hole conducting ability and the nonplanar structure that could facilitate the fast photoelectron transfer and reduce electron−hole recombination as well as dye aggregation.19,24−26 The carboxyl group was selected as the electron acceptor because of its strong electronwithdrawing property and stable binding with TiO2 via the bridging ester-like or bidentate interaction, and more importantly, acrylic moiety could provide good reactivity for Michael addition reaction.27 In the designed TTA, a thiophene group was used to improve the molar absorption coefficient due to its π-conjugated structure.28 Thus, TTA could theoretically satisfy the aforementioned requirements. The synthesis procedure of TTA is shown in Scheme 2. After thorough separation and purification, the mass spectrum of synthesized TTA showed a molecular ion peak of 397.1142 (Figure S-1 in Supporting Information). Its structure was demonstrated by NMR (Figure S-2 in Supporting Information). 5924



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Figure 2. (A) Fluorescent spectra of 10 μM of TTA (9:1 CH3CN/H2O) before (a) and after (b) reaction with 0.2 mM of Cys for 10 min, and (B) plot of fluorescence intensity change vs reaction time of 10 μM TTA with 0.2 mM of Cys. Inset: Fluorescent spectra at different reaction times.

Properties of TTA and Its Reaction with Cys. To evaluate the feasibility of using TTA as a PEC probe, its PEC properties and specific reactivity to the model molecule, Cys, were characterized by UV−vis spectrum, fluorescence spectrum, and electrochemical methods. The UV−vis spectrum of TTA in CH2Cl2 showed one broad absorption band centered at 300 nm and an intense absorption band at 410 nm (curve a in Figure 1A), which corresponded to the locally excited π−π* transition of TPA group and the ICT transition of TTA, respectively.28 The molar absorption coefficient of the intense absorption peak was calculated to be 1.1 × 104 L·mol−1·cm−1 from the plot of absorbance versus TTA concentration (Figure S-3 in Supporting Information), indicating good visible light harvesting capability. The fluorescence spectrum of TTA excited at 410 nm showed a strong ICT emission peak at 560 nm (curve b in Figure 1A), providing a good evidence that TTA possessed good photoelectron emission ability. The UV− vis spectrum and fluorescent spectrum overlapped at 480 nm. According to the equation of E0−0 = 1240/λ,29 where E0−0 and λ, respectively, represent the 0−0 excitation energy and crossover point wavelength of the UV−vis spectrum and fluorescent spectrum, the E0−0 of TTA was calculated to be 2.58 eV. The cyclic voltammogram of TTA, using ferrocene as an internal standard, showed a couple of redox peaks at +0.988 and +0.876 V (vs SCE). The corresponding formal potential of TTA was +1.095 V (versus normal hydrogen electrode, NHE), which could be used to estimate the ground state potential (Eox) of TTA.25 In principle, by neglecting any entropy change during the light absorption, the excited state potential (ES+/S*) of TTA can be derived from the equation of ES+/S* = Eox − E0−0. In this case, the ES+/S* of TTA was −1.488 V (vs NHE). The value was more negative than the conduction-band-edge energy level of TiO2 (−0.5 V vs NHE),25 suggesting a sufficient driving force for electron injection into the TiO2 conduction band from the excited dyes. To investigate the chemical interaction between TTA and Cys, the fluorescent spectra of TTA in 9:1 CH3CN/H2O before and after the addition of Cys were recorded. Before the addition of Cys, the TTA showed a strong ICT fluorescence emission peak at 525 nm (curve a, Figure 2A). Upon addition of 20 equiv of Cys (dissolved in PBS) to the TTA solution, the fluorescence peak decreased significantly (curve b, Figure 2A). Control experiment using the same volume of PBS, without Cys, did not result in any observable fluorescence change (data now shown). The reaction between TTA and Cys was very fast

and could get stable within 15 min (Figure 2B). To investigate the reason leading to the fluorescence quench, the final product of 0.1 mM of TTA and 5 mM of Cys was characterized by HPLC/MS (Figure S4 in Supporting Information). A new product with a quasi-molecular ion peak of 517.2 was observed, which was equal to “M-1” of the Michael product of TTA and Cys. Therefore, this observation indicated the fluorescence quench was a consequence of the Michael addition of Cys to TTA, which could destroy the conjugated π−bridge of TTA and result in inhibited ICT process from TPA to the carboxyl group. Characterization of Interaction between TTA and TiO2. To evaluate the interaction between TTA and TiO2 nanoparticles, the IR spectra and UV−vis spectra of TiO2 nanoparticle, TTA, and TTA-TiO2 nanocomposites powders were recorded (Figure 3A). The TiO2 nanoparticles showed

Figure 3. (A) IR and (B) UV−vis spectra of (a) TiO2, (b) TTA, and (c) TTA-TiO2 powders.

two absorption bands at 3433 and 1630 cm−1 (curve a), corresponding to the surface-adsorbed water and hydroxyl groups,31,32 respectively. Two absorption peaks centered at 1685 and 1411 cm−1 were observed in the IR spectrum of TTA (curve b), corresponding to the typical stretching vibration of CO group and the bending vibration of O−H group, respectively.30 After TTA was mixed with TiO2, the two absorption peaks centered at 1685 and 1411 cm−1 disappeared, while two new peaks centered at 1629 and 1394 cm−1 appeared (curve c), which corresponded to the asymmetric and symmetric stretching of carboxylate units. These phenomena were consistent with the results reported when organic dye was bonded to TiO2 via carboxyl group by ester-like or bidentate interactions,30 suggesting the successful binding of TTA to TiO2 nanoparticles. 5925



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photocurrent change before and after the Michael addition of Cys. The Michael addition based PEC sensing principle was beneficial to improve the selectivity of the sensor. Figure 5A

The UV−vis spectrum of TiO2 powder did not show any absorption above 410 nm, while the TTA powder showed a wide absorption peak centered at 426 nm, corresponding to the D−π−A ICT process. In comparison to its counterpart in CH2Cl2, the ICT absorption peak of TTA shifted to a longer wavelength and was much wider, which may be probably due to the aggregation of TTA in solid state. After TTA was mixed with TiO2 nanoparticles, the absorption peak corresponding to the ICT process shifted from 426 to 404 nm. The blue-shift was attributed to the deprotonation of the carboxylic acid group, resulted from the binding of carboxylic acid to the TiO2 nanoparticle which decreased the strength of the electron acceptor.28 Therefore, the UV−vis spectra further confirmed the successful anchoring of TTA to TiO2 surface. Highly Selective PEC Sensing of Cys. The inhibition of Michael addition reaction at the active site of acrylic moiety to the ICT process could be used for selective PEC sensing of small biomolecules. Using Cys as a model analyte, Figure 4

Figure 5. (A) PCDF of TTA-TiO2/FTO of 1.0 mM of different compounds or 0.2 mM of Cys, and (B) fluorescence intensity change of 1.0 mM of different compounds or 0.2 mM of Cys in 9:1 CH3CN/ H2O solution on the fluorescence of 10 μM of TTA. The photocurrent curve was recorded under optimal conditions. The fluorescent spectra were excited at 410 nm.

showed the PCDF obtained at the TTA-TiO2/FTO in the presence of different interferents, including natural amino acids and biologically relevant reducing reagents, such as AA, UA, AD, GSH and Hcy. Except Hcy that showed a PCDF of 31.6%, the PCDF originated from other interferents were all below 9%, less than 18% of that induced by Cys, although their concentrations were all 5 times higher. The specificity was far more superior to the reported electrochemical methods34−36 and PEC methods14−16 in excluding interferences from the coexisted oxidizable species, and was much better than some of the fluorescence methods37,38 in excluding interference from GSH. Control experiments using TTA as a fluorescent probe showed that TTA could not exclude the interference from GSH, Ala, Glu, L-Asp, AA, UA, and AD (Figure 5B). Thus, the high specificity was attributed to the synergistic action of Michael addition reaction and steric hindrance. Sensitive PEC Detection of Cys. To improve the performance of the PEC sensor, key factors that will affect the sensitivity of the PEC sensor, such as the concentration of sacrificing reagent, applied detection potential, irradiation wavelength and time required for Cys to react with TTATiO2/FTO, were optimized. After optimization (Figure S-5 in Supporting Information), the best conditions for performing the PEC analysis were fixed at AA concentration of 2.0 mM, applied detection potential of −0.1 V and irradiation wavelength of 410 nm, after the PEC sensor has been reacted with Cys for 10 min. Under optimal conditions, the PCDF displayed a linear increase as the Cys concentration increased from 1.0 μM to 200 μM with a limit of detection of 0.17 μM (Figure 6). The limit of detection was 294 times lower than that observed at the FeTPPS-TiO2/ITO for GSH detection,15 and 3.53 times lower than that obtained at the free-base-porphyrin-functionalized ZnO/ITO for Cys detection.16 Reproducibility, Stability and Application in Real Sample Analysis. The reproducibility and stability of the

Figure 4. Photocurrent responses of (a) TiO2/FTO and (b−d) TTATiO2/FTO in 0.1 M PBS containing 2.0 mM of AA at −0.1 V under irradiation of 410 nm before (b) and after (c) reaction with 0.2 mM of Cys for 10 min or (d) immersion in 0.1 M PBS for 30 min.

showed the typical photocurrent response. Before Michael addition, an anodic photocurrent of 25.8 μA (curve b) was observed at the TTA-TiO2/FTO, which was 78 times of 330 nA observed at the TiO2/FTO (curve a), indicating the strong visible light harvesting ability and highly efficient photoelectron injection efficiency of TTA. After the PEC sensor was incubated with 0.2 mM of Cys for 10 min, the corresponding anodic photocurrent decreased to 12.8 μA (curve c), only 49.6% of the initial value. Two possible reasons may be responsible for the photocurrent decrease: TTA detachment and decreased photoelectron injection efficiency of TTA originated from the Michael addition of Cys to TTA.33 To explore the main reason, the stability of TTA-TiO2/FTO was evaluated by comparing the photocurrent of TTA-TiO2/FTO before (curve b) and after a 30 min immersion in PBS (curve d). The corresponding photocurrent after incubation was almost the same as its original value, although the immersion time was 3 times longer the Cys reaction time of 10 min. Thus, the possibility from the TTA detachment could be excluded and the photocurrent decrease was attributed to the specific chemical reaction between Cys and TTA (Scheme 1). Clearly, TTA can serve as a good recognition element to specifically bind with Cys. The photocurrent decrease obtained from the PEC sensor was directly associated with the amount of “non-active” TTA on the TiO2/FTO. Thus, the concentration of Cys could be accurately determined by the 5926



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Figure 6. (A) Photocurrent response of TTA-TiO2/FTO toward Cys concentration (from upper to lower, 0, 1, 10, 30, 70, 90, 120, 200, 500, and 1000 μM) recorded in optimal conditions, and (B) the corresponding calibration curve.

materials are available free of charge via the Internet at http:// pubs.acs.org.

PEC sensor are crucial for their practical applications. The reproducibility was estimated by determining the PCDF to 50 μM of Cys at six different TTA-TiO2/FTO electrodes. Under optimal conditions, the R.SD was calculated to be 7.4%. The stability was evaluated by monitoring the photocurrent decrease as a function of storing time. No obvious photocurrent decrease was observed after a two-week storage at room temperature. The urine Cys concentrations of two men and two women were 108 ± 4.67, 105 ± 8.43, 98.7 ± 4.96, and 113 ± 7.94 μM (Supporting Information Table S1), respectively, in well agreement with the results reported in the literature.39 The recovery of these four urine samples was from 102% to 115%, indicating the proposed PEC method was highly accurate. The RSD was from 4.72% to 7.28%, indicating the acceptable reproducibility.

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*Phone: 86-411-84986044. Fax: 86-411-84986044. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Nos. 21275024, 21205008, 21101020, and 21107010), the Natural Science Foundation of Liaoning Province and the Fundamental Research Funds for the Central Universities (No. DUT12LK35).

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CONCLUSION This work designed and synthesized a TPA-based organic dye, TTA, for sensitive and selective PEC sensing. The TTA molecule with a typical D−π−A structure possessed a high molar absorption coefficient, strong photoelectron emission ability, and sufficient driving force for photoelectron injection, which were beneficial to improve the photocurrent transfer efficiency of TiO2. The TTA could bind to TiO2 nanoparticles via carboxyl group by dentate interaction to ascertain fast photoelectron injection into the conduction band of TiO2 nanoparticles. The presence of acrylic acid moiety provided an active site for Michael addition, which can destroy the πbridge of TTA and thus reduce the photocurrent response. The Michael addition induced photocurrent decrease led to a novel PEC sensing strategy for small molecule detection. The effectiveness of this strategy was demonstrated in an experiment using Cys as a model analyte, in which high detection sensitivity and good selectivity were obtained, and the interferences from amino acids and biologically relevant reducing species were avoided. This study provides insights into the importance of functional design of dye molecules to the performance of PEC sensor and would promote the optimization of relevant PEC sensing systems.

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AUTHOR INFORMATION

Corresponding Author

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Mass spectrum, NMR spectrum, plot of UV−vis absorbance of TTA at 410 nm vs its concentration, condition optimization of the TTA-TiO2/FTO, and data of real sample analysis. These 5927



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