Visible light photoelectrochemical sensor for ultrasensitive determination of dopamine based on synergistic effect of graphene quantum dots and TiO2 nanoparticles.

G Model ACA 233530 No. of Pages 7

Analytica Chimica Acta xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca

Visible light photoelectrochemical sensor for ultrasensitive determination of dopamine based on synergistic effect of graphene quantum dots and TiO2 nanoparticles Yuting Yan, Qian Liu, Xiaojiao Du, Jing Qian, Hanping Mao, Kun Wang * Key Laboratory of Modern Agriculture Equipment and Technology, School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China

H I G H L I G H T S

G R A P H I C A L A B S T R A C T

The GQDs–TiO2 could prevent GQDs assembling and achieve the immobilization of GQDs. The GQDs–TiO2 showed synergistic amplification on the PEC in the visible region. The PEC sensor for sensitive and selective detection of dopamine was constructed. The sensor showed wide linear response and low detection limit for dopamine detection.

A R T I C L E I N F O

A B S T R A C T

Article history: Received 29 July 2014 Received in revised form 9 October 2014 Accepted 13 October 2014 Available online xxx

We have demonstrated a facile approach for fabricating graphene quantum dots–TiO2 (GQDs–TiO2) nanocomposites by a simple physical adsorption method. Compared with pure GQDs and TiO2 nanoparticles (NPs), the as-prepared GQDs–TiO2 nanocomposites showed enhanced photoelectrochemical (PEC) signal under visible-light irradiation. The photocurrent of GQDs–TiO2/GCE was nearly 30-fold and 12-fold enhancement than that of GQDs/GCE and TiO2/GCE, respectively, which was attributed to the synergistic amplification between TiO2 NPs and GQDs. More interestingly, the photocurrent of GQDs– TiO2 nanocomposites was selectively sensitized by dopamine (DA), and enhanced with the increasing of DA concentration. Further, a new PEC methodology for ultrasensitive determination of DA was developed, which showed linearly enhanced photocurrent by increasing the DA concentration from 0.02 to 105 mM with a detection limit of 6.7 nM (S/N = 3) under optimized conditions. This strategy opens up a new avenue for the application of GQDs-based nanocomposites in the field of PEC sensing and monitoring. ã 2014 Elsevier B.V. All rights reserved.

Keywords: Graphene quantum dot TiO2 nanoparticle Synergistic effect Photoelectrochemical sensor Dopamine

1. Introduction Photoelectrochemical (PEC) measurement, as a promising electroanalytical technique, has received increasing attention and been applied in the fields of biology, medicine, environment

* Corresponding author. Tel.: +86 511 88791800; fax: +86 511 88791708. E-mail address: [email protected] (K. Wang).

monitoring, etc. [1,2]. As a sensitizer, semiconductors have been widely employed in the field of PEC owing to their size-tunable optical and electronic properties [3–5]. However, most of the semiconductors under investigation are wide-band gap transition metal oxides and only active under ultraviolet (UV) light, which limits their potential application in PEC fields because the biologic samples may be damaged by UV light in the process of the experiment [6–8]. Meanwhile, it is well known that semiconductors also suffer from the problems of instability and

http://dx.doi.org/10.1016/j.aca.2014.10.021 0003-2670/ ã 2014 Elsevier B.V. All rights reserved.

Please cite this article in press as: Y. Yan, et al., Visible light photoelectrochemical sensor for ultrasensitive determination of dopamine based on synergistic effect of graphene quantum dots and TiO2 nanoparticles, Anal. Chim. Acta (2014), http://dx.doi.org/10.1016/j.aca.2014.10.021


2

Y. Yan et al. / Analytica Chimica Acta xxx (2014) xxx–xxx

photocorrosion [6]. Hence, in order to overcome these problems, various nanomaterials, especially carbonaceous materials (such as graphene, carbon nanotube and carbon quantum dot) were used to doping with those semiconductors to improve their visible-light PEC properties, and prevent the photocorrosion of them [7,9–13]. For instance, Vietmeyer et al. [13] described a methodology to anchor ZnO NPs onto the functionalized single-walled carbon nanotubes (SWCNTs), and the result indicated that the ZnO/ SWCNTs nanocomposite film could harvest visible light energy quite efficiently than ZnO film. In addition, our group found that graphene-TiO2 nanohybrids possessed enhanced photocurrent under visible light [7], which was nearly 5 times higher than that of pure TiO2 nanocrystals. All these results above indicate that the PEC properties of semiconductors can be improved effectively by appropriate doping with carbonaceous materials. And designing various attempts to prepare novel carbonaceous materials doped semiconductor nanohybrids for PEC sensing under visible-light is still highly desirable and technologically important. Graphene quantum dots (GQDs), as a new kind of quantum dots and rising star in the graphene family, have ignited tremendous interests due to their novel chemical/physical properties [14–18]. In particular, due to their better surface grafting, outstanding low cytotoxicity, high fluorescent activity, stable photoluminescence, and excellent solubility, GQDs have been also successfully applied in optical and imaging analysis fields [14–16,19,20]. Zhao et al. [21] demonstrated that GQD modified pyrolytic graphite electrode coupled with probe single-stranded DNA could be used as a platform to develop different kinds of electrochemical biosensors for the sensitive and selective detection of various target molecules. Recently, based on GQDs’ excellent electrochemical activity, Razmi and Mohammad-Rezaei [22] have further developed a new, simple and low cost glucose biosensor based on glucose oxidase–GQD/CCE. The high level stability, sensitivity and accuracy of the biosensor in determination of glucose confirmed the outstanding function of GQD in electrochemical biosensors. However, due to the high cohesive van der Waals energy [23], GQDs adhere easily to one another, making it difficult to construct the electrochemical biosensing platforms, thus there was still few reports about GQDs research in the electroanalysis field. As an alternative strategy, it comes to be a good idea that forming composites with other nano-materials, which further develop the electrochemical activity of GQDs. Dopamine (DA) is an important neurotransmitter in the mammalian central nervous system and plays a very important role in the renal, hormonal, and cardiovascular systems [24]. Many efforts are focused on the development of simple, rapid, and accurate methods for DA determination. Then, a number of analytical methods have been developed to provide fast but sensitive, selective and reliable quantification in complex biological samples, including capillary electrophoresis [25], liquid chromatography [26], spectrofluorometry [27], electrochemical methods [28] and microchip electrophoresis [29]. Until now, there is still rare attention been paid to the application of the PEC for determination of DA [30]. Thus, it is just the beginning of this fantastic topic for DA determination using PEC measurement. In the present research, on one hand, TiO2 as the carrier material of the GQDs could prevent GQDs assembling and achieve the immobilization of GQDs, which show better performance excellent electrical conductivity of the GQDs. On the other hand, the excellent electrical conductivity of the GQDs in the GQDsnanocomposites also improved the PEC performance of the GQDsTiO2/GCE consequently.

2. Experimental 2.1. Reagents DA was obtained from Sigma–Aldrich. Pitch carbon fibers were directly purchased from Zhenjiang Hengshen Co., Ltd. PBS (0.1 M, pH 7.4) was used as the supporting electrolyte, which was prepared by mixing stock standard solutions of NaH2PO4 and Na2HPO4, and adjusted the pH with 0.1 M H3PO4 or NaOH solution. Other reagents were of analytical grade and used as received without further purification, and all solutions were prepared with twicedistilled water. 2.2. Apparatus Transmission electron microscopy (TEM) image was taken with a JEOL 2100 transmission electron microscopy (JEOL, Japan) operated at 200 kV, and the energy dispersive spectrum (EDS) was obtained from an EDS spectrometer. X-ray diffraction (XRD) analysis was conducted on a Bruker D8 diffractometer with highintensity Cu Ka (l = 1.54 Å). The UV–vis diffuse reflectance spectra (DRS) of the samples were measured by a PerkinElmer Lambda 18 UV–vis-NIR spectrometer (PerkinElmer, USA). The FT-IR spectra of the samples were obtained from FT-IR spectrometer (Nicolet Nexus 470 FT-IR). All the electrochemical and PEC measurements were conducted using CHI660 B electrochemical analyzer (Chen Hua Instruments, Shanghai, China) and recorded by a conventional three-electrode system where a glassy carbon electrode (GCE, 3 mm in diameter) was used as working electrode, a Ag/ AgCl (saturated KCl solution) as reference electrode and platinum wire as counter electrode, respectively. The PEC measurement was performed in 0.1 M PBS at 0 V, and a 250 W Xe lamp (Beijing Trusttech Co., Ltd.) was used as the visible light source with an intensity (passing through a 400 nm UV-cut filter) of 100 mW cm2. 2.3. Preparation of GQDs–TiO2 nanocomposites Preparation of GQDs: according to the previous literature [18], 0.15 g pitch carbon fibers were added into a mixture of concentrated H2SO4 (98%, 30 mL) and HNO3 (65%, 10 mL). The solution was sonicated for 2 h and stirred for 24 h at 100 C. The mixture was cooled and diluted with water. Then, the pH was adjusted to 8.0 with Na2CO3. The final product solution was further dialyzed in a dialysis bag (retained molecular weight: 2000 Da) for 3 days. Preparation of TiO2 NPs [31]: typically, the solution (8.5 mL) containing Si(OC2H5)4, Ti(OC4H9)4 and anhydrous ethanol (volume ratio 1:10:60, respectively) was formed. Then, the mixture of anhydrous ethanol (8.5 mL), 28% NH3 H2O (0.8 mL), 30% H2O2 (8.5 mL) was added slowly to the former solution and further stirred at room temperature. The final mixture was transferred into the 20 mL Teflon-sealed autoclave and heated at 170 C for 30 h. After cooling to room temperature, the precipitation was collected by centrifugation and washed several times with water and anhydrous ethanol. The washed precipitation was dried in vacuum oven at 50 C. Preparation of GQDs–TiO2 nanocomposites: 14 mg GQDs was dispersed into 20 mL water by sonication to give a yellow solution. Then, under sonication, 20 mg TiO2 NPs was added slowly into the above solution. The final mixture was stirred for 30 min. At last, the precipitate was centrifuged and dried in vacuum at 45 C for 24 h to obtain the GQDs–TiO2 nanocomposites.




3

Fig. 1. (A) TEM image of GQDs, (B) size distribution of GQDs, (C) UV–vis (a) and PL spectra (b) of GQDs.

2.4. Fabrication of the modified electrodes

3. Results and discussion

Prior to each modification, the GCE was firstly polished with sand paper followed by 1.0, 0.3, and 0.05 mm alumina slurry, respectively, and then sonicated in water to remove any residues. The procedure for the preparation of the modified electrodes was described as follows: 1.0 mg GQDs–TiO2 nanocomposites were dispersed in 0.5 mL water to make a GQDs–TiO2 nanocomposites homogeneous suspension, then 6 mL of above suspension was cast onto the pretreated GCE surface and dried in air at room temperature to form GQDs–TiO2 nanocomposites modified GCE (GQDs–TiO2/GCE). The GQDs–TiO2/GCE were rinsed with water for several times prior to use. As comparison, 6 mL of 2.0 mg mL1 GQDs and TiO2 suspension were used to fabricate GQDs/GCE and TiO2/GCE, respectively.

3.1. Characterization of the GQDs and GQDs–TiO2 nanocomposites Fig. 1A shows the TEM image of the as-prepared GQDs and the inset was the related TEM image in higher magnification. The distribution of the diameters size of GQDs was evaluated statistically through measuring the diameter of 100 GQDs in the selected TEM images. It was noted that the diameters size of GQDs was distributed mainly between 1.0 and 5.0 nm with an average diameter of about 3 nm (Fig. 1B). Fig. 1C shows the UV–vis absorption of GQDs, a typical absorption peak at ca. 300 nm can be observed. The emission spectra of as-prepared GQDs showed a strong peak at 500 nm. According to previous report [19], it is a relationship between the PL peak and band gap, and the band gap

Fig. 2. (A) TEM image of GQDs–TiO2 nanocomposites, inset: EDS of the GQDs–TiO2 nanocomposites; (B) FT-IR of TiO2 NPs (a), GQDs (b) and GQDs–TiO2 nanocomposites (c); (C) XRD patterns of standard values of JCPDS 21-1272 (a), TiO2 NPs (b) and GQDs–TiO2 nanocomposites (c); (D) plots of (A hv)2 versus hv of TiO2 NPs (a) and GQDs–TiO2 nanocomposites (b).



4


-0.75

-0.8

photocurrent / μA

photocurrent / μA

b -0.6

-0.4

-0.2

c

a

-0.60 -0.45 -0.30 -0.15

0.0 0.00

Time

150

300

of GQDs decreased with the PL peak of GQDs shifts to longer wavelengths. Thus, the band gap of the as-prepared GQDs can be estimated to be smaller than 3.01 eV. Fig. 1C inset shows the optical images of the GQDs with beautiful green emission under UV illumination. By simple physical adsorption, GQDs–TiO2 nanocomposites were also characterized by TEM (Fig. 2A) and EDS (inset of Fig. 2A). It was found that the surface of TiO2 NPs were not observed clearly, the result was maybe small-sized GQDs coated on the surface of TiO2 NPs. And the EDS measurement confirmed the existence of C, O, and Ti elements in the GQDs–TiO2 nanocomposites. Fig. 2B shows the FT-IR of TiO2 NPs, GQDs and GQDs–TiO2 nanocomposites. In the FT-IR analysis of TiO2 NPs (curve a), the peak lower than 900 cm1 was attributed to the stretching vibration of TiO2 NPs [32]. The GQDs (curve b) exhibited free O—H bonding, C— O stretching vibration, C—H vibration, C¼O vibration and aromatic ring stretch C¼C at 3416, 1079, 1353, 1724 and 1579 cm1, respectively. It was noted that the above characteristic peaks of TiO2 NPs and GQDs were also observed in the FT-IR of the GQDs– TiO2 (curve c), which indicated that GQDs–TiO2 were obtained successfully. The XRD patterns of the TiO2 and GQDs–TiO2 nanocomposites are displayed in Fig. 2C. The main peaks can be indexed to the anatase TiO2 phase (JCPDS, card no: 21-1272), which suggest that the anatase TiO2 NPs was exclusively formed during the process of solvothermal treatment (curve b). In addition, GQDs–TiO2 nanocomposites (curve c) showed similar XRD pattern with pure TiO2 NPs, suggesting that the presence of GQDs in the GQDs–TiO2 nanocomposites did not affect the crystalline structure of TiO2 NPs. However, the peak of GQDs was not observed in the GQDs–TiO2 nanocomposites, which might be due to the low amount and relatively low diffraction intensity of GQDs [33].

A

-0.6

where hn is the photo energy, A is the absorption coefficient, C is a constant for the material, and n is 2 for a direct transition or 1/2 for an indirect transition [35]. Furthermore, the band gap of pure TiO2 NPs and GQDs–TiO2 nanocomposites could be calculated according to the Eq. (1). Fig. 2C displays plots the relationship of (A hv)2 versus photon energy, which showed that the band gap of TiO2 NPs was 3.34 eV, whereas the band gap of the GQDs–TiO2 nanocomposites had been reduced to 2.50 eV. This result revealed that the band gap of the GQDs–TiO2 nanocomposites was much narrower than that of TiO2 NPs. 3.2. PEC behaviors of the GQDs–TiO2/GCE The PEC behaviors of different modified electrodes were investigated in 0.1 M PBS (pH 7.4) under visible irradiation. As shown in Fig. 3, the small photocurrent was observed at GQDs/ GCE (curve a), which was due to the GQDs adhere easy to one another. This result was in agreement with previous report [36]. It is clearly that the photocurrent of the GQDs–TiO2/GCE (curve b) was much higher than that of GQDs/GCE (curve a) and TiO2/GCE (curve c), which was due to the synergistic effect of GQDs and TiO2 in the GQDs–TiO2 film, which was ascribed to two aspects as follows: on one hand, TiO2 as the carrier material of the GQDs could prevent GQDs assembling and achieve the immobilization

-1.6

pH=8

pH=6

-0.4 pH=5 -0.2

0.0

pH=9

750

In addition, the Eg value of semiconductor material could be calculated from the following equation [34]: n (1) ðAhV Þ ¼ C hv Eg

pH=7.4 pH=7

600

Fig. 4. Time-based photocurrent response of GQDs–TiO2/GCE in 0.1 M PBS (pH 7.4) under visible irradiation repeated every 100 s.

photocurrent / μA

photocurrent / μA

-0.8

450

Time / sec

Fig. 3. Photocurrent responses of different modified material: GQDs (a), GQDs–TiO2 nanocomposites (b) and TiO2 NPs (c).

B 0.3 V

0.4 V

0.2 V

-1.2

0.1 V -0.8

0V

-0.4

0.0

Time

Time

Fig. 5. Effect of pH values (A) and the applied potential (B) on the photocurrent response of GQDs–TiO2/GCE.




photocurrent / μA

-2.0

5

Table 1 The analytical performances for DA detection by various methods.

b

-1.5

-1.0

a -0.5

0.0

Time Fig. 6. Photocurrent intensity of the GQDs–TiO2/GCE in PBS (pH 7.4) without (a) and with (b) 5 mM DA.

Method

Linear range

Detection limit

PEC [28] ECL [41] ECL [42] Electrochemical Electrochemical Electrochemical Electrochemical Present work

0.4 mM 10 mM 2.5 47.5 mM 0.5 19 mM 5 200 mM 0.05 3 mM 1 100 mM 0.5 110 mM 0.02 105 mM

0.17 mM – 0.1 mM – 15 nM 0.37 mM 0.29 mM 6.7 nM

[6] [43] [44] [45]

Fig. 4 shows the PEC stability of GQDs–TiO2/GCE in 0.1 M PBS (pH 7.4). The photocurrent response of GQDs–TiO2 nanocomposites was repeated for 5 times every 100 s under visible irradiation, and the photocurrent response intensity of GQDs–TiO2 nanocomposites was ca. 0.6 mA and did not show obvious change. The result indicated that the photocurrent intensity of the modified electrode was very stable and beneficial for the construction of PEC sensor.

photocurrent / μA

-12

photocurrent / μA

16

-16

3.3. Optimization of conditions

12 8 4 0

-8

0

40

80

C(DA) / μM

120

160

-4 0

a

c

b

d

e

g

f

Time Fig. 7. Photocurrent intensity of GQDs–TiO2/GCE in the presence of different DA concentrations: 0 mM (a), 0.02 mM (b), 0.1 mM (c), 5 mM (d), 45 mM (e), 105 mM (f) and 145 mM (g); Inset: plot of photocurrent versus CDA on GQDs–TiO2/GCE (n = 3).

of GQDs, which show better performance and excellent electrical conductivity of the GQDs. On the other hand, the excellent electrical conductivity of the GQDs in the GQDs-nanocomposites also improved the PEC performance of the GQDs–TiO2/GCE consequently. Furthermore, it was found that the photocurrent of GQDs–TiO2/GCE was nearly 30-fold and 12-fold enhancement than that of GQDs/GCE and TiO2/GCE, respectively. The photocurrent enhancement of GQDs–TiO2 nanocomposites also indicated a higher separation efficiency of photo-induced electrons and holes [37], which could be attributed to the electronic interaction between GQDs and TiO2 NPs.

The effect of pH on the PEC behavior of GQDs–TiO2/GCE was performed in 0.1 M PBS with the pH range of 5.0 9.0. As shown in Fig. 5A, the PEC intensity increased with the increasing of pH from 5.0 to 7.4, and then dropped at high pH from 7.4 to 9.0. Thus, pH 7.4 was chosen as the optimal pH value in the following experiments. Then, the effect of the applied potential on the photocurrent response of GQDs–TiO2 nanocomposites modified electrode was also investigated with the potential range from 0 to 0.4 V in our previous work. As shown in Fig. 5B, it is obvious that the photocurrent response increased with the potential increasing from 0 to 0.4 V. In order to exclude the interference of other reductive species coexisting in the samples [38], therefore, 0 V was chosen for the PEC detection of DA. 3.4. PEC sensing of DA The PEC behaviors of GQDs–TiO2/GCE in the absence and presence of DA were investigated in 0.1 M PBS (pH 7.4) under visible irradiation. As shown in Fig. 6, the photocurrent of GQDs– TiO2/GCE in 0.1 M PBS (pH 7.4) could be observed obviously (curve a). More interestingly, when 5 mM DA was injected into the above electrolyte, the photocurrent of GQDs–TiO2/GCE was enhanced to 2.8 times (curve b), indicating the addition of DA has an effect on photocurrent of GQDs–TiO2/GCE. Based on this, the PEC sensor for DA determination was constructed. The possible mechanism for

-1.8

A

-1.0

B

photocurrent / μA

photocurrent / μA

-1.5 -1.2 -0.9 -0.6 -0.3

-0.8 -0.6 -0.4 -0.2 0.0

0.0 nk Bl a

UA

AA

+

Na

+

K

e n AP lucos ophe DA g in tam e c a

0

4

8

12

16

20

Time / day

Fig. 8. (A) Photocurrent intensity of the GQDs–TiO2/GCE in the absence and presence of interfering agents; (B) stability of the PEC sensor stored at ambient conditions over 20 days in 0.1 M PBS with addition of 5 mM DA.



6


Table 2 Determination of DA in human plasma samples (n = 3). Sample number

Detected (mM)

Added (mM)

Total found (mM)

Recovery (%)

RSD (%)

1 2 3

1.2 1.5 2.3

3.0 3.0 3.0

4.4 4.3 5.4

104.7 95.6 98.1

1.4 2.1 1.7

the determination of DA via the PEC is as follows: upon irradiation with visible light, TiO2 NPs were excited and the electrons transformed from valance band to conduction band, and generated electron-hole pairs, the electrons then transferred from TiO2 NPs to GQDs, and at last to the GCE. Due to the GQDs in the GQDs–TiO2 nanocomposites had excellent electrical conductivity, which consequently improved the photoelectrochemical performance of the GQDs–TiO2/GCE. At the same time, when DA existed in the solution, the oxidation product of DA by the photogenerated holes could effectively avoid electron-hole recombination effectively, thus increased the photocurrent intensity [39,40]. Based on the results above, we explored the quantitative range of photoelectrochemical detection for DA. Fig. 7 presents the effect of different concentrations of DA on the photocurrent response of GQDs–TiO2/GCE. It is clearly that the photocurrents increased with the increasing of DA concentration, and the standard calibration curve for DA detection was illustrated in the inset of Fig. 7. The photocurrents displayed a linear range of 0.02 105 mM with a correlation coefficient of 0.998. The detection limit was 6.7 nM (S/N = 3). Compared with the previous reports for biosensor toward DA [6,28,41–45], this work shows wider liner response with lower detection limit (Table 1). Especially, compared with the PEC for DA biosensing based on the CdTe [30], the sensitivity of the as-prepared biosensor was 10 times higher. 3.5. Selectivity and stability A number of oxidizable species usually co-exist with DA in human plasma, such as AA, AP, UA, and other compounds including glucose, acetaminophen, inorganic ions and so on. The results are summarized in Fig. 8A, it is clear that the species described above have no obvious effect on the DA detection, indicating that GQDs– TiO2/GCE is highly specific to DA even in the presence of several interfering species. The long-term stability of the PEC biosensor was investigated by measuring the photocurrent responses upon 5 mM DA in 0.1 M PBS, as shown in Fig. 8B. The response upon 5 mM DA in 0.1 M PBS of the GQDs–TiO2/GCE remained 91% of the initial response after 20 days, indicating the reliability of PEC biosensor. 3.6. Real sample analysis In order to investigate the possible application of the PEC biosensor in clinical analysis, the GQDs–TiO2 nanocomposites modified electrode has been employed to the detection of DA in human plasma utilizing standard addition method without sample pretreatment. The concentration of DA was determined to be 1.7 mM, and the recovery tests were performed by adding 3 mM DA to the sample. The corresponding results are listed in Table 2, and the average recoveries were ranged between 95.6% and 104.7%, ascertaining the practical application of the proposed DA biosensor in clinical analysis. 4. Conclusions In this paper, a novel carbonaceous materials doped semiconductor nanocomposites GQDs–TiO2 were successfully fabricated.

Compared with GQDs and TiO2 NPs, the GQDs–TiO2 nanocomposites could largely enhance the photocurrent due to the synergistic amplification of TiO2 NPs and GQDs. In addition, the PEC intensity of GQDs–TiO2/GCE could be selectively sensitized by DA, based on which a PEC biosensor for DA was constructed. The PEC biosensor showed acceptable accuracy and precision for the detection of DA with a wide linear range from 0.02 mM to 105 mM and a low detection limit of 6.7 nM (S/N = 3). All these discoveries showed that GQDs–TiO2 nanocomposites exhibited great potential in PEC applications. Since the proposed GQDs–TiO2 nanocomposites were highly photosensitive, they could provide a new platform for designing a variety of bioelectrochemical devices. Acknowledgements The present work was supported by the National Natural Science Foundation of China (Nos. 21175061, 21375050, and 21405063), Key Laboratory of Modern Agriculture Equipment and Technology (No. NZ201109), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (No. PAPD-2014-37), the Natural Science Foundation of Jiangsu province (No. BK20130481). References [1] X.R. Zhang, S.G. Li, X. Jin, S.S. Zhang, A new photoelectrochemical aptasensor for the detection of thrombin based on functionalized graphene and CdSe nanoparticles multilayers, Chem. Commun. 47 (2011) 4929–4931. [2] Y.T. Long, C. Kong, D.W. Li, Y. Li, S. Chowdhury, H. Tian, Ultrasensitive determination of cysteine based on the photocurrent of nafion-functionalized CdS–MV quantum dots on an ITO electrode, Small 7 (2011) 1624–1628. [3] W.W. Zhao, Z.Y. Ma, P.P. Yu, X.Y. Dong, J.J. Xu, H.Y. Chen, Highly sensitive photoelectrochemical immunoassay with enhanced amplification using horseradish peroxidase induced biocatalytic precipitation on a CdS quantum dots multilayer electrode, Anal. Chem. 84 (2012) 917–923. [4] W.W. Tu, J.P. Lei, P. Wang, H.X. Ju, Photoelectrochemistry of free-baseporphyrin-functionalized zinc oxide nanoparticles and their applications in biosensing, Chem. Eur. J. 17 (2011) 9440–9447. [5] W.W. Tu, Y.T. Dong, J.P. Lei, H.X. Ju, Low-potential photoelectrochemical biosensing using porphyrin-functionalized TiO2 nanoparticles, Anal. Chem. 82 (2010) 8711–8716. [6] H.J. Chen, L.Z. Wang, Nanostructure sensitization of transition metal oxides for visible-light photocatalysis, Beilstein J. Nanotechnol. 5 (2014) 696–710. [7] K. Wang, J. Wu, Q. Liu, Y.C. Jin, J.J. Yan, J.R. Cai, Ultrasensitive photoelectrochemical sensing of nicotinamide adenine dinucleotide based on graphene-TiO2 nanohybrids under visible irradiation, Anal. Chim. Acta 745 (2012) 131–136. [8] Y. Li, Y. Hu, Y. Zhao, G. Shi, L. Deng, Y. Hou, L. Qu, An electrochemical avenue to green-luminescent graphene quantum dots as potential electron-acceptors for photovoltaics, Adv. Mater. 23 (2011) 776–780. [9] B. Sun, K. Zhang, L.J. Chen, L.T. Guo, S.Y. Ai, A novel photoelectrochemical sensor based on PPIX-functionalized WO3–rGO nanohybrid-decorated ITO electrode for detecting cysteine, Biosens. Bioelectron. 44 (2013) 48–51. [10] X.M. Zhao, S.W. Zhou, L.P. Jiang, W.H. Hou, Q.M. Shen, J.J. Zhu, Graphene–CdS nanocomposites: facile one-step synthesis and enhanced photoelectrochemical cytosensing, Chem. Eur. J. 18 (2012) 4974–4981. [11] T.T. Duong, Q.D. Nguyen, S.K. Hong, D. Kim, S.G. Yoon, T.H. Pham, Enhanced photoelectrochemical activity of the TiO2/ITO nanocomposites grown onto single-walled carbon nanotubes at a low temperature by nanocluster deposition, Adv. Mater. 23 (2011) 5557–5562. [12] X. Zhang, F. Wang, H. Huang, H.T. Li, X. Han, Y. Liu, Z.H. Kang, Carbon quantum dot sensitized TiO2 nanotube arrays for photoelectrochemical hydrogen generation under visible light, Nanoscale 5 (2013) 2274–2278. [13] F. Vietmeyer, B. Seger, P.V. Kamat, Anchoring ZnO particles on functionalized single wall carbon nanotubes. Excited state interactions and charge collection, Adv. Mater. 19 (2007) 2935–2940. [14] Y.Q. Dong, G.L. Li, N.N. Zhou, R.X. Wang, Y.W. Chi, G.N. Chen, Graphene quantum dot as a green and facile sensor for free chlorine in drinking water, Anal. Chem. 84 (2012) 8378–8382. [15] S.J. Zhu, J.H. Zhang, C.Y. Qiao, S.J. Tang, Y.F. Li, W.J. Yuan, B. Li, L. Tian, F. Liu, R. Hu, H.N. Gao, H.T. Wei, H. Zhang, H.C. Sun, B. Yang, Strongly green-photoluminescent graphene quantum dots for bioimaging applications, Chem. Commun. 47 (2011) 6858–6860. [16] L.A. Ponomarenko, F. Schedin, M.I. Katsnelson, R. Yang, E.W. Hill, K.S. Novoselov, A.K. Geim, Chaotic dirac billiard in graphene quantum dots, Science 320 (2008) 356–358. [17] L.S. Li, X. Yan, Colloidal graphene quantum dots, J. Phys. Chem. Lett. 1 (2010) 2572–2576.



Y. Yan et al. / Analytica Chimica Acta xxx (2014) xxx–xxx [18] J. Peng, W. Gao, B.K. Gupta, Z. Liu, R. Romero-Aburto, L.H. Ge, L. Song, L.B. Alemany, X.B. Zhan, G.H. Gao, S.A. Vithayathil, B.A. Kaipparettu, A.A. Marti, T. Hayashi, J.J. Zhu, P.M. Ajayan, Graphene quantum dots derived from carbon fibers, Nano Lett. 12 (2012) 844–849. [19] D.Y. Pan, J.C. Zhang, Z. Li, M.H. Wu, Hydrothermal route for cutting graphene sheets into blue-luminescent graphene quantum dots, Adv. Mater. 22 (2010) 734–738. [20] J.H. Shen, Y.H. Zhu, C. Chen, X.L. Yang, C.Z. Li, Facile preparation and upconversion luminescence of grapheme quantum dots, Chem. Commun. 47 (2011) 2580–2582. [21] J. Zhao, G.F. Chen, L. Zhu, G.X. Li, Graphene quantum dots-based platform for the fabrication of electrochemical biosensors, Electrochem. Commun. 13 (2011) 31–33. [22] H. Razmin, R. Mohammad-Rezaei, Graphene quantum dots as a new substrate for immobilization and direct electrochemistry of glucose oxidase: application to sensitive glucose determination, Biosens. Bioelectron. 41 (2013) 498–504. [23] Y.C. Si, E.T. Samulski, Synthesis of water soluble graphene, Nano Lett. 8 (2008) 1679–1682. [24] S.J. Clarke, C.A. Hollmann, Z. Zhang, D. Suffern, S.E. Bradforth, N.M. Dimitrijevic, W.G. Minarik, J.L. Nadeau, Photophysics of dopamine-modified quantum dots and effects on biological systems, Nat. Mater. 5 (2006) 409–417. [25] Y.M. Liu, C.Q. Wang, H.B. Mu, J.T. Cao, Y.L. Zheng, Determination of catecholamines by CE with direct chemiluminescence detection, Electrophoresis 28 (2007) 1937–1941. [26] P. Song, O.S. Mabrouk, N.D. Hershey, R.T. Kennedy, In vivo neurochemical monitoring using benzoyl chloride derivatization and liquid chromatography– mass spectrometry, Anal. Chem. 84 (2012) 412–419. [27] H. Huang, Y. Gao, F.P. Shi, G.N. Wang, S.M. Shah, X.G. Su, Determination of catecholamine in human serum by a fluorescent quenching method based on a water-soluble fluorescent conjugated polymer-enzyme hybrid system, Analyst 137 (2012) 1481–1486. [28] Y. Mao, Y. Bao, S.Y. Gan, F.H. Li, L. Niu, Electrochemical sensor for dopamine based on a novel graphene-molecular imprinted polymers composite recognition element, Biosens. Bioelectron. 28 (2011) 291–297. [29] C.M. Kang, S. Joo, J.H. Bae, Y.R. Kirn, Y. Kim, T.D. Chung, In-channel electrochemicaldetection in the middle of microchannel under high electric field, Anal. Chem. 84 (2012) 901–907. [30] Q. Hao, P. Wang, X.Y. Ma, M.Q. Su, J.P. Lei, H.X. Ju, Charge recombination suppression-based photoelectrochemical strategy for detection of dopamine, Electrochem. Commun. 21 (2012) 39–41. [31] K. Wang, H.N. Li, J. Wu, C. Ju, J.J. Yan, Q. Liu, B.J. Qiu, TiO2-decorated graphene nanohybrids for fabricating an amperometric acetylcholinesterase biosensor, Analyst 136 (2011) 3349–3354.

7

[32] L.H. Lee, W.C. Chen, High-refractive-index thin films prepared from trialkoxysilane-capped poly(methyl methacrylate)-titania materials, Chem. Mater. 13 (2001) 1137–1142. [33] H. Zhang, X.J. Lv, Y.M. Li, Y. Wang, J.H. Li, P25-graphene composite as a high performance photocatalyst, ACS nano 4 (2010) 380–386. [34] W.Q. Fan, Q.H. Lai, Q.H. Zhang, Y. Wang, Nanocomposites of TiO2 and reduced graphene oxide as efficient photocatalysts for hydrogen evolution, J. Phys. Chem. C 115 (2011) 10694–10701. [35] J. Tauc, A. Menth, States in the gap, J. Non-Cryst. Solids 8–10 (1972) 569–585. [36] J.H. Shen, Y.H. Zhu, X.L. Yang, J. Zong, J.M. Zhang, C.Z. Li, One-pot hydrothermal synthesis of graphene quantum dots surface-passivated by polyethylene glycol and their photoelectric conversion under near-infrared light, New. J. Chem. 36 (2012) 97–101. [37] S.J. Zhuo, M.W. Shao, S.T. Lee, Upconversion and downconversion fluorescent graphene quantum dots: ultrasonic preparation and photocatalysis, ACS Nano 6 (2012) 1059–1064. [38] P.P. Wang, W.J. Dai, L. Ge, M. Yan, S.G. Ge, J.H. Yu, Visible light photoelectrochemical sensor based on Au nanoparticles and molecularly imprinted poly(o-phenylenediamine)-modified TiO2 nanotubes for specific and sensitive detection, Analyst 138 (2013) 939–945. [39] X.M. Zhao, S.W. Zhou, Q.M. Shen, L.P. Jiang, J.J. Zhu, Fabrication of glutathione photoelectrochemical biosensor using graphene–CdS nanocomposites, Analyst 137 (2012) 3697–3703. [40] L.L. Xue, L.H. Guo, B. Qiu, Z.Y. Lin, G.N. Chen, Mechanism for inhibition of Ru (bpy)32+/DBAE electrochemiluminescence system by dopamine, Electrochem. Commun. 11 (2009) 1579–1582. [41] L.L. Li, H.Y. Liu, Y.Y. Shen, J.R. Zhang, J.J. Zhu, Electrogenerated chemiluminescence of Au nanoclusters for the detection of dopamine, Anal. Chem. 83 (2011) 661–685. [42] R. Cui, Y.P. Gu, L. Bao, J.Y. Zhao, B.P. Qi, Z.L. Zhang, Z.X. Xie, D.W. Pang, Nearinfrared electrogenerated chemiluminescence of ultrasmall Ag2Se quantum dots for the detection of dopamine, Anal. Chem. 84 (2012) 8932–8935. [43] A. Kutluay, M. Aslanoglu, An electrochemical sensor prepared by sonochemical one-pot synthesis of multi-walled carbon nanotube-supported cobalt nanoparticles for the simultaneous determination of paracetamol and dopamine, Anal. Chim. Acta 839 (2014) 59–66. [44] P. Rattanarat, W. Dungchai, W. Siangproh, O. Chailapakul, C.S. Henry, Sodium dodecyl sulfate-modified electrochemical paper-based analytical device for determination of dopamine levels in biological samples, Anal. Chim. Acta 744 (2012) 1–7. [45] Q.W. Lian, Z.F. He, Q. He, A. Luo, K.W. Yan, D.X. Zhang, X.Q. Lu, X.B. Zhou, Simultaneous determination of ascorbic acid, dopamine and uric acid based on tryptophan functionalized graphene, Anal. Chim. Acta 823 (2014) 32–39.


A visible-light-driven composite photocatalyst of TiO2 nanotube arrays and graphene quantum dots.

boron doped graphene composite.

Engineered photoelectrochemical platform for rational global antioxidant capacity evaluation based on ultrasensitive sulfonated graphene-TiO2 nanohybrid.

Photovoltammetric behavior and photoelectrochemical determination of p-phenylenediamine on CdS quantum dots and graphene hybrid film.

Quantum dots sensitized titanium dioxide decorated reduced graphene oxide for visible light excited photoelectrochemical biosensing at a low potential.

Graphitic Carbon Nitride Sensitized with CdS Quantum Dots for Visible-Light-Driven Photoelectrochemical Aptasensing of Tetracycline.

Graphene quantum dots enhanced electrochemiluminescence of cadmium sulfide nanocrystals for ultrasensitive determination of pentachlorophenol.

Using graphene-based plasmonic nanocomposites to quench energy from quantum dots for signal-on photoelectrochemical aptasensing.

Improving photocatalytic performance of ZnO via synergistic effects of Ag nanoparticles and graphene quantum dots.

Enhance photoelectrochemical hydrogen-generation activity and stability of TiO2 nanorod arrays sensitized by PbS and CdS quantum dots under UV-visible light.

Highly luminescent S, N co-doped graphene quantum dots with broad visible absorption bands for visible light photocatalysts.

Highly Visible Light Responsive, Narrow Band gap TiO2 Nanoparticles Modified by Elemental Red Phosphorus for Photocatalysis and Photoelectrochemical Applications.

Photodynamic antibacterial effect of graphene quantum dots.

Visible-Light-Activated Bactericidal Functions of Carbon "Quantum" Dots.

Graphene-palladium nanowires based electrochemical sensor using ZnFe2O4-graphene quantum dots as an effective peroxidase mimic.

Photoelectrochemical biosensor using enzyme-catalyzed in situ propagation of CdS quantum dots on graphene oxide.

Turn-on fluorescent dopamine sensing based on in situ formation of visible light emitting polydopamine nanoparticles.

Graphene-CdS quantum dots-polyoxometalate composite films for efficient photoelectrochemical water splitting and pollutant degradation.

Quantum dots: electrochemiluminescent and photoelectrochemical bioanalysis.

Electrochemical sensor for ultrasensitive determination of isoquercitrin and baicalin based on DM-β-cyclodextrin functionalized graphene nanosheets.

SnO(2) quantum dots-reduced graphene oxide composite for enzyme-free ultrasensitive electrochemical detection of urea.

Enhanced photoelectrochemical properties of nanocrystalline TiO2 electrode by surface sensitization with CuxO quantum dots.

ZnS colloidal nanocrystal quantum dots.

Fabrication of graphene oxide decorated with nitrogen-doped graphene quantum dots and its enhanced electrochemiluminescence for ultrasensitive detection of pentachlorophenol.