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Polydopamine interconnected graphene quantum dots and gold nanoparticles for enzymeless H2O2 detection
Journal of Electroanalytical Chemistry 796 (2017) 75–81
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Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Polydopamine interconnected graphene quantum dots and gold nanoparticles for enzymeless H2O2 detection
MARK
Ye Zhua, Shun Lub, A. Gowri Manoharia, Xiuxiu Donga, Feng Chena, Wei Xua, Zengliang Shia, Chunxiang Xua,⁎ a b
State Key Laboratory of Bioelectronics, School of Biological Science & Medical Engineering, Southeast University, Nanjing 210096, China Faculty of Materials and Energy, Southwest University, Chongqing 400715, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Peroxidase-like activity Au-PDA-GQD nanocomposites Non-enzymatic H2O2 biosensors
Graphene quantum dots (GQDs) offer new opportunities for the applications of electrochemical sensing because of their intrinsic peroxidase-like activity. To develop a sensitive electrochemical sensor with better stability and reproducibility, polydopamine (PDA) has been employed as an adhesive agent to integrate the GQDs and gold nanoparticles (Au NPs) tightly through a green, facile and rapid method. The synergistic effect of Au-PDA-GQD nanocomposite presented an excellent electrocatalytic activity towards the reduction of hydrogen peroxide (H2O2). Hence, the H2O2 biosensor based on Au-PDA-GQD exhibited better sensing performances, such as wide linear range (0.1–40 μM and 40–20,000 μM), low detection limit (5.8 nM) and fast response time (< 2 s) at low applied potential (− 0.1 V). Also, it presented a wonderful stability, reproducibility (RSD = 1.01%) and antiinterference. These results can be demonstrated that a novel Au-PDA-GQD is one of the most promising nanocomposites for the fabrication of non-enzymatic H2O2 biosensors.
1. Introduction The horseradish peroxidase (HRP) enzyme has been immobilized on sonogel-carbon electrode composite and used in the application of third generation H2O2 biosensor [1]. This kind of enzyme-based H2O2 biosensor has been received considerable attention because of their remarkable selectivity [2] and they can be applied in various fields such as biology, environmental protection, pharmaceutics, food security and chemistry [3]. However, some of the terrible disadvantages of natural enzymes limit their applications [4]. Therefore, the development of an artificial non-enzymatic H2O2 sensor is an essential one. With rapid developments in materials science and technology, previous metal nanoparticles [5], metal hexacyanoferrates [6] and carbon nanomaterials [7] have been widely employed to replace the enzyme-based H2O2 biosensors. Among various nanomaterials, carbon (like carbon nanotubes and graphene) based electrochemical sensors are considered as promising candidates because of their superior electron transfer kinetics, better chemical stability and biocompatibility and low cost [8]. Graphene quantum dots (GQDs), as a kind of zero dimensional materials have been obtained from graphene sheets or carbon dots [9], which caused an abundant attention because of their excellent physical and chemical properties, including low cytotoxicity, biocompatibility,
⁎
Corresponding author. E-mail address: [email protected] (C. Xu).
http://dx.doi.org/10.1016/j.jelechem.2017.04.017 Received 16 January 2017; Received in revised form 10 April 2017; Accepted 11 April 2017 Available online 12 April 2017 1572-6657/ © 2017 Published by Elsevier B.V.
tunable luminescence and anti-photobleaching [10]. Meanwhile, GQDs possess a higher peroxidase-like activity in comparison with that of micrometer-sized graphene oxide [11] and they have a considerable scope for the applications in electrochemical sensing. From the recent reports, it can be identified that both covalently assembled GQDs on Au electrode [11] and drop-casted Au-N-GQD hybrid on glass carbon electrode [12] showed high sensitivity and selectivity in electrochemical detection of H2O2. An integration of carbon-based materials and metal nanoparticles can be exhibited synergistic effects in electrocatalytic performance thus leading to enhanced catalytic activity [13]. It demonstrated a non-enzymatic composite for H2O2 electrocatalysis with low cost, easy preparation and excellent catalytic activity. Even though, some possible problems can be occurred in the nanocomposites including irreversible aggregation of GQDs [14], weak adhesion between GQDs and metal nanoparticles, poor operational stability and reproducibility [15]. It is still a challenge for solving the above problems simultaneously and improving the performance of nonenzymatic biosensor with broad linear response, low detection limit, good stability and reproducibility. In addition, a bioinspired adhesive with good conductivity and stability is an indispensable one. Polydopamine (PDA) as a musselinspired polymer has been prepared via simple self-polymerization of dopamine in a weak alkaline pH media [16]. The self-polymerization is
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a very mild reaction without any complicated instruments or harsh reaction conditions [17,18]. Interestingly, PDA is considered as a novel and an amazing material which adhered easily on any smooth or rough substrates due to its high concentrations of amino and catechol functional groups [18,19]. The PDA layer also offers the advantages in one-step surface functionalization and allows an introduction of new paradigm in the field of surface modification [17]. In this work, PDA has been employed as an adhesive agent to integrate the GQDs and Au NPs tightly and stably. The PDA provides a multifunctional platform and it is not only used for an immobilization of GQDs but also for absorption of H2O2 molecule. The Au-PDA-GQD nanocomposites have been successfully prepared by using a green, facile and rapid method. The synergistic effect of Au-PDA-GQD can be possessed an excellent peroxidase-like activity. Compared with other GQD-based H2O2 sensors, the non-enzymatic H2O2 biosensor based on Au-PDA-GQD nanocomposites has superior performances including lower detection limit, wider linear range, better stability and reproducibility, higher sensitivity and selectivity at lower applied potential.
Au-PDA/ITO electrodes were also prepared by using a similar procedure. The morphologies, sizes and structures of samples were characterized by the scanning electron microscopy (SEM, Hitachi S-4800) which equipped with X-ray energy dispersive spectrometer (EDS, Oxford XMax 50) and transmission electron microscopy (TEM, JEM-2100). Fourier transform infra-red analysis (FTIR, Nicolet5700) was carried out to provide the information about functional groups of PDA-GQD. UV–visible spectrophotometer (Shimadzu UV-2450, Japan) was used to analyze the absorption properties of the samples. Electrochemical measurements were performed by using a CHI630D Electrochemical Workstation with 0.1 M of oxygen free phosphate buffer solution (PBS) at room temperature. A conventional three-electrode system was equipped with the working electrode of Au-PDA-GQD modified ITO, reference electrode of saturated calomel (SCE) and Pt wire as counter electrode.
2. Experimental details
3.1. Characterization of Au-PDA-GQD nanocomposites
All the chemicals have an analytical grade and they were used in the experiment without any further purification process. All the solutions were prepared with deionized water. The graphite oxide was synthesized by using a famous modified Hummers method as described in our previous work [20]. GQDs were prepared from GO according to a previously reported methods with minor modifications [21]. Briefly, the mixture of 40 mL of H2O2 (30%) and 10 mL of ammonia (25%–28%) solutions were added into 10 mL of as-prepared GO suspension under vigorous stirring at 80 °C for 8 h. The brown mixture was gradually turned into colorless, which indicates a formation of smaller sized graphenes. Then, the obtained colorless solution was heated at 90 °C in an autoclave to remove the unreacted H2O2 and ammonia. Upon cooling to room temperature, the solution was ultrafiltered through mixed cellulose membrane (PES membrane, Millipore) with hole diameter of 0.22 μm. Finally, the obtained GQDs solution was further dialyzed by using a dialysis tubing membrane with molecular mass cut-off of 3500 Da for 3 days to remove the remaining tiny fragments. The preparation of modified electrodes is schematically illustrated in Scheme 1. Initially, substrate of indium tin oxide (ITO) was washed with acetone, ethanol and distilled water under ultrasonification. After that, Au NPs were deposited on a well cleaned ITO (with active area of 1 cm2) substrate by using an ion beam sputtering system. Concomitantly, 10 mg of dopamine was dissolved in 10 mL of 10 mM Tris-HCl solution with pH of 8.5. The obtained Au/ITO electrodes were then simply dipped into dopamine solution for 1 h thus forming PDA thin film. The coated surfaces were rinsed with water and dried at N2 atmosphere. Finally, an aqueous solution of 10 μL of GQDs was dropcasted on the surfaces of Au-PDA/ITO electrodes by using a micropipette and dried at room temperature. For comparison, the Au/ITO and
Fig. 1(a) represents the SEM image of ion sputtered Au NPs with average diameters of 35 nm. From Fig. 1(b), it can be suggested that well dispersing nature of GQDs in aqueous solution with narrow sized distribution (2–3 nm). The inset of Fig. 1(d) presents a high resolution TEM (HRTEM) image of GQDs, which has a good crystallinity with lattice diameter of 0.21 nm. An existence of (102) diffraction plane of sp2 graphitic carbon can be indicated that the GQDs have quite similar crystallinity with graphene [22]. Moreover, a thin film is obviously noticed on the surface of sputtered Au NPs as shown in Fig. 1(c) which denotes a successful formation of PDA thin film layer through selfpolymerization of dopamine. Meanwhile, the Fig. 1(d) yields no significant aggregation of GQDs, which demonstrates a unique biocompatibility and high water solubility of PDA thus leading to well absorption of GQDs on the surface of PDA film. In addition, FTIR spectroscopic analysis has been carried out to provide the information about functional groups of PDA-GQD. A broad range of absorption between 3500 and 3100 cm− 1 is assigned to NeH/OeH stretching vibrations of PDA [23]. The observed peaks at around 1630, 1551 and 1296 cm− 1 indicate an existence of C]C, C]N and CeOH, respectively [24,25]. After the coating of GQDs, the peak intensity of C-OH at around 1330 cm− 1 is increased obviously while the characteristic peaks of PDA are almost disappeared. UV–visible absorption spectra were further used to study the absorption properties of GQDs, PDA and Au NPs as shown in Fig. 1(f). A broad range of absorption is observed in GQDs due to π-π⁎ electron transition of aromatic sp2 domains. An obvious absorption peak at 280 nm in the black curve can be attributed to π-π⁎ electron transition of PDA, and peak at about 550 nm in the blue curve is assigned to the surface plasmon absorption of Au NPs. These results can be provided a clear evidence for the formation of Au NPsPDA-GQDs nanocomposites.
3. Results and discussion
Scheme 1. A schematic illustration for the preparation of Au-PDA-GQD nanocomposites.
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Fig. 1. (a) SEM image of Au NPs; (b) TEM image of GQDs and inset shows the diameter distribution of GQDs; (c) TEM image of Au-PDA nanocomposite; (d) TEM image of PDA-GQD nanocomposite and inset shows HRTEM image of GQDs; (e) FTIR spectra of PDA (red line) and PDA-GQD nanocomposite (black line); (f) UV–visible absorption spectra of PDA (black line), GQDs (red line) and Au (blue line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
reduction ability and fast electron transfer rate of the composites. When rising the scan rate from 10 to 300 mV s− 1, a peak current is increased linearly with respect to scan rate (ν), which possesses a classical surface-controlled electrode process [26]. Fig. 3 represents comparison of the CV response for different electrodes with and without H2O2 in 0.1 M PBS solution and pH of 7.0 at the scan rate of 50 mV s− 1. An electrocatalytic activity of bare blank ITO electrode is fairly low
3.2. Electrochemical performance to H2O2 Fig. 2(a) shows the typical cyclic voltammetry (CVs) of Au-PDAGQD modified ITO electrode in 0.1 M PBS with various scan rates ranging between − 0.8 to + 0.2 V versus SCE. From the figure, it can be observed that the potential of cathodic peaks are almost unchanged with increase in scan rate, which indicates well electrochemical
Fig. 2. (a) CVs of Au-PDA-GQD/ITO electrode in 0.1 M PBS with different scan rates (a–i: 10, 20, 40, 80, 120, 160, 200, 250, 300 mV s− 1). (b) A linear dependent plot of peak current versus scan rate.
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Fig. 3. CVs of (a) bare ITO electrode, (b) Au/ITO, (c) Au-PDA/ITO and (d) Au-PDA-GQD/ITO electrodes in absence (black curves) and presence (red curves) of 2 mM H2O2 in 0.1 M PBS solution at the scan rate of 50 mV s− 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
electrochemical potential of a LUMO of H2O2, due to GQDs with ntype incorporation of charge transfer process which accelerates a transfer of electron from GQDs into H2O2 [29]. This reaction equation can be expressed as follows:
with respect to H2O2. An onset potential of a generated cathodic current is observed at − 0.2 V for Au/ITO electrode as shown in Fig. 3(b), which denotes an enhanced electrocatalytic activities of Au NPs on ITO electrode in H2O2 reduction [12]. As introducing a thin film layer of PDA on Au/ITO electrode, the cathodic peak current becomes higher at 0.1 V when compared to that of without PDA and shown in Fig. 3(c). It indicates the importance of PDA in an elevation of catalysis via absorption of more H2O2 molecules [19]. In addition, a response current is increased significantly when decorating the GQDs on AuPDA/ITO electrode, which is an interesting phenomenon. The response current is 7 times higher than that of those obtained from both Au/ITO and Au-PDA/ITO electrodes after an addition of same concentration of H2O2. Furthermore, the onset reduction potential of H2O2 on Au-PDAGQD/ITO electrode is shifted to − 0.1 V. It is important to note that the Au-PDA-GQD nanocomposites can be involved in decrement of overvoltage for H2O2 reduction efficiently because common electroactive species can jam the measurements at higher overpotential [27]. These results can be suggested that the Au-PDA-GQD modified ITO electrode has a superior electrocatalytic performances and synergistic effect towards the reduction of H2O2, and it can be considered as a promising platform for the non-enzymatic H2O2 sensor.
GQDs → GQDs+ +e−
(1)
H2 O2 +2e− → 2OH−
(2)
Secondly, PDA is a conductive polymer which provides an excellent interfacial contact between GQDs and Au NPs for a fast electron transfer. Herein, PDA thin film prevented an aggregation of GQDs, thus resulting in large surface for H2O2 interaction. Also, it offered considerable active sites for an attachment of H2O2 molecule owing to its high concentrations of amino and catechol functional groups [18]. Thirdly, Au NPs exhibited an additional peroxidase-like activity and electrocatalytic activity in the reduction of H2O2 [12]. Due to their high surface area to volume ratio, Au NPs provided an extensive platform for the immobilization of GQDs and PDA film. Au NPs also enhanced a transfer of electron between electrode and detection molecules effectively due to their good conducting nature. These features can be contributed a synergistic enhancement in electrocatalytic performances of Au-PDA-GQD nanocomposites for H2O2 reduction. It is expected that an engineered catalytic GQD-based material can be act as a potential electrochemical sensing material for the detection of H2O2 in biological system.
3.3. Electrocatalytic mechanism of H2O2 reduction An enhanced electrocatalytic performance of Au-PDA-GQD modified ITO electrode can be attributed to three possible aspects. Firstly, a higher peroxidase-like activity of GQDs plays a key role in the detection of H2O2 [11,28]. Moreover, the H2O2 reduction is mainly based on previously suggested mechanism in which an electron transfer occurred from top of the valence band of GQDs into the lowest unoccupied molecular orbital (LUMO) of H2O2 [29,30]. In addition, PDA denoted its lone-pair electrons of amino groups to GQDs when GQDs are absorbed on the surface of PDA, which improve the electron density and mobility in GQDs. Thus, increase in Fermi level as well as
3.4. Amperometric response to H2O2 A rapid and an accurate H2O2 detection capability of Au-PDA-GQD modified ITO electrode have been evaluated by utilizing an amperometric response with stepwise injection of different concentrations of H2O2. Fig. 4 depicts the amperometric I-t curves of Au-PDA-GQD/ITO electrode with successive addition of various concentrations of H2O2 into 0.1 M PBS solution at − 0.1 V under stirring condition. When adding the H2O2, the response current is increased in a stepwise 78
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ITO electrode is more sensitive to H2O2 at lower concentrations. In H2O2 sensor, an estimated sensitivity value for first and second linear region is 48.6 and 8.08 μA mM− 1 cm− 2, respectively. The limit of detection (LOD) of H2O2 sensor was estimated to be 5.8 nM (S/N = 3) and it is much lower than that of previously reported GQD-based biosensors. The comparison of variable parameters for Au-PDA-GQD/ITO electrode with other reported non-enzymatic H2O2 sensors based on GQDs is obtained in Table 1. Obviously, the yielded LOD and response time of this work are superior when compared to other reported values as listed. Moreover, the proposed sensor is displayed a wider linear range in comparison with that of other sensors whereas an applied potential is much lower than that of other reported values. An excellent electrochemical performance is mainly attributed to synergistic effect of Au-PDA-GQD nanocomposites as mentioned above. 3.5. Reproducibility, stability and anti-interference property Fig. 4. Amperometric responses of Au-PDA-GQD/ITO electrode for the successive addition of H2O2 (applied potential: − 0.1 V). Inset shows the magnified amperometric response to a lower concentration of H2O2.
It is very important to test the reproducibility and storage stability of electrochemical sensors. Fig. 6 displays the relative standard deviation (RSD) for response currents of Au-PDA-GQD/ITO electrodes. The RSD for five measurements of same electrode and five independent electrodes was evaluated to be 1.01% and 1.14% under the same condition which denotes an excellent reproducibility of H2O2 biosensor. In addition, the Au-PDA-GQD/ITO electrode was stored at 4 °C for two weeks and still it retained 95% of its initial current which clearly demonstrates a good stability of H2O2 sensor. Moreover, some possible physiological-level electroactive species of 300 μM ascorbic acid (AA), uric acid (UA), glucose (GLU) and 0.9% NaCl have also been interfered in the detection of H2O2 and investigated by using an amperometric experiment at the potential of − 0.1 V. There is an occurrence of distinct response current for added 10 μM of H2O2 but no response current with presence of other compounds as shown in Fig. 7. The results can be indicated that a good antiinterference performance of Au-PDA-GQD/ITO electrode in the detection of H2O2. A better selectivity of this non-enzymatic sensor is attributed to relatively low working potential (−0.1 V) which efficiently inhibits an oxidation of potential interfering substances.
Fig. 5. The calibration curves for Au-PDA-GQD/ITO electrode in H2O2 detection.
manner. An increase in current is observed obviously even at a lower concentration of H2O2 like 0.1 μM and shown in inset of Fig. 4. The current reached a maximum steady-state within 2 s after an addition of H2O2 which indicates a very fast amperometric response behavior. From Fig. 5, it can be noted that the response currents are exhibited a good linear behavior ranging between 0.1 and 20,000 μM for two regions and they are much wider than that of GQD-based electrodes which has been reported previously. At relatively lower concentrations (from 0.1 to 40 μM), the calibration plot is steeped in comparison with that of higher concentrations (from 40 to 20,000 μM as shown in inset of Fig. 5 due to an occurrence of substrate inhibition effect at higher concentrations of H2O2. It can also be indicated that the Au-PDA-GQD/
4. Conclusions In summary, Au-PDA-GQD nanocomposites have been successfully prepared by using a green, facile and rapid method. The PDA acted as a binder between GQDs and Au NPs as well as provided a multifunctional platform. It is not only used for an immobilization of GQDs but also for adsorption of H2O2 molecule. An electrochemical performance of this non-enzymatic H2O2 biosensor is better than that of GQDs and other materials based sensors. The fabricated H2O2 biosensor with low cost, simple preparation method and excellent electrocatalysis properties possessed a wide detection linear range of 0.1–40 μM and 40–20,000 μM, low detection limit of 5.8 nm and fast response time
Table 1 Comparison of non-enzymatic H2O2 biosensors based on GQDs nanostructures. Materials
Potentials (V)
LOD (μM)
Sensitivity (μA mM− 1 cm− 2)
Response time (s)
Linear range (μM)
Reference
Au NPs-N-GQDs rGO QDs/ZnO HRP/GQD GQD/Au electrode CQDs/octahedral Cu2O Chit-GQDs/Ag NCs PVA/GQD GQD/GCE Au-PDA-GQD
− 0.3 − 0.4 − 0.445 − 0.4 − 0.2 − 0.45 − 0.5 − 0.2 − 0.1
0.12 0.025 0.53 and 2.16 0.7 2.8 0.15 0.53 20 0.0058
186.22 101,460 0.905 and 7.057 51 130 111 – – 48.6 and 8.08
5 3 2–3 10 – 8 10 – 2
0.25–13,327 1–22.48 100–1300 and 1700–2600 2–8000 5–5300 10–7380 100–200,000 20–10,000 0.1–40 and 40–20,000
[12] [31] [32] [11] [33] [34] [35] [36] This work
HRP: horseradish peroxidase; PVA: polyvinyl alcohol.
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Fig. 6. (a) The repeatability for five measurements of same modified electrode. (b) Electrode-to-electrode reproducibility of five independent modified electrodes. In each case, 2 mM of H2O2 was added. Error bars indicate the standard deviation of five measurements.
[7]
[8]
[9]
[10] [11]
[12]
[13] Fig. 7. Effects of 300 μM AA, 300 μM UA, 0.9% NaCl and 300 μM glucose (GLU), in amperometric responses of Au-PDA-GQD/ITO electrode with 2 mM H2O2.
[14]
[15]
less than 2 s at relatively lower applied potential (− 0.1 V). The results can be demonstrated that these novel nanocomposites are promising candidates for the fabrication of non-enzymatic H2O2 biosensors and also GQDs may be play an essential role in the next generation nonenzymatic biosensor.
[16] [17]
Acknowledgment
[18]
This work was supported by NSFCs (61475035, 61275054), the Opened Fund of the State Key Laboratory on Integrated Optoelectronics (2011KFJ004), the General Project of Education Department of Hunan Province (15C0251), the Research of Key Technologies to New Type Nano-lasers (BE2016177) and Collaborative Innovation Center of Suzhou Nano Science and Technology (SX21400213).
[19]
[20]
[21]
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Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Polydopamine interconnected graphene quantum dots and gold nanoparticles for enzymeless H2O2 detection
MARK
Ye Zhua, Shun Lub, A. Gowri Manoharia, Xiuxiu Donga, Feng Chena, Wei Xua, Zengliang Shia, Chunxiang Xua,⁎ a b
State Key Laboratory of Bioelectronics, School of Biological Science & Medical Engineering, Southeast University, Nanjing 210096, China Faculty of Materials and Energy, Southwest University, Chongqing 400715, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Peroxidase-like activity Au-PDA-GQD nanocomposites Non-enzymatic H2O2 biosensors
Graphene quantum dots (GQDs) offer new opportunities for the applications of electrochemical sensing because of their intrinsic peroxidase-like activity. To develop a sensitive electrochemical sensor with better stability and reproducibility, polydopamine (PDA) has been employed as an adhesive agent to integrate the GQDs and gold nanoparticles (Au NPs) tightly through a green, facile and rapid method. The synergistic effect of Au-PDA-GQD nanocomposite presented an excellent electrocatalytic activity towards the reduction of hydrogen peroxide (H2O2). Hence, the H2O2 biosensor based on Au-PDA-GQD exhibited better sensing performances, such as wide linear range (0.1–40 μM and 40–20,000 μM), low detection limit (5.8 nM) and fast response time (< 2 s) at low applied potential (− 0.1 V). Also, it presented a wonderful stability, reproducibility (RSD = 1.01%) and antiinterference. These results can be demonstrated that a novel Au-PDA-GQD is one of the most promising nanocomposites for the fabrication of non-enzymatic H2O2 biosensors.
1. Introduction The horseradish peroxidase (HRP) enzyme has been immobilized on sonogel-carbon electrode composite and used in the application of third generation H2O2 biosensor [1]. This kind of enzyme-based H2O2 biosensor has been received considerable attention because of their remarkable selectivity [2] and they can be applied in various fields such as biology, environmental protection, pharmaceutics, food security and chemistry [3]. However, some of the terrible disadvantages of natural enzymes limit their applications [4]. Therefore, the development of an artificial non-enzymatic H2O2 sensor is an essential one. With rapid developments in materials science and technology, previous metal nanoparticles [5], metal hexacyanoferrates [6] and carbon nanomaterials [7] have been widely employed to replace the enzyme-based H2O2 biosensors. Among various nanomaterials, carbon (like carbon nanotubes and graphene) based electrochemical sensors are considered as promising candidates because of their superior electron transfer kinetics, better chemical stability and biocompatibility and low cost [8]. Graphene quantum dots (GQDs), as a kind of zero dimensional materials have been obtained from graphene sheets or carbon dots [9], which caused an abundant attention because of their excellent physical and chemical properties, including low cytotoxicity, biocompatibility,
⁎
Corresponding author. E-mail address: [email protected] (C. Xu).
http://dx.doi.org/10.1016/j.jelechem.2017.04.017 Received 16 January 2017; Received in revised form 10 April 2017; Accepted 11 April 2017 Available online 12 April 2017 1572-6657/ © 2017 Published by Elsevier B.V.
tunable luminescence and anti-photobleaching [10]. Meanwhile, GQDs possess a higher peroxidase-like activity in comparison with that of micrometer-sized graphene oxide [11] and they have a considerable scope for the applications in electrochemical sensing. From the recent reports, it can be identified that both covalently assembled GQDs on Au electrode [11] and drop-casted Au-N-GQD hybrid on glass carbon electrode [12] showed high sensitivity and selectivity in electrochemical detection of H2O2. An integration of carbon-based materials and metal nanoparticles can be exhibited synergistic effects in electrocatalytic performance thus leading to enhanced catalytic activity [13]. It demonstrated a non-enzymatic composite for H2O2 electrocatalysis with low cost, easy preparation and excellent catalytic activity. Even though, some possible problems can be occurred in the nanocomposites including irreversible aggregation of GQDs [14], weak adhesion between GQDs and metal nanoparticles, poor operational stability and reproducibility [15]. It is still a challenge for solving the above problems simultaneously and improving the performance of nonenzymatic biosensor with broad linear response, low detection limit, good stability and reproducibility. In addition, a bioinspired adhesive with good conductivity and stability is an indispensable one. Polydopamine (PDA) as a musselinspired polymer has been prepared via simple self-polymerization of dopamine in a weak alkaline pH media [16]. The self-polymerization is
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a very mild reaction without any complicated instruments or harsh reaction conditions [17,18]. Interestingly, PDA is considered as a novel and an amazing material which adhered easily on any smooth or rough substrates due to its high concentrations of amino and catechol functional groups [18,19]. The PDA layer also offers the advantages in one-step surface functionalization and allows an introduction of new paradigm in the field of surface modification [17]. In this work, PDA has been employed as an adhesive agent to integrate the GQDs and Au NPs tightly and stably. The PDA provides a multifunctional platform and it is not only used for an immobilization of GQDs but also for absorption of H2O2 molecule. The Au-PDA-GQD nanocomposites have been successfully prepared by using a green, facile and rapid method. The synergistic effect of Au-PDA-GQD can be possessed an excellent peroxidase-like activity. Compared with other GQD-based H2O2 sensors, the non-enzymatic H2O2 biosensor based on Au-PDA-GQD nanocomposites has superior performances including lower detection limit, wider linear range, better stability and reproducibility, higher sensitivity and selectivity at lower applied potential.
Au-PDA/ITO electrodes were also prepared by using a similar procedure. The morphologies, sizes and structures of samples were characterized by the scanning electron microscopy (SEM, Hitachi S-4800) which equipped with X-ray energy dispersive spectrometer (EDS, Oxford XMax 50) and transmission electron microscopy (TEM, JEM-2100). Fourier transform infra-red analysis (FTIR, Nicolet5700) was carried out to provide the information about functional groups of PDA-GQD. UV–visible spectrophotometer (Shimadzu UV-2450, Japan) was used to analyze the absorption properties of the samples. Electrochemical measurements were performed by using a CHI630D Electrochemical Workstation with 0.1 M of oxygen free phosphate buffer solution (PBS) at room temperature. A conventional three-electrode system was equipped with the working electrode of Au-PDA-GQD modified ITO, reference electrode of saturated calomel (SCE) and Pt wire as counter electrode.
2. Experimental details
3.1. Characterization of Au-PDA-GQD nanocomposites
All the chemicals have an analytical grade and they were used in the experiment without any further purification process. All the solutions were prepared with deionized water. The graphite oxide was synthesized by using a famous modified Hummers method as described in our previous work [20]. GQDs were prepared from GO according to a previously reported methods with minor modifications [21]. Briefly, the mixture of 40 mL of H2O2 (30%) and 10 mL of ammonia (25%–28%) solutions were added into 10 mL of as-prepared GO suspension under vigorous stirring at 80 °C for 8 h. The brown mixture was gradually turned into colorless, which indicates a formation of smaller sized graphenes. Then, the obtained colorless solution was heated at 90 °C in an autoclave to remove the unreacted H2O2 and ammonia. Upon cooling to room temperature, the solution was ultrafiltered through mixed cellulose membrane (PES membrane, Millipore) with hole diameter of 0.22 μm. Finally, the obtained GQDs solution was further dialyzed by using a dialysis tubing membrane with molecular mass cut-off of 3500 Da for 3 days to remove the remaining tiny fragments. The preparation of modified electrodes is schematically illustrated in Scheme 1. Initially, substrate of indium tin oxide (ITO) was washed with acetone, ethanol and distilled water under ultrasonification. After that, Au NPs were deposited on a well cleaned ITO (with active area of 1 cm2) substrate by using an ion beam sputtering system. Concomitantly, 10 mg of dopamine was dissolved in 10 mL of 10 mM Tris-HCl solution with pH of 8.5. The obtained Au/ITO electrodes were then simply dipped into dopamine solution for 1 h thus forming PDA thin film. The coated surfaces were rinsed with water and dried at N2 atmosphere. Finally, an aqueous solution of 10 μL of GQDs was dropcasted on the surfaces of Au-PDA/ITO electrodes by using a micropipette and dried at room temperature. For comparison, the Au/ITO and
Fig. 1(a) represents the SEM image of ion sputtered Au NPs with average diameters of 35 nm. From Fig. 1(b), it can be suggested that well dispersing nature of GQDs in aqueous solution with narrow sized distribution (2–3 nm). The inset of Fig. 1(d) presents a high resolution TEM (HRTEM) image of GQDs, which has a good crystallinity with lattice diameter of 0.21 nm. An existence of (102) diffraction plane of sp2 graphitic carbon can be indicated that the GQDs have quite similar crystallinity with graphene [22]. Moreover, a thin film is obviously noticed on the surface of sputtered Au NPs as shown in Fig. 1(c) which denotes a successful formation of PDA thin film layer through selfpolymerization of dopamine. Meanwhile, the Fig. 1(d) yields no significant aggregation of GQDs, which demonstrates a unique biocompatibility and high water solubility of PDA thus leading to well absorption of GQDs on the surface of PDA film. In addition, FTIR spectroscopic analysis has been carried out to provide the information about functional groups of PDA-GQD. A broad range of absorption between 3500 and 3100 cm− 1 is assigned to NeH/OeH stretching vibrations of PDA [23]. The observed peaks at around 1630, 1551 and 1296 cm− 1 indicate an existence of C]C, C]N and CeOH, respectively [24,25]. After the coating of GQDs, the peak intensity of C-OH at around 1330 cm− 1 is increased obviously while the characteristic peaks of PDA are almost disappeared. UV–visible absorption spectra were further used to study the absorption properties of GQDs, PDA and Au NPs as shown in Fig. 1(f). A broad range of absorption is observed in GQDs due to π-π⁎ electron transition of aromatic sp2 domains. An obvious absorption peak at 280 nm in the black curve can be attributed to π-π⁎ electron transition of PDA, and peak at about 550 nm in the blue curve is assigned to the surface plasmon absorption of Au NPs. These results can be provided a clear evidence for the formation of Au NPsPDA-GQDs nanocomposites.
3. Results and discussion
Scheme 1. A schematic illustration for the preparation of Au-PDA-GQD nanocomposites.
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Fig. 1. (a) SEM image of Au NPs; (b) TEM image of GQDs and inset shows the diameter distribution of GQDs; (c) TEM image of Au-PDA nanocomposite; (d) TEM image of PDA-GQD nanocomposite and inset shows HRTEM image of GQDs; (e) FTIR spectra of PDA (red line) and PDA-GQD nanocomposite (black line); (f) UV–visible absorption spectra of PDA (black line), GQDs (red line) and Au (blue line). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
reduction ability and fast electron transfer rate of the composites. When rising the scan rate from 10 to 300 mV s− 1, a peak current is increased linearly with respect to scan rate (ν), which possesses a classical surface-controlled electrode process [26]. Fig. 3 represents comparison of the CV response for different electrodes with and without H2O2 in 0.1 M PBS solution and pH of 7.0 at the scan rate of 50 mV s− 1. An electrocatalytic activity of bare blank ITO electrode is fairly low
3.2. Electrochemical performance to H2O2 Fig. 2(a) shows the typical cyclic voltammetry (CVs) of Au-PDAGQD modified ITO electrode in 0.1 M PBS with various scan rates ranging between − 0.8 to + 0.2 V versus SCE. From the figure, it can be observed that the potential of cathodic peaks are almost unchanged with increase in scan rate, which indicates well electrochemical
Fig. 2. (a) CVs of Au-PDA-GQD/ITO electrode in 0.1 M PBS with different scan rates (a–i: 10, 20, 40, 80, 120, 160, 200, 250, 300 mV s− 1). (b) A linear dependent plot of peak current versus scan rate.
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Fig. 3. CVs of (a) bare ITO electrode, (b) Au/ITO, (c) Au-PDA/ITO and (d) Au-PDA-GQD/ITO electrodes in absence (black curves) and presence (red curves) of 2 mM H2O2 in 0.1 M PBS solution at the scan rate of 50 mV s− 1. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
electrochemical potential of a LUMO of H2O2, due to GQDs with ntype incorporation of charge transfer process which accelerates a transfer of electron from GQDs into H2O2 [29]. This reaction equation can be expressed as follows:
with respect to H2O2. An onset potential of a generated cathodic current is observed at − 0.2 V for Au/ITO electrode as shown in Fig. 3(b), which denotes an enhanced electrocatalytic activities of Au NPs on ITO electrode in H2O2 reduction [12]. As introducing a thin film layer of PDA on Au/ITO electrode, the cathodic peak current becomes higher at 0.1 V when compared to that of without PDA and shown in Fig. 3(c). It indicates the importance of PDA in an elevation of catalysis via absorption of more H2O2 molecules [19]. In addition, a response current is increased significantly when decorating the GQDs on AuPDA/ITO electrode, which is an interesting phenomenon. The response current is 7 times higher than that of those obtained from both Au/ITO and Au-PDA/ITO electrodes after an addition of same concentration of H2O2. Furthermore, the onset reduction potential of H2O2 on Au-PDAGQD/ITO electrode is shifted to − 0.1 V. It is important to note that the Au-PDA-GQD nanocomposites can be involved in decrement of overvoltage for H2O2 reduction efficiently because common electroactive species can jam the measurements at higher overpotential [27]. These results can be suggested that the Au-PDA-GQD modified ITO electrode has a superior electrocatalytic performances and synergistic effect towards the reduction of H2O2, and it can be considered as a promising platform for the non-enzymatic H2O2 sensor.
GQDs → GQDs+ +e−
(1)
H2 O2 +2e− → 2OH−
(2)
Secondly, PDA is a conductive polymer which provides an excellent interfacial contact between GQDs and Au NPs for a fast electron transfer. Herein, PDA thin film prevented an aggregation of GQDs, thus resulting in large surface for H2O2 interaction. Also, it offered considerable active sites for an attachment of H2O2 molecule owing to its high concentrations of amino and catechol functional groups [18]. Thirdly, Au NPs exhibited an additional peroxidase-like activity and electrocatalytic activity in the reduction of H2O2 [12]. Due to their high surface area to volume ratio, Au NPs provided an extensive platform for the immobilization of GQDs and PDA film. Au NPs also enhanced a transfer of electron between electrode and detection molecules effectively due to their good conducting nature. These features can be contributed a synergistic enhancement in electrocatalytic performances of Au-PDA-GQD nanocomposites for H2O2 reduction. It is expected that an engineered catalytic GQD-based material can be act as a potential electrochemical sensing material for the detection of H2O2 in biological system.
3.3. Electrocatalytic mechanism of H2O2 reduction An enhanced electrocatalytic performance of Au-PDA-GQD modified ITO electrode can be attributed to three possible aspects. Firstly, a higher peroxidase-like activity of GQDs plays a key role in the detection of H2O2 [11,28]. Moreover, the H2O2 reduction is mainly based on previously suggested mechanism in which an electron transfer occurred from top of the valence band of GQDs into the lowest unoccupied molecular orbital (LUMO) of H2O2 [29,30]. In addition, PDA denoted its lone-pair electrons of amino groups to GQDs when GQDs are absorbed on the surface of PDA, which improve the electron density and mobility in GQDs. Thus, increase in Fermi level as well as
3.4. Amperometric response to H2O2 A rapid and an accurate H2O2 detection capability of Au-PDA-GQD modified ITO electrode have been evaluated by utilizing an amperometric response with stepwise injection of different concentrations of H2O2. Fig. 4 depicts the amperometric I-t curves of Au-PDA-GQD/ITO electrode with successive addition of various concentrations of H2O2 into 0.1 M PBS solution at − 0.1 V under stirring condition. When adding the H2O2, the response current is increased in a stepwise 78
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ITO electrode is more sensitive to H2O2 at lower concentrations. In H2O2 sensor, an estimated sensitivity value for first and second linear region is 48.6 and 8.08 μA mM− 1 cm− 2, respectively. The limit of detection (LOD) of H2O2 sensor was estimated to be 5.8 nM (S/N = 3) and it is much lower than that of previously reported GQD-based biosensors. The comparison of variable parameters for Au-PDA-GQD/ITO electrode with other reported non-enzymatic H2O2 sensors based on GQDs is obtained in Table 1. Obviously, the yielded LOD and response time of this work are superior when compared to other reported values as listed. Moreover, the proposed sensor is displayed a wider linear range in comparison with that of other sensors whereas an applied potential is much lower than that of other reported values. An excellent electrochemical performance is mainly attributed to synergistic effect of Au-PDA-GQD nanocomposites as mentioned above. 3.5. Reproducibility, stability and anti-interference property Fig. 4. Amperometric responses of Au-PDA-GQD/ITO electrode for the successive addition of H2O2 (applied potential: − 0.1 V). Inset shows the magnified amperometric response to a lower concentration of H2O2.
It is very important to test the reproducibility and storage stability of electrochemical sensors. Fig. 6 displays the relative standard deviation (RSD) for response currents of Au-PDA-GQD/ITO electrodes. The RSD for five measurements of same electrode and five independent electrodes was evaluated to be 1.01% and 1.14% under the same condition which denotes an excellent reproducibility of H2O2 biosensor. In addition, the Au-PDA-GQD/ITO electrode was stored at 4 °C for two weeks and still it retained 95% of its initial current which clearly demonstrates a good stability of H2O2 sensor. Moreover, some possible physiological-level electroactive species of 300 μM ascorbic acid (AA), uric acid (UA), glucose (GLU) and 0.9% NaCl have also been interfered in the detection of H2O2 and investigated by using an amperometric experiment at the potential of − 0.1 V. There is an occurrence of distinct response current for added 10 μM of H2O2 but no response current with presence of other compounds as shown in Fig. 7. The results can be indicated that a good antiinterference performance of Au-PDA-GQD/ITO electrode in the detection of H2O2. A better selectivity of this non-enzymatic sensor is attributed to relatively low working potential (−0.1 V) which efficiently inhibits an oxidation of potential interfering substances.
Fig. 5. The calibration curves for Au-PDA-GQD/ITO electrode in H2O2 detection.
manner. An increase in current is observed obviously even at a lower concentration of H2O2 like 0.1 μM and shown in inset of Fig. 4. The current reached a maximum steady-state within 2 s after an addition of H2O2 which indicates a very fast amperometric response behavior. From Fig. 5, it can be noted that the response currents are exhibited a good linear behavior ranging between 0.1 and 20,000 μM for two regions and they are much wider than that of GQD-based electrodes which has been reported previously. At relatively lower concentrations (from 0.1 to 40 μM), the calibration plot is steeped in comparison with that of higher concentrations (from 40 to 20,000 μM as shown in inset of Fig. 5 due to an occurrence of substrate inhibition effect at higher concentrations of H2O2. It can also be indicated that the Au-PDA-GQD/
4. Conclusions In summary, Au-PDA-GQD nanocomposites have been successfully prepared by using a green, facile and rapid method. The PDA acted as a binder between GQDs and Au NPs as well as provided a multifunctional platform. It is not only used for an immobilization of GQDs but also for adsorption of H2O2 molecule. An electrochemical performance of this non-enzymatic H2O2 biosensor is better than that of GQDs and other materials based sensors. The fabricated H2O2 biosensor with low cost, simple preparation method and excellent electrocatalysis properties possessed a wide detection linear range of 0.1–40 μM and 40–20,000 μM, low detection limit of 5.8 nm and fast response time
Table 1 Comparison of non-enzymatic H2O2 biosensors based on GQDs nanostructures. Materials
Potentials (V)
LOD (μM)
Sensitivity (μA mM− 1 cm− 2)
Response time (s)
Linear range (μM)
Reference
Au NPs-N-GQDs rGO QDs/ZnO HRP/GQD GQD/Au electrode CQDs/octahedral Cu2O Chit-GQDs/Ag NCs PVA/GQD GQD/GCE Au-PDA-GQD
− 0.3 − 0.4 − 0.445 − 0.4 − 0.2 − 0.45 − 0.5 − 0.2 − 0.1
0.12 0.025 0.53 and 2.16 0.7 2.8 0.15 0.53 20 0.0058
186.22 101,460 0.905 and 7.057 51 130 111 – – 48.6 and 8.08
5 3 2–3 10 – 8 10 – 2
0.25–13,327 1–22.48 100–1300 and 1700–2600 2–8000 5–5300 10–7380 100–200,000 20–10,000 0.1–40 and 40–20,000
[12] [31] [32] [11] [33] [34] [35] [36] This work
HRP: horseradish peroxidase; PVA: polyvinyl alcohol.
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Fig. 6. (a) The repeatability for five measurements of same modified electrode. (b) Electrode-to-electrode reproducibility of five independent modified electrodes. In each case, 2 mM of H2O2 was added. Error bars indicate the standard deviation of five measurements.
[7]
[8]
[9]
[10] [11]
[12]
[13] Fig. 7. Effects of 300 μM AA, 300 μM UA, 0.9% NaCl and 300 μM glucose (GLU), in amperometric responses of Au-PDA-GQD/ITO electrode with 2 mM H2O2.
[14]
[15]
less than 2 s at relatively lower applied potential (− 0.1 V). The results can be demonstrated that these novel nanocomposites are promising candidates for the fabrication of non-enzymatic H2O2 biosensors and also GQDs may be play an essential role in the next generation nonenzymatic biosensor.
[16] [17]
Acknowledgment
[18]
This work was supported by NSFCs (61475035, 61275054), the Opened Fund of the State Key Laboratory on Integrated Optoelectronics (2011KFJ004), the General Project of Education Department of Hunan Province (15C0251), the Research of Key Technologies to New Type Nano-lasers (BE2016177) and Collaborative Innovation Center of Suzhou Nano Science and Technology (SX21400213).
[19]
[20]
[21]
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