N-doped graphene quantum dots as an effective photocatalyst for the photochemical synthesis of silver deposited porous graphitic C₃N₄ nanocomposites for nonenzymatic electrochemical H₂O₂ sensing

Abstract

In this work, nitrogen-doped graphene quantum dots (N-GQDs) were proved to be an efficient photocatalyst for photochemical synthesis of Ag nanoparticle loaded porous graphitic C₃N₄ (p-g-C₃N₄) for the first time. The Ag nanoparticles were well-formed in high yield and closely anchored at the surface of p-g-C₃N₄ with uniform size distribution. Importantly, the as-prepared Ag/p-g-C₃N₄ nanocomposites exhibited excellent catalytic activity towards electrocatalytic reduction of hydrogen peroxide (H₂O₂). The cyclic voltammetry and amperometry results show that the sensor of the as-prepared Ag/p-g-C₃N₄ composite exhibits excellent analytical response to H₂O₂ with fast response, wide linear range and low detection limit.

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1 Introduction

Graphitic carbon nitride, referred to as g-C₃N₄, possesses a stacked two-dimensional structure and is the most stable allotrope of carbon nitride under ambient conditions.¹ In addition, g-C₃N₄ is abundant and easily-synthesized via one-step polymerization of cheap feedstocks like cyanamide, dicyandiamide, melamine, thiourea and urea.^2–5 Its graphene-like sp² bonding structure, together with abundant in-plane nitrogen dopants and defects, gives it an appealing electronic structure, which is responsible for its multifunctional catalytic activity for a broad variety of reactions, including water splitting, CO₂ reduction, organic degradation, etc.^6–8 In our previous reports, heterojunction photocatalysts (BiOI/porous g-C₃N₄, Ag₃PO₄/porous g-C₃N₄) with excellent visible-light-driven photocatalytic performance have been successfully fabricated.^9,10

On the other hand, N-containing carbons have received considerable attentions due to the strong electron donor nature of N which should promote enhancement in π bonding, leading to improved stability, electron transfer rate, and enhance durability of the carbon supports during electrocatalytic processes.¹¹ Because of its rich pyridine-like nitrogens content and facile synthesis procedure, g-C₃N₄ may provide more active reaction sites than other N-carbon materials towards electrocatalytic applications.¹² Recently, g-C₃N₄ as electrocatalyst in fuel cells and other electrochemical applications has been gradually studied extensively.^13–16 Barman and co-workers fabricated two-dimensional C₃N₄ sheets through a bottom-up method for electrochemical sensing of mercuric ions.¹⁷ Sun et al. reported that ultrathin g-C₃N₄ nanosheets directly prepared by ultrasonication-assisted liquid exfoliation of bulk g-C₃N₄ can be used as a low-cost and efficient electrocatalyst toward the reduction of H₂O₂.¹⁸ However, these synthetic approaches suffer from relatively low yield and poor reproducibility. Furthermore, despite these efforts, g-C₃N₄ alone still shows limited electrocatalytic activity.

Fluorescent carbon nanodots (CDs) and graphene quantum dots (GQDs) have attracted much attention due to their unique properties and wide applications in photocatalyst, bio-imaging, ion detection, and electrochemical luminescence.^19–22 It has been found recently that CDs or GQDs could also be used as a green reductant or support to synthesize CDs–metal nanocomposites.^23–25 We have successfully synthesized Ag/C/Ag core–shell–satellite nanocomposites by using nitrogen-doped CDs prepared from biomass as reductant, which exhibited excellent surface-enhanced Raman scattering properties.²⁶ Kim and co-workers synthesized carbon dots-supported silver nanoparticles (NPs) (CDs–Ag NPs) using the carbon dots both as a reducing agent and a template under ultraviolet irradiation to fabricate solution-processable polymer light-emitting diodes and polymer solar cells.²⁷ The excellent electron-donating capability of photoexcited CDs enables fast reduction of metal salts to corresponding metal nanoparticles on the surfaces of the CDs. Sun et al. demonstrated that CDs can be served as an effective photocatalyst for fabricating AuNPs–rGO nanocomposites by UV irradiation of a mixture of GO and HAuCl₄ aqueous solution in the presence of CDs.²⁸

Herein, we report a facile and green synthesis of silver NPs deposited porous g-C₃N₄ (Ag/p-g-C₃N₄) using nitrogen-doped GQDs (N-GQDs) as effective photocatalyst. The Ag/p-g-C₃N₄ nanocomposites were obtained simply by visible light irradiation of the mixture of g-C₃N₄ and AgNO₃ aqueous solution in the presence of N-GQDs. To the best of our knowledge, the use of N-GQDs as a photocatalyst for photocatalytic deposition has never been reported before. Importantly, the resultant Ag/p-g-C₃N₄ nanocomposites exhibited excellent catalytic activities towards electrocatalytic hydrogen peroxide (H₂O₂) reduction. In addition, the as-fabricated Ag/p-g-C₃N₄ modified glassy carbon electrode (GCE) as a nonenzymatic H₂O₂ sensor showed high sensitivity and low detection limit.

2 Experimental

2.1 Materials

Citric acid, urea, hydrogen peroxide (H₂O₂, 30 wt%) and ethanol were purchased from Sinopharm Chem. Reagent Co., Ltd (Shanghai, China). Silver nitrate (AgNO₃) were purchased from Aldrich Chemical Comp. All chemicals were used as received without further purification. The water used was double-distilled after a deionized exchange.

2.2 Preparation of p-g-C₃N₄

p-g-C₃N₄ were prepared according to the method we reported previously,⁹ and the details are described as follows. p-g-C₃N₄ was synthesized by thermal treatment of 10 g of urea in a crucible with a cover under ambient pressure in air. After drying at 80 °C for 24 h, the precursor was heated to 550 °C at a heating rate of 2.3 °C min⁻¹ in a tube furnace for 4 h in air. The resulting final light yellow powder was washed with nitric acid (0.1 mol L⁻¹) and distilled water to remove any residual alkaline species (e.g. ammonia) adsorbed on the sample surface, and then dried at 80 °C for 24 h.

2.3 Preparation of N-GQDs

N-GQDs were synthesized according to the method reported previously.²⁹ Typically, 0.525 g (2.5 mmol) citric acid and 0.45 g (7.5 mmol) urea were dissolved into 12 mL water, and stirred to form a clear solution. Then the solution was transferred into 50 mL Teflon lined stainless autoclave. The sealed autoclave was heated to 160 °C in an electric oven and kept for additional 8 hours. The final product was collected by adding ethanol into the solution and centrifuged at 5000 rpm for 5 min. The solid can be easily redispersed into water.

2.4 Preparation of Ag/p-g-C₃N₄

In a typical preparation, 0.1 g of p-g-C₃N₄ powders were added into 10 mL of 0.5 mg mL⁻¹ as-prepared N-GQDs dispersion. After 30 min stirring at room temperature, the suspension were added into a quartz tube, followed by addition of 50 mL of 11.8 mM AgNO₃ solution. Prior to irradiation, the suspension was magnetically stirred in the dark for another 30 min. The mixture was irradiated under visible light obtained by using cut off filters to remove light (150 W, xenon lamp) of λ < 420 nm for 1 h under magnetic stirring (note: cooling water was circulated outside the reactor to maintain the reaction temperature at 25 °C). After irradiation, the brown precipitates were collected by centrifugation at the speed of 8000 rpm and washed with deionized water and ethanol for 3 times, respectively, finally dried under vacuum at 60 °C for 12 h before further characterization. For comparison, the Ag deposited p-g-C₃N₄ in the absence of N-GQDs (denoted as a-Ag/p-g-C₃N₄) was also synthesized using the same method except for prolonging the irradiation time to 6 h.

2.5 Preparation of the modified electrode and electrochemical measurements

1.0 mg of Ag/p-g-C₃N₄ composite and 10 μL of Nafion solution (5 wt%) were dispersed in 1 mL water–isopropanol mixed solvent (3 : 1 v/v) by at least 30 min sonication to form a homogeneous catalyst solution. Then 10 μL of the catalyst solution was dropped onto the surface of pretreated GCE and left to dry at room temperature to get Ag/p-g-C₃N₄-modified GCE, referred to as Ag/p-g-C₃N₄/GCE. Amperometric and cyclic voltammetry (CV) experiments were carried out by a CHI 650D electrochemical analyzer (CH Instruments, Inc., Shanghai), in a home-made three-electrode electrochemical cell consisting of a twisted platinum wire as an auxiliary electrode, a saturated calomel electrode (SCE) as a reference electrode and a modified-GCE (0.07 cm²) as a working electrode. All experiments were carried out at the ambient temperature. Prior to the experiment, the electrolyte, consisting of a solution of 0.2 M phosphate buffer solution (PBS, pH 7.4), was purged with high-purity nitrogen for at least 30 min and a nitrogen atmosphere was maintained over the solution.

2.6 Characterization

Purity and crystallization of the samples were characterized by powder X-ray diffraction on a D/max-γA X-ray spectrometer (Rigaku, Japan) at 40 kV and 200 mA with monochromatic Cu Kα (λ = 1.5418 Å) radiation. Transmission electron microscopy (TEM) images were taken on a TECNAI12 TEM instrument (Philips) with an accelerating voltage of 200 kV. Scanning electron micrograph (SEM) and energy dispersive X-ray analysis (EDX) spectroscopy were taken on a field emission SEM (FESEM) instrument (Hitachi S-4800 II, Japan). The surface areas of the sample was measured by a TriStar II 3020-BET/BJH Surface Area. X-ray photoelectron spectroscopy (XPS) was performed on a VG-ADES 400 instrument with Mg K-ADES source. Elemental analyses (C, N and O) were performed with a Vario MICRO CHNOS Elemental Analyzer. Metal elemental analyses were carried out by inductively coupled plasma spectroscopy (ICP, Optima2000DV, USA) analysis. Electrochemical measurements were performed with a CHI 650D electrochemical analyzer (CH Instruments, Inc., Shanghai).

3 Results and discussion

3.1 Morphology and structure of the Ag/p-g-C₃N₄ composite

XRD was used to identify and determine the phase structures of the as-prepared samples. As shown in Fig. 1a, the XRD pattern of the p-g-C₃N₄ sample reveals two distinct diffraction peaks at 27.50° and 13.04° corresponding to the (002) and (100) diffraction planes of the graphite-like carbon nitride, which is consistent with the standard value (JCPDS 87-1526).³⁰ Fig. 1b shows the XRD pattern of the Ag deposited p-g-C₃N₄ synthesized using the same method in the absence of N-GQDs (denoted as: a-Ag/p-g-C₃N₄). It can be found that the diffraction intensity of the peaks at 27.50° and 13.04° become weaker, additionally, related to the metallic Ag are hardly found, which may be due to the low doping levels of Ag NPs in a-Ag/p-g-C₃N₄. In spite of shorter irradiation time, compared with a-Ag/p-g-C₃N₄, as-prepared Ag/p-g-C₃N₄ (curve c) exhibit four additional peaks located at 38.0, 44.2, 64.3, and 77.3° corresponding to 111, 200, 220, and 311 faces of Ag, respectively. These observations suggest that N-GQDs serve as an effective photocatalyst for the formation of Ag photo-deposited p-g-C₃N₄.

Fig. 1 XRD patterns for the as-prepared samples: pure p-g-C₃N₄ (a), a-Ag/p-g-C₃N₄ (b) and Ag/p-g-C₃N₄ (c).

Fig. 2 shows FE-SEM images of the pure p-g-C₃N₄, a-Ag/p-g-C₃N₄ and Ag/p-g-C₃N₄ nanocomposites. The morphology of the pure p-g-C₃N₄ is consisted of thin sheet-like with wrinkles and irregular shape (Fig. 2a), which is the typical structure characteristic of g-C₃N₄ synthesized by the polymerization of urea.³¹ After photo-deposition of silver NPs on the p-g-C₃N₄, it can be seen that the morphology of p-g-C₃N₄ remained intact and a few small sphere-like nanoparticles were dispersed on its surface (Fig. 2b). Fig. 2c and d show the FE-SEM images of the Ag/p-g-C₃N₄ synthesized in the presence of N-GQDs. It is observed that a large amount of Ag NPs with diameters in the range 10–20 nm are uniformly and densely generated on the p-g-C₃N₄. In addition, the EDX spectrum of Ag/p-g-C₃N₄ reveals the presence of elements of C, N and Ag (note that: the presence of element Si was generated from the supporting Si substrate for decreasing the charging effects under the SEM-imaging conditions). These results provide another piece of evidence to support the formation of Ag nanoparticles after irradiation.

Fig. 2 FE-SEM images of the pure p-g-C₃N₄ (a), a-Ag/p-g-C₃N₄ (b) and Ag/p-g-C₃N₄ (c and d), and EDX spectrum of Ag/p-g-C₃N₄ (e).

In order to study the microstructure of the Ag/p-g-C₃N₄ in more detail, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) observations were carried out. As presented in Fig. 3a, the morphology of the pure p-g-C₃N₄ is composed of a large lamellar structure, and a number of mesopores of several tens of nanometres in size can be observed in the p-g-C₃N₄ sheets, which was highly analogous to previously reported results.³² In comparison with TEM image of a-Ag/p-g-C₃N₄ (Fig. 3b), more Ag NPs with a narrow size distribution were observed on the surface of p-g-C₃N₄ in Fig. 3c. At high magnification TEM image (Fig. 3d), it can be found that the Ag nanoparticles are approximately spherical in shape and anchored on the p-g-C₃N₄. The HRTEM image taken from the nanoparticle (insert in Fig. 3d) reveals clear lattice fringes with an interplane distance of 0.238 nm corresponding to the (111) lattice space of metallic Ag, further revealing the nanoparticle is Ag. These results are consistent with above XRD and EDX observations.

Fig. 3 TEM images of the pure p-g-C₃N₄ (a), a-Ag/p-g-C₃N₄ (b) and Ag/p-g-C₃N₄ (c and d), and HRTEM image of one particle of Ag/p-g-C₃N₄ (insert).

To learn more about the porous nature and specific surface area of the as-prepared Ag/p-g-C₃N₄ composite, a nitrogen adsorption–desorption isotherm was measured. Fig. 4 shows the isotherm for the sample, which exhibits a type IV with a H3 hysteresis loop according to the IUPAC classification, reflecting the presence of a mesoporous structure of the composites. It can be found that the synthesized Ag/p-g-C₃N₄ composite shows a relatively large specific surface area (21.98 m² g⁻¹), which is close to the BET surface area of the pure p-g-C₃N₄ synthesized according to our previous reports (23.19 m² g⁻¹).^9,10 The pore-size distribution of the sample is also estimated using the Barrett–Joyner–Halenda (BJH) method from the desorption branch of the isotherm, as shown in the inset of Fig. 4. The calculated pore size distribution using BJH method indicates that the size of mesopores is not uniform ranging from 8 to 50 nm, which is consistent with the TEM observation. And the average pore size of the Ag/p-g-C₃N₄ composite is estimated to be 14.59 nm, which is also close to the average pore size of the p-g-C₃N₄ (13.64 nm).^9,10

Fig. 4 Nitrogen sorption isotherm and Barrett–Joyner–Halenda (BJH) pore size distribution plot (inset) of the Ag/p-g-C₃N₄ composite.

X-ray photoelectron spectroscopy (XPS) surface probe technique can further confirm the surface composition and elements of the resultant Ag/p-g-C₃N₄ composite. Fig. 5 shows the survey XPS of the hybrid, which is mainly dominated by the signals of C, N, O, and Ag. The accurate percentage ratio of C : N : O : Ag further determined by elemental analyzer and ICP analysis was estimated to be 40.12 : 50.09 : 5.71 : 4.08. Fig. 6a presents the Ag XPS spectrum of the sample. The strong peaks at 368.3 and 374.4 eV can be ascribed to the binding energies of the Ag 3d_5/2 and 3d_3/2 electrons of metallic Ag(0). Fig. 6b shows the XPS of Ag/p-g-C₃N₄ in the N 1s binding energy regions, and the spectrum can be fitted into three peaks at 398.9, 399.7 and 401.1 eV, which are attributed to pyridinic, pyrrolic and quaternary N, respectively.^33–35 The C1s spectrum (Fig. 6c) has been deconvoluted into four peaks at 284.9, 287.6, 288.5, 289.0 eV, assigned to sp² C atoms bonded to N in an aromatic ring (N–C=N), sp³ hybridized C atoms (C–(N)₃), C–O (hydroxyl and epoxy carbon) and C=O (carboxylic carbon),respectively.^36,37 And the O 1s spectrum (Fig. 6d) decomposed at 531.2, 532.8 eV are assigned to O–C and O=C, which can be ascribed to the oxygenous groups of the residual N-GQDs on the surface of Ag/p-g-C₃N₄. These results also indicate the strong interaction between the N-GQDs and p-g-C₃N₄.

Fig. 5 XPS survey spectrum of the Ag/p-g-C₃N₄ composite.

Fig. 6 XPS spectra of the Ag/p-g-C₃N₄: (a) Ag 3d; (b)N 1s; (c) C 1s and (d) O 1s.

Based on all of above experimental results, the photochemical synthesis of Ag/p-g-C₃N₄ nanocomposites can be possibly explained as follows. Firstly, the N-GQDs can be adsorbed effectively onto p-g-C₃N₄ due to their similar CN heterocyclic structures and the strong interaction between carboxyl groups of N-GQDs and amino groups of p-g-C₃N₄.^26,29,38,39 These oxygenous groups and amino groups also played an important role in adsorbing and stabilizing Ag⁺ ions,^27,39,40 thus Ag⁺ ions can be anchored uniformly on the surface of p-g-C₃N₄. During the photochemical synthetic process, N-GQDs acted as a photosensitizer because of their broad visible light absorption under visible light.²⁹ N-GQDs can absorb the visible light, and then the electron is excited to excited state.^{28,29,41–43} Subsequently, the photogenerated electrons from N-GQDs inject to the p-g-C₃N₄ and simultaneously reduce the surrounding Ag⁺ to Ag⁰, finally resulting in the formation of Ag NPs deposited p-g-C₃N₄ composite.^28,29 Actually, this successful and efficient preparation can be ascribed to the excellent electron-donating capability of photoexcited N-GQDs.²⁷ In the photochemical synthetic process, the N-GQDs essentially not only act as a catalyst but also perform as a reducing agent. The schematic illustration of the proposed mechanism for the photochemical synthesis of the Ag/p-g-C₃N₄ nanocomposite is shown in Scheme 1.

Scheme 1 A schematic diagram illustrating the proposed mechanism for the photochemical synthesis of Ag/p-g-C₃N₄ nanocomposites.

3.2 Electrochemical sensing of H₂O₂

Hydrogen peroxide (H₂O₂), a well-known oxidizing agent, is not only a by-product of a large number of oxidase enzymes, but also an essential mediator in food, pharmaceutical, clinical, industrial and environmental analysis.⁴⁴ Electrochemical H₂O₂ sensors, especially nonenzymatic amperometric biosensors, have attracted extensive attention due to their high sensitivity, simplicity, good compatibility and long term stability.^45–47 The promising application as an enzyme-free biosensor for H₂O₂ was selected as a preliminary test of the electrocatalytic behaviors of as-prepared Ag/p-g-C₃N₄ nanocomposites.

To testify the sensing application of Ag/p-g-C₃N₄ nanocomposites, an enzymeless H₂O₂ sensor was constructed by direct deposition of the aqueous dispersion of Ag/p-g-C₃N₄ nanocomposites on a bare GCE surface. The cyclic voltammograms of the bare GCE, p-g-C₃N₄/GCE, a-Ag/p-g-C₃N₄/GCE and Ag/p-g-C₃N₄/GCE in N₂ saturated 0.2 M pH 7.4 PBS containing 5.0 mM H₂O₂ are shown in Fig. 7. It can be obviously seen that the responses of both the bare GCE and p-g-C₃N₄/GCE towards the reduction of H₂O₂ are quite weak. The a-Ag/p-g-C₃N₄/GCE shows a good response towards the reduction of H₂O₂ due to the presence of Ag nanoparticles, but no obvious current peak is observed. In contrast, the Ag/p-g-C₃N₄/GCE exhibits a remarkable catalytic current peak about 238 μA in intensity at −0.52 V vs. SCE. The above results indicate that as-prepared Ag/p-g-C₃N₄ has a good electrocatalytic activity towards the H₂O₂ reduction and the high number density Ag NPs with narrow size distribution anchored on the p-g-C₃N₄ play an important role as the main electroactive materials.

Fig. 7 Cyclic voltammograms (CVs) of bare GCE, p-g-C₃N₄/GCE, a-Ag/p-g-C₃N₄/GCE and Ag/p-g-C₃N₄/GCE in N₂-saturated 0.2 M PBS at pH 7.4 in the presence of 5.0 mM H₂O₂ (scan rate: 50 mV s⁻¹).

It should be noted that the Ag/p-g-C₃N₄/GCE exhibits a 260 mV and 100 mV positive shift of the peak potential compared to Ag NPs-modified GCE by electrodeposition technique⁴⁸ and hydrothermal synthesis of PQ11-Ag NPs decorated GCE,⁴⁹ respectively. In addition, the as-prepared Ag/p-g-C₃N₄ modified GCE shows good repeatability with a relative standard deviation (RSD) of 3.8% for five successive detections at 5.0 mM H₂O₂. Also, the current response retained 92% of its initial value after a storage period of 3 weeks, indicating the good long-term stability of the Ag/p-g-C₃N₄ composites.

Under optimal conditions, a typical current–time plot of a Ag/p-g-C₃N₄ composite modified electrode upon successive additions of aliquots of H₂O₂ is shown in Fig. 8a. To avoid or decrease the interference of other electro-active species and minimize the background current, an applied potential of −0.3 V was used for the amperometric determination of H₂O₂.⁵⁰ As H₂O₂ is added to the stirred PBS solution, well-defined steady state current responses are obtained within 2 s at the applied potential, and the current increases stepwise with successive additions of H₂O₂.

Fig. 8 (a) Typical steady-state response of the Ag/p-g-C₃N₄/GCE to successive injection of H₂O₂ into the stirred N₂-saturated 0.2 M PBS at pH 7.4 (applied potential: −0.30 V), and (b) the corresponding calibration curve.

The current response displays a good liner behavior in a concentration range from 0.1 to 39.5 mM with a correlation coefficient of 0.999 (as shown in Fig. 8b). And the detection limit is estimated to be 0.6 μM at a signal-to-noise ratio of 3. Notably, our present sensing system shows a lower detection limit than most of those sensors for detection of H₂O₂, such as Ag NPs (1.0 μM),⁵¹ PEDOT/Ag NPs (7.0 μM),⁵² Ag NPs/DNA (1.7 μM),⁵³ Ag NPs/PQ11/GN (28 μM),⁵⁴ Ag NPs/rGO (31.3 μM),⁵⁵ Ag NPs–CDs (0.5 μM),²⁵ nitrogen-doped carbon nanotubes (0.37 μM).⁵⁶ Taking advantage of its cheaper and easier preparation approach, as-prepared Ag/p-g-C₃N₄ composite is an excellent choice for construction of enhanced electrochemical sensing platform.

From these results, the well-dispersed Ag NPs and the p-g-C₃N₄ support show a synergistic effect on the superior electrocatalytic activity. The mechanism for H₂O₂ electro-reduction can be expressed as follows:⁵⁶

H₂O₂ + e⁻ → OH_ad + OH⁻ (1)

OH_ad + e⁻ → OH⁻ (2)

2OH⁻ + 2H⁺ → 2H₂O (3)

The formation of OH_ad (i.e. chemisorbed OH) is a key step controlling the reaction rate in the reduction of H₂O₂.⁵⁷ For as-prepared Ag/p-g-C₃N₄ composite modified electrode, the well-dispersed Ag NPs with high number density and their surfactant-free surface attributed to the green synthetic approach could make the main contributions to the observed excellent catalytic activity.³⁸ The pyridinic nitrogen in the p-g-C₃N₄ support also play an important role in enhancing its electrocatalytic activity.^58,59 In addition, the excellent electrochemical sensing performance of as-synthesized Ag/p-g-C₃N₄ composite could be also attributed to its relatively large specific surface area and porosity.

For practical applications, we also investigate the selectivity of the Ag/p-g-C₃N₄ composite modified electrode for H₂O₂ detection. Ascorbic acid (AA), uric acid (UA) and glucose (Glu) are three common electroactive substances and commonly present in physiological samples, so interfering effect of AA, UA and Glu on the amperometric response of Ag/p-g-C₃N₄/GCE toward H₂O₂ was investigated. As shown in Fig. 9 a well-defined current response is observed for H₂O₂ while no obvious current responses to 1.0 mM AA, 1.0 mM UA and 2.0 mM Glu are observed, which is largely attributed to the relatively low applied potential of −0.3 V used in the determination of H₂O₂.⁵⁰ The observation indicates that the electrode could be applied to the selective determination of H₂O₂ in real samples.

Fig. 9 Current–time curves for the Ag/p-g-C₃N₄/GCE exposed to AA, UA (1.0 mM), glucose (2 mM) and H₂O₂ (1 mM) each.

4 Conclusions

In summary, N-doped GQDs has been proved to be an efficient photocatalyst for photochemical synthesis of Ag NPs loaded p-g-C₃N₄ for the first time. The FESEM and TEM images of Ag/p-g-C₃N₄ composite revealed that Ag NPs were well-formed in high yield and closely anchored at the surface of p-g-C₃N₄ with uniform size distribution. Moreover, the cyclic voltammetry and amperometry results show that the electrochemical sensor of as-prepared Ag/p-g-C₃N₄ composite exhibit excellent analytical response to H₂O₂ with fast response, wide linear range and low detection limit. Importantly, this method opens up a green photocatalytic synthesis of noble metal NPs/p-g-C₃N₄ nanocomposites for sensing and other applications.

Acknowledgements

This work was supported by the financial supports of Jiangsu Planned Projects for Postdoctoral Research Funds (1202040C), College Natural Science Research Program of Jiangsu Province (13KJB610003), Zhenjiang Industry Supporting Plan (GY2013023), Zhenjiang Social Developing Plan (SH2013002), Research Foundation for Talented Scholars of Jiangsu University (11JDG149 and 10JDG133) and Jiangsu Innovation Training Program Plan for College Students (201310299055Y).

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