Rutin detection using highly electrochemical sensing amplified by an Au–Ag nanoring decorated N-doped graphene nanosheet

Abstract

A hybrid nanostructure of Au–Ag nanorings prepared by decorating the surface of N-doped graphene (NG) was utilized as an electrocatalyst to construct a novel electrochemical sensor. Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) were used to characterize the as-prepared composites. The Au–Ag nanorings/NG modified electrode exhibited a much better electrochemical response of rutin than that of Au/NG, Ag/NG and NG due to the synergetic catalytic effect between the Au–Ag nanorings and NG. Under optimal conditions, the electrochemical sensor of the Au–Ag nanorings/NG exhibited a wide linear range from 0.05 μM to 241.2 μM (S/N = 3) with a low detection limit of 0.01 μM. In addition, the proposed sensor also displayed good anti-interference ability and long-term stability, which had promising applications in bioassay analysis.

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1. Introduction

Rutin is one of the most commonly found flavone glycosides identified as vitamin P with quercetin and hesperidin. It is presented in numerous typical nutrimental plants and is an important dietary constituent of food and plant-based beverages.¹ Some related investigations show that rutin exerts an extensive range of clinically relevant functions, including hypotensive effect, anti-inflammatory, anti-hypertensive, hemostat, anti-bacterial, anti-tumor, anti-allergic and anti-oxidant.^2–4 Furthermore, rutin also plays a crucial role in the regulation of the capillary permeability and the stabilization of platelets.⁵ In this case, it is of great significance to exploit an efficient method for sensitive rutin determination in pharmaceutical drugs and biological samples. To date, many detection assays, including high-performance liquid chromatography,⁶ capillary electrophoresis,⁷ chemiluminescence,⁸ and electrochemistry, have been established to determine rutin. However, some of these methods are long time-consuming, require sophisticated instrumentation and tedious sample preparation procedures, which limit their practical applications. Among them, electrochemical methods have the preferable advantages of simple pretreatment procedure, fast analytical time, and precise and sensitive current measurements, and different modified electrodes have been electrochemically attempted for the determination of rutin because it is an electroactive compound.^9–12

In order to amplify the signal and improve the detection sensitivity, it is necessary to explore new electrochemical and biological sensors. The development of biocompatible nanomaterials has recently opened as a new horizon for achieving powerful biosensors based on direct electron transfer between the biomolecule and electrode. Recently, Au-based nanomaterials, especially Au–Ag bimetallic nanoparticles, are promising catalysts that can enhance the catalytic activity and sensitivity of nanomaterials, thereby leading to an improvement in the signal amplification of the sensor.^13–17 For instance, Yu et al. developed the dendritic Au–Ag bimetallic nanoparticles with chemiluminescence (CL) activity by the seed-assisted approach, which presented a new assay for the detection of D-glucose with high sensitivity and reproducibility.¹⁸ In addition, finely manipulating the morphology and structure of Au–Ag bimetallic can offer a great opportunity to achieve enhanced catalytic performance. Recent research found that Ag/Au alloy NPs provided greater catalytic activities for H₂O₂ reduction compared to NPs of Au or Ag alone, especially in a shape-controlled manner.¹⁹

In electrocatalysis and electrochemical sensors applications, to improve the electrocatalytically active surface area, catalytic activity and stability, these Au nanoparticles are designed for introducing a support material to form hybrids, which subsequently avoid them from aggregating. Recently, nitrogen-doped graphene (NG), as a novel type of graphene derived 2D materials, has receive considerable attention by virtue of its excellent properties owing to the introduction of nitrogen atoms.^20,21 Compared to pristine graphene, the special 2D structure of NG with heteroatomic defects and disordered surface morphology shows improved electrical conductivity, thermal stability, and specific surface area.²² In addition, recent studies have shown that doping N in graphene can enhance the interaction between graphene and nano-materials, as well as the active site for the adsorption of metal nanomaterial on graphene and their catalytic activity.^23,24 For example, Jain fabricated the N-doped GNs with Au NPs by an ethylene glycol reduction process and further used it for the biosensing HbA1c.²⁵ To this end, the combination of metal-based NPs and the special 2D carbon-based material NG at the nanoscale dimension may attract a flurry of activities. In this study, we developed a nanohybrid of Au–Ag nanorings and NG via a two-step method, in which the Ag NPs/NG hybrid was first obtained by using trisodium citrate dehydrate, then Au–Ag nanorings were developed for assembly on NG by a galvanic replacement method with AA as the reducing agent. The morphology, structure and electrochemical properties of the as-prepared Au–Ag nanorings/NG were examined in detail. It was found that the Au–Ag nanorings/NG modified glass carbon electrode (GCE) exhibited excellent sensing performances for electrochemical detection of rutin.

2. Experimental section

2.1 Materials and reagents

Trisodium citrate dehydrate (Na₃C₆H₅O₇·2H₂O), chloroauric acid (HAuCl₄), silver nitrate (AgNO₃), cetyltrimethylammonium chloride (CTAC), disodium hydrogen phosphate (Na₂HPO₄), sodium dihydrogen phosphate (NaH₂PO₄), sodium chloride (NaCl), calcium carbonate (CaCO₃), ammonium hydroxide (NH₃·H₂O), ammonium phosphate ((NH₄)₃PO₄), tannic acid (TA), glucose, glycine and rutin were purchased from Sinopharm Chemicals Reagent Co., Ltd. Ascorbic acid (AA), uric acid (UA), and dopamine (DA) were obtained from Acros Organics. N-doped graphene was bought from Xianfeng Reagent Co. Ltd (Nanjing, China). All reagents were of analytical grade and used as received. Deionized water (DI) was used throughout the experiments.

2.2 Apparatus

The morphologies of the products were characterized on a TECNAI-G20 electron microscope (TEM) at an accelerating voltage of 200 kV. High-resolution TEM images were obtained on a JEM-2100F high-resolution transmission electron microscope operating 200 KV. X-ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al Kα X-ray radiation for excitation. All the electrochemical experiments were performed in a conventional three-electrode system at room temperature using a CHI 760E potentiostat/galvanostat (CH Instrumental Co. Ltd, China). The glassy carbon electrodes (GCEs) with a diameter of 3 mm and their modified electrodes were used as the working electrodes, a Pt wire and saturated calomel electrode (SCE) as the counter electrode and reference electrode, respectively. All the measurements were carried out at room temperature.

2.3 Synthesis of silver/N-doped graphene (Ag/NG), Au–Ag nanorings/NG and Au/NG composites

Ag nanoparticles supported on N-doped graphene (NG) were prepared based on the following method. Primarily, NG (2 mg, 1 mg mL⁻¹) was dispersed in DI water (20 mL), and the solution was then added to a three-neck round flask. After adding sodium citrate (21.6 mg), the solution was heated with an oil bath at 100 °C under magnetic stirring. Upon boiling, silver nitrate (6 mg) was added, and the system was refluxed and stirred for 1.5 h, resulting in a solution color change into blackish green. After cooling to room temperature, the resulting dispersion was centrifuged at 6000 rpm for 10 min and washed with DI water three times. The as-prepared Ag/NG product obtained was re-dispersed in DI water (10 mL) for further use. Second, we prepared Au–Ag nanorings containing NG supported Ag-seeds via a galvanic replacement reaction between the Ag nanocrystal seeds and aqueous HAuCl₄ solution. 15 mL of aqueous CTAC (1.75 mmol) and AA (0.35 mmol) were mixed in a 50 mL vial, and 1 mL HAuCl₄ aqueous solution (24.3 mM) was then added (the corresponding molar ratio of Au to Ag is 1 : 1.5). The abovementioned mixture was stirred for 5 min, followed by the injection of the as-prepared Ag/NG solution. The reaction vial was gently shaken then capped and allowed to sit undisturbed in an oil bath at 30 °C for 14 hours. The obtained precipitate was collected by centrifugation and then washed with saturated NaCl solution, ammonia solution, ethanol and DI water. Finally, the product was dispersed in ethanol–water (5 mL) to obtain the catalyst “ink” by sonication. The described processes of the synthesis of the Au–Ag nanorings/NG are shown in Scheme 1. Similarly, the Au₁Ag₁–NG and Au_1.5Ag₁–NG samples were prepared under the same condition by varying different volumes of Ag-seeds. In addition, Au/NG with the same manner was prepared without the introduction of Ag-seeds. For comparison, 2 mg NG was dispersed in 5 mL in ethanol–water (5 mL) for further use.

Scheme 1 Illustration of the synthesis of the Au–Ag nanorings/NG composite.

3. Results and discussion

3.1 Characterization

The structure and morphology of the as-prepared Au–Ag nanorings/NG, Au/NG, and Ag/NG composites were investigated by TEM, as shown in Fig. 1. Fig. 1A and B show typical TEM images of the as-prepared Au–Ag/NG composite with different magnifications, clearly finding that a pile of Au–Ag nanorings particles were dispersed on the NG surface. However, as presented in Fig. 1C and D, without the presence of initial Ag/NG, it was interesting to observe that dendritic-like structure Au was obtained by serving AA as the reductant and CTAC as a structure directing agent. In order to investigate the structural evolution of the Au–Ag nanorings/NG composite, the morphology of the initially generated Ag/NG is shown in Fig. 1E and F. As observed, a group of cumulated Au nanoparticles were deposited on the NG surface, which has been employed to successfully produce these special Au–Ag nanorings/NG through the galvanic replacement method. In addition, Fig. S1† shows the HR-TEM and HAADF-STEM-EDS mapping images. As observed, Fig. S1(B and C)† show the lattice-resolved TEM images from regions A; lattice fringes are separated by 0.23 nm, corresponding to (111) planes of face-centered cubic (fcc) Au–Ag. As shown in Fig. S1(D–G),† the elemental distribution of Au and Ag was mainly focused on the center of Au–Ag nanorings and could be overlapped, indicating the formation of Au–Ag alloy.

Fig. 1 TEM images of the as-synthesized nanomaterials with different magnification. (A and B) Au–Ag nanorings/NG, (C and D) Au/NG, (E and F) the initial Ag/NG.

In order to obtain the chemical state and the atomic surface composition of Au–Ag nanorings/NG, XPS was performed. As shown in Fig. 2A, the XPS spectrum suggested that the sample contained C, N, O, Au and Ag, which provided evidence for nitrogen doping into the graphene sheets. The deconvoluted C 1s XPS spectra in Fig. 2B showed four peaks at 284.7, 285.2, 286.8, and 288.4 eV, representing four types of carbon originating from C–C, C–N, C–O, and O–C=O.^26,27 The high-resolution O 1s spectra of Au–Ag nanorings/NG are displayed in Fig. 2C, in which the deconvoluted three peaks around 531.4, 532.4 and 533.6 eV can be assigned to C–O, C=O and O–C=O groups, respectively.^28,29 Fig. 2D shows the N 1s spectra of the catalysts deconvoluted with three types of nitrogen: pyridinic type N (397.8 eV), amino-type N (399.7 eV) and graphitic-type N (402.3 eV).³⁰ Fig. 2E and F present the high-resolution XPS spectra of Au 4f and Ag 3d, respectively. In the spectrum of Au 4f, the two peaks that appeared at 83.9 and 87.5 eV could be attributed to Au 4f7/2 and Au 4f5/2 peaks, respectively, which confirmed the existence of metallic Au in the composite. Moreover, the observation of the Ag 3d5/2 and 3d3/2 peaks at 367.8 and 373.8 eV also indicated that Ag is mainly present in the metallic state.³¹

Fig. 2 Survey XPS of the Au–Ag nanorings/NG composites (A), XPS core level spectra of C 1s (B), O 1s (C), N 1s (D), Au 4f (E) and Ag 3d (F) in the composite.

3.2 Electrocatalytic activity on the different modified electrodes

Cyclic voltammetry (CV) is an efficient, well-accepted analytical method to monitor surface modification. Fig. 3A shows the voltammograms obtained at NG, Ag/NG, Au/NG, Au–Ag/NG electrodes in 5 mM K₃Fe(CN)₆ containing 0.1 M KCl solution. As observed, the Au–Ag nanorings/NG composite exhibited a tremendously increased peak current (I_pa: 153.6 μA) compared to the bare NG (85.3 μA), Ag/NG (100.7 μA), Au/NG (131.8 μA) nanostructures, demonstrating better catalytic ability of Au–Ag/NG composite that can promote the electron transfer rate of Fe(CN)₆^3−/4−. These results indicated that the combination of Au–Ag nanorings and NG could generate an advanced and sensitive electrode surface, which might be a better choice for the fabrication of electrochemical sensors. The electrochemical response of different electrodes to 0.1 mM rutin in 0.1 M PBS (pH = 3.0) was investigated by CV and the corresponding curves are shown in Fig. 3B. Since rutin was an electroactive compound with four hydroxyl groups present on its molecular structure, it was anticipated that a pair of redox peaks appear at all the electrodes, which suggested that the electrochemical reaction of rutin can be successfully achieved. Moreover, at the Au–Ag/NG electrode, rutin obtained a remarkably larger oxidation peak current (48.5 μA) than those obtained on NG (19.6 μA), Ag/NG (22.8 μA), and Au/NG (42.4 μA) electrodes. For the Au–Ag/NG composites, the oxidation peak currents increased, indicating that the electrochemical reaction of rutin could be efficiently carried out due to the excellent conductivity, special nanorings structure, rapid electron transfer and large surface area of Au–Ag nanorings and NG. In addition, we have recorded the electrochemical response for rutin of Au₁–Ag₁/NG and Au_1.5–Ag₁/NG electrode. As shown in Fig. S2,† although the Au_1.5–Ag₁/NG displayed a higher current density when the Au to Ag molar ratio was set at 1.5 : 1, all these catalysts exhibited a higher electrochemical response than that of the Au/NG, Ag/NG and NG catalysts.

Fig. 3 (A) CVs of NG, Ag/NG, Au/NG, Au–Ag/NG electrodes recorded in 5 mM Fe(CN)₆^3−/4− + 0.1 M KCl solution (A), 0.1 M PBS (pH = 3.0) containing 0.1 mM rutin (B). Scan rate: 50 mV s⁻¹.

3.3 Effect of the scan rate

To verify the fast electron-transfer properties of the electrode, Fig. 4 displays the CVs of 0.1 mM rutin on the Au–Ag/NG modified electrode at different scan rates. It was found that both the anodic peak current (I_pa) and cathodic peak current (I_pc) of rutin increased linearly with increasing scan rates ranging from 20 to 200 mV s⁻¹. As shown in Fig. 4B, the relationship of redox peak current (I) versus scan rate (υ) was carried out with two linear regression equations as I_pa (μA) = 0.9608υ (mV s⁻¹) + 8.4318 (R² = 0.998) and I_pc (μA) = −0.8692υ (mV s⁻¹) − 5.0601 (R² = 0.998), demonstrating that the electrode reaction of rutin was a predominantly adsorption-controlled process. Moreover, the redox peak potentials shifted slightly along with the increase of the scan rate, as depicted in Fig. 4C, suggesting a quasi-reversible electrochemical process.³² The anodic (E_pa) and cathodic (E_pc) peak potentials have a linear relationship with the natural logarithm of scan rate (ln υ), which could be expressed as E_pa (V) = 0.0305 ln υ + 0.4663 (R² = 0.983) and E_pc (V) = −0.0286 ln υ + 0.6329 (R² = 0.983), respectively. According to the following Laviron's equation,³³where α is the electron transfer coefficient, n is the number of electron transferred, υ is the scan rate, and E′⁰ is the formal potential. R, T and F have their conventional meanings. Together with eqn (1) and (2), α and n were obtained as 0.5161 and 1.74, respectively, indicating that there were two approaching electrons involved in the reaction of rutin, which was also consistent with the previously reported result.

Fig. 4 (A) CV curves of Au–Ag/NG electrode in 0.1 M PBS (pH = 3.0) solution containing 0.1 mM rutin at different scan rates of 20, 40, 60, 80, 100, 120, 140, 160, 180, and 200 mV s⁻¹. (B) Plots of anodic and cathodic peak currents versus the scan rate. (C) Anodic and cathodic peak potentials versus ln υ.

3.4 Effect of buffer pH

To evaluate the analytical performance of the Au–Ag/NG composite sensor, a series of various buffer pH solution was measured under the optimized conditions. Fig. 5A describes the CVs of 0.1 M PBS solutions containing 0.1 mM rutin with the pH ranging from 2.0 to 6.0. It can be observed that a pair of well-defined redox peaks appeared with the change of the electrochemical response at different pH values, and the redox peak potentials shifted to the negative direction with increasing buffer pH, indicating that protons took part in the electrode reaction. The linear relationship between the anodic peak potential (E_pa) and pH was presented as E_pa (V) = −0.0642pH + 0.7730 (R² = 0.998) with a slope value of −0.0642 V pH⁻¹, which was close to the anticipated Nernstian value of −0.0591 V pH⁻¹ at 25 °C, indicating that electron transfer was accompanied by an equal number of protons in electrode reaction, as shown in Fig. 5B.³⁴ Furthermore, as shown in Fig. 5C, the maximum value of I_pa appeared at a pH of 3.0 and subsequently decreased with the further decrease in buffer pH, which provide suitable evidence to choose the optimal pH for rutin detection.

Fig. 5 (A) CVs of the Au–Ag/NG electrode in 0.1 mM rutin with different pH PBS solution (from a to e is 2.0, 3.0, 4.0, 5.0, 6.0), scan rate: 50 mV s⁻¹. (B) Relationship between the oxidation peak potential (E_pa) and pH value. (C) Relationship between the oxidation peak current and pH value.

3.5 Calibration curve

Under the optimal conditions, differential pulse voltammetry (DPV) was used to determine rutin due to its high sensitivity and selectivity, and the result is displayed in Fig. 6A. As observed, the response of the sensor toward oxidation peak current of rutin increased with its concentration ranged from 0.05 μM to 241.2 μM. As shown in Fig. 6B, the relationship between oxidation peak current and rutin concentration was divided into two linear ranges. In the range from 0.05 μM to 152.8 μM, the linear regression equation was fitted as I_pa (rutin) = 1.4333C + 114.28, R² = 0.946, and in the range from 152.8 μM to 241.2 μM, its linear regression equation was obtained as I_pa (rutin) = 0.1249C + 148.24, R² = 0.981. As a result, this curve reached a relatively low detection limit of 0.01 μM with a linear range from 0.05 μM to 241.2 μM (S/N = 3). Based on different modified materials, a comparison for rutin detection is listed in Table 1. As shown in Table 1, the Au–Ag/NG electrode exhibited a relatively broader detection range and lower detection limit.

Fig. 6 (A) DPV curves of Au–Ag/NG electrode in different concentrations of DA (0.05–241.2 μM) containing 0.1 M PBS (pH = 3.0) solution. (B) Plots of oxidation currents versus the concentration of rutin.

Table 1 Comparison of different electrochemical sensor proposed for the determination of rutin

Electrodes	Linear range (μM)	LOD (μM)	Reference
a Multiwalled carbon nanotubes.b Carbon nanotube paste electrodes.
Au/en/MWNT^a	0.048–0.96	0.032	35
Fc–SAc/AuNPs/GCE	0.05–30	0.01	36
PtNP/RGO/GCE	0.05–10	0.015	37
GR/CILE	0.07–100.0	0.024	38
MIP–MWNPE	0.08–1.4	0.05	39
Nafion–GO–IL/CILE	0.08–100.0	0.016	40
CNTPE^b	0.199–9.99	0.0339	41
g-C3N4/TEA	0.2–45.0	0.14	8
IL/GCE	0.3–100	0.024	42
Au–Ag/NG	0.05–241.2	0.01	This work

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3.6 Long-term stability, interference studies and real samples analysis for the Au–Ag/NG electrode

The long-term stability to electrochemical response of Au–Ag/NG electrode was further evaluated by 500 successive CV numbers in 0.1 M PBS (pH = 3.0) containing 0.1 mM rutin. As shown in Fig. 7A, using the oxidation peak current of the 2^nd cycle as the reference, the peak current of rutin had a dramatic decrease in the initial 200 cycles, which subsequently continued to decrease from 200 cycles to 500 cycles. Finally, it was found that the oxidation current response of the sensor still remains up to 87.30% of the 2^nd cycle value at the end of 500 cycles, indicating that the Au–Ag/NG electrode has good long-term stability. To investigate the morphological change after long-term use, Fig. S3† shows TEM images of Au–Ag/NG catalyst after 500 cycles of CV in 0.1 M PBS (pH = 3.0) containing 0.1 mM rutin. It reveals that the Au–Ag structure was almost unchanged, i.e., it still maintained the previous nanoring structure after 500 cycles, further indicating the long-term stability. Furthermore, the interference from coexisting species of rutin was also investigated, and the results are shown in Fig. 7B. As it can be seen, 100-fold KCl, Na₂SO₄, CaCO₃ and (NH₄)₃PO₄, 10-fold AA, UA, TA, glycine and glucose were chosen as the interferential species in the tests. Noticeably, the relative errors of KCl, Na₂SO₄, CaCO₃, (NH₄)₃PO₄, AA, UA, DA, TA, glycine, and glucose were −0.2%, −8.2%, −1.2%, 4.3%, 0.5%, −6.5, 6.6%, 3.8%, 2.7% and −5.8%, respectively, which was no more than 10%, demonstrating a good anti-interference ability of Au–Ag/NG toward rutin detection. To evaluate the practicality and reliability of the proposed method, a medicinal rutin tablet (label amount: 20 mg per tablet) was employed as standard sample for the determination of its rutin content. Three parallel determinations were performed and the results are listed in Table 2. As expected, the standard addition recoveries in rutin tablet solution are 98.4%, 99.4%, and 105.7%, indicating that the Au–Ag NTs/NG can be used efficiently for the determination of a real sample.

Fig. 7 (A) 500 cycle numbers of stability test in 0.1 M PBS (pH = 3.0) containing 0.1 mM rutin. (B) Interference study on the response of 100-fold KCl, Na₂SO₄, CaCO₃ and (NH₄)₃PO₄, 10-fold AA, UA, DA, TA, glycine and glucose.

Table 2 Determination of rutin in commercial tablets

Sample no.	Declared (mg per table)	Found (mg per table)	RSD (%)	Recovery (%)
1	20	19.69	3.2	98.4
2	20	19.87	1.7	99.4
3	20	21.14	5.7	105.7

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4. Conclusions

Au–Ag nanorings supported on the surface of NG were prepared via a facile and galvanic replacement method using AA as the reducing agent in the presence of CTAC. The electrochemical response of the elaborately-designed Au–Ag nanorings/NG composite toward rutin detection was evaluated by a comparison with the catalytic ability of Au/NG, Ag/NG and NG. Moreover, the proposed hybrid electrode was utilized for rutin sensing and exhibited a detection limit of 0.01 μM with a broad linear range from 0.05 μM to 241.2 μM. Therefore, this novel material could be anticipated for electrochemical sensing and biosensing applications involving the detection of different biomolecules.

Acknowledgements

The authors gratefully acknowledged the support from the National Natural Science Foundation of China (Grant No. 51373111), the Local and State Joint Engineering Laboratory for the Priority Academic Program Development (PAPD) and the Novel Functional Polymeric Materials of Jiangsu Higher Education Institutions.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra22264a

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