Preparation of β-cyclodextrin functionalized reduced graphene oxide: application for electrochemical determination of paracetamol

Abstract

β-Cyclodextrin functionalized reduced graphene oxide (β-CD/RGO) was successfully prepared using a simple wet chemical method. The β-CD/RGO nanohybrid was characterized by UV-vis spectroscopy, FTIR, Raman spectroscopy, TEM and SEM. The results confirmed that β-CD had effectively covered the RGO surface. The β-CD/RGO nanohybrid modified glassy carbon electrode was employed for the sensitive electrochemical determination of paracetamol. Cyclic voltammetry measurements indicated that β-CD/RGO could significantly enhance the electrochemical response of paracetamol due to the outstanding electronic properties of RGO sheets and the high supramolecular recognition and enrichment capability of β-CD. The experimental factors were investigated and optimized. Under optimized conditions, the amperometric oxidation currents of paracetamol were linearly proportional to the concentration in the range of 0.01 to 0.8 mM with a detection limit of 2.3 μM (S/N = 3). Furthermore, the proposed sensor exhibited an excellent anti-interference property and acceptable reproducibility.

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1. Introduction

Paracetamol (acetaminophen, N-acetyl-p-aminophenol) is an effective antipyretic and analgesic drug used for the widespread relief of pain and fever, which has been recognized as an alternative to aspirin.^1,2 Although paracetamol shows no propensity to be addictive over long term use and no side effects in the proper therapeutic dose,³ an overdose of paracetamol can produce toxic metabolite accumulation leading to liver and kidney damage.^4,5 Thus, it is very valuable to develop a simple, inexpensive, rapid, sensitive and precise determination method for general public health purposes. To date, several analytical methods such as spectrophotometry,⁶ spectrofluorometry,⁷ high performance liquid chromatography,⁸ titrimetry,⁹ chemiluminescence,¹⁰ gas chromatography,¹¹ capillary electrophoresis¹² and electrochemical methods^1,13–18 have been studied for the determination of paracetamol. Among them, electrochemical technique is more attractive compared with other methods due to the relatively simple sample preparation, fast response and low detection limits.

In order to improve the performance of traditional electrodes, many kinds of materials such as carbon materials,^13–17 noble metal nanoparticles,^1,19,20 conductive polymers¹⁷ and metal oxides¹⁸ have been extensively employed as electrocatalysts for the construction of paracetamol sensors. Among various carbon materials, graphene, a two-dimensional sheet of sp² hybridized carbon, have attracted tremendous attention since physicist Geim and co-workers isolated in 2004.^21,22 The graphene materials possess extraordinary mechanical strength, large specific surface area and high conductivity. It has showed great performance in developing various electrochemical sensors.^23–27 However, one severe problem that may limit its applications is most graphene used in electrode modification is reduced graphene oxide (RGO) which is reduced from graphene oxide (GO). RGO is prone to irreversible spontaneous agglomeration in water, which highly restricts its application in water based systems and also further modification with other compounds. To overcome this problem, poly(diallyl dimethyl ammonium chloride) (PDDA) has been used as a dispersive reagent for stopping RGO sheets agglomeration. Study showed that the PDDA functionalized RGO exhibits excellent conductivity, biocompatibility and solubility.^28,29

Meanwhile, β-cyclodextrin (β-CD) is a cyclic oligosaccharide consisting of seven glucose units and being toroidal in shape with a hydrophobic inner cavity and a hydrophilic exterior. Due to its unique ability to form stable host–guest inclusion complexes, it has been immobilized on electrodes to interact with many organic, inorganic and biological guest molecules.^30–32 Till now, β-CD has been applied in sensors for detecting various molecules such as methyl parathion,³³ doxorubicin, methotrexate,³⁴ 4-nitrophenol,³⁵ dopamine³⁶ and imidacloprid.³⁷

We believe the integration of β-CD with PDDA functionalized graphene might possess both unique properties together and service as a promising composite material for sensing purpose. Herein, we report a wet chemical strategy to synthesize the β-CD/RGO nanohybrid with excellent dispersity. The obtained β-CD/RGO nanohybrid is characterized by SEM, FTIR, UV-vis spectroscopy and Raman spectroscopy. The composite is then modified onto an electrode for the development a sensitive electrochemical sensor for the measurement of paracetamol. The electrochemical enhancement of paracetamol at β-CD/RGO modified electrode due to the synergic effect of graphene and β-CD is studied in detail.

2. Experimental

2.1 Chemicals

Poly(diallyl dimethyl ammonium chloride) (PDDA, 20 wt%, M_w = 100 000–200 000 g mol⁻¹), hydrazine hydrate, paracetamol, uric acid (99%), L-ascorbic acid (99%), 3-hydroxytyramine hydrochloride (DA), and glucose were purchased from Sigma-Aldrich. Graphene oxide (GO) powder was purchased from JCNANO, INC. β-Cyclodextrin (β-CD) was purchased from SANGHAIHUSHI. All other chemicals used were analytical grade reagents without further purification. Phosphate buffer solution (PBS) was prepared by mixing 0.2 M KH₂PO₄ and K₂HPO₄ solution to appropriate pH. Milli-Q water (18.2 MΩ cm) was used throughout the experiments.

2.2 Preparation of β-CD/RGO nanohybrid

In a typical synthesis of β-CD/RGO nanohybrid, 10 mg GO was firstly dispersed in 10 mL water facilitated by sonication. Then, 1 mL PDDA and 0.5 g β-CD were successively added to the GO dispersion. After being vigorously stirred for 2 h, 500 μL ammonia and 300 μL hydrazine were added into the solution. The mixture was then heated to 90 °C and maintained at the temperature for 4 h. Finally, a black precipitation was collected by centrifugation followed by three times water washing. RGO sample was also synthesized by similar method without adding PDDA and β-CD.

2.3 Characterization

The surface functional groups present on the samples were analyzed by Fourier transform infrared spectroscopy (FTIR, Nicolet iS5, Thermo Scientific, USA). The optical characterizations were obtained by UV-vis spectrophotometer in the wavelength range from 190 to 800 nm. The surface morphology of the samples was analyzed by a field emission scanning electron microscope (FESEM, ZEISS SUPRA 40VP). The structure of the products was analyzed by transmission electron microscopy (TEM: JEOL JEM-2100 Japan Optics Laboraory CO., Ltd, Japan) with an acceleration voltage of 200 kV. Raman spectroscopy was performed at room temperature using a Raman microscope (Renishaw, inVia) with 514 nm laser light.

2.4 Electrochemical detection of paracetamol

A glassy carbon electrode (GCE) was polished by 0.3 and 0.05 μm alumina slurry followed by thoroughly rinsing with ethanol and water. For the electrode surface modification, 5 μL of as-prepared β-CD/RGO dispersion (0.5 mg mL⁻¹) was dropped onto the GCE surface and dried at room temperature.

All electrochemical measurements were performed on a CHI430a electrochemical workstation (USA) at room temperature. A conventional three electrode system containing a modified GCE as working electrode, a platinum wire as auxiliary electrode and a Ag/AgCl (3 M KCl) electrode as reference electrode was used throughout the electrochemical experiments.

3. Results and discussion

3.1 Characterization of β-CD/RGO nanohybrid

Fig. 1 illustrates the procedure for the synthesis of β-CD/RGO nanohybrid and the sensing mechanism for guest molecules (paracetamol). Briefly, the homogeneous GO dispersion was mixed with β-CD and PDDA. GO was then reduced by hydrazine under an alkaline condition. The introduction of PDDA was to improve the dispersity of β-CD/RGO nanohybrid after the reduction of GO. The resulting dispersion remains stable for more than one month.

Fig. 1 Schematic diagram of the formation of β-CD/RGO which was used for sensing paracetamol molecules via host–guest interaction.

The obtained nanohybrid was confirmed by the UV-vis spectroscopy. Fig. 2A displays the UV-vis spectra of GO and β-CD/RGO dispersions. It can be clearly seen that GO exhibits a characteristic absorption peak at 227 nm, corresponding to the π → π* transition of the C=C bonds.³⁸ After the chemical reduction, this peak gradually red-shifted to 264 nm, suggesting the restoration of sp²-carbon network.

Fig. 2 (A): UV-vis spectra and (B): Raman spectra of GO and β-CD/RGO dispersions.

Raman spectroscopy was used to characterize the structural changes from GO to β-CD/RGO. As shown in Fig. 2B, the spectrum of GO has two bands at 1592 and 1350 cm⁻¹, corresponding to the graphite (G) and diamondoid (D) bands, respectively. The G band is a characteristic feature of graphitic carbon layers corresponding to the tangential vibration of the carbon atoms, whereas the D band is a typical sign of the presence of defective graphitic carbon.³⁹ The intensity ratio of the D and G peaks (I_D/I_G) increases from 0.896 to 0.961 for the β-CD/RGO, indicating the restoration of sp² domains on reduction with hydrazine.⁴⁰

FTIR analysis is used to analyse the surface functionalization process of graphene sheets. The FTIR spectra of GO, β-CD, PDDA and β-CD/RGO are depicted in Fig. 3. The spectrum of GO displays peaks at 1710, 1592, 1398 and 1024 cm⁻¹, which belong to the C=O stretching of COOH groups, C=C vibrations, C–OH stretching vibrations and C–O vibrations from alkoxy groups, respectively.^41–43 After hydrazine reduction, the intensity of these characteristic peaks significantly decreased or even vanished, indicating the GO sheets have been reduced successfully. Compared with the β-CD spectrum, the spectrum of β-CD/RGO exhibits typical β-CD absorption peaks of CH_n stretching vibrations at 2925 cm⁻¹, O–H bending vibrations at 1140 cm⁻¹ and the coupled C–O/C–C stretching at 1026 cm⁻¹, which confirm the successful functionalization of β-CD to the RGO sheets.^44–46 In addition, compared with the spectrum of PDDA, β-CD/RGO spectrum shows the absorption peaks at 2925, 1480 and 882 cm⁻¹ correspond to CH_n, CH₂ and C–N vibrations of the nitroso groups of PDDA, respectively.^44,47 These absorption peaks confirmed the presence of PDDA. In summary, the spectrum of β-CD/RGO shows both features of β-CD and PDDA, indicating the RGO sheets were successfully functionalized with β-CD and PPDA.

Fig. 3 FTIR spectra of GO, β-CD, PDDA and β-CD/RGO.

The morphology of the GO, RGO and β-CD/RGO were observed under SEM characterization. As shown in Fig. 4, the GO (Fig. 4A) displays a typical wrinkled structure. After hydrazine reduction, the RGO (Fig. 4B) shows a clearly restacking effect due to the removal of surface negatively charged oxygen containing functional groups. However, the β-CD/RGO (Fig. 4C) shows a flatter surface with very low level of agglomeration in comparison to RGO, indicating the surface functionalization could actually prevent the restacking effect of RGO and remain the specific surface area of the material. The morphology of GO and β-CD/RGO was also characterized by TEM. According to the images shown in Fig. S1,† the surface of GO is smooth like silk fabric (Fig. S1A†). The surface contrast apparently increases after the modification of β-CD (Fig. S1B†). The increase of surface roughness is an indication that β-CD have been coated on the surface of RGO which is consistent with the literature.⁴⁸

Fig. 4 SEM images of (A) GO, (B) RGO and (C) β-CD/RGO.

3.2 Electrochemical activities of paracetamol at various electrodes

The electrochemical responses of paracetamol at various electrodes were investigated using cyclic voltammetry. As shown in Fig. 5A, paracetamol only shows an irreversible oxidation peak at a bare GCE (curve a). In contract, RGO modified GCE exhibits a pair of well-defined redox peak corresponding to the oxidation (0.40 V) and reduction (0.35 V) of paracetamol (curve b). This enhancement is probably due to the large surface area and outstanding conductivity which is provided by the RGO sheets. Moreover, when the as-prepared β-CD/RGO nanohybrid was modified onto the GCE surface, the current responses of this pair of well-defined redox peaks from paracetamol increased greatly although the background current of the β-CD/RGO modified GCE also increase in some degree (curves c and d). The reasons that β-CD could further increase the current response of paracetamol are mainly due to two aspects. Firstly, the adsorption of paracetamol on pure RGO is a single-layer adsorption, while on the β-CD/RGO nanohybrid it is a combined process of surface envelope behavior between cyclodextrin and paracetamol together with a single layer adsorption. Thus β-CD/RGO allows adsorbing more drug molecules on the electrode surface resulting in higher sensitivity. Secondary, the introducing β-CD onto the surface of RGO can enhance the sensitivity of detection for drug molecules through the formation of supramolecular complexes between β-CD and paracetamol molecules.⁴⁹ Therefore, the synergistic effect of β-CD and RGO provides a superior interface for performing the sensitive electrochemical measurement of paracetamol.

Fig. 5 (A) Cyclic voltammograms of (a) bare, (b) RGO and (d) β-CD/RGO modified GCE in 0.2 M pH 7.0 PBS with 0.1 mM paracetamol and (c) β-CD/RGO without paracetamol. Scan rate: 50 mV s⁻¹. (B) Cyclic voltammograms of β-CD/RGO modified GCE in 0.1 mM paracetamol at scan rates from 20 to 200 mV s⁻¹. The inset is the corresponding plots of current responses vs. scan rate.

The effect of scan rate on the redox current of paracetamol using β-CD/RGO modified GCE was investigated in Fig. 5B. It can be seen that both anodic and cathodic peak currents increase along with increasing scan rate accompanied with enlarged peak separation. Furthermore, both the anodic and cathodic peak current showed linear relationship with the scan rate from 20 to 200 mV s⁻¹ (inset of Fig. 5B), suggesting that the reaction of paracetamol at β-CD/RGO modified GCE is an adsorption controlled process.^19,20 The regression equations are E_pa = 0.32105 + 0.06482 log v (R = 0.996) and E_pc = 0.43301 − 0.05114 log v (R = 0.990), respectively. According to Laviron's model,^50,51 the number of electrons involved in the reaction and the charge-transfer coefficient constant can be calculated as 1.963 and 0.441, respectively. The results indicate that two electrons are involved in the electrochemical redox process of paracetamol.

3.3 Optimization of the detection conditions

Fig. 6A shows the effect of pH on the redox behavior of paracetamol using cyclic voltammetry at β-CD/RGO modified GCE. It can be clearly observed that both oxidation and reduction potentials shift to negative region with increasing pH, suggesting that the electrochemical reaction of paracetamol involves transfer of protons.^14,20 Moreover, as shown in the inset of Fig. 6A (black line), the oxidation potentials have a linear relationship with pH change. The regression equation can be expressed as E_pa (V) = 0.684 − 0.051 pH (R = 0.989). The slope value of 0.051 V pH⁻¹ is very close to the theoretical value of 0.059 V pH⁻¹ based on the Nernst equation,^14,17 indicating that the same proportion of protons and electrons involved in the redox process. Thus according to the calculated numbers of electrons (Section 3.2), the redox mechanism of paracetamol involves 2 protons and 2 electrons (Fig. 6B). In addition, the inserted Fig. 6A (blue line) shows the relationship between the oxidation peak currents and pH values. The peak currents increase from 5 to 7 and then decrease from 7 to 10. Therefore, pH 7 was chosen for the measurement of paracetamol, which is well fit to the real physiological conditions.

Fig. 6 (A): Cyclic voltammograms of 0.1 mM paracetamol at β-CD/RGO modified GCE in 0.2 M PBS at different pH values. pH: 5.0, 6.0, 7.0, 8.0, 9.0, 10.0. Scan rate: 50 mV s⁻¹. Inset: the relationship between pH and E_pa (black line) and the effect of pH on the oxidation peak currents (blue line). (B): Proposed redox reaction mechanism of paracetamol.

The effect of the amount of β-CD/RGO modified on the electrode surface on the current responses of paracetamol was investigated. As shown in Fig. 7A, the oxidation peak current increases remarkably when the amount of β-CD/RGO increases from 2 μL to 4 μL and reach to the maximum at 5 μL. After that, the peak current decreases, especially when the amount of β-CD/RGO exceeded 6 μL. Consequently, 5 μL of β-CD/RGO dispersion was utilized to modify the GCE.

Fig. 7 The plot of current response vs. (A) the amount of β-CD/RGO and (B) pre-adsorption time at β-CD/RGO modified GCE towards 0.1 mM paracetamol.

The pre-adsorption time is an important factor during the electrochemical determination of paracetamol. The relationship between oxidation peak currents and the pre-adsorption time was investigated from 0 to 12 min and depicted in Fig. 7B. The response current increases clearly from 0 to 4 min and remains similar responses after 4 min. Thus, 4 min pre-adsorption time was used throughout the measurements.

3.4 Amperometric determination of paracetamol

The amperometric response of β-CD/RGO modified GCE upon the successive addition of paracetamol in pH 7.0 PBS was shown in Fig. 8A. As can be seen, the β-CD/RGO modified GCE responds quickly on the concentration change of paracetamol. The steady-state current was achieved within 7 s after the addition of paracetamol. The inset of Fig. 8A represents the calibration curve between the current response and the concentration of paracetamol from 0.01 to 0.1 mM. The corresponding linear equation is I (μA) = 0.0278 C_paracetamol (μM) + 0.695 with a correlation coefficient of 0.998. The detection limit was calculated to be 2.3 μM at the signal to noise ratio of 3 (S/N = 3). The analytical performances of β-CD/RGO modified GCE were compared with other nitrite sensors in the literature and the results are shown in Table S1.†

Fig. 8 (A) Typical current–time response of β-CD/RGO modified GCE upon the successive addition of: 0.01 and 0.1 mM paracetamol. (B) Amperometric response of β-CD/RGO modified GCE upon the addition of 0.1 mM of paracetamol, uric acid, dopamine, ascorbic acid and glucose in PBS. Applied voltage: +0.4 V.

The influence of potential interferences on the electrochemical measurement of paracetamol was studied. Fig. 8B shows the amperometric response of β-CD/RGO modified GCE upon addition of paracetamol and various interfering species including uric acid, dopamine, ascorbic acid and glucose. As shown in Fig. 8B, the addition of 0.01 mM paracetamol results in a quick current response while the addition of 0.8 mM uric acid, dopamine, ascorbic acid and glucose does not affect the current response. The results reveal that the β-CD/RGO modified GCE has excellent selectivity even the sample contains 10-fold concentration of interference species.

The reproducibility of the β-CD/RGO modified GCE was investigated by 7 successive scans in the PBS containing 0.1 mM paracetamol. A relative standard deviation of 2.9% was obtained, indicating the β-CD/RGO modified GCE owing a good reproducibility.

4 Conclusions

In this work, we have demonstrated a novel paracetamol electrochemical sensor based on β-CD/RGO modified electrode. β-CD functionalized RGO was synthesized via a simple wet chemical method. The electrochemical behavior of paracetamol was greatly enhanced by the senor due to the combination of the advantages of both β-CD and RGO materials. Specifically, the fabricated electrode showed a liner detection range from 0.01 to 0.8 mM with a low detection limit of 2.3 μM. Moreover, the proposed paracetamol sensor exhibited an excellent anti-interference behavior.

Acknowledgements

Swinburne University Postgraduate Research Award (SUPRA) and the National Natural Science Foundation of China (21475033) are acknowledged for partially supporting this work.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra12520k

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