Determination of Bisphenol A in Beverages by an Electrochemical Sensor Based on Rh₂O₃/Reduced Graphene Oxide Composites

Abstract

A novel electrochemical sensor, based on a Rh₂O₃–reduced graphene oxide (rGO) composite modified carbon electrode, has been developed for detecting bisphenol A (BPA) in beverages. The prepared Rh₂O₃/rGO and its precursor materials were characterized by scanning electron microscope (SEM) and X-ray diffraction (XRD). Under optimum conditions, the sensor presented good electrochemical performance for analyzing BPA, with a linear range of 0.6–40 μM, detection limit of 0.12 μM, good reproducibility, and excellent stability. The good performance can be attributed to the combination of the good catalytic properties of Rh₂O₃ and good conductivity of rGO. The sensor is directly used for detecting BPA in the residual solutions of four beverages after simple filtration, with satisfactory recoveries of 93–99%.

1. Introduction

Bisphenol A (BPA), one of the most produced and used chemicals, has been widely used for preparing industrial polymers, such as polycarbonate, epoxy resin, and thermosensitive paper [1]. Furthermore, it is often involved in various food contact materials (i.e., canned packaging, bottles, and lacquer coating). Its widespread usage, especially in these food contact materials, has inevitably lead to its exposure to human beings via food and drinking water [2,3]. As an endocrine-disrupting chemical (EDC) [4,5], it can destroy natural or synthetic compounds with endocrine function by simulating or blocking endogenous hormones [6]. To date, a temporary tolerable daily intake (t-TDI) of 4 μg kg⁻¹ b.w. day⁻¹ for BPA has been proposed by the European Food Safety Authority (EFSA) [7]. Legislation against BPA usage in some infant food package and bottles has also been enacted, in many countries all over the world [8].

For monitoring the pollution levels and ecological risks of BPA, many methods based on capillary electrophoresis [9,10,11], gas chromatography coupled with mass spectrometry (GC–MS) [12,13,14,15], high performance liquid chromatography mass spectrometry (LC–MS) [16,17], enzyme linked immune sorbent assay (ELISA) [18,19,20], and electrochemical methods [21,22,23,24] have been developed. Pretreatment procedures, such as solid-phase extraction [24,25,26], liquid-liquid extraction [27,28], and immunoaffinity chromatography [29,30], are often involved when using GC–MS, HPLC–MS, and ELISA for analyzing BPA in complex matrices [31,32]. Although these common methods can’t do without complementary pretreatments and expensive equipment, they are still the reliable strategies for analyzing environmental BPA.

Compared with the methods mentioned above, electrochemical methods, with the advantages of easy preparation, high sensitivity, simplicity for operators, and in-situ monitoring [33], are often proposed as one of the preferable techniques for routine analysis. In order to analyze trace levels of chemicals, improving the response signals of substances is a vital part of these electrochemical methods. Modification of the electrodes with various functional materials is one of the most recommended strategies. For example, electrodes modified with graphene nanomaterials present excellent electric conductivity as a result of their good conductivity [33,34,35]. Molybdenum–selenide (MoSe₂) electrodes showed much more sensitivity to BPA, as a result of increasing surface active sites [36]. Noble metal oxides, such as RuO₂, PdO, IrO₂, and Rh₂O₃, are widely used in heterogeneous catalysis systems. Rh₂O₃ has been proven to be an excellent catalyst for the oxidation of CO [37,38] and the deposition of H₂O₂ [39] and N₂O [40], but it is still rarely reported for sensors.

In this paper, a Rh₂O₃–GO composite nanomaterial was chosen, and a novel electrochemical sensor was fabricated with a modified glassy carbon electrode (GCE) for excellent catalytic performance and conductivity. The modified glassy carbon electrode (GCE), coated with a Rh₂O₃–rGO membrane with excellent stability and selectivity, was synthesized and was then used for analyzing BPA dissolved in drinks for the first time after simple filtration.

2. Materials and Methods

2.1. Materials and Reagents

Poly (allylamine hydrochloride) (Mw = 15,000) was purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Rhodium (III) chloride trihydrate (RhCl₃·3H₂O), formaldehyde solution (HCHO, 37%), sodium hypochlorite (NaClO), bisphenol A (BPA), phosphotungstic acid (H₃[PW₁₂O₄]), graphite powders, potassium hydroxide (KOH), sodium hydroxide (NaOH), potassium permanganate (KMnO₄), sulfuric acid (H₂SO₄, 98%), sodium nitrate (NaNO₃), and phosphoric acid (H₃PO₄) were purchased from Sinopharm Group Chemical Reagent Co. Ltd. (Shanghai, China). All the chemicals were analytical grade. BPA was dissolved in ethanol at 1 mM and kept at −4 °C. Phosphate buffer solution (PBS, 0.1 M, pH = 7.0) was used reduced, and a supporting electrolyte (containing 0.1 mM phosphotungstic acid) was used.

2.2. Instruments

All electrochemical experiments were carried out on a CHI660e electrochemical workstation (Chenhua Instruments, Shanghai, China) with a conventional three-electrode cell. A saturated Ag/AgCl electrode was used as reference electrode, platinum wire as a counter electrode, and a bare glassy carbon electrode (GCE) (or modified GCE) as the working electrode.

The morphology of the prepared nanocomposites was characterized by scanning electron microscopy (SEM, Hitachi S-4800, Tokyo, Japan) and X-ray diffraction (XRD, DX-2700 Bruker D8 Advance).

2.3. Synthesis of Rh₂O₃ Nanoparticles (Rh₂O₃–NPs)

Rh₂O₃ nanoparticles were prepared, according to the method in the literature [41] with slight modification. Typically, RhCl₃ (63 mg) and poly (allylamine hydrochloride) (200 mg) were dissolved in 100 mL of water. The pH of the solution was adjusted to 7.0 by KOH. After that, 10 mL of a HCHO solution was added to the solution. Then, the reaction solution was transferred to a teflon-lined high-pressure vessel and heated at 120 °C for 6 h. Then, 15 mL of NaClO solution was added to the mixture and mechanically stirred for 72 h at room temperature. Finally, the suspension was centrifuged and then washed with water several times, to eliminate the residues and obtain Rh₂O₃–NPs.

Graphene oxide (GO) was prepared, according to previous reports [42]. Briefly, graphite (0.5 g) and NaNO₃ (0.5 g) were added into concentrated H₂SO₄ (23 mL) in an ice bath, followed by slow addition of 3 g KMnO₄. The mixture was heated at 35 °C for two hours, and then 40 mL of H₂O was added. After that, the reaction temperature was raised to 95 °C, kept for 30 min, and then followed by the addition of 100 mL H₂O and 20 mL of H₂O₂. The final suspension was filtered, washed with 1.2 M of HCl and ultrapure water three times, and dried at 60 °C to obtain graphene oxide (GO).

2.4. Preparation of Reduced Graphene Oxide–Rhodium Nanoparticle Electrode (Rh₂O₃–rGO/GCE)

The schematic diagram of the preparation and action mechanism of the Rh₂O₃–GO/GCE is given as Scheme 1. The Rh₂O₃–NP solution was prepared by dispersing the above NPs into 10 mL water, then ultrasound treatment for 2 h. The GO suspension was dispersed into water to prepare 1 mg mL⁻¹ GO solution. Then, 100 μL of Rh₂O₃ nanoparticle solution was dispersed into 10 mL GO solution (1 mg mL⁻¹). After ultrasound treatment for 30 min, the Rh₂O₃–GO nanoparticle composites were synthesized successfully. Before modifying the electrode, the GCE was polished to a mirror-like finish using 0.3 mm alumina slurry, and sonicated in distilled water and ethanol. The Rh₂O₃–GO solution (5 μL) was coated on the GCE surface and left to dry in air. Reduction of GO/GCE to rGO/GCE and Rh₂O₃–GO/GCE to Rh₂O₃–rGO/GCE was performed by cyclic voltammetry (CV) at a reduced potential of −1.2 V for 120 s in phosphate buffer solution.

2.5. Real Sample Preparation

Four beverage samples and one thermal paper sample were purchased from a local supermarket, to evaluate the performance of Rh₂O₃–rGO/GCE. After simple filtration for removing the fruit flesh, 800 μL beverage was mixed with PBS solution (9.2 mL) and directly analyzed by the proposed sensors. Thermal paper (1 g) was cut to pieces and extracted by ultrasound with 100 mL ethanol for 2 h. Then, 100 μL of extraction agent was mixed with PBS (9.9 mL) and analyzed by the proposed sensors. Afterward, the samples were spiked with BPA at three levels (5, 10 and 15 μM), and treated, following the above procedures, for recovery evaluation.

3. Results

3.1. Material Characterization

Pure GO exhibited a typical silk fold surface (Figure 1a), corresponding to a much higher surface area than GO. As for Rh₂O₃–NP, it displayed uniform three-dimensional characteristics composed of network structures (Figure 1b). The Rh₂O₃ nanoparticles were wrapped by the gauze-like GO, while a relatively smoother surface was observed in the Rh₂O₃–GO composite, compared with that of the Rh₂O₃–NPs (Figure 1c). In XRD spectra of rGO (Figure 1d), the peak at 2θ = 28.4° belonged to the characteristic reflection of graphite (002). The wide diffraction peaks at 2θ = 40° and 70° can be attributed to the diffractions of (100) and (214) planes of Rh₂O₃ (Figure 1d). Totally, the XRD pattern of Rh₂O₃–rGO indicated the successful preparation of the Rh₂O₃–rGO composite.

3.2. Electrochemical Properties of the Modified Electrodes

Electrochemical characterizations of the electrodes were investigated using [Fe(CN)₆]^3−/4− as probes. The cyclic voltammograms (CVs) of different electrodes in 1.0 mM [Fe(CN)₆]^3−/4− solution containing 0.1 M KCl at a scan rate of 100 mV s⁻¹ are shown in Figure 2A. The smaller peak-to-peak separations (Ep) of Rh₂O₃–rGO/GCE (90 mV) than that of bare GCE (149 mV) and rGO/GCE (148 mV) can be referred to the faster electron transfer on the surface of Rh₂O₃–rGO/GCE. Additionally, the redox current of rGO/GCE was much larger than GCE. When Rh₂O₃–NPs were modified on rGO, the peak current increased rapidly. This indicated that Rh₂O₃–NPs should be involved for the enhanced electron transfer rate.

3.3. Electrocatalytic Behaviors of BPA

The CVs were performed in 0.1 M PBS (pH = 7.0) solutions containing 20 μM BPA, to investigate the electrochemical behaviors of the prepared electrodes at a scan rate of 100 mV s⁻¹. There was only a relatively small oxidation peak (4.5 μA) at the potential of 0.57 V (Figure 2B, curve a). After coating with rGO (curve b), the peak current dramatically increased, due to the good adsorption efficiency of BPA onto rGO. Rh₂O₃–rGO/GCE (curve c) presented the greatest peak current of 30.5 μA, at the peak potential of about 0.52 V. Though peak potential shifted from 0.57 to 0.52 V, the results still indicated that Rh₂O₃–rGO could accelerate the electrocatalytic oxidation of BPA. This enhancement can be attributed to both the effective specific surface area of rGO, and the good electrocatalytic properties of Rh₂O₃–NPs.

3.4. Effects of Solution pH

The electrochemical responses of BPA on prepared Rh₂O₃–rGO/GCE under different pH were also studied by CV (Figure 3). The oxidation peak current increased gradually with pH from 5.5 to 7.0, and then it began to decrease with the pH continually increasing to 8.5. So, in the following studies, a pH of 7.0 was selected for the supporting electrolyte.

3.5. Effects of Scan Rate

To assess the electrocatalytic behaviors of BPA at Rh₂O₃–rGO/GCE, scan rates from 10 to 270 mV s⁻¹ were studied (Figure 4). Oxidation peak current (I, μA) of BPA was linearly increased with the scan rate (Ʋ, mV s⁻¹) with R² = 0.9938, and the relationship can be expressed by the linear regression Equation (1).

I = 0.0489 × Ʋ + 2.7458,

(1)

The oxidation of BPA on the Rh₂O₃–rGO/GCE surface was an adsorption-controlled process. Meanwhile, the oxidation peak potential (E, V) increased linearly with natural logarithm of the scan rate (lnƲ, mV s⁻¹) with R² = 0.9931, and the Equation (2) is as follows:

E = 0.0246 × lnƲ + 0.4543,

(2)

As for an adsorption-controlled and totally irreversible electrode process, E was defined by Equation (3) [43].

E = E^θ + (RT/αnF) × ln((RTk^θ)/αnF) + (RT/αnF) × lnƲ,

(3)

where α, k^θ, n, υ, E^θ, R, T and F represent transfer coefficient, standard rate constant of the reaction, electron transfer number (involved in rate determining step), scan rate, formal redox potential, the gas constant, the absolute temperature, and the Faraday constant, respectively. According to Equation (2), the value of RT/αnF was 0.0246. Generally, α is assumed to be 0.5 in a totally irreversible electrode process [44]. Therefore, n was calculated to be 2.09, indicating that the electron transfer number for BPA oxidation is 2.

3.6. Analytical Performance of the Rh₂O₃–rGO/GCE

As shown in Figure 5, a linear relationship between oxidation current and BPA concentration within the range of 0.6–40 μM could be expressed as:

I = 3.1569 × C + 0.1002,

(4)

where I is oxidation peak current of BPA (μA), and C is the concentration of BPA in solution (μM). The limit of detection (LOD) was estimated to be 0.12 μM using the Equation (5)

LOD = (k × SD)/b,

(5)

where k is a constant, SD is the standard deviation of the intercept, and b is the slope of the calibration plot. In this paper, the values of k, SD, and b are 3, 0.1315 and 3.1569, respectively. Compared with some other electrodes, based on gold and copper oxide–graphene nanoparticles [42,45], the Rh₂O₃–rGO/GCE presented comparable performance in similar linear detection ranges (

Table 1. Comparison of the Rh₂O₃–rGO and other sensors for BPA determination.

Sensors	Linear Range (μM)	LOD (μM)	Reference
Residual metal impurities within SWCNT electrode	10–100	7.3	[46]
Gold nanoparticles supported carbon nanotubes electrode	0.87–87	0.13	[47]
Gold nanoparticles dotted graphene electrode	0.01–10	0.005	[45]
Exfoliated Ni2Al layered double hydroxide nanosheets electrode	0.02–1.51	0.007	[48]
Copper oxide and graphene electrode	0.1–80	0.053	[42]
Rhodium oxide and graphene electrode	0.6–40	0.12	This work

).

The stability, reproducibility, and selectivity of the Rh₂O₃–rGO/GCE were also evaluated. To test the stability of prepared electrode, Rh₂O₃–rGO/GCE was stored at 4 °C and examined three times per week by CV. After two weeks, the peak current retained 77.2% of the initial response. The Rh₂O₃–GO solution displayed excellent stability, being deposited at 4 °C within 30 days. Using this Rh₂O₃–GO solution to prepare Rh₂O₃–rGO/GCE, the response current for detecting BPA still remained within 98% of the initial currents. The good stability and reproducibility indicated the sensor should be appropriate for routine measurements of real samples. The relative standard deviation (RSD) for detecting BPA (n = 7) by one as-prepared sensor was less than 2.73%. The RSD for seven Rh₂O₃–rGO/GCEs fabricated in parallel was less than 3.22%.

The i–t curves of bisphenol A, by the addition of different inorganic ions and BPA analogs, are shown in Figure 6. The concentrations of inorganic ions, BPA, and BPA analogs are 200 µM, 20 µM, and 20 µM, respectively. Common inorganic ions (Ba²⁺, Cu²⁺, Ca²⁺, K⁺, Na⁺, OH⁻, SO₄²⁻ and Cl⁻) have no effect on analyzing BPA, even when their levels are 10 times higher than BPA. Meanwhile, no obvious interferences from BPA analogs have been observed, such as phenol, hydroquinone, bisphenol B, bisphenol F, and bisphenol S. The RSDs for different interferences were all less than 3.53%.

3.7. Sample Analysis

The real samples spiked with BPA at 0–15 μM were successfully analyzed by the prepared Rh₂O₃–rGO/GCE. Before analysis, these drinks were filtered with an 0.45 μM hydrophilic membrane to remove the suspended particles. The detailed procedure has been described in Section 2.5. The recoveries of BPA from four kinds of beverages and thermal paper ranged from 93% to 98% (

Table 2. The results of determined BPA in practical samples.

Sample	Added (μM)	Found (μM)	Recovery (%)
Beverage 1	0	3.22 ± 0.09 1	-
	5	8.10 ± 0.18	98
	10	12.66 ± 0.43	94
	15	17.22 ± 0.62	93
Beverage 2	0	4.69 ± 0.09	-
	5	9.46 ± 0.18	95
	10	14.34 ± 0.37	97
	15	19.49 ± 0.43	99
Beverage 3	0	3.96 ± 0.04	-
	5	8.63 ± 0.09	93
	10	13.85 ± 0.21	98
	15	17.80 ± 0.49	93
Beverage 4	0	0	-
	5	4.86 ± 0.10	97
	10	9.57 ± 0.30	96
	15	14.73 ± 0.24	98
Thermal paper	0	6.46 ± 0.23	-
	5	11.33 ± 0.18	97
	10	13.85 ± 0.21	98
	15	20.95 ± 0.54	97

¹ Average ± Standard deviation.

), indicating that the prepared Rh₂O₃–rGO modified electrode should be a useful sensor for analyzing BPA in aqueous solutions containing relatively complex interferences.

4. Conclusions

In this work, a Rh₂O₃–rGO modified GCE has been successfully fabricated for detecting BPA, which presents good catalytic performance. The enhanced electrocatalytic property can be attributed to the synergism of rGO and Rh₂O₃–NPs. Wide linear range, low detection limit, and good stability for detecting BPA have been obtained for this prepared Rh₂O₃–rGO/GCE sensor. More excitingly, BPA in some drinks could be directly and successfully detected after simple filtration. As a whole, a Rh₂O₃–rGO/GCE sensor, combining the catalytic properties of Rh₂O₃ and the conductivity of rGO, should be a useful supplemental strategy to common methods based on GC–MS and HPLC–MS, especially for rapid field detection.