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. 2023 Jan 3;8(2):2564–2574. doi: 10.1021/acsomega.2c07095

Electrochemical Sensor Based on a Composite of Reduced Graphene Oxide and Molecularly Imprinted Copolymer of Polyaniline–Poly(o-phenylenediamine) for Ciprofloxacin Determination: Fabrication, Characterization, and Performance Evaluation

Jedsada Chuiprasert †, Sira Srinives ‡, Narin Boontanon §, Chongrak Polprasert ∥, Nudjarin Ramungul ⊥, Napat Lertthanaphol ‡, Apisit Karawek ‡, Suwanna Kitpati Boontanon †,#,*
PMCID: PMC9850462  PMID: 36687093

Abstract

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Contamination of antibiotics in water is a major cause of antibiotic resistance (ABR) in pathogens that endangers human health and food security worldwide. Ciprofloxacin (CIP) is a synthetic fluoroquinolone (FQ) antibiotic and is reportedly present in surface water at a concentration exceeding the ecotoxicological predicted no-effect concentration in some areas. This study fabricated a CIP sensor using an electropolymerized molecularly imprinted polymer (MIP) of polyaniline (PANI) and poly(o-phenylenediamine) (o-PDA) with CIP recognition sites. The MIP was coated on a reduced graphene oxide (rGO)-modified glassy carbon electrode (rGO/GCE) and operated under a differential pulse voltammetry (DPV) mode for CIP detection. The sensor exhibited an excellent response from 1.0 × 10–9 to 5.0 × 10–7 mol L–1 CIP, showing a sensor detection limit and sensitivity of 5.28 × 10–11 mol L–1 and 5.78 μA mol–1 L, respectively. The sensor’s sensitivity for CIP was 1.5 times higher than that of the other tested antibiotics, including enrofloxacin (ENR), ofloxacin (OFX), sulfamethoxazole (SMZ), and piperacillin sodium salt (PIP). The reproducibility and reusability of the sensor devices were also studied.

1. Introduction

Ciprofloxacin (CIP) is a synthetic second-generation fluoroquinolone (FQ) antibiotic used to prevent and treat infectious bacterial diseases. It is a widely used medicine for humans and veterinary use, providing excellent bactericidal responses against gram-negative and gram-positive pathogens. Generally, more than 75% of the consumed CIP is unmetabolized and excreted into the environment, polluting surface water, groundwater, and drinking water.1,2 Several reports reveal the presence of ng L–1 to μg L–1 CIP in wastewater at water treatment facilities, which contributes to antibiotic resistance (ABR) in pathogens and leads to difficult-to-treat or even untreatable infections. The issue of ABR has become a global concern that requires urgent action to reduce the impact and control the expansion of resistance.3

Several analytical methods were employed to determine CIP in water, including high-performance liquid chromatography (HPLC),4 fluorescence spectrophotometry,5 atomic absorption spectrometry, and capillary electrophoresis.6 Although these techniques are standardized and reliable, they require costly instruments, are time-consuming, and are impractical for on-site monitoring. Therefore, a mobile sensing device that provides quick concentration measurements at a reasonable price is essential. An electrochemical sensor is a monitoring device that relies on redox reactions to yield sensing responses to a target analyte. Such a device can be used as an alternative to conventional techniques for detecting CIP with a quick response time, the potential to be installed in a compact area, and excellent mobility for on-site measurements.

The molecularly imprinted polymer (MIP) is a technique that carries specific sites for a target molecule using the molecule as the template. It has recently received significant attention owing to its high selectivity and sensitivity toward targets. Applications of MIP have been demonstrated in drug delivery,7,8 solid-phase extraction coupled with liquid chromatography,9,10 catalysis,11,12 and environmental and biomedical sensing.13−15 Various techniques have been utilized to synthesize MIP layers, including free-radical polymerization, chemical grafting,16 soft lithographies,17 molecular self-assembly,18 and electropolymerization.19,20 Among these techniques, electropolymerization is a practical approach for MIP synthesis because a target molecule can be added to the electrolyte and entrapped in a conductive polymeric film. The molecules are sequentially extracted or leached out of the external polymeric surfaces to create recognition sites for MIP. Yahyapour et al.21 synthesized MIP from the 4-aminobenzoic acid in the presence of a cometal organic framework nanofiber. The composite was coated on a glassy carbon electrode (GCE) and operated in differential pulse voltammetry (DPV) to exhibit sensitivity and a limit of detection (LOD) of 0.016 μA μmol–1 L and 1.7 × 10–8 mol L–1 for CIP detection. Jiang et al.22 electropolymerized o-phenylenediamine (o-PDA) and l-lysine using cyclic voltammetry (CV) mode with a moxifloxacin template. The poly l-lysine-o-PDA copolymer MIP showed sensitivity and LOD of 11.3 μA mol–1 L and 5.12 × 10–10 mol L–1, respectively, for moxifloxacin. The synergic effects of l-lysine-o-PDA were the key to the excellent performance of the MIP as a sensitive element. El Azab et al.23 synthesized a poly(levodopa) and poly(o-phenylenediamine) composite onto gold nanoparticles. The composite was utilized as a sensor in the detection of levofloxacin. The sensor operated in the DPV mode provided a sensitivity of 356,955 μA mol–1 L and had an LOD of 4.62 × 10–7 mol L–1.

Polyaniline (PANI) is a popular conductive polymer, offering a wide range of tunable conductivities, and is electroactive in acidic to neutral pH ranges.24,25 It can be copolymerized with other conductive polymers, such as o-PDA, to combine functionality and adjust the conductivity of the copolymer film. The copolymer of PANI and o-PDA integrates the excellent conductivity of PANI and the thin-film-forming nature of o-PDA, making it a great candidate for MIP. An issue regarding the film is the passivation of the entrapped nonconductive molecules during electropolymerization. This causes an over-oxidation of the conductive polymer film, which leads to poor surface electroactivity at the external layer.

Carbon nanostructures with high charge-transfer ability, such as carbon nanotubes and reduced graphene oxide (rGO), can be implemented in the MIP to enhance the electroactivity of the MIP film. Graphene is a two-dimensional nanostructure with chemical stability, good charge-transfer ability, and outstanding electroactivity. It can be synthesized on a gram scale following a chemical exfoliation method that yields GO, a graphene sheet with few carbon layers and many functional groups. The functional groups on GO are structural defects that inhibit the charge mobility through the GO sheet and cause GO to be electrically insulating. GO can be chemically reduced to remove some functional groups and form rGO, a semiconductive material. rGO is a popular transducer and has been demonstrated as a sensitive element for charge-transfer enhancement in electrochemical sensors.26−31

In this study, we synthesized PANI–o-PDA MIP/rGO on a GCE and used it as the working electrode (WE) in an electrochemical sensor. The rGO was coated on GCE followed by the electro-copolymerization of PANI–o-PDA MIP film. CIP was added to the electrolyte during the electro-copolymerization and later removed to create recognition sites on the MIP. The sensor was characterized using CV and exploited in the DPV mode for CIP measurement. We compared CIP measurement results from the MIP/rGO/GCE with those of the non MIP (NIP)/rGO/GCE. In addition, the MIP/rGO/GCE sensor was tested against other antibiotics, including enrofloxacin (ENR), ofloxacin (OFX), sulfamethoxazole (SMZ), and piperacillin sodium salt (PIP), for cross-sensitivity studies.

2. Results and Discussion

2.1. Physical and Chemical Characterizations

2.1.1. Physical Characterizations

CIP, rGO, pre-elution MIP/rGO, and post-elution MIP/rGO were observed by field-emission scanning electron microscopy (FE-SEM). CIP (Figure 1A) appears to be a rod-shaped crystal structure with an average diameter and length of 368.0 ± 101.6 and 1088.0 ± 130.8 nm, respectively. rGO (Figure 1B) is a sheet-like structure that promotes the formation of self-assembled films. The pre-elution MIP/rGO (Figure 1C) shows the MIP copolymer film coated on rGO sheets, in which the CIP structure is hardly distinguishable from other substances. Surface roughness and thickness increase slightly compared to rGO owing to the film coverage of the CIP–PANI–o-PDA copolymer. The post-elution MIP/rGO exhibits wrinkles, roughness (Figure 1D), and a sheet-like structure resulting from CIP elution.

Figure 1.

Figure 1

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FE-SEM images of (A) CIP, (B) rGO, (C) pre-elution MIP/rGO, and (D) post-elution MIP/rGO.

2.1.2. FT-IR Spectral Studies

FT-IR spectra revealed the presence of functional groups in the synthesized GO, rGO, NIP, post-elution NIP/rGO, and post-elution MIP/rGO (Figure 2A) samples. The spectra of GO and rGO exhibited a broad peak at 3408 cm–1, corresponding to the stretching vibrations of hydroxyl (O–H) and moisture. Weak bands at 2919 and 2851 cm–1 were attributed to the residual alkyl groups (C–H).33 FT-IR spectrum of rGO is slightly less intense than that of the GO because of the partial removal of the functional groups from the GO. NIP was the PANI–o-PDA copolymer with no CIP and was tested as a control sample. The FT-IR peaks for the NIP in the 3200–3500 cm–1 were attributed to the N–H stretching vibrations of secondary amine on the PANI–o-PDA.34 The peaks at 2600, 1625, and 1598 cm–1 corresponded to O–H of the carboxyl group, C=C vibrations of quinoid, and C=C vibrations of benzenoid, respectively. The absorption band at 1497 cm–1 could be ascribed to the phenazine group,35 whereas the peak at 756 cm–1 was associated with the substituted phenyl ring. The post-elution NIP/rGO and post-elution MIP/rGO samples provided a similar IR pattern. The peaks at 3424, 2919, 2851, and 2600 cm–1 corresponded to N–H, C–H, C–H, and O–H groups, respectively, and were contributed by the functional groups on the copolymer and rGO. The copolymer (NIP and MIP) of the NIP/rGO and MIP/rGO provided peak signals at 1625, 1598, 1497, and 756 cm–1, corresponding to the C=C vibrations of quinoid, C=C vibrations of benzenoid, phenazine, and phenyl ring, respectively.

Figure 2.

Figure 2

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(A) FT-IR spectra of GO, rGO, NIP, post-elution NIP/rGO, and post-elution MIP/rGO; (B) Raman spectra of GO, rGO, NIP, pre-elution MIP, and post-elution MIP/rGO; and (C) XRD spectra of GO, rGO, NIP, pre-elution MIP, and post-elution MIP/rGO.

2.1.3. Raman and X-ray Diffraction (XRD) Analysis

Raman spectroscopy is an effective tool for characterizing rGO because it provides information on the disordered amorphous structure and ordered graphitic structure ratio (ID/IG). For GO, rGO, and post-elution MIP/rGO, the IG and ID bands were present at 1340 and 1589 cm–1. The ID/IG for GO, rGO, and post-elution MIP/rGO was 0.98, 0.91, and 0.94, respectively (Figure 2B), revealing an equivalent level of crystallinity. For NIP, the spectral peak at 1360 and 1514 cm–1 corresponded to the C–N•+ stretching of delocalized polaronic charge carriers and the N–H bending deformation of protonated amines.36 For the pre-elution MIP, the peaks at 1618 and 1379 cm–1 corresponded to the C–C stretching of an aromatic ring and a combination of CH2 wagging and C–C stretching. The Raman spectra confirm that the composite contains copolymer and graphene; both phases maintain their functionalities and crystallinities.

The crystallographic structures of GO, rGO, NIP, pre-elution MIP, and post-elution MIP/rGO were studied using XRD (Figure 2C). For GO, the pattern showed only one broad peak at 2θ = 12.3°, corresponding to the (001) plane of the graphene structure.37 For rGO, two broad peaks were observed at 14° and 24°, corresponding to the (001) and (002) planes of rGO. For NIP, the XRD patterns at 14.6°, 22.1°, 25.3°, and 33.3° were observed, suggesting the presence of PANI and poly(o-PDA).38−40 For pre-elution MIP, the XRD patterns were observed at 6.3°, 9.4°, and 18.9°, indicating the presence of the copolymer PANI and poly(o-PDA). Notably, the copolymer pattern was shifted from its initial position, and other small peaks were observed. The shift can be attributed to the presence of CIP in the copolymer matrix.41 For the post-elution MIP/rGO, signals from the rGO pattern overlapped with the copolymer, resulting in only one broad peak at 15.1°.

2.2. Electrochemical Characterization

Electrochemical currents from CV analysis were used to investigate the electroactivity of the modified electrodes in a background mediator of [Fe(CN)6]3–/4–. The anodic and cathodic peaks of the bare GCE were observed at 0.26 and 0.11 V (Figure 3A). The height of the anodic peak was extracted as the response (Ip). The Ip values for the bare GCE and rGO/GCE were 38.27 and 58.08 μA, respectively, indicating a significant enhancement in charge-transfer ability owing to the incorporation of rGO (Figure 3B). The post-elution rGO/GCE (Figure 3C) exhibited an Ip value of 56.92 μA, indicating no effect from the elution process on the electroactivity of rGO/GCE. For NIP/rGO/GCE (Figure 3D), Ip was 60.79 μA, slightly higher than that of rGO/GCE and post-elution rGO/GCE. We believe that NIP, a copolymer film of PANI–o-PDA, can provide an extra surface area for the electrode or a reduction in the double layer of the electrode/copolymer interfaces. The post-elution MIP/rGO/GCE (Figure 3E) produced the highest Ip of 77.77 μA, which is attributed to an additional surface area from wrinkles and a sheet-like copolymer of the MIP. We determined the electroactive area of the electrode following the Randles–Sevcĭk equation (eq 1):32,42

2.2. 1

where Ip denotes the anodic peak current (A), A denotes the electroactive area (cm2), D denotes the diffusion coefficient (4.0 × 10–6 cm2 s–1), n denotes the number of electrons participating in the redox reaction (n = 1), v denotes the scan rate (V s–1), and C denotes the concentration of the probe molecule (5.0 × 10–3 mol L–1 [Fe(CN)6]3–/4–). The electroactive areas of bare GCE, rGO/GCE, post-elution rGO/GCE, post-elution NIP/rGO/GCE, and post-elution MIP/rGO/GCE were 0.07, 0.09, 0.09, 0.10, and 0.14 cm2, respectively.

Figure 3.

Figure 3

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Electroactivity of the bare GCE (A), rGO/GCE (B), post-elution rGO/GCE (C), post-elution NIP/rGO/GCE (D), post-elution MIP/rGO/GCE (E), and in [Fe(CN)6]3–/4– mediator evaluated by CV at scan rate of 50 mV s–1 (n = 5).

2.3. Variation in MIP/rGO/GCE Synthesis Conditions

2.3.1. Effect of the Suspension Medium of rGO

The rGO can be suspended in water and a polar aprotic solvent such as dimethylformamide (DMF). The effect of the suspension medium was studied by suspending rGO in either ultrapure water (UPW) or DMF at a concentration of 1.0 mg mL–1. The suspension was drop-coated onto GCE. The electroactivity of rGO/GCE was investigated in the CV mode (Figure S1a). The rGO (UPW)/GCE and rGO (DMF)/GCE provided Ip values of 54.56 and 48.42 μA, respectively. The GO (UPW)/GCE was studied as a control sample and yielded an Ip of 1.25 μA. Based on the Ip results, rGO suspension in UPW medium was used for subsequent experiments.

2.3.2. Effect of pH in Electropolymerization

The pH of electrolytes plays a critical role in the electropolymerization of PANI and poly(o-PDA). Because the ANI and o-PDA are electroactive in the pH range of 1–4, the experiments were performed accordingly. The MIP was electropolymerized on rGO/GCE from the electrolytes in the pH range of 1–4 to obtain the MIP/rGO/GCE. The MIP/rGO/GCE was characterized using a CV to receive the Ip responses. The Ip of MIP/rGO/GCE (Figure S1b) slightly increased from 55.46 to 59.43 and 60.71 μA as the pH increased from 1 to 3. Furthermore, the Ip decreased to 55.87 μA when the pH of the electrolyte reached beyond 3.5. The results support the hypothesis that the copolymer is only well electropolymerized within the potential pH range. The dependence of MIP electropolymerization on the pH of electrolytes results from a surface cavity, oxidative state, and surface structure of the copolymer film. Therefore, a medium solution (pH 3.0) was selected for the electropolymerization of the MIP.

2.3.3. Effect of CIP Concentration

CIP was used as a template for MIP synthesis. The concentration of CIP in the electrolytes can significantly affect the number of cavities on MIP. To investigate the effect of template concentration, MIP was polymerized on rGO/GCE at different CIP concentrations, ranging from 1.0 × 10–3 to 10.0 × 10–3 mol L–1 (Figure S1c) (pH of electrolytes = 3). At CIP concentrations of 1.0 × 10–3 and 2.5 × 10–3 mol L−1, the Ip values were 60.02 and 60.29 μA, respectively. The highest Ip of 63.71 μA was obtained for MIP/rGO synthesized at a CIP concentration of 5.0 × 10–3 mol L–1. As the CIP concentration increased to 7.5 × 10–3 and 10 × 10–3 mol L–1, the Ip value decreased to 59.61 and 57.66 μA, respectively. This observation can be explained since CIP is less electroactive than the rGO and MIP, and excessive CIP coverage on MIP passivates the electroactive area and reduces the Ip value.

2.3.4. Effect of the Monomer Concentration

The interaction between the monomers and CIP is essential to form a MIP film. During the electropolymerization, the ANI to o-PDA ratio varied from 2:1 to 1:1, 1:2, 1:3, 1:4, and 1:5. As the ANI to o-PDA ratio decreased from 2:1 to 1:1, Ip increased from 56.88 to 59.88 μA (Figure S1d). This result explains the higher electroactivity of PANI than that of poly(o-PDA), which leads to a thicker copolymer film and lower Ip value. A decrease in Ip value (52.26 μA) occurred as the ratio increased to 1:2. The Ip value decreased to 52.62, 48.16, and 48.94 μA as the ratio decreased from 1:2 to 1:3, 1:4, and 1:5, respectively. This lowering of Ip was attributed to the incorporation of poly(o-PDA) in the MIP/rGO, which reduced the electroactivity of the copolymer film. The ANI:o-PDA ratio of 1:1 was selected for the electropolymerization of the MIP.

2.3.5. Effect of the Number of CV Cycles and Scan Rate

In the electropolymerization of PANI–o-PDA copolymer, the number of cycles for the CV scan is crucial for the characteristics of the film. Electropolymerization was performed with cycle numbers ranging from 5 to 30 (Figure S1e). The film was employed as WE and studied for electroactivity in [Fe(CN)6]3–/4– mediator. The number of cycles in electropolymerization process controls the thickness and electroactivity of the external layer of the MIP film.33 As the number rose from 5 to 10 and 15, Ip increased from 58.50 to 59.65 and 64.10 μA. This increasing trend ceased as Ip decreased to 61.07, 57.15, and 50.57 μA, corresponding to the cyclic number of 20, 25, and 30, respectively. To investigate the effect of the scan rate on electropolymerization, MIP was electropolymerized in CV mode at different scan rates in the 30–100 mV s–1 window. The Ip was insensitive to the scan rate changes at 30, 40, 50, and 60 mV s–1, a relatively low scan rate range (Figure S1f), yielding Ip values of 64.23, 64.10, 64.50, and 64.20 μA, respectively. However, the Ip rose more intensely at a high scan rate change of 70 to 80 mV s–1, yielding Ip values of 65.52 and 66.52 μA, respectively. A decrease in Ip was observed as the scan rate increased to 100 mV s–1 owing to insufficient monomer diffusion to the WE and over-oxidation of the external layer of the copolymer film. Thus, for the MIP synthesis, the number of CV cycles and the scan rate were set at 15 and 80 mV s–1, respectively.

2.3.6. Template Removal Eluents

Removal of CIP from MIP was required to create CIP recognition sites on the MIP film. The removal process can be studied using EDS spectra comparing binding energy signals from the pre-eluted and post-eluted MIP/rGO/GCE. The signal from fluorine (F) revealed the presence of the CIP (Figure 4A). The CIP removal was performed by submerging MIP/rGO/GCE in a 1.0 mol L–1 HCl solution for 180 s. The F signal was spotted at 0.70 eV for the pre-elution MIP/rGO and disappeared for the post-elution MIP/rGO (Figure 4B). The disappearance of the F signal indicates the absence of CIP on the MIP after the removal step. In this study, water, ethanol, water–ethanol mixture (1:3, v:v), 1.0 mol L–1 NaOH, 1.0 mol L–1 NaOH solution–ethanol mixture (1:3, v:v), 1.0 mol L–1 HCl, and 1.0 mol L–1 HCl solution–ethanol mixture (1:3, v:v) were tested as eluting solutions (eluents) for template removal (Figure 4C). Water, ethanol, and the water–ethanol mixture provided MIP with Ip values of 65.87, 65.41, and 66.87 μA, respectively. The 1.0 mol L–1 NaOH yielded MIP with the lowest Ip of 29.77 μA. However, MIP eluted with 1.0 mol L–1 NaOH solution–ethanol mixture, 1.0 mol L–1 HCl, and 1.0 mol L–1 HCl solution–ethanol mixture provided Ip values of 69.87, 71.60, and 65.49 μA, respectively. In this case, the solution interacts with copolymeric chains and causes the chains to release CIP. Differences in MIP electroactivities correspond to different eluents used in the CIP removal and correlate with the interactions between the copolymeric chains and the eluents.

Figure 4.

Figure 4

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EDS pattern of the pre-elution MIP/rGO (A) and post-elution MIP/rGO (B); changes in Ip corresponding to different template removal eluents used in the preparation of MIP/rGO/GCE (C).

2.3.7. Time for CIP Removal

Elution time is an essential factor for CIP removal and MIP synthesis. Figure S1g shows changes in the Ip as the elution time varied from 30 to 300 s. The Ip response plateaued quickly in the 70–72.55 μA range as the elution time rose from 30 to 150 s. The Ip reached the maximum value of 76.42 μA at the elution time of 180 s and reduced to 67.44 and 66.59 μA as the elution time rose to 240 and 300 s.

2.4. Optimization of Conditions for CIP Determination

2.4.1. Effect of the Incubation pH on the CIP Detection Performance

For CIP sensing, the MIP/rGO/GCE electrode should rebind with CIP before the CIP determination via the DPV technique. In this study, the CIP solution was prepared by diluting a stock solution of CIP in 0.05 mol L–1 phosphate buffer solution (PBS) at pH values ranging from 6.0 to 8.0, creating 1.0 × 10–9 mol L–1 of CIP. The electrode was further used in the electrochemical cell and operated in the DPV mode to obtain a responding current. Results were reported in the form of current variation (ΔI), in which ΔI was defined as I0 – Ic, a subtraction of baseline current (I0) and corresponding current (Ic) (Figure S2a). The ΔI increased as the pH increased from 5.0 to 8.0 and was unaffected within a narrow pH range (6.5–7.0). Notably, although MIP/rGO/GCE was introduced into the incubation medium at different pH values, it was operated in a DPV mode in the medium of pH 6.5.43−45

2.4.2. Incubation Time

The effects of CIP incubation time on the response of the sensor were investigated by introducing MIP/rGO/GCE to 1.0 × 10–9 mol L–1 CIP solution at pH 6.5. The time was varied in the range of 1–20 min. As the time increased from 1 to 3, 5, and 7 min, the current variation (ΔI = I0 – Ic) increased from 37.65 to 49.46, 76.54, and 109.74 μA, respectively (Figure S2b). The ΔI plateaued as the rebinding time lasted longer than 8.0 min. The ΔI values were 110.37, 121.37, 116.18, 120.17, and 119.64, corresponding to the rebinding time of 10, 13, 15, 18, and 20 min, respectively. The results state that adsorption sites on the MIP reach equilibrium at the CIP concentration of 1.0 × 10–9 mol L–1 after the incubation time of 7.0 min. An optimal incubation time of 7.0 min was used for this study.

2.5. Electrochemical Characteristics of MIP/rGO/GCE

The redox characteristics of the MIP/rGO/GCE sensor were analyzed using CV analysis. The CV curves were acquired in a [Fe(CN)6]3–/4– mediator at scan rates ranging from 10 to 150 mV s–1 (Figure 5A). Anodic and cathodic (Ipc) currents were monitored from the peaks of a CV curve and reported corresponding to the scan rate (v1/2) (Figure 5B). The CV curves of the peak currents of the reduction and oxidation of [Fe(CN)6]3–/4– were plotted against the square roots of the scan rates (Ipa vs v1/2 and Ipc vs v1/2). The anodic and cathodic currents increased linearly with an increase in the scan rate. The correlations were: Ipa (μA) = 3.19 v1/2 + 11.48 (R2 = 0.9900) and Ipc (μA) = −4.04 v1/2 – 14.65 (R2 = 0.9814). The distinctive peaks of Ipa and Ipc indicated favorable electron transfer between MIP/rGO/GCE and [Fe(CN)6]3–/4– redox species. The characteristic of the MIP/rGO/GCE resembles that of a diffusion-controlled electrochemical reaction model, in which the sensing signal is directly proportional to the diffusion rate of the target analyte.

Figure 5.

Figure 5

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(A) CV of MIP/rGO/GCE at scan rates of 10 to 150 mV s–1 and (B) plot of Ip vs v1/2 (n = 5).

2.6. Analytical Performances toward CIP Detection

The MIP/rGO/GCE sensor was operated in the DPV mode and introduced to CIP at different concentrations. The normalized peak current decreased as the CIP concentration increased (Figure 6A). The relation between the normalized current (ΔI) and CIP concentration was linear over two dynamic ranges of the MIP/rGO/GCE sensor. The first range was at 1.0 × 10–9 to 1.0 × 10–8 mol L–1 with a regression equation of ΔI (μA) = 33.27 logCCIP (M) + 342.86 with an R2 of 0.9613. The second range was at 2.5 × 10–8 to 5.0 × 10–7 mol L–1 with a regression equation of ΔI (μA) = 5.78 logCCIP (M) + 121.96 and a correlation coefficient (R2) of 0.8835. An LOD was determined within a linear correlation range, following the equation LOD = 3.3 SD/S, where SD is the standard deviation of the intercept and S is the slope of the calibration curve.46 The sensitivity of 5.78 μA mol–1 L (n = 5) was obtained from the calibration plot of ΔI and logCCIP. The LOD was 5.28 × 10–11 mol L–1, indicating that the sensor can detect the CIP of 5.28 × 10–11 mol L–1 with 95% confidence.

Figure 6.

Figure 6

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Performance of MIP/rGO/GCE sensor: (A) DPV of different CIP concentrations. Inset: plot of ΔI vs (a–k) logarithmic of 1.0 × 10–9 to 5.0 × 10–7 mol L–1 CIP (ΔI = difference in peak current in absence and presence of the CIP) and (B) sensor selectivity upon introduction to 1.0 × 10–6 mol L–1 of CIP, ENR, OFX, SMZ, and PIP in PBS at pH 6.5.

We compared our results with other MIP reports as summarized in Table 1 and other electrochemical methods in Table S1. Okan et al. developed MIP nanoparticles to modify a micromechanical cantilever sensor for CIP detection. The MIP microcantilever provided a sensitivity of 2.6 Hz pg–1 with a LOD of 0.8 μmol L–1 in the linear range of 1.5–150.9 μmol L–1.47 Surya et al. reported uses of chitosan-gold nanoparticle/MIP (Ch-AuMIP) to modify GCE for CIP detection. In the DPV mode, the sensor exhibited a LOD of 0.21 μmol L–1 and sensitivity of −47.6 × 106 μA μmol–1 L within 1 to 100 μmol L–1 of the linear response range.48 Wu et al. synthesized MIP on fluorescein isothiocyanate-modified silica (MIPs@SiO2-FITC). The detection was operated via the fluorescence techniques to obtain the LOD of 0.004 μmol L–1 and a linear range of 0.004–0.25 μmol L–1.49 Bagheri et al. combined iron nanoparticles and MWCNT with MIP to synthesize magnetic MWCNT. The MWCNT–MIP composite was coated on the GCE. The electrode exhibited a LOD of 0.0017 μmol L–1, a sensitivity of 22.766 μA μmol–1 L, and a linear range of 0.005–0.85 μmol L–1.50 Yan et al. modified pencil graphite electrodes (PGE) with pyrrole and o-PDA MIP. In a square wave voltammetry (SWV) mode, the MIP/PGE provided a LOD of 0.000076 μmol L–1 and a sensitivity of 100 × 106 μA μmol–1 L in the linear range of 0.001–1000 μmol L–1.51 In this work, we synthesized the copolymer film of PANI and poly(o-PDA) MIP on rGO/GCE using the electropolymerization process. The electrode was operated in the DPV mode, yielding a LOD of 0.00005 μmol L–1, a sensitivity of 5.78 × 106 μA μmol–1 L, and a linear range of 0.001–0.5 μmol L–1.

Table 1. Comparison of the MIP Analytical Techniques for the Detection of CIP.

electrode detection method linear range (μmol L–1) LOD (μmol L–1) sensitivity (μA μmol–1 L) references
MIP microcantilever dynamic 1.5–150.9 0.8 2.6 Hz pg–1 (47)
Ch-AuMIP/GCE DPV 1–100 0.21 –47.6 × 106 (48)
MIPs@SiO2–FITC fluorescent 0.004–0.25 0.004   (49)
MMWCNTs@MIP/CPE DPV 0.005–0.85 0.0017 22.766 (50)
MIP/PGE SWV 0.001–1000 0.000076 100 × 106 (51)
MIP/rGO/GCE DPV 0.001–0.5 0.00005 5.78 × 106 this study
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2.7. Reproducibility and Repeatability

Five different MIP/rGO/GCE sensors (n = 5) were prepared independently under identical conditions and tested for the net current responses to 1.0 × 10–9 mol L–1 CIP. The responses were collected and compared to determine the reproducibility of sensor fabrication. The responses from the five sensors showed a relative standard deviation (RSD) of 4.9%, indicating good reproducibility of the fabrication process. The repeatability of the sensor was determined by averaging sensing responses five times from the same sensor to the same concentration of CIP (1.0 × 10–9 mol L–1) (Figure S3). The sensor revealed an RSD of 0.5%, indicating good repeatability in accurate readouts without pretreatment.

2.8. Selectivity of MIP/rGO/GCE

The selectivity of the MIP/rGO/GCE sensor was examined by comparing the DPV response (ΔI) to CIP in the presence of other interferences. The selected interferences in aquatic environments include antibiotic pollutants, such as ENR, OFX, SMZ, and PIP. The MIP/rGO/GCE sensors were fabricated and exposed to ENR, OFX, SMZ, and PIP at the same concentration of 1.0 × 10–6 mol L–1. Figure 6B displays the current intensity responses corresponding to CIP, ENR, OFX, SMZ, and PIP, in which the sensing response values to CIP are 1.5, 1.5, 1.6, and 1.9 times higher than those of ENR, OFX, SMZ, and PIP, respectively. The NIP-rGO/GCE sensor was tested as a control, revealing no sensing selectivity for the CIP compared to ENR, OFX, SMZ, and PIP. The imprinting factor (α) and selectivity factor (β) were computed using eqs 2 and 3:

2.8. 2
2.8. 3

Both factors provide information regarding the sensitivity of the sensor toward a target analyte as calculated in Table S2. The α suggests the superiority of MIP over NIP, whereas the β focuses on the relative sensitivity of the target analyte over other interferences. The α for CIP, ENR, OFX, SMZ, and PIP are 1.7, 1.1, 1.0, 1.1, and 1.1, respectively. The αCIP is significantly greater than the other antibiotics, indicating a high MIP/rGO/GCE sensitivity toward the target analyte (CIP). The β for CIP, ENR, OFX, SMZ, and PIP are 1.0, 1.6, 1.6, 1.6, and 1.4, indicating that the sensor is more sensitive to CIP than other interference.

2.9. Determination of CIP in Actual Water Samples

The MIP/rGO/GCE sensor was designed to function as a real-time sensor for CIP detection. We investigated the performance of the sensor against tap and pond water samples. The diluted water solution (actual water: PBS; 1:5; v:v) was transferred to the electrochemical cell and spiked with different concentrations of CIP. Recovery tests were performed using DPV to determine CIP concentrations in water. The CIP concentration data from the sensor were compared with the actual concentrations in the water sample (Table S3). The precision of the sensor was determined using recovery (%), which is defined as the obtained data divided by the actual CIP concentration and multiplied by 100 (Table S3). For tap water, the recovery (%) of 84.2, 87.8, 87.1, and 87.8 corresponded to the CIP spike of 2.5, 5.0, 7.5, and 10.0 mol L–1. The RSD remained less than 5% for all the data. For the pond water, the recovery (%) of 124.8, 123.4, 122.1, and 118.4 corresponded to the CIP spike of 2.5, 5.0, 7.5, and 10.0 mol L–1. The RSD was slightly higher than that of the tap water measurements and lower than 7.5% for all the data. Notably, the sensing data exhibited ∼15% reduction for tap water and ∼20% increment for pond water. This result requires further tests and analyses for water compositions and thus is part of our future experiments.

3. Conclusions

The MIP/rGO/GCE-based electrochemical sensor was synthesized by electropolymerization in the presence of a CIP molecular template. The copolymer film of PANI–o-PDA functions as the MIP with recognition sites for the CIP. Subsequently, an MIP/rGO composite was fabricated to promote carrier mobility. The sensor was operated in the DPV mode and exhibited high sensitivity and selectivity for CIP with good reproducibility and repeatability. Furthermore, key factors for the synthesis of MIP and CIP measurements were studied. The MIP/rGO/GCE sensor exhibited a high recovery rate, indicating a substantial potential to be used in realistic environments. Our future studies will focus on analyzing components in water samples, including surface water, groundwater, and tap water.

4. Experimental Section

4.1. Chemicals and Reagents

Graphite flakes (10 mesh, 99.9%) were purchased from Alfa Aesar (Tewksbury, USA). Sulfuric acid (H2SO4, analytical grade, 98%) was purchased from RCI Labscan Ltd. (Rongmuang, Thailand). Potassium permanganate (KMnO4, 99.0%) and l-ascorbic acid (C6H8O6, 99.0%) were purchased from Ajax Finechem Pty Ltd. (Seven Hills, Australia). Sodium nitrate (NaNO3, Fluka Chemika, 99%), hydrogen peroxide (H2O2, Mercks, 30%), CIP (HPLC grade), ENR (HPLC grade), OFX (HPLC grade), SMZ (HPLC grade), and PIP (HPLC grade) were purchased from Sigma-Aldrich (St. Louis, USA) and used as received. Aniline (ANI, ACS reagent), o-PDA (HPLC grade), potassium ferricyanide (K3Fe(CN)6, ACS reagent), potassium ferrocyanide (K4Fe(CN)6·3H2O, ACS reagent), potassium chloride (KCl, ReagentPlus), and N,N-DMF (C3H7NO, anhydrous) were obtained from Sigma-Aldrich (St. Louis, USA). Hydrochloric acid (HCl, ACS reagent, 37.0%), sodium hydroxide (NaOH, ACS reagent), and ethyl alcohol (C2H5OH, ACS reagent) were purchased from Merck (Darmstadt, Germany). The PBS (0.05 mol L–1) was prepared by mixing solutions of potassium dihydrogen phosphate (KH2PO4, reagent grade) and dipotassium hydrogen phosphate (K2HPO4, reagent grade), which were purchased from Merck (Darmstadt, Germany). A stock solution of 0.1 mol L–1 CIP was prepared by dissolving CIP in a 0.1 N HCl solution. All the aqueous solutions were prepared using UPW (Millipore water, USA, 18.2 MΩ cm–1).

4.2. Synthesis of Reduced Graphene Oxide

rGO was supplied by the Nanocomposite Engineering (NanoCEN) Laboratory, Department of Chemical Engineering, Mahidol University. GO powder was synthesized following the chemical exfoliation method presented in a previous study.37 Briefly, we mixed 2.0 g of graphite flakes with 1.0 g of NaNO3 in 50 mL of H2SO4. The mixture was stirred in an ice bath at 0 °C for 2 h while 7.3 g of KMnO4 was added. The mixture was transferred from the bath and stirred at 30 °C for 2.5 h. The viscous mixture was mixed with 55.0 mL of DI water and 7.0 mL of H2O2 to terminate the oxidation reaction. The mixture was filtered using a vacuum filtration apparatus to obtain a brownish GO slurry. The GO was rinsed sequentially with 3% (v/v) HCl and DI water until the pH 7 supernatant was achieved. GO powder was dried in a vacuum oven at 60 °C for 24 h. For a synthesis of rGO, GO suspension (1.0 mg mL–1) was prepared in an aqueous solution of 10 mg mL–1 C6H8O6. The mixture was stirred for 24 h to reduce GO to rGO. The rGO powder was obtained using a vacuum filtration apparatus, rinsed with DI water, and dried at 100 °C for 24 h. The powder was kept in a desiccator for future use.

4.3. Preparation of the rGO/GCE

The GCE (3 mm diameter carbon disk) was sequentially polished with 0.3, 0.1, and 0.05 μm alumina powder on polishing pads (CH Instruments, Inc.). It was rinsed with 1:1 HNO3/H2O (v/v), ethyl alcohol, and UPW, dried under a nitrogen stream, and heated to 60 °C for 10 min. The rGO suspension was prepared by suspending rGO (1.0 mg mL–1) in UPW with assistance from ultrasonication. The rGO suspension of 5.0 μL was dropped on the GCE and further dried in an oven (Memmert, Schwabach, Germany) at 60 °C to create a uniform rGO film (1.42 μg mm–2).

4.4. Fabrication of MIP/GCE and NIP/GCE

The rGO/GCE was employed in an electrochemical cell as a WE along with a counter electrode (CE, CHI115, CH Instruments Inc., Austin, USA) and reference electrode (RE, CHI111, CH Instruments Inc., USA). For the electropolymerization of MIP, the electrolyte was 1.0 × 10–3 mol L–1 ANI, 1.0 × 10–3 mol L–1o-PDA, and 5.0 × 10–3 mol L–1 CIP as a template in a background medium of HCl solution (pH 3). The electropolymerization was performed in a cyclic voltammogram. A potential window of 0.0 to 0.7 V was applied to WE at 80 mV s–1 scan rate for 15 cycles (Figure S4a). The MIP-modified electrode was eluted by immersion in 1.0 mol L–1 HCl for 3 min, creating imprinted cavities on the MIP. MIP/rGO/GCE was thoroughly rinsed with UPW and stored in a desiccator for future use. In addition, NIP/rGO/GCE was fabricated following a similar procedure with no CIP during the electropolymerization. The CV curve obtained from the electropolymerization is illustrated in Figure S4b.

4.5. Material Characterizations

The physical, chemical, and crystallographic properties were analyzed using FE-SEM (JSM 7610F Plus, JEOL Ltd., Tokyo, Japan) equipped with an energy-dispersive X-ray spectrometer, FT-IR (Nicolet iS50 spectrometer, Thermo Fisher Scientific, Madison, USA), Raman spectroscopy (Jobin Yvon XploRA Plus, HORIBA, Kyoto, Japan), and XRD (D2 PHASER, Bruker, Bremen, Germany). For FE-SEM analysis, CIP, rGO, pre-elution MIP/rGO, and post-elution MIP/rGO were deposited in powder form on an adhesive carbon tape. The samples were dried in a vacuum oven sputter-coated with Pt (JEC-3000FC, JEOL, Tokyo, Japan) at 20 mA for 25 s for better visualization. CIP was sprinkled onto the grid and sputter-coated with Pt film. FE-SEM images were acquired using a 5.0 keV electron beam. FT-IR spectra were achieved over 4000–400 cm–1 wavelength range, 4 cm–1 resolution, and 64 scanning cycles per spectrum using the KBr-disc technique. The solid powder was diluted in KBr diluent and pressed using a Perkin Elmer hydraulic device at an applied pressure of five tons. Raman spectra were created in the 200–2000 cm–1 range using Raman lasers of 532 nM at 0.70 mW and 785 nm at 7.18 mW. For XRD analysis, the spectra were recorded from a 5–90° (2θ) window with a Cu-Kα X-ray radiation source.

4.6. Electrochemical Procedures

The electrochemical studies were performed on a PalmSens4 Potentiostat/Galvanostat electrochemical system (PalmSens BV, Utrecht, The Netherlands) using a PSTrace 5.9 electrochemistry software PC interface. The sensitive element on the GCE was used as the WE, consisting of silver/silver chloride (Ag/AgCl) RE and platinum (Pt) wire CE. For the CV analysis, the sensor was scanned within a potential window of −0.2 to 0.6 V at a scan rate of 50 mV s–1. An aqueous solution of 5.0 × 10–3 mol L–1 [Fe(CN)6]3–/4– (1:1) and 0.1 mol L–1 KCl was used as a medium. For the DPV experiment, the WE was applied with a step potential (Estep) of 5.0 mV, pulse potential (Epulse) of 60.0 mV, pulse time (tpulse) of 0.02 s, and at a scan rate of 50 mV s–1. High-purity nitrogen gas was used for degassing the electrolyte medium for 10 min before the experiments. For the statistical analysis, we used regression analysis to determine key parameters for the sensor at the 95% confidence level. Precision of electrochemical analysis was quantified where n = 5. The error bars of the results represent the SD of the mean by measurement.

4.7. Preparation of Actual Water Samples

Samples from the tap and pond water (13.796512N, 100.326857E) were collected during the summer time (April 8, 2022, 9:00 a.m.) at Mahidol University, Salaya campus, Thailand. The values of temperature (26.0 and 28.7 °C), pH (7.19 ± 0.17 and 8.11 ± 0.06), conductivity (0.28 and 0.88 mS cm–1), salinity (0 and 0 g L–1), and dissolved oxygen (2.0 and 0.5 mg L–1) were averaged from three measurements for the tap and pond water, respectively. All samples were filtered with a glass fiber membrane (GF/B, 1.0 μm pore size, Whatman, U.K.) and diluted at a ratio of 1:5 in PBS (pH 6.5) to a total volume of 10.0 mL. CIP stock solutions were diluted to CIP-containing water samples with concentrations of 2.5, 5.0, 7.5, and 10.0 nM.

Acknowledgments

J.C. is grateful for the full Ph.D. scholarship from the Thailand Research Fund through the Royal Golden Jubilee (RGJ) Ph.D. program (PHD/0051/2561) with the National Research Council of Thailand (NRCT). The authors thank the On-site Laboratory Initiative of the Graduate School of Global Environmental Studies, Kyoto University, for their financial support. We also acknowledge the Mahidol University Frontier Research Facility (MU-FRF) for instrumental support and the MU-FRF scientists Nawapol Udpuay, Dr. Suwilai Chaveanghong, and Chawalit Takoon for their kind assistance in material analysis of Raman spectroscopy, XRD, and energy-dispersive X-ray spectrometer-equipped FE-SEM.

Glossary

Abbreviations

ABR

antibiotic resistance

ANI

aniline

CE

counter electrode

CIP

ciprofloxacin

CV

cyclic voltammetry

DMF

dimethylformamide

DPV

differential pulse voltammetry

ENR

enrofloxacin

FQ

fluoroquinolone

FT-IR

Fourier transform infrared

GCE

glassy carbon electrode

HPLC

high-performance liquid chromatography

LOD

limit of detection

MIP

molecularly imprinted polymer

NIP

nonmolecular imprinted polymer

OFX

ofloxacin

o-PDA

o-phenylenediamine

PANI

polyaniline

PBS

phosphate buffer solution

PGE

pencil graphite electrode

PIP

piperacillin sodium salt

RE

reference electrode

rGO

reduced graphene oxide

RSD

relative standard deviation

SD

standard deviation

SMZ

sulfamethoxazole

SWV

square wave voltammetry

UPW

ultrapure water

WE

working electrode

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.2c07095.

  • It covers results from variation in synthesis parameter of the MIP/rGO/GCE; conditions for CIP sensing; CV from electropolymerization of MIP and NIP; comparison of the sensor with other electrochemical sensors; selectivity of the sensor; and signal recovery in actual water samples of the MIP/rGO/GCE sensor (PDF)

Author Contributions

J.C.: conceptualization, data curation, formal analysis, visualization, methodology, and writing—original draft and editing; S.S.: supervision, methodology, resources, investigation, and writing—review and editing; N.B.: methodology and supervision; C.P.: conceptualization and supervision; N.R.: funding acquisition and project administration; N.L.: resources; A.K.: formal analysis; and S.K.B.: supervision, formal analysis, funding acquisition, validation, and writing—review and editing.

The authors declare no competing financial interest.

Supplementary Material

ao2c07095_si_001.pdf (593.5KB, pdf)

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