A novel sensor based on bifunctional monomer molecularly imprinted film at graphene modified glassy carbon electrode for detecting traces of moxifloxacin

Abstract

A novel selective and sensitive electrochemical sensor for moxifloxacin (MFLX) detection based on bifunctional monomer molecularly imprinted polymer (MIP) membranes on a glassy carbon electrode (GCE) modified with graphene was constructed. A suspension of graphene was deposited on the GCE surface. Subsequently, a molecularly imprinted film was prepared by the electropolymerization, via cyclic voltammetry, of o-phenylenediamine and L-lysine as functional monomers in the presence of MFLX as the template molecule. A control electrode (NIP) was also prepared. The electrochemical properties of the MIP and non-molecularly imprinted polymer (NIP) sensors were investigated via cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), in which [Fe(CN)₆]^3−/4− was used as an electrochemical active probe. The surface morphology of the imprinted film was characterized by scanning electron microscopy (SEM). The fabrication conditions that affect the performance of the imprinted sensor are discussed. Under the optimal experimental conditions, the imprinted sensor had good linear current responses to moxifloxacin concentrations in the ranges from 1.0 × 10⁻⁹ to 1.0 × 10⁻⁸ M and from 1.0 × 10⁻⁸ to 5.0 × 10⁻⁵ M, with a detection limit of 5.12 × 10⁻¹⁰ M (S/N = 3). The developed sensor was successfully applied to detect moxifloxacin in tablets and human urine samples. Moreover, the fabricated sensor possessed good selectivity and stability, providing a promising tool for immunoassays and clinical applications.

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1. Introduction

Moxifloxacin (MFLX) is a fourth-generation fluoroquinolone antibacterial agent that is active against a broad spectrum of Gram-positive and Gram-negative ocular pathogens, typical microorganisms and anaerobes.¹ Moxifloxacin is mainly applied in the treatment of acute bacterial sinusitis caused by sensitive microbes, acute bacterial chronic bronchitis, mild to moderate community intravenous pneumonia, and skin and soft tissue infection without complications.^2,3 Moxifloxacin has been detected by various methods, such as spectrophotometry,^4,5 spectrofluorimetry,⁶ atomic absorption spectrometry,⁷ high performance liquid chromatography (HPLC),^8–11 capillary electrophoresis (CE)¹² and electrochemical methods.^13–15 Electrochemical sensors, one of the electrochemical methods, have been reported to be eco-friendly and are considered to be highly sensitive, selective and convenient tools with fast response and low cost compared to the other common analytical techniques. Various electrochemical sensors have been used for moxifloxacin determination.^16–18 However, the presence of higher concentrations of some structurally related analogues, such as gatifloxacin, ciprofloxacin, ofloxacin and norfloxacin, strongly interferes with the selective determination of moxifloxacin in biological samples. Thus, the aim of this study was to prepare a sensor for the selective and sensitive determination of moxifloxacin in human biological fluids.

As a typical approach for high affinity and specific recognition, molecularly imprinted polymers (MIPs) have gained considerable attention in recent years and have been found to be very promising in the field of electrochemical sensors.^19,20 The integration of electrochemical devices and MIPs, which demonstrates good sensitivity and selectivity, is an attractive approach for the development of biochemical sensors.^21–23 Most MIPs commonly prepared with strategies such as bulk polymerization, precipitation polymerization and sol–gels often have limitations, including slow mass transfer, incomplete template removal and heterogeneous distribution of binding sites.^24–26 Therefore, the use of electropolymerization for the proper design of MIP-modified electrodes is an efficient way to solve these limitations by generating a rigid, uniform and compact molecularly imprinted film with controlled thickness.^27,28 For the construction of a molecularly imprinted polymer (MIP)-based sensor by an electropolymerization technique, the choice of functional monomer is important. The electropolymerization of o-phenylenediamine (OPD) has been widely used for the preparation of molecularly imprinted electrochemical sensors,^29,30 due to its excellent biocompatibility and the feasibility of immobilizing different compounds. L-Lysine is an essential α-amino acid with basic properties. L-Lysine modified electrodes have the advantages of stability and positive surfaces,^31,32 which can provide fast electron transfer. At the same time, those charged molecules are more easily adsorbed on the surface of the sensor. To further increase the amount of effective binding sites in the sensor, an attempt was made to use both of these monomers to form MIPs.

Although MIPs are excellent for improving selectivity, sensitivity is also a fundamentally important feature of an electrochemical sensor. In some cases, MIPs have resulted in reduced sensitivity. Therefore, materials such as multi-walled carbon nanotubes (MWCNTs),^33,34 metallic nanomaterials^35,36 and, more recently, graphene,^37,38 have been used as a substrate layer in the preparation of MIPs. Among these materials, graphene is considered an ideal supporting material because it promotes electron transfer reactions due to its significant mechanical strength, high electrical conductivity, high surface area and good chemical stability.

Herein, for the first time, we designed a rapid, selective and sensitive sensor based on MIP for the determination of moxifloxacin. GR as the supporting material, moxifloxacin as the template molecule, and OPD and L-lysine as functional monomers have been used to construct the MIP film on the surface of glassy carbon electrode by electropolymerization. After the removal of the embedded template moxifloxacin by extraction with a 50% ethanol (V/V = 1 : 1) solution, the MIP/GR/GCE sensor was finally obtained. The adsorbed moxifloxacin is detected by the electrochemical signal of [Fe(CN)₆]^3−/4− due to the binding of moxifloxacin blocking electron transfer of [Fe(CN)₆]^3−/4− at the electrode surface. The electrochemical signal intensity is related to the concentration of moxifloxacin. The whole preparation procedure is shown in Scheme 1. The sensor could differentiate the template molecule from its analogs with good selectivity and sensitively detect moxifloxacin with a wide linear range and a low detection limit. Meanwhile, the sensor was used to detect moxifloxacin in real samples with satisfactory results.

Scheme 1 Schematic of stepwise electrode modification.

2. Experimental

2.1. Reagents and apparatus

Moxifloxacin, gatifloxacin, ciprofloxacin, ofloxacin and norfloxacin were purchased from Wuhan Yuancheng Gongchuang Technology Co., Ltd. (China). L-Lysine was purchased from Aladdin Chemical Reagent Co., Ltd. (Shanghai, China). Graphene (1.0 mg mL⁻¹) was purchased from XFNANO, INC (Nanjing, China). o-Phenylenediamine (OPD), potassium ferricyanide, potassium ferrocyanide and ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd. (China). The phosphate buffer solution (PBS) was prepared by mixing stock solutions of NaH₂PO₄ and Na₂HPO₄ and adjusting the pH values with either 0.10 M HCl or NaOH solutions. Tablets containing moxifloxacin manufactured by Bayer Pharma AG (Germany) were purchased from the local market of Chong Qing. Fresh urine samples obtained from healthy persons were supplied by Southwest University Hospital. All other chemicals and solvents used in the experiment were of analytical grade, and double distilled water was used throughout the experiments.

Electrochemical experiments, including cyclic voltammetry (CV) and differential pulse voltammetry (DPV), were performed on a LK 2006AZ electrochemical workstation (Tianjin Lanlike Co., Ltd., China), with a conventional three-electrode system including a MIP/GR/GCE working electrode, a Pt wire counter electrode and a saturated calomel electrode (SCE) reference electrode. All potential values given below were in reference to SCE. The scanning electron micrograph (SEM) measurements were carried out on a scanning electron microscope (JSM-6510, Japan). Electrochemical impedance spectroscopy (EIS) was performed on a CHI 660D electrochemical workstation (Chenhua Corp. Shanghai, China). A digital pH/mV/ion meter (CyberScan model 2500, USA) was used for the preparation of the buffer solution.

2.2. Preparation of the graphene-modified electrode

The bare GCE (3 mm in diameter) was polished with 0.05 μm Al₂O₃ slurry before it was used and was rinsed ultrasonically with 1 : 1 HNO₃, ethanol and ultrapure water respectively until a mirror-like surface was obtained. The electrode was then washed with ultrapure water and allowed to dry at room temperature before use. Two microliters of the graphene suspension (1.0 mg mL⁻¹) was dropped onto the surface of the GCE and dried in a vacuum oven at 60 °C for 1 h.

2.3. Construction of MIP/GR/GCE, MIP/GCE and NIP/GR/GCE

GR/GCE was immersed in 10.0 mL phosphate buffer solution (pH = 5.7) containing 1.0 mM OPD and 1.0 mM L-lysine as functional monomers and 0.10 mM of template moxifloxacin; electrochemical polymerization was performed via cyclic voltammetry (CV) in the potential range of −0.2 to 0.8 V at a scan rate of 50 mV s⁻¹ for 20 cycles. After the electropolymerization, the polymer modified electrode was incubated in 50% ethanol (V/V = 1 : 1) solution for 3 min to extract the template moxifloxacin to obtain MIP/GR/GCE. The procedure for the preparation of MIP/GR/GCE is depicted in Scheme 1. As a control, non-imprinted polymer sensor (NIP/GR/GCE) and MIP/GCE were prepared by the same procedure but without the addition of moxifloxacin or GR, respectively.

2.4. Electrochemical characterization and measurements

Different modified electrodes were characterized by EIS in a solution of 5.0 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] (1 : 1) containing 0.10 M KCl using an alternating current voltage of 10 mV and recorded at a bias potential of 200 mV within a frequency range of 10⁻¹ to 10⁵ Hz.

[Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ was also chosen as an electrochemically active probe to study the performance of the prepared sensor, due to the poor electroactivity of moxifloxacin. Imprinted cavities formed in MIP/GR can provide pathways for the diffusion of probe into and out of the MIP matrix; the probe is then oxidized or reduced at the electrode and produces an electrochemical signal. MIP/GR/GCE was immersed in moxifloxacin solutions with different concentrations and incubated for 10 min to ensure rebinding of the moxifloxacin molecules by MIP/GR/GCE. Then, CV and DPV were conducted for the electrochemical determination of moxifloxacin in 2.0 mM [Fe(CN)₆]^3−/4− containing 0.10 M KCl (pH = 7.0). CV was performed over a potential range from −0.2 to 0.6 V with a scan rate of 100 mV s⁻¹. DPV measurements were carried out between −0.2 and 0.6 V at a pulse width of 50 ms and amplitude of 50 mV. All the electrochemical experiments were conducted at room temperature (RT, 25 ± 1 °C).

2.5. Preparation and determination of real samples

A proposed sensor for evaluating the accuracy of the content of moxifloxacin in commercial tablets (400 mg of moxifloxacin in each tablet, from Bayer Pharma AG) was determined using the DPV method. Ten tablets of moxifloxacin drug were accurately weighed in order to find the average weight of each tablet. Then the tablets were powdered in a mortar and carefully mixed. A quantity equivalent to one tablet was weighed and dissolved in double distilled water which was transferred to a 100 mL volumetric flask and diluted to the mark with double distilled water. The resulting solution was centrifuged at 5000 rpm; then the supernatant was collected and diluted to 100 mL and used as a stock solution of sample.

Urine samples were collected in sterile bottles. The samples were spiked with a known concentration of moxifloxacin, centrifuged (3000 rpm, 10 min) to remove proteins and diluted to 50% with 0.10 M phosphate buffer solution (pH = 7.0). The samples were then analyzed without further treatment, using the conditions described in Section 2.4.

3. Results and discussion

3.1. Characterization of the different modified electrodes

SEM was performed to obtain an insight into the surface morphology of the different modified electrodes, as shown in Fig. 1. Fig. 1A–C show SEM images of GR/GCE, MIP/GR/GCE, and NIP/GR/GCE, respectively. It can be seen from Fig. 1A that GR has a large surface area, which facilitates electron transfer. A rough multihole structure can be observed in Fig. 1B, which provides a large number of recognition sites for MIP/GR/GCE. However, there are no imprinted cavities in NIP/GR/GCE.

Fig. 1 SEM images of GR/GCE (A), MIP/GR/GCE (B) and NIP/GR/GCE (C).

The extraction of moxifloxacin from the MIP layer on the surface of the electrode has resulted in the formation of imprinted cavities in MIP/GR/GCE. The formed imprinted cavities can act as channels and allow access for the diffusion of [Fe(CN)₆]^3−/4− into and out of the polymeric network, which can be oxidized or reduced at the electrode and produce an electrochemical signal. As an effective method for probing the features of a surface modified electrode, electrochemical impedance spectroscopy (EIS) was employed to characterize the stepwise construction process of the sensor. Fig. 2A shows the EIS curves of different electrodes. Bare GCE (a) shows a very low charge transfer resistance. With the modification of the MIP before elution (b), the resistance to charge transfer (R_et) is large because the film modified on the electrode is nonconductive. When the template moxifloxacin was removed from the imprinted film (c), the R_et reduced significantly, which suggested that the removal of moxifloxacin from the MIP film decreased the electron transfer resistance. This phenomenon could be attributed to the formation of imprinted cavities after the removal of template moxifloxacin, leaving channels for the penetration of [Fe(CN)₆]^3−/4− through the MIP film to reach the GCE for the further oxidation. The R_et of MIP/GR/GCE after elution (d) is less than that of MIP/GCE after elution (c), which verifies that the graphene facilitates electron transfer. Compared with MIP/GR/GCE after elution (d), the NIP/GR/GCE after elution (e) shows a larger diameter semicircle, which is related to the presence of template moxifloxacin.

Fig. 2 (A) EIS of different modified electrodes in 5.0 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] containing 0.10 M KCl and (B) CV of different modified electrodes in 2.0 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] containing 0.10 M KCl: bare GCE (a), MIP/GCE before elution (b), MIP/GCE after elution (c), MIP/GR/GCE after elution (d) and NIP/GR/GCE after elution (e). Inset: bare GCE.

The current change of [Fe(CN)₆]^3−/4− on the different electrodes recorded by the CV method confirmed the same result. As shown in Fig. 2B, MIP/GCE (b) before elution has a current response. After removal of the template, the current response of MIP/GCE (c) increases, which suggests that cavities are formed in the MIP membranes. The current response of the CV of [Fe(CN)₆]^3−/4− at MIP/GR/GCE (d) is greater than that of MIP/GCE (c) after the addition of graphene. Graphene shows high conductivity, which allows [Fe(CN)₆]^3−/4− to reach the electrode surface easily. Compared with MIP/GR/GCE (d), NIP/GR/GCE (e) without the template has a very small current response. These results may be attributed to the NIP membranes, which block electron transfer.

3.2. Choice of electropolymerized monomer for MIP/GR/GCE

In order to choose an efficient monomer, OPD, L-lysine, and OPD–lysine were employed as monomers to prepare three different sensors, and the specific rebinding properties were investigated. From Fig. S2,† the sensors prepared using OPD or L-lysine as monomer showed specific adsorption, since the current of [Fe(CN)₆]^3−/4− on NIP/GR/GCE was very low; this implies that OPD or L-lysine could been applied individually in the MIP preparation for the determination of moxifloxacin. The sensor prepared using OPD and L-lysine as monomers showed the highest current of [Fe(CN)₆]^3−/4− compared with the other sensors, which could be because the synergistic effects of OPD and L-lysine in the MIP film could rebind many moxifloxacin molecules. The imprinted factors (IF, IF = ΔI_MIPs/ΔI_NIPs) have been calculated and compared for the three sensors; the results were 2.6, 5.5, and 10.2, corresponding to the sensors prepared using L-lysine, OPD, and OPD–lysine as monomers, respectively. These results fully illustrated the advantage of OPD–lysine as a polymerized monomer.

3.3. Optimization of conditions for MIP/GR/GCE preparation

3.3.1. Effect of the volume of graphene suspension. The effect of the volume of the graphene suspension on the peak current in MIP/GR/GCE was initially studied. As shown in Fig. S3(A),† the [Fe(CN)₆]^3−/4− current response increased from 0 to 2.0 μL and then decreased sharply with further increase of the graphene volume. A large volume of graphene on the GCE can increase the sensor response. However, an increase of the graphene volume above the threshold value leads to a decrease, probably because of the thick graphene membrane, which decreases the electrode surface conductivity.

3.3.2. Effect of function monomer to template ratio. The monomer concentration in the electropolymerization process could affect not only the thickness of the polymer matrix but also the amount of imprinted molecule, which in turn influences the electrochemical behavior of the sensor. To investigate the effect of monomer concentration on the MIP/GR/GCE, the electrodes were electropolymerized in different monomer concentrations in the range of 0.80 to 2.0 mM with a constant moxifloxacin concentration of 0.10 mM. As shown in Fig. S3(B),† the highest peak current of [Fe(CN)₆]^3−/4− on the MIP/GR/GCE was observed when the concentration of monomer was at 1.0 mM. A lower peak current was found when the monomer concentration was lower than 1.0 mM, which may be due to the reduced capture of moxifloxacin during electropolymerization. Additionally, a considerable decrease in the current response on MIP/GR/GCE was observed when the concentration of monomer was above 1.0 mM because the electropolymerized film was too compact to form imprinted caves after elution. Thus, the optimum concentration of monomer for preparing MIP/GR/GCE was 1.0 mM.

3.3.3. Scan cycles and scan rate of electropolymerization. Scan cycles and the scan rate of electropolymerization are both important factors for the fabrication of MIP/GR/GCE, influencing the thickness and compactness of the imprinted polymers, respectively. To investigate the effect of scan cycles on the polymer thickness, 5 to 30 scan cycles were carried out. From the results of Fig. S3(C),† higher cycles lead to thicker films with fewer accessible imprinted sites. The optimum number of polymerization cycles was selected as 20 according to the peak current of [Fe(CN)₆]^3−/4−. Fig. S3(D)† showed the influence of scan rate on electropolymerization. On the one hand, at a slower scan rate, the imprinted polymer formed a tight polymer that decreased the accessibility for removing the template moxifloxacin to form imprinted sites. On the other hand, a loose and rough film with a low recognition capacity was formed at a faster scan rate. Thus, the optimum scan rate of electropolymerization was set to be 50 mV s⁻¹.

3.3.4. Template removal treatment. Complete removal of the template molecules is a very important step in the preparation of molecularly imprinted electrochemical sensors. A 50% ethanol (V/V = 1 : 1) solution was used to elute the template molecules. [Fe(CN)₆]^3−/4− was used as a probe molecule and the differential pulses were scanned corresponding to different elution times. Fig. S3(E)† shows the elution curve of MIP/GR/GCE. As shown in Fig. S3(E),† as the elution time increased, the current gradually increased until it approached a stable value after more than 3 min of elution time. This indicated that the template molecules were removed completely from the MIP. Therefore, we choose 50% ethanol (V/V = 1 : 1) solution and 3 min as the best solvent and time for template removal. Fig. S3(F)† shows the elution curve of MIP/GCE. Compared with MIP/GCE, the current response of MIP/GR/GCE after elution is greater. These results further indicate that graphene could improve the conductivity of molecularly imprinted polymers.

3.4. Optimization of determination conditions

3.4.1. The pH effect of rebinding solution for MIP/GR/GCE. The pH effect of rebinding solution was investigated by the DPV method at a constant concentration of moxifloxacin (1.0 × 10⁻⁸ M) in PBS with pH values ranging from 5.7 to 7.4. As shown in Fig. S4(A),† the response current increased from pH 5.7 to 6.5 and decreased above pH 6.5. The highest current change (ΔI) of [Fe(CN)₆]^3−/4− was observed when the pH value of the rebinding solution was adjusted to 6.5. Thus, we chose 6.5 as the pH value.

3.4.2. The effect of incubation time. The incubation time is important for the sensitivity of the sensor. After removal of the template molecule, MIP/GR/GCE was incubated in 4.0 × 10⁻⁹ M moxifloxacin solution for different time periods. The test results are shown in Fig. S4(B).† The peak current decreased sharply with incubation time from 0 to 15 min, which indicates the rapid and effective recognition ability of the MIP film for the target molecule. When the incubation time reached 10 min, the oxidation peak current levelled off gradually. Therefore, 10 min was chosen as the incubation time in this experiment.

3.5. Electrochemical behavior of the electrochemical active probe

The electrochemical mechanism can usually be obtained from the relationship between the peak current and the scan rate. The CV curves of the imprinted sensors in the [Fe(CN)₆]^3−/4− solution at different scan rates were investigated in the range of 10 to 100 mV s⁻¹. As seen in Fig. S5,† the peak currents of the CV in the imprinted sensor increased with the increment of the scan rate. The anodic (I_pa) and cathodic (I_pc) peak currents were nearly independent of the scan rate and can be expressed as: I_pa (μA) = 8.51 + 5.17v^1/2 (R² = 0.998) and I_pc (μA) = −9.40 − 4.03v^1/2 (R² = 0.998) (where v is the scan rate with unit mV s⁻¹), suggesting typical surface controlled electrochemical behavior.

3.6. Calibration curve

Under the optimum conditions, the detection of various concentrations of moxifloxacin was investigated with DPV using the MIP/GR/GCE sensor. As shown in Fig. 3, the peak current decreased as the moxifloxacin concentration increased, and the reduction in ΔI for [Fe(CN)₆]^3−/4− was proportional to the moxifloxacin concentration for the ranges of 1.0 × 10⁻⁹ to 1.0 × 10⁻⁸ M and 1.0 × 10⁻⁸ to 5.0 × 10⁻⁵ M, respectively. The linear regression equations are: ΔI (μA) = 11.3 log C_MFLX (M) + 106.3 (R² = 0.998) and ΔI (μA) = 1.59 log C_MFLX (M) + 28.81 (R² = 0.997). The imprinted sensor had a detection limit (S/N = 3) of 5.12 × 10⁻¹⁰ M for moxifloxacin. Table S1† shows the comparison of the performance of this sensor with other sensors for moxifloxacin detection. The results indicated that the prepared MIP/GR/GCE possessed excellent sensitivity and high selectivity for moxifloxacin determination.

Fig. 3 Different pulse voltammograms of different moxifloxacin concentrations on the sensor in 2.0 mM [Fe(CN)₆]^3−/4− containing 0.10 M KCl. Inset: plot of ΔI vs. (a–r) concentrations of moxifloxacin from 1.0 × 10⁻⁹ M to 5.0 × 10⁻⁵ M.

3.7. Repeatability and stability

To evaluate the reproducibility of the MIP/GR/GCE sensor, the net response of the sensor before and after incubation in 1.0 × 10⁻⁸ M moxifloxacin solution was measured with five replicates. The relative standard deviation (RSD) was 1.2% for the five successive assays. On the other hand, five sensors were prepared and tested under the same conditions, and the RSD of the five tests was 3.3%. Furthermore, the storage stability of the sensor was investigated. The results showed that the sensor lost only 6.0% of its initial response after it was stored in a refrigerator for 25 days. Therefore, the MIP/GR/GCE sensor has good reproducibility and stability.

3.8. Selectivity

The selectivity of the sensor towards moxifloxacin (MFLX) was evaluated by DPV using compounds with structures similar to moxifloxacin, such as gatifloxacin (GFLX), ciprofloxacin (CPLX), ofloxacin (OFLX) and norfloxacin (NFLX). As shown in Fig. S1,† the current variation (ΔI) (ΔI = I₀ − I_c, where I₀ is the original current and I_c denotes the current response of MIP/GR/GCE incubated in a solution of a particular concentration) of MIP/GR/GCE was greater than that of NIP/GR/GCE. The current response of the sensor to different analytes was measured at a concentration of 4.0 × 10⁻⁹ M. It was found that the sensor had a stronger response towards the moxifloxacin template than the structurally related analogues, suggesting that the sensor had special recognition and selectivity to moxifloxacin due to the imprinted effect. The imprinting and selecting factors are defined as

α = (ΔI/I₀)_MIP/(ΔI/I₀)_NIP (1)

β = α_MFLX/α_analog (2)

where the α value of the sensor to the template molecule is much greater than that of the other substances. The calculated results are given in Table S2,† which suggests that the size and the conformation of the cavities match with moxifloxacin in the MIP network.

3.9. Applications

The sensor was evaluated by carrying out the determination of moxifloxacin in real sample solutions obtained from tablets and human urine samples using the standard addition method under optimized conditions. The moxifloxacin content of the real samples was determined using MIP/GR/GCE, and the results are shown in Table 1. The recoveries of 96 to 103% and the relative standard deviation of less than 2.0% for the proposed sensor in real sample analysis indicate acceptable precision for the voltammetric determination of moxifloxacin using the MIP sensor. Therefore, MIP/GR/GCE has been successfully applied to the monitoring of moxifloxacin in biological and pharmaceutical samples.

Table 1 Determination of moxifloxacin in real samples (n = 3)

Sample	Added (μM)	Found^a (μM)	Recovery^b (%)	RSD^c (%)
a Average value of three determinations.b Recovery (%) = (found concentration/added concentration) × 100.c RSD: relative standard deviation.
Tablet	0	5.36	—	1.9
	2.00	7.40	102	1.1
	4.00	9.33	99	1.3
	6.00	11.4	101	1.7
Human urine	2.00	1.91	96	0.8
	4.00	4.13	103	1.0
	6.00	5.80	97	0.6
	8.00	7.85	98	0.9

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4. Conclusions

In this study, we have developed a new electrochemical sensor for moxifloxacin determination using a novel graphene-molecular imprinted polymer composite as the recognition element. The developed MIP/GR/GCE sensor has several advantages. First, the preparation of MIP/GR/GCE sensor simply involved the electrochemical polymerization of o-phenylenediamine and L-lysine in the presence of moxifloxacin on the surface of GR/GCE, which is very convenient and inexpensive. Second, the resultant MIP/GR/GCE sensor selectively recognized the template moxifloxacin and revealed a remarkably wide linear range, with a low detection limit of 5.12 × 10⁻¹⁰ M. Third, short response periods and satisfactory reproducibility and stability were also demonstrated. Moreover, MIP/GR/GCE sensor has been successfully used to determine moxifloxacin in real samples, with satisfactory results.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra01494a

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