Electrochemical synthesis of molecularly imprinted poly(p-aminobenzene sulphonic acid) on carbon nanodots coated pencil graphite electrode for selective determination of folic acid

https://doi.org/10.1016/j.jelechem.2019.113518Get rights and content

Highlights

  • Electrochemical synthesis of carbon nanodots (CNDs).

  • Electropolymerization of o-phenylenediamine on CNDs coated pencil graphite electrode (PGE).

  • Electrochemical folic acid (FA) sensor based on molecularly imprinted polymer electrode.

  • Selective voltammetric determination of FA by using FA-imp/CNDs/PGE.

Abstract

In this work, a new electrochemical sensor based on molecular imprinting was developed by electrochemical polymerization of p-aminobenzenesulphonic acid (ABSA) on a pencil graphite electrode modified with carbon nanodots (CNDs) for detection of folic acid (FA) selectively and sensitively. The detection of FA is crucial since the deficiency of FA causes many diseases in humans and due to its antioxidant property, FA prevents the free radical attacks on DNA. CNDs were synthesized electrochemically by applying a constant potential to the two graphite pencil rods, and characterized by TEM, UV–visible and fluorescence measurements. CNDs were utilized to improve the electrochemical signal enlarging the rate of electron-transfer, electroactive surface area and intensity of the sensor. Folic acid imprinted (FA-imp) and non-imprinted (N-imp) electropolymerized films on electrodes were characterized by SEM, optical profilometry and voltammetry measurements. The electropolymerization of ABSA and optimizations of experimental steps were conducted by cyclic voltammetry and differential pulse voltammetry. The increase in reduction peak current was proportional to the concentration of FA in the range of 2.2–30.8 ng/mL and the limit of detection was calculated as 2.02 ng/mL. The proposed sensor with long term stability and satisfactory reproducibility was effectively implemented for selective and sensitive detection of FA in pharmaceutical and human urine samples.

Introduction

Folic acid (FA) is the synthetic form of folate; vitamin B9 and found in vitamin tablets and fortified foods. FA is vital for making red blood cells, helping rapid cell division and growth, enhancing brain health, age-related hearing loss as well as the synthesis and repair of DNA and RNA. FA is especially important for women who are pregnant or planning to become pregnant since it might help prevent the fetus from developing major congenital deformities of the brain or spine, including neural tube defects, such as spina bifida and anencephaly. Therefore, the selective and sensitive determination of FA in biological system is very important task. However, the analysis of FA in low concentrations is not easy procedure due to the low solubility in acidic conditions, light and temperature sensitive properties, and complex extraction and detection methods [1]. Although there are several analytical techniques for the determination of FA such as high-performance liquid chromatography [2], capillary electrophoresis [3], LC-MS [4], spectrophotometry [5], fluorimetric [6], and flow injection chemiluminescence [7], these methods are time-consuming, expensive and requires complicated pre-treatment steps. On the other hand, the electrochemical methods are very promising way for determination of FA [[8], [9], [10]] due to easy fabrication, low cost and rapid analysis compared to other analytical methods. However, the lack of selectivity is an important aspect in most of the sensors fabricated by electrochemical methods. Therefore, the molecular imprinted polymers (MIPs) based electrochemical sensors have taken great attention recently due to their high selectivity to the target molecule. MIPs are synthetic polymers that have remarkable specific adsorption capability for the analyte detected under interference effect of complex matrix [11]. The molecular imprinting procedure includes the polymerization of a functional monomers in the presence of a template (target) molecule, and then removal of the template creates highly selective recognizing cavities with appropriate size and shape in the polymer backbone [12,13].

Molecular Imprinting Technology (MIT), which concerns formation of selective sites in a polymer matrix with the memory of a template, is a technique to design artificial receptors called MIPs. This technology usually refers to a progress that the template molecules interact with the selected functional monomer to form main host-guest complexes and then a certain amount of cross-linking agent and initiator were added together into the complexes to obtain macromolecule polymers [14]. MIPs have several advantages such as stability at different temperatures and pH’s, relatively inexpensive synthesis and facile preparation compared with natural recognition receptors like antibodies [15,16]. Moreover, to some extent, MIPs are similar to natural recognition receptors due to the high selectivity to the target molecule and the recognition mechanism [17]. These superior features allow MIPs to be used as a recognition element or modifying agents in various nanosensor applications such as ultra-trace level determination of antibiotics [18,19], antioxidants [20], drugs [21,22], and toxic compounds [23,24] with a long lifetime, high reproducibility and repeatability. The conversion of the specific binding to electrical signals is affirmative for this type of sensor applications [25,26].

MIPs could be synthesized by photopolymerization, free radical polymerization or electropolymerization [27]. Comparing to other methods, the electropolymerization have some superior features since the thickness and morphology of the polymer film could be controlled through electropolymerization conditions such as applied cycle number and voltage. However, the relatively low conductivity and electrocatalytic activity of MIPs reduce the sensitivity of the electrochemical sensor [28]. Therefore, the electrodes are modified with conductive nanomaterials especially carbon-based materials and a thin MIP layer formed on the surface of the electrodes increase the conductivity of the sensor [[29], [30], [31], [32]]. Additionally, the electrochemical sensor combined with the MIP could effectively prevent the interference of impurity, which is the major problem of electrochemical detection.

Recently, carbon nanodots (CNDs) have attracted growing interest as a new carbon nanomaterial to fabricate the electrochemical sensor due to the suitable conductivity, small particle size, increasing surface area, easy synthesis, low toxicity and cost [[33], [34], [35], [36]]. CNDs could be synthesized by different methods such as electrochemical oxidation of graphene oxide [37], microwave synthesis [38], and from natural sources like corn [39], orange peel [40] and sugarcane [41]. However, these methods require high temperature and acid usage in large quantities that cause environmental pollution by producing chemical waste. Unlike the other methods, the synthesis of CNDs with electrochemical exfoliation of graphite is a green method that could be carried out at room temperature with high yield [42]. In spite of the superiority of MIPs for electrochemical sensor applications, the most of the studies about the synthesis of MIP for FA sensing are still in initial stage since the large size of FA molecule could inhibit the creation of active sites in the polymer backbone during the polymer formation [43,44].

Herein, a new electrochemical sensor based on MIP, which was synthesized by electropolymerization of p-aminobenzosulphonic acid on CNDs modified pencil graphite electrode, was developed for sensitive and selective recognition of FA. The electrochemical polymerization was achieved by employing of ABSA as a monomer and FA as a template. CNDs were synthesized in an electrochemical cell containing two graphite pencil rods as anode and cathode by applying a constant potential. The double modification of the electrode with CNDs and MIP increased the electroactive surface area, and therefore electrochemical signal of the electrode. To our knowledge, no prior study was done with CNDs and PABSA, which were used at the same time for developing a FA-imprinted electrochemical sensor. Therefore, this paper addresses the need for selective and sensitive determination of FA, so far lacking in the scientific literature. The results showed that the developed sensor provides a successful way in producing effective solution to selective and sensitive determination of FA in drug and human urine samples compared to other analytical techniques as well as rapid analysis time, low cost, and efficient analytical signal.

Section snippets

Reagents and solutions

All chemicals used in study were of analytical reagent grade and used as received without further purification. Folic acid (FA) and p-aminobenzosulfonic acid (ABSA) were purchased from Alfa Aesar (Lancaster, UK). Methotrexate, folinic acid, tetrahydofolic acid, pyridoxine, 5-methyltetrahydrofolate were obtained from Sigma-aldrich (Darmstadt, Germany). Acetonitrile and triethanolamine were purchased from Merck (Darmstadt, Germany). Deionized water from Millipore Milli-Q system (resistivity

Spectral characterizations of CNDs

UV–visible absorption and fluorescence spectra of electrochemically produced CNDs were depicted in Fig. 1. UV–visible spectrum of CNDs exhibited two absorption bands appeared at 283 nm and broad shoulder around 360 nm. When excited at 360 nm, the fluorescence spectrum of CNDs exhibits intense and symmetrical emission peak at 470 nm.

The quantum yield of CNDs was calculated by measuring the integrated fluorescence area of spectrum in aqueous dispersion against quinine sulphate (as a standard one

Conclusion

In this study, a new electrochemical sensor for selective and sensitive determination FA was developed by electro-polymerization of FA-imprinted film on carbon nanodots coated pencil graphite electrode. The experiments revealed that FA-imprinted electrode generated by electro-polymerization of ABSA exhibited a superior selectivity and stability compared to both of the electrodes modified with only carbon nanodots and non-imprinted film. After incubation with FA molecules, the reduction peak

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