Laponite synthetic clay known as nanoclay consists of

Laponite, synthetic clay, known as 2:1 nanoclay consists of two tetrahedral silicate sheets sandwiching an octahedral sheet and separated from another unit via electrostatic interaction. The geometrical structure is disk-like with diameter of ∼30nm and thickness of 1nm. Clays are used in biosensors as immobilization matrices due to their large specific surface area, good absorbance ability and high cationic exchange capacity [3–6]. Due to the property of laponite to form non-ergodic soft solids in the aqueous dispersions, they have been used to cast films onto electrodes for the immobilization of proteins [7–11] Rawat et al. have studied the aspect ratio and concentration dependent cytotoxicity and antimicrobial activity of laponite and Montmorillonite nanoclay [12,13]. Ionic liquids (ILs) are class of compounds that are widely used in the development of electrochemical sensors [14,15]. ILs are defined as those which consist of an organic cation and an organic/inorganic anion which are molten salts at low temperature [16,17]. Maleki et al incorporated ILs into carbon paste electrode which exhibited excellent electrochemical behavior [18]. Safavi et al used carbon ionic liquid electrode where ionic liquid are basically used to improve the reversibility and kinetics of electrochemical reaction for the simultaneous determination of dopamine, ascorbic pi3k inhibitor and uric acid [19]. A biosensor is usually defined as an analytical device through which a biological response is converted into signal that can be quantified and processed [20,21]. Clay colloid provides a favorable microenvironment for electron transfer and catalytic reactions on the electrode [22,23]. Electrodes modified with clay minerals have attracted the attention of many electrochemists because of their unique layered structure and their ion exchange properties [24]. The entropic ionogels of laponite prepared in IL solutions causes the formation of self-assembled colloidal networks [25]. The aim of the present work was to make a biosensor using laponite (L), and [C2mim][Cl] ionic liquids (IL), and to study their sensing behavior towards different bio-analytes. The determination of oxalic acid in food and urine is utmost importance as this toxic compound can cause kidney stones. Various instrumental techniques such as gas chromatography, liquid chromatography, spectroscopy, enzymatic method were developed but due to their high cost with low sensitivity, it is convenient to use electrochemical techniques. Thus, electrode materials are used for constructing oxalic acid sensor. Shimohigoshi and Karube [26] construct a multi-functional bio-thermo chip system for the determination of kidney calculus indices oxalic acid. Liu et al. [27] developed an HRP-immobilized RTIL based sol–gel matrix for constructing an electrochemical biosensor which exhibit excellent sensitivity as well as stability. Maiyalagam et al. [28] demonstrated the determination of oxalic acid in real samples, specifically in tomato extract using the tungsten carbide with tube like nanostructures supported platinum nanoparticles. A method for the determination of oxalic acid based on the use of oxalate oxidase where injection of oxalate oxidase in a buffer solution into an internal chamber of the SIRE biosensor by Hong et al. [29]. In our work we are using laponite–IL electrode for determining oxalic acid which is simple to construct and most important is that it does not require any enzymes. The introduction of Ionic Liquid into laponite makes it more stable film and enhanced the conductivity as compared to pure laponite. A lot of work has been done in literature for oxalic acid sensor using enzymes as well as enzymes free some of which is shown in Table 2.
Materials and method The IL, 1-ethyl-3-methyl imidazolium chloride [C2mim][Cl], was purchased from Sigma–Aldrich and was used as received. Deionized water from Organo Biotech Laboratories, India, was used to prepare the solutions. laponite RD® clay was purchased from Southern Clay Products, USA. Clay dispersions (2% w/v) were prepared by dissolving laponite in deionized water with constant stirring for 2h, to which a given amount of IL was added, and stirred for 10min. The solutions of different analytes i.e. glucose, urea, citric acid, ascorbic acid, cholesterol and oxalic acid were prepared as stock solutions (20mM) and stored in refrigerator at 4 °C. Indium-tin-oxide (ITO) coated glass plates obtained from Balzers, UK, (Baltracom 247 ITO, 1.1mm thick) with a sheet resistance and transmittance of 25Ωsq−1 and 90%, respectively were used as electrode substrates.