Glucose biosensors based on platinum nanoparticles-deposited carbon nanotubes in sol–gel chitosan/silica hybrid

Abstract

A new strategy for fabricating a sensitivity-enhanced glucose biosensor was presented, based on multi-walled carbon nanotubes (CNT), Pt nanoparticles (PtNP) and sol–gel of chitosan (CS)/silica organic–inorganic hybrid composite. PtNP-CS solution was synthesized through the reduction of PtCl₆²⁻ by NaBH₄ at room temperature. Benefited from the amino groups of CS, a stable PtNP gel was obtained, and a CNT–PtNP–CS solution was prepared by dispersing CNT functionalized with carboxylic groups in PtNP–CS solution. The CS/silica hybrid sol–gel was produced by mixing methyltrimethoxysilane (MTOS) with the CNT–PtNP–CS solution. Then, with the immobilization of glucose oxidase (GOD) into the sol–gel, the glucose biosensor of GOD–CNT–PtNP–CS–MTOS–GCE was fabricated. The properties of resulting glucose biosensor were measured by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). In phosphate buffer solutions (PBS, pH 6.8), nearly interference free determination of glucose was realized at low applied potential of 0.1 V, with a wide linear range of 1.2 × 10⁻⁶ to 6.0 × 10⁻³ M, low detection limit of 3.0 × 10⁻⁷ M, high sensitivity of 2.08 μA mM⁻¹, and a fast response time (within 5 s). The results showed that the biosensor provided the high synergistic electrocatalytic action, and exhibited good reproducibility, long-term stability. Subsequently, the novel biosensor was applied for the determination of glucose in human serum sample, and good recovery was obtained (in the range of 95–104%).

Introduction

Since Clark and Lyons [1] reported their enzyme electrode for measuring glucose, great attention has been devoted to glucose biosensors [2], [3], [4], [5], [6]. It is important to develop the methods with high sensitivity, good reproducibility, fast response, and easy fabrication for the determination of glucose in both clinical diagnostics and food industry analysis. In recent years, nanomaterials, such as carbon nanotubes (CNT) and metal nanoparticles, have been widely applied to biosensors. CNT have good performances of high surface-to-volume ratios and fast electron-transfer kinetic for a wide range of electroactive species in the construction of biosensor [7], [8], [9], [10], [11], [12]. Metal nanoparticles can display following unique advantages over macroelectrodes when used for electroanalysis: enhancement of mass transport, catalysis, high effective surface area and control over electrode microenviroment [13]. For example, Pt and Au nanoparticles are very effective as a matrix of enzyme sensors by taking advantage of the biocompatibility and huge surface [14], [15]. Most work has been carried out using transition metal nanoparticles for biosensors, such as platinum [3], copper [4], silver [16] palladium [17], and gold [18]. Recently, there are some interesting researches on integrating nanoparticles with CNT to fabricate biosensors and easily to gain synergistic action with the hybrid of nanomaterials. The methods of nanohybridization are completed by solvent DMF [19], polymers, such as CS [20], [21], [22], Nafion [23], [24], [25], [26], and an appropriate surfactant dihexadecyl hydrogen phosphate (DHP) [27]. The used techniques included sol–gel [3] process, metal nanoparticles self-assembly [28], [29], [30], [31] and electrodeposition on the surface of CNT from metal salt solutions [32], [33], [34], [35].

The immobilization of enzymes is a key step for the fabrication of biosensors. The common methods for immobilizing enzymes on electrodes include entrapment techniques [3], electrochemical copolymerization [36], covalent or cross-linking [24], and adsorption [32]. Lately, sol–gel-derived materials have been applied for the immobilization of biomolecules to improve the performance of the biosensor [2], [3], [9], [37], [38]. The biosensor fabricated by this method possesses several advantages, such as high stability, wide potential window, and easy fabrication at room temperature and encapsulation for electrocatalysts and biocatalysts. Furthermore, porous and open frameworks of sol–gel ensure that the analytes can effectively and fast access to the active functionalities immobilized at the interface between electrode and solution, which results in a high sensitivity to the electrochemical response, as most electron transfer processes are diffusion-controlled. However, the silica sol–gel, which is common used, is brittle and low biocompatible. In order to overcome these obstacles, silica-based organic–inorganic hybrids are adopted as attractive composite materials for the fabrication of biosensors [39]. Some polymers are hybridized into a silica sol–gel matrix, such as polyhydroxyle (PVA–g–PVP) [2], CS [9], [40], poly(vinyl alcohol)(PVA) [41], which can effectively avoid the brittleness of pure inorganic sol–gel and the swelling of some pure polymers.

In this work, a new glucose biosensor was developed, based on the signal-enhanced effect of CNT, PtNP and sol–gel of CS/silica organic–inorganic hybrid composite film. The properties of resulting glucose biosensor were characterized by EIS and CV. The resulting biosensor exhibited several advantages, such as high sensitivity at a low potential, good stability, and easy fabrication at room temperature and encapsulation for nanomaterials and biocatalysts. Furthermore, the biosensor was applied for the determination of glucose in human serum sample.