Gold coated electrospun polymeric fibres as new electrode platform for glucose oxidase immobilization

Highlights

• Gold-coated electrospun polymeric fibres as flexible electrodes.

• Morphological, electrochemical characterization, comparison with planar electrodes.

• Sensitive electrochemical detection of hydrogen peroxide.

• Immobilization of glucose oxidase on gold-coated electrospun polymeric fibres.

Abstract

Fibres of poly(methyl methacrylate) were obtained by electrospinning, subjected to coating with a gold layer and then attached on a thin polyethylene terephthalate substrate in order to obtain flexible electrodes for biosensing applications. The morphology of these electrodes, investigated by scanning electron microscopy showed multi-layers of random oriented fibres of approx. 400 nm diameter. The electrochemical characterization of these flexible electrodes was performed by cyclic voltammetry and electrochemical impedance spectroscopy in acid and neutral media, in the absence and in the presence of redox probes, proving their superior performance (e.g. 5-fold current density value) when compared to planar gold electrodes obtained on silicon wafers. The electrodes obtained from conductive electrospun polymeric fibres nets were tested by cyclic voltammetry and amperometry for the detection of hydrogen peroxide with a sensitivity of 0.84 mA cm⁻² mM⁻¹ and a detection limit of 20.40 μM. The immobilization of the model enzyme glucose oxidase at the surface of the gold-coated electrospun polymeric fibres electrode was investigated and the obtained biosensor was applied for glucose determination in aqueous solutions by fixed potential amperometry with a sensitivity of 3.10 μA cm⁻² mM⁻¹, a detection limit of 0.33 mM, and reduced interferences. Also, the practical applicability of the biosensor was tested for the detection of glucose in artificial sweat and serum samples.

Introduction

The demand for devices able to provide continuous point-of-care screening of biologically active compounds has been consistently increasing over the last years, due to their applicability in the medical field with the intent to prevent, diagnose and/or treat various medical conditions [1], [2], [3]. The continuous monitoring of various analytes in body fluids is able to provide in real time a profile of the parameters related to the physiological status of an organism [4], [5], [6], [7]. Referring to body fluids, sweat contains information on electrolytes such as pH and salts or biomolecules such as glucose, lactate and uric acid among others, and their concentrations are directly related to the processes that take place in the human body [4], [8], [9], [10], [11]. With the intent of measuring biomarkers in sweat, wearable sensor for pH and electrolytes in sweat are readily developed by using metal/metal oxide electrodes [12], [13]. On their turn, determination of biomolecules such as glucose, uric acid or lactate can be performed by modifying the electrode surface with the specific oxidase enzymes [14], [15], [16]. However, the most important features of devices able to perform continuous monitoring are sensitivity, selectivity and flexibility, which is usually achieved by employing polymeric substrates for the development of the electronic component [17], [18].

Nanostructured materials brought enormous advantages in electroanalytical devices, due to their specific characteristics such as the high surface to volume ratio and electrocatalytic properties [19], [20], [21], [22], [23]. Different morphologies such as nanoparticles, nanowires, nanorods among others, can be obtained by several methodologies including chemical and physical synthesis.

Electrospinning allows obtaining multi-layers of submicron fibre-like structures [24], [25], [26] with applications from actuators to electrochemical sensors [12], [27], [28]. This technique can be applied to various polymeric solutions including nylon [29], polycaprolactone [30], poly(methyl methacrylate) [31], polyaniline [28] and polypyrrole [27]. When used in biosensing applications, the production of electrospun polymeric fibres at an electrode surface was seen as a strategy to increase the surface area but the conductivity of the fibre is usually low and needs to be enhanced [32]. There are several approaches useful for this purpose including electrospinning blends of polymers with conductive carbon black, carbon nanotubes, graphene or metallic nanoparticles or covering the electrospun polymeric fibre with conductive nanostructures. For example, blends of cross-linkable polymers with multi-wall carbon nanotubes and glucose oxidase were electrospun in order to obtain a glucose biosensor with a linear range up to 4 mM and a detection limit of 2 μM [33]. In another example, a mixt solution of glucose oxidase with polyethylenimine and polyvinyl alcohol was electrospun and then decorated with Au nanoparticles, achieving a lower detection limit of 0.9 μM with the compromise of a narrower linear range of up to 400 μM [34]. Also, a fructose dehydrogenase biosensor obtained by electroless deposition of gold nanoparticles on an electrospun poly(acrylonitrile)-HAuCl4 fibres [35] or a sensor for H₂O₂ detection through gold nanoparticles-poly(vinyl alcohol) (Au NPs-PVA) [36] were reported. Apart from gold and carbon nanostructures, some other (semi)conductive materials used to enhance the conductivity of the electrospun fibres are Ag, ZnO or TiO₂. There is a large number of publications that dealt with the applications of electrospun fibres in biosensing which may be reviewed in reference [32]. Although very innovative, most of the work referenced in the scientific literature involve difficult/time-consuming steps for fabrication and moreover, deposition/dropcasting of these fibres on conductive electrode support such as glassy carbon, indium-tin oxide or gold, the fibres being used in these conditions mainly as an immobilization support [37], [38], [39], [40], [41]. Besides, neither of the above mention supports allows flexibility, the most important feature of a wearable (bio)sensor.

In this context, the aim of this work is the development of an original biosensing platform, based on electrodes manufactured from electrospuned polymeric fibres coated with a thin metal layer allowing to obtain a conductive path in which the charge transfer mechanism is different, and occurs along the metal-coated fibre [12], [13], [42]. It is envisioned that this architecture plays a dual role: as an immobilization support for the biomolecule and, at the same time, as a transducer of the biochemical interaction, while ensuring its flexibility. The resulting electrodes were used as electrochemical sensors for H₂O₂ detection. Their applicability in enzyme biosensor construction was verified by employing glucose oxidase as a model enzyme.