Gold coated electrospun polymeric fibres as new electrode platform for glucose oxidase immobilization
Graphical abstract
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.
Section snippets
Materials and reagents
Poly(methyl methacrylate) (PMMA), N,N-dimethylformamide (DMF), sulphuric acid (H2SO4), sodium phosphate monobasic (NaH2PO4), sodium phosphate dibasic (Na2HPO4), potassium hexacyanoferrate (II) (K4Fe(CN)6), NaCl, KCl, hydrogen peroxide (H2O2), glutaraldehyde (GA), bovine serum albumin (BSA), 11-mercaptoundodecanoic acid (MUA), N-(3-dimethylaminopropyl)-N’-ethylcarbodiimide hydrochloride (EDC), N-hydroxysulfosuccinimide sodium salt (NHS), glucose oxidase (GOx) from Aspergillus niger (>100 kU/g
Results and discussion
The objective of this research is the development and characterization of flexible electrodes obtained from metal coated conductive polymeric fibres, and their applications for biosensing. Such kind of flexible electrodes can be further integrated into wearable devices for continuous monitoring of analytes in body fluids. In this context, the first part of the manuscript deals with the characterization of these electrodes by scanning electron microscopy (SEM) and electrochemical methods. The
Conclusion
This manuscript describes the electrochemical properties of electrodes obtained from conductive polymeric fibres and their applications for biosensing. These electrodes were obtained by electrospinning a poly(methyl methacrylate) solution, leading to submicrometer fibres which were further subjected to coating with a gold layer and attached on a flexible polyethylene terephthalate substrate. The morphology of these electrodes, investigated by scanning electron microscopy showed multi-layers of
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Financial support from the Executive Agency for Higher Education, Research, Development and Innovation Funding (UEFISCDI), Romania, Project code: PN-III-P4-ID-PCE-2016-0580, the Romanian Ministry of Research and Innovation through Operational Programme Competitiveness 2014-2020, Project: NANOBIOSURF-SMIS 103528, and PN19-03 (contract no. 21 N/08.02.2019).
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