BIOSENSORS BASED ON GOLD NANOELECTRODE ENSEMBLES AND SCREEN PRINTED ELECTRODES
Nanostructured materials have proven as one of the most powerful tool in new technologies and research, due to their absolutely peculiar properties at nanometer size scale. Many studies have shown that optical, mechanical, photo-catalytic and transport properties drastically changes, depending on quantum size effect, as the mean diameter of the particles is in the exciton size regime (i.e. 10nm). As a matter of fact, metallic and semiconductor nanosized materials have found large applications in biochemistry, bioanalytical techniques as well as in photocatalysis, optoelectronics, photo-electrochemistry.
The unique chemical and physical properties of nanoparticles make them suitable for designing new and improved sensing devices, especially electrochemical sensors and biosensors. The main functions of nanostructured particles for biosensing purposes include: immobilization of biomolecules, catalysis of electrochemical reactions, enhancement of electron transfer mechanism between electrode surfaces and proteins, labeling of biomolecules and even acting as reactant. On the other hand, the efficient assembly of biochemical recognizing systems on such sensors requires essentially: - the control of the surface activation (i.e. using self-assembled monolayers SAM); - the possible modification of the substrate; - the immobilization process of biomolecules.
Due to its special characteristic of easily forming SAM through thio-derivative compounds, gold electrode has been often used as a transducer in electrochemical biosensors. For our purposes, nanowires of gold were synthesized into polycarbonate membranes with controlled pore size using an electroless deposition method, obtaining nanoelectrode ensembles (NEEs) with special electrochemical features.
Such electrode ensembles can be either used as platform for immobilization of biomolecules or as free standing NEEs, i.e. a large assembly of very small ultramicrolectrodes confined in a rather small space.
NEEs can exhibit three distinct voltammetric response regimes, depending on the scan rate and reciprocal distance between the nanoelectrode elements, which is a function of the pore density of the template: total overlap regime, pure radial regime and linear regime. It was demonstrated that for electroanalytical applications the total overlap regime is the most advantageous one because of the higher faradaic-to-capacitive current ratios. Under such regime, Au NEEs fabricated from commercial track-etched membranes present a signal to noise ratio in the order of 100 respect to a conventional Au macro electrode with the same geometrical area.
NEEs were deposited on the working probe and coupled with carbon screen printed electrode (SPE) to give a novel easy to be used tool for disposable biosensors, which has been defined as nanoelectrode ensemble on screen printed substrate (NEE/SPS). Features of NEE/SPS based biosensors were electrochemically compared to unmodified carbon or conventional gold SPEs using immobilized glucose oxidase as biochemical model system.
Sequential deposition of SAM or polyelectrolyte onto NEE/SPS has been experimented to extend the potential range of applicability for third generation biosensors where direct electron transfer between red-ox proteins and polymer modified gold nanoelectrodes has been achieved. The NEE/SPS preparation is aimed to couple the high electroanalytical sensitivities, deriving from the nanosized properties, with the feasibility and versatility of screen printing technology in easy to be used sensor fabrications.
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