Aptamers have been widely used as recognition elements for biosensor construction, especially in the detection of proteins or small molecule targets, and regarded as promising alternatives for antibodies in bioassay areas. In this review, we present an overview of reported design strategies for the fabrication of biosensors and classify them into four basic modes: target-induced structure switching mode, sandwich or sandwich-like mode, target-induced dissociation/displacement mode and competitive replacement mode. In view of the unprecedented advantages brought about by aptamers and smart design strategies, aptamer-based biosensors are expected to be one of the most promising devices in bioassay related applications.

The design and characterization of a microneedle array-based carbon paste electrode towards minimally invasive electrochemical sensing are described. Arrays consisting of 3 × 3 pyramidal microneedle structures, each with an opening of 425 µm, were loaded with a metallized carbon paste transducer. The renewable nature of carbon paste electrodes enables the convenient packing of hollow non-planar microneedles with pastes that contain assorted catalysts and biocatalysts. Smoothing the surface results in good microelectrode-to-microelectrode uniformity. Optical and scanning electron micrographs shed useful insights into the surface morphology at the microneedle apertures. The attractive performance of the novel microneedle electrode arrays is illustrated in vitro for the low-potential detection of hydrogen peroxide at rhodium-dispersed carbon paste microneedles and for lactate biosensing by the inclusion of lactate oxidase in the metallized carbon paste matrix. Highly repeatable sensing is observed following consecutive cycles of packing/unpacking the carbon paste. The operational stability of the array is demonstrated as well as the interference-free detection of lactate in the presence of physiologically relevant levels of ascorbic acid, uric acid, and acetaminophen. Upon addressing the biofouling effects associated with on-body sensing, the microneedle carbon paste platform would be attractive for the subcutaneous electrochemical monitoring of a number of physiologically relevant analytes.

This article describes the design of a new and attractive minimally-invasive bicomponent microneedle sensing device for the electrochemical monitoring of the excitatory neurotransmitter glutamate and glucose. The new device architecture relies on the close integration of solid and hollow microneedles into a single biosensor array device containing multiple microcavities. Such microcavities facilitate the electropolymeric entrapment of the recognition enzyme within each microrecess. The resulting microneedle biosensor array can be employed as a minimallyinvasive on-body transdermal patch, obviating the extraction/sampling of the biological fluid, thereby simplifying device requirements. The new concept is demonstrated for the electropolymeric entrapment of glutamate oxidase and glucose oxidase within a poly(o-phenylenediamine) (PPD) thin film. The PPD-based enzyme entrapment methodology enables the effective rejection of coexisting electroactive interferents without compromising the sensitivity or response time of the device. The resulting microneedle-based glutamate and glucose biosensors thus exhibit high selectivity, sensitivity, speed, and stability in both buffer and undiluted human serum. High-fidelity glutamate measurements down to the 10 mM level are obtained in serum. The attractive recess design also serves to protect the enzyme layer upon insertion into the skin. This simple, yet robust microneedle design is well-suited for diverse biosensing applications in which real-time metabolite monitoring is a core requirement.