Microcantilever-Based Label-Free Characterization of Temperature-Dependent Biomolecular Affinity Binding

Soft and flexible material-based affinity sensors

Recent advances in biosensors and point-of-care (PoC) devices are poised to change and expand the delivery of diagnostics from conventional lateral-flow assays and test strips that dominate the market currently, to newly emerging wearable and implantable devices that can provide continuous monitoring. Soft and flexible materials are playing a key role in propelling these trends towards real-time and remote health monitoring. Affinity biosensors have the capability to provide for diagnosis and monitoring of cancerous, cardiovascular, infectious and genetic diseases by the detection of biomarkers using affinity interactions. This review tracks the evolution of affinity sensors from conventional lateral-flow test strips to wearable/implantable devices enabled by soft and flexible materials. Initially, we highlight conventional affinity sensors exploiting membrane and paper materials which have been so successfully applied in point-of-care tests, such as lateral-flow immunoassay strips and emerging microfluidic paper-based devices. We then turn our attention to the multifarious polymer designs that provide both the base materials for sensor designs, such as PDMS, and more advanced functionalised materials that are capable of both recognition and transduction, such as conducting and molecularly imprinted polymers. The subsequent content discusses wearable soft and flexible material-based affinity sensors, classified as flexible and skin-mountable, textile materials-based and contact lens-based affinity sensors. In the final sections, we explore the possibilities for implantable/injectable soft and flexible material-based affinity sensors, including hydrogels, microencapsulated sensors and optical fibers. This area is truly a work in progress and we trust that this review will help pull together the many technological streams that are contributing to the field.

Epitope Binning Assay Using an Electron Transfer-Modulated Aptamer Sensor

Surface plasmon resonance and quartz crystal microbalance are workhorses of protein-DNA interaction research for over 20 years, providing ways to quantitatively determine the protein-DNA binding. However, the cost, necessary technical expertise, and severe nonspecific adsorption poses barriers to their use. Convenient and effective techniques for the measurement of protein-DNA binding affinity and the epitope binning between DNA and proteins for developing highly sensitive detection platform remain challenging. Here, we develop a binding-induced alteration in electron transfer kinetics of the redox reporter labeled (methylene blue) on DNA aptamer to measure the binding affinity between prostate-specific antigen (PSA) and aptamer. We demonstrate that the binding of PSA to aptamer decreases the electron transfer rate of methylene blue for ∼45%. Further, we identify the best pairwise selection of aptamers for developing sandwich assay by sorting from 10 pairwise modes with the PSA detection limit of 500 ng/mL. Our study provides promising ways to analyze the binding affinity between ligand and receptor and to sort pairwise between aptamers or antibodies for the development of highly sensitive sandwich immunoassays.

SELEX Modifications and Bioanalytical Techniques for Aptamer-Target Binding Characterization

The quest to improve the detection of biomolecules and cells in health and life sciences has led to the discovery and characterization of various affinity bioprobes. Libraries of synthetic oligonucleotides (ssDNA/ssRNA) with randomized sequences are employed during Systematic Evolution of Ligands by Exponential Enrichment (SELEX) to select highly specific affinity probes called aptamers. With much focus on the generation of aptamers for a variety of target molecules, conventional SELEX protocols have been modified to develop new and improved SELEX protocols yielding highly specific and stable aptamers. Various techniques have been used to analyze the binding interactions between aptamers and their cognate molecules with associated merits and limitations. This article comprehensively reviews research advancements in the generation of aptamers, analyses physicochemical conditions affecting their binding characteristics to cellular and biomolecular targets, and discusses various field applications of aptameric binding. Biophysical techniques employed in the characterization of the molecular and binding features of aptamers to their cognate targets are also discussed.