Surface Functionalized Anodic Aluminum Oxide Membrane for Opto-Nanofluidic SARS-CoV-2 Genomic Target Detection

Influence of crosslinker on aptamer immobilization and aptasensor sensing response for non-metallic surfaces

Aptasensors transduce the specific binding of the target biomolecules with an aptamer into a measurable signal and enable rapid detection of biomolecules. Aptamer immobilization on the sensing surface through covalent bonding is preferred as it ensures stable and repeatable attachment of the aptamers in the desired configuration. Previous sensing reports have utilized homo-bifunctional crosslinkers like glutaraldehyde to immobilize aptamers to amine-terminated surfaces, but the efficacy of attachment chemistries and their influence on sensor response have not been evaluated. Here, we compare sensing and immobilization density results for four separate crosslinkers - one homo-bifunctional and three hetero-bifunctional - to immobilize the Ebola soluble Glycoprotein (sGP) binding DNA aptamer on the amine-terminated non-metallic surfaces - nanoporous anodized alumina oxide (NAAO) membranes and polystyrene beads. In each case, the aptamer-functionalized NAAO membranes were exposed to a range of Ebola virus sGP concentrations to obtain sensing responses. Measured sensing responses were fit to the Langmuir-Hill's equation. The maximum possible sensor response, Fmax, obtained for each attachment chemistry is compared to determine the effectiveness of the different attachment chemistries. We also used crosslinkers to attach radiolabeled aptamers to amine modified polystyrene particles to determine the attachment efficiency based on the acquired aptamer density on the particle for each crosslinker. Sensing and attachment efficiency results show that aptamer attachment on the sensor surface using a homo-bifunctional crosslinker led to inconsistent sensing response for sensing of sGP across experiments and lower aptamer densities on polystyrene particles. In contrast, the results from aptasensors with aptamers attached through hetero-bifunctional crosslinkers provided consistent sensing responses across experiments and higher aptamer densities on the surface.

Rapid Detection of SARS-CoV-2 Genetic Targets Using Nanoporous Waveguide Based Competitive Displacement Assay

Rapid detection of unlabeled SARS-CoV-2 genetic target was demonstrated using a competitive displacement hybridization assay made by a nanostructured anodized alumina oxide (AAO) membrane. The assay applied the toehold-mediated strand displacement reaction. The nanoporous surface of the membrane was functionalized with a complementary pair consisting of Cy3-labeled probe and quencher-labeled nucleic acids through a chemical immobilization process. In the presence of the unlabeled SARS-CoV-2 target, the quencher-tagged strand of the immobilized probe-quencher duplex was separated from the Cy3-modifed strand. A stable probe-target duplex formed and regained a strong fluorescence signal, thus enabling real-time and label-free SARS-CoV-2 detection. Assay designs with different numbers of base pair (bp) matches were synthesized to compare their affinities. Because of the large surface of a free-standing nanoporous membrane, two orders enhancement of the fluorescence was observed, where the detection limit of the unlabeled concentration can be improved to 1 nM. The assay was miniaturized by integrating a nanoporous AAO layer onto an optical waveguide device. The detection mechanism and the sensitivity improvement of the AAO-waveguide device were illustrated from the finite difference method (FDM) simulation and the experimental results. Light-analyte interaction was further improved due to the presence of the AAO layer, which created an intermediate refractive index and enhanced the waveguide's evanescent field. Our competitive hybridization sensor is an accurate and label-free testing platform applicable to the deployment of compact and sensitive virus detection strategies.

Nanoscale self-assembly: concepts, applications and challenges

Self-assembly offers unique possibilities for fabricating nanostructures, with different morphologies and properties, typically from vapour or liquid phase precursors. Molecular units, nanoparticles, biological molecules and other discrete elements can spontaneously organise or form via interactions at the nanoscale. Currently, nanoscale self-assembly finds applications in a wide variety of areas including carbon nanomaterials and semiconductor nanowires, semiconductor heterojunctions and superlattices, the deposition of quantum dots, drug delivery, such as mRNA-based vaccines, and modern integrated circuits and nanoelectronics, to name a few. Recent advancements in drug delivery, silicon nanoelectronics, lasers and nanotechnology in general, owing to nanoscale self-assembly, coupled with its versatility, simplicity and scalability, have highlighted its importance and potential for fabricating more complex nanostructures with advanced functionalities in the future. This review aims to provide readers with concise information about the basic concepts of nanoscale self-assembly, its applications to date, and future outlook. First, an overview of various self-assembly techniques such as vapour deposition, colloidal growth, molecular self-assembly and directed self-assembly/hybrid approaches are discussed. Applications in diverse fields involving specific examples of nanoscale self-assembly then highlight the state of the art and finally, the future outlook for nanoscale self-assembly and potential for more complex nanomaterial assemblies in the future as technological functionality increases.