Electrokinetic Mixing for Improving the Kinetics of an HbA1c Immunoassay

Glycated Hemoglobin and Methods for Its Point of Care Testing

Glycated hemoglobin (HbA1c) is a product of the spontaneous reaction between hemoglobin and elevated glucose levels in the blood. It is included among the so-called advanced glycation end products, of which is the most important for the clinical diagnosis of diabetes mellitus, and it can serve as an alternative to glycemia measurement. Compared to the diagnosis of diabetes mellitus by glycemia, the HbA1c level is less influenced by a short-term problem with diabetes compensation. Mass spectroscopy and chromatographic techniques are among the standard methods of HbA1c level measurement. Compared to glycemia measurement, there is lack of simple methods for diabetes mellitus diagnosis by means of the HbA1c assay using a point-of-care test. This review article is focused on the surveying of facts about HbA1c and its importance in diabetes mellitus diagnosis, and surveying standard methods and new methods suitable for the HbA1c assay under point-of-care conditions. Various bioassays and biosensors are mentioned and their specifications are discussed.

Probe-target hybridization depends on spatial uniformity of initial concentration condition across large-format chips

Diverse assays spanning from immunohistochemistry (IHC), to microarrays (protein, DNA), to high-throughput screens rely on probe-target hybridization to detect analytes. These large-format 'chips' array numerous hybridization sites across centimeter-scale areas. However, the reactions are prone to intra-assay spatial variation in hybridization efficiency. The mechanism of spatial bias in hybridization efficiency is poorly understood, particularly in IHC and in-gel immunoassays, where immobilized targets are heterogeneously distributed throughout a tissue or hydrogel network. In these systems, antibody probe hybridization to a target protein antigen depends on the interplay of dilution, thermodynamic partitioning, diffusion, and reaction. Here, we investigate parameters governing antibody probe transport and reaction (i.e., immunoprobing) in a large-format hydrogel immunoassay. Using transport and bimolecular binding theory, we identify a regime in which immunoprobing efficiency (η) is sensitive to the local concentration of applied antibody probe solution, despite the antibody probe being in excess compared to antigen. Sandwiching antibody probe solution against the hydrogel surface yields spatially nonuniform dilution. Using photopatterned fluorescent protein targets and a single-cell immunoassay, we identify regimes in which nonuniformly distributed antibody probe solution causes intra-assay variation in background and η. Understanding the physicochemical factors affecting probe-target hybridization reduces technical variation in large-format chips, improving measurement precision.

Electrokinetic mixing in electrode-embedded multiwell plates to improve the diffusion limited kinetics of biosensing platforms

Rapid and accurate biosensing with low concentrations of the analytes is usually challenged by the diffusion limited reaction kinetics. Thus, as a remedy, long incubation times or excess amounts of the reagents are employed to ensure the reactions to go to completion. Therefore, mixing becomes both a serious problem and necessity to overcome that diffusion limitation and homogenize the samples, especially for the biochemical reactions that take place in multiwell plates. Because the current mixing platforms such as shakers/vortexers, sonicators, magnetic stirrers and acoustic mixers have disadvantages including, but not limited to, being invasive/harfmul to the samples, causing the samples to splash out or stick to the walls of the wells and allowing foreign compartments to enter the solutions in the wells. Here we propose a noninvasive and safer (considering the risk of sample loss) technology that provides electrokinetic-mixing (EKM) of the reagents placed in electrode-embedded multiwell plates where the incubation times, or in other words, the time required for the desired molecules to meet in stationary solutions, can be reduced substantially. In order to demonstrate the power of this innovation, in this specific case, a simple Förster resonance energy transfer (FRET) based quenching bioplatform was adopted, where a molecular beacon DNA (MB) modified with sulfhydryl (-SH) and fluorescein (FITC) dye at opposite terminals was incubated with 10 nm sized gold nanoparticles (AuNPs) in the wells of an electrode-embedded multiwell plate, in which a printed circuit board (PCB) was attached at the bottom to control the liquid flows by EKM. When the MB binds to AuNPs through thiolate chemistry in the solution, FITC dye comes in close proximity to the AuNP surface and the emission is quenched via FRET principle. Thus, this quenching percentage over time was our comparison parameter for the mixing and no mixing cases to demonstrate the impact of mixing on the quenching kinetics. This reaction was conducted with different concentrations of AuNPs to observe the impact of mixing on MB quenching kinetics when the concentrations of the AuNPs were increased. Total quenching efficiency could go up to 90% in the presence of the AuNPs and it took about 60 min to reach stability. When the EKM was involved, fluorescence quenching time for the MBs could be reduced by up to 4.1 times. Thus, it was demonstrated that this technology may improve the kinetics of the diffusion limited biological reactions take place in multiwell plates substantially so that it may be adopted in various different sensing platforms for rapid measurements.