Development of aptamer-based affinity assays using temperature gradient focusing: minimization of the limit of detection

Microfluidic Techniques for Analytes Concentration

Microfluidics has been undergoing fast development in the past two decades due to its promising applications in biotechnology, medicine, and chemistry. Towards these applications, enhancing concentration sensitivity and detection resolution are indispensable to meet the detection limits because of the dilute sample concentrations, ultra-small sample volumes and short detection lengths in microfluidic devices. A variety of microfluidic techniques for concentrating analytes have been developed. This article presents an overview of analyte concentration techniques in microfluidics. We focus on discussing the physical mechanism of each concentration technique with its representative advancements and applications. Finally, the article is concluded by highlighting and discussing advantages and disadvantages of the reviewed techniques.

Optimization of a microfluidic electrophoretic immunoassay using a Peltier cooler

Successful analysis of electrophoretic affinity assays depends strongly on the preservation of the affinity complex during separations. Elevated separation temperatures due to Joule heating promotes complex dissociation leading to a reduction in sensitivity. Affinity assays performed in glass microfluidic devices may be especially prone to this problem due to poor heat dissipation due to the low thermal conductivity of glass and the large amount of bulk material surrounding separation channels. To address this limitation, a method to cool a glass microfluidic chip for performing an affinity assay for insulin was achieved by a Peltier cooler localized over the separation channel. The Peltier cooler allowed for rapid stabilization of temperatures, with 21°C the lowest temperature that was possible to use without producing detrimental thermal gradients throughout the device. The introduction of cooling improved the preservation of the affinity complex, with even passive cooling of the separation channel improving the amount of complex observed by 2-fold. Additionally, the capability to thermostabilize the separation channel allowed for utilization of higher separation voltages than what was possible without temperature control. Kinetic CE analysis was utilized as a diagnostic of the affinity assay and indicated that optimal conditions were at the highest separation voltage, 6 kV, and the lowest separation temperature, 21°C, leading to 3.4% dissociation of the complex peak during the separation. These optimum conditions were used to generate a calibration curve and produced 1 nM limits of detection, representing a 10-fold improvement over non-thermostated conditions. This methodology of cooling glass microfluidic devices for performing robust and high sensitivity affinity assays on microfluidic systems should be amenable in a number of applications.

Using electrophoretic exclusion to manipulate small molecules and particles on a microdevice

Electrophoretic exclusion, a novel separations technique that differentiates species in bulk solution using the opposing forces of electrophoretic velocity and hydrodynamic flow, has been adapted to a microscale device. Proof-of-principle experiments indicate that the device was able to exclude small particles (1 μm polystyrene microspheres) and fluorescent dye molecules (rhodamine 123) from the entrance of a channel. Additionally, differentiation of the rhodamine 123 and polystyrene spheres was demonstrated. The current studies focus on the direct observation of the electrophoretic exclusion behavior on a microchip.

DNA adsorption to the reservoir walls causing irreproducibility in studies of protein-DNA interactions by methods of kinetic capillary electrophoresis

Methods of kinetic capillary electrophoresis (KCE) facilitate kinetic studies of protein-DNA interactions and highly efficient selection of DNA aptamers for protein targets. Here, we report a previously unnoticed source of error that affects the precision and accuracy of KCE-based measurements. The error manifests itself in cases that require the use of low concentrations of DNA. In such measurements, the reproducibility of the signal generated by the same fluorescently labeled DNA sample can have a relative standard deviation (RSD) as high as 40%. We have investigated the cause of the irreproducibility and found that it is attributed to DNA adsorption to the surface of the sample vials, in which protein-DNA mixtures are prepared prior to a KCE experiment. The use of commercially available "high DNA recovery" sample vials does not resolve the problem. We have found that the problem can be significantly alleviated by the passivation of the vial surface with blocking agents, such as masking DNA or bovine serum albumin (BSA). The described adsorption of DNA to the surface of sample vials may also be important in other procedures that deal with low DNA concentrations, such as aptamer selection and quantitative PCR.

Continuous signal enhancement for sensitive aptamer affinity probe electrophoresis assay using electrokinetic concentration

We describe an electrokinetic concentration-enhanced aptamer affinity probe electrophoresis assay to achieve highly sensitive and quantitative detection of protein targets in a microfluidic device. The key weaknesses of aptamer as a binding agent (weak binding strength/fast target dissociation) were counteracted by continuous injection of fresh sample while band-broadening phenomena were minimized due to self-focusing effects. With 30 min of continuous signal enhancement, we can detect 4.4 pM human immunoglobulin E (IgE) and 9 pM human immunodeficiency virus 1 reverse transcriptase (HIV-1 RT), which are among the lowest limits of detection (LOD) reported. IgE was detected in serum sample with a LOD of 39 pM due to nonspecific interactions between aptamers and serum proteins. The method presented in this paper also has broad applicability to improve sensitivities of various other mobility shift assays.

Development of a membrane-less dynamic field gradient focusing device for the separation of low-molecular-weight molecules

Dynamic field gradient focusing uses an electric field gradient generated by controlling the voltage profile of an electrode array to separate and concentrate charged analytes according to their individual electrophoretic mobilities. This study describes a new instrument in which the electrodes have been placed within the separation channel. The major challenge faced with this device is that when applied voltages to the electrodes are larger than the redox potential of water, electrolysis will occur, producing hydrogen ions (H+) plus oxygen gas on the anodes and hydroxide (OH(-)) plus hydrogen gas on the cathodes. The resulting gas bubbles and pH excursions can cause problems with system performance and reproducibility. An on-column, degassing system that can remove gas bubbles "on-the-fly" is described. In addition, the use of a high capacity, low-conductivity buffer to address the problem of the pH shift that occurs due to the production of H+ on the anodes is illustrated. Finally, the successful separation of three, low-molecular-weight dyes (amaranth, bromophenol blue and methyl red) is described.