Improvement in the sensitivity of microfluidic ELISA through field amplified stacking of the enzyme reaction product

Electrokinetic stacking of particle zones in confined channels enabling their UV absorbance detection on microchips

In this article, we report a simple approach to stacking micro- and nanoparticle zones by electrokinetically migrating them through moderately confined channels of uniform cross-section. Experiments show the reported pre-concentration process to initiate at the tail end of the zone following its electrokinetic injection, with the stacked region migrating faster than the rest of the sample band. This effect causes the particles traveling in front to merge into the stacked region making it grow both in size and concentration. Because the stacked zone also gradually loses particles from its trailing edge, it eventually disintegrates upon running out of particles at its front end. Nevertheless, enhancements in peak height by over 100-fold were recorded using the reported approach for polystyrene beads with diameters comparable to the channel depth. This enhancement however, exhibited a temporal variation as the particle band migrated through the analysis column reaching a maximum value that depended on the particle diameter, particle concentration, channel depth, electric field strength, electroosmotic mobility, etc. Interestingly, the peak area recorded by the detector remained relatively constant during this particle migration period allowing reliable sample quantitation. Moreover, upon incubating antibody-coated particles against an antigen sample, the peak area for the particle zone was seen to scale linearly with the antigen concentration establishing the utility of the reported focusing phenomenon for chemical/biochemical analysis. The noted stacking technique was further applied to enabling UV absorbance detection of particle zones on microchips which then allowed us to determine the colloidal content in actual natural water samples. .

Rapid Mouse Follicle Stimulating Hormone Quantification and Estrus Cycle Analysis Using an Automated Microfluidic Chemiluminescent ELISA System

Follicle stimulating hormone (FSH) plays a critical role in female reproductive development and homeostasis. The blood/serum concentration of FSH is an important marker for reporting multiple endocrinal functions. The standardized method for mouse FSH (mFSH) quantification based on radioimmunoassay (RIA) suffers from long assay time (∼2 days), relatively low sensitivity, larger sample volume (60 μL), and small dynamic range (2-60 ng/mL); thus, it is insufficient for monitoring fast developing events with relatively small mFSH fluctuations (e.g., estrous cycles of mammals). Here, we developed an automated microfluidic chemiluminescent ELISA device along with the disposal sensor array and the corresponding detection protocol for rapid and quantitative analysis of mFSH from mouse tail serum samples. With this technology, highly sensitive quantification of mFSH can be accomplished within 30 min using only 8 μL of the serum sample. It is further shown that our technique is able to generate results comparable to RIA but has a significantly improved dynamic range that covers 0.5-250 ng/mL. The performance of this technology was evaluated with blood samples collected from ovariectomized animals and animals with reimplanted ovarian tissues, which restored ovarian endocrine function and correlated with estrus cycle analysis study.

Microfluidic ELISA employing an enzyme substrate and product species with similar detection properties

The requirement for an enzyme label to carry out a chemical reaction directly at the signaling region of the enzyme substrate in order to produce a large change in its detectability places a significant constraint on the scope of enzyme-linked immunosorbent assays (ELISAs). In particular, this requirement limits the kinds of enzyme label-substrate couples employable in ELISAs and prevents their independent optimization with respect to the enzyme reaction and the detectability of the enzyme reaction substrate/product. The detection limit and multiplexing capabilities of the assay are consequently restricted in addition to rendering the technique applicable to a narrow range of assay conditions/samples. Attempting to address some of these limitations, the current article describes a microfluidic ELISA method that does not require the enzyme label to act around the signaling region of the substrate molecule. A highly detectable rhodamine based substrate was synthesized to demonstrate the reported assay which upon cleavage by the enzyme label, alkaline phosphatase, transformed from a monoanionic to a monocationic species, both of which had nearly identical fluorescence properties. These species were later separated based on their charge difference using capillary zone electrophoresis in an integrated device yielding a quantitative measure for the analyte (human TNF-α) in our sample. Impressively, the noted approach not only enabled the use of a new kind of enzyme substrate for ELISAs but also allowed the detection of human TNF-α at concentrations over 54-fold lower than that possible on commercial microwell plates primarily due to the better detectability of the rhodamine dye.

Pre-concentration by liquid intake by paper (P-CLIP): a new technique for large volumes and digital microfluidics

Microfluidic platforms are an attractive option for incorporating complex fluid handling into low-cost and rapid diagnostic tests. A persistent challenge for microfluidics, however, is the mismatch in the "world-to-chip" interface - it is challenging to detect analytes present at low concentrations in systems that can only handle small volumes of sample. Here we describe a new technique termed pre-concentration by liquid intake by paper (P-CLIP) that addresses this mismatch, allowing digital microfluidics to interface with volumes on the order of hundreds of microliters. In P-CLIP, a virtual microchannel is generated to pass a large volume through the device; analytes captured on magnetic particles can be isolated and then resuspended into smaller volumes for further processing and analysis. We characterize this method and demonstrate its utility with an immunoassay for Plasmodium falciparum lactate dehydrogenase, a malaria biomarker, and propose that the P-CLIP strategy may be useful for a wide range of applications that are currently limited by low-abundance analytes.

Undergraduate Laboratory Module for Implementing ELISA on the High Performance Microfluidic Platform

Implementing enzyme-linked immunosorbent assays (ELISA) in microchannels offers several advantages over its traditional microtiter plate-based format, including a reduced sample volume requirement, shorter incubation period, and greater sensitivity. Moreover, microfluidic ELISA platforms are inexpensive to fabricate and allow integration of analytical procedures, such as sample preconcentration, that further enhance the performance of the immunoassay. In view of the scientific potential of microfluidic ELISAs, inclusion of this technique into an undergraduate curriculum is valuable in preparing the next generation of scientists and engineers. Here, an experimental module is presented for this immunoassay method that can be completed in an undergraduate laboratory setting within two 3-h periods (including all incubation and data analyses procedures) using only a microliter of sample and reagents per assay. In addition to acquainting students with the microfluidic technology, the reported module provides training in quantitating ELISAs using the kinetic format of the assay. Furthermore, it offers a useful educational tool for introducing undergraduates to basic image analysis techniques, as well as signal-to-noise ratio and limit of detection calculations that are valuable in characterizing any analytical method.