An integrated microfluidic platform for sensitive and rapid detection of biological toxins

Integrated SPPS on continuous-flow radial microfluidic chip

A novel integrated continuous-flow microfluidic system was designed and fabricated for solid phase peptide synthesis (SPPS) using conventional reactants. The microfluidic system was composed of a glass-based radial reaction chip, a diffluent chip, amino acid feeding reservoirs and continuous-flow reagent pathways. A tri-row cofferdam-fence structure was designed for solid phase supports trapping. Highly cross-linked, porous and high-loading 4-(hydroxymethyl)phenoxymethyl polystyrene (HMP) beads were prepared for microfluidic SPPS. The transfer losses, hazardous handling and time-consuming processes in traditional peptide cleavage steps were avoided by being replaced with the on-chip cleavage treatment. Six peptides from an antibody affinity peptide library against β-endorphin with different lengths and sequences were obtained simultaneously on the constructed continuous-flow microfluidic system within a short time. This microfluidic system is automatic, integrated, effective, low-cost, recyclable and environment-friendly for not only SPPS but also other solid phase chemical syntheses.

Heterogeneous immunoassays in microfluidic format using fluorescence detection with integrated amorphous silicon photodiodes

Miniaturization of immunoassays through microfluidic technology has the potential to decrease the time and the quantity of reactants required for analysis, together with the potential of achieving multiplexing and portability. A lab-on-chip system incorporating a thin-film amorphous silicon (a-Si:H) photodiode microfabricated on a glass substrate with a thin-film amorphous silicon-carbon alloy directly deposited above the photodiode and acting as a fluorescence filter is integrated with a polydimethylsiloxane-based microfluidic network for the direct detection of antibody-antigen molecular recognition reactions using fluorescence. The model immunoassay used consists of primary antibody adsorption to the microchannel walls followed by its recognition by a secondary antibody labeled with a fluorescent quantum-dot tag. The conditions for the flow-through analysis in the microfluidic format were defined and the total assay time was 30 min. Specific molecular recognition was quantitatively detected. The measurements made with the a-Si:H photodiode are consistent with that obtained with a fluorescence microscope and both show a linear dependence on the antibody concentration in the nanomolar-micromolar range.

Design and operation of a portable scanner for high performance microchip capillary array electrophoresis

We have developed a compact, laser-induced fluorescence detection scanner, the multichannel capillary array electrophoresis portable scanner (McCAEPs) as a platform for electrophoretic detection and control of high-throughput, integrated microfluidic devices for genetic and other analyses. The instrument contains a confocal optical system with a rotary objective for detecting four different fluorescence signals, a pneumatic system consisting of two pressure/vacuum pumps and 28 individual addressable solenoid valves for control of on-chip microvalves and micropumps, four Polymerase Chain Reaction (PCR) temperature control systems, and four high voltage power supplies for electrophoresis. The detection limit of the instrument is ~20 pM for on-chip capillary electrophoresis of fluorescein dyes. To demonstrate the system performance for forensic short tandem repeat (STR) analysis, two experiments were conducted: (i) electrophoretic separation and detection of STR samples on a 96-lane microfabricated capillary array electrophoresis microchip. Fully resolved PowerPlex(®) 16 STR profiles amplified from 1 ng of 9947A female standard DNA were successfully obtained; (ii) nine-plex STR amplification, sample injection, separation, and fluorescence detection of 100-copy 9948 male standard DNA in a single integrated PCR- capillary electrophoresis microchip. These results demonstrate that the McCAEPs can be used as a versatile control and detection instrument that operates integrated microfluidic devices for high-performance forensic human identification.

Aptamers as affinity reagents in an integrated electrophoretic lab-on-a-chip platform

Nucleic acid based affinity reagents (e.g., aptamers) offer several possible advantages over antibodies as specific recognition elements in biochemical assays. Besides offering improved cost and stability, aptamers are ideal for rapid electrophoretic analysis due to their low molecular weight and high negative charge. While aptamers have proven well-suited for affinity-shift electrophoretic analysis, demonstrating a fully integrated aptamer-based assay platform represents an important achievement toward low-cost point-of-care analysis, particularly for remote or resource poor settings where cost and ambient stability of reagents is a key consideration. Here we perform and evaluate the suitability of aptamer-based affinity assays for two clinically relevant target analytes (IgE using a known aptamer and NF-κB using a thio-modified aptamer) in an integrated electrophoretic gel-shift platform. Key steps of (i) mixing sample with aptamer, (ii) buffer exchange, and (iii) preconcentration of sample were successfully integrated on-chip upstream of a fluorescence-based gel-shift analysis step. This approach, utilizing a size-exclusion membrane optimized here for aptamer retention and preconcentration with sample, enables automated sample-to-answer for trace analytes in 10 min or less. We addressed notable nonspecific interference from serum proteins by adding similar nucleic acid competitors to suppress such interactions with the aptamer. Nanomolar sensitivities were demonstrated and integrated preconcentration of sample provides an important means of further improving detection sensitivities. Aptamers proved superior in many respects to antibody reagents, particularly with regard to speed and resolution of gel-shifts associated with specific binding to target.

Miniaturized system for isotachophoresis assays

We present an inexpensive hand-held device (240 g) that implements microchip isotachophoresis (ITP) with laser induced fluorescence (LIF) detection. This self-contained instrument integrates the functionality required for high voltage generation onto a microelectronic chip, includes LIF detection and is powered by a universal serial bus (USB) link connected to a laptop computer. Using this device we demonstrate focusing and detection of a fluorescent species with a limit of detection of 100 pM. We show that the response of the detector is linear with the initial analyte concentration, making this device suitable for quantitative analysis. We also demonstrate the use of our simulation tools for design and prediction of ITP assays, and validate these results with a demonstration of multiplexed indirect detection of (unlabeled) analytes performed using the device. We find good agreement between simulations and experimental results. Using a label-free isotachaphoresis assay implemented in the hand-held device we detect two explosives and an endocrine disruptor spiked in river water, with no prior sample processing.

ELISA-LOC: lab-on-a-chip for enzyme-linked immunodetection

A miniature 96 sample ELISA-lab-on-a-chip (ELISA-LOC) was designed, fabricated, and tested for immunological detection of Staphylococcal Enterotoxin B (SEB). The chip integrates a simple microfluidics system into a miniature ninety-six sample plate, allowing the user to carry out an immunological assay without a laboratory. Assay reagents are delivered into the assay plate without the need for separate devices commonly used in immunoassays. The ELISA-LOC was constructed using Laminated Object Manufacturing (LOM) technology to assemble six layers with an acrylic (poly(methyl methacrylate) (PMMA)) core and five polycarbonate layers micromachined by a CO(2) laser. The ELISA-LOC has three main functional elements: reagent loading fluidics, assay and detection wells, and reagent removal fluidics, a simple "surface tension" valve used to control the flow. To enhance assay sensitivity and to perform the assay without a lab, ELISA-LOC detection combines several biosensing elements: (1) carbon nanotube (CNT) technology to enhance primary antibody immobilization, (2) sensitive ECL (electrochemiluminescence) detection, and (3) a charge-coupled device (CCD) detector for measuring the light signal generated by ECL. Using a sandwich ELISA assay, the system detected SEB at concentrations as low as 0.1 ng ml(-1), which is similar to the reported sensitivity of conventional ELISA. The fluidics system can be operated by a syringe and does not require power for operation. This simple point-of-care (POC) system is useful for carrying out various immunological assays and other complex medical assays without a laboratory.

Ultrashort separation length homogeneous electrophoretic immunoassays using on-chip discontinuous polyacrylamide gels

To realize efficient homogeneous electrophoretic immunoassays, we introduce discontinuous polyacrylamide gels that enable quantitative assay completion in separation lengths as short as 350 mum in <10 s. The discontinuous cross-linked gels reduce the required electrophoretic separation lengths and thereby significantly reduce the required applied electrical potentials needed to achieve 100's V/cm electric field strengths for rapid electrophoresis. To optimize the discontinuous polyacrylamide gel assay format, we demonstrate development of a two-color homogeneous electrophoretic immunoassay for concurrent quantitation of C reactive protein (CRP) and tumor necrosis factor-alpha (TNF-alpha) for monitoring inflammatory response. To achieve necessary pore-size control at the gel discontinuity, an optimized mask-based fabrication protocol is introduced. The fabrication approach improves electrophoretic separations using the discontinuous separation gels by eliminating two confounding phenomena: (1) smaller than desired pores at the discontinuity which result in undesired physical exclusion of large-species and (2) an associated transition from small to large pores aft of the interface which acts to "destack" analyte bands during the separation. With the use of the optimized discontinuous separation gels, both assays were linear and quantitative over a two-log detection range, with a lower limit of detection of 11 ng/mL for CRP and 40 ng/mL for TNF-alpha. An optimal single-point detector location was identified by balancing the separation resolution and assay duration constraints. The ultrashort separation distance electrophoretic assays developed here provide flexibility in chip and instrument design by relaxing electrical potential requirements and expanding the possibilities for assay multiplexing, therefore addressing important design considerations when developing field-portable diagnostic assays for near-patient environments.

Integrated Multi-process Microfluidic Systems for Automating Analysis

Microfluidic technologies have been applied extensively in rapid sample analysis. Some current challenges for standard microfluidic systems are relatively high detection limits, and reduced resolving power and peak capacity compared to conventional approaches. The integration of multiple functions and components onto a single platform can overcome these separation and detection limitations of microfluidics. Multiplexed systems can greatly increase peak capacity in multidimensional separations and can increase sample throughput by analyzing many samples simultaneously. On-chip sample preparation, including labeling, preconcentration, cleanup and amplification, can all serve to speed up and automate processes in integrated microfluidic systems. This paper summarizes advances in integrated multi-process microfluidic systems for automated analysis, their benefits and areas for needed improvement.

Immunoassays in microfluidic systems

Immunoassays have greatly benefited from miniaturization in microfluidic systems. This review, which summarizes developments in microfluidics-based immunoassays since 2000, includes four sections, focusing on the configurations of immunoassays that have been implemented in microfluidics, the main fluid handling modalities that have been used for microfluidic immunoassays, multiplexed immunoassays in microfluidic platforms, and the emergence of label-free detection techniques. The field of microfluidic immunoassays is continuously improving and has great promise for the future.

Bioaffinity detection of pathogens on surfaces

The demand for improved technologies capable of rapidly detecting pathogens with high sensitivity and selectivity in complex environments continues to be a significant challenge that helps drive the development of new analytical techniques. Surface-based detection platforms are particularly attractive as multiple bioaffinity interactions between different targets and corresponding probe molecules can be monitored simultaneously in a single measurement. Furthermore, the possibilities for developing new signal transduction mechanisms alongside novel signal amplification strategies are much more varied. In this article, we describe some of the latest advances in the use of surface bioaffinity detection of pathogens. Three major sections will be discussed: (i) a brief overview on the choice of probe molecules such as antibodies, proteins and aptamers specific to pathogens and surface attachment chemistries to immobilize those probes onto various substrates, (ii) highlighting examples among the current generation of surface biosensors, and (iii) exploring emerging technologies that are highly promising and likely to form the basis of the next generation of pathogenic sensors.

Toward one-step point-of-care immunodiagnostics using capillary-driven microfluidics and PDMS substrates

Point-of-care diagnostics will strongly benefit from miniaturization based on microfluidics because microfluidics integrate functions that can together preserve valuable samples and reagents, increase sensitivity of a test, and accelerate mass transport limited reactions. But a main challenge is to incorporate reagents into microfluidics and to make microfluidics simple to use. Here, we integrate microfluidic functional elements, some of which were developed earlier, and reagents such as detection antibodies (dAbs), capture antibodies (cAbs) and analyte molecules for making one-step immunoassays: the integrated device only requires the addition of sample to trigger a cascade of events powered by capillary forces for effecting a sandwich immunoassay that is read using a fluorescence microscope. The microfluidic elements comprise a sample collector, delay valves, flow resistors, a deposition zone for dAbs, a reaction chamber sealed with a polydimethylsiloxane (PDMS) substrate, and a capillary pump and vents. Parameters for depositing 3.6 nL of a solution of dAb on the chip using an inkjet are optimized and the PDMS substrate is patterned with analytes, which provide a positive control, and cAbs. Various storage conditions of the patterned PDMS are investigated for up to 6 months revealing that storage with a desiccant preserved at least 51% of the activity of the cAbs. C-reactive protein (CRP), a general inflammation and cardiac marker, is detected using this one-step chip using only 5 microL of human serum by measuring fluorescent signals from 30 x 100 microm(2) areas of the PDMS substrate in the wet reaction chamber. The one-step chip can detect CRP at a concentration of 10 ng mL(-1) in less than 3 min and below 1 ng mL(-1) within 14 min. The work presented here may spur the adoption of fluorescence immunoassays using capillary driven microfluidics and PDMS substrates for point-of-care diagnostics.

Sensitive giant magnetoresistive-based immunoassay for multiplex mycotoxin detection

Rapid and multiplexed measurement is vital in the detection of food-borne pathogens. While highly specific and sensitive, traditional immunochemical assays such as enzyme-linked immunosorbent assays (ELISAs) often require expensive read-out equipment (e.g. fluorescent labels) and lack the capability of multiplex detection. By combining the superior specificity of immunoassays with the sensitivity and simplicity of magnetic detection, we have developed a novel multiplex magnetic nanotag-based detection platform for mycotoxins that functions on a sub-picomolar concentration level. Unlike fluorescent labels, magnetic nanotags (MNTs) can be detected with inexpensive giant magnetoresistive (GMR) sensors such as spin-valve sensors. In the system presented here, each spin-valve sensor has an active area of 90 microm x 90 microm, arranged in an 8 x 8 array. Sample is added to the antibody-immobilized sensor array prior to the addition of the biotinylated detection antibody. The sensor response is recorded in real time upon the addition of streptavidin-linked MNTs on the chip. Here we demonstrate the simultaneous detection of multiple mycotoxins (aflatoxins B(1), zearalenone and HT-2) and show that a detection limit of 50 pg/mL can be achieved.