Assay technologies capable of detecting low biomarker concentrations in complex biological samples are fundamental for biological research and for applications in medical diagnostics. In this paper we address the challenge to perform protein biomarker detection homogeneously in one single step, applying a minute amount of reagent directly into whole human blood plasma, avoiding any sample dilution, separation, amplification, or fluid manipulation steps. We describe a one-step homogeneous assay technology based on antibody-coated magnetic nanoparticles that are spiked in very small amount directly into blood plasma. Pulsed magnetic fields and a double-linker molecular architecture are used to generate high biomarker-induced binding and low nonspecific binding between the nanoparticles. We demonstrate dose-response curves for prostate specific antigen (PSA) measured in undiluted human blood plasma with a detection limit of 400-500 femtomol/L, in a total assay time of 14 min and an optically probed volume of only 1 nL. We explain the dose-response curves with a model based on discrete binding of biomarker molecules onto the nanoparticles, which allows us to extract reaction parameters for the binding of biomarker molecules onto the nanoparticles and for the biomarker-induced binding between nanoparticles. The demonstrated analytical performance and understanding of the nanoparticle assay technology render it of interest for a wide range of applications in quantitative biology and medical diagnostics.

A comparison is made of the quantitative detection of a labeled antibody by surface-enhanced resonance Raman scattering (SERRS) and by fluorescence using the same instrument with the same laser excitation source. The area under the curve for the fluorescence band is greater than for any single peak in the SERRS spectrum, but the broad fluorescence band is more difficult to discriminate from the background at low concentrations. Using the peak height of one SERRS band and the peak height at the fluorescence maximum, the detection limit for SERRS was lower (1.19 x 10-11 mol.dm-3) than that obtained using fluorescence (3.46 x 10-10 mol.dm-3). The SERRS detection limit is calculated for the concentration of the sample added, but compared to fluorescence, there is an additional dilution step due to the addition of the colloid and the extent of this dilution is dependent on assay format. For comparison with the detection limits found earlier with labeled oligonucleotides, SERRS was remeasured with a 10 s accumulation time, and the final concentration in the cuvette after colloid addition and before any adsorption to the silver was used to calculate a detection limit of 2.79 x 10-13 mol.dm-3. This is comparable to the detection limit found using a similar SERRS procedure for an oligonucleotide labeled with the same dye. This experiment is dependent on many parameters that could affect this result, including the nature of the SERRS substrate, the excitation wavelength, and the dye chosen. However, the result indicates that SERRS can give assay sensitivities comparable or better than fluorescence for quantitative direct assay determination, suggesting that the much greater potential for multiple analyte detection could be exploited.

We describe the magneto-capillary valve (MCV) technology, a flexible approach for integrated biological sample preparation within the concept of stationary microfluidics. Rather than moving liquids in a microfluidic device, discrete units of liquid are present at fixed positions in the device and magnetic particles are actuated between the fluids. The MCV concept is characterized by the use of two planar surfaces at a capillary mutual distance, with specific features to confine the fluids by capillary forces, and the use of a gas or a phase-change material separating the stationary aqueous liquids. We have studied the physics of magneto-capillary valving by quantifying the magnetic force as a function of time and position, which reveals the balance of magnetic, capillary and frictional forces in the system. By purification experiments with a fluorescent tracer we have measured the amount of co-transported liquid, which is a key parameter for efficient purification. To demonstrate the versatility of the technology, several MCV device architectures were tested in a series of biological assays, showing the purification and enrichment of nucleic acids and proteins. Target recovery comparable to non-miniaturized commercial kits was observed for the extraction of DNA from human cells in buffer, using a device architecture with patterned air valves. Experiments using an enrichment module and patterned air valves demonstrate a 40-fold effective enrichment of DNA in buffer. DNA was also successfully purified from blood plasma using paraffin phase-change valves. Finally, the enrichment of a protein biomarker (prostate-specific antigen) using geometrical air valves resulted in a 7-fold increase of detection signal. The MCV technology is versatile, offers extensive freedom for the design of fully integrated systems, and is expected to be manufacturable in a cost-effective way. We conclude that the MCV technology can become an important enabling technology for point-of-care systems with sample in-result out performance.

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