(41d) Microfabricated Retroreflectors for Biomedical Assays and Diagnostics

Many bioanalytical technologies employ a label, typically a dye, enzyme or fluor, to signal the presence of analyte. While these labels are common and well-developed, such systems can suffer from low signal strength and label instability, and elaborate instrumentation is often required for detection. This work introduces magnetic sample-prep particles and nanoparticles as light-blocking labels in optical assays based on micron-scale microfabricated retroreflectors. Retroreflectors return light directly to its source and are readily detectable with inexpensive optics. We use detector tetrads composed of four retroreflectors; one assay retroreflector decorated with target specific antibodies is surrounded by three always-on reference reflectors without antibodies. When the target is present, magnetic sample-prep particles or gold nanoparticles decorated with anti-target antibodies can assemble on the surface in front of the assay reflector and substantially reduce reflectivity. The light intensity returned by the assay reflector is compared to that of the three reference reflectors to determine the presence of the analyte in ?digital? fashion without needing to optically calibrate the device. The assay can easily detect the presence of a single magnetic sample-prep particle bound to the surface. Results show that one or two Rickettsia conorii bacteria can hold down a 1.0 µm magnetic particle coated with rabbit polyclonal anti-Rickettsia antibodies, supporting the possibility of detecting a single target pathogen (though ROC analysis would be used to choose a larger number as the criterion for a positive). The detectability of gold particles can be greatly enhanced by incorporating silver intensification using the silver/hydroquinone chemistry of black and white photography. This reaction and subsequent darkening can take place over a few minutes. The magnetic properties of the particles are useful in sample preparation and concentration, and magnetic force, as well as microfluidic fluid flow shear, are used to increase specificity by discriminating against non-specific interactions. Shear force discrimination, reproducibility, and convenience are enhanced by the implementation of the technology in a microfluidic cartridge format.