Label-free multi-analyte detection using a BioCD

We previously reported the application of spinning-disk interferometry, implemented in a compact optical sensor format called the BioCD, in the detection of antigen-antibody recognition. The BioCD consists of interferometers micro-fabricated on the surface of a 2" laser mirror disk, which can spin up to 6000 rpm resulting in high data acquisition rates. The interferometric elements are fabricated by evaporating gold ridges on the mirror substrate operating in the linear sensitivity regime of the interferometer defined as quadrature. Antibodies or proteins are immobilized on the gold interferometric structures through alkanethiols, and the target molecules are immobilized by application of reagents or samples to the disk while it is spinning. The centrifugal force distributes the sample over the sensor surface, causing a change in the optical phase of the interferometric elements, which is detected in real time using a lock-in amplifier with small detection bandwidth. We detected the binding of Mouse IgG by immobilized Anti-Mouse IgG using the BioCD with a detection limit of 1 ng/ml and low non-specific binding. Furthermore, the selectivity of specific binding was found to be greater than 1 in 10000, determined using the response curve of the BioCD to exposures of specific and non-specific analytes of varying concentrations. This opens up the possibility of simultaneous detection of several analytes with the same sensor while maintaining high selectivity. In this paper we demonstrate simultaneous detection of Rabbit and Mouse IgG on the same disk. The sensitivity limit for multi-analyte detection remains the same as that for a single analyte. In addition to the ability to do simultaneous detection, the current detection scheme presents a way to reference the results of one track with respect to others, thus increasing the reliability of the data. Used in conjunction with high-density protein patterning techniques, the BioCD has the potential to be a highly multiplexed label-free high-speed sensor.