User:ProfPhaseSpace/sandbox/The BioCD

=The BioCD=

The BioCD (biological compact disc) is an optical biosensor that shares features in common with the digital compact disc including a focused laser spot that interrogates a spinning disk. However, instead of measuring binary data or music as the disk spins beneath the laser, it measures biological molecules bound to the disc surface. The primary application of the BioCD is as a protein microarray for serum diagnostics.

Optical biosensors seek to measure small numbers of biological target molecules, such as antigen molecules arising from bacterial infections or cancer that are found in patient biofluids such as the blood or urine. Because molecules are so much smaller than the wavelength of light, detecting them presents a serious measurement challenge. The role of the spinning disk is to enable high-speed repetitive measurements of surface-bound molecules as they pass repeatedly through the focused laser. The high-speed repetitive sampling improves the signal-to-noise ratio of the measurement, and small amounts of bound target molecules can be measured with high precision.

BioCD Classes
There are several classes of BioCD that are defined by their detection mechanism. Many operate as a label-free common-path interferometer, while others rely on fluorescent label molecules.


 * Microdiffraction BioCD: This configuration is a wavefront-splitting common-path interferometer  that is closest to the digital compact disc in its optical principles.  A micro-scale feature, called a pit or a ridge, passes through the focal plane of the laser beam.  While the digital compact disc operates on the principle of a null interferometer, the BioCD operates in phase quadrature.  This requires


 * Phase-Contrast BioCD: This interferometric configuration uses principles of phase-contrast microscopy to measure the phase modulation caused by the bound molecules on the disk surface.  Phase shifts on a light beam are made visible by converting them to intensity shifts using the principle of phase quadrature.


 * In-Line BioCD: This configuration is an amplitude-splitting interferometer that uses bio-layer interferometry of molecules on surfaces to convert the molecular phase modulation to intensity that is detected or imaged directly


 * Fluorescence BioCD: This BioCD approach uses fluorescent tag molecules that fluoresce when excited by the scanning laser.  This configuration is neither interferometeric nor label-free, but has the advantage of low background noise and high specificity.