The transmission interferometric adsorption sensor

This paper describes a high-speed adsorption sensor based on thin-film interference at the interfaces. The sensor can be used as a stand-alone instrument or in combination with a direct surface force measurement, which yields a wide range of additional information on molecular interactions on adsorbed films. The achieved mass resolution of the presented method (1–10 ng cm<SUP>−2</SUP> Hz<SUP>−1/2</SUP>) is comparable to or better than other modern bio-sensors. The dependence of mass resolution on various factors is presented and demonstrated in a number of relevant examples. The described method is suitable for the implementation of a low-cost bio-sensor with a minimal number of optical elements.<BR/>The measurement spot size is one micrometre or more and sampling rates >10 Hz are readily possible. In contrast to other bio-sensors, the signal baseline has a remarkable long-term stability since the measured signal is virtually independent of refractive index changes in the fluid medium above the sensor surface.<BR/>In combination with an optical spectral correlation method, the classical computer calculations are substituted by an optical calculator and a label-free real-time imaging adsorption sensor is realized. We demonstrate sensor operation both inside the extended surface forces apparatus as well as in a stand-alone bio-sensor configuration. As a final point, we illustrate the imaging capability of this new sensor technology on a patterned bio-functionalized surface.