Scaling issues in chemical and biological sensors

When a system is reduced isomorphically in size (i.e., scaled down with all dimensions of the system decreased uniformly, or isomorphic scale reduction), the changes in length, area, and volume ratios alter the relative influence of various physical effects that determine the overall operation - often in unexpected ways. As objects shrink, the ratio of surface area to volume increases, rendering surface forces more important. More generally, as the size of an object decreases, forces scaling with a lower power of the linear dimension dominate over the ones scaling with a higher power (e.g., surface tension gains over gravity, electrostatics over magnetics, etc.) [see M. J. Madou, Fundamentals of Microfabrication, 2nd ed. (Boca Raton, FL: CRC, 2002)]. In this paper, we are investigating the influence of miniaturization on various aspects of chemical and biological sensors; we review scaling issues faced in sensor construction, the importance of sample size and the effect of sensor size on detection sensitivity in some of the most popular sensing approaches.