Nanoporous waveguides as versatile, high-sensitivity biosensors

This chapter introduces the application of nanoporous oxide thin films as optical waveguides for high-sensitivity biosensing. We discuss the waveguiding properties of nanoporous oxide thin films in the visible wavelengths when pore dimensions are much smaller (<1/10) than the guided light, and we show that the refractive index of such a nanopore-oxide matrix composite film is very well described by effective medium theory (EMT). Moreover, the optical response arising from binding of an analyte to a (suitably functionalized) nanoporous thin film is also described by EMT and forms the basis of analysis for high-sensitivity biosensing. Recent results on probing the sensitivity limits of label-free detection and the characterization of different molecular architectures are also presented. Particular attention is paid to the ability to differentiate, in situ, processes that occur on pore surfaces from those occurring on the top surface of the film. Moreover, we were able to directly measure the permeation of proteins within a nanoporous structure by simultaneous in-situ waveguide and electrochemical measurements.