Differential adsorption of small molecules in spatially functionalized porous silicon nanostructures

An experimental approach to rapidly quantify the relative affinity of a small molecule analyte for two different surfaces is described. The method uses optical measurements of high surface area porous Si thin films that contain two spatially distinct surface chemistries. The chemistries are placed on the walls of the ∼10 nm diameter pores of the porous Si film by means of microdroplet patterning, where a chemical resist is drop-coated on the porous Si sample to define distinct regions across the plane of the chip. In this work, the two chemistries consist of a hydrophilic silicon oxide surface and a hydrophobic methyl-terminated silicon surface. Detection is achieved by simultaneous optical reflectance measurements of both regions, where the reflectance spectrum contains a convolution of the Fabry-Pérot interference spectrum of both the oxide and the methyl-grafted layers. The differential partitioning of a test analyte (2-acetoxybenzoic acid or diphenyl ether) from aqueous solution is determined from the Fourier transform of the optical interference spectrum. The approach is rapid and nondestructive, and it can be performed on a small sample volume as a means to quantify the partition behavior of small molecules.