Superhydrophobic Properties of Nanostructured–Microstructured Porous Silicon for Improved Surface-Based Bioanalysis

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Wettability is a fundamental property of a solid surface, which plays important roles in many industrial applications. The possibility to create well-controlled nonwetting states on silicon surfaces without photolithography-based processing can bring many advantages in the biotechnology and microfluidics areas. In this paper, superhydrophobic properties of macroporous–nanoporous structured silicon are reported. The superhydrophobic porous silicon layers are prepared by electrochemical etching of bulk crystalline silicon wafers. Altered anodization conditions provide surfaces with varying pore morphologies, yielding different wetting properties, ranging from highly wetting (nanoporous morphologies) to water-repellent surfaces (macroporous morphologies). Subsequent surface modification with a fluorocarbon coupling agent can further improve nonwetting properties and stabilize the surface for a long term. Contact angles as high as 176° were achieved on macroporous silicon and superhydrophobicity was maintained for several months without degradation. The porous surfaces have proven to be a very attractive substrate for protein microarrays. Fluorescence-based assay of immunoglobulin G in plasma is reported with a limit of detection of 1 pM, a spot size of 50 μm, and an array density of 15,625 spots per square centimeter. Macroporous surfaces have also been developed for matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) applications, where the intrinsic hydrophobic surface properties confine the deposited sample to MALDI spots of less than 200 μm with well-defined MALDI crystals, providing a high-sensitivity readout. Furthermore, a superhydrophobic MALDI-TOF MS target anchor chip composed of nonporous anchor points surrounded by superhydrophobic porous areas for sample deposition and on anchor point confinement is reported. Such anchor chips allowed localized crystallization of large sample volumes (5 μL) improving the hydrophobic spot confinement strategy in terms of final MALDI crystal localization and readout sensitivity.

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