Laser-programmable wettability on porous silicon visualized by photoacoustic microscopy

Abstract

This work introduces a novel, mask-free approach to achieve programmable wettability control on porous silicon (PSi) through laser writing, with the process visualized by optical-resolution photoacoustic microscopy (OR-PAM). Unlike conventional lithography methods, this approach enables direct, site-specific conversion of hydrophobic PSi into hydrophilic regions with micrometer precision. Hydrophobic PSi was first fabricated via electrochemical etching and 1-octene hydrosilylation, forming hierarchical porous structures terminated with alkyl chains. Focused nanosecond pulsed laser irradiation locally ablated and oxidized the porous layer, disrupting surface alkyl termination and allowing water infiltration into the irradiated pores—thus realizing a controllable hydrophobic-to-hydrophilic transition. The wettability modification was quantitatively analyzed by water contact angle and photoacoustic (PA) imaging, visualizing a minimum line width of 40 μm. Leveraging the preserved photoluminescence (PL) of PSi, laser-defined hydrophilic wells were employed as miniaturized biochemical reactors for protein detection. A new PL quenching–based bioassay achieved a detection limit of 96.5 ng/mL for HRP–streptavidin without labeling or complex instrumentation. Moreover, interconnected laser-written hydrophilic channels demonstrated open-surface microfluidic diffusion and mixing. This study pioneers a direct, optically programmable platform that unifies laser patterning, PA imaging, and PL sensing on PSi—offering an innovative route toward next-generation lab-on-a-chip systems.