Fabrication of IR-transparent microfluidic devices by anisotropic etching of channels in CaF2

Abstract

A simple fabrication method for generating infrared (IR) transparent microfluidic devices using etched CaF₂ is demonstrated. To etch microfluidic channels, a poly(dimethylsiloxane) (PDMS) microfluidic device was reversibly sealed on a CaF₂ plate and acid was pumped through the channel network to perform anisotropic etching of the underlying CaF₂ surface. To complete the CaF₂ microfluidic device, another CaF₂ plate was sealed over the etched channel using a 700 nm thick layer of PDMS adhesive. The impact of different acids and their concentrations on etching was studied, with HNO₃ giving the best results in terms of channel roughness and etch rates. Etch rate was determined at etching times ranging from 4–48 hours and showed a linear correlation with etching time. The IR transparency of the CaF₂ device was established using a Fourier Transform IR microscope and showed that the device could be used in the mid-IR region. Finally, utility of the device was demonstrated by following the reaction of N-methylacetamide and D₂O, which results in an amide peak shift to 1625 cm⁻¹ from 1650 cm⁻¹, using an FTIR microscope.