Spray-Coated Liquid Metal Reflectors for Transparent Hydrogel Atomic Force Microscope Cantilevers

This letter reports optical-lever detection with spray-coated liquid metal reflectors for transparent hydrogel atomic force microscope (AFM) cantilevers. Eutectic gallium-indium (EGaIn) filled in an air brush was locally sprayed over poly(ethylene) glycol-diacrylate (PEG-DA) AFM cantilevers with a shadow mask. Oxygen plasma treatment for bare hydrogel cantilevers prior to the EGaIn spray was shown to improve the wetting characteristics between EGaIn and PEG-DA, thus enabled relatively thin coating of ~800 nm within ~100 ms. PEG-DA AFM cantilevers coated with EGaIn were underwent mechanical characterization, and were employed for AFM imaging in both air and deionized (DI) water. Except the reflectivity degraded down to 40% of the initial value after 1-h dwell in DI water, their overall performance was comparable with those of PEG-DA AFM cantilevers with solid metal reflectors.