All-in-one square capillary platform for nanoliter sampling and sample-efficient micro-scale optical measurements

We propose the use of a square capillary as an all-in-one analytical platform for handling extremely rare and/or hazardous samples, where the sample volume is geometrically confined to the nanoliter scale and the capillary functions both as a sampling tool and as a measurement cell. Although various techniques such as photothermal spectroscopy and cavity-enhanced spectroscopy have been developed to reduce the optical probe volume down to the pico- or femtoliter scale, the actual sample consumption in these techniques typically exceeds microliters or even milliliters. This is due to the need to fill microchannels, the difficulty of reproducibly preparing sub-microliter liquid samples, and sample loss during transfer and handling. This discrepancy between probe volume and actual sample consumption limits measurement efficiency for scarce and/or hazardous samples. In this study, by combining capillary sampling with photothermal detection, optical absorption-based detection of uranyl ions in aqueous solution was achieved with a limit of detection of 0.2 mM using only 50 nL of sample per measurement, demonstrating sample-efficient optical measurements enabled by capillary sampling.