In the analysis of radioactive liquid waste, safety considerations limit the amount of sample that can be handled, creating a need for techniques capable of accurately measuring components in very small volumes of dilute aqueous solutions. However, dilute and nonfluorescent species are difficult to detect conventional absorption measurements lack sufficient sensitivity, and fluorescence cannot be used. Although highly sensitive photothermal spectroscopic methods exist, they typically require complex optical setups involving beam alignment and adjustments of interferometric components, which makes them challenging to operate.

In this study, we utilized the fact that slight temperature increases induced by optical absorption can change the reflectivity at the glass/aqueous interface, enabling sensitive measurements with a simple optical configuration (Fig. 1). The method requires only that the heating and probe beams be focused on the same spot, eliminating the need for precise beam offsetting or specialized optical elements. Experiments demonstrated that even extremely dilute dye solutions in microchannels could be detected at concentrations as low as 75 nM, corresponding to femtoliter-scale volumes of nonfluorescent analytes (Fig. 2).

With further adaptation to microchannels made from different materials and strategies to suppress background light, this technique is expected to evolve into a safer and more reliable analytical method for small aqueous samples, including those containing radioactive species.

This work was supported by the MEXT Leading Initiative for Excellent Young Researchers Grant Number JPMXS0320230068.

Paper URL: https://doi.org/10.1039/D4AN01247J

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