Sensitive detection of nonfluorescent solutes in diluted aqueous solutions through photothermally induced reflectivity modulation

Urashima, Shuhei; Kusaka, Ryoji  

This work introduces a novel adaptation of Photothermal reflectivity (PTR) spectroscopy for sensitive detection of non-luminescent solutes in dilute aqueous solutions. Traditional PTR relies on measuring the change in reflectance of a solid sample upon illumination with a pump beam resonant with a coating film, which acts as a light absorber and a reflective layer whose reflectivity is sensitive to temperature changes. In contrast, our approach focuses on analyzing the thermal response at the interface between a transparent material and an aqueous solution containing the target solute. A pump beam resonant with the solutes is focused onto this interface, inducing localized heating due to optical absorption. This minute temperature increase alters the refractive index at the interface, changing the reflectivity of a probe beam that is also focused on the same spot. The resulting modulation in reflected light intensity directly correlates with the solute concentration. This novel PTR approach offers several advantages: Sensitivity can be enhanced by increasing the pump beam power as other photothermal techniques. The technique relies solely on the interaction at the material/solution interface, enabling analysis of samples confined within microchannels or pits. Furthermore, its straightforward optical configuration eliminates complex alignment systems, making it cost-effective and reproducible. We demonstrate this novel PTR approach using a dye solution in a microchannel as a model system. We achieve a limit of detection of 75 nM, showcasing the remarkable sensitivity achievable with the simple optical configuration. This work paves the way for utilizing PTR spectroscopy as a versatile tool for analyzing small, non-fluorescent, and diluted solutions in diverse fields such as lab-on-a-chip devices, forensic analysis, and handling hazardous materials.