Sensitive Detection of Nonfluorescent Solutes in Small Amounts of Diluted Aqueous Solutions Through Photothermally Induced Reflectivity Modulation at Glass/Aqueous Solution Interfaces

This study proposes to employ photothermal reflectance (PTR) spectroscopy, typically used to assess the thermal properties of solid materials, for detecting nonluminescent solutes in small amounts of dilute aqueous solutions. In this spectroscopy, a probe beam nonresonant and a pump beam resonant to the solution are focused on a transparent material/aqueous solution interface. Since the temperature coefficients of refractive indexes depend on the media, the heat from the pump beam absorption changes the reflectivity at the material/solution interface. A limit of detection of 75 nM was achieved for an interface of a glass and an aqueous solution of a dye. Among various photothermal spectroscopic techniques, a notable advantage of reflectance spectroscopy is its straightforward optical configuration, where neither the precisely aligned beam expander nor differential beam splitting is required, making the optical system inexpensive and reproducible. Since reflectance spectroscopy only requires a material/solution interface, it is applicable to solutions confined in micrometer-sized channels or pits, allowing us to analyze samples with tiny amounts.