Biocompatible fluorescein-based temperature sensor for applications in endoscopic surgery

Temperature monitoring is critical for endoscopic laser surgery to prevent tissue damage. Thermal injuries of the urothelium have been reported as a known complication of laser lithotripsy, however, in most cases real time temperature measurement is not available due to the absence of sensors in conventional endoscopes. In this study, we present an all-optical fluorescein-based thermal sensing approach enabling temperature measurement via an optical fiber, independent of the endoscope model, using a biocompatible fluorescent probe. The temperature-dependent fluorescence response originates from the redistribution between the monoanionic and dianionic forms of fluorescein, which is influenced by albumin through different mechanisms depending on the solution environment. A comparative analysis in water and in saline solution, which is commonly used for irrigation during surgery, revealed distinct environmental contributions to the temperature response of fluorescein emission. In water, a stable fluorescein-albumin complex was formed, accompanied by a systematic shift in the monoanion-dianion equilibrium, leading to strong temperature-dependent spectral changes. In saline solution, which is more relevant to intraoperative irrigation, no stable complex was observed, as shown by time-resolved fluorescence techniques; nevertheless, a reproducible temperature response was retained and was attributed to weaker, non-specific microenvironmental effects, including local pH variations near the protein surface. These findings demonstrate a reproducible temperature-dependent fluorescence response under defined solution conditions and provide a proof of principle for further development of optical temperature-monitoring approaches relevant to minimally invasive procedures.