Multi-fluorescent micro-sensor for accurate measurement of pH and temperature variations in micro-environments

In this study, a novel multi-fluorescent micro-sensor that can respond to both pH and temperature was designed and synthesized. It is based on amino-polystyrene microbeads, and two different fluorescent indicators (Rhodamine B and Fluorescein isothiocyanate) were used simultaneously to synthesize the fluorescent micro-sensor. Rhodamine B was embedded inside the amino-polystyrene microbeads, whereas Fluorescein isothiocyanate (FITC) was used to modify the surface of the beads. Because of the different fluorescence wavelengths and measurement positions of FITC and Rhodamine B, interference from each fluorescent indicator should be avoided. The fluorescence responses of FITC and Rhodamine B to pH and temperature, respectively, were detected. Rhodamine B demonstrates an excellent linear relationship between relative fluorescence intensity and temperature, while the relative fluorescence intensity was found to be independent of the pH. The calibrated sensitivity of Rhodamine B is −3.4%/°C, with a temperature accuracy of 0.1 °C. Therefore, changes in the temperature information can be calibrated based on the relative fluorescence intensity changes in Rhodamine B. On the other hand, FITC is sensitive to both pH and temperature. We propose a temperature compensation method for pH calibration. After temperature compensation, the pH accuracy calibrated based on the pH sensitivity of FITC improves from 1.5 to 0.2.