Carbon-dot-based ratiometric fluorescent pH sensor for the detections of very weak acids assisted by auxiliary reagents that contribute to the release of protons

In this work, a carbon-dot-based ratiometric fluorescent pH sensor was fabricated by conjugating a pH-sensitive fluorescein, fluoresceinisothiocyanate (FITC), onto the surface of carbon dots (CD-FITCs). The fabrication condition was optimized so as to make the pH sensor possess a typical character of ratiometic probe that the two emission intensities ratio is independent on its concentration. As a result, the CD-FITCs exhibited high sensitivity and selectivity to pH, and the as-proposed method for pH sensing is of considerable repeatability. However, the CD-FITCs sensor is still not an effective tool for the direct detections of very weak acids based on pH sensing because of their extremely low dissociated constants. Interestingly, with the help of the auxiliary reagents that contribute to the release of protons, the sensitivities of the CD-FITCs sensor to the very weak acids can be considerably enhanced. For example, the very weak acids, boric acid and hydroxylamine hydrochloride, which can be transformed into relatively strong acids assisted by D-fructose and acetone, was successfully measured using the carbon-dot-based ratiometric fluorescent pH sensor in the real water samples (tap water and lake water). Moreover, the proposed method can be extended to sensing any analytes in chemical reactions associated with proton release.