Rationally Designed Efficient Dual-Mode Colorimetric/Fluorescence Sensor Based on Carbon Dots for Detection of pH and Cu2+ Ions

A dual-mode colorimetric/fluorescence pH sensor was fabricated from carbon dots (CDs). It exhibited a pH response via both fluorescence and visible colorimetric observation. The CDs solution changed from red to yellow when the pH changed from acid to alkaline. Meanwhile, the color of pH test paper that incorporated the CDs changed from purple-red to orange and then to yellow. The CD fluorescence maximum was at 630 nm in acid solution and 590 nm under neutral and alkaline conditions. The fluorescence of the pH test paper changed from red to orange and then to yellow, which was better than that of the pure CDs solution. The CDs exhibited highly selective and reversible fluorescence quenching with respect to Cu2+ ions because of strong binding and fast chelating kinetics. There was a linear relationship between fluorescence quenching and Cu2+ concentration, suggesting the other promising practical usage of this sensing system. The versatile CDs-based pH sensor thus provides a basis for developing sustainable and fast-response dual-mode pH meters as well as Cu2+ sensing.