Nickel doping: Improving the optical properties of carbon dots and increasing the sensitivity for detecting pH and water

pH and water play a crucial role in chemical, environmental and physiological monitoring processes, so it is of great importance to establish a simple and sensitive multi-mode detection method. In this study, we fabricated nickel-doped carbon dots (Ni-CDs) by a hydrothermal method using nickel chloride (NiCl2 center dot 6 H2O) and m-phenylenediamine (m-PDA) as precursors, which exhibits excellent photostability, outstanding resistance to photobleaching and ultra-low cytotoxicity. Ni doping increased the fluorescence quantum yield (from 21.4% to 30.3%) and H content (from 44.4% to 57.1%) of the Ni-CDs, making them highly sensitive to pH and water, which facilitated the detection of pH and water based on deprotonation and aggregation mechanisms, respectively. Ni-CDs can detect pH in the ranges of 6.0-9.0 and 6.0-11.0 via fluorometric and smartphone methods, respectively. The limits of detection (LODs) of the Ni-CDs in DMSO, DMF and THF for the detection of water were 0.0112%, 0.0144% and 0.0060%, respectively. Moreover, the Ni-CDs have been effectively utilized for pH detection in real samples as well as for pH in cells and zebrafish with satisfactory results. More importantly, the Ni-CDs/hydrogel remains perfectly the sensitive to pH and water, broadening the applications of the CDs in sensing, bioimaging and information encryption.