A tunable pH-sensing system based on Ag nanoclusters capped by hyperbranched polyethyleneimine with different molecular weights

In this assay, a tunable pH sensing system was developed based on Ag nanoclusters (Ag NCs) capped by hyperbranched polyethyleneimine (PEI) with different molecular weights (abbreviated as Ag NC-PEIs). For instance, when the molecular weight of PEI was 600 or 1800, the fluorescence intensities of Ag NCs exhibited a linear fashion over the pH range 4.10–7.96; when the molecular weight of PEI was 25,000, the pH linear range was from 4.78 to 7.96; when the molecular weight of PEI was 70,000, the pH linear range was 6.09–8.95. According to the molecular weight of PEI 600/1800, 25,000, and 70,000, the color change point was pH 4.10–4.78, 5.33–6.09, and 6.09–6.80, respectively. Therefore, Ag NC-PEI 600 and 1800 were proper to acid conditions; Ag NC-PEI 25,000 was sensitive to weak acid media; while Ag NC-PEI 70,000 was adapted to neutral solution. The tunable and selective color change points brought an excellent feature of Ag NC-PEIs as visual pH indicators, which was flexible and applicable to a variety of environments. Besides, the ratios of absorbance at 415   nm and 268   nm of Ag NCs also showed linear relationships with pH variations. Therefore, there were three ways of this system for sensing pH values, including fluorescence assay, ultraviolet–visible measurement, and visual detection, suggesting that this tunable pH-sensing platform was more feasible, reliable, and accurate.