Citric acid coated K2GdF7:Ce/Tb nanocrystals for broad and reversible photoluminescence pH sensing

An pH-sensitive citric acid coated Ce/Tb:KGd 2 F 7 nanocrystals with a broad detection range from 3 to 10 is established. Under the 254 nm excitation, the Tb 3+ emission intensity originated from Ce 3+ → Gd 3+ →Tb 3+ decreases gradually with the increase of pH, while its intensity almost keeps unchanged in pH 3 to 6 and increases in pH 6 to 10 under the 311 nm excitation, which activates the ratiometric pH sensing with high sensitivity.