Colorimetric detection of serum doxycycline with d-histidine-functionalized gold nanoclusters as nanozymes

Gold nanoclusters (AuNCs) have emerged as a new generation of "nanozymes" based on their intrinsic catalytic activity. However, highly selective and colorimetric detection of drugs is still far from adequately developed due to the lack of means of regulating the catalytic activity of nanozymes. Herein, d-histidine stabilized AuNCs (d-His@AuNCs) were synthesized and their nanozyme ability was demonstrated in the catalytic oxidation of the peroxidase substrate, 3,3',5,5'-tetramethylbenzidine, for the promotion of hydrogen peroxide. Copper ions led to the aggregation of d-His@AuNCs and inhibited their peroxidase-like activity. The addition of doxycycline restored the enzyme-mimicking catalytic activity of d-His@AuNCs, which was based on the strong coordination interaction between copper ions and doxycycline. A highly sensitive and colorimetric assay for determining the amount of doxycycline was developed at a detection wavelength of 650 nm. The color intensity and ultraviolet-visible absorbance intensity of the testing assay displayed a good linear relationship in the doxycycline concentration range of 5.0-12.5 μM, with a limit of detection of 1.0 μM. Moreover, the metabolic process of doxycycline in serum was further investigated with the proposed monitoring system after the drug was abdominally injected into rats. Notably, the tunable catalytic activity performance of the nanozymes indicates their significant potential in clinical application.