Discrimination of cysteamine from mercapto amino acids through isoelectric point-mediated surface ligand exchange of β-cyclodextrin-modified gold nanoparticles.

The discrimination of cysteamine (CA) from mercapto amino acids (including glutathione, cysteine and homocysteine, GSH/Cys/Hcy) with high efficiency is essential to detect biothiols in a physiological environment. Herein, a nano-assembly was constructed by combining one-pot synthesized β-cyclodextrin-modified AuNPs and encapsulated rhodamine 6G (R6G-β-CD@AuNPs) without covalent modification. Interestingly, when the pH value of the tested solution was approximately the isoelectric point (pI) of the biothiol, the quenched fluorescence of R6G-β-CD@AuNPs was obviously restored by the introduction of the biothiol, which was due to the replacement of the R6G-β-CD composite by the biothiol on the surfaces of AuNPs via a stronger S-Au bond. The pI-mediated surface ligand exchange process was designed for the first time for the discrimination and detection of CA and GSH/Cys/Hcy at the pH values of 9.5 and 5.5, respectively. Under the optimized conditions, the enhanced fluorescence intensity displayed a good linear response to the concentration of each biothiol, with the detection limits of CA, GSH, Cys and Hcy calculated as 66.8, 417.6, 652.7 and 603.4 nM, respectively. The ingenious fabrication of the R6G-β-CD@AuNPs sensing platform eliminated the interferences from cholesterol by host-guest interactions and enabled high selectivity for the detection of biothiols in human serum samples. The pI-mediated sensing platform exhibits great potential for biothiol-related therapeutic drug monitoring.