A New Ratiometric Fluorescent Probe for Specific Monitoring of hROS under Physiological Conditions Using Boric Acid-Protected l-DOPA Gold Nanoclusters.

The specific monitoring of physiological highly reactive oxygen species (hROS) using fluorescent gold nanoclusters (AuNCs) remains a challenge for scientists. Herein, SLB-AuNC was first synthesized via an ecofriendly one-pot method using starch as a template, l-3,4-dihydroxyphenylalanine (l-DOPA) as a reducing and a capping agent, and boric acid as a protecting agent for the catechol moiety of l-DOPA. The ingenious introduction of starch and boric acid enhanced the dispersibility, quantum yield, and photostability of fluorescent SLB-AuNCs. The obtained SLB-AuNCs possessed good monodispersity with an average diameter of 2.9 ± 0.8 nm and exhibited highly stable fluorescence with maximum emission at 480 nm under physiological conditions. A ratiometric fluorescent probe for hROS was developed through an oxidization-regulated Förster-resonance-energy-transfer process between SLB-AuNCs and 2,3-diaminophenazine (the oxidative product of hROS and o-phenylenediamine, with maximum fluorescence emission at 560 nm). With increasing amount of hROS, the outstanding fluorescence variation of the probe (I560 nm/I480 nm) enhanced about 300-fold, accompanied with a distinguishable color change from cyan to yellow. The detection limits of •OH, ClO-, and ONOO- were calculated as 0.11, 0.50, and 0.69 µM, respectively. High selectivity was achieved using o-phenylenediamine as a specific signal response for hROS to enable no interference reaction of other ROS toward SLB-AuNCs. The practicability of the proposed probe with super biocompatibility was evaluated by measuring exogenous and endogenous hROS levels in HeLa cells through fluorescence imaging. This work provides a novel strategy to design fluorescent AuNC probes for physiological hROS with great potential for the application of bioassay and bioimaging.

Discrimination of cysteamine from mercapto amino acids through isoelectric point-mediated surface ligand exchange of ß-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 ß-cyclodextrin-modified AuNPs and encapsulated rhodamine 6G (R6G-ß-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-ß-CD@AuNPs was obviously restored by the introduction of the biothiol, which was due to the replacement of the R6G-ß-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-ß-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.