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L-Histidine (L-His) is an essential amino acid, which is used to synthesize proteins and enzymes. The concentration of L-His in the body is controlled to regulate tissue growth and repair of tissues. In this study, a rapid and sensitive method was developed for colorimetric L-his detection using Cu2+ ions to inhibit the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB)–H2O2 system. H2O2 can oxidize TMB to oxTMB in the presence of copper, and the change in color from colorless (TMB) to blue (oxTMB) is similar to that observed in the presence of peroxidase. However, because the imidazole ring and carboxyl group of L-His can coordinate with Cu2+ ions to form stable L-His–Cu2+ complexes, the color of the TMB–H2O2 solution remains unchanged after the addition of L-His. Therefore, because L-His effectively hinders the colorimetric reaction of TMB with H2O2, this assay can be used to quantitatively determine the concentration of L-His in samples. Under optimized conditions, our colorimetric sensor exhibited two linear ranges of 60 nM to 1 μM and 1 μM to 1 mM for L-His detection and a detection limit of 50 nM (S/N = 3); furthermore, the assay can be performed within 20 min. Moreover, the proposed assay was used to determine the concentration of L-His in urine samples, suggesting that this convenient and label-free colorimetric method presents promising applications in bioanalytical chemistry and clinical diagnosis.

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In order to conveniently and rapidly detect Cr[Formula: see text] ions in aqueous solution, novel functionalized gold nanoparticles (AuNPs) were designed with 4-mercaptobenzoic acid (4-MBA) and polyethyleneimine (PEI). In addition, we also compared the response signal of various functional AuNPs to Cr[Formula: see text] ions on the detection system. The modifiers included 4-MBA, 4-nitrobenzene mercaptan (4-NTP) and PEI, respectively. The results showed that 4-MBA and PEI co-functionalized had better sensitiveness than others. Cr[Formula: see text] ions in aqueous solution could be detected by naked eye and UV-Vis absorption spectrum and the limit of detection was 1[Formula: see text][Formula: see text]M. The response was found to be proportional to Cr[Formula: see text] ions concentration in the range from 2[Formula: see text][Formula: see text]M to 10[Formula: see text][Formula: see text]M. The developed platform with high selectiveness was successfully utilized to detect Cr[Formula: see text] ions in simulated samples.