Chuanxia Chen

Integrated Logic Gate for Fluorescence Turn-on Detection of Histidine and Cysteine Based on Ag/Au Bimetallic Nanoclusters–Cu2+ Ensemble

By means of employing 11-mercaptoundecanoic acid (11-MUA) as a reducing agent and protecting ligand, we present straightforward one-pot preparation of fluorescent Ag/Au bimetallic nanoclusters (namely AgAuNCs@11-MUA) from AgNO3 and HAuCl4 in alkaline aqueous solution at room temperature. It is found that the fluorescence of AgAuNCs@11-MUA has been selectively quenched by Cu(2+) ions, and the nonfluorescence off-state of the as-prepared AgAuNCs@11-MUA-Cu(2+) ensemble can be effectively switched on upon the addition of histidine and cysteine. By incorporating Ni(2+) ions and N-ethylmaleimide, this phenomenon is further exploited as an integrated logic gate and a specific fluorescence turn-on assay for selectively and sensitively sensing histidine and cysteine has been designed and established based on the original noncovalent AgAuNCs@11-MUA-Cu(2+) ensemble. Under the optimal conditions, histidine and cysteine can be detected in the concentration ranges of 0.25-9 and 0.25-7 μM; besides, the detection limits are found to be 87 and 111 nM (S/N = 3), respectively. Furthermore, we demonstrate that the proposed AgAuNCs@11-MUA-based fluorescent assay can be successfully utilized for biological fluids sample analysis.

A simple and sensitive assay for the determination of nitrite using folic acid as the fluorescent probe

Nitrite, which poses a serious threat to human health despite finding wide application in industry and architecture, has aroused more and more attention. In this study, a simple yet sensitive and selective fluorimetric assay for the determination of trace nitrite using folic acid as the probe has been established. Folic acid exhibited weak fluorescence in strong acidic conditions. However, a dramatic enhancement in the fluorescence intensity occurred when folic acid was exposed to ultraviolet (UV) radiation with the addition of nitrite. In particular, the UV radiation time played a vital role. Furthermore, the fluorescence intensity could reach the maximum by adjusting the pH value of the solution within a certain range. The excellent selectivity was attributed to the specificity of diazo reaction. The “turn-on” fluorescence strategy endows this assay with high sensitivity down to the 18 nM detection limit and a wide detection range from 50 nM to 32 μM. Finally, this assay was applied to detect nitrite in real samples with satisfactory results.

A Fluorescence Immunoassay Based on the Phosphate-Triggered Fluorescence Turn-on Detection of Alkaline Phosphatase

A simple and cost-effective fluorescence immunoassay for the sensitive quantitation of disease biomarker α-fetoprotein (AFP) has been developed based on the phosphate-triggered fluorescence turn-on detection of alkaline phosphatase (ALP), with the reversible binding between calcein and Ce3+ as a signaling element. In this immunoassay, fluorescent calcein is readily quenched by Ce3+ via a coordination process. The ALP-catalyzed hydrolysis of p-nitrophenyl phosphate leads to the formation of p-nitrophenol and inorganic orthophosphate, and the newly formed orthophosphate could potently combine with Ce3+ due to the higher affinity, thus, recovering the fluorescence of calcein. The corresponding fluorescence signal triggered by phosphate is related to ALP activities labeled on antibody, and thus could be applied to detect target antigen in an enzyme-linked immunosorbent assay (ELISA) platform. The fluorescence intensity correlated well to the AFP concentration ranges of 0.2-1.0 and 1.0-4.0 ng/mL, with a detection limit of 0.041 ng/mL. The proposed fluorescence ELISA possesses convincing recognition mechanism and exhibits excellent assay performance in the evaluation of the AFP level in serologic test, which unambiguously reveals great application potential in the clinic diagnosis of disease biomarkers.