"Turn-on" fluorometric probe for α-glucosidase activity using red fluorescent carbon dots and 3,3',5,5'-tetramethylbenzidine.

A turn-on method for determining α-glucosidase activity is described using a chemical redox strategy in which the fluorescence of red fluorescent carbon dots (CDs) is modulated. The red fluorescent CDs were prepared using a solvothermal method with p-phenylenediamine and sodium citrate. The excitation and emission maxima of the CDs were 490 and 618 nm, respectively. Ce4+ ions catalyze the oxidation of the colorless substrate 3,3',5,5'-tetramethylbenzidine (TMB) to give a blue oxidized TMB product (oxTMB). Absorption by oxTMB overlaps with the red light emitted by the CDs because of the fluorescence inner filter effect; therefore the presence of oxTMB decreases the intensity of fluorescence emission by the CDs. However, hydrolysis of L-ascorbic acid-2-O-α-D-glucopyranosyl by the enzyme α-glucosidase causes formation of ascorbic acid . Ascorbic acid reduces oxTMB to TMB, so that the inner filter effect disappeared and the fluorescence recovered. The strategy allows α-glucosidase activity to be successfully determined down to 0.02 U mL-1 and gives a dynamic linear range of 0-5.5 U mL-1. The strategy is very selective for α-glucosidase activity in the presence of potentially interfering substances. The method has been successfully applied to the determination of α-glucosidase activity in spiked human serum samples and gave satisfactory results. Graphical Abstract Schematic of the method used to prepare the carbon dots and the mechanisms involved in determining α-glucosidase activity.

Preparation of nonconjugated fluorescent polymer nanoparticles for use as a fluorescent probe for detection of 2,4,6-trinitrophenol.

Water-soluble nonconjugated fluorescent polymer nanoparticles (NFPNs) were prepared from branched polyethylenimine (PEI) and citric acid through an amide condensation reaction in the aqueous phase. The NFPNs were characterized using a transmission electron microscope, Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectra (XPS). The NFPN fluorescence (with excitation/emission peaks at 360/450 nm) was quenched by 2,4,6-trinitrophenol (TNP) at trace concentrations through the inner filter effect and the formation of self-assembled non-fluorescent Meisenheimer complexes of TNP on the NFPN surfaces through acid-base interactions. The complexes effectively enriched TNP from the bulk solution on the NFPN surfaces through acid-base interactions, and the strong overlap between NFPN excitation and TNP absorption peaks contributed to NFPNs having good sensitivity and selectivity for TNP. The method was selective for TNP and was not sensitive to other interfering species. The calibration plot of log(F0/F) versus TNP concentration shows a linear relationship (R2 = 0.999) for TNP concentration in the range of 0.5-150 µM. The detection limit for TNP was 0.7 µM. The assay was successfully used to determine TNP in spiked lake water samples, and the recoveries were 96.6-102.7%.

A novel fluorescent probe for ascorbic acid based on seed-mediated growth of silver nanoparticles quenching of carbon dots fluorescence.

A novel, economic, and eco-friendly method of detecting ascorbic acid (AA) with excellent sensitivity was developed. The method took advantage of the fluorescence of carbon quantum dots (CDs) decreasing as the AA concentration increased through interactions between AA and Ag(I) in the presence of silver nanoparticle (AgNP) seeds, producing more AgNPs and allowing fluorescence resonance energy transfer between the AgNPs and the CDs. The change in the fluorescence intensity when AA was added was proportional to the AA concentration over the range 0-9.0 µM. The AA detection limit was 0.2 µM. This approach is a new method for determining the concentration of AA.

Silver nanoclusters functionalized with Ce(III) ions are a viable "turn-on-off" fluorescent probe for sulfide.

The authors show that silver nanoclusters functionalized with Ce(III) ions are a viable fluorescent probe for selective and sensitive detection of sulfide at pH 7.0. The blue fluorescence of silver nanoclusters (with excitation/emission peaks at 358/426 nm) is enhanced on the addition of Ce(III) ions but is quenched in the presence of a trace concentrations of sulfide. A fluorometric assay was worked out using the Ce(III)/AgNCs as the probe. Sulfide can be detected in concentrations up to 2.0 µM, and the detection limit is 15 nM. The method was successfully applied to the determination of sulfide in spiked real samples. Graphical abstract Silver nanoclusters functionalized with Ce(III) ions are a viable "turn-on-off" fluorescent probe for selective and sensitive detection of sulfide at pH 7.0.