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In this work, we designed a simple, convenient and high sensitive “turn-on” fluorescence platform for trypsin assay based on N -acetyl- l -cysteine capped CdTe Quantum Dots (NAC-CdTe QDs). A series of NAC-CdTe QDs with different sizes were synthesized in aqueous via refluxing routes. The fluorescence of NAC-CdTe QDs could be effectively quenched by hemoglobin (Hb) via the electron transfer interactions from NAC-CdTe QDs to hemoglobin due to the electrostatic attraction of QDs/Hb complex. In the presence of trypsin, hemoglobin would be hydrolyzed to small peptides, releasing inactive hemin molecules.Hemin has slight effect on the fluorescent intensity of NAC-CdTe QDs, so the combination of hemoglobin and NAC-CdTe QDs would be dissociated, leading to the fluorescence recovery of NAC-CdTe QDs. Therefore, we could monitor the trypsin activity by utilizing the different fluoresence responses of NAC-CdTe QDs to hemoglobin and hemin. The recovered fluorescence intensity of NAC-CdTe QDs was proportional to the logarithm of trypsin concentration in the range of 0.05–10 mU mL −1 and the detection limit for trypsin was 0.036 mU mL −1 . A model for trypsin inhibition was further established to verify the feasibility of the system. The IC 50 value is 3.06 μg mL −1 for the inhibitor from soybean. Thus, a label free real time fluorometric assay for trypsin and inhibitor screening has been developed. The established method showed a high selectivity and sensitivity for trypsin over other biological relevant enzyme, and it was applied to the determination of trypsin in human urine sample with satisfactory results. [ABSTRACT FROM AUTHOR]

In this paper, we developed a simple, ultra-sensitive and label-free fluorescent probe for trypsin and its inhibitor based on DNA hosted Cu nanoclusters (Cu NCs). Fluorescent Cu nanoclusters with maximum emission wavelength of 591 nm are formed by using double-stranded DNA (dsDNA) as templates. With the addition of protamine, the fluorescence intensity (FL) of Cu NCs could be enhanced obviously through forming protamine/DNA complexes between Cu NCs and protamine. The fluorescence intensity of Cu NCs would decrease in the present of trypsin due to the protamine hydrolysis, which allows the sensitive detection of trypsin in the range of 0.1–1000 ng mL −1 . The detection limit for trypsin is 0.048 ng mL −1 . And this system was further used for trypsin inhibitor detection. Compared with the previous reports, this method does not need complex DNA sequence design, fluorescence label and has a much lower detection limit. [ABSTRACT FROM AUTHOR]

In this work, we have designed a simple and sensitive fluorescence probe for adenosine 5′-triphosphate (ATP) detection. A series of l -cysteine capped CdTe quantum dots (QDs) with different sizes were synthesized in aqueous phase. The effects of Zn 2+ on different sizes of l -cysteine capped CdTe QDs were investigated and the fluorescence of CdTe with small size QDs could be effectively quenched due to the binding of Zn 2+ to the l -cysteine on the surface of the QDs and the electron transfer from the photoexcited QDs to Zn 2+ . The phosphate groups of ATP have a high affinity for zinc ions through Zn O P bonds, with the addition of ATP, Zn 2+ could bind with ATP based on the metal–ligand coordinative effect and the modulation of Zn 2+ to CdTe QDs would be inhibited. Hence the quenched fluorescence of the CdTe QDs can be recovered. The recovered fluorescence of QDs was proportional to the concentration of ATP in the range of 5–50 μmol L −1 and the detection limit for ATP was 2.07 μmol L −1 . The proposed sensing assay showed a good selectivity for ATP over other biologically important phosphates, and it was applied to the determination of ATP in human serum sample with satisfactory results. [ABSTRACT FROM AUTHOR]

Highlights: [•] The water-soluble CuInS2 QDs was functionalized with dopamine molecules. [•] The fluorescence of the dopamine functionalized CuInS2 QDs was sensitive to pH value changes. [•] Urea would release OH− to change the pH value with urase as the catalyst. [•] The dopamine functionalized CuInS2 QDs could be utilized as a fluorescence probe for urea. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, a new one-step turn-on sensor was developed for the detection of glutathione (GSH) and cysteine (Cys). The photoluminescence (PL) intensity of the water-soluble fluorescent conjugated polymers PPESO3 could be quenched by Cu(II) due to the strong electrostatic interaction and electron transfer between PPESO3 and Cu2+. In the presence of biothiols, such as GSH and Cys, Cu(II) preferred to react with biothiols to form the Cu(II)e to the strong affinity between Cu(II) and thiols. The recovered PL intensity of PPESO3 was proportional to the concentration of GSH or Cys in the concentration ranges of 1.0×10−7 to 1.5×10−5 mol/L and 1.0×10−7 to 2.0×10−5 mol/L, respectively. The detection limit for GSH and Cys were 4.0×10−8 and 4.5×10−8 mol/L, respectively. The established method shows a high selectivity for biothiols compared with other twelve amino acids without thiols group. Furthermore, the PPESO3–Cu(II) system as a fluorescence probe was successfully used for fluorescence imaging of biothiols in the HepG2 cells, which presents a potential application in bioimaging field. [Copyright &y& Elsevier]

Herein, we firstly used double-strand-templated DNA Cu nanoclusters (Cu NCs) as a simple, lable-free and sensitive fluorescence (FL) probe to detect protein kinase (PKA) activity. This method was based on the strong interaction between adenosine triphosphate (ATP) aptamer and ATP. When ATP was added, fluorescent Cu NCs could not be formed due to the lack of effective substrate and the fluorescence of Cu NCs decreased. However, when PKA was added, the fluorescence of Cu NCs recovered because that ATP had been translated into ADP by PKA and ADP can not combine with ATP aptamer. We can effective monitored the activity of protein kinase according to the variation of fluorescence signal in the range of 0.1–1000 mU/μL. The detection limit (LOD) is 0.041 mU/μL. We also used this method to detect protein kinase inhibitor H-89. In addition, this method was also used to explore the activity of drug-induced PKA in Hela cells, which makes the sensor had a great application value in biochemistry and targeted kinase drug discovery research. [ABSTRACT FROM AUTHOR]

Picric acid (PA) was sensitively detected via changed fluorescence of nitrogen-doped graphene quantum dots (N-GQDs) solid film in this paper. The mixture of N-GQDs and chitosan was firstly deposited on the silica slides to form N-GQDs solid film. Then creatinine was capped outside the solid film to enhance the selectivity to PA according to Jaffé reaction. The prepared solid film can provide strong fluorescence signal due to the strong blue emission of N-GQDs. PA can quench the fluorescence signal of creatinine/N-GQDs-chitosan solid film greatly due to the electron transfer among PA, creatinine and N-GQDs. Under the optimal condition, the fluorescence intensity of solid film was related linearly to the concentration of PA in the range of 2.0–200.0 ng/mL with a detection limit of 1.24 ng/mL. The selectivity of fluorescence quenching of the fluorescent solid film for PA was enhanced. Moreover, the potential application of the proposed methodology for the sensing of PA in water was demonstrated with a satisfactory result. The fluorescence intensity of film sensor was investigated and showed high stability in four weeks. To the best of our knowledge, it was the first report of PA detection by using the quenched fluorescence of N-GQDs solid film. [ABSTRACT FROM AUTHOR]