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Herein, we developed a pH sensing platform based on reduced graphene quantum dots (rGQDs) and chitosan (CS). CS was used as the recognition element in the self-assembly of the rGQD/CS nanocomposite because of its fascinating pH-sensitivity, induced by the protonation and deprotonation of the –NH2 group on its edge. The –NH2 was easily protonated under acidic conditions, making CS positively charged. The negatively-charged rGQDs under weak acidic conditions thus could couple with CS through electrostatic attraction, leading to fluorescence quenching. When the pH was changed to basic, the CS became negatively charged, resulting in the disassembly of the rGQD/CS system and causing the system to exhibit a turn-on fluorescence signal. The proposed pH-sensing nanocomposite was successfully applied for sensitive and reliable pH measurements from 5.0 to 9.0. This nanocomposite system was further utilized for the sensitive detection of pH changes caused by the enzymatic activity of urease, thereby proving its utility as a fluorescence turn-on sensor for urease in the field of biochemical and environmental analysis. The photoluminescence (PL) intensity of the rGQD/CS system increased as the pH increased. The increased intensity is directly related to the urease activity in the assay system. Thus, a novel fluorescence turn-on biosensor for urease based on the disassembly of the rGQD/CS composite is proposed. The system response exhibited a nearly linear relationship with urease concentration in the range of 0.05–0.75 U mL−1. The detection limit for urease was 0.036 U mL−1. This is the first report of this type of sensor for urease detection. When applied to real sample analysis, the present strategy exhibited satisfactory results. [ABSTRACT FROM AUTHOR]

In this work, we designed a simple and sensitive fluorescence probe for sequential detection of lysozyme and trypsin. Firstly, l-cysteine was chosen as a stabilizer to obtain l-cysteine-capped CdTe QDs. Lysozyme with positive charges can interact with l-cysteine capped CdTe quantum dots (QDs) via the special assembly between lysozyme and l-cysteine, resulting in the quenching of the fluorescence of CdTe QDs. Lysozyme can be hydrolyzed into small fragments in the presence of trypsin, and the interaction between l-cysteine-capped CdTe QDs and lysozyme would be inhibited, which could be used for trypsin quantification. There was a linear relationship between the fluorescence intensity of CdTe QDs and the lysozyme concentration in the range of 75–1000 ng mL−1 and another linear relationship between the fluorescence intensity of CdTe QDs and the logarithm of the trypsin concentration in the range of 10–500 ng mL−1. The detection limits were 28.33 ng mL−1 for lysozyme and 8.35 ng mL−1 for trypsin, respectively. The established method showed a good selectivity for lysozyme and trypsin detection. [ABSTRACT FROM AUTHOR]

A facile and rapid fluorescence assay based on a redox reaction for successively detecting ascorbic acid and acid phosphatase was developed via Cr(vi)-modulated graphene quantum dots (GQDs). Graphene quantum dots with yellow-green emission were first synthesized via a one-pot hydrothermal method. Based on the electrostatic adsorption of Cr3+ on GQDs and the strong chelation between Cr3+ and the –COOH and –OH groups on the surface of GQDs, the fluorescence of GQDs could be greatly quenched by Cr3+ ions. By the introduction of ascorbic acid, Cr2O72− could be reduced to Cr3+, which resulted in quenching of the fluorescence signal of GQDs. The degree of quenching of the fluorescence intensity of GQDs was proportional to the concentration of ascorbic acid. The dynamic detection range for ascorbic acid was from 0.5 to 250 μmol L−1 with a limit of detection (LOD) of 0.28 μmol L−1. Moreover, this phenomenon was further exploited for the sensitive and selective detection of acid phosphatase (ACP). l-Ascorbic acid-2-phosphate (AAP), which is a more stable phosphatase substrate, could be hydrolyzed by ACP to give ascorbic acid. Ascorbic acid then reduced Cr2O72− to Cr3+, leading to quenching of the fluorescence of GQDs. Thus, the amount of ACP could be indirectly detected in the range from 0.02 to 3 mU mL−1 with a LOD of 8.9 μU mL−1. Thus, a Cr(vi)-modulated GQDs “turn-off” fluorescence sensor for ascorbic acid and ACP was constructed. The present strategy showed high selectivity for ascorbic acid and ACP. The feasibility of the proposed sensing system in a real sample assay was also studied and satisfactory results were obtained. [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]

In this paper, we constructed a novel fluorescence resonance energy transfer (FRET) system between CdTe quantum dots (QDs) and graphene oxide (GO) for sensitive detection of concanavalin A (Con A). In this system, CdTe QDs as an energy donor and GO as an energy acceptor were both covalently functionalized by glucosamine via an amide linkage. The specific combination of Con A with glucosamine could bring CdTe QDs and GO into appropriate proximity and therefore induce efficient energy transfer. The concentration of Con A caused modulation of the energy transfer efficiency between GO and QDs, thus enabling Con A detection. Under the optimized experimental conditions, the decrease of fluorescence intensity ratio I/I0 (I0 and I are the fluorescence intensity of the sensing system in the absence and presence of Con A, respectively) is proportional to the concentration of Con A in the range of 9.8–196.1 nM. This method has high sensitivity with the detection limit as low as 3.3 nM. The results suggested that the graphene FRET platform could be used for sensitive and selective sensing of Con A and could be applied for protein–carbohydrate interaction studies. [ABSTRACT FROM AUTHOR]

We have developed a simple and efficient fluorescence detection system for lysozyme utilizing I–III–VI type Cu–In–S ternary quantum dots (CuInS2 QDs) as the probe. Water-soluble near-infrared CuInS2 QDs capped with 3-mercaptopropionic acid were directly synthesized by using a hydrothermal method. Poly(dimethyl diallyl ammonium chloride), as a cationic polyelectrolyte, could bind to 3-mercaptopropionic acid-capped CuInS2 QDs via electrostatic interactions that would lead to fluorescence quenching of CuInS2 QDs. And the addition of a lysozyme aptamer could effectively turn on the quenched fluorescence due to the stronger interaction between the cationic polyelectrolyte and negatively charged aptamer. The addition of lysozyme would result in fluorescence quenching of CuInS2 QDs again due to the specific binding between lysozyme and the lysozyme aptamer. Therefore, this label-free fluorescence system could be applied to selectively detect lysozyme. [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 novel near-infrared fluorescence probe for the determination of 2,4,6-trinitrophenol (TNP) was developed based on bovine serum albumin (BSA) coated CuInS2 quantum dot (QD). Water-soluble CuInS2 QDs were directly synthesized in aqueous solution with mercaptopropionic acid (MPA) as stablizers, and it was coated by BSA via an amide link interacting with carboxyl of the MPA-capped CuInS2 QDs. The obtained BSA coated CuInS2 QDs (BSA-CuInS2 QDs) can bind TNP in water via the acid–base pairing interaction between electron-rich amino groups of BSA and electron-deficient nitroaromatic rings, and leading the fluorescence quenching of BSA-CuInS2 QDs. The quenched fluorescence intensity of BSA-CuInS2 QDs was proportional to the concentration of TNP in the concentration ranges from 50nmol/L to 3.0μmol/L. The detection limit (LOD) for TNP was 28nmol/L. The developed fluorescence probe showed a excellent resistant to interference of metal ions such as K+, Na+, Zn2+, Cu2+, Hg2+, and Pb2+. The developed biosensor was applied to the determination of TNP in spiked water samples with satisfactory results. [Copyright &y& Elsevier]

A novel, rapid and highly sensitive fluorescence turn-on assay for the detection of biothiols (glutathione, and l-cysteine), histidine and threonine was developed. Water-soluble CuInS2 ternary quantum dots (QDs) capped by mercaptopropionic acid (MPA) were directly synthesized in aqueous solution, and then functionalized using tyrosine molecules to form tyrosine-functionalized CuInS2 QDs (T-CuInS2 QDs). The fluorescence of T-CuInS2 QDs would decrease in the presence of Cu2+ due to the coordination effect of phenolic hydroxyls of the tyrosine molecules. Subsequently, the addition of biothiols (glutathione and l-cysteine), histidine or threonine could turn on the fluorescence of the T-CuInS2 QDs–Cu2+ system due to their strong affinity for Cu2+. The proposed method was simple in design and fast in operation, and it was applied for the detection of glutathione, l-cysteine, histidine, and threonine in human serum samples with satisfactory results. [ABSTRACT FROM AUTHOR]

Abstract: Water-soluble CuInS2 ternary quantum dots (QDs) capped by mercaptopropionic acid were directly synthesized in aqueous solution. Consequently, the CuInS2 QDs were covalently linked to 3-aminophenyl boronic acid molecules to form the 3-aminophenyl boronic acid-functionalized CuInS2 QDs (F-CuInS2 QDs). The F-CuInS2 QDs had a fairly symmetric fluorescence emission centered at 736nm that was in the near-infrared region (NIR). The F-CuInS2 QDs containing boronic acid functional groups were reactive toward vicinal diols to form five- or six-member cyclic esters in an alkaline aqueous solution. The reaction would cause the fluorescence quenching, which could be used as a fluorescence probe for the determination of dopamine (DA). This assay could also probe other vicinal diols such as catechol, pyrogallol, and gallate, based on the fluorescence quenching of the F-CuInS2 QDs, and this assay was nearly unaffected by other phenol compounds such as phenol, resorcinol, and hydroquinone without the vicinal diol structures. The developed F-CuInS2 QDs were applied to the detection of DA in human serum samples with satisfactory results. Therefore, this experment provided a simple and sensitve NIR fluorescence probe for the detection of DA, catechol, pyrogallol, and gallate. [Copyright &y& Elsevier]

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]

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]

A label-free fluorescence assay for rapid and sensitive detection of heparin (Hep) or chondroitin sulfate (CS) was developed by guanidine-terminated poly (amidoanime) (PAMAM-Gu + ) dendrimers induced aggregation of graphene quantum dots (GQDs). The fluorescence of GQDs was obviously quenched after mixing with PAMAM-Gu + . However, the addition of highly negatively charged Hep or CS into the fluorescence sensing system resulted in the fluorescence recovery. Because the multi-positively charged PAMAM-Gu + would prefer to bind with highly negatively charged Hep or CS, resulting in the deaggregation of GQDs. Under the optimized experimental conditions, the recovery of fluorescence intensity ratio I / I 0 ( I 0 and I were the fluorescence intensity of the sensing system in the absence or presence of target analytes, respectively) was proportional to the concentration of target analytes in the range of 0.04–1.6 μg mL −1 for Hep and 0.1–2.5 μg mL −1 for CS. In addition, this method afforded high sensitivity with the detection limit as low as 0.02 μg mL −1 and 0.05 μg mL −1 for Hep and CS, respectively. All results suggested that the fluorescence turn-on method could be successfully employed for sensitive and selective detection of heparin analogs. [ABSTRACT FROM AUTHOR]

We have developed a simple and efficient fluorescence detection system for lysozyme utilizing I-III-VI type Cu-In-S ternary quantum dots (CuInS2 QDs) as the probe. Water-soluble near-infrared CuInS2 QDs capped with 3-mercaptopropionic acid were directly synthesized by using a hydrothermal method. Poly(dimethyl diallyl ammonium chloride), as a cationic polyelectrolyte, could bind to 3-mercaptopropionic acid-capped CuInS2 QDs via electrostatic interactions that would lead to fluorescence quenching of CuInS2 QDs. And the addition of a lysozyme aptamer could effectively turn on the quenched fluorescence due to the stronger interaction between the cationic polyelectrolyte and negatively charged aptamer. The addition of lysozyme would result in fluorescence quenching of CuInS2 QDs again due to the specific binding between lysozyme and the lysozyme aptamer. Therefore, this label-free fluorescence system could be applied to selectively detect lysozyme.

A novel, rapid and highly sensitive fluorescence turn-on assay for the detection of biothiols (glutathione, and L-cysteine), histidine and threonine was developed. Water-soluble CuInS2 ternary quantum dots (QDs) capped by mercaptopropionic acid (MPA) were directly synthesized in aqueous solution, and then functionalized using tyrosine molecules to form tyrosine-functionalized CuInS2 QDs (T-CuInS2 QDs). The fluorescence of T-CuInS2 QDs would decrease in the presence of Cu(2+) due to the coordination effect of phenolic hydroxyls of the tyrosine molecules. Subsequently, the addition of biothiols (glutathione and L-cysteine), histidine or threonine could turn on the fluorescence of the T-CuInS2 QDs-Cu(2+) system due to their strong affinity for Cu(2+). The proposed method was simple in design and fast in operation, and it was applied for the detection of glutathione, L-cysteine, histidine, and threonine in human serum samples with satisfactory results.

In this paper, a sensitive water-soluble fluorescent conjugated polymer biosensor for catecholamine (dopamine DA, adrenaline AD and norepinephrine NE) was developed. In the presence of horse radish peroxidase (HRP) and H(2)O(2), catecholamine could be oxidized and the oxidation product of catecholamine could quench the photoluminescence (PL) intensity of poly(2,5-bis(3-sulfonatopropoxy)-1,4-phenylethynylenealt-1,4-poly(phenylene ethynylene)) (PPESO(3)). The quenching PL intensity of PPESO(3) (I(0)/I) was proportional to the concentration of DA, AD and NE in the concentration ranges of 5.0 × 10(-7) to 1.4 × 10(-4), 5.0 × 10(-6) to 5.0 × 10(-4), and 5.0 × 10(-6) to 5.0 × 10(-4) mol L(-1), respectively. The detection limit for DA, AD and NE was 1.4 × 10(-7) mol L(-1), 1.0 × 10(-6) and 1.0 × 10(-6) mol L(-1), respectively. The PPESO(3)-enzyme hybrid system based on the fluorescence quenching method was successfully applied for the determination of catecholamine in human serum samples with good accuracy and satisfactory recovery. The results were in good agreement with those provided by the HPLC-MS method.