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Herein, a sodium hexametaphosphate ((NaPO 3 ) 6 ) modulated fluorescence responsive probe based on the integration of reduced graphene quantum dots (rGQDs) and chitosan (CS) via self-assembly/disassembly for label-free alkaline phosphatase assay was constructed. The cationic-charged CS can couple with anionic rGQDs and quench their fluorescence intensity through electrostatic attraction and structure transformation. This self-assembly system above could be decomposed when (NaPO 3 ) 6 present, because (NaPO 3 ) 6 could competes with rGQDs for the binding sites on the CS, leading to the disassembly of the rGQDs/CS system, as well as to the system exhibiting a turn-on fluorescence signal. By introducing alkaline phosphatase (ALP) into the system, (NaPO 3 ) 6 can be hydrolyzed to give phosphate anions. The decomposition effect of enzymatic products on the rGQDs/CS system is weakened. Thus the self-assembling system shows a decreasing photoluminescence (PL) signal compared with the rGQDs/CS-(NaPO 3 ) 6 disassembling system. The concentration of ALP can be reflected by the variation of the PL intensity of rGQDs/CS system mixed with the enzymatic hydrolysis products. The dynamic detection range for ALP is 20-500 mU mL -1 , with a detection limit (LOD) of 7.8 mU mL -1 . The present fluorescence probe based on the rGQDs/CS system for ALP has excellent selectivity and strong anti-interference capability. When applied to real samples analysis, the present strategy exhibits satisfactory results. In addition, the rGQDs/CS system was used to fabricate paper-based test strips for visual detection of ALP activity, validating its great potential in the application of on-site ALP assays., (Copyright © 2019 Elsevier B.V. All rights reserved.)

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 Cr 3+ on GQDs and the strong chelation between Cr 3+ and the -COOH and -OH groups on the surface of GQDs, the fluorescence of GQDs could be greatly quenched by Cr 3+ ions. By the introduction of ascorbic acid, Cr 2 O 7 2- could be reduced to Cr 3+ , 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 Cr 2 O 7 2- to Cr 3+ , 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.

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/I0 (I0 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., (Copyright © 2015 Elsevier B.V. All rights reserved.)

In this paper, we developed dopamine functionalized-CdTe quantum dots as fluorescence probes for the determination of l-histidine. Firstly, CdTe was covalently linked to dopamine to form a kind of fluorescence sensor with pyrocatechol structure on the surface. The photoluminescence intensity of CdTe-dopamine (QDs-DA) could be quenched by Ni(2+) due to the strong coordination interaction between the pyrocatechol structure of QDs-DA and Ni(2+). In the presence of l-histidine, Ni(2+) preferred to bind with l-histidine due to high affinity of Ni(2+) to l-histidine and the photoluminescence intensity of QDs-DA was recovered. The recovered photoluminescence intensity of QDs-DA was proportional to the concentration of l-histidine in the ranges of 1.0×10(-6)-1.0×10(-4)mol L(-1) and the detection limit was 5.0×10(-7)mol L(-1) respectively. The established method showed a good selectivity for l-histidine among other common amino acids, and it was applied for determination of l-histidine in human serum sample with satisfactory results., (Copyright © 2014 Elsevier B.V. All rights reserved.)

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.

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 50 nmol/L to 3.0 μmol/L. The detection limit (LOD) for TNP was 28 nmol/L. The developed fluorescence probe showed a excellent resistant to interference of metal ions such as K(+), Na(+), Zn(2+), Cu(2+), Hg(2+), and Pb(2+). The developed biosensor was applied to the determination of TNP in spiked water samples with satisfactory results., (© 2013 Elsevier B.V. All rights reserved.)

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.

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 © 2013 Elsevier B.V. All rights reserved.)

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.