Construction of a dual-signal readout platform for effective glutathione S-transferase sensing based on polyethyleneimine-capped silver nanoclusters and cobalt-manganese oxide nanosheets with oxidase-mimicking activity.

A novel dual-mode fluorometric and colorimetric sensing platform is reported for determining glutathione S-transferase (GST) by utilizing polyethyleneimine-capped silver nanoclusters (PEI-AgNCs) and cobalt-manganese oxide nanosheets (CoMn-ONSs) with oxidase-like activity. Abundant active oxygen species (O2•-) can be produced through the CoMn-ONSs interacting with dissolved oxygen. Afterward, the pink oxDPD was generated through the oxidation of colorless N,N-diethyl-p-phenylenediamine (DPD) by O2•-, and two absorption peaks at 510 and 551 nm could be observed. Simultaneously, oxDPD could quench the fluorescence of PEI-AgNCs at 504 nm via the inner filter effect (IFE). However, in the presence of glutathione (GSH), GSH prevents the oxidation of DPD due to the reducibility of GSH, leading to the absorbance decrease at 510 and 551 nm. Furthermore, the fluorescence at 504 nm was restored due to the quenching effect of oxDPD on decreased PEI-AgNCs. Under the catalysis of GST, GSH and1-chloro-2,4-dinitrobenzo (CDNB) conjugate to generate an adduct, initiating the occurrence of the oxidation of the chromogenic substrate DPD, thereby inducing a distinct colorimetric response again and the significant quenching of PEI-AgNCs. The detection limits for GST determination were 0.04 and 0.21 U/L for fluorometric and colorimetric modes, respectively. The sensing platform illustrated reliable applicability in detecting GST in real samples.

A ratiometric fluorescence strategy based on polyethyleneimine surface-modified carbon dots and Eosin Y for the ultrasensitive determination of protamine and trypsin.

In this work, we report a novel and ultrasensitive dual-signal fluorescence emission detection system for protamine and trypsin based on the electrostatic interaction between polyethyleneimine (PEI) surface-modified positively charged carbon quantum dots (CDs-PEI) and the anionic fluorescent dye Eosin Y. The fluorescence system exhibited yellow-green fluorescence from Eosin Y and blue fluorescence from CDs-PEI. As a cationic peptide, protamine quenched the yellow-green fluorescence of Eosin Y at 542 nm through electrostatic interaction. In the presence of trypsin, protamine was specifically hydrolyzed by trypsin, which led to the subsequent recovery of the fluorescence of Eosin Y. Simultaneously, the blue fluorescence emission of CDs-PEI at 452 nm remained constant during the whole process. Hence, a ratiometric fluorescent nanoprobe for protamine and trypsin detection with high sensitivity was successfully constructed based on CDs-PEI and Eosin Y. For protamine detection, the ratiometric fluorescence intensity (I542/I452) exhibited an excellent linear relationship in the range of 0.1-5.2 µg mL-1 with a limit of detection (LOD) of 0.03 µg mL-1. And the linear relationship between I542/I452 and trypsin concentration ranged from 0.4 to 56 ng mL-1 with an LOD of 0.21 ng mL-1. Upon evaluating the performance of this method for the detection of trypsin in actual human urine samples, satisfactory results were finally obtained.

Determination of catecholamine in human serum by a fluorescent quenching method based on a water-soluble fluorescent conjugated polymer-enzyme hybrid system.

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.

Multicolor quantum dot-encoded microspheres for the fluoroimmunoassays of chicken newcastle disease and goat pox virus.

Semiconductor nanocrystals (or quantum dots, QDs) have the potential to overcome some of the limitations encountered by traditional fluorophores in fluorescence labeling applications. The unique spectroscopic properties of QDs make them hold immense promise as versatile labels for biological applications. In this work, we employ the layer-by-layer (LbL) method for the construction of bio-functional multicolor QD-encoded microspheres. Polystyrene microspheres with diameter of 3 microm were used as templates for the deposition of different sized CdTe QDs/polyelectrolyte multilayers. Two different antigens, Chicken newcastle disease (CND) antigen and goat pox virus (GPV) antigen, were conjugated to two kinds of biofunctional multicolor microspheres with different optical encoding. The multicolor microspheres can capture corresponding antibodies labeled with QDs, QDs-CND antibody and QDs-GPV antibody in the fluoroimmunoassays. The microspheres can be distinguished from each other based on their optical encoding.

A molybdenum disulfide quantum dots-based ratiometric fluorescence strategy for sensitive detection of epinephrine and ascorbic acid.

In this paper, a novel and sensitive ratiometric fluorescence strategy for the detection of epinephrine (EP) and ascorbic acid (AA) was established based on the fluorescence resonance energy transfer (FRET) between the molybdenum disulfide quantum dots (MQDs) and the fluorescent oxidative polymerization product (PEP-PEI) of EP in polyethyleneimine (PEI) aqueous solution. The continuous formation of PEP-PEI can lead to the fluorescence quenching of MQDs at 414 nm while the fluorescence of PEP-PEI at 522 nm gradually increased. The introduction of AA can inhibit the oxidative polymerization process of EP due to the strong reducibility of AA, resulting in the fluorescence recovery of MQDs at 414 nm and the fluorescence decreasing of PEP-PEI at 522 nm. Therefore, EP and AA can be monitored by measuring the ratio of the fluorescence intensities at 522 nm and 414 nm. A good linear calibration of I522/I414 versus EP and AA concentrations were obtained within 0.2-40 µM and 0.5-40 µM, respectively. And the detection limit was 0.05 µM for EP and 0.2 µM for AA. Furthermore, the developed ratiometric fluorescence method with high sensitivity and selectivity was applied for EP in human urine samples and AA in human serum samples determination with satisfactory results obtained.

Copper nanoclusters/polydopamine nanospheres based fluorescence aptasensor for protein kinase activity determination.

A fluorescence aptasensor was constructed for protein kinase (PKA) activity detection by utilizing copper nanoclusters (CuNCs) and polydopamine nanospheres (PDANS). Through the π-π stacking interactions between adenosine triphosphate (ATP) aptamer and PDANS, the ATP aptamer modified CuNCs (apt-CuNCs) were absorbed onto PDANS surface, thus the fluorescence of apt-CuNCs were quenched through fluorescence resonance energy transfer (FRET) from apt-CuNCs to PDANS. In the presence of ATP, ATP specifically bound to aptamer, causing the dissociation of apt-CuNCs from PDANS surface and restoring the fluorescence of apt-CuNCs. However, PKA translated ATP into adenosine diphosphate (ADP), and ADP had no competence to combine with ATP aptamer, thus, apt-CuNCs were released and absorbed onto the PDANS surface to cause the fluorescence quenching of apt-CuNCs again. Therefore, PKA activity was conveniently detected via the fluorescence signal change. Under the optimal conditions, PKA activity was detected in the range of 0.05-4.5 U mL-1 with a detection limit of 0.021 U mL-1. Furthermore, the feasibility of the aptasensor for kinase inhibitor screening was explored via assessment of kinase inhibitor H-89 as one model. This aptasensor was also performed for PKA activity determination in HepG2 cell lysates with satisfactory results.

Fluorescence turn-off-on probe based on polypyrrole/graphene quantum composites for selective and sensitive detection of paracetamol and ascorbic acid.

In this work, we presented a novel biosensor for rapid detection of paracetamol and ascorbic acid. The novel biosensor was based on the fluorescent "turn off-on" of polypyrrole/graphene quantum dots (PPy/GQDs) composites. The composites exhibit strong fluorescence emission, which is dramatically enhanced as high as three times than that of pure GQDs. It is found that the fluorescence intensity of PPy/GQDs can be efficiently quenched by N-acetyl-p-benzoquinone (4-AOBQ), the oxidation product of paracetamol (PAR). And a turn-on fluorescence signal was observed when 4-AOBQ is reduced by ascorbic acid (AA). The quenched and recovered fluorescence intensity of PPy-GQDs was proportional to the concentration of PAR (0.067-233µg/L) and AA (3.33-997.5µg/L) respectively. The limit of detection is 0.022µg/L for PAR and 1.05µg/L for AA. The present method was applied to the determination of PAR and AA in human serum samples with satisfactory results.

A novel magnetic/photoluminescence bifunctional nanohybrid for the determination of trypsin.

In this work, we have designed a novel kind of nanohybrid with magnetic and photoluminescence (PL) property for trypsin detection. The modified magnetic Fe3O4 nanoparticles (MNPs) with polydopamine (PDA) and human serum albumin (HSA) were prepared through a one step self-polymerization under mild condition. The polydopamine (PDA) coating on MNPs can improve the biocompatibility of the MNP-PDA-HAS composite due to its hydrophilicity and multifunctional groups. When MNP-PDA-HSA composite was added into the Anti-HSA modified CdTe QDs (anti-HSA-QDs), HSA on the MNP-PDA-HSA composite was captured by the site of anti-HSA-QDs to form MNP-PDA-HSA/anti-HSA-QDs nanohybrid. Therefore, the photoluminescence of QDs can be quenched by Fe3O4 nanoparticles due to the electron transfer. In the presence of trypsin, the protein (anti-HSA) was digested by trypsin and QDs was separated from the nanohybrid surface. As a result, the photoluminescence intensity of QDs was recovered. The magnetic/luminescent bifunctional nanohybrid displayed excellent analytical performance for the detection of trypsin in the range of 0.5-30µg/mL with a low detection limit of 0.25µg/mL.

Detection of bisphenol A in food packaging based on fluorescent conjugated polymer PPESO3 and enzyme system.

Bisphenol A (BPA) is a kind of carcinogen, which can interfere with the body's endocrine system. In this paper, a new kind of fluorescent sensor for BPA detection was established based on the fluorescent conjugated polymer PPESO3. The oxidative product of BPA is able to quench PPESO3 in the presence of HRP and H2O2, and the quenched PL intensity of PPESO3 was proportionally to the concentration of BPA in the range of 1-100 µmol/L with a detection limit of 4 × 10(-7) mol/L. The proposed method has been applied to detect BPA in eight food packaging samples with satisfactory results. The proposed method has the potential for the assay of BPA in food or food packaging samples.

Polystyrene microspheres based sandwich immunosensor using CdTe nanoparticles amplification and ultrasensitive flow-injection chemiluminescence detection.

In this paper we propose a specific sandwich immunoassay method for human-immunoglobulin G (HIgG). This immunoassay protocol takes advantage of sandwich binding of primary and secondary antibodies for increased specificity. Polystyrene microspheres (PS) serve as immobilizing support, site for sandwich immunoassay and then subsequently used for chemiluminescence (CL) detections. In this sandwich immunoassay, PS microspheres were modified with the primary anti-HIgG (Ab1) via electrostatic interaction, while CdTe nanoparticles (CdTeNPs) were modified with horseradish peroxidase labeled anti-HIgG (Ab2) via covalent binding. Antigen HIgG (Ag) was specifically captured by the first and secondary antibody and form sandwich immunoassay format. Combination of the remarkable sensitivity of CL method and the use of CdTe NPs as anti-HIgG-HRP carrier for the enzymatic signal amplification, provide a linear response range of HIgG from 0.01 to 300 ng mL(-1) with an extremely low detection limit of 0.3 pg mL(-1). This immunoassay system has many desirable merits including sensitivity, accuracy, and little required instrumentation. The assay results were compared with enzyme-linked immunosorbent assay (ELISA), and showed relatively good reliability. Significantly the new protocol may become quite promising technique for protein immune-detection as well as DNA analysis and other biological analyses.

Study the damage of DNA molecules induced by three kinds of aqueous nanoparticles.

In this paper, the interaction of DNA molecules with aqueous CdTe quantum dots (CdTe QDs), CdTe/SiO(2) composite nanoparticles (CdTe/SiO(2) NPs), and Mn-doped ZnSe quantum dots (Mn:ZnSe d-dots) was studied with ethidium bromide as a probe. The purpose of this work was to study the damage of DNA molecules induced by these three kinds of water-soluble nanoparticles. It was found that ionic strength, pH value and UV irradiation influenced the PL emission properties of CdTe QDs, CdTe/SiO(2) NPs and Mn:ZnSe d-dots, and also influenced the interaction of DNA molecules with them. Among the three kinds of nanoparticles, DNA molecules were most easily damaged by CdTe QDs whether in the dark or under UV irradiation. The CdTe/SiO(2) NPs led to much less DNA damage when compared with CdTe QDs, as a silica overcoating layer could isolate the QDs from the external environment. Mn:ZnSe d-dots as a new class of non-cadmium doped QDs demonstrated almost no damage for DNA molecules, which have great potentials as fluorescent labels in the applications of biomedical assays, imaging of cells and tissues, even in vivo investigations.

Highly sensitive gaseous formaldehyde sensor with CdTe quantum dots multilayer films.

A novel method for the direct detection of gaseous formaldehyde based on the fluorescence quenching of CdTe quantum dots (QDs)/polyelectrolyte multilayer films (QDMF) was proposed in this paper. The functional multilayer films were assembled by layer-by-layer (LBL) deposition of oppositely charged CdTe QDs and poly(dimethyldiallylemmonium chloride) (PDDA). Formaldehyde can quench the fluorescence of CdTe QDs multilayer films effectively. Under the optimization conditions, the fluorescence intensity of QDs decreased linearly with the increase of formaldehyde concentration in the range of 5-500ppb. The detection limit for formaldehyde was 1ppb. The quenching mechanism of CdTe QDs multilayer films by formaldehyde was also studied in detail. This proposed approach was simple, rapid and had excellent selectivity and sensitivity for the detection of gaseous formaldehyde.

The preparation of CdTe nanoparticles and CdTe nanoparticle-labelled microspheres for biological applications.

Different sizes of CdTe semiconductor nanoparticles were prepared in aqueous solution. These nanoparticles exhibit narrow fluorescence with full width at half-maximum (FWHM) of 35-45 nm that spans the visible spectrum, and they also have high PL quantum yield with high resistance to photodegradation. In addition, CdTe quantum dot (QD)-labelled microspheres, comprising polystyrene (PS) cores and CdTe/polyelectrolyte (PE) shells, were also prepared by the layer-by-layer technique in this paper. The optical properties of the CdTe nanoparticles and CdTe-labelled microspheres were investigated by UV-Visible absorption and luminescence spectroscopy, and fluorescence microscopy was employed for microscopic identification behaviour of the luminescent microspheres.

The use of CdTe quantum dot fluorescent microspheres in fluoro-immunoassays and a microfluidic chip system.

Polystyrene fluorescent microspheres prepared by deposition of CdTe quantum dots (QDs) are used in an immunoassay in this study. CdTe QDs/polyelectrolyte multilayers on the surface of polystyrene microspheres have been formed by layer-by-layer self-assembly via electrostatic interactions. As a model antigen, rabbit IgG has been bound to the outermost layer of the fluorescent microspheres. The immunoreaction between fluorescent microspheres/rabbit IgG and the corresponding antibody was confirmed by change of the fluorescence spectrum and competitive immunoassay. This approach allowed detection of the antigen (rabbit IgG) in the range 1-500 mg/L, based on the change in the fluorescence intensity of the reporter (fluorescent microspheres/rabbit IgG). A novel microfluidic chip device with a laser-induced fluorescence system was established and used for the detection of fluorescent microspheres in this study.