Sensitive and selective DNA probe based on "turn-on" photoluminescence of C-dots@RGO.

In this study, highly hydrophilic and photoluminescent sheets of reduced graphene oxide decorated with carbon dots (C-dots@RGO), methylene blue (MB), and a probe DNA have been used for the detection of DNA. The photoluminescence of C-dots@RGO is quenched by MB, which is restored in the presence of a target DNA. The combination of the C-dots@RGO, MB, and a DNA probe is selective for perfectly matched DNA over mismatched DNA, mainly because relative to single-stranded DNA, double-stranded DNA intercalates more strongly with MB, but interacts more weakly with RGO. In the presence of a target DNA, MB intercalates with the as-formed double-stranded DNA and is released from the surface of C-dots@RGO, leading to "turn-on" photoluminescence. The practicality of this assay has been validated by the determination of tumor suppressor gene BRCA1, with linearity over the concentration range from 25 to 250 nM and a limit of detection (LOD, at a signal-to-noise ratio of 3) of 14.6 nM. The C-dots@RGO probe provides higher specificity towards target DNA than towards common salts, carbohydrates, amino acids, and proteins found in real samples. Having the advantages of simplicity, cost-effectiveness, selectivity, and sensitivity, the DNA-P/C-dots@RGO-MB probe on microwells has been successfully employed for the detection of DNA, suggesting its potential for multiple analyses of DNA targets when various DNA probes are employed.

Photoluminescent C-dots@RGO for sensitive detection of hydrogen peroxide and glucose.

We have demonstrated sensitive detections of hydrogen peroxide (H2O2) and glucose using reduced graphene oxide decorated with carbon dots (C-dots@RGO). The C-dots@RGO prepared from catechin (reducing agent and carbon source) and graphene oxide via hydrothermal routes possesses excitation-wavelength-dependence photoluminescence (PL) characteristics, with maximum excitation and emission wavelengths of 365 and 440 nm, respectively. The C-dots@RGO is stable in solution containing NaCl up to 350 mM, but is quenched by reactive oxygen species (ROS). ROS reacts with H2O2 and thus its PL quenching toward the C-dots@RGO is minimized. When using C-dots@RGO and glucose oxidase (GOx), the PL assay allows detection of glucose in the presence of 10 µM of bovine serum albumin, with linearity over a concentration range from 1 to 60 µM (r=0.99) and a limit of detection (at a signal-to-noise ratio of 3) of 140 nM. The practicality of this assay has been validated by determining the concentrations of glucose in serum and saliva samples, with results of 5.1 ± 0.6mM (n=3) and 117.9 ± 8.1 µM (n=3), respectively. Our simple and sensitive assay opens a new avenue of developing assays for various analytes using C-dots@RGO in conjunction with different enzymes.

Photoluminescent C-dots@RGO probe for sensitive and selective detection of acetylcholine.

We have developed a sensitive and selective photoluminescence (PL) quenching assay for the detection of acetylcholine (ACh) that uses reduced graphene oxide decorated with carbon dots (C-dots@RGO). The highly stable C-dots@RGO synthesized from catechin and graphene oxide through a hydrothermal reaction displays excitation-wavelength dependence of PL. Acetylcholinesterase (AChE) converts ACh to choline, which in turn is oxidized by choline oxidase (ChOx) to produce betaine and H2O2 that generates the reactive oxygen species (ROS). The as-produced ROS induces PL quenching of the C-dots@RGO through an etching process. With respect to sensitivity, the optimal reaction/sensing temperature and pH are 37 °C and 9.0, respectively, using C-dots@RGO (0.4 mg·mL(-1)) and AChE and ChOx at the activities of 0.5 and 0.1 unit·mL(-1), respectively. The PL intensity (excitation/emission wavelengths 365/440 nm) of the C-dots@RGO is inversely proportional to the concentration of ACh over a range of 0.05-10 nM (r = 0.997), with a limit of detection (signal-to-noise ratio 3) of 30 pM. We have validated this assay by determination of concentrations of ACh in plasma and blood samples, with results of 2.6 ± 0.8 nM (n = 5) and 6.8 ± 0.4 nM (n = 5), respectively. Our study opens an avenue for the detection of various analytes by use of C-dots@RGO in conjunction with different enzymes, substrates, and/or inhibitors.

Enzyme mimics of Au/Ag nanoparticles for fluorescent detection of acetylcholine.

We have developed a highly sensitive and selective fluorescent assay for the detection of acetylcholine (ACh) based on enzyme mimics of Au/Ag nanoparticles (NPs). These NPs were prepared via a one-step solution phase reaction between 13 nm Au NPs and Ag(+) ions in the presence of stabilizing agents such as adenosine triphosphate (ATP) and polyethylene glycol (PEG). Our sensing strategy involves reacting ACh with acetylcholinesterase (AChE) to form choline that is in turn oxidized by choline oxidase (ChOx) to produce betaine and H(2)O(2), which reacts with Amplex UltraRed (AUR) in the presence of bimetallic NPs catalyst to form a fluorescent product. The fluorescence intensity (excitation/emission wavelengths of 540/592 nm) is proportional to the concentration of ACh over a range of 1-100 nM (R(2) = 0.998), with a limit of detection of 0.21 nM (signal/noise = 3). When compared with Au NPs and horseradish peroxidase, the Au/Ag NPs provide 150- and 115-fold higher catalytic activity toward the H(2)O(2)-mediated AUR reaction. The practicality of the assay has been validated by determining the concentrations of ACh in plasma and blood samples, with results of 2.69 ± 0.84 nM (n = 5) and 6.75 ± 1.42 nM (n = 5), respectively. Thus, the present assay holds great potential for the analysis of ACh in biological samples.

Catalytic gold nanoparticles for fluorescent detection of mercury(II) and lead(II) ions.

In this study, we developed a fluorescence assay for the highly sensitive and selective detection of Hg(2+) and Pb(2+) ions using a gold nanoparticle (Au NP)-based probe. The Hg-Au and Pb-Au alloys that formed on the Au NP surfaces allowed the Au NPs to exhibit peroxidase-mimicking catalytic activity in the H(2)O(2)-mediated oxidation of Amplex UltraRed (AUR). The fluorescence of the AUR oxidation product increased upon increasing the concentration of either Hg(2+) or Pb(2+) ions. By controlling the pH values of 5mM tris-acetate buffers at 7.0 and 9.0, this H(2)O(2)-AUR-Au NP probe detected Hg(2+) and Pb(2+) ions, respectively, both with limits of detection (signal-to-noise ratio: 3) of 4.0 nM. The fluorescence intensity of the AUR oxidation product was proportional to the concentrations of Hg(2+) and Pb(2+) ions over ranges 0.05-1 µM (R(2)=0.993) and 0.05-5 µM (R(2)=0.996), respectively. The H(2)O(2)-AUR-Au NP probe was highly selective for Hg(2+) (>100-fold) and Pb(2+) (>300-fold) ions in the presence of other tested metal ions. We validated the practicality of this simple, selective, and sensitive H(2)O(2)-AUR-Au NP probe through determination of the concentrations of Hg(2+) and Pb(2+) ions in a lake water sample and of Pb(2+) ions in a blood sample. To the best of our knowledge, this system is the first example of Au NPs being used as enzyme-mimics for the fluorescence detection of Hg(2+) and Pb(2+) ions.