A "turn-off" fluorescent biosensor for the detection of mercury (II) based on graphite carbon nitride.

A novel and simple fluorescent biosensor has been realized through the fluorescence quenching of graphite carbon nitride (g-C3N4) for mercuric ion (Hg2+) detection. In this assay, the g-C3N4 sheets which were functionalized with single-stranded DNA (ssDNA) aptamer showed strong fluorescence emission at 440nm under the excitation of 380nm in the absence of Hg2+. When added to the assay solution, Hg2+ was embedded in the hairpin-shaped double-stranded DNA (dsDNA) due to the formation of the thymine-Hg2+-thymine (T-Hg2+-T) complex, which made the Hg2+ close to the surface of g-C3N4 sheet. Therefore, the fluorescence of g-C3N4 was quenched. This sensor has good selectivity with a limit of detection as low as 0.17nM under optimal conditions. The present work demonstrates that the g-C3N4-based fluorescent sensor has a promising application for detection of metal ions in real samples.

Highly selective fluorescent chemosensor for detection of Fe(3+) based on Fe3O4@ZnO.

The combination of fluorescent nanoparticles and specific molecular probes appears to be a promising strategy for developing fluorescent nanoprobes. In this work, L-cysteine (L-Cys) capped Fe3O4@ZnO core-shell nanoparticles were synthesized for the highly selective detection of Fe(3+). The proposed nanoprobe shows excellent fluorescent property and high selectivity for Fe(3+) due to the binding affinity of L-Cys with Fe(3+). The binding of Fe(3+) to the nanoprobe induces an apparent decrease of the fluorescence. Thus a highly selective fluorescent chemosensor for Fe(3+) was proposed based on Fe3O4@ZnO nanoprobe. The magnetism of the nanoprobe enables the facile separation of bound Fe(3+) from the sample solution with an external magnetic field, which effectively reduces the interference of matrix. The detection limit was 3 nmol L(-1) with a rapid response time of less than 1 min. The proposed method was applied to detect Fe(3+) in both serum and wastewater samples with acceptable performance. All above features indicated that the proposed fluorescent probe as sensing platform held great potential in applications of biological and analytical field.

Ultrasensitive sandwich-type electrochemical immunosensor based on a novel signal amplification strategy using highly loaded palladium nanoparticles/carbon decorated magnetic microspheres as signal labels.

An ultrasensitive sandwich-type electrochemical immunosensor for quantitative detection of alpha fetoprotein (AFP) was proposed based on a novel signal amplification strategy in this work. Carbon decorated Fe3O4 magnetic microspheres (Fe3O4@C) with large specific surface area and good adsorption property were used as labels to anchor palladium nanoparticles (Pd NPs) and the secondary antibodies (Ab2). Pd NPs were loaded on Fe3O4@C to obtain Fe3O4@C@Pd with core-shell structure by electrostatic attraction, which were further used to immobilize Ab2 due to the bonding of Pd-NH2. A signal amplification strategy was the noble metal nanoparticles, such as Pd NPs, exhibiting high electrocatalytic activities toward hydrogen peroxide (H2O2) reduction. This signal amplification was novel not only because of the great capacity, but also the ease of magnetic separation from the sample solution based on their magnetic property. Moreover, carboxyl-functionalized multi-walled carbon nanotubes (MWCNTs-COOH) were used for the immobilization of primary antibodies (Ab1). Therefore, high sensitivity could be realized by the designed immunosensor based on this novel signal amplification strategy. Under optimal conditions, the immunosensor exhibited a wide linear range of 0.5 pg/mL to 10 ng/mL toward AFP with a detection limit of 0.16 pg/mL (S/N=3). Moreover, it revealed good selectivity, acceptable reproducibility and stability, indicating a potential application in clinical monitoring of tumor biomarkers.

Sandwich-type electrochemical immunosensor using dumbbell-like nanoparticles for the determination of gastric cancer biomarker CA72-4.

A novel and sensitive nonenzymatic sandwich-type electrochemical immunosensor for the detection of gastric cancer biomarker CA72-4 was fabricated using dumbbell-like PtPd-Fe3O4 nanoparticles (NPs) as a novel kind of label. The signal amplification strategy, using the synergetic effect present in PtPd-Fe3O4 to increase the reduction ability of the NPs toward H2O2, improved the sensitivity of the immunosensor. The immunosensor was constructed by modifying glassy carbon electrode with reduced graphene oxide-tetraethylene pentamine (rGO-TEPA) for effective immobilization of primary anti-CA72-4 antibody (Ab1). Secondary anti-CA72-4 antibody (Ab2) was adsorbed onto the PtPd-Fe3O4 NPs. The proposed immunosensor displayed a wide linear range (0.001-10 U/mL) with the low detection limit (0.0003 U/mL). The immunosensor was evaluated for serum samples, receiving satisfactory results. Therefore, the immunosensor possesses excellent clinical value in cancer screening as well as convenient point-of-care diagnostics.

Ultrasensitive electrochemical immunosensor for carbohydrate antigen 72-4 based on dual signal amplification strategy of nanoporous gold and polyaniline-Au asymmetric multicomponent nanoparticles.

A sandwich electrochemical immunosensor is described for carbohydrate antigen 72-4 (CA72-4) based on a dual amplification strategy with nanoporous gold (NPG) film as the sensor platform and polyaniline-Au asymmetric multicomponent nanoparticles (PANi-Au AMNPs) as labels. In this study, the second anti-CA72-4 antibody (Ab2) adsorbed onto the Au of the PANi-Au AMNPs, which could be simply synthesized by interfacial reaction and have many characteristics of polyaniline and Au nanoparticle, such as well-controlled size, high conductivity, biocompatibility and catalysis. NPG film was used as electrode substrate material to fix a large number of antibodies, due to its unique properties: good biocompatibility, high conductivity, large surface area, and stability. The synergetic of NPG film and PANi-Au AMNPs could increase signal response, and significantly improve sensitivity of the immunosensor. The proposed immunosensor exhibited a wide linear range from 2 to 200 U/mL, with a detection limit of 0.10 U/mL CA72-4, good reproducibility, selectivity and stability. This new type of labels for immunosensors may provide many potential applications in the detection of carbohydrate antigen in immunoassays.

Ultrasensitive electrochemical immunoassay for CEA through host-guest interaction of ß-cyclodextrin functionalized graphene and Cu@Ag core-shell nanoparticles with adamantine-modified antibody.

A novel non-enzymatic immunoassay was designed for ultrasensitive electrochemical detection of carcino-embryonic antigen (CEA) using ß-cyclodextrin functionalized Cu@Ag (Cu@Ag-CD) core-shell nanoparticles as labels and ß-cyclodextrin functionalized graphene nanosheet (CD-GN) as sensor platform. CD-GN has excellent conductivity which promoted the electric transmission between base solution and electrode surface and enhanced sensitivity of immunosensor. In addition, owing to supramolecular recognition of CD-GN for the guest molecule, quite a few synthesized adamantine-modified primary antibodies (ADA-Ab1) were immobilized on the CD-GN by supramolecular host-guest interaction between CD and ADA. Cu@Ag-CD as a signal tag could be captured by ADA-modified secondary antibody (ADA-Ab2) through a host-guest interaction, leading to a large loading of Cu@Ag nanoparticles with high electrical conductivity and catalytic activity. The fabricated immunosensor exhibits excellent analytical performance for the measurement of CEA with wide range linear (0.0001-20 ng/mL), low detection limit (20 fg/mL), good sensitivity, reproducibility and stability, which provide an enormous application prospect in clinical diagnostics.

Quenched electrochemiluminescence of Ag nanoparticles functionalized g-C3N4 by ferrocene for highly sensitive immunosensing.

In this paper, a novel sandwich electrochemiluminescence (ECL) immunosensor was constructed by ferrocene for quenching Ag nanoparticles functionalized g-C3N4 (Ag@g-C3N4) emission. The prepared Ag@g-C3N4 had strong and stable ECL signals compared to pure g-C3N4 and primary antibody (Ab1) can be immobilized on Ag@g-C3N4 by adsorption of Ag nanoparticles. Ferrocene carboxylic acid (Fc-COOH) labeled secondary antibody was immobilized on Au doped mesoporous Al2O3 nanorods (Au@Al2O3-Fc-COOH@Ab2) as labels through adsorption ability of Au toward proteins. After a sandwich-type immunoreaction, a remarkable decrease of ECL signal was observed due to the ECL quenching of Ag@g-C3N4 by Au@Al2O3-Fc-COOH@Ab2. As a result, the change of ECL intensity has a direct relationship with the logarithm of CEA concentrations in the range of 1 pg mL(-1)-100 ng mL(-1) with a detection limit of 0.35 pg mL(-1) (S/N=3). Additionally, the proposed immunosensor shows high specificity, good reproducibility, and long-term stability.

A novel controlled release system-based homogeneous immunoassay protocol for SCCA using magnetic mesoporous Fe3O4 as a nanocontainer and aminated polystyrene microspheres as a molecular gate.

A novel controlled release system-based homogeneous immunoassay protocol (CRSHIP) was developed for the detection of squamous cell carcinoma antigen (SCCA). The positively charged aminated polystyrene microsphere (APSM) used as molecular gate was capped on the pores of the negatively charged antibody-conjugated magnetic mesoporous Fe3O4 (Ab-Fe3O4) by electrostatic interaction to achieve encapsulation of toluidine blue (TB). With the addition of SCCA, APSM was replaced from Ab-Fe3O4 due to the specific binding of antibody to antigen, and then the trapped TB was released from the pores. The released TB could be detected by square wave voltammetry and the signal could be correlated with the amount of SCCA. The developed CRSHIP displayed a high current response, a relatively wide linear range (from 0.001 to 4.0ng/mL) and a low detection limit (0.33pg/mL) for the detection of SCCA. The method showed high selectivity and good reproducibility. In addition, the CRSHIP provided a simple, fast and sensitive approach for the detection of SCCA in serum sample.

Simultaneous electrochemical detection of cervical cancer markers using reduced graphene oxide-tetraethylene pentamine as electrode materials and distinguishable redox probes as labels.

A novel, highly sensitive electrochemical immunoassay was proposed for the simultaneous determination of carcinoembryonic antigen (CEA) and squamous cell carcinoma antigen (SCCA) for the diagnosis of cervical cancer. Using an electrochemical analysis technique, two well-separated peaks were generated by neutral red and thionine, making the simultaneous detection of the two analytes on the electrode possible. Reduced graphene oxide-tetraethylene pentamine (rGO-TEPA), containing more amino groups, was of benefit to immobilize the primary antibody (Ab1) through an amidation reaction. Au@mesoporous carbon CMK-3 was synthesized and incubated with two secondary antibodies (Ab2) and different redox probes (neutral red and thionine) to fabricate the electrochemical immunosensor label intending to improve the analytical performance of the immunosensor. The immunosensor was prepared with a sandwich structure based on the peak current change of neutral red and thionine before and after the antigen-antibody reaction. The results showed that the immunosensor had a wide linear range, low detection limit, good reproducibility and stability. The method has been applied to the analysis of serum samples with satisfactory results.