ReS2@Au NPs as signal labels quenching steady photocurrent generated by NiCo2O4/CdIn2S4/In2S3 heterojunction for sensitive detection of CYFRA 21-1.

In order to achieve rapid and sensitive detection of CYFRA 21-1, a signal-off photoelectrochemical (PEC) immunosensor was devised with NiCo2O4/CdIn2S4/In2S3 heterojunction photoactive materials as sensing platform and ReS2@Au NPs as the secondary antibody labels amplifying signal based on the energy band-matching cascade structure and double suppression effect. NiCo2O4 possessed a faster charge transfer rate due to the abundance of redox electron pairs (Co3+/Co2+ and Ni3+/Ni2+). To further improve the PEC properties of NiCo2O4 under visible light, CdIn2S4 with matching bandgap energy was selected to form heterojunction with NiCo2O4 and sensitized with In2S3. The proposed heterojunctions with well-matched band structure promoted the transfer of photo-generated carriers and were exploited as signal transducers for immobilization of antibodies and recognition of CYFRA 21-1. Furthermore, a novel urchin-like p-type ReS2 semiconductor nanostructure functionalized by Au NPs was firstly used as a nanolabel to quench the signal. On the one hand, the Schottky heterojunction generated by ReS2 and Au NPs could compete with the transducer substrate for both light and electron donors. On the other hand, the large space steric hindrance of ReS2 prevented contact between the matrix and AA. Subsequently, the sensor was sensitive in a wide range of concentrations for CYFRA 21-1 (0.0001-50 ng/mL), and the detection limit was 0.05 pg/mL.

Fabrication of a novel electrochemical immunosensor for the sensitive detection of carcinoembryonic antigen using a double signal attenuation strategy.

Carcinoembryonic antigen (CEA), an acidic protein, is a characteristic antigen produced by the tumor of various cancers (eg, breast, cervical, rectal, lung, etc.). Therefore, the detection of cancer antigens is very important for the early diagnosis and treatment of cancer. In this study, a novel of "signal off" strategy for electrochemical immunosensor was developed to detect CEA. To this end, Prussian blue nanoparticles (PB NPs), an electroactive substance, were used as the immunological platform. In addition, CuO2@SiO2 nanocomposites, which release Cu2+ and H2O2 under acidic conditions, were synthesized. The generated Cu2+ can replace the high spin iron (FeIII) in PB NPs, which in turn reduces the oxidation peak current of PB NPs. Due to the peroxidase-like nature of PB NPs, they can react with self-generated H2O2 to generate hydroxyl radicals (·OH), which can further convert 4-chloro-1 naphthol (4-CN) into a non-conductive polymer that accumulates on the electrode surface, this leads to a further reduction in the electrical signal of the PB NPs. Moreover, the self-generated Cu2+ and H2O2 can reduce the introduction of exogenous substances and improve the detection accuracy. Square wave voltammetry (SWV) revealed that the electrical signal of PB NPs gradually decreased with increasing CEA concentration. In addition, the electrical signal of PB NPs exhibited a good linearity in the range from 0.01 pg mL-1 to 80 ng mL-1, where in the logarithm of CEA concentration and the detection limit was as low as 0.0032 pg mL-1.

Dual-modal label-free genosensor based on hemoglobin@gold nanocluster stabilized graphene nanosheets for the electrochemical detection of BCR/ABL fusion gene.

For the first time, we have successfully synthesized stable graphene nanosheets from graphite powder through sonication in the hemoglobin-capped gold nanoclusters (Hb@AuNCs) solution for biosensing application. This approach, as a simple method for the exfoliation and fragmentation of graphite in a nanocluster solution, enabled us to produce stable aqueous graphene dispersions at low cost and without the need for hazardous chemicals or tedious experimental procedures. In this method, Hb@AuNCs were used not only as stabilizing agent of graphene through non-covalent bonding, but also as dispersing agent of few-layer graphene nanosheets. The Hb@AuNCs stabilized graphene (Hb@AuNCs-G) was characterized by high resolution transmission electron microscopy (HRTEM), zeta-sizer and Raman spectroscopy. Then, the graphene nanosheets were applied as a novel versatile electrochemical platform for ultrasensitive biosensing of short DNA species of chronic myelogenous leukemia (CML) based on the "signal off" and "signal on" strategies. For this purpose, a single strand DNA (ssDNA) was immobilized on the Hb@AuNCs-G/AuNPs modified electrode surface and acted as the biorecognition element. Methylene blue (MB), as the signaling probe, was then intercalated into the ssDNA. The intercalated MB was liberated upon interaction with the synthetic complementary DNA (cDNA, target), thereby resulting in the apparent reduction of MB redox signal. This designed "signal off" sensing system enabled the voltammetric determination of the target cDNA over a dynamic linear range (DLR) of 0.1 fM to 10 pM with a limit of detection (LOD) of 0.037 fM. In the "signal on" strategy, the response to the cDNA was detected by monitoring the change in the electron transfer resistance (Rct) using the ferro/ferricyanide system as a redox probe. The charge transfer resistance of the probe was found to increase linearly with increasing concentration of target cDNA in the range of 0.1 fM-10 pM with a limit of detection of 0.030 fM. Finally, the selectivity and feasibility of genosensor was evaluated by the analysis of derived nucleotides from mismatched sequences and the clinical samples of patients with leukemia as real samples, respectively.

Detection of microRNA using enzyme-assisted amplifying and DNA-templated silver nanoclusters signal-off fluorescence bioassay.

A Simple and fast analysis strategy of fluorescence quenching based on DNA-templated silver nanoclusters was developed for detection of miR-122 related to diseases such as human liver. We used Exo III to cleave the silver cluster template and assist in the DNA-RNA complex cycle. When the target is absent, the silver cluster template remains intact, and DNA-AgNCs are generated under the action of AgNO3/NaBH4, producing a strong background fluorescence signal. Once the target is added, the site of the Exo III occurs after a series of hybridization cycles, the Exo III acts, the template DNA is continuously hydrolyzed, and the fluorescence intensity of the system is significantly reduced. By comparing the changes in the fluorescence signal, we found that this strategy has good sensitivity and the detection limit is as low as 84.0 pM. The strategy also has excellent discriminating ability and good selectivity, it can provide a persuasive reference for the early diagnosis of liver cancer and hepatitis.

A novel signal-off photoelectrochemical biosensor for M.SssI MTase activity assay based on GQDs@ZIF-8 polyhedra as signal quencher.

DNA methylation catalyzed by M.SssI methyltransferases (MTase) has important roles in gene expression and other cellular activities, and relates to some diseases, especially cancers. Therefore, it is important to develop a sensitive sensing platform for M.SssI MTase activity assay. Here, taking zeolitic imidazolate framework-8 (ZIF-8) polyhedra as the carriers of graphene quantum dots (GQDs), GQDs-embedded ZIF-8 polyhedra (denoted as GQDs@ZIF-8 polyhedra) were successfully prepared and used as the multi-functional signal quencher to construct a novel signal-off photoelectrochemical (PEC) biosensor for M.SssI MTase activity assay. Firstly, the indium tin oxide (ITO) slice was modified with TiO2, poly(diallyldimethylammonium chloride) and CdTe quantum dots (QDs). The obtained electrode was used as the photoelectrode and labeled as ITO/TiO2/CdTe QDs. Then, single-stranded DNA (S1) was anchored on the photoelectrode surface via S-Cd bond. After hybridization between S1 and biotinylated single-stranded DNA (S2), the streptavidin (SA)-labeled GQDs@ZIF-8 polyhedra were introduced to the modified electrode via the specific reaction between biotin and SA. As the signal quencher, GQDs@ZIF-8 polyhedra could not only inhibit the photocurrent signal of the ITO/TiO2/CdTe QDs electrode due to the steric hindrance effect, but also act as peroxidase mimetics to catalyze precipitation reaction of 4-chloro-1-naphthol, resulting in the evident depression of the photocurrent signal. For the specially designed S1/S2 double-strand DNA, the decreased photocurrent was quantitatively correlated with the M.SssI MTase activity (linear response range, 0.005-150 U mL-1; detection limit, 0.004 U mL-1). The developed GQDs@ZIF-8 polyhedra and related PEC biosensor may have potential applications in clinical research and disease diagnosis.

L-Cysteine modified gold nanoparticles for tube-based fluorometric determination of mercury(II) ions.

A fluorescent probe is described for detection of mercury(II) ion by using L-cysteine-modified gold nanoparticles (Cys-AuNP). These were fabricated by a tube-based redox reaction where Cys acts as both the reducing reagent and capping ligand. The Cys-AuNP display red fluorescence, with excitation/emission peaks at 373/625 nm. Owing to the high-affinity of the Hg(II)-Au(I) interaction and the Hg(II)/carboxy or amino group interaction, the presence of Hg(II) cause selective quenching the fluorescence, while other metal ions do not give such an effect. Based on these findings, a method was designed for the determination of Hg(II) that has attractive figures of merit. These include a low limit of detection (1.3 nM), a wide detection range (from 2 nM to 30µM), and excellent specificity. The method was applied to Hg(II) screening in (spiked) tap and river water, and it gave satisfactory results. Graphical abstract Schematic representation of the application of L-cysteine modified gold nanoparticles (Cys-AuNP) for qualitative and quantitative detection of mercury(II) ions. Based on the interaction between Cys-AuNP and mercury(II) ion to quench the red fluorescence of Cys-AuNP, the target mercury(II) can in turn be determined by a fluorometric method.

Ionic liquid and spatially confined gold nanoparticles enhanced photoelectrochemical response of zinc-metal organic frameworks and immunosensing squamous cell carcinoma antigen.

Metal-organic framework nanocrystal (Zn-MOF) was synthesized by using 3,3'-{(propane-1,3-diyl)bis[1-(4-carboxybenzyl)-1H-imidazol-3-ium]} hexafluorophosphate ionic liquid as the functional monomer and Zn2+ as the central metal ion under hydrothermal conditions. Spatially confined gold nanoparticles (Au-NP) were prepared by in-situ reduction of chloroauric acid in the nanopores of Zn-MOF using acetic acid as the reducing agent to fabricate Au-NP@Zn-MOF nanocomposites. Au-NPs@Zn-MOF was further functionalized with 1H-imidazolium-1,3-bis(2-aminoethyl)bromide ionic liquid (IBABr) to prepare IBABr-Au@Zn-MOF nanocomposites. All abovementioned nanomaterials were thoroughly characterized by TEM, SEM, XPS, FTIR, and nitrogen-adsorption surface area analysis. IBABr-Au@Zn-MOFnanocomposites were then deposited onto a glassy carbon electrode and used as the photoactive element to fabricate a label-free photoelectrochemical (PEC) immunosensor by immobilizing anti-squamous cell carcinoma antigen (anti-SCCA). The PEC sensing principle is based on the photocurrent decline due to the blocking effect of SCCA on the electron and mass transfer after binding SCCA to anti-SCCA. The photocurrent variation related to the specific recognition of SCCA shows a linear relationship to the logarithm of SCCA concentration in the range of 5.0 pg mL-1 to 15.0 ng mL-1. The detection limit is as low as 2.34 pg mL-1. Such a signal-off PEC immunosensor is highly selective, sensitive, stable, and reproducible towards SCCA detection. Its performance is comparable to enzyme-linked immunosorbent assay from the studies on clinical samples. This immunosensor is promising for the label-free determining SCCA in clinical human serum samples.

A signal-off type photoelectrochemical immunosensor for the ultrasensitive detection of procalcitonin: Ru(bpy)32+ and Bi2S3 co-sensitized ZnTiO3/TiO2 polyhedra as matrix and dual inhibition by SiO2/PDA-Au.

A signal-off type photoelectrochemical (PEC) immunosensor based on Ru(bpy)32+ and Bi2S3 co-sensitized ZnTiO3/TiO2 polyhedra as matrix and SiO2/PDA-Au as the label of second antibody was proposed for the ultrasensitive detection of procalcitonin (PCT). The hollow ZnTiO3/TiO2 polyhedra called zinc titanium composite hollow structures (ZTCHS) was prepared by hydrothermal method to enhance the utilization of visible light comparing with pure ZnTiO3 or TiO2. The unique structure endowed large specific surface areas and satisfying load capacity to ZTCHS. Ru(bpy)32+ and Bi2S3 were utilized to co-sensitize ZTCHS to further strengthen the PEC performance of ZTCHS. It was interesting that the dual-inhibition effects of SiO2/PDA-Au further enhanced the sensitivity of proposed PEC immunosensor. On the one hand, the steric hindrance of SiO2/PDA effectively restricted electron transfer. On the other hand, Au nanoparticles could absorb visible light to compete with the matrix material. Based on the above aspects, the proposed signal-off type PEC immunosensor possessed a wide linear range from 0.0001 ng mL-1 to 100 ng mL-1 and low detection limit (about 0.03 pg mL-1, S/N = 3), demonstrating the promising application of PEC immunosensor in analysis fields.

An immunosensing device based on inhibition of mediator's faradaic process for early diagnosis of prostate cancer using bifunctional nanoplatform reinforced by carbon nanotube.

In this study, a simple and lable-free voltammetric immunosensor was successfully developed for the ultrasensitive detection of prostate specific antigen (PSA). To do this, multiwalled carbon nanotube (MWCNT)/L-histidine functionalized reduced graphene oxide (His-rGO) was demonstrated as a bifunctional nanoplatform for covalently attaching thionine redox indicator and anti-PSA antibody (Ab). The MWCNT enhanced electrical conductivity and facilitated the electron transfer between thionine and the glassy carbon electrode. While, the presence of anti-PSA antibody blocked the electron transfer of thionine and decreased redox signals. The principle response of proposed immunosensor was based on the selective interaction of PSA with thionine-NH2-GO-COOH-Ab. This selective interaction led to further decrease of response current of attached electrochemical probe. The liner calibration curve for tumor marker determination was 10 fg mL-1-20 ng mL-1 (R2 = 0.996). Under optimized conditions, the immunosensor was able to selectively detect PSA with a limit of detection (LOD) of 2.8 fg mL-1 at 3σ. The relative standard deviations (RSDs) for single-electrode repeatability and electrode-to-electrode reproducibility were less than 2.9% and 5.7% (n = 5), respectively. Furthermore, the as-proposed immunosensor showed excellent performance in detection of PSA in the human serum and saliva samples, which implies that the current strategy has a promising feature for the clinical assessment of tumor marker status in patients with prostate cancer.

A highly sensitive VEGF165 photoelectrochemical biosensor fabricated by assembly of aptamer bridged DNA networks.

We developed a novel "signal-off" photoelectrochemical (PEC) aptasensor based on an aptamer bridged DNA network structure for the sensitive detection of vascular endothelial growth factor (VEGF165), using g-C3N4 as photoactive material. The DNA network provides an excellent platform for the immobilization of methylene blue (MB), which can facilitate the electron transport through the DNA helix structure and suppress the recombination of electron-hole pairs generated by g-C3N4. In the presence of the target VEGF165, the DNA network can be destroyed adequately by the recognition between VEGF165 and the aptamer, resulting in the release of MB. Therefore, the originally enhanced electron transfer process could be inhibited, leading to a remarkable decrease of the photocurrent. A good linear relationship between the PEC signal and the logarithm of VEGF165 concentration over the range of 100fM to 10nM with a detection limit of 30 fM can be obtained. Our concept can be easily extended to develop aptasensors for the sensitive detection of different targets by triggering the release of the payloads from their corresponding aptamer bridged DNA networks.

CdTe amplification nanoplatforms capped with thioglycolic acid for electrochemical aptasensing of ultra-traces of ATP.

A "signal off" voltammetric aptasensor was developed for the sensitive and selective detection of ultra-low levels of adenosine triphosphate (ATP). For this purpose, a new strategy based on the principle of recognition-induced switching of aptamers from DNA/DNA duplex to DNA/target complex was designed using thioglycolic acid (TGA)-capped CdTe quantum dots (QDs) as the signal amplifying nano-platforms. Owing to the small size, high surface-to-volume ratio and good conductivity, quantum dots were immobilized on the electrode surface for signal amplification. In this work, methylene blue (MB) adsorbed to DNA was used as a sensitive redox reporter. The intensity of voltammetric signal of MB was found to decrease linearly upon ATP addition over a concentration range of 0.1nM to 1.6µM with a correlation coefficient of 0.9924. Under optimized conditions, the aptasensor was able to selectively detect ATP with a limit of detection of 45pM at 3σ. The results also demonstrated that the QDs-based amplification strategy could be feasible for ATP assay and presented a potential universal method for other small biomolecular aptasensors.

Single electrode biosensor for simultaneous determination of interferon gamma and lysozyme.

Simultaneous detection of multiple biomarkers holds great promise for acute leukemia evaluation. Here, a novel biosensor is developed for simultaneous electrochemical detection of interferon gamma (IFN-γ) and lysozyme (Lys) based on aptamer recognition by coupling "signal-on" and "signal-off" modes. On one Au electrode, two kinds of signaling probes labeled by the thiolated ferrocene (Fc)- and methy blue (MB)- were designed to hybridize with IFN-γ and Lys aptamers respectively to form partial complementary DNA duplexes. In the presence of IFN-γ and Lys, the target-aptamer interaction led to the release of aptamer from duplex DNA structure. The single-stranded signaling probes thus suffered from the conformation changes, which resulted in the decreased (or increased) oxidation peak current of Fc (or MB) according to the "signal-off (or signal-on)" mode. Electrodes were characterized using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Under the optimized conditions, the signal changes were quantified using square wave voltammetry (SWV). This proposed biosensor for IFN-γ and Lys possessed linear detection range from 0.01 to 10 nM and 0.1 to 100 nM, with the detection limits of 1.14×10(-3) nM and 0.0164 nM, respectively. Moreover, this biosensor was readily regenerated and proved successful toward the practical analysis. The proposed strategy could provide more integrated and reliable information for acute leukemia evaluation.