We developed a novel electrochemical sensor for Hg2+ detection using two mercury-specific oligonucleotide probes and streptavidin-horseradish peroxidase (HRP) enzymatic signal amplification. The two mercury-specific oligonucleotide probes comprised a thiolated capture probe and a biotinated signal probe. The thiolated capture probe was immobilized on a gold electrode. In the presence of Hg2+, the thymine–Hg2+–thymine (T–Hg2+–T) interaction between the mismatched T–T base pairs directed the biotinated signal probe hybridizing to the capture probe and yielded a biotin-functioned electrode surface. HRP was then immobilized on the biotin-modified substrate via biotin–streptavidin interaction. The immobilized HRP catalyzed the oxidation of hydroquinone (H2Q) to benzoquinone (BQ) by hydrogen peroxide (H2O2) and the generated BQ was further electrochemically reduced at the modified gold electrode, producing a readout signal for quantitative detection of Hg2+. The results showed that the enzyme-amplified electrochemical sensor system was highly sensitive to Hg2+ in the concentration of 0.5 nM to 1 μM with a detection limit of 0.3 nM, and it also demonstrated excellent selectivity against other interferential metal ions.

A novel scheme for scanning electrochemical microscopy (SECM) assay of DNA based on hairpin probe and enzymatic amplification biosensor was described. In this method, streptavidin–horseradish peroxidase (HRP) was captured by double-stranded DNA (ds-DNA) modified gold substrate via biotin–streptavidin interaction after hybridization of target DNA to the immobilized hairpin probe functioned with a biotin at its 3′ end. In the presence of H2O2, hydroquinone (H2Q) was oxidized to benzoquinone (BQ) at the modified substrate surface through the HRP catalytic reaction, and the generated BQ corresponding to the amount of target DNA was reduced in solution by a SECM tip. The resulting reduction current allowed concentration detection of target DNA and SECM imaging of hybridization between the target DNA and the immobilized hairpin probe. The detection limit of this method was as low as 17 pM for complementary target DNA and it had good selectivity to discriminate between the complementary sequence and one containing base mismatches.

A convenient microwave plasma treatment method with ammonia precursor was proposed to enhance the solubility of carbon nanotubes (CNTs). The SEM, XRD and FTIR spectra clearly demonstrated that the carbon skeleton structure of the resultant ammonia plasma-treated CNTs (ammonia PT-CNTs) was not destroyed and amine groups of different forms were successfully coupled to CNTs in the MWP treatment process. The ammonia PT-CNTs have excellent solubility in water and are insoluble in nonpolar tetrahydrofuran, and the cyclic voltammograms suggest that the enhanced wetting properties clearly favor faster electron transfer kinetics on the ammonia PT-CNT electrodes. By choosing glucose oxidase as a model enzyme, the application of the ammonia PT-CNTs in construction of biosensors was further investigated. Due to the biocompatibility and electron transfer capability of the ammonia PT-CNTs, the resultant GOD biosensor displayed a good sensing performance. The biosensor has a fast response of less than 10 s, and the response current linearly increases with the glucose concentration in the range of 1.2 × 10−4 to 7.5 × 10−3 M with a detection limit of 1.0 × 10−5 M.

A novel platinum nanoparticle-modified carbon fiber ultramicroelectrode (Pt/CFUME) is proposed for the construction of a mediator-free amperometric ultramicroelectrode biosensor. The platinum nanoparticles electrodeposited on the ultramicroelectrodes greatly increase the surface areas with enhanced electron transfer characteristics between the electroactive centers of enzymes and the ultramicroelectrodes. By using horseradish peroxidase (HRP) as an enzyme model, we construct a mediator-free amperometric ultramicroelectrode HRP sensor for detecting H2O2. The resultant biosensor well displays the direct electrochemical behavior of HRP on the Pt/CFUME and the electrocatalytic response to the reduction of H2O2 without the aid of electron mediators. The steady-state response current is linearly related to the H2O2 concentration in the range of 0.64 μM–3.6 mM with a detection limit of 0.35 μM (S/N = 3) and a correlation coefficient of 0.9953.

An ultrasensitive piezoelectric immunosensor using an amplification path based on an insoluble biocatalyzed precipitation product has proposed for Schistosoma japonicum. A mercapto Schistosoma japonicum antigen was self-assembled onto the quartz crystal surface via an Au nanoparticle mediator monolayer to sense the Schistosoma japonicum antibody (SjAb). And the horseradish peroxidase labeled protein A conjugate which was bounded to the SjAb by a “sandwich” format was used as a biocatalyst for the oxidative precipitation of 4-chloro-1-naphthol by H2O2 to yield the insoluble product benzo-4-chlorohexadienone, resulting in an amplified mass sensing of antigen–antibody interaction. The amount of the precipitate accumulated on the quartz crystal is controlled by the antibody concentration. The SjAb can be linearly determined in the range of 10–200 ng ml−1 and the detection limit reaches as low as 5 ng ml−1.