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Sequence-specific DNA detection is a routine job in medical diagnostics and genetic screening. Alternative to a fluorescence readout scheme or electrophoresis approach, various kinds of rapid, low-cost, facile, and label-free methods have also been developed in last decades. Among these, direct electrical detection of DNA received increasing attention but more research is desirable. Particularly, enhancement with high discrimination must be employed to selectively amplify the responding signal. A chip-based biosensor was developed in this work to electrically detect 22-mer oligonucleotide DNA at low concentration, from 50 fM to 10 pM. First, a gold nanoparticle (NP) was capped with 3-mercaptopropionic acid through a thiol-gold bond. The derivatized carboxylic acid group showed strong complex interaction with an inorganic linker, [Zr.sup.4+]. As a result, [Zr.sup.4+] could link several hundreds of individual gold NPs together to form an aggregate of nanoparticles (ANP), which was capable of being used as a conductive tag for the electrical detection of DNA. Second, in order to achieve the discriminative localization of ANP to bridge two comb-shaped electrodes (with height of ~50 nm and interdistance of 300-350 nm) gapped with insulative material of silicon oxide, peptide nucleic acids were covalently bonded to the silicon oxide in the gap as capture sites for DNA. After hybridization with target DNA, the charged phosphate-containing backbone of DNA was introduced into the gap. Phosphate groups also exhibited strong complex interaction with the linker of [Zr.sup.4+] and could react with the residual [Zr.sup.4+] on the ANP surface. As a consequence, the conductive tags were linked to the phosphate groups and localized into the gap, which could modify the conductance between the two comb-shaped electrodes in turn. The degree of modification correlated directly to the amount of hybridized DNA and to the concentration of target DNA in sample solution. Compared with the individual NPs used as the tag, a strong enhancement from the gold ANP was obtained.

An ultrasensitive electrical detection method of nucleic acids has been developed on a nanogapped biosensor. In this study, peptide nucleic acid (PNA) probes were immobilized in the gaps of a pair of finger microelectrodes first and were then hybridized with their complementary target DNA. After that, pectin molecules were introduced into the DNA strand via zirconium-phosphate and zirconium-carbonate chemistries and were oxidated by periodate in acetate buffer (pH 3.98). The newly produced aldedyde groups act as a reactant to reduce ammoniacal silver ion to produce silver nanoparticles, which bridged the gap of the interdigitated microelectrode. The conductance of the metallic nanoparticles correlated directly with the amount of the hybridized DNA. A much higher sensitivity was achieved at 3 femtomolar (S/N > 3) under optimal conditions. This biosensor is also applicable to the direct detection of RNA.

Arrays of highly ordered n-type silicon nanowires (SiNW) are fabricated using complementary metal-oxide semi-conductor (CMOS) compatible technology, and their applications in biosensors are investigated. Peptide nucleic acid (PNA) capture probe-functionalized SiNW arrays show a concentration-dependent resistance change upon hybridization to complementary target DNA that is linear over a large dynamic range with a detection limit of 10 fM. As with other SiNW biosensing devices, the sensing mechanism can be understood in terms of the change in charge density at the SiNW surface after hybridization, the so-called 'field effect'. The SiNW array biosensor discriminates satisfactorily against mismatched target DNA. It is also able to monitor directly the DNA hybridization event in situ and in real time. The SiNW array biosensor described here is ultrasensitive, non-radioactive, and more importantly, label-free, and is of particular importance to the development of gene expression profiling tools and point-of-care applications.

Cobalt-mediated radical polymerization is noted for its great level of control over the polymerization of acrylic and vinyl esters monomers, even at high molar mass. Vitamin B 12 , a natural bionic enzyme cobalt complex, involves the conversion of organic halides to olefins through chain-growth polymerization. In this work, the notion of R-Co(III) free radical persistent free radical effect and vitamin B 12 circulation were first reported for the perception of ultralow abundance of microRNA-21, a lung cancer biomarker. Indeed, most Co-containing catalytic reactions can occur under mild conditions due to their minimal bond dissociation of the C-Co bond, with blue light irradiation. Based on the intrinsic stability of the vitamin B 12 framework and recycling of the catalyst, it is evident that this natural catalytic scheme has potential applications in medicinal chemistry and biomaterials. In addition, this strategy, combined with highly specific recognition probes and vitamin B 12 circulation-mediated chain-growth polymerization, has a detection limit as low as 910 aM. Furthermore, it is sensitive for sensing in serum samples containing biomarkers and shows great potential for RNA selection and amplification sensing in clinical samples.

Sensitive and selective sensing of biological molecules is fundamental to disease diagnosis and infectious disease surveillance. Herein, an ultrasensitive and highly selective electrochemical DNA biosensor is described by exploiting the electrochemically controlled reversible addition-fragmentation chain-transfer (eRAFT) polymerization as a signal amplification strategy and the peptide nucleic acid (PNA) probes as the recognition elements. Specifically, the PNA probes with a thiol at their 5'-terminals are anchored to a gold electrode surface (via gold-sulfur self-assembly) for sequence-specific recognition of target DNA (tDNA) fragments, of which the phosphate sites serve as the anchorages for the targeted labeling (via the well-established phosphate-Zr 4+ -carboxylate chemistry) of the carboxyl-group-containing chain-transfer agents (CTAs) for the succedent eRAFT polymerization, wherein the initiating radicals are generated through electrochemical reduction of aryl diazonium salts under a potentiostatic condition. In the presence of ferrocenylmethyl methacrylate (FcCH═CH 2 ) as the monomer, the grafting of polymer chains from the CTA-anchored sites as a result of the eRAFT polymerization brings numerous electroactive Fc tags to the electrode surface, outputting a high electrochemical sensing signal even in the presence of trace amounts of tDNA fragments. Under the optimized conditions, the linear range of the described electrochemical DNA biosensor spans from 10 aM to 10 pM ( R 2 = 0.998), with an attomolar detection limit (4.1 aM) being achieved. Moreover, the described electrochemical DNA biosensor is highly selective and applicable to the sensing of tDNA fragments in complex serum samples. Given its high efficiency, easy operation, and low cost, this biosensor shows great promise in real applications.

The development of convenient and efficient strategies without involving any complex nanomaterials or enzymes for signal amplification is of great importance in bioanalytical applications. In this work, we report the use of electrochemically mediated surface-initiated atom transfer radical polymerization (SI-eATRP) as a novel amplification strategy based on the de novo growth of polymers (dnGOPs) for the electrochemical detection of DNA. Specifically, the capture of target DNA (tDNA) by the immobilized peptide nucleic acid (PNA) probes provides a high density of phosphate groups for the subsequent attachment of ATRP initiators onto the electrode surface by means of the phosphate-Zr 4+ -carboxylate chemistry, followed by the de novo growth of electroactive polymer via the SI-eATRP. De novo growth of long polymeric chains enables the labeling of numerous electroactive probes, which in turn greatly improves the electrochemical response. Moreover, it circumvents the slow kinetics and poor coupling efficiency encountered when nanomaterials or preformed polymers are used and features sufficient flexibility and simplicity in controlling the degree of signal amplification. Under optimal conditions, it allows a highly sensitive and selective detection of tDNA within a broad linear range from 0.1 fM to 0.1 nM (R 2 = 0.996), with the detection limit down to 0.072 fM. Compared with the unamplified method, more than 1.2 × 10 6 -fold sensitivity improvement in DNA detection can be achieved. By virtue of its simplicity, high efficiency, and cost-effectiveness, the proposed dnGOPs-based signal amplification strategy holds great potential in bioanalytical applications for the sensitive detection of biological molecules.

A nanogapped microelectrode-based biosensor array is fabricated for ultrasensitive electrical detection of microRNAs (miRNAs). After peptide nucleic acid (PNA) capture probes were immobilized in nanogaps of a pair of interdigitated microelectrodes and hybridization was performed with their complementary target miRNA, the deposition of conducting polymer nanowires, polyaniline (PAn) nanowires, is carried out by an enzymatically catalyzed method, where the electrostatic interaction between anionic phosphate groups in miRNA and cationic aniline molecules is exploited to guide the formation of the PAn nanowires onto the hybridized target miRNA. The conductance of the deposited PAn nanowires correlates directly to the amount of the hybridized miRNA. Under optimized conditions, the target miRNA can be quantified in a range from 10 fM to 20 pM with a detection limit of 5.0 fM. The biosensor array is applied to the direct detection of miRNA in total RNA extracted from cancer cell lines.