Arrays of highly ordered n-type silicon nanowires (SiNW) are fabricated using complementary metal-oxide semiconductor (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.

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-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 = 0.996), with the detection limit down to 0.072 fM. Compared with the unamplified method, more than 1.2 × 10-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.

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Phosphorylation of proteins catalyzed by protein kinases (PKs) is essential to many biological processes; the sensitive detection of PK activity and the screening of PK inhibitors are thus integral to disease diagnosis and drug discovery. Herein, a highly sensitive biosensor has been fabricated for the electrochemical detection of PK activity by exploiting the electrochemically controlled reversible addition−fragmentation chain transfer (eRAFT) polymerization as a novel amplification strategy. The fabrication of the eRAFT-polymerization-based electrochemical biosensor involves (1) the immobilization of substrate peptides onto a gold electrode by way of gold− sulfur self-assembly, (2) the site-specific phosphorylation of substrate peptides by PKs, (3) the anchoring of carboxyl-group-containing chain transfer agents (CTAs) to the phosphorylated sites, and (4) the eRAFT polymerization under a potentiostatic condition, using ferrocenylmethyl methacrylate (FcMMA) as the monomer. Through the eRAFT polymerization, long polymer chains containing numerous electroactive Fc tags can be de novo grafted from each phosphorylated site, resulting in significant amplification of the electrochemical detection signal. The asfabricated biosensor is highly selective and features a very low detection limit of 1.02 mU mL −1 , in the presence of adenosine 3′,5′-cyclic monophosphate (cAMP)-dependent PK (PKA) as the model target. Results also demonstrate that it can be applied to the screening of PK inhibitors and the detection of PK activity in complex serum samples and cell lysates. Moreover, it holds the merits of easy fabrication, high efficiency, and low cost, which make it a promising tool for the detection of PK activity and the screening of potential PK inhibitors.

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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(4+). As a result, Zr(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 approximately 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(4+) and could react with the residual Zr(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.