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Detection of specific DNA sequences in clinical samples is a key goal of studies on DNA biosensors and gene chips. Herein we present a highly sensitive electrochemical genosensor for direct measurements of specific DNA sequences in undiluted and untreated human serum and urine samples. Such genosensing relies on a new ternary interface involving hexanedithiol (HDT) co-immobilized with the thiolated capture probe (SHCP) on gold surfaces, followed by the incorporation of 6-mercapto-1-hexanol (MCH) as diluent. The performance of ternary monolayers prepared with linear dithiols of different lengths was systematically examined, compared and characterized by cyclic voltammetry and electrochemical impedance spectroscopy, with HDT exhibiting the most favorable analytical performance. The new SHCP/HDT+MCH monolayer led to a 80-fold improvement in the signal-to-noise ratio (S/N) for 1 nM target DNA in undiluted human serum over the common SHCP/MCH binary alkanethiol interface, and allowed the direct quantification of the target DNA down to 7 pM (28 amol) and 17 pM (68 amol) in undiluted/ untreated serum and urine, respectively. It also displayed attractive antifouling properties, as indicated from the favorable S/N obtained after a prolonged exposure (24 h) to untreated biological matrices. These attractive features of the SHCP/HDT+MCH sensor interface indicate considerable promise for a wide range of clinical applications.

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The ability of proteins to catalyze hydrogen evolution has been known for more than 80 years, but the poorly developed d.c. polarographic "pre-sodium wave" was of little analytical use. Recently, we have shown that by using constant current chronopotentiometric stripping analysis, proteins produce a well-developed peak H at hanging mercury drop and solid amalgam electrodes. Peak H sensitively reflects changes in protein structures due to protein denaturation, single amino acid exchange, etc. at the picomole level. Unmodified DNA and RNA do not yield such a peak, but they produce electrocatalytic voltammetric signals after modification with osmium tetroxide complexes with nitrogen ligands [Os(VIII)L], binding covalently to pyrimidine bases in nucleic acids. Recently, it has been shown that six-valent [Os(VI)L] complexes bind to 1,2-diols in polysaccharides and oligosaccharides, producing voltammetric responses similar to those of DNA-Os(VIII)L adducts. Electrocatalytic peaks produced by Os-modified nucleic acids, proteins (reaction with tryptophan residues) and carbohydrates are due to the catalytic hydrogen evolution, allowing determination of oligomers at the picomolar level.

DNA methylation, i.e., addition of methyl group to 5′-carbon of cytosine residues in CpG dinucleotides, is an important epigenetic modification regulating gene expression, and thus implied in many cellular processes. Deregulation of DNA methylation is strongly associated with onset of various diseases, including cancer. Here, we review how DNA methylation affects carcinogenesis process and give examples of solid tumors where aberrant DNA methylation is often present. We explain principles of methods developed for DNA methylation analysis at both single gene and whole genome level, based on (i) sodium bisulfite conversion, (ii) methylation-sensitive restriction enzymes, and (iii) interactions of 5-methylcytosine (5mC) with methyl-binding proteins or antibodies against 5mC. In addition to standard methods, we describe recent advances in next generation sequencing technologies applied to DNA methylation analysis, as well as in development of biosensors that represent their cheaper and faster alternatives. Most importantly, we highlight not only advantages, but also disadvantages and challenges of each method.

We show, for the first time, that DNA produces an electrocatalytic square wave voltammetric reduction peak or chronopotentiometric peak HDNA at mercury and solid amalgam electrodes. These peaks are attributed to the catalytic hydrogen evolution reaction, allowing label‐free detection of chromosomal DNA and oligodeoxynucleotides (ODNs) below parts per million and nanomolar levels, respectively. By using peak HDNA at high current densities, native chromosomal DNA was distinguished from its denatured form. With this method, damage of DNA from human cancer cells by ionizing radiation or sonication was detected. Sensitivity of the DNA determination was greatly enhanced in the presence of [Co(NH3)6]3+. Chromosomal DNA and ODNs can be detected at concentrations 2–3 orders lower than any earlier voltammetric or chronopotentiometric label‐free methods. In the presence of 4 mM [Co(NH3)6]3+, DNA from human cancer cells was easily detected at 5 ng/mL, that is, below the concentration level of cell‐free DNA in body fluids.

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