Genosensors oxidation

Moreover, the unique adsorption properties of GEC allowed the very sensitive electrochemical detection of DNA based on its intrinsic oxidation signal that was shown to be strongly dependent of the multi-site attachment of DNA and the proximity of G residues to GEC [100]. The thick layer of DNA adsorbed on GEC was more accessible for hybridization than those in nylon membranes obtained with genosensors based on nylon/GEC with a changeable membrane [99,101,102]. Allhough GEC has a rough surface, it is impermeable, while nylon is more porous and permeable. DNA assays made on an impermeable support are less complex from a theoretical standpoint [7] the kinetics of the interactions are not compUcated by the diffusion of solvent and solutes into and out of pores or by multiple interactions that can occur once the DNA has entered a pore. This explained the lower hybridization time, the low nonspecific adsorplion and the low quantity of DNA adsorbed onto GEC compared to nylon membranes. 

Physical or electrochemical adsorption uses non-covalent forces to affix the nucleic acid to the solid support and represents a relatively simple mechanism for attachment that is easy to automate. Adsorption was favoured and described in some chapters as suitable immobilization technique when multisite attachment of DNA is needed to exploit the intrinsic DNA oxidation signal in hybridization reactions. Dendrimers such as polyamidoamine with a high density of terminal amino groups have been reported to increase the surface coverage of physically adsorbed DNA to the surface. Furthermore, electrochemical adsorption is described as a useful immobihzation strategy for electrochemical genosensor fabrication. 

Lucarelli et al. describe a disposable indicator-free screen-printed genosensor applied to the detection of apoE sequences in PCR samples [26]. The biosensor format involved the immobilization of an inosine-modified (guanine-free) probe onto a SPE transducer and the detection of the duplex formation in connection with the square-wave voltammetric measurement of the guanine oxidation peak of the target sequence. 

Intercalated doxorubicin has been also used as an electrochemical label in the detection of DNA hybridization events in a genosensor built by layer-by-layer covalent attachment of multiwalled carbon nanotubes and Au-NPs [46]. The oxidation peak current obtained by differential pulse voltammetry showed a linear relationship with the logarithm of the target DNA concentration in the range 5.0 x 10 to 1.0 x 10 M, with a detection limit of 6.2 pM. 

Amperometric or voltammetric biosensors typically rely on an enzyme system that catalyt-ically converts electrochemically non-active analytes into products that can be oxidized or reduced at a working electrode. Although these devices are the most commonly reported class of biosensors, they tend to have a small dynamic range due to saturation kinetics of the enzyme, and a large overpotential is required for oxidation of the analyte this may lead to oxidation of interfering compounds as well (e.g., ascorbate in the detection of hydrogen peroxide). In addition to the use in enzyme-based biosensors, amperometric transducers have also been used to measure enzyme-labelled tracers for affinity-based biosensor (mainly immunosensors and genosensors). Enzymes which are commonly used for this purpose include horseradish peroxidase (HRP) [17] and alkaline phosphatase (AP) [18,19,21]. 

Loaiza, O.A., Jubete, E., Ochoteco, E. et al. (2011) Gold coated ferric oxide nanoparticles based disposable magnetic genosensors for the detection of DNA hybridization processes. Biosens. Bioelectron., 26 (5), 2194-2200. 

---