Biomedical Engineering Reference
In-Depth Information
4.3.2 Graphene-BasedPlatforms
Graphene is a one-atom-thick layer of graphite with a honey-comb lattice
structure. Since its successful isolation in 2004 [108], numerous studies
have been conducted to explore and understand its superior electrical [109],
electrochemical [110], optical [111] and mechanical [112] properties. Even
though the earliest samples were produced by micromechanical cleavage,
recent ef orts have been focused on the production of large-scale amounts
of graphene. So far three main approaches have been used to this end: direct
growth of graphene on metallic surfaces by chemical vapor deposition
method (CVD) [113]; direct exfoliation of graphite by using dif erent sol-
vents [114]; reduction of the graphene oxide obtained by oxidation and fur-
ther exfoliation of graphite, thus obtaining the so-called chemically modii ed
graphenes (CMGs) [115]. While with the i rst two protocols a higher percent-
age of monolayers of pristine graphene are present in the product, the third
protocol is highly recommended for the production of bulk amounts. In this
case, the presence of a few layers of structure in the i nal product is dominant.
Graphene-based nanomaterials exhibit advantages over other electrode
materials in terms of high 2D electrical conductivity, very fast heteroge-
neous electron transfer, and high surface area [110, 116, 117]. For these
reasons a large number of research reports have recently emerged, suggest-
ing the importance of utilizing graphene for electrochemical sensing and
biosensing [17, 118].
In the impedimetric detection of DNA sequences, linear or hairpin-
shaped DNA probes have been immobilized on the graphene surface either
by physical adsorption [18, 119, 120] or by chemical binding [121, 122]. In
a typical example of physical adsorption, Bonanni and Pumera [18] used
hairpin DNA sequences as DNA probes to be immobilized onto graphene
platforms consisting of dif erent numbers of same-sized graphene layers.
h e π-stacking interactions between the nucleobases and the hexagonal
cells of graphene make the platform a stable substrate for genosensing.
When the hybridization with a complementary target takes place, a par-
tial release of the hpDNA probes from the graphene surface occurs, as
also coni rmed by dif erent authors [123]. h is is due to the formation of
stable hydrogen bonds among the nucleobases and their shielding inside
the phosphate backbone at er hybridization. h e above-mentioned release
generates a signii cant decrease in the charge transfer resistance value.
h is decrease is less signii cant when hybridization is performed with a
sequence containing one mismatch, and is almost negligible in the case of
a non-complementary sequence (see Figure 4.5).
With this label-free protocol the presence of a single nucleotide poly-
morphism correlated to the development of Alzheimer's disease was
