Nanobiosensors - Nanomaterials for Medical Applications - page 133

FIGURE 4.3 Schematic diagram of AuNP with ssDNA attached through charge-charge interac-
tion through the negative phosphate backbone of the DNA. (For color version of this figure, the
reader is referred to the online version of this topic)
without analyte tagging such as SPR, imaging ellipsometry, and sandwich
assays using chemically functionalized AuNPs have been invented. 13,21,60,66
These approaches involve surface modification, require expensive transduction
instrument, and hybridization is separate from detection.
One of the most studied NMs for medical applications is gold. AuNPs are
very versatile because biomolecules can easily adsorb through electrostatic
interaction. Colloidal AuNPs prepared as gold citrate are stabilized by adsorbed
negative ions (e.g. citrate) that prevent the strong van der Waals attraction
between gold particles from aggregation. 59,67 These AuNPs have been used
to directly adsorb ssDNA to be used to create dsDNA ( Figure 4.3 ). ssDNA
has the bases exposed whereas dsDNA has stable double-helix geometry with
the negatively charged phosphate backbone exposed. Repulsion between the
charged phosphate backbone of dsDNA and the adsorbed citrate ions domi-
nates the electrostatic interaction between AuNPs and dsDNA so that dsDNA
will not adsorb. Under these conditions, the negative charge on the backbone
is sufficiently distant so that attractive van der Waals forces between the bases
and the AuNP are sufficient to cause ssDNA to stick to the gold. The same
mechanism is not operative with dsDNA because the duplex structure does not
have the bases readily exposed to AuNPs. Li and Rothberg 60 used adsorption
of ssDNA on colloidal AuNP solution to create a hybridization assay based
on color changes associated with gold aggregation. The assay took only 5 min
detecting as low as <100 fmol of target without instrumentation. It can be used
to detect single-base pair mismatches.
Titanium dioxide nanoparticles are accepted to be environmentally non-
threatening, chemically benign in physiological fluids, and possess good
mechanical strength. The titanate surface contains functional hydroxyl groups
making it hydrophilic. This titanate hydrophilic surface is expected to provide
an aqueous-like environment that facilitates the stabilization of the adsorbed
immobilized proteins on nanocrystalline TiO 2 which exhibited less denatur-
ation. 52,68 Titanate nanotubes (TNTs) have been used to develop a reagentless
electrochemical biosensor for lactate detection. 69 Their studies demonstrated
that the nanotubes formed a porous three-dimensional (3D) network that served
as the enzyme support matrix. The negatively charged groups that are present