Biomedical Engineering Reference
In-Depth Information
This demonstrates that single QDs can still be observed in nanobiosensor at
conditions such as those of a monolayer of molecules or atoms, with an ulti-
mate sensitivity limit of one QD per target molecule. Additionally, QDs are
available in a virtually unlimited number of well-separated colors that are all
excitable by a single wavelength. Aside from simplifying image acquisition,
this property could be used in confocal microscopy to perform nanometer reso-
lution with colocalization of multiple-color individual QDs.
23-25
In addition,
just like other NMs, QDs offer an increased surface area that is available for
bioapplications.
The application of NMs in biosensor development is strongly related
to their properties that depend on the mode of synthesis that is responsi-
ble for the quality of the resulting NPs and the postsynthesis modifications,
both chemical and biological. The preparation procedures for NMs, either
in colloidal solutions or grown on solid substrates, have been extensively
reviewed.
26
Along with synthetic advances for varying the size, shape, and
composition of nanostructured materials is the ability to tailor their bind-
ing affinities for various biomolecules through surface modification and
meticulous engineering.
Thus, NM-based biosensors take advantage of the optical and electronic
properties of NMs for a sensitive response to biomolecular binding events.
15,18
The sensor signal comes in the form of a change in the intensity or the peak
position of optical absorption, fluorescence emission, reflection, surface plas-
mon resonance (SPR), surface-enhanced Raman scattering (SERS), and electro-
chemical potential/current under various conditions leading to the development
of corresponding biosensors.
6,7,11,15,19,27,28
The relationship between protein adsorption to particle surfaces and
association of particles can be influenced by properties of the NMs, the cell
surfaces, and the environment. Binding of NPs to a cell surface can involve
adsorption of the proteins on the cell surface to the NM surface. For neutral
chemistries such as poly(ethylene glycol) (PEG) and CH
3
on NMs surface,
low protein binding is observed indicating that such groups have low affinity
for both proteins and cells. For basic or acidic groups on NM surfaces, two
mechanisms of protein or cell binding occur: specific or nonspecific binding
could occur. Specific binding is the mechanism of choice for controlled and
quantitative processes while nonspecific binding is the choice for quick and
qualitative processes.
During specific interactions, NMs surfaces are modified so that proteins
attached to the NMs dock within the binding sites of receptor proteins on
cell surfaces. In nonspecific interactions, random binding between proteins
on particles and any component of cell surfaces occurs. In both processes,
the orientation of proteins on NMs surfaces plays an important role in deter-
mining the extent and stability of binding. In either case, both mechanisms
hold for small to large protein molecules as wells as cells, DNA, virus, or
microorganisms.
