Biocompatibility and Functionalization - Nanomaterials for Medical Applications - page 99

are stained with organic dyes because QDs are resistant to photobleaching, have
high brightness, and are stable in cells for over six months at ambient RT.
NM bioconjugation through covalent binding is preferred to avoid desorption
of the biomolecule from the surface of the NMs. 109,146,160-162,164-179 Bioconjuga-
tion of NMs with carboxyl (-COOH) functional groups on the surface often uses
the carbodiimide derivative 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide
(EDC) for linking to the amine (-NH 2 ) groups on the biomolecules. 161,162,164,165
NMs functionalized with sulfyhydryl (-SH) groups are activated with the linker
4-( N -maleimidomethyl)-cyclohexanecarboxylic acid N -hydroxysuccinimide
ester (SMCC) for linking with the -NH 2 groups of the biomolecule.
One example of the carbodiimide chemistry in infectious diseases diag-
nostics is the conjugation of the pathogen biomarkers hepatitis B surface anti-
gen (HBsAg), HCV nonstructural protein 4 (NSP4), and HIV glycoprotein
41 (gp41) to the surface of quantum dot barcodes (QdotBs). 58 The use of the
QdotBs along with advances in microfluidic and photon detection technologies,
signal processing, and proteomic biomarkers of infection, have shown potential
to be developed as a handheld device for POC diagnostics. 180 Agrawal et al. 181
showed how carbodiimide chemistry was also used to conjugate oligonucle-
otides to green and red NP probes for the determination of intact respiratory
syncytial virus (RSV) and that the use of bioconjugation of two-color NPs to
simultaneously recognize two binding sites in a single target allows for detec-
tion of intact viruses without the need for target amplification or the need to
separate probe from target molecules.
The same carbodiimide chemistry was also used to conjugate Abs against epi-
thelial cell adhesion molecules (EpCAM) to detect breast cancer cells 161,164,165
through QD-based specific imaging ( Figure 3.6 ). The breast cancer cells under
white light were more difficult to observe under the microscope than when
these were stained specifically with the Ab-conjugated QDs. Furthermore, the
FIGURE 3.6 Photograph of SK-BR3 cells stained with green emitting QDs emitting at 530 nm
attached to Ab against cell membrane proteins. The QDs-Ab are seen as dots along the cell mem-
brane to bring fluorescence color to the cells. Courtesy of Ocean NanoTech, LLC. (For color version
of this figure, the reader is referred to the online version of this topic)