Biomedical Engineering Reference
In-Depth Information
[
37
,
40
]. Except Si QDs, C-dots, or GQDs, other kinds of cadmium-free QDs have
also been well developed due to their good biocompatibility and excellent optical
properties, such as InP [
41
], InP/ZnS [
42
], CuInS
2
/ZnS [
43
,
44
], Ag
2
Se [
45
], Ag
2
S
[
46
]. They all showed promising potentials in biological imaging applications.
2.2.3 Metal Nanoclusters
As a viable alternative to QDs, fluorescent metal nanoclusters, known as ultras-
mall size, good biocompatibility, and excellent photostability, have become a new
class of fluorescent labels for biological applications. Among them, Au and Ag
nanoclusters attract much more attentions. Actually, at the beginning of the obser-
vation of photoluminescence from the noble metals, little attention was paid due
to the extremely low quantum yield (QY) of 10
−
10
and much more interests have
been attracted by researchers until the much enhanced QY reached to the range
of 10
−
3
to 10
−
1
[
47
]. Until now, a lot of Au and Ag NCs stabilized with different
scaffolds (protein, peptide, and oligonucleotide) have been developed and applied
for the detection of thiol compounds [
48
], metal ions [
49
,
50
], protein [
51
,
52
],
DNA [
53
], RNA [
54
] as well as intracellular and in vivo bioimaging. Dickson and
coworkers successfully transferred poly (acrylic acid)-stabilized Ag NCs (PA-SCs)
to anti-actin Ab/C12 and anti-
α
-tubulin/C12 conjugates to obtain fluorogenic sil-
ver cluster biolabels for cell surface labeling [
55
]. Wang et al. reported fluorescent
Au NCs could be spontaneously biosynthesized by cancerous cell incubated with
micromolar chloroauric acid solutions, a biocompatible molecular Au (III) species,
which could not occur in noncancerous cells. They further realized in vivo self-
bioimaging of tumors by subcutaneous injections of millimolar chloroauric acid
solution near xenograft tumors of the nude mouse model of hepatocellular carci-
noma or chronic myeloid leukemia. This opens up promising opportunities of fluo-
rescent metal nanoclusters for in vivo bioimaging [
56
]. Specially, DNA-stabilized
Ag NCs possess obvious advantage in DNA biosensing because of the easy assem-
bly of DNA sequence. Werner and colleagues designed a nanocluster beacon to
detect a DNA sequence related to the human
Braf
oncogene based on an inter-
esting phenomenon that the red fluorescence of DNA-stabilized Ag NCs could be
enhanced 500-fold when placed in proximity to guanine-rich DNA sequences [
57
].
2.3 Preparation and Functionalization
2.3.1 Cadmium-Based Quantum Dots
A lot of synthetic methods have been developed for QD preparation, which can
be divided into two classifications: physical approach and chemical approach.
Physical approach, which was mainly referred to epitaxial growth and/or
