Biomedical Engineering Reference
In-Depth Information
degradation in the biological environment. For a solution, surface modification
is necessary. Erogbogbo et al. prepared Si QDs through a nanoparticle synthesis,
surface functionalization, PEGylated micelle encapsulation, and bioconjugation
process. The obtained Si QDs could be used in multiple cancer-related in vivo
applications, including tumor vasculature targeting, sentinel lymph node mapping,
and multicolor NIR imaging in live mice, which showed great potentials of Si QDs
as biocompatible fluorescent probes for both in vitro and in vivo imaging [
18
].
2.2.2 Carbon Dots
C-dots are a new class of carbon nanomaterials with sizes below 10 nm, which
were first obtained during purification of single-walled carbon nanotubes through
preparative electrophoresis in 2004 [
19
]. Since the discovery of their excellent
optical property, C-dots have attracted wide attentions and displayed great poten-
tials in biological applications. A special optical property of C-dots is that besides
normal or down-converted photoluminescence, they were shown to possess excel-
lent up-converted PL (UCPL), which enables the design of high-performance,
complex catalyst systems based on C-dots for efficient utilization of the full spec-
trum of sunlight [
20
-
23
]. Additionally, C-dots can exhibit PL emission in the near-
infrared (NIR) spectral region under NIR light excitation, which is particularly
significant and useful for in vivo bionanotechnology because of the low autofluo-
rescence and high tissue transparency in the NIR region [
24
,
25
]. Except strong
fluorescence, C-dots also own other properties such as electrochemical lumines-
cence [
26
-
28
], photoinduced electron transfer property [
29
,
30
], photocatalysis
[
22
], optoelectronics [
31
,
32
], which all extend their applications in various areas.
As a type of C-dots, the GQDs have also attracted a lot of interest from
researchers over the past few decades because of their fascinating optical and elec-
tronic properties. As graphene is a zero-bandgap material, in principle, the band-
gap of graphene can be tuned from 0 eV to that of benzene by varying their sizes
[
33
,
34
]. The 1D graphene sheets could be converted into 0D GQDs, which assume
numerous novel chemical and physical properties due to the pronounced quantum
confinement and edge effects [
35
,
36
]. Although GQDs are considered as a mem-
ber of C-dot family, there are still some differences between them [
37
]. The C-dots
are either amorphous or crystalline, while GQDs possess graphene lattices inside
the dots, regardless of the dot sizes [
38
]. Additionally, luminescent C-dots com-
prise discrete, quasi-spherical carbon nanoparticles with sizes below 10 nm, while
GQDs are always defined as the graphene sheets with lateral dimensions than
100 nm in single, double, and few (3 to <10) layers [
31
,
34
]. In general, the aver-
age sizes of GQDs are mostly below 10 nm, and up to now, the largest diameter
of GQDs reported is 60 nm, which is dependent on the preparation methods [
39
].
Similar to C-dots, the properties of photoluminescence, good electron mobility
and chemical stability, electrochemical luminescence, and photocatalyst of GQDs
have been widely employed in the fabrication of numerous sensors and bioimaging
