Biomedical Engineering Reference
In-Depth Information
QDs on an extremely low level of surface modification, thus providing the data
for a further development of defined multi-component structures for exploitation
as artificial light-harvesting complexes, electro- and photochemical devices or
nanosensors.
4.1
Introduction to Nanoassemblies
At present, nanotechnology is emerging as an integrated research field aimed at
investigating and controlling nanomaterials by combining concepts from chemistry,
physics, biology, and engineering. Nanostructured materials with tunable morphol-
ogy have attracted exceptional interest because of their unique architectures, tailored
physicochemical properties, and central role in fabricating nano-electronics, and
potential applications in nano-biomedicine [
1
-
7
]. Very often, the bottom-up forma-
tion of nanomaterials is based on self-assembly approaches being the fundamental
phenomenon that generates structural organization on all scales and may be realized
in solutions and solid state via various basic interactions: hydrogen bonding,
coordination bonding, electrostatic and donor-acceptor interactions, or metal-ion
binding [
8
]. The designed self-assembled process, the information necessary to
initiate such a process, and the algorithm behind must be stored in the components
and must be operative via selective/specific interactions [
9
].
One direction in the field of nanoscience and nanotechnology is connected
with the study and applications of inorganic semiconductor materials of nanoscale
dimensions. Semiconductor nanocrystals [often referred to as quantum dots (QD),
e.g. CdSe or CdSe/ZnS and other II-VI compounds] represent a specific class of
matter between atomic clusters and bulk materials with well-defined size-dependent
tunable photophysical properties [
10
-
14
]. Moreover, based on self-assembly ideas
discussed above, the anchoring of functional organic molecules, molecular com-
plexes, and biostructures to QDs is of considerable scientific and a wide practical
interest including material science and biomedical applications [
15
-
24
]. In this
respect, there are few a principal aspects which should be taken into account
upon analysis of photoluminescence (PL) characteristics for QDs being a part of
heterogeneous nanoassemblies.
Optical properties of colloidal semiconductor QDs have been investigated in-
tensively during the last two decades [
10
-
14
,
25
,
26
] including design strategies
of, e.g. of core-shell systems [
27
], ligand chemistry [
28
-
34
], and surface func-
tionalization [
23
,
35
-
40
]. Tuneable band position and PL high quantum yields
are of crucial importance for envisaged applications [
41
,
42
]. Notably, because
of the increased surface-to-volume ratio relative to bulk materials, QD surfaces
are subject to chemical and structural disorder. Thus, along with size distribution,
surface chemistry is the major source of heterogeneity in the optical properties
of QDs both in time, which manifests as PL intermittency or “blinking” for
single QDs, and within an ensemble of particles. It means that the heterogeneity
and dynamics of QD surface complicate efforts to understand the mechanisms
