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Semiconductor Nanocrystals
Quantum Dots
- Properties and Biological Applications
Semiconductor physics
 Electrons are confined to a number of bands of energy,
and forbidden from other regions.
 The term "band gap" refers to the energy difference
between the top of the valence band and the bottom of
the conduction band; electrons are able to jump from one
band to another.
 Valence band is the highest range of electron energies in
which electrons are normally present at absolute zero
temperature.
 Conduction band is the range of electron energies enough
to free an electron from binding with its atom to move
freely within the atomic lattice of the material.
 In order for an electron to jump from a valence band to
a conduction band, it requires a specific amount of
energy for the transition.
 The required energy differs with different materials.
 The conductivity of intrinsic semiconductors is strongly
dependent on the band gap.
 The only available carriers for conduction are the electrons
which have enough thermal energy to be excited across
the band gap.
Band gaps
 Atomic orbital: a discrete set of energy levels.
 If several atoms are brought together into a molecule, their atomic
orbitals split, as in a coupled oscillation
 A number of molecular orbitals proportional to the number of atoms.
 When a large number of atoms (of order 1020 or more) are brought
together to form a solid, the number of orbitals becomes exceedingly
large.
 Difference in energy between them becomes very small
 The levels may be considered to form continuous bands of energy
rather than the discrete energy levels of the atoms in isolation.
 However, some intervals of energy contain no orbitals, no matter how
many atoms are aggregated, forming band gaps.
A stimulus (assume electromagnetic radiation)
of bandgap energy or higher can
excite an electron into the
conduction band.

In reality and at room temperature, there are practically no electrons
in the conduction band compared to the number in the valence band.
 In reality, the distance between energy levels in a band is practically
zero compared to the size of the band gap (in this diagram, the distance between energy
levels has been blown up for visual ease).
Bulk Semiconductors
- A Fixed Range of Energies
Relaxing Electron Emits Fixed Radiation
 Electrons relax back to the top edge of the valence band
from the bottom edge of the conduction band.
 This causes the fixed emission peak of semiconductors.
Quantum Confinement
 If the size of a semiconductor crystal becomes small enough that it approaches the size




of the material's Exciton Bohr Radius, the electron energy levels can no longer be treated
as continuous
- treated as discrete, namely there is a small and finite separation between energy levels.
This situation of discrete energy levels is called quantum confinement.
Bohr radius: Physical constant : the most probable distance between the proton
and electron in a hydrogen atom in its ground state
Exciton : A bound state of an electron and hole which are attracted to each other by the
electrostatic Coulomb force : Electron-hole pair
An exciton can form when a photon is absorbed by a semiconductor.
 This excites an electron from the valence band into the conduction band
A Feel for the Size of a Quantum Dot
Each dot is between 2 and 10 nm (10 and 50 atoms) in diameter.
Lined end to end, 2 million dots would be 1 cm long.
In reality, most applications of quantum dots involve attaching molecules
to their surface and suspending the dots in a liquid, gel, or solid matrix.
A Tunable Bandgap
According to size of quantum dot
semiconductor will measurably alter the
bandgap energy
– a tunable bandgap!
This is possible as long as the size of
the dot is close to or below the Exciton
Bohr Radius.
Size dependent color
 Quantum dots, the size of the bandgap is controlled simply
by adjusting the size of the dot.
 Because the emission frequency of a dot is dependent on the bandgap,
it is therefore possible to control the output wavelength of a dot
Ref. http://www.evidenttech.com/qdot-definition/quantum-dot-introduction.php
Quantum Dots – Semiconductor Nanocrystals
QDs: Artificial Atom - Nanosized semiconductor materials
 II-VI Semiconductor
CdS CdSe CdTe
PbS PbSe PbTe
 III-V Semiconductor
AlSb
GaP GaAs GaSb
InP InAS InSb
65Å
500Å
CdSe Nanocrystal
Structure of CdSe Quantum Dots
- CdSe core crystal are capped with
surface stabilizing capping
molecules
Synthesis of CdSe/ZnS (Core/Shell) QDs
Step 1
CdO
+ Se
CdSe
Solvent : TOPO, HAD, TOP
Surfactant : TDPA, dioctylamine
Step 2
CdSe/ZnS
5.5 nm
(red)
ZnS
ZnEt2 + S(TMS)2
CdSe
Growth temperature
 140 ℃ (green)
200 ℃ (red)
Ar
Thermocouple
Se solution
CdO solution
320 ℃
20 nm
Bawendi et al. J. Am. Chem. Soc. (1994)
Optical Properties Of Quantum Dots
b) Photostability
a) Multiple colors
(Tellurium)
c) Wide absorption and narrow emission d) High quantum yield
Quantum Yield ≥ 60 ~ 70 %
Single source excitation
Biotechnological applications of QDs
 Requirements under aqueous biological conditions
- efficient fluorescence
- colloidal stability
- low non-specific adsorption
 Main challenge
- QDs have hydrophobic organic ligands coating their surface
 Organophilic ligands should be exchanged with more polar ones
to make QDs biocompatible
 Approach
- Monolayer shells: reproducible, rapid, well-oriented, thin-coating
low colloidal stability
- Multilayer shells : high stable in vitro, long coating process,
difficult to control the coating process
ex)
overcoating with proteins followed by other layers for bioconjugation
overcoating of the outer shell with surfactants or polymers
 Drawbacks : tends to aggregate and adsorb non-specifically
QD Surface Coating for Biocompatibility ①
NH2
NH2
Encapsulation with the hydrophobic core of a micell
NH2
+
Coating with PC
P P
O O
P
O
CdSe O P
OP
O
O P
P
CdSe QDs
N
+
P
P
O O
ZnS
P
O OP
ZnS
O
O
P
Coating of the outer shell
with ZnS
OP
OP
O
P
O
P
NH2
OP
CdSe
CdSe
NH2
N
+
OP
O
P
O
N
NH2
P
+
N
P
CdSe/ZnS core-shell
+
N
NH2
NH2
Quantum Dots Encapsulated in
Phospholipid Micelles
PEG-PE (n-poly(ethylenglycol)phosphatidylethanolamine): micell-forming hydrophilic
polymer-grafted lipids  comparable to natural lipoproteins
PEG : low immunogenic and antigenic, low non-specific protein binding
PC : Phosphatidylcholine
Dubertret et al. Science (2002)
QD Surface Coating for Biocompatibility ②
(Trioctylphosphine Oxide)
1-Ethyl-3-(3-dimethylaminopropyl)-carbodiimide
N-hydroxysulfosuccinimide
EDC/NHS
Wu et al. Nature Biotech. (2003) ; QdotTM Corporation
Bioconjugation Method with Quantum Dots
S. Nie, Science 1998, 281, 1016
H. Mattoussi, J. Am. Chem. Soc. 2000, 122, 12142
X. Wu, Nature Biotech. 2003, 21, 41
S. Nie, Nature Biotech. (2001)
A. P. Alivisatos, Science (1998)
-1-Ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) : a water- soluble
derivative of carbodiimide.
-Carbodiimide catalyzes the formation of amide bonds between carboxylic
acids or phosphates and amines by activating carboxyl or phosphate to
form an O-urea derivative
- EDC coupling is enhanced in the presence of N-hydroxysulfosuccinimide
(Sulfo-NHS)
Bioconjugaion Using Chemical Linker Molecules
IgG
CdSe
sulfo-NHS / EDC
CdSe
O
N
H
C
streptavidin
CdSe
sulfo-NHS / EDC
CdSe
C
O
EDC : Ethyl-3-(dimethylaminopropyl)carbodiimide
NHS (Sulfo-NHS) : N-hydroxysulfosuccinimide
H
N
Commercially Available QD Bioconjugates
Core
Core
Shell
Polymer Coating
Biomolecule
Shell
Polymer Coating
Streptavidin
525
(Biotin, Protein A…)
565 585 605
655
From QdotTM Corporation
In Vivo Cell Imaging
Y
+
QD
Organelle
QD-Antibody
conjugates
Antigen
Y
▲ 3T3 cell nucleus stained
with red QDs and
microtubules with green QDs
QD
Organelle
- Multiple Color Imaging
- Stronger Signals
Wu et al. Nature Biotech. (2003)
In Vivo Cell Imaging
Live Cell Imaging
Quantum Dot Injection
▶ Red Quantum Dot locating
a tumor in a live mouse
Cell Motility Imaging
10um
◀ Green QD filled
vesicles move toward to
nucleus (yellow arrow) in
breast tumor cell
Alivisatos et al., Adv. Mater.(2002)
–
In vivo Cell Imaging
Xenopus embryos
B ~E : different stages
from injection into a cell
F : axon & somites
G : Nucleus
H: Neural crest cells
I: Gut of an embryo
“In Vivo Imaging of Quantum Dots Encapsulated in Phospholipid Micelles”
Science (2002)
Quantum Dot Microarrays
DNA-microarray based application
QD-DNA conjugates
QD
QD
DNA chip
Human oncogene p53
 Very High Signal to Noise ratio (>100)
 No Cross talk
 High Sensitivity
Human hepatitis virus B
Human hepatitis virus C
Gerion et al., Anal. Chem.(2003)
Biobarcode Made of Quantum Dots
Fluorescence Intensity-based Molecular Probes
 QDs with different color ratio coated with
silica beads and are linked to probe DNA
which can hybridize with target DNA.
 These QD barcodes can be read by
fluorescence profiles
QDs in Silica Beads
Nie et al., Nature Biotech.(2001)
Particle in a box
 In many species, the lowest excited state (the lowest unfilled orbital (LUO)) is more
than 300 kJ/mole above the ground state (the highest filled orbital (HFO)) and no
visible spectrum is observed.
 Application of the Schroedinger equation to this problem results in the
well known expressions for the wavefunctions and energies, namely:
n ; the quantum number (n= 1, 2, 3,....)
L ; the 'length' of the (one dimensional) molecular box
m ; the mass of the particle (electron)
h ; Planck's constant
 The particle may only occupy certain positive energy levels.
Likewise, it can never have zero energy, meaning that the particle can never "sit still".
 It is more likely to be found at certain positions than at others, depending on its energy level
.