biological applications and non- semiconductor materials
TRANSCRIPT
Nanoparticles 2
Biological applications and non-semiconductor materials
Biological applications of fluorescent QDs
God made the bulk and the devil the surfaces (Pauli)
The surface is 50% of atoms
Problems are weak fluorescence or loss of fluorescence, cap decay, aggregation, toxicity
Dozens of suggestions have been proposed; none is perfect
Properties of QDs
Fluorescence (photoluminescence, PL) results from trapped or free carrier recombination
Radiative recombination of the free exciton is desired; traps red-shift or eliminate fluorescence
The surface is not round and smooth, so the optical properties are highly dependent upon its features!
TOPO provides “passivation” c.b.
v.b.
shallow trap states
deep trap states
radiative
radiative &
non-radiative
non-radiative
500 550 600 650 700 7500
20
40
60
80
100F
luore
scence (
arb
. units)
Emission (nm)
Trap states arise from defects and un-coordinated atoms on the nanocrystal surface.
CdSe CdSe
Coating the surface with a higherband gap material or a polymerincreases the fluorescence.
Weller, 1993
Add an excess of thiol to the organic solvent; reflux for 2- 24
hours; add aqueous buffer at basic pH; QDs will partition into
the aqueous phase and can be removed and further purified by centrifugation and washing
The original solubilization method
“Cap exchange” using mercaptoacetic acid to replace
trioctylphosphine oxide
P
R R
RO
P
RR
RO
PR
R R
O
PR
RR
O
PR
RR
O
SHOH
O
OH
OS
OH
O
S
OH
O
S
OH
O
S
OHO
S
(CdSe)TOPOin CH2Cl2
+CdSeprecipitates from CH2Cl2
shake 1 hour in dark
MAAmercaptoacetic acid
(CdSe)MAA,rinsed and stored in H2O
CdSe
Chan and Nie 1998
Other chain lengths can be used
In general, all this will change is the dissociation constant and
the degree of hydrophilicity
Mercaptoundecanoic acid
Probably the most popular
Mercaptohexadecanoic acid
See e.g.
Huge problem: loss of fluorescence and instability!
CdSe CdSe
+ O2
Occurs over days to weeks,depending on solvent, lightexposure, temperature, etc...
oxidizedsurface layer
SeO2
oxidized non-oxidized
500 550 600 650 700 750
0
500
1000
1500
2000
2500
3000
3500
Flu
ore
scence (
arb
. units)
Emission (nm)
after 3 days light exposure
fresh prep.
(CdSe)MAA
Overall “cap decay” scheme Dabbousi et al. 1997
Aldana et al. 2001
What can be done?
Problem: the degree of fluorescence quenching is even
greater than with mono-dentate thiols! CdSe solubilized by
dihydrolipoic acid is nonfluorescent
Another classic
Bruchez et al Science 25 September 1998
Problem: difficult to perform; leads to a great reduction in
fluorescence
Oligomeric phosphines
Kim and Bawendi, J. Am. Chem. Soc., 125 (48), 14652 -14653, 2003
Triblock copolymers
Problem:
requires
elaborate synthesis;
greatly
increases size
of the particle
Gao et al
Nature
Biotechnology
22, 969 - 976 (2004)
Phospholipid micelles
Dubertret et al, Science 29 November 2002:
Vol. 298. no. 5599, pp. 1759 - 1762
An even worse problem for biologists!
QDs that do get into cells just sit in endosomes
Suggested solution: hyperbranched copolymer
Duan and Nie, J. Am. Chem. Soc., 129 (11), 3333 -3338, 2007
And the latest
Liu et al, J. Am. Chem. Soc., Web Release Date: November 6, 2007
•CdSe(ZnCdS)
•Very small
•Cleared by kidneys (rat)
Sub-conclusions
Everyone recognizes the problem
Many people have tried to fix it
The majority of researchers
continue to use MAA, MUA, or DHLA because these methods are easy and generally acceptable
Toxicity
“Quantum dots are nontoxic” is a lie
The most obvious (but not most common) source of toxicity is Cd ions
The most common source is reactive oxygen species (ROS) OR direct oxidation (unclear which!)
Hepatocytes hate Cd
Derfus et al
Nano Letters, 4
(1), 11 -18, 2004.
Some, but not all QDs (might) make ROS
Expose to UV/blue light
Measure direct singlet oxygen luminescence (1271 nm) or use
terephthalate
CdS, CdSe yes
CdSe/ZnS no
Ipe et al, Small Volume 1, Issue 7, Pages
706-709
ROS leads to lipid peroxidation and loss of mitochondrial membrane
potential
mitochondrial
membrane lipid
peroxidation
Caspase 8
FADD
NAC
NAC
plasma membrane
lipid peroxidation
metabolic
activity CELL DEATH
cytochrome c caspase
cascade
Fas
ROS
M
QD
cardiolipin
peroxidation
Proposed mechanism
(D. Maysinger, Pharmacology)
Simple molecules can photosensitize QDs
Clarke et al. NATURE MATERIALS 5 (5): 409-417 MAY 2006; and Clarke et al in progress
Really ROS or just oxidation?
False positive results with oxidation-based dyes using
fullerenes (Lyon et al, Nano Letters 8(5):1539)
Using a reduction-based dye (XTT), no ROS was detected
Nevertheless, oxidative damage to cells can occur
Apart from oxidation, charge matters
Rasmussen et al, Journal of Investigative Dermatology (2007) 127, 143-153
Is there a solution?
Toxicity can be used to kill unwanted cells
In vitro, QDs must be passivated to prevent toxicity; antioxidants also help
It’s very unlikely that Cd-containing particles will be used in vivo; other types are needed!
The ideal particle
Made of non-toxic materials
Small
Multifunctional (e.g. fluorescent and provides MR contrast)
Easy to make
Many types of nanoparticles fit one or more of these criteria: ZnS, Au, Ag, TiO2, …
Fluorescent but nontoxic
ZnO
ZnS
Problems:not a wide range of
fluorescence; very blue, excite with UV
Nonfluorescent but interesting
TiO2
Ag
Au
FeO3
etc
Ag is antimicrobial
Sondi Journal of Colloid and Interface Science Volume 275, Issue 1, 1 July 2004, Pages 177-182
Au has many interesting properties It all started in 1994: Brust et al, JOURNAL OF THE CHEMICAL
SOCIETY-CHEMICAL COMMUNICATIONS (7): 801-802 APR 7 1994
Colloids stabilized with alkanethiols as in CdSe
Au-S bond is very stable
Simple synthesis: reduction of AuCl4- by sodium borohydride in the presence of the thiol
All the “particle in a box” quantum mechanics discussed for QDs still applies
All of the solubilization/encapsulation methods also apply!
Daniel and Astruc, Chem. Rev., 104 (1), 293 -346, 2004
The surface plasmon
Au colloids are deep red due to surface plasmon band absorbance
Collective oscillations of 6s electrons in conduction band in response to EM field of visible light
Varies with shape and especially size: for particles of 9, 15, 22, 48, and 99 nm, the SPB maximum max was observed at 517, 520, 521, 533, and 575 nm. Absent in bulk gold and in particles < 2 nm
Some remarkable features
Aggregation causes colour change
This can be used as a basis for
sensing
Electron transfer to semiconductors
Enhancement of CdSe
Hsieh et al, Nanotechnology 18 (2007) 415707
Au toxicity
Pan et al, Small Volume 3, Issue 11 , Pages 1941 - 1949
CT contrast agents
Usually iodine-based (high X-ray absorption coefficient)
Problems of toxicity (kidneys)
Fast clearance--> short imaging times
Alternatives of nontoxic materials include bismuth and gold
PEG-SH coated Au
CT images of rat hepatoma
using Au-PEG
nanoparticles injected by tail vein as a contrast agent Kim et al, J. Am. Chem. Soc., 129 (24), 7661
-7665, 2007
Multifunctional QDs
a,b, Quantum dots having
different molecules for target-specific interaction,
and, attached to the surface, paramagnetic lipids
(a) and chelators (b) for
nuclear-spin labelling. (c) The silica sphere has QDs
and paramagnetic nanoparticles inside and
target-specific groups
attached to the outside. (d) The structure of a
multimodal QD probe, based on silica-shelled
single-QD micelles
Bakalova et al, Nature
Photonics 1, 487 - 489 (2007)
Iron oxide FDA approved as MR contrast agent “SPIO”
Superparamagnetic
Unlike ferromagnetic materials, paramagnetic
materials do not retain magnetization when B = 0.
Superparamagnetism is the ability to exhibit paramagnetism below the Curie temperature. In
nanoparticles, thermal energy is sufficient to change
the direction of all atoms in the crystallite.
A quick synthesis method
0.2 mL of Fe(CO)5 to 10mL octyl ether and 478 μL oleic acid at 100 ºC, reflux at 280 ºC for 1 h; should turn black.
Let cool, add 0.34 g of trimethyl-N-oxide, heat to 125 ºC for 2 h under nitrogen and then reflux at 280 ºC for 1 h.
Micelle encapsulation: dilute original solution tenfold, add 100 μL to 900 μL chloroform and 1 mg mPEG-2000 PE, 0.25 mg of DPPC, 34 μL of a 5.4 mM DiI solution. Allow solvents to evaporate, resuspend in 1 mL H2O.
Appearance
Usefulness in MR and fluorescence imaging
0
20
40
60
80
100
120
0 200 400 600 800
Conc (uM)
1/T
2 (
se
c-1
)
1
T 2=
1
T 2H2O
+ R2 Fe[ ]
MR imaging
Conclusions Quantum confinement not restricted to semiconductors
Metals and insulators show interesting properties at the nanoscale that are not observed in bulk materials
Wet-chemical synthesis methods are similar to those for QDs
Methods of solubilization, encapsulation, etc are also similar, aided by the Au-S bond strength for Au
Multifunctional particles can permit several different applications at once
Downloading papers
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