Chiral gold nanoparticles
Cyrille Gautier and Thomas Bürgi
Institut de Chimie, Université de Neuchâtel, CH-2001 Neuchâtel, Switzerland.
Introduction
Gold Monolayer Protected Nanoparticles, MPNs, are of considerable interest in
various fields such as bio-sensing, catalysis, electronics and nanotechnology1.
We prepared and size separated small gold MPNs protected with chiral thiols,
such as N-acetyl-L-cysteine, N-isobutyryl-L-cysteine or N-isobutyryl-D-cysteine.
The physical properties of these MPNs are largely determined by their metal
core, whereas their chemical properties depend on the protection layer. These
particles were characterized by Vibrational and Electronic Circular Dichroism
(VCD and ECD). Density Functional theory (DFT) calculations of N-acetyl-Lcysteine adsorbed on gold clusters were used to simulate VCD spectra in order
to determine the conformation of the adsorbed thiols2. ECD spectra reveal
optical activity in transitions located in the metal core, indicating that the latter is
intrinsically chiral.
Comparison between calculated
and experimental VCD
These MPNs exhibited vibrational circular dichroism activity in vibrations
associate with the adsorbed NALC. DFT calculations of the most stable
conformers of NALC adsorbed on gold clusters were used to simulate VCD
spectra. These calculations revealed a strong dependence of the VCD
spectra on the conformation of the adsorbed thiol, whereas the structure of
the underlying gold cluster was less important.
Calculated
conformers
VCD
Preparation of gold MPNs
Cl
NaBH4
Cl
Au
Cl
Cl
3 RSH
ΔA
H
RSSR
+ 4HCl
HS
HN
O
O
O
O
(R)
HS
HN
NILC
NALC
5x10-5
HO
HO
HO
(R)
RSH =
(S)
HS
O
O
HN
NIDC
MPNs are prepared by the
conventional chemical route based
on nucleation of zerovalent metal
atoms in the presence of thiols3.
Such a method inevitably produces
a distribution in the core sizes due to
statistical
fluctuations
in
the
nucleation
process
(see
Transmission Electron Microscopy
on right).
exp
1800
1700
1600
1400
Calculations:
DFT b3pw91
6-31G(d,p)/LanL2DZ
1300
Wavenumber / cm-1
The calculated VCD spectrum of the most stable conformer (green)
compared well with the experimental spectrum (black). This structure is
characterized by an interaction of the carboxylate group with the metal, which
represents a second anchoring point besides the strong sulfur-gold bond.
This structure is further favored at high coverage, as it uses only little surface
space and allows dense packing. The latter property increases the MPNs
stability.
SDS-PAGE electrophoresis
Isolation of MPNs with well-defined
sizes was achieved by sodium
dodecyl sulfate-polyacrylamide gel
electrophoresis (SDS-PAGE). Such
separations reveal a non-uniform
size-distribution
with
a
high
abundance of certain particle sizes
(magic numbers). A photograph of a
typical gel separation is shown (on
right).
1500
Experimental ECD
Chirality is often associated with organic molecules, inorganic salts and
biological materials. However, chirality can also be bestowed to metal
surfaces. The latter can be intrinsically chiral4 or become chiral through
adsorption of chiral molecules5. MPNs can be viewed as the nanometer size
analogues of extended metal surfaces. Optical activity in circular dichroism of
MPNs are associated with transitions located in the metal core indicating that
the latter is chiral.
Electronic circular dichroim
3
2
1
0
Orange particles protected with
a monolayer of N-Isobutyryl-LCysteine
-1
-2
Ellipticity / mdeg
-3
-4
10
Yellow particles protected with a
monolayer
of
N-Isobutyryl-LCysteine
N-Isobutyryl-L-Cysteine
0
Yellow particles protected with a
monolayer of N-Isobutyryl-DCysteine
-10
References:
300
350
400
450
500
550
Wavelength / nm
[1] Daniel, M. C. l; Astruc, D. Chem. Rev.,2004, 104, 293-346
[2] Nafie, L. A., Annu. Rev. Phy. Chem., 1997,48,357-386
[3] Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc. Chem. Commun. 1994, 801.
[4] McFadden, C. F.; Cremer, P.S.; Gellman, A. J.; Langmuir, 1996, 12, 2483-2487.
[5] Lorenzo, M. O.; Baddeley, C. J.; Muryn, C.; Raval R. Nature, 2000, 404, 376-379.
Contact: [email protected]
Acknowledgments
Financial support by the Swiss National Science Foundation and grants of
computer time by the Swiss National supercomputer Centre (Manno) are kindly
acknowledged. We thank Massoud Dadras, Institute of Microtechnology, for TEM
measurements.
Conclusions and Perspectives
MPNs can be viewed as the nanometer-size analogues of selfassembled monolayers (SAMs). Our results indicate that VCD
spectroscopy may greatly help elucidating the structure of chiral
molecules adsorbed on metal particles and that ECD should help in the
same way to understand surface chirality.