APPLIED PHYSICS LETTERS 95, 073109 共2009兲
Strong UV absorption and emission from L-cysteine capped monodispersed
gold nanoparticles
S. N. Sarangi, A. M. P. Hussain, and S. N. Sahua兲
Institute of Physics, Sachivalaya Marg, Bhubaneswar 751005, India
共Received 1 June 2009; accepted 29 July 2009; published online 20 August 2009兲
We report a synthesis of L-cysteine capped monodispersed gold 共Au兲 nanoparticles 共NPs兲 with size
⬃2.0 nm exhibiting a strong surface plasmon resonance optical absorption at 3.13 eV, which is
blueshifted by 1.01 eV compared to the uncapped Au NPs of size 20.0 nm. A strong fluorescence
共FL兲 of the capped Au NPs appears at 3.25 eV, whereas the uncapped Au NPs do not show any FL
in this range. The L-cysteine concentration has been optimized to achieve one of the strongest
ultraviolet absorption and luminescence. The capping of Au NPs has been confirmed by Fourier
transform infra red measurement. © 2009 American Institute of Physics. 关DOI: 10.1063/1.3210788兴
Metallic nanoparticles 共NPs兲 recently have received
much attention due to their wide range of applications in
biological markers,1 DNA sensors,2,3 molecular recognition
systems,4,5 nanoscale electronics,6,7 and catalytic activities.8
Structurally, Au NPs are reported to exhibit a hexagonal
phase9,10 apart from a few cases of the fcc phase11 where the
lattice constant is 76.6 nm.12 Recent developments in Au
NPs synthesis showed the use of alkanethiolets as a capping
agent to achieve a size ⬍5.0 nm 共Ref. 13兲 with the surface
plasmon resonance 共SPR兲 absorption peak appearing in the
range of 2.3–2.39 eV.14 The optical absorption spectra of Au
NPs or clusters exhibit pronounced resonance lines caused
by the coherent electron motion of the free electron gas.15
The coherent electron motion gives rise to the SPR absorption and decay nonradiatively by electron-electron collisions
with a lifetime of a few femtoseconds.16 The position and the
width of the SPR absorption band and the luminescence of
Au NPs are reported to be size and shape dependent.17 The
band generally appears in the visible or in the near infrared
共IR兲 region, but are redshifted with increasing particle size or
increasing aspect ratio in the case of Au nanorod.17 For large
size 共bulk兲 Au particles and Au NPs with size ⬍2.0 nm either there is no or very weak photoluminescence in the
longer wavelength region is observed.15,16 However, Bigioni
et al.18 reported that Au NPs of size 1.1–1.7 nm show weak
near-infrared luminescence with a quantum yield in the range
of 10−4 – 10−5 m, which is higher than the bulk gold’s
共10−10兲.19 A luminescence as high as six orders of magnitude
compared to bulk Au has been reported for Au NPs 共Ref. 20兲
and nanorods.17 In this communication, we report a technique to synthesize nearly monodispersed highly stable
L-cysteine capped spherical Au NPs, which show SPR absorption as well as FL in the UV region only. The structural
and optical properties are evaluated by UV-visible absorption, FL emission, high resolution transmission electron microscopy 共HRTEM兲, and Fourier transform infra red 共FTIR兲
spectroscopic studies.
Precursors aurochloric acid 共HAuCl4兲, L-cysteine and
sodium borohydride 共NaBH4兲, all 99.9% pure, were used as
received and double distilled deionized water was used as the
solvent. The synthesis of Au NPs is a simple reduction reaca兲
Electronic mail: sahu@iopb.res.in.
0003-6951/2009/95共7兲/073109/3/$25.00
tion of HAuCl4 with a reducing agent NaBH4 in presence
of L-cysteine. 25 ml aqueous solution of 2 mM L-cysteine
was prepared by stirring for 30 min. To this solution of
L-cysteine, 250 ␮l of 0.05M stock solution HAuCl4 was
added under constant stirring and then 350 ␮l of 0.01M
NaBH4 solution was added. The solution is then stirred for
further 2 h for complete reduction and formation of
L-cysteine capped Au NPs 共here after referred as Au NPs兲 at
300 K and then washed and centrifuged to remove any unreacted precursors. The Au NPs thus, prepared was found to
be stable at least for more than two months as confirmed
from repeated absorption and FL measurements. The
L-cysteine concentration was varied from 0 共uncapped兲 to
4.0 mM to optimize the average size of the capped Au NPs
size, the size distribution and their optical properties. The
UV-visible spectra of the NPs were recorded with a Shimadzu UV-3101PC spectrophotometer. The FL behavior of
the NPs was studied at 300 K using Oriel PL setup with 250
nm incident excitation from the Hg–Xe lamp. The HRTEM
of the Au NPs were carried out with a JEOL JEM 2010
electron microscope operating at 200 keV.
The wide SPR absorption of the uncapped Au NPs appearing as 540 nm 共2.3 eV兲, shown in Fig. 1, indicates the
presence of a size distribution and an energy gap in the Au
NPs. Considering that the energy of bulk Au phonons is of
the order of 10 meV, an energy gap of 2.3 eV or more should
provide sufficient decoupling from phonon-mediated nonra-
FIG. 1. 共Color online兲 UV-visible absorption spectra of Au NPs synthesized
with different concentration of L-cysteine.
95, 073109-1
© 2009 American Institute of Physics
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073109-2
Appl. Phys. Lett. 95, 073109 共2009兲
Sarangi, Hussain, and Sahu
TABLE I. Absorption peak position, FWHM of absorption peaks and particle size with size distributions at different L-cysteine concentration.
L-cysteine
concentration
共mM兲
Absorption
peak position
共nm兲
FWH
共nm兲
Particle size
共from TEM兲
Size distribution
共nm兲
0.0
0.5
1.0
2.0
4.0
541
378
375
365
371
53
51
45
33
36
20
9
5
2
2
⫾5
⫾4
⫾3
⫾1
⫾1
diative process for radiative process to become significant,
which in turn would enhance the luminescence yield. This
effect would be more pronounced for smaller NPs with still
larger energy gaps and this we can see with L-cysteine
capped Au NPs in our subsequent discussions. The SPR absorption peak 共Fig. 1兲 of the Au NPs for 0.5 mM L-cysteine
concentration appears at 375 nm. This peak shows low peak
intensity with reduced width and a large blueshift compared
to corresponding peak of the uncapped Au NPs. This suggests that the NPs are very small and have a narrow size
distribution. On further increasing the L-cysteine concentration to 1.0 mM, there is a little increase in the SPR absorption but blueshifted to 365 nm. However, for 2.0 mM
L-cysteine concentration, there is a sharp increase in the Au
NPs SPR absorption, which is symmetric but slightly blueshifted without any change in the spectral shape. This suggests the presence of monodispersed spherical NPs21,22 in the
colloid. Interestingly, for 4.0 mM L-cysteine concentration,
one can also achieve sharp and symmetric SPR absorption
but slightly redshifted compared to the 2.0 mM L-cysteine
capped Au NPs. The peak positions and the full width at half
maxima 共FWHM兲 of the SPR absorption peaks given in
Table I clearly demonstrate the high quality of the Au NPs
whose SPR absorption is being observed in the UV region.
This has not been reported earlier. The spectral position of
the SPR absorption band depends on the dielectric properties, the crystalline size and shape of the clusters. It is also
influenced by the surrounding medium, the resonance energy
generally shifts with a change in the refractive index of the
surrounding medium, due to the screening of the Coulomb
attraction between the oscillating electrons and the positively
charged cluster ions,22 also be influenced by chemical interactions with the environment.23 The small blueshift with increasing capping agent concentration is due to the smaller
particle size with narrower size distributions arising from a
better confinement of the Au NPs by the capping materials.
The latter essentially forms a shell-like structure around the
metal NPs. The organic shell-like coating may become sufficiently thick at higher L-cysteine concentrations, which
may exert stress on the Au NPs. Further, as the L-cysteine
concentration is increased, the refractive index of the solution containing the capped Au NPs also probably increased
and this result in a small redshift of the plasmon absorption
peak17,22 in case of samples prepared with 4.0 mM
L-cysteine compared to 2.0 mM. The decrease in absorption
at the highest concentration of L-cysteine may be attributed
to the less absorption by the surface plasmon due to the
change in the dielectric properties of the surroundings of the
Au NPs. The underlying idea of the blueshift is based on the
assumption that the screening effects are reduced over a sur-
FIG. 2. 共Color online兲 FL spectra of Au NPs at different L-cysteine
concentration.
face layer inside the metallic particles due to localized character of the core-electron wave function.
The uncoated Au NPs do not exhibit any luminescence
under 250 nm excitation. The FL spectra of the Au NPs 共Fig.
2兲 recorded with 250 nm excitation show emission peak at
382 nm 共3.25 eV兲 with increase in intensity 共without any
change in peak position兲 and a small decrease in FWHM as
the L-cysteine concentration is increased. The decrease in the
FWHM with increasing L-cysteine concentration can be attributed to the narrower size distribution of the Au NPs,
which also contributes to the increase in the FL emission
intensity. For Au NPs prepared at 2.0 mM L-cysteine concentration the FWHM is 11.0 nm. We do not believe the
observed FL involves the ligand shell as the FL emission
studies of the organic ligand alone did not exhibit any emission signature in this range. The vibrational mode of the
ligand shell too is an unlikely source since the transitions are
of very low energy as is the case of Au–S linkage. Combination bands as seen in the transmission spectrum of the
solvent where typical transitions are quite discrete and
unique to molecules.24 FL possibility due to surface related
mechanism such as those involving charge transfer or localized states can be ruled out as x-ray photoelectron spectroscopy studies of thiol capped Au NP indicate that the core Au
is metallic.25 Hence, the origin of FL of L-cysteine capped
Au NPs appearing at 382 nm 共3.25 eV兲 can be ascribed to the
localized surface plasmon only in contrast to the reported
visible FL arising due to the interband transition between the
sp conduction band and occupied d band.26 FL of Au NPs in
the near infrared region is ascribed to the intraband transition
inside the sp conduction band.27 A small shift in the FL peak
position due to a change in the L-cysteine concentration follows a similar trend as is the case with the plasmon
FIG. 3. TEM image of Au NP synthesized without any capping agent.
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073109-3
Appl. Phys. Lett. 95, 073109 共2009兲
Sarangi, Hussain, and Sahu
FIG. 4. TEM image of 2.0 mM L-cysteine capped Au NPs.
absorption.28 For the FL emission due to a transition between
a d band hole and an sp conduction band electron, the peak
position does not change with a small change in the NP
radius or a change in the aspect ratio in case of Au
nanorods.8,29 This is not the case in the present work. Attempts have been made to excite the capped Au NPs with
two excitation energies, 4.97 and 3.82 eV, but no change in
the peak position could be detected but the intensities do
change. As we observe a strong FL in the UV regime, which
is not expected from metallic NPs, we expect the L-cysteine
capped Au system could be the Au cluster instead of Au NPs.
Since an eight atom cluster is a stable structure of the smallest cubic unit cell, we strongly feel that the 2 nm particles
could be Au clusters. However, the time of flight measurement can identify such a cluster and the same has been
planned as a future work.
The TEM micrograph of uncapped Au NPs 共Fig. 3兲 gave
the average size as 20 nm. Figure 4 shows the HRTEM image of the L-cysteine 共2.0 mM concentration兲 coated Au
NPs. The crystallites are fairly isolated and the size variation
from 2.0 to 3.0 nm implies a monodispersed NPs system.
The inset in Fig. 4 is the selected area electron diffraction
pattern, which shows the crystallites to be small and monocrystalline in character. The FTIR spectra of L-cysteine and
L-cysteine coated Au NPs are shown in Figs. 5共a兲 and 5共b兲,
respectively. The S-H bond occurring in the spectra of
L-cysteine at 2550 cm−1 is absent in the L-cysteine coated
Au NPs indicating a breaking of the S-H bond and removal
of the hydrogen from L-cysteine to form the Au-S bond in
the surface passivation process of Au NPs at the sulfur site of
the coating materials.30 This confirms the coating of the Au
NPs by L-cysteine. All other peaks occurring in the
FIG. 5. FTIR spectra of 共a兲 L-cysteine and 共b兲 L-cysteine capped Au NPs.
L-cysteine 关Fig. 4共a兲兴 are also present in the spectra of the
L-cysteine coated Au NPs 关Fig. 4共b兲兴. High quality Au NPs
giving emission in the UV region are expected to be useful in
biological applications.
The present work reports a synthesis of highly stable
L-cysteine capped Au NPs of very narrow 共⬃2 nm兲 size
distribution. The NPs exhibit a strong SPR absorption as well
as emission in the UV region. The intense luminescence observed in the UV region in this study is unique to its material
property. The work further shows that the localized plasmon
resonance emission causing the FL emission is due to the
localized surface plasmon only and not due to the sp and d
inter- and intraband transition electron-hole recombination.
The L-cysteine capping of Au NPs was also confirmed from
FTIR measurement.
The authors would like to acknowledge Professor P. V.
Satyam, Umananda, and Ashutosh of Institute of Physics for
their help in the HRTEM study.
J. W. Slot and H. J. Geuze, J. Cell Biol. 90, 533 共1981兲.
R. Elghanian, J. J. Storhoff, R. C. Mucic, R. L. Letsinger, and C. A.
Mirkin, Science 277, 1078 共1997兲.
3
J. J. Storhoff, R. Elghanian, R. C. Mucic, C. A. Mirkin, and R. L.
Letsinger, J. Am. Chem. Soc. 120, 1959 共1998兲.
4
J. Liu, R. Xu, and A. E. Kaifer, Langmuir 14, 7337 共1998兲.
5
A. Labande and D. Astruc, Chem. Commun. 共Cambridge兲 2000, 1007.
6
R. P. Andres, J. D. Bielefeld, J. I. Henderson, D. B. Janes, V. R. Kolagunta, C. P. Kubiak, W. J. Mahoney, and R. G. Osifchin, Science 273,
1690 共1996兲.
7
T. Sato, H. Ahmed, D. Brown, and B. F. G. Johnson, J. Appl. Phys. 82,
696 共1997兲.
8
M. S. Chen and D. W. Goodman, Science 306, 252 共2004兲.
9
Y. Zhao, Y. Qi, Y. Zhang, S. Zhang, and Z. Liu, Mater. Lett. 62, 1197
共2008兲.
10
L. Zhai and R. D. McCullough, J. Mater. Chem. 14, 141 共2004兲.
11
K. Y. Lee, J. Hwang, Y. W. Lee, J. Kim, and S. W. Han J. Colloid Interface
Sci. 316, 476 共2007兲.
12
H. D. Hill, R. J. Macfarlane, A. J. Senesi, B. Lee, S. Y. Park, and C. A.
Mirkin, Nano Lett. 8, 2341 共2008兲.
13
M. Brust, M. Walker, D. Bethel, D. J. Schiffrin, and R. J. Whyman, Chem.
Commun. 共Cambridge兲 1994, 801.
14
J. Lee, P. Hernandez, J. Lee, A. O. Govorov, and N. A. Kotov, Nature
Mater. 6, 291 共2007兲.
15
G. C. Papavassiliou, Prog. Solid State Chem. 12, 185 共1979兲.
16
U. Kreibig and M. Volmer, Optical Properties of Metal Chusters
共Springer, Berlin, 1995兲.
17
M. B. Mohamed, V. Volkov, S. Link, and M. A. El-Sayed, Chem. Phys.
Lett. 317, 517 共2000兲.
18
T. P. Bigioni, R. L. Whettero, and O. Dag, J. Phys. Chem. 104, 6983
共2000兲.
19
A. Mooradian, Phys. Rev. Lett. 22, 185 共1969兲.
20
J. P. Wilcoxon, J. E. Martin, F. Parsapour, B. Wiedenman, and D. F.
Kelley, J. Chem. Phys. 108, 9137 共1998兲.
21
D. Gao, Y. Tian, S. Bi, Y. Chen, A. Yu, and H. Zhang, Spectrochim. Acta,
Part A 62, 1203 共2005兲.
22
J. Müller, C. Sönnichsen, H. von Poschinger, G. von Plessen, T. A. Klar,
and J. Feldmann, Appl. Phys. Lett. 81, 171 共2002兲.
23
P. Mulvaney, Langmuir 12, 788 共1996兲.
24
R. F. Goddu, Near-Infrared Spectroscopy 共Wiley Interscience, New York,
1960兲, Vol. 1.
25
J. T. Khoury, “Colours of Nanometric Gold,” Ph.D. thesis, UCLA, 1999.
26
L. Liu, H. Z. Zheng, Z. J. Zhang, Y. M. Huang, S. M. Chen, and Y. F. Hu,
Spectrochim. Acta, Part A 69, 701 共2008兲.
27
H. Liao, W. Wen, and G. K. Wong, J. Opt. Soc. Am. B 23, 2518 共2006兲.
28
C. Sönnichsen, T. Franzl, T. Wilk, G. von Plessen, and J. Feldmann, New
J. Phys. 4, 93 共2002兲.
29
O. P. Varnavski, M. B. Mohamed, M. A. El-Sayed, and T. Goodson III, J.
Phys. Chem. B 107, 3101 共2003兲.
30
S. Wang and D. Du, Sensors 2, 41 共2002兲.
1
2
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