Supplemental Material
Reactions and Adsorption at the Surface of Silver Nanoparticles probed by
Second Harmonic Generation
Wei Gan, Grazia Gonella, Min Zhang and Hai-Lung Dai*
Department of Chemistry, Temple University, Philadelphia, PA 19122
Email: hldai@temple.edu
Experimental and Sample Information
The experiments were performed by focusing the 800 nm output (pulse duration 130 femtosecond, 78 MHz
repetition rate, average power 200 mW) from a Ti:Saphire laser (Coherent, Micra V oscillator) onto a liquid jet
produced in a micropump-driven flow system. The polarization of the pump laser was set by a half-wave plate and
polarizing cube beamsplitter. The signal in a wide solid angle (within 20° centered around the fundamental beam
propagation direction) was collected for all the curves shown in this Letter. As it is expected that SHG from 40 nm
radius particles may have a wide distribution of scattering angles, experiments have been performed with other
collecting angles such as 30°. All experiments gave the same observed spectral pattern and dependence of thiol
concentration.
The collected light passed through a band pass filter (Schott BG39, 340 nm - 600 nm) that blocks the
fundamental beam, and was directed into a monochromator (Princeton Instrument, Acton SP2300) for spectral
analysis and detected by a Photomultiplier tube (R1527 Hamamatsu). The PMT output was amplified by a fast
preamplifier (350 MHz, Stanford Research) and analyzed by a photon counting system (Stanford Research SR400)
which was coupled to a microcomputer.
The citrate stabilized colloidal Ag with average particle diameter of 80 nm (original concentration 1.1x1010
particles/ml, Microspheres-Nanospheres) was used after being diluted to 2 times volume with water (Millipore,
18.2 M ). Ethanethiol (Sigma-Aldrich, 97%), 1,2-benzenedithiol (Sigma-Aldrich, 96%) and sodium
3-mercapto-1-propanesulfonate (Sigma-Aldrich, 90%) were used after being dissolved in water. Sodium citrate
(Sigma-Aldrich, 99%) was used to keep a stable citrate concentration of 5 μM in all the experiments.
Sample stability during experiments
FIG S1. TEM images of the silver nanoparticles.
FIG S1 shows Transmission Electron Microscopy (TEM) images of the silver nanoparticles used in the
experiments. Silver nanoparticles were supported with thin carbon film coated copper grids for TEM imaging with
a FEI Tecnai 12T Transmission Electron Microscope. The average diameter of the particles in the picture is close
to 80 nm as labeled by the manufacturer. It is apparent from FIG S1 that there are particles with shapes deformed
from spherical. These particles, however, still have an inversion symmetry center, as described in the main text.
S1
To ensure that the fundamental light would not cause any physical changes to the Ag particles, we have
measured the average diameter of silver nanoparticles using dynamic light scattering (90Plus/BI-MAS,
Brookhaven Instrument) before and after the laser irradiation as used in the SHG experiments. No change of the
average diameter can be detected. The emission spectral band from the Ag colloids did not change upon laser
irradiation as well. It has been recognized that the two-photon fluorescence band from the nanpoparticles changes
its peak position noticeably with the particle size in the 10-9 to 10-7 m range. Both observations indicate that there
is no fragmentation of Ag particles upon laser irradiation in our experiment.
Dynamic light scattering was also used to monitor the average diameter of the particles during the surface
modification with thiols. The average diameter during the process did not change more than the experimental
uncertainty which is less than ±2% of standard deviation. There was no detectable coagulation or fragmentation of
the particles. This observation shows that the decrease in the SH signal detected upon thiol addition is not due to
the changes in the physical size of the particles.
The extinction spectra of a silver colloid sample before and after adding Ethanethiol are shown in FIG S2. A
minor (<5%) change in intensity is observed with no peak position shift. For colloids with nanoparticles with
diameter ~80 nm, the extinction of intensity is primarily due to scattering and the extinction band shape depends
on particle size, as was discussed by Sosa et al.S1 With the adsorption of thiol molecules on the particle surface, the
diameter of the particle does not change much, due to the fact that thiol molecules used here are small. It is then
reasonable to expect only a minor change in the extinction spectra. This experimental observation is also consistent
with the anticipation that no coagulation or fragmentation occurred for the particles during thiol adsorption. The
peak position in the extinction spectra is at 440 nm. This peak position is also an indication that most of the
particles are spherical in shape, Otherwise the peak position should have considerably red shifted, as showed in
literaturesS1,S2.
FIG S2. UV-VIS extinction spectra of a silver colloid before and after addition of Ethanethiol.
Influence of surface adsorbed citrate molecules on SHG generated from particle surface
Sodium citrate has been the common surfactant used to stabilize the Ag colloid; the citrate ions adsorb onto the
surface of the Ag particles to prevent them from coagulating. An intuitive question is “what is the effect of the
citrate ions adsorbed at the particle surface, if the SHG signal is generated from the particle surface”. The samples
used in all experiments are stablized by adding 5 μM sodium citrate in the colloids. We have also conducted
experiments using samples with sodium citrate concentration as high as 1 mM. No change of the SHG intensity is
observed for samples with different sodium citrate concentrations. It is expected that the citrate coverage at the
nanoparticle surface is higher in colloids with higher citrate concentrations. These experiments indicate that citrate
ions at the surface are not affecting the surface susceptibility of the silver particle: neither contributing to nor
decreasing the surface susceptibility.
Furthermore, even higher concentration of sodium citrate was also added into the silver colloids already treated
with thiols. The decreased SHG signal showed no recovery. This observation shows that once the thiols adsorb
onto the surface, through chemical bonding, citrate ions cannot displace these molecules. This means that the
interaction between silver and citrate is much weaker than that between silver and thiol.
S2
Based on the above observations it is clear that citrate ions weakly interact with Ag particle surface and have
little effect on the particle surface generated SHG. Upon addition of thiols, chemical bonds are formed between
thiol and silver. This bonding reduces the surface generated SHG.
In order to assess the strength of the 2nd order susceptabilities of the citrate and thiols, in comparison with the
dye molecule Malachite Green (MG), at the wavelength of interest here, we have measured the hyper Rayleigh
scattering intensities from solutions of the four molecules. The measurements showed that nonlinear
hyperpolarizability of the sodium citrate and thiols are at least 2 orders of magnitude smaller than MG. We have
noted that the SH signal detected from polystyrene particles with saturated adsorption of MG molecules is smaller
than those observed from silver nanoparticles in this work. These observations together prove that the SH signal
from the Ag colloids cannot be from the citrate ions or thiol molecules at the particle surface, and that the change
of the SH intensity due to thiol addition is solely due to the change of the surface susceptibility of the silver surface
due to chemical bonding.
The Langmuir model analysis of the SH intensity as a function of thiol concentration
With the equilibrium established between thiol molecules in the colloidal solution, T, the thiols bonded at the
surface, TS, and the surface empty sites ES described as： k
1

 TS
T + ES 
(S1)

k -1
the number of molecules adsorbed to the particle surface per unit volume solution, N, can be related to the total
concentration C of thiols in the colloid, and the maximum number of thiol molecules on the surface per unit
volume Nmax as:
(S2)
k1 (C  N )( N max  N )  k1 N
From this equation, it can be deduced that (the only physically allowed solution):
N
(C  N max  K 1 )  (C  N max  K 1 ) 2  4* C * N max
(S3)
2
So the surface coverage  , i.e. N/Nmax , can be expressed asS3

(C  N max  K 1 )  (C  N max  K 1 ) 2  4* C * N max
(S4)
2* N max
where K is the equilibrium constant, i.e. K=k1/k-1. Since the SH intensity is expressed as I SHG | B  A ei |2 , the
SH intensity dependence of C should be:
I SHG | B  A
(C  N max  K 1 )  (C  N max  K 1 ) 2  4* C * N max
2* N max
ei |2
(S5)
Eqn. S5 can be used to fit the SH intensity dependence on thiol concentration. Good fittings can only be
achieved with the K (k1/k-1) value  3*106 M-1, with B as 1.01  0.01, A as 0.47  0.02,  as 0.59  0.01 and
Nmax as 1.30.1. Trials with smaller K values produce fitting results that have much larger reduced-χ squared
values. On the other hand, use of bigger K values results in similar fitting curves. These observations lead to the
lower limit of K value as 3*106 M-1.
REFERENCES
(S1)
Sosa, I. O.; Noguez, C.; Barrera, R. G. J. Phys. Chem. B 2003, 107, 6269-6275.
(S2)
Mock, J. J.et al. J. Chem. Phys. 2002, 116, 6755-6759.
(S3)
Wang, H. F.; Troxler, T.; Yeh, A. G.; Dai, H. L. Langmuir 2000, 16, 2475-2481.
S3