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Optical trapping and light-induced agglomeration of gold nanoparticle aggregates
Yi Zhang and Claire Gu*
Department of Electrical Engineering, University of California, Santa Cruz, California 95064, USA
Adam M. Schwartzberg, Shaowei Chen, and Jin Z. Zhang†
Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, USA
共Received 6 September 2005; revised manuscript received 28 February 2006; published 6 April 2006兲
This paper demonstrates the optical trapping of micron-sized gold nanoparticle aggregates 共GNAs兲with a
TEM00 mode laser at 532 nm and reports the observation of an unusual light-induced agglomeration phenom-
enon that occurs besides the trapping of the GNAs. The observed agglomerate has a 60– 100
m donut-shaped
metal microstructure with the rate of formation dependent on the laser power used. Citrate capped gold
nanoparticles also show light-induced agglomeration, yielding different sized microstructures from those pro-
duced with GNAs. While the observed trapping can be explained by a model including the optical radiation
and radiometric forces, the light-induced agglomeration cannot be explained by these two forces alone as the
size of the agglomerate is much greater than the waist of the Gaussian beam used in the optical trapping. We
attribute the additional cause of the light-induced agglomeration to ion detachment from the surface of the
nanoparticles 共aggregates兲due to light illumination or heating. This is supported by the observation of revers-
ible electrical conductivity changes of the solution of the nanoparticles 共aggregates兲upon laser illumination or
direct heating. Light-induced agglomeration can be useful in the design and fabrication of microstructures from
nanomaterials for various device applications.
DOI: 10.1103/PhysRevB.73.165405 PACS number共s兲: 61.46.⫺w, 36.40.⫺c, 81.07.⫺b
I. INTRODUCTION
Metal nanoparticles have attracted significant attention re-
cently due to their unique optical,1–4 chemical,5,6 and other
physical properties.7–9 Gold and silver particles are of par-
ticular interest due to their potential applications in various
areas including surface enhanced Raman scattering
共SERS兲.10–15 SERS possesses the molecular specificity of
Raman scattering, while magnifying the nominally weak sig-
nal by as much as ten orders of magnitude due to the greatly
enhanced electromagnetic field at the surface of the nanopar-
ticles upon resonant excitation.16–19 It has been found that
the majority of the enhancement takes place at the junction
of aggregated particles20–23 bringing to light the importance
of interparticle interaction, in particular, under photoirradia-
tion.
Recently, we have shown that surface chemistry is an im-
portant factor in nanoparticle aggregation that affects the
physical properties of the aggregates.5,6 Stable nanoparticle
solution requires a repulsive surface capping layer of ions or
molecules. If the capping layer is interrupted, nanoparticles
will agglomerate to form aggregates. This can be induced
experimentally in several ways. The most common aggrega-
tion method for SERS studies is the addition of sodium chlo-
ride or pyridine to silver or gold nanoparticles stabilized by
negatively charged citrate ions. Sodium chloride induces ag-
gregation by screening surface charges, while neutral pyri-
dine displaces the charged citrate and decreases particle-
particle repulsion. Aggregation or agglomeration is a
dynamic process that depends sensitively on many factors
such as light, heat, embedding environment, and, most im-
portantly, the chemical nature of the surface capping ions or
molecules. However, the interplay among these factors and
their effects on the agglomeration of nanoparticles has not
yet been fully explored.
Optical trapping24,25 is a promising technique for probing
the fundamental properties of metal nanoparticles 共aggre-
gates兲and for enhancing SERS detection in chemical and
biochemical analysis.26 Optical trapping of nanometer-sized
Rayleigh 共
Ⰶ, where
is the size of the particle and is
the wavelength of the trapping light兲metal particles27,28 has
been achieved in three dimensions using TEM00 beams.
However, micron-sized metal Mie particles29,30 共
Ⰷ兲can
only be trapped in two dimensions with the light focused
below the center of the particle 共Fig. 1兲with TEM00 beams.
Three-dimensional trapping of Mie metal particles has been
achieved in a TEM01-mode trapping beam or an obstructed
laser beam, where the ring-shaped intensity profile increases
the axial trapping efficiency.31–33
Optical trapping force has been analyzed in detail in con-
junction with the applications of optical tweezers. When a
particle is near the focus of a Gaussian beam, it experiences
a repulsive scattering force and an attractive gradient force
simultaneously.24 When the two forces balance each other
out, the particle will be trapped. Two theoretical models have
been developed to evaluate the trapping force, based on ray
optics24,31 and electromagnetic wave theory,33–35 respec-
tively. Both models agree with experimental measurements
of dielectric Rayleigh and Mie particles. In the case of trap-
ping micron-sized metal particles, both models could be used
but ray optics is more suitable for analyzing the radiation
force due to the large particle size.
In addition, when optical tweezers are used to trap par-
ticles, heat can be generated due to light absorption. As a
result, the particles can be heated to high temperatures, es-
pecially at the laser focus and when the nanoparticles have
strong absorption at the laser wavelength. In this case,
aberration-radiometric force, generated by the heat flow at
the focus due to nonuniform heating of the nanoparticles,
PHYSICAL REVIEW B 73, 165405 共2006兲
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becomes important and needs to be taken into consideration.
Trapping of large metal nanoparticle aggregates has been
challenging due to the irregular shapes of the aggregates. In
this study, we have found that only a small number of aggre-
gates can be trapped in a stable manner. While regularly
shaped particles such as spherical or elliptical particles could
be readily trapped, most irregularly shaped aggregates were
repelled or spun around at the focus of the laser beam.
Interestingly, in the process of observing the trapping of
gold nanoparticle aggregates 共GNAs兲, we have found that
light can induce further agglomeration of the aggregates.
While optical trapping can be explained by a model based on
the radiation and radiometric 共thermal兲forces,36–38 the light-
induced agglomeration indicates an additional mechanism in
operation. We attribute the additional cause for the observed
agglomeration to an optical or thermal detachment of ions
from the nanoparticles 共aggregate兲surface. This suggestion
is supported by an independent study of reversible changes
of the electrical conductivity of the nanoparticles 共aggregate兲
solution upon laser illumination or direct heating.
II. EXPERIMENTAL METHODS
A. Nanoparticle synthesis
GNAs were synthesized by the method of Schwartzberg
et al.5Briefly, 500
l of a 0.02 M HAuCl4stock solution
was diluted to 5⫻10−4 M with Milli-Q water in glassware
cleaned in aquaregia and rinsed with Milli-Q water to avoid
contamination. To this, 40–60
l of a 0.1 M solution of
Na2S aged 2 –3 months was added. After approximately
60– 120 minutes, the color changed from a straw yellow to
deep purple with the extended plasmon band 共EPB兲growing
in between 600– 1000 nm, indicating reaction completion.
The aggregate formation is signified by a strong near infrared
共NIR兲absorption band at wavelengths longer than 600 nm.
This synthesis yields gold aggregates capped with sulfur-
oxygen species, the nature of which is not entirely known.
However, it is believed that these species not only stabilize
the particles, but also induce stable aggregation.
Gold nanoparticles of approximately 40 nm were synthe-
sized by the Turkevich method,39 and silver nanoparticles
were synthesized by the Lee method.40 These procedures in-
volve the reduction of metal salts by sodium citrate in a
boiling aqueous solution resulting in citrate capped nanopar-
ticles.
B. UV-visible spectroscopy, TEM, and AFM measurements
Transmission electron microscopy 共TEM兲measurements
were carried out with a JEOL transmission electron micro-
scope 共Model JEM-1200EX兲. The samples were prepared by
drying one drop of the GNAs solution on a sample holder.
The measurements were run at 80 KV with a Gatan Bioscan
Model 792 camera. A Veeco Bioscope 2 atomic force micro-
scope 共AFM兲was used to take the three-dimensional 共3D兲
pictures of a single GNA. The sample was scanned using a
commercially available piezoelectric scanner 共Physik Instru-
ment兲and control electronics 共Digital Instruments兲. To fur-
ther test the property of the GNAs, optical absorption mea-
surements were performed on an HP 89532A spectrometer
with 2 nm resolution.
C. Optical trapping and light-induced agglomeration
Figure 2 shows a schematic of the optical trapping instru-
mentation. A cw Verdi laser at 532 nm with a maximum laser
power of 2 W was used in the experiment as the trapping
light source. The diameter of the laser beam was reduced
FIG. 1. Two different configurations of light focused on a
spherical metal particle. 共a兲The focal plane is below the center of
the metal particles and a net attractive force is generated to pull the
particle to the focus. 共b兲The focal plane is above the center of the
metal particle and a repulsive force is generated to push the particle
out of the focus 共adapted from Ref. 30兲.
FIG. 2. 共Color online兲Schematic of experimental setup for the
optical trapping and light-induced agglomeration studies.
ZHANG et al. PHYSICAL REVIEW B 73, 165405 共2006兲
165405-2
before entering the 60⫻objective lens 共numerical aperture
=0.85兲, however, the effective numerical aperture 共NA兲was
0.64 because the objective lens was pointed into the water
solution 共with GNAs兲. The trapping and agglomeration were
observed with a charge coupled device 共CCD兲camera
mounted behind a cubic beam splitter. Another lens 共f
=10 cm兲was placed in front of the camera in order to ob-
serve the image at the focus of the objective lens. The lamp
on the right bottom was used for imaging. The green light
共532 nm兲was filtered out by a bandpass filter.
D. Electrical conductivity measurement
Figure 3 shows the experimental setup of the electrical
conductivity measurements. The electrical conductivity of a
solution of GNAs was measured under various conditions
with a computer-controlled potentiostat 共SI-1280B兲.Two
gold electrodes were inserted into the GNA solution with a
spacing of 2 cm and an external 3 V potential was applied.
The electrodes were about 2 cm long and the diameter was
about 1 mm with a tip diameter of 50
m共Fig. 3 inset兲.
About 75% of the entire electrode was immersed into the
solution. A thermometer was also inserted into the solution
between the electrodes to trace the temperature change of the
solution.
At first, the sample was illuminated with another cw Verdi
laser with an output power 3 W at 532 nm between the elec-
trodes with constant stirring to ensure thermal consistency
throughout the solution. Afterward, the solution was cooled
down back to its original temperature. Next, the solution was
heated with a hot plate without laser light. The current
change was recorded as a function of time in the cases of
both laser illumination and direct heating.
III. SIMULATION METHOD
Accurate modeling of the radiation force exerted on nano-
particle aggregates requires the knowledge and consideration
of the size, shape, and arrangement of the constituent nano-
particles. Such an analysis would allow us to examine the
details of forces acting upon every point on the surface of an
aggregate. However, as we are mostly interested in the over-
all force experienced by an aggregate whose detailed feature
size is on the order of a few nanometers 共5– 10 nm as in Fig.
5 and related discussions兲, which is much smaller than the
wavelength of light used in our experiments 共532 nm兲,we
treat each aggregate as a metal particle with an effective
complex refractive index. In our experiments, we also ob-
served various irregular shapes of the aggregates. To simplify
our analysis, we analyze the forces exerted on a spherical
metal particle. These approximations allow us to obtain an
estimate of the radiation force.
The mechanism of trapping a metal particle in two dimen-
sions with a fixed TEM00 mode laser beam focused at the
bottom of the particle is shown schematically in Fig. 1. Be-
cause of the large imaginary part of the refractive index of
metal, the reflectivity of the metal surface is so large that
most of the light will be reflected 共reflected rays not shown
in Fig. 1兲. When the light is focused at the bottom left of the
particle 共top figure in Fig. 1兲, the optical radiation force ex-
erted on the metal particle, indicated by the small arrows
pointing to the center inside the sphere, is larger on the right
than on the left side. In other words, the lateral net force is
pointing toward the focus. However, when the light is fo-
cused at the upper left of the particle 共bottom figure in Fig.
1兲, the net force is pointing away from the focus. As the
nonuniformly reflected 共and absorption as will be shown be-
low兲light will only provide lateral restoring force when the
beam is focused below the particle center, the metal particle
can only be trapped two dimensionally.29
By using the ray optics method and considering the focus
below the particle center, detailed calculations of the trap-
ping force of micron-sized metal particles have been carried
out by Gu,31,32 For GNAs, the refractive index is 0.62 +2.08i
at 532 nm. Both reflection and absorption are strong at this
wavelength and need to be taken into account. For instance,
at normal incidence, 34% of the light will be absorbed and
could result in a temperature change of 200 °C or more de-
pending on the particle size and photon flux.41 Therefore,
both reflection and absorption are included in our calculation
to improve the accuracy.
For the refracted light, only the first refraction is neces-
sary because of the thin skin depth of the metal, which is
only a few nanometers and smaller than the size of the gold
nanoparticles. In the 1970s, Roosen and Imbert33 introduced
another method to evaluate the optical trapping force expe-
rienced by metal particles in TEM00 and TEM01 mode33
beams in which the Gaussian mode profile was included for
the calculation, but they did not consider the case in which
the light could be focused at the bottom of the particle 共Fig.
1兲. We base our analysis on Roosen’s expressions of radia-
tion force, including absorption, and consider that the focus
of the Gaussian beam is below the center of the metal par-
ticle 共Fig. 1兲. The radiation forces are expressed below with
the parameters defined by Roosen and Imbert.33
Fy=
冕
0
/2
d
冕
0
2
E2
2sin
cos
2
0c2Im关
共
兲兴sin
d
,
FIG. 3. 共Color online兲Experimental setup for the electrical con-
ductivity measurement. The change of conductivity is induced by
either laser illumination or hot plate heating. The temperature is
recorded for both cases to make sure that the temperature changes
are the same. The inset shows the scanning electron microscopy
共SEM兲picture of the tip of gold electrode and how it is assembled
with the holder. This type of probe is used for the electrical con-
ductivity measurement.
OPTICAL TRAPPING AND LIGHT-INDUCED¼PHYSICAL REVIEW B 73, 165405 共2006兲
165405-3
Fx=
冕
0
/2
d
冕
0
2
E2
2sin
cos
2
0c2Im关
共
兲兴cos
d
,共1兲
where Eis the electric field,
E2=E0
2exp
冉
−2共
2sin2
+
0
2−2
0sin
sin
兲
w2
冊
共2兲
and
is the radius of the metal sphere,
0is the distance
between the laser axis and the sphere center, wis the beam
waist,
is the in-plane Euler angle in the local spherical
coordinate system,
is the incident angel for a single ray,
and
共
兲is the transmission intensity factor depending on the
ray path which is defined as
共
兲= 1 + Rexp共−2j
兲−T2exp共−2j
兲
exp共−2j
1兲+R.共3兲
The TEM00 mode in our system is described as a Gaussian
function instead of using a plane wave approximation, since
the beam waist is about 1
m and the trapped particles are
similar in size.
For each incident angle
, there will be a complex refrac-
tive angle,
1, because of the complex refractive index of the
metal particles. The reflectivity Rand transmittance Tare
calculated using the complex refractive index and the inci-
dent angle of each ray, respectively. Because of the strong
absorption of GNAs at 532 nm, it is necessary to include the
refraction and absorption. Furthermore, because of the strong
absorption, it is not appropriate to only consider the trapping
force; the light-induced heating must also be considered, as
has been pointed out and considered by others.28–32
The light-induced heating results in the radiometric
force36–38 that is another important factor in the observed
trapping and agglomeration. The metal nanoparticles inside
the trapping beam will be heated nonuniformly due to the
nonuniform intensity distribution of the Gaussian beam. The
particles in the higher intensity region 共near the beam center兲
are heated to a higher temperature, therefore they move
faster, recoil back, are pushed out of the focus.36 Previously,
levitation by radiometric forces was observed with glycerol
spheres that were impregnated with dye in the air at low
pressure.36 For metal particles, the absorption coefficient is
large, especially for gold and silver at 532 nm, and radiomet-
ric force is thus expected to be strong.
Yalamov42 has shown the expression of radiometric force
Fras
F
ជ
r=−4
Rp
e
2JKI
ជ
gTsKi
,共4兲
where all the parameters were given in detail by Arnold.37
Briefly, Rpis the radius of the particle, and Tsis the surface
temperature,
eis the viscosity of the water, and
gis the
density of metal particles. Kiis the interior thermal conduc-
tivity, which is related to the thermal property of the active
material.
IV. EXPERIMENTAL RESULTS
A. UV-visible spectrum, TEM, and AFM of
gold nanoparticles (aggregates)
The UV-visible electronic absorption spectrum 共Fig. 4兲of
the GNAs shows two bands in the visible to near IR region,
one peaked at 530 nm and another near 760 nm. The 530 nm
band is the transverse surface plasmon absorption, typical for
gold nanoparticles, while the 760 nm band has been attrib-
uted to strong interaction between nanoparticles in the aggre-
gates, termed extended plasmon band 共EPB兲.43 The width
and location of the EPB strongly depends on the size and the
shape of the aggregates.
Representative TEM and AFM images of the GNAs are
shown in Fig. 5 Based on the TEM and AFM data, the aver-
age size of the aggregates varies from 100 nm to over
2000 nm and each aggregate is composed of many strongly
interacting gold nanoparticles with an average diameter be-
tween 5 and 10 nm.
B. Observation of optical trapping of
nanoparticles (aggregates)
Figure 6 shows a single GNA, 2
m in diameter, trapped
stably in two dimensions. Larger cylindrical, elliptical, or
spherical aggregates up to 3
m were also trapped with a
TEM00 mode laser beam 共532 nm兲with approximately
50 mW power and a beam waist of about 1
m. The micron-
sized metal structure could only be trapped in the periphery
of the beam when focused at the bottom of the aggregate
共Fig. 6兲.
Although our experimental setup does not allow direct
observation of trapping of individual isolated gold or silver
nanoparticles smaller than a few hundred nanometers since
they are much smaller than the diffraction limit of the experi-
mental system, we did observe trapping of aggregates with
regular shapes and diameters about 1– 2
m that were
formed from the isolated particles. The mechanism for ag-
gregation is believed to be largely caused by light- or heat-
induced ion detachment, as discussed in Sec. III C.
C. Observation of light-induced agglomeration
Besides the successful optical trapping of GNAs, agglom-
eration of the GNAs was also observed under light illumina-
FIG. 4. UV-visible absorption spectrum of gold nanoparticle
aggregate solution. Concentration was as prepared and as used in
trapping experiments.
ZHANG et al. PHYSICAL REVIEW B 73, 165405 共2006兲
165405-4
tion. This agglomeration only took place at some particular
locations, where the GNAs of about 1– 2
m are deposited
on the glass slide. The agglomeration occurred more quickly
for larger sized aggregates, e.g., in the 1– 2
m range. Fig-
ure 7 shows images of the agglomeration process. Typically,
some aggregates were first attracted and trapped close to the
focus region of the laser beam 共as what happened in Fig. 6兲,
since the metal particles could not be stably trapped in the
focus and the particles would be pushed away to the periph-
eral region. Within 10 to 15 s, more aggregates were at-
tracted to the laser beam and collected in the peripheral re-
gion of the laser beam 关Fig. 7共b兲兴. Agglomeration began to
take place when the laser beam was shifted a few microme-
ters to the peripheral region of the collected aggregates. We
observed that the GNAs as far as 100 micrometers away
were sucked into the agglomeration region and began to
form a ring microstructure 关Fig. 7共c兲兴. Eventually, on the
time scale of 1– 5 min, a large donut-shaped microstructure
with an outer diameter approximately 60
m was formed
and the hole in the center had a diameter of about 15
m,
and it became stabilized 关Fig. 7共d兲兴. The bright spot in the
center of the agglomerate was the focal spot of the laser
beam. After the donut-shaped microstructure was fully
grown and the laser beam was blocked, it would be stable for
10– 20 min before it eventually broke apart or redispersed
into smaller aggregates or nanoparticles again. This photoin-
duced agglomeration process was easily reproducible. Occa-
sionally, the microstructure was stable for longer periods of
time. If one of these stable samples was left to dry, a disk of
gold nanoparticles 共aggregates兲with a hole in the center
could be clearly observed on the glass substrate under a mi-
croscope.
To determine the effect of the laser beam shape and power
on the agglomeration, two different beam patterns, circular
and elliptical, with various power levels were used. An ellip-
tical beam pattern was obtained by replacing one of the con-
vex spherical lenses of the beam expander with a cylindrical
lens. The microstructure formed became elliptical rather than
circular when a cylindrical lens was used 共Fig. 7兲. The size
of this agglomerate structure as well as the rate of formation
increased when the power of the laser was increased.
Possible manipulation of the agglomerate with a laser
beam was also investigated. After the formation of the ag-
glomerate, if the glass slide was moved laterally by 2– 5
m,
the metal agglomerate initially moved together with the glass
slides, then jumped rapidly to the shifted new focal point,
showing that it was trapped. The agglomerate did not follow
if the laser beam was moved farther away 共e.g., ⬃20
m兲.
As mention in Sec. III B, large agglomerates can also be
observed if the starting sample solution contains isolated sil-
ver or gold nanoparticles. These large agglomerates are be-
lieved to originate from smaller aggregates that were formed
from isolated particles due to surface ion detachment by the
laser light or heat. The experimental setup only allows us to
observe the aggregates 共agglomerates兲when they reach a
certain size 共over several hundreds of nanometers兲.
D. Measurement of electrical conductivity of gold
aggregates solution
It observed that when the GNAs solution was illuminated
with a laser beam at 532 nm, the electrical conductivity of
FIG. 5. 共Color online兲共a兲AFM image showing the three-
dimensional structure of the gold nanoparticle aggregates. 共b兲A
representative TEM image of aggregates formed by gold nanopar-
ticles 共around 10 nm in diameter兲, with a variety of aggregate sizes
and shapes.
FIG. 6. 共Color online兲Microscopic images of a two-
dimensionally trapped GNA 共approximately 2
m兲by using a fixed
Verdi laser beam of TEM00 mode 共a兲before and 共b兲after the move-
ment of the glass slide. The arrow points to the darker aggregate
trapped at the periphery of the beam. The bright spot is the laser
focus which is not totally filtered out. The larger aggregate on the
left is shown as a reference point.
OPTICAL TRAPPING AND LIGHT-INDUCED¼PHYSICAL REVIEW B 73, 165405 共2006兲
165405-5
the solution increased quite drastically 共Fig. 8兲. During the
illumination, the temperature of the solution increased from
30 ° C to 50 ° C in 5 min while the electrical current in-
creased from 15
Ato22
A, indicating an increase of the
electrical conductivity of the solution.
To determine if the conductivity increase was due to heat-
ing as a result of laser illumination, a hot plate was utilized
as a direct heat source. The current was found to increase
from 15
Ato18
A when the solution temperature was
changed from 30 ° C to 50 ° C 共Fig. 8兲.
V. SIMULATION RESULTS
Figure 9 shows the simulation results of the optical trap-
ping force as well as the radiometric force exerted on a 1
m
gold particle by using the model introduced above. By focus-
ing the Gaussian beam 0.5
m below the center of the par-
ticle, it can be seen that there is an attractive force in the
peripheral region of the beam, although in the center of the
beam the radiation force is still repulsive. One can see that
the maximum trapping force 共with the radiometric force兲is
about 4.8 pN and it has an effective region about twice the
size of the beam waist. In the center there is a 4 pN repulsive
force. Only the lateral force is calculated in this two-
dimensional trapping case. A 0.7 pN repulsive radiometric
共thermal兲force is also added to the radiation 共trapping兲force.
Compared with the radiation force, the radiometric force is
smaller but it is not negligible and it is repulsive near the
center of the laser beam.
FIG. 7. 共Color online兲Microscopic images of the formation process of agglomeration induced by light from a Verdi laser at 50 mW. 共a兲
Beginning of the trapping and agglomeration process, there are many aggregates 共1–2
m in size兲in the center region. 共b兲t= 15 s of light
illumination, more aggregates of 1– 2
m in size were attracted and deposited to the bottom of solution on the glass slide; 共c兲t=20 s of
illumination: a large agglomerate formed suddenly at the consumption of the small aggregates located within a radius of about 50
m. The
formed agglomerate then continued to grow by attracting more aggregates from the solution. 共d兲t=60 s of illumination: final state of the
agglomerate, almost all the small aggregates 共1⬃2
m兲in the viewing area on the glass slide were attracted to and became part of the large
donut-shaped agglomerate, which has an elliptical shape because a cylindrical lens was used resulting in an elliptical focus.
FIG. 8. Comparison of the current change of the gold nanopar-
ticle aggregate solution due to heating by the hot plate or by laser
illumination. The temperature was monitored to ensure that the tem-
perature change was the same or very similar for the two cases.
ZHANG et al. PHYSICAL REVIEW B 73, 165405 共2006兲
165405-6
VI. DISCUSSION
A. Mechanism of optical trapping
As mentioned earlier, two forces, radiation force and ra-
diometric 共thermal兲force, are usually involved in the optical
trapping of particles. For micron-sized metal particles, ray-
optics approximation can be used to evaluate the radiation
force and multiple internal reflections do not need to be con-
sidered because of the strong absorption of the metal. Radio-
metric force caused by the strong absorption of metal par-
ticles could be generated by the asymmetric heat flow. This
may destabilize the trapping.
As shown by the simulation results in Fig. 9, there is a
maximum of 4.8 pN attractive radiation force in the periph-
eral area and a 3.5 pN repulsive force in the center of the
Gaussian beam 共without radiometric force兲. The force distri-
bution is central symmetric, which is consistent with the ex-
perimental observation of both trapping and agglomeration.
The radiation force will help hold together the nanoparticles
共aggregates兲and possibly initiate further agglomeration.
On the other hand, the radiometric force36–38 is generally
repulsive, as shown in Fig. 9. As a result, particles 共aggre-
gates兲will be repelled from the center of the laser focus. This
explains our observation 共shown in Fig. 6兲that a micron-
sized GNA can only be trapped at the peripheral region of
the laser focus spot. In our simulation results shown in Fig.
9, the radiometric force adds to the repulsive radiation 共trap-
ping兲force in the center of the beam, resulting in a hole in
the central region. This is consistent with what has been
predicted theoretically by Sato et al.29 previously. To our
knowledge, Sato et al.29 is the first experimental observation
to confirm this prediction.
To fully understand the details of the radiation and radio-
metric forces, a model that takes into account the detailed
size, shape, and arrangement information of the constituent
nanoparticles, as well as the irregular shape of the aggregate,
needs to be established. The results reported here provide an
order of magnitude estimate and indicate that there are other
factors involved in the observed agglomeration.
B. Mechanism of agglomeration
To understand the agglomeration phenomenon observed,
it is not sufficient to consider only the radiation force and the
radiometric force, because the optical trapping force extends
to only twice the size of the beam waist 共Fig. 9兲. For the
large agglomerates, the size is about 40–50 times larger than
the beam waist 共w=1
m兲. Here we suggest another factor
to consider, namely photothermal ion detachment from the
surface of nanoparticles 共aggregates兲.
The GNAs consists of many 5– 10 nm nanoparticles as
the fundamental components 共Fig. 4兲. The surface chemistry
of the Au particles is quite complex and not well understood.
However, it has been suggested that the capping layer of the
gold nanoparticles 共aggregates兲consists of negatively
charged ions containing sulfur and oxygen that stabilizes the
nanoparticles 共aggregates兲.44 As a result, the surface of the
nanoparticles 共aggregates兲is negatively charged. This has
been confirmed recently using gel electrophoresis measure-
ment in our lab. The negative charges on the nanoparticles
共aggregates兲prevent them from forming larger aggregates or
agglomerating due to repulsive forces between them. Any
removal of the surface ions would destabilize the nanopar-
ticles 共aggregates兲and result in agglomeration due to re-
duced charge screening.
Due to its strong optical absorption at the laser wave-
length used and the high intensity of the focused laser beam,
the nanoparticles 共aggregates兲quickly convert the light en-
ergy into thermal energy, initially resulting in a substantial
increase in the local temperature of the particles, and even-
tually causing a global temperature increase of the colloidal
solution.41 This quick and substantial increase in temperature
could liberate a significant amount of capping ions from the
particle surface. We suggest that removal of surface ions,
which help to stabilize the original nanoparticles and aggre-
gates, due to laser-induced heating results in the observed
light-induced agglomeration of the nanoparticles 共aggre-
gates兲.
If this suggestion is correct, one could expect a change in
the electrical conductivity of the nanoparticles 共aggregate兲
solution upon laser illumination due to ions released from the
particle surface. To test this idea, the electrical conductivity
change of the metal nanoparticles 共aggregate兲solution has
been measured 共Fig. 3 and Fig. 8兲. As expected, a noticeable
increase in the conductivity 共current兲was measured upon
light illumination of the nanoparticles 共aggregate兲solution.
The temperature of the solution was monitored at the same
time and found to increase with the laser illumination. The
increase in global 共solution兲temperature alone could in-
crease the ion mobility and hence solution conductivity. In
order to decouple the effect of global heating 共averaged for
the entire solution sample兲from that of local heating 关nano-
particles 共aggregates兲heated by the laser light directly before
equilibrating with the entire solution兴, a similar experiment
FIG. 9. Simulation results of transverse trapping forces Fx 共pN兲
as a function of the transverse displacement for a single gold par-
ticle 共radius=1
m兲from the center of the focal point of the laser
beam. The radiometric force is included for correction. There is a
net repulsive force at the beam center when laser beam is focused
z=0.5
m below the center of the particle.
OPTICAL TRAPPING AND LIGHT-INDUCED¼PHYSICAL REVIEW B 73, 165405 共2006兲
165405-7
was performed with direct heating without light.
By heating the solution on a hot plate, we examined the
effect of global heating on ion mobility in solution and pos-
sible ion liberation from the nanoparticles 共aggregates兲, both
of which can contribute toward the solution conductivity
change. As shown in Fig. 8, the conductivity clearly in-
creased with heating of the solution in the same temperature
range as in the laser illumination experiment 共30– 50 ° C兲.
However, the increase in conductivity due to thermal heating
by the hot plate is noticeably less than that due to laser illu-
mination. The difference between the two measurements is
likely due to the liberation of capping ions from the particle
surface by laser-induced heating, which results in a local
temperature of the particles much higher than the average
temperature measured for the entire solution by a thermom-
eter.
In the case of direct thermal heating by a hot plate, ion
mobility is expected to increase with the increased tempera-
ture, which results in an increase of the conductivity or cur-
rent. Liberation of capping ions from the nanoparticles 共ag-
gregate兲surface is possible but not very significant within
the temperature range under study.45 Therefore, the increase
in ion mobility is suggested as the predominant factor in the
observed increase in conductivity with heating.
In the case of laser illumination, the average solution tem-
perature increase was the same as in the thermal heating
experiment. One would expect the same conductivity in-
crease if the effect of the laser is the same as direct thermal
heating. The fact that the conductivity increase is noticeably
more with laser illumination than with thermal heating sug-
gests that additional contributions must be taken into ac-
count. One possible explanation is that, besides heat gener-
ated from the laser, direct detachment or liberation of ions is
induced by photoexcitation. Another possible explanation is
that the local temperature of the nanoparticles 共aggregates兲
might be much higher with the initial light absorption than
the average temperature measured using a thermometer glo-
bally. A higher local temperature would also result in the
release of more ions from the particle surface. The liberation
from the particle surface, due to direct photoexcitation or
photoinduced local heating, was consistent with the larger
increase in conductivity or current in the laser illumination
experiment. Very importantly, this explanation is consistent
with the light-induced agglomeration observed, since ion lib-
eration from the surface of nanoparticles 共aggregates兲is ex-
pected to result in the formation of agglomerates.
It was noticed that the microstructured agglomerates
could be redissolved or redispersed into nanoparticles 共ag-
gregates兲after the laser illumination was stopped if the so-
lution was not dried up. This observation is consistent with
the suggestion of photothermal ion detachment from the
metal surface. It is a dynamic process for the capping ions to
leave or attach to the surface of the nanoparticles 共aggre-
gates兲. Upon laser illumination or direct heating, surface ions
detach from the particle surface, resulting in agglomeration.
When laser illumination is stopped, the ions can attach to the
surface of the agglomerates and shift the equilibrium toward
nanoparticles and aggregates.
VII. CONCLUSION
The optical trapping of micron-sized metal gold nanopar-
ticle aggregates 共GNAs兲with a TEM00 mode laser light at
532 nm has been demonstrated. Besides the successful opti-
cal trapping of 1– 3
m GNAs, an unusual light-induced ag-
glomeration of GNAs has also been observed. By illuminat-
ing the nanoparticles with a 50 mW focused laser beam, a
60– 100
m donut-shaped metal microstructure was formed
in a GNA solution. The size of the agglomerate microstruc-
ture is proportional to the power of the laser. This agglom-
eration phenomenon, along with optical trapping of the nano-
particles 共aggregates兲, was analyzed by considering radiation
force, radiometric force, together with surface ion detach-
ment caused by laser-induced heating. The observed electri-
cal conductivity changes of the nanoparticles 共aggregate兲so-
lution upon light illumination supports the suggestion of
photothermal ion detachment. The results indicate the possi-
bility of using light to control the surface properties of na-
nomaterials and thereby to design and fabricate microstruc-
tures from nanomaterials for device applications in
nanotechnology.
ACKNOWLEDGMENTS
This research is supported by the National Science Foun-
dation 共Grants No. ECS-0401206 and CHE-0456130兲and by
University of California at Santa Cruz, Special Research
Grants.
*Electronic address: claire@soe.ucsc.edu
†Electronic address: zhang@chemistry.ucsc.edu
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