Journal of Materials Science and Chemical Engineering, 2014, 2, 20-25
Published Online January 2014 (http://www.scirp.org/journal/msce)
http://dx.doi.org/10.4236/msce.2014.21004
OPEN ACCESS MSCE
Gold Nanorods: Near-Infrared Plasmonic Photothermal
Conversion and Surface Coating
Bo Cong1, Caixia Kan1,2*, Honggeng Wang1, Jinsheng Liu1, Haiying Xu1, Shanlin Ke1
1College of Science, Nanjing University of Aeronautics and Astronautics, Nanjing, China
2Key Laboratory for Intelligent Nano Materials and Devices of the Ministry of Education, China
Received October 2013
ABSTRACT
In this paper, AuNRs colloids with SPRL located at ~810 nm and ~1100 nm were synthesized using an improved
seed method. Based on the NIR lasers available, photothermal conversion of AuNRs were systematically studied
compared with that of water. Under low power irradiation, the highest temperature is obtained when the SPRL
wavelength of AuNRs is equal to the laser wavelength, and temperature of colloid increases from ~20˚C to ~65˚C.
With increasing laser power (such as 6 W), the AuNRs colloid boils within a few minutes, and nanorods undergo
a shape deformation from rod to spherical particle and even fusion, and the SPRL disappears. For further inves-
tigation, the obtained AuNRs were coated with SiO2 shell to form a core-shell nanostructure (Au@SiO2). The
surface coating can be used not only in keeping the stability of AuNRs for further treatment, but also in increas-
ing plasmonic property and biocompatibility . This work will be useful for designing plasmonic photothermal
properties and further applications in nanomedicine.
KEYWORDS
Gold Nanorods; Surface Plasmon Resonance; Photothermal Effect; Core-Shell
1. Introduction
Gold nanorod is one of the most studied colloidal nanos-
tructures for its two distinct surface plasma resonances
(SPR), known as the transverse mode (SPRT) and longi-
tudinal mode (SPRL) [1-4]. The SPRT is weak and lo-
cated at ~520 nm, while the strong SPRL can be tuned in
the visible-near infrared (Vis-NIR) region [5-7]. Wet
chemical fabrication of AuNRs was pioneered by Mur-
phy and coworkers through a citrate-capped seed tech-
nique [8,9]. Improvement of this method for producing
AuNRs with high yield was achieved by introduction of
CTAB [10-12].
The optical tunability of AuNRs has brought several
advantages to AuNRs. For example, using the tunability
(and sensitivity) of SPR to aspect ratios of AuNRs and
polarization of light, five-d i m e nsi onal optical recording-
reading process was realized [13]. The changes of den-
sity of surface charge and surrounding index can be used
for the direct observation of chemical reactions [14,15].
The high dependence of SPRL on aspect ratios, surface
coa ti ng, and aggregation degree or assembly is promising
for chemical and biological sensing. The NIR SPRL ex-
tinction is consistent with the human tissue facilitating
photothermal therapy and solar energy harvesting [16-
19]. For in-vivo therapy in cancer research, small diame-
ter AuNRs are more ideal metal nanostructures for can-
cer diagnosis and therapy for their tunable SPRL and op-
timal penetration through biological skin and tissue in the
NIR region [20-25]. And the photothermal conversion ef-
ficiency is an essential parameter in the therapy [26,27].
However, due to the toxicity of CTAB covered on
AuNRs, surface coupling with biological compatibility
molecular was usually proposed for application research
[28]. On the other hand, surface coating with medium to
form a core-shell structure also provides an effective way
to keep the stability in further treatment. Silica (SiO2), as
a gain medium for plasmonic property and advantage of
biological compatibility and chemical stability, was a
promising coating medium, especially for surface coating
of AuNRs [29]. The SiO2 coating can be used not only in
keeping the stability of AuNRs for further treatment, but
also in increasing plasmonic property and biocompatibil-
ity.
In this work, AuNRs colloids with SPRL absorption
peaks were synthesized using an improved “seed”
method. Based on the NIR lasers available, such as 808
*Corresponding author.

B. CONG ET AL.
OPEN ACCESS MSCE
21
nm and 1064 nm, the photothermal conversion of AuNRs
colloids was studied under different irradiation perform-
ance. [30] Under laser radiation, temperatures of AuNRs
colloids rise obviously. With increasing the laser power
(such as 6 W), the AuNRs colloid boils within a few
minutes, and nanorods undergo a shape deformation
from rod to spherical particle and even fusion. For fur-
ther research, SiO2 coating on AuNRs to form Au@SiO2
core-shell structure was also carried out.
2. Experiment Section
2.1. Synthesis of AuNRs
AuNRs colloids with desired SPRL wavelengths (such as
810 nm and 1100 nm) can readily be synthesized through
a similar process, as reported previously [31,32]. Briefly,
in the first step, the seed solution for AuNRs was pre-
pared by dissolving HAuCl4 (0.05 mL, 0.05 M) in an
aqueous solution of CTAB (10 mL, 0.1 M). Then, ice-
cold NaBH4 (0.6 mL, 0.01 M) was injected into the solu-
tion under vigorous stirring. In the second step of the
growth solution fo r GNRs with SPRL at 805 nm, 0.912 g
CTAB together with 0.110 g 5-bromosalicylic acid was
dissolved in 49 mL water. Then AgNO3 (0.24 mL - 1 mL,
0.02 M) was added under stirring, followed by addition
of HAuCl4 (0.5 mL, 0.05 M). Then 0.13 mL (0.1 M) AA
solution was added with st ir r ing until the solution be-
came colorless. After aging 30 min, 0.06 mL “seed” so-
lution was added into the as-prepared growth solution,
and AuNRs can be obtained after 12 h.
2.2. Photothermal Conversion of AuNRs
The setup for the measuring the photothermal conversion
was composed of a 10 mm path length quartz cuvette, a
sensitive digital thermometer (TM-902C), and continu-
ous lasers (including 808 nm laser coupled out through
an optical fiber, GKFCM-808; semiconductor diode 980
nm laser; and 1064 nm, GKNQL-1064). The laser power
was obtained from the P-I curves. The laser light illumi-
nated on the AuNRs colloid in the cuvette. After each
irradiation, the probe head of the thermocouple was
completely submerged in the colloid immediately. The
temperature of the colloid is reasonably assumed to be
uniform in the solution for the same testing data at dif-
ferent positions.
2.3. Surface Silica Coating AuNRs
0.04 mL NaOH (0.1 M) solution was added into the
as-obtained Au NRs colloid (4 mL) with a certain CTAB
concentration under stirring. Then 0.04 mL TEOS (in
ethanol) was injected into Au NRs colloid under gentle
rotary shaking for the hydrolysis of TEOS and SiO2
coating on the surface of Au NRs. Au@SiO2 core-shell
structure was obtained after 20 h.
2.4. Characterization
UV-Vis-NIR spectra of prepared solution were collected
by spectrophotometer (UV-6300) in the wavelength ran-
ge of 200 - 1100 nm. The products were purified by first
centrifugating at 3000 RPM for 20 min to remove the
large sized particles deposited on the bottom of centrifu-
gal tube. Then the colloids were centrifugated repeatedly
at 14000 RPM with deionized water. Then the samples
were deposited on copper grids covered by an amorphous
carbon film for transmission electron microscope (TEM:
JEOL-100CX) measurements.
3. Results and Discussion
The microstructure and growth mechanism of the AuNRs
were studied in detail, as reported previously [31]. Fig.
1a shows the normalized UV -Vis-NIR absorption spectra
for the AuNRs colloids. In addition to the strong and
tunable SPRL absorption peak located at ~810 and 1100
nm, respectively, there is a weak resonance peak located
at ~520 nm, originated from the SPRT absorption of
nanorods and possible existence of spherical nanoparti-
cles and nanocubes. TEM images of the AuNRs with
SPRL located at ~810 nm, and ~1100 nm are shown in
Figures 1(b) and (c). The sizes of the AuNRs are nearly
unifo rm.
The effects of the SPR wavelength and NIR laser on
the photothermal conversion efficiencies of AuNRs are
studied by measuring the temperature increase, compared
with that of water. Under low power irradiation, it is
shown that the highest temperature is obtained when the
SPRL wavelength of AuNRs is equal to the illumination
laser wavelength, and temperature of the colloid rises
from ~20˚C to ~65˚C, which is accordant with the re-
ported work [32]. However, the photothermal efficiency
Figure 1. UV-Vis-NIR absorption spectra (a) and TEM
images for the as-prepared AuNRs colloids with SPRL ab-
sorption located at ~810 (b) and ~1100 nm (c).

B. CONG ET AL.
OPEN ACCESS MSCE
22
decreased with increasing irradiating time. As shown in
Tables 1 and 2 for the photothermal effect of AuNRs
colloid (1 mL) with SPRL peaks located at ~805 and
1100 nm irradiated, respectively, under 808 and 1064 nm
lasers. Since AuNRs were dispersed in deionized water,
we used the heat capacity of water 4.2 J/g in calculating
the efficiency of AuNRs colloid.
Figures 2(a) and (b) showed the optical absorption
spectra of AuNRs colloids before and after irradiation
with high laser power. After the AuNRs colloid with
SPRL at 810 nm was irradiated under 808 nm laser (6 W)
for 3 min, the intensity of SPRL decreased sharply, and
the absorption damped over the whole region. The SPRL
at 1100 nm disappeared after the AuNRs colloid was
irradiated by 1064 nm laser (7 W) for 8 min, and only the
absorption peak of gold spherical nanoparticles was ob-
served, as inserted one in Figure 2(b).
With increasing the laser power (such as 6W for 808
nm laser), the AuNRs colloid would boil within a few
minutes. After the strong irradiation, the color of the
colloid faded with products deposited on the bottom.
Corresponding to the optical spectra changes of the
AuNRs colloid under strong laser irradiation, TEM im-
ages indicated that nanorods undergo deformation from
rod to spherical particle and even fusion under irradiation
of high power. Figures 3(a), (b) presents the TEM im-
ages of the AuNRs (with SPRL at 1100 nm) before and
after laser irradiations. After the AuNRs colloid was ir-
radiated with 1064 nm laser (7 W) for 8 min, the aspect
ratio decreased and large sized nanoparticles were obser-
ved. The morphology evolution of the AuNRs under high
power laser irradiation can be used to explain the disap-
pearance of SP R L and the photothermal efficiency de-
crease of AuNRs colloid with increasing irradiating time.
Table 1. Phot o-thermal effect of AuNRs colloid (1 mL) with SPRL peaks located at ~810 nm under 808 nm lasers (laser spot
size: 2 mm) irradiation. (In calculating the efficiency of AuNRs colloid, the heat capacity of water is 4.2 J/g).
Power
(W) Irradiation time (min) T(˚C) of sample before and after irradiation Efficiency of AuNRs colloid T(˚C) of pure water before and after irradiation Efficiency of pure wa ter
0.333
2 22/27 0.525 18/18 0
5 22/31 0.372 18/18 0
10 22/35 0.269 18/19 0.021
0.667
2 22/35 0.682 19/20 0.0525
5 22/46 0.503 19/22 0.063
10 22/52 0.315 19/23 0.042
0.800
2 22/36 0.612 20/21 0.0438
5 22/48 0.455 20/24 0.07
10 22/57 0.30625 20/27 0.0613
Table 2. Ph ot o-thermal effect of AuNRs colloid (1 mL) with SPRL peaks located at 1100 nm under 1064 nm lasers (laser spot
size:r = 1 mm) irra diation.
Power
(W) Irradiation time (min) T(˚C) of sample before and after irradiation Efficiency of AuNRs colloid T(˚C) of water before and after irradiation
0.8
2 23/29 0 .2625 23/24
5 23/36 0 .2275 23/25
10 23/41 0.1 575 23/26
1.2006
2 23/38 0 .4375 22/23
5 23/46 0.268 22/25
10 23/53 0.175 22/27
1.6328
2 23/42 0.407 23/25
5 23/53 0.257 23/27
10 23/63 0.171 23/30
7 2 22/74 0.52

B. CONG ET AL.
OPEN ACCESS MSCE
23
Figure 2. UV-Vis-NIR absorption spectra of AuNRs colloids
before and after irradiation with high laser power, insets
are the spectra of AuNRs after irradiation. AuNRs colloids
with SPRL located at ~810 nm (a) and 1100 nm (b) before
and after irradiation under 808 nm laser (6 W, 3 min) and
1064 nm laser (7 W, 2 min), respectively.
Figure 3. (a) , (b) TEM images of the AuNRs (with SPRL at
1100 nm) before and after irradiations under 1064 nm laser
(7 W) for 8 mi n .
Figure 4(a) presents the TEM image of AuNRs after
SiO2 coating. It can be seen that most of the AuNRs, to-
gether with some Au nanoparticles, have been success-
fully sealed by homogeneous SiO2, and formed core-
shell Au@SiO2 nanostructure. Also, there are a few SiO2
nanospheres without AuNRs core. By controlling the
Figure 4. (a)TEM images of silica-coated AuNRs. Insertion
of (a) is the magnified image of core-shell structured
Au@S i O 2. (b-c) TEM images of AuNRs coated with silica
shells of different thickness.
coating time, volume of TEOS/ethanol, and the concen-
tration of AuNRs, we were able to vary the shell thick-
ness (Figure s 4(b) and (c)). In the experiment, it is found
that certain CTAB coverage on the surface of AuNRs is
very important for the SiO2 capping. And further ex-
periments are now being carried out.
4. Conclusion
In summary, we have demonstrated the synthesis of
AuNRs with SPRL in the NIR region. The photothermal
conversion efficiencies of AuNRs were studied. An ob-
vious temperature increasing was observed in the photo-
th ermal c o nver s i o ns of AuNRs under laser irradiation.
The highest temperature is obtained when the SPRL
wavelength of AuNRs is equal to the laser wavelength,
and temperature of the colloid increased up to ~65 ˚C
even at low irradiating power. At a relative high power
(such as 6 W), nanorods undergo deformation from rod
to spherical particle and even fusion Au@SiO2 core-shell
structure was obtained, and further comparing experi-
ments, such as photothermal conversions and stability are
now being carried out.
Acknow l edge ments
This study was financially supported by National Natural
Science Foundation of China (Nos. 11274173, 51032002,
61222403).
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