Magnetic tuning of plasmonic excitation of gold nanorods

Article abstract of DOI:10.1021/ja408289b

By using gold nanorods as an example, we report the dynamic and reversible tuning of the plasmonic property of anisotropically shaped colloidal metal nanostructures by controlling their orientation using external magnetic fields. The magnetic orientationa

Full text of DOI:10.1021/ja408289b

Communication
pubs.acs.org/JACS
Magnetic Tuning of Plasmonic Excitation of Gold Nanorods
†
†
†
†
†
‡
§
Mingsheng Wang, Chuanbo Gao, Le He, Qipeng Lu, Jinzhong Zhang, Chi Tang, Serkan Zorba,
,†
and Yadong Yin*
†
‡
Department of Chemistry and Department of Physics and Astronomy, University of California, Riverside, California 92521, United
States
§
Department of Physics and Astronomy, Whittier College, Whittier, California 90608, United States
S Supporting Information
*
in which electrons oscillate along the short axes, whereas the
band at a longer wavelength results from the excitation of
longitudinal plasmon, in which electrons oscillate along the
longer axis. The excitation of transverse and longitudinal modes
is determined by the orientation of the nanorods relative to the
direction of oscillating electric field of incident light, in other
words, the polarization of light. When the polarization of light
is parallel to the long axis of the nanorods, only the longitudinal
plasmon is excited. Similarly, only the transverse mode will be
excited if the polarization of light is parallel to the short axes of
AuNRs. Figure 1 schematically illustrates the excitation of
ABSTRACT: By using gold nanorods as an example, we
report the dynamic and reversible tuning of the plasmonic
property of anisotropically shaped colloidal metal nano-
structures by controlling their orientation using external
magnetic fields. The magnetic orientational control
enables instant and selective excitation of the plasmon
modes of AuNRs through the manipulation of the field
direction relative to the directions of incidence and
polarization of light.
31
ocalized surface plasmon resonance (LSPR) of noble metal
L
nanostructures has been intensively studied for a wide
range of applications, such as chemical sensing, optoelectronics,
1
−5
catalysis, and photothermal therapy.
As the oscillation of
electrons is sensitive to the size, shape, and surrounding of the
particles, many efforts have been devoted to tuning the
plasmonic property by controlling these parameters during
6
−14
chemical synthesis of the metal nanostructures
or by
assembling them into defined secondary structures to take
advantage of the near-field plasmonic coupling between
1
5−19
neighboring particles
or controlling their alignment if
2
0−26
the nanostructures are anisotropic in shape.
In spite of
these prior studies, instantaneous and reversible tuning of the
plasmonic property of metal nanostructures remains a
challenge, which however holds great promise for developing
novel optoelectronic devices and more effective chemical and
biomedical sensors by allowing instant selective excitation or
Figure 1. Plasmon excitation of AuNRs under ordinary light. The
black arrows indicate the polarization of light, and blue curves
represent longitudinal plasmon resonance, while red curves represent
transverse plasmon resonance.
27−29
quenching of specific plasmon modes.
For the first time,
we report the dynamic and reversible tuning of the plasmonic
property of colloidal anisotropic metal nanostructures by
controlling their orientation using external magnetic fields. By
using gold nanorods (AuNRs) as an example, we demonstrate
that magnetic orientational control of the nanostructures can be
achieved by binding them to superparamagnetic iron oxide
nanorods in a parallel manner so that the resulting hybrid
nanostructures tend to align along the external magnetic field in
order to minimize the magnetic potential energy, thus enabling
selective excitation of the plasmon modes of AuNRs through
the manipulation of the field direction relative to the directions
of incidence and polarization of light.
plasmon modes of AuNRs upon the incidence of ordinary light.
For the simplicity in discussion, the direction of light is fixed in
y axis, and then the electric field of light oscillates in xz plane as
it is always perpendicular to the propagation direction of light.
If AuNRs are aligned along z axis, both the transverse and the
longitudinal plasmon along the x and z axes will be excited,
displaying two typically observed bands in the extinction
spectrum. Aligning AuNRs along the x axis leads to a similar
result as both longitudinal and transverse modes can be excited.
However, a dramatic difference occurs when AuNRs are aligned
AuNRs usually display two resonance modes: transverse and
longitudinal modes, which correspond to two bands of different
30
wavelengths in extinction spectrum. The band at a shorter
wavelength is attributed to the excitation of transverse plasmon,
Received: August 9, 2013
©
XXXX American Chemical Society
A
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Journal of the American Chemical Society
Communication
along the y axis. In this case, electron oscillation only occurs
along the short axes, therefore results in the excitation of
transverse plasmon only.
The above analysis suggests a new mechanism that allows
dynamic tuning of the optical property of AuNRs even under
normal light illumination. A critical question is then how to
enable instant and reversible alignment of AuNRs in three
dimensions. Magnetic field is an ideal tool for this purpose
32−34
because it can be applied instantly and remotely.
Another
important advantage of magnetic control is the anisotropic
nature of magnetic interactions, which allow effective alignment
of magnetic dipoles along the external fields. However, as Au is
nonmagnetic, additional magnetically active material needs to
be incorporated to enable the desired magnetic orientational
control of AuNRs. This challenge can be overcome by attaching
AuNRs to the surface of magnetic nanorods in a parallel
manner so that orientational control of AuNRs can be simply
achieved by aligning the magnetic nanorods. As magnetic
nanorods tend to orient themselves parallel to the direction of
the external magnetic field to minimize their magnetic potential
35
energy and dipole−dipole interaction, the AuNRs attached to
the magnetic nanorods will be aligned along the same direction,
thus enabling magnetic control of their plasmonic excitation.
In order for the designed scheme to work, there are a few
requirements on the magnetic nanorods. The most important
requirement is that their net magnetization should be small
enough to avoid magnetically induced aggregation, but their
magnetic response should be strong enough to guarantee an
effective reorientation under normal magnetic fields. The
diameter of the magnetic nanorods should be kept close to
that of the AuNRs so that a parallel attachment is highly
preferred when the two types of nanorods are brought together
through coordination covalent bonds. In order to produce
magnetic nanorods with the required features, we have
developed a unique solution phase synthesis which involves
first preparation of nonmagnetic iron oxyhydroxide (FeOOH)
Figure 2. (a) TEM images of the as-assembled structures; scale bar:
100 nm. (b) Spectra of a dispersion of the hybrid nanostructures under
magnetic fields with different directions relative to that of the incident
light (from perpendicular (90°) to parallel (0°)). (c) Spectra of the
dispersion under magnetic fields with varying strengths controlled by
the sample-magnet distance. The field direction is parallel to the
incident direction of light.
3
6,37
nanorods,
coating with a thin layer of silica through a sol−
gel process and then dehydration and reduction in diethylene
glycol to produce magnetite (Fe O ) nanostructures of a similar
3
4
incident light and the magnetic field changes from 90° to 0°,
the excitation of longitudinal mode decreases in magnitude,
leading to an attenuated band at the wavelength of around 700
nm, as shown in Figure 2b. Meanwhile, only a slight change
over the plasmon band at 520 nm can be observed as transverse
plasmonic mode is excited all the time. Importantly, only a
relatively weak magnetic field is required to drive the rotation of
nanorods. As shown in Figure 2c, when a magnet moves along
the incident direction toward the sample, the resonance band at
700 nm becomes significantly suppressed. A change in the
extinction spectrum can be observed when the magnet is 20 cm
away from the sample, which corresponds to a field strength of
8 G, indicating an acute response of the sample to magnetic
fields. The band at 700 nm reaches a minimum at a sample-
magnet distance of 4 cm, corresponding to a field strength of 50
G.
Magnetic orientational control of AuNRs under the
illumination of linearly polarized light provides more
opportunity in tuning their plasmon resonance, e.g., by
completely suppressing either the transverse or longitudinal
modes. As schematically illustrated in Figure 3a, under the
illumination of z-polarized light incident along y axis, the
plasmon excitation of AuNRs changes differently when they are
rotated in the yz and xy planes. When AuNRs are aligned along
the z axis, the transverse mode resonance is completely
morphology. The silica layer on the surface plays important
roles: it helps maintaining the rod-like morphology during
reduction, prevents the as-reduced magnetic Fe O4 from
3
aggregation, and also facilitates the surface functionalization
of magnetic nanorods with amino-groups for linking to AuNRs.
AuNRs stabilized by polyvinylpyrrolidone (PVP) were
3
8
synthesized and then allowed to bind to magnetic nanorods
through the amino groups grafted to the Fe O surface. Due to
3
4
the high affinity between gold surface and amino groups,
AuNRs tend to attach to the magnetic nanorods by maximizing
the contact area, as confirmed by the TEM observation shown
in Figure 2a.
Magnetic tuning of the optical property of the AuNRs under
ordinary light is then investigated. In accordance with the above
discussion, the perceived color of AuNRs changes with the
direction of magnetic field (see Supporting video). When the
field is parallel to the incident light, AuNRs are aligned along
the same direction so that only transverse plasmon can be
excited, resulting in a red color of the dispersion; when the field
is tuned perpendicular to the direction of incidence, the color
turns to green, owing to the dominant excitation of the
longitudinal plasmon mode. The plasmon response is very
sensitive to the external magnetic field, including both its
direction and strength. As the angle between the directions of
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Figure 3. (a) Scheme showing the plasmon excitation of AuNRs under polarized light. (b) Spectra of a dispersion of the hybrid nanostructures under
a magnetic field with its direction varying from perpendicular to parallel within the yz plane relative to the incident light. The incident light is
polarized along the z axis. The inset shows digital images of the dispersion under a magnetic field with its direction parallel (bottom) and
perpendicular (up) to the incident beam. (c) Spectra of the dispersion under a magnetic field with its direction varying within the xy plane from
perpendicular to parallel relative to the incident light.
suppressed, and only a strong band for longitudinal mode can
be observed (Figure 3b). Rotating the nanorod orientation
from z toward y axis gradually enhances the transverse plasmon
excitation while at the same time suppressing the longitudinal
mode, as evidenced by the gradual decrease of the band at 700
nm and rise of the one at 520 nm. In the end, the band at 700
nm disappears almost completely, indicating very good
alignment of AuNRs along the y direction. The color of
nanorod dispersion changes accordingly, from green to red, as
shown in the inset in Figure 3b. When the orientation of
AuNRs is tuned within the xy plane, the longitudinal mode
cannot be excited so that only the band at 520 nm dominates
the absorption spectra (Figure 3c) and the AuNRs dispersion
remains in a red color.
In order to obtain a quantitative understanding of the rate of
optical response, we illuminated the system with laser beams
39,40
under alternating magnetic fields.
Upon the application of
an alternating field parallel to the incident direction, the AuNRs
oscillates, leading to alternating changes in their orientation and
consequently extinction spectra. As indicated in Figure 4, in a
∼
200 Hz magnetic field, AuNRs showed a 6% modulation to
the intensity of a laser beam at 650 nm (red curve), which is
close to the excitation wavelength of longitudinal mode. To
confirm that this effect resulted from AuNRs, another laser
beam at 532 nm (corresponding to the resonant wavelength of
the transverse mode) was chosen, and only a 0.15% modulation
was achieved. The modulation percentage shows no changes
after many cycles, indicating good reversibility of the magnetic
tuning. The optical response of the system to a rotating
magnetic field has also been tested by placing the colloidal
dispersion on top of a magnetic stirrer and illuminating it with
light. The nanorods self-rotate just like a stir bar, displaying
alternating green and red colors under ordinary light with an
incident direction parallel to the rotating plane of stirrer.
Interestingly, no color change can be observed when the sample
is illuminated under lights polarized perpendicular to the
rotating plane, in great accordance with the above discussions.
In summary, we have successfully demonstrated the magnetic
manipulation of plasmonic excitation of AuNRs by controlling
Figure 4. (a) Scheme showing optical modulation using AuNRs
oscillating under an alternating magnetic field with field direction
parallel to the incident laser beam. Longitudinal plasmon can only be
excited when AuNRs are displaced from the parallel position and
illuminated by light with comparable wavelength. The resulting change
in extinction thus leads to intensity modulation of the laser beam. (b)
Optical modulation of AuNRs to laser beams with different
wavelengths under a ∼200 Hz alternating magnetic field: 650 nm
(red) and 532 nm (black).
their orientation relative to the incident lights. Such tuning is
enabled by attaching AuNRs to Fe O4 nanorods whose
3
orientation can be magnetically controlled. By tuning the
direction of magnetic field, we are then able to control the
excitation of plasmonic modes of AuNRs under the incidence
of both ordinary and polarized light. The colloidal dispersion of
AuNRs shows instant color switching in response to the
changes in the orientation or strength of external magnetic
fields. The optical switching is extremely sensitive and can
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Journal of the American Chemical Society
Communication
operate under considerably weak magnetic fields or alternating
magnetic fields with high frequency. The same strategy can be
extended to dynamic tuning of the optical property of other
anisotropic plasmonic nanostructures by using magnetic fields.
These tunable plasmonic hybrid nanostructures not only
enhance our understanding of plasmonic tuning but also
provide a new platform for building novel active optical
components, color presentation and display devices, and highly
sensitive and selective chemical and biomedical sensors.
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ASSOCIATED CONTENT
Supporting Information
Experimental procedures and setups, TEM images, UV−vis
spectra, and captions for videos. This material is available free
of charge via the Internet at http://pubs.acs.org.
■
(
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AUTHOR INFORMATION
Corresponding Author
yadong.yin@ucr.edu
Chem. B 2003, 107, 668.
31) Chen, H. J.; Shao, L.; Li, Q.; Wang, J. F. Chem. Soc. Rev. 2013,
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32) Ge, J. P.; Hu, Y. X.; Yin, Y. D. Angew. Chem., Int. Ed. 2007, 46,
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4
(
Notes
7428.
The authors declare no competing financial interest.
(33) He, L.; Wang, M. S.; Ge, J. P.; Yin, Y. D. Acc. Chem. Res. 2012,
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34) Wang, M. S.; He, L.; Yin, Y. D. Mater Today 2013, 16, 110.
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36) Peng, Z. M.; Wu, M. Z.; Xiong, Y.; Wang, J.; Chen, Q. W. Chem.
ACKNOWLEDGMENTS
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We thank the U.S. National Science Foundation for support of
this research (CHE-1308587). Y.Y. also thanks the Research
Corporation for Science Advancement for the Cottrell Scholar
Award and DuPont for the Young Professor Grant. C.T. is
supported by DARPA/Defense Microelectronics Activity
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