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Raman scattering (CARS), pump-probes, and the photothermal
effect (Fig. 1). Many nanostructures exhibit strong intrinsic nonlinear
optical (NLO) signals under tight focusing conditions. Combining
NLO signals with scanning microscopy has generated an array of
imaging modalities for material and biological studies1,2. Unlike
linear optical microscopy, NLO microscopy offers inherent 3D spatial
resolution, relatively large optical penetration into tissues with near
infrared (NIR) excitation, and reduced photodamage due to reduced
optical interaction with endogenous molecules. Many NLO contrasts
have been discovered across the broad spectrum of nanomaterials, as
summarized in Table 1.
Strong intrinsic nonlinear optical (NLO) signals not only make nanostructures promising agents for bio-imaging, but also advance NLO microscopy for the study of interactions between nanomaterials and live cells. Single beam modalities such as multiphoton luminescence, second harmonic generation, and third harmonic generation provide a simple way to probe many types of nanostructures. As for more advanced modalities, photothermal heterodyne imaging provides improved detection sensitivity for smaller objects, and transient absorption microscopy provides structural information to distinguish metal from semiconducting carbon nanotubes, and eumelanin from pheomelanin. The four-wave mixing signal achieves chemical selectivity in the presence of either vibrational or electronic resonance, as used in coherent Raman scattering imaging of molecules and in electronically resonance enhanced four-wave mixing imaging of nanostructures.
Ling Tonga and Ji-Xin Chenga,b* aDepartment of Chemistry, Purdue University, West Lafayette, IN 47907, USAbWeldon School of Biomedical Engineering, Purdue University, West Lafayette, IN 47907, USA
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JUNE 2011 | VOLUME 14 | NUMBER 6 265
In this article, we review recent advances in the exploration
of intrinsic NLO signals from metallic and semiconducting
nanostructures. We also review how NLO signals can be applied to
provide an imaging contrast for the study of cellular interactions with
nanomaterials.
Multiphoton luminescence from gold nanostructuresFirst reported by Mooradian in 196962, photoluminescence from
noble metals is generated in a three-step process: (i) excitation of
electrons from the d- to the sp- band to generate electron-hole pairs,
(ii) scattering of electrons and holes on the picosecond timescale with
partial energy transfer to the phonon lattice, and (iii) electron-hole
recombination resulting in photoemission. Photoluminescence can
be generated by single or multi-photon excitation, and the efficiency
can be enhanced by several orders via resonant coupling with
localized plasmons from nanostructures63,64. Herein we focus on the
multiphoton luminescence from gold nanostructures.
Two-photon luminescence Two-photon luminescence (TPL) from nanostructured gold was first
demonstrated by Boyd et al. in 198664, but the efficiency from the gold
film was low. With the longitudinal plasmon resonance peaks shifted to
NIR wavelengths, gold nanorods (GNRs)6,7, gold nanoshells8 and gold
nanocages12 are particularly well suited for use as TPL-based imaging
agents. The mechanism and characteristics of TPL from GNRs have
been investigated by both far-field6,65 and near-field5,7 microscopy. A
broad luminescence emission from 400 – 650 nm has been observed
in both cases6,7, corresponding to electron-hole recombination near
the X and L symmetry points of the Brillouin zone. In the far field
study, Wang et al.6 investigated the relationship between TPL and
the longitudinal plasmon resonance mode of GNRs. The excitation
spectrum overlaps with the longitudinal plasmon band, indicating that
plasmon-enhanced two-photon absorption contributes to the TPL from
GNRs. Due to the rod shape, TPL from a single GNR is sensitive to the
polarization of the incident beam, as shown in Figs. 2a,b. The individual
Fig. 1 Nonlinear optical (NLO) modalities employed for the visualization of nanomaterials. Solid lines represent electronic and vibrational states of molecules. Dashed lines are virtual states. The straight and wavy arrows are excitation and output signal beams, respectively. The gray arrows represent relaxation in electronically excited states. ω1 and ω2 are excitation beams. ωp and ωs are pump and Stokes beams for CARS. Ω is the frequency of vibrational transition between vibrational ground state and vibrationally excited state. ωp and ωpr are the pump and probe beams for pump-probe and photothermal modalities. ΔT is the temperature change due to excitation. n1 and n2 are the refractive indices of the material before and after excitation, respectively. TPEF: Two-photon excited fluorescence. TPL: Two-photon luminescence. SHG: Second harmonic generation. THG: Third harmonic generation. 3PL: Three-photon luminescence. CARS: coherent anti-Stokes Raman scattering. FWM: Four-wave mixing.
NLO modalities Nanomaterials
Multiphoton luminescence from gold nanostructures Gold nanoparticles3,4, gold nanorods5-7, gold nanoshells8, gold nanowires9,10, and Au-Ag nanocages11,12.
SHG ZnO nanowires13, GaN nanowires14, KNbO3 nanowires15, metallic nanoparticles16-18, nanocrystals19-22, and quantum dots23.
FWM Semiconducting and metallic nanostructures30 (e.g., gold nanowires31, gold nanoparticles32, gold nanoparticle antennas33, gold nanorods34, Si nanowires28, TiO2, ZnO, CeO2, Fe2O3 nanoparticles35-37, quantum dots38 ), SWNT39, and graphene flakes40.
CARS Polymeric nanoparticles41,42.
Transient absorption Semiconducting and metallic nanostructures41,42 (e.g., gold and silver nanoparticles41,43-45, nanoparticle dimers46, CdTe nanowire47, and silver nanowire48), and SWNT49.
Photothermal imaging Metal nanoparticles (e.g., gold nanoparticles50-58, gold nanorods59), quantum dots60, and SWNT61.
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JUNE 2011 | VOLUME 14 | NUMBER 6 271
band, and by increasing the probe power. Thirdly, photothermal
detection is especially sensitive to small objects for which absorption
dominates the scattering58, and is able to measure the absorption
spectral profile and the absorption cross-section of individual
nanoparticles58. The method has been used to detect the absorption
of single nonfluorescent azo dye molecules117, and to directly visualize
biological components, such as mitochondria in live cells118 and red
blood cells in live animals119.
Photothermal microscopy has also been widely applied to study
the nanostructures50,120. The first photothermal imaging of nanosized
absorbers, in this case 2.5 nm diameter gold nanoparticles, was
demonstrated by Boyer et al. using a photothermal interference
contrast method57. In their work, the signal was generated by a phase
shift between two orthogonally polarized and spatially separated
beams of an interferometer, with only one beam propagating through
the heated region. Later, the sensitivity was improved by two orders of
magnitude through use of the photothermal heterodyne imaging (PHI)
method121. In PHI, the probe beam generates a modulated scattered
field when it interacts with the time-modulated variations of the
refractive index around the sample caused by the time-modulated
pump beam. The scattered field is then detected by a lock-in
amplifier with the probe field as a local oscillator. The theoretical
framework of PHI is discussed and compared with experimental data53.
Photothermal spectroscopy and microscopy has been applied to metal
nanoparticles55,58,59, quantum dots60, and carbon nanotubes61. For
example, photothermal spectroscopy has been used to estimate the
concentration of nanosized particles in solvents122 and to measure the
absorption of individual SWNTs61. By the far-field polarization sensitive
photothermal imaging, Link and co-workers probed the anisotropic
absorption properties of small GNRs and measured the nanorod
orientation based on the transverse plasmon mode which is difficult to
detect with other optical imaging tools59. Recently, the detection limits
of photothermal microscopy have been discussed and the SNR has
been optimized as a function of probe power, as well as the optical and
thermal properties of the embedding medium of the absorbers, and the
isolation of the absorbers from the glass substrate50. Examples of PHI
imaging of 20 nm and 5 nm gold nanoparticles are shown in Figs. 6a-d.
With the development of photothermal correlation spectroscopy, the
diffusion of single gold nanoparticles, gold-protein complex, and gold
labeled bacteriophage has been investigated123-125.
Because the photothermal signal from nanostructures is not prone
to photobleaching or blinking, it has also been used for sensing and
imaging in biological applications. Blab et al.52 demonstrated the
potential of PHI as a reliable quantification of DNA hybridization in
gold nanoparticle-labeled DNA microarrays as it has a high sensitivity
and a dynamic range. Kulzer et al. applied photothermal detection
of individual gold nanoparticles in high-throughput screening51.
Cognet et al.56 visualized membrane proteins labeled with 10 nm gold
nanoparticles in cells by photothermal imaging, providing an efficient
and reproducible alternative to traditional electron microscopy.
Fig. 6 Photothermal imaging of gold nanoparticles. (a-d) Characterization of photothermal signals from gold nanoparticles. Reproduced from50 by permission of The Royal Society of Chemistry. (a) Photothermal microscopy image of 20 nm and 5 nm gold nanoparticles (marked with circles) immobilized on a glass slide in glycerol. The signal is color-coded on a logarithmic scale. (b) Histograms of the photothermal signal-to-noise ratios (SNRs) of gold nanoparticles. The population of 5 nm and 20 nm nanoparticles have SNR =12 ± 4 and SNR = 421 ± 92, respectively. (c) Photothermal image of a single 20 nm gold nanoparticle. (d) Photothermal image of a single 20 nm gold nanoparticle taken along the vertical z-axis, perpendicular to the glass surface. Gaussian fits of the shape of the signal along the z-, y-, and x-axes gives FWHM values of 730 nm, 250 nm, and 220 nm, respectively. Vertical dashed lines indicate the position of the glass-glycerol interface (glass on the right hand). (e-g) Photothermal imaging and single particle tracking of gold nanoparticles in a live neuron. Reprinted from54, Copyright 2006, with permission from Elsevier. (e) White light image and (f) PHI image of a live neuron labeled with gold nanoparticles. PHI image exhibits signals from one stationary (spot) and two moving (strips) membrane receptors labeled with gold nanoparticles (arrows). (g) Trajectory of an individual 5 nm gold nanoparticle (5 min, 9158 data points) acquired at video rate on a live neuron.
(a) (b) (e) (f)
(c) (d)(g)
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Applications of PHI have been extended to track single nanoparticles
in live cells by Lasne et al.54. The authors overcame the challenge of
slow imaging speed in normal photothermal microscopy by designing
a single particle photothermal tracking scheme. Using this system,
Lasne et al. were able to track membrane proteins labeled with 5 nm
gold nanoparticles in live neutrons at video rates for several minutes
(Figs. 6e-g).
Discussion and outlook Many nanostructures exhibit multiple NLO signals which could provide
imaging contrast to track them. For example, GNR exhibits TPL, THG,
FWM, and photothermal signals. The rational choice for application
depends on the advantages and limitations of each modality as
well as the availability of instrument. Single beam modalities (e.g.,
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