32 research outputs found
Enhancing the Sensitivity of Single-Particle Photothermal Imaging with Thermotropic Liquid Crystals
Individual molecules and nanoparticles can be imaged based on their
absorption using photothermal microscopy. This technique relies on
the heating-induced changes in the refractive index of the surrounding
medium. Here, we demonstrate an order of magnitude larger enhancement
of the signal-to-noise ratio in photothermal imaging of 20 nm gold
nanoparticles when using a thermotropic liquid crystal (5CB). We show
quantitatively that this increase is due to the large change in the
thermo-optical properties of 5CB mainly along the nematic director.
Enhancing the sensitivity is important for the further development
of absorption-based single-molecule spectroscopy techniques
Relaxation of Plasmon-Induced Hot Carriers
Plasmon-induced hot carrier generation
has attracted much recent attention due to its promising potential
in photocatalysis and other light harvesting applications. Here we
develop a theoretical model for hot carrier relaxation in metallic
nanoparticles using a fully quantum mechanical jellium model. Following
pulsed illumination, nonradiative plasmon decay results in a highly
nonthermal distribution of hot electrons and holes. Using coupled
master equations, we calculate the time-dependent evolution of this
carrier distribution in the presence of electronâelectron,
electronâphoton, and electronâphonon scattering. Electronâelectron
relaxation is shown to be the dominant scattering mechanism and results
in efficient carrier multiplication where the energy of the initial
hot electronâhole pair is transferred to other multiple electronâhole
pair excitations of lower energies. During this relaxation, a small
but finite fraction of electrons scatter into luminescent states where
they can recombine radiatively with holes by emission of photons.
The energy of the emitted photons is found to follow the energies
of the electrons and thus redshifts monotonically during the relaxation
process. When the energies of the electrons approach the Fermi level,
electronâphonon interaction becomes dominant and results in
heating of the nanoparticle. We generalize the model to continuous-wave
excitation and show how nonlinear effects become important when the
illumination intensity increases. When the temporal spacing between
incident photons is shorter than the relaxation time of the hot carriers,
we predict that the photoluminescence will blueshift with increasing
illumination power. Finally, we discuss the effect of the photonic
density of states (Purcell factor) on the luminescence spectra
Influence of the Substrate on the Mobility of Individual Nanocars
We monitored the mobility of individual fluorescent nanocars on three surfaces: plasma cleaned, reactive ion etched, and amine-functionalized glass. Using single-molecule fluorescence imaging, the percentage of moving nanocars and their diffusion constants were determined for each substrate. We found that the nanocar mobility decreased with increasing surface roughness and increasing surface interaction strength
Mechanistic Study of Bleach-Imaged Plasmon Propagation (BlIPP)
Bleach-imaged plasmon propagation, BlIPP, is a far-field
microscopy
technique developed to characterize the propagation length of surface
plasmon polaritons in metallic waveguides. To correctly extract the
propagation length from the measured photobleach intensity, it is
necessary to understand the mechanism by which dye photobleaching
occurs. In particular, 1- vs 2-photon bleaching reactions yield different
propagation lengths based on a kinetic model for BlIPP. Because a
number of studies have reported on the importance of 2-photon processes
for dye photobleaching, we investigate here the nature of the photobleaching
step in BlIPP. We are able to demonstrate that only 1-photon absorption
is relevant for typical BlIPP conditions as tested here for a thin
film of indocyanine green fluorescent dye molecules coated over gold
nanowires and excited at a wavelength of 785 nm. These results are
obtained by directly measuring the excitation intensity dependence
of the photobleaching rate constant of the dye in the presence of
the metallic waveguide
In Situ Measurement of Bovine Serum Albumin Interaction with Gold Nanospheres
We present in situ observations of adsorption of bovine
serum albumin
(BSA) on citrate-stabilized gold nanospheres. We implemented scattering
correlation spectroscopy as a tool to quantify changes in the nanoparticle
Brownian motion resulting from BSA adsorption onto the nanoparticle
surface. Protein binding was observed as an increase in the nanoparticle
hydrodynamic radius. Our results
indicate the formation of a protein monolayer at similar albumin concentrations
as those found in human blood. Additionally, by monitoring the frequency
and intensity of individual scattering events caused by single gold
nanoparticles passing the observation volume, we found that BSA did
not induce colloidal aggregation, a relevant result from the toxicological
viewpoint. Moreover, to elucidate the thermodynamics of the gold nanoparticleâBSA
association, we measured an adsorption isotherm which was best described
by an anticooperative binding model. The number of binding sites based
on this model was consistent with a BSA monolayer in its native state.
In contrast, experiments using polyÂ(ethylene glycol)-capped gold nanoparticles
revealed no evidence for adsorption of BSA
Adsorption of a Protein Monolayer via Hydrophobic Interactions Prevents Nanoparticle Aggregation under Harsh Environmental Conditions
We
find that citrate-stabilized gold nanoparticles aggregate and
precipitate in saline solutions below the NaCl concentration of many
bodily fluids and blood plasma. Our experiments indicate that this
is due to complexation of the citrate anions with Na<sup>+</sup> cations
in solution. A dramatically enhanced colloidal stability is achieved
when bovine serum albumin is adsorbed to the gold nanoparticle surface,
completely preventing nanoparticle aggregation under harsh environmental
conditions where the NaCl concentration is well beyond the isotonic
point. Furthermore, we explore the mechanism of the formation of this
albumin âcoronaâ and find that monolayer protein adsorption
is most likely ruled by hydrophobic interactions. As for many nanotechnology-based
biomedical and environmental applications, particle aggregation and
sedimentation are undesirable and could substantially increase the
risk of toxicological side effects; the formation of the BSA corona
presented here provides a low-cost biocompatible strategy for nanoparticle
stabilization and transport in highly ionic environments
Plasmon Emission Quantum Yield of Single Gold Nanorods as a Function of Aspect Ratio
We report on the one-photon photoluminescence of gold nanorods with different aspect ratios. We measured photoluminescence and scattering spectra from 82 gold nanorods using single-particle spectroscopy. We found that the emission and scattering spectra closely resemble each other independent of the nanorod aspect ratio. We assign the photoluminescence to the radiative decay of the longitudinal surface plasmon generated after fast interconversion from excited electronâhole pairs that were initially created by 532 nm excitation. The emission intensity was converted to the quantum yield and was found to approximately exponentially decrease as the energy difference between the excitation and emission wavelength increased for gold nanorods with plasmon resonances between 600 and 800 nm. We compare this plasmon emission to its molecular analogue, fluorescence
Chemical Interface Damping Depends on Electrons Reaching the Surface
Metallic
nanoparticles show extraordinary strong light absorption
near their plasmon resonance, orders of magnitude larger compared
to nonmetallic nanoparticles. This âantennaâ effect
has recently been exploited to transfer electrons into empty states
of an attached material, for example to create electric currents in
photovoltaic devices or to induce chemical reactions. It is generally
assumed that plasmons decay into hot electrons, which then transfer
to the attached material. Ultrafast electronâelectron scattering
reduces the lifetime of hot electrons drastically in metals and therefore
strongly limits the efficiency of plasmon induced hot electron transfer.
However, recent work has revived the concept of plasmons decaying
directly into an interfacial charge transfer state, thus avoiding
the intermediate creation of hot electrons. This direct decay mechanism
has mostly been neglected, and has been termed chemical interface
damping (CID). CID manifests itself as an additional damping contribution
to the homogeneous plasmon line width. In this study, we investigate
the size dependence of CID by following the plasmon line width of
gold nanorods during the adsorption process of thiols on the gold
surface with single particle spectroscopy. We show that CID scales
inversely with the effective path length of electrons, i.e., the average
distance of electrons to the surface. Moreover, we compare the contribution
of CID to other competing plasmon decay channels and predict that
CID becomes the dominating plasmon energy decay mechanism for very
small gold nanorods
Dye-Assisted Gain of Strongly Confined Surface Plasmon Polaritons in Silver Nanowires
Noble
metal nanowires are excellent candidates as subwavelength
optical components in miniaturized devices due to their ability to
support the propagation of surface plasmon polaritons (SPPs). Nanoscale
data transfer based on SPP propagation at optical frequencies has
the advantage of larger bandwidths but also suffers from larger losses
due to strong mode confinement. To overcome losses, SPP gain has been
realized, but so far only for weakly confined SPPs in metal films
and stripes. Here we report the demonstration of gain for subwavelength
SPPs that were strongly confined in chemically prepared silver nanowires
(mode area = λ<sup>2</sup>/40) using a dye-doped polymer film
as the optical gain medium. Under continuous wave excitation at 514
nm, we measured a gain coefficient of 270 cm<sup>â1</sup> for
SPPs at 633 nm, resulting in partial SPP loss compensation of 14%.
This achievement for strongly confined SPPs represents a major step
forward toward the realization of nanoscale plasmonic amplifiers and
lasers
Turning the Corner: Efficient Energy Transfer in Bent Plasmonic Nanoparticle Chain Waveguides
For
integrating and multiplexing of subwavelength plasmonic waveguides
with other optical and electric components, complex architectures
such as junctions with sharp turns are necessary. However, in addition
to intrinsic losses, bending losses severely limit plasmon propagation.
In the current work, we demonstrate that propagation of surface plasmon
polaritons around 90° turns in silver nanoparticle chains occurs
without bending losses. Using a far-field fluorescence method, bleach-imaged
plasmon propagation (BlIPP), which creates a permanent map of the
plasmonic near-field through bleaching of a fluorophore coated on
top of a plasmonic waveguide, we measured propagation lengths at 633
nm for straight and bent silver nanoparticle chains of 8.0 ±
0.5 and 7.8 ± 0.4 Όm, respectively. These propagation lengths
were independent of the input polarization. We furthermore show that
subradiant plasmon modes yield a longer propagation length compared
to energy transport via excitation of super-radiant modes