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

Resolving resonance effects in the theory of single particle photothermal imaging

Photothermal spectroscopy and microscopy provides a route to measure the spectral and spatial properties of individual nanoscopic absorbers, independent from scattering, extinction, and emission. The approach relies upon use of two light sources, one that resonantly excites and heats the target and its surrounding environment and a second off-resonant probe that scatters from the resulting volume of thermally modified refractive index. Over the past twenty years, considerable effort has been extended to apply photothermal methods to detect, spatially resolve, and perform absorption spectroscopy on single non-emissive molecules and other absorbers like plasmonic nanoparticles at room temperature conditions. Companion theoretical models have been developed to interpret these experimental advances, yet it is not clear how they are related to each other nor how the effects of lock-in detection modify the theory. For larger target systems that host their own intrinsic scattering resonances as well as for background media that do not instantaneously thermalize with the absorbing target, additional dependencies arise that are yet to be explored theoretically. The aim of this Perspective is to overview the theory of photothermal spectroscopy and microscopy and present a unifying theoretical approach that recovers past models in certain limits while explicitly including the effects of target scattering resonances, thermal and optical retardation, and lock-in detection. Focus is made on plasmonic particles to interpret the photothermal signal, yet all results are applicable equally to individual molecules or nanoparticle absorbers. Consequently, we expect this review to provide a useful foundation for the understanding of photothermal measurements independent of target identity

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

Fluorescence Correlation Spectroscopy: Criteria for Analysis in Complex Systems

We have evaluated the effect of varying three key parameters for Fluorescence Correlation Spectroscopy analysis, first in the context of a one species/one environment system, and then in a complex system composed of two species, or conversely, two environments. We establish experimentally appropriate settings for the (1) minimum lag time, (2) maximum lag time, and (3) averaging times over which an autocorrelation is carried out, as a function of expected diffusion decay time for a particular solute, and show that use of appropriate settings plays a critical role in recovering accurate and reliable decay times and resulting diffusion constants. Both experimental and simulated data were used to show that for a complex binary system, to extract accurate diffusion constants for both species, decay times must be bounded by adequate minimum and maximum lag times as dictated by the fast and slow diffusing species, respectively. We also demonstrate that even when constraints on experimental conditions do not permit achieving the necessary lag time limits for both of the species in a binary system, the accuracy of the recovered diffusion constant for the one species whose autocorrelation function is fully time-resolved is unaffected by uncertainty in fitting introduced by the presence of the second species

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

Particle Plasmons as Dipole Antennas: State Representation of Relative Observables

The strong interactions between light and plasmons (in metal nanoparticles) allow to observe chemical and physical processes on and around the particle on nanometer length scales as well as they allow to use plasmons for various applications. While electrodynamic theory predicts such effects very accurately, it is too complex to intuitively connect the underlying processes with the observed changes. The much simpler description of particle plasmons as harmonically driven dipole antennas has already been successfully used to describe many plasmonic effects. Here, these insights are combined and complemented to form a coherent and simple description of particle plasmons as dipole antennas. Unlike electrodynamic theory, this description connects fundamental plasmon properties such as shape, charges, environment, or surface coverage with spectroscopic properties like line width or resonance position. Therefore, this connection uniquely allows to identify the chemistry and physics behind the plasmonic responses and to intuitively predict the effects of various processes. The intuitive understanding is important to estimate the effects of complex processes, which occur in applications employing plasmonic nanostructures. To demonstrate the utility of this description, we untangled physical and chemical processes of changes in the plasmon observables during alkanethiol adsorption

Active Modulation of Nanorod Plasmons

Confining visible light to nanoscale dimensions has become possible with surface plasmons. Many plasmonic elements have already been realized. Nanorods, for example, function as efficient optical antennas. However, active control of the plasmonic response remains a roadblock for building optical analogues of electronic circuits. We present a new approach to modulate the polarized scattering intensities of individual gold nanorods by 100% using liquid crystals with applied voltages as low as 4 V. This novel effect is based on the transition from a homogeneous to a twisted nematic phase of the liquid crystal covering the nanorods. With our method it will be possible to actively control optical antennas as well as other plasmonic elements

Naturally Occurring Proteins Direct Chiral Nanorod Aggregation

Serum albumin can template gold nanorods into chiral assemblies, but the aggregation mechanism is not entirely understood. We used circular dichroism spectroscopy and scanning electron microscopy to investigate the role of protein identity/shape, protein/nanorod ratio, and surfactants on chiral protein–nanorod aggregation. Three globular proteinsserum albumin, immunoglobulin, and transferrinproduced similarly sized chiral protein–nanorod aggregates. In solution these aggregates exhibited circular dichroism at the plasmon resonance that switched direction at specific protein/nanorod concentration ratios. Our explanation is that the extent of protein crowding influences protein conformation and therefore protein–protein interactions, which in turn direct nanorod aggregation into preferentially left- or right-handed structures. The fibrous proteins fibrinogen and fibrillar serum albumin also produced chiral nanorod aggregates but did not exhibit a ratio-dependent switch in the circular dichroism direction. In addition, cetyltrimethyl­ammonium bromide micelles prevented all aggregation, providing compelling evidence that protein–protein interactions are crucial for chiral protein–nanorod aggregate formation. The protein-dependent variations in circular dichroism and aggregation reported here present opportunities for future chiral nanostructure engineering and biosensing applications

Heterogeneity and Hysteresis in the Polymer Collapse of Single Core–Shell Stimuli-Responsive Plasmonic Nanohybrids

Broad application of polymeric stimuli-responsive smart nanohybrids requires understanding the mechanisms governing active control. Ensemble techniques have identified inhomogeneous polymer collapse in microgels that potentially arise from heterogeneous interchain interactions and differences in core size. A single-particle examination would establish the influence of core size and internal polymer network heterogeneity on local interactions that contribute to the observed inhomogeneous polymer collapse dynamics of nanohybrids. Using single-particle dark-field spectroscopy, we investigated the complex polymer collapse profiles of core–shell plasmonic nanohybrids comprising thermoresponsive poly­(N-isopropylacrylamide) (pNIPAM)-encapsulated gold nanorods (AuNRs). We report that the polymer collapse behavior was independent of the core size. For thinner polymer shells, we observed hysteresis in the collapse of AuNR@pNIPAMs, likely related to local pNIPAM aggregation due to interchain hydrogen bonding. For thicker polymer shells, we observed a broad polymer collapse distribution that we attributed to a two-step phase transition that arises from a polymer network density gradient. Our single-particle approach relates the internal heterogeneity of the polymer network of nanohybrids to the mechanisms underlying heterogeneous phase transitions that traditional, ensemble-averaged approaches are unable to discern

Fluorescence Correlation Spectroscopy of Magnetite Nanocrystal Diffusion

We have measured the hydrodynamic radii of magnetite nanocrystals (NCs) with an 11 nm core by fluorescence correlation spectroscopy (FCS). We found that the sizes determined from particle diffusion varied by as much as an order of magnitude for the same magnetite NC sample due to the presence of a small number of larger and brighter aggregates, which bias the fluorescence autocorrelation. By analyzing the fluorescence intensity distributions and applying a magnetic field we were able to gain insight into the size distribution of the magnetite NCs and estimate the percentage of larger aggregates present. Size-selective separation of aggregates larger than about 60 nm in diameter was achieved by applying a magnetic field of 0.24 T