Properties of Carbon Dots versus Small Molecules from "Bottom-up" Synthesis.

A major challenge in the "bottom-up" solvothermal synthesis of carbon dots (CDs) is the removal of small-molecule byproducts, noncarbonized polyamides, or other impurities that confound the optical properties. In previously reported benzene diamine-based CDs, the observed fluorescence signal already has been shown to arise from free small molecules, not from nanosized carbonized dots. Here we have unambiguously identified the small-molecule species in the synthesis of CDs starting with several isomers of benzene diamine by directly matching their NMR, mass spectrometry, and optical data with commercially available small organic molecules. By combining dialysis and chromatography, we have sufficiently purified the CD reaction mixtures to measure the CD size by TEM and STM, elemental composition, optical absorption and emission, and single-particle blinking dynamics. The results can be rationalized by electronic structure calculations on small model CDs. Our results conclusively show that the purified benzene diamine-based CDs do not emit red fluorescence, so the quest for full-spectrum fluorescence from isomers of a single precursor molecule remains open.

Plasmon Energy Transfer Driven by Electrochemical Tuning of Methylene Blue on Single Gold Nanorods.

Plasmonic photocatalysis has attracted interest for its potential to generate energy-efficient reactions, but ultrafast internal conversion limits efficient plasmon-based chemistry. Resonance energy transfer (RET) to surface adsorbates offers a way to outcompete internal conversion pathways and also eliminate the need for sacrificial counter-reactions. Herein, we demonstrate RET between methylene blue (MB) and gold nanorods (AuNRs) using in situ single-particle spectroelectrochemistry. During electrochemically driven reversible redox reactions between MB and leucomethylene blue (LMB), we show that the homogeneous line width is broadened when spectral overlap between AuNR scattering and absorption of MB is maximized, indicating RET. Additionally, electrochemical oxidative oligomerization of MB allowed additional dipole coupling to generate RET at lower energies. Time-dependent density functional theory-based simulated absorption provided theoretical insight into the optical properties, as MB molecules were electrochemically oligomerized. Our findings show a mechanism for driving efficient plasmon-assisted processes by RET through the change in the chemical states of surface adsorbates.

Active Control of Energy Transfer in Plasmonic Nanorod-Polyaniline Hybrids.

The hybridization of plasmonic energy and charge donors with polymeric acceptors is a possible means to overcome fast internal relaxation that limits potential photocatalytic applications for plasmonic nanomaterials. Polyaniline (PANI) readily hybridizes onto gold nanorods (AuNRs) and has been used for the sensitive monitoring of local refractive index changes. Here, we use single-particle spectroscopy to quantify a previously unreported plasmon damping mechanism in AuNR-PANI hybrids while actively tuning the PANI chemical structure. By eliminating contributions from heterogeneous line width broadening and refractive index changes, we identify efficient resonance energy transfer (RET) between AuNRs and PANI. We find that RET dominates the optical response in our AuNR-PANI hybrids during the dynamic tuning of the spectral overlap of the AuNR donor and PANI acceptor. Harnessing RET between plasmonic nanomaterials and an affordable and processable polymer such as PANI offers an alternate mechanism toward efficient photocatalysis with plasmonic nanoparticle antennas.

Progress and Prospects in Optical Ultrafast Microscopy in the Visible Spectral Region: Transient Absorption and Two-Dimensional Microscopy.

Ultrafast optical microscopy, generally employed by incorporating ultrafast laser pulses into microscopes, can provide spatially resolved mechanistic insight into scientific problems ranging from hot carrier dynamics to biological imaging. This Review discusses the progress in different ultrafast microscopy techniques, with a focus on transient absorption and two-dimensional microscopy. We review the underlying principles of these techniques and discuss their respective advantages and applicability to different scientific questions. We also examine in detail how instrument parameters such as sensitivity, laser power, and temporal and spatial resolution must be addressed. Finally, we comment on future developments and emerging opportunities in the field of ultrafast microscopy.

Site-Specific Chemistry on Gold Nanorods: Curvature-Guided Surface Dewetting and Supracolloidal Polymerization.

Control of interparticle interactions in terms of their direction and strength highly relies on the use of anisotropic ligand grafting on nanoparticle (NP) building blocks. We report a ligand deficiency exchange strategy to achieve site-specific polymer grafting of gold nanorods (AuNRs). Patchy AuNRs with controllable surface coverage can be obtained during ligand exchange with a hydrophobic polystyrene ligand and an amphiphilic surfactant while adjusting the ligand concentration (CPS) and solvent condition (Cwater in dimethylformamide). At a low grafting density of ≤0.08 chains/nm2, dumbbell-like AuNRs with two polymer domains capped at the two ends can be synthesized through surface dewetting with a high purity of >94%. These site-specifically-modified AuNRs exhibit great colloidal stability in aqueous solution. Dumbbell-like AuNRs can further undergo supracolloidal polymerization upon thermal annealing to form one-dimensional plasmon chains of AuNRs. Such supracolloidal polymerization follows the temperature-solvent superposition principle as revealed by kinetic studies. Using the copolymerization of two AuNRs with different aspect ratios, we demonstrate the design of chain architectures by varying the reactivity of nanorod building blocks. Our results provide insights into the postsynthetic design of anisotropic NPs that potentially serve as units for polymer-guided supracolloidal self-assembly.

d-Band Holes React at the Tips of Gold Nanorods.

Reactive hot spots on plasmonic nanoparticles have attracted attention for photocatalysis as they allow for efficient catalyst design. While sharp tips have been identified as optimal features for field enhancement and hot electron generation, the locations of catalytically promising d-band holes are less clear. Here we exploit d-band hole-enhanced dissolution of gold nanorods as a model reaction to locate reactive hot spots produced from direct interband transitions, while the role of the plasmon is to follow the reaction optically in real time. Using a combination of single-particle electrochemistry and single-particle spectroscopy, we determine that d-band holes increase the rate of gold nanorod electrodissolution at their tips. While nanorods dissolve isotropically in the dark, the same nanoparticles switch to tip-enhanced dissolution upon illimitation with 488 nm light. Electron microscopy confirms that dissolution enhancement is exclusively at the tips of the nanorods, consistent with previous theoretical work that predicts the location of d-band holes. We, therefore, conclude that d-band holes drive reactions selectively at the nanorod tips.

Chiral Plasmonic Pinwheels Exhibit Orientation-Independent Linear Differential Scattering under Asymmetric Illumination.

Plasmonic nanoantennas have considerably stronger polarization-dependent optical properties than their molecular counterparts, inspiring photonic platforms for enhancing molecular dichroism and providing fundamental insight into light-matter interactions. One such insight is that even achiral nanoparticles can yield strong optical activity when they are asymmetrically illuminated from a single oblique angle instead of evenly illuminated. This effect, called extrinsic chirality, results from the overall chirality of the experimental geometry and strongly depends on the orientation of the incident light. Although extrinsic chirality has been well-characterized, an analogous effect involving linear polarization sensitivity has not yet been discussed. In this study, we investigate the differential scattering of rotationally symmetric chiral plasmonic pinwheels when asymmetrically irradiated with linearly polarized light. Despite their high rotational symmetry, we observe substantial linear differential scattering that is maintained over all pinwheel orientations. We demonstrate that this orientation-independent linear differential scattering arises from the broken mirror and rotational symmetries of our overall experimental geometry. Our results underscore the necessity of considering both the rotational symmetry of the nanoantenna and the experimental setup, including illumination direction and angle, when performing plasmon-enhanced chiroptical characterizations. Our results demonstrate spectroscopic signatures of an effect analogous to extrinsic chirality for linear polarizations.

d-Band Hole Dynamics in Gold Nanoparticles Measured with Time-Resolved Emission Upconversion Microscopy.

The performance of photocatalysts and photovoltaic devices can be enhanced by energetic charge carriers produced from plasmon decay, and the lifetime of these energetic carriers greatly affects overall efficiencies. Although hot electron lifetimes in plasmonic gold nanoparticles have been investigated, hot hole lifetimes have not been as thoroughly studied in plasmonic systems. Here, we demonstrate time-resolved emission upconversion microscopy and use it to resolve the lifetime and energy-dependent cooling of d-band holes formed in gold nanoparticles by plasmon excitation and by following plasmon decay into interband and then intraband electron-hole pairs.

Single-Particle Photoluminescence and Dark-Field Scattering during Charge Density Tuning.

Single-particle spectroelectrochemistry provides optical insight into understanding physical and chemical changes occurring on the nanoscale. While changes in dark-field scattering during electrochemical charging are well understood, changes to the photoluminescence of plasmonic nanoparticles under similar conditions are less studied. Here, we use correlated single-particle photoluminescence and dark-field scattering to compare their plasmon modulation at applied potentials. We find that changes in the emission of a single gold nanorod during charge density tuning of intraband photoluminescence can be attributed to changes in the Purcell factor and absorption cross section. Finally, modulation of interband photoluminescence provides an additional constructive observable, giving promise for establishing dual channel sensing in spectroelectrochemical measurements.

Mechanism for plasmon-generated solvated electrons.

Solvated electrons are powerful reducing agents capable of driving some of the most energetically expensive reduction reactions. Their generation under mild and sustainable conditions remains challenging though. Using near-ultraviolet irradiation under low-intensity one-photon conditions coupled with electrochemical and optical detection, we show that the yield of solvated electrons in water is increased more than 10 times for nanoparticle-decorated electrodes compared to smooth silver electrodes. Based on the simulations of electric fields and hot carrier distributions, we determine that hot electrons generated by plasmons are injected into water to form solvated electrons. Both yield enhancement and hot carrier production spectrally follow the plasmonic near-field. The ability to enhance solvated electron yields in a controlled manner by tailoring nanoparticle plasmons opens up a promising strategy for exploiting solvated electrons in chemical reactions.

Nonlinear effects in single-particle photothermal imaging.

Although photothermal imaging was originally designed to detect individual molecules that do not emit or small nanoparticles that do not scatter, the technique is now being applied to image and spectroscopically characterize larger and more sophisticated nanoparticle structures that scatter light strongly. Extending photothermal measurements into this regime, however, requires revisiting fundamental assumptions made in the interpretation of the signal. Herein, we present a theoretical analysis of the wavelength-resolved photothermal image and its extension to the large particle scattering regime, where we find the photothermal signal to inherit a nonlinear dependence upon pump intensity, together with a contraction of the full-width-at-half-maximum of its point spread function. We further analyze theoretically the extent to which photothermal spectra can be interpreted as an absorption spectrum measure, with deviations between the two becoming more prominent with increasing pump intensities. Companion experiments on individual 10, 20, and 100 nm radius gold nanoparticles evidence the predicted nonlinear pump power dependence and image contraction, verifying the theory and demonstrating new aspects of photothermal imaging relevant to a broader class of targets.

Single-Particle Insights into Plasmonic Hot Carrier Separation Augmenting Photoelectrochemical Ethanol Oxidation with Photocatalytically Synthesized Pd-Au Bimetallic Nanorods.

Understanding the nature of hot carrier pathways following surface plasmon excitation of heterometallic nanostructures and their mechanistic prevalence during photoelectrochemical oxidation of complex hydrocarbons, such as ethanol, remains challenging. This work studies the fate of carriers from Au nanorods before and after the presence of reductively photodeposited Pd at the single-particle level using scattering and emission spectroscopy, along with ensemble photoelectrochemical methods. A sub-2 nm epitaxial Pd0 shell was reductively grown onto colloidal Au nanorods via hot carriers generated from surface plasmon resonance excitation in the presence of [PdCl4]2-. These bimetallic Pd-Au nanorod architectures exhibited 14% quenched emission quantum yields and 9% augmented plasmon damping determined from their scattering spectra compared to the bare Au nanorods, consistent with injection/separation of intraband hot carriers into the Pd. Absorbed photon-to-current efficiency in photoelectrochemical ethanol oxidation was enhanced 50× from 0.00034% to 0.017% due to the photodeposited Pd. Photocurrent during ethanol oxidation improved 13× under solar-simulated AM1.5G and 40× for surface plasmon resonance-targeted irradiation conditions after photodepositing Pd, consistent with enhanced participation of intraband-excited sp-band holes and desorption of ethanol oxidation reaction intermediates owing to photothermal effects.

Towards scalable plasmonic Fano-resonant metasurfaces for colorimetric sensing.

Transitioning plasmonic metasurfaces into practical, low-cost applications requires meta-atom designs that focus on ease of manufacturability and a robustness with respect to structural imperfections and nonideal substrates. It also requires the use of inexpensive, earth-abundant metals such as Al for plasmonic properties. In this study, we focus on combining two aspects of plasmonic metasurfaces-visible coloration and Fano resonances-in a morphology amenable to scalable manufacturing. The resulting plasmonic metasurface is a candidate for reflective colorimetric sensing. We examine the potential of this metasurface for reflective strain sensing, where the periodicity of the meta-atoms could ultimately be modified by a potential flexion, and for localized surface plasmon resonance refractive index sensing. This study evaluates the potential of streamlined meta-atom design combined with low-cost metallization for inexpensive sensor readout based on human optical perception.

Spectroscopic signatures of plasmon-induced charge transfer in gold nanorods.

Plasmon-induced charge transfer has been studied for the development of plasmonic photodiodes and solar cells. There are two mechanisms by which a plasmonic nanoparticle can transfer charge to an adjacent material: indirect transfer following plasmon decay and direct transfer as a way of plasmon decay. Using single-particle dark-field scattering and photoluminescence imaging and spectroscopy of gold nanorods on various substrates, we identify linewidth broadening and photoluminescence quantum yield quenching as key spectroscopic signatures that are quantitatively related to plasmon-induced interfacial charge transfer. We find that dark-field scattering linewidth broadening is due to chemical interface damping through direct charge injection via plasmon decay. The photoluminescence quantum yield quenching reveals additional mechanistic insight into electron-hole recombination as well as plasmon generation and decay within the gold nanorods. Through these two spectroscopic signatures, we identify charge transfer mechanisms at TiO2 and indium doped tin oxide interfaces and uncover material parameters contributing to plasmon-induced charge transfer efficiency, such as barrier height and resonance energy.

Toward Quantitative Nanothermometry Using Single-Molecule Counting.

Photothermal heating of nanoparticles has applications in nanomedicine, photocatalysis, photoelectrochemistry, and data storage, but accurate measurements of temperature at the nanoparticle surface are lacking. Here we demonstrate progress toward a super-resolution DNA nanothermometry technique capable of reporting the surface temperature on single plasmonic nanoparticles. Gold nanoparticles are functionalized with double-stranded DNA, and the extent of DNA denaturation under heating conditions serves as a reporter of temperature. Fluorescently labeled DNA oligomers are used to probe the denatured DNA through transient binding interactions. By counting the number of fluorescent binding events as a function of temperature, we reconstruct DNA melting curves that reproduce trends seen for solution-phase DNA. In addition, we demonstrate our ability to control the temperature of denaturation by changing the Na+ concentration and the base pair length of the double-stranded DNA on the nanoparticle surface. This degree of control allows us to select narrow temperature windows to probe, providing quantitative measurements of temperature at nanoscale surfaces.

Nanophotonic Approaches for Chirality Sensing.

Chiral nanophotonic materials are promising candidates for biosensing applications because they focus light into nanometer dimensions, increasing their sensitivity to the molecular signatures of their surroundings. Recent advances in nanomaterial-enhanced chirality sensing provide detection limits as low as attomolar concentrations (10-18 M) for biomolecules and are relevant to the pharmaceutical industry, forensic drug testing, and medical applications that require high sensitivity. Here, we review the development of chiral nanomaterials and their application for detecting biomolecules, supramolecular structures, and other environmental stimuli. We discuss superchiral near-field generation in both dielectric and plasmonic metamaterials that are composed of chiral or achiral nanostructure arrays. These materials are also applicable for enhancing chiroptical signals from biomolecules. We review the plasmon-coupled circular dichroism mechanism observed for plasmonic nanoparticles and discuss how hotspot-enhanced plasmon-coupled circular dichroism applies to biosensing. We then review single-particle spectroscopic methods for achieving the ultimate goal of single-molecule chirality sensing. Finally, we discuss future outlooks of nanophotonic chiral systems.

Light emission from plasmonic nanostructures.

The mechanism of light emission from metallic nanoparticles has been a subject of debate in recent years. Photoluminescence and electronic Raman scattering mechanisms have both been proposed to explain the observed emission from plasmonic nanostructures. Recent results from Stokes and anti-Stokes emission spectroscopy of single gold nanorods using continuous wave laser excitation carried out in our laboratory are summarized here. We show that varying excitation wavelength and power change the energy distribution of hot carriers and impact the emission spectral lineshape. We then examine the role of interband and intraband transitions in the emission lineshape by varying the particle size. We establish a relationship between the single particle emission quantum yield and its corresponding plasmonic resonance quality factor, which we also tune through nanorod crystallinity. Finally, based on anti-Stokes emission, we extract electron temperatures that further suggest a hot carrier based mechanism. The central role of hot carriers in our systematic study on gold nanorods as a model system supports a Purcell effect enhanced hot carrier photoluminescence mechanism. We end with a discussion on the impact of understanding the light emission mechanism on fields utilizing hot carrier distributions, such as photocatalysis and nanothermometry.

Wavelength-Dependent Photothermal Imaging Probes Nanoscale Temperature Differences among Subdiffraction Coupled Plasmonic Nanorods.

Plasmonic structures confine electromagnetic energy at the nanoscale, resulting in local, inhomogeneous, controllable heating, but reading out the temperature using optical techniques poses a difficult challenge. Here, we report on the optical thermometry of individual gold nanorod trimers that exhibit multiple wavelength-dependent plasmon modes resulting in measurably different local temperature distributions. Specifically, we demonstrate how photothermal microscopy encodes different wavelength-dependent temperature profiles in the asymmetry of the photothermal image point spread function. These asymmetries are interpreted through companion numerical simulations to reveal how thermal gradients within the trimer can be controlled by exciting its hybridized plasmon modes. We also find that plasmon modes that are optically dark can be excited by focused laser beam illumination, providing another route to modify thermal profiles beyond wide-field illumination. Taken together these findings demonstrate an all-optical thermometry technique to actively create and measure nanoscale thermal gradients below the diffraction limit.