Plasmon Energy Transfer in Hybrid Nanoantennas.

Plasmonic metal nanoparticles exhibit large dipole moments upon photoexcitation and have the potential to induce electronic transitions in nearby materials, but fast internal relaxation has to date limited the spatial range and efficiency of plasmonic mediated processes. In this work, we use photo-electrochemistry to synthesize hybrid nanoantennas comprised of plasmonic nanoparticles with photoconductive polymer coatings. We demonstrate that the formation of the conductive polymer is selective to the nanoparticles and that polymerization is enhanced by photoexcitation. In situ spectroscopy and simulations support a mechanism in which up to 50% efficiency of nonradiative energy transfer is achieved. These hybrid nanoantennas combine the unmatched light-harvesting properties of a plasmonic antenna with the similarly unmatched device processability of a polymer shell.

Interfacial States Cause Equal Decay of Plasmons and Hot Electrons at Gold-Metal Oxide Interfaces.

We compare the decay of plasmons and "conventional" hot electrons within the same series of gold/metal oxide interfaces. We found an accelerated decay of hot electrons at gold-metal oxide interfaces with decreasing band gap of the oxide material. The decay is accelerated by the increased phase space for electron scattering caused by additional interfacial states. Since plasmons decay faster within the same series of gold-metal oxide interfaces, we propose plasmons are able to decay into the same interfacial states as hot electrons. The similarity of plasmon damping to conventional hot electron decay implies that many classical surface analysis techniques and theoretical concepts are transferable to plasmonic systems. Our results support the mechanism of direct decay of plasmons into interfacial hot electron pairs but the utility of these interfacial states for charge transfer reactions remains to be investigated.

Nanoelectrode-emitter spectral overlap amplifies surface enhanced electrogenerated chemiluminescence.

Electrogenerated chemiluminescence (ECL) is a promising technique for low concentration molecular detection. To improve the detection limit, plasmonic nanoparticles have been proposed as signal boosting antennas to amplify ECL. Previous ensemble studies have hinted that spectral overlap between the nanoparticle antenna and the ECL emitter may play a role in signal enhancement. Ensemble spectroscopy, however, cannot resolve heterogeneities arising from colloidal nanoparticle size and shape distributions, leading to an incomplete picture of the impact of spectral overlap. Here, we isolate the effect of nanoparticle-emitter spectral overlap for a model ECL system, coreaction of tris(2,2'-bipyridyl)dichlororuthenium(ii) hexahydrate and tripropylamine, at the single-particle level while minimizing other factors influencing ECL intensities. We found a 10-fold enhancement of ECL among 952 gold nanoparticles. This signal enhancement is attributed exclusively to spectral overlap between the nanoparticle and the emitter. Our study provides new mechanistic insight into plasmonic enhancement of ECL, creating opportunities for low concentration ECL sensing.

Hot Holes Assist Plasmonic Nanoelectrode Dissolution.

Strong light-absorbing properties allow plasmonic metal nanoparticles to serve as antennas for other catalysts to function as photocatalysts. To achieve plasmonic photocatalysis, the hot charge carriers created when light is absorbed must be harnessed before they decay through internal relaxation pathways. We demonstrate the role of photogenerated hot holes in the oxidative dissolution of individual gold nanorods with millisecond time resolution while tuning charge-carrier density and photon energy using snapshot hyperspectral imaging. We show that light-induced hot charge carriers enhance the rate of gold oxidation and subsequent electrodissolution. Importantly, we distinguish how hot holes generated from interband transitions versus hot holes around the Fermi level contribute to photooxidative dissolution. The results provide new insights into hot-hole-driven processes with relevance to photocatalysis while emphasizing the need for statistical descriptions of nonequilibrium processes on innately heterogeneous nanoparticle supports.

Single Gold Nanorod Charge Modulation in an Ion Gel Device.

A reliable and reproducible method to rapidly charge single gold nanocrystals in a solid-state device is reported. Gold nanorods (Au NRs) were integrated into an ion gel capacitor, enabling them to be charged in a transparent and highly capacitive device, ideal for optical transmission. Changes in the electron concentration of a single Au NR were observed with dark-field imaging spectroscopy via localized surface plasmon resonance (LSPR) shifts in the scattering spectrum. A time-resolved, laser-illuminated, dark-field system was developed to enable direct measurement of single particle charging rates with time resolution below one millisecond. The added sensitivity of this new approach has enabled the optical detection of fewer than 110 electrons on a single Au NR. Single wavelength resonance shifts provide a much faster, more sensitive method for all surface plasmon-based sensing applications.

Hydrogen Spillover between Single Gold Nanorods and Metal Oxide Supports: A Surface Plasmon Spectroscopy Study.

We used dark field spectroscopy to monitor the dissociation of hydrogen on single gold nanoparticles embedded in metal oxide supports. Individual gold nanorods were monitored in real time to reveal the peak position, the full width at half-maximum, and the relative intensity of the surface plasmon resonances during repeated N2-H2-N2 and air-H2-air cycles. Shifts in the spectra are shown to be due to changes in electron density and not to refractive index shifts in the environment. We demonstrate that hydrogen does not dissociate on gold nanorods (13 nm × 40 nm) at room temperature when in contact with silica and that electrons or hydrogen atoms migrate from Pt nanoparticles to Au nanoparticles through the supporting metal oxide at room temperature. However, this spillover mechanism only occurs for semiconducting oxides (anatase TiO2 and ZnO) and does not occur for Au and Pt nanoparticles embedded in silica. Finally, we show that hydrogen does dissociate directly on anatase surfaces at room temperature during air-H2-air cycles. Our results show that hydrogen spillover, surface dissociation of reactants, and surface migration of chemical intermediates can be detected and monitored in real time at the single particle level.

Spectroelectrochemistry of Silver Deposition on Single Gold Nanocrystals.

We report the electrodeposition of metallic silver onto gold nanostars adsorbed to ITO electrodes. The electrochemical process was studied at the single particle level by correlated in situ dark field spectroscopy and scanning electron microscopy (SEM). Underpotential deposition avoids bulk silver formation on the ITO substrates. SEM proves that deposition occurs on all surfaces of the gold nanostars when polyvinylpyrrolidone (PVP) is stabilizing the nanostars or preferentially at the nanostar tips when the ligand is removed. The surface plasmon resonance blue-shifts by more than 100 nm following the formation of a 5 nm Ag film on PVP stabilized gold nanostars, moving the scattered color from the near-infrared to red or orange. The spectral shifts can be accurately modeled using finite element simulations. These results demonstrate that the morphology and composition of individual bimetallic nanocrystals can be engineered electrochemically.

In situ SERS probing of nano-silver coated individual yeast cells.

For understanding cells functionalities and their communications, there is a need for highly sensitive cell analysis platforms capable of assessing non-specific chemicals on the surface and in the vicinity of cells. We report a microfluidic system integrating dielectrophoresis and surface enhanced Raman scattering (SERS) for the trapping and real time monitoring of cell functions in isolated and grouped cell clusters. Yeast cells are coated with silver nanoparticles to enable highly sensitive SERS analysis. The SERS responses of cells are examined under various conditions: live vs. dead and isolated vs. grouped. This work illustrates the feasibility of the system for in situ cell monitoring and analysis of secreted chemicals during their growth, metabolism, proliferation and apoptosis.