Collective Electron Oscillations Research Articles

Symmetry-breaking-induced plasmonic exceptional points and nanoscale sensing

Singularities of open systems, known as exceptional points (EPs), have been shown to exhibit increased sensitivities, but the observation of EPs has so far been limited to wavelength-scaled systems subject to the diffraction limit. Plasmons, the collective oscillations of free electrons coupled to photons, shrink the wavelength of light to electronic and molecular length scales. We propose a novel approach to EPs based on spatial symmetry breaking and report their observation in plasmonics at room temperature. The plasmonic EPs are based on the hybridization of detuned resonances in multilayered plasmonic structures to reach a critical complex coupling rate between nanoantenna arrays, resulting in the simultaneous coalescence of the resonances and loss rates. Their utility as sensors of anti-immunoglobulin G, the most abundant immunoglobulin isotype in human serum, is evaluated. Our work opens the way to a new class of nanoscale devices, sensors and imagers based on topological polaritonic effects. The hybridized modes of an asymmetric plasmonic dimer show avoided crossing of both the real and imaginary parts. This can lead to plasmonic exceptional points, which are used for biosensing with very high sensitivity.

Localized Surface Plasmon Resonance Spectroscopy for the Detection of Microtubule Nucleation

Localized surface plasmon resonance (LSPR) is a collective oscillation of conduction electrons in metal nanoparticles. The excitation of LSPR induces a large electromagnetic field enhancement near the metal surfaces, making it highly sensitive to the local refractive index surrounding the nanoparticles. LSPR spectroscopy uses this localized sensitivity to achieve label-free detection of biological molecules. Previously, LSPR spectroscopy has been employed to study various biomolecular interactions. However, it has not been used to detect biopolymer nucleation. Here, we present a LSPR spectroscopy approach for the detection of microtubule nucleation. Using a combination of an extended Mie theory and computational simulations, we investigate theoretically how the optical responses of spherical and nonspherical gold nanoparticles change when microtubules form around them. Our results indicate that the extinction peak wavelength is sensitive to the formation of short microtubules around the nanoparticles, but is insensitive to their elongation. We also demonstrate that LSPR spectroscopy can detect microtubule nucleation experimentally by inducting spontaneous microtubule formation around spherical gold nanoparticles. Consistent with our theoretical predictions, our experiments show that the peak wavelength changes significantly when short microtubules form around the nanoparticles, but not when they are elongated. We also show that the peak wavelength is sensitive to the formation of microtubule nuclei even when they are too small to be detectable with a turbidity measurement.

Plasmonic nanoparticles for environmental analysis

Gold and silver nanoparticles have unique optical properties. For instance, the intense colour of nanoparticle suspensions results from the excitation of a collective oscillation of surface conduction electrons, named surface plasmons. This excitation is done using an electromagnetic radiation that interacts with nanoparticles having a negative real and small positive imaginary dielectric constant, such as nanoparticles of gold or silver. The plasmonic optical properties of metal nanostructures are dependent on their shape and size, the dielectric properties of the metal and the surroundings and on the possible electromagnetic coupling with the localized surface plasmons in nearby other plasmonic objects. The other important consequence of the excitation of surface plasmons is a local significant enhancement of the electromagnetic field at some places of the illuminated nanoparticles. Specific plasmonic properties of gold and silver nanoparticles have allowed the development of many sensors for chemical analysis, including sensors dedicated for environmental analysis. Some of these sensors are so sensitive that recording of the reliable analytical signal of a single molecule is possible. Here, we review analytical techniques based on plasmonic properties of metallic nanoparticles for environmental analysis. We present the theory and mechanism of interaction of the electromagnetic radiation with the plasmonic nanoparticles. We detail analytical techniques including methods utilizing local enhancement of the intensity of the electromagnetic field induced by plasmons, and hence increase in the efficiency of some optical processes in the proximity of the plasmonic nanoparticles. Those techniques are surface-enhanced Raman scattering, surface-enhanced infrared absorption and metal-enhanced fluorescence and methods based on the change in the optical properties of plasmonic nanoparticles caused by the analyte-induced aggregation or by analyte-influenced growth or etching of plasmonic nanostructures. Environmental compounds include heavy metal cations, metallo-organic compounds, polycyclic aromatic hydrocarbons, pesticides, nitrite ions, bacterial cells and bacterial pathogens.

New Strategy for Streptavidin Detection Using AuNRs/PAAm Hydrogel Composites

Localized surface plasmon resonance (LSPR) is a collective oscillation of free electrons induced by the incident light in nanometric surfaces of conductive materials. Gold nanorods (AuNRs) have been studied for plasmonic devices due to their anisotropic properties. Herein, we report the construction of a specific biosensor using polyacrylamide (PAAm) as the gold nanorods support and test its efficiency with specific streptavidin interaction using biotinylated substrates. First, the hydrogels were synthesized by electropolymerization using cyclic voltammetry on ITO/glass substrates and swallowed with AuNRs. The AuNRs were characterized by visible absorption spectroscopy and transmission electron microscopy. The substrates were characterized by scanning electron microscopy and their plasmonic sensitivities were evaluated using sucrose solutions with different concentrations. The results showed a sensitivity of ca. 226.4 nm RIU-1 with linear correlation of 0.9974. Furthermore, this composite presented reliable detection of the streptavidin biomolecule in the gold surface with good signal-to-noise ratio (SNR).

Aluminium metal-insulator-metal structure fabricated by the bottom-up approach.

Plasmonic color is an elegant color resulting from light absorption and emission induced by collective oscillation of free electrons in a metal and enables unprecedented new color expression. In particular, Al plasmonic color is highly desirable because of the low cost and high stability of Al. Here, we report a new cost-effective, wide-area fabrication method for an Al metal–insulator–metal (MIM) plasmonic nanostructure using a vapor deposition and sintering process.

Surface plasmon induced transparency in coupled microcavities assisted by slits

The coupled waveguide-microcavity structure has a wide range of applications in optical filters and optical modulators. The optical transmission properties of structure are mostly determined by the coupling strength of the modes. In the conventional waveguide-microcavity structure, the mode coupling is finished by the form of evanescent field, which is usually achieved by controlling the geometric spacing between waveguide and microcavity. Surface plasmon polaritons are the excitations of the electromagnetic waves coupled to collective oscillations of free electrons in metal. Since the electromagnetic waves are attenuated sharply in the metal, this requires precise control of the spacing between the waveguide and the metal microcavity, and poses a great challenge for controlling the coupling of modes in the metal waveguide-cavity structure. In this paper, we proposed a scheme of using a metal-dielectric-metal waveguide side coupling metal microcavities to overcome this limit. Based on the resonant characteristics of the Fabry–Pérot mode in the metal microcavity, a slit is introduced to connect the waveguide and microcavities. By adjusting the width and the offset location of slits, the leakage rate and coupling strength of the mode in metal microcavity can be controlled. The finite difference frequency domain (FDFD) method was used to numerically simulate the electromagnetic properties of structure. First, we have studied the transmission behaviors of surface plasmon polaritons in the system consisted by metal waveguide and single microcavity. As other microcavity is introduced to the structure and connected the original microcavity by slit, the electromagnetically induced transparency phenomena based on surface plasmon polaritons are demonstrated in the coupled metal waveguide and double microcavities structure. As the width of slit connected the microcavity is increased, the transmission peak of structure and the full width at half maximum of the transparency window also increase accordingly. The change of the geometric parameters of slit will modulate the resonance characteristics of structure, and the corresponding physical mechanism is explained by the temporal coupled mode theory. In our works, the metal waveguide and microcavities are coupled by the energy leakage of microcavities assisted by slits, which breaks the limit of separation distance between metal waveguide and microcavity, and contributes to the manufacture of devices. The results of the paper will have applications in designing the compact photonic devices based on surface plasmon polaritons.

Feasibility of Plasmonic Circuits in Nanophotonics

The configuration of plasmonic circuits comprising SiO2-load waveguides and their characteristics within the nanophotonic range are presented and compared with electronic and lightwave circuits in 1300 and 1550 nm wavelength bands. In the nanophotonic range of less than $1~\mu \text{m}$ , plasmonic signals propagate in narrow waveguides with cross-sections less than a few hundred square nanometers, while lightwaves exhibit only slight propagation in high-refractive-index (i.e., Si) waveguides owing to the transmission loss increase via the cut-off wavelength of the waveguide. Additionally, the plasmonic signal transmission loss is lower than that of electric signals for transmission lengths less than a few hundred micrometers. During signal transmission, a narrow spectral width of the plasmonic signals is needed to suppress any signal shape deformation induced by the frequency dependence of the collective oscillation of electrons in plasmonic signals. In the nanophotonic range, the degree of integration for plasmonic circuits is not governed by the transmission loss but by the leak distance of the plasmonic signal optical field from the side-walls of the waveguides and components. Employing a metal/insulator/metal structure in plasmonic circuits is a valid way to heighten the integration density, and its effectiveness is numerically and experimentally confirmed in a plasmonic multiplexer less than $1~\mu \text{m}$ in length. On the basis of these results, the feasibility of plasmonic circuits is discussed and it can be said that plasmonic circuits including waveguides and components are promising photonic techniques for signal transmission in the nanophotonic range

Nonequilibrium hot-electron-induced wavelength-tunable incandescent-type light sources

The collective oscillation of electrons located in the conduction band of metal nanostructures being still energized, with the energy up to the bulk plasmon frequency, are called nonequilibrium hot electrons. It can lead to the state-filling effect in the energy band of the neighboring semiconductor. Here, we report on the incandescent-type light source composed of Au nanorods decorated with single Ga-doped ZnO microwire (AuNRs@ZnO:Ga MW). Benefiting from Au nanorods with controlled aspect ratio, wavelength-tunable incandescent-type lighting was achieved, with the dominating emission peaks tuning from visible to near-infrared spectral regions. The intrinsic mechanism was found that tunable nonequilibrium distribution of hot electrons in ZnO:Ga MW, injected from Au nanorods, can be responsible for the tuning emission features. Apart from the modification over the composition, bandgap engineering, doping level, etc., the realization of electrically driving the generation and injection of nonequilibrium hot electrons from single ZnO:Ga MW with Au nanostructure coating may provide a promising platform to construct electronics and optoelectronics devices, such as electric spasers and hot-carrier-induced tunneling diodes.

Solid-state tube wakefield accelerator using surface waves in crystals

Solid-state or crystal acceleration has for long been regarded as an attractive frontier in advanced particle acceleration. However, experimental investigations of solid-state acceleration mechanisms which offer [Formula: see text] acceleration gradients have been hampered by several technological constraints. The primary constraint has been the unavailability of attosecond particle or photon sources suitable for excitation of collective modes in bulk crystals. Secondly, there are significant difficulties with direct high-intensity irradiation of bulk solids, such as beam instabilities due to crystal imperfections and collisions etc. Recent advances in ultrafast technology with the advent of submicron long electron bunches and thin-film compressed attosecond x-ray pulses have now made accessible ultrafast sources that are nearly the same order of magnitude in dimensions and energy density as the scales of collective electron oscillations in crystals. Moreover, nanotechnology enabled growth of crystal tube structures not only mitigates the direct high-intensity irradiation of materials, with the most intense part of the ultrafast source propagating within the tube but also enables a high degree of control over the crystal properties. In this work, we model an experimentally practicable solid-state acceleration mechanism using collective electron oscillations in crystals that sustain propagating surface waves. These surface waves are driven in the wake of a submicron long particle beam, ideally also of submicron transverse dimensions, in tube shaped nanostructured crystals with tube wall densities, [Formula: see text]. Particle-In-Cell (PIC) simulations carried out under experimental constraints demonstrate the possibility of accessing average acceleration gradients of several [Formula: see text] using the solid-state tube wakefield acceleration regime. Furthermore, our modeling demonstrates the possibility that as the surface oscillations and resultantly the surface wave transitions into a nonlinear or “crunch-in” regime under [Formula: see text], not only does the average gradient increase but strong transverse focusing fields extend down to the tube axis. This work thus demonstrates the near-term experimental realizability of Solid-State Tube Wakefield Accelerator (SOTWA). The ongoing progress in nanoengineering and attosecond source technology thereby now offers the potential to experimentally realize the promise of solid-state or crystal acceleration, opening up unprecedented pathways in miniaturization of accelerators.

Plasmon-Induced Charge Transfer: Challenges and Outlook

The decay of a surface plasmon, a collective electron oscillation at the surface of a metal, can generate hot charge carriers that may be transferred to an adjacent semiconductor. This plasmon-induced chargetransfer process can be used to enhance photocatalysis, to create photodetectors, or to drive selective photochemistry. However, the charge transfer efficiency in many fabricated devices remains too low for practical applications, typically <1%. In this Perspective, I discuss critical aspects of designing plasmonic systems for improved performance and highlight important findings for maximizing the transfer efficiency. In particular, I draw attention to the article by Ma and Gao in this issue ofACS Nano that describes using real-time time-dependent density functional theory to give a detailed and informative look at the charge transfer dynamics at a TiO2-Ag nanocluster interface.

Impact of a charged neighboring particle on Förster resonance energy transfer (FRET)

Förster resonance energy transfer (FRET) is an important physical phenomenon which demands precise control over the FRET rate for its wide range of applications. Hence, enhancing the FRET rate using different techniques has been extensively studied in the literature. Research indicates that introducing additional particles to a system consisting of a donor-acceptor pair can change the behaviour of FRET in the system. One such technique is to utilize the collective oscillations of the surface electrons of a neighboring electrically-neutral metal nanoparticle (MNP). However, the perceived changes on the FRET rate between the donor and the acceptor, when the MNP carries excess electrical charges are yet unknown. In this paper, we study these changes by introducing a charged MNP, in the proximity of an excited donor and a ground state acceptor. We deploy the classical Green’s tensor to express the FRET rate in the system. We consider an effective dielectric response for the MNP, which accounts for the extraneous surface charge effects. We analyze the electrical potential at the acceptor position due to the changed dipole moment of the donor molecule as a result of the electric field induced at the donor position, and obtain the FRET rate of the system. This model considers arbitrary locations and orientations of the two molecular dipole moments with regard to the position of the spherical MNP. We present the enhancement of the FRET rate, predominantly caused by both the surface plasmon excitations and the extraneous surface electrical charges carried by the neighboring MNP. We obtain the results by varying the separation distance between the molecules and the MNP, the transition frequency of the donor-acceptor pair and the size of the metallic sphere. Specifically, we demonstrate that a donor-acceptor pair placed in the vicinity of an electrically-charged Silver MNP exhibits a remarkable improvement in the FRET rate. Furthermore, the aggregate FRET enhancement is determined by other characteristics such as the location of the donor, transition frequency, separation distances and the radius of the MNP. In essence, these findings reveal an approach to realize the enhanced FRET rate in a larger span in a more controlled manner that is desirable in many FRET-based applications including spectroscopic measurements.

Compact High-Efficiency Resonator Antennas Based on Dispersion Engineering of Even-Mode Spoof Surface Plasmon Polaritons

This article proposes a compact high-efficiency metallic resonator antenna operating at even-mode spoof surface plasmon polaritons (SSPPs). The collective electron oscillations within the structure increase the effective relative permittivity that can be tuned by engineering the dispersion of SSPP modes on the structure. The even-mode SSPPs are excited by placing a radiator at their vicinity. As an example, a resonator antenna is designed using the comb-like SSPP structure. The proposed metallic resonator antenna achieves low loss and hence high radiation efficiency because majority of fields of SSPP modes concentrates in air rather than in dielectric compared with conventional dielectric resonator antennas (DRAs). The proposed design also employs compact size due to the high effective relative permittivity and planar configuration of the SSPP resonator. A prototype was fabricated and measured. Both the simulated and measured results show the advantages of higher efficiency of 95.2% and more compact volume of $0.0002{\lambda }^{3}$ over a DRA. This method provides a good alternative for designing DRAs.

Electrochemical and Photoelectrochemical Applications of Plasmonic Metal and Compound Nanoparticles

Metal nanoparticles (NPs) that exhibit bright colors have been used as coloring agents of glass artifacts since before Christ. After the origin of the coloration, localized surface plasmon resonance (LSPR), was elucidated, their application has been expanded widely. This account focuses on electrochemical and photoelectrochemical applications of the plasmonic metal and compound NPs. Because LSPR is based on collective oscillation of free electrons in a NP in resonance with electric field oscillation of light, the resonant wavelength can be tuned electrochemically by transferring electrons to or from the NP. On this basis, we developed a new class of plasmonic sensors without gratings, and near infrared electrochromic smart windows. Meanwhile, resonant NPs exhibit charge separation at the interface with a semiconductor such as TiO2. We studied this phenomenon, plasmon-induced charge separation (PICS), in particular about mechanisms including behavior of energetic carriers (hot holes and electrons), by using a wide variety of plasmonic NPs with different morphologies and compositions. Knowledge thus obtained was exploited for nanofabrication and development of near infrared PICS devices.

On measuring the structure stability for small silver clusters to use them in plasmonics

One of the new applications of silver nanoparticles is their use in plasmonic applications determined by the strong interaction of the electromagnetic wave and free electrons in nanostructures. Silver particles of a size smaller than the visible light wavelength can strongly absorb light due to the surface plasmonic resonance caused by the collective oscillation of the conduction electrons. The frequency and intensity of the plasmonic resonance depends on the distribution of the nanostructure polarized charge, which is determined by the shape and structure of the nanoparticle. But the rapid oxidation/sulfidation due to the ambient atmosphere dramatically reduces all the advantages of silver and causes difficulties from the view point of practical applications. Possible solution to this problem could be the formation of very pure particles of a perfect crystal structure, which should be more resistant to the abovementioned phenomena. We believe that unaccounted possibility of increasing the plasmon efficiency can be the usage of silver nanoparticles with a size equal to the magic numbers of various structures. To test this hypothesis, computer simulation was performed to determine the stability of the structure of silver clusters with a size of up to 2.0 nm. It shows that the use of small silver clusters in plasmonic applications strongly requires considering the problem of the thermal stability of their cluster structure with consideration of various kinds of “magic” numbers.

Tailored plasma-density profiles for enhanced energy extraction in passive plasma beam dumps

Plasma-based deceleration could greatly improve the overall compactness of accelerator facilities. In the so-called passive plasma beam dump, collective oscillations of plasma electrons are used to absorb kinetic energy of spent high-energy electron beams. Moreover, due to the high-amplitude decelerating gradient and the low-density plasma medium, deceleration is achieved in a compact and safer way, if compared to conventional beam dumps. Adoption of this novel decelerating scheme might be critical for facilities aiming for high-energy, high-repetition-rates, as well as for transportable applications built upon plasma-based accelerator technology. However, key issues such as, for example, particle re-acceleration, need to be addressed. In this work, tailored plasma-density profiles are used to shift the defocusing phase of the beam self-driven transverse wakefield towards re-accelerated beam particles, and transversely eject them. PIC simulations are used to evaluate the total beam energy, energy deposited in the plasma, and energy transversely ejected, for each of the investigated plasma density profiles. Moreover, the average energy per ejected particle is estimated for each case. In particular, it is shown that the energy of ejected particles is affected by the rate at which the plasma wavelength decreases, since it determines the time that particles experience re-acceleration before being defocused.

Plasmon-Mediated Chemical Reactions on Nanostructures Unveiled by Surface-Enhanced Raman Spectroscopy.

Surface plasmons (SPs) originating from the collective oscillation of conduction electrons in nanostructured metals (Au, Ag, Cu, etc.) can redistribute not only the electromagnetic fields but also the excited carriers (electrons and holes) and heat energy in time and space. Therefore, SPs can engage in a variety of processes, such as molecular spectroscopy and chemical reaction. Recently, plenty of demonstrations have made plasmon-mediated chemical reactions (PMCRs) a very active research field and make it as a promising approach to facilitate light-driven chemical reactions under mild conditions. Concurrently, making use of the same SPs, surface-enhanced Raman spectroscopy (SERS) with a high surface sensitivity and energy resolution becomes a powerful and commonly used technique for the in situ study of PMCRs. Typically, various effects induced by SPs, including the enhanced electromagnetic field, local heating, excited electrons, and excited holes, can mediate chemical reactions. Herein, we use the para-aminothiophenol (PATP) transformation as an example to elaborate how SERS can be used to study the mechanism of PMCR system combined with theoretical calculations. First, we distinguish the chemical transformation of PATP to 4,4'-dimercaptoazobenzene (DMAB) from the chemical enhancement mechanism of SERS through a series of theoretical and in situ SERS studies. Then, we focus on disentangling the photothermal, hot electrons, and "hot holes" effects in the SPs-induced PATP-to-DMAB conversion. Through varying the key reaction parameters, such as the wavelength and intensity of the incident light, using various core-shell plasmonic nanostructures with different charge transfer properties, we extract the key factors that influence the efficiency and mechanism of this reaction. We confidently prove that the transformation of PATP can occur on account of the oxygen activation induced by the hot electrons or because of the action of hot holes in the absence of oxygen and confirm the critical effect of the interface between the plasmonic nanostructure and reactants. The products of these two process are different. Furthermore, we compare the correlation between PMCRs and SERS, discuss different scenario of PMCRs in situ studied by SERS, and provide some suggestions for the SERS investigation on the PMCRs. Finally, we comment on the mechanism studies on how to distinguish the multieffects of SPs and their influence on the PMCRs, as well as on how to power the chemical reaction and regulate the product selectivity in higher efficiencies.

Near-Field Imaging of Surface Plasmons from the Bulk and Surface State of Topological Insulator Bi2Te2Se

Surface plasmons are collective oscillations of electrons that can enable confinement of electromagnetic energy to subwavelength scales. Recent progress in plasmonics has largely relied on noble me...

LSPR Biosensor Based on Au@Ag Core-Shell Nanobipyramids

A highly precise and sensitive immune monitor is required to be biosensors for detecting various bio-marks from human blood. Localized surface plasmon resonance (LSPR) microfluidic optical biosensors allow label-free, high throughput, and real-time detection. Gold and silver nanoparticles have a collective oscillation of free electrons at their surface, where they can resonate with the incident light resulting in changes of wavelength. The higher sensitivity particles have, the more changes in wavelength are shown. In this project, for developing sensitivity of LSPR biosensor, Au@Ag core-shell nanobipyramids has been successfully synthesized with a LSPR peak at about 610 nm and relatively high refractive index sensitivity as 254 nm/RIU, where the ultralow concentration of different cytokines can be detected.

Nonlinear Graphene Nanoplasmonics.

Nonlinear optics is limited by the weak nonlinear response of available materials, a problem that is generally circumvented by relying on macroscopic structures in which light propagates over many optical cycles, thus giving rise to accumulated unity-order nonlinear effects. While this strategy cannot be extended to subwavelength optics, such as in nanophotonic structures, one can alternatively use localized optical resonances with high quality factors to increase light-matter interaction times, although this approach is limited by inelastic losses partly associated with the nonlinear response. Plasmons-the collective oscillations of electrons in conducting media-offer the means to concentrate light into nanometric volumes, well below the light-wavelength-scale limit imposed by diffraction, amplifying the electromagnetic fields upon which nonlinear optical phenomena depend. Due to their abundant supply of free electrons, noble metals are the traditional material platform for plasmonics and have thus dominated research in nanophotonics over the past several decades, despite exhibiting large ohmic losses and inherent difficulties to actively modulate plasmon resonances, which are primarily determined by size, composition, and morphology. Highly doped graphene has recently emerged as an appealing platform for plasmonics due to its unique optoelectronic properties, which give rise to relatively long-lived, highly confined, and actively tunable plasmon resonances that mainly appear in the infrared and terahertz frequency regimes. Efforts to extend graphene plasmonics to the near-infrared and visible ranges involve patterning of graphene into nanostructured elements, thus facilitating the optical excitation of localized resonances that can be blue-shifted through geometrical confinement while maintaining electrical tunability. Besides these appealing plasmonic attributes, the conical electronic dispersion relation of graphene renders its charge carrier motion in response to light intrinsically anharmonic, resulting in a comparatively intense nonlinear optical response. The combined synergy of extreme plasmonic field enhancement and large intrinsic optical nonlinearity are now motivating intensive research efforts in nonlinear graphene plasmonics, the recent progress of which we discuss in this Account. We start with a description of the appealing properties of plasmons in graphene nanostructures down to molecular sizes, followed by a discussion of the unprecedented level of intrinsic optical nonlinearity in graphene, its enhancement by resonant coupling to its highly confined plasmons to yield intense high harmonic generation and Kerr nonlinearities, the extraordinary thermo-optical capabilities of this material enabling large nonlinear optical switching down to the single-photon level, and its strong interaction with quantum emitters.

Virus-Sized Gold Nanorods: Plasmonic Particles for Biology.

Plasmons, collective oscillations of conduction-band electrons in nanoscale metals, are well-known phenomena in colloidal gold and silver nanocrystals that produce brilliant visible colors in these materials that depend on the nanocrystal size and shape. Under illumination at or near the plasmon bands, gold and silver nanocrystals exhibit properties that enable fascinating biological applications: (i) the nanocrystals elastically scatter light, providing a straightforward way to image them in complex aqueous environments; (ii) the nanocrystals produce local electric fields that enable various surface-enhanced spectroscopies for sensing, molecular diagnostics, and boosting of bound fluorophore performance; (iii) the nanocrystals produce heat, which can lead to chemical transformations at or near the nanocrystal surface and can photothermally destroy nearby cells. While all the above-mentioned applications have already been well-demonstrated in the literature, this Account focuses on several other aspects of these nanomaterials, in particular gold nanorods that are approximately the size of viruses (diameters of ∼10 nm, lengths up to 100 nm). Absolute extinction, scattering, and absorption properties are compared for gold nanorods of various absolute dimensions, and references for how to synthesize gold nanorods with four different absolute dimensions are provided. Surface chemistry strategies for coating nanocrystals with smooth or rough shells are detailed; specific examples include mesoporous silica and metal-organic framework shells for porous (rough) coatings and polyelectrolyte layer-by-layer wrapping for "smooth" shells. For self-assembled-monolayer molecular coating ligands, the smoothest shells of all, a wide range of ligand densities have been reported from many experiments, yielding values from less than 1 to nearly 10 molecules/nm2 depending on the nanocrystal size and the nature of the ligand. Systematic studies of ligand density for one particular ligand with a bulky headgroup are highlighted, showing that the highest ligand density occurs for the smallest nanocrystals, even though these ligand headgroups are the most mobile as judged by NMR relaxation studies. Biomolecular coronas form around spherical and rod-shaped nanocrystals upon immersion into biological fluids; these proteins and lipids can be quantified, and their degree of adsorption depends on the nanocrystal surface chemistry as well as the biophysical characteristics of the adsorbing biomolecule. Photothermal adsorption and desorption of proteins on nanocrystals depend on the enthalpy of protein-nanocrystal surface interactions, leading to light-triggered alteration in protein concentrations near the nanocrystals. At the cellular scale, gold nanocrystals exert genetic changes at the mRNA level, with a variety of likely mechanisms that include alteration of local biomolecular concentration gradients, changes in mechanical properties of the extracellular matrix, and physical interruption of key cellular processes-even without plasmonic effects. Microbiomes, both organismal and environmental, are the likely first point of contact of nanomaterials with natural living systems; we see a major scientific frontier in understanding, predicting, and controlling microbe-nanocrystal interactions, which may be augmented by plasmonic effects.