Plasmonic Nickel Nanoantennas

The fundamental optical properties of pure nickel nanostructures are studied by far‐field extinction spectroscopy and optical near‐field microscopy, providing direct experimental evidence of the existence of particle plasmon resonances predicted by theory. Experimental and calculated near‐field maps allow for unambiguous identification of dipolar plasmon modes. By comparing calculated near‐field and far‐field spectra, dramatic shifts are found between the near‐field and far‐field plasmon resonances, which are much stronger than in gold nanoantennas. Based on a simple damped harmonic oscillator model to describe plasmonic resonances, it is possible to explain these shifts as due to plasmon damping.     

Second Near-Infrared Plasmonic Nanomaterials for Photoacoustic Imaging and Photothermal Therapy

Photoacoustic imaging (PAI) and imaging-guided photothermal therapy (PTT) in the second near-infrared window (NIR-II, 1000–1700 nm) have received increasing attention owing to their advantages of greater penetration depth and higher signal-to-noise ratio. Plasmonic nanomaterials with tunable optical properties and strong light absorption provide an alternative to dye molecules, showing great prospects for phototheranostic applications. In this review, the research progress in principally modulating the optical properties of plasmonic nanomaterials, especially affecting parameters such as size, morphology, and surface chemical modification, is introduced. The commonly used plasmonic nanomaterials in the NIR-II window, including noble metals, semiconductors, and heterostructures, are then summarized. In addition, the biomedical applications of these NIR-II plasmonic nanomaterials for PAI and PTT in phototheranostics are highlighted. Finally, the perspectives and challenges for advancing plasmonic nanomaterials for practical use and clinical translation are discussed.     

Surface Assembly and Plasmonic Properties in Strongly Coupled Segmented Gold Nanorods

An assembly strategy is reported such that segmented nanorods fabricated through template‐assisted methods can be robustly transferred and tethered to a pre‐functionalized substrate with excellent uniformity over large surface areas. After embedding the rods, sacrificial nickel segments were selectively etched leaving behind strongly coupled segmented gold nanorods with gaps between rods below 40 nm and as small as 2 nm. Hyper‐spectral imaging is utilized to measure Rayleigh scattering spectra from individual and coupled nanorod elements in contrast to common bulk measurements. This approach discerns the effects of not only changing segment and gap size but also the presence of characteristic defects on the plasmonic coupling between closely spaced nanorods. Polarized hyper‐spectral measurements are conducted to provide direct observation of the anisotropic plasmonic resonance modes in individual and coupled nanorods, which are close to those predicted by computer simulations for nanorods with ideal shapes. Some common deviations from ideal shape such as non‐flat facets and asymmetric tails are demonstrated to result in the appearance of characteristic plasmon resonances, which have not been considered before. The large‐scale assembly of coupled noble nanostructures with fine control over geometry and high uniformity provides means to strongly tune the scattering, absorption, and near‐field plasmonic properties through the geometric arrangement of precisely controlled nanorod segments.     

A Self-Assembled Plasmonic Substrate for Enhanced Fluorescence Resonance Energy Transfer

Fluorescence resonance energy transfer (FRET) has found widespread uses in biosensing, molecular imaging, and light harvesting. Plasmonic metal nanostructures offer the possibility of engineering photonic environment of specific fluorophores to enhance the FRET efficiency. However, the potential of plasmonic nanostructures to enable tailored FRET enhancement on planar substrates remains largely unrealized, which are of considerable interest for high-performance on-surface bioassays and photovoltaics. The main challenge lies in the necessitated concurrent control over the spectral properties of plasmonic substrates to match that of fluorophores and the fluorophore–substrate spacing. Here, a self-assembled plasmonic substrate based on polydopamine (PDA)-coated plasmonic nanocrystals is developed to effectively address this challenge. The PDA coating not only drives interfacial self-assembly of the nanocrystals to form closely packed arrays with customized optical properties, but also can serve as a tailored nanoscale spacer between the fluorophores and plasmonic nanocrystals, which collectively lead to optimized fluorescence enhancement. The biocompatible plasmonic substrate that allows convenient bioconjugation imparted by PDA has afforded improved FRET efficiency in DNA microarray assay and FRET imaging of live cells. It is envisioned that the self-assembled plasmonic substrates can be readily integrated into fluorescence-based platforms for diverse biomedical and photoconversion applications.     

Visible to Near‐Infrared Fluorescence Enhanced Cellular Imaging on Plasmonic Gold Chips

Rapid and sensitive detections of a variety of surface and intracellular proteins, nucleic acids, and other cellular biomarkers are important to elucidating biological signaling pathways and to devising disease diagnostics and therapeutics. Here, sensitive imaging and detection of cellular proteins on fluorescence‐enhancing, nanostructured plasmonic gold (pGold) chips is presented. Imaging of fluorescently labeled cellular biomarkers on pGold is enhanced by 2–30‐fold in the visible to near infrared (NIR) range of ≈500–900 nm. The high fluorescence enhancement of >700 nm significantly improves the dynamic range and signal/background ratios of NIR imaging, allowing high‐performance multicolor imaging in the visible–NIR range using high quantum yield (QY) visible dyes and lower QY NIR fluorophores. Further, multiple cellular proteins of single cells of various cell types can be detected through microarraying of cells, useful for potentially hundreds and thousands different types of cells assayed on a single chip down to small cell numbers. This work suggests a simple, high throughput, high sensitivity, and multiplexed single‐cell analysis method on fluorescence enhancing plasmonic substrates in the entire visible to NIR window.     

Plasmonic Materials

We provide an overview of the way in which different approaches to nanostructuring metals can lead to a wealth of interesting optical properties and functionality through manipulation of the plasmon modes that such structures support, a field known as plasmonics. The increasing interest in plasmonics derives in large measure from the interplay between better fabrication techniques and an awareness of the potential that controlled plasmon modes have to offer. The combination of nanometer‐scale fabrication techniques and increasingly sophisticated numerical modeling capabilities thus enables a significant advance in our understanding of the science underlying plasmonics. Here, we survey some of the different structures that have been explored. We hope that this Review will spur others to continue the exploration of this fascinating topic.     

Oscillatory Dynamics and In Vivo Photoacoustic Imaging Performance of Plasmonic Nanoparticle‐Coated Microbubbles

Microbubbles bearing plasmonic nanoparticles on their surface provide contrast enhancement for both photoacoustic and ultrasound imaging. In this work, the responses of microbubbles with surface‐bound gold nanorods—termed AuMBs—to nanosecond pulsed laser excitation are studied using high‐speed microscopy, photoacoustic imaging, and numerical modeling. In response to laser fluences below 5 mJ cm^?2, AuMBs produce weak photoacoustic emissions and exhibit negligible microbubble wall motion. However, in reponse to fluences above 5 mJ cm^?2, AuMBs undergo dramatically increased thermal expansion and emit nonlinear photoacoustic waves of over 10‐fold greater amplitude than would be expected from freely dispersed gold nanorods. Numerical modeling suggests that AuMB photoacoustic responses to low laser fluences result from conductive heat transfer from the surface‐bound nanorods to the microbubble gas core, whereas at higher fluences, explosive boiling may occur at the nanorod surface, producing vapor nanobubbles that contribute to rapid AuMB expansion. The results of this study indicate that AuMBs are capable of producing acoustic emissions of significantly higher amplitude than those produced by conventional sources of photoacoustic contrast. In vivo imaging performance of AuMBs in a murine kidney model suggests that AuMBs may be an effective alternative to existing contrast agents for noninvasive photoacoustic and ultrasound imaging applications.     

High-Speed Parallel Plasmonic Direct-Writing Nanolithography Using Metasurface-Based Plasmonic Lens

Simple and efficient nanofabrication technology with low cost and high flexibility is indispensable for fundamental nanoscale research and prototyping. Lithography in the near field using the surface plasmon polariton (i.e., plasmonic lithography) provides a promising solution. The system with high stiffness passive nanogap control strategy on a high-speed rotating substrate is one of the most attractive high-throughput methods. However, a smaller and steadier plasmonic nanogap, new scheme of plasmonic lens, and parallel processing should be explored to achieve a new generation high resolution and reliable efficient nanofabrication. Herein, a parallel plasmonic direct-writing nanolithography system is established in which a novel plasmonic flying head is systematically designed to achieve around 15 nm minimum flying-height with high parallelism at the rotating speed of 8–18 m·s⁻¹. A multi-stage metasurface-based polarization insensitive plasmonic lens is proposed to couple more power and realize a more confined spot compared with conventional plasmonic lenses. Parallel lithography of the nanostructures with the smallest (around 26 nm) linewidth is obtained with the prototyping system. The proposed system holds great potential for high-freedom nanofabrication with low cost, such as planar optical elements and nano-electromechanical systems.     

In Vivo Volumetric Photoacoustic Molecular Angiography and Therapeutic Monitoring with Targeted Plasmonic Nanostars

Photoacoustic (PA) imaging promises deeper tissue penetration while maintaining rich optical contrast as compared to other high resolution optical imaging techniques. In this report, a near‐infrared pulse laser serves as the excitation source, and 128 ultrasonic transducers are spirally distributed on a hemispherical surface to receive PA signals for three‐dimensional (3D) image reconstruction. With these attributes, the unique modality produces an isotropic and homogeneous spatial resolution (～200 μm) with penetration depth of centimeters. Cyclic Arg‐Gly‐Asp (RGD) peptides conjugated plasmonic gold nanostars (RGD‐GNS) are designed to specifically target over‐expressed integrin α_vβ₃ on tumor neovasculature, enabling highly sensitive angiography and photothermal therapy (PTT). After the administration of RGD‐GNS, tumor angiogenesis is clearly imaged with enhanced contrast, and the growth of tumor is effectively inhibited by PTT after laser irradiation. This study suggest that the PA angiography with plasmonic RGD‐GNS can be applied as a triple functional platform for tumor diagnosis, PTT, and treatment monitoring. This PA technique offers deeper imaging depth with homogeneous resolution over existing optical imaging techniques for early diagnosis of tumor angiogenesis as well as on‐the‐spot nanotherapeutic evaluation.     

Nanoporous Plasmonic Metamaterials

We review different routes for the generation of nanoporous metallic foams and films exhibiting well‐defined pore size and short‐range order. Dealloying and templating allows the generation of both 2D and 3D structures that promise a plasmonic response determined by material constituents and porosity. Viewed in the context of metamaterials, the ease of fabrication of samples covering macroscopic dimensions is highly promising, and suggests more in‐depth investigations of the plasmonic and photonic properties of this material system for photonic applications.     

Lateral Magnetic Near‐Field Imaging of Plasmonic Nanoantennas With Increasing Complexity

The design of many promising, newly emerging classes of photonic metamaterials and subwavelength confinement structures requires detailed knowledge and understanding of the electromagnetic near‐field interactions between their building blocks. While the electric field distributions and, respectively, the electric interactions of different nanostructures can be routinely measured, for example, by scattering near‐field microscopy, only recently experimental methods for imaging the magnetic field distributions became available. In this paper, we provide direct experimental maps of the lateral magnetic near‐field distributions of variously shaped plasmonic nanoantennas by using hollow‐pyramid aperture scanning near‐field optical microscopy (SNOM). We study both simple plasmonic nanoresonators, such as bars, disks, rings and more complex antennas. For the studied structures, the magnetic near‐field distributions of the complex resonators have been found to be a superposition of the magnetic near‐fields of the individual constituting elements. These experimental results, explained and validated by numerical simulations, open new possibilities for engineering and characterization of complex plasmonic antennas with increased functionality.     

Performance enhancements to absorbance-modulation optical lithography. II. Plasmonic superlenses

The ability to improve the transmission and intensity profiles in absorbance-modulation optical lithography (AMOL) [J. Opt. Soc. Am. A 23, 2290 (2006) and Phys. Rev. Lett. 98, 043905 (2007)] through the introduction of a plasmonic metal layer is investigated. In this part of the work, a plasmonic layer is placed between the absorbance-modulation layer and the photoresist layer. Transmission through this layer is possible due to the ability of thin plasmonic layers to act as near-field analogues of negative refraction materials. The superlens performance is best with a thin layer of 10-20 nm, although this causes a full width at half-maximum increase of ~50%. The introduction of the plasmonic layers allows dichroic filtering of the two wavelengths, with a difference of a factor of 10 in the transmitted intensity ratio, reducing undesirable exposure of the resist. The presented work demonstrates that a plasmonic layer can be interfaced with an AMOL system, but that further optimization and material development are needed to allow substantial performance improvements.     

Plasmonic Metallic Heteromeric Nanostructures

Binary, ternary, and other high‐order plasmonic heteromers possess remarkable physical and chemical properties, enabling them to be used in numerous applications. The seed‐mediated approach is one of the most promising and versatile routes to produce plasmonic heteromers. Selective growth of one or multiple domains on desired sites of noble metal, semiconductor, or magnetic seeds would form desired heteromeric nanostructures with multiple functionalities and synergistic effects. In this work, the challenges for the synthetic approaches are discussed with respect to tuning the thermodynamics, as well as the kinetic properties (e.g., pH, temperature, injection rate, among others). Then, plasmonic heteromers with their structure advantages displaying unique activities compared to other hybrid nanostructures (e.g., core–shell, alloy) are highlighted. Some of the main most recent applications of plasmonic heteromers are also presented. Finally, perspectives for further exploitation of plasmonic heteromers are demonstrated. The goal of this work is to provide the current know‐how on the synthesis routes of plasmonic heteromers in a summarized manner, so as to achieve a better understanding of the resulting properties and to gain an improved control of their performances and extend their breadth of applications.