Probing quantum plasmon coupling using gold nanoparticle dimers with tunable interparticle distances down to the subnanometer range

Plasmon-mediated photocatalytic conversion in Au or Ag nanorod aggregates by surface-enhanced Raman spectroscopy

Plasmonic nanoparticles (NPs) hold considerable potential as photocatalysts owing to their robust light-matter interactions across diverse electromagnetic wavelengths, which significantly influence the photophysical characteristics of the adjacent molecular entities. Despite the widespread use of noble-metal NPs in surface-enhanced Raman scattering (SERS) applications, little is known about the kinetics of nanoparticle aggregation and how it affects their configurations. This study investigates the plasmon-driven photochemical conversion of 4-nitrobenzenethiol (NBT) to 4,4'-dimercaptoazobenzene (DMAB) on Au and Ag nanorods (NRs) through SERS. Significantly, photoconversion phenomena were observed on Ag NRs but not on Au NRs upon laser excitation at 633 nm. Finite-difference time-domain simulations revealed the presence of stronger electromagnetic fields on Ag NRs than on Au NRs. The aspect ratios and gaps between individual NPs in dimer configurations were determined to elucidate their effects on electromagnetic fields. The Ag NR dimer with an end-to-end configuration, an aspect ratio of 3.3, and a 1-nm gap exhibited the highest enhancement factor of 1.05 × 1012. Our results demonstrate that the primary contribution from diverse configurations in NR aggregates is the end-to-end configuration. The proposed NP design with adjustable parameters is expected to advance research in plasmonics, sensing, and wireless communications. These findings also contribute to the understanding of plasmon-driven photochemical processes in metallic nanostructures.

Plasmonic nanoparticle sensors: current progress, challenges, and future prospects

Plasmonic nanoparticles (NPs) have played a significant role in the evolution of modern nanoscience and nanotechnology in terms of colloidal synthesis, general understanding of nanocrystal growth mechanisms, and their impact in a wide range of applications. They exhibit strong visible colors due to localized surface plasmon resonance (LSPR) that depends on their size, shape, composition, and the surrounding dielectric environment. Under resonant excitation, the LSPR of plasmonic NPs leads to a strong field enhancement near their surfaces and thus enhances various light-matter interactions. These unique optical properties of plasmonic NPs have been used to design chemical and biological sensors. Over the last few decades, colloidal plasmonic NPs have been greatly exploited in sensing applications through LSPR shifts (colorimetry), surface-enhanced Raman scattering, surface-enhanced fluorescence, and chiroptical activity. Although colloidal plasmonic NPs have emerged at the forefront of nanobiosensors, there are still several important challenges to be addressed for the realization of plasmonic NP-based sensor kits for routine use in daily life. In this comprehensive review, researchers of different disciplines (colloidal and analytical chemistry, biology, physics, and medicine) have joined together to summarize the past, present, and future of plasmonic NP-based sensors in terms of different sensing platforms, understanding of the sensing mechanisms, different chemical and biological analytes, and the expected future technologies. This review is expected to guide the researchers currently working in this field and inspire future generations of scientists to join this compelling research field and its branches.

Innovative Synthesis of Au Nanoparticle-Trapped Flexible Macrocrystals: Achieving Stable Black Crystal Wires with Broadband Absorption

In optical materials, the development of absorbers for a wide spectrum is a focal point of research. A pivotal challenge lies in ensuring the stability and durability of optical absorbers, particularly at elevated temperatures. This study introduces a novel approach to creating absorbers with diverse colors, focusing on the synthesis and properties of black crystal wires. In contrast to black gold nanoparticle (Au NP) precipitates, which change color within hours under similar conditions, the method involves strategically trapping Au NPs within defects during the growth of single crystals. This results in black crystal wires that not only exhibit broadband absorption but also maintain exceptional stability even under prolonged exposure to high temperatures. The method also involves the controlled synthesis of colorless and red crystal wires. As a proof of concept, these stable black Au crystal wires demonstrate superior performance in photothermal conversion applications. The methodology, derived from the crystal growth process, presents a defect template that offers a novel approach to material design. Furthermore, these unique crystals, available in various colors, hold significant promise for a range of unexplored applications.

Active control of plasmon coupling via simple electrochemical surface oxidation/reduction of Au nanoparticle agglomerates

Reversible tuning of plasmon coupling of Au nanoparticle (AuNP) agglomerates containing dimers as the main component was achieved via electrochemical surface oxidation/reduction of the AuNP surface. The system required no reactant except for water and was almost finished within a unit second, which leads to novel active plasmonic devices.

Rapid formation of gold core-satellite nanostructures using Turkevich-synthesized satellites and dithiol linkers: the do's and don'ts for successful assembly

Turkevich syntheses represent a foundational approach for forming colloids of monodisperse gold nanoparticles where the use of these structures as building blocks when forming multicomponent nanoassemblies is pervasive. The core-satellite motif, which is characterized by a central core structure onto which satellite structures are tethered, distinguishes itself in that it can realize numerous plasmonic nanogaps with nanometer scale widths. Established procedures for assembling these multicomponent structures are, to a large extent, empirically driven, time-consuming, difficult to reproduce, and in need of a strong mechanistic underpinning relating to the close-range electrostatic interactions needed to secure satellite structures onto core materials. Described herein is a rapid, repeatable procedure for assembling core-satellite structures using Turkevich-grown satellites and dithiol linkers. With this successful procedure acting as a baseline for benchmarking modified procedures, a rather complex parameter space is understood in terms of timeline requirements for various processing steps and an analysis of the factors that prove consequential to assembly. It is shown that seemingly innocuous procedures realize sparsely populated cores whereas cores initially obstructed with commonly used capping agents lead to few disruptions to satellite attachment. Once these factors are placed under control, then it is the ionic strength imposed by the reaction biproducts of the Turkevich synthesis that is the critical factor in assembly because they decide the spatial extent of the electrical double layer surrounding each colloidal nanoparticle. With this understanding, it is possible to control the ionic strength through the addition or subtraction of various ionic species and assert control over the assembly process. The work, hence, advances the rules for a robust core-satellite assembly process and, in a broader sense, contributes to the knowhow required for the precise, programmable, and controllable assembly of multicomponent systems.

Versatile Ce(III)‐Terephthalic Acid@Au Metal Organic Frameworks for ROS Elimination and Photothermal Sterilization

Nanozymes have been widely used for treating reactive oxygen species (ROS) caused diseases. However, the ROS‐dependent antibacterial property is inevitably damaged during the process of scavenging ROS, which is unfavorable for the treatment of diseases related to both ROS accumulation and bacterial infections. To address the issues, biomedical materials with both ROS‐elimination ability and ROS‐independent antibacterial capacity are fabricated via in situ depositing spherical Au nanoparticles (Au NPs) on rough surface of metal organic frameworks composed of Ce(III) and terephthalic acid (Ce‐BDC@Au MOFs). The synthesized Ce‐BDC@Au MOFs show multi‐enzymatic activities owing to the reversible conversion between Ce3+ and Ce4+, and can significantly scavenge ROS in cells. The deposition of spherical Au NPs on surface of Ce‐BDC MOFs causes Au NPs to come close proximity for forming plasmon resonance coupling, inducing the resonance wavelength of Au NPs red shifted to NIR region. Based on this, Ce‐BDC@Au MOFs show good photothermal conversion efficiency under NIR laser (808 nm) irradiation. Benefitting from rough surface and photothermal conversion ability, Ce‐BDC@Au MOFs have high antibacterial efficiency against staphylococcus aureus through both mechanically damaging and photothermal destruction. This strategy is biosafety and effectiveness for treating diseases related to both ROS accumulation and bacterial infections.

Singular and Nonsingular Transitions in the Infrared Plasmons of Nearly Touching Nanocube Dimers

Narrow gaps between plasmon-supporting materials can confine infrared electromagnetic energy at the nanoscale, thus enabling applications in areas such as optical sensing. However, in nanoparticle dimers, the nature of the transition between touching (zero gap) and nearly nontouching (nonzero gap ≲15 nm) regimes is still a subject of debate. Here, we observe both singular and nonsingular transitions in infrared plasmons confined to dimers of fluorine-doped indium oxide nanocubes when moving from touching to nontouching configurations depending on the dimensionality of the contact region. Through spatially resolved electron energy-loss spectroscopy, we find a continuous spectral evolution of the lowest-order plasmon mode across the transition for finite touching areas, in excellent agreement with the simulations. This behavior challenges the widely accepted idea that a singular transition always emerges in the near-touching regime of plasmonic particle dimers. The apparent contradiction is resolved by theoretically examining different types of gap morphologies, revealing that the presence of a finite touching area renders the transition nonsingular, while one-dimensional and point-like contacts produce a singular behavior in which the lowest-order dipolar mode in the touching configuration, characterized by a net induced charge in each of the particles, becomes unphysical as soon as they are separated. Our results provide valuable insights into the nature of dimer plasmons in highly doped semiconductors.

Lateral flow immunoassay based on surface-enhanced Raman scattering using pH-induced phage-templated hierarchical plasmonic assembly for point-of-care diagnosis of infectious disease

The outbreak of emerging infectious diseases gave rise to the demand for reliable point-of-care testing methods to diagnose and manage those diseases in early onset. However, the current on-site testing methods including lateral flow immunoassay (LFIA) suffer from the inaccurate diagnostic result due to the low sensitivity. Herein, we present the surface-enhanced Raman scattering-based lateral flow immunoassay (SERS-LFIA) by introducing phage-templated hierarchical plasmonic assembly (PHPA) nanoprobes to diagnose a contagious disease. The PHPA was fabricated using gold nanoparticles (AuNPs) assembled on bacteriophage MS2, where inter-particle gap sizes can be adjusted by pH-induced morphological alteration of MS2 coat proteins to provide the maximum SERS amplification efficiency via plasmon coupling. The plasmonic probes based on the PHPA produce strong and reproducible SERS signal that leads to sensitive and reliable diagnostic results in SERS-LFIA. The developed SERS-LFIA targeting severe acute respiratory syndrome-coronavirus 2 (SARS-CoV-2) antibodies for a proof of concept had <100 pg/mL detection limits with high specificity in serum, proving it as an effective diagnostic device for the infectious diseases. Clinical validation using human serum samples further confirmed that the PHPA-based SERS-LFIA can distinguish the patients with COVID-19 from healthy controls with significant accuracy. These outcomes prove that the developed SERS-LFIA biosensor can be an alternative point-of-care testing (POCT) method against the emerging infectious diseases, in combination with the commercially available portable Raman devices.

Combinatorial Approach to Find Nanoparticle Assemblies with Maximum Surface-Enhanced Raman Scattering

Plasmonic nanoparticles exhibit unique properties that distinguish them from other nanomaterials, including vibrant visible colors, the generation of local electric fields, the production of hot charge carriers, and localized heat emission. These properties are particularly enhanced in the narrow nanogaps formed between nanostructures. Therefore, creating nanogaps in a controlled fashion is the key to achieving a fundamental understanding of plasmonic phenomena originating from the nanogaps and developing advanced nanomaterials with enhanced performance for diverse applications. One of the most effective approaches to creating nanogaps is to assemble individual nanoparticles into a clustered structure. In this study, we present a fast, facile, and highly efficient method for preparing core@satellite (CS) nanoassembly structures using gold nanoparticles of various shapes and sizes, including nanospheres, nanocubes (AuNCs), nanorods, and nanotriangular prisms. The sequential assembly of these building blocks on glass substrates allows us to obtain CS nanostructures with a 100% yield within 4 h. Using 9 different building blocks, we successfully produce 16 distinct CS nanoassemblies and systematically investigate the combinations to search for the highest Raman enhancement. We find that the surface-enhanced Raman scattering (SERS) intensity of AuNC@AuNC CS nanoassemblies is 2 orders of magnitude larger than that of other CS nanoassemblies. Theoretical analyses reveal that the intensity and distribution of the electric field induced in the nanogaps by plasmon excitation, as well as the number of molecules in the interfacial region, collectively contribute to the unprecedentedly large SERS enhancement observed for AuNC@AuNC. This study not only presents a novel assembly method that can be extended to produce many other nanoassemblies but also identifies a highly promising SERS material for sensing and diagnostics through a systematic search process.

Assembly and Healing: Capacitive and Conductive Plasmonic Interfacing via a Unified and Clean Wet Chemistry Route

Solution-based nanoparticle assembly represents a highly promising way to build functional metastructures based on a wealth of synthetic nanomaterial building blocks with well-controlled morphology and crystallinity. In particular, the involvement of DNA molecular programming in these bottom-up processes gradually helps the ambitious goal of customizable chemical nanofabrication. However, a fundamental challenge is to realize strong interunit coupling in an assembly toward emerging functions and applications. Herein, we present a unified and clean strategy to address this critical issue based on a H2O2-redox-driven "assembly and healing" process. This facile solution route is able to realize both capacitively coupled and conductively bridged colloidal boundaries, simply switchable by the reaction temperature, toward bottom-up nanoplasmonic engineering. In particular, such a "green" process does not cause surface contamination of nanoparticles by exogenous active metal ions or strongly passivating ligands, which, if it occurs, could obscure the intrinsic properties of as-formed structures. Accordingly, previously raised questions regarding the activities of strongly coupled plasmonic structures are clarified. The reported process is adaptable to DNA nanotechnology, offering molecular programmability of interparticle charge conductance. This work represents a new generation of methods to make strongly coupled nanoassemblies, offering great opportunities for functional colloidal technology and even metal self-healing.

Mode-Selective Plasmon Coupling between Au Nanorods and Au Nanospheres

Plasmons play a central role in the properties of gold nanoparticles (AuNPs). Plasmons in a AuNP are influenced by neighboring ones, resulting in hybridized bonding dipole modes and red-shifted resonance peaks in the extinction spectra. Previous studies have mainly focused on plasmon coupling among spherical AuNPs (AuNSs). Here, we explore plasmonic interactions between AuNSs and anisotropic gold nanorods (AuNRs), which have longitudinal (LO) and transverse (TR) plasmon modes. We successfully assemble AuNSs around AuNRs ("AuNR@AuNS"), observing shifts in both the LO and TR modes in the extinction spectra due to directional coupling. Selectively binding AuNSs to the ends of AuNRs ("AuNR═AuNS") leads to predominant plasmon coupling along the LO direction. Our simulation studies reveal that exclusive LO or TR coupling occurs only when AuNSs attach to the center of either the end or the side of AuNRs. This study provides a valuable guideline for selectively exciting plasmons in desired nanogaps when multiple nanogaps are present.

Plasmonic Nanocrystal Assembly-Semiconductor Hybrids for Boosting Visible to Near-Infrared Photocatalysis

Plasmonic metal-semiconductor hybrid photocatalysts have received much attention because of their wide light harvesting range and efficient charge carrier generation capability originating from plasmon energy transfer. Here, we introduce a plasmonic metal-semiconductor hybrid nanostructure consisting of a Au core-satellite assembly and crystalline TiO2. The formation of Au@TiO2-Au core-satellite assemblies using TiO2 as a spacer and the subsequent growth of outer TiO2 shells on the core-satellite assemblies, followed by calcination, successfully generated Au core-satellite assembly@TiO2 nanostructures. Exquisite control over the growth of the TiO2 interlayer enabled the regulation of the gap distance between the core and satellite Au nanocrystals within the same hybrid morphology. Due to the structural controllability of the present approach, the gap-distance-dependent plasmonic and photocatalytic properties of the hybrid nanostructures could be explored. The nanostructures possessing the most closely arranged Au nanocrystals showed high photocatalytic activity under visible to near-infrared light irradiation, which can be attributed to strong plasmon coupling between the core and satellite Au nanocrystals that can expedite the formation of intense plasmon energy and its transfer to TiO2.

Gap-enhanced gold nanodumbbells with single-particle surface-enhanced Raman scattering sensitivity

Gap-enhanced Raman tags (GERTs) have been widely used for surface-enhanced Raman scattering (SERS) imaging due to their excellent SERS performances. Here, we reported a synthetic strategy for novel gap-enhanced dumbbell-like nanoparticles with anisotropic shell coatings. Controlled shell growth at the tips of gold nanorods was achieved by using cetyltrimethylammonium bromide (CTAB) as a capping agent. A mechanism related to the shape-directing effects of CTAB was proposed to explain the findings. Optimized gap-enhanced gold dumbbells exhibited highly enhanced SERS responses compared to rod cores, with an enhancement ratio of 101.5. We further demonstrated that gap-enhanced AuNDs exhibited single-particle SERS sensitivity with an acquisition time as fast as 0.1 s per spectrum, showing great potential for high-speed SERS imaging.

Chemical and Physical Properties of Photonic Noble-Metal Nanomaterials

Colloidal noble metal nanoparticles (NPs) are composed of metal cores and organic or inorganic ligand shells. These NPs support size- and shape-dependent plasmonic resonances. They can be assembled from dispersions into artificial metamolecules which have collective plasmonic resonances originating from coupled bright and dark optical electric and magnetic modes that form depending on the size and shape of the constituent NPs and their number, arrangement, and interparticle distance. NPs can also be assembled into extended 2D and 3D metamaterials that are glassy thin films or ordered thin films or crystals, also known as superlattices and supercrystals. The metamaterials have tunable optical properties that depend on the size, shape, and composition of the NPs, and on the number of NP layers and their interparticle distance. Interestingly, strong light-matter interactions in superlattices form plasmon polaritons. Tunable interparticle distances allow designer materials with dielectric functions tailorable from that characteristic of an insulator to that of a metal, and serve as strong optical absorbers or scatterers, respectively. In combination with lithography techniques, these extended assemblies can be patterned to create subwavelength NP superstructures and form large-area 2D and 3D metamaterials that manipulate the amplitude, phase, and polarization of transmitted or reflected light.

Controlling Atomic-Scale Restructuring and Cleaning of Gold Nanogap Multilayers for Surface-Enhanced Raman Scattering Sensing

We demonstrate the reliable creation of multiple layers of Au nanoparticles in random close-packed arrays with sub-nm gaps as a sensitive surface-enhanced Raman scattering substrate. Using oxygen plasma etching, all the original molecules creating the nanogaps can be removed and replaced with scaffolding ligands that deliver extremely consistent gap sizes below 1 nm. This allows precision tailoring of the chemical environment of the nanogaps which is crucial for practical Raman sensing applications. Because the resulting aggregate layers are easily accessible from opposite sides by fluids and by light, high-performance fluidic sensing cells are enabled. The ability to cyclically clean off analytes and reuse these films is shown, exemplified by sensing of toluene, volatile organic compounds, and paracetamol, among others.

Smart Delivery Systems Responsive to Cathepsin B Activity for Cancer Treatment

Cathepsin B is a lysosomal cysteine protease, contributing to vital cellular homeostatic processes including protein turnover, macroautophagy of damaged organelles, antigen presentation, and in the extracellular space, it takes part in tissue remodeling, prohormone processing, and activation. However, aberrant overexpression of cathepsin B and its enzymatic activity is associated with different pathological conditions, including cancer. Cathepsin B overexpression in tumor tissues makes this enzyme an important target for smart delivery systems, responsive to the activity of this enzyme. The generation of technologies which therapeutic effect is activated as a result of cathepsin B cleavage provides an opportunity for tumor-targeted therapy and controlled drug release. In this review, we summarized different technologies designed to improve current cancer treatments responsive to the activity of this enzyme that were shown to play a key role in disease progression and response to the treatment.

Plasmonic nanoparticle's anti-aggregation application in sensor development for water and wastewater analysis

Colorimetric sensors have emerged as a powerful tool in the detection of water pollutants. Plasmonic nanoparticles use localized surface plasmon resonance (LSPR)-based colorimetric sensing. LSPR-based sensing can be accomplished through different strategies such as etching, growth, aggregation, and anti-aggregation. Based on these strategies, various sensors have been developed. This review focuses on the newly developed anti-aggregation-based strategy of plasmonic nanoparticles. Sensors based on this strategy have attracted increasing interest because of their exciting properties of high sensitivity, selectivity, and applicability. This review highlights LSPR-based anti-aggregation sensors, their classification, and role of plasmonic nanoparticles in these sensors for the detection of water pollutants. The anti-aggregation based sensing of major water pollutants such as heavy metal ions, anions, and small organic molecules has been summarized herein. This review also provides some personal insights into current challenges associated with anti-aggregation strategy of LSPR-based colorimetric sensors and proposes future research directions.

SERS-ELISA using silica-encapsulated Au core-satellite nanotags for sensitive detection of SARS-CoV-2

The sensitive detection of viruses is key to preventing the spread of infectious diseases. In this study, we develop a silica-encapsulated Au core-satellite (CS@SiO₂) nanotag, which produces a strong and reproducible surface-enhanced Raman scattering (SERS) signal. The combination of SERS from the CS@SiO₂ nanotags with enzyme-linked immunosorbent assay (ELISA) achieves a highly sensitive detection of SARS-CoV-2. The CS@SiO₂ nanotag is constructed by assembling 32 nm Au nanoparticles (AuNPs) on a 75 nm AuNP. Then the core-satellite particles are encapsulated with SiO₂ for facile surface modification and stability. The SERS-ELISA technique using the CS@SiO₂ nanotags provides a great sensitivity, yielding a detection limit of 8.81 PFU mL-1, which is 10 times better than conventional ELISA and 100 times better than lateral flow assay strip method. SERS-ELISA is applied to 30 SARS-CoV-2 clinical samples and achieved 100% and 55% sensitivities for 15 and 9 positive samples with cycle thresholds < 30 and > 30, respectively. This new CS@SiO₂-SERS-ELISA method is an innovative technique that can significantly reduce the false-negative diagnostic rate for SARS-CoV-2 and thereby contribute to overcoming the current pandemic crisis.

Cucurbit[8]uril-mediated SERS plasmonic nanostructures with sub-nanometer gap for the identification and determination of estrogens

The SERS intensity of analytes is primarily influenced by the density and distribution of hotspots, which are often difficult to manipulate or regulate. In this study, cucurbit[8]uril (CB[8]), a kind of rigid macrocyclic molecule, was introduced to achieve ~ 1-nm nanogap between gold nanoparticles to increase the density of SERS hotspots. Three kinds of estrogens (estrone (E1), bisphenol A (BPA), and hexestrol (DES)) which are molecules with weak SERS signals were targeted in the hotspots by CB[8] to further improve the sensitivity and selectivity of SERS. It was demonstrated that CB[8] can link gold nanoparticles together through carbonyl groups. In addition, the host-guest interaction of CB[8] and estrogens was proved from the nuclear magnetic resonance hydrogen and infrared spectra. In the presence of CB[8], the SERS intensities of E1, BPA, and DES were increased to 19-fold, 74-fold, and 4-fold, respectively, and the LOD is 3.75 µM, 1.19 µM, and 8.26 µM, respectively. Furthermore, the proposed SERS method was applied to actual milk sample analysis with recoveries of E1 (85.0 ~ 112.8%), BPA (83.0 ~ 103.7%), and DES (62.6 ~ 132.0%). It is expected that the proposed signal enlarging strategy can be applied to  other analytes after further development.

Dimers of Plasmonic Nanocubes to Reach Single-Molecule Strong Coupling with High Emission Yields

Reaching reproducible strong coupling between a quantum emitter and a plasmonic resonator at room temperature, while maintaining high emission yields, would make quantum information processing with light possible outside of cryogenic conditions. We theoretically propose to exploit the high local curvatures at the tips of plasmonic nanocubes to reach Purcell factors of >10⁶ at visible frequencies, rendering single-molecule strong coupling more easily accessible than with the faceted spherical nanoparticles used in recent experimental demonstrations. In the case of gold nanocube dimers, we highlight a trade-off between coupling strength and emission yield that depends on the nanocube size. Electrodynamic simulations on silver nanostructures are performed using a realistic dielectric constant, as confirmed by scattering spectroscopy performed on single nanocubes. Dimers of silver nanocubes feature Purcell factors similar to those of gold while allowing emission yields of >60%, thus providing design rules for efficient strongly coupled hybrid nanostructures at room temperature.

Particle Size-Dependent Onset of the Tunneling Regime in Ideal Dimers of Gold Nanospheres

We report on the nanoparticle-size-dependent onset of quantum tunneling of electrons across the subnanometer gaps in three different sizes (30, 50, and 80 nm) of highly uniform gold nanosphere (AuNS) dimers. For precision plasmonics, the gap distance is systematically controlled at the level of single C-C bonds via a series of alkanedithiol linkers (C₂-C₁₆). Parallax-corrected high-resolution transmission electron microscope (HRTEM) imaging and subsequent tomographic reconstruction are employed to resolve the nm to subnm interparticle gap distances in AuNS dimers. Single-particle scattering experiments on three different sizes of AuNS dimers reveal that for the larger dimers the onset of quantum tunneling regime occurs at larger gap distances: 0.96 ± 0.04 nm (C₆) for 80 nm, 0.83 ± 0.03 nm (C₅) for 50 nm, and 0.72 ± 0.02 nm (C₄) for 30 nm dimers. 2D nonlocal and quantum-corrected model (QCM) calculations qualitatively explain the physical origin for this experimental observation: the lower curvature of the larger particles leads to a higher tunneling current due to a larger effective conductivity volume in the gap. Our results have possible implications in scenarios where precise geometrical control over plasmonic properties is crucial such as in hybrid (molecule-metal) and/or quantum plasmonic devices. More importantly, this study constitutes the closest experimental results to the theory for a 3D sphere dimer system and offers a reference data set for comparison with theory/simulations.

Core-satellite-satellite hierarchical nanostructures: assembly, plasmon coupling, and gap-selective surface-enhanced Raman scattering

The plasmonic properties of gold nanoparticles (AuNPs), such as color tunability, electric field generation, hot carrier generation, and localized heating, are significantly enhanced in the nanogaps between AuNPs. Therefore, the creation and control of nanogaps are key to developing advanced plasmonic nanomaterials. Most AuNP nanoassemblies, including dimers, trimers, and core-satellites, have a single type of nanogap within the assembly. In this study, we construct core-satellite-satellite (CSS) hierarchical, fractal-like nanostructures featuring two types of nanogaps, namely first generation nanogaps (Gap1) between the core and first satellite (Sat1) AuNPs and second generation nanogaps (Gap2) between Sat1 and second satellite (Sat2) AuNPs. The sequential and alternating immersion of glass slides in different-sized AuNPs and linkers forms CSS with perfect yield. The UV-vis spectroscopy, combined with charge density distribution calculations, reveals the nature of the plasmon coupling between the AuNPs that constitute CSS nanoassemblies. The plasmon coupling can be tuned by independently varying Gap1 and Gap2. Furthermore, we explore the electric field amplification in Gap1 and Gap2 by comparing the surface-enhanced Raman scattering signal intensity selectively from each nanogap. This new type of nanostructure provides a great flexibility to control and enhance the plasmonic properties of noble metal nanoparticles.

Point-of-Care Biosensing of Urinary Tract Infections Employing Optoplasmonic Surfaces Embedded with Metal Nanotwins

We report the synthesis of gold nanotwins (Au NTs) on a solid and transparent glass substrate which in turn has been employed for the selective optoplasmonic detection of Escherichia coli (EC) bacteria in human urine for the point-of-care diagnosis of urinary tract infections (UTIs). As compared to the single nanoparticle systems (Au NPs), the Au NTs show an enriched localized surface plasmon resonance (LSPR) due to the enhancement of the electric field under electromagnetic irradiation, e.g., photon, which helps in improving the limits of detection. For this purpose, initially a simple glass surface has been coated with Au NPs, with the help of the linker 3-aminopropyl-triethoxysilane - APTES. The surface has been linked further with another Au NP with the help of the 1,10-alkane-dithiol linker with two thiol ends, which eventually leads to the development of the optoplasmonic surface with Au NTs and an enhanced LSPR response. Subsequently, the EC specific aptamer has been chemically immobilized on the surface of Au NTs with the blocking of free sites via bovine serum albumin (BSA). Remarkably, Raman spectroscopy unfolds a 7-fold increase in the peak intensities with the Au NTs on the glass surface as compared to the surface coated with isolated Au NPs. The enhancement in the LSPR response of glass substrates coated with Au NTs and the EC specific aptamer has been further utilized for the selective and sensitive detection of UTIs. The results have been verified with the help of UV-visible spectroscopy to establish the utility of the proposed sensing methodology. An extensive interference study with other bacterial species unveils the selectivity and specificity of the proposed optoplasmonic sensors toward EC with a detection range of 5 × 10³ to 10⁷ CFU/mL. Intuitively, the method is more versatile in a sense that the sensor can be made specific to any other pathogens by simply changing the design of the aptamer. Finally, a low-cost, portable, and point-of-care optoplasmonic transduction setup is designed with a laser light illumination source, a sample holder, and a sensitive photodetector for the detection of UTIs in human urine.

Mass-determining role in the electrophoretic separation of colloidal plasmonic nanoparticle oligomers

Gel electrophoresis techniques have been commonly applied in sieving plasmonic nanoparticle oligomers, while the intrinsic role in determining their phoresis velocity differences through the gel remains debatable. In this work, we explore the components and yield in each gel band after bundling two rationally designed types of nanoparticles in a system for electrophoretic separation. All results indicate that the mass property of plasmonic oligomers plays an essential role in determining their phoresis velocity divergences during separation. Further theoretical simulations reveal that the grounds for the mass-determining role stemmed from the random inelastic collisions among the oligomers and the gel-network microchannel. Moreover, under the guidance of such a mass-determining role, it is easy to achieve the direct electrophoretic separation of hetero-structured plasmonic dimers with high purity and high yield. This work will not only facilitate the precise nano-engineering of complex plasmonic oligomers with unique optical properties, but also might remove the obstacles toward their industrial manufacture with high purity.

Hydrogen-Induced Aggregation of Au@Pd Nanoparticles for Eye-Readable Plasmonic Hydrogen Sensors

Plasmonic materials provide a promising platform for optical hydrogen detection, but their sensitivities remain limited. Herein, a new type of eye-readable H₂ sensor based on Au@Pd core-shell nanoparticle arrays (NAs) is reported. After exposed to 2% H₂, Au@Pd (16/2) NAs demonstrate a dramatic decrease in the optical extinction intensity, along with an obvious color change from turquoise to gray. Experimental results and theoretical calculations prove that the huge optical change resulted from the H₂-induced aggregation of Au@Pd nanoparticles (NPs), which remarkably alters the plasmon coupling effect between NPs. Moreover, we optimize the sensing behavior from two aspects. The first is selecting appropriate substrates (either rigid glass substrate or flexible polyethylene terephthalate substrate) to offer moderate adhesion force to NAs, ensuring an efficient aggregation of Au@Pd NPs upon H₂ exposure. The second is adjusting the Pd shell thickness to control the extent of NP aggregation and thus the detection range of the as-prepared sensors. This work highlights the advantage of designing eye-readable plasmonic H₂ sensors from the aspect of tuning the interparticle plasmonic coupling in NP assemblies. Au@Pd NAs presented here have several advantages in terms of simple fabrication method, eye-readability in air background at room temperature, tunable detection range, and high cost-effectiveness.

Plasmonic Gold Trimers and Dimers with Air-Filled Nanogaps

The subwavelength confinement of light energy in the nanogaps formed between adjacent plasmonic nanostructures provides the foundational basis for nanophotonic applications. Within this realm, air-filled nanogaps are of central importance because they present a cavity where application-specific nanoscale objects can reside. When forming such configurations on substrate surfaces, there is an inherent difficulty in that the most technologically relevant nanogap widths require closely spaced nanostructures separated by distances that are inaccessible through standard electron-beam lithography techniques. Herein, we demonstrate an assembly route for the fabrication of aligned plasmonic gold trimers with air-filled vertical nanogaps having widths that are defined with spatial controls that exceed those of lithographic processes. The devised procedure uses a sacrificial oxide layer to define the nanogap, a glancing angle deposition to impose a directionality on trimer formation, and a sacrificial antimony layer whose sublimation regulates the gold assembly process. By further implementing a benchtop nanoimprint lithography process and a glancing angle ion milling procedure as additional controls over the assembly, it is possible to deterministically position trimers in periodic arrays and extend the assembly process to dimer formation. The optical response of the structures, which is characterized using polarization-dependent spectroscopy, surface-enhanced Raman scattering, and refractive index sensitivity measurements, shows properties that are consistent with simulation. This work, hence, forwards the wafer-based processing techniques needed to form air-filled nanogaps and place plasmonic energy at site-specific locations.

Controlled synthesis of gold nanorod dimers with end-to-end configurations

End-to-end gold nanorod dimers provide unique plasmonic hotspots with extremely large near-field enhancements in the gaps. Thereby they are beneficial in a wide range of applications, such as enhancing the emissions from ultra-weak emitters. For practical purposes, synthesis of gold nanorod dimers with high yield, especially on the substrates, is essential. Here, we demonstrate two controllable strategies to synthesize gold nanorod dimers based on the self-assembly of gold nanorods, either in bulk solution or on the surface of a glass substrate directly. Both methods can give a high yield of gold nanorod dimers, yet, assembling them directly on the substrate provides more flexibility in controlling the shape and size of each nanorod within the dimer. We also show that these gold nanorod dimers can be used to enhance two-photon-excited fluorescence signals at the single-molecule level.

Influence of spatial dispersion on phase and lateral shifts near the reflection dip in the Kretschmann-Raether structure

The influence of spatial dispersion of metals on phase and Goos-Hänchen (GH) shifts near the reflection dip has been investigated in the Kretschmann-Raether configuration, within the hydrodynamic model framework. We have derived an analytical expression of the reflection coefficient and discussed the optical properties when the nonlocality of metals based on the phenomenological model and Kretchmann's theory is taken into account. Our results show that nonlocality has a significant impact for large wavevectors and causes a shift of the critical point corresponding to the total absorption. Furthermore, these changes also lead to diverse changes in the optical properties including amplitude, phase and GH shift close to the conditions of excitation for the surface plasmon. Our work provides a solid foundation for the understanding of nonlocality in multilayered plasmonic structures and paves the way for future experiments.

Switching plasmonic nanogaps between classical and quantum regimes with supramolecular interactions

In the realm of extreme nanophotonics, nanogap plasmons support reliable field enhancements up to 1000, which provide unique opportunities to access a single molecule for strong coupling and a single atom for quantum catalysis. The quantum plasmonics are intriguing but difficult to modulate largely because of the lack of proper spacers that can reversibly actuate the sub-1-nm gaps. Here, we demonstrate that supramolecular systems made of oligoamide sequences can reversibly switch the gap plasmons of Au nanoparticles on mirror between classical and quantum tunneling regimes via supramolecular interactions. The results reveal detailed plasmon shift near the quantum tunneling limit, which fits well with both classical- and quantum-corrected models. In the quantum tunneling regime, we demonstrate that plasmonic hot electron tunneling can further blue shift the quantum plasmons because of the increased conductance in the nanogaps, making it a promising prototype of optical tunable quantum plasmonic devices.

Quantum Plasmonics: Energy Transport Through Plasmonic Gap

At the interfaces of metal and dielectric materials, strong light-matter interactions excite surface plasmons; this allows electromagnetic field confinement and enhancement on the sub-wavelength scale. Such phenomena have attracted considerable interest in the field of exotic material-based nanophotonic research, with potential applications including nonlinear spectroscopies, information processing, single-molecule sensing, organic-molecule devices, and plasmon chemistry. These innovative plasmonics-based technologies can meet the ever-increasing demands for speed and capacity in nanoscale devices, offering ultrasensitive detection capabilities and low-power operations. Size scaling from the nanometer to sub-nanometer ranges is consistently researched; as a result, the quantum behavior of localized surface plasmons, as well as those of matter, nonlocality, and quantum electron tunneling is investigated using an innovative nanofabrication and chemical functionalization approach, thereby opening a new era of quantum plasmonics. This new field enables the ultimate miniaturization of photonic components and provides extreme limits on light-matter interactions, permitting energy transport across the extremely small plasmonic gap. In this review, a comprehensive overview of the recent developments of quantum plasmonic resonators with particular focus on novel materials is presented. By exploring the novel gap materials in quantum regime, the potential quantum technology applications are also searched for and mapped out.

Synthesis, Assembly, Optical Properties, and Sensing Applications of Plasmonic Gap Nanostructures

Plasmonic gap nanostructures (PGNs) have been extensively investigated mainly because of their strongly enhanced optical responses, which stem from the high intensity of the localized field in the nanogap. The recently developed methods for the preparation of versatile nanogap structures open new avenues for the exploration of unprecedented optical properties and development of sensing applications relying on the amplification of various optical signals. However, the reproducible and controlled preparation of highly uniform plasmonic nanogaps and the prediction, understanding, and control of their optical properties, especially for nanogaps in the nanometer or sub-nanometer range, remain challenging. This is because subtle changes in the nanogap significantly affect the plasmonic response and are of paramount importance to the desired optical performance and further applications. Here, recent advances in the synthesis, assembly, and fabrication strategies, prediction and control of optical properties, and sensing applications of PGNs are discussed, and perspectives toward addressing these challenging issues and the future research directions are presented.

Controlled Assembly of Plasmonic Nanoparticles: From Static to Dynamic Nanostructures

The spatial arrangement of plasmonic nanoparticles can dramatically affect their interaction with electromagnetic waves, which offers an effective approach to systematically control their optical properties and manifest new phenomena. To this end, significant efforts were made to develop methodologies by which the assembly structure of metal nanoparticles can be controlled with high precision. Herein, recent advances in bottom-up chemical strategies toward the well-controlled assembly of plasmonic nanoparticles, including multicomponent and multifunctional systems are reviewed. Further, it is discussed how the progress in this area has paved the way toward the construction of smart dynamic nanostructures capable of on-demand, reversible structural changes that alter their properties in a predictable and reproducible manner. Finally, this review provides insight into the challenges, future directions, and perspectives in the field of controlled plasmonic assemblies.

Design and Preparation of "corn-like" SPIONs@DFK-SBP-M13 Assembly for Improvement of Effective Internalization

Purpose

Superparamagnetic iron oxide nanoparticles (SPIONs) have exhibited preeminent diagnosis and treatment performances, but their low internalization severely limits predesigned functions. The low cell internalization is now an urgent bottleneck problem for almost all nanomaterials. To achieve more internalization of SPIONS, recombinant M13 phage was designed for targeted delivery and smart release.

Methods

M13 phages were designed to co-express exogenous SPARC binding peptide (SBP) and cathepsin B cleavage peptide (DFK), formed recombinant DFK-SBP-M13. 3.37± 0.06 nm of SPIONs were modified by 3, 4-dihydroxyhydrocinnamic acid (DHCA) to gain 10.80 ± 0.21 nm of DHCA-coated SPIONs, i.e., DHCA@SPIONs. Upon adjusting the proportions of DHCA@SPIONs and DFK-SBP-M13, the multi-carboxyl SPIONs assembled onto recombinant M13 phages via covalent bonding. The assemblies were co-cultured with MDA-MB-231 cells to interpret their internalization and smart release.

Results

The “corn-like” SPIONs@DFK-SBP-M13 (261.47±3.30 nm) assemblies have not been reported previously. The assembly was stable, dispersible, superparamagnetic and biocompatible. After co-cultivation with MDA-MB-231 cells, the SPIONs@DFK-SBP-M13 assemblies quickly bond to the cell surface and are internalized. The enrichment rate of SPIONs@DFK-SBP-M13 assembly was 13.9 times higher than free SPIONs at 0.5 h, and intracellular Fe content was 3.6 times higher at 1 h. Furthermore, the DFK peptides favored cathepsin B to cleave SPIONs from the M13 templates resulting in release of SPIONs inside cells.

Conclusion

The novel SPIONs@DFK-SBP-M13 assembly can rapidly deliver SPIONs to the targeted sites and enabled smart release. The combination of genetic recombination and nanotechnology is beneficial for designing and optimizing some new nanomaterials with special functions to achieve wider applications.

Few-Molecule Strong Coupling with Dimers of Plasmonic Nanoparticles Assembled on DNA

Hybrid nanostructures, in which a known number of quantum emitters are strongly coupled to a plasmonic resonator, should feature optical properties at room temperature such as few-photon nonlinearities or coherent superradiant emission. We demonstrate here that this coupling regime can only be reached with dimers of gold nanoparticles in stringent experimental conditions, when the interparticle spacing falls below 2 nm. Using a short transverse DNA double-strand, we introduce five dye molecules in the gap between two 40 nm gold particles and actively decrease its length down to sub-2 nm values by screening electrostatic repulsion between the particles at high ionic strengths. Single-nanostructure scattering spectroscopy then evidence the observation of a strong-coupling regime in excellent agreement with electrodynamic simulations. Furthermore, we highlight the influence of the planar facets of polycrystalline gold nanoparticles on the probability of observing strongly coupled hybrid nanostructures.

Heterodimers of metal nanoparticles: synthesis, properties, and biological applications

Heterodimers of metal nanoparticles consist of two metals, come in many sizes and adopt various shapes. They offer unique properties due to the presence of two metals and have the extraordinary flexibility needed to serve as a multipurpose platform for diverse applications in areas including photonics, sensing, and catalysis. Heterodimer nanoparticles contain different metals that contribute to extraordinary surface plasmon resonance (SPR), surface-enhanced Raman scattering (SERS), and catalytic properties. These properties make them versatile molecules that can be used in intracellular imaging, as antibacterial agents, as photocatalytic and biological macromolecules and for the detection of chemical substances. Moreover, heterodimer nanoparticles are composed of the two metals within larger molecules that provide more choices for modification and application. In this review, we briefly summarize the lesser-known aspects of heterodimers, including some of their properties, and present concrete examples of recent progress in synthesis and applications. This review provides a perspective on achievements and suggests a framework for future research with a focus on the synthesis and application of heterodimers. We also explore the possible applications of heterodimer nanoparticles based on their unique properties.

Plasmon Couplings from Subsystem Time-Dependent Density Functional Theory

Many applications in plasmonics are related to the coupling between metallic nanoparticles (MNPs) or between an emitter and a MNP. The theoretical analysis of such a coupling is thus of fundamental importance to analyze the plasmonic behavior and to design new systems. While classical methods neglect quantum and spill-out effects, time-dependent density functional theory (TD-DFT) considers all of them and with Kohn-Sham orbitals delocalized over the whole system. Thus, within TD-DFT, no definite separation of the subsystems (the single MNP or the emitter) and their couplings is directly available. This important feature is obtained here using the subsystem formulation of TD-DFT, which has been originally developed in the context of weakly interacting organic molecules. In subsystem TD-DFT, interacting MNPs are treated independently, thus allowing us to compute the plasmon couplings directly from the subsystem TD-DFT transition densities. We show that subsystem TD-DFT, as well as a simplified version of it in which kinetic contributions are neglected, can reproduce the reference TD-DFT calculations for gap distances greater than about 6 Å or even smaller in the case of hybrid plasmonic systems (i.e., molecules interacting with MNPs). We also show that the subsystem TD-DFT can be also used as a tool to analyze the impact of charge-transfer effects.

Impact of Nonlocality on Group Delay and Reflective Behavior Near Surface Plasmon Resonances in Otto Structure

In this work, we study the effects of nonlocality on the optical response near surface plasmon resonance of the Otto structure, and such nonlocality is considered in the hydrodynamic model. Through analyzing the dispersion relations and optical response predicted by the Drude’s and hydrodynamic model in the system, we find that the nonlocal effect is sensitive to the large propagation wavevector, and there exists a critical incident angle and thickness. The critical point moves to the smaller value when the nonlocal effect is taken into account. Before this point, the absorption of the reflected light pulse enhances; however, the situation reverses after this point. In the region between the two different critical points in the frequency scan calculated from local and nonlocal theories, the group delay of the reflected light pulse shows opposite behaviors. These results are explained in terms of the pole and zero phenomenological model in complex frequency plane. Our work may contribute to the fundamental understanding of light–matter interactions at the nanoscale and in the design of optical devices.

Protection of lead-induced cytotoxicity using paramagnetic nickel–insulin quantum clusters

Pb-toxicity is associated with inflammation which leads to delay in wound healing. Pb²⁺ utilizes calcium ion channels to enter the cell. Therefore, to achieve effective healing in a Pb-poisoned system, capturing Pb²⁺ from the circulatory system would be an effective approach without hampering the activity of the calcium ion channel. In this work insulin–nickel fluorescent quantum clusters (INiQCs) have been synthesized and used for the specific detection of Pb²⁺ ions in vitro and in cell-free systems. INiQCs (0.09 μM) can detect Pb²⁺ concentrations as low as 10 pM effectively in a cell-free system using the fluorescence turn-off method. In vitro INiQCs (0.45 μM) can detect Pb²⁺ concentrations as low as 1 μM. INiQCs also promote wound healing which can easily be monitored using the bright fluorescence of INiQCs. INiQCs also help to overcome the wound recovery inhibitory effect of Pb²⁺in vitro using lead nitrate. This work helps to generate effective biocompatible therapeutics for wound recovery in Pb²⁺ poisoned individuals.

Receptor targeted ferromagnetic Insulin–Nickel Quantum fluorescence Clusters (INiQCs) can specifically detect Pb²⁺ and prevents Pb²⁺ poisoning.

Biomacromolecular-Assembled Nanoclusters: Key Aspects for Robust Colloidal SERS Sensing

Superstructures of gold nanospheres offer augmented surface-enhanced Raman scattering (SERS) activities beyond the limits of their individual building blocks. However, for application as reliable and quantitative colloidal SERS probes, some key aspects need to be considered to combine efficiency and robustness with respect to hotspot excitation, analyte adsorption, signal stability, and colloidal stability. For this purpose, we studied core/satellite superstructures with spherical cores as a simple optically isotropic model system. Superstructures of different core sizes were assembled using bovine serum albumin (BSA), which serves as a non-specific biomacromolecular linker and provides electrosteric stabilization. We show that the "noisy" spectral footprint of the protein coating may serve as an internal standard, which allows accurate monitoring of the adsorption kinetics of analytes. The SERS activity was quantified using 4-mercaptobenzoic acid (MBA) as an aromatic low-molecular-weight model analyte. The molar SERS efficiency was studied by variation of the particle (Au⁰) and analyte concentrations with a limit of detection of 10^-7 M MBA. The practical importance of colloidal stability for robust measurement conditions was demonstrated by comparing the superstructures with their citrate-stabilized or protein-coated building blocks. We explain the theoretical background of hotspot formation by a leader/follower relationship of asymmetric control between the core and the satellites and give practical guidelines for robust colloidal SERS sensing probes.

Gold Nanoparticle Self-Aggregation on Surface with 1,6-Hexanedithiol Functionalization

Here we study the morphology and the optical properties of assemblies made of small (17 nm) gold nanoparticles (AuNPs) directly on silicon wafers coated with (3-aminopropyl)trimethoxysilane (APTES). We employed aliphatic 1,6-hexanedithiol (HDT) molecules to cross-link AuNPs during a two-stage precipitation procedure. The first immersion of the wafer in AuNP colloidal solution led mainly to the attachment of single particles with few inclusions of dimers and small aggregates. After the functionalization of precipitated NPs with HDT and after the second immersion in the colloidal solution of AuNP, we detected a sharp rise in the number of aggregates compared to single AuNPs and their dimers. The lateral size of the aggregates was about 100 nm, while some of them were larger than 1μm. We propose that the uncompensated dipole moment of the small aggregates appeared after the first precipitation and acts further as the driving force accelerating their further growth on the surface during the second precipitation. By having such inhomogeneous surface coating, the X-ray reciprocal space maps and modulation polarimetry showed well-distinguished signals from the single AuNPs and their dimers. From these observations, we concluded that the contribution from aggregated AuNPs does not hamper the detection and investigation of plasmonic effects for AuNP dimers. Meantime, using unpolarized and polarized light spectroscopy, the difference in the optical signals between the dimers, being formed because of self-aggregation and the one being cross-linked by means of HDT, was not detected.

The IR plasmonic properties of sub-wavelength ITO rod arrays predicted by anisotropic effective medium theory

Simple three-layer Fresnel equations combined with Maxwell-Garnett approximation were applied to study the IR plasmonic properties of indium-tin-oxide (ITO) nanorods. By treating the anisotropic nanorod layer as a layer with an effective dielectric constant, and using anisotropic effective medium theory, we were able to accurately predict the surface plasmon resonance behavior of ITO nanorods with different nanorod length, spacing, and tilt angle. This model allows a fast and computationally inexpensive calculation to predict the plasmonic properties of arrayed nanorods.

Optical Coulomb blockade lifting in plasmonic nanoparticle dimers

If two metal nanoparticles are ultimately approached, a tunneling current prevents both an infinite redshift of the bonding dipolar plasmon and an infinite increase of the electric field in the hot spot between the nanoparticles. We argue that a Coulomb blockade suppresses the tunneling current and sustains a redshift even for sub-nanometer approach up to moderate fields. Only for stronger optical fields, the Coulomb blockade is lifted and a charge transfer plasmon is formed. Numerical simulations show that such scenarios are well in reach with manageable nanoparticle dimensions, even at room temperature. Applications may include ultrafast, optically driven switches, photo detectors operating at 500 THz, or highly nonlinear devices.

Insulin-copper quantum clusters preparation and receptor targeted bioimaging

Protein embedded fluorescence quantum clusters (QCs) have received a great amount of interest among the researchers because of their high aqueous solubility, stability, cost efficiency, and target specificity. Considerable advancement has happened in making functional quantum clusters with target specificity. This work reports the simple synthesis of insulin protected copper quantum clusters (ICuQCs) and its receptor-targeted bioimaging applications. The preparation of copper quantum clusters (CuQCs) was done simply by one-pot synthesis method by changing the pH of the insulin protein firstly to 10.5 basic pH than physiological pH. At physiological pH, the mixture incubated in oven 37 ⁰C at 240 rpm has been developed to process initially polydisperse, non-fluorescent, and unstable CuDs into monodispersed (∼2-3 nm), highly fluorescent, and extremely stable ICuQCs in the same phase (aqueous) using insulin as protein. HRTEM image show uniform distribution of CuDs within the protein matrix. Metal ion binding site prediction and docking server (MIB) results show that chain B of insulin contains 3 templates contains 5 amino acid residues which bind with Cu²⁺ metal ion. Groove 1 contains GLY8 and HIS10 bind has the highest binding potential towards Cu metal ions. The methodology adopted in this study should largely contribute to the practical applications of this new class of QCs. In view of the protein protection, coupled with direct synthesis and easy functionalization, this hybrid QC-protein system is expected to have numerous optical and bioimaging applications in the future.

Tailor-engineered plasmonic single-lattices: harnessing localized surface plasmon resonances for visible-NIR light-enhanced photocatalysis

A platform material composed of 2D gold (Au) nanodot plasmonic single-lattices (Au-nD-PSLs) featuring tailor-engineered geometric features for visible-NIR light-driven enhanced photocatalysis is presented.

Self-assembly of spherical and rod-shaped nanoparticles with full positional control

The controlled positioning of spherical gold nanoparticles and gold nanorods upon self-assembly on a substrate is of great interest for the fabrication of tailored plasmonic devices. Here, an electrostatic approach with a sequential two-step assembly protocol is presented as a cost-effective and high-yield alternative to previously presented, more complex proof of concepts. Three different geometries can be separately produced in large quantities relying on electrostatic attraction and repulsion of the charge-carrying building blocks: a single gold nanoparticle at the tip, the side or on top of a gold nanorod. DLVO theory is used to explain the electrostatic assembly strategy. The process is highly efficient and assembly yields between 79% (at the tip) and 94% (for the nanoparticle at the long side of the nanorod) are achieved.

Manipulating the confinement of electromagnetic field in size-specific gold nanoparticles dimers and trimers

The intriguing light-matter interactions can be governed by controlling the particle size and shape, electromagnetic interactions and dielectric properties and local environment of the metal nanostructures. Amongst the different approaches that have been engendered to manipulate light at the nanoscale, the self-assembly of metallic nanostructures with controllable interparticle distances and angular orientations, which strongly impact their optical attributes, is one of the viable avenues to exploit their utility in a diverse range of niche applications. The simplest geometrical architectures that enable such modulations are dimers with changeable interparticle distances and trimers with an additional degree of angular orientation to correlate the plasmonic observables with the observed spectral characteristics. Wet chemical approaches have been adopted in this study for the synthesis of size-selective gold nanoparticles, and appropriate organic linkers have judiciously been employed to induce plasmonic interactions amongst the gold nanoparticles in close proximity to each other. The combination of experimental observations and electromagnetic simulations adopted to probe the plasmonic interactions revealed that the electrodynamic coupling effect was very sensitive to particle size, interparticle distances and angular orientations in these simple nanoassemblies. The capability to precisely manipulate the electric field at the junctions between these plasmon-coupled nanoparticles could pave the way for the application of these nanoassemblies in surface-enhanced spectroscopies and sensing applications.

Ag Ion Soldering: An Emerging Tool for Sub-nanomeric Plasmon Coupling and Beyond

Self-assembly represents probably the most flexible way to construct metastructured materials and devices from a wealth of colloidal building blocks with synthetically controllable sizes, shapes, and elemental compositions. In principle, surface capping is unavoidable during the synthesis of nanomaterials with well-defined geometry and stability. The ligand layer also endows inorganic building blocks with molecular recognition ability responsible for their assembly into desired structures. In the case of plasmonic nanounits, precise positioning of them in a nanomolecule or an ordered nanoarray provides a chance to shape their electrodynamic behaviors and thereby assists experimental demonstration of modern nanoplasmonics toward practical uses. Despite previous achievements in bottom-up nanofabrication, a big challenge exists toward strong coupling and facile charge transfer between adjacent nanounits in an assembly. This difficulty has impeded a functional development of plasmonic nanoassemblies. The weakened interparticle coupling originates from the electrostatic and steric barriers of ionic/molecular adsorbates to guarantee a good colloidal stability. Such a dilemma is rooted in fundamental colloidal science, which lacks an effective solution. During the past several years, a chemical tool termed Ag ion soldering (AIS) has been developed to overcome the above situation toward functional colloidal nanotechnology. In particular, a dimeric assembly of plasmonic nanoparticles has been taken as an ideal model to study plasmonic coupling and interparticle charge transfer. This Account starts with a demonstration of the chemical mechanism of AIS, followed by a verification of its workability in various self-assembly systems. A further use of AIS to realize postsynthetic coupling of DNA-directed nanoparticle clusters evidences its compatibility with DNA nanotechnology. Benefiting from the sub-nanometer interparticle gap achieved by AIS, a conductive pathway is established between two nanoparticles in an assembly. Accordingly, light-driven charge transfer between the conductively bridged plasmonic units is realized with highly tunable resonance frequencies. These situations have been demonstrated by thermal/photothermal sintering of silica-isolated nanoparticle dimers as well as gap-specific electroless gold/silver deposition. The regioselective silver deposition is then combined with galvanic replacement to obtain catalytically active nanofoci (plasmonic nanogaps). The resulting structures are useful for real time and on-site Raman spectroscopic tracking of chemical reactions in the plasmonic hotspots (nanogaps) as well as for study of plasmon-mediated/field-enhanced catalysis. The Account is concluded by a deeper insight into the chemical mechanism of AIS and its adaption to conformation-rich structures. Finally, AIS-enabled functional pursuits are suggested for self-assembled materials with strongly coupled and easily reshapable physicochemical properties.

Effect of Nanogap Morphology on Plasmon Coupling

Plasmon coupling is the fundamental principle by which the optical resonances in nanoparticle assemblies are tuned. Interactions of plasmons among nanoparticles in close proximity create plasmon coupling modes whose energies are sensitive to the nanogap parameters. Whereas many studies have focused on the gap distances, we herein probe the effect of gap morphology on plasmon coupling. Dimers that are prepared by adsorbing perfectly round ultrauniform Au nanospheres (AuNSs) onto the faces, edges, and vertices of Au nanocubes (AuNCs) present distinctly different nanogap morphologies. Dark-field single-particle scattering spectroscopy reveals that the longitudinal plasmon coupling mode shifts to lower energies as the AuNS forms a nanogap with parts of the AuNC with higher curvature. Simulation spectra are also consistent with this observation. Our calculations indicate that the much larger charge density at the vertex or edge of a AuNC lowers the plasmon coupling energy through the contribution of the Coulomb interaction when the AuNC combines with the AuNS. In comparison, the plasmon energies or anisotropic polarizability along the face, edge, and vertex directions of a AuNC differ only slightly and thus do not cause a shift in the plasmon coupling mode.