Collective multipole oscillations direct the plasmonic coupling at the nanojunction interfaces

Polymer/AgPt bimetallic nanoparticle synergy: optimizing plasmonic durability through controlled synthesis and matrix integration

An innovative approach combining UV-induced polymerization and ultra-high vacuum (UHV) atomic vapor deposition was developed to synthesize and disperse 2-3 nm AgPt bimetallic nanoparticles (BNPs) within a non-porous poly (dipropylene glycol diacrylate) (PDGDA) matrix, surpassing conventional porous polymer strategies. This method offers unprecedented control over the structural properties of BNPs and in general nanoalloys, with the polymer matrix playing a critical role in regulating nanoparticle formation, spatial arrangement, and size uniformity. The PDGDA matrix enhances nanoparticle stability through steric stabilization and controlled diffusion, effectively maintaining small nanoparticle sizes (∼2.4-2.8 nm) and low dispersity (σ D/D = 0.16) during extended high-temperature annealing. Confinement of nanoparticles (NPs) was significantly accelerated by successive thermal annealing to 320 °C, which increased polymer chain mobility and reduced viscosity, enabling rapid diffusion while preserving the structural integrity of the polymer matrix. This process dramatically reduced the embedding time from 12 days at room temperature to near-instantaneous incorporation upon heating. Successful confinement is attributed to key thermodynamic factors that promote interfacial interactions and particle mobility within the polymer network. Experimental results reveal distinctive UV plasmonic properties of the embedded AgPt BNPs with long-term stability. The produced AgPt BNPs exhibit significantly stronger localized surface plasmon resonances (LSPRs) than pure platinum nanoparticles, attributed to synergistic effects between the two metals. Factors contributing to this enhancement include silver's high electrical conductivity and relatively low optical losses, electromagnetic coupling, and localized electric field enhancement, highlighting the potential of these BNPs for advanced plasmonics. This research addresses the growing demand for surface-enhanced Raman scattering (SERS) detection of UV-absorbing biospecies and the development of more efficient broad-spectrum solar cells.

Air sampling and simultaneous detection of airborne influenza virus via gold nanorod-based plasmonic PCR

Reliable and sensitive virus detection is essential to prevent airborne virus transmission. The polymerase chain reaction (PCR) is one of the most compelling and effective diagnostic techniques for detecting airborne pathogens. However, most PCR diagnostics rely on thermocycling, which involves a time-consuming Peltier block heating methodology. Plasmonic PCR is based on light-driven photothermal heating of plasmonic nanostructures to address the key drawbacks of traditional PCR. This study introduces a methodology for plasmonic PCR detection of air-sampled influenza virus (H1N1). An electrostatic air sampler was used to collect the aerosolized virus in a carrier liquid for 10 min. Simultaneously, the viruses collected in the liquid were transferred to a tube containing gold (Au) nanorods (aspect ratio = 3.6). H1N1 viruses were detected in 12 min, which is the total time required for reverse transcription, fast thermocycling via plasmonic heating through gold nanorods, and in situ fluorescence detection. This methodology showed a limit of detection of three RNA copies/μL liquid for H1N1 influenza virus, which is comparable to that of commercially available PCR devices. This methodology can be used for the rapid and precise identification of pathogens on-site, while significantly reducing the time required for monitoring airborne viruses.

Finite element modeling of plasmonic resonances in photothermal gold nanoparticles embedded in cells

The use of plasmonic nanoparticles in performing photothermal treatments in cancer cells requires a full knowledge about their optical properties. The surface plasmon resonance is easily foreseen and measurable in colloidal suspensions, however it can be strongly modified when located inside cells. Assessing the optical behavior of plasmonic nanoparticles in cells is essential for an efficient and controlled treatment. This requires the combination of experimental data and computational models to understand the mechanisms that cause the change in their optical response. In this work, we investigate the plasmonic response of Au nanospheres (AuNSs) internalized into cancer cells (MCF-7). Experimental data are compared to the simulations provided by a 3D model based on a finite element method. We demonstrate the impact of physical parameters such as the type of NS assembly, the surrounding medium and the interparticle gap, in the photothermal efficiency of AuNSs. Results open the avenue to predict, by numerical calculations, the optical properties of plasmonic nanoparticles inside cells to minimize treatment costs and times in photothermal therapies.

Realization of ultrastrong coupling between LSPR and Fabry-Pérot mode via self-assembly of Au-NPs on p-NiO/Au film

The realization of higher coupling strengths between coupled resonant modes enables exploration of compelling phenomena in diverse fields of physics and chemistry. In this study, we focus on the modal coupling between localized surface plasmon resonance (LSPR) of Au nanoparticles (Au-NPs) and Fabry-Pérot mode (p-NiO/Au film). The effects of nanoparticle size, projected surface coverage (PSC), interparticle distance (IPD), and arrangement to the coupling strength between the two modes are theoretically investigated using finite-difference time-domain (FDTD) method. Au-NPs/p-NiO/Au film (ANA) nanostructures with NPs size of 10 nm, 30 nm, and 50 nm are considered. Numerical calculations point to larger size and higher projected surface coverage (also smaller IPD) of NPs as pre-eminent factors in enhancing the strength of modal coupling. ANA nanostructure with NPs size of 30 nm (ANA-30) and 50 nm (ANA-50) are experimentally fabricated via a facile air-liquid interface self-assembly. The fabricated nanodevices exhibit immense Rabi splitting energies of 655 meV (ANA-30) and 770 meV (ANA-50), and thus fulfill the ultrastrong coupling condition with remarkable splitting energy to bare (plasmon) energy ratio of 0.35 (ANA-30) and 0.4 (ANA-50). The physical insights presented in this study, together with the simple and scalable fabrication process, establish a viable approach to realize stronger coupling between LSPR and Fabry-Pérot mode in metal NPs/dielectric/metal film systems. This will be vital to take advantage of the promising performance enhancements of plasmonic-based nanostructures under strongly coupled regimes in areas such as solar to fuel conversion, sensing, opto-electronics, and quantum applications.

A calculation method for optical properties of yolk shell based on deep learning

The yolk shell is widely used in optoelectronic devices due to its excellent optical properties. Compared to single metal nanostructures, yolk shells have more controllable degrees of freedom, which may make experiments and simulations more complex. Using neural networks can efficiently simplify the computational process of yolk shell. In our work, the relationship between the size and the absorption efficiency of the yolk-shell structure is established using a backpropagation neural network (BPNN), significantly simplifying the calculation process while ensuring accuracy equivalent to discrete dipole scattering (DDSCAT). The absorption efficiency of the yolk shell was comprehensively described through the forward and reverse prediction processes. In forward prediction, the absorption spectrum of yolk shell is obtained through its size parameter. In reverse prediction, the size parameters of yolk shells are predicted through absorption spectra. A comparison with the traditional DDSCAT demonstrated the high precision prediction capability and fast computation of this method, with minimal memory consumption.

Metal-Organic Framework-Enabled Trapping of Volatile Organic Compounds into Plasmonic Nanogaps for Surface-Enhanced Raman Scattering Detection

Utilizing electromagnetic hotspots within plasmonic nanogaps is a promising approach to create ultrasensitive surface-enhanced Raman scattering (SERS) substrates. However, it is difficult for many molecules to get positioned in such nanogaps. Metal-organic frameworks (MOFs) are commonly used to absorb and concentrate diverse molecules. Herein, we combine these two strategies by introducing MOFs into plasmon-coupled nanogaps, which has so far remained experimentally challenging. Ultrasensitive SERS substrates are fabricated through the construction of nanoparticle-on-mirror structures, where Au nanocrystals are encapsulated with a zeolitic imidazolate framework-8 (ZIF-8) shell and then coupled to a gold film. The ZIF-8 shell, as a spacer that separates the Au nanocrystal and the Au film, can be adjusted in thickness over a wide range, which allows the electric field enhancement and plasmon resonance wavelength to be varied. By trapping Raman-active molecules within the ZIF-8 shell, we show that our plasmon-coupled structures exhibit a superior SERS detection performance. A range of volatile organic compounds at the concentrations of 10-2 mg m-3 can be detected sensitively and reliably. Our study therefore offers an attractive route for synergistically combining plasmonic electric field enhancement and MOF-enabled molecular enrichment to design and create SERS substrates for ultrasensitive detection.

Colorimetric Pressure Sensing by Plasmonic Decoupling of Silver Nanoparticles Confined within Polymeric Nanoshells

Employing a plasmonic decoupling mechanism, we report the design of a colorimetric pressure sensor that can respond to applied pressure with instant color changes. The sensor consists of a thin film of stacked uniform resorcinol-formaldehyde nanoshells with their inner surfaces functionalized with silver nanoparticles. Upon compression, the flexible polymer nanoshells expand laterally, inducing plasmonic decoupling between neighboring silver nanoparticles and a subsequent blue-shift. The initial color of the sensor is determined by the extent of plasmonic coupling, which can be controlled by tuning the interparticle distance through a seeded growth process. The sensing range can be conveniently customized by controlling the polymer shell thickness or incorporating hybrid nanoshells into various polymer matrices. The new colorimetric pressure sensors are easy to fabricate and highly versatile, allow for convenient tuning of the sensing range, and feature significant color shifts, holding great promise for a wide range of practical applications.

Leveraging Coupling Effect-Enhanced Surface Plasmon Resonance of Ruthenium Nanocrystal-Decorated Mesoporous Silica Nanoparticles for Boosted Photothermal Immunotherapy

Photothermal immunotherapy (PTI) has emerged as a promising approach for cancer treatment, while its efficacy is often hindered by the immunosuppressive tumor microenvironment (TME). Here, this work presents a multifunctional platform for tumor PTI based on ruthenium nanocrystal-decorated mesoporous silica nanoparticles (RuNC-MSN). By precisely regulating the distance between RuNC on MSN, this work achieves a remarkable enhancement in surface plasmon resonance of RuNC, leading to a significant improvement in the photothermal efficiency of RuNC-MSN. Furthermore, the inherent catalase-like activity of RuNC-MSN enables effective modulation of the immunosuppressive TME, thereby facilitating an enhanced immune response triggered by the photothermal effect-mediated immunogenic cell death (ICD). As a result, RuNC-MSN exhibits superior PTI performance, resulting in pronounced inhibition of primary tumor and metastasis. This study highlights the rational design of PTI agents with coupling effect-enhanced surface plasmon resonance, enabling simultaneous induction of ICD and regulation of the immunosuppressive TME, thereby significantly boosting PTI efficacy.

Effect of plasmonic excitation on mature insulin amyloid fibrils

Interactions between amyloid protein structures and nanomaterials have been extensively studied to develop effective inhibitors of amyloid aggregation. Limited investigations are reported on the impact of nanoparticles on mature fibrils. In this work, gold nanoparticles are used as photothermal agents to alter insulin fibrils. To this end, gold colloids bearing a negatively charged capping shell, with an average diameter of 14 nm and a plasmon resonance maximum at 520 nm are synthesized. The effects on mature insulin fibril morphology and structure upon plasmonic excitation of the nanoparticles-fibril samples have been monitored by spectroscopic and microscopic methods. The obtained data indicate that an effective destruction of the amyloid aggregates occur upon irradiation of the plasmonic nanoparticles, allowing the development of emerging strategies to alter the structure of amyloid fibrils.

Anisotropic metallic heterotrimer systems for an ultrahigh plasmonic-based improvement of hyper-Raman scattering signal

An anisotropic metallic trimer is proposed as an active plasmonic substrate for an ultrahigh enhancement in the spectroscopic signal of the hyper-Raman scattering (HRS) process. The suggested three-particle system is composed from non-aligned asymmetric nanoparticles of a cubic shape. The interacting resonators are made of gold material and illuminated by a longitudinally polarized light. The non-alignment condition in the heterotrimer is achieved by shifting the intermediate cube transversely away from the interparticle axis. Optical cross-section, nearfield distribution and charge density are calculated by using the finite-difference time-domain electrodynamic simulation tool. The enhancement factor of the HRS is calculated theoretically from the nearfield intensity associated with the resonance phenomenon of the considered trimer. The extinction profile of the illuminated system exhibits the excitation of two plasmonic modes. A superradiant mode observed in the longer wavelength region which resulted from the in-phase coupling between the plasmonic modes excited in each one of the three resonators. The second mode is a subradiant band emerged from the interference between bright and dark modes. The resonance wavelength of these two modes matches the excitation one and the second-order Stockes condition, respectively. After optimizing the value of both the transverse shift and the gap spacing, the enhancement factor of the HRS can reach as high as a value never reported before of 1 × 10¹⁸.

Heterostructures of Cu2-xS/Cu2-xTe plasmonic semiconductors: disappearing and reappearing LSPR with anion exchange

Localized surface plasmon resonance (LSPR) of Cu_2-xS nanorods is quenched during the initial Cu_2-xS/Cu_2-xTe core/shell stage of anion exchange then returns as Cu_2-xTe progresses into the nanorod. Phase change within the core accounts for this behaviour illustrating the complexity emergent from anion exchange.

Colloidal silver-based lateral flow immunoassay for detection of profenofos pesticide residue in vegetables

A colloidal silver nanoparticle (AgNP)-based lateral flow immunoassay (LFIA) was evaluated in terms of the rapid detection of profenofos (PEO) pesticide residue in vegetables. Colloidal AgNPs, of a diameter of approximately 20 nm, were surface-modified with trisodium citrate dehydrate (TSC) in order to improve their stability and dispersion. An anti-profenofos polyclonal antibody (pAb) was successfully immobilized on the surface of the AgNPs by ionic interaction and characterized using UV-vis, SEM, TEM, FTIR and XPS analyses. Surface modification of Ag-pAb conjugates of varying pH, pAb content and cross-reactivity was employed to design and prepare labels for use in an LFIA to examine whether these factors affect the performance of the assay. The visible detection limit and optical detection limit of the PEO test strip were 0.20 and 0.01 ppm, respectively, in PEO standard solution. This assay showed no cross-reaction with omethoate, methamidophos or pyraclofos. Finally, the PEO test strip was effectively applied for the detection of PEO in liquid vegetables A and B, with optical detection limits of 0.09 and 0.075 ppm, respectively.

Microscopic Understanding of Reaction Rates Observed in Plasmon Chemistry of Nanoparticle-Ligand Systems

Surface-enhanced Raman scattering (SERS) is an effective and widely used technique to study chemical reactions induced or catalyzed by plasmonic substrates, since the experimental setup allows us to trigger and track the reaction simultaneously and identify the products. However, on substrates with plasmonic hotspots, the total signal mainly originates from these nanoscopic volumes with high reactivity and the information about the overall consumption remains obscure in SERS measurements. This has important implications; for example, the apparent reaction order in SERS measurements does not correlate with the real reaction order, whereas the apparent reaction rates are proportional to the real reaction rates as demonstrated by finite-difference time-domain (FDTD) simulations. We determined the electric field enhancement distribution of a gold nanoparticle (AuNP) monolayer and calculated the SERS intensities in light-driven reactions in an adsorbed self-assembled molecular monolayer on the AuNP surface. Accordingly, even if a high conversion is observed in SERS due to the high reactivity in the hotspots, most of the adsorbed molecules on the AuNP surface remain unreacted. The theoretical findings are compared with the hot-electron-induced dehalogenation of 4-bromothiophenol, indicating a time dependency of the hot-carrier concentration in plasmon-mediated reactions. To fit the kinetics of plasmon-mediated reactions in plasmonic hotspots, fractal-like kinetics are well suited to account for the inhomogeneity of reactive sites on the substrates, whereas also modified standard kinetics model allows equally well fits. The outcomes of this study are on the one hand essential to derive a mechanistic understanding of reactions on plasmonic substrates by SERS measurements and on the other hand to drive plasmonic reactions with high local precision and facilitate the engineering of chemistry on a nanoscale.

Design considerations of gold nanoantenna dimers for plasmomechanical transduction

Internal optical forces emerging from plasmonic interactions in gold nanodisc, nanocube and nanobar dimers were studied by the finite element method. A direct correlation between the electric-field enhancement and optical forces was found by observing the largest magnitude of optical forces in nanocube dimers. Moreover, further amplification of optical forces was achieved by employing optical power of the excitation source. The strength of optical forces was observed to be governed by the magnitude of polarisation density on the nanoparticles, which can be varied by modifying the nanoparticle geometry and source wavelength. This study allows us to recognise that nanoparticle geometry along with the inter-dimer distance are the most prominent design considerations for optimising optical forces in plasmonic dimers. The findings facilitate the realisation of all-optical modulation in a plasmomechanical nanopillar system, which has promising applications in ultra-sensitive nanomechanical sensing and building reconfigurable metamaterials.

How the Physicochemical Properties of Manufactured Nanomaterials Affect Their Performance in Dispersion and Their Applications in Biomedicine: A Review

The growth in novel synthesis methods and in the range of possible applications has led to the development of a large variety of manufactured nanomaterials (MNMs), which can, in principle, come into close contact with humans and be dispersed in the environment. The nanomaterials interact with the surrounding environment, this being either the proteins and/or cells in a biological medium or the matrix constituent in a dispersion or composite, and an interface is formed whose properties depend on the physicochemical interactions and on colloidal forces. The development of predictive relationships between the characteristics of individual MNMs and their potential practical use critically depends on how the key parameters of MNMs, such as the size, shape, surface chemistry, surface charge, surface coating, etc., affect the behavior in a test medium. This relationship between the biophysicochemical properties of the MNMs and their practical use is defined as their functionality; understanding this relationship is very important for the safe use of these nanomaterials. In this mini review, we attempt to identify the key parameters of nanomaterials and establish a relationship between these and the main MNM functionalities, which would play an important role in the safe design of MNMs; thus, reducing the possible health and environmental risks early on in the innovation process, when the functionality of a nanomaterial and its toxicity/safety will be taken into account in an integrated way. This review aims to contribute to a decision tree strategy for the optimum design of safe nanomaterials, by going beyond the compromise between functionality and safety.

Plasmon resonance and enhanced near-field of anisotropic nanoparticle systems: unified analysis by factorization of light-excited dipole distribution

We develop a simple factorization scheme to analyze the mechanism of dipole-plasmon resonance, which is controlled by the particle shape or the gap distance of neighboring particles. The method focuses on extracting the motion of local induced dipoles based on the discrete dipole approximation (DDA) and is applied to silver nanoparticles. Our analysis clarifies that the particle shape effect is characterized quantitatively by the oscillation of a small number of collective dipoles when the inhomogeneity of the distribution of induced dipoles is weak. Our factorization scheme is also applicable to a system consisting of neighboring nanoparticles and explains the relationship between the gap distance of neighboring nanoparticles and near-field enhancement. Our theoretical approach is useful for understanding the optical response of anisotropic- and multi-nanoparticle systems in a unified manner, and it provides a convenient view for the design of optical materials of nanoparticles.

Plasmonic Coupling Architectures for Enhanced Photocatalysis

Plasmonic photocatalysis is a promising approach for solar energy transformation. Comparing with isolated metal nanoparticles, the plasmonic coupling architectures can provide further strengthened local electromagnetic field and boosted light-harvesting capability through optimal control over the composition, spacing, and orientation of individual nanocomponents. As such, when integrated with semiconductor photocatalysts, the coupled metal nanostructures can dramatically promote exciton generation and separation through plasmonic-coupling-driven charge/energy transfer toward superior photocatalytic efficiencies. Herein, the principles of the plasmonic coupling effect are presented and recent progress on the construction of plasmonic coupling architectures and their integration with semiconductors for enhanced photocatalytic reactions is summarized. In addition, the remaining challenges as to the rational design and utilization of plasmon coupling structures are elaborated, and some prospects to inspire new opportunities on the future development of plasmonic coupling structures for efficient and sustainable light-driven reactions are raised.

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.

Self-Assembled Ag Nanocomposites into Ultra-Sensitive and Reproducible Large-Area SERS-Active Opaque Substrates

This work describes a novel, one-shot strategy to fabricate ultrasensitive SERS sensors based on silver/poly(methyl methacrylate) (PMMA) nanocomposites. Upon spin coating of a dispersion of PMMA and silver precursor on N-doped silicon substrate, closely separated silver nanoparticles were self-assembled into uniform nanospheres. As a result, a thin hydrophobic PMMA layer embedded with Ag nanoparticles (AgNPs) was obtained on the whole silicon substrate. Consequently, a large-scale, reproducible SERS platform was produced through a rapid, simple, low-cost, and high-throughput technology. In addition, reproducible SERS features and high SERS enhancement factors were determined (SEF ~10¹⁵). This finding matches the highest SEF reported in literature to date (10¹⁴) for silver aggregates. The potential and novelty of this synthesis is that no reducing agent or copolymer was used, nor was any preliminary functionalization of the surface carried out. In addition, the AgNPs were fabricated directly on the substrate’s surface; consequently, there was no need for polymer etching. Then, the synthetic method was successfully applied to prepare opaque SERS platforms. Opaque surfaces are needed in photonic devices because of the absence of secondary back reflection, which makes optical analysis and applications easier.

Dual-Mode Plasmonic Coupling-Enhanced Color Conversion of Inorganic CsPbBr3 Perovskite Quantum Dot Films

Plasmonic coupling has been demonstrated to be an effective manipulation strategy for emission enhancement in low-dimensional semiconductor materials. Here, dual-mode plasmonic resonances based on a metal dimer structure were proposed to simultaneously enhance the absorption under short-wavelength excitation and excitons' emission at longer wavelengths for CsPbBr₃ perovskite quantum dots (QDs). Large-area metal nanodimer arrays with well-controlled local surface plasmon resonance were facilely fabricated by a simple method combined with metal angular deposition and nanosphere lithography. With the addition of an optimized polymethyl methacrylate spacer, the effective plasmonic coupling and interfacial passivation of QDs were successfully achieved in the hybrid system. As a result, the QD films exhibited a significant and approximately 3.95-fold overall fluorescence enhancement when using blue light excitation, showing the novel advantages of dual-mode plasmonic coupling of semiconductor quantum structures for color conversion applications.

Localized Plasmonic Photothermal Therapy as a Life-saving Treatment Paradigm for Hospitalized COVID-19 Patients

Lung failure is the main reason for mortality in COVID-19 patients, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). To date, no drug has been clinically approved for treatment of COVID-19. Nanotechnology has a great potential in contributing significantly to the fight against COVID-19 by developing effective therapies that can selectively eradicate the respiratory virus load. We propose a novel COVID-19 management approach that is efficient in eliminating the virus load from the airways and protecting the lungs from the fatal effects of the virus. This approach relies on targeting the virus using ACE-2-functionalized gold nanorods (AuNRs) followed by irradiation with near-infrared (NIR) light for the selective eradication of SARS-CoV-2 without off-target effects, i.e., targeted plasmonic photothermal therapy. Using discrete dipole approximation (DDA), we quantitatively determined the efficiency of AuNRs (31 nm × 8 nm) in absorbing NIR when present at different orientations relative to one another on the surface of the virus. The safety and the local administration of AuNRs using a well-tolerated flexible bronchoscopy technique, commonly used for hospitalized COVID-19 patients, ensure feasibility and clinical translation. While requiring further research, we anticipate this approach to result in a first-line treatment for hospitalized COVID-19 patients that are experiencing severe respiratory conditions or belong to a high-risk population, e.g., seniors and diabetic patients.

Plasmonic Titanium Nitride Facilitates Indium Oxide CO2 Photocatalysis

Nanoscale titanium nitride TiN is a metallic material that can effectively harvest sunlight over a broad spectral range and produce high local temperatures via the photothermal effect. Nanoscale indium oxide-hydroxide, In₂ O_{3- x} (OH)_y , is a semiconducting material capable of photocatalyzing the hydrogenation of gaseous CO₂ ; however, its wide electronic bandgap limits its absorption of photons to the ultraviolet region of the solar spectrum. Herein, the benefits of both nanomaterials in a ternary heterostructure: TiN@TiO₂ @In₂ O_{3- x} (OH)_y are combined. This heterostructured material synergistically couples the metallic TiN and semiconducting In₂ O_{3- x} (OH)_y phases via an interfacial semiconducting TiO₂ layer, allowing it to drive the light-assisted reverse water gas shift reaction at a conversion rate greatly surpassing that of its individual components or any binary combinations thereof.

Cooperative nanoparticle self-assembly and photothermal heating in a flexible plasmonic metamaterial

We theoretically investigate equilibrium behaviors and photothermal effects of a flexible plasmonic metamaterial composed of aramid nanofibers and gold nanoparticles. The fiber matrix is considered as an external field to reconfigure a nanoparticle assembly. We find that the heating process tunes particle–particle and fiber–particle interactions, which alter adsorption of nanoparticles on fiber surfaces or clustering in pore spaces. Thus, it is possible to control the nanoparticle self-assembly by laser illumination. Gold nanoparticles strongly absorb radiations and efficiently dissipate absorbed energy into heat. By solving the heat transfer equation associated with an effective medium approximation, we calculate the spatial temperature rise. Remarkably, our theoretical results quantitatively agree with prior experiments. This indicates that we can ignore plasmonic coupling effects induced by particle clustering. Effects of the laser spot size and intensity on the photothermal heating are also discussed.

We theoretically investigate equilibrium behaviors and photothermal effects of a flexible plasmonic metamaterial composed of aramid nanofibers and gold nanoparticles.

Plasmene nanosheets as optical skin strain sensors

Skin-like optoelectronic sensors can have a wide range of technical applications ranging from wearable/implantable biodiagnostics, human-machine interfaces, and soft robotics to artificial intelligence. The previous focus has been on electrical signal transduction, whether resistive, capacitive, or piezoelectric. Here, we report on "optical skin" strain sensors based on elastomer-supported, highly ordered, and closely packed plasmonic nanocrystal arrays (plasmene). Using gold nanocubes (AuNCs) as a model system, we find that the types of polymeric ligands, interparticle spacing, and AuNC sizes play vital roles in strain-induced plasmonic responses. In particular, brush-forming polystyrene (PS) is a "good" ligand for forming elastic plasmenes which display strain-induced blue shift of high-energy plasmonic peaks with high reversibility upon strain release. Further experimental and simulation studies reveal the transition from isotropic uniform plasmon coupling at a non-strained state to anisotropic plasmon coupling at strained states, due to the AuNC alignment perpendicular to the straining direction. The two-term plasmonic ruler model may predict the primary high-energy peak location. Using the relative shift of the averaged high-energy peak to the coupling peak before straining, a plasmene nanosheet may be used as a strain sensor with the sensitivity depending on its internal structures, such as the constituent AuNC size or inter-particle spacing.

Preserving the shape of silver nanocubes under corrosive environment by covering their edges and corners with iridium

Silver nanocubes have found use in an array of applications but their performance has been plagued by the shape instability arising from the oxidation and dissolution of Ag atoms from the edges and corners. Here we demonstrate that the shape of Ag nanocubes can be well preserved by covering their edges and corners with a corrosion-resistant metal such as Ir. In a typical process, we titrate a Na3IrCl6 solution in ethylene glycol (EG) into a suspension of Ag nanocubes in an EG solution in the presence of poly(vinylpyrrolidone) (PVP) held at 110 °C. The Ir atoms derived from the reduction of Na3IrCl6 by EG and Ag are deposited onto the edges and then corners for the generation of Ag-Ir core-frame nanocubes. Remarkably, our results indicate that a small amount of Ir atoms on the edges and corners is adequate to prevent the Ag nanocubes from transforming into nanospheres when heated in a PVP/EG solution up to 110 °C. We further demonstrate that these Ag-Ir nanocubes embrace plasmonic properties comparable to those of the original Ag nanocubes, making them immediately useful in a variety of applications. This strategy for stabilizing the shape of Ag nanocubes should be extendible to Ag nanocrystals with other shapes or nanocrystals comprised of other metals.

Silver nanoparticles: Synthesis, investigation techniques, and properties

The unique silver properties, especially in the form of nanoparticles (NPs), allow to utilize them in numerous applications. For instance, Ag NPs can be utilized for the production of electronic and solar energy harvesting devices, in advanced analytical techniques (NALDI, SERS), catalysis and photocatalysis. Moreover, the Ag NPs can be useful in medicine for bioimaging, biosensing as well as in antibacterial and anticancer therapies. The Ag NPs utilization requires comprehensive knowledge about their features regarding the synthesis approaches as well as exploitation conditions. Unfortunately, a large number of scientific articles provide only restricted information according to the objects under investigation. Additionally, the results could be affected by artifacts introduced with exploited equipment, the utilized technique or sample preparation stages. However, it is rather difficult to get information about problems, which may occur during the studies. Thus, the review provides information about novel trends in the Ag NPs synthesis, among which the physical, chemical, and biological approaches can be found. Basic information about approaches for the control of critical parameters of NPs, i.e. size and shape, was also revealed. It was shown, that the reducing agent, stabilizer, the synthesis environment, including trace ions, have a direct impact on the Ag NPs properties. Further, the capabilities of modern analytical techniques for Ag NPs and nanocomposites investigations were shown, among other microscopic (optical, TEM, SEM, STEM, AFM), spectroscopic (UV-Vis, IR, Raman, NMR, electron spectroscopy, XRD), spectrometric (MALDI-TOF MS, SIMS, ICP-MS), and separation (CE, FFF, gel electrophoresis) techniques were described. The limitations and possible artifacts of the techniques were mentioned. A large number of presented techniques is a distinguishing feature, which makes the review different from others. Finally, the physicochemical and biological properties of Ag NPs were demonstrated. It was shown, that Ag NPs features are dependent on their basic parameters, such as size, shape, chemical composition, etc. At the end of the review, the modern theories of the Ag NPs toxic mechanism were shown in a way that has never been presented before. The review should be helpful for scientists in their own studies, as it can help to prepare experiments more carefully.

Synthesis and Multipole Plasmon Resonances of Spherical Aluminum Nanoparticles

In comparison to Au and Ag, the high plasma frequency of Al allows multipole plasmon resonances from the ultraviolet to visible (UV-vis) range to be achieved by its nanoparticles with much smaller sizes and even a spherical shape. Herein, we report the high-supersaturation growth of monodisperse spherical Al nanoparticles (Al NPs) from 84 to 200 nm and their distinctive size-dependent multipole plasmon resonance properties in the UV-vis range. Linear relationships between the particle diameter and resonance peak positions of the dipole, quadrupole, and octupole were observed experimentally and confirmed by finite-difference time-domain (FDTD) calculations. FDTD calculations further reveal the high scattering-to-extinction ratio of multipole modes for the particle diameters >100 nm. The extinction coefficients of spherical Al NPs with different diameters were also determined. The excellent matching between the experimental and simulated results in the present work not only offers a standard for the synthesis and characterization of high-quality Al NPs but also provides new insight into the multipole plasmonic properties of Al NPs for advanced optical and sensing applications.

Silver nanoparticle-base lateral flow immunoassay for rapid detection of Staphylococcal enterotoxin B in milk and honey

A silver nanoparticle (AgNP)-based sandwich-type lateral flow immunoassay (LFIA) was evaluated for rapid detection of Staphylococcal enterotoxin B (SEB) in milk and honey. The role of trisodium citrate dihydrate (TSC) in the formation of Ag/TSC nanoparticles was established using UV-Vis spectroscopy. The association of silver with TSC in Ag/TSC nanoparticles was studied by the decrease in the intensity of anodic peak potential at 0.47 V and shift to 0.30 V in cyclic voltammetry (CV). The morphological, compositional and interaction studies of the AgNPs conjugated with the anti-SEB polyclonal antibody (Ag-sAb) was established using transmission electron microscopy (TEM) and X-ray photo electron spectroscopy (XPS) measurements. The visible detection limit and optical detection limit of the SEB test strip were 0.5 and 0.125 ppm, respectively, in SEB standard solution. This assay showed no cross-reaction with Staphylococcal enterotoxin A, Staphylococcal enterotoxin C or Salmonella typhi. Finally, the SEB test strip was effectively applied for the detection of SEB in spiked liquid milk and viscous honey, with optical detection limits of 0.25 and 0.5 ppm, respectively.

Simulation guided design of silver nanostructures for plasmon-enhanced fluorescence, singlet oxygen generation and SERS applications

Plasmonic nanostructures such as gold and silver could alter the intrinsic properties of fluorophores, photosensitizers or Raman reporters in their close vicinity. In this study, we have conducted systematic simulations to provide insight for the design of silver nanostructures with appropriate geometrical features for metal-enhanced fluorescence (MEF), metal-enhanced singlet oxygen generation (ME-SOG) and surface-enhanced Raman scattering (SERS) applications. The size-dependent optical properties and electric field enhancement of single and dimeric nanocubes were simulated. The extinction spectra of silver nanocubes were analysed by the multipole expansion method. Results show that a suitable size of Ag nanocubes for MEF and ME-SOG can be selected based on their maximum light scattering yield, the excitation and emission wavelengths of a particular fluorophore/photosensitizer and their maximum spectral overlap. Simulations of the 'hot-spot' or gap distance between two silver nanocubes with different configurations (i.e., face-to-face, edge-to-edge and corner-to-corner) were also performed. A direct correlation was found between the size and enhanced electric field around the Ag nanocubes simulated under 15 common Raman laser wavelengths from the UV to near-infrared region. The maximum SERS enhancement factor can be achieved by selecting the silver nanocubes with the right orientation, suitable edge length and gap distance that give the highest electric field at a specific Raman laser wavelength. It was also found that the higher order of silver nanostructures, e.g., trimer and tetramer, can lead to better enhancement effects. These simulation results can serve as generic guidelines to rationally design metal-enhancement systems including MEF, ME-SOG and SERS for different application needs without cumbersome optimization and tedious trial-and-error experimentation.

The Response of UV/Blue Light and Ozone Sensing Using Ag-TiO2 Planar Nanocomposite Thin Film

We successfully fabricated a planar nanocomposite film that uses a composite of silver nanoparticles and titanium dioxide film (Ag-TiO₂) for ultraviolet (UV) and blue light detection and application in ozone gas sensor. Ultraviolet-visible spectra revealed that silver nanoparticles have a strong surface plasmon resonance (SPR) effect. A strong redshift of the plasmonic peak when the silver nanoparticles covered the TiO₂ thin film was observed. The value of conductivity change for the Ag-TiO₂ composite is 4–8 times greater than that of TiO₂ film under UV and blue light irradiation. The Ag-TiO₂ nanocomposite film successfully sensed 100 ppb ozone. The gas response of the composite film increased by roughly six and four times under UV and blue light irradiation, respectively. We demonstrated that a Ag-TiO₂ composite gas sensor can be used with visible light (blue). The planar composite significantly enhances photo catalysis. The composite films have practical application potential for wearable devices.