Unconventional Breathing Currents Far beyond the Quantum Tunneling Distances in Large-Gapped Nanoplasmonic Systems.

Localized surface plasmon resonance (LSPR) in plasmonic nanoparticles propels the field of plasmo-electronics, holding promise for transformative optoelectronic devices through efficient light-to-current conversion. Plasmonic excitations strongly influence the charge distribution within nanoparticles, giving rise to electromagnetic fields that can significantly impact the macroscopic charge flows within the nanoparticle housing material. In this study, we present evidence of ultralow, unconventional breathing currents resulting from dynamic irradiance interactions between widely separated nanoparticles, extending far beyond conventional electron (quantum) tunneling distances. We develop an electric analogue model and derive an empirical expression to elucidate the generation of these unconventional breathing currents in cascaded nanoplasmonic systems under irradiance modulation. This technique and theoretical model have significant potential for applications requiring a deeper understanding of current dynamics, particularly on large nanostructured surfaces relevant to photocatalysis, energy harvesting, sensing, imaging, and the development of future photonic devices.

Plasmonic effect of metal nanoparticles on the photocatalytic properties of TiO2/rGO composite.

A comparative study of the plasmon effect of Ag and Au nanoparticles on TiO2/rGO nanocomposite was carried out. The synthesis of Au and Ag nanoparticles was carried out by laser ablation. The morphology and structure of the nanocomposites were studied by EDA, HRTEM, XRD and Raman spectroscopy. It was shown that the absorption capacity of the nanocomposite material was increased in the visible range of the spectrum when Ag and Au nanoparticles were added to TiO2/rGO. This leads to an increase in their photocatalytic activity. The photocurrent generated by NC/Au 10-11films is in 3.8 times and NC/Ag 10-12is in 2 times higher compared to pure TiO2/rGO film. Similar results were obtained from experimental data on the dyes photodegradation. In the presence of plasmon nanoparticles a significant enhancement in the electrical properties of the TiO2/rGO nanocomposite was recorded. The charge carrier transfer resistance in nanocomposites was decreased by almost ∼7 times for NC/Au,10-11and ∼4 times for NC/Ag,10-12films compared to pure TiO2/rGO. In addition, for nanocomposites with Ag or Au nanoparticles, a decrease in the effective electron lifetime was observed. The data obtained allow us to conclude that plasmonic NPs have a synergistic effect in TiO2/rGO nanocomposites, which consists in modifying both their light-harvesting properties and charge-transport characteristics. The results obtained can be used for the design of materials with improved photocatalytic and optoelectronic characteristics.

Energy Transfer to Molecular Adsorbates by Transient Hot Electron Spillover.

Hot electron (HE) photocatalysis is one of the most intriguing fields of nanoscience, with a clear potential for technological impact. Despite much effort, the mechanisms of HE photocatalysis are not fully understood. Here we investigate a mechanism based on transient electron spillover on a molecule and subsequent energy release into vibrational modes. We use state-of-the-art real-time Time Dependent Density Functional Theory (rt-TDDFT), simulating the dynamics of a HE moving within linear chains of Ag or Au atoms, on which CO, N2, or H2O are adsorbed. We estimate the energy a HE can release into adsorbate vibrational modes and show that certain modes are selectively activated. The energy transfer strongly depends on the adsorbate, the metal, and the HE energy. Considering a cumulative effect from multiple HEs, we estimate this mechanism can transfer tenths of an eV to molecular vibrations and could play an important role in HE photocatalysis.

Plexcitonic Nanorattles as Highly Efficient SERS-Encoded Tags.

Plexcitonic nanoparticles exhibit strong light-matter interactions, mediated by localized surface plasmon resonances, and thereby promise potential applications in fields such as photonics, solar cells, and sensing, among others. Herein, these light-matter interactions are investigated by UV-visible and surface-enhanced Raman scattering (SERS) spectroscopies, supported by finite-difference time-domain (FDTD) calculations. Our results reveal the importance of combining plasmonic nanomaterials and J-aggregates with near-zero-refractive index. As plexcitonic nanostructures nanorattles are employed, based on J-aggregates of the cyanine dye 5,5,6,6-tetrachloro-1,1-diethyl-3,3-bis(4-sulfobutyl)benzimidazolocarbocyanine (TDBC) and plasmonic silver-coated gold nanorods, confined within mesoporous silica shells, which facilitate the adsorption of the J-aggregates onto the metallic nanorod surface, while providing high colloidal stability. Electromagnetic simulations show that the electromagnetic field is strongly confined inside the J-aggregate layer, at wavelengths near the upper plexcitonic mode, but it is damped toward the J-aggregate/water interface at the lower plexcitonic mode. This behavior is ascribed to the sharp variation of dielectric properties of the J-aggregate shell close to the plasmon resonance, which leads to a high opposite refractive index contrast between water and the TDBC shell, at the upper and the lower plexcitonic modes. This behavior is responsible for the high SERS efficiency of the plexcitonic nanorattles under both 633 nm and 532 nm laser illumination. SERS analysis showed a detection sensitivity down to the single-nanoparticle level and, therefore, an exceptionally high average SERS intensity per particle. These findings may open new opportunities for ultrasensitive biosensing and bioimaging, as superbright and highly stable optical labels based on the strong coupling effect.

Editorial for focus on manipulations of atomic and molecular layers and its applications in energy, environment sciences and optoelectronic devices.

This Focus aims at showcasing the significance of manipulating atomic and molecular layers for various applications. To this end, this Focus collects 15 original research papers featuring the applications of atomic layer deposition, chemical vapor deposition, wet chemistry, and some other methods for manipulations of atomic and molecular layers in lithium-ion batteries, supercapacitors, catalysis, field-effect transistors, optoelectronics, and others.

Sensitivity-enhanced nanoplasmonic biosensor using direct immobilization of two engineered nanobodies for SARS-CoV-2 spike receptor-binding domain detection.

Sensitive, rapid, and easy-to-implement biosensors are critical in responding to highly contagious and fast-spreading severe acute respiratory syndrome coronavirus (SARS-CoV-2) mutations, enabling early infection screening for appropriate isolation and treatment measures to prevent the spread of the virus. Based on the sensing principle of localized surface plasmon resonance (LSPR) and nanobody immunological techniques, an enhanced sensitivity nanoplasmonic biosensor was developed to quantify the SARS-CoV-2 spike receptor-binding domain (RBD) in serum within 30 min. The lowest concentration in the linear range can be detected down to 0.01 ng/mL by direct immobilization of two engineered nanobodies. Both the sensor fabrication process and immune strategy are facile and inexpensive, with the potential for large-scale application. The designed nanoplasmonic biosensor achieved excellent specificity and sensitivity for SARS-CoV-2 spike RBD, providing a potential option for accurate early screening of the novel coronavirus disease 2019 (COVID-19).

Nanoplasmonic biosensor for rapid detection of multiple viral variants in human serum.

As viruses constantly change due to mutation, variants are expected to emerge demanding development of sensors capable of detecting multiple variants using one single sensor platform. Herein, we report the integration of a synthetic binder against SARS-CoV-2 with a nanoplasmonic-based sensing technology, which enables the successful detection of spike proteins of Alpha, Beta and Gamma variants of SARS CoV-2. The recognition event is achieved by specific nanostructured molecularly imprinted polymers (nanoMIPs), developed against a region of the receptor binding domain (RBD) of the SARS CoV-2 spike protein. The transduction is based on the principle of localized surface plasmon resonance (LSPR) associated with silver nanostructures. The nanoMIPs-functionalised LSPR sensor allows for the detection of all 3 protein variants with a limit of detection of 9.71 fM, 7.32 fM and 8.81 pM using wavelength shifts respectively for Alpha, Beta and Gamma spike protein variants. This can be achieved within 30 min from the sample collection, both from blood and using nasal swab, thus making this sensor suitable for rapid detection of COVID-19. Additionally, the turnaround time for sensor development and validation can be completed in less than 8 weeks, making it suitable for addressing future pandemic needs without the requirement for biological binding agents, which is one of the bottlenecks to the supply chain in diagnostic devices.

Strong Coupling between Localized Surface Plasmons and Molecules by Coupled Cluster Theory.

Plasmonic nanocavities enable the confinement of molecules and electromagnetic fields within nanometric volumes. As a consequence, the molecules experience a remarkably strong interaction with the electromagnetic field to such an extent that the quantum states of the system become hybrids between light and matter: polaritons. Here, we present a nonperturbative method to simulate the emerging properties of such polaritons: it combines a high-level quantum chemical description of the molecule with a quantized description of the localized surface plasmons in the nanocavity. We apply the method to molecules of realistic complexity in a typical plasmonic nanocavity, featuring also a subnanometric asperity (picocavity). Our results disclose the effects of the mutual polarization and correlation of plasmons and molecular excitations, disregarded so far. They also quantify to what extent the molecular charge density can be manipulated by nanocavities and stand as benchmarks to guide the development of methods for molecular polaritonics.

Detection of amyloid fibrils in Parkinson's disease using plasmonic chirality.

Amyloid fibrils, which are closely associated with various neurodegenerative diseases, are the final products in many protein aggregation pathways. The identification of fibrils at low concentration is, therefore, pivotal in disease diagnosis and development of therapeutic strategies. We report a methodology for the specific identification of amyloid fibrils using chiroptical effects in plasmonic nanoparticles. The formation of amyloid fibrils based on α-synuclein was probed using gold nanorods, which showed no apparent interaction with monomeric proteins but effective adsorption onto fibril structures via noncovalent interactions. The amyloid structure drives a helical nanorod arrangement, resulting in intense optical activity at the surface plasmon resonance wavelengths. This sensing technique was successfully applied to human brain homogenates of patients affected by Parkinson's disease, wherein protein fibrils related to the disease were identified through chiral signals from Au nanorods in the visible and near IR, whereas healthy brain samples did not exhibit any meaningful optical activity. The technique was additionally extended to the specific detection of infectious amyloids formed by prion proteins, thereby confirming the wide potential of the technique. The intense chiral response driven by strong dipolar coupling in helical Au nanorod arrangements allowed us to detect amyloid fibrils down to nanomolar concentrations.

Randomly positioned gold nanoparticles as fluorescence enhancers in apta-immunosensor for malaria test.

A plasmon-enhanced fluorescence-based antibody-aptamer biosensor - consisting of gold nanoparticles randomly immobilized onto a glass substrate via electrostatic self-assembly - is described for specific detection of proteins in whole blood. Analyte recognition is realized through a sandwich scheme with a capture bioreceptor layer of antibodies - covalently immobilized onto the gold nanoparticle surface in upright orientation and close-packed configuration by photochemical immobilization technique (PIT) - and a top bioreceptor layer of fluorescently labelled aptamers. Such a sandwich configuration warrants not only extremely high specificity, but also an ideal fluorophore-nanostructure distance (approximately 10-15 nm) for achieving strong fluorescence amplification. For a specific application, we tested the biosensor performance in a case study for the detection of malaria-related marker Plasmodium falciparum lactate dehydrogenase (PfLDH). The proposed biosensor can specifically detect PfLDH in spiked whole blood down to 10 pM (0.3 ng/mL) without any sample pretreatment. The combination of simple and scalable fabrication, potentially high-throughput analysis, and excellent sensing performance provides a new approach to biosensing with significant advantages compared to conventional fluorescence immunoassays.

Plasmonic Circular Dichroism in Chiral Gold Nanowire Dimers.

We report a computational study at the time-dependent density functional theory (TDDFT) level of the chiro-optical spectra of chiral gold nanowires coupled in dimers. Our goal is to explore whether it is possible to overcome destructive interference in single nanowires that damp chiral response in these systems and to achieve intense plasmonic circular dichroism (CD) through a coupling between the nanostructures. We predict a huge enhancement of circular dichroism at the plasmon resonance when two chiral nanowires are intimately coupled in an achiral relative arrangement. Such an effect is even more pronounced when two chiral nanowires are coupled in a chiral relative arrangement. Individual component maps of rotator strength, partial contributions according to the magnetic dipole component, and induced densities allow us to fully rationalize these findings, thus opening the way to the field of plasmonic CD and its rational design.

Rapid and Digital Detection of Inflammatory Biomarkers Enabled by a Novel Portable Nanoplasmonic Imager.

New point-of-care diagnostic devices are urgently needed for rapid and accurate diagnosis, particularly in the management of life-threatening infections and sepsis, where immediate treatment is key. Sepsis is a critical condition caused by systemic response to infection, with chances of survival drastically decreasing every hour. A novel portable biosensor based on nanoparticle-enhanced digital plasmonic imaging is reported for rapid and sensitive detection of two sepsis-related inflammatory biomarkers, procalcitonin (PCT) and C-reactive protein (CRP) directly from blood serum. The device achieves outstanding limit of detection of 21.3 pg mL-1 for PCT and 36 pg mL-1 for CRP, and dynamic range of at least three orders of magnitude. The portable device is deployed at Vall d'Hebron University Hospital in Spain and tested with a wide range of patient samples with sepsis, noninfectious systemic inflammatory response syndrome (SIRS), and healthy subjects. The results are validated against ultimate clinical diagnosis and currently used immunoassays, and show that the device provides accurate and robust performance equivalent to gold-standard laboratory tests. Importantly, the plasmonic imager can enable identification of PCT levels typical of sepsis and SIRS patients in less than 15 min. The compact and low-cost device is a promising solution for assisting rapid and accurate on-site sepsis diagnosis.

Quantum Plasmonic Immunoassay Sensing.

Plasmon-polaritons are among the most promising candidates for next-generation optical sensors due to their ability to support extremely confined electromagnetic fields and empower strong coupling of light and matter. Here we propose quantum plasmonic immunoassay sensing as an innovative scheme, which embeds immunoassay sensing with recently demonstrated room-temperature strong coupling in nanoplasmonic cavities. In our protocol, the antibody-antigen-antibody complex is chemically linked with a quantum emitter label. Placing the quantum-emitter-enhanced antibody-antigen-antibody complexes inside or close to a nanoplasmonic (hemisphere dimer) cavity facilitates strong coupling between the plasmon-polaritons and the emitter label resulting in signature Rabi splitting. Through rigorous statistical analysis of multiple analytes randomly distributed on the substrate in extensive realistic computational experiments, we demonstrate a drastic enhancement of the sensitivity up to nearly 1500% compared to conventional shifting-type plasmonic sensors. Most importantly and in stark contrast to classical sensing, we achieve in the strong-coupling (quantum) sensing regime an enhanced sensitivity that is no longer dependent on the concentration of antibody-antigen-antibody complexes down to the single-analyte limit. The quantum plasmonic immunoassay scheme thus not only leads to the development of plasmonic biosensing for single molecules but also opens up new pathways toward room-temperature quantum sensing enabled by biomolecular inspired protocols linked with quantum nanoplasmonics.

Generating and Detecting High-Frequency Liquid-Based Sound Resonances with Nanoplasmonics.

We use metal nanostructures (nanoplasmonics) excited with dual frequency lasers to generate and detect high-frequency (>10 GHz) sound wave resonances in water. The difference frequency between the two lasers causes beating in the intensity, which results in a drop in the transmission through the nanostructure when an acoustic resonance is excited. By observing the resonance frequency shifts with changing nanostructure size, the transition from slow to fast sound in water is inferred, which has been measured by inelastic scattering methods in the past. The observed behavior shows remarkable similarities to finite element simulations using a simple Debye model for sound velocity (without fitting parameters).

New Trends in the Simulation of Nanosplasmonic Optical D-Type Fiber Sensors.

This article presents a review of the numerical techniques employed in simulating plasmonic optical sensors based on metal-dielectric nanostructures, including examples, ranging from conventional D-type fiber sensors, to those based on photonic crystal D-type fibers and incorporating metamaterials, nanowires, among other new materials and components, results and applications. We start from the fundamental physical processes, such as optical and plasmonic mode coupling, and discuss the implementation of the numerical model, optical response customization and their impact in sensor performance. Finally, we examine future perspectives.

Lorentz Nanoplasmonics for Nonlinear Generation.

While past works have suggested that the Lorentz magnetic contribution to second harmonic generation from metal nanostructures is negligible as compared to other terms, here we demonstrate a dominant Lorentz contribution from T-shaped apertures in a gold film. The apertures are designed to have overlapping magnetic and electric near-field intensities at the plasmonic resonance. This gives 65% greater nonlinear generation from the Lorentz term than the sum of the other two terms. We demonstrate this effect experimentally by milling of nanoapertures of different size and orientation in a metal film and measuring their second harmonic generation. Good agreement is seen between the experiments and comprehensive calculations. In the development of highly efficient nonlinear metasurfaces, careful optimization of the Lorentz contribution should be considered in addition to all other contributions. Following the approach of this work, the Lorentz contribution may also be optimized for THz generation.

Coupling of Molecular Emitters and Plasmonic Cavities beyond the Point-Dipole Approximation.

As the size of a molecular emitter becomes comparable to the dimensions of a nearby optical resonator, the standard approach that considers the emitter to be a point-like dipole breaks down. By adoption of a quantum description of the electronic transitions of organic molecular emitters, coupled to a plasmonic electromagnetic field, we are able to accurately calculate the position-dependent coupling strength between a plasmon and an emitter. The spatial distribution of excitonic and photonic quantum states is found to be a key aspect in determining the dynamics of molecular emission in ultrasmall cavities both in the weak and strong coupling regimes. Moreover, we show that the extreme localization of plasmonic fields leads to the selection rule breaking of molecular excitations.

Simultaneous Local Heating/Thermometry Based on Plasmonic Magnetochromic Nanoheaters.

A crucial challenge in nanotherapies is achieving accurate and real-time control of the therapeutic action, which is particularly relevant in local thermal therapies to minimize healthy tissue damage and necrotic cell deaths. Here, a nanoheater/thermometry concept is presented based on magnetoplasmonic (Co/Au or Fe/Au) nanodomes that merge exceptionally efficient plasmonic heating and simultaneous highly sensitive detection of the temperature variations. The temperature detection is based on precise optical monitoring of the magnetic-induced rotation of the nanodomes in solution. It is shown that the phase lag between the optical signal and the driving magnetic field can be used to detect viscosity variations around the nanodomes with unprecedented accuracy (detection limit 0.0016 mPa s, i.e., 60-fold smaller than state-of-the-art plasmonic nanorheometers). This feature is exploited to monitor the viscosity reduction induced by optical heating in real-time, even in highly inhomogeneous cell dispersions. The magnetochromic nanoheater/thermometers show higher optical stability, much higher heating efficiency and similar temperature detection limits (0.05 °C) compared to state-of-the art luminescent nanothermometers. The technological interest is also boosted by the simpler and lower cost temperature detection system, and the cost effectiveness and scalability of the nanofabrication process, thereby highlighting the biomedical potential of this nanotechnology.

Label-Free Optofluidic Nanobiosensor Enables Real-Time Analysis of Single-Cell Cytokine Secretion.

Single-cell analysis of cytokine secretion is essential to understand the heterogeneity of cellular functionalities and develop novel therapies for multiple diseases. Unraveling the dynamic secretion process at single-cell resolution reveals the real-time functional status of individual cells. Fluorescent and colorimetric-based methodologies require tedious molecular labeling that brings inevitable interferences with cell integrity and compromises the temporal resolution. An innovative label-free optofluidic nanoplasmonic biosensor is introduced for single-cell analysis in real time. The nanobiosensor incorporates a novel design of a multifunctional microfluidic system with small volume microchamber and regulation channels for reliable monitoring of cytokine secretion from individual cells for hours. Different interleukin-2 secretion profiles are detected and distinguished from single lymphoma cells. The sensor configuration combined with optical spectroscopic imaging further allows us to determine the spatial single-cell secretion fingerprints in real time. This new biosensor system is anticipated to be a powerful tool to characterize single-cell signaling for basic and clinical research.

Quantitative Comparison of Protein Adsorption and Conformational Changes on Dielectric-Coated Nanoplasmonic Sensing Arrays.

Nanoplasmonic sensors are a popular, surface-sensitive measurement tool to investigate biomacromolecular interactions at solid-liquid interfaces, opening the door to a wide range of applications. In addition to high surface sensitivity, nanoplasmonic sensors have versatile surface chemistry options as plasmonic metal nanoparticles can be coated with thin dielectric layers. Within this scope, nanoplasmonic sensors have demonstrated promise for tracking protein adsorption and substrate-induced conformational changes on oxide film-coated arrays, although existing studies have been limited to single substrates. Herein, we investigated human serum albumin (HSA) adsorption onto silica- and titania-coated arrays of plasmonic gold nanodisks by localized surface plasmon resonance (LSPR) measurements and established an analytical framework to compare responses across multiple substrates with different sensitivities. While similar responses were recorded on the two substrates for HSA adsorption under physiologically-relevant ionic strength conditions, distinct substrate-specific behavior was observed at lower ionic strength conditions. With decreasing ionic strength, larger measurement responses occurred for HSA adsorption onto silica surfaces, whereas HSA adsorption onto titania surfaces occurred independently of ionic strength condition. Complementary quartz crystal microbalance-dissipation (QCM-D) measurements were also performed, and the trend in adsorption behavior was similar. Of note, the magnitudes of the ionic strength-dependent LSPR and QCM-D measurement responses varied, and are discussed with respect to the measurement principle and surface sensitivity of each technique. Taken together, our findings demonstrate how the high surface sensitivity of nanoplasmonic sensors can be applied to quantitatively characterize protein adsorption across multiple surfaces, and outline broadly-applicable measurement strategies for biointerfacial science applications.