Collective chiroptical activity through the interplay of excitonic and charge-transfer effects in localized plasmonic fields.

The collective light-matter interaction of chiral supramolecular aggregates or molecular ensembles with confined light fields remains a mystery beyond the current theoretical description. Here, we programmably and accurately build models of chiral plasmonic complexes, aiming to uncover the entangled effects of excitonic correlations, intra- and intermolecular charge transfer, and localized surface plasmon resonances. The intricate interplay of multiple chirality origins has proven to be strongly dependent on the site-specificity of chiral molecules on plasmonic nanoparticle surfaces spanning the nanometer to sub-nanometer scale. This dependence is manifested as a distinct circular dichroism response that varies in spectral asymmetry/splitting, signal intensity, and internal ratio of intensity. The inhomogeneity of the surface-localized plasmonic field is revealed to affect excitonic and charge-transfer mixed intermolecular couplings, which are inherent to chirality generation and amplification. Our findings contribute to the development of hybrid classical-quantum theoretical frameworks and the harnessing of spin-charge transport for emergent applications.

Lateral Flow Assay Biotesting by Utilizing Plasmonic Nanoparticles Made of Inexpensive Metals─Replacing Colloidal Gold.

Nanoparticles (NPs) can be conjugated with diverse biomolecules and employed in biosensing to detect target analytes in biological samples. This proven concept was primarily used during the COVID-19 pandemic with gold-NP-based lateral flow assays (LFAs). Considering the gold price and its worldwide depletion, here we show that novel plasmonic NPs based on inexpensive metals, titanium nitride (TiN) and copper covered with a gold shell (Cu@Au), perform comparable to or even better than gold nanoparticles. After conjugation, these novel nanoparticles provided high figures of merit for LFA testing, such as high signals and specificity and robust naked-eye signal recognition. Since the main cost of Au NPs in commercial testing kits is the colloidal synthesis, our development with the Cu@Au and the laser-ablation-fabricated TiN NPs is exciting, offering potentially inexpensive plasmonic nanomaterials for various bioapplications. Moreover, our machine learning study showed that biodetection with TiN is more accurate than that with Au.

Emergence of a Low-Energy Peak in the Smoothed Absolute Value Circular Dichroism Spectra of Small Helical Gold Nanorods.

We calculate, using time-dependent density functional theory, absorption and circular dichroism (CD) spectra for a series of small helical gold nanorod structures with a width of 0.6 nm and length increasing from 0.7 nm for Au24 to 1.9 nm for Au56. For a low-energy window, ranging from 1.7 to 4.1 eV, broadening the lines in the absorption spectra results in a low energy peak which previous studies have identified as the (localized) plasmon resonance. As expected, the absorption peak position of the plasmon resonance systematically redshifts as the length of the nanorod increases. However, trends in the CD and straightforwardly broadened CD spectra are more difficult to discern. We introduce the idea of an absolute value CD spectrum and show that broadening the lines results in a low energy peak that has not previously been reported. The peak position systematically redshifts as the length of the nanorod increases but over a significantly smaller range than that for the absorption spectrum.

Reshaping and induction of optical activity in gold@silver nanocuboids by chiral glutathione molecules.

Core-shell gold-silver cuboidal nanoparticles were produced, with either concave or straight facets. Their incubation with a low concentration of chiral l-glutathione (GSH) biomolecules was found to produce near UV plasmonic extinction and induced circular dichroism (CD) peaks. The effect is sensitive to the silver shell thickness. The GSH molecules were found to cause redistribution of silver in the shell, removing silver atoms from edges/corners and re-depositing them at the nanocuboid facets, probably through some redox and complexation processes between the silver and thiol group of the GSH. Other thiolated chiral biomolecules (and drug molecules) did not show this effect. The emerging near UV surface plasmon resonance is a silver slab resonance, which might also possess some multipolar resonance nature. The concave-shaped nanocuboids exhibited stronger induced plasmonic CD relative to the nanocuboids with straight facets.

Quantum plasmonics pushes chiral sensing limit to single molecules: a paradigm for chiral biodetections.

Chiral sensing of single molecules is vital for the understanding of chirality and their applications in biomedicine. However, current technologies face severe limitations in achieving single-molecule sensitivity. Here we overcome these limitations by designing a tunable chiral supramolecular plasmonic system made of helical oligoamide sequences (OS) and nanoparticle-on-mirror (NPoM) resonator, which works across the classical and quantum regimes. Our design enhances the chiral sensitivity in the quantum tunnelling regime despite of the reduced local E-field, which is due to the strong Coulomb interactions between the chiral OSs and the achiral NPoMs and the additional enhancement from tunnelling electrons. A minimum of four molecules per single-Au particle can be detected, which allows for the detection of an enantiomeric excess within a monolayer, manifesting great potential for the chiral sensing of single molecules.

Lateral Flow Assays Biotesting by Utilizing Plasmonic Nanoparticles Made of Inexpensive Metals - Replacing Colloidal Gold.

Nanoparticles (NPs) can be conjugated with diverse biomolecules and employed in biosensing to detect target analytes in biological samples. This proven concept was primarily used during the COVID-19 pandemic with gold NPs-based lateral flow assays (LFAs). Considering the gold price and its worldwide depletion, here we show that novel plasmonic nanoparticles (NPs) based on inexpensive metals, titanium nitride (TiN) and copper covered with a gold shell (Cu@Au), perform comparable or even better than gold nanoparticles. After conjugation, these novel nanoparticles provided high figures of merit for LFA testing, such as high signals and specificity and robust naked-eye signal recognition. To the best of our knowledge, our study represents the 1st application of laser-ablation-fabricated nanoparticles (TiN) in the LFA and dot-blot biotesting. Since the main cost of the Au NPs in commercial testing kits is in the colloidal synthesis, our development with TiN is very exciting, offering potentially very inexpensive plasmonic nanomaterials for various bio-testing applications. Moreover, our machine learning study showed that the bio-detection with TiN is more accurate than that with Au.

Unraveling the Chirality Transfer from Circularly Polarized Light to Single Plasmonic Nanoparticles.

Due to their broken symmetry, chiral plasmonic nanostructures have unique optical properties and numerous applications. However, there is still a lack of comprehension regarding how chirality transfer occurs between circularly polarized light (CPL) and these structures. Here, we thoroughly investigate the plasmon-assisted growth of chiral nanoparticles from achiral Au nanocubes (AuNCs) via CPL without the involvement of any chiral molecule stimulators. We identify the structural chirality of our synthesized chiral plasmonic nanostructures using circular differential scattering (CDS) spectroscopy, which is correlated with scanning electron microscopy imaging at both the single-particle and ensemble levels. Theoretical simulations, including hot-electron surface maps, reveal that the plasmon-induced chirality transfer is mediated by the asymmetric distribution of hot electrons on achiral AuNCs under CPL excitation. Furthermore, we shed light on how this plasmon-induced chirality transfer can also be utilized for chiral growth in bimetallic systems, such as Ag or Pd on AuNCs. The results presented here uncover fundamental aspects of chiral light-matter interaction and have implications for the future design and optimization of chiral sensors and chiral catalysis, among others.

Exploration of the origin of the excellent charge-carrier dynamics in Ruddlesden-Popper oxysulfide perovskite Y2Ti2O5S2.

Although the efficient separation of electron-hole (e-h) pairs is one of the most sought-after electronic characteristics of materials, due to thermally induced atomic motion and other factors, they do not remain separated during the carrier transport process, potentially leading to rapid carrier recombination. Here, we utilized real-time time-dependent density functional theory in combination with nonadiabatic molecular dynamics (NAMD) to explore the separated dynamic transport path within Ruddlesden-Popper oxysulfide perovskite Y2Ti2O5S2 caused by the dielectric layer and phonon frequency difference. The underlying origin of the efficient overall water splitting in Y2Ti2O5S2 is systematically explored. We report the existence of the bi-directional e-h separate-path transport, in which, the electrons transport in the Ti2O5 layer and the holes diffuse in the rock-salt layer. This is in contrast to the conventional e-h separated distribution with a crowded transport channel, as observed in SrTiO3 and hybrid perovskites. Such a unique feature finally results in a long carrier lifetime of 321 ns, larger than that in the SrTiO3 perovskite (160 ns) with only one carrier transport channel. This work provides insights into the carrier transport in lead-free perovskites and yields a novel design strategy for next-generation functionalized optoelectronic devices.

Theoretical Dissection of Cation-Deficient Layered Ruddlesden-Popper Oxysulfide Perovskites with a High-Efficiency Carrier Transport Channel.

The search for lead-free perovskite materials has triggered intensive interest. Here, we study the electronic structures and optical properties of cation-deficient Ruddlesden-Popper oxysulfide perovskites Ln2Ti2O5S2 (Ln = Sc, Y, or La), with a tunable band gap of 1.45-2.1 eV and a small exciton binding energy of ∼0.1 eV, among which Y2Ti2O5S2 has been synthesized experimentally. Sc2Ti2O5S2 possesses the largest light absorbance in the visible region. We further rationalize the light absorption via the transition dipole moment and suggest potential applications of Sc2Ti2O5S2 in solar cells and Y2Ti2O5S2 and La2Ti2O5S2 in water splitting. In addition, this family exhibits small effective masses within the x-y plane and large ones along the z direction. Most importantly, electron gas-like carrier behaviors are observed within the Ti-O bond region, offering a diffusion channel for electron transport. These findings greatly advance our understanding of lead-free perovskites and offer a novel material platform for future optoelectronic devices.

Nonclassical Mechanism of Metal-Enhanced Photoluminescence of Quantum Dots.

Metal-enhanced photoluminescence is able to provide a robust signal even from a single emitter and is promising in applications in biosensors and optoelectronic devices. However, its realization with semiconductor nanocrystals (e.g., quantum dots, QDs) is not always straightforward due to the hidden and not fully described interactions between plasmonic nanoparticles and an emitter. Here, we demonstrate nonclassical enhancement (i.e., not a conventional electromagnetic mechanism) of the QD photoluminescence at nonplasmonic conditions and correlate it with the charge exchange processes in the system, particularly with high efficiency of the hot-hole generation in gold nanoparticles and the possibility of their transfer to QDs. The hole injection returns a QD from a charged nonemitting state caused by hole trapping by surface and/or interfacial traps into an uncharged emitting state, which leads to an increased photoluminescence intensity. These results open new insights into metal-enhanced photoluminescence, showing the importance of the QD surface states in this process.

Balancing Near-Field Enhancement and Hot Carrier Injection: Plasmonic Photocatalysis in Energy-Transfer Cascade Assemblies.

Photocatalysis stands as a very promising alternative to photovoltaics in exploiting solar energy and storing it in chemical products through a single-step process. A central obstacle to its broad implementation is its low conversion efficiency, motivating research in different fields to bring about a breakthrough in this technology. Using plasmonic materials to photosensitize traditional semiconductor photocatalysts is a popular strategy whose full potential is yet to be fully exploited. In this work, we use CdS quantum dots as a bridge system, reaping energy from Au nanostructures and delivering it to TiO2 nanoparticles serving as catalytic centers. The quantum dots can do this by becoming an intermediate step in a charge-transfer cascade initiated in the plasmonic system or by creating an electron-hole pair at an improved rate due to their interaction with the enhanced near-field created by the plasmonic nanoparticles. Our results show a significant acceleration in the reaction upon combining these elements in hybrid colloidal photocatalysts that promote the role of the near-field enhancement effect, and we show how to engineer complexes exploiting this approach. In doing so, we also explore the complex interplay between the different mechanisms involved in the photocatalytic process, highlighting the importance of the Au nanoparticles' morphology in their photosensitizing capabilities.

Local Photochemical Nanoscopy of Hot-Carrier-Driven Catalytic Reactions Using Plasmonic Nanosystems.

Nanoscale investigation of the reactivity of photocatalytic systems is crucial for their fundamental understanding and improving their design and applicability. Here, we present a photochemical nanoscopy technique that unlocks the local spatial detection of molecular products during plasmonic hot-carrier-driven photocatalytic reactions with nanometric precision. By applying the methodology to Au/TiO2 plasmonic photocatalysts, we experimentally and theoretically determined that smaller and denser Au nanoparticle arrays present lower optical contribution with quantum efficiency in hot-hole-driven photocatalysis closely related to the population heterogeneity. As expected, the highest quantum yield from a redox probe oxidation is achieved at the plasmon peak. Investigating a single plasmonic nanodiode, we unravel the areas where oxidation and reduction products are evolved with subwavelength resolution (∼200 nm), illustrating the bipolar behavior of such nanosystems. These results open the way to quantitative investigations at the nanoscale to evaluate the photocatalytic reactivity of low-dimensional materials in a variety of chemical reactions.

Investigating Plasmonic Catalysis Kinetics on Hot-Spot Engineered Nanoantennae.

Strong hot-spots can facilitate photocatalytic reactions potentially providing effective solar-to-chemical energy conversion pathways. Although it is well-known that the local electromagnetic field in plasmonic nanocavities increases as the cavity size reduces, the influence of hot-spots on photocatalytic reactions remains elusive. Herein, we explored hot-spot dependent catalytic behaviors on a highly controlled platform with varying interparticle distances. Plasmon-meditated dehalogenation of 4-iodothiophenol was employed to observe time-resolved catalytic behaviors via in situ surface-enhanced Raman spectroscopy on dimers with 5, 10, 20, and 30 nm interparticle distances. As a result, we show that by reducing the gap from 20 to 10 nm, the reaction rate can be sped up more than 2 times. Further reduction in the interparticle distance did not improve reaction rate significantly although the maximum local-field was ∼2.3-fold stronger. Our combined experimental and theoretical study provides valuable insights in designing novel plasmonic photocatalytic platforms.

Onset of Chirality in Plasmonic Meta-Molecules and Dielectric Coupling.

Chirality is a fundamental feature in all domains of nature, ranging from particle physics over electromagnetism to chemistry and biology. Chiral objects lack a mirror plane and inversion symmetry and therefore cannot be spatially aligned with their mirrored counterpart, their enantiomer. Both natural molecules and artificial chiral nanostructures can be characterized by their light-matter interaction, which is reflected in circular dichroism (CD). Using DNA origami, we assemble model meta-molecules from multiple plasmonic nanoparticles, representing meta-atoms accurately positioned in space. This allows us to reconstruct piece by piece the impact of varying macromolecular geometries on their surrounding optical near fields. Next to the emergence of CD signatures in the instance that we architect a third dimension, we design and implement sign-flipping signals through addition or removal of single particles in the artificial molecules. Our data and theoretical modeling reveal the hitherto unrecognized phenomenon of chiral plasmonic-dielectric coupling, explaining the intricate electromagnetic interactions within hybrid DNA-based plasmonic nanostructures.

Mach-Zehnder interferometer based integrated-photonic acetone sensor approaching the sub-ppm level detection limit.

The detection of acetone in the gaseous form in exhaled breath using an integrated sensor can provide an effective tool for disease diagnostics as acetone is a marker for monitoring human metabolism. An on-chip acetone gas sensor based on the principle of Mach-Zehnder interferometer is proposed and demonstrated. The sensing arm of the device is activated with a composite film of polyethyleneimine and amido-graphene oxide as the gas-sensitive adsorption layer. The composite film demonstrates good selectivity to acetone gas, can be used repeatedly, and is stable in long-term use. Room temperature operation has been demonstrated for the sensor with high sensitivity under a 20 ppm acetone environment. The detection limit can reach 0.76 ppm, making it feasible to be used for the clinical diagnosis of diabetes and the prognosis of heart failure.

Unraveling the Complex Chirality Evolution in DNA-Assembled High-Order, Hybrid Chiroplasmonic Superstructures from Multi-Scale Chirality Mechanisms.

Hierarchical, chiral hybrid superstructures of chromophores and nanoparticles are expected to give rise to intriguing unveiled chiroptical responses originating from the complex chiral interactions among the components. Herein, DNA origami cavity that could self-assemble into one-dimensional (1D) DNA tubes was employed as a scaffold to accurately organize metal nanoparticles and chromophores. The chiral interactions were studied at the level of individual hybrid particles and their 1D hybrid superstructures. Complex chirality mechanisms involving global structural chirality, plasmon-induced circular dichroism (PICD) and exciton-coupled circular dichroism (ECCD) were disentangled. The multiplexed CD spectrum superposition revealed the chirality evolution at different length scales. These results can offer a model for boosting the theoretical understanding of classical-quantum hybrid systems, and would inspire the future design of optically-active substances across length scales.

Synergistic Combination of Charge Carriers and Energy-Transfer Processes in Plasmonic Photocatalysis.

Important efforts are currently under way in order to develop further the nascent field of plasmonic photocatalysis, striving for improved efficiencies and selectivities. A significant fraction of such efforts has been focused on distinguishing, understanding, and enhancing specific energy-transfer mechanisms from plasmonic nanostructures to their environment. Herein, we report a synthetic strategy that combines two of the main physical mechanisms driving plasmonic photocatalysis into an engineered system by rationally combining the photochemical features of energetic charge carriers and the electromagnetic field enhancement inherent to the plasmonic excitation. We do so by creating hybrid photocatalysts that integrate multiple plasmonic resonators in a single entity, controlling their joint contribution through spectral separation and differential surface functionalization. This strategy allows us to create complex hybrids with improved photosensitization capabilities, thanks to the synergistic combination of two photosensitization mechanisms. Our results show that the hot electron injection can be combined with an energy-transfer process mediated by the near-field interaction, leading to a significant increase in the final photocatalytic response of the material and moving the field of plasmonic photocatalysis closer to energy-efficient applications. Furthermore, our multimodal hybrids offer a test system to probe the properties of the two targeted mechanisms in energy-related applications such as the photocatalytic generation of hydrogen and open the door to wavelength-selective photocatalysis and novel tandem reactions.

Plasmonic photocatalysis in aqueous solution: assessing the contribution of thermal effects and evaluating the role of photogenerated ROS.

Plasmon-induced photocatalysis can drive photochemical processes with an unprecedented control of reactivity, using light as sole energy source. Nevertheless, disentangling the relative importance of thermal and non-thermal features upon plasmonic excitation remains a difficult task. In this work we intend to separate the role played by the photogenerated charge carriers from thermal mechanisms in the plasmonic photo-oxidation of a model organic substrate in aqueous solution and using a metal-semiconductor hybrid as model photocatalyst. Accordingly, we present a simple set of experimental procedures and simulations that allow us to discard the thermal dissipation upon plasmonic excitation as the main driving force behind these chemical reactions. Moreover, we also study the photogeneration of reactive oxygen species (ROS), discussing their fundamental role in photo-oxidation reactions and the information they provide regarding the reactivity of the photogenerated electrons and holes.

DNA-Assembled Chiral Satellite-Core Nanoparticle Superstructures: Two-State Chiral Interactions from Dynamic and Static Conformations.

A significant challenge exists in obtaining chiral nanostructures that are amenable to both solution-phase self-assembly and solid-phase preservation, which enable the observation of unveiled optical responses impacted by the dynamic or static conformation and the incident excitations. Here, to meet this demand, we employed DNA origami technology to create quasi-planar chiral satellite-core nanoparticle superstructures with an intermediate geometry between the monolayer and the double layer. We disentangled the complex chiral mechanisms, which include planar chirality, 3D chirality, and induced chirality transfer, through combined theoretical studies and thorough experimental measurements of both solution- and solid-phase samples. Two distinct states of optical responses were demonstrated by the dynamic and static conformations, involving a split or nonsplit circular dichroism (CD) line shape. More importantly, our study on chiral nanoparticle superstructures on a substrate featuring both a dominant 2D geometry and a defined 3D represents a great leap toward the realization of colloidal chiral metasurfaces.