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.

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.

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.

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.

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.

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.

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.

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.

Local Growth Mediated by Plasmonic Hot Carriers: Chirality from Achiral Nanocrystals Using Circularly Polarized Light

Plasmonic nanocrystals and their assemblies are excellent tools to create functional systems, including systems with strong chiral optical responses. Here we study the possibility of growing chiral plasmonic nanocrystals from strictly nonchiral seeds of different types by using circularly polarized light as the chirality-inducing mechanism. We present a novel theoretical methodology that simulates realistic nonlinear and inhomogeneous photogrowth processes in plasmonic nanocrystals, mediated by the excitation of hot carriers that can drive surface chemistry. We show the strongly anisotropic and chiral growth of oriented nanocrystals with lowered symmetry, with the striking feature that such chiral growth can appear even for nanocrystals with subwavelength sizes. Furthermore, we show that the chiral growth of nanocrystals in solution is fundamentally challenging. This work explores new ways of growing monolithic chiral plasmonic nanostructures and can be useful for the development of plasmonic photocatalysis and fabrication technologies.

Characterization of UVB and UVA-340 Lamps and Determination of Their Effects on ER Stress and DNA Damage

Ultraviolet B-light (UVB) has been often used as a "physiological" UV in photobiology studies. How representative and equivalent these studies are compared to the effect of the sunlight is always of great interest. We now characterized the spectrum and intensity of two commonly used UV sources, a UVB lamp and a UVA-340 lamp which simulate the solar spectrum in the UVB/UVA range in the presence or absence of a UVB band pass filter that reduces >80% UVA from the UVA-340 lamp. The spectrum of each lamp was used in computational modeling for skin penetration. The effects of the lamps on endoplasmic reticulum (ER)-stress response and DNA damage in cultured keratinocytes HaCaT cells were analyzed. Our data show that the UVB lamp is a better inducer for both eIF2α phosphorylation and PERK modification, as well as a better reducer of ATF6 expression. The UVB lamp is also the best inducer of gamma-H2AX expression and cyclobutane pyrimidine dimers formation. However, the UVA-340 lamp is a better inducer for ATF4 expression. Our results indicate that different spectral characteristics of UV lamps can produce different results for the activation of the ER-stress responses and the differences do not always follow a defined pattern.

Chiral Optofluidics with a Plasmonic Metasurface Using the Photothermal Effect

Plasmonic metasurfaces with the photothermal effect have been increasingly investigated for optofluidics. Meanwhile, along with the expanding application of circularly polarized light, a growing number of investigations on chiral plasmonic metasurfaces have been conducted. However, few studies have explored the chirality and the thermal-induced convection of such systems simultaneously. This paper aims to theoretically investigate the dynamics of the thermally induced fluid convection of a chiral plasmonic metasurface. The proposed metasurface exhibits giant circular dichroism in absorption and thus leads to a strong photothermal effect. On the basis of the multiphysical analysis, including optics, thermodynamics, and hydrodynamics, we propose a concept of chiral spectroscopy termed optofluidic circular dichroism. Our results show that different fluid velocities of thermally induced convection appear around a chiral plasmonic metasurface under different circularly polarized excitation. The chiral fluid convection is induced by an asymmetric heat distribution generated by absorbed photons in the plasmonic heater. This concept can be potentially used to induce chiral fluid convection utilizing the chiral photothermal effect. Our proposed structure can potentially be used in various optofluidics applications related to biochemistry, clinical biology, and so on.

Broadband thin-film and metamaterial absorbers using refractory vanadium nitride and their thermal stability

Strong absorption of the full spectrum of sunlight at high temperatures is desired for photothermal devices and thermophotovoltaics. Here, we experimentally demonstrate a thin-film broadband absorber consisting of a vanadium nitride (VN) film and a SiO₂ anti-reflective layer. Owing to the intrinsic high loss of VN, the fabricated absorber exhibits high absorption over 90% in the wide range of 400-1360 nm. To further enhance the near-infrared absorption, we also propose a metamaterial absorber by depositing patterned VN square patches on the thin-film absorber. An average absorption of 90.4% over the range of 400-2500 nm is achieved due to the excitation of broad electric dipole resonance. Both thin-film and metamaterial absorbers are demonstrated to possess excellent incident angle tolerances (up to 60°) and superior thermal stability at 800 ℃. The proposed refractory VN absorbers may be potentially used for solar energy harvesting, thermal emission, and photodetection.

Rational synthesis of novel "giant" CuInTeSe/CdS core/shell quantum dots for optoelectronics

"Giant" core/shell quantum dots (g-QDs) are promising candidates for emerging optoelectronic technologies thanks to their facile structure/composition-tunable optoelectronic properties and outstanding photo-physical/chemical stability. Here, we synthesized a new type of CuInTeSe (CITS)/CdS g-QDs and regulated their optoelectronic properties by controlling the shell thickness. Through increasing the shell thickness, as-prepared g-QDs exhibited tunable red-shifted emission (from 900 to 1200 nm) and prolonged photoluminescence (PL) lifetimes (up to ∼14.0 μs), indicating a formed band structure showing efficient charge separation and transfer, which is further testified by theoretical calculations and ultrafast time-resolved transient absorption (TA) spectroscopy. These CITS/CdS g-QDs with various shell thicknesses can be employed to fabricate photoelectrochemical (PEC) cells, exhibiting improved photoresponse and stability as compared to the bare CITS QD-based devices. The results indicate that the rational design and engineering of g-QDs is very promising for future QD-based optoelectronic technologies.

Chiral Photomelting of DNA-Nanocrystal Assemblies Utilizing Plasmonic Photoheating

Chiral plasmonic nanostructures exhibit anomalously strong chiroptical signals and offer the possibility to realize asymmetric photophysical and photochemical processes controlled by circularly polarized light. Here, we use a chiral DNA-assembled nanorod pair as a model system for chiral plasmonic photomelting. We show that both the enantiomeric excess and consequent circular dichroism can be controlled with chiral light. The nonlinear chiroptical response of our plasmonic system results from the chiral photothermal effect leading to selective melting of the DNA linker strands. Our study describes both the single-complex and collective heating regimes, which should be treated with different models. The chiral asymmetry factors of the calculated photothermal and photomelting effects exceed the values typical for the chiral molecular photochemistry at least 10-fold. Our proposed mechanism can be used to develop chiral photoresponsive systems controllable with circularly polarized light.

Plasmonic Chirality and Circular Dichroism in Bioassembled and Nonbiological Systems: Theoretical Background and Recent Progress

Nature is chiral, thus chirality is a key concept required to understand a multitude of systems in physics, chemistry, and biology. The field of optics offers valuable tools to probe the chirality of nanosystems, including the measurement of circular dichroism, the differential interaction strength between matter and circularly polarized light with opposite helicity. Simultaneously, the use of plasmonic systems with giant light-interaction cross-sections opens new paths to investigate and manipulate systems on the nanoscale. Consequently, the interest in chiral plasmonic and hybrid systems has continually grown in recent years, due to their potential applications in biosensing, polarization-encoded optical communication, polarization-selective chemical reactions, and materials with polarization-dependent light-matter interaction. Experimentally, chiral properties of nanostructures can be either created artificially using modern fabrication techniques involving inorganic materials, or borrowed from nature using bioassembly or biomolecular templating. Herein, the recent progress in the field of plasmonic chirality is summarized, with a focus on both the theoretical background and the experimental advances in the study of chirality in various systems, including molecular-plasmonic assemblies, chiral plasmonic nanostructures, chiral assemblies of interacting plasmonic nanoparticles, and chiral metal metasurfaces and metamaterials. The growth prospects of this field are also discussed.

Manipulating the Optoelectronic Properties of Quasi-type II CuInS2/CdS Core/Shell Quantum Dots for Photoelectrochemical Cell Applications

Colloidal core/shell heterostructured quantum dots (QDs) possessing quasi-type II band structure have demonstrated effective surface passivation and prolonged exciton lifetime, leading to enhanced charge separation/transfer efficiencies that are promising for photovoltaic device applications. Herein, we synthesized CuInS₂ (CIS)/CdS core/shell heterostructured QDs and manipulated the optoelectronic properties via controlling the CdS shell thickness. The shell-thickness-dependent optical properties indicate the existence of a quasi-type II band structure in such core/shell QDs, which was verified by ultrafast spectroscopy and theoretical simulations. These quasi-type II core/shell QDs having various shell thicknesses are used as light absorbers for the fabrication of solar-driven QDs-based photoelectrochemical (PEC) devices, exhibiting an optimized photocurrent density of ∼6.0 mA/cm² and excellent stability under simulated AM 1.5G solar illumination. The results demonstrate that quasi-type II CIS/CdS core/shell heterostructured QDs with tailored optoelectronic properties are promising to realize high-performance QDs-based solar energy conversion devices for the production of solar fuels.

Special topic on emerging directions in plasmonics

Plasmonics enables a wealth of applications, including photocatalysis, photoelectrochemistry, photothermal heating, optoelectronic devices, and biological and chemical sensing, while encompassing a broad range of materials, including coinage metals, doped semiconductors, metamaterials, 2D materials, bioconjugates, and chiral assemblies. Applications in plasmonics benefit from the large local electromagnetic field enhancements generated by plasmon excitation, as well as the products of plasmon decay, including photons, hot charge carriers, and heat. This special topic highlights recent work in both theory and experiment that advance our fundamental understanding of plasmon excitation and decay mechanisms, showcase new applications enabled by plasmon excitation, and highlight emerging classes of materials that support plasmon excitation.

Chiral Assembly of Gold-Silver Core-Shell Plasmonic Nanorods on DNA Origami with Strong Optical Activity

The spatial organization of metal nanoparticles has become an important tool for manipulating light in nanophotonic applications. Silver nanoparticles, particularly silver nanorods, have excellent plasmonic properties but are prone to oxidation and are therefore inherently unstable in aqueous solutions and salt-containing buffers. Consequently, gold nanoparticles have often been favored, despite their inferior optical performance. Bimetallic, i.e., gold-silver core-shell nanoparticles, can resolve this issue. We present a method for synthesizing highly stable gold-silver core-shell NRs that are instantaneously functionalized with DNA, enabling chiral self-assembly on DNA origami. The silver shell gives rise to an enhancement of plasmonic properties, reflected here in strongly increased circular dichroism, as compared to pristine gold nanorods. Gold-silver nanorods are ideal candidates for plasmonic sensing with increased sensitivity as needed in pathogen RNA or antibody testing for nonlinear optics and light-funneling applications in surface enhanced Raman spectroscopy. Furthermore, the control of interparticle orientation enables the study of plasmonic phenomena, in particular, synergistic effects arising from plasmonic coupling of such bimetallic systems.

Experimental and Theoretical Observation of Photothermal Chirality in Gold Nanoparticle Helicoids

Single-particle spectroscopy is central to the characterization of plasmonic nanostructures and understanding of light-matter interactions in chiral nanosystems. Although chiral plasmonic nanostructures are generally characterized by their circular differential extinction and scattering, single-particle absorption studies can extend our understanding of light-matter interactions. Here, we introduce an experimental observation of photothermal chirality which originated from circular differential absorption of chiral plasmonic nanostructures. Using luminescence ratio thermometry, we identify the optical and photothermal handedness and an absolute temperature difference of 6 K under the right and left circularly polarized light. We observe a circular differential extinction parameter (g_ext) of -0.13 in colloidally prepared gold helicoids and compare our findings with numerical simulations using finite element methods. The simulated data showed that circular differential absorption and the maximum temperature of a small cluster of helical nanoparticles are polarization-dependent. We observed an intensity-dependent photothermal g-factor from chiral helicoids that decreases slightly at higher temperatures. We also measure a range of optical g-factors from several gold helicoids, which are attributed to the heterogeneity of helicoids in nanoparticles during synthesis. The principles of differential photothermal response of chiral nanomaterials and heat generation described here can be potentially used for thermal photocatalysis, energy conversion, and electronic applications.

Terahertz three-dimensional monitoring of nanoparticle-assisted laser tissue soldering

In view of minimally-invasive clinical interventions, laser tissue soldering assisted by plasmonic nanoparticles is emerging as an appealing concept in surgical medicine, holding the promise of surgeries without sutures. Rigorous monitoring of the plasmonically-heated solder and the underlying tissue is crucial for optimizing the soldering bonding strength and minimizing the photothermal damage. To this end, we propose a non-invasive, non-contact, and non-ionizing modality for monitoring nanoparticle-assisted laser-tissue interaction and visualizing the localized photothermal damage, by taking advantage of the unique sensitivity of terahertz radiation to the hydration level of biological tissue. We demonstrate that terahertz radiation can be employed as a versatile tool to reveal the thermally-affected evolution in tissue, and to quantitatively characterize the photothermal damage induced by nanoparticle-assisted laser tissue soldering in three dimensions. Our approach can be easily extended and applied across a broad range of clinical applications involving laser-tissue interaction, such as laser ablation and photothermal therapies.

Plasmonic Copper Sulfide Nanoparticles Enable Dark Contrast in Optical Coherence Tomography

Optical coherence tomography (OCT) is an imaging technique affording noninvasive optical biopsies. Like for other imaging techniques, the use of dedicated contrast agents helps better discerning biological features of interest during the clinical practice. Although bright OCT contrast agents have been developed, no dark counterpart has been proposed yet. Herein, plasmonic copper sulfide nanoparticles as the first OCT dark contrast agents working in the second optical transparency window are reported. These nanoparticles virtually possess no light scattering capabilities at the OCT working wavelength (≈1300 nm); thus, they exclusively absorb the probing light, which in turn results in dark contrast. The small size of the nanoparticles and the absence of apparent cytotoxicity support the amenability of this system to biomedical applications. Importantly, in the pursuit of systems apt to yield OCT dark contrast, a library of copper sulfide nanoparticles featuring plasmonic resonances spanning the three optical transparency windows is prepared, thus highlighting the versatility and potential of these systems in light-controlled biomedical applications.

Hot Electrons Generated in Chiral Plasmonic Nanocrystals as a Mechanism for Surface Photochemistry and Chiral Growth

The realization of chiral photochemical reactions at the molecular level has proven to be a challenging task, with invariably low efficiencies originating from very small optical circular dichroism signals. On the contrary, colloidal nanocrystals offer a very large differential response to circularly polarized light when designed with chiral geometries. We propose taking advantage of this capability, introducing a novel mechanism driving surface photochemistry in a chiral nanocrystal. Plasmonic nanocrystals exhibit anomalously large asymmetry factors in optical circular dichroism (CD), and the related hot-electron generation shows in turn a very strong asymmetry, serving as a mechanism for chiral growth. Through theoretical modeling, we show that chiral plasmonic nanocrystals can enable chiral surface growth based on the generation of energetic (hot) electrons. Using simple and realistic phenomenological models, we illustrate how this kind of surface photochemistry can be observed experimentally. The proposed mechanism is efficient if it operates on an already strongly chiral nanocrystal, whereas our proposed mechanism does not show chiral growth for initially nonchiral structures in a solution. The asymmetry factors for the chiral effects, driven by hot electrons, exceed the values observed in chiral molecular photophysics at least 10-fold. The proposed chiral-growth mechanism for the transformation of plasmonic colloids is fundamentally different to the traditional schemes of chiral photochemistry at the molecular level.

DNA-Enabled Chiral Gold Nanoparticle-Chromophore Hybrid Structure with Resonant Plasmon-Exciton Coupling Gives Unusual and Strong Circular Dichroism

Circular dichroism (CD) from hybrid complexes of plasmonic nanostructures and chiral molecules has recently attracted significant interest. However, the hierarchical chiral self-assembly of molecules on surfaces of metal nanostructures has remained challenging. As a result, a deep understanding of plasmon-exciton coupling between surface plasmons and chiral collective molecular excitations has not been achieved. In particular, the critical impact of resonant plasmon-exciton coupling within the hybrid is unclear. Here, we employed DNA-templated strategies to control the chiral self-assembly of achiral chromophores with rationally tuned exciton transitions on gold nanosphere (AuNP) or gold nanorod (AuNR) surfaces. Unlike many previous chiral plasmonic hybrids utilizing chiral biomolecules with CD signals in the UV range, we designed structures with the chiral excitonic resonances at visible wavelengths. The constructed hybrid complexes displayed strong chiroptical activity that depends on the spectral overlap between the chiral collective molecular excitations and the plasmon resonances. We find that when spectral overlap is optimized, the molecular CD signal originating from the chiral self-assemblies of chromophores was strongly enhanced (maximum enhancement of nearly an order of magnitude) and a plasmonic CD signal was induced. Surprisingly, the sign of the molecular CD was reversed despite different self-assembly mechanisms of the Au nanoparticle-chromophore hybrids. Our results provide new insight into plasmonic CD enhancements and will inspire further studies on chiral light-matter interactions in strongly coupled plasmonic-excitonic systems.

Chiral Restructuring of Peptide Enantiomers on Gold Nanomaterials

The use of biomolecules has been invaluable at generating and controlling optical chirality in nanomaterials; however, the structure and properties of the chiral biotemplate are not well understood due to the complexity of peptide-nanoparticle interactions. In this study, we show that the complex interactions between d-peptides and gold nanomaterials led to a chiral restructuring of peptides as demonstrated by circular dichroism and proteolytic cleavage of d-peptides via gold-mediated inversion of peptide chirality. The gold nanoparticles synthesized using d-peptide produce a highly ordered atomic surface and restructured peptide bonds for enzyme cleavage. Differences in gold nanoparticle catalyzed reduction of 4-nitrophenol were observed on the basis of the chiral peptide used in nanoparticle synthesis. Notably, the proteolytic cleavage of d-peptides on gold provides an opportunity for designing nanoparticle based therapeutics to treat peptide venoms, access new chemistries, or modulate the catalytic activity of nanomaterials.

Chiral Plasmonic Nanostructures Enabled by Bottom-Up Approaches

We present a comprehensive review of recent developments in the field of chiral plasmonics. Significant advances have been made recently in understanding the working principles of chiral plasmonic structures. With advances in micro- and nanofabrication techniques, a variety of chiral plasmonic nanostructures have been experimentally realized; these tailored chiroptical properties vastly outperform those of their molecular counterparts. We focus on chiral plasmonic nanostructures created using bottom-up approaches, which not only allow for rational design and fabrication but most intriguingly in many cases also enable dynamic manipulation and tuning of chiroptical responses. We first discuss plasmon-induced chirality, resulting from the interaction of chiral molecules with plasmonic excitations. Subsequently, we discuss intrinsically chiral colloids, which give rise to optical chirality owing to their chiral shapes. Finally, we discuss plasmonic chirality, achieved by arranging achiral plasmonic particles into handed configurations on static or active templates. Chiral plasmonic nanostructures are very promising candidates for real-life applications owing to their significantly larger optical chirality than natural molecules. In addition, chiral plasmonic nanostructures offer engineerable and dynamic chiroptical responses, which are formidable to achieve in molecular systems. We thus anticipate that the field of chiral plasmonics will attract further widespread attention in applications ranging from enantioselective analysis to chiral sensing, structural determination, and in situ ultrasensitive detection of multiple disease biomarkers, as well as optical monitoring of transmembrane transport and intracellular metabolism.

Gap-plasmon enhanced water splitting with ultrathin hematite films: the role of plasmonic-based light trapping and hot electrons

Hydrogen is a promising alternative renewable fuel for meeting the growing energy demands of the world. Over the past few decades, photoelectrochemical water splitting has been widely studied as a viable technology for the production of hydrogen utilizing solar energy. A solar-to-hydrogen (STH) efficiency of 10% is considered to be sufficient for practical applications. Amongst the wide class of semiconductors that have been studied for their application in solar water splitting, iron oxide (α-Fe₂O₃), or hematite, is one of the more promising candidate materials, with a theoretical STH efficiency of 15%. In this work, we show experimentally that by utilizing gold nanostructures that support gap-plasmon resonances together with a hematite layer, we can increase the water oxidation photocurrent by two times over that demonstrated by a bare hematite film at wavelengths above the hematite bandgap. Moreover, we achieve a six-fold increase in the oxidation photocurrent at near-infrared wavelengths, which is attributed to hot electron generation and decay in the gap-plasmon nanostructures. Theoretical simulations confirmed that the metamaterial geometry with gap plasmons that was used allows us to confine electromagnetic fields inside the hematite semiconductor and to enhance the surface photochemistry.

Chiral Plasmonic Nanocrystals for Generation of Hot Electrons: Toward Polarization-Sensitive Photochemistry

The use of biomaterials, with techniques such as DNA-directed assembly or biodirected synthesis, can surpass top-down fabrication techniques in creating plasmonic superstructures in terms of spatial resolution, range of functionality, and fabrication speed. In particular, by enabling a very precise placement of nanoparticles in a bioassembled complex or through the controlled biodirected shaping of single nanoparticles, plasmonic nanocrystals can show remarkably strong circular dichroism (CD) signals. We show that chiral bioplasmonic assemblies and single nanocrystals can enable polarization-sensitive photochemistry based on the generation of energetic (hot) electrons. It is now established that hot plasmonic electrons can induce surface photochemistry or even reshape plasmonic nanocrystals. We show that merging chiral plasmonic nanocrystal systems and the hot-election generation effect offers unique possibilities in photochemistry, such as polarization-sensitive photochemistry promoting nonchiral molecular reactions, chiral photoinduced growth of a colloid at the atomic level, and chiral photochemical destruction of chiral nanocrystals. In contrast, for chiral molecular systems, the equivalent of the described effects is challenging to observe because molecular species typically exhibit very small CD signals. Moreover, we compare our findings with traditional chiral photochemistry at the molecular level, identifying new, different regimes for chiral photochemistry with possibilities that are unique for plasmonic colloidal systems. In this study, we bring together the concept of hot-electron generation and the field of chiral colloidal plasmonics. Using chiral plasmonic nanorod complexes as a model system, we demonstrate remarkably strong CD in both optical extinction and generation rates of hot electrons. Studying the regime of steady-state excitation, we discuss the influence of geometrical and material parameters on the chiral effects involved in the generation of hot electrons. Optical chirality and the chiral hot-electron response in the nanorod dimers result from complex interparticle interactions, which can appear in the weak coupling regime or in the form of Rabi splitting. Regarding practical applications, our study suggests interesting opportunities in polarization-sensitive photochemistry, in chiral recognition or separation, and in promoting chiral crystal growth at the nanoscale.

Spectrally Resolved Ultrafast Exciton Transfer in Mixed Perovskite Quantum Wells

Solution-processed perovskite quantum wells have been used to fabricate increasingly efficient and stable optoelectronic devices. Little is known about the dynamics of photogenerated excitons in perovskite quantum wells within the first few hundred femtoseconds-a crucial time scale on which energy and charge transfer processes may compete. Here we use ultrafast transient absorption and two-dimensional electronic spectroscopy to clarify the movement of excitons and charges in reduced-dimensional perovskite solids. We report excitonic funneling from strongly to weakly confined perovskite quantum wells within 150 fs, facilitated by strong spectral overlap and orientational alignment among neighboring wells. This energy transfer happens on time scales orders of magnitude faster than charge transfer, which we find to occur instead over 10s to 100s of picoseconds. Simulations of both Förster-type interwell exciton transfer and free carrier charge transfer are in agreement with these experimental findings, with theoretical exciton transfer calculated to occur in 100s of femtoseconds.

Generation of hot electrons in nanostructures incorporating conventional and unconventional plasmonic materials

The choice of materials for hot electron generation and injection: peak efficiency or broadband.

Highly Efficient Copper Sulfide-Based Near-Infrared Photothermal Agents: Exploring the Limits of Macroscopic Heat Conversion

Among the foreseeable therapeutic approaches at the cellular level, nanoplatform-driven photothermal therapy is a thriving tool for the selective eradication of malignant tissues with minimal side effects to healthy ones. Hence, chemically versatile, near-infrared absorbing plasmonic nanoparticles are distinctly appealing and most sought after as efficient photothermal agents. In this work, a straightforward method to synthesize monodisperse PEGylated copper sulfide nanoparticles of pure covellite (CuS) phase, featuring strong localized surface plasmonic resonance absorption in the near-infrared and flexible surface chemistry, imparted by monomethyl ether polyethylene glycol molecules, is developed and optimized. These nanoparticles show a remarkable photothermal heat conversion efficiency (HCE) of 71.4%, which is among the highest for CuS systems and rivals that of plasmonic noble metal nanostructures. Moreover, through critical evaluation and mathematical modeling of the material's properties and measurement methodology, it is assessed that the calculated HCE values drastically depend on experimental conditions such as wavelength-dependent solvent absorption properties, sol concentration, and optical path. These findings are of paramount relevance to the photothermal community, since they call for a standardization of the procedure for the evaluation of the HCE of proposed photothermal agents, in order to make the reported values universally and reliably comparable.

Circular Dichroism of Chiral Molecules in DNA-Assembled Plasmonic Hotspots

The chiral state of a molecule plays a crucial role in molecular recognition and biochemical reactions. Because of this and owing to the fact that most modern drugs are chiral, the sensitive and reliable detection of the chirality of molecules is of great interest to drug development. The majority of naturally occurring biomolecules exhibit circular dichroism (CD) in the UV range. Theoretical studies and several experiments have demonstrated that this UV-CD can be transferred into the plasmonic frequency domain when metal surfaces and chiral biomolecules are in close proximity. Here, we demonstrate that the CD transfer effect can be drastically enhanced by placing chiral molecules, here double-stranded DNA, inside a plasmonic hotspot. By using different particle types (gold, silver, spheres, and rods) and by exploiting the versatility of DNA origami, we were able to systematically study the impact of varying particle distances on the CD transfer efficiency and to demonstrate CD transfer over the whole optical spectrum down to the near-infrared. For this purpose, nanorods were also placed upright on DNA origami sheets, forming strong optical antennas. Theoretical models, demonstrating the intricate relationships between molecular chirality and achiral electric fields, support our experimental findings. From both experimental measurements and theoretical considerations, we conclude that the transferred CD is most intensive for systems with strong plasmonic hotspots, as we find them in relatively small gaps (5-12 nm) between spherical nanoparticles and preferably between the tips of nanorods.

DNA-Guided Plasmonic Helix with Switchable Chirality

The ability to dynamically tune the self-assembled structures of nanoparticles is of significant interest in the fields of chemistry and material studies. However, it continues to be challenging to dynamically tune the chiral superstructures of nanoparticles and actively switch the chiral optical properties thereof. Here, we dynamically controlled a gold nanorod 3D chiral plasmonic superstructure (a stair helix with a pinwheel end view) templated by a DNA origami supramolecular polymer, using DNA-toehold-mediated conformational change in the DNA template. The gold nanorod chiral plasmonic helix was controllably reconfigured between a tightly folded state (with a small inter-rod angle) and an extended state (with a wide inter-rod angle) of the same handedness, or between two mirror-image-like structures of opposite handedness. As a result, the chiral plasmonic properties of the gold nanorod helix superstructures, in terms of the circular dichroism amplitude, peak response frequency, and signature of chirality, were actively switched upon the DNA-guided structural reconfiguration. We envision that the strategy demonstrated here will boost the advancement of reconfigurable chiral materials with increased complexity for active light control applications through rational molecular design and predictable self-assembly procedures.

Metamaterial perfect absorber with unabated size-independent absorption

Metamaterial absorbers open a new door for the design of optical harvesting devices ranging from the microwave to optical regimes. The top resonator in these structures is critical for the function of metamaterial absorbers. The resonant frequency, bandwidth, and maximum absorption mainly depend on the choice of material, shape, and size of the top resonator. The maximum absorption is generally impaired as the size of the resonator changes, due to the high sensitivity of impedance matching with the medium. In this paper, we experimentally demonstrate a metamaterial perfect absorber with unabated absorption as its resonator's size changes. The perfect absorber is based on an array of metal squares inscribed with a hollow square. The absorption maxima stay above 98% as the size changes from 600 to 1500 nm in the mid-infrared region, agreeing with simulated results yielding an absorption of ~100%. The unabated absorption properties can be interpreted by the equivalent circuit theory. Moreover, the experimental absorption remains above 91% for incident angles change up to 50°, both for TE and TM polarization. Our work offers a method for achieving stable perfect absorption in sensing, filtering, and selective thermal emission.

Optoelectronic Properties in Near-Infrared Colloidal Heterostructured Pyramidal "Giant" Core/Shell Quantum Dots

Colloidal heterostructured quantum dots (QDs) are promising candidates for next-generation optoelectronic devices. In particular, "giant" core/shell QDs (g-QDs) can be engineered to exhibit outstanding optical properties and high chemical/photostability for the fabrication of high-performance optoelectronic devices. Here, the synthesis of heterostructured CuInSe x S2-x (CISeS)/CdSeS/CdS g-QDs with pyramidal shape by using a facile two-step method is reported. The CdSeS/CdS shell is demonstrated to have a pure zinc blend phase other than typical wurtzite phase. The as-obtained heterostructured g-QDs exhibit near-infrared photoluminescence (PL) emission (≈830 nm) and very long PL lifetime (in the microsecond range). The pyramidal g-QDs exhibit a quasi-type II band structure with spatial separation of electron-hole wave function, suggesting an efficient exciton extraction and transport, which is consistent with theoretical calculations. These heterostructured g-QDs are used as light harvesters to fabricate a photoelectrochemical cell, exhibiting a saturated photocurrent density as high as ≈5.5 mA cm-2 and good stability under 1 sun illumination (AM 1.5 G, 100 mW cm-2). These results are an important step toward using heterostructured pyramidal g-QDs for prospective applications in solar technologies.

Tunable Nonthermal Distribution of Hot Electrons in a Semiconductor Injected from a Plasmonic Gold Nanostructure

For semiconductors photosensitized with organic dyes or quantum dots, transferred electrons are usually considered thermalized at the conduction band edge. This study suggests that the electrons injected from a plasmonic metal into a thin semiconductor shell can be nonthermal with energy up to the plasmon frequency. In other words, the electrons injected into the semiconductor are still hot carriers. Photomodulated X-ray absorption measurements of the Ti L_2,3 edge are compared before and after excitation of the plasmon in Au@TiO₂ core-shell nanoparticles. Comparison with theoretical predictions of the X-ray absorption, which include the heating and state-filling effects from injected hot carriers, suggests that the electrons transferred from the plasmon remain nonthermal in the ∼10 nm TiO₂ shell, due in part to a slow trapping in defect states. By repeating the measurements for spherical, rod-like, and star-like metal nanoparticles, the magnitude of the nonthermal distribution, peak energy, and number of injected hot electrons are confirmed to be tuned by the plasmon frequency and the sharp corners of the plasmonic nanostructure. The results suggest that plasmonic photosensitizers can not only extend the sunlight absorption spectral range of semiconductor-based devices but could also result in increased open circuit voltages and elevated thermodynamic driving forces for solar fuel generation in photoelectrochemical cells.

Determination of hot carrier energy distributions from inversion of ultrafast pump-probe reflectivity measurements

Developing a fundamental understanding of ultrafast non-thermal processes in metallic nanosystems will lead to applications in photodetection, photochemistry and photonic circuitry. Typically, non-thermal and thermal carrier populations in plasmonic systems are inferred either by making assumptions about the functional form of the initial energy distribution or using indirect sensors like localized plasmon frequency shifts. Here we directly determine non-thermal and thermal distributions and dynamics in thin films by applying a double inversion procedure to optical pump-probe data that relates the reflectivity changes around Fermi energy to the changes in the dielectric function and in the single-electron energy band occupancies. When applied to normal incidence measurements our method uncovers the ultrafast excitation of a non-Fermi-Dirac distribution and its subsequent thermalization dynamics. Furthermore, when applied to the Kretschmann configuration, we show that the excitation of propagating plasmons leads to a broader energy distribution of electrons due to the enhanced Landau damping.

The determination of thermal and non-thermal carrier populations in plasmonic systems generally requires assumptions on the types of distributions present. Here, Heilpern et al. directly determine such populations in thin film pump-probe measurements using a double inversion procedure.

Plasmonic Glasses and Films Based on Alternative Inexpensive Materials for Blocking Infrared Radiation

The need for energy-saving materials is pressing. This Letter reports on the design of energy-saving glasses and films based on plasmonic nanocrystals that efficiently block infrared radiation. Designing such plasmonic composite glasses is nontrivial and requires taking full advantage of both material and geometrical properties of the nanoparticles. We compute the performance of solar plasmonic glasses incorporating a transparent matrix and specially shaped nanocrystals. This performance depends on the shape and material of such nanocrystals. Glasses designed with plasmonic nanoshells are shown to exhibit overall better performances as compared to nanorods and nanocups. Simultaneously, scalable synthesis of plasmonic nanoshells and nanocups is technologically feasible using gas-phase fabrication methods. The computational simulations were performed for noble metals (gold and silver) as well as for alternative plasmonic materials (aluminum, copper, and titanium nitride). Inexpensive plasmonic materials (silver, copper, aluminum, and titanium nitride) show an overall good performance in terms of the commonly used figures of merit of industrial glass windows. Together with numerical data for specific materials, this study includes a set of general rules for designing efficient plasmonic IR-blocking media. The plasmonic glasses proposed herein are good candidates for the creation of cheap optical media, to be used in energy-saving windows in warm climates' housing or temperature-sensitive infrastructure.

Photothermal Circular Dichroism Induced by Plasmon Resonances in Chiral Metamaterial Absorbers and Bolometers

Chiral photochemistry remains a challenge because of the very small asymmetry in the chiro-optical absorption of molecular species. However, we think that the rapidly developing fields of plasmonic chirality and plasmon-induced circular dichroism demonstrate very strong chiro-optical effects and have the potential to facilitate the development of chiral photochemistry and other related applications such as chiral separation and sensing. In this study, we propose a new type of chiral spectroscopy-photothermal circular dichroism. It is already known that the planar plasmonic superabsorbers can be designed to exhibit giant circular dichroism signals in the reflection. Therefore, upon illumination with chiral light, such planar metastructures should be able to generate a prominent asymmetry in their local temperatures. Indeed, we demonstrate this chiral photothermal effect using a chiral plasmonic absorber. Calculated temperature maps show very strong photothermal circular dichroism. One of the structures computed in this Letter could serve as a chiral bolometer sensitive to circularly polarized light. Overall, this chiro-optical effect in plasmonic metamaterials is much greater than the equivalent effect in any chiral molecular system or plasmonic bioassembly. Potential applications of this effect are in polarization-sensitive surface photochemistry and chiral bolometers.