All-State Switching of the Mie Resonance of Conductive Polyaniline Nanospheres.

Polyaniline (PANI), a conductive polymer, is a promising active material for optical switching. In most studies, active switching has so far been realized only between two states, whereas PANI has a total of six states. The optical properties of nanoscale PANI in all six states have remained unclear. Herein we report on all-state switching of the Mie resonance on PANI nanospheres (NSs) and active plasmon switching on PANI-coated Au nanodisks (NDs). All-state switching of differently sized PANI NSs is achieved by proton doping/dedoping and electrochemical methods. Theoretical studies show that the scattering peaks of the individual PANI NSs originate from Mie resonances. All-state switching is further demonstrated on PANI-coated circular Au NDs, where an unprecedentedly large plasmon peak shift of ∼200 nm is realized. Our study not only provides a fundamental understanding of the optical properties of PANI but also opens the probability for developing high-performance dynamic media for active plasmonics.

Colloidal Gold Nanorings and Their Plasmon Coupling with Gold Nanospheres.

Gold nanorings are attractive as plasmonic metal nanocrystals because they have a hollow inner cavity. Their enhanced electric field inside the ring cavity is accessible, which is highly desirable for assembling with other optical components and studying their plasmon-coupling behaviors. However, the lack of robust methods for synthesizing size-controllable and uniform Au nanorings severely impedes the study of their attractive plasmonic properties and plasmon-driven applications. Herein, an improved wet-chemistry method is reported for the synthesis of monodisperse colloidal Au nanorings. Using circular Au nanodisks with different thicknesses and diameters as templates, Au nanorings are synthesized with thicknesses varied from ≈30 to ≈50 nm and cavity sizes varied from ≈90 to ≈40 nm. The produced Au nanorings are assembled with colloidal Au nanospheres to yield Au nanoring-nanosphere heterodimers in sphere-in-ring and sphere-on-ring configurations on substrates. The sphere-in-ring heterodimers exhibit the interesting feature of plasmonic Fano resonance upon the excitation of the dark quadrupolar plasmon mode of the Au nanorings. The open cavity in a nanoring holds a great promise for studying plasmon-coupled systems, which will facilitate the construction of advanced metamaterials and high-performance Fano-based devices.

High-Efficiency "Working-in-Tandem" Nitrogen Photofixation Achieved by Assembling Plasmonic Gold Nanocrystals on Ultrathin Titania Nanosheets.

The fixation of atmospheric N2 to NH3 is an essential process for sustaining life. One grand challenge is to develop efficient catalysts to photofix N2 under ambient conditions. Herein we report an all-inorganic catalyst, Au nanocrystals anchored on ultrathin TiO2 nanosheets with oxygen vacancies. It can accomplish photodriven N2 fixation in the "working-in-tandem" pathway at room temperature and atmospheric pressure. The oxygen vacancies on the TiO2 nanosheets chemisorb and activate N2 molecules, which are subsequently reduced to NH3 by hot electrons generated from plasmon excitation of the Au nanocrystals. The apparent quantum efficiency of 0.82% at 550 nm for the conversion of incident photons to NH3 is higher than those reported so far. Optimizing the absorption across the overall visible range with the mixture of Au nanospheres and nanorods further enhances the N2 photofixation rate by 66.2% in comparison with Au nanospheres used alone. This work offers a new approach for the rational design of efficient catalysts toward sustainable N2 fixation through a less energy-demanding photochemical process compared to the industrial Haber-Bosch process.

Environmental permittivity-asymmetric BIC metasurfaces with electrical reconfigurability.

Achieving precise spectral and temporal light manipulation at the nanoscale remains a critical challenge in nanophotonics. While photonic bound states in the continuum (BICs) have emerged as a powerful means of controlling light, their reliance on geometrical symmetry breaking for obtaining tailored resonances makes them highly susceptible to fabrication imperfections, and their generally fixed asymmetry factor fundamentally limits applications in reconfigurable metasurfaces. Here, we introduce the concept of environmental symmetry breaking by embedding identical resonators into a surrounding medium with carefully placed regions of contrasting refractive indexes, activating permittivity-driven quasi-BIC resonances (ε-qBICs) without altering the underlying resonator geometry and unlocking an additional degree of freedom for light manipulation through active tuning of the surrounding dielectric environment. We demonstrate this concept by integrating polyaniline (PANI), an electro-optically active polymer, to achieve electrically reconfigurable ε-qBICs. This integration not only demonstrates rapid switching speeds and exceptional durability but also boosts the system's optical response to environmental perturbations. Our strategy significantly expands the capabilities of resonant light manipulation through permittivity modulation, opening avenues for on-chip optical devices, advanced sensing, and beyond.

Spectral CT in the evaluation of perineural invasion status in rectal cancer.

PURPOSE: To investigate whether preoperative spectral CT quantitative parameters can assess perineural invasion (PNI) status in rectal cancer. METHODS: Sixty-two patients diagnosed with rectal cancer who underwent preoperative spectral CT were retrospectively enrolled and divided into positive and negative PNI groups according to histopathologic results. The CT attenuation value (HU) of virtual monochromatic images (40-70 keV), spectral curve slope (K(HU)), effective atomic number (Zeff), and iodine concentration (IC) from spectral CT were compared between these two groups using t test or rank sum test. A nomogram was established by incorporating the independent predictors to assess the overall diagnostic efficacy. The area under the ROC curves (AUCs) were compared using the DeLong test. RESULTS: The preoperative spectral CT parameters (40-70 keV attenuation, K(HU), Zeff, and IC) were significantly higher in the PNI-positive group compared to the PNI-negative group (all p < 0.05). The highest predictive efficiency of PNI was observed at 40 keV attenuation, with an area under the curve (AUC), sensitivity, specificity, and accuracy of 0.847, 81.8%, 72.5%, and 75.8%, respectively. Binary logistic regression demonstrated that the clinical feature (cN stage) and 40 keV attenuation were independent predictors of PNI status. The nomogram incorporating these two predictors (cN stage and 40 keV attenuation) exhibited the best evaluation efficacy, with an AUC, sensitivity, specificity, and accuracy of 0.885, 86.4%, 77.5%, and 80.6%. CONCLUSION: Spectral CT quantitative parameters proved valuable in the preoperative assessment of PNI status in rectal cancer patients. The combination of spectral CT parameters and clinical features could further enhance the diagnostic efficiency.

A new cobalt(II) complex nanosheet as an electroactive medium for plasmonic switching on Au nanoparticles.

2D metal-organic complex nanosheets with the merits of high stability and structure tunability are an emerging topic in recent years. To extend the promising ultrathin architectures, a new Co(II) complex nanosheet (Co-nanosheet) is designed and prepared via a readily operated interface-assisted coordination reaction between the ligand 4,4'',4'''-(2,4,6-trimethylbenzene-1,3,5-triyl)tris(2,2':6',2''-terpyridyl) (L) and Co2+ ions. The as-formed Co(II) complex nanosheet exhibits both a uniform layered structure and good thermostability as proposed, which were verified by various chemical and physical analytical methods. Moreover, it is first utilized as an electroresponsive medium to tune the surface plasmon resonance behavior of Au nanoparticles, expanding the applicable fields of this type of 2D materials.

Electrochemically controlled metasurfaces with high-contrast switching at visible frequencies.

Recently in nanophotonics, a rigorous evolution from passive to active metasurfaces has been witnessed. This advancement not only brings forward interesting physical phenomena but also elicits opportunities for practical applications. However, active metasurfaces operating at visible frequencies often exhibit low performance due to design and fabrication restrictions at the nanoscale. In this work, we demonstrate electrochemically controlled metasurfaces with high intensity contrast, fast switching rate, and excellent reversibility at visible frequencies. We use a conducting polymer, polyaniline (PANI), that can be locally conjugated on preselected gold nanorods to actively control the phase profiles of the metasurfaces. The optical responses of the metasurfaces can be in situ monitored and optimized by controlling the PANI growth of subwavelength dimension during the electrochemical process. We showcase electrochemically controlled anomalous transmission and holography with good switching performance. Such electrochemically powered optical metasurfaces lay a solid basis to develop metasurface devices for real-world optical applications.

Electrochemical coating of different conductive polymers on diverse plasmonic metal nanocrystals.

Conductive polymers are attracting much attention for realizing active plasmonics on conventional static plasmonic nanostructures because of their variable dielectric functions. Combining organic conductive polymers with inorganic plasmonic nanostructures allows for the creation of active devices, such as active metasurfaces, reconfigurable metalenses and dynamic plasmonic holography. However, the complexity of such a combination, together with the poor control in polymer thickness and morphology, has limited the advancement of active plasmonics. Herein we report on the electrochemical coating of conductive polymers on pre-grown metal nanocrystals. Robust control of the polymer thickness and morphology is accomplished through the variation of the applied electrochemical potential. Various types of conductive polymers are coated on different metal nanocrystals, including Au, Pd and Pt. Active plasmonic color switching and H2O2 sensing are demonstrated with polyaniline-coated Au nanorods.

Electrochemical Switching of Plasmonic Colors Based on Polyaniline-Coated Plasmonic Nanocrystals.

Plasmonic color generation has attracted much research interest because of the unique optical properties of plasmonic nanocrystals that are promising for chromatic applications, such as flat-panel displays, smart windows, and wearable devices. Low-cost, monodisperse plasmonic nanocrystals supporting strong localized surface plasmon resonances are favorable for the generation of plasmonic colors. However, many implementations so far have either a single static state or complexities in the particle alignment and switching mechanism for generating multiple displaying states. Herein, we report on a facile and robust approach for realizing the electrochemical switching of plasmonic colors out of colloidal plasmonic nanocrystals. The metal nanocrystals are coated with a layer of polyaniline, whose refractive index and optical absorption are reversibly switched through the variation of an applied electrochemical potential. The change in refractive index and optical absorption results in the modulation of the plasmonic scattering intensity with a depth of 11 dB. The electrochemical switching process is fast (∼5 ms) and stable (over 1000 switching cycles). A device configuration is further demonstrated for switching plasmonic color patterns in a transparent electrochemical device, which is made from indium tin oxide electrodes and a polyvinyl alcohol solid electrolyte. Our control of plasmonic colors provides a favorable platform for engineering low-cost and high-performance miniaturized optical devices.

(Gold nanorod core)/(poly(3,4-ethylene-dioxythiophene) shell) nanostructures and their monolayer arrays for plasmonic switching.

(Gold nanorod core)/(poly(3,4-ethylene-dioxythiophene) (PEDOT) shell) nanostructures are prepared by the surfactant-assisted oxidative polymerization of 3,4-ethylene-dioxythiophene on the surface of gold nanorods (NRs). The PEDOT shell exhibits distinct dielectric properties at doped and undoped states, which allows the manipulation of plasmonic responses of the Au nanorod core. The shift in plasmon resonance induced by the dedoping of PEDOT is found to be associated with the overlap between the plasmon resonance band of the core/shell nanostructure and the spectral region where the largest refractive index variation of PEDOT occurs, as well as with the type of the dedopant. Macroscopic two-dimensional (2D) monolayer arrays of core/shell nanostructures with controlled particle number densities are fabricated on indium tin oxide (ITO)-coated glass substrates by electrophoretic deposition. A reversible plasmonic shift of about 70 nm is obtained on the core/shell nanostructure monolayer array with a number density of around 18 particles per µm2. Our design of colloidal (Au nanorod core)/(PEDOT shell) nanostructures and their 2D monolayer arrays paves the way for the fabrication of high-performance plasmonic switches in large-scale practical usages as well as for the preparation of advanced, programmable chromic materials for a broad range of applications, such as smart windows, anti-counterfeiting tags, and medical and environmental sensors.

Switching plasmonic Fano resonance in gold nanosphere-nanoplate heterodimers.

The interference between spectrally overlapping superradiant and subradiant plasmon resonances generates plasmonic Fano resonance, which allows for attractive applications such as electromagnetically induced transparency, light trapping, and refractometric sensing with high figures of merit. The active switching of plasmonic Fano resonance holds great promise in modulating optical signals, dynamically harvesting light energy, and constructing switchable plasmonic sensors. However, structures enabling the active control of plasmonic Fano resonance have rarely been achieved because of the fabrication complexity and cost. Herein we report on the realization of active plasmonic Fano resonance switching on Au nanosphere-nanoplate heterodimers. The active switching is enabled by varying the refractive index of a layer of polyaniline that fills in the gap between the Au nanosphere and the Au nanoplate. A reversible spectral shift of 20 nm is observed on the individual heterodimers during switching. The maximal spectral shift decreases as the interparticle gap distance is enlarged, showing a strong dependence of the spectral shift on the local electric field intensity enhancement in the gap region. This trend agrees with the predicted dependence of the refractive index sensitivity on the local field intensity enhancement. Our results provide insights into the development of plasmonic structures supporting actively switchable Fano resonances, which can lead to new technological applications, such as switchable cloaking and display, dynamic coding of optical signals, color sorting and filtering. The Au heterodimers with polyaniline in the gap can also be applied for the sensing of local environmental parameters such as pH values and heavy metal ions.

Emerging Applications of Plasmons in Driving CO2 Reduction and N2 Fixation.

The photochemical production of fuels using sunlight is an innovative way for meeting the quickly increasing energy demands. One of the largest challenges is to develop high-performance photocatalysts that can meet the requirements of practical applications. Owing to their intriguing localized surface plasmon resonances, noble metal nanoparticles and nanostructures show a great potential for enhancing the photocatalytic efficiency and thereby have attracted rapidly growing interest recently. Here, for the first time, the latest achievements in the utilization of plasmons in driving CO2 reduction and N2 fixation into high-value products are comprehensively described. The involved plasmonic enhancement mechanisms in the two types of reactions are fully illustrated. A particular emphasis is given to the outlook on the direction and prospects for future work in this topic.

Plasmonic and sensing properties of vertically oriented hexagonal gold nanoplates.

Plasmonic metal nanocrystals, owing to their high sensitivity to the dielectric changes in the surrounding environment, can allow for the direct probing and monitoring of molecular binding on their surfaces. Anisotropic Au nanoplates possess high refractive index sensitivities, with their nanoscale sensing volumes located at their sharp tips and edges. One of the main challenges in molecular detection based on localized plasmon resonance is to increase the detection capability at the single-particle level. Vertically oriented Au nanoplates are more attractive candidates for developing ultrasensitive plasmonic sensors than horizontally oriented ones, as vertical Au nanoplates allow for more analyte molecules to access their sharp tips and edges. However, few reports have studied the sensing performance of vertically oriented, elongated, individual metal nanocrystals. Herein we report on the deposition of vertically oriented, hexagonal Au nanoplates on substrates and the study of their plasmonic and sensing properties. The vertically aligned nanoplates are compared with the horizontally oriented ones both experimentally and through numerical simulations. The vertically oriented nanoplates possess shorter plasmon resonance wavelengths and narrower peak widths than the horizontally oriented ones of similar sizes. The shorter plasmon wavelengths and smaller peak widths are also confirmed by knocking down the vertical Au nanoplates through mild perturbation and performing the scattering measurements beforehand and afterwards. Further scattering measurements of the Au nanoplates upon the adsorption of positively charged polyelectrolyte reveal that the vertical Au nanoplates are more sensitive to the polyelectrolyte molecules than the horizontal ones.

Active Electrochemical Plasmonic Switching on Polyaniline-Coated Gold Nanocrystals.

High-performance electrochemical plasmonic switching is realized on both single-particle and ensemble levels by coating polyaniline on colloidal gold nanocrystals through surfactant-assisted oxidative polymerization. Under small applied potentials, the core@shell nanostructures exhibit reversible plasmon shifts as large as 150 nm, a switching time of less than 10 ms, and a high switching stability.