Nanostructured Dielectric Fractals on Resonant Plasmonic Metasurfaces for Selective and Sensitive Optical Sensing of Volatile Compounds

Ab initio investigation of hot electron transfer in CO2 plasmonic photocatalysis in the presence of hydroxyl adsorbate

Photoreduction of carbon dioxide (CO2) on plasmonic structures is of great interest in photocatalysis to aid selectivity. While species commonly found in reaction environments and associated intermediates can steer the reaction down different pathways by altering the potential energy landscape of the system, they are often not addressed when designing efficient plasmonic catalysts. Here, we perform an atomistic study of the effect of the hydroxyl group (OH) on CO2 activation and hot electron generation and transfer using first-principles calculations. We show that the presence of OH is essential in breaking the linear symmetry of CO2, which leads to a charge redistribution and a decrease in the OCO angle to 134°, thereby activating CO2. Analysis of the partial density of states (pDOS) demonstrates that the OH group mediates the orbital hybridization between Au and CO2 resulting in more accessible states, thus facilitating charge transfer. By employing time-dependent density functional theory (TDDFT), we quantify the fraction of hot electrons directly generated into hybridized molecular states at resonance, demonstrating a broader energy distribution and an 11% increase in charge-transfer in the presence of OH groups. We further show that the spectral overlap between excitation energy and plasmon resonance plays a critical role in efficiently modulating electron transfer processes. These findings contribute to the mechanistic understanding of plasmon-mediated reactions and demonstrate the importance of co-adsorbed species in tailoring the electron transfer processes, opening new avenues for enhancing selectivity.

Advances in flame synthesis of nano-scale architectures for chemical, biomolecular, plasmonic, and light sensing

Flame spray pyrolysis (FSP), a key technique under the broader category of flame aerosol synthesis, is being increasingly explored for the design of advanced miniaturized sensor architectures with applications including chemical, biomolecular, plasmonic, and light sensing. This review provides an overview of the advantages of FSP for the fabrication of nanostructured materials for sensing, delving into synthesis strategies and material structures that meet the increasing demands for miniaturized sensor devices. We focus on the fundamentals of FSP, discussing reactor configurations and how process parameters such as precursor compositions, flow rates, and temperature influence nanoparticle characteristics and their sensing performance. A detailed analysis of nanostructures, compositions, and morphologies made by FSP and their applications in chemical, chemiresistive, plasmonic, biosensing, and light sensing is presented. This review identifies the challenges and opportunities of FSP, exploring current limitations and potential improvements for industrial translation. We conclude by highlighting future research directions aiming to establish guidelines for the flame-based design of nano-scale sensing architectures.

Structural Engineering Three-Dimensional Nano-Heterojunction Networks for High-Performance Photochemical Sensing

Nanoscale heterojunction networks are increasingly regarded as promising functional materials for a variety of optoelectronic and photocatalytic devices. Despite their superior charge-carrier separation efficiency, a major challenge remains in the optimization of their surface properties, with surface defects playing a major role in charge trapping and recombination. Here, we report the effective engineering of the photocatalytic properties of nanoscale heterojunction networks via deep ultraviolet photoactivation throughout their cross-section. For the first time, in-depth XPS analysis of very thick (∼10 μm) NixOy-ZnO films reveals localized p-n nanoheterojunctions with tunable oxygen vacancies (Vo) originating from both NixOy and ZnO nanocrystals. Optimizing the amount of oxygen vacancies leads to a 30-fold increase in the photochemoresistive response of these networks, enabling the detection of representative analyte concentrations down to 2 and 20 ppb at an optimal temperature of 150 °C and room temperature, respectively. Density functional theory calculations reveal that this performance enhancement is presumably due to an 80% increase in the analyte adsorption energy. This flexible nanofabrication approach in conjunction with straightforward vacancy control via photoactivation provides an effective strategy for engineering the photocatalytic activity of porous metal oxide semiconductor networks with applications in chemical sensors, photodetectors, and photoelectrochemical cells.

Mechanically-Guided 3D Assembly for Architected Flexible Electronics

Architected flexible electronic devices with rationally designed 3D geometries have found essential applications in biology, medicine, therapeutics, sensing/imaging, energy, robotics, and daily healthcare. Mechanically-guided 3D assembly methods, exploiting mechanics principles of materials and structures to transform planar electronic devices fabricated using mature semiconductor techniques into 3D architected ones, are promising routes to such architected flexible electronic devices. Here, we comprehensively review mechanically-guided 3D assembly methods for architected flexible electronics. Mainstream methods of mechanically-guided 3D assembly are classified and discussed on the basis of their fundamental deformation modes (i.e., rolling, folding, curving, and buckling). Diverse 3D interconnects and device forms are then summarized, which correspond to the two key components of an architected flexible electronic device. Afterward, structure-induced functionalities are highlighted to provide guidelines for function-driven structural designs of flexible electronics, followed by a collective summary of their resulting applications. Finally, conclusions and outlooks are given, covering routes to achieve extreme deformations and dimensions, inverse design methods, and encapsulation strategies of architected 3D flexible electronics, as well as perspectives on future applications.

Environmental formaldehyde sensing at room temperature by smartphone-assisted and wearable plasmonic nanohybrids

Formaldehyde is a toxic and carcinogenic indoor air pollutant. Promising for its routine detection are gas sensors based on localized surface plasmon resonance (LSPR). Such sensors trace analytes by converting tiny changes in the local dielectric environment into easily readable, optical signals. Yet, this mechanism is inherently non-selective to volatile organic compounds (like formaldehyde) and yields rarely detection limits below parts-per-million concentrations. Here, we reveal that chemical reaction-mediated LSPR with nanohybrids of Ag/AgOx core-shell clusters on TiO2 enables highly selective formaldehyde sensing down to 5 parts-per-billion (ppb). Therein, AgOx is reduced by the formaldehyde to metallic Ag resulting in strong plasmonic signal changes, as measured by UV/Vis spectroscopy and confirmed by X-ray diffraction. This interaction is highly selective to formaldehyde over other aldehydes, alcohols, ketones, aromatic compounds (as confirmed by high-resolution mass spectrometry), inorganics, and quite robust to relative humidity changes. Since this sensor works at room temperature, such LSPR nanohybrids are directly deposited onto flexible wristbands to quantify formaldehyde between 40-500 ppb at 50% RH, even with a widely available smartphone camera (Pearson correlation coefficient r = 0.998). Such chemoresponsive coatings open new avenues for wearable devices in environmental, food, health and occupational safety applications, as demonstrated by an early field test in the pathology of a local hospital.

Plasmonic Biosensors with Nanostructure for Healthcare Monitoring and Diseases Diagnosis

Nanophotonics has been widely utilized in enhanced molecularspectroscopy or mediated chemical reaction, which has major applications in the field of enhancing sensing and enables opportunities in developing healthcare monitoring. This review presents an updated overview of the recent exciting advances of plasmonic biosensors in the healthcare area. Manufacturing, enhancements and applications of plasmonic biosensors are discussed, with particular focus on nanolisted main preparation methods of various nanostructures, such as chemical synthesis, lithography, nanosphere lithography, nanoimprint lithography, etc., and describing their respective advances and challenges from practical applications of plasmon biosensors. Based on these sensing structures, different types of plasmonic biosensors are summarized regarding detecting cancer biomarkers, body fluid, temperature, gas and COVID-19. Last, the existing challenges and prospects of plasmonic biosensors combined with machine learning, mega data analysis and prediction are surveyed.

Surface Functionalization and Texturing of Optical Metasurfaces for Sensing Applications

Optical metasurfaces are planar metamaterials that can mediate highly precise light-matter interactions. Because of their unique optical properties, both plasmonic and dielectric metasurfaces have found common use in sensing applications, enabling label-free, nondestructive, and miniaturized sensors with ultralow limits of detection. However, because bare metasurfaces inherently lack target specificity, their applications have driven the development of surface modification techniques that provide selectivity. Both chemical functionalization and physical texturing methodologies can modify and enhance metasurface properties by selectively capturing analytes at the surface and altering the transduction of light-matter interactions into optical signals. This review summarizes recent advances in material-specific surface functionalization and texturing as applied to representative optical metasurfaces. We also present an overview of the underlying chemistry driving functionalization and texturing processes, including detailed directions for their broad implementation. Overall, this review provides a concise and centralized guide for the modification of metasurfaces with a focus toward sensing applications.

Morphology-driven gas sensing by fabricated fractals: A review

Fractals are intriguing structures that repeat themselves at various length scales. Interestingly, fractals can also be fabricated artificially in labs under controlled growth environments and be explored for various applications. Such fractals have a repeating unit that spans in length from nano- to millimeter range. Fractals thus can be regarded as connectors that structurally bridge the gap between the nano- and the macroscopic worlds and have a hybrid structure of pores and repeating units. This article presents a comprehensive review on inorganic fabricated fractals (fab-fracs) synthesized in labs and employed as gas sensors across materials, morphologies, and gas analytes. The focus is to investigate the morphology-driven gas response of these fab-fracs and identify key parameters of fractal geometry in influencing gas response. Fab-fracs with roughened microstructure, pore-network connectivity, and fractal dimension (D) less than 2 are projected to be possessing better gas sensing capabilities. Fab-fracs with these salient features will help in designing the commercial gas sensors with better performance.

Plasmonic Metasurface Resonators to Enhance Terahertz Magnetic Fields for High-Frequency Electron Paramagnetic Resonance

Nanoscale magnetic systems play a decisive role in areas ranging from biology to spintronics. Although, in principle, THz electron paramagnetic resonance (EPR) provides high-resolution access to their properties, lack of sensitivity has precluded realizing this potential. To resolve this issue, the principle of plasmonic enhancement of electromagnetic fields that is used in electric dipole spectroscopies with great success is exploited, and a new type of resonators for the enhancement of THz magnetic fields in a microscopic volume is proposed. A resonator composed of an array of diabolo antennas with a back-reflecting mirror is designed and fabricated. Simulations and THz EPR measurements demonstrate a 30-fold signal increase for thin film samples. This enhancement factor increases to a theoretical value of 7500 for samples confined to the active region of the antennas. These findings open the door to the elucidation of fundamental processes in nanoscale samples, including junctions in spintronic devices or biological membranes.

Photonic Fractal Metamaterials: A Metal-Semiconductor Platform with Enhanced Volatile-Compound Sensing Performance

Advance of photonics media is restrained by the lack of structuring techniques for the 3D fabrication of active materials with long-range periodicity. A methodology is reported for the engineering of tunable resonant photonic media with thickness exceeding the plasmonic near-field enhancement region by more than two orders of magnitude. The media architecture consists of a stochastically ordered distribution of plasmonic nanocrystals in a fractal scaffold of high-index semiconductors. This plasmonic-semiconductor fractal media supports the propagation of surface plasmons with drastically enhanced intensity over multiple length scales, overcoming the 2D limitations of established metasurface technologies. The fractal media are used for the fabrication of plasmonic optical gas sensors, achieving a limit of detection of 0.01 vol% at room temperature and sensitivity up to 1.9 nm vol%^-1 , demonstrating almost a fivefold increase with respect to an optimized planar geometry. Beneficially to their implementation, the self-assembly mechanism of this fractal architecture allows fabrication of micrometer-thick media over surfaces of several square centimeters in a few seconds. The designable optical features and intrinsic scalability of these photonic fractal metamaterials provide ample opportunities for applications, bridging across transformation optics, sensing, and light harvesting.

Hierarchical Metal-Organic Framework Films with Controllable Meso/Macroporosity

The structuring of the metal-organic framework material ZIF-8 as films and membranes through the vapor-phase conversion of ZnO fractal nanoparticle networks is reported. The extrinsic porosity of the resulting materials can be tuned from 4% to 66%, and the film thickness can be controlled from 80 nm to 0.23 mm, for areas >100 cm2. Freestanding and pure metal-organic frameworks (MOF) membranes prepared this way are showcased as separators that minimize capacity fading in model Li-S batteries.

Molecularly imprinted sol-gel/Au@Ag core-shell nano-urchin localized surface plasmon resonance sensor designed in reflection mode for detection of organic acid vapors

A molecularly imprinted sol-gel (MISG)/Au@Ag core-shell NU sensor is proposed for organic vapor detection in an optical fiber-based reflection mode. The compact structure design of the system in the reflection model is promising for practical use as a portable and rapid responsivity sensing probe. Volatile organic acids (OAs) are analogs to biogenetic volatile organic vapors related to specific human diseases. Here, Au@Ag core-shell nano-urchins exhibiting branched tips were synthesized and deposited on indium tin oxide (ITO) glass in small dimer and trimmer clusters to generate an enhanced electric field. A MISG solution was then spin-coated on the substrate to fabricate MISG-LSPR sensors, and three types of MISGs were developed for the detection of hexanoic acid, heptanoic acid and octanoic acid. The normalized spectral response indicated selectivity of the MISG-LSPR sensors for the corresponding template OAs. With Native Bayes and linear discriminant analysis of the sensor responses, where the latter were detected by the proposed system, single- and mixed-OA vapors could be classified into separate clusters. This signified that the proposed MISG-LSPR sensor can be applied toward pattern recognition of single vapors or multiple vapor mixtures.

Erbium emission in Er:Y2O3 decorated fractal arrays of silicon nanowires

Disordered materials with new optical properties are capturing the interest of the scientific community due to the observation of innovative phenomena. We present the realization of novel optical materials obtained by fractal arrays of silicon nanowires (NWs) synthesized at low cost, without mask or lithography processes and decorated with Er:Y2O3, one of the most promising material for the integration of erbium in photonics. The investigated structural properties of the fractal Er:Y2O3/NWs demonstrate that the fractal morphology can be tuned as a function of the sputtering deposition angle (from 5° to 15°) of the Er:Y2O3 layer. We demonstrate that by this novel approach, it is possible to simply change the Er emission intensity by controlling the fractal morphology. Indeed, we achieved the increment of Er emission at 560 nm, opening new perspectives on the control and enhancement of the optical response of novel disordered materials.

Breakthroughs in the Design of Novel Carbon-Based Metal Oxides Nanocomposites for VOCs Gas Sensing

Nowadays, the detection of volatile organic compounds (VOCs) at trace levels (down to ppb) is feasible by exploiting ultra-sensitive and highly selective chemoresistors, especially in the field of medical diagnosis. By coupling metal oxide semiconductors (MOS e.g., SnO₂, ZnO, WO₃, CuO, TiO₂ and Fe₂O₃) with innovative carbon-based materials (graphene, graphene oxide, reduced graphene oxide, single-wall and multi-wall carbon nanotubes), outstanding performances in terms of sensitivity, selectivity, limits of detection, response and recovery times towards specific gaseous targets (such as ethanol, acetone, formaldehyde and aromatic compounds) can be easily achieved. Notably, carbonaceous species, highly interconnected to MOS nanoparticles, enhance the sensor responses by (i) increasing the surface area and the pore content, (ii) favoring the electron migration, the transfer efficiency (spillover effect) and gas diffusion rate, (iii) promoting the active sites concomitantly limiting the nanopowders agglomeration; and (iv) forming nano-heterojunctions. Herein, the aim of the present review is to highlight the above-mentioned hybrid features in order to engineer novel flexible, miniaturized and low working temperature sensors, able to detect specific VOC biomarkers of a human's disease.

Non-Periodic Epsilon-Near-Zero Metamaterials at Visible Wavelengths for Efficient Non-Resonant Optical Sensing

Epsilon-near-zero (ENZ) materials offer unique properties for applications including optical clocking, nonlinear optics, and telecommunication. To date, the fabrication of ENZ materials at visible wavelengths relies mostly on the use of periodic structures, providing some manufacturing and material challenges. Here, we present the engineering of nonperiodic sodium tungsten bronzes (Na_xWO₃) metamaterials featuring ENZ properties in the visible spectrum. We showcase their use as efficient optical sensors, demonstrating a nonresonant sensing mechanism based on refractive index matching. Our optimized ENZ metamaterials display an unconventional blue-shift of the transmittance maximum to increasing refractive index of the surrounding environment, achieving sensitivity as high as 150 nm/RIU. Our theoretical and experimental investigations provide first insights on this sensing mechanism, establishing guidelines for the future engineering and implementation of efficient ENZ sensors. The unique optoelectronic properties demonstrated by this class of tunable Na_xWO₃ materials bear potential for various applications ranging from light-harvesting to optical photodetectors.

One-Step Synthesis of Porous Transparent Conductive Oxides by Hierarchical Self-Assembly of Aluminum-Doped ZnO Nanoparticles

Transparent conductive oxides (TCOs) are highly desirable for numerous applications ranging from photovoltaics to light-emitting diodes and photoelectrochemical devices. Despite progress, it remains challenging to fabricate porous TCOs (pTCOs) that may provide, for instance, a hierarchical nanostructured morphology for the separation of photoexcited hole/electron couples. Here, we present a facile process for the fabrication of porous architectures of aluminum-doped zinc oxide (AZO), a low-cost and earth-abundant transparent conductive oxide. Three-dimensional nanostructured films of AZO with tunable porosities from 10 to 98% were rapidly self-assembled from flame-made nanoparticle aerosols. Successful Al doping was confirmed by X-ray photoemission spectroscopy, high-resolution transmission electron microscopy, elemental mapping, X-ray diffraction, and Fourier transform infrared spectroscopy. An optimal Al-doping level of 1% was found to induce the highest material conductivity, while a higher amount led to partial segregation and formation of aluminum oxide domains. A controllable semiconducting to conducting behavior with a resistivity change of more than 4 orders of magnitudes from about 3 × 10² to 9.4 × 10⁶ Ω cm was observed by increasing the AZO film porosity from 10 to 98%. While the denser AZO morphologies may find immediate application as transparent electrodes, we demonstrate that the ultraporous semiconducting layers have potential as a light-driven gas sensor, showing a high response of 1.92-1 ppm of ethanol at room temperature. We believe that these tunable porous transparent conductive oxides and their scalable fabrication method may provide a highly performing material for future optoelectronic devices.

Ultrasensitive Transmissive Infrared Spectroscopy via Loss Engineering of Metallic Nanoantennas for Compact Devices

Miniaturized infrared spectroscopy is highly desired for widespread applications, including environment monitoring, chemical analysis, and biosensing. Nanoantennas, as a promising approach, feature strong field enhancement and provide opportunities for ultrasensitive molecule detection even in the nanoscale range. However, current efforts for higher sensitivities by nanogaps usually suffer a trade-off between the performance and fabrication cost. Here, novel crooked nanoantennas are designed with a different paradigm based on loss engineering to overcome the above bottleneck. Compared to the commonly used straight nanoantennas, the crooked nanoantennas feature higher sensitivity and a better fabrication tolerance. Molecule signals are increased by 25 times, reaching an experimental enhancement factor of 2.8 × 10⁴. The optimized structure enables a transmissive CO₂ sensor with sensitivities up to 0.067% ppm^-1. More importantly, such a performance is achieved without sub-100 nm structures, which are common in previous works, enabling compatibility with commercial optical lithography. The mechanism of our design can be explained by the interplay of radiative and absorptive losses of nanoantennas that obeys the coupled-mode theory. Leveraging the advantage of the transmission mode in an optical system, our work paves the way toward cheap, compact, and ultrasensitive infrared spectroscopy.

Shape-engineered silver nanocones for refractive index plasmonic nanosensors

Silver nanocones with tunable plasmon resonances and high refractive index (RI) sensitivity have attracted much attention. Herein, through systematic measuring of the RI sensitivities of silver nanocones with different geometric parameters, the size and shape effects are investigated. The results show that RI sensitivities increase as silver nanocones become longer and the widths of their heads become smaller. Through engineering of the outline symmetry, the silver nanocones exhibit RI sensitivity as high as 910 nm/RIU (RI unit) and the figure of merit arrives at 3.8.