Controlled Patterning of Plasmonic Dimers by Using an Ultrathin Nanoporous Alumina Membrane as a Shadow Mask

Plasmonic trimers designed as SERS-active chemical traps for subtyping of lung tumors

Plasmonic materials can generate strong electromagnetic fields to boost the Raman scattering of surrounding molecules, known as surface-enhanced Raman scattering. However, these electromagnetic fields are heterogeneous, with only molecules located at the 'hotspots', which account for ≈ 1% of the surface area, experiencing efficient enhancement. Herein, we propose patterned plasmonic trimers, consisting of a pair of plasmonic dimers at the bilateral sides and a trap particle positioned in between, to address this challenge. The trimer configuration selectively directs probe molecules to the central traps where 'hotspots' are located through chemical affinity, ensuring a precise spatial overlap between the probes and the location of maximum field enhancement. We investigate the Raman enhancement of the Au@Al2O3-Au-Au@Al2O3 trimers, achieving a detection limit of 10-14 M of 4-methylbenzenethiol, 4-mercaptopyridine, and 4-aminothiophenol. Moreover, single-molecule SERS sensitivity is demonstrated by a bi-analyte method. Benefiting from this sensitivity, our approach is employed for the early detection of lung tumors using fresh tissues. Our findings suggest that this approach is sensitive to adenocarcinoma but not to squamous carcinoma or benign cases, offering insights into the differentiation between lung tumor subtypes.

Open Cross-gap Gold Nanocubes with Strong, Large-Area, Symmetric Electromagnetic Field Enhancement for On-Particle Molecular-Fingerprint Raman Bioassays

Plasmonic nanoparticles with an externally open nanogap can localize the electromagnetic (EM) field inside the gap and directly detect the target via the open nanogap with surface-enhanced Raman scattering (SERS). It would be beneficial to design and synthesize the open gap nanoprobes in a high yield for obtaining uniform and quantitative signals from randomly oriented nanoparticles and utilizing these particles for direct SERS analysis. Here, we report a facile strategy to synthesize open cross-gap (X-gap) nanocubes (OXNCs) with size- and EM field-tunable gaps in a high yield. The site-specific growth of Au budding structures at the corners of the AuNC using the principle that the Au deposition rate is faster than the surface diffusion rate of the adatoms allows for a uniform X-gap formation. The average SERS enhancement factor (EF) for the OXNCs with 2.6 nm X-gaps was 1.2 × 109, and the EFs were narrowly distributed within 1 order of magnitude for ∼93% of the measured OXNCs. OXNCs consistently displayed strong EM field enhancement on large particle surfaces for widely varying incident light polarization directions, and this can be attributed to the symmetric X-gap geometry and the availability of these gaps on all 6 faces of a cube. Finally, the OXNC probes with varying X-gap sizes have been utilized in directly detecting biomolecules with varying sizes without Raman dyes. The concept, synthetic method, and biosensing results shown here with OXNCs pave the way for designing, synthesizing, and utilizing plasmonic nanoparticles for selective, quantitative molecular-fingerprint Raman sensing and imaging applications.

Exploring and Engineering 2D Transition Metal Dichalcogenides toward Ultimate SERS Performance

Surface-enhanced Raman spectroscopy (SERS) is an ultrasensitive surface analysis technique that is widely used in chemical sensing, bioanalysis, and environmental monitoring. The design of the SERS substrates is crucial for obtaining high-quality SERS signals. Recently, 2D transition metal dichalcogenides (2D TMDs) have emerged as high-performance SERS substrates due to their superior stability, ease of fabrication, biocompatibility, controllable doping, and tunable bandgaps and excitons. In this review, a systematic overview of the latest advancements in 2D TMDs SERS substrates is provided. This review comprehensively summarizes the candidate 2D TMDs SERS materials, elucidates their working principles for SERS, explores the strategies to optimize their SERS performance, and highlights their practical applications. Particularly delved into are the material engineering strategies, including defect engineering, alloy engineering, thickness engineering, and heterojunction engineering. Additionally, the challenges and future prospects associated with the development of 2D TMDs SERS substrates are discussed, outlining potential directions that may lead to significant breakthroughs in practical applications.

Probing Oxidation Mechanisms in Plasmonic Catalysis: Unraveling the Role of Reactive Oxygen Species

Plasmon-induced oxidation has conventionally been attributed to the transfer of plasmonic hot holes. However, this theoretical framework encounters challenges in elucidating the latest experimental findings, such as enhanced catalytic efficiency under uncoupled irradiation conditions and superior oxidizability of silver nanoparticles. Herein, we employ liquid surface-enhanced Raman spectroscopy (SERS) as a real-time and in situ tool to explore the oxidation mechanisms in plasmonic catalysis, taking the decarboxylation of p-mercaptobenzoic acid (PMBA) as a case study. Our findings suggest that the plasmon-induced oxidation is driven by reactive oxygen species (ROS) rather than hot holes, holding true for both the Au and Ag nanoparticles. Subsequent investigations suggest that plasmon-induced ROS may arise from hot carriers or energy transfer mechanisms, exhibiting selectivity under different experimental conditions. The observations were substantiated by investigating the cleavage of the carbon-boron bonds. Furthermore, the underlying mechanisms were clarified by energy level theories, advancing our understanding of plasmonic catalysis.

Templated Synthesis of Diamond Nanopillar Arrays Using Porous Anodic Aluminium Oxide (AAO) Membranes

Diamond nanostructures are mostly produced from bulk diamond (single- or polycrystalline) by using time-consuming and/or costly subtractive manufacturing methods. In this study, we report the bottom-up synthesis of ordered diamond nanopillar arrays by using porous anodic aluminium oxide (AAO). Commercial ultrathin AAO membranes were adopted as the growth template in a straightforward, three-step fabrication process involving chemical vapor deposition (CVD) and the transfer and removal of the alumina foils. Two types of AAO membranes with distinct nominal pore size were employed and transferred onto the nucleation side of CVD diamond sheets. Subsequently, diamond nanopillars were grown directly on these sheets. After removal of the AAO template by chemical etching, ordered arrays of submicron and nanoscale diamond pillars with ~325 nm and ~85 nm diameters were successfully released.

Significantly increased Raman enhancement enabled by hot-electron-injection-induced synergistic resonances on anisotropic ReS2 films

Two-dimensional (2D) materials are an excellent platform for surface-enhanced Raman spectroscopy (SERS). However, a poor detection sensitivity hinders their practical application. Exciton resonance (μ_ex) can improve SERS significantly by lending intensity to nearby charge-transfer resonance. Coincidentally, for ReS₂, the enhanced μ_ex can be achieved through the injection of excited-state electrons which can adjust the energy band to the SERS detection range. Moreover, ReS₂ has strong anisotropic properties, which adds an additional dimension for SERS. Therefore, ReS₂ is an ideal candidate to realize highly sensitive anisotropic SERS. In this paper, the metallic T phase of ReS₂ is introduced to the semiconducting Td phase by phase engineering. The photoinduced electron tunneling from the T phase to the Td phase can tune exciton emissions to the visible region, which effectively facilitates the photoinduced charge transfer processes. With RhB as the probe molecule, the synergistic resonance effects improve the limit of detection to 10^-9 M with the enhancement factor up to about 10⁸. Meanwhile, the obtained ultrasensitive SERS substrates also show good uniformity, stability as well as unique anisotropy. Our results open a new perspective in the improvement of the SERS performance.

Ultra-dense plasmonic nanogap arrays for reorientable molecular fluorescence enhancement and spectrum reshaping

Understanding interactions between molecular transition and intense electromagnetic fields confined by plasmon nanostructures is of great significance due to their huge potential in fundamental cavity quantum electrodynamics and practical applications. Here, we report reorientable plasmon-enhanced fluorescence leveraging the flexibilities in densely-packed gold nanogap arrays by template-assisted depositions. By finely adjusting the symmetry of the unit structure, arrays of nanogaps along two nearly-orthogonal axes can be tailored collectively with spacing down to sub-10 nm on a single chip, facilitating distinct "inter-cell" and "intra-cell" plasmon couplings. Through engineering two sets of nanogaps, the varying hybridization-induced plasmonic bonding modes lead to adjustable splitting of the fluorescence emission peak with a width up to 81 nm and narrowing of linewidths up to a factor of 3. Besides, polarization anisotropy with a ratio up to 63% is obtained on the basis of spectrally separated local hotspots with discrepant oscillation directions. The developed plasmonic nanogap array is envisaged to provide a promising chip-scale, cost-effective platform for advancing fluorescence-based detection and emission technologies in both classical and quantum regimes.

Fabrication of gold nanoparticles tethered in heat-cooled calf thymus-deoxyribonucleic acid Langmuir-Blodgett film as effective surface-enhanced Raman scattering sensing platform

SERS active substrate fabricated through self-assembly of Gold nanoparticles on the disjointed networks of Heat-cooled Calf Thymus DNA (HC-Ct DNA) Langmuir-Blodgett (LB) film has been reported. Adsorption kinetics of HC-Ct DNA molecules at the air-water interface has been studied explicitly. The UV-Vis electronic absorption spectra in conjunction with the FESEM images collectively suggest the presence of H- type aggregated domains most likely owing to plane-to-plane self-association of the HC-Ct DNA molecules aligned vertically on the surface of the LB film. Elemental composition and the morphological features of the as-prepared substrate (APS) are explored from XPS analysis and the FESEM, AFM images respectively. The SERS efficacy of the APS has been tested with trace concentrations of 4-Mercaptopyridine molecule. Finally, this SERS active substrate has also been used for the detection of malathion at ultrasensitive concentrations.

Nanoscale engineering of ring-mounted nanostructure around AAO nanopores for highly sensitive and reliable SERS substrates

Surface-enhanced Raman scattering (SERS) is recognized as one of the most favored techniques for enhancing Raman signals. The morphology of the SERS substrate profoundly affects molecular Raman spectra. This study aimed to construct a ring-mounted nanostructured substrate via liquid-liquid two-phase self-assembly incorporated with anodic aluminum oxide (AAO) membrane transfer techniques. High-density nanoparticles (NPs) assembled on AAO membranes were ascribed to reduce the diameters of the nanopores, with Au-Ag alloy NPs to regulate the dielectric constant so as to reveal the local surface plasmon resonance tunability. SERS engineered in this way allowed for the fabrication of a ring-mounted nanostructured substrate where the distribution density of NPs and dielectric constant could be independently fine-tuned. High SERS activity of the substrate was revealed by detecting the enhanced factor of crystal violet and rhodamine 6G molecules, which was up to 1.56 × 10⁶. Moreover, SERS of thiram target molecules confirmed the supersensitivity and repeatability of the substrate as a practical application. The results of this study manifested a low-cost but high-efficiency ring-mounted nanostructured SERS substrate that might be suitable in many fields, including biosensing, medical research, environmental monitoring, and optoelectronics.

Au Polyhedron Array with Tunable Crystal Facets by PVP-Assisted Thermodynamic Control and Its Sharp Shape As Well As High-Energy Exposed Planes Co-Boosted SERS Activity

A route is developed for directly growing 2D Au polyhedron arrays with controllable exposed facets of polyhedron by utilizing the substrate-supported 2D Au quasi-spherical nanoparticle arrays as the Au seed arrays, which cannot be realized by traditional lithography. In the reaction system, polyvinyl pyrrolidone (PVP) plays a vital role in guiding the reduced Au atoms and stabilizing the substrate-supported Au seeds. More importantly, by thermodynamic control, PVP as a capping agent can further direct the formation of {111} facets. The key to guarantee the integrity and periodicity of array is a proper reduction of Au ions and low growth rate of crystal. Benefiting from the higher electric field intensity near the sharp vertexes and edges of Au polyhedra and the exposed {110} facets with high energy, the Au polyhedron array with {110} facets encasing polyhedron exhibits good, stable surface enhanced Raman scattering activity toward 4-aminothiophenol among the involved arrays. The proposed fabrication approach tremendously enriches the structural diversity of Au nanoarrays on substrates and greatly overcomes the shortcoming of traditional lithography.

How gap distance between gold nanoparticles in dimers and trimers on metallic and non-metallic SERS substrates can impact signal enhancement

The impact of variation in the interparticle gaps in dimers and trimers of gold nanoparticles (AuNPs), modified with Raman reporter (2-MOTP), on surface-enhanced Raman scattering (SERS) intensity, relative to the SERS intensity of a single AuNP, is investigated in this paper. The dimers, trimers, and single particles are investigated on the surfaces of four substrates: gold (Au), aluminium (Al), silver (Ag) film, and silicon (Si) wafer. The interparticle distance between AuNPs was tuned by selecting mercaptocarboxylic acids of various carbon chain lengths when each acid forms a mixed SAM with 2-MOTP. The SERS signal quantification was accomplished by combining maps of SERS intensity from a Raman microscope, optical microscope images (×100), and maps/images from AFM or SEM. In total, we analysed 1224 SERS nanoantennas (533 dimers, 648 monomers, and 43 trimers). The average interparticle gaps were measured using TEM. We observed inverse exponential trends for the Raman intensity ratio and enhancement factor ratio versus gap distance on all substrates. Gold substrate, followed by silicon, showed the highest Raman intensity ratio (9) and dimer vs. monomer enhancement factor ratio (up to 4.5), in addition to the steepest inverse exponential curve. The results may help find a balance between SERS signal reproducibility and signal intensity that would be beneficial for future agglomerated NPs in SERS measurements. The developed method of 3 to 1 map combination by an increase in image transparency can be used to study structure-activity relationships on various substrates in situ, and it can be applied beyond SERS microscopy.

Gas-Flow-Assisted Wrinkle-Free Transfer of a Centimeter-Scale Ultrathin Alumina Membrane onto Arbitrary Substrates

The transfer of an ultrathin membrane onto arbitrary substrates is important in different practical fields. Conventional wet-transfer methods inevitably induce wrinkle defects as a result of the large contact angle of the trapped droplet between the membrane and the substrate. Here, we demonstrate a gas flow-assisted method (GFAM) to transfer centimeter (cm)-scale ultrathin membranes onto arbitrary substrates (including a curved substrate) without wrinkles. GFAM makes use of contact angle hysteresis to bulge the trapped droplet between the substrate and the ultrathin membrane and simultaneously stretch the ultrathin membrane during rapid dewetting driven by gas flow. Moreover, GFAM can be easily fulfilled by using compressed air for seconds. Compared with conventional hydrophilic treatments or organic liquid wetting, this method has no durability concern and does not change the surface nature of substrates. Taking a widely used ultrathin anodic aluminum oxide (AAO) membrane as an example, we successfully demonstrate the application of a large-area wrinkle-free ultrathin AAO membrane to defect-free ordered nanostructure array fabrication and investigate the micro-scale details of macro-scale wrinkles generated by the conventional ways. In addition, its corresponding superiority over the defective counterpart is further studied in optical sensing. This method is highly valuable for promoting the simplicity of large-area ultrathin membrane transfer in practice.

Recent Advances in Nanoporous Anodic Alumina: Principles, Engineering, and Applications

The development of aluminum anodization technology features many stages. With the story stretching for almost a century, rather straightforward-from current perspective-technology, raised into an iconic nanofabrication technique. The intrinsic properties of alumina porous structures constitute the vast utility in distinct fields. Nanoporous anodic alumina can be a starting point for: Templates, photonic structures, membranes, drug delivery platforms or nanoparticles, and more. Current state of the art would not be possible without decades of consecutive findings, during which, step by step, the technique was more understood. This review aims at providing an update regarding recent discoveries-improvements in the fabrication technology, a deeper understanding of the process, and a practical application of the material-providing a narrative supported with a proper background.

Hotspots on the Move: Active Molecular Enrichment by Hierarchically Structured Micromotors for Ultrasensitive SERS Sensing

Surface-enhanced Raman scattering (SERS) is recognized as one of the most sensitive spectroscopic tools for chemical and biological detections. Hotspots engineering has expedited promotion of SERS performance over the past few decades. Recently, molecular enrichment has proven to be another effective approach to improve the SERS performance. In this work, we propose a concept of "motile hotspots" to realize ultrasensitive SERS sensing by combining hotspots engineering and active molecular enrichment. High-density plasmonic nanostructure-supporting hotspots are assembled on the tubular outer wall of micromotors via nanoimprint and rolling origami techniques. The dense hotspots carried on these hierarchically structured micromotors (HSMs) can be magnet-powered to actively enrich molecules in fluid. The active enrichment manner of HSMs is revealed to be effective in accelerating the process of molecular adsorption. Consequently, SERS intensity increases significantly because of more molecules being adjacent to the hotspots after active molecular enrichment. This "motile hotspots" concept provides a synergistical approach in constructing a SERS platform with high performance. Moreover, the newly developed construction method of HSMs manifests the possibility of tailoring tubular length and diameter as well as surface patterns on the outer wall of HSMs, demonstrating good flexibility in constructing customized micromotors for various applications.

Twin-ZnSe nanowires as surface enhanced Raman scattering substrate with significant enhancement factor upon defect

Semiconductor-based surface enhanced Raman scattering (SERS) substrate design has attracted much interest due to the excellent photoelectronic and biochemical properties. The structural change caused by twin in semiconductor will have an influence on improving the Raman signals enhancement based on the chemical mechanism (CM). Here, we demonstrated the twin in semiconductor ZnSe nanowires as an ultrasensitive CM-based SERS platform. The SERS signals of the rhodamine 6G (R6G) and crystal violet (CV) molecules adsorbed on twin-ZnSe nanowires could be easily detected even with an ultralow concentration of 10^-11 M and 10^-8 M, respectively, and the corresponding enhancement factor (EF) were up to 6.12 × 10⁷ and 3.02 × 10⁵, respectively. In addition, the charge transfer (CT) between the twin-ZnSe nanowires and R6G molecule has been demonstrated theoretically with first-principles calculations based on density-functional theory (DFT). These results demonstrated the proposed ZnSe nanowires with twin as SERS substrate has a broader application in the field of biochemical sensing.

Fabrication of a Silica-Silica Nanoparticle Monolayer Array Nanocomposite Film on an Anodic Aluminum Oxide Substrate and Its Optical and Tribological Properties

The fabrication and properties of silica nanoparticle monolayer arrays (SNMAs) immobilized on silica films on nanoporous anodic aluminum oxide (AAO) substrates by polymerization of silicic acid and a two-step spin-coating technique are reported. Reflection spectra of the obtained silica-SNMA nanocomposite films on AAO substrates were almost the same as those of the original AAO substrate. The coefficient of friction at an applied load of 0.98 N under dry conditions for a film fabricated under optimal conditions was significantly decreased by 76% with respect to that without a silica-SNMA nanocomposite film on an AAO substrate. The results also showed a lower coefficient of friction than that for MoS₂ nanoparticles (commonly used for self-lubricating films) deposited on an AAO substrate. We demonstrate that the silica-SNMA nanocomposite film with an optimal nanoroughness, thickness, and wear resistance can be used as a novel coating film for AAO substrates with both a high color degree of freedom and a low coefficient of friction at a high applied load (ca. 1 N).

Large Scale Fabrication of Ordered Gold Nanoparticle-Epoxy Surface Nanocomposites and Their Application as Label-Free Plasmonic DNA Biosensors

A robust and scalable technology to fabricate ordered gold nanoparticle arrangements on epoxy substrates is presented. The nanoparticles are synthesized by solid-state dewetting on nanobowled aluminum templates, which are prepared by the selective chemical etching of porous anodic alumina (PAA) grown on an aluminum sheet with controlled anodic oxidation. This flexible fabrication technology provides proper control over the nanoparticle size, shape, and interparticle distance over a large surface area (several cm²), which enables the fine-tuning and optimization of their plasmonic absorption spectra for LSPR and SERS applications between 535 and 625 nm. The nanoparticles are transferred to the surface of epoxy substrates, which are subsequently selectively etched. The resulting nanomushrooms arrangements consist of ordered epoxy nanopillars with flat, disk-shaped nanoparticles on top, and their bulk refractive index sensitivity is between 83 and 108 nm RIU^-1. Label-free DNA detection is successfully demonstrated with the sensors by using a 20 base pair long specific DNA sequence from the parasite Giardia lamblia. A red-shift of 6.6 nm in the LSPR absorbance spectrum was detected after the 2 h hybridization with 1 μM target DNA, and the achievable LOD was around 5 nM. The reported plasmonic sensor is one of the first surface AuNP/polymer nanocomposites ever reported for the successful label-free detection of DNA.

Controlling Gold Electrodeposition on Porous Polymeric Templates Produced by the Breath-Figure Method: Fabrication of SERS-Active Surfaces

Porous structured surfaces decorated with gold nanoparticles can be fabricated in an economical two-step process. Sub-micrometric porous polymethyl methacrylate (PMMA) templates are formed by using the breath-figure method (BFM) with a single-step spin-coating onto a conducting substrate to result in a set of delimited microdomains for gold electrodeposition. By controlling electrochemical parameters, distinct morphologies are engendered, among them, nanoparticles that present useful local Raman enhancement in the order of up to 10⁶ with stability of a solid-phase platform and characteristic chemical resistance of gold. The spatial confinement of metallic structures resulted in electromagnetic field enhancement, here compared to flat metallic surfaces where contributions of area effect and fluorescence quenching are responsible for a significant apparent enhancement to Raman spectra that has not been frequently reported to date.

Recent advances in cancer bioimaging using a rationally designed Raman reporter in combination with plasmonic gold

Gold nanomaterials (AuNMs) have been widely implemented for the purpose of bioimaging of cancer and tumor cells in combination with Raman spectral markers.

Arrays of Plasmonic Nanoparticle Dimers with Defined Nanogap Spacers

Plasmonic molecules are building blocks of metallic nanostructures that give rise to intriguing optical phenomena with similarities to those seen in molecular systems. The ability to design plasmonic hybrid structures and molecules with nanometric resolution would enable applications in optical metamaterials and sensing that presently cannot be demonstrated, because of a lack of suitable fabrication methods allowing the structural control of the plasmonic atoms on a large scale. Here we demonstrate a wafer-scale "lithography-free" parallel fabrication scheme to realize nanogap plasmonic meta-molecules with precise control over their size, shape, material, and orientation. We demonstrate how we can tune the corresponding coupled resonances through the entire visible spectrum. Our fabrication method, based on glancing angle physical vapor deposition with gradient shadowing, permits critical parameters to be varied across the wafer and thus is ideally suited to screen potential structures. We obtain billions of aligned dimer structures with controlled variation of the spectral properties across the wafer. We spectroscopically map the plasmonic resonances of gold dimer structures and show that they not only are in good agreement with numerically modeled spectra, but also remain functional, at least for a year, in ambient conditions.

Three-Dimensional Microtubular Devices for Lab-on-a-Chip Sensing Applications

The rapid advance of micro-/nanofabrication technologies opens up new opportunities for miniaturized sensing devices based on novel three-dimensional (3D) architectures. Notably, microtubular geometry exhibits natural advantages for sensing applications due to its unique properties including the hollow sensing channel, high surface-volume ratio, well-controlled shape parameters and compatibility to on-chip integration. Here the state-of-the-art sensing techniques based on microtubular devices are reviewed. The developed microtubular sensors cover microcapillaries, rolled-up nanomembranes, chemically synthesized tubular arrays, and photoresist-based tubular structures via 3D printing. Various types of microtubular sensors working in optical, electrical, and magnetic principles exhibit an extremely broad scope of sensing targets including liquids, biomolecules, micrometer-sized/nanosized objects, and gases. Moreover, they have also been applied for the detection of mechanical, acoustic, and magnetic fields as well as fluorescence signals in labeling-based analyses. At last, a comprehensive outlook of future research on microtubular sensors is discussed on pushing the detection limit, extending the functionality, and taking a step forward to a compact and integrable core module in a lab-on-a-chip analytical system for understanding fundamental biological events or performing accurate point-of-care diagnostics.

Plasmon-coupled charge transfer in WO3-x semiconductor nanoarrays: toward highly uniform silver-comparable SERS platforms

Transition metal oxide semiconductors have been explored in surface-enhanced Raman scattering (SERS) active substrates, yet their detection sensitivity and enhancement effects are inferior. What's more, the reported fabrication technique ignored the effects of the electromagnetic mechanisms and was far from satisfactory for practical applications. Herein, we report on a convenient nanotechnique to fabricate large-area hexagon plum-blossom-like WO3-x nanoarrays based on aluminum nanobowl array substrates. Localized surface plasmon resonance can be increased via adjusting the time of tungsten magnetron sputtering with H2 annealing treatment. The introduction of a double-switch experiment demonstrates that localized surface plasmon-coupled photoinduced charge transfer can not only increase SERS enhancement comparable to similar silver nanostructures but also implement a low limit of detection below 10-9 M. A triple-switch experiment offers specific rules in the molecular detection of WO3-x semiconductors and important guidance for the fabrication of SERS-active semiconducting platforms.

Large-scale flexible metal-covered polymer nanopillar arrays as highly uniform and reproducible SERS substrates for trace analysis

While surface-enhanced Raman scattering (SERS) spectroscopy has been studied extensively due to its sensitivity to molecular vibration states, the fabrication of SERS substrates with homogeneous activity over a large area still remains difficult. Here, a facial fabrication of large-scale metal-covered polymer nanopillar arrays used as reliable SERS substrates was developed by nano-pouring process using porous alumina membranes. Our strategy integrates the advantages of low-cost production, high reproducibility and good biocompatibility because the flexible transparent polymer nanopillars are nondestructive to biomaterial surfaces. The as-prepared SERS substrates exhibit highly uniform Raman signals with low relative standard deviation values of 4% at 1169 cm^-1 of CV and 8.1% at 1355 cm^-1 of R6G, both with the concentration of 10^-5 M. The results also show that more than 95% intensity within the limits of the ±10% deviation of the average intensity of signals, indicating excellent stability of SERS signals. In addition, the mechanically flexible substrates can be attached to surfaces with complex morphologies and the good transparency makes the excitation and collection of signals from the backside of the substrates possible, showing great potential in analytical chemistry, food safety, medical diagnostics and other practical SERS detections.

Arrays of highly complex noble metal nanostructures using nanoimprint lithography in combination with liquid-phase epitaxy

Current best-practice lithographic techniques are unable to meet the functional requirements needed to enable on-chip plasmonic devices capable of fully exploiting nanostructure properties reliant on a tailored nanostructure size, composition, architecture, crystallinity, and placement. As a consequence, numerous nanofabrication methods have emerged that address various weaknesses, but none have, as of yet, demonstrated a large-area processing route capable of defining organized surfaces of nanostructures with the architectural diversity and complexity that is routinely displayed in colloidal syntheses. Here, a hybrid fabrication strategy is demonstrated in which nanoimprint lithography is combined with templated dewetting and liquid-phase syntheses that is able to realize periodic arrays of complex noble metal nanostructures over square centimeter areas. The process is inexpensive, can be carried out on a benchtop, and requires modest levels of instrumentation. Demonstrated are three fabrication schemes yielding arrays of core-shell, core-void-shell, and core-void-nanoframe structures using liquid-phase syntheses involving heteroepitaxial deposition, galvanic replacement, and dealloying. With the field of nanotechnology being increasingly reliant on the engineering of desirable physicochemical responses through architectural control, the fabrication strategy provides a platform for advancing devices reliant on addressable arrays or the collective response from an ensemble of identical nanostructures.

Periodic Porous Alloyed Au-Ag Nanosphere Arrays and Their Highly Sensitive SERS Performance with Good Reproducibility and High Density of Hotspots

Periodic porous alloyed Au-Ag nanosphere (NS) arrays with different periodic lengths and tunable composition ratios were prepared on Si substrates on a large scale (∼cm²) using stepwise metal deposition-annealing and subsequent chemical corrosion from a monolayer of colloidal polystyrene (PS) microspheres as the initial template. The porous alloyed Au-Ag NSs possessed a high porosity and bicontinuous morphology composed of hierarchically interconnected ligaments, which were obtained from an optimized dealloying process in nitric acid. Interestingly, when the dealloying time was prolonged, the average size of the porous alloyed NSs slightly decreased, and the width of the ligaments gradually increased. The periodic length of the array could be facilely changed by controlling the initial particle size of the PS template. Moreover, the porous alloyed Au-Ag NS arrays were explored as a platform for the surface-enhanced Raman scattering (SERS) detection of 4-aminothiophenol (4-ATP) and exhibited excellent reproducibility and high sensitivity because of the periodic structure of the arrays and the abundance of inherent "hotspots". After optimization experiments, a low concentration of 10^-10 M 4-ATP could be detected on these porous Au-Ag NS array substrates. Such highly reproducible SERS activity is meaningful for improving the practical application of portable Raman detection equipment.

VO2 /TiN Plasmonic Thermochromic Smart Coatings for Room-Temperature Applications

Vanadium dioxide/titanium nitride (VO₂ /TiN) smart coatings are prepared by hybridizing thermochromic VO₂ with plasmonic TiN nanoparticles. The VO₂ /TiN coatings can control infrared (IR) radiation dynamically in accordance with the ambient temperature and illumination intensity. It blocks IR light under strong illumination at 28 °C but is IR transparent under weak irradiation conditions or at a low temperature of 20 °C. The VO₂ /TiN coatings exhibit a good integral visible transmittance of up to 51% and excellent IR switching efficiency of 48% at 2000 nm. These unique advantages make VO₂ /TiN promising as smart energy-saving windows.