Surface-Enhanced Infrared Absorption Using Individual Cross Antennas Tailored to Chemical Moieties

Mid-infrared enhanced spectrochemical detection using azide vibrational probes

Spectrochemical analysis of trace elements in complex matrices is crucial across various fields of science, industry, and technology. However, this analysis is often hindered by background interference and the challenge of detecting ultralow analyte concentrations. Surface Enhanced Infrared Absorption (SEIRA) spectroscopy is emerging as a viable technique to address these challenges as it can successfully reveal soluble and unmodified analytes in a label-free manner through their interactions with a bioreceptor following site-specific labeling with small infrared-active probes. In this study, we present and demonstrate an advanced method for mid-infrared spectroscopy utilizing a pixeled SEIRA substrate coupled with a peculiar infrared-active vibrational probe. We select a small azide moiety as the vibrational tag since its signature around 2100 cm-1 is in the cell- and protein-silent window and its small size preserves the structure and biological function of the protein it integrates into. As model bioreceptor, we utilize an antigen-binding fragment (Fab') derived from the therapeutic antibody trastuzumab, modified with azidoacetic acid, and its Her2 antigen as the soluble analyte. Employing mid-infrared SEIRA spectroscopy, we are able to monitor the immobilization of the azide-modified Fab', and demonstrate the detection of analyte quantities as low as 83 amol within an area of 100 μm2.

On-chip phonon-enhanced IR near-field detection of molecular vibrations

Phonon polaritons - quasiparticles formed by strong coupling of infrared (IR) light with lattice vibrations in polar materials - can be utilized for surface-enhanced infrared absorption (SEIRA) spectroscopy and even for vibrational strong coupling with nanoscale amounts of molecules. Here, we introduce and demonstrate a compact on-chip phononic SEIRA spectroscopy platform, which is based on an h-BN/graphene/h-BN heterostructure on top of a metal split-gate creating a p-n junction in graphene. The metal split-gate concentrates the incident light and launches hyperbolic phonon polaritons (HPhPs) in the heterostructure, which serves simultaneously as SEIRA substrate and room-temperature infrared detector. When thin organic layers are deposited directly on top of the heterostructure, we observe a photocurrent encoding the layer's molecular vibrational fingerprint, which is strongly enhanced compared to that observed in standard far-field absorption spectroscopy. A detailed theoretical analysis supports our results, further predicting an additional sensitivity enhancement as the molecular layers approach deep subwavelength scales. Future on-chip integration of infrared light sources such as quantum cascade lasers or even electrical generation of the HPhPs could lead to fully on-chip phononic SEIRA sensors for molecular and gas sensing.

Surface-enhanced optical-mid-infrared photothermal microscopy using shortened colloidal silver nanowires: a noble approach for mid-infrared surface sensing

We propose surface-enhanced optical-mid-infrared photothermal (MIP) microscopy using highly crystalline silver nanowires, acting as a Fabry-Perot resonator, and demonstrate its applicability to enhanced mid-infrared surface sensing of thin polymer layers as thin as 20 nm.

Surface Enhanced Infrared Absorption Using Single Conducting Polymer Antennas

Infrared absorption provides the intrinsic vibrational information on chemical bonds, which is important for identifying molecular moieties. To enhance the sensitivity of infrared absorption, plasmonic antennas have been widely used to localize and concentrate mid-infrared light into nanometer-scale hotspots at desired wavelengths. Here, instead of inorganic plasmonic antennas, we have demonstrated surface-enhanced infrared absorption (SEIRA) using single plasmonic antennas based on a conducting polymer. With commercially available PEDOT:PSS (poly(ethylenedioxythiophene):poly(styrenesulfonate)), the organic plasmonic antennas are in the fashion of single PEDOT:PSS micropillars. The plasmonic resonance of single PEDOT:PSS micropillar antennas can be easily tuned by the micropillar diameter or by the interantenna gap across the mid-infrared frequencies. These organic plasmonic antennas show the ability to enhance the molecular vibrations of CBP (4,4'-bis(N-carbazolyl)-1,1'-biphenyl) molecules with a thickness of about 50 nm, illustrating the good SEIRA sensitivity (with SEIRA sensitivity up to ∼7800) at the single antenna level. Our findings provide another material choice for mid-infrared plasmonic antennas toward SEIRA applications.

Oxygen Vacancies Alter Methanol Oxidation Pathways on NiOOH

A thorough comprehension of the mechanism underlying the methanol oxidation reaction (MOR) on Ni-based catalysts is critical for future electrocatalytic design and development. However, the mechanism of MOR on these materials remains a matter of controversy. Herein, we combine in situ surface-enhanced infrared absorption spectroscopy (SEIRAS) and density functional theory (DFT) calculations to identify the active sites and determine the mechanism of MOR on monometallic Ni-based catalysts in alkaline media. The SEIRAS results show that formate and (bi)carbonate are formed after the commencement of the MOR with potential-dependent relative distributions. These spectroscopic results are in good agreement with the DFT-computed reaction profiles over an oxygen vacancy, suggesting that the MOR mainly proceeds through the formate-involving pathway, in which the early consumption of methanol yields formate as the major product, while increasing potential drives further oxidation of formate to (bi)carbonate. We also find a parallel pathway for the generation of (bi)carbonate at high potentials that bypasses the formation of formate. The two main pathways are thermodynamically more feasible than the one predominantly reported in the literature for MOR on NiOOH that involves CHO and/or CO as key intermediates. These DFT results are supported by spectroscopic evidence showing that no band associated with CHO or CO can be detected by SEIRAS, which is attributed to the nature of the oxygen vacancies as the active sites, suppressing deep dehydrogenation of CH2O to CHO. This work thus shows the promising role of defect engineering in promoting the electrocatalytic MOR activity and selectivity.

Plasmonic nanosensors for pharmaceutical and biomedical analysis

The detection and identification of clinical biomarkers with related sensitivity have become a source of considerable concern for biomedical analysis. There have been increasing efforts toward the development of single-molecule analytical platforms to overcome this concern. The latest developments in plasmonic nanomaterials include fascinating advances in energy, catalyst chemistry, optics, biotechnology, and medicine. Nanomaterials can be successfully applied to biomolecule and drug detection in plasmonic nanosensors for pharmaceutical and biomedical analysis. Plasmonic-based sensing technology exhibits high sensitivity and selectivity depending on surface plasmon resonance (SPR) or localized surface plasmon resonance (LSPR) phenomena. In this critical paper, we offer an overview of the methodology of the SPR, LSPR, surface-enhanced Raman scattering (SERS), surface-enhanced infrared absorption (SEIRA), surface-enhanced fluorescence (SEF), and plasmonic nanoplatforms advanced for pharmaceutical and biomedical applications. First of all, we present here a brief discussion of the above trends. We have devoted the last section to the explanation of SPR, LSPR, SERS, SEIRA, and SEF platforms, which have found a wide range of applications, and reviewed recent advances for biomedical and pharmaceutical analysis.

A super asymmetric cross antenna structure with tunable dual-frequency resonances

The detection performance of traditional infrared spectroscopy can be very limited in the case of molecular vibrational modes with low absorption cross-sections. On account of its electric field enhancement, plasmonic antenna can be combined with infrared spectroscopy to realize surface enhanced infrared detection and characterization of molecules. In this work, a super asymmetric cross antenna structure with tunable dual-frequency resonance and a high enhancement factor is designed. By systematically studying the transmission spectrum and charge distribution of this super asymmetric cross antenna structure, the physical origin of the dual-frequency resonance and its tunability are characterized in detail. In addition, in order to target desired molecular ensembles, the relationship between the resonance frequency and electric-field intensity of the two resonance modes and the parameters of structure and incident light are examined, yielding an enhancement factor close to 100 in the desired frequency region. Finally, the experimental results show that the proposed super asymmetric cross antenna structure can indeed generate dual-frequency resonances, agreeing reasonably with the theoretical results. It is believed that the super asymmetric cross antenna structure can be widely used to sensitively detect trace molecules, and in monolayered chemistry and bio-molecules, allowing their structures and dynamics to be studied using nonlinear infrared spectroscopy.

Over-coupled resonator for broadband surface enhanced infrared absorption (SEIRA)

Detection of molecules is a key issue for many applications. Surface enhanced infrared absorption (SEIRA) uses arrays of resonant nanoantennas with good quality factors which can be used to locally enhance the illumination of molecules. The technique has proved to be an effective tool to detect small amount of material. However, nanoresonators can detect molecules on a narrow bandwidth so that a set of resonators is necessary to identify a molecule fingerprint. Here, we introduce an alternative paradigm and use low quality factor resonators with large radiative losses (over-coupled resonators). The bandwidth enables to detect all absorption lines between 5 and 10 μm, reproducing the molecular absorption spectrum. Counterintuitively, despite a lower quality factor, the system sensitivity is improved and we report a reflectivity variation as large as one percent per nanometer of molecular layer of PMMA. This paves the way to specific identification of molecules. We illustrate the potential of the technique with the detection of the explosive precursor 2,4-dinitrotoluene (DNT). There is a fair agreement with electromagnetic simulations and we also introduce an analytic model of the SEIRA signal obtained in the over-coupling regime.

Metasurface-Enhanced Infrared Spectroscopy: An Abundance of Materials and Functionalities

Infrared spectroscopy provides unique information on the composition and dynamics of biochemical systems by resolving the characteristic absorption fingerprints of their constituent molecules. Based on this inherent chemical specificity and the capability for label-free, noninvasive, and real-time detection, infrared spectroscopy approaches have unlocked a plethora of breakthrough applications for fields ranging from environmental monitoring and defense to chemical analysis and medical diagnostics. Nanophotonics has played a crucial role for pushing the sensitivity limits of traditional far-field spectroscopy by using resonant nanostructures to focus the incident light into nanoscale hot-spots of the electromagnetic field, greatly enhancing light-matter interaction. Metasurfaces composed of regular arrangements of such resonators further increase the design space for tailoring this nanoscale light control both spectrally and spatially, which has established them as an invaluable toolkit for surface-enhanced spectroscopy. Starting from the fundamental concepts of metasurface-enhanced infrared spectroscopy, a broad palette of resonator geometries, materials, and arrangements for realizing highly sensitive metadevices is showcased, with a special focus on emerging systems such as phononic and 2D van der Waals materials, and integration with waveguides for lab-on-a-chip devices. Furthermore, advanced sensor functionalities of metasurface-based infrared spectroscopy, including multiresonance, tunability, dielectrophoresis, live cell sensing, and machine-learning-aided analysis are highlighted.

Honeycomb-like aluminum antennas for surface-enhanced infrared absorption sensing

Surface-enhanced infrared absorption (SEIRA) spectroscopy is a competent method to detect trace quantity of molecules and even protein conformational flexibility by enhancing their vibrational modes. To improve the spectroscopy features, we propose a surface with honeycomb-like (HC) arrangement of aluminum equilateral triangles within a metal-insulator-metal configuration. With adjustable geometric parameters, the HC nanoantennas allow a tunable and wide spectral coverage in the IR. The reflectance measurements correlate extremely well with the numerical simulations. Being compact and insensitive to the light polarization, the HC are appealing for boosting the signal-to-noise ratio and increasing the number of hotspots as required for sensing applications. These nanoantennas are thus suitable for accurate and broadband SEIRA sensing via a spectral overlap between the large plasmonic resonances and the narrow IR vibrational modes of our analyte (vanillin). In line with our previously studied bowties nanoantennas, we demonstrate, using HC, SEIRA enhancement factors greater than 107 achieved at a tuning ratio below 1 stating the best spectral overlap. Around 104 molecules are sensed per HC tip. The investigation results are matching the best-reported SEIRA studies. These findings pave the way toward sensitive, adaptable, and miniaturized IR spectroscopy devices for vital applications like biosensing and environmental monitoring.

Nanointerfaces: Concepts and Strategies for Optical and X-ray Spectroscopic Characterization

Interfaces at the nanoscale, also called nanointerfaces, play a fundamental role in physics and chemistry. Probing the chemical and electronic environment at nanointerfaces is essential in order to elucidate chemical processes relevant for applications in a variety of fields. Many spectroscopic techniques have been applied for this purpose, although some approaches are more appropriate than others depending on the type of the nanointerface and the physical properties of the different phases. In this Perspective, we introduce the major concepts to be considered when characterizing nanointerfaces. In particular, the interplay between the characteristic length of the nanointerfaces, and the probing and information depths of different spectroscopy techniques is discussed. Differences between nano- and bulk interfaces are explained and illustrated with chosen examples from optical and X-ray spectroscopies, focusing on solid-liquid nanointerfaces. We hope that this Perspective will help to prepare spectroscopic characterization of nanointerfaces and stimulate interest in the development of new spectroscopic techniques adapted to the nanointerfaces.

Highly sensitive terahertz fingerprint sensing based on the quasi-guided modes in a distorted photonic lattice

Using photonic structures resonating at the characteristic absorption frequency of the target molecules is a widely-adopted approach to enhance the absorption and improve the sensitivity in many spectral regions. Unfortunately, the requirement of accurate spectral matching poses a big challenge for the structure fabrication, while active tuning of the resonance for a given structure using external means like the electric gating significantly complicates the system. In this work, we propose to circumvent the problem by making use of quasi-guided modes which feature both ultra-high Q factors and wavevector-dependent resonances over a large operating bandwidth. These modes are supported in a distorted photonic lattice, whose band structure is formed above the light line due to the band-folding effect. The advantage and flexibility of this scheme in terahertz sensing are elucidated and exemplified by using a compound grating structure on a silicon slab waveguide to achieve the detection of a nanometer scale α-lactose film. The spectral matching between the leaky resonance and the α-lactose absorption frequency at 529.2 GHz by changing the incident angle is demonstrated using a flawed structure which exhibits a detuned resonance at normal incidence. Based on the high dependence of the transmittance at the resonance on the thickness of α-lactose, our results show it is possible to achieve an exclusive detection of α-lactose with the effective sensing of thickness as small as 0.5 nm.

Direct Determination of Plasmon Enhancement Factor and Penetration Depths in Surface Enhanced IR Absorption Spectroscopy

Surface Enhanced Infrared Absorption Spectroscopy (SEIRAS) is a powerful tool for studying a wide range of surface and electrochemical phenomena. For most electrochemical experiments the evanescent field of an IR beam partially penetrates through a thin metal electrode deposited on top of an attenuated total reflection (ATR) crystal to interact with molecules of interest. Despite its success, a major problem that complicates quantitative interpretation of the spectra from this method is the ambiguity of the enhancement factor due to plasmon effects in metals. We developed a systematic method for measuring this, which relies upon independent determination of surface coverage by Coulometry of a surface-bound redox-active species. Following that, we measure the SEIRAS spectrum of the surface bound species, and from the knowledge of surface coverage, retrieve the effective molar absorptivity, ε_SEIRAS. Comparing this to the independently determined bulk molar absorptivity leads us to the enhancement factor f = ε_SEIRAS/ε_bulk. We report enhancement factors in excess of 1000 for the C-H stretches of surface bound ferrocene molecules. We additionally developed a methodical approach to measure the penetration depth of the evanescent field from the metal electrode into a thin film. Such systematic measure of the enhancement factor and penetration depth will help SEIRAS advance from a qualitative to a more quantitative method.

Controlling Water Delivery to an Electrochemical Interface with Surfactants

Most electrochemical reactions require delivery of protons, often from water, to surface-adsorbed species. However, water also acts as a competitor to many such processes by directly reacting with the electrode, which necessitates using water in small amounts. Controlling the water content and structure near the surface is an important frontier in directing the reactivity and selectivity of electrochemical reactions. Surfactants accumulate near surfaces, and therefore, they can be used as agents to control interfacial water. Using mid-IR spectro-electrochemistry, we show that a modest concentration (1 mM) of the cationic surfactant CTAB in mixtures of 10 M water in an organic solvent (dDMSO) has a large effect on the interfacial water concentration, changing it by up to ∼35% in the presence of an applied potential. The major cause of water content change is displacement due to the accumulation or depletion of surfactants driven by potential. Two forces drive the surfactants to the electrode: the applied potential and the hydrophobic interactions with the water in the bulk. We have quantified their competition by varying the water content in the bulk. To our knowledge, for the first time, we have identified the electrochemical equivalent of the hydrophobic drive. For our system, a change in applied potential of 1 V has the same effect as adding a 0.55 mole fraction of water to the bulk. This work illustrates the significance of surfactants in the partitioning of water between the bulk and the surface and paves the way toward engineering interfacial water structures for controlling electrochemical reactions.

Epsilon-near-zero substrate-enabled strong coupling between molecular vibrations and mid-infrared plasmons

As the strong light-matter interaction between molecular vibrations and mid-infrared optical resonant modes, vibrational strong coupling (VSC) has the potential to modify the intrinsic chemistry of molecules, leading to the control of ground-state chemical reactions. Here, by using quartz as an epsilon-near-zero (ENZ) substrate, we have realized VSC between organic molecular vibrations and mid-infrared plasmons on metallic antennas. The ENZ substrate enables sharp mid-infrared plasmonic resonances (Q factor ∼50) which efficiently couple to the molecular vibrations of polymethyl methacrylate (PMMA) molecules with prominent mode splitting. The coupling strength is proportional to the square root of the thickness of the PMMA layer and reaches the VSC regime with a thickness of ∼300 nm. The coupling strength also depends on the polarization of the incident light, illustrating an additional way to control the molecule-plasmon coupling. Our findings provide a new, to the best of our knowledge, possibility to realize VSC with metallic antennas and pave the way to increase the sensitivity of molecular vibrational spectroscopy.

Plasmonic Hybrid Nanostructures in Photocatalysis: Structures, Mechanisms, and Applications

(Sun)Light is an abundantly available sustainable source of energy that has been used in catalyzing chemical reactions for several decades now. In particular, studies related to the interaction of light with plasmonic nanostructures have been receiving increased attention. These structures display the unique property of localized surface plasmon resonance, which converts light of a specific wavelength range into hot charge carriers, along with strong local electromagnetic fields, and/or heat, which may all enhance the reaction efficiency in their own way. These unique properties of plasmonic nanoparticles can be conveniently tuned by varying the metal type, size, shape, and dielectric environment, thus prompting a research focus on rationally designed plasmonic hybrid nanostructures. In this review, the term "hybrid" implies nanomaterials that consist of multiple plasmonic or non-plasmonic materials, forming complex configurations in the geometry and/or at the atomic level. We discuss the synthetic techniques and evolution of such hybrid plasmonic nanostructures giving rise to a wide variety of material and geometric configurations. Bimetallic alloys, which result in a new set of opto-physical parameters, are compared with core-shell configurations. For the latter, the use of metal, semiconductor, and polymer shells is reviewed. Also, more complex structures such as Janus and antenna reactor composites are discussed. This review further summarizes the studies exploiting plasmonic hybrids to elucidate the plasmonic-photocatalytic mechanism. Finally, we review the implementation of these plasmonic hybrids in different photocatalytic application domains such as H₂ generation, CO₂ reduction, water purification, air purification, and disinfection.

Classical Model of Surface Enhanced Infrared Absorption (SEIRA) Spectroscopy

The molecule-plasmon interaction is the key to the mechanisms of surface enhanced infrared absorption (SEIRA) and surface enhanced Raman scattering (SERS). Since plasmons are well described by Maxwell's equations, one fundamental treatment involves the classical interpretation of infrared absorption and resonance Raman spectroscopies. We can understand the molecule-plasmon interaction using electromagnetic theory if the classical field effect on a transition dipole moment or transition polarizability is properly described. In previous work, we derived the Raman excitation profile of a model molecule using a classical driven spring attached to a charged mass with a perturbative force constant due to vibrational oscillations. In this study we generalize the interactions of plasmons with molecules by considering the N₂O asymmetric stretch SEIRA signal on a Dy doped CdO (CdO:Dy) film. This semiconductor has tunable plasmon dispersion curves throughout the near-and mid-infrared that can interact directly with vibrational absorption transitions. We have demonstrated this using the Kretschmann configuration with a CaF₂ prism and a MgO substrate. The model predicts the phase behavior of SEIRA. The calculated enhancement factor relative to an Au control is 6.2, in good agreement with the value of 6.8 ± 0.5 measured under the same conditions.

Plasmonic Resonant Nanoantennas Induce Changes in the Shape and the Intensity of Infrared Spectra of Phospholipids

Surface enhanced infrared absorption spectroscopic studies (SEIRAS) as a technique to study biological molecules in extremely low concentrations is greatly evolving. In order to use the technique for identification of the structure and interactions of such biological molecules, it is necessary to identify the effects of the plasmonic electric-field enhancement on the spectral signature. In this study the spectral properties of 1,2-Dipalmitoyl-sn-glycero-3 phosphothioethanol (DPPTE) phospholipid immobilized on gold nanoantennas, specifically designed to enhance the vibrational fingerprints of lipid molecules were studied. An AFM study demonstrates an organization of the DPPTE phospholipid in bilayers on the nanoantenna structure. The spectral data were compared to SEIRAS active gold surfaces based on nanoparticles, plain gold and plain substrate (Si) for different temperatures. The shape of the infrared signals, the peak positions and their relative intensities were found to be sensitive to the type of surface and the presence of an enhancement. The strongest shifts in position and intensity were seen for the nanoantennas, and a smaller effect was seen for the DPPTE immobilized on gold nanoparticles. This information is crucial for interpretation of data obtained for biological molecules measured on such structures, for future application in nanodevices for biologically or medically relevant samples.

Monitoring the effects of chemical stimuli on live cells with metasurface-enhanced infrared reflection spectroscopy

Infrared spectroscopy has found wide applications in the analysis of biological materials. A more recent development is the use of engineered nanostructures - plasmonic metasurfaces - as substrates for metasurface-enhanced infrared reflection spectroscopy (MEIRS). Here, we demonstrate that strong field enhancement from plasmonic metasurfaces enables the use of MEIRS as a highly informative analytic technique for real-time monitoring of cells. By exposing live cells cultured on a plasmonic metasurface to chemical compounds, we show that MEIRS can be used as a label-free phenotypic assay for detecting multiple cellular responses to external stimuli: changes in cell morphology, adhesion, and lipid composition of the cellular membrane, as well as intracellular signaling. Using a focal plane array detection system, we show that MEIRS also enables spectro-chemical imaging at the single-cell level. The described metasurface-based all-optical sensor opens the way to a scalable, high-throughput spectroscopic assay for live cells.

Ultrasensitive Molecule Detection Based on Infrared Metamaterial Absorber with Vertical Nanogap

Surface-enhanced infrared absorption (SEIRA) spectroscopy is a powerful methodology for sensing and identifying small quantities of analyte molecules via coupling between molecular vibrations and an enhanced near-field induced in engineered structures. A metamaterial absorber (MA) is proposed as an efficient SEIRA platform; however, its efficiency is limited because it requires the appropriate insulator thickness and has a limited accessible area for sensing. SEIRA spectroscopy is proposed using an MA with a 10 nm thick vertical nanogap, and a record-high reflection difference SEIRA signal of 36% is experimentally achieved using a 1-octadecanethiol monolayer target molecule. Theoretical and experimental comparative studies are conducted using MAs with three different vertical nanogaps. The MAs with a vertical nanogap are processed using nanoimprint lithography and isotropic dry etching, which allow cost-effective large-area patterning and mass production. The proposed structure may provide promising routes for ultrasensitive sensing and detection applications.

Gold nanorod arrays enable highly sensitive bacterial detection via surface-enhanced infrared absorption (SEIRA) spectroscopy

Infrared (IR) spectroscopy is a unique and powerful method in the identification, characterization, and classification of chemical and biological molecules. However, the low absorbance of biological molecules has arisen as a major bottleneck and inhibits the application of IR in practical applications. To overcome this limitation, in the last four decades, surface-enhanced IR absorption (SEIRA) spectroscopy has been proposed and has become the focus of interest in various applications. In this study, for the first time, we proposed the employment of 3D anisotropic gold nanorod arrays (GNAs) as a highly active SEIRA platform in bacterial detection. For this, GNA platforms were fabricated through an oblique angle deposition (OAD) approach by using a physical vapor deposition (PVD) system. OAD of gold at proper deposition angle (10°) created closely-packed and columnar gold nanorod structures onto the glass slides in a well-controlled manner. GNA platform was tested as a SEIRA system in three different species of bacteria (Escherichia coli, Staphylococcus aureus, and Bacillus subtilis) by collecting IR spectra of each bacteria from different parts of GNA. The employment of GNA provided robust IR spectra with high reproducibility and signal-to-noise ratio. For the comparison, IR spectra of each bacteria were collected from aluminum foil and a smooth gold surface (SGS). No or very low IR spectra were observed in comparison to the GNA platform for these substrates. Unsupervised (PCA, HCA) and supervised (SIMCA, LDA, and SVM classification) machine learning analysis of bacteria spectra obtained from GNA substrate indicated that all bacteria samples can be detected and identified without using a label-containing biosensor, in a fast and simple manner.

Nanoporous Metals: From Plasmonic Properties to Applications in Enhanced Spectroscopy and Photocatalysis

The field of plasmonics is capable of enabling interesting applications in different wavelength ranges, spanning from the ultraviolet up to the infrared. The choice of plasmonic material and how the material is nanostructured has significant implications for ultimate performance of any plasmonic device. Artificially designed nanoporous metals (NPMs) have interesting material properties including large specific surface area, distinctive optical properties, high electrical conductivity, and reduced stiffness, implying their potentials for many applications. This paper reviews the wide range of available nanoporous metals (such as Au, Ag, Cu, Al, Mg, and Pt), mainly focusing on their properties as plasmonic materials. While extensive reports on the use and characterization of NPMs exist, a detailed discussion on their connection with surface plasmons and enhanced spectroscopies as well as photocatalysis is missing. Here, we report on different metals investigated, from the most used nanoporous gold to mixed metal compounds, and discuss each of these plasmonic materials' suitability for a range of structural design and applications. Finally, we discuss the potentials and limitations of the traditional and alternative plasmonic materials for applications in enhanced spectroscopy and photocatalysis.

Tuning gold nanostar morphology for the SERS detection of uranyl

The impact of tunable morphologies and plasmonic properties of gold nanostars are evaluated for the surface enhanced Raman scattering (SERS) detection of uranyl. To do so, gold nanostars are synthesized with varying concentrations of the Good's buffer reagent, 2-[4-(2-hydroxyethyl)-1-piperazinyl]propanesulfonic acid (EPPS). EPPS plays three roles including as a reducing agent for nanostar nucleation and growth, as a nanostar-stabilizing agent for solution phase stability, and as a coordinating ligand for the capture of uranyl. The resulting nanostructures exhibit localized surface plasmon resonance (LSPR) spectra that contain two visible and one near-infrared plasmonic modes. All three optical features arise from synergistic coupling between the nanostar core and branches. The tunability of these optical resonances are correlated with nanostar morphology through careful transmission electron microscopy (TEM) analysis. As the EPPS concentration used during synthesis increases, both the length and aspect ratio of the branches increase. This causes the two lower energy extinction features to grow in magnitude and become ideal for the SERS detection of uranyl. Finally, uranyl binds to the gold nanostar surface directly and via sulfonate coordination. Changes in the uranyl signal are directly correlated to the plasmonic properties associated with the nanostar branches. Overall, this work highlights the synergistic importance of nanostar morphology and plasmonic properties for the SERS detection of small molecules.

A target-activated plasmon coupling surface-enhanced Raman scattering platform for the highly sensitive and reproducible detection of miRNA-21

We have developed an SERS-based platform for the miRNA-21 assay with nucleic acid and Raman dye-modified AuNPs as capture substrates.

Dispersion-based intertwined SEIRA and SPR effect detection of 2,4-dinitrotoluene using a plasmonic metasurface

Surface enhanced infrared absorption (SEIRA) spectroscopy and surface plasmon resonance (SPR) make possible, thanks to plasmonics nanoantennas, the detection of low quantities of biological and chemical materials. Here, we investigate the infrared response of 2,4-dinitrotoluene deposited on various arrays of closely arranged metal-insulator-metal (MIM) resonators and experimentally show how the natural dispersion of the complex refractive index leads to an intertwined combination of SEIRA and SPR effect that can be leveraged to identify molecules. They are shown to be efficient for SEIRA spectroscopy and allows detecting of the dispersive explosive material, 2,4-dinitrotoluene. By changing the in-plane parameters, a whole spectral range of absorptions of 2,4-DNT is scanned. These results open the way to the design of sensors based on SEIRA and SPR combined effects, without including a spectrometer.

Nanometer-Scale Heterogeneous Interfacial Sapphire Wafer Bonding for Enabling Plasmonic-Enhanced Nanofluidic Mid-Infrared Spectroscopy

As one of the most effective surface-enhanced infrared absorption (SEIRA) techniques, metal-insulator-metal structured metamaterial perfect absorbers possess an ultrahigh sensitivity and selectivity in molecular infrared fingerprint detection. However, most of the localized electromagnetic fields (i.e., hotspots) are confined in the dielectric layer, hindering the interaction between analytes and hotspots. By replacing the dielectric layer with the nanofluidic channel, we develop a sapphire (Al₂O₃)-based mid-infrared (MIR) hybrid nanofluidic-SEIRA (HN-SEIRA) platform for liquid sensors with the aid of a low-temperature interfacial heterogeneous sapphire wafer direct bonding technique. The robust atomic bonding interface is confirmed by transmission electron microscope observation. We also establish a design methodology for the HN-SEIRA sensor using coupled-mode theory to carry out the loss engineering and experimentally validate its feasibility through the accurate nanogap control. Thanks to the capillary force, liquid analytes can be driven into sensing hotspots without external actuation systems. Besides, we demonstrate an in situ real-time dynamic monitoring process for the acetone molecular diffusion in deionized water. A small concentration change of 0.29% is distinguished and an ultrahigh sensitivity (0.8364 pmol^-1 %) is achieved. With the aid of IR fingerprint absorption, our HN-SEIRA platform brings the selectivity of liquid molecules with similar refractive indexes. It also resolves water absorption issues in traditional IR liquid sensors thanks to the sub-nm long light path. Considering the wide transparency window of Al₂O₃ in MIR (up to 5.2 μm), the HN-SEIRA platform covers more IR absorption range for liquid sensing compared to fused glass commonly used in micro/nanofluidics. Leveraging the aforementioned advantages, our work provides insights into developing a MIR real-time liquid sensing platform with intrinsic IR fingerprint selectivity, label-free ultrahigh sensitivity, and ultralow analyte volume, demonstrating a way toward quantitative molecule identification and dynamic analysis for the chemical and biological reaction processes.

Gold nanonails for surface-enhanced infrared absorption

Surface-enhanced infrared absorption (SEIRA) can dramatically enhance the vibrational signals of analyte molecules owing to the interaction between plasmons and molecular vibrations. It has huge potential for applications in various detection and diagnostic fields. High-aspect-ratio rod-like metal nanostructures have been the most widely studied nanomaterials for SEIRA. However, nearly all of the rod-like nanostructures reported previously are fabricated using physical methods. They suffer from damping and low areal number densities. In this work, high-aspect-ratio Au nanorods are synthesized, and Au nanonails are prepared through Au overgrowth on the as-prepared Au nanorods. The aspect ratios of the Au nanorods and nanonails can be varied in the range of ∼10 to ∼60, and their longitudinal dipolar plasmon resonance wavelengths can be correspondingly tailored from ∼1.6 to ∼8.3 μm. The Au nanonails exhibit superior SEIRA performance with 4-aminothiophenol used as the probe molecules. They are further used to detect the common biomolecule l-cysteine. Numerical simulations are further performed to understand the experimental results. They match well with the experimental observations, revealing the mechanism of the SEIRA enhancement. Our study demonstrates that colloidal high-aspect-ratio Au nanonails and nanorods can function as SEIRA nanoantennas for highly sensitive molecular detection in various situations.

Surface-Enhanced Infrared Absorption of Ligands on Colloidal Gold Nanowires through Resonant Coupling

Pushing the detection limit of infrared absorption (IR) through surface-enhanced (SEIRA) approaches have far-reaching prospect for related applications in molecular analysis and detection. Specifically engineered Au nanowires (NWs) can be applied as the surface-enhancing substrates in colloidal solution, given their longitudinal surface plasmon resonance (SPR) being aspect-ratio dependent and extendable into the infrared region. Through carefully designed control experiments, we realized resonant coupling between the longitudinal surface plasmons of Au NWs and the vibration modes of the bonded oleylamine (OA) ligands. In our system, after deliberately tuning thickness of the OA ligands and ratio of the detached/attached ligands in the solution, the apparent enhancement factor of IR signal from ligands around Au NWs could be pushed up to 5.29 × 10⁴. Given the facile tuning of SPR properties of Au NWs in the colloidal solution and the performance demonstrated in the report, our work could be an intriguing platform for SEIRA implementations in a broad spectrum of circumstances.

Surface-Enhanced Absorption Spectroscopy for Optical Fiber Sensing

Visible and near-infrared spectroscopy are widely used for sensing applications but suffer from poor signal-to-noise ratios for the detection of compounds with low concentrations. Enhancement by surface plasmon resonance is a popular technique that can be utilized to increase the signal of absorption spectroscopy due to the increased near-field created close to the plasmons. Despite interest in surface-enhanced infrared absorption spectroscopy (SEIRAS), the method is usually applied in lab setups rather than real-life sensing situations. This study aimed to achieve enhanced absorption from plasmons on a fiber-optic probe and thus move closer to applications of SEIRAS. A tapered coreless fiber coated with a 100 nm Au film supported signal enhancement at visible wavelengths. An increase in absorption was shown for two dyes spanning concentrations from 5 × 10⁻⁸ mol/L to 8 × 10⁻⁴ mol/L: Rhodamine 6G and Crystal Violet. In the presence of the Au film, the absorbance signal was 2–3 times higher than from an identically tapered uncoated fiber. The results confirm that the concept of SEIRAS can be implemented on an optical fiber probe, enabling enhanced signal detection in remote sensing applications.

Target-activated DNA nanomachines for the ATP detection based on the SERS of plasmonic coupling from gold nanoparticle aggregation

The self-assembly of plasmonic nanoparticles provides a powerful approach to generate surface-enhanced Raman scattering (SERS), which promotes the actual applications in chemical and biomolecular analyses. Herein, we developed a facile SERS sensing strategy for an ATP assay with a 3-D DNA nanomachine that walks by the Exo III cleavage, leading to the formation of AuNP aggregates, which resulted in the enhancement of the electromagnetic field. Depending on the target-activated Exo III cleavage, the 3-D nanomachine can walk along the 3-D track on the surface of AuNPs and generate self-assembled hot-spots to enhance the SERS signal of a Raman dye, allowing a homogenous assay of the ATP concentration with high sensitivity and reproducibility. Under optimized experimental conditions, the biosensor detected ATP with a widened dynamic range from 1 pM to 1 × 10⁵ pM with a limit of detection of up to 0.29 pM. Hence, the novel strategy provides a useful and practical platform for the SERS assay of ATP with high sensitivity and repeatability. Besides, this platform shows great potential for applications in high-throughput assays for drug screening and clinical diagnostics.

Ultrasensitive and Selective Gas Sensor Based on a Channel Plasmonic Structure with an Enormous Hot Spot Region

We present experimental and theoretical studies of a metamaterial-based plasmonic structure to build a plasmonic-molecular coupling detection system. High molecular sensitivity is realized only when molecules are located in the vicinity of the enhanced field (hot spot region); thus, introducing target molecules in the hot spot region to maximize plasmonic-molecular coupling is crucial to developing the sensing technology. We design a metamaterial consisting of a vertically oriented metal insulator metal (MIM) structure with a 25 nm channel sandwiched between two metal films, which enables the delivery of molecules into the large ravinelike hot spot region, offering an ultrasensitive platform for molecular sensing. This metamaterial is applied to carbon dioxide and butane detection. We design the structure to exhibit resonances at 4033 and 2945 cm^-1, which overlap with the C═O and -CH₂ vibration modes, respectively. The mutual coupling of these two resonance modes creates a Fano resonance, and their distinct peaks are clearly observed in the corresponding transmission dips. In addition, owing to its small footprint, such a vertical-oriented MIM structure enables us to increase the integration density and allows the detection of a 20 ppm concentration with negligible background noise and high selectivity in the mid-infrared region.

Plasmon-Enhanced Electrochemiluminescence of Silver Nanoclusters for microRNA Detection

Surface plasmon-enhanced electrochemiluminescence (SPEECL) with excellent sensitivity and simplicity has attracted increasing attention. In this work, we reported a novel SPEECL with DNA templated silver nanoclusters (DNA-AgNCs) as ECL emitters and gold nanoparticles (AuNPs) as localized surface plasmon resonance (LSPR) source. The SPEECL with DNA-AgNCs as ECL luminophores possessed low toxicity and avoided the labeling process, which is favorable for its further sensing application. In addition, by investigation of the SPEECL under different distances between DNA-AgNCs and AuNPs, it was demonstrated that the SPEECL was distance dependent. Meanwhile, the SPEECL intensity changed with the sizes and interdistance of AuNPs under different electrodeposition time. Furthermore, by the combination of a cyclic amplification process with enzyme-free catalytic hairpin DNA, a sensitive SPEECL biosensor was proposed for the detection of microRNA (miRNA-21) successfully with a wide linear range from 1 aM to 10⁴ fM and a relatively low detection limit of 0.96 aM, which was applied in the detection of miRNA-21 in real samples with satisfying results. This novel, simple, sensitive, and selective SPEECL with label-free and low-toxic ECL emitters displayed a great potential for bioassay application.

Tailoring grating strip widths for optimizing infrared absorption signals of an adsorbed molecular monolayer

Metal structures with resonances in the mid-infrared spectral range enable an increased sensitivity for detecting molecular vibrational signals. 1D gold strip gratings have already proven potential in surface-enhanced infrared absorption (SEIRA) experiments, as grating resonances and local electric field enhancement can be spectrally tuned by changing the grating period. Here, we identify the grating strip width as another important design parameter, which is investigated for further optimization of molecular absorption signal enhancement in SEIRA experiments. Previous literature used gratings to increase light absorption in relatively thick polymer layers. Here, we demonstrate the capability of gold strip gratings fabricated on a CaF₂ substrate to enhance the CH₂ vibrational modes of a thiol-based monolayer of MHDA. An optimal choice of the strip width w = 1.33 μm enables a maximum vibrational signal enhancement factor of around 84, when normalized to microscopic GIR measurements of an MHDA monolayer on an extended gold surface. Numerical simulations demonstrate the broadband local field enhancement of gold strip gratings, which are suitable for enhancing multiple vibrational modes in a large hot-spot volume.

Split aptamer-based detection of adenosine triphosphate using surface enhanced Raman spectroscopy and two kinds of gold nanoparticles

An ultrasensitive and highly selective method is described for the determination of adenosine triphosphate (ATP) via surface-enhanced Raman scattering (SERS). Two split aptamers are used for specific recognition of ATP. They were attached to two SERS substrates. The first was placed on a nanolayer of gold nanoparticle-decorated graphene oxide (GO/Au3), and the other on gold nanoparticles (Au2). When ATP is introduced, it will interact with the split aptamers on the gold nanostructures to form a sandwich structure that brings the GO/Au3 nanolayer and the Au2 nanoparticle in close proximity. Consequently, the SERS signal, best measured at 1072 cm^-1, is strongly enhanced. The sandwich structure also displays good water solubility and stability. Under optimized conditions, the SERS signal increases in the 10 pM - 10 nM ATP concentration range, and the limit of detection (LOD) is 0.85 pM. The method was applied to the determination of ATP in spiked human serum, and the LODs in serum and buffer are comparable. In our perception, the method has a wide scope in that numerous other aptamers may be used. This may result in a variety of other highly sensitive aptasensors for use in in-vitro diagnostics. Graphical abstract Schematic presentation of a self-assembly sandwich nanostructure as unique SERS assay platform for the sensitive detection of ATP.

Exciton-enhanced infrared spectroscopy with organometallic perovskite nanoplatelets

Organometallic perovskite nanoplatelets enhance molecular infrared absorption as their excitons can be excited by infrared photons and amplify molecular polarization.

Plasmonic mid-infrared third harmonic generation in germanium nanoantennas

We demonstrate third harmonic generation in plasmonic antennas consisting of highly doped germanium grown on silicon substrates and designed to be resonant in the mid-infrared frequency range that is inaccessible with conventional nonlinear plasmonic materials. Owing to the near-field enhancement, the result is an ultrafast, subdiffraction, coherent light source with a wavelength tunable between 3 and 5 µm, and ideally overlapping with the fingerprint region of molecular vibrations. To observe the nonlinearity in this challenging spectral window, a high-power femtosecond laser system equipped with parametric frequency conversion in combination with an all-reflective confocal microscope setup is employed. We demonstrate spatially resolved maps of the linear scattering cross section and the nonlinear emission of single isolated antenna structures. A clear third-order power dependence as well as mid-infrared emission spectra prove the nonlinear nature of the light emission. Simulations support the observed resonance length of the double-rod antenna and demonstrate that the field enhancement inside the antenna material is responsible for the nonlinear frequency mixing.

Two-Dimensional Infrared Spectroscopy with Local Plasmonic Fields of a Trimer Gap-Antenna Array

Half-wavelength plasmonic antennas tuned to resonance with molecular vibrational excitations have been demonstrated to enhance 2DIR signals by multiple orders of magnitude. We design doubly degenerate in-plane plasmonic normal modes of the symmetric trimer gap-antenna, which have orthogonal dipole moments excited by light of the appropriate polarization, to localize the enhanced field into the antenna's gap. Vibrational excitations serve as sensitive probes of the plasmonic fields. 2DIR spectroscopy of thin molecular films indicates that molecules emitting enhanced signals experience an electric field with a direction independent of the excitation laser pulse polarization. Our results illustrate the trade-off between the large signal amplification in molecules close to the antenna surface by resonant plasmons, where the direction of the enhanced fields follows metal surface boundary conditions, and the associated limitations for the polarization-selective spectroscopy. The ultrafast quantum dynamics reported by the enhanced signals is not affected by its interaction with plasmonic excitation.

Disc Antenna Enhanced Infrared Spectroscopy: From Self-Assembled Monolayers to Membrane Proteins

Plasmonic surfaces have emerged as a powerful platform for biomolecular sensing applications and can be designed to optimize the plasmonic resonance for probing molecular vibrations at utmost sensitivity. Here, we present a facile procedure to generate metallic microdisc antenna arrays that are employed in surface-enhanced infrared absorption (SEIRA) spectroscopy of biomolecules. Transmission electron microscopy (TEM) grids are used as shadow mask deployed during physical vapor deposition of gold. The resulting disc-shaped antennas exhibit enhancement factors of the vibrational bands of 4 × 10⁴ giving rise to a detection limit <1 femtomol (10^-15 mol) of molecules. Surface-bound monolayers of 4-mercaptobenzoic acid show polyelectrolyte behavior when titrated with cations in the aqueous medium. Conformational rigidity of the self-assembled monolayer is validated by density functional theory calculations. The membrane protein sensory rhodopsin II is tethered to the disc antenna arrays and is fully functional as inferred from the light-induced SEIRA difference spectra. As an advance to previous studies, the accessible frequency range is improved and extended into the fingerprint region.

Perfect-absorption graphene metamaterials for surface-enhanced molecular fingerprint spectroscopy

Graphene plasmon with extremely strong light confinement and tunable resonance frequency represents a promising surface-enhanced infrared absorption (SEIRA) sensing platform. However, plasmonic absorption is relatively weak (approximately 1%-9%) in monolayer graphene nanostructures, which would limit its sensitivity. Here, we theoretically propose a hybrid plasmon-metamaterial structure that can realize perfect absorption in graphene with a low carrier mobility of 1000 cm² V^-1 s^-1. This structure combines a gold reflector and a gold grating to the graphene plasmon structures, which introduce interference effect and the lightning-rod effect, respectively, and largely enhance the coupling of light to graphene. The vibration signal of trace molecules can be enhanced up to 2000-fold at the hotspot of the perfect-absorption structure, enabling the SEIRA sensing to reach the molecular level. This hybrid metal-graphene structure provides a novel path to generate high sensitivity in nanoscale molecular recognition for numerous applications.

Nanomaterial-Based Plasmon-Enhanced Infrared Spectroscopy

Surface-enhanced infrared absorption (SEIRA) has attracted increasing attention due to the potential of infrared spectroscopy in applications such as molecular trace sensing of solids, polymers, and proteins, specifically fueled by recent substantial developments in infrared plasmonic materials and engineered nanostructures. Here, the significant progress achieved in the past decades is reviewed, along with the current state of the art of SEIRA. In particular, the plasmonic properties of a variety of nanomaterials are discussed (e.g., metals, semiconductors, and graphene) along with their use in the design of efficient SEIRA configurations. To conclude, perspectives on potential applications, including single-molecule detection and in vivo bioassays, are presented.

High-Contrast Infrared Absorption Spectroscopy via Mass-Produced Coaxial Zero-Mode Resonators with Sub-10 nm Gaps

We present a wafer-scale array of resonant coaxial nanoapertures as a practical platform for surface-enhanced infrared absorption spectroscopy (SEIRA). Coaxial nanoapertures with sub-10 nm gaps are fabricated via photolithography, atomic layer deposition of a sacrificial Al₂O₃ layer to define the nanogaps, and planarization via glancing-angle ion milling. At the zeroth-order Fabry-Pérot resonance condition, our coaxial apertures act as a "zero-mode resonator (ZMR)", efficiently funneling as much as 34% of incident infrared (IR) light along 10 nm annular gaps. After removing Al₂O₃ in the gaps and inserting silk protein, we can couple the intense optical fields of the annular nanogap into the vibrational modes of protein molecules. From 7 nm gap ZMR devices coated with a 5 nm thick silk protein film, we observe high-contrast IR absorbance signals drastically suppressing 58% of the transmitted light and infer a strong IR absorption enhancement factor of 10⁴∼10⁵. These single nanometer gap ZMR devices can be mass-produced via batch processing and offer promising routes for broad applications of SEIRA.

Triangle-, tripod-, and tetrapod-branched ITO nanocrystals for anisotropic infrared plasmonics

While many sophisticated geometries of metallic nanoparticles have been explored for plasmon resonance in the visible light waveband, similar structures for infrared plasmonics have seldom been studied. Herein, for the first time, we report branched colloidal Sn-doped In₂O₃ (ITO) NCs prepared through heating metal organic precursors in a high boiling solvent and surfactants via a simple one-pot method and their interesting anisotropic infrared plasmonics. Shaped ITO NCs were anisotropically grown into triangles, tripods, and tetrapods from spherical particles by tuning the composition of the surfactants. Interestingly, such anisotropic ITO NCs exhibit shape-dependent localized surface plasmon resonances. A new peak emerged at a low energy band, and the peak intensity increases when the particles become more anisotropic. Meanwhile, a redshift of this newly emerging peak and a blueshift of the original peak can also be observed. The proposed overlapping spheroid model successfully explained those features and the calculated extinction spectra matched excellently with the experimental data. Moreover, the origins of those peaks were also revealed and assigned to in-plane longitudinal and out-of-plane transverse modes, respectively. This work may help design more sophisticated structures for infrared plasmonics and broaden its applications in various areas.

Plasmonics-Nanofluidics Hydrid Metamaterial: An Ultrasensitive Platform for Infrared Absorption Spectroscopy and Quantitative Measurement of Molecules

One of the most attractive potentials of plasmonic metamaterials is the amplification of intrinsically weak signals such as molecular infrared absorption or Raman scattering for detection applications. This effect, however, is only effective when target molecules are located at the enhanced electromagnetic field of the plasmonic structures (i.e., hot-spots). It is thus of significance to control the spatial overlapping of molecules and hot-spots, yet it is a long-standing challenge, since it involves the handling of molecules in nanoscale spaces. Here a metamaterial consisting of a nanofluidic channel with a depth of several tens of nanometers sandwiched between plasmonic resonators and a metal film enables the controllable delivery of small molecules into the most enhanced field arising from the quadrupole mode of the structures, forming a plasmon-molecular coupled system. It offers an ultrasensitive platform for detection of IR absorption and molecular sensing. Notably, the precise handling of molecules in a fixed and ultrasmall (10-100 nm) gap also addressed some critical issues in IR spectroscopy such as quantitative measurement and measurement in aqueous solution. Moreover, a drastic change in the reflectance characteristic resulting from the strong coupling between molecules and plasmonic structures indicates that molecules can also be utilized as triggers for actively switching the optical property of metamaterials.