Surface-Enhanced Infrared Absorption: Pushing the Frontier for On-Chip 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.

Advances and Applications of Metal-Organic Frameworks (MOFs) in Emerging Technologies: A Comprehensive Review

Metal-organic frameworks (MOFs) that are the wonder material of the 21st century consist of metal ions/clusters coordinated to organic ligands to form one- or more-dimensional porous structures with unprecedented chemical and structural tunability, exceptional thermal stability, ultrahigh porosity, and a large surface area, making them an ideal candidate for numerous potential applications. In this work, the recent progress in the design and synthetic approaches of MOFs and explore their potential applications in the fields of gas storage and separation, catalysis, magnetism, drug delivery, chemical/biosensing, supercapacitors, rechargeable batteries and self-powered wearable sensors based on piezoelectric and triboelectric nanogenerators are summarized. Lastly, this work identifies present challenges and outlines future opportunities in this field, which can provide valuable references.

Research Progress in Surface-Enhanced Infrared Absorption Spectroscopy: From Performance Optimization, Sensing Applications, to System Integration

Infrared absorption spectroscopy is an effective tool for the detection and identification of molecules. However, its application is limited by the low infrared absorption cross-section of the molecule, resulting in low sensitivity and a poor signal-to-noise ratio. Surface-Enhanced Infrared Absorption (SEIRA) spectroscopy is a breakthrough technique that exploits the field-enhancing properties of periodic nanostructures to amplify the vibrational signals of trace molecules. The fascinating properties of SEIRA technology have aroused great interest, driving diverse sensing applications. In this review, we first discuss three ways for SEIRA performance optimization, including material selection, sensitivity enhancement, and bandwidth improvement. Subsequently, we discuss the potential applications of SEIRA technology in fields such as biomedicine and environmental monitoring. In recent years, we have ushered in a new era characterized by the Internet of Things, sensor networks, and wearable devices. These new demands spurred the pursuit of miniaturized and consolidated infrared spectroscopy systems and chips. In addition, the rise of machine learning has injected new vitality into SEIRA, bringing smart device design and data analysis to the foreground. The final section of this review explores the anticipated trajectory that SEIRA technology might take, highlighting future trends and possibilities.

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.

Highly Responsive Plasmon Modulation in Dopant-Segregated Nanocrystals

Electron transfer to and from metal oxide nanocrystals (NCs) modulates their infrared localized surface plasmon resonance (LSPR), revealing fundamental aspects of their photophysics and enabling dynamic optical applications. We synthesized and chemically reduced dopant-segregated Sn-doped In₂O₃ NCs, investigating the influence of radial dopant segregation on LSPR modulation and near-field enhancement (NFE). We found that core-doped NCs show large LSPR shifts and NFE change during chemical titration, enabling broadband modulation in LSPR energy of over 1000 cm^-1 and of peak extinction over 300%. Simulations reveal that the evolution of the LSPR spectra during chemical reduction results from raising the surface Fermi level and increasing the donor defect density in the shell region. These results establish dopant segregation as a useful strategy to engineer the dynamic optical modulation in plasmonic semiconductor NC heterostructures going beyond what is possible with conventional plasmonic metals.

A Review of Gas Measurement Practices and Sensors for Tunnels

The concentration of pollutant gases emitted by traffic in a tunnel affects the indoor air quality and contributes to structural deterioration. Demand control ventilation systems incur high operating costs, so reliable measurement of the gas concentration is essential. Numerous commercial sensor types are available with proven experience, such as optical and first-generation electrochemical sensors, or novel materials in detection methods. However, all of them are subjected to measurement deviations due to environmental conditions. This paper presents the main types of sensors and their application in tunnels. Solutions will also be discussed in order to obtain reliable measurements and improve the efficiency of the extraction systems.

Modeling Interfacial Interaction between Gas Molecules and Semiconductor Metal Oxides: A New View Angle on Gas Sensing

With the development of internet of things and artificial intelligence electronics, metal oxide semiconductor (MOS)-based sensing materials have attracted increasing attention from both fundamental research and practical applications. MOS materials possess intrinsic physicochemical properties, tunable compositions, and electronic structure, and are particularly suitable for integration and miniaturization in developing chemiresistive gas sensors. During sensing processes, the dynamic gas-solid interface interactions play crucial roles in improving sensors' performance, and most studies emphasize the gas-MOS chemical reactions. Herein, from a new view angle focusing more on physical gas-solid interactions during gas sensing, basic theory overview and latest progress for the dynamic process of gas molecules including adsorption, desorption, and diffusion, are systematically summarized and elucidated. The unique electronic sensing mechanisms are also discussed from various aspects including molecular interaction models, gas diffusion mechanism, and interfacial reaction behaviors, where structure-activity relationship and diffusion behavior are overviewed in detail. Especially, the surface adsorption-desorption dynamics are discussed and evaluated, and their potential effects on sensing performance are elucidated from the gas-solid interfacial regulation perspective. Finally, the prospect for further research directions in improving gas dynamic processes in MOS gas sensors is discussed, aiming to supplement the approaches for the development of high-performance MOS gas sensors.

Towards multi-molecular surface-enhanced infrared absorption using metal plasmonics

Surface-enhanced infrared absorption (SEIRA) leads to a largely improved detection of polar molecules, compared to standard infrared absorption. The enhancement principle is based on localized surface plasmon resonances of the substrate, which match the frequency of molecular vibrations in the analyte of interest. Therefore, in practical terms, the SEIRA sensor needs to be tailored to each specific analyte. We review SEIRA sensors based on metal plasmonics for the detection of biomolecules such as DNA, proteins, and lipids. We further focus this review on chemical SEIRA sensors, with potential applications in quality control, as well as on the improvement in sensor geometry that led to the development of multiresonant SEIRA substrates as sensors for multiple analytes. Finally, we give an introduction into the integration of SEIRA sensors with surface-enhanced Raman scattering (SERS).

MOF/Polymer-Integrated Multi-Hotspot Mid-Infrared Nanoantennas for Sensitive Detection of CO2 Gas

Metal-organic frameworks (MOFs) have been extensively used for gas sorption, storage and separation owing to ultrahigh porosity, exceptional thermal stability, and wide structural diversity. However, when it comes to ultra-low concentration gas detection, technical bottlenecks of MOFs appear due to the poor adsorption capacity at ppm-/ppb-level concentration and the limited sensitivity for signal transduction. Here, we present hybrid MOF-polymer physi-chemisorption mechanisms integrated with infrared (IR) nanoantennas for highly selective and ultrasensitive CO₂ detection. To improve the adsorption capacity for trace amounts of gas molecules, MOFs are decorated with amino groups to introduce the chemisorption while maintaining the structural integrity for physisorption. Additionally, leveraging all major optimization methods, a multi-hotspot strategy is proposed to improve the sensitivity of nanoantennas by enhancing the near field and engineering the radiative and absorptive loss. As a benefit, we demonstrate the competitive advantages of our strategy against the state-of-the-art miniaturized IR CO₂ sensors, including low detection limit, high sensitivity (0.18%/ppm), excellent reversibility (variation within 2%), and high selectivity (against C₂H₅OH, CH₃OH, N₂). This work provides valuable insights into the integration of advanced porous materials and nanophotonic devices, which can be further adopted in ultra-low concentration gas monitoring in industry and environmental applications.

Surface-enhanced mid-infrared absorption spectroscopy using miniaturized-disc metasurface

Surface-enhanced infrared spectroscopy is an important technique for improving the signal-to-noise ratio of spectroscopic material identification measurements in the mid-infrared fingerprinting region. However, the lower bound of the fingerprinting region receives much less attention due to a scarcity of transparent materials, more expensive sources, and weaker plasmonic effects. In this paper, we present a miniaturized metasurface unit cell for surface-enhanced infrared spectroscopy of the 15-[Formula: see text]m vibrational band of CO[Formula: see text]. The unit cell consists of a gold disc, patterned along the edge with fine gaps/wires to create a resonant metamaterial liner. In simulation, our plasmonic metamaterial-lined disc achieves greater than [Formula: see text] the average field intensity enhancement of a comparable dipole array and a miniaturized size of [Formula: see text] using complex, 100-nm features that are patterned using 100-kV electron-beam lithography. In a simple experiment, the metamaterial-lined disc metasurface shows a high tolerance to fabrication imperfections and enhances the absorption of CO[Formula: see text] at 15 [Formula: see text]m. The resonant wavelength and reflection magnitude can be tuned over a wide range by adjusting the liner feature sizes and the metasurface array pitch to target other vibrational bands. This work is a step toward low-cost, more compact on-chip integrated gas sensors.

Metal-Organic Framework-Surface-Enhanced Infrared Absorption Platform Enables Simultaneous On-Chip Sensing of Greenhouse Gases

Simultaneous on-chip sensing of multiple greenhouse gases in a complex gas environment is highly desirable in industry, agriculture, and meteorology, but remains challenging due to their ultralow concentrations and mutual interference. Porous microstructure and extremely high surface areas in metal-organic frameworks (MOFs) provide both excellent adsorption selectivity and high gases affinity for multigas sensing. Herein, it is described that integrating MOFs into a multiresonant surface-enhanced infrared absorption (SEIRA) platform can overcome the shortcomings of poor selectivity in multigas sensing and enable simultaneous on-chip sensing of greenhouse gases with ultralow concentrations. The strategy leverages the near-field intensity enhancement (over 1500-fold) of multiresonant SEIRA technique and the outstanding gas selectivity and affinity of MOFs. It is experimentally demonstrated that the MOF-SEIRA platform achieves simultaneous on-chip sensing of CO2 and CH4 with fast response time (<60 s), high accuracy (CO2: 1.1%, CH4: 0.4%), small footprint (100 × 100 µm2), and excellent linearity in wide concentration range (0-2.5 × 104 ppm). Additionally, the excellent scalability to detect more gases is explored. This work opens up exciting possibilities for the implementation of all-in-one, real-time, and on-chip multigas detection as well as provides a valuable toolkit for greenhouse gas sensing applications.

Ultracompact gas sensor with metal-organic-framework-based differential fiber-optic Fabry-Perot nanocavities

Refractive-index (RI)-based sensing is a major optical sensing modality that can be implemented in various spectral ranges. While it has been widely used for sensing of biochemical liquids, RI-based gas sensing, particularly small-molecule gases, is challenging due to the extremely small RI change induced by gas concentration variations. We propose a RI-based ultracompact fiber-optic differential gas sensor that employs metal-organic-framework (MOF)-based dual Fabry-Perot (FP) nanocavities. A MOF is used as the FP cavity material to enhance the sensitivity as well as the selectivity to particular gas molecules. The differential sensing scheme leverages the opposite change in the cavity-length-dependent reflection of the two FP cavities, which further enhances the sensitivity compared with single FP cavity based sensing. For proof-of-concept, a fiber-optic CO₂ sensor with ZIF-8-based dual FP nanocavities was fabricated. The effective footprint of the sensor was as small as 157 µm² and the sensor showed an enhanced sensitivity of 48.5 mV/CO₂Vol%, a dynamic range of 0-100 CO₂Vol%, and a resolution of 0.019 CO₂Vol% with 1 Hz low-pass filtering. Although the current sensor was only demonstrated for CO₂ sensing, the proposed sensor concept can be used for sensing of a variety of gases when different kinds of MOFs are utilized.

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.

Realization of an Ultrasensitive and Highly Selective OFET NO2 Sensor: The Synergistic Combination of PDVT-10 Polymer and Porphyrin-MOF

Organic field-effect transistors (OFETs) are emerging as competitive candidates for gas sensing applications due to the ease of their fabrication process combined with the ability to readily fine-tune the properties of organic semiconductors. Nevertheless, some key challenges remain to be addressed, such as material degradation, low sensitivity, and poor selectivity toward toxic gases. Appropriately, a heterojunction combination of different sensing layers with multifunctional capabilities offers great potential to overcome these problems. Here, a novel and highly sensitive receptor layer is proposed encompassing a porous 3D metal-organic framework (MOF) based on isostructural-fluorinated MOFs acting as an NO₂ specific preconcentrator, on the surface of a stable and ultrathin PDVT-10 organic semiconductor on an OFET platform. Here, with this proposed combination we have unveiled an unprecedented 700% increase in sensitivity toward NO₂ analyte in contrast to the pristine PDVT-10. The resultant combination for this OFET device exhibits a remarkable lowest detection limit of 8.25 ppb, a sensitivity of 680 nA/ppb, and good stability over a period of 6 months under normal laboratory conditions. Further, a negligible response (4.232 nA/%RH) toward humidity in the range of 5%-90% relative humidity was demonstrated using this combination. Markedly, the obtained results support the use of the proposed novel strategy to achieve an excellent sensing performance with an OFET platform.

Metasurface Color Filters Using Aluminum and Lithium Niobate Configurations

Two designs of metasurface color filters (MCFs) using aluminum and lithium niobate (LN) configurations are proposed and numerically studied. They are denoted as tunable aluminum metasurface (TAM) and tunable LN metasurface (TLNM), respectively. The configurations of MCFs are composed of suspended metasurfaces above aluminum mirror layers to form a Fabry-Perot (F-P) resonator. The resonances of TAM and TLNM are red-shifted with tuning ranges of 100 nm and 111 nm, respectively, by changing the gap between the bottom mirror layer and top metasurface. Furthermore, the proposed devices exhibit perfect absorption with ultra-narrow bandwidth spanning the whole visible spectral range by composing the corresponding geometrical parameters. To increase the flexibility and applicability of proposed devices, TAM exhibits high sensitivity of 481.5 nm/RIU and TLNM exhibits high figure-of-merit (FOM) of 97.5 when the devices are exposed in surrounding environment with different refraction indexes. The adoption of LN-based metasurface can enhance FWHM and FOM values as 10-fold and 7-fold compared to those of Al-based metasurface, which greatly improves the optical performance and exhibits great potential in sensing applications. These proposed designs provide an effective approach for tunable high-efficiency color filters and sensors by using LN-based metamaterial.

Metal–Organic Framework Thin Films: Fabrication, Modification, and Patterning

Metal–organic frameworks (MOFs) have been of great interest for their outstanding properties, such as large surface area, low density, tunable pore size and functionality, excellent structural flexibility, and good chemical stability. A significant advancement in the preparation of MOF thin films according to the needs of a variety of applications has been achieved in the past decades. Yet there is still high demand in advancing the understanding of the processes to realize more scalable, controllable, and greener synthesis. This review provides a summary of the current progress on the manufacturing of MOF thin films, including the various thin-film deposition processes, the approaches to modify the MOF structure and pore functionality, and the means to prepare patterned MOF thin films. The suitability of different synthesis techniques under various processing environments is analyzed. Finally, we discuss opportunities for future development in the manufacturing of MOF thin films.

Lab-on-Fiber Nanoprobe with Dual High-Q Rayleigh Anomaly-Surface Plasmon Polariton Resonances for Multiparameter Sensing

Surface plasmon resonance (SPR) based sensing is an attractive approach for realizing lab-on-fiber nanoprobes. However, simultaneous measurement of multiple parameters (e.g., refractive index and temperature) with SPR-based nanoprobes, although highly desirable, is challenging. We report a lab-on-fiber nanoprobe with dual high-Q Rayleigh anomaly (RA)-surface plasmon polariton (SPP) resonances for multiparameter sensing. To achieve high-Q RA-SPP resonance the nanoprobe employs a plasmonic crystal cavity enhanced by distributed Bragg reflector (DBR) gratings on the end-face of a single-mode optical fiber. By tailoring the grating periods of the plasmonic crystal cavity and DBRs, two spatially separated high-Q RA-SPP resonance modes are designed within a 50 nm spectral range in C + L band. The fabricated nanoprobe demonstrates two RA-SPP resonances near 1550 nm with high Q-factors up to 198. These two high-Q resonances are further showed to exhibit distinctive responses to the changes of refractive index and temperature, which enables simultaneous measurements of both parameters. The proposed lab-on-fiber nanoprobes will pave the way for realizing compact multiparameter sensing solutions compatible with optical communication infrastructures.

All-Dielectric Surface-Enhanced Infrared Absorption-Based Gas Sensor Using Guided Resonance

The surface-enhanced infrared absorption (SEIRA) technique has been focusing on the metallic resonator structures for decades, exploring different approaches to enhance sensitivity. Although the high enhancement is achieved, the dissipative loss and strong heating are the intrinsic drawbacks of metals. Recently, the dielectric platform has emerged as a promising alternative. In this work, we report a guided resonance-based all-dielectric photonic crystal slab as the platform for SEIRA. The guided resonance-induced enhancement in the effective path length and electric field, together with gas enrichment polymer coating, leads to a detection limit of 20 ppm in carbon dioxide (CO₂) sensing. This work explores the feasibility to apply low loss all-dielectric structures as a surface enhancement method in the transmission mode.