High-performance plasmonics nanostructures in gas sensing: a comprehensive review

Plasmonic nanostructures have emerged as indispensable components in the construction of high-performance gas sensors, playing a pivotal role across diverse applications, including industrial safety, medical diagnostics, and environmental monitoring. This review paper critically examines seminal research that underscores the remarkable efficacy of plasmonic materials in achieving superior attributes such as heightened sensitivity, selectivity, and rapid response times in gas detection. Offering a synthesis of pivotal studies, this review aims to furnish a comprehensive discourse on the contemporary advancements within the burgeoning domain of plasmonic gas sensing. The featured investigations meticulously scrutinize various plasmonic structures and their applications in detecting gases like carbon monoxide, carbon dioxide, hydrogen and nitrogen dioxide. The discussed frameworks encompass cutting-edge approaches, spanning ideal absorbers, surface plasmon resonance sensors, and nanostructured materials, thereby elucidating the diverse strategies employed for advancing plasmonic gas sensing technologies.

Mid-Infrared Response from Cr/n-Si Schottky Junction with an Ultra-Thin Cr Metal

Infrared detection technology has been widely applied in many areas. Unlike internal photoemission and the photoelectric mechanism, which are limited by the interface barrier height and material bandgap, the research of the hot carrier effect from nanometer thickness of metal could surpass the capability of silicon-based Schottky devices to detect mid-infrared and even far-infrared. In this work, we investigate the effects of physical characteristics of Cr nanometal surfaces and metal/silicon interfaces on hot carrier optical detection. Based on the results of scanning electron microscopy, atomic force microscopy, and X-ray diffraction analysis, the hot carrier effect and the variation of optical response intensity are found to depend highly on the physical properties of metal surfaces, such as surface coverage, metal thickness, and internal stress. Since the contact layer formed by Cr and Si is the main role of infrared light detection in the experiment, the higher the metal coverage, the higher the optical response. Additionally, a thicker metal surface makes the hot carriers take a longer time to convert into current signals after generation, leading to signal degradation due to the short lifetime of the hot carriers. Furthermore, the film with the best hot carrier effect induced in the Cr/Si structure is able to detect an infrared signal up to 4.2 μm. Additionally, it has a 229 times improvement in the signal-to-noise ratio (SNR) for a single band compared with ones with less favorable conditions.

Rapid Assay for the Therapeutic Drug Monitoring of Edoxaban

Edoxaban is a direct oral anticoagulant (DOAC) that has been recently indicated for the treatment of pulmonary embolism (PE) in SARS-CoV-2 infections. Due to its pharmacokinetic variability and a narrow therapeutic index, the safe administration of the drug requires its therapeutic drug monitoring (TDM) in patients receiving the treatment. In this work, we present a label-free method for the TDM of edoxaban by surface enhanced Raman spectroscopy (SERS). The new method utilises the thiol chemistry of the drug to chemisorb its molecules onto a highly sensitive SERS substrate. This leads to the formation of efficient hotspots and a strong signal enhancement of the drug Raman bands, thus negating the need for a Raman reporter for its SERS quantification. The standard samples were run with a concentration range of 1.4 × 10⁻⁴ M to 10⁻¹² M using a mobile phase comprising of methanol/acetonitrile (85:15 v/v) at 291 nm followed by the good linearity of R² = 0.997. The lowest limit of quantification (LOQ) by the SERS method was experimentally determined to be 10⁻¹² M, whereas LOQ for HPLC-UV was 4.5 × 10⁻⁷ M, respectively. The new method was used directly and in a simple HPLC-SERS assembly to detect the drug in aqueous solutions and in spiked human blood plasma down to 1 pM. Therefore, the SERS method has strong potential for the rapid screening of the drug at pathology labs and points of care.

Development of spray-drying-based surface-enhanced Raman spectroscopy

We report a spray-drying method to fabricate silver nanoparticle (AgNP) aggregates for application in surface-enhanced Raman spectroscopy (SERS). A custom-built system was used to fabricate AgNP aggregates of four sizes, 48, 86, 151, and 218 nm, from drying droplets containing AgNPs atomized from an AgNP suspension. Sample solutions of Rhodamine B (RhB) at 10^-6, 10^-8, and 10^-10 M concentrations were dropped onto the AgNP aggregates as probe molecules to examine the enhancement of the Raman signals of the RhB. The ordering of the analytical enhancement factors (AEFs) by aggregate size at a 10^-6 M RhB was 86 nm > 218 nm > 151 nm > 48 nm. When RhB concentrations are below 10^-8 M, the 86 and 151 nm AgNP aggregates show clear RhB peaks. The AEFs of the 86 nm AgNP aggregates were the highest in all four aggregates and higher than those of the 218-nm aggregates, although the 218-nm aggregates had more hot spots where Raman enhancement occurred. This finding was attributable to the deformation and damping of the electron cloud in the highly aggregated AgNPs, reducing the sensitivity for Raman enhancement. When RhB was premixed with the AgNP suspension prior to atomization, the AEFs at 10^-8 M RhB rose ~ 100-fold compared to those in the earlier experiments (the post-dropping route). This significant enhancement was probably caused by the increased opportunity for the trapping of the probe molecules in the hot spots.

Selectivity in trace gas sensing: recent developments, challenges, and future perspectives

Selectivity is one of the most crucial figures of merit in trace gas sensing, and thus a comprehensive assessment is necessary to have a clear picture of sensitivity, selectivity, and their interrelations in terms of quantitative and qualitative views.

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.

Post-synthetically modified metal-organic frameworks for sensing and capture of water pollutants

Thanks to a bottom-up design of metals and organic ligands, the library of metal-organic frameworks (MOFs) has seen a conspicuous growth. Post-synthetically modified MOFs comprise a relatively smaller subset of this library. Whereas the approach of post-synthetic modification was seminally introduced for MOFs in the early 1990s, the earliest examples of post-synthetically modified MOFs are only congruous with adsorption and catalysis. The utility of PSM-derived MOFs for the sensing and capture of water contaminants is relatively niche. Arguably though, an increasing number of post-synthetically modified MOFs are finding relevance in the context of water pollutant remediation. In this article, we review the recent advances in this area and propose a structure-function relationship-guided blueprint for the future outlook.

Surface-enhanced Raman scattering sensors for biomedical and molecular detection applications in space

The detection of molecular traces in the environment is a technical problem that is critical in pollutant control procedures at all stages of spacecraft assembly, in space flight, as well as in other technological processes such as food production, medical diagnostics, environmental control, warfare. However, in the aerospace industry, it is necessary to detect molecular traces of contaminants with extreme sensitivity, as even concentrations as low as part-per-billion (ppb) can be critical during long missions. The high sensitivity of the Volatile Organic Compounds (VOCs) detection within the air can be a challenge because of the poor affinity of VOC's to the metal surface of the sensor substrate. In this work, we present a surface-enhanced Raman scattering (SERS) spectroscopy technique as a highly sensitive and selective molecular sensor for gas trace detection not sensitive to molecules adsorbtion on sensing element. The developed hybrid SERS platform for molecular trace detection is supported by the hybrid nanoplasmonic porous silicon membrane in conjunction with micropump to achieve the trace level detection of VOCs in the environment. The combination of silicon membrane, made by electrochemical etching of the microchannels in the silicon chip, with chemical deposition of the silver nanoparticles inside the channels, produce a porous Ag nanoparticles membrane with a high density of plasmonic nanostructures ("hot spots"). The micropump integrated with the SERS sensor, pump the air with VOC's molecules through the plasmonic membrane "hot spots" to increase the probability of interaction of VOC's molecules with SERS substrate and to increase the enhancement factor. The sensor chip structure was designed, gas flow in the sensor was simulated, and the sensor was fabricated using 3D printing. The limit of detection of hydrazine with concentration level 10^-12 M from solution and the vapor phase 0.1 ppm was demonstrated. The anisole vapors with concentration 0.5 ppb spectra in the air were recorded. Our results demonstrate that plasmonic membrane can be used as a high enhancement factor SERS sensor for many pollutants molecules detection with the nanomolar sensitivity and can be applied in the design of sensors for space applications, environment control, biomedical diagnostic.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12567-021-00356-6.

Nanoengineering Approaches Toward Artificial Nose

Significant scientific efforts have been made to mimic and potentially supersede the mammalian nose using artificial noses based on arrays of individual cross-sensitive gas sensors over the past couple decades. To this end, thousands of research articles have been published regarding the design of gas sensor arrays to function as artificial noses. Nanoengineered materials possessing high surface area for enhanced reaction kinetics and uniquely tunable optical, electronic, and optoelectronic properties have been extensively used as gas sensing materials in single gas sensors and sensor arrays. Therefore, nanoengineered materials address some of the shortcomings in sensitivity and selectivity inherent in microscale and macroscale materials for chemical sensors. In this article, the fundamental gas sensing mechanisms are briefly reviewed for each material class and sensing modality (electrical, optical, optoelectronic), followed by a survey and review of the various strategies for engineering or functionalizing these nanomaterials to improve their gas sensing selectivity, sensitivity and other measures of gas sensing performance. Specifically, one major focus of this review is on nanoscale materials and nanoengineering approaches for semiconducting metal oxides, transition metal dichalcogenides, carbonaceous nanomaterials, conducting polymers, and others as used in single gas sensors or sensor arrays for electrical sensing modality. Additionally, this review discusses the various nano-enabled techniques and materials of optical gas detection modality, including photonic crystals, surface plasmonic sensing, and nanoscale waveguides. Strategies for improving or tuning the sensitivity and selectivity of materials toward different gases are given priority due to the importance of having cross-sensitivity and selectivity toward various analytes in designing an effective artificial nose. Furthermore, optoelectrical sensing, which has to date not served as a common sensing modality, is also reviewed to highlight potential research directions. We close with some perspective on the future development of artificial noses which utilize optical and electrical sensing modalities, with additional focus on the less researched optoelectronic sensing modality.

Silver holographic gratings as substrates for surface-enhanced Raman scattering gas analysis

This work is devoted to the investigation of the enhancement of Raman signals of nonadsorbed gases in the vicinity of corrugated metallic surfaces supporting propagating surface plasmon-polaritons (PSPPs). Simulation of the PSPP excitation efficiency on holographic gratings coated with silver films of various thicknesses at different groove heights was carried out. Verification showed good agreement between theory and experiment. Also, it was found that an increase of the PSPP excitation efficiency may not lead to an increase in the enhancement factor of Raman signals of gases located near the surface-enhanced Raman scattering active surface. For a holographic grating with a period of 667 nm, a groove height of 70 nm, and a silver film thickness of 30 nm coated with a protective ${{\rm Al}_2}{{\rm O}_3}$Al₂O₃ layer, the enhancement factor of Raman signals of nonadsorbed nitrogen molecules was $\sim{{\rm 4\cdot10}^3}$∼4⋅10³.

Features and advantages of flexible silicon nanowires for SERS applications

The paper reports on the features and advantages of horizontally oriented flexible silicon nanowires (SiNWs) substrates for surface-enhanced Raman spectroscopy (SERS) applications. The novel SERS substrates are described in detail considering three main aspects. First, the key synthesis parameters for the flexible nanostructure SERS substrates were optimized. It is shown that fabrication temperature and metal-plating duration significantly influence the flexibility of the SiNWs and, consequently, determine the SERS enhancement. Second, it is demonstrated how the immersion in a liquid followed by drying results in the formation of SiNWs bundles influencing the surface morphology. The morphology changes were described by fractal dimension and lacunar analyses and correlated with the duration of Ag plating and SERS measurements. SERS examination showed the optimal intensity values for SiNWs thickness values of 60-100 nm. That is, when the flexibility of the self-assembly SiNWs allowed hot spots occurrence. Finally, the test with 4-mercaptophenylboronic acid showed excellent SERS performance of the flexible, horizontally oriented SiNWs in comparison with several other commercially available substrates.

Three-Dimensional-Stacked Gold Nanoparticles with Sub-5 nm Gaps on Vertically Aligned TiO2 Nanosheets for Surface-Enhanced Raman Scattering Detection Down to 10 fM Scale

Seeking for ultrasensitive and low-cost substrates is highly demandable for practical applications of surface-enhanced Raman scattering (SERS) technology. In this work, we report an ultrasensitive SERS-active substrate based on wet-chemistry-synthesized vertically aligned large-area TiO₂ nanosheets (NSs) decorated by densely packed gold nanoparticles (Au NPs) with sub-5 nm gaps. Via a multistep successive deposition process, three-dimensional-stacked Au NPs sandwiched by a 3 nm SiO₂ layer were assembled onto the TiO₂ NS, enabling numerous hotspots due to the formation of both ultratiny plasmonic gaps and semiconductor/metal interfaces. Experimental results show that the fabricated substrate displays a detection limit down to 10 fM (10^-14 M) without involving any condensation process by using the crystal violet as probe molecules. Control experiments and electromagnetic simulations indicate that the nanogaps defined by the 3 nm spacer are essential for the obtained excellent SERS performance. With its ultrasensitive detection capability, we demonstrate that the fabricated SERS substrate can be used for the trace analysis of melamine in milk.

SERS based monitoring of toluene vapors at ambient and elevated temperatures by using a ruffled silver nanolayer as a substrate

The authors describe a Surface enhanced Raman spectroscopy (SERS)-based method for the detection of gaseous toluene at different temperature regimes using 3D ruffled silver SERS substrates and a commercially available handheld Raman system equipped with a 785 nm laser. The 3D silver SERS substrates were synthesized via electroless deposition of silver on the ruffled sandpaper and HF-etched silicon wafers. The morphological characterization of the silver SERS substrates was carried out by atomic force microscopy and scanning electron microscopy. UV-Vis spectroscopy absorption spectra of the silver nanostructures showed plasmonic peaks at 522 nm and 731 nm. Toluene vapors were collected with a syringe at ambient temperature and at 100 °C, while SERS detection was always performed at room temperature. Toluene detection was based on the measurement of the Raman bands at 787 cm^-1 and 1003 cm^-1 (in the fingerprint region). The method allow gaseous toluene to be detected at its vapor concentrations of 522 ppm (mg/L), 261 ppm (mg/L) and 26 ppm (mg/L). Graphical abstract Schematic presentation of an original method for the detection of toluene vapors by SERS technique. The collection of toluene vapors was carried out at room and at high temperatures. The vapors were transferred to methanol by bubbling. The SERS measurements were carried out at room temperature.

Compact dual-fiber surface-enhanced Raman scattering sensor with monolayer gold nanoparticles self-assembled on optical fiber

Employing a self-assembly method and dual-fiber structure, a compact, low-cost, highly sensitive, fast surface-enhanced Raman scattering (SERS) optical fiber sensor was realized. Through detecting a SERS signal of 10 ppm rhodamine B analyte, sensor parameters such as coupling angle, fiber types, and corresponding glass substrate structure were optimized. The ratio of the SERS signal intensity to excitation light residual peak intensity was used as a significant parameter during the optimization process. Sensor characteristics such as the SERS signal dependence on excitation power, stability related to excitation time, and reusability were studied. Its compatibility in size with microfluidic structure would make it a prospective candidate for integrating into a microfluidic chip.

Highly sensitive and on-site NO2 SERS sensors operated under ambient conditions

This paper presents the first demonstration of highly sensitive and on-site SERS-based NO₂ sensors with a detection limit of 0.1 ppm operated under ambient conditions.

Sensitive Surface-Enhanced Raman Scattering Detection Using On-Demand Postassembled Particle-on-Film Structure

Highly sensitive and low-cost surface-enhanced Raman scattering (SERS) substrates are essential for practical applications of SERS. In this work, we report an extremely simple but effective approach to achieve sensitive SERS detection of molecules (down to 10^-10 M) by using a particle/molecule/film sandwich configuration. Compared to conventional SERS substrates which are preprepared to absorb analyte molecules for detection, the proposed sandwich configuration is achieved by postassembling a flexible transparent gel tape embedded with plasmonic nanoparticles onto an Au film decorated with to-be-detected analyte molecules. In such a configuration, the individual plasmonic gel tape and Au film have low or no SERS activity but the final assembled sandwich structure shows strong SERS signal due to the formation of numerous hot spots at the particle-film interface, where the analyte molecules themselves serve as both spacer and signal probes. Because of its simple configuration, we demonstrate that the proposed SERS substrate can be obtained over a large area with extremely low cost. Particularly, because of the on-demand nature and the flexibility, such a postassembly strategy provides an ideal solution to detect the pesticide residue on fruit surfaces with significantly enhanced sensitivity.

Large-volume constant-concentration sampling technique coupling with surface-enhanced Raman spectroscopy for rapid on-site gas analysis

In this work, a portable large-volume constant-concentration (LVCC) sampling technique coupling with surface-enhanced Raman spectroscopy (SERS) was developed for the rapid on-site gas analysis based on suitable derivatization methods. LVCC sampling technique mainly consisted of a specially designed sampling cell including the rigid sample container and flexible sampling bag, and an absorption-derivatization module with a portable pump and a gas flowmeter. LVCC sampling technique allowed large, alterable and well-controlled sampling volume, which kept the concentration of gas target in headspace phase constant during the entire sampling process and made the sampling result more representative. Moreover, absorption and derivatization of gas target during LVCC sampling process were efficiently merged in one step using bromine-thiourea and OPA-NH₄⁺ strategy for ethylene and SO₂ respectively, which made LVCC sampling technique conveniently adapted to consequent SERS analysis. Finally, a new LVCC sampling-SERS method was developed and successfully applied for rapid analysis of trace ethylene and SO₂ from fruits. It was satisfied that trace ethylene and SO₂ from real fruit samples could be actually and accurately quantified by this method. The minor concentration fluctuations of ethylene and SO₂ during the entire LVCC sampling process were proved to be <4.3% and 2.1% respectively. Good recoveries for ethylene and sulfur dioxide from fruit samples were achieved in range of 95.0-101% and 97.0-104% respectively. It is expected that portable LVCC sampling technique would pave the way for rapid on-site analysis of accurate concentrations of trace gas targets from real samples by SERS.

Synthesis of oxocarbon-encapsulated gold nanoparticles with blue-shifted localized surface plasmon resonance by pulsed laser ablation in water with CO2 absorbers

Colloidal suspensions of oxocarbon-encapsulated gold nanoparticles have been synthesized in a one-step procedure by pulsed-laser ablation (PLA) at 532 nm of a solid gold target placed in aqueous solution containing CO2 absorbers, but without any stabilizing agent. Multi-wavelength surface enhanced Raman spectroscopy allows the identification of adsorbed amorphous carbon and graphite, Au-carbonyl, Au coordinated CO2-derived bicarbonates/carbonates and hydroxyl groups around the AuNPs core. Scanning electron microscopy, energy dispersive x-ray analysis and high resolution transmission electron microscopy highlight the organic shell structure around the crystalline metal core. The stability of the colloidal solution of nanocomposites (NCs) seems to be driven by solvation forces and is achieved only in neutral or basic pH using monovalent hydroxide counter-ions (NaOH, KOH). The NCs are characterized by a blue shift of the localized surface plasmon resonance (LSPR) band typical of metal-ligand stabilization by terminal π-back bonding, attributed to a core charging effect caused by Au-carbonyls. Total organic carbon measurements detect the final content of organic carbon in the colloidal solution of NCs that is about six times higher than the value of the water solution used to perform PLA. The colloidal dispersions of NCs are stable for months and are applied as analytical probes in amino glycoside antibiotic LSPR based sensing.

Short-range plasmonic nanofocusing within submicron regimes facilitates in situ probing and promoting of interfacial reactions

In this study, a simple configuration, based on high-index dielectric nanoparticles (NPs) and plasmonic nanostructures, is employed for the nanofocusing of submicron-short-range surface plasmon polaritons (SPPs). The excited SPPs are locally bound and focused at the interface between the dielectric NPs and the underlying metallic nanostructures, thereby greatly enhancing the local electromagnetic field. Taking advantage of the surface properties of the dielectric NPs, this system performs various functions. For example, the nanofocusing of submicron-short-range SPPs is used to enhance the Raman signals of gas molecules adsorbed on the dielectric NPs. In addition, the presence of the local strong electromagnetic field accelerates the rates of interfacial reactions on the surfaces of the dielectric NPs. Therefore, the proposed nanofocusing configuration can both promote and probe interfacial reactions simultaneously. Herein, the promotion and probing of the desorption of EtOH vapor are described, as well as the photodegradation of methylene blue. Moreover, the nanofocusing of SPPs is demonstrated on an aluminum surface in both the visible and UV regimes, a process that has not been achieved using conventional tapered waveguide nanofocusing structures. Therefore, the nanofocusing of submicron-short-range SPPs by dielectric NPs on plasmonic nanostructures is not limited to low-loss noble metals. Accordingly, this system has potential for use in light management and on-chip green devices and sensors.

Review of Recent Progress of Plasmonic Materials and Nano-Structures for Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) has demonstrated single-molecule sensitivity and is becoming intensively investigated due to its significant potential in chemical and biomedical applications. SERS sensing is highly dependent on the substrate, where excitation of the localized surface plasmons (LSPs) enhances the Raman scattering signals of proximate analyte molecules. This paper reviews research progress of SERS substrates based on both plasmonic materials and nano-photonic structures. We first discuss basic plasmonic materials, such as metallic nanoparticles and nano-rods prepared by conventional bottom-up chemical synthesis processes. Then, we review rationally-designed plasmonic nano-structures created by top-down approaches or fine-controlled synthesis with high-density hot-spots to provide large SERS enhancement factors (EFs). Finally, we discuss the research progress of hybrid SERS substrates through the integration of plasmonic nano-structures with other nano-photonic devices, such as photonic crystals, bio-enabled nanomaterials, guided-wave systems, micro-fluidics and graphene.