Volatile Organic Compound Vapour Measurements Using a Localised Surface Plasmon Resonance Optical Fibre Sensor Decorated with a Metal-Organic Framework

Formation of Gold Nanoclusters from Goldcarbonyl Chloride inside the Metal-Organic Framework HKUST-1

Gas-phase infiltration of the carbonylchloridogold(I), Au(CO)Cl precursor into the pores of HKUST-1 ([Cu3(BTC)2(H2O)2], Cu-BTC) SURMOFs (surface-mounted metal-organic frameworks; BTC = benzene-1,3,5-tricarboxylate) leads to Au(CO)Cl decomposition within the MOF through hydrolysis with the aqua ligands on Cu. Small Aux clusters with an average atom number of x ≈ 5 are formed in the medium-sized pores of the HKUST-1 matrix. These gold nanoclusters are homogeneously distributed and crystallographically ordered, which was supported by simulations of the powder X-ray diffractometric characterization. Aux@HKUST-1 was further characterized by scanning electron microscopy (SEM) and infrared reflection absorption (IRRA) as well as Raman spectroscopy, time-of-flight secondary ion mass spectrometry (ToF-SIMS), X-ray photoelectron spectroscopy (XPS) and inductively coupled plasma optical emission spectroscopy (ICP-OES).

Artificial Olfactory Biohybrid System: An Evolving Sense of Smell

The olfactory system can detect and recognize tens of thousands of volatile organic compounds (VOCs) at low concentrations in complex environments. Bioelectronic nose (B-EN), which mimics olfactory systems, is becoming an emerging sensing technology for identifying VOCs with sensitivity and specificity. B-ENs integrate electronic sensors with bioreceptors and pattern recognition technologies to enable medical diagnosis, public security, environmental monitoring, and food safety. However, there is currently no commercially available B-EN on the market. Apart from the high selectivity and sensitivity necessary for volatile organic compound analysis, commercial B-ENs must overcome issues impacting sensor operation and other problems associated with odor localization. The emergence of nanotechnology has provided a novel research concept for addressing these problems. In this work, the structure and operational mechanisms of biomimetic olfactory systems are discussed, with an emphasis on the development and immobilization of materials. Various biosensor applications and current developments are reviewed. Challenges and opportunities for fulfilling the potential of artificial olfactory biohybrid systems in fundamental and practical research are investigated in greater depth.

Electrochemical Signal Amplification Strategies and Their Use in Olfactory and Taste Evaluation

Biosensors are powerful analytical tools used to identify and detect target molecules. Electrochemical biosensors, which combine biosensing with electrochemical analysis techniques, are efficient analytical instruments that translate concentration signals into electrical signals, enabling the quantitative and qualitative analysis of target molecules. Electrochemical biosensors have been widely used in various fields of detection and analysis due to their high sensitivity, superior selectivity, quick reaction time, and inexpensive cost. However, the signal changes caused by interactions between a biological probe and a target molecule are very weak and difficult to capture directly by using detection instruments. Therefore, various signal amplification strategies have been proposed and developed to increase the accuracy and sensitivity of detection systems. This review serves as a reference for biosensor and detector research, as it introduces the research progress of electrochemical signal amplification strategies in olfactory and taste evaluation. It also discusses the latest signal amplification strategies currently being employed in electrochemical biosensors for nanomaterial development, enzyme labeling, and nucleic acid amplification techniques, and highlights the most recent work in using cell tissues as biosensitive elements.

Optical Fiber Sensor for Monitoring the Evaporation of Ethanol–Water Mixtures

An inline optical fiber sensor is proposed to monitor in real time the evaporation process of ethanol–water binary mixtures. The sensor presents two interferometers, a cladding modal interferometer (CMI) and a Mach–Zehnder interferometer (MZI). The CMI is used to acquire the variations in the external medium refractive index, presenting a maximum sensitivity of 387 nm/RIU, and to attain the variation in the sample concentration profile, while the MZI monitors temperature fluctuations. For comparison purposes, an image analysis is also conducted to obtain the droplet profile. The sensor proposed in this work is a promising alternative in applications where a rigorous measurement of volatile organic compound concentrations is required, and in the study of chemical and physical properties related to the evaporation process.

Materials for Chemical Sensing: A Comprehensive Review on the Recent Advances and Outlook Using Ionic Liquids, Metal–Organic Frameworks (MOFs), and MOF-Based Composites

The ability to measure and monitor the concentration of specific chemical and/or gaseous species (i.e., “analytes”) is the main requirement in many fields, including industrial processes, medical applications, and workplace safety management. As a consequence, several kinds of sensors have been developed in the modern era according to some practical guidelines that regard the characteristics of the active (sensing) materials on which the sensor devices are based. These characteristics include the cost-effectiveness of the materials’ manufacturing, the sensitivity to analytes, the material stability, and the possibility of exploiting them for low-cost and portable devices. Consequently, many gas sensors employ well-defined transduction methods, the most popular being the oxidation (or reduction) of the analyte in an electrochemical reactor, optical techniques, and chemiresistive responses to gas adsorption. In recent years, many of the efforts devoted to improving these methods have been directed towards the use of certain classes of specific materials. In particular, ionic liquids have been employed as electrolytes of exceptional properties for the preparation of amperometric gas sensors, while metal–organic frameworks (MOFs) are used as highly porous and reactive materials which can be employed, in pure form or as a component of MOF-based functional composites, as active materials of chemiresistive or optical sensors. Here, we report on the most recent developments relative to the use of these classes of materials in chemical sensing. We discuss the main features of these materials and the reasons why they are considered interesting in the field of chemical sensors. Subsequently, we review some of the technological and scientific results published in the span of the last six years that we consider among the most interesting and useful ones for expanding the awareness on future trends in chemical sensing. Finally, we discuss the prospects for the use of these materials and the factors involved in their possible use for new generations of sensor devices.

Advanced microstructure, morphology and CO gas sensor properties of Cu/Ni bilayers at nanoscale

In this study, we investigated the morphology of synthesized Cu/Ni nanoparticles in trace of carbon sources by the co-deposition process of RF sputtering and RF-PECVD methods and localized surface plasmon resonance of CO gas sensing of Cu/Ni nanoparticles. The surface morphology was studied by analyzing 3D micrographs of atomic force microscopy using image processing techniques and fractal/multifractal analyses. The MountainsMap® Premium software with the two-way ANOVA (Variance analysis) and least-significant differences tests were used for statistical analysis. The surface nano-patterns have a local and global particular distribution. Experimental and simulated Rutherford backscattering spectra confirm the quality of nanoparticles. Then, prepared samples were exposed to CO gas flue to study their gas sensor application using the localized surface plasmon resonance method. Increasing the Ni layer over Cu one shows an interesting result in both morphology and gas sensing sides. Advanced stereometric analyses for the surface topography of thin films in conjunction with Rutherford backscattering spectrometry and Spectroscopic analysis make a unique study in the field.

Reversible and Irreversible Laser Interference Patterning of MOF Thin Films

Laser interference patterning on top of a thin film and inside a crystal is a powerful tool today to create the desired patterns for optical data processing. Here, we demonstrate reversible and irreversible laser interference patterning on a metal-organic framework (MOF) thin film through the water desorption and thermal decomposition processes, respectively. The irreversible interference pattern with a period of the strips of up to 5 µm has been realized, and its morphology has been characterized using confocal Raman and reflection spectroscopy as well as atomic force microscopy. We revealed that reducing the distance between the interference maxima from 10.5 to a record of 5 µm for MOFs yields a 10-fold increase in the surface roughness of the irreversible pattern; on the other hand, the reversible laser pattern provides a completely non-destructive effect of variable optical contrast. The experimental results obtained open up prospects for the use of MOF crystals as photosensitive materials in the template drawing of the desired patterns for different application scopes.

Sensors for Volatile Organic Compounds

This paper provides an overview of recent developments in the field of volatile organic compound (VOC) sensors, which are finding uses in healthcare, safety, environmental monitoring, food and agriculture, oil industry, and other fields. It starts by briefly explaining the basics of VOC sensing and reviewing the currently available and quickly progressing VOC sensing approaches. It then discusses the main trends in materials' design with special attention to nanostructuring and nanohybridization. Emerging sensing materials and strategies are highlighted and their involvement in the different types of sensing technologies is discussed, including optical, electrical, and gravimetric sensors. The review also provides detailed discussions about the main limitations of the field and offers potential solutions. The status of the field and suggestions of promising directions for future development are summarized.

Localised plasmonic hybridisation mode optical fibre sensing of relative humidity

This work reports an optical fibre probe functionalised with 'cotton-shaped' gold-silica nanostructures for relative humidity (RH) monitoring. The sensor response utilises the localised surface plasmon resonance (LSPR) of self-assembled nanostructures: gold nanospheres (40 nm) surrounded by one layer of poly (allylamine hydrochloride) and hydrophilic silica nanoparticles (10-20 nm) on the end-facet of an optical fibre via a wavelength shift of the reflected light. Sensor optimisation is investigated by varying the density of gold nanoparticles on the end-facet of an optical fibre. It is demonstrated that the plasmonic hybridisation mode appearing when the average gold interparticle distance is small (Median: 7.5 nm) is more sensitive to RH after functionalisation than the singular plasmonic mode. The plasmonic hybridisation mode sensor demonstrates a high linear regression to RH with a sensitivity of 0.63 nm/%RH and excellent reversibility. The response time (T_10-90%) and recovery time (T_90-10%) are calculated as 1.2 ± 0.4 s and 0.95 ± 0.18 s. The sensor shows no measurable cross-talk to temperature in the tested range between 25 °C to 40 °C and the 95% limit of agreement is 3.1%RH when compared to a commercial reference sensor. Simulation with finite element analysis reveals a polarisation-dependent plasmonic hybridisation with a redshift of plasmonic wavelength as a decrease of the interparticle distance and a higher refractive index sensitivity, which results in a high sensitivity to RH as observed in the experiment.

Polymer Microtip on a Multimode Optical Fiber as a Threshold Volatile Organic Compounds Sensor

Polymer microtips are 3D microstructures manufactured on the end face of an optical fiber by using the photopolymerization process. Such micro-optic elements made on a multi-mode optical fiber were previously tested as a transducer of refractive index sensor. These studies were an inspiration to investigate the possibility of using this type of transducer to measure the presence of volatile organic compounds in the air. The experimental results of microtips polymerized with UV and VIS were reported. It was possible to detect the presence of five different volatile compounds in the air due to the sensitivity of the transducer to the refractive indices changes. These changes were induced by the vapors condensed on the microtip surface. The measured time responses have shown that the return loss decreases rapidly as the microtip is inserted inside a glass vial filled with the tested compound. Moreover, correlations between calculated dynamic ranges and refractive indices and volumes of the volatile compounds inside the vials were negligible. Therefore, this type of sensor can be categorized as a condensed material threshold sensor. This sensor can be used in warning systems for monitoring leakages of pipelines carrying volatile chemicals.

Exploring breath biomarkers in BLM-induced pulmonary fibrosis mice with associative ionization time-of-flight mass spectrometry

Pulmonary fibrosis (PF) is a common but fatal disease that threatens human health worldwide. Developing effective diagnostic methods is of great importance for the early detection of PF in patients. In this study, bleomycin (BLM) was used in mice to mimic idiopathic pulmonary fibrosis (IPF). The exhaled breath of BLM-induced PF, PF plus DDAH1 overexpression, and healthy control mice were analyzed in real-time using a newly developed associative ionization time-of-flight mass spectrometry method (AI-TOFMS), which is uniquely sensitive, especially to oxygenated volatile organic compounds (VOCs). Multivariate data analyses and discriminant modeling analyses revealed that four exhaled compounds, i.e., acrolein, ethanol, nitric oxide, and ammonia, had a strong correlation with PF symptoms. An Orthogonal Partial Least Square Discriminant Analysis (OPLS-DA) score plot showed an excellent separation between these three groups. The area under the receiver operating characteristic (ROC) curve for these four compounds distinguished PF mice from healthy controls at 0.989. In addition, the degrees of acute inflammation and fibrosis were assessed with Hematoxylin and Eosin (H&E) staining and Masson's Trichrome staining. Finally, combined with pathological characteristics and mRNA expression levels, the formation of the above-mentioned volatile compounds was explored. The obtained experimental results indicated that these four breath compounds, acrolein, ethanol, nitric oxide, and ammonia, were potential exhaled biomarkers for pulmonary fibrosis.

Surface-Enhanced Raman Sensing of Semi-Volatile Organic Compounds by Plasmonic Nanostructures

Facile detection of indoor semi-volatile organic compounds (SVOCs) is a critical issue to raise an increasing concern to current researchers, since their emissions have impacted the health of humans, who spend much of their time indoors after the recent incessant COVID-19 pandemic outbreaks. Plasmonic nanomaterial platforms can utilize an electromagnetic field to induce significant Raman signal enhancements of vibrational spectra of pollutant molecules from localized hotspots. Surface-enhanced Raman scattering (SERS) sensing based on functional plasmonic nanostructures has currently emerged as a powerful analytical technique, which is widely adopted for the ultra-sensitive detection of SVOC molecules, including phthalates and polycyclic aromatic hydrocarbons (PAHs) from household chemicals in indoor environments. This concise topical review gives updated recent developments and trends in optical sensors of surface plasmon resonance (SPR) and SERS for effective sensing of SVOCs by functionalization of noble metal nanostructures. Specific features of plasmonic nanomaterials utilized in sensors are evaluated comparatively, including their various sizes and shapes. Novel aptasensors-assisted SERS technology and its potential application are also introduced for selective sensing. The current challenges and perspectives on SERS-based optical sensors using plasmonic nanomaterial platforms and aptasensors are discussed for applying indoor SVOC detection.

On-chip integration of a metal-organic framework nanomaterial on a SiO2 waveguide for sensitive VOC sensing

In silicon photonic waveguides, the on-chip integration of high-performance nanomaterials is considerably important to enable the waveguide sensing function. Herein, the in situ self-assembly of the low refractive index (RI) metal-organic framework nanomaterial ZIF-8 with a large surface area and high porosity on the surface of a designated SiO₂ waveguide for evanescent wave sensing is demonstrated. The surface morphology and transmission loss of the nano-functionalized waveguide are investigated. The specific design and fabrication of asymmetric Mach-Zehnder interferometers (AMZIs) are performed based on the optical properties of ZIF-8. Such efforts in waveguide engineering result in an output interfering spectrum of nano-functionalized AMZI with an ultra-high extinction ratio (28.6 dB), low insertion loss (∼13 dB) and suitable free spectral range (∼30 nm). More significantly, the outstanding sensing features of ZIF-8 are successfully realized on the SiO₂ waveguide chip. The results of ethanol detection show that the AMZI sensor has a large detection range (0 to 1000 ppm), high sensitivity (19 pm ppm^-1 from 0 to 50 ppm or 41 pm ppm^-1 from 600 to 1000 ppm) and low detection limit (1.6 ppm or 740 ppb). This combination of nanotechnology and optical waveguide technology is promising to push forward lab-on-waveguide technology for volatile organic compound (VOC) detection.

An Overview of Artificial Olfaction Systems with a Focus on Surface Plasmon Resonance for the Analysis of Volatile Organic Compounds

The last three decades have witnessed an increasing demand for novel analytical tools for the analysis of gases including odorants and volatile organic compounds (VOCs) in various domains. Traditional techniques such as gas chromatography coupled with mass spectrometry, although very efficient, present several drawbacks. Such a context has incited the research and industrial communities to work on the development of alternative technologies such as artificial olfaction systems, including gas sensors, olfactory biosensors and electronic noses (eNs). A wide variety of these systems have been designed using chemiresistive, electrochemical, acoustic or optical transducers. Among optical transduction systems, surface plasmon resonance (SPR) has been extensively studied thanks to its attractive features (high sensitivity, label free, real-time measurements). In this paper, we present an overview of the advances in the development of artificial olfaction systems with a focus on their development based on propagating SPR with different coupling configurations, including prism coupler, wave guide, and grating.

Biochemical Methanol Gas Sensor (MeOH Bio-Sniffer) for Non-Invasive Assessment of Intestinal Flora from Breath Methanol

Methanol (MeOH) in exhaled breath has potential for non-invasive assessment of intestinal flora. In this study, we have developed a biochemical gas sensor (bio-sniffer) for MeOH in the gas phase using fluorometry and a cascade reaction with two enzymes, alcohol oxidase (AOD) and formaldehyde dehydrogenase (FALDH). In the cascade reaction, oxidation of MeOH was initially catalyzed by AOD to produce formaldehyde, and then this formaldehyde was successively oxidized via FALDH catalysis together with reduction of oxidized form of β-nicotinamide adenine dinucleotide (NAD+). As a result of the cascade reaction, reduced form of NAD (NADH) was produced, and MeOH vapor was measured by detecting autofluorescence of NADH. In the development of the MeOH bio-sniffer, three conditions were optimized: selecting a suitable FALDH for better discrimination of MeOH from ethanol in the cascade reaction; buffer pH that maximizes the cascade reaction; and materials and methods to prevent leaking of NAD+ solution from an AOD-FALDH membrane. The dynamic range of the constructed MeOH bio-sniffer was 0.32-20 ppm, which encompassed the MeOH concentration in exhaled breath of healthy people. The measurement of exhaled breath of a healthy subject showed a similar sensorgram to the standard MeOH vapor. These results suggest that the MeOH bio-sniffer exploiting the cascade reaction will become a powerful tool for the non-invasive intestinal flora testing.