Infrared Spectroscopy in Aqueous Solutions: Capabilities and Challenges

Fourier-transform infrared (FTIR) spectroscopy provides rapid, reliable, quantitative, and qualitative analysis of samples in different aggregation states, i.e., gases, thin films, solids, liquids, etc. However, when analyzing aqueous solutions, particular issues associated with the rather pronounced IR absorption characteristics of water appear to interfere with the solute determination. In this review, Fourier-transform infrared spectroscopic techniques and their analytical capabilities for analyzing aqueous solutions are reviewed, and highlight examples are discussed.

Toward Chalcogenide Platform Infrared Sensor Dedicated to the In Situ Detection of Aromatic Hydrocarbons in Natural Waters via an Attenuated Total Reflection Spectroscopy Study

The objective of this study is to demonstrate the successful functionalization of the surface of a chalcogenide infrared waveguide with the ultimate goal of developing an infrared micro-sensor device. First, a polyisobutylene coating was selected by testing its physico-chemical compatibility with a Ge-Sb-Se selenide surface. To simulate the chalcogenide platform infrared sensor, the detection of benzene, toluene, and ortho-, meta- and para-xylenes was efficaciously performed using a polyisobutylene layer spin-coated on 1 and 2.5 µm co-sputtered selenide films of Ge₂₈Sb₁₂Se₆₀ composition deposited on a zinc selenide prism used for attenuated total reflection spectroscopy. The thickness of the polymer coating was optimized by attenuated total reflection spectroscopy to achieve the highest possible attenuation of water absorption while maintaining the diffusion rate of the pollutant through the polymer film compatible with the targeted in situ analysis. Then, natural water, i.e., groundwater, wastewater, and seawater, was sampled for detection measurement by means of attenuated total reflection spectroscopy. This study is a valuable contribution concerning the functionalization by a hydrophobic polymer compatible with a chalcogenide optical sensor designed to operate in the mid-infrared spectral range to detect in situ organic molecules in natural water.

Calibration of systems for quantitative fluorescence analysis of thin layers

Imaging fluorescence analysis is a powerful tool for the characterization of thin functional layers. Due to the development of new components such as cost-efficient and long life diode lasers and LEDs as well as sensitive cameras, the number of industrial in situ sensors based on fluorescence analysis technology increased rapidly in recent years. Of crucial importance for all these new sensors are efficient and robust methods for calibration. Although there are many examples for the calibration of laboratory setups for single specialized applications, there is no standardized method for the traceable device independent calibration of imaging fluorescence systems. This paper presents the evaluation of five different methods for the calibration of systems for quantitative fluorescence analysis. Each method is applied for the calibration of an imaging fluorescence laser scanner. In addition to characterizing the precision of the methods, the work analyzes the usability of the methods for different applications. The results show for the first time that a calibrated IR point sensor can be used for the auto calibration of high resolution imaging inline fluorescence sensors. In addition, we present a novel method for the transfer of calibration data between analysis systems with different optical setups by using a solid material fluorescence standard.

Mid-IR evanescent-field fiber sensor with enhanced sensitivity for volatile organic compounds

The increasing awareness of the harsh environmental and health risks associated with air pollution has placed volatile organic compounds (VOCs) sensor technologies in elevated demand. While the currently available VOC-monitoring technologies are either bulky and expensive, or only capable of measuring a total VOC concentration, the selective detection of VOCs in the gas-phase remains a challenge. To overcome this, a novel method and device based on mid-IR evanescent-wave fiber-optic spectroscopy, which enables enhanced detection of VOCs, is hereby proposed. This is achieved by increasing the number of analyte molecules in the proximity of the evanescent field via capillary condensation inside nano-porous microparticles coated on the fiber surface. The nano-porous structure of the coating allows the VOC analytes to rapidly diffuse into the pores and become concentrated at the surface of the fiber, thereby allowing the utilization of highly sensitive evanescent-wave spectroscopy. To ascertain the effectiveness and performance of the sensor, different VOCs are measured, and the enhanced sensitivity is analyzed using a custom-built gas cell. According to the results presented here, our VOC sensor shows a significantly increased sensitivity compared to that of an uncoated fiber.

Single-system based discriminative optical sensors: different strategies and versatile applications

Discriminative optical sensors with pattern recognition properties and high-throughput ability have been widely developed as they can distinguish multiple chemically similar analytes. Compared to traditional sensor arrays composed of a series of sensor elements, single-system based discriminative sensors using an array of optical changes at different wavelengths to provide input signals have drawn intensive attention recently. On the one hand, they can provide discrimination ability that is lack in using selective sensors; on the other hand, they can simplify the complex data acquisition process accompanied by multiple-element-based sensor arrays and reduce consumption of sensor samples. This tutorial review gives an overview of the development of single-system based discriminative optical sensors. Different strategies for the construction of single-system based discriminative sensors including dynamic combinatorial libraries, cross-reactive conjugated polymers, DNA G-quadruplex ensembles, combinatorial fluorescent molecular sensors, and fluorophore/surfactant aggregate ensembles are particularly introduced.

Gradient Polymer Nanofoams for Encrypted Recording of Chemical Events

We have fabricated gradient-grafted nanofoam films that are able to record the presence of volatile chemical compounds in an offline regime. In essence, the nanofoam film (100-300 nm thick) is anchored to a surface cross-linked polymer network in a metastable extended configuration that can relax back to a certain degree upon exposure to a chemical vapor. The level of the chain relaxation is associated with thermodynamic affinity between the polymer chains and the volatile compounds. In our design, the chemical composition of the nanofoam film is not uniform; therefore, the film possesses a gradually changing local affinity to a vapor along the surface. Upon vapor exposure, the nonuniform changes in local film morphology provide a permanent record or "fingerprint" for the chemical event of interest. This permanent modification in the film structure can be directly detected via changes not only in the film surface profile but also in the film optical characteristics. To this end, we demonstrated that sensing/recording nanofoam films can be prepared and interrogated on the surfaces of optical waveguides, microring optical resonators. It is important that the initial surface profile and structure of the nanofoam film are encrypted by the distinctive conditions that were used to fabricate the film and practically impossible to replicate without prior knowledge.

Waveguide-enhanced mid-infrared chem/bio sensors

Despite providing the opportunity for directly sensing molecular constituents with inherent fingerprint specificity in the 2.5-20 μm spectral regime, mid-infrared optical sensing technologies have not yet achieved the same penetration in waveguide-based chem/bio sensing compared to related sensing schemes operating at visible and near-infrared frequencies. In this review, current advances in mid-infrared chem/bio sensor technology will be highlighted and contrasted with the prevalent bottlenecks that have to date limited a more widespread adoption of mid-infrared sensing devices. However, with the increasing availability of advanced light sources such as quantum cascade lasers and the advent of on-chip semiconductor waveguide technologies, a prosperous future of this sensing concept for label-free detection in environmental analysis, process monitoring, and bioanalytics is perceived.

Determination of chlorinated hydrocarbons in water using highly sensitive mid-infrared sensor technology

Chlorinated aliphatic hydrocarbons and chlorinated aromatic hydrocarbons (CHCs) are toxic and carcinogenic contaminants commonly found in environmental samples, and efficient online detection of these contaminants is still challenging at the present stage. Here, we report an advanced Fourier transform infrared spectroscopy-attenuated total reflectance (FTIR-ATR) sensor for in-situ and simultaneous detection of multiple CHCs, including monochlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, trichloroethylene, perchloroethylene, and chloroform. The polycrystalline silver halide sensor fiber had a unique integrated planar-cylindric geometry, and was coated with an ethylene/propylene copolymer membrane to act as a solid phase extractor, which greatly amplified the analytical signal and contributed to a higher detection sensitivity compared to the previously reported sensors. This system exhibited a high detection sensitivity towards the CHCs mixture at a wide concentration range of 5~700 ppb. The FTIR-ATR sensor described in this study has a high potential to be utilized as a trace-sensitive on-line device for water contamination monitoring.

Mid-infrared materials and devices on a Si platform for optical sensing

In this article, we review our recent work on mid-infrared (mid-IR) photonic materials and devices fabricated on silicon for on-chip sensing applications. Pedestal waveguides based on silicon are demonstrated as broadband mid-IR sensors. Our low-loss mid-IR directional couplers demonstrated in SiN x waveguides are useful in differential sensing applications. Photonic crystal cavities and microdisk resonators based on chalcogenide glasses for high sensitivity are also demonstrated as effective mid-IR sensors. Polymer-based functionalization layers, to enhance the sensitivity and selectivity of our sensor devices, are also presented. We discuss the design of mid-IR chalcogenide waveguides integrated with polycrystalline PbTe detectors on a monolithic silicon platform for optical sensing, wherein the use of a low-index spacer layer enables the evanescent coupling of mid-IR light from the waveguides to the detector. Finally, we show the successful fabrication processing of our first prototype mid-IR waveguide-integrated detectors.