Significant advancements in waveguide technology in the mid-infrared (MIR) regime during recent decades have assisted in establishing MIR spectroscopic and sensing technologies as a routine tool among nondestructive analytical methods. In this review, the evolution of MIR waveguides along with state-of-the-art technologies facilitating next-generation MIR chem/bio sensors will be discussed introducing a classification scheme defining three "generations" of MIR waveguides: (1) conventional internal reflection elements as "first generation" waveguides; (2) MIR-transparent optical fibers as "second generation" waveguides; and most recently introduced(3) thin-film structures as "third generation" waveguides. Selected application examples for these each waveguide category along with future trends will highlight utility and perspectives for waveguide-based MIR spectroscopy and sensing systems.

An on-line infrared sensor system for monitoring greenhouse gases in brine environments is demonstrated. Evident changes of distinctive infrared signatures of dissolved methane and carbon dioxide under conditions relevant for greenhouse gas storage are shown, which are of particular relevance for understanding their subsurface behaviour after injection during carbon capture and storage (CCS) processes.

An infrared attenuated total reflection (IR-ATR) method for detecting, differentiating, and quantifying hydrocarbons dissolved in water relevant for oil spills by evaluating the "fingerprint" of the volatile organic compounds (VOCs) associated with individual oil types in the mid-infrared spectral range (i.e., 800-600 cm(-1)) is presented. In this spectral regime, these hydrocarbons provide distinctive absorption features, which may be used to identify specific hydrocarbon patterns that are characteristic for different crude and refined oils. For analyzing the "VOC fingerprint" resulting from various oil samples, aqueous solutions containing the dissolved hydrocarbons from different crude oils (i.e., types "Barrow", "Goodwyn", and "Saladin") and refined oils (i.e., "Petrol" and "Diesel") were analyzed using a ZnSe ATR waveguide as the optical sensing element. To minimize interferences from the surrounding water matrix and for amplifying the VOC signatures by enrichment, a thin layer of poly(ethylene-co-propylene) was coated onto the ATR waveguide surface, thereby enabling the establishment of suitable calibration functions for the quantification of characteristic concentration patterns of the detected VOCs. Multivariate data analysis was then used for a prelininary classification of various oil-types via their VOC patterns.

A portable infrared attenuated total reflection (IR-ATR) spectrometer was developed for analyzing CO2 and CH4 in geosequestration scenarios. This infrared-based online sensor system is suitable for monitoring, detecting, and differentiating carbon dioxide and methane at different pressures (i.e., up to 11 MPa) in saline aquifer and/or synthetic brine environments. The design of the sensor system eliminates the present problems in such measurement scenarios of either portability or capability operating at harsh conditions, and especially at elevated pressures for in-field deployment of current available IR systems. It is demonstrated that the detection and quantification of dissolved CO2 and CH4 at pressurized conditions is feasible at relevant saline downhole conditions present within the piping of the present injection wells serving as an online/in-line monitoring tool.