Basic and Advanced Logical Concept Derived from Surface Enhanced Infrared Spectroscopy (SEIRS) as Sensing Probe for Analysis of Chemical Species: A Brief Review

Shubhra Sinha¹, Manas Kanti Deb¹*, Indrapal Karbhal¹, Suryakant Manikpuri¹, Rajiv Nayan¹, Babita Markande¹

¹School of Studies in Chemistry, Pt. Ravishankar Shukla University, Raipur-492010 Chhattisgarh, India

Abstract

The worldwide concern for environmental pollution, climate change and health hazards caused by various pollutants has significantly increased in the recent past. Various techniques have so far been employed for sensing applications of such organic as well as inorganic pollutants. Amongst the different techniques, surface enhanced infrared spectroscopy (SEIRS) is a powerful tool which is utilized for label-free and unambiguous identification of molecular species. SEIRS overcomes the limitations of the conventional infrared spectroscopy and has emerged as a potential technique with high surface sensitivity by enhancing the signals by many folds and also facilitates new studies from the fundamental aspect to applied sciences. The current review is dedicated to a comprehension of the SEIRS technique to provide a critical overview of its application as sensing probe for analysis of chemical species. The major features of Fourier Transform Infrared spectroscopy and SEIRS have been critically discussed.

Keywords: Vibrational Spectroscopy; FTIR; SEIRS; Pollutants; Functional Group.

Infrared (IR) spectroscopy is an immensely important technique based on the vibrations of constituent atoms of a molecule which involves the interaction of IR radiation with matter. Its major concern is to measure vibrational frequencies in any molecule and especially specific functional groups present in a molecule. The applicability of IR spectrometry to qualitative and quantitative determination of chemical functionality of a gamut of chemical substances, including polymers, liquids, gases, and solids (crystalline as well as amorphous) is its major strength [Mohamed et al., 2017]. This method is conducted with the help of IR spectrometers which produce an IR spectrum. The IR spectrum is a visualized plot between transmitted/absorbed frequency and the intensity of transmission/absorption.

IR is basically a part of the electromagnetic spectrum, ranging from 50 to 12500 cm^-1. It is further divided into three main regions namely, Near IR (14000–4,000 cm⁻¹), Mid IR (4000–400 cm⁻¹), and Far IR (400–50 cm⁻¹). As compared to other spectroscopic techniques, IR is on the upper hand with the facts of being almost a universal technique, relatively fast and inexpensive, highly sensitive, easy to handle and giving rich spectral information. But the picture has a darker side too as, only those species are considered detectable and IR responsive whose IR photons alter the dipole moment of the molecule [Kurrey et al., 2019a, Rodriguez-Saona et al., 2011]. Also, it cannot detect certain compounds, mixtures and water since IR wavelengths correspond to only those sizes of molecular bonds which have lighter elements.

According to a famous saying, “Necessity is the mother of invention,” the drawbacks of IR spectroscopy led to the urge of using technique like Fourier Transform Infrared (FTIR) spectroscopy. FTIR spectroscopy is a technique used to obtain an IR spectrum of absorption of a solid, liquid or gas. An FTIR spectrometer collects high-spectral-resolution data over a wide spectral range concurrently. The emergence of FT instrumentation facilitated the increase of speed and accuracy of the conventional IR technique by replacing the use of traditional prism and grating monochromators with an interferometer. FTIR spectroscopy at present is an appealing technique due to its remarkable characteristics such as little sample preparation, rapid analysis, better signal-to-noise (SNR) ratio and minimized use of hazardous solvents [Rodriguez-Saona et al., 2011]. It utilizes the radiation’s interferometric modulation to measure multiple frequencies synchronously and produces an interferogram which is then recalculated using complicated algorithms that give the original spectrum. It is a well-established technique much earlier than Raman spectroscopy and provides a greater sensitivity and reliability compared to Surface Enhanced Raman Spectroscopy (SERS). This technique is used for the determination of variety of analytes in environmental and biological samples as its high sensitivity allows to detect the analyte at very low concentration [Kurrey et al., 2019a].

This analytical technique is commonly used for characterization and monitoring of organic and some cases of inorganic pollutants based on molecular structure and chemical bonding [Mohamed et al., 2017]. Pollutants are usually the substances that are introduced into the environment that have undesired effects on the usefulness of various resources. Organic pollutants are a class of environmental pollutants which are basically carbon pollutants which include a variety of chemical compounds depicting variant structures, origin, functions, and properties and may cause long-term or short-term detriment to the surroundings and its living beings [Mas et al., 2010]. In the last few years, the various techniques have been developed for the analysis of organic and inorganic pollutants in different composition and origins. Literatures report that these pollutants have previously been detected in the environmental samples by chromatographic and hyphenated techniques such as gas chromatography (GC), liquid chromatography-mass spectrometry (LC-MS), gas chromatography-mass spectrometry (GC-MS) and ion chromatography (IC) with low limits of detection (LOD) [Wille et al., 2012; Petrovic et al., 2010; Lim et al., 2003; Santos et al., 2003; Santos et al., 2002]. However, it is seen that such methodologies are extortionate, tedious, time consuming and require sample pre-treatment. Other proposed detection techniques for the afore detections are based on fluorimetry and phosphorimetry which too have the limitations of being sensitive to presence of even traces of heavy metals, halides, or dissolved oxygen and to slight changes in the pH. Therefore, to overcome these cruxes, FTIR spectroscopy proves out to be an excellent technique of detection for the same.

Furthermore, surface enhanced infrared spectroscopy (SEIRS) is an advanced version of attenuated total reflectance FTIR in which a nanostructured metal film is at the prism–sample interface, providing enhancement in the detection of vibrational modes of molecules adsorbed on the metal film via surface plasmon resonances. It is a strictly surface sensitive technique that exploits the electromagnetic properties of nanostructured metal films to enhance the vibrational bands of a molecular adlayer [Haung et. al., 2023]. It is noteworthy that, the phenomenon of SEIR absorption (SEIRA) was first reported by Hartstein and co-workers in 1980.  The SEIRA spectra follow the surface selection rule in a similar way as the reflection‐absorption spectra of thin films do on smooth metal substrates. It is seen that when the metal nanoparticles (MNPs) are in close contact, i.e., they begin to exceed the percolation limit, the bands in the adsorbate spectra start to suppose a dispersive shape [Khalkho et al., 2021].

In this context, the present review is dedicated to a comprehension of the SEIRS technique to provide a critical overview of its application as sensing probe for analysis of chemical species. The major features of FTIR and SEIRS have been critically discussed.

Assay for advanced logical concept derived from SEIRS as sensing probe over conventional IR

SEIRA includes the equivalent advantages with that to the conventional FTIR spectroscopy in addition to coupling with nanotechnology and chemometric tools. Additionally, it is a non-destructive technique which allows further experimental actions on the IR analyzed samples [Rytwo et al., 2015]. The phenomenon of SEIRA involves the intensity enhancement of vibrational bands of adsorbates that usually bond through carboxylic acid or thiol groups onto the thin nanoparticulate metallic films that have been deposited on an appropriate substrate. SEIRA spectra obey the surface selection rule similar to the reflection‐absorption spectra of thin films on smooth metal substrates. When the MNPs come in close contact, i.e., start to exceed the percolation limit, the bands in the adsorbate spectra start to assume a dispersive shape. Unlike SERS, which is usually only observed with silver, gold and, albeit less frequently, copper, SEIRA is observed with most metals, including platinum and even zinc [Griffiths, 2013]. Distinct from the surface‐enhanced Raman scattering i.e. SERS, (usually only observed with silver, gold and, though less frequently with copper), SEIRA is observed with majority of metals, including platinum and even zinc. In case of SERS, the Raman signal intensity of a molecule gets augmented by many orders of magnitude when adsorbed to metallic nanostructures showing atomic scale roughness. It is seen that SERS shows sensitivity only to vibrational modes with changes in the polarizability of the molecules, while on the other hand, vibrations resulting in an alteration of the dipolar moment are reported by FTIR spectroscopy. Hence, SEIRS technique is used for the determination of diversity of analytes in environmental as well as biological samples, since, its ultra sensitivity allows detection of analyte at trace concentrations [Yang et al., 2008].

IR spectral information may be used for the identification of the presence and amount of a particular compound in any sample mixture. IR spectroscopy is based on vibrations of constituent atoms of a molecule which involves interaction of IR radiation with matter. Its major concern is to measure vibrational frequencies in any molecule and especially specific functional groups present in a molecule. The IR region in the electromagnetic spectrum ranges from 50 to 12500 cm^-1 and is further divided into three main regions, Near IR (14000–4000 cm⁻¹), Mid IR (4000–400 cm⁻¹), and Far IR (400–50 cm⁻¹). The mid IR spectrum has following two regions, namely, the fingerprint region (400-1500 cm⁻¹), which is exclusive for a molecule and the functional group region (1500-4000 cm⁻¹), which is analogous for molecules having same functional groups (Figure 1).

Figure 1. Classification of IR regions

The molecules in the Finger print region show Stretching and Bending modes of vibrations which are further classified into Symmetric-Antisymmetric stretching and In plane- Out of plane bending (further diversified as scissoring, rocking, twisting and wagging) correspondingly (Figure 2 and 3). Symmetric stretching occurs when the two attached atoms move away and towards the central atom at the same time while Antisymmetric stretching occurs when the two attached atoms move away and towards the central atom at different times.

Figure 2. Symmetric and antisymmetric stretching

While in case of Scissoring, just like the name suggests, it occurs when the two atoms move away and towards each other. Rocking, on the other hand refers to the motion like a pendulum on a clock going back and forth. Here, an atom is the pendulum and there are two instead of one. Wagging can be explained by assuming a person holding up their hand in front of them and putting their two fingers in a "V" sign and bending wrist towards and away from them (the tips of the fingers are the attached atoms and the wrist is the central atom).

Figure 3. In plane and out of plane bending

Twisting can be depicted as a person walking on a treadmill where waist of the person is the central atom and their feet are the two attached atoms [Skoog et al., 2007]. The samples for analysis may be of any state, liquids, solids, or gases. However, it is noteworthy that molecules which under ordinary conditions are transparent to IR radiations are monatomic and homonuclear molecules (like Ne, He, O₂, N₂, and H₂). Diverse classes of chemical compounds contain structural units which absorb IR radiation at essential similar frequencies and intensities within that class of compound and these bands are known as “group frequencies”. The knowledge of these obtained group frequencies is used to predict the structures of unknown molecules when the standard IR spectra are not available. Sample collection and presentation accessories are present which allow collection of spectra as solids, liquids, vapours and in solution, at various temperatures, and also while undergoing mechanical deformation. The experiments conducted under such conditions assist in the determination of the structures of molecules in diverse phases as well as the structure-property relationships of materials. It is notable that the modern instrumentation allows collection of IR spectra of materials even trace levels. The capability of IR spectroscopy to examine and identify materials under such a broad diversity of conditions has made this technique the foremost position as the “work horse” of analytical science [McKelvy et al., 1998].

The conventional IR spectrometers are being used for research purposes since 1940’s [Kirk-Othmer, 2007]. However, the most remarkable development in the field of IR spectroscopy started with the emergence of the FTIR. FTIR is an analytical technique used to identify organic and inorganic materials which measures the absorption of IR radiation by the sample material versus wavelength. The IR absorption bands help identifying molecular structures and components. Advantages of FTIR over the conventional dispersive IR technique include low mechanical wear on equipment as FTIR spectroscopy does not use moving grating parts; enhancing the SNR; increased incident beam intensity going, giving a higher throughput; superior wavelength resolution and advanced wavelength accuracy. Whether it is the analysis of biological matrices or estimation of toxic elements in environmental and biological solid/liquid samples, FTIR always proves out to be an outstanding tool for quantitative as well as qualitative analysis.

The secret behind the supremacy of FTIR as a potent quantitative tool lies in its knack to carry multicomponent analyses. This multiple component quantitation is based on the additive nature of the Beer-Lambert’s law [Baravkar et al., 2011]. Fourier transformation is a mathematical method which is capable of interconversion of the two domains of distance and frequency. In a common FTIR spectrometer, the radiation emerging from the IR source is allowed to pass through an interferometer and then to the sample before reaching the detector. Figure 4 illustrates the instrumentation of FTIR spectrophotometer. The energy from the source strikes the beamsplitter and produces two beams of almost same intensity. One of the beams strikes the fixed mirror and returns to the beamsplitter while the other goes to the moving mirror. The motion of this moving mirror makes the total path length variable which is taken by the stationary mirror beam. Some of the characteristic peaks for different functional groups have been listed in Table 1 [Saha et al., 2022; Khan et al., 2018; Fuente et al, 2003].

Table 1. Characteristic IR peaks for different functional groups

Figure 4. Optical system of a FTIR spectrometer

When these two beams convene for the second time at the beamsplitter, they recombine, and the difference in their path lengths creates constructive as well as destructive interference, which is called as an interferogram. The recombined beam passes through the sample that absorbs all the wavelengths characteristic of the spectrum and then deducts specific wavelengths from the interferogram. The detector now accounts variation in energy-versus-time for all the wavelengths simultaneously. A laser beam is superimposed to serve as a reference for the operation of the instrument. Upon amplification of the signal, in which a filter priorly eradicates high frequency contributions, the data are converted to a digital form by an analogue-to-digital converter and is then transferred to the computer for Fourier transformation (FT). The most widespread interferometer that is used in FTIR spectrometry is a Michelson interferometer.

Recently, the Miniaturized Micro Electro-Mechanical Systems (MEMS) IR gas sensors were introduced as strong applicants for dense and low-cost solutions with a MEMS FTIR spectrometer as the central building block. MEMS interferometer can be regarded as a novel optical interferometer, which is based on the spatial splitting and combining of optical beams and it uses the imaging properties of multimode interference (MMI) waveguides. In principle, the light proliferates in air, allowing operation over a gamut, covering both the IR as well as the visible ranges. The overall interferometer and the beam splitter are made-up using deep reactive ion etching technology on silicon-on-insulator wafer, which are characterized in the visible and near-IR spectral ranges. The interferometer is distinguished versus the wavelength and tested as a FT spectrometer, (the spectral resolution being 2.5 nm at 635-nm wavelength). Advantages of MEMS include low cost and compact size, improved sensitivity, shorter response time, absence of interaction with the detected gas and ability to detect and quantify a broad range of gases at the same time. MEMS spectrometers can be considered as promising components in fields including future healthcare and environmental monitoring applications, where Michelson interferometers serve as the core optical engine. It is a noteworthy fact that majority of the MEMS FTIR spectrometers function in the Near-IR (NIR) with satisfactory performance taking the benefits of the high performance of the light sources and detectors along with the lesser cost and availability of the NIR optical fibres and lenses.

The strength of an IR signal relies on number of factors, namely, the electromagnetic (EM) field strength, number of molecules and corresponding absorption cross-section. It is noteworthy that these factors are quite frequent among the optical absorption spectroscopy. In principle, mid-IR spectroscopy is an essential, label-free, non-disturbing as well as appropriate optical method since the signal origins from matter-light interactions. Therefore, by using mid-IR spectroscopy, the structural and functional changes of target molecules during various biological events can be directly differentiated [Li et al., 2017]. In addition to the conventional transmission FTIR (T-FTIR) methods (eg. KBr-pellet or mull techniques) modern reflectance techniques such as Attenuated total reflectance (ATR) and Diffuse Reflectance (DRS)/DRIFT- FTIR are widely used presently in various scientific fields such as environmental, agricultural, pharmaceuticals, medicine, and food studies.

Transmission spectroscopy

It is a bygone and most commonly used method for purposes like identifying various organic/inorganic chemicals and also for providing specific information on molecular structure and chemical bonding. It can be applied to study of all types of (solids, liquids or gaseous) and is therefore an influential tool for qualitative and quantitative studies. With numerous advantages like high spectral quality, appreciable sensitivity, and speed, easy to handle, reduced analysis time, this non-destructive technique has applications in various fields like remote sensing, measurement and analysis of atmospheric spectra, analysis of solids, liquids, gases and organic/inorganic compounds. However, it has certain limitations such as the spectral quality is affected by the thickness of KBr pellets, air bubbles might disrupt liquid analysis and inconsistent liquid cell may produce irreproducible spectra. Also, water may dissolve NaCl windows and hence, an alternative water tolerant window material such as CaF₂ for specific liquid sample is required to be used.

Attenuated total reflectance (ATR)

ATR spectroscopy is a non-destructive, versatile sampling technique, helpful for surface studies, films, and solutions. It employs the phenomenon of total internal reflection (which takes place when the angle of incidence at the interface between the sample and the crystal is greater than the critical angle). ATR is a swift analytical tool which is on the upper hand to the conventional IR transmission spectroscopy as it requires less sample preparation, improves sample-to-sample reproducibility and results in better quality data for more precise material verification and identification [Rytwo et al., 2015].

In case of ATR, a beam of radiation entering the zinc selenide (ZnSe) crystal undergoes total internal reflection. It is a reflection technique which involves internal reflection of the IR light off the back surface of an internal reflection element with high refractive index, which is in contact with the sample. The IR beam travelling inside the crystal results in a standing wave of radiation which is known as evanescent wave [Elmer, 2004] and a sample in contact with this crystal can interact with the evanescent wave, absorb IR radiations and get its IR spectrum detected. The sample’s absorbance attenuates the evanescent wave which originates its name ATR. This avails the ATR with a diversified enhancement in the sample’s response in comparison with the other singe-reflection crystals [Rodriguez-Saona et al., 2011]. It allows measurements of samples like gels, dispersions, liquids and pastes.

Different designs of ATR cells enable examination of both liquid as well as solid samples. Placing a flow-through ATR cell is also likely by including an inlet and outlet source into the apparatus [Ma et. al., 2024]. This permits continuous flow of the solutions through the cell and allows monitoring the spectral changes with time. ATR-FTIR proves out to be a novel and straightforward technique with lower susceptibility to over and underestimations for the investigation of reactions at drying interfaces [Dowding et al., 2005].

The small depth of penetration attained through ATR implicates that it measures a relatively thinner layer of the sample which is in contact with the surface of an ATR element. Hence, it is not just a surface technique. The same depth of penetration is applied to ATR imaging, in which a layer of the sample, which is a few micrometres thick, adjacent to the surface of the ATR crystal is scrutinised by IR light within the imaging field of vision. In this, the rest of the sample (numerous micrometres beyond the surface) will not be quantified and remarkably, it will not affect the measurement of the layer being studied. This phenomenal feature of ATR analysis allows direct measurement of the samples without any requirement for prior cross sectioning, microtoming or polishing [Kazarian and Chan, 2013].

Diffuse Reflectance (DRS) FTIR spectroscopy

DRS is simple, sensitive and speedy analytical technique used for the characterization and quality assurance of materials on the basis of functional group, chemical bonding and molecular structures of chemical substances. It is a reproducible technique which requires little to no sample preparation with easy cleanup and automation. It is a dominant computer-based technique which runs the rapid Fourier transform algorithm and removes limitations like CO₂/H₂O vapour, water-present broad interfering absorption bands [Kurrey et al., 2019b]. Recently, Kurrey and co-workers have quantitatively analysed metal ions, food adulterants and ionic species using different complexing agents in environmental, biological and food samples based on the measurement of selected absorption peak of analytes in FTIR spectra with the help of DRS-FTIR and have statistically validated the same for its accuracy and precision [Kurrey et al., 2018]. Figure 5 compiles various characteristic features, advantages, disadvantages and applications of FTIR spectroscopy.

The measurement of powdered samples typically results in relatively long path lengths that increase the interaction of IR light and samples. It is to be noted that concentrated samples may have absorbance values beyond the dynamic range of the instrument which eventually result in higher noise. Therefore, to obtain the absorbance in linear range, the samples need pre-dilution with non-absorbing and diffusely reflecting salts like KBr. It is a viable alternative to the traditional sampling techniques for paint and varnish surfaces, tablets and rigid polymers [Selvasembian et. al., 2024].

DRS is employed in high throughput monitoring, screening and compositional analysis of solid samples – from soils and sediments to plants and wood. However, the technique has disadvantages like even trace amounts of impurities might disturb the signal; highly absorbing samples often need to be mixed with IR transparent diluters like KBr; grinding is often required to attain small particle size (which is laborious and can affect the sample due to the heat generated or the bonds may also be broken). These spectra are capable of exhibiting both absorbance and reflectance features caused by the contributions from transmission, specular and internal reflectance components and the scattering phenomena in the collected radiation [Schmitt and Flemming, 1998].

Figure 5. Features and applications of FTIR spectroscopy.

Some analytical aspects for separation and detection of chemical species using SEIRS method

In case of environmental studies, SEIRS is widely used to acquire significant compositional and structural information. FTIR spectrometers are capable for detection of over a hundred volatile organic compounds (VOCs) which are emitted from various industrial and biogenic sources. It is notable that the concentration of gases in the stratosphere and troposphere were also ascertained using FTIR spectrometers.

FTIR along with partial least square (PLS) techniques has been used in the concurrent on-line determination of gases in smoke from burning textiles [Bulien, 1996]. The various compounds that have been determined using the afore technology comprise water, CO₂, CO, NO, NO₂, SO­₂, C₃H₄O, HCl, HCN, HBr, and HF. Lindblom and co-workers have determined the shelf life of nitrocellulose containing single base propellants using FTIR and PLS calibration techniques [Lindblom et al., 1995]. A PLS method using transmission FTIR spectroscopy has recently been developed for the analysis of aldehyde formation and anisidine value of thermally stressed oils. A PLS method has also been developed for the quantitative FTIR analysis of fatty acid estersin the recent years. [Haines et al., 1997; Dubois et al., 1996]. Evaluation of polyolefin formulations using a multiple model approach and discriminant analysis with process, chemistry, and spectroscopic information have also been done [Van Every et al., 1996].

Sugarcane juices have been analysed by developing principal component regression (PCR) and Principal component analysis (PCA) [Cadet et al., 1997]. This method avails the qualitative classification of spectra without the knowledge of their chemical composition. The effect of PCA on mid-IR spectroscopy data has been inspected to determine the effects of instrumental instability on results [Defernez and Wilson, 1997]. Recently, Kurrey and co-workers have developed a novel technique of SEIRS with silver nanoparticles (AgNPs) assisted single drop microextraction (SDME) for the detection of total mixed quaternary ammonium cationic surfactants (QACS) in water samples. They used SDME to separate and preconcentrate QACS from water samples into organic solvent containing citrate-capped AgNPs through the electrostatic and hydrophobic force of interactions and abbreviated the overall procedure as an “AgNPs-SDME/(SEIRS)” method. In this method, AgNPs served to augment the signal intensity of QACS through the aggregation of NPs which resulted in the enhancement in the hot-spot density for effective absorption of the IR radiation. For the determination of total mixed QACS in water sample. They obtained linearity range of 1-20 μg L^-1 with the limit of detection (LOD) and limit of quantification (LOQ) as 0.03 μg L^-1 and 2.0 μg L^-1, respectively. [Kurrey et al., 2019a]

FTIR spectroscopy has been employed in monitoring of gases generated during chemical inhibition of fuel pool fires burning in the air. This technique, FTIR was employed in the analysis of acid gases formed when Halon 1301 was used as fire extinguishers. FTIR spectroscopy has also been exploited to study the nitric acid ices that are formed from the vapours containing water. Also, passive FTIR remote sensing has been utilized for the analysis of effluent clouds like controlled gas releases chemical manufacturing facilities and power plant emission stacks [Modiano et al., 1996].

Bacsik and co-workers have reported that FTIR has always been a promising technique for the non-destructive, simultaneous and real-time measurement of multiple gas phase compounds in multifaceted mixtures like cigarette smoke [Bacsik and Mink, 2007]. In their study, Jager and co-workers have reported that FTIR spectroscopy has the capability of measuring trace concentrations of CO₂, CH₄, N₂O and CO and also isotope ratios (especially that of ¹³CO₂) in gaseous samples [Simonescu, 2012], Ni and Cr are omnipresent heavy metal pollutants in the aquatic environments which bring out toxicities to aquatic organisms including microbes. FTIR spectroscopy was used to examine the interaction of these two heavy metals on the toxicity in Escherichia coli (E. coli). The study revealed that binding of Ni(II) to E. coli was brawnier than that for Cr(VI). It was observed that in the presence of Ni in E. coli, Cr showed aggressive effects. FTIR analysis exhibited a decrease in lipid content in the presence of Ni and not for Cr. Also, a reduction in the band area was observed in the region of 3000–2800 cm^-1 and at 1455 cm^-1 (because of a decrease in fatty acids and lipid molecules). Principle component method from the FTIR data was used to distinguish the consequences between control and metal toxicities in E. coli. [Gupta, and Karthikeyan, 2016].

Kardas and co-workers have examined and reported the alterations in cobalt-acclimated bacteria using ATR-FTIR spectroscopy on feasible samples. They investigated Bacillus sp. and Pseudomonas sp. isolated from a temperate shallow lake and a finely established strain of E. coli. This study provides updates at molecular level, supplying insight into how dissimilar kinds of bacteria develop adaptations for survival in aquatic environments exposed to heavy metals [Lewis and McElhaney, 2013]. Figure 6 compiles the steps involved in sample analysis of organic and inorganic pollutants using FTIR spectroscopy.

Figure 6. Steps involved in sample analysis of organic and inorganic pollutants using FTIR spectroscopy.

FTIR spectroscopy has been engaged broadly for the characterization of nanodiamonds (NDs) since last two decades. Features like ultra-sensitivity to the surface functional groups of NDs, non-destructive nature and usually easy sample preparation have established FTIR as a reference method for the characterization of NDs surface chemistry. However, it is noteworthy that the FTIR spectra of NDs can vary considerably between two studies, depending on the type of NDs, their surface treatments and environmental conditions [Petit and Puskar, 2018].

SEIRS is a flexible tool for the characterization of soil mineral components, including mineral identification, structural assessment, and in situ monitoring of the pedogenic processes (for instance, mineral formation). It harmonizes other analytical techniques, most remarkably, X-ray diffraction (XRD) employed in mineral identification. Specific absorption fingerprints are adequately sensitive to differentiate among shared bond types (e.g., Si-O, Al-O) by the local structural environment and hence facilitating soil mineral identification and characterization [Margenot et al., 2016].

Erfan and co-workers reported that the environmental gas sensing can be done in the NIR but it requires comparatively very high SNR for detection of traces concentrations of carbon dioxide (CO₂) with the subsistence of the water vapour (H₂O) in the ambient air. However, the environmental sensing in the Mid-IR (MIR) proves out to be advantageous since it shows strong CO₂ absorption and the absence (or the weakness) of the H₂O absorption as compared to the NIR. In their work, they have reported environmental sensing of the ambient air (i.e. CO₂ concentration) [Erfan et al., 2017; Mortada et al., 2016; Elsayed et al., 2016; Mortada et al., 2014; Al-Demerdash et al., 2014] using the MIR MEMS FTIR spectrometer.

Studies reveal that in the past times, reflectance FTIR spectroscopy has been employed for the examination of the electrochemical mechanism for ethylene glycol oxidation by polycrystalline platinum. IR spectroscopy has lately been utilized in the study of fullerene and information related to intermolecular interactions have also been discussed. In the past decade, the oxidation of mesocarbon microbeads has been followed by thermogravimetric and FTIR spectroscopic techniques. The quantitative determination of fluconazole has also been discussed using KBr pellets of the material and the transmission FTIR technique [McKelvy et al., 1998].

Chen and co-workers have characterized and investigated the adsorption mechanism of well-structured cotton derived porous carbon (CDPC) along with the cotton derived porous carbon oxide (CDPCO) which were fabricated via a simplistic and economic alkaline etching method and exploited as adsorbents for waste water cleanup. The natural cotton waste, as a carbon source, was dehydrated with sodium hydroxide (NaOH) at low temperatures and was further etched in a thermal treatment process at elevated temperatures. This blended CDPCO exhibited a phenomenal adsorption performance of organic pollutants and heavy metal ions such as methylene blue (MB), 1-naphthylamine, Cd(II) and Co(II) in aqueous solutions. The FTIR spectra of the adsorbents prior and subsequent to the adsorption were attained to identify the respective adsorption mechanism. It was reported that CDPC shows no expected adsorption peaks, signifying a pure carbon sample. In the CDPCO spectrum, the peaks of the functional groups –OH at 3360 cm^-1, C=O at 1450 cm^-1 and C–O at 1067 cm^-1 were obtained, which recommended the subsistence of –OH and –COOH groups. It is seen that these oxygen functional groups on the surface of CDPCO significantly augment its hydrophilic properties and also serve as binding sites for the organic pollutant molecules. A shift from 1318 cm^-1 to 1324 cm^-1 was observed for the adsorption of MB, attributing to the surface complexation of MB on CDPCO [Chen et al., 2015].

Moreover, SEIRS has also been employed in the analysis of pertinent amount of compositional and structural information concerning the environmental samples [Grube et al., 2008]. The analysis is not only performed to establish the nature of pollutants, but also in the determination of the bonding mechanism in case of pollutants removal by sorption processes. The techniques like continuous air pollutants analyzer (used for gases like SO₂, NO₂, O₃, NH₃), on-line gas chromatography (GC) which were employed in measurement of gas pollutants made use of simple real-time instruments to quantify the gas pollutants. It is a notable fact that innumeral sensors are needed to be used for the analysis of multiple gas pollutants simultaneously. In recent decades, FTIR spectroscopy coupled with erstwhile spectroscopic techniques like AAS (atomic absorption spectroscopy) have been exploited to investigate the impact of industrial as well as natural activities on air quality [Simonescu, 2012; Kumar et al., 2005; Childers et al., 2001].

FTIR spectroscopy has broadly been utilized for the characterization and identification of microorganisms like bacteria and yeasts, by reason of the fact that they are hydrophilic microorganisms and thus can effortlessly be suspended in water for sample preparation. Fischer and co-workers described the identification of airborne fungi employing FTIR spectroscopy and reported that the method was appropriate to reproducibly distinguish between Aspergillus and Penicillium species [Simonescu, 2012, Duygu et al., 2009; Fischer et al., 2006; Essendoubi et al., 2005].

In case of air pollution, SEIRS is productively employed for the measurement of gas pollutants due to its advantages like real time monitoring of multiple gases; prevention and analysis of IR spectra of sample can be done for a prolonged time; direct detection and measurement of criteria of toxic pollutants in ambient air and measurement of both organic as well as inorganic compounds. In addition, it can also be utilised in the characterization and analysis of microorganisms and monitoring of biotechnological processes. It was reported that, FTIR is usually installed at one fixed location, but is sometimes portable and can be operated using battery for certain short-term surveys [Simonescu, 2012; Santos et al., 2010].

FTIR spectroscopy has also been employed in the identification of the nature of possible interactions between sorbent (biosorbent) and pollutants (including heavy metals, inorganic compounds and organic compounds). Biomass FTIR spectra before and after the biosorption were taken to determine the characteristic functional groups involved in copper removal by fungal biomass which were responsible for biosorption. Interpretation of IR absorption spectra can enable [Simonescu, 2012] the determination of the bonding mechanism between copper and biomass (fungal strain, cyanobacteria or other microorganism) [Burnett et al., 2006; Ye et al., 2004;].

Flores-Jardines and co-workers, in their study, have specially focussed on FTIR emission spectroscopy, the purported “passive technique” because there are several originally hot gaseous samples like volcanic plumes, stack gas plumes, automobile gases or flames. They have abridged the basic literature in the field of special environmental applications of FTIR spectroscopy, including power plants, petrochemical and waste disposals, natural gas plants, agricultural, and industrial sites and also the detection of gases produced in biomass burning, in flames and in flares. Figure 7 summarizes the wide applicability of SEIRS as a sensing probe for various chemical species.

Figure 7. Various applications of SEIRS as a sensing probe for chemical species.

In order to evaluate the impact of air traffic on the upper and lower layers of the troposphere, it is essential to discover an effectual remote-sensing method for the measurement of the authentic gas emissions of aircraft engines at all attitudes and at defined thrust level. Numerous scientists around the world are investigating the eventual impact of such emissions on the Earth’s atmosphere [Dai et al., 2015; Flores-Jardineset al., 2007; Bacsik et al., 2005; Flores-Jardines et al., 2005;]. It has been reported that Fourier transform spectrometers have been employed in a diversity of suitable techniques, ranging from high-precision measurements of a single emission-rotation vibrational band (i.e. laser spectroscopy) to the measurement of broadband spectra.  It is seen that FTIR emission spectroscopy (FTIRES) method appears to accomplish all the afore-mentioned requirements by detecting the thermal radiations of the hot (temp ~ 300–500ºC) exhaust gases yielding details about the compounds present during the measurements [Bacsik et al., 2005].

Shrivas and co-workers have developed a smartphone-paper based sensor impregnated with AgNPs/CTAB for uncomplicated determination of ferric iron (Fe³⁺) from water and blood plasma samples. They made use of a smartphone to record an image of the paper substrate past the deposition of analyte followed by quantitative determination of Fe³⁺. They have exploited FTIR spectroscopy to illustrate and validate the sensing mechanism for the determination of Fe³⁺ employing the smartphone coupled with paper-based sensor. They believe that in the upcoming decades, these smartphone-paper-based chemical sensors can prove out to be extremely useful for the determination of iron in environmental as well as biological samples [Shrivas et al., 2020].

Khalkho and co-workers have exploited FTIR spectroscopic the detection of vitamin B₁ in food and water samples by making use of L-cysteine modified AgNPs, using plasmonic colorimetric sensor and visual (naked eye) methods under the optimized conditions. These methods were based on change in colour and red shift of the Localized Surface Plasmon Resonance (LSPR) band from 390 nm to 580 nm, the reason being the interaction of vitamin B1 towards the L-cysteine through strong electrostatic interaction disquieting the stability of AgNPs that further directed the aggregation of particles. The size, shape, diameter distribution and optical properties of AgNPs were also investigated by FTIR [Khalkho et al., 2020].

Evolved gas analysis (EGA) from thermal analyzers such as thermogravimetry (TG) or concurrent thermal analysis (STA) which refers to simultaneous TG-DSC is well recognized because it significantly augments the value of TG or TG-DSC results. The coupling interface between thermal analyzers and FTIR spectrometers frequently consists of heated adapters and a flexible, heated transfer line. Schindler and co-workers developed a unique direct coupling of an STA instrument and an FTIR spectrometer without a transfer line. They directly mounted a tiny FTIR spectrometer on top of the STA furnace leading to a compact and fully integrated STA-FTIR coupling system and concluded that the time delay caused by the volume of the transfer line itself is slightly negligible while a considerably better correlation between gas detection and TG results was observed in case of certain highly condensable decomposition gases [Risoluti et al., 2017].

In the recent years, Jin and co-workers prepared a nitrocellulose aerogel by sol-gel synthetic approach and supercritical carbon dioxide drying method as a latest energetic matrix for the nano-composite energetic materials. They derived the decomposition mechanism by making use of the TG-FTIR coupled analysis of the condensed phase and on the basis of the experimental results obtained [Jin et al., 2015].

Wang and co-workers recently experimentally examined the behaviour of pollutant gas emissions during the firing of wheat straw and coal blends by employing thermogravimetric analysis (TGA). They determined the emission characteristics of gas pollutants including HCl, SO₂, CO₂ and NO_x by exploiting coupled FTIR measurements and concluded that HCl, SO₂, CO₂ and NO_x emissions were intimately related to the volatile combustion along with char reacting stages [Wang et al., 2011].

Textile dyes symbolize some of the most intricate environmental pollutants because of their diversity and complex structure. In the recent years, plasma oxidation methods have evolved as practicable techniques for effective decay of these pollutants. Tichonovas and co-workers have examined the degradation of a gamut (in total 13) of industrial textile dyes in a pilot dielectric barrier discharge (DBD) semi-continuously operated plasma reactor. They generated plasma in a quartz tube with central liquid-filled electrode engrossed in wastewater while ambient air was employed as a feeding gas for the reactor. They used the production of ozone (gas and liquid phase) as a basis for the evaluation of performance of the reactor and exploited FTIR analysis and toxicity tests to determine the kinetics and by-products of the oxidation process [Tichonovas et al., 2013].

Recently, Cincinelli and co-workers have surveyed and investigated for the first time, the occurrence and extent of microplastic (MPs) contamination in sub surface waters which is gathered near-shore and off-shore the coastal area of the Ross Sea (Antarctica). Furthermore, they proposed a non-invasive method employing FTIR 2D Imaging, using an FPA detector for the analysis of the MPs, consisting in filtration after water sampling and also the analysis of the dried filter. They confirmed the presence of variant types of MPs using FTIR spectroscopy, with principal profusion of polyethylene and polypropylene. The study also evidenced the potential environmental impact occurring from scientific activities which include marine activities for scientific purposes and from the sewage treatment plant [Cincinelli et al., 2017].

In the recent years, thermogravimetric simultaneous analyzer coupled with a FTIR measurements have been employed in the investigation of the kinetic thermal behaviour and gaseous pollutant emissions of co-combustion between paper sludge and oil-palm solid wastes with a complete range of amalgamation ratio. Lin and co-workers reported that the co-combustion paper sludge along with oil-palm solid wastes produced noteworthy alterations in the thermal behaviours. They observed and suggested that the blending of paper sludge with oil-palm solid wastes enhanced the comprehensive combustion performance. Moreover, the analysis of the emission profiles of gaseous pollutants further exposed that the co-combustion paper sludge and oil-palm solid wastes cause a reduction in gaseous emissions (SO₂, NO and CO₂). In addition, co-combustion endorsed the KCl of oil-palm solid wastes to convert into HCl in fuel gas, which otherwise, had the capability of reducing the possibility of slagging, corrosion as well as fouling during co-combustion. They employed the nth order reaction model by the Coats–Redfern method for the determination of the kinetics parameters for the co-combustion of paper sludge, oil-palm solid wastes and their individual blended fuels. The results of the analysis illustrated that the nth order reaction model could fit the co-combustion process thoroughly [Lin et al., 2015].

In last few decades, microplastics (MPs) have been identified as promising marine pollutants of momentous concern, because of their characteristics including their persistence, toxic potential and ubiquity [Engler, 2012]. It is seen that the bulky plastic debris crumble and become smaller and also the analysis of MPs in a variety of environmental samples necessitate the identification of the same from natural materials. However, the identification technique is short of a standardized protocol. Therefore, in their recent study, Song and co-workers employed stereomicroscope and FTIR identification methods for MPs (<0.05) underestimated and fibre was considerably overestimated using the stereomicroscope both in the SML as well as beach samples. By FTIR studies, they concluded that the overall abundance was higher than by microscope in both, the SML as well as beach samples [Song et al., 2015].

Table 2. Comparison table for detection of various chemical species employing SEIRS and FTIR technique