4.2.2. Clad/Functionalized

The use of polymer cladding, with the ability to preconcentrate gases and, thus, increase the Raman signal, has been shown recently in several reports. Holmstrom et al. [193] covered Si3N4 rib waveguides with a hyperbranched carbosilane fluoroalcohol-based sorbent polymer (HCFSA2). These sensing properties were first tested with ethyl acetate (EA), methyl salicylate (MeS), and dimethyl sulfoxide (DMSO), listed in increasing order of their partitioning coefficient into HCSFA2 (Figure 5a,b). These analytes, exhibiting carboxylate and sulfoxide structures, work as substitutes for the more toxic phosphonate esters, which are well-known warfare agents with a major effect on the nervous system. Later on, the detection of dimethyl methyl phosphonate (DMMP), diethyl methane phosphate (DEMP), trimethyl phosphate (TMP), and triethyl phosphate (TEP) in HCSFA2 over a single-mode waveguide was also studied by Tyndall and collaborators [221]. They reported a limit of detection of approximately 5, 10, 50 and 50 ppb, respectively. The limit of detection was related to the increase in the basicity of the compounds and the affinity to the acid–polymer cladding. The same group recently proposed alternative coatings for the waveguide-enhanced Raman spectroscopy of trace chemical warfare agent simulants. DMMP was chosen to compare the performance of three alternative coatings to HCSFA2: carbosilane chain polymer poly(methyl 2-butanol, 1,1,1-trifluoro-2-(trifluoromethyl)) siloxane (PMBTTS); 2,2-bis(4-hydroxy-3-propyl phenyl) hexafluoropropane (o1pBPAF); and fluoro-polyol (FPOL). Each sorbent is a hydrogen-bond acid designed to target hydrogenbond basic vapors. The group proved for this particular analyte that o1pBPAF has a better performance over HCSFA2 [222].

Alternatively to polymer cladding, sol-gel-based materials were tested for WERS by Haolan Zhao et al. [194]. Motivated by the two orders of magnitude of improvement for the determination of benzonitrile, valeronitrile and cyclohexanone from water, they tested the performance of a mesoporous silica cladding post-grafted with hexamethyldisilazane against solvent vapors (VOCs) in a gaseous matrix. The resultant hydrophobic coating could absorb VOCs, particularly ethanol, acetone and isopropyl alcohol (Figure 5c,d). This cladding was used to coat a single- and a double-slot silicon nitride waveguide. The confinement factor was estimated to be 36% and 32%, respectively, at a 785 nm wavelength. The single-slot waveguide LOD for isopropanol, ethanol and acetone was 53, 157 and 594 ppm, respectively [194]. The double-slot waveguide device could determine the concentration of isopropanol vapors down to 808 ppm [203]. Table 2 summarizes the most relevant information from clad WERS gas sensors discussed so far.

Despite the undeniable increase in sensitivity enabled by the use of enrichment cladding, several drawbacks can be identified for both polymers and sol-gel layers. VOCs at standard pressure and temperature are liquids according to the phase diagram, readily susceptible to condensation in the cladding, as can be seen in the broad absorption bands analyzed in most of these reports [223]. The broad spectral features are susceptible to cross-sensitivity and limit the sensor selectivity, i.e., the capability to discriminate different compounds in complex matrices [194]. In addition, signal enhancement comparable to VOC cannot be easily achieved for gases with critical temperatures below or near ambient temperature, such as methane, carbon dioxide, or acetylene.

**Figure 5.** Examples of WERS spectra. Experimental Raman spectrum after background subtraction for vapors absorbed in the cladding (HCSFA2 (**a**,**b**), Ms-HMDS (**c**,**d**)). In each plot, the reference spectrum is placed above the experimental one, and the selected peak used to track different concentrations is shown to the right. Reproduced with permission from reference [193], © 2021, Optical Society of America, and reference [194], © 2021, Optical Society of America.

**Table 2.** Overview of works using clad waveguides for WERS.


\* The group reported 3.1–5.7-cm long spirals in another publication [224].
