3.2.2. Cladding

Top cladding on single-mode waveguides for IR spectroscopy has been only used in aqueous solutions to suppress water background and increase the concentration of the analyte close to the waveguide. Although no reports have been made on the gas phase, the use of cladding to preconcentrate organic compounds from aqueous solution also implies the ability to preconcentrate the same analyte from vapors, as has been described for ATR crystals and QCM sensors [190,191]. Therefore, it is of relevance to cover the topic briefly, as cladding on integrated gas sensors can further push the sensitivity of miniature sensors. Here, we stress that LODs in liquid environments are expected to be much lower than in gasses for the same volumetric unit as the density of molecules per volume is several orders of magnitude higher in the condensed phase.

Polymers and porous silica cladding on integrated waveguides have been reported for the detection of pollutants in water solutions. A PDMS cladding on photonic crystal slot waveguides was tested to detect xylene in water down to 100 ppb *v/v* [185]. The group tested the same device with a 2 μm Su-8 coating to measure xylene and trichloroethylene in water, with a detection limit of 1 and 10 ppb (*v/v*), respectively [186]. Polyisobutylene as a sorbent cladding on chalcogenide strip waveguides was proposed but was not experimentally evaluated [192]. According to their calculations, the cladding can decrease the limit of detection by two orders of magnitude, compared to the waveguide without cladding, due to water background suppression and organic analyte diffusion into the cladding. In another work, germanium rib waveguides were coated with a mesoporous silica coating templated with cetyltrimethylammonium bromide (CTAB) and post-grafted with hexamethyldisilazane [177]. This sensor was used to determine the concentration of toluene in water in the 6.5–7.5 μm wavelength range. It is noteworthy to mention that the use of high refractive-index core material (GOS) led to a very low field confinement factor in the cladding of only around 1% when deposited on a strip waveguide. The resulting sensitivity is consequently considerably lower than with other platforms, e.g., silicon nitride rib waveguides are able to confine light in the mesoporous cladding up to 25% [193] and slot waveguides are able to increase this percentage to 36% [76,194].

Both organic polymers and mesoporous inorganic coatings were previously deposited on ATR crystal for the detection of volatile organic compounds (VOC) from vapors [107,195] and no limitation exists to perform the same experiments on single-mode waveguides. However, the possibility of increasing sensitivity for low-weight gases other than VOC by absorption or preconcentration is dependent on the availability of material designs specific to the task. Typically, these materials hold pockets properly matching the targeted molecule size and functional groups. Examples of such materials are molecularly imprinted polymers or composite materials with cage-like organic molecules, such as cryptophanes, able to match the size of methane and halogenated analogs [196]. Alternatively, reactive centers with high selectivity toward specific reactants such as platinum nanoparticles have been described [99]. Although, still, no reports can be found on specific cladding for integrated infrared spectroscopy for the pre-concentration of gases such as carbon dioxide, methane, or acetylene, some examples can be found in works on ATR crystal [197], quartz crystal microbalance [198,199], and refractive index gas sensors [96,99,200].
