*2.3. Cladding*

Functional coatings, ranging from monolayers to films of several micrometers thick, have been used for decades as a route to increasing the sensitivity and/or specificity of integrated (bio)sensors. These layers work as molecular recognition coatings, serving as a solid-phase enrichment matrix for the targeted analyte, while simultaneously excluding undesirable molecules and avoiding unspecific binding. In other words, these layers enhance the signal to noise ratio by decreasing the background signal due to unspecific binding, they reduce the cross-interference with other molecules, and, at the same time, they increase the concentration of the targeted analyte relative to that of the surrounding media (i.e., solutions, atmosphere). The design of the coating layer can be adapted for a specific molecule through recognition sites, partitioning, and charge- or size exclusion [91]. This strategy has become of the utmost importance for highly sensitive transducers with low selectivity or specificity and has been adopted in a number of chemical-sensing devices such as opto-chemical, electro-chemical, plasmonic sensors, and refractive index waveguidebased sensors [92–96]. In the latter case, the analyte either increases the refractive index of the cladding or motivates a change in the thickness of the cladding itself [97–99], which is then detected by a phase-sensitive device such as a ring resonator, Mach-Zehnder interferometer, or a Bragg grating [95,97,100]. The advantages of functional cladding have also proven to be highly useful for already selective transducers, such as Raman and IR spectroscopy sensors, as a means to increase the sensitivity by analyte up-concentration and the reduction of the background signal in complex matrices.

The enrichment cladding layers can be oxides, polymers, silanes, specific biological molecules, or composite materials where more than one element is present. Polymers have been widely used due to simple processing, availability, tuneability (functional groups, molecular weight, ramification, backbone structure, or crosslinking degree), and their behaviour as extraction materials, with their enrichment properties mainly dependent on their polarity, free volume, pore size, and pore distribution [101]. A great variety of polymeric materials have already been investigated, including polyisobutylene [102], ethylene/propylene copolymer [103], low-density polyethylene [104], Teflon®AF [105], poly(dimethylsiloxane) [106,107], poly(acrylonitrile-co-butadiene) [107], poly(styrene-cobutadiene) [107], poly(vinyl chloride) [108], polystyrene, and poly(methyl methacrylate) [83,109,110]. Among these, fluorinated polymers have shown good transparency up to MIR wavelengths due to the substitution of C-H bonds, high free volume, and outstanding thermal and mechanical properties [57]. Although mostly used on waveguidebased refractive index sensors and ATR crystals, some specific polymers have been tested on integrated single-mode waveguides for Raman and IR absorption spectroscopy (see Sections 3.2.2 and 4.2.2).

Mesoporous inorganic and mesoporous hybrid inorganic-organic cladding, based on sol-gel chemistry, represent a robust alternative to polymers [111]. These materials are equally capable of providing partitioning for custom analytical tasks, while they exhibit advantageous optical, dielectric and thermal properties [112–115]. Mesoporous inorganicbased materials show robust mechanical and chemical stability capable of sustaining harsh environmental conditions such as high-energy radiation, acid, or alkaline media, as well as oxidative chemicals. Furthermore, these materials have tunable pore volume that can surpass 50% and a decreased response time (in the order of seconds) in comparison to many polymers (typically in the order of minutes). Optical losses in VIS-NIR in these materials are generally low, due to their amorphous structure and small pore size [111]. Nevertheless, the transparency at longer wavelengths suffers due to OH groups and the adsorption of water on the large surface area of the pore network. In addition, clad sensors normally need calibration due to variations in the material properties attributed to minimal changes in the process/environment.

Although cladding brings about improvements in specificity and sensitivity, calibration is mandatory for clad systems as the up-concentration factors are difficult to quantify analytically. Cladding properties may also change over time, due to phenomena such as thermal instability, dehydration, or reconfiguration. Increased response time and aging are among other limitations of clad sensor systems, as well as the potentially reduced reversibility of the system after exposure to the analyte, discriminating between disposable and reusable devices.
