# Chiral Chromonics Confined in Spherical Geometries
## 6 Applications  
The applications in optics for chromonic liquid crystals in planar geometries have been extensively reported [54]. In the form of dried thin films, chromonics can be exploited as linear polarizers, optical compensators, retarders, alignment layers and color filters [tam chang]. As an example, a shear-induced orientation can be used to align the chromonic N phase. Following the solvent evaporation, the polarizing films with a dichroic ratio as high as 30 can be obtained [55; 56] and, using the appropriate materials, the films may also show a polarized fluorescent emission [57]. Figure 21: XRD pattern of SSY doped with trans-Hyp (black), crystallographic peaks of trans.Hyp (red). Reprinted with permission from ref. [50]. Copyright 2022, Elsevier. Figure 20: XRD pattern of (**a**) pure SSY and (**b**) SSY doped with L-lysine. In general, all the applications demand a homogeneous and stable alignment on anisotropic surfaces, a condition that is difficult to achieve [58]. At present, a good alignment of chromonics, both planar and homeotropic, on large areas cannot be obtained without the presence of defects and stability issues. Recently, [PERSON] and co-workers presented a technique that could overcome this limit [59]. The authors supposed that the alignment problem was mainly related to the hydrophobic/hydrophilic mismatch between the substrates and chromonics, since the alignment surfaces are often hydrophobic while the in-plane aligned liquid crystalline assemblies are covered in hydrophilic groups on the outside. Following this idea, the quality and stability of the planar alignment of chromonics can be improved, adding a small amount of a non-ionic surfactant. The samples with surfactant showed an excellent alignment that was stable for months without any loss in the alignment quality. Using the same approach, well-aligned dry films of chromonics can also be obtained. Chromonics have also been proposed for applications as biosensors. In 2005, [PERSON] et al. presented a sensor composed of a chromonic liquid crystal sandwiched between two rubbed polyimide coated glasses that was observed between crossed polarizers [60]. The authors showed that the distortions of the nematic chromonic director around a spherical particle with a planar surface orientation and the consequent light transmittance through the distorted region was a steep function of the particle's size. For their experiments, they used a chromonic nematic liquid crystal containing antigen-coated latex beads and an antibody. The specific recognition of the antigen by the antibody resulted in the agglomeration of the latex beads in the large particles. The presence of these agglomerates caused a director distortion around them that could be easily detected using polarized light microscopy. Recently, the use of chromonic thin films as humidity sensors was proposed [61]. [PERSON] and co-workers demonstrated that the sensing properties related to the self-assembling properties of solid chromonic films as a function of the humidity of the surrounding air. In the dry films, the stacking distance in the molecular aggregates depended on the water content of the ambient atmosphere that modified the weak non-covalent hydrophobic and hydrophilic interactions between the molecular groups. Applying an electric field, a variation of the humidity and, consequently, the stacking distance, caused a dramatic change in the electric current passing through the aggregate that could be easily detected. The application of chiral chromonics in spherical geometries has not yet been investigated. However, it could also be foreseen in the field of sensors, for example, as time temperature indicators (TTIs). A TTI is a device or smart label that accumulates the time-temperature history of a material. This type of devices is extremely important, as an example, for the monitoring of the food cold chain to ensure food safety and freshness. There are two types of TTIs, namely, diffusion-based and chemical reaction-activated [62].