# Nano- and Micropatterning on Optical Fibers by Bottom-Up Approach: The Importance of Being Ordered
## 3 Bottom-Up Fabrication of Periodic Structures on OFs
### Colloidal Self-Assembly  
For the greatest part, the processes employed so far to accomplish ordered patterns on OF are based on the self-assembly of colloidal particles. In fact, the auto-organization of colloids naturally brings to the formation of compact ordered structures. If the building blocks are monodispersed nanobeads and the process is controlled enough in order to minimize defect formation, the result is a crystal-like lattice, which represents the starting structure to obtain different nanopatterns. Self-assembly can occur directly on the fiber, as well as on a different substrate and then transferred on the fiber. In the latter case, the preferred substrate is water surface, which is easy to clean, and allows for easy transfer to any substrate. The generally used method for the assembly at the air-water interface consists of preparing a hydroalcoholic colloidal dispersion and depositing it on the water surface through a piece of a silicon wafer or plasma-treated glass slide partially immersed in the water, but other set-ups are also possible. The resulting photonic structure can be 3D PCs (where multilayer assemblies are packed), inverse opals (where the 3D assembly is first infiltrated with a different material than the one constituting the spheres, and then the spheres are removed by dissolution), 2D photonic crystals (2 DPCs), and its more popular variations gathered under the name of nanosphere lithography (NSL). The first attempt to integrate colloidal crystals with the OF dates back to 2005 when [PERSON] et al. coated a single-mode OF with a monolayer of silica beads [43] by an isothermal heating evaporation method. A similar procedure was adopted by [PERSON] et al. [44]. They used a microstructured OF featuring two 110 \(\upmu\)m large holes. The fiber was immersed in a vial containing a poly(methylmethacrylate) (PMMA) colloidal suspension in water. The size of the colloidal PMMA spheres was 300 nm. A face-centered-cubic lattice-type structure was achieved inside the twin-hole OF for a length of 2.5 cm. [PERSON] et al. fabricated the colloidal crystal directly on the tip of an OF [45]. Preliminarily, wet chemical etching technique was employed to fabricate a cavity in one side of the fiber while keeping the coating intact. The fiber was immersed with the cavity upward in the PMMA colloidal dispersion (microspheres of 690 nm diameter in water/ethanol mixture). After 2-3 h at 50 \({}^{\circ}\)C, the colloidal PC self-assembled on the tip by isothermal heating evaporation [45]. Successively, a colloidal crystal of polystyrene (PS) nanospheres with a 200 nm mean diameter was self-assembled onto the tip of a UV-VIS multimode OF by means of a dip-coating approach. The obtained fiber probes were used for label-free biosensor applications and their performances were evaluated using the highly stable streptavidin-biotin binding [46]. Self-organized colloids provide also a large-scale template for infiltrating different materials and obtaining inverse opals. In 2005, [PERSON] et al. exploited a cylindrical shell of colloidal crystals assembled around an OF by a dip-coating method in order to create microporous arrays of interconnected air cavities [47]. A UV-curable polymer precursor was infiltrated and polymerized into the interstices of the colloidal crystal template and the microporous polymer structures around the OF were obtained by dissolving the PS spheres into toluene. Similarly, the process was demonstrated by using a silica colloid with 2 \(\upmu\)m diameter. Here, in order to obtain the inverse structure, after the infiltration and cure of the polymer precursor, the silica spheres and the fiber itself were removed simultaneously by chemical etching in a diluted hydrofluoric acid solution, leaving an inverted colloidal crystal with hollow cores. The formation of inverse opals on the internal and external surface of a glass capillary was similarily carried out by [PERSON] et al. [48], employing a sol-gel co-assembly method previously demonstrated on planar substrates[49].