**4.2 BioPhotonic devices**

274 Photonic Crystals – Innovative Systems, Lasers and Waveguides

growth and Bosch Reactive Ion Etching (RIE) techniques. Non conventional biophotonic nanostructures as those described below, conveniently positioned upon the pillars, would complete a hierarchical structure thus permitting the identification of proteins in the single molecule regime (Fig. 4.3C). These 'two-stages' micro nano structures function as SHSs with

Fig. 4.3. SEM image of a photonic crystal for superhydrophobicity applications.

effect, and here suitable superhydrophobic substrates come into play.

Many applications exist revolving around the use of superhydrophobic surfaces and mainly founding upon two key features of these, that are the limited contact area and the ability of a drop to slide or slip upon such surfaces (Li et al., 2007). Water repellent coatings for radar domes, satellite dishes and glass are widely reported and currently in use. These coatings provide self-cleaning under the action of rain in that they mimic the self-cleaning action of the Lotus leaf. Other utilizations include antibiofouling paints for boats, antisticking of snow for antennas and windows, self-cleaning windshields for automobiles, the separation of water and oil, and the manufacture of water-proof fireretardant clothes. The latter would represent examples of a target market that is the huge household-commodity sector, nevertheless new analytical potentials also do exist which carry the promise of unparalleled opportunities in life sciences and in the high tech field of biotechnology. In Biotechnology many efforts are devoted to manipulate, control and analyse relevant molecules as DNA and proteins, and artificially (nanotech) fabricated superhydrophobic surfaces could be an effective advance towards this objective. Let's consider, for instance, the case of microarrays. These are libraries of biological or chemical entities (probes) immobolised in a grid on a solid surface, the probes would be then interacting with targets (i.e., geneses, proteins, cDNA...) to provide deep insight into DNA sequences. It is desirable that the drop containing the targets would evaporate avoiding pinning and the formation of noxious ring like structures (it is in fact well known that pinning during evaporations causes final solute formations disposed in doughnut shape, and this is commonly known as the coffee-ring

The transport, separation and mixing of moieties dispersed in droplets has an enormous interest likewise. Superhydrophobic surfaces are slippy, and thus a droplet would move upon the application of an infinitesimal external force field: tilting the surface (and thus gravitation) or electrostatic or surface waves induced force fields would move the droplet in a controllable way (see also below in materials and methods). On the other hand, variations in the patterning or texture of the surface would generate regions where the adhesive

an increased contact angle ranging from 155° to about 175°.

**4.1.2 Current applications of SHSs** 

Biophotonic nanostructures can be integrated to SHSs to obtain devices with advanced sensing capabilities. Here we shall recapitulate in short specific types of nano-geometry based plasmonic device, and namely (i) electroless grown random assemblies of silver nanograins; (ii) regular arrays of metallic nano dots; (iii) adiabatic nanofocusing cones. Nevertheless, the method can be extended to a number of different plasmonic nanostructures, as those broadly described in the paragraphs above (De Angelis et al., 2011; Accardo et al., 2011; Gentile et al., 2011a; Gentile et al., 2011b; Di Fabrizio et al., 2008; Gentile et al., 2010).
