**4.3 The device as a whole**

The above analysis of SHSs and of the related properties thereof may be summarized as follows: (i) SHSs retain unique properties in terms of wettability, in particular a certain mass of water, in shape of a drop, would be repelled by such surfaces; (ii) SHSs have superior adhesive properties, in the sense that they exhibit vanishing friction coefficients; (iii) a droplet, post upon these surfaces, would accordingly preserve a quasi-spherical shape while evaporates, and the contact area at the interface would thus progressively reduce. Node (iii), above, is the key feature for such surfaces, in that it would enable to concentrate tiny amounts of agents (biomolecules) over micrometric areas. Imagine to deposit a drop of an extremely diluted solution upon a textured, superhydrophobic substrate. The drop would evaporate over time and thus the solution would get more and more concentrated. At the late stage of evaporation, the residual solute would be confined within an incredibly small region of the plane. With an appropriate design, few molecules may be conveniently enforced to confine into the smallest area conceivable, at the limit upon a sole pillar. Nano geometry based biophotonic devices, conveniently tiling these surfaces, would probe/detect the moieties with heretofore unattainable resolution limits (the process, as a whole, is recapitulated in Fig. 4.1).

The devices introduced would perform SERS measurements as well as (and definitely not better than) conventional SERS substrates. Nevertheless, here, the beneficial effects of super hydrophobicity and nanogeometry based spectroscopy are combined and conveyed into a unique platform, and from the combination of the two novel properties arise permitting the identification of proteins or analytes in the single molecule regime.

#### **4.4 Measurements**

Here, we report briefly on some experiments that would demonstrate the potentials of the method. Small drops of D.I. water containing Rhodamine molecules were gently positioned upon the substrates as in Fig. 4.4A. The evaporation process was followed over time until an irreversible transition to a pinning (Wenzel) state occurred. Few molecules were conveniently enforced to confine into a small area. Solutions were investigated with concentration as small as 10-18 M, that is, in the atto molar range. Figs. from 4.5A to C are SEM images of the residual solute of Rhodamine at the end of the process of evaporation. The magnification of the images is different: Fig. 4.5A was captured at the low magnification factor of 150, whereby the initial footprint of the drop is clearly visible; Fig. 4.5B and C were acquired setting higher magnification factors as 800 and 1500, respectively. Notice, from these, the solute extremely concentrated to the extent that the Rhodamine is accumulated into a small area clearly bridging the pillars. In any event, the residual solute is smaller than few tens of microns. Considering that the initial diameter of deposition is about 1200 m, while the final deposit is as large as 40 m, the concentration capability of the substrate is, at the very least, in the order of, roughly, (1200/40)2~1000, that is, the problem is scaled down by three orders of magnitude. Notice that this analysis relies upon geometric effects solely.

Fig. 4.5. SEM images of a metallic photonic crystal for SERS measurement of Rhodamine.

Fig. 4.5D is a microscope image reporting the deposit of a Rhodamine evaporation process as above. To prove that the identification of the residue as Rhodamine is correct and it is not instead constituted by debris or other refuses, SERS spectroscopy measurements were carried out on the sample. While conventional Raman intensity is directly proportional to the number of molecules probed, in the case of SERS, solely the molecules that are in close proximity of the nano-metallic substrate assure the enhancement of the electric field. In the contour and 3D plot as in Fig. 4.5E and F the SERS intensity signal is consistent with the matter distribution as in Fig. 4.5D, and this would prove the hypothesis above. The mapping analysis was performed by referring the band centred at 1650 cm-1.
