Scientific Highlights

The evaporation of a lysozyme solution drop containing Ca²⁺ ions on a superhydrophobic poly(methyl methacrylate) surface was probed by X-ray microbeam raster-diffraction. We observe the rapid deposition of fibrillar lysozyme with cross-b amyloidic structure at the interface of the drop with the substrate under conditions of weak acidity, close to physiological. The flows generated inside evaporating drops on superhydrophobic surfaces with very low adhesion forces can also be used for manipulating of matter at interfaces. Indeed, fibrillar lysozyme observed at the interface of the drop-residue to the substrate has been related to a local convective flow field developed during pinning of the drying drop. Aggregation into a fibrillar morphology with cross-b amyloidic structure has been observed by X-ray microdiffraction for small, natural peptides at the rim of drop-residues. These observations suggest that convective flow fields could be systematically explored for manipulating and probing of biopolymers such as proteins at interfaces. As an example we report here on the evaporation of drops of lysozyme solution, an approximately 14.7 kDa, 130 residue globular protein which is involved in egg shell formation. Flow-related shear-stress in blood vessels could well contribute to amyloid-associated diseases as suggested by b-peptide aggregation in microfluidic channels.

Core-shell colloidal nanorods are a very interesting material for light emitting applications due to their bright and polarized luminescence. This system has great advantages for optically pumped light emitting devices since the excitation light is efficiently harvested by the large rod-shaped shell, and the photo-excited carriers recombine after fast relaxation under emission of light from the strongly quantum confined exciton ground state of the spherical core. We recently discovered that dense layers of such nanorods do not only exhibit amplified spontaneous emission from the core, but also from the shell material.

The detection of few molecules from highly diluted solution is of extreme interest in different fields such as biomedicine, safety and eco pollution from rare and dangerous chemicals. Nano sensors based on plasmonics are the most promising devices that combine high sensitivity, label free detection and miniaturization. However, plasmonic based nanosensors, and more in general sensors whose sensitive area is in the nanometer scale, cannot directly be used for detecting molecules dissolved in femto/atto molar solutions. In other words, they are diffusion limited and their detection time becomes unpractical at those concentrations. We recently demonstrated that super hydrophobic artificial surfaces can be exploited to drive molecules present in highly diluted solution toward a specific point with accuracy of few hundreds of nanometers. At the end of the evaporation process few molecules initially dispersed in volumes of mm3 (nano/micro liters, atto molar concentration) are concentrated over the surface of plasmonic nanosensors able to detect up to few molecules by means of Raman Scattering. Results were published on Nature Photonics (Sept. 2011). Moreover, we demonstrated that devices can be used to create networks of suspended l-DNA molecules.

The fields of Plasmonics, Raman Spectroscopy, and Atomic Force Microscopy experienced a huge but independent development in the last decade. The potential progress derived by unifying these different techniques is of primary importance for obtaining simultaneous and complementary information at level of single molecule detection. We demonstrated that the convergence of those distant fields can open access to topographic, chemical and structural information on a spatial scale of few nanometers. The results were published in Nature Nanotechnology and the paper obtained the cover of January 2010, and a dedicated highlight. The work reported on the design, fabrication and measurements of a photonic-plasmonic device that is fully compatible with atomic force microscopy and Raman spectroscopy. By combining advanced fabrication methods we were able to fabricate three-dimensional nanostructures with a full control over geometries and optical properties. Imaging resolution (AFM topography) and chemical sensitivity (Raman analysis) with a spatial resolution up to 7 nm were demonstrated. The physical mechanism exploited is the generation of surface plasmon polaritons by means of a tapered nanostructure that causes Raman excitation through an adiabatic concentration of electromagnetic field in a spatial region of few nanometers at the tip apex.