The size of an ion affects everything from the structure of water to life itself. In this report, to gauge their size, ions dissolved in water are forced electrically through a sub-nanometer-diameter pore spanning a thin membrane and the current is measured. The measurements reveal an ion-selective conductance that vanishes in pores <0.24 nm in diameter—the size of a water molecule—indicating that permeating ions have a grossly distorted hydration shell. Analysis of the current noise power spectral density exposes a threshold, below which the noise is independent of current, and beyond which it increases quadratically. This dependence proves that the spectral density, which is uncorrelated below threshold, becomes correlated above it. The onset of correlations for Li + , Mg 2+ , Na + and K + -ions extrapolates to pore diameters of 0.13 ± 0.11 nm, 0.16 ± 0.11 nm, 0.22 ± 0.11 nm and 0.25 ± 0.11 nm, respectively—consonant with diameters at which the conductance vanishes and consistent with ions moving through the sub-nanopore with distorted hydration shells in a correlated way.

The modifications to the vibrational spectra produced by inclusion into cyclodextrins on the vibrational spectra of of the non-steroidal anti-inflammatory drug ibuprofen, by inclusion into cyclodextrins have been investigated by means of Raman scattering and numerical simulation. These changes are discussed and explained by comparison with the theoretical vibrational wavenumbersfrequencies and Raman intensities obtained by quantum and classical numerical simulations, disentangling the effects directly related to the complexation process, from those to be ascribed to non-covalent dimerization of ibuprofen due to hydrogen bonding.

Slot waveguides are very promising for optical sensing applications because of their peculiar spatial mode profile. In this paper we have carried out a detailed analysis of mode confinement properties in slot waveguides realized in very low refractive index materials. We show that the sensitivity of a slot waveguide is not directly related to the refractive index contrast of high and low materials forming the waveguide. Thus, a careful design of the structures allows the realization of high sensitivity devices even in very low refractive index materials (e.g., polymers) to be achieved. Advantages of low index dielectrics in terms of cost, functionalization and ease of fabrication are discussed while keeping both CMOS compatibility and integrable design schemes. Finally, applications of low index slot waveguides as substitute of bulky fiber capillary sensors or in ring resonator architectures are addressed. Theoretical results of this work are relevant to well established polymer technologies.

Channel waveguide based evanescent-field optical sensors were developed to make a fully integrated chip biosensor. The optical system senses fluorescent analytes immobilized within a micrometric sized bio-reactor well realized within an optical waveguide. The main novelty of this work is related to the fact that, within the bioreactor well, the excitation of the fluorescent signal is achieved by means of the evanescent field propagating through a silicon oxynitride waveguide. The immobilization of the emitting molecules has been realized by functionalization of the waveguide surface by a wet chemical method. These photonic biosensors are successfully applied to detect low surface concentration (10 -11 mol/cm 2 ) of a green emitting organic dye. This approach could permit the selective detection of a wide range of chemical and biological species in complex matrices and can be exploited to set-up array-based screening devices. In this regards, the preferential excitation of the dye molecules in the close vicinity of the exposed waveguide core is also analyzed.