Trap and track: designing self-reporting porous Si photonic crystals for rapid bacteria detection.

The task of rapid detection and identification of bacteria remains a major challenge in both medicine and industry. This work introduces a new concept for the design of self-reporting optical structures that can detect and quantify bacteria in real-time. The sensor is based on a two-dimensional periodic structure of porous Si photonic crystals in which the pore size is adjusted to fit the target bacteria cells (Escherichia coli). Spontaneous bacteria capture within the pores induces measurable changes in the zero-order reflectivity spectrum collected from the periodic structure. Confocal laser microscopy and electron microscopy confirm that the Escherichia coli cells are individually imprisoned within the porous array. A simple model is suggested to correlate the optical readout and the bacteria concentration and its predictions are found to be in good agreement with experimental results. In addition, we demonstrate that sensing scheme can be easily modified to potentially allow monitoring of concentration, growth and physiological state of bacteria cells. This generic platform can be tailored to target different microorganisms by tuning the array periodicity and its surface chemistry for rapid and label-free detection outside the laboratory environment.

Mathematical modeling of drug release from nanostructured porous Si: combining carrier erosion and hindered drug diffusion for predicting release kinetics.

A novel, empirical, macroscopic model is developed to describe the release of a model anticancer drug, Mitoxantrone, from native and chemically modified porous Si (PSi) thin films. Drug release from these carriers results from a combination of two mechanisms, i.e. out-diffusion of the drug molecules and erosion of the Si scaffold. Thus, the proposed mathematical model adapts the Crank model to lump the effects of temporal changes in molecular interactions and carrier scaffold erosion into a comprehensive model of hindered drug diffusion from nanoscale porous systems. Careful characterization of pore size, porosity, surface area, drug loading, as well as Si scaffold degradation profiles, measured over the same time-scale as drug release, are incorporated into the model parameter estimation. A comparison of the experimental and model results shows accurate representation of the data, emphasizing the reliability of the model. The proposed model shows that drug diffusivity values significantly vary with time for the two studied carriers, which are ascribed to the distinctive role of the prevailing physical mechanisms in each system. Finally, secondary validation of the proposed model is demonstrated by showing adequate fit to published data of the release of dexamethasone from similar mesoporous Si carriers.