Victor M. Pantojas

Monitoring air quality is crucial for understanding and improving public health. There is interest in developing ultra-sensitive, low-power, cost-effective sensors. This work demonstrates that structural modulation of Sn nanoparticles through controlled deposition and oxidation enables a transition from metallic to semiconducting percolative networks, significantly enhancing NO₂ sensing performance at room temperature. The proposed percolation-driven sensing mechanism provides a new framework for understanding charge transport and gas interaction in nanostructured metal oxide systems. The nanoparticles are deposited near the percolation threshold for electrical conduction and, upon exposure to air, consist of a tin core and an amorphous Sn₃O₄ surface. Post-deposition heating in air at 320 °C for two hours forms SnO and Sn₃O₄ on top of the gold electrodes and polycrystalline SnO in the tetragonal litharge phase, known as Romarchite, on the glass between the electrodes. Both as-deposited and heat-treated sensors were capable of detecting NO₂ at room temperature, with a limit of detection in the parts-per-billion range. A percolation model is used to explain their operating currents, in which NO₂ reacts at nanoparticle gaps and intra-grain boundaries to form charge-depletion regions that primarily determine their resistance. Heat treatment has also been found to cause disproportionation of SnO, resulting in tin-rich precipitates and increasing the operating current to the milliampere range. These precipitates, although oxidized on their surfaces when exposed to air, may serve as bridges that reduce the total resistance of the percolating paths.

Scaffolds composed of a mixture of poly (L-lactic) acid (PLLA) and polyethylene glycol (PEG) biodegradable polymers were prepared by electrospinning. Three-dimensional scaffolds of highly porous non-woven fibers were produced for biomedical applications and coated with calcium phosphate for bone tissue engineering. A mixture (80/20) of PLLA/PEG was dissolved at 5.7%, 7%, 8% and 9% blend solution concentrations. The structure and morphology of the scaffolds were investigated by scanning electron microscopy. Average fiber diameters ranging from 600 nm to 800 nm were obtained as result of the change in viscosity. The low polymer concentration fibers were found to be flat with fused junctions between fibers. For high polymer concentrations fibers they were cylindrical with fibers overlaying each other. For samples deposited at 9% concentration, individual fibers contained pores on their surface with nanometric dimensions. In addition, thin films of calcium deficient hydroxyapatite were prepared by rf magnetron sputtering on silicon substrates heated to temperatures between 300-6000C. These results suggest that it is feasible to fabricate biopolymer scaffolds using methods combining electrospinning and sputtering techniques.