Materials

Ayusman Sen
Department of Chemistry
The Pennsylvania State University
University Park, Pennsylvania 16802, USA
E-mail: asen@psu.edu

Bottom-up Assembly of Ordered Metamaterials:
In conventional solids nature does not typically allow for much tuning of the bonding and electronic structure. Considerable interest has arisen in ordered arrays of quantum structures, such as quantum dots, that are linked together by molecules that facilitate electronic, magnetic and thermal communication. Such materials might be engineered to have desired properties, such as superior thermoelectric transport or exotic magnetic behavior. With such a “metamaterial” approach there is much more flexibility in designing the electronic structure by varying the linker molecules, the terminating end groups and the semiconductor quantum structures than there is for conventional solids. The number of possible quantum structure metamaterials and the opportunities to controllably tune them for optimal properties are endless. Successful implementation of this concept, in particular realization of metamaterial energy bands, could allow for a profound new paradigm in solid state materials. Metamaterial energy bands arise because of the order in the linked quantum structures, rather than the order in a conventional solid. Our focus is on synthesis of well-ordered and structurally well-characterized 2 and 3-dimensional nanocomposite metamaterials. We have begun with an investigation of the synthesis of smaller, soluble clusters of ordered metamaterials, followed by extension of these studies to extended crystalline arrays. We have adapted a modular approach to the synthesis of metamaterials using interchangeable building blocks.

Novel Antimicrobial Polymers and Composites:
We have initiated research aimed at the design of novel ionic polymers and composites with antimicrobial properties. Obvious applications include the fabrication of sterile devices and surfaces, and as material for antiseptic containers and packagings. Military applications include decontamination kits, gas masks and other intake filters. It may also be possible to incorporate these materials as part of protective clothing. The target materials are expected to have the following combination of properties: (a) Killing of up to 99% of air and waterborne gram-positive and gram-negative bacteria on contact. (b) Synthetic techniques applicable to coating of a wide range of surfaces.

Polycations with pendant alkyl tails are highly effective bactericides. It is speculated that the bactericidal mechanism involves penetration into cell membranes leading to cell damage and death. Two important consequences of using an appropriate polycation rather than the monomeric analog are a significant increase in potency as judged by minimum inhibitory concentration (MIC) and the insensitivity to the action of multidrug resistant (MDR) pumps. Consequently, the development of bacterial resistance to polycations appears to be unlikely and materials and surfaces fabricated from these materials should resist biofilm formation.

In our initial studies, we have devised a simple method of fabricating highly potent antibacterial materials consisting of a cationic polymer matrix and, optionaly, embedded silver halide nanoparticles. The synthesized composites have potent antibacterial activity towards both gram-positive and gram-negative bacteria. The materials form good coatings on surfaces and kill both airborne and waterborne bacteria. We have also demonstrated the ability to tune the release of biocidal Ag+ ions from these composites by controlling the size of the embedded silver halide nanoparticles.