(80b) Comparison of Uniform and Structured Particles for the Protection of Ultraviolet Sensitive Molecules

Photodegradation is a problem observed in a wide array of systems including pharmaceuticals, paints, textiles, and even human skin. While many protection methods focus on protecting an entire system (such as applying sunscreen to skin), another approach is to protect the UV sensitive molecules themselves (such as a pharmaceutical product). Beta-carotene, the pigment that gives carrots their orange color and is also present in human skin, has a tendency to photodegrade and its degradation mechanism has been the subject of many studies. The wealth of information available on beta-carotene photodegradation along with its prominent absorption peaks which allow degradation to be quantified makes it an ideal molecule to compare UV protection mechanisms.

Particles comprised of a polymer, beta-carotene, and a UV absorber were synthesized. Uniform particles were produced in the range of 800-900 nanometers in diameter and contained a uniform distribution of the components.  Structured particles were microcapsules with a shell of polymer and absorber and a core comprised of an aqueous suspension of beta-carotene and polymer uniform particles. Microcapsules were approximately 200 micrometers in diameter. The microcapsule geometry allows a much larger separation distance between the beta-carotene molecules and protective molecules than is typically found in uniform particles.

Both types of particles were exposed to UV radiation for various intervals and the amount of beta-carotene left undegraded was determined by use of an ultraviolet visible spectrophotometer since absorption is proportional to concentration. UV protection was measured by beta-carotene decay curves of normalized absorption over increased exposure time and quantified by decay rate constants. These tools make it possible to determine which polymer structure offers superior protection from photo-oxidation.