Adsorption characteristics of a cationic porphyrin on nanoclay at various pH.
Natural and synthetic porphyrin derivatives offer a range of applications including enzymatic catalysis, photosensitizers for light harvesting and chemical reactions, and molecular electronics. They exhibit unique optical spectra dominated by the presence of Soret and Q-band structures whose position and shape offer a straightforward method to characterize porphyrins in various surroundings. In many applications it is often beneficial to have porphyrins adsorbed onto a solid matrix. Applications of porphyrin-clay complexes extend to numerous biological applications including pharmaceutical drug delivery, cosmetics, and agricultural applications and thus a full understanding of porphyrin-clay surface interactions are essential. Here we investigated the adsorption behavior of meso-tetra(4-N,N,N-trimethylanilinium) porphine (TMAP) onto sodium containing, natural montmorillonite clay (Cloisite Na(+)) in characteristic biological buffers over a range of pHs (approximately 2-9). Spectroscopic analyses show a linear absorption response at acidic and basic pHs but a slight deviation at intermediate pHs. Absorption spectra for TMAP on clay showed distinct red shifts of the Soret and Q-bands compared to free TMAP for all buffer conditions indicating core pi-electron delocalization into the substituent rings. At intermediate pHs, a gradual transition between protonated/deprotonated states were seen, presumably due to higher H(+) concentration at the surface than in bulk. Results indicate TMAP adsorption to clay occurs in a monolayer fashion at low/high pH while slightly acidic/neutral pH possibly rearrange on the surface and/or form aggregates. AFM images of clay saturated with TMAP are reported and show single isolated clay sheets without aggregation, similar to clay without TMAP.
Rice
Zachary
Z
University at Albany, State University of New York, College of Nanoscale Science and Engineering, Albany, NY 12203, USA.
Bergkvist
Magnus
M
D000327:Adsorption; D000538:Aluminum Silicates; D002412:Cations; D000077215:Clay; D006863:Hydrogen-Ion Concentration; D018625:Microscopy, Atomic Force; D036103:Nanotechnology; D011166:Porphyrins / Q000737:chemistry*; D013057:Spectrum Analysis
D000538:Aluminum Silicates; D002412:Cations; D011166:Porphyrins; D000077215:Clay