Growth of calcium phosphate on surface-modified cotton

Abstract

A study of the growth of amorphous calcium phosphate on surface-modified cotton fibres by a combination of scanning electron microscopy/electron diffraction X-ray analysis, micro-FTIR and X-ray photoelectron spectroscopy is reported. Cotton fibres phosphorylated by the urea/phosphorous acid method and then soaked in saturated Ca(OH)2 for approximately one week were found to stimulate the growth of a calcium phosphate coating on their surfaces after soaking in 1.5×SBF for as little as 1 day. Ca(OH)2 soaking of the fibres is found to produce highly crystalline clusters lodged in the fibres which were confirmed by micro-FTIR to be calcium phosphite monohydrate (CaHPO3·H2O). In contrast, phosphorylated fibres not subjected to the Ca(OH)2 treatment did not exhibit calcium phosphate growth upon immersion in 1.5×SBF solution. Soaking of the Ca(OH)2-treated fibres with time in the 1.5×SBF solution produced progressively thicker layers of calcium phosphate on the fibres as confirmed by scanning electron microscopy and X-ray photoelectron spectroscopy. In general, calcium phosphate coatings formed over 1 1–5 day period soaking in 1.5×SBF solution appeared to consist of agglomerations of a large number of small spherical particles, while coatings formed after 17 days of soaking were distinctly chunky, thick and non-uniform in appearance. Micro-FTIR indicated that CaHPO3·H2O clusters were still present in cotton samples even after 4 days of soaking, while after 17 days, only the infrared spectrum typical of calcium phosphate was observed. EDX-measured Ca:P ratios of the coatings, although variable, suggested amorphous calcium phosphate. The mechanism of formation of the coating is believed to involve dissolution of the CaHPO3.H2O clusters upon introduction of the Ca(OH)2-treated phosphorylated cotton into the 1.5×SBF solution which elevates the Ca2+ ion concentration in the vicinity of the fibres so stimulating calcium phosphate formation. It is postulated that phosphite groups chemically bound to the cotton fibre surface or a calcium phosphite coating on the fibres act as nucleation sites for calcium phosphate growth in 1.5×SBF solution.