Elucidation of adsorption mechanism of bone-staining agent alizarin red S on hydroxyapatite by FT-IR microspectroscopy.
To elucidate adsorption mechanism of alizarin red S (ARS), which is often used for staining bones in histology, adsorption of ARS on hydroxyapatite, Ca10(PO4)6(OH)2 (HAP), was investigated by a batch method, compared with alizarin, phenols, and benzenesulfonates. We found that ionized 1-, 2-OH groups (1-, 2-O(-)) of ARS can be electrostatically bound to Ca2+ on HAP, but that the 3-SO3(-) group of ARS hardly participates in adsorption on HAP. ARS-adsorbed HAP (ARS-HAP) in dark reddish violet was also prepared and analyzed by FT-IR microspectroscopy to gain structural information on bonding between ARS and HAP. The obtained spectrum, which was converted to difference spectra, indicated a single band of nu(C=O) at 1627 cm(-1) and two types of symmetric C=O stretching bands of nu(s)(C=O) + nu(C=C) at 1345 cm(-1) and nu(s)(C=O) + delta(O-C=C) at 1272 cm(-1). These bands imply the existence of a salt form in ARS-HAP via 1-, 2-OH groups of ARS. As a result of the existence of a chelate form in ARS-HAP via 1-OH and 9-C=O groups of ARS, two bands of nu(C=C) + nu(C=O) at 1572 cm(-1) and nu(C=O) + nu(C=C) at 1537 cm(-1) were also observed. In addition, ARS was almost desorbed from colored ARS-HAP at 50 degrees C by using neutral phosphate buffer to recover slightly pale pinkish HAP, or De-ARS-HAP. The desorbed ARS belongs to ARS previously adsorbed on HAP by salt formation, while the remaining color on De-ARS-HAP indicates ARS still adsorbed on HAP by chelate formation. Consequently, we elucidated two adsorption mechanisms of ARS on HAP: The major adsorption is salt formation made up with 1-, 2-O(-) of ARS and Ca2+ on HAP, and the minor adsorption is chelate formation made up with 1-O(-) and 9-C=O of ARS and Ca2+ on HAP.
Department of Chemistry, Saitama Medical School, 981 Kawakado, Moroyama-machi, Iruma-gun, Saitama 350-0496, Japan. tksmorig@saitama-med.ac.jp
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Moriguchi
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Yano
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Nakagawa
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Kaji
D000327:Adsorption; D000818:Animals; D000880:Anthraquinones / Q000737:chemistry*; D001842:Bone and Bones / Q000737:chemistry*; D002118:Calcium / Q000737:chemistry; D002413:Cations, Divalent; D017886:Durapatite / Q000737:chemistry*; D006878:Hydroxides / Q000737:chemistry; D007659:Ketones / Q000737:chemistry; D017550:Spectroscopy, Fourier Transform Infrared; D013194:Staining and Labeling; D013696:Temperature
D000880:Anthraquinones; D002413:Cations, Divalent; D006878:Hydroxides; D007659:Ketones; C004468:Alizarin Red S; C031356:hydroxide ion; D017886:Durapatite; D002118:Calcium