Department of Chemistry, Barnard College, New York, New York 10027, USA.
Environ Sci Technol. 2011 May 1;45(9):3959-66. doi: 10.1021/es1042832. Epub 2011 Apr 7.
Dihydroxyphenylalanine (DOPA) and similar molecules are of considerable interest in studies of bioadhesion to minerals, solar cells involving titanium dioxide, and biomedical imaging. However, the extent and mechanisms of DOPA adsorption on oxides in salt solutions are unknown. We report measurements of DOPA adsorption on well-characterized rutile (α-TiO₂) particles over a range of pH, ionic strength, and surface coverage as well as a surface complexation model analysis establishing the stoichiometry, model surface speciation, and thermodynamic equilibrium constants, which permits predictions in more complex systems. DOPA forms two surface species on rutile, the proportions of which vary strongly with pH but weakly with ionic strength and surface loading. At pH < 4.5 a species involving four attachment points ("lying down") is important, whereas at pH > 4.5 a species involving only two attachment points via the phenolic oxygens ("standing up") predominates. Based on evidence of strong attachment of DOPA to titanium dioxide from single molecule AFM (Lee, H. et al., Proc. Natl. Acad. Sci.2006, 103, 12999-12003) and studies of catechol adsorption, one or more of the DOPA attachments for each species is inner-sphere, the others are likely to be H-bonds.
二羟苯丙氨酸（DOPA）和类似的分子在矿物生物黏附、涉及二氧化钛的太阳能电池以及生物医学成像等研究中具有相当大的兴趣。然而，DOPA 在盐溶液中的氧化物上的吸附程度和机制尚不清楚。我们报告了在一系列 pH 值、离子强度和表面覆盖率下，在经过良好表征的金红石（α-TiO₂）颗粒上 DOPA 吸附的测量结果，以及一个表面络合模型分析，确定了化学计量、模型表面形态和热力学平衡常数，从而可以在更复杂的系统中进行预测。DOPA 在金红石上形成两种表面物种，其比例强烈地随 pH 值变化，但随离子强度和表面负载变化较弱。在 pH < 4.5 时，涉及四个附着点的物种（“躺下”）很重要，而在 pH > 4.5 时，仅通过酚氧基涉及两个附着点的物种（“站立”）占主导地位。基于单分子 AFM（Lee, H. et al., Proc. Natl. Acad. Sci.2006, 103, 12999-12003）对 DOPA 与二氧化钛的强附着的证据以及儿茶酚吸附的研究，每个物种的 DOPA 附着之一或多个是内球的，其余的可能是氢键。
