SURFACE FUNCTIONAL-GROUP DEPENDENCE ON APATITE FORMATION ON SELF-ASSEMBLED MONOLAYERS IN A SIMULATED BODY-FLUID

Self-assembled monolayers (SAMs) of alkanethiols having CH3, PO4H2, CO OH, CONH2, OH, and NH2 terminal groups formed on a gold surface via su lfur attachment were soaked in a simulated body fluid (SBF), whose ion concentrations were nearly equal to those of human blood plasma, at 3 7 degrees C for up to 40 days. The effect of their terminal functional groups on apatite formation was assessed using X-ray photoelectron sp ectroscopic (XPS) measurement and a quartz crystal microbalance (QCM) technique. The Ca and P atoms were detected, of which element intensit ies increased with time, on SAMs except for the alkanethiol having the methyl terminal group. The Ca/P atomic ratios of the apatites formed on the SAMs ranged from around 1.0 to around 1.3. The most potent indu cer for apatite formation, judged from the growth rate (micrometers pe r day) calculated from the weight change during QCM measurement, was t he SAM of the alkanethiol with the PO4H2 group, followed by that of th e alkanethiol with the COOH group. The SAMs of the alkanethiols with t he CONH2, OH, and NH2 groups possessed much weaker inducing powers tha n the former two SAMs. Little weight change was observed for the methy l-group-terminated alkanethiol SAM. The growth rates increased with ti me, irrespective of the terminal group species among apatite formation -inducing groups. During the experimental observation period, the foll owing relationship held. The growth rate decreased in the order PO4H2 > COOH much greater than CONH2 similar or equal to OH > NH2 much great er than CH3 similar or equal to 0. Since negatively charged groups str ongly induced apatite formation but the positively charged group did n ot, it can be said that the apatite formation initiated via calcium io n-adsorption upon complexation with a negative surface-charged group m ay be dominant in biomaterial calcification where ionic species direct ly contact the biomaterial surface in body fluids. (C) 1997 John Wiley & Sons, Inc.