Vera P, Gallardo V, Salcedo J, Delgado AV
Facultad de Ciencias, Universidad de Granada, Granada, 18071, Spain
J Colloid Interface Sci. 1997 Mar 15;187(2):429-34. doi: 10.1006/jcis.1996.4714.
An electrokinetic study of the adsorption of a glucocorticoid drug, betamethasone disodium phosphate (BMP), on hematite (alpha-Fe2O3) particles is presented in this work. We analyze the electrophoretic mobility of both spherical and elongated hematite as a function of the concentration of BMP in the dispersion medium; it is found that, even for the lowest BMP concentration, the drug molecules adsorb onto the particles, changing their surface charge from positive to negative. When the drug concentration is increased between 10(-5) and 5 x 10(-3) M, the mobility of spherical hematite shows a monotonous increase from approximately -1.5 to approximately -3.5 &mgr;ms-1/Vcm-1; in the case of elongated particles, the mobility increases with BMP concentration up to 10(-4) M, larger concentrations of the corticoid having little effect on the electrokinetic properties of the particles. The effect of pH on the electrophoretic mobility of hematite is also investigated for two constant drug concentrations (0.05 and 0.5 mM); the isoelectric point of the particles is shifted to a pH below 3, another indication of adsorption of the negatively charged phosphate groups of BMP. These qualitative reasonings are confirmed by direct spectrophotometric determinations of the amount of BMP adsorbed on both hematite samples: spherical and ellipsoidal. The effect of pH on the adsorption density, Gamma, suggests that electrostatic interactions are not the only forces responsible for adsorption. In fact, although Gamma decreases with pH for both types of particles, it remains measurable up to pH 10, when strong electric repulsion is to be expected between hematite and phosphate groups. The effect of initial BMP concentration on Gamma is also studied. It is shown that a plateau region is reached in both samples, this indicating that their surfaces get saturated with BMP: the saturation begins when the coverage is of the order of approximately 1/8 (spherical particles) and approximately 1/40 (elongated particles) of a drug monolayer. Such difference might be due to the ellipsoidal hematite particles having a lower positive surface charge, since they are synthesized in a different dispersion medium.
本文介绍了对糖皮质激素药物倍他米松磷酸二钠（BMP）在赤铁矿（α-Fe₂O₃）颗粒上吸附的电动学研究。我们分析了球形和细长形赤铁矿的电泳迁移率随分散介质中BMP浓度的变化情况；结果发现，即使在最低的BMP浓度下，药物分子也会吸附到颗粒上，使它们的表面电荷从正变为负。当药物浓度在10⁻⁵至5×10⁻³ M之间增加时，球形赤铁矿的迁移率从约-1.5单调增加至约-3.5 μms⁻¹/Vcm⁻¹；对于细长形颗粒，迁移率随BMP浓度增加至10⁻⁴ M，更高浓度的皮质激素对颗粒的电动学性质影响很小。还研究了pH对两种恒定药物浓度（0.05和0.5 mM）下赤铁矿电泳迁移率的影响；颗粒的等电点移至pH低于3处，这是BMP带负电荷的磷酸基团吸附的另一个迹象。这些定性推理通过直接分光光度法测定BMP在球形和椭圆形两种赤铁矿样品上吸附量得到证实。pH对吸附密度Γ的影响表明，静电相互作用不是吸附的唯一作用力。事实上，尽管两种颗粒的Γ均随pH降低，但在pH高达10时仍可测量，而此时赤铁矿与磷酸基团之间预计会有强烈电排斥。还研究了初始BMP浓度对Γ的影响。结果表明，两个样品均达到一个平稳区域，这表明它们表面被BMP饱和：当覆盖率约为药物单分子层的1/8（球形颗粒）和约1/40（细长形颗粒）时开始饱和。这种差异可能是由于椭圆形赤铁矿颗粒表面正电荷较低，因为它们是在不同的分散介质中合成的。
