Adsorption and release of ampicillin antibiotic from ordered mesoporous silica

In this work the adsorption and the release of ampicillin - a β-lactam penicillin-like antibiotic - from MCM-41, SBA-15, and (amino functionalized) SBA-15-NH₂ ordered mesoporous silica (OMS) materials were investigated. The silica matrices differ for their pore size (SBA-15 vs. MCM-41) mainly, and also for surface charge (SBA-15 and MCM-41, vs. SBA-15-NH₂). OMS samples were characterized through small-angle X-rays scattering (SAXS), transmission electron microscopy (TEM), N₂ adsorption-desorption isotherms, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and potentiometric titrations. The quantification of immobilized and released ampicillin was monitored by mean of UV-Vis spectroscopy. Experimental adsorption isotherms evidenced that ampicillin's loading is not related to the pore size (d_BJH) of the adsorbent. Indeed the maximal loadings were 237mg/g for SBA-15 (d_BJH=6.5nm), 278mg/g for MCM-41 (d_BJH=2.2nm), and 333mg/g for SBA-15-NH₂ (d_BJH=5.6nm). Loading seems, instead, to be related to the surface charge density (σ) of the sorbent surface. Indeed, at pH 7.4 ampicillin drug is negatively charged and likely prefers to interact with SBA-15-NH₂ (σ_SBA-15-NH2=+0.223Cm^-2) rather than the slightly negatively charged silicas (σ_SBA-15=-0.044Cm^-2 and σ_MCM-41=-0.033Cm^-2). Similarly, ampicillin release is affected by interfacial interactions. Indeed, we found a burst release from pure silica samples (SBA-15 and MCM-41), whereas a sustained one from SBA-15-NH₂ sample. We explain this behavior as a result of an attractive interaction between the protonated amino group of SBA-15-NH₂ and the negatively charged carboxylate group of ampicillin. In summary, in order to obtain a sustained drug release, the chemical nature of the matrix's surface plays a role which is more important than its textural features. SBA-15-NH₂ matrix is hence a suitable candidate for local sustained release of antibiotic drugs.