Improved stability of the carbon nanotubes-enzyme bioconjugates by biomimetic silicification

Nano-materials have been applied in many fields due to their excellent characteristics, such as the high surface area-to-volume ratio, excellent physicochemical properties and biological compatibility. In this study, multi-walled carbon nanotubes (MWCNTs) were utilized to prepare MWCNTs-papain bioconjugates and then realized the immobilization of papain. MWCNTs functionalized with carboxyl- and amine- groups on their surface were used as immobilization carriers. The immobilization of papain on the functionalized MWCNTs through physical absorption was examined. The conjugates were denoted as MWCNTs-papain bioconjugates. To improve the stability, the bioconjugates were further coated by silica through the biomimetic silicification process that induced by papain (denoted as silica-coated bioconjugates). The as-prepared MWCNTs-papain bioconjugates and the silica-coated bioconjugates were characterized by scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy. The preliminary results showed that the bioconjugates could retain most of the initial activity of papain. Compared to free papain and MWCNTs-papain bioconjugates, the silica-coated bioconjugates exhibited significantly improved thermal, pH and recycling stability. Comparisons of the kinetic parameters between MWCNTs-papain bioconjugates and the silica-coated bioconjugates revealed that the K_m value of the immobilized papain experienced a slight increase after silica coating, which suggested that the silica coating did not significantly hinder papain's access to substrate or release of product.