Magnetic restricted access carbon nanotubes for smooth Cu and Zn extraction from Cu, Zn-superoxide dismutase

In this paper we proposed the synthesis, characterization and application of magnetic restricted access carbon nanotubes (M-RACNTs) to smoothly capture Cu and Zn from Cu, Zn superoxide dismutase (Cu,Zn-SOD). Fe3O4 magnetic nanoparticles were incorporated on the external surface of carbon nanotubes. The obtained material was encapsulated with a chemically crosslinked bovine serum albumin (BSA) layer, resulting in the M-RACNTs. The BSA capsule acts as a biocompatible barrier that prevents the proteins binding on the M-RACNTs surface. In a pH higher than the isoelectric point of BSA, the proteins from the sample and from the BSA layer, are negatively charged, resulting in an exclusion mechanism by electrostatic repulsion. The characterization tests confirmed the presence of the Fe3O4 magnetic nanoparticles in the interior, and the BSA layer on the surface of the M-RACNTs. The kinetic and isotherm tests were carried out for the Cu and Zn sorption by M-RACNTs, and the results confirmed the fast mass transference rate, and the metal retention in monolayers (Sips model, Cu and Zn maximum adsorption capacities of about 65 mg g−1), respectively. M-RACNTs were able to remove 64 and 62% of Cu and Zn, respectively, from a 0.140 mol L−1 Cu,Zn-SOD aqueous solution, with only one magnetic dispersive solid phase extraction cycle. During this process, M-RACNTs retained less than 5% of the enzyme, attesting the efficiency of the BSA layer to prevent protein binding. The Cu,Zn-SOD submitted to the extraction lost about 15% of its activity, which was totally recovered when Cu and Zn were reincorporated in the enzyme structure by addiction of an aqueous solution of both metals. Thus, we are sure that the M-RACNTs are promising sorbents to obtain apoproteins from metalloproteins in biotechnological applications.