A comparative study on mesocellular foam silica with different template removal methods and their effects on enzyme immobilization

Mesocellular foam silica (MCF) was synthesized using Pluronic P-123 triblock copolymer as a template. Calcination and solvent extraction were employed to remove templates and compared. Through physical, chemical and functional characterization interesting results have been observed. Microstructures of the two MCFs were very different. Pore structure of the calcined MCF was found to be more ordered than that of the solvent extracted MCF. However, pores of the solvent extracted MCF seemed more defined and spongy under SEM observations. Surface functional groups and their concentrations characterized by ToF-SIMS were also found to be different. Surface silanol groups were found to be more concentrated in the calcined MCF. Each type of MCFs was subsequently grafted by an amine functional group, adsorbed by Au ion precursors, and reduced to form Au nanoparticles (AuNPs) on the MCF surfaces. These decorated MCF surfaces were immobilized with enzyme acetylcholinesterase (AChE), modified on screen-printed carbon electrodes and tested for electrochemical responses with acetylthiocholine. AuNPs were successfully distributed within the pores for both types of MCFs. Before the functional biosensor electrochemical tests, calcination seemed more promising than solvent extraction due to periodic structure of pores, high surface area, less contamination and higher concentration of surface silanol groups available for grafting of the amine functional group. However, after immobilization of the AuNPs for electron transport improvement and enzyme immobilization, a few observations of biosensing performances were noteworthy. Firstly, having AuNPs distributed throughout the MCF structure did not help the electron transport. Results of reduced response currents from both types of MCFs compared to MCFs without AuNPs indicated that MCF walls were too thick to allow electron transport. Secondly, the higher concentration of surface silanol groups in calcined MCFs resulted in Ostwald's ripening of small AuNPs giving bigger AuNPs than the case of solvent extraction. Therefore, although the AuNPs in enzyme biosensors are expected to help electron transport and enzyme immobilization, this study showed that although the former did not occur, the latter was well demonstrated. The smaller AuNPs in the solvent extraction case resulted in higher surface area than the bigger AuNPs in the calcination case. This was the reason why the response current was higher for the solvent extracted MCFs compared to the calcined MCFs.