Self assembly for surface functionalization to improve biocompatibilities in Ti-based implants and enzyme immobilization in biofuel cells
Self-assembly is an effective biomimetic technique for surface functionalization and nanostructural synthesis. 3-Aminopropyltriethoxysilane (APTES) is a popular molecule that can assemble over substrates to modify surface properties. Titanium is a structural material with high weight-specific mechanical properties, corrosion-resistance and bioinertness. Here, a systematic investigation was carried out to optimize the self-assembly of an APTES-modified film on an oxidized titanium surface in order to improve its biocompatibility as an implant material and molecular selectivity, e.g. for CO2 capture. A clean TiOx layer was formed on titanium after the treatment in a Piranha solution of H2SO4 : H2O2 = 3:1. The IR spectra confirmed that the formation of the APTES-modified film (called APS film) on the surface by the presence of the Si-O-Ti and Si-O-Si covalent bonds. The ordering of the self-assembled film did not show strong temperature dependence from 30 to 70ºC, although a thicker film was noted at a higher temperature. Anhydrous toluene as the solvent is essential to the formation of a well-ordered and thin film, compared with hydrous toluene. The well-assembled film was formed on the oxidised titanium surface in the anhydrous toluene solution of ~0.2 v% APTES at 30°C for 16 hours. A higher APTES concentration leads to a disorder film with protonated –NH3 + groups, whereas a lower concentration causes end groups of the adsorbed APTES to loop with the -OH groups on the surface. The APS film with the free –NH2 functional groups is more stable in aqueous solution with pH 10, although it is still hydrolyzed according to the intensity of the –Si-O-Si- bond in the IR spectra. The well-ordered APS film with the –NH2 groups cannot induce heterogeneous nucleation in a simulated body fluid (SBF), because the –NH2 groups are neutral in the solution and the –CH2- hydrophobic groups are exposed in the disordered structure of the APS film. In the application of biofuel cells, the laccase from Trametes versicolor as an enzyme was immobilized on titanium and graphite with the APS film by the covalent bond, respectively. Compared with the native laccase, optimum pH of the immobilized laccase decreased to 3 because of the increase of turnover number (Kcat). Further comparison of Michaelis-Menten constant (Km) of the immobilized laccase with the native one clearly shows that the increase of Km value is mainly due to the change of configuration of the active site, further leading to the lower affinity of immobilized laccase towards the substrate. The laccase on graphite shows higher optimum temperature and twice lower the Km value, compared with the laccase on titanium, which results from the surface morphology of graphite after oxidation. For electrochemical behaviour, graphite with the laccase as electrode does not show direct electron transfer (DET), due to the long electron tunnel between the T1 centre and electrode surface. However, the electrode with laccase shows good mediator electron transfer (MET) in the presence of mediator.
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