Water dispersible 1-one-butyric acid-functionalised multi-walled carbon nanotubes for enzyme immobilisation and glucose sensing

A new approach to synthesising 1-one-butyric acid-functionalised multi-walled carbon nanotubes (MWCNTs-BA) is presented involving the surface modification of multi-walled carbon nanotubes (MWCNTs) via a Friedel-Crafts chemical oxidative reaction using succinic anhydride in aluminium chloride. The MWCNTs-BA disperse homogeneously in water and exhibit very good solubility. Fourier transform infrared, Raman and X-ray photoelectron spectra indicated that the 1-one-butyric acid (BA) groups had formed on the sidewalls of the MWCNTs. The percentage of BA groups in the MWCNTs-BA was 7.80 mol% as determined using a simple acid-base titration method. The electrical conductivity, morphology and structure of the MWCNTs-BA were characterised with a four-point probe, field-emission scanning electron microscopy, transmission electron microscopy and X-ray diffraction. Furthermore, the sensor created by the immobilisation of glucose oxidase (GOD) onto the surface of MWCNTs-BA was sensitive toward glucose over a linear range from 10 μM to 2.5 mM with a correlation coefficient of 0.999 (n = 5), and it showed good sensitivity (23.5 μA mM ^-1 cm ^-2), enzyme affinity, reproducibility and storage stability. This immobile MWCNTs-BA matrix acts as an electron bridge between the enzyme molecules and the electrode, and it has the potential to allow the construction of useful sensors through enzyme immobilisation.