Novel lactate and pH biosensor for skin and sweat analysis based on single walled carbon nanotubes

Carbon nanotubes (CNTs) were functionalized and employed in an electrochemical cell to serve as a biosensor to specifically detect either lactate or pH in an electrolyte solution of artificial sweat. They were functionalized with the carboxyl group (COOH) to detect pH and the enzyme lactate oxidase (LOX) to detect lactate. All CNT samples were characterized to compare the electrodes before and after functionalization. Fourier transform infrared spectroscopy (FTIR) was used to verify the attachment of both COOH and LOX to the respective carbon nanotubes samples. Scanning electron microscopy (SEM) was used to examine the structure of the CNT–lactate electrode. Square wave voltammetry proved to be the best template to use to sense these target analytes. The functionalized CNT–COOH electrode displayed a linear response to pH 1–10, with a negative voltage shift corresponding to an increase in pH. Two types of lactate sensors were fabricated, both of which exhibited an increase in current corresponding to an increase in lactate concentration. The functionalized CNT–LOX on a glassy carbon electrode displayed an amperometric response in the range of 1–4 mM lactate. The CNT–LOX on a silicon/indium tin oxide (Si/ITO) substrate displayed an amperometric response in the range of 0.01–0.05 M lactate.