Page 244 - Carbon Nanotube Fibres and Yarns

234 Carbon Nanotube Fibers and Yarns

9.7 Biochemical sensors
CNT fibers have good electro-catalytic properties that are ideal for use in
electrochemical devices. The porosity and high surface area of CNT fi-
bers have been exploited to drive molecular-scale interactions with en-
zymes and other chemicals to efficiently capture and promote electron
transfer reactions [77]. CNT fiber-based electrodes are being consid-
ered as alternatives to carbon fiber microelectrodes for the detection of
neurotransmitters [78,79]. CNT fibers are more sensitive, and the micro-
electrodes could exhibit fast electron transfer kinetics. Zhu et al. demon-
strated that microelectrodes made from CNT fibers could accelerate the
redox process of these biomolecules allowing for high sensitivity of low-
potential detection [80,81]. The glucose-sensing electrodes were built by
adsorption of a mediator on the surface of a CNT fiber.
Bourourou et al. produced bio-electrochemical sensing electrodes by
electrospinning a fiber from a solution containing MWCNTs and polyac-
rylonitrile (PAN) [82]. The nitrile groups of these fibers were chemically
reduced to amines groups that were protonated at pH 5. The resulting pos-
itively charged nanofibers swell in aqueous solutions increasing the exposed
surface of CNT and facilitating the diffusion of small molecules and ions
to the conducting CNTs. The electrodes were biofunctionalized through
the activation of the amino groups while polyphenol oxidase (PPO) was
immobilized onto the PAN-MWCNTs nanofiber through covalent bind-
ings. The PPO-PAN–NH 2 –CNT nanofiber electrodes could sense organic
compounds due to their efficient covalent binding. When the electrodes
were used for electro-enzymatic detection of catechol, a sensitivity of
−1
118 mA mol L was obtained at saturated catechol concentration while
maximum current values reached 10.66 μA and a limit of detection of
−1
0.9 μmol L .
Kim et al. [83] showed that when coated or submerged in an electro-
lyte, CNT yarns are charged by the electrolyte to form supercapacitors.
Fig. 9.12 shows the configuration, structure, and performance of a Twistron
(made by spinning thousands of CNT bundles) harvester for tensile energy
harvesting in 0.1 M HCl [83]. By applying a twist to the yarns as illustrated
in Fig. 9.12A, charges on the yarn are amplified by the compaction of the
CNT fiber bundles, reducing the overall capacitance. The capacitance of
the CNT yarn could be correlated with the applied strain (Fig. 9.12D).
Additionally, the movement of electrical charges by the twisting motion of
the fibers within the supercapacitor, could be used to attain and store poten-
tial. Energy could be harvested by twisting CNT yarns from both torsional