BIOMEDICAL APPLICATIONS VI: CARBON NANOTUBES AS BIOSENSING AND BIO-INTERFACIAL MATERIALS - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 189

Besides transistors, a chemicapacitor was constructed from SWNT
electrodes, and it showed the features of stability, high sensitivity to broad
range of analytes and fast response time. Snow et al. 24 designed a nanoscale
electrode by using an SWNT network. The electrode served as one plate of
the capacitor, while Si was another electrode. Fringing electric ields radiated
outward from the SWNTs under an applied bias voltage. The strongest
fringing ields at the surface of the SWNTs produced a net polarisation of
the adsorbates, which changed the capacitance of the system. The SWNT
capacitor responded to a number of chemical molecules, including N , N -
dimethylformamide (DMF), benzene, hexane, chloroform, isopropyl alcohol,
tetrahydrofuran, acetone, dimethylmethylphosphonate and so on. Sensing
chemicals by the capacitor was quick and reversible, and the response was
proportional to the analyte concentration. The magnitude of the capacitance
response depended on two factors. The irst one was the dipole moment of
the analytes. Non-polar molecules such as hexane produced small responses,
whereas polar molecules like DMF produced a large capacitance response.
The other factor was the surface interaction between the analytes and the
capacitor. Because of the surface interactions between the analytes and the
capacitor, chlorobenzene, 1,2-dichlorobenzene, 4-dinitrotoluene and water
showed a low response. Covering the SWNT electrode dramatically increased
the sensitivity for detecting chemicals, while introducing functional groups
on the SWNTs greatly shortened the response time by hundreds of times and
improved the selectivity of the sensor.
Further study on the mechanism of the CNT chemicapacitor was carried
out by exploring the electronic response of SWNTs to trace levels of chemical
vapours; Robinson et al. 25 found that adsorption at defect sites produced a
large electronic response, which resulted from increased adsorbate binding
energy and charge transfer and dominated the SWNTs' capacitance and
conductance sensitivity. The sensitivity of an SWNT network sensor could be
enhanced by controlled introduction of oxidation defects on the tubes.
Besides the pristine CNTs, modiied CNTs appeared to be more functional
in chemical and biological sensing. Functionalised CNTs could be used
as electron conductors in enzyme-based electrochemical sensors. 26 The
electrocatalytic oxidation o(1,4-dihydronicotinamide adenine dinucleotide
[phosphate]) cofactors and the reduction/oxidation of H 2 O 2 stimulated by
CNTs are particularly important, since these electrocatalytic reactions may be
easily coupled to enzymatic transformations. 27-28 Amperometric biosensors
have been prepared by encapsulating alcohol dehydrogenase or glucose
oxidase in CNT/Telon composite material to analyse ethanol and glucose.