BIOMEDICAL APPLICATIONS VI: CARBON NANOTUBES AS BIOSENSING AND BIO-INTERFACIAL MATERIALS - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 203

7.2.3.5 Photoconducvity
Semiconducting CNTs are possible photo-sensors because their conductivity
was reported to be sensitive to photoradiation. Freitag and coworkers 69
prepared single nanotube ambipolar FETs and then excited them by infrared
light. The incident infrared light that was used excites the second exciton
state of the semiconducting nanotube (i.e., the n = 2 → n ′ = 2 (E 2-2 ) transition).
For SWNTs with diameters centred around 1.3 nm, the energy of E 2-2 is
centred at about 1.35 eV. This excited state decayed and gave an energetic
electron-hole pair within the nanotube molecule 70,71 and the charge carriers
could diffuse within the channel or be separated by an applied electric
ield between the source and drain contacts. With a diameter 1000 times
smaller than the wavelength of the light, the nanotube FET had an estimated
quantum eficiency of >10%. Upon irradiation, the current was increased by
an order of magnitude and the gate voltage shifted signiicantly. Therefore
the semiconductor-based FET could act as a photodetector.
Qiu et al. 72 obtained correlated results on CNT-based transistors but found
a weaker absorption band, which correlated with an energy 200 meV higher
than that of the E 2-2 resonance band. That sideband was attributed to the C-C
bond stretching phonon. Qiu and coworkers also showed clear evidence of the
dependence of current quantum yield on the polarisation of the laser. There
was no photocurrent generated when the polarisation was perpendicular to
the carbon axis.
7.3 BIOINTERFACE
CNTs can be used to prepared bio-compatible materials, which may possess
advantages such as high tensile strength and low cytotoxicity. Therefore CNTs
can be used for many kinds of biomaterials for tissue/cell engineering.
7.3.1 The Fundamental Properes for Bio-interface Applicaon
Before the application of CNTs for bio-interface purposes, cytotoxicity
is the irst issue to be considered. Studies on CNTs' cytotoxicity showed
controversial results. 73-74 However some recent evidence showed that the
CNTs are versatile for chemical modiications, which can introduce some
functional groups, ligands and even biomolecules. 75 The functionalised CNTs
might display a very low cytotoxicity, 76 suggesting that they are possible bio-
engineering materials for bio-interface applications. More details will be
described in the chapter for cytotoxicity (Chapter 8).