Biomedical Engineering Reference
In-Depth Information
M 1 , respectively. Xu et al. have explored the pH-sensi-
tive property of the CNT-modifi ed electrode and immobilized the enzymes with sol-gel
hybrid material on the CNT-modifi ed electrode to construct voltammetric urease and
acetylcholinesterase biosensors based on pH detection [116]. Luong et al. [117] have
constructed a putrescine biosensor based on putrescine oxidase (POx) immobilized on
CNTs modifi ed by APTES, which is able to effi ciently monitor the direct electroactivity
of POx at the electrode surface. Rubianes and Rivas [118] have demonstrated that the
strong electrocatalytic activity of CNTs towards the reduction of hydrogen peroxide and
quinones and the oxidation of NADH has allowed an effective low potential ampero-
metric determination of lactate, phenols, catechols and ethanol, with the incorporation
of lactate oxidase, polyphenol oxidase, and alcohol dehydrogenase/NAD , respectively,
within the composite matrix.
CNT-based DNA sensors are also important in the application of CNTs in bioelec-
trochemistry. Due to the electrocatalytic activity of CNTs and interfacial accumulation,
the direct electrochemical oxidation of natural DNA with an enhanced signal has
been demonstrated on either multi-wall [119] or single-wall [120] CNT-modifi ed elec-
trodes. Cai et al. have described a sensitive electrochemical DNA sensor based on
CNTs with a carboxylic acid group for covalent DNA immobilization and enhanced
hybridization. The CNT-based assay with its large surface area and good charge trans-
fer characteristics dramatically improves DNA attachment and complementary DNA
detection sensitivity compared with the previous DNA sensors which directly incor-
porate oligononucleotides on carbon electrodes [121]. A nanoelectrode array based on
vertically aligned multi-wall CNTs embedded in SiO 2 as an ultrasensitive DNA detec-
tion device has been developed. The hybridization of subattomole DNA target can
be detected by combining with Ru(bpy) 3 2 which mediates guanine oxidation [45].
Kerman et al. [122] have demonstrated that the combination of sidewall- and end-
functionalization of CNTs provides a signifi cant enhancement in the voltammetric
signal of guanine oxidation and creates a large surface area for DNA immobilization.
Wang et al. have described that DNA biosensors based on self-assembled CNTs have a
higher hybridization effi ciency compared with those based on random CNTs [123]. In
addition, combining CNTs with other nanoparticles such as CdS [124], Pt [125], and
magnetite nanoparticles [126] has offered great promise for constructing ultrasensitive
DNA electrochemical biosensors.
sensitivities of 25 and 6 nA
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15.4 SPECTROSCOPIC CHARACTERIZATION OF CARBON
NANOTUBE SENSORS
15.4.1 Raman spectroscopy of carbon nanotubes
15.4.1.1 General features of Raman spectra from carbon nanotubes
Raman scattering is one of the most useful and powerful techniques to characterize
carbon nanotube samples. Figure 15.17 shows the Raman spectrum of a single SWNT
[127]. The spectrum shows four major bands which are labeled RBM, D, G, and G
.
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