Biomedical Engineering Reference
In-Depth Information
procedures CNT played a dual role in both the recognition and transduction events,
namely as carriers for numerous enzyme tags and for accumulating the electroactive
-
naphthol product of the enzyme-hydrolysis reaction. CNTs were loaded with around 9,600
alkaline phosphatase enzyme molecules per CNT by covalently coupling the enzyme to
the carboxylated CNTs. The enzyme-loaded CNTs were further modified with DNA or
antibody for DNA sensing or immunosensing, respectively. For example, probe DNA-
functionalized magnetic particles were hybridized with the complementary target DNA
leading to the DNA duplex. Free ends of target DNA were then further hybridized with
the complementary DNA, which was linked to the enzyme-functionalized CNT. The enzy-
matic hydrolysis of
-naphthylphosphate led to the production of electrochemically
detectable
-naphthol product. Such CNT-derived amplification method allowed the
detection of DNA and proteins down to 1.3 and 160 zmol, respectively, in 25-50
L sam-
ples and indicates great promise for PCR-free DNA analysis.
13.4.4
DNA Hybridization Sensor Based on Impedance Measurement
Cai et al. [48] reported on a direct electrochemical detection of DNA hybridization by AC
impedance measurement. An indicator or label-free detection method was based on the
probe-DNA-doped ppy-conducting film formed on MWCNT-modified GC electrode. The
hybridization event with a complementary target DNA led to a decrease in impedance val-
ues as a result of the reduction of the electrode resistance. The detection limit of this label-
free method was 10 nM of oligonucleotide. The sensitivity was around fivefold higher
than that obtained with analogous measurements without CNT. This method offers great
promise for simple and sensitive reagentless DNA hybridization analysis.
13.5
Field-Effect Transistors Based on Biomolecule-Functionalized CNTs
13.5.1
Glucose Detection
Dekker and his coworkers [49] reported on a SWCNT FET (field-effect transistor) for glu-
cose biosensor. GOx was attached onto the sidewalls of SWCNT through a bifunctional
linking molecule containing on one side a pyrene group that strongly adsorbs onto the
sidewalls of CNT due to
interactions and on the other side an active ester group for
covalent binding to the lysine residues of GOx. The GOx-functionalized CNT was placed
at the gate between two electrodes of source and drain, and the conductance was meas-
ured as shown in Figure 13.5 (left).
The addition of glucose to the CNTFET led to increased conductivity. This result indi-
cates that an enzyme-activity sensor can be constructed at the single-molecule level of an
individual SWCNT. The mechanism of the conductance change of the CNT as a result of
the biocatalytic process is not well understood yet. Further theoretical study is necessary
to explain such phenomenon.
13.5.2
Assay of Protein Bindings
Surface-protein and protein-protein binding have been studied based on the changes in
the conductive properties of the single-wall CNT by Chen et al [50]. Specific antigen-anti-
body (associated with human autoimmune disease) interactions have been analyzed using
