BIOMEDICAL APPLICATIONS VI: CARBON NANOTUBES AS BIOSENSING AND BIO-INTERFACIAL MATERIALS - Carbon Nanotubes: From Bench Chemistry to Promising Biomedical Applications - page 202

Reversible quenching of CNT luorescence was applied also for macro-
biomolecular sensing. Satishkumar and coworkers 66 designed a bi osensing
device, which takes advantage of the reversible luorescence quenching. In
their study, nanotube-based optical sensing was used to detect dye-ligand
conjugates (DLCs) (Fig. 7.10). The DLCs used in this study consisted of
biotinylated anthracene and biotinylated phenylazoaniline, respectively.
The luorescence spectra of doped samples showed that the nanotube
luorescence was quenched substantially relative to the undoped sample.
The luorescence quench caused by the DLCs may be due to the transfer of
photo-induced excited-state electrons from the nanotube conduction band
to the lowest unoccupied molecular orbital of the DLCs. This is supported
by the evidence that B-nicotinamide adenine dinucleotide (NADH), which is
a reducing agent and can inhibit the excited-state charge transfer through
competitive reduction of DLCs, recovered the nanotube luorescence. The
target protein, avidine, served as the model analyte of the SWNT-DLC system.
After successive addition of a nanomolar amount avidin, signiicant recovery of
luorescence was observed, suggesting a very sensitive response to the target
protein. The recovery was complete, and the luorescence almost reached the
original level. The possible mechanism was that the binding afinity between
biotin and avidin disrupted the weak non-covalent interaction between DLC
and SWNT and diminished the quenching effect of the DLC. Although BSA
could also react with biotin and repristinate the luorescence, the recovery
eficiency was two to three orders lower than that of avidin, indicating the
system's high selectivity.
Figure 7.10 The dye-ligand conjugates (DLCs) quenched the luorescence of CNTs.
Upon addition of avidin, the DLC detached from CNTs and the luorescence was
reversed. Redrawn from Barone et al. 67
Barone et al. 67,68 explored a method to detect the glucose concentration
by NIR of CNTs. Glucose oxidase was immobilised on CNTs. The enzyme
catalyses β--glucose to the -glucono -1,5-lactone with H 2 O 2 as co-product.
The subsequent reduction of Fe(CN) 6 3- resulted in a signiicant change in the
luorescence strength, making the system a sensitive detection device.