Biomedical Engineering Reference
In-Depth Information
CNTs themselves are not able to both perform speciic molecular recognition
and be a signal transducer, but they seem particularly suitable for these tasks
because their electrical conductance is very sensitive and changes with the
adsorption of charged molecules on CNTs' surface. As previously mentioned,
their high aspect ratio, together with their chemical and electronic properties,
makes them critically important in the development of such devices. They
can have a high loading of biomolecules, with a consequent high sensitivity
as biosensors, but there are still problems about the stability, reusability
and repeatability of the systems. 76 The exploitation of electrochemistry is
advantageous because it monitors the redox activity of species and is apt in
surface science and solid state chemistry. CNTs play a crucial role in the ield
of sensors because of their ability to facilitate electron transfer when acting
as modiied electrodes.
CNT-based detection devices can be classiied by considering (i) the
functionalisation (covalent or non-covalent), (ii) the detection system and
(iii) the detected molecule. In fact CNTs can recognise proteins by speciic
or non-speciic interactions. Non-covalent attachment of antibodies to CNTs
is strictly related to the size of biomolecules: molecules much larger than
SWCNTs (7-8 nm, e.g. immunoglobulin) reveal dificulties in non-speciic
binding (NSB). It has been reported that tryptophan and histidine are
responsible for most interactions in non-speciic protein binding. 77 The main
reason for this phenomenon is the hydrophobic interaction between CNT
sidewalls and proteins, and it is possible to prevent it by using polyethylene
oxide and polyethylene glycol, which wrap the tubes and render them more
hydrophilic and thus prevent hydrophobic interactions. Dai et al. offered a
detailed study about non-speciic binding of proteins on the SWCNT surface
(Table 3.1). 78 Moreover, they suggested that, since the electrical properties
are guaranteed by the integrity of an electronic structure, non-covalent
functionalisation be preferred, considering that it can prevent the remarkable
inluence of non-speciic binding. They prepared a dense ilm of SWCNTs by
chemical vapour deposition growth on quartz substrates. Subsequent metal
evaporation served to obtain the two electrodes of Ti/Au connected by the
SWCNT network to use ield effect transistor (FET) and quartz microbalance
together. The CNTs were non-covalently functionalised through a linker with
auto-antigens to generate an immunosensor for autoimmune diseases. This
system was used to detect many proteins and the binding was demonstrated
by atomic force microscopy, quartz crystal microbalance and conductance
measurements in buffer.
By discovering the important effect of non-speciic binding, the authors
studied the possibility of producing a functionalised derivative for NSB
resistance (Table 3.1).
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