Biomedical Engineering Reference
In-Depth Information
by self-assembly of shortened CNTs and functionalized after the assembly.
Despite the number of ways of assembling CNTs on electrode surfaces and the
well-documented electron-transfer reactions on their surfaces, the application
of CNTs in amperometric sensors remains mostly limited to small molecules.
Wang et al.
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have shown CNT-modified transducers can detect proteins and
DNA down to 1.3 and 160 zmol, respectively in 25-50 µL samples indicating
great promise for PCR-free DNA analysis. Yet, despite this promise, little work
has been done for the application of amperometric biosensors on infectious dis-
eases diagnostics. This has been attributed to lack of works to establish that
amperometric biosensors do not suffer from biofouling.
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Potentiometric transducers are not as sensitive as amperometric trans-
ducers but architectures for immunosensing of pathogens involving the use
of nanowires as FETs have been considered as viable candidates for ultra-
sensitive biosensing applications.
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Reliable ultrasensitive biosensors using
FETs are fabricated by the expensive top-down approaches such as EBL and
molecular beam epitaxy, and then functionalized with biorecognition ele-
ments using bottom-up approaches as discussed by He et al.
250
They have
also pointed out that bottom-up approaches of fabricating FETs offer flex-
ibility of surface functionalization but suffer from low reproducibility and
reliability. Among the mentioned bottom-up assembly of nanowires FETs are
electric field and magnetic field controlled deposition, Langmuir-Blodgett
method, alignment and selective growth on the device, programmed dip coat-
ing, growth substrate contact printing, blown bubble film, and electroplating
in nanochannels.
Silicon nanowire FETs have been employed for the detection of influenza A
and adenovirus in a study by Patolsky et al.
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Their studies showed that single
viruses can be detected directly with high selectivity and their methods allowed
parallel detection of different viruses. Using virtually unpurified samples, the
method exceeded the capabilities of existing methods such as PCR.
In the case of optical sensors (especially SPR-based and SERS-based),
research on improving sensitivity is focused on sensor design. Conventional
SPR-based or SERS-based optical sensors used planar transducer surfaces made
from bulk materials or colloidal materials. Surfaces roughened with nanosized
materials were found to increase the sensitivity of signal transduction. In this
section, examples of nanomanipulations of the optical sensor surface to improve
the sensitivity of diagnostic sensing devices are presented.
NSL involves assembly of 2D hexagonally close packed nanosphere array
and then thermally depositing metals on the sphere mask. The nanospheres are
removed in the LSPR applications whereas they are kept intact in the SERS appli-
cation. The technique has been shown to provide a sensing platform that can detect
anthrax spores well below the infectious dose and with a faster detection time.
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OAD involves physical vapor deposition creating NMs on tilted substrate
that causes nanorods to grow on the substrate in the direction of the deposition.
This process is used to make silver nanorod arrays that serve as SERS substrates
