Chemistry Reference
In-Depth Information
is another technique that measures the change of the conductivity, commonly used in applications such as
environmental monitoring and clinical analysis.
Field effect
transistors represent other interesting transducer ISFETs (ion sensitive field effect transistors)
or FETs with open gates, with the purpose of using the sum of external gate tension with its own electroactive
potential or ionic solution (sample to detect and quantify) have been studied. Most of the reported devices
using FET based detectors are fabricated by using conventional photolithography process [10,11,12].
An important transducer used in mass sensors as cantilever, is the
piezoelectric based transducer
. The
piezoelectric effect is related to a linear electromechanical interaction between the mechanical and electrical
state in crystalline materials. This process is reversible, and is used to transform the variation in the deflection
in a micro/nanocantilever where the response toward an analyte reacting with a biorecognizer is measured.
These kinds of measurements can also take the advantages of surface acoustic wave detection by means of an
interdigitated transducer [13].
Some of the most reported optical and electrochemical nano(bio)systems/nano(bio)sensors based on the
various transducing mechanisms already mentioned in this section will be shown and discussed in the
following part. The operation of these devices will be discussed in relation also to various kinds of
nanomaterials (i.e. nanoparticles) due to their particularities in advantages that these bring in terms of
detection performance as well as the variability of assays/detection systems.
18.2.1 Opticalnano(bio)sensors
The intrinsic optical properties (UV-Vis light absorption and auto-fluorescence properties) of nanoparticles
(NPs) in addition to their ability to change optical properties of sensor surfaces (i.e. in surface plasmon
resonance and scattering light based devices) have been approached for their sensible detection in many
analytical applications (i.e. detection of DNA, proteins, cells, heavy metals etc.). Various kinds of nanoparticles
and other nanomaterials (i.e. nanotubes) are being used in the fabrication of optical sensors.
Core-shell nanoparticles
such as magnetic/luminescent Fe
3
O
4
/Eu:Gd
2
O
3
synthesized by spray pyrolysis
were used by Son
et al
. as both platforms for the hybridization reactions and as fluorescence labels. The
developed DNA hybridization sensor is capable of perfectly distinguishing of matching targets from
two-base pair mismatching, allowing the discrimination of different bacterial species [14] (Figure 18.1a).
Some nanostructures are and optimally active have great interest for their potential applications thanks to
their optical properties; these are the nanoshells [15]. They consist of small silica spheres covered by a thin
gold layer of approximately 100 nm in diameter. Besides their composition, two fundamental factors exist for
the optical response of nanoshell: its geometric structure and its dimensions. Modifying these two parameters
makes it possible to control the wavelength of the emitted light when the particle scatters the light or when it
absorbs radiation and consequently temperature increases. These structures have provided hopeful results in
experiments carried out in animal weaves for diagnosis and treatment in cancer patients [16].
Also, simple
gold nanoparticles
(AuNPs) are a promising approach for the cancer cells detection. The
University of San Francisco showed that by conjugating gold nanoparticles with specific antibodies, it is
possible to detect cancer cells and further remove them by applying ordinary light using a simple optical
microscope [17].
It is well known, for example, that the NP plasmon band shifts when AuNPs are aggregated, due to a
decrease in the interparticle's distance. The maximum of absorbance shifts from 520 nm to 580 nm (red to
blue colour) when the aggregation takes place and the monitoring of this phenomenon can be used for
biosensing purposes. For example, AuNPs suspension previously modified with ssDNA probe has been
hybridized with a DNA target (complementary in its two ends with ssDNA) producing NPs aggregation with
the consequent change in colour in this way. This principle was pioneered by Mirkin's group and applied for
the detection of DNA characteristic of anthrax, a biological warfare agent [18] (Figure 18.1b).
