nanoparticles come in close proximity, a strong electric field is developed between
them, which were used to enhance Raman signals. As for the surface plasmons,
the strength of the electric field depends on the shape and size of the nanoparticles
that are mostly synthesized by the condensation method, which is based on the
reduction of metal salts. By the addition of different seeds and surface modifiers,
a high variety of shapes can be achieved, such as rods, triangles, stars, cubes, and
cages. It has been shown that nanostars give higher intensities in Raman signal
than spheres, sphere aggregates, or triangles [28]. Talemi et al. [29] developed an
impedimetric aptasensor for dopamine detection based on gold nanostars deposited
on a pencil graphite electrode with the expectation that the charge transfer will
be also enhanced, leading to a significant amplification of the impedance signal.
Indeed, they were able to decrease impedance from 12 kΩ to 5 kΩ by the deposition
of gold nanostars and achieved a low detection limit of 0.3 ng/L, which was one
magnitude lower compared to the same sensor with gold nanoparticles (209 ng/L).
To reduce the non-specific binding but retain high surface-to-volume ratio and
high electrical conductivity, the fabrication of more complex nanocomposites gained
interests. Nanocomposites are made from two or more different materials with the
aim to combine their properties to improve the characteristics of the transducer
surface, that is, in impedimetric biosensors the electrode.
By using a nanocomposite made of reduced graphene oxide, iron particles,
polyaniline, and gold nanoparticles, Hashemi et al. [30] were able to detect cocaine
down to 29 pM and could also detect 25 nM cocaine in urine and serum samples.
Another example for nanocomposites is described by Aghajari et al. [31] who
modified a glassy carbon electrode with a mixture of chitosan, carbon nanotubes, and
palladium. Streptomycin aptamers were immobilized on the created nanocomposite
via glutaraldehyde. Streptomycin is an antibiotic which is used in human therapy as
well as in agriculture and as veterinary drug (e.g., in beekeeping). As it has serious
side effects, the detection of trace amounts in patients or food products is required.
Aghajari et al. were able to detect down to 18 pM of streptomycin by the use of
nanocomposites.
In this example, the natural polymer chitosan improved the stability of the
biorecognition element, but recently silk was discovered as the material for biosensors due to its mechanical and optical properties, elasticity, and biocompatibility.
In the impedimetric aptasensor developed by Benvidi et al. [32], silk fibroin
was combined with TiO 2 to increase conductivity and decrease solubility. This
mixture was dropped on a glassy carbon electrode, and gold nanoparticles
were electrodeposited to enable immobilization of thiol-modified aptamers. This
aptasensor increased the sensitivity for the impedimetric detection of prostatespecific antigen (PSA) to a detection limit of 33 aM. Compared to the impedimetric
aptasensor developed by [33], who used a nanocomposite of multiwalled carbon
nanotubes and gold nanoparticles on reduced graphene oxide to sensitively detect
PSA down to 33 fM, the nanocomposites with silk led to an increase of sensitivity of
three magnitudes.
Impedimetric Aptamer-Based Biosensors: Principles and Techniques
27
them, which were used to enhance Raman signals. As for the surface plasmons,
the strength of the electric field depends on the shape and size of the nanoparticles
that are mostly synthesized by the condensation method, which is based on the
reduction of metal salts. By the addition of different seeds and surface modifiers,
a high variety of shapes can be achieved, such as rods, triangles, stars, cubes, and
cages. It has been shown that nanostars give higher intensities in Raman signal
than spheres, sphere aggregates, or triangles [28]. Talemi et al. [29] developed an
impedimetric aptasensor for dopamine detection based on gold nanostars deposited
on a pencil graphite electrode with the expectation that the charge transfer will
be also enhanced, leading to a significant amplification of the impedance signal.
Indeed, they were able to decrease impedance from 12 kΩ to 5 kΩ by the deposition
of gold nanostars and achieved a low detection limit of 0.3 ng/L, which was one
magnitude lower compared to the same sensor with gold nanoparticles (209 ng/L).
To reduce the non-specific binding but retain high surface-to-volume ratio and
high electrical conductivity, the fabrication of more complex nanocomposites gained
interests. Nanocomposites are made from two or more different materials with the
aim to combine their properties to improve the characteristics of the transducer
surface, that is, in impedimetric biosensors the electrode.
By using a nanocomposite made of reduced graphene oxide, iron particles,
polyaniline, and gold nanoparticles, Hashemi et al. [30] were able to detect cocaine
down to 29 pM and could also detect 25 nM cocaine in urine and serum samples.
Another example for nanocomposites is described by Aghajari et al. [31] who
modified a glassy carbon electrode with a mixture of chitosan, carbon nanotubes, and
palladium. Streptomycin aptamers were immobilized on the created nanocomposite
via glutaraldehyde. Streptomycin is an antibiotic which is used in human therapy as
well as in agriculture and as veterinary drug (e.g., in beekeeping). As it has serious
side effects, the detection of trace amounts in patients or food products is required.
Aghajari et al. were able to detect down to 18 pM of streptomycin by the use of
nanocomposites.
In this example, the natural polymer chitosan improved the stability of the
biorecognition element, but recently silk was discovered as the material for biosensors due to its mechanical and optical properties, elasticity, and biocompatibility.
In the impedimetric aptasensor developed by Benvidi et al. [32], silk fibroin
was combined with TiO 2 to increase conductivity and decrease solubility. This
mixture was dropped on a glassy carbon electrode, and gold nanoparticles
were electrodeposited to enable immobilization of thiol-modified aptamers. This
aptasensor increased the sensitivity for the impedimetric detection of prostatespecific antigen (PSA) to a detection limit of 33 aM. Compared to the impedimetric
aptasensor developed by [33], who used a nanocomposite of multiwalled carbon
nanotubes and gold nanoparticles on reduced graphene oxide to sensitively detect
PSA down to 33 fM, the nanocomposites with silk led to an increase of sensitivity of
three magnitudes.
Impedimetric Aptamer-Based Biosensors: Principles and Techniques
27