experiments was only 1.47%. With an incubation time of 40 min, this biosensor
represents an attracting alternative for routinely food control purposes.
At last, we want to report on the biosensor developed by Sheikhzadeh et al. as this
is a truly label-free impedimetric aptasensor that does not require any redox probe
but enables faradaic measurements by the use of a conductive polypyrrole copolymer [128]. They reached a LoD of 3 CFU/ml and a wide linear range from 10
2 to
10
8 CFU/ml. The biosensor showed good selectivity (150%) and repeatability
(5.2%, each concentration was repeated three times), but performed poorly on spiked
apple juice samples with recovery rates of 140–410%.
Escherichia coli is a bacteria that is found in the intestines of humans and most
types are harmless, but some cause food borne diseases such as the Shiga toxin
producing E. coli (STEC). According to EFSA (European Food Safety Authority),
there was an outbreak of STEC in Germany in 2011 that caused ~4,000 cases of
bloody diarrhea and hemolytic uremic syndrome. The EC sets limits for relevant
foodborne bacteria in products placed on the market and defines process hygiene
criteria to ensure food safety [142]. To this regulation, STEC should be absent in
25 g of sprouts placed on the market during their shelf-life. Besides, the detection of
E. coli is used as an indicator for fecal contamination, products with low hygiene or
high microbial contamination.
Burrs et al. [135] developed an impedimetric biosensor based on a RNA aptamer
that specifically binds to the O-antigen on the cell surface of the STEC stem O157:
H7 [143]. They increased the electroactive surface of graphene paper by
functionalization with fractal platinum nanocauliflower and obtained a roughness
factor R (¼ geometrical area/ electroactive surface) of 10. Immobilized thiolated
aptamers enabled the binding of E. coli from the sample resulting in an increase of
impedance. They achieved a LoD of 4 CFU/ml and a linear range from 4 to 10
5 CFU/
ml with a response time of 12 min by a low cost of 4 $ per sample. However, the
developed aptasensor needs to be verified in real samples with complex matrices as
no prior sample preparation step is intended.
Dua et al. developed a non-faradaic impedance biosensor based on a 2
0 -fluoropyrimidines modified RNA aptamer selected by Cell-SELEX against E. coli DH5α
[134]. They used a gap sensor of two gold electrodes modified with the aptamers
with a distance of 4 μm in a microfluidic channel. The sample is incubated for 1 h
resulting not only in an increase of the impedance at low frequencies representing the
solution resistance, but also in an increase of the impedance at high frequencies
(10 kHz). With this sensor a LoD of 104 CFU/ml was achieved.
Yao et al. were able to detect bacteria concentrations as low as 12 CFU/ml by a
procedure combining antibody-modified magnetic nanoparticles for sample preparation and aptamer-modified gold nanoparticles with immobilized urease
[136]. With the addition of the substrate urea, ammonium carbonate is produced.
The supernatant of the enzymatic reaction will be transferred into a microfluidic cell
with interdigitated microelectrodes and the impedance at 15 kHz is measured every
2 s. The impedance decreases with increasing bacteria concentration due to the
higher conductivity of the product (see Fig. 13). The procedure was verified with
spiked pasteurized milk. This procedure contains a lot of premeasurement steps,
Impedimetric Aptamer-Based Biosensors: Applications
75
represents an attracting alternative for routinely food control purposes.
At last, we want to report on the biosensor developed by Sheikhzadeh et al. as this
is a truly label-free impedimetric aptasensor that does not require any redox probe
but enables faradaic measurements by the use of a conductive polypyrrole copolymer [128]. They reached a LoD of 3 CFU/ml and a wide linear range from 10
2 to
10
8 CFU/ml. The biosensor showed good selectivity (150%) and repeatability
(5.2%, each concentration was repeated three times), but performed poorly on spiked
apple juice samples with recovery rates of 140–410%.
Escherichia coli is a bacteria that is found in the intestines of humans and most
types are harmless, but some cause food borne diseases such as the Shiga toxin
producing E. coli (STEC). According to EFSA (European Food Safety Authority),
there was an outbreak of STEC in Germany in 2011 that caused ~4,000 cases of
bloody diarrhea and hemolytic uremic syndrome. The EC sets limits for relevant
foodborne bacteria in products placed on the market and defines process hygiene
criteria to ensure food safety [142]. To this regulation, STEC should be absent in
25 g of sprouts placed on the market during their shelf-life. Besides, the detection of
E. coli is used as an indicator for fecal contamination, products with low hygiene or
high microbial contamination.
Burrs et al. [135] developed an impedimetric biosensor based on a RNA aptamer
that specifically binds to the O-antigen on the cell surface of the STEC stem O157:
H7 [143]. They increased the electroactive surface of graphene paper by
functionalization with fractal platinum nanocauliflower and obtained a roughness
factor R (¼ geometrical area/ electroactive surface) of 10. Immobilized thiolated
aptamers enabled the binding of E. coli from the sample resulting in an increase of
impedance. They achieved a LoD of 4 CFU/ml and a linear range from 4 to 10
5 CFU/
ml with a response time of 12 min by a low cost of 4 $ per sample. However, the
developed aptasensor needs to be verified in real samples with complex matrices as
no prior sample preparation step is intended.
Dua et al. developed a non-faradaic impedance biosensor based on a 2
0 -fluoropyrimidines modified RNA aptamer selected by Cell-SELEX against E. coli DH5α
[134]. They used a gap sensor of two gold electrodes modified with the aptamers
with a distance of 4 μm in a microfluidic channel. The sample is incubated for 1 h
resulting not only in an increase of the impedance at low frequencies representing the
solution resistance, but also in an increase of the impedance at high frequencies
(10 kHz). With this sensor a LoD of 104 CFU/ml was achieved.
Yao et al. were able to detect bacteria concentrations as low as 12 CFU/ml by a
procedure combining antibody-modified magnetic nanoparticles for sample preparation and aptamer-modified gold nanoparticles with immobilized urease
[136]. With the addition of the substrate urea, ammonium carbonate is produced.
The supernatant of the enzymatic reaction will be transferred into a microfluidic cell
with interdigitated microelectrodes and the impedance at 15 kHz is measured every
2 s. The impedance decreases with increasing bacteria concentration due to the
higher conductivity of the product (see Fig. 13). The procedure was verified with
spiked pasteurized milk. This procedure contains a lot of premeasurement steps,
Impedimetric Aptamer-Based Biosensors: Applications
75