which enable a 5 times concentration of the bacteria but taking all together around
2 h which is hampering the development of point-of-care tests. However, purification steps will be necessary as the different ionic strength of samples will complicate
the measurement.
Listeria, Staphylococci, and Enterobacteriaceae are also listed in the EC regulation [142]. There are two publications on aptamer-based impedimetric biosensors for
the detection of Listeria reaching LoDs of 100 CFU/ml [137] and 5 CFU/ml [138]
but without verification on real samples.
Rubab et al. published a review about biosensors for the detection of S. aureus in
food, which is a main representative of Staphylococci in food poisoning due to the
production of several exo- and enterotoxins such as staph. enterotoxin A and B
[144]. However, an ingestion of 144 ng staph. enterotoxin B (SEB) can cause
gastroenteritis with symptoms as vomiting and diarrhea [145]. Thus, the EC postulates the absence of staph. enterotoxins in 25 g cheese, milk powder, and whey
powder.
Xiong et al. developed an aptamer-based impedimetric biosensor for the sensitive
detection of SEB as low as 0.17 ng/ml by the simple modification of gold electrodes
with thiolated aptamers and mercaptohexanol as backfiller [141]. They could detect
SEB in a linear range from 0.5 to 500 ng/ml within 100 min and proved a high
selectivity with a 9 times higher signal for SEB compared to other toxins found in
food such as OTA, SEA, and AFB1. Furthermore, they verified the biosensor in
spiked milk samples and obtained recovery rates of 77–119%, which were compared
to measurements with an ELISA kit resulting in relative deviations of À8 to 8%.
The EC also demands a limit of coagulase-positive Staphylococci that is 10 CFU/
g in different cheese products, milk powder, and whey powder. The coagulase-test is
used to differentiate between toxic strains like S. aureus and harmless strains like
S. epidermidis, which are coagulase-negative. Jia et al. developed an aptamer-based
impedimetric biosensor for the detection of S. aureus by using a nanocomposite of
reduced graphene and gold nanoparticles to increase the effective surface
[139]. They reached a detection of 10 CFU/ml within 60 min and verified the
performance in fish and water samples with recovery rates of 92–114%. Reich
et al. performed impedimetric detection of S. aureus in buffer using aptamers
immobilized on a gold electrode in a flow-through chamber [140]. They reached a
LoD of 10 CFU/ml within 10 min, which is significantly faster than most other
methods, but they did not show any verification in real samples.
In conclusion, the development of impedimetric aptasensors for food control has
advanced in recent years. The reviewed biosensors showed sufficient detection limits
and proved functionality in real samples. However, most are still time-consuming
taking several hours and thus offer no advantage over ELISA and PCR. Although by
miniaturization of the electrochemical setups, space saving and cost-efficient solutions will be developed in the near future.
Impedimetric Aptamer-Based Biosensors: Applications
77
2 h which is hampering the development of point-of-care tests. However, purification steps will be necessary as the different ionic strength of samples will complicate
the measurement.
Listeria, Staphylococci, and Enterobacteriaceae are also listed in the EC regulation [142]. There are two publications on aptamer-based impedimetric biosensors for
the detection of Listeria reaching LoDs of 100 CFU/ml [137] and 5 CFU/ml [138]
but without verification on real samples.
Rubab et al. published a review about biosensors for the detection of S. aureus in
food, which is a main representative of Staphylococci in food poisoning due to the
production of several exo- and enterotoxins such as staph. enterotoxin A and B
[144]. However, an ingestion of 144 ng staph. enterotoxin B (SEB) can cause
gastroenteritis with symptoms as vomiting and diarrhea [145]. Thus, the EC postulates the absence of staph. enterotoxins in 25 g cheese, milk powder, and whey
powder.
Xiong et al. developed an aptamer-based impedimetric biosensor for the sensitive
detection of SEB as low as 0.17 ng/ml by the simple modification of gold electrodes
with thiolated aptamers and mercaptohexanol as backfiller [141]. They could detect
SEB in a linear range from 0.5 to 500 ng/ml within 100 min and proved a high
selectivity with a 9 times higher signal for SEB compared to other toxins found in
food such as OTA, SEA, and AFB1. Furthermore, they verified the biosensor in
spiked milk samples and obtained recovery rates of 77–119%, which were compared
to measurements with an ELISA kit resulting in relative deviations of À8 to 8%.
The EC also demands a limit of coagulase-positive Staphylococci that is 10 CFU/
g in different cheese products, milk powder, and whey powder. The coagulase-test is
used to differentiate between toxic strains like S. aureus and harmless strains like
S. epidermidis, which are coagulase-negative. Jia et al. developed an aptamer-based
impedimetric biosensor for the detection of S. aureus by using a nanocomposite of
reduced graphene and gold nanoparticles to increase the effective surface
[139]. They reached a detection of 10 CFU/ml within 60 min and verified the
performance in fish and water samples with recovery rates of 92–114%. Reich
et al. performed impedimetric detection of S. aureus in buffer using aptamers
immobilized on a gold electrode in a flow-through chamber [140]. They reached a
LoD of 10 CFU/ml within 10 min, which is significantly faster than most other
methods, but they did not show any verification in real samples.
In conclusion, the development of impedimetric aptasensors for food control has
advanced in recent years. The reviewed biosensors showed sufficient detection limits
and proved functionality in real samples. However, most are still time-consuming
taking several hours and thus offer no advantage over ELISA and PCR. Although by
miniaturization of the electrochemical setups, space saving and cost-efficient solutions will be developed in the near future.
Impedimetric Aptamer-Based Biosensors: Applications
77