active surface [110]. The achieved detection limit is even lower than the LoD
obtained with real-time PCR (1 fg/ml), although such sensitivities are not necessary
for the application in food control. The developed aptasensor was tested on soybean
samples and recoveries of 95–108% were obtained. Compared to the simple sensor
surface of Castillo’s aptasensor [105], the signal ratio of OTA to AFB1 is only ~2.5.
It is reasonable to presume that by increasing the active surface, the unspecific
binding is increased as well.
In conclusion, impedimetric aptasensors for mycotoxin detection are welldeveloped, providing detection limits and ranges comparable with HPLC and
sufficient for the legislative limits set by the EC. They were validated with real
samples and complex food matrices with short measurement times of 5–90 min and
have mechanisms for regeneration. For the transfer of these biosensors into commercial products, handheld impedance analyzers with microfluidic cartridges are
needed.
4.2 Microorganisms
Besides mycotoxins from fungi, another major cause of foodborne diseases are the
toxins produced by bacteria like Salmonella, Escherichia coli, Staphylococcus
aureus, and Bacillus cereus. Although it is more accurate to directly determine the
toxins, often in food control the microorganisms are detected.
Optical biosensors for the detection of pathogenic bacteria are reviewed in Yoo
and Lee [120], whereas electrochemical biosensors are reviewed by Amiri et al.
[121] who concluded that aptamers, imprinted polymers, and bacteriophages are
promising alternatives to antibodies and that improvements are needed in reproducibility and reliability as well as in minimizing non-specific binding. In comparison
with clinical applications that require spotting of a single bacterium, in food control
higher concentrations are sufficient, as for example 10
5 colony forming units (CFU)
per ml of Staphylococcus aureus (S. aureus) are producing toxins in a quantity that
can cause food poisoning. A general review of biosensors for the detection of
foodborne pathogens is given by Arora et al. [122], whereas Teng et al. [123]
reviewed aptamer-based biosensors and Kant et al. [124] concentrate on microfluidic
devices. In this chapter, we will review aptamer-based impedimetric biosensors for
the detection of food borne pathogens (see Table 10).
Salmonella are gram-negative, flagellated, and facultative anaerobic bacteria
from the family Enterobacteriaceae. More than 2,600 serotypes are distinguished
by the existence of different antigens, like the somatic O-antigen, also known as
lipopolysaccharide or endotoxin (see Sect. 2.3) and the flagellar H antigen. However, the most common serovar found in humans and animals are S. typhimurium and
S. enteritidis that also the major pathogens in gastrointestinal infections. Salmonella
food poisoning is mainly caused not only by contaminated poultry and eggs, but also
by the feces of infected humans and animals, insufficient hygiene in food handling,
or contaminated surface water. Thus, the EC regulation No. 2073/2005 demands the
Impedimetric Aptamer-Based Biosensors: Applications
73
obtained with real-time PCR (1 fg/ml), although such sensitivities are not necessary
for the application in food control. The developed aptasensor was tested on soybean
samples and recoveries of 95–108% were obtained. Compared to the simple sensor
surface of Castillo’s aptasensor [105], the signal ratio of OTA to AFB1 is only ~2.5.
It is reasonable to presume that by increasing the active surface, the unspecific
binding is increased as well.
In conclusion, impedimetric aptasensors for mycotoxin detection are welldeveloped, providing detection limits and ranges comparable with HPLC and
sufficient for the legislative limits set by the EC. They were validated with real
samples and complex food matrices with short measurement times of 5–90 min and
have mechanisms for regeneration. For the transfer of these biosensors into commercial products, handheld impedance analyzers with microfluidic cartridges are
needed.
4.2 Microorganisms
Besides mycotoxins from fungi, another major cause of foodborne diseases are the
toxins produced by bacteria like Salmonella, Escherichia coli, Staphylococcus
aureus, and Bacillus cereus. Although it is more accurate to directly determine the
toxins, often in food control the microorganisms are detected.
Optical biosensors for the detection of pathogenic bacteria are reviewed in Yoo
and Lee [120], whereas electrochemical biosensors are reviewed by Amiri et al.
[121] who concluded that aptamers, imprinted polymers, and bacteriophages are
promising alternatives to antibodies and that improvements are needed in reproducibility and reliability as well as in minimizing non-specific binding. In comparison
with clinical applications that require spotting of a single bacterium, in food control
higher concentrations are sufficient, as for example 10
5 colony forming units (CFU)
per ml of Staphylococcus aureus (S. aureus) are producing toxins in a quantity that
can cause food poisoning. A general review of biosensors for the detection of
foodborne pathogens is given by Arora et al. [122], whereas Teng et al. [123]
reviewed aptamer-based biosensors and Kant et al. [124] concentrate on microfluidic
devices. In this chapter, we will review aptamer-based impedimetric biosensors for
the detection of food borne pathogens (see Table 10).
Salmonella are gram-negative, flagellated, and facultative anaerobic bacteria
from the family Enterobacteriaceae. More than 2,600 serotypes are distinguished
by the existence of different antigens, like the somatic O-antigen, also known as
lipopolysaccharide or endotoxin (see Sect. 2.3) and the flagellar H antigen. However, the most common serovar found in humans and animals are S. typhimurium and
S. enteritidis that also the major pathogens in gastrointestinal infections. Salmonella
food poisoning is mainly caused not only by contaminated poultry and eggs, but also
by the feces of infected humans and animals, insufficient hygiene in food handling,
or contaminated surface water. Thus, the EC regulation No. 2073/2005 demands the
Impedimetric Aptamer-Based Biosensors: Applications
73