15
major advantage as recognition elements. This advantage stems from the AMPs
semi-selective binding nature to target cells of a variety of pathogens.
The bioactivity of AMPs toward microbial cells can be classified according to
their secondary structures [24, 25]. Many AMPs adopt amphipathic conformations
that spatially shield the hydrophobic group of the cationic amino acids so it can
target the anionic head groups of lipids in the bacterial membrane. On the other
hand, the membranes of plants and animals separate negative charges to the inner
leaflet and contain cholesterols that decrease the activity of the AMP [25]. The
AMPs, linear cationic peptides such as magainins, are in particular, attractive for the
microbial sensing applications due to their small molecular size and intrinsic
stability [26, 27] Especially, the positively charged AMP magainin I
(GIGKFLHSAGKFGKAFVGEIMKS) binds most selectively to the bacterial cell
E. coli O157∶H7 as a precursor to bactericidal activity [25–27].
2.3.1.1 Development of an AMP-Based, Label-Free Electronic Biosensor
A robust and portable biosensor for the pathogenic bacteria detection that could
impact water quality monitoring for bacterial contamination was developed by
McAlpine and coworkers [23–25]. The particular interest of the developed biosensor
was that it combined the natural specificity of biological recognition to label-free
sensors providing sensitive electronic readout. Accordingly, McAlpine et al.
reported the selective and sensitive detection of infectious agents via electronic
detection based on antimicrobial peptide-functionalized micro capacitive electrode
arrays [25].
The semi-selective antimicrobial peptide magainin I, which occurs naturally on
the skin of African clawed frogs, was immobilized on gold microelectrodes via a
C-terminal cysteine residue. Remarkably, exposing the sensor to different pathogenic
Escherichia coli concentrations showed detection limits of approximately 1
bacterium∕μL, a clinically acceptable detection limit. The peptide-micro capacitive
hybrid device was additionally able to exhibit both Gram-selective detection as well
as interbacterial strain discrimination, while maintaining recognition capabilities
toward pathogenic strains of E. coli and Salmonella [25].
The first step toward the development of an AMP-based, label-free electronic
biosensor is to target the microbial cells by using magainin I through the use of the
impedance spectroscopy. It should be noted that electrical impedance measures the
total opposition to a circuit or a part of the circuit presented to an electric current.
Usually, the impedance is the results for both resistance and reactance. It is important
to remember that the resistance component is formed from collisions of the currentcarrying charged particles with the internal structure of the conductor. Furthermore,
the reactance component is an additional opposition to the movement of the electric
charge that results from the changing magnetic and electric fields in circuits carrying
alternating current.
Figure 2.2 outlines the sensing platform. First, the AMPs are immobilized on the
micro-fabricated interdigitated gold electrodes (Fig. 2.2a). It should be specified
2 Detection of Biological Warfare Agents Using Biosensors
major advantage as recognition elements. This advantage stems from the AMPs
semi-selective binding nature to target cells of a variety of pathogens.
The bioactivity of AMPs toward microbial cells can be classified according to
their secondary structures [24, 25]. Many AMPs adopt amphipathic conformations
that spatially shield the hydrophobic group of the cationic amino acids so it can
target the anionic head groups of lipids in the bacterial membrane. On the other
hand, the membranes of plants and animals separate negative charges to the inner
leaflet and contain cholesterols that decrease the activity of the AMP [25]. The
AMPs, linear cationic peptides such as magainins, are in particular, attractive for the
microbial sensing applications due to their small molecular size and intrinsic
stability [26, 27] Especially, the positively charged AMP magainin I
(GIGKFLHSAGKFGKAFVGEIMKS) binds most selectively to the bacterial cell
E. coli O157∶H7 as a precursor to bactericidal activity [25–27].
2.3.1.1 Development of an AMP-Based, Label-Free Electronic Biosensor
A robust and portable biosensor for the pathogenic bacteria detection that could
impact water quality monitoring for bacterial contamination was developed by
McAlpine and coworkers [23–25]. The particular interest of the developed biosensor
was that it combined the natural specificity of biological recognition to label-free
sensors providing sensitive electronic readout. Accordingly, McAlpine et al.
reported the selective and sensitive detection of infectious agents via electronic
detection based on antimicrobial peptide-functionalized micro capacitive electrode
arrays [25].
The semi-selective antimicrobial peptide magainin I, which occurs naturally on
the skin of African clawed frogs, was immobilized on gold microelectrodes via a
C-terminal cysteine residue. Remarkably, exposing the sensor to different pathogenic
Escherichia coli concentrations showed detection limits of approximately 1
bacterium∕μL, a clinically acceptable detection limit. The peptide-micro capacitive
hybrid device was additionally able to exhibit both Gram-selective detection as well
as interbacterial strain discrimination, while maintaining recognition capabilities
toward pathogenic strains of E. coli and Salmonella [25].
The first step toward the development of an AMP-based, label-free electronic
biosensor is to target the microbial cells by using magainin I through the use of the
impedance spectroscopy. It should be noted that electrical impedance measures the
total opposition to a circuit or a part of the circuit presented to an electric current.
Usually, the impedance is the results for both resistance and reactance. It is important
to remember that the resistance component is formed from collisions of the currentcarrying charged particles with the internal structure of the conductor. Furthermore,
the reactance component is an additional opposition to the movement of the electric
charge that results from the changing magnetic and electric fields in circuits carrying
alternating current.
Figure 2.2 outlines the sensing platform. First, the AMPs are immobilized on the
micro-fabricated interdigitated gold electrodes (Fig. 2.2a). It should be specified
2 Detection of Biological Warfare Agents Using Biosensors