Biomedical Engineering Reference
In-Depth Information
Berry ( 2008 ). The p-type semiconducting CMG sheets attach easily to microscale
bacterial cells or proteins and allow hybridization of DNA on their surface, the
interaction with these biological molecules modulating the electronic properties
of CMG. For example, about 1,400 conducting holes are generated when a single
bacterium attaches to the graphene-amine compound, while the hybridization of six
complementary DNA strands on graphene oxide sheets produces one hole. DNA
tethering on the latter material occurred predominantly on wrinkles and thicker parts
of the sheets. The CMG/bacteria hybrids were fabricated by selectively assembling
bacterial cells extracted from Bacillus cereus , which are negatively charged, on
graphene-amine scaffolds, which are positively charged. Amazingly, most bacterial
cells were alive after deposition but died after 4 h. Graphene-amine is obtained by
aminization of graphene oxide sheets immobilized on silica substrates. The bacterial
cells and proteins induced a sharp decrease of the resistance of a graphene-amine
transistor with about two orders of magnitude, from 800 to 5:34 M due to an
increase in the number of holes of 1:8 10 11 cm 2 .
Engineered protein pores, which behave as diodes, can be inserted into interface
bilayers between droplets in a droplet network that shows collective properties. Such
networks can implement full- and half-wave current rectifiers or current limiters
( Maglia et al. 2009 ). The protein diode-like pore is a modified staphylococcal '-
hemolysin, which can be introduced vectorially into lipid bilayers that form when
two droplets enclosed in lipid monolayers are bought in contact (see Fig. 9.9 ),
displaying full rectification of the ionic current in a 1 M KCl solution. The droplets
are contacted by Ag/AgCl electrodes. More precisely, as shown in Fig. 9.9 , the pore
closes for negative voltages after an exponential time delay with a time constant of
the order of 200 ms necessary for changing the conformation of the protein, and no
ionic current is measured. In the absence of the nanopore, the current collected
by the electrodes that penetrate the droplets simply follows the voltage signal.
A measure of the rectification property is, for example, the ratio of conductance
measured for the positive and negative polarities of the same applied bias; this
rectification ratio at 50 mV is about 60.
I
droplet
droplet
electrode
electrode
V
pore
Fig. 9.9 To p : engineered protein pores ( left ) with diode-like electrical behavior ( right ). Bottom :
networks of engineered nanopores that act as a half-wave rectifier ( left ) and full-wave rectifier
( right )
 
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