Nano-Bio Integration - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 244

When a sinusoidal voltage is applied on this diode-like nanopore, it works as a
half-wave rectifier, allowing the passage of applied voltages with positive polarity
only. However, due to the decay time constant of the nanopore and the capacitance
of the system, half-wave rectifier operation is possible only for voltage signals with
frequencies below 0.2 Hz. In networks formed by chains of three touching droplets,
the output current depends on the orientation of the pores that penetrate the two
bilayers: the network acts as a half-wave rectifier if the two pores have the same
orientation and as a full-wave rectifier when the pores are oriented in opposite
directions, case in which the current is not allowed to pass through the device
irrespective of its polarity. The orientation of the nanopore can be manipulated by
selecting first the droplets that incorporate the protein and controlling the order in
which the bilayers form. A more complicated, four-droplet network that acts as a
bridge rectifier was also demonstrated. In reality, these devices are not ideal; brief
current spikes appear, especially on closing and opening events of the nanopores.
Optoelectronic devices can also benefit from the incorporation of biomolecules.
The efficient photosynthesis properties of plants and bacteria, which can harvest
photons with almost optimum yield, prompted the research of composites that
could enhance the efficiency of photovoltaic cells and photodetectors. At least two
biological photosynthetic complexes that fulfill this aim have been identified: a bac-
terial reaction center extracted from the purple bacteria Rhodobacter sphaeroides
and a larger complex isolated from spinach chloroplasts ( Das et al. 2004 ). These
complexes, after a careful processing, can be incorporated as interfacial materials in
light-harvesting devices since they are able to sustain open circuit voltages of about
1 V without significant damage and can contribute to loss reduction by acting as a
self-assembled insulating membrane. The hybrid devices have an internal quantum
efficiency of about 12%.
Recently, another bioinspired conversion mechanism of light power into elec-
tricity in a hybrid solar cell based on proton transport across a membrane with
smart-gating nanochannels, which mimic the ion channels in photosynthetic mem-
branes in nature, has been reported in We n e t a l . ( 2010 ) but showed extremely
low efficiencies. The conical photon-driven channels had negatively charged DNA
molecules attached on it, the conformation of which depends on the pH of
a solution of the photo-acid molecule 8-hydroxypyrene-1,3,6-trisulfonate. This
molecule releases protons when irradiated with UV light.
The fluorescence of hybrid bioinorganic surfaces can be modulated by an applied
alternating electric field ( Rant et al. 2004 ). As shown in Fig. 9.10 , the polarity
change of the applied bias switches the orientation of a dye-labeled biological
molecule tethered on the surface between a standing and a lying state, which in turn
modulates the fluorescence of the surface. The fluorescence in the case of the
biological nano-electro-mechanical-optical system in Rant et al. ( 2004 ) is due to
the Cy3 dye that binds to the 3 0 end of a DNA molecule (denoted by F in Fig. 9.10 ),
the 5 0 end of which chemically grafts to gold surfaces.
In solution, the negatively charged DNA is driven away from the surface if the
latter is negatively charged, adopting an upright position, and is pulled toward it if
the surface is positively charged, case in which the DNA molecule is considerably