250 6.6  Electrical Force Tools

These soft-​matter nanopores can also be used in constructing complex nanodroplet

systems. Here, 200 nL droplets have an internal aqueous phase separated by an artificial

phospholipid monolayer that remains structurally stable due to centrally acting hydrophobic

forces imposed from an external oil phase (Figure 6.11b). These droplets can be

positioned directly by capturing onto the tip of an agarose-​coated Ag/​AgCl 100 μm diameter

electrode using surface tension from the aqueous phase, which in turn is connected to

a micromanipulator. Multiple droplets may be positioned adjacent to each other relatively

easily in a 2D array, with droplets sharing common phospholipid bilayer interfaces and joined

by one or more α-​hemolysin nanopores integrated in the bilayer.

By modifying the amino acid residues in the pore lumen to give all positive charges, it was

found that these nanopores would be open in the presence of a positive voltage potential,

but closed in the presence of a negative potential, presumably due to some induced con­

formational change blocking the pore lumen (Maglia et al., 2009). This modified nanopore

is therefore voltage gated and acts as an electrical diode. As a proof of principle, it was

possible to join four such nanodroplets to form a full-​wave AC–​DC rectification system

(Figure 6.11c).

Other complex arrangements of nanodroplets have led to a tiny nanodroplet biobattery

(Figure 6.11d), in its simplest form made from a linear arrangement of three nanodroplets

in which the central droplet is connected to the others via either a positive or a negative ion

selective nanopore, resulting in a small current flow between the electrode termini of ~50

pA located at the outer two nanodroplets. There is also a nanoeye in which photons of light

can be detected (Holden et al., 2007). This biomimetic system consists of five nanodroplets

FIGURE 6.11  Synthetic soft nanopores using protein adapters in lipid bilayers. (a) Structure of

protein complex α-​hemolysin shown in a side view (left panel, integrated in a lipid bilayer) and

plan view (right panel), which can (b) form a link between two adjacent aqueous droplets with

a lipid monolayer border that becomes a bilayer where the droplets touch, surrounded by oil

that stabilizes the lipid monolayer. The α-​hemolysin protein complex allows flux of ions, water,

and narrow molecules between the two droplets. These droplets can form complex, functional

devices by linking together multiples of droplets, such as (c) an AC–​DC voltage rectifier, (d) a

biobattery, and (e) a nanodroplet photoreceptor (or nanoeye).