Chapter 6 – Forces  251

(Figure 6.11e) with nanopores consisting either of α-​hemolysin or the photosensitive pro­

tein bacteriorhodopsin. Bacteriorhodopsin is a cell membrane nanopore in bacteria, which

utilizes the absorbed energy from a single green wavelength photon of light to pump a single

proton across a phospholipid bilayer. This constitutes a small current, which the nanodroplet

arrangement can detect. Although it is possible to controllably implement such a system with

only one to two α-​hemolysin complexes in each common phospholipid bilayer interface, the

number of bacteriorhodopsin molecules required to generate a measurable current is of the

order of thousands, but as a proof of principle, this shows great promise. Currently, most

nanodroplet arrangements are 2D, but there are recent developments toward implementing

more complex nanoscale biosynthetic systems in 3D. Although nanodroplet systems are

clearly not natively cellular, they represent a synthetic biological system that is moving in the

direction of an exceptionally cell-​like physiological behavior.

6.6.5  ELECTROROTATION

An electric dipole can be induced on an electrically polarizable particle between

microelectrodes generating an electric field in a bespoke microscope flow cell (Figure 6.12a).

A suitable particle could include a latex microsphere that has been functionalized on its

surface with electrically charged chemical groups, such as negatively charged carboxyl or

amino groups that become protonated and hence positively charged in aqueous solution (see

Chapter 2). In a nonuniform electric field, there is a nonzero E-​field gradient that imparts

a force on the particle in a direction parallel to the E-​field gradient, termed “dielectrophor­

esis” (Figure 6.12b), the same driving force of electrical molecular mobility used in gel

electrophoresis.

FIGURE 6.12  Electrorotation. (a) Microfabricated electrodes in quadrature using for

electrorotation. (Courtesy of Hywel Morgan, University of Southampton, Southampton, U.K.)

(b) Induced electrical dipole by dielectrophoresis. (c) Generation of torque during electrorotation

due to phase lag between direction of driving E-​field and induced dipole in bead.