Figure 4 | Full-wave rectification in a four-droplet network. a, Bridge rectifier using four diodes to convert an alternating current into a direct current. b,c, Schematic and photograph of a four-droplet network using 7R-aHL to create a bridge rectifier. d, The triangular 0.1 Hz input (top), and the current output from a semiconductor diode bridge rectifier (middle) and the 7R-aHL network (bottom). The input waveforms were generated with a custom signal generator (see Supplementary Fig. S4). The current outputs were measured with a patch-clamp amplifier in voltage-clamp mode acting as an ammeter (no applied potential). The input waveform was +1.5 V for the semiconductor device, with the output measured across a 100 MV resistor. The input waveform was +100 mV for the droplet network.

only operate efficiently when the diameter of the nanopore is comparable to the Debye length in the solution, the 7R-aHL nano-pores work efficiently at all ionic strengths tested up to 3 M KCl. In addition, single-channel recording showed that at negative bias the current through the nanopore is reduced from the open value to almost zero in a single step after an exponentially distributed delay with a time constant consistent with a change in protein conformation (Fig. 2d). Further, occasional reopenings occur, which is also consistent with a conformational change such as a collapse of the transmembrane b barrel (see Supplementary Fig. S1). If the rectification were due to the formation of a depletion zone inside the nanopore, as proposed for np junctions, the current block would be instantaneous and no re-openings would occur.

Although the response time of 7R-aHL is slow, it can be harnessed to build devices that resemble traditional electronic circuits based on semiconductors. We first used 7R-aHL to build a half-wave rectifier, which is the simplest device that converts alternating current to direct current, by permitting current flow under applied potentials of one polarity only. The device was built by reconstituting 7R-aHL into a droplet interface bilayer (DIB) and applying a sinusoidal voltage waveform with an amplitude of + 100 mV (Fig. 3a). At frequencies below 0.2 Hz, the output current from the input sinusoidal waveform was mainly positive (Fig. 3a). The efficiency of rectification at higher frequencies is limited by the closing decay constant at negative potentials (see Supplementary Fig. S2) and a smaller contribution from the capacitance of the system (see Supplementary Fig. S5). Hence, above 0.5 Hz, the rectification efficiency degrades substantially (see Supplementary Fig. S3).

More complex electronic functions were produced by using DIB networks. The formation of DIB networks is straightforward; droplets that have been penetrated with agarose-tipped electrodes can be moved into contact with a micromanipulator without damaging other parts of a network. The majority of insertion events arising from pores encapsulated within the droplets occur within a few minutes of bilayer formation. Therefore, the orientation of the protein diodes at each interface in a network can be controlled by selecting which droplets contain protein and the order in which the bilayers are formed. We applied a 0.5 Hz, +100 mV, square voltage wave to four different three-droplet DIB networks (Fig. 3). In the protein-free network, no current was observed as expected (Fig. 3b). When WT-aHL was incorporated into both bilayers from the middle droplet, a symmetrical output current trace that matched the input waveform was obtained (Fig. 3c). In contrast, we found that the orientation of the 7R-aHL pore in each bilayer changes the overall behaviour of the network (Fig. 3d,e). When the 7R-aHL pores in each bilayer are oriented in the same direction, current passes when the network is forward-biased, but not when it is reverse-biased, again constituting a half-wave rectifier (Fig. 3d). On the other hand, when the 7R-aHL pores in each bilayer are oriented in opposite directions, the network behaves as a current limiter, with little current passing in either direction (Fig. 3e).

The droplet technique is suitable for building networks with complex connectivity in two and potentially three dimensions to produce systems with higher-level properties. With these ideas in mind, we also built a four-droplet network, in which the compartments communicated through four DIBs. This device functioned as a bridge rectifier (Fig. 4a). Bridge rectifiers make use of four diodes

NATURE NANOTECHNOLOGY | VOL 4 | JULY 2009 | www.nature.com/naturenanotechnology

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