Focusing a laser on a semiconductor nanoparticle film causes it to conduct electricity to neurons, and such devices might eventually be capable of electrically exciting the nervous system. As a result, they could one day be used to alleviate pain or to restore function to patients with neural damage.Researchers at the University of Michigan in Ann Arbor and at the University of Texas’ Center for Biomedical Engineering in Galveston created the semiconductor nanoparticle film and determined whether neural cells could respond to it. The device, which consists solely of nanoparticle layers, requires no wires or batteries because it is light-activated.To make the metal film, the scientists employed mercury telluride because it exhibited the greatest photoexcitation and lowest toxicity in comparison with CdSe and other heavy metals in the same chemical group, according to study author Nicholas A. Kotov. Furthermore, both green and IR light can stimulate them.They added nanoparticles by using the layer-by-layer assembly technique. Kotov said that this method produces materials with a high nanoparticle content, improves the charge transport between the particles and increases light absorption within the film. In addition, it enabled the researchers to change the properties of the device simply by adding strata of alternate materials. For example, they added a layer each of poly(acrylic acid) and polylysine because they enabled the nanoparticle film to adhere to neurons.In their experiment, they incubated neurons from a neuroblastoma on a nanoparticle film. To detect voltage changes characteristic of an action potential, they began voltage-clamp recordings with a Molecular Devices amplifier. To activate the metal film, they employed a 532-nm laser operating at a fluence of 800 mW/cm2.Researchers focused a 532-nm laser on a nanoparticle film that transduced the optical energy to electrical energy, thereby electrically stimulating the neurons on top of them. Nanoparticle films may one day be used to modulate neuronal functions in patients. In this phase-contrast image, one can see not only a neuron, but also the outline of the patch-clamp electrode used for electrophysiology recordings.With light stimulation, an inward current traveled through the neurons. Those that strongly adhered to the film had depolarizations as high as >10 mV. Three percent of the neurons exhibited an action potential. In other words, several neurons responded to stimulation, and 3 percent of them manifested the desired result. The results inspired the investigators to add clay nanoparticles to their film to increase its ability to excite neurons. Kotov said that clay nanoparticles are compatible with living cells and have unique ion transport properties that allow cells to better interface with them. With a clay monolayer, neurons had a greater mean depolarization, and 11 percent of them generated an action potential. Therefore, the single layer of clay nanoparticles greatly increased the percentage of neurons that had an action potential.The scientists did not see evidence of thermal or oxidative damage. Kotov said that they proved that the device works in concept. In the future, they will experiment with additional layers that could increase its ability to interface with neurons.Nano Letters, online publication, doi: 10.1021/nl062513v.