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IR-absorbing lead selenide particles form the basis of a method for the study of neuronal activation in samples of brain tissues without the need for hard-wired electrodes. The technique instead utilises light-triggered nanostructured semiconductor photoelectrodes to probe activity. Philip Larimer, Richard Todd Pressler, and Ben Strowbridge of the Department of Neurosciences, at Case Western Reserve University, in Cleveland, Ohio, working with Yixin Zhao and Clemens Burda in CWRU's Center for Chemical Dynamics and Nanomaterials Research explain their approach in the current issue of Angewandte Chemie. Understanding brain function remains one of the great challenges facing science. For example, simply understanding how brain regions process synaptic inputs to generate defined responses is a puzzle. One particularly promising avenue of research in this area remains the study of the electrical conduction of stimuli by nerve cells, neurons. However, in order to study neuronal circuits in detail, a sharp metal electrode is usually introduced into the living brain or a brain slice to introduce a current. Such a crude approach is too blunt a probe to discern the highly complex activation patterns of natural nerve stimuli. Moreover, this approach causes direct damage to tissue because of unwanted electrochemical side reactions. Alternative approaches involve genetically encoded light switches or bath-applied caged compounds for neuronal stimulation, all of which add a layer of complexity to any given experiment. Now, neuroscientists and nanomaterials researchers at CWRU have developed an approach that is infinitely gentler on the tissue being probed and elicits a more natural response from nerve impulses. The approach is based on a micropipette coated with semiconductor nanoparticles (composed of PbSe). PbSe is a Group IV-Group VI semiconductor with a narrow bulk band-gap energy of just 0.26 electronVolts, the researchers explain; as such it is commonly used in infrared photodetectors. The team characterised their PbSe films using scanning electron microscopy (SEM) and X-ray diffraction (XRD), with the crystal structure results revealing it to have the rock salt structure known for bulk PbSe. Stimulating PbSe with IR light allows them to activate neurons in brain tissue with visible or infrared (IR) light. In contrast to conventional electrodes, these new photoelectrodes require neither wires nor electrical power. "With these photoelectrodes, no molecular biology manipulations are necessary, and no direct contact of the neurons with nanoparticle-coated surfaces is required (with the inside-coated tips)," the researchers say. The team led by Strowbridge and Burda coated the interior of extremely finely drawn-out glass micropipettes with lead selenide nanoparticles. Short bursts of laser light can be used to create an electrical pulse in the micropipette which the researchers then use to stimulate neurons in rat brain samples. The use of a laser pulse to generate the stimulating electrical field means that a high degree of temporal resolution is possible. The researchers add that this could allow them to record the natural activation patterns of very similar nerve impulses. The researchers hope their new photoelectrodes will allow researchers to study cooperation of neurons. They have now tested their wireless probes on the olfactory bulb. This is a region of the brain involved in processing smell. The neurobiology of the olfactory bulb is especially intriguing because it utilises unusual synaptic connections the physiological role in facilitating olfactory discrimination and learning are yet to be revealed. They have also probed the hippocampus, a part of the cerebrum important in the transfer of contents from short-term to the long-term memory. The team is particularly interested in its so-called "mossy" cells which are thought to have a connection with epilepsy. They saw no toxicity problems nor damage to neurons even after repeated stimulation. Additional work could also point the way to therapeutic applications where the wireless probes might be used to activate individual regions of the brain, stimulate damaged or cut nerves and perhaps restore function, without the need for electrical wiring. Related links:
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