Diamonds reveal neurological secrets

Diamonds reveal neurological secrets

A prototype of a diamond voltage imaging microscope made by physicists at the University of Melbourne. A small electrode is suspended over the diamond chip to test the performance of the device. The bottom-lit green laser provides fluorescence excitation to the chip. Credit: Author Submitted, University of Melbourne

The brain is arguably one of the most complex structures in the known universe.

Continuing advances in our understanding of the brain and our ability to effectively treat a range of neurological diseases depend on examining the brain’s neural microcircuits in ever-increasing detail.

One class of methods for studying neural circuits is called Voltage Photography. These technologies allow us to see the voltage generated by the neurons in our brain – they tell us how networks of neurons develop, function, and change over time.

Today, voltage imaging of cultured neurons is performed using dense arrays of electrodes on which cells are grown (or implanted), or by applying light-emitting dyes that optically respond to changes in voltage at the cell surface.

But the level of detail we can see using these techniques is limited.

The smallest electrodes cannot reliably distinguish individual neurons, which are about 20 millionths of a meter in diameter, not to mention the dense network of nanojunctions that form between them, and no significant technological progress has been made in this field for more than two decades.

Furthermore, each electrode requires its own wire connection and amplifier, which places significant limitations on the number of electrodes that can be measured simultaneously.

A small electrode, eight millionths of a meter wide, is used to locally inject a cloud of electric charge into a fluid placed on top of a diamond chip. The brilliance of diamonds reflects the propagation of this charge through the liquid in real time. Credit: Author provided

The dyes can overcome these limitations by wirelessly imaging the voltage in the form of light – meaning that complex electronics can be placed far from the cells inside the camera.

The result is high resolution over large areas, able to distinguish every single neuron in a large network. But there are limitations here too, voltage responses to the latest dyes are slow and unstable.

Our latest research was published in Nature Photonicsexplores a new type of high-speed, high-resolution, scalable voltage imaging platform created with the goal of overcoming these limitations—the diamond voltage imaging microscope.

Developed by a team of physicists from the University of Melbourne and RMIT University, the device uses a diamond-based sensor that converts voltage signals on its surface directly into light signals – meaning we can see electrical activity as it happens.

The conversion uses the properties of an atom scale defect in the crystal structure of diamond known as the nitrogen vacuum (NV).

NV defects can be engineered by bombarding diamond with a nitrogen ion beam using a special type of particle accelerator. Sensor fabrication begins using this process to create a high-density, ultra-thin layer of NV defects near the surface of the diamond.

You can think of each NV defect as a bucket that holds up to two electrons. When this bucket is empty, the NV defect is dark. With one electron, the NV defect emits an orange light when illuminated by the laser – this property is known as fluorescence. With two electrons, the color of the fluorescence becomes red.

As the voltage varies in a uniformly conductive solution, the brightness of the light emitted by the diamond wafer follows an almost instantaneous response. Here, the diamond surface has been patterned in an array of nanopillars to increase the detected light signal. Credit: Author provided

a NV . Defect Detected is that the number of electrons they hold—and the resulting fluorescence—can be controlled by an electrical voltage. In contrast to dyes, the voltage response of the NV defect is very fast and stable.

Our research aims to overcome the challenge of making this effect sensitive enough for imaging neural activity.

On the surface of diamond, the crystal structure ends with a layer one atom thick, consisting of hydrogen and oxygen atoms. NV defects closest to the surface are most sensitive to voltage changes outside diamond, but they are also very sensitive to the atomic structure of the surface layer.

So much hydrogen and NVs are so dark that the light signals we’re looking for can’t be seen. So little hydrogen and NVs are so bright that the little signals we’re after just disappear.

So, there is a “golden zone” for voltage imaging, where the surface contains the right amount of hydrogen.

To access this region, our team developed an electrochemical method for controlled hydrogen removal. By doing so, we were able to achieve voltage sensitivities that are 2 times better than previously reported.

We tested the sensor in salt water using a microscopic wire ten times thinner than a human hair. By applying a current, the wire can produce a small cloud of charge in the water over the diamond. The formation and subsequent diffusion of this charge cloud results in small stresses on the surface of the diamond.

By capturing these voltages through a high-speed recording of the NV fluorescence, we can determine the speed, sensitivity and resolution of our diamond imaging slice.

We were able to further enhance the sensitivity by shaping the diamond surface into ‘nanopillars’ – conical structures with NV centers embedded at their tips. These pillars divert the light emitted by the NVs toward the camera, which greatly increases the amount of signal we can collect.

With the development of a diamond voltage imaging microscope to detect neuronal activity, the next step is to record the activity from neurons cultured in the lab – these are experiments on cells grown outside of their normal biological context, known as a test tube or petri dish experiment.

What sets this technique apart from the latest in the lab is the combination of high spatial resolution (on the order of a millionth of a meter or less), large spatial scale (a few millimeters in each direction – which a network of neurons in mammals is very vast), and complete stability by the time.

No other system can present these three qualities simultaneously, and it is this combination that will allow the Made in Melbourne technology to make a valuable contribution to the work of neuroscientists and neuropharmacologists globally.

Our system will help these researchers pursue basic knowledge and the next generation of treatments for neurological and neurodegenerative diseases.

The new method enables long-term imaging of rapid brain activity in individual cells deep in the cortex

more information:
DJ McCloskey et al., Diamond Voltage Imaging Microscope, Nature Photonics (2022). DOI: 10.1038 / s41566-022-01064-1

Introduction of
University of Melbourne

the quote: Diamond Reveals Neuronal Secrets (2022, September 8) Retrieved September 9, 2022 from

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