Engineers develop new sensor to detect tiny individual nanoparticles

The image shows arrays of self-referenced and self-heterodyned Whispering-Gallery Raman microlasers for single nanoparticle detection. A "pump" laser generates a single Raman lasing mode inside the silica resonators. Upon landing of a nanoparticle on the resonator, Raman laser circulating inside the resonator undergo mode splitting leading to two new lasing modes in different colors. Monitoring the changes in the color difference (frequency difference) enables detecting and measuring of nanoparticles with single particle resolution. Credit: J. Zhu, B. Peng, S.K. Ozdemir, L. Yang

Nanoparticles, engineered materials about a billionth of a meter in size, are around us every day. Although they are tiny, they can benefit human health, as in some innovative early cancer treatments, but they can also interfere with it through viruses, air pollution, traffic emissions, cosmetics, sunscreen and electronics.

A team of researchers at Washington University in St. Louis, led by Lan Yang, PhD, the Das Family Career Development Associate Professor in Electrical & Systems Engineering, and their collaborators at Tsinghua University in China have developed a new sensor that can detect and count nanoparticles, at sizes as small as 10 nanometers, one at a time. The researchers say the sensor could potentially detect much smaller particles, viruses and small molecules.

The research appears in the Proceedings of the National Academy of Sciences online Early Edition Sept. 1, 2014.

Yang and her colleagues have created the Raman microlaser sensor in a silicon dioxide chip to find individual nanoparticles without the need to "dope" the chip with chemicals called rare-earth ions to provide optical gain for the microlaser. Incorporating additions to the microresonator creates the need for more processing steps and increased costs and invites biocompatibility risks. In addition, the use of rare-earth ions requires specific "pump" lasers matching the energy transitions of the ions to generate optical gain, so for different rare-earth ions, different pump lasers must be used. Using the Raman process loosens the requirement of specific wavelength bands for pump lasers because Raman gain can be obtained using pump at any wavelength band, Yang says.

"This gives us the advantage of using the same dopant-free sensor at different sensing environments by tailoring the lasing frequency for the specific environment, for example, at the band where the environment has minimum absorption, and for the properties of the targeted nanoparticles by just changing the wavelength of the pump laser," says Sahin Kaya Ozdemir, PhD, a research scientist in Yang's group and the first author of the paper.

Yang's team integrated Raman lasing in a silica microcavity with the mode splitting technique pioneered by her group to develop a new, powerful sensor that more readily detects nanoparticles. The technology will benefit the electronics, acoustics, biomedical, plasmonics, security and metamaterials fields.

Yang's microsensor is in a class called whispering gallery mode resonators (WGMRs) because it works similarly to the renowned whispering gallery in London's St. Paul's Cathedral, where a person on one side of the dome can hear a message spoken to the wall by another person on the other side. Yang's device does much the same thing with light frequencies rather than audible ones.

One of the main differences between early resonators and the novel resonator, known as a morphology dependent resonator, was they didn't use mirrors to reflect light. Yang's WGMR is an actual mini-laser that supports "frequency degenerate modes," patterns of excitation inside the mini-laser's doughnut-shaped ring that are of the same frequency. One portion of light beamed by the Raman laser goes counterclockwise, another goes clockwise. When a particle lands on the ring and scatters energy between these modes, the single Raman lasing line splits into two lasing lines with different frequencies.

When a Raman laser beam is generated in the resonator, it likely will encounter a particle, such as a virus nanoparticle, on the circle. When the beam initially sees the particle, the beam splits into two, generating two lasing lines that serve as reference to the other to form a self-referenced sensing technique.

"Our new sensor differs from the earlier whispering gallery sensors in that it relies on Raman gain, which is inherent in silica, thereby eliminating the need for doping the microcavity with gain media, such as rare-earth ions or optical dyes, to boost detection capability," Ozdemir says. "This new sensor retains the biocompatibility of silica and could find widespread use for sensing in biological media."

"It doesn't matter what kind of wavelength is used, once you have the Raman laser circulating inside and there is a molecule sitting on the circle, when the beam sees the particle it will scatter in all kinds of directions," Yang says. "Initially you have a counterclockwise mode, then a clockwise mode, and by analyzing the characterization of the two split modes, we confirm the detection of nanoparticles."

In addition to the demonstration of Raman microlasers for particle sensing, the team says their work shows the possibility of using intrinsic gain mechanisms, such as Raman and parametric gain, instead of optical dyes, rare-earth ions or quantum dots, for loss compensation in optical and plasmonic systems where dissipation hinders progress and limits applications.

Source: Washington University in St. Louis

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Mode of action of protein channelrhodopsin-2 decoded: Findings facilitate manufacture of optogenetic tools

The pore of the ion channel is opened by removing the amino acid E90. Water molecules enter and tilt Helix H2, thus opening the continuous channel. Credit: © RUB, graphics: Eisenhauer

Researchers have shed light upon the mode of action of the light-controlled channelrhodopsin-2 with high spatiotemporal resolution. This biomolecule is used in optogenetic applications, which is deployed to control the activity of living cells with light.

"The model we developed makes it possible to create customised optogenetic tools for individual applications," says Prof Dr Klaus Gerwert from the Department of Biophysics at the Ruhr-Universität Bochum. Together with colleagues at the Humboldt Universität zu Berlin from the team headed by Prof Dr Peter Hegemann, the Bochum researchers report about their finding in the magazine "Angewandte Chemie."

Channelrhodopsin-2 has revolutionised optogenetics

Discovered by Peter Hegemann in green algae, channelrhodopsin-2 is the central light-activated channel protein in optogenetics. If this ion channel is applied to nerve cells, the channels can be opened by light, thus activating the cell. "The application of channelrhodopsin-2 in optogenetics has revolutionised neurobiology in the recent years," says Klaus Gerwert. The magazine "Nature Methods" awarded this process as "Method of the Year" in 2010. "However, scientists had not been aware of what is actually happening inside a protein and thus ultimately triggers its activation," continues the Bochum researcher. But it is the understanding of processes on the atomic level that is essential for optimising the protein specifically for its applications.

"EHT" model describes the mode of action of channelrhodopsin-2

With time-resolved vibrational spectroscopy and bio-molecular simulations, the Bochum-Berlin team has now closed that gap. The EHT (E90-Helix2-tilt) model describes the mode of action of channelrhodopsin-2 as follows: the light-sensitive group of the protein, i.e. the retinal, is twistedunder incidence of light. This twist then continues in the protein and opens a pore ultra fast, which is closed by amino acid E90 in the dark. E90 marks the narrowest place in the pore and opens it through a downward move , similar to the motion of a swing door, so that water can enter an empty vestibule above the narrowest place in the pore. The entering water then tilts the protein helix H2, which eventually triggers a protein-traversing open ion channel. When forming this model, the Bochum researchers benefitted from their comprehensive experience that they had gained resolving the mechanism of light-driven proton pump bacteriorhodopsin in detail.

"Protein engineering": pioneering novel optogenetic tools

"With this structural model, the next step, i.e. protein engineering, will become possible," explains Klaus Gerwert. Through mutation of the amino acid E90, the protein's properties can be controlled in a targeted manner. The conductivity or the selectivity for certain ions could be customised for specific applications, and the protein could be specifically activated with different wavelengths.

Source: Ruhr-Universitaet-Bochum

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A new, tunable device for spintronics

Tunable spin Hall angle device based on GaAs through field induced intervalley repopulation.
Credit: Copyright Jairo Sinova

An international team of scientists including physicist Jairo Sinova from the University of Mainz has developed a tunable spin-charge converter made of GaAs.

Spin-charge converters are important devices in spintronics, an electronic which is not only based on the charge of electrons but also on their spin and the spin-related magnetism. Spin-charge converters enable the transformation of electric into magnetic signals and vice versa. Recently, the research group of Professor Jairo Sinova from the Institute of Physics at Johannes Gutenberg University Mainz in collaboration with researchers from the UK, Prague, and Japan, has for the first time realised a new, efficient spin-charge converter based on the common semiconductor material GaAs. Comparable efficiencies had so far only been observed in platinum, a heavy metal. In addition, the physicists demonstrated that the creation or detection efficiency of spin currents is electrically tunable in a certain regime. This is important when it comes to real devices. The underlying mechanism, that was revealed by theoretical works of the Sinova group, opens up a new approach in searching and engineering spintronic materials. These results have recently been published in the journal Nature Materials.

Spintronics does not only make use of the electron's charge to transmit and store information but it takes also advantage of the electron's spin. The spin can be regarded as a rotation of the electron around its own axis, and generates a magnetic field like a small magnet. In some materials, electron spins spontaneously align their direction, leading to the phenomenon of ferromagnetism which is well known e.g. in iron. Additionally, "spin-up" or "spin-down" directions can be used to represent two easily distinguishable states -- 0 and 1 -- used in information technology. This is already used for memory applications such as computer hard discs.

Making use of electron spin for information transmission and storage, enables the development of electronic devices with new functionalities and higher efficiency. To make real use of the electron spin, it has to be manipulated precisely: it has to be aligned, transmitted and detected. The work of Sinova and his colleagues shows, that it is possible to do so using electric fields rather than magnetic ones. Thus, the very efficient, simple and precise mechanisms of charge manipulation well established in semiconductor electronics can be transferred to the world of spintronic and thereby combine semiconductor physics with magnetism.

Spin-charge converters are essential tools for that. They can transform charge currents into spin currents, and vice versa. The main principle behind these converters is the so called spin-Hall effect. Jairo Sinova had already been involved in the prediction and discovery of this relativistic phenomenon in 2004.

The spin-Hall effect appears when an electric field drives electrons through a (semi-) conductor plate. Taking a look at the classical Hall effect that is known from undergraduate physics, the interaction of moving electrons and an external magnetic field forces the electrons to move to one side of the plate, perpendicular to their original direction. This leads to the so called Hall voltage between both sides of the plate. For the spin-Hall effect electron-spins are generated by irradiating the sample with circularly polarised light. The electron spins are then parallel or antiparallel, and their direction is perpendicular to the plate and the direction of movement. The moving electron spins are now forced to one or the other side of the plate, depending on the spin orientation. The driving force behind this is the so called spin-orbit coupling, a relativistic electromagnetic effect which influences moving electron spins. This leads to the separation of both spin orientations.

To make practical use of this effect, it is essential to get a highly efficient spin separation. Up to now, platinum has been the most efficient spin-charge converter material, as it is a heavy metal, and the spin-orbit coupling of heavy metals is known to be especially strong due to the large amount of protons (positive charge) in their core.

Now, Sinova and his colleagues have shown that gallium-arsenide (GaAs), a very common and widely used semiconductor material, can be an as efficient spin-charge converter as platinum, even at room temperature, which is important for practical applications. Moreover, the physicists have demonstrated for the first time that the efficiency can be tuned continuously by varying the electric field that drives the electrons.

The reason for this -- as theoretical calculations of the Sinova group have shown -- lies in the existence of certain valleys in the conduction band of the semiconductor material. One can think of the conduction band and its valleys as of a motor highway with different lanes, each one requiring a certain minimum velocity. Applying a higher electric field enables a transition from one lane to the other.

Since the spin-orbit coupling is different in each lane, a transition also affects the strength of the spin-hall effect. By varying the electric field, the scientists can distribute the electron spins on the different lanes, thus varying the efficiency of their spin-charge converter.

By taking into account the valleys in the conduction band, Sinova and his colleagues open up new ways to find and engineer highly efficient materials for spintronics. Especially, since current semiconductor growth technologies are capable of engineering the energy levels of the valleys and the strength of spin-orbit coupling, e.g. by substituting Ga or As with other materials like Aluminum.

Source: Universität Mainz

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