Physicists use light to probe deeper into the ‘invisible’ energy states of molecules

31 July 2024 University of Bath

A team led by scientists at the University of Bath discovers how light particles can be used to reveal the ‘hidden’ energy states of molecules.

The nanotechnological revolution requires standardised ‘screws’ – here is a way to measure them

17 June 2024 University of Bath

Physicists at the University of Bath lead on the discovery of a new optical property that measures the twist in tiny helices.

Can rainbows monitor the environment?

3 August 2023 University of Bath

Keeping up with the first law of robotics: A new photonic effect for accelerated drug discovery

13 January 2022 University of Bath

Physicists at the University of Bath and University of Michigan demonstrate a new photonic effect in semiconducting nanohelices.

A new photonic effect in semiconducting helical particles with nanoscale dimensions has been discovered by an international team of scientists led by researchers at the University of Bath. The observed effect has the potential to accelerate the discovery and development of life-saving medicines and photonic technologies.

In his Robot series, science-fiction writer Isaac Asimov imagined a future where robots grew into trustworthy companions for humans. These robots were guided by the laws of robotics, the first of which states that 'a robot may not injure a human being or, through inaction, allow a human being to come to harm'. Thanks to the new photonic discovery, robots may get a chance to prevent humans from coming to harm in a very meaningful way – by greatly speeding up the development of important drugs, such as new antibiotics.

Currently, the World Health Organisation regards antibiotic resistance (the growing ineffectiveness of drugs currently on the market) as one of the top 10 threats to humanity. Moreover, globalisation coupled with human encroachment into wildlife habitats increases the risk of new infectious diseases emerging. It is widely recognised that the cost of discovering and development new drugs for these and other conditions using today’s technology is unsustainable. The need for pharmaceutical research to be accelerated has never been more pressing and it would benefit hugely from the help of artificial intelligence (AI).

Bath Physics professor Ventsislav Valev, who headed the research, said: “Although we are a long way still from Asimov’s positronic robot brains, our latest finding does have the potential to link AI algorithms that analyse chemical reactions and robotic arms that prepare chemical mixtures – a process known as high-throughput screening.”

Meeting the needs of robotised chemistry

High-throughput screening (HTS) is an experimental method that uses robots to discover new drugs. Some labs have adopted it already, to help them analyse vast libraries of molecules. In the future, however, discovering new drugs could happen entirely through HTS. Using this method, robots simultaneously operate a large number of syringes, preparing thousands of chemical mixtures that are then robotically analysed. The results are fed back to AI algorithms, which then determine what mixtures to prepare next, and so on until a useful drug is discovered.

The analytical step is key, since without it, the robots cannot know what they have prepared.

HTS happens on microplates (or tablets) that are about the size of a chocolate bar. Each tablet contains wells into which the chemical mixtures are poured. The more wells found on a tablet, the more chemicals can be analysed in one hit. But though a modern tablet can host thousands of wells, the size of the table does not change.

“To meet the requirements of the emerging robotised chemistry, wells are getting really tiny – too small for current analytical methods,” said Professor Valev. “So, fundamentally new methods are needed to analyse would-be drugs.

“Currently, most new drugs that are entering the market and the majority of old drugs are chiral (their chemical formula lacks mirror symmetry). Therefore it is especially important to be able to measure chirality in tiny volumes of less than 1 mm3 which is about the size of a cube with sides of the thickness of a credit card.”

The effect discovered by the researchers allows chirality to be measured in volumes that are 10,000 times smaller than 1 mm³.

“We have used a very exciting new material developed by our colleagues at the University of Michigan in the US, led by Professor Nicholas Kotov,” explained Professor Valev. “It’s a biomimetic structure (i.e. one that simulates biological phenomena) that chemically assembles into semiconducting helices, at the nanoscale, similarly to the way proteins assemble.”

Professor Kotov said: “Being illuminated with red light, the small semiconductor helices generate new light that is blue and twisted. The blue light is also emitted in a specific direction, which makes it easy to collect and analyse. The trifecta of unusual optical effects drastically reduce the noise that other nanoscale molecules and particles in biological fluids may cause.”

Professor Valev added: “This means that by carefully measuring the blue light, we can ascertain the direction of twist (or chirality) of the structures we’re studying.”

The twist of the nanohelices can change dramatically depending on the kind of biomolecules that were present when these helixes formed, providing a wealth of information about the biological samples.

“Our results open the way for measuring chirality in volumes potentially 10-million times smaller than 1 mm³. Although the structures that we measured so far are much larger than typical pharmaceuticals, we have proven that the physical effect is real, so in principle, applications to molecules and especially drugs are now only a question of technological development. Our next step is to seek funding for this development,” said Professor Valev.

PhD student Lukas Ohnoutek, also involved in the research, said: “In nanotechnology, one of the big challenges is to be able to see the properties of tiny things. Nowadays, this is easy for stationary objects but it’s still hard for an object that freely floats in a liquid.

“It has been extremely gratifying to reduce our volume of study so successfully – we now focus light to a spot that would be invisible to most people’s eyes. And within that volume, we can determine the direction of twist of helices that are much smaller still.”

The research is published in the journal Nature Photonics. It was funded by The Royal Society, the Science and Technology Facilities Council (STFC) and the Engineering and Physical Science Research Council (EPSRC). Read full paper.

The nanophotonics orchestra presents: Twisting to the light of nanoparticles

23 September 2021 University of Bath

Physicists at the University of Bath observe a new physical effect in chiral (twisted) nanoparticles.

Physics researchers at the University of Bath discover a new physical effect relating to the interactions between light and twisted materials – an effect that is likely to have implications for emerging new nanotechnologies in communications, nanorobotics and ultra-thin optical components.

In the 17th and 18th centuries, the Italian master craftsman Antonio Stradivari produced musical instruments of legendary quality, and most famous are his (so-called) Stradivarius violins. What makes the musical output of these musical instruments both beautiful and unique is their particular timbre, also known as tone colour or tone quality. All instruments have a timbre – when a musical note (sound with frequency fs) is played, the instrument creates harmonics (frequencies that are an integer multiple of the initial frequency, i.e. 2fs, 3fs, 4fs, 5fs, 6fs, etc.).

Similarly, when light of a certain colour (with frequency fc) shines on materials, these materials can produce harmonics (light frequencies 2fc, 3fc, 4fc, 5fc, 6fc, etc.). The harmonics of light reveal intricate material properties that find applications in medical imaging, communications and laser technology.

For instance, virtually every green laser pointer is in fact an infrared laser pointer whose light is invisible to human eyes. The green light that we see is actually the second harmonic (2fc) of the infrared laser pointer and it is produced by a special crystal inside the pointer.

In both musical instruments and shiny materials, some frequencies are ‘forbidden’ – that is, they cannot be heard or seen because the instrument or material actively cancels them. Because the clarinet has a straight, cylindrical shape, it supresses all of the even harmonics (2fs, 4fs, 6fs, etc.) and produces only odd harmonics (3fs, 5fs, 7fs, etc.). By contrast, a saxophone has a conical and curved shape which allows all harmonics and results in a richer, smoother sound. Somewhat similarly, when a specific type of light (circularly polarised) shines on metal nanoparticles dispersed in a liquid, the odd harmonics of light cannot propagate along the direction of light travel and the corresponding colours are forbidden.

Now, an international team of scientists led by researchers from the Department of Physics at the University of Bath have found a way to reveal the forbidden colours, amounting to the discovery of a new physical effect. To achieve this result, they ‘curved’ their experimental equipment.

Professor Ventsislav Valev, who led the research, said: “The idea that the twist of nanoparticles or molecules could be revealed through even harmonics of light was first formulated over 42 years ago, by a young PhD student – David Andrews. David thought his theory was too elusive to ever be validated experimentally but, two years ago, we demonstrated this phenomenon. Now, we discovered that the twist of nanoparticles can be observed in the odd harmonics of light as well. It’s especially gratifying that the relevant theory was provided by none other than our co-author and nowadays well-established professor – David Andrews!

“To take a musical analogy, until now, scientists who study twisted molecules (DNA, amino acids, proteins, sugars, etc) and nanoparticles in water – the element of life – have illuminated them at a given frequency and have either observed that same frequency or its noise (inharmonic partial overtones). Our study opens up the study of the harmonic signatures of these twisted molecules. So, we can appreciate their ‘timbre’ for the first time.

“From a practical point of view, our results offer a straightforward, user-friendly experimental method to achieve an unprecedented understanding of the interactions between light and twisted materials. Such interactions are at the heart of emerging new nanotechnologies in communications, nanorobotics and ultra-thin optical components. For instance, the ‘twist’ of nanoparticles can determine the value of information bits (for left-handed or right-handed twist). It is also present in the propellers for nanorobots and can affect the direction of propagation for a laser beam. Moreover, our method is applicable in tiny volumes of illumination, suitable for the analysis of natural chemical products that are promising for new pharmaceuticals but where the available material is often scarce.

PhD student Lukas Ohnoutek, also involved in the research, said: “We came very close to missing this discovery. Our initial equipment was not ‘tuned’ well and so we kept seeing nothing at the third-harmonic. I was starting to lose hope but we had a meeting, identified potential issues and investigated them systematically until we discovered the problem. It is wonderful to experience the scientific method at work, especially when it leads to a scientific discovery!”

Professor Andrews added: ‘‘Professor Valev has led an international team to a real first in the applied photonics. When he invited my participation, it led me back to theory work from my doctoral studies. It has been amazing to see it come to fruition so many years later.”

The research is published in the journal Laser & Photonics Reviews. It was funded by The Royal Society, the Science and Technology Facilities Council (STFC) and the Engineering and Physical Science Research Council (EPSRC). Read full paper.

Parallel universes cross in Flatland

10 May 2021 University of Bath

Physicists at the University of Bath observe modified energy landscapes at the intersection of 2D materials.

In 1884, Edwin Abbott wrote the novel Flatland: A Romance in Many Dimensions as a satire of Victorian hierarchy. He imagined a world that existed only in two dimensions, where the beings are 2D geometric figures. The physics of such a world is somewhat akin to that of modern 2D materials, such as graphene and transition metal dichalcogenides, which include tungsten disulfide (WS₂), tungsten diselenide (WSe₂), molybdenum disulfide (MoS₂) and molybdenum diselenide (MoSe₂).

Modern 2D materials consist of single-atom layers, where electrons can move in two dimensions but their motion in the third dimension is restricted. Due to this 'squeeze', 2D materials have enhanced optical and electronic properties that show great promise as next-generation, ultrathin devices in the fields of energy, communications, imaging and quantum computing, among others.

Typically, for all these applications, the 2D materials are envisioned in flat-lying arrangements. Unfortunately, however, the strength of these materials is also their greatest weakness – they are extremely thin. This means that when they are illuminated, light can interact with them only over a tiny thickness, which limits their usefulness. To overcome this shortcoming, researchers are starting to look for new ways to fold the 2D materials into complex 3D shapes.

In our 3D universe, 2D materials can be arranged on top of each other. To extend the Flatland metaphor, such an arrangement would quite literally represent parallel worlds inhabited by people who are destined to never meet.

Now, scientists from the Department of Physics at the University of Bath have found a way to arrange 2D sheets of WS₂ (previously created in their lab) into a 3D configuration, resulting in an energy landscape that is strongly modified when compared to that of the flat-laying WS₂ sheets. This particular 3D arrangement is known as a 'nanomesh': a webbed network of densely-packed, randomly distributed stacks, containing twisted and/or fused WS₂ sheets.

Modifications of this kind in Flatland would allow people to step into each other's worlds. "We didn't set out to distress the inhabitants of Flatland," said Professor Ventsislav Valev who led the research, "But because of the many defects that we nanoengineered in the 2D materials, these hypothetical inhabitants would find their world quite strange indeed.

"First, our WS₂ sheets have finite dimensions with irregular edges, so their world would have a strangely shaped end. Also, some of the sulphur atoms have been replaced by oxygen, which would feel just wrong to any inhabitant. Most importantly, our sheets intersect and fuse together, and even twist on top of each other, which modifies the energy landscape of the materials. For the Flatlanders, such an effect would look like the laws of the universe had suddenly changed across their entire landscape."

Dr Adelina Ilie, who developed the new material together with her former PhD student and post-doc Zichen Liu, said: "The modified energy landscape is a key point for our study. It is proof that assembling 2D materials into a 3D arrangement does not just result in 'thicker' 2D materials – it produces entirely new materials. Our nanomesh is technologically simple to produce, and it offers tunable material properties to meet the demands of future applications."

Professor Valev added: "The nanomesh has very strong nonlinear optical properties – it efficiently converts one laser colour into another over a broad palette of colours. Our next goal is to use it on Si waveguides for developing quantum optical communications."

PhD student Alexander Murphy, also involved in the research, said: "In order to reveal the modified energy landscape, we devised new characterisation methods and I look forward to applying these to other materials. Who knows what else we could discover?"

The research is published in the journal Laser & Photonics Reviews. It was funded by The Royal Society, the Science and Technology Facilities Council (STFC), the Engineering and Physical Science Research Council (EPSRC) and the UK Quantum Technology Hub Sensors and Timing.

A plot twist in pharmaceuticals: single nanoparticles could pave the way for medicines on demand

20 July 2020 University of Bath

For the first time, a single, twisted nanoparticle has been accurately measured and characterised in a lab.

RFor the first time, a single, twisted nanoparticle has been accurately measured and characterised in a lab, taking scientists one vital step closer to a time when medicines will be produced and blended on a microscopic scale.

Physicists at the University of Bath who study materials on the nanoscale – that is, molecules 10,000 smaller than a pinhead – made their groundbreaking observations using a new method for examining the shape of nanoparticles in 3D. This technique, called the hyper-Rayleigh scattering optical activity (HRS OA) technique, was used to examine the structure of gold (among other materials), resulting in an exceptionally clear image of the ‘screw thread’ twist in the metal’s shape.

Understanding the twists within a material (known as its chirality) is vital in industries that produce medicines, perfumes, food additives and pesticides, as the direction in which a molecule twists determines some of its properties. For instance, a molecule that twists clockwise will produce the smell of lemons while the identical molecule twisting anticlockwise (the mirror image of the lemon-smelling molecule) smells of oranges.

“Chirality is one of the most fundamental properties of nature. It exists in sub-atomic particles, in molecules (DNA, proteins), in organs (the heart, the brain), in bio-materials (such as seashells), in storm clouds (tornadoes) and in the shape of galaxies (spirals hurling through space).” said Professor Ventsislav Valev, who led the project.

Until now, physicists have relied on 200-year-old optical methods for determining the chiral properties of molecules and materials, but these methods are weak and require large amounts of molecules or materials to work. Through their use of a technique based on powerful laser pulses, Professor Valev and his team at Bath’s Centre for Photonics and Photonic Materials have produced a far more sensitive probe for chirality, one that can detect a single nanoparticle as it floats freely in a liquid.

This discovery was made by Bath’s Department of Physics in collaboration with the Department of Chemistry. The researchers’ findings are published in Nano Letters.

“This is both a record and a milestone in nanotechnology,” said Professor Valev. “Pursuing this line of research has been one of the most rewarding achievements in my career.”

“The observation by Valev’s group is historic, and scientifically it inspires us in our work to synthesise new chiral 3D nanomaterials,” said study co-author Professor Ki Tae Nam from Material Science and Engineering at the Seoul National University in Republic of Korea.

The potential applications for ultra-sensitive chiral sensing are many. For instance, many pharmaceuticals are chiral. Local pharmacists will be able to harness the technology to mix substances in a completely new way, producing pharmaceuticals from minute droplets of active ingredients rather than from large beakers of chemicals.

“You’ll be able to go to the chemist with a prescription and instead of receiving a medicine that has to be mixed from bottles of chemicals and then stored in the fridge for several days, you’ll walk away with pills that are mini-labs. Upon cracking the pill, a precise number of micro-droplets will flow through microchannels to mix and produce the needed medicine.” said Professor Valev.

“For these mini-labs to produce chiral drugs, you’ll need to know the number of molecules and catalysts within each micro droplet, as well as their chirality.” said PhD student Lukas Ohnoutek, who is the first author on the paper. “This is where our result is really important. We can now aim to produce microdroplets containing a single chiral nanoparticle, to use as catalysts in chemical reactions.”

Professor Valev added: “Looking ahead, we can imagine building up chiral materials and even machines, one nanoparticle at a time, from such microdroplets. To do so would be amazing.”

Dialling up the heat on nanoparticles

21 January 2020 University of Bath

The ability of metallic nanoparticles to harvest and control light is transforming scientific research, according to physicists from the University of Bath.The ability of metallic nanoparticles to harvest and control light is transforming scientific research, according to physicists from the University of Bath.

Rapid progress in the field of metallic nanotechnology is sparking a science revolution that is likely to impact all areas of society, according to professor of physics Ventsislav Valev and his team at the University of Bath.

Metallic nanotechnology is an area that allows microscopic particles of metals, such as gold and silver, to be manipulated with heat and light. The potential applications are vast, ranging from optimising the way we harvest renewable energy to overhauling our treatment of cancerous tumours.

Writing in Advanced Optical Materials, Prof Valev’s team reviews the current state of nanotechnology research and discusses its likely applications in the near and medium future.

PhD student Lukas Ohnoutek sees nanomedicine – a branch of medicine that uses nanotechnology to improve the diagnosis and treatment of disease – as a particularly buoyant area of research. On-command delivery of drugs has already proven successful in several animal trials, he says. Using this technology, medicines encapsulated in nanomaterials are directed to a specific site in the body before releasing their active ingredients in a highly controlled manner.

“It is crucial to increase the efficiency of drugs and to reduce side effects, and this is something that can be achieved with on-command drug delivery,” said Mr Ohnoutek. “By illuminating metal nanoparticles, it is possible to control the location, time, and amount of drug released in a patient.”

Research fellow Dr Kristina Rusimova says dramatic improvements are expected in the treatment of cancer, thanks both to on-command drug delivery and photothermal cancer therapy (PTT). PTT involves injecting nanoparticles into a patient’s body, where they accumulate in the tumour. When the particles are then subjected to radiation, they heat up and destroy the tumour with very little damage to surrounding tissue. In animal trials, advanced tumours have completely disappeared following photothermal therapy.

“We have looked at animal trials conducted on mice, cats and dogs,” said Dr Rusimova. “In each case, the treatment seemed successful, which is very encouraging for treatment in humans. We know that human trials have been approved and are currently ongoing, so we are cautiously optimistic.”

Other research is focused on finding nanotechnology solutions to the climate crises. There is hope that non-radiative plasmonic decay will provide a new method for improving solar cells and for producing hydrogen fuel directly from water. This process is known as ‘water splitting’. The result will be an efficient and economical low-carbon fuel, particularly suitable for heating homes and other spaces.

Other tantalising applications for metallic nanoparticles technology include advanced biomedical imaging, improved magnetic storage and nanorobotics, where robots are manufactured with components on the nanoscale.

Prof Valev said: “The tiniest metal pieces can now be formed, cut and joined with light. This allows us to integrate humanity’s knowledge of metal working with our understanding of molecular self-assembly and nanoscale biotechnology. This research field offers some truly amazing perspectives for the future.”

These prospects are all built around the ability of metallic nanoparticles to harvest and control light at the subwavelength scale.

Scientists illuminate secrets of a nanomaterial that is both twisted and untwisted at the same time

13 September 2019 University of Bath

The material developed at University of Bath allows for incredibly sensitive detection of the direction molecules twist.

A new nanomaterial developed by scientists at the University of Bath could solve a conundrum faced by scientists probing some of the most promising types of future pharmaceuticals.

Scientists who study the nanoscale - with molecules and materials 10,000 smaller than a pinhead - need to be able to test the way that some molecules twist, known as their chirality, because mirror image molecules with the same structure can have very different properties. For instance one kind of molecule smells of lemons when it twists in one direction, and oranges when twisted the other way.

Detecting these twists is especially important in some high-value industries such as pharmaceuticals, perfumes, food additives and pesticides.

Recently, a new class of nanoscale materials have been developed to help distinguish the chirality of molecules. These so-called ‘nanomaterials’ usually consist of tiny twisted metal wires, that are chiral themselves.

However, it has become very hard to distinguish the twist of the nanomaterials from the twist of the molecules they are supposed to help study.

To solve this problem the team from the University of Bath’s Department of Physics created a nanomaterial that is both twisted and it is not. This nanomaterial has equal number of opposite twists - meaning they cancel each other out. Usually, upon interacting with light, such material appears without any twist; how then could it be optimised to interact with molecules? Using a mathematical analysis of the material’s symmetry properties, the team discovered a few special cases, which can bring the ‘hidden’ twist to light and allow very sensitive detection of chirality in molecules.

Lead author Professor Ventsislav Valev, from the University of Bath Department of Physics, said: “This work removes an important roadblock for the entire research field and paves the way to ultra-sensitive detection of chirality in molecules, using nanomaterials.”

PhD student Alex Murphy, who worked on the study, said: “Molecular chirality is an amazing property to study. You can smell chirality, since the same but oppositely twisted molecules smell of lemons and oranges. You can taste chirality, since one twist of Aspartame is sweet and the other is tasteless. You can feel chirality, since one twist of menthol gives a cool sensation to the skin while the other does not. You touch chirality expressed in the twist of seashells. And it is great to see chirality expressed in its interactions with the colours of laser light.”

The study is published in the journal Nanoscale Horizons. The research was funded by the Royal Society, the Engineering and Physical Sciences Research Council, and the Science and Technology Facilities Council.

Quantum computing boost from vapour stabilising technique

30 May 2019 University of Bath

The patented technique allows electrons to be used for quantum computing, high-precision measurements and other applications.

A technique to stabilise alkali metal vapour density using gold nanoparticles, so electrons can be accessed for applications including quantum computing, atom cooling and precision measurements, has been patented by scientists at the University of Bath.

Alkali metal vapours, including lithium, sodium, potassium, rubidium and caesium, allow scientists to access individual electrons, due to the presence of a single electron in the outer ‘shell’ of alkali metals.

This has great potential for a range of applications, including logic operations, storage and sensing in quantum computing, as well as in ultra-precise time measurements with atomic clocks, or in medical diagnostics including cardiograms and encephalograms.

However, a serious technical obstacle has been reliably controlling the pressure of the vapour within an enclosed space, for instance the tube of an optical fibre. The vapour needs to be prevented from sticking to the sides in order to retain its quantum properties, but existing methods to do this, including directly heating vapour containers are slow, costly, and impractical at scale.

Scientists from the University of Bath, working with a colleague at the Bulgarian Academy of Sciences, have devised an ingenious method of controlling the vapour by coating the interior of containers with nanoscopic gold particles 300,000 times smaller than a pinhead.

When illuminated with green laser light the nanoparticles rapidly absorb and convert the light into heat, warming the vapour and causing it to disperse into the container more than 1,000 times quicker than with other methods. The process is highly reproducible and, in addition, the new nanoparticle coating was found to preserve the quantum states of alkali metal atoms that bounce from it.

The study is published in Nature Communications.

Professor Ventsislav Valev, from the University of Bath’s Department of Physics led the research. He said: “We are very excited by this discovery because it has so many applications in current and future technologies! It would be useful in atomic cooling, in atomic clocks, in magnetometry and in ultra-high-resolution spectroscopy.

“Our coating allows fast and reproducible external control of the vapour density and related optical depth, crucial for quantum optics in these confined geometries.”

Assoc. Prof Dimitar Slavov, from the Institute of Electronics in the Bulgarian Academy of Sciences, added “In this proof of principle, it was demonstrated that illuminating our coating significantly outperforms conventional methods and is compatible with standard polymer coatings used to preserve quantum states of single atoms and coherent ensembles.”

Dr Kristina Rusimova, a prize fellow in the Department of Physics, added: “Further improvements of our coating are possible by tuning particle size, material composition and polymer environment. The coating can find applications in various containers, including optical cells, magneto-optical traps, micro cells, capillaries and hollow-core optical fibres.”

The research was funded by the Engineering and Physical Sciences Research Council (EPSRC) UK Quantum Technology Hub “Networked Quantum Information Technologies” and the Royal Society.

Electrode's 'hot edges' convert CO₂ gas into fuels and chemicals

15 May 2019 University of Bath

The bowl-shaped design can efficiently convert CO₂ from gas into carbon based fuels and chemicals, helping combat the threat of atmospheric carbon dioxide.

A team of scientists has created a bowl-shaped electrode with ‘hot edges’ which can efficiently convert CO₂ from gas into carbon based fuels and chemicals, helping combat the climate change threat posed by atmospheric carbon dioxide.

The research team, from the University of Bath, Fudan University, Shanghai, and the Shanghai Institute of Pollution Control and Ecological Security, hopes the catalyst design will eventually allow the use of renewable electricity to convert CO₂ into fuels without creating additional atmospheric carbon - essentially acting like an electrochemical ‘leaf’ to convert carbon dioxide into sugars.

Using this reaction, known as the reduction of carbon dioxide, has exciting potential but two major obstacles are poor conversion efficiency of the reaction and a lack of detailed knowledge about the exact reaction pathway.

This new electrode addresses these challenges with higher conversion efficiency and sensitive detection of molecules created along the reaction’s progress – thanks to its innovative shape and construction. The bowl shaped electrode works six times faster than standard planar - or flat - designs.

The bowl-like shape of the design, technically known as an “inverse opal structure” concentrates electric fields on its hot edges - the rim of the bowl - which then concentrates positively charged potassium ions on the active sites of the reaction, reducing its energy requirements.

The Copper-Indium alloy electrode can also be useful to sensitively study the reaction process via measuring the Raman signal, which is higher compared to a typical electrode.

The study is published in the Royal Society of Chemistry’s Journal of Materials Chemistry A.

Professor Ventsislav Valev, from the University of Bath’s Department of Physics, said: “There is no more pressing human need than breathing. Yet for hundreds of million people this most basic activity is a source of anxiety over lowering life expectancy, rising child mortality and climate change. There is evidence that CO₂ increases surface ozone, carcinogens, and particulate matter, thereby increasing death, asthma, hospitalization, and cancer rates. It is therefore crucial to keep researching new ways for lowing the CO₂ levels in the atmosphere.”

The team wants to continue research to develop the most efficient catalyst to perform carbon reduction.

Professor Liwu Zhang, from Fudan University, said: “CO₂ is causing climate change, making our planet warmer. By using clean electricity, we can convert CO₂ into chemical fuels, which can be used again. This builds a cycle of CO₂, with no increment of CO₂ concentration and will help save our world.

“However, to improve the efficiency of transforming CO₂ into chemical fuels, it is extremely important to know the reaction pathway, and find the most suitable catalyst.

“Just as plants transform CO₂ into sugar we are finding suitable electrochemical ‘leaf’ for CO₂ conversion.”

The study: "Hot edges" in an inverse opal structure enable efficient CO₂ electrochemical reduction and sensitive in situ Raman characterization” is published in Journal of Materials Chemistry A.

The study was funded by the Ministry of Science and Technology of the People's Republic of China, and National Natural Science Foundation of China, the Engineering and Physical Sciences Research Council(EPSRC) Centre for Doctoral Training in Condensed Matter Physics (CDT-CMP), and the Royal Society.

Read the research paper.

New tunable nanomaterials possible via flexible process invented by University of Bath physicists

11 April 2019 University of Bath

Physicists have developed a flexible way to synthesise a wide range of novel nanomaterials with potential applications in areas including optics and sensors.

Physicists at the University of Bath have developed a flexible process allowing the synthesis in a single flow of a wide range of novel nanomaterials with various morphologies, with potential applications in areas including optics and sensors.

The nanomaterials are formed from Tungsten Disulphide - a Transition Metal Dichalcogenide (TMD) - and can be grown on insulating planar substrates without requiring a catalyst. TMDs are layered materials, and in their two-dimensional form can be considered the inorganic analogues of graphene.

The various Tungsten Disulphide morphologies synthesized - two-dimensional sheets growing parallel to the substrate, nanotubes, or a nanomesh resembling a ‘field of blades’ growing outwards from the substrate - are possible due to Dr. Zichen Liu’s PhD research at Bath to split the growth process into two distinct stages. Through this decoupling, the growth process could be routed differently than in more conventional approaches, and be guided to produce all these material morphologies.

So far, the ‘field of blades’ morphology has shown powerful optical properties, including strong non-linear effects such as Second Harmonic Generation, that is, doubling the frequency and halving the wavelength of laser light, changing its colour as it does so. The strength of these effects opens up a range of optical applications for the material.

The research is published in ACS Nano.

Dr Adelina Ilie, from the University of Bath’s Department of Physics, who led the research, said: “The simplicity of this process is important from the standpoint that it allows us to obtain practically all phases of this Transition Metal Dichalcogenide, from in-plane to out-of-plane, as well as from two-dimensional sheets to one-dimensional nanotubes and everything between. Usually different processes are used to create two-dimensional or one-dimensional morphologies. Our process, instead, leads to tunable materials with tunable properties.

“The ‘field of blades’ morphology is entirely new, and due to its very large effective surface area, might be of interest not only for the non-linear optical properties we showed so far, but also for application in various sensing technologies. We are exploring all these avenues now.”

Professor Ventsislav Valev, who tested the nanomesh for optical properties added: “We haven’t actually been able to test the upper limits of the optical effects yet because the signal is too strong for the equipment we used to probe it. We are talking about a material that is one or two atoms in thickness; it is quite extraordinary. Its arrangement into a ‘field of blades’ clearly increases the signal.”

The team plans to continue to explore the properties of the materials.

The research was supported by the University of Bath, the Engineering and Physical Sciences Research Council, and the Royal Society.

Read the reseach paper.

New physical effect demonstrated by University of Bath scientists after a 40 year search

6 February 2019 University of Bath

The discovery could lead to advancements in diverse fields including chemical manufacturing, pharmaceuticals and miniaturisation

A new physical effect has been demonstrated at the University of Bath after 40 years of pursuit by physicists around the world, which could lead to advancements in chemical manufacturing efficiency, miniaturisation and quality control in personalised pharmaceuticals.

For the first time ever the research team in the Department of Physics was able to use a physical effect - specifically the colour-changing of light scattered from chiral molecules - to measure the chirality present, confirming predictions of theoretical work from the 1970s. The technique is 100,000 times more sensitive than standard methods used today.

Chirality describes the orientation of molecules, which can exist in left or right ‘handed’ forms depending on how they twist in three dimensions. Many molecules essential to life, including DNA, amino acids and proteins, exhibit chirality and the handedness can totally change their function or properties. Therefore knowing the chirality of a substance is often critically important.

For decades scientists had sought to prove that you could accurately determine the chirality of molecules by measuring a colour-changing (nonlinear) effect upon illumination with twisted (circularly polarised) light. In theory, twisted light could change colour and then scatter differently from differently handed molecules - but this had never been demonstrated experimentally.

Dr Ventsislav Valev, who leads the research group in the Department of Physics at the University of Bath, said: “We’ve demonstrated a new physical effect - you don’t get to say that every day. This is exactly why I got into science.

“We began thinking about the problem 13 years ago, together with Prof Thierry Verbiest, at KU Leuven, Belgium. Because the effect was so elusive, I knew half of the solution would be to develop a very sensitive experimental setup. This is what I did for many years. The other half was finding the right samples and I was really excited to discover the nanoscopic silver springs (nano-helices) fabricated by Prof Peer Fischer’s group, at the Max Planck Institute for Intelligent Systems, in Stuttgart, Germany.”

PhD student Joel Collins had an incredible moment when running a series of tests on these springs.

He said: “To be honest my attitude was almost ‘OK let’s get this out the way to make sure it doesn’t work and we can move onto something else’. Then, together with my colleague Dr Kristina Rusimova, we noticed that there did actually seem to be an effect, and I thought ‘Hmmmm, that’s interesting.’

“We kept repeating the experiment to make sure it was actually a real effect and we saw that not only it’s there but it’s huge – we were only using really low concentrations of our nano-helices.

“For my part, I didn’t really recognise how important it is, and was expecting someone to come along and rip it to shreds, to say - ‘you haven’t thought of that’ or ‘you’ve missed this’. But over time it has dawned on me - that this is actually a fantastic result.”

The experimental geometry is in fact quite simple; the nano-springs are dispersed in water within a glass container where they spread randomly. Then a laser is aimed at them. The twist (circular polarisation) of the laser is switched periodically and light scattered from the container at 90° is analysed to determine the chirality of the springs present. The research is published in Physical Review X.

Dr Valev added: “It’s taken 40 years, people have been looking for this without success, and not for lack of trying. It’s amazing. The theory was quite controversial, people thought that maybe the effect was impossible to observe, maybe something else was there, blocking it.

“For 200 years, scientists have been using the same method to measure chirality. It’s not very sensitive, but it’s robust and simple, however precise measurements of chirality have become a major hurdle for human-made chiral nanotechnology because of false positives.

“Now we have a method 100,000 times more sensitive, free from false positives. There’s a new kind of manufacturing process currently emerging. It is called “lab-on-a-chip” and our effect fits very well with it.

“A more sensitive test means you can use lower quantities in quality control and reduce waste, there are applications in chemical and pharmaceutical manufacturing, as well as in microfluidics, in miniaturisation and for developing personal pharmaceutical technologies.”

Advanced laser sources, sensitive detection equipment and state-of-the-art nanofabrication techniques have all come together to enable the experimental observation of the new effect.

Professor David Andrews, from the University of East Anglia, theorised the effect 40 years ago. He said: “Dr Valev’s pioneering work is a clever and highly significant achievement, for he has realised a kind of application that could never have been imagined when the theory was first laid, forty years ago.

“His results serve as an encouragement to all pure theorists!”

Next, the researchers will be using their findings to characterise chiral molecules and to develop its technological applications. The paper “First observation of optical activity in hyper-Rayleigh scattering” is published in Physical Review X. DOI: 10.1103/PhysRevX.9.011024.

The research was funded by The Royal Society, the Science and Technology Facilities Council (STFC) and the Engineering and Physical Science Research Council (EPSRC).

Read the research paper.

Ultra-sensitive sensor with gold nanoparticle array

9 January 2019 University of Bath

Physicists from Bath have developed a new type of sensor platform using a gold nanoparticle array, that's 100 times more sensitive than current similar sensors

The sensor is made up of a series of gold disk-shaped nanoparticles on a glass slide. The team at Bath discovered that when they shone an infra-red laser at a precise arrangement of the particles, they started to emit unusual amounts of ultra violet (UV) light.

This mechanism for generating UV light is affected by molecules binding to the surface of the nanoparticles, providing a means of sensing a very small amount of material.

The researchers, from the University of Bath’s Department of Physics, hope that in the future they can use the technology to develop new ultra-sensitive sensors for air pollution or for medical diagnostics.

Dr Ventsislav Valev, Royal Society Research Fellow and Reader in Physics at the University of Bath, led the work with Research Associate David Hooper.

He explained: “This new mechanism has great potential for detecting small molecules. It is 100 times more sensitive than current methods.

“The gold nanoparticle disks are arranged on a glass slide in a very precise array – changing the thickness and separation of the disks completely changes the detected signal.

“When molecules bind to the surface of a gold nanoparticle, they affect the electrons at the gold surface, causing them to change the amount of UV light they emit.

“The amount of UV light emitted would depend on the type of molecules that bind to the surface.

“This technique could enable ultra-sensitive detection of molecules in tiny volumes. It could in the future be used for detecting very low concentrations of biological markers for the early diagnostic screening for diseases, such as cancer.”

The study has demonstrated the proof of principle for this new sensing mechanism. The team would next like to test the sensing of various types of chemicals and expects the technique to be available to other scientists to use within five years.

The nanoparticles were fabricated by researchers at Northwestern University, Illinois (USA).

David C. Hooper, Christian Kuppe, Danqing Wang, Weijia Wang, Jun Guan, Teri W. Odom, and Ventsislav K. Valev (2018) “Second Harmonic Spectroscopy of Surface Lattice Resonances” is published in Nano Letters DOI: 10.1021/acs.nanolett.8b03574

Twisted meta-molecules as they really are

26 June 2018 University of Bath

Physicists at the University of Bath have devised a new method to truly test the chirality of a material, eliminating the risk of false positives

Physicists at the University of Bath have devised a new and highly sensitive method to truly test the chirality of a material, eliminating the risk of false positives from competing effects.

Chiral molecules exist in different forms, even when they are made of the same atoms, those atoms can be arranged differently – twisting one way or another. This crucial difference can affect the properties of the molecules and has applications in fields such as telecommunications, nanorobotics, pharmaceuticals and industrial chemicals.

However, because of the nanoscopic nature of many of these molecules and materials it can be difficult for scientists to be certain they are working with chiral molecules of a particular twist (known as ‘handedness’). Some tests which are used can produce false positives, meaning scientists could be left working with the wrong samples.

The University of Bath team, working with colleagues at the Max Planck Institute for Intelligent Systems in Germany, demonstrated a method to separate the chirality of a substance from sources of false positives. In order to achieve this they used the way light interacts with artificial molecules (known as ‘meta-molecules’) that were made of tiny gold helices.

The team directed a powerful laser beam at samples of chiral meta-molecules, which caused it to change colour: from red to blue. Because the sample chirality imprints a twist on light, the blue light was also twisted: 45° left or right, depending on the sample handedness. Previously, false positives (such as anisotropy) have been known to affect chiral optical measurements.

Joel Collins, who conducted the experiments said: “Some other techniques can produce a similar effect but without this being due to the chirality which can be misleading. By being able to rotate the sample and retain the optical effects we have a true test of chirality, and you can be sure what you’re seeing is due to chirality and not some other property.”

Dr Ventsislav Valev who led the research said: “Meta-molecules are scientifically very exciting. Our demonstration of this effect, free from anisotropy contributions, is a first in the research field. Plus, we were astounded by how large it was. A 45° twist usually requires travelling through 5 cm of concentrated sugar syrup. Our material is half a million times thinner.”

The research is published in the journal ACS Nano.

The Research was funded by The Royal Society, Science and Technologies Facilities Council (STFC) and the Engineering and Physical Sciences Research Council (EPSRC).

Morphing twisted nanoscale objects opens up a new way to tailor applications in future technologies

10 April 2018 University of Bath

Scientists have created a way to model interactions between light and twisted molecules, as they transition from left to right-handed versions, or vice versa.

For the first time scientists have created a way to model the interaction between light and twisted molecules, as these molecules transition from left- to right-handed versions, or vice versa. The transitional forms offer a deeper insight into material symmetries and their unexpected behaviour could lead to improved design of telecoms components.

Many molecules, including important pharmaceuticals and valuable chemicals, exist in two “chiral” forms – they have the same chemical structure arranged in mirror images, termed left-handed and right-handed forms. This can alter their properties and is therefore important to fully understand how the compound interacts with other molecules, or light.

Typically, it has only been possible to study either the left- or right-handed chiral form but nothing in between, however ideally scientists would like to gradually morph a shape from one handedness to the other and observe how the effects of this change translate into physical properties.

Now a research team from the Department of Physics at the University of Bath, working with colleagues at University College London, Belgium and China, has created a way to do exactly that.

Their unique method involves manufacturing metallic nano-scale “artificial molecules” representative of 35 intermediate stages along the way of a geometric transformation, from one handedness to the other. At this nano-scale, the shape of the artificial molecule affects its optical properties, so by using twisted laser light the team studied the properties of the various stages, as the artificial molecules morphed from left to right handedness.

PhD student Joel Collins said: “We were able to follow the properties of a chiral artificial molecule, as it was morphed from left- to right-handed form, through two different routes. No-one has done this before. Surprisingly, we found that each route leads to a different behavior.

"We measured the difference in absorption of left and right circularly polarized light, known as circular-dichroism (CD). Along one route, the artificial molecules behaves as might be expected, with progressively decreasing CD, and eventually a reversal of the CD, for the mirrored structure. However, along the second route, the CD reversed several times, even before the structure changed handedness.”

The research is published in the journal Advanced Optical Materials.

Dr Ventsislav Valev who led the research said: “This is actually a very elegant idea but it has only become a possibility thanks to the recent advances in nanofabrication.

“In chemistry, you can’t tune the twist of a chiral molecule, so every scientist who studies such molecules needs to tune the wavelength of light. We have demonstrated a new, complementary physical effect, where we fix the wavelength and tune the twist of the chiral artificial molecule. In many cases, our approach is more practical; for instance, when we’re designing telecoms components, where the optical wavelength is pre-determined.”

The research was funded by the Royal Society and the Engineering and Physical Sciences Research Council.

Twisting laser light to probe the nanoscale

05 April 2018 University of Bath

A new method to sensitively measure the structure of molecules has been demonstrated by twisting laser light and aiming it at miniscule gold gratings.

A new method to sensitively measure the structure of molecules has been demonstrated by twisting laser light and aiming it at miniscule gold gratings to separate out wavelengths.

The technique could potentially be used to probe the structure and purity of molecules in pharmaceuticals, agrochemicals, foods and other important products more easily and cheaply than existing methods.

Developed by physicists at the University of Bath, working with colleagues at the University of Cambridge and University College London, the technique relies on the curious fact that many biological and pharmaceutical molecules can be either ‘left-handed’ or ‘right-handed’. Although such molecules are built from exactly the same elements they can be arranged in mirror images of each other, and this configuration sometimes changes their properties drastically.

Notoriously the morning sickness drug Thalidomide caused birth defects and deaths in babies before it was pulled from the market in the 1960s. Investigation showed that the drug existed in two mirror images – the right-handed form was effective as a morning sickness drug, but the left-handed form was harmful to foetuses. This is one example of why testing what ‘handedness’, or chirality, a molecule has is essential for many valuable products.

The research team from the Centre for Photonics and Photonic Materials, and the Centre for Nanoscience and Nanotechnology at the University of Bath, used a special white-light laser built in-house and directed it through several optical components to put a twist on the beam. The twisted laser beam then hits a nano-scopic U-shaped gold grating which serves as a template for the light, further twisting the beam in either a right or left-handed direction. This deflects the beam in many directions and further splits it into its constituent wavelengths across the colour spectrum.

By carefully measuring the deflected light scientists can detect tiny differences in intensity across the spectrum which inform them about the chirality of the grating the laser beam interacts with.

The study, published in the journal Advanced Optical Materials, demonstrates the technique as a proof of principle.

Christian Kuppe, the PhD student who conducted the experiments, said: “At the moment chiral sensing requires high molecular concentrations because you’re looking for tiny differences in how the light interacts with the target molecule.

“By using our gold gratings we aim to use a much smaller amount of molecules to conduct a very sensitive test of their handedness. The next step will be to continue to test the technique with a range of well-known chiral molecules.

“We hope that this will become a valuable way to perform really important tests on all sorts of products including pharmaceuticals and other high-value chemicals.”

Dr Ventsislav Valev, who oversaw the work, said: “There’s a great deal of scientific excitement about miniaturisation and working on nano-sized dimensions at the very small scale. However, in the rush to go as small as possible, some opportunities have been overlooked. Working with chiral nano-gratings is a great example of that.”

The researchers were funded by grants from the Royal Society, European Research Council, China Scholarship Council and Engineering and Physical Sciences Research Council (EPSRC).

Public unwittingly buying dangerous laser pointers warn scientists

09 October 2017 University of Bath

Dangerous laser pointers are being sold to the public which unwittingly believes them to be safe, scientists from the University of Bath have warned.

Misleading and mistaken labelling, drastic variations in laser strength under different conditions, and emissions of invisible infrared light all pose risks they say.

The researchers have responded to a Government consultation on laser pointers calling for better labelling and new easy-to-understand warning signs for all laser pointers.

Physicist Dr Ventsislav Valev, who has experienced a laser pointer attack, and who led tests at the University of Bath on a range of laser pointers, said: “Our main concern is laser pointers which seem safe to the user but are in fact very powerful. These lasers can lull people into a false sense of security and lead to injury.

“Some lasers that are labelled as safe turn out to be highly dangerous because they can emit highly powerful invisible laser light. Moreover, the visible laser power can vary a lot, depending on temperature.

“Other labels are mistaken, and the maximum power output of the laser is often higher than the safety class on the label, again putting the user and others in danger.”

Dr Valev and PhD student Christian Kuppe in the University of Bath Department of Physics, collaborated with Dr Dimitar Slavov from the Bulgarian Academy of Sciences to develop scientific equipment for carefully testing the laser diodes in the heart of every pointer.

“Our results show that the so-called ‘frequency doubled’ laser pointers, usually green, blue and violet pointers, can be particularly dangerous even if they seem safe to the user,” he said.

“For example, some laser pointers can output widely different laser power depending on the temperature. They can appear perfectly safe at room temperature only to become much more dangerous outside and vice-versa. Moreover, as pointers are being used they heat up, so a pointer that initially seems safe can subsequently become highly powerful and dangerous.

“Other lasers can produce safe levels of coloured light, but at the same time emit high power invisible infrared light. A person looking at the visible green light would estimate the laser to be safe and the much greater power and danger would go unnoticed until injury occurs.”

Laser pointers have been controversial, in particular because they have been shined into the eyes of plane and helicopter pilots and train drivers, with an average of 1,500 reported attacks per year in the UK. They can cause permanent or temporary eye damage, and it is a criminal offence to do so.

Dr Valev added: “I know from personal experience what it is like to be the victim of a laser pointer attack. The circumstances were particularly aggravating as I had my baby daughter in my arms at the time and so she was exposed too. Fortunately she was just falling asleep so she had her eyes closed.

“The pointer was powerful, but poor quality so the laser spot was about a meter in size when it reached me and my daughter. Because of this huge spread, the amount of laser light entering my eyes was much reduced from the original laser pointer output. So, I got only momentarily dazed, but suddenly everything became red. I was thinking that perhaps I was experiencing a medical condition, but my wife saw someone shining a pointer at me from outside our home.

“Then I got very angry that someone would direct a high power laser pointer at a person holding a baby. These are not toys and can be very dangerous, especially to babies’ eyes. We need to make sure laser pointers being sold are properly labelled and safe to use.”

Nanoscopic golden springs could unravel twisted molecules

03 March 2017 University of Bath

University of Bath scientists have used gold spring-shaped coils 5,000 times thinner than human hairs and powerful lasers to enable the detection of twisted molecules, and the applications could improve pharmaceutical design, telecommunications and nanorobotics. Molecules, including many pharmaceuticals, twist in certain ways and can exist in left or right ‘handed’ forms depending on how they twist. This twisting, called chirality, is crucial to understand because it changes the way a molecule behaves, for example within our bodies.

Scientists can study chiral molecules using particular laser light, which itself twists as it travels. Such studies get especially difficult for small amounts of molecules. This is where the minuscule gold springs can be helpful. Their shape twists the light and could better fit it to the molecules, making it easier to detect minute amounts.

Using some of the smallest springs ever created, the researchers from the University of Bath Department of Physics, working with colleagues from the Max Planck Institute for Intelligent Systems, examined how effective the gold springs could be at enhancing interactions between light and chiral molecules. They based their study on a colour-conversion method for light, known as Second Harmonic Generation (SHG), whereby the better the performance of the spring, the more red laser light converts into blue laser light.

They found that the springs were indeed very promising but that how well they performed depended on the direction they were facing.

Physics PhD student David Hooper who is the first author of the study, said: “It is like using a kaleidoscope to look at a picture; the picture becomes distorted when you rotate the kaleidoscope. We need to minimise the distortion.”

In order to reduce the distortions, the team is now working on ways to optimise the springs, which are known as chiral nanostructures.

“Closely observing the chirality of molecules has lots of potential applications, for example it could help improve the design and purity of pharmaceuticals and fine chemicals, help develop motion controls for nanorobotics and miniaturise components in telecommunications,” said Dr Ventsislav Valev who led the study and the University of Bath research team.

The research is published in the journal Advanced Materials.

The research is funded by the Royal Society as part of a the University Research Fellowships scheme, as well as by the UK Engineering and Physical Sciences Research Council (EPSRC) through the Centre for Doctoral Training in Condensed Matter Physics.

Full bibliographic information
D. C. Hooper, A. G. Mark, C. Kuppe, J. T. Collins, P. Fischer, V. K. Valev, Strong Rotational Anisotropies Affect Nonlinear Chiral Metamaterials Adv. Mater. 29, 1605110 (2017).

Little ANTs: researchers build the world's tiniest engine.

02 May 2016 University of Bath

Researchers have developed the world's tiniest engine - just a few billionths of a metre in size - which uses light to power itself. The nanoscale engine, developed by scientists at the University of Cambridge and University of Bath, could form the basis of future nano-machines that can navigate in water, sense the environment around them, or even enter living cells to fight disease.

A new technique which uses light like a needle to thread long chains of particles could help bring sci-fi concepts such as cloaking devices one step closer to reality.

28 June 2014 University of Cambridge

A new method of building materials using light, developed by researchers at the University of Cambridge, could one day enable technologies that are often considered the realm of science fiction, such as invisibility cloaks and cloaking devices.

A combination of nanotechnology and a unique twisting property of light could lead to new methods for ensuring the purity and safety of pharmaceuticals

16 May 2014 University of Cambridge

A direct relationship between the way in which light is twisted by nanoscale structures and the nonlinear way in which it interacts with matter could be used to ensure greater purity for pharmaceuticals, allowing for "evil twins" of drugs to be identified with much greater sensitivity.

Researchers from the University of Cambridge have used this relationship, in combination with powerful lasers and nanopatterned gold surfaces, to propose a sensing mechanism that could be used to identify the right-handed and left-handed versions of molecules.

Some molecules are symmetrical, so their mirror image is an exact copy. However, most molecules in nature have a mirror image that differs - try putting a left-handed glove on to your right hand and you'll see that your hands are not transposable one onto the other. Molecules whose mirror-images display this sort of "handedness" are known as chiral.

The chirality of a molecule affects how it interacts with its surroundings, and different chiral forms of the same molecule can have completely different effects. Perhaps the best-known instance of this is Thalidomide, which was prescribed to pregnant women in the 1950s and 1960s. One chiral form of Thalidomide worked as an effective treatment for morning sickness in early pregnancy, while the other form, like an "evil twin", prevented proper growth of the foetus. The drug that was prescribed to patients however, was a mix of both forms, resulting in more than 10,000 children worldwide being born with serious birth defects, such as shortened or missing limbs.

When developing new pharmaceuticals, identifying the correct chiral form is crucial. Specific molecules bind to specific receptors, so ensuring the correct chiral form is present determines the purity and effectiveness of the end product. However, the difficulty with achieving chiral purity is that usually both forms are synthesised in equal quantities.

Researchers from the University of Cambridge have designed a new type of sensing mechanism, combining a unique twisting property of light with frequency doubling to identify different chiral forms of molecules with extremely high sensitivity, which could be useful in the development of new drugs. The results are published in the journal Advanced Materials.

The sensing mechanism, designed by Dr Ventsislav Valev and Professor Jeremy Baumberg from the Cavendish Laboratory, in collaboration with colleagues from the UK and abroad, uses a nanopatterned gold surface in combination with powerful lasers.

Currently, differing chiral forms of molecules are detected by using beams of polarised light. The way in which the light is twisted by the molecules results in chiroptical effects, which are typically very weak. By using powerful lasers however, second harmonic generation (SHG) chiroptical effects emerge, which are typically three orders of magnitude stronger. SHG is a quantum mechanical process whereby two red photons can be annihilated to create a blue photon, creating blue light from red.

Recently, another major step towards increasing chiroptical effects came from the development of superchiral light - a super twisty form of light.

The researchers identified a direct link between the fundamental equations for superchiral light and SHG, which would make even stronger chiroptical effects possible. Combining superchiral light and SHG could yield record-breaking effects, which would result in very high sensitivity for measuring the chiral purity of drugs.

The researchers also used tiny gold structures, known as plasmonic nanostructures, to focus the beams of light. Just as a glass lens can be used to focus sunlight to a certain spot, these plasmonic nanostructures concentrate incoming light into hotspots on their surface, where the optical fields become huge. Due to the presence of optical field variations, it is in these hotspots that superchiral light and SHG combine their effects.

"By using nanostructures, lasers and this unique twisting property of light, we could selectively destroy the unwanted form of the molecule, while leaving the desired form unaffected," said Dr Valev. "Together, these technologies could help ensure that new drugs are safe and pure."

Full bibliographic information
V. K. Valev, J. J. Baumberg, B. De Clercq, N. Braz, X. Zheng, E. J. Osley, S. Vandendriessche, M. Hojeij, C. Blejean, J. Mertens, C. G. Biris, V. Volskiy, M. Ameloot, Y. Ekinci, G. A. E. Vandenbosch, P. A. Warburton, V. V. Moshchalkov, N. C. Panoiu, T. Verbiest, Nonlinear superchiral meta-surfaces: tuning chirality and disentangling non-reciprocity at the nanoscale. Adv. Mater. 26, 4074-4081 (2014).

"Loops of light" promising for optical detection of individual molecules

18 July 2012 KU Leuven University

KU Leuven researcher Ventsislav Valev and an international team of colleagues have developed a new method for manipulating light at the nanoscale in order to optically detect single molecules. By shining circularly polarised light on a gold, square-ring shaped nanostructure, the researchers were able to "activate" the entire surface of the nanostructure, thereby significantly increasing the opportunity for interaction with molecules. The method has a broad range of potential applications in nanoscale photochemistry and could assist in the advancement of technologies for visualising single molecules and multiple-molecule interactions.

Nanotechnology researchers around the world are exploring ways to optically detect single molecules, but progress can be hindered by the fact that single molecules have extremely weak optical responses. Thus far, scientists have developed a way to use metal nanostructures to focus light into tiny spots called "hotspots". The hotspots excite electrons on the surface of the nanostructure, causing them to oscillate coherently. When shone on a molecule, and with the help of these oscillating electrons, the focused light can increase a molecule's optical signal to 100 billion times its normal strength. This signal can then be detected with an optical microscope. 

But there are two limitations to the current method: hotspots can become too hot, and they are just spots. That is, the heat from hotspots can melt the nanostructures, thus destroying their ability to channel light effectively, and hotspots produce only a very small cross-section in which interaction with molecules can take place. Additionally, for a single molecule to become detectable, it needs to find the hotspot.

Loops of light

In order to overcome these limitations, Dr. Valev and his colleagues sought out to nanoengineer larger spots. They  began by shining circularly polarised light rather than linearly polarised light on the nanostructures and found that this could increase the useful area of these nanostructures. More importantly, when shone on square-ring shaped gold nanostructures, the scientists observed that the entire surface of the nanostructures was successfully activated.

Dr. Valev explains: "Essentially, light is constituted of electric and magnetic fields moving through space. While with linearly polarised light, the fields move in a linear, forward direction, with circularly polarised light, they rotate in a spiral-like motion." The circularly polarised light imparts a sense of rotation on the electron density in ring-shaped gold nanostructures, thus trapping the light in the rings and forming "loops of light". The loops of light cause excited electrons to oscillate coherently on the full surface of the square-ringed nanostructures - rather than in a few concentrated hotspots. This increases the opportunity for interaction with molecules: "The trick is to try to activate the whole surface of the nanostructure so that whenever a molecule attaches, we will be able to see it," says Dr. Valev. "That is precisely what we did." 

The method has a broad range of potential applications in nanoscale photochemistry and could assist in the advancement of technologies for visualising single molecules and multiple-molecule interactions. The findings were published in the scientific journal Advanced Materials.

Full bibliographic information
V. K. Valev, B. De Clercq, C. G. Biris, X. Zheng, S. Vandendriessche, M. Hojeij, D. Denkova, Y. Jeyaram, N. C. Panoiu, Y. Ekinci, A. V. Silhanek, V. Volskiy, G. A. E. Vandenbosch, M. Ameloot, V. V. Moshchalkov, and T. Verbiest, Distributing the Optical Near-Field for Efficient Field-Enhancements in Nanostructures. Adv. Mater. 24, OP208-OP215, (2012).

Perfectly spherical gold nanodroplets produced with the smallest-ever nanojets

13 January 2012 KU Leuven University

KU Leuven researcher Ventsislav Valev and an international team of scientists have developed a new method for optical manipulation of matter at the nanoscale. Using "plasmonic hotspots" - regions with electric current that heat up very locally - gold nanostructures can be melted and made to produce the smallest nanojets ever observed. The tiny gold nanodroplets formed in the nanojets, are perfectly spherical, which makes them interesting for applications in medicine.

The "backjet" phenomenon on which the method turns can be compared to a pebble being dropped into water. Tightly focused ultrafast laser pulses carry sufficient energy to locally melt the surface of a gold film. When a laser pulse of light hits the film, a nanoscale backjet - a nanojet - of molten gold surges upward.  

As the name suggests, nanojets on the surface of a homogeneous gold film are incredibly small, their size being determined by the distribution of energy in the light pulse. This distribution of energy is in turn dependent on the wavelength of light. Initially, scientists anticipated that nanojets could not be significantly smaller than the wavelength of light. In this study however, Ventsislav Valev and his colleagues show that nanojets can in fact be made much smaller with the help of "plasmonic hotspots".

 Plasmonic hotspots are regions on the surface of metal nanostructures where light causes very strong oscillation of the electrons. Because electron oscillations constitute an electric current and because electric currents heat up the material the same way an electric stove heats up in the kitchen, the plasmonic hotspots are extremely hot. So hot that they can melt the gold in a spot much smaller than the wavelength of light. Dr. Valev and his colleagues were successfully able to demonstrate that this tiny little pool of molten gold can give rise to the smallest nanojets ever observed.

The gold nanodroplets propelled upward by the nanojets solidify in flight, producing perfectly spherical nanoparticles. These gold nanodroplets can be collected and used for medical applications including cancer treatment. The nanoparticles can be attached to molecules and injected in the blood. Once the molecules attach to cancer cells, light can be used to heat up the gold nanodroplets and destroy the cancer cells. Currently, the gold nanoparticles used in medications are chemically synthesised. These chemically synthesised gold nanoparticles have an unavoidably granular aspect. Conversely, gold nanodroplets created by the plasmonic nanojet method detailed by Dr. Valev and his colleagues are perfectly spherical, ensuring a better efficiency.

Full bibliographic information
V. K. Valev, D. Denkova, X. Zheng, A. I. Kuznetsov, C. Reinhardt, B. N. Chichkov, G. Tsutsumanova, E.J. Osley, V. Petkov, B. De Clercq, A. V. Silhanek, Y. Jeyaram, V. Volskiy, P. A. Warburton, G. A. E. Vandenbosch, S. Russev, O. A. Aktsipetrov, M. Ameloot, V. V. Moshchalkov, T. Verbiest, Plasmon-enhanced sub-wavelength laser ablation: plasmonic nanojets. Adv. Mater. 24, OP29-OP35 (2012).

Imaging of surface plasmons may be a lot easier than you thought

06 June 2011 KU Leuven University

An unusual observation turned into a scientific breakthrough when KU Leuven researchers investigating the optical properties of nanomaterials discovered that so-called surface plasmons leave imprints on the surface of the nanostructures. This leads to a new type of high resolution microscopy for imaging the electric fields of nanostructures.

Nanomaterials, consisting of extremely small particles or thin layers, tend to acquire unexpected properties. Optical nanomaterials are a class of materials that have emerged over the last ten years and that have quickly become a hot topic in material science due to their counterintuitive optical behavior and revolutionary potential applications. Optical nanomaterials are mainly based on surface plasmon resonances - the property whereby, in metallic nanostructures, light can collectively excite surface electron waves. These electron waves have the same frequency as light, but much shorter wavelengths, which allow their manipulation at the nanoscale. In other words, with the help of plasmons, light can be captured, modified and even stored in nanostructures. This emerging technology finds applications in surprising areas, ranging from cancer treatment (by targeting cancer cells with nanoparticles that will produce heat when excited) to invisibility (by causing light to follow a trail of nanoparticles, that acts as an invisibility cloak to whatever is underneath them).

The imaging of surface plasmons provides a direct way to map and understand the local electric fields that are responsible for the unusual electromagnetic properties of optical nanomaterials. However, the imaging of surface plasmons is quite challenging. While there are methods to image plasmons with high resolution, they come at a considerable increase in both cost and complexity. But now, Ventsislav K. Valev and his colleagues have demonstrated a powerful and user friendly method for imaging plasmonic patterns in nanostructures.

"We were performing routine characterization of freshly grown samples, when I asked Yogesh, one of our Ph.D. students, to look at a sample that had already been studied. There was absolutely no reason to do this; I just had a hunch," sais Ventsislav Valev. "Surprisingly, this sample appeared to be decorated and I immediately recognized the pattern. Somehow, the optical properties have been imprinted on the surface of the nanostructures."

The scientists indeed found out that upon illuminating nanostructures made of nickel or palladium, the resulting surface plasmon pattern is imprinted on the structures themselves. This imprinting is done through displacing material from the nanostructure to the regions where the plasmon enhancements are the largest. In this manner, the plasmons are effectively decorated, allowing for subsequent imaging with standard surface probe techniques, such as scanning electron microscopy or atomic force microscopy. The imprinting method is quite unique, combining aspects of both imaging and writing techniques.

This research is described in an upcoming paper in the journal Physical Review Letters.

Full bibliographic information
V. K. Valev, A. V. Silhanek, Y. Jeyaram, D. Denkova, B. De Clercq, V. Petkov, X. Zheng, V. Volskiy, W. Gillijns, G. A. E. Vandenbosch, O. A. Aktsipetrov, M. Ameloot, V. V. Moshchalkov, and T. Verbiest, Hotspot Decorations map plasmonic patterns with the resolution of scanning probe techniques. Phys. Rev. Lett. 106, 226803 (2011).