Graphene microbubbles make perfect lenses – And Much More … Drug Delivery .. Water Treatment

In situ optical microscopic images showing the process of the microbubble generation and elimination. Credit: H. Lin et al

Tiny bubbles can solve large problems. Microbubbles—around 1-50 micrometers in diameter—have widespread applications. They’re used for drug delivery, membrane cleaning, biofilm control, and water treatment. They’ve been applied as actuators in lab-on-a-chip devices for microfluidic mixing, ink-jet printing, and logic circuitry, and in photonics lithography and optical resonators. And they’ve contributed remarkably to biomedical imaging and applications like DNA trapping and manipulation.

Given the broad range of applications for microbubbles, many methods for generating them have been developed, including air stream compression to dissolve air into liquid, ultrasound to induce bubbles in water, and laser pulses to expose substrates immersed in liquids. However, these bubbles tend to be randomly dispersed in liquid and rather unstable.

According to Baohua Jia, professor and founding director of the Centre for Translational Atomaterials at Swinburne University of Technology, “For applications requiring precise bubble position and size, as well as high stability—for example, in photonic applications like imaging and trapping—creation of bubbles at accurate positions with controllable volume, curvature, and stability is essential.” Jia explains that, for integration into biological or photonic platforms, it is highly desirable to have well controlled and stable microbubbles fabricated using a technique compatible with current processing technologies.

Balloons in graphene

Jia and fellow researchers from Swinburne University of Technology recently teamed up with researchers from National University of Singapore, Rutgers University, University of Melbourne, and Monash University, to develop a method to generate precisely controlled graphene microbubbles on a glass surface using laser pulses. Their report is published in the peer-reviewed, open-access journal, Advanced Photonics.

Photonic jet focused by a graphene oxide microbubble lens. Credit: H. Lin et al., doi 10.1117/1.AP.2.5.055001

The group used graphene oxide materials, which consist of graphene film decorated with oxygen functional groups. Gases cannot penetrate through graphene oxide materials, so the researchers used laser to locally irradiate the graphene oxide film to generate gases to be encapsulated inside the film to form microbubbles—like balloons. Han Lin, Senior Research Fellow at Swinburne University and first author on the paper, explains, “In this way, the positions of the microbubbles can be well controlled by the laser, and the microbubbles can be created and eliminated at will. In the meantime, the amount of gases can be controlled by the irradiating area and irradiating power. Therefore, high precision can be achieved.”

Such a high-quality bubble can be used for advanced optoelectronic and micromechanical devices with high precision requirements.

The researchers found that the high uniformity of the graphene oxide films creates microbubbles with a perfect spherical curvature that can be used as concave reflective lenses. As a showcase, they used the concave reflective lenses to focus light. The team reports that the lens presents a high-quality focal spot in a very good shape and can be used as light source for microscopic imaging.

Lin explains that the reflective lenses are also able to focus light at different wavelengths at the same focal point without chromatic aberration. The team demonstrates the focusing of a ultrabroadband white light, covering visible to near-infrared range, with the same high performance, which is particularly useful in compact microscopy and spectroscopy.

Jia remarks that the research provides “a pathway for generating highly controlled microbubbles at will and integration of graphene microbubbles as dynamic and high precision nanophotonic components for miniaturized lab-on-a-chip devices, along with broad potential applications in high resolution spectroscopy and medical imaging.”

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Monolayer transition metal dichalcogenide lens for high resolution imaging

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More information: Han Lin et al, Near-perfect microlenses based on graphene microbubbles, Advanced Photonics (2020). DOI: 10.1117/1.AP.2.5.055001

Provided by SPIE

Graphene solar heating film offers new opportunity for efficient thermal energy harvesting

Credit: CC0 Public Domain

Researchers at Swinburne University of Technology’s Centre for Translational Atomaterials have developed a highly efficient solar absorbing film that absorbs sunlight with minimal heat loss and rapidly heats up to 83°C in an open environment.

The graphene metamaterial film has great potential for use in solar thermal energy harvesting and conversion, thermophotovoltaics (directly converting heat to electricity), solar seawater desalination, wastewater treatment, light emitters and photodetectors.

The researchers have developed a prototype to demonstrate the photo-thermal performance and thermal stability of the film. They have also proposed a scalable and low-cost manufacturing strategy to produce this graphene metamaterial film for practical applications.

“In our previous work, we demonstrated a 90 nm graphene metamaterial heat-absorbing film,” says Professor Baohua Jia, founding Director of the Centre for Translational Atomaterials.

“In this new work, we reduced the film thickness to 30 nm and improved the performance by minimising heat loss. This work forms an exciting pillar in our atomaterial research.”

Lead author Dr. Keng-Te Lin says: “Our cost-effective and scalable structured graphene metamaterial selective absorber is promising for energy harvesting and conversion applications. Using our film an impressive solar to vapour efficiency of 96.2 percent can be achieved, which is very competitive for clean water generation using renewable energy source.”

Co-author Dr. Han Lin adds: “In addition to the long lifetime of the proposed graphene metamaterial, the solar-thermal performance is very stable under working conditions, making it attractive for industrial use. The 30 nm thickness significantly reduced the amount of the graphene materials, thus saving the costs, making it accessible for real-life applications.”

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Novel form of graphene-based optical material developed

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More information: Keng-Te Lin et al. Structured graphene metamaterial selective absorbers for high efficiency and omnidirectional solar thermal energy conversion, Nature Communications (2020). DOI: 10.1038/s41467-020-15116-z

Journal information: Nature Communications

Provided by Swinburne University of Technology

Graphene-based discs ensure safe storage

(Phys.org) —Swinburne University of Technology researchers have shown the potential of a new material for transforming secure optical information storage.

In their latest research paper published in Scientific Reports, researchers Xiangping Li, Qiming Zhang, Xi Chen and Professor Min Gu demonstrated the potential to record holographic coding in a graphene oxide polymer composite.

“Conventionally, information is recorded as binary data in a disc. If the disc is broken, the information cannot be retrieved,” Director of the Centre for Micro-Photonics at Swinburne, Professor Min Gu, said.

“This is a major operation cost in big data centres, which consist of thousands of disc arrays with multiple physical duplicates of data. The new material allows the development of super-discs, which will enable information to be retrieved – even from broken pieces.”

Graphene oxide is similar to graphene, discovered by Andre Geim and Konstantin Novoselov, who received the 2010 Nobel Prize in Physics for this groundbreaking discovery. Graphene is very strong, light, flexible, nearly transparent, and is an excellent conductor of heat and electricity.

Graphene oxide has similar properties, but also has a fundamental fluorescent property that can be used in bioimaging and for multimode optical recording.

By focusing an ultrashort laser beam onto the graphene oxide polymer, the researchers created a 10-100 times increase in the refractive-index of the graphene oxide along with a decrease in its fluorescence. (The refractive index is the measure of the bending of light as it passes through a medium.)

“The unique feature of the giant refractive-index modulation together with the fluorescent property of the graphene oxide polymer offers a new mechanism for multimode optical recording,” Professor Gu said.

To demonstrate the feasibility of the mechanism, the researchers encoded the image of a kangaroo in a computer generated hologram. The hologram was then rendered as a three-dimensional recording to the graphene oxide polymer. The encrypted patterns in the hologram could not be seen as a normal microscope image, but could be retrieved in the diffracted mode.

“The giant refractive index of this material shows promise for merging data storage with holography for security coding,” Professor Gu said.

“This exciting feature not only boosts the level of storage security, but also helps to reduce the operation costs of big data centres that rely on multiple physical duplicates to avoid data loss.”

The researchers say it could also revolutionise flat screen TV and solar cell technology.

“More importantly, graphene has been deemed as a revolutionary replacement for silicon, which is the platform for current information technologies based on electronics,” Dr Xiangping Li said.

“The giant refractive index we discovered shows the promise of graphene to merge electronics and photonics for the platform of the next generation information technologies.”

Read more at: http://phys.org/news/2013-10-graphene-based-discs-safe-storage.html#jCp

Read more at: http://phys.org/news/2013-10-graphene-based-discs-safe-storage.html#jCp

Nanopillars and a Disinfected World

The microbial world is ever-present and unrelenting.  The enormity of it is hard to fathom, with facts like ‘there are 10  bacterial cells living in or on you for every one cell that is you’  and ‘estimates suggest there are five million trillion trillion bacteria  on this planet’, that’s hard to predict, it may be plus or minus  a few. Controlling our interactions with this world may seem futile  but we do so everyday.

Bacteria come in all shapes, sizes and types with  some beneficial, others pathogenic and others insignificant (to our  health at least) so being able to regulate our microbial environments  is vitally important. It is to our advantage to foster the beneficial  species and inhibit the species that are less so. We do this every day  by eating certain food, taking certain supplements and, of course, enlisting  the support of drugs and medications, all of which affect the bacteria  inside and on you. Controlling our own microbial microenvironments is  only part of the story though, what about controlling the bacterial  reservoirs we interact with, the tables, handrails, chairs, the surfaces  of our lives? That employs a whole range of other techniques.

Disinfecting a surface can be done in many ways. By  far the most common are the chemical disinfectant sprays and aerosols.  Disinfectant sprays contain active ingredients that effect either the  walls or metabolism of microbes. By disturbing the stability of bacterial  membranes or metabolic pathways they kill indiscriminately but they  have their drawbacks. Many bacteria sporulate and disinfectants are useless against them and to differences in virus  and fungus make-up they can also be less effective against these agents  too, but, most importantly, are often toxic.

Toxicity is not the only problem. Spreading these  agents around can cause a range of issues and as we have seen with antibiotics,  resistance to disinfecting agents can and is occurring. That ‘kills  99.9%’ label hides the problem of the 0.1% that survive, divide, and  pass on the ability to survive the disinfectant attack to their daughters.

An alternative to disinfectants is UV light. UV light  is very good at disinfecting solid surfaces. UV light mutates the nucleic  acids in DNA, which results in an inability to divide easily or continue  making important proteins. Having a surface disinfection system that  works by inducing mutations has its own problems and the known ability  of UV to cause mutations in any DNA means that this method has the potential  to cause cancers long term.

There is another problem shared by systems such as  spray disinfectants and UV lights, a reliance on continuing human involvement.  What would be really great would be a disinfection system that is included  as part of a products manufacture. Such systems exist and are part of  a growing field of ‘passive antimicrobial agents’.

Many metals are known to possess antimicrobial properties.  Products made with silver, despite there short shelf life, are thought  to be effective, although there are conflicting data on this. A particular  form of silver (a chelated form called silver dihydrogen citrate, SDC)  is thought to work in two main ways, by interfering with the way membrane  proteins work and by denaturing DNA after being taken-up by the bacterial  cells.

 

Another example is surfaces containing copper alloys.  Copper, in much the same way as silver, can interrupt protein form and  function as well as being able to interact with lipids and other cellular  architecture and by doing so inhibit bacterial population growth. Copper  also acts as a potent catalyst of redox reactions and so acts to increase  free radicals and oxidative stress.

 

With more support for copper than silver it seems  like the best way to go but copper is expensive. Reserves are dwindling  and some predictions suggest we could run out of economically viable  reserves within 60 years. The major reservoir of copper now lies in  recycled materials and these are increasingly re-used in electronics.  Dumping copper into surfaces is perhaps not the best use of it.

Passive antimicrobial surfaces have a new hero. Recent  work from Swinburne University in Australia has found that ‘nanopillars’  on the surface of the wings of an insect-like locust (the clanger cicada)  give it the ability to fight bacterial colonisation. The arrangement  of these hexagonal pillars is much like a bed of nails, as a bacterial  cell lies on top of them it spreads out and the pillars push against  the membrane. The parts of the membrane that sag between the pillars  are stretched and when weakened the bacterial membrane cannot keep the  liquid insides of the bacteria, well, inside. As the inside leaks out  the bacterial cell dies.

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This arrangement is mechanical, not chemical, and  so is completely non-toxic and safe for humans. Finding a cheap and  effective way to build these structures on surfaces would result in  a microenvironment imperceptible to us but lethal to bacteria that happen  upon it and inducing this microenvironment on hospital surfaces like  door handles, bed rails and tables can help prevent hospital-acquired  infections which are a huge issue in hospitals all around the world.  Being passive means it takes the risk of not quite cleaning that spot  out of the equation and being mechanical means it need never be replaced.

As the research pointed out, the more rigid a bacterial  membrane (rigidity was increased as a result of microwaving them) the  less effective this approach as the membrane doesn’t sag between the  pillars. This suggests that there may be a selectable trait for evolving  around this strategy long-term but as it is the only mechanical antimicrobial  surface structure to be observed so far it presents an interesting opportunity  to think differently about disinfection.

About the Author: Dr James Byrne has a PhD in Microbiology and works as a science communicator at the Royal Institution of Australia (RiAus), Australia’s unique national science hub, which showcases the importance of science in everyday life. Follow on Twitter @JB_blogs.