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Published a paper in Nano Letters as a cover article

Jul 16, 2020

Upconversion Nonlinear Structured Illumination Microscopy

Video-rate super-resolution imaging through biological tissue can visualize and track biomolecule interplays and transportations inside cellular organisms. Structured illumination microscopy allows for wide-field super resolution observation of biological samples but is limited by the strong extinction of light by biological tissues, which restricts the imaging depth and degrades its imaging resolution. Here we report a photon upconversion scheme using lanthanide-doped nanoparticles for wide-field super-resolution imaging through the biological transparent window, featured by near-infrared and low-irradiance nonlinear structured illumination. We demonstrate that the 976 nm excitation and 800 nm upconverted emission can mitigate the aberration. We found that the nonlinear response of upconversion emissions from single nanoparticles can effectively generate the required high spatial frequency components in the Fourier domain. These strategies lead to a new modality in microscopy with a resolution below 131 nm, 1/7th of the excitation wavelength, and an imaging rate of 1 Hz.

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Published a paper in Nanoscale as a cover article

Jun 29, 2020

Video-rate upconverting display by optimizing lanthanide doped nanoparticles

Volumetric displays that create bright image points within a transparent bulk is one of the most attracting technologies in everyday life. Lanthanide ions doped upconversion nanoparticles (UCNPs) is an promising luminescent nanomaterial for the background free, full-colour volumetric displaying within transparent bulk materials. However, video-rate display using UCNPs was limited from their low emission intensity. Here we developed a video-rate upconverting display system with much enhanced brightness. The integral emission intensity of the single UCNPs was fully employed for video-rate display. It was maximized by optimizing the emitter concentration and, more imporatantly, by temporally synchronizing the scanning time of the excitation light to the emission raising time of single UCNPs. The excitation power dependent emission response and emission’s time decay curves were systematically characterized for single UCNPs with varied emitter concentrations from 0.5% to 6%. 1%Tm3+ doped UCNPs presented highest integral emission intensity. By embedding this UCNPs into a PVA film, we achieved a two-dimensional (2D) upconverting display with frame rate of 29 Hz for 35 by 50 pixels. This work demonstrates that the temporal response as well as the integral emission intensity enable video-rate upconverting display.

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Published a paper in Nature Nanotechnology

Advance in 'optical tweezers' to boost biomedical research

March, 2021

Much like the Jedis in Star Wars use 'the force' to control objects from a distance, scientists can use light or 'optical force' to move very small particles.

The inventors of this ground-breaking laser technology, known as 'optical tweezers', were awarded the 2018 Nobel Prize in physics.

Optical tweezers are used in biology, medicine and materials science to assemble and manipulate nanoparticles such as gold atoms. However, the technology relies on a difference in the refractive properties of the trapped particle and the surrounding environment.

Now scientists have discovered a new technique that allows them to manipulate particles that have the same refractive properties as the background environment, overcoming a fundamental technical challenge.

The study 'Optical tweezers beyond refractive index mismatch using highly doped upconversion nanoparticles' has just been published in Nature Nanotechnology.

"This breakthrough has huge potential, particularly in fields such as medicine," says leading co-author Dr Fan Wang from the University of Technology Sydney (UTS).

"The ability to push, pull and measure the forces of microscopic objects inside cells, such as strands of DNA or intracellular enzymes, could lead to advances in understanding and treating many different diseases such as diabetes or cancer.

"Traditional mechanical micro-probes used to manipulate cells are invasive, and the positioning resolution is low. They can only measure things like the stiffness of a cell membrane, not the force of molecular motor proteins inside a cell," he says.

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The research team developed a unique method to control the refractive properties and luminescence of nanoparticles by doping nanocrystals with rare-earth metal ions.

Having overcome this first fundamental challenge, the team then optimised the doping concentration of ions to achieve the trapping of nanoparticles at a much lower energy level, and at 30 times increased efficiency.

"Traditionally, you need hundreds of milliwatts of laser power to trap a 20 nanometre gold particle. With our new technology, we can trap a 20 nanometre particle using tens of milliwatts of power," says Xuchen Shan, first co-author and UTS PhD candidate in the UTS School of Electrical and Data Engineering.

"Our optical tweezers also achieved a record high degree of sensitivity or 'stiffness' for nanoparticles in a water solution. Remarkably, the heat generated by this method was negligible compared with older methods, so our optical tweezers offer a number of advantages," he says.

Fellow leading co-author Dr Peter Reece, from the University of New South Wales, says this proof-of-concept research is a significant advancement in a field that is becoming increasingly sophisticated for biological researchers.

"The prospect of developing a highly-efficient nanoscale force probe is very exciting. The hope is that the force probe can be labelled to target intracellular structures and organelles, enabling the optical manipulation of these intracellular structures," he says.

Distinguished Professor Dayong Jin, Director of the UTS Institute for Biomedical Materials and Devices (IBMD) and a leading co-author, says this work opens up new opportunities for super resolution functional imaging of intracellular biomechanics.

"IBMD research is focused on the translation of advances in photonics and material technology into biomedical  applications, and this type of technology development is well aligned to this vision," says Professor Jin.

"Once we have answered the fundamental science questions and discovered new mechanisms of photonics and material science, we then move to apply them. This new advance will allow us to use lower-power and less-invasive ways to trap nanoscopic objects, such as live cells and intracellular compartments, for high precision manipulation and nanoscale biomechanics measurement."

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Published a paper in Nature Communications

Nanoparticles reveal their location via mirror SELFI

April, 2021

Can a mirror turn an orange into a doughnut? The answer is definitely no in the real (macro) world. But at the nanoscale, a mirror can turn an "orange" shaped pattern into a "doughnut" shaped pattern by overlapping the "orange" with its reflected mirror image.

A team of researchers from the University of Technology Sydney (UTS) has shown for the first time that fluorescent nanoparticles placed near a mirror generate unique patterns that can be used to pinpoint their location.

The researchers attribute this effect to the light emitting nanoparticle's interference with its own mirror image. Using this method they can also detect the size of particles to a resolution of one nanometre - or around 1/80,000th of the diameter of a human hair.

This breakthrough in ultra-sensitive measuring technology, published in Nature Communications, could have many applications including tracking and analysing disease causing viruses and other pathogens.

"When we look in a mirror it doesn't change our physical shape, but that's not the case with emission patterns of nanoparticles," says leading co-author Dr Fan Wang from the UTS Institute for Biomedical Materials and Devices.

"If you put a nanoparticle in front of a mirror, it will change its image by itself, and the image shape reflects the spacing between the particle and the mirror. This is due to the phase difference between the emitter and its image," he says.

The researchers describe this encoding of position information from a particle emission's self-interference as the "SELFI effect". The resulting patterns include Gaussian, doughnut and archery target shapes.

"To the best of our knowledge, the spatial distribution of the spontaneous emission's SELFI from multiple emitters at the nanoscale has not been reported," says leading co-author Professor Dayong Jin.

"This SELFI leads to a fast, high-resolution and anti-drift sensing method to accurately resolve the position of a single nanoparticles."

The nanoparticles are doped with many rare-earth element ions to achieve the necessary luminescence to create an effective SELFI.

The authors note this new method is suitable for conventional widefield fluorescence microscopy setups without requiring system modification.

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Published a paper in Advanced Materials as a cover article

Heterochromatic Nonlinear Optical Responses in Upconversion Nanoparticles for Super‐Resolution Nanoscopy

April, 2021

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Point spread function (PSF) engineering by an emitter's response can code higher‐spatial‐frequency information of an image for microscopy to achieve super‐resolution. However, complexed excitation optics or repetitive scans are needed, which explains the issues of low speed, poor stability, and operational complexity associated with the current laser scanning microscopy approaches. Here, the diverse emission responses of upconversion nanoparticles (UCNPs) are reported for super‐resolution nanoscopy to improve the imaging quality and speed. The method only needs a doughnut‐shaped scanning excitation beam at an appropriate power density. By collecting the four‐photon emission of single UCNPs, the high‐frequency information of a super‐resolution image can be resolved through the doughnut‐emission PSF. Meanwhile, the two‐photon state of the same nanoparticle is oversaturated, so that the complementary lower‐frequency information of the super‐resolution image can be simultaneously collected by the Gaussian‐like emission PSF. This leads to a method of Fourier‐domain heterochromatic fusion, which allows the extended capability of the engineered PSFs to cover both low‐ and high‐frequency information to yield optimized image quality. This approach achieves a spatial resolution of 40 nm, 1/24th of the excitation wavelength. This work suggests a new scope for developing nonlinear multi‐color emitting probes in super‐resolution nanoscopy.