The research group led by Prof. DU Guoguang from the University of Science and Technology of China have recently proposed and implemented a momentum-space polarization filter. When the filter is installed at the output of a conventional label-free optical microscope, it effectively suppresses background noise interference in the output light field. As a result, it enables the acquisition of high-contrast, high-signal-to-noise-ratio optical microscopy images of single nanoscale objects. The research was recently published online in the Proceedings of the National Academy of Sciences of the United States of America.
Precise characterization of the physicochemical properties, evolution processes, and motion behaviors of single nanoscale objects, such as ultrafine atmospheric particulate matters, metal/dielectric nanoparticles, and biomolecules, is crucial to understanding and eventually manipulating the properties and functions of nanoscale substances. Such studies are significant both in basic scientific research and industrial applications.
Label-free optical microscopy offers unique advantages of non-destructive, non-invasive, and fast detection, making it widely used in imaging and sensing studies of microscopic objects. However, due to the much weaker scattering light signal intensity of a single nanoscale object compared to background noise, conventional label-free optical microscopy finds it difficult to achieve high-contrast, high-signal-to-noise-ratio imaging. It is even more challenging to record the evolution process and trajectory of a single object in real time.
To address this issue, Prof. DU's research group designed and implemented a momentum-space polarization filter based on the principle of vector light manipulation. The filter dramatically filters and suppresses various types of background noise, allowing the scattering light signal of a single nanoscale object to pass through and be detected by the detector, resulting in high-contrast, high-signal-to-noise-ratio imaging detection.
One of the remarkable features of the momentum-space filter is that it can be directly installed at the output of conventional total internal reflection microscopy or surface plasmon resonance microscopy without changing the internal structure of the conventional label-free optical microscope. As a result, the detection sensitivity of single nanoscale objects is significantly enhanced.
Before adding the filter, the microscope images captured for a single protein molecule were filled with various background noises, making it difficult to identify the protein molecule. After loading the filter, the conventional total internal reflection microscope was transformed into a dark-field optical microscope with lower background noise and higher detection sensitivity. As a result, high-signal-to-noise-ratio and high-contrast optical microscopy images of single protein molecules, gold nanoparticles, and perovskite nanocrystals can be captured in real time.
Furthermore, hydrochloric acid gas and hydrogen iodide gas were introduced into the sample chamber. These two gases can induce ion-exchange chemical reactions with single perovskite nanocrystals, leading to changes in the nanocrystal's morphology and refractive index, and thus causing changes in the nanocrystal's scattering light signal. The dark-field optical microscope can image this change process in real time, demonstrating its potential for the real-time detection of single-nanoparticle chemical reaction processes, providing a novel photonic technique for studying the evolution process of material properties of single nanoscale objects.
"The dark-field microscope developed in this study provides a new platform for the analysis of single nanoparticles, and it is expected to find wide applications in biology, physics, environmental science, and materials science," said Prof. DU.
Relevant paper link: https://doi.org/10.1073/pnas.2321825121
