In a study published in Proceedings of the National Academy of Sciences, a research team led by Prof. Dongguo Zhang of the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS) developed an optical module consisting of cascaded momentum-space polarization filters based on vector light field modulation.
Label-free optical microscopy of single nano-objects. Image Credit: Prof. ZHANG’s team
This optical module effectively suppresses background noise while capturing high signal-to-noise ratio (SNR) microscopy images of individual nano-objects.
Precise imaging of the evolution and motion behavior of individual nano-objects, such as ultrafine atmospheric particles, is crucial when it comes to understanding their functionalities and properties. Label-free optical microscopy has been widely used in imaging and sensing of nanoparticles due to its unique non-destructive, non-invasive and fast detection characteristics.
However, the strength of light scattering by individual nanoparticles in air drops significantly with particle size to the sixth power. As a result, the scattered light intensity is significantly lower than the background noise, making it difficult for traditional label-free optical microscopes to accomplish high SNR imaging of individual nanoparticles.
To address this issue, Prof. Zhang’s team developed a momentum-space polarization filter that can be used to modify vector field polarization.
Only scattering light from single nano-objects can pass through the filter and be caught by the detector, while varied background noise is significantly filtered and suppressed, therefore achieving high contrast and high SNR imaging of nano-objects.
The filter was placed at the exit end of a total internal reflection microscopy (TIRM) to show how it is used. In comparison to traditional label-free black-field microscopy, black-field microscopy, which has reduced (blacker) background noise and greater detection sensitivity, was achieved by converting TIRM into black-field microscopy once the filter was installed.
As such, it was made clear that black-field microscopy can be used to obtain real-time high SNR and high-contrast optical microscopy images of individual protein molecules, gold nanoparticles, and perovskite nanocrystals.
Additionally, single perovskite nanocrystals undergo anion-exchange reactions upon the sequential introduction of HCl and HI vapor. These processes alter the nanocrystals' shape and refractive index, hence causing differences in the scattered light signals of the individual nanocrystals.
Black-field microscopy can capture this process in real-time, offering a novel photonic technique for the detection of physical and chemical reactions taking place as a nanoobject’s properties evolve.
The momentum-space polarization filter device is unique in that it can be used to provide black-field imaging capability to conventional label-free optical microscopy, including surface plasmon resonance microscopy (SPRM), TIRM, and other near-field optical microscopy, without altering the internal structure of the microscopes. This greatly increases the sensitivity of the microscopes in detecting individual nano-objects.
In summary, the team created a novel platform for analyzing individual nanoparticles using black-field microscopy, which holds significant promise for use in the fields of materials science, biology, physics, and environmental science.
Journal Reference:
Liu, Y., et. al. (2024) Cascaded momentum-space polarization filters enabled label-free black-field microscopy for single nanoparticles analysis. Proceedings of the National Academy of Sciences. doi:10.1073/pnas.232182512