nanoparticles and applications in super-resolution imaging
With the rapid development of social science and technology, optical microscopes have gone through the development process of ordinary optical microscopes with limited diffraction to the fluorescence super-resolution microscopes that break the diffraction limit, as well as non-fluorescent, label-free super-resolution optical microscopes. The non-fluorescent super-resolution imaging based on the nonlinear scattering of plasmonic nanoparticles is an interesting research topic and has been successfully applied to the imaging of biological cells. However, it has been found in the research that gold nanoparticles have severe overheating problems, aggregation and coupling between nanoparticles are prone to occur, and high temperature melting at high laser power is likely to affect practical applications.
The photothermal effect of nanoparticles is closely related to the thermal conductivity of the surrounding medium. Therefore, it is of great significance to study the key role of local thermal environment in the nonlinear scattering of plasmonic nanoparticles, and to further develop the label-free super-resolution imaging.
Recently, the research group led by Prof. Xiangping Li at Jinan University (JNU) proposed the strategy of Au@SiO2 core-shell particles, in order to improve the above-mentioned shortcomings of gold nanoparticles, and studied the mechanism of the nonlinear scattering saturation of gold nanoparticles based on the photothermal effect, and they also explored the application of Au@SiO2 in super-resolution imaging. Benefitting from the enhanced thermal stability and the reduced plasmonic coupling provided by the shell-isolated nanoparticles, the super-resolution imaging with a feature size of 52 nm (λ/10) can be achieved, and pairs of nanoparticles with a gap-to-gap distance of 5 nm can be readily resolved. This work was published in Chinese Optics Letters 2023, Vol. 21, No. 10 Tianyue Zhang, Zhiyuan Wang, Xiangchao Zhong, Ying Che, Xiangping Li. Photothermal nonlinear scattering of shell-isolated gold nanoparticles and applications in super-resolution imaging[J]. Chinese Optics Letters, 2023, 21(10): 103601) and was selected as the cover of the issue.
Principle: Gold nanoparticles are irradiated by incident light field, resulting in the temperature rising due to the photothermal effect. When the temperature reaches a certain threshold, the permittivity of the gold material is no longer constant, but instead becomes dependent on the temperature changes, leading to the nonlinear relationship between the temperature elevation and the excitation intensity. In turn, the change of optical constant of the material will further affect the plasmonic resonance of the particle, and consequently induces the scattering modulation. The optical super-resolution imaging of gold nanoparticles based on saturation scattering properties is realized by using the point spread function modulation technique of suppressed scattering imaging (SUSI) microscope.
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Figure 1 (a) Illustration of optical heating of the gold nanospheres that converts light into temperature rises. Due to the difference in the local thermal media, the gold nanospheres (top: in immersion oil, middle: in SOG materials, and bottom: with silica coatings) undergo various temperature rising. (b) Experimental measurements of nonlinear plasmonic scattering of the five prepared samples, showing the nonlinear dependency of scattering on irradiance intensities for CW excitations at the wavelength of 532 nm. (c)The gap-to-gap distances of two nanoparticles are shown in the SEM images. The super-resolution imaging of the shell-isolated nanoparticles clearly shows the resolving ability to distinguish two nanoparticles that are closely located in sub-diffraction spaces as small as 5 nm.
The photothermal nonlinear scattering efficiency of gold nanoparticles is related to the thermal conductivity of the local environment related to heat dissipation. Five different samples were prepared with 70 nm diameter gold nanospheres immersed in oil, coated with silica shells with various thicknesses (shell thickness of 10 nm, 23 nm and 40 nm, respectively), and encapsulated in spin-on glass (SOG) material. To this extent, the gold spheres in all cases are in the same optical environment due to the homogenous medium dielectric constant, but they experience different local thermal environments caused by varied immediate surroundings during optical heating. The experimental results have proved that the local heat dissipation of the particles can be effectively improved by coating the SiO2 layer, therefore reduce the heat accumulation, and avoid the melting of gold cores. Compared with the bare gold nanospheres, the shell-isolated gold nanoparticles are not only photothermally more stable but also can effectively prevent plasmon coupling due to particle aggregations. This enables us to access the super resolving of two individual shell-isolated nanoparticles with the gap-to-gap distance
of 5 nm. The full-width at half-maximum (FWHM) of the measured image spot from the single core-shell nanoparticle (70 nm gold core with 40 nm silica shell) can be squeezed to 52 nm (∼λ/10).
The JNU researchers believe that the proposed method of using shell-isolated gold nanoparticles with a silica coating for photothermal nonlinear scattering represents a new principle and technology for non-fluorescence super-resolution imaging. It has potentials to be applied to the imaging of microscopic structures such as cells and tissues, which is helpful to the study of biological processes and the pathogenesis of diseases.
