基于点扫描的超分辨显微成像进展

光学显微镜一直推动着现代科学技术的发展.随着科学的进步,对显微成像分辨率的要求在生物、材料等领域日渐凸显,而常规宽场显微成像一直面临着成像分辨率衍射受限的问题.1968年出现的共聚焦显微镜作为点扫描显微镜的开端第一次实现了远场下成像分辨率的突破,它具有层切性好、信噪比高等优点.在1994年出现的受激辐射荧光损耗显微镜将显微成像能力突破到2.8 nm左右,并成为目前效果最佳、应用较广泛的超分辨显微技术.荧光差分显微和饱和荧光吸收竞争等点扫描技术具有无荧光染剂限制、饱和光强低、光路简单等优势,并且能取得1/6波长的分辨能力,进而在超分辨显微领域仍有着发挥空间.Airyscan技术作为以上方法的补充可以弥补点扫描系统中由于探测小孔半径减小而带来的信号丢失,从而提高成像信噪比和分辨率,但阵列探测器成本较高.上述点扫描显微镜通过改变照明或者探测的方式实现了分辨率突破.本文详细讨论了点扫描超分辨方法的原理、成像效果及面临的瓶颈,并分析了点扫描超分辨显微镜在应用和技术上的趋势.

Progress of point-wise scanning superresolution methods

Optical microscope has been giving impetus to the development of modern technology. As the advancement of these techniques, high resolution microscopy becomes crucial in biological and material researches. However, the diffraction limit restricts the resolution of conventional microscopy. In 1968, confocal microscopy, the first pointwise scanning superresolution method, appeared. It improves the imaging resolution, enhances the contrast, and thus breaks through the diffraction limit. Since then many superresolution methods have come into being, among which the pointwise scanning superresolution method earns reputation for its high imaging resolution and contrast. The stimulated emission depletion microscopy becomes the most prominent method with an achievable resolution of about 2.4 nm and then widely used. Besides, the newly developed fluorescence emission difference microscopy (FED) and the saturated absorption competition microscopy (SAC) have their advantages of non-constraint on fluorescent dyes, low saturated beam power, simplified optical setups, while they achieve a resolution of lower than λ/6. Further explorations of FED will be keen on vivo biological observations by using it, while that of SAC can concentrate on enhancing the resolution on a nanoscale and reducing the signal-to-noise ratio. In addition, the Airyscan technique in which a detector array is used for image acquisition, can serve as a complementary tool to further enhance the imaging quality of pointwise scanning superresolution method. The detector-array enables both the narrowed size of pinhole and the increasing of the acquired signal intensity by 1.84 folds. The other methods, e.g. superoscillation lens and high-index resolution enhancement by scattering, have the potentialities to obtain superresolved image in material science or deep tissues. After being developed in the past three decades, the superresolution methods now encounter a new bottleneck. Further improvement of the current methods is aimed at imaging depth, and being used more practically and diversely. In this review, we detailedly describe the above pointwise scanning superresolution methods, and explain their principles and techniques. In addition, the deficiencies and potentialities of these methods are presented in this review. Finally, we compare the existing methods and envision the next generation of the pointwise scanning superresolution methods.