Fernandez Ainaga Dario Luis, Roncal-Herrero Teresa, Ilett Martha, Aslam Zabeada, Cheng Cheng, Hitchcock James P, Cayre Olivier J, Hondow Nicole
School of Chemical and Process Engineering, University of Leeds, Leeds, UK.
J Microsc. 2025 Apr;298(1):92-105. doi: 10.1111/jmi.13391. Epub 2025 Jan 31.
Transmission electron microscopy can be used for the characterisation of a wide range of thin specimens, but soft matter and aqueous samples such as gels, nanoparticle dispersions, and emulsions will dry out and collapse under the microscope vacuum, therefore losing information on their native state and ultimately limiting the understanding of the sample. This study examines commonly used techniques in transmission electron microscopy when applied to the characterisation of cryogenically frozen Pickering emulsion samples. Oil-in-water Pickering emulsions stabilised by 3 to 5 nm platinum nanoparticles were cryogenically frozen by plunge-freezing into liquid ethane to retain the native structure of the system without inducing crystallisation of the droplet oil cores. A comparison between the droplet morphology following different sample preparation methods has confirmed the effectiveness of using plunge-freezing to prepare these samples. Scanning transmission electron microscopy imaging showed that dry droplets collapse under the microscope vacuum, changing their shape and size (average apparent diameter: ∼342 nm) compared to frozen samples (average diameter: ∼183 nm). Cryogenic electron tomography was used to collect additional information of the 3D shape and size of the emulsion droplets, and the position of the stabilising nanoparticles relative to the droplet surface. Cryogenic energy dispersive X-ray and electron energy loss spectroscopy were used to successfully obtain elemental data and generate elemental maps to identify the stabilising nanoparticles and the oil phase. Elemental maps generated from spectral data were used in conjunction with electron tomography to obtain 3D information of the oil phase in the emulsion droplets. Beam-induced damage to the ice was the largest limiting factor to the sample characterisation, limiting the effective imaging resolution and signal-to-noise ratio, though careful consideration of the imaging parameters used allowed for the characterisation of the samples presented in this study. Ultimately this study shows that cryo-methods are effective for the representative characterisation of Pickering emulsions.
透射电子显微镜可用于表征各种薄样品，但软物质和水性样品，如凝胶、纳米颗粒分散体和乳液，在显微镜真空下会干燥并塌陷，从而失去其原始状态的信息，最终限制了对样品的理解。本研究考察了透射电子显微镜在用于表征低温冷冻的皮克林乳液样品时常用的技术。通过将由3至5纳米铂纳米颗粒稳定的水包油型皮克林乳液快速冷冻到液态乙烷中进行低温冷冻，以保留系统的原始结构，而不会诱导液滴油核结晶。不同样品制备方法后的液滴形态比较证实了使用快速冷冻制备这些样品的有效性。扫描透射电子显微镜成像显示，干燥的液滴在显微镜真空下会塌陷，与冷冻样品（平均直径：约183纳米）相比，其形状和尺寸发生了变化（平均表观直径：约342纳米）。低温电子断层扫描用于收集乳液液滴的三维形状和尺寸以及稳定纳米颗粒相对于液滴表面位置的额外信息。低温能量色散X射线和电子能量损失谱用于成功获取元素数据并生成元素图，以识别稳定纳米颗粒和油相。从光谱数据生成的元素图与电子断层扫描结合使用，以获得乳液液滴中油相的三维信息。束流对冰的损伤是样品表征的最大限制因素，限制了有效成像分辨率和信噪比，不过仔细考虑所使用的成像参数使得本研究中呈现的样品得以表征。最终，本研究表明低温方法对于皮克林乳液的代表性表征是有效的。
