Department of Physics, University of Illinois, Chicago, IL, United States of America.
Department of Mechanical Engineering, San Diego State University, San Diego, CA, United States of America.
Nanotechnology. 2021 Sep 7;32(48). doi: 10.1088/1361-6528/ac1ebb.
Liquid cell electron microscopy is an imaging technique allowing for the investigation of the interaction of liquids and solids at nanoscopic length scales. Suchobservations are increasingly in-demand in an array of fields, from biological sciences to medicine to batteries. Graphene liquid cells (GLCs), in particular, have generated a great interest as a low-scattering window material with the potential for increasing the quality of both imaging and spectroscopy. However, preserving the stability of the liquid and of the sample in the GLC remains a considerable challenge. In the present work we encapsulate water and hydroxyapatite (HAP), a pH-sensitive biological material, in GLCs to observe the interactions between the graphene, HAP, and the electron beam. HAP was chosen for several reasons. One is its ubiquity in biological specimens such as bones and teeth, and the second is the presence of phosphate ions in common buffer solutions. Finally, there is its sensitivity to changes in pH, which result from beam-induced hydrolysis in liquid cells. A dynamic process of dissolution and recrystallization of HAP was observed, which correlated with the production of Hions by the beam during imaging. In addition, a large increase in the stability of the GLC under irradiation was noted. Specifically, no stable hydrogen bubbles were detected under the electron fluxes routinely exceeding 170 eÅs. With the measured threshold dose for the bubble formation in pure water equaling 9 eÅs, it was concluded that the presence of HAP increases the resistance of water against radiolysis in the GLC by more than an order of magnitude. These results confirm the possibility of using biological materials, such as HAP, as stabilizers in liquid cell electron microscopy. They outline a potential route for stabilization of specimens in liquid cells through the addition of a scavenger of reactive species generated by the beam-induced hydrolysis of water. These improvements are essential for enhancing both the resolution of imaging and the available imaging time, as well as avoiding the beam-induced artifacts.
液相电子显微镜技术是一种可以在纳米尺度上研究液体和固体相互作用的成像技术。这种观察在从生物科学到医学再到电池等一系列领域都越来越受到需求。特别是石墨烯液相池（GLC）作为一种低散射窗口材料，具有提高成像和光谱质量的潜力，因此引起了极大的兴趣。然而，保持 GLC 中液体和样品的稳定性仍然是一个相当大的挑战。在本工作中，我们将水和羟磷灰石（HAP）——一种对 pH 敏感的生物材料——封装在 GLC 中，以观察石墨烯、HAP 和电子束之间的相互作用。选择 HAP 有几个原因。一是它在骨骼和牙齿等生物标本中普遍存在，二是在常见的缓冲溶液中存在磷酸根离子。最后，它对 pH 值的变化很敏感，这是由于液体池中的束诱导水解作用导致的。观察到 HAP 的溶解和再结晶的动态过程，这与成像过程中束产生的 H 离子有关。此外，还注意到 GLC 在辐照下的稳定性有了很大提高。具体来说，在电子束通量通常超过 170 eÅs 的情况下，没有检测到稳定的氢气泡。由于在纯水中形成气泡的测量阈值剂量等于 9 eÅs，可以得出结论，HAP 的存在使水在 GLC 中的抗辐射分解能力提高了一个数量级以上。这些结果证实了使用生物材料（如 HAP）作为液体池电子显微镜中的稳定剂的可能性。它们为通过添加束诱导水解产生的活性物质的清除剂来稳定液体池中的样品提供了一种潜在的途径。这些改进对于提高成像分辨率和可用成像时间，以及避免束诱导的伪影都是至关重要的。
