Neodymium as an alternative contrast for uranium in electron microscopy
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Neodymium as an alternative contrast for uranium in electron microscopy Jeroen Kuipers1 · Ben N. G. Giepmans1 Accepted: 18 January 2020 / Published online: 1 February 2020 © The Author(s) 2020
Abstract Uranyl acetate is the standard contrasting agent in electron microscopy (EM), but it is toxic and radioactive. We reasoned neodymium acetate might substitute uranyl acetate as a contrasting agent, and we find that neodymium acetate indeed can replace uranyl acetate in several routine applications. Since neodymium acetate is not toxic, not radioactive and easy to use, we foresee neodymium will replace uranyl in many EM sample preparation applications worldwide. Keywords Contrast · Electron microscopy · Neodymium · Alternative contrast · Neodymium acetate · Uranyl acetate
Introduction Uranyl acetate has been the standard contrasting agent in transmission electron microscopy (TEM) for decades (Watson 1958a, b). However, its use is increasingly hampered by regulations by governments due to its radioactive properties as well as its high toxicity. Therefore, alternatives are being searched for, including lanthanides or platinum blue (Hosogi et al. 2015; Ikeda et al. 2011; Inaga et al. 2007; Yamaguchi et al. 2010) as well as the use of less defined substances such as oolong tea extract (Sato et al. 2008; He and Liu 2017). Despite these published alternatives, uranyl acetate (UAc) is still the standard for EM contrasting. We tested published lanthanides and commercial solutions as a replacement for UAc, but were not satisfied since these alternatives were prone to contamination and lead to variable results. In classical TEM, where area selection is done by the operator, recording artefacts may be circumvented by choosing contamination-free areas, but in combination with large-scale EM showing the whole section the presence of drying spots and precipitates will be recorded (Kuipers et al. 2015). Here the use of non-radioactive neodymium acetate as an alternative for uranyl acetate is advocated. In the periodic table the vertical ordering of elements in groups is based on the * Ben N. G. Giepmans [email protected] 1
Department of Biomedical Sciences of Cells and Systems, University of Groningen, University Medical Center Groningen, Groningen, The Netherlands
presence of the same number of electrons in their outermost shell, which determines their chemical and physical properties. Because neodymium (Nd) is right above uranium (U) we reasoned that the chemical properties of UAc and NdAc would be very similar in binding to tissue in ultrathin sections thus leading to a similar amount of contrast.
Material and methods Neodymium (III) acetate (Sigma Aldrich) was dissolved as a 4% solution in water by heating in a water bath at 60° for 10 min. After vortexing and cooling to room temperature the pale pink solution was spun in an Eppendorf centrifuge at maximal speed for 5 min before use. Uranyl acetate (Merck) was prepared as a 2% solution in water and spun as above before use. Fresh cut 100 nm secti
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