High-resolution imaging of organic pharmaceutical crystals by transmission electron microscopy and scanning moiré fringes

Our plain-English summary of High-resolution imaging of organic pharmaceutical crystals by transmission electron microscopy and scanning moiré fringes, by M. S'ari, N. Koniuch, R. Brydson, et al (Journal of Microscopy, 2020).

The study

• Authors: M. S'ari, N. Koniuch, R. Brydson, N. Hondow, A. Brown
• Journal: Journal of Microscopy
• Published: 2020;279(3):197-206
• Cited by: 8 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Formulation processing of organic crystalline compounds can have a significant effect on drug properties, such as dissolution rate or tablet strength/hardness. Transmission electron microscopy (TEM) has the potential to resolve the atomic lattice of these crystalline compounds and, for example, identify the defect density on a particular crystal face, provided that the sensitivity of these crystals to irradiation by high-energy electrons can be overcome. Here, we acquire high-resolution (HR) lattice images of the compound furosemide using two different methods: low-dose HRTEM and bright-field (BF) scanning TEM (STEM) scanning moiré fringes (SMFs). Before acquiring HRTEM images of furosemide, a model system of crocidolite (asbestos) was used to determine the electron flux/fluence limits of low-dose HR imaging for our scintillator-based, complementary metal-oxide semiconductor (CMOS) electron camera by testing a variety of electron flux and total electron fluence regimes. An electron flux of 10 e- /(Å2 s) and total fluence of 10 e- /Å2 was shown to provide sufficient contrast and signal-to-noise ratio to resolve 0.30 nm lattice spacings in crocidolite at 300 kV. These parameters were then used to image furosemide which has a critical electron fluence for damage of ≥10 e- /Å2 at 300 kV. The resulting HRTEM image of a furosemide crystal shows only a small portion of the total crystal exhibiting lattice fringes, likely due to irradiation damage during acquisition close to the compound's critical fluence. BF-STEM SMF images of furosemide were acquired at a lower electron fluence (1.8 e- /Å2 ), while still indirectly resolving HR details of the (001) lattice. Several different SMFs were observed with minor variations in the size and angle, suggesting strain due to defects within the crystal. Overall BF-STEM SMFs appear to be more useful than BF-STEM or HRTEM (with a CMOS camera) for imaging the crystal lattice of very beam-sensitive materials since a lower electron fluence is required to reveal the lattice. BF-STEM SMFs may thus prove useful in improving the understanding of crystallization pathways in organic compounds, degradation in pharmaceutical formulations and the effect of defects on the dissolution rate of different crystal faces. Further work is, however, required to quantitatively determine properties such as the defect density or the amount of relative strain from a BF-STEM SMF image.

© 2020 The Authors. Journal of Microscopy published by John Wiley & Sons Ltd on behalf of Royal Microscopical Society.

In plain English

This was a laboratory microscopy methods study, not a study of asbestos health effects. The authors tested ways to image furosemide, a drug crystal easily damaged by the microscope's electron beam, at very high magnification. They first used a purchased crocidolite (blue asbestos) sample, which is relatively stable under the beam, to find the beam flux and total dose needed for low-dose imaging with their CMOS camera. With a beam flux of 10 electrons per square ångström per second and a total dose of 10 electrons per square ångström, they resolved 0.30 nm spacings in crocidolite. With those settings, only patches of the furosemide crystal showed its regular structure. The authors attributed this most likely to beam damage. A second technique, scanning moiré fringes, was used at a lower dose (1.8 electrons per square ångström). Its patterns suggested strain and possible defects in the crystal. The authors concluded that, for imaging the crystal lattice of very beam-sensitive materials, this technique appears more useful than bright-field STEM or high-resolution TEM with a CMOS camera, because it needs a lower dose. They added that measuring defects and strain quantitatively needs further work.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory electron microscopy methods study (imaging technique development); not a human, animal or health study
• Who was studied: No people or animals. The samples were a commercially purchased crocidolite (fibrous riebeckite) sample, used as a test material, and laboratory-grown crystals of furosemide form I made from powder supplied by AstraZeneca. All samples were examined in a transmission electron microscope at the University of Leeds operated at 300 kV.
• Exposure: No asbestos exposure of people was studied. Crocidolite was used only as a test material to find the minimum electron beam flux and total dose (fluence) needed for low-dose high-resolution imaging with the authors' CMOS camera. The combinations tried were fluxes of 1, 10 and 100 electrons per square ångström per second and total fluences of 1, 10 and 100 electrons per square ångström.
• Main results:
  • In crocidolite, a flux of 10 electrons per square ångström per second with a total fluence of 10 electrons per square ångström gave enough contrast and signal-to-noise ratio to resolve 0.30 nm lattice spacings.
  • In crocidolite at the lowest flux (1 electron per square ångström per second), only the larger (020) and (040) spacings could be resolved, and these were difficult to see.
  • In the furosemide high-resolution TEM image, lattice fringes appeared only in patches of the crystal. The authors considered this most likely due to electron beam damage.
  • Scanning moiré fringe images of furosemide (001) planes, taken at a total fluence of 1.8 electrons per square ångström, showed variations suggesting strain in the crystal. They also showed defects resembling edge dislocations.
  • A second furosemide crystal, imaged using the (010) spacing, showed larger variations in fringe size. The authors interpreted this as suggesting more defects or more strain than in the (001)-oriented crystal.
  • The scanning moiré fringe image of the (001) planes used less than one fifth of the electron fluence needed for the high-resolution TEM image.
• The authors' conclusion: For imaging the crystal lattice of very beam-sensitive materials, the authors concluded that bright-field STEM scanning moiré fringes appear more useful than bright-field STEM or high-resolution TEM with a CMOS camera, because a lower electron fluence is needed.
• Limitations the authors state:
  • The authors could not yet quantify the number of defects or the amount of strain from the scanning moiré fringe images.
  • The furosemide crystal had already received some beam exposure during searching and focusing before the image was taken.
  • The authors stated that many more areas would need to be sampled to obtain more representative high-resolution TEM results.

Topics

Medical terms in this study, explained in our Lexicon: Transmission Electron Microscopy (TEM), Crystalline, Hardness (Mohs Scale), HRTEM, STEM, Crocidolite, Lattice Fringe.

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