Ultra-thin Metal-Organic Layers Prevent Ice Crystal Formation
Small amounts of nanometer-thin metal-organic layers efficiently
protect red blood cells during freezing and thawing, as a team of
researchers writing in the journal Angewandte Chemie has
discovered. The nanolayers, made from metal-organic frameworks based on
the metal hafnium, prevent ice crystal formation at very low
concentrations. This effective novel cryoprotection mode could be used
to develop new and more efficient cryoprotectants for the biosciences.
© Wiley-VCH, re-use with credit to 'Angewandte Chemie' and a link to the original article.
Cryoprotectants prevent ice crystals from forming when samples are
frozen. Growing crystals can damage delicate cell membranes and cell
components and disrupt cell integrity. Some solvents or polymers make
good cryoprotectants; they keep ice crystal formation in check by
binding water molecules and disrupting their ordered assembly during ice
formation.
Synthetic chemistry has yet more tricks up its sleeve for targeting
and influencing ice formation in a more effective way. Metal-organic
frameworks (MOFs) are three-dimensional crystalline networks of metal
ions linked by organic ligands. These ligands can be tailored to bind
small molecules such as water, allowing the assembly of the water
molecules into ice crystals to be very precisely tuned.
Wei Zhu from the South China University of Technology in Guangzhou
(China) and colleagues have now discovered that as MOFs based on hafnium
and organic ligands become thinner, their ability to bind and influence
water molecules increases, mainly because more ligand sites are
available. The team therefore developed a method for controlled
deconstruction of the three-dimensional metal-organic frameworks until
only two-dimensional thin nanolayers remain.
To test the suitability of their hafnium-MOLs (MOL stands for
metal-organic layer, to distinguish them from three-dimensional MOFs) as
cryoprotectants, the team froze red blood cells, a type of cell that
needs to be stored in large numbers for medical purposes but is easily
destroyed by ice crystal formation. Compared to the hydroxyethyl starch
(HES), which is commonly used as a cryoprotectant, hafnium MOLs showed
excellent cryoprotection at a minimal concentration of less than 0.1%,
whereas the HES solutions are typically used at concentrations of up to
30%.
Zhu and the team explained that the MOLs are so effective because the
irregular two-dimensional structure of the ligands binding the water
molecules prevents the formation of regular ice crystal nuclei. The team
suggest that the dimensional reduction of MOFs is an interesting new
perspective for obtaining highly efficient cryoprotectants.
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About the Author
Wei Zhu, Ph.D., is a
Professor of chemistry at the School of Biology and Biological
Engineering of the South China University of Technology, Guangzhou,
China. His research interests focus on nanoporous materials, biological
and nanomaterial interfaces, biosilicification, and their related
biomedical applications, including biosensing, biopreservation, and
nanomedicine.
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