Water molecules keep nanoparticles apart

07 Apr 2016 An in situ Transmission Electron Microscopy (TEM) study reveals that water molecules bound to nanoparticle surfaces serve as a barrier that stall their pairwise attachment.

When most surfaces are placed in water they are instantly coated with a layer of water molecules, referred to as a hydration layer. These surface-bound water molecules give rise to repulsive steric forces that can keep any two surfaces from contacting each other, unless these water molecules are drained. Also when two surfaces are separated by a distance of a few water molecules, other intermolecular interactions such as an attractive van der Waals (vdW) force kick in.

Mr Utkarsh ANAND and Dr Jingyu LU supervised by Prof Utkur MIRSAIDOV from the Department of Biological Sciences in NUS used in situ TEM to show that the repulsive forces generated by the nanoparticle’s hydration layer in solution can balance the attractive vdW forces for a short period of time. This delicate balance between two opposing forces locks the nanoparticle into a nanoparticle pair, which merges only after the hydration layer between nanoparticle surfaces is removed.

The unexpected formation of transiently stabilised nanoparticle pairs in water is important because the delay associated with the dehydration of interacting nanoscale surfaces may be very important in many natural processes. For example, in the case of two fusing nanocrystals in solution, the short delay time prior to their contact may be all that is need to reorient themselves to form larger defect free-crystals, a process known as oriented attachment. In a biological system, the delay time associated with surface dehydration may provide an opportunity for two interacting proteins to explore optimal binding conformations.

This approach of probing interactions between nanoparticles based on advanced imaging and image processing tools now opens the possibility of exploring a wide range of interactions between nanoscale bodies in solution. Detailed understanding of these interactions that cannot be probed by other conventional methods will be the basis of our future research, that aims to explores the self-assembly of functional nanodevices from different nanoparticles.

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Figure shows the balance between an attractive van der Waals force and a repulsive hydration force results in formation of transiently stabilised nanoparticle pair [Image credit: Utkur Mirsaidov].

 

Reference

Anand U, Lu J, Loh D, Aabdin Z, Mirsaidov U. “Hydration Layer-Mediated Pairwise Interaction of Nanoparticles”. Nano Letters 16 (2016) 786.