J. Am. Chem. Soc. 138, 11623–11632 (2016)

The coffee-ring effect — the formation of a particle corona from an evaporating droplet of a colloidal suspension — is typically a hindrance in printing and patterning applications. While many strategies have been sought to suppress this effect, Damien Baigl and colleagues at the Sorbonne, CNRS and PSL Research University in Paris now report a method that exploits it for protein analysis.

By examining the drying of drops containing polystyrene nanoparticles and proteins (bovine serum albumin, BSA, and haemoglobin), the researchers found a clear relationship between drying patterns and the protein's adsorption and reorganization behaviour. Coffee-ring formation is observed when the protein's (BSA) and nanoparticle's charges match, whereas it is suppressed when they neutralize each other. This behaviour is believed to result from the formation of favourable electrostatic interactions, as well as hydrophobic interactions between the protein surface and unfunctionalized nanoparticle regions, rendering the particle–protein assembly hydrophobic.

The tetrameric protein haemoglobin does not follow as simple adsorption behaviour, suggesting protein reorganization also plays a role in protein–particle interactions. The researchers use their method to detect a single point mutation in a protein, as demonstrated by the distinct patterns of drying drops containing haemoglobin or the mutant responsible for sickle cell anaemia. As well as a way of suppressing the coffee-ring effect, this work offers a simple method for investigating protein folding and screening protein interactions for nanotoxicology and nanomedicine.