Unravelling the nanometre-scale stimuli-responsive properties of natural rubber latex particles using atomic force microscopy

Abstract

Natural rubber (NR) latex particles exhibit a core–shell structure, with a polyisoprene elastomer as the core and a thin layer made of proteins and lipids as the shell. Knowledge of the surface properties and interactions of core–shell NR particles is a key to gain a detailed understanding of their colloidal behaviour and to efficiently exploit them in industrial processes. Here, we report the first nanoscale measurement of the stimuli-responsive behaviour of single NR latex particles by atomic force microscopy (AFM), revealing that their molecular interactions are tightly controlled by salt conditions. High-resolution imaging, combined with protease treatments, confirmed the presence of a soft layer of proteins on the surface of the NR particles. Upon approach of the particles in diluted solutions of monovalent salts (1 mM NaCl), the AFM tip experienced a long-range repulsive force, reflecting electrostatic surface forces and compression of the soft, loosely structured protein layer. Upon retraction, sharp adhesion events were observed due to strong attractive interactions between the tip and proteins. When increasing the ionic strength (100 mM NaCl) or ion valency (1 mM and 100 mM MgCl₂), the long-range repulsion was no longer seen, while nanotubes made of proteins and/or lipids were found to strengthen the tip–particle adhesive bonds. These findings, which correlate with the macroscopic aggregation of the particles, indicate that high ionic strength or ion valency leads to more compact shells by lowering repulsive intermolecular forces. The nanoscale measurements presented here provide new opportunities for quantifying the stimuli-responsive properties of soft colloids, like NR particles, and for tuning their interfacial interactions for industrial applications.