Schematic view of the repolymerization triggered by green light on polymer patches grafted on metal nanoparticles.
Schematic view of the repolymerization triggered by green light on polymer patches grafted on metal nanoparticles.

Researchers have designed a reaction process that uses light to trigger the growth of polymer layers on a metal nanoparticle [Kameche et al., Materials Today (2020),].

A process known as reversible deactivation radical polymerization (RDRP) allows a high degree of control over the molecular structure of a polymer. Different triggers can be used to kick start the process, but light is a particularly attractive option. A photoactive catalyst such as an organometallic catalyst or metal ion like copper, zirconium, iron, cobalt, gold, manganese, or iridium, which responds to light, such as an LED, laser or lamp, initiates the reaction. Now the researchers at the Institut de Science des Matériaux de Mulhouse, Univ. de Technologie de Troyes, Univ. Aix Marseille, Univ. Paris-Saclay, and Shanghai University have taken this one stage further, creating a photopolymerization process that can be started, stopped and restarted at will.

“We have succeeded in demonstrating that it is possible to deposit polymer patches locally on the surface of metallic nanoparticles using light and that these polymer patches, known as ‘living’ polymer patches, can be reactivated by a second irradiation to graft on their surface a second layer of polymer of nanometric thickness of arbitrary composition,” says Olivier Soppera, who led the work. “Our aim is to develop a new tool to prepare new functional nanoparticles that are impossible to obtain by current methods.”

He and his colleagues exploit the concept of optical near-field (ONF) photopolymerization, based on the local electromagnetic field induced around a metal nanostructure excited by light. The surface plasmon resonance generated in the metal nanostructure confines the polymerization in very small nanoscale volumes surrounding the structure.

“Using a living polymerization molecular system, it is possible to restart, by visible laser irradiation, the polymerization from the first polymer surface, in order to increase the polymer layer and/or graft a second monomer,” explains Soppera. “The near-field optical response of the nanoparticle is used to define the areas where the polymer patches are grafted.”

Using an iridium complex with spherical and cubic gold nanoparticles on a Si3N4 membrane irradiated with a Nd-YAG laser, the researchers demonstrate the synthesis of polymer patches. Subsequent bursts of irradiation add further polymer layers to the nanostructures. Moreover, the polymer layers can be fabricated asymmetrically – only on the sides of a nanocube and not the top surface, for example.

“The second monomer is covalently bounded to the first layer, which makes these particles very stable and usable in different contexts,” says Soppera. “Such a route opens almost unlimited possibilities to achieve complex hybrid metal-polymer nanoparticles with well-controlled geometry and chemistry.”

The approach could be highly valuable wherever hybrid nanoparticles are needed, plasmonics, photonics, nanosensors and nanomedicine, suggests Soppera.