Catalysis by metallic nanoparticles in solution: Thermosensitive microgels as nanoreactors

TL;DR

This paper reviews how thermosensitive microgels, specifically PNIPAM, can act as nanoreactors by controlling nanoparticle catalysis through temperature-induced swelling and shrinking, enabling external regulation of catalytic activity.

Contribution

It introduces the concept of thermosensitive microgels as active nanoreactors that modulate catalytic activity via temperature-triggered changes in substrate diffusion.

Findings

PNIPAM microgels enable temperature-controlled catalysis.

Swelling state allows hydrophilic substrate diffusion; shrinking favors hydrophobic substrates.

Thermosensitive microgels can serve as switchable nanoreactors for catalysis.

Abstract

Metallic nanoparticles have been used as catalysts for various reactions, and the huge literature on the subject is hard to overlook. In many applications, the nanoparticles must be affixed to a colloidal carrier for easy handling during catalysis. These "passive carriers" (e.g., dendrimers) serve for a controlled synthesis of the nanoparticles and prevent coagulation during catalysis. Recently, hybrids from nanoparticles and polymers have been developed that allow us to change the catalytic activity of the nanoparticles by external triggers. In particular, single nanoparticles embedded in a thermosensitive network made from poly(N-isopropylacrylamide) (PNIPAM) have become the most-studied examples of such hybrids: Immersed in cold water, the PNIPAM network is hydrophilic and fully swollen. In this state, hydrophilic substrates can diffuse easily through the network, and react at the surface of the nanoparticles. Above the volume transition located at 32{\deg}C, the network becomes hydrophobic and shrinks. Now hydrophobic substrates will preferably diffuse through the network and react with other substrates in the reaction catalyzed by the enclosed nanoparticle. Such "active carriers", may thus be viewed as true nanoreactors that open new ways for the use of nanoparticles in catalysis.