Semiconductor nanocrystals are promising photocatalysts for a wide range of applications, ranging from alternative fuel generation to biomedical and environmental applications. This stems from their diverse properties, including flexible spectral tunability, stability, and photocatalytic efficiencies. Their functionality depends on the complex influence of multiple parameters, including their composition, dimensions, architecture, surface coating, and environmental conditions. A particularly promising direction for rapid adoption of these nanoparticles as photocatalysts is their ability to act as photoinitiators (PIs) for radical polymerization. Previous studies served to demonstrate the proof of concept for the use of quantum confined semiconductor nanocrystals as photoinitiators, coining the term Quantum PIs, and provided insights for their photocatalytic mechanism of action. However, these early reports suffered from low efficiencies while requiring purging with inert gases, use of additives, and irradiation by high light intensities with very long excitation durations, which limited their potential for real-life applications. The progress in nanocrystal syntheses and surface engineering has opened the way to the introduction of the next generation of Quantum PIs. Herein, we introduce the research area of nanocrystal photocatalysts, review their studies as Quantum PIs for radical polymerization, from suspension polymerization to novel printing, as well as in a new family of polymerization techniques, of reversible deactivation radical polymerization, and provide a forward-looking view for the challenges and prospects of this field.
Three-dimensional printing (3DP) is considered among the key-technologies for the next industrial revolution, with considerable effects on production processes, economy, and society. In this context, the most relevant part of the market consists of polymeric 3D printing. The 3D printable liquids are composed of various components, among them dyes are usually underrated because they are introduced merely for aesthetical reasons or to enhance the objects' resolution. In recent years, the capability of specific dyes to go beyond conventional use and to confer functional properties to 3D printed objects has become an emerging research area. Modifying elastic moduli upon light irradiation, inducing optical and emitting properties in the matrices or conferring temperature responsivity are just few examples of innovative stimuli-responsive materials that can be produced by combining well-designed dyes with the appropriate 3DP printed matrices. In this Review, we discuss and critically analyze the most relevant recent results achieved in the use of smart dyes in the synthesis of stimuli responsive 3D printed polymers.