One- and Two-Photon Induced Polymerization of Methylmethacrylate Using Colloidal CdS Semiconductor Quantum Dots

Ag-In-Zn-S alloyed nanocrystals as photocatalysts of controlled light-mediated radical polymerization

We report on the first case of the use of nonstoichiometric ternary (Ag-In-Zn-S) semiconductor nanocrystals as photoinitiators and photocatalysts of methyl methacrylate (MMA) polymerization. Two types of nanocrystals were tested, differing in their composition and characterized by red (λmax = 731 nm) and green (λmax = 528 nm) photoluminescence, respectively. Exploiting their reducing properties and capability of free radical generation we demonstrate that under ultraviolet (UV) radiation they effectively photoinitiate radical polymerization of MMA whereas under visible light (blue or green) they act as photocatalysts of living radical polymerization.

One-Step Photochemical Preparation of CdS/Poly(MMA-co-MAA) Composite with Enhanced Photocatalytic Activity

This paper presents a one-step photochemical method for the preparation of CdS/Poly(MMA-co-MAA) composite photocatalyst, based on the concept of simultaneous photocatalytic polymerization of organic monomers during UV-light induced formation of CdS. The preparation is carried out in an aqueous solution of Na2S2O3, CdSO4, methyl methacrylate (MMA) and methacrylic acid (MAA), under a UV lamp. The continuously formed CdS particles with photocatalytic activity act the role of initiator to directly initiate the copolymerization of MMA and MAA, resulting in the in situ formation of the composite and full contact of the CdS particles with the oxygen-containing groups in the polymer. Taking the photocatalytic degradation of methylene blue as a case study, the composite exhibited significantly higher activity under simulated solar light compared to the pure CdS. By analysis on various data, the enhanced photocatalytic activity is attributed to the enhanced visible light absorption, and especially the high electron-hole separation efficiency caused by the electrostatic interaction between photogenerated holes and carbonyl oxygen atoms with negatively charged features. Furthermore, the composite displays excellent sunlight activity and recyclability, suggesting its potential for practical applications. Such a one-step construction strategy relying only on photo-energy is green, low-cost and promising in obtaining high-performance semiconductor/polymer composite photocatalysts.

EPR spin trapping of nucleophilic and radical reactions at colloidal metal chalcogenide quantum dot surfaces

The participation of the surfaces of colloidal semiconductor nanocrystal quantum dots (QDs) in QD-mediated photocatalytic reactions is an important factor that distinguishes QDs from other photosensitizers (e.g. transition metal complexes or organic dyes). Here, we probe nucleophilic and radical reactivity of surface sulfides and selenides of metal chalcogenide (CdSe, CdS, ZnSe, and PbS) QDs using chemical reactions and NMR spectroscopy. Additionally, the high sensitivity of EPR spectroscopy is adapted to study these surface-centered reactions through the use of spin traps like 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) under photoexcitation and thermal conditions. We demonstrate that DMPO likely adds to CdSe QD surfaces under thermal conditions by a nucleophilic mechanism in which the surface chalcogenides add to the double bond, followed by further oxidation of the surface-bound product. In contrast, CdS QDs more readily form surface sulfur-centered radicals that can perform reactions including alkene isomerization. These results indicate that QD surfaces should be an important consideration for the design of photocatalysis beyond simply tuning QD semiconductor band gaps.

Rich Landscape of Colloidal Semiconductor-Metal Hybrid Nanostructures: Synthesis, Synergetic Characteristics, and Emerging Applications

Nanochemistry provides powerful synthetic tools allowing one to combine different materials on a single nanostructure, thus unfolding numerous possibilities to tailor their properties toward diverse functionalities. Herein, we review the progress in the field of semiconductor-metal hybrid nanoparticles (HNPs) focusing on metal-chalcogenides-metal combined systems. The fundamental principles of their synthesis are discussed, leading to a myriad of possible hybrid architectures including Janus zero-dimensional quantum dot-based systems and anisotropic quasi 1D nanorods and quasi-2D platelets. The properties of HNPs are described with particular focus on emergent synergetic characteristics. Of these, the light-induced charge-separation effect across the semiconductor-metal nanojunction is of particular interest as a basis for the utilization of HNPs in photocatalytic applications. The extensive studies on the charge-separation behavior and its dependence on the HNPs structural characteristics, environmental and chemical conditions, and light excitation regime are surveyed. Combining the advanced synthetic control with the charge-separation effect has led to demonstration of various applications of HNPs in different fields. A particular promise lies in their functionality as photocatalysts for a variety of uses, including solar-to-fuel conversion, as a new type of photoinitiator for photopolymerization and 3D printing, and in novel chemical and biomedical uses.

Development of Synthesis and Application of High Molecular Weight Poly(Methyl Methacrylate)

Poly(methyl methacrylate) (PMMA) is widely used in aviation, architecture, medical treatment, optical instruments and other fields because of its good transparency, chemical stability and electrical insulation. However, the application of PMMA largely depends on its physical properties. Mechanical properties such as tensile strength, fracture surface energy, shear modulus and Young’s modulus are increased with the increase in molecular weight. Consequently, it is of great significance to synthesize high molecular weight PMMA. In this article, we review the application of conventional free radical polymerization, atom transfer radical polymerization (ATRP) and coordination polymerization for preparing high molecular weight PMMA. The mechanisms of these polymerizations are discussed. In addition, applications of PMMA are also summarized.

Chromophoric Dendrimer-Based Materials: An Overview of Holistic-Integrated Molecular Systems for Fluorescence Resonance Energy Transfer (FRET) Phenomenon

Dendrimers (from the Greek dendros → tree; meros → part) are macromolecules with well-defined three-dimensional and tree-like structures. Remarkably, this hyperbranched architecture is one of the most ubiquitous, prolific, and recognizable natural patterns observed in nature. The rational design and the synthesis of highly functionalized architectures have been motivated by the need to mimic synthetic and natural-light-induced energy processes. Dendrimers offer an attractive material scaffold to generate innovative, technological, and functional materials because they provide a high amount of peripherally functional groups and void nanoreservoirs. Therefore, dendrimers emerge as excellent candidates since they can play a highly relevant role as unimolecular reactors at the nanoscale, acting as versatile and sophisticated entities. In particular, they can play a key role in the properties of light-energy harvesting and non-radiative energy transfer, allowing them to function as a whole unit. Remarkably, it is possible to promote the occurrence of the FRET phenomenon to concentrate the absorbed energy in photoactive centers. Finally, we think an in-depth understanding of this mechanism allows for diverse and prolific technological applications, such as imaging, biomedical therapy, and the conversion and storage of light energy, among others.

Tunable Quantum Photoinitiators for Radical Photopolymerization

This review describes the use of nanocrystal-based photocatalysts as quantum photoinitiators, including semiconductor nanocrystals (e.g., metal oxides, metal sulfides, quantum dots), carbon dots, graphene-based nanohybrids, plasmonic nanocomposites with organic photoinitiators, and tunable upconverting nanocomposites. The optoelectronic properties, cross-linking behavior, and mechanism of action of quantum photoinitiators are considered. The challenges and prospects associated with the use of quantum photoinitiators for processes such as radical polymerization, reversible deactivation radical polymerization, and photoinduced atom transfer radical polymerization are reviewed. Due to their unique capabilities, we forsee a growing role for quantum photoinitiators over the coming years.

Light-Mediated Polymerization Induced by Semiconducting Nanomaterials: State-of-the-Art and Future Perspectives

Direct capture of solar energy for chemical transformation via photocatalysis proves to be a cost-effective and energy-saving approach to construct organic compounds. With the recent growth in photosynthesis, photopolymerization has been established as a robust strategy for the production of specialty polymers with complex structures, precise molecular weight, and narrow dispersity. A key challenge in photopolymerization is the scarcity of effective photomediators (photoinitiators, photocatalysts, etc.) that can provide polymerization with high yield and well-defined polymer products. Current efforts on developing photomediators have mainly focused on organic dyes and metal complexes. On the other hand, nanomaterials (NMs), particularly semiconducting nanomaterials (SNMs), are suitable candidates for photochemical reactions due to their unique optical and electrical properties, such as high absorption coefficients, large charge diffusion lengths, and broad absorption spectra. This review provides a comprehensive insight into SNMs' photomediated polymerizations and highlights the roles SNMs play in photopolymerizations, types of polymerizations, applications in producing advanced materials, and the future directions.

Quantum Photoinitiators: Toward Emerging Photocuring Applications

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.

Thiol ligand capped quantum dot as an efficient and oxygen tolerance photoinitiator for aqueous phase radical polymerization and 3D printing under visible light

We report here a rapid visible-light-induced radical polymerization in aqueous media photoinitiated by only ppm level thiol ligand capped cadmium selenide quantum dots. The photoinitiation system could be readily employed for photo 3D printing.

Heterogeneous photocatalytic reversible deactivation radical polymerization

Photocatalytic reversible deactivation radical polymerization (RDRP) permits the use of sustainable solar light for spatiotemporal regulation of radical polymerization under mild conditions.

Role of External Field in Polymerization: Mechanism and Kinetics

The past decades have witnessed an increasing interest in developing advanced polymerization techniques subjected to external fields. Various physical modulations, such as temperature, light, electricity, magnetic field, ultrasound, and microwave irradiation, are noninvasive means, having superb but distinct abilities to regulate polymerizations in terms of process intensification and spatial and temporal controls. Gas as an emerging regulator plays a distinctive role in controlling polymerization and resembles a physical regulator in some cases. This review provides a systematic overview of seven types of external-field-regulated polymerizations, ranging from chain-growth to step-growth polymerization. A detailed account of the relevant mechanism and kinetics is provided to better understand the role of each external field in polymerization. In addition, given the crucial role of modeling and simulation in mechanisms and kinetics investigation, an overview of model construction and typical numerical methods used in this field as well as highlights of the interaction between experiment and simulation toward kinetics in the existing systems are given. At the end, limitations and future perspectives for this field are critically discussed. This state-of-the-art research progress not only provides the fundamental principles underlying external-field-regulated polymerizations but also stimulates new development of advanced polymerization methods.

Size effect of semiconductor quantum dots as photocatalysts for PET-RAFT polymerization

Larger QDs result in a higher polymerization rate and a better fit ofM_n,GPCwithM_{n,theoretical}for PET-RAFT polymerization using CdSe QDs photocatalysts.

PET-RAFT polymerization catalyzed by cadmium selenide quantum dots (QDs): Grafting-from QDs photocatalysts to make polymer nanocomposites

We report herein the first example of light-controlled radical reversible addition–fragmentation chain transfer polymerization facilitated by cadmium selenide quantum dots and the grafting-from CdSe QDs to create polymer-QDs nanocomposites.

A clear solution: semiconductor nanocrystals as photoinitiators in solvent free polymerization

Semiconductor nanocrystals have been shown to have unique advantages over traditional organic photoinitiators for polymerization in solution. However, efficient photoinitiation with such nanoparticles in solvent-free and additive-free formulations so far has not been achieved. Herein, the ability to use semiconductor nanocrystals for efficient bulk polymerization as sole initiators is reported, operating under modern UV-blue-LED light sources found in 3D printers and other photocuring applications. Hybrid semiconductor-metal nanorods exhibit superior photoinitiation capability to their pristine semiconductor counterparts, attributed to the enhanced charge separation and oxygen consumption in such systems. Moreover, photoinitiation by semiconductor nanocrystals overcoated by inorganic ligands is reported, thus increasing the scope of possible applications and shedding light on the photoinitiation mechanism; in light of the results, two possible pathways are discussed - ligand-mediated and cation-coordinated oxidation. A demonstration of the unique attributes of the quantum photoinitiators is reported in their use for high-resolution two-photon printing of optically fluorescing microstructures, demonstrating a multi-functionality capability. The bulk polymerization demonstrated here can be advantageous over solvent based methods as it alleviates the need of post-polymerization drying and reduces waste and exposure to toxic solvents, as well as broadens the possible use of quantum photoinitiators for industrial and research uses.

Semiconductor Quantum Dots as Photocatalysts for Controlled Light-Mediated Radical Polymerization

Light-mediated radical polymerization has benefited from the rapid development of photoredox catalysts and offers many exceptional advantages over traditional thermal polymerizations. Nevertheless, the majority of the work relies on molecular photoredox catalysts or expensive transition metals. We exploited the capability of semiconductor quantum dots (QD) as a new type of catalyst for the radical polymerization that can harness natural sunlight. Polymerizations of (meth)acrylates, styrene, and construction of block copolymers were demonstrated, together with temporal control of the polymerization by the light source. Photoluminescence experiments revealed that the reduction of alkyl bromide initiator by photoexcited QD is the key to this light-mediated radical polymerization.

Photocatalysis in organic and polymer synthesis

This review, with over 600 references, summarizes the recent applications of photoredox catalysis for organic transformation and polymer synthesis. Photoredox catalysts are metallo- or organo-compounds capable of absorbing visible light, resulting in an excited state species. This excited state species can donate or accept an electron from other substrates to mediate redox reactions at ambient temperature with high atom efficiency. These catalysts have been successfully implemented for the discovery of novel organic reactions and synthesis of added-value chemicals with an excellent control of selectivity and stereo-regularity. More recently, such catalysts have been implemented by polymer chemists to post-modify polymers in high yields, as well as to effectively catalyze reversible deactivation radical polymerizations and living polymerizations. These catalysts create new approaches for advanced organic transformation and polymer synthesis. The objective of this review is to give an overview of this emerging field to organic and polymer chemists as well as materials scientists.

Homogenous photopolymerization of acrylic monomers initiated with ZnO-methacrylate in non-aqueous medium and production of luminescent nanocomposites

Luminescent ZnO quantum dots coated with methacrylic acid initiate photopolymerization of methylmethacrylate in the absence of hole-scavengers producing luminescent nanocomposites.

Photoinduced Electron Transfer Reactions for Macromolecular Syntheses

Photochemical reactions, particularly those involving photoinduced electron transfer processes, establish a substantial contribution to the modern synthetic chemistry, and the polymer community has been increasingly interested in exploiting and developing novel photochemical strategies. These reactions are efficiently utilized in almost every aspect of macromolecular architecture synthesis, involving initiation, control of the reaction kinetics and molecular structures, functionalization, and decoration, etc. Merging with polymerization techniques, photochemistry has opened up new intriguing and powerful avenues for macromolecular synthesis. Construction of various polymers with incredibly complex structures and specific control over the chain topology, as well as providing the opportunity to manipulate the reaction course through spatiotemporal control, are one of the unique abilities of such photochemical reactions. This review paper provides a comprehensive account of the fundamentals and applications of photoinduced electron transfer reactions in polymer synthesis. Besides traditional photopolymerization methods, namely free radical and cationic polymerizations, step-growth polymerizations involving electron transfer processes are included. In addition, controlled radical polymerization and "Click Chemistry" methods have significantly evolved over the last few decades allowing access to narrow molecular weight distributions, efficient regulation of the molecular weight and the monomer sequence and incredibly complex architectures, and polymer modifications and surface patterning are covered. Potential applications including synthesis of block and graft copolymers, polymer-metal nanocomposites, various hybrid materials and bioconjugates, and sequence defined polymers through photoinduced electron transfer reactions are also investigated in detail.

Two-photon excited quantum dots with compact surface coatings of polymer ligands used as an upconversion luminescent probe for dopamine detection in biological fluids

Two-photon excited CdTe quantum dots were developed as a novel upconversion luminescent probe for dopamine detection in biological fluids.

Magnetic iron oxide nanoparticles as long wavelength photoinitiators for free radical polymerization

Iron oxide nanoparticles (Fe₃O₄ NPs) capped with lauric acid agents were synthesized and their photocatalytic activity was investigated in free radical photopolymerization of vinyl monomers.

Two-photon lithography in the future of cell-based therapeutics and regenerative medicine: a review of techniques for hydrogel patterning and controlled release

In recent decades, there has been considerable interest in using photochemistry to produce biomaterials, owing to their ability to be used in the presence of biological material. Two-photon-induced photoreactions have been used to produce materials for optical data storage and microfabrication and, recently, researchers have exploited two-photon-induced chemical processes to create biomaterials. Researchers have used two-photon-induced lithography to fabricate hydrogels with well-defined chemical and physical properties in 3D through network polymerization, functionalization, uncaging and degradation, as described in this article. Fabrication and modification of chemical and physical architecture of biomaterials in 3D with submicron resolution will allow the elucidation of more complex relationships in cell behavior and tissue development and introduce pathways to engineering complex tissues.

In situ photopolymerization of pyrrole in mesoporous TiO2

We examined mesoporous TiO(2) as a photosensitizer and template for creating hybrid TiO(2)-polypyrrole materials. Optical excitation of mesoporous TiO(2) was used to generate the electronic potential necessary for the oxidation and polymerization of the pyrrole monomer. The photopolymerization process was monitored by the quartz crystal microbalance, nitrogen sorption, and thermogravimetric techniques. In situ generation of polypyrrole was observed to be self-limiting after approximately 20-30% filling of the mesoporous TiO(2) network. In situ generation of a complementary phase by means of charge transfer from an active host phase represents an alternative means of assembling hybrid inorganic-organic materials with potential applications ranging from electrocatalysis to photovoltaics.

Modulation of the surface charge on polymer-stabilized gold nanoparticles by the application of an external stimulus

A new approach to controlling the charge on gold nanoparticle surfaces is described. The method exploits the simultaneous coattachment of both charged and neutral polymers onto gold surfaces. The charged and neutral polymers were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization, and the RAFT end-group functionality was used as the anchor for attachment to gold. The approach described is general and can be applied to a wide range of monomers; those exemplified in the paper are poly(2-aminoethyl methacrylamide) (P(AEA)), poly(acrylic acid) (PAA), and poly(N,N-diemthylaminoethyl acrylate) (P(DMAEA)) together with neutral polymers based on poly(oligoethylene oxide) acrylate (P(OEG-A)). The hybrid polymer-stabilized GNPs thus formed were characterized in solution using dynamic light scattering and zeta potential measurements, transmission electron microscopy, UV-visible spectroscopy, X-ray photoelectron spectroscopy, and attenuated total reflection-Fourier-transform IR spectroscopy. The grafting densities of the polymers on GNPs were measured using thermal gravimetric analyses (TGA), as 0.4 chains/nm(2) (for PAA), 0.9 chains/nm(2) (for neutral polymers, such as P(NIPAAm), and 0.6 chain/nm(2) for the positive charged polymers P(AEA) and P(DMAEA). The directed coassembly of two different polymers (one charged and one noncharged) on the gold nanoparticle surfaces provided a mechanism (dependent on molecular weight) for shielding the surface charge imparted by the charged polymer component, allowing for a range of surface charges on the GNPs from -30 to +39 mV. In further work, the surface-charges were modulated by an external stimulus (temperature). The charge-modulation was controlled by the use of thermosensitive neutral polymers coassembled with charged polymers. The thermosensitive polymers exemplified in this paper are P(oligoethylene oxide acrylate-co-diethylene oxide acrylate) (P(OEG-A-co-DEG-A)) and P(N-isopropyl acrylamide) (P(NIPAAm). The temperature of the aqueous phase (from 15 to 70 degrees C) was then adjusted to tune the zeta potentials of the hybrid GNPs from +39 or -30 to approximately 0 mV. Finally, by manipulating the solution pH, reversible aggregation behavior of the hybrid GNPs could be induced.

Two-color single-photon photoinitiation and photoinhibition for subdiffraction photolithography

Controlling and reducing the developed region initiated by photoexposure is one of the fundamental goals of optical lithography. Here, we demonstrate a two-color irradiation scheme whereby initiating species are generated by single-photon absorption at one wavelength while inhibiting species are generated by single-photon absorption at a second, independent wavelength. Co-irradiation at the second wavelength thus reduces the polymerization rate, delaying gelation of the material and facilitating enhanced spatial control over the polymerization. Appropriate overlapping of the two beams produces structures with both feature sizes and monomer conversions otherwise unobtainable with use of single- or two-photon absorption photopolymerization. Additionally, the generated inhibiting species rapidly recombine when irradiation with the second wavelength ceases, allowing for fast sequential exposures not limited by memory effects in the material and thus enabling fabrication of complex two- or three-dimensional structures.