Materials Horizons Blog

The synthesis and control of the performance of chiral superstructures are intriguing because they bring us closer to the magical concept of chirality, exemplified by the dichotomy between our hands. In the long run, scientists aim to control the chirality of matter, particularly molecules. The enduring challenge is to overcome the mismatch between the molecular work function and magnetic quadrupole transitions and/or the wavelength of light. “Due to this size mismatch, the molecule sees uniform electric and magnetic fields, just as we see the Earth as locally flat,” said Prof. Adam E. Cohen in one of his pioneering reviews on chirality (Nano Today (2009) 4, 269—279). The design of chiral superstructures allows us to sculpt the three-dimensional shape of the electromagnetic field at the size scale of an individual molecule. The most rapidly developing strategy to prepare these nanostructures involves using diverse nanoscale templates, such as origami techniques or lithography. However, the preparation of such nanostructures is very challenging, which slows down the field’s development. Another significant limitation is the inability to dynamically control the handedness and spectral characteristics.

Recent work by Chaolumen Wu and Yadong Yin suggests a magnetic assembly strategy to overcome these limitations. They prepared Ag@Fe3O4 nanoparticles through a relatively simple procedure and further assembled them by introducing a chiral magnetic field from a cubic permanent magnet. This magnet was placed beside the Ag@Fe3O4 suspension and was controlled by two parameters: rotation angle and distance to the suspension (shorter separation distance results in a stronger field strength). The formed assemblies are chains of nanoparticles oriented in a chiral structure. The main advantage of the suggested strategy is the very smoothly controlled dynamicity of the system: spectral position, handedness, and intensity of the chiral signal can be controlled by the magnetic angle and distance relative to the sample. Previously reported approaches required time-consuming simulations and production of plasmonic substrates with those varied chiral characteristics. The authors could fix the chiral superstructures in polymer films with precise control of the handedness, position of optical absorption, and degree of plasmonic coupling. The dynamic optical rotation enables the authors to demonstrate distinguishable color switching. Without the magnet, no color is observed; however, variation of magnet positions gives a wide palette from purple to pink, and from yellow to blue. Therefore, the authors mainly envision the application of this tunability for color-changing optical devices used in anti-counterfeiting and stress sensors.

Figure 1: (a) Simulation of the chiral field distribution from a cubic permanent magnet and schematic illustration of the chirality transfer from a chiral magnetic field to magnetic nanoparticle assembly. (b) TEM image of Ag@Fe3O4 hybrid nanoparticles. (c) Schematic illustration of the setup for measuring the extinction and CD spectrum of particle dispersion under a chiral magnetic field. (d)–(f) Extinction (d), CD (e), and the corresponding g-factor (f) spectra of the Ag@Fe3O4 nanoparticle dispersion without a magnetic field or under a field along the X- and Z-axes. Reproduced from DOI: 10.1039/D3MH01597A with permission from the Royal Society of Chemistry

However, the author of this blog highlights alternative applications. Controlling chiroptical properties is a direct way to enable enantioselective sensing and catalysis by transferring chirality to small molecules. While the application of a chiral quadrupole magnetic field induces the assembly of Ag@Fe3O4, simultaneous irradiation of these structures with a wavelength corresponding to the maximum absorption, due to the presence of Ag, could enhance interactions with small molecules. Photocatalytic reactions performed under a magnetic field, which couple magnetic and light fields, are a novel concept. Although magnetic field-enhanced photocatalysis is relatively new and has mostly been applied to dye degradation, controlling chiral photochemistry with a magnetic field would significantly advance interactions with chiral molecules. In this scenario, applied magnetic fields to Ag@Fe3O4 could serve a dual role of (i) chirality induction and (ii) plasmonic carrier generation and prolongation of the lifetime of excited plasmons.

Research on chirality remains niche due to the complex preparation techniques of chiral superstructures. The published research opens new possibilities for more scientific groups to work in this direction, thanks to the simplicity of using cubic permanent magnets. However, measurement techniques, such as optical rotation density measurement setups, may still pose challenges for the wider community. Increased availability of these techniques should spur more investigations into various applications of chiral nanostructures, including color displays, anti-counterfeiting measures, and chiral sensing and catalysis.

To find out more, please read:

Magnetic assembly of plasmonic chiral superstructures with dynamic chiroptical responses
Chaolumen Wu, Qingsong Fan, Zhiwei Li, Zuyang Yea and Yadong Yin
Mater. Horiz., 2024,11, 680-687, DOI: 10.1039/D3MH01597A

About the blogger

Dr Olga Guselnikova is a member of the Materials Horizons Community Board. She recently joined the Center for Electrochemistry and Surface Technology (Austria) to work on functional materials as a group leader. Dr. Guselnikova received her PhD degree in chemistry from the University of Chemistry and Technology Prague (Czech Republic) and Tomsk Polytechnic University (Russia) in 2019. Her research interests are related to surface chemistry for functional materials. This means that she is applying her background in organic chemistry to materials science: plasmonic and polymer surfaces are hybridized with organic molecules to create high-performance elements and devices.

Polymeric particles represent a widely utilized class of materials due to their adjustable size and shape, high volume-surface area ratio, customizable properties and ease of surface modification. These attributes make them indispensable across diverse fields, finding applications in coatings, pigments, drug delivery, nanomedicine, catalysis and more. Traditional methods of synthesis, predominantly via thermal heterogeneous polymerization like emulsion or dispersion polymerization, often necessitate the use of surfactants/stabilizers and thermal initiators, risking material contamination and complicating purification processes. In recent years, light polymerization has gained significant attention due to its ability to operate under milder conditions and offer temporal regulation. However, employing the photo-flow reaction, which is commonly viewed as an appealing method for upscaling reactions, proves challenging for heterogeneous systems due to constraints such as limited light penetration and scattering.

In a recent study by Holloway et al., an upscaled, photochemical synthesis of polymeric particles via flow chemistry was achieved based on precipitation step-growth polymerization without radical or surfactant sources. This innovative approach optimized an easy, scalable, and fast synthetic procedure and produced functional polymeric materials with chemiluminescent self-reporting properties and chemical-stimulated degradability, thus opening novel avenues for scaled-up synthesis of functional polymeric particles (Figure 1).

Figure 1: (A) Overview of the particle formation with AA and BB2 monomers and subsequent cleavage of the oxalate bond in presence of H2O2, resulting in the degradation of the particles and light emission. (B) SEM image of the particles after synthesis. (C) SEM image of the particles after adding H2O2. (D) Photocount recorded at various particles concentrations. Reproduced from DOI: 10.1039/D4MH00106K with permission from the Royal Society of Chemistry.

Harnessing their expertise in photochemical reactions, the team synthesized functional AA and BB monomers capable of Diels-Alder step-growth polymerization under light irradiation. The oligomers, with limited solubility in the reaction solvent, precipitated out upon reaching a critical molecular weight, facilitating particle formation via precipitation polymerization mechanism. Thorough optimization of polymerization parameters enabled precise control over particle size, yield, and shape, with solvent selection and flow rate playing crucial roles in the process. The solvent selection significantly affected polymer solubility, with dramatic effects on yield and particle size. The reaction yield increased impressively, from 1% in acetonitrile, a commonly used solvent, to up to 60% in the optimized water/methanol mixture. Since the reactions took place in a flow system, the flow rate was of paramount importance. In-depth investigation revealed that tuning the flow rate allowed manipulation of reaction yield, particle size, and shape, with the ability to tune the particle size over 545 nm (ranging from 185 to 730 nm). Furthermore, the initial monomer concentration played an important role in the reaction, as the limited solubility of the monomers prohibited equimolar monomer ratios at higher concentrations, significantly affecting particle morphology.

After successfully demonstrating the photo-flow reaction, the authors took advantage of the polymer’s nature, demonstrating its potential application in point-of-care devices. The Diels-Alder adduct exhibited intrinsic fluorescence, serving as a fluorophore for the peroxylate chemiluminescent reaction. Specifically, upon the addition of H₂O₂, the particles degraded via oxalate bond cleavage, resulting in the excitation of the fluorophore and subsequent photon emission with a strong signal, allowing for degradation monitoring.

In summary, this work surpasses previous approaches by developing a scalable photo-flow system capable of producing polymeric particles with tuneable properties within minutes, free from additives or radical initiators. The molecular design of these polymers enables the synthesis of functional and degradable particles responsive to chemical stimuli, featuring the potential for diverse applications. Overall, this study elegantly combines fundamental aspects of polymerization and materials design to pave the way for a plethora of applications in various fields.

To find out more, please read:

Photo-induced synthesis of polymeric nanoparticles and chemiluminescent degradable materials via flow chemistry
Joshua O. Holloway, Laura Delafresnaye, Emily M. Cameron, Jochen A. Kammerer and Christopher Barner-Kowollik
Mater. Horiz., 2024, DOI:10.1039/D4MH00106K

About the blogger

Dr. Kostas Parkatzidis is a Swiss National Science Foundation Postdoc Fellow in the group of Professor Zhenan Bao at Stanford University (United States), working on the molecular design of polymer-based skin-inspired materials for various applications. Kostas obtained his PhD from ETH Zurich (Switzerland) under the supervision of Professor Athina Anastasaki where he focused on the development of advanced polymer synthesis and chemical recycling methodologies. He also holds MSc in Organic Chemistry and BSc in Materials Science and Technology obtained from the University of Crete (Greece). Since 2023, Kostas has served as a Materials Horizons Community Board member.