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.

Electrification of transportation has become one of the most important central themes of modern society to build a sustainable system. The market for electric vehicles (EVs), which are powered by Li-ion batteries (LIBs), has been growing very rapidly and continuously. While EVs have become more popular on the road, there are concerns about their safety and somewhat limited driving ranges. In this respect, solid-state Li metal batteries have been extensively explored because of the following merits. First, solid-state electrolytes are more resistant to catching fire than liquid organic electrolytes that are flammable. Second, solid-state electrolytes are expected to be more stable against lithium metal dendrite penetration, which allows the use of energy-dense lithium metal anodes instead of graphitic carbon. However, it is still challenging to achieve “practical” high-energy density of solid-state Li metal batteries, which requires thick composite cathodes enabled by super-ionic conductors (>10 mS/cm). Although we have observed significant progress in improving the ionic conductivities of solid-state electrolytes, more fundamental studies to understand important parameters promoting lithium-ion conductivity and further exploration in new chemical spaces are needed.

A recent study by Han and colleagues developed a new borohydride/halide dual-substituted argyrodite-type solid-state electrolyte, which achieves an ionic conductivity of > 26 mS/cm after low-temperature sintering. They synthesized the borohydride/halide dual-substituted argyrodite solid-state electrolytes using a two-step ball-milling method from β-Li₃PS₄, LiBH₄, and LiCl. While they also tried to incorporate Br^– and I^– instead of Cl^– in the argyrodite solid-state electrolytes, they always produced unknown impurity phases and exhibited lower ionic conductivity. When they varied the compositions (2.5 ≥ x ≥ 1.5, Li₃PS₄ + xLiBH₄ + 0.5LiCl), it was found that the composition of Li₃PS₄ + 2LiBH₄ + 0.5LiCl (x = 2) exhibits the highest ionic conductivity of 16.4 mS/cm without sintering and 26.1 mS/cm after the sintering. They claimed that the increase of occupancy of BH₄^– in 4a may harm the ionic conductivity when x is larger than 2, but it still remains an open question why x = 2 composition has a sweet spot for the highest ionic conductivity in their system.

Figure 1: (a) Nyquist plots of Li3PS4 + xLiBH4 + 0.5LiCl (2.5 ≥ x ≥ 1.5) and commercial Li6PS5Cl. (b) EIS spectra of Li5.35PS4.35(BH4)1.15Cl0.5 at various temperatures. (c) Arrhenius plots and the activation energy of Li3PS4 + xLiBH4 + 0.5LiCl (2.5 ≥ x ≥ 1.5) and commercial Li6PS5Cl. (d) BH4- occupancy of 4a and 4d sites for each electrolyte and its ionic conductivity. The dashed lines are the trend lines of each site. Reproduced from DOI: 10.1039/D3MH01450A with permission from the Royal Society of Chemistry.

They further investigated the electrochemical properties, including oxidation stability limit, Li metal deposition/stripping cycles, and full cell tests. While they confirmed that their borohydride/halide dual-substituted argyrodite-type solid-state electrolytes are stable up to 5.0 V (vs. Li/Li⁺) using cyclic voltammetry (CV) technique, they used a simple cell configuration of Li-metal/solid-state electrolyte/current collector, which often overestimates oxidation limits. In the Li/solid-state electrolyte/Li symmetric cycling tests, they achieved stable cycling for up to 2000 hours (at 1mA/cm², for 1 hour charge – 1 hour discharge cycles) and high critical current density (CCD) of 2.5 mA/cm². They also confirmed the practical feasibility of their solid-state electrolytes in full-cell tests where Li metal and LiNbO₃-coated LiNi_0.8Co_0.1Mn_0.1O₂ are used as an anode and a cathode, respectively.

In conclusion, Han et al. demonstrated that the dual substitution of borohydride and halide enhances lithium-ionic conductivity of argyrodite-type solid-state electrolytes significantly. Their work sheds light on a new strategy of dual substitution to achieve super-ionic conductivity although there are remaining important questions to understand (i) why only Cl^– could be soluble in the argyrodite structure along with borohydride and (ii) why a specific composition shows the best ionic conductivity. Answering the questions above will give us better insights to design even better super-ionic conductors in the future.

To find out more, please read:

Borohydride and halide dual-substituted lithium argyrodites
Ji-Hoon Han, Do Kyung Kim, Young Joo Lee, Young-Su Lee, Kyung-Woo Yi and  Young Whan Cho.
Mater. Horiz., 2024, 11, 251-261, DOI: 10.1039/D3MH01450A

About the blogger

Haegyeom Kim is a Staff Scientist at Materials Sciences Division of Lawrence Berkeley National Laboratory (LBNL) and a Community Board member of Materials Horizons. In 2016-2019, he worked as a postdoctoral researcher at the LBNL (Supervisor: Prof. Gerbrand Ceder). He also spent 1 year as a postdoctoral researcher at Research Institute of Advanced Materials in Seoul National University (SNU) (Supervisor: Prof. Kisuk Kang) in 2015-2016. Dr. Kim completed his PhD degree in Materials Science and Engineering from SNU in 2015 (Advisor: Prof. Kisuk Kang), Master’s Degree in EEWS (Energy, Environment, Water and Sustainability) from Korea Advanced Institute of Science and Technology (KAIST) in 2011, and Bachelor’s Degree in Materials Science and Engineering from Hanyang University in 2009. At LBNL, Dr. Kim runs Renewable Energy Storage Lab, which designs and develops efficient and cost-effective energy storage and conversion materials based on the fundamental understanding of synthesis-structure-performance relationships. More information about Dr. Kim and his research group can be found here: https://kimhaegyeom1.wixsite.com/kim1