Silk fibroin (SF) is a biopolymer from Bombyx mori mulberry silkworm that has been utilized as textile for millennia. Recent advancement has emphasized to the biodegradation and biocompatibility of silk for medical applications. 

Due to the hydrogen bonding in silk fibroin, SF interacts with water molecules through the random coil conformation, while the β-sheet conformation provides mechanical strength to the entire spherical structure. 

Figure 1. Optical (a), fluorescence (b), and SEM (c) images of the SF microspheres. Scale bars: 5 µm. (d) PL spectra of a single SF microsphere upon excitation with cw laser (λex = 450 nm). Each WGM peak is assigned as shown on the bottom. (e) PL spectra of a single SF microsphere upon excitation with fs pulsed laser.

Recently, the group of Prof. Yohei Yamamoto and collaborators of University of Tsukuba have demonstrated that self-assembled SF microspheres, doped with ionic fluorescent dye, display resonant luminescence, which shifts in response to the humidity change. The resonant peaks result from the total internal reflection of the fluorescence, causing interference at the circumference of the microspheres. The SF microspheres display lasing property upon femto-second laser pumping (Figure 1).

The WGM peaks respond to the change of the ambient humidity. When the humidity is low, SF microsphere release water molecules, leading to shrinkage of the SF microsphere and thereby causing the WGM peaks shift to the lower wavelength. In the reversal process where ambient humidity is high, SF microsphere is hydrated, causing the shift WGM of the WGM peaks to the higher wavelength (Figure 2).

Figure 2. (a) Humidity-dependent PL spectra of a single SF microsphere upon excitation with cw laser (λex = 450 nm). (b) Plot of the wavelength of the resonant peak of TE26 upon increasing (filled circle) and decreasing (open circle) the surrounding humidity. (c) Plot of the wavelength of the resonant peak of TE26 upon 6 cycles of hydration (red) and dehydration (blue) between 93 and 25 %RH.

It is interesting to highlight the secondary structure of silk fibroin. Upon treated with alcohol, β-sheet conformation is increased, providing rigid structure for the SF microspheres while the remaining random coil structure in the SF microspheres is interacting with ambient moisture. This combination contributes to the SF microsphere for obtaining a high responsivity and high sensing range toward humidity.

These properties have given a new prospect or direction for designing next generation microresonators for optical sensing or lasing applications.

Prof. Yohei Yamamoto, University of Tsukuba

Yohei Yamamoto is a professor in the department of materials Science, faculty of Pure and applied sciences, University of Tsukuba. He received his doctor degree in 2003 from Osaka University. After post-doctoral researcher term, he was appointed as an associate professor in University of Tsukuba at 2010. on 2018, he promoted to full professor in University of Tsukuba. His research interests are self-assembly of π-conjugated molecules, polymers and biomolecules to construct electronically and optically active nano/micrometer-scale materials. He is the author of more than 90 articles and cited more than 2900 times with an index H = 28.

https://publons.com/researcher/1757952//

Polymer membranes are an important class of materials that find use in a wide variety of fields. The suitability of a polymer membrane often requires careful tuning of their properties to the target application. This must be balanced with the cost of any modification. Hence the non-solvent induced phase separation method (NIPS) is a common route of manufacture for polymer membranes, as it is easy to accomplish on a commercial scale at low-cost.’

In the NIPS method, the polymer of choice is first dissolved in a good solvent, along with any additives, before its immersion in a non-solvent to produce the membrane morphology. This morphology typically shows a dense skin-layer with smaller pores above a layer of larger finger-like vertical pores. By careful choice of additive, some of the membrane properties, including hydrophilicity and microstructure, can be modified.

Poly(ethersulfone) (PES) is a material commonly used for water filtration membranes, chosen for its good mechanical, thermal and chemical properties. Use of an amphiphilic surfactant additive has been shown to produce a membrane with a larger and more well-defined microstructure in the larger finger-like pore regime, as well as increasing the surface hydrophilicity, a key requirement for reduction in biological fouling.

This work by Southern and Evans of the University of Cambridge introduces an additional level of structural hierarchy by the use of a second template molecule, 4-(phenylazo)benzoic acid (PABA), as well as the surfactant Pluronic® F127 (F127) to allow templating of both the dense skin layer and the larger pores. This addition of PABA leads to a more fibrous structure at the 1μm level, leading to higher pore connectivity and permeability, compared to membranes templated only with F127 (Figure 1).

Figure 1a. shows the poor connectivity of the skin layer of a membrane templated with only F127, compared to the fibrous structure of a membrane templated with both F127 and PABA shown in Figure 1b.

Their work demonstrates that this fibrous structure leads to a remarkable increase in flow rate that is improved further by the subsequent removal of the PABA. Extraction using ethanol is shown to provide an excellent approach for removal. This extraction method also allows recycling of the PABA for further membrane manufacture.

This dual-template approach, as part of the NIPS process, can be used to easily modify membrane manufacture, producing membranes exhibiting a hierarchical structure with improved pore connectivity, which could find use as energy materials or in water filtration.

Authors:

Dr Rachel C. Evans

Dr Rachel Evans obtained her MChem and PhD in Physical Chemistry from Swansea University. She was a Marie Curie Postdoctoral Fellow at the Université Paris-Sud, France and subsequently held an FCT research fellowship between the University of Coimbra and the University of Aveiro, Portugal. From 2009-2016, she was an Assistant, then Associate Professor in Physical Chemistry at Trinity College Dublin (TCD). In 2017, Rachel moved back to the UK to take up a University Lectureship at the University of Cambridge in the Department of Materials Science and Metallurgy. Rachel’s research is multidisciplinary and involves polymer, colloid and photophysical chemistry. Her current work is focused on the development of photoactive polymer-hybrid materials for luminescent solar devices, organic photovoltaics and stimuli-responsive membranes. She is a Fellow of the Royal Society of Chemistry and the Institute of Materials, Minerals and Mining and. In 2017, she was awarded the Dillwyn Medal for STEMM from the Learned Society of Wales and the MacroGroup UK Young Researcher’s Medal.

Thomas Southern graduated from the University of Cambridge with an MSci and B.A. in Materials Science. In 2017, Thomas began his PhD as part of the Functional Photoactive Materials group at the Department of Materials Science and Metallurgy, within the University of Cambridge. Thomas’ work, funded by an EPSRC studentship, focuses on hierarchically porous membranes for environmental remediation.

Article information:

Dual-template approach to hierarchically porous polymer membranes
Thomas J. F. Southern and Rachel C. Evans
Mater. Chem. Front., 2021, Advance Article
https://doi.org/10.1039/D0QM00610F

Nanotechnology is an indispensable item in advanced bio-related and life sciences, but a novel concept is necessary to bridge gaps between nanotechnology and biology/materials chemistry. The most suitable concept for this task would be nanoarchitectonics. In this short review article, our recent accomplishments of nanoarchitectonics approaches on cell functions including gene delivery and controlled differentiation are summarized. Regulations of cell activities by nanoarchitected materials are carried out through their interfacial contacts. Our accomplishments are here described according to types of material structural motif, (i) nanotopography, (ii) self-assembled structures, and (iii) composite materials. Finally, several challenging approaches are introduced as frontiers of cell fate regulation at the interfacial media. Developments of artificial materials and systems to regulate bio-organizations including living cells will give intuitions and ideas even to the design of general functional systems. Interfacial nanoarchitectonics could be an important key concept for future advanced life technologies as well as currently required biomedical applications.

Figure 1. Outline of nanoarchitectonics and application to life science at interfaces.

Interfacial structures with various topological and mechanical features affect significantly cell behaviours including cell fates. At insides of living cells, sophisticated mechanisms are working upon relays of functional elements, and these mechanisms can be triggered by the input of external stimuli at the surfaces of cells. Control of surface contact can lead to the regulation of complicated cell functions. Interfacial nanoarchitectonics would be an important key concept for cell regulations for biomedical applications and life sciences.

Article Information

Life science nanoarchitectonics at interfaces
Katsuhiko Ariga, Kun-Che Tsai, Lok Kumar Shrestha and Shan-hui Hsu
Mater. Chem. Front., 2020, Accepted Manuscript
https://doi.org/10.1039/D0QM00615G

Authors Information

Katsuhiko Ariga

National Institute for Materials Science & University of Tokyo

Katsuhiko Ariga received his Ph.D. from Tokyo Institute of Technology in 1990. He is currently the Leader of the Supermolecules Group and Principal Investigator at the World Premier International Research Centre for Materials Nanoarchitectonics, NIMS. He has also been appointed as Professor at the University of Tokyo. He is the author of more than 700 articles indexed by SCI and cited more than 40000 times with an index H = 106 (Sept., 2020)

https://publons.com/researcher/2767466/katsuhiko-ariga/

Shan-hui Hsu

National Taiwan University

Shan-hui Hsu received her Ph.D. degree from Case Western Reserve University (USA) in 1992. She is now the Director for the Doctoral Program of Green Sustainable Materials and Precision Devices and Distinguished Professor at the Institute of Polymer Science and Engineering, National Taiwan University.

The efficiency of water splitting is severely limited by the oxygen evolution reaction (OER), due to sluggish kinetics and a substantial overpotential. To overcome this challenge, precious-metal based catalysts, such as IrO₂ and RuO₂, have been investigated and confirmed to exhibit good OER performance. However, the scarcity and high cost of these materials restrict their large-scale application.

Recently, the group of Derek Ho and collaborators of the City University of Hong Kong have demonstrated a one-step method for the synthesis of Mn doped ultrathin nickel-iron oxide (Mn-Ni-Fe-O) nanosheets, which simultaneously achieves an abundance of oxygen vacancies and high valance Ni³⁺ catalytic sites (Fig. 1). The Mn dopant exists in the form of mixed-valence Mn cations, which contributes to tailoring the electronic structure of the Ni and Fe sites, leading to outstanding OER catalytic performance.

Figure 1. Schematic of the preparation procedure of Mn-Ni-Fe-O nanosheets.

SEM and TEM images of the Mn-Ni-Fe-O hybrid shows 100 – 300 nm interconnected nanosheet structures, having an ultrathin and veil-like morphology (Fig. 2). AFM images show a nanosheet thickness of approximately 3.2 nm. EDX mapping presents that Ni, Fe, Mn, and O elements are uniformly dispersed throughout the nanosheets.

Figure 2. (a) SEM image, (b, c) TEM images and the inset in (c) is the corresponding SAED patterns, (d) HRTEM image, (e) AFM image and the corresponding thickness curve, (f) STEM image and the corresponding element mapping, and (g) EDX spectrum of the Mn-Ni-Fe-O nanosheets.

XRD, XPS, CV, and EPR are also performed (Fig. 3). From XPS, after Mn doping, the Fe 2p_3/2 XPS peak of the Mn-Ni-Fe-O nanosheets shifts to a higher binding energy as compared to that of undoped Ni-Fe-O nanosheets, suggesting that Mn dopant can modulate the charge density of Fe atom sites. Compared to the Ni 2p XPS spectrum of pristine Ni-Fe-O, the Ni 2p XPS spectrum of Mn-Ni-Fe-O nanosheets exhibits an obvious positive shift of 0.3 eV in binding energy, which is attributed to Mn incorporation. From CV curves, the Ni²⁺ oxidation peaks appear at 1.40 and 1.36 V versus RHE for the undoped and doped samples, respectively, indicating the oxidation of Ni species is enhanced upon Mn doping. Also, the O2 ratio (51.0 %) for the Mn-Ni-Fe-O nanosheets is higher than that of the Ni-Fe-O nanosheets (41.9 %), which indicates that Mn dopants can create an enhanced oxygen vacancies concentration.

Figure 3. Characterization data of the Mn-Ni-Fe-O and Ni-Fe-O: (a) XRD patterns, (b) XPS survey spectra, (c) high-resolution XPS spectra for Mn 2p region for Mn-Ni-Fe-O, (d) XPS for the Fe 2p region, (e) XPS for the Ni 2p region, CV curves (scan rate of 50 mV s^-1) of (f) Ni-Fe-O  and (g) Mn-Ni-Fe-O, (h) XPS for the O 1s region, and (i) electron paramagnetic resonance (EPR) spectra of Mn-Ni-Fe-O (2 wt%).

OER electrochemical performance has been investigated in an O₂-saturated KOH (1 M) solution. Upon doping of Mn, polarization curves show an OER overpotential of only 225 mV (vs. undoped at 250 mV) (Fig. 4). Remarkably, these two as-prepared ultrathin nanosheets, with or without Mn doping, exhibit faster OER than the commercial RuO₂. The Mn doped nanosheets exhibit a turnover frequency (TOF) of 0.063 s⁻¹ at the overpotential of 300 mV, which is 3.5 and 12 times higher than that of the undoped sample and commercial RuO₂, respectively. The Tafel slope is 38.2 mV dec^-1 (vs. 65.8 mV dec^-1 undoped and 72.0 mV dec^-1 from RuO₂). Electrochemical impedance spectroscopy (EIS) reveals that the Mn dopants can effectively improve the electrical conductivity.

Figure 4. (a) Polarization curves, (b) TOF, and (c) Tafel slope of Mn-Ni-Fe-O, Ni-Fe-O, and RuO₂. (d) Nyquist slopes of Ni-Fe-O and Mn-Ni-Fe-O, (e) overpotential at 10 mA cm^-2 and Tafel slope of Ni-Fe-O nanosheets with different Mn doping levels, and (f) chronopotentiometry curves of Mn-Ni-Fe-O nanosheets at 30 mA cm^-2.

This work demonstrated a facile method in synthesizing ultrathin Mn-Ni-Fe-O nanosheets that achieve highly efficient OER catalytic performance, providing a sound strategy for the design and synthesis of multi-metallic, atomically-thin oxides nanosheets to mitigate the catalytic limitation of OER, thereby rendering the electrolysis of water a practical form of alternative fuel production.

Information on Corresponding Author

Derek Ho

City University of Hong Kong

Derek Ho is currently an associate professor at the Department of Materials Science and Engineering at City University of Hong Kong. He directs the Atoms to Systems Laboratory. He received his B.A.Sc. (first class) and M.A.Sc. in Electrical and Computer Engineering from the University of British Columbia (UBC), Vancouver, Canada, in 2005 and 2007 respectively. At UBC, he focused his study on microelectronics. He received his Ph.D. in Electrical and Computer Engineering from the University of Toronto, Toronto, Canada in 2013, where he worked on sensors incorporating nanomaterials and CMOS electronics for chemical detection and DNA biosensing applications. Professor Ho’s research interest is in the synthesis of electronic nanomaterials and fabrication of advanced devices. His current research focuses on sensing and energy applications, mainly in the form of stretchable and healable electronics. www.atomstosystems.com

Article information:

Mn dopant induced high-valence Ni3+ sites and oxygen vacancies for enhanced water oxidation

Yu Zhang, Zhiyuan Zeng and Derek Ho

Mater. Chem. Front., 2020, Advance Article

https://doi.org/10.1039/D0QM00300J