Excellent Woman Researcher Award of The Electrochemical Society of Japan
Electrode Applications Using Organic Thin Films with Three-Dimensional Structures
2023 Volume 91 Issue 10 Pages 101008
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2023 Volume 91 Issue 10 Pages 101008
The influence of nano- and micro-structures on various physicochemical phenomena is a subject of interest to many researchers. Physicochemical reactions at solid/electrolyte interfaces, such as crystallization, metal complex formation, adsorption, and electrochemical reactions are influenced by surface modifications with organic thin films. In this study, we review and generalize the findings of our research on the application of organic thin films with or without three-dimensional structures to chiral sensors, interactive motions of nano/micro materials, and electrodes for electrochemical energy devices.
The development of nano- and micro-technology in materials science and technology has had a great impact on both the electric and electronic industries and electrochemical-related fields. These developments have included numerous investigations regarding the effect of surface modifications on physicochemical reactions.
Among the various surface modification methods, the self-assembly method spontaneously modifies the surface by using an organic monolayer, or a self-assembled monolayer (SAM). SAMs were first fabricated more than 75 years ago,1 and research on their application has accelerated over the last 30 years ago because a system capable of fabricating more stable film was conceived.2,3 SAMs typically consist of three groups: (1) a binding group that binds to the substrate surface, (2) a chain group, and (3) a functional group whose intermolecular interactions stabilize the film structure.
In this paper, we present a review of the generalized findings of our research on the application of organic thin films, which mainly includes SAMs with or without three-dimensional structures, to chiral sensors, interactive motions of nano/micro materials, and electrodes for electrochemical energy devices.
Proteins and DNA are essential functional molecules in vivo and possess chirality, and most enantiomers of chiral molecules exhibit different biological activities owing to the prevailing biological homochirality; therefore, when one is physiologically active, the other is inactive or sometimes even toxic. Thus, enantiomer discrimination is an important issue in medical, pharmaceutical, and food-related fields. A chiral sensor is a chemical sensor that discriminates between and detects the quality of enantiomers. Within the chiral sensor, the chiral recognition sensor discriminates between the enantiomers, and the transducer converts the recognition event into a signal. In this study, we investigate the utilization of SAMs for both the recognition sensor4–14 and transducer6,8,12,15 in electrochemical/electrical chiral sensor applications.
SAM is usually used as a platform to immobilize enantioselective biological molecules such as enzymes in the fabrication of chiral sensors.16,17 Although biomolecules generally show high substrate specificity, their structures are often unstable under harsh environments, for example, where the temperature and pH differ from those in vivo. Some of our studies focused on using the thin films of chiral amino acids rather than biological molecules as the recognition sensor. In these studies, various physicochemical reactions, such as crystallization,4,9,13 electrochemical reaction,5,7,10,11 and complex formation6,8 have been investigated as recognition events.
For crystallization, enantioselective crystallization was initialized by an enantiomer of leucine (Leu) attached to the end of a SAM on Au,4 and the enantioselective mass change on the surface was monitored by a quartz crystal microbalance (QCM), which acted as a transducer. Here, enantioselective crystallization is the result of matching the two-dimensional arrangement of an immobilized enantiomer with that of a crystal nucleus.4 The rate of mass change, which is considered to be due to crystal nucleation, increased without decreasing the enantioselectivity by increasing the root-mean-square (RMS) roughness of a Au substrate from 2 to 17 nm.13 Here, the roughness factor, i.e., the real surface area/geometrical area, of each Au substrate with an RMS roughness of 2 and 17 nm was 1.3 and 2.5, respectively.13 Thus, one possible explanation for the increase in the mass change rate is that the enantioselective nucleation rate is increased by the large number of an immobilized enantiomer per unit geometrical area owing to the large real surface area. Surface roughening enables the clear and quantitative observation of the enantioselective mass change, which is used as the sensing signal.13
For an electrochemical reaction, Au(111) electrodes are modified with cysteine7 and homocysteine (Hcy)5,10,11 SAMs with their thiol group directly bound to the Au(111) surface; these electrodes are used for chiral discrimination. The enantioselectivity of the redox reaction of 3,4-dihydroxy phenylalanine (DOPA) is significantly influenced by the surface coverage of the electrode using a SAM,7 concentration of target molecules in the solution,10 and pH of the solution.5 Detailed investigations, including a structural analysis of the SAM for each pH solution, suggested that high enantioselectivity occurs when a chiral space similar in size to that of the target molecule exists in a stable film structure, such as the SAM of Hcy, on Au(111) in an acidic solution, as shown in Fig. 1.5 A key factor for high enantioselectivity is the steric and chemical interaction of DOPA with the Hcy SAM on Au(111) in the redox process. The chiral sensing achieved overwhelmingly high selectivity under the optimal conditions investigated, where d-form was detected as a current peak and no apparent current peak was detected for l-form with l-Hcy, and vice-versa for d-Hcy, as shown in Figs. 1a and 1b.
Cyclic voltammograms for the DOPA redox reactions on Au electrodes modified with (a) l- and (b) d-Hcy in an aqueous 0.25 M H2SO4 solution. Red and blue curves represent the voltammograms for 40 μM l- and d-DOPA, respectively. (c) Cyclic voltammograms obtained with a bare Au electrode for l-DOPA. Scan rate of 5 mV s−1. (d) In situ scanning tunneling microscope (STM) images of the l-Hcy monolayer on Au(111) in 0.05 M H2SO4 (pH = 0.6, recorded at different magnifications; Itip. = 0.84 nA; Etip. = −0.11 V; Eelectrode = 0.40 V). The corresponding structural model, ($2\sqrt{3} $ × $3\sqrt{3} $)R30°, is presented in (e, f, g). Each model is depicted by a (e) top and (f) side views of an Hcy dimer and (g) a DOPA molecule on the same scale. Arrows indicate the 121 direction of the Au(111) surface. (a, b, c) are reproduced with the permission of The America Chemical Society.10 (d, e, f, g) are reproduced with the permission of Wiley-VCH.5
For metal complex formation, the Hcy SAM was used in a 0.25 M K2SO4 solution with a pH of 5.5. Metal complex formation has been applied in chiral discrimination using chromatography, commonly referred to as chiral chromatography, in the field of chiral separation.18 In chiral chromatography using metal complex formation, target enantiomers are eluted out of a column at different rates through ligand exchange with metal ions and the chiral molecules immobilized on the column. We propose a method in which enantioselective metal complex formations are applied to electrochemical6 and electrical8 chiral sensing. In electrochemical detection, the difference in chemical stability of the metal complexes between enantiomers can be clearly discriminated through the electrochemical reduction of Cu(II) as a difference in peak current.6 Notably, the Hcy SAM functions not only as a chiral ligand for the metal complex but also as a catalyst for the underpotential deposition of Cu, thus enabling the highly sensitive detection of Cu(II). However, practically, this system is unsuitable for multiple uses because complete regeneration of the surface of the electrodes after detection is difficult. In electrical detection, the electric charge of copper ions with the accompanying complex formation on the surface can be used to quantitatively detect target amino acids owing to the change in gate potential via field-effect transistor (FET).8 In the development of FET as a transducer for sensing in a solution, the operation was stabilized by forming a SAM using a SiO2 gate terminated with the thiol group of (3-mercaptopropyl)trimethoxysilane (MPTMS) to strengthen the adhesion between the Au and SiO2 surfaces, as shown in Fig. 2a, and prevent water from entering through the gate. Figures 2b and 2c shows the time course of the enantioselective responses of FETs whose gates were modified with l- and d-Hcy, to d- and l-alanine (Ala). For l-Hcy, the gate potential changed only in the l-Ala solution when Cu(II) was added, whereas change occurred in the d-Ala solution with d-Hcy.
(a) Schematic diagram of the FET with a gate composed of Hcy/Au/MPTMS/SiO2. The expanded view of the gate shows the Cu-complex formation on Hcy with Ala. Relationship between Vg and time after the addition of Ala and Cu(II), (b) l-Hcy SAM-modified gate, and (c) d-Hcy SAM-modified gate. Dashed and dotted vertical lines indicate the addition of Ala and Cu(II), respectively. The final concentration of Ala and Cu(II) is 40 and 20 μM, respectively. Reproduced with permission of Elsevier Ltd.8
Thus, while the l-Hcy SAM forms a stable or rigid Cu(II) complex with l-Ala to achieve a Cu(II) charge effect at the gate surface, the same outcome is not obtained with d-Ala. The opposite result is observed in the case with the d-Hcy SAM. The above-mentioned electrochemical and electrical sensing of chiral molecules using Cu(II) as a detection probe expands the range of applicable target chiral molecules and enables chiral sensing, even when the target molecules are not electrochemically active.
The electronic sensing principle that focuses on potential distribution changes at the electrode/electrolyte interface has been expanded to chiral sensing, even with a protein.12,19 In these studies, serum albumin, which is a main component of the chiral recognition sensor, was chemically immobilized on an amino-terminated SAM-modified indium tin oxide (ITO) electrode, as shown in Fig. 3a. The electrode quantitatively responded to certain indole compounds, including tryptophan (Trp), indicating enantioselectivity to the potential change, where the shift to the base potential was greater for d-form than l-form. This enantioselectivity is based on the enantioselective interaction of human serum albumin (HSA) with Trp, which was also confirmed in studies on chromatography.20,21 An enantiomer, whose retention time is shorter than another enantiomer on the column in studies on chromatography,20,21 is found to have a more intense potential response.19 The highly selective response of this electrode to the indole moiety12 is unexpected, and the cause remains unclear; however, the binding site of HSA to indoles could be a factor responsible for this phenomenon.
(a) Schematic diagram of the ITO electrode at each modification step. 3-aminopropyltriethoxysilane (APTES) and disuccinimidyl suberate (DSS) are used. (b) Potential response of the HSA-modified ITO electrode for Trp enantiomers at various concentrations. (c) Time dependence of the potential shift after injecting the Trp solution. Reproduced with permission of Elsevier Ltd.6
Based on our investigations described above, the concentration detection range tends to be narrower for sensing with a thin-film modified flat electrode when electrochemical redox reactions are used rather than electrical detection. Because increasing the surface area is one approach to expand the detection concentration range, especially for detection using the redox reaction of target molecules, our attention has recently been focused on mesoporous Pt electrodes with a large electrochemically-active area and abundant high-index surfaces.22 These electrodes can be applied in various electrochemical systems, including chemical/chiral sensors,14,23–26 because the high-index metal surfaces with significant surface energy exhibit different electrochemical activities from that low-index surfaces did,22 and certain high-index surfaces are enantioselective.27–30 The detailed mechanism of chiral sensing using this system will be reported elsewhere. The effects of mesoporous film morphology14 and the type and concentration of electrolytes23–25 on analyte sensitivity and selectivity in chemical sensing have also been investigated to determine its applicability as an electrode for a non-enzymatic chemical sensor of glucose. In an alkaline solution, the electrode became more sensitive, that is, a much larger oxidation peak current can be observed for glucose of the same concentration by maintaining substrate-selectivity for glucose sensing than in acidic solutions.25
For the electrochemical chiral sensor mentioned in section 2, a three-dimensional structure is expected to be an attractive material for expanding the surface area to widen the detection concentration range. Nowadays, various nanosized or mesosized materials are utilized to increase surface area. Bristles, or pillars in other words, are examples. We focus our attention on their unique self-assembly behaviors in liquid volatilization31 because many properties including mechanical, chemical (/electrochemical), and physical (/optical) can be changed with the use of either a pillar, cluster, or in-between phase structure. Here, we investigate the motion control of micropillars with a high aspect ratio, via surface modification with organic thin films.32
Figure 4 shows a schematic diagram of the experimental system using micropillars. To investigate the effect of surface chemistry, the surface of micropillars made from epoxy resin is completely covered by a flat Au film before modification with organic thin films to maximally exclude the effects of chemicals other than an organic thin film. Then, the Au surface was modified with an organic thin film using the self-assembled method. A pillar, having any aspect ratio, in a solvent will start clustering during solvent volatilization when the surface tension, which causes an attraction with the pillar, overwhelms the elastic force of the pillar (Fig. 4c).31 Our investigations, using various SAMs with different functional groups and chain lengths, demonstrate that the combination of a solvent with SAM, which modifies the pillar surface, changes the possibility, reversibility, and timing of the transition of the pillar to cluster structures.32 For example, stable clusters can be formed using SAMs with either carboxyl or hydroxy groups, which form hydrogen bonds, and are easily disassembled using water or organic solvents, such as ethanol, acetone, and chloroform (Fig. 4i). A solvent with a lower dielectric constant will disassemble the clusters faster than one with a higher dielectric constant. In addition, clusters can be reformed and disassembled by repeating the treatment with an appropriate solvent, followed by drying. In contrast, stable clusters can be formed by using a thiol-terminated SAM, which forms a disulfide bond and is stable, even in solvents (Fig. 4j). Furthermore, pillars can be converted to clusters position-selectively via micropattern technology, as shown in Figs. 5b and 5c.
Schematic diagram of the experimental system depicting cluster formation. (a) Initial upright position. (b) Start of solvent evaporation, where C is the capillary force, E is the elastic force, and A is the adhesion force. If C > E, (c) the pillars are brought together, and if E > C, (d) the pillars are left standing. After solvent evaporation, A maintains the clusters instead of C. Here, the probability of stable cluster formation depends on the magnitude of A, or A1, relative to E. The introduction of the solvent first changes A from A1 to A2, and thereafter the probability of cluster formation changes on demand. Reproduced with permission of The American Chemical Society from Scheme 1.32
Cluster patterning by stamping 11-mercaptoundecanoic acid (C10-COOH) or 1-dodecanethiol (C11-CH3) on a Au surface through microcontact printing. (a) The right half is modified by C10-COOH. (b) The right half is modified by C11-CH3. (c) Patterning of C10-COOH in an array of circles. White outlines indicate the stamped areas (diameter: 100 µm). Reprinted with permission of The American Chemical Society from Fig. 4.32
This simple adjustment method to balance the adhesion and elasticity will enable the design of dynamic, responsive, and reversible self-assembled systems using various micro-structured materials on the mesoscale for various applications.
The use of electrochemical energy conversion systems such as lithium-ion batteries (LIBs), capacitors, fuel cells, dye-sensitized solar cells, and photocatalysts including green hydrogen generation are expected to play an active role in energy savings toward a low-carbon society. It is imperative that these applications are safe, improve their energy conversion efficiency consumption, and extend their life to reduce life cycle costs.
Organic thin films formed at the electrode/electrolyte interface, commonly known as the solid electrolyte interphase (SEI), of the LIB anode contribute to the electrode stabilization, and thus safety. Because the physicochemical changes of the film are possibly related to the lifespan of the LIBs, an analysis of the change is important. Electrochemical impedance spectroscopy is a non-destructive method to analyze electrochemical properties. In addition, the electrical information of each elementary process of the electrochemical reaction can be investigated by using an equivalent circuit. Figures 6a and 6b show the equivalent circuit and Nyquist plots, respectively, of a LIB at different temperatures. Our investigation33 demonstrated that new information can be obtained through impedance spectrum analyses at low temperatures below 0 °C. At such low temperatures, certain reactions, which may have time constants similar to those of other reactions, and/or exhibit relatively low resistance, can be distinguished using Nyquist plots, as shown in Fig. 6b. Thus, an examination of the temperature characteristics over a wide temperature range that includes low temperatures will expand the current knowledge base.
(a) Equivalent circuit designed to analyze the LIB impedance, where L and RI are the inductance and resistance, respectively, of the battery lead and connected cable; RS and RF are the resistances of electrolyte and SEI, respectively; CPEF is the constant phase element of the SEI; RA and RC are the charge transfer resistances of the anode and cathode, respectively; and CPEA and CPEC are the constant phase elements of the electrode surface of the anode and cathode, respectively. (b) Nyquist plots obtained via electrochemical AC impedance for an LIB at −20, 0, 5, 10, 15, and 20 °C. Each parallel line represents 0 Ω of Z′′ at each temperature. Inset shows the magnified Nyquist plots for 20 °C. Reproduced with permission of Elsevier Ltd.33
To improve energy conversion efficiency, the electrode must decrease its total resistance. One approach to achieve this is to increase in the reaction surface area by using a porous material. We investigated the effective utilization of electro-deposition methods to prepare an electrode with three-dimensional structures. To change the surface morphology of a three-dimensional structure, zinc oxide nanorods and nanotubes with different surface morphologies were prepared by individually controlling the chemical etching and zinc oxide formation rates by optimizing the reaction time and temperature (Fig. 7).34
Scanning electron microscope (SEM) images of ZnO nanotubes electrodeposited on fluorine-doped tin oxide (FTO) glass at −1.0 V (vs. Ag/AgCl (KCl saturated)) under the following conditions of (a) 30 min and 70 °C, (b) 60 min and 70 °C, (c) 30 min and 90 °C, and (d) 60 min and 90 °C. (a, c) are reprinted from the permission of The Electrochemical Society, Inc.34 © The Electrochemical Society, Inc. [2015]. All rights reserved. Except where provided under U.S. copyright law, this work may not be reproduced, resold, distributed, or modified without the express permission of The Electrochemical Society (ECS). The archival version of this work was previously published.34
Another approach to decrease the total resistances is the development of electrodes using a combination of various conductive materials. Among such materials, nanocarbons have attracted increasing attention in recent years as electrode components for energy conversion and storage applications owing to their excellent electrical properties, mechanical strength, chemical stability, and large surface area.35 The high conductivity of carbon nanotubes (CNTs) was reported in 1996,36 thereafter, CNTs were applied to electrodes in fuel cells and transistors at the beginning of the 21st century. The number of reports on hybrid film electrodes with CNTs has increased significantly in the last decade. Our investigations demonstrated that thin-film electrodes composed of nanocarbons with high structural stability can be prepared by using a solvent containing silane-coupling agents through various methods, including electrophoretic deposition. Remarkably, the performance of electrodes prepared using highly dispersible nanocarbons like carbon Nano-horn is superior.37 Various thin-film electrode structures have been prepared through electrophoretic deposition by using nanocarbons and have been used in many energy systems.38–40 In other studies, various surface modification methods were applied, and the electrochemical properties of the modified nano-carbons were investigated.41–44 Further results, which have been reported in various conference papers, will be explained in future papers upon extensive analysis.
Various physicochemical phenomena of two- and three-dimensional structures modified with organic thin monolayers have been investigated for electrode applications for several electrochemical devices. This paper reviewed our investigations on the effect of organic thin films on chiral discrimination, electrical/electrochemical properties, and pillar aggregation. These properties change, in combination, an electrode performance for each application. The knowledge obtained from these studies can not only be utilized for the development of specific electrochemical devices but can also be applied for promoting research in a wider range of fields in the near future.
The author is deeply grateful to Professor Tetsuya Osaka at Waseda University, Professor Joanna Aizenberg at Harvard University, and the co-authors in related research for their helpful input at each stage. The author also expresses her gratitude to the past and present members of her laboratory for together pushing forward our research. The author acknowledges financial support by the Grant-in-Aid for JSPS Fellows, MEXT, Marubun Research Promotion Foundation, JGC-S Scholarship Foundation, the Chuo University Joint Research Grant (2013–2014), Grant for Special Research (2017–2018), and JSPS KAKEN (No. 12024046), MEXT.
Mariko Matsunaga: Investigation (Lead), Writing – original draft (Lead)
The authors declare no conflict of interest in the manuscript.
A part of this paper has been presented in the, 64th Annual Meeting of the International Society of Electrochemistry in 2013 (Presentation #S12-010), The 2015 Spring Meeting of the European Material Research Society, ECSJ Fall Meeting in 2016 (Presentation #PS31), Chemical Sensor Symposium in 2017 (Presentation #25), 2018 (Presentation #1G03), and 2019 (Presentation #1Q02, #2M01). The 2017 and 2018 Fall Meeting of the European Material Research Society.
M. Matsunaga: ECSJ Active Member
Mariko Matsunaga (Associate Professor, Chuo University)
Mariko Matsunaga received her Ph.D. in engineering from Waseda University in 2008. She worked as a JSPS Research Fellow in 2005–2008 during doctoral course. After the postdoctoral researcher at Waseda University, and Harvard University, she moved to Chuo University as an Assistant Professor in 2012 and promoted to Associate Professor in 2017.
