Yongmin Ko

Electrically bistable properties of layer-by-layer assembled multilayers based on protein nanoparticles

Electrochemical properties of redox proteins, which can cause the reversible changes in the resistance according to their redox reactions in solution, are of the fundamental and practical importance in bioelectrochemical applications. These redox properties often depend on the chemical activity of transition metal ions as cofactors within the active sites of proteins. Here, we demonstrate for the first time that the reversible resistance changes in dried protein films based on ferritin nanoparticles can be caused by the externally applied voltage as a result of charge trap/release of Fe(III)/Fe(II) redox couples. We also show that one ferritin nanoparticle of about 12 nm size can be operated as a nanoscale-memory device, and furthermore the layer-by-layer assembled protein multilayer devices can be extended to bioinspired electronics with adjustable memory performance via molecular level manipulation.

Hydrophobic nanoparticle-based nanocomposite films using in situ ligand exchange layer-by-layer assembly and their nonvolatile memory applications.

A robust method for preparing nanocomposite multilayers was developed to facilitate the assembly of well-defined hydrophobic nanoparticles (i.e., metal and transition metal oxide NPs) with a wide range of functionalities. The resulting multilayers were stable in both organic and aqueous media and were characterized by a high NP packing density. For example, inorganic NPs (including Ag, Au, Pd, Fe₃O₄, MnO₂) dispersed in organic media [corrected]were shown to undergo layer-by-layer assembly with amine-functionalized polymers to form nanocomposite multilayers while incurring minimal physical and chemical degradation of the inorganic NPs. In addition, the nanocomposite multilayer films formed onto flat and colloidal substrates could directly induce the adsorption of the electrostatically charged layers without the need for additional surface treatments. This approach is applicable to the preparation of electronic film devices, such as nonvolatile memory devices requiring a high memory performance (ON/OFF current ratio >10(3) and good memory stability).

Organic Field-Effect Transistor Memory Devices Using Discrete Ferritin Nanoparticle-Based Gate Dielectrics

Organic field-effect transistor (OFET) memory devices made using highly stable iron-storage protein nanoparticle (NP) multilayers and pentacene semiconductor materials are introduced. These transistor memory devices have nonvolatile memory properties that cause reversible shifts in the threshold voltage (Vth ) as a result of charge trapping and detrapping in the protein NP (i.e., the ferritin NP with a ferrihydrite phosphate core) gate dielectric layers rather than the metallic NP layers employed in conventional OFET memory devices. The protein NP-based OFET memory devices exhibit good programmable memory properties, namely, large memory window ΔVth (greater than 20 V), a fast switching speed (10 μs), high ON/OFF current ratio (above 10(4)), and good electrical reliability. The memory performance of the devices is significantly enhanced by molecular-level manipulation of the protein NP layers, and various biomaterials with heme Fe(III) /Fe(II) redox couples similar to a ferrihydrite phosphate core are also employed as charge storage dielectrics. Furthermore, when these protein NP multilayers are deposited onto poly(ethylene naphthalate) substrates coated with an indium tin oxide gate electrode and a 50-nm-thick high-k Al2 O3 gate dielectric layer, the approach is effectively extended to flexible protein transistor memory devices that have good electrical performance within a range of low operating voltages (<10 V) and reliable mechanical bending stability.

Flexible supercapacitor electrodes based on real metal-like cellulose papers

The effective implantation of conductive and charge storage materials into flexible frames has been strongly demanded for the development of flexible supercapacitors. Here, we introduce metallic cellulose paper-based supercapacitor electrodes with excellent energy storage performance by minimizing the contact resistance between neighboring metal and/or metal oxide nanoparticles using an assembly approach, called ligand-mediated layer-by-layer assembly. This approach can convert the insulating paper to the highly porous metallic paper with large surface areas that can function as current collectors and nanoparticle reservoirs for supercapacitor electrodes. Moreover, we demonstrate that the alternating structure design of the metal and pseudocapacitive nanoparticles on the metallic papers can remarkably increase the areal capacitance and rate capability with a notable decrease in the internal resistance. The maximum power and energy density of the metallic paper-based supercapacitors are estimated to be 15.1 mW cm−2 and 267.3 μWh cm−2, respectively, substantially outperforming the performance of conventional paper or textile-type supercapacitors.With ligand-mediated layer-by-layer assembly between metal nanoparticles and small organic molecules, the authors prepare metallic paper electrodes for supercapacitors with high power and energy densities. This approach could be extended to various electrodes for portable/wearable electronics.

Amphiphilic Layer-by-Layer Assembly Overcoming Solvent Polarity between Aqueous and Nonpolar Media

We introduce a general and versatile methodology that allows a facile incorporation of the functional components with completely different chemistry of hydrophilic/hydrophobic properties within nanocomposite films, and furthermore combine a number of the distinctive advantages of traditional electrostatic layer-by-layer (LbL) assembly in aqueous media and covalent LbL assembly in nonpolar media. Our approach, amphiphilic LbL assembly, is based on the high affinity between sulfonic (or phosphonic) acid-functionalized materials in aqueous media and hydrophobic metal oxide (or metal) NPs stabilized by oleic acid (OA) in nonpolar solvent. For demonstrating the effectiveness of our approach, we show that amphiphilic LbL assembly can be easily applied to the preparation of functional colloid materials allowing the reversible phase transfer between aqueous and nonpolar media, and supercapacitor electrodes with high volumetric capacitance (280 F·cm(-3) at 10 mV·s(-1)) using reduced graphene oxide with sulfonic acid moieties and well-defined OA-Fe3O4 NPs.

Electrochemical sensors based on porous nanocomposite films with weak polyelectrolyte-stabilized gold nanoparticles

Porous hybrid multilayer films composed of cationic poly(allylamine hydrochloride) (PAH)- and anionic poly(acrylic acid) (PAA)-stabilized gold nanoparticles (AuNPs) (i.e., PAH-AuNPs and PAA-AuNPs) were prepared on indium tin oxide (ITO) electrodes using pH-controlled layer-by-layer (LbL) assembly method with subsequent acid treatment. The exponential growth of AuNP deposition layers was caused by the “in-and-out” diffusion of PAH and PAA chains not bound to AuNPs. Immersion of the films in an acidic solution (pH 2.4) converted the nonporous films to porous films via the disruption of ionic bonds and the rearrangement of free PE chains. In this case, the pH-induced porous films showed high electrochemical activity. Nonporous/dense films were found to prevent direct contact between probe molecules in solution and the catalytic components immobilized on an electrode. Electrodes coated with porous films, however, exhibited higher electrocatalytic activity toward nitric oxide oxidation compared with electrodes coated with nonporous films, despite the same levels of AuNP loading. This work demonstrates that structural transformations via a facile pH treatment can significantly improve electrode sensitivity without the aid of porous supports or additional catalytic components.

Layer-by-layer assembled (high-energy carbon nanotube/conductive carbon nanotube)n nanocomposites for high volumetric capacitance supercapacitor electrodes

We introduce high-performance ultrathin electrochemical electrodes based on multi-stacking of high-energy multiwall carbon nanotube (MWCNT) hybrids and conductive MWCNTs. The MWCNT hybrids coated with oleic acid-stabilized pseudocapacitive nanoparticles (i.e., OA-PC–MWCNTs) were assembled via a sequential covalent-bonded layer-by-layer (LbL) approach with amine-functionalized MWCNTs (NH2–MWCNT) in organic media, generating a highly porous structure and allowing for precise nanoscale control of the electrode thickness. The resultant NH2–MWCNT/OA-PC–MWCNT multilayer electrodes exhibited a high energy capacity and remarkable operational stability, considerably higher than the capacity and stability of conventional blended nanocomposite or electrostatic LbL-assembled electrodes. The volumetric capacitances of the (NH2–MWCNT/OA–Fe3O4–MWCNT)20 and (NH2–MWCNT/OA–MnO–MWCNT)20 were approximately 394 ± 10, and 674 ± 13 F cm−3 at 1 A cm−3, respectively. Additionally, these electrodes maintained their high volumetric capacitances without loss of initial capacitance even after 10000 cycles; this cycling stability stemmed from the formation of chemically stable covalent bonds between the MWCNT hybrids and NH2–MWCNTs and between the PC NPs and NH2–MWCNTs. Given that a variety of PC NPs can be used to prepare MWCNT hybrids and that this approach can be further expanded to nanocomposite films including LbL-assembled multilayers, our approach may provide a promising platform for designing electrodes for use as thin film-type energy storage devices.