Jinhan Cho

Fabrication of Highly Ordered Multilayer Films Using a Spin Self‐Assembly Method

This work was supported by the Ministry of Education through the Brain Korea 21 Program at Seoul National University and by the National Program for Tera-level Nano-devices of the Ministry of Science and Technology as one of the 21st century Frontier Programs as well as by the Korean Ministry of Science and Technology (MOST) under Grant 99-07. X-ray reflectivity experiments performed at the Pohang Light Source (PLS) were supported in part by MOST and POSCO. We are very grateful to S.-H. Lee, H. Kang, J. Koo, and B. H. Seung for their assistance during the X-ray reflectivity experiments.

Stabilization of Polymer‐Hydrogel Capsules via Thiol–Disulfide Exchange

Polymer hydrogels are used in diverse biomedical applications including drug delivery and tissue engineering. Among different chemical linkages, the natural and reversible thiol-disulfide interconversion is extensively explored to stabilize hydrogels. The creation of macro-, micro-, and nanoscale disulfide-stabilized hydrogels commonly relies on the use of oxidizing agents that may have a detrimental effect on encapsulated cargo. Herein an oxidization-free approach to create disulfide-stabilized polymer hydrogels via a thiol-disulfide exchange reaction is reported. In particular, thiolated poly(methacrylic acid) is used and the conditions of polymer crosslinking in solution and on colloidal porous and solid microparticles are established. In the latter case, removal of the core particles yields stable, hollow, disulfide-crosslinked hydrogel capsules. Further, a procedure is developed to achieve efficient disulfide crosslinking of multilayered polymer films to obtain stable, liposome-loaded polymer-hydrogel capsules that contain functional enzymatic cargo within the liposomal subcompartments. This approach is envisaged to facilitate the development of biomedical applications of hydrogels, specifically those including fragile cargo.

Multifunctional Colloids with Optical, Magnetic, and Superhydrophobic Properties Derived from Nucleophilic Substitution-Induced Layer-by-Layer Assembly in Organic Media

We demonstrate the successful preparation of multifunctional silica colloids by coating with 2-bromo-2-methylpropionic acid (BMPA)-stabilized quantum dots (BMPA-QDs) and BMPA-stabilized iron oxide particles (BMPA-Fe(3)O(4)), along with amine-functionalized poly(amidoamine) (PAMA) dendrimers, using layer-by-layer (LbL) assembly based on a nucleophilic substitution (NS) reaction between the bromo and amine groups in organic media. The QDs and Fe(3)O(4) nanoparticles used in this study were directly synthesized in a nonpolar solvent (chloroform or toluene), and the oleic acid stabilizers were exchanged with BMPA in the same solvent to minimize chemical and physical damage to the nanoparticles. The direct adsorption of nanoparticles via an NS reaction in organic solvent significantly increased the packing density of the nanoparticles in the lateral dimensions because electrostatic repulsion between neighboring nanoparticles was absent. The multifunctional colloids densely coated with nanoparticles showed excellent characteristics (i.e., superparamagnetism, photoluminescence, and magneto-optical tuning properties) with long-term stability in nonpolar solvents. Furthermore, deposition of the nanocomposite colloids onto flat substrates, followed by coating with a low-surface-energy fluoroalkylsilane polymer, produced a densely packed rugged surface morphology in the colloidal films that displayed superhydrophobic properties with water contact angles greater than 150°.

Free-standing nanocomposite multilayers with various length scales, adjustable internal structures, and functionalities.

We introduce an innovative and robust method for the preparation of nanocomposite multilayers, which allows accurate control over the placement of functional groups as well as the composition and dimensions of individual layers/internal structure. By employing the photocross-linkable polystyrene (PS-N(3), M(n) = 28.0 kg/mol) with 10 wt % azide groups (-N(3)) for host polymer and/or the PS-N(3)-SH (M(n) = 6.5 kg/mol) with azide and thiol (-SH) groups for capping ligands of inorganic nanoparticles, nanocomposite multilayers were prepared by an efficient photocross-linking layer-by-layer process, without perturbing underlying layers and nanostructures. The thickness of individual layers could be controlled from a few to hundreds of nanometers producing highly ordered internal structure, and the resulting nanocomposite multilayers, consisting of polymer and inorganic nanoparticles (CdSe@ZnS, Au, and Pt), exhibit a variety of interesting physical properties. These include prolonged photoluminescent durability, facile color tuning, and the ability to prepare functional free-standing films that can have the one-dimensional photonic band gap and furthermore be patterned by photolithography. This robust and tailored method opens a new route for the design of functional film devices based on nanocomposite multilayers.

Water-Based Thixotropic Polymer Gel Electrolyte for Dye-Sensitized Solar Cells

For the practical application of dye-sensitized solar cells (DSSCs), it is important to replace the conventional organic solvents based electrolyte with environmentally friendly and stable ones, due to the toxicity and leakage problems. Here we report a noble water-based thixotropic polymer gel electrolyte containing xanthan gum, which satisfies both the environmentally friendliness and stability against leakage and water intrusion. For application in DSSCs, it was possible to infiltrate the prepared electrolyte into the mesoporous TiO2 electrode at the fluidic state, resulting in sufficient penetration. As a result, this electrolyte exhibited similar conversion efficiency (4.78% at 100 mW cm(-2)) and an enhanced long-term stability compared to a water-based liquid electrolyte. The effects of water on the photovoltaic properties were examined elaborately from the cyclic voltammetry curves and impedance spectra. Despite the positive shift in the conduction band potential of the TiO2 electrode, the open-circuit voltage was enhanced by addition of water in the electrolyte due to the greater positive shift in the I(-)/I3(-) redox potential. However, due to the dye desorption and decreased diffusion coefficient caused by the water content, the short-circuit photocurrent density was reduced. These results will provide great insight into the development of efficient and stable water-based electrolytes.

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.

Layer-by-Layer Growth of Polymer/Quantum Dot Composite Multilayers by Nucleophilic Substitution in Organic Media†

Layer-by-layer (LbL) self-assembly is a versatile and simple methodology for growing polymer and polymer/inorganic nanoparticle hybrid multilayer thin films with controlled chemical composition and thickness on the nanometer scale. Traditional LbL assembly is carried out in aqueous media and is based on the electrostatic attraction between two oppositely charged materials, such as polycations and polyanions. The recent progress in utilizing hydrogen bonding, click chemistry, disulfide bonding, silanization, esterification, urethane linking, amidation, and so forth, for LbL self-assembly has allowed the growth of multilayer thin films in polar solvents, mainly water and/or alcohols. To our knowledge, LbL self-assembly for functional organic/inorganic nanocomposites has not yet been implemented in nonpolar solvents. Herein we report the first success in using a nucleophilic substitution reaction for LbL self-assembly of organic/inorganic multilayers in nonpolar solvents. Based on a nucleophilic substitution reaction between Br and NH2, alternating layers of highly hydrophobic CdSe@ZnS quantum dots (QDs) capped with 2-bromo-2-methylpropionic acid (BMPA) in toluene or hexane and poly(amidoamine) dendrimer (PAMA) in ethanol were deposited to form QD/ PAMA composite multilayer thin films. The resulting thin films exhibited more robust photoluminescence (PL) in air (oxidation) and in the presence of moisture (hydrolysis) than those obtained by electrostatic LbL self-assembly. These results also demonstrate the possibility of LbL growth of patterned films based on nucleophilic substitution with the aid of microcontact printing. Photoluminescent (and electroluminescent) polymer/QD nanocomposite films are quite important in technical applications and may be used as functional components in electronic devices, such as optical thin films, or for biomedical imaging. Nevertheless, there has been limited success in fabricating polymer/QD composite thin films using the LbL self-assembly techniques developed to date, because the PL properties of the embedded QDs are usually poor. Conventional LbL self-assembly techniques are carried out in aqueous or polar media, which means that the QDs, which are produced either directly in aqueous or polar media or obtained through ligand exchange or phase transfer, have poor surface passivation, which makes the PL of the resulting QDs vulnerable either during LbL self-assembly or during the thin film storage. Recent studies have shown that a high packing density of small and hydrophilic thiol ligands reduces the quantum yield of QDs significantly. Kotov et al. reported that the PL intensity of composite multilayer thin films of polyelectrolyte and citrate-stabilized CdSe@CdS QDs was increased by 50–500 times after ambient light irradiation for several days owing to surface oxidization on the QDs with ambient oxygen for 3 days, which was accompanied by a notable blue shift in the PL bands with exposure time. To date, the growth of polymer/QD multilayer thin films that preserve the original PL behavior of the QDs is a significant challenge for LbL self-assembly. To circumvent this challenge, we prepared LbL-assembled highly hydrophobic BMPA-stabilized CdSe@ZnS QDs in nonpolar solvents based on nucleophilic substitution of the terminal Br groups of a BMPA coating with the NH2 groups of PAMA. CdSe@ZnS QDs consisting of 4 nm CdSe cores and 1 nm ZnS shells stabilized by oleic acid were prepared in hexane or toluene according to a reported method. The original oleic acid stabilizer ligands were replaced with BMPA through ligand exchange, leading to BMPA-stabilized CdSe@ZnS QDs, denoted BMPA-QDs (see the Supporting Information, Figure S1). Ligand exchange reduced the quantum yield of the QDs (relative to coumarin 545) from 59 % to 30%. For comparison, the oleic acid stabilizers of the QDs were also replaced with mercapto acetic acid (MAA) through phase transfer (from toluene to aqueous media) to form negatively charged MAA-coated QDs, denoted MAA-QDs (see experimental details in the Supporting Information). In this case, the relative quantum yield of the resulting MAA-QDs at pH 9 was approximately 9 %, which shows that using hydrophilic thiol ligands to replace the original hydrophobic ligands caused a dramatic decrease in the PL quantum yield. [*] B. Lee, Y. Kim, S. Lee, Prof. J. Cho School of Advanced Materials Engineering Kookmin University, Seoul 136-702 (Korea) E-mail: jinhan@kookmin.ac.kr

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).

Desalination membranes from pH-controlled and thermally-crosslinked layer-by-layer assembled multilayers

We introduce a novel and facile approach to improve the desalination performance of pressure-driven layer-by-layer (LbL) assembled membranes. Electrostatic LbL multilayers composed of weak polyelectrolytes (PEs), e.g., cationic poly(allylamine hydrochloride) (PAH) and anionic poly(acrylic acid) (PAA), were prepared on commercial polysulfone substrates. In order to measure the ion rejection and permeate flux of these membranes, the ionic concentration of the feed solution and operating pressure were fixed at 2000 ppm NaCl and 20 bar, respectively. It was observed that the crosslinked (PAH pH 7.5/PAA pH 3.5)n=10,20 multilayers, which were assembled at the pH conditions allowing a low charge density of the respective PEs, show relatively high ion rejection compared to other multilayers. This result suggests that the optimal structures for desalination membranes should contain a large amount of freely charged groups with densely-packed structures via crosslinking. Finally, the recycling process was employed to further improve the desalination performance. In this case the (PAH pH 7.5/PAA pH 3.5)n=10,20 multilayers exhibited the ion rejection up to 99.8%. This pH-controlled and thermal crosslinking method suggests a new route for the design of well-defined desalination reverse osmosis membranes based on LbL multilayers.

Resistive switching memory devices composed of binary transition metal oxides using sol-gel chemistry

We describe a novel and versatile approach for preparing resistive switching memory devices based on binary transition metal oxides (TMOs). Titanium isopropoxide (TIPP) was spin-coated onto platinum (Pt)-coated silicon substrates using a sol-gel process. The sol-gel-derived layer was converted into a TiO2 film by thermal annealing. A top electrode (Ag electrode) was then coated onto the TiO2 films to complete device fabrication. When an external bias was applied to the devices, a switching phenomenon independent of the voltage polarity (i.e., unipolar switching) was observed at low operating voltages (about 0.6 VRESET and 1.4 VSET). In addition, it was confirmed that the electrical properties (i.e., retention time, cycling test and switching speed) of the sol-gel-derived devices were comparable to those of vacuum deposited devices. This approach can be extended to a variety of binary TMOs such as niobium oxides. The reported approach offers new opportunities for preparing the binary TMO-based resistive switching memory devices allowing a facile solution processing.