Won Gun Koh

Periodic Array of Polyelectrolyte-Gated Organic Transistors from Electrospun Poly(3-hexylthiophene) Nanofibers

High-performance organic field-effect transistors (OFETs) based on polyelectrolyte gate dielectric and electrospun poly(3-hexylthiophene) (P3HT) nanofibers were fabricated on a flexible polymer substrate. The use of UV-crosslinked hydrogel including ionic liquids for the insulating layer enabled fast and large-area fabrication of transistor arrays. The P3HT nanofibers were directly deposited on the methacrylated polymer substrate. During UV irradiation through a patterned mask, the methacrylate groups formed covalent bonds with the patterned polyelectrolyte dielectric layer, which provides mechanical stability to the devices. The OFETs operate at voltages of less than 2 V. The average field-effect mobility and on/off ratio were approximately 2 cm(2)/(Vs) and 10(5), respectively.

Photosensitizing hollow nanocapsules for combination cancer therapy

The social and economic burden of cancer demands a spectrum of therapeutic methodologies. Current options include surgery, chemotherapy, radiotherapy, hyperthermia, and photodynamic therapy (PDT). Only rarely, however, is a single methodology sufficient to overcome cancer. This requirement has inspired combination regimens that overcome the additive, synergistic, and complementary interactions between treatments. An important advance in combination cancer therapy was achieved with the fabrication of multifunctional nanomaterials, including polymeric micelles and nanoparticles (NPs), which may be used to simultaneously perform more than one therapy. Polymeric multilayer capsules present several advantages in combination cancer therapy, including a relatively high capacity for the active substance and versatility in fabrication of the capsule shell. The hollow capsules are assembled in a layer-by-layer (LbL) process onto a sacrificial template followed by dissolution of the template. Hollow polymeric capsules can be fabricated by using templates that vary in size from a few nanometers to hundreds of micrometers, and their chemical and mechanical properties can be precisely tailored by modulating the thickness and composition of the shell. Polymeric multilayer capsules attract interest in various fields of research, and most recently for their high loading capacity as vehicles in drug delivery systems (DDSs). Based on this background, we developed a new type of hollow nanocapsule (NC) for use in combining PDT with chemotherapy. To produce the photosensitizer, we synthesized a negatively charged dendritic porphyrin (DP) that was shown to be effective photosensitizer for PDT, and combined it as a bilayer component with poly(allylamine hydrochloride) (PAH) to fabricate hollow NCs. In general, photosensitizers have large p-conjugation domains for high quantum yields and effective energy absorption. Therefore, many photosensitizers can easily form aggregates in aqueous media because of their abilities to form p–p interactions and their hydrophobic characteristics, which provide a self-quenching effect of the excited state. Unlike conventional photosensitizers, DP has large dendritic wedges that effectively prevent self-quenching phenomena. Moreover, when the DP forms self-assembled nanostructures such as polymeric micelles, large numbers of DPs can effectively generate a high concentration of singlet oxygen at local sites in order to overcome the threshold concentration for oxidative damage. The (PAH/DP)n multilayer nanocapsules were filled with doxorubicin (DOX), a model anticancer drug, in order to implement chemotherapy. While most NC shells used in DDS are prepared from linear polyelectrolytes that lack any function other than drug container, our system employs DP as not only a polyelectrolyte for the formation of NC shells but also as photosensitizing units for photodynamic therapy. Figure 1 shows the preparation of hollow NCs by alternating deposition of PAH and DP onto a negatively charged polystyrene (PS) NP and the subsequent removal of the template PS NP. The average molecular weight of PS was determined to be 70 kDa by GPC analysis. It has been reported that dissolved PS (Mw 10 Da) can diffuse through the multilayer shells when they were extracted with organic solvents such as chloroform or tetrahydrofuran. The DP used in this study bears 32 carboxylate groups on its periphery and a negative zeta potential ( 31.0 mV) at pH 7.4. We expected that multilayer shells would be formed by this LbL deposition technique based on the electrostatic interaction between positively charged PAH and negatively charged DP. The stepwise formation of multilayer shells onto PS NPs was monitored by observing zeta potential changes of particles after each deposition step (Figure 2a). The bare PS NPs have a zeta potential of approximately 55 mV. The PS NPs coated with layers of PAH and DP showed discrete zeta potentials that alternate between positive or negative, depending on the outer layer type. This observation showed that the multilayer surface was charge-overcompensated in each adsorption step, which facilitated adsorption of the next oppositely charged capsule shell layer. Owing to the strong UV/Vis absorbance and fluorescence (FL) emission of DP, multilayer formation could be monitored by changes in UV/Vis absorbance and FL emission from the multilayer-coated PS particles (NCn PS; n = numbers of LbL bilayer, n = 1–3). As shown in Figure 2b, the UV/Vis absorbance and FL emission increased with the number of bilayers. Quantities of DP deposited in NCs were determined to be (106.2 0.4), (212.6 0.4), and (366.1 [*] K. J. Son, Y. Lee, Prof. W.-G. Koh Department of Chemical and Biomolecular Engineering Yonsei University 50 Yonsei-ro, Seodaemoon-Gu, Seoul 120-749 (Korea) E-mail: wongun@yonsei.ac.kr H.-J. Yoon, J.-H. Kim, Prof. W.-D. Jang Department of Chemistry, Yonsei University 50 Yonsei-ro,Seodaemoon-Gu, Seoul 120-749 (Korea) E-mail: wdjang@yonsei.ac.kr [] These authors contributed equally to this work.

Efficiency improvement of dye-sensitized solar cells using graft copolymer-templated mesoporous TiO2 films as an interfacial layer

Organized mesoporous TiO2 films with high porosity and good connectivity were synthesized via sol–gel by templating an amphiphilic graft copolymer consisting of poly(vinyl chloride) backbone and poly(oxyethylene methacrylate) side chains, i.e., PVC-g-POEM. The randomly microphase-separated graft copolymer was self-reorganized to exhibit a well-ordered micellar morphology upon controlling polymer–solvent interactions, as confirmed by atomic force microscope (AFM) and glazing incidence small-angle X-ray scattering (GISAXS). These organized mesoporous TiO2 films, 550 nm in thickness, were used an an interfacial layer between a nanocrystalline TiO2 thick layer and a conducting glass in dye-sensitized solar cells (DSSC). Introduction of the organized mesoporous TiO2 layer resulted in the increased transmittance of visible light, decreased interfacial resistance and enhanced electron lifetime. As a result, an energy conversion efficiency of DSSC employing polymer electrolyte was significantly improved from 3.5% to 5.0% at 100 mW cm−2.

Fabrication of multifunctional layer-by-layer nanocapsules toward the design of theragnostic nanoplatform.

Self-assembled polymeric nanocapsules (NCs) that incorporate dendrimer porphyrin (DP) in the shells and superparamagnetic iron oxide nanoparticles (SPIONs) in the cores are fabricated to create a theragnostic platform for the application in photodynamic therapy (PDT) and magnetic resonance imaging (MRI). SPIONs-embedded polystyrene NPs (SPIONs@PS) are used as a template to build up multilayered NCs. The formation of PAH/DP multilayer on the SPIONs@PS is monitored by zeta-pential and fluorescence emission measurement, because the porphyrin unit in the core of DP has strong red fluorescence emission. NCs have strong enough magnetic property (>20 emu/g) for MRI application with typical superparamagnetic behavior, where the linear correlation of R2 and Fe concentration at diluted conditions led to corresponding T2 relaxivity coefficient (r2) value of 93.5 mM(-1) s(-1). Cell viability study upon light irradiation reveals that NCs can successfully work in photosensitizer formulation for PDT.

Fabrication of hydrogel-micropatterned nanofibers for highly sensitive microarray-based immunosensors having additional enzyme-based sensing capability

Nanofiber-based protein microarrays were fabricated through a combination of electrospinning and hydrogel lithography. Electrospinning generated polystyrene (PS)/poly(styrene-alt-maleic anhydride) (PSMA) fibers with diameters ranging from 0.5 to 1.0 µm and photopatterning of poly(ethylene glycol) (PEG) hydrogel on the electrospun fibers created clearly defined hydrogel microstructures with incorporated nanofibers. The resultant micropatterned nanofibrous substrates were obtained as freestanding and bidirectionally porous sheets, where most of the nanofibers were inserted through the side walls of the hydrogel microstructures. Because of the protein-repellent nature of PEG hydrogels, IgG was selectively immobilized only within the nanofibrous region, creating an IgG microarray. Due to increased surface area, IgG loading in nanofibrous substrates was about six times greater than on planar substrates, which consequently yielded a higher fluorescence signal and faster reaction rate in immunoassays. The capability of encapsulating enzymes made it possible for PEG hydrogels to be used not only for defining protein micropatterns but also for additional biosensor elements. Based on this result, micropatterned nanofibrous substrates consisting of IgG-immobilized nanofibers and enzyme-entrapping PEG hydrogels were fabricated, and their potential to simultaneously carry out both immunoassays and enzyme-based assays was successfully demonstrated.

Microfluidic bioassay system based on microarrays of hydrogel sensing elements entrapping quantum dot-enzyme conjugates.

This paper presents a simple method to fabricate a microfluidic biosensor that is able to detect substrates for H(2)O(2)-generating oxidase. The biosensor consists of three components (quantum dot-enzyme conjugates, hydrogel microstructures, and a set of microchannels) that were hierarchically integrated into a microfluidic device. The quantum dot (QD)-enzyme conjugates were entrapped within the poly(ethylene glycol) (PEG)-based hydrogel microstructures that were fabricated within the microchannels by a photopatterning process. Glucose oxidase (GOX) and alcohol oxidase (AOX) were chosen as the model oxidase enzymes, conjugated to carboxyl-terminated CdSe/ZnS QDs, and entrapped within the hydrogel microstructures, which resulted in a fluorescent hydrogel microarray that was responsive to glucose or alcohol. The hydrogel-entrapped GOX and AOX were able to perform enzyme-catalyzed oxidation of glucose and alcohol, respectively, to produce H(2)O(2), which subsequently quenched the fluorescence of the conjugated QDs. The fluorescence intensity of the hydrogel microstructures decreased as the glucose and alcohol concentrations increased, and the detection limits of this system were found to be 50 μM of glucose and 70 μM of alcohol. Because each microchannel was able to carry out different assays independently, the simultaneous detection of glucose and alcohol was possible using our novel microfluidic device composed of multiple microchannels.

Mutiscale substrates based on hydrogel-incorporated silicon nanowires for protein patterning and microarray-based immunoassays

Here, protein micropatterns were prepared on micropatterned nanostructures for potential applications in microarray-based multiplex bioassays with enhanced protein-loading capacity and detection sensitivity. Vertically-aligned silicon nanowires (SiNWs) that were about 8 μm in height and 150 nm in diameter were prepared using an etching process and were surface-modified with aminopropyltriethoxysilane (APTES) to allow them to covalently immobilize proteins. The SiNW substrate was then overlaid with a micropattern of poly(ethylene glycol) (PEG) hydrogel to create defined arrays of microwells consisting of APTES-modified SiNW on the bottom of the wells, with hydrogel on the walls of the wells. Due to the non-adhesiveness of PEG hydrogels toward proteins, proteins were selectively immobilized on the surface-modified SiNW regions to create protein micropatterns. The increase in surface area increased the protein loading capacity of the SiNWs by more than 10 times the capacity of a planar silicon substrate. Immunobinding assays between IgG and anti-IgG and between IgM and anti-IgM that were performed on micropatterned SiNWs emitted stronger fluorescent signals and showed higher sensitivity than assays performed on planar silicon substrates. Finally, microfluidic channels were successfully integrated into the micropatterned SiNWs to enable the simultaneous performance of multiple immunoassays on a single microarray platform.

Multiplex Immunoassay Platforms Based on Shape-Coded Poly(ethylene glycol) Hydrogel Microparticles Incorporating Acrylic Acid

A suspension protein microarray was developed using shape-coded poly(ethylene glycol) (PEG) hydrogel microparticles for potential applications in multiplex and high-throughput immunoassays. A simple photopatterning process produced various shapes of hydrogel micropatterns that were weakly bound to poly(dimethylsiloxane) (PDMS)-coated substrates. These micropatterns were easily detached from substrates during the washing process and were collected as non-spherical microparticles. Acrylic acids were incorporated into hydrogels, which could covalently immobilize proteins onto their surfaces due to the presence of carboxyl groups. The amount of immobilized protein increased with the amount of acrylic acid due to more available carboxyl groups. Saturation was reached at 25% v/v of acrylic acid. Immunoassays with IgG and IgM immobilized onto hydrogel microparticles were successfully performed with a linear concentration range from 0 to 500 ng/mL of anti-IgG and anti-IgM, respectively. Finally, a mixture of two different shapes of hydrogel microparticles immobilizing IgG (circle) and IgM (square) was prepared and it was demonstrated that simultaneous detection of two different target proteins was possible without cross-talk using same fluorescence indicator because each immunoassay was easily identified by the shapes of hydrogel microparticles.

Preparation of photolithographically patterned inverse opal hydrogel microstructures and its application to protein patterning

Protein pattern has played an important role in biosensors, bioMEMS, tissue engineering, fundamental studies of cell biology, and basic proteomics research. Here, we developed a straightforward and effective protein patterning technique using macroporous poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogel micropatterns as a three-dimensional (3D) template for protein immobilization. Micropatterns of macroporous hydrogels with inverse opal structures were prepared on poly(ethylene glycol) (PEG)-coated silicon substrates by combining a colloidal crystal templating method with photopatterning. The resultant inverse opal hydrogel (IOH) micropatterns were modified with 3-aminopropyltriethoxysilane using the hydroxyl groups in PHEMA for the covalent immobilization of proteins. Proteins were selectively immobilized only on the hydrogel micropatterns, while the PEG regions served as an effective barrier to protein adsorption. Because of their highly ordered and interconnected 3D macroporous structures and large internal surface areas, protein loading in the IOH micropattern was about six times greater than that on a non-porous hydrogel micropattern, which consequently improved the protein activity. The porosity of the hydrogel micropatterns could be controlled using different sizes of colloidal nanoparticles, and using smaller nanoparticles produced hydrogel micropatterns with higher protein loading capacities and activities. To demonstrate the potential use of IOH micropatterns in biosensor systems, biotin was micropatterned on the hydrogels and the specific binding of streptavidin was successfully assayed using IOH micropatterns with better fluorescence signals and sensitivity than that of the corresponding non-porous hydrogel micropatterns.

Dendrimer porphyrin-terminated polyelectrolyte multilayer micropatterns for a protein microarray with enhanced sensitivity

Through a combination of layer-by-layer (LbL) self-assembly (SA) and lift-off methods, a dendrimer-coated polyelectrolyte multilayer micropattern was prepared for protein microarrays. A silicon substrate was patterned with a photoresist thin film using conventional photolithography, and then poly(ethyleneimine) (PEI) and poly(sodium 4-styrenesulfonate) (PSS) were alternatively deposited onto the substrate surface using spin-assisted self-assembly. A well-defined multilayer microarray was produced by subsequent removal of the photoresist template by a lift-off process. Dendrimer porphyrin (DP) was successively immobilized onto the PEI-terminated micropatterns via electrostatic interactions between the negatively-charged DPs and positively-charged PEI segments. Because of strong fluorescence from focal porphyrins, the homogeneous covering of DPs onto the multilayer micropatterns was easily confirmed using fluorescence microscopy. Atomic force microscopy (AFM) also showed morphological change of micropatterned surfaces by DP immobilization. Based on these results, IgG was immobilized on the DP-coated protein microarrays, and immunoassays were performed to demonstrate that the DP-coated microarrays yielded a higher fluorescence signal and were more sensitive than the control microarrays that were coated with linear PAA polymer instead of DP due to the multiple functional groups present on the DP-coated arrays and their increased surface area relative to control microarrays.