Jungkyu K. Lee

Surface-initiated, ring-opening polymerization of p-dioxanone from gold and silicon oxide surfaces

A polymeric thin film of a biodegradable poly(p-dioxanone) (PPDX) was grown from flat solid surfaces by a combination of formation of self-assembled monolayers terminating in hydroxyl groups and surface-initiated, ring-opening polymerization (ROP) of p-dioxanone (PDX). Tin(II) octoate [Sn(Oct)2] was used as a catalyst, and gold and silicon oxide surfaces were chosen as model surfaces. A molecularly-ordered film (self-assembled monolayer, SAM) was formed by immersing gold and silicon oxide substrates in a solution of alkanethiolates and in a solution of triethoxysilanes, respectively. The polymerization was achieved by heating a mixture of Sn(Oct)2, PDX, and the SAM-coated substrate in anhydrous toluene for 24 h at 55 °C (for a gold substrate) or 70 °C (for a silicon oxide substrate). The characteristic IR peaks of PPDX (C–H stretching at ∼2923 cm−1 and the CO ester at ∼1748 cm−1) were observed in polarized infrared external reflectance spectroscopy (PIERS) spectra, indicating the presence of a PPDX film on the gold and silicon oxide surfaces. The average thickness of the PPDX film was measured to be 90 nm on the gold substrate and 46 nm on the silicon oxide substrate by ellipsometry. The PPDX films were further characterized by X-ray photoelectron spectroscopy (XPS), contact angle measurement, and atomic force microscopy (AFM).

Systematic study of fluorescein-functionalized macrophotoinitiators for colorimetric bioassays.

We report a systematic investigation of a set of photoreducible macrophotoinitiators for use in polymerization-based signal amplification. To test the dependence of photopolymerization responses on the number of photoinitiators localized per molecular recognition event, we gradually increased the number of photoinitiator molecules coupled to a constant scaffold macromolecule from an average of 7 per polymer to an average of 168 per polymer. To evaluate the capacity of the macrophotoinitiators to detect molecular recognition, we coupled neutravidin to these molecules to recognize biotin-labeled DNA immobilized on biochip test surfaces. Fluorescein macroinitiators were found to be useful in detecting molecular recognition above a threshold number of initiators per polymer. Above this threshold, increasing the number of initiators per macroinitiator resulted in increased signal strength. These findings demonstrate the feasibility of increasing the number of photoreducible initiators per binding event beyond three, the number used in previous studies, that the initiation reaction remains limiting in the range we investigated, and that the number of initiators per binding event in this system has a clear impact on assay sensitivity and signal strength.

Syntheses of organic molecule-DNA hybrid structures.

Investigation of robust and efficient pathways to build DNA-organic molecule hybrid structures is fundamentally important to realize controlled placement of single molecules for potential applications, such as single-molecule electronic devices. Herein, we report a systematic investigation of synthetic processes for preparing organic molecule-DNA building blocks and their subsequent elongation to generate precise micrometer-sized structures. Specifically, optimal cross-coupling routes were identified to enable chemical linkages between three different organic molecules, namely, polyethylene glycol (PEG), poly(p-phenylene ethynylene) (PPE), and benzenetricarboxylate, with single-stranded (ss) DNA. The resulting DNA-organic molecule hybrid building blocks were purified and characterized by both denaturing gel electrophoresis and electrospray ionization mass spectrometry (ESI-MS). The building blocks were subsequently elongated through both the DNA hybridization and ligation processes to prepare micrometer-sized double-stranded (ds) DNA-organic molecule hybrid structures. The described synthetic procedures should facilitate future syntheses of various hybrid DNA-based organic molecular structures.

Synthesis of DNA-Organic Molecule-DNA Triblock Oligomers Using the Amide Coupling Reaction and Their Enzymatic Amplification

Precise electrical contact between single-molecule and electrodes is a first step to study single-molecule electronics and its application such as (bio)sensors and nanodevices. To realize a reliable electrical contact, we can use DNA as a template in the field of nanoelectronics because of its micrometer-scaled length with the thickness of nanometer-scale. In this paper, we studied the reactivity of the amide-coupling reaction to tether oligodeoxynucleotides (ODNs) to organic molecules and the elongation of the ODNs by the polymerase chain reaction (PCR) to synthesize 1.5 kbp dsDNA-organic molecule-1.5 kbp dsDNA (DOD) triblock architecture. The successful amide-coupling reactions were confirmed by electrospray ionization mass spectrometry (ESI-MS), and the triblock architectures were characterized by 1% agarose gel electrophoresis and atomic force microscope (AFM). Our result shows that this strategy is simple and makes it easy to construct DNA-organic molecule-DNA triblock architectures and potentially provides a platform to prepare and investigate single molecule electronics.

Binding behaviors of protein on spatially controlled poly[oligo(ethylene glycol) methacrylate] brushes grafted from mixed self-assembled monolayers on gold

Binding behaviors of streptavidin were investigated with different lateral packing densities of biotin-functionalized, non-biofouling pOEGMA brushes, synthesized by surface-initiated polymerization from mixed SAMs with different mole fractions of the polymerization initiator on gold surfaces.

Conduction and Low-Frequency Noise Analysis in Bipolar Switching Resistance Random Access Memory Devices

We investigated the low-frequency noise (LFN) properties of the bipolar switching resistance random access memories (RRAMs) for the first time with amorphous TiOx -based RRAM devices. The LFNs are proportional to 1/f for both high-resistance (HRS) and low-resistance states (LRS). The normalized noise in HRS is around an order of magnitude higher than that in LRS. The random telegraph noise (RTN) is observed only in HRS, which represents that the dominant trap causing the RTN becomes electrically inactive by being filled with electrons in LRS. The voltage dependence Si/I2 of shows that the noise can be used to elucidate the operation mechanism of RRAM devices.

The shear flow processing of controlled DNA tethering and stretching for organic molecular electronics.

DNA has been recently explored as a powerful tool for developing molecular scaffolds for making reproducible and reliable metal contacts to single organic semiconducting molecules. A critical step in the process of exploiting DNA-organic molecule-DNA (DOD) array structures is the controlled tethering and stretching of DNA molecules. Here we report the development of reproducible surface chemistry for tethering DNA molecules at tunable density and demonstrate shear flow processing as a rationally controlled approach for stretching/aligning DNA molecules of various lengths. Through enzymatic cleavage of λ-phage DNA to yield a series of DNA chains of various lengths from 17.3 μm down to 4.2 μm, we have investigated the flow/extension behavior of these tethered DNA molecules under different flow strengths in the flow-gradient plane. We compared Brownian dynamic simulations for the flow dynamics of tethered λ-DNA in shear, and found our flow-gradient plane experimental results matched well with our bead-spring simulations. The shear flow processing demonstrated in our studies represents a controllable approach for tethering and stretching DNA molecules of various lengths. Together with further metallization of DNA chains within DOD structures, this bottom-up approach can potentially enable efficient and reliable fabrication of large-scale nanoelectronic devices based on single organic molecules, therefore opening opportunities in both fundamental understanding of charge transport at the single molecular level and many exciting applications for ever-shrinking molecular circuits.

Micrometer-sized DNA-single-fluorophore-DNA supramolecule: synthesis and single-molecule characterization.

The synthesis of single-fluorophore-bis(micrometer-sized DNA) triblock supramolecules and the optical and structural characterization of the construct at the single-molecule level is reported. A fluorophore-bis(oligodeoxynucleotide) triblock is synthesized via the amide-coupling reaction. Subsequent protocols of DNA hybridization/ligation are developed to form the supramolecular triblock structure with lambda-DNA fragments on the micrometer length scale. The successful synthesis of the micrometer-sized DNA-single-fluorophore-DNA supramolecule is confirmed by agarose gel electrophoresis with fluorescence imaging under UV excitation. Single triblock structures are directly imaged by combined scanning force microscopy and single-molecule fluorescence microscopy, and provide unambiguous confirmation of the existence of the single fluorophore inserted in the middle of the long DNA. This type of triblock structure is a step closer to providing a scaffold for single-molecule electronic devices after metallization of the DNAs.

Backfilling‐Free Strategy for Biopatterning on Intrinsically Dual‐Functionalized Poly[2‐Aminoethyl Methacrylate‐co‐Oligo(Ethylene Glycol) Methacrylate] Films

We demonstrated protein and cellular patterning with a soft lithography technique using poly[2-aminoethyl methacrylate-co-oligo(ethylene glycol) methacrylate] films on gold surfaces without employing a backfilling process. The backfilling process plays an important role in successfully generating biopatterns; however, it has potential disadvantages in several interesting research and technical applications. To overcome the issue, a copolymer system having highly reactive functional groups and bioinert properties was introduced through a surface-initiated controlled radical polymerization with 2-aminoethyl methacrylate hydrochloride (AMA) and oligo(ethylene glycol) methacrylate (OEGMA). The prepared poly(AMA-co-OEGMA) film was fully characterized, and among the films having different thicknesses, the 35 nm-thick biotinylated, poly(AMA-co-OEGMA) film exhibited an optimum performance, such as the lowest nonspecific adsorption and the highest specific binding capability toward proteins.

Direct Patterning and Biofunctionalization of a Large‐Area Pristine Graphene Sheet

Direct patterning of streptavidin and NIH 3T3 fibroblast cells was successfully achieved over a large-area pristine graphene sheet on Si/SiO2 by aryl azide-based photografting with the conventional UV lithographic technique and surface-initiated, atom transfer radical polymerization of oligo(ethylene glycol) methacrylate.