Young Shik Chi

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).

Formation of carbon nanotube/glucose-carrying polymer hybrids by surface-initiated, atom transfer radical polymerization

ConclusionsIn conclusion, we demonstrated that SI-ATRP could be utilized for generating CNT/glycopolymer hybrid nanocomposites. As a proof-of-concept, we grafted MAIpGlc, a monomer that carries protected glucose ring, onto s-MWCNTs. After coating s-MWCNTs with pMAIpGlc, the MAIpGlc moiety was deprotected to extrude the glucose group onto the surface of s-MWCNTs. LAMA (and its derivatives) would have advantages over MAIpGlc, because various monosaccharides and a sequence of saccharides could be incorporated into the polymer backbone and polymerization does not need protection/deprotection steps. Considering the biological importance of polysaccharides, the method described herein would be beneficial in many areas, such as pathogen detection and biosensors.

pH-Dependent rectification in self-assembled monolayers based on electrostatic interactions

Asymmetric electrostatic interactions dependent on pH between the redox molecules and the terminal group on the top of the self-assembled monolayer (SAM) afford control of the electron transfer property of the SAM having the imidazole terminal group.

A noncovalent approach to the construction of tween 20-based protein microarrays

In the development of efficient chips capable of recognizing biospecific interactions, such as DNA hybridization and peptide–antibody and protein–protein interactions, one of the most critical issues yet to be improved is how to prevent nonspecific binding (NSB) of incoming target biomolecules, while maximizing biospecific bindings of targets to the surface-immobilized probes. In the cases of protein and peptide chips, the minimization of NSBs of target proteins has generally been attempted by coating chip surfaces with bovine serum albumin (BSA). Although BSA protects the surface from nonspecifically binding proteins reasonably well, this approach still causes several undesirable problems, especially those involving the sizes of proteins. The efficiency of biospecific binding becomes quite low when the sizes of probe molecules are small. For example, when FLAG peptides were immobilized as a probe and the chip surface was covered with BSA, the large BSA spatially shielded the much smaller FLAG peptides from the incoming counterpart, anti-FLAG, resulting in inefficient biospecific binding. As an alternative approach, strategically designed, small organic molecules have been developed. These are coated as a monolayer onto various solid substrates in order to provide biologically “inert” proteinor cell-resistant surfaces. Compounds containing oligoor poly(ethylene glycol) have been widely used for this purpose. Recently, commercially available Tween 20 has drawn much attention as a potential coating material for hydrophobic surfaces, since it contains a long alkyl chain of twelve carbons (dodecyl chain), through which Tween 20 can effectively interact with other hydrophobic surfaces, along with twenty ethylene glycol groups and three terminal hydroxy ( OH) groups that should provide effective nonbiofouling properties and gateways for introducing bioactive probe molecules, respectively. Thanks to its structural characteristics, especially the dodecyl chain, Tween 20 has been utilized for carbon nanotube-based (CNT-based) protein sensors, in which CNTs were coated with Tween 20 through hydrophobic interactions between the surface of the CNTs and the ACHTUNGTRENNUNGdodecyl chain of Tween 20. More recently, we have also shown that single-walled CNT films effectively coated with Tween 20 were suitable for the highly efficient discrimination of antigen–antibody and small peptide–antibody pair recognition. Although Tween 20 is stably adsorbed onto CNTs through parallel hydrophobic interactions along the CNT growth axis, it is still a challenging task to apply Tween 20 to more common chip platforms such as Au and SiO2, due to the instability of immobilized Tween 20 on flat surfaces. Whitesides and coworkers studied the stability of various detergents, including Tween 20, adsorbed onto the self-assembled monolayer (SAM) of hexadecanethiol. They found that Tween 20 was eliminated easily from the hydrophobic SAM surface upon simple washing with water, which indicated that the attractive interactions between Tween 20 and the hydrophobic methyl-terminated surface were not sufficient for the stable immobilization of Tween 20. In this article we report a simple but versatile method for the formation of stable Tween 20 layers on flat surfaces. Because simple hydrophobic interactions were not strong enough to maintain the adsorbed Tween 20 on hydrophobic surfaces, we introduced additional attractive interactions—van der Waals interactions—into the system in order to increase the stability of the adsorbed Tween 20. Specifically, we designed the substrate surface in such a way as to present a SAM of loosely packed long alkyl chain molecules, through which the dodecyl chain of Tween 20 could be spontaneously guided and squeezed into the SAM through van der Waals interactions. The key strategy for the formation of the stable Tween 20 layer began with the generation of a smart molecular layer on a solid substrate, composed of a loosely packed SAM of long alkyl chains. We reasoned that the loosely packed SAM should provide adequate spaces for the dodecyl chain of Tween 20 to be squeezed and accommodated through van der Waals interactions (Scheme 1). The construction of the loosely packed SAM on gold, without phase segregation, has been rather challenging, and the reported examples are scarce. Langer and coworkers ingeniously designed a precursor of self-assembling thiols, in which a bulky (2-chlorophenyl)diphenylmethyl group was coupled with 16-mercaptohexadecanoic acid (MHDA) through the ester linkage. After the formation of SAMs of the MHDA derivative, the selective cleavage of the bulky group generated the loosely packed SAM of MHDA. Recently, by adopting a similar strategy, Jennings and a co-worker reported that loosely packed, hydroxyl-terminated SAMs on gold could be prepared by the base-mediated cleavage of the large fluorocarbon terminal group. We devised a rather simpler approach for the formation of the loosely packed SAM. The loosely packed SAM was prepared by allowing long alkyl amines to react with an interchain carboxylic anhydride-terminated (ICA-terminated) SAM on gold. 28] The SAM of MHDA was first prepared by the reported procedure, and the resulting SAM was characterized by el[a] Y. S. Chi, Prof. I. S. Choi Department of Chemistry and School of Molecular Science (BK21), KAIST Daejeon 305-701 (Korea) Fax: (+82)42-869-2810 E-mail : ischoi@kaist.ac.kr [b] H. R. Byon, Prof. H. C. Choi Department of Chemistry, Pohang University of Science and Technology (POSTECH) San 31, Hyoja-Dong, Nam-Gu, Pohang 790-784 (Korea) Fax: (+82)54-279-3399 E-mail : choihc@postech.edu Supporting information for this article is available on the WWW under http://www.chembiochem.org or from the author.

Local scanning probe polymerization of an organic monolayer covalently grafted on silicon.

The possibility of lateral extension of conjugation within a covalently grafted molecular layer by a scanning probe-based method was tested. A molecular layer derived from ω-(N-pyrrolyl)propanol was formed on n-type Si(111) surface. Application of large sample biases greater than ±4 V during conductive atomic force microscope (AFM) scans under vacuum resulted in changes of mechanical and electrical characteristics of the molecular layer: the tip-sample conductance was increased greatly, the friction was reduced significantly, and the surface potential of the scanned area was increased. The reduction in friction could be attributed to molecular linking formed within the layer. The increased conductance suggested extended conjugation among the pyrrolyl end groups. Therefore, it was inferred that the biased AFM scan successfully induced local polymerization/oligomerization within the covalently grafted molecular layer.

Disorder–order phase change of ω-(N-pyrrolyl)alkanethiol self-assembled monolayers on gold induced by STM scans and thermal activation

Molecular ordering of pyrrolyl-terminated alkanethiol self-assembled monolayers (PyC(n)SH SAMs) on Au(111) substrates (n = 11 or 12) was investigated by scanning tunneling microscopy (STM) and various spectroscopic methods. The SAMs, which were in a disordered state when formed at room temperature, could be ordered either globally by thermal annealing at 70 degrees C, or locally via stimulation with repetitive STM scans. The ordered phase was characterized by small domains of molecular rows formed along 112[combining macron] directional set with an inter-row corrugation period close to 1.44 nm, in which defects were abundant. Based on the experimental results, the molecular arrangement in the ordered PyC(n)SH SAM was proposed to be a (5x radical3)rect structure with a molecular deficiency >or=10%. While mechanical interactions between molecules and scanning probe tips had been pointed out as the major cause of scan-induced phase transformations in other SAM systems, electronic or electrostatic factors were thought to affect considerably the scan-induced ordering process in this SAM system. From comparison of surface molecular coverage between disordered and thermally ordered SAMs of PyC(12)SH, it was inferred that the disorder could be ascribed to both kinetic and thermodynamic factors. The kinetic barrier to the ordered phase was supposed to result from strong dipole-dipole interactions among the pyrrolyl endgroups.

High voltage-derived enhancement of electric conduction in nanogap devices for detection of prostate-specific antigen

We report a simple method of enhancing electric conductance in nanogap devices without any additional treatments, such as silver-enhancing process. The low electric conductance after selective immobilization of biofunctionalized gold nanoparticles in the gap region was greatly enhanced by repeated I-V scans at relatively high voltage ranges of −5 to 5 V, which was attributed to the formation of a new conduction pathway across the gap. The higher conduction state of the nanogap device showed a very stable I-V curve, which was used as an excellent measure of the existence of prostate-specific antigen.

Asymmetrically Functionalized, Four-Armed, Poly(ethylene glycol) Compounds for Construction of Chemically Functionalizable Non-Biofouling Surfaces

Asymmetrically functionalized, four-armed, Tween 20 derivatives that formed stable monomolecular films on solid substrates were designed and synthesized. Thiol-modified Tween 20 was used for forming self-assembled monolayers (SAMs) on gold, and maleimide-modified Tween 20 was introduced onto SiO(2) surfaces with SAMs of (3-mercaptopropyl)trimethoxysilane through Michael addition. These structurally modified Tween 20 compounds gave the original characteristics of Tween 20, non-biofouling (from ethylene glycol groups) and functionalizable (from OH groups) properties, to each substrate. The non-biofouling properties of the Tween 20-coated gold and SiO(2) surfaces were investigated by surface plasmon resonance spectroscopy and ellipsometry, and these surfaces showed strong resistance against nonspecific adsorption of proteins. In addition, the biospecific binding of streptavidin was achieved after coupling of (+)-biotinyl-3,6,9-trioxaundecanediamine onto the non-biofouling surfaces through amide-bond formation.

Uniform grafting of poly(1,5-dioxepan-2-one) by surface-initiated, ring-opening polymerization

A polymeric film of a biodegradable poly(1,5-dioxepan-2-one) (PDXO) was formed on a gold surface by a combination of the formation of self-assembled monolayers (SAMs) presenting hydroxyl groups and the surfaceinitiated, ring-opening polymerization (SI-ROP) of 1,5-dioxepan-2-one (DXO). The SI-ROP of DXO was achieved by heating a mixture of Sn(Oct)2, DXO, and the SAM-coated substrate in anhydrous toluene at 55°C. The resulting PDXO film was quite uniform. The PDXO film was characterized by polarized infrared external reflectance spectroscopy, X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, atomic force microscopy, ellipsometry, and contact angle goniometry.