Polymeric Self-Assembled Monolayers Research Articles

An AIE‐Active Ultrathin Polymeric Self‐Assembled Monolayer Sensor for Trace Volatile Explosive Detection

Front Cover: Based on the aggregation-induced emission (AIE) effect, an ultra-thin self-assembled monolayer (SAM) sensor for trace nitro explosives is designed by Ziwei Li, Feng Wei, Jinyao Tang, and co-workers, in article 2100551. The polymeric SAMs are fabricated by covalent bonds to the substrate, and prove to show good stability, sensitivity, selectivity, and reversibility. The sensing properties are further improved by being coated on Al2O3 fiber paper, which realizes the detection by the naked eye.

An AIE-Active Ultrathin Polymeric Self-Assembled Monolayer Sensor for Trace Volatile Explosive Detection.

This work has prepared polymeric self-assembled monolayer (SAM) sensors for the detection of trace volatile nitroaromatic compound (NAC) explosives by fluorescence quenching. A typical aggregation-induced emission (AIE) luminogen 1,1,2,2-tetraphenylethene (TPE) polymerizes into PTPE to increase the fluorescence intensity in the SAMs, and the phosphoric acid acts as the anchor group to form stable covalent bonds with the Al2 O3 substrate. This design takes advantage of the high sensitivity and good stability of SAMs, and high fluorescence intensity, and "wire effect" of the conjugated polymers. The polymeric SAM sensors are prepared on the Al2 O3 silicon wafer and testing paper. Both of them show good response speed, reversibility, selectivity, and sensitivity. The detection limits down to 0.07, 0.35, and 4.11ppm for TNT, DNB, and NB, respectively, are achieved on the inorganic testing paper. Furthermore, due to the higher fluorescence intensity by interlacing and overlapping of fibers, the detection of the paper can be distinguished by naked eyes even with a low-power handheld UV lamp, which provides an experimental basis for the development of cheap and easy trace NAC explosive sensors.

Facile and effective antibacterial coatings on various oxide substrates

This work reports a facile and effective antibacterial coating for oxide substrates. As a coating material, a random copolymer, abbreviated as poly(TMSMA-r-PEGMA), was synthesized by radical polymerization of 3-(trimethoxysilyl)propyl methacrylate (TMSMA) and poly(ethylene glycol) methyl ether methacrylate (PEGMA). Polymeric self-assembled monolayers of poly(TMSMA-r-PEGMA) were formed on various inorganic oxide substrates, including silicon oxide, titanium dioxide, aluminum oxide, and glass, via the simple dip-coating process. The polymer-coated substrates were characterized by ellipsometry, contact angle measurements, and X-ray photoelectron spectroscopy. The bacterial adhesion on the polymer-coated substrates was completely suppressed compared to that on the uncoated substrates.

Polymeric Self-Assembled Monolayers Anomalously Improve Thermal Transport across Graphene/Polymer Interfaces.

Ultralow thermal conductivities of bulk polymers greatly limit their applications in areas demanding fast heat dissipation, such as flexible electronics and microelectronics. Therefore, polymeric composites incorporating highly thermally conductive filler materials (e.g., graphene and carbon nanotubes) have been produced to address the issue. However, despite some enhancement, thermal conductivities of these materials are still far below theoretical predictions, mainly due to the inefficient thermal transport across material interfaces. Here, using molecular dynamics simulations, we demonstrate that polyethylene (PE) self-assembled monolayer (SAM) functionalized graphene surfaces at a high grafting density can drastically improve interfacial thermal conduction between graphene and the matrix of poly(methyl methacrylate) (PMMA). In contrast to abrupt temperature drop across pristine graphene/PMMA interfaces, temperature field in the vicinity of a PE-grafted graphene/PMMA interface is continuous with a smoother transition and higher thermal conductance. This anomalous improvement is attributed to three factors that closely relate to the grafting density of the SAM of PE. First, the SAM with high grafting densities features highly extended chains that enhance along-chain thermal conduction. Second, the strong covalent bonding between the SAM and the graphene facilitates heat transfer at their joints. Third, the SAM and the PMMA matrix are both organic materials, leading to enhanced interfacial vibrational coupling. Molecular mechanisms underpinning these phenomena are systematically elucidated by analyzing the temperature field, density distribution, Herman's orientation factor, the vibrational density of states, cumulative correlation factor, the integrated autocorrelation of interfacial heat power, and interfacial adhesion. All results suggest the incorporation of SAMs at a high grafting density or extremely extended polymer brushes for drastically improved interfacial thermal transport between hard and soft materials toward a wide range of applications.

Multiscale treatment of mechanical contact problems involving thin polymeric layers

We propose a strategy to obtain a hyperelastic constitutive law for film-like systems from molecular dynamics (MD) simulations. The aim is to furnish a computationally efficient continuum model with this description of the material. In particular, two different methods are suggested, both of which consist of virtual experiments that are performed on the material to sample systematically the stress–strain relation. The latter is then fitted to a suitable functional form. We use a polymeric self-assembled monolayer, which spans a height of only a few nanometers, as a test case. Having determined the response function, we then apply it within a finite-element simulation of a continuum mechanical nanoindentation problem. Several contact quantities such as normal reaction forces and the contact geometry are extracted from these calculations and are compared to those from an analogous, fully atomistic nanoindentation simulation. We find that the considered benchmark quantities as obtained from the continuum surrogate model reproduce well the corresponding values of the MD simulation.

Direct, Noncovalent Coating of a Gold Surface with Polymeric Self-Assembled Monolayers

Department of Biological Sciences, KAIST, Daejeon 305-701, KoreaReceived September 4, 2013, Accepted September 12, 2013Key Words : Biomolecule immobilization, Gold surface, Micropatterns, Non-biofouling, Polymeric self-as-sembled monolayersThe spatio-selective immobilization of biomolecules, suchas DNAs, antibodies, or aptamers, onto a solid surface isrequired for the development of bioanalytical and bio-medical devices that interface the immobilized probe withthe target biospecifically.

Synthesis of Novel Copolymer for Selective Biomolecule Immobilization on Gold Surface

Various types of nanoparticles such as quantum dots, magnetic nanoparticles, and liposomes have been studied for numerous biomedical applications. Among these nanoparticles, gold nanoparticles have been extensively exploited for various applications including drug delivery, imaging agents, biosensors, and computed tomography (CT) contrast agents because they have many advantages such as low cytotoxicity, easy size control, high x-ray absorption, unique optical and electrical properties. Stability of gold nanoparticles under physiological conditions is key step for biomedical application. Another key is the immobilization or conjugation of bioactive molecules on gold nanoparticles for targeted drug delivery, molecular imaging, and in vitro diagnosis. To satisfy these key requirements, we developed a novel random copolymer composed of a “surface anchor part” (thiol group), “antibiofouling part” (PEG), and “bioreactivity part” (carboxyl group) for gold surface modification in particular. A random polymer to modify the gold surface for targeting ligand conjugation or SPR (surface plasmon resonance) chip was synthesized using radical polymerization. The chemical structure of poly(SVE-r-mPEGMA-r-MA) is shown in Figure 1(a). This polymer consists of three parts: a S-4vinylbenzyl ethanethionate (SVE) monomer as the surfaceanchor part, poly(ethylene glycol) methyl ether methacrylate (mPEGMA) as the protein-resistant part, and methacrylic acid (MA) monomer as a functional group to conjugate biomolecules. The initial feed ratio (SVE: mPEGMA:MA, 2:2:1) was chosen for synthesizing polymer that have high antibiofouling effect and strong surface binding capacity. After radical polymerization, unreacted monomers were removed by dialysis. The structure of the polymer was confirmed using NMR. Peaks for the methoxy of the mPEGMA and the methyl of the SVE monomer were observed at 3.4 ppm and 2.3 ppm, respectively (Figure S1). The molecular weight of the polymer was Mn = 16 000, as measured by gel permeation chromatography (GPC). Next, we examined the ability of the polymer to coat the gold surface. First, the acetylated thiol of the polymer was deprotected using hydroxylamine to yield a free thiol group. Because several thiol groups in the polymer provide multipoint attachments to the gold surface, the polymer spontaneously forms monolayers on a gold surface. Because of the multiple attachments, the stability of the polymeric monolayers can be significantly improved when exposed to a buffer with a non-physiological pH, or oxidative chemical and electrochemical environments. A polymeric self-assembled monolayer (SAM) of the polymer on the gold surface was prepared by immersing the substrates in an ethanol solution of 0.2 wt % polymer. The polymer monolayer thickness was measured using ellipsometry. The ellipsometric thickness increased after immersion times of up to 4 h. After 4 h, the thickness of the films remained unchanged, and the thickness of the monolayer was approximately

Adsorption/desorption-induced phase transformation in the self-assembled monolayers of an amphiphilic rod–coil block copolymer

In the polymeric self-assembled monolayer of an amphiphilic rod–coil copolymer consisting of a planar-adsorbed rod-like block and an anchoring coil block, phase transformation from an isotropic to nematic monolayer and then to a crystalline monolayer occurs during adsorption of chains to the surface in solution, whereas the monolayer reorganizes to anisotropic surface micelles when immersed in a coil-selective solvent.

Surface-Grafted Rodlike Polymers: Adaptive Self-Assembled Monolayers and Rapid Photo-Patterning of Surfaces

We present a new concept of functionalizing solid surfaces using polymeric self-assembled monolayers (PSAM) that are obtained by grafting onto solid surfaces an asymmetric block copolymer composed of a long rodlike block and a short surface-reactive block. Poly(n-hexylisocyanate)-b-poly[3-(trimethoxysilyl)propyl methacrylate] (PHIC-b-PTMSM) is synthesized via a living anionic polymerization and an atom transfer radical polymerization. The new rod–coil block copolymer forms a polymeric self-assembled monolayer (PSAM) through covalent bonding of the sticky PTMSM block and planar adsorption of PHIC rodlike chains onto the substrate surface. The uniform PSAM with a thickness identical to the diameter of the rodlike chain is produced by the immersion coating method in a range of immersion solution concentrations and coating times. The PSAMs present unique properties thanks to the freedom of rotation of the end-grafted rodlike chains. The PSAMs exhibit nematic liquid crystalline ordering when the monolayer is f...

Ionic Topochemical Tuned Biosensor Interface

Two new hydrophilic, poly(ethylene glycol) (PEG)-based redox copolymers bearing electrochemically active ferrocene (Fc) and thiol/disulfide anchoring functionalities were synthesized. These copolymers are shown to adsorb on gold surfaces causing polymeric self-assembled monolayers (pSAMs) that possess triple functions: "redox-active", "ionic-tunable", and "bio-inert". Both immobilized polymers showed redox potentials at +400 mV (Ag|AgCl), and facilitate the electrocatalytical oxidation of NADH. Additionally, interfacial architecture of the polymers is affected by an increase in Ca(2+) concentration, which leads to an amplification of the electrochemical response. The electrode current, measured for NADH-oxidation, increased by 80% after addition of 10 mM Ca(2+) ions. Considering the Ca(2+) influence on the heterogeneous electron transfer a structural model of the immobilized polymers is proposed based on the strong chelating ability of noncyclic PEG moieties.

Polymeric self-assembled monolayers derived from surface-active copolymers: a modular approach to functionalized surfaces

Physical or covalent adsorption of surface-active block or random copolymers consisting of anchoring units and functional units onto solid surfaces yields polymeric self-assembled monolayers (PSAMs) with designed chemical and morphological structures and functionalities. This tutorial review provides an overview of PSAMs derived from various types of copolymers. It also discusses design principles for PSAMs to tune interfacial and surface interactions of materials for potential applications.

Comparison of the Nonspecific Binding of DNA-Conjugated Gold Nanoparticles between Polymeric and Monomeric Self-Assembled Monolayers

The nonspecific binding of DNA-conjugated gold nanoparticles (AuNPs) to solid surfaces is more difficult to control than that of DNA molecules due to the more attractive interactions from the large number of DNA molecules per AuNP. This paper reports that the polymeric self-assembled monolayers (SAMs) formed on indium-tin oxide (ITO) electrodes significantly inhibit the nonspecific binding of DNA-conjugated AuNPs. The random copolymers used to prepare the polymeric SAMs consist of three functional parts: an ITO-reactive silane group, a DNA-blocking poly(ethylene glycol) (PEG) group, and an amine-reactive N-acryloxysuccinimide group. In order to compare the polymeric SAMs with various monomeric SAMs, the relative nonspecific binding of the DNA-conjugated AuNPs to the ITO electrodes modified with (3-aminopropyl)triethoxysilane (APTES), 3-aminopropylphosphonic acid, 3-phosphonopropionic acid, or 11-phosphonoundecanoic acid is examined by measuring the electrocatalytic anodic current of hydrazine caused by the nonspecifically absorbed AuNPs and by counting the AuNPs adsorbed onto modified ITO electrodes. Carboxylic-acid-terminated and amine-terminated monomeric SAMs cause high levels of nonspecific binding of DNA-conjugated AuNPs. The monomeric SAM modified with the carboxylic-acid-terminated poly(amidoamine) dendrimer shows low levels of nonspecific binding (2.0% nonspecific binding relative to APTES) due to the high surface density of the negative charge. The simply prepared polymeric SAM produces the lowest level of nonspecific binding (0.8% nonspecific binding relative to APTES), resulting from the combined effect of (i) DNA-blocking PEG and carboxylic acid groups and (ii) dense polymeric SAMs. Therefore, thin and dense polymeric SAMs may be effective in electrochemical detection and easy DNA immobilization along with low levels of nonspecific binding.

Dual Functional, Polymeric Self-Assembled Monolayers as a Facile Platform for Construction of Patterns of Biomolecules

We report a facile approach to the construction of patterns of biomolecules based on polymeric self-assembled monolayers (pSAMs) that possess dual functions: "bio-reactive (post-functionalizable)" and "bio-inert (anti-biofouling)" properties. To prepare pSAMs on Si/SiO2 wafers were synthesized new random copolymers by radical polymerization of poly(ethylene glycol) methyl ether methacrylate (PEGMA), 3-(trimethoxysilyl)propyl methacrylate (TMSMA), and N-acryloxysuccinimide (NAS), and referred to as poly(TMSMA-r-PEGMA-r-NAS). Poly(TMSMA-r-PEGMA-r-NAS) was designed to play triple roles: "surface-reactive", "bio-reactive", and "bio-inert" ones. The pSAMs of poly(TMSMA-r-PEGMA-r-NAS) were formed on Si/SiO2 wafers with 1 h incubation of the substrates in the polymer solution, which showed approximately a 1 nm-thick film as measured by ellipsometry. After the formation of the pSAMs, the feasibility of the pSAMs as a dual functional surface (bio-inert and bio-reactive properties) was examined. The ability of the pSAMs to block nonspecific adsorption of proteins was evaluated against bovine serum albumin as a model protein. High-resolution N(1s) X-ray photoelectron spectroscopy (XPS) analysis on the protein adsorption revealed that significant reduction up to approximately 97% was observed compared to the unmodified Si/SiO2 wafer. In addition, micropatterns of streptavidin with high signal-to-noise ratios were achieved using microcontact printing (microCP) of NH2-bearing biotin onto the pSAMs of poly(TMSMA-r-PEGMA-r-NAS) on glass slides, which suggests that other biomolecules could also be efficiently immobilized onto the pSAMs with high specificity while minimizing nonspecific adsorption. On the other hand, the surface density of both bio-reactive and anti-biofouling functionality could be tailored by simply changing initial feed ratios of each monomer in the polymer synthesis: different molar ratios of the bio-reactive group (NAS: 33%, 20%, and 10%, respectively) were employed. When micropatterns of streptavidin were constructed, the pSAMs with 33% NAS moiety showed the highest immobilization of the protein. Taken together, the present dual functional, random copolymers may have warrant applications in the field of biosensors and biochips.

Water-repellent coating: formation of polymeric self-assembled monolayers onnanostructured surfaces

In this paper, we suggest a facile and effective method for water-repellent coating of oxidesurfaces. As a coating material, we synthesized a new random copolymer, referred to aspoly(TMSMA-r-fluoroMA), by the radical polymerization of 3-(trimethoxysilyl)propyl methacrylate (TMSMA) anda fluoromonomer® bearing methacrylate moiety (fluoroMA). The random copolymer was designed to consist of a‘surface-reactive part’ (trimethoxysilyl group) for anchoring onto oxide-based surfaces and a‘functional part’ (perfluoro group) for water repellency. The polymeric self-assembled monolayers(pSAMs) ofpoly(TMSMA-r-fluoroMA) were constructed on three different aluminum oxide substrates,such as flat, concave-textured, and nanoporous plates, and the staticwater contact angle of each surface before and after the formation ofpSAMs was measured.The formation of pSAMs resulted in significantly enhanced hydrophobicity compared withthe corresponding bare surfaces. In particular, among threepoly(TMSMA-r-fluoroMA)-coated surfaces, the nanoporous plate showed thehighest water-repellent property, with a static contact angle of∼163°, which is indicative of superhydrophobic surfaces.

Construction of Nonbiofouling Surfaces by Polymeric Self-Assembled Monolayers

The synthesis of a poly(ethylene glycol) (PEG)-grafted surface-reactive random copolymer and its self-assembled structure on Si/SiO2 substrates for construction of nonbiofouling surfaces are reported. The copolymer, poly(TMSMA-r-PEGMA), which is comprised of an “anchor part” (trimethoxysilane) and a “function part” (PEG), was synthesized by a radical polymerization reaction. The copolymer spontaneously formed monolayers on Si/SiO2 wafers with average thicknesses of 11 A. Tapping mode atomic force microscopy (AFM) revealed that the surface of the polymer monolayers was smooth with an average roughness of 1.3 A (root-mean-square). The protein resistance of the polymer monolayers on Si/SiO2 wafers was evaluated using insulin, lysozyme, and fibrinogen. For all tested proteins, the polymer monolayers showed significant reduction (up to 98%) in nonspecific protein adsorption compared to the unmodified Si/SiO2 wafers. In addition, cell adhesion as probed using 3T3 fibroblasts was significantly reduced on the pol...

Polymeric Self-Assembled Monolayers. 4. Chemical, Electrochemical, and Thermal Stability of ω-Functionalized, Self-Assembled Diacetylenic and Polydiacetylenic Monolayers

Here we contrast the stability of monomeric and photopolymerized self-assembled monolayers (SAMs) containing a diacetylene group (HS(CH2)10C⋮CC⋮C(CH2)10COOH) with SAMs prepared from simple n-alkanethiols. The polymerized SAMs are extremely durable compared to either the n-alkanethiol SAMs or the unpolymerized diacetylenic SAMs. For example, they are stable to repeated electrochemical cycling to extreme potentials, thermal excursions to 200 °C, and exposure to hot base (1:1 mixture of ethanol and 1.0 M aqueous KOH at 100 °C). All of these conditions completely strip n-alkanethiol SAMs from Au substrates. These high-performance materials are suitable for applications in lubrication, adhesion, corrosion passivation, and chemical sensing.

Polymeric Self-Assembled Monolayers. 5. Synthesis and Characterization of ω-Functionalized, Self-Assembled Diacetylenic and Polydiacetylenic Monolayers

Here we discuss the preparation and characterization of photopolymerizable, self-assembled monolayers (SAMs) that consist of acid-, hydroxyl-, and methyl-terminated n-alkanethiols containing a diacetylene group (HS(CH2)10C⋮CC⋮C(CH2)10X; X = COOH, CH2OH, and CH3, respectively). The acid and hydroxyl surfaces are readily amenable to further synthetic elaboration, while the methyl-terminated SAM results in a clean, low-energy surface. As demonstrated by Fourier transform infrared external reflectance spectroscopy (FTIR-ERS), surface-enhanced Raman spectroscopy (SERS), ellipsometry, UV−vis spectroscopy, and electrochemical methods, all three materials self assemble onto Au surfaces to form ordered monolayers that can be photopolymerized with UV light.

Polymeric Self-Assembled Monolayers. 3. Pattern Transfer by Use of Photolithography, Electrochemical Methods, and an Ultrathin, Self-Assembled Diacetylenic Resist

We report herein that a substrate can be patterned using a diacetylenic, self-assembled monolayer (SAM) resist and photolithographic and electrochemical methods. Our proof-of-concept experiments result in micron-scale pattern transfer onto Au substrates, but extension to other materials, including Si, Al, and GaAs, and nanometer-scale patterning will be straightforward. It is demonstrated that a SAM composed of diacetylenic organomercaptans can be used as a negative photolithographic resist. The importance of this method results from the fact that the resist is highly organized, thin, and largely defect free. These factors ensure high-resolution patterning and control over the surface energy and reactivity of the resist itself, which permits an added dimension of control over surface processing. In addition, resist stripping involves only gentle and easily controlled electrochemical methods. Moreover, we have previously shown that organized multilayers of polydiacetylenic SAMs can be easily formed, so resist thickness can be controlled over a broad range. Finally, unique electronic and photonic properties of the polymeric SAMs might themselves be integrated into Si-based devices using this lithographic approach in the future. 34 refs., 2 figs.

Polymeric Self-Assembled Monolayers. 2. Synthesis and Characterization of Self-Assembled Polydiacetylene Mono- and Multilayers

We report the synthesis and characterization of self-assembled mono- and multilayers of {omega}-functionalized diacetylenic thiols. We used the carboxylic acid-terminated alkanethiol diacetylene HS(CH{sub 2}){sub 10}C=CC=VC(CH{sub 2}){sub 10}-COOH as a block for preparing multilayers. Individual layers are covalently bonded through a thioester linkage. These two orthogonal polymerization steps yield a highly structured three-dimensional, surface-confined polymer. The results of infrared external reflectance spectroscopy and ellipsometry clearly indicate layer-by-layer growth up to at the least 6 layers. The multilayers can be polymerized by UV irradiation either after the deposition of each individual layer or after all layers are deposited. The appearance of bands attributable to C=C vibrational modes in resonance Raman spectra confirms polymerization. Transmission UV-vis spectroscopy indicates the films consist of the highly conjugated blue polydiacetylenic polymer, and that polymerization is complete within 5 min of initial irradiation. 26 refs., 4 figs.