Biosurfaces Fabricated by Polymerization-Induced Surface Self-Assembly

Surface biofunctionalization provides an approach to the fabrication of surfaces with improved biological and clinical performances. Biosurfaces have found increasing applications in many areas such as sensing, cell growth, and disease detection. Efficient synthesis of biosurfaces without damages to the structures and functionalities of biomolecules is a great challenge. Polymerization-induced surface self-assembly (PISSA) provides an effective approach to the synthesis of surface nanostructures with different compositions, morphologies, and properties. In this research, application of PISSA in the fabrication of biosurfaces is investigated. Two different reversible addition-fragmentation chain transfer (RAFT) agents, RAFT chain transfer agent (CTA) on silica particles (SiO₂-CTA) and CTA on bovine serum albumin (BSA-CTA), were employed in RAFT dispersion polymerization of N-isopropylacrylamide (NIPAM) in water at a temperature above the lower critical solution temperature (LCST) of poly-(isopropylacrylamide) (PNIPAM). After polymerization, PNIPAM layers with BSA on the top surfaces are fabricated on the surfaces of silica particles. Transmission electron microscopy results show that the average PNIPAM layer thickness increases with monomer conversion. Kinetics study indicates that there is a turn point on a plot of ln([M]₀/[M]_t) versus polymerization time. After the critical point, surface coassembly of PNIPAM brushes and BSA-PNIPAM bioconjugates is performed on the silica particles. The secondary structure and the activity of BSA immobilized on top of the PNIPAM layers are basically kept unchanged in the PISSA process. To prepare permanently immobilized protein surfaces, PNIPAM layers on silica particles are cross-linked. BSA on the top surfaces presents a reversible "on-off" switching property. At a temperature below the LCST of PNIPAM, the activity of the immobilized BSA is retained; however, the BSA activity decreases significantly at a temperature above the LCST because of the hydrophobic interaction between PNIPAM and BSA. Based on this approach, many different biosurfaces can be fabricated and the materials will find applications in many fields, such as enzyme immobilization, drug delivery, and tissue engineering.

Squish and CuAAC: additive-free covalent monolayers of discrete molecules in seconds

A terminal alkyne is immobilized rapidly into a full monolayer by squishing a small volume of a solution of the alkyne between an azide-modified surface and a copper plate. The monolayer is covalently attached to the surface through a copper-catalyzed alkyne-azide cycloaddition (CuAAC) reaction, and the coverages of the immobilized electroactive alkyne species are quantified by cyclic voltammetry. A reaction time of less than 20 s is possible with no other reagents required. The procedure is effective under aerobic conditions using either an aqueous or aprotic organic solution of the alkyne (1-100 mM).

Double protein functionalized poly-ε-caprolactone surfaces: in depth ToF-SIMS and XPS characterization

In biomaterial research, great attention has focussed on the immobilization of biomolecules with the aim to increase cell-adhesive properties of materials. Many different strategies can be applied. In previously published work, our group focussed on the treatment of poly-ε-caprolactone (PCL) films by an Ar-plasma, followed by the grafting of 2-aminoethyl methacrylate (AEMA) under UV-irradiation. The functional groups introduced, enabled the subsequent covalent immobilisation of gelatin. The obtained coating was finally applied for the physisorption of fibronectin. The successful PCL surface functionalization was preliminary confirmed using XPS, wettability studies, AFM and SEM. In the present article, we report on an in-depth characterization of the materials developed using ToF-SIMS and XPS analysis. The homogeneous AEMA grafting and the subsequent protein coating steps could be confirmed by both XPS and ToF-SIMS. Using ToF-SIMS, it was possible to demonstrate the presence of polymethacrylates on the surface. From peak deconvoluted XPS results (C- and N-peak), the presence of proteins could be confirmed. Using ToF-SIMS, different positive ions, correlating to specific amino-acids could be identified. Importantly, the gelatin and the fibronectin coatings could be qualitatively distinguished. Interestingly for biomedical applications, ethylene oxide sterilization did not affect the surface chemical composition. This research clearly demonstrates the complementarities of XPS and ToF-SIMS in biomedical surface modification research.

Electrochemical release of amine molecules from carbamate-based, electroactive self-assembled monolayers

In this paper, carbamate-based self-assembled monolayers (SAMs) of alkanethiolates on gold were suggested as a versatile platform for release of amine-bearing molecules in response to the electrical signal. The designed SAMs underwent the electrochemical oxidation on the gold surface with simultaneous release of the amine molecules. The synthesis of the thiol compounds was achieved by coupling isocyanate-containing compounds with hydroquinone. The electroactive thiol was mixed with 11-mercaptoundecanol [HS(CH(2))(11)OH] to form a mixed monolayer, and cyclic votammetry was used for the characterization of the release behaviors. The mixed SAMs showed a first oxidation peak at +540 mV (versus Ag/AgCl reference electrode), indicating the irreversible conversion from carbamate to hydroquinone groups with simultaneous release of the amine molecules. The analysis of ToF-SIMS further indicated that the electrochemical reaction on the gold surface successfully released amine molecules.

Label-free biological and chemical sensors

Highly sensitive, label-free biodetection methods have applications in both the fundamental research and healthcare diagnostics arenas. Therefore, the development of new transduction methods and the improvement of the existing methods will significantly impact these areas. A brief overview of the different types of biosensors and the critical parameters governing their performance will be given. Additionally, a more in-depth discussion of optical devices, surface functionalization methods to increase device specificity, and fluidic techniques to improve sample delivery will be reviewed.

Sulfonation of alkyl phenyl ether self-assembled monolayers

The sulfonation of phenyl ether decorated self-assembled monolayers (SAMs) was studied with an eye toward creating surfaces with a particularly high negative charge density based on a close-packed array of phenyl rings with more than one sulfonic acid group per molecule. The product distribution and kinetics of this process were studied by ultraviolet, infrared, and photoelectron spectroscopies and by monitoring changes in the thickness and wetting properties of the SAM. The sulfonation chemistry could be effected without undermining monolayer integrity and the isomer distribution of ortho- and para-monosulfonated material, along with the percentages of mono- and disulfonated molecules could be established throughout the process. As doubly sulfonated molecules appeared, the reaction slowed drastically. Ultimately, sulfonation stops completely with approximately 60% of the molecules disulfonated and 20% each of the two monosulfonated isomers. This striking constraint on monolayer reactivity and the relationship between the surface chemistry and variations in SAM structure are discussed.

Kinetics of enzyme attack on substrates covalently attached to solid surfaces: influence of spacer chain length, immobilized substrate surface concentration and surface charge

The use of alpha-chymotrypsin to cleave covalently bound N-acetyl- l-tryptophan (Ac-Trp-OH) from the surfaces of aminopropylated controlled pore glass (CPG) and the polymer PEGA 1,900 was investigated. Oligoglycine spacer chains were used to present the covalently attached Ac-Trp-OH substrate to the aqueous enzyme. In the absence of the oligoglycine spacer chain, the rate of release was relatively slow, especially from the PEGA 1,900. These slow rates reflect the position of the amino group to which Ac-Trp-OH is covalently attached. On the glass there was a clear optimum with a chain of four glycine residues. For PEGA 1,900 there is no real apparent change beyond two glycine residues. The decline in rate beyond these optima are a possible result of changes in oligoglycine structure. Comparing different surface loadings of bound substrate the rate of release of Ac-Trp-OH from CPG with a pore diameter of 1,200 A was optimal when using 83% of the maximum that can be coupled, then fell again at higher loading. The rate of Ac-Trp-OH release from CPG was the same for surface coverages of 0.4 and 1.0. The introduction of permanent surface charges on CPG 1,200 exhibits a distinct influence on enzymatic cleavage with an increase in the rate of biocatalysis at the surface. Optimal presentation of covalently immobilized substrate on different supports by use of appropriate linkers leads to favorable biocatalysis from the support.

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

In this paper, we suggest a facile and effective method for water-repellent coating of oxide surfaces. As a coating material, we synthesized a new random copolymer, referred to as poly(TMSMA-r-fluoroMA), by the radical polymerization of 3-(trimethoxysilyl)propyl methacrylate (TMSMA) and a 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) of poly(TMSMA-r-fluoroMA) were constructed on three different aluminum oxide substrates, such as flat, concave-textured, and nanoporous plates, and the static water contact angle of each surface before and after the formation of pSAMs was measured. The formation of pSAMs resulted in significantly enhanced hydrophobicity compared with the corresponding bare surfaces. In particular, among three poly(TMSMA-r-fluoroMA)-coated surfaces, the nanoporous plate showed the highest water-repellent property, with a static contact angle of ∼163°, which is indicative of superhydrophobic surfaces.

Reactivity of acid fluoride-terminated self-assembled monolayers on gold

This report describes the reactivity of acid fluoride (AF)-terminated self-assembled monolayers (SAMs) on gold toward amine and alcohol compounds and the potentiality of AF as a reactive intermediate for surface functionalizations. The AF group was generated in situ on a gold surface by reacting the terminal carboxylic acid group in the SAM of 16-mercaptohexadecanoic acid with cyanuric fluoride and pyridine under the optimized conditions. AF was found to be highly reactive toward various amine groups, such as primary and secondary amines, but it did not react effectively with alcohol. In addition, the amide coupling reaction by microcontact printing (microCP) was compared with the solution-based reaction: when amine-derivatized ferrocene compound was used for 1-min microCP on the AF-activated surface, the surface coverage of the reaction product was about 83% of 3.45 x 1014 cm-2, the coverage obtained in the solution-based reaction. On the basis of the high reaction efficiency of microCP, the AF-activated surface was also used as a platform for patterning a biological ligand, biotin.

Fluoro-N,N,N',N'-tetramethylformamidinium hexafluorophosphate: a reagent for formation of interchain carboxylic anhydrides on self-assembled monolayers

In this paper, we report the reactivity of fluoro-N,N,N',N'-tetramethylformamidinium hexafluorophosphate (TFFH), a reagent for transformation of carboxylic acids into acid fluorides in solution, toward self-assembled monolayers (SAMs) of 16-mercaptohexadecanoic acid on gold. Contrary to the solution-based reactions, we found that only interchain carboxylic anhydrides (ICAs), not acid fluorides (AFs), were obtained at surfaces by the facile interchain reaction under most reaction conditions studied. AFs were found to be formed only when tetrabutylammonium fluoride, a reagent inducing fast decomposition of ICAs, was added to the reaction mixture. The reactivity of TFFH toward carboxylic acid-terminated SAMs was different from that of cyanuric fluoride, which has been reported previously (Langmuir 2005, 21, 11765-11772). This study provides more insight into the role of the proximity effect in SAM-based reactions as well as another approach to the formation of ICAs from carboxylic acid-terminated SAMs.

Reactivity control of carboxylic acid-terminated self-assembled monolayers on gold: acid fluoride versus interchain carboxylic anhydride

Reactions that occur at interfaces often show different behaviors from their solution analogues. In this paper, we demonstrated how proximity effect, one of the unique phenomena at interfaces, could control the product distributions of interfacial reactions. Self-assembled monolayers (SAMs) of 16-mercaptohexadecanoic acid on gold surfaces were treated with cyanuric fluoride and pyridine, which are generally used for forming acid fluorides from carboxylic acids in the solution-based reaction. After the treatment, two different products, acid fluorides (AFs) and interchain carboxylic anhydrides (ICAs), were controllably obtained at surfaces under different reaction conditions with keeping the reagents the same. Various factors, such as the concentrations of reagents, reaction time, and additives, affected the product distribution (or the reaction pathway) at surfaces. We found that one of the key factors in controlling the reaction pathway was a relative contribution from the proximity effect of adjacent carboxylic acid chains in the SAMs (kinetic control) and the equilibrium shift (thermodynamic control). The relative reactivity of AF- and ICA-presenting surfaces toward primary amines, such as undecylamine and [((6-aminohexyl)amino)carbonyl]ferrocene, was also investigated, in terms of the number and the ordering of the amines coupled onto the surfaces.

In-plane enyne metathesis and subsequent Diels-Alder reactions on self-assembled monolayers

We report in-plane enyne metathesis and subsequent Diels-Alder reactions on self-assembled monolayers (SAMs) terminating in vinyl and acetylenyl groups on gold. After the formation of SAMs of vinyl and acetylenyl group-containing dithiols on gold, in-plane enyne metathesis of the vinyl and acetylenyl groups, leading to the formation of 1,3-diene, was achieved on the SAMs, and Diels-Alder reactions were then successfully performed with tetracyanoethylene, maleic anhydride, and maleimide. The reactions were confirmed by FT-IR spectroscopy, X-ray photoelectron spectroscopy, and time-of-flight secondary-ion mass spectrometry. In-plane enyne metathesis developed herein would offer a versatile platform for the functionalization of surfaces with mild reaction conditions and a high compatibility in functional groups.

Surface-initiated growth of poly d(A-T) by Taq DNA polymerase

In this paper, we report surface-initiated d(A-T) polymerization by Taq DNA polymerase as a method for constructing DNA-tethered surfaces using an enzyme. The enzymatic polymerization was conducted successfully via two steps: tethering of oligo d(A-T)s onto the surface presenting carboxylic acids by amide coupling and surface-initiated polymerization using Taq DNA polymerase. In this enzymatic polymerization process, the design and construction of carboxylic acid-presenting surfaces were found to be an important factor: DNA growth did not occur on the gold surface coated only with the self-assembled monolayer (SAM) of 16-mercaptohexadecanoic acid (MHDA), but effectively proceeded on the surfaces presenting mixed SAMs of MHDA and 1-pentadecanethiol. The coupling of oligo d(A-T)s and the subsequent DNA polymerization reaction were characterized by polarized infrared external reflectance spectroscopy, ellipsometry, X-ray photoelectron spectroscopy, and atomic force microscopy.