Route to smooth silica-based surfaces decorated with novel self-assembled monolayers (SAMs) containing glycidyl-terminated very long hydrocarbon chains

Ultrastable Glassy Polymer Films with an Ultradense Brush Morphology

Glassy polymer films with extreme stability could enable major advancements in a range of fields that require the use of polymers in confined environments. Yet, from a materials design perspective, we now know that the glass transition temperature (T_g) and thermal expansion of polymer thin films can be dramatically different from those characteristics of the bulk, i.e., exhibiting confinement-induced diminished thermal stability. Here, we demonstrate that polymer brushes with an ultrahigh grafting density, i.e., an ultradense brush morphology, exhibit a significant enhancement in thermal stability, as manifested by an exceptionally high T_g and low expansivity. For instance, a 5 nm thick polystyrene brush film exhibits an ∼75 K increase in T_g and ∼90% reduction in expansivity compared to a spin-cast film of similar thickness. Our results establish how morphology can overcome confinement and interfacial effects in controlling thin-film material properties and how this can be achieved by the dense packing and molecular ordering in the amorphous state of ultradense brushes prepared by surface-initiated atom transfer radical polymerization in combination with a self-assembled monolayer of initiators.

Fabrication of Biosensing Interface with Monolayers

Surface modification is recognized as one of the fundamental techniques to fabricate biosensing interfaces. This review focuses on the surface modification of carbon substrates (GC and HOPG) and silica with a close-packed monolayer, in particular. In the cases of carbon substrates, GC and HOPG, it was demonstrated that surface modification of carbon substrates with diazonium derivatives could create a close-packed monolayer similar to the self-assembled monolayer (SAM) formation with mercapto derivatives. Similarly, the potential of trialkoxysilanes to form a close-packed monolayer was evaluated, and modification with a close-packed monolayer tended to occur under milder conditions when the trialkoxysilanes had a longer alkyl chain. In these studies, we synthesized surface modification materials having ferrocene as a redox active moiety to explore features of the modified surfaces by an electrochemical method using cyclic voltammetry, where surface concentrations of immobilized molecules and blocking effect were studied to obtain insight for density leading to a close-packed layer. Based on those findings, fabrication of a biosensing interface on the silica sensing chip of the waveguide-mode sensor was carried out using triethoxysilane derivatives bearing succinimide ester and oligoethylene glycol moieties to immobilize antibodies and to suppress nonspecific adsorption of proteins, respectively. The results demonstrate that the waveguide-mode sensor powered by the biosensing interface fabricated with those triethoxysilane derivatives and antibody has the potential to detect several tens ng/mL of biomarkers in human serum with unlabeled detection method.

Epoxy-terminated self-assembled monolayers containing internal urea or amide groups

We report the synthesis of new coupling agents with internal amide or urea groups possessing an epoxy-terminal group and trimethoxysilyl-anchoring group. The structural characterizations of the corresponding self-assembled monolayers (SAMs) were performed by polarization modulation infrared reflection adsorption spectroscopy (PM-IRRAS). The molecular assembly is mainly based on the intermolecular hydrogen-bonding between adjacent amide or urea groups in the monolayers. Because of the steric hindrance of amide or urea groups, the distance between the alkyl chains is too large to establish van der Waals interactions, inducing their disorder. The reactivity of the epoxy-terminal groups was successfully investigated through reaction with a fluorescent probe. We show that SAMs containing internal urea or amide groups exhibited a higher density of accessible epoxide groups than the corresponding long-chain (C22) glycidyl-terminated SAM.

Layer-by-layer generation of PEG-based regenerable immunosensing surfaces for small-sized analytes

Small molecules (haptens) like pharmaceuticals or peptides can serve as targets for antibody binding in competitive immunoassay-based flow-through assays. In this work, a strategy for preparing polyethylene glycol (PEG) coatings for subsequent hapten immobilization on glass-type silica surfaces is presented and characterized in detail. Two substrates bearing terminal silanol groups were utilized, a glass slide and a silicon wafer. First, surfaces were thoroughly cleaned and pretreated to generate additional silanol groups. Then, a silane layer with terminal epoxy groups was created using 3-glycidyloxypropyltrimethoxysilane (GOPTS). Epoxy groups were used to bind a layer of diamino-poly(ethylene glycol) (DAPEG) with terminal amino groups. Finally, the low molecular weight compound diclofenac was bound to the surface to be used as model ligand for competitive biosensing of haptens. The elementary steps were characterized using atomic force microscopy (AFM), water contact angle measurement, grazing-angle attenuated total reflection (GA-ATR) FT-IR spectroscopy, and X-ray photoelectron spectroscopy (XPS). The data collected using these techniques have confirmed the successive grafting of the molecular species, evidencing, that homogeneous monolayers were created on the silica surfaces and validated the proposed mechanism of functionalization. The resulting surfaces were used to investigate polyclonal anti-diclofenac antibodies recognition and reversibility using quartz crystal microbalance with dissipation (QCM-D) measurements or an automated flow-through immunoassay with chemiluminescence (CL) read-out. For both techniques, recognition and reversibility of the antibody binding were observed. The stability of sensors over time was also assessed and no decrease in CL response was observed upon 14 days in aqueous solution. The herein presented strategy for surface functionalization can be used in the future as reproducible and reusable universal platform for hapten biosensors.

Surface modification of silicon oxide with trialkoxysilanes toward close-packed monolayer formation

In order to scrutinize potential of trialkoxysilanes to form close-packed monolayer, surface modification of silicon oxide was carried out with the trialkoxysilanes bearing a ferrocene moiety for analysis by electrochemical methods. As it was found that hydrogen-terminated silicon reacts with trialkoxysilane through natural oxidation in organic solvents, where the silicon oxide layer is thin enough to afford conductivity for electrochemical analysis, hydrogen-terminated silicon wafer was immersed in trialkoxysilane solution for surface modification without oxidation treatment. Cyclic voltammetry measurements to determine surface concentrations of the immobilized ferrocene-silane on silicon surface were carried out with various temperature, concentration, solvent, and molecular structure, while the blocking effect in the cyclic voltammogram was investigated to obtain insight into density leading to the close-packed layer. The results suggested that a monolayer modification tended to occur under milder conditions when the ferrocene-silane had a longer alkyl chain, and formation of a close-packed layer to show significant blocking effect was observed. However, the surface modification proceeded even when surface concentration of the immobilized ferrocene-silane was greater than that expected for the monolayer. On the basis of these tendencies, the surface of silicon oxide modified with trialkoxysilane is considered to be a partial multilayer rather than monolayer although a close-packed layer is formed. This result is supported by the comparison with carbon surface modified with ferrocene-diazonium, in which a significant blocking effect was observed when surface concentrations of the immobilized ferrocene moiety are lower than that for silicon oxide modified with ferrocene-silane.

Characterization of Polymer Brush Membranes via HF Etch Liftoff Technique

Surface modification using end-tethered polymer brushes is an attractive, versatile, and effective method of tailoring the surface properties of a material. However, because the chains are covalently attached, characterization of these films is limited. When polymer brushes are detached in their native state, as opposed to fabricating a cross-linked initiator support, additional analytical techniques can be employed. We report lifting off patterned polymer brush membranes from a silicon oxide surface via a hydrofluoric acid etch. This method allows examination of polymer brushes via TEM and thus provides information regarding the perfection of initiator self-assembled monolayer formation and brush growth, as well as the effect of different cross-linking procedures.

PM-IRRAS investigation of self-assembled monolayers grafted onto SiO2/Au substrates

Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was used to characterize self-assembled monolayers (SAMs). Novel ester-terminated organosilicon coupling agents possessing a trialkoxysilyl headgroup and a urea group in the linear alkyl chains (4) were synthesized and grafted onto SiO(2)/Au substrates (SiO(2) film of 200 Å thickness deposited on gold mirror). This composite substrate allowed the anchoring of SAMs and preserved the high reflectivity for infrared radiation. PM-IRRAS spectra with very high signal-to-noise ratios have been obtained in the mid-infrared spectral range allowing monitoring of the grafted SAMs. Quantitative analysis of the measured signal is described to compare PM-IRRAS and conventional IRRAS spectra. This quantitative analysis has been validated since the band intensities in the corrected PM-IRRAS and conventional IRRAS spectra are identical. Orientation information on the different functional groups has been obtained comparing the corrected PM-IRRAS spectrum with the one calculated using isotropic optical constants of ester-terminated organosilicon coupling agents 4. The carbonyls of the urea groups are preferentially parallel to the substrate surface favoring intermolecular hydrogen bonding and consequently a close packing of the molecules attached to the surface. By contrast, the alkyl chains present gauche defects and are poorly oriented.