Azide-terminated Self-assembled Monolayers Research Articles

Electrochemical Characterization of Redox Probes Confined in 3D Conducting Polymer Networks.

In this manuscript we present a versatile platform for introducing functional redox species into tailor‐made 3D redox polymer networks. Electrochemical characterization based on cyclic voltammetry is applied to verify the immobilization of the redox species within the conducting networks. Ultimately this strategy shall be extended to (photo)electrocatalytic applications which will profit from the conducting polymer matrix. Soluble precursor copolymers are synthesized via radical copolymerization of vinyltriphenylamine (VTPA) with chloromethylstyrene (CMS) in different ratios, whereas CMS is subsequently converted into azidomethylstyrene (AMS) to yield poly(VTPA‐co‐AMS) copolymers. Spin‐coating of poly(VTPA‐co‐AMS) on gold electrodes yields thin films which are converted into stable polymer network structures by electrochemical crosslinking of the polymer chains via their pendant triphenylamine groups to yield N,N,N′,N′‐tetraphenylbenzidine (TPB) crosslinking points. Finally, the resulting redox‐active, TPB‐crosslinked films are functionalized with ethynylferrocene (EFc) as a representative redox probe using a click reaction. Main experimental tools are polarization modulation infrared reflection absorption spectroscopy and scan rate dependent cyclic voltammetry. Especially the latter proves the successful conversion and the immobilization of redox probes in the polymer matrix. The results are compared with the reference system of azide‐terminated self‐assembled monolayers on gold substrates, allowing to distinguish between free and immobilized EFc species.

Maximizing Conversion of Surface Click Reactions for Versatile Molecular Modification on Metal Oxide Nanowires.

Click reactions (e.g., Huisgen cycloaddition) on metal oxide nanostructures offer a versatile and robust surface molecular modification for various applications because they form strong covalent bonds in a wide range of molecular substrates. This study reports a rational strategy to maximize the conversion rate of surface click reactions on single-crystalline ZnO nanowires by monitoring the reaction progress. p-Polarized multiple-angle incidence resolution spectrometry (pMAIRS) and Fourier-transformed infrared (FT-IR) spectroscopy were employed to monitor the reaction progress of an azide-terminated self-assembled monolayer (SAM) on single-crystalline ZnO nanowires. Although various reaction parameters including the concentration of Cu(I) catalysts, triazolyl ligands, solvents, and target alkynes were systematically examined for the surface click reactions, 10-30% of terminal azide on the nanowire surface remained unreacted. Temperature-dependent FT-IR measurements revealed that such unreacted residual azides deteriorate the thermal stability of the nanowire molecular layer. To overcome this observed conversion limitation of click reactions on nanostructure surfaces, we considered the steric hindrance around the closely packed SAM reaction points, then experimented with dispersing the azide moiety into a methyl-terminated SAM. The mixed-SAM method significantly improved the azide conversion rate to almost 100%. This reaction method enables the construction of spatially patterned molecular surface modifications on metal oxide nanowire arrays without detrimental unreacted azide groups.

DNA Loading and Release Using Custom-Tailored Poly(l-lysine) Surfaces.

This Article describes the generation and study of surfaces modified with custom-crafted poly(l-lysine) (PLL) coatings for use in the loading and delivery of single-stranded DNA (ssDNA). The experimental strategy utilizes bidentate dithiol adsorbates to generate stably bound azide-terminated self-assembled monolayers (SAMs) on gold possessing an oligo(ethylene glycol) (OEG) spacer. Consequent to the molecular assembly on gold, the azide termini are covalently attached to a maleimide linker moiety via a copper-catalyzed azide-alkyne "click" reaction. Functionalization with maleimide provides a platform for the subsequent attachment of cysteine-terminated poly(l-lysine) (PLL), thus forming a suitable surface for the loading of ssDNA via electrostatic interactions. In efforts to maximize DNA loading, we generate SAMs containing mixtures of short and long PLL segments and explore the DNA-loading capability of the various PLL SAMs. We then use thermal increases to trigger the release of the ssDNA from the surface. By examining the loading and release of ssDNA using these new two-dimensional systems, we gain preliminary insight into the potential efficacy of this approach when using three-dimensional gold nanostructure systems in future gene-delivery and biosensing applications.

Click functionalization of phenyl-capped bithiophene on azide-terminated self-assembled monolayers

We immobilized tetra(ethylene glycol)-substituted phenyl-capped bithiophene with alkyne terminals (Ph2TPh-alkyne) on azide-terminated self-assembled monolayers (N3-SAMs) by Cu-catalyzed azide-alkyne cycloaddition reaction. Ph2TPh-functionalized SAMs on a gold substrate showed reversible electrochemical response. The surface densities of the azide groups in N3-SAMs and Ph2TPh units in Ph2TPh-functionalized SAMs were estimated to be 7.3±0.3×10−10molcm−2 and 4.6±0.3×10−10molcm−2, respectively, by quartz crystal microbalance (QCM). Most of Ph2TPh-alkynes are considered to be anchored on N3-SAMs via both terminal groups. Ph2TPh-functionalized SAMs exhibited reversible redox peaks in cyclic voltammetry (CV). In redox reaction, reversible capture and release of the counter anion could be monitored by electrochemical QCM (E-QCM).

Fibrillar structures formed by covalently bound, short, β-stranded peptides on self-assembled monolayers.

The ability to maintain or reproduce biomolecular structures on inorganic substrates has the potential to impact diverse fields such as sensing and molecular electronics, as well as the study of biological self-assembly and structure-function relationships. Because the structure and self-assembly of biomolecules are exquisitely sensitive to their local chemical and electrostatic environment, the goal of reproducing or mimicking biological function in an abiological environment, including at a surface, is challenging. However, simple and well-characterized chemical modifications of prepared surfaces can be used to tune surface chemistry, structure, electrostatics, and reactivity of inorganic materials to facilitate biofunctionalization and function. Here, we describe the covalent attachment of 13-residue β-stranded peptides containing alkyne groups to a flat gold surface functionalized with an azide-terminated self-assembled monolayer through a Huisgen cycloaddition, or "click", reaction. The chemical composition and structural morphology of these surfaces were characterized using X-ray photoelectron spectroscopy, grazing incidence angle reflection-absorption infrared spectroscopy, surface circular dichroism, and atomic force microscopy. The surface-bound β-strands self-assemble into antiparallel β-sheets to form fibrillar structures 24.9 ± 1.6 nm in diameter and 2.83 ± 0.74 nm in height on the reactive surface. The results herein provide a platform for studying and controlling the self-assembly process of biomolecules into larger supermolecular structures while allowing tunable control through chemical functionalization of the surface. Interest in the mechanisms of formation of fibrillar structures has most commonly been associated with neurodegenerative diseases, such as Alzheimer's and Parkinson's, but fibrils may actually represent the thermodynamic low-energy conformation of a much larger class of peptides and proteins. The protocol developed here is an important step toward uncovering not only the factors that dictate self-assembly but also the mechanisms by which this fibrillar class of superstructures forms.

Synthesis and Micropatterning of Photocatalytically Reactive Self-Assembled Monolayers Covalently Linked to Si(100) Surfaces via a Si–C Bond

Selective generation of an amine-terminated self-assembled monolayer bound to silicon wafers via a silicon-carbon linkage was realized by photocatalytically reducing the corresponding azide-terminated, self-assembled monolayers (Az-SAMs). The Az-SAM was obtained by thermal deposition of 11-chloroundecene onto a hydrogen-terminated silicon wafer followed by nucleophilic substitution of the chloride with the azide ion in warm N,N'-dimethylformamide (DMF). The presence of the terminal azide group on the SAM was confirmed by reflection absorption infrared spectroscopy (RAIRS), by X-ray photoelectron spectroscopy (XPS), and by detecting the formation of a triazole upon reaction of the azide with an activated alkyne. The desired terminal amine groups were generated by photocatalytic reduction of the Az-SAM with cadmium selenide quantum dots (CdSe Qdots) using λ > 400 nm. Analysis of the reduced SAM by XPS gave results that were consistent with those obtained with an amine-terminated surface obtained by reducing the Az-SAM with triphenylphosphine. To demonstrate the feasibility of using the Az-SAM for surface patterning, a sample was coated with adsorbed CdSe Qdots and exposed to the output of a diode laser at λ = 407 nm through a micropatterned mask. Using a SEM, the pattern formed in this manner was revealed after removing the CdSe Qdots and subsequently adsorbing 10 nm gold nanoparticles (AuNPs) to the positively charged terminal-amine groups. The formation of the pattern by CdSe-photocatalyzed reduction of the azide demonstrates a novel route to create features by selective modification of organic monolayers on silicon wafers.

Immobilization of Liposomes and Vesicles on Patterned Surfaces by a Peptide Coiled‐Coil Binding Motif

Patchy surfaces: An azide-terminated self-assembled monolayer was patterned with the peptide sequence (EIAALEK)(3) by using microcontact printing. This sequence forms stable coiled-coil heterodimers with the complementary peptide (KIAALKE)(3). By introducing this peptide to the surface of phospholipid liposomes and cyclodextrin vesicles, liposomes and vesicles can be immobilized at the patterned surface.

Evidence of click and coordination reactions on a self-assembled monolayer by synchrotron radiation based XPS and NEXAFS

An ethynylterpyridine was “clicked” to an azide-terminated self-assembled monolayer on gold and characterized by synchrotron radiation based surface analysis as NEXAFS and XPS. The detection of azide and terpyridine signatures confirmed a partial click reaction at room temperature. The absence of the azides after reaction at 50°C indicates an almost complete conversion. For the latter case successful Pd(II) coordination has been proven. The Au–S interface of the SAMs has been characterized by S 1s and S 2p XPS.

Orthogonal, metal-free surface modification by strain-promoted azide–alkyne and nitrile oxide–alkene/alkyne cycloadditions

In this article we present a fast and efficient methodology for biochemical surface patterning under extremely mild conditions. Micropatterned azide/benzaldoxime-surfaces were prepared by microcontact printing of a heterobifunctional cyclooctyne oxime linker on azide-terminated self-assembled monolayers (SAMs). Strain-promoted azide–alkyne cycloaddition (SPAAC) in combination with microcontact printing allows fast and effective surface patterning. The resulting bifunctional azide/oxime substrates could successfully be used for metal-free, orthogonal immobilization of various biomolecules by 1,3-dipolar cycloadditions at room temperature. Azide-decorated areas were modified by reaction with a cyclooctyne-conjugate using SPAAC, while benzaldoxime-decorated areas were activated by in situ oxidation to the reactive nitrile oxides and subsequent nitrile oxide cycloaddition with alkene- and alkyne-functionalized bioconjugates. In addition, orthogonal double immobilization was achieved by consecutive and independent SPAAC and nitrile oxide cycloadditions.

Magnetic Nanoparticle Assembly on Surfaces Using Click Chemistry

Controlled assembly of ferromagnetic nanoparticles on surfaces is of crucial importance for a range of spintronic and data storage applications. Here, we present a novel method for assembling monolayers of ferromagnetic FePt nanoparticles on silicon oxide substrates using "click chemistry". Reaction of alkyne-functionalized FePt nanoparticles with azide-terminated self-assembled monolayers (SAMs), on silicon oxide, leads to the irreversible attachment of magnetic nanoparticles to the surface via triazole linkers. Based on this covalent interaction, well-packed monolayers of FePt nanoparticles were prepared and nanoparticle patterns are generated on surfaces via microcontact printing (μCP).

Tethering Hydrophobic Peptides to Functionalized Self-Assembled Monolayers on Gold through Two Chemical Linkers Using the Huisgen Cycloaddition

Gold surfaces functionalized with an α-helical peptide have been generated by reacting an azide-terminated self-assembled monolayer with structured peptides containing two cyanophenylalanines through a Huisgen cycloaddition. Mixed monolayers of a reactive bromine-terminated thiol and inert alkane thiol were prepared at various concentrations of the Br-terminated moiety. These were reacted with sodium azide to form azide-terminated monolayers with controlled concentration of the reactive azide. These surfaces were studied through ellipsometry and X-ray photoelectron spectroscopy, which demonstrated that the concentration of the reactive azide group on the surface is controlled by the chemical conditions under which the monolayer is prepared. Grazing incident angle surface infrared spectroscopy (GRAS-IR) of the azide-terminated surface demonstrated that the azide is approximately perpendicular to the plane of the surface, as expected. These surfaces were then exposed to an α-helical peptide composed of alternating leucine and lysine residues, with two residues replaced with cyanophenylalanine to react with two neighboring surface-bound azide groups to bind the peptide to the surface through two covalent bonds. The yield of this reaction was quantified through monitoring the absorption of the azide group by GRAS-IR. Despite damage to the monolayer during the reaction, reaction yields of 80-98% were determined for optimized reaction conditions. Although the peptide retains its α-helical configuration under the reaction conditions, GRAS-IR analysis of the amide I and II modes of the surface-bound peptide showed that it is probably randomly oriented on the surface.

Heterogeneous Catalysis of a Copper-Coated Atomic Force Microscopy Tip for Direct-Write Click Chemistry

We report a constructive scanning probe lithography method that uses heterogeneous copper-coated atomic force microscopy tips to catalyze azide-alkyne cycloadditions (CuAAC) between solvated terminal alkyne molecules and azide-terminated self-assembled monolayers on silicon surfaces. Spatially controlled surface functionalization was carried out successfully with 50 mM ethanolic solutions of small molecules bearing terminal alkyne groups--propargylamine, 4-pentynoic acid, and an alkynyl-oligoethyleneoxide. We observed that reaction occurs only where the copper tip is in contact with an azide-terminated surface resulting in features with linewidths on the order of 50 nm. The extent of surface functionalization, as measured by changes in surface topography and lateral force microscopy, depends on the scanning force (31-350 nN) and scanning speed, with significant surface patterning observed even at speeds as high as 64 microm/s. In contrast with related SPL techniques, this approach affords a direct-write lithographic approach to constructively modifying and patterning surfaces at the nanoscale without the need for auxiliary reagents. All that is required is (1) an azide surface, (2) a solution of a terminal alkyne, and (3) a copper-coated AFM tip. These advantages allow the direct attachment of a potentially limitless library of molecules that bear terminal alkyne functionalities, including biomolecules, under relatively mild conditions, with sub-100 nm spatial resolution.

Reversible Supramolecular Functionalization of Surfaces: Terpyridine Ligands as Versatile Building Blocks for Noncovalent Architectures

We report on the reversible and selective functionalization of surfaces by utilizing supramolecular building blocks. The reversible formation of terpyridine bis-complexes, based on a terpyridine ligand-functionalized monolayer, is used as a versatile supramolecular binding motif. Thereby, click chemistry was applied to covalently bind an acetylene functionalized Fe(II) bis-complex onto azide-terminated self-assembled monolayers. By decomplexation of the formed supramolecular complex, the ligand modified monolayer could be obtained. These monolayers were subsequently used for additional complexation reactions, resulting in the reversible functionalization of the substrates. The proper choice of the coordinating transition metal ions allows the tuning of the binding strength, as well as the physicochemical properties of the formed complexes and thus an engineering of the surface properties.

Photocatalytic Reduction of an Azide-Terminated Self-Assembled Monolayer Using CdS Quantum Dots

Ordered, tightly packed aryl-azide-terminated, self-assembled monolayers (SAMs) were created on gold substrates from a new disulfide precursor. These monolayers were reduced at least partially in an aqueous environment using approximately 2 nm CdS quantum dots (Qdots) as photocatalysts to give mixed monolayers of arylamine- and aryl azide-terminated species. The CdS photocatalysts were made available for the reaction by exposure of the azide-terminated SAM to Qdots initially in solution or by preadsorption of the CdS nanoparticles on the SAM. In either case, X-ray photoelectron spectroscopy (XPS), grazing angle Fourier transform infrared spectroscopy (FTIR), and contact angle measurements were used to show the occurrence of the photocatalytic reduction. As further evidence for the presence of arylamine-terminated thiolate in the reduced SAM, these arylamine groups were successfully tagged with fluorescein isothiocyanate (FITC). The use of Qdot photocatalysts to functionalize surfaces may lead to a means to pattern surfaces at the nanoscale.