Entropy-Driven Conformational Control of α,ω-Difunctional Bidentate-Dithiol Azo-Based Adsorbates Enables the Fabrication of Thermally Stable Surface-Grafted Polymer Films

Thermally stable radical initiator monolayers were prepared from uniquely designed α,ω-difunctional adsorbates with bidentate headgroups for the growth of nanoscale polymer films on metal surfaces. The length of the spacer separating the bidentate headgroups was varied to afford 4,4′-(diazene-1,2-diyl)­bis­(<i>N</i>-(16-(3,5-bis­(mercaptomethyl)­phenoxy)­hexadecyl)-4-cyanopentanamide) (<b>B16</b>), 4,4′-(diazene-1,2-diyl)­bis­(<i>N</i>-(16-(3,5-bis­(mercapto-methyl)­phenoxy)­decyl)-4-cyanopentanamide) (<b>B10</b>), and 4,4′-(diazene-1,2-diyl)­bis­(<i>N</i>-(4-(3,5-bis­(mercaptomethyl)­phenoxy)­butyl)-4-cyanopentanamide) (<b>B4</b>). The structural features of the self-assembled monolayers (SAMs) derived from <b>B16</b>, <b>B10</b>, and <b>B4</b> were characterized by X-ray photoelectron spectroscopy (XPS), ellipsometry, and polarization modulation infrared reflection–absorption spectroscopy (PM-IRRAS) and compared to those derived from an analogous α,ω-difunctional adsorbate with monodentate headgroups, 4,4′-(diazene-1,2-diyl)­bis­(4-cyano-<i>N</i>-(16-mercaptohexadecyl)­pentanamide (<b>M</b>). These studies demonstrate that the conformation (i.e., hairpin vs standing up) of the bidentate initiator adsorbates on gold surfaces was easily controlled by adjusting the concentration of the adsorbates in solution. The results of solution-phase thermal desorption tests revealed that the radical initiator monolayers generated from <b>B16</b>, <b>B10</b>, and <b>B4</b> exhibit an enhanced thermal stability when compared to those generated from <b>M</b>. Furthermore, a study of the growth of polymer films was performed to evaluate the utility of these new bidentate adsorbate SAMs as film-development platforms for new functional materials and devices. Specifically, surface-grafted polystyrene films were successfully generated from SAMs derived from <b>B16</b>. In contrast, attempts to grow polystyrene films from SAMs derived from <b>M</b> under a variety of analogous conditions were unsuccessful

Two Are Better than One: Bidentate Adsorbates Offer Precise Control of Interfacial Composition and Properties

Mixed self-assembled monolayers (SAMs) generated from perfluoro-, <i>n</i>-alkyl-, and oligo­(ethylene glycol)-terminated alkanedithiols were utilized to prepare two-dimensional interfaces with precise composition and wettability. Interfacial control was afforded simply by adjusting the mole fraction of the adsorbates present in the development solutions and was modulated by the dual contributions of the tailgroups of the bidentate thiols. In contrast, the composition and wettability of mixed SAMs generated from traditional monodentate thiols bearing analogous tailgroups failed to track systematically with the mole fractions of the adsorbates in the development solutions, reflecting a greater dependence on solvent/adsorbate interactions. All of the SAMs were thoroughly characterized by contact angle goniometry, ellipsometry, X-ray photoelectron spectroscopy (XPS), and polarization modulation infrared reflection–absorption spectroscopy (PM-IRRAS). Furthermore, solution-phase displacement tests performed to evaluate the stability of the adsorbates as a function of the adsorbate composition of the mixed alkanedithiolate films, provided evidence of the markedly enhanced stability associated with mixed SAMs formed from bidentate adsorbates

In Situ Vibrational Study of the Reductive Desorption of Alkanethiol Monolayers on Gold by Sum Frequency Generation Spectroscopy

In situ sum frequency generation vibrational spectroscopy (SFG) was used to monitor the reductive desorption of decanethiol (DT) and octadecanethiol (ODT) self-assembled monolayers (SAMs) grown on evaporated gold on silica. At negative potentials, the alkyl chains of both monolayers became disordered as monitored by the appearance of methylene symmetric stretching modes in the collected spectra. The increased tilting of the terminal methyl groups on the chains of the DT monolayer further support this observation. The disappearance of the methyl C–H stretching vibrational modes at the reductive potential suggests that DT molecules diffused away from the surface after reduction. ODT molecules, on the other hand, retained their two-dimensional structure near the gold surface, implied by the strong methyl vibrational modes at the reductive potential. After four reductive cycles, a large portion of the DT SAM was fully desorbed, while the ODT monolayer existed as a combination of physisorbed and chemisorbed molecules on the gold surface, held in place by the van der Waals interactions between the alkyl chains

Robust Maleimide-Functionalized Gold Surfaces and Nanoparticles Generated Using Custom-Designed Bidentate Adsorbates

A series of custom-designed alkanethioacetate ligands were synthesized to provide a facile method of attaching maleimide-terminated adsorbates to gold nanostructures via thiolate bonds. Monolayers on flat gold substrates derived from both mono- and dithioacetates, with and without oligo­(ethylene glycol) (OEG) moieties in their alkyl spacers, were characterized using X-ray photoelectron spectroscopy, polarization modulation infrared reflection–absorption spectroscopy, ellipsometry, and contact angle goniometry. For all adsorbates, the resulting monolayers revealed that a higher packing density and more homogeneous surface were generated when the film was formed in EtOH, but a higher percentage of bound thiolate was obtained in THF. A series of gold nanoparticles (AuNPs) capped with each adsorbate were prepared to explore how adsorbate structure influences aqueous colloidal stability under extreme conditions, as examined visually and spectroscopically. The AuNPs coated with adsorbates that include OEG moieties exhibited enhanced stability under high salt concentration, and AuNPs capped with dithioacetate adsorbates exhibited improved stability against ligand exchange in competition with dithiothreitol (DTT). Overall, the best results were obtained with a chelating dithioacetate adsorbate that included OEG moieties in its alkyl spacer, imparting improved stability via enhanced solubility in water and superior adsorbate attachment owing to the chelate effect

Line Tension and Line Activity in Mixed Monolayers Composed of Aliphatic and Terphenyl-Containing Surfactants

Custom-designed surfactants, known as “linactants”, have the ability to reduce the line tension between coexisting phases within mixed monolayers of chemically dissimilar compounds at the air–water interface. Thus far, linactants have been successfully identified for only one type of chemical dissimilarity, involving mixed monolayers of hydrocarbon and fluorocarbon surfactants. In the present work, we have pursued a more general interpretation of linactant compounds by extending the concept to a new system that is comprised of a mixture of aliphatic (pentadecanoic acid) and aromatic (<i>p</i>-terphenyl carboxylic acid) compounds. We found that the “bare” line tension between phases of this mixed monolayer was ∼4 pN, and within the same order of magnitude as our previous measurement in mixed monolayers containing hydrocarbons and fluorocarbons. Furthermore, we examined a homologous series of potential linactant compounds possessing an aliphatic tail of variable length and a <i>p-</i>terphenyl block. We determined that linactants with longer tails were able to reduce the line tension more efficiently and effectively. In particular, the addition of only 0.14% of a linactant with an 11-carbon chain reduced the line tension by more than a factor of 2. We hypothesize that the efficiency of this particular linactant is associated with its long tail; this creates strong van der Waals interactions with the aliphatic chains and enables the tail to adopt conformations that facilitate π-stacking interactions with the aromatic compounds within the monolayer

Quantitatively Resolving Ligand–Receptor Bonds on Cell Surfaces Using Force-Induced Remnant Magnetization Spectroscopy

Molecule-specific noncovalent bonding on cell surfaces is the foundation for cellular recognition and functioning. A major challenge in probing these bonds is to resolve the specific bonds quantitatively and efficiently from the nonspecific interactions in a complex environment. Using force-induced remnant magnetization spectroscopy (FIRMS), we were able to resolve quantitatively three different interactions for magnetic beads bearing anti-CD4 antibodies with CD4⁺ T cell surfaces based upon their binding forces. The binding force of the CD4 antibody–antigen bonds was determined to be 75 ± 3 pN. For comparison, the same bonds were also studied on a functionalized substrate surface, and the binding force was determined to be 90 ± 6 pN. The 15 pN difference revealed by high-resolution FIRMS illustrates the significant impact of the bonding environment. Because the force difference was unaffected by the cell number or the receptor density on the substrate, we attributed it to the possible conformational or local environmental differences of the CD4 antigens between the cell surface and substrate surface. Our results show that the high force resolution and detection efficiency afforded by FIRMS are valuable for studying protein–protein interactions on cell surfaces

Robust Thick Polymer Brushes Grafted from Gold Surfaces Using Bidentate Thiol-Based Atom-Transfer Radical Polymerization Initiators

A new bromoisobutyrate-terminated alkanethiol, 16-(3,5-bis­(mercaptomethyl)­phenoxy)­hexadecyl 2-bromo-2-methylpropanoate (BMTBM), was designed as a bidentate adsorbate to form thermally stable bromoisobutyrate-terminated self-assembled monolayers (SAMs) on flat gold surfaces to conduct atom-transfer radical polymerizations (ATRPs). The monolayers derived from BMTBM were characterized by ellipsometry, X-ray photoelectron spectroscopy (XPS), and polarization modulation infrared reflection–absorption spectroscopy (PM-IRRAS) and compared to the monolayers formed from 16-mercaptohexadecyl 2-bromo-2-methylpropanoate (MBM), 16-(3-(mercaptomethyl)­phenoxy)­hexadecyl 2-bromo-2-methyl-propanoate (MTBM), and octadecanethiol (C18SH). In this study, although the monolayer derived from BMTBM was less densely packed than those derived from MBM and MTBM, the bidentate adsorbates demonstrated much higher thermal stability in solution-phase thermal desorption tests, owing to the “chelate effect”. The enhanced stability of the BMTBM SAMs ensured the development of thick brushes of poly­(methyl methacrylate) and polystyrene at elevated temperatures (60, 90, 105, and 120 °C). In contrast, SAMs derived from MBM and MTBM failed to grow polymer brushes at temperatures above 100 °C

Self-Assembled Monolayer Films Derived from Tridentate Cyclohexyl Adsorbates with Alkyl Tailgroups of Increasing Chain Length

Tridentate cyclohexyl-based alkanethiolate SAMs generated from a series of adsorbates of the form R₃C₆H₆(CH₂SH)₃, where R = −(CH₂)_<i>n</i>H and <i>n</i> = 3, 8, and 13 (<b>3CnCyTSH</b>), were examined. Characterization of the SAMs by X-ray photoelectron spectroscopy (XPS) revealed that all sulfur atoms of the tridentate adsorbates were bound to the surface of gold, and that the tailgroups were in general less densely packed than the SAM derived from octadecanethiol (<b>C18SH</b>). For each of the SAMs, the relative molecular coverage on the surface was estimated from the XPS-derived C_1s/Au_4f ratios. The trend in conformational order for these SAMs as determined by the surface interactions with contacting liquids and the relative crystallinity of the alkyl chains as revealed by the PM-IRRAS spectra were found to decrease as follows: <b>C18SH</b> ≫ <b>3C13CyTSH</b> > <b>3C8CyTSH</b> > <b>3C3CyTSH</b>. A preliminary study of the thermal stability of the SAMs as evaluated by XPS indicates that the SAM generated from the cyclohexyl-based adsorbate with the longest alkyl chain, <b>3C13CyTSH</b>, is markedly more stable than the SAM generated from <b>C18SH</b>. Overall, the results suggest that the stability of the SAMs are influenced by both the length of the alkyl chains and the chelate effect associated with the tridentate adsorbates

Synthesis, Characterization, and Relative Stabilities of Self-Assembled Monolayers on Gold Generated from Bidentate <i>n</i>‑Alkyl Xanthic Acids

A series of self-assembled monolayers (SAMs) on gold were generated by the adsorption of <i>n</i>-alkyl xanthic acids (NAXAs) having the general formula CH₃(CH₂)_<i>n</i>OCS₂H (<i>n</i> = 12–15). The structural features of these SAMs were characterized by optical ellipsometry, contact angle goniometry, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS). This series of xanthate SAMs were compared to SAMs generated from the corresponding <i>n</i>-alkanethiols and aliphatic dithiocarboxylic acids (ADTCAs). The collected data indicate that the NAXAs generate densely packed and well-ordered monolayers. The contact angles of hexadecane on the xanthate monolayers exhibited a large “odd–even” effect similar to that produced by the ADTCA SAMs. The relative stability of these bidentate xanthate SAMs was evaluated by monitoring the changes in ellipsometric thicknesses and wettability as a function of time under various conditions. The results demonstrate that SAMs formed from NAXAs are much less stable than analogous <i>n</i>-alkanethiolate and ADTCA SAMs

In Situ Growth of Hollow Gold–Silver Nanoshells within Porous Silica Offers Tunable Plasmonic Extinctions and Enhanced Colloidal Stability

Porous silica-coated hollow gold–silver nanoshells were successfully synthesized utilizing a procedure where the porous silica shell was produced prior to the transformation of the metallic core, providing enhanced control over the structure/composition of the bimetallic hollow core. By varying the reaction time and the precise amount of gold salt solution added to a porous silica-coated silver-core template solution, composite nanoparticles were tailored to reveal a readily tunable surface plasmon resonance that could be centered across the visible and near-IR spectral regions (∼445–800 nm). Characterization by X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, scanning electron microscopy, and transmission electron microscopy revealed that the synthetic methodology afforded particles having uniform composition, size, and shape. The optical properties were evaluated by absorption/extinction spectroscopy. The stability of colloidal solutions of our composite nanoparticles as a function of pH was also investigated, revealing that the nanoshells remain intact over a wide range of conditions (i.e., pH 2–10). The facile tunability, enhanced stability, and relatively small diameter of these composite particles (∼110 nm) makes them promising candidates for use in tumor ablation or as photothermal drug-delivery agents