Alkylthiol Self-assembled Monolayers Research Articles

Energetics and Redox Kinetics of Pure Ferrocene-Terminated N-Heterocyclic Carbene Self-Assembled Monolayers on Gold.

N-heterocyclic carbene (NHC) self-assembled monolayers (SAMs) on gold have received considerable attention, but little is known about the lateral interactions between neighboring NHC molecules, their stability when subjected to aggressive oxidizing/reducing conditions, and their interactions with solution ions, all of which are essential for their use in a wide range of applications. To address these deficiencies, we present a comprehensive investigation of two different ferrocene (Fc)-terminated NHC SAMs with different chain lengths and linking groups. Pure monolayers of Fc-terminated NHCs display only a single, symmetrical pair of redox peaks, implying the formation of a homogeneous SAM structure with uniformly distributed Fc/Fc+ redox centers. By comparison, pure Fc-alkylthiol SAMs exhibit complex and impractical redox chemistry and require surface dilution in order to achieve reproducible properties. The NHC SAMs examined in this study exhibit very fast Fc redox kinetics and comparable or even superior stability against the application of multiple potential cycles or long-time holding at constant potential compared to alkylthiol SAMs. Furthermore, ion pairing of Fc+ and hydrophobic perchlorate and other hydrophilic anions is observed with Fc-NHC SAMs, highlighting conditions favorable for future applications of these monolayers. This study should therefore shed light on the very promising characteristics of redox-active NHC SAMs as an alternative to traditional Fc-alkylthiol SAMs for multiple practical applications, including in sensors and electrocatalysis.

Photoswitchable Machine-Engineered Plasmonic Nanosystem with High Optical Response for Ultrasensitive Detection of microRNAs and Proteins Adaptively

Modulating optoelectronic properties of inorganic nanostructures tethered with light-responsive molecular switches by their conformational change in the solid state is fundamentally important for advanced nanoscale-device fabrication, specifically in biosensing applications. Herein, we present an entirely new solid-state design approach employing the light-induced reversible conformational change of spiropyran (SP)-merocyanine (MC) covalently attached to gold triangular nanoprisms (Au TNPs) via alkylthiolate self-assembled monolayers to produce a large localized surface plasmon resonance response (∼24 nm). This shift is consistent with the increase in thickness of the local dielectric shell-surrounded TNPs and perhaps short-range dipole-dipole (permanent and induced) interactions between TNPs and the zwitterionic MC form. Water contact angle measurement and Raman spectroscopy characterization unequivocally prove the formation of a stable TNP-MC structural motif. Utilizing this form, we fabricated the first adaptable nanoplasmonic biosensor, which uses an identical structural motif for ultrasensitive, highly specific, and programmable detection of microRNAs and proteins at attomolar concentrations in standard human plasma and urine samples, and at femtomolar concentrations from bladder cancer patient plasma (n = 10) and urine (n = 10), respectively. Most importantly, the TNP-MC structural motif displays a strong binding affinity with receptor molecules (i.e., single-stranded DNA and antibody) producing a highly stable biosensor. Taken together, the TNP-MC structural motif represents a multifunctional super biosensor with the potential to expand clinical diagnostics through simplifying biosensor design and providing highly accurate disease diagnosis.

Modeling of Quasi-Static Floating-Gate Transistor Biosensors.

Floating-gate transistors (FGTs) are a promising class of electronic sensing architectures that separate the transduction elements from molecular sensing components, but the factors leading to optimum device design are unknown. We developed a model, generalizable to many different semiconductor/dielectric materials and channel dimensions, to predict the sensor response to changes in capacitance and/or charge at the sensing surface upon target binding or other changes in surface chemistry. The model predictions were compared to experimental data obtained using a floating-gate (extended gate) electrochemical transistor, a variant of the generic FGT architecture that facilitates low-voltage operation and rapid, simple fabrication using printing. Self-assembled monolayer (SAM) chemistry and quasi-statically measured resistor-loaded inverters were utilized to obtain experimentally either the capacitance signals (with alkylthiol SAMs) or charge signals (with acid-terminated SAMs) of the FGT. Experiments reveal that the model captures the inverter gain and charge signals over 3 orders of magnitude variation in the size of the sensing area and the capacitance signals over 2 orders of magnitude but deviates from experiments at lower capacitances of the sensing surface (<1 nF). To guide future device design, model predictions for a large range of sensing area capacitances and characteristic voltages are provided, enabling the calculation of the optimum sensing area size for maximum charge and capacitance sensitivity.

Detection and amplification of capacitance- and charge-based signals using printed electrolyte gated transistors with floating gates

The electrolyte gated transistor with a floating gate (FGT) is a promising sensing platform for both chemical and biodetection applications due to its fast, label-free response, low voltage operation, and simple fabrication by printing and conventional lithography. We present here a unified framework for understanding how FGTs measure changes in interfacial capacitance and surface charge, using self-assembled monolayers (SAMs) on the FGT detection area as model systems. The capacitance measurements take advantage of alkyl thiol SAMs with different chain lengths, while the charge experiments deprotonate 11-mercaptoundecanoic acid by changing solution pH. The effects of capacitance and surface charge are identified readily by analysis of the changes in the quasi-static current–voltage (ID–VG) characteristics of a stand-alone FGT in response to changes in the surface properties; changes in capacitance produce changes in slope, whereas changes in surface charge cause horizontal shifts in the transfer curves. For sensing applications, it is preferable to integrate the FGT into a resistor-loaded inverter to take advantage of the amplified voltage output relative to a stand-alone FGT. For inverters, changes in capacitance lead to changes in inverter gain, whereas changes in surface charge produce horizontal shifts in the inverter curves. A capacitance sensitivity of 70 mV/(μF/cm2) and a charge sensitivity of 40 mV/(μC/cm2) are obtained from the inverter output voltages. The ability to sense both capacitance and charge, and to distinguish between them, makes FGTs attractive for the detection of a wide variety of targets in chemical and biological sensing applications.

Effect of amino, hydroxyl, and carboxyl terminal groups of alkyl chains of self‐assembled monolayers on the adsorption pattern of gold nanoparticles

The adsorption pattern of gold nanoparticles (AuNPs) on functionalized self‐assembled monolayers (SAMs) produced on a Au(111) surface was characterized. The Au(111) was modified with 11‐amino‐1‐undecanethiol hydrochloride (AUT), 11‐mercapto‐1‐undecanol (MUT), or 11‐mercaptoundecanoic acid (MUA) at an elevated temperature and pressure. The AuNPs aggregated on the AUT‐SAM surface, whereas they were well dispersed on the MUT‐SAM surface and localized on the MUA‐SAM surface. The results suggest that interactions between AuNPs differ according to the degree of peeling of citrate‐layer‐capped AuNPs. The degree of peeling, which is related to both the surface randomness of the SAMs and the functional characteristics of the terminal group of each SAM, was discussed on the basis of scanning tunneling microscopy observations, X‐ray photoelectron spectroscopic analyses, and contact angle measurements. Our study shows that AuNP patterns can be controlled by changing the terminal group of the alkyl thiol SAM on a Au(111) surface.

Realizing high aspect ratio silver micro and nanostructures by microcontact printing of alkyl thiol self-assembled monolayers

ABSTRACTThe patterning of gold and silver micro and nanostructures on rigid and flexible substrates is investigated by microcontact printed thiol based self-assembled monolayers. The aspect ratio of the noble metal micro and nanostructures is determined by interaction of the -SH head group of the CH3(CH2)19SH molecules and the surface of the noble metal. Silver micro and nanostructures with &gt;10 times higher aspect ratios can be realized in comparison to commonly realized gold micro and nanostructures. The printing process is described, and the etching process is characterized in terms of etching window and etching selectivity. Potential electronic and photonic applications of the micro and nanostructures are discussed taking the boundary conditions of the printing process and the selected material system into consideration.

Direct Measurement of Water Permeation in Submerged Alkyl Thiol Self-Assembled Monolayers on Gold Surfaces Revealed by Neutron Reflectometry.

Self-assembled monolayers (SAMs) of alkyl thiols are frequently used to chemically functionalize gold surfaces for applications throughout materials chemistry, electrochemistry, and biotechnology. Despite this, a detailed understanding of the structure of the SAM-water interface generated from both formation and use of the SAM in an aqueous environment is elusive, and analytical measurements of the structure and chemistry of the SAM-water interface are an ongoing experimental challenge. To address this, we used neutron reflectometry (NR) to measure water association with both hydrophobic and hydrophilic SAMs under both wet and dry conditions. SAMs used for this study were made from hydrophobic decanethiol mixed with hydrophilic 11-azido-1-undecanethiol with compositions of 0-100% of the azide-terminated thiol. All SAMs were formed by conventional solution incubation of a Au substrate immersed in ethanol. Each SAM was characterized by grazing incidence angle reflection-absorption Fourier transfer infrared spectroscopy, contact angle goniometry, and electrochemical methods to confirm it was a completely formed monolayer with evidence of extensive crystalline-like domains. NR measured significant absorption of water into each SAM, ranging from 1.6 to 5.7 water molecules per alkyl thiol, when SAMs were immersed in water. Water infiltration was independent of SAM composition and terminal group hydrophilicity. These results demonstrate that water accesses defects, fluid regions, and heterogeneous domains inherent to even well-formed SAMs.

Quantifying the Selective Modification of Au(111) Facets via Electrochemical and Electroless Treatments for Manipulating Gold Nanorod Surface Composition.

Manipulating the composition of a mixed alkylthiol self-assembled monolayer (SAM) modified gold surface using both electrochemical and electroless methods is demonstrated. Through the use of fluorophore labeled thiolated DNA and in situ fluorescence microscopy with a gold single crystal bead electrode, a procedure was developed to study and quantify the selective desorption of an alkylthiolate SAM. This method enabled a self-consistent measurement of the removal of the SAM from the 111 surface compared to the 100 surface region at various potentials. A 20-fold increase in the electrochemical removal and replacement of the SAM from the 111 surface over the 100 surface was realized at -0.8 V/AgAgCl. A related procedure was developed for the solution-based electroless removal of the SAM using NaBH4 achieving a similar selectivity at the same potential. Unfortunately, in the electroless process fine control over the reducing potential was difficult to achieve. In addition, working in the presence of O2 complicates the solution potential measurement due to depolarization by the reduction of O2, resulting in a less clear relationship between selectivity and measured solution potential. Interestingly, the electrochemical method was not disturbed by the presence of O2. In preparation for work with Au nanorods, electrochemical measurements were performed in electrolyte that included 1 mM CTAB and was found to not interfere with this method. Preliminary results are promising for using this methodology for treatment of acid-terminated alkylthiol modified Au nanorods.

Simulating the Effect of Charge State on Reactive Landing of a Cyclic Tetrapeptide on Chemically Modified Alkylthiolate Self-Assembled Monolayer Surfaces

Previous simulations have provided insight into reactive landing (RL) of protonated peptides on chemically modified organic self-assembled monolayer (SAM) surfaces. However, they have been limited to small diglycine peptides with a single binding site on the surface. Here we present results of QM/MM direct dynamics simulations of c(-GGKG-), a small cyclic, lysine-containing peptide in both a protonated and deprotonated state colliding with a surface that includes nine binding sites. We find that when the peptide is deprotonated, RL efficiency is increased and preferential to the nitrogen of the lysine side chain. Qualitatively similar RL efficiency is seen in the present study and the experimental work of Laskin and co-workers.

Alkylthiol self-assembled monolayers on Au(111) with tailored tail groups for attaching gold nanoparticles

Self-assembled monolayers (SAMs) on Au(111) are able to control the functionality of a gold surface. We use scanning tunnelling microscopy (STM) in air and contact angle measurements to compare the morphology and the chemistry of three alkylthiol SAMs differing by their tail groups: 1,9-nonanedithiol (NDT), 1,4-butanedithiol (BDT) and 11-mercaptoundecanol (MUOH). STM reveals very different morphologies: a hexagonal lattice for MUOH and parallel rows for NDT and BDT. In the case of NDT, we find that the thiol tail groups may form disulfide bridges with long immersion times. The availability of the –SH group for chemical reactions is demonstrated by attaching gold nanoparticles (AuNPs). When the thiol tail group is available, AuNPs readily attach as shown with atomic force microscopy (AFM). When disulfide bridges are formed, the gold surface is not able to bind nanoparticles.

Molecular Dynamics Simulation of Alkylthiol Self-Assembled Monolayers on Liquid Mercury

We report computer simulation of the self-assembly of alkylthiols on the surface of liquid mercury. Here we focus mainly on the alkylthiol behavior on mercury as a function of the surfactant surface coverage, which we study by means of large-scale molecular dynamics simulations of the equilibrium structure at room temperature. The majority of the presented results are obtained for octa- and dodecanethiol surfactants. This topic is particularly interesting because the properties of the alkylthiol self-assembled monolayers on liquid mercury are relevant for practical applications (e.g., in organic electronics) and can be controlled by mechanically manipulating the monolayer, i.e., by changing its structure. Our computer simulation results shed additional light on the alkylthiol self-assembly on liquid mercury by revealing the coexistence of a dense agglomerated laying-down alkylthiols with a very dilute 2D vapor on mercury surface rather than a single vapor phase in the low surface coverage regime. In the regimes of the high surface coverage we observe the coexistence of the laying-down liquid phase and crystalline phases with alkylthiols standing tilted at a sharp angle to the surface normal, which agrees with the phase behavior previously seen in X-ray and tensiometry experiments. We also discuss the influence of finite-size effects, which one inevitably encounters in molecular simulations. Our findings agree well with the general predictions of the condensation/evaporation theory for finite systems. The temperature dependence of the stability of the crystalline alkylthiol phases and details of the surfactant chemical binding to the surface are discussed. The equilibrium structure of the crystalline phase is investigated in detail for the alkylthiols of various tail lengths.

Does the S-H Bond Always Break after Adsorption of an Alkylthiol on Au(111)?

The reaction mechanism for the formation of alkyl thiol self-assembled monolayers (SAM) on Au(111) is still not clearly understood. Especially, the role of defects on the chemisorption process is an important goal to be addressed. In this work, different minimum energy reaction paths for R-SH dissociation of thiols (with long and short chains and dithiol species) adsorbed on gold adatom are calculated by using periodic density functional theory (DFT). Our results show a lower energy barrier for the RS-H bond dissociation when two thiols are adsorbed per adatom. In addition, in contrast with the formation of an adatom at the Au(111) which has been shown to depend on the alkyl chain length, the activation energy of the RS-H bond dissociation of thiols adsorbed on an adatom was shown to be independent of the alkyl chain length. The presented results and derived hypothesis support the model that thiols with long alkyl chain thiols mainly adsorb molecularly on Au(111), while for short alkyl chain thiols the S-H bond breaks. This result is explained by the fact that short-chain thiols have lower interchain interaction energies and are thus more mobile compared to the long alkyl chain thiols on the Au(111) surface. This feature enables the short chains to reach adequate geometries, driven by entropy, which could deform the Au(111) more drastically and probably pull Au atoms out from surface to form adatoms. With these results a new mechanism is proposed for the formation of alkyl chain thiols on Au(111).

Comparative anti-corrosion properties of alkylthiols SAMs and mercapto functional silica sol–gel coatings on copper surface in sodium chloride solution

In this paper, we tried to fabricate a compact and thick mercapto functional silica sol–gel coating to achieve excellent corrosion resistance performance on copper. A detailed comparative study of alkylthiol self-assembled monolayers (SAMs) and mercapto functional silica sol–gel coating prepared by 1-octanethiol, n-dodecanethiol and (3-mercaptopropyl)trimethoxysilane (MPTMS) was carried out. The elements composition and chemical bonding of the protective coatings were characterized by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. In addition, the cross section morphologies of Cu–MPTMS sol–gel coating were investigated by field emission scanning electron microscopy. The wettability of various coatings was investigated by contact angle measurement instrument. Finally, the anti-corrosion ability of the coatings was characterized using an electrochemical workstation. Results demonstrated that compared to other alkylthiol SAMs, the Cu–MPTMS sol–gel coating showed the best anti-corrosion property in sodium chloride solution because it owned a multiple hierarchic Si–O–Si network structure, thus to achieve a higher range of thickness and lengthen the diffusion path way of corrosive medium. It was also found that compact and thick mercapto functional silica sol–gel coating shows better corrosion resistance performance than other SAMs of the same type.

Alkyl-Based Surfactants at a Liquid Mercury Surface: Computer Simulation of Structure, Self-Assembly, and Phase Behavior.

Self-assembled organic films on liquid metals feature a very rich phase behavior, which qualitatively differs from the one on crystalline metals. In contrast to conventional crystalline supports, self-assembled alkylthiol monolayers on liquid metals possess a considerably higher degree of molecular order, thus enabling much more robust metal-molecule-semiconductor couplings for organic electronics applications. Yet, compared to crystalline substrates, the self-assembly of organic surfactants on liquid metals has been studied to a much lesser extent. In this Letter we report the first of its kind molecular simulation investigation of alkyl-based surfactants on a liquid mercury surface. The focus of our investigation is the surfactant conformations as a function of surface coverage and surfactant type. First, we consider normal alkanes because these systems set the basis for simulations of all other organic surfactants on liquid mercury. Subsequently, we proceed with the discussion of alkylthiols that are the most frequently used surfactants in the surface science of hybrid organometallic interfaces. Our results indicate a layering transition of normal alkanes as well as alkylthiols from an essentially bare substrate to a completely filled monolayer of laying molecules. As the surface coverage increases further, we observe a partial wetting of the laying monolayer by the bulk phase of alkanes. In the case of alkylthiols, we clearly see the coexistence of molecules in laying-down and standing-up conformations, in which the sulfur headgroups of the thiols are chemically bound to mercury. In the standing-up phase, the headgroups form an oblique lattice. For the first time we were able to explicitly characterize the molecular-scale structure and transitions between phases of alkyl-based surfactants and to demonstrate how the presence of a thiol headgroup qualitatively changes the phase equilibrium and structure in these systems. The observed phenomena are consistent with available direct and indirect experimental evidence.

An experimental and theoretical approach to investigate the effect of chain length on aminothiol adsorption and assembly on gold.

Despite the numerous studies on the self-assembled monolayers (SAMs) of alkylthiols on gold, the mechanisms involved, especially the nature and influence of the thiol-gold interface are still under debate. In this work the adsorption of aminothiols on Au(111) surfaces has been studied by using surface IR and X-ray photoelectron spectroscopy (XPS) as well as by density functional theory (DFT) modeling. Two aminothiols were used, cysteamine (CEA) and mercaptoundecylamine (MUAM), which contain two and eleven carbon atoms, respectively. By combining experimental and theoretical methods, it was possible to draw a molecular picture of the thiol-gold interface. The long-chain aminothiol produced better ordered SAMs, but, interestingly, the XPS data showed different sulfur binding environments depending on the alkyl chain length; an additional peak at low binding energy was observed upon CEA adsorption, which indicates the presence of sulfur in a different environment. DFT modeling showed that the positions of the sulfur atoms in the SAMs on gold with similar unit cells [(2√3×2√3)R30°] depended on the length of the alkyl chain. Short-chain alkylthiol SAMs were adsorbed more strongly than long-chain thiol SAMs and were shown to induce surface reconstruction by extracting atoms from the surface, possibly forming adatom/vacancy combinations that lead to the additional XPS peak. In the case of short alkylthiols, the thiol-gold interface governs the layer, CEA adsorbs strongly, and the mechanism is closer to single-molecule adsorption than self-assembly, whereas for long chains, interactions between alkyl chains drive the system to self-assembly, leading to a higher level of SAM organization and restricting the influence of the sulfur-gold interface.

Self-assembled monolayers of alkyl-thiols on InAs: A Kelvin probe force microscopy study

Abstract We report on the preparation and characterization of self-assembled monolayers from aliphatic thiols with different chain length and termination on InAs (100) planar surfaces. This included as first step the development and investigation of a thorough chemical InAs surface preparation step using a dedicated bromine/NH4OH-based etching process. Ellipsometry, contact angle measurements and atomic force microscopy (AFM) indicated the formation of smooth, surface conforming monolayers. The molecular tilt angles were obtained as 30 ± 10° with respect to the surface normal. Kelvin probe force microscopy (KPFM) measurements in hand with Parameterized Model number 5 (PM5) calculations of the involved molecular dipoles allowed for an estimation of the molecular packing densities on the surface. We obtained values of up to n = 1014 cm− 2 for the SAMs under study. These are close to what is predicted from a simple geometrical model that would calculate a maximum density of about n = 2.7 × 1014 cm− 2. We take this as additional conformation of the substrate smoothness and quality of our InAs–SAM hybrid layer systems.

Adsorption of Alkylthiol Self-Assembled Monolayers on Gold and the Effect of Substrate Roughness: A Comparative Study Using Scanning Tunneling Microscopy, Cyclic Voltammetry, Second-Harmonic Generation, and Sum-Frequency Generation

Self-assembled monolayers (SAMs) of alkylthiols on gold have called considerable attention due to several applications in the modification and control of surface properties, such as wettability, tribology, and biocompatibility. Therefore, understanding the adsorption and molecular structure of these SAMs is crucial to optimize their quality for a given application. Although many studies in this area have been performed, the effects of substrate morphology on the quality of long chain alkylthiols on gold have not been satisfactorily clarified. This study focuses on the adsorption and conformation of SAMs of 1-hexadecanethiol (C16SH) as a function of adsorption time, substrate roughness, and morphology. The adsorption of C16SH on atomically flat Au surfaces was analyzed via cyclic voltammetry (CV), scanning tunneling microscopy (STM), and second-harmonic generation (SHG), while the molecular conformation and orientation were characterized via sum-frequency generation (SFG) vibrational spectroscopy. We show ...

Construction of Binding Group Terminated Self-Assembled Monolayers for Multilayer Formation

Introduction. The self-assembly technique is one of the best methods to arrange the molecule in order onto the solid substrate surface and the alkylthiol self-assembled monolayers (SAMs) on gold and alkyl SAMs on silicon have been widely studied because of their high stability, high packing density, and high orientation [1,2]. Thus, they are expected to apply to the modern nanotechnology such as sensors, wetting control, corrosion inhibition, and biomolecular and molecular electric devices [1,2]. Moreover, using the binding group terminated SAM as a substrate, multilayer with various functionalities can be constructed. In order to control the multilayer thickness and functionality, we have to construct highly oriented and well-packed SAMs with binding group as a terminal group [3]. In this study, we constructed four kinds of binding group terminated SAMs such as 4-mercaptobenzoic acid (4-MBA) and 3-mercaptopropionic acid (3-MPA) SAMs on Au(111) and 2-methoxy ethyl (2-ME) and 11-ethoxy undecyl (11-EU) SAMs on Si(111) by various preparation conditions and their characteristics were estimated by electrochemical measurements for the reductive desorption, angle-resolved X-ray photoelectron spectroscopy (ARXPS), attenuated total reflection - Fourier transformed infrared (ATR-FTIR) spectroscopy, and contact angle measurements. Experimentals. After Au(111) single crystal disks were annealed by a gas burner and quenched with ultrapure water, they were immersed in ethanol solution containing 1 mM 4-MBA or 1 mM 3-MPA for certain periods at room temperature. From the results of the electrochemical measurements for the reductive desorption and of ARXP specta, density, stability, and orientation of the constructed 4-MBA and 3-MPA SAMs were investigated in order to find the optimum preparation conditions of the densely-packed and well-oriented SAMs on Au(111). After etching of n-typed Si(111) prisms, 2-ME and 11-EU SAMs were prepared on Si(111) by dipping Si prism into liquid state of 2-ME or 11-EU under UV light irradiation and nitrogen atmosphere for several hours. From the results of the ATR-FTIR spectra and contact angle measurement, density, stability, and orientation of the constructed 2-ME and 11-EU SAMs were investigated in order to find the optimum preparation conditions of the densely-packed and well-oriented SAMs on Si(111). Results and Discussion. Linear sweep voltammograms (LSVs) measured in a deaerated 0.1 M KOH at Au(111) electrodes modified with the 4-MBA SAMs, which were prepared for several dipping periods, showed cathodic peaks due to the reductive desorption of 4-MBA, around -0.60 V. With increasing of the dipping period, the peak potential shifted to negative, namely stability increased, and the surface coverage of 4-MBA, namely density, increased. However, full width of half maximum (fwhm) of the cathodic peak, which shows homogeneity of SAM’s orientation, decreased until the dipping period of 1 h and then increased with increasing of the dipping period. This result indicated that repulsion interaction between carboxylate groups should increase with increasing of the dipping period, namely density. For 5 h dipping, the surface coverage was saturated to 0.52 nmol cm-2. From the XP spectrum of the 4-MBA SAM, we confirmed that the 4-MBA SAM prepared by the dipping period of 5 h is highly stable. As compared with 4-MBA SAM on Au(111) prepared for the same dipping period, 3-MPA SAM on Au(111) was less stable, less densely-packed, and less highly oriented, because van der Waals interaction between 4-MBA molecules, namely hydrophobic interaction between phenyl groups, should be stronger than that between 3-MPA, namely hydrophobic interaction between propyl chains. Modification of metal complex molecules such as metallo-porphyrin and metallo-phthalocyanine on the 4-MBA SAM modified Au(111) is now under investigation. On the other hand, 2-ME and 11-EU SAMs on Si(111) was relatively stable, because C-Si bonding energy is higher than S-Au bonding energy. As compared with 2-ME SAM on Si(111) for the same preparation period, 11-EU SAM was more stable, more densely-packed, and more highly oriented on Ai(111), because van der Waals interaction between 11-EU, namely hydrophobic interaction between longer chains, should be higher than those between shorter chains. Now we are trying to find preparation condition to construct more stable, densely-packed, and highly oriented 2-ME SAM on Si(111) because shorter chain SAM is easy to use as a substrate for multilayer formation. References. [1] A. Ulman, An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly, Academic Press, New York (1990).[2] T. Kondo, R. Yamada, and K. Uosaki, in Organized Organic Ultrathin Films – Fundamentals and Applications, Wiley-VCH, Chap 2 (2012).[3] T. Arisaka, B. Zhang, N. Aoki, and T. Kondo, 224th ECS Meeting, San Francisco, #44 (2013).

Fast, simple, combinatorial routes to the fabrication of reusable, plasmonically active gold nanostructures by interferometric lithography of self-assembled monolayers.

We describe a fast, simple method for the fabrication of reusable, robust gold nanostructures over macroscopic (cm(2)) areas. A wide range of nanostructure morphologies is accessible in a combinatorial fashion. Self-assembled monolayers of alkylthiolates on chromium-primed polycrystalline gold films are patterned using a Lloyd's mirror interferometer and etched using mercaptoethylamine in ethanol in a rapid process that does not require access to clean-room facilities. The use of a Cr adhesion layer facilitates the cleaning of specimens by immersion in piranha solution, enabling their repeated reuse without significant change in their absorbance spectra over two years. A library of 200 different nanostructures was prepared and found to exhibit a range of optical behavior. Annealing yielded structures with a uniformly high degree of crystallinity that exhibited strong plasmon bands. Using a combinatorial approach, correlations were established between the preannealing morphologies (determined by the fabrication conditions) and the postannealing optical properties that enabled specimens to be prepared "to order" with a selected localized surface plasmon resonance. The refractive index sensitivity of gold nanostructures formed in this way was found to correlate closely with measurements reported for structures fabricated by other methods. Strong enhancements were observed in the Raman spectra of tetra-tert-butyl-substituted phthalocyanine. The shift in the position of the plasmon band after site-specific attachment of histidine-tagged green fluorescent protein (His-GFP) and bacteriochlorophyll a was measured for a range of nanostructured films, enabling the rapid identification of the one that yielded the largest shift. This approach offers a simple route to the production of durable, reusable, macroscopic arrays of gold nanostructures with precisely controllable morphologies.

One-Pot Electrografting of Mixed Monolayers with Controlled Composition

Surface functionalization with ultrathin layers exhibiting a highly robust interface is of paramount importance for designing materials with tailored properties or operating functions, without modifying drastically the material’s bulk structures. A fine-tuning of the surface composition obtained, for instance from binary mixed layers, is also a key issue for developing high value-added applications like efficient sensors. Herein, binary mixtures of calix[4]arene-tetra-diazonium salts generated in situ from their corresponding calix[4]tetra-anilines are electrografted to form covalently bound monolayers onto substrates for yielding versatile functionalizable molecular platforms. Wettability studies, X-ray photoelectron spectroscopy analyses, and scanning electrochemical microscopy show the formation of homogeneous mixed monolayers. The distribution of the two calixarenes on the surface is directed by their relative molar fraction in the deposition solution. The strategy allows the control of the composition of mixed monolayers in a one-step approach. Postfunctionalization of the mixed layers with ferrocene centers is performed to exemplify the benefit of a dilution procedure when functional groups are introduced at the calix[4]arene small rim. This study highlights the potential of diazonium salt electrografting as a competitive alternative to chemisorption strategies such as self-assembled monolayers of alkyl thiols in the field of surface functionalization.