Self-assembled Monolayers Chain Length Research Articles

Probing charge transfer through antifouling polymer brushes by electrochemical methods: The impact of supporting self-assembled monolayer chain length

Ultrathin surface-tethered polymer brushes represent attractive platforms for a wide range of sensing applications in strategically vital areas such as medicine, forensics, or security. The recent trends in such developments towards “real world conditions” highlighted the role of zwitterionic poly(carboxybetaine) (pCB) brushes which provide excellent antifouling properties combined with bio-functionalization capacity. Highly dense pCB brushes are usually prepared by the “grafting from” polymerization triggered by initiators on self-assembled monolayers (SAMs). Here, multi-methodological experimental studies are pursued to elucidate the impact of the alkanethiolate SAM chain length (C6, C8 and C11) on structural and functional properties of antifouling poly(carboxybetaine methacrylamide) (pCBMAA) brush. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in a custom-made 3D printed cell employing [Ru(NH3)6]3+/2+ redox probe were used to investigate penetrability of SAM/pCBMAA bilayers for small molecules and interfacial charge transfer characteristics. The biofouling resistance of pCBMAA brushes was characterized by surface plasmon resonance; ellipsometry and FT-IRRAS spectroscopy were used to determine swelling and relative density of the brushes synthesized from initiator-bearing SAMs with varied carbon chain length. The SAM length was found to have a substantial impact on all studied characteristics; the highest value of charge transfer resistance (Rct) was observed for denser pCBMAA on longer-chain (C11) SAM when compared to shorter (C8/C6) SAMs. The observed high value of Rct for C11 implies a limitation for the analytical performance of electrochemical sensing methods. At the same time, the pCBMAA brushes on C11 SAM exhibited the best bio-fouling resistance among inspected systems. This demonstrates that proper selection of supporting structures for brushes is critical in the design of these assemblies for biosensing applications.

Weak interaction engineering on thermal transport in metal-organic semiconductor nanocomposites with self-assembled monolayers

The engineering of weak interactions between metals and organic semiconductors is crucial in regulating energy carrier transport within organic semiconductor electronics. In this study, we introduce self-assembled monolayers (SAMs) to improve thermal transport in metal-organic semiconductor nanocomposites. SAM molecules with different terminal groups (-CH3, -COOH, and -C6H5) and chain lengths are utilized to modify the surface of Au nanoparticles with multiple core diameters. The introduction of SAMs significantly boosts the thermal transport of metal-organic semiconductor nanocomposites, which can be attributed to improved vibrational coupling and stronger nonbonding interactions. Furthermore, longer SAM chains broaden the area of van der Waals regions and further strengthen the intermolecular interaction across the interface, reducing the interfacial thermal transport barrier. Additionally, quantum chemistry calculation and evaluations of interfacial adhesion energy indicate that the π-π stacking interaction between TPD and SAM(-C6H5) molecules forms the best interfacial combination of nanoparticles and TPD molecules, resulting the greatest thermal transport characteristics. Finally, free energy calculations demonstrate that the variation of nanoparticle surface topology with the increase of the SAM chain length induces more obvious free energy fluctuation, which leads to tighter connections between TPD and SAM molecules under weak interaction.

Electrochemical and surface enhanced infrared absorption spectroscopy studies of TEMPO self-assembled monolayers

The electron transfer kinetics of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) monolayers have been studied using both a chronocoloumetric method and surface sensitive infrared spectroelectrochemistry (attenuated total reflection surface enhanced infrared absorption spectroscopy, ATR-SEIRAS). Redox active self-assembled monolayers (SAMs) are constructed by covalently attaching 4-amino-TEMPO to preformed monolayers of different chain length alkanoic acid thiol monolayers. The density of TEMPO groups on the resulting Cn-TEMPO (n = 6, 8, 11, 12, 16) SAMs is approximately 20% of a full monolayer coverage and the resulting SAMs exhibit very little kinetic heterogeneity. Standard heterogeneous rate constants are measured as a function of SAM chain length. Tafel plots constructed for n = 12 show asymmetry between the forward (oxidation of the radical to the nitrosonium cation) and reverse (reduction to the TEMPO radical) reactions. The reorganization energy of the nitrosonium cation is estimated to be 1.0 ± 0.1 eV for the C16-TEMPO SAM. ATR-SEIRAS studies reveal that the oxidation of the TEMPO radical leads to a conformational change (chair-chair flip) in the piperdinyl ring. The nitrosonium conformer pushes the N=O+ center away from the electrode and into a polar environment. Conformational differences in the reduced and oxidized species are cited as the reason for kinetic asymmetry in the TEMPO/TEMPO+ redox system. Time-resolved ATR-SEIRAS is used to extract the standard rate constant for C12-TEMPO on gold island films deposited on an indium tin oxide (ITO) surface. Standard rate constants for SAMs formed on Au@ITO are found to be higher than those formed on gold beads due to greater kinetic heterogeneity caused by the larger roughness of the gold islands.

Impact of Self-Assembled Monolayer Design and Electrochemical Factors on Impedance-Based Biosensing.

Real-time sensing of proteins, especially in wearable devices, remains a substantial challenge due to the need to convert a binding event into a measurable signal that is compatible with the chosen analytical instrumentation. Impedance spectroscopy enables real-time detection via either measuring electrostatic interactions or electron transfer reactions while simultaneously being amenable to miniaturization for integration into wearable form-factors. To create a more robust methodology for optimizing impedance-based sensors, additional fundamental studies exploring components influencing the design and implementation of these sensors are needed. This investigation addresses a sub-set of these issues by combining optical and electrochemical characterization to validate impedance-based sensor performance as a function of (1) biorecognition element density, (2) self-assembled monolayer chain length, (3) self-assembled monolayer charge density, (4) the electrochemical sensing mechanism and (5) the redox reporter selection. Using a pre-existing lysozyme aptamer and lysozyme analyte combination, we demonstrate a number of design criteria to advance the state-of-the-art in protein sensing. For this model system we demonstrated the following: First, denser self-assembled monolayers yielded substantially improved sensing results. Second, self-assembled monolayer composition, including both thickness and charge density, changed the observed peak position and peak current. Third, single frequency measurements, while less informative, can be optimized to replace multi-frequency measurements and in some cases (such as that with zwitterionic self-assembled monolayers) are preferred. Finally, various redox reporters traditionally not used in impedance sensing should be further explored. Collectively, these results can help limit bottlenecks associated with device development, enabling realization of next-generation impedance-based biosensing with customize sensor design for the specific application.

New self assembly monolayer onto SiGe as a selective biosensor for single-strand DNA

An important step of fabrication of selective DNA probe/receptor is the functionalization of semiconducting surfaces with a self-assembled monolayer (SAM) with an appropriate surface termination to interact with DNA. In this work, we studied an immobilization of single-strand DNA (ssDNA) onto self-assembly monolayer of both the heavily exploited aminopropyltriethoxy silane (APTES) and n-(2-aminoethyl)-11-aminoundecyltrimethoxysilane (NAATS), a bi-functional amino silane onto the surface of SiGe substrate. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) techniques were employed to characterize the silanization of SAMs on SiGe surface. The immobilization process was confirmed using fluorescence intensity measurements. The coverage values of APTES and NAATS SAMs were estimated to be 71.3 and 83 XPS% at 78°, respectively from the C/Ge ratio. Comparing the performance towards both specific and non-specific ssDNA complements for both silanes, it was observed that a higher selectivity and sensitivity was obtained by using NAATS SAM. This work highlighted the importance of SAM chain length by comparing one short alkyl chains, APTES with that long-chain counterpart, NAATS.

Structural Changes of 4,4′-(Dithiodibutylene)dipyridine SAM on a Au(111) Electrode with Applied Potential and Solution pH and Influence of Alkyl Chain Length of Pyridine-Terminated Thiolate SAMs on Cytochrome c Electrochemistry

From the standpoint of cytochrome c electrochemistry, it is important to understand the relationship between the electron transfer reaction and alkyl chain length of pyridine-terminated self-assembled monolayers (SAMs) at the molecular level. In the present study, SAMs of 4,4′-(dithiodibutylene)dipyridine ((PyC4S)2) formed on a Au(111) surface were investigated in aqueous electrolyte solutions by cyclic voltammetry (CV), surface-enhanced infrared absorption spectroscopy (SEIRAS), and in situ scanning tunneling microscopy (STM). Structural changes in the PyC4S-SAM were driven by changes in the applied electrode potential or pH of the electrolyte solution. In 0.05 M HClO4, a structural change in PyC4S-SAM was drastically induced by the potential manipulation from the open circuit potential to 0.10 V versus a reversible hydrogen electrode (RHE). A high-resolution STM image showed that the PyC4S-SAMs formed a p(12 × √3 R–30°) adlattice at ∼0.15 V versus RHE. A semisquarely arranged adlattice was newly formed ...

InGaZnO Thin-Film Transistors Modified by Self-Assembled Monolayer With Different Alkyl Chain Length

InGaZnO (IGZO) thin-film transistors (TFTs) modified by self-assembled monolayers (SAMs) based on triethoxysilane (TES) with three different alkyl chain lengths were fabricated and the relationship between the SAM chain length and the TFT electrical properties was investigated. The mobility increased and the hysteresis of transfer curves was reduced after SAM-modification, owing to less adsorption/ desorption effect on the IGZO surface. IGZO-TFTs modified by TES with longer alkyl chain lengths exhibited better electrical performance, which was attributed to the formation of a more hydrophobic and higher ordered monolayer due to the cohesive interaction between the molecules.

Vascular endothelial growth factor attachment to hydroxyapatite via self-assembled monolayers promotes angiogenic activity of endothelial cells

Currently, tissue engineered constructs for critical sized bone defects are non-vascularized. There are many strategies used in order to promote vascularization, including delivery of growth factors such as vascular endothelial growth factor (VEGF). In this study, hydroxyapatite (HA) was coated with self-assembled monolayers (SAMs). The SAMs were in turn used to covalently bind VEGF to the surface of HA. The different SAM chain length ratios (phosphonoundecanoic acid (11-PUDA):16-phosphonohexadecanoic acid (16-PHDA) utilized in this study were 0:100, 25:75, 50:50, 75:25, and 100:0. Surfaces were characterized by contact angle (CA) and atomic force microscopy, and an in vitro VEGF release study was performed. It was observed that CA and root-mean-squared roughness were not significantly affected by the addition of SAMs, but that CA was significantly lowered with the addition of VEGF. VEGF release profiles of bound VEGF groups all demonstrated less initial burst release than adsorbed control, indicating that VEGF was retained on the HA surface when bound by SAMs. An in vitro study using human aortic endothelial cells (HAECs) demonstrated that bound VEGF increased metabolic activity and caused sustained production of angiopoietin-2, an angiogenic marker, over 28days. In conclusion, SAMs provide a feasible option for growth factor delivery from HA surfaces, enhancing angiogenic activity of HAECs in vitro.

Single Molecule Dynamics on Hydrophobic Self-Assembled Monolayers

The interactions between adsorbate molecules and hydrophobic surfaces are of significant interest due to their importance in a variety of biological and separation processes. However, it is challenging to extrapolate macroscopic ensemble-averaged force measurements to molecular-level phenomena. Using total internal reflection fluorescence microscopy to image individual molecules at hydrophobic solid-aqueous interfaces, we directly observed dynamic behavior associated with the interactions between fluorescently labeled dodecanoic acid (our probe molecules) and self-assembled monolayers (SAM) comprising n-alkyltriethoxysilanes with systematically increasing chain length (from n = 4-18). In all cases, we observed at least two characteristic surface residence times and two diffusive modes, suggesting the presence of multiple distinct adsorbed populations. In general, the mean surface residence time increased and the mobility decreased with increasing SAM chain length, consistent with stronger probe-surface interactions. However, these trends were not primarily due to changes in characteristic residence times or diffusion coefficients associated with the individual populations but rather to a dramatic increase in the fraction associated with the long-lived slow-moving population(s) on long-chain SAMs. In particular, on longer (16-18 carbon) alkylsilane monolayers, the probe molecule exhibited far fewer desorption-mediated "flights" than on short (4-6 carbon) monolayers. Additionally, probes on the longer chain surfaces were much more likely to exhibit extended surface residence times as opposed to short transient surface visits.

Characterizing the Molecular Order of Phosphonic Acid Self-Assembled Monolayers on Indium Tin Oxide Surfaces

Self-assembled monolayers (SAMs) of alkanephosphonic acids with chain lengths between 8 and 18 carbon units were formed on thin films of indium tin oxide (ITO) sputter-deposited on silicon substrates with 400 nm thermally grown SiO(2). The silicon substrates, while not intended for use in near-IR or visible optics applications, do provide smooth surfaces that permit systematic engineering of grain size and surface roughness as a function of the sputter pressure. Argon sputter pressures from 4 to 20 mTorr show systematic changes in surface morphology ranging from smooth, micrometer-sized grain structures to <50 nm grains with 3× higher surface roughness. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy experiments are conducted for alkanephosphonic acids deposited on these wide range of ITO surfaces to evaluate the effects of these morphological features on monolayer ordering. Results indicate that long-chain SAMs are more highly ordered, and have a smaller tilt angle, than short-chain SAMs. Surprisingly, the 1-octadecyl phosphonic acids maintain their order as the lateral grain dimensions of the ITO surface shrink to ∼50 nm. It is only when the ITO surface roughness becomes greater than the SAM chain length (∼15 Å) that SAMs are observed to become relatively disordered.

Effect of chain length of self-assembled monolayers in dip-pen nanolithography using molecular dynamics simulations

The pattern transfer mechanism of an alkanethiol self-assembled monolayer (SAM) with different chain lengths during the dip-pen nanolithography (DPN) process and pattern characterizations are studied using molecular dynamics (MD) simulations. The mechanisms of molecular transference, alkanethiol meniscus characteristics, surface adsorbed energy, transfer number, and pattern formation are evaluated during the DPN process at room temperature. The simulation results clearly show that the molecular transfer ability in DPN is strongly dependent on the chain length. Shorter molecules have significantly better transport and diffusion abilities between the meniscus and substrate surface, and the transport period can be maintained longer. The magnitude of adsorbed energy increases with chain length, so many more molecules can be transferred to the surface when shorter molecules are used. After deposition, the magnitude of the adsorbed area and pattern height decrease with increasing chain length.

Molecular dynamics study of interfacial bonding strength of self-assembled monolayer-coated Au-epoxy and Au–Au systems

Interfacial adhesion between metals and organic polymers plays a crucial role in the mechanical properties and reliability performance of multiplayer thin film structures. To improve their interfacial bonding strength and so the reliability, the self-assembled monolayer (SAM) method is considered as an effective means. The present study is devoted to studying the effects of SAM coating on the interfacial bonding strength of the Au-epoxy and the Au–Au bonding structures through molecular dynamics (MD) simulation. Three different types of functionalized alkanethiol SAMs (SH(CH 2) n X, X = CH 3, OH, NH 2) chemisorbed onto two different Au crystal planes, i.e., (1 0 0) and (1 1 1), are considered. The study starts from the characterization of the interfacial bonding strength of both the SAM-coated Au-epoxy and Au–Au systems, followed by the investigation of the dependence of the interfacial bonding strength on the chain lengths and tail groups of the n-alkanethiolates. A comparative study of the effects of the crystal orientation of Au substrate on the bonding strength is reported, and the elastic moduli of these SAMs through uniaxial tensile simulation are also examined. The calculated results are compared with the published experimental data, and also with each other to identify the optimal SAM candidate. Results show that the interfacial bonding strength of the SAM-coated Au-epoxy and Au–Au systems exhibits a strong dependency on the crystal orientation of Au substrate and also on the chain length of the monolayer where it tends to increase with an increasing SAM chain length. In specific, the interfacial bonding strength of the SH(CH 2) n CH 3 SAM-coated Au–Au joint would reach a maximal value at the chain length n = 8 while that of the SAM/epoxy interface in the SH(CH 2) n CH 3 SAM-coated Au-epoxy system attains a minimal value at n = 4 and becomes the maximum at n = 10, regardless of the crystal orientation of the Au substrates. Besides, the Au substrate with (1 1 1) crystal orientation would outperform the Au(1 0 0) substrate in the SAM/epoxy interfacial bonding strength of the SAM-coated Au-epoxy system while there is a totally opposite result for that of the SAM-coated Au–Au joint.

Control and stability of self-assembled monolayers under biosensing conditions

The ability to stabilize and control the attachment of cells on the surfaces of a variety of inorganic materials is important for the development of biomedical devices and sensors. An important intermediate step is the functionalization of semiconducting surfaces with a self-assembled monolayer (SAM) with an appropriate surface termination to interact with proteins. The stability of such SAMs is critical to withstanding subsequent processing and measurement conditions (e.g. long exposure to a buffer solution) to avoid artifacts resulting from such deterioration during electrical measurements. This work highlights the importance of surface cleaning and SAM chain length by comparing two commonly used short alkyl chains, aminopropyltriethoxysilane (APTES) or 3-(trimethoxysilyl)propyl aldehyde (C4-ald) molecules, with their long-chain counterparts, amino-undecilenyltriethoxysilane (AUTES) and 11-(triethoxysilyl)undecanal (C11-ald). Using IR spectroscopy, spectroscopic ellipsometry, and electrical measurements, a cleaning method is developed, based on a short room temperature (RT) SC-1 treatment, to remove photoresist without degrading device performance as is the case with currently used oxygen plasma methods. The spectroscopic and electrical measurements also show that short-chain SAMs, typically used for pH- or bio-sensing, do not have the stability suitable for biosensor environments. In contrast, long-chain SAMs display much higher stability and can be reproducibly grafted. These findings are the basis for a reliable preparation and robust operation of biosensors.

Effects of the alkyl chain length in phosphonic acid self-assembled monolayer gate dielectrics on the performance and stability of low-voltage organic thin-film transistors

We have fabricated pentacene organic thin-film transistors (TFTs) using self-assembled monolayers (SAMs) based on alkyl-phosphonic acids with five different alkyl chain lengths as the gate dielectric and investigated the relationship between the SAM chain length and the electrical performance and stability of the transistors. A SAM chain length of 14 carbon atoms provides a maximum TFT mobility of 0.7 cm2/V s, along with an on/off current ratio greater than 105. We have also investigated the bias stress effect in these TFTs and found that the change in drain current is substantially less severe than in pentacene TFTs with polymer gate dielectrics.

Characterization of self-assembled monolayers by using a near-field microwave scanning microprobe

A near-field microwave scanning microprobe (NSMM) technique has been used to investigate the material properties of n-alkanethiol self-assembled monolayers (SAMs) on a gold (Au) surface. We demonstrate that near-field microwave probing technique can achieve the noncontact detection of the thickness of SAMs by measuring the microwave reflection coefficient S 11 at an operating frequency near 5.3 GHz. We also directly image the patterned SAMs by NSMM. The thickness (chain length) of SAMs is determined from the visualized microwave reflection coefficient changes on the Au surface with high sensitivity. This nano-scale measurement of SAMs has a great potential for investigating the surface profile with high sensitivity.

Role of SAM Chain Length in Enhancing the Sensitivity of Nanopillar Modified Electrodes for Glucose Detection

In this report, alkanethiol self assembled monolayers (SAM) with two different chain lengths were used to immobilize the functionalizing enzyme (glucose oxidase) onto gold nanopillar modified electrodes and the electrochemical processes of these functionalized electrodes in glucose detection were investigated. First, the formation of these SAMs on the nanopillar modified electrodes was characterized by the cyclic voltammetry and electrochemical impedance spectroscopy techniques, and then the detection sensitivity of these functionalized electrodes to glucose was evaluated by the amperometry technique. Results showed that the SAM of alkanethiols with a longer chain length resulted in a higher degree of surface coverage with less defect and a higher electron transfer resistance, whereas the SAM of alkanethiols with a shorter chain length gave rise to a higher detection sensitivity to glucose. This study sheds some new insight into how to enhance the sensing performance of nanopillar modified electrodes.

Formula omitted] as gate dielectric for organic transistors: Charge transport phenomena in poly-(3-hexylthiophene) based devices

We studied organic thin film transistors based on poly-(3-hexylthiophene) having as gate dielectric Al2O3, which was prepared by atomic layer deposition (ALD) technique, that provides films with very good electrical properties, roughness below 1nm and compatibility with virtually any type of substrate, including polymeric ones. High-k gate oxides such as Al2O3 are advantageous since they enable a reduction of operating voltages, but when used in conjunction with organic semiconductors, they induce worse transport properties if compared to low-k dielectrics. To address this issue, we focused on the interface between the gate dielectric and the active material and we explored the effects of functionalizing the Al2O3 surface by means of self-assembled monolayers (SAM). We studied and compared a set of SAMs differing in the ligand groups and in the chain lengths. We show that the most important parameter is the SAM chain length, the longer SAM being more effective in giving better charge carrier mobility. The microscopical origins of this finding are analyzed exploiting and comparing the dependence of the mobility on the gate voltage in the transistors based on the variously functionalized Al2O3. With n-octadecyltrichlorosilane (OTS) the mobility reaches the value of 10−2cm2/Vs, comparable to the mobility on transistors with properly functionalized SiO2 gate dielectric.

Effect of chain length of self-assembled monolayers on adhesion force measurement by AFM

It has been confirmed experimentally that the adhesion force measured between an atomic force microscope (AFM) tip and the self-assembled monolayers (SAMs) has a direct correlation with the chain length of SAMs, and that the adhesion force decreases with the increase of the chain length. In this paper, a theoretical model is put forward to calculate the adhesion force between the AFM tip and the SAMs by integrating the Lennard–Jones potential. The theoretical results are in good agreement with the existing experimental results.

Self-Assembled Monolayers on Engineering Metals: Structure, Derivatization, and Utility

A methodology for the formation and derivatization of self-assembled monolayers (SAMs) of alkyl phosphonic acids on common engineering metals such as steel, stainless steel, aluminum, copper, and brass is described. This methodology is shown to be a versatile route for surface modification of such substrates. α,ω-Metal bisphosphonate SAMs are shown to be receptive to complexation by organic acids and acid-containing polymers such as fluoropolymers and ethylene-co-methacrylic acid. This latter attribute is exploited in the construction of polymer/SAM/metal interfaces of surprising durability. The durability of the interface is a strong function of the SAM chain length.

Blocking behavior of self-assembled monolayers on gold electrodes

Self-assembled monolayers (SAMs) with metal electrodes, especially thiols on gold, are the subject of this investigation because of the unique properties of SAM-modified surfaces. Normal alkanethiols are used to modify the surface of a conventional gold electrode to block certain ions such as Pb(II) and Cu(II) from the surface of the electrode. Normal alkanethiols are also used to study the SAM-gold interfacial adsorption-desorption behavior of the self-assembled monolayer. The effects of varying chain length of SAMs, varying concentration of the alkanethiol solutions, immersion time of the pure gold electrode in the SAM solution, and the stability of a SAM-modified gold electrode in fresh chloroform are investigated using the oxidation-reduction peaks of gold. Conditions that optimize the surface coverage and the uniformity of the SAMs have been determined. Normal alkanethiols proved to be a good insulator on the electrode surface.