Charged Self-assembled Monolayers Research Articles

Interaction Forces between Hydrophobic and Hydrophilic Self-Assembled Monolayers

An investigation is presented of the interaction of charged self-assembled monolayers (SAMs) with a monoprotic ionizable acid functional group (–COOH) and uncharged SAMs with a methyl terminated functional group (–CH3). The strength of the interactions are determined using an atomic force microscope. For all electrolyte conditions investigated the interactions are not well described by a summation of van der Waals attractions and electrostatic repulsions in a manner suggesting that van der Waals attractions are screened. The repulsions are accurately described as corresponding to two surfaces of different charge interacting with surface charges that are independent of separation (i.e., the constant charge model). A small adhesion force was observed under all conditions and its magnitude increased with NaCl concentration.

Size does matter: electrostatically determined surface coverage trends in protein and colloid adsorption

The surface coverage of charged colloidal particles such as proteins adsorbed at solid surfaces is determined by a variety of properties of the particles, surface, solution and the process itself. A review is presented of the mechanisms determining the ultimate coverage based on the energetics of interaction of the particles with one another and with the adsorbent surface. In particular, two limiting cases are identified. In the first, in which adsorption is irreversible, the coverage is determined largely by interparticle repulsion, and can be modelled by a variant of the random sequential adsorption (RSA) approach; this leads to a prediction of increasing coverage with increasing ionic strength. In the second limiting situation the particle–surface interactions are weaker and may be attenuated by increasing ionic strength, leading to a more complex balance with interparticle interactions. The situation is modelled using a mechanistically based isotherm in which various trends with both particle and salt concentration are possible. The first limiting case tends to occur more frequently with larger particles and the second with smaller ones such as proteins. Experimental data are presented in the intermediate range for the large globular protein catalase adsorbed on negatively charged self-assembled monolayers (SAM), which was studied by liquid tapping mode atomic force microscopy (LTM-AFM). The influence of increasing ionic strength on surface coverage varies, showing increasing coverage at low ionic strength, then a drop at intermediate ionic strength, and another increase at high ionic strength. The initial increase is interpreted as being consistent with the modified RSA mechanism and the subsequent decrease with screening of particle-surface attraction in the isotherm model, while the final increase is thought to be caused by depletion forces. The results indicate the potential complexity of surface coverage trends that may occur in different experimental situations.

Use of Self-Assembled Monolayers as Substrates for Atomic Force Imaging of Hydroxyapatite Crystals from Mammalian Skeletal Tissues

Atomic force microscopy (AFM) has recently been successfully used to describe the surface topography of hydroxyapatite crystals from mammalian skeletal tissues. To further characterize the growth mechanisms of skeletal hydroxyapatite crystals and the role of adsorbed proteins in these processes, imaging under biological fluids is essential. However, under aqueous solutions, these crystals do not bind to the usual AFM substrates such as mica and graphite and therefore alternative substrates are necessary. The aim of the present study was to evaluate the use of self-assembled monolayer technology with controllable chemical functionality to provide “designer surfaces” for crystal binding in fluid environments which simulate the normal physiological milieu. We have found that hydroxyapatite crystals from developing enamel are bound most effectively by negatively charged self-assembled monolayer (COO- and SO3-) surfaces, demonstrating an important role for such substrates in AFM imaging of biological samples under aqueous fluids and suggesting that the prevalent charge on enamel crystal surfaces is positive.

DNA Adsorption and Cationic Bilayer Deposition on Self-Assembled Monolayers

A cationic bilayer adsorbed to a self-assembled monolayer (SAM) of alkylthiols could be a useful substrate for DNA immobilization and patterning. Therefore, we studied the layer formation of a self-assembled system consisting of a base layer of a negatively charged SAM chemisorbed on gold, a middle layer of an electrostatically adsorbed cationic bilayer, and a top layer of double-stranded DNA that electrostatically adsorbs to the cationic bilayer. The formation of DNA, lipid, and alkylthiol layers was monitored by surface plasmon spectroscopy. Cationic lipids readily formed layers with thickness between 32 and 33 Å on self-assembled alkylthiols possessing terminal carboxylic acid groups within 24 h and at pH > 2. Fluorescence bleaching experiments indicated that these layers were homogeneous and relatively immobile. For comparison, we found that cationic lipids do not form layers on alkylthiols possessing a terminal alcohol group, while zwitterionic lipids formed bilayers on these surfaces and on the carboxylated surfaces with a thickness of approximately 38−44 Å. The use of self-assembled alkylthiols with diethylene glycol groups prohibited the formation of both cationic and zwitterionic lipid layers and also prevented DNA adsorption. Finally, DNA was adsorbed to cationic lipid bilayers which were electrostatically attached to negatively charged SAMs. The results indicate that DNA forms a layer of 8 Å calculated thickness, which is consistent with a monolayer possessing average interhelical distances of 50 Å, in agreement with other studies using different techniques. Hence this surface is useful for immobilizing DNA. No differences were observed in kinetics of deposition or the thickness of the DNA monolayer when different cationic lipids were used.

Two-Dimensional Polyaniline Thin Film Electrodeposited on a Self-Assembled Monolayer

A two-dimensional conducting polyaniline (PAN) monolayer has been formed on an electrically insulating monolayer. The approach is based on the electrochemical polymerization of surface-confined anilinium ions that were electrostatically attached to a negatively charged self-assembled monolayer of ω-mercaptodecanesulfonate (MDS), HS(CH2)10SO3-, on a gold surface. The formation and characterization of the two-dimensional film and the MDS monolayer have been examined by cyclic voltammetry, Fourier transform IR spectroscopy, X-ray photoelectron spectroscopy, wettability, and scanning electrochemical microscope. The formation of a capacitor-like assembly, in which electron transfer was blocked between PAN and the gold surface, was accomplished by electrochemically incorporating hexadecanethiol (C16) into the MDS monolayer. The PAN monolayer exhibits properties similar to those of a thin polymer film.

Microfabrication through Electrostatic Self-Assembly

We use electrostatic interactions to direct the patterning of gold disks having ∼10-μm diameters on functionalized surfaces. Planar and curved substrates with patterned surface charge were generated either by microcontact printing or by photolithography. Small charged gold disks were generated by electroplating gold into photoresist molds and derivatizing these disks with charged self-assembled monolayers. When agitated as a suspension in contact with the patterned surfaces, the charged gold disks deposited specifically but as disordered aggregates on regions presenting the opposite charge. Positively-charged disks deposited on phosphonate-, carboxylate-, and SiOH-terminated surfaces but not on trimethylammonium- and dimethylammonium-terminated surfaces, and vice versa for negatively-charged disks. Methyl- and CF3-terminated surfaces resisted deposition of disks of either charge. Selective and dense assembly was achieved in methanol, ethanol, 2-propanol, and dioxane; in water, deposition was nonspecific. The overlap of disks was eliminated by using disks with ∼1:1 aspect ratios.

Surface Plasmon Resonance Imaging Measurements of Electrostatic Biopolymer Adsorption onto Chemically Modified Gold Surfaces

A combination of in situ and ex situ surface plasmon resonance (SPR) imaging experiments is used to characterize the differential electrostatic adsorption of proteins and synthetic polypeptides onto photopatterned monolayers at gold surfaces. The nonspecific electrostatic adsorption of proteins onto negatively charged self-assembled monolayers (SAMs) of 11-mercaptoundecanoic acid (MUA) is found to depend on the protein pI, solution ionic strength, and solution pH. The pH dependence of the electrostatic adsorption of the protein avidin onto a MUA SAM indicates that a full monolayer adsorbs at a solution pH greater than 5.0, and an "effective pK(a)" of 3.6 is determined for the avidin adsorption. This effective pK(a) is a combination of the pK(a) of the MUA monolayer and the ion pairing adsorption coefficient for the avidin. Additional SPR imaging experiments show that the electrostatic adsorption of the synthetic polypeptide poly-l-lysine (PL) onto a MUA SAM varies with molecular weight, forming a full PL monolayer for polypeptides with more than 67 lysine residues.