Self-assembled Monolayer Layer Research Articles

Investigating Electrochemical Behavior of Antibacterial Polyele-ctrolyte-Coated Magnesium Alloys for Biomedical Applications

This study presents corrosion rates of layer-by-layer (LBL) and self-assembled monolayer (SAM) coated magnesium (Mg) alloys, and their antibacterial properties. Mg alloy samples were coated with cationic (chitosan - CHI) and anionic polyelectrolyte (carboxymethyl cellulose-CMC) LBL coating, and phosphonic acid self-assembled monolayer (SAM) coating methods. Electrochemical impedance spectroscopy (EIS) was employed for analyzing these samples in order to detect their corrosion properties. During the electrochemical analysis, a corrosion rate of 72 milli inch per year (mpy) was found for the sample coated with a 12 deionized phosphonic acid SAM and 9 CHI/CMC multilayers. During the antibacterial tests, gentamicin was investigated about how it would adhere on the Mg substrate surface and how it would be an antibacterial agent against Escherichia coli (E. coli) bacteria. From a coating point of view, a bare layer, self-assembly monolayer, polyelectrolyte layer, and combination of LBL and SAM were analyzed. From an antibacterial treatment point of view, samples with no antibacterial treatment, 10% gentamicin sulfate, UV treatment and 0% gentamicin sulfate, anti-anti and 10% gentamicin sulfate, and 70% ethanol and 10% gentamicin sulfate were individually studied. Duration of the incubation was 7 days at 35°C. Antibacterial sensitivity was tested using the disk diffusion method in Petridish. Based on the standard diameter of the zone of inhibition chart, the growth of bacteria was inhibited relatively with those Mg substrates. The largest recorded diameter of the zone of inhibition was 50 mm for the pre-UV treated and gentamicin-loaded sample, which is more than the standard inhibition diameter.

Self-assembled monolayer modified MoO3/Au/MoO3 multilayer anodes for high performance OLEDs

We control the work function and the surface energy of the MoO3/Au/MoO3 (MAM) anode of OLEDs by modifying the top MoO3 layer via vapor phase deposition. The performance and stability of the device are significantly altered depending on the dipole direction of the selfassembled monolayer (SAM) with permanent dipole moment inserted between N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) film and a MAM anode as well as on the interfacial wetting between the SAM and NPB layer. A CF3-terminated monolayer on a MAM electrode improved the performance and stability of the OLEDs relative to a reference device with only a MAM electrode, demonstrating that coating with SAMs via vapor phase deposition is an effective method to engineer the interface of MAM electrode optoelectronic devices.

Modeling of self-assembled monolayers (SAMs) of Octadecanethiol and Hexadecanethiol on gold (Au) and silver (Ag)

This work proposes a modeling of self-assembled monolayers (SAMs) to assess the quality of SAM layers in terms of their leaky behavior. In the modeling, electrochemical impedance spectroscopy (EIS) and a conceptual and simplified resistance-resistance-capacitance (RRC) electrical model from an electrochemical setup were demonstrated. Based on the model, Matlab-based simulation was carried out to find out the suggestive low frequency measurement for analyzing the defective monolayers. Finally, the developed electrical model facilitated the interpretation of the EIS spectra measured in the range of 10Hz to 100kHz and provided the capacitance, resistance and thickness of the monolayer. Thus the proposed model could be considered as a powerful tool to characterize the defective SAMs, which was very specific to determine the resistive behavior of the self-assembled monolayers prepared for shorter adsorption time.

Advanced drug delivery via self-assembled monolayer-coated nanoparticles

Nanotechnology has greatly enhanced the field of medicine over the last decade. Within this field, advances in nanoparticle research have rendered them attractive candidates for drug delivery. Consequently, controlling the chemistry that occurs at the nanoparticle interface influences the efficiency of the drug-delivery system. In this review, we explore the role of coating materials, in the form of self-assembled monolayers (SAMs), in enhancing the interfacial properties of nanoparticles. We discuss how SAMs enhance the properties of particles, such as stability and dispersibility, as well as provide a platform for delivering biomolecules and other therapeutic agents. In addition, we describe recent methods for generating nanoparticles with targeted surface functionality using custom-designed SAMs. These functionalities offer marked advances to the three stages of a drug-delivery system: loading, delivery, and release of therapeutic molecules. A suitable functionalization strategy can provide a means for covalent or non-covalent immobilization of drug molecules. Moreover, a robust coating layer can aid the encapsulation of drug molecules and inhibit molecular degradation during the delivery process. A stimuli-responsive SAM layer can also provide an efficient release mechanism. Thus, we also review how stimuli-responsive coatings allow for the controlled release of therapeutic agents. In addition, we discuss the merits and limitations of stimuli-responsive SAMs as well as possible strategies for future delivery systems. Overall, advances in this research area allow for developing novel drug delivery methodologies with high efficiency and minimal toxicity.

Controlling recombination kinetics of hybrid poly-3-hexylthiophene (P3HT)/titanium dioxide solar cells by self-assembled monolayers

Self-assembled monolayers (SAMs) of benzoic acid based molecules are used to modify the metal oxide–polymer interface in a hybrid poly-3-hexylthiophene (P3HT)/TiO2 photovoltaic device structure. The effect of SAMs on current density is in accordance with expectation from the driving force for charge separation of metal oxide–polymer interface in a hybrid poly-3-hexylthiophene (P3HT)/TiO2 photovoltaic device. However, the effect of monolayers on open circuit voltage is quite unexpected from the interfacial energetics as all the monolayers improve the open circuit voltage in spite of different sign of the interfacial dipole for different SAMs. This suggests that the monolayers have additional functions. Overall device performance is enhanced by more than a factor of two using a SAM with permanent dipole pointing towards the TiO2 surface or pointing towards polymer when compared to a control device with no interface modifiers. This study concludes that the SAM layer has two functions that are to shift the position of the conduction band of the porous TiO2 relative to the polymer HOMO level so as to influence interfacial charge separation and to act as a barrier layer, insulating back electron transfer from the TiO2 to the polymer. Both effects can benefit the performance of hybrid polymer metal oxide solar cells.

ARM-microcontroller based portable nitrite electrochemical analyzer using cytochrome c reductase biofunctionalized onto screen printed carbon electrode

Nitrite (NO2−) supplementation limits hypoxia-induced oxidative stress and activates the alternate NO pathway which may partially account for the nitrite-mediated cardioprotection. So, sensitive and selective biosensors with point-of-care devices need to be explored to detect the physiological nitrite level due to its important role in human pathophysiology. In this work, cytochrome c reductase (CcR) biofunctionalized self assembled monolayer (SAM) functionalized on gold nanoparticles (GNPs) in polypyrrole (PPy) nanocomposite onto the screen printed carbon electrode (SPCE) was investigated as a biosensor for the detection of nitrite based on its electrochemical and catalytic properties. CcR was covalently coupled with SAM layers on GNPs by using EDC and NHS. Direct electrochemical response of CcR biofunctionalized electrodes showed a couple of well-defined and nearly reversible cyclic voltammetric peaks at −0.34 and −0.45 vs. Ag/AgCl. Under optimal conditions, the biosensor could be used for the determination of NO2− with a linear range from 0.1–1600µm and a detection limit of 60nM with a sensitivity of 0.172µAµM−1cm–2. Further, we have designed and developed a novel and cost effective portable electrochemical analyzer for the measurement of NO2− in hypoxia induced H9c2 cardiac cells using ARM microcontroller. The results obtained here using the developed portable electrochemical nitrite analyzer were also compared with the standard cyclic voltammetry instrument and found in agreement with each other.

Optimization and upscaling of spin coating with organosilane monolayers for low-k pore sealing

For porous low-k film to be integrated into the next generation of interconnects, the pores need to be sealed against metal ions and barrier precursors. Self-assembled monolayers (SAMs) from organosilane precursor are spin coated onto 300mmk=2.2 low-k wafers. Two solvents, propylene glycol monomethyl ether acetate (PGMEA) and methanol with different dielectric constant of 8.3 and 20.1, are evaluated in terms of SAMs layer quality and sealing efficiency at coupon level. SAMs deposited from PGMEA show better sealing than SAMs deposited from methanol and therefore are selected for upscaling. Full wafer spin coating results show that a concentration of 0.05mM or below results in a partial coverage and a tilt angle as high as 70° from the backbone to the normal. Aggregation is observed for all tested concentrations and is worse for higher concentrations, which is possibly induced by the non-negligible presence of water in PGMEA solvents. In order to test the sealing efficiency of the SAMs layer against metal barrier precursors, MnN films by chemical vapor deposition (CVD) and TaNx/Ta (TNT) films by physical vapor deposition (PVD) are deposited on SAM coated low-k wafers. HfO2 is also deposited by Atomic layer deposition (ALD), which is not considered as a barrier but to test the sealing against ALD precursors. Depth profiling Rutherford Backscattering Spectrometry (RBS) measurements indicate an effective sealing of SAMs against CVD and ALD precursors but not against PVD barrier.

Application of Self-Assembled Monolayers to the Electroless Metallization of High Aspect Ratio Vias for Microelectronics

All-wet electroless metallization of through-silicon vias (TSVs) with a width of 5 μm and a 1:10 aspect ratio was carried out. Immersion in a n-(2-aminoethyl) 3-aminopropyl-trimethoxysilane (AEAPTMS) self-assembled monolayer (SAM) was used to enhance the adhesion between the metal film and substrate. Contact angle variation and atomic force microscopy were used to verify the formation of a SAM layer. A PdCl2 solution was later used to activate the silanized substrates, exploiting the affinity of the –NH3 functional group of AEAPTMS to palladium. A nickel-phosphorus-boron electroless bath was employed to deposit the first barrier layer onto silicon. The NiPB growth rate was evaluated on flat silicon wafers, while the structure of the coating obtained was investigated via glow discharge optical emission spectroscopy. Cross-sectional scanning electron microscope observations were carried out on metallized TSVs to characterize the NiPB seed, the Cu seed layer deposited with a second electroless step, and the Cu superfilling obtained with a commercial solution. Complete filling of TSV was achieved.

Simple, Reversible, and Fast Modulation in Superwettability, Gradient, and Adsorption by Counterion Exchange on Self-Assembled Monolayer.

A simple fabrication method for preparation of surfaces able to switch from superhydrophobic to superhydrophilic state in a reversible and fast way is described. A self-assembled monolayer (SAM) consisting of quaternary ammonium group with aliphatic tail bearing terminal thiol functionality was created on gold nano/microstructured and gold planar surfaces, respectively. A rough nano/microstructured surface was prepared by galvanic reaction on a silicon wafer. The reversible counterion exchange on the rough surface resulted in a switchable contact angle between <5° and 151°. The prewetted rough surface with Cl(-) as a counterion possesses a superoleophobic underwater character. The kinetics of counterion exchanges suggests a long hydration process and strong electron ion pairing between quaternary ammonium group and perfluorooctanoate counterion. Moreover, a wettability gradient from superhydrophobic to superhydrophilic can be formed on the modified rough gold surface in a robust and simple way by passive incubation of the substrate in a counterion solution and controlled by ionic strength. Furthermore, adsorption of gold nanoparticles to modified plain gold surface can be controlled to a high extent by counterions present on the SAM layer.

Investigation of functionalized silicon nanowires by self-assembled monolayer

The functionalization using self assembled monolayer (SAM) of silicon nanowires (SiNW) fabricated by plasma enhanced chemical vapor deposition (PECVD) is reported here. The SAM is being utilized as the first building block in the functionalization process. The morphology of the SiNW comprises a polycrystalline core wrapped by an hydrogenated amorphous silicon (α-Si:H) shell. Since most of the available methods for SAM verification and characterization are suitable only for flat substrates; therefore, in addition to the SiNW α-Si:H on flat samples were produced in the same system as the SiNWs. First we confirmed the SAM's presence on the flat α-Si:H samples using the following methods: contact angle measurement to determine the change in surface energy; atomic force microscopy (AFM) to determine uniformity and molecular coverage. Spectroscopic ellipsometry and X-ray reflectivity (XRR) were performed to measure SAM layer thickness and density. X-ray photoelectron spectroscopy (XPS) was applied to study the chemical states of the surface. Next, SiNW/SAM were tested by electrochemical impedance spectroscopy (EIS), and the results were compared to α-Si:H/SAM. The SAM electrical coverage on SiNW and α-Si:H was found to be ∼37% and ∼65±3%, respectively. A model, based on transmission line theory for the nanowires is presented to explain the disparity in results between the nanowires and flat surface of the same materials.

Corrosion Processes Controlled by Phosphonic Acid Nano-layers

Self assembled monolayers (SAM) formed by fluoro-phosphonic and undecenyl-phosphonic acids on carbon steel surfaces as anticorrosive nanocoatings were investigated. The anticorrosive efficacy of these SAM layers was followed by atomic force microscopy, when the change in the surface morphology caused by layer deposition and corrosion processes was monitored. The corrosion process was determined by electrochemical potentiodynamic polarization and cyclic voltammetry. Results showed that both chemicals produced good protection against corrosion as anodic inhibitors specially with increasing the time of layer formation which results in a more compact molecular film. According to the AFM images, the fluoro-phosphonic acid self-assembled molecular layer can control the general as well as the pitting corrosion, but the SAM layers of the undecenyl-phosphonic acid cannot inhibit the pitting corrosion. The AFM measurements confirmed the electrochemical results.

Characteristics of AlGaN/GaN HEMTs for Detection of MIG

The aim of this research work is to analyze the surface characteristics of an improved AlGaN/GaN HEMT biosensor. The investigation leads to analyze the transistor performance to detect human MIG with the help of an analytical model and measured data. The surface engineering includes the effects of repeatability, influence of the substrate, threshold shifting, and floating gate configuration. A numerical method is developed using the charge-control model and the results are used to observe the changes in the device channel at the quantum level. A Self-Assembled Monolayer (SAM) is formed at the gate electrode to allow immobilization and reliable cross-linking between the surface of the gate electrode and the antibody. The amperometric detection is realized solely by varying surface charges induced by the biomolecule through capacitive coupling. The equivalent DC bias is 6.99436 × 10-20 V which is represented by the total number of charges in the MIG sample. The steady state current of the clean device is 66.89 mA. The effect of creation and immobilization of the protein on the SAM layer increases the current by 80 - 150 μA which ensures that successful induction of electrons is exhibited.

Density Analysis on Self-Assembled Monolayers (SAMs) of Functionalized Alkanethiol on Gold over Polyethylene Terephthalate (PET) Substrate

In this work, an equivalent circuit from electrochemical impedance spectroscopy (EIS) has been designed and executed for qualitative analyzing of leaky behavior of self‐assembled monolayers (SAMs) of octadecanethiol [CH3(CH2)17‐SH] on gold (Au) suface. Polyethylene terephthalate (PET), the cost‐effective substrate has been utilized for metalization of thin Au layer with subsequent formation of octadecanethiol SAM on it with systematic variation of adsorption time. The SAM layer shows its highly dependency on the adsorption time of the surfactants of octadecanethiol with the metal surface.

Low-Temperature Solution-Processed Gate Dielectrics for High-Performance Organic Thin Film Transistors.

A low-temperature solution-processed high-k gate dielectric layer for use in a high-performance solution-processed semiconducting polymer organic thin-film transistor (OTFT) was demonstrated. Photochemical activation of sol-gel-derived AlOx films under 150 °C permitted the formation of a dense film with low leakage and relatively high dielectric-permittivity characteristics, which are almost comparable to the results yielded by the conventionally used vacuum deposition and high temperature annealing method. Octadecylphosphonic acid (ODPA) self-assembled monolayer (SAM) treatment of the AlOx was employed in order to realize high-performance (>0.4 cm2/Vs saturation mobility) and low-operation-voltage (<5 V) diketopyrrolopyrrole (DPP)-based OTFTs on an ultra-thin polyimide film (3-μm thick). Thus, low-temperature photochemically-annealed solution-processed AlOx film with SAM layer is an attractive candidate as a dielectric-layer for use in high-performance organic TFTs operated at low voltages.

Optimisation and Characterisation of Anti-Fouling Ternary SAM Layers for Impedance-Based Aptasensors.

An aptasensor with enhanced anti-fouling properties has been developed. As a case study, the aptasensor was designed with specificity for human thrombin. The sensing platform was developed on screen printed electrodes and is composed of a self-assembled monolayer made from a ternary mixture of 15-base thiolated DNA aptamers specific for human thrombin co-immobilised with 1,6-hexanedithiol (HDT) and further passivated with 1-mercapto-6-hexanol (MCH). HDT binds to the surface by two of its thiol groups forming alkyl chain bridges and this architecture protects from non-specific attachment of molecules to the electrode surface. Using Electrochemical Impedance Spectroscopy (EIS), the aptasensor is able to detect human thrombin as variations in charge transfer resistance (Rct) upon protein binding. After exposure to a high concentration of non-specific Bovine Serum Albumin (BSA) solution, no changes in the Rct value were observed, highlighting the bio-fouling resistance of the surface generated. In this paper, we present the optimisation and characterisation of the aptasensor based on the ternary self-assembled monolayer (SAM) layer. We show that anti-fouling properties depend on the type of gold surface used for biosensor construction, which was also confirmed by contact angle measurements. We further studied the ratio between aptamers and HDT, which can determine the specificity and selectivity of the sensing layer. We also report the influence of buffer pH and temperature used for incubation of electrodes with proteins on detection and anti-fouling properties. Finally, the stability of the aptasensor was studied by storage of modified electrodes for up to 28 days in different buffers and atmospheric conditions. Aptasensors based on ternary SAM layers are highly promising for clinical applications for detection of a range of proteins in real biological samples.

Self-Assembled Monolayers Get Their Final Finish via a Quasi-Langmuir-Blodgett Transfer.

The growth of self-assembled monolayers (SAMs) of octadecylphosphonic acid (ODPA) molecules on α-Al2O3(0001) and subsequent dewetting of the SAMs were studied with a combination of in situ sum-frequency generation (SFG) and molecular dynamics (MD) simulations. Although SAM growth after deposition times >8 h reduces to nearly negligible values, the resultant ODPA SAMs in solution are still not in a well-ordered state with the alkyl chains in all-trans configurations. In fact, in situ SFG spectroscopy revealed a comparatively high concentration of gauche defects of the SAM in the ODPA 2-propanol solution even after a growth time of 16 h. Here, results of the MD simulations strongly suggest that defects can be caused by ODPA molecules which are not attached to the substrate but are incorporated into the SAM layer with the polar headgroup oriented into the 2-propanol solvent. This inverted adsorption geometry of additional ODPA molecules blocks adsorption sites and thus stabilizes the SAM without improving ordering to an extent that all molecules are in the all-trans configuration. While persistent in solution, the observed defects can be healed out when the SAMs are transferred from the solvent to a gas phase. During this process, a quasi-Langmuir-Blodgett transfer of molecules takes place which drives the SAM into a higher conformational state and significantly improves its quality.

A novel electrochemical piezoelectric label free immunosensor for aflatoxin B1 detection in groundnut

An electrochemical quartz crystal microbalance (EQCM) based label-free immunosensor has been developed for the quantitative detection of aflatoxin B1 (AFB1) in groundnut. The gold (Au) coated quartz crystal (6 MHz) functionalized with self-assembled monolayer (SAM) of 4-aminothiophenol (4-ATP) has been utilized to immobilize anti-aflatoxin antibodies (aAFB1). The quartz crystal microbalance QCM frequency and EQCM cyclic voltammetry measurements have been used to optimize the deposition of SAM layer on Au electrode (4-ATP/Au), antibody immobilization (aAFB1/4-ATP/Au), bovin serum albumin (BSA) adsorption (BSA/aAFB1/4-ATP/Au) and immuno interaction (AFB1/BSA/aAFB1/4-ATP/Au). Interestingly, the EQCM studies conducted on this immunoelectrode provide wider linear range of 0.1–4.0 ng mL−1 and higher sensitivity than those obtained using under identical condition. The high association constant of 9.4 × 10−2 ng mL−1 of the EQCM immunosensor can be attributed to molecularly oriented self assembled monolayer of 4-ATP on QCM gold electrode that acts as a nanowire between aAFB1 and electrode through amide linkage formed by the NH2 group of 4-ATP and COOH group of Fc part of aAFB1. This linkage also helps to project paratope part of aAFB1 to interact with AFB1. The developed label free EQCM based immunosensor was tested for the AFB1 detection in the extract of contaminated groundnut and the results were compared with Liquid chromatography-tandem mass spectrometry (LC-MS/MS) method. The comparative results demonstrate the suitability of this sensor for routine screening of groundnut and the percent error is found to be ±12%.

Optical imaging beyond the diffraction limit by SNEM: effects of AFM tip modifications with thiol monolayers on imaging quality.

Tip-enhanced nanoscale optical imaging techniques such as apertureless scanning near-field optical microscopy (a-SNOM) and scanning near-field ellipsometric microscopy (SNEM) applications can suffer from a steady degradation in performance due to adhesion of atmospheric contaminants to the metal coated tip. Here, we demonstrate that a self-assembled monolayer (SAM) of ethanethiol (EtSH) is an effective means of protecting gold-coated atomic force microscopy (AFM) probe tips from accumulation of surface contaminants during prolonged exposure to ambient air. The period over which they yield consistent and reproducible results for scanning near-field ellipsometric microscopy (SNEM) imaging is thus extended. SNEM optical images of a microphase separated polystyrene-block-poly (methylmethacrylate) (PS-b-PMMA) diblock copolymer film, which were captured with bare and SAM-protected gold-coated AFM probes, both immediately after coating and following five days of storage in ambient air, were compared. During this period the intensity of the optical signals from the untreated gold tip fell by 66%, while those from the SAM protected tip fell by 14%. Additionally, gold coated AFM probe tips were modified with various lengths of alkanethiols to measure the change in intensity variation in the optical images with SAM layer thickness. The experimental results were compared to point dipole model calculations. While a SAM of 1-dodecanethiol (DoSH) was found to strongly suppress field enhancement we find that it can be locally removed from the tip apex by deforming the molecules under load, restoring SNEM image contrast.

Slip behavior in polymethylmethacrylate films in dependence of self-assembled monolayer wettability

The slip behavior of polymethylmethacrylate (PMMA) films prepared on self-assembled monolayers (SAM) supported by Si substrate was investigated according to the wettability of the SAM. To this end, the dewetting of thermally annealed PMMA films was measured varying the surface energies of underlyed SAM. Total surface energy (γt) of SAM increased with increasing UV exposure time. At surface energy of <31.3 mJ/m2, the PMMA films showed slip behavior; radius of holes (R) ∝ t0.68−0.72, where t is dewetting time. The slip behavior seems to be dominated by the viscous friction between the PMMA films and the SAM. As t increased, slip behavior disappeared, R ∝ t0.34−0.39. At surface energy of >42.0 mJ/m2, the PMMA films did not show slip behavior, R ∝ t0.45. It means that interface effects between PMMA films and SAM layers increase with increasing of γ of SAM layers, and then the molecular interaction in the PMMA films increases. The spreading coefficient (S) of the PMMA films approached positive number for UV exposure times between 0 s and 300 s. We concluded that the stability of the PMMA films can be simply controlled by UV-ozone treatment on the SAM layers.

Improving area-selective molecular layer deposition by selective SAM removal.

Area selective molecular layer deposition (MLD) is a promising technique for achieving micro- or nanoscale patterned organic structures. However, this technique still faces challenges in attaining high selectivity, especially at large MLD cycle numbers. Here, we illustrate a new strategy for achieving high quality patterns in selective film deposition on patterned Cu/Si substrates. We employed the intrinsically selective adsorption of an octadecylphosphonic acid self-assembled monolayer (SAM) on Cu over Si surfaces to selectively create a resist layer only on Cu. MLD was then performed on the patterns to deposit organic films predominantly on the Si surface, with only small amounts growing on the Cu regions. A negative potential bias was subsequently applied to the pattern to selectively desorb the layer of SAMs electrochemically from the Cu surface while preserving the MLD films on Si. Selectivity could be enhanced up to 30-fold after this treatment.