Degradation Of Self-assembled Monolayers Research Articles

Photocatalytic Lithography

Patterning, the controlled formation of ordered surface features with different physico-chemical properties, is a cornerstone of contemporary micro- and nanofabrication. In this context, lithographic approaches owe their wide success to their versatility and their relative ease of implementation and scalability. Conventional photolithographic methods require several steps and the use of polymeric photoresists for the development of the desired pattern, all factors which can be deleterious, especially for sensitive substrates. Efficient patterning of surfaces, with resolution down to the nanometer scale, can be achieved by means of photocatalytic lithography. This approach is based on the use of photocatalysts to achieve the selective chemical modification or degradation of self-assembled monolayers, polymers, and metals. A wide range of photoactive compounds, from semiconducting oxides to porphyrins, have been demonstrated to be suitable photocatalysts. The goal of the present review is to provide a comprehensive state-of-the-art photocatalytic lithography, ranging from approaches based on semiconducting oxides to singlet oxygen-based lithography. Special attention will be dedicated to the results obtained for the patterning of polymer brushes, the sculpturing of metal nanoparticle arrays, and the patterning of graphene-based structures.

Degradation of alkanethiol self-assembled monolayers in mesenchymal stem cell culture.

Alkanethiol self-assembled monolayers (SAMs) have been used extensively in studying the role of surface functionality and geometry on stem cell differentiation; however, the effects of stem cell culture conditions on SAM stability over time are not well understood. In this work, we examined the physical and chemical changes occurring on gold (Au)-SAM surfaces over time as a function of Au thickness. Within a narrow range of thicknesses (4, 8, and 10 nm), we observed significant differences in temporal SAM stability for a commonly utilized, hydrophilic, protein and cell repulsive oligo(ethylene) glycol alkanethiol (HS-(CH2 )11 (O(CH2 )2 )6 OH) SAM and the hydrophobic, protein adhesive hexadecanethiol (SH-(CH2 )15 CH3 ) SAM. Within both acellular and stem cell culture conditions, 8 nm Au resulted in the most stable SAMs (∼7 days). The 4 and 10 nm Au SAMs exhibited loss in stability following 5 days at varying degradation rates, showing 4 nm Au to be the least stable. Migration of human mesenchymal stem cells seeded on SAM surfaces showed that SAM degradation rates in acellular conditions were directly correlated with the cellular migration behavior. Findings of this study can be used to develop SAM surfaces with controlled degradation rates for applications in stem cell engineering and regenerative medicine. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 464-474, 2017.

Degradation of self-assembled monolayers in organic photovoltaic devices

Self-assembled monolayers (SAMs) based on n-octylphosphonic acid (C8PA) and 1H,1H,2H,2H-perfluorooctanephosphonic acid (PFOPA) were investigated for application as an anode buffer layer in C60-based organic photovoltaic (OPV) devices. We found that the degradation of the OPV efficiency with respect to air exposure was significantly reduced with the perfluorinated PFOPA compared to the aliphatic C8PA. We attribute the OPV degradation to moisture diffusion from the top aluminum electrode and the lowering of the anode work function as a result of hydrolysis of the SAM buffer layer.

Increased Stability of Glycol-Terminated Self-Assembled Monolayers for Long-Term Patterned Cell Culture

Self-assembled monolayers (SAMs) are widely used to confine proteins and cells to a pattern to study cellular processes and behavior. To fully explore some of these phenomena, it is necessary to control cell growth and confinement for several weeks. Here, we present a simple method by which protein and cellular confinement to a pattern can be maintained for more than 35 days. This represents a significant increase in pattern stability compared to previous monolayer systems and is achieved using an amide-linked glycol monomer on 50 Å titanium/100 Å gold-coated glass coverslips. In addition, this study provides insight into the method of SAM degradation and excludes interfacial mixing of the monomers and blooming of the adlayer as major mechanisms for SAM degradation.

The stability of self‐assembled monolayers with time and under biological conditions

The stability of self-assembled monolayers (SAMs) of different functionalities (CH(3)-, OH-, and EG4-) over time under appropriate storage conditions and when immersed in cell culture media was evaluated. X-ray photoelectron spectroscopy (XPS) was used to detect the oxidized sulfur species (S(2p) binding energy from 167 to 168 eV) resulting from oxidation of the surfaces. CH(3)-terminated SAMs stored for a period of 9 weeks in a nitrogen chamber were not altered. The same did not happen with OH- and EG4-SAMs, for which the XPS spectra evidenced oxidized peaks after 2 weeks. Regarding the stability of these surfaces under biological conditions, 30 min of immersion at 37 degrees C in serum-free or 10% fetal bovine serum (FBS) supplemented medium did not induce detectable oxidation. However, a small percentage of oxidized sulfur could have been washed out by the media, as confirmed in studies using SAMs immersed in water. Despite the possible rinsing out of oxidized thiols, high amounts of oxidation can still be detected by XPS. SAMs degradation during ethanol sterilization was not detectable by XPS, although a small increase on the wettability of OH-SAMs was observed. The data suggest that SAMs must be used freshly prepared, being recommended for short-term biological studies.

Thermal stability of thiol and silane monolayers: A comparative study

The stability of self-assembled monolayers (SAMs) at elevated temperatures is of considerable technological importance. The thermal stability of 1-octadecanethiol (ODT), 16-mercaptohexadecanoic acid (MHDA) and 1H,1H,2H,2H-perfluorodecanethiol (PFDT) SAMs on gold surfaces, and of 4-aminobutyltriethoxysilane (ABTES) and 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane (PFDS) assembled on hydroxylated silicon surfaces, was studied by X-ray photoelectron spectroscopy (XPS). The samples were heated in ultrahigh vacuum to temperatures in excess of that required for SAM degradation. ODT monolayers were stable to ca. 110 °C, while MHDA and PFDT SAMs were stable to ca. 145 °C. ABTES SAMs were found to be indefinitely stable to 250 °C, while PFDS SAMs were stable to 350 °C. These studies demonstrate the advantages of using silane monolayers for moderate to high temperature applications and illustrate differences that arise due to the nature of the tail group. To demonstrate the feasibility of silanes for template-directed patterning, a hydroxylated silicon oxide surface containing microcontact-printed PFDS patterns was spin-coated with a mainly hydrophilic block copolymer. Annealing the surface at 90 °C for 2 h caused the block copolymer to dewet the hydrophobic PFDS-patterned regions and adsorb exclusively on the unpatterned regions of the surface.

Electron Beam-Induced Damage of Alkanethiolate Self-Assembled Monolayers Adsorbed on GaAs (001): A Static SIMS Investigation

We have investigated the reaction pathways involved in the electron-beam-induced damage of −CH3, −OH, and −COOH terminated alkanethiolate self-assembled monolayers (SAMs) adsorbed on GaAs (001) using time-of-flight secondary ion mass spectrometry. Upon electron beam exposure, the monolayers dehydrogenate, leading to the formation of C═C bonds, cross-links, and polycyclic aromatic hydrocarbons (PAHs). We also observe C−S bond scission. The data suggest that the electron beam damage is not uniform along the alkanethiol backbone. Upon electron beam exposure, we observe the loss of CO2 and H2O from the −COOH and −OH terminated SAMs, respectively, indicating that the terminal groups decompose. Although the SAM degradation mechanism is similar to that previously reported for alkanethiolate SAMs adsorbed on metals, there are some important differences. First, the electron dose required to form C═C bonds and PAHs is much smaller than for SAMs adsorbed on metals. Second, although we observe C−S bond scission, we d...

Enhanced Stability of Thiolate Self-Assembled Monolayers (SAMs) on Nanostructured Gold Substrates

Degradation of thiolate self-assembled monolayers (SAMs) in ambient conditions and liquid environments seriously limits the fabrication of thiol-based devices. Here, we demonstrate that nanostructured gold exhibits higher resistance to SAM degradation and increased electrochemical stability against thiolate desorption in relation to polycrystalline preferred oriented Au(111). The increased stability can be related to the presence of a large number of defects, such as adatoms, vacancies, and steps where the thiolate binding energy is stronger than at terraces. The nanostructured Au is an interesting platform because it can be easily prepared, has surface enhanced Raman spectroscopy (SERS) activity, and exhibits a high signal/noise ratio for amperometric detection because of its large real surface area.

Area Selective Atomic Layer Deposition of Cobalt Thin Films

Area selective atomic layer deposition (ALD) of Co thin films was investigated by using octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) as a blocking layer. Selective deposition was not observed in Co plasma enhanced ALD (PE-ALD) by CoCp2 and NH3 plasma, which was found to be due to the rapid degradation of OTS SAM. In contrast to PE-ALD, thermal ALD Co by Co(AMD)2 and H2 produced area selective deposition up to 1000 cycles.

Polar and Azimuthal Alignment of a Nematic Liquid Crystal by Alkylsilane Self-Assembled Monolayers: Effects of Chain-Length and Mechanical Rubbing

Alkylsilane self-assembled monolayers (SAMs) on oxide substrates are commonly used as liquid crystal (LC) alignment layers. We have studied the effects of alkyl chain length, photolytic degradation, and mechanical rubbing on polar and azimuthal LC anchoring. Both gradient surfaces (fabricated using photolytic degradation of C18 SAMs) and unirradiated SAMs composed of short alkyl chains show abrupt transitions from homeotropic to tilted alignment as a function of degradation or chain length. In both cases, the transition from homeotropic to tilted anchoring corresponds to increasing wettability of the SAM surfaces. However, there is an offset in the critical contact angle for the transition on gradient vs unirradiated SAMs, suggesting that layer thickness is more relevant than wettability for LC alignment. Mechanical rubbing can induce azimuthal alignment along the rubbing direction for alignment layers sufficiently near the homeotropic-to-planar transition. Notably, mechanical rubbing causes a small but significant shift in the homeotropic-to-tilted transition, e.g., unrubbed C5 SAMs induce homeotropic anchoring, but the same surface after rubbing induces LC pretilt.

Stability of Mixed PEO−Thiol SAMs for Biosensing Applications

The secret of a successful affinity biosensor partially hides in the chemical interface layer between the transducer system and the biological receptor molecules. Over the past decade, several methodologies for the construction of such interface layers have been developed on the basis of the deposition of self-assembled monolayers (SAMs) of alkanethiols on gold. Moreover, mixed SAMs of polyethylene oxide (PEO) containing thiols have been applied for the immobilization of biological receptors. Despite the intense research in the field of thiol SAMs, relatively little is known about their biosensing properties in correlation with their long-term stability. Especially the impact of the storage conditions on their biosensing characteristics has not been reported before to our knowledge. To address these issues, we prepared mixed PEO SAMs and tested their stability and biosensing performance in several storage conditions, i.e., air, N2, ethanol, phosphate buffer, and H2O. The quality of the SAMs was monitored as a function of time using various characterization techniques such as cyclic voltammetry, contact angle, grazing angle Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy. In addition, the impact of the different storage conditions on the biosensor properties was investigated using surface plasmon resonance. Via the latter technique, the receptor immobilization, the analyte recognition, and the nonspecific binding were extensively studied using the prostate specific antigen as a model system. Our experiments showed that very small structural differences in the SAM can have a great impact in their final biosensing properties. In addition it was shown that the mixed SAMs stored in air or N2 are very stable and retain their biosensor properties for at least 30 days, while ethanol appeared to be the worst storage medium due to partial oxidation of the thiol headgroup. In conclusion, care must be taken to avoid SAM degradation during storage to retain typical SAM characteristics, which is very important for their general use in many proposed applications.

In situ AFM investigations on degradation of self-assembled monolayers on mica: Effect of humidity

The structure and stability of self-assembled monolayers (SAMs) on substrates under certain forces are of great importance to thin films for their applications such as lubrication and adhesion in micromachines, corrosion inhibition, biological sensors, etc. In this paper, we investigate the degradation of arachidic acid (AA) monolayers on the mica surface via in situ tapping mode atomic force microscopy (TM-AFM) by continuously scanning at different humidities. Our results indicate that the degradation of AA monolayers varied along with the changing environmental humidity. The curves on the number of pinholes as a function of scan times and the dependence of the surface coverage on the scan times at varied humidity are also presented in this report. It was also found that the average apparent height of AA monolayers decreased with the increasing relative humidity.

Mechanism of UV Photoreactivity of Alkylsiloxane Self-Assembled Monolayers

A molecular level understanding of the photoreactivity of self-assembled monolayers (SAMs) becomes increasingly important as the spatial resolution starts to be limited by the size of the resist and the spatial extent of the photochemical reactions in photoresist micropatterning. To this end, a number of surface characterization techniques were combined to understand the reactive agents, reactive sites, kinetics, and reaction pathways in the UV photoreactivity of octadecylsiloxane (ODS) SAMs. Quantitative analysis of our results provides evidence that ground state atomic oxygen is the primary reactive agent for the UV degradation of ODS SAMs. UV degradation, which follows zero-order kinetics, results in the scission of alkyl chains instead of the siloxane headgroups. Our results suggest that the top of the ODS SAMs is the preferential reactive site. Using a novel, highly surface sensitive technique, fluorescence labeling of surface species, we identified the presence of submonolayer quantities chemical functional groups formed by the UV degradation. These groups are intermediates in a proposed mechanism based on hydrogen abstraction.

Rapid degradation of alkanethiol-based self-assembled monolayers on gold in ambient laboratory conditions

Self-assembled monolayers (SAMs) consisting of alkanethiols and similar sulfur-containing molecules on noble metal substrates are extensively used and explored for various chemical and biological surface-functionalization in the scientific community. SAMs consisting of thiol- or disulfide-containing molecules adsorbed on gold are commonly used due to their ease of preparation and stability. However, the gold–thiolate bond is easily and rapidly oxidized under ambient conditions, adversely affecting SAM quality and structure. Here, the oxidation of dodecanethiol on gold is explored for various 12-h exposures to ambient laboratory air and light. SAM samples are freshly prepared, air-exposed, and stored in small, capped vials. X-ray photoelectron spectroscopy (XPS) reveals nearly complete oxidation of the thiolate in air-exposed samples, and a decrease in carbon signal on the surface. Near-edge X-ray absorption fine structure spectroscopy (NEXAFS) at the carbon K-edge shows a loss of upright orientational order upon air exposure. Alternatively, the oxidation of the thiolate is minor when SAMs are stored in limited-air-containing small 15 ml vials. Thus, care must be taken to avoid SAM degradation by ensuring alkanethiolates on gold have sufficient durability for each intended environment and application.

Photocatalytic Degradation of Self-Assembled Monolayers Anchored at the Vicinity of Titanium Dioxide Domains

AbstractIn order to study the photodegradation of molecules located at the vicinity of a titanium dioxide photocatalyst, a well-defined structure comprised of alternating micro-stripes of TiO

Photoreactivity of Alkylsiloxane Self-Assembled Monolayers on Silicon Oxide Surfaces

We have combined contact angle measurements, Fourier transform infrared spectroscopy (FT-IR), and atomic force microscopy (AFM) to investigate the UV photoreactivity of octadecylsiloxane (ODS) SAMs (self-assembled monolayers) in air. Contact angle measurements present evidence that ozone is not the active agent in alkylsiloxane SAM degradation under UV illumination in air. A combination of UV and oxygen is necessary for monolayer degradation to proceed. AFM measurements on submonolayer coverage SAMs provide direct evidence of the photodegradation of ODS SAMs and reveal the role of defects in the degradation process. FT-IR and AFM results suggest that the hydrocarbon chain is the reactive site of the monolayers. A microscopic mechanism of the photoreactivity involving hydrogen abstraction is suggested based on the mechanism of gas-phase oxidation of alkanes. Our results have implications in clarifying the contribution from the reactivity of alkyl chains during the photomodification of SAMs. This clarificat...