Solvent-controlled organization of self-assembled polymeric monolayers on gold: an easy approach for the construction of protein resistant surfaces

Protein resistant surfaces based on poly(ethylene glycol) (PEG) coatings are extensively applied in the fields of biosensors, tissue engineering, fundamental cell-surface interaction research, and drug delivery systems. The structural organization of the PEG film on the surface has a significant effect on the performance of the film to resist protein adsorption. In this paper, we report an approach using solvent to control the organization of the polymeric monolayer on gold. A water soluble copolymer with grafted PEG side chains and alkyl disulfide side chains was synthesized. A polymeric monolayer was fabricated on a gold surface from different solutions (water- and toluene-based) of the copolymer. The organization of the polymeric monolayers was characterized by means of ellipsometry, cyclic voltammetry, contact angle, X-ray photoelectron spectroscopy, and atomic force microscopy. It was proven that the structural organization of the polymeric monolayer on a gold surface could be controlled by the solvent. A polymeric monolayer with PEG enriched at the outer level is obtained when water is used as the solvent. Various types of proteins, including fibrinogen, albumin, and normal human serum, were used to test the protein resistance of the gold surfaces modified by the polymeric monolayers. The polymeric monolayer formed from a water solution of the copolymer showed excellent protein resistance. In addition, by using water as the solvent, patterning of the polymeric monolayer could easily be achieved through a combination of lift-off and self-assembly. We believe that the approach reported here provides an easy, fast, and efficient way to fabricate a robust protein resistant surface.     

Andreev reflection at high magnetic fields: evidence for electron and hole transport in edge states

We have studied magnetotransport in arrays of niobium filled grooves in an InAs/Al(x)Ga(1-x)Sb heterostructure. The critical field of up to 2.6 T permits one to enter the quantum Hall regime. In the superconducting state, we observe strong magnetoresistance oscillations, whose amplitude exceeds the Shubnikov-de Haas oscillations by a factor of about 2, when normalized to the background. Additionally, we find that above a geometry-dependent magnetic field value the sample in the superconducting state has a higher longitudinal resistance than in the normal state. Both observations can be explained with edge channels populated with electrons and Andreev-reflected holes.     

Observation of optical Ramsey interference fringes in collinear ion beam-laser beam interaction by Doppler switching

We report the observation of optical Ramsey interference fringes in collinear ion beam-laser beam interaction. The analogue of separated fields is created by Doppler switching the ions in two successive zones of the common beam path. New conditions to study quantum interference effects are offered by this method.     

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.     

Mössbauer spectroscopy investigation of the persistent photoionization of the DX-center in129mTe-implanted GaAs

Persistent photoionization of the DX-center in GaAs implanted with^129mTe isotopes is observed with Mössbauer spectroscopy. The time constant of the relaxation from the donor site to the DX-center defect site, after the illumination is stopped, is measured as a function of temperature. A small energy barrier for capture (1 meV) is derived from these measurements.     

Formation of dense self-assembled monolayers of (n-decyl)trichlorosilanes on Ta/Ta2O5

Tantalum pentoxide (Ta2O5) is a promising material for the realization of biological interfaces because of its high dielectric constant, its high chemical stability, and its excellent passivating properties. Nevertheless, the deposition of highly organized silane SAMs to realize well-defined and tailored Ta2O5-based (bio)interfaces, has not been studied in great detail as of yet. In this work, we have investigated the formation of a highly ordered, dense monolayer of trichlorosilanes on Ta2O5 surfaces. Specifically, two different cleaning procedures for Ta2O5 were compared and (n-decyl)trichlorosilane (DTS) was used to study the effect of both cleaning methods on the silanization of Ta2O5. Both types of cleaning allowed the formation of complete and crystalline DTS monolayers on Ta2O5, in contrast with the incomplete, disordered silane layer assembled on uncleaned Ta2O5. The deposited self-assembled monolayers were studied by means of contact angle goniometry, Brewster angle FTIR, X-ray photoelectron spectroscopy, cyclic voltammetry, and ellipsometry. Infrared analysis exhibited a highly ordered DTS silane film on Ta2O5 and indicated a larger tilt angle of the alkyl chains on this substrate by comparison to DTS on SiO2. Furthermore, with use of ellipsometry and XPS, the silane film thickness on Ta2O5 was determined to be substantially smaller than that reported in the literature for DTS on SiO2, supporting the observations of an increased tilt angle (approximately 45 degrees ) on Ta2O5 than on SiO2 (approximately 10 degrees ). By means of cyclic voltammetry, the formation of a dense, essentially pinhole-free, silane film was observed on the cleaned samples. In conclusion, the fully characterized and optimized procedure for the silanization of Ta2O5 surfaces with trichlorosilanes will allow the formation of well-defined, reproducible, and controllable chemical interfaces on Ta2O5.