The preparation of self-assembled monolayers (SAMs) of organophosphonic acids on indium tin oxide (ITO) surfaces from different solvents (triethylamine, ethyl ether, tetrahydofuran (THF), pyridine, acetone, methanol, acetonitrile, dimethyl sulfoxide (DMSO), or water) has been performed with some significant differences observed. Cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and contact angle measurement demonstrated that the quality of SAMs depends critically on the choice of solvents. Higher density, more stable monolayers were formed from solvents with low dielectric constants and weak interactions with the ITO. It was concluded low dielectric solvents that were inert to the ITO gave monolayers that were more stable with a higher density of surface bound molecules because higher dielectric constant solvents and solvents that coordinate with the surface disrupted SAM formation.
[Show abstract][Hide abstract] ABSTRACT: The formation of non-electroactive self-assembled monolayers (SAMs) at the electrode / electrolyte interface was characterized with simultaneous impedance, gravimetric and direct current measurements. In the presence of specifically adsorbing inorganic ions, this provides key information about the formation of SAMs. Gravimetric measurements allow an estimation of the adsorbate surface coverage; and completion of the assembly process can then be monitored in real-time. Electrochemical impedance spectroscopy measurements play a multifunctional role: they enable elucidation of the physical models of the interface, provide the information about the effective capacitance of SAMs thus probing the dielectric properties of the adsorbed layers, and evaluate the ability of charged electrolyte components to approach the electrode surface through the SAM (using adsorbing / desorbing SO42- as an electroactive probe). The latter is important to assess the extent of defects in the formed organic layers. Finally, monitoring the direct current during SAM formation together with the collected gravimetric data can give additional important information about the process. A series of n-mercaptoalcohols with different hydrocarbon chain length adsorbing at Au electrodes was used as the model objects to evaluate the proposed approach.
[Show abstract][Hide abstract] ABSTRACT: Self-assembled monolayers (SAMs) have been used for the preparation of functional microtools consisting of encoded polysilicon barcodes biofunctionalized with proteins of the lectin family. These hybrid microtools exploit the lectins ability for recognizing specific carbohydrates of the cell membrane to give an efficient system for cell tagging. This work describes how the control of the methodology for SAM formation on polysilicon surfaces followed by lectin immobilization has a crucial influence on the microtool biofunction. Several parameters (silanization time, silane molar concentration, type of solvent or deposition methodology) have been studied to establish optimal function. Furthermore, silanes incorporating different terminal groups, such as aldehyde, activated ester or epoxide groups were tested in order to analyze their chemical coupling with the biomolecules, as well as their influence on the biofunctionality of the immobilized protein. Two different lectins - wheat germ agglutinin (WGA) and phytohemagglutinin (PHA-L) - were immobilized, because they have different and specific cell recognition behaviour and exhibit different cell toxicity. In this way we can assess the effect of intrinsic bulk toxicity with that of the cell compatibility once immobilized as well as the importance of cell affinity. A variety of nanometrical techniques were used to characterize the active surfaces, and lectin immobilization was quantified using ultraviolet-visible absorption spectroscopy (UV-vis) and optical waveguide light mode spectroscopy (OWLS). Once the best protocol was found, WGA and PHA were immobilized on polysilicon coded barcodes, and these microtools showed excellent cell tagging on living mouse embryos when WGA was used.
[Show abstract][Hide abstract] ABSTRACT: It is essential to obtain stable self-assembled monolayers (SAMs) for their applications to electrochemical (bio)sensors. Two commonly used methods for preparing phosphonate SAMs were compared in detail for the first time. SAMs of phosphonates with different alkyl chains were formed on indium–tin oxide (ITO) electrodes using the dipping method and the T-BAG method (tethering by aggregation and growth). In addition, two different post-assembly washing methods were assessed. The stability of the SAMs measured by their charge-transfer blocking abilities were investigated using cyclic voltammetry and electrochemical impedance spectroscopy. The SAMs were tested to assess their stability against ultrasonic washing and their long-term stability in phosphate-buffered saline. Only the phosphonate with the longest alkyl chain (octadecylphosphonic acid, ODPA) was stable to the ultrasonic washing, with the charge-transfer blocking ability of SAMs prepared from decyl- and hexadecylphosphonic acid (DPA and HDPA) being significantly reduced after the process owing to damage of the monolayer. Moreover, the ODPA SAMs gave similar X-ray photoelectron spectroscopic data, irrespective of the method of preparation and washing process used, providing further evidence of the stability of this monolayer. An increase in the length of the alkyl chain of the phosphonate (i.e., the length of the dielectric SAMs) decreased the double-layer capacitance and increased the charge-transfer resistance (blocking ability) against a redox reaction of Fe(CN)63-/Fe(CN)64-. After 7 days of immersion in phosphate-buffered saline, the ODPA SAMs prepared by the dipping method maintained their blocking ability to a greater extent than those prepared using the T-BAG method.
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