Thioctic Acid Self-assembled Monolayer Research Articles

Electrochemical Impedance Spectroscopy for Investigation on Ion Permeation in Thioctic Acid Self-Assembled Monolayer

Electrochemical impedance spectroscopy was employed to investigate the permeation of electrolyte ions in thioctic acid self-assembled monolayer when its structure was changed by the interaction of copper ions. The ion permeation was evaluated by using relatively low excitation frequencies, 0.2 Hz to 1000 Hz, and quantified by an extra resistive component in the equivalent circuit (RSAM). The extent of ion permeation affected by the electrode potentials and the electrolyte concentration were investigated. The experimental results verified that RSAM decreased ~70% by interaction with copper ions and that RSAM increased ~2 - 3 times when the electrolyte concentration was decreased by 10 times. This analysis can be performed without addition of redox species.

Anionic Functionalized Gold Nanoparticle Continuous Full Filling Separations: Importance of Sample Concentration

Electrically driven separations which contain nanoparticles offer detection and separation advantages but are often difficult to reproduce. To address possible sources of separation inconsistencies, anionic functionalized gold nanoparticles are thoroughly characterized and subsequently included in continuous full filling capillary electrophoresis separations of varying concentrations of three small molecules. Citrate stabilized gold nanospheres are functionalized with 11-mercaptoundecanoic acid, 6-mercaptohexanoic acid, or thioctic acid self-assembled monolayers (SAMs) and characterized using dynamic light scattering, extinction spectroscopy, zeta potential, and X-ray photoelectron spectroscopy prior to use in capillary electrophoresis. Several important trends are noted. First, the stability of these anionic nanoparticles in the capillary improves with increased ligand packing density as indicated by a ratio of absorbance collected at 520 to 600 nm. Second, increasing nanoparticle concentration from 0 to 2 nM (0-0.002(5)%, w/w) minimally impacts analyte migration times; however, when higher nanoparticle concentrations are included within the capillary, nanoparticle aggregation occurs which induces separation inconsistencies. Third, analyte peak areas are most significantly impacted as their concentration decreases. These trends are attributed to both sample enrichment and electrostatic interactions between the anionic carboxylic acid functionalized gold nanoparticles and sample. These important findings suggest that sample concentration-induced conductivity differences between the sample matrix and separation buffer as well as SAM packing density are important parameters to both characterize and consider when nanoparticles are used during continuous full filling separations and their subsequent use to enhance spectroscopic signals to improve in-capillary analyte detection limits.

Salt-mediated self-assembly of thioctic acid on gold nanoparticles.

Self-assembled monolayer (SAM) modification is a widely used method to improve the functionality and stability of bulk and nanoscale materials. For instance, the chemical compatibility and utility of solution-phase nanoparticles are often improved using covalently bound SAMs. Herein, solution-phase gold nanoparticles are modified with thioctic acid SAMs in the presence and absence of salt. Molecular packing density on the nanoparticle surfaces is estimated using X-ray photoelectron spectroscopy and increases by ∼20% when molecular self-assembly occurs in the presence versus the absence of salt. We hypothesize that as the ionic strength of the solution increases, pinhole and collapsed-site defects in the SAM are more easily accessible as the electrostatic interaction energy between adjacent molecules decreases, thereby facilitating the subsequent assembly of additional thioctic acid molecules. Significantly, increased SAM packing densities increase the stability of functionalized gold nanoparticles by a factor of 2 relative to nanoparticles functionalized in the absence of salt. These results are expected to improve the reproducible functionalization of solution-phase nanomaterials for various applications.

Aryl Diazonium for Biomolecules Immobilization onto SPRi Chips

A method for the immobilization of proteins at the surface of surface plasmon resonance imaging (SPRi) chips is presented. The technology, based on the electro-deposition of a 4-carboxymethyl aryl diazonium (CMA) monolayer is compared to a classical thioctic acid self-assembled monolayer. SPRi live recording experiments followed by the quantification of the diazonium surface coverage demonstrate the presence of a monolayer of electro-deposited molecules (11*10(12) molecules mm(-2)). This monolayer, when activated through a classical carbodiimide route, generates a surface suitable for the protein immobilization. In the present study, protein A and BSA are immobilized as specific and control spots (150 microm id), respectively. The AFM characterization of the spots deposited onto CMA or thioctic acid modified chips prove the presence of 4.7 nm protein monolayers. Finally, the SPRi detection capabilities of the two surface chemistries are compared according to specific signal, non-specific interaction and regeneration possibilities. Advantages are given to the CMA surface modification since no measurable non-specific signal is obtained while reaching a higher specific signal.

A piezoelectric immunosensor for the determination of pesticide residues and metabolites in fruit juices

A quartz crystal microbalance (QCM) immunosensor was developed for the determination of the insecticide carbaryl and 3,5,6-trichloro-2-pyridinol (TCP), the main metabolite of the insecticide chlorpyrifos and of the herbicide triclopyr. The detection was based on a competitive conjugate-immobilized immunoassay format using monoclonal antibodies (MAbs). Hapten conjugates were covalently immobilized, via thioctic acid self-assembled monolayer (SAM), onto the gold electrode sensitive surface of the quartz crystal. This covalent immobilization allowed the reusability of the modified electrode surface for at least one hundred and fifty assays without significant loss of sensitivity. The piezoimmunosensor showed detection limits (analyte concentrations producing 10% inhibition of the maximum signal) of 11 and 7 μg l −1 for carbaryl and TCP, respectively. The sensitivity attained ( I 50 value) was around 30 μg l −1 for both compounds. Linear working ranges were 15–53 μg l −1 for carbaryl and 13–83 μg l −1 for TCP. Each complete assay cycle took 20 min. The good sensitivity, specificity, and reusability achieved, together with the short response time, allowed the application of this immunosensor to the determination of carbaryl and TCP in fruits and vegetables at European regulatory levels, with high precision and accuracy.

Electrochemical Immunosensor for Detection of Celiac Disease Toxic Gliadin in Foodstuff

Celiac disease is a gluten-sensitive enteropathy that affects as much as 1% of the population. Patients with celiac disease should maintain a lifelong gluten-free diet, in order to avoid serious complications and consequences. It is essential to have methods of analysis to reliably control the contents of gluten-free foods, and there is a definitive need for an assay that is easy to use, and can be used on site, to facilitate the rapid testing of incoming raw materials or monitoring for gluten contamination, by industries generating gluten-free foods. Here, we report on the development of an electrochemical immunosensor exploiting an antibody raised against the putative immunodominant celiac disease epitope, for the measurement of gliadin content and potential celiac toxicity of a foodstuff. To develop the gliadin immunosensor, we explored the use of two surface chemistries, based on the use of dithiols, 22-(3,5-bis((6-mercaptohexyl)oxy)phenyl)-3,6,9,12,15,18,21-heptaoxadocosanoic acid (1) and 1,2-dithiolane-3-pentanoic acid (thioctic acid) (2), for anchoring of the capture antibody. The different surface chemistries were evaluated in terms of time required for formation of self-assembled monolayers, stability, susceptibility to nonspecific binding, reproducibility, and sensitivity. The thioctic acid self-assembled monolayer took more than 100 h to attain a stable surface and rapidly destabilized following functionalization with capture antibody, while the heptaoxadocosanoic acid surface rapidlyformed (less than 3 h) and was stable for at least 5 days, stored at room temperature, following antibody immobilization. Both surface chemistries gave rise to highly sensitive immunosensors, with detection limits of 5.5 and 11.6 ng/mL being obtained for 1 and 2, respectively, with nonspecific binding of just 2.7% of the specific signal attained. The immunosensors were extremely reproducible, with RSD of 5.2 and 6.75% obtained for 1 and 2 (n = 5, 30 ng/mL), respectively. Finally, the immunosensor was applied to the analysis of commercial gluten-free and gluten-containing raw and processed foodstuffs, and excellent correlation achieved when its performance compared to that of an ELISA.

Interfacial pH and surface pKa of a thioctic acid self-assembled monolayer

A self-assembled acid-functionalised monolayer on a gold surface has an interfacial pH 2.93 more acidic than the bulk and surface pK(a) very similar to that of the free acid.

Application of N-PLS calibration to the simultaneous determination of Cu2+, Cd2+ and Pb2+ using peptide modified electrochemical sensors

The simultaneous determination of Cu(2+), Cd(2+) and Pb(2+) is demonstrated at four modified gold electrodes using N-PLS calibration. Three of the electrodes were modified with the peptides Gly-Gly-His, gamma-Glu-Cys Gly and human angiotensin I which were covalently attached to thioctic acid self-assembled monolayers and the fourth electrode was modified with thioctic acid only. Voltammetry at the modified electrodes in the presence of the three metal ions revealed one peak due to the reduction of copper and another due to the overlapping peaks of cadmium and lead which made quantification using conventional methods difficult. N-PLS was used to calibrate and predict trace concentrations (100 nM to 10 microM) of mixtures of Cu(2+), Cd(2+) and Pb(2+).

Electrochemical detection of lead ions via the covalent attachment of human angiotensin I to mercaptopropionic acid and thioctic acid self-assembled monolayers

An electrochemical sensor for the detection of lead ions is described which is made by modifying a gold electrode substrate with self-assembled monolayers (SAMs) of 3-mercaptopropionic acid (MPA) or thioctic acid (TA) followed by covalent attachment of a lead binding peptide, human angiotensin I. Cyclic voltammetry of MPA–angiotensin modified gold electrodes complexed with lead displayed voltammograms with prominent lead peaks at E 0′, −0.29 V. A detection limit of 1 nM was achieved using Osteryoung square wave voltammetry. However, the electrodes were not stable over repeated electrochemical cycles due to partial electrochemical desorption of the SAM. The TA–angiotensin modified gold electrode showed greater stability and were able to be regenerated several times. Using Osteryoung square wave voltammetry for TA–angiotensin modified electrodes, lead concentrations down to 1.9 nM were detected. Although the detection limit of the TA–angiotensin modified electrode is higher than achieved with MPA–angiotensin, it is still well below Australian drinking water guidelines. Studies of interference effects on the Pb 2+ current showed Hg 2+ as a significant interferent but only at levels significantly greater than those found in natural waters.

Direct application strategy to immobilise a thioctic acid self-assembled monolayer on a gold electrode

Immobilisation of a self-assembled monolayer (SAM) onto an electrode surface is often achieved by immersing it in a solution for over 24 h. A biological or biologically derived recognition component can then be linked to the SAM in fabricating a biosensor. This time consuming immobilisation step can be a drawback in biosensor development, especially when repeated preparations of the biosensor are required. In this work, an alternative immobilisation strategy involving the direct application of a known quantity of the ethanolic solution of the alkanethiol, thioctic acid, on a gold electrode surface was studied. The solution was left to dry at room temperature for approximately 20 min. Comparable results including the relative percentage decrease in double layer capacitance, the surface coverage and the percentage of binding to the bacterial protein, Protein A, were obtained relative to those obtained with SAM formed by the immersion method. Shewhart’s statistical analysis technique was applied to examine the stability in terms of the relative percentage decrease in double layer capacitance. In these tests, within 99.7% confidence control limits, only a 1% deterioration was observed over a 3-month period. Therefore, all these results have demonstrated that the direct application method yields a stable thioctic acid SAM on a gold electrode surface with characteristics similar to those obtained with an immersion method. However, formation of a SAM using direct application can be achieved within a significantly shorter period of time compared to immersion method.

Sensitively Detecting Recombinant Hirudin Variant-2 with Capacitive Immunoassay Based on Self-Assembled Monolayers

Recombinant hirudins have a definite role in patients with heparin-induced thrombocytopenia, and prevent coronary thrombosis and recurrences. Here, capacitive immunosensors based on self-assembled monolayers (SAMs) were developed to immobilize polyclonal recombinant hirudin antibodies on gold electrodes for detecting recombinant hirudin variant-2 (r-HV2), a low-weight protein, directly and sensitively. A fast electrochemical measurement, potentiostatic step method, was applied to measure the capacitance changes caused by the interaction between the antigens and antibodies. The insulating properties of the immunosensors were investigated by cyclic voltammetry in a buffer solution containing (Fe(CN)6)3−/4−. The detection capability of the immunosensors modified by short-chain alkanethiol, thioctic acid (TA), or mercaptoacetic acid (MAA), were compared. Based on TA SAMs, the immunosensor could detect r-HV2 in the range from 100 pg/mL to 5 ng/mL, with a detection limit of 50 pg/mL. For the MAA modified immunosensor, a linear detection range from 5 pg/mL to 1 ng/mL was obtained, with a detection limit of 2.5 pg/mL. This indicated capacitive immunosensors with thinner SAMs could detect the antigens with higher sensitivity. The success of detecting r-HV2 showed that the capacitive immunoassay might be employed in the practical application with characteristics of simplicity and sensitivity.

Effect of Hexacyanoferrate(II/III) on Self-Assembled Monolayers of Thioctic Acid and 11-Mercaptoundecanoic Acid on Gold

The change in characteristics of self-assembled monolayers (SAMs) of thioctic acid (TA) and 11-mercaptoundecanoic acid (MUA) with time in buffered hexacyanoferrate(II/III) solutions of pH 7.4 has been studied with electrochemical impedance spectroscopy. Frequency scans were recorded at regular time intervals at +0.2 V versus SCE and a superimposed sinusoidal potential with an amplitude of 10 mV, in the frequency range from 10 kHz to 50 mHz. The stability of the SAMs was studied in the dark and during subsequent exposure of the solution to light. A TA SAM, a MUA SAM, and clean gold can be modeled with the same equivalent circuit. From this model, it was found that, as expected, charge transfer and diffusion of the redox probe through the MUA SAM are more inhibited than through the TA SAM. Further, the MUA SAM has a larger mean monolayer thickness, as is revealed from the lower value of the capacitive component. After exposure of the solution to light, a relatively rapid decrease in the resistances and incr...

Formation and Electrochemical Characterization of Self-Assembled Monolayers of Thioctic Acid on Polycrystalline Gold Electrodes in Phosphate Buffer pH 7.4

Formation of self-assembled monolayers (SAMs) of thioctic acid (TA) on polycrystalline gold electrodes has been investigated. The Au electrodes are polished, first mechanically and subsequently electrochemically, until the roughness of the surface is minimal. The best TA SAMs are formed onto these electropolished electrodes from stirred solutions, while a potential E = 0.0 V or E = +0.2 V versus saturated calomel reference electrode (SCE) is applied during the adsorption process. Alternating current (ac) impedance methods are used to monitor the formation of the SAMs and to characterize these. Potentiostatically formed TA SAMs have better characteristics than those formed at open circuit. The TA SAMs are stable in 100 mM phosphate buffer pH 7.4, and when protected from light, also in buffered 2 mM hexacyanoferrate(II/III) solution. However, when the hexacyanoferrate(II/III) solutions are exposed to light, the TA SAMs are not stable. Probably, damaging of the SAM proceeds via etching of the gold surface by photochemically released cyanide ions. Using potential pulses (+1.6, 0.0, −0.8 V versus SCE, each 0.1 s) for 15 min in the flow (0.5 mL min-1), the TA SAM can be removed completely, recovering the initial state of the gold electrode, as deduced from potential-step experiments. This way TA SAMs can be formed and removed repeatedly and reproducibly on a single electrode.