Mercaptopropionic Acid Self-assembled Monolayer Research Articles

Exploring the enhancement of electron tunneling induced by intermolecular interactions on surface of self-assembled monolayer

The topic of intermolecular interactions enhancing electron tunneling between protein residues on biofilm surface has been investigated by some theoretical studies. However, few of experimental studies could confirm this viewpoint. In this work, the Mercaptopropionic acid self-assembled monolayer (MPA SAM) with terminal carboxyl groups was constructed, as a simplified model for functionalized biomimic membrane. The electron tunneling rate constant of ferrocenylmethanol redox process on SAM was obtained from experimental results of scanning electrochemical microscopy. It is found that the electron tunneling across MPA SAM is faster than that of contrastive Methyl mercaptopropionate SAM. This result is attributed to the intermolecular interactions between the redox and MPA molecules. The conditions of lower pH and weaker ion pair effect all benefit the formation of the intermolecular interactions, resulting the enhancement of electron tunneling more obvious. Based on this, this kind of intermolecular interaction is inferred to be intermolecular hydrogen bond. Our work may have contributions to exploring electron tunneling between protein residues on biofilm surface.

Layer-By-Layer Construction of Three-Dimensional Metal Organic Frameworks with Porphyrin on the Solid Surfaces

Introduction Porphyrins have planar structure and effectively absorb visible-light normal to the ring plane. Thus, they can be applied to artificial photosynthetic devices and photovoltaic cells when the porphyrin ring is flatly adsorbed on the solid substrate [1]. On the other hand, metal-organic frameworks (MOFs), which have a high surface area, flexibility, and designability, are one of the highly promissing materials to construct the highly oriented three-dimensional structures [2]. Thus, it is expected to construct three-dimentional MOFs with porphyrin, in which porphyrin ring is highly oriented, on the solid surfaces, in order to construct artifical photosynthetic devices. We have succeeded to construct the three-dimensional MOFs on the solid surfaces by a layer-by-layer method [3,4] and also to construct self-assembled monolayers (SAMs) with porphyrin [5-10]. Thus, in this study, we tried to construct the three-dimensional MOFs with porphyrin group, in which porphyrin ring is oriented parallel to the solid surfaces, using various SAM modified solid surfaces, such as iso-nicotinic acid (IA) SAM on TiO2(110), mercapto-propionic acid (MPA) SAM on Au(111), acrylic acid (AA) SAM on Si(111), and amino-propyl tri-methoxy silane (APTMS) SAM on glass substrate. At each step to construct these MOFs with tetra-carboxyphenyl porphyrin (TCPP), Zn tetra-phenyl porphyrin (Zn-TPP), and bipyridine (Bipy) by a layer-by-layer fasion, the adsorbed amounts and orientation were evaluated by x-ray photoelectron spectroscopy (XPS), polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS), attenuated total refection infrared spectroscopy (ATR-IRS), and ultraviolet/visible absorption spectroscopy (UV-vis). Experiment IA SAM on TiO2(110), MPA SAM on Au(111), AA SAM on Si(111), and APTMS SAM on glass were prepared by dipping each substrate into DMF solution with saturated IA at 110 °C for 10 h, ethanol solution with 1 mM MBA, ethanol solution with saturated methoxy acrylate and then hydrolysis of methoxy group to carboxylate, and ethanol solution with saturated ATPMS at 40 °C for 10h, respectively. After these SAM preparations, two-dimentional TCPP MOF sheets, which is prepared on the water surface [11], was transferred onto IA SAM on TiO2(110) and glass substrate. On the MPA SAM on Au(111) and AA SAM on Si(111), the substrates were dipped instead of the ethanol solutions with Zn-TPP and Bipy. At each step, layered structures and orientation were evaluated XPS, PM-IRRAS, ATR-IRS, and UV-vis. Results and discussion As an example, it is explained that IA SAM was confirmed to construct the TiO2(110) surface. When the pre-treated TiO2(110) substrate was dipped into DMF solution with saturated IA at 110 °C for 10 h, XP peaks around 401 eV in the N1s region and 532 eV in the O1s region due to the aniline ring and calboxylate, respectively, were observed. From these XP peaks, we confirmed that the IA SAM is formed on the TiO2(110) surface. After that, two-dimensional porphyrin sheet was transferred onto this SAM modified TiO2(110) surface by a layer-by-layer fasion.

Formation of supported lipid bilayers of charged E. coli lipids on modified gold by vesicle fusion

We describe a simple way of fusing E. coli lipid vesicles onto a gold surface. Supported lipid bilayers on metal surfaces are interesting for several reasons: transducing a biological signal to an electric readout, using surface analytical tools such as Surface Plasmon Resonance (SPR), Infrared Reflection Absorption Spectroscopy, Neutron Reflectivity or Electrochemistry. The most widely used method to prepare supported lipid membranes is fusion of preexisting liposomes. It is quite efficient on hydrophilic surfaces such as glass, mica or SiO2, but vesicle fusion on metals and metal oxide surfaces (as gold, titanium oxide or indium tin oxide), remains a challenge, particularly for vesicles containing charged lipids, as is the case of bacterial lipids. We describe a simple method based on modifying the gold surface with a charged mercaptopropionic acid self-assembled monolayer and liposomes partially solubilized with detergent. The formed bilayers were characterized using a Quartz Crystal Microbalance with dissipation (QCM-D) and Atomic Force Microscopy (AFM). Some advantages of this protocol are that the stability of the self-assembled monolayer allows for repeated use of the substrate after detergent removal of the bilayer and that the amount of detergent required for optimal fusion can be determined previously using the lipid-detergent solubility curve.

Comparative Electrochemical Behavior of Proteins; Cytochrome c, Agaricus Bisporus Laccase, and Glucose Oxidase, Immobilized onto Gold-Thiol Self-Assembled Monolayer via Electrostatic, Covalent, and Covalent Coordinate Bond Methods

Comparative electrochemical behavior of three types of proteins; cytochrome c (Cyt c), Agaricus Bisporus laccase (LacAB) and glucose oxidase (GOx) immobilized on gold mercaptopropionic acid self-assembled monolayer via three different methods, including (a) covalent attachment by using EDC/NHS organic activators, (b) electrostatic interactions, and (c) covalent-coordinate binding by using Zr(IV) ion glue is investigated. Immobilization steps and electrochemical behavior of the immobilized proteins are traced by means of cyclic and differential pulse voltammetry (CV and DPV), chronoamperometry (CA) and electrochemical impedance spectroscopy (EIS). The results indicate that characteristics of the immobilized systems including; surface coverage, electrocatalytic activity and stability of the proteins are considerably affected by type of the bond formed between electrode and protein (the immobilization method), and nature of the protein. Immobilization via Zr(IV) ion resulted in higher activities for metalloproteins, i.e. Cytc with Heme active center and LacAB with T copper active center, while EDC/NHS method was more efficient for GOx having FAD cofactor. The experimental data will be presented and discussed from which the efficient immobilizing method for each case is introduced.

Functionalization of gold mercaptopropionic acid self-assembled monolayer with 5-amino-1,10-phenanthroline: Interaction with iron(II) and application for selective recognition of guanine

A novel method is developed for selective electrochemical detection of guanine (GA) based on activity of gold mercaptopropionic acid self-assembled monolayers (Au-MPA SAMs) functionalized via covalent binding of 5-amino-1,10-phenanthroline-Fe(II) complex (5Aphen-Fe(II)). Modification of the electrode and its activity for oxidation of GA are investigated using differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS). The voltammetric results are supported by the EIS findings. All the results indicate that modification of the Au-MPA SAM electrode surface with 5Aphen-Fe(II) assemblies results in amplification of guanine (GA) oxidation compared with Au and Au-MPA electrodes. The oxidation potential of GA is shifted 0.130V to more facile (negative) direction compared with the previously studied electrodes for this case. The Au-MPA-5Aphen-Fe(II) electrode shows a wide dynamic range of response (1.0 to 136.0μM GA), including a linear range (1.0 to 100.0μM GA) with RSDs varied from 2.1% to 4.2% for n=4 at each point, and a detection limit of 0.17(±0.01)μM GA. The results show the sensor benefits of a very good stability, repeatability, and excellent selectivity. The GA is determined in the presence of various inorganic ions and biological species especially adenine. A calf thymus double strand DNA sample is successfully tested.

Electrocatalytic determination of H2O2 on the electrode modified by LBL assembly of polyoxometalates via zirconium ion glue

Layer-by-layer (LBL) assembly of polyoxometalates (POM) on gold mercaptopropionic acid self-assembled monolayer (Au-MPA SAM) via zirconium ion (Zr) glue was performed for the first time. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used for LBL characterization of the modified electrode. The Au-MPA-(Zr-POM)n electrode exhibited fast surface-confined reversible electron transfer up to 5 bilayer (n=5). The Au-MPA-(Zr-POM)5 electrode exhibited remarkable electrocatalytic activity toward the reduction of H2O2 with a high sensitivity and low detection limit. Because of the simplicity of the preparation method, stable and reproducible electrochemical properties of the modified electrode, the proposed strategy shows remarkable advantages and should be important for practical applications.

Bienzyme self-assembled monolayer on gold electrode: An amperometric biosensor for carbaryl determination

A new electrochemical biosensor based on the covalent immobilization of two enzymes on self-assembled monolayer attached to a polycrystalline gold electrode is proposed, experimentally. The acetylcholinesterase (AChE) and cholineoxidase (ChO) are two enzymes that covalently co-immobilized on the mercaptopropionic acid self-assembled monolayer on polycrystalline gold electrode (Au-MPA-AChE/ChO SAM). Fabrication steps and electrochemical interaction of the Au-MPA-AChE/ChO SAM with carbaryl were monitored by general electrochemical methods like cyclic voltammetry (CV) and chronoamperometry (CA), and by a more advanced method, electrochemical impedance spectroscopy (EIS) in the presence of parabenzoquinone (PBQ) or K3[Fe(CN)6] redox probes. The close distances between two immobilized enzymes on Au-MPA SAM result in preconcentration of choline, the product of acetylcholine hydrolysis by AChE, in ChO nano-environment. Enzyme activity was measured by chronoamperometric tracing of hydroquinone from the reduction of PBQ mediator which in turn is oxidized at Au electrode in diffusion layer. Carbaryl was chosen as a model toxin and its inhibition characteristics were utilized for the toxin detection. The linear range for the determination of Carbaryl was 10–1000 nmol l−1. A limit of detection 5.96 nmol l−1 was obtained.

Electronic and Magnetic Properties of Mn12 Molecular Magnets on Sulfonate and Carboxylic Acid Prefunctionalized Gold Surfaces

Structural, electronic, and magnetic properties of [Mn12O12(bet)16(EtOH)4](PF6)14·4CH3CN·H2O (in short Mn12bet, bet = betaine = +N(CH3)3-CH2–COO–) single-molecule magnets (SMMs) deposited on previously functionalized gold surfaces have been investigated. Self-assembled monolayers (SAMs) either of sodium mercaptoethanesulfonate (MES) or mercaptopropionic acid (MPA) are used as functionalization to avoid the direct interaction between the Mn12bet molecules and the Au surface with the aim of preserving the main functional properties of the molecules. Scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS) analysis show deposited Mn12bet SMMs well-isolated from each other and uniformly distributed on both MES and MPA SAMs. X-ray absorption spectroscopy (XAS) studies show that the oxidation state of the mixed-valence Mn12bet core is largely reduced to Mn2+ when molecules are deposited on MES-SAM, whereas in the case of MPA-SAM the relative weights of Mn2+, Mn3+, and Mn4+ in the Mn12bet core are preserved. Despite the substantial retaining of their electronic properties, the magnetization of Mn12bet molecules deposited on MPA-SAM measured by X-ray magnetic circular dichroism (XMCD) is perturbed with respect to the pristine molecules.

Immobilization and nanoscale film structure of bacterial reaction centers on gold

Photosynthetic bacterial reaction centers (BRC) have shown great potential for bioengineering applications. In the present work we demonstrate their preferentially oriented immobilization on top of a mercaptopropionic acid (MPA) self assembled monolayer (SAM) formed on gold screen printed electrodes (Au-SPE). The AFM of the self assembled biofilm reveals formation of nanohole features with the depths of 2.2–2.5nm, consistent with the thickness of the BRC (~3nm). Thus, BRCs organize into a sub-monolayer film on top of the MPA SAM layer. The square-wave voltammogram of the BRC films exhibits features due to the redox reactions for the bacteriochlorophyll dimer (the strongest), bacteriopheophytin, and the quinones (weak), indicating that the BRCs are more likely immobilized with the dimer side towards the electrode surface. Non-saturating illumination of the BRC film at open circuit conditions leads to a sharp spike of negative current which could be due to the electron acceptance by the oxidized dimer.

Electrochemical probe of natural DNA attached onto the gold–thiol SAMs via Zr(IV) ion glue

Attachment of natural double strand calf thymus DNA onto gold–mercaptopropionic acid self-assembled monolayer electrode via Zr(IV) ion glue, Au–MPA–Zr(IV)-ds-ct-DNA SAM, is studied. The sequential assembly of the layers on the Au surface is traced by electrochemical methods including cyclic and square wave voltammetries in the presence of methylene blue. The results show that DNA is attached to Zr(IV) via its phosphate groups without any contribution from its bases. Effects of the MB concentration on the behavior of adsorbed MB, ionic strength on the MB adsorption, and pH on the amount of immobilized DNA are studied. The proposed method for the preparation of the new template, the Au–MPA–Zr(IV)-ds-ct-DNA, is relatively simple and fast. Long-term stability of this electrode is tested by keeping the template at pH 3, and carrying out a daily electrochemical study at pH 7.4. Under these circumstances, the Au–MPA–Zr(IV)-ds-ct-DNA electrode showed stable behavior for one week.

Electrochemical determination of calf thymus DNA on Zr(IV) immobilized on gold–mercaptopropionic-acid self-assembled monolayer

An electrochemical biosensor, constructed by immobilization of Zr(IV) on the topside of gold–mercaptopropionic acid self-assembled monolayer (Au–MPA–Zr SAM), is developed for the sensitive quantification of calf thymus DNA (ct-DNA). The sensor is based on ionic adsorption of ct-DNA from its phosphate backbone onto the Au–MPA–Zr(IV) SAM electrode. Preparation, characterization, and application of the sensor for determination of ct-DNA are described by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and Osteryoung square wave voltammetry (OSWV) in the presence of an appropriate redox reaction probe. Parameters influencing the method have been tested. A linear range calibration curve from 1.0 × 10 − 4 to 5.0 × 10 − 7 g mL − 1 ct-DNA with a detection limit of 9.5 × 10 − 8 g mL − 1 and mean of relative standard deviations (R.S.D) of 2.5% for n = 4 at each point was observed in the best conditions by EIS. Regeneration of the surface was carried out successfully by 5 min sonication in 0.1 M KOH solution and then 1 min incubation in 1.0 × 10 − 3 M Zr(IV) with a good reproducibility, R.S.D = 1.5% for n = 4 as detected by EIS. The long-term storage stability of the electrode was also studied.

Zirconium immobilized on gold–mercaptopropionic acid self-assembled monolayer for trace determination of phosphate in blood serum by using CV, EIS, and OSWV

Preparation, characterization, and application of a new sensor for the determination of phosphate in blood serum is described by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and osteryoung square wave voltammetry (OSWV) in the presence of an appropriate redox reaction probe. The sensor was based on ionic adsorption of phosphate on Zr(IV) immobilized on gold–mercaptopropionic acid self-assembled monolayer (Au–MPA–Zr(IV) SAM) electrode. Parameters influencing the method were optimized. A linear range calibration curve from 1.0×10−7 to 1.0×10−6M PO43− with a detection limit of 5.30×10−8M and mean of relative standard deviations (R.S.D.) of 2.75% for n=4 was observed in the best conditions by OSWV. Possible interferences from the coexisting ions were also investigated. The results demonstrated that sensor could be used for the determination of phosphate in the presence of various ions. Regeneration of the surface was carried out successfully by 5-min sonication in 0.1M KOH solution and then 1-min incubation in 1.0×10−3M Zr(IV) with a good reproducibility, R.S.D.=1.47% for n=4 by OSWV. The validity of the method and applicability of the sensor were successfully tested by detection of phosphate in blood serum after deproteinization of sample without interference from sample matrix. The long-term storage stability of the electrode was studied. The experimental data is presented and discussed from which the new sensor is characterized.

Immunosensor for the determination of Staphylococcus aureus using a tyrosinase–mercaptopropionic acid modified electrode as an amperometric transducer

An amperometric immunosensor for the quantification of Staphylococcus aureus based on the coimmobilization of rabbit immunoglobulin G (RbIgG) and tyrosinase on a mercaptopropionic acid self-assembled monolayer modified gold electrode is reported. A competitive mode in which protein-A-bearing S. aureus cells and antiRbIgG labeled with alkaline phosphatase (AP) compete for the binding sites of immobilized RbIgG was used. Monitoring of the affinity reaction was carried out by the amperometric detection at -0.15 V of phenol generated in the enzyme reaction with AP, at the tyrosinase-modified electrode through the electrochemical reduction of the o-quinone formed. Optimization of the working variables, such as the immunosensor composition and incubation times, the applied potential, the working pH and the concentration of phenyl phosphate used as the AP substrate, was carried out. Under the optimized conditions, both the repeatability of the measurements and the reproducibility of the responses obtained with different immunosensors yielded relative standard deviation values for the steady-state current lower than 10%. The immunosensor showed a dynamic range from 4.4x10(5) to 1.8x10(7) S. aureus cells mL(-1), with a detection limit of 1.7x10(5) cells mL(-1). The limit of detection was remarkably improved by subjecting S. aureus cells to wall lysis by heat treatment. The value obtained was 2.3x10(3) cells mL(-1), which is adequate for the monitoring of S. aureus contamination levels in some foodstuffs. As an application, milk samples spiked with bacteria at the 4.8x10(3) cells mL(-1) level were analyzed.

A novel method for glucose determination based on electrochemical impedance spectroscopy using glucose oxidase self-assembled biosensor

A method is developed for quantitative determination of glucose using electrochemical impedance spectroscopy (EIS). The method is based on immobilized glucose oxidase (GOx) on the topside of gold mercaptopropionic acid self-assembled monolayers (Au-MPA-GOx SAMs) electrode and mediation of electron transfer by parabenzoquinone (PBQ). The PBQ is reduced to hydroquinone (H 2Q), which in turn is oxidized at Au electrode in diffusion layer. An increase in the glucose concentration results in an increase in the diffusion current density of the H 2Q oxidation, which corresponds to a decrease in the faradaic charge transfer resistance ( R ct) obtained from the EIS measurements. Glucose is quantified from linear variation of the sensor response (1/ R ct) as a function of glucose concentration in solution. The method is straightforward and nondestructive. The dynamic range for determination of glucose is extended to more than two orders of magnitude. A detection limit of 15.6 μM with a sensitivity of 9.66 × 10 − 7 Ω − 1 mM − 1 is obtained.

The influence of gold nanoparticle modified electrode on the structure of mercaptopropionic acid self-assembly monolayer

The comparison of assembly structure and property of mercaptopropionic acid (MPA) self-assembled monolayers (SAMs) on gold nanoparticle modified electrode (nanogold electrode) with that on planar gold electrode was studied by cyclic voltammetry (CV). The electron transfer of through the MPA SAMs and Cu underpotential deposition (UPD) on MPA-covered electrode indicated that MPA molecules assemblied on the planar gold electrode could form a very compact layer, which could surpass the electron transfer K 3Fe(CN) 6 greatly, whereas on the surface on the nanogold, which curvature make the compact packing loose. The reductive desorption in 0.5 M KOH and oxidative desorption in phosphate buffer solution (PBS) (pH 6.8) showed that gold nanoparticles could enhance the Au–S bond and stabilize the MPA molecules.