Immobilisation of cobaltferritin onto gold electrode based on self-assembled monolayers

The iron storage protein, ferritin, has a cavity of ～7?nm in diameter in which iron is oxidised and stored as a hydrated oxide core. Electron transfer is known to be an important step in the sequestering of iron by cellular ferritin. The cavity was used as a nanocontainer to grow cobalt nanoparticles. The immobilisation of ferritin on the electrode surface is essential for various bioelectronic applications. A cobaltferritin-immobilised electrode based on self-assembled monolayer (SAM)-modified gold electrode was developed. The cobaltferritin-immobilised SAM-modified electrode was characterised by electrochemical and atomic force microscopy (AFM) techniques. The results indicated that cobaltferritin was selectively immobilised onto succinimidyl alkanedisulfide-modified Au electrode by the covalent interaction between cobaltferritin and the terminal functional groups of the SAMs. The cobaltferritin immobilised modified electrode showed a direct electron transfer reaction between cobaltferritin and the electrode. The electrochemically regulated uptake and release of cobalts for cobaltferritin immobilised on the SAMs were demonstrated. The results obtained in this study indicate that cobaltferritin has potential for a biomaterial in nanoscale synthesis for potential magnetic, catalytic and biomedical-sensing applications.     

Discrete charging effects in gold nanoclusters grown on self-assembled monolayers

Single electron charging effects were observed for gold nanoclusters grown on octanedithiol self-assembled monolayers by scanning tunneling microscopy and X-ray photoelectron spectroscopy (XPS). Strong interaction of gold with the terminal sulfur atoms of dithiol molecules on Au(111) suppresses effectively the penetration of deposited gold atoms through the dithiol layer and results in the formation of uniform metal nanoclusters. Decoupling of the clusters from Au(111) by the octanedithiol layer and the small self-capacitance of the nanoclusters realize the observation of the Coulomb blockade in scanning tunneling spectroscopy and the Au 4f core level shifts in XPS at room temperature. Both phenomena originate from a common physics, the Coulomb energy of charged particles.     

Forming microstructured alkanethiol self-assembled monolayers on gold by laser ablation

A process to form microstructured alkanethiol self-assembled monolayers (SAMs) on gold is described. It is well known that alkanethiols spontaneously form homogenous SAMs on gold surfaces. By means of laser ablation, the exposed areas of alkanethiol monolayers can be removed from the gold surface. Free gold is obtained which can react further with second and third thiols. By this technique, structured alkanethiol SAMs are obtained reliably and easily. In a rather narrow window of pulse intensities, in our example 120$hboxMW/cm^2pm hbox10%$from a frequency-doubled Nd : YVO$_4$laser with 6-ns pulsewidth operating at a repetition rate of 20 kHz, ablation of alkanethiol monolayers is obtained without causing any damage to the gold substrate. Examples are presented where lines down to 10$muhbox m$in width were laser ablated into an SAM formed either from a hydrophilic or a hydrophobic alkanethiol and filled with a monolayer of a second alkanethiol of opposite hydrophilicity. The patterned structures were examined by optical and fluorescence microscopy as well as by lateral force microscopy. The presented method enables the preparation of microstructured SAMs on gold and probably on a wide variety of other substrates.     

Selective gold deposition via CVD onto self-assembled organic monolayers

We demonstrate a selective deposition of ultrathin gold layers via CVD onto self-assembled dithiols. Dithiols have been self-assembled to produce a thiolated surface. Metallic gold was deposited from the gas phase by using a volatile organometallic molecular gold precursor. The gold layer is bound to the exposed thiol groups. We demonstrate a selective deposition of the gold only at the areas where the binding thiol groups are located and characterized the deposition process and the obtained coatings with spontaneous desorption time of flight mass spectrometry and Rutherford backscattering spectroscopy.     

Electron permeable self-assembled monolayers of dithiolated aromatic scaffolds on gold for biosensor applications

Self-assembled monolayers (SAMs) of thiolated compounds are formed by the spontaneous chemisorption of thiolate groups on metal surfaces. In biosensors, they are most commonly used to covalently immobilize a biorecognition molecule onto the surface of the transducer, thus offering the possibility of controlling the orientation, distribution, and spacing of the sensing element while reducing nonspecific interactions. In this paper, self-assembled monolayers of dithiolated derivatives of 3,5-dihydroxybenzyl alcohol containing carboxyl and hydroxyl end groups have been prepared on gold surfaces and characterized by cyclic voltammetry and electrochemical impedance spectroscopy. Impedance measurements revealed that SAM formation is essentially completed after 3-5 h of exposure by observing the successive blocking of the faradic response of ferricyanide anion due to the adsorption of the dithiol molecules. The surface coverage of these molecules, estimated by reductive desorption experiments, was in the range of (1.1-2.8) x 10-10 mol/cm2. To demonstrate the potential of the dithiol SAM, a model system for detection of a tumor marker, prostate-specific antigen (PSA), was developed. The carboxyl groups of the SAM were succinimide-activated, and an anti-PSA antibody was covalently immobilized via amide bonds. The modified SAM was used for the label-free detection of prostate-specific antigen using EIS with a detection limit of 9 ng/mL. The results described here demonstrate that this kind of dithiol-modified SAM can be used as supports in electrochemical biosensors and the results are explained in terms of the structural features of these dithiols.     

Inkjet printed electrode arrays for potential modulation of DNA self-assembled monolayers on gold

In this paper, we report a novel and cost-effective fabrication technique to produce electrode arrays that can be used for monitoring and electrical manipulation of the molecular orientation of DNA self-assembled monolayers (SAMs) on gold. The electrode arrays were prepared from gold coated glass sides or compact discs (CD-Rs) by using standard office inkjet printers without any hardware or software modifications. In this method, electrode arrays of varied shape and size (from submillimeter to centimeter) can be rapidly fabricated and are suitable for standard electrochemical measurements. We were able to use a dual-channel potentiostat to control the electrodes individually and a fluorescence (FL) scanner to image the electrode array simultaneously. With such an integrated modulation setup, the structural switching behavior (from "lying" to "standing" position) and the enhanced hybridization reactivity of thiolate DNA SAMs on gold under potential control have been successfully demonstrated.     

Diffusion of self-assembled monolayers of thiols on the gold surfaces covered with polydimethylsiloxane stamps

Thiols can diffuse and form self-assembled monolayers (SAMs) on the gold surfaces covered with polydimethylsiloxane (PDMS) stamps. For the first time, with cells as the indicator of how far alkanethiols had diffused to form SAM, we studied the growth dynamics of SAMs of HS(CH₂)₁₁(OCH₂CH₂)₃OH (EG₃) and HS(CH₂)₁₁(OCH₂CH₂)₆OH (EG₆) on the gold surfaces covered with PDMS stamps. The growth of SAMs is well described by one-dimensional diffusion from a line source of concentration, with surface diffusion coefficient of 193.4 ± 19.2 μm²/min (EG₃) and 95.8 ± 18.9 μm²/min (EG₆).     

Nanopatterned self-assembled monolayers

We report on the fabrication of chemically nanopatterned gold surfaces by combining electron-beam lithography with gas and liquid phase thiolization. The line-edge roughness of the patterns is ～4?nm, corresponding to a limiting feature size in the range of 15?nm. Indications for a lower packing density of the self-assembled monolayers grown in the nanofeatures are given, and evidences for the bleeding of thiols along the grain boundaries of the gold substrate are displayed. A comparison is provided between nanopatterned thiol and silane monolayers on gold and on silicon wafers, respectively. The line-edge roughnesses are shown to be close to each other for these two systems, indicating that the limiting step is currently the lithography step, suggesting possible improvement of the resolution. The advantages and drawbacks of thiol versus silane monolayers are finally discussed with respect to the formation of chemically nanopatterned surfaces.     

Attenuated total reflection surface-enhanced infrared absorption spectroscopy of carboxyl terminated self-assembled monolayers on gold

A new recipe for surface-enhanced infrared absorption (SEIRA) active island Au films with improved adhesion in aqueous solution, low resistivity, and enhancement of the infrared (IR) absorption of about 300 was developed. The Au films prepared were utilized in studies of the ionization of self-assembled monolayers of 11-mercaptoundecanoic acid in Na2SO4 aqueous solutions by attenuated total reflection surface-enhanced infrared absorption (ATR-SEIRA) spectroscopy. It was found that the carboxyl end groups of the self-assembled monolayer turn into carboxylate anions on going from anodic to cathodic potentials or from acidic to alkaline pH. The water molecules close to the self-assembled monolayer in acidic solutions or at anodic potentials are preferentially aligned with their dipole moments parallel to the interface. This type of alignment can be ascribed to the dipole-dipole interaction between the carboxyl groups and the water molecules. On the other hand, in alkaline solutions or at cathodic potentials the structure of water close to the self-assembled monolayer is essentially bulk-like, with randomly oriented water molecules. This observation suggests that in alkaline solutions or at cathodic potentials the charge of the carboxylate anions is almost completely compensated for by strongly adsorbed counter cations. As a result, the electric field close to the surface of the ionized self-assembled monolayer is weak and has little influence on the orientation and hydrogen bonding of the water molecules.     

Stripe domain analysis of alkanethiol and alkynethiol monolayers self-assembled on the surfaces of gold grains

The self-assembled monolayers (SAMs) of alkanethiols such as 1-dodecanethiol (DDT) on the surface of polycrystalline gold grain on quartz crystal microbalance (QCM) at room temperature and alkynethiols such as phenylethynyl thiol (PET) on the surface of Au(111)/mica under − 65 °C have been fabricated. A multimode scanning probe microscope (SPM) investigation for the first time reveals the formation of stripe domain of DDT SAMs together with roundish domain. This feature was not caused by an annealing treatment but might be induced by coexistence of a few different facets and reconstruction of the surface on the grain. The stripe domain of PET SAMs could also be found on the surface of the grain, which was within the substrate of the gold/mica. It shows that the feature of the stripe domain for organothiol monolayers self-assembled on gold grain surfaces possesses a certain generality. Furthermore, the arrangement of organothiol stripe domain can be well understood from the interpretation of the relationship between the stripe width and width direction, molecule interval distance, molecule length and tilt angles of the thiols.     

Analysis of self-assembled monolayers on gold surfaces using direct analysis in real time mass spectrometry

Mass spectrometry was performed on self-assembled monolayers (SAMs) of dodecanethiol on gold using the new direct analysis in real time (DART) ionization technique. Observed peaks for the SAMs included monomers, dimers, and trimers of the SAM molecules, with the dimer and trimer relative peak heights enhanced as compared to the spectra for neat dodecanethiol. The possibility that the observed peaks were due to residual (noncovalently bonded) material on the surface was tested by attempting to observe residual dodecanol. No peaks corresponding to dodecanol were observed. These results indicate that DART is an excellent ionization method for the direct and unambiguous mass analysis of chemical species in self-assembled monolayers.     

Simulation and modeling of self-assembled monolayers of carboxylic acid thiols on flat and nanoparticle gold surfaces

Quantitative analysis of the 16-mercaptohexadecanoic acid self-assembled monolayer (C16 COOH-SAM) layer thickness on gold nanoparticles (AuNPs) was performed using simulation of electron spectra for surface analysis (SESSA) software and X-ray photoelectron spectroscopy (XPS) experimental measurements. XPS measurements of C16 COOH-SAMs on flat gold surfaces were made at nine different photoelectron emission angles (5-85° in 10° increments), corrected using geometric weighting factors and then summed together to approximate spherical AuNPs. The SAM thickness and relative surface roughness (RSA) in SESSA were optimized to determine the best agreement between simulated and experimental surface composition. On the basis of the glancing-angle results, it was found that inclusion of a hydrocarbon-contamination layer on top the C16 COOH-SAM was necessary to improve the agreement between the SESSA and XPS results. For the 16 COOH-SAMs on flat Au surfaces, using a SAM thickness of 1.1 ?/CH(2) group, an RSA of 1.05, and a 1.5 ? CH(2)-contamination overlayer (total film thickness = 21.5 ?) for the SESSA calculations provided the best agreement with the experimental XPS data. After applying the appropriate geometric corrections and summing the SESSA flat-surface compositions, the best fit results for the 16 COOH-SAM thickness and surface roughness on the AuNPs indicated a slightly thinner overlayer with parameters of 0.9 ?/CH(2) group in the SAM, an RSA of 1.06 RSA, and a 1.5 ? CH(2)-contamination overlayer (total film thickness = 18.5 ?). The 3 ? difference in SAM thickness between the flat Au and AuNP surfaces suggests that the alkyl chains of the SAM are slightly more tilted or disordered on the AuNP surfaces.     

A method to study the phase transition and desorption of self-assembled monolayers on planar gold surfaces

In-plane resistance of thin gold films (thickness approximately 1,000 A) after the formation of self-assembled monolayers has been measured as a function of temperature. We attribute the observed increase in the slope of the resistance vs temperature plot in the case of octadecanethiol and octanethiol monolayers at characteristic temperatures to phase transitions. The structural transition of 1,4-benzenedimethanethiol is also manifested similarly. Propanethiol does not show any phase transition, as expected. The observed changes in the alkanethiol monolayers are related to the decrease in the lifetime of the parallel vibrational motion of the adsorbate on the gold surface. The increase in slope is followed by a decrease which is attributed to the commencement of desorption of monolayers from the gold surface.     

Photo-deprotection patterning of self-assembled monolayers

Photo-deprotectable self-assembled monolayers (SAMs) provide a versatile platform for creating functional patterned surfaces. In this study, we present nanoscale photo-patterning, multi-component patterning, and a method for producing molecular gradients using photo-deprotectable SAMs. Nanoscale patterning of photo-deprotectable SAMs was achieved by coupling a UV laser (365 nm) through a scanning near field probe to produce nanoscale lines of ～40 nm, i.e. λ/9. Multi-component patterning was achieved by a two-stage method combining both microcontact printing and soft-UV photo-patterning. The example demonstrated in this study produced a three-component patterned surface with regions of CF₃, CH₃ and COOH/CF₃ functionality. The versatility of these photocleavable SAMs is further demonstrated by creating linear molecular gradients of two functionalities along a distance of ～25 mm. The use of ‘soft’ UV gives several advantages including the ability to pattern SAMs with micron-scale features over large areas quickly, with greater control over the photochemical reactions, and compatibility with existing lithographic facilities thus offering an effective alternative to other patterning methods such microcontact-printing or deep UV patterning.     

Influence of an external magnetic field on the formation of self-assembled monolayers of dodecanethiol on polycrystalline gold electrode

This paper reports the preparation of dodecanethiol self-assembled monolayers (C₁₂SH-SAMs) on polycrystalline gold electrodes in a magnetic field. The qualities of C₁₂SH-SAMs were characterized by both cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results show that highly dense and well-ordered SAMs could form in a relatively short time (3 h), indicating that the rate of SAMs formation increased under an external magnetic field compared with the natural self-assembly process. Moreover, the results of CV and EIS measurements also suggested that the presence of a magnetic field had influenced the qualities of the SAMs; the stronger magnetic intensity can help to obtain much denser and well-ordered SAMs.     

Fabrication of carbohydrate microarrays on gold surfaces: direct attachment of nonderivatized oligosaccharides to hydrazide-modified self-assembled monolayers

This paper describes a new and simple microarray platform for presenting multiple nonderivatized oligosaccharides to protein targets, with utility for mapping carbohydrate-protein recognition events. The approach is based on the creation of a hydrazide-derivatized, self-assembled monolayer on a gold surface in a single or two-step procedure, for efficient and selectively oriented anchoring of oligosaccharide probes via their reducing ends, with detection using fluorescence detection of bound proteins. The biggest hurdles in employing gold-based substrate for fluorescence-based microarray detection include fluorescence quenching and nonspecific surface adsorption of proteins. We found that the quenching effect could be minimized by introducing a omega-thiolated fatty acid (C16) self-assembled monolayer between the gold surface and hydrazide groups, followed by detection involving three successive binding protein layers covering the gold surface. In addition, an effective blocking scheme involving poly(ethylene glycol) aldehyde and bovine serum albumin was employed to reduce nonspecific protein adsorption to the chip surface. As proof of principle, we demonstrate here that sulfated oligosaccharide probes from heparin can be effectively and covalently attached without prior derivatization onto the hydrazide-modified, self-assembled monolayer on gold-coated slide surfaces in a microarray format. This platform is used to assess binding of specific heparin-binding protein targets at very high sensitivity, and we also demonstrate that the approach can be extended to nonsulfated sugars. Direct attachment of nonderivatized sugar probes on the chip is advantageous since it avoids the need for laborious prederivatization and cleanup steps. This versatile fluorescence microarray platform provides a facile approach for interrogating multiple carbohydrate-protein interactions in a high-throughput manner and has potential as a common gold surface platform for other diverse interrogations by MALDI-MS, surface plasmon resonance, and quartz crystal microbalances.     

Immobilization of Trichoderma viride for enhanced methylene blue biosorption: Batch and column studies

An efficient dye biosorbent was developed by entrapping a fungus mold, Trichoderma viride, within loofa sponge (LS) matirx. Immobilization enhanced the sorption of dye by 30% at equilibrium as compared with T. viride free biomass (TVFB). The maximum dye biosorption capacity of T. viride immobilized onto loofa sponge (TVILS) and TVFB was found to be 201.52 and 155.06 mg g⁻¹ biomass, respectively. The kinetics of dye removal by TVILS was rapid, with 84.3% sorption within the first 30 min and equilibrium after 90 min, whereas sorption by TVFB was slower as 61.4% dye was removed in first 30 min and equilibrium was achieved in 120 min. Biosorption kinetics and equilibria followed the pseudo-second-order and Langmuir adsorption models. FTIR spectroscopy of T. viride biomass showed that amine, hydroxyl, carbonyl and amide bonds were involved in the sorption of dye. Dye desorption from dye-laden TVILS with 0.1 M HCl was 99%. Regenerated TVILS was reusable without any appreciable decrease in its biosorption capacity during five repeated cycles. The dye removing capacity of TVILS in a continuous-flow column bioreactor was better than in batch-scale procedures. The study shows that TVILS has the potential of application as an efficient biosorbent for the removal of methylene blue from aqueous solutions.     

Silver and gold nanocluster catalyzed reduction of methylene blue by arsine in micellar medium

Arsenic can be determined in parts-per-million (ppm) level by absorbance measurement. This method is based on the quantitative colour bleaching of the dye, methylene blue by arsine catalyzed by nanoparticles in micellar medium. The arsine has been generated in situ from sodium arsenate by NaBH₄ reduction. The absorbance measurement was carried out at the λ_max of the dye at 660 nm. The calibration graph set-up for three linear dynamic ranges (LDR) are 0-8.63 ppm, 0-1.11 ppm and 0-0.11 ppm and limit of detections (LODs) are 1.3, 0.53 and 0.03 ppm, respectively. This method is simple, sensitive and easy to carry out. It is free from phosphate and silicate interference and applicable to real sample analysis.