Inkjet-printed thiol self-assembled monolayer structures on gold: quality control and microarray electrode fabrication

Laterally structured, self-assembled monolayers (SAMs) of different thiols (HS-R-X, R = (CH 2) 3-16, X = -CH 3, -COOH, -NH 2) on gold have been prepared by inkjet printing. The printer is a modified, low-cost desktop printer (Epson Stylus Photo R200), the ink is a 1 mM solution of the thiol in ethanol/glycerol (6:1). The quality of inkjet-printed large area SAMs obtained in this study is between that of a layer self-assembled from a thiol solution and that obtained by soft lithography, according to cyclic voltammetry, electrochemical impedance spectroscopy, scanning electrochemical microscopy (SECM), and polarization-modulated Fourier transform infrared reflection-absorption spectroscopy (PM IRRAS). For the first time, simultaneous printing of two different thiols in a single print job as an alternative to sequential printing and backfilling is demonstrated. The smallest structures consisting of conductive disks of 40 microm diameter were analyzed as single spots by SECM and as random array electrodes with different average disk-disk distance. Conductive band electrodes with variable bandwidth (300 microm to 1 cm) are presented, as well as a pH switchable band structure. As compared to stamping, inkjet printing allows for simultaneous multiple thiol printing in a single print job with the resolution limited only by the droplet size and the precision of the translation stage.     

Enzymatic nanolithography of a self-assembled oligonucleotide monolayer on gold

This paper describes a simple strategy to biochemically manipulate a surface at the nanoscale by enzyme dip-pen nanolithography using an endonuclease (DNase I) that is directly patterned on a self-assembled monolayer presenting a terminal oligonucleotide. Physisorbed nanopatterns of DNase I carried out nanoscale enzymology at the surface creating oligonucleotide patterns with the fidelity of the patterned enzyme because of the affinity of the enzyme for the immobilized, oligonucleotide substrate.     

FTIR-ATR study of self-assembled thiol monolayers on gold

Self-assembled monolayers of 1-teradecanethiol on gold were characterized by means of FTIR-ATR measurements, XPS and contact angle measurements. Linear dichroism measurements using FTIR-ATR are used to estimate the orientation of the alkyl chains. An equation for calculating the orientation angles of the alkyls chains was deduced.     

Stereo-electrochemistry by a self-assembled monolayer of sulfur-bridged calixthiophene on gold

Sulfur-bridged calixthiophene formed a self-assembled mono-molecular layer on polycrystalline gold, and it regulated an electrochemical electron transfer by the host–guest interaction between the cavity and reactants. 1,7,13,19,25-Tetrathia[1.₅](2,5)thiophenophane (thiacalix[5]thiophene) perfectly passivated the gold electrode for relatively large reversible metal complexes: [Fe(CN)₆]^4−/3− and [IrCl₆]^3−/2−. However, for mono-atomic ions, such as silver and some of the halogen ions, the electrode behaved reversibly. For copper reduction, a large activation overpotential was observed to induce an initial copper reduction in the cavity.     

Two-dimensional crystal structure of a quaterthiophene-alkanethiol self-assembled monolayer on gold

We investigated the fine structure of a self-assembled monolayer of dodecanethiol functionalized by alpha-quaterthiophene on gold (alpha-4TC 12H 24SH). The molecular orientation, quantified using polarization modulation infrared reflection-absorption spectroscopy, was studied as a function of the adsorption time. The alpha-4T moieties arrange in the upright position on the surface as the adsorption time increases, while the alkyl chain organization remains poor. Here we quantify the orientation of the self-assembled monolayer and, more significantly, reveal through surface X-ray diffraction that after a long incubation period (12 h) the alpha-4T on the gold surface adopts a 2D crystal structure.     

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.     

Study on self-assembled monolayer of functionalized thiol on gold electrode forming capacitive sensor for chromium(VI) determination

A capacitive sensor based on S-{12-[1-(pyridin-4-ylmethyl)-1H-1,2,3-triazol-4-yl]dodecyl} ethanethioate (FT), a compound with a functional group exhibiting selective affinity towards Cr(VI) ions, was developed. FT was mixed with shorter-chain thiol-decanethiol (DT), to obtain an Au electrode surface well covered by a thiol monolayer. The composition and high quality of self-assembled monolayers (SAMs) were crucial factors influencing the performance of the capacitive sensor. In this work, SAMs formed from FT and DT mixtures with different compositions were studied. For physicochemical characterization of SAMs X-ray photoelectron spectroscopy (XPS), contact angle measurements as well as atomic force microscopy (AFM) were used. Cyclic voltammetry was employed to estimate an electrode surface coverage. Based on the obtained results, the composition of thiol layer providing the best parameters for capacitive sensing of chromium(VI) was chosen. Moreover, the analytical performance of sensor was verified.

     

Mediating electron transfer from bacteria to a gold electrode via a self-assembled monolayer

Numerous bacterial genera are known to respire anaerobically using macroscopic electrodes as electron acceptors. Typically, inexpensive graphite electrodes, which are readily colonized, are used to monitor electrogenic bacterial metabolism for microbial fuel cell and bioelectronics studies. We compare current production by electrogenic bacteria on gold electrodes coated with various alkanethiol self-assembled monolayers to current production on glassy carbon electrodes. Current production is correlated to chain length and headgroup of the monolayer molecules as expected. Relative to graphite, the coated gold electrodes achieve more reproducible experimental conditions and certain headgroups enhance electronic coupling to the bacteria.     

Protein recognition by a self-assembled deep cavitand monolayer on a gold substrate

This paper details the first use of a self-folding deep cavitand on a gold surface. A sulfide-footed deep, self-folding cavitand has been synthesized, and its attachment to a cleaned gold surface studied by electrochemical and SPR methods. Complete monolayer formation is possible if the cavitand folding is templated by noncovalent binding of choline or by addition of space-filling thiols to cover any gaps in the cavitand adsorption layer. The cavitand is capable of binding trimethylammonium-tagged guests from an aqueous medium and can be deposited in 2 × 2 microarrays on the surface for characterization by SPR imaging techniques. When biotin-labeled guests are used, the cavitand:guest construct can recognize and immobilize streptavidin proteins from aqueous solution, acting as an effective supramolecular biosensor for monitoring protein recognition.     

Fractional coverage of defects in self-assembled thiol monolayers on gold

Au electrodes are alkylated by self-assembled organic monolayers of octadecanethiol from alcohol solution. The electron tunnelling resistance of a monolayer-coated gold electrode has been investigated by ac impedance. The relation between the fractional coverage of different defects and the corresponding film thickness at these ‘collapsed’ sites has been deduced from electron tunnelling theory. By using the concepts of average film thickness at defect (d_a) and average fractional coverage of defect (θ_a), we have obtained the θ_ad_a plot. The influence of the apparent standard rate constant on the shape of the θ_ad_a plot has been discussed. In our experiments, Fe(CN)₆^3−/4− is used as a redox probe to study the θ_ad_a plot of an octadecanethiol monolayer. The θ_a versus d_a plot indicates that the defects with d_a<6 methylene groups and θ_a<0.1 can increase the apparent standard rate constant from 1.9×10⁻¹⁰ cm s⁻¹, which is the theoretical value calculated from electron tunnelling theory, to 2.9×10⁻⁷ cm s⁻¹. The average thickness of the whole monolayer (ATWM), which is obtained from the θ_a versus d_a plot and which can indicate the blocking property of the monolayer, is 11 methylene groups.     

A spectroscopic study of self-assembled monolayer of porphyrin-functionalized oligo(phenyleneethynylene)s on gold: the influence of the anchor moiety

Porphyrin-functionalized oligo(phenyleneethynylene)s (OPE) are promising molecules for molecular electronics applications. Three such molecules () with the common structure P-OPE-AG (P and AG are a porphyrin and anchor group, respectively) and different anchor groups, viz. an acetyl protected thiol, -S-COCH(3) (), an acetyl protected thiol with methylene linker, -CH(2)-S-COCH(3) (), and a trimethylsilylethynyl group, -C[triple bond, length as m-dash]C-Si(CH(3))(3) () have been synthesized and the corresponding self-assembled monolayers (SAMs) on Au(111) substrates have been prepared. The integrity and structural properties of these films were studied by X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. The results suggest that the films formed from have a high orientational order with an almost upright orientation and dense packing of the molecular constituents, i.e. represent a high quality SAM. In contrast, molecule formed disordered molecular layers on Au, even though the molecule-surface bonding (thiolate) is the same as in the case of molecule . This suggests that the methylene linker in molecule has a strong impact on the quality of the resulting film, so that a well-ordered SAM cannot be formed. The silane system, , is also able to bind to the gold surface but the resulting SAM has a poor quality, being significantly disordered and/or comprised of strongly inclined molecules. The above results suggest that the nature of the anchor group along with a possible linker is an important parameter which, to a high extent, predetermines the entire quality of OPE-based molecular layers.     

Infrared detection of a phenylboronic acid terminated alkane thiol monolayer on gold surfaces

Polarization modulation infrared reflectance absorption spectroscopy (PM-IRRAS) and infrared reflectance absorption spectroscopy (IRRAS) have been used to characterize the formation of a self-assembled monolayer of N-(3-dihydroxyborylphenyl)-11-mercaptoundecanamide) (abbreviated PBA) on a gold surface and the subsequent binding of various sugars to the PBA adlayer through the phenylboronic acid moiety to form a phenylboronate ester. Vibrationally resonant sum frequency generation (VR-SFG) spectroscopy confirmed the ordering of the substituted phenyl groups of the PBA adlayer on the gold surface. Solution FTIR spectra and density functional theory were used to confirm the identity of the observed vibrational modes on the gold surface of PBA with and without bound sugar. The detection of the binding of glucose on the gold surface was confirmed in part by the presence of a C-O stretching mode of glucose and the observed O-H stretching mode of glucose that is shifted in position relative to the O-H stretching mode of boronic acid. An IR marker mode was also observed at 1734 cm(-1) upon the binding of glucose. Additionally, changes in the peak profile of the B-O stretching band were observed upon binding, confirming formation of a phenylboronate ester on the gold surface. The binding of mannose and lactose were also detected primarily through the IR marker mode at approximately 1736 to 1742 cm(-1) depending on the identity of the bound sugar.     

Development of a myoglobin impedimetric immunosensor based on mixed self-assembled monolayer onto gold

Immunosensors rely on antibody-antigen binding with a range of possible detection methodologies. In this study, electrochemical impedance spectroscopy was used to monitor the sensor surface assembly and recognition of the analyte (myoglobin). Myoglobin is rapidly released into the circulatory system after an acute myocardial infarction and rapidly rising levels make it the first biochemical marker of myocardial damage. The immunosensor fabrication steps comprised the steps of (a) formation of mixed self-assembled monolayers (mSAM) on gold electrodes using a mixture of biotinyl-phospholipid and mer captohexadecanoic acid; (b) neutravidin functionalisation and (c) attachment of biotinyl anti-myoglobin antibodies. A range of analyte concentration (10⁻¹²–10⁻⁶ M) was successfully detected in phosphate buffered saline and in serum concentration ranging from 10% (v/v) to 100% (v/v) serum. Quartz crystal microbalance and atomic force microscopy studies were carried out to study each step of fabrication to elucidate binding characteristics and surface topography. Correspondence: Morsaline Billah, Institute of Membrane and Systems Biology, Garstang Building, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, United Kingdom     

Voltammetric investigation of cytochrome c on gold coated with a self-assembled glutathione monolayer

The direct, reversible electrochemistry of horse-heart cytochrome c (cyt. c) was realized on a self-assembled glutathione (GSH) monolayer modified Au electrode. The voltammetric responses of cyt. c on GSH/Au electrode were found to be affected by pH during the electrode modification, metal ions and surfactants. Using potassium ferricyanide [K4Fe(CN)6] as a probe, these effects on the voltammetric responses of cyt. c were characterized by electrochemical methods. It was found that the pH during the electrode modification, metallic ions and surfactants changed GSH monolayer's charge state and the conformation on the electrode surface, and resulted in the influence on the voltammetric responses of cyt. c. The experimental results provided us information to understand the mechanism of the interfacial electron transfer of electrode-protein, as well as the electron transfer of cyt. c in life system.     

Formation process and solvent-dependent structure of a polyproline self-assembled monolayer on a gold surface

The formation process and structure of a self-assembled monolayer (SAM) of lipoic-acid-terminated polyproline on a gold surface in aqueous solution were investigated by several techniques. The amount of polyproline molecules on the gold surface was determined from the area of the reductive desorption peak, and orientation and thickness of the polyproline SAM were determined in situ by attenuated total reflection infrared (ATR-IR) spectroscopy and ellipsometry. The kinetics of the polyproline SAM formation process were discussed on the basis of these results. The in situ IR study confirmed that the conformation of the polyproline SAM was changed by changing the solvent from water to methanol and methanol to water, as is the case for polyproline dissolved in solution.     

The influence of a self-assembled monolayer on the activity of rough gold for glucose oxidation

Gold surface has been roughened by amalgamation and tested for self-assembled monolayer (SAM) formation and glucose oxidation. The rough gold undergoes structural changes at elevated temperatures, which lead to loss of activity for glucose oxidation and lower affinity for the self-assembly. The transformation of “active” into “inactive” surface can be prevented by SAM formation. The SAM modified gold exhibits high activity for glucose oxidation. These results are important especially for those studies, which use rough gold modified by SAM for enzyme immobilisation and further glucose oxidation, since the gold activity itself was so far neglected.     

Application of 2-(3,4-dihydroxyphenyl)-1,3-dithialone self-assembled monolayer on gold electrode as a nanosensor for electrocatalytic determination of dopamine and uric acid

The electrooxidation of dopamine (DA), uric acid (UA) and their mixture on a gold electrode modified by a self-assembled monolayer of 2-(3,4-dihydroxyphenyl)-1,3-dithialone has been studied by cyclic voltammetry (CV), chronoamperometry and differential pulse voltammetry (DPV). CV was used to investigate the redox properties of the modified electrode at various scan rates and the apparent charge transfer rate constant (k(s)), and transfer coefficient (α) were calculated. The mediated oxidation of DA at the modified electrode under the optimum condition (pH = 7.0) in CV occurs at a potential about 220 mV less positive than that of the unmodified gold electrode. The values of electron transfer coefficients (α), catalytic rate constant (k) and diffusion coefficient (D) were calculated for DA, using electrochemical methods. DPV exhibited a linear dynamic range over the concentration range of 0.2-250.0 μM and a detection limit (3σ) of 0.07 μM for DA. The modified electrode was used for simultaneous determination of DA and UA by DPV. The results showed that the electrode is highly efficient for the catalytic electrooxidation of DA and UA, leading to a remarkable peak resolution (~350 mV) for two compounds. The electrode was used for the determination of DA in an injection sample.     

The nature of alkanethiol self-assembled monolayer adsorption on sputtered gold substrates

A detailed study of the self-assembly and coverage by 1-nonanethiol of sputtered Au surfaces using molecular resolution atomic force microscopy (AFM) and scanning tunneling microscopy (STM) is presented. The monolayer self-assembles on a smooth Au surface composed predominantly of [111] oriented grains. The domains of the alkanethiol monolayer are observed with sizes typically of 5-25 nm, and multiple molecular domains can exist within one Au grain. STM imaging shows that the (4 x 2) superlattice structure is observed as a (3 x 2) structure when imaged under noncontact AFM conditions. The 1-nonanethiol molecules reside in the threefold hollow sites of the Au[111] lattice and aligned along its [112] lattice vectors. The self-assembled monolayer (SAM) contains many nonuniformities such as pinholes, domain boundaries, and monatomic depressions which are present in the Au surface prior to SAM adsorption. The detailed observations demonstrate limitations to the application of 1-nonanethiol as a resist in atomic nanolithography experiments to feature sizes of approximately 20 nm.     

多巴胺在DTNB自组装膜上的电催化研究

在金电极表面制备了DTNB(5,5′ Di thiobis(2 nitrobenzoicacid))自组装单分子层膜(DTNB/AuSAM)。多巴胺在DTNB自组装膜上有一对可逆性良好的氧化还原峰,其氧化峰电流与多巴胺的浓度在5.0×10-6mol/L～1.0×10-4mol/L的范围内呈线性关系,检出限为1.0×10-6mol/L。在pH3.5的缓冲溶液中,在DTNB自组装膜上多巴胺和抗坏血酸的电化学响应可以明显区分,氧化峰电位分离达276mV。可用于抗坏血酸存在下多巴胺的检测。测定了盐酸多巴胺注射液中多巴胺的含量,其平均回收率为104%。