Monolayers Of Thiophenol Research Articles

Benchtop Fabricated Nano-Roughened Microstructured Electrodes for Electrochemical and Surface-Enhanced Raman Scattering Sensing.

Tailoring a material's surface with hierarchical structures from the micro- to the nanoscale is key for fabricating highly sensitive detection platforms. To achieve this, the fabrication method should be simple, inexpensive, and yield materials with a high density of surface features. Here, using benchtop fabrication techniques, gold surfaces with hierarchically structured roughness are generated for sensing applications. Hierarchical gold electrodes are prepared on pre-stressed polystyrene substrates via electroless deposition and amperometric pulsing. Electrodes fabricated using 1mm H[AuCl₄] and roughened with 80 pulses revealed the highest electroactive surface area. These electrodes are used for enzyme-free detection of glucose in the presence of bovine serum albumin and achieved a limit of detection of 0.36mm, below glucose concentrations in human blood. The surfaces nanoroughened with 100 pulses also showed excellent surface-enhanced Raman scattering (SERS) response for the detection of rhodamine 6G, with an enhancement factor of ≈2×106 compared to detection in solution, and for the detection of a self-assembled monolayer of thiophenol, with an enhancement factor of ≈30 compared to the response from microstructured gold surfaces. It is envisioned that the simplicity and low fabrication cost of these gold-roughened structures will expedite the development of electrochemical and SERS sensing devices.

Synthesis and comparison of gold nanoparticles coated with silicon, manganese and titanium dioxides

The synthesized gold nanoparticles (50–60 nm) were coated with thin (1.5–8 nm) layers of silicon, manganese and titanium dioxides. The obtained three species of nanospheres were applied for the analysis of self-assembled monolayer of thiophenol by shell-isolated nanoparticleenhanced Raman spectroscopy method. Detailed synthesis and comparison of the particle properties are provided in this report.

Silica shell-isolated gold nanospheres: Synthesis, investigation of stability in organic solvents, and application in shell-isolated nanoparticle-enhanced Raman spectroscopy

Abstract Shell-isolated nanoparticles spread over the surface of sample material are widely used in various fields of analysis. Their stability has not yet been well established. In this report, the silica shell-isolated gold nanospheres were synthesized in aqueous medium. Their stability in water and 17 organic solvents was observed visually and employing spectroscopic UV and Raman techniques. The nanoparticles onto self-assembled monolayer of thiophenol were used to amplify Raman signal.

Wavelength and refractive index dependence of the geometrical enhancement in surface‐enhanced Raman scattering

The overall enhancement in surface‐enhanced Raman scattering (SERS) is considered to originate from a combination of electromagnetic and chemical effects. However, various additional enhancements associated with SERS have been reported to arise from geometrical factors such as excitation direction and thin film interference. Here, we theoretically and experimentally study the wavelength and refractive index dependence of the geometrically enhanced SERS signal. Monolayers of thiophenol on thermally evaporated Ag‐island films on glass and sapphire substrates were analysed as a model system. The effective optical constants of the nanostructured metal film, as determined by Mueller matrix ellipsometry, were used in an electric‐field model that predicts the enhanced SERS signal arising at the boundary of the metal film–substrate interface. SERS experimental measurements were found to exhibit a geometrically enhanced signal in broad agreement with the theoretical model, with the additional enhancements peaking at more than three for both substrates. Designing plasmonic structures by carefully controlling their effective optical constants could lead to increased enhancement factors for SERS sensing on transparent substrates such as optical fibre probes or observation windows. Copyright © 2017 John Wiley & Sons, Ltd.

The effect of STM parameters on tip-enhanced Raman spectra.

In this work, we evaluate the dependence of tip-enhanced Raman (TER) spectra of a monolayer of thiophenol at a Au(111) electrode on the scanning tunneling microscope's tunneling current set-point and bias voltage parameters. We find an increase of the TER intensity upon set-point increase or bias decrease as expected from a gap-distance reduction. The relations obtained follow a theoretical model considering a simple gap-distance change when tuning the mentioned parameters. We find that the value of the bias voltage affects the TER intensity to a larger extent than the current set-point. Therefore it is advisable to work in a low-bias regime when aiming for ultrasensitive TER measurements.

Versatile Side-Illumination Geometry for Tip-Enhanced Raman Spectroscopy at Solid/Liquid Interfaces.

In situ characterization of surfaces with tip-enhanced Raman spectroscopy (TERS) provides chemical and topographic information with high spatial resolution and submonolayer chemical sensitivity. To further the versatility of the TERS approach toward more complex systems such as biological membranes or energy conversion devices, adaptation of the technique to solid/liquid working conditions is essential. Here, we present a home-built side-illumination TERS setup design based on a commercial scanning tunneling microscope (STM) as a versatile, cost-efficient solution for TERS at solid/liquid interfaces. Interestingly, the results obtained from showcase resonant dye and nonresonant thiophenol monolayers adsorbed on Au single crystals suggest that excitation beam aberrations due to the presence of the aqueous phase are small enough not to limit TER signal detection. The STM parameters are found to play a crucial role for solid/liquid TERS sensitivity. Raman enhancement factors of 10(5) at μW laser power demonstrate the great potential the presented experimental configuration holds for solid/liquid interfacial spectroscopic studies.

Semiconductor-Metal Nanofloret Hybrid Structures by Self-Processing Synthesis.

We present a synthetic strategy that takes advantage of the inherent asymmetry exhibited by semiconductor nanowires prepared by Au-catalyzed chemical vapor deposition (CVD). The metal-semiconductor junction is used for activating etch, deposition, and modification steps localized to the tip area using a wet-chemistry approach. The hybrid nanostructures obtained for the coinage metals Cu, Ag, and Au resemble the morphology of grass flowers, termed here Nanofloret hybrid nanostructures consisting of a high aspect ratio SiGe nanowire (NW) with a metallic nanoshell cap. The synthetic method is used to prepare hybrid nanostructures in one step by triggering a programmable cascade of events that is autonomously executed, termed self-processing synthesis. The synthesis progression was monitored by ex situ transmission electron microscopy (TEM), in situ scanning transmission electron microscopy (STEM) and inductively coupled plasma mass spectrometry (ICP-MS) analyses to study the mechanistic reaction details of the various processes taking place during the synthesis. Our results indicate that the synthesis involves distinct processing steps including localized oxide etch, metal deposition, and process termination. Control over the deposition and etching processes is demonstrated by several parameters: (i) etchant concentration (water), (ii) SiGe alloy composition, (iii) reducing agent, (iv) metal redox potential, and (v) addition of surfactants for controlling the deposited metal grain size. The NF structures exhibit broad plasmonic absorption that is utilized for demonstrating surface-enhanced Raman scattering (SERS) of thiophenol monolayer. The new type of nanostructures feature a metallic nanoshell directly coupled to the crystalline semiconductor NW showing broad plasmonic absorption.

Comparison of a Fluorinated Aryl Thiol Self-Assembled Monolayer with Its Hydrogenated Counterpart on Polycrystalline Ag Substrates

The effects of perfluorination of aryl thiols on surface coverage, surface electronic properties, and molecular orientation of self-assembled monolayers of thiophenol (TP) and pentafluorothiophenol (F(5)TP) on polycrystalline Ag were evaluated using linear sweep voltammetry, ultraviolet photoelectron spectroscopy (UPS), and surface Raman spectroscopy, respectively. Electrochemical reductive desorption by linear sweep voltammetry indicates a surface coverage for the TP monolayer of (5.07 +/- 1.29) x 10(-10) mol/cm(2), equating to a molecular area of 32.8 +/- 8.3 A(2), and a surface coverage for the F(5)TP monolayer of (1.95 +/- 0.59) x 10(-10) mol/cm(2), equating to an area of 85.2 +/- 25.8 A(2)/molecule. TP-modified Ag exhibits a change in work function (DeltaPhi) of -0.64 eV relative to bare Ag, whereas F(5)TP-modified Ag exhibits a DeltaPhi of +0.54 eV relative to bare Ag. Quantitative analysis of the UPS and reductive desorption results yields molecular pictures of the proposed interfaces with TP molecules tilted <20 degrees from the surface normal in a herringbone pattern spaced 6.4 A apart and F(5)TP molecules in a more disordered arrangement tilted 67 degrees from the surface normal with an intermolecular distance of 10.4 A. Qualitative surface Raman spectroscopic analysis of in-plane and out-of-plane modes for these systems confirms that TP molecules are oriented more vertical than F(5)TP molecules in these monolayers.

A high‐throughput method for controlled hot‐spot fabrication in SERS‐active gold nanoparticle dimer arrays

AbstractWe present a high‐throughput method for fabricating large arrays of surface‐enhanced Raman scattering (SERS) active gold dimers. Using a large‐area/low‐cost nanopatterning method in conjunction with a meniscus force deposition technique, we were able to create large arrays of uniformly spaced nanoclusters comprising two 60‐nm gold nanospheres. Raman measurements of a thiophenol monolayer deposited on smaller scale arrays of aligned dimers yielded enhancement factors as high as 109. Polarization‐controlled measurements show spectral peak heights to be 10–100 times smaller when the incident beam is polarized perpendicularly to the dimer axis, confirming that the measured enhancements arise from the ‘hot spots’ between the two nanospheres. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Electron transfer studies on cholesterol LB films assembled on thiophenol and 2-naphthalenethiol self-assembled monolayers

We have formed the cholesterol monolayer and multilayer LB films on the self-assembled monolayers of 2-naphthalenethiol (2-NT) and thiophenol (TP) and studied the electrochemical barrier properties of these composite films using cyclic voltammetry and electrochemical impedance spectroscopy. We have also characterized the cholesterol monolayer film using grazing angle FTIR, scanning tunneling microscopy (STM) and atomic force microscopy (AFM). Cholesterol has a long hydrophobic steroid chain, which makes it a suitable candidate to assemble on the hydrophobic surfaces. We find that the highly hydrophobic surface formed by the self-assembled monolayers (SAM) of 2-NT and TP act as effective platforms for the fabrication of cholesterol monolayer and multilayer films. The STM studies show that the cholesterol monolayer films on 2-NT form striped patterns with a separation of 1.0 nm between them. The area per cholesterol molecule is observed to be 0.64 nm2 with a tilt angle of about 28.96 degrees from the surface normal. The electrochemical studies show a large increase in charge transfer resistance and lowering of interfacial capacitance due to the formation of the LB film of cholesterol. We have compared the behavior of this system with that of cholesterol monolayer and multilayers formed on the self-assembled monolayer of thiophenol.

Electron transfer studies through mixed self-assembled monolayers of thiophenol and thioctic acid

Mixed self-assembled monolayer of thiols were formed on the electrode surface using aromatic and aliphatic thiols and the modified interface were used to demonstrate charge trapping behavior at the modified interface. The electrode surface was modified by two schemes. In Scheme 1, electrode surface modified with methylene blue (MB) shows charge trapping behavior when contacts the electrolyte containing potassium ferricyanide. In Scheme 2, a redox-active bilayer is constructed using hexamineruthenium (II) chloride and methylene blue. This redox-active bilayer assembly exhibits unidirectional flow of current.

Molecule orientation in self-assembled monolayers determined by infrared-visible sum-frequency generation spectroscopy

The molecular orientation in self-assembled monolayers of thiophenol (TP) on Ag(1 1 1) and of para-nitroanilino-dodecane-thiol (NAT) on Au was investigated by infrared-visible sum-frequency generation between 500 and 1500 cm−1. Ab initio calculations were performed to determine the interface nonlinear response. The combined experimental and theoretical results reveal that the S–C bond of TP is tilted 37∘ from the Ag(1 1 1) surface normal, with the aromatic ring plane perpendicular to the surface. On Au, the tilt angle of the NAT molecule stabilizes below 60∘ at the end of film growth.

Electroactive gold nanoparticles protected by 4-ferrocene thiophenol monolayer

Numerous reports have focused on ferrocene-terminated electroactive self-assembled monolayers (SAMs) on a flat Au surface but only a few on ferrocene SAMs on Au colloid. In this paper, we employ 4-ferrocene thiophenol as a novel capping agent to produce electroactive gold nanoparticles in consideration of the peculiar π-conjugated structure. Transmission electron microscopy shows the narrow-dispersed gold core with an average core diameter of ca. 2.5 nm. UV/vis spectra examine the π-conjugated structure of 4-ferrocene thiophenol and surface plasmon absorbance of the indicated gold nanoparticles. X-ray photoelectron spectroscopy reveals electronic properties of the Au core and thiol ligands. Electrochemical measurement shows that the oxidation peak current is proportional to the scan rate, indicating the electrode process is controlled by adsorbed layer reaction. The formal potential of the Fc-MPCs is compared with that of free ferrocene in MeCN solution and the Fc-SAMs. The shifts are attributed to the phenyl moiety in the 4-ferrocene thiophenol and dielectric constant of the solvation environment.

Electrochemical study of 4-ferrocene thiophenol monolayers assembled on gold nanoparticles

In this paper, 4-ferrocene thiophenol was employed as a novel capping agent to synthesize electroactive gold nanoparticles. Transmission electron microscopy showed an average core diameter of 2.5 nm. The optical and electrochemical properties of the 4-ferrocene thiophenol capped gold nanoparticles were characterized by UV–Vis spectroscopy and cyclic voltammograms. Surface plasmon absorbance was detected at 522 nm. Cyclic voltammograms revealed the adsorbed layer reaction controlled electrode process, and the formal potential of electroactive ferrocene centers shifted anodically compared with ferrocene in solution, which could be attributed to the electron-withdrawing phenyl moiety linked to ferrocene.

Self-Assembled Monolayers of Thiophenol on Gold as a Novel Substrate for Surface-Enhanced Infrared Absorption

A unique method of obtaining surface-enhanced infrared absorption (SEIRA) spectra for chemicals that will not chemically attach to a metal surface has been investigated. Surface enhancements are greatest for molecules that bind to metals. In order to achieve greater enhancement for those analytes that do not bind to SEIRA metals, we have investigated self-assembled monolayers as a means of linking analytes to a gold substrate. Monolayers of thiophenol were formed onto sputter-coated gold–silicon substrates. Analytes were deposited onto the thiophenol-coated gold–silicon wafers, and external reflection SEIRA spectra were then measured. Enhancement factors as high as 30-fold compared to those for conventional SEIRA substrates are demonstrated. The self-assembled monolayers on gold substrates are shown to change both relative intensities and band positions of the adsorbed analyte. These intensity changes and frequency shifts show strong interaction of selected analytes with the self-assembled monolayer. This study of phthalates and nitrosubstituted aromatic compounds demonstrates the usefulness of the technique. Spectral changes evident through the use of the thiophenol monolayer are discussed.

Surface Enhancement Factors for Ag and Au Surfaces Relative to Pt Surfaces for Monolayers of Thiophenol

Raman signal intensities from the 997 cm−1 ring breathing mode of thiophenol monolayers adsorbed at Ag, Au, and Pt surfaces were employed for determination of absolute surface enhancement factors (SEFs). Unlike previous estimations of SEFs, these SEFs are determined by referencing the surface-enhanced Raman scattering (SERS) intensities to the unenhanced Raman scattering at Pt surfaces. The surfaces studied include those commonly prepared in a laboratory ambient and those prepared in vacuum. Surfaces prepared in ambient include polycrystalline Ag electrochemically roughened by an oxidation-reduction cycle (ORC), mechanically polished (MP) polycrystalline Ag, chemically polished (CP) polycrystalline Ag, Ag (111), MP polycrystalline Au, and MP polycrystalline Pt. Vacuum environment surfaces include coldly deposited Ag films (cold Ag) and room temperature-deposited (RT) “thick” Ag films. Each of these thiophenol/metal systems was sampled with an excitation wavelength (λex) of 514.5 nm; MP Au surfaces were also studied with λex of 720 nm. SEFs of 2.0 × 104 for ORC Ag, 5.3 × 103 for MP Ag, 160 for cold Ag, 64 for MP Au720 (λex = 720 nm), 69 for Ag (111), 39 for CP Ag, 7.9 for RT “thick” Ag, and 2.2 for MP Au514.5 (λex 514.5 nm) are observed relative to the SEF of MP Pt, which is assigned as 1. For practical purposes, the significance of the magnitudes of these SEFs is discussed in terms of estimated surface Raman limits of detection (LODs).

Sum-frequency generation from thiophenol on silver in the mid and far-IR

Sum-frequency (SF) spectra of a monolayer of thiophenol on silver are reported in the mid and far-IR (infrared). The free-electron laser FELIX was used to reach wavelengths up to 54 μm. Molecular vibrations of thiophenol are observed at wavelengths near 10 μm (three modes), 14 μm (1 mode), and 24 μm (1 mode). The appearance of the different vibrational modes in the spectra varies dramatically due to interference between the resonant sum-frequency signal and the nonresonant sum-frequency signal from silver. The standard model used to describe line shapes in SF spectra is shown to be insufficient to explain the different line shapes for the various vibrational modes of thiophenol on silver.

Characterization of self-assembled monolayer of thiophenol on gold by Penning ionization electron spectroscopy

The molecular orientation in self-assembled monolayers (SAMs) of thiophenol chemisorbed on polycrystalline gold from aqueous solutions has been investigated by Penning ionization electron spectroscopy (PIES) and ultraviolet photoelectron spectroscopy (UPS). The analysis of the relative band intensity of the Penning spectrum indicates that the phenyl ring in phenyl thiolate stands perpendicular to the substrate plane at room temperature. Upon heating, the phenyl ring becomes tilted in the layer.