SIMS of organic anions adsorbed onto an aminoethanethiol self-assembled monolayer: an approach for enhanced secondary ion emission

Secondary ion mass spectrometry (SIMS) was used to monitor the uptake of organic anions from solution by aminoethanethiol (AET) monolayers on Au substrates, as a test of the applicability of this monolayer as a substrate for organic SIMS analysis. Event-by-event bombardment and detection mode coupled with coincidence counting allowed the atomic and polyatomic projectile impacts on a particular sample surface to be compared simultaneously and under the same experimental conditions. The mass spectra produced from the monolayer surface and those from Au and Si blanks demonstrate that the AET monolayer is important to the uptake of the organic anion. The exchanged monolayer surfaces were used to measure secondary ion yields, defined as the number of secondary ions detected per incident primary ion, produced from ultrathin films by (CsI)nCs+ (n = 0-2) projectiles at the limit of single-ion impacts. The yield of a tetradecyl sulfate (IDS) anion was improved by a factor of 200 using the AET substrate instead of the thick salt target. The intact ion and fragment ion yield trends produced from the AET surface were measured as a function of number of atoms in the primary projectile and energy. We observed a yield increase for both the intact ion and the fragment ion with the projectile complexity and energy. The increase in yield per projectile atom was linear for the emission of intact TDS and intact dodecyl sulfate from the AET surfaces. A supralinear yield enhancement, however, was observed for the fragment ion SO3- when the three-atom (CsI)Cs+ cluster was used. The experiments demonstrate that the various organosulfate and suffonates are weakly bound to the AET surface and their adsorption to the AET monolayer is reversible. The utility of the AET monolayer on Au was also tested as a general substrate for the characterization of derivatized organic molecules with biological and industrial importance by TOF-SIMS.     

Fabrication of a surface plasmon resonance biosensor based on gold nanoparticles chemisorbed onto a 1,10-decanedithiol self-assembled monolayer

We have investigated the fabrication of surface plasmon resonance (SPR) biosensors using self-assembled monolayers (SAMs) and adsorbed gold nanoparticles. The SAM of 1,10-decanedithiol was first fabricated onto a gold substrate. Gold nanoparticles were then chemisorbed onto the SAM surface by bonding with the terminal thiol groups, forming a sensor that can be used to immobilize proteins. Bovine serum albumin (BSA) was used as a test protein in this study. Several spectroscopic and microscopic techniques were used to investigate both the SAM and the chemisorption of gold nanoparticles at the SAM surface. Our results confirm the covalent bonding of the gold nanoparticles onto the SAM. Surface plasmon resonance (SPR) was used to study both the adsorption of BSA to the SAM surface and to the gold nanoparticle-coated SAM. For SAM surfaces with adsorbed gold nanoparticles a larger SPR response to BSA than to the sensors with a bare SAM is observed.     

A new pathway to GaN: deposition of GaN-clusters on functionalized thiol-SAMs on gold

We demonstrate the synthesis of a gallium and nitrogen containing film onto the surface of a hydroxyl-terminated self-assembled monolayer (SAM) on gold. This layer was produced by rinsing the sample in a solution of a donor-stabilized galliumtriazid, Ga(N₃)₃NEt₃. Surface-coverage in dependence on reaction time was analyzed. Repeatingly rinsing the SAMs in Ga(N₃)₃NEt₃-solution in toluene, cleaning with toluene, drying in vacuum and exposure to NH₃ allows a stepwise growth of the film, i.e. increasing of the surface-coverage. These films were compared to films, which were not exposed to ammonia but rather to moisture. Hydrolytic cleavage of the Ga-N₃-bond was the dominant process in this case. The films were analyzed with spontaneous desorption time of flight mass spectrometry (SDMS) and Rutherford backscattering spectroscopy (RBS).     

Determination of the ionization state of 11-thioundecyl-1-phosphonic acid in self-assembled monolayers by chemical force microscopy

The present article describes the preparation and preliminary characterization of a novel phosphate-functionalized self-assembled monolayer (SAM) and the determination of the surface ionization states of the phosphate headgroup in aqueous solutions by chemical force microscopy (CFM). The phosphate headgroup used was PO(OH)2, a diprotic acid. The adhesion force between an AFM probe and a flat substrate, both of which were chemically modified with the same phosphate SAM, was also measured as a function of pH and ionic strength. At low ionic strength (10(-4) M), two peaks were observed in the force titration curve (adhesion force versus pH) at pH 4.5 and 8.4. The two peaks are positioned 2.4 and 1.2 pH units higher, respectively, than the acid dissociation constants obtained for the phosphate group free in aqueous solution. At high ionic strength (10(-1) M), the adhesion forces were reduced by 1 order of magnitude and the peaks were replaced by shoulders similar to those previously reported for acid force titrations. On the basis of JKR theory, the surface pKa values of the phosphate group in high ionic strength solutions were found to be 4.5 and 7.7, respectively. However, in light of the effects of ionic strength on the force titration curves, we discuss the applicability of JKR theory to nanoscopic measurements of adhesion force and surface pKa.     

Electron transfer reactions of glucose oxidase at Au111 electrodes modified with phenothiazine derivatives

The catalytic reaction of glucose oxidase (GOx) mediated by 3-(10-phenothiazyl)propionic acid (PT-PA) and phenothiazine-labeled poly(ethylene oxide) (PT-PEO1000) that are covalently bonded to Au(111) electrodes has been investigated. The PT-PA and PT-PEO1000 are reacted with 2-aminoethanethiol (AET), followed by the formation of a self-assembled monolayer (SAM) onto the Au surface. The PT group immobilized on the SAM of AET acts as an effective mediator for the electron transfer (ET) between the electrode and the FAD center of freely diffusing GOx in solution. The ET rate constant estimated from the catalytic current using a newly derived equation is larger by 1 order of magnitude for the PT-PA-modified system (1.1 x 10(5) dm(3) mol(-1) s(-1)) than for the PT-PEO1000 system (1.4 x 10(4) dm(3) mol(-1) s(-1)). The order of the magnitude of the ET rate constant clearly contrasts with the GOx hybrid systems that we previously investigated (Anal. Chem. 2003, 75, 910-917), in which the presence of the PEO spacer enhances the ET reaction rate. The reduction in the apparent PT concentration at the electrode interface due to the high mobility of the PEO chain, leading to low efficiency in the formation of an enzyme-mediator complex, is a possible reason for the lower mediation ability of PT-PEO1000 than that of PT-PA for the ET between the FAD group and PT(+) immobilized on the electrode. Inhibition of the penetration of GOx molecules into the monolayer and of the accessibility of some part of PT groups to GOx molecules could also be reasons for the lower mediation ability of PT-PEO1000 thickly modified on the electrode.     

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.     

Dynamics of single-protein molecules at a liquid/solid interface: implications in capillary electrophoresis and chromatography

The behavior of individual molecules of R-phycoerythrin (RPE) was monitored by fluorescence imaging at various pHs and ionic strengths within the evanescent-field layer (EFL) at a water/fused-silica interface. Above the isoelectric point (pI), the individual protein molecules moved between exposures with random motion. As the pH approached the pI of the protein, the RPE molecules were partially adsorbed onto the fused-silica surface. The residence time and the number of molecules within the EFL also increased near the pI. Below the pI, the protein molecules were completely and permanently adsorbed onto the surface. However, the observed number of distinct molecule spots was decreased somewhat because of aggregation. At a given buffer condition, plots of residence times and molecule numbers exhibit asymmetry nearly identical to the corresponding elution peaks of the proteins in capillary electrophoresis and capillary liquid chromatography. These results provide insights into the fundamental interactions for the adsorption/desorption of proteins at the liquid/solid interface.     

Enlargement of gold nanoparticles on the surface of a self-assembled monolayer modified electrode: a mode in biosensor design

Gold nanoparticle (Au-NP) seeds were adsorbed onto the surface of a self-assembled monolayer (SAM)-modified electrode. With the treatment of this modified electrode by Au-NPs growth solution containing different concentrations of H2O2 or cholesterol along with cholesterol oxidase (ChOx), the Au-NP seeds on the electrode surface were enlarged in varying degrees. As a result, the peak currents in corresponding cyclic voltammograms were inversely proportional to the concentration of H2O2 or cholesterol. ChOx was also further modified onto the surface of Au/SAM/Au-NP electrode to prepare Au/SAM/Au-NP/ChOx electrode. Using the enzyme-modified electrode to detect cholesterol, which also utilized the enlargement of the NPs, an extraordinary low detection limit of 5 x 10(-9) M was achieved and two linear dependence ranges of 7.5 x 10(-8)-1 x 10(-6) and 1 x 10(-6)-5 x 10(-5) M were obtained. Consequently, new kinds of H2O2 and cholesterol biosensors could be fabricated.     

High-resolution TOF-SIMS study of varying chain length self-assembled monolayer surfaces

A high-resolution time-of-flight secondary ionization mass spectrometer (TOF-SIMS) has been used to investigate chain length effects in hydrocarbon seff-assembled monolayer (SAM) surfaces on gold substrates. A wide range of n-alkanethiols was used to make homogeneous SAM surfaces, which included both odd and even hydrocarbon chain length thiols. Variations in coverage, extent of oxidation, and high-mass cluster formation as a function of hydrocarbon chain length of the alkanethiol SAM surfaces were investigated. Long-short chain length effects were observed for the relative coverage of the SAM surfaces, which directly influences the extent of oxidation for the thin films. The formation of gold-sulfur and gold-adsorbate cluster ions was also observed, since the mass range of the TOF-SIMS made it possible to monitor all of the cluster ions that were formed following the high-energy ion/surface interactions.     

Monolayer detection on flat metal surface via surface enhanced Raman spectroscopy and sum frequency generation spectroscopy

Monolayer detection on metal surface requires ultra high sensitivity. Sum Frequency Generation Spectroscopy (SFG) and Surface Enhanced Raman Spectroscopy (SERS) are regarded as two powerful techniques with submolecular sensitivity to detect adsorbents on metal surface. However, in some cases it's still challenge to characterize molecules or groups with relatively high intramolecular symmetry, such as 4-Nitrothiophenol (4NTP), on flat metal surface even combining these two techniques. Basically, this is due to that 4NTP with para-substituted phenol groups is SFG insensitive while flat metal surface is unfavorable to yield strong SERS enhancement. In this concern, a simple and efficient method, silver mirror method, was employed to facilitate the detection of 4NTP SAM on flat gold surface. Silver nanopheres with diameters around 300 nm was fabricated through silver mirror reaction and in situ formed milky overlayer on top of 4NTP SAM adsorbed on gold surface. Significant enhancement on SERS signal can be achieved with such special assembly structure of the "metal-molecule-metal" system. Generally, the silver mirror method provided a complementary approach to facilitate the spectroscopic applications of molecule level detection on various metal surfaces in situ.     

Assembly behavior and monolayer characteristics of OH-terminated alkanethiol on Au(111): in situ scanning tunneling microscopy and electrochemical studies

Self-assembled monolayers (SAMs) of 6-mercapto-1-hexanol (MHO) on an Au(111) electrode were prepared in an electrochemical system. The adsorption behavior of MHO and the time-dependent organization of the SAM were investigated by in situ scanning tunneling microscopy (STM) and cyclic voltammetry (CV). The results show that a potential higher than 0.28?V (relative to RHE) is required to induce the adsorption of MHO. At 0.28?V, the MHO molecules adsorb in a flat-lying orientation, forming an ordered striped phase with a molecular arrangement of ([Formula: see text]). However, the adlayer is not stable at this potential. The adsorbed striped phase may recover to the herringbone feature of the gold substrate due to the desorption of adsorbed MHO. At a higher potential (0.35?V), the adlayer becomes stable and can undergo a phase evolution from the striped phase to a condensed structure, identified as c([Formula: see text]). This structure can also be described as a c(4 × 2) superlattice of a [Formula: see text] hexagonal adlattice. The surface coverage of the MHO SAM is identical to the saturated structure of an 11-mercapto-1-undecanol (MUO) SAM reported in a previous work, [Formula: see text]. However, the STM image of MHO adlayer shows a modulation in intensity, reflecting the presence of various conformations of adsorbed molecules. This result is attributed to the shorter chain length of MHO, which gives a weaker?van der Waals interaction between adsorbed molecules. This effect also results in a higher charge permeability across the adlayer and a lower striping potential to an MHO SAM.     

The Adsorption of Gelatin on Silver Bromide

The adsorption heat of gelatin on AgBr was determined calorimetrically in function of the pH for gelatins with high and low isoelectric point (IEP). The adsorption heat increases for pH-values lower than the IEP and remains constant for the higher pH-values. The differences in adsorption heat are also revealed by competitive adsorption with dyes. The amount of adsorbed gelatin is also strongly pH-dependent. The results obtained completely confirm the hypothesis of Pouradier and Roman concerning the structure of the adsorbed gelatin layer; the first molecules are spread over the AgBr surface so as to form a monolayer. If more gelatin is added a reorganisation of the adsorbed molecules occurs. Segments of one definite molecule are replaced by segments of another one. In this way the quantity of gelatin adsorbed can be multiplied.     

Formation of chiral surface with enantiomeric tartaric acid on gemini-structured self-assembled monolayers

Chiral surfaces were prepared by L, D, and Meso-tartaric acids (TAs) adsorbed on gemini-structured self-assembled monolayers (SAMs) composed of ethylenebis [(12-mercaptododecyl) dimethyl ammonium bromide] (HS-gQA-SH). The formation and structure of the chiral surfaces were characterized by surface plasmon resonance spectroscopy (SPR) and Fourier transform infrared-reflection adsorption spectroscopy (FTIR-RAS). The thickness of enantiomeric TA layers on the HS-gQA-SH SAM was estimated to be c.a. 5-6 angstroms regardless of their chirality, in good agreement with the height of TA molecules anchoring on the surface with two COOH groups. All the TAs on the HS-gQA-SH SAM exhibit the same ionization state independent of their chirality in their vibration bands of carboxylic groups. We attempted a second-layer adsorption of the enantiomeric TAs on L-TA monomolecular layer (L-TA SAM) precomposed on the HS-gQA-SH. A strong affinity between first and second TA layers resulted in the film growth when their chirality is identical (i.e., L-TA on L-TA SAM). We found the structure of second L-TA layer was completely different from that of the first layer, where a crystalline-like L-TA phase was found as a predominant component. Our results imply a preferential crystalline growth of chiral molecules on the same chiral surface, which may lead to a work for optical resolution into two enantiomers at a solid-liquid interface.     

Surface mass spectrometry of biotinylated self-assembled monolayers

Biotin and biotinylated self-assembled monolayers (SAMs) on gold have been investigated using time-of-flight secondary ion mass spectrometry, direct laser desorption, laser desorption with 193 nm photoionization of ion- and laser-desorbed species, and laser desorption with vacuum ultraviolet (VUV, 118 nm) photoionization. Our results indicate that direct laser desorption and laser desorption combined with 193 nm multiphoton ionization can detect a chromophoric molecule like biotin that is covalently bound to a SAM. However, secondary ion mass spectra were dominated by fragmentation, and ion desorption/193 nm photoionization detected no species related to biotin. The dominant features of the laser desorption/VUV mass spectra were neat and Au-complexed dimers of intact and fragmented biotinylated SAM molecules. Multiphoton and single-photon ionization of laser-desorbed neutrals from biotinylated SAMs both led to the production of ions useful for chemical analysis of the monolayer. Multiphoton ionization with ultraviolet radiation was experimentally less challenging but required a chromophore for ionization and resulted in significant fragmentation of the adsorbate. Single-photon ionization with VUV radiation was experimentally more challenging but did not require a chromophore and led to less fragmentation. X-ray photoelectron spectra indicated that the biotinylated SAM formed a disordered, 40-60 ? thick monolayer on Au. Additionally, projection photolithography with a Schwarzschild microscope was used to pattern the biotinylated SAM surface and laser desorption/photoionization was used to detect biotinylated adsorbates from the ～10 μm sized pattern.     

Surface plasmon fluorescence measurements of human chorionic gonadotrophin: role of antibody orientation in obtaining enhanced sensitivity and limit of detection

This paper describes the determination of limits of detection (LODs) of interactions between an antigen, human chorionic gonadotrophin (hCG), and antibodies, anti-alpha-hCG and anti-beta-hCG, using a sandwich assay by surface plasmon field-enhanced fluorescence spectroscopy (SPFS). Randomly biotinylated antibodies were adsorbed onto a structured self-assembled monolayer (SAM)-streptavidin matrix, tethered to gold via a SAM consisting of biotinylated thiol molecules interspersed with hydroxyalkanethiol molecules. The influence of the concentration of biotinylated thiol on the binding of biotinylated antibody and its functionality, in terms of its ability to bind to the hCG antigen, was studied. This allowed determination of the optimum biotin-thiol mole fraction in the mixed thiol solution and consequently in the SAM, to maximize binding of hCG of the streptavidin-bound antibody. SPFS studies of the binding of a secondary fluorescently labeled antibody to hCG immobilized on the optimized SAM-streptavidin-antibody layer showed that a LOD of hCG of 2 mIU mL(-1) (4 x 10(-12) mol L(-1)) could be realized. The system was further optimized by using a more oriented and organized surface by adsorbing monobiotinylated Fab-hCG in place of the whole antibody. A LOD of 0.3 mIU mL(-1) (6 x 10(-13) mol L(-1)) was achieved for this system. This work illustrates the importance of antibody orientation, both on the planar surface and in terms of position of binding site, in maximizing sensor sensitivity.     

Gold nanoparticle-enhanced secondary ion mass spectrometry imaging of peptides on self-assembled monolayers

We demonstrate the use of gold nanoparticles (AuNPs) to enhance the secondary ion emission of peptides in time-of-flight secondary ion mass spectrometry (TOF-SIMS). The signal intensity of peptides adsorbed onto AuNPs was significantly increased when compared to that of self-assembled monolayers (SAMs). This gold nanoparticle-enhanced SIMS, termed NE-SIMS, enabled the sensitive detection of subtle modifications of peptides, such as phosphorylation. From a quantitative analysis of the amounts of adsorbed peptides and AuNPs on SAMs using quartz crystal microbalance and surface plasmon resonance spectroscopy, the ratio of peptide molecule to AuNP on amine-SAMs was revealed to be 18-19:1. When considering the ratio of peptide to matrix (1:10(3)-10(6)) employed in a matrix-enhanced SIMS, the use of AuNPs gave rise to a significantly increased secondary ion emission of peptides. Peptides were adsorbed onto patterned AuNPs on SAMs using a microfluidic system, and well-contrasted molecular ion images were obtained. NE-SIMS is expected to be applied to a chip-based analysis of modification of biomolecules in a label-free manner.     

Mass spectrometry of chlorine monolayers on solid substrates

A new method is proposed for determining chlorine traces in monolayer films adsorbed on solid substrates. The method is based on secondary-ion mass spectrometry (SIMS) with the formation of positive chlorine-containing molecular ions. In contrast to the traditional method making use of inert gases, the secondary ions are generated by bombarding the chlorine-containing substrate with a beam of methane molecules. This leads to the formation of CCl ₂ ⁺ ions with molecular masses of 82, 84, and 85 amu. These peaks are reliably observed in the form of a doublet not masked by the peaks of ions of the substrate material and the other background reaction products.     

Engineering peptide linkers for scFv immunosensors

Using A10B single-chain fragment variable (scFv) as a model system, we demonstrated that the flexibility of scFv linker engineering can be combined with the inherent quick and adaptable characters of surface coupling chemistry (e.g., electrostatic, hydrogen bonding, or covalent attachment) to attach scFv to preformed functionalized self-assembled monolayers (SAMs). Six arginines, which were separated by glycine or serine as spacer, were incorporated in the peptide linker to form a 15-mer peptide linker (RGRGRGRGRSRGGGS). The polycationic arginine peptide was engineered into the A10B scFv-RG3 to favor its adsorption at anionic charged template surface (11-mercaptoundecanoic acid (MUA) and poly(sodium 4-styrenesulfonate (PSS))). This new approach was compared with the other engineered scFv constructs. Our results demonstrated that the anionic charged SAM template facilitated the oriented immobilization of scFvs on the SAM template surface as well as reduced the possibility of protein denaturation when directly immobilized on the solid surface. A 42-fold improvement of detection limits using MUA/A10B scFv-RG3 (less than 0.2 nM experimentally determined) was achieved compared to A10B Fab antibody and a 5-fold improvement was observed compared to A10B scFv that was engineered with a cysteine in the linker sequence. Using protein A-coated gold nanoparticles, a picomolar experimental detection limit was achieved. With 20 amino acids to choose from, engineered recombinant scFv in combination with SAM technology and nanoparticle mass amplification provide an emerging strategy for the development of highly sensitive and specific scFv immunosensors.     

Simulating the effects of surface diffusion on electron beam induced deposition via a three-dimensional Monte Carlo simulation

The effects that adsorbed precursor surface diffusion has on electron beam induced deposition are explored via a three-dimensional Monte Carlo simulation. Initially the growth rate and resolution are compared for a common set of deposition conditions with a variable surface diffusion coefficient ranging from 0 to 1 × 10(-8)?cm(2)?s(-1). The growth rate and resolution are shown to both be enhanced as the growth changes from a mass transport limited regime to a reaction rate limited regime. The complex interplay between the vertical growth rate, the lateral growth rate, the interaction volume and the adsorbed and diffused precursor species are discussed. A second scenario is also simulated in which only gas diffused from a constant source at the perimeter of the simulation boundary is assumed (no gas phase adsorption). At low diffusion coefficients, the diffusing gas is consumed by secondary and backscattered electrons and experimentally observed ring-like structures are generated. At higher diffusion coefficients, the diffusion length is sufficient for the precursor atoms to diffuse to the center (and up the pillar sidewalls) to generate nanowires.     

Electrochemical behavior of two and one electron redox systems adsorbed on to micro- and mesoporous silicate materials: Influence of the channels and the cationic environment of the host materials

Electrochemical behavior of two electron redox system, phenosafranine (PS⁺) adsorbed on to micro- and mesoporous materials is investigated by cyclic voltammetry and differential pulse voltammetry using modified micro- and mesoporous host electrodes. Two redox peaks were observed when phenosafranine is adsorbed on the surface of microporous materials zeolite-Y and ZSM-5. However, only a single redox peak was observed in the modified electrode with phenosafranine encapsulated into the mesoporous material MCM-41 and when adsorbed on the external surface of silica. The observed redox peaks for the modified electrodes with zeolite-Y and ZSM-5 host are suggested to be primarily due to consecutive two electron processes. The peak separation ΔE and peak potential of phenosafranine adsorbed on zeolite-Y and ZSM-5 were found to be influenced by the pH of the electrolyte solution. The variation of the peak current in the cyclic voltammogram and differential pulse voltammetry with scan rate shows that electrodic processes are controlled by the nature of the surface of the host material. The heterogeneous electron transfer rate constants for phenosafranine adsorbed on to micro- and mesoporous materials were calculated using the Laviron model. Higher rate constant observed for the dye encapsulated into the MCM-41 indicates that the one-dimensional channel of the mesoporous material provides a more facile micro-environment for phenosafranine for the electron transfer reaction as compared to the microporous silicate materials. The stability of the modified electrode surface was investigated by multisweep cyclic voltammetry.