Marc D. Porter

The electrochemical desorption of n-alkanethiol monolayers from polycrystalline Au and Ag electrodes

Monolayers formed at Au and Ag surfaces by the spontaneous adsorption of n-alkanethiols were characterized voltammetrically to examine the chemistry of the bound thiol head group. Electrode reactions that correspond to the oxidative- and reductive-desorption of the adsorbed n-alkanethiol monolayer are reported for the first time. Analysis of the data indicates that upon adsorption at both Au and Ag surfaces the hydrogen of the thiol group is lost and the sulfur atom is oxidized by one electron. Based on the charge required for the reductive-desorption of the monolayer, the surface coverage of the oxidized n-alkanethiol species is 9.3×10−10 mol/cm2 and 7.0 ×10−10 mol/cm2 on Au and Ag, respectively. The value of the surface coverage at Au is slightly greater than that expected for a closest-packed overlayer commensurate with a Au(111) substrate, whereas the value at Ag is somewhat less than that expected for layers commensurate at any of the low index Ag surface planes. The low apparent coverage observed for Ag electrodes is attributed to a portion of the monolayer being electroinactive at accessible applied voltages. As a probe of the mechanism for monolayer formation, films that were deposited on Ag and Au surfaces from solutions containing sodium n-octadecanethiolate were also characterized. Examination of the resulting layers with infrared reflection spectroscopy and optical ellipsometry indicates that only at Ag substrates were the films deposited from the thiolate solution complete and structurally similar to monolayers formed from n-octadecanethiol solutions. Based on this observation, it is postulated that the thiol hydrogen of the n-alkanethiol molecule participates in the reduction reaction that is concomitant to the thiol oxidation during adsorption on Au, whereas adsorption on Ag proceeds through the reduction of the Ag(I) surface species of the native oxide. A model for the electric double layer at the monolayer-coated-electrode/solution interface is also suggested based on the observed chain-length dependence of both the capacitance and the potential for the reductive desorption.

SERS as a bioassay platform: fundamentals, design, and applications

Bioanalytical science is experiencing a period of unprecedented growth. Drivers behind this growth include the need to detect markers central to human and veterinary diagnostics at ever-lower levels and greater speeds. A set of parallel arguments applies to pathogens with respect to bioterrorism prevention and food and water safety. This tutorial review outlines our recent explorations on the use of surface enhanced Raman scattering (SERS) for detection of proteins, viruses, and microorganisms in heterogeneous immunoassays. It will detail the design and fabrication of the assay platform, including the capture substrate and nanoparticle-based labels. The latter, which is the cornerstone of our strategy, relies on the construction of gold nanoparticles modified with both an intrinsically strong Raman scatterer and an antibody. This labelling motif, referred to as extrinsic Raman labels (ERLs), takes advantage of the well-established signal enhancement of scatterers when coated on nanometre-sized gold particles, whereas the antibody imparts antigenic specificity. We will also examine the role of plasmon coupling between the ERLs and capture substrate, and challenges related to particle stability, nonspecific adsorption, and assay speed.

Electrochemical and X-ray photoelectron spectroscopic evidence for differences in the binding sites of alkanethiolate monolayers chemisorbed at gold

Abstract Voltammetric data for the reductive desorption of alkanethiolate monolayers from gold electrodes reveal the presence of different adsorbate binding sites. This conclusion is based on a series of characterizations of monolayers formed from butanethiol and octanethiol at annealed mica-supported gold (Au/mica) and at gold supported by both chromium-primed silicon (Au/Si) and glass (Au/glass). At Au/mica, a single wave is observed for the desorption induced by a linear voltage scan. The voltammetry at Au/Si and Au/glass is, however, comparatively more complex, as reflected by the presence of at least one additional desorption wave. Results from structural characterizations (IR and X-ray photoelectron spectroscopies and electrochemical capacitance measurements) of the monolayers formed at each type of substrate and from morphological assessments (X-ray diffraction, underpotential metal deposition, and scanning tunneling microscopy) of the underlying substrates reveal that the differences in the voltammetry originate from differences in the microscopic roughnesses of the two types of substrates. Importantly, the morphological data indicate that the bulk of the gold films on all three supports has a pronounced (111) crystallinity, that the surface at Au/Mica is strongly (111) terraced, but that the surfaces at Au/Si and Au/glass have a much higher density of steps. The voltammetry at Au/mica, therefore, represents desorption from sites at Au(111) terraces. On the contrary, the voltammetry at Au/glass or Au/Si reflects desorption from both terrace and step sites with the adsorbates at step sites bound by as much as 25 kJ mol −1 more strongly than at terrace sites. The implications of our findings for descriptions of the nucleation and growth of these systems are discussed.

Detection of the Potential Pancreatic Cancer Marker MUC4 in Serum Using Surface-Enhanced Raman Scattering

Pancreatic cancer (PC) is one of the most lethal malignancies. It has a 5-year survival rate of only 6%, owing in part to the lack of a reliable tumor marker for early diagnosis. Recent research has shown that the mucin protein MUC4 is aberrantly expressed in pancreatic adenocarcinoma cell lines and tissues but is undetectable in normal pancreas and chronic pancreatitis. Thus, the level of MUC4 in patient sera has the potential to function as a diagnostic and prognostic marker for PC. However, the measurement of MUC4 in sera using conventional test platforms (e.g., enzyme linked immunosorbent assay (ELISA) and radioimmunoassay (RIA)) has been unsuccessful. This has prevented the assessment of the utility of this protein as a possible PC marker in sera. In addressing this obstacle, the work herein examines the potential to create a simple diagnostic test for MUC4 through the development of a surface-enhanced Raman scattering (SERS)-based immunoassay, which was then used to demonstrate the first ever detection of MUC4 in cancer patient serum samples. Importantly, these measurements showed that sera from patients with PC produced a significantly higher SERS response for MUC4 compared to sera from healthy individuals and from patients with benign diseases. These results indicate that a SERS-based immunoassay can monitor MUC4 levels in patient sera, representing a much needed first step toward assessing the potential of this protein to serve as a serum marker for the early stage diagnosis of PC. This paper details these and other findings (i.e., the detection of the mucin protein CA19-9), which demonstrate that our SERS assay outperforms conventional assays (i.e., RIA and ELISA) with respect to limits of detection, readout time, and required sample volume.

Fine structure in the voltammetric desorption curves of alkanethiolate monolayers chemisorbed at gold

This paper presents the preliminary results from a study of the chain length dependence of the reductive desorption and oxidative re-deposition processes of alkanethiolate monolayers on Au(111) electrodes. In contrast to the more extensively studied short chain length monolayers, we report a previously undetected fine structure in the voltammetric waves for monolayers composed of alkyl chains of more than 10 carbons. The short chain systems in general exhibit a single voltammetric wave for the desorption and re-deposition processes. In contrast, multiple voltammetric waves separated by 20 to 100 mV are found for both the desorption and the re-deposition of the long chain systems. The origin of these results is examined in terms of possible differences in the adlayer domain structures and in the binding modes between sulfur and gold.

Mixed monolayers on gold nanoparticle labels for multiplexed surface-enhanced Raman scattering based immunoassays.

This paper describes a new approach, based on self-assembled mixed monolayers, to the design and preparation of extrinsic Raman labels (ERLs). ERLs function as spectroscopic tags for the readout of sandwich-type immunoassays using surface-enhanced Raman scattering (SERS). They are created by coating gold nanoparticles with Raman reporter molecules and antibodies specific for the target analyte. Mixed-monolayer ERLs are formed by covering gold nanoparticles with a mixture of two different thiolates. One thiolate serves to covalently bind antibodies to the particles, imparting biospecificity to the ERLs, while the other thiolate produces a strong Raman signal. Mixed-monolayer ERLs can be prepared in a few relatively simple steps using readily available materials. The SERS intensity of each type of ERL can be tuned to match other ERLs by adjusting the mixed monolayer composition, greatly facilitating the generation of sets of ERLs for multiplexed applications. The work herein not only describes the new pathway for ERL production, but also demonstrates the simultaneous qualitative and quantitative multiplexed detection using a set of four mixed-monolayer ERLs.

Formation and structure of a spontaneously adsorbed monolayer of arachidic on silver

Abstract A stable monolayer assembly can be formed on silver by the spontaneous adsorption (SA) of arachidic acid (CH 3 (CH 2 ) 18 COOH) from hexadecane. IR spectroscopy and optical ellipsometry indicate that the structure is similar to the Langmuir-Blodgett (LB) multilayer assembly of cadmium arachidate on silver. This is a definitive example of the correspondence between the structure of LB and SA assemblies.

Model for detection of immobilized superparamagnetic nanosphere assay labels using giant magnetoresistive sensors

Commercially available superparamagnetic nanospheres are commonly used in a wide range of biological applications, particularly in magnetically assisted separations. A new and potentially significant technology involves the use of these particles as labels in magnetoresistive assay applications. In these assays, magnetic bead labels are used like fluorescent labels except that the beads are excited and detected with magnetic fields rather than with photons. A major advantage of this technique is that the means for excitation and detection are easily integrable on a silicon circuit. A preliminary study of this technique demonstrated its basic feasibility, and projected a sensitivity of better than 10−12 molar [Baselt et al., Biosensors Bioelectronic 13, 731 (1998)]. In this article we examine the theoretical signal to noise ratio of this type of assay for the special case of a single magnetic bead being detected by a single giant magnetoresistive (GMR) detector. Assuming experimentally observed and reasona...

Media ion composition controls regulatory and virulence response of Salmonella in spaceflight.

The spaceflight environment is relevant to conditions encountered by pathogens during the course of infection and induces novel changes in microbial pathogenesis not observed using conventional methods. It is unclear how microbial cells sense spaceflight-associated changes to their growth environment and orchestrate corresponding changes in molecular and physiological phenotypes relevant to the infection process. Here we report that spaceflight-induced increases in Salmonella virulence are regulated by media ion composition, and that phosphate ion is sufficient to alter related pathogenesis responses in a spaceflight analogue model. Using whole genome microarray and proteomic analyses from two independent Space Shuttle missions, we identified evolutionarily conserved molecular pathways in Salmonella that respond to spaceflight under all media compositions tested. Identification of conserved regulatory paradigms opens new avenues to control microbial responses during the infection process and holds promise to provide an improved understanding of human health and disease on Earth.

Voltammetric reductive desorption characteristics of alkanethiolate monolayers at single crystal Au(111) and (110) electrode surfaces

Abstract This note describes the results of a comparative study of the reductive desorption characteristics of alkanethiolate monolayers at single crystal Au(111) and Au(110) electrodes. The voltammetric data reveal that the reductive desorption potential is dependent on the surface crystallinity of the underlying gold substrate. The observed difference in potential between these two substrates is indicative of a difference in binding strength of the monolayer at different binding sites in which the gold-sulfur bonding is stronger at the Au(110) than at the Au(111) single crystal. Spectroscopic data (X-ray photoelectron and infrared reflection spectroscopies) provide additional comparisons for the monolayer structures at the two substrates. These findings substantiate our recent study on the voltammetric differences between the atomically smooth surfaces of annealed Au(111) films and the micro-topographically rougher as-evaporated Au(111) films.