Identification of Nucleophilic and Electrophilic Binding Sites on Gold

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Identification of Nucleophilic and Electrophilic Binding Sites on Gold Surface-Enhanced Raman Substrates Ashish Tripathi, Erik D. Emmons, Augustus W. Fountain, Steven D. Christesen, and Jason A. Guicheteau J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b06717 • Publication Date (Web): 21 Sep 2016 Downloaded from http://pubs.acs.org on September 26, 2016

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The Journal of Physical Chemistry

Identification of Nucleophilic and Electrophilic Binding Sites on Gold Surface-Enhanced Raman Substrates Ashish Tripathi, Erik D. Emmons, Augustus W. Fountain III, Steven D. Christesen, and Jason A. Guicheteau* Research and Technology Directorate, Edgewood Chemical Biological Center, Aberdeen Proving Ground, MD 21010-5424 Email: [email protected]

ABSTRACT We have studied the binding of several nitrogen containing aromatic chemicals to nanostructured gold surfaces from aqueous media in order to better understand the equilibrium thermodynamics and nature of the binding. These studies have revealed that a complex equilibrium state exists between the molecules and the gold surface that cannot be explained by a simple Langmuir isotherm. Instead, the concentration dependence of the SERS response suggests two equilibrium states. We present evidence that these two equilibrium states correspond to nucleophilic and electrophilic adsorption sites, both of which can be filled depending on the state of the analyte molecule.

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1. INTRODUCTION We have studied the binding of various probe molecules to SERS substrates in an effort to understand the factors that influence the strength of that binding and to develop predictive models of SERS behavior.1-2

To this end, kinetics, reaction mechanisms, and equilibrium

behaviors have been examined. Langmuir originally described equilibrium isotherms in the early twentieth century and mentioned several possible results dependent on the type of binding sites available.3-4 His observations of the simplest equilibrium are when a molecule adsorbs on a surface to a single type of site without changing its orientation. When additional types of binding sites or molecular orientations are included, Langmuir derived additional equations describing the binding behavior including the equilibria change. While fitting these models to experimental data can imply one binding condition over another, additional experimental evidence is often necessary for confirmation. In a previously published study, we observed a dual-Langmuir type equilibrium isotherm for the binding of a series of nitrogen-containing aromatic molecules (azarenes and aromatic amines) on gold substrates.2 As a result, two equilibrium constants and associated Gibbs free energies of binding were determined for each analyte. Although a clear deviation from simple Langmuir behavior was experimentally observed, it was difficult to empirically determine the mechanism leading to this behavior. Two competing mechanisms were proposed in which there are either one molecular orientation and two types of binding sites or two types of molecular orientations binding at a single type of site.3-6 In the dual site hypothesis, molecules independently bind at the different types of sites with different equilibrium constants and Gibbs free energies of binding. In the dual arrangement hypothesis, the molecules all bind at the same type of site, but

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with different molecular orientations that may depend on intramolecular interactions (for example, by tilting their axes more perpendicular to the surface to allow for denser packing). In this study we have collected additional experimental data that favors the dual-site hypothesis for the probe molecules examined.

This evidence was found by performing

equilibrium isotherm measurements at varying pH values and with SERS substrates possessing different levels of surface roughness. Additional insight into the mechanism was discovered by partially pre-coating the surfaces with thiophenol before measuring the isotherms in order to glean more information on the nature of the sites.

Other groups have also examined the

mechanisms of binding of these nitrogen-containing aromatic molecules to gold and silver surfaces, particularly two azarenes, pyridine and BPE (1,2-di(4-pyridyl) ethylene).7-13 However, these studies focused heavily on the involvement of the nitrogen lone pair and aromatic π orbitals with respect to the surfaces.

This study goes beyond the previously published research by

providing additional insights into the nature of the surface-adsorbate interactions, including the possibility of molecular rearrangement and different types of binding sites.

2. EXPERIMENTAL METHODS 2.1 Materials The probe molecules used were BPE (1,2-di(4-pyridyl) ethylene), quinoline, and isoquinoline.

Solvents and solvent modifiers were water, ethanol, acetonitrile, sodium

hydroxide, and hydrochloric acid. These chemicals were all purchased from Sigma-Aldrich and used without further purification. Stock BPE solutions were prepared in ethanol at 0.1 M, and then diluted to obtain aqueous solutions at different pH values with concentrations down to 5×109

M. Ultrapure water was obtained from an in-house water purification system, and the pH was 3



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adjusted with either sodium hydroxide or hydrochloric acid.

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All of the experiments were

performed with the solutions at 23 °C. Three different types of SERS substrates were used in this study: commercially purchased Klarite (Cat. No. KLA-312, Renishaw Diagnostics Ltd., UK), silicon microstructured square grids (Cat. No. 79502, Electron Microscopy Sciences, Hatfield, PA) coated in-house with ~0.5 µm of evaporated gold (denoted in this article as GP substrates), and micro-roughened substrates that were also coated in-house with ~0.5 µm of evaporated gold (denoted here as MR substrates). The evaporated gold coatings were produced with a Denton Explorer thermal evaporation system. Klarite has been used by our group and others extensively in the past.1-2, 14-16 The other types of substrates were chosen for their surface morphologies, primarily the ratio of flat-torough areas. The silicon microstructured grids were very flat except for raised ridges in an approximately 10 µm square grid pattern. These substrates would be expected to give a SERS response primarily around the ridges. The three different types of substrates gave varying SERS intensities when excited with 785 nm laser light. Scanning-electron microscope (SEM) images of the surfaces were obtained with a Phenom Pro-X desktop SEM and a JEOL JSM-7401F field emission scanning electron microscope.

2.2 Raman microscopy The Raman spectroscopy measurements were performed with a JASCO NRS-3200 Raman micro-spectrometer equipped with a thermoelectrically cooled Andor CCD camera. Measurements for all three types of investigated substrates were interrogated with 785 nm excitation operating at ~4 mW power in order to avoid sample damage and a Semrock 785 nm notch filter to reject Rayleigh scattered light. A 10× objective was used to generate a spot size on 4


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the substrate on the order of a few microns in diameter. The spectrometer entrance slit was set at 100 µm, and a spectral resolution of approximately 8 cm-1 was obtained using a 600 grooves/mm grating. An automated stage was used to obtain 36 measurements across the face of the SERS substrate in a 6×6 grid pattern. These individual measurements were then averaged to compensate for any possible substrate inhomogeneity.

2.3 Isotherm Measurements Equilibrium isotherms were measured using the technique described in a previous study.2 Briefly, the substrates were immersed in a sufficient volume of solution for a long enough time period to ensure that equilibrium was achieved without a significant decrease in the analyte concentration in solution. During this time period the solution was continuously agitated to ensure it remained of uniform concentration.

When the substrates were ready for Raman

spectroscopy measurements, they were transferred to a small Petri dish along with a small quantity of the solution in which they had been immersed. During the transfer the substrates remained wet in order to maintain the equilibrium conditions. The Petri dish was maintained at room temperature, 23°C, during the Raman measurements using a temperature controlled system, and was continuously stirred with a magnetic stirrer. The previously mentioned 6×6 grid of points was then measured with the Raman microscope to obtain an average spectrum for that concentration. Additional experimental details can be found in previous articles.1-2

3. RESULTS AND DISCUSSION

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3.1 Complex Langmuir Isotherms SERS based adsorption isotherms for five azarenes and four anilines with Klarite as the substrate and deionized water as the solvent (near neutral pH) were acquired in a previous study.2 Three examples of the azarene response on Klarite are shown in Figure 1.

a

b

c

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Figure 1: Concentration-dependent SERS response of three azarenes: (a) BPE, (b) isoquinoline, and (c) quinoline, on Klarite.

Traditionally, concentration dependent isotherms observed in SERS have been fit to a single Langmuir isotherm (SLI), as given in Equation 1:17-18

 KC s  ICs = Imax    1 + KC s 

(1)

where K is the equilibrium constant, CS is the analyte concentration, IC s is the peak area of a spectral feature at the concentration CS, and Imax is the maximum observed peak area of that spectral feature. As seen in Figure 1, the azarene data are fit poorly (r2

Identification of Nucleophilic and Electrophilic Binding Sites on Gold

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