Probing Interfacial Organization in Surface Monolayers Using Tethered Pyrene. 1. Structural Mediation of Electron and Proton Access to Adsorbates

Electrochemical Investigation and Molecular Docking Techniques on the Interaction of Acridinedione Dyes with Water-Soluble Nonfluorophoric Simple Amino Acids

Electrochemical studies of resorcinol-based acridinedione (AD) dyes with nonfluorophoric simple amino acids, glycine, alanine, and valine, were carried out in water. AD probes are classified into photoinduced electron transfer (PET) and non-PET-based dyes, wherein the electrochemical properties and photophysical and photochemical behavior vary significantly based on the nature of substituent groups and the nature of the solute. The oxidation potential of PET dye (ADR1) to that of non-PET-based dye (ADR2) differs significantly such that the addition of amino acids results in a shift of the oxidation peak to a less positive potential and the reduction peak to a lesser negative potential. The extent of shift of oxidation and reduction potential in PET dye is more pronounced than that of non-PET dye on the addition of valine rather than glycine. The variation in the shift is attributed to the presence of an electron-donating moiety (OCH₃) group in the ninth position of ADR1 dye. Consequently, the quenching of fluorescence is observed in ADR2 with non fluorophoric amino acids that are authenticated by the shift of the anodic and cathodic peaks toward a lesser positive potential. Molecular docking (MD) studies of PET and non-PET dye with amino acids portray that neither hydrophobic interactions nor electrostatic or weak interactions such as van der Waals and pi-pi interactions govern the electrochemical nature of dye on the addition of amino acids. Furthermore, the formation of a conventional hydrogen bond between dye and amino acid is established from MD studies. The existence of dye-water-amino acid competitive hydrogen-bonding interactions is presumably well-oriented throughout the aqueous phase as observed through photophysical studies which support our electrochemical investigation.

Fluorescent-Labeled Octasilsesquioxane Nanohybrids as Potential Materials for Latent Fingerprinting Detection

The recent demand for fluorescent-labeled materials (FLMs) in forensic security concepts such as latent fingerprints (LFs) that encode information for anti-counterfeiting and encryption of confidential data makes necessary the development of building new and innovative materials. Here, novel FLMs based on polyhedral oligomeric silsesquioxanes (POSS) functionalized with fluorophores via "click" reactions have been successfully synthesized and fully characterized. A comprehensive study of their photophysical properties has displayed large Stokes's shift together with good photostability in all cases, fulfilling the fundamental requisites for any legible LF detection on various surfaces. The excellent performance of the hetero-bifunctional FLM in the visualization of LF is emphasized by their legibility, selectivity, sensitivity and temporal photostability. In this study, development mechanisms have been proposed and the overall concept constitute a novel approach for vis-à-vis forensic investigations to trace an individual's identity.

Intramolecular Proton and Charge Transfer of Pyrene-based trans-Stilbene Salicylic Acids Applied to Detection of Aggregated Proteins

Two analogues to the fluorescent amyloid probe 2,5-bis(4'-hydroxy-3'-carboxy-styryl)benzene (X-34) were synthesized based on the trans-stilbene pyrene scaffold (Py1SA and Py2SA). The compounds show strikingly different emission spectra when bound to preformed Aβ1-42 fibrils. This remarkable emission difference is retained when bound to amyloid fibrils of four distinct proteins, suggesting a common binding configuration for each molecule. Density functional theory calculations show that Py1SA is twisted, while Py2SA is more planar. Still, an analysis of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of the two compounds indicates that the degree of electronic coupling between the pyrene and salicylic acid (SA) moieties is larger in Py1SA than in Py2SA. Excited state intramolecular proton transfer (ESIPT) coupled-charge transfer (ICT) was observed for the anionic form in polar solvents. We conclude that ICT properties of trans-stilbene derivatives can be utilized for amyloid probe design with large changes in emission spectra and decay times from analogous chemical structures depending on the detailed physical nature of the binding site.

Functionalization of amorphous SiO₂ and 6H-SiC(0001) surfaces with benzo[ghi]perylene-1,2-dicarboxylic anhydride via an APTES linker

The successful covalent functionalization of quartz and n-type 6H-SiC with organosilanes and benzo[ghi]perylene-1,2-dicarboxylic dye is demonstrated. In particular, wet-chemically processed self-assembled layers of aminopropyltriethoxysilane (APTES) and benzo[ghi]perylene-1,2-dicarboxylic anhydride are investigated. The structural and chemical properties of these layers are studied by contact angle measurements, attenuated total reflection infrared (ATR-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The optical properties are measured by confocal microscopy. The wetting angles observed for the organic layers are α = 68° for the APTES-functionalized surface, while angles of α = 85° and 78° are determined for dye-functionalized quartz and 6H-SiC surfaces, respectively. However, not all amino groups of the APTES-functionalized surfaces react to bind dye molecules. Further dye functionalization is not uniform throughout the surface, showing different island sizes of the dye and including different chemical environments. The quartz surface exhibits a higher packing density of dyes than the 6H-SiC surface. The fluorescence lifetimes of the surface-attached dye show double exponential decays of about 1.4 and 4.2 ns, largely independent of the substrates.

Fluorescent film sensor for copper ion based on an assembled monolayer of pyrene moieties

We previously reported the construction of a family of fluorescent film sensors for organic copper salts by covalently coupling polycyclic aromatic hydrocarbons on epoxy-terminated self-assembled monolayers on glass plate surfaces. Here we investigate the sensing properties and mechanism of covalently coupling pyrene on a glass plate surface via a long flexible "Y" type spacer. X-ray photoelectron spectroscopy (XPS) and fluorescence spectra measurements demonstrate the covalent attachment of pyrene in our adlayer. Compared with those results obtained in the previous studies, this new film sensor did not show highly selectivity for organic copper salts, which can be attributed to the introduction of sulfonyl groups connecting the pyrene moieties and the spacers. The presence of sulfonyl units made the microenvironments of pyrene relatively hydrophilic and thus showed less screening effect for inorganic ions. The specificity and reversibility of the film sensor toward Cu (II) made it attractive for sensing applications.

Probing the microenvironment of surface-attached pyrene formed by a thermo-responsive oligomer

We have constructed a novel molecular assembly attached to quartz, oxidized silicon and indium-doped tin oxide coated substrates, where a tethered pyrene derivative is co-immobilized with oligo-N-isopropyl acrylamide (oligo-NIPAM). The addition of tethered oligo-NIPAM to the adlayer creates two different, temperature-dependent microenvironments for the surface-bound pyrene. X-ray photoelectron spectroscopy (XPS) and ellipsometry measurements demonstrate the covalent attachment of both oligo-NIPAM and pyrene in our adlayers. Contact angle results confirm the thermo-responsive nature of the oligo-NIPAM on the substrate surface. Steady-state fluorescence data show that the presence of oligo-NIPAM moieties reduces the extent of pyrene excimer formation and provides different environments for the chromophore at temperatures above and below the phase transition. Fluorescence lifetime decay data on surface-bound pyrene are biexponential, consistent with multiple local environments, regardless of whether tethered oligo-NIPAM is present or not. Quenching studies reveal that we can manipulate the sensing properties of this new film simply by adjusting the conformations of oligo-NIPAM.

Interrogating interfacial organization in planar bilayer structures

The field of biomimetic planar lipid membranes is finding increased importance as the need to devise sensing systems for biologically important species increases. We approach this area with an eye toward understanding how to interrogate local organization in these complex media. Our primary tools for this purpose are spectroscopy and electrochemistry, where imbedded reporter molecules serve as the information transducers. We use Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) methods to construct planar lipid membranes on hydrophilic solid substrates (Au for electrochemistry, SiO x for spectroscopy). Pyrene tethered to the substrate acts as our probe and AC voltammetry was used to evaluate its redox kinetics, showing slow, distance independent electron transfer between the pyrene moieties and the electrode for both monolayer and bilayer systems. Time-resolved fluorescence data indicate that tethered pyrene resides in a highly rigid environment and that the addition of the top lipid leaflet improves the organization of the bottom lipid leaflet. These data point to the cooperative effect of the bilayer leaflets in creating a system that is comparatively rigid on short length scales, and capable of mediating motion and accessibility of imbedded species.

Surface-confined energy transfer in mixed self-assembled monolayers

We have fabricated a set of self-assembled monolayers consisting of naphthalene and dansyl derivatives in a range of surface loading ratios for the purpose of examining excitation transport in mixed self-assembled monolayer systems. Both tethered chromophores were immobilized on an epoxide-terminated adlayer on silica via an identical spacer, where the linking chemistry produced an amide linkage. X-ray photoelectron spectroscopy (XPS), ellipsometry, and contact angle measurements were used to characterize these chromophore-containing layers. The excitation transfer behavior of these monolayers has been examined using steady-state and time-resolved fluorescence spectroscopy. Steady-state fluorescence measurements show that excitation transfer from the naphthalene to dansyl chromophores occurs, with the efficiency of excitation transport scaling with chromophore surface loading densities, as expected. The donor lifetimes decrease with increasing acceptor loading density, and the functional form of the acceptor decay was independent of the donor/acceptor ratio. Our findings are not consistent with a homogeneous adlayer, but do provide information on the structural heterogeneity that is characteristic of these interfaces.

Probing the effects of cholesterol on pyrene-functionalized interfacial adlayers

We have synthesized a novel set of pyrene-functionalized, covalently bound surface adlayers with and without cholesterol derivatives coadded to the adlayer. We have deposited these adlayers on quartz, oxidized silicon wafers, and indium-doped tin oxide coated substrates. The addition of tethered cholesterol to the adlayer creates a hydrophobic, likely disordered, microenvironment in which the surface-bound pyrene resides. X-ray photoelectron spectroscopy measurements demonstrate the covalent attachment of both cholesterol and pyrene in our adlayers. The presence of the cholesterol moieties gives rise to a reduction in film thickness, as measured ellipsometrically, and contact angle data indicate significant surface heterogeneity. Steady-state fluorescence data show that the presence of cholesterol moieties reduces the extent of pyrene excimer formation and provides a less polar environment for the chromophore. Fluorescence lifetime measurements on surface-bound pyrene were biexponential, consistent with multiple local environments, regardless of whether tethered cholesterol was present or not. Cyclic voltammetry reveals competition between the pyrene and cholesterol moieties for binding to available surface sites on the epoxide-terminated surface-binding layer we use.

Immobilization of laccase on gold, silver and indium tin oxide by zirconium–phosphonate–carboxylate (ZPC) coordination chemistry

In this paper we present a simple method allowing for stable laccase immobilization on various conducting surfaces that retains the activity of the enzyme. The strategy for laccase immobilization presented in this paper relies on Zr(4+) ion coordination chemistry that involves -COO- terminal groups present on the protein. Using a host of techniques, including surface plasmon resonance (SPR), quartz crystal microbalance (QCM) gravimetry, atomic force microscopy (AFM), surface enhanced Raman scattering (SERS), resonance Raman scattering (RR) and electrochemical techniques, we show that laccase bound to a surface coordinatively through zirconium phosphonate/carboxylate (ZPC) functionalities forms a stable enzymatic layer with the enzyme retaining its activity to a significant extent.

Probing organization and communication at layered interfaces

We have investigated the local organization intrinsic to a variety of interfacial structures, by both electrochemical and spectroscopic means. Our focus has been on the design and construction of biomimetic interfaces, where a lipid bilayer or a hybrid bilayer membrane can be bound to an interface. The goal of this work is ultimately to create an interface on a transducer surface that can support an enzyme in its active form. To this point, we have examined the extent of organization that is achievable in monolayers that will be used to bind bilayer structures to a transducer surface. Our electrochemical data point to the important role of the substrate surface in determining adlayer organization. We have also investigated the fluidity and structural heterogeneity of lipid bilayers using time-resolved and steady state fluorescence spectroscopy. Our data point to the highly interactive nature of lipid bilayer constituents, where perturbations introduced to one region have significant consequences on other regions of the bilayer. Such information is directly relevant to the existence and properties of lipid raft structures in both model and biological bilayers.