Fiber-optic surface plasmon resonance for vapor phase analyses

Fiber-optic sensors based on surface plasmon resonance (SPR) for direct refractive index (RI) measurements of samples with the RI between 1.00 and 1.30 are described. Most applications of SPR sensors are designed to function near the refractive index of water (1.3330 RI). The RI changes of aqueous solution (RI, ca. 1.34) can easily be monitored by silica-fiber (RI, 1.4601 at 550 nm) based SPR sensor. With regard to gas species detection, the fiber-optic SPR sensor must be modified for sensitivity to changes in refractive index near 1.0008 (i.e., RI of air). However, the silica waveguide has a prohibitively high RI for unmodified monitoring of the RI changes of gas. The silica-fiber based SPR probe design presented here is based upon the modification of the probe geometry to the ability to tune the SPR coupling wavelength/angle pair. In this study, the tapered silica-based fiber SPR sensors are shown to directly determine the RI changes of gas species and the density change of dry air.     

Characterization of oxidoreductase-redox polymer electrostatic film assembly on gold by surface plasmon resonance spectroscopy and Fourier transform infrared-external reflection spectroscopy

The electrostatic assembly of nanocomposite thin films consisting of alternating layers of an organometallic redox polymer (RP) and oxidoreductase enzymes, glucose oxidase (GOX), lactate oxidase (LOX) and pyruvate oxidase (PYX), was investigated. Multilayer nanostructures were fabricated on gold surfaces by the deposition of an anionic self-assembled monolayer of 11-mercaptoundecanoic acid, followed by the electrostatic attachment of a cationic RP, poly(vinylpyridine Os(bis-bipyridine)₂Cl-co-allylamine) (PVP-Os-AA), and anionic oxidoreductase enzymes. Surface plasmon resonance (SPR) spectroscopy, Fourier transform infrared external reflection spectroscopy (FT-IR-ERS) and electrochemistry were employed to characterize the assembly of these nanocomposite films. The surface concentration of GOX was found to be 2.4 ng/mm² for the first enzyme layer and 1.96 ng/mm² for the second enzyme layer, while values of 10.7 and 1.3 ng/mm² were obtained for PYX and LOX, respectively. The apparent affinity constant for GOX adsorption was found to be 8×10⁷ M⁻¹. FT-IR-ERS was used to verify the incorporation of GOX and its conformational stability inside of these nanocomposite thin films. An SPR instrument with a flow-through cell was modified by additions of Ag/AgCl reference and Pt counter electrodes, with the gold-coated SPR surface film serving as the working electrode. This enabled real-time observation of the assembly of sensing components and immediate, in situ electrochemical verification of substrate-dependent current upon the addition of enzyme to the multilayer structure. A glucose-dependant amperometric response with sensitivity of 0.197 μA/cm²/mM for a linear range of 1-10 mM of glucose was obtained. The SPR and FT-IR-ERS studies also showed no desorption of polymer or enzyme from the nanocomposite RP-GOX structure when stored in aqueous environment occurred over the period of 3 weeks, suggesting that decreasing substrate sensitivity with time was due to loss of enzymatic activity rather than loss of film compounds from the nanostructure.     

Biological sensing using transmission surface plasmon resonance spectroscopy

Ultrathin gold island films evaporated on transparent substrates offer promising transducers for chemical and biological sensing in the transmission surface plasmon resonance (T-SPR) mode. In the present work, the applicability of T-SPR-based systems to biosensing is demonstrated, using a well-established biological model system. Au island films were evaporated on polystyrene slides and modified with a biotinylated monolayer via a multistep surface reaction, the latter assisted by the good adhesion of metal islands to polystyrene. The biotin-derivatized Au island film was then used as a biological recognition surface for selective sensing of avidin binding, distinguishing between specific and nonspecific binding to the substrate. Transduction of the binding event into an optical signal was achieved by T-SPR spectroscopy, using plasmon intensity measurements, rather than wavelength change, for maximal sensitivity and convenience. T-SPR spectroscopy of Au island films is shown to be an effective tool for monitoring the binding of biological molecules to receptor layers on the Au surface and a promising approach to label-free optical biosensing.     

Localized surface plasmon resonance spectroscopy near molecular resonances

The peak location of the localized surface plasmon resonance (LSPR) of noble metal nanoparticles is highly dependent upon the refractive index of the nanoparticles' surrounding environment. In this study, new phenomena are revealed by exploring the influence of interacting molecular resonances and nanoparticle resonances. The LSPR peak shift and line shape induced by a resonant molecule vary with wavelength. In most instances, the oscillatory dependence of the peak shift on wavelength tracks with the wavelength dependence of the real part of the refractive index, as determined by a Kramers-Kronig transformation of the molecular resonance absorption spectrum. A quantitative assessment of this shift based on discrete dipole approximation calculations shows that the Kramers-Kronig index must be scaled in order to match experiment.     

Anisotropic light absorption by localized surface plasmon resonance in a thin film of gold nanoparticles studied by visible multiple-angle incidence resolution spectrometry

Anisotropic light absorptions via localized surface plasmon resonance in a gold-evaporated film parallel (in-plane; IP) and perpendicular (out-of-plane; OP) to the film surface are studied using visible multiple-angle incidence resolution spectrometry (Vis-MAIRS). When the thin film was aged for eighteen days, the time-dependent Vis-MAIRS IP spectra exhibited significantly different variation from that of the OP spectra: the IP spectra exhibited a large shift to the shorter wavelength side, whereas the OP spectra were explained by a linear combination of three-constituent spectra. The surface topographical analysis of the film revealed that a continuous film coalesced to form aggregates of metal particles. The intrinsic difference between the IP and OP spectra was readily elucidated by considering the surface-parallel and -perpendicular dipoles interaction depending on the topographical changes, which was confirmed by performing spectral simulation using metal particle array models.     

Modeling protein binding and elution over a chromatographic surface probed by surface plasmon resonance

Surface plasmon resonance (SPR) spectroscopy is used as a scaled-down, analytical, pseudo-chromatography tool for analyzing protein binding and elution over an ion-exchange surface under cyclic sorption conditions. A micrometric-scale adsorption surface was produced by immobilizing a typical ion exchange ligand – diethylaminoethyl (DEAE) – onto commercially available planar gold sensor chip surfaces pre-derivatized with a self-assembled monolayer of 11-mercaptoundecanoic acid with known density. An explicit mathematical formulation is provided for the deconvolution and interpretation of the SPR sensorgrams. An adsorption rate model is proposed to describe the SPR sensorgrams for bovine serum albumin, used here as model protein, when the DEAE surface is subjected to a cyclic series of binding and elution steps. Overall, we demonstrate that the adsorption rate model is capable of quantitatively describing BSA binding and elution for protein titers from dilute conditions up to overloaded conditions and a broad range of salt concentrations.     

Recent progress in the studies of electrochemical interfaces by surface plasmon resonance spectroscopy and microscopy

Surface plasmon resonance (SPR) is a label-free spectroscopic technique that is highly sensitive to various surface reactions. Incorporating SPR into electrochemical measurements has emerged as a powerful method to study both faradaic and non-faradaic processes. SPR microscopy (SPRM) integrates an optical microscope into SPR detection, which further offers high throughput detection and spatially resolved information at an electrode surface and thus, has attracted attention especially in single entity electrochemical studies. In this review, the progress in the studies of electrochemical interfaces by SPR and SPRM during the past two years will be discussed.     

Layer-by-layer composite films of polyaniline and vanadium pentoxide sol gels studied by electrochemical surface plasmon spectroscopy

Electroactive multilayer films of polyaniline (PAni) and vanadium pentoxide (V₂O₅) sol-gels were prepared on gold surfaces by means of the layer-by-layer (LBL) method. The stepwise buildup and the optoelectrochemical behaviour of the composite films were investigated in-situ by electrochemical surface plasmon spectroscopy (ESPS). A thickness increment of 2.3 nm per deposited bilayer was obtained by modelling the surface plasmon spectra. The composite multilayer films display charging-discharging characteristics similar to a bare vanadium pentoxide xerogel network along with an optoelectrochemical response which is dominated by the conducting polymer component. Electrochemical impedance spectroscopy (EIS) experiments confirm the findings from ESPS with regard to the presence of different conductive states of the composite.     

Characterization of pore formation by streptolysin O on supported lipid membranes by impedance spectroscopy and surface plasmon resonance spectroscopy

We report the study of the interactions of bacterial toxin streptolysin O (SLO) and cholesterol-containing membranes using electrochemical impedance and surface plasmon resonance (SPR) spectroscopy at low hemolytic units on a novel supported membrane interface. The detailed understanding of the process will aid significantly the construction of nanoscale transport channels for biosensing applications. Cholesterol-containing egg PC vesicles, pristine and incubated with SLO toxin, were fused onto a hexyl thioctate (HT)-modified gold substrate. The charge-transfer resistance of the resulting lipid membrane, which is related to the formation of the transmembrane pores, is measured with the aid of an electroactive probe. Impedance spectra were collected over a range of 0.1-100 kHz, and the obtained complex resistance was fit to an equivalent circuit. The charge-transfer resistance decreases for increasing SLO concentration, following a first-order exponential decay. The two-part membrane interface was further characterized with SPR spectroscopy. For the hexyl thioctate support layer, an equivalent monolayer thickness of 1.3 nm was determined. This value suggests a loosely packed structure of the monolayer on gold, presenting an ideal platform for permeability studies. A comparative study on the fusion behavior of vesicles with and without SLO induced pores revealed no substantial difference for the two systems, indicating that the pore formation has no adverse effect on the integrity of the vesicles. The resulting lipid membrane thickness from pre-perforated lipids was found to be 3.2 nm, suggesting that one leaflet is knocked off during the fusion process and a hybrid membrane is formed. A slightly higher thickness value of 3.4 nm was obtained for membranes from non-perforated vesicles. Deposition of lipids and subsequent incubation with SLO, as monitored by SPR, shows that the HT surface chemistry allows partial insertion of the toxin into the membrane, indicating unique properties as compared to the previously explored long-chain alkylthiols.     

Detection of adrenaline on poly(3-aminobenzylamine) ultrathin film by electrochemical-surface plasmon resonance spectroscopy

In this Article, we present a novel method to detect adrenaline on poly(3-aminobenzylamine) (PABA) ultrathin films by electrochemical-surface plasmon resonance (EC-SPR) spectroscopy. We prepared a PABA film, which specifically reacts with adrenaline, on a gold electrode by electropolymerization of 3-aminobenzylamine. The specific reaction of benzylamine within the PABA structure with adrenaline was studied by XPS, UV-vis spectroscopy, and EC-SPR techniques. Adrenaline was detected in real time by EC-SPR spectroscopy, which provides simultaneous monitoring of both optical SPR reflectivity and electrochemical current responses upon injecting adrenaline into the PABA thin film. The number of changes in both current and SPR reflectivity on the injection of adrenaline exhibited the linear relation to the concentration, and the detection limit was 100 pM. The responses were distinctive to those for uric acid and ascorbic acid, which are major interferences of adrenaline.     

Interaction of proflavine with DNA studied by colloid surface enhanced resonance Raman spectroscopy

The interaction of the mutagenic highly fluourescing proflavine (3,6-diaminoacridine: PF) dye with calf thymus DNA has been studied by Surface Enhanced Resonance Raman Scattering (SERRS). Since the Ag-colloids almost completely quenche the strong fluorescence it is possible to obtain excellent vibrational spectra in a wide frequency range providing valuable information about the intercalation. The intercalation does not affect the vibrational frequencies of the proflavine dye. On the other hand, intensity changes are observed in some of the ring- and NH₂-modes of proflavine upon intercalation. This Raman hypochromism is characteristic for ring stacking interactions and in the SERRS spetroscopy for an additional effects of the dye orientation to the surface.     

Detection of transglucosidase-catalyzed polysaccharide synthesis on a surface in real time using surface plasmon resonance spectroscopy

Biological systems that involve enzyme catalysis at surfaces, particularly strategically important ones that involve insoluble substrates/products such as the cell wall and the starch granule, require analyses beyond classical solution state enzymology. Using a model system, we have demonstrated the real-time measurement of transglucosidase activity on a surface using surface plasmon resonance (SPR) spectroscopy. We monitored the extension of a (partially carboxymethylated) dextran surface with alternansucrase and sucrose as a glycosyl donor. Conditions were used where surface polymer synthesis rates were a function of enzyme concentration and proportional to the extent of enzyme binding to the surface. A method to determine the turnover number of the enzyme on the surface was also developed. The presence of a new amorphous polysaccharide was observed optically, detected by lectin binding and imaged by atomic force microscopy. This surface method will have utility in a wide range of carbohydrate enzyme systems including screens.     

Ultrathin molecularly imprinted polymer sensors employing enhanced transmission surface plasmon resonance spectroscopy

An ultrathin novel nanosensor (31.5 +/- 4.1 nm thick in the absence of analytes), employing a molecularly imprinted polymer as a recognition element for cholesterol and gold nanoparticle enhanced transmission surface plasmon resonance spectroscopy for detection, was constructed.     

Determination of the surface pK of carboxylic- and amine-terminated alkanethiols using surface plasmon resonance spectroscopy

When using self-assembled monolayers (SAMs) with ionizable functional groups, such as COOH and NH2, the dissociation constant (pKd) of the surface is an important property to know, since it defines the charge density of the surface for a given bulk solution pH. In this study, we developed a method using surface plasmon resonance (SPR) spectroscopy for the direct measurement of the pKd of a SAM surface by combining the ability of SPR to detect the change in mass concentration close to a surface and the shift in ion concentration over the surface as a function of surface charge density. This method was then applied to measure the pKd values of both COOH- and NH2-functionalized SAM surfaces using solutions of CsCl and NaBr salts, respectively, which provided pKd values of 7.4 and 6.5, respectively, based on the bulk solution pH. An analytical study was also performed to theoretically predict the shape of the SPR plots by calculating the excess mass of salt ions over a surface as a function of the difference between the solution pH and surface pKd. The analytical relationships show that the state of surface charge also influences the local hydrogen ion concentration, thus resulting in a substantial local shift in pH at the surface compared to the bulk solution as a function of the difference between the bulk solution pH and the pKd of the surface.     

Ammonia sorption by Dawson acid studied by IR spectroscopy and microbalance

Using in-situ microbalance and infrared spectroscopy techniques the double ammonia proton complexation was traced. The results confirm the formation of N₂H₇⁺ dimer in solid Dawson acid H₆P₂W₁₈O₆₂, previously reported only for N₂H₇I and for (N₂H₇)₄SiW₁₂O₄₀. The formation of such dimers was evidenced by the microbalance results, the molar ratio of ammonia to proton was measured as 2:1 at 10.7 kPa and 298 K. The formation of NH₄⁺ monomer (band at 1410 cm⁻¹) and N₂H₇⁺ dimer (1460 cm⁻¹) was revealed by IR spectroscopy. Enthalpy of ammonia sorption on Dawson structure was calculated to be −127.9 kJ mol⁻¹, indicating the lower acid strength of Dawson-type compared to that of the Keggin-type heteropolyacids, like H₄SiW₁₂O₄₀.     

A novel polymerization of ultrathin sensitive imprinted film on surface plasmon resonance sensor

A new surface-initiated polymerization based on pasting the initiator on a sensor chip surface was applied to prepare a malachite green (MG) imprinted ultrathin film on a surface plasmon resonance (SPR) sensor. First, the initiator (2,2-azoisobutyronitrile) was pasted on the gold surface using polyvinyl chloride (PVC). The initiator-covered gold chip was then soaked in a pre-polymerization solution prepared by dissolving methacrylic acid (functional monomer), ethylene glycol dimethacrylate (cross-linker), and MG (template) in DMSO in a weighing bottle. Finally, the weighing bottle was placed in a vacuum oven and thermal-initiated polymerization was conducted at 60 °C for 16 h. This method was simple and time-saving compared with the commonly used surface-initiated polymerization. More importantly, the molecularly imprinted polymer (MIP) film prepared using this method was much thicker than that of commonly used methods; the adsorption quantity was also much larger. The MIP modified SPR sensor showed high sensitivity and selectivity as well as good stability in detecting MG. The results suggest that the ultrathin MIP film prepared using the new method in this study is suitable to serve as the recognition element of the SPR sensor.     

Lectin recognition of a new SOD mimic bioconjugate studied with surface plasmon resonance imaging

Surface plasmon resonance imaging is used to demonstrate the recognition by the Ricinus communis agglutinin of a new SOD mimic, a bioconjugate of the manganese(II) complex of 1,4,7,10,13-pentaazacyclopentadecane with galactose.     

Molecular structure of polymeric materials and sorption of water vapor

Methods for calculating the coefficients of the thermal equation of water vapor sorption by polymeric materials were developed. These methods enable calculation of the equilibrium amounts of sorption in a wide temperature range from the cohesion energy of a polymer.