Carbon nanotube-assisted enhancement of surface plasmon resonance signal

We describe a method of amplifying the biosensing signal in surface plasmon resonance (SPR)-based immunoassays using an antibody–carbon nanotube (CNT) conjugate. As a model system, human erythropoietin (EPO) and human granulocyte macrophage colony-stimulating factor (GM–CSF) were detected by sandwich-type immunoassays using an SPR biosensor. For the amplification of the SPR signal, the CNT was conjugated with a polyclonal antibody, and then the conjugates were reacted with antibodies coupled with the target proteins. This amplification strategy increases the dynamic range of the immunoassays and enhances the detection sensitivity. The SPR immunoassays, combined with the CNT-assisted signal amplification method, provided a wide dynamic range over four orders of magnitude for both EPO and GM–CSF (0.1–1000 ng/ml). The CNT amplification method is expected to realize the detection of picogram levels and a wide dynamic detection range of multiple proteins, enabling it to offer a robust analysis tool for the development of biopharmaceutical production.     

Surface plasmon resonance based pesticide assay on a renewable biosensing surface using the reversible concanavalin A monosaccharide interaction

A competitive immunoassay based on surface plasmon resonance (SPR) for the detection of the pesticide 2,4-dichlorophenoxyacetic acid (2,4-D) is reported. The novelty of the assay is based on the regeneration of the chip surface by the reversible interaction between monosaccharide (D-glucose) and lectin (Concanavalin A). Concanavalin A-2,4-D conjugate was chemically synthesized, purified and used for binding to the SPR chip modified with covalently bound alpha-D-glucose. The interaction between anti-2,4-D antibody and the surface-bound concanavalin A-2,4-D conjugate was monitored by surface plasmon resonance and the response was used for the quantification of 2,4-D. The dynamic range of the calibration curve was between 3 and 100 ng/ml. The demonstrated principle of surface regeneration based on the reversible sugar-lectin interaction may be of more general applicability in immunoassays.     

Transmitive Spectral SPR Droplet Biosensing: Principle,Sensor Design,and Experimental Demonstration

Surface plasmons resonance (SPR) architectures based on grating coupler/disperser combination is an attractive alternative for spectral-based biochemical sensing. In this paper, we investigate theoretically and experimentally a new concept where the plasmon coupling occurs through a thin film grating and sensing occurs via the first evanescent diffraction order in transmitive mode. The surface plasmon wave excitation induces a peak in the wavelength as well as in the angular spectra of the detected first transmitted diffraction order. Accordingly, a change in SPR spectrum of the detected diffraction order can be used to quantify the amount of the target molecules immobilized on the sensor surface, and therefore, the concentration of these molecules in the analyte solution. The developed sensor architecture is dedicated to droplet biochemical sensing and appears to be especially suitable for biosensor integration and miniaturization. The presented sensor concept is perfectly suited for mass production of low-cost and reproducible SPR sensor chip for biochemical analysis. The implemented setup gives access to multichannel biosensing with the potential for efficient internal referencing essential to achieve sufficiently high reproducibility and accuracy of the measurements.     

Multi-analyte surface plasmon resonance biosensing

Surface plasmon resonance (SPR) biosensors are affinity sensing devices exploiting a special mode of electromagnetic field-surface plasmon-polariton-to detect the binding of analyte molecules from a liquid sample to biomolecular recognition elements immobilized on the surface of the sensor. In this paper, we review advances of SPR biosensor technology towards detection systems for the simultaneous detection of multiple analytes (multi-analyte detection). In addition, we report application of a recently developed multichannel SPR sensor based on spectroscopy of surface plasmons and wavelength division multiplexing of sensing channels to multi-analyte detection.     

A sensitivity comparison of optical biosensors based on four different surface plasmon resonance modes

Current surface plasmon resonance (SPR) modes based on the attenuated total reflection (ATR) method can broadly be categorized as: conventional SPR, long-range SPR (LRSPR), coupled plasmon-waveguide resonance (CPWR), and waveguide-coupled SPR (WCSPR). Although the features of optical biosensors are dependent upon their particular SPR mode, a common requirement for all biosensors utilized for biomolecular interaction analysis (BIA) is a high degree of sensitivity. The current paper presents a theoretical analysis and comparison of the sensitivity and resolution of these four types of SPR biosensors when employed in three of the most prevalent detection methods, namely angular interrogation, wavelength interrogation, and intensity measurement. This study develops a detailed understanding of the influences of various biosensor design parameters in order to enhance the sensitivity and detection limit capabilities of such devices.     

Detection of antigen-antibody interactions by surface plasmon resonance. Application to epitope mapping.

Surface plasmon resonance (SPR) detection requires no labeling of antigen or antibodies and allows quantification of two or more interacting molecular species. The automated SPR instrument used here consists of an optical detection unit, an integrated liquid handling unit, and an autosampler. A first molecule is immobilized to the dextran modified surface of the sensor chip. By sequential introduction, the stepwise formation of multimolecular complexes can then be monitored. A two-site binding assay which allows characterization of MoAb epitope specificities is described. A polyclonal rabbit anti-mouse IgG1 (RAMG1) immobilized to the dextran surface is used to capture the first MoAb from unprocessed hybridoma culture supernatants. After introducing the antigen, the ability of a second MoAb to bind to the antigen is tested. The analysis cycle which is fully automated can be performed more than 100 times using the same RAMG1 surface. Since the detection principle allows monitoring of each reactant in the consecutive formation of a multimolecular complex, multi-site binding experiments can be performed. Five MoAbs recognizing different epitopes on an antigen were shown to bind sequentially, forming a hexamolecular complex. MoAbs were further characterized by inhibition analysis using synthetic peptides derived from the primary structure of their antigen. As a model system MoAbs against recombinant HIV-1 core protein p24 were used in all experiments.     

Self-Referencing Plasmonic Array Sensors

A self-referencing plasmonic platform is proposed and analyzed. By introducing a thin gold layer below a periodic two-dimensional nano-grating, the structure supports multiple modes including localized surface plasmon resonance (LSPR), surface plasmon resonance (SPR), and Fabry-Perot resonances. These modes get coupled to each other creating multiple Fano resonances. A coupled mode between the LSPR and SPR responses is spatially separated from the sensor surface and is not sensitive to refractive index changes in the surrounding materials or surface attachments. This mode can be used for self-referencing the measurements. In contrast, the LSPR dominant mode shifts in wavelength when the refractive index of the surrounding medium is changed. The proposed structure is easy to fabricate using conventional lithography and electron beam deposition methods. A bulk sensitivity of 429 nm/RIU is achieved. The sensor also has the ability to detect nanometer thick surface attachments on the top of the grating.

     

Enhanced SPR Sensitivity with Nano-Micro-Ribbon Grating—an Exhaustive Simulation Mapping

In this study, we theoretically investigate the sensing potential of 2D nano- and micro-ribbon grating structuration on the surface of Kretschmann-based surface plasmon resonance (SPR) biosensors when they are employed for detection of biomolecular binding events. Numerical simulations were carried out by employing a model based on the hybridization of two classical methods, the Fourier modal method and the finite element method. Our calculations confirm the importance of light manipulation by means of structuration of the plasmonic thin film surfaces on the nano- and micro-scales. Not only does it highlight the geometric parameters that allow the sensitivity enhancement compared with the response of the conventional SPR biosensor based on a flat surface but also describes the transition from the regime where the propagating surface plasmon mode dominates to the regime where the localized surface plasmon mode dominates. An exhaustive mapping of the biosensing potential of the 2D nano- and micro-structured biosensors surface is presented, varying the structural parameters related to the ribbon grating dimensions, i.e., the widths and thicknesses. The nano- and micro-structuration also leads to the creation of regions on biosensor chips that are characterized by strongly enhanced electromagnetic (EM) fields. New opportunities for further improving the sensitivity are offered if localization of biomolecules can be carried out in these regions of high EM fields. The continuum of nano- and micro-ribbon structured biosensors described in this study should prove a valuable tool for developing sensitive and reliable 2D-structured plasmonic biosensors.     

Surface plasmon resonance for high-throughput ligand screening of membrane-bound proteins

Technologies based on surface plasmon resonance (SPR) have allowed rapid, label-free characterization of protein-protein and protein-small molecule interactions. SPR has become the gold standard in industrial and academic settings, in which the interaction between a pair of soluble binding partners is characterized in detail or a library of molecules is screened for binding against a single soluble protein. In spite of these successes, SPR is only beginning to be adapted to the needs of membrane-bound proteins which are difficult to study in situ but represent promising targets for drug and biomarker development. Existing technologies, such as BIAcoreTM, have been adapted for membrane protein analysis by building supported lipid layers or capturing lipid vesicles on existing chips. Newer technologies, still in development, will allow membrane proteins to be presented in native or near-native formats. These include SPR nanopore arrays, in which lipid bilayers containing membrane proteins stably span small pores that are addressable from both sides of the bilayer. Here, we discuss current SPR instrumentation and the potential for SPR nanopore arrays to enable quantitative, high-throughput screening of G protein coupled receptor ligands and applications in basic cellular biology.     

Colloidal Au replacement assay for highly sensitive quantification of low molecular weight analytes by surface plasmon resonance

A novel sensing method based on surface plasmon resonance (SPR) was developed for the highly sensitive quantification of low molecular weight (LMW) analytes (colloidal Au replacement assay). Gold nanoparticles (diameter = 20 nm) functionalized with lactosyl-poly(ethylene glycol) (PEG) were prepared and were specifically adsorbed onto a Ricinus communis agglutinin (RCA120)-immobilized SPR sensor chip surface. Subsequent injection of free d-galactose elicited the elution of the preadsorbed lactosyl-PEGylated gold nanoparticles in a manner proportional to the galactose concentration, achieving a substantial and quantitative analysis over a wide range of galactose concentrations (0.1-50 ppm). This method of d-galactose sensing through the substituted elution of preadsorbed nanoparticles from the sensor chip surface would be applicable for the highly sensitive SPR quantification of various LMW analytes, which are known to be difficult to detect by the conventional SPR sensing regime.     

Sensitive and Label-Free Detection of DNA by Surface Plasmon Resonance

While an array of technologies based on radioactive labels or luminescent tags are dominant in modern biomedical research on DNA, surface plasmon resonance (SPR) and SPR imaging measurements are sensitive, rapid, and label-free. This review summarizes recent advances in the development of SPR and coupled techniques and their applications in DNA research, including the gene analysis at trace levels and studies of DNA–protein and DNA–drug interactions.     

Rapid and label-free bacteria detection by surface plasmon resonance (SPR) biosensors

Surface Plasmon Resonance (SPR) biosensor technology has been successfully used for the detection of various analytes such as proteins, drugs, DNA, and microorganisms. SPR-based immunosensors that coupled with a specific antigen-antibody reaction, have become a promising tool for the quantification of bacteria as it offers sensitive, specific, rapid, and label-free detection. In this paper, we review the important issues in the development of SPR-based immunoassays for bacteria detection, concentrating on instrumentation, surface functionalization, liquid handling, and surface regeneration. In addition, this review touches on the recent advances in SPR biosensing for sensitivity enhancement.     

Real-time monitoring of odorant-induced cellular reactions using surface plasmon resonance

Surface plasmon resonance (SPR) is a powerful technique for measuring molecular interaction in real-time. SPR can be used to detect molecule to cell interactions as well as molecule to molecule interactions. In this study, the SPR-based biosensing technique was applied to real-time monitoring of odorant-induced cellular reactions. An olfactory receptor, OR I7, was fused with a rho-tag import sequence at the N-terminus of OR I7, and expressed on the surface of human embryonic kidney (HEK)-293 cells. These cells were then immobilized on a SPR sensor chip. The intensity of the SPR response was linearly dependent on the amount of injected odorant. Among all the aldehyde containing odorants tested, the SPR response was specifically high for octanal, which is the known cognate odorant for the OR I7. This SPR response is believed to have resulted from intracellular signaling triggered by the binding of odorant molecules to the olfactory receptors expressed on the cell surface. This SPR system combined with olfactory receptor-expressed cells provides a new olfactory biosensor system for selective and quantitative detection of volatile compounds.     

Surface plasmon resonance (SPR) based binding studies of refolded single chain antibody fragments

Recent advances in Recombinant antibody technology / Antibody Engineering has given impetus to the genetic manipulation of antibody fragments that has paved the way for better understanding of the structure and functions of immunoglobulins and also has escalated their use in immunotherapy. Bacterial expression system such as Escherichia coli has complemented this technique through the expression of recombinant antibodies. Present communication has attempted to optimize the expression and refolding protocol of single chain fragment variable (ScFv) and single chain antigen binding fragment (ScFab) using E.coli expression system. Efficiency of refolding protocol was validated by structural analysis by CD, native folding by fluorescence and functional analysis by its binding with full length HIV-1 gp120 via SPR. Results show the predominant β–sheet (CD) as secondary structural content and native folding via red shift (tryptophan fluorescence). The single chain fragments have shown good binding with HIV-1 gp120 thus validating the expression and refolding strategy and also reinstating E.coli as model expression system for recombinant antibody engineering. SPR based binding analysis coupled with E.coli based expression and purification will have implication for HIV therapeutics and will set a benchmark for future studies of similar kind.     

Surface plasmon resonance analysis of ricin binding to plasma membranes isolated from NIH 3T3 cells

As the potential for bioterrorism has appeared to increase, the need for simple systems for identifying potential inhibitors of the binding of such biological agents to cell membranes has increased. In this work, surface plasmon resonance (SPR) was used to monitor binding of ricin, a ribosome-inactivating protein, to the plasma membranes of NIH 3T3 cells. Once conditions were established, efficacy of the system for monitoring effectiveness of compounds at inhibiting ricin binding was ascertained by determining the IC₅₀ values for asialofetuin (ASF) and for bovine serum albumin derivatized with an average of 34 lactosyl moieties (BSA-Lac₃₄). Results indicated that SPR is an efficient method for measuring adherence of a toxin to isolated cell plasma membranes. SPR can also indicate whether a compound that is an effective inhibitor of binding when a single ligand such as ASF is used will be as effective when used in studies with cells that may express multiple cell surface ligands for ricin and/or the inhibitor.     

Surface Plasmon Resonance Imaging Sensors: A Review

Surface plasmon resonance (SPR) imaging sensors realize label-free, real-time, highly sensitive, quantitative, high-throughput biological interaction monitoring and the binding profiles from multi-analytes further provide the binding kinetic parameters between different biomolecules. In the past two decades, SPR imaging sensors found rapid increasing applications in fundamental biological studies, medical diagnostics, drug discovery, food safety, precision measurement, and environmental monitoring. In this paper, we review the recent advances of SPR imaging sensor technology towards high-throughput multi-analyte screening. Finally, we describe our multiplex spectral-phase SPR imaging biosensor for high-throughput biosensing applications.     

Surface plasmon resonance imaging analysis of hexahistidine-tagged protein on the gold thin film coated with a calix crown derivative

A surface plasmon resonance (SPR) imaging system was constructed and used to detect the hexahistidine-ubiquitin-tagged human parathyroid hormone fragment (His₆-Ub-hPTHF(1–34)) expressed inEscherichia coli. The hexahistidine-specific antibody was immobilized on a thin gold film coated with ProLinker^TM B, a novel calixcrown derivative with a bifunctional coupling property that permits efficient immobilization of capture proteins on solid matrices. The soluble and insoluble fractions of anE. coli cell lysate were spotted onto the antibody-coated gold chip, which was then washed with buffer (pH 7.4) solution and dried. SPR imaging measurements were carried out to detect the expressed His₆-Ub-hPTHF (1–34). There was no discernible protein image in the uninduced cell lysate, indicating that non-specific binding of contaminant proteins did not occur on the gold chip surface. It is expected that the approach used here to detect affinity-tagged recombinant proteins using an SPR imaging technique could be used as a powerful tool for the analyses of a number of proteins in a high-throughput mode.     

Numerical Simulation of Extinction Spectra of Plasmonically Coupled Nanospheres Using Discrete Dipole Approximation: Influence of Compositional Asymmetry

We demonstrate plasmon coupling phenomenon between equivalent (homodimer) and non-equivalent (heterodimer) spherical shape noble metal nanoparticle (Ag, Au and Al). A systematic comparison of surface plasmon resonance (SPR) and extinction properties of various configurations (monomer, homodimer and heterodimer) has been investigated to observe the effect of compositional asymmetry. Numerical simulation has been done by using discrete dipole approximation method to study the optical properties of plasmonically coupled metal nanoparticles (MNPs). Plasmon coupling between similar nanoparticles allows only higher wavelength bonding plasmon mode while both the plasmon modes lower wavelength antibonding mode as well as higher wavelength bonding mode in the case of heterodimer. Au monomer of radius 50 nm shows resonance peak at 518 nm while plasmon coupling between Au-Au homodimer results in a spectral red shift around 609 nm. Au-Ag plasmonic heterodimer (radius 50 nm) reveals two resonant modes corresponding to higher energy antibonding mode (422 nm) as well as lower energy bonding mode (533 nm). Further, we have shown that interparticle edge-to-edge separation is the most significant parameter affecting the surface plasmon resonances of MNPs. As the inter particle separation decreases, resonance wavelength shows red spectral shift which is maximum for the touching condition. It is shown that plasmon coupling is a reliable strategy to tune the SPR.

     

Adenovirus type 5 intrinsic adsorption rates measured by surface plasmon resonance

Intrinsic adsorption rates of whole adenovirus type 5 (Ad5) onto a diethylaminoethyl (DEAE) anion exchange surface are measured for the first time by surface plasmon resonance (SPR). Fitting SPR sensorgrams to a two-compartment mass transport reaction model distinguishes intrinsic adsorption rates from slow diffusive Ad5 mass transport. Ad5 is a widely used viral vector for gene therapy that binds electrostatically to surfaces of cells and synthetics such as membranes, chromatographic resins, and glass. Increasing NaCl concentration from 4.8 to 14.4mM shifts binding of whole Ad5 from diffusion control to a regime where both sorption and diffusion affect binding. Intrinsic adsorption rates for Ad5-DEAE interaction are 16 times faster than intrinsic adsorption rates for Ad5 fiber knob interacting with soluble extracellular domain of coxsackievirus adenovirus receptors (s-CAR).     

A novel nanolayer biosensor principle

A method for eliminating the mass transport limitation on biosensor surfaces is introduced. The measurement of macromolecular binding kinetics on plane surfaces is the key objective of many evanescent wave (e.g. total internal reflection fluorescence (TIRF)), and surface plasmon resonance (SPR) based biosensor systems, allowing the determination of binding constants within minutes or hours. However, these methods are limited in not being rigorously applicable to large macromolecules like proteins or DNA, since the on-rates are transport limited due to a Nernst diffusion layer of 5-10 microm thickness. Thus, for the binding of fibrinogen (340 kDa) to a surface current SPR biosensors will show a mass transport coefficient of ca. 2 x 10(-6) m/s. In a novel approach with an immiscible fluid vesicle (e.g. air bubble), it has been possible to generate nanoscopic fluid films of ca. 200 nm thickness on the sensor surface of an interfacial TIRF rheometer system. The thickness of the liquid film can be can be easily probed and measured by evanescent wave technology. This nanofilm technique increases the mass transport coefficient for fibrinogen to ca. 1 x 10(-4) m/s eliminating the mass transport limitation, making the binding rates reaction-rate limited. From the resulting exponential kinetic functions, lasting only 20-30s, the kinetic constants for the binding reaction can easily be extracted and the binding constants calculated. As a possible mechanism for the air bubble effect it is suggested that the aqueous fluid flow in the rheometer cell is separated by the air bubble below the level of the Nernst boundary layer into two independent laminar fluid flows of differing velocity: (i) a slow to stationary nanostream ca. 200 nm thick strongly adhering to the surface; and (ii) the bulk fluid streaming over it at a much higher rate in the wake of the air bubble. Surprising properties of the nanofluidic film are: (i) its long persistence for at least 30-60s after the air bubble has passed (2.5s); and (ii) the absence of solute depletion. It is suggested that a new liquid-liquid interface (i.e. a "vortex sheet") between the two fluid flows plays a decisive role, lending metastability to the nanofluidic film and replenishing its protein concentration via the vortices-thus upholding exponential binding kinetics. Finally, the system relaxes via turbulent reattachment of the two fluid flows to the original velocity profile. It is concluded that this technique opens a fundamentally novel approach to the construction of macromolecular biosensors.