Enzyme-free surface plasmon resonance aptasensor for amplified detection of adenosine via target-triggering strand displacement cycle and Au nanoparticles

Herein, we combine the advantage of aptamer technique with the amplifying effect of an enzyme-free signal-amplification and Au nanoparticles (NPs) to design a sensitive surface plasmon resonance (SPR) aptasensor for detecting small molecules. This detection system consists of aptamer, detection probe (c-DNA1) partially hybridizing to the aptamer strand, Au NPs-linked hairpin DNA (Au-H-DNA1), and thiolated hairpin DNA (H-DNA2) previously immobilized on SPR gold chip. In the absence of target, the H-DNA1 possessing hairpin structure cannot hybridize with H-DNA2 and thereby Au NPs will not be captured on the SPR gold chip surface. Upon addition of target, the detection probe c-DNA1 is forced to dissociate from the c-DNA1/aptamer duplex by the specific recognition of the target to its aptamer. The released c-DNA1 hybridizes with Au-H-DNA1 and opens the hairpin structure, which accelerate the hybridization between Au-H-DNA1 and H-DNA2, leading to the displacement of the c-DNA1 through a branch migration process. The released c-DNA1 then hybridizes with another Au-H-DNA1 probe, and the cycle starts anew, resulting in the continuous immobilization of Au-H-DNA1 probes on the SPR chip, generating a significant change of SPR signal due to the electronic coupling interaction between the localized surface plasma of the Au NPs and the surface plasma wave. With the use of adenosine as a proof-of-principle analyte, this sensing platform can detect adenosine specifically with a detection limit as low as 0.21 pM, providing a simple, sensitive and selective protocol for small target molecules detection.     

A Au nanoparticle and polydopamine co-modified biosensor: A strategy for in situ and label-free surface plasmon resonance immunoassays

This article reports a surface plasmon resonance (SPR) strategy capable of label-free yet amplified in situ immunoassays for sensitive and specific detection of human IgG (hIgG), a serum marker that is important for the diagnosis of certain diseases. Primarily, a wavelength-modulated Kretschman configuration SPR analyzer was constructed, and Au film SPR biosensor chips were fabricated. Specifically, based on Au nanoparticles (AuNPs) adsorbed on the surface of the Au film, the AuNP/Au film was coated with polydopamine (PDA) to fix streptavidin (SA), and then the biotinylated antibodies were connected to the surface of the biosensor chip. The SPR analyzer was utilized for in situ real-time monitoring of hIgG. Due to the immunological recognition between the receptor and target, the surface plasmon waves produced by the attenuated total reflection were affected by the changes in the surface of the biosensor chip. The resonance wavelength (λ_R) of the output spectra gradually redshifted, and the redshift degrees were directly related to the target concentration. The biosensor can realize the in situ detection of hIgG, displaying satisfactory sensitivity, excellent specificity and stability. Briefly, by monitoring the shift in λ_R after specific binding, a new SPR immunoassay can be customized for label-free, in situ and amplified hIgG detection. The operating principle of this research could be extended as a common protocol for many other targets of interest.     

Development of surface chemistry for surface plasmon resonance based sensors for the detection of proteins and DNA molecules

The immobilisation of biological recognition elements onto a sensor chip surface is a crucial step for the construction of biosensors. While some of the optical biosensors utilise silicon dioxide as the sensor surface, most of the biosensor surfaces are coated with metals for transduction of the signal. Biological recognition elements such as proteins can be adsorbed spontaneously on metal or silicon dioxide substrates but this may denature the molecule and can result in either activity reduction or loss. Self assembled monolayers (SAMs) provide an effective method to protect the biological recognition elements from the sensor surface, thereby providing ligand immobilisation that enables the repeated binding and regeneration cycles to be performed without losing the immobilised ligand, as well as additionally helping to minimise non-specific adsorption. Therefore, in this study different surface chemistries were constructed on SPR sensor chips to investigate protein and DNA immobilisation on Au surfaces. A cysteamine surface and 1%, 10% and 100% mercaptoundecanoic acid (MUDA) coatings with or without dendrimer modification were utilised to construct the various sensor surfaces used in this investigation. A higher response was obtained for NeutrAvidin immobilisation on dendrimer modified surfaces compared to MUDA and cysteamine layers, however, protein or DNA capture responses on the immobilised NeutrAvidin did not show a similar higher response when dendrimer modified surfaces were used.     

Characterizing the interaction between aptamers and human IgE by use of surface plasmon resonance

Human immunoglobulin E (hIgE) is such an important protein, because of its involvement in allergic disease, that it is of significance to study the interactions between it and its recognizing elements. In this report an analytical strategy based on surface plasmon resonance (SPR) was developed to probe the pattern of interaction between hIgE and its recognizing molecules, including aptamers and antibodies. The affinity constants of hIgE for the antibody and the aptamer were compared first; the aptamer has more affinity than the antibody for human IgE. To study their pattern of interaction, three different binding approaches, including adding the antibody and the streptavidin-coupled aptamer to the sensing surface, were designed. The results showed that hIgE captured on the sensing surface could form a multivalent complex with the aptamer. An ELISA-like assay using the aptamer as both capture and detection probes was then developed. This work highlights an SPR method for characterizing the interaction between the protein and aptamers that is useful for study of biomolecular interaction patterns and binding properties. Figure Schematic diagram of the use of surface plasmon resonance for detection of the pattern of interaction of human IgE with its DNA aptamer and antibody     

Optical detection of phenolic compounds based on the surface plasmon resonance band of Au nanoparticles

An indirect colorimetric method is presented for detection of trace amounts of hydroquinone (1), catechol (2) and pyrogallol (3). The reduction of AuCl4(-) to Gold nanoparticles (Au-NPs) by these phenolic compounds in the presence of cetyltrimethylammonium chloride (CTAC) produced very intense surface plasmon resonance peak of Au-NPs. The plasmon absorbance of Au-NPs allows the quantitative colorimetric detection of the phenolic compounds. The calibration curves derived from the changes in absorbance at lambda = 568 nm were linear with concentration of hydroquinone, catechol and pyrogallol in the range of 7.0 x 10(-7) to 1.0 x 10(-4)M, 6.0 x 10(-6) to 2.0 x 10(-4)M and 6.0 x 10(-7) to 1.0 x 10(-4)M, respectively. The detection limits were 5.3 x 10(-7), 2.5 x 10(-6) and 3.2 x 10(-7)M for the hydroquinone, catechol and pyrogallol, respectively. The method was applied satisfactorily to the determination of phenolic compounds in water samples and pharmaceutical formulations.     

Recent advances in surface plasmon resonance based techniques for bioanalysis

Surface plasmon resonance (SPR) is a powerful and versatile spectroscopic method for biomolecular interaction analysis (BIA) and has been well reviewed in previous years. This updated 2006 review of SPR, SPR spectroscopy, and SPR imaging explores cutting-edge technology with a focus on material, method, and instrument development. A number of recent SPR developments and interesting applications for bioanalysis are provided. Three focus topics are discussed in more detail to exemplify recent progress. They include surface plasmon fluorescence spectroscopy, nanoscale glassification of SPR substrates, and enzymatic amplification in SPR imaging. Through these examples it is clear to us that the development of SPR-based methods continues to grow, while the applications continue to diversify. Major trends appear to be present in the development of combined techniques, use of new materials, and development of new methodologies. Together, these works constitute a major thrust that could eventually make SPR a common tool for surface interaction analysis and biosensing. The future outlook for SPR and SPR-associated BIA studies, in our opinion, is very bright. Surface plasmon resonance (SPR) is a powerful and versatile spectroscopic method for biomolecular interaction analysis (BIA) and has been well reviewed in previous years. This updated 2006 review of SPR, SPR spectroscopy, and SPR imaging explores cutting-edge technology with a focus on material, method, and instrument development. A number of recent SPR developments and interesting applications for bioanalysis are provided. Three focus topics are discussed in more detail to exemplify recent progress. They include surface plasmon fluorescence spectroscopy, nanoscale glassification of SPR substrates, and enzymatic amplification in SPR imaging. Through these examples it is clear to us that the development of SPR-based methods continues to grow, while the applications continue to diversify. Major trends appear to be present in the development of combined techniques, use of new materials, and development of new methodologies. Together, these works constitute a major thrust that could eventually make SPR a common tool for surface interaction analysis and biosensing. The future outlook for SPR and SPR-associated BIA studies, in our opinion, is very bright.      

Studies on surface plasmon resonance and photoluminescence of silver nanoparticles

Silver nanoparticles of different sizes were prepared by citrate reduction and characterized by UV-vis absorbance spectra, TEM images and photoluminescence spectra. The morphology of the colloids obtained consists of a mixture of nanorods and spheres. The surface plasmon resonance (SPR) and photoemission properties of Ag nanoparticles are found to be sensitive to citrate concentration. A blue shift in SPR and an enhancement in photoluminescence intensity are observed with increase in citrate concentration. Effect of addition of KCl and variation of pH in photoluminescence was also studied.     

Surface plasmon resonance-based trace detection of small molecules by competitive and signal enhancement immunoreaction

A surface plasmon resonance (SPR)-immunosensor for detection of the low molecular weight compound 2,4-dinitorophenol (DNP) at ultra-low concentration has been developed. The sensor strategy is based on a competitive immunoreaction between DNP and a DNP-protein conjugate, namely DNP-bovine serum albumin conjugate (DNP-BSA). Anti-DNP monoclonal antibody was immobilized on a gold thin-film coated SPR-sensor chip by means of a chemical coupling process. DNP-BSA, on contact with the anti-DNP antibody immobilized SPR-immunosensor chip causes an increase in the resonance angle of the sensor chip. The optimum concentration of immobilized antibody on the SPR-sensor chip is 100 μg mL⁻¹. The SPR-immunosensor response for free DNP determination using the competitive immunoreaction had a response time of ca. 15 min. Using this method, DNP could be determined in the concentration range 1 ppt to 1 ppb. The SPR signal for ppt levels of DNP was enhanced by a factor of three by subsequently treating immuno-bound DNP-BSA with a secondary anti-DNP antibody.     

A novel low-cost and easy to develop functionalization platform. Case study: Aptamer-based detection of thrombin by surface plasmon resonance

A novel low-cost platform to assess biomolecular interactions was investigated using surface plasmon resonance and an aptamer-based assay for thrombin detection. Gold SPR surface functionalized with a carboxylated cross-linked BSA film (cBSA) and commercially available carboxymethylated dextran chip (CM5) were used as immobilization platforms for the thrombin binding aptamer. The high end commercial instrument Biacore 3000 and a custom made FIA set-up involving TI Spreeta sensor (TSPR2K23) were used to assess different concentrations of thrombin within the range 0.1-150 nM both in buffer and in a complex matrix (plasma) using the obtained aptasensors. Based on data derived from both CM5 and cBSA platforms, the cBSA aptasensor exhibited good selectivity, stability and regeneration ability, both in buffer and in complex matrices (plasma), comparable with CM5.     

Au nanoparticle-enhanced surface plasmon resonance sensing of biocatalytic transformations

N-(3-Aminopropyl)-N'-methyl-4,4'-bipyridinium is coupled to tiopronin-capped Au nanoparticles (diameter ca. 2 nm) to yield methyl(aminopropyl)viologen-functionalized Au nanoparticles (MPAV(2+)-Au nanoparticles). In situ electrochemical surface plasmon resonance (SPR) measurements are used to follow the electrochemical deposition of the bipyridinium radical cation modified Au nanoparticles on an Au-coated glass surface and the reoxidation and dissolution of the bipyridinium radical cation film. The MPAV(2+)-functionalized Au nanoparticles are also employed for the amplified SPR detection of NAD(+) and NADH cofactors. By SPR monitoring the partial biocatalyzed dissolution of the bipyridinium radical cation film in the presence of diaphorase (DP) NAD(+) is detected in the concentration range of 1x10(-4) M to 2x10(-3) M. Similarly, the diaphorase-mediated formation of the bipyridinium radical cation film on the Au-coated glass surface by the reduction of the MPAV(2+)-functionalized Au nanoparticles by NADH is used for the amplified SPR detection of NADH in the concentration range of 1x10(-4) M to 1x10(-3) M.     

Molecularly imprinted polymers for highly sensitive detection of morphine using surface plasmon resonance spectroscopy

Molecular imprinting technology is applied in surface plasmon resonance spectroscopy for highly sensitive and selective detection of morphine（MO）.As SPR-based sensor of MO,the preparation of molecular imprinted polymer is as follows: methacryhc acids（MAA）,ethylene glycol dimethacrylate（EGDMA）,azodiisobutyronitrile（AIBN） were used as functional monomer,cross-linker and initiator,respectively.The experiment results showed that morphine imprinted polymer had the performance of high sensitivity and specificity,i.e.the relative signal of SPR response was proportional to the concentration of morphine in acetonitrile in the range of 10^-9mol/L to 10^-6mol/L（1 ppb-1 ppm） with LOD of 10^-10mol/L,and MO was distinguished from its analogs,such as codeine.     

Rapid detection of nivalenol and deoxynivalenol in wheat using surface plasmon resonance immunoassay

A surface plasmon resonance (SPR) immunoassay using a monoclonal antibody was developed to measure nivalenol (NIV) and deoxynivalenol (DON) contamination in wheat. A highly sensitive and stable DON-immobilized sensor chip was prepared, and an SPR detection procedure was developed. The competitive inhibition assay used a monoclonal antibody that cross-reacts with NIV and DON. The half maximal inhibitory concentration (IC₅₀) values of the SPR assay were 28.8 and 14.9 ng mL⁻¹ for NIV and DON, respectively. The combined responses of NIV and DON in wheat were obtained using a simultaneous detection assay in a one-step cleanup procedure. NIV and DON were separated using a commercial DON-specific immunoaffinity column (IAC) and their responses were obtained using an independent detection assay. Spiked tests using these toxins revealed that recoveries were in the range 91.5-107% with good relative standard deviations (RSDs) (0.40-4.1%) and that detection limits were 0.1 and 0.05 mg kg⁻¹ for NIV and DON, respectively. The independent detection using IAC showed detection limits of 0.2 and 0.1 mg kg⁻¹ for NIV and DON, respectively. SPR analysis results were correlated with those obtained using a conventional LC/MS/MS method for wheat co-contaminated with NIV and DON. These results suggested that the developed SPR assay is a practical method to rapidly screen the NIV and DON co-contamination of wheat and one of a very few immunoassays to detect NIV directly.     

A surface plasmon resonance probe without optical fibers as a portable sensing device

A surface plasmon resonance (SPR) sensor integrating a small sensor probe, a laser emission diode, a photo detector, and a polarizer was developed as a portable sensing device. The sensor probe was made with a glass cylinder, 50 mm long and 1.5 mm in diameter, that was connected directly to a beam splitter without optical fibers. The SPR spectrum obtained with this probe system showed a 10% reflectivity minimum at 690 nm. Shifts of the SPR spectrum induced by refractive index (RI) changes in the sample were measured by detecting the reflection light intensity at 670 nm. When the sensitivity was compared using a BIAcore™ SPR instrument, the lowest sensor response of 1 mV observed with the SPR probe system coincided with 1.4 × 10⁻⁶ of the RI changes. The RI resolution of the SPR probe was estimated with experimentally evaluated noise on the signal, and, consequently, it was concluded that the RI resolution was 1.2 × 10⁻⁵. Moreover, immunoreaction was demonstrated with adsorbed bovine serum albumin (BSA) and anti-BSA antibody as an analyte. As a result, 50 ng mL⁻¹ of the lower detection limit was estimated.     

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.     

Spin-coated methacrylic acid copolymer thin films for covalent immobilization of small molecules on surface plasmon resonance substrates

Hydrophilic copolymers of methacrylic acid and ethylene glycol dimethacrylate are simply and rapidly prepared as thin films by spin-coating on gold-coated glass slides with a concurrent photo-crosslinking step. Coating techniques were optimized for use on gold surfaces both separately and as part of surface plasmon resonance (SPR) sensor chips. The population of carboxylic acid functional groups as binding sites in the polymer matrix, as reflected in the corresponding hydrophilicity, could be easily adjusted through changes to the stoichiometric ratio of the monomers, allowing for good control of immobilization capacity. The polymers used adhered to the gold surfaces both with and without use of thiol moieties. Coating thickness was measured by ellipsometry and coatings of 30–40 nm thickness were routinely achieved on gold-coated slides. This dimension is dependent on the spin speed and the viscosity of the polymerization mixture applied. The polymers were further characterized by contact angle measurements and infrared spectroscopy before being applied to immobilization of the steroid cortisol in a BIAcore SPR instrument, where binding to a monoclonal antibody was studied and the surface coatings optimized for maximum specific binding capacity. Optimized surfaces could be regenerated and re-used, and have potential applications as immobilization matrices in plasmonic biosensors with a very rapid coating technique.     

Label-free detection of cancer biomarker candidates using surface plasmon resonance imaging

In this work, we present an antibody array for the detection of cancer biomarker candidates by a surface plasmon resonance (SPR) imaging sensor with polarization contrast. Responses from the SPR imaging sensor are shown to be similar to those from a conventional spectroscopy-based SPR sensor. Antibodies are spotted onto a self-assembled monolayer (SAM) composed of oligo(ethylene glycol) (OEG)-containing alkanethiol chains. Detection of two cancer biomarker candidates, activated leukocyte cell adhesion molecule/CD 166 (ALCAM) and transgelin-2 (TAGLN2), is demonstrated. Limits of detection for ALCAM and TAGLN2 are established at 6 ng/mL and 3 ng/mL, respectively, in buffer. No cross-reactivity is observed between immobilized antibodies and nonspecific antigen. Biomarker candidates are also detected in a 10% human serum solution. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Janus particles with a functional gold surface for control of surface plasmon resonance

We report in this study the presence of Janus particles, which are candidates for use with electronic color papers. We used negatively charged polystyrene particles (370 nm) as the core particles, and gold was then sputtered onto their packed monolayer under several conditions. The sputtered particles were next redispersed into the aqueous medium by gentle sonication. Gold nanoparticles localized on one side of the cores could also serve as seeds for subsequent shell growth by electroless gold plating. Through these treatments, a series of well-dispersed Janus particles were obtained with gold nanostructures of different size and shape only on one side. Their dispersions showed different colors originating from the surface plasmon resonance absorption of gold nanoparticles localized on the hemisphere. The particles obtained by this approach have potential applications such as in sensors and electronic color paper.     

Comparison of biosensor platforms for surface plasmon resonance based detection of paralytic shellfish toxins

Paralytic shellfish poisoning (PSP) toxins are produced by certain marine dinoflagellates and may accumulate in bivalve molluscs through filter feeding. The Mouse Bioassay (MBA) is the internationally recognised reference method of analysis, but it is prone to technical difficulties and regarded with increasing disapproval due to ethical reasons. As such, alternative methods are required. A rapid surface plasmon resonance (SPR) biosensor inhibition assay was developed to detect PSP toxins in shellfish by employing a saxitoxin polyclonal antibody (R895). Using an assay developed for and validated on the Biacore Q biosensor system, this project focused on transferring the assay to a high-throughput, Biacore T100 biosensor in another laboratory. This was achieved using a prototype PSP toxin kit and recommended assay parameters based on the Biacore Q method. A monoclonal antibody (GT13A) was also assessed. Even though these two instruments are based on SPR principles, they vary widely in their mode of operation including differences in the integrated μ-fluidic cartridges, autosampler system, and sensor chip compatibilities. Shellfish samples (n = 60), extracted using a simple, rapid procedure, were analysed using each platform, and results were compared to AOAC high performance liquid chromatography (HPLC) and MBA methods. The overall agreement, based on statistical 2 × 2 comparison tables, between each method ranged from 85% to 94.4% using R895 and 77.8% to 100% using GT13A. The results demonstrated that the antibody based assays with high sensitivity and broad specificity to PSP toxins can be applied to different biosensor platforms.     

Surface plasmon resonance biosensor for genetically modified organisms detection

The development of a surface plasmon resonance (SPR) affinity biosensor based on DNA hybridisation is described. This biosensor has been applied to genetically modified organisms (GMOs) detection. Single stranded DNA (ssDNA) probes were immobilised on the sensor chip of an SPR device and the hybridisation between the immobilised probe and the complementary sequence (target) was monitored. The probe sequences were internal to the sequence of 35S promoter and NOS terminator which are inserted sequences in the genome of GMO regulating the transgene expression. The system has been optimised using synthetic oligonucleotides, then applied to real samples analysis. Samples, containing the transgenic target sequences, were amplified by polymerase chain reaction (PCR) and then detected with the SPR biosensor.