Grating-coupled Surface Plasmon Resonance Research Articles

Biosensor platform for parallel surface plasmon-enhanced epifluorescence and surface plasmon resonance detection

For the first time, plasmonic diffraction grating is implemented for the combined direct and epifluorescence-based readout of assays in real-time. This plasmonic structure serves for grating-coupled surface plasmon resonance (SPR) monitoring of molecular binding-induced changes in the refractive index. In parallel, it allows to simultaneously perform plasmonically enhanced fluorescence (PEF) analysis of affinity binding of molecules that are labeled with fluorophores. This configuration offers facile readout of e.g. magnetic nanoparticle-enhanced assays which is not possible with more conventional Kretschmann geometry. The performance characteristics of this combined approach are discussed by using two types of assays. In the first assay a fluorophore-labeled protein with a medium molecular weight of 55kDa was affinity captured on the plasmonic sensor grating. The associated PEF signal in this assay showed a signal-to-noise ratio that was 140-fold higher compared to that of the SPR detection channel. In the second assay, extracellular vesicles were detected by using antibodies against CD81 attached to the plasmonic grating. These vesicles were pre-concentrated by their coupling to magnetic nanoparticles with cholera toxin B chain. This assay exploited magnetic nanoparticles as labels enabling rapid collection of analyte at the sensor surface and for the enhancement of the SPR sensor response. In this case the label-free SPR detection channel outperformed the fluorescence-based detection, as the SPR signal-to-noise ratio was 2.4-fold higher than that of PEF.

Grating-coupled surface plasmon resonance on bulk stainless steel.

Grating-coupled surface plasmon resonance (SPR) is demonstrated with one-dimensional gratings fabricated on the surface of bulk stainless steel using imprinting combined with electrochemical etching. The extent of light coupling and the wavelengths of SPR peaks were characterized with respect to the incident angle and polarization states of the light. When the plane of incidence was orthogonal to the grating grooves, only TM polarization was absorbed at two different wavelengths. In the plane of incidence parallel to the grooves, a single resonance peak was observed only when the incident light was TE-polarized. The dependence of SPR wavelengths on the incident angle was in good agreement with theoretical consideration.

Detection of biomolecules and bioconjugates by monitoring rotated grating-coupled surface plasmon resonance

Plasmonic bio-sensing chips were prepared by fabricating wavelength-scaled sinusoidal dielectric-metal interfacial gratings on polymer film covered bimetal layers. Lysozyme biomolecules (LYZ) and gold nanoparticle bioconjugates (AuNP-LYZ) were seeded onto the biochip surfaces. Comparison of the reflectance curves measured in a modified Kretschmann arrangement and computed numerically proved that monitoring of the narrower secondary reflectance minima under optimal rotated-grating coupling condition makes it possible to achieve enhanced sensitivity in biodetection. The enlarged reflectance minimum shift is due to the horizontally and vertically antisymmetric long-range plasmonic mode, which originates from surface plasmon polariton Bragg scattering and propagates at the border of the valley and hill. The sensitivity is significantly increased in case of bioconjugates due to the coupled localized resonances on the gold nanoparticles.

Lipopolysaccharides detection on a grating-coupled surface plasmon resonance smartphone biosensor

We report a smartphone label-free biosensor platform based on grating-coupled surface plasmon resonance (GC-SPR). The sensor system relies on the smartphone's built-in flash light source and camera, a disposable sensor chip with Au diffraction grating and a compact disk (CD) as the spectra dispersive unit. The Au grating sensor chip was modified with a synthetic peptide receptor and employed on the GC-SPR detection of lipopolysaccharides (known as endotoxins) with detection limit of 32.5ng/mL in water. Upon incubation of various small and macro-molecules with the synthetic peptide modified sensor chips, we concluded the good selectivity of the sensor for LPS detection. In addition, the sensor shows feasibility for the detection of LPS in commonly used clinical injectable fluids, such as clinical-grade 0.9% sodium chloride intravenous infusion, compound sodium lactate intravenous infusion and insulin aspart. The developed sensor platform offers the advantage of portability and simplicity, which is attractive for point-of-care and remote detection of biomedical and environmental targets.

Grating-coupled surface plasmon resonance enhanced organic photovoltaic devices induced by Blu-ray disc recordable and Blu-ray disc grating structures.

In this work, we studied the performance enhancement of organic thin-film solar cells (OSCs) originating from the presence of diffraction gratings on the surface of the active layer. Two types of diffraction gratings, periodic gratings (Blu-ray disc recordable: BD-R) and quasi-random gratings (Blu-ray disc: BD), were employed as master templates for grating structures. The grating structures were introduced to the surfaces of poly(3-hexylthiophene) (P3HT):phenyl-C61-butyric acid methyl ester (PCBM) films, which were the active layers of the solar cells. The addition of the grating structures led to an increase of light absorption in the absorption region of P3HT:PCBM induced by light scattering. Furthermore, the grating-coupled surface plasmon resonance generated additional light absorption peaks. With illumination of non-polarized light at a normal incident angle, the short-circuit current densities of the BD-R and BD solar cells improved by 11.05% and 10.6%, respectively. Efficiency improvements of 19.28% and 3.21% were also observed for the BD-R and BD devices, respectively. Finally, the finite-difference time-domain simulation results revealed an enhanced electric field in the P3HT:PCBM layer, especially in the BD-R OSC devices.

Magnetic nanoparticle-enhanced surface plasmon resonance biosensor for extracellular vesicle analysis

The sensitive analysis of small lipid extracellular vesicles (EVs) by using a grating-coupled surface plasmon resonance (GC-SPR) biosensor has been reported. In order to enable the analysis of trace amounts of EVs present in complex liquid samples, the target analyte is pre-concentrated on the sensor surface by using magnetic nanoparticles and its affinity binding is probed by wavelength interrogation of SPR. The GC-SPR has been demonstrated to allow for the implementation of efficient pulling of EVs to the sensor surface by using magnetic nanoparticles and an external magnetic field gradient applied through the sensor chip. This approach overcomes slow diffusion-limited mass transfer and greatly enhances the measured sensor response. The specific detection of different EV populations secreted from mesenchymal stem cells is achieved with a SPR sensor chip modified with antibodies against the surface marker CD81 and magnetic nanoparticles binding the vesicles via annexin V and cholera toxin B chain.

Transmission light property due to grating-coupled long-range surface plasmon resonance

Transmission light property due to surface plasmon resonance (SPR) was investigated for glass substrate/500-nm-thick fluoropolymer (CYTOP) with a grating structure/thin metal film/mixed solution of water and glycerol system. Transmission light peaks were observed due to grating-coupled SPR, with a remarkable peak intensity when pure water exists as an ambient dielectric. CYTOP exhibits a refractive index close to that of water, and the remarkable peak intensity was attributed to long-range SPR. The monitoring of polyelectrolyte layer-by-layer film deposition using transmission light was also investigated.

Fluorescence microscopy imaging of cells with a plasmonic dish integrally molded

A plastic dish with a wavelength-scale periodic structure at a bottom panel was integrally molded and coated with thin metal films. The integrally molded dish called plasmonic dish was applied to bioimaging under a fluorescence microscope. On the plasmonic substrate, the enhanced electric field based on a grating-coupled surface plasmon resonance (GC-SPR) can provide an enhanced fluorescence. In this study, two kinds of cells, human embryonic kidney (HEK) cells and neuronal cells, were observed in our plasmonic dish. Fluorescence images of HEK cells were above 10 times brighter than those obtained on a conventional glass-bottomed dish. Neuronal cells were successfully cultured for 10 d on the plasmonic dish integrally molded, and in fluorescence images with transmitted light, a higher contrast was obtained than in epifluorescence images. The plasmonic dish integrally molded, as well as that fabricated by the UV nanoimprint method, was also found to be useful for sensitive bioimaging.

SPR Enhanced molecular imprinted sol–gel film: A promising tool for gas-phase TNT detection

An innovative surface plasmon resonance (SPR) approach for the detection of 2,4,6-trinitrotoluene (TNT) gas traces, obtained using an engineered molecular imprinted sol–gel film as recognition layer onto a plasmonic gold grating, is here proposed. The SPR substrate combined with the sol–gel matrix is able to trap gas phase TNT molecules from a TNT-saturated environment (4.9ppb). Taking advantage of the azimuthally-controlled grating-coupled SPR, this sensing platform demonstrates the potentiality to detect TNT traces <1ppb with a sensitivity of 0.47°/ppb.

Enhancement and control of surface plasmon resonance sensitivity using grating in conical mounting configuration.

In this work we propose a method to enhance and control the angular sensitivity of a grating coupled surface plasmon resonance (GCSPR) sensor. We lighted a silver grating, mounted in conical configuration, with a laser source and we measured the transmittance of the grating as a function of the azimuthal angle. To evaluate the sensitivity, grating surface was functionalized with four different alkanethiol self assembled monolayers (SAM) and the correspondent azimuthal transmittance peak shifts were measured. The sensitivity control was performed by simply change the light incident angle. This method offers the possibility to design dynamic GCSPR sensor benches that can be used to amplify the SPR angle shift at any step of a biological detection process.

Label-free efficient and accurate detection of cystic fibrosis causing mutations using an azimuthally rotated GC-SPR platform.

Plasmonic nanosensors are candidates for the development of new sensors with low detection limits, high sensitivity, and specificity for target detection: these characteristics are of critical importance in the screening of mutations responsible for inherited diseases. In this work, we focused our study on the detection of some of the most frequent mutations responsible for cystic fibrosis (CF) among the Italian population. For the detection of the CF mutations we adopted a recently developed and highly sensitive Grating Coupled-Surface Plasmon Resonance (GC-SPR) enhanced spectroscopy method for label-free molecular identification exploiting a conical illumination configuration. Gold sinusoidal gratings functionalized with heterobifunctional PEG were used as sensing surfaces, and the specific biodetection was achieved through the coupling with DNA hairpin probes designed for single nucleotide discrimination. Such substrates were used to test unlabeled PCR amplified homozygous wild type (wt) and heterozygous samples, deriving from clinical samples, for the screened mutations. Hybridization conditions were optimized to obtain the maximum discrimination ratio (DR) between the homozygous wild type and the heterozygous samples. SPR signals obtained from hybridizing wild type and heterozygous samples show DRs able to identify univocally the correct genotypes, as confirmed by fluorescence microarray experiments run in parallel. Furthermore, SPR genotyping was not impaired in samples containing unrelated DNA, allowing the platform to be used for the concomitant discrimination of several alleles also scalable for a high throughput screening setting.

Integrated sorting and detection of circulating tumor cells in blood using microfluidic cell sorting and surface plasmon resonance

ABSTRACTMetastatic tumors can spread via release of circulating tumor cells (CTCs) into the bloodstream. Early detection of these CTCs could greatly improve cancer survival rates by enabling diagnosis, and therefore treatment, before secondary tumors arise. However, tumor cells are typically present in very low concentrations, making them difficult to detect in a fluid dominated by red blood cells (RBCs), leukocytes and serum proteins. Separation of CTCs from blood plasma, leukocytes and RBCs is predicted to improve cell capture via antibody-based methods and reduce interference in capture/detection assays. Previously, members of our team have demonstrated microfluidic, size-based separation of blood components, but have yet to integrate this sorting capability with an affinity-based detection technology. To this end, we have developed a microfluidic platform to separate CTCs from mouse blood and detect them using grating coupled surface plasmon resonance (GCSPR). We have implemented a size-based sorting array, which separates objects based upon their diameter, within a microfluidic channel. Separation of beads (2 μm, 6 μm, 10 μm) has been demonstrated, as well as separation of white blood cells and CTCs from blood. The resulting stream of large blood cells (including CTCs) is then directed onto an integrated SPR grating for affinity based capture and detection. Using GCSPR vs. conventional SPR enables detection of multiple cell types across the grating in an array-based format. We have demonstrated differential capture and detection of cells on GCSPR gratings following size-based separation of blood. Using capture antibodies specific to unique CTC surface proteins enables identification of cell types and may provide prognostic capability, beyond the diagnostic capacity of this system.

Rapid and Sensitive Detection of Brain-Derived Neurotrophic Factor with a Plasmonic Chip

Plasmonic chips, which are grating replicas coated with thin metal layers and overlayers such as ZnO, were applied in immunosensors to improve their detection sensitivity. Fluorescence from labeled antibodies bound to plasmonic chips can be enhanced on the basis of a grating-coupled surface plasmon resonance (GC-SPR) field. In this study, as one of the representative candidate protein markers for brain disorders, the brain-derived neurotrophic factor (BDNF) was quantitatively measured by sandwich assay on a plasmonic chip and detected on our plasmonic chip in the concentration of 5–7 ng/mL within 40 min. Furthermore, BDNF was detected in the blood sera from three types of mice: wild-type mice and two types of mutant mice. This technique is promising as a new clinical diagnosis tool for brain disorders based on scientific evidence such as blood test results.

Optical characteristics of subwavelength metallic grating coupled porous film surface plasmon resonance sensor with high sensitivity

A high performance sub-wavelength metallic grating coupled surface plasmon resonance (SWMGCSPR) sensor with metal and porous composite layer is proposed. Rigorous coupled-wave analysis (RCWA) is conducted to prove the design feasibility, characterize the sensor's performance and determine geometric parameters of the structure, which is also employed to compute the electromagnetic (EM) field distributions at the resonant wavelengths. Parameters of sensing platform are optimized to achieve the best performance of the SPR sensor. Obtained results reveal that the proposed structure can excite SPR with negative diffraction order of SWMG. Both wavelength and angular sensitivities are greatly enhanced because surface plasmon wave (SPW) exhibits a large penetration depth which will enlarge the distance of interactions between SP and analytes. The detection sensitivities and quality parameters are estimated to be 700nm/RIU and 509°/RIU with full width at half maximum (FWHM) less than 2.5nm using the same optimized structure.

Implementation and testing of a compact and high-resolution sensing device based on grating-coupled surface plasmon resonance with polarization modulation

An experimental setup exploiting grating-coupled surface plasmon resonance (GCSPR) technique based on polarization modulation in conical mounting is presented for the first time. A metallic grating is azimuthally rotated in order to support excitation of surface plasmon polaritons (SPPs) with enhanced sensitivity. In correspondence of SPP resonance, a polarization scan of the incident light is performed and reflectivity is collected. Output signal exhibits harmonic dependence on polarization and the phase term is exploited as a parameter for sensing analysis. Since the sensing configuration is kept fixed during the analysis and the only degree of freedom is represented by the incident polarization, the mechanical complexity of this SPR system is significantly reduced with respect to other configurations. Moreover the setup assures competitive performance in refractive index sensitivity and resolution. The assembled prototype is tested for fluid analysis with an embedded glass/PDMS microfluidic cell fabricated by soft-lithography procedure. A model biorecognition assay based on avidin/biotin reaction was considered in order to test the detection performance of the presented setup.

Use of Dispersion Imaging for Grating-Coupled Surface Plasmon Resonance Sensing of Multilayer Langmuir–Blodgett Films

We report grating-coupled surface plasmon resonance measurements involving the use of dispersion images to interpret the optical response of a metal-coated grating. Optical transmission through a grating coated with a thin, gold film exhibits features characteristic of the excitation of surface plasmon resonance due to coupling with the nanostructured grating surface. Evidence of numerous surface plasmon modes associated with coupling at both front (gold/air) and back (gold/substrate) grating interfaces is observed. The influence of wavelength and angle of incidence on plasmon coupling can be readily characterized via dispersion images, and the associated image features can be indexed to matching conditions associated with several diffracted orders at both the front and back of the grating. These features collapse onto a set of global dispersion curves when plotted as peak energy versus the grating wavevector, with feature locations clustered according to the refractive index values of the neighboring dielectric material, either air or polycarbonate. Coating of the grating with multilayer arachidic acid films via Langmuir-Blodgett deposition results in red-shifting of some, but not all, of the plasmon features. The magnitude of the shift is a function of the film thickness, wavelength, and angle of incidence. Dispersion images clearly depict the red-shifting and also broadening of the front side features with increasing film thickness. In contrast, little change is observed in features associated with the back-side of the grating. The nature and magnitude of the interaction between the plasmon modes appearing at the front and back sides of the grating are discussed and analyzed in terms of the predicted interactions determined via optical modeling calculations.

Plasmonic platforms for innovative surface plasmon resonance configuration with sensing applications

An experimental prototype exploiting Grating-Coupled Surface Plasmon Resonance (GCSPR) based on polarization modulation has been assembled and tested for sensing purposes. The plasmonic gratings are azimuthally rotated in order to exploit the symmetry breaking for the excitation of highly sensitive Surface Plasmon Polaritons in conical mounting. By exploiting the optimal-polarization shift, a scan of the incident polarization is performed and reflectivity data are collected. The output signal exhibits a harmonic dependence on polarization and the phase term is considered as a parameter for sensing. Since the optical configuration is fixed during the analysis and the only degree of freedom is represented by the incident polarization, this setup provides a more compact and simplified architecture with respect to other commercial SPR techniques, however assuring at the same time competitive performances in refractive index sensitivity and resolution. The employed metallic gratings are fabricated by interferential lithography and replicated onto resin substrates by soft-lithography techniques, thus thermally evaporated.

Enhanced sensitivity azimuthally controlled grating-coupled surface plasmon resonance applied to the calibration of thiol-poly(ethylene oxide) grafting

Azimuthally controlled grating-coupling surface plasmon resonance (φ≠0° GC-SPR) is known to exhibit refractive index sensitivity (SRI) up to one order of magnitude higher with respect to classic φ=0° GC-SPR. Here, grated gold surfaces (period of 500nm and amplitude of 40nm) were covered with different amounts of thiol-poly(ethylene oxide) (PEO, Mw 5kDa) through a 24h functionalization process using polymer solutions at increasing concentration (from 500nM to 2.5mM). The GC-SPR output signal generated by each PEO layer was measured in both null and rotated configuration. The number of PEO molecules adsorbed onto each surface (cm−2) and the corresponding molecular surface densities (σ [ng/cm2]) were calculated from the angle shifts collected after PEO film formation, by combining the model shown by Jung and the theory of PEO layer formation. In the end, we obtained a calibration curve that correlates the amount of PEO tethered onto the surface with the SPR measurement output signal, that will be useful for future reference. At the PEO surface saturation condition (PEO σ=592ng/cm2, obtained using PEO concentration ≥2mM), a SRI of 603°/RIU was reached (φ=45°), which is ≈10-fold higher than what obtained through the classic GC-SPR (φ=0°). This enhanced instrumental sensitivity allowed detecting tiny amounts of molecules adsorbed onto the surface (3.1ng vs 59.7ng of classic φ=0° GC-SPR), a property that is extremely valuable for the application of SPR sensing to quantitative determinations.

Grating-Coupled Surface-Plasmon-Resonance Biosensor Discs with a C-Type Fluidic Channel for Monitoring Growth of Self-Assembled Monolayer

In this paper, we present a grating-coupled surface plasmon resonance (SPR) biosensor with a c-type fluidic channel on a grating disc. For obtaining the kinetic information of molecular interaction, we combined a c-type fluidic channel on disc to drive testing solution forward by gravity via turning the grating disc. The disc biosensor can react with probe molecules on the different sensing surfaces in the following time. The repeatability testing shows the resolution of angle of the measuring system was enhanced by threefold comparing to that without the automation improvement. In order to identify the accuracy of angle changes of SPR due to the chemical reaction on surface, experiment for monitoring a growth self-assembled monolayer (SAM) immobilized on gold surface was performed. The measured response curves show that the saturation time of the SAM formation, which thickness is smaller than 2 nm, is roughly 5 hours when 1 mM MHDA growing on gold at room temperature. The demonstration reveals that the disc biosensor with c-type fluidic channels can be a promising tool for a kinetic analysis of bimolecular interaction without any external fluid pumping systems.

Phase-detection-sensitivity enhancement of grating-coupled surface plasmon resonance sensor with light incident at nonzero azimuth angle

We have numerically and experimentally demonstrated the enhancement of phase detection sensitivity of a grating-coupled surface plasmon resonance (GCSPR) sensor by using incident light at a nonzero azimuth angle. Phase detection measurements were performed using an electro-optic heterodyne interferometer. When the GCSPR sensor was rotated azimuthally by 0 deg and 58 deg, the phase detection sensitivities were approximately 3.2×10−7 RIU and 5.5×10−8 RIU, respectively. The azimuth angle of 58 deg was found to enhance the sensor sensitivity by a factor of 5.87 relative to the azimuth angle of 0 deg.