High-resolution Surface Plasmon Resonance Research Articles

High spatial resolution surface plasmon resonance imaging using a plasmonic chip.

The surface plasmon resonance (SPR) technique has been widely applied to biosensing technologies for the rapid quantification of biomolecules without enzyme and fluorescent labeling. However, the conventional prism-coupling SPR method generally has a detection area of a few mm2, and the large contribution of the background signal forms a barrier to highly sensitive detection. Based on a highly spatially resolved SPR method, the present study constructed a scanning GC-SPR imaging instrument using an objective lens with a high numerical aperture and a plasmonic chip that could be used for grating-coupled SPR. Focusing light on the diffraction limit can suppress background signals and improve detection sensitivity. SPR imaging can also be performed by scanning a focal spot. Using this method, the refractive index of a mixture of water and dimethyl sulfoxide was measured with a detection accuracy of 2.43 × 10-3 RIU. Polydopamine films prepared with a thickness of <5nm were also measured, and each film thickness was evaluated with high sensitivity from the effective refractive index detected in a small area of <1 µm2.

Chitosan Micro-Membranes with Integrated Gold Nanoparticles as an LSPR-Based Sensing Platform.

Currently, there is an increasing need to develop highly sensitive plasmonic sensors able to provide good biocompatibility, flexibility, and optical stability to detect low levels of analytes in biological media. In this study, gold nanoparticles (Au NPs) were dispersed into chitosan membranes by spin coating. It has been demonstrated that these membranes are particularly stable and can be successfully employed as versatile plasmonic platforms for molecular sensing. The optical response of the chitosan/Au NPs interfaces and their capability to sense the medium's refractive index (RI) changes, either in a liquid or gas media, were investigated by high-resolution localized surface plasmon resonance (HR-LSPR) spectroscopy, as a proof of concept for biosensing applications. The results revealed that the lowest polymer concentration (chitosan (0.5%)/Au-NPs membrane) presented the most suitable plasmonic response. An LSPR band redshift was observed as the RI of the surrounding media was incremented, resulting in a sensitivity value of 28 ± 1 nm/RIU. Furthermore, the plasmonic membrane showed an outstanding performance when tested in gaseous atmospheres, being capable of distinguishing inert gases with only a 10-5 RI unit difference. The potential of chitosan/Au-NPs membranes was confirmed for application in LSPR-based sensing applications, despite the fact that further materials optimization should be performed to enhance sensitivity.

High-resolution surface plasmon resonance holographic microscopy based on symmetrical excitation

Surface plasmon resonance holographic microscopy (SPRHM) is able to simultaneously obtain the amplitude- and phase-contrast surface plasmon resonance (SPR) images, showing great potentials in imaging near-field targets with high sensitivity. However, suffered by the decaying length of surface plasmon wave which can be as long as tens of microns, the spatial resolution of SPRHM is lower than that of traditional holographic microscopy. In this work, we propose to enhance the spatial resolution in SPRHM by exciting surface plasmon resonance in two symmetrical directions and detecting the complex amplitudes of the reflected light symmetrically. Through the Fourier analysis of the recorded composite hologram, the reconstruction schemes for high-resolution amplitude- and phase-contrast SPR images are established, respectively. The feasibility and advantages of the proposed method is verified by numerical simulations and experimental demonstrations of small-size particles and micro-structures.

Discrimination of Bulk and Surface Refractive Index Change in Plasmonic Sensors with Narrow Bandwidth Resonance Combs.

A method to enable surface plasmon resonance (SPR) sensors to discriminate between bulk and surface-localized refractive index changes is demonstrated with modified gold-coated tilted fiber Bragg grating SPR sensors (TFBG-SPR). Without this capability, all high-resolution SPR sensors should be using reference channels and strict temperature control to prevent the contamination of the desired detection of surface-localized chemical or binding events by drift of the refractive index of the medium, in which the experiment is carried out. The very fine comb of high-quality-factor resonances of a TFBG-SPR device coupled to the large differential sensitivity of some of the resonances to various perturbations is used to measure unambiguously the refractive index changes within a surface layer thinner than 25 nm from those of the bulk surrounding. The enabling modification of the conventional TFBG-SPR is a reduction of the gold coating from its optimum value near 50-30 nm: at this lower thickness, a surface plasmon wave can still be excited by a limited number of cladding mode resonances, but at the same time, the metal is thin enough to allow modes away from the SPR to tunnel across the metal and probe the bulk RI value. Measurements and simulations of the deposition of a self-assembled monolayer of 1-dodecanethiol in ethanol show that the bulk refractive index changes as small as 0.0004 can be distinguished from the formation of a 1 nm thick coating on the surface of the fiber.

Carbon Monoxide (CO) Sensor Based on Au Nanoparticles Embedded in a CuO Matrix by HR-LSPR Spectroscopy at Room Temperature

Optical gas sensors have emerged as an alternative to conventional conductometric sensors, opening new possibilities, especially for the detection of polluting and/or toxic molecules, such as carbon monoxide (CO). In this letter, an optimized plasmonic thin film, composed of Au nanoparticles embedded in a CuO matrix, was tested as a CO sensor, using high-resolution localized surface plasmon resonance (LSPR) spectroscopy. The Au:CuO thin film system was found to be highly sensitive, exhibiting a fast (~14 s) and reversible (~50 s) response when exposed to 50 ppm of CO, working at room temperature. Furthermore, the Au:CuO thin film prototype sensor presented a signal-to-noise ratio of 11, calculated at the transmittance minimum of the LSPR band, which allowed estimation of a detection limit as low as 14 ppm for CO gas.

Characterization and Sensing of Inert Gases with a High-Resolution SPR Sensor.

It is generally difficult to characterize inert gases through chemical reactions due to their inert chemical properties. The phase interference-sensing system based on high-resolution surface plasmon resonance (SPR) has an excellent refractive index detection limit. Based on this, this paper presents a simple and workable method for the characterization and detection of inert gases. The phase of light for the present SPR sensor is more sensitive to the change in the external dielectric environment than an amplitude SPR sensor. The limit of detection (LOD) is usually in the order of 10−6 to 10−7 RIU, which is superior to LSPR (Localized Surface Plasmon Resonance) sensors and traditional SPR sensors. The sensor parameters are simulated and optimized. Our simulation shows that a 36 nm-thick gold film is more suitable for the SPR sensing of inert gases. By periodically switching between the two inert gases, helium and argon, the resolution of the system is tested. The SPR sensing system can achieve distinguishable difference signals, enabling a clear distinction and characterization of helium and argon. The doping of argon in helium has a detection limit of 1098 ppm.

Binding and Release of Reactive Oxygen Species-Responsive Charge Reversal Cationic Polymers with DNA Studied by Surface Plasmon Resonance

We have designed and synthesized a novel cationic polymer, poly[(2-acryloyl)ethyl (p-boronic acid)benzyl diethylammonium bromide], whose p-boronic acid benzyl can be converted to anionic poly(acrylic acid). Herein, the complexation of the cationic polymer with DNA and their disassembly triggered by reactive oxygen species were analyzed in real time by a high-resolution surface plasmon resonance. We have found that both single-stranded DNA and double-stranded DNA could bind to cationic polymer and immobilized onto the Au surface via electrostatic interactions with the binding constant 1.4 × 1011 M–1 for double-stranded DNA and 1.0 × 1011 M–1 for single-stranded DNA. Upon adding H2O2, the adsorbed DNA was quickly released due to cationic polymer turned into negatively charged poly(acrylic acid). Thus, this study confirmed that the polymer that can undergo positive to negative charge-reversal and provide the quick release of DNA for effective gene transfection.

High-resolution fiber optic surface plasmon resonance sensor for biomedical applications

This paper presents a modified design of a high-resolution fiber optic sensor that operates on the surface plasmon resonance effect. The sensor is based on the well-known method of generating surface plasmons with the help of an inscribed tilted fiber Bragg grating that excites the cladding modes. Because the original design solution used a polarizing fiber, it was possible to significantly improve the stability of the sensor readings. The specialized mathematical apparatus was used to determine the surface plasmon resonance spectral position. It was experimentally shown that the limit of detection to the refractive index of such a sensor is 2 × 10−6 refractive index units. The sensor's response to the investigated medium temperature change is presented and analyzed. The high resolution of the sensor in detecting protein molecules was demonstrated. Such sensors open wide perspectives for their application in real high-sensitivity sensor systems as biosensors for immune analysis in medical diagnostics.

Spatial resolution versus contrast trade-off enhancement in high-resolution surface plasmon resonance imaging (SPRI) by metal surface nanostructure design.

Surface plasmon resonance imaging (SPRI) is an optical near-field method used for mapping the spatial distribution of chemical/physical perturbations above a metal surface without exogenous labeling. Currently, the majority of SPRI systems are used in microarray biosensing, requiring only modest spatial resolution. There is increasing interest in applying SPRI for label-free near-field imaging of biological cells to study cell/surface interactions. However, the required resolution (sub-µm) greatly exceeds what current systems can deliver. Indeed, the attenuation length of surface plasmon polaritons (SPP) severely limits resolution along one axis, typically to tens of µm. Strategies to date for improving spatial resolution result in a commensurate deterioration in other imaging parameters. Unlike the smooth metal surfaces used in SPRI that support purely propagating surface modes, nanostructured metal surfaces support "hybrid" SPP modes that share attributes from both propagating and localized modes. We show that these hybrid modes are especially well-suited to high-resolution imaging and demonstrate how the nanostructure geometry can be designed to achieve sub-µm resolution while mitigating the imaging parameter trade-off according to an application-specific optimum.

Mass Measurements of Focal Adhesions in Single Cells Using High Resolution Surface Plasmon Resonance Microscopy.

Surface plasmon resonance microscopy (SPRM) is a powerful label-free imaging technique with spatial resolution approaching the optical diffraction limit. The high sensitivity of SPRM to small changes in index of refraction at an interface allows imaging of dynamic protein structures within a cell. Visualization of subcellular features, such as focal adhesions (FAs), can be performed on live cells using a high numerical aperture objective lens with a digital light projector to precisely position the incident angle of the excitation light. Within the cell-substrate region of the SPRM image, punctate regions of high contrast are putatively identified as the cellular FAs. Optical parameter analysis is achieved by application of the Fresnel model to the SPRM data and resulting refractive index measurements are used to calculate protein density and mass. FAs are known to be regions of high protein density that reside at the cell-substratum interface. Comparing SPRM with fluorescence images of antibody stained for vinculin, a component in FAs, reveals similar measurements of FA size. In addition, a positive correlation between FA size and protein density is revealed by SPRM. Comparing SPRM images for two cell types reveals a distinct difference in the protein density and mass of their respective FAs. Application of SPRM to quantify mass can greatly aid monitoring basic processes that control FA mass and growth and contribute to accurate models that describe cell-extracellular interactions.

High-resolution surface plasmon resonance sensor with Fano resonance in waveguide-coupled multilayer structures

An ultra-high resolution refractive-index sensor with the Kretschmann configuration was proposed and experimentally demonstrated. The Fano resonance (FR) in the attenuated total reflection curve arose from the interactions between the surface plasmon polariton and planar waveguide modes. It was shown to depend strongly on the structural parameters that governed the position of the FR and to be in good agreement with the results of electromagnetic calculations. The sensitivity by intensity was estimated to be 3.56 × 102-fold higher than that of conventional surface plasmon resonance sensors.

Achieving High Spatial Resolution Surface Plasmon Resonance Microscopy with Image Reconstruction.

Surface plasmon resonance microscopy (SPRM) is a powerful platform for biomedical imaging and molecular binding kinetics analysis. However, the spatial resolution of SPRM along the plasmon propagation direction (longitudinal) is determined by the decaying length of the plasmonic wave, which can be as large as tens of microns. Different methods have been proposed to improve the spatial resolution, but each at the expense of decreased sensitivity or temporal resolution. Here we present a method to achieve high spatial resolution SPRM based on deconvolution of complex field. The method does not require additional optical setup and improves the spatial resolution in the longitudinal direction. We applied the method to image nanoparticles and achieved close-to-diffraction limit resolution in both longitudinal and transverse directions.

Theoretical design of a surface plasmon resonance sensor with high sensitivity and high resolution based on graphene–WS2 hybrid nanostructures and Au–Ag bimetallic film

We proposed a high sensitivity and a high resolution surface plasmon resonance sensor composed of graphene–WS2 hybrid nanostructure and Au–Ag bimetallic-layers film.

Two-Dimensional Surface Plasmon Resonance Imaging System for Cellular Analysis.

Optical biosensors based on surface plasmon resonance (SPR) phenomenon have received a great deal of attention in cellular analysis applications. Sensitive and high-resolution SPR imaging (SPRi) platforms are very useful for real-time monitoring and measurement of individual cell responses to various exogenous substances. In cellular analysis, mainstream SPR-based sensors have potential for investigations of cell responses under ambient conditions. Evaluations that account only for the average response of cell monolayers mask the understanding of precise cell-molecular interactions or intracellular reactions at the level of individual cells. SPR/SPRi technology has attracted a great deal of attention for detecting the response of cell monolayers to various substances cultivated on the gold sensor chip. To unleash the full strength of SPRi technology in complex cell bio-systems, the applied SPR imaging system needs to be sufficiently effective to allow evaluation of a compound's potency, specificity, selectivity, toxicity, and effectiveness at the level of the individual cell. In our studies, we explore the utility of high-resolution 2D-SPR imaging for real-time monitoring of intracellular translocation of protein kinase C (PKC), and detection of neuronal differentiation in live cells at the level of individual cells. The PC12 cell line, which is one of the most commonly used neuronal precursor cell lines for research on neuronal differentiation, was chosen as a nerve cell model. Two dimensional SPR (2D-SPR) signals/images are successfully generated. We have found that cells treated with the differentiation factor nerve growth factor (NGF) showed a remarkable enhancement of SPR response to stimulation by muscarine, a nonselective agonist of the muscarinic acetylcholine receptor.

The Potential Prognostic Markers for Myelodysplatic Syndromes Studied By Surface Plasmon Resonance Imaging and Mass Spectrometry

IntroductionMyelodysplastic syndromes are clonal hematopoetic stem cell disorders characterized by refractory cytopenias with displasia as a result of ineffective hematopoiesis. Approximately 30 percent of MDS cases progress to acute myeloid leukemia. Excess blasts are the strongest predictors for poor outcome and are associated with disease progression. Because of its heterogeneity and lack of molecular and protein markers that effectively monitor disease progression, clinical management of MDS patients is challenging.The concentrations of several proteins were found elevated during MDS disease. Their interaction with receptors/ligands is a part of signaling pathways that may play a crucial role in pathogenesis of the disease. Serum levels of vascular cell adhesion protein 1 (VCAM1) and intercellular adhesion molecule 1 (ICAM1) were found higher in MDS patients in comparison with control group and were related to MDS severity. Alpha-2-HS-glycoprotein (FETUA) was analyzed in plasma of MDS patients. Leucine-rich alpha-2-glycoprotein (LRG) was found increased in RAEB-I and RAEB-II patients by proteomic studies.In this work we developed a protein chip for study of molecular interactions in plasma of MDS patients using surface plasmon resonance (SPR) imaging. Interacting proteins were analyzed and identified using LC-MS/MS.MethodsPatients with diagnosis of refractory anemia and refractory anemia with ringed sideroblasts (RA/RARS, n=13), refractory cytopenia with multilineage dysplasia (RCMD, n=16), and refractory anemia with excess blasts (RAEB, n=15) as a subgroup of MDS, and MDS patients who progress to acute myeloid leukemia (AML, n=8) were chosen for this study.The high-resolution SPR imaging (SPRi) system with polarization contrast and internal referencing combined with dispersionless microfluidics was used to study protein interactions. Passive mixing structures were employed to further improve the sensing performance of this SPRi biosensor.The respective biomarkers (ICAM1, VCAM1, FETUA and LRG) were covalently attached to the functionalized sensor surface and interaction with their counterparts in MDS plasma samples were monitored.Interacting proteins were removed from the chip surface by diluted NaOH, treated by trypsin, analyzed by nanoLC-MS/MS (TripleTOF 5600) and identified using ProteinPilot database.Results and ConclusionSignificant differences were observed between analyzed MDS subtypes for VCAM1, FETUA, and LRG (ANOVA: P = 0,00755, P = 0,0306 and P < 0.001 respectively). Using post hoc Tukey contrasts tests we ascertained that sensor responses were significantly higher in group RCMD and AML with respect to the healthy donors for immobilized VCAM1 and in group RA/RARS, RCMD, RAEB and AML with respect to the healthy donors for LRG. Moreover sensor responses were significantly higher in group AML with respect to the RA, RCMD or RAEB for immobilized LRG. Based on our results LRG seems to be a suitable marker of MDS progression.The identified proteins presented in MDS plasma samples unlike in healthy donor plasma are clusterin, protein S100-A8, serum amyloid A-1 protein, annexin A2, and zinc-alpha-2-glycoprotein among others. S100-A8 and clusterin have been connected with MDS or AML previously; it could be tested as a new protein marker of MDS by SPRi. The results show that SPR biosensors combined with nanoLC-MS/MS are a promising tool for improvement of the diagnosis in complex heterogeneous malignancies. DisclosuresNo relevant conflicts of interest to declare.

Rational Design of Thermodynamic and Kinetic Binding Profiles by Optimizing Surface Water Networks Coating Protein-Bound Ligands.

A previously studied congeneric series of thermolysin inhibitors addressing the solvent-accessible S2' pocket with different hydrophobic substituents showed modulations of the surface water layers coating the protein-bound inhibitors. Increasing stabilization of water molecules resulted in an enthalpically more favorable binding signature, overall enhancing affinity. Based on this observation, we optimized the series by designing tailored P2' substituents to improve and further stabilize the surface water network. MD simulations were applied to predict the putative water pattern around the bound ligands. Subsequently, the inhibitors were synthesized and characterized by high-resolution crystallography, microcalorimetry, and surface plasmon resonance. One of the designed inhibitors established the most pronounced water network of all inhibitors tested so far, composed of several fused water polygons, and showed 50-fold affinity enhancement with respect to the original methylated parent ligand. Notably, the inhibitor forming the most perfect water network also showed significantly prolonged residence time compared to the other tested inhibitors.

The effects of magnetic fields exposure on relative permittivity of saline solutions measured by a high resolution SPR system.

A measurement system for the relative permittivity of a physiological solution under 50 Hz magnetic fields (MF) is presented. It is based on a phase-sensitive surface plasmon resonance (SPR) system. Relative permittivity was analyzed for different solute concentrations of sodium chloride under various MF exposure parameters. We found that MF exposure at 0.2–4.0 mT step-wise decreased significantly the SPR phase signal of a 0.9% sodium chloride solution while 0.1 mT of MF exposure did not. The decreases in the SPR phase signal depended on the duration of MF exposure, and the signal reached a plateau after 15 min of exposure. Interestingly, the decreased SPR phase signal showed a gradual increase and approached the background level when the exposure was drawn off. In addition, we found that the response of the sodium chloride solution to MF also depended on its concentration. In brief, the relative permittivity of sodium chloride in solutions appears to be practically affected by 50 Hz MF exposure. Our data indicates that the relative permittivity of the saline solution influenced by MF exposure should be considered when investigating the biological effects of MF exposure on organisms in experimental study.

Resolution enhancement of surface plasmon resonance sensors with spectral interrogation: resonant wavelength considerations

Surface plasmon resonance (SPR) has been used extensively in biosensing implementation as a high-sensitivity and label-free sensing technique. For SPR sensors based on spectral interrogation, their performance in terms of refractive index (RI) resolution differs substantially when operating at different resonant wavelengths. This paper presents a feasible scheme of enhancing the sensor resolution by simulating measured SPR curves using an experiment-assisted simulation model to effectively explore the optimal resonant wavelength for the sensing system. It shows that the variation of experimentally obtained RI resolution with the resonant wavelength is in good accordance with the simulation results. In addition, the RI resolution at the derived optimal resonant wavelength is at least 2 times better than that at the commonly used resonant wavelength region both experimentally and theoretically. The methods of the proposed scheme potentially facilitate optimization of various sensor instrumentation for high-resolution SPR sensing.

Highly sensitive biochemical sensor comprising rectangular nanometal arrays

This paper proposes a novel design for a highly sensitive high-resolution localized surface plasmon resonance (LSPR) biochemical sensor. The geometrical structure of the sensor consists of three segments. The first segment comprises single-mode fibers located in the output and input ends of the sensor. The second and third segments comprise rectangular nanometal arrays and serve as the analyte regions of the sensor; the amount of displacement between these two segments is one rectangular nonmetal particle. We integrated two breakthrough methods, the object meshing method and the boundary meshing method, with the finite element method to effectively improve the accuracy of simulation outcomes and reduce the amount of time and memory required for performing calculations. Subsequently, we constructed the proposed LSPR biochemical sensor, and the results indicated that the proposed sensor exhibited excellent geometric structure and spectral characteristics. Specifically, the sensor is short (approximately 250μm) and features high resolution (approximately −130dB) and high sensitivity (approximately 126,849.1333nm/RIU).

Applications of High Resolution Surface Plasmon Resonance Imaging to Adherent Cells: Single Mammalian Cells to Bacterial Biofilms

High resolution surface plasmon resonance imaging (SPRI) allows label-free imaging of subcellular features when performed using a high numerical aperture objective lens with a digital light projector to precisely position incident angle excitation. The SPRI signal is a result of the mass of material within the evanescent field, and can be interpreted as a density or a refractive index of the cellular material. The quantitative nature of SPR images and the direct relationship to refractive index changes at the surface sensor allow for visualization new insights into mechanisms of cell biology at an interface. When applied to mammalian cells, such as rat aortic smooth muscle cells, cellular components near the sensor surface such as the cell membrane, focal adhesions, and cell nucleus are visualized in the SPRI image. Focal adhesion sizes measured by SPRI are similar with those highlighted with fluorescent antibody stained vinculin. In addition, a positive correlation between focal adhesion size and protein density is observed by SPR imaging. When SPRI is applied to pathogenic biofilms of Streptococcus mutans, distinct components of the bacterial biofilm at the surface including individual bacteria, bacterial microcolonies, and extracellular polymeric substance (EPS) are observed. SPRI shows that the refractive index of bacteria in a biofilm increases over time compared to that of bacteria not in a biofilm, which remains constant. SPRI also indicates that the EPS material generated in the biofilm at early time points is thicker near the bacterial microcolony periphery. This suggests that the EPS matrix is generated at the colony edge and that SPRI can be used to monitor the dynamics of EPS production in biofilms.