Surface Plasmon Resonance Response Research Articles

Highly sensitive SPR response of Au/chitosan/graphene oxide nanostructured thin films toward Pb (II) ions

Optical sensors based on surface plasmon resonance (SPR) are utilized for detecting toxic heavy metals in solutions. To improve the sensitivity of SPR sensors, nanostructured thin films with active layers can be synthesized. In this study, the response to Pb (II) was measured and compared for SPR sensors incorporating gold–chitosan–graphene oxide (Au/CS/GO) nanostructured thin films and Au/CS films. The characterization of Au/CS/GO using FESEM analysis revealed a film composed of nanosheets with wrinkled, rough surfaces. The results from XRD analysis confirmed the successful incorporation of GO in the prepared films. Additionally, AFM analysis determined that the Au/CS/GO films had a root mean square (rms) roughness of 28.38nm and were considerably rougher than the Au/CS films. Upon exposure to a 5ppm Pb (II) ion solution, the Au/CS/GO films exhibited higher SPR sensitivity, as much as 1.11200ppm−1, than Au/CS films, 0.77600ppm−1. This enhancement of the SPR response was attributed to strong covalent bonding between CS and GO in these films. These results indicated that the Au/CS/GO films show potential for the detection of heavy metal pollution in environmental applications.

Protein–Conjugated Quantum Dots Interface: Binding Kinetics and Label-Free Lipid Detection

We propose a label-free biosensor platform to investigate the binding kinetics using antigen-antibody interaction via electrochemical and surface plasmon resonance (SPR) techniques. The L-cysteine in situ capped cadmium sulfide (CdS; size < 7 nm) quantum dots (QDs) self-assembled on gold (Au) coated glass electrode have been covalently functionalized with apolipoprotein B-100 antibodies (AAB). This protein conjugated QDs-based electrode (AAB/CysCdS/Au) has been used to detect lipid (low density lipoprotein, LDL) biomolecules. The electrochemical impedimetric response of the AAB/CysCdS/Au biosensor shows higher sensitivity (32.8 kΩ μM(-1)/cm(2)) in the detection range, 5-120 mg/dL. Besides this, efforts have been made to investigate the kinetics of antigen-antibody interactions at the CysCdS surface. The label-free SPR response of AAB/CysCdS/Au biosensor exhibits highly specific interaction to protein (LDL) with association constant of 33.4 kM(-1) s(-1) indicating higher affinity toward LDL biomolecules and a dissociation constant of 0.896 ms(-1). The results of these studies prove the efficacy of the CysCdS-Au platform as a high throughput compact biosensing device for investigating biomolecular interactions.

On-line surface plasmon resonance biosensing of vascular endothelial growth factor signaling in intact-human hepatoma cell lines

Surface plasmon resonance (SPR) monitoring of biorecognition events at intracellular levels is a valuable tool for studying the angiogenic response of carcinoma living cells during tumor growth and proliferation. We report here a comparative study of two different strategies to detect human hepatoma cell interactions between transmembrane vascular endothelial growth factor receptor (VEGFR2) and vascular endothelial growth factor (VEGF). To monitor VEGFR2 activation after VEGF stimulation, intact hepatocellular carcinoma HepG2 or Huh7 cells (2 × 10(5) cells per mL) were directly immobilized on the sensor chip. Distinguishable SPR sensorgrams were obtained for each cell line depending on the time required for VEGFR2 activation. SPR signals for VEGF-VEGFR2 binding were inhibited by the VEGFR inhibitor, CBO-P11. The SPR response after VEGF stimulation/inhibition was in good agreement with the results observed by immunoblotting analysis. In a second approach we used intact cell lines as analytes. SPR analysis was done by injecting HepG2 and HuH7 cell suspensions (2-4 × 10(4) cells per mL) onto a sensor surface previously immobilized with VEGF via a thiol self-assembled monolayer (SAM). Specificity and reproducibility were evaluated reusing the same chip surface over more than 60 complete regeneration cycles. Comparison between both methods yielded differences in terms of reliability, making the latter strategy more effective for the analysis of real samples. The investigation of VEGF signaling in intact human hepatoma living cells by SPR monitoring comprises a novel and promising design for the study of tumor angiogenesis via downregulation of VEGF and VEGFR2 pathways. Further investigation on VEGFR activation and vascular function could contribute to establish a robust and meaningful tool for early cancer diagnostics.

Influence of the Debye length on the interaction of a small molecule-modified Au nanoparticle with a surface-bound bioreceptor

We report that a shorter Debye length and, as a consequence, decreased colloidal stability are required for the molecular interaction of folic acid-modified Au nanoparticles (Au NPs) to occur on a surface-bound receptor, human dihydrofolate reductase (hDHFR). The interaction measured using surface plasmon resonance (SPR) sensing was optimal in a phosphate buffer at pH 6 and ionic strength exceeding 300 mM. Under these conditions, the aggregation constant of the Au NPs was approximately 10(4) M(-1) s(-1) and the Debye length was below 1 nm, on the same length scale as the size of the folate anion (approximately 0.8 nm). Longer Debye lengths led to poorer SPR responses, revealing a reduced affinity of the folic acid-modified Au NPs for hDHFR. While high colloidal stability of Au NPs is desired in most applications, these conditions may hinder molecular interactions due to Debye lengths exceeding the size of the ligand and thus preventing close interactions with the surface-bound molecular receptor.

Coupling surface plasmon resonance of gold nanoparticles with slow-photon-effect of TiO2 photonic crystals for synergistically enhanced photoelectrochemical water splitting

The slow photon effect of a photonic crystal (PC) is a promising characteristic for tuning light–matter interactions through material structure designing. A TiO2 bi-layer structure photoanode was constructed by fabricating a TiO2 PC layer through a template-assisted sol–gel process on a TiO2 nanorod array (NR) layer. Gold nanoparticles (Au NPs) with an average size of about 10 nm were deposited in situ into the TiO2 bi-layer structure. The extended photoelectrochemical (PEC) water splitting activity in visible light was ascribed to the energetic hot electrons and holes that were generated in the Au NPs through the excitation and decay of surface plasmons. By alternating the characteristic pore size of the TiO2 PC layer, the slow photon region at the red edge of the photonic band gap could be purposely tuned to overlap with the strong localized surface plasmon resonance (SPR) region of Au NPs. The matching slow photon effect of TiO2 PC (with a characteristic pore size of 250 nm) intensified the SPR responses (central at 536 nm) of Au NPs. Consequently, more hot electrons were generated in the Au NPs and injected into the conduction band of TiO2, resulting in improved PEC water splitting efficiency in the visible light region. Under simulated sunlight illumination, the photoconversion efficiency of the well matching Au/TiO2 photoanode approached 0.71%, which is one of the highest values ever reported in Au/TiO2 PEC systems. The work reported here provides support for designing coupling plasmonic nanostructures with PC-based materials to synergistically enhance PEC water splitting efficiency.

Novel pH-responsive nanoplasmonic sensor: controlling polymer structural change to modulate localized surface plasmon resonance response

The detection of chemical or biological analytes in physiological media remains a great challenge and current methods suffer from low sensitivity, reproducibility, and require expensive instruments. Here we report the design of a simple, pH-responsive nanoplasmonic sensor utilizing polymer structural changes to induce localized surface plasmon resonance (LSPR) shifts. The sensors were fabricated by chemical attachment of poly(allylamine) onto ∼28 nm gold nanoprisms bound to a silanized glass surface. The reversible change of polymer structure upon protonation and deprotonation of its amine groups alters the nanoprisms' LSPR properties. A spectral shift of the nanoprisms' dipole peak was observed because of changes in thickness of local dielectric environment, which are caused by shrinking and swelling of the pH-responsive polymer. The pH-induced shrinking and the swelling transition provided the opportunity to design ultrasensitive glucose sensors. The pH change from oxidation of β-glucose by glucose oxidase, resulted in up to a 17 nm LSPR peak shift because of the 3.7 nm change in polymer thickness measured by in situ atomic force microscopy. The lowest concentration of glucose that can be repeatedly detected in bovine plasma with this sensor was 25 μM. This nanoplasmonic sensor exhibited simplicity of operation and excellent reproducibility. The polymer-functionalized sensor provided a powerful avenue for simple, ultrasensitive, and cost effective detection of target analytes, which can be translated to clinical application.

Surface Plasmon Resonance Sensor Based on Magnetic Molecularly Imprinted Polymers Amplification for Pesticide Recognition

We reported here a method to enhance detection sensitivity in surface plasmon resonance (SPR) spectroscopy integrated with a surface molecular imprinting recognition system and employing magnetic molecular imprinting polymer nanoparticles for amplifying SPR response. The proposed magnetic molecular imprinting polymer was designed by self-polymerization of dopamine on the Fe3O4 NPs surface in weak base aqueous solution in the presence of template chlorpyrifos (CPF). The imprinted Fe3O4@polydopamine nanoparticles (Fe3O4@PDA NPs) were characterized by Fourier transform infrared spectroscopy, UV-vis absorption spectroscopy, and transmission electron microscopy. The biosensor showed a good linear relationship between the SPR angle shift and the chlorpyrifos concentration over a range from 0.001 to 10 μM with a detection limit of 0.76 nM. A significant increase in sensitivity was therefore afforded through the use of imprinted Fe3O4@PDA NPs as an amplifier, and meanwhile, the imprinted Fe3O4@PDA NPs had an excellent recognition capacity to chlorpyrifos over other pesticides. The excellent sensitivity and selectivity and high stability of the designed biosensor make this magnetic imprinted Fe3O4@PDA NP an attractive recognition element for various SPR sensors for detecting pesticide residuals and other environmentally deleterious chemicals.

Temperature-Controlled Reversible Localized Surface Plasmon Resonance Response of Polymer-Functionalized Gold Nanoprisms in the Solid State

Solid-state temperature responsive localized surface plasmon resonance (LSPR)-based nanosensors were constructed by functionalizing the glass substrate-attached gold nanoprisms with the thermoresponsive polymer poly(allylamine hydrochloride)-co-poly(N-isopropylacrylamide). The robustness of the sensor was enhanced by chemically attaching polymer to the nanoprism surface through an amide coupling reaction versus simple physisorption of polymer onto nanoprism. The highest sensitivity of this kind of solid sensing platform was obtained by employing chemically synthesized gold nanoprisms for fabrication. The surface ligand chemistry significantly influenced the swelling and shrinking transition of the polymer during the temperature variation, which resulted in the alteration of the local dielectric environment of the nanoprisms, modulation of their LSPR properties, and enhancement of sensing efficiency of the nanosensors. Importantly, we have shown for the first time that the dimension of the nanostructure plays an important role in achieving the highest sensitivity for these types of sensors. For example, the edge length of the nanoprisms plays a critical role in the temperature sensitivity of the nanosensors where nanoprisms with ∼28 and ∼40 nm edge lengths displayed ∼10.9 and ∼18.2 nm LSPR dipole peak red-shift, respectively, as the solution temperature increased from 18 to 56 °C. We believe the higher temperature sensitivity for larger edge-length nanoprisms was achieved due to their larger sensing volume. The nanosensors were found to be very stable and displayed high reversibility, which suggests that our temperature-dependent nanosensors can potentially be used as a reversible thermal switch.

Rapid detection of aflatoxin B1 by a bifunctional protein crosslinker-based surface plasmon resonance biosensor

The aim of this study was the development of a bifunctional protein crosslinker-based surface plasmon resonance (SPR) biosensor for rapid detection of aflatoxin B1 (AFB1), a potent carcinogen. A fusion protein was obtained by genetically fusing gold binding protein (GBP) that binds strongly to gold surfaces to protein G (ProG) that interacts with the Fc portion of antibodies. It was used as a bifunctional crosslinker for rapid self-oriented immobilization of antibodies on gold substrates without any chemical treatment. SPR analyses demonstrated the binding of the GBP-ProG crosslinker to the gold surface was superior to that of an only ProG via currently used self-assembled monolayers of alkanethiol due to the GBP property. As a result, anti-AFB1 antibodies were 36% more immobilized on the GBP-ProG layer than the ProG layer. When the GBP-ProG crosslinker-based SPR chips were fabricated with the best density (100 μg/mL) of anti-AFB1 antibodies, they could detect AFB1 as low as 1 μg/mL in both buffer and corn extracts and selectively detect it with negligible SPR responses in control toxins (zearalenone and ochratoxin A). These results mean the GBP-ProG is more useful than the thiolated chemical linkers for development of gold substrate-based immunosensors, and this GBP-ProG crosslinker-based immunosensor could detect small molecules effectively.

Elucidating the Signal Responses of Multi-Parametric Surface Plasmon Resonance Living Cell Sensing: A Comparison between Optical Modeling and Drug–MDCKII Cell Interaction Measurements

In vitro cell-based assays are widely used during the drug discovery and development process to test the biological activity of new drugs. Most of the commonly used cell-based assays, however, lack the ability to measure in real-time or under dynamic conditions (e.g. constant flow). In this study a multi-parameter surface plasmon resonance approach in combination with living cell sensing has been utilized for monitoring drug-cell interactions in real-time, under constant flow and without labels. The multi-parameter surface plasmon resonance approach, i.e. surface plasmon resonance angle versus intensity plots, provided fully specific signal patterns for various cell behaviors when stimulating cells with drugs that use para- and transcellular absorption routes. Simulated full surface plasmon resonance angular spectra of cell monolayers were compared with actual surface plasmon resonance measurements performed with MDCKII cell monolayers in order to better understand the origin of the surface plasmon resonance signal responses during drug stimulation of cells. The comparison of the simulated and measured surface plasmon resonance responses allowed to better understand and provide plausible explanations for the type of cellular changes, e.g. morphological or mass redistribution in cells, that were induced in the MDCKII cell monolayers during drug stimulation, and consequently to differentiate between the type and modes of drug actions. The multi-parameter surface plasmon resonance approach presented in this study lays the foundation for developing new types of cell-based tools for life science research, which should contribute to an improved mechanistic understanding of the type and contribution of different drug transport routes on drug absorption.

An electrochemical surface plasmon resonance imaging system targeting cell analysis

This paper presents an electrochemical-surface plasmon resonance imaging (EC-SPRI) system, enabling the characterization of optical and electrical properties of cells, simultaneously. The developed surface plasmon resonance (SPR) imaging system was capable of imaging micro cavities with a dimension of 10 μm × 10 μm and differentiated glycerol solutions with a group of refractive indices (RIs). Furthermore, the EC-SPRI system was used to image A549 cells, suggesting corresponding RI and morphology changes during the cell death process. In the end, electrochemical and SPR methods were used in combination, recording oxidation peaks of A549 cells in the cyclic voltage curves and SPR response unit increase, simultaneously.

An In-situ Real-Time Optical Fiber Sensor Based on Surface Plasmon Resonance for Monitoring the Growth of TiO2 Thin Films

An optical fiber sensor based on surface plasmon resonance (SPR) is proposed for monitoring the thickness of deposited nano-thin films. A side-polished multimode SPR optical fiber sensor with an 850 nm-LD is used as the transducing element for real-time monitoring of the deposited TiO2 thin films. The SPR optical fiber sensor was installed in the TiO2 sputtering system in order to measure the thickness of the deposited sample during TiO2 deposition. The SPR response declined in real-time in relation to the growth of the thickness of the TiO2 thin film. Our results show the same trend of the SPR response in real-time and in spectra taken before and after deposition. The SPR transmitted intensity changes by approximately 18.76% corresponding to 50 nm of deposited TiO2 thin film. We have shown that optical fiber sensors utilizing SPR have the potential for real-time monitoring of the SPR technology of nanometer film thickness. The compact size of the SPR fiber sensor enables it to be positioned inside the deposition chamber, and it could thus measure the film thickness directly in real-time. This technology also has potential application for monitoring the deposition of other materials. Moreover, in-situ real-time SPR optical fiber sensor technology is in inexpensive, disposable technique that has anti-interference properties, and the potential to enable on-line monitoring and monitoring of organic coatings.

Charge-Based Detection of Small Molecules by Plasmonic-Based Electrochemical Impedance Microscopy

Charge-based detection of small molecules is demonstrated by plasmonic-based electrochemical impedance microscopy (P-EIM). The dependence of surface plasmon resonance (SPR) on surface charge density is used to detect small molecules (60-120 Da) printed on a dextran-modified sensor surface. Local variations in charge density on an electrode surface are manifest in an optical SPR signal. The SPR response to an applied ac potential measures the sensor surface impedance which is a function of the surface charge density. This optical signal is comprised of a dc and an ac component, and is measured with high spatial resolution. The dc element of the SPR signal represents conventional SPR imaging information. The amplitude and phase of local surface impedance is provided by the ac component. The phase signal of the small molecules is a function of their charge status, which is manipulated by the pH of a solution. Small molecules with positive, neutral, and negative charge are detected by P-EIM. This technique is used to detect and distinguish small molecules based on their charge status, thereby circumventing the mass limitation (~100 Da) of conventional SPR measurement.

Low-cost, high performance surface plasmon resonance-compatible films characterized by the surface plasmon resonance technique

A new analytical method based on the surface plasmon resonance (SPR) technique is presented, with which SPR curves for both wavelength and angular modulations can be obtained simultaneously via only a single scan of the incident angle. Using this method, the SPR responses of TiO2-coated Cu films are characterized in the wavelength range from 600 nm to 900 nm. For the first time, we determine the effective optical constants and the thicknesses of TiO2-coated Cu films using the SPR curves of wavelength modulation. The sensitivities of prism-based SPR refractive index sensors using TiO2-coated Cu films are investigated theoretically for both wavelength and angular modulations, the results show that in the case of sensitivity with wavelength modulation, TiO2-coated Cu films are not as good as the Au film, however, they are more suitable than the Au film for SPR refractive index sensors with angular modulation because a higher sensitivity can be achieved.

Identification of the molecular mechanisms in cellular processes that elicit a surface plasmon resonance (SPR) response using simultaneous surface plasmon-enhanced fluorescence (SPEF) microscopy

Surface plasmon resonance (SPR) has developed into a powerful approach for label-free monitoring of cellular behavior. Most cellular responses, however, involve a complex cascade of molecular events which makes identifying the specific components of cellular behavior dynamics contributing to the aggregate SPR signal problematic. Recently, a number of groups have used surface plasmon-enhanced fluorescence (SPEF) microscopy on living cells. In this work, we show that SPEF microscopy can be used to identify the molecular mechanisms responsible for SPR detection of cellular processes. By specifically labeling the actin cytoskeleton in human epithelial kidney cells (HEK 293) and rat vascular smooth muscle cells (A7r5), we correlate cell reorganization observed in SPEF with SPR signal variations reflecting aggregate cellular changes. HEK 293 cells stimulated with angiotensin-II exhibited transient contraction, appearing as an SPR signal decrease with a subsequent increase above the initial baseline. SPEF micrographs showed a decrease in cellular area followed by actin densification and cell spreading. A7r5 stimulated with Latrunculin A showed actin cytoskeleton depolymerization, generating a steady SPR signal decrease, with SPEF micrographs showing extensive collapse of cell actin structures. We observed that SPR monitoring of cellular response is strongly dependent on minute variations in cellular footprint on the substrate as well as changes in the molecular density in the basal portions of the cells. Therefore, combining SPR with imaging of selective fluorescent markers by SPEF allows a more comprehensive deconvolution of the cellular signal in relation to molecular events within the cells.

Enhanced Stability in Surface Plasmon Resonance Sensor Using Prism Coupler Based on Au/Bi2O3 Bilayer Film

Surface Plasmon Resonance (SPR) sensing has become a comprehensive utilized technology for detection, measurement and analysis in a wide spectrum of fields, ranging from biotechnology, environmental monitoring to food and drug monitoring. In this study, Au/Bi 2O3 bilayer films with various layer thicknesses were deposited by thermal evaporation method on BK7 prism substrates and then post-annealing was conducted under ambient conditions. The adhesive strength of Au/Bi2O3 and Bi2O3/prism was measured with different layer thicknesses. Also, the SPR responses (reflectance vs. incident angle) were investigated as a function of the thickness of Bi2O3 layer in the Kretschmann geometry using ethanol as dielectric. The results indicate that the adhesive strength between Au and prism was improved more than 3 times by introducing the Bi 2O3 as buffer layer. And the SPR dips also demonstrate that SPR sensor based on Au/Bi 2O3 bilayer films is practical, although the height of SPR dip with about 6nm Bi2O3 is approximately 4 times weaker than that of monolayer Au and the width (defined as FWHM) broadens from 9° to 11°.

Au Nanocrystal Array/Silicon Nanoantennas as Wavelength-Selective Photoswitches

Au nanocrystal array/silicon nanoantennas exhibiting wavelength-selective photocurrent enhancement were successfully fabricated by a facile and inexpensive method combining colloidal lithography (CL) and a metal-assisted chemical etching (MaCE) process. The localized surface plasmon resonance (LSPR) response and wavelength-selective photocurrent enhancement characteristics were achieved by tuning the depth of immersion of Au nanocrystal arrays in silicon through a MaCE process. The wavelength selectivity of photocurrent enhancement contributed by LSPR induced local field amplification was confirmed by simulated near-field distribution. In addition, it can be integrated to well-developed Si-based manufacturing process. These characteristics make Au nanocrystal array/Si nanoantennas promising as low power-consumption photoswitches and nano-optoelectronic and photonic communication devices.

Optimization of the refractive index plasmonic sensing of gold nanorods by non-uniform silver coating

Localized surface plasmon resonance (LSPR) response sensitivity to the environmental refractive index (RI) has been studied for Ag coated Au nanorods both theoretically and experimentally. Quasistatic calculation results indicate that the Ag coating induced increasing of RI sensitivity only takes place when the Ag shell at the ends is much thicker than that at the side facets. Because of the corporate effects from the coating thickness dependent sensitivity and band width changing, the Ag coating always leads to an improvement of the figure of merit (FOM) of Au nanorod sensing, and the increase of FOM becomes faster as the Ag coating is non-uniform. Furthermore, higher RI sensitivity with the same resonance wavelength could be obtained by Ag coating, and the improvement of RI sensitivity with the same resonance wavelength could be fine tuned by changing the non-uniformity ratio of the Ag coating. These calculation results have also been verified experimentally. And this Ag coating dependent optimization of RI sensitivity of Au–Ag bimetallic nanorod makes them potential candidates for nanoscale biochemical sensing applications.

SPR studies of the adsorption of silver/bovine serum albumin nanoparticles (Ag/BSA NPs) onto the model biological substrates

The primary objective of this study is to investigate the interactive forces that promote the adsorption of bio-conjugated nanoparticles onto proteins. To elucidate the interactive forces, we demonstrate an approach using synthetic and model biological surfaces to study adsorption of bio-conjugated nanoparticles. Real-time adsorption of BSA conjugated silver nanoparticles (Ag/BSA NPs) on the immobilized substrates was followed by surface plasmon resonance (SPR). The extent of adsorption of the nanoparticles on the synthetic surface was found to be larger for self-assembled monolayers (SAMs) with ionizable terminal groups and lower for SAMs with unionizable terminal groups. For model biological substrate, the extent of nanoparticles adsorption was found to relate to the pKa of immobilized proteins. For collagen immobilized substrate, the adsorption of Ag/BSA nanoparticles showed a significantly higher SPR response than that of free BSA. The extent of nanoparticles adsorption on the collagen immobilized substrate was also influenced by the type and concentration of electrolyte used in dispersing nanoparticles. Our findings indicate that the adsorption of nanoparticles to immobilized surface has contributions from electrostatic interactions, hydrophobic, and/or hydrogen bonding. This work provides the framework to study interactions that may arise when bio-conjugated nanoparticles are transported in biological systems.

A gravity-induced flow injection system for surface plasmon resonance biosensor

A number of portable surface plasmon resonance (SPR) devices have been developed for point-of-care (POC) testing. Meanwhile, micropumps have been fabricated to be integrated into these devices for flow injection analysis (FIA). However, the (micro) pumps, the tubes and their external control units were space-consuming. Here we developed a power-free flow injection analysis (FIA) method for SPR detection based on a gravity-induced flow injection (gFI) system. The gFI system was tubeless and did not need to be controlled. The fluid was driven into the detection areas by its own gravitational force. A transition channel was used to increase the liquid-level difference between the inlet reservoir and the outlet reservoir. After a liquid sample was placed in the inlet reservoir, the flow rate of the liquid sample was increased in the transition channel. Before it arrived at the sensing surface, the flow rate of the sample was steady (with an error of less than 10%). The fluctuation of the flow rate had an influence on the SPR response signal, which was successfully denoised using an internal reference. With the gFI system, the SPR imaging biosensor was able to perform real-time detection manually. The SPR responses of DNA hybridization and protein immobilization were successfully obtained.