Time-resolved Surface Plasmon Resonance Research Articles

Direct Measurement of Near-Wall Molecular Transport Rate in a Microchannel and Its Dependence on Diffusivity.

Solute transport in a narrow space is the most elemental process in chromatography and biological pattern formation. However, the observation of such transport has been quite difficult, and theoretical investigations have therefore preponderated. Here, using a space- and time-resolved surface plasmon resonance (SPR) method, we measured the nanoscale near-wall (next to the wall) transport rate in a narrow channel after a solution and its solvent had come into contact. By combining the SPR method with a capillary injection method, which enables two solution plugs to flow immediately after they have made contact, we were able to measure the solute concentration evolution at the channel wall. We tested three combinations of two plugs of solution-water-glucose, sodium chloride-water, and glucose-sodium chloride-and succeeded in measuring diffusion-coefficient-dependent changes in the concentration of solute flowing through a rectangular microchannel in less than 0.4 s. A numerical analysis of this system revealed the acceleration of the solute/solution boundary moving on the wall and its deceleration at the center of the channel cross section. The observed experimental transport rate agreed with the numerical result quantitatively. These results show that the solute transport followed a laminar flow with a no-slip model and that the molecules were transported in the order of their diffusivity. In the third combination, when the two solutions made contact and started flowing, the interdiffusion of the solutes resulted in temporal concentrations lower than either of the solutions before contact, which indicated that the contact between the two solutions quickly led to separation by the advection-diffusion processes. We found that such a concentration profile could actually be measured. Our techniques are simple and applicable to a wide range of molecules; the method opens the way to direct observation of the space-time near-wall solute transport process and can be used for the rapid determination of diffusivity.

Deciphering the stepwise binding mode of HRG1β to HER3 by surface plasmon resonance and interaction map.

For the development of efficient anti-cancer therapeutics against the HER receptor family it is indispensable to understand the mechanistic model of the HER receptor activation upon ligand binding. Due to its high complexity the binding mode of Heregulin 1 beta (HRG1β) with its receptor HER3 is so far not understood. Analysis of the interaction of HRG1β with surface immobilized HER3 extracellular domain by time-resolved Surface Plasmon Resonance (SPR) was so far not interpretable using any regular analysis method as the interaction was highly complex. Here, we show that Interaction Map (IM) made it possible to shed light on this interaction. IM allowed deciphering the rate limiting kinetic contributions from complex SPR sensorgrams and thereby enabling the extraction of discrete kinetic rate components from the apparently heterogeneous interactions. We could resolve details from the complex avidity-driven binding mode of HRG1β with HER3 by using a combination of SPR and IM data. Our findings contribute to the general understanding that a major conformational change of HER3 during its activation is induced by a complex sequential HRG1β docking mode.

Combination and Applications of Time-Resolved Surface Plasmon Resonance Spectroscopy and Electrochemical Methods

Combination and Applications of Time-Resolved Surface Plasmon Resonance Spectroscopy and Electrochemical Methods

Development and Application of Time-Resolved Surface Plasmon Resonance Spectrometer

Surface plasmon resonance (SPR) sensor, an optical sensor exploiting special electromagnetic waves-surface plasmon polaritons to probe interactions between an analyte in solution and a molecular recognition element immobilized on the SPR sensor surface, has been widely used in various realms, such as investigating biomolecular interactions and binding properties, detection of biological and chemical analytes, environmental monitoring, food safety and medical diagnostics. This paper reviews the development of SPR sensors and SPR commercial instruments, and emphatically introduces the time-resolved surface plasmon resonance (TR-SPR) techniques. The excellent performances of high sensitivity and rapid detection are easily achieved with TR-SPR spectrometer, whereas the traditional SPR spectrometer cannot be accomplished. Therefore, TR-SPR spectrometer is appropriate for real time analysis of bio-recognition events and small molecular dynamics. However, only two commercial TR-SPR instruments have been exhibited at present, Thermo FT-SPR 100 and Autolab ESPRIT (SPRINGLE). Unfortunately, the high-priced instruments are not suitable for extensive applications in laboratories at present. Herein, a novel commercial TR-SPR spectrometer has been introduced in this review.

Real-time detection of Cu 2+ sequestration and release by immobilized apo-metallothioneins using SECM combined with SPR

Scanning electrochemical microscopy (SECM) combined with surface plasmon resonance (SPR), SECM–SPR, was applied for real-time detection of the incorporation of Cu 2+ by apo-metallothionein (apo-MT) immobilized on the SPR substrate and release of Cu 2+ from surface-confined metallothionein (MT). Cu 2+ anodically stripped from a Cu-coated SECM Au tip was sequestered by apo-MT upon its diffusion to the SPR substrate, and release of Cu 2+ by MT was accomplished by generating protons via oxidation of hydroquinone at the tip. The high sensitivity of the SPR instrument is capable of following the structural and compositional changes of MT molecules during the metal sequestration and release processes. Due to the enhanced mass transfer rate at the SECM tip, the complication of mass transfer limitation on kinetic measurements, commonly encountered in flow injection SPR, is circumvented. The time-resolved SPR response reveals stepwise changes among three stable MT structures and allows the number of copper ions coordinated in each structure to be determined. The numbers of copper ions incorporated by each MT molecule in the three structures were determined to be 5, 9, and 12. This work expands the SECM–SPR approach to assessments of the dynamics and affinity of binding of small ions to surface-confined proteins and to studies of proteins that do not undergo facile electron transfer reactions.

PH-triggered ionization of self-assembled monolayers investigated byin-situ surface plasmon resonance measurements

Self-assembled monolayers (SAMs) of 2-aminoethanethiolhydrochloride (AET) or 2-aminoethanethiol-hydrochloride (AHT) were formed on gold substrate. Deprotonation and reprotonation of SAMs of AET and AHT on gold substrate were investigated by using time-resolved surface plasmon resonance (SPR) measurements. The changes of the terminal functional groups from -NH3 +Cl-to -NH2 (vice versa) via the HCl and NaOH treatment were successfully detected, based on the fact that the initially formed AET and AHT SAMs were assumed to be a zwitterion-like species,-NH3 +Cl-.

In situ sensing of metal ion adsorption to a thiolated surface using surface plasmon resonance spectroscopy

The kinetics of the adsorption of metal ions onto a thiolated surface and the selective and quantitative sensing of metal ions were explored using surface plasmon resonance (SPR) spectroscopy. The target metal ion was an aqueous solution of Pt 2+ and a thin-gold-film-coated glass substrate was modified with 1 , 6 -hexanedithiol (HDT) as a selective sensing layer. SPR spectroscopy was used to examine the kinetics of metal ion adsorption by means of the change in SPR angle. The selectivity of the thiolated surface for Pt 2+ over other divalent metal ions such as Cu 2+, Ni 2+, and Cd 2+ was evident by the time-resolved SPR measurement. SPR angle shift, Δ θ SPR , was found to increase logarithmically with increasing concentration of Pt 2+ in the range of 1.0 × 10 −5 – 1.0 mM . The rate of Pt 2+ adsorption on HDT observed at both 0.1 and 1 mM Pt 2+ accelerates until the surface coverage reaches approximately 17%, after which the adsorption profile follows Langmuirian behavior with the surface coverage. The experimental data indicated that heavy metal ions were adsorbed to the hydrophobic thiolated surface by a cooperative mechanism. A mixed self-assembled monolayer (SAM) composed of HDT and 11-mercaptoundecanoic acid was used to reduce the hydrophobicity of the thiol-functionalized surface. The addition of hydrophilic groups to the surface enhanced the rate of adsorption of Pt 2+ onto the surface. The findings show that the adsorption of metal ions is strongly dependent upon the hydrophilicity/hydrophobicity of the surface and that the technique represents an easy method for analyzing the adsorption of metal ions to a functionalized surface by combining SPR spectroscopy with a SAM modification.

Surface plasmon resonance transients at an electrochemical interface: time resolved measurements using a bicell photodiode

Thin film gold electrodes are frequently used for various sensor applications, and often it is necessary to know how the optical parameters of such an electrode depend on external electrochemical variables. In this work, we demonstrate a time resolved surface plasmon resonance (SPR) technique for studying the correlation between the optical and electrochemical parameters of a thin (∼590 Å) film gold electrode in a neutral aqueous electrolyte of 0.05 M NaNO 3. The electrochemical double layer is charged by a potential step, generating a current transient. A bicell photodiode is used to record the resulting SPR transient, simultaneously with electrochemical measurement of the electrode current. A recently reported SPR detection technique is further extended for this purpose, and the working principle of this method is discussed in detail. The results show how voltage steps of different signs and magnitudes affect the optical properties of the Au surface.

Ionization and Reprotonation of Self-Assembled Mercaptopropionic Acid Monolayers Investigated by Surface Plasmon Resonance Measurements

We have investigated self-assembled monolayers (SAMs) of 3-mercaptopropionic acid (MPA) on gold substrates by using angle and time-resolved surface plasmon resonance (SPR) measurements. The angle-resolved SPR measurements, carried out in two solvents (ethanol and water) and at two wavelengths (632.8 and 850.0 nm), lead to a value of 0.50 ± 0.02 nm for the thickness of the MPA SAM. Employing a high-resolution bicell photodetector for time-resolved SPR, we have determined the rate constant for the formation of MPA SAM in water to be 7.54 × 10-3 min-1. By combining the results of the two types of SPR experiments, we determined the change of refractive index (∼10-3 refractive index units) as a function of MPA ionization and reprotonation.

Incorporation of rhodopsin in laterally structured supported membranes: observation of transducin activation with spatially and time-resolved surface plasmon resonance.

Rhodopsin-transducin coupling was used as an assay to investigate a laterally patterned membrane reconstituted with a receptor and its G protein. It served as a model system to show the feasibility to immobilize G protein-coupled receptors on solid supports and investigate receptor activation and interaction with G proteins by one-dimensional imaging surface plasmon resonance. Supported membranes were formed by the self-assembly of lipids and rhodopsin from detergent solution onto functionalized gold surfaces. They formed micrometer-sized alternating regions of pure fluid phospholipid bilayers separated by bilayers composed of an outer phospholipid leaflet on a gold-attached inner thiolipid. Rhodopsin was found to incorporate preferentially into the phospholipid bilayer regions, whereas transducin was uniformly distributed over the entire outer surface of the supported patterned membrane. The influence of rhodopsin on the dark binding of transducin to lipid membranes was described quantitatively and compared with previously published data. Coupling reactions with transducin resembled closely the native system, indicating that the native functionality of rhodopsin was preserved in the supported membranes. The spatially varying properties of the membranes resulted in a pattern of rhodopsin activity on the surface. This combination of techniques is very promising for the investigation of the lateral diffusion of transducin, can be extended to include signalling proteins downstream of the G protein, and may be applied to functional screening of other G protein-coupled receptors. In the future, it may also serve as a basis for constructing biosensors based on receptor proteins.