Waveguide surface plasmon resonance sensor for electrochemically controlled surface reactions

Electrochemical oxidation of the surface of a thin gold film overlaid on an optical waveguide and placed in a cell containing sulfuric acid is monitored by integrated optical surface plasmon resonance (IOSPR) combined with cyclic voltammetry. Waveguide transmittance correlates well with the charge transferred to and from the electrode to oxidize and reduce the surface, with a 60% reduction in transmittance being observed for complete formation of the surface oxide. IOSPR sensors combined with electrochemical measurement and control show potential for sensitive and robust integrated multisensors for electroactive species.     

Encoded and multiplexed surface plasmon resonance sensor platform

We present a flexible new sensor system that combines the joint advantages of (i) discretely functionalized, code-bearing, microparticles and (ii) label-free detection using grating-coupled surface plasmon resonance. This system offers the possibility of simultaneously investigating the real-time binding kinetics of a variety of molecular interactions. One single multiplexed assay could employ a wide range of immobilization chemistries, surface preparation methods, and formats. Thus, the new system offers a very high level of assay conformability to the end user, particularly when compared to fixed microarrays.     

Hybrid surface platform for the simultaneous detection of proteins and DNAs using a surface plasmon resonance imaging sensor

In this work, we present a novel surface and assay for the simultaneous detection of DNA and protein analytes on a surface plasmon resonance (SPR) imaging sensor. A mixed DNA/oligo (ethylene glycol) (OEG) self-assembled monolayer (SAM) is created using a microarrayer. Thiol-modified single-stranded DNA sequences are spotted onto a gold-coated glass substrate. Backfilling with an OEG-modified alkanethiol creates a protein-resistant surface background. Antibodies conjugated to complementary single-stranded DNA sequences are immobilized on the surface through DNA hybridization. By converting only part of the DNA array into a protein array, simultaneous detections of DNA and protein analytes are possible. A model system of two cDNA sequences and two human pregnancy hormones are used to demonstrate the assay. No cross-reactivity was observed between DNA or protein analytes and nontargeted immobilized cDNA sequence or antibodies. A response from a detection of a single analyte in a mixture of protein and DNA analytes corresponds well with that of a single-analyte solution.     

Optical phase-shift detection of surface plasmon resonance

A heterodyne optical measurement system for studying the phase shift of surface plasmon resonance (SPR) is presented. The system utilizes a frequency-stabilized Zeeman laser as a detection light source and is suitable for real-time phase measurement in SPR-sensing applications. The phase shift in an angular dispersion SPR excitation setup was measured ranging from +180 degrees to -120 degrees around the SPR excitation region. The experimental results fit well with the theoretical analysis. Compared with the reflection coefficient variation that is widely investigated in SPR studies, phase shift is estimated to provide a higher sensitivity to sensor systems and more information about the resonance phenomenon.     

Phase detection sensitivity enhancement of surface plasmon resonance sensor in a heterodyne interferometer system

We propose a method of rotating the analyzer in front of the photodetector in a heterodyne interferometer system to implement optical phase shift and detection sensitivity enhancement of surface plasmon resonance (SPR) sensors. When the analyzer is rotated to shift the phase curve to be near the phase jump point, the phase detection sensitivity of the SPR sensor can be greatly enhanced. Theoretical calculations of a prism-coupled SPR device were performed using this method. Experimental result using an electro-optic heterodyne interferometer is reported.     

Low density lipoprotein sensor based on surface plasmon resonance

Biotinylated heparin has been immobilized onto self-assembled monolayer of 4-aminothiophenol using avidin–biotin specific binding. The modified electrodes have been characterized using surface plasmon resonance technique (SPR), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), atomic force microscopy (AFM) and contact angle (CA) measurements. The interaction of immobilized biotinylated heparin with low density lipoprotein (LDL) has been studied using surface plasmon resonance technique. The biotinylated heparin modified electrode can be used to detect LDL in the range of 20 to 100 mg/dl with the sensitivity of 513.3 m°/μM.     

Postcolumn renewal of sensor surfaces for high-performance liquid chromatography-surface plasmon resonance detection

The combination of high-performance liquid chromatography (HPLC) with surface plasmon resonance (SPR) for continuous separation and label-free detection of protein samples is described. The detection was realized by electrostatic adsorption of proteins bearing positive and negative charges onto chemically modified SPR sensors in two separate SPR channels. One SPR channel is coated with carboxymethylated dextran which facilitates the detection of positively charged proteins, whereas the other, devoted to the monitoring of negatively charged proteins, is covered with ethylenediamine molecules attached onto a dextran surface. Renewal of the sensor surface in the channels can be accomplished by introducing regeneration solutions through two six-port valves. The coupled technique (HPLC-SPR) was assessed for its analytical figures of merit and applied to the quantification of lysozyme in human milk samples. Unlike the SPR detection of bulk solution refractive index changes during chromatographic peak elutions, the highest sensitivity of SPR is retained in this work since the measurement is performed at the SPR sensor surface where the evanescent field is the strongest. The renewable SPR detection of continuous separations is reproducible and versatile and does not require the separated proteins to contain chromophores or to be prelabeled with a tag (e.g., a redox-active or fluorescent molecule). Such generality makes SPR complementary to other types of chromatographic detectors.     

Characterization of swellable molecularly imprinted polymer particles by surface plasmon resonance spectroscopy

Surface plasmon resonance (SPR) has been used to investigate template binding at sites in theophylline-imprinted poly-[N-(N-propyl) acrylamide] particles. At concentrations as low as 10(-6) M theophylline, particle swelling is detected as a shift in the angle of minimum reflectance. The binding constant of theophylline estimated from the inflection point of the theophylline calibration curve is approximately 10,000. The imprinted polymer particles do not respond to caffeine or theobromine (which differs from theophylline by a single methyl group) at concentrations as high as 10(-2) M. Full-scale response of the imprinted polymer particles to theophylline (template) occurs in less than 15 minutes, and swelling is reversible. The immobilized imprinted polymer particles can undergo approximately 20 to 25 swelling and shrinking cycles before there is significant loss in functionality. A unique aspect of these imprinted polymer particles is that template binding causes the angle of minimum reflectance to decrease, not increase, in magnitude. Adsorption, which causes an increase in the angle of minimum reflectance, can be readily discriminated from template binding.     

Analysis of biomolecular interactions using a miniaturized surface plasmon resonance sensor

A commercially available miniaturized surface plasmon resonance sensor has been investigated for its applicability to biological interaction analysis. The sensor was found to exhibit excellent repeatability and linearity for high-refractive index solutions and good reproducibility for the binding of proteins. Its detection limit for the monoclonal antibody M1 was found to be 2.1 fmol, which corresponds to a surface concentration of 21 pg/mm2. Simple surface immobilization procedures relying on biotin/avidin or glycoprotein/lectin chemistry have been explored. Equilibrium dissociation constants for the binding of the FLAG peptide to its monoclonal antibody (M1) and for the binding of concanavalin A to a glycoprotein have been determined. The close agreement of these measurements with values obtained by surface fluorescence microscopy and fluorescence correlation spectroscopy helps to validate the use of this device. Thus, this sensor shows promise as an inexpensive, portable, and accurate tool for bioanalytical applications in laboratory and clinical settings.     

Theoretical limit of localized surface plasmon resonance sensitivity to local refractive index change and its comparison to conventional surface plasmon resonance sensor

In this paper, the theoretical sensitivity limit of the localized surface plasmon resonance (LSPR) to the surrounding dielectric environment is discussed. The presented theoretical analysis of the LSPR phenomenon is based on perturbation theory. Derived results can be further simplified assuming quasistatic limit. The developed theory shows that LSPR has a detection capability limit independent of the particle shape or arrangement. For a given structure, sensitivity is directly proportional to the resonance wavelength and depends on the fraction of the electromagnetic energy confined within the sensing volume. This fraction is always less than unity; therefore, one should not expect to find an optimized nanofeature geometry with a dramatic increase in sensitivity at a given wavelength. All theoretical results are supported by finite-difference time-domain calculations for gold nanoparticles of different geometries (rings, split rings, paired rings, and ring sandwiches). Numerical sensitivity calculations based on the shift of the extinction peak are in good agreement with values estimated by perturbation theory. Numerical analysis shows that, for thin (≤10 nm) analyte layers, sensitivity of the LSPR is comparable with a traditional surface plasmon resonance sensor and LSPR has the potential to be significantly less sensitive to temperature fluctuations.     

Concentration detection of quantum dots in the visible and near-infrared range based on surface plasmon resonance sensor

We employ an optical sensor based on surface plasmon resonance (SPR) operating in the near-infrared and in the visible range to determine the concentration of CdSe/ZnS core-shell quantum dots (QDs) which are embedded in the SU8 organic films. Attenuated total reflection (ATR) measurements show that the amplitude of the shift of the resonance dip is closely related to the concentration variation of QDs in the organic films and the incident laser. The sensitivity is enhanced by 1.5-time and the detect limitation is expanded to 10⁻⁵ μmol/L in the visible range as compared to that in the near-infrared. The sensitivity enhancement and the expansion of detect limitation of the visible SPR sensor may originate from the coupling of surface plasmons to luminescence from QDs.     

Single-layer graphene-based surface plasmon resonance sensor with dynamic evanescent field enhancement for biomarker study

ABSTRACT

In this work, we have developed a kind of single-layer graphene-based surface plasmon resonance (SLG-SPR) biosensor to detect C-reactive protein (CRP) and Prostate-specific antigen (PSA). In the experiment of testing CPR, the results obtained revealed that the changes in resonance wavelength of SLG-SPR biosensors are higher than that of the gold-film based SPR (Au-SPR) biosensors. Moreover, for the experiment of testing PSA, due to the dynamic evanescent field enhancement produced by a strong electric field coupling between the localized SPR (LSPR) of AuNPs and SPR of single-layer graphene-based film (SLG-film) that further amplify the evanescent field signal. We verified the SLG-SPR biosensors exhibited higher sensitivity than the Au-SPR biosensors and the SLG-SPR biosensor exceeded the traditional biosensor detection limit. Accordingly, the SLG-SPR biosensor based on dynamic optical enhancement can realize high sensitivity detection of low concentration biomarkers and can be applied to most of the trace biomarkers in theory.     

A graphene-based surface plasmon sensor

We present the application of graphene as a plasmon sensor. It was found that the electronic transport of chemical vapor deposition CVD-synthesized graphene is sensitive to surface plasmons generated by the illumination of metal nanoparticles. The observed change in electronic conduction can be up to seven times larger than the intrinsic photoresponse of graphene. A study of the mechanism revealed local field-assisted oxygen desorption induced by surface plasmons to be the cause of this intriguing behavior. A detailed investigation of the wavelength and spacing dependence of the plasmon-graphene coupling proves that graphene can be used as a sensitive, high resolution electronic plasmon detector. This finding shows the potential of devices exploiting the novel properties of graphene and surface plasmons.      

High-sensitivity small-angle sensor based on surface plasmon resonance technology and heterodyne interferometry

A high-sensitivity small-angle sensor based on surface plasmon resonance technology and heterodyne interferometry is proposed that uses a new technique with two right-angle prisms. Interestingly, the technique provides a novel method for designing small-angle sensors with high sensitivity and high resolution. Its theoretical resolution can reach 1.2x10(-7) rad over the measurement range of -0.15 degrees < or =theta< or =0.15 degrees . The method has some merits, e.g., a simple optical setup, easy operation, high resolution, high sensitivity, and rapid measurement. Its feasibility is demonstrated.     

Optimized angle scanning method for array sample detection in surface plasmon resonance biosensor

An optimized angle scanning method is presented for array sample detection in a surface plasmon resonance biosensor. It provides a way to find the optimal rotation axis in the prism to resolve the drifting problem of the light incidence point on samples in the plane prism-coupling mode. The detection of array samples can be achieved by the translation of the prism along a particular direction. The validity of this method is theoretically analyzed and demonstrated by experiments.     

Very thin spin-coated silver films via transparent silver ink for surface plasmon resonance sensor applications

We fabricated very thin silver films with thicknesses of 20 nm, 40 nm, and 60 nm on a prism using a spin coating method for surface plasmon resonance (SPR) image sensor module applications. An aqueous silver ionic complex solution was spin-coated and then thermally cured for 10 minutes at 150 degrees C in an oven. The spin-coated solid silver films possessed silver crystallinity. The prism modules with the 20-nm-, 40-nm- and 60-nm-thick thin silver films were applied to an SPR image sensor system. The coefficients of determination for the 20-nm-, 40-nm- and 60-nm-thick silver films were 0.923, 0.990 and 0.989, respectively when standard ethanol solutions with 0.1% intervals in the range of 20.0% to 20.5% were applied. The correlation is high-performed and the coefficients of determination are as close as 1. The spin coating method of very thin silver films for SPR image sensor modules is expected to be a very cost-effective solution because the films can be formed at a low temperature in a short period of time without requiring a vacuum system.     

Differential-phase surface plasmon resonance biosensor

In this paper, a novel differential-phase-sensitive surface plasmon resonance biosensor (DP-SPRB) is proposed and developed, in which a two-frequency laser is integrated with a differential amplifier in order to analytically convert the phase modulation into amplitude modulation. With the use of the conventional envelope detection technique, the differential phase is precisely decoded in real time in terms of the demodulated amplitude. In order to verify high detection sensitivity of the DP-SPRB, a sucrose-water solution and glycerin-water solution at low concentrations were both tested, and the experimental results confirm that the detection sensitivity on wt % concentration of the sucrose solution is 0.00001%. Moreover, the real-time monitoring mouse IgG/antimouse IgG interaction shows the minimum concentration of mouse IgG to be at 10 fg/mL. To our knowledge, this is the highest sensitivity ever measured by a surface plasmon resonance biosensor. However, because of the limited dynamic range of DP-SPRB, it can only apply to biomolecule interactions at extremely low concentration.     

Wavelength-scanning surface plasmon resonance imaging

A new surface plasmon resonance (SPR) imaging technique was proposed. After measurements were conducted at varying wavelengths, the wavelength affording the minimum brightness (SPR wavelength) was determined at each pixel of the image. A two-dimensional map of the SPR wavelength could be converted to a thickness profile by use of a nonlinear calibration curve, which was obtained by Fresnel calculation. An array of protein thin layers on a gold film was evaluated in air to present the layers' surface structure in nanometer scale.     

Temperature sensor based on surface plasmon resonance within selectively coated photonic crystal fiber

We demonstrate a temperature sensor based on surface plasmon resonances supported by photonic crystal fibers (PCFs). Within the PCF, to enhance the sensitivity of the sensor, the air holes of the second layer are filled with a large thermo-optic coefficient liquid and some of those air holes are selectively coated with metal. Temperature variations will induce changes of coupling efficiencies between the fundamental core mode and the plasmonic mode, thus leading to different loss spectra that will be recorded. In this paper, variations of the dielectric constants of all components, including the metal, the filled liquid, and the fused silica, are considered. We conduct numerical calculations to analyze the mode profile and evaluate the power loss, demonstrating a temperature sensitivity as high as 720 pm/°C.     

Silicon-based surface plasmon resonance sensing with two surface plasmon polariton modes

Surface plasmon resonance (SPR) sensing on a silicon-based platform is considered. We have studied properties of SPR in a combined silicon-dielectric layer-gold film-sample medium structure and established conditions of the simultaneous excitation of two plasmon polariton modes that provide narrow and well-separated minima of the reflected intensity. It has been shown that the external mode over the gold-sample medium interface demonstrates a highly sensitive response to a change in the refractive index of the sample medium, whereas the internal mode over the dielectric-gold interface is almost insensitive to medium parameters. We propose that the internal mode can be used as an effective reference zero point for miniature and portable SPR-based systems designed for field and multichannel sensing.