Surface plasmon resonance-based biosensors: From the development of different SPR structures to novel surface functionalization strategies

Surface plasmon resonance (SPR)-based biosensors are very powerful tools for the study of biomolecular interactions, chemical detection and immunoassays. This paper reviews the performance of various SPR structures and detection schemes focusing on propagating surface plasmons generated in planar structures. Some aspects of their surface functionalization, the key element which imparts biofunctionality to these structures and hence transforming them into biosensors, will also be discussed accordingly. The ultimate performance of SPR-based biosensors will thus be determined by both their inherent optical performance and suitable surface functionalization.     

Multiple label-free detection of antigen-antibody reaction using localized surface plasmon resonance-based core-shell structured nanoparticle layer nanochip

In this research, a localized surface plasmon resonance (LSPR)-based bioanalysis method for developing multiarray optical nanochip suitable for screening bimolecular interactions is described. LSPR-based label-free monitoring enables to solve the problems of conventional methods that require large sample volumes and time-consuming labeling procedures. We developed a multiarray LSPR-based nanochip for the label-free detection of proteins. The multiarray format was constructed by a core-shell-structured nanoparticle layer, which provided 300 nanospots on the sensing surface. Antibodies were immobilized onto the nanospots using their interaction with Protein A. The concentrations of antigens were determined from the peak absorption intensity of the LSPR spectra. We demonstrated the capability of the array measurement using immunoglobulins (IgA, IgD, IgG, IgM), C-reactive protein, and fibrinogen. The detection limit of our label-free method was 100 pg/mL. Our nanochip is readily transferable to monitor the interactions of other biomolecules, such as whole cells or receptors, with a massively parallel detection capability in a highly miniaturized package. We anticipate that the direct label-free optical immunoassay of proteins reported here will revolutionize clinical diagnosis and accelerate the development of hand-held and user-friendly point-of-care devices.     

Localized surface plasmon resonance-based hybrid Au–Ag nanoparticles for detection of Staphylococcus aureus enterotoxin B

A triangular hybrid Au–Ag nanoparticles array was proposed for the purpose of biosensing in this paper. Constructing the hybrid nanoparticles, an Au thin film is capped on the Ag nanoparticles which are attached on glass substrate. The hybrid nanoparticles array was designed by means of finite-difference and time-domain (FDTD) algorithm-based computational numerical calculation and optimization. Sensitivity of refractive index of the hybrid nanoparticles array was obtained by the computational calculation and experimental detection. Moreover, the hybrid nanoparticles array can prevent oxidation of the pure Ag nanoparticles from atmosphere environment because the Au protective layer was deposited on top of the Ag nanoparticles so as to isolate the Ag particles from the atmosphere. We presented a novel surface covalent link method between the localized surface plasmon resonance (LSPR) effect-based biosensors with hybrid nanoparticles array and the detected target molecules. The generated surface plasmon wave from the array carries the biological interaction message into the corresponding spectra. Staphylococcus aureus enterotoxin B (SEB), a small protein toxin was directly detected at nanogramme per milliliter level using the triangular hybrid Au–Ag nanoparticles. Hence one more option for the SEB detection is provided by this way.     

Evaluation of multi-layered graphene surface plasmon resonance-based transmission type fiber optic sensor

Graphene is a zero band-gap semi-metal with remarkable electromagnetic and mechanical characteristics. This study is the first ever attempt to use graphene in the surface plasmon resonance (SPR) sensor as replacement material for gold/silver. Graphene, comprised of a single atomic layer of carbon, is a purely two-dimensional material and it is an ideal candidate for use as a biosensor because of its high surface-to-volume ratio. This sensor is based on the resonance occasion of the surface plasmon wave (SPW) according to the dielectric constants of each metal film and detected material in gas or aqueous phase. Graphene in the SPR sensor is expected to enlarge the range of analyte to bio-aerosols based on the superior electromagnetic properties of graphene. In this study, a SPR-based fiber optic sensor coated with multi-layered graphene is described. The multi-layered graphene film synthesized by chemical vapor deposition (CVD) on Ni substrate was transferred on the sensing region of an optical fiber. The graphene coated SPR sensor is used to analyze the interaction between structured DNA biotin and Streptavidin is analyzed. Transmitted light after passing through the sensing region is measured by a spectrometer and multimeter. As the light source, blue light which of 450 to 460 nm in wavelength was used. We observed the SPR phenomena in the sensor and show the contrary trends between bare fiber and graphene coated fiber. The fabricated graphene based fiber optic sensor shows excellent detection sensitivity of the interaction between structured DNA and Streptavidin.     

Improving the sensitivity limit of surface plasmon resonance biosensors by detecting mixed interference signals

We demonstrate that the sensitivity limit of intensity-based surface plasmon resonance (SPR) biosensors can be enhanced when we combine the effects of the phase and amplitude contributions instead of detecting the amplitude variation only. Experimental results indicate that an enhancement factor of as much as 20 times is achievable, yet with no compromise in measurement dynamic range. While existing SPR biosensor systems are predominantly based on the angular scheme, which relies on detecting intensity variations associated with amplitude changes only, the proposed scheme may serve as a direct system upgrade approach for these systems. The new measurement scheme may therefore lead to a strong impact in the design of SPR biosensors.     

Effect of resonant localized plasmon coupling on the sensitivity enhancement of nanowire-based surface plasmon resonance biosensors

A nanowire-based surface plasmon resonance (SPR) is investigated as a structure that offers improved sensor performance. The results calculated by rigorous coupled-wave analysis on a model using a hexanedithiol self-assembled monolayer (SAM) indicate that the resonant coupling between localized surface plasmons (LSPs) of nanowires affects the sensitivity enhancement substantially, while the LSP resonance in a single nanowire also contributes. SPR characteristics change significantly by applying a SAM, which can give rise to zero sensitivity for a given SAM. The results suggest that a properly designed nanowire-based SPR biosensor can enhance sensitivity by an order of magnitude with reasonable detection properties.     

Target dependence of the sensitivity in periodic nanowire-based localized surface plasmon resonance biosensors

We investigate the target dependence of the sensitivity in a localized surface plasmon resonance (LSPR) biosensor and compare it with that of a conventional thin-film-based plasmon resonance structure. An LSPR biosensor was modeled as subwavelength periodic nanowires on a metal/dielectric substrate and targets either as bulk refractive index changes or as a biomolecular interaction that forms a monolayer. The results found that significant target-dependent variation arises in sensitivity and sensitivity enhancement by LSPR. The variation is attributed to the nonlinearity in the plasmon dispersion relation as well as the effective permittivity due to strong LSPR signals. The target dependence suggests that an LSPR structure be designed based on estimated index changes induced by target interactions. Associated broadening of resonance width can be controlled by way of profile engineering, which is discussed in connection with experimental data.     

Field-matter integral overlap to estimate the sensitivity of surface plasmon resonance biosensors

We have analyzed the effectiveness of field-matter integral overlap between target index distribution and local near-fields to assess detection sensitivity of surface plasmon resonance (SPR) biosensors. The correlation of the overlap with sensitivity was clear. An overlap integral defined with lateral electric field intensity produced the highest correlation due to tangential continuity across a boundary. Among the three detection scenarios considered, the correlation for localized SPR sensing was slightly lower than that of thin film-based detection and improved with an increased fill factor in the structure. The results will be useful to maximize the optical signature created by target interactions and to produce highest sensitivity of SPR detection to variations when target or field distribution is not uniform.     

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.     

Designed biointerface using near-infrared quantum dots for ultrasensitive surface plasmon resonance imaging biosensors

The surface plasmon resonance imaging chip biointerface is fully designed using near-infrared (NIR) quantum dots (QDs) for the enhancement of surface plasmon resonance imaging (SPRi) signals in order to extend their application for medical diagnostics. The measured SPRi detection signal following the QD binding to the surface was amplified 25-fold for a 1 nM concentration of single-stranded DNA (ssDNA) and 50-fold for a 1 μg/mL concentration of prostate-specific antigen (PSA), a cancer biomarker, thus substantiating their wide potential to study interactions of a diverse set of small biomolecules. This significant enhancement is attributed to the QD's mass-loading effect and spontaneous emission coupling with propagating surface plasmons, which allowed the SPRi limit of detection to be reduced to 100 fM and 100 pg/mL for ssDNA and PSA, respectively. Furthermore, this study illustrates the potential of SPRi to be easily integrated with fluorescent imaging for advanced correlative surface-interaction analysis.     

Surface plasmon resonance biosensor screening method for paralytic shellfish poisoning toxins: a pilot interlaboratory study

A surface plasmon resonance (SPR) optical biosensor method was developed for the detection of paralytic shellfish poisoning (PSP) toxins in shellfish. This application was transferred in the form of a prototype kit to seven laboratories using Biacore Q SPR optical biosensor instrumentation for interlaboratory evaluation. Each laboratory received 20 shellfish samples across a range of species including blind duplicates for analysis. The samples consisted of 4 noncontaminated samples spiked in duplicate with a low level of PSP toxins (240 μg STXdiHCl equivalents/kg), a high level of saxitoxin (825 μg STXdiHCl/kg), 2 noncontaminated, and 14 naturally contaminated samples. All 7 participating laboratories completed the study, and HorRat values obtained were <1 demonstrating that the method performance was acceptable. Mean recoveries expressed as STXdiHCl equivalents/kg were 94.6 ± 16.8% for the low level PSP toxin mix and 98.6 ± 5.6% for the high level of saxitoxin. Relative standard deviations for within-laboratory variations (RSD(r): repeatability) and between-laboratory variations (RSD(R) = reproducibility) ranged from 1.8 to 9.6% and 2.9 to 18.3% respectively. This first ever reported SPR biosensor interlaboratory study demonstrated this PSP application to be an empowering tool in the drive toward the reduction and replacement of the mouse bioassay within Europe.     

A kinetic study of analyte-receptor binding and dissociation for surface plasmon resonance biosensors applications

A fractal analysis, which takes into account the effect of surface heterogeneity brought about by ligand immobilization on the reaction kinetics in surface plasmon resonance (SPR) biosensors, is presented. The binding and dissociation of estrogen receptors (ERs), ERa and ER/spl alpha/ and ER/spl beta/, in solution to different ligands immobilized on the SPR biosensor is analyzed within the fractal framework. The heterogeneity on the biosensor surface is made quantitative by using a single number, the fractal dimension D/sub f/. The analysis provides physical insights into the binding of these receptors to different ligands and compounds, particularly the endocrine disrupting compounds (EDCs). These EDCs have deleterious effects on humans and on wildlife. Single- and dual-fractal models were employed to fit the ER-binding data obtained from the literature. Values of the binding and dissociation rate coefficient and fractal dimensions were obtained from a regression analysis provided by Corel Quattro Pro, 8.0. Values for the affinity K/sub D/(=k/sub d//k/sub a/) were also calculated. This provides us with some extra flexibility in designing biomolecular assays. The analysis should provide further information on the mode of action and interaction of EDCs with the ERs. This would help in the design of agents and modulators against these EDCs.     

Nonregeneration protocol for surface plasmon resonance: study of high-affinity interaction with high-density biosensors

Surface plasmon resonance (SPR) has been used in determining kinetics and thermodynamics of biological interaction in the past decades. One difficulty encountered in this technology is the need for a proper regeneration, which means the removal of analytes from the bound complexes to regenerate the activity of the ligands. Regeneration is not always practical since the harsh regeneration reagents may destroy the bioactivity of the ligands. It is even more difficult for complexes with high affinity constants. In this paper, we report a nonregeneration protocol for SPR techniques in which subsequent ligand/analyte interactions can be measured without regeneration; thus ligand biological activity could be retained. Kinetics, binding models, and mathematics of this protocol are discussed in detail using rabbit IgG as the analyte and engineered recombinant antibody A10B single-chain fragment variables (scFv) as the ligand. The affinity constant of rabbit IgG binding with A10B scFv measured by using a nonregeneration protocol was (2.5 +/- 0.2) x 10(7) M(-1), which was comparable with the value determined with a conventional regeneration SPR method ((2.2 +/- 1.5) x 10(7) M(-1)) and quartz crystal microbalance (1.9 x 10(7) M(-1)). A paradigm of streptavidin-biotin binding was analyzed to validate this protocol. The affinity constant for each binding subunit of streptavidin to the immobilized biotin was determined to be (7.3 +/- 0.2) x 10(6) M(-1), which was comparable with the solution-based value of 2 x 10(7) M(-1). The nonregeneration protocol requires a relatively high ligand density on the biosensor surface so that more data points can be obtained before surface saturation. The small size of scFv enables them to be constructed in the biosensors for such purpose.     

Influence of the metal film thickness on the sensitivity of surface plasmon resonance biosensors

The influence of the metal film thickness (i.e., the chromium adhesion promoting film and the gold film) on the sensitivity of surface plasmon resonance (SPR) signals (i.e., resonance angle shift and reflectance change) towards the thickness variation of the nonabsorbing dielectric film is investigated. The sensitivity of reflectance change decreases when a thick chromium film or a thin gold film is employed. Its linear range becomes narrower as the thickness of the metal films increases. The sensitivity and linear range of the resonance angle shift are not affected by the thickness variation of the metal films. The phenomena were theoretically explained based on the attenuated total reflection (ATR) generated evanescent field at the prism/metal interface and the SPR-generated evanescent field at the metal/dielectric interface.     

Comparative study of Ag and Au nanoparticles biosensors based on surface plasmon resonance phenomenon

The specific sensitivity of surface plasmon resonance to changes in the local environment of nanoparticles allows their use as platforms to probe chemical and biochemical binding events on their surfaces without any labeling [1], [2], [3], [4]. In this paper, we perform a comparative study of gold and silver nanoparticle based biosensors, prepared within the same conditions, in order to determine which metal seems the best for biological sensing. The prototypical biocytin–avidin interaction is used to study gradual changes over time and with avidin concentration in the absorption spectra bands of biocytinylated 10 nm silver and gold nanospheres. First, the Ag nanoparticles plasmon resonance absorbance signal is about 10 times larger than the Au one. Secondly, for an equivalent concentration of avidin, the optical property modifications are more pronounced for silver nanoparticles than for gold ones of the same geometry. These observations attest the superiority of Ag on Au nanoparticles when optical considerations are only taken into account. Finally, with both biosensors, the specificity of the interaction, checked by replacing avidin with bovine serum albumin, is relatively poor and needs to be improved.     

Sensing characteristics of NIR localized surface plasmon resonances in gold nanorings for application as ultrasensitive biosensors

The optical responses of 75-150 nm diameter gold nanorings to changes in local refractive index have been quantified by near-infrared extinction spectroscopy and compared to DDA calculations and an analytical approach. The "bulk" refractive index sensitivities of gold nanorings are substantially (>5 times) larger than those of nanodisks with similar diameters. Nanorings retain a significantly larger sensitivity than nanodisks at the same spectral position, demonstrating a clear shape dependence that may correlate to a systematic difference in the influence of the dielectric substrate. The nanoring bulk refractive index sensitivity scales linearly with plasmon peak position. The spectral sensitivity to thin films of alkanethiols gave a shift of approximately 5.2 nm/CH2 unit while bulk sensitivities as high as 880 nm/RIU were observed, the highest such reported sensitivities. Both bulk and thin dielectric film sensitivities correlated well with theory. Real-time label-free monitoring of protein binding via molecular recognition was demonstrated.     

Profile effect on the feasibility of extinction-based localized surface plasmon resonance biosensors with metallic nanowires

We investigate the effect of the cross-sectional profile of an array of metallic nanowires on the feasibility of a localized surface plasmon resonance (LSPR) biosensor. Calculations were performed using rigorous coupled-wave analysis with an emphasis on the extinction properties of the LSPR structure. The results indicate that the nanowire structure, particularly that of a T-profile, delivers an extremely linear sensing performance over a wide range of the target refractive index with much enhanced sensitivity. The extinction-based LSPR structure also involves a relatively large dimension and thus is expected to provide a feasible biosensor using current semiconductor technology.     

Microcontact imprinted surface plasmon resonance sensor for myoglobin detection

In this study, we prepared surface plasmon resonance (SPR) sensor using the molecular imprinting technique for myoglobin detection in human serum. For this purpose, we synthesized myoglobin imprinted poly(hydroxyethyl methacrylate-N-methacryloyl-l-tryptophan methyl ester) [poly(HEMA-MATrp)] nanofilm on the surface of SPR sensor. We also synthesized non-imprinted poly(HEMA-MATrp) nanofilm without myoglobin for the control experiments. The SPR sensor was characterized with contact angle measurements, atomic force microscopy, X-ray photoelectron spectroscopy, and ellipsometry. We investigated the effectiveness of the sensor using the SPR system. We evaluated the ability of SPR sensor to sense myoglobin with myoglobin solutions (pH 7.4, phosphate buffer) in different concentration range and in the serum taken from a patient with acute myocardial infarction. We found that the Langmuir adsorption model was the most suitable for the sensor system. The detection limit was 87.6 ng/mL. In order to show the selectivity of the SPR sensor, we investigated the competitive detection of myoglobin, lysozyme, cytochrome c and bovine serum albumin. The results showed that the SPR sensor has high selectivity and sensitivity for myoglobin.     

Enhancement of localized surface plasmon resonance detection by incorporating metal-dielectric double-layered subwavelength gratings

In this study, we investigated the enhanced sensing performance of a localized surface plasmon resonance (LSPR) biosensor by employing metal-dielectric double-layered subwavelength grating structures. The numerical results showed that the LSPR substrate with a dielectric spacer can provide not only a better sensitivity but also a significantly improved reflectance characteristic. While the presence of metallic gratings leads to a broad and shallow reflectance curve inevitably, the dielectric spacer can prevent the propagating surface plasmons from being interfered by the locally enhanced fields excited at the gold gratings, finally resulting in a strong and deep absorption band at resonance. Therefore, the proposed structure could potentially open a new possibility of the enhanced LSPR detection for monitoring biomolecular interactions of low molecular weights.     

Determination of ribonuclease H surface enzyme kinetics by surface plasmon resonance imaging and surface plasmon fluorescence spectroscopy

The kinetics of the ribonuclease H (RNase H) surface hydrolysis of RNA-DNA heteroduplexes formed on DNA microarrays was studied using a combination of real-time surface plasmon resonance imaging (SPRI) and surface plasmon fluorescence spectroscopy (SPFS). Time-dependent SPRI and SPFS data at various enzyme concentrations were quantitatively analyzed using a simple model that couples diffusion, enzyme adsorption, and surface enzyme kinetics. This model is characterized by a set of three rate constants, enzyme adsorption (k(a)), enzyme desorption (k(d)), enzyme catalysis (k(cat)), and one dimensionless diffusion parameter (beta). Values of k(a) = 3.15 (+/-0.20) x 10(6) M(-1).s(-1), k(d) = 0.10 (+/-0.05) s(-1), and k(cat) = 0.95 (+/-0.10) s(-1) were determined from fitting all of the SPRI and SPFS data sets. One of the most interesting kinetic parameters is the surface RNase H hydrolysis reaction rate constant (k(cat)), which was found to be approximately 10 times slower than that observed in solution, but approximately 100 times faster than that recently observed for the exonuclease III surface hydrolysis of double-stranded DNA microarrays (k(cat) = 0.009 s(-1)). Moreover, the surface coverage of the intermediate enzyme-substrate complex (ES) was found to be extremely small during the course of the reaction because k(cat) is much larger than the product of k(a) and the bulk enzyme concentration.