Surface functionalization for self-referencing surface plasmon resonance (SPR) biosensors by multi-step self-assembly

Recently, a novel SPR sensor with on-chip referencing has been realized. In this sensor, one-half of the gold sensing surface is covered with a high refractive index overlayer of tantalum pentoxide (Ta₂O₅). When polychromatic beam illuminates the sensing surface, surface plasmon resonance in the areas with and without the overlayer occur at different wavelengths. Therefore, the reflected light exhibits two dips associated with SPRs in those two areas. When functionalized properly, one of the areas can be used as a specific sensing channel for detection of specific bio-interactions and the other can act as a reference channel for compensation for background refractive index fluctuations. In this paper we present a new functionalization approach for these mixed architecture chips. The gold side of the chip is functionalized with a mixed self-assembled monolayer of polyethylene oxide (PEO) and biotin terminated (BAT) thiols whereas the Ta₂O₅ side is coated with PEO terminated silanes. The PEO terminated thiols and silanes serve as a protein resistant background, while the biotin terminated thiols are used to bind streptavidin, which in turn immobilizes biotinylated antibodies. Hence, the gold side of the chip is used for the binding and detection of target analytes and the Ta₂O₅ side functions as a reference channel that monitors bulk refractive index changes and temperature drift. We have studied human chorionic gonadotropin (hCG) as a model system, currently detecting down to 5 ng/ml. In addition, we demonstrate the power of the on-chip reference channel for compensating for refractive index changes and eliminating false alarms.     

Surface plasmon resonance (SPR) sensors for the rapid, sensitive detection of the cellular response to osmotic stress

Cells regulate their volume in response to changes in osmolarity of both, their extracellular and intracellular environments. As stability of the cell volume is a compelling exigency for cellular integrity, techniques for a sensitive, time-resolved volume measurement of adherently grown mammalian cells attract considerable interest, especially in the field of cell physiology and biology. In this study we apply a surface plasmon resonance (SPR) based biosensor for the comparative analysis of the volume responses of two renal epithelial cell types to non-isotonic challenges. The on-line, label-free and non-invasive biosensor format shows distinct similarities and differences in the reaction kinetics of the two cell types. Furthermore regulatory volume responses to the osmotic stimuli as well as their inhibition by Gd³⁺ ions can be observed with a high time-resolution. Limit-of-detection measurements indicate the high sensitivity of the sensor capable of detecting cellular volume responses of adherently grown mammalian cells to osmotic stimuli well below a bioanalytical relevant value of 5 mOsm/kg.     

Surface plasmon resonance based fiber optic sensor for the detection of low water content in ethanol

A fiber optic sensor utilizing surface plasmon resonance (SPR) has been fabricated for the detection of low content of water in ethanol. The sensor utilizes spectral interrogation technique for operation. The resonance wavelength has been found to vary linearly with water content in the range 0-10% with sensitivity of 1.149 nm per percentage of water. The results are in agreement with the refractive index variation of ethanol-water mixture. The sensor has a water resolution of 0.145% which is better than the evanescent wave absorption sensor reported for the similar study. The sensor will find application in determining the low water content in ethanol which is used as a bio-fuel and in the field of medicine and organic chemistry.     

Surface plasmon resonance immunosensor using Au nanoparticle for detection of TNT

In order to develop the fully integrated portable surface plasmon resonance (SPR) system for detection of explosives, the amplification strategy of SPR signal was investigated. Indirect competitive inhibition method allowed the middle-sized SPR sensor to detect trinitrotoluene (TNT) at ppt level. However, this enhanced SPR signal was not high enough to detect TNT at ppt level by a miniaturized SPR sensor. Therefore, localized surface plasmon resonance (LSPR) effect using Au nanoparticle as further signal amplification approach was used. The amplification method of indirect competitive inhibition and LSPR were combined together for fabrication of the immunosurface using Au nanoparticle. TNT detectable range of this immunosurface was from 10 ppt (10 pg/ml) to 100 ppb (100 ng/ml), which was almost comparable to that without Au nanoparticle. The observed resonance angle change due to binding monoclonal TNT antibody (M-TNT Ab) with the immunosurface modified with Au nanoparticle was amplified to four times higher than that in absence of Au nanoparticle.     

Label-free sensitive optical detection of polychlorinated biphenyl (PCB) in an aqueous solution based on surface plasmon resonance measurements

A label-free, ultrasensitive method for the optical detection of polychlorinated biphenyl (PCB) is described. The detection mechanism for PCB is based on PCB-induced conformational changes of immobilized Cytochrome c (Cyt c) on an Au thin film altering the local dielectric function of the supported Cyt c, which can be detectable by surface plasmon resonance (SPR) spectroscopy. Results showed that the confirmations of Cyt c changed in a very sensitive way depend on PCB concentrations as confirmed by circular dichroism (CD) measurements and atomic force microscope (AFM) observations. Based on the results, we investigated the PCB-sensitive detection performance of the self-assembling Cyt c monolayer surface by SPR. On exposure to PCB, the reflectance R at the SPR angle of the supported Cyt c showed a sensitive and systematic increase with increasing PCB concentrations. The limit of detection was as low as 0.1 ppb and the responses completed within 10 min.     

The reversible binding of hydrophobically end-capped poly(ethylene glycol)s to poly-β-cyclodextrin-coated gold surfaces: a surface plasmon resonance investigation

Polymers of β-cyclodextrin (β-CD) bearing amino groups (poly-β-CD-NH₂) were grafted to gold surfaces by two different procedures. Hydrophobically end-capped model-polymers were adsorbed onto resulting (poly-β-CD-NH₂)-coated sensor chips by formation of inclusion complexes. Their adsorption onto the surface was followed in real time by surface plasmon resonance (SPR). Multilayered structures obtained by inclusion of adamantyl-modified poly(ethylene glycol)s (Ad-PEG) into immobilized β-cyclodextrin cavities were highly stable in aqueous media. Conditions leading to the regeneration of initial (poly-β-CD-NH₂)-coated surfaces were studied by SPR. Regeneration by competitors such as polymers of β-cyclodextrin was possible. However, it was found to be a complex mechanism involving two opposite phenomena depending on the structure and composition of the (poly-β-CD-NH₂) layer. Complete desorption of the bound Ad-PEG layer was also observed after exposure of the multilayered structures to organic solvents or sodium dodecylsulfate (SDS).     

Surface characterisation of the Dome Concordia area (Antarctica) as a potential satellite calibration site, using Spot 4/Vegetation instrument

A good calibration of satellite sensors is necessary to derive reliable quantitative measurements of the surface parameters or to compare data obtained from different sensors. In this study, the snow surface of the high plateau of the East Antarctic ice sheet, particularly the Dome C area (75°S, 123°E), is used first to test the quality of this site as a ground calibration target and then to determine the inter-annual drift in the sensitivity of the VEGETATION sensor, onboard the SPOT4 satellite. Dome C area has many good calibration site characteristics: The site is very flat and extremely homogeneous (only snow), there is little wind and a very small snow accumulation rate and therefore a small temporal variability, the elevation is 3200 m and the atmosphere is very clear most of the time. Finally, due to its location, it is frequently within view of many satellites. VEGETATION visible blue channel data (0.43-0.47 μm) of a 716×716 km² area centred on the French-Italian Dome Concordia station, during the 1998-1999, 1999-2000, 2001-2001, and 2001-2002 austral summers were cloud masked and atmospherically corrected. The snow surface Bidirectional Reflectance Distribution Function is very high with little spatial and seasonal variability, which is a major advantage for sensor calibration. The inter-annual variation is found to be very small, proving that the stability of the site is very good.     

Comparison of two hyperspectral imaging and two laser-induced fluorescence instruments for the detection of zinc stress and chlorophyll concentration in bahia grass (Paspalum notatum Flugge.)

Bahia grass (Paspalum notatum Flugge.) plants were grown in silica sand and irrigated daily with one of five levels of Zn (0, 0.5, 25, 50, or 100 mg l⁻¹) to determine the effects of the heavy metal on the growth and development of plant canopies. Healthy and stressed plants were measured with two hyperspectral imagers, laser-induced fluorescence spectroscopy (LIFS), and laser-induced fluorescence imaging (LIFI) systems in order to determine if the four handheld remote sensing instruments were equally capable of detecting plant stress and measuring canopy chlorophyll levels in bahia grass. Symptoms of bahia grass plants grown at deficient (0 mg l⁻¹) or toxic (25, 50, or 100 mg l⁻¹) concentrations of Zn were dominated by leaf chlorosis and plant stunting. Leaf fresh weight, leaf dry weight, CO₂ assimilation, total chlorophyll, and leaf thickness followed (+) quadratic models in which control plants (0.5 mg l⁻¹ Zn) exhibited higher responses than plants grown at either deficient or toxic levels of Zn. Normalized difference vegetation index [NDVI=(NIR−Red)/(NIR+Red)] and ratio vegetation index [RVI=R₇₅₀/R₇₀₀, in which R denotes reflectance] values were calculated for calibrated digital images from both hyperspectral imagers. The NDVI and RVI values from both hyperspectral imagers were fit best by (+) quadratic models when treatments were constrained between 0 and 100 mg l⁻¹ Zn, but were fit best by linear regression models with (−) slopes when treatments were constrained between 0.5 and 100 mg l⁻¹ Zn. Furthermore, both NDVI and RVI algorithms were effective in predicting the concentrations of chlorophyll in canopies of bahia grass grown at the various levels of Zn. In contrast, red/far-red (R/FR) fluorescence ratios estimated from leaf fluorescence values measured with the LIFS and LIFI instruments were fit best by (−) quadratic models when treatments were constrained between 0 and 100 mg l⁻¹ Zn, but were fit best by linear regression models with (+) slopes when treatments were constrained between 0.5 and 100 mg l⁻¹ Zn. A series of regression analyses were conducted among plant biometric, biochemical, and leaf anatomical parameters (treated as independent variables) and the remote sensing algorithms, NDVI, RVI, blue/green (BL/GR), and R/FR (treated as dependant variables). In general, residuals were significantly higher for NDVI and RVI models compared to the BL/GR and R/FR models indicating that the NDVI and RVI algorithms were able to measure total chlorophyll and plant biomass more accurately than the BL/GR and R/FR algorithms. However, unique capabilities of LIFS and LIFI instruments continue to argue for the development of laser-induced fluorescence remote sensing technologies.