Soame Banerji

Tapered fiber optic surface plasmon resonance sensor for analyses of vapor and liquid phases

We report a new approach to analyze both vapor and liquid phases by utilizing a tapered fiber optic surface plasmon resonance (SPR) probe. This technique employs a fiber optic SPR probe with a modified geometry to tune the SPR coupling wavelength-angle pair. The observed composite spectrum included two distinct SPR dips associated with surface plasmons excited in the gas and liquid active regions. This sensor is able to detect refractive index changes in both vapor and liquid phases individually by simultaneous monitoring SPR coupling wavelengths from the two sensing surfaces.

Investigation of dual-channel fiber-optic surface plasmon resonance sensing for biological applications

A dual-channel fiber-optic sensor based on surface plasmon resonance (SPR) for self-referencing refractive-index measurements has been proposed. Most applications of fiber-optic SPR sensors are designed to measure the refractive index of a liquid or gas sample by measuring the signal from a single surface, the sensitivity and stability of which is easily affected by the fluctuation of external environmental conditions. We have designed a dual-channel fiber-optic surface sensor with two independent SPR signals from two areas of the same probe. A prototype sensor was fabricated and characterized. The preliminary experimental results demonstrate the characteristic responses of both SPR signals from two channels that independently correspond to the refractive index changes in the liquid samples with which they are in contact. The design could be extended to a multichannel sensor with further developments. The experimental results confirmed that one channel can be used as a reference sensor that could compensate for unexpected changes in bulk refraction or temperature and develop this sensor as a practicable high-sensitivity biosensing device.

Discourse on the utilization of polyaniline coatings for surface plasmon resonance sensing of ammonia vapor.

Surface plasmon resonance spectroscopy is sensitive to near-surface (<300 nm) chemical and physical events that result in refractive index changes. The non-specific nature of the stimulus implies that chemical selectivity in SPR sensing configurations entirely relies upon the chemical recognition scheme employed. Biosensing applications commonly use surface layers composed of antibodies or enzymes for biomolecular recognition. Monitoring of volatile compounds with SPR spectroscopy, however, has not been widely discussed due to the difficulty in selectively responding to small molecules (<100 Da) in addition to the limited refractive index changes resulting from the interaction between the plasmon wave and volatile compounds. Different strategies explored thus far for sensing of small molecules have relied on optical and electrical changes of the recognition layer upon exposure to the analyte, yielding an indirect measurement. Examples of coatings used for gas-phase sensing with SPR include conducting metal oxides, polymers and organometallic dyes. Electrically conducting polymers, like polyaniline and polypyrrole, display dramatic conductivity changes in the presence of certain compounds. This property has resulted in their routine incorporation into different sensing schemes. However, application of electrically conducting polymers to SPR gas-phase sensing has been limited to a few examples, despite encouraging results. The emeraldine salt form of polyaniline (PAni) demonstrates a decreased electrical conductivity correlated to NH(3) concentration. In this contribution, PAni doped with camphorsulfonic acid (PAni-CSA) was applied to gas-phase sensing of NH(3) by way of SPR spectroscopy. Spectroscopic ellipsometry was used to determine the optical constants (n and k) for emeraldine salt and emeraldine base forms of PAni, confirming the wavelength-dependent response observed via SPR. The analytical performance of the coatings show that a limit of detection of 32 ppm NH(3) based on precision of the mass-flow controllers used and an estimated method limit of detection of ∼0.2 ppm based on three standard deviations of the blank. This is directly comparable to other, more established sensing architectures.

Glucose detection with surface plasmon resonance spectroscopy and molecularly imprinted hydrogel coatings.

Molecularly imprinted hydrogel membranes were developed and evaluated for detection of small analytes via surface plasmon resonance spectroscopy. Imprinting of glucose phosphate barium salt into a poly(allylamine hydrochloride) network covalently bound to gold surfaces yielded a selective sensor for glucose. Optimization of relative amounts of chemicals used for preparation of the hydrogel was performed to obtain highest sensitivity. Addition of gold nanoparticles into the hydrogel matrix significantly amplified its response and sensitivity due to the impact of gold nanoparticles on the refractive index of the sensing layer. Evaluation of its selectivity showed that the sensor displayed preferential recognition to glucose compared to structurally related sugars in addition to being unaffected by phosphate as well as compounds containing amine groups, like creatinine. The detection limit of glucose in deionized water was calculated to be 0.02 mg/mL. The developed sensor was finally exposed to human urine spiked with glucose illustrating the coatings ability to re-bind the analyte in complex matrices. While the working concentration range in water was determined to be suitable for glucose monitoring in diabetic individuals at physiological levels, the detection in urine was determined to be 0.12 mg/mL. The decreased performance in urine provided an initial perspective on the difficulties associated with measurements in complex media.

Investigation of in Situ Surface Plasmon Resonance Spectroscopy for Environmental Monitoring in and around Deep-Sea Hydrothermal Vents

We have developed a fiber optic Surface Plasmon Resonance (SPR) sensor capable of continuous in situ and real-time monitoring of physical or chemical properties in and around deep-sea hydrothermal vents. The spectroscopic sensor system was deployed on three Alvin cruises to hydrothermal fields in the eastern Pacific: cruise AT11-20 to the East Pacific Rise (EPR) and cruises AT11-31 and AT15-09 to the Juan de Fuca Ridge. The SPR-based sensor was integrated with a thermocouple employed to measure the densities of hydrothermal vent fluid and seawater surrounding the vent. The system was battery powered and placed in a pressure vessel capable of operating six kilometers below the ocean surface in and around hydrothermal vents. Results show the potential of in situ SPR analysis for monitoring in and around hydrothermal vents, including the ability to characterize spatial gradients of the dissolved hydrocarbons and mixing of vent fluid.

Chemically responsive hydrogel with nanoparticle enhanced detection for small biomolecules

A surface plasmon resonance (SPR) sensor to quantify glucose using a molecularly imprinted polymer was developed. The polymer was prepared by crosslinking poly(allylamine) in the presence of glucose phosphate, monobarium salt (GPS-Ba) and attached to a thin film of gold (50 nm) which had been sputtered on top of a glass slide, via amide coupling. Upon removal of the template, this sensor was used to detect glucose in human urine in physiologically significant levels (1-20 mg/ml). Signal enhancement of the glucose sensor was made possible by incorporating gold nanoparticles in the polymer.

Fiber-optic-based surface plasmon resonance (SPR) sensors for the detection of toxic nerve agents

Analytical instruments capable of detecting nerve agents in battlefield conditions where speed, accuracy and ease of operation are a must in todays military. Fast detection and decontamination of nerve agents in very low concentrations is the primary focus of our research. The method presented here focuses on optimizing polymer stabilized sensing elements on the surface of SPR fiber-optic probes. A number of polymers & polymer supported metal complexes capable of reversibly binding to the species of interest & which have robust operation in hostile environments are incorporated with the fiber optic sensing elements. An optical technique, such as Surface Plasmon Resonance (SPR), better suited to rapid data collection without sample pretreatment is employed. The approach using polymer-based optical fibers with off-the-shelf SPR system components has been tested for the detection of Pinacolyl methylphosphonate (PMP), a simulant for nerve agent Soman. Surface initiated polymeric sensors have higher sensitivity toward detecting PMP than bulk-polymerized sensors.

Novel dual-channel fiber-optic surface plasma resonance sensors for biological monitoring

Most applications of fiber optic SPR sensors are designed to measure refractive index (RI) of biological sample by single channel, also the sensitivity and stability of the sensor system is easily affected by the unexpected effects from instrumentation and external environment conditions. In this study, we presented two dual-channel fiber optic SPR sensors based on flat-tip or tetra-taper tip structure with two SPR spectrum located on separate wavelengths that can be used for self-compensating RI measurements of more than one biological samples. The prototyped sensors were fabricated and laboratory characterized. The preliminary experimental results demonstrate the characteristic responses of both SPR wavelengths from two channels are independently correspond to the RI changes of the detected samples or the temperature characters of external environment. Both of these two designs could be extend practicable highly sensitive multi-channel sensor systems that will have extensive applications for biological monitoring.

Molecularly Imprinted Polymerization based Surface Plasmon Resonance Sensing for Glucose Detection in Human Urine

A novel Surface Plasmon Resonance (SPR) sensor to detect glucose using molecularly imprinted polymer (MIP) will be presented in this paper. SPR has been traditionally used as a probe for surface interaction of large molecules but harder to measure small molecules since the effective change in the SPR condition becomes smaller. The accurate measurement of glucose in complex physiological fluids like urine is particularly challenging since the constituents of these fluids vary significantly from person to person and even throughout the day for a particular individual. The polymer was prepared by crosslinking polyallyamine in the presence of Glucose Phosphate, monobarium salt (GPS-Ba) and attached to a 50 nm thin film of gold which had been sputtered on top of a glass slide, via amide coupling. Upon removal of the template, this sensor was used to detect glucose in human urine in physiologically significant levels (1-20 mg/ml). Enhancement of the glucose sensor was made possible by incorporating gold nanoparticles which improved the signal. This study has demonstrated the specific detection of glucose in a complex physiological fluid using SPR spectroscopy. The association of glucose to the imprinted polymer results in the swelling of the polymer that can be tracked by the minima in SPR spectra. The sensitivity of this method, while lower than protein based detection schemes, is sufficient for quantitative measurement of glucose in urine at physiologically significant levels without extensive pre-treatment of the sample. Given the nature of the weak non-covalent binding of glucose to the amine functional groups, the scheme used here can be adapted to detect a number of different molecular species of sizes comparable to that of glucose without the need for extensive sample preparation or use of chemicals with limited shelf life.

Screening of polymer coatings for surface plasmon resonance sensing of ammonia vapor

Several polymeric membranes were evaluated for their potential to improve the sensitivity and impart chemical selectivity to surface plasmon resonance (SPR)-based sensors. The membranes tested encompass a variety of deposition methods, providing an insight of the contact requirements between polymers and the plasmon supporting metal. Among the membranes evaluated, preliminary results utilizing polyelectrolyte multilayer membranes displayed reliable detection of vapor-phase ammonia at ~40 ppm levels. Chemically synthesized polyaniline also presented encouraging results, responding to ammonia gas at 48 ppm. This is in sharp contrast to the electropolymerized counterpart, which showed minor wavelength shifts even at elevated ammonia levels (4 %). SPR has been adopted by the bioanalytical community to probe biomolecular interactions and obtain information relating to binding kinetics. Similarly, modifying plasmon-supporting surfaces with bioreceptors enables access to biosensing applications. Gas-phase sensing with SPR has largely remained unexplored primarily due to the small changes in refractive index from low molecular weight molecules. Coating SPR sensors with tailored polymers has been discussed as a viable approach to amplifying refractive index changes related to low molecular weight analytes. Ammonia is a low molecular weight analyte that is ubiquitously present in the gas phase. Industrial and medical interest in ammonia at low ppm level yielded numerous scientific contributions describing diverse sensing approaches. Hence, ammonia is a good candidate to provide a baseline for immediate comparison with other approaches for evaluation of the polymers with regards to their susceptibility to undergo changes in dielectric properties and chemical affinity for the analyte.