A simple but quantitative mathematical formalism for interpretation of surface plasmon resonance (SPR) signals from adsorbed films of a wide variety of structures is presented. It can be used to estimate adsorbed film thicknesses, surface coverages, or surface concentrations from the SPR response over the entire range of film thicknesses without relying on calibration curves of response versus known thicknesses or surface concentrations. This formalism is compared to more complex optical simulations. It is further tested by (1) calibrating the response of two SPR spectrometers to changes in bulk index of refraction, (2) using these calibrations with this formalism to predict responses to several well-characterized adlayer structures (alkanethiolates and serum albumin on gold, propylamine on COOH-functionalized gold), and then (3) comparing these predictions to measured SPR responses. Methods for estimating the refractive index of the adlayer material are also discussed. Detection limits in both bulk and adsorption-based analyses are discussed. The planar system used here has a detection limit of ∼0.003 nm in average film thickness for adsorbates whose refractive index differs from that of the solvent by only 0.1. The temperature sensitivities of these two SPR spectrometers are characterized and discussed in terms of detection limits.

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A surface plasmon resonance fiber optic sensor has been used to monitor the deposition of a multilayered cadmium arachidate Langmuir-Blodgen 6lm. Measurements were taken directly as the monolayers were deposited onto the sensor. Experimental results indicate that there are constant shifts in surface plasmon resanance coupling wavelength as the number of transferred monolayers is increased.Film thicknesses were then calculated by using the measured changes in the surface plasmon resonance spectra Multilayered Langmuu-Blodgett (LB) films are defined as built up assemblies of ordered monolayers formed at an air-water interface and deposited onto a solid substrate, Deposition is achieved by subsequent dipping and/or withdrawing of the substrate through the interface while maintaining a constant surface pressure. Often, biosensing applications utilize LB fihs as organic transducing structures. Examples include: immunosensing. using LB deposited antibody films (11; odor sensing, using LB deposited phospholipids 121; and glucose sensing, using LB deposited lipid-glucose oxidase films [3]. In the study of LB 6hs, it is of great interest to sharacterize both the thickness and refractive index of the film.Surface plasmon resonance (SPR) refers to optical excitation of charge density waves at a metal-dielectric interface. The detection of SPR allows for the optical characterization of both materials at this interface. This measurement technique has previously been applied to the characterization of LB films [4]. However, the technique has been limited to relatively large and expensive bulk optic systems not suitable for in-situ sensing applications. A recently developed fiber optic based SPR sensor allows for a more practical method for performing such measurements [51.In this papzr. results are presented from the use of the SPR 6ber optic sensor as the solid substrate onto which cadmium arachidate LB 6 h s are deposited. Analysis of measured data allows for determina. tion of the film thickness for each sucsessive deposition. ~ SPW r Core Figure 1. Illustration of the SPR fiber optic sensor. 0-7803-1377-1/93 $3.00 01993 IEEE Figure 2. SPR fiber optic sensing system and LE depositon trough.The theory and results of the fiber optic sensor discussed have been previously reported [5]. The fiber optic sensor is fundamentally simple in structure and is ilhstrated in Figure 1. Fabrication of the sensor begins by removing 10 mm of both the buffer and cladding from a multimode Eber with a 400 micron core diameter. A 2000 Angstrom thick silver mirror is added to the tip of the fiber by electron-beam deposition. Gold is additonally electron-beam deposited while rotating the fiber along its radial axis to achieve a uniform 550 Angstrom thick layer. Centered at the gold-chemical sample interface is the area of surface plasmon wave excitation.The fiber optic probe is connected to the sensor measurement system as illustrated in Figure 2. White light is coupled into the fiber optic splitter, with 50% of this Light transmitted to the probe...

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