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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The self-assembly of streptavidin onto biotinylated alkylthiolate monolayers on gold has served as an important model system for protein immobilization at surfaces. Here, we report a detailed study of the surface composition and structure of mixed self-assembled monolayers (SAMs) containing biotinylated and diluent alkylthiolates and their use to specifically immobilize streptavidin. X-ray photoelectron spectroscopy (XPS), angle-resolved XPS (ARXPS), near-edge X-ray absorption fine structure (NEXAFS), and surface plasmon resonance (SPR) have been used to characterize the films produced on gold from a range of binary mixtures of a biotinylated alkylthiol (BAT) and either a C16 methyl-terminated thiol (mercaptohexadecane, MHD) or a C11-oligo(ethylene glycol)-terminated (OEG) thiol in ethanol. The correlation between the solution mole fraction of BAT and its surface mole fraction (χBAT,sur) indicates that it adsorbs ∼4-fold faster than OEG but slightly slower than MHD. ARXPS analysis demonstrates that the biotin terminus of the BAT is exposed at the surface of mixed monolayers with χBAT,sur < 0.5 but is randomly distributed through BAT-rich films. Thus, the OEG diluent not only adds nonfouling properties but induces an improved concentration of biotin at the surface and reduces the exposure of the methylene segments of BAT. NEXAFS characterization demonstrates that pure OEG and mixed BAT/OEG SAMs do not show significant anisotropy in C−C bond orientation, in contrast to MHD and mixed BAT/MHD SAMs, whose aliphatic segments exhibit pseudo-crystalline packing. SPR measurements of streptavidin binding to and competitive dissociation from the different mixed SAMs indicate that streptavidin binds both specifically and nonspecifically to the BAT/MHD SAMs but purely specifically to BAT/OEG SAMs with χBAT,sur < 0.5. For BAT/OEG mixtures with χBAT,sur = 0.1−0.5, specifically bound streptavidin coverages of ∼80% of the C(2,2,2) two-dimensional streptavidin crystalline density (∼280 ng/cm2) can be reproducibly achieved. These composite results clarify the relationship between the specificity of streptavidin recognition and the surface architecture and properties of the mixed SAMs.

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The kinetics of adsorption and competitive desorption of wild-type streptavidin (WT SA) and three genetically engineered mutants (S27A, N23E, and W120A) was studied at gold surfaces functionalized with mixed alkylthiolates, some terminated with biotin headgroups and the rest with oligo(ethylene oxide) using surface plasmon resonance (SPR). The saturation coverage of the protein varied strongly with surface biotin concentration (XBAT) and was independent of mutation (except at very low and very high XBAT, where a weak dependence was seen). Initial adsorption rates were nearly diffusion-limited except at extremely low XBAT, where the rate varied weakly between mutants in accordance with their differing strengths of binding to biotin. Initial sticking probabilities were estimated to be between ∼1-6 × 10 -6 per collision with the surface. The adsorbed SA desorbs upon introduction of solution-phase biotin. For XBAT below 1%, the desorption rate constants of the SA variants closely follow their off-rate constants measured in homogeneous solution (which at 25 °C are WT ) 4 × 10 -6 sec -1 , N23E ) 1.6 × 10 -3 sec -1 , S27A ) 1.2 × 10 -3 sec -1 , and W120A estimated to be ca. 23 s -1 ). This proves that SA is mainly bound to the surface by a single SA-biotin link at very low XBAT. Importantly, for XBAT between 10 and 40%, where desorption is 30-to >1000-fold slower and the saturation coverage maximizes, the ratios of off-rate constants between mutants (W120A/N23E and W120A/S27A) are approximately the square of their ratios for XBAT below 1%. This squaring strongly suggests that the dominant species at these coverages is doubly bounded SA (i.e., immobilized via two surface biotins). The kinetics are explained with a mechanism involving only two first-order rate constants, that is, for (1) the slow dissociation of any bond between an SA site and a surface-immobilized biotin and (2) the fast reforming of this bond in the special case that it was released from a doubly bonded SA whose other site is still linked to one surface-immobilized biotin. The rate constant for (2) is almost independent of the SA mutant, as it is for adsorption. For XBAT > 60%, the desorption rates again approach the singly bound SA values, and the ratios of rate constants for the SA variants drop to slightly less than below 1% biotin. This is due to the dominance of singly bonded SA, plus a contribution from nonspecific binding, consistent with structural studies of these alkylthiolate films.