Patterned and switchable surfaces for biomaterial applications

Chapter 5 – Surface plasmon resonance imaging of polymer microarraysAs a proof of principle approach a PAAc array spotted from polymer solutionswith a concentration of 1.0, 0.5 and 0.25 mg/ml was prepared corresponding toaverage thicknesses of 4.2, 3.4 and 2.2 nm respectively, determined from themeasured shift in resonance angle after formation of the polymer spot using theWinspall modelling software. An SPR image of the array is shown as Figure 5.9.Reflectivity and the change of reflectivity versus angle of incidence for each of thespots as well as the unmodified bare gold background were measured after primingthe array in phosphate buffer as shown in Figure 5.10.The resonance angle for the PAAc was higher than for unmodified gold (50.4°,50.2° and 50.1° for spots from polymer concentrations of 2.0 mg/ml, 1.0 mg/ml, 0.5mg/ml, respectively, compared to 49.8° for uncoated gold) (Figure 5.10A). This isdue to the introduction of inhomogeneities in the surface coating by the addition ofPAA. Peak minimums for the differential of the reflectivity were found in the rangeof 48.9°-49.4° (Figure 5.10B). An angle of 48.9° was selected for subsequent fixedangle measurements.After a stabilising phase in phosphate buffer for 1000 s, a solution of 1% ethanolin water was injected into the SPR flowcell at 3 l/s and the SPR signal intensity wasmeasured over time for the PAAc array (Figure 5.11A). The exchange of phosphatebuffer for 1% ethanol would result in an equivalent change in refractive index acrossthe entire surface, thus, an equivalent SPR response was desired at each spot. Adecrease in the reflectivity was observed corresponding to a negative shift in theresonance angle. Notably, however, the change in reflectivity was significantlydifferent for the different spots, ranging from -13 – -27 units. Clearly, theinhomogeneities and thickness differences between each spot prevent measuring aconstant change in reflectivity for a constant change in refractive index for each spot.5-182