Design of a high-throughput device for screening surface modification protocols
The development of new high performance, ultra-thin organic coatings requires a strategy that has to consider a large number of surface treatment variables such as binding moieties, substrate, and adsorption conditions (e.g. temperature, solvent, concentration, pH, salt). The optimization of the latter is often the bottleneck of the entire development process and restricts the number of parameters that can be tested with acceptable effort. Here we present a screening platform for the efficient, parallel testing of various surface modification protocols, based on an array of 70 wells for individual adsorption experiments with a volume of 20 μL each (SuMo device). The device performance was validated using the copolymer poly(l-lysine)- grafted-poly(ethylene glycol) (PLL-g-PEG) that adsorbs on negatively charged surfaces, rendering them non-fouling in contact with proteins such as fibrinogen. The latter functionality was tested by a second adsorption step of FITC-labelled fibrinogen; polymer and protein thickness values, measured by spectroscopic ellipsometry were used as a measure for the quality of the polymer adlayer. The results obtained are in excellent agreement with traditional coating methods using single chips for each parameter set. A further improvement in the efficiency of the surface modification experiments resulted from the use of a fluorescence read out of the fibrinogen adsorption. Measurements with a microarray scanner proved to be very fast providing uniform fluorescence images with low bleaching rate and high detection sensitivity. The results of the fluorescence readout correlated with the ellipsometry data with a lower limit of detection of ca. 2% of a saturated layer for both techniques. The readout data of the SuMo device were further compared with the quantitative results from in situ optical waveguide lightmode spectroscopy (OWLS) and successfully validated by testing the dependence of fibrinogen coverage as a function of fibrinogen solution concentration. Finally, to demonstrate its application feasibility, the array device was applied to study the polymeric surface layer stability under a range of harsh conditions (14 > pH > 1, ionic strength up to 5.3 M NaCl).