Mechanistic understanding of the discrete morphology formed by multi-cycle assembly of tannic acid with Poloxamer 188 on silicon using QMC-D

Abstract

Formation of discrete microdome-shaped features on planar silicon surfaces through multi-cycles of alternate adsorption of tannic acid (TA) and Poloxamer 188 of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (P188) has been reported recently. To further our understanding of the underlying fundamentals, we turned to quartz crystal microbalance with dissipation (QCM-D) for non-invasive monitoring of the process using SiO 2 -coated quartz crystals. Different concentrations of TA and P188 solutions were used for the step-by-step assembly, and the corresponding QCM-D responses were investigated. Additional TA-P188 assemblies were prepared on silicon substrates using the same concentration settings and subjected to surface morphology characterizations before and after drying. The results showed that increasing the concentration of P188 causes increased restructuring of the constructs during the assembling process, and leads to the formation of well-hydrated but less spherically shaped morphology with more continuity in coverage. In contrast, increasing TA concentration helps the assembled discrete features grow in size without inducing drastic change in morphology, and the large dissipation shift recorded is most likely related to water trapped in between the features. Furthermore, the study revealed that, besides the amphiphilic nature of P188 and the right solution concentration of TA, the surface of the underlying silicon substrate being hydrophilic is also instrumental in achieving discrete features of TA-P188 assembly. Collectively, findings reported herein allow for further tailored design of functional surfaces using TA and P188 as building blocks.