Thesis title: From Thermoresponsive Microgels at Interfaces to Hybridization on DNA-Coated Surfaces: a Single-Molecule Level Numerical Study
This thesis describes the research work conducted during my PhD within the SuperCol network. The primary focus was on the numerical study of functionalized colloids of biomedical interest, in some cases for their potential use as biosensors, in other cases as drug-delivery agents. The results were frequently compared with measurements of single-particle experiments.
Within the simulated systems, microgels were the colloidal particles that received the most attention. Significant effort was dedicated to studying the conformation of microgels, and their response with temperature, when placed at interfaces. First, in collaboration with the SupreCol node in Fribourg, where I spent a secondment of approximately one month, we examined the volume phase transition of microgels near solid surfaces, in particular, one hydrophilic and one hydrophobic surface. Our results, compared with super-resolution microscopy measurements, showed no noticeable effects on structure or thermo-responsiveness when the microgel was near a hydrophilic surface relatively to the bulk. Instead, when the surface is hydrophobic, the microgel was found to spread over it adopting a shape that resembles a fried-egg, a conformation commonly found in liquid-liquid interfaces. Furthermore, when near a hydrophobic surface, the microgels collapses at slightly lower temperatures with respect to the other cases.
We then directed our investigation toward the behavior of microgels at liquid-liquid interfaces. We compared two strategies for modeling the effect of temperature on microgels when located in between two solvents, discovering the existence of a mapping between the parameters of both approaches. Intriguingly, there seems to be a linear dependence between the parameters, making the back and forth between the models a rather simple task. Then, we also tested our results with experimental data, this time from our colleagues at ETH Zurich, finding qualitative agreement. Finally, we inquired on the relatively similar fried-egg shape microgels adopt at the liquid-liquid and solid-liquid interfaces, addressing the trending topic of whether deposition on substrates altered the native liquid-liquid configuration. Our comparison between interfaces favors the idea that the microgels suffer further deformation due to the deposition.
A somewhat different but nonetheless related topic examined within the SuperCol project was the DNA hybridization on densely coated surfaces. In collaboration with our colleagues from TUE, where I spent another secondment, we explored the effects on the binding and unbinding kinetics of surfaces according to their surface density either due to an increase on the number of decorating strands, or due to the fraction of replaced strands with non-complementary spacer ones of different lengths. On the one hand, we find that the dissociation process is independent of the surface density or type of spacer strand used. On the other hand, for these densely coated surfaces, a clear dependence of the association is spotted, where the efficiency to bind per strand decreases with density and with the length of the spacer strand. These findings are crucial to consider when designing DNA-based sensors.
In summary, the numerical work conducted over these three years, in collaboration with our colleagues, served to corroborate and unveil fascinating features that colloidal particles possess. Our ultimate goal will be harnessing these features for biomedical applications.