Growing up on the coast of Maine in the United States, Nick was inspired to study physical sciences by the powerful winter cyclones that strike his home town each year. Northeastern snowstorms are a rare example of how physics through a continuum of scales—microscopic to global—act in coincidence to form a beautiful, yet fleeting, natural structure that affects all five senses of a human being in a profound way. Long histories and mythologies written by colonial American scholars add a poetical and artistic dimension to their study.
Building on this inspiration, Nick enjoys solving challenging problems in chemistry and physics, with the goal of discovering new mathematical and physical beauty in nature.
Nick is a post-doctoral research associate in the Frenkel Group at Cambridge University, and an Experienced Researcher in the SassyPol network. Presently, he is studying how “multivalent” design strategies can be employed to tune the binding sensitivity of polymers and nanoparticles to other microscopic entities.
Research
The following are the projects in which I am currently involved. Links to relevant papers are included.
Surface-Adsorbing Multivalent Particles
Multivalent polymers are macromolecules containing multiple chemical entities that are able to bind to target chemical entities on another microscopic structure. A common example of a multivalent polymer is a protein having multiple sites of interaction with an enzyme. A more general example of a multivalent entity is a virus, which may use many molecular structures to firmly attach to a cell membrane.
Multivalent interactions can allow for exceptionally strong binding even when each individual interaction is weak. This is because of the entropy associated with binding; there are many different ways to form multiple weak bonds between two structures, but only one way to form a single strong bond.
Systems with a high level of multivalency can exhibit “super-selective” binding. For example, consider a system of particles coated with ligands, capable of binding to receptors on a surface. If one doubles the concentration of receptors on a surface, the number of bound particles may increase ten-fold. (This is in contrast to if the number of bound particles increases only two-fold or less, which is not super-selective.)
For multivalent structures, the adsorption-desorption transition can be extremely sharp in response to system conditions, such as temperature, pH, ionic strength, etc. This effect can be exploited to create materials that are highly responsive to a delicate perturbation in their environment.
Multivalency also introduces the possibility of binding specificity, in which particular binding entities on the multivalent object may only bind to specific entities on a target structure. This idea has been utilised to create self-assembling DNA-coated colloids, in which the self-assembled structure is “encoded” within the base-pair sequences comprising each DNA chain. Creating large structures has been a challenge, however, due to the kinetics of assembling these large structures.
I am studying the behaviour of multivalent polymers and particles when they adsorb to a receptor-coated surface. In particular, I am interested in understanding how the properties of the particle, such as ligand arrangement, chain length / topology (for polymers), and the presence of competing adsorbers, affects the super-selectivity of adsorption.
Publications:
Optimizing the Selectivity of Surface-Adsorbing Multivalent Polymers.
Tito, N. B.; Frenkel, D. Macromolecules, 2014, 47, 7496-7509.
Efficient Simulations of Dense Polymer Systems
Polymer brushes are systems comprised of many chains, chemically tethered by one end to a substrate. They find wide use in the materials community; for example, in sensors, as chemical switches, and to reduce drag in fluid flow.
Simulation is a powerful tool often termed a “virtual experiment”, as it allows one to examine microscopic processes at a level of detail that is not possible in experiment. However, the amount of time it takes to carry out a simulation can vary broadly depending on the level of detail in the model. The length of “microscopic time” that can be studied by the simulation is also dependent on the simulation complexity.
I am working to develop a very efficient method of simulating dense polymer systems (such as a polymer brush) that combines Monte Carlo techniques with ideas from mean-field self-consistent field theory (SCFT). The result is a model that can rapidly sample the equilibrium configurations of the system, as well as detailed information about the conformations of the chains within the system.
Former Work
Enhanced Diffusion and Mobile Fronts in a Simple Lattice Model of Glass-Forming Liquids
Tito, N. B.; Milner, S. T.; Lipson, J. E. G. Soft Matter 2015, Advance Article
Lattice Model of Mobility at Interfaces: Free Surfaces, Substrates, and Bilayers.
Tito, N. B.; Lipson, J. E. G.; Milner, S. T. Soft Matter 2013, 9, 9403-9413.
Lattice Model of Dynamic Heterogeneity and Kinetic Arrest in Glass-Forming Liquids.
Tito, N. B.; Lipson, J. E. G.; Milner, S. T. Soft Matter 2013, 9, 3173-3180.
Ball-of-Yarn Conformation of a Linear Gradient Copolymer in a Hompolymer Melt.
Tito, N. B.; Milner, S. T.; Lipson, J. E. G. Macromolecules 2012, 45, 7607-7620.
Self-Assembly of Lamellar Microphases in Linear Gradient Copolymer Melts.
Tito, N. B.; Milner, S. T.; Lipson, J. E. G. Macromolecules 2010, 43, 10612-10620.
Application of a coarse-grained model for DNA to homo- and heterogeneous melting equilibria.
Tito, N. B.; Stubbs, J. M. Chem. Phys. Lett. 2010, 485, 354-359.
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