Structure and stimuli-responsiveness of all-DNA dendrimers: theory and experiment

We present a comprehensive theoretical and experimental study of the solution phase properties of DNA-based family of nanoparticles - dendrimer-like DNA molecules (DL-DNA). These charged DNA dendrimers are novel macromolecular aggregates, which hold high promise in targeted self-assembly of soft matter systems in the bulk and at interfaces. To describe the behavior of this family of dendrimers (with generations ranging from G1 to G7), we use a theoretical model in which base-pairs of a single DL-DNA molecule are modeled by charged monomers, whose interactions are chosen to mimic the equilibrium properties of DNA correctly. Experimental results on the sizes and conformations of DL-DNA are based on static and 1dynamic light scattering; at the same time, Molecular Dynamics simulations are employed to model the equilibrium properties of DL-DNA, which compare favorably with the findings from experiments while at the same time providing a host of additional information and insight into the molecular structure of the nanostructures. We also examine the salt-responsiveness of these macromolecules, finding that despite the strong screening of electrostatic interactions, brought about by the added salt, the macromolecules shrink only slightly, their size robustness stemming from the high bending rigidity of the DNA-segments. The study of these charged dendrimer systems is an important field of research in the area of soft matter due to their potential role for various interdisciplinary applications, ranging from molecular cages and carriers for drug delivery in a living organism to the development of dendrimer- and dendron-based ultra-thin films in the area of nanotechnology. These findings are essential to determine if DL-DNA is a viable candidate for the experimental realization of cluster crystals in the bulk, a novel form of solids with multiple site occupancy.Comment: 38 pages, 17 figure

Model-free Rheo-AFM probes the viscoelasticity of tunable DNA soft colloids

Atomic force microscopy rheological measurements (Rheo‐AFM) of the linear viscoelastic properties of single, charged colloids having a star‐like architecture with a hard core and an extended, deformable double‐stranded DNA (dsDNA) corona dispersed in aqueous saline solutions are reported. This is achieved by analyzing indentation and relaxation experiments performed on individual colloidal particles by means of a novel model‐free Fourier transform method that allows a direct evaluation of the frequency‐dependent linear viscoelastic moduli of the system under investigation. The method provides results that are consistent with those obtained via a conventional fitting procedure of the force‐relaxation curves based on a modified Maxwell model. The outcomes show a pronounced softening of the dsDNA colloids, which is described by an exponential decay of both the Young's and the storage modulus as a function of the salt concentration within the dispersing medium. The strong softening is related to a critical reduction of the size of the dsDNA corona, down to ≈70% of its size in a salt‐free solution. This can be correlated to significant topological changes of the dense star‐like polyelectrolyte forming the corona, which are induced by variations in the density profile of the counterions. Similarly, a significant reduction of the stiffness is obtained by increasing the length of the dsDNA chains, which we attribute to a reduction of the DNA density in the outer region of the corona

Molecular engineering of model soft-matter systems using DNA

DNA is widely recognized as a promising engineering material in the field of nanotechnology and material science. Here, I will show that using DNA as building block for constructing molecules with complex architecture in the field of soft matter can offer a great opportunity to obtain well-defined monodisperse systems with a wide variety of particle shape, stiffness, charge, etc. A small library of well-defined DNA-based macromolecular assemblies will be presented, focused on an all-DNA chain-stick construct which consists of a gapped DNA duplex - with contour length near DNA’s persistence length - where two stiff double-stranded DNA segments of equal length are connected by a single-stranded flexible spacer. Trough a combined experimental and numerical study, we provide unambiguous evidence for the stabilization of an unconventional smectic-A liquid crystal phase, where most of the constituent molecules attain a folded configuration. Our results demonstrate that DNA as a building block offers an exquisitely tunable means to engineer a potentially rich assortment of lyotropic liquid crystals that can be precisely designed on subnanometer level

Interactions, structure and rheology of ultrasoft colloidal spheres and mixtures

Η χρήση καλά χαρακτηρισμένων (από πλευράς χημείας) σύνθετων μακρομορίων με μεταβλητό δυναμικό αλληλεπίδρασης αποτελεί το κλειδί για την διερεύνηση της περιοχής χαλαρών υλικών με συμπεριφορά ανάμεσα σε σκληρές σφαίρες και πολυμερικά κουβάρια. Πολύκλωνα αστεροειδή πολυμερή αποτελούν ιδανική επιλογή συστημάτων για το σκοπό αυτό και ειδικά για την μελέτη σχηματισμού και ιδιοτήτων κινητικά παγιδευμένων καταστάσεων. Στην παρούσα εργασία παρουσιάσαμε ένα μεγάλο εύρος από φαινόμενα κινητικής παγίδευσης σε πυκνά διαλύματα των παραπάνω συστημάτων μέσω μεταβολής του χαλαρού απ ωστικού δυναμικού αλληλεπίδρασης τους. Συγκεκριμένα μελετήσαμε την ρευστοποίηση υάλου πολύκλωνων αστεροειδών πολυμερών με την προσθήκη γραμμικών αλυσίδων και την αντιστρεπτή μετάβαση στερεοποίησης πυκνών διαλυμάτων τους με την αύξηση της θερμοκρασίας. Στην πρώτη περίπτωση ο μηχανισμός αποκλεισμού φάνηκε να είναι το κλειδί για την κατανόηση του φαινομένου αυτού ο οποίος σχετίζεται με τον λόγο μεγέθους αστεριού- γραμμικής αλυσίδας. Λεπτομερή μελέτη έδειξε ότι η αύξηση των οσμωτικών δυνάμεων (μέσω της προσθήκης γραμμικών αλυσίδων) έχει σαν αποτέλεσμα την συρρίκνωση των αστεριών οδηγόντας τελικά σε μεγάλες συγκεντρώσεις γραμμικών αλυσίδων στον σχηματισμό συσσωματωμάτων αστεριών. Στην δεύτερη περίπτωση σε πυκνά διαλύματα πολύκλωνων αστεροειδών πολυμερών διεσπαρμένα σε διαλύτη ενδιάμεσης ποιότητας (όπου τα αστέρια μπορούν να διογκωθούν με την αύξηση της θερμοκρασίας) υπόκεινται σε μια αντιστρεπτή μετάβαση στερεοποίησης με την αύξηση της θερμοκρασίας. Αυτό το ενδιαφέρον φαινόμενο αποδώθηκε στο σχηματισμό συσσωματωμάτων λόγω αλληλοδιείσδυσης των διογκωμένων αστεριών με αποτέλεσμα την δυναμική παγίδευση τους. Λόγω της παρουσίας αργών δυναμικών διαδικασιών, στα φαινόμενα που μελετήσαμε, οδηγηθήκαμε στην τροποποίηση μιας από τις πειραματικές τεχνικές που χρησιμοποιήσαμε. Αυτή ήταν μια τροποποίηση της συμβατικής φασματοσκοπίας συσχέτισης φωτονίων (Δυναμική Σκέδαση Φωτός) χρησιμοποιώντας μια ψηφιακή CCD κάμερα σαν ανιχνευτή (Multispeckle Autocorrelation Spectroscopy). Δείξαμε ότι μέθοδος αυτή μας δίνει την δυνατότητα να μετρηθούν σωστά άκρως αργές διαδικασίες χαλάρωσης σε μή-εργοδικά ή εργοδικά συστήματα με αργές δυναμικές λόγω της στατιστικής υπεροχής της στους αργούς χρόνους (1-104sec) σε σχέση με τον συμβατικό τρόπο. Η εφαρμογή της για τη μελέτη του φαινόμενου της θερμοαντιστερπτής μετάβασης πήκτωσης (ή υάλου) αποκάλυψε την εμφάνιση μια αργής διαδικασίας χαλάρωσης, η οποία φθίνει λογαριθμικά και φαίνεται να ευθύνεται για την μακροσκοπική ακινητοποίηση του συστήματος.The use of well defined macromolecular assemblies with tunable interactions represents the key for exploring the regime of soft-material behavior between hard spheres and polymer coils. Colloidal stars are ideal choices for such a formidable task and especially for shedding light on the formation and properties of dynamically arrested states. In this thesis, we demonstrate the rich variety of kinetic frustration phenomena that can be encountered with such ultrasoft particles. We address two specific examples in particular: the melting of star glass upon the addition of linear polymers and the reversible vitrification upon heating. In the first, star depletion appears to be the key to this effect and is related to the starlinear polymer size ratio. A detailed investigation indicates that with increasing the osmotic forces (due to the added polymer) these soft colloids first shrink and then form stable clusters. In the second, upon heating, concentrated solutions of colloidal stars in a solvent of intermediate quality (between θ and athermal) undergo a reversible glass like kinetic transition. This phenomenon can be attributed to the increase in the effective volume fraction of the stars with the temperature, which leads to the formation of clusters and causes the dynamic arrest of the solution. Due to the slow dynamics, unsteady and nonergodic behavior are typically present in this kind of transitions. To resolve these problems and obtain reliable information, we modified the classical Dynamic Light Scattering technique and developed a version of Multispeckle Autocorrelation Spectroscopy instrumentation. The application of this technique in the reversible vitrification phenomenon upon heating reveal that a logarithmic decay govern this glass-like transition

Direct Visualization of Conformation and Dense Packing of DNA-Based Soft Colloids

Soft colloids—such as polymer-coated particles, star polymers, block-copolymer micelles, microgels—constitute a broad class of materials where microscopic properties such as deformability and penetrability of the particle play a key role in tailoring their macroscopic properties which is of interest in many technological areas. The ability to access these microscopic properties is not yet demonstrated despite its great importance. Here we introduce novel DNA-coated colloids with star-shaped architecture that allows accessing the above local structural information by directly visualizing their intramolecular monomer density profile and arm’s free-end locations with confocal fluorescent microscopy. Compression experiments on a two-dimensional hexagonal lattice formed by these macromolecular assemblies reveal an exceptional resistance to mutual interpenetration of their charged corona at pressures approaching the MPa range. Furthermore, we find that this lattice, in a close packing configuration, is surprisingly tolerant to particle size variation. We anticipate that these stimuli-responsive materials could aid to get deeper insight in a wide range of problems in soft matter, including the study and design of biomimetic lubricated surfaces