Wiesner Group - Research

Current research in the Wiesner group at Cornell University focuses on the control of materials assembly at very small length scales, towards the development of novel functional materials. Our work interfaces polymer science with inorganic and solid-state chemistry to create materials tailored to applications including separation, catalysis, fuel cells and nanobiotechnology.

Towards that end, we have developed expertise in a number of areas including: the synthesis and characterization of macromolecular amphiphiles (block copolymers, etc), the growth of hybrid materials from polymer-type structure-directing agents, the sol-gel growth and application of functional hybrid silica nanoparticles and the synthesis and characterization of complex soft materials such as polymer hydrogels.

Macromolecular Amphiphiles and Bio-Inspired Hybrid Materials

As a particular model system to understand structure formation principles, hybrid silica-based glasses from block copolymer mesophases have been studied extensively over the past decade. One of the main working principles involves utilizing the thermodynamics of amphiphilic block copolymers, i.e., knowledge about their self-assembly behavior (bottom-up) to structure direct precursors for inorganic materials like oxides (e.g., silica) or non-oxide ceramics (e.g., SiCN).

Conceptually, the work combines structure control of soft materials with functionality of inorganic materials in ways that are similar to what is found in natural systems ('bio-inspired'). In the case of sol-gel derived silica hybrids the fundamental building blocks co-assembling with block copolymers are sol nanoparticles, similar to what is found in biomineralized silica. The structure formation mechanism for the synthetic materials was first elucidated in a 2004 Macromolecules paper and the effect of nanoparticle size on hybrid materials assembly and disassembly was revealed in a 2007 Nature Materials publication.

All polymers are synthesized in the group using living polymerization techniques like anionic polymerization and atom transfer radical polymerization (ATRP). Besides employing typical polymer characterization techniques like GPC, DSC, and NMR, mainly small angle x-ray scattering (SAXS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Nitrogen sorption/desorption, atomic force microscopy (AFM), and dynamic mechanical spectroscopy are used to study polymer & hybrid behavior. Research directions that are currently pursued in the group include: (i) nanostructured polymer-inorganic hybrid materials from diblock- and triblock-copolymers and organo-silicon derivatives; (ii) mesoporous aluminosilicates and iron oxide aluminosilicates; (iii) novel polymer/ceramic precursor nanocomposites based on dendrimers; (iv) non-oxide ceramic materials from polysilazanes; and (v) nanostructured block copolymer-silica hybrid thin films.

Furthermore, recent research results of the Wiesner group on ABC triblock copolymers suggest that in analogy to biology, the sequence information of higher order blocked synthetic macromolecular architectures may be used to encode information about hierarchical structure of co-assemblies with silica or other ceramic materials. These principles may permit the design of entirely new classes of functional materials that have no analogue in the natural world with potential applications ranging from materials for energy generation and storage all the way to the life sciences.

Synthesis results in mesostructured hybrid materials with structure control down to the nanometer length scale that upon, e.g., thermal processing can subsequently be converted into purely ceramic materials with preserved structure. With a 1997 Science paper on mesostructured silica from block copolymers the Wiesner research group is one of the pioneers in this field. In 2004 a similar approach lead to the first high-temperature non-oxide ceramic material mesostructured via a bottom-up block copolymer approach published in JACS. In 2001 and 2005 two Angewandte papers reported on hybrids with the Plumber's Nightmare and its inverse morphology, novel bicontinuous cubic structures which were not known in polymer science.

Functional Hybrid Silica Nanoparticles

Another current research direction of the Wiesner research group at Cornell grew from experience with silica hydrolysis and condensation reactions. In a 2005 Nano Letters publication, the group reported on a novel class of silica-based fluorescent core-shell nanoparticles, now known as C dots, with potential applications ranging from information technology to the life sciences. To the best of our knowledge this was the first time that fluorescent core-shell silica nanoparticles in the 20-30 nm size range with narrow size distributions were described with brightness levels reaching those of semiconductor quantum dots and simultaneously enhanced photostability. C dots are synthesized through a modified Stöber process, are non-toxic, and as a result of their optical property profiles constitute an attractive alternative to existing materials platforms for applications requiring bright fluorescent probes.

Fundamental studies are now underway in collaborations with various other groups at Cornell and beyond funded by the National Science Foundation (NSF) as an Interdisciplinary Research Group (IRG) of the Cornell Center for Materials Research (CCMR). The aim of this group of faculty is to understand and control optical phenomena of this novel class of radiative nanoparticles and of optical structures and devices that integrate them. The work follows the reoccurring nanotechnology theme of improving materials properties. By encapsulating organic fluorophores into a water-soluble solid-like silica environment, the photophysical properties of the dyes are significantly improved as compared to the free dye in water.

Because of the high demand for visualization of biological processes down to the molecular or near-molecular level, C dots constitute an excellent materials platform to develop nanobiotechnological tools. For example, in a 2006 Small paper, the Wiesner group showed for the first time that the original core-shell architecture could be further explored to design and synthesize C dot core-shell sensors that permit quantitative ratiometric sensing down to the single particle level.

In a proof-of-principle experiment, the concept was validated to quantitatively measure intracellular pH. These results are very promising since the concept can potentially be generalized towards multiple other analytes, thereby enabling high throughput pharmaceutical screening providing real-time, spatially resolved information about multiple metabolic intracellular parameters including pH, redox chemistry, or oxygen status. Furthermore, the core-shell architecture can be generalized to multiple functional shells with the required silica chemistry towards incorporation of other functionalities being well established.

For example, in a 2006 Chemical Society Reviews article the group described the concept of a "lab on a particle" or "single particle laboratory" as a multifunctional silica core-shell particle where the capabilities of a biological probe, sensor (or sensors), and drug delivery or other remediative methods can be integrated into specific shells on a single particle.

Our work is now expanding to encompass in vivo studies of C dots in living systems to probe their biocompatibility and biodistribution as well as their efficacy as probes for nanomedicine applications in a variety of fields.