research: crystalline colloidal arrays

norris nature
Though synthetic colloidal crystals were identified early in the 20th century with the emulsion polymerization of styrene, there has been a resurgence of colloidal crystals in research in order to establish fundamental insights into colloidal forces and self-assembly. In addition, these highly ordered structures have been exploited as precursors for the next generation of advanced materials. If the dimensionally periodic dielectric structure of the crystal exhibits a sufficient dielectric contrast between the particles and its interstitial spaces and has a periodicity that is on the order of the wavelength of light, the colloidal crystal may act as a 3-dimensional diffraction grating for visible light and “opalesce”. The attached upper-right figure demonstrates such a periodic structure with a blue opalescence (Nature 414, 289-293 (2001)). These systems have been labeled photonic crystals since they can exhibit a photonic bandgap (or more typically, a stop band). Colloidal crystals have been exploited in this field since they may undergo self-assembly at a nanometer length scales, resulting in spatial periodicities that may range from ca. 100 - 1000 nm; a number of review articles on colloidally-based photonic bandgap materials has been recently presented that discusses the underlying physics of these interesting structures.
opaline tiger paw
In general, there are two approaches for manipulating monodisperse spherical colloidal particles for the generation of photonic crystals. One approach involves the assembly of the particles into close-packed arrays through sedimentation and typically relies on non-specific particle-particle “hard sphere” packing to induce order. The attractive aspects of this assembly approach is both its simplicity and versatility. The second approach utilizes the long range electrostatic repulsive interactions of charged colloidal spheres suspended in a liquid medium to procure order. These latter systems will often adopt a minimum energy crystal structure with either bcc or fcc symmetry. The attached left figure demonstrates such an electrostatically stabilized "colloidal crystal" that has been templated into a Clemson University tiger paw. The Foulger group continues to pursue a number of different research thrusts into crystalline colloidal arrays that are presented by a review of our publications.

research: polymeric memristors


A new device has been recently realized, though it was postulated years ago as one of the fundamental passive circuit elements. This new device alters its conductivity based on its electronic history and is referred to as a memory resistor or "memristor". In this implementation, instead of encoding "0" and "1" as the amount of charges stored in a traditional silicon-based cell, the low and high conductivity condition is interpreted as the OFF and ON states. The growing interest in memristors has resulted in the development of several concepts for the use of these devices, such as utilizing memristive systems for reconfigurable logic circuits, or new computer memory concepts. One of the very appealing aspects of this device is that both information retention and processing could be combined in a single device, a potentially massive advantage for achieving a reduced decoder size. Its not just academia that is interested, the majority of efforts in memristors are being advanced by industry and can be read about here: CNN Report

Just as living creatures alter their decision making process based on past experiences, memristors alter their response to a probe voltage based on their previous voltage exposure. This voltage path dependence can be exploited in a collection of circuit elements to achieve adaptive network systems though one of the most promising applications for memristors is the emulation of synaptic behavior.

The task of building an electronic version of a biological brain is a daunting task. Researchers are beginning to appreciate the complexity inherent in the "soft" aspects of adaptive memory and accept their lack of understanding at this point in time, but in addition, the "hard'' aspects of circuit density and power use for a biological brain are beyond the realm of current silicon technology. For example, how can the biological synapses, which occupy 10 Giga synapses per cubic cm in the cortex, be mimicked with an electronic version. In addition, the biological synapses consume minuscule power; have complex, non-linear dynamics; and, in some cases, can maintain their memory for decades. These characteristics, when combined with additional problems in our current understanding in the formalisms for adaptive learning, have until recently translated into making an electronic facsimile of a biological brain an unreachable goal. The advent of memristive devices has offered an alternative path to build an electronic brain architecture that can adaptively interact with the world.

As early as the late 1960's and early 1970's, a number of researchers presented the conductance switching properties of thin organic films and since then, organic bistable switching devices have been presented that were based on small molecules, polymers, as well composite constructs. These system were predominately developed to serve as a memory component in a von Neumann computer architecture. The mechanism by which the transitions occur are under much debate and is speculated to be vastly disparate for differing materials and/or device constructs. The mechanism for the switching in organic systems has been attributed to a range of phenomena, including filamentary conduction, charge transfer, ionic conduction, space charge and traps, as well as conformational change. The attached figure presents the hysteretic current-voltage response of poly(9-(9H-Carbazol-9-yl)nonyl methacrylate) [inset presents the corresponding monomer], where the directionality of the voltage scans indicated by red arrows. The Foulger group continues to pursue a number of different research thrusts into memristors that are presented by a review of our publications.

research: activated bio-imaging

Cancer is the number one cause of death among adults, and, as such, it is important to discover better drugs and imaging agents that allow for the early detection and treatment of cancer. The Fougler group has been very active in generating polymeric, biocompatible nanoparticles functionalized with various moieties and fluorescent dyes. To enhance the target to background ratio, the nanoparticles are functionalized in such a way that they only become fluorescent upon some environmental stimulus, which could be a change in pH, something on the cellular surface, or a digestive enzyme located inside the cell. This creates a brighter image with more contrast, which can provide more information about the tumor. Recently, the Foulger group has coupled an imaging agent and a therapeutic agent to actively image and treat cancer in one sitting. The figure shows fluorescence activation upon contact with the cellular surface with fluorescence silencing upon internalization by the cell.
Up to this point, the Foulger group has focused on passive targeting of the nanoparticles containing imaging and therapeutic agents. Passive targeting occurs when the enhanced permeation and retention effect (EPR) is utilized; the EPR effect essentially says that cancer cells suffer from an irregular cell membrane which allows nanoparticles, among other things, into the cell much more easily than a normal cell would. This effect is further compounded because cancer cells are known to have poor lymphatic drainage, so, once the nanoparticle is inside the cell, it stays there quite some time before it is able to escape. By increasing the time the nanoparticle is actually in the cell, we can create a possibly better image and have a far greater therapeutic effect. The next stage of this research will focus on active targeting. The way we foresee utilizing active targeting is by attaching a conjugate along with an imaging and/or therapeutic agent to the nanoparticle surface that will bind specifically with an over-expressed surface receptor on the cell membrane to ensure delivery of the imaging and/or therapeutic moiety to the cancer site.