Richard M. Crooks
Professor of Chemistry
Research topics in the Crooks group
The group has broad interests in electrochemistry, catalysis, nanomaterials, and biological and chemical microsensors. At present, projects are focused in three main areas: (1) synthesis and characterization of very well-defined mono- and multimetallic catalysts in the 1 - 2 nm size range, (2) design and fabrication of a new family of array-based electrochemical microsensors, and (3) development of a novel means for replicating DNA and RNA microarrays. The most recent information about these and related projects can be found in the group's publications, but a brief introduction to each is provided below.
Dendrimer-encapsulated catalysts. A central challenge in the field of catalysis is to predict effective catalyst nanostructures based on first-principles calculations. Such calculations exist for nanoparticle catalyst, but at present there are few (if any) adequate experimental models that would provide a means for these calculations to be tested. However, we recently developed an approach for synthesizing very well-defined mono- and multimetallic catalysts using dendrimer templates that might provide a first step toward addressing this challenge. This synthesis relies on two steps. First, metal ions are extracted into dendrimers and coordinate in fixed stoichiometries with interior functional groups. Second, the intradendrimer metal ions are reduced to yield dendrimer-encapsulated nanoparticles (DENs). This process leads to stable, nearly size-monodisperse, catalytically active nanoparticles composed of Pt, Pd, Au, Ag, Ni, Fe, or Cu. It is also possible to prepare alloy and core/shell bimetallic DENs using a slight variation of this basic approach. We have shown that these materials are catalytically active for homogeneous hydrogenation and carbon-carbon coupling reactions as well as for heterogeneous catalytic reactions. Presently, however, our main focus is on electrocatalytic reactions, and particularly the oxygen reduction reaction (an important fuel cell reaction). An illustration of a catalytically active DEN attached to a glassy carbon electrode (GCE) is shown in the figure. Current challenges in this project include the invention of improved synthetic methods, particularly for preparing core/shell catalysts, development of analytical methods, which are especially challenging for particles containing only 30-250 atoms, and improvements in catalytic measurements that will allow direct correlation of structure and function. More information about this project is available in a recent review article: "Synthesis, Characterization, and Applications of Dendrimer-Encapsulated Nanoparticles" J. Phys. Chem. B 2005, 109, 692-704.
Microelectrochemical Array Sensors. The group has an interest in the development of microsensors stretching back more than 15 years. One current project is particularly interesting to think about. At present, all (or nearly all) array-type sensors are based on fluorescence detection. There are two principle reasons for this. First, signals from fluorescence-based sensors can be detected in parallel using a CCD, and this greatly improves their speed. Second, it is possible to detect fluorescently labeled analytes at very low concentrations. In contrast to fluorescence, electrochemical methods are typically less expensive and require less power, and 
this means they are better-suited for most point-of-care applications. However, electrochemical methods usually have high limits of detection and there is no convenient method for detecting the output from thousands of electrodes in parallel. We have set out to address this problem by designing, fabricating, and understanding massively parallel arrays of electrodes that can be interrogated simultaneously. Our approach for the design of a DNA sensor is shown in the figure. The important aspect of this design is that the individual electrodes do not need to be connected to a potentiostat or current measurement system. Rather, just a single potential source is required to control all the electrodes in the array, and the output is measured in parallel using a phenomenon called electrogenerated chemiluminescence (ECL). For example, complementary DNA may bind to the cathode end of the electrodes in the sensor shown here. If it does, the system is designed to turn on an ECL reaction at the anode end of the bipolar electrode. The light from this reaction is then detected using a CCD. As part of this project, we are also developing methods for lowering the limit of detection for electrochemical methods. More information about this approach is available in this article: "Electrochemical Sensing in Microfluidic Systems Using Electrogenerated Chemiluminescence as a Photonic Reporter of Redox Reactions" J. Am. Chem. Soc. 2002, 124, 13265-13270.
Parallel replication of DNA and RNA arrays. We recently discovered a new method for replicating nucleic acid microarrays. Such arrays are presently used for genomic analysis, and within the next few years are expected to find their way into doctor's offices as part of the new era of personalized medicine. Our approach for replicating microarrays is illustrated in the figure. In the first step, a DNA master array is prepared using suitable templates. Next, a primer bearing a biotin group is annealed to the distal end of the template DNA, and then it is extended by a polymerase-catalyzed surface reaction (step b). In step c, a streptavidin-modified poly(dimethylsiloxane) (PDMS) monolith is brought into contact with the DNA master microarray. Finally, the PDMS and master-array surfaces are mechanically separated. This results in transfer of the DNA complements to the PDMS replica surface (step d). Many such replicas can be prepared from a single master array. This approach has been improved in many ways since it was reported. For example, replica DNA microarrays can be prepared from a zip code master array having short zip code oligonucleotides. The use of a zip code master provides a means to fabricate replica DNA microarrays having any configuration from the single, universal zip code master array. This same approach can be used to replicate other types of arrays. For example, we recently reported fabrication of RNA replica arrays from DNA masters. Because this is a new analytical method, there many unanswered questions that we are just starting to think about: what biomaterials, other than nucleic acids, can be used with this method, what happens on the molecular level during transfer, and how many of the complements are transferred to the replica? More information about this method is available in the following article: "Transfer of Surface Polymerase Reaction Products to a Secondary Platform with Conservation of Spatial Registration" J. Am. Chem. Soc. 2006, 128, 12076-12077.