Richard M Crooks

University of Texas at Austin, Austin, Texas, United States

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Publications (300)1960.34 Total impact

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    ABSTRACT: In this report we present the synthesis and characterization of Pt and Pd dendrimer-encapsulated nanoparticles (DENs) using the method of galvanic exchange. Sixth-generation hydroxyl-terminated poly(amidoamine) dendrimers were used to prepare Cu DENs composed of 55 atoms. In the presence of either PtCl 4 2À or PdCl 4 2À , the less noble Cu DENs oxidize to Cu 2+ leaving behind an equal-sized DEN of Pt or Pd, respectively. DENs prepared by direct reduction with BH 4 À , which is the common synthetic route, and those prepared by galvanic exchange have the same composition, structure, and properties as judged by UV-vis spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and electrochemical methods. However, the galvanic exchange synthesis is much faster (3 h vs. 96 h), and the yield of reduced DENs is significantly higher (nearly 100% in the case of galvanic exchange).
    New Journal of Chemistry 01/2054; 35(35):2054-2060. DOI:10.1039/C1NJ20083F · 3.16 Impact Factor
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    ABSTRACT: The synthesis and characterization of Sn nanoparticles in organic solvents using sixth-generation dendrimers modified on their periphery with hydrophobic groups as stabilizers are reported. Sn(2+):dendrimer ratios of 147 and 225 were employed for the synthesis, corresponding to formation of Sn147 and Sn225 dendrimer-stabilized nanoparticles (DSNs). Transmission electron microscopy analysis indicated the presence of ultrasmall Sn nanoparticles having an average size of 3.0-5.0 nm. X-ray absorption spectroscopy suggested the presence of Sn nanoparticles with only partially oxidized surfaces. Cyclic voltammetry studies of the Sn DSNs for Li alloying/dealloying reactions demonstrated good reversibility. Control experiments carried out in the absence of DSNs clearly indicated that these ultrasmall Sn DSNs react directly with Li to form SnLi alloys.
    Langmuir 06/2015; DOI:10.1021/acs.langmuir.5b01383 · 4.38 Impact Factor
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    ABSTRACT: The objective of the research described in this Account is the development of high-throughput computational-based screening methods for discovery of catalyst candidates and subsequent experimental validation using appropriate catalytic nanoparticles. Dendrimer-encapsulated nanoparticles (DENs), which are well-defined 1-2 nm diameter metal nanoparticles, fulfill the role of model electrocatalysts. Effective comparison of theory and experiment requires that the theoretical and experimental models map onto one another perfectly. We use novel synthetic methods, advanced characterization techniques, and density functional theory (DFT) calculations to approach this ideal. For example, well-defined core@shell DENs can be synthesized by electrochemical underpotential deposition (UPD), and the observed deposition potentials can be compared to those calculated by DFT. Theory is also used to learn more about structure than can be determined by analytical characterization alone. For example, density functional theory molecular dynamics (DFT-MD) was used to show that the core@shell configuration of Au@Pt DENs undergoes a surface reconstruction that dramatically affects its electrocatalytic properties. A separate Pd@Pt DENs study also revealed reorganization, in this case a core-shell inversion to a Pt@Pd structure. Understanding these types of structural changes is critical to building correlations between structure and catalytic function. Indeed, the second principal focus of the work described here is correlating structure and catalytic function through the combined use of theory and experiment. For example, the Au@Pt DENs system described earlier is used for the oxygen reduction reaction (ORR) as well as for the electro-oxidation of formic acid. The surface reorganization predicted by theory enhances our understanding of the catalytic measurements. In the case of formic acid oxidation, the deformed nanoparticle structure leads to reduced CO binding energy and therefore improved oxidation activity. The final catalytic study we present is an instance of theory correctly predicting (in advance of the experiments) the structure of an effective DEN electrocatalyst. Specifically, DFT was used to determine the optimal composition of the alloy-core in AuPd@Pt DENs for the ORR. This prediction was subsequently confirmed experimentally. This study highlights the major theme of our research: the progression of using theory to rationalize experimental results to the more advanced goal of using theory to predict catalyst function a priori. We still have a long way to go before theory will be the principal means of catalyst discovery, but this Account begins to shed some light on the path that may lead in that direction.
    Accounts of Chemical Research 05/2015; 48(5). DOI:10.1021/acs.accounts.5b00125 · 24.35 Impact Factor
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    ABSTRACT: We present a method for quantifying the accuracy of extended X-ray absorption fine structure (EXAFS) fitting models. As a test system, we consider the structure of bare Au147 nanoparticles as well as particles bound with thiol ligands, which are used to systematically vary disorder in the atomic structure of the nanoparticles. The accuracy of the fitting model is determined by comparing two distributions of bond lengths: (1) a direct average over a molecular dynamics (MD) trajectory using forces and energies from density functional theory (DFT) and (2) a fit to the theoretical EXAFS spec- tra generated from that same trajectory. Both harmonic and quasi-harmonic EXAFS fitting models are used to characterize the first-shell Au-Au bond length distribution. The harmonic model is found to significantly underestimate the coordination number, disorder, and bond length. The quasi-harmonic model, which includes the third cu- mulant of the first-shell bond length distribution, yields accurate bond lengths, but incorrectly predicts a decrease in particle size and little change in the disorder with increasing thiol ligands. A direct analysis of the MD data shows that the particle surfaces become much more disordered with ligand binding and the high disorder is incorrectly interpreted by the EXAFS fitting models. Our DFT calculations compare well with experimental EXAFS measurements of Au nanoparticles, synthesized using a dendrimer encapsulation technique, showing that systematic errors in EXAFS fitting models apply to nanoparticles on the size of 1-2 nm. Finally we show that a combina- tion of experimental EXAFS analysis with candidate models from DFT is a promising strategy for a more accurate determination of nanoparticle structures.
    ACS Nano 04/2015; 9(4). DOI:10.1021/acsnano.5b00090 · 12.03 Impact Factor
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    ABSTRACT: This paper summarizes several studies correlating the structure and function of nanoparticle catalysts. Three types of alloy nanoparticles are considered, random alloy, core@shell and alloy-core@shell structures. In the first two cases, the focus is to build theoretical models to understand previous experimental results. In the latter case, calculations play a greater role in leading the development of nanoparticle catalysts. We demonstrate that iteration between theory and experiment can facilitate an understanding of nanoparticle catalysts and reduce the time and effort involved in the design of new catalysts.
    Surface Science 04/2015; DOI:10.1016/j.susc.2015.03.018 · 1.87 Impact Factor
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    ABSTRACT: We report electrochemical catalytic amplification of individual collisions between ∼57 nm diameter Pt nanoparticles (Pt NPs) and 12.5 μm diameter Au ultramicroelectrodes modified with passivating, electrostatically assembled polyelectrolyte multilayer (PEM) films prepared by the layer-by-layer deposition method. Two key findings are reported. First, despite the thicknesses of the insulating PEM films, which range up to 5 nm, electrons are able to tunnel from the Pt NPs to the electrode resulting in electrocatalytic N2H4 oxidation at the PEM film-solution interface. These single-particle measurements are in accord with prior reports showing that the electrochemical activity of passive PEM films can be reactivated by adsorption of metallic NPs. Second, it is possible to control the frequency of the collisions by manipulating the net electrostatic charge present on the outer surface of the PEM thin film. These results, which demonstrate that chemistry can be used to control electrocatalytic amplification, set the stage for future sensing applications.
    Langmuir 01/2015; DOI:10.1021/la5043124 · 4.38 Impact Factor
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    Long Luo, Xiang Li, Richard M Crooks
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    ABSTRACT: We present an origami paper-based electrophoretic device (oPAD-Ep) that achieves rapid (∼5 min) separation of fluorescent molecules and proteins. Due to the innovative design, the required driving voltage is just ∼10 V, which is more than 10 times lower than that used for conventional electrophoresis. The oPAD-Ep uses multiple, thin (180 μm/layer) folded paper layers as the supporting medium for electrophoresis. This approach significantly shortens the distance between the anode and cathode, and this, in turn, accounts for the high electric field (>1 kV/m) that can be achieved even with a low applied voltage. The multilayer design of the oPAD-Ep enables convenient sample introduction by use of a slip layer as well as easy product analysis and reclamation after electrophoresis by unfolding the origami paper and cutting out desired layers. We demonstrate the use of oPAD-Ep for simple separation of proteins in bovine serum, which illustrates its potential applications for point-of-care diagnostic testing.
    Analytical Chemistry 12/2014; DOI:10.1021/ac503976c · 5.83 Impact Factor
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    ABSTRACT: Here we outline a new method for synthesizing fully reduced Pt dendrimer-encapsulated nanoparticles (DENs). This is achieved by first synthesizing Cu DENs of the appropriate size through sequential dendrimer loading and reduction steps, and then galvanically exchanging the zerovalent Cu DENs for Pt. The properties of Pt DENs having an average of 55, 140, and 225 atoms prepared by direct chemical reduction and by galvanic exchange are compared. Data obtained by UV-vis spectroscopy, X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and high-resolution electron microscopy confirm only the presence of fully reduced Pt DENs when synthesized by galvanic exchange, while chemical reduction leads to a mixture of reduced DENs and unreduced precursor. These results are significant because Pt DENs are good models for developing a better understanding of the effects of finite size on catalytic reactions. Until now, however, the results of such studies have been complicated by a heterogeneous mixture of Pt catalysts.
    Langmuir 12/2014; DOI:10.1021/la503956h · 4.38 Impact Factor
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    ABSTRACT: Here we report on the electrochemical properties of carbon electrodes coated with thin layers of Al2O3 and SnO2. These oxide films were deposited using atomic layer deposition (ALD) and range in thickness from 1 to 6 nm. Electrochemical experiments show that the thinnest oxide layers contain defects that penetrate to the underlying carbon electrode. However, oxygenation of the carbon surface prior to ALD increases the surface concentration of nucleation sites for oxide growth and suppresses the defect density. Films of Al2O3 just ∼3-4 nm in thickness are free of pinholes. Slightly thicker coatings of SnO2 are required for equivalent passivation. Both Al2O3 and SnO2 films are stable in both neutral and acidic electrolytes even after repeated voltammetric scanning. The results reported here open up the possibility of studying the effect of oxide supports on electrocatalytic reactions.
    Langmuir 11/2014; DOI:10.1021/la503232m · 4.38 Impact Factor
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    ABSTRACT: We report on the effect of convection on electrochemically active collisions between individual Pt nanoparticles (PtNPs) and Hg and Au electrodes. Compared to standard electrochemical cells utilizing Hg and Au ultramicroelectrodes (UMEs) used in previous studies of electrocatalytic amplification, microelectrochemical devices offer two major advantages. First, the PtNP limit of detection (0.084 pM) is ∼8 times lower than the lowest concentration measured using UMEs. Second, convection enhances the mass transfer of PtNPs to the electrode surface, which enhances the collision frequency from ∼0.02 pM(-1) s(-1) on UMEs to ∼0.07 pM(-1) s(-1) in microelectrochemical devices. We also show that the size of PtNPs can be measured in flowing systems using data from collision experiments and then validate this finding using multiphysics simulations.
    Langmuir 10/2014; DOI:10.1021/la503628h · 4.38 Impact Factor
  • Morgan J Anderson, Richard M Crooks
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    ABSTRACT: Here we report on the development of a high-efficiency, dual channel-electrode (DCE) generation-collection system and its application for interrogating redox-active surface-adsorbed thin films. DCE systems consist of two electrodes configured on the base of a microfluidic channel. Under laminar flow conditions, a redox reaction can be driven on the upstream generator electrode, and the products carried by convection to the downstream collector electrode where the reverse redox reaction occurs. One significant outcome of this report is that simple fabrication techniques can be used to prepare DCE systems that have collection efficiencies of up to 97%. This level of efficiency makes it possible to quantitatively measure the charge associated with redox-active thin films interposed between the generator and collector electrodes. This is important, because it provides a means for interrogating species that are not in sufficiently close proximity to an electrode to enable direct electron transfer or electroactive films adsorbed to insulating surfaces. Here, the method is demonstrated by comparing results from this indirect surface interrogation method, using Fe(CN)6(3-) as the redox probe, and direct electroreduction of Au oxide thin films. These experimental results are further compared to finite-element simulations.
    Analytical Chemistry 09/2014; 86(19). DOI:10.1021/ac502869j · 5.83 Impact Factor
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    ABSTRACT: Membraneless desalination: A simple power supply is used to apply a 3.0 V potential bias across a microelectrochemical cell comprising two microchannels spanned by a single bipolar electrode (BPE) to drive chloride oxidation and water electrolysis at the BPE poles. The resulting ion depletion zone and associated electric field gradient direct ions into a branching microchannel, consequently producing desalted water. Gnd=ground.
    Angewandte Chemie International Edition 09/2014; 52(31). DOI:10.1002/anie.201302577 · 11.34 Impact Factor
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    ABSTRACT: We report a new type of paper analytical device that provides quantitative electrochemical output and detects concentrations as low as 767 fM. The model analyte is labeled with silver nanoparticles (AgNPs), which provide 250 000-fold amplification. AgNPs eliminate the need for enzymatic amplification, thereby improving device stability and response time. The use of magnetic beads to preconcentrate the AgNPs at the detection electrode further improves sensitivity. Response time is improved by incorporation of a hollow channel, which increases the flow rate in the device by a factor of 7 and facilitates the use of magnetic beads. A key reaction necessary for label detection is made possible by the presence of a slip layer, a fluidic switch that can be actuated by manually slipping a piece of paper. The design of the device is versatile and should be useful for detection of proteins, nucleic acids, and microbes.
    Analytical Chemistry 06/2014; 86(13). DOI:10.1021/ac501004a · 5.83 Impact Factor
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    ABSTRACT: In this paper we describe a method for three-dimensional wax patterning of microfluidic paper-based analytical devices (μPADs). The method is rooted in the fundamental details of wax transport in paper and provides a simple way to fabricate complex channel architectures such as hemichannels and fully enclosed channels. We show that three-dimensional μPADs can be fabricated with half as much paper by using hemichannels rather than ordinary open channels. We also provide evidence that fully enclosed channels are efficiently isolated from the exterior environment, decreasing contamination risks, simplifying the handling of the device, and slowing evaporation of solvents.
    Langmuir 06/2014; 30(23). DOI:10.1021/la501212b · 4.38 Impact Factor
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    ABSTRACT: Here, we report a strategy for the design of an inexpensive paper analytical device (PAD) for quantitative detection of oligonucleotides and proteins. Detection is based on the principle of target-induced conformational switching of an aptamer linked to an electrochemical label. This simple and robust method is well matched to the equally simple and robust characteristics of the PAD platform. The demonstrated limits of detection for DNA and thrombin are 30 nM and 16 nM, respectively, and the device-to-device reproducibility is better than ±10%. The PAD has a shelf life of at least 4 weeks, involves little user intervention, and requires a sample volume of just 20 μL.
    Analytical Chemistry 05/2014; 86(12). DOI:10.1021/ac501438y · 5.83 Impact Factor
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    ABSTRACT: We present an introduction to the broad field of desalination by providing a brief review of modern thermal, membrane, and electrochemical technologies. However, the main focus of this article is to introduce a fundamentally new and potentially powerful electrochemical approach to desalination: electrochemically mediated desalination (EMD). EMD is a membraneless desalination method that uses a simple power supply to oxidize a small fraction of the chloride ions present in seawater. This results in the generation of a local electric field gradient, which consequently separates ions to produce partially desalted water.
    05/2014; 1(5). DOI:10.1002/celc.201300236
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    ABSTRACT: We report electrochemical detection of collisions between individual magnetic microbeads, present at subattomolar concentrations, and electrode surfaces. This limit of detection is 4 orders of magnitude lower than has been reported previously, and it is enabled by using a magnetic field to preconcentrate the microbeads prior to detection in a microfluidic electrochemical cell. Importantly, the frequency of collisions between the microbeads and the electrode is not compromised by the low concentration of microbeads. These findings represent an unusual case of detecting individual electrochemical events at very low analyte concentration. In addition to experiments supporting these claims, finite-element simulations provide additional insights into the nature of the interactions between flowing microbeads and their influence on electrochemical processes.
    Analytical Chemistry 04/2014; 86(9). DOI:10.1021/ac404093c · 5.83 Impact Factor
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    ABSTRACT: Here, we report the development of a parallel electrocatalyst screening platform for the hydrogen evolution reaction (HER) using bipolar electrodes (BPEs). Electrocatalyst candidates are subjected to screening in a N2-purged bipolar electrochemical cell where a pair of driving electrodes produce an electric field in the electrolyte solution. The HER occurring at the BPE cathodes is electrically coupled to the electrodissolution of an array of Cr microbands present at the BPE anodes. The readout of this device is simple, where the species that dissolve the most Cr microbands are identified as the most promising electrocatalyst candidates for further evaluation. We demonstrate the utility of this technique by comparing several bi- and trimetallic systems involving Co, Fe, Ni, Mo, and W, which are compared directly with pure Pt. Of all the compositions tested, Ni8–Mo2 is demonstrated to be the most active for the HER in a neutral electrolyte solution.
    ACS Catalysis 03/2014; 4(5):1332–1339. DOI:10.1021/cs500168t · 7.57 Impact Factor
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    ABSTRACT: In the present article we provide a detailed analysis of fundamental electrochemical processes in a new class of paper-based analytical devices (PADs) having hollow channels (HCs). Voltammetry and amperometry were applied under flow and no flow conditions yielding reproducible electrochemical signals that can be described by classical electrochemical theory as well as finite-element simulations. The results shown here provide new and quantitative insights into the flow within HC-PADs. The interesting new result is that despite their remarkable simplicity these HC-PADs exhibit electrochemical and hydrodynamic behavior similar to that of traditional microelectrochemical devices.
    Journal of the American Chemical Society 03/2014; 136(12). DOI:10.1021/ja4118544 · 11.44 Impact Factor
  • 247th National Spring Meeting of the American-Chemical-Society (ACS); 03/2014

Publication Stats

14k Citations
1,960.34 Total Impact Points


  • 2006–2015
    • University of Texas at Austin
      • • Institute for Computational Engineering and Sciences
      • • Department of Chemistry and Biochemistry
      • • Center for Nano- and Molecular Science and Technology
      Austin, Texas, United States
  • 1994–2010
    • Texas A&M University
      • • Department of Chemistry
      • • Department of Chemical Engineering
      College Station, Texas, United States
  • 2009
    • University of California, Riverside
      • Department of Chemistry
      Riverside, California, United States
  • 2008
    • University of Chicago
      • Department of Chemistry
      Chicago, Illinois, United States
  • 1999
    • Sandia National Laboratories
      Albuquerque, New Mexico, United States
  • 1996
    • Hallym University
      Sŏul, Seoul, South Korea
  • 1991–1994
    • University of New Mexico
      Albuquerque, New Mexico, United States