Robert D. Tilton

 

Center for Complex Fluids Engineering, Department of Biomedical Engineering and Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213.  tilton@andrew.cmu.edu

 

Microfluidic Biotechnologies

        Proteins and Protein-Conjugates at Interfaces

                Polymer and Surfactant Co-Adsorption

                        Nanomaterials for Environmental Remediation

 

The main theme underlying the research in our group is the pervasive influence of complex fluid interfacial phenomena in nature and technology. The group maintains a long-standing interest in the structure, dynamics and function of macromolecules or supramolecular assemblies at interfaces.  In addition, a variety of concepts based on colloidal and interfacial phenomena are being developed for new biotechnological and environmental technologies. A selection of projects in the group is presented below. A breakdown of publications in different areas is included at the end.

 

Microfluidic Biotechnologies – Electric Field-Driven Cell Separations

We are developing techniques based on a coupling of dielectrophoresis (DEP) and electrohydrodynamic flows to manipulate colloidal particles and biological cells using micropatterned electrodes.  Particles and cells migrate to either high field or low field traps, according to their induced dipole strengths.  This offers the opportunity to separate mixtures according to differences in dielectric properties.  Whereas DEP separations are typically operated at MHz frequencies, we operate at frequencies ranging from ~ 10 Hz to 1 kHz where AC electroosmotic flows are established along the edges of the microelectrode features.  The simultaneous action of DEP and AC electroosmotic drag forces on particles provides a finer level of discrimination to laterally separate particles, making it possible to separate mixtures that are not well separated by DEP (high frequency operation) methods alone. The effect is easily visualized for the separation of  low dielectric colloids from yeast cells in suspension.

 

                

 

Lateral separation of polystyrene colloids and yeast cells.  Low frequency (60 Hz) operation shown at left aligns yeast cells along the microelectrode strip centers, with polystyrene aligned along the strip edges. This placement is dictated by the DEP/AC electroosmotic coupling.  Operation at high frequencies (1 MHz) shown at right instead aligns yeast in the high field traps along the edges and polystyrene in the low field traps in the strip centers and the centers of the insulating glass regions between strips.  This placement is dictated by DEP alone.  We have used this technique to separate mixtures of suspension cells and to pattern vesicles.  This work is an ongoing collaboration with Professor Lee White.  See Zhou et al. J. Colloid Interface Sci. 285, 179-191 (2005).  From the Ph.D. Research of Hao Zhou.

 

Proteins at Interfaces – Effect of Poly(Ethylene Glycol) Modification on Protein Adsorption

Covalently grafting PEG chains to protein therapeutics has been demonstrated in the literature to improve pharmaceutical efficacy by significantly increasing protein clearance times in the body.  In collaboration with Professor Todd Przybycien, we are investigating the effects of PEG grafts on the extent, kinetics and reversibility of protein adsorption to solid surfaces, including chromatographic materials such as will ultimately be used in the commercial manufacture of PEGylated protein therapeutics and biodegradable poly(lactide-co-glycolide) (PLG) materials that are used for sustained release of proteins.  Whereas the protein-repellant nature of PEG has led to its perception as a biologically “stealthy” polymer, PEG is surface active on many types of surfaces. From a fundamental perspective, this makes PEG-protein conjugates an intriguing adsorption system in that it can be considered as a complex block copolymer with multiple surface active blocks.  Under some conditions, the conjugate can be adsorbed to a surface via the protein “block” while under other conditions, the PEG block(s) anchor the conjugate.  We have found that on PLG surfaces, PEGylation decreases the extent of ribonuclease A (a protein with anti-tumoral effect) adsorption relative to the unmodified protein via an excluded area effect on the surface.  PEGylation also reverses the way in which the protein responds to aging of the PLG material. The diminished adsorption correlates with more complete release, and improved retention of biological activity of ribonuclease A, from PLG microspheres. See Daly et al. “Adsorption of poly(ethylene glycol)-modified ribonuclease A to a poly(lactide-co-glycolide) surface,”  Biotechnology and Bioengineering (2005)

 

On silica surfaces, steric interactions among PEG grafts on lysozyme distort the shape of the adsorption isotherm and flip the preferred orientation of the protein relative to the surface.  PEG grafts also increase the reversibility of adsorption (even though PEG homopolymer adsorption and unconjugated lysozyme adsorption are both effectively irreversible on silica). They inhibit the lateral aggregation of lysozyme, thereby reducing a prevalent mode of surface-induced denaturation of protein therapeutics.  The complexity of protein adsorption phenomena mandates the use of multiple experimental probes in parallel. The conclusions described here were based on several novel variants of total internal reflection fluorescence (TIRF) spectroscopy, flouorescence spectroscopy, optical reflectometry, AFM colloidal force measurements, electrokinetic (streaming current) measurements and attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR). See Daly et al. “Adsorption of poly(ethylene glycol) modified lysozyme to silica,” Langmuir 21, 1328-1337 (2005).

Adsorption isotherms on silica for lysozyme (squares), monoPEG-lysozyme (triangles) and diPEG-lysozyme (circles).

From the Ph.D. research of Susan Daly DiPasquo

 

 

Co-Adsorption of Polymers and Surfactants – Control of Persistent Non-Equilibrium States in Thin Films

The properties of colloidal materials are engineered by controlling interfacial chemistry.  Mixed self-assembly and/or co-adsorption of polymers and surfactants exert powerful controls over the macroscopic properties of complex fluids that are found in the manufacture and application of materials such as pharmaceutical suspensions or solid dosage forms, ceramics, paints, inks, and personal care products.    The tendency of surfactants and polymers to exert strongly coupled influences on complex fluid properties makes these fluids particularly difficult to formulate.  A particular emphasis of our research in this area concerns the prevalence of persistent non-equilibrium, or hysteretic, states in polymer layers.  We have found that it is possible to control the composition and structure of polymer layers by controlling the order in which surfactants and polymers are exposed to a surface. For example, dense films of Pluronic® PEO-PPO-PEO copolymers can be prepared on silica by co-adsorption with sodium dodecyl sulfate (SDS), followed by removal of the surfactant from the system.  Similar SDS-processing can be used to to condense, or to stretch, cellulosic polymers on a silica surface depending on conditions.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The force between two silica surfaces coated with PEO-PPO-PEO layers, measured by atomic force microscopy, on the left is for a layer prepared by simple adsorption from an SDS-free solution.  The force profile on the right, which is consistent with a thinner layer, is for a PEO-PPO-PEO layer produced by co-adsorption with SDS followed by surfactant removal.  Although the SDS-processed layer is thinner, it contains approximately 30% more polymer per unit area – the processed layer is ~ 50 % denser, due to a condensation of polymer around bound SDS micelles that occurs during adsorption.  Once deposited on the surface, the condensed polymer configuration does not relax sufficiently to match the configuration of the “ordinary” polymer layer. See Braem et al. “Control of Persistent Nonequilibrium Adsorbed Polymer Layer Structure by Transient Exposure to Surfactants,” Langmuir 19, 2736 (2003).  From the Ph.D. Research of Alan Braem and Derek Berglund.

 

 

Nanomaterials for Environmental Remediation – Development of Amphiphilic Polymers for Targeted Delivery of Iron Nanoparticles to DNAPL/Water Interfaces

Organic contamination of subsurface soil and groundwater is an extensive and vexing environmental problem that stands to benefit from nanotechnology. The Environmental Protection Agency reports that contamination by organic pollutants, especially chlorinated volatile organic compounds (CVOCs), are primary concerns at over half of the Superfund National Priorities List sites. (See Common Chemicals Found at Superfund Sites, last update January 11, 2005; http://www.epa.gov/superfund/resources/chemicals.htm).  As concluded by the National Research Council of the National Academy of Sciences, the prevailing “pump-and-treat” technologies cannot meet cleanup targets in a reasonable amount of time in most cases. (Water Science Technology Board. Contaminants in the subsurface:  Source zone assessment and remediation; Division of Earth and Life Sciences, National Research Council of the National Academies, The National Academies Press: Washington, DC, 2004.)  This is because they address primarily the plume, not the source.

 

In collaboration with Professors Gregory Lowry (Dept. of Civil & Environmental Engineering), Krzysztof Matyjaszewski (Dept. of Chemistry) and Sara Majetich (Dept. of Physics), we are developing the concept of targeted delivery of remediation agents to the contamination source – underground residuals of dense nonaqueous phase liquids (DNAPL) trapped in microporous aquifers.  We have synthesized amphiphilic poly(methacrylic acid)-b-poly(methylmethacrylate)-b-poly(styrenesulfonate) triblock copolymers that adsorb to reactive nanoparticulate Fe(0) (RNIP, particles commercially available from Toda America.  See Liu et al. “TCE Dechlorination Rates, Pathways, and Efficiency of Nanoscale Iron Particles with Different Properties,” Environmental Science and Technology, 39, 1338-1335 (2005).) The adsorbed polymers stably disperse the particles in water, maximize their transportability through saturated sand columns, and cause the particles to adsorb to the DNAPL/water interface, all as designed.

 

 

The macroscale problem.  Illustration of DNAPL distribution as residual saturation and a plume of dissolved contaminants in an aquifer. Pump and treat remediation methods require multiple wells to pump groundwater from the plume to the surface for chemical treatment, leaving the residual saturation source behind to continually replenish the plume.  Our proposed method will target the chemical treatment to the residual saturation zone in situ.  Drawing from S.G. Huling and J.W. Weaver, “Dense Nonaqueous Phase Liquids,” EPA Groundwater Issue EPA/540/4-91-002, 1991.

 

 

 

 

 

 

 

The nanoscale solution.  Residual DNAPL (shown in gray) is trapped in and between soil grains (shown in black) in pores that range from micrometers to as small as 2 – 50 nm in diameter.  Targeted iron nanoparticles, by virtue of their size and surface functionalization, gain access to the trapped DNAPL in micropores and adsorb with high affinity to the DNAPL/water interface.  There, the accumulated iron can reduce DNAPL to nontoxic products.

 

 

The targeting mechanism.  Particle-grafted block copolymers contain a hydrophilic (blue) block that has an affinity for water and a hydrophobic (red) block that has an affinity for DNAPL. In water, the hydrophilic block swells and the hydrophobic block collapses.  The reverse happens in the DNAPL phase. This amphiphilicity anchors the particle at the DNAPL/water interface.  The hydrophilic block is large enough to stably suspend particles in water without aggregating. Incorporating strong negative charge in the hydrophilic block minimizes particle adhesion to negatively charged mineral or natural organic matter surfaces in the soil before reaching the DNAPL.

 

 

Pickering Emulsions Stabilized by Poly(Styrenesulfonate)-Grafted Nanoparticles


 

As a sub-set of this project, we developed poly(styrenesulfonate)-grafted silica nanoparticles that are excellent stabilizers of non-invertable oil-in-water emulsions.  These “Pickering emulsions” are stable for months, because the surface activity of the polyelectrolyte shell surrounding the silica tightly anchors the nanoparticles to the oil/water interface.  Trichloroethylene-in-water emulsions are shown: a typical emulsion on the left at pH 7 and a structured emulsion on the right at pH ~ 2.

 

From the Ph.D. research of Kevin Sirk and Navid Saleh.

                                                                                                                       

 

 

YKI Ambassador Program

Professor Tilton maintains an active collaboration with the Institute for Surface Chemistry (Ytkemiska Institutet; YKI) in Stockholm, Sweden.  The Institute for Surface Chemistry (official name: YKI, Ytkemiska Institutet AB) is an internationally leading industrial research institute with deep knowledge in applied surface and colloid chemistry. YKI serves many industrial branches including pharmaceuticals, biotechnology, food, industrial chemicals, household products, engineering, pulp and paper, coatings, ceramics, concrete, adhesives, paint, ink and printing. There are currently 85 member companies, 50% of which are located outside Sweden. See http://www.surfchem.kth.se for more information, or send email to tilton@andrew.cmu.edu.

 

As a YKI Ambassador, Prof. Tilton is available to facilitate industrial research collaborations with YKI, including joint collaborations that leverage the knowledge and resources of YKI and Prof. Tilton’s Complex Fluid and Bio-Interfaces Group at Carnegie Mellon.

 

 

Group Publications

Polymers and Surfactants: Adsorption, Self-Assembly, Wetting, Surface Forces

K. D. Berglund, M. T. Truong, R. D. Tilton and L. M. Walker, “Rheology of Transient Networks Containing Hydrophobically Modified Cellulose, Anionic Surfactant and Colloidal Silica: Role of Selective Adsorption,” Rheol. Acta 43, 50 – 61 (2004)

O.J. Rojas, P.M. Claesson, K.D. Berglund, and R.D. Tilton, “Coadsorption and Surface Forces for Selective Surfaces in Contact with Aqueous Mixtures of Oppositely Charged Surfactants and Low Charge Density Polyelectrolytes,” Langmuir, 20, 3221-3230 (2004).

N. Kumar, S. Garoff, and R.D. Tilton, “Experimental Observations on the Scaling of Adsorption Isotherms for Nonionic Surfactants at a Hydrophobic Solid-Water Interface,” Langmuir 20, 4446-4451 (2004).

N. Kumar and R.D. Tilton, “A Unified Model to Predict Self-Assembly of Nonionic Surfactants in Solution and Adsorption on Solid or Fluid Hydrophobic Surfaces: Effect of Molecular Structure,” Langmuir 20, 4452-4464 (2004).

I.N. Nnebe, R.D. Tilton, and J.W. Schneider, “Direct Measurement of the Stability of Poly(ethylene glycol)-Poly(ethylenimine) Graft Films,” Journal of Colloid and Interface Science 276, 306-316 (2004).

R.D. Tilton, “Adsorption from Mixtures of Polymers and Surfactants,” in Encyclopedia of Surface and Colloid Science, 8th Updated Edition, P. Somasundaran, Ed., Marcel-Dekker, Inc., New York  (2004)

K.D. Berglund, A. Timko, T.M. Przybycien, R.D. Tilton, “Use of Nonionic Ethylene Oxide Surfactants as Phase Transfer Catalysts for Poly(Acrylic Acid) Adsorption to Silica against an Electrostatic Repulsion,” Progress in Colloid and Polymer Science 122, 56-66 (2003).

K.D. Berglund, T.M. Przybycien, R.D. Tilton, “Coadsorption of Sodium Dodecylsulfate with Hydrophobically Modified Nonionic Cellulose Polymers I: Role of Polymer Hydrophobic Modification,” Langmuir 19, 2705-2713 (2003).

K.D. Berglund, T.M. Przybycien, R.D. Tilton, “Coadsorption of Sodium Dodecylsulfate with Hydrophobically Modified Nonionic Cellulose Polymers II: Role of Surface Selectivity in Adsorption Hysteresis,” Langmuir 19, 2714-2721 (2003).

A.D. Braem, S. Biggs, D.C. Prieve, R.D. Tilton, Control of Persistent Nonequilibrium Adsorbed Polymer Layer Structure by Transient Exposure to Surfactants,” Langmuir 19, 2736 (2003).

N. Kumar, K. Varanasi, R.D. Tilton, S. Garoff, “Surfactant Self-Assembly ahead of the Contact Line on a Hydrophobic Surface and its Implications for Wetting,” Langmuir 19, 5366 (2003).

S.B. Velegol, R.D. Tilton, “Specific Counterion Effects on the Competitive Co-Adsorption of Polyelectrolytes and Ionic Surfactants,” Journal of Colloid and Interface Science 249, 282-289 (2002).

E.A.S. Doherty, K.D. Berglund, B.A. Buchholz, I.V. Kourkine, T.M. Przybycien, R.D. Tilton, A.E. Barron, “Critical Factors for High-Performance Physically Adsorbed (Dynamic) Polymeric Wall Coatings for Microchannel Electrophoresis of DNA,” Electrophoresis 23, 2766-2776 (2002).

S.B. Velegol, R.D. Tilton, “A Connection between Interfacial Self-Assembly and the Inhibition of Hexadecyltrimethylammonium Bromide Adsorption on Silica by Poly-L-lysine,” Langmuir 17, 219 (2001).

A.D. Braem, D.C. Prieve, R.D. Tilton, “Electrostatically Tunable Coadsorption of Sodium Dodecylsulfate and Poly(Ethylene Oxide)-b-Poly(Propylene Oxide)-b-Poly(Ethylene Oxide) Triblock Copolymer to Silica,” Langmuir 17, 883 (2001).

B.B. Luokkala, S. Garoff, R.D. Tilton, R.M. Suter, “Interfacial Structure and Rearrangement of Nonionic Surfactants near a Moving Contact Line,” Langmuir 17, 5917 (2001).

S.B. Velegol, B.D. Fleming, S. Biggs, E.J. Wanless, R.D. Tilton, “Counterion Effects on Hexadecyltrimethylammonium Surfactant Adsorption and Self-Assembly on Silica,” Langmuir 16, 2548 (2000).

J.-H. Kim, M.M. Domach, R.D. Tilton, “Effect of Electrolytes on the Pyrene Solubilization Capacity of Dodecyl Sulfate Micelles,” Langmuir 16, 10037 (2000).

J.-H. Kim, M.M. Domach and R.D. Tilton, “Solubilization Capacity of Pyrene in Octaethylene Glycol Monododecyl Ether Micelles,” Colloids Surf. A Physicochem. Eng. Aspects 150, 55 (1999).

J.-H. Kim, M.M. Domach, R.D. Tilton, “Pyrene Micropartitioning and Solubilization by Sodium Dodecyl Sulfate Complexes with Poly(ethylene glycol),” J. Phys. Chem. B, 103, 10582 (1999).

R.D. Tilton, “Scanning Angle Reflectometry and its Application to Polymer Adsorption and Coadsorption with Surfactants,” in Polymer-Colloid Interactions: Techniques and Applications, P. Dubin and R. Farinato, Eds., pp. 331-363, John Wiley and Sons, Inc., New York (1999).

E.S. Pagac, D.C. Prieve and R.D. Tilton, “Kinetics and Mechanism of Cationic Surfactant Adsorption and Coadsorption with Cationic Polyelectrolytes at the Silica-Water Interface,” Langmuir 14, 2333 (1998).

E.S. Pagac, R.D. Tilton and D.C. Prieve, “Depletion Attraction Caused by Unadsorbed Polyelectrolytes,” Langmuir 14, 5106 (1998).

E.S. Pagac, D.C. Prieve, Y. Solomentsev and R.D. Tilton, “A Comparison of Polystyrene-Poly(ethylene oxide) Diblock Copolymer and Poly(ethylene oxide) Homopolymer Adsorption from Aqueous Solutions,” Langmuir, 13, 2993-3001 (1997).

J.R. Charron and R.D. Tilton, “Penetration of Insoluble Lipid Monolayers at the Air-Water Interface by Water-Soluble Block Copolymers and Homopolymers,” Langmuir 13,  5524 (1997).

J.R. Charron and R.D. Tilton, “A Scanning Angle Reflectometry Investigation of Block Copolymer Adsorption to Insoluble Lipid Monolayers at the Air-Water Interface,” J. Phys. Chem. 100, 3179 (1996).

E.M. Furst, E.S. Pagac, and R.D. Tilton, “Coadsorption of Polylysine and the Cationic Surfactant Cetyltrimethylammonium Bromide on Silica,” Ind. Eng. Chem. Res. 35, 1566 (1996).

E.S. Pagac, R.D. Tilton and D.C. Prieve, “Hindered Mobility of a Rigid Sphere Near a Wall,” Chem. Eng. Commun. 148-150, 105 (1996).

 

Colloidal Dispersions

N. Saleh, T. Sarbu, K. Sirk, G.V. Lowry, K. Matyjaszewski, and R.D. Tilton, “Oil-in-water emulsions stabilized by highly charged polyelectrolyte-grafted silica nanoparticles,” Langmuir in press (available on-line July 2005).

 

Proteins at Interfaces

S.M. Daly, T.M. Przybycien and R.D. Tilton, “Adsorption of poly(ethylene glycol)-modified ribonuclease A to a poly(lactide-co-glycolide) surface,” Biotechnology and Bioengineering 90, 856-868 (2005).

S. Daly, T.M. Przybycien, and R.D. Tilton, “Adsorption of poly(ethylene glycol) modified lysozyme to silica,” Langmuir 21, 1328-1337 (2005).

S. Daly, T.M. Przybycien, R.D. Tilton, “Coverage Dependent Orientation of Lysozyme Adsorbed on Silica,” Langmuir 19, 3848 (2003).

E. J. Choi, M.D. Foster, S. Daly, R. Tilton, T. Przybycien, C.F. Majkrzak, P. Witte, H. Menzel, "Effect of Flow on Human Serum Albumin Adsorption to Self-Assembled Monolayers of Varying Packing Density, " Langmuir 19, 5464  (2003).

R.D. Tilton, “Mobility of Biomolecules at Interfaces,” in Biopolymers at Interfaces, Surfactant Science Series vol. 75, M. Malmsten, Ed., pp. 363-407, Marcel Dekker, New York (1998).  Reproduced in Biopolymers at Interfaces, 2nd Edition Revised and Expanded, Surfactant Science Series vol. 110, M. Malmsten, Ed., pp. 221-257, Marcel Dekker, New York (2003).

M.L. Sun, R.D. Tilton “Coadsorption of Protein/Surfactant Complexes to the Air/Water Interface,” Colloids and Surfaces B: Biointerfaces 20, 281 (2001).

S. M. Butler, M. A. Tracy and R.D. Tilton, “Adsorption of Serum Albumin to Thin Films of Poly(lactide-co-glycolide),” J. Controlled Release 58, 335 (1999).

E.F. Tsung and R.D. Tilton, “Measurement of Catalytic Reaction Kinetics for Adsorbed Enzyme Monolayers,” J. Colloid Interface Sci. 213, 208 (1999).

J.L. Robeson and R.D. Tilton, “Spontaneous Reconfiguration of Adsorbed Lysozyme Layers Observed by Total Internal Reflection Fluorescence with a pH-Sensitive Fluorophore,” Langmuir, 12, 6104 (1996).

J.L. Robeson and R.D. Tilton, “Effect of Concentration Quenching on Fluorescence Recovery After Photobleaching Measurements,” Biophys. J. 68, 2145 (1995).

E. Blomberg, P.M. Claesson, J.C. Fröberg and R.D. Tilton, “The Interaction between Adsorbed Layers of Lysozyme Studied with the Surface Force Technique,” Langmuir 10, 2325 (1994).

E. Blomberg, P.M. Claesson and R.D. Tilton, “Short-Range Interaction between Adsorbed Layers of Human Serum Albumin,” J. Colloid Interface Sci 166, 427 (1994).

T.E. Rabe and R.D. Tilton, “Surface Diffusion of Adsorbed Proteins in the Vicinity of the Substrate Glass Transition Temperature,” J. Colloid Interface Sci. 159, 243 (1993).

R.D. Tilton, E. Blomberg and P.M. Claesson, “The Effect of Anionic Surfactant Binding on the Interaction between Adsorbed Layers of Lysozyme on Mica,” Langmuir 9, 2102 (1993).R.D. Tilton, C.R. Robertson and A.P. Gast, “Manipulation of Hydrophobic Interactions in Protein Adsorption,” Langmuir 7, 2710 (1991).

R.D. Tilton, C.R. Robertson and A.P. Gast, “Lateral Diffusion of Bovine Serum Albumin Adsorbed at the Solid-Liquid Interface,” J. Colloid Interface Sci. 137, 192 (1990).

R.D. Tilton, A.P. Gast and C.R. Robertson, “Surface Diffusion of Interacting Proteins: Effect of Concentration on the Lateral Mobility of Adsorbed Bovine Serum Albumin,” Biophys. J. 58, 1321 (1990).

 

 

Dielectrophoresis and Electrohydrodynamics

H. Zhou, M.A. Preston, R.D. Tilton, and L.R. White, “Calculation of the dynamic impedance of the double layer on a planar electrode by the theory of electrokinetics,” Journal of Colloid and Interface Science in press (available on-line July 5, 2005)

H. Zhou, L.R. White and R.D. Tilton, “Lateral separation of colloids or cells by dielectrophoresis augmented by AC electroosmosis,” Journal of Colloid and Interface Science 285, 179-191 (2005).

H. Zhou, M.A. Preston, R.D. Tilton, and L.R. White, “Calculation of the electric polarizability of a charged spherical dielectric particle by the theory of colloidal electrokinetics,” Journal of Colloid and Interface Science 285, 845-856 (2004).

V. Brisson, R.D. Tilton, “Self-assembly and two-dimensional patterning of cell arrays by electrophoretic deposition,” Biotechnology and Bioengineering 77, 290-295 (2002)

 

 

Environmental Applications

Y.Liu, S. Majetich, K. Matyjaszewski, R.D. Tilton, D. Sholl, and G.V. Lowry, “TCE Dechlorination Rates, Pathways, and Efficiency of Nanoscale Iron Particles with Different Properties,” Environmental Science and Technology, 39, 1338-1335 (2005).