Director of Graduate Students,
Oliver Hall 4025
B.S., Universidad del Tolima, Ibague, Colombia, 1989
M.S., Universidad del Valle, Cali, Colombia, 1996
Ph.D., University of Miami, Coral Gables, FL., 2000
Postdoctoral Research, Texas A&M University, College Station, TX, 2000-2004
National Science Foundation Career Award 2007
Excellence in Scholarship Award, College of Humanities and Sciences, VCU, 2007
Outstanding Graduate Student Award, University of Miami, 2000
Research in our lab revolves around using electrochemistry and other electroanalytical methodologies to address diverse problems in analytical chemistry and mechanisms of complex redox reactions. Our aim is to investigate systems for which current analytical approaches become inadequate or fundamental understanding of reactivity is lacking. For instance, we are interested in uncovering physicochemical factors that affect the mechanistic pathways of Proton-Coupled Electron Transfer (PCET), which is a special type of redox reaction underpinning natural and artificial processes for the interconversion of chemical and electrochemical energy. We also try to apply novel methods like Particle Collision Electrochemistry to investigate chemical reactivity of PCET and other complex reactions relevant to organic synthesis, at the organic-water interface in aqueous emulsions and suspensions. Finally, we employ conventional electrokinetic methods like streaming potentials in microfluidic channels to study ion and molecular binding on thin films coated on microchannel surfaces.
This type of reaction entails the sequential or concerted occurrence of proton transfer (PT) and electron transfer (ET) in a coupled manner, such that the interdependence of the two events appears to control the kinetics and thermodynamics of the overall reaction. Given that PCET underlies natural processes that produce energy (respiration) and chemical fuels (photosynthesis) with remarkable efficiency, the effort to discover the mechanistic underpinnings that lead to fast and efficient PCET reactions has intensified greatly in the last 20 years. Of particular interest, is the concerted pathway, which appears to be responsible for the effectiveness of photosynthesis due to the avoidance of charged and unstable intermediates when both PT and ET happen in concert. Despite the great progress in theoretical understanding of PCET, the success in implementing the putative advantage of the concerted pathway in technologically relevant processes for renewable energy like the electrochemical reduction of CO2 and O2, has come up short. This stems from the still imprecise knowledge about the role that the interplay between PT and ET has in rendering a concerted pathway, which demands the need to distinguish between the sequential routes (ET-PT or PT-ET) and the concerted one. One goal of this research is to investigate how experimental parameters like pKa and redox potential control the rate and dominance of the concerted pathway, as well as the conditions whereby this route becomes faster and more energetically efficient than the sequential mechanisms. To this end, we try to select model PCET systems that can be studied electrochemically and use digital simulations to back up the proposed mechanisms. For instance, we recently reported on the oxidation of glutathione (GSH) by oxidants of variable strength in the presence of bases of different pK. Data analysis shows that despite parallel mechanisms, the concerted one seems to predominate for the oxidant-base pair that renders the most isoenergetic coupled state, whereby a PT with DG˚′PT à 0 is capable of producing an ET with DG˚′ET à 0, as a result of the Nernstian shift of the redox potential of GSH with pKa. In contrast, the stepwise PT–ET appears to dominate when the deprotonated intermediate GS- grows in stability as DG˚′PT becomes more negative.
This is an emerging methodology that allows characterization of microscopic particles in solution as they collide with the surface of an ultramicroelectrode (UME) by virtue of Brownian motion. Typically, a signal in the form of a spike or step in electrochemical current, can be generated if the collision encounter is coupled to an electrochemical reaction during the impact with the surface of the UME, which customarily has a disk diameter between ~6 to 15 mm. This size of UME can detect objects as small as ~0.3 mm depending on the mechanism of electrochemical coupling. The signal is often the result of a perturbation in the local concentration of the electrolyzed species, due to consumption, production or just temporary blockage of the active area of the UME by the colliding particle. The ability of PCE to detect individual impacts of particles, rendering information like particle size and chemical reactivity from spikes of electrochemical current, has inspired fundamental studies in detection of cells, bacteria, viruses, nanoparticles, emulsion droplets, etc. For instance, we recently detected hydrogen bonding of carboxylic acids with a quinone species trapped inside toluene droplets (~1.4 mm in diameter) that were dispersed in water and studied by PCE. The effect was revealed by comparing collision spikes from the reduction of quinone-loaded droplets, after addition of carboxylic acids of varying hydrophobicity and pKa, which showed higher current at lower potential than pristine droplets. Our ongoing goal in this project is to use PCE for investigating reactivity and mechanisms at the organic-water interface of emulsions for reactions relevant to the organic synthesis industry (i.e “on-water catalysis” and others). Furthermore, we would like to expand the scope of PCE by coming up with strategies to detect inherently non-electroactive particles like bacteria and cells.
Streaming potentials (SPs) are differences in potential (DE) spontaneously developed at the outlets of a microchannel when a liquid is forced through it by pressure-driven flow. This DE, which can be measured with two electrodes placed along the flow axis at the outlets of the channel, is a result of the downstream motion of the counterions closest to the surface and carried by the liquid flow. The stream of surface counterions moving forward gives rise to a streaming current, IS, which is counterbalanced by a conduction current IC in the opposite direction, so that when steady state is attained, a net excess of counterions is accumulated at the downstream end of the channel whereas a reverse condition appears on the other side. Because this is a surface induced phenomenon, SPs are only measurable (0.01 to 10 mV) in microchannels with volumes small enough for the excess of ions to make an impact in the channel concentration and potential. However, the principle of SPs has been known for a century and can be used to determine the charge of different materials represented in a parameter called z-potential (z). For channels of micrometer dimensions and no overlap of the electric double layers from the walls, the magnitude of DE is linearly related to the pressure and z, such that when the latter reflects the actual charge of the surface. We have employed this notion for tracking in real time, the adsorption of analytes like heparin and lysozyme, using channels with selective receptors attached to their walls. The signal arises from the binding of the analyte with the attached receptor, whereby a concomitant change in surface charge induces a proportional variation in DE.
Our current goal in this project is to build a SP instrument that can measure changes in DE induced by surface reactions different than adsorption, particularly in biofilms.
Meng, K., Medina-Ramos, J., Yibeltal-Ashenafi, E., Alvarez, J.C.; “Interplay of Proton and Electron Transfer to Determine Concerted Behavior in the Proton-Coupled Electron Transfer of Glutathione Oxidation”, Phys. Chem. Chem. Phys., 2018, 20, 17666 – 17675.
Medina-Ramos, J., Oyesanya, O., Alvarez, J. C.; “Buffer Effects in the Kinetics of Concerted Proton-Coupled Electron Transfer: The Electrochemical Oxidation of Glutathione Mediated by [IrCl6]2−at Variable Buffer pKa and Concentration”, J. Phys. Chem. C, 2013, 117, 902-912.
Paul, D., Meng, K., Omanovic, D., Alvarez, J.C.; “Hydrogen Bonding and Proton Transfer in Aqueous Toluene Microdroplets Studied by Particle Collision Electrochemistry”, ChemElectroChem, 2018, DOI: 10.1002/celc.201800542, in press.
Luna-Vera, F., Ferguson, J. D., Alvarez, J.C, “Real Time Detection of Lysozyme by Pulsed Streaming Potentials Using Polyclonal Antibodies Immobilized on a Renewable Nonfouling Surface Inside Plastic Microfluidic Channels”, Anal. Chem., 2011, 83, 2012-2019.
Pu, Q., Elazazy, M., Alvarez, J. C., “Label- Free Detection of Heparin, Streptavidin, and Other Probes By Pulsed Streaming Potentials in Plastic Microfluidic Channels”. Anal. Chem., 2008, 80, 6532-6536.