1. Electrochemical energy storage and conversion

The CO2 concentration in the Earth’s atmosphere has been on a dramatic rise over the past century (currently > 400 ppm). The rise in CO2 levels has often been correlated with an increase in temperature anomalies and deleterious climate change effects. As a result, developing sustainable technologies that can stabilize and eventually reduce the rising levels of CO2 emissions remains one of the grand challenges of the 21st century. In our group, we try to provide a solution to the problem by developing: (a) electrochemical systems that can convert excess CO2 emissions to value added chemicals (such as CO, HCOOH, CH3OH, C2H4) thereby closing the carbon emission loop; and (b) portable microscale fuel cells that can provide clean power for a variety of stationary as well as transport applications.

Electrochemical reduction of CO2

For the electrochemical reduction of CO2, our group aims at improving the product selectivity (Faradaic efficiency), energetic efficiency and conversion rate (current density) through the development of novel catalysts, application of suitable electrolytes, and optimization of the electrode structure and the reactor design. For example, we decreased the cell overpotential to less than 0.2 V by using an aqueous solution containing 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM BF4), which presumably stabilizes a reaction intermediate (Rosen et al. Science, 2011). We also developed silver-based organometallic catalysts that exhibit high catalytic activity at low Ag loading (Thorson et al., J. Am. Chem. Soc., 2012). As a support material, TiO2 was used to minimize Ag particle size and increase catalyst activity, resulting in a drastically lower Ag loading without sacrificing the performance towards reducing CO2 to CO (Ma et al., ChemSusChem, 2014). The optimization of the anode also has shown significant improvement in the overall system energy efficiency (Ma et al., JECS, 2014). We developed active and durable Cu nanoparticle-based catalysts, which are able to efficiently convert CO2 to C2 chemicals at unprecedented high production rates at low overpotentials (Ma et al., J. Power Sources, 2016). The electrolyte composition (cations, anions, concentration, and electrolyte mixtures) was also found to play a significant role in the electroreduction of CO2 to CO, with the optimal electrolyte composition exhibiting partial current density for CO as high as 440 mA cm-2 at an energy efficiency of 42% (Thorson et al., J. Electrochem. Soc., 2013 and Verma et al., Phys. Chem. Chem. Phys., 2016).

Engineering the catalyst layer structure provides an additional approach to maximize catalyst utilization and overall performance. An automated airbrushed catalyst deposition method led to high performance on CO2 reduction with reduced catalyst loading while unwanted H2 evolution was suppressed (Jhong et al., Adv. Energy Mater., 2013). We also optimized gas diffusion electrodes (e.g., studying the effect of micro porous layer and substrates) explicitly for CO2 reduction. These optimized gas diffusion electrodes outperform commercially available electrodes and exhibit no decay in performance during continuous operation (Kim et al., J. Power Sourc., 2016). We also incorporated multi-walled carbon nanotubes (MWCNTs) in the catalyst layer to improve charge transfer within the catalyst layer, thereby increasing the current density by a factor of two, while at the same time decreasing the noble metal loading to only 1/5th compared to the case without the incorporation of MWCNTs (Ma et al., J. Mater. Chem. A, 2016). In other work, we performed a technoeconomic analysis on the process of CO2 electroreduction using a gross marging model (Verma et al., ChemSusChem, 2016). We worked on a systematic analysis of the influence of dilute CO2 feeds on CO2 reduction to determine the practical feasibility of the direct use of flue gas as a feed for electroreduction of CO2 (Kim et al., Electrochim. Acta, 2015). Currently we continue to pursue research towards better catalysts, electrodes, and operation conditions for the electrochemical conversion of CO2 into different value added carbon chemicals and fuels. Some of this work is in collaboration with other groups such as: Dr. Gewirth at UIUC; Dr. Nakashima, Dr. Fujigaya, Dr. Fujigawa, Dr. Lyth, and Dr. Yamauchi at Kyushu University in Japan, Dr. Ohno at Kyushu Institute of Technology in Japan; and Dr. Rich Masel at Dioxide Materials.

Microscale Fuel Cells

Fuel cells have the potential to serve as an alternative, highly efficient, and clean power source for a range of applications. Since membrane-related issues such as water management and fuel crossover often hinder the development of traditional cells, we have developed membraneless laminar flow-based fuel cells (LFFCs) in which the fuel and electrolyte streams are compartmentalized in a single channel (Choban et al., J. Power Sources, 2004). Subsequently w developed air-breathing LFFCs by integrating a gas diffusion electrode (GDE) as the cathode (Jayashree et al., J. Am. Chem. Soc., 2005). More recently, we created a microfluidic H2/O2 fuel cell that can serve as an excellent platform for the characterization and optimization of catalysts and electrodes. (Jayashree et al., Langmuir, 2007). Nowadays, the limitation of the application of fuel cell is dominated by the performance of the cathode reaction - the sluggish oxygen reduction reaction. In collaboration with Dr. Gewirth in the Department of Chemistry, we have developed a Cu-DAT catalyst, which shows comparable cathode performance to expensive Pt based materials, but at much lower loading (Brushett et al., J. Am. Chem. Soc., 2010). Currently further research is ongoing to optimize the oxygen reduction reaction.

2. Microfluidics for studying pharmaceuticals and proteins

In our group, we develop microscale approaches to study the structures and properties of pharmaceuticals and biomolecules. Analysis can quickly become very expensive because of large amounts of material needed to run and optimize experiments. Many of our microfluidic platforms enable in situ analysis of proteins or pharmaceutical compounds from experiments run at the nanoliter scale, allowing high-throughput methods while efficiently utilizing precious sample.

Crystallization of membrane proteins

Membrane proteins (MPs) reside within the cellular membrane and act as mediators for signal, energy, and material transduction into and out of the cell. Not surprisingly, the malfunction of membrane proteins has been linked to numerous diseases. MPs are thus common drug targets, and despite their overwhelming abundance cells (~30% of all proteins), MPs account for less than 1% of protein structures deposited in the Protein Databank. Structure determination of membrane proteins has been hampered by difficulties in obtaining sufficient quantities of the proteins due to low abundance and their inherent amphiphilicity, and subsequent difficulties in crystallization. In our group, we have developed X-ray transparent microfluidic platforms for in surfo (Guha et al., Sens. & Act. B, 2012) and in meso MP crystallization (Khvostichenko et al., Crys. Growth & Des., 2014). In addition, our research includes X-ray transparent platforms that enable the study of lipidic cubic phase diagrams (Khvostichenko et al., Analyst, 2013) and microseed matrix screening, two powerful yet typically inaccessible crystallization techniques for membrane proteins. The overall goal of our research is to crystallize large, well-ordered ("diffraction-quality") crystals for X-ray analysis and structure elucidation. We have crystallized several targets and solved their structures using data collected solely on chip (Perry et al., LOC, 2013). This work is in collaboration with Prof. Robert Gennis, Department of Biochemistry.

Solid form screening of candidate pharmaceuticals

During the early stages of pharmaceutical drug discovery, scientists search for solid forms of active pharmaceutical ingredients (APIs) that have appropriate physical and chemical properties (i.e. solubility, bioavailability, stability) that can later move through the drug development pipeline. Unfortunately, success in finding a crystalline solid form of an API with optimized properties using conventional screening procedures (well plates) is limited by small quantity of API available during the early stages of drug discovery. To address this issue we have developed microfluidic platforms for pharmaceutical solid form screening with the goals of (i) reducing the quantity of active pharmaceutical ingredients (APIs) needed for solid form screening, (ii) increasing compatibility between solid form screening platform and analytical instruments, and (iii) determining if a microfluidic approach to solid form screening allows for elucidation of novel solid forms. We have validated microfluidic platforms based on free interface diffusion (Thorson et al., LOC, 2011) and controlled evaporation (Goyal et al., LOC, 2013) that reduce the amount of API needed per solid form screening condition by an order of magnitude (from 5 mg to 5 µg for each conditions), with comparable results to traditional evaporation based solid form screening experiments. We designed the microfluidic platforms to be optically transparent allowing for easy identification of crystalline solids with optical microscopy, and to show minimal signal in Raman spectroscopy (Goyal et al., Crys. Growth & Des., 2012) and X-ray diffraction allowing for on-chip identification of solid forms (Horstman et al., Crys. Growth & Des., 2015). Additionally, we have demonstrated that solid form screening on-chip can be used to grow and characterize novel cocrystals (Horstman et al., CrystEngComm, 2015) demonstrating the unique crystallization within the microfluidic platform. This work is in collaboration with AbbVie, Inc.

Time-resolved FTIR spectroscopy of proteins

Our overall goal is to develop an innovative microfluidic technology for time-resolved Fourier-transform infrared (FT-IR) spectroscopy of biomolecular reactions or interactions. Protein folding, enzyme catalysis, and protein-ligand interactions are critical to maintaining healthy cells and tissues. The root of many chronic or genetic diseases can be traced back to the malfunction of such reactions in proteins - e.g., plaque formation by misfolded beta-amyloid peptide in Alzheimer's disease. Investigations to reveal reaction mechanisms at the molecular and intermolecular level are essential for developing novel therapeutics from rational drug design as well as to their testing - e.g., beta-amyloid folding pathways can reveal targets on which candidate drugs against plaque formation can be tested and optimized. Fourier transform infrared (FTIR) spectroscopy provides several advantages compared to other spectroscopy techniques, including non-requirement of extrinsic labeling, simple sample preparation, and easy acquisition of a range of information (high-resolution molecular details to low-resolution protein-protein interactions). However, several limitations with current FTIR flow cells, including low time-resolution, cost, and requirement of large sample volumes, have prevented the wide-spread use of FTIR. We address these issues by developing microfluidic FITR flow cells out of low-cost, IR-transparent materials. Preliminary results with ubiquitin have validated our approach and we are optimizing the flow cell for conducting experiments with clinically relevant proteins. This project is in collaboration with Prof. Rohit Bhargava in the Department of Bioengineering.

Freeze-quench EPR studies of membrane proteins

Most of the interesting phenomena in many biochemical reactions occur during the first few milliseconds of the reactions, e.g., ATP synthesis mediated by the cytochrome bc1 complex. Structural and functional studies of these early-stage intermediate products will not only elucidate the mechanism of these reactions, but will also enable rational design of drugs to treat diseases and disorders associated with the malfunctioning of these reactions. Freeze-quench electron paramagnetic resonance (EPR) is a powerful technique to study these reactions, where the intermediate products of these reactions are rapidly frozen to prevent further reactions and later analyzed using EPR. However, the limitations of the current apparatus for freeze-quench EPR, mainly the slow mixing of reagents, has prevented the application of this technique to study ultra-fast biochemical reactions. In our research group, we are developing a microfluidic device for rapid mixing of reagents (~20 µs) and subsequent ejection of the mixed reagents in the form of an ultra-thin jet onto a frozen copper wheel set-up. We have validated this approach with a model biochemical reaction and are exploring the application of clinically relevant biochemical reactions. This project is in collaboration with Prof. Tony Crofts from the Department of Biochemistry.

Determining interactions between pharmaceuticals and target proteins

All of biology, and by extension all of pharmacology, depends on the interaction of proteins with other molecules. Electron Paramagnetic Resonance (EPR) combined with Spin Labeling (SLEPR) can be used to detect such interactions in real time, in vitro or in vivo, and to track the ratio of bound to unbound proteins, with minimal perturbation of the biology. This makes it an ideal tool to directly study the effects of pharmaceutical agents on their biological target and on related biochemical systems, improving the accuracy of early stage development predictions of efficacy and toxicity of drug candidates. However, current wet-lab methods for preparation of the small samples required by EPR spectrometers tend to be wasteful, imprecise, and slow (taking 24 hours or more). In our group we are developing devices for rapid and precise labeling of proteins, taking full advantage of the combinatorial nature of microfluidic chips to create a series of samples at multiple concentrations or with a variety of partners, and incorporating on-chip cell culture when necessary. This project is in collaboration with New Liberty Proteomics.

3. Radiopharmaceutical and Quantum dot synthesis

In our group, we develop microreactors and scalable continuous flow reactors for the synthesis of radiopharmaceuticals and semiconductor nanocrystals. Microreactors provide exquisite control over flow profiles, temperature profiles, and mixing, resulting in high-quality, consistent products.

Continuous flow synthesis of quantum dots

Fluorescent semiconductor nanoparticles show promise in solid-state lighting and display technology due to significantly higher photoluminescence and better spectral behavior than conventional phosphor technology. These nanoparticles also have potential uses in medical imaging and quantum computing. High production costs due in part to a lack of reliable methods for the production of high quality, monodisperse nanoparticles currently greatly inhibit their widespread usage. Conventional batch synthesis methods suffer especially from batch-to-batch variation of nanomaterial quality. Batch syntheses, owing to slow heat and mass transfer lack the ability to precisely control on size, morphology and composition of nanoparticles. Continuous flow reactors provide potential solution to these problems. The efforts in Kenis group are focused on development of high throughput continuous reactors affording fast mixing and heating times at high temperatures to synthesize high quality semiconductor nanoparticles of varying composition and morphology. For example, we successfully synthesized nanorods using one of our continuous flow reactors (Naughton, et al., Nanoscale, 2015). We are studying both Cd containing and Cd-free systems, reaching quantum yields as high as 60%, which is comparable to commercial products.

Synthesis of radiopharmaceuticals

Radiopharmaceuticals are a class of drugs used in the diagnosis and treatment of several diseases and disorders, including certain types of cancer and heart diseases. The amounts of the precursors for the synthesis of these drugs are typically small (a few microliters) due to limited availability, high costs, and upper limits on the amount of radioactivity that can be handled safely. The inability of the conventional 'wet-lab' methods to efficiently manipulate low reagent volumes not only leads to synthesis of low-quality drugs for clinical applications, but also hampers the development of new drugs. We try to address these issues by developing microfluidic technologies, or better microreactors, for the synthesis of these radiopharmaceuticals. We have shown that the microfluidic technologies provide several advantages for each step compared to conventional methods, including improved reaction yields, reduced consumption of reagents, and amenability for automation (Wright et al., J. Nucl. Med., 2016; Li et al., RSC Adv., 2015; Goyal et al., Sens. & Act. B, 2014; Li et al., Bioconj. Chem., 2014; Zeng et al., Nuc. Med. & Bio., 2013; Wheeler et al., Lab Chip, 2010). This project is in collaboration with Prof. David Reichert's research group in the department of Radiological Chemistry at Washington University in St. Louis.

4. Biological study platforms

Microfluidic platforms provide several characteristics that better facilitate studying cellular and inter-cellular processes compared to traditional petri dish- or well plate-based techniques. Examples include the ability to study single cells in highly controlled environments, superior control over the cellular microenvironment in space and time, and convenient integration with different types of microscopy.

Antibiotic susceptibility testing

Effective treatment of clinical infections is critically dependent on the ability to rapidly screen patient samples to identify the susceptibility of the infecting pathogens to antibiotics. Existing methods for antibiotic susceptibility testing (AST) suffer from several issues, including long turnaround times (days), excess sample and reagent consumption, poor detection sensitivity, and limited combinatorial capabilities. These factors preclude the timely administration of appropriate antibiotics, complicating management of infections and exacerbating the development of antibiotic resistance. To address these issues, we develop microfluidic platforms for AST that provides several advantages compared to conventional methods, including higher detection sensitivity, rapid results (less than 6 hours), reduced consumption of reagents, and more quantitative results. For example, in collaboration with Prof. Schroeder we have used our microfluidic platforms for studying the susceptibility of various pathogenic bacteria, such as E. coli, P. aeruginosa, and K. pneumoniae, against different antibiotics (Mohan et al. Biosens. & Bioelect., 2013). We have also used the platform to study the interaction amongst different species of bacteria (polymicrobial cultures) and the effect of these interactions on antibiotic susceptibility (Mohan et al., RSC Adv., 2015).

Studying cells under controlled oxygen conditions

As tumors grow outward away from the local vascular architecture formation of variable hypoxic (sub-physiologic tissue oxygenation) regions occur throughout the solid mass. These hypoxic regions have been associated with therapeutic resistance, metabolic reprogramming, and the epithelial-mesenchymal transition. Many questions remain regarding the effects of hypoxia on these outcomes, yet only few methods enable both precise control over oxygen concentration and real-time imaging of cell behavior. Microfluidic platforms are particularly well suited to control oxygen concentration while enabling real-time imaging due to their control over temporal and spatial chemical conditions. In addition to control over the local microenvironment, the reduced length scale in microfluidic platforms compared to conventional methods provides shorter equilibration times. Utilizing the advantages of microfluidic platforms, we have developed an arrayed device capable of controlling oxygen concentration from 0.5% to 21%. In collaboration with Professor Rex Gaskins (Department of Animal Sciences), we utilize these platforms to study real-time changes of organellar redox potential in cancer cells under hypoxia.

5. Other Efforts

Development and characterization of microfluidic valves

Pneumatic normally-closed (NC) valves: Devices employing conventional normally-open (NO) valves have limited portability in applications that require continuous closed state for operation, as these valves need bulky ancillaries (pumps, nitrogen gas cylinders, pneumatic peripherals) for actuation. NC valves not only address the above limitation of restricted portability, but also retain the ease of fabrication and integration into microfluidic devices. To enable integration of NC valves, we used a combination of analytical and computational modeling, and systematic experiments to formulate design rules for developing optimal NC valves with the objective of minimizing actuation pressures and facilitating fabrication of these valves (Mohan et al., Sens. & Act. B, 2011). The figure shows the actuation pressure needed as a function of the width of the fluid channel for different microvalve shapes (straight, v-shaped, and diagonal). We have used these valves for a variety of applications, such as protein-antibody interactions virus detection, protein crystallization, solid form screening, and exploring other applications (Schudel et al., LOC, 2011; Thorson et al., CrystEngComm, 2012; Guha et al., Sens, & Act. B, 2012; Mohan et al., Biosens. & Bioelect., 2013; Tice et al., JMEMS, 2013).

Electrostatic microvalves: Our microvalves based on electrostatic actuation retain the small footprint (less than 500 mm) and fast response time (less than 1 s) of their pneumatic equivalents, while requiring less bulky ancillaries (no need for a pressure source), which greatly improves portability and scalability. To integrate these electrostatic microvalves, we used analytical modeling followed by experimental validation to formulate design rules (Desai et al., LOC, 2012). Electrostatic microvalves that actuate with low potentials have applications in digital microfluidics that typically require fluid manipulation at low pressures and devices employing NC valves. The Figure shows the combinations of microvalve diameters (D) and channel heights (g), indicated by the shaded region, that can be actuated with potentials less than 300 V, while not collapsing during operation (i.e., greater than 1), for membrane thicknesses (tm) of 5 µm. The design parameter space is estimated for the presence of air (darker), oil (hatched), or water (lighter) in the fluidic channel (Tice et al., RSC Adv., 2014). Another interesting application that we are exploring is the use of electrostatic microvalves to control pneumatic microvalves. This combination of pneumatic and electrostatic microvalves will greatly simplify the ancillaries, and aid in realizing the goal of 'lab-in-a-chip' rather than the 'chip-in-a-lab'.

Alternative materials and fabrication processes for microfluidics

Poly(dimethylsiloxane) or PDMS has been the preferred material for fabrication of microfluidics devices, mainly because the use of PDMS allows for simple, rapid, and inexpensive fabrication of devices with varying degrees of complexity. However, PDMS suffers from several limitations, a key one being incompatibility with a wide range of organic solvents and analytical techniques. In our research group, we are exploring a variety of polymeric materials as an alternative to PDMS to manufacture microfluidics devices; some of these materials are thiolene, cyclic-olefin copolymer and Teflon. We used these materials to develop microfluidic devices that are compatible with a range of organic solvents and analytical techniques, such as X-ray and Raman. We also show that hybrid devices, can provide more control over solvent evaporation rates leading to better crystallization outcomes (Goyal et al., RSC Adv., 2016).