Current status of work in the project
Microfluidics is a field of science investigating processes and technologies for handling liquids in volumes in range of nano- to microliters. Thanks to the small volumes, precise control and manipulation of liquids is possible on a scale where surface forces prevail over volumetric ones. Microfluidics covers a wide spectrum of research applications and is firmly embedded in the trends towards miniaturization and automation of systems. Microfluidic tools have been progressively implemented in many laboratories, including electrochemical ones. The miniaturized electrochemical platforms help to achieve high repeatability of results and integrate many elements to create intelligent multi-task electrochemical analysis platforms. In light of trends associated with the growing use of measurements in microflows, we are actively developing a multiparametric instrumentation. Our plan is to integrate advanced, thin film microelectrodes and such in situ analyses methods as UV-Vis, FTIR/IR, Raman, NMR and ultra-fast laser spectroscopy in microfluidic electrochemical cells.
The work carried out in UPTURN project was divided into several main threads, including design and fabrication of miniaturized electrochemical cells with integrated thin-film electrodes, design of chip electro-mechanical interface, and finally development of two demonstrators showcasing the developed technologies.
One of the first goals of the UPTURN project was to design and fabricate miniaturized electrochemical cells on glass or silicon substrates. The cells contain microfluidic chambers and channels with thin-film electrodes. It was crucial to carefully consider the design of channels and electrodes to ensure that these electrochemical flow microreactors could find widespread application in the future. The mask design includes the following concepts of microfluidic electrochemical cells: basic static processes, basic flow processes, custom-deposited thin film, spectrometric measurements, photoelectrochemical measurements, impedance spectroscopy of bulk electrolysis. The design variants consider the dimensions of the electrodes and the distances between them to ensure efficient and sensitive electrochemical measurements. For example, in the flow cell (Figure 1), the surface of the counter electrode is twice higher as the working electrode, which is intended to ensure a larger charge capacity. An alternative reference pseudoelectrode was also designed. One of the possible applications of this system is flow voltammetry, which can be used, among others, in electrochemical sensors. Additionally, the synthesis and detection of short-lived species may have on-chip applications. The fact that each designed cell has two sets of electrodes located inside the channel, and each chip has up to five channels, opens for numerous multiparametric applications and synergies with microfluidics. For example, flow systems have been designed for electrochemical measurements with the simultaneous use of UV-Vis-NIR spectrometry, or a system for impedance spectroscopy, where an ion-exchange membrane is placed between the electrodes.
Figure 1 The demonstrative drawings of the chip (functional design) for basic flow microfluidic electrochemical cell presenting top view (A), cross-sectional view on electrode arrangement (B).
Thanks to the broad experience of consortium members in electrochemistry, a variety of interesting functional designs were developed. Based on the functional designs, mask designs were prepared, and SINTEF has further focused on the fabrication process to bring these ambitious concepts to life on a chip. An eight-mask process has been developed, relying on lithography processes, wet and dry etching, metal sputtering, patterning, and finally, bonding and dicing of silicon and glass wafers. We have worked with both thin film Pt and Au electrodes. Downscaling of an electrochemical cell to a microfluidic system involves several technical risks. Some of the aspects that the consortium is currently working to improve are associated with failures due to mechanical instability and variations in the performance of microelectrodes. The performance of miniaturized electrochemical cells can only be predicted to a certain degree. To truly understand how they function, they need to be thoroughly tested and compared to macroscopic cells. For this purpose, the consortium has performed many tedious electrochemical measurements of individual chips, and also constructed the demonstrators comprising ion-exchange membranes and spectroelectrochemical modalities.
Electrochemical testing of individual chips was performed in various inorganic environments. These included repeated cyclic voltammetry (CV) measurements for different scan rate values and long-term measurements in which 1000 cyclic voltammetry measurements were made at a determined scan rate. After the measurements, the repeatability of the results was determined, and the quality of the electrodes was observed. Figure 2 presents CV curves obtained in 0.1M HCl electrolyte for wide potential ranges, collected at different scan rates. These measurements were performed on planar glass substrate (without microchannels) (A) and non-planar (electrodes are placed inside microfluidic channels) (B) devices. The repeatability of the obtained curves is visible, and after series of measurements, the Pt microelectrodes were of good quality.
Figure 2 Effect of scan rates on the electrochemical response in 0.1 M HCl registered for the Pt microelectrodes on planar chip (A) and chip with channels (B).
One of the project tasks in UPTURN involves developing a microfluidic cell demonstrator for electrochemical measurements. This microfluidic cell demo aims to facilitate efficient characterization of membranes and other separators in a liquid environment. This system allows for measurements of resistance/conductivity, permeability, and selectivity of membranes. It is addressed particularly to the energy industry (flow electrolytes for redox batteries, liquid fuel cells, and batteries) and other electrochemical processes.
The research work showing this demonstrator in UPTURN was divided into two stages. The first was focused on the macro-scale measurements using electrochemical cells dedicated to testing membranes: (a) Devanathan-Stachurski Permeation Cell and (b) Magnetic Mount Electrochemical H-Cell. The second stage involved the analogous measurements in a microfluidic format and using miniaturized electrochemical cells. The main components of this demonstration system are microfluidic flow devices with integrated microelectrodes that are assembled with an ion exchange membrane of interest. In this approach, it was possible to verify and compare the obtained results. We have demonstrated the resistivity measurements of the Nafion™ membranes in a range of selected inorganic electrolytes. The demonstration was carried out using such techniques as cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy (EIS). For example, Figure 3 presents the EIS spectrum for a microflow electrochemical cell obtained in 0.1M HCl electrolyte. The cell was equipped with a Nafion™ 211 ion exchange membrane, and during the measurement, electrolyte was passed through the channel at a rate of 10 µl/min. The impedance Z was determined to be equal to 85 Ohm and Zfit =88 Ohm.
Figure 3 Electrochemical impedance spectroscopy results obtained in 0.1M HCl in the microfluidic reactor with Nafion™ 211 membrane. Flow rate = 10 µl/min.
In parallel to the work carried out on demonstrator with membranes, we are working on a second demonstrator intended for photoelectrochemical measurements, which is based on a TiO2 photoanode. Potential application is removing organic pollutants from aqueous solutions. Under the illumination of light of a specific wavelength, the titanium oxide photoanode generates reactive oxygen molecules, such as the hydroxyl radical, which is a strong oxidant useful in the decomposition of organic compounds. Analogously to the previously described research work, we carry out measurements on a macroscopic scale. TiO2 layers were made on an FTO glass substrate using titanium dioxide paste, which is heated to burn off the polymer binder. The measurement system consists of a spectro-electrochemical flow cell, an LED light source, a potentiostat and a UV-vis spectrophotometer. Methylene blue solution in 0.1M H2SO4 was used as the electrolyte for a model reaction of removal of organic pollutants. Example results of electrochemical measurements are shown in Figure 4. The photoelectrochemical effect can be observed as the difference in current measured at a given potential with and without light. If we perform CV scanning of the TiO2 layer electrode without light, the current will be almost zero. If we carry out the same measurement with light exposure, a much larger current (called photocurrent) will be recorded (Figure 4A). The same with chronoamperometry – if the electrode is kept at a constant voltage (in this case 0 V), it can be observed that current only flows when the light source is turned on (Figure 4B).
Figure 4 Photoelectrochemical effect observed during the cyclic voltammetry measurements (A) and chronoamperometry (B).
Microfluidic reactors are placed in a holder specially designed considering the chip geometry and functionality. We are currently at the stage of holder design screening and microfluidic experiments. The challenge is to design and create a holder that must meet several demanding requirements. It is necessary to ensure the flow of electrolytes through the channels and the electrical contact of all electrodes.
In the UPTURN project, we adopted one of the Agile methodologies – Scrum, which consists of successive, fast-delivered iterations of the design to create a better product for our customers. The method involves working in successive stages, called “sprints”, which consist of specific stages: planning, design, development, testing and implementation. After each sprint, the team draws conclusions from subsequent product iterations and makes corrections and modifications, if necessary, so that the holder can meet the expectations of future customers.
A digital CAD design is created in the initial stage, and the holder is manufactured using a high-precision 3D SLA printing method. The printed holder is equipped with elements such as luer fittings, electrical contacts, and gaskets and is sent to electrochemical laboratories. Tests are carried out on the analysis of several types of microfluidic electrochemical cell designs described previously. In case of any problems related to the use of the holder in practice, feedback is provided to designers. In this way, several product versions were created, each time with improved functionality. We are currently working on additional elements of the measurement system, such as temperature control. Our research is currently focused on improving the holder design and collection of good quality data from both demonstrators. We want to test the demonstrators in a wide range of functionalities, i.e., using various electrochemical environments, measurement methods, and parameters. In turn, the holder should fulfill basic functions and be user-friendly. Our goal is to create a system for multiparametric measurements in microflows. By testing various types of microfluidic cells, we expand the potential application of our products so that they can be used in many electrochemical and analytical laboratories. The UPTURN project truly lives up to the ambitious of multiparametric analysis, and we are looking forward to what the final phase of the project will bring.