Research

 Electrochemiluminescence (ECL) and its application in biochemical analysis

Electrochemiluminescence also known as Electrogenerated Chemiluminescence (ECL), is light emission produced through the secondary chemical reaction of radical intermediates that are primarily generated by an electrochemical redox process (as shown in Figure 1). This phenomenon has a wide range of applications in the fields of optoelectronic devices and chemical and biological analysis.

My research group has conducted a detailed study of the mechanism of ECL, based on both material and physical chemistry. We aim to apply this knowledge towards the development of next-generation bioanalytical instruments.

 A)    Developing new forms of biosensor based on the ECL analysis

In essence, ECL is a light emission from chemical materials triggered by an electrochemical process. Light, or photons, plays a critical role in signal transduction in spectroscopic analysis. Thus, as a spectroscopic detection method, ECL offers several advantages over conventional spectroscopic analysis (as shown in Figure 2).

One of my research focuses is to develop ECL molecular sensors for biologically and clinically significant molecules such as ions, amino acids, peptides, proteins, phosphates, pyrophosphates, carbohydrates, nucleotides, and nucleic acids. Our ongoing work is directed towards bioanalytical applications such as enzyme assays, DNA/RNA sequencing techniques, disease diagnosis, and bioimaging. In our approach, we employ a combination of molecular recognition and ECL analysis. We synthesize a specific molecule that selectively binds to the target molecule within a sample mixture. This binding can occur through hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, π-π interactions, halogen bonding, or electrostatic and/or electromagnetic effects. The binding of the target to the molecular probe results in a conformational change that generates a photophysical or electrochemical signal from the binding adduct. Therefore, when the synthetic molecule recognizes and binds to the target, it changes ECL emission properties, providing qualitative and quantitative analysis without needing further sample preparation (as shown in Figure 3).

 B)    Electrochemical biosensor based on supramolecular recognition.

 The integration of molecular recognition and electrochemistry provides a significant opportunity to create novel biosensors for the diagnosis of various diseases. The concept of biosensors, which measure glucose levels with simple electrochemical equipment, was first proposed by Clarke and Lyons in 1962 (Ann. N. Y. Acad. Sci., 102, 29-45). Since then, biosensors have been utilized in fields such as medicine, pharma, food and process control, environmental monitoring, defense, and security. While electrochemical biosensors are currently the dominant type, they primarily focus on metabolite monitoring. On the other hand, bioaffinity monitoring is primarily performed using optical techniques. However, both transducers have utility across the field, as well as piezoelectric, thermometric, magnetic, and micromechanical transducers. The emergence of semi-synthetic and synthetic receptors has led to the creation of more robust, versatile, and widely applicable sensors, and the use of nanomaterials has increased the sensitivity and ease of 

Our group has developed an electrochemical biosensor using a synthetic molecular probe as the bioreceptor. The molecular probe was designed and synthesized based on the principle of molecular recognition and is capable of specifically binding to small molecular targets that cannot be recognized by typical bioreceptors such as enzymes, antibodies, or nucleic acids. The synthesized probe offers consistent binding properties, and the electrochemical approach enables the creation of easy-to-use, portable devices for decentralized, in situ, or home analysis by non-specialists. For instance, we recently developed a rapid, chip-based electrochemical biosensor for protein kinase activity assay (Anal. Chem., 2014, 86, 10992-10995). The synthetic probe on the electrode surface specifically recognizes the enzymatic product from protein kinase A (PKA) and successfully reports the activity in blood serum on a plastic chip. Additionally, we developed a new homogeneous electrochemical assay method for exonuclease activity, where the DNA substrate is hydrolyzed by the exonuclease in the solution, and the enzyme product is then actively immobilized on the electrode surface, generating an electrochemical current signal. The method allows for the direct quantification of endonuclease and exonuclease enzymes by simple voltammetric measurement, and the results are comparable with conventional methods such as gel-based or fluorescence-based. We are currently collaborating with an industry partner to develop a high-precision, portable electrochemical sensor based on this technology.

C)    Operation dynamics in electrochemical devices

Recently, there has been significant interest in studying electrochemical devices, such as secondary batteries, fuel cells, and electrochromic devices. These devices are typically composed of a two-terminal electrode system and operate through voltage or current application between the anode and cathode. However, electrochemical devices often face operational stability and long-term reliability issues due to the intense, high-powered energy consumption during their operation. Although efforts by many groups to solve these issues through new materials, only a few studies have investigated the underlying device physics.

Our research focuses on the driving dynamics of electrochemical devices, intending to provide insights into the current reliability issues. We started with studying light-emitting devices based on ECL, as they provide a model for energy conversion through electrolytic redox processes. Unlike OLEDs, which consist of multiple layers of organic materials, ECL devices only require a single emissive layer between two electrodes to support the entire luminescent process. This simple device architecture allows for a better understanding of the underlying mechanics during fabrication.

We have found that the operation of ECL devices is similar to that of an electrolytic cell system, where an external electric field is applied to the anode and cathode, forcing electrolytic redox reactions to occur at the electrode surfaces. ECL devices are also prone to operational stability issues and can easily degrade under external voltage, which has been a critical challenge for commercialization. However, we have found that ECL devices can operate with improved light-emitting performance and longer lifetimes by applying current instead of voltage. Our ongoing research aims to understand the appropriate conditions and mechanisms for this improved performance, and we are considering future research for commercialization. Our investigation has revealed that the interfacial redox reactions are highly dependent on the driving conditions and that the concentration depletion of redox species and their diffusion at the electrode surfaces are critical in determining the device's operation.

D)    Next-generation batteries of high energy density and large scale 

Traditional battery chemistry and cell architectures for lithium-ion batteries (LIBs) achieve excellent power and energy density, allowing LIBs to become the most appropriate one among any existing technologies for electrical energy storage. However, on the other hand, they have faced technical challenges in the increasing demand for electrified transportation and large grid applications. Researches are actively underway on new "beyond lithium-ion" battery technologies that can meet the needs of more sustainable and more decarbonized energy grids. We are an research group that develops new highly functional materials that can realize "beyond lithium-ion" technologies such as high-capacity silicon and all-solid-state battery. Our materials will be the key technology to accelerate the realization of the next-generation batteries of high-energy-density and large-scale and lead the world.

As the next generation of batteries that exceed the capacity limits of commercial lithium-ion batteries, silicon batteries are the closest technology to practical use. The silicon battery has an energy density of a maximum of 10 times that of a conventional lithium-ion battery and is capable of charging and discharging faster. However, solving the large volume expansion issues of 4-5 times the size of the battery during the charging/discharging process is a major challenge in this R&D field. Our only patented technology provides a solution to these critical problems and provides a critical foundation for the realization of next-generation silicon batteries.

The most important technical challenges in electric vehicle batteries are high energy density, fast charging time, low-temperature performance and safety to improve driving mileage. We develop technologies that can improve battery performance most effectively without compromising existing standard manufacturing processes. Through the development of new highly functional additive materials, we successfully inhibit the generation of the dendrite, which was a chronic problem of lithium metal batteries, and thus improve the lifetime and stability of the battery performance.

The design of an electrolyte system, such as a highly functional electrolyte additive or electrolyte salt, is one of the low-cost, high-effectiveness solutions that can improve battery performance most effectively while minimizing current process changes. Based on proprietary patent and material technologies, we design and develop stable next-generation electrolyte systems that can be applied to a wide range of secondary batteries from lithium-ion to lithium-metal batteries.