⚡ Part 1 : Surface Functionalization

Multi-functionalization of solid surface is of importance in the field of multiplexed biochips, drug delivery, and various medical devices. Especially, spatially selective surface functionalization is prerequisite for successful fabrication of the multiplexed biochips. My main work was following two topics: (1) Production of multifunctional groups using electrochemically active linkage molecules which are pre-patterned electrode surfaces; (2) Surface functionalization of gold-plated magnetic polymers for highly specific enrichment of blood biomarkers.

New electroactive linkage molecules for spatially selective bio-functionalization

Organic thiols (R-SH) are versatile functional groups in biological systems and bio-conjugation chemistries because they provide site-specific covalent coupling pathways between a solid surface and biomolecules (e.g. thiol-maleimide coupling reaction). 1,3-Dithiane is generally utilized to protect carbonyl groups such as aldehyde during organic synthesis. Dithiane group can be deprotected by means of electrochemical oxidation or organic reactions, rendering disulfide and carbonyl group. We demonstrated a new strategy for immobilizing multiple bio-probes onto a microelectrode array. Electrochemical deprotection of 1,3-dithiane combined with aryldiazonium cation generates aldehyde functionality and then biomolecules can be attached (Fig 1). The dithiane-coupled aryldiazonium can reduce much time to produce multiple bioprobe-functionalized surfaces.

We next reported orthogonal immobilization of biomolecules onto pre-patterned microelectrodes by using 1,3-dithiane group. In contrast to the previous report, disulfides and oxidized derivatives of disulfide were functionalized while removing out carbonyl groups after electrochemical oxidations for the first time (Fig. 2A).

We found that the generation of either disulfides or oxide derivatives of disulfide depend on water content in the electrolyte solution. The generated multifunctional surfaces via the water-controlled differential oxidation enabled orthogonal immobilization of multiple probes for the purpose of multiplex biomarker detection (Fig. 2B and 2C). The orthogonal immobilization approach in this study is very useful since thiol and biotin functionalities are common techniques used for bio-immobilization.

As a serial study, we also utilized cyclic disulfide-ended silane linkage for preparation of multi-biofunctional electrode surfaces. Cyclic disulfide is applicable not only for the preparation of multiple biomolecule-immobilized surfaces but also for the detection of Cu2+ ions.

Label-free detection of blood biomarkers under physiological conditions using gold-plated magnetic polymers

In label-free assays using suspension arrays, nonspecific binding prevents proper signal identification because any biomolecule that binds to sensors will generate signal. Furthermore, in clinical settings where physiological fluid should be analyzed, avoiding such a nonspecific binding is extremely difficult. As an efficacious remedy for the pandemic nonspecific contamination, we suggested a magnetic microsphere of which surfaces are coated by gold nanoparticles. The gold nanoparticular surface readily adopt multifunctional polyethylene glycol (PEG)-alkanethiolates with an exceptional density due to extended actual surface area and self-assembly of aliphatic backbones and thus effectively insulate the solid supports from the impeding biomolecules. In this study, we found that the magnetic gold microspheres (MGMs) effectively prevented nonspecific bindings, leading to a superior enrichment of biomarkers for quantitative analyses in neat serum. Furthermore, by virtue of the innate ability of MALDI MS for simultaneous detection of multiple biomolecules, we successfully demonstrated that the MGMs are also able to serve as affinity probes for a label-free multiplex detection of biomarkers in physiological condition.

⚡ Part 2 : Power generation from salinity gradient and its application

Electrical power generation from renewable energy resources have attracted much attention as environmental contamination accelerates with increasing energy demands. Reverse electrodialysis (RED) is regarded as a practical alternative for generating electrical power from renewable energy resources (Fig. 4). My research since the middle stage of my graduate course has been focused on the development of electrochemical energy conversion systems and its applications to iontronic circuits and drug delivery systems.

Ion-selective polyelectrolyte-stuffed nanochannel arrays for power generation from salinity gradient

Functionalized nanochannels have been recently utilized for electrical power generation systems, e.g. miniaturized power generators and micro batteries. When a surface-charged nanochannel is situated between two different ionic solutions, counter-ions preferentially pass through the nanochannel from the concentrated solution to the diluted one while excluding co-ions out of the channel due to overlap of electrical double layer (EDL) in the nanochannel. Such charge-selective ion transports generate the electrical potential gradient originating from the salinity differences. However, under the conditions of highly concentrated solutions such as seawater, the EDL could become too thin to be overlapped so that the charge-selective ion permeation as well as the generation of the electrical potential should be greatly reduced. In this work, to retain high ion-selectivity even in a concentrated electrolyte solution, an anodized aluminum oxide (AAO) membrane consisting of nanochannel arrays was fully stuffed with polyelectrolytes, which possess low resistivity, ion selectivity, and biocompatible properties. The polyelectrolytic AAO membranes exhibit much better properties when compared with bare AAO membranes for the purpose of electrical power production (Fig. 5). The ion-selective PAMs suggest a new strategy for portable energy harvesting devices in the presence of salinity gradients. It may also be utilized as miniaturized power supply systems, e.g. micro batteries, to operate biosensors for the point-of-care system.

Ionic circuits powered by a miniaturized power source

I am currently developing iontronic circuits operated by a miniaturized power supply. The iontronic circuits fully consist of ionic current through the whole system. This study originates from asking that how neurons in organisms can interact with themselves without any electronic circuits involved. A neuron transports its information by propagating an action potential whose process proceeds by involving specific ion movements, i.e. K+ and Na+, across ion channels. We paid attention to the mechanism of the neuron transmission in which the electrical potential propagates only by the ionic currents. Inspired by the neuron signaling, I am manufacturing iontronic circuits consisting of only ionic currents without using any electronic currents. The biomimetic ionic circuits in this study may provide a better insight to understand the neurotransmission systems and open up a new field for developing such as aqueous computers in the future.

Further direction

The group will be participating in various fields related to electrochemical principles, e.g. chemical/bio sensors, neural interfaces, aqueous circuits, and energy conversion systems.