Sangjun Lee

Bio: Sangjun Lee is an academic researcher from Seoul National University. The author has an hindex of 3, co-authored 4 publications receiving 78 citations.

Engineered nanofluidic preconcentration devices by ion concentration polarization

Abstract: A nanofluidic preconcentration device utilizing an ion concentration polarization (ICP) phenomenon has been regarded as one of the most efficient mechanism for preconcentrating low abundant molecules to be detected. In this review, a short fundamental aspect behind ICP was introduced and detailed engineering endeavors to enhance the performance of the preconcentration device were followed. While a conventional nanostructure based on silicon or glass substrate lose its ion-selectivity at a physiologically relevant electrolyte concentration, Nafion as a highly charged nanoporous material would maintain its permselectivity at the concentration so that various fabrication processes utilizing Nafion were introduced. In order to extend the capability of the device in terms of preconcentration factor and throughput, dual gates, “U”-shaped and one-channel device were shown with their pros and cons. Last section would introduce the most recent development of preconcentration mechanism; the simultaneous preconcentration with separation, the radial preconcentration and the two stress-free preconcentration mechanisms. Conclusively, this review would not only provide the key insight of development history of the nanofluidic preconcentration device but also contribute for creating the next generation preconcentration mechanisms.

Enhancing the sensitivity of point-of-use electrochemical microfluidic sensors by ion concentration polarisation – A case study on arsenic

Abstract: Point of use (POU) sensors are extremely relevant, being capable of providing fast and reliable analysis in remote and resource-limited settings. Of all the diverse techniques utilised for POU sensors, a combination of electrochemistry and microfluidics may have the greatest potential towards quantitative assessment of heavy metal ions. The major challenge in combining these for sensing applications lies in the complexity of fabricating integrated devices and the insufficient quantity of analytes in the sample volume. To address these issues, we have developed a radial microfluidic device capable of electrokinetic preconcentration by ion concentration polarization (ICP) and integrated it with electroactive surfaces. The proposed sensor is the first demonstration of concentration of heavy metal ions by ICP and its quantitative assessment by voltammetry. Utilising the integrated sensor, we have shown the sensing of As3+ down to 1 ppb by linear sweep voltammetry with ∼40 μL of sample. The sensor was also tested successfully for sensing As3+ in a field sample from an arsenic affected region of India. The sensor was also tested for the detection of other metal ions too. This work would facilitate the development of highly sensitive POU hand-held sensors for water quality monitoring in resource-limited areas.

Nanoelectrokinetic bufferchannel-less radial preconcentrator and online extractor by tunable ion depletion layer.

Abstract: Among various preconcentration strategies using nanofluidic platforms, a nanoscale electrokinetic phenomenon called ion concentration polarization (ICP) has been extensively utilized due to several advantages such as high preconcentration factor and no need of complex buffer exchange process. However, conventional ICP preconcentrator had difficulties in the recovery of preconcentrated sample and complicated buffer channels. To overcome these, bufferchannel-less radial micro/nanofluidic preconcentrator was developed in this work. Radially arranged microchannel can maximize the micro/nano membrane interface so that the samples were preconcentrated from each microchannel. All of preconcentrated plugs moved toward the center pipette tip and can be easily collected by just pulling out the tip installed at the center reservoir. For a simple and cost-effective fabrication, a commercial printer was used to print the nanoporous membrane as “Nafion-junction device.” Various analytes such as polystyrene particle, fluorescent dye, and dsDNA were preconcentrated and extracted with the recovery ratio of 85.5%, 79.0%, and 51.3%, respectively. Furthermore, we used a super inkjet printer to print the silver electrode instead of nanoporous membrane to preconcentrate either type of charged analytes as “printed-electrode device.” A Faradaic reaction was used as the main mechanism, and we successfully demonstrated the preconcentration of either negatively or positively charged analytes. The presented bufferchannel-less radial preconcentrator would be utilized as a practical and handy platform for analyzing low-abundant molecules.

Nanoelectrokinetic radial preconcentrator/extractor based on ion concentration polarization

Abstract: This paper reported the experimental demonstration of a radial preconcentrator/extractor in a micro/nanofluidic platform based on ion concentration polarization (ICP) phenomena. While various biomolecular preconcentration mechanisms have been developed for BioMEMS applications, an integration with downstream analyzer or an efficient recovery of concentrated sample still remain challenging. Here we designed a simple radial device without complex channel network and one can complete sample preparation steps from the preconcentration of samples to the recovery of concentrated samples with this single platform. Various charged species such as fluorescent dye, dsDNA and polystyrene microparticle are exemplified for the process (preconcentration factor ∼ 10 and throughput ∼ 1 μL/min). This simple device configuration would serve a key component for point-of-care-test applications and biomedical analysis tools.

Ion Concentration Polarization by Bifurcated Current Path.

Abstract: Ion concentration polarization (ICP) is a fundamental electrokinetic process that occurs near a perm-selective membrane under dc bias. Overall process highly depends on the current transportation mechanisms such as electro-convection, surface conduction and diffusioosmosis and the fundamental characteristics can be significantly altered by external parameters, once the permselectivity was fixed. In this work, a new ICP device with a bifurcated current path as for the enhancement of the surface conduction was fabricated using a polymeric nanoporous material. It was protruded to the middle of a microchannel, while the material was exactly aligned at the interface between two microchannels in a conventional ICP device. Rigorous experiments revealed out that the propagation of ICP layer was initiated from the different locations of the protruded membrane according to the dominant current path which was determined by a bulk electrolyte concentration. Since the enhancement of surface conduction maintained the stability of ICP process, a strong electrokinetic flow associated with the amplified electric field inside ICP layer was significantly suppressed over the protruded membrane even at condensed limit. As a practical example of utilizing the protruded device, we successfully demonstrated a non-destructive micro/nanofluidic preconcentrator of fragile cellular species (i.e. red blood cells).

Selection of Non-Equilibrium Over-Limiting Currents: Universal Depletion Layer Formation Dynamics and Vortex Instability

Abstract: The ion flux dynamics across a straight nanoslot is imaged to understand the nonequilibrium phenomenon of overlimiting current density across a nanoporous membrane. With a slow ac field, an ion-depletion front is generated intermittently from one end of the nanochannel, and a vortex instability first predicted by Rubinstein, Staude, and Kedem [Desalination 69, 101 (1988).10.1016/0011-9164(88)80013-4] is found to arrest the self-similar diffusive front growth. This electrokinetic instability evolves into a stationary interfacial vortex array that specifies the overlimiting current, independent of external stirring or convective flow.