Tae Hyun Yoon

Graphene Oxide Thin Films for Flexible Nonvolatile Memory Applications

There has been strong demand for novel nonvolatile memory technology for low-cost, large-area, and low-power flexible electronics applications. Resistive memories based on metal oxide thin films have been extensively studied for application as next-generation nonvolatile memory devices. However, although the metal oxide based resistive memories have several advantages, such as good scalability, low-power consumption, and fast switching speed, their application to large-area flexible substrates has been limited due to their material characteristics and necessity of a high-temperature fabrication process. As a promising nonvolatile memory technology for large-area flexible applications, we present a graphene oxide based memory that can be easily fabricated using a room temperature spin-casting method on flexible substrates and has reliable memory performance in terms of retention and endurance. The microscopic origin of the bipolar resistive switching behavior was elucidated and is attributed to rupture and formation of conducting filaments at the top amorphous interface layer formed between the graphene oxide film and the top Al metal electrode, via high-resolution transmission electron microscopy and in situ X-ray photoemission spectroscopy. This work provides an important step for developing understanding of the fundamental physics of bipolar resistive switching in graphene oxide films, for the application to future flexible electronics.

Assessment of cytocompatibility of surface-modified CdSe/ZnSe quantum dots for BALB/3T3 fibroblast cells

With the widespread use of quantum dots (QDs), the likelihood of exposure to QDs has been assumed to have increased substantially. Recently, QDs have been employed in numerous biological and medical applications. However, there is a lack of toxicological data pertaining to QDs. In this study, we aimed to investigate the cytocompatibility of surface-modified CdSe/ZnSe QDs for BALB/3T3 fibroblast cells. The ligands used for surface modification are mercaptopropionic acid (MPA) and Gum arabic (GA)/tri-n-octylphosphine oxide (TOPO). Cells were exposed to different concentrations of QDs followed by illustrative cytotoxicity analyses. Furthermore, we used a confocal microscope to assess intracellular uptake of QDs. Confocal images showed that MPA-coated QDs were distributed inside the cytoplasmic region of cells. In contrast, GA/TOPO-coated QDs were not found inside cells. MPA-coated QDs were highly cytocompatible, whereas GA/TOPO-coated QDs were toxic to the cells. Cells treated with GA/TOPO-coated QDs showed altered morphology, decreased viability, significant concentrations of intracellular free cadmium, detectable reactive oxygen species (ROS) formation, depolymerized cytoskeleton, and irregular-shaped nuclei. This study suggests that surface modification by ligands plays a significant role in the prevention of cytotoxicity of QDs.

Acute toxicity of two CdSe/ZnSe quantum dots with different surface coating in Daphnia magna under various light conditions.

With an increasing use of quantum dots (QDs) in many applications, their potential hazard is of growing concern. However, little is known about their ecotoxicity, especially in vivo. In the present study, we employed freshwater macroinvertebrate, Daphnia magna, to evaluate toxicity characteristics of cadmium selenide/zinc selenide (CdSe/ZnSe) in relation to surface coatings, e.g., mercaptopropionic acid QD (MPAQD), and gum arabic/tri‐n‐octylphosphine oxide QD (GA/TOPOQD), and light conditions, i.e., dark, fluorescent light, environmental level of ultraviolet (UV) light, and sunlight. The results of the present study showed that D. magna was more susceptible to GA/TOPOQD exposure compared to MPAQD. The surface coating of QD appeared to determine the stability of QDs and hence the toxicity, potentially by size change of or the release of toxic components from QDs. However, GA/TOPOQD was still less toxic than the equivalent level of CdCl2. The toxicity of all the tested compounds increased by changing the light condition from dark to white fluorescence to UV‐B light, and to natural sunlight. The effect of light condition on QDs toxicity could also be explained by photostability of the QDs, which would affect size of the particle, release of toxic component ions, and generation of reactive oxygen species. Considering increasing use of QDs in various applications, their environmental fates and corresponding toxic potentials deserve further investigation.

A new perspective on in vitro assessment method for evaluating quantum dot toxicity by using microfluidics technology.

In this study, we demonstrate a new perspective on in vitro assessment method for evaluating quantum dot (QD) toxicity by using microfluidics technology. A new biomimetic approach, based on the flow exposure condition, was applied in order to characterize the cytotoxic potential of QD. In addition, the outcomes obtained from the flow exposure condition were compared to those of the static exposure condition. An in vitro cell array system was established that used an integrated multicompartmented microfluidic device to develop a sensitive flow exposure condition. QDs modified with cetyltrimethyl ammonium bromide∕trioctylphosphine oxide were used for the cytotoxicity assessment. The results suggested noticeable differences in the number of detached and deformed cells and the viability percentages between two different exposure conditions. The intracellular production of reactive oxygen species and release of cadmium were found to be the possible causes of QD-induced cytotoxicity, irrespective of the types of exposure condition. In contrast to the static exposure, the flow exposure apparently avoided the gravitational settling of particles and probably assisted in the homogeneous distribution of nanoparticles in the culture medium during exposure time. Moreover, the flow exposure condition resembled in vivo physiological conditions very closely, and thus, the flow exposure condition can offer potential advantages for nanotoxicity research.

L-cysteine-induced photoluminescence enhancement of CdSe/ZnSe quantum dots in aqueous solution.

L-cysteine molecules dramatically enhance the photoluminescence of colloidal CdSe/ZnSe quantum dots (i.e., (CTAB/TOPO)QD). Based on our spectroscopic studies of temporal variations in QD quantum yields as well as the in situ infrared spectral features of QDs, we propose that adsorption and rearrangement of L-cysteine molecules at the QD-water interface induces the observed unusual enhancement of the photoluminescence quantum yield. Upon addition of L-cysteine to the (CTAB/TOPO)QD solution, the adsorption of L-cysteine to the (CTAB/TOPO)QD colloidal particles is driven by the formation of a kinetically favorable intermediate species, which is formed by the coordination of thiol groups to the QD surface Cd atoms. The above species then reacts further to form a thermodynamically stable QD species, which probably involves coordination of both the amine and thiol groups of L-cysteine on the QD surface. Additional comparison studies using (MPA)QD and other small ligands (i.e., L-alanine, L-serine, and MPA) confirmed our proposed mechanism of L-cysteine adsorption at the (CTAB/TOPO)QD-water interfaces. In addition to these adsorption structures, we also propose that the dramatic enhancement of QY observed in this study is probably induced by the rearrangement and structural organization of L-cysteine and CTAB molecules at the QD-water interface, which improves the homogeneity and self-organization of the interfacial molecules.

Multicompartmented microfluidic device for characterization of dose-dependent cadmium cytotoxicity in BALB/3T3 fibroblast cells

This paper describes the development of a miniaturized multicompartmented microfluidic device for high-throughput cell cytotoxicity assays and its applicability to the investigation of cadmium-induced cytotoxicity. A steady gradient of cadmium was generated inside the compartments to study the effects of cadmium ion on BALB/3T3 fibroblast cells in a dose-dependent fashion. The device allowed the performance of multiplexed assays to probe the dosage effect of cadmium, morphological alterations of live cells, regulation of proliferation and viability of cells, determination of reactive oxygen species, mechanisms of cell death, i.e. apoptosis and/or necrosis, and immunocytochemical staining of cells in parallel and/or serially, or on a single population simultaneously. The outcomes of all the microfluidic assays were compared to conventional plates-based cytotoxicity assays. The results indicated that the cells cultured in this device were morphologically healthy with greater than 90% viability. They further suggested that the basic mode of cell death behind cadmium-induced cytotoxicity was apoptosis, which was regulated by intracellular oxidative stress via cytoskeleton disorganization and nuclear condensation. Such microenvironments resemble the in vivo physiological conditions very closely and thus offer a unique platform for more accurate observations of cytotoxicity assays and more precise estimation of the IC50 value in comparison to conventional analytical assays.

Multiparametric assessment of Cd2+ cytotoxicity using MTT-based microfluidic image cytometry

A modified MTT protocol‐based microfluidic image cytometry (μFIC) was performed to assess Cd2+ induced cytotoxicity. The expanded capabilities of μFIC, such as in situ measurement, high‐throughput, and multiparametric analysis of adherent cells under precisely controlled chemical environments of microfluidic channels, were demonstrated in this study. Multiparametric analysis of μFIC data has enabled us to categorize the progress of cell death into at least four different subgroups based on their morphology and metabolic activity. These advantages of the MTT‐based μFIC as a simpler, cheaper, and faster in vitro cell‐based assay tool have many implications in biomedical, pharmaceutical, toxicological, and biological application areas, and we propose this technique as a future high throughput–high content screening (HT‐HCS) platform for cytotoxicity assays and drug screening.

High throughput cell cycle analysis using microfluidic image cytometry (μFIC)

Microfluidic image cytometry (μFIC) is a novel approach for the cytotoxicity assessment of the cells cultured and treated within microfluidic channels under a precisely controlled chemical environment. Here, following our previous morphology‐, and MTT absorbance‐based μFIC, we are presenting our recent effort to develop, evaluate, and apply a high throughput cell cycle analysis method using fluorescence‐based μFIC. A microfluidic device with a concentrantion gradient generator (CGG) and eight straight cell culture channels was fabricated, optimized, and applied for the assessment of paclitaxel‐induced cell cycle changes of HeLa cells. Throughout this study, we have shown that the cell cycle analysis using fluorescence‐based μFIC was able to provide comparable experimental data with those of flow cytometry. Moreover, cell cycle analysis using μFIC can also provide further advantages over flow cytometry, such as higher throughput, lower assay cost, less generation of toxic waste, and etc., which should have significant implications in pharmaceutical and biological applications as a future high throughput cell cycle analysis platform.

Scanning transmission X-ray microscopy probe for in situ mechanism study of graphene-oxide-based resistive random access memory.

Here, an inxa0situ probe for scanning transmission X-ray microscopy (STXM) has been developed and applied to the study of the bipolar resistive switching (BRS) mechanism in an Al/graphene oxide (GO)/Al resistive random access memory (RRAM) device. To perform inxa0situ STXM studies at the C K- and O K-edges, both the RRAM junctions and the I0 junction were fabricated on a single Si3N4 membrane to obtain local XANES spectra at these absorption edges with more delicate I0 normalization. Using this probe combined with the synchrotron-based STXM technique, it was possible to observe unique chemical changes involved in the BRS process of the Al/GO/Al RRAM device. Reversible oxidation and reduction of GO induced by the externally applied bias voltages were observed at the O K-edge XANES feature located at 538.2u2005eV, which strongly supported the oxygen ion drift model that was recently proposed from exxa0situ transmission electron microscope studies.

Effects of surface-modifying ligands on the colloidal stability of ZnO nanoparticle dispersions in in vitro cytotoxicity test media

The extrinsic physicochemical properties of nanoparticles (NPs), such as hydrodynamic size, surface charge, surface functional group, and colloidal stabilities, in toxicity testing media are known to have a significant influence on in vitro toxicity assessments. Therefore, interpretation of nanotoxicity test results should be based on reliable characterization of the NPs’ extrinsic properties in actual toxicity testing media. Here, we present a set of physicochemical characterization results for commercially available ZnO NPs, including core diameter, hydrodynamic diameter, surface charges, and colloidal stabilities, in two in vitro toxicity testing media (Roswell Park Memorial Institute [RPMI] and Dulbecco’s Modified Eagle’s Medium [DMEM]), as well as simple cell viability assay results for selected ZnO NPs. Four commercially available and manufactured ZnO NPs, with different core sizes, were used in this study, and their surface charge was modified with five different surface coating materials (sodium citrate, tris(2-aminoethyl)amine, poly(acrylic acid), poly(allylamine hydrochloride), and poly-L-lysine hydrochloride). The results showed that ZnO NPs were better dispersed in cell culture media via surface modification with positively or negatively charged molecules. Moreover, in the presence of fetal bovine serum (FBS) in RPMI and DMEM media, ZnO NPs were found even better dispersed for a longer period (at least 48 hours). For the HeLa cells exposed to ZnO NPs in DMEM media without FBS, surface charge-dependent cytotoxicity trends were observed, while these trends were not observed for those cells cultured in FBS-containing media. This confirmed the important roles of surface-modifying compounds and of surface charge on the resultant cytotoxicities of NPs.