Jun Seop Lee

Multidimensional Conducting Polymer Nanotubes for Ultrasensitive Chemical Nerve Agent Sensing

Tailoring the morphology of materials in the nanometer regime is vital to realizing enhanced device performance. Here, we demonstrate flexible nerve agent sensors, based on hydroxylated poly(3,4-ethylenedioxythiophene) (PEDOT) nanotubes (HPNTs) with surface substructures such as nanonodules (NNs) and nanorods (NRs). The surface substructures can be grown on a nanofiber surface by controlling critical synthetic conditions during vapor deposition polymerization (VDP) on the polymer nanotemplate, leading to the formation of multidimensional conducting polymer nanostructures. Hydroxyl groups are found to interact with the nerve agents. Representatively, the sensing response of dimethyl methylphosphonate (DMMP) as a simulant for sarin is highly sensitive and reversible from the aligned nanotubes. The minimum detection limit is as low as 10 ppt. Additionally, the sensor had excellent mechanical bendability and durability.

Flexible FET-Type VEGF Aptasensor Based on Nitrogen-Doped Graphene Converted from Conducting Polymer

Graphene-based field-effect transistors (FETs) have been developed rapidly and are currently considered as an alternative for postsilicon electronics. In this study, polypyrrole-converted nitrogen-doped few-layer graphene (PPy-NDFLG) was grown on Cu substrate by chemical vapor deposition combined with vapor deposition polymerization and then transferred onto a flexible substrate. Furthermore, antivascular endothelial growth factor (VEGF) RNA aptamer conjugated PPy-NDFLG was integrated into a liquid-ion gated FET geometry to fabricate a high-performance VEGF aptamer-based sensor. Field-induced high sensitivity was observed for the analyte-binding events, eventually leading to the recognition of the target molecules at an unprecedentedly low concentration (100 fM). Additionally, the aptasensor had excellent reusability, mechanical bendability, and durability in the flexible process. The developed methodology describes, for the first time, the fabrication of N-doped graphene using conducting polymers including heteroatoms in their structures as the carbonization precursor and demonstrates its use in a high-performance, flexible FET-type aptasensor to detect vascular endothelial growth factor as a cancer biomarker.

Fabrication of Ultrafine Metal-Oxide-Decorated Carbon Nanofibers for DMMP Sensor Application

Ultrafine metal-oxide-decorated hybrid carbon nanofibers (CNFs) were fabricated by a single-nozzle co-electrospinning process using a phase-separated mixed polymer composite solution and heat treatment. To decorate metal oxides on the CNF surface, core (PAN) and shell (PVP) structured nanofibers (NFs) were fabricated as starting materials. The core-shell NF structure was prepared by single-nozzle co-electrospinning because of the incompatibility of the two polymers. Ultrafine hybrid CNFs were then formed by decomposing the PVP phase, converting the metal precursors to metal oxide nanonodules, and transforming the PAN to CNFs of ca. 40 nm diameter during heat treatment. The decoration morphology of the metal oxide nanonodules could be controlled by precursor concentration in the PVP solution. These ultrafine hybrid CNFs were applied to a dimethyl methylphosphonate (DMMP) chemical sensor at room temperature with excellent sensitivity. The minimum detectable level (MDL) of hybrid CNFs was as low as 0.1 ppb, which is 10-100 times higher than for a chemical sensor based on carbon nanotubes. This is because the metal oxide nanonodules of hybrid CNFs increase the surface area and affinity to DMMP vapor. Our new synthetic methodology promises to be an effective approach to fabricating hybrid CNF/inorganic nanostructures for future sensing technologies.

Fabrication of Graphene Sheets Intercalated with Manganese Oxide/Carbon Nanofibers: Toward High‐Capacity Energy Storage

Herein, 3D nanohybrid architectures consisting of MnO(x) nanocrystals, carbon nanofibers (CNFs), and graphene sheets are fabricated. MnO(x) -decorated CNFs (MCNFs) with diameters of about 50 nm are readily obtained via single-nozzle co-electrospinning, followed by heat treatment. The MCNFs are then intercalated between graphene sheets, yielding the ternary nanohybrid MCNF/reduced graphene oxide (RGO). This straightforward synthesis process readily affords product on a scale of tens of grams. The ultrathin CNFs, which might be a promising alternative to carbon nanotubes (CNTs), overcome the low electrical conductivity of the excellent pseudocapacitive component, MnO(x) . Furthermore, the graphene sheets separated by the MCNFs boost the electrochemical performance of the nanohybrid electrodes. These nanohybrid electrodes exhibit enhanced specific capacitances compared with a sheet electrode fabricated of MCNF-only or RGO-only. Evidently, the RGO sheet acts as a conductive channel inside the nanohybrid, while the intercalated MCNFs increase the efficiency of the ion and charge transfer in the nanohybrid. The proposed nanohybrid architectures are expected to lay the foundation for the design and fabrication of high-performance electrodes.

Polypyrrole-coated manganese dioxide with multiscale architectures for ultrahigh capacity energy storage

For practical nanoscale development, multiscale architectures composed of uniformly patterned nanomaterials are attractive for various applications in electrochemical devices owing to the benefits derived from their unique structures: relatively large surface area, low tortuousness and interconnected pores. However, although various morphological modifications exist, there are no reports about one-dimensional (1-D) nanomaterial based multiscale architectures because of difficulties in the fabrication of uniform 1-D nanostructures. Here we report the demonstration of one-dimensional metal oxide nanostructure-based multiscale architectures that are micronodules composed of ca. 30 nm-diameter MnO2 nanofibers on carbon cloth to produce energy storage devices. In addition, partially carbonized polypyrrole (CPPy) was coated on the MnO2 surface through vapour deposition polymerization (VDP) and subsequent heat annealing to give the multiscale micronodules a low resistance and high rate performance. Then, the PPy-coated MnO2-based multiscale micronodules were assembled within a PVA–KOH polymer electrolyte as the positive-electrodes of solid-state asymmetric supercapacitors (ASCs). This multiscale architecture based device displays ultrahigh performance (59.5 F cm−3 of capacitance and 27.0 mW h cm−3 of energy density). Furthermore, the as prepared ASCs show high cycle stability and an enhanced charge transfer rate owing to the coating layers.

One-pot synthesis of silver nanoparticles decorated poly(3,4-ethylenedioxythiophene) nanotubes for chemical sensor application

Ag nanoparticle (NP) decorated PEDOT nanotubes (NTs) were constructed using metal ion reduction-mediated vapor deposition polymerization (VDP). Ag NPs/PEDOT NTs were obtained by one-pot synthesis because PEDOT can reduce Ag cations to Ag NPs. Ag NPs/PEDOT NTs efficiently sensed NH3 due to an increased surface area and improved conductivity originating from oxidation of PEDOT by Ag NPs. This study evaluated increasing concentrations of AgNO3 in Ag NPs/PEDOT NTs to determine the optimal amount for NH3 sensing. We found that as the Ag NP concentration increased and passed a specific threshold amount, they aggregated and reduced the gas-sensing performance. Additionally, the gas response of Ag NPs/PEDOT NTs to NH3 was evaluated in depth.

Fabrication of amorphous carbon-coated NiO nanofibers for electrochemical capacitor applications

Amorphous carbon-coated nickel oxide nanofibers (NiC NFs) were fabricated using vapor deposition polymerization (VDP) on electrospun nickel oxide nanofibers (NiO NFs), followed by carbonization. To decorate the surface with amorphous carbon, the NiO NF starting materials were prepared by electrospinning a PVP solution containing a nickel oxide precursor (NiAc2·4H2O) and calcining the electrospun NFs. Then, polypyrrole (PPy)-coated NiO nanofibers (NiP NFs) were fabricated as intermediate materials using the pyrrole monomer in the VDP method. Finally, carbonization of the NiP NFs converted the PPy into amorphous carbon and thereby formed the NiC NFs. According to X-ray diffraction (XRD) and Barrett–Joyner–Halenda (BJH) analyses, the NiO structure was maintained during the PPy coating and heat treatment processes. Furthermore, a new pore structure was formed with each fabrication step. The NiC NFs were used as electrochemical capacitors (ECs) with 1 M KOH as the electrolyte. The electrochemical results show that NiC NFs with a thin coating (NiC_L) had a higher specific capacitance (288 F g−1 at 0.3 A g−1) and longer cycle stabilization (89% capacitance maintained after 3000 cycles) than pristine NiO NFs (221 F g−1 at 0.3 A g−1; 56% capacitance maintained after 3000 cycles). Herein, the synthetic methodology is an effective route to obtain hybrid core (inorganic)–shell (organic) nanostructures for electrochemical applications.

Ultrasensitive and selective recognition of peptide hormone using close-packed arrays of hPTHR-conjugated polymer nanoparticles.

Recognition of diverse hormones in the human body is a highly significant challenge because numerous diseases can be affected by hormonal imbalances. However, the methodologies reported to date for detecting hormones have exhibited limited performance. Therefore, development of innovative methods is still a major concern in hormone-sensing applications. In this study, we report an immobilization-based approach to facilitate formation of close-packed arrays of carboxylated polypyrrole nanoparticles (CPPyNPs) and their integration with human parathyroid hormone receptor (hPTHR), which is a B-class family of G-protein-coupled receptors (GPCRs). Our devices enabled use of an electrically controllable liquid-ion-gated field-effect transistor by using the surrounding phosphate-buffered saline solution (pH 7.4) as electrolyte solution. Field-induced signals from the peptide hormone sensors were observed and provided highly sensitive and selective recognition of target molecules at unprecedentedly low concentrations (ca. 48 fM). This hormone sensor also showed long-term stability and excellent selectivity in fetal bovine serum. Importantly, the hormone receptor attached on the surface of CPPyNPs enabled GPCR functional studies; synergistic effects corresponding to increased hPTH peptide length were monitored. These results demonstrate that close-packed CPPyNP arrays are a promising approach for high-performance biosensing devices.

Wireless Hydrogen Smart Sensor Based on Pt/Graphene-Immobilized Radio-Frequency Identification Tag

Hydrogen, a clean-burning fuel, is of key importance to various industrial applications, including fuel cells and the aerospace and automotive industries. However, hydrogen gas is odorless, colorless, and highly flammable; thus, appropriate safety protocol implementation and monitoring are essential. Highly sensitive hydrogen-gas leak detection and surveillance systems are needed; additionally, the ability to monitor large areas (e.g., cities) via wireless networks is becoming increasingly important. In this report, we introduce a radio frequency identification (RFID)-based wireless smart-sensor system, composed of a Pt-decorated reduced graphene oxide (Pt_rGO)-immobilized RFID sensor tag and an RFID-reader antenna-connected network analyzer to detect hydrogen gas. The Pt_rGOs, produced using a simple chemical reduction process, were immobilized on an antenna pattern in the sensor tag through spin coating. The resulting Pt_rGO-based RFID sensor tag exhibited a high sensitivity to hydrogen gas at unprecedentedly low concentrations (1 ppm), with wireless communication between the sensor tag and RFID-reader antenna. The wireless sensor tag demonstrated flexibility and a long lifetime due to the strong immobilization of Pt_rGOs on the substrate and battery-independent operation during hydrogen sensing, respectively.

WO3 nanonodule-decorated hybrid carbon nanofibers for NO2 gas sensor application

WO3 nanonodule-decorated carbon nanofibers (CNFs) of various diameters were fabricated by single-nozzle co-electrospinning using two phase-separated polymer solutions. Using more polyvinylpyrrolidone (PVP) solution decreased the CNF diameter from 130 to 40 nm and increased the Brunauer–Emmett–Teller (BET) surface area from 147 to 276 m2 g−1. A spin-coating method was used to deposit the hybrid CNFs on the sensor substrate to minimize the contact resistance between them. In addition, ultraviolet (UV) irradiation lowered the desorption energy level of the NO2 gas between the transducer materials during the recovery time. As a result, the recovery time decreased to ca. 7 min using UV light with an intensity of 75 mW cm−2. The sensitivity of the hybrid CNF gas sensors increased with decreasing diameter of the CNFs; the minimum detectable level (MDL) was 1 ppm at room temperature for 40 nm hybrid CNFs dispersed uniformly on the electrode. Furthermore, increasing the amount of decorated WO3 nanonodules on the CNF surface enhanced the sensitivity to NO2 gas. The NO2 sensor made with the hybrid CNFs had sensing performance comparable to those made with conventional metal oxide-based nanomaterials and pristine carbon nanotubes (CNTs).