Hyun-U Ko

Disposable chemical sensors and biosensors made on cellulose paper

Most sensors are based on ceramic or semiconducting substrates, which have no flexibility or biocompatibility. Polymer-based sensors have been the subject of much attention due to their ability to collect molecules on their sensing surface with flexibility. Beyond polymer-based sensors, the recent discovery of cellulose as a smart material paved the way to the use of cellulose paper as a potential candidate for mechanical as well as electronic applications such as actuators and sensors. Several different paper-based sensors have been investigated and suggested. In this paper, we review the potential of cellulose materials for paper-based application devices, and suggest their feasibility for chemical and biosensor applications.

Transparent and Flexible Cellulose Nanocrystal/Reduced Graphene Oxide Film for Proximity Sensing

The rapid development of touch screens as well as photoelectric sensors has stimulated the fabrication of reliable, convenient, and human-friendly devices. Other than sensors that detect physical touch or are based on pressure sensing, proximity sensors offer controlled sensibility without physical contact. In this work we present a transparent and eco-friendly sensor made through layer-by-layer spraying of modified graphene oxide filled cellulose nanocrystals on lithographic patterns of interdigitated electrodes on polymer substrates, which help to realize the precise location of approaching objects. Stable and reproducible signals generated by keeping the finger in close proximity to the sensor can be controlled by humidity, temperature, and the distance and number of sprayed layers. The chemical modification and reduction of the graphene oxide/cellulose crystal composite and its excellent nanostructure enable the development of proximity sensors with faster response and higher sensitivity, the integration of which resolves nearly all of the technological issues imposed on optoelectronic sensing devices.

Fabrication of Cellulose ZnO Hybrid Nanocomposite and Its Strain Sensing Behavior

This paper reports a hybrid nanocomposite of well-aligned zinc oxide (ZnO) nanorods on cellulose and its strain sensing behavior. ZnO nanorods are chemically grown on a cellulose film by using a hydrothermal process, termed as cellulose ZnO hybrid nanocomposite (CEZOHN). CEZOHN is made by seeding and growing of ZnO on the cellulose and its structural properties are investigated. The well-aligned ZnO nanorods in conjunction with the cellulose film shows enhancement of its electromechanical property. Strain sensing behaviors of the nanocomposite are tested in bending and longitudinal stretching modes and the CEZOHN strain sensors exhibit linear responses.

Preparation and characterization of hydrogels from polyvinyl alcohol and cellulose and their electroactive behavior

ABSTRACT This paper reports polyvinyl alcohol/cellulose-based electroactive hydrogels for actuator applications. The polyvinyl alcohol/cellulose electroactive hydrogels were obtained by physical crosslinking of polyvinyl alcohol and cellulose. The formation of the polyvinyl alcohol/cellulose hydrogel structure, its thermal stability, crystallinity, and mechanical properties were studied by using the Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric analysis, differential scanning calorimetry, X-ray diffraction, and mechanical test. The studies reveal that the cellulose is uniformly reacted with hydroxyl groups of polyvinyl alcohol by intermolecular bond formation. The cellulose content along with actuation voltage and frequency of electroactive hydrogels influence their displacement behavior.

Preparation and characterization of Cellulose-ZnO nanolayer film by blending method

Cellulose-ZnO nanolayer film (CZNF) was prepared by simply mixing ZnO nanopowder with cellulose solution and sodium dodecyl sulphate dispersing agent. CZNF was cured in a mixture of isopropyl alcohol and deionized water and a freestanding film was obtained. CZNF was prepared with different weight percentage of ZnO and all materials were characterized by optical, thermal, mechanical and electrical methods. The morphology shows that ZnO nanoparticles are evenly dispersed between cellulose nanolayers of CZNF. Thermal and mechanical test results exhibit that CZNF possesses two distinctive characteristics of regenerated cellulose and ZnO. Its complex electrical impedance exhibits very similar to ionic membranes for fuel cells.

Electroactive and Optically Adaptive Bionanocomposite for Reconfigurable Microlens

This paper introduces an electroactive bionanocomposite based on poly(diethylene glycol adipate) (PDEGA) and cellulose nanocrystals (CNCs). The bionanocomposites were made using CNCs extracted from cotton and by optimizing its concentration in terms of the optical transmittance and viscosity. The characteristic properties of the materials were analyzed using contact angle measurements and Fourier transformation infrared spectra. Using the PDEGA/CNC bionanocomposite at a very low concentration of CNCs, a configurable lens having a robust, self-contained tunable optical structure was developed. The shape and curvature of the soft PDEGA/CNC device were controlled by applying voltage, and the focal length was measured. The simple structure, high optical transparency, biodegradability, thermal stability, high durability, and low power consumption make the new material particularly useful in fabricating a reconfigurable lens for future electronic and optical devices.

Investigation of Cellulose and Tin Oxide Hybrid Composite as a Disposable pH Sensor

Abstract This paper reports a low cost and disposable pH sensor made with a cellulose and tin oxide hybrid composite. Cellulose- tin oxide hybrid composite was prepared by wet chemical method, which involves immersion of a wet regenerated cellulose film in to a mixture of deionized water and SnF2 (5 mM concentration), maintained at 90 ºC for a given time; 6, 12, 18 and 24 h with stirring. The proposed pH sensor showed a consistent increase of its conductance, and the sensitivity defined as the change of resistance with respect to the pH level portrays three regions with different slope rates depending on the pH value: 0.4 MΩ/pH, 31.2 kΩ/pH and 0.2 MΩ/pH in pH range of 1–4, 4–8 and 8–12, respectively. The high sensitivity at the pH range of 4–8 is very useful for industrial applications. Also, Since the proposed pH sensor is made with cellulose film, it is disposable, cheap and environment-friendly.

Flexible cellulose and ZnO hybrid nanocomposite and its UV sensing characteristics

Abstract This paper reports the synthesis and UV sensing characteristics of a cellulose and ZnO hybrid nanocomposite (CEZOHN) prepared by exploiting the synergetic effects of ZnO functionality and the renewability of cellulose. Vertically aligned ZnO nanorods were grown well on a flexible cellulose film by direct ZnO seeding and hydrothermal growing processes. The ZnO nanorods have the wurtzite structure and an aspect ratio of 9 ~ 11. Photoresponse of the prepared CEZOHN was evaluated by measuring photocurrent under UV illumination. CEZOHN shows bi-directional, linear and fast photoresponse as a function of UV intensity. Electrode materials, light sources, repeatability, durability and flexibility of the prepared CEZOHN were tested and the photocurrent generation mechanism is discussed. The silver nanowire coating used for electrodes on CEZOHN is compatible with a transparent UV sensor. The prepared CEZOHN is flexible, transparent and biocompatible, and hence can be used for flexible and wearable UV sensors.

Preparation and characterisation of cellulose ZnO hybrid film by blending method and its glucose biosensor application

Abstract This paper reports an inexpensive, flexible and disposable cellulose ZnO hybrid film (CZHF) and its feasibility for a conductometric glucose biosensor. CZHF is fabricated by simply blending ZnO nanoparticles with cellulose solution prepared by dissolving cotton pulp with lithium chloride/N, N-dimethylacetamide solvent. After curing in isopropyl alcohol and water mixture, the CZHF is obtained. The enzyme glucose oxidase is immobilised into the CZHF by physical adsorption method. The enzyme activity of the glucose biosensor increases as the ZnO weight ratio increases linearly. The CZHF can detect glucose in the range of 1–12 mM.

Transparent and semi-interpenetrating network P(vinyl alcohol)- P(Acrylic acid) hydrogels: pH responsive and electroactive application

ABSTRACT In this paper, poly(vinyl alcohol)-poly(acrylic acid) based transparent semi-interpenetrating network (semi-IPN) hydrogels were synthesized by using a solvent mixture of dimethyl sulfoxide and deionized water via free radical polymerization and subsequent freeze-thaw technique. The formation of the semi-IPN hydrogels was conformed from FT-IR spectra. The acrylic acid concentration effect on the hydrogels was investigated in terms of transparency, crystalline structure and thermal stability by using UV-visible spectroscopy, X-ray diffraction and thermogravimetric analysis. Swelling behaviours of the semi-IPNPAP hydrogels were studied in deionized water and different pH solutions. The compression and electroactive behaviour was tested in fully hydrated stage by using compression test and by applying electrical voltage. The hydrogels showed displacements under the applying voltage and detailed experiment is illustrated.