Yeongjin Lim

Monolithic carbon structures including suspended single nanowires and nanomeshes as a sensor platform

With the development of nanomaterial-based nanodevices, it became inevitable to develop cost-effective and simple nanofabrication technologies enabling the formation of nanomaterial assembly in a controllable manner. Herein, we present suspended monolithic carbon single nanowires and nanomeshes bridging two bulk carbon posts, fabricated in a designed manner using two successive UV exposure steps and a single pyrolysis step. The pyrolysis step is accompanied with a significant volume reduction, resulting in the shrinkage of micro-sized photoresist structures into nanoscale carbon structures. Even with the significant elongation of the suspended carbon nanowire induced by the volume reduction of the bulk carbon posts, the resultant tensional stress along the nanowire is not significant but grows along the wire thickness; this tensional stress gradient and the bent supports of the bridge-like carbon nanowire enhance structural robustness and alleviate the stiction problem that suspended nanostructures frequently experience. The feasibility of the suspended carbon nanostructures as a sensor platform was demonstrated by testing its electrochemical behavior, conductivity-temperature relationship, and hydrogen gas sensing capability.

Glucose sensor based on redox-cycling between selectively modified and unmodified combs of carbon interdigitated array nanoelectrodes.

We present a novel electrochemical glucose sensor employing an interdigitated array (IDA) of 1:1 aspect ratio carbon nanoelectrodes for the electrochemical-enzymatic redox cycling of redox species (ferricyanide/ferrocyanide) between glucose oxidase (GOx) and the two comb-shaped nanoelectrodes of the IDA. The carbon nanoelectrodes were fabricated using a simple, cost-effective, reproducible microfabrication technology known as the carbon-microelectromechanical-systems (C-MEMS) process. One comb (comb 1) of the IDA was selectively modified with GOx via the electrochemical reduction of an aryl diazonium salt, while the other comb (comb 2) remained unmodified; this facilitates electrochemically more active surface of comb 2, resulting in sensitive glucose detection. Ferricyanide is reduced to ferrocyanide by the GOx in the presence of glucose, and ferrocyanide diffuses to both combs of the IDA where it is oxidized. The limited electrochemical current collection at the surface-modified comb 1 is counterbalanced by the efficient redox cycling between the enzyme sites at comb 1 and the bare carbon surface of comb 2. Reducing the electrode-to-electrode gap between the two combs (gap = 1.9 μm) increases the diffusion flux of redox species at comb 2 hence, enhanced the sensitivity and limit of detection of the glucose sensor by ∼2.3 and ∼295 times, respectively at comb 2 compared to comb 1. The developed IDA-based glucose sensor demonstrated good amperometric response to glucose, affording two linear ranges from 0.001 to 1 mM and from 1 to 10 mM, with limits of detection of 0.4 and 61 μM and sensitivities of 823.2 and 70.0 μA mM(-1) cm(-2), respectively.

Scalable suspended carbon nanowire meshes as ultrasensitive electrochemical sensing platforms

This paper presents novel electrochemical sensing platforms consisting of a set of a suspended carbon nanowire mesh electrode of deliberately controlled shape (hexagonal or diamond) and a planar carbon electrode located just below the suspended electrode that achieved the electrochemical current signal amplification of 120 times in a micro-channel by redox cycling of redox species. The carbon sensing platform was fabricated using only series of photolithography and pyrolysis processes which are known as Carbon-MEMS processes enabling production of carbon nano-structures in designed manner without using expensive nanolithography tools. The functionality of the stacked carbon nanoelectrodes were studied using cyclic voltammetry.

A Three-Step Resolution-Reconfigurable Hazardous Multi-Gas Sensor Interface for Wireless Air-Quality Monitoring Applications

This paper presents a resolution-reconfigurable wide-range resistive sensor readout interface for wireless multi-gas monitoring applications that displays results on a smartphone. Three types of sensing resolutions were selected to minimize processing power consumption, and a dual-mode front-end structure was proposed to support the detection of a variety of hazardous gases with wide range of characteristic resistance. The readout integrated circuit (ROIC) was fabricated in a 0.18 μm CMOS process to provide three reconfigurable data conversions that correspond to a low-power resistance-to-digital converter (RDC), a 12-bit successive approximation register (SAR) analog-to-digital converter (ADC), and a 16-bit delta-sigma modulator. For functional feasibility, a wireless sensor system prototype that included in-house microelectromechanical (MEMS) sensing devices and commercial device products was manufactured and experimentally verified to detect a variety of hazardous gases.

Electrical conductivity enhancement of 1D glassy carbon nanostructure using rapid thermal annealing

Rapid thermal annealing (RTA) process enabling significant enhancement of electrical conductivity (up to 2.3 times greater than currently reported conductivity values) of 1D glassy carbon nanostructures fabricated using carbon-MEMS was developed. After the RTA process, the carbon/oxygen and G-/D-band ratios that are strongly correlated to the electrical conductivity were changed depending on the pyrolysis temperature. The architecture of a suspended carbon nanowire also plays strong role on RTA-based conductivity enhancement by up to 14 % compared to carbon nanowire built on the substrate. In addition, electrochemical reactivity can be enhanced via RTA enabling better redox current collection.

Circumferentially grown ZnO nanowire forest on a suspended carbon nanowire for a highly sensitive gas sensor

We present a suspended ZnO nanowire forest as a highly sensitive gas sensor. ZnO nanowires were grown selectively on a suspended single glassy carbon nanowire using hydrothermal method so that the detrimental effects from the substrate inclusive of contamination, stagnant layer and limited mass transfer could be alleviated. The novel geometry of the radially grown ZnO nanowires resembling burs of a chestnut is expected to enhance the gas sensing capability because of enhanced mass transfer.

Fabrication of a monolithic carbon mold for producing a mixed-scale PDMS channel network using a single molding process

We introduce a novel batch fabrication technique of a monolithic carbon mold for producing a mixed-scale polydimethylsiloxane (PDMS) channel network consisting of nanochannels and microchannels with micro-pillars. Nanofluidics has attracted much attention because of their distinguishing properties. However, the research on the nanofluidics is limited by complex nanofabrication techniques. In this paper, a mixed-scale monolithic carbon mold was simply fabricated using a batch carbon-MEMS process consisting of two successive UV-lithography processes and a single pyrolysis. Then, PDMS channel networks was completed by soft molding process. By modulating pyrolysis conditions, the surface energy of the pyrolyzed carbon mold could be optimized for efficient PDMS channel demolding.

Alignment-less microchannel integration onto a stacked carbon electrode set for highly sensitive electrochemical sensor applications

Herein, we present simple integration of the microchannel and stacked carbon electrode set enabling high electrochemical current signal amplification due to redox cycling effect. The stacked electrode set consists of a substrate-bound carbon electrode and a suspended carbon nanomesh electrode (width = ~300 nm, thickness = ~600 nm) with a narrow electrode gap (~4 μm) achieved using a polymerization-stop layer. A planar PDMS plate ceiling and the side walls of the carbon posts that support the carbon nanomesh enclose the electrode set and thus the sensor-integrated microchannel is completed by simple PDMS bonding process without any complex alignment process. The stacked electrode set embedded in the microchannel showed ~1,300 times amplified current signal in chronoamperometry.