Letter pubs.acs.org/NanoLett
Tunable Volatile Organic Compounds Sensor by Using Thiolated Ligand Conjugation on MoS2 Jong-Seon Kim,† Hae-Wook Yoo,† Hyung Ouk Choi, and Hee-Tae Jung* Department of Chemical and Biomolecular Engineering (BK-21 plus), Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, , Daejeon 305-701, Korea S Supporting Information *
ABSTRACT: One of the most important issues in the development of gas sensors for breath analysis is the fabrication of gas sensor arrays that possess different responses for recognizing patterns for volatile organic compounds (VOCs). Here, we develop a high-performance chemiresistor with a tunable sensor response and high sensitivity for representative VOC groups by using molybdenum disulfide (MoS2) and by conjugating a thiolated ligand (mercaptoundecanoic acid (MUA)) to MoS2 surface. Primitive and MUA-conjugated MoS2 sensing channels exhibit distinctly different sensor responses toward VOCs. In particular, the primitive MoS2 sensor presents positive responses for oxygen-functionalized VOCs, while the MUA-conjugated MoS2 sensor presents negative responses for the same analytes. Such characteristic sensor responses demonstrate that ligand conjugation successfully adds functionality to a MoS2 matrix. Thus, this will be a promising approach to constructing a versatile sensor array, by conjugating a wide variety of thiolated ligands on the MoS2 surface. Furthermore, these MoS2 sensors in this study exhibit high sensitivity to representative VOCs down to a concentration of 1 ppm. This approach to fabricating a tunable and sensitive VOC sensor may lead to a valuable real-world application for lung cancer diagnosis by breath analysis. KEYWORDS: Chemiresistor, molybdenum disulfides, MoS2, volatile organic compound, VOC, gas sensor
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rapid adsorption/desorption rates for target analytes with reversible and repeatable reactions. Importantly, adding functionality to the channel material is highly necessary for achieving selectivity for various target applications.7,11 Various channel materials, including metal oxides,8−10 monolayer-capped metal nanoparticles (MCNPs),11−13 conducting polymers,14 carbon nanotubes,15 and reduced graphene oxide,16−18 have been used to achieve high sensitivity, high selectivity, and high stability of sensor response. Semiconducting metal oxides, such as SnO2, ZnO, and TiO2, are the most widely studied channel materials because they have an easy fabrication process and can be miniaturized for portable devices, as well as high sensitivity toward specific gas molecules.8−10 Chemiresistors based on metal oxides rely on charge transfer between the oxygen adsorbed metal oxide surface and analyte molecules, which act as electron donors or electron acceptors for the metal oxide sensing channels.8 Recently, MCNPs have been regarded as another promising material for sensing channels.11−13 MCNPs consist of a molecular monolayer (thiolated ligands) around a metal core. As the analyte adsorbs onto the MCNP film, it causes a change in resistance by increasing the interparticle distance via a
arly diagnosis of lung cancer is very important for enabling effective treatment and for improving the survival rates of patients.1,2 To enhance the accuracy of early diagnosis, it is essential to develop highly efficient breath analysis tools that can detect specific volatile organic compounds (VOCs) in exhaled breath from potential patients. Several VOC detection tools have been suggested, including gas chromatography−mass spectrometry,3 ion flow-tube mass spectrometry,4 surface acoustic wave sensors,5 quartz crystal microbalance,6 and chemiresistors.7−18 Among the various methods, the chemiresistor is particularly interesting because it is portable, costeffective, power-efficient, rapid, reversible, and highly sensitive. These properties make it suitable for VOC sensors for breath analysis.7,11 In previous spectrometric studies, more than 1000 trace VOCs are detected in human breath at concentration from part per million (ppm) to parts per trillion (ppt) level. For this reason, in breath analysis research, gas sensors that are considered as possessing high sensitivity could detect below few ppm.2 Detecting target gas molecules in a chemiresistor relies on a change in electrical resistance of channel materials interacting with analytes.11 Thus, developing a highly efficient channel material is very important for realizing a high-performance chemiresistor. Channel materials in a chemiresistor must possess a moderate baseline resistance (104−107 Ω), high stability (i.e., low noise level), high surface-to-volume ratio, and © XXXX American Chemical Society
Received: July 29, 2014 Revised: August 26, 2014
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dx.doi.org/10.1021/nl502906a | Nano Lett. XXXX, XXX, XXX−XXX
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Figure 1. Schematic description for the preparation of MoS2 chemiresistor from powder state material. (a) MoS2 in pristine powder. (b) MoS2 flakes are well dispersed in ethanol/water solvent through grinding and ultrasonication process. Some of sulfur atoms are naturally detached from the primitive MoS2 flakes (schematic illustration in red rectangle). (c) Thiolated ligand (MUA) conjugated MoS2 flakes after solution based mixing process. (d) Loading the primitive and MUA-conjugated MoS2 films on the microelectrode printed SiO2/Si substrate through vacuum filtration. (e) The prepared MoS2 chemiresistors are mounted on the customized reaction chamber with a gas delivery system and data acquisition module.
Herein, we present a highly sensitive, stable, and inexpensive chemiresistor using MoS2 bulk films, which can be functionalized via thiolated ligand conjugation. After preparation of a MoS2 dispersion by grinding and sonication, MoS2 was conjugated with the thiolated ligand, mercaptoundecanoic acid (MUA), through simple solution mixing. This step was followed by vacuum filtration to generate sensing films from primitive MoS2 and MUA-conjugated MoS2 (MUA-MoS2) solution. Both films displayed high sensitivity (down to 1 ppm) and selectivity toward representative VOC groups (toluene, hexane, ethanol, propionaldehyde (propanal), and acetone). However, we found that the sensing behaviors were significantly different, depending on the surface functionalization of the MoS2 film and the type of VOC. In the case of primitive MoS2 film, we observed an increase in resistance (positive response) for all target VOCs. In contrast, the MUAMoS2 film displayed a decrease in resistance (negative response) with oxygen-functionalized molecules (ethanol, propanal, and acetone); other VOCs (toluene and hexane) elicited a similar but reduced positive response compared with that of the primitive MoS2 sensor. Such difference in sensing
swelling effect and/or increasing the permittivity of the organic matrix surrounding the metal cores.13 However, despite intensive efforts to develop channel materials, it is still very difficult to achieve high performance of sensitivity, selectivity, stability, and response/recovery time that are required for practical applications. Molybdenum disulfide (MoS2 ) is another promising candidate for channel material in gas sensors, as it possesses low band gap energy in the bulk (1.2 eV) and monolayer (1.8 eV). Furthermore, it has high mobility, large specific surface area, and low cost, and it is advantageous for surface functionalization.19−21 However, there are only few reports on MoS2-based gas sensors,22−24 primarily because most studies focused on devices based on a single and/or on few-layers MoS2. This lack of data makes it difficult to achieve reproducible and reliable sensing performance. To produce a practical chemiresistive gas sensor using MoS2 as a channel material, the development of a new fabrication method that provides stable film formation, a simple process, costeffectiveness, high reproducibility, and surface functionalization for selective sensing ability is necessary. B
dx.doi.org/10.1021/nl502906a | Nano Lett. XXXX, XXX, XXX−XXX
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behaviors toward different kinds of analytes might originate from the modified functionality of the MoS2 surface, which plays a key role in interacting with VOCs and causes changes in the resistance of sensing channels. Figure 1 illustrates the overall scheme for the fabrication of a MoS2-based chemiresistor. First, the MoS2 powder (SigmaAldrich, powder
