Article pubs.acs.org/ac
Group 6 Layered Transition-Metal Dichalcogenides in Lab-on-a-Chip Devices: 1T-Phase WS2 for Microfluidics Non-Enzymatic Detection of Hydrogen Peroxide Rou Jun Toh,†,Δ Carmen C. Mayorga-Martinez,† Jongyoon Han,Δ,‡ Zdenek Sofer,□ and Martin Pumera*,† †
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Science, Nanyang Technological University, 637371 Singapore Δ BioSystems & Micromechanics IRG (BioSyM), Singapore-MIT Alliance for Research and Technology (SMART) Centre, S16-05-08, 3 Science Drive 2, 117543 Singapore ‡ Department of Electrical Engineering and Computer Science, Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States □ Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technicka 5, 166 28 Prague 6, Czech Republic S Supporting Information *
ABSTRACT: Two-dimensional (2D) layered transition-metal dichalcogenides (TMDs) have been placed in the spotlight for their advantageous properties for catalytic and sensing applications. However, little work is done to explore and exploit them in enhancing the performance of analytical lab-on-a-chip (LOC) devices. In this work, we demonstrate a simple, sensitive, and low-cost fabrication of electrochemical LOC microfluidic devices to be used for enzymatic detection. We integrated four t-BuLi exfoliated, group 6 TMD materials (MoS2, MoSe2, WS2, and WSe2) within the LOC devices by the drop-casting method and compared their performance for H2O2 detection. The 1T-phase WS2-based LOC device outperformed the rest of the TMD materials and exhibited a wide range of linear response (20 nM to 20 μM and 100 μM to 2 mM), low detection limit (2.0 nM), and good selectivity for applications in real sample analysis. This work may facilitate the expanded use of electrochemical LOC microfluidics, with its easier integrability, for applications in the field of biodiagnostics and sensing.
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patternability and device fabrication due to their 2D atomically layered structures.22 Graphene and graphene oxide-modified transducing platforms have been reported widely to provide highly sensitive detection in LOC systems.23−28 Moreover, transition-metal dichalcogenides (TMDs), possessing 2D layered structures, similar to that of graphene, and desirable properties such as high surface-to-volume ratio, excellent conductivity, and capacitive behavior,29−31 present as promising candidates for enhancing the performance of electrochemical LOC devices. Sarkar et al. demonstrated the high performance of a MoS2based field effect transistor (FET) biosensor, surpassing the sensitivity of that based on graphene by more than 74-fold, and its ability to be scaled down for integration into LOC systems without compromising its sensitivity.17 Highlighting the promising potential of TMDs for sensing applications, it is surprising that they remain underexplored in the field of analytical LOC devices with only one significant work published at the current time to the best of our knowledge.
ab-on-a-chip (LOC) devices have been developed extensively to improve the sensing speed and efficiency, by miniaturizing chemical and biological analytical instruments. They bring benefits such as portability, field deployability, low sample consumption, and low costs.1 However, it remains a challenge in the field of analytical LOC devices to provide a downsized, yet highly sensitive detector for the minute sample amounts often encountered in real-world applications. In this aspect, electrochemical detection is advantageous, compared with more conventional optical detection methods, due to its ease of miniaturization, low energy consumption, and compatibility with advanced microfabrication schemes.2 For the purpose of improving the sensitivity and detection limits of LOC devices, various nanomaterial-based detectors,3−7 have been integrated within LOC systems.8,9 While carbon nanotubes are the most extensively studied due to their excellent mechanical properties, high thermal and electrical conductivity, and electrochemical properties,10−18 the onedimensional (1D) nature of the material poses a huge challenge in fabrication.19,20 On the other hand, increasing attention is being placed on two-dimensional (2D) nanomaterials21 with advantageous electrochemical properties which can offer easy © XXXX American Chemical Society
Received: January 24, 2017 Accepted: March 27, 2017
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DOI: 10.1021/acs.analchem.7b00302 Anal. Chem. XXXX, XXX, XXX−XXX
Article
Analytical Chemistry
pump (Chemxy Inc., U.S.A.) and a Pump 11 Elite syringe pump, respectively. The Fusion 200 syringe pump has a flow performance with accuracy and smooth flow ranging from 100 pL min−1 to 128 mL min−1 while the Pump 11 Elite syringe pump performs in the range of 1.28 pL min−1 to 88.28 mL min−1. Flow rates were monitored with a SLI-0430 liquid flow meter (Sensirion AG, Switzerland) which works in a flow range of −80 μL min−1 to 80 μL min−1. Chronoamperometric experiments were carried out using a μAutolab type III electrochemical analyzer (Eco Chemie, The Netherlands) connected to a computer and controlled by the NOVA, Version 1.8.17, software. Synthesis of 2D Layered Transition-Metal Dichalcogenides. Exfoliation of bulk MoS2, MoSe2, WS2, and WSe2 materials was performed by tert-butyllithium exfoliation. Suspensions were prepared by adding a particular TMD powder (3 g) to tert-butyllithium (20 mL; t-BuLi) in pentane (1.7 M). Subsequently, the suspensions were stirred at 25 °C under argon atmosphere for 72 h. The resulting Li-intercalated materials were separated by suction filtration under argon atmosphere and washed three times with hexane (dried over sodium). After which, the exfoliated material was dispersed in water (100 mL) and subjected to ultrasonication for 15 min. Centrifugation at 18 000g, and redispersion in water was carried out in order to achieve purification of the samples. This process was repeated until the conductivity falls below 20 μS. Finally, the exfoliated TMD materials were dried in a vacuum oven at 25 °C for 48 h prior to further use. Electrode Preparation. The electrodes were designed using AutoCAD (Autodesk, U.S.A.) and patterned on a glass substrate by laser cutting a polyimide tape mask. Next, carbon conductive paste was spread evenly over the patterned stencil and allowed to cure for 1 h at 100 °C. A three-electrode setup was employed. The carbon electrode serves as the counter and working electrode, as well as a base material for obtaining the pseudo Ag/AgCl reference electrode. The pseudo Ag/AgCl reference electrode was obtained by depositing a thin layer of Ag conductive ink over the carbon material. The Ag conductive ink was allowed to cure for 30 min at 100 °C. Subsequently, the Ag electrode was treated with bleach for 1 min to produce the pseudo Ag/AgCl electrode. Modification of the working electrode was performed by drop-casting a suspension of the exfoliated TMD materials (5 μL) onto the carbon electrode and allowing it to dry for 30 min at room temperature. Chip Fabrication. All experiments were carried out in microfluidic chips made of polydimethylsiloxane (PDMS). The fabrication process can be broken down into three major steps: (1) fabrication of the SU-8 master, (2) casting of PDMS, and (3) plasma bonding of PDMS over the prepared electrodes on the glass substrate. The master for the PDMS device was fabricated using standard photolithography techniques with SU8 2035 photoresist. The positive master mold for the device contained channels that were 49 μm deep. In order to prevent adhesion with PDMS, the master mold was treated with hexamethyldisilane for 1 h. After the silane treatment, PDMS was poured onto the master mold, which was degassed in a desiccator with a ∼5 psi vacuum for 30 min before pouring. After curing in an oven at 80 °C for 3 h, the PDMS layer was peeled from the silicon master. Inlet and outlet ports were created with a 2 mm cutting tip. It was treated with oxygen plasma in a plasma cleaner for 40 s before it was bonded to the patterned glass substrate. To ensure strong bonding between the PDMS channel and the glass substrate, curing is performed
As a major reactive oxygen species (ROS) in living organisms and an oxidative stress marker,32−34 highly sensitive and inexpensive point-of-care testing of H2O2 is of paramount importance. Considering batch measurements, Wang et al. reported the high electrocatalytic performance of MoS2 nanoparticles toward H2O2 reduction.35 In this work, we integrate 2D layered TMDs (MoS2, MoSe2, WS2, WSe2) as detector surfaces for hydrogen peroxide (H2O2) in LOC devices. Moreover, a simple and reproducible method of preparing TMDs-based LOC microfluidic devices is presented. This method allows investigators without access to state-of-theart fabrication equipment to explore electrochemical LOC microfluidics and diagnostics with ease and at a low cost. Herein, the TMD materials were prepared by tert-butyllithium exfoliation and consequently, we demonstrate the high performance of 2D layered, 1T-phase WS2-based LOC device for the detection of H2O2 (Figure 1) compared to that based on MoS2, MoS2, and WSe2. The robust 1T-phase WS2-based LOC device is furthermore practical for H2O2 determination in human serum, indicating its feasibility for applications in real biosample analysis.
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EXPERIMENTAL SECTION Materials. Carbon conductive paste (BQ242) and polyimide tape (0.5 mil) were purchased from DuPont, U.S.A. and CAPLINQ, Canada, respectively. Conductive silver epoxy (CW2400) was acquired from Chemtronics, U.S.A. SU-8 2035 photoresist and hexamethyldisilane were obtained from MicroChem, U.S.A. and Sigma-Aldrich, St. Louis, MO respectively. Harris Uni-Core cutting tip (2 mm) was obtained from Ted Pella, Inc., U.S.A. Polydimethylsiloxane (Sylgard 184) was acquired from Dow Corning Inc., U.S.A. Liquid chlorine bleach was purchased from GB Chemicals Pte. Ltd., Singapore. Clear silicone sealant was obtained from Sellys, Australia. MoS2, MoSe2, WS2, WSe2 (
