Article Cite This: ACS Sens. 2018, 3, 2518−2525
pubs.acs.org/acssensors
Laser-Cut Polymer Tape Templates for Scalable Filtration Fabrication of User-Designed and Carbon-Nanomaterial-Based Electrochemical Sensors Mengmeng Song, Lantu Dang, Juan Long, and Chengguo Hu* Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China
ACS Sens. 2018.3:2518-2525. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/14/19. For personal use only.
S Supporting Information *
ABSTRACT: We report here a simple filtration method for the scalable fabrication of user-designed and carbon-nanomaterial-based electrode arrays using laser-cut poly(vinyl chloride) (PVC) tape templates. This method can produce electrode arrays with high uniformity and low resistance from the dilute dispersions of single-walled carbon nanotubes (SWNTs) and graphene nanoplatelets (GNPs). For these two carbon arrays, the SWNT array is demonstrated to possess several interesting properties, e.g., good mechanical properties, excellent flexibility, and favorable electrochemical behavior. Moreover, its porous structure enables the construction of a paperlike solid-state electrochemical sensor using Nafion electrolytes, which is suitable for the on-site monitoring of trace phenol pollutants in electrolyte-free water. Besides, an electrochemically addressable 36-zone sensor was constructed by this method. With the aid of an inexpensive 3D printer, the addressable sensor can achieve the semiautomatic and high-throughput evaluation of antioxidant capacity on a series of vegetables and fruits using a single-channel electrochemical analyzer. KEYWORDS: template filtration, electrode arrays, electrochemical sensors, phenol pollutants, food antioxidant capacity
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sensitive devices,21,26 and stretchable multilayer circuits.22,24 Meanwhile, although the template filtration method has been used for the construction of electrochemical sensing arrays for small biomolecules,29,30 the detailed cutting procedures for fabricating the nonadhesive polymer templates are generally not mentioned. Here, we establish a new template filtration method for the scalable fabrication of user-designed and carbon-nanomaterialbased electrode arrays using laser-cut adhesive PVC patterns. The PVC templates can be produced by an inexpensive commercial laser engraving machine (95 wt %) were purchased from XFNANO Materials Tech Co., Ltd. (Nanjing, China). Spectroscopically pure graphite powder, 1methyl-2-pyrrolidone (NMP), phenol, hydroquinone (HQ), pnitrophenol (PNP), o-aminophenol (OAP), chloroauric acid (HAuCl4·4H2O), hydrogen peroxide (H2O2), gallic acid (GA), (+)-catechin hydrate (CT), caffeic acid (CA), ascorbic acid (AA), trisodium citrate dehydrate (cit), hydroxylamine hydrochloride (NH2OH·HCl), sodium dodecyl sulfate (SDS), potassium ferricyanide (K3Fe(CN)6), and potassium ferrocyanide (K4Fe(CN)6) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Dopamine (DA) and bisphenol A (BPA) were obtained from J&K Scientific Co., Ltd. (Beijing, China). Nafion electrolytes (Nafion 117 solution, 5 wt % in a mixture of lower aliphatic alcohols and water, lot no. BCBC0802), the Folin−Ciocalteu (F−C) phenol reagent, and bovine serum albumin (BSA) were purchased from Sigma-Aldrich. Carbon paste (Electrodag 423SS) for the electric connection of three-electrode arrays was a product of Henkel Co. Epoxy glue was purchased from Hu’nan Magic Power Industrial Co., China. Polyimide (PI), poly(methyl methacrylate) (PMMA), biaxially oriented polypropylene (BOPP), paper, and conductive copper tapes were purchased from local markets. PVC tapes (thickness 0.15 mm) with polyacrylate-based pressure-sensitive adhesives were purchased from Junye Adhesive Tape Technology Co., Ltd. (Shenzhen, China). Nitrogen (N2, purity >99.9%) was purchased from WISCO Oxygen, Wuhan, China. All chemicals were of analytical grade and used without further purification. All aqueous solutions were prepared using ultrapure deionized (DI) water (>18 MΩ·cm) produced on Heal Force from Nison Instrument Ltd., Shanghai, China. Phosphatebuffered solutions (PBS, 0.1 M, pH 7.4) were prepared from Na2HPO4·12H2O and NaH2PO4·2H2O and adjusted to the desired pH values with 1.0 M HCl or NaOH using a pH meter (PB-10, Sartorius). Apparatus. All electrochemical experiments were performed with a CHI 660B electrochemical analyzer. Sheet resistance was measured with a four-point probe sheet resistance tester (ST-21H, China). Scanning electron microscope (SEM) images were collected on a Zeiss sigma field emission system (FEI Nova NanoSEM 450, USA). Transmission electron microscope (TEM) images were characterized with an FEI tecnai G2 F30 (USA). UV−vis spectra were collected on a UV 2550 spectrometer (Shimadzu, Japan). The ultrasonic dispersion of carbon materials was accomplished with an SB-80 sonicator (Ningbo Scientz Biotechnology Co., Ltd., China). A laser engraving machine (JL-K3020, Liaocheng Julong Laser Equipment Co., Ltd., China), equipped with a 50 W CO2 laser and driven with LaserDRW 2013.02 software, was used for cutting user-designed PVC tape patterns. A 3D printer with a laser engraving function (CR-8, Shenzhen Creality 3D Technology Co., Ltd., China) was employed to assist the addressing detection of the 36-zone SWNT sensor. A XHFDY high-speed disperser (Ningbo Scientz Biotechnology Co., Ltd., China) was used to pulverize food samples, and a TGL-16G highspeed centrifuge (Shanghai Anting Scientific Instrument Factory, China) was used to separate plant juice extractions from the solid residuals. Unless otherwise stated, the potential of each threeelectrode array was reported versus the reference electrode part of the array itself. All electrochemical data were reported as the means of at least three tests. Preparation of Carbon Nanomaterial Dispersion Solutions. The dispersion solution of SWNTs was prepared as follows: 60 mg of SWNTs was added to a 500 mL aqueous solution of 5.0 mM SDS,
Figure 1. Schematic representations (A, B) and digital photographs (C−I) for fabricating user-designed conductive patterns by template filtration. (C) Laser cutting of electrode array patterns on PVC tapes. (D) Easy peeling off of a PVC pattern from the silicone film support. (E) PVC-tape-patterned PVDF filter membrane. (F) Stainless steel filter for filtration. (G) Diluted SWNT dispersion in SDS aqueous solution for filtration. (H) SWNT electrode arrays on PVDF membranes. (I) SWNT arrays transferred onto a plant leaf with the aid of double-sided tape. Scale bars are 5 mm. cut by the JL-K3020 laser engraving machine on a PVC tape (step 1), which was attached to the surface of a PVDF membrane (step 2) and then used for the region-selective filtration deposition of conductive nanomaterials from their diluted solutions (step 3). The resulting composite film was immersed in ethanol to peel off the PVC tape pattern (step 4). This conductive nanomaterial−PVDF composite film was then dried, and individual electrode arrays were cut for electrochemical tests (step 5). To fabricate a gold-nanoparticle-coated SWNT (AuNP−SWNT) electrode array, 25 nm AuNPs were further filtered onto the SWNT−PVDF composite film and then underwent two seeded growth cycles according to our previous works.32,33 The detailed procedures for the fabrication of conductive electrode arrays and electrochemical sensors using SWNTs, GNPs, and AuNPs− SWNTs can be found in the Supporting Information. Fabrication of Solid-State Sensors for Phenols and Addressable Sensors for Food Antioxidant Capacity. The solid-state electrochemical sensor for the detection of phenols in electrolyte-free water was fabricated using Nafion as the solid electrolyte; i.e., 3 μL of a 5 wt % Nafion 117 stock solution was dropped on the sensing zone (4 mm × 6 mm) of an SWNT threeelectrode array, which was allowed to dry under ambient conditions 2519
DOI: 10.1021/acssensors.8b00639 ACS Sens. 2018, 3, 2518−2525
Article
ACS Sensors
Figure 2. Top (A, B, C) and cross-sectional-view SEM images (D, E, F) of the SWNTs (A, D), GNPs (B, E), and AuNP−SWNTs arrays (C, F) on PVDF membranes. Scale bars are (A−C) 1 μm and (D−F and insets of A−C) 200 nm. for 6 min. The resulting Nafion-coated SWNT three-electrode array (Nafion−SWNTs) was denoted as the solid-state sensor for phenol pollutants. To construct the addressable 36-zone SWNT sensor, a conductive SWNT film on PVDF membranes was prepared by filtering an SWNT dispersion on PVDF membranes without any templates. After being washed with water and dried, a PVC tape with 36 square patterns (size 2 mm × 2 mm, interval 3 mm) was used to cover the SWNT film. Then, the four exposed edges of the SWNT film were coated with silver paste and adhesive copper tape to ensure sufficient conductivity, which produces an addressable SWNT sensor with 36 separated sensing zones. Detailed procedures for the fabrication, characterization, and analytical applications of these electrochemical sensors can be found in the Supporting Information.
user-designed and surface-contamination-free SWNTs arrays are fabricated on flexible PVDF membranes (Figure 1H), which can be fully transferred onto the desired substrates using double-sided tapes (Figures 1I and S4). Although the precision of our template filtration method is relatively lower than that of some previous methods such as wax printing21,22 and lithography,23,24 the reproducibility of the produced SWNT electrode array is good enough for electrochemical sensing applications (Figure S5). On the basis of this method, several conductive materials including SWNTs and GNPs were employed for fabricating carbon nanomaterial electrode arrays from their diluted solutions. As expected, the filtration method is able to produce the nanomaterial films or patterns with high smoothness and uniformity (Figures S6−S9). Moreover, the surface morphology of the filtration films is strongly dependent on the employed nanomaterials. In particular, the SWNT film consists of plenty of seriously tangled nanotube bundles of about 50 nm diameter, and the GNP film is made up of compact and stacked nanoplatelets (Figure 2A,B and D,E). The SWNT film has a unique nanoporous structure and better mechanical properties (Figure S6), which therefore is a promising nanofiltration membrane for the further deposition of functional nanomaterials. For instance, 25 nm AuNPs were filtered onto the surface of the SWNT film and further enlarged by a seeded growth method,32,33 producing an interesting dot-on-sheet microstructure of an SWNT film decorated with plenty of ∼100 nm AuNPs (Figure 2C,F). Considering the unique properties of AuNPs for electrochemical biosensing, e.g., facile biolabeling and good catalytic activity,34,35 the successful preparation of the AuNP−SWNT film may further expand the function of the SWNT array and its application fields.
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RESULTS AND DISCUSSION Fabrication and Characterization of Conductive Carbon Nanomaterial Electrodes by Template Filtration. Because the cutting precision of the adhesive tape patterns determines the quality of the final electrode arrays, we tested the performance of a series of polymer tapes for preparing the adhesive patterns, including PVC, PI, PMMA, BOPP, and paper tapes (Figure S1). The result shows that the PVC tape has the best performance with respect to its clear cutting edge, low ash contamination, modest adhesion strength, and good solvent resistance (Figure 1C) and can be freely peeled off from the silicone film support and attached to the surface of PVDF membranes due to its excellent flexibility (Figure 1D,E). The resulting PVC patterned membrane has a size, shape, and flexibility similar to those of the original PVDF membrane and thus is applicable to most traditional filters (Figures 1F and S2). Then, through a simple filtration process, conductive materials such as SWNTs are selectively deposited onto the hollowed-out areas from their diluted solutions (Figures 1G and S3). The PVC template is readily removed by immersion in ethanol. As a consequence, 2520
DOI: 10.1021/acssensors.8b00639 ACS Sens. 2018, 3, 2518−2525
Article
ACS Sensors Electrochemical Behaviors of Carbon Nanomaterial Three-Electrode Arrays. The electrochemical behaviors of the SWNTs, GNPs, and AuNP−SWNT three-electrode arrays toward several typical electrochemical probes such as DA, BPA, and H2O2 were tested (Figure 3A−C). The results
chemical responses toward the above probes (Figure S10). Moreover, the AuNP−SWNT array exhibits obvious catalytic behavior for the oxidation of H2O2 (Figures 3C and S11). At the same time, although the GNP electrode possesses a high signal-to-background ratio for the detection of analytes such as BPA, the mechanical strength of the conductive GNP coating is not as good as that of the SWNT coating (Figure S6). Therefore, compared to the GNP electrode array, the SWNT array has several advantages, including facile fabrication, good mechanical properties, favorable electrochemical behavior, and low cost (
