Laser-Cut Polymer Tape Templates for Scalable Filtration Fabrication

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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 ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.8b00639 • Publication Date (Web): 07 Nov 2018 Downloaded from http://pubs.acs.org on November 13, 2018

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Laser-Cut Polymer Tape Templates for Scalable Filtration Fabrication of User-Designed and Carbon Nanomaterial-Based Electrochemical Sensors Mengmeng Song, Lantu Dang, Juan Long, 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 * To whom correspondence should be addressed to: Dr. Chengguo Hu, Email: [email protected] (C. Hu).

ABSTRACT: We report here a simple filtration method for the scalable fabrication of user-designed and carbon nanomaterial-based electrode arrays using laser-cut polyvinyl 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 SWNTs array is demonstrated to possess several interesting properties, e.g., good mechanical property, excellent flexibility and favorable electrochemical behaviors. Moreover, its porous structure enables the construction of a paper-like and 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 semi-automatic and high-throughput evaluation of antioxidant capacity on a series of vegetable and fruit foods using a single-channel electrochemical analyzer.

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KEYWORDS: Template filtration; electrode arrays; electrochemical sensors; phenol pollutants; food antioxidant capacity.

Carbon nanomaterials such as carbon nanotubes (CNTs) and graphene are extensively employed in electrochemical sensing fields, which can remarkably improve the responses of a wide range of analytes from inorganic ions to biological cells.1-8 Moreover, these materials generally possess high conductivity, and are demonstrated to be the promising conductive components of transparent electrodes.9,10 Therefore, various methods have been developed for the construction of CNTs or graphene conductive arrays with high uniformity and precision, e.g., inkjet printing,11,12 transfer printing,13 patterned CVD growth,14 lithography,15 oxygen plasma,16,17 and filtration.1820

Among these methods, the filtration method has the advantages of material saving,

high uniformity as well as simple devices and operations. Moreover, this method is able to precisely control the composition and thickness of the films by simply changing the volume or composition of filtration solutions. In consequence, filtration is frequently employed for the fabrication of CNTs and graphene films with high conductivity and transparency.18-20 Since the employment of templates during filtration is essential to the construction of electrode arrays with desired patterns, several strategies are proposed to form conductive electrode arrays by template filtration, including wax printing,21,22 lithography23,24 and non-adhesive film masks.25-28 The prepared electrode arrays are applied in electronic fields such as flexible electrodes,23,25,28 touch 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


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biomolecules,29,30 the detailed cutting procedures for fabricating the non-adhesive polymer templates are generally not mentioned. Here, we establish a new template filtration method for the scalable fabrication of user-designed and carbon nanomaterial-based electrode arrays using laser-cut adhesive PVC patterns. The PVC templates can be produced by an inexpensive commercial laser engraving machine (< $450) with high speed and low cost. Different from the previous unremovable21-24 or non-adhesive patterns25-30, the adhesive PVC templates can be firmly attached onto or freely peeled off from the hydrophobic PVDF membranes. This unique property not only enables us to produce carbon nanomaterial-based electrode arrays with performance similar to screen printed electrodes (SPEs),31 but also allows the facile construction of addressable electrochemical sensors by a simple coating process. In addition, we propose two novel sensing applications for the fabricated electrodes: (1) the on-site detection of phenol pollutants in electrolyte-free water by a Nafion electrolyte-based solid-state SWNTs sensor; (2) the semi-automatic and highthroughput detection of food antioxidant capacity on an addressable SWNTs sensor with the aid of an inexpensive 3D printer.

EXPERIMENTAL SECTION Chemicals PVDF membranes (pore size 0.22 µm) were obtained from Mosu Science Equipment Co., Ltd. (Shanghai, China). SWNTs (diameter 1 - 2 nm, length 5 ~ 30 µm, purity > 95wt%) were purchased from XFNANO Materials Tech Co., Ltd. (Nanjing, China). Spectroscopically pure graphite powder, 1-methyl-2-pyrrolidone (NMP), phenol, hydroquinone (HQ), p-nitrophenol (PNP), o-aminophenol (OAP), chlorchloric


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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, 5wt% in a mixture of lower aliphatic alcohols and water, Lot#BCBC0802), the Folin-Ciocalteu (F-C) phenol reagent and bovine serum albumin (BSA) were purchased from Sigma-Aldrich. Carbon paste (Electrodag 423SS) for electric connection of three-electrode arrays was the product of Henkel Co. Epoxy glue was purchased from Hu'nan Magic Power Industrial Co., China. Polyimide (PI), polymethyl 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, Nison Instrument Ltd., Shanghai, China. Phosphate buffer solutions (PBS, 0.1 M, pH 7.4) were prepared from Na2HPO4·12H2O and NaH2PO4·2H2O and adjusted to desired pH values with 1.0 M HCl or NaOH by a pH meter (PB-10, Sartorius). Apparatus All electrochemical experiments were performed with a CHI 660B electrochemical analyzer (CHI). Sheet resistance was measured with a four-point probe sheet resistance


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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 by FEI tecnai G2 F30, USA. UV-vis spectra were collected on a UV 2550 spectrometer (Shimadzu, Japan). Ultrasonic dispersion of carbon materials was accomplished by a SB-80 sonicator (Ningbo Scientz Biotechnology Co., Ltd., China). A laser engraving machine, which is equipped with a 50 W CO2 laser (JL-K3020, Liaocheng Julong Laser Equipment Co., Ltd., China) and driven by a LaserDRW 2013.02 software, was used for cutting userdesigned PVC tape patterns. A 3D printer with laser engraving function (CR-8, Shenzhen Creality 3D Technology Co., Ltd., China) was employed to assist the addressing detection of the 36-zone SWNTs sensor. A XHF-DY high-speed disperser (Ningbo Scientz Biotechnology Co., Ltd., China) was used to pulverize food samples, and a TGL-16G high-speed 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 three-electrode array was reported versus the reference electrode part of the array itself. All electrochemical data were reported as means of at least three tests. Preparation of Carbon Nanomaterial Dispersion Solutions The dispersion solution of SWNTs was prepared as follows: 60 mg SWNTs was added into 500 mL aqueous solution of 5.0 mM SDS, and the mixture solution was sonicated for 2 h at ambient conditions. Then, the solution was kept quiet for 2 days to ensure sufficient settlement of large SWNTs particles. Finally, the upper stable dispersion solution was collected and used for filtration. The resulting SWNTs dispersion was sonicated for additional 10 min before each filtration when it was reused, in order to ensure the good reproducibility of the SWNTs array. The procedures for


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preparing the dispersion of GNPs were similar to those of SWNTs, except for the addition of 2.5 g graphite into 500 mL NMP and the sonication for 6 h. Filtration Fabrication of Conductive Carbon Nanomaterial Electrode Arrays Schematic procedures for fabricating conductive electrode arrays by template filtration using laser-cut PVC tape patterns are shown in Figure 1A and 1B. Briefly, a user-defined pattern was cut by the JL-K3020 laser engraving machine on a PVC tape (step 1), which was attached onto 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 nanoparticles coated SWNTs (AuNPs-SWNTs) electrode array, 25 nm AuNPs were further filtrated onto the SWNTs-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 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 electrolytefree water was fabricated using Nafion as the solid electrolyte, i.e., 3 μL of 5wt% Nafion 117 stock solution was dropped on the sensing zone (4 mm × 6 mm) of a SWNTs three-electrode array, which was allowed to dry at ambient conditions for 6 min. The resulting Nafion coated SWNTs three-electrode array (Nafion-SWNTs) was denoted as


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the solid-state sensor for phenol pollutants. To construct the addressable 36-zone SWNTs sensor, a conductive SWNTs film on PVDF membranes was prepared by filtrating SWNTs dispersion on PVDF membranes without any templates. After water washing and drying, a PVC tape with 36 square patterns (size 2 mm × 2 mm, interval 3 mm) was covered onto the SWNTs film. Then, the exposed four edges of the SWNTs film were coated by silver paste and adhesive copper tapes to ensure sufficient conductivity, which produces an addressable SWNTs sensor with 36 separated sensing zones. The detailed procedures for fabrication, characterization and analytical applications of these electrochemical sensors can be found in Supporting Information.

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 such as clear cutting edge, low ash contamination, modest adhesion strength and good solvent resistance (Figure 1C), which can be freely peeled off from the silicone film support and attached onto the surface of PVDF membranes due to its excellent flexibility (Figure 1D and 1E). The resulting PVC patterned membrane has size, shape and flexibility similar to the original PVDF membrane, thus applicable to most traditional filters (Figures 1F and S2). Then, through a simple filtration process, conductive materials such as SWNTs are selectively deposited on the hollow out areas from their


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diluted solutions (Figures 1G and S3). The PVC template is readily removed by immersing in ethanol. In consequence, user-designed and surface contamination-free SWNTs arrays are fabricated on flexible PVDF membranes (Figure 1H), which can be fully transferred onto desired substrates using double-side tapes (Figures 1I and S4). Although the precision of our template filtration method is relatively lower than some previous methods such as wax printing21,22 and lithography,23,24 the reproducibility of the produced SWNTs electrode array is good enough for electrochemical sensing applications (Figure S5).

Figure 1. Schematic representations (A-B) and digital photos (C-I) for fabricating userdesigned 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) A PVC tape-patterned PVDF filter membrane. (F) A stainless steel filter for filtration. (G) Diluted SWNTs dispersion in SDS aqueous solution for filtration. (H) SWNTs electrode arrays on PVDF membranes. (I) SWNTs arrays transferred onto a plant leave with the aid of a double-side tape. Scale bars are 5 mm.


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Figure 2. Top (A, B, C) and cross section-view SEM images (D, E, F) of the SWNTs (A, D), GNPs (B, E) and AuNPs-SWNTs arrays (C, F) on PVDF membranes. Scale bars are 1 μm for Parts A-C, and 200 nm for Parts D-F and the insets of Parts A-C. Based on 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. Particularly, the SWNTs film consists of plenty of seriously tangled nanotube bundles of about 50 nm in diameter while the GNPs film is made up of compact and stacked nanoplatelets (Figure 2A-2B and 2D-2E). The SWNTs film has a unique nanoporous structure and a better mechanical property (Figure S6), which therefore is a promising nanofiltration membrane for the further deposition of functional nanomaterials. For instance, 25 nm AuNPs were filtrated onto the surface of


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the SWNTs film and further enlarged by a seeded growth method,32,33 producing an interesting dot-on-sheet microstructure of a SWNTs film decorated with plenty of ~ 100 nm AuNPs (Figure 2C and 2F). Considering the unique properties of AuNPs for electrochemical biosensing, e.g., facile biolabeling and good catalytic activity,34,35 the successful preparation of the AuNPs-SWNTs film may further expand the function of the SWNTs array and its application fields.

Electrochemical Behaviors of Carbon Nanomaterial Three-Electrode Arrays

Figure 3. Cyclic voltammograms of 20.0 mM DA (A), 5.0 mM BPA (B) and 10.0 mM H2O2 (C) in 0.1 M PBS (pH 7.4) on the SWNTs, GNPs and AuNPs-SWNTs arrays. D. Cyclic voltammograms of 5.0 mM Fe(CN)63-/4- in 0.1 M KCl on nine different SWNTs arrays. The voltammograms were collected by drop casting a 20 μL electrolyte solution on these three-electrode arrays. The potentials were reported versus the reference electrode part of each array. Scan rate, 100 mV/s.

The electrochemical behaviors of the SWNTs, GNPs and AuNPs-SWNTs threeelectrode arrays toward several typical electrochemical probes such as DA, BPA and


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H2O2 were tested (Figure 3A-3C). The results indicate these electrodes generally have wide available potential windows, simple backgrounds and favorable electrochemical responses towards the above probes (Figure S10). Moreover, the AuNPs-SWNTs array shows an 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 GNPs coating is not as good as the SWNTs one (Figure S6). Therefore, compared to the GNPs electrode array, the SWNTs array has several advantages including facile fabrication, good mechanical property, favorable electrochemical behaviors and low cost (< $0.2), which is therefore selected as a preferred carbon electrode array for the following electrochemical sensing applications. Since conductivity is usually a critical factor that determines the electrochemical performance of carbon nanotube thin-film electrodes,8 we examined the influence of filtration volume on the sheet resistance and the voltammetric response of the SWNTs three-electrode array. When increase the filtration volume of the SWNTs dispersion in SDS aqueous solution (cal. 0.074 mg/mL for SWNTs), the sheet resistance of the SWNTs array rapidly decreases from 446.3 to 28.0 ohm/sq in the volume range of 1 ~ 10 mL, and then slowly decreases to 11.7 ohm/sq at 25 mL (Table S1). Moreover, the capacitive background of the array at 0.0 V almost linearly increases with its filtration volume. In the case of the voltammetric response of Fe(CN)63-/4- at the array, the peakto-peak separation firstly decreases from 1 mL to 5 mL, and then tends to be stable thereafter (Figure S12). Considering both the redox behaviors of Fe(CN)63-/4- and the background, a volume of 5 mL is optimal for preparing the SWNTs array with high signal-to-noise ratios. We further examined the reproducibility of our method. A low relative standard


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deviation (RSD) of 1.2% was obtained for the voltammetric responses of 5.0 mM Fe(CN)63-/4- on nine identical arrays (Figure 3D). Moreover, the voltammograms of Fe(CN)63-/4- at the SWNTs array in a 20 μL droplet and a 10 mL solution are highly overlapped (Figure S13). Besides, the replacement of either the counter or reference electrode of the SWNTs array by traditional platinum (Pt) counter or saturated Ag/AgCl reference electrodes hardly influenced the voltammogram shape of [Fe(CN)6]3-/4-, expect that the change of the reference electrode led to an apparent shift of redox potentials (Figure S14). These results are in good accordance with our previously reported inkjet-printed gold arrays,33 which suggest the good electrochemical sensing performance of the flexible SWNTs array.

Electrochemical Detection of Phenol Pollutants in Electrolyte-Free Water by the Solid-State Sensor The SWNTs array has a porous structure for both the SWNTs conductive layer and the beneath PVDF membrane. This unique structure allows the controllable infiltration of functional materials into the whole array by a simple modification method, i.e., the drop casting of a 3 μL 5wt% Nafion 117 stock solution on the SWNTs three-electrode array produced a paper-like and solid-state electrochemical sensor, which employs Nafion as a solid electrolyte.36,37 The resulting sensor is expected to be able to detect electroactive phenols in electrolyte-free aqueous solutions, and therefore is especially suitable for on-site monitoring of environmental water (Figures 4A and S15). As revealed by Figure 4B and 4C, a thin polymer film is clearly observed on the NafionSWNTs composite. Moreover, in contrast to the poor response of BPA in water on the naked SWNTs array, a well-defined oxidation peak of BPA at 0.38 V is observed on the Nafion-SWNTs composite array (Figure 4D and 4E), which exhibits a calibration


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range of 0.2 ~ 10.0 μM with a detection limit of 0.1 μM (S/N=3) (Figure 4F). These results clearly demonstrate the good performance of Nafion as a solid-state electrolyte for the detection of electroactive species in electrolyte-free aqueous solutions.

Figure 4. Detection of phenol pollutants in electrolyte-free water by the NafionSWNTs solid-state electrochemical sensor. (A) Schematic procedures for preparing the solid-state sensor. SEM images of the naked (B) and the Nafion coated SWNTs arrays (C). Cyclic voltammograms of water and 1.0 μM BPA in water on the naked (D) and the Nafion coated SWNTs arrays (E). (F) Linear sweep voltammograms of BPA at different concentrations on the solid-state sensor (inset shows the calibration plot). (G) Schematic structure of a home-made FIA system using the solid-state sensor for the real-time amperometric detection of phenols in electrolyte-free water. (H) FIA amperometric responses at 0.38 V for BPA with different concentrations on the solidstate sensor. (I) FIA amperometric responses of 15.0 μM different phenols on the solidstate sensor. The detection potentials for BPA, phenol, HQ, PNP and OAP are 0.38, 0.70, 0.10, 0.80 and 0.60 V, respectively. Scan rates are 100 mV/s for Parts 4D-4F.


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To mimic the on-site real-time monitoring of phenol pollutants in electrolyte-free environmental water, a simple home-made flow injection assay (FIA) system was constructed for the detection of various phenol pollutants by amperometric methods (Figures 4G and S16). As indicated by Figure 4H, the solid-state sensor shows a sensitive and rapid response towards BPA in water, which has a wide calibration range from 0.05 to 25.0 μM and a low detection limit of 37 nM (S/N=3). We further examined the FIA responses of 15.0 μM phenol pollutants including BPA, phenol, HQ, PNP and OAP on the sensor. Interestingly, although some phenols such as BPA have been demonstrated to seriously foul carbon electrodes (Figures S17 and S18),38 the five phenols all shows rapid and sensitive amperometric responses in the FIA system with acceptable reproducibility (Fig. 4I). It should be pointed out that although the SWNTs sensor exhibits no selectivity toward different phenols (Figure S19), this disadvantage is conversely favorable to its practical applications, i.e., the sensor may monitor as many as possible phenols of different types when operated at a relatively high positive potential, which can reduce the risk of missing any phenol pollutant during practical applications. Moreover, as indicated by Figure S20, the employment of the Nafion electrolyte was found to apparently improve the anti-fouling ability of the SWNTs sensor in protein-rich aqueous solutions. In addition, although the sensitivity of our solid-state sensor is slightly lower than some previously reported carbon nanotube-based electrochemical sensors of BPA (Table S2), the performance of the sensor is apparently better than the widely used naked glassy carbon electrode (GCE) (Figure S17B). The stability of the solid-state sensor was also examined using HQ as a reversible model. The result reveals that the oxidation current of HQ hardly changes for a continuous 500 s flux of 10.0 μM


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HQ or the intermittent injection of 15.0 μM HQ during a long period of about 3 h (Figure S21). The strong hydrophobic interactions between the sidewalls of SWNTs and PVDF membranes may contribute to the stable adhesion of the SWNTs array on its PVDF support. These results, together with Figure S6C, suggest the good mechanical and chemical stability of the SWNTs-PVDF composite. What’s more, the RSD for the response of 25.0 μM HQ at seven identical arrays is 2.1% (Figure S22), suggesting the good reproducibility of this electrochemical FIA system. All these results verify the promising application of the Nafion electrolyte-based solid-state electrochemical sensor for the on-site and real-time monitoring of phenol pollutants in environmental water.

High-Throughput Electrochemical Screening of Food Antioxidant Capacity by an Addressable SWNTs Sensor On the basis of the above discussions, it is clear that the SWNTs array exhibits favorable electrochemical performance for the detection of various phenols, which may also find potential applications in food sciences, e.g., electrochemical evaluation of food antioxidant capacity.39,40 However, traditional three-electrode cell configurations are not suitable for the rapid screening of a large amount of food samples. Recently, Levkin et al reported a high-density droplet microarray of individually addressable electrochemical cells for the simultaneous detection of analytes by both cyclic voltammetry and chronoamperometry using multi-channel analyzers.41 Zhang et al. developed a reliable single molecule electrochemical detection system by integration of enzyme-induced metallization (EIM) together with addressable droplet cells on microelectrode arrays.42,43 These works demonstrate the promising application of


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addressable droplet cell arrays for rapid and high throughput biosensing.

Figure 5. Schematic representation (A) and test setup (B) for the addressing detection of antioxidants in droplet cell arrays on a 36-zone SWNTs sensor. (C) Detection principle and amperometric responses of 0.1 M pH 7.4 PBS droplet cell arrays containing 0 or 0.2 mM GA (the blue curve is a portion of the data in the inset). (D) Antioxidant capacity of 0.2 mM organic antioxidants and several food samples, which was obtained by amperometry at different detection potentials and by the UV-vis based F-C method. The antioxidant capacity of each compound or sample at different detection potentials was evaluated using the calibration plots at the corresponding detection potentials. (E) Amperometric responses and calibration plots of GA at 0.4 V. (F) Screening of antioxidant capacity for various fruits, vegetables and fruit-based commercial drinks by the addressable sensor.

Inspired by these pioneering works, we developed a new addressable sensing platform for the screening of food antioxidant capacity using a single-channel analyzer. We constructed an electrochemically addressable 36-zone sensor by simply patterning a SWNTs conductive film with a PVC template (Figure S23). Then, a silver (Ag) wire


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and a Pt wire were assembled into a reference/counter (Ag/Pt) electrode couple, and then fixed onto the printing head of the CR-8 3D printer. The Ag/Pt electrode couple, together with the addressable SWNTs sensor, enables the semi-automatic detection of analytes in droplet cell arrays on the sensor by computer-controlled addressing (Figures 5A, 5B and S24). That is to say, when the Ag/Pt electrode couple comes into one electrolyte droplet of GA on the working electrode during addressing, the three electrodes temporarily form a complete electrochemical cell, which produces an instantaneous amperometric response directly related to the concentration of GA in the droplet (inset of Figure 5C). Here, GA was chosen as a standard for evaluating the antioxidant capacity of various foods.44,45 It should be pointed out that although the Ag/Pt electrode couple has a relatively small effective area compared to commercial Ag/AgCl or Pt wire electrodes, their electrochemical performance is comparable to traditional three-electrode systems (Figure S25). Clearly, the addressable SWNTs sensor exhibits acceptable reproducibility for different sensing areas on a single electrode using both amperometric and voltammetric methods (Figure S26). Moreover, although no additional washing treatments were performed for the Ag/Pt electrode couple, the proceeded detection of one droplet hardly influenced the responses of the following ones (Figure S27). This may arise from the minimized areas of the Ag/Pt electrode couple, the short addressing period of a single droplet (< 3 s), and the much smaller diffusion distance of possible contaminants on the Ag/Pt electrode couple (< 100 μm) as compared with their distance to the SWNTs working electrode (> 1 mm). On the basis of the above discussions, it is clear that the established addressing detection system allows the rapid and reproducible detection of analytes in the droplet cell arrays on a single sensor, which therefore is suitable for the high-throughput electrochemical evaluation of antioxidant capacity on a large amount of food samples.


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We examined the influence of detection potential on the amperometric results using the widely used F-C method as a reference (Figure S28). As shown in Figures 5D and S29, the results obtained by our amperometric method at 0.4 V are well consistent with those by the F-C method using different standard antioxidants, e.g., CT, CA, AA and several real food samples. Therefore, we established the calibration plot for the evaluation of antioxidant capacity using the oxidation current of GA at 0.4 V, which shows a linear relationship with its concentration in the range of 5.0 ~ 500.0 μM and a detection limit of 0.6 μM (S/N=3) (Figure 5E). We also evaluated the antioxidant capacity of a series of foods such as fruits, vegetables and fruit-based commercial drinks (Figure 5F). The results are generally in accordance with the F-C method (Table S2). In summary, the combination of droplet cell arrays on addressable SWNTs sensors with inexpensive addressing instruments (e.g., a 3D printer reported here or previously reported computer-controlled stepper motors46), enables the construction of high-throughput electrochemical sensing systems with simple instrument, high detection speed and low cost.

Conclusions A simple template filtration method for the scalable fabrication of user-designed and carbon nanomaterial-based electrodes using laser-cut PVC tape patterns is reported. This method can produce SWNTs arrays in large scale with several attractive properties, e.g., free of templates or surface contamination, good mechanical property, high uniformity and reproducibility, tunable material composition and favorable electrochemical behaviors. Moreover, the unique porous structure of the SWNTs array allows its further modification by either filtration or drop casting to expand its application fields. For instance, the drop casting of Nafion 117 electrolytes on a three-


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electrode SWNTs array can produce a paper-like and solid-state electrochemical sensor, which is suitable for on-site detection of trace phenol pollutants in electrolyte-free water. In addition, an addressable 36-zone SWNTs sensor was constructed by simply coating a uniformly filtrated SWNTs film with a PVC pattern. Together with an inexpensive 3D printer, the addressable sensor provides a facile approach to the rapid and highthroughput screening of food antioxidant capacity using traditional single-channel electrochemical analyzers.

ACKNOWLEDGMENTS. We thank the financial support of the National Nature Science Foundation of China (No. 21675116), the Natural Science Foundation of Hubei Province (No. 2015CFB538) and the Fundamental Research Funds for the Central Universities (No. 2042014kf0244).

Supporting Information Available. The Supporting Information is available free of charge on the ACS Publications website. Additional text describing detail experimental procedures, and twenty-nine figures and three tables showing the fabrication procedures, structures and electrochemical behaviors of the fabricated electrode arrays, as well as the setups and electrochemical performance of the two sensors for the real-time monitoring of phenol pollutants and the screening of food antioxidants.

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for TOC only


Laser-Cut Polymer Tape Templates for Scalable Filtration Fabrication

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