Flexible Superwettable Tapes for On-Site Detection of Heavy Metals

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Flexible Superwettable Tapes for On-Site Detection of Heavy Metals Xuecheng He, Tailin Xu, Wei Gao, Li-Ping Xu, Tingrui Pan, and Xueji Zhang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b04536 • Publication Date (Web): 09 Nov 2018 Downloaded from http://pubs.acs.org on November 10, 2018

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Analytical Chemistry

Flexible Superwettable Tapes for On-Site Detection of Heavy Metals Xuecheng He,† Tailin Xu,*,† Wei Gao,‡ Li-Ping Xu,† Tingrui Pan,§,|| Xueji Zhang*,†,# †Research

Center for Bioengineering and Sensing Technology, University of Science and Technology Beijing, Beijing 100083, P. R. China #Beijing

Advanced Innovation Center for Materials Genome Engineering, University of Science & Technology Beijing, 30 Xueyuan Road, Beijing 100083, P.R. China ‡Division

of Engineering and Applied Science, California Institute of Technology, 1200 E California Blvd, Pasadena, California 91125, United States §Department ||Shenzhen

of Biomedical Engineering, University of California, Davis, California 95616, United States

Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen, Guangdong 518055, P. R. China

ABSTRACT: Bioinspired superwettable micropatterns that combine superhydrophobicity and superhydrophilicity have been proved to exhibit outstanding capacity in controlling and patterning microdroplets and possessed new functionalities and possibilities in emerging sensing applications. Here, we introduce a flexible tape-based superhydrophilic-superhydrophobic tape toward on-site heavy metals monitoring. On such a superwettable tape, capillarity-assisted superhydrophilic microwells allow directly anchoring indicators in fixed location and sampling into a test zone via simple dip-pull from an origin specimen solution. In contrast, the superhydrophobic substrate could confine the microdroplets in the superhydrophilic microwells for reducing the amount of analytical solution. The tape-based microchip also displays excellent flexibility against stretching, bending, and torqueing for expanding wearable and portable sensing devices. Qualitative and quantitative colorimetric assessments of multiplex heavy metal analyses (chromium, copper, and nickel) by naked eyes are also achieved. The superwettable tape-based platforms with a facile operation mode and accessible signal read-out represent unrevealed potential for on-site environmental monitoring.

controllable cell patterns,31,32 high-throughput cell 33,34 screening. open-channel diagnosis.27,35,36 Importantly, under the comprehensive effect of geometry, roughness and surface energy, superwettable materials have also demonstrated excellent capacity toward microdroplet manipulations, such as directional aqueous transportation on anisotropic wettable surface,37,38 and anchoring/driving liquid microdroplets even against the gravity.39,40 The above encouraging results have shown great potential to manipulate microdroplet for on-site chemical analysis. A flexible tape-based superwettable microchip toward on-site detection of heavy metals has been proposed in this paper. A flexible adhesive tape is chosen as the substrate that is further micropatterned with a superwettable nanodendritic silica coating along with a superhydrophobic background. Such a superwettability-enabled tape with high affinity of superhydrophilic microwells to aqueous molecules are capable of capturing water droplets, while the water-repellent superhydrophobic background effectively restrict water spreading. The on-site sample collection can be performed by direct dip-pull from an original solution, leading to a decentralized colorimetric assay toward common heavy metal detections (chromium, copper, and nickel) on an indicator

Continuous pressure on diseases and health catastrophes have aroused global concerns about water, air and food security. Driven by the widely spreading use of point-of-care testing (POCT), considerable efforts have been contributed to fabricate flexible, wearable and portable devices for in situ or on-site analysis.1-8 Heavy metal ions, as a typical source of water pollution, can cause a series of physiological problems.9 Therefore, several platforms toward environmental heavy metals monitoring have been explored recently.10-23 Unfortunately, most of them still rely on tedious functional modification,16,17 multifarious channel/reservoir design,18,23 or additional optical,19,20 electronic,21,22 to manipulate the liquid sample handling or optimize signal processing, which are not desirable for rapid on-site analysis to potentially improve specific patient-care outcomes in certain circumstances.24 Droplet-based assays have gained tremendous progresses due to their advantages of compartmentalization, miniaturization, and high-throughput.25-28 By integrating two extreme wettabilities of superhydrophobicity and superhydrophilicity in well-defined micropatterns, the superwettable microchips exhibit new functionalities and possibilities as microdroplet-based platform for widespread biological and biomedical applications,29,30 including

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zone. Compared to conventional microfluidic and paper-based analytical system, the flexible superwettable tapes with significant benefits of direct sample collection and transfer can greatly reduce the required sampling time of liquid flowing or wicking, which can meet rapid on-site sensing demand. The present flexible superwettable tape offers on-site fingertip analysis of heavy metals, which involves in colorimetric indicator functionalization and direct on-glove sampling as schematically illustrated in Figure 1. The superhydrophilic microwell is served as independent “microcontainer” under the aid of water-repellent superhydrophobic surface for indicator immobilization and downstream droplet-based analysis. Detailedly, on the tape, color indicator allows to be physically immobilized in the superhydrophilic microwells through natural evaporation processes as shown in Figure 1a, avoiding complicated chemical modification process. The adhesive tapes are employed as a wettable substrate, which is convenient to attach onto an optional device such as a latex glove. Such a system offer a simple approach to capture and hold a drop of analytic solution by pulling it out from the pristine analytes without extra protocol as demonstrated in Figure 1b. The chemical interactions between heavy metal ions and active components from deposited indicators occur in the captured microdroplets, and produce concentration-dependent colorimetric signals.

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on a conceptual substrate of flat tweezers, curved glass rod or uneven glove cover, to carry out the sample harvest process as demonstrated in Figure 3b. Furthermore, Figure 3c illustrates the tapes that are cut into different sizes with 1×3, 2×2 and 2×3 superhydrophilic microarrays for multiplex detection. We further investigated the splendid capability of capturing microdroplets of superhydrophilic microwells. Gradually taking the tapes away from the sample results in stretching of the water and a thin neck next to the tape side was formed in Figure S2, suggesting the high capillary force of porous superhydrophilic microwell that ensure its micropipette-free sampling for on-site detection activity. Based on the simplified capillary model between liquid and porous solid surfaces, the volume of captured microdroplet increases with the diameters of superhydrophilic microwells.39 Therefore, the amount of sample solution can be precisely tailored via altering the microwell size. To better understand the its feasibility for on-site sensing applications, a simulated on-site heavy metals monitoring study (Nickel ion, 70 mg/L) is performed as demonstrated in Figure 4 and corresponding SI Video 1. Firstly, the assay reagent comprising buffer solution and color indictor are carefully drop-casted into the superhydrophilic microwell and dried at room temperature (25 °C, Humidity: 40%). Then the tape is adhered to a latex glove to perform the analysis process. Prior to measurement, the obtained superhydrophilic test zone and the nickel ion standard solution both present a quasicolorless status with black arrows as highlighted in Figure 4a (SI Video 1, 0s). Upon lightly touching the nickel sample solution surface (Figure 4b, SI Video 1, 3s), and pulling it out immediately, a sample droplet selectively stays in indicatorfunctionalized superhydrophilic test site without observable lateral flow. As a result, the captured droplet become into a distinct pink within few second (Figure 4c, SI Video 1, 6s), and finally turned into stable deep magenta after 10 seconds as demonstrated in Figure 4d (SI Video 1, 16s). Compared to the common dipstick-style sensors on which the analyte solution randomly spreads that produce low-density colorimetric signal,43-45 such superhydrophobic-superhydrophilic micropatterns coordinately restrict the microdroplets in superhydrophilic microwell, which can dramatically reduce the required sample amount and concentrate the colorimetric signal. The whole on-glove decentralized sampling/sensing operation is quite rapid (about 16 s) that facilitates in-time and qualitative heavy metals risk alert in practical scene. As a concept-of-proof, three common heavy metal ions (Chromium, copper and nickel) closed to environmental pollution are chosen to verify the compatibility and versatility of such superwettable tape toward multivariate chemical threat quantification. Similarly, the color reagent component are firstly added in the superhydrophilic microwell (Diameter: 1 mm) and dried as test zone prior to detection. Then the standard solutions of chromium, copper and nickel drop-cast into the superhydrophilic assay zones, respectively. The stable color intensity of chromogenic water droplets are recorded by digital camera and the grayscale value are measured by ImageJ analysis software (Detailed in Supporting Information). Each values are averaged by three independent measurements for reliable results. As expected, chromogenic microdroplets in Figure 4 e-g demonstrate that high abundance of the metal ions lead to increased color intensity. The calibration plot of

The flexible superwettable tapes are fabricated through the direct roll-to-roll coating of a silica suspension, which is systematically characterized as illustrated in Figure 2 and Figure S1. Briefly, the superhydrophobic silica suspension is prepared by the co-reactions between fluoroalkylsilane (FAS) and hydrophobic silica particles,41 and then evenly applied onto the tape matrix by using a scraping ink bar. Such direct coating method provides a simple and low-cost strategy for large-scale preparing superhydrophobic materials. The resulted superhydrophobic coating consists of nanodendritic network of cross- linked silica nanoparticles (Diameter: 20~40 nm) (Figure 2a) with a total thickness of 3.7 ± 0.3 μm (Figure 2b). Subsequently, the superhydrophilic microwells are patterned by an oxygen plasma etching step through a custommade photomask.42 Such a treatment result in wettability switching from superhydrophobicity (Contact angle of 152.5 ± 1.9°) to superhydrophilicity (Contact angle of 0°) as shown in Figure 2 c and d. Allura-red labeled microdroplets exhibit totally contrary behaviors on the tape surfaces with different surface treatments illustrated in Figure 2e: on FAS modifiedsilica surface the droplet is quasi-spherical, while on oxygenetched spot it tends to be flattened. The tape-based microchips exhibit unique property of mechanical flexibility, physical adhesion as well as unique capacity of capturing microdroplets. As shown in Figure 3a, the water droplet keeps pinning in the superhydrophilic microwell regardless of stretching, bending, and torqueing examination. These results reveal the high flexibility of such superwettable tapes without sacrificing superwettable characteristic, and could be expanded to design wearable and portable sensing devices. The adhesive tape matrix of the superwettable tapes offers sufficient physical integration with alternative equipment for more complex applications in different settings. For example, we can optionally attach tapes


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Analytical Chemistry principal, it is expected that the facile, low-cost and easy-tooperate adhesive chips also have desirable commercialization potential (similar to the typical case of Band Aid) in “singleuse” and POCT diagnostics application including wound monitoring, biofluid analysis and transdermal drug delivery.

color intensities versus the Cr (VI), Cu (Ⅱ) and Ni (Ⅱ) ion concentration is almost linear ranging from 0.5 to 9, 1.0 to 12 and 5 to 40 mg/L, with correlation coefficients of 0.989, 0.995, 0.974, respectively, and corresponding limit of detection at a confidence factor of three was calculated to be about 0.36, 1.28 and 2.39 mg/L. In addition, direct naked-eye observation toward sample standard solutions in opposite rows of almost the same color intensity reveals their appreciative sensing reproducibility. Therefore, such flexible superwettable tapes suggest infinite opportunity of manipulating microdroplets for multivariate heavy ions sensing. Following the simulation behaviors and calibration plot of standard solution, significant considerations environmentally relevant to real water sample on the flexible superwettable was also investigated. Firstly, Figure 5a shows that measured heavy metals concentration show good agreement with spiked heavy metal ions content using the aforementioned calibration. Therefore, the measured heavy metals concentration could roughly represent the actual analytes abundance in the subsequent experiment. To confirm the interference tolerance of the superwettable tapes for the analysis of target heavy metal ions, as depicted in Figure 5b, water samples containing target metal ions of Cr(VI), Cu(II) and Ni(II) are then compared to those spiked with 200 mg/L Ca(II), Mg(II) and Na(I) (commonly existed in natural water body). The color intensity signals of interference group exhibit negligible decrease compared to the target metal ions group, suggesting the fine interference tolerance. We finally employs the superwettable tapes to quantify heavy metals in real water samples collected from the Qinghe River, Haidian Park and Olympic Forest Park in Beijing. All the measured heavy metals concentration is around 0 in Figure 5c, implying the good water quality around Beijing. By comparison, standard solution is split into the raw sample to form artificial specimen of 0.5 mg/L Cr(VI), 1.0 mg/L Cu(II) and 1.0 mg/L Ni(II). These values are selected since they are defined as threshold of sewage discharged from electroplating enterprises according to current Chinese national standard. Figure 5c demonstrates that heavy metals concentration in water body with added ions were significantly higher than in raw sample as expected (p < 0.001). Overall, the proposed tapes display favorable feasibility of semi-quantitatively evaluating real water sample. In conclusion, we have demonstrated a portable tape-based flexible superwettable microchip for on-site heavy metal detection. The mechanical flexibility and physical adhesion of the soft tape substrate is experimentally confirmed. Compared to other flexible sensors (e.g. paper-based and microfluidic devices), our tape-based superwettable sensors allow universally adhesive integration onto other substrate as a promising candidate for wearable and portable devices to achieve direct sample immobilization onto assay zone via direct dip-pull from an origin specimen solution, remarkably overcoming multistep sample manipulation. The indicatormodified superwettable tapes coupled with wearable glove permit proof-of-concept analysis of routine heavy metals including chromium, copper, and nickel. The droplet-based sensors have fine interference tolerance for the analysis of target ions, capable of discriminating the heavy metal content among different water samples. Due to the versatile design

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Detailed materials and instruments, fabrication of superwettable tapes, thin neck phenomenon produced by the superhydrophilic microwell, additional figures (PDF), simulated on-site detection of nickel ion standard solution (70 mg/L) using the superwettable tapes (AVI)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge funding from National Natural Science Foundation of China (21804007), Beijing Natural Science Foundation (2184109), Fundamental Research Funds for Central Universities (FRF-TP-17-066A1), National Postdoctoral Innovative Talents Support Program of China (BX20180036), and Program for Guangdong Introducing Innovative and Entrepreneurial Teams (2016ZT06D631), and the Shenzhen Fundamental Research Program (JCYJ20170413164102261).

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from 1.0 to 12.0 mg/L, and (g) divalent nickel from 5 to 40.0 mg/L, with correlation coefficient of 0.989, 0.995, and 0.974, respectively.

FIGURE CAPTIONS:

Figure 1. Design the superwettable tapes toward heavy metals monitoring. (a) Schematic of indicator immobilization processes on a superhydrophilic-superhydrophobic nanodendritic substrate (b) Schematic of on-site fingertip analysis of heavy metals via dip-pull manipulation.

Figure 5. (a) Relation between added heavy metals concentration and measured heavy metals concentration that converted using corresponding calibration plots. (b) Interference study for the analysis of Cr(VI), Ni(II), Cu(II). Color intensity contrast is presented between individual metal ions and those doped with 200 mg/L Ca(II), Mg(II) and Na(I). (c) Semi-quantitative analysis of different water samples. Heavy metals abundance was higher in artificial liquid (raw water sample spiked with 0.5 mg/L Cr(VI), 1.0 mg/L Cu(II) and 1.0 mg/L Ni(II) , respectively) than in raw water sample (n = 6, *: p < 0.05, **: p < 0.01, ***: p < 0.001, unpaired t test).

Figure 2. Characterization of the superwettable tapes. Top-view (a) and side-view (b) of the nanodendritic silica coating. Water contact angles of superwettable tape before (c) and after (d) oxygen plasma etching. (e) Behaviors of two allura-red-labeled microdroplet on the superhydrophobic substrate and the superhydrophilic microwell.

Figure 3. (a) Flexible superwettable tapes steadily anchor microdroplets regardless of stretched, bended and twisted state (Microwells diameter: 1 mm, microdroplets: 5 μL). (b) Adhesive superwettable tapes adhered on flat tweezers, curved glass rod and uneven bumpy latex glove ensures its excellent feasibility of capturing microdroplets under complex setting. (c) 1×3, 2×2 and 2×3 superhydrophilic microarrays on fingertip tapes for multiplex detection (Scale bar: 5 mm).

Figure 4. (a-d) Time-lapse images of the simulated manipulation of on-site monitoring of nickel ion standard solution (Microwells diameter: 1 mm; solution concentration: 70 mg/L). All the photographs were extracted from SI Video 1. Calibration plot of superwettable tapes for quantitative heavy metals sensing. Color intensity versus heavy metals concentration including (e) hexavalent chromium varying from 0.5 to 9.0 mg/L, (f) bivalent copper 5 ACS Paragon Plus Environment


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Figure 1. Design the superwettable tapes toward heavy metals monitoring. (a) Schematic of indicator immobilization processes on a superhydrophilic-superhydrophobic nanodendritic substrate (b) Schematic of on-site fingertip analysis of heavy metals via dip-pull manipulation. 215x128mm (300 x 300 DPI)

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Figure 2. Characterization of the superwettable tapes. Top-view (a) and side-view (b) of the nanodendritic silica coating. Water contact angles of superwettable tape before (c) and after (d) oxygen plasma etching. (e) Behaviors of two allura-red-labeled microdroplet on the superhydrophobic substrate and the superhydrophilic microwell. 207x122mm (300 x 300 DPI)


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Figure 3. (a) Flexible superwettable tapes steadily anchor microdroplets regardless of stretched, bended and twisted state (Microwells diameter: 1 mm, microdroplets: 5 μL). (b) Adhesive superwettable tapes adhered on flat tweezers, curved glass rod and uneven bumpy latex glove ensures its excellent feasibility of capturing microdroplets under complex setting. (c) 1×3, 2×2 and 2×3 superhydrophilic microarrays on fingertip tapes for multiplex detection (Scale bar: 5 mm). 151x118mm (300 x 300 DPI)



Figure 4. (a-d) Time-lapse images of the simulated manipulation of on-site monitoring of nickel ion standard solution (Microwells diameter: 1 mm; solution concentration: 70 mg/L). All the photographs were extracted from SI Video 1. Calibration plot of superwettable tapes for quantitative heavy metals sensing. Color intensity versus heavy metals concentration including (e) hexavalent chromium varying from 0.5 to 9.0 mg/L, (f) bivalent copper from 1.0 to 12.0 mg/L, and (g) divalent nickel from 5 to 40.0 mg/L, with correlation coefficient of 0.989, 0.995, and 0.974, respectively. 301x138mm (300 x 300 DPI)


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Figure 5. (a) Relation between added heavy metals concentration and measured heavy metals concentration that converted using corresponding calibration plots. (b) Interference study for the analysis of Cr(VI), Ni(II), Cu(II). Color intensity contrast is presented between individual metal ions and those doped with 200 mg/L Ca(II), Mg(II) and Na(I). (c) Semi-quantitative analysis of different water samples. Heavy metals abundance was higher in artificial liquid (raw water sample spiked with 0.5 mg/L Cr(VI), 1.0 mg/L Cu(II) and 1.0 mg/L Ni(II) , respectively) than in raw water sample (n = 6, *: p < 0.05, **: p < 0.01, ***: p < 0.001, unpaired t test). 313x84mm (300 x 300 DPI)


Flexible Superwettable Tapes for On-Site Detection of Heavy Metals

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