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Low-cost Chemical-responsive Adhesive Sensing Chips Weirui Tan, Liyuan Zhang, and Wei Shen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14122 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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ACS Applied Materials & Interfaces

Low-cost Chemical-responsive Adhesive Sensing Chips Weirui Tan, Liyuan Zhang, Wei Shen* Department of Chemical Engineering, Monash University, Wellington Road, Clayton, VIC 3800, Australia

ABSTRACT: Chemical-responsive adhesive sensing chip is a new low-cost analytical platform, which uses adhesive tape loaded with indicator reagents to detect or quantify the target analytes by directly sticking the tape to the samples of interest. The chemical-responsive adhesive sensing chips can be used with paper to analyse aqueous samples; they can also be used to detect and quantify solid, particulate and powder analytes. The colorimetric indicators become immediately visible as the contact between the functionalized adhesives and target samples was made. The chemical-responsive adhesive sensing chip expands the capability of paper-based analytical devices to analyse solid, particulate or powder materials via one-step operation. It is also a simpler alternative way, to the covalent chemical modification of paper, to eliminate indicator leaching from the dip-stick style paper sensors. Chemical-responsive adhesive chips can display analytical results in the form of colorimetric dot patterns, symbols and texts; enabling clear understanding of assay results by even non-professional users. In this work, we demonstrate the analyses of heavy metal salts in silica powder matrix, heavy metal ions in water, as well as bovine serum albumin in aqueous solution. The detection is low-cost, one-step, specific, sensitive and easy-to-operate.

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KEYWORDS: Chemical-responsive adhesive sensing chip, Low-cost, Adhesive tape, Chemical analysis of powder, Colorimetric method, Text reporting paper sensor

INTRODUCTION The awareness of our society to health, food safety and environmental issues has increased significantly, so also is people’s desire to monitor their own health condition, water and food qualities, etc. Research on low-cost analytical sensing technology has been in a booming trend over the past decade. Low-cost sensing technologies show the potential to fill the gap between the advanced analytical technologies in hospitals and laboratories, and the needs of end users to perform point-of-care tests from home.1 To date, concepts of various low-cost analytical sensing devices manufactured based on substrates of paper,2-5 thread,6-7 textile,8-9 polymer membrane,10 multi-material composition,11-12 have been published. These low-cost sensing concepts are dominantly based on microfluidic or dip-stick principles. In microfluidic paper sensors, aqueous samples and indicators can transport through porous and hydrophilic channels of patterned paper without the need of external pumps. In sensor operation, samples and indicators are introduced to the microfluidic channels at different times; they are then mixed in porous fibre matrices without any aid and then display analytical results colorimetrically. The principle of passive fluid transport significantly reduces the fabrication cost of point-of-care sensors. At the same time, however, these sensors have also shown certain limitations in their applications, which remain as technical challenges to researchers working on microfluidic paper-based analytical devices (μPADs). One limitation of current low-costing sensing devices is that μPADs driven by passive microfluidic channels are capable of analysing only liquid samples of low viscosity. In order to analyse solid and particulate samples, additional (and in certain cases, complex) pretreatments of the samples are normally required to convert them into the liquid form. Another undesirable technical issue is that some μPADs are often affected by the “coffee stain” effect.13 2


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The introduction of liquid sample could displace and change the distribution of the indicator that had been deposited on detection zones of a paper sensor; open drying of the paper sensor often exacerbates the non-uniform distribution of both the analyte and indicator, negatively affecting the accuracy of colorimetric evaluation. To address this problem, some researchers have used polymers to immobilize the indicators chemically or physically to prevent the indicator displacement caused by the introduction of the samples.14 However, in some cases, modifications of paper could affect the performance of paper microfluidic device, due, for example, to decrease in paper wettability.15 While a sensor designed based on a single platform (e.g. paper) may have the above-mentioned limitations, these limitations may be overcome through combining concepts and methods of different platforms. Since food and environmental samples are not always in liquid form, direct assays, regardless of sample forms, are desirable for home- and field-based sensing. For dip-stick style indicator papers, a basic requirement is that the indicator must not be leached out of the paper by the sample liquid. Conventional method to meet this requirement is to chemically or physically immobilize the indicator to paper.16 However, in designing this type of paper sensors for multiple-analyte analysis, it is challenging to introduce different chemistries to immobilize different indicators on one paper sensor.4 By adopting new concepts of different platforms, sensors can be designed with flexibility to directly test samples in various forms and to overcome problems such as indicator-leaching and sample/indicator displacement or “coffee stain” in paper-based sensors. In this work, we report a new platform concept of developing low-cost sensors using common adhesives (e.g. tape). These common adhesives can stick to a wide range of materials immediately after being applied to the surfaces.17 Adhesive tapes have been reported as tools for biological and forensic analysis, but only for collecting samples18-22 or for wound dressing.23 To the authors’ best knowledge, there has been no report on using adhesives for 3



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direct chemical sensing. We conceived the idea of chemical responsive adhesive tape (CAT) for sensing applications, since they can deliver chemicals and indicators to a surface without relying on channel flow of liquids. CAT can be obtained by modifying common adhesive tapes with colorimetric or biological indicators to enable them to detect and report the presence of target analytes. Selection of adhesives is important, they should not contain compounds which interfere with the intended analysis. Indicators can be introduced onto the adhesive surface of the tape in the form of dot patterns, symbols and texts, depending on the nature and the requirement of the test. Indicators can be introduced by deposition, pen-writing, or even formulated into the glue component of the tape, depending on the application. We show using CAT to detect and report the presence and concentrations of chemical analytes (e.g. solid heavy metal salts, metal ions and biomolecules in solution). When CATs are used in combination with the dip-stick style paper sensors to analyse heavy metal ions (copper(II), nickel(II) and chromium(VI)) in water, the problem of indicator-leaching from paper can be eliminated. EXPERIMENTAL METHODS Materials and equipment Copper(II) sulfate (CuSO4), nickel(II) chloride hexahydrate (NiCl2·6H2O), potassium dichromate

(K2Cr2O7),

bathocuproine,

chloroform,

hydroxylamine

hydrochloride

(NH2OH·HCl), glacial acetic acid (CH3COOH), dimethylglyoxime (DMG), ethanol, sodium fluoride (NaF), sodium thiosulfate (Na2S2O3), 1,5-diphenylcarbazide (DPC), acetone, sulfuric acid (H2SO4), bovine serum albumin (BSA), tetrabromophenol blue, citrate acid, glycerol, zinc sulfate heptahydate (ZnSO4·7H2O), cobalt(II) chloride hexahydrate (CoCl2·6H2O), manganese chloride tetrahydrate (MnCl2·4H2O), iron chloride (FeCl3), magnesium sulfate (MgSO4), calcium chloride dihydrate (CaCl2·2H2O), sodium chloride (NaCl), potassium chloride (KCl), silica (SiO2) and Whatman #1 qualitative-grade filter paper were purchased from Sigma Aldrich (St. Louis, MO, USA). Scotch Magic Tape (3M) and Brush Pen were purchased from 4


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an Office Works outlet in Melbourne, Australia. Color intensity of the assays were measured and recorded usng a scanner (EPSON Perfection V370). Device fabrication and operation In our design, the CATs were fabricated by pen-writing method: indicator “inks” were transferred using a brush pen onto the adhesive surface of the Scotch Magic Tape in dot or text pattern (with volume 15~20 μL). Briefly, measurement was then conducted by adhering the adhesive side of the tape to the surface of a substrate containing target analytes (e.g. powders or dried liquid samples). The development of analyte-specific colorimetric results or text readouts are perceivable by naked eyes (Fig. 1). For detection of analytes in aqueous samples, a pre-selected sized filter paper (2 cm  3 cm) was dipped in the aqueous sample containing the target analyte for 3~5 seconds; the paper absorbed ~200 μL of liquid sample. After dried, analytes on the filter paper can be measured by CATs (Fig. 1B). The feasibility of chemical-responsive adhesive sensing chips is determined by the requirement that the adhesives must not contain compounds that can interfere with the detection of the target analytes. In this work, we chose Scotch Magic Tape (3M) because of its inertness to the indicators and analytes. The adhesive material of the Scotch Magic Tape is a mixture of acrylate polymers, however, no further information about the structures of the polymer is available in the literature because of trade secrecy.24 Acrylate polymers could have ester and carboxylic acid termini in the molecule. Among the acrylate polymers, sodium polyacrylate could potentially react with heavy metal ions. A pH measurement of the adhesive surface of the 3M Magic Tape showed neutral pH (Supporting Information), suggesting none or negligible presence of sodium polyacrylate in the adhesive, if at all. Furthermore, even if a trace amount of sodium polyacrylate was present in the adhesives, its lower chelating stability constant with the target metal ions compared to those with their corresponding indicators makes the polyacrylate impossible to hinder the detection of the heavy metal ions. Taking Cu2+ for 5



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example, the stability constant of polyacrylate – Cu2+ chelate is 6.98,25 while that of the Cu and bathocuproine chelate is 19.1.26

Figure 1. Schematics of fabrication and application of a CAT: A. a CAT for detection of analytes in powder form. A CAT carrying the indicator ink is directly affixed onto a powder matrix, which contains the target heavy metal salt; B. a CAT for metal ion (e.g. Cu2+) measurement in a liquid sample by the text-reporting method. A piece of filter paper is dipped into a sample solution containing Cu2+, then taken out of the solution and let dry. A CAT carrying the indicator is then affixed onto the filter paper to test and read out the assay result. The indicator ink for assays A and B is the same; it contains the indicator reagent, masking reagent, buffer and humectant. The indicator was drawn or written on the tape by using a brush pen. Detection of metal salt in a powder matrix

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Each metal salt/powder mixture was prepared by mixing metal salt (e.g. CuSO4, NiCl2, K2Cr2O7) with a SiO2 powder (with molar ratio from 1:50000 to 1:100). The indicator ink contained a specific indicator reagent to its corresponding metal ion species of a salt, a masking reagent for interference suppression and glycerol as the humectant, with the volume ratio of 1:1:2. For CuSO4 detection, the ink contained 0.05 g/mL bathocuproine in chloroform as the indicator reagent, 0.1 g/L hydroxylamine in acetic buffer (6.3 M) as the masking reagent. For NiCl2 measurement, the ink contained 120 mM dimethylglyoxime (DMG) in ethanol; a solution of NaF and Na2S2O3 (20 and 80 mg/mL, respectively) was used as the masking reagent. For K2Cr2O7 assay, 1 mg/mL 1,5-diphenylcarbazide (DPC) in 50% acetone and 1% H2SO4 were used as the indicator and masking reagent, respectively. The CAT was fabricated by transferring the indiator inks on the adhesive surface of the Scotch Magic Tape. Assays were conducted by placing 0.05 g metal salt/SiO2 powder samples of a series of molar ratios (1:50000 to 1:100) on filter paper sheets, then affixed with the corresponding CAT. After the development of color, a scanner or camera was employed to record the color intensity. Interference tolerance of metal ions in powder analysis Interference tolerance of non-target metal ions in the powder samples was examined. Powder matrices including CuSO4/SiO2, NiCl2/SiO2, K2Cr2O7/SiO2, CoCl2/SiO2 and ZnSO4/SiO2 (all in the same molar ratio of salt/SiO2) were mixed together with the mass ratio of 1:1:1:1:1. 0.05 g of such sample was tested and compared with sample that contained only the target salt (e.g. CuSO4/SiO2, NiCl2/SiO2 or K2Cr2O7/SiO2) and had the same molar ratio of metal salt/SiO2, to investigate the interference tolerance of the CAT sensing system. Detection of metal ions in aqueous solutions In this test, filter paper pieces (2 cm  3 cm) were dipped into different metal salt solutions to collect metal ions. After dried at room temperature, the metal ion was deposited on paper. Each specific indicator ink (same as those in metal powder detection) was then written on a piece of 7



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Scotch Magic Tape in the chemical symbols of the target metal ions to form text-reporting CATs. Detection was carried out simply by sticking CATs onto the filter paper. Presence of each metal ion was instantly and clearly shown by the chemical symbols revealed. For quantitative analysis, a scanner was used to record the color intensity of the symbols. RESULTS AND DISCUSSION Quantitative determination of metal salt in SiO2 powder matrix By using the CAT, one-step detection of metal salt in SiO2 powder matrix can be easily realized, since the indicator ink on the tape surface was kept wet by a humectant. As the indicator ink contacted metal salt in the powder matrix, dissolution of the metal salt by the indicator ink released the metal ions, which reacted specifically with the indicator, leading to the specific colorimetric change. To quantify the content of metal salt, a calibration curve was established by measuring a series powder samples of different molar ratios of metal salt to SiO2. The presence of solid metal salt in silica powder was revealed by the indicator as small dots (Fig. 2). For powder matrices with lower content of metal salt, the number of dots is less than those with high metal content. Accordingly, the average color intensity of CATs can be measured and correlated with the metal salt content of the samples. A linear relationship between the color intensity and the natural logarithm of the powder molar ratio was obtained, indicating the CAT method can achieve quantification for metal salt content in SiO2 powder matrices. For CuSO4 and NiCl2, the limits of detection are 1:30000 and 1:20000 in molar ratios with SiO2 powder, respectively, while for K2Cr2O7, the detection limit obtained by the scanner is 1:30000, but for the naked eye the detection limit is much lower, being 1:50000.

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Figure 2. Calibration curves fitted by the measured color intensity versus the natural logarithm of the molar ratio of each kind of metal salt/SiO2 powder: a) CuSO4/SiO2, molar ratios from 9



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1:30000 to 1:100; b) NiCl2/SiO2, molar ratios from 1:20000 to 1:400; c) K2Cr2O7/SiO2, molar ratios from 1:50000 to 1:400. Four to five replicates were performed for each assay. Identification of metal ion species in powder Fig. 3 presents clear qualitative tests of metal ion species in powder samples using textreporting by CAT. In this method indicators were written on the adhesive tape in the chemical symbols of the ions.This method enables the identification of the metal ion species with certainty, which is highly desirable for qualitative tests. It removes the chance of result misinterpretation due to similar color changes of different metal ions, e.g. those for Ni(II) and Cr(VI).

Figure 3. CAT assays of Cr-, Cu-, and Ni- salt/SiO2 powder by text-reporting. Interference tolerance on metal salt quantification by CAT Interference of the metal salt analysis in silica powder matrix was investigated by using the interference tolerance method, with the tolerance ratio set to be 5%.27 This tolerance ratio defines that changes in analytical signal (i.e. measured color intensity) of the target metal salt in the presence of interfering metal salt must be within 5% of that of the control. CuSO4, NiCl2, K2Cr2O7, ZnSO4 and CoCl2 were investigated as the interfering salts. With Cu(II), Ni(II) and Cr(VI) as the target ions, each target salt was investigated with four interfering transition metal salts. The molar ratios of target metal salts in silica powder were: 1:2000, 1:5000 and 1:30000 for Cu; 1:2000, 1:5000 and 1:20000 for Ni; 1:2000, 1:5000 and 1:50000 for Cr. The molar ratio of each interfering metal salt was made the same as that of the target salt (mol/mol silica). Fig. 4 shows that non-target metal salts present in silica powder matrices in the same 10


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molar ratio as the target salts, do not have interference to the assay results at the tolerance ratio of 5%.

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Figure 4. Interference tolerance studies of the CAT device for Cu(II), Ni(II) and Cr(VI) salts/SiO2 powder in the presence of four non-target transition metal salts (see text, NiCl2/SiO2, CuSO4/SiO2, K2Cr2O7/SiO2, ZnSO4/SiO2, and CoCl2/SiO2) as the interference salts. The molar ratios and analytes to silica were: a) 1:20000, 1:5000, 1:2000 for NiCl2/SiO2; b) 1:30000, 1:5000, 1:2000 for CuSO4/SiO2; and c) 1:50000, 1:5000, 1:2000 for K2Cr2O7/SiO2. The molar ratios of each interfering salt to silica were the same as that of the target ion. Four to five replicate tests were performed for each assay (data seen in Table S1). Liquid sample analysis CAT provides a simple alternative method to overcome the drawbacks of the μPADs – it delivers the indictor without relying on channel flow. This method effectively reduces the nonuniform color display of an assay caused by the displacement of the pre-deposited indicator in μPADs by the channel flow of sample, and also by the ‘‘coffee stain’’ effect by opening drying.13 CAT also overcomes the problem of indicator-leaching from the dip-stick style of paper sensors, without the need to immobilize indicators to paper. An advantage of the dipstick style paper sensor is that it eliminates the reliance of micropipette for quantitative delivery of samples on paper-based sensors.

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Fig. 5(a) shows the literature data of using a polymer to reduce the non-uniform distribution of the indicator in the detection zones of a PAD.14 Fig. 5(b) demonstrates our alternative method of using CAT to achieve the same effect. CAT achieves more uniform sample/indicator distribution on paper by eliminating channel flow and suppressing open drying.

Figure 5. The study of weakening the non-uniform color-distribution of an assay by: a) adding PDDA (Reprinted with permission from ref 14. Copyright 2013 Elsevier); b) the CAT method. Quantitative assays Metal ions including Cu(II), Ni(II), Cr(VI) were tested using CAT. Results are shown in Fig. 6 and Fig. 7. The color intensity is linearly correlated with the concentration of metal ions in the samples over the ranges of: 0 to 10 ppm for Cu(II), 0 to 10 ppm for Ni(II) and 0 to 4 ppm for Cr(VI). In all tests, paper was used as a dip-stick sample collector, indicators were deposited or written on the adhesive tape. In a Cu(II) assay, a CAT carrying the Cu(II) indictor was applied to the paper and then a brown ‘‘Cu’’ symbol instantly became visible as the Cu(II) concentration reached 0.5 mg/L (Fig. 7a). Similarly, the detection limits for Cr(VI) and Ni(II) are 0.5 mg/L (Fig. 7b) and 2 mg/L (Fig. 7c), respectively. The interference tolerance studies of the target heavy metal ions, Ni(II), Cu(II) and Cr(VI), are illustrated in Fig. 8. For all interference tolerance study, the concentration of the target ions was made to 2 mg/L. Samples of the individual target ions were compared with those doped with different concentrations of the non-target metal ions (10 times for non-target transition metal ions, Cu(II), Cr(VI), Co(II), Fe(III), Zn(II), Mn(II) and Ni(II); 100 times for Ca(II) and 13 ACS Paragon Plus Environment


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Mg(II) and 200 times for Na(I) and K(I)). The color intensity signals show that the average level of the interference is lower than the 10% interference tolerant limit. A significant advantage of CAT is that it eliminates indicator-leaching from paper sensor, which is critical to dip-stick style of paper sensors. An example that clearly demonstrates this point is the Cr(VI) assay.4 In our previous study, we developed a dip-stick style text-reporting method for heavy metal detection. The indicator and the indicator-Cr(VI) complex have serious leaching-out problem from the paper and therefore could not meet the design requirement of a dip-stick style paper sensor. This problem can be easily overcome by using the CAT; paperbased sensors can be designed to unambiguously identity Cr(VI) via dipping the paper in the sample solution and then stick the CAT on paper to obtain colorimetric (Fig. 6b) and text (Fig. 7b) signals. This sensor design approach does not rely on micro-pipetting of samples; it also removes the necessity of chemically immobilization of the indicator onto paper surface, therefore significantly simplifies the fabrication of paper-based sensors.

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Figure 6. Calibration curves fitted by the measured color intensity versus the concentration of each metal ion: a) Cu(II), from 0 to 10 mg/L; b) Cr(VI), from 0 to 4 mg/L ; (c) Ni(II), from 0 to 10 mg/L; five replicate tests were performed for each assay.

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Figure 7. Colorimetric text-reporting assays by CAT of different concentrations of: (a) Cu(II), (b) Cr(IV)and (c) Ni(II).

Figure 8. Interference tolerance study of the CAT device for analysis of Cu(II), Ni(II) and Cr(VI) in aqueous solutions. The concentration of the target heavy metal ions was made to 2 mg/L. Comparisons are then made between individual target metal ions and those doped with 20 mg/L of non-target transition metal ions Cu(II), Cr(VI), Ni(II) Co(II), Fe(III), Zn(II), Mn(II); 200 mg/L Ca(II) and Mg(II) and 400 mg/L Na(I) and K(I). Average level of interference of all non-target metal ions are presented. Five replicate tests were performed for each assay (data seen in Table S2 and S3).

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The CAT device can also be used for biological assays, for example, testing protein in solution. The biological assays can be performed in a similar way to those of heavy metal ions and results can be reported by either colorimetric change or text. BSA was used to demonstrate the biological assay by CAT; the limitation of detection is 0.5 mg/mL (Fig. S2). CONCLUSIONS The chemical-responsive adhesive sensing chip is a novel and simple platform technique for point-of-use assays. This platform is adapted in this work to suit various detection chemistries and multiple analytes, including those of complicated environmental and biological sample matrices. Compared with paper-based and thread-based microfluidics, chemical-responsive adhesive sensing chips have a clear advantage in that they can be applied to solid surfaces, powder materials and liquid samples to obtain a rapid analytical appraisal of specific target analytes. The detection operation is easily realized by sticking CAT onto target analytes, without need of laboratory equipment. This novel chip device is portable, low-cost, equipmentfree and user-friendly, satisfying the ‘‘ASSURED’’ criteria.28 This method provides an alternative sensing platform which does not rely on the mixing of sample and indictor via capillary channel flow, instead, indicators can be delivered onto the paper sensor through contact with adhesives; reducing the displacement of the indicator by sample introduction and vice versa on paper sensors. As a result the distribution of color in the sensing zone triggered by CAT is more uniform, which increases the accuracy of the subsequent color analysis. A further advantage of the CAT technique is that indicators can be patterned on a tape in any shape by writing method, offering easy options to any form of assay result reporting. In addition, CAT eliminates the indicator-leaching problem of the dip-stick style of paper sensors, without the need of chemical immobilization of indicators to paper. It is expected that the chemical-responsive adhesive sensing chips should have commercialization potential in pointof-use detection and point-of-care diagnostics.

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ASSOCIATED CONTENT Supporting Information Additional supporting figures showing the test results of BSA by CAT. Tabulated data of the interference tolerance study results of metal salts in powder matrix and in solution. The Supporting Information is available free of charge on the ACS Publications website. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT W.T. gratefully acknowledges a scholarship from the Chinese Scholarship Council. Authors gratefully acknowledge Australian Research Council Discovery Project (ARC DP1094179 and 140100052) and Zhejiang International Science and Technology Cooperation Project (2015C34014). REFERENCES 1. Nilghaz, A.; Guan, L.; Tan, W.; Shen, W., Advances of Paper‐Based Microfluidics for Diagnostics—The Original Motivation and Current Status. ACS Sensors 2016, 1 (12), 1382‐1393. 2. Martinez, A. W.; Phillips, S. T.; Butte, M. J.; Whitesides, G. M., Patterned paper as a platform for inexpensive, low‐volume, portable bioassays. Angew. Chem. Int. Ed. 2007, 46 (8), 1318‐1320. 3. Martinez, A. W.; Phillips, S. T.; Carrilho, E.; Thomas, S. W.; Sindi, H.; Whitesides, G. M., Simple Telemedicine for Developing Regions: Camera Phones and Paper‐Based Microfluidic Devices for Real‐ Time, Off‐Site Diagnosis. Anal. Chem. 2008, 80 (10), 3699‐3707. 4. Li, M.; Cao, R.; Nilghaz, A.; Guan, L.; Zhang, X.; Shen, W., "Periodic‐table‐style" paper device for monitoring heavy metals in water. Anal. Chem. 2015, 87 (5), 2555‐2559. 5. Kang, S.‐M.; Jang, S.‐C.; Huh, Y. S.; Lee, C.‐S.; Roh, C., A highly facile and selective Chemo‐ Paper‐Sensor (CPS) for detection of strontium. Chemosphere 2016, 152, 39‐46. 6. Li, X.; Tian, J.; Shen, W., Thread as a versatile material for low‐cost microfluidic diagnostics. ACS applied materials & interfaces 2010, 2 (1), 1‐6. 7. Nilghaz, A.; Ballerini, D. R.; Fang, X.‐Y.; Shen, W., Semiquantitative analysis on microfluidic thread‐based analytical devices by ruler. Sensor Actuat B‐Chem 2014, 191, 586‐594. 8. Vatansever, F.; Burtovyy, R.; Zdyrko, B.; Ramaratnam, K.; Andrukh, T.; Minko, S.; Owens, J. R.; Kornev, K. G.; Luzinov, I., Toward fabric‐based flexible microfluidic devices: pointed surface modification for pH sensitive liquid transport. ACS applied materials & interfaces 2012, 4 (9), 4541‐ 4548.

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