Chemically Resistant Perfluoroalkoxy Nanoparticle-Packed Porous

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Interface-Rich Materials and Assemblies

Chemically Resistant Perfluoroalkoxy Nanoparticle-Packed Porous Substrates and Their Use in Colorimetric Sensor Arrays Virendra Patil, Myung-Goo Lee, Jaesub Yun, Jong-Seok Lee, Sung H. Lim, and Gi-Ra Yi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b02481 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 6, 2018

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Chemically Resistant Perfluoroalkoxy Nanoparticle-Packed Porous Substrates and Their Use in Colorimetric Sensor Arrays Virendra S. Patil,† Myung-Goo Lee,† Jaesub Yun,‡ Jong-Seok Lee,‡ Sung H. Lim,*,†,§ and Gi-Ra Yi*,† †

School of Chemical Engineering and ‡Department of Industrial Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea § iSense LLC, Mountain View, CA 94043, USA *Corresponding authors Email: [email protected], [email protected] Tel:+ 82-31-290-7289

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ABSTRACT

As a printing substrate for colorimetric sensor arrays, chemically resistant membranes are prepared by coating perfluoroalkoxy (PFA) polymer nanoparticles onto a cellulose filter paper. A water-based fluorothermoplastic polymer dispersion was diluted with an organic solvent that causes weak aggregation of polymer nanoparticles. The resulting solution improved adhesion between the polymer and the cellulose membrane, providing more mechanically stable substrate. These PFA polymer-coated substrates demonstrated superior chemical resistance against strong alkalines, and had relatively uniform nanoporous structures that substantially improved the printability of a colorimetric sensor array. Finally, colorimetric sensor arrays printed on these substrates were evaluated for the detection of four different toxic industrial chemicals (e.g., ammonia, hydrogen sulfide, nitrogen dioxide, and sulfur dioxide) at or below their permissible exposure limits.

KEYWORDS: Perfluoroalkoxy nanoparticles, Colorimetric sensor array, Polymer substrate, Detection of toxic gases


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INTRODUCTION A colorimetric sensor array (CSA) is a polymer sheet embedded with an array of chemoresponsive indicators that differentially change color when exposed to different analytes.1-3 The core sensing element of the technology is a set of indicators, which spans a diverse set of strong chemical interactions with various classes of analytes. An overall pattern of color changes embodies a chemical fingerprint of the analyte, which can be acquired using an optical scanner or camera.4, 5 The fingerprint is computed by subtracting the post-exposure image from the preexposure image, which constitute a signal similar to that generated by the mammalian olfaction system. The mammalian nose has a large number of different odor receptors that can sense odorants based on cross-responsive patterns. Similarly, many indicators in the CSA are cross responsive to a given analyte, and the resulting chemical fingerprint can be matched against a database of reference fingerprints to identify the given analyte. While the sensor technology is not designed for component-by-component analysis like GC-MS, the sensor technology has been successfully demonstrated for identification of various classes of analytes, including explosives, toxic gases, food and beverages, microorganisms, serum antioxidants and, organic antifreezes.2, 3, 6-16

While the primary sensing element of the CSA technology is the chemo-responsive indicators, the overall sensor performance also depends on the chemical and physical properties of the printing substrate and the ink formulations.4 In order to make more reproducible CSAs, all components of the ink composition must not react with each other, and the indicator chemistry should be chemically compatible with the substrate.4 This is particularly important for indicators immobilized in either highly acidic or alkaline formulations. Most prior CSAs have been printed


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on polyvinylidene difluoride (PVDF) membranes, which slowly degrades under strong alkaline conditions. Furthermore, the substrate must also provide excellent print quality in terms of spot uniformity, size and consistency for more reproducible sensor responses.1 More consistent spots can reduce the noise from printing variability and effectively increase the signal to noise ratio, which is particularly important for analytes with low signal intensities.17 Further, the printing substrates should be produced cost effectively, considering that the CSAs are disposable.18

Various substrates have been used to print CSAs, including silica gel plates (C2 reverse phase),2 glass slides,3 cellulose-based membranes,19,

20

and polymeric microporous membranes.4,

8, 21

According to a recent report on effects of substrates and ink formulations on the CSA performance,4 cellulose-based CSAs had large inconsistent spots which often overlapped with the neighboring spots. Impermeable substrates, such as glass or polyethylene terephthalate (PET), resulted in small dense spots that had much slower response time. Polymeric microporous membranes exhibited superior performance among the substrates studied. In particular, the PVDF membrane was compatible with most ink formulations, and provide the best overall sensor performance.5, 8, 22-24 Unfortunately, the PVDF membrane is not chemically compatible with an ink formulation requiring strong alkaline conditions.6, 25

Considering the importance of solid support for CSAs,26-28 we have developed low cost and highly

chemical

resistant

nanoparticle-packed

porous

substrates.

A

perfluoroalkoxy (PFA) polymer nanoparticles dispersion solution was dip coated on non-woven fibrous substrates, in which nanoparticles become interconnected. Chemically inert PFA polymer provided high chemical resistance, while improving the ink printability. Nanoporosity of the


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PFA polymer coating allowed the ink formulations to be printed more uniformly. Finally, we demonstrated the CSA printed on this substrate is highly effective for rapid detection and identification of toxic industrial chemicals (TICs).


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EXPERIMENT Preparation of the Nanoparticle-packed Porous Substrates. An aqueous PFA dispersion solution (3MTM DyneonTM Fluoroplastic PFA 6900GZ, 50 w/v%) in a 100 mL glass bottle was ultrasonicated for a few minutes to degas the solution. Resulting air bubbles on the surface was carefully removed with a dropper. Freshly plasma treated (2.5 cm × 3.0 cm) cellulose paper (Whatman CHR-1, GE Healthcare, UK) was dipped coated in the PFA solution and placed on a clean glass plate. The coated substrate was dried inside an oven at 27 oC for 24 hours, which was further dried under a stream of nitrogen for additional 48 hours (flow rate 500 sccm). The substrate prepared by this protocol was denoted an AQ-50 substrate. For the acetone-treated PFA substrates, the 50 w/v% PFA solution was diluted with water to make a desired concentration of aqueous PFA solution. Thereafter, the PFA solution was mixed with acetone at a volume ratio of 2:1 and used to coat the cellulose paper in a similar way applied for the AQ-50 substrate (Table 1).

Table 1. Concentrations of PFA polymer nanoparticle dispersion with an acetone used for fabrication of nanoparticle-packed porous substrates. Substrate Initial PFA PFA volume Acetone volume Notation concentration (w/v%) (mL) (mL) AQ-50 50 60 0 AA-33 50 40 20 AA-27 40 40 20 AA-20 30 40 20 AA-13 20 40 20 *Volume ratio of PFA nanoparticle dispersion to acetone = 2:1

Final PFA concentration 50% 33% 27% 20% 13%

Printing of Colorimetric Sensor Arrays. First, 12 indicator formulations shown in Table S1 were loaded into a custom Teflon inkwell with 50 µL capacity. A robotic manipulator (Musashi


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Shot Mini 200S) holding 12 slotted pins (V&P Scientific FP100S) was used to simultaneously print all formulations on the PFA polymer-coated substrates by dipping the floating pins into the inkwell and then transfer the formulations to the substrate. For comparison, a commercially available PVDF membrane (Pall FluoroTrans W 0.45 µm) was also printed. Resulting sensor arrays were analyzed to evaluate the printability of different substrates. For the colorimetric sensing experiments, separate 2×10 arrays with a more diverse set of indicators were printed and tested against four different TICs as shown in Table 2. Table 2. List of indicators used for TICs detection. Spot Name #1 1,8-Diaminonaphthalene #2 2,3-Diaminonaphthalene #3 5,10,15,20-Tetraphenyl-21H,23H-porphine zinc(II)/Bromophenol blue #4 5,10,15,20-tetraphenyl-21H,23H-porphine cobalt(II)/Bromophenol blue #5 Bismuth neodecanoate #6 Lead (II) napthalanate #7 Lead (II) 2-ethylhexanoate #8 Copper (II) neodecanoate #9 TBAOH solution #10 HgCl2 + Bromophenol Blue + TBAOH #11 N,N,N’N’-tetramethyl-p-phenylenediamine #12 Bromothymol Blue + TBAOH #13 Zn(II)acetate + m-cresol purple + TBAOH #14 Pyrocatechol violet + TBAOH #15 Xylenol orange disodium salt + TBAOH #16 o-phenylenediamine #17 2,4-Dinitrophenylhydrazine/ phenyl red #18 2,4-Dinitrophenylhydrazine/ acridine orange #19 Thymol blue #20 N-phenyl-1, 2-phenylenediamine * Spot numbering from left to right, top to bottom for 2×10 array

The printed sensor arrays were stored under nitrogen for at least three days prior to any sensing experiments. For the sensing experiments, the relevant gas streams were generated below


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or at their permissible exposure limits (PEL) by diluting premixed gas tanks with dry nitrogen using a series of mass flow controllers. Final gas concentrations were verified with an FTIR gas analyzer (MKS Instruments MultiGas 2030).

Chemical Resistance Tests. Chemical resistance of the coated substrates was tested against the seven common organic solvents, one acid, and four alkaline solutions by measuring color changes with a digital camera. For an easy comparison, 12 pieces of substrates (~1 cm × ~1.5 cm) was placed inside a glass Petri dish, and a few drops of selected chemicals were deposited on each substrate. In addition, an unexposed substrate was placed as a control. Chemical resistance tests were performed at temperature ranging from 20 oC to 50 oC for a period of 12 hours.

Material Characterization. Surface morphology of the coated substrates was analyzed by a scanning electron microscope (SEM, Hitachi S-4300) operating at 3 KV. For the SEM analysis, samples were fixed on a metallic holder using a conductive carbon tape. All substrates were cleaned with a plasma cleaner for 30 seconds to remove atmospheric dust. The samples were then sputter coated with 10 nm of gold/palladium (Au/Pd) to render them conductive. PFA nanoparticles size distributions were measured with dynamic light scattering (DLS) technique (Malvern Zetasizer Nano ZS90). The solution pH’s were measured with Mettler Toledo digital pH meter. The viscosities of PFA nanoparticle solutions were measured at 25 oC by rotational Brookfield LVDV-E viscometer. Specific surface area and mean pore volume of the PFA nanoparticles-packed nanoporous substrates were measured by the Brunauer, Emmett, and Teller (BET) analysis using a Belsorp-mini II apparatus (BEL-Japan Inc., Osaka, Japan) operated at −196 °C. The substrates were vacuum dried at room temperature for 24 hours before BET estimations.


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Qualitative and Quantitative Analysis of CSA. For all sensing experiments, CSA images were acquired with a flatbed scanner (Epson Perfection V600) with the following settings: 800 dpi, 100 brightness, 48-color bit depth. For each experiment, the CSA was incubated under a stream of nitrogen for five minutes prior to the chemical exposure. The sensor array was exposed for 15 minutes and the sensor images were acquired at 15 second interval. For each image, a 60dimensional vector of 16-bit RGB colors was extracted by taking the median of all pixels within a 10-pixel-radius disk centered in each of the 20 indicator spots. The last pre-exposure image was used as a reference image, and all subsequent images were then subtracted from it, creating a time series of color differences. The color difference maps were generated by expanding the RGB color range to 4-30 from 0-255.8 Each difference map is an average of multiple trials. Color difference digitalization was done with the R software with a customized software package. The Wards method was applied for hierarchical cluster analysis (HCA) as previously reported.29, 30 Furthermore, pixel-based spot analysis and ferret diameter-based spot circularity analysis were performed with R31and imageJ software.32


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RESULTS AND DISCUSSION Nanoparticle-packed Porous Substrates. For chemically resistant nanoporous substrate, a cellulose paper was dip-coated with a PFA polymer dispersion, which provided nanoparticlepacked porous substrate.

Bare cellulose papers consist of interconnected polydisperse

macropores as shown in the SEM image of Figure S1. The nanoporous membrane AQ-50 was conveniently prepared by dip-coating a cellulose paper in the 50 w/v% PFA dispersion as shown in Figure 1b. Using the 50 w/v% PFA dispersion solution resulted in many cracks as shown in Figure 1a, which may be ascribed to an excessive stress generated by the surplus surface tension of water (76 mN/m). The cross-sectional SEM image of AQ-50 (Figure 1c) revealed that PFA nanoparticles was only packed on the outer portion of the cellulose paper. Additionally, the PFA nanoparticles were interconnected with each other as shown in the high-resolution crosssectional SEM micrograph (Figure 1d).

Figure 1. (a) Low-magnification and (b) high-magnification SEM micrographs of AQ-50 substrate. (c) Low-magnification cross-section and (d) high-magnification cross-section SEM micrographs of AQ-50 substrate.


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Furthermore, the PFA nanoparticles or aggregates easily separated from the substrates due to a poor adhesion between the nanoparticles and the substrate. As such AQ-50 is not a feasible substrate for CSAs. In order to resolve this problem, we added an appropriate amount of acetone to the aqueous 50 w/v% PFA dispersion. Acetone has lower surface tension (25.1 mN/m) that would diminish the residual strain in the coating. More importantly, acetone caused PFA nanoparticles to partially swell, which would readily fuse in the coating process.

A PFA nanoparticles-based coating solution was prepared by diluting commercially available aqueous PFA suspension with acetone. As the acetone concentration increased, the PFA nanoparticles swelling also increased proportionally. Average size and size distribution of PFA nanoparticles increased from 212.0 nm to 223.8 nm, 327.9 nm and 351.0 nm for 4:1, 2:1 and 4:3 mixtures of 50 w/v% PFA suspension and acetone (Figure S2a). When acetone was added to the PFA solution, the resulting solutions became more turbid for 4:1 and 2:1 mixtures (Figure S3). At higher acetone concentrations, the solution became either semi-gel for 4:3 mixtures or gel for 1:1 mixture (Video S1) after two hours. With the 4:1 mixture, the substrate was sparsely coated (Figure S4). In contrast, 4:3 and 1:1 mixtures were too viscous to dip-coat the cellulose paper (Figure S3, Video S1). Therefore, the 2:1 mixture was selected for further evaluation, which provided uniform and dense-packed coating on macroporous cellulose substrate (Figure S3, Video S1).

As summarized in Table 3, we measured the viscosities of parent PFA solutions (20, 30, 40, and 50 w/v%) and two hours after acetone addition. As can be predicted in the Einstein equation (ߟ = ߟ଴ ሺ1 + 2.5߶ሻ), the solution viscosity increased as the acetone concentration increased.33, 34


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As nanoparticles swell, the effective viscosity of the suspension increases as the volume fraction increases. Interestingly, for concentrations greater than 30 w/v% of suspension, the increase of effective viscosity was much more than one for the 20 w/v% of solution, which may be ascribed to shear thickening due to shear-induced agglomeration of soft swollen particles.

Table 3. Viscosity of PFA nanoparticles dispersion after acetone treatment at 25 oC. Initial PFA concentration (w/v%) 20 30 40 50

Viscosity (mPa⋅s) 3.2 3.7 3.8 4.1

Viscosity after acetone addition (mPa⋅s) 3.6 4.5 4.9 5.2

Four aqueous PFA nanoparticle dispersions (20, 30, 40 and 50 w/v%) were mixed with acetone in 2:1 ratio as shown in Figure S5. Original particle size was about 190 nm as measured by a dynamic light scattering (DLS) instrument. However, average particle sizes increased with acetone, except for the 20 w/v% solution as summarized in Table 4 and Figure S2b-e. Similarly, viscosity values showed a similar trend as summarized in Table 3. For all four concentrations, no significant change was observed in average particle sizes (Table 4) and the solutions viscosity (Table 3) even when the aging period was extended from 2 hours to 24 hours. Therefore, we conclude that acetone swelling equilibrates within two hours. Table 4. PFA particle sizes in aqueous and acetone treated dispersions. Initial PFA Average particle size Average particle size concentration 2 hours after acetone 1 day after acetone (w/v%) addition1 (nm) addition1 (nm) 20 193.3 194.5 30 273.8 273.6 40 299.6 301.2 50 326.9 327.9


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1

PFA was mixed with acetone in 2:1 ratio.

Surface Texture and Morphology of Fabricated Substrates. Acetone-swollen PFA polymer dispersion was utilized for the fabrication of nanoparticle-packed porous substrates by a simple dip-coating method as illustrated in Figure 2. The swollen PFA nanoparticles gave rise to improved adherence of the PFA polymer nanoparticles to the cellulose paper and enhanced mechanical stability on the substrates. This approach eliminated the large cracks, which would otherwise form without the acetone treatment (Figure 1a). As with the AQ-50 substrate (Figure 1b), the acetone treated PFA polymer nanoparticles shrunken and interconnected after drying, which can be clearly observed in the SEM micrographs for AA-13 to AA-33 substrates (Figure 3 e-h). All swollen PFA nanoparticles in the range of 200 to 328 nm (Table 4 and Figure S1) shrunk to approximately 160 nm after drying on the substrates. Also, the PFA nanoparticles coated only outer half of the cellulose paper. This observation can be seen from the cross-section SEM micrographs (×500) (Figure 3i-l). Moreover, all acetone treated substrates (AA-13, AA-20, AA-27, AA-33) showed aggregation and interconnectivity of polymer nanoparticles in the highresolution cross-sectional SEM micrographs (×20,000) (Figure 3m-p).


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Figure 2. Schematic overview of ‘nanoparticle-packed porous substrate’ fabrication process.

Figure 3. (a-h) Low-magnification (×500) and high-magnification (×50,000) SEM micrographs of AA-13, AA-20, AA-27 and AA-33 substrates. (i-p) Low-magnification cross-section (×500) and high-magnification cross-section (×20,000) SEM micrographs of AA-13, AA-20, AA-27 and AA-33 substrates.


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Sensor Array Printing. To compare print quality of each substrate, 12 different indicator formulations were printed on AQ-50, AA-33 and PVDF membranes. Table S1 provides the list of printed indicators. Printed spot sizes and shapes are significantly influenced by the pore size and structure of the substrate.13 Even though the AQ-50 substrate has nanoporosity, it was covered with many cracks (Figure 4a,d), which led to unpredicted flow of the ink, leading to a web-like spot as shown in Figure 4g. In contrast, a uniform circular spot was obtained on a PVDF membrane as previously reported.8

Figure 4. (a-c) High-resolution SEM micrographs of AQ-50, PVDF and AA-33 substrates. (d-f) Optical microscope images for AQ-50, PVDF and AA-33 substrates. (g-i) RGB line scans across the center of the spot # 5 (Methyl red). Insets are the magnified images of printed spot # 5 on AQ-50, PVDF and AA-33 substrates.


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While the bare cellulose paper and AQ-50 showed poor print quality, AA-33 substrate had a high print quality comparable to that of PVDF. High-resolution SEM micrographs of AA-33 substrate demonstrates uniform nanoscale porosity throughout the substrate surface (Figure 4c). Tightly packed PFA nanoparticles coated macropores of the cellulose paper. This nanoporous and uniform texture of the AA-33 substrate provided controlled wetting. In addition, the printing performance for the AA-33 substrate found to be uniform for spot size and consistency across the sensors. However, the AA-33 substrate still contained uncoated cellulose surface, which had more rough surface than the PVDF membrane (Figure 4e-f), and it affected printed spot smoothness (Figure 4h-i). Successive printing consistency of the AA-33 substrate is summarized in the supporting information (Figure S6). In particular, the circularity of printed spot #1, #4, #5, #6, and #8 was calculated using the ImageJ software (Table S2). The circularity of printed spots on the AA-33 substrate perceived in the range of 0.850 to 0.950 as calculated by ferret diameter.35

Advantageously, spot diameter can be controlled with the coating formulation as shown in Figure 5. As the PFA solution concentration increased, the spot size decreased proportionally due to increasing coverage of the PFA polymer nanoparticles on the cellulose paper (Figure 3ad). With the AA-13 to AA-33 substrates, the cellulose paper was not completely covered and partially exposed cellulose surface allowed more facile indicators spreading. This effect can be quantified with line scan profiles of printed spots as shown in Figure 5f-i. The close-up views for spot #5 also provided better visualization of the concentration effect on the spot size (insets


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of Figure 5f-i). This advantageous factor provides the ability to design CSAs with the required spot size by simply controlling the PFA nanoparticles concentration. At low concentrations, limited artifacts around the boundary of the spots on the AA-13 and AA-27 substrates. Therefore, it was necessary to add additional amount of acetone to minimize the surface tension stress for the AA-13 to AA-27 substrates to obtain equivalent spot size and similar printing results as the AA-33 substrate. Moreover, controlling aggregation is another criterion for a better coating of PFA nanoparticles over cellulose paper.

Figure 5. Printability in AA-13, AA-20, AA-27 and AA-33 substrates. (a) Schematic presenting spot positions in 3×4 printed array of four substrates. (b-e) Scanned images for (3×4) printed arrays. (f-i) RGB line scans across the center of spot # 5 (Methyl red) for each array. Insets of figure (f-i) consist of magnified images of printed spot # 5.

Chemical Resistance Test. Chemical resistance of the printing substrates is an important aspect of making CSAs with a prolonged shelf-life and reproducible sensor responses. In order to test the chemical resistance of the coated substrates, bare cellulose, AQ-50, PVDF and AA-33 substrates were assessed for their chemical resistance. The shortlisted chemicals for the


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resistance test included seven organic solvents and five acidic and basic solutions commonly used in the colorimetric indicator formulations.23 Chemical resistance was assessed by measuring color changes as an outcome of chemical reactivity, which were evident by the photographs taken before and after the chemical resistance tests as shown in Figure 6. Table 5 provides the list of chemicals used for a chemical resistance test, in which substrate numbers correspond to the labels in Figure 6a.


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Table 5. List of chemicals used for chemical resistance test. Substrate No. Chemicals 1 Acetone 2 Propylene glycol monomethyl ether acetate 3 Tetrahydrofuran 4 Cyclohexanone 5 1,2 Dichlorobenzene 6 2-Pentanone 7 2-Methoxyethanol 8 1.0 M TsOH in 2-methoxyethanol 9 1.0 M Sodium hydroxide in 2-methoxyethanol 10 1.0 M TBAOH in 2-methoxyethanol 11 50 wt% TBAOH in propylene glycol monomethyl ether acetate 12 Triethylamine TsOH: p-Toluenesulfonic acid; TBAOH: Tetrabutylammonium hydroxide

Figure 6. Digital photographs acquired in 12 hours after chemical exposure at 20 oC to 50 oC for (a-d) bare cellulose paper, (e-h) AQ-50 substrate, (i-l) PVDF membrane and (m-p) AA-33 substrate. Samples with visible chemical degradation are outlined in red.


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The AA-33 substrate showed no visible chemical degradation against every tested chemical at the temperature ranging from 20 oC to 50 oC (Figure 6m-p). The pH values of acid and base solutions were evaluated to know the acidic and basic strength of chemicals. Measured pH values of variable concentration acids and base solutions provided in supporting information Table S3. While the bare cellulose paper and the AQ-50 substrate also passed the chemical resistance test, they lack the printability required for the CSA applications. On the other hand, the PVDF membrane, which are often used as the base substrate for CSAs, was not chemically compatible with strong alkalines (spot #9, #10, and #11), which restricts the selection of indicators in designing the sensor. The reactivity of the PVDF membrane with sodium hydroxide (NaOH) was the subsequent results of the mechanochemical solid-phase response by substitution of fluorides on the PVDF polymer with the hydroxide.36-38 The reactivity of NaOH with PVDF can be visualized by discoloration of PVDF, which increases with the temperature.38 Tetrabutylammonium hydroxide (TBAOH), which is a strong organic base, also demonstrated similar reactivity (Figure 6i-l). By contrast, triethylamine, another organic base, did not show any chemical degradation with all evaluated substrates (Figure 6). Exceptional chemical steadiness of our designed AA-33 substrate against the chemical attack of frequently used acidic or basic additives in colorimetric indicator formulations makes it an advantageous substrate for CSAs.23

Detection of Toxic Industrial Chemicals (TICs). The gas sensing experiments were performed with the CSAs printed on the PFA polymer coated substrates. The printed CSA consist of a diverse set of chemically responsive dyes, which includes pH indicators, redox-sensitive dyes,


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metalloporphyrins, metal salts, nucleophilic dyes and strong bases for detection of screened TICs22, 23, 39-45. Four different TICs namely hydrogen sulfide (H2S), ammonia (NH3), nitrogen dioxide (NO2) and sulfur dioxide (SO2) were evaluated at or below their PEL concentrations.46 The type of screened chemicals responsive dyes and their chemical reactivity with four screened TIC analytes to produce a color response is provided in Table 6 below.

Table 6. Type of dyes used for CSA and their chemical reactivity with H2S, NH3, NO2 and SO2 Spot Dye Type Reactivity #1 Redox-sensitive dyes Redox reaction #2 Redox-sensitive dyes Redox reaction #3 Metalloporphyrins/pH indicator Ligation and pH #4 Metalloporphyrins/pH indicator Ligation and pH #5 Organometallic Chelation with reduced sulfur compounds #6 Organometallic Chelation with reduced sulfur compounds #7 Organometallic Chelation with reduced sulfur compounds #8 Organometallic Chelation with reduced sulfur compounds #9 Strong base pH and Lewis acidity/basicity #10 Metal salt/ pH indicator Metal complexation #11 Redox-sensitive dyes Redox reaction #12 Base treated pH indicator Acid indicator #13 Metal salt/ pH indicator Metal complexation #14 Chelating dye Metal chelation and pH #15 Chelating dye Metal chelation and pH #16 Redox-sensitive dyes Redox reaction #17 Nucleophilic dye/ pH indicator Nucleophilic addition and pH #18 Nucleophilic dye/fluorescent dye Nucleophilic addition and pH #19 pH indicator pH #20 Redox-sensitive dyes Redox reaction * Spot numbering from left to right, top to bottom for 2×10 array

All four PFA nanoparticle-packed substrates shows fairly high sensitivities towards the selected TICs (Figure 7). AQ-50 substrate was not evaluated due to its brittle nature that delaminates PFA nanoparticles during substrate handling. In addition, several spots on the cellulose paper merged


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with the neighboring printed spots, making it impractical to use the cellulose paper-based sensors (Figure S7). 47 Remarkably, in quadruplicate runs, all four TICs accurately classified without any misclassification with the AA-13 to AA-33 substrates-based CSAs (Figure 7). Further, a PVDF based sensor had much weaker response to SO2 at 5 ppm. Hierarchical cluster analysis (HCA) was performed for each sensor type using response data in the 60-dimensional RGB color space (Figure 8a-e). This method used to group the TICs (H2S, NH3, NO2, and SO2) in a hierarchical manner and calculate the distance between analytes by square Euclidean distance.28 Furthermore, principal component analysis (PCA) was performed to reduce the dimensionality of sensing array response dataset by decomposing the data set into eigenvectors and eigenvalues.48 PCA was further used to discriminate and to visualize the data as shown in Figure 9.

Figure 7. Color difference maps for AA-13 to AA-33 and PVDF based CSAs, which include H2S, NH3, and NO2 below their PEL concentrations and SO2 at PEL concentration. Color difference maps visualized by the expansion of RGB color range from 4-30 to 0-255.


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Figure 8. (a-e) Hierarchical cluster analysis of (a) PVDF membrane, (b) AA-13, (c) AA-20, (d) AA-27 and (e) AA-33 substrate based colorimetric sensor array responses for H2S (2 ppm), NO2 (25 ppm), NH3 (0.5 ppm), and SO2 (5 ppm).

PCA plots demonstrated close clustering of repeated trials (Figure 9). A separation between the data points from different classes is also observable in Figure 10. Interestingly, the separation between the data points for different analytes (NH3, H2S, and SO2) was perceived minimum with the PVDF based sensor (Figure 9a). The similar observation is evident in the HCA dendrogram (Figure 8a). Moreover, NO2 had substantially different response than other analytes (NH3, H2S, and SO2) with the PVDF-based CSA. The separation of representative clusters belongs to NH3, H2S, and SO2 increased in order of AA-13 < AA-20 < AA-27 < AA-33 (Figure 10b-e). However, SO2 and NO2 analyte clusters found to be less diverse from each other. Still, they remained separate from each other in PCA graphs (Figure 9b-e).


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Figure 9. Principal component analysis score plot of (a) PVDF membrane, (b) AA-13, (c) AA20, (d) AA-27 and (e) AA-33 substrate based colorimetric sensor array responses for H2S (2 ppm), NO2 (25 ppm), NH3 (0.5 ppm), and SO2 (5 ppm).

The data analysis also revealed that the colorimetric responses were correlated to the PFA polymer concentrations. At low PFA polymer concentrations, the coated substrate (AA-13 to AA-27) had reduced number of interconnected pores due to low nanoparticle density over coated cellulose paper (Table 7 and Figure S8). However, at a higher PFA concentration, completely interconnected particles over the cellulose paper was obtained (AA-33) (Table 7 and Figure S8). Sensor performance was influenced by the concentration of the nanoparticles covering the base cellulose paper (Figure S8). The resulting variations on the nanoporosity and surface area lead to the different sensing responses (Figure 7b-e). Nanoporosity supported higher surface areas of


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AA-33 showed improved colorimetric sensing performance (Figure 7) in comparison to AA-13 to AA-27 and PVDF membrane.

Figure 10. Nitrogen adsorption-desorption isotherm plots for PFA nanoparticles-packed AA-13, AA-20, AA-27 and AA-33 substrates.

Table 7. BET surface area and mean pore diameter for PFA nanoparticle coated substrates. Entry Substrate Surface area Mean pore diameter 2 (m /g) (nm) 1 AA-13 5.99 60.494 2 AA-20 6.45 48.503 3 AA-27 6.99 44.534 4 AA-33 7.29 42.806 PFA nanoparticles-packed AA-13 to AA-33 substrates displayed the type III IUPAC isotherm (Figure 10). Our fabrication process for PFA polymer coated substrates involves a concentrationdependent coating method. Low concentration solution yielded PFA polymer coated substrates with large pores while high concentration solution provided the coated substrates with small pores. The observation can be evident from the obtained mean pore diameter values (60.494 nm, 48.503 nm, 44.534 nm, and 42.806 nm) for each prepared sample (Table 7). However, BET


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study for AA-13 to AA-33 revealed other pores sizes along with mean pore diameter values in the range of 10 nm to 120 nm (Figure S8). A dropping in mean pore sizes of the substrates outcomes into higher surface areas (5.99 m2/g, 6.45 m2/g, 6.99 m2/g, 7.29 m2/g) can be seen in Figure 10 and Table 7. Analyzed BET surface area values for AA-13 to AA-33 substrates were not significantly different because of similar fabrication conditions instead of a single change in coating solution concentration.

The improved colorimetric sensing results of PFA nanoparticles coated porous cellulose substrates for four TICs detection was the outcome of the nanoporous nature of PFA nanoparticles coated substrates (Figure 3e-h).

Nanoporosity of PFA substrates and their

improved surface area through interconnected pores provides more interaction of TICs with chromogenic indicators over the surface of the substrate during sensing experiment. Excellent sensing performance and chemical stability with co-solvent based improved mechanical strength of AA-33 substrate make it as an ideal candidate to use as a substrate for colorimetric gas sensing applications.

CONCLUSIONS An aqueous PFA polymer nanoparticles solution coated AQ-50 substrate had micron size cracks on the surface due to a high surface strain of water. Addition of a suitable amount of acetone caused the swelling induced aggregation of the PFA nanoparticles, which mitigated the cracking problem. Aggregation of the blend is controllable with the amount of acetone ratio and the solution aging time. This co-dissolvable treatment was utilized to minimize the surface tension of


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the solution and manufacture of more mechanically stable and uniform nanoporous film on the cellulose filter paper. Further, it is conceivable to control the printed spot size by adjusting the concentration of PFA polymer nanoparticles solution. Our designed substrate observed steady against the chemical reactivity with screened organic solvents, acidic and basic solutions at working temperatures from 20 oC to 50 oC contrasted with commercial PVDF membrane utilized before for colorimetric sensor array application. Acetone-swollen PFA nanoparticles-based AA33 substrate demonstrated superior response to the detection of four different toxic industrial chemicals (H2S, NO2, NH3, and SO2). The wide accessibility of chemically inert PFA polymer nanoparticles dispersion and simplicity of substrate manufacturing by dip-coating strategy has an enhanced future ahead to raise the utilization of this substrate for fabrication of colorimetric sensor array.


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AUTHORS INFORMATIONS Corresponding Authors *Email: [email protected], [email protected] Notes The author declares no competing financial interest

AKNOWLEGEMENT We acknowledge support from the by the National Research Foundation of Korea (NRF) (NRF2017R1A5A1070259, NRF-2018M3D1A1058624 and NRF-2014M3A9B8023471).

ORCID ID Virendra S. Patil: 0000-0002-1105-2100 Myung-Goo Lee: 0000-0001-9145-7986 Gi-Ra Yi: 0000-0003-1353-8988


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Chemically Resistant Perfluoroalkoxy Nanoparticle-Packed Porous Substrates and Their Use in Colorimetric Sensor Array Virendra S. Patil,† Myung-Goo Lee,† Jaesub Yun,‡ Jong-Seok Lee,‡ Sung H. Lim,*,†,§ and Gi-Ra Yi*,†


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