Paper-Based Assay for Ascorbic Acid Based on the Formation of Ag

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Paper-Based Assay for Ascorbic Acid Based on the Formation of Ag Nanoparticles in Layer-by-Layer Multilayers Yuki Tokura, Yukari Moriyama, Yuki Hiruta, and Seimei Shiratori ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01782 • Publication Date (Web): 13 Dec 2018 Downloaded from http://pubs.acs.org on December 18, 2018

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Paper-Based Assay for Ascorbic Acid Based on the Formation of Ag Nanoparticles in Layer-byLayer Multilayers Yuki Tokura,† Yukari Moriyama,† Yuki Hiruta, † and Seimei Shiratori*,†

†Center

for Material Design Science, School of Integrated Design Engineering, Keio

University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522 Japan

Corresponding Author: [email protected]

ABSTRACT A simple, portable and cost-effective paper-based colorimetric assay is developed for ascorbic acid (AA) with excellent properties, such as fast response, high sensitivity, high selectivity and good stability. AA is an important nutrient for human life and adequate quantification of AA is required for controlling the intake of AA for health management. Moreover, point-of-care analysis, which is a simple, easy-to-use and cost-effective analysis method for unskilled users, has been a focus in the medical and health care fields. We fabricate a colorimetric paper-based assay with poly(allylamine hydrochloride) (PAH)/poly(acrylic acid) (PAA) layer-by-layer self-assembly containing silver ions for the point-of-care analysis of AA. The color of the assay is changed from white to yellow, and then to dark-brown after immersion in AA solution because of the reduction of silver ions to

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silver nanoparticles by AA and the localized surface plasmon resonance of the generated silver nanoparticles. The assay shows a fast response to AA of only 60 sec and a sensitivity to AA (limit of detection is 0.4 ppm). Furthermore, the colorimetric assay exhibits a high selectivity for AA compared with other compounds contained in human fluid and various drinks. The concentration of AA in various commercial products is quantified by the assay with the same accuracy as high-performance liquid chromatography. The design of the PAH/PAA multilayers containing silver ions for a fast response, highly sensitive and selective colorimetric assay will contribute to the development of point-of-care analysis for the early detection of diseases as well as controlling the intake of nutrients.

KEYWORDS: layer-by-layer, silver nanoparticle, colorimetric sensor, paper-based assay, ascorbic acid

INTRODUCTION Ascorbic acid (AA), which is known as vitamin C and exist in physiological fluids, is an important nutrient for the metabolic process of human. Nevertheless, humans must achieve an adequate intake of AA from only eating or drinking, such as from foods, drinks and tablets, as they are unable to produce AA themselves.1, 2 A deficiency of AA causes scurvy which caused bleeding and impaired wound healing.1-5 Furthermore, an excess intake of this nutrient also causes gastric irritation and renal disorder.6 Conversely, AA plays significant roles in the treatment or prevention of various diseases, such as common cold, scurvy, cancer, Alzheimer’s disease and various kinds of infections because of these strong antioxidant

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property that protect human body from oxidative stress.2, 3, 6 Therefore, an accurate and rapid quantification of AA is required for the accurate intake of AA and the control of nutrients in the human body. There are several methods for the detection and quantification of AA in solution, including liquid chromatography,7 electrochemical devices,3-5, 8, 9 optical devices1012

and colorimetric sensors.6,

13-21

Recently, point-of-care (POC) diagnostics have been a

focus in the medical and health care fields as a simple and cost-effective monitoring method of chemicals by unskilled home-users.22-24 Therefore, though liquid chromatography, electrochemical and optical devices show high sensitivity and selective detection, they are not applicable for POC diagnostics because they involve laboratory operation, large size, high cost and require expertise for use. Conversely, colorimetric-based sensors or test assays for the detection of AA have been reported as simple, rapid and cost-effective methods for POC analysis. For example, colorimetric sensors for AA based on a metal nanoparticles growth solution have been reported.6, 13, 14 Metal nanoparticles show a unique optical property of scattering light of specific wavelengths that is called localized surface plasmon resonance (LSPR).25-28 Especially, the scattering wavelengths of silver and gold nanoparticles are in the range of visible light. Moreover, the scattering wavelength of LSPR can be changed by the growth of metal nanoparticles, tuning size, shape and aggregation of metal nanoparticles with interaction of nanoparticles and analytes; therefore, target analytes can be detected by a color change with LSPR. In general, AA, which is a reducing material, can be detected by the growth of metal nanoparticles in a solution and the color change of the solution, which are caused by the change of wavelength or/and intensity of LSPR of the metal nanoparticles.6, 13, 14

However, because these methods are based on solution, they do not have portability and

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are difficult to use of POC analysis. Conversely, various membrane-based (mainly paperbased) colorimetric assays have been widely reported to detect various analytes like toxic chemicals, nutrients and metal ions for POC analysis.15-21, 29-35 Especially, membrane-based colorimetric assays, such as polyaniline/polyamide 66 nanofibers15 and membranes that consist of a membrane, metal oxide nanoparticles and metal ion,16-21 were reported to be simple, portable and easy-to-use sensors to detect AA. However, these membrane-based assays still have the drawbacks of low sensitivity and selectivity to AA and color unevenness, as well as slow response time. The layer-by-layer (LbL) self-assembly technique is a useful method for fabricating uniform and thickness-controllable polyelectrolyte films on various substrates through the adsorption of alternating polyanions and polycations by electrostatic force,36-38 and can be widely applied to chemical sensors for the detection of various chamicals.39-41 Recently, a LbL film containing a metal ion by coordination of the metal ion and polyelectrolytes has been reported.42-47 In these films, we reported a novel method to fabricate poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA) LbL multilayers containing a metastable metal ion through coordination between the amine groups of PAH and metal ions.45 These coating films have been applied for various applications, such as slippery film46 and gas sensors,47 by using the redox property of the metal ion contained in PAH/PAA LbL multilayers. Since the metastable metal ion exhibits a redox reaction with reducing agents, they are promising materials for the highly sensitive and selective detection of AA because AA is a reducing agent. In this study, we fabricated a PAH/PAA LBL film containing silver ions on filter paper and applied this as a paper-based assay for the sensitive, selective and fast detection of AA (Scheme 1). This assay showed a color change from white to dark-

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yellow or black after detection of AA, owing to the LSPR of silver nanoparticles from the silver ions contained in the LbL film by the redox reaction with AA. This enabled a highly sensitive and selective detection of AA by the fast color change. The design of the LbL film with silver ions on filter paper will have the potential for application in point-of-care diagnostics as health management in daily life.

Scheme 1. Schematic representation of approaches for the detection of AA by a paper-based colorimetric assay.

EXPERIMENTAL SECTION Materials. PAH (MW = 120,000–200,000, Polysciences, Warrington, PA, USA) and PAA (MW = 250,000, 35% aqueous solution, Sigma-Aldrich Co., St. Louis, MO, USA) were used for the fabrication of the LbL film. Poly(diallyldimethylammonium chloride) (PDDA, MW = 200,000–350,000, 20% aqueous solution, Sigma-Aldrich) and poly(4-styrenesulfonic acid) (PSS, MW = 70,000, Sigma-Aldrich) were used for the fabrication of an LbL buffer layer on the paper substrate. Silver acetate (Wako Pure Chemical Industries, Ltd., Osaka, Japan) was used as the source of silver ions for the LbL films. L-ascorbic acid (AA, Junsei Chemical Co., Ltd., Tokyo, Japan) was used as the target analyte in the detection test. Filter paper (diameter

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of 110 mm, pore size of 3 m, thickness of 0.18 mm, GE Healthcare UK Ltd., Buckinghamshire, England) were used as assay substrates and glass substrates (76 × 26 mm, thickness of 1.0 mm, Matsunami Glass Ind., Ltd., Kishiwada, Japan) were used for optical analysis. Fabrication of PAH/PAA LbL film containing silver ions. PAH, PAA, PDDA and PSS solutions of 10 mM were respectively prepared with pure water. The pH values of the solution were adjusted to 11.5 for PAH and 9.0 for PAA using a 0.25 M NaOH solution. The pH values of PDDA and PSS solutions were not adjusted. LbL films were fabricated with an automatic LbL fabrication machine (Nano Film Maker, SNT Co., Tokyo, Japan). The paper substrate was immersed three times in a cationic PDDA solution and anionic PSS solution for 5 min and then rinsed with water (one time for 2 min and two times for 1 min each) after deposition of each layer for the fabrication of the buffer layer and dried in room temperature. After fabrication of the buffer layers, the paper substrate was immersed in cationic PAH solution and anionic PAA solution for 5 min and then rinsed with water (one time for 2 min and two times for 1 min each) after deposition of each layer. This procedure was repeated several times. After the deposition of the LbL multilayer, the sample was dried at room temperature for one night and subsequently dried at 60 oC for 1 h under vacuum condition for removing the water reside. To fabricate the PAH/PAA multilayer on a glass substrate, PAH and PAA were coated in the same way after glass substrate was rinsed in water for 10 min with ultrasonication. After the deposition of the LbL multilayer, the sample was dried at 60 oC for 30 min under a vacuum condition. We described the PAH/PAA multilayers as (PAH/PAA)n, where n is the number of bilayers in the multilayers. The paper substrate was cut into a circle with a diameter of 11 mm. After preparation of PAH/PAA films, each

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substrate was immersed in several concentrations of silver acetate aqueous solution for 4 h and dried at room temperature. We expressed PAH/PAA multilayers containing silver ions as (PAH/PAA)n-Agm, where n is the number of bilayers in the multilayers and m is the concentration of silver acetate solution (unit is mM). Detection measurement of AA with LbL films. Each sample of the paper substrate and the glass substrate was immersed in AA aqueous solution. After immersion in the AA solution, the substrate was rinsed with water and dried at room temperature. After drying for a day, the optical property of the glass substrate was measured by a UV-vis spectrophotometer (UV3600Plus, Shimadzu Corporation, Kyoto, Japan). The filter paper was photographed by using a scanner (Lide 220, Canon Inc., Tokyo, Japan) to remove the effect of the light by the environment. Then, the red, green and blue (R, G and B) digital values were measured by the image J software. R, G and B values expressed the intensity of red, green and blue where (R, G, B) = (255, 255, 255) meant the color of the image was white. Conversely, (R, G, B) = (0, 0, 0) meant the color of the image was black. We calculated the −RGB value (255 − RGB value after immersion in the AA solution) as the sensor response because the initial color of the paper-based assay was white before immersion in AA solution. Therefore, the color change from white to black meant a high −RGB value. The error bars were obtained from standard deviation calculated by the results of same experiments with three fabricated paperbased assays. For the quantification of AA in commercial products, the fabricated colorimetric paper-based assay was immersed in 100 mL of each product. After immersion and drying at room temperature, the filter paper was photographed and the R, G and B digital values were measured by the image J software.

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Analysis of PAH/PAA LbL film. The morphology of the fabricated films was observed by field emission scanning electron microscopy (FE-SEM, Sirion, FEI Company, Tokyo, Japan) and FE-SEM with secondary electron detection and back-scattered detection (MERLIN Compact, ZEISS, Oberkochen, Germany). The surface elemental constituents of the fabricated films on the filter paper after immersion in the AA solution were determined by using energy dispersive X-ray spectroscopy (EDX) combined with MERLIN Compact and X-ray photoelectron spectroscopy (XPS, JPS-9010TR, JEOL, Tokyo, Japan) with a MgK laser.

RESULTS AND DISCUSSION Response of (PAH/PAA)7-Ag2 to AA and sensing mechanism. We used a paper-based assay with (PAH/PAA)7-Ag2 to investigate the color change of the paper filter with the films after immersion in AA solution and to clarify the response mechanism of the films to AA. The surface morphology of (PAH/PAA)7 on the filter paper was smoother compared with the surface of the bare filter paper (Figure 1). Also, the surface of the filter paper was filled with PAH and PAA after layer-by-layer deposition as shown in Figure S1. This result indicated that PAH/PAA multilayers were coated on the filter paper. Moreover, the surface structure of (PAH/PAA)7 did not change after immersion of the paper filter with the (PAH/PAA)7 film in silver acetate solution. The color did not change and kept white after fabrication of the (PAH/PAA)7 films and (PAH/PAA)7-Ag2 on the filter paper. After immersion of the filter paper with (PAH/PAA)7-Ag2 in 100 ppm AA solution, the color of the filter paper was changed from white to dark brown. Conversely, the color of the filter paper with (PAH/PAA)7 without silver ions did not change after immersion in 100 ppm AA solution

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(Figure S2). Furthermore, the filter paper of (PAH/PAA)7-Ag2 did not change color after immersion in water of pH 3.8, which was the same pH of the AA solution. Therefore, the fabricated paper-based assay with (PAH/PAA)7-Ag2 showed a response to AA in the aqueous solution and the silver ions in the PAH/PAA multilayers responded not to the pH in the solution but to the 100 ppm AA. From the SEM images of the filter paper with (PAH/PAA)7Ag2 after immersion into a 100 ppm AA solution, the nanoparticles with sizes of several tens of nanometers were observed on the filter paper with the LbL film. It was assumed that the nanoparticles were generated by the reaction between silver ions and AA.

Figure 1. SEM images and photographs of bare filter paper, (PAH/PAA)7 films on filter paper, (PAH/PAA)7-Ag2 on filter paper, and (PAH/PAA)7-Ag2 films on filter paper after immersion in an AA aqueous solution.

To further investigate the response for AA and determine the response mechanism of (PAH/PAA)7-Ag2 film to AA, the optical properties and chemical characteristics of the (PAH/PAA)7-Ag2 film were analyzed. Figure 2(a) shows the absorbance of (PAH/PAA)7Ag2 on the glass substrate after immersion in various concentrations of the AA solution for 20 seconds. From this result, the intensity of the peaks at around 420 nm gradually increased

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as the concentration of the AA solution increased. Moreover, when the concentration of the silver acetate aqueous solution for containing silver ions in the PAH/PAA multilayers was increased, the peaks at 420 nm also increased after immersion into the 100 ppm AA solution for 20 seconds (Figure 2(b)). In previous studies, it has been reported that the absorbance peak at 420 nm corresponded to the LSPR scattering peak of silver nanoparticles.25-27, 42 Therefore, because the intensity of the peaks was increased with an increasing concentration of AA or silver ions, it was proposed that the color change of (PAH/PAA)7-Ag2 was caused by the generation of silver nanoparticles through the reaction of AA and silver ions. From Figure S3, the surface of (PAH/PAA)7 and (PAH/PAA)7-Ag2 were flat, and aggregation of polymers or silver nanoparticles generated by oxygen were partly observed. After immersion in 100 ppm AA solution, approximately 100 nm silver nanoparticles were generated on the surface of (PAH/PAA)7-Ag2 multilayers, which was similar with the case of filter paper. However, the color of (PAH/PAA)7-Ag2 film on glass substrate were dark yellow after immersion in 100 ppm AA though that on filter paper was dark blown. It was assumed that these results were caused by the microstructure of the substrates and the difference of substrate color (glass substrate was transparent and filter paper was white).

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Figure 2. UV-visible absorption spectra of (a) (PAH/PAA)7-Ag2 on the glass substrate after immersion in various concentrations of AA solution and (b) (PAH/PAA)7, (PAH/PAA)7Ag0.5, (PAH/PAA)7-Ag2, (PAH/PAA)7-Ag5, and (PAH/PAA)7-Ag10 on the glass substrate after immersion in a 100 ppm AA solution.

Figure 3(a) and (b) show the surface morphology of (PAH/PAA)7-Ag2 after immersion into a 100 ppm AA solution measured by each secondary electron mode and backscattered electron mode of the MERLIN Compact SEM. In the back-scattered electron mode of the SEM, the larger the atomic weight, the brighter was the SEM image. The paper-based assay with PAH/PAA multilayers containing silver ions contained H, C, O, N and Ag atoms. This indicated that the bright nanoparticles in the SEM image were silver nanoparticles and not an aggregation of PAH/PAA because silver had the largest atomic weight in the used materials. From the SEM image of back-scattered electron mode under high magnification (Figure 3(c)), the distribution of the Feret diameter was analyzed by the image J software (Figure S4). The size of the nanoparticles was approximately 20–60 nm (Figure 3(d)). This

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result of the nanoparticles size was consistent with the optical property of LSPR scattering of silver nanoparticles, which was reported in a previous study.25-27, 42 To analyze the chemical components on the surface of (PAH/PAA)7-Ag2 after immersion in the 100 ppm AA solution, EDX analysis and XPS analysis were performed. From the EDX spectra of (PAH/PAA)7-Ag2 after immersion in the 100 ppm AA solution, a small peak attributed to silver was observed at around 3 keV (Figure 3(e)).48, 49 This peak was derived from silver nanoparticles on the surface of the (PAH/PAA)7-Ag2 films. In addition, from the XPS spectra of the filter paper with (PAH/PAA)7-Ag2 before and after immersion in the 100 ppm AA solution, the peaks in the Ag (3d5/2, 3/2) binding energy were slightly shifted to a higher binding energy after immersion in the AA solution (Figure 3(f)), which demonstrated that the silver ions in the PAH/PAA multilayers were reduced to silver after immersion in the 100 ppm AA solution.46, 47, 50 In summary, the color change mechanism after immersion in the AA solution consists of a redox reaction between a silver ion and AA, and LSPR scattering of the silver nanoparticles (Scheme 2). AA is well known as a reducing agent and is used in the synthesis of metal and metal oxide nanoparticles.6,

13, 14

Additionally, silver ions in the PAH/PAA

multilayers show a redox property with reducing agents and are reported to be reduced to silver nanoparticles in previous studies.42,

44, 46

Therefore, because silver ions in the

PAH/PAA LbL film showed a redox property with a reducing material, the silver ions in (PAH/PAA)7 multilayers were reduced to silver nanoparticles by AA after immersion of the filter paper with (PAH/PAA)7 multilayers containing silver ions into the AA solution. After preparation of silver nanoparticles on the surface of the filter paper, the color of the filter paper was changed to yellow or dark brown by the LSPR scattering of the silver nanoparticles.

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Moreover, it has been reported that the wavelength of LSPR scattering is changed by size, shape and aggregation of silver nanoparticles.25-27 Therefore, in the case of immersion in a high concentration AA solution, the color of the filter paper was changed to black because the size of the silver nanoparticles was larger than that at a low concentration of the AA solution and a lot of silver nanoparticles were aggregated on the filter paper.

Figure 3. Surface morphology of (PAH/PAA)7-Ag2 on the filter paper observed by (a) secondary electron mode and (b) back-scattered electron mode of SEM, and (c) magnified image of (b) and (d) Feret diameter distribution of nanoparticles on the filter paper with (PAH/PAA)7-Ag2 analyzed by the image J software. (e) EDX spectra of (PAH/PAA)7-Ag2 on the filter paper after immersion in a 100 ppm AA solution. (f) XPS spectra of (PAH/PAA)7-Ag2 on the filter paper before and after immersion in a 100 ppm AA solution.

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Scheme 2. Scheme of the response mechanism of the paper-based assay with (PAH/PAA) layer-by-layer self-assembly containing silver ions to AA.

Optimization of paper-based assay with PAH/PAA multilayers containing silver ions. For the fabrication of a paper-based assay with PAH/PAA LbL self-assembly containing silver ions for detection of AA, the effect of the response time, number of bilayers of PAH/PAA LbL self-assembly and concentration of silver acetate solution for containing silver ions on the color change were investigated. First, we investigated the dependence of the immersion time on the response time and the R, G and B digital value of (PAH/PAA)7Ag2. All values were decreased with increasing immersion time in the 100 ppm AA solution and became saturated after a response time of one minute (Figure 4(a)). At times longer than one minute, the color of the filter paper did not change. This result indicated that silver ions in LbL film had almost completely reacted with AA after approximately one minute. The reaction time of the fabricated paper-based assay was fastest in the colorimetric assays for AA as shown in Table S1 and this result indicated that the paper-based assay can detect AA immediately for POC analysis. In the subsequent experiments, the immersion time in AA solution was one minute.

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When the number of bilayers of the PAH/PAA LbL film was increased, a large color change of the filter paper with the PAH/PAA film containing silver ions was assumed to be observed because more silver ions were contained as the number of bilayers increased. However, the color change of the filter paper with (PAH/PAA)9-Ag2 was largest in the LbL film with the other bilayers (Figure 4(b)). In the case of (PAH/PAA)30-Ag2 which had most silver ions inside LbL multilayers, the color was gradually changed to dark-brown during the immersion in AA solution for 720 min, which was indicated that silver nanoparticles inside PAH/PAA multilayers were gradually reduced to silver nanoparticles (Figure S5). On the other hand, it took long time to generate silver nanoparticles because of the slow penetration of AA into LbL film. It was assumed that most of the silver nanoparticles were firstly generated on and near the surface of the film for several minutes, and subsequently silver ions inside LbL film were gradually reduced to silver nanoparticles. And, the color change of the paper-based assay was mainly related to the first response. Therefore, the color change was correlated with surface area of the paper-based assay. From the SEM image of the PAH/PAA film of 3, 7, 9, 15, 20, 30 bilayers with silver ions, the porosity of the filter paper was filled with the PAH and PAA polymers and the surface area of the filter paper was decreased as number of bilayers was increased (Figure S6). Therefore, the PAH/PAA film of 9 bilayers with silver ions showed the greatest response to AA owing to the effects of the number of silver ions and the surface area of the filter paper after coating of the LbL film with silver ions. According to the above results, the number of bilayers of the PAH/PAA LbL film was decided to be 9 bilayers. Finally, the effect of the concentration of the silver acetate aqueous solution for containing silver ions in the PAH/PAA multilayers was investigated. The color change of the

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filter paper with the (PAH/PAA)9 LbL film containing silver ions gradually increased as the concentration of the silver acetate solution increased (Figure 4(c)). Moreover, R, G and B values became saturated at over a 5 mM silver acetate solution. This indicated that the amine groups of PAH might almost be coordinated with silver ions in the over 5 mM silver acetate aqueous solution. When a concentration of 5 mM or more of silver acetate was used as the source of silver ions, the color change of the filter paper was almost the same because the number of silver ions in each PAH/PAA LbL film were the same. From the optimization of the parameters of response time, number of bilayers of PAH/PAA LbL film and concentration of silver acetate solution, a paper-based assay with (PAH/PAA)9-Ag5 showed a color change from colorless to dark brown after immersion into an AA solution for one minute.

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Figure 4. Optimization results of (a) response time, (b) number of bilayers of PAH/PAA multilayers and (c) concentration of silver acetate solution compared with the color change of the filter paper with PAH/PAA multilayers containing silver ions.

Sensing property of paper-based assay with PAH/PAA multilayers containing silver ions. There are various sensing factors to evaluate the sensing performances for detection of chemicals, for example, portability, response time, sensitivity, relationship between concentration of analytes and color change, selectivity, storage condition and reproducibility. These factors of the paper-based assay with (PAH/PAA)9-Ag5 were investigated. The 17



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fabricated assay showed portability because the assay was based on a small filter paper. Moreover, the response time of the fabricated assay was one minute, which was very fast, as shown in Table S1. Therefore, the fabricated paper-based assay showed good portability and a fast response. The paper-based assay with (PAH/PAA)9-Ag5 showed a nonlinear curve for the R, G and B values against the concentration of the AA solution from 0 ppm to 1000 ppm (Figure 5(a)). From Figure 2, the peak of localized surface plasmon resonance of silver nanoparticles corresponded to the absorbance peak of 420 nm, that is yellow. Therefore, R and G values were lower than B value in the range of from 0 ppm to 300 ppm. Moreover, the R, G and B values gradually decreased and the color was gradually close to black ((R, G, B) = (0, 0, 0)) as the concentration of the AA solution was increased from 0 ppm to 300 ppm because generated silver nanoparticles were aggregated in LbL multilayers. Therefore, the curve of the relationship between the R value and the concentration of the AA solution showed good linearity (R2 = 0.96). Above the region of 300 ppm, the color change of the paper-based assay was almost the same. These results demonstrated that almost all of silver ions in PAH/PAA LbL multilayers were reduced to silver nanoparticles over 300 ppm AA in aqueous solution. From the relationship between the concentration of AA solution and -R value, the equation was described as follows: ―∆R = 0.8C + 15.2

(1)

in which C is concentration of AA. -R was defined by R parameter (R) as following formula: ―∆R = 255 ― 𝑅

(2)

Therefore, the concentration of AA was shown as below C = 311.4 ― 1.3𝑅

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(3)

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From Figure 5(a), the error bars (standard deviation) of each plot were increased as increasing concentration of AA, and the largest error bar was 15.6 at 180 ppm of concentration. Therefore, the concentration of AA was shown as below C = (311.4 ― 1.3𝑅 ) ± 20.3

(4)

From this equation, the largest standard deviation of the concentration of AA was 20.3 ppm. Based on the above results, the concentration of AA in the solution was calculated by the R value of paper-based assay with (PAH/PAA)9-Ag5. Here, we investigated the selectivity of a paper-based assay with (PAH/PAA)9-Ag5 (Figure 5(b)). The fabricated paper-based assay responded to only 100 ppm AA and the color change of the paper-based assay was negligible after immersion in aqueous solutions of 100 ppm vitamin B1, fructose, glucose, calcium chloride, magnesium chloride, potassium nitrate, sodium sulfate, trisodium citrate, ethanol, caffeine, and albumin from bovine serum (BSA) which are mainly included in foods, drinks, blood and bodily fluid. From these results, the paper-based assay with (PAH/PAA)9-Ag5 showed excellent selectivity to AA over other nutrients and chemicals in the human body and demonstrated the potential for analysis of AA in foods, drinks, blood and bodily fluid. Finally, the stability of the paper-based assay with (PAH/PAA)9-Ag5 was investigated. The RGB values of the paper-based assay stored in a shaded metal box for several days were almost the same after immersion in a 100 ppm AA solution (Figure 6). This result demonstrated that the fabricated paper-based assay showed stability and reproducibility by storing the assay in a shaded box and preventing the reaction of silver ions. From this result, the fabricated paper-based assay can be stored at room temperature and at normal pressure.

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In summary, the paper-based assay with (PAH/PAA)9 containing silver ions showed excellent sensing properties, such as high sensitivity and selectivity to AA, very fast response and stable response compared with a membrane-based assay to AA in a previous report (Table S1). The fabricated simple, portable and easy-to-use paper-based assay will have potential for application in POC analysis of AA in various solutions.

Figure 5. (a) Relationship between concentration of AA solution and the color change of paper-based assay with (PAH/PAA)9-Ag5 after immersion in AA solution, and inset is the relationship between the concentration of AA solution from 0 ppm to 300 ppm and R value of the color change of the paper-based assay with (PAH/PAA)9-Ag5 after immersion in an AA solution. (b) RGB values of the paper-based assay with (PAH/PAA)9-Ag5 after immersion in 100 ppm AA, vitamin B1, fructose, glucose, calcium ion, magnesium ion, potassium ion and trisodium citrate solutions.

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Figure 6. RGB digital values of the paper-based assay with (PAH/PAA)9-Ag5 after storing in a shaded metal box and immersion in 100 ppm AA solution.

Analysis of AA in commercial products by paper-based assay with PAH/PAA multilayers containing silver ions. Although the HPLC had high reliability to adequately quantify concentration of AA at wide concentration range (Figure S7), the paper-based assay with (PAH/PAA)9-Ag5 also showed high reliability to analyze concentration of AA between 0 ppm and 300 ppm because of linearity of calibration curve (Figure 5(a)). Moreover, the paper-based assay with (PAH/PAA)9-Ag5 was used to directly detect AA in three commercial products, such as green tea, straight tea and oolong tea, and the calculated concentration of AA by the paper-based assay was compared with that obtained by HPLC (Analysis method by HPLC is described in Supporting Information). From this result, the color of the fabricated paper-based assay was changed by AA in samples and the calculated concentration of AA in green tea and straight tea from the color of the fabricated paper-based assay was almost the same as that determined by HPLC (Table 1). This demonstrated that the paper-based assay has the potential to quantify the concentration of AA in various drinks between 0 ppm and

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300 ppm by using the R value of the color. However, in the case of oolong tea with a concentration of AA over 300 ppm from HPLC, it was still challenging to quantify the concentration of AA because of the range of the calibration curve. Though it is challenging to widen the range of the concentration, the concentration of AA in various commercial tea products were calculated directly by the fabricated paper-based assay without diluting the commercial samples and in the future, the paper-based assay with a layer-by-layer film containing silver ions will be widely applied to POC analysis in the health care and medical fields.

Table 1. Concentration of AA in commercial products calculated by HPLC and fabricated paper-based assay.

CONCLUSIONS A paper-based colorimetric assay was fabricated with PAH/PAA layer-by-layer selfassembly containing silver ions for a portable, cost-effective and easy-to-use analysis of AA in solution. The paper-based assay showed an excellent performance to detect AA. The response time of the assay to AA was 60 sec, which was very fast. The assay showed high sensitivity for AA and the limit of detection was 0.4 ppm, based on a curve fit, and three times the standard deviation of the −R value of the blank samples. Moreover, this assay had high selectivity toward AA over several materials, which are mainly included in foods, drinks, blood and bodily fluid. The relationship between the R value of the paper-based assay and

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the concentration of AA showed linearity from 0 ppm to 300 ppm, which meant that it is possible to quantify the amount of AA in the solution. Actually, the concentration of AA in various commercially products was directly calculated by the fabricated paper-based assay between 0 ppm and 300 ppm. The mechanism of the color change from white to dark-brown of fabricated paper-based assay was the reduction of silver ions to silver nanoparticles by AA and the LSPR scattering of the generated silver nanoparticles. In the future, though it is challenging to widen the range of concentration, the paper-based assay, with the design of PAH/PAA layer-by-layer self-assembly containing silver ions on the filter paper, will be potentially applied to POC analysis of AA in various drinks and tablets. Furthermore, the design of the LbL film containing metal ions like not only silver ions but also gold ions or copper ions will be potentially used for the analysis of reducing materials such as AA.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/XXXXXXX. Low resolution SEM images of the paper-based assay before and after immersion in AA aqueous solution, a color change of the paper-based assay, SEM images of LbL film with silver ions on glass substrate, back-scattered electron mode of SEM of the assay after immersion in AA solution and the image after threshold treatment, RGB digital values of (PAH/PAA)30-Ag2 film after immersion in AA, SEM images of PAH/PAA multilayers as increasing number of bilayers, comparison of membranebased colorimetric sensor for AA, and quantification of AA in the commercial products by HPLC.

AUTHOR INFORMATION Corresponding Author

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E-mail: [email protected] Fax: +81-45-566-1602

ORCID Seimei Shiratori: 0000-0001-9807-3555 Yuki Hiruta: 0000-0001-7303-4189

Author Contributions Y. T. and Y. M. conceived and carried out the experiments. Y. T. designed equipment and wrote the paper. Y. H. provided experimental support and commented on the manuscript. S. S. supervised the project and commented on the manuscript.

ACKNOWLEDGMENT We are deeply grateful to Dr. Yosuke Tsuge and Dr. Kengo Manabe, whose insightful comments and suggestions were of inestimable value for our study. We thank Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript.

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For Table of Contents (TOC) graphic use only

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Paper-Based Assay for Ascorbic Acid Based on the Formation of Ag

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