A portable lateral flow distance-based paper sensor for drinking water hardness test

Yulin Liu; Longzhan Dong; Wenli Wu; Jiantao Ping; Jingbo Chen; Qiongzheng Hu

doi:10.1371/journal.pone.0308424

Abstract

Hardness is one of the basic parameters of water, and a high-level hardness of drinking water may be harmful to human health. Thus, it is very important to monitor drinking water hardness. In this work, a portable lateral flow distance-based paper sensor for the semi-quantitative detection of drinking water hardness is demonstrated. In the presence of Ca²⁺/Mg²⁺, the hydrogel can be formed via the chelation between sodium alginate and Ca²⁺/Mg²⁺, inducing a phase separation process. The viscosity change of the sodium alginate solution is directly related to the Ca²⁺/Mg²⁺ concentration and can be determined by the water lateral flow distance on test strips. The sensor successfully realizes the quantification of Ca²⁺ and Mg²⁺ in the range of 0–10 mmol L^-1 and 4–20 mmol L^-1, respectively. The recoveries are found varied from 95% to 108.9%. The water hardness is acceptable for drinking if the Cr values lies in the range of 0.259 to 0.419, and it is high with the Cr value above 0.595. Remarkably, the performance of the sensor is comparable with the commercial kit for real water samples, which avoids the subjective judgment. Overall, this method provides a portable approach for semi-quantitative detection of drinking water hardness with the merits of convenience and low cost, which shows great potential for the potential application.

Citation: Liu Y, Dong L, Wu W, Ping J, Chen J, Hu Q (2024) A portable lateral flow distance-based paper sensor for drinking water hardness test. PLoS ONE 19(9): e0308424. https://doi.org/10.1371/journal.pone.0308424

Editor: Elingarami Sauli, Nelson Mandela African Institute of Science and Technology, UNITED REPUBLIC OF TANZANIA

Received: January 30, 2024; Accepted: July 23, 2024; Published: September 6, 2024

Copyright: © 2024 Liu et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Data Availability: All relevant data are within the manuscript and its Supporting Information files.

Funding: Natural Science Foundation of Shandong Province (ZR2021QH106, ZR2020QB153, ZR2022YQ123); the Taishan Scholars Program (tsqn201812088); the Education and Industry Integration Pilot Project of Qilu University of Technology (2023PY058, 2022PY036) The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

Introduction

Water is an indispensable natural resource for human beings. Drinking water quality is vital to human health. Water hardness is one of the basic parameters for evaluating water quality, and a high-level drinking water hardness may cause diseases [1, 2]. Water hardness is generally referred to the sum content of calcium, magnesium, iron, aluminum, zinc and other ions contained in water, and usually calculated by Ca²⁺ and Mg²⁺ contents considering the lower concentrations of other ions, which is also called as Ca²⁺ hardness. Although the Ca²⁺ concentration in drinking water is not strictly defined, the World Health Organization recommends that calcium ion level in drinking water should be no more than 5 mmol L^-1 [3, 4]. Therefore, it is very important to detect Ca²⁺ hardness in drinking water samples.

Until now, several methods for monitoring Ca²⁺ have been reported despite its difficulty to be differentiated from other interfering ions. Among them, atomic absorption spectrometry and complexometric titration are the classic methods used for calcium quantification [5, 6]. However, they often suffer from the disadvantages such as complex procedures, bulky sample requirement, complicated instrumentation, and trained operators. For instance, the commercial colorimetric kit has already been accessible, which is developed based on the titration method. However, a large amount sample of 10 mL was generally required. Meanwhile, the color change of titration terminal is subjectively judged by operators, which may inevitably lead to systematic errors. Additionally, the fluorescence method is available for the simplified detection process with high sensitivity [7–9], but it still requires the usage of large-scale instrument and trained operators. Ionophore-based ion-selective optode also have been used for the colorimetric detection of Ca²⁺ [10, 11]. Nevertheless, the results are greatly influenced by the pH of the samples, which hampers their broad application. Thus, it is greatly demanded to construct a portable sensor for monitoring Ca²⁺ in water.

Paper-based detection methods have become appealing in recent years, with the merits of convenient operation, fast response, and easy modification. They are extensively applied in clinical diagnostics, food quality management, and environmental monitoring [12–15]. In particular, the distance-based paper sensor can quantify the analyte by measuring the change of water flow distance, which has shown great application prospect as point-of-care test, because of its advantages of simple portability, visualization, easy quantification, and short analysis time [16–18]. The pH indicator papers can be employed as test strips because they are cheap and can clearly show the water flow marks. In addition, analyte-responsive hydrogels are series of polymers with three dimensional network structures. Various hydrogels have been designed and developed for biosensing of metal ions [19, 20], nucleic acids, proteins [21], and microorganisms [22, 23]. With the specific experimental design, hydrogels can respond to analytes and induce physical or chemical changes that generate subsequent readable signals. Among these methods, the gel-sol transition triggered by external stimuli is a most commonly used detection principle [23–25]. Therefore, it possesses great potential for the exploration of the distance-based lateral flow sensor using stimuli-responsive polymers for the evaluation of drinking water hardness.

Herein, a portable lateral flow sensor with the distance readout signal for the semi-quantitative determination of drinking water hardness on the paper strip was developed (Scheme 1). In the presence of Ca²⁺/Mg²⁺, the sodium alginate (Alg) hydrogel network with “egg-box” structure can be formed during the phase separation process. Correspondingly, the viscosity of the Alg solution drops sharply due to reduction of the Alg concentration. Thus, the concentration of Ca²⁺/Mg²⁺ can be determined via the measurement of water lateral flow distance on paper strips of the residual Alg solution. This method offers a simple and convenient method for the water Ca²⁺ hardness evaluation using a small amount of samples with satisfacoty accuracy, which also avoids the subjective color endpoint judgment.

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Scheme 1. The principle of the portable distance readout paper sensor for evaluating drinking water hardness.

https://doi.org/10.1371/journal.pone.0308424.g001

Experimental

Materials

Sodium alginate (CP, viscosity 200 ± 20 mPa·s), (NH₄)₂S₂O₈ (AR, 98.1%), and NaHSO₃ (AR, 99.99%) were obtained from Macklin. Hydrochloric acid (AR) for pH adjustment was obtained from Sinopharm Group. NaH₂PO₄ (AR, 99.0%) and Na₂HPO₄ (AR, 99%) were purchased by Aladdin Technology Co., Ltd., China. Water hardness test kit was bought from Lohand Biological. Co., Ltd. The pH strips with the dimension of 60 mm × 5 mm (length × width) were used in this investigation. The PVC plates were cleaned by ethanol thoroughly and dried before use. Tris-HCl buffer (pH = 7.4) was offered from Sangon Biotech Co., Ltd., China.

Optimization of the sodium alginate concentration

Tris-HCl buffer (100 mmol L^-1, pH = 7.4) was used in this study. The Alg solutions with different concentrations from 0.1 wt% to 0.5 wt% were firstly prepared at 25°C. The solutions of CaCl₂ with various concentrations of 0, 2, 4, 10, 20 mmol L^-1 were then obtained. According to our previous study, the mixture of CaCl₂ and Alg solution was incubated at 25°C after vortex for 30s [26]. Then, 30 μL of the supernatant solution obtained by centrifuging for 1 min was transferred onto the left side of the test strip. After waiting for 2 min, the images of the paper sensor were captured by smartphone and analyzed by Adobe Photoshop software. All of the experiments were conducted at least three times to obtain the standard deviations.

Determination of the water hardness using the commercial test kit

Firstly, 10 mL of the test solution was accurately pipetted into the cleaned conical flask, followed by the addition of a package of total hardness reagent I. If the solution color is pure blue after reagent I dissolved, the hardness value of the water sample is 0 mg/L. If the solution exhibits purple red, hardness reagent II aqueous solution was then vertically added with the continuous shaking of the conical flask. The dropping speed should be controlled as 1 drop every 3 s until the solution changed from purple red to pure blue. Then, the number of drops consumed (N) was recorded and hardness (mg L^-1, calculated as CaCO₃, 1 mg L^-1 = 0.01 mmol L^-1) was calculated by the following equation, the detection range of this kit is 30–600 mg L^-1:

Data analysis

The pH indicator papers were photographed with a smartphone. Then, the pixel values of areas including water marked and whole test strips were obtained by Adobe Photoshop and recorded as P_mark and P_total, respectively. Finally, the ratio of P_mark and P_total was calculated as the water trace coverage ratio (Cr) as follows:

Results and discussion

Feasibility of the paper sensor for the evaluation of Ca²⁺/Mg²⁺

The responses of the paper sensor in different conditions were recorded. As shown in Fig 1, the flow distance of the Ca²⁺ solution was obviously longer than that of the 0.2 wt% Alg solution. The Cr values were 0.8 and 0.5, respectively. A similar phenomenon also occurred for the Mg²⁺ solution (S1 Fig). The difference was mainly ascribed to the viscosity discrepancy caused by Alg itself. Upon the Alg solution was incubation with the 10 mmol L^-1 Ca²⁺ solution, the Cr value increased to 0.65 with the longer water flow distance as shown in Fig 1A and 1B. The solution viscosity was increased obviously in the existence of Alginate compared with that of Ca²⁺ and Mg²⁺ existing alone in Fig 1C. These results are consistent with the proposed principle above. The viscosity decrease of the Alg solution was caused by the consumption of Alg via cross-linking with Ca²⁺/Mg²⁺ during the formation of the hydrogel [26–29]. Overall, this method provides an effective means to monitor Ca²⁺/Mg²⁺, which is candidate for the drinking water hardness determination.

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Fig 1. Feasibility of the paper sensor for the evaluation of Ca²⁺/Mg²⁺.

(A) The photopgraphs and (B) the Cr values of the paper sensor in the Ca²⁺ solution, the Alg solution, and the mixture of Alg and Ca²⁺, respectively. The concentrations of Ca²⁺ and Alg are 10 mmol L^-1 and 0.2 wt%, respectively. (C) The viscosities of Ca²⁺, Mg²⁺, Alg/ Ca²⁺ and Alg/Mg²⁺ solution, respectively.

https://doi.org/10.1371/journal.pone.0308424.g002

Optimization of condition for the paper sensor

The performance of Alg solutions at different concentrations on pH test strips was studied. As shown in Fig 2A and S2 Fig, with the concentration of Alg solution elevating from 0.1 wt% to 0.5 wt%, the water flow distance descended obviously due to the increasing viscosity. The concentration of Ca²⁺ from 0 to 20 mmol L^-1 was evaluated at a fixed Alg concentration of 0.1 wt%. Surprisingly, the water flow distance becomes shorter with the concentration increase of Alg from 0–4 mmol L^-1, while further distance increases with the Alg concentration from 4 to 20 mmol L^-1 (Fig 2B and S3 Fig). Moreover, the similar variation tendency was found for the solution and the mixture of Alg and Mg²⁺ (S1 Fig), respectively. The concentrations of Ca²⁺ and Alg were 10 mmol L^-1 and 0.2 wt%, respectively. Moreover, the similar variation tendency was observed with different concentrations of Alg (Fig 2C–2F). The results are reasonable considering the crosslinking process. At the initial stage, the addition of Ca²⁺ into Alg solution can cause the rapid formation of the hydrogel based on the chelation between them, leading to the increased viscosity. While the continued increase of the Ca²⁺ concentration contributed to increase of the cross-linking degree of the hydrogel, which was separated from the aqueous phase. As only the remainly Alg was in the aqueous phase, the viscosity is decreased with further increase of the Ca²⁺ concentration. Thus, the viscosity change can be coupled to the concentration change of Ca²⁺. As concluded from Fig 2, the fixed 0.2 wt% Alg solution was chosen for following experiments, because it can clearly distinguish different concentrations of Ca²⁺.

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Fig 2. Optimization of conditions for the paper sensor.

(A) The Cr values of the paper sensor responses towards the Alg solutions with different concentrations. (B)-(F) The responses of the paper sensor towards different concentrations of Alg with different concentrations of Ca²⁺ (0, 2, 4, 10 and 20 mmol L^-1).

https://doi.org/10.1371/journal.pone.0308424.g003

Detection of Ca²⁺ and Mg²⁺

Inspired by the excellent performance of the paper sensor above, the study investigating determination of the concentration of Ca²⁺ and Mg²⁺ was also conducted. Based on the results in Fig 2, the Cr value droped at the Ca²⁺ concentration from 0–4 mmol L^-1 and increased at the Ca²⁺ concentration from 4–20 mmol L^-1, which makes it challenging for the direct quantitative detection of Ca²⁺. Thus, 4 mmol L^-1 Ca²⁺ was initially added directly into the test sample to address this issue. In this way, the concentration of Ca²⁺ can be quantitatively determined. As shown in Fig 3A, the water flow distance increased complying with the progressive concentration increase in the range of 4–14 mmol L^-1. Thus, the function between Cr value and the Ca²⁺ concentration from 4–14 mmol L^-1 was plotted as Fig 3B. A satisfactory linear relationship was obtained with R² as 0.990. With the initially added 4 mmol L^-1 Ca²⁺ case, the function exhibited the same trendency in the range of 0–10 mmol L^-1 (Fig 3C, 3D). Besides, the responses of the paper sensor to Mg²⁺ were also studied and the pictures were showed in S3 Fig. The Cr value exhibited decreasing trendy accompanied by the Mg²⁺ concentration rising from 0 to 4 mmol L^-1. Following, the Cr values increased from 4–16 mmol L^-1 and reached a plateau until the concentration of Mg²⁺ increased to 20 mmol L^-1 (Fig 3E and 3F). A linear relationship was also plotted with R² calculated to be 0.993. The results showed the potential of the paper sensor for Ca²⁺ and Mg²⁺ hardness detection in water.

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Fig 3. Detection of Ca²⁺ and Mg²⁺.

(A) The images of the sensors and (B) the linear plot between the Cr and Ca²⁺ concentration from 4–14 mmol L^-1. (C) The images of the sensors and (D) the linear plot between the Cr and Ca²⁺ concentration from 0–10 mmol L^-1. (E) The images of the sensors and (F) the linear plot between the Cr and Mg²⁺ concentration from 4–20 mmol L^-1.

https://doi.org/10.1371/journal.pone.0308424.g004

The selectivity of the paper sensor

The Ca²⁺ of 10 mmol L^-1 was chosen in this section. The water flow distance remained almost same with different pH values from 5–9 (Fig 4A), which indicated the pH stability of the paper sensor. Subsequently, the influence of ionic strength of the solution was also considered (Fig 4B). The actual drinking water samples rarely contains heavy metal ions. Therefore, common anions were selected for selectivity detection. The results show that ignorable difference was observed at a fixed Ca²⁺ concentration of 10 mmol L^-1 in the presence of NaCl solution (0–200 mmol L^-1). The corresponding photograph is shown in S4 Fig. Finally, the potential interfering ions in real samples including NaCl, KCl, NaH₂PO₄, NaH₂PO₄, (NH₄)₂S₂O₈, Na₂S₂O₃, NaHSO₃ and NaHCO₃ were mixed with Ca²⁺. As shown in Fig 4C and 4D, the water flow distances exhibited no obvious alternation. All of these results suggest the satisfactory selectivity of the paper sensor.

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Fig 4. The selectivity of the paper sensor.

(A) The Cr values responses for the mixture solution of Alg and Ca²⁺ with different pH values, (B) for aqueous solutions with NaCl solutions (0 to 200 mmol L^-1) and Ca²⁺, (C) for aqueous solutions with other ions and Ca²⁺, and (D) the corresponding photohraphs, respectively. The concentration of each ion was 10 mmol L^-1.

https://doi.org/10.1371/journal.pone.0308424.g005

The semi-quantitative detection of water hardness using the paper sensor

The World Health Organization recommends that the Ca²⁺ level in drinkable water should be less than 5 mmol L^-1. Thus, the corresponding Cr value of drinkable water sample detected by the paper sensor should be in the range of 0.259 to 0.595 in the absence of Mg²⁺ according to Fig 3B. If only Mg²⁺ is existing in the tested sample, the concentration of Mg²⁺ should be below than 12.5 mmol L^-1 Mg²⁺ for drinkable water, which means the same water hardness and calculated by the equation:

According to Fig 5, the Cr values should be below 0.419. In short, the water hardness is suitable for drinking if the Cr values lie in the range of 0.259 to 0.419, and unqualified with Cr value above 0.595. Thus, the proposed sensor has been successfully applied for the semi-quantitative detection of water hardness.

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Fig 5. The illustration of the semi-quantitative detection of water hardness using the paper sensor.

(A) The Cr values and (B) corresponding images of the paper sensor under different conditions: in the presence of 4 mmol L^-1 Ca²⁺ + 12.5 mmol L^-1 Mg²⁺, 4 mmol L^-1 Ca²⁺ + 5 mmol L^-1 Ca²⁺, and 4mmol L^-1 Ca²⁺. (C) The Cr values ranges illustration of the sensor for the water hardness detection for the drinking water.

https://doi.org/10.1371/journal.pone.0308424.g006

Comparison of the paper sensor with the commercial kit

The applicability of the paper sensor was also evaluated in water. The standard addition method was used herein. After 4 mmol L^-1 of Ca²⁺ was initially added into the tested purified water, the additional different concentrations of Ca²⁺ with 0, 2, 4 and 6 mmol L^-1 were spiked respectively. The concentrations of Ca²⁺ were determined by the paper sensor (S5 Fig), the water hardness kit and inductively coupled plasma-mass spectrometry (ICP-MS), respectively. The recoveries were found varied from 95% to 108.9% and from 105% to 145% for the paper-based sensor and the commercial kit, respectively (Table 1). The results also exhibited the comparable accuracy of the proposed paper sensor with the commercial kit. Test-t analysis was performed between the proposed paper-based method and the ICP-MS method which is the gold standard for metal content measurement. The p values were calculated as 0.317, 0.353, and 0.596 respectively, corresponding to the marked samples with Ca²⁺ concentrations of 2, 4, 6 mM. No significant difference were observed, which demonstrated the reliability of the paper method. More importantly, only 30 μL of the test sample is required for the paper sensor, which is much less than the amount of the test sample (10 mL) used for the commercial kit. In addition, the comparison of this paper-based sensor with different methods for the detection of Ca²⁺ and Mg²⁺ were performed (S1 and S2 Tables), which clearly demonstrated the merits of low-cost, convince, and easily-read for the paper-based sensor. Thus, this method works as an effective means for the quantification of Ca²⁺ in water.

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Table 1. Hardness detection in purified water by the paper-based sensor, commercial kit and ICP-MS.

https://doi.org/10.1371/journal.pone.0308424.t001

Conclusions

In summary, this work reported a portable distance-based lateral flow paper-based sensor for drinking water hardness semi-quantitative detection based on the phase separation induced by the gel-sol transition. Briefly, the chelation between sodium alginate and Ca²⁺/Mg²⁺ triggers the formation of hydrogel to cause the viscosity changes of sodium alginate solution. The viscosity change can be quantified by the water flow distance on test strips. This method effectively avoids the usage of a large amount of samples, complicated operation, and high cost, which also exhibits high accuracy performance. Though the paper sensor could not exclude thoroughly the interference produced by other metal ions in some specific water samples, this approach can eliminate the error caused by the subjective color endpoint judgment in tested commercial kit, which shows the great potential of the paper sensor for further application.

Supporting information

S1 Fig. The photographs of the paper-based sensors in response to different concentrations of Mg²⁺ in the presence of 0.2 wt% Alg.

https://doi.org/10.1371/journal.pone.0308424.s001

(TIF)

S2 Fig. The condition optimization of the sensor.

(A) The performance of the sensor towards different concentrations of sodium alginate alone (0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt% and 0.5 wt%), (B-F) the images of the sensors towards the different concentrations of sodium alginate with different concentrations Ca²⁺ (0, 2, 4, 10 and 20 mmol L^-1).

https://doi.org/10.1371/journal.pone.0308424.s002

(TIF)

S3 Fig. PThe images of the paper-based sensor in response to different concentrations of Ca²⁺.

https://doi.org/10.1371/journal.pone.0308424.s003

(TIF)

S4 Fig. The selectivity evaluation of the paper-based sensor.

(A) The images of the paper-based sensor towards the aqueous solution with different pH value; (B) The images of the paper-based sensor with the coexistence of different concentrations NaCl (0, 50, 100 and 200 mmol L^-1) with Ca²⁺ (10 mmol L^-1), respectively.

https://doi.org/10.1371/journal.pone.0308424.s004

(TIF)

S5 Fig. The images of the paper-based sensors in response to different spiked concentration of Ca²⁺.

https://doi.org/10.1371/journal.pone.0308424.s005

(TIF)

S1 Table. Comparison of this work with different methods for the detection of Ca²⁺.

https://doi.org/10.1371/journal.pone.0308424.s006

(DOCX)

S2 Table. Comparison of this work with different methods for the detection of Mg²⁺.

https://doi.org/10.1371/journal.pone.0308424.s007

(DOCX)

Acknowledgments

We thank Yanhui Bi and Binglu Zhao for expert technical assistance.

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Figures

Abstract

Introduction

Experimental

Materials

Optimization of the sodium alginate concentration

Determination of the water hardness using the commercial test kit

Data analysis

Results and discussion

Feasibility of the paper sensor for the evaluation of Ca2+/Mg2+

Optimization of condition for the paper sensor

Detection of Ca2+ and Mg2+

The selectivity of the paper sensor

The semi-quantitative detection of water hardness using the paper sensor

Comparison of the paper sensor with the commercial kit

Conclusions

Supporting information

S1 Fig. The photographs of the paper-based sensors in response to different concentrations of Mg2+ in the presence of 0.2 wt% Alg.

S2 Fig. The condition optimization of the sensor.

S3 Fig. PThe images of the paper-based sensor in response to different concentrations of Ca2+.

S4 Fig. The selectivity evaluation of the paper-based sensor.

S5 Fig. The images of the paper-based sensors in response to different spiked concentration of Ca2+.

S1 Table. Comparison of this work with different methods for the detection of Ca2+.

S2 Table. Comparison of this work with different methods for the detection of Mg2+.

Acknowledgments

References

Feasibility of the paper sensor for the evaluation of Ca²⁺/Mg²⁺

Detection of Ca²⁺ and Mg²⁺

S1 Fig. The photographs of the paper-based sensors in response to different concentrations of Mg²⁺ in the presence of 0.2 wt% Alg.

S3 Fig. PThe images of the paper-based sensor in response to different concentrations of Ca²⁺.

S5 Fig. The images of the paper-based sensors in response to different spiked concentration of Ca²⁺.

S1 Table. Comparison of this work with different methods for the detection of Ca²⁺.

S2 Table. Comparison of this work with different methods for the detection of Mg²⁺.