Visual detection of lead(II) using a simple device based on P(NIPAM-co-B18C6Am) hydrogel

Abstract

A simple and portable device equipped with poly(N-isopropylacrylamide-co-benzo-18-crown-6-acrylamide) (P(NIPAM-co-B18C6Am)) hydrogel is developed for visual detection of Pb²⁺ ions. The device consisted of a compartment for incorporating the hydrogel, a flow channel and an indicating liquid chamber with colored solution, and a dense and elastic polyethylene (PE) membrane to separate the hydrogel and the colored solution. The device works like a mercury thermometer, in which the P(NIPAM-co-B18C6Am) hydrogel and the PE membrane act as the Pb²⁺ sensor and actuator and the colored liquid column in the flow channel acts as the indicator. The volume of P(NIPAM-co-B18C6Am) hydrogel changes in response to Pb²⁺ concentration at a certain temperature due to the formation of B18C6Am/Pb²⁺ host–guest complexes, and the volume change of the hydrogel causes elastic deformation of the PE diaphragm, which pushes out the colored solution from the liquid chamber into the flow channel to form a liquid column. Therefore, the Pb²⁺ concentration can be detected quantitatively by simply and easily measuring of the liquid column length. The device shows excellent sensitivity towards Pb²⁺ ions. The detection performances of the proposed devices are satisfactorily stable and repeatable. This simple and portable device enables visual detection of Pb²⁺ concentration without the help of electrical amplification or spectroscopic measurements.

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Introduction

Lead is a common toxic heavy metal,^1,2 which can accumulate in the human body through water intake or food chains.^3–6 Even ingestion of low-level lead can cause neurological, cardiovascular, and immunological disorders, and especially has a great influence on children's intellectual development.^4–6 However, lead is inevitably used in modern industry, and high concentrations of lead are commonly found in industrial wastewater from lead mining, battery manufacturing plants, lead smelteries, and printed circuit board factories, etc.^2,3 Direct discharge of wastewater with high Pb²⁺ concentration that does not meet the industrial emissions standards will cause serious pollution to environment and do harm to human health. Currently, many analytical methods have been developed for the detection of lead ions, including inductively coupled plasma optical emission spectrometry analysis,^7,8 atomic absorption spectrometry,⁹ anodic stripping voltammetry,^10–12 and so on. However, these methods are usually sophisticated and require accurate instruments that should be operated by professionals. Therefore, a simple, portable and visual detection device is desired for on-the-spot detection of high concentration of lead ions.

The chemical and biological sensors based on the stimuli-responsive hydrogels have attracted an extensive attention in the detection of lead ions. Up to now, hydrogel-based sensors have mostly focused on microcantilever sensors,¹³ crystalline colloidal array reflection,^14–18 fluorescence,^19,20 and so on.^21,22 The key to microcantilever sensors is to coat the microcantilever with stimuli-responsive hydrogel. The microcantilever undergoes bending deflection upon exposure to solutions containing Pb²⁺ as the result of a swelling of the hydrogel, and thus the Pb²⁺ concentration can be detected by measuring the deflection of the microcantilever.¹³ However, microcantilever deflection is usually measured by the optical beam deflection method, which requires accurate and expensive instruments. Optical sensors such as polymerized crystalline colloidal arrays and hydrogel diffraction gratings also have some limitations. For example, the polymerized crystalline colloidal arrays attach recognition agents (crown ether, single-stranded adapter) to the hydrogel, which can capture Pb²⁺ ions to cause the hydrogel to swell or shrink.^14–18 As a result it can actuate a lattice constant change that shifts the diffraction wavelength in proportion to Pb²⁺ concentration. As for hydrogel diffraction gratings, the detection of Pb²⁺ ions is carried out by monitoring the variation of diffraction efficiency of the gratings when exposed to Pb²⁺ solutions.²² Optical sensors need spectral analysis which should be carried out by using spectrometer and other precision instruments. Fluorescent sensors detect the target analytes by changing the fluorescence intensity.¹⁹ A hydrogel-based fluorescent sensor has been developed for visual detection of Pb²⁺ by immobilizing a DNA probe in the hydrogel, which shows a yellow-to-green fluorescence change in the presence of Pb²⁺.²⁰ However, it is hard to accurately recognize the colour change by naked eyes. Moreover, DNA probe is difficult to be stored. Therefore, it is desired and necessary to develop a easy method for quantitative detection of lead without any help of electrical amplification or spectroscopic measurement.

Here, we report a simple and portable hydrogel-based device for visual detection of Pb²⁺. The structure construction and the Pb²⁺ detection mechanism of the proposed device is schematically illustrated in Fig. 1. The device consists of a block of poly(N-isopropylacrylamide-co-benzo-18-crown-6-acrylamide) (P(NIPAM-co-B18C6Am)) hydrogel, a upper poly(dimethylsiloxane) (PDMS) module, a lower PDMS module, and a dense and elastic polyethylene (PE) membrane to separate the upper and lower PDMS modules (Fig. 1a). The upper PDMS module is consisted of an indicating liquid chamber and a flow channel, and the lower PDMS module has a compartment for holding hydrogel and through-holes in the bottom of the module (Fig. 1a). The PDMS modules are fabricated by the well-known soft lithography method. A block of P(NIPAM-co-B18C6Am) hydrogel is cut and embedded into the compartment in a shrunken state at 65 °C. Then, the PDMS modules and PE membrane are assembled, and methylene blue solution is injected into the indicating liquid chamber (Fig. 1b, c, f and g). The through-holes in the bottom of the lower PDMS module are designed for intaking Pb²⁺ ions into the hydrogel (Fig. 1c). When crown ether units capture Pb²⁺ and form B18C6Am/Pb²⁺ host–guest complexes,^23,24 the volume phase transition temperature (VPTT) of the P(NIPAM-co-B18C6Am) hydrogel can shift from a low temperature to a higher temperature due to the repulsion among charged complex groups and the enhancement of hydrophilicity of the hydrogel.^25,26 When the ambient temperature is set at a certain temperature between the two VPTT values, the P(NIPAM-co-B18C6Am) hydrogel can change from a shrunk state to a swollen state in response to recognizing Pb²⁺. Compared with P(NIPAM-co-B18C6Am) linear copolymers²⁶ or gating membranes with grafted P(NIPAM-co-B18C6Am) copolymers,^27–29 the crosslinked P(NIPAM-co-B18C6Am) hydrogels provide better mechanical strength and flexibility for Pb²⁺ response and can be used as sensors and actuators by changing their volume, shape or mechanical properties.³⁰ The volume change of the hydrogel can be transduced into the movement of the colored solution in the flow channel with the help of the elastic diaphragm.^31,32 The volume change of the hydrogel causes elastic deformation of the PE diaphragm, which pushes out the colored solution from the liquid chamber into the flow channel to form liquid column (Fig. 1d and e). Thus, the Pb²⁺ concentration can be detected quantitatively by measuring the liquid column length. To verify the efficiency and performance repeatability of the proposed device, we first equip the device with a block of thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) hydrogel, which is much more cheaper and easier to be fabricated than the P(NIPAM-co-B18C6Am) hydrogel and used to detect temperature. Then, the device is equipped with P(NIPAM-co-B18C6Am) hydrogel and used to detect the Pb²⁺ concentration. The detection performance of the device is satisfactorily stable and repeatable. The results verify that the proposed simple and portable device in this study is an efficient and reliable device for visual detection of Pb²⁺.

Fig. 1 Schematic illustrations of the structure construction and the Pb²⁺ detection mechanism of the proposed device. (a–c) Structure construction. The device consists of a compartment for holding the hydrogel, a flow channel and an indicating liquid chamber with colored solution, and a dense and elastic polyethylene (PE) membrane to separate the hydrogel and the colored solution. (d and e) Pb²⁺ detection mechanism. (f and g) Photographs of top view (f) and bottom view (g) of the device.

Experimental

Structure design and fabrication of device

The flow channel (700 μm in width and 700 μm in depth) and the indicating liquid chamber (12 mm × 8 mm × 1 mm or 10 mm × 5 mm × 1 mm) are formed in the 3.5 mm-thick upper PDMS module (Fig. 1a). A compartment to hold the hydrogel (10 mm × 5 mm × 1 mm, 12 mm × 8 mm × 1 mm or 12 mm × 8mm × 2 mm) and 20 through-holes with diameter of 0.8 mm for intaking environmental solution into the hydrogel are formed in the 3.5 mm-thick lower PDMS module (Fig. 1a). The PDMS modules are formed by the well-known soft lithography method with solid molds. For preparing the modules, PDMS prepolymer is prepared by mixing the elastomer base with curing agent (Sylgard 184, Dow Corning) at a ratio of 10 : 1 (weight/weight). PDMS modules are obtained by curing in the molds at 90 °C for 1.5 h. The thickness of the PE membrane for separating the compartments of hydrogel and colored solution is 50 μm. Methylene blue (Tianjin Bodi Chemicals) is diluted to 1 mM with deionized water (18.2 MΩ, 25 °C) from a Milli-Q Plus water purification system (Millipore) and used as a colored solution filled in the indicating liquid chamber.

Syntheses of PNIPAM and P(NIPAM-co-B18C6Am) hydrogels

PNIPAM hydrogel is prepared by thermally initiated free-radical polymerization using 2,2′-azobis(2-amidi-nopropane dihydrochloride) (V50, Qingdao Runxing Photoelectric Materials) as initiator and N,N′-methylene-bis-acrylamide (MBA, Chengdu Kelong Chemicals) as cross-linker. The molar ratios of MBA and V50 to the total monomer are 1 mol% and 0.5 mol% respectively. The concentration of monomer N-isopropylacrylamdie (NIPAM, TCI) is 1.0 mol L⁻¹. For synthesis of P(NIPAM-co-B18C6Am) hydrogel, the molar ratio of benzo-18-crown-6-acrylamide (B18C6Am, synthesized according to previously reported procedures^33,34) to NIPAM is 15 mol%. The molar ratios of MBA and V50 are the same as mentioned above. Nitrogen gas is bubbled into the solution for 10 min to remove dissolved oxygen in the system. The polymerization is carried out at 70 °C for 8 h. After the gelation is completed, the prepared hydrogels are purified by immersing the hydrogels in an excess of deionized water and replacing the water every 12 h to remove residual unreacted components. The washing process lasts for 7 days. The obtained hydrogels are characterized by an FT-IR spectrometer (IR Prestige-21, Shimadzu).

Performance characterization of the device equipped with PNIPAM hydrogel

To verify the efficiency and performance repeatability of the proposed device, we first equip the device with a block of thermo-responsive PNIPAM hydrogel, and use it for detecting temperature. Thermo-responsive volume change behaviors of PNIPAM hydrogel are studied by evaluating the volume-phase transition behaviors in deionized water at various desired ambient temperatures in the range from 20 to 50 °C. The temperature is controlled in a water bath with an accuracy of ±0.1 °C. The measurements are performed according to previously reported procedures.³⁵

Then, a block of PNIPAM hydrogel is cut and embedded into the compartment in a shrunken state at 50 °C. The performance of the device is examined by immersing the lower PDMS device with PNIPAM hydrogel in deionized water at different temperatures. The PE membrane is deformed as the volume of hydrogel changes, which pushes out the colored solution from the chamber into the flow channel. The end of the flow channel is covered with a paraffin film to prevent the evaporation of the colored solution before the detection. Effects of hydrogel sizes on the changes in liquid column length (ΔL) are investigated. The liquid column length (ΔL) in the flow channel is measured with a calibrated scale by naked eyes. Three hydrogel sizes (10 × 5 × 1, 12 × 8 × 1 and 12 × 8× 2, unit: mm) are studied and the optimum size is chosen for subsequent experiments. To investigate the repeatability and stability of the change in the column length of colored solution, the ΔL values in deionized water at 25 °C (<VPTT) and 40 °C (>VPTT) are measured alternately and repeatedly. Furthermore, the same device is used to repeatedly measure the ΔL values at various desired ambient temperatures in the range from 20 to 40 °C. Finally, different devices of the same construction and the same dimension are prepared and used to examine the reproducibility of the proposed device.

Detection of Pb²⁺ with the device equipped with P(NIPAM-co-B18C6Am) hydrogel

The Pb²⁺-responsive and thermo-responsive behaviors of P(NIPAM-co-B18C6Am) hydrogel are first studied by evaluating the volume-phase transition behaviors in Pb²⁺ solutions with different ion concentrations at various ambient temperatures in the range from 20 to 65 °C. The P(NIPAM-co-B18C6Am) hydrogel is cut into thin pieces and then immersed in Pb²⁺ solutions with different concentrations before measurements. Subsequently, the device is equipped with P(NIPAM-co-B18C6Am) hydrogel and used for Pb²⁺ detection. The through-holes of device are initially immersed in deionized water and then in Pb²⁺ solutions from low concentration to high concentration orderly at various ambient temperatures in the range from 20 to 48 °C.

K⁺ and Na⁺ are chosen as the reference ions to investigate the selectivity of the device to different cations. We test the device with different concentrations of Pb²⁺ in the presence of excess Na⁺ or K⁺ at 24 °C.

Results and discussion

Efficiency and performance repeatability of the device equipped with PNIPAM hydrogel

The temperature dependence of the volume change of PNIPAM hydrogel in water is shown in Fig. 2. The temperature-induced changes in volumes of hydrogel pieces in deionized water are expressed with the volume (V_T) (Fig. 2a) and volume ratio (V_T/V₅₁) (Fig. 2b), where V_T represents the volume of hydrogel in the equilibrium state at a certain test temperature (T, °C) and V₅₁ represents that at 51 °C. The PNIPAM hydrogel pieces undergo volume shrinkage when the environmental temperature increases from 20 to 51 °C. The temperature, at which the value of V_T or V_T/V₅₁ decreases to half of the total change, is taken as the VPTT of the cross-linked hydrogel. The VPTT of the prepared PNIPAM hydrogel is about 30 °C.

Fig. 2 Temperature dependence of volume changes of PNIPAM hydrogels in deionized water, where V_T is the volume of hydrogel at a certain test temperature T °C (a), and V_T/V₅₁ is the ratio of volume of the hydrogel at temperature T °C to that at 51 °C (b).

Fig. 3 shows the photographs of PNIPAM-hydrogel-based device in deionized water at different temperatures. The column length of the colored solution in the flow channel changes in response to the environmental temperature. A calibrated scale is fixed at the bottom of the device. The change in column length is obvious, which can be observed by naked eyes without any help of optical or electronic instruments.

Fig. 3 Photographs of a PNIPAM-hydrogel-based device for detecting temperature of deionized water.

Fig. 4 shows the reproducibility and performance stability of the PNIPAM-hydrogel-based device for detecting temperature. No matter how large the PNIPAM hydrogel in the device is, the temperature detection performance with the device is reproducible and stable (Fig. 4a). However, the larger the hydrogel size is, the more obvious the thermo-responsive change of the liquid column (ΔL). Therefore, the hydrogel size of 12 × 8 × 2 mm is chosen for the subsequent experiments. Fig. 4b shows the result of the temperature dependence of ΔL in the same device upon repeatedly heating up and cooling down the temperature of deionized water. The ΔL value reduces when environmental temperature increases from 20 to 40 °C and rises when environmental temperature decreases from 40 to 20 °C, and the four curves can overlap very well. Fig. 4c shows the temperature dependence of ΔL in two different devices with the same construction and the same dimension. The two curves also overlap with each other very well. In addition, the tendency of the temperature-dependent ΔL value agrees well with the thermo-responsive volume change behaviors of PNIPAM hydrogel shown in Fig. 2. The results confirm that the proposed device possesses a good reproducibility and performance stability.

Fig. 4 Repeatability and performance stability of the PNIPAM-hydrogel-based device for detecting temperature. (a) Effect of hydrogel size on the indicating liquid column variation (ΔL) at 25 °C (<VPTT) and 40 °C (>VPTT), in which L, W and H are the length, width and height of the hydrogel piece. (b) Temperature dependence of ΔL in the same device upon repeatedly heating up (no. 1 and no. 3) and cooling down (no. 2 and no. 4) the temperature of deionized water. (c) Temperature dependence of ΔL in two different devices with the same construction and the same dimension.

Detection of Pb²⁺ using the P(NIPAM-co-B18C6Am)-hydrogel-based device

The volume changes of P(NIPAM-co-B18C6Am) hydrogel pieces in Pb²⁺ solutions with different ion concentrations at different temperatures are shown in Fig. 5. The volume of P(NIPAM-co-B18C6Am) hydrogel responds to both Pb²⁺ concentration and temperature. When the environmental temperature increases from 20 to 65 °C, the P(NIPAM-co-B18C6Am) hydrogel shrinks. With increasing the Pb²⁺ concentration, the VPTT of P(NIPAM-co-B18C6Am) hydrogel shifts to a higher temperature and the volume at lower temperature increases to some extent, which is due to the repulsion among the charged B18C6Am/Pb²⁺ complex groups and osmotic pressure within the P(NIPAM-co-B18C6Am) hydrogel.

Fig. 5 Temperature dependence of volume changes of P(NIPAM-co-B18C6Am) hydrogels in Pb²⁺ solutions with different ion concentrations, where V_T is volume of the hydrogel piece at temperature T °C (a), and V_T/V₆₅ is the volume ratio of the hydrogel piece at temperature T °C to that at 65 °C (b).

Fig. 6 shows the detection performance of Pb²⁺ using the P(NIPAM-co-B18C6Am)-hydrogel-based device. Temperature dependence of change in the column length in Pb²⁺ solutions with different concentrations (ΔL_Pb²⁺) is shown in Fig. 6a. The tendency of the ΔL_Pb²⁺ curves agrees well with the Pb²⁺-responsive volume change behaviors of P(NIPAM-co-B18C6Am) hydrogel shown in Fig. 5. Fig. 6b shows a three-dimensional diagram of ΔL_Pb²⁺ as a function of both Pb²⁺ concentration and temperature, and Fig. 6c shows the corresponding contour diagram. With the contour diagram of ΔL_Pb²⁺ as a function of both Pb²⁺ concentration and temperature, the Pb²⁺ concentration can be conveniently and quickly determined if environmental temperature and the ΔL_Pb²⁺ have been measured.

Fig. 6 Detection of Pb²⁺ concentration using the P(NIPAM-co-B18C6Am)-hydrogel-based device. (a) Temperature dependence of change in the column length in Pb²⁺ solutions with different concentrations (ΔL_Pb²⁺). (b) Three-dimensional diagram of ΔL_Pb²⁺ as a function of both Pb²⁺ concentration and temperature. (c) Contour diagram of ΔL_Pb²⁺ as a function of both Pb²⁺ concentration and temperature. (d) Effect of interference ions on the change in the column length (ΔL) at 24 °C.

Different ion solutions containing some obstructive ions (e.g., Na⁺, K⁺) are used to confirm the selectivity of the device to detect Pb²⁺. Fig. 6d shows the effect of interference ions on the change in the column length (ΔL) at 24 °C. Excess Na⁺ (40 mM) or K⁺ (40 mM) causes nearly no obvious change of the ΔL whether they are in the pure water or in the Pb²⁺ solutions with different ion concentrations. Due to the binding constant between Pb²⁺ and benzo-18-crown-6 (log K = 3.19 in water at 25 °C) is much higher than K⁺ (log K = 1.74 in water at 25 °C) and Na⁺ (log K = 1.38 in water at 25 °C),²⁴ the benzo-18-crown-6 is highly selective to forming complexes with Pb²⁺. The results verify that the visual detection of Pb²⁺ by the device is not interfered by the presence of certain interference metal ions.

Calibration of Pb²⁺ concentration with measured column length in the device

Temperature dependence of change in the column length of the P(NIPAM-co-B18C6Am) hydrogel-based device in deionized water (ΔL_w) is shown in Fig. 7a. When environmental temperature increases from 20 to 48 °C, the relationship between ΔL_w and temperature can be described with the eqn (1):

Fig. 7 Temperature dependence of change in the column length of the P(NIPAM-co-B18C6Am) hydrogel-based device in deionized water (ΔL_w). (a) The indicating liquid column variation (ΔL_w) as a function of temperature of deionized water. (b) Comparison of the calculated ΔL_w value from eqn (1) and the experimental ΔL_w value.

A comparison of the calculated ΔL_w value from eqn (1) and the experimental ΔL_w value is shown in Fig. 7b. Obviously, the experimental data fit well with the calculated data, and the relative error is less than 3%.

Because the P(NIPAM-co-B18C6Am) hydrogel exhibits both thermo-responsive and Pb²⁺-responsive properties, the relationship between Pb²⁺ concentration and the change in the column length should be established at certain temperature. An index is defined as the Pb²⁺-responsive column length variation, which is calculated from ΔX = ΔL_Pb²⁺ − ΔL_w, where ΔL_Pb²⁺ is the measured column length and ΔL_w is calculated from eqn (1). Fig. 8a shows the Pb²⁺-concentration dependence of the ΔX value at different temperatures, in which the curves are very close in the temperature range from 22 to 28 °C. Fig. 8b shows the relationship between Pb²⁺ concentration and the ΔX value at 24 °C. The relationship between Pb²⁺ concentration and the ΔX is in accordance with the cubic polynomial, which can be described with the eqn (2):

C₂₄ = 0.0092ΔX³ − 0.2514ΔX² + 2.5567ΔX + 0.104 (2)

Fig. 8 Detection of Pb²⁺ concentration with the P(NIPAM-co-B18C6Am)-hydrogel-based device and eqn (2). (a) The indicating liquid column variation ΔX (ΔX = ΔL_Pb²⁺ − ΔL_w) as a function of Pb²⁺ concentration at different temperatures. (b) The relationship between the Pb²⁺ concentration and the ΔX value at 24 °C. (c) Comparison of the calculated Pb²⁺ concentration from eqn (2) at 24 °C with the experimental Pb²⁺ concentration.

A comparison of the calculated Pb²⁺ concentration from eqn (2) and the experimental Pb²⁺ concentration at 24 °C is shown in Fig. 8c. Obviously, the experimental data fit well with the calculated data and the relative error is less than 2%.

Because the Pb²⁺-concentration-dependent ΔX curves are very close in the temperature range from 22 to 28 °C, the relationship between Pb²⁺ concentration and the ΔX value at different temperatures in this temperature range can be expressed with a general eqn (3):

C = aΔX³ + bΔX² + cΔX + d (3)

where, the parameters a, b, c and d, and correlation coefficient R² are listed in Table 1. The results show that the parameters a, b, c and d are slightly temperature-dependent, and all the R² values are larger than 0.996, which indicates a satisfactory accuracy.

Table 1 The parameters and correlation coefficient R² of eqn (3)

T (°C)	a	b	c	d	R ^2
22	0.0089	−0.2347	2.4213	0.0997	0.9961
24	0.0092	−0.2514	2.5567	0.1040	0.9993
26	0.0082	−0.2315	2.4713	0.2461	0.9971
28	0.0085	−0.2400	2.5590	0.1920	0.9980

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In a word, with measured column length in the device, the Pb²⁺ concentration can be accurately calculated with eqn (1) and (3).

Due to the intrinsic nature of the P(NIPAM-co-B18C6Am) hydrogel, there are still some limitations of the current device in terms of operational and functional parameters such as temperature and concentration, requiring further developments. The optimal temperature range of the current device for detecting Pb²⁺ is from 20 to 36 °C. Because the salting-out effect of increased amount of anions in solutions would counteract the swelling of the P(NIPAM-co-B18C6Am) hydrogel to some extent,^36,37 and the P(NIPAM-co-B18C6Am) hydrogel begins to shrink when Pb²⁺ concentration is larger than 30 mM,²⁶ so the upper limit of the current device for Pb²⁺ detection is about 30 mM. The lower limit of the current device for Pb²⁺ detection is about 1 mM at present stage, which can be further extended by improving the Pb²⁺-responsive characteristics of the hydrogel, improving the sensitivity of the elastic diaphragm, or reducing the dimension of the flow channel, etc.

Conclusions

Visual detection of Pb²⁺ concentration is successfully achieved by developing a simple and portable P(NIPAM-co-B18C6Am)-hydrogel-based device. The device is designed to work like a mercury thermometer and can be fabricated easily. The device consists of a compartment for holding a block of P(NIPAM-co-B18C6Am) hydrogel, a flow channel and an indicating liquid chamber with colored solution, and a dense and elastic PE membrane to separate the hydrogel and the colored solution. The P(NIPAM-co-B18C6Am) hydrogel and the PE membrane act as the Pb²⁺ sensor and actuator, and the colored liquid column in the flow channel acts as the indicator. The volume of P(NIPAM-co-B18C6Am) hydrogel changes in response to Pb²⁺ concentration and temperature; therefore, the Pb²⁺ concentration at a certain temperature can be detected by simply and easily measuring the liquid column length in the device. When the Pb²⁺ concentration is in the range of 6 to 21 mM in this study, the Pb²⁺ concentration at different temperatures in the range from 22 to 28 °C can be accurately calculated with two equations. The device shows excellent sensitivity to Pb²⁺ ion. The detection performances of the prepared devices are satisfactorily stable and repeatable. Future work must focus on lowering the detection limit of the device proposed herein to allow the full potential of the technique to be realized, which can be achieved by improving the Pb²⁺-responsive characteristics of the hydrogel, improving the sensitivity of the elastic diaphragm, or reducing the dimension of the flow channel. Nevertheless, the results in this study provide an essential guide to assess the utility of the structures that can be produced.

Acknowledgements

The authors gratefully acknowledge support from the National Natural Science Foundation of China (21136006), Program for Changjiang Scholars and Innovative Research Team in University (IRT1163), the Key Laboratory for Green Chemical Technology of Ministry of Education and the Specialized Research Fund for the Doctoral Program of Higher Education by the Ministry of Education of China (20120181110074).

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