Ultrasensitive Pb(II) Potentiometric Sensor Based ... - ACS Publications

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Ultrasensitive Pb(II) Potentiometric Sensor Based on Copolyaniline Nanoparticles in a Plasticizer-Free Membrane with a Long Lifetime Xin-Gui Li,*,†,‡ Hao Feng,† Mei-Rong Huang,*,†,‡ Guo-Li Gu,† and Mark G. Moloney‡ †

Institute of Materials Chemistry, Key Laboratory of Advanced Civil Engineering Materials, College of Materials Science and Engineering, Tongji University, 1239 Si-Ping Road, Shanghai 200092, China ‡ Department of Chemistry, University of Oxford, Mansfield Road, Oxford OX1 3TA, U.K.

bS Supporting Information ABSTRACT: A newly designed Pb(II) potentiometric sensor based on intrinsically conducting nanoparticles of solid poly(aniline-co-2-hydroxy-5-sulfonic aniline) possessing many ligating functional groups like
NH
,
N=,
OH,
SO3H,
NH2 as ionophores in plasticizer-free vinyl resin solid membranes has been fabricated. A linear Nernstian response is obtained within a wide Pb(II) activity range from 1.0  10
3 to 1.0  10
10 M with a detection limit as low as 2.2  10
11 M. The pH independent plateau ranges between 3.5 and 7.0. After 15 months’ usage, the sensor maintains 95% performance parameters. Its anti-interference ability to Cu(II), Cd(II), Ag(I), and Hg(II) is much stronger than other sensors with a detection limit at (sub)nanomolar level. Electrochemical impedance spectroscopy reveals that the solid sensing membrane has a diffusion coefficient of around 5  10
14 to 1  10
13 cm2 s
1. The much lower diffusion coefficient for Pb(II) is highly beneficial for the elimination of Pb(II) flux across the membrane. The wide detection concentration range, low detection limit, high selectivity, extensive pH window, and long lifetime make for a robust sensor giving reliable measurement of Pb(II) content with potential application in real-world samples at trace levels.

T

he detection limit of conventional liquid-contact poly(vinyl chloride) (PVC)-based potentiometric sensors has been restricted to the micromolar range1
6 due to primary ion leaching from the inner filling solution (IFS) across the membrane into the sample. Even a minor primary ion flux from IFS into the sample compromises the lower detection limit by building up an extensive diffusion layer at the membrane surface in the sample side, in which the local primary ion activity is higher than that in the bulk sample. Furthermore, the primary ion flux from the sample into the IFS would give a super-Nernstian slope due to the uptake effect and thus depletion of the primary ion in the sample solution. Thus, either the undesired leaching or uptake effect should be kept as low as possible so as to achieve an improved lower detection limit required for trace analysis. More and more approaches for improving the detection limit have been investigated,7,8 including optimizing ion buffer by (1) adding EDTA9
12 and NTA13,14 into the IFS that maintains a constant concentration of trace primary ions at 10
13 to ∼10
11 M, (2) using ion-exchange resins Dowex C-350,15
18 (3) introducing interfering ions Et4NNO3,19
21 (4) simply reducing primary ions in IFS to 10
7 M,22 (5) applying an external current to the sensing membrane,23,24 (6) rotating the membrane electrode sensor,25 (7) covalently bonding ionophore to polymer backbones,26
28 and (8) doping PVC with ionic liquids.29 With these techniques, a linear response range of the potentiometric sensor could be extended into the nanomolar or even subnanomolar concentration, and the lifetime could also be prolonged to some extent. However, these approaches cannot simultaneously achieve r 2011 American Chemical Society

excellent lower detection limit and long usage time. For example, the Pb(II) potentiometric sensors with EDTA buffered IFS show a picomolar detection limit, but their lifetime was not more than 1 week,30 exhibiting super-Nernstian response due to depletion of primary ions from the analyte solution to the membrane surface. Reducing the diffusion coefficient of the sensing membrane by using plasticizer-free matrix is an alternative way to suppress transmembrane ion flux. Polyacrylate-based membranes possessing an approximately thousand times lower diffusion coefficient than PVC have found application as a matrix31,32 to try to prolong lifetime and improve the lower detection limit, but this approach did not work well for Pb(II) potentiometric sensors.33 It seems that a high-performance electrode sensor for trace level measurement in practical use could not be achieved by using only one individual technique. In the present work, we considered that significant improvement of the detection limit and lifetime simultaneously by application of two completely new materials into the sensing membrane for Pb(II) potentiometric sensor may be possible. The sensors were designed to immobilize intrinsically conducting nanoparticles of poly(aniline-co-2-hydroxy-5-sulfonic aniline) having many functional groups such as
NH
,
N=,
OH,
SO3H, and
NH 2 as ionophores in a plasticizer-free vinyl resin membrane. The resulting whole-solid sensing membrane Received: August 7, 2011 Accepted: November 20, 2011 Published: November 20, 2011 134

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Scheme 1. Robust Sensing Membrane without Transmembrane Pb(II) Flux Due to the Whole Solid Plasticizer-Free Vinyl Resin Matrix Immobilizing Poly(AN-co-HSA) Nanoparticles as Ionophores

would be expected to expose multiple functionalized sites without ion flux across the membrane while allowing solution samples with concentration diversity to contact each side of it. This robust sensor gives a detection limit of 2.2  10
11 M, lifetime of 15 months, and reliable analysis for even trace Pb(II) in real samples. To the best of our knowledge, this is the first reported strategy of enhancing the sensitivity and durability of a potentiometric sensor at the same time.

LS230 laser particle-size analyzer from Beckman Coulter, Inc. The bulk electrical conductivity of pressed sheets of the copolymer particles with a thickness of ∼200 μm and a constant efficient area of 0.785 cm2 was examined by a two-disk method at room temperature with a UT 70A multimeter made in China. Nitrogen adsorption/desorption isotherms were measured with a Micromeritics Tristar 3000. The surface area of the particles was calculated by the Barrett-Emmett-Teller (BET) method. The sensing membranes were observed under a Leica XS-402P microscope made in Germany. Sensing Membrane Preparation and Sensor Assembly. A typical procedure is as follows: 5 mg of poly(AN-co-HSA) nanoparticles and 10 mg of NaTPB were together dissolved in 3 mL of THF by an intermittently ultrasonic treatment for 10 min. Meanwhile, 150 mg of the vinyl resin was ultrasonically dissolved in 5 mL of THF for 10 min. These were mixed together with intermittent ultrasonic treatment for 30 min to ensure complete blending. The uniform cocktail was poured onto a smooth PTFE plate and allowed to evaporate at 35 °C until a solid membrane with a thickness of about 60 μm was formed. Upon total evaporation of THF, a solid translucent membrane was obtained. A circular membrane of 5
15 mm diameter was carefully cut out to prepare a solid sensing membrane and glued to one end of a plastic tube that would be filled with Pb(NO3)2 solution. The as-prepared electrodes were conditioned in 1.0  10
3 M Pb(NO3)2 for 24 h and finally washed with water until a stable potential (drift < 1 mV/15 min) was reached before initial use, and the electrode also needed 6 h of conditioning before reusing after storage for 1 week or longer. Potential Measurement. The representative electrochemical cell for EMF measurement is as follows:

’ MATERIALS AND METHODS Materials. The vinyl resin powder (degree of polymerization 360; molecular weight 22 000; vinyl chloride/vinyl acetate/vinyl alcohol weight ratio of 90/4/6; glass transition temperature 77 °C) is a UCAR VAGD vinyl resin produced by the Dow Chemical Company. Aniline (AN), 2-hydroxy-5-sulfonic aniline (HSA), ammonium persulfate ((NH4)2S2O8), hydrochloric acid (HCl), sodium tetraphenylborate (NaTPB), polyvinyl chloride (PVC), dioctylphthalate (DOP), tetrahydrofuran (THF), lead nitrate (Pb(NO3)2), and other nitrates or chloride salts of cations are all commercial reagents in analytical grade from China Chemicals Market and used as received. The working solutions with variety of Pb(II) concentrations were confected by gradually diluting 1.0  10
2 M Pb(NO3)2 stock solution. Deionized water from an RO-1000 type Nanopure water system was used throughout. Synthesis of Poly(AN-co-HSA) Nanoparticles. Poly(AN-coHSA) copolyaniline particles as an ionophore were simply prepared by a chemically oxidative copolymerization of AN and HSA comonomers (Scheme 1a). A typical preparation procedure of the poly(AN-co-HSA) nanoparticles is as follows: AN (0.729 mL, 8 mmol) and HSA (0.378 g, 2 mmol) were added and mixed in a glass flask which contains 100 mL of 1.0 M HCl. Ammonium persulfate (2.28 g, 10 mmol) was dissolved separately in 50 mL of water to prepare an oxidant solution. The oxidant was then dropped into the mixed comonomer at a rate of one drop every 3 s at 10 °C. The mixture was vigorously magnetically stirred for 24 h at 10 °C. After reaction, the resulting copolymer particles were isolated by centrifugation, washed with ethanol and water, and left to dry in ambient air for 3 days. Measurements. The elemental analysis of the poly(AN-coHSA) copolymers was carried out on a Carlo Erba 1106 element analyzer. NMR spectra were obtained on the Bruker spectrometers: AVC500 for 1D 1H NMR at 500 MHz and DQX400 for 2D 1H
1H COSY NMR at 400 MHz in [D6]DMSO at the University of Oxford, U.K. The MALDI-TOF MS of the poly(AN-co-HSA) in THF with dithranol as the matrix in the presence of silver nitrate was recorded on a Waters Micromass MALDI micro MX mass spectrometer. The size of the poly(ANco-HSA) copolymer particles in THF was analyzed with an

AgjAgCljjPbðNOÞ3 ðconventional concentrationÞ jvinyl resin solid membranejtest solutionjjSCE All potentiometric measurements were performed by a PHS3C digital pH meter in magnetically stirred solution at least three times. Activity coefficients were calculated according to the Debye
Huckel approximation. Response time of the electrode sensor was determined by measuring the time required to achieve a steady potential (drift < 1 mV/5 min). The pH of the lead-cation solution was adjusted with 1.0 M HNO 3 and 1.0 M NaOH. Lifetime Evaluation of Potentiometric Sensor. According to the IUPAC, the lifetime of a potentiometric sensor was defined as “the time interval between the conditioning of the membrane and the moment when at least one parameter of the functionality characteristics of the device changes detrimentally”.34,35 On the basis of this definition, we designed a series of experiments to evaluate the lifetime that is defined as “the time interval between the 135

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conditioning of the membrane and the moment when the slope of the potential response curve of the device decreases to 95% of the original slope”. The lifetime of the potentiometric sensor was evaluated by inspecting the variation of the calibration curve at a frequency of twice a week in the first 3 months and then once a week. Before every use, the electrode was conditioned in freshly prepared 1.0  10
4 M Pb(NO3)2 solution for 6 h and then washed with water until a stable potential was reached. The recorded response potential was repeated three times. Measurement of Sensor Selectivity Coefficient. The fixed interference method (FIM) was employed to assess the selectivity of the fabricated potentiometric sensor for Pb(II) over the potential interfering ions. The concentration of interfering ions was fixed at 1.0  10
3 M. The selectivity coefficients were calculated according to the following equation: pot

KPb, J ¼ αPb ðDLÞ=ðαJ Þ2=z

underwent assimilation by a following procedure: 2 g of green gram or 10 mL of urine was treated with a mixture of 10 mL of HNO3 and 30 mL of 30% H2O2 at a constant temperature of 50 °C for 24 h. After complete assimilation, the solution was filtered and the resulting digested solution was made up to a volume of 1.0 L. Lead ion concentrations were measured by the proposed potentiometric sensor for Pb(II) in a direct potentiometry way, and every sample was repetitively tested 10 times. The Pb(II) concentrations were independently analyzed by atomic absorption spectroscopy to give true values for calculation of the relative error.

’ RESULTS AND DISCUSSION Strategy of Designing Ultrasensitive and Durable Membrane. We consider quite otherwise to greatly improve the

ð1Þ

detection limit by applying two completely new materials into the sensing membrane for potentiometric sensor at the same time. One is a vinyl resin material, a new type of PVC with a small amount of vinyl acetate and vinyl alcohol segments in the backbone, giving an actual hydroxyl-functionalized vinyl chloride/ vinyl acetate resin. Relative to the conventional PVC consisting of only whole vinyl chloride segments, a few acetate and hydroxyl groups would be expected to substantially weaken interactions among the vinyl chloride macromolecular chains, enhance slippage of the backbones, and thus properly soften the polymer materials. These appendents on the backbone of the vinyl resin indeed act like a plasticizer, but in this circumstance they are covalently bonded onto the macromolecular chains rather than an external additive. The greatest advantage of this matrix is that a flexible membrane is obtained without the addition of any other external plasticizer. Such an inherently self-plasticizing vinyl resin not only keeps the original electric properties of PVC and possesses much better durability than the externally plasticized PVC, but it also suppresses or even eliminates the undesired transmembrane ion flux that happens in the presence of the plasticizer droplet. Therefore, a better detection limit could be anticipated once a selective ionophore is embedded (Scheme 1b). Moreover, a small amount of hydroxyl functional groups on the vinyl resin would appropriately ameliorate the hydrophilicity of the membrane, giving a quick response in aqueous solution. The other new material which we intended to include is intrinsically semiconducting amorphous nanoparticles of aniline (AN) and 2-hydroxy-5-sulfonic aniline (HSA) copolymer [i.e., poly(AN-co-HSA)] that was newly synthesized by a chemically oxidative copolymerization with a synthetic yield of above 70% in surfactant-free acid media (Scheme 1a). The reactivity ratios of AN and HSA comonomers are found to be 1.26 and 0.26, respectively, by element analysis. 1H NMR, 1H
1H 2D COSY, and MALDI-MS methods all confirm the formation of a real copolymer consisting of average AN/HSA unit molar ratio of 5/1. Careful assignment analysis of the proton resonance peaks indicates that the additional weak peaks at 6.54, 7.30, 7.48, and 9.70 ppm are attributable to the free
OH and
SO3H (j) protons, aromatic δ proton, aromatic ε proton, and confined
OH and
SO3H (η) protons in the HSA units, respectively, because pure PAN does not demonstrate any significant resonance peaks in this region. The moderate resonance peaks at around 7.43 and 7.37 ppm, which showed correlation in the H, H-2D-COSY spectra of both PAN and copolyaniline, were ascribed to aromatic α and β protons in the AN repetitive units.

where αPb(DL) is the lower detection limit of the potentiometric sensor for Pb(II) when interfering ions existed, αJ the activity of the interfering ion, and z the charge of the interfering ion. On the basis of the recommendations from IUPAC,36 true selectivity coefficients could be obtained since the response toward these potential interfering ions, such as Na(I) is also Nernstian between 1.0  10
4 and 1.0  10
2 M. Electrochemical Impedance Spectroscopy Measurement. The carbon electrode (4 mm in diameter) was dipped into the uniform cocktail to form a solid sensing membrane with the thickness of 100 μm and the surface area of 1.38 cm2 on the carbon electrode surface at 35 °C. The carbon electrode coated with the sensing membrane was used as the working electrode for testing. After the modified electrode was conditioned in 1.0  10
3 M Pb(NO3)2 solution for 24 h and washed with deionized water, the adjusted working electrode was immersed in 1.0  10
5 M Pb(NO3)2 solution for 5 min until the open-circuit potential was stable, and then electrochemical impedance spectroscopy (EIS) was measured at an amplitude of 5 mV and frequency range from 0.01 Hz to 100 kHz. The measurements were performed in a three-electrode system: counter electrode (platinum electrode), reference electrode (calomel electrode), and working electrode (investigated electrode). The Autolab General Purpose Electrochemical System and Autolab Frequency Response Analyzer System (CH Instruments model 660A) were used for the EIS measurement. Impedance Spectrum Analyzer software “Zsimpwin” was used to analyze the impedance spectra. A DOP-plasticized PVC membrane containing poly(AN-coHSA) ionophores was also prepared for an EIS comparison with the solid vinyl resin membrane containing the same poly(AN-coHSA) ionophores. A solid vinyl resin membrane entrapping unmodified pure polyaniline (PAN) microparticles was also used to inspect the importance of poly(AN-co-HSA) nanoparticles as ionophores. Real-World Sample Analysis. The real-world samples used in this study included environmental waters, e.g., tap water, river water, rainwater, and wastewater from the Printing House of Fudan University, Shanghai, China. These environmental waters were not pretreated before measurement except for the wastewater that was filtered through filter paper 3
4 times to obtain clear water. The green gram was obtained from a farmland in Jiangxi Province China, and urine was obtained from a hospital affiliated with Tongji University, China. The latter two samples 136

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Table 1. Comparison of Membrane Parameters between the Plasticizer-Free Solid Membrane and Other Sensing Membrane Systems ionophore/membrane matrix poly(AN-co-HSA)/PVC+DOP (this work) polyaniline/plasticizer-free vinyl resin

membrane

membrane

bulk resistance

capacitance

diffusion

thickness (μm)

surface (cm2)

(MΩ)

(F)

coefficient (cm2 s
1)

100 100

1.38 1.38

0.71 14.0

3.3  10
9 4.7  10
10

N/A N/A

100

1.38

36.0

1.7  10
10

5  10
14∼1  10
13

200

0.20

0.05

1.8  10
11

N/A

200

0.20

1.70

1.1  10
11

N/A

200

0.07

100

1.0  10
10

3  10
10a

1.60

0.40

N/A

N/A

10
9∼10
8b

1.60

0.40

N/A

N/A

10
12∼10
11b

(this work) poly(AN-co-HSA)/plasticizer-free vinyl resin (this work) N,N-diheptyl-N,N0 ,6,6-tetramethyl-4,833

dioxaundecanediamide/PVC+o-NPOE N,N-diheptyl-N,N0 ,6,6-tetramethyl-4,8dioxaundecanediamide/copolyacrylate IDA/ACN/HDDA+o-NPOE33

t-butylcalix[4]arene-tetrakis N,N-dimethyl thioacetamide/plasticizer-free pBA-HEMA chromogenic/PVC-o-NPOE43 43

chromogenic/plasticizer-free MMA/nBA a

42

Determined by LA-ICPMS technique. Determined by spectrophotometry for optode film. b

initial concentration decreased from 10
5 to 10
6 M, the selectivity to Pb(II) is even more outstanding. At the same time, such binding ability to aqueous Pb(II) just requires 1
2 min to reach equilibrium (Figure S3 in the Supporting Information). Diffusion Coefficient of Pb(II) in the Sensing Membrane. To exert a synergistic effect of the two materials, an entirely new sensing membrane containing the solid copolymer ionophores embedded in vinyl resin was designed and assembled. Such copolymer ionophores are anchored in the sensing membrane matrix and thus possess an intrinsic ability to inhibit the mobility of primary ions and therefore reducing or even eliminating transmembrane ion fluxes (Scheme 1). This has been confirmed by an earlier report on a covalent binding of an ionophore onto gold nanoparticles in order to reduce ion fluxes in the membrane.39 Electrochemical impedance spectroscopy (Figure S4 in the Supporting Information and Table 1) revealed that the plasticizer-free solid sensing membrane entrapping copolyaniline has a bulk resistance of 3.6  107 Ω which is about 2 orders of magnitude higher than the PVC one containing DOP plasticizer and the same copolyaniline ionophore. The diffusion of Pb(II) ions in the plasticizer-free solid membrane entrapping copolyaniline can be approximately described by a Warburg diffusion process, as shown in Figure S4c in the Supporting Information, by a line in a Nyquist plot at low frequencies with a slope of about 0.6. Other linear behavior was observed (Figure S5 in the Supporting Information) by fitting the real part of impedance resistance vs (angular frequency)
1/2, from which the Warburg coefficient of 1.0  107 Ω s
1/2 can be obtained based on the slope of the latter line. The Pb(II) diffusion coefficient in the membrane estimated based on this Warburg coefficient40 is in a range from 5  10
14 to 1  10
13 cm2 s
1, which is about 6 orders of magnitude lower than typical PVC formulations of 4.0  10
8 cm2 s
1.41 It must be appreciated that such a low diffusion coefficient could not be achieved by a copolyacrylate-based membrane whose diffusion coefficient is 10
12∼10
10 cm2 s
1,42,43 though there is no plasticizer in this membrane either, but the ionophore added is a liquid compound. The much lower diffusion coefficient for Pb(II) confirms the suppression effect of transmembrane ion flux. As a result, leakage of primary ions from the sensing membrane to

In particular, their cross peak at close chemical shifts (7.43, 7.37 ppm) due to the correlation between aromatic α and β protons has been revealed in the H,H-2D-COSY spectra of both PAN and copolyaniline. This verifies the important fact that the HSA monomer has copolymerized with AN monomer and thus formed a true AN/HSA copolyaniline. MALDI-MS results summarized in Table S1 in the Supporting Information also demonstrated the synthesis of copolyaniline containing an AN/HSA molar ratio of 5/1 and with a molecular weight up to 2019.6. Moreover, the fine copolymer particles have an ellipsoid shape with an average diameter of 250
300 nm, and the nanoparticles are porous with a BET area of 25 m2 g
1, average pore-diameter of nearly 30 nm, total micropore volume of 0.15 cm3 g
1 (Figure S1 in the Supporting Information), and intrinsic electrical conductivity of 10
3∼10
2 S cm
1. The accomplishment of the electrically conducting nanoparticles is mainly attributable to the unique self-stabilized effect originated from intrinsic static repulsion among negatively charged
SO3
groups on the HSA moiety.37,38 These physical properties, along with many ligating functional groups such as
NH
,
N=,
OH,
SO3H, and
NH2 on the copolymer nanoparticles, give them the possibility to act as a kind of powerful cation carrier. These five types of ligating functional groups can form complexes with Pb(II) ions (Scheme 1c). The porous, loose, and amorphous morphological structure gives a high surface area, so that more functional groups are exposed, resulting in more efficient interaction between the copolymer chains and metal ions. However, only for the copolymer to be a highly selective ionophore, a few kinds of metal ions or even one kind of metal ion are expected to be favorably complexed. In order to examine this vital possibility, a small amount of poly(AN-co-HSA) was incubated in an individual metal ion aqueous solution for a period of time to give distribution of various metal ions in the copolymer (Figure S2 in the Supporting Information). It was surprising but very pleasing to find that poly(AN-coHSA) nanoparticles demonstrated prior and high selectivity for Pb(II) and Hg(II) compared to other metal ions at the same initial micromolar concentration with the distribution percentage of Pb(II) in the region of 96.7
98.1%. Moreover, when the 137

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concentration is 104.5, about 30 000 times higher than the Pb(II) concentration, so many H+ ions did not interfere with the potential response. On the other hand, for Pb(II) solution at pH 4.0
7.0, since not all lead ions are in the free form, the complexed lead form can also result in Nernstian response. This unique feature may be attributed to the specific solid ionophore, i.e., HCl-doped copolyaniline that can release H+ to the sample solution near the membrane side. Moreover, the OH
from complexed species including Pb(OH)(I) near the membrane will promote the release of H+ on the copolyaniline ionophores in the membrane to some extent, resulting in the recovery and response of free Pb(II) as the sample solution pH increases slightly. Meanwhile, the influences of ionophore particle size and membrane composition were further explored. In all cases investigated, small size (Figure 2a) and the membrane composition of 1:2:30 are optimal (Figure 2b). In fact, this potentiometric sensor shows much stronger improvement than the other ones also based on solid ionophores (i.e., polyaminoanthraquinone44 and polyphenylenediamine45 particles) but dispersed in the externally plasticized PVC membrane with the detection limit of only at 10
7 M order of magnitude. Other conducting polymers have been used to construct a solid-contact potentiometric sensor, but they act as an ion-to-electron transducer rather than an ionophore in potentiometric sensor.46 On the other hand, a pure vinyl resin membrane without any copolyaniline ionophores gives a very narrow linear response and poor detection limit (Figure 2b) because the interaction between matrix polymer and Pb(II) is much weaker than that with the copolyaniline ionophores. All these results indicate that the combination of the solid ionophores and plasticizer-free vinyl resin is responsible for the great enhancement of the sensitivity of the electrode sensor. Sensor Selectivity. The selectivity of the potentiometric sensor for Pb(II) is also excellent with a logarithmic selectivity coefficient of
3.1 to
6.7 for metal ions (Table 2). Comparison of selectivity coefficients of the potentiometric sensor for Pb(II) based on the new sensing membrane with the other ones having improved detection limits of better than 10
8 M is summarized in Table S2 in the Supporting Information. It can be seen that except for good selectivity of Pb(II) over alkali metal ion and alkaline earth metal ions, the new sensing membrane-based potentiometric sensor for Pb(II) has an excellent selectivity over Cu(II) and Cd(II), which is apparently superior to the PVC membrane based on ETH5435 ionophores and 1.0  10
12 M

samples and vice versa are eliminated and a robust potentiometric sensor for Pb(II) with subnanomolar detection limits is developed. Sensor Response. The potential responses of the potentiometric sensor with the smooth solid sensing membrane, but still using a conventional inner reference system, are shown in Figure 1a. A linear Nernstian response range for Pb(II) is extended to 1.0  10
10 M and the detection limit is lowered to 2.2  10
11 M when the concentration of Pb(II) in IFS is 10
5 M, which is better by about 5 orders of magnitude than that of the conventional PVC membrane with the same inner electrolytes. The linear analysis fit in the Pb(II) concentration range from 1.0  10
10 M to 1.0  10
3 M gives a perfect linear relationship with a Nernstian slope of 29.3 mV decade
1 at a correlation coefficient of 0.9994 and standard deviation of 2.95. The linear range spans 7 orders of magnitude. At the same time, a very quick response with a response time of 22 s (Figure S6 in the Supporting Information) and an adequate pH window from pH 3.5 to pH 7.0 (Figure 1b) are observed. Such a high performance sensor for Pb(II) at subnanomolar levels is rare. It should be emphasized that the Pb(II)-ISE shows a wide working pH range of 3.5
7.0, which means the sensor possesses an excellent actual anti-interference ability to H+. For example, for the Pb(II) solution of 1.0  10
8 M at pH 3.5, whose H+

Figure 1. (a) Response curves and (b) pH windows of a potentiometric sensor based on the solid poly(AN-co-HSA) ionophores dispersed in vinyl resin membrane filling with conventional inner electrolytes of 1.0  10
5 M Pb(NO3)2 rather than metal-ion buffer. The sensing membrane without any plasticizer is composed of poly(AN-co-HSA) nanoparticles/NaTPB/vinyl resin of 1:2:30 in weight ratio with a membrane thickness of ∼60 μm.

Figure 2. Potentiometric response curves of solid membrane Pb(II)-ISE based on poly(AN-co-HSA) copolyaniline ionophores with (a) three copolyaniline particle sizes (synthesized at three polymerization temperatures) in the membrane composition of (poly(AN-co-HSA)/NaTPB/vinyl resin = 1:2:30) with a thickness of ∼60 μm and (b) five membrane compositions with a membrane thickness of ∼60 μm and the same inner filling solution is 1.0  10
5 M Pb(NO3)2. 138

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Table 2. Logarithmic Selectivity Coefficients Measured by Fixed Interference Method (Fixed at 10
3 M) for the Potentiometric Sensor for Pb(II) interfering ion

H+

Li(I)

Na(I)

NH4(I)

K(I)

Mn(II)

Ca(II)

Mg(II)

Ba(II)

Co(II)

logKpot Pb,J


3.24


3.23


3.12


3.10


2.98


5.37


5.65


5.60


5.50


5.53

interfering ion

Al(III)

Fe(III)

Cr(III)

Sr(II)

Ni(II)

Cd(II)

Cu(II)

Zn(II)

Ag(I)

Hg(II)

logKpot Pb,J


6.70


6.64


6.39


5.36


5.31


5.27


5.19


5.09


1.39


4.02

Table 3. Measurement of Pb(II) Content in Real Samples by a Direct Potentiometric Method real samples

pH

by potentiometric sensor (nM)

RSD (%)

river water

6.9

10.2

4.69

rainwater

6.7

26.2

2.34

25.9

1.16

tap water

7.3

37.0

4.14

35.9

3.06

wastewater

6.1

green gram

4.1

human urine

3.9

159

1.82

71.6

6.31

4.08

Pb(II) IFS,9 and its selectivity is comparable to that of the potentiometric sensor for Pb(II) based on ETH5234-polyurethane ionophore.26 As for Ag(I), the new potentiometric sensor for Pb(II) shows better anti-interference ability than that based on the ETH5234-polyurethane ionophore.26 As for Hg(II), which appears to be a severe interfering ion for the potentiometric sensor for Pb(II), the new electrode sensor fabricated here also possesses stronger anti-interference than others in Table S2 in the Supporting Information. The selectivity toward Pb(II) over Hg(II) is comparable with that of the highly Pb(II) selective electrode based on crown DMCDA18C6.47 It is appreciated that the new electrode sensor exhibits an excellent selectivity over H+, which is consistent with the sufficient pH plateau for convenient potential measurement in Figure 1b. Sensor Lifetime. Long-term inspection of the potentiometric sensor for Pb(II) gives a long lifetime (Figure S7 and Table S3 in the Supporting Information). The promising sensor can be used for at least 3 months without any measurable response decay. After 6 months’ usage, the slope was the same, although the detection limit shifted slightly upward. After 15 months, the detection limit still reaches down to 6.0  10
10 M, and the response slope maintains 95% of the original one with a little deterioration of the response time and pH window. The durability is much superior to both liquid-contact30,48 and solidcontact29,47,49 potentiometric sensors for Pb(II) with a detection limit below 10
8 M (Table S4 in the Supporting Information). Obviously, it is the solid sensing membrane system suffering leaching of neither ionophore nor plasticizer that guarantees a long-term usage. Real Sample Assay. By the optimized potentiometric sensor for Pb(II) with very low detection limits, wide linear range, and excellent selectivity along with long lifetime, the measurements of trace Pb(II) in real-world samples have become feasible and samples ranging from environmental waters, food, and human urine were examined. The Pb(II) concentrations in the samples were also determined by atomic absorption spectrometry (AAS) and used as the true value for calculation of relative error. As summarized in Table 3, both relative standard deviation (RSD) for 10 times determination and relative errors are within 10% for various real samples at trace Pb(II), which presents a reliable measurement by the direct potentiometry and may extend the

8.13

by AAS (nM) 9.91

158 68.1 3.81

relative error (%) 2.93

0.633 5.14 7.09

scope in clinical analysis using a potentiometric sensor for Pb(II).50

’ CONCLUSIONS We have developed a plasticizer-free solid sensing membrane simply by immobilizing conductive copolyaniline nanoparticles in a vinyl resin matrix, significantly addressing intractable problems of conventional PVC membranes, including transmembrane ion fluxes due to mobility of lipophilic ionophores in the plasticizer phase and short lifetime due to leakage of plasticizer but without compromise. The Pb(II) potentiometric sensor has unique comprehensive performance which allows the analysis of Pb(II) in real samples in the subnanomolar region precisely, efficiently, and conveniently. It can be expected that picomolar or even femtomolar detection limits could be achieved after further optimization, especially by using proper metal-ion buffer, and potential applications such as monitoring and screening Pb(II) in drinking water or even blood seem prospective. Additionally, the unique methodology for the zero primary ion flux sensing membrane by the combination of conducting nanoparticles and plasticizer-free matrix may be expanded to fabricate durable ultrasensitive potentiometric sensors for other metal ions besides lead ions. ’ ASSOCIATED CONTENT

bS

Supporting Information. SEM image, size/pore distribution, and nitrogen adsorption
desorption isotherm plots of poly(AN-co-HSA) particles; distribution percentage of metal ions on poly(AN-co-HSA) particles; complexation equilibrium time of Pb(II) onto poly(AN-co-HSA) particles; Bode and Nyquist form diagrams of impedance spectra of the carbon electrode coated by DOP-plasticized PVC entrapping poly(AN-co-HSA) ionophores, plasticizer-free solid vinyl resin membrane entrapping PAN ionophores, and plasticizer-free solid vinyl resin membrane entrapping poly(AN-co-HSA) ionophores; plot of impedance Zre as function of inverse square root of angular frequency ω
1/2 for the electrodes based on the membrane containing the poly(AN-co-HSA) ionophore; response time, stability of the potentiometric sensor with usage time;

139

dx.doi.org/10.1021/ac2028886 |Anal. Chem. 2012, 84, 134–140

Analytical Chemistry

ARTICLE

proposed structure of poly(AN-co-HSA) molecules; performance comparison between the sensor and other ones; variation of performance and corresponding parameters of the proposed potentiometric sensor for Pb(II) with usage time. This material is available free of charge via the Internet at http://pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (M.-R.H.); adamxgli@ yahoo.com (X.-G.L.).

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