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Sulfur-Containing Functional Groups
CHINESE JOURNAL OF ANALYTICAL CHEMISTRY Volume 40, Issue 1, January 2012 Online English edition of the Chinese language journal
Cite this article as: Chin J Anal Chem, 2012, 40(1), 50–58.
REVIEW
Development of Potentiometric Lead Ion Sensors Based on Ionophores Bearing Oxygen/Sulfur-Containing Functional Groups HUANG Mei-Rong1,*, GU Guo-Li1, SHI Feng-Ying2, LI Xin-Gui1 1
Institute of Materials Chemistry, Key Laboratory of Advanced Civil Engineering Materials of the Ministry of Education, College of Materials Science and Engineering, Tongji University, Shanghai 200092, China 2 Architectural Engineering College, Shanghai Normal University, Shanghai, 201418, China
Abstract: Potentiometric Pb2+ sensors based on ionophores bearing oxygen/sulfur-containing functional groups, which include anthraquinone, aromatic carboxylic acids, carboxylates, phosphates, and macrocyclic compounds containing amide or thioamide groups, are systematically summarized. Anthraquinone is found to be quickly responsive to lead ions, with the shortest response time of two seconds. Aromatic carboxylic acids with a sulfur atom as an ionophore in an asymmetric membrane respond sensitively to lead ions, with the lowest detection limit of 6.0 u 10–10 M. Phosphate has the advantage of better anti-interference ability, with a logarithmic selectivity coefficient, logK, smaller than –3.0. Macrocyclic amides, after assembly into microelectrodes, have been used as ionophores for the analysis of samples confined in cells of volume 3 PL, with a superior lower detection limit of 2.7 u 10–9 M. Key Words: Potentiometric sensor; Lead ion sensor; Ionophore; Anthraquinone; Aromatic carboxylic acid; Phosphate; Thioamide; Review
1
Introduction
Many analytical techniques, such as atomic absorption spectrometry[1], atomic fluorescence spectrometry[2], inductively coupled plasma mass spectrometry[3], dithizone spectrophotometry[4], and colorimetry[5], can be used for the quantitative analysis of lead ions. Especially, in the case of fluorescent chemosensors[6–8] reported in recent years, the lower detection limits for lead ions are improved and reach 5 × 10–8 M (10 parts per billion, ppb), which is lower than the maximum contamination level for lead (72 nM, 15 ppb) in drinking water as defined by the US Environmental Protection Agency. However, the prohibitive cost of the use and maintenance of equipment, together with the requirement for the professional skills of an operator, make these methods almost remain confined to the laboratory. On the contrary,
potentiometric sensors based on the trapping of ionophores by a sensing membrane, a fast, efficient, and low-cost analytical technique, are particularly suitable for outdoor operations, and therefore, for the on-line monitoring of ions. Since the late 1960s, a variety of high-performance potentiometric sensors, such as potentiometric sensors optimized by using an ionic buffer as an inner filling solution[9,10] and polyacrylate as the matrix[11–13], have been developed for lead ions. Numerous applications have been found for these sensors in the real-time monitoring of wastewater discharged from storage batteries, cables, and metallurgical industries, in addition to applications in the context of safety inspection of mineral water and foodstuffs. In recent years, potentiometric sensors have not been limited only to single head-mode analysis; a multiprobe automatic potentiometric sensor array[14] has already been used in the
Received 30 June 2011; accepted 26 August 2011 * Corresponding author. Email: [email protected] This work was supported by the National Natural Sciencpe Foundation of China (No. 20974080). Copyright © 2012, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-2040(11)60521-5
HUANG Mei-Rong et al. / Chinese Journal of Analytical Chemistry, 2012, 40(1): 50–58
instant quantitative analysis of heavy metal ions in submicromolar concentrations. The considerable progress and development achieved in these fields are inextricably related to the development of excellent ionophores. The ionophore is a vital component embedded in the sensing membrane, which largely determines the latter’s performance, especially the anti-interference capability of the sensor. Undoubtedly, the ionophore is the only guarantee for the anti-interference property of the sensor. Excellent ionophores enable the sensor to respond exclusively to the target ions while being almost irresponsive to other coexisting ions. Up to now, the ionophores developed for the selective coordination of Pb2+ include macrocyclic compounds such as crown ethers, calixarenes with the right cavity size[15,16], nitrogen compounds[17], polymers[18], and some O/S atom-enriched organic compounds. Based on the hard and soft acid-base theory, the O/S atom-enriched organic compounds are the soft-base ligands that can easily form stabler complexes with soft-acid metal ions such as Pb2+; therefore, they are sensitive to even trace amounts of Pb2+. These soft-base compounds, which have attracted increased attention recently, include anthraquinones, aromatic carboxylic acids, carboxylates, phosphates, and macrocyclic compounds containing an amide or thioamide group.
2 Potentiometric sensors based on anthraquinone Anthraquinones used as ionophores for potentiometric Pb2+ sensors are 9,10-anthraquinone derivatives, which mainly have the hydroxyl, alkenyl, or alkoxyl groups. The carbonyl oxygen on the quinone, along with the hydroxyl and ether groups, forms a cavity structure, which can coordinate with metal ions, particularly Pb2+, in nonaqueous solvents in the ratio 2:1 (ligand: metal ion) to form stable complexes. Additionally, the alkalinity or acidity and the lipophilicity of the anthraquinones can be adjusted by controlling the lengths of the side chains. Therefore, the use of anthraquinone derivatives as lead ion ionophores in potentiometric sensors has increased recently, as shown in Table 1. Comparative studies have shown that 9,10-anthraquinone derivatives bearing only one or two hydroxyl groups do not function well as lead ion ionophores. Only by the introduction of a certain length of double bond-containing aliphatic chains in a position adjacent[19] to the hydroxyl group, or the allyl ether groups[20], do these derivatives show a significantly improved ability to coordinate with lead ions. Furthermore, two aliphatic side chains result in better performance than only one[19,20], owing to both the lipophilicity of the electron-donating S-bonds in the side chain and the synergistic effect of the phenolic hydroxyl groups while chelating the lead ions. The introduced double bond-containing aliphatic chains are mostly allyl groups, and the double bonds at the terminal position enable them to easily complex with the metal ions to
form a stabler cyclic cavity structure[19]. Thus, among the series of Pb2+ ionophores, 1,8-dihydroxy-2,7-dipropenyl-9,10anthraquinone has the best performance (Table 1). Regarding the anti-interference performance, a ring structure formed between the ionophores and the metal ions seems selective toward lead ions due to the formation of cyclic complexes. Except for the common interfering ions, such as Hg2+ (logarithmic selectivity coefficient, logK = –2.0), Cu2+ (logK = –1.7), and Na+ (logK = –2.2), which have a little interference, other metal ions hardly ever show any interference. Although 1,4-bis(prop-2ƍ-enyloxy)-9,10-anthraquinone, containing allyl ether groups, has a performance similar to 1,8-dihydroxy-2,7-diallyl-9,10-anthraquinone in terms of the ability to detect lead ions, its anti-interfering ability is one order of magnitude worse. Most metal ions, whether alkaline metal ions, alkaline earth metal ions, or transition metal ions, can cause interference in the detection of lead ions, with their logK values ranging from –1.3 to –2.9[20]. Among the interfering ions, Ag+ (logK = –1.3) and Tl+ (logK = –1.9) have the strongest interference. Obviously, it mainly arises from the unstable cyclic coordination of this type of structure with metal ions because of the presence of a single complexation site on the single oxygen functional group. Therefore, most metal ions can readily interfere with the ionophore’s detection ability, consequently leading to lower selectivity. An effective approach to improve the lipophilicity and enhance the capability of coordinating with lead ions is introducing an ether bond-containing aliphatic chain in the ortho-position of the phenolic hydroxyl group of the anthraquinone molecule. The introduced aliphatic chain can not only alter its acid-base character greatly and improve the lipophilicity, but also exert its lariat effect in the process of coordination with metal ions and hasten the coordination between the compound and the metal ions. At the same time, the anti-interference capability is additionally enhanced. For OH
example, the introduction of a side chain [21]
O
n
(n =
1–5) onto the position ‘2’ of 1-hydroxy-9,10anthraquinone generates five types of anthraquinone derivatives with polyether chains, all of which have excellent lead ion–detecting performance; moreover, their lower detection limits are around 10–6 M, with a Nernstian slope above 26.0 mV/decade for all derivatives. However, too long a side chain will produce steric hindrance and impede coordination. Therefore, among the derivatives with polyether side chains, those with n = 3 have the best Nernstian response (Table 1), with a response time as short as two seconds, which is the fastest potentiometric sensing time for lead ions until date. Regarding the anti-interfering capability of these sensors against divalent metal ions, such as Cd2+, Cu2+, and Hg2+ that cause the strongest interference, the logarithmic selectivity coefficients are –2.0, –1.9 and –1.7, respectively, whereas most other metal ions have little or negligible interference.
HUANG Mei-Rong et al. / Chinese Journal of Analytical Chemistry, 2012, 40(1): 50–58
Table 1 Response characteristics of potentiometric lead ion sensors based on anthraquinone derivatives as ionophores Ionophore
Slope (mV/decade)
Molecular structure
Linear range (M)
Detection limit (M)
Response time (s)
pH
Lifetime (month)
29.0
2.0 × 10–6–2.0 × 10–3
1.1 × 10–6
30
6.0
4
29.8
2.5 × 10–6–1.0 × 10–2
1.5 × 10–6
15
4.7–6.8
4
28.9
1.0 × 10–6–1.0 × 10–2
6.7 × 10–7
2
1.5–6.8
4
29.5
1.0 × 10–7–1.0 × 10–2
8.0 × 10–8
10
3.5–6.8
2
28.9
2.5 × 10–6–1.0 × 10-1
7.8 × 10–7
12
2.8–5.2
4
O
1,8-Dihydroxy-2,7-bis(prop-2’enyl)-9,10-anthraquinone[19] O
OH O
OH O
1,4-Bis(prop-2’-enyloxy)-9,10anthraquinone[20] O O
O
OH
1-Hydroxy-2-({2-[2-(2-hydrox yethoxy)ethoxy]ethoxy}methyl )-9,10- anthraquinone[21]
OH 3
O
O
O
1-Hydroxy-2-(2-ethanoloxymet hyl)-9,10-anthraquinone[22]
OH OH
O
O
Polyaminoanthraquinone[23]
O H
O H N
n
H
The potentiometric sensors for lead ions mentioned herein are liquid-contact sensors based on conventional plasticized polyvinyl chloride (PVC) membranes. For derivatives with polyether chains (when n = 1), such as substituted anthraquinone, which may be used as the ionophore, if the plasticized PVC-based sensing membrane is assembled on the surface of a graphite electrode to form a solid-contact sensor[22], the performance can be substantially improved, with the Nernstian slope increasing from 26.4 to 29.5 mV/decade, and the corresponding lower detection limit is lowered from 7.8 u 10–6 to 8.0 ² 10–8 M. This improvement may be attributed to the additive, oleic acid, which not only acts as a phase-conversion catalyst, but also is involved in the coordination with Pb2+. This deduction can be verified by the influence of its dosage on the performance of the resulting sensor. On varying the amounts of oleic acid, ranging from one to three times the ionophore concentration, the Nernstian slope increases from 24.0 to 29.5 mV/decade. In addition, compared with the liquid-contact PVC-membrane sensor, this solid-contact graphite sensor has better anti-interference performance, except for the interference from Hg2+ (logK = 2.0); however, the interference from other ions could be ignored. Poly(1-amino anthraquinone), synthesized by simple chemical oxidative polymerization, also shows a strong coordinating capability with lead ions. With poly(1-amino anthraquinone) as the lead ion ionophore, the liquid-contact sensor based on plasticized PVC membrane has a relatively wide linear range, excellent lower detection limit, and short response time. Furthermore, the sensor has a good selectivity for lead ions in relation to alkali and alkaline earth metal ions,
with a logarithmic selectivity coefficient less than –1[23].
3
Potentiometric sensors based on carboxylic acid derivatives
Carboxylic acid derivatives can coordinate with metal ions and form stable complexes because the carbonyl group of –COOH and the O atom on the hydroxyl group can offer lone pairs of electrons for the empty orbitals of the metal ions. Similar to anthraquinone ionophores, the formation of a cavity structure, lipophilicity, and chain length are the main factors influencing their selectivity to lead ions (Table 2). Carboxylic acid derivatives may vary their ratio in relation to lead ions from 1:1 to 1:2 during coordination, which is related to the specific unit structure of the molecule. Fatty acids, as one of the important groups of carboxylic acids, have been successfully used in potentiometric sensors, showing the most sensitive response to Pb2+ compared to that for all other cations. However, not all the fatty acids have an ideal Nernstian response to Pb2+ due to the different lengths of the molecular chains. For example, among the four fatty acids oflengths C7, C10, C14 and C18, capric acid (C10) has the best sensing performance over a wide pH range, a relatively high Nernstian slope, and a three-month lifetime when used in PVC-membrane-based potentiometric sensors[24]. Because fatty acids coordinate with Pb2+ through their single O-atom functional groups, the resulting complexes cannot stabilize in this type of cyclic or semicyclic structures, and, thus, alkali metal ions can easily interfere with the coordination process. However, for the same reason, the transition metal ions (except for Ag+) only slightly affect the detection of lead ions.
HUANG Mei-Rong et al. / Chinese Journal of Analytical Chemistry, 2012, 40(1): 50–58
Table 2 Response characteristics of potentiometric lead ion sensors based on aromatic carboxylic acids as ionophores Ionophore Capric acid
Molecular structure
[24]
CH3(CH2)8COOH O
C
SH
O
2,2’-Dithiodibenzoic acid
C
O C
[26]
O2 N
8-(Dodecyloxy)quinoline-2-carbox ylic acid[27]
Trans-1,2-diaminocyclohexane N,N,N’N’-tetra acetic acid[28] Humic acid[29]
Detection limit (M) 6.0 × 10–-6
4.5–7.0
23.8
1.9 × 10–6–6.2 × 10–4
4.0 × 10–8
4.0–8.0
21.8
8.7 × 10–8–6.2 × 10–4
5.0 × 10–-8
4.4–5.5
29.1
1.0 × 10–-9–1.0 × 10–3
6.0 × 10–10
3.5–9.0
33.0
1.0 × 10–6–1.0 × 10–2
5.0 × 10–7
4.5–6.5
51.7
1.0 ×10–5–1.0 ×10–2
8.9 × 10–5
5.5–10
28.6
3.2 × 10–6–6.3 × 10–3
2.0 × 10–6
4.5
pH
OH S
S HO
5,5’-Dithiobis(2-nitrobenzoic acid)
Linear range (M) 1.0 ×10–5–1.0 × 10–2
OH
2-Mercaptobenzoic acid[25]
[25]
Slope (mV/decade) 29.0
C
O
NO2
OH S
S
HO
C O
O N C O OH CH3(CH2)11
N(CH2COOH)2 N(CH2COOH)2
Cross linking organic macrocompounds
Therefore, the ionophore has great advantages in the detection of lead ions in aqueous solutions containing various heavy metal ions. The pH can also affect the performance of sensors based on aliphatic carboxylic acids in different ways from the influence of pH on sensors based on other ionophores. In addition to its interference with lead ion detection, H+ is also in an ion-exchange chemical equilibrium, namely, the equilibrium between Pb2+ and H+. Higher H+ concentration causes the equilibrium to shift toward the formation of acid rather than toward release of Pb2+. This circumstance is not conducive to the formation of complexes. As a result, the sensor is unsuitable for the detection of lead ions in acidic solutions. Coordination to Pb2+ can be enhanced by the introduction of a soft active-site S atom into carboxylic acid derivatives, such as thiosalicylic acid and dithiodibenzoic acid. After dipping in the lead ion solution for a period of time, aggregates of white needles on the surface of the sensing membrane are observed under a scanning electron microscope, which confirms the complex formation between lead(II) ions and the carbon paste modifier[25] with different atomic ratios of S:Pb:thiosalicylic acid, the ratio is 1:1, and for dithiodibenzoic acid, the ratio is 1:2. Because dithiodibenzoic acid is equivalent to two dehydrogenized and bridge-connected thiosalicylic acid molecules, the former contains twice as much carboxyl groups and S atoms as that of the latter. Moreover, both can coordinate with Pb2+ in a stable cyclic structure, resulting in an improved sensing performance when compared with aliphatic acids. For example, their detection limits are as low as 10–8 M[25], which is at the cost of their Nernstian slopes (Table 2). In terms of selectivity, their logarithmic selectivity
coefficients, logK, are both lower than –4, which is much better than those of aliphatic acids. However, their anti-interference property with reference to transition metal ions, such as Fe3+, Hg2+ and Cu2+, is worse than that of the other ionophores, because the soft active sites can not only coordinate with Pb2+, but also with other heavy metal ions and, thus, form stabler complexes. The selectivity coefficients could move up to 103, which indicates that Fe3+ and Hg2+, even in low concentrations, will cause a big influence on the detection of Pb2+. From another viewpoint, these types of ionophores may be more suitable for the detection of Fe3+ or Hg2+ by the sensors. Thus, 5,5-dithiobis(2-nitrobenzoic acid)[26], an ionophore with similar structure, can be used to prepare an asymmetric sensing membrane in a microelectrode sensor by dip-coating PVC on a cross-section of a gold microwire electrode, which is sealed in a soft glass capillary. This sensor is much better than a graphite-paste electrode-based membrane sensor in terms of the potential-response slope, linear range, and particularly, the lower detection limit (Table 2). It indicates another worthwhile research direction toward the fabrication of sensing membranes having an asymmetric structure. Carboxylic acids containing quinoline substituent groups can also coordinate with a variety of metal ions and form stable complexes. The stability of these complexes is related greatly to the substituents connected to the aromatic ring. For example, quinoline-2-carboxylic acids substituted by alkyl or alkoxyl groups have a better extraction ability toward Cu2+ and Zn2+; 8-benzoylmethyl-quinolinic acid has a stronger extraction ablity toward Cd2+ and Cu2+ in relation to other metal ions; and 8-(dodecyloxy)quinoline-2-carboxylic acid
HUANG Mei-Rong et al. / Chinese Journal of Analytical Chemistry, 2012, 40(1): 50–58
has an excellent extraction ability toward Pb2+ and Cu2+ [27]. The alkoxyl group on quinoline, an electron donor, changes both the cloud density over the aromatic ring and the character of the group on the ȕ-position of the quinoline ring and, thus, changes the stability and alkalinity or acidity. Similarly, the steric effect of the substituent group also has the same effect. Moreover, 8-(dodecyloxy) quinoline-2-carboxylic acid, an ionophore, coordinates with Pb2+ in the ratio of 1:2, based on which the sensor has a relatively high Nernstian slope of 33.0 mV/decade[27], a lower detection limit, and excellent selectivity. Except for Cu2+, the log K of other metal ions ranges from –3.1 to –4.5, having almost no interference. However, no reports on the interference by Hg2+ and Ag+ are found thus far. In addition, the service life of the sensor is only 40 d. Recently, multiple carboxyl group-containing chelating agents, such as trans-1,2-diamino cyclohexane N,N,Nƍ,Nƍtetra-acetic acid (DCTA), have been applied as lead ion ionophores[28]. With a well-dispersed DCTA ionophore, the sensor based on a spectrum-level active graphite paste electrode has an excellent linear response to lead ions, with the detectable concentration ranging from 10–5 to 10–2 M, and the response slope is as high as 51.7 mV/decade (Table 2). However, its anti-interference capability toward Hg2+ and Ca2+ is degraded due to its strong coordinating ability. Therefore, choosing a right chelating agent as ionophore is highly important. Aquatic humic substances, when used as ionophores in PVC membrane–based potentiometric sensors[29], have theoretical Nernstian slope and a better lower detection limit toward lead ions but a narrow linear range extending only within three orders of magnitude, and they can only be used in a medium with pH around 4.5. In a medium with lower pH, the existence of large amounts of H+ will reduce the stability of the complexes, whereas a medium with higher pH, lead ions are prone hydrolysis or even settle to form Pb(OH)2. However, a sensor with humic substance as the ionophore has excellent anti-interference capability, and the selectivity coefficients toward alkali and most of the alkali metal and transition metal ions are less than 10–2. Only Hg2+ has unignorable interference, and its selectivity coefficient is 10–0.2. In addition, the dosage of humic substance also heavily affects the sensing performance. Too low a dosage will decrease the sensing performance, and excessive dosage will easily generate complexes between the charged humic substance and Pb2+ within the membrane phase in the concentrated Pb2+ solution, resulting in electrostatic repulsion toward the sample solution and, therefore, the deterioration of the sensing performance. The greatest advantage of humic substance over other ionophores is its natural source, that is, it can be obtained without a complicated synthesis process. Although the sensing performance of sensors based on humic substance as a potentiometric lead-ion ionophore, which has not been studied well until date, is not particularly spectacular, more studies on its sensing performance as lead ion ionophore are worth
carrying out considering its excellent lower detection limit.
4
Potentiometric sensors based on carboxylate and phosphate
In recent years, several studies have been reported about the electron donating character of the phosphoryl groups present in phosphates and carboxylates used in potentiometric cation-selective sensors, and the main issue of concern in all the reports is the coordinating performance of the P- and O-atom-containing groups toward metal ions. The earliest report is about the application of dihydroxy phsphate in Ca2+-selective electrode sensors[30]. A later study showed that organophosphates have a higher extraction constant for Pb2+, Cu2+, and Ag+ than for Ca2+, implying that they have stabler coordination and better sensing performances compared to the performances for Ca2+ when used in ion-selective electrode sensors. It also means that severe interference may originate from Cu2+ and Ag+ when they are used in lead ion sensor. Therefore, Cu2+ and Ag+ are always the biggest interferences in all organophosphate-based potentiometric sensors. Especially, in the case of Ag+, the selectivity coefficient is in the range 1–10. The second-strongest interference is from alkaline metal ions, such as Na+ and K+, which have selectivity cofficients of around 0.01–0.1. Fortunately, other divalent ions have almost no interference. Although the selectivity of the phosphates toward Pb2+ is achieved by the stable coordination between phosphoryl groups and Pb2+, an organophosphate with two phosphoryl groups does not definitely have a better sensing performance than that with only one group[31,32] (Table 3). Their detection ranges are in the range 10–5–10–2 M, and their lower detection limits are around 10–6 M. Dibenzyl phosphate has a relatively outstanding performance, whose detection ranges between 10–6 and 10–2 M, the detection limit is as low as 9.0 u 10–7 M, the response time is dozens of seconds, and the adapted pH range is 4–6. It reflects the fact that the number of phosphoryl groups is not a critical factor for sensing performance and that the synergistic effect of O and P atoms during coordination may play a key role in achieving the detection of lead ions. In recent years, aromatic carboxylic acid esters have also been used as potential lead ion ionophores. A PVC membrane-forming solution containing 2% of benzyl-2-(3(phenylhydrazone)-2-oxygenindole)phenyl acetic acid ester and 4% of carbon powder is dip-coated five times to modify the platinum wire electrode. The resultant sensor has a moderate linear range from 7.2 u 10–5 M to 1.0 u 10–2 M but a poor response slope of 26.6 mV/decade. Apart from this study, no other carboxylic acid ester ionophores have been reported yet for the fabrication of lead ion sensors. Further study on this type of ionophore needs to be carried out in the context of its response performance to lead ions. The capability of organophosphates to coordinate with lead
HUANG Mei-Rong et al. / Chinese Journal of Analytical Chemistry, 2012, 40(1): 50–58
Table 3 Response characteristics of potentiometric lead ion sensors based on phosphates and carboxylates as ionophores Ionophore
Molecular structure
Triphenylphosphine oxide[31]
P 3
Slope (mV/decade)
Linear range (M)
Detection limit (M)
28.0
5.0 × 10–5–1.0 × 10–2
6.0 × 10–6
O
Dibenzyl phosphate[32]
OH CH2O P O 2
30.1
3.0 × 10–6–1.0 × 10–2
9.0 × 10–7
Diphenyl phosphate[32]
OH O P O 2
30.0
1.0 × 10–5–1.0 × 10–2
4.0 × 10–6
31.3
1.0 × 10–-4–1.0 × 10–2
4.0 × 10–5
26.6
7.2 × 10–5–1.0 × 10–2
—
Bis(2-ethylhexyl) phosphate[32]
O CH3CHCH2CH2CH2CH2 CH2CH3
Benzyl 2-(3(phenylhydrazono)2-oxoindolin-1-yl) acetate[33]
O P OH 2
N=NH O N O C O
ions promotes the development of phosphorus-containing inorganophosphate ionophores. An ion-exchange resin with a chelating effect, which is obtained by the adsorption of Į-nitroso-ȕ-naphthol to zirconium tungstate phosphate[34], has excellent response to lead ions, and there is no need to add any anion excluder when it is used in a sensing membrane. Although its Nernstian slope is as low as 25.6 mV/decade and its response time is relatively long, its detection limit is very low, namely, 4 u 10–6 M. In addition, its anti-interference is moderate because the main interference is from Cd2+ and Ba2+, which are not very common in most lead-ion-containing solutions.
5
Potentiometric sensors based on macrocyclic compounds containing thioamide groups
Recent studies show that a thioamide connected at the lower rim of calix[4]arene can strengthen the interaction between the calixarene cavity and lead ions when the former is used as
ionophore and, thus, achieve significantly improved sensing function. Benzo-substituted dual-thioamide macrocyclic crown ether and open-chained compounds have the same behavior and function, as summarized in Table 4. So far, these types of potentiometric sensors have been prepared in two forms: one is the liquid-contact form, which is prepared by adhering a PVC membrane to an end of a tube that is filled with the electrolyte solution; the other is the solid-contact form, which is prepared by sandwiching an ion-to-electron transducer as an intermediate layer between the ion-selective membrane and the underlying metal electrode. Because carbon nanotubes[35], especially electrically conducting polymers[36–40] with intrinsically high redox capacity, possess excellent ion-to-electron transducing properties, the transducing sandwich layer is mainly composed of conducting polymers, such as poly(3-octylthiophene), poly(3,4-ethylenedioxythiophene (PEDOT), and polyaniline, and sometimes, poly(hydroxyethyl methacrylate). Compared with a liquidcontact sensor, a solid-contact sensor has better lead ion-sensing
Table 4 Response characteristics of potentiometric lead ion sensors based on macrocyclic compounds containing amide or thioamide groups as ionophores Ionophore
tert-Butyl-calix[4]arene thioamides[41]
tert-Butyl-calix[4]arene-tetra kis(N,N-dimethylthioacetami de) [42-45]
Benzo-substituted macrocyclic diamides[46]
Molecular structure
S O HN
O S O O HN S S NH HN
S O
O
N
N O
O S N
N
O O HN
O NH O
N,N,N,N-Tetradodecyl-3,6-di oxaoctanedithioamide[9]
O S S
O
Contact form
Slope (mV/decade)
Linear range (M)
Detection limit (M)
Liquid contact
26.8
1.0 × 10–6.3–1.0 × 10–1.9
10–6.3
PEDOT Solid contact PEDOT Solid contact
29.1[44] 33.1[45]
1.0 × 10–8–1.0 × 10–2 1.0 × 10–6–1.0 × 10–3
10–8.1 10–6
Liquid contact
29[46]
1.0 × 10–9–1.0 × 10–3
1.7 u 10–9
Microsensor
29[47]
1.0 × 10–8–1.0 × 10–6
2.7 u 10–9
Liquid contact
27.2
3.6 u 10–6–1.3 u 10–2
2.0 u 10–-6
Liquid contact
29
1.0 × 10–5–1.0 × 10–2
4 u 10–6
O
C12H25 S S N O O C12H25
C12H25 N C12H25
HUANG Mei-Rong et al. / Chinese Journal of Analytical Chemistry, 2012, 40(1): 50–58
function, including more excellent lower detection limit and higher sensitivity (Table 4). Among all the solid-contact sensors, those with PEDOT as an ion-to-electron conductivity transducer show better performance, with a detection limit as low as 10–8 M and a linear range spanning seven orders of magnitude. Solid-contact potentiometric sensors are expected to replace liquid-contact sensors and become the next-generation sensors. Although a solid-contact sensor was reported 30 years ago, it was practically used as a lead ion-potentiometric sensor only in recent years. Note that solid-contact lead ion sensors still have some problems, for example, the transducer layer is prone to absorption of moisture to form a water layer; and redox reactions occur easily at the inner side of the sensing membrane. All these problems invariably lead to potential drifts and limit the repeatability and lifetime of the sensors in practical applications. If these problems are overcome successfully, solid-contact sensors can fully display their potential advantages, such as high sensitivity, ease of miniaturization, and portability, and will undoubtedly become the main type of potentiometric lead sensors. Thioacetyl amines can be introduced onto macromolecular chains by polymerization between the functional groups introduced at the upper rim of calixarene and a second monomer. For instance, one of the four t-butyl groups on the upper rim of calix[4]arene tetra(N,N-dimethyl thioacetamide) can be replaced with dihydroxy ether chains, which can undergo polycondensation with diisocyanate monomers and, thereby, the ionophore is covalently bonded to the main chain of polyurethane. When this ionophore is utilized in liquid-contact potentiometric sensor, even if the concentration of the inner liquid Pb(NO3)2 is as high as 0.1 mol/L, the lower detection limit is still as low as 10–9 M[44] (Table 4). The miniaturization of the potentiometric sensor based on the ionophore 4-t-butyl calix[4]arene tetra(N,N-dimethylthioacetamide) enables the successful detection of sample liquids confined in a measuring cell of volume 3 PL[45], and, therefore, this sensor has become one of the important electrochemical measuring methods that require the least quantity of sample. Unlike the solid-contact sensor reported previously[26], this microelectrode sensor is based on the liquid-contact mode and is made of a medical micropipette filled with the electrolyte solution. It can be inserted into a 1-mm-inner-diameter silicone rubber tube to contact and detect the aqueous sample plug. The sensor enables the detection of space-confined samples and has all the great advantages of microelectrode sensors. Considering the great potential applications of microelectrode sensors in the detection of Ca2+, Ag+, Na+ [47,48], and K+ [49] in human blood and urine, in addition to detecting trace amounts of lead ions existing in an organism, the research and development of microelectrode sensors will be of great significance in environmental monitoring, particularly in the field of life
science.
6 Applications of potentiometric lead ion sensors The anti-interference ability of potentiometric lead ion sensors, that is, selectivity, is a crucial indicator for the potentiometric lead ion sensor to move out of the laboratory and be successfully applied for real-time ion detection in samples. The anti-interference ability of an ion-selective sensor can be described by the potentiometric selectivity coefficient, KA,Bpot, for the primary ion A against the interfering ion B. The parameter defines the ability of an ion-selective electrode to distinguish a particular ion from others, and its value can be expressed by the modified Nicolsky-Eisenman equation. The numerical value of the selectivity coefficient can be understood to denote an activity ratio of the primary ion A over the interfering ion B when they produce the same potential response. Obviously, the smaller the value of the selectivity coefficient, the greater is the electrode preference for the primary ion. However, the true selectivity coefficient can be determined only when the electrode sensor shows a linear response to both primary and interfering ions[50]. Most of the reports on selectivity coefficients do not seem to pay sufficient attention to these prerequisites and do not give any specification regarding their assessment of selectivity coefficients. Thus, most reported selectivity coefficients may not be accurate enough. Additionally, there are several methods for the determination of the selectivity coefficient and many parameters for choice during the measurement procedure. Therefore, a direct comparison of the selectivity coefficients from different research groups will perhaps lead to misunderstanding. Under these circumstances, by using the herein-developed electrode for the practical sensing of lead ions in real-world samples and thereafter checking the determined results with those based on instrumental analysis having high accuracy, we can indeed evaluate the anti-interference ability. There are many reports on the determination of lead ion content by electrode sensors based on ionophores bearing oxygen- and sulfur-containing functional groups. All these sensors can be used as indicator electrodes in the potentiometric titration of lead ions, most of which are titrations involvingethylenediamine tetraacetic acid[19–22,25] and some involving chromate[24,46]. This lead ion potentiometric sensor can accurately measure trace amounts of lead in lakes, rivers, mineral water, and other environmental waters by the direct potential method[22,23]. These lead ion sensors with a strong anti-interference in relation to other metal ions are also sufficiently competent to determine lead ions in water with complex backgrounds[21]. After dissolving rock minerals,[34] soil[29], solder alloys[25], black tea[26], and hot and black pepper[26] samples in nitric acid solutions, the lead ion content can be determined by this potentiometric sensor by the
HUANG Mei-Rong et al. / Chinese Journal of Analytical Chemistry, 2012, 40(1): 50–58
standard addition method. Most of these test results are very close to those obtained from instrumental analysis. Lead ion potentiometric sensors can also be applicable to the determination of the stability constant of lead ions when complexed with humic acid. Humic acid is an important natural complexing agent of metal ions. Any information about lead ion coordination with humic acid is valuable in the study of the origin of species. The stability constant of lead ions complexed with humic acid can be obtained by the potential changes observedduring the titration process[25].
7
Future prospects
Compared to the instruments used for large-scale analysis based on techniques such as atomic absorption spectrometry, fluorescence spectrometry, and inductively coupled plasma mass spectrometry, potentiometry has lower sensitivity and selectivity. Nonetheless, the high tolerance to the tested sample; the low cost of the instrument, its maintenance, and measurement; and the easy operation and portability of the potentiometric sensor facilitate its extensive application in areas including basic medical treatment, general scientific research, and mass-oriented determination. Furthermore, these superior qualities will drive the potentiometric sensor to further enhancement of its features. Both theoretical and practical studies have repeatedly proven that the ionophores in potentiometric sensors not only affect the sensitivity, but also determine the selectivity of the sensor. The ionophores are the key elements determining the quality of potentiometric sensors, and bad ionophores will lead to failure of the sensor in more complicated samples containing various interfering ions. Therefore, developing new and excellent ionophores for the assembly of superior lead ion sensors is important and necessary. Among these, ionophores for lead ions made from oxygen/sulfur-containing functional groups constitute just one class of the developed ionophores. However, they attract much attention due to their high potential response slope, short response time, excellent detection limits, and simple sample preparation. These types of sensors could be further developed to contribute to a breakthrough in the following aspects: (1) Synthesis of new types of ionophores bearing oxygen/sulfur-containing functional groups; these ionophores can chelate lead ions to form a ring-or even several rings-to greatly enhance the coordination ability of the ionophore, which can thus be sufficiently sensitive to capture trace amounts of lead ions. (2) Breaking through the current sensors that are limited to the simple three-component system “ionophore + plasticizer + PVC,” by applying an ionic liquid-based doping process to improve the lower detection limit. (3) Breaking through the current sensors, which are confined to a single PVC-based matrix material that contains a plasticizer, by making use of an acrylic acid copolymer, polyurethane, and silicone rubber
matrix without any plasticizer so that the service life may be largely extended. (4) Using hot forming polymer matrix to get rid of organic solvents and developing green-film formation. (5) Breaking through the most current sensors that are limited to a routine contact form-liquid-contact form-by using substrate electrodes such as graphite, wire, strip, sheet, plate, and other forms of electrodes to build solid-contact sensors and get rid of the inner filling solution, thus laying a solid foundation for the microelectrode. (6) While studying the traditional microelectrodes based on the micropipette and the gold wire, the building of print electrodes and chip electrodes should also be given adequate attention to build microscale, and even nanoscale, electrodes with the characteristics of miniaturization, integration, and flexibility. Once some breakthroughs in the above investigations of lead ion potentiometric sensors have been achieved and high-performance sensors are obtained, they can be successfully applied to the assay of trace levels of lead ions in biological samples, such as human blood and urine, in addition to fluids with complex compositions.
References [1]
Jiang W, Tang H, Liu Z C, Wang K, Chen B G, Li C H. Chin. J. Health. Lab. Technol., 2008, l8(3): 464–465
[2]
Wu T, Zhang L. J. Nanjing Univ. Traditional. Chin. Medicine (Natural Sci.), 2006, 22(3): 101–103
[3]
Wang L L, Lin L, Chen Y H. Environ. Chem., 2011, 30(2): 571–573
[4]
Hu L, Chen J X, Wang L M, Wang Q, Lin L D. J. Kunming Univ. Sci. Technol. (Sci. Technol.), 2007, 32(6): 81–83
[5]
Wang Z, Lee J H, Lu Y. Adv. Mater., 2008, 20(17): 3263–3267
[6]
Marbella L, Serli-Mitasev B, Basu P. Angew. Chem. Int. Ed., 2009, 48(22): 3996–3998
[7]
Son H, Lee H Y, Lim J M, Kang D, Han W S, Lee S S, Jung J H. Chem. Eur. J., 2010, 16(38): 11549–11553
[8]
Huang M R, Huang S J, Li X G. J. Phys. Chem. C, 2011, 115(13): 5301–5315
[9]
Sokalski T, Ceresa A, Zwickl T, Pretsch E. J. Am. Chem. Soc., 1997, 119(46): 11347–11348
[10]
Sokalski T, Ceresa A, Fibbioli M, Zwickl T, Bakker E,
[11]
Sutter J, Radu A, Peper S, Bakker E, Pretsch E. Anal. Chim.
[12]
Michalska A, Wojciechowski M, Bulska E, Mieczkowski J,
[13]
Lisak
Pretsch E. Anal. Chem., 1999, 71(6): 1210–1214 Acta, 2004, 523(1): 53–59 Maksymiuk K. Talanta, 2009, 79(5): 1247–1251 G,
Grygolowicz-Pawlak
E,
Mazurkiewicz
M,
Malinowska E, Sokalski T, Bobacka J, Lewenstam A. Microchim. Acta, 2009, 164(3-4): 293–297 [14]
Mimendia A, Legin A, Merkoci A, del Vallea M. Sensors and Actuators B, 2010, 146(2): 420–426
[15]
Huang M R, Ma X L, Li X G. Chin. Sci. Bull., 2008, 53(21):
HUANG Mei-Rong et al. / Chinese Journal of Analytical Chemistry, 2012, 40(1): 50–58
[34]
3255–3266 [16]
[35]
Huang M R, Rao X W, Li X G. Chin. J. Anal. Chem., 2008, 36(12): 1735–1741
[18]
2004, 81(8): 666–669
Kulesza J, Bochenska M. Eur. J. Inorg. Chem., 2011, 2011(6): 777–783
[17]
Agarwal H, Gupta A P, Verma G L. J. Indian Chem. Soc., Crespo G A, Macho S, Rius F X. Anal. Chem., 2008, 80(4): 1316–1322
[36]
Huang M R, Rao X W, Li X G, Ding Y B. Talanta, 2011,
Bobacka J, Ivaska A, Lewenstam A. Anal. Chim. Acta, 1999, 385(1-3): 195–202
85(3): 1575–1584
[37]
Lindfors T, Ivaska A. Anal. Chem., 2004, 76(15): 4387–4394
[19]
Tavkkoli N, Khojasteh Z, Sharghi H, Shamsipur M. Anal.
[38]
Bobacka J. Electroanalysis, 2006, 18(1): 7–18
Chim. Acta, 1998, 360(1-3): 203–208
[39]
[20]
Rahmani A, Barzegar M, Shamsipur M, Sharghi H, Mousavi
[21]
Barzegar M, Mousavi M F, Khajehsharifi H, Shamsipur M,
Michalska
Sharghi H. IEEE Sensors J., 2005, 5(3): 392–397
1568–1571
M F. Anal. Lett., 2000, 33(13): 2611–2629
[22]
[40]
Riahi S, Mousavi M F, Shamsipur M, Sharghi H.
Kisiel A, Mazur M, KuĞnieruk S, Kijewska K, KrysiĔski P, A.
Electrochem.
Commun.,
2010,
12(11):
[41]
Bochenska M, Guzinski M, Kulesza J. Electroanalysis, 2009,
[42]
Michalska A, Wojciechowski M, JĊdral W, Bulska E,
Electroanalysis, 2003, 15(19): 1561–1565
21(17-18): 2054–2060
[23]
Li X G, Ma X L, Huang M R. Talanta, 2009, 78(2): 498–505
[24]
Mousavi M F, Barzegar M B, Sahari S. Sensors and Actuators
[25]
Gismera M J, Sevilla M T, Procopio J R. Anal. Sci., 2006,
[26]
[26] Faridbod F, Ganjali M R, Larijani B, Hosseini M,
Maksymiuk K. J. Solid. State Electrochem., 2009, 13(1):
B, 2001, 73(2-3): 199–204
99–106 [43]
Michalska A, Wojciechowski M, Bulska E, Maksymiuk K.
[44]
Püntener M, Vigassy T, Baier E, Ceresa A, Pretsch E. Anal.
22(3): 405–410
Talanta, 2010, 82(1): 151–157
Alizadeh K, Norouzi P. Norouzi. Int. J. Electrochem. Sci., 2009, 4(11): 1528–1540
Chim. Acta, 2004, 503(2): 187–194 [45]
[27]
Casado M, Daunert S, Valiente M. Electroanalysis, 2001,
[28]
Alasier A S, Entisar S A, Elbishti H A. Arabian. J. Chem.,
13(1): 54–60
Malon A, Vigassy T, Bakker E, Pretsch E. J. Am. Chem. Soc., 2006, 128(25): 8154–8155
[46]
2010, 3(2): 89–94 [29]
Pawáowski P, Michalska A, Maksymiuk K. Electroanalysis, 2006, 18(13-14): 1339–1346
Kazemi S Y, Shamsipur M, Sharghi H. J. Hazard. Mater., 2009, 172(1): 68–73
[47]
Wygladacz K, Durnas M, Parzuchowski P, Brzózka Z,
Lu X Q, Chen Z L, Hall S B. Anal. Chim. Acta, 2000, 418(2):
Malinowska E. Sensors and Actuators B, 2003, 95(1-3):
205–212
366–372
[30]
Ross J W. Science, 1967, 156(3780): 1378–1379
[31]
Xu D, Katsu T. Talanta, 2000, 51(2): 365–371
[32]
Xu D, Katsu T. Anal. Chim. Acta, 1999, 401(1-2): 111–115
[33]
Abbaspour
A,
Mirahmadi
E,
Khalafi-nejad
Vigassy T, Huber C G, Wintringer R, Pretsch E. Anal. Chem.,
[49]
Uhlig A, Lindner E, Teutloff C, Schnakenberg U, Hintsche R.
[50]
Bakker E, Pretsch E, Buhlmann P. Anal. Chem., 2000, 72(6):
2005, 77(13): 3966–3970 A,
Babamohammadi S. J. Hazard. Mater., 2010, 174(1-3): 656–661
[48]
Anal. Chem., 1997, 69(19): 4032–4038 1127–1133
Cite this article as: Chin J Anal Chem, 2012, 40(1), 50–58.
REVIEW
Development of Potentiometric Lead Ion Sensors Based on Ionophores Bearing Oxygen/Sulfur-Containing Functional Groups HUANG Mei-Rong1,*, GU Guo-Li1, SHI Feng-Ying2, LI Xin-Gui1 1
Institute of Materials Chemistry, Key Laboratory of Advanced Civil Engineering Materials of the Ministry of Education, College of Materials Science and Engineering, Tongji University, Shanghai 200092, China 2 Architectural Engineering College, Shanghai Normal University, Shanghai, 201418, China
Abstract: Potentiometric Pb2+ sensors based on ionophores bearing oxygen/sulfur-containing functional groups, which include anthraquinone, aromatic carboxylic acids, carboxylates, phosphates, and macrocyclic compounds containing amide or thioamide groups, are systematically summarized. Anthraquinone is found to be quickly responsive to lead ions, with the shortest response time of two seconds. Aromatic carboxylic acids with a sulfur atom as an ionophore in an asymmetric membrane respond sensitively to lead ions, with the lowest detection limit of 6.0 u 10–10 M. Phosphate has the advantage of better anti-interference ability, with a logarithmic selectivity coefficient, logK, smaller than –3.0. Macrocyclic amides, after assembly into microelectrodes, have been used as ionophores for the analysis of samples confined in cells of volume 3 PL, with a superior lower detection limit of 2.7 u 10–9 M. Key Words: Potentiometric sensor; Lead ion sensor; Ionophore; Anthraquinone; Aromatic carboxylic acid; Phosphate; Thioamide; Review
1
Introduction
Many analytical techniques, such as atomic absorption spectrometry[1], atomic fluorescence spectrometry[2], inductively coupled plasma mass spectrometry[3], dithizone spectrophotometry[4], and colorimetry[5], can be used for the quantitative analysis of lead ions. Especially, in the case of fluorescent chemosensors[6–8] reported in recent years, the lower detection limits for lead ions are improved and reach 5 × 10–8 M (10 parts per billion, ppb), which is lower than the maximum contamination level for lead (72 nM, 15 ppb) in drinking water as defined by the US Environmental Protection Agency. However, the prohibitive cost of the use and maintenance of equipment, together with the requirement for the professional skills of an operator, make these methods almost remain confined to the laboratory. On the contrary,
potentiometric sensors based on the trapping of ionophores by a sensing membrane, a fast, efficient, and low-cost analytical technique, are particularly suitable for outdoor operations, and therefore, for the on-line monitoring of ions. Since the late 1960s, a variety of high-performance potentiometric sensors, such as potentiometric sensors optimized by using an ionic buffer as an inner filling solution[9,10] and polyacrylate as the matrix[11–13], have been developed for lead ions. Numerous applications have been found for these sensors in the real-time monitoring of wastewater discharged from storage batteries, cables, and metallurgical industries, in addition to applications in the context of safety inspection of mineral water and foodstuffs. In recent years, potentiometric sensors have not been limited only to single head-mode analysis; a multiprobe automatic potentiometric sensor array[14] has already been used in the
Received 30 June 2011; accepted 26 August 2011 * Corresponding author. Email: [email protected] This work was supported by the National Natural Sciencpe Foundation of China (No. 20974080). Copyright © 2012, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-2040(11)60521-5
HUANG Mei-Rong et al. / Chinese Journal of Analytical Chemistry, 2012, 40(1): 50–58
instant quantitative analysis of heavy metal ions in submicromolar concentrations. The considerable progress and development achieved in these fields are inextricably related to the development of excellent ionophores. The ionophore is a vital component embedded in the sensing membrane, which largely determines the latter’s performance, especially the anti-interference capability of the sensor. Undoubtedly, the ionophore is the only guarantee for the anti-interference property of the sensor. Excellent ionophores enable the sensor to respond exclusively to the target ions while being almost irresponsive to other coexisting ions. Up to now, the ionophores developed for the selective coordination of Pb2+ include macrocyclic compounds such as crown ethers, calixarenes with the right cavity size[15,16], nitrogen compounds[17], polymers[18], and some O/S atom-enriched organic compounds. Based on the hard and soft acid-base theory, the O/S atom-enriched organic compounds are the soft-base ligands that can easily form stabler complexes with soft-acid metal ions such as Pb2+; therefore, they are sensitive to even trace amounts of Pb2+. These soft-base compounds, which have attracted increased attention recently, include anthraquinones, aromatic carboxylic acids, carboxylates, phosphates, and macrocyclic compounds containing an amide or thioamide group.
2 Potentiometric sensors based on anthraquinone Anthraquinones used as ionophores for potentiometric Pb2+ sensors are 9,10-anthraquinone derivatives, which mainly have the hydroxyl, alkenyl, or alkoxyl groups. The carbonyl oxygen on the quinone, along with the hydroxyl and ether groups, forms a cavity structure, which can coordinate with metal ions, particularly Pb2+, in nonaqueous solvents in the ratio 2:1 (ligand: metal ion) to form stable complexes. Additionally, the alkalinity or acidity and the lipophilicity of the anthraquinones can be adjusted by controlling the lengths of the side chains. Therefore, the use of anthraquinone derivatives as lead ion ionophores in potentiometric sensors has increased recently, as shown in Table 1. Comparative studies have shown that 9,10-anthraquinone derivatives bearing only one or two hydroxyl groups do not function well as lead ion ionophores. Only by the introduction of a certain length of double bond-containing aliphatic chains in a position adjacent[19] to the hydroxyl group, or the allyl ether groups[20], do these derivatives show a significantly improved ability to coordinate with lead ions. Furthermore, two aliphatic side chains result in better performance than only one[19,20], owing to both the lipophilicity of the electron-donating S-bonds in the side chain and the synergistic effect of the phenolic hydroxyl groups while chelating the lead ions. The introduced double bond-containing aliphatic chains are mostly allyl groups, and the double bonds at the terminal position enable them to easily complex with the metal ions to
form a stabler cyclic cavity structure[19]. Thus, among the series of Pb2+ ionophores, 1,8-dihydroxy-2,7-dipropenyl-9,10anthraquinone has the best performance (Table 1). Regarding the anti-interference performance, a ring structure formed between the ionophores and the metal ions seems selective toward lead ions due to the formation of cyclic complexes. Except for the common interfering ions, such as Hg2+ (logarithmic selectivity coefficient, logK = –2.0), Cu2+ (logK = –1.7), and Na+ (logK = –2.2), which have a little interference, other metal ions hardly ever show any interference. Although 1,4-bis(prop-2ƍ-enyloxy)-9,10-anthraquinone, containing allyl ether groups, has a performance similar to 1,8-dihydroxy-2,7-diallyl-9,10-anthraquinone in terms of the ability to detect lead ions, its anti-interfering ability is one order of magnitude worse. Most metal ions, whether alkaline metal ions, alkaline earth metal ions, or transition metal ions, can cause interference in the detection of lead ions, with their logK values ranging from –1.3 to –2.9[20]. Among the interfering ions, Ag+ (logK = –1.3) and Tl+ (logK = –1.9) have the strongest interference. Obviously, it mainly arises from the unstable cyclic coordination of this type of structure with metal ions because of the presence of a single complexation site on the single oxygen functional group. Therefore, most metal ions can readily interfere with the ionophore’s detection ability, consequently leading to lower selectivity. An effective approach to improve the lipophilicity and enhance the capability of coordinating with lead ions is introducing an ether bond-containing aliphatic chain in the ortho-position of the phenolic hydroxyl group of the anthraquinone molecule. The introduced aliphatic chain can not only alter its acid-base character greatly and improve the lipophilicity, but also exert its lariat effect in the process of coordination with metal ions and hasten the coordination between the compound and the metal ions. At the same time, the anti-interference capability is additionally enhanced. For OH
example, the introduction of a side chain [21]
O
n
(n =
1–5) onto the position ‘2’ of 1-hydroxy-9,10anthraquinone generates five types of anthraquinone derivatives with polyether chains, all of which have excellent lead ion–detecting performance; moreover, their lower detection limits are around 10–6 M, with a Nernstian slope above 26.0 mV/decade for all derivatives. However, too long a side chain will produce steric hindrance and impede coordination. Therefore, among the derivatives with polyether side chains, those with n = 3 have the best Nernstian response (Table 1), with a response time as short as two seconds, which is the fastest potentiometric sensing time for lead ions until date. Regarding the anti-interfering capability of these sensors against divalent metal ions, such as Cd2+, Cu2+, and Hg2+ that cause the strongest interference, the logarithmic selectivity coefficients are –2.0, –1.9 and –1.7, respectively, whereas most other metal ions have little or negligible interference.
HUANG Mei-Rong et al. / Chinese Journal of Analytical Chemistry, 2012, 40(1): 50–58
Table 1 Response characteristics of potentiometric lead ion sensors based on anthraquinone derivatives as ionophores Ionophore
Slope (mV/decade)
Molecular structure
Linear range (M)
Detection limit (M)
Response time (s)
pH
Lifetime (month)
29.0
2.0 × 10–6–2.0 × 10–3
1.1 × 10–6
30
6.0
4
29.8
2.5 × 10–6–1.0 × 10–2
1.5 × 10–6
15
4.7–6.8
4
28.9
1.0 × 10–6–1.0 × 10–2
6.7 × 10–7
2
1.5–6.8
4
29.5
1.0 × 10–7–1.0 × 10–2
8.0 × 10–8
10
3.5–6.8
2
28.9
2.5 × 10–6–1.0 × 10-1
7.8 × 10–7
12
2.8–5.2
4
O
1,8-Dihydroxy-2,7-bis(prop-2’enyl)-9,10-anthraquinone[19] O
OH O
OH O
1,4-Bis(prop-2’-enyloxy)-9,10anthraquinone[20] O O
O
OH
1-Hydroxy-2-({2-[2-(2-hydrox yethoxy)ethoxy]ethoxy}methyl )-9,10- anthraquinone[21]
OH 3
O
O
O
1-Hydroxy-2-(2-ethanoloxymet hyl)-9,10-anthraquinone[22]
OH OH
O
O
Polyaminoanthraquinone[23]
O H
O H N
n
H
The potentiometric sensors for lead ions mentioned herein are liquid-contact sensors based on conventional plasticized polyvinyl chloride (PVC) membranes. For derivatives with polyether chains (when n = 1), such as substituted anthraquinone, which may be used as the ionophore, if the plasticized PVC-based sensing membrane is assembled on the surface of a graphite electrode to form a solid-contact sensor[22], the performance can be substantially improved, with the Nernstian slope increasing from 26.4 to 29.5 mV/decade, and the corresponding lower detection limit is lowered from 7.8 u 10–6 to 8.0 ² 10–8 M. This improvement may be attributed to the additive, oleic acid, which not only acts as a phase-conversion catalyst, but also is involved in the coordination with Pb2+. This deduction can be verified by the influence of its dosage on the performance of the resulting sensor. On varying the amounts of oleic acid, ranging from one to three times the ionophore concentration, the Nernstian slope increases from 24.0 to 29.5 mV/decade. In addition, compared with the liquid-contact PVC-membrane sensor, this solid-contact graphite sensor has better anti-interference performance, except for the interference from Hg2+ (logK = 2.0); however, the interference from other ions could be ignored. Poly(1-amino anthraquinone), synthesized by simple chemical oxidative polymerization, also shows a strong coordinating capability with lead ions. With poly(1-amino anthraquinone) as the lead ion ionophore, the liquid-contact sensor based on plasticized PVC membrane has a relatively wide linear range, excellent lower detection limit, and short response time. Furthermore, the sensor has a good selectivity for lead ions in relation to alkali and alkaline earth metal ions,
with a logarithmic selectivity coefficient less than –1[23].
3
Potentiometric sensors based on carboxylic acid derivatives
Carboxylic acid derivatives can coordinate with metal ions and form stable complexes because the carbonyl group of –COOH and the O atom on the hydroxyl group can offer lone pairs of electrons for the empty orbitals of the metal ions. Similar to anthraquinone ionophores, the formation of a cavity structure, lipophilicity, and chain length are the main factors influencing their selectivity to lead ions (Table 2). Carboxylic acid derivatives may vary their ratio in relation to lead ions from 1:1 to 1:2 during coordination, which is related to the specific unit structure of the molecule. Fatty acids, as one of the important groups of carboxylic acids, have been successfully used in potentiometric sensors, showing the most sensitive response to Pb2+ compared to that for all other cations. However, not all the fatty acids have an ideal Nernstian response to Pb2+ due to the different lengths of the molecular chains. For example, among the four fatty acids oflengths C7, C10, C14 and C18, capric acid (C10) has the best sensing performance over a wide pH range, a relatively high Nernstian slope, and a three-month lifetime when used in PVC-membrane-based potentiometric sensors[24]. Because fatty acids coordinate with Pb2+ through their single O-atom functional groups, the resulting complexes cannot stabilize in this type of cyclic or semicyclic structures, and, thus, alkali metal ions can easily interfere with the coordination process. However, for the same reason, the transition metal ions (except for Ag+) only slightly affect the detection of lead ions.
HUANG Mei-Rong et al. / Chinese Journal of Analytical Chemistry, 2012, 40(1): 50–58
Table 2 Response characteristics of potentiometric lead ion sensors based on aromatic carboxylic acids as ionophores Ionophore Capric acid
Molecular structure
[24]
CH3(CH2)8COOH O
C
SH
O
2,2’-Dithiodibenzoic acid
C
O C
[26]
O2 N
8-(Dodecyloxy)quinoline-2-carbox ylic acid[27]
Trans-1,2-diaminocyclohexane N,N,N’N’-tetra acetic acid[28] Humic acid[29]
Detection limit (M) 6.0 × 10–-6
4.5–7.0
23.8
1.9 × 10–6–6.2 × 10–4
4.0 × 10–8
4.0–8.0
21.8
8.7 × 10–8–6.2 × 10–4
5.0 × 10–-8
4.4–5.5
29.1
1.0 × 10–-9–1.0 × 10–3
6.0 × 10–10
3.5–9.0
33.0
1.0 × 10–6–1.0 × 10–2
5.0 × 10–7
4.5–6.5
51.7
1.0 ×10–5–1.0 ×10–2
8.9 × 10–5
5.5–10
28.6
3.2 × 10–6–6.3 × 10–3
2.0 × 10–6
4.5
pH
OH S
S HO
5,5’-Dithiobis(2-nitrobenzoic acid)
Linear range (M) 1.0 ×10–5–1.0 × 10–2
OH
2-Mercaptobenzoic acid[25]
[25]
Slope (mV/decade) 29.0
C
O
NO2
OH S
S
HO
C O
O N C O OH CH3(CH2)11
N(CH2COOH)2 N(CH2COOH)2
Cross linking organic macrocompounds
Therefore, the ionophore has great advantages in the detection of lead ions in aqueous solutions containing various heavy metal ions. The pH can also affect the performance of sensors based on aliphatic carboxylic acids in different ways from the influence of pH on sensors based on other ionophores. In addition to its interference with lead ion detection, H+ is also in an ion-exchange chemical equilibrium, namely, the equilibrium between Pb2+ and H+. Higher H+ concentration causes the equilibrium to shift toward the formation of acid rather than toward release of Pb2+. This circumstance is not conducive to the formation of complexes. As a result, the sensor is unsuitable for the detection of lead ions in acidic solutions. Coordination to Pb2+ can be enhanced by the introduction of a soft active-site S atom into carboxylic acid derivatives, such as thiosalicylic acid and dithiodibenzoic acid. After dipping in the lead ion solution for a period of time, aggregates of white needles on the surface of the sensing membrane are observed under a scanning electron microscope, which confirms the complex formation between lead(II) ions and the carbon paste modifier[25] with different atomic ratios of S:Pb:thiosalicylic acid, the ratio is 1:1, and for dithiodibenzoic acid, the ratio is 1:2. Because dithiodibenzoic acid is equivalent to two dehydrogenized and bridge-connected thiosalicylic acid molecules, the former contains twice as much carboxyl groups and S atoms as that of the latter. Moreover, both can coordinate with Pb2+ in a stable cyclic structure, resulting in an improved sensing performance when compared with aliphatic acids. For example, their detection limits are as low as 10–8 M[25], which is at the cost of their Nernstian slopes (Table 2). In terms of selectivity, their logarithmic selectivity
coefficients, logK, are both lower than –4, which is much better than those of aliphatic acids. However, their anti-interference property with reference to transition metal ions, such as Fe3+, Hg2+ and Cu2+, is worse than that of the other ionophores, because the soft active sites can not only coordinate with Pb2+, but also with other heavy metal ions and, thus, form stabler complexes. The selectivity coefficients could move up to 103, which indicates that Fe3+ and Hg2+, even in low concentrations, will cause a big influence on the detection of Pb2+. From another viewpoint, these types of ionophores may be more suitable for the detection of Fe3+ or Hg2+ by the sensors. Thus, 5,5-dithiobis(2-nitrobenzoic acid)[26], an ionophore with similar structure, can be used to prepare an asymmetric sensing membrane in a microelectrode sensor by dip-coating PVC on a cross-section of a gold microwire electrode, which is sealed in a soft glass capillary. This sensor is much better than a graphite-paste electrode-based membrane sensor in terms of the potential-response slope, linear range, and particularly, the lower detection limit (Table 2). It indicates another worthwhile research direction toward the fabrication of sensing membranes having an asymmetric structure. Carboxylic acids containing quinoline substituent groups can also coordinate with a variety of metal ions and form stable complexes. The stability of these complexes is related greatly to the substituents connected to the aromatic ring. For example, quinoline-2-carboxylic acids substituted by alkyl or alkoxyl groups have a better extraction ability toward Cu2+ and Zn2+; 8-benzoylmethyl-quinolinic acid has a stronger extraction ablity toward Cd2+ and Cu2+ in relation to other metal ions; and 8-(dodecyloxy)quinoline-2-carboxylic acid
HUANG Mei-Rong et al. / Chinese Journal of Analytical Chemistry, 2012, 40(1): 50–58
has an excellent extraction ability toward Pb2+ and Cu2+ [27]. The alkoxyl group on quinoline, an electron donor, changes both the cloud density over the aromatic ring and the character of the group on the ȕ-position of the quinoline ring and, thus, changes the stability and alkalinity or acidity. Similarly, the steric effect of the substituent group also has the same effect. Moreover, 8-(dodecyloxy) quinoline-2-carboxylic acid, an ionophore, coordinates with Pb2+ in the ratio of 1:2, based on which the sensor has a relatively high Nernstian slope of 33.0 mV/decade[27], a lower detection limit, and excellent selectivity. Except for Cu2+, the log K of other metal ions ranges from –3.1 to –4.5, having almost no interference. However, no reports on the interference by Hg2+ and Ag+ are found thus far. In addition, the service life of the sensor is only 40 d. Recently, multiple carboxyl group-containing chelating agents, such as trans-1,2-diamino cyclohexane N,N,Nƍ,Nƍtetra-acetic acid (DCTA), have been applied as lead ion ionophores[28]. With a well-dispersed DCTA ionophore, the sensor based on a spectrum-level active graphite paste electrode has an excellent linear response to lead ions, with the detectable concentration ranging from 10–5 to 10–2 M, and the response slope is as high as 51.7 mV/decade (Table 2). However, its anti-interference capability toward Hg2+ and Ca2+ is degraded due to its strong coordinating ability. Therefore, choosing a right chelating agent as ionophore is highly important. Aquatic humic substances, when used as ionophores in PVC membrane–based potentiometric sensors[29], have theoretical Nernstian slope and a better lower detection limit toward lead ions but a narrow linear range extending only within three orders of magnitude, and they can only be used in a medium with pH around 4.5. In a medium with lower pH, the existence of large amounts of H+ will reduce the stability of the complexes, whereas a medium with higher pH, lead ions are prone hydrolysis or even settle to form Pb(OH)2. However, a sensor with humic substance as the ionophore has excellent anti-interference capability, and the selectivity coefficients toward alkali and most of the alkali metal and transition metal ions are less than 10–2. Only Hg2+ has unignorable interference, and its selectivity coefficient is 10–0.2. In addition, the dosage of humic substance also heavily affects the sensing performance. Too low a dosage will decrease the sensing performance, and excessive dosage will easily generate complexes between the charged humic substance and Pb2+ within the membrane phase in the concentrated Pb2+ solution, resulting in electrostatic repulsion toward the sample solution and, therefore, the deterioration of the sensing performance. The greatest advantage of humic substance over other ionophores is its natural source, that is, it can be obtained without a complicated synthesis process. Although the sensing performance of sensors based on humic substance as a potentiometric lead-ion ionophore, which has not been studied well until date, is not particularly spectacular, more studies on its sensing performance as lead ion ionophore are worth
carrying out considering its excellent lower detection limit.
4
Potentiometric sensors based on carboxylate and phosphate
In recent years, several studies have been reported about the electron donating character of the phosphoryl groups present in phosphates and carboxylates used in potentiometric cation-selective sensors, and the main issue of concern in all the reports is the coordinating performance of the P- and O-atom-containing groups toward metal ions. The earliest report is about the application of dihydroxy phsphate in Ca2+-selective electrode sensors[30]. A later study showed that organophosphates have a higher extraction constant for Pb2+, Cu2+, and Ag+ than for Ca2+, implying that they have stabler coordination and better sensing performances compared to the performances for Ca2+ when used in ion-selective electrode sensors. It also means that severe interference may originate from Cu2+ and Ag+ when they are used in lead ion sensor. Therefore, Cu2+ and Ag+ are always the biggest interferences in all organophosphate-based potentiometric sensors. Especially, in the case of Ag+, the selectivity coefficient is in the range 1–10. The second-strongest interference is from alkaline metal ions, such as Na+ and K+, which have selectivity cofficients of around 0.01–0.1. Fortunately, other divalent ions have almost no interference. Although the selectivity of the phosphates toward Pb2+ is achieved by the stable coordination between phosphoryl groups and Pb2+, an organophosphate with two phosphoryl groups does not definitely have a better sensing performance than that with only one group[31,32] (Table 3). Their detection ranges are in the range 10–5–10–2 M, and their lower detection limits are around 10–6 M. Dibenzyl phosphate has a relatively outstanding performance, whose detection ranges between 10–6 and 10–2 M, the detection limit is as low as 9.0 u 10–7 M, the response time is dozens of seconds, and the adapted pH range is 4–6. It reflects the fact that the number of phosphoryl groups is not a critical factor for sensing performance and that the synergistic effect of O and P atoms during coordination may play a key role in achieving the detection of lead ions. In recent years, aromatic carboxylic acid esters have also been used as potential lead ion ionophores. A PVC membrane-forming solution containing 2% of benzyl-2-(3(phenylhydrazone)-2-oxygenindole)phenyl acetic acid ester and 4% of carbon powder is dip-coated five times to modify the platinum wire electrode. The resultant sensor has a moderate linear range from 7.2 u 10–5 M to 1.0 u 10–2 M but a poor response slope of 26.6 mV/decade. Apart from this study, no other carboxylic acid ester ionophores have been reported yet for the fabrication of lead ion sensors. Further study on this type of ionophore needs to be carried out in the context of its response performance to lead ions. The capability of organophosphates to coordinate with lead
HUANG Mei-Rong et al. / Chinese Journal of Analytical Chemistry, 2012, 40(1): 50–58
Table 3 Response characteristics of potentiometric lead ion sensors based on phosphates and carboxylates as ionophores Ionophore
Molecular structure
Triphenylphosphine oxide[31]
P 3
Slope (mV/decade)
Linear range (M)
Detection limit (M)
28.0
5.0 × 10–5–1.0 × 10–2
6.0 × 10–6
O
Dibenzyl phosphate[32]
OH CH2O P O 2
30.1
3.0 × 10–6–1.0 × 10–2
9.0 × 10–7
Diphenyl phosphate[32]
OH O P O 2
30.0
1.0 × 10–5–1.0 × 10–2
4.0 × 10–6
31.3
1.0 × 10–-4–1.0 × 10–2
4.0 × 10–5
26.6
7.2 × 10–5–1.0 × 10–2
—
Bis(2-ethylhexyl) phosphate[32]
O CH3CHCH2CH2CH2CH2 CH2CH3
Benzyl 2-(3(phenylhydrazono)2-oxoindolin-1-yl) acetate[33]
O P OH 2
N=NH O N O C O
ions promotes the development of phosphorus-containing inorganophosphate ionophores. An ion-exchange resin with a chelating effect, which is obtained by the adsorption of Į-nitroso-ȕ-naphthol to zirconium tungstate phosphate[34], has excellent response to lead ions, and there is no need to add any anion excluder when it is used in a sensing membrane. Although its Nernstian slope is as low as 25.6 mV/decade and its response time is relatively long, its detection limit is very low, namely, 4 u 10–6 M. In addition, its anti-interference is moderate because the main interference is from Cd2+ and Ba2+, which are not very common in most lead-ion-containing solutions.
5
Potentiometric sensors based on macrocyclic compounds containing thioamide groups
Recent studies show that a thioamide connected at the lower rim of calix[4]arene can strengthen the interaction between the calixarene cavity and lead ions when the former is used as
ionophore and, thus, achieve significantly improved sensing function. Benzo-substituted dual-thioamide macrocyclic crown ether and open-chained compounds have the same behavior and function, as summarized in Table 4. So far, these types of potentiometric sensors have been prepared in two forms: one is the liquid-contact form, which is prepared by adhering a PVC membrane to an end of a tube that is filled with the electrolyte solution; the other is the solid-contact form, which is prepared by sandwiching an ion-to-electron transducer as an intermediate layer between the ion-selective membrane and the underlying metal electrode. Because carbon nanotubes[35], especially electrically conducting polymers[36–40] with intrinsically high redox capacity, possess excellent ion-to-electron transducing properties, the transducing sandwich layer is mainly composed of conducting polymers, such as poly(3-octylthiophene), poly(3,4-ethylenedioxythiophene (PEDOT), and polyaniline, and sometimes, poly(hydroxyethyl methacrylate). Compared with a liquidcontact sensor, a solid-contact sensor has better lead ion-sensing
Table 4 Response characteristics of potentiometric lead ion sensors based on macrocyclic compounds containing amide or thioamide groups as ionophores Ionophore
tert-Butyl-calix[4]arene thioamides[41]
tert-Butyl-calix[4]arene-tetra kis(N,N-dimethylthioacetami de) [42-45]
Benzo-substituted macrocyclic diamides[46]
Molecular structure
S O HN
O S O O HN S S NH HN
S O
O
N
N O
O S N
N
O O HN
O NH O
N,N,N,N-Tetradodecyl-3,6-di oxaoctanedithioamide[9]
O S S
O
Contact form
Slope (mV/decade)
Linear range (M)
Detection limit (M)
Liquid contact
26.8
1.0 × 10–6.3–1.0 × 10–1.9
10–6.3
PEDOT Solid contact PEDOT Solid contact
29.1[44] 33.1[45]
1.0 × 10–8–1.0 × 10–2 1.0 × 10–6–1.0 × 10–3
10–8.1 10–6
Liquid contact
29[46]
1.0 × 10–9–1.0 × 10–3
1.7 u 10–9
Microsensor
29[47]
1.0 × 10–8–1.0 × 10–6
2.7 u 10–9
Liquid contact
27.2
3.6 u 10–6–1.3 u 10–2
2.0 u 10–-6
Liquid contact
29
1.0 × 10–5–1.0 × 10–2
4 u 10–6
O
C12H25 S S N O O C12H25
C12H25 N C12H25
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function, including more excellent lower detection limit and higher sensitivity (Table 4). Among all the solid-contact sensors, those with PEDOT as an ion-to-electron conductivity transducer show better performance, with a detection limit as low as 10–8 M and a linear range spanning seven orders of magnitude. Solid-contact potentiometric sensors are expected to replace liquid-contact sensors and become the next-generation sensors. Although a solid-contact sensor was reported 30 years ago, it was practically used as a lead ion-potentiometric sensor only in recent years. Note that solid-contact lead ion sensors still have some problems, for example, the transducer layer is prone to absorption of moisture to form a water layer; and redox reactions occur easily at the inner side of the sensing membrane. All these problems invariably lead to potential drifts and limit the repeatability and lifetime of the sensors in practical applications. If these problems are overcome successfully, solid-contact sensors can fully display their potential advantages, such as high sensitivity, ease of miniaturization, and portability, and will undoubtedly become the main type of potentiometric lead sensors. Thioacetyl amines can be introduced onto macromolecular chains by polymerization between the functional groups introduced at the upper rim of calixarene and a second monomer. For instance, one of the four t-butyl groups on the upper rim of calix[4]arene tetra(N,N-dimethyl thioacetamide) can be replaced with dihydroxy ether chains, which can undergo polycondensation with diisocyanate monomers and, thereby, the ionophore is covalently bonded to the main chain of polyurethane. When this ionophore is utilized in liquid-contact potentiometric sensor, even if the concentration of the inner liquid Pb(NO3)2 is as high as 0.1 mol/L, the lower detection limit is still as low as 10–9 M[44] (Table 4). The miniaturization of the potentiometric sensor based on the ionophore 4-t-butyl calix[4]arene tetra(N,N-dimethylthioacetamide) enables the successful detection of sample liquids confined in a measuring cell of volume 3 PL[45], and, therefore, this sensor has become one of the important electrochemical measuring methods that require the least quantity of sample. Unlike the solid-contact sensor reported previously[26], this microelectrode sensor is based on the liquid-contact mode and is made of a medical micropipette filled with the electrolyte solution. It can be inserted into a 1-mm-inner-diameter silicone rubber tube to contact and detect the aqueous sample plug. The sensor enables the detection of space-confined samples and has all the great advantages of microelectrode sensors. Considering the great potential applications of microelectrode sensors in the detection of Ca2+, Ag+, Na+ [47,48], and K+ [49] in human blood and urine, in addition to detecting trace amounts of lead ions existing in an organism, the research and development of microelectrode sensors will be of great significance in environmental monitoring, particularly in the field of life
science.
6 Applications of potentiometric lead ion sensors The anti-interference ability of potentiometric lead ion sensors, that is, selectivity, is a crucial indicator for the potentiometric lead ion sensor to move out of the laboratory and be successfully applied for real-time ion detection in samples. The anti-interference ability of an ion-selective sensor can be described by the potentiometric selectivity coefficient, KA,Bpot, for the primary ion A against the interfering ion B. The parameter defines the ability of an ion-selective electrode to distinguish a particular ion from others, and its value can be expressed by the modified Nicolsky-Eisenman equation. The numerical value of the selectivity coefficient can be understood to denote an activity ratio of the primary ion A over the interfering ion B when they produce the same potential response. Obviously, the smaller the value of the selectivity coefficient, the greater is the electrode preference for the primary ion. However, the true selectivity coefficient can be determined only when the electrode sensor shows a linear response to both primary and interfering ions[50]. Most of the reports on selectivity coefficients do not seem to pay sufficient attention to these prerequisites and do not give any specification regarding their assessment of selectivity coefficients. Thus, most reported selectivity coefficients may not be accurate enough. Additionally, there are several methods for the determination of the selectivity coefficient and many parameters for choice during the measurement procedure. Therefore, a direct comparison of the selectivity coefficients from different research groups will perhaps lead to misunderstanding. Under these circumstances, by using the herein-developed electrode for the practical sensing of lead ions in real-world samples and thereafter checking the determined results with those based on instrumental analysis having high accuracy, we can indeed evaluate the anti-interference ability. There are many reports on the determination of lead ion content by electrode sensors based on ionophores bearing oxygen- and sulfur-containing functional groups. All these sensors can be used as indicator electrodes in the potentiometric titration of lead ions, most of which are titrations involvingethylenediamine tetraacetic acid[19–22,25] and some involving chromate[24,46]. This lead ion potentiometric sensor can accurately measure trace amounts of lead in lakes, rivers, mineral water, and other environmental waters by the direct potential method[22,23]. These lead ion sensors with a strong anti-interference in relation to other metal ions are also sufficiently competent to determine lead ions in water with complex backgrounds[21]. After dissolving rock minerals,[34] soil[29], solder alloys[25], black tea[26], and hot and black pepper[26] samples in nitric acid solutions, the lead ion content can be determined by this potentiometric sensor by the
HUANG Mei-Rong et al. / Chinese Journal of Analytical Chemistry, 2012, 40(1): 50–58
standard addition method. Most of these test results are very close to those obtained from instrumental analysis. Lead ion potentiometric sensors can also be applicable to the determination of the stability constant of lead ions when complexed with humic acid. Humic acid is an important natural complexing agent of metal ions. Any information about lead ion coordination with humic acid is valuable in the study of the origin of species. The stability constant of lead ions complexed with humic acid can be obtained by the potential changes observedduring the titration process[25].
7
Future prospects
Compared to the instruments used for large-scale analysis based on techniques such as atomic absorption spectrometry, fluorescence spectrometry, and inductively coupled plasma mass spectrometry, potentiometry has lower sensitivity and selectivity. Nonetheless, the high tolerance to the tested sample; the low cost of the instrument, its maintenance, and measurement; and the easy operation and portability of the potentiometric sensor facilitate its extensive application in areas including basic medical treatment, general scientific research, and mass-oriented determination. Furthermore, these superior qualities will drive the potentiometric sensor to further enhancement of its features. Both theoretical and practical studies have repeatedly proven that the ionophores in potentiometric sensors not only affect the sensitivity, but also determine the selectivity of the sensor. The ionophores are the key elements determining the quality of potentiometric sensors, and bad ionophores will lead to failure of the sensor in more complicated samples containing various interfering ions. Therefore, developing new and excellent ionophores for the assembly of superior lead ion sensors is important and necessary. Among these, ionophores for lead ions made from oxygen/sulfur-containing functional groups constitute just one class of the developed ionophores. However, they attract much attention due to their high potential response slope, short response time, excellent detection limits, and simple sample preparation. These types of sensors could be further developed to contribute to a breakthrough in the following aspects: (1) Synthesis of new types of ionophores bearing oxygen/sulfur-containing functional groups; these ionophores can chelate lead ions to form a ring-or even several rings-to greatly enhance the coordination ability of the ionophore, which can thus be sufficiently sensitive to capture trace amounts of lead ions. (2) Breaking through the current sensors that are limited to the simple three-component system “ionophore + plasticizer + PVC,” by applying an ionic liquid-based doping process to improve the lower detection limit. (3) Breaking through the current sensors, which are confined to a single PVC-based matrix material that contains a plasticizer, by making use of an acrylic acid copolymer, polyurethane, and silicone rubber
matrix without any plasticizer so that the service life may be largely extended. (4) Using hot forming polymer matrix to get rid of organic solvents and developing green-film formation. (5) Breaking through the most current sensors that are limited to a routine contact form-liquid-contact form-by using substrate electrodes such as graphite, wire, strip, sheet, plate, and other forms of electrodes to build solid-contact sensors and get rid of the inner filling solution, thus laying a solid foundation for the microelectrode. (6) While studying the traditional microelectrodes based on the micropipette and the gold wire, the building of print electrodes and chip electrodes should also be given adequate attention to build microscale, and even nanoscale, electrodes with the characteristics of miniaturization, integration, and flexibility. Once some breakthroughs in the above investigations of lead ion potentiometric sensors have been achieved and high-performance sensors are obtained, they can be successfully applied to the assay of trace levels of lead ions in biological samples, such as human blood and urine, in addition to fluids with complex compositions.
References [1]
Jiang W, Tang H, Liu Z C, Wang K, Chen B G, Li C H. Chin. J. Health. Lab. Technol., 2008, l8(3): 464–465
[2]
Wu T, Zhang L. J. Nanjing Univ. Traditional. Chin. Medicine (Natural Sci.), 2006, 22(3): 101–103
[3]
Wang L L, Lin L, Chen Y H. Environ. Chem., 2011, 30(2): 571–573
[4]
Hu L, Chen J X, Wang L M, Wang Q, Lin L D. J. Kunming Univ. Sci. Technol. (Sci. Technol.), 2007, 32(6): 81–83
[5]
Wang Z, Lee J H, Lu Y. Adv. Mater., 2008, 20(17): 3263–3267
[6]
Marbella L, Serli-Mitasev B, Basu P. Angew. Chem. Int. Ed., 2009, 48(22): 3996–3998
[7]
Son H, Lee H Y, Lim J M, Kang D, Han W S, Lee S S, Jung J H. Chem. Eur. J., 2010, 16(38): 11549–11553
[8]
Huang M R, Huang S J, Li X G. J. Phys. Chem. C, 2011, 115(13): 5301–5315
[9]
Sokalski T, Ceresa A, Zwickl T, Pretsch E. J. Am. Chem. Soc., 1997, 119(46): 11347–11348
[10]
Sokalski T, Ceresa A, Fibbioli M, Zwickl T, Bakker E,
[11]
Sutter J, Radu A, Peper S, Bakker E, Pretsch E. Anal. Chim.
[12]
Michalska A, Wojciechowski M, Bulska E, Mieczkowski J,
[13]
Lisak
Pretsch E. Anal. Chem., 1999, 71(6): 1210–1214 Acta, 2004, 523(1): 53–59 Maksymiuk K. Talanta, 2009, 79(5): 1247–1251 G,
Grygolowicz-Pawlak
E,
Mazurkiewicz
M,
Malinowska E, Sokalski T, Bobacka J, Lewenstam A. Microchim. Acta, 2009, 164(3-4): 293–297 [14]
Mimendia A, Legin A, Merkoci A, del Vallea M. Sensors and Actuators B, 2010, 146(2): 420–426
[15]
Huang M R, Ma X L, Li X G. Chin. Sci. Bull., 2008, 53(21):
HUANG Mei-Rong et al. / Chinese Journal of Analytical Chemistry, 2012, 40(1): 50–58
[34]
3255–3266 [16]
[35]
Huang M R, Rao X W, Li X G. Chin. J. Anal. Chem., 2008, 36(12): 1735–1741
[18]
2004, 81(8): 666–669
Kulesza J, Bochenska M. Eur. J. Inorg. Chem., 2011, 2011(6): 777–783
[17]
Agarwal H, Gupta A P, Verma G L. J. Indian Chem. Soc., Crespo G A, Macho S, Rius F X. Anal. Chem., 2008, 80(4): 1316–1322
[36]
Huang M R, Rao X W, Li X G, Ding Y B. Talanta, 2011,
Bobacka J, Ivaska A, Lewenstam A. Anal. Chim. Acta, 1999, 385(1-3): 195–202
85(3): 1575–1584
[37]
Lindfors T, Ivaska A. Anal. Chem., 2004, 76(15): 4387–4394
[19]
Tavkkoli N, Khojasteh Z, Sharghi H, Shamsipur M. Anal.
[38]
Bobacka J. Electroanalysis, 2006, 18(1): 7–18
Chim. Acta, 1998, 360(1-3): 203–208
[39]
[20]
Rahmani A, Barzegar M, Shamsipur M, Sharghi H, Mousavi
[21]
Barzegar M, Mousavi M F, Khajehsharifi H, Shamsipur M,
Michalska
Sharghi H. IEEE Sensors J., 2005, 5(3): 392–397
1568–1571
M F. Anal. Lett., 2000, 33(13): 2611–2629
[22]
[40]
Riahi S, Mousavi M F, Shamsipur M, Sharghi H.
Kisiel A, Mazur M, KuĞnieruk S, Kijewska K, KrysiĔski P, A.
Electrochem.
Commun.,
2010,
12(11):
[41]
Bochenska M, Guzinski M, Kulesza J. Electroanalysis, 2009,
[42]
Michalska A, Wojciechowski M, JĊdral W, Bulska E,
Electroanalysis, 2003, 15(19): 1561–1565
21(17-18): 2054–2060
[23]
Li X G, Ma X L, Huang M R. Talanta, 2009, 78(2): 498–505
[24]
Mousavi M F, Barzegar M B, Sahari S. Sensors and Actuators
[25]
Gismera M J, Sevilla M T, Procopio J R. Anal. Sci., 2006,
[26]
[26] Faridbod F, Ganjali M R, Larijani B, Hosseini M,
Maksymiuk K. J. Solid. State Electrochem., 2009, 13(1):
B, 2001, 73(2-3): 199–204
99–106 [43]
Michalska A, Wojciechowski M, Bulska E, Maksymiuk K.
[44]
Püntener M, Vigassy T, Baier E, Ceresa A, Pretsch E. Anal.
22(3): 405–410
Talanta, 2010, 82(1): 151–157
Alizadeh K, Norouzi P. Norouzi. Int. J. Electrochem. Sci., 2009, 4(11): 1528–1540
Chim. Acta, 2004, 503(2): 187–194 [45]
[27]
Casado M, Daunert S, Valiente M. Electroanalysis, 2001,
[28]
Alasier A S, Entisar S A, Elbishti H A. Arabian. J. Chem.,
13(1): 54–60
Malon A, Vigassy T, Bakker E, Pretsch E. J. Am. Chem. Soc., 2006, 128(25): 8154–8155
[46]
2010, 3(2): 89–94 [29]
Pawáowski P, Michalska A, Maksymiuk K. Electroanalysis, 2006, 18(13-14): 1339–1346
Kazemi S Y, Shamsipur M, Sharghi H. J. Hazard. Mater., 2009, 172(1): 68–73
[47]
Wygladacz K, Durnas M, Parzuchowski P, Brzózka Z,
Lu X Q, Chen Z L, Hall S B. Anal. Chim. Acta, 2000, 418(2):
Malinowska E. Sensors and Actuators B, 2003, 95(1-3):
205–212
366–372
[30]
Ross J W. Science, 1967, 156(3780): 1378–1379
[31]
Xu D, Katsu T. Talanta, 2000, 51(2): 365–371
[32]
Xu D, Katsu T. Anal. Chim. Acta, 1999, 401(1-2): 111–115
[33]
Abbaspour
A,
Mirahmadi
E,
Khalafi-nejad
Vigassy T, Huber C G, Wintringer R, Pretsch E. Anal. Chem.,
[49]
Uhlig A, Lindner E, Teutloff C, Schnakenberg U, Hintsche R.
[50]
Bakker E, Pretsch E, Buhlmann P. Anal. Chem., 2000, 72(6):
2005, 77(13): 3966–3970 A,
Babamohammadi S. J. Hazard. Mater., 2010, 174(1-3): 656–661
[48]
Anal. Chem., 1997, 69(19): 4032–4038 1127–1133