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<record><TEI><teiHeader><fileDesc><titleStmt><title xml:lang="en">Lanthanide Recognition: an Asymetric Erbium Microsensor Based on a Hydrazone Derivative</title>
<author><name sortKey="Faridbod, Farnoush" sort="Faridbod, Farnoush" uniqKey="Faridbod F" first="Farnoush" last="Faridbod">Farnoush Faridbod</name>
<affiliation><nlm:aff id="af1-sensors-07-03119"> Center of Excellence in Electrochemistry, Faculty of Chemistry, University of Tehran, Tehran, Iran, Tel.: +9861112788, Fax: +982166495291</nlm:aff>
</affiliation>
</author>
<author><name sortKey="Ganjali, Mohammad Reza" sort="Ganjali, Mohammad Reza" uniqKey="Ganjali M" first="Mohammad Reza" last="Ganjali">Mohammad Reza Ganjali</name>
<affiliation><nlm:aff id="af1-sensors-07-03119"> Center of Excellence in Electrochemistry, Faculty of Chemistry, University of Tehran, Tehran, Iran, Tel.: +9861112788, Fax: +982166495291</nlm:aff>
</affiliation>
<affiliation><nlm:aff id="af2-sensors-07-03119"> Endocrinology & Metabolism Research Center, Medical Sciences/University of Tehran, Tehran, Iran</nlm:aff>
</affiliation>
</author>
<author><name sortKey="Larijani, Bagher" sort="Larijani, Bagher" uniqKey="Larijani B" first="Bagher" last="Larijani">Bagher Larijani</name>
<affiliation><nlm:aff id="af2-sensors-07-03119"> Endocrinology & Metabolism Research Center, Medical Sciences/University of Tehran, Tehran, Iran</nlm:aff>
</affiliation>
</author>
<author><name sortKey="Norouzi, Parviz" sort="Norouzi, Parviz" uniqKey="Norouzi P" first="Parviz" last="Norouzi">Parviz Norouzi</name>
<affiliation><nlm:aff id="af1-sensors-07-03119"> Center of Excellence in Electrochemistry, Faculty of Chemistry, University of Tehran, Tehran, Iran, Tel.: +9861112788, Fax: +982166495291</nlm:aff>
</affiliation>
<affiliation><nlm:aff id="af2-sensors-07-03119"> Endocrinology & Metabolism Research Center, Medical Sciences/University of Tehran, Tehran, Iran</nlm:aff>
</affiliation>
</author>
<author><name sortKey="Riahi, Siavash" sort="Riahi, Siavash" uniqKey="Riahi S" first="Siavash" last="Riahi">Siavash Riahi</name>
<affiliation><nlm:aff id="af3-sensors-07-03119"> Institute of Petroleum Engineering, Faculty of Engineering, University of Tehran, Tehran, Iran</nlm:aff>
</affiliation>
</author>
<author><name sortKey="Mirnaghi, Fatemeh Sadat" sort="Mirnaghi, Fatemeh Sadat" uniqKey="Mirnaghi F" first="Fatemeh Sadat" last="Mirnaghi">Fatemeh Sadat Mirnaghi</name>
<affiliation><nlm:aff id="af1-sensors-07-03119"> Center of Excellence in Electrochemistry, Faculty of Chemistry, University of Tehran, Tehran, Iran, Tel.: +9861112788, Fax: +982166495291</nlm:aff>
</affiliation>
</author>
</titleStmt>
<publicationStmt><idno type="wicri:source">PMC</idno>
<idno type="pmc">3841884</idno>
<idno type="url">http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3841884</idno>
<idno type="RBID">PMC:3841884</idno>
<idno type="pmid">NONE</idno>
<date when="2007">2007</date>
<idno type="wicri:Area/Pmc/Corpus">000395</idno>
<idno type="wicri:explorRef" wicri:stream="Pmc" wicri:step="Corpus" wicri:corpus="PMC">000395</idno>
</publicationStmt>
<sourceDesc><biblStruct><analytic><title xml:lang="en" level="a" type="main">Lanthanide Recognition: an Asymetric Erbium Microsensor Based on a Hydrazone Derivative</title>
<author><name sortKey="Faridbod, Farnoush" sort="Faridbod, Farnoush" uniqKey="Faridbod F" first="Farnoush" last="Faridbod">Farnoush Faridbod</name>
<affiliation><nlm:aff id="af1-sensors-07-03119"> Center of Excellence in Electrochemistry, Faculty of Chemistry, University of Tehran, Tehran, Iran, Tel.: +9861112788, Fax: +982166495291</nlm:aff>
</affiliation>
</author>
<author><name sortKey="Ganjali, Mohammad Reza" sort="Ganjali, Mohammad Reza" uniqKey="Ganjali M" first="Mohammad Reza" last="Ganjali">Mohammad Reza Ganjali</name>
<affiliation><nlm:aff id="af1-sensors-07-03119"> Center of Excellence in Electrochemistry, Faculty of Chemistry, University of Tehran, Tehran, Iran, Tel.: +9861112788, Fax: +982166495291</nlm:aff>
</affiliation>
<affiliation><nlm:aff id="af2-sensors-07-03119"> Endocrinology & Metabolism Research Center, Medical Sciences/University of Tehran, Tehran, Iran</nlm:aff>
</affiliation>
</author>
<author><name sortKey="Larijani, Bagher" sort="Larijani, Bagher" uniqKey="Larijani B" first="Bagher" last="Larijani">Bagher Larijani</name>
<affiliation><nlm:aff id="af2-sensors-07-03119"> Endocrinology & Metabolism Research Center, Medical Sciences/University of Tehran, Tehran, Iran</nlm:aff>
</affiliation>
</author>
<author><name sortKey="Norouzi, Parviz" sort="Norouzi, Parviz" uniqKey="Norouzi P" first="Parviz" last="Norouzi">Parviz Norouzi</name>
<affiliation><nlm:aff id="af1-sensors-07-03119"> Center of Excellence in Electrochemistry, Faculty of Chemistry, University of Tehran, Tehran, Iran, Tel.: +9861112788, Fax: +982166495291</nlm:aff>
</affiliation>
<affiliation><nlm:aff id="af2-sensors-07-03119"> Endocrinology & Metabolism Research Center, Medical Sciences/University of Tehran, Tehran, Iran</nlm:aff>
</affiliation>
</author>
<author><name sortKey="Riahi, Siavash" sort="Riahi, Siavash" uniqKey="Riahi S" first="Siavash" last="Riahi">Siavash Riahi</name>
<affiliation><nlm:aff id="af3-sensors-07-03119"> Institute of Petroleum Engineering, Faculty of Engineering, University of Tehran, Tehran, Iran</nlm:aff>
</affiliation>
</author>
<author><name sortKey="Mirnaghi, Fatemeh Sadat" sort="Mirnaghi, Fatemeh Sadat" uniqKey="Mirnaghi F" first="Fatemeh Sadat" last="Mirnaghi">Fatemeh Sadat Mirnaghi</name>
<affiliation><nlm:aff id="af1-sensors-07-03119"> Center of Excellence in Electrochemistry, Faculty of Chemistry, University of Tehran, Tehran, Iran, Tel.: +9861112788, Fax: +982166495291</nlm:aff>
</affiliation>
</author>
</analytic>
<series><title level="j">Sensors (Basel, Switzerland)</title>
<idno type="eISSN">1424-8220</idno>
<imprint><date when="2007">2007</date>
</imprint>
</series>
</biblStruct>
</sourceDesc>
</fileDesc>
<profileDesc><textClass></textClass>
</profileDesc>
</teiHeader>
<front><div type="abstract" xml:lang="en"><p><italic>N</italic>
'-(2-hydroxy-1,2-diphenylethylidene)benzohydrazide (HDB) was found to have a very selective and sensitive behavior towards erbium(III) ions, in comparison to thirteen lanthanide ions, inner transition and representative metal ions and was hence used as a neutral ion carrier in construction of an Er(III) microelectrode. Theoretical calculations and conductance studies of HDB to erbium and some other metal ions were carried out and confirmed selectivity toward Er(III) ions.</p>
<p>The best performance was obtained with a membrane contain 3% potassium tetrakis(<italic>p</italic>
-chlorophenyl)borate (KTpClPB) as an anionic additive, 72% dibutyl phthalate (DBP) as solvent mediator, 5% HDB, and 20% poly(vinyl chloride) (PVC). The proposed Er(III) microelectrode exhibits a near Nernstian response of 17.5±0.5 mV per decade of erbium activity, and a very wide linear range 1.0×10<sup>-3</sup>
-3.0×10<sup>-10</sup>
M. It can work well in the pH range of 3.0-9.0. The lower detection limit (LDL) of the microelectrode was calculated to be 2.0×10<sup>-10</sup>
M.</p>
</div>
</front>
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</analytic>
</biblStruct>
<biblStruct><analytic><author><name sortKey="Buck, P R" uniqKey="Buck P">P. R. Buck</name>
</author>
<author><name sortKey="Lindneri, E" uniqKey="Lindneri E">E. Lindneri</name>
</author>
</analytic>
</biblStruct>
<biblStruct><analytic><author><name sortKey="Greenwood, N N" uniqKey="Greenwood N">N. N. Greenwood</name>
</author>
<author><name sortKey="Earnshaw, A" uniqKey="Earnshaw A">A. Earnshaw</name>
</author>
</analytic>
</biblStruct>
</listBibl>
</div1>
</back>
</TEI>
<pmc article-type="research-article"><pmc-dir>properties open_access</pmc-dir>
<front><journal-meta><journal-id journal-id-type="nlm-ta">Sensors (Basel)</journal-id>
<journal-id journal-id-type="iso-abbrev">Sensors (Basel)</journal-id>
<journal-title-group><journal-title>Sensors (Basel, Switzerland)</journal-title>
</journal-title-group>
<issn pub-type="epub">1424-8220</issn>
<publisher><publisher-name>Molecular Diversity Preservation International (MDPI)</publisher-name>
</publisher>
</journal-meta>
<article-meta><article-id pub-id-type="pmc">3841884</article-id>
<article-id pub-id-type="publisher-id">sensors-07-03119</article-id>
<article-categories><subj-group subj-group-type="heading"><subject>Full Research Paper</subject>
</subj-group>
</article-categories>
<title-group><article-title>Lanthanide Recognition: an Asymetric Erbium Microsensor Based on a Hydrazone Derivative</article-title>
</title-group>
<contrib-group><contrib contrib-type="author"><name><surname>Faridbod</surname>
<given-names>Farnoush</given-names>
</name>
<xref ref-type="aff" rid="af1-sensors-07-03119"><sup>1</sup>
</xref>
</contrib>
<contrib contrib-type="author"><name><surname>Ganjali</surname>
<given-names>Mohammad Reza</given-names>
</name>
<xref ref-type="aff" rid="af1-sensors-07-03119"><sup>1</sup>
</xref>
<xref ref-type="aff" rid="af2-sensors-07-03119"><sup>2</sup>
</xref>
<xref rid="c1-sensors-07-03119" ref-type="corresp"><sup>*</sup>
</xref>
</contrib>
<contrib contrib-type="author"><name><surname>Larijani</surname>
<given-names>Bagher</given-names>
</name>
<xref ref-type="aff" rid="af2-sensors-07-03119"><sup>2</sup>
</xref>
</contrib>
<contrib contrib-type="author"><name><surname>Norouzi</surname>
<given-names>Parviz</given-names>
</name>
<xref ref-type="aff" rid="af1-sensors-07-03119"><sup>1</sup>
</xref>
<xref ref-type="aff" rid="af2-sensors-07-03119"><sup>2</sup>
</xref>
</contrib>
<contrib contrib-type="author"><name><surname>Riahi</surname>
<given-names>Siavash</given-names>
</name>
<xref ref-type="aff" rid="af3-sensors-07-03119"><sup>3</sup>
</xref>
</contrib>
<contrib contrib-type="author"><name><surname>Mirnaghi</surname>
<given-names>Fatemeh Sadat</given-names>
</name>
<xref ref-type="aff" rid="af1-sensors-07-03119"><sup>1</sup>
</xref>
</contrib>
</contrib-group>
<aff id="af1-sensors-07-03119"><label>1</label>
Center of Excellence in Electrochemistry, Faculty of Chemistry, University of Tehran, Tehran, Iran, Tel.: +9861112788, Fax: +982166495291</aff>
<aff id="af2-sensors-07-03119"><label>2</label>
Endocrinology & Metabolism Research Center, Medical Sciences/University of Tehran, Tehran, Iran</aff>
<aff id="af3-sensors-07-03119"><label>3</label>
Institute of Petroleum Engineering, Faculty of Engineering, University of Tehran, Tehran, Iran</aff>
<author-notes><corresp id="c1-sensors-07-03119"><label>*</label>
Author to whom correspondence should be addressed. E-mail: <email>ganjali@khayam.ut.ac.ir</email>
</corresp>
</author-notes>
<pub-date pub-type="collection"><month>12</month>
<year>2007</year>
</pub-date>
<pub-date pub-type="epub"><day>05</day>
<month>11</month>
<year>2007</year>
</pub-date>
<volume>7</volume>
<issue>12</issue>
<fpage>3119</fpage>
<lpage>3135</lpage>
<history><date date-type="received"><day>09</day>
<month>8</month>
<year>2007</year>
</date>
<date date-type="accepted"><day>26</day>
<month>10</month>
<year>2007</year>
</date>
</history>
<permissions><copyright-statement>© 2007 by MDPI (<ext-link ext-link-type="uri" xlink:href="http://www.mdpi.org">http://www.mdpi.org</ext-link>
).</copyright-statement>
<copyright-year>2007</copyright-year>
<license><license-p>Reproduction is permitted for noncommercial purposes.</license-p>
</license>
</permissions>
<abstract><p><italic>N</italic>
'-(2-hydroxy-1,2-diphenylethylidene)benzohydrazide (HDB) was found to have a very selective and sensitive behavior towards erbium(III) ions, in comparison to thirteen lanthanide ions, inner transition and representative metal ions and was hence used as a neutral ion carrier in construction of an Er(III) microelectrode. Theoretical calculations and conductance studies of HDB to erbium and some other metal ions were carried out and confirmed selectivity toward Er(III) ions.</p>
<p>The best performance was obtained with a membrane contain 3% potassium tetrakis(<italic>p</italic>
-chlorophenyl)borate (KTpClPB) as an anionic additive, 72% dibutyl phthalate (DBP) as solvent mediator, 5% HDB, and 20% poly(vinyl chloride) (PVC). The proposed Er(III) microelectrode exhibits a near Nernstian response of 17.5±0.5 mV per decade of erbium activity, and a very wide linear range 1.0×10<sup>-3</sup>
-3.0×10<sup>-10</sup>
M. It can work well in the pH range of 3.0-9.0. The lower detection limit (LDL) of the microelectrode was calculated to be 2.0×10<sup>-10</sup>
M.</p>
</abstract>
<kwd-group><kwd>Er(III)</kwd>
<kwd>Hydrazone derivative</kwd>
<kwd>Microelectrode</kwd>
<kwd>Potentiometry</kwd>
<kwd>Sensor</kwd>
</kwd-group>
</article-meta>
</front>
<body><sec><label>1.</label>
<title>Introduction</title>
<p>Erbium is one of the rare chemicals that can be found in houses in equipment such as color televisions, fluorescent lamps, energy-saving lamps and glasses. All rare chemicals have comparable properties.</p>
<p>Erbium is dumped in the environment in many different places, mainly by petrol-producing industries. It can also enter the environment when household equipment is thrown away. Erbium will gradually accumulate in soils and water soils and this will eventually lead to increasing concentrations in humans, animals and soil particles. The use of erbium is still growing, due to the fact that it is suited to produce catalysers and to polish glass. Erbium is mostly dangerous in the working environment, due to the fact that damps and gasses can be inhaled with air. This can cause lung embolisms, especially during long-term exposure. Erbium can be a threat to the liver when it accumulates in the human body [<xref rid="b1-sensors-07-03119" ref-type="bibr">1</xref>
,<xref rid="b2-sensors-07-03119" ref-type="bibr">2</xref>
].</p>
<p>All erbium compounds should be regarded as highly toxic because the biological properties of the lanthanides, primarily based on their similarity to calcium, have a high affinity for Ca<sup>2+</sup>
sites on biological molecules and hence can act as either Ca<sup>2+</sup>
inhibitors or probes. Although the Lanthanide cannot gain access to intracellular organelles, they have been used as biochemical probes to study calcium transport in mitochondria and other organelles [<xref rid="b3-sensors-07-03119" ref-type="bibr">3</xref>
].</p>
<p>So ability to determine low amount of this ion in environmental solution or biological fluid is inevitable.</p>
<p>Up to now, voltammetry, mass spectroscopy and Rutherford back-scattering techniques, higher order derivative spectrometry, ICP-MS, NAA (Neutron Activation Analysis) and spectrofluorometric methods have been used for the analysis of erbium [<xref rid="b4-sensors-07-03119" ref-type="bibr">4</xref>
-<xref rid="b7-sensors-07-03119" ref-type="bibr">7</xref>
]. These methods are either time consuming, involving multiple sample manipulations, or too expensive for most analytical laboratories.</p>
<p>Potentiometric electrodes, offer several advantages such as fast and ease of preparations and procedures, simple instrumentations, relatively fast responses, very low detection limit, wide dynamic ranges, reasonable selectivity and low costs. This has led to increasing the number of available electrodes and microelectrodes over the last few years [<xref rid="b8-sensors-07-03119" ref-type="bibr">8</xref>
].</p>
<p>Although the neutral carrier-type ISEs have been successful for determination of a wide variety of metal ions [<xref rid="b8-sensors-07-03119" ref-type="bibr">8</xref>
], including the alkali, alkaline-earth, transition, and some other heavy metal ions, there are only a limited number of reports on the development of highly selective electrodes for eleven lanthanide ions based on different noncyclic and macrocyclic ionophores such as (2-mercaptoanil)diacetyl [<xref rid="b9-sensors-07-03119" ref-type="bibr">9</xref>
], aza-crown [<xref rid="b10-sensors-07-03119" ref-type="bibr">10</xref>
], dicyclohexano-18-crown-6 [<xref rid="b11-sensors-07-03119" ref-type="bibr">11</xref>
], 1,3,5-trithiacyclohexane [<xref rid="b12-sensors-07-03119" ref-type="bibr">12</xref>
], N-N Schiff's base [<xref rid="b13-sensors-07-03119" ref-type="bibr">13</xref>
], bis(thiophenal)phenylen-1,3-diamine [<xref rid="b14-sensors-07-03119" ref-type="bibr">14</xref>
], 1-phenyl-3-methyl-4-octadecanoyl-5-pyrazolone [<xref rid="b15-sensors-07-03119" ref-type="bibr">15</xref>
], 4-methyl-2-hydrazinobenzothiazole [<xref rid="b16-sensors-07-03119" ref-type="bibr">16</xref>
], 5,14-N,N'-hydroxyphenyl-4,15-dioxo-1,5,14,18-tetraaza hexacosane [<xref rid="b17-sensors-07-03119" ref-type="bibr">17</xref>
], on N-(2-pyridyl)-N'-(2-methoxyphenyl)-thiourea [<xref rid="b18-sensors-07-03119" ref-type="bibr">18</xref>
], gliclazide [<xref rid="b19-sensors-07-03119" ref-type="bibr">19</xref>
] and bis(5-nitro-2-furaldehyde)butane-2,3-dihydrazone [<xref rid="b20-sensors-07-03119" ref-type="bibr">20</xref>
] for lanthanum, 1,3,5-trithiane [<xref rid="b21-sensors-07-03119" ref-type="bibr">21</xref>
,<xref rid="b22-sensors-07-03119" ref-type="bibr">22</xref>
], azomethine [<xref rid="b23-sensors-07-03119" ref-type="bibr">23</xref>
], N'-[(2-Hydroxyphenyl) methylidene]-2-furohydrazide [<xref rid="b24-sensors-07-03119" ref-type="bibr">24</xref>
], and 2-aminobenzothiazole [<xref rid="b25-sensors-07-03119" ref-type="bibr">25</xref>
] for cerium, bis(thiaalkylxanthato)alkanes [<xref rid="b26-sensors-07-03119" ref-type="bibr">26</xref>
], glipizide [<xref rid="b27-sensors-07-03119" ref-type="bibr">27</xref>
,<xref rid="b28-sensors-07-03119" ref-type="bibr">28</xref>
], 3-{[2-oxo-1(2H)-acenaphthylenyliden]amino}-2-thioxo-1,3-thiazolidin-4-one [<xref rid="b29-sensors-07-03119" ref-type="bibr">29</xref>
], Tin (IV)Boratophosphate [<xref rid="b30-sensors-07-03119" ref-type="bibr">30</xref>
], and 4,5,6,7-tetrathiocino[1,2-b:3,4-b′]diimidazolyl-1,3,8,10-tetraethyl-2, 9-dithione (Et4todit) [<xref rid="b31-sensors-07-03119" ref-type="bibr">31</xref>
] for samarium, omeprazole [<xref rid="b32-sensors-07-03119" ref-type="bibr">32</xref>
], S-N Schiff's base [<xref rid="b33-sensors-07-03119" ref-type="bibr">33</xref>
], and N-(2-pyridyl)-N'-(4-nitrophenyl)thiourea [<xref rid="b34-sensors-07-03119" ref-type="bibr">34</xref>
] for gadolinium, N-(2-pyridyl)-N'-(2-methoxyphenyl)-thiourea [<xref rid="b35-sensors-07-03119" ref-type="bibr">35</xref>
], 1-phenyl-3-(2-thiazolyl)-2-thiourea [<xref rid="b36-sensors-07-03119" ref-type="bibr">36</xref>
], (3-hydroxy-N'-[(2-hydrox yphenyl) methylene]-2-naphthohydrazide [<xref rid="b37-sensors-07-03119" ref-type="bibr">37</xref>
], cefixime [<xref rid="b38-sensors-07-03119" ref-type="bibr">38</xref>
], and 6-methy-4-{[1-(1H-pyrrol-2-yl)methylidene]amino}-3-thioxo-3,4-dihydro-1,2,4-triazin-5(2H)-one [<xref rid="b39-sensors-07-03119" ref-type="bibr">39</xref>
] for ytterbium, N,N-diethyl-N-(4-hydroxy-6-methylpyridin-2-yl)guanidine [<xref rid="b40-sensors-07-03119" ref-type="bibr">40</xref>
], S-N hexadentates schiff's base [<xref rid="b41-sensors-07-03119" ref-type="bibr">41</xref>
], and 4-(2-hydroxybenzylideneamino)-6-methyl-3-thioxo-3,4-dihydro-1,2,4-triazin-5(2H)-one [<xref rid="b42-sensors-07-03119" ref-type="bibr">42</xref>
] for europium, bispyrrolidene Schiff's base [<xref rid="b43-sensors-07-03119" ref-type="bibr">43</xref>
] and 4-Amino-3-{2-[4-amino-6-methyl-5-oxo-4,5-dihydro-1,2,4-triazin-3(2H)-yliden] hydrazono}-6-methyl-3,4-dihydro-1,2,4-triazin-5(2H)-one [<xref rid="b44-sensors-07-03119" ref-type="bibr">44</xref>
] for terbium, bis-pyrrolidene Schiff's base [<xref rid="b45-sensors-07-03119" ref-type="bibr">45</xref>
] and asymmetrical Schiff's base [<xref rid="b46-sensors-07-03119" ref-type="bibr">46</xref>
] for dysprosium, N'-(2-hydroxy-1,2-diphenylethylidene) benzohydrazide [<xref rid="b47-sensors-07-03119" ref-type="bibr">47</xref>
] for erbium, N-(1-thien-2-ylmethylene)-1,3-benzothiazol-2-amine [<xref rid="b48-sensors-07-03119" ref-type="bibr">48</xref>
] and N,N'-bis(2-quinolinecarboxamide)-1,2-benzene [<xref rid="b49-sensors-07-03119" ref-type="bibr">49</xref>
] for holmium, hydrazide derivative [<xref rid="b50-sensors-07-03119" ref-type="bibr">50</xref>
] for presidium, thiophene-2-carbaldehyde-(7-methyl-1,3-benzothiazol-2-yl)hydrazone [<xref rid="b51-sensors-07-03119" ref-type="bibr">51</xref>
] for thulium, asymmetrically S–N Schiff's base for lutetium [<xref rid="b52-sensors-07-03119" ref-type="bibr">52</xref>
].</p>
<p>Among these reported lanthanide sensors, there is no report on fabrication an asymmetric Er(III) sensor for the fast monitoring of very low level of erbium ion. In this work we wish to report the first erbium(III) microelectrode based on <italic>N</italic>
'-(2-hydroxy-1,2-diphenylethylidene) benzohydrazide (HDB) for the monitoring of sub-nanomolar concentration of erbium(III) ions.</p>
</sec>
<sec><label>2.</label>
<title>Experimental</title>
<sec><label>2.1.</label>
<title>Reagents</title>
<p>Potassium tetrakis(4-chlorophenyl) borate (KTpClPB), PVC of high relative molecular weight, <italic>o</italic>
-nitrophenyloctyl ether (NPOE), nitrobenzene (NB), dibutyl phthalate (DBP), tetrahydrofuran (THF), chloride and nitrate salts of cations were of the highest purity available (from Merck and Aldrich), and were used without further purification. All aqueous solutions were prepared with deionized distilled water. The pH of all solutions, were adjusted with dilute nitric acid and sodium hydroxide. The ionophore HDB (<xref rid="f1-sensors-07-03119" ref-type="fig">Fig. 1</xref>
) was synthesized as described elsewhere [<xref rid="b47-sensors-07-03119" ref-type="bibr">47</xref>
].</p>
</sec>
<sec><label>2.2.</label>
<title>Electrode preparation</title>
<p>To prepare the PVC membrane, we used dipping method [<xref rid="b8-sensors-07-03119" ref-type="bibr">8</xref>
], after thoroughly mixing 20 mg of powdered PVC, 72 mg of DBP, 3 mg of additive KTpClPB, and 5 mg of HDB and 3 ml of THF, the resulting mixture was transferred into a glass dish with a 2 cm in diameter. The solvent was slowly evaporated until a relative oily concentrated was obtained. The gold electrode was prepared by sealing gold micro-wire (Goodfellow Metals Ltd., UK) into a soft glass capillary. The capillary was then cut perpendicular to its length to expose the gold wire. Electrical contact was made using silver epoxy (Johnson Matthey Ltd., UK). Before each experiment the electrode surface was polished for 1 min, using extra fine carborundum paper and then for 10 min with 0.3 □m alumina, sonicated in distilled water and dried in air. The polished gold electrode was dipped into the membrane solution mentioned above and the solvent was evaporated. A membrane was formed on the gold surface and the electrode was allowed to set overnight. The electrode was finally conditioned for 48 h by soaking in a 1.0 × 10<sup>-3</sup>
M of ErCl<sub>3</sub>
. The surfaces of the Er(III) microelectrode membrane sensors before and after conditioning in a Er(III) solution are shown in <xref rid="f2-sensors-07-03119" ref-type="fig">Fig. 2</xref>
. As seen, after the conditioning of the membrane in the erbium solution, most of the sites in the membrane form the Er(III)-HDB complex and the surface of the membrane look likes homogeneous.</p>
</sec>
<sec><label>2.3.</label>
<title>Apparatus</title>
<p>Potentials were measured with a Corning ion analyzer Model 250-pH/mV meter. The pH of the sample solutions was monitored simultaneously with a conventional glass pH electrode.</p>
</sec>
<sec><label>2.4.</label>
<title>EMF-Measurement</title>
<p>All emf measurements were carried out with the following assembly:
<disp-formula id="FD1"><mml:math id="mm1"><mml:mrow><mml:msub><mml:mtext>Hg</mml:mtext>
<mml:mn>2</mml:mn>
</mml:msub>
<mml:msub><mml:mtext>Cl</mml:mtext>
<mml:mn>2</mml:mn>
</mml:msub>
<mml:mo>,</mml:mo>
<mml:mtext>KCl</mml:mtext>
<mml:mspace width="0.2em"></mml:mspace>
<mml:mo stretchy="false">(</mml:mo>
<mml:mtext>satd</mml:mtext>
<mml:mo>.</mml:mo>
<mml:mo stretchy="false">)</mml:mo>
<mml:mo>|</mml:mo>
<mml:mtext>Sample solution</mml:mtext>
<mml:mo>|</mml:mo>
<mml:mtext>PVC membrane</mml:mtext>
<mml:mo>|</mml:mo>
<mml:mtext>gold surface</mml:mtext>
</mml:mrow>
</mml:math>
</disp-formula>
</p>
<p>In all measurements, the ionic strength of the solutions was maintained by using 10<sup>-4</sup>
M of NaCl.</p>
</sec>
</sec>
<sec><label>3.</label>
<title>Results and Discussions</title>
<p>Regarding that some neutral ion carriers, containing nitrogen, sulfur or both nitrogen and sulfur donor atoms, have been reported, to be used in construction of selective and sensitive transition and heavy metal ion membrane microelectrodes, lately [<xref rid="b17-sensors-07-03119" ref-type="bibr">17</xref>
-<xref rid="b25-sensors-07-03119" ref-type="bibr">25</xref>
,<xref rid="b53-sensors-07-03119" ref-type="bibr">53</xref>
,<xref rid="b54-sensors-07-03119" ref-type="bibr">54</xref>
] and considering the existence of two nitrogen and one oxygen donor atoms in the semi cavity of HDB, it was expected to act as a suitable ion carrier for special transition and heavy metal ions (specially, higher charge density), in the PVC membranes.</p>
<p>The microelectrode sensor is an asymmetrical sensor without any internal reference electrode and internal solution. The main problem of the symmetric ion selective liquid membrane electrodes is the leaking of the internal solution to the outer surface of the membrane, causing changes in the surface potential. Therefore, the detection limit of this kind of electrode is about 10<sup>-6</sup>
M. In the case of asymmetric sensors, the wire coated and the graphite coated detection limit is about 10<sup>-8</sup>
-10<sup>-11</sup>
M (on the grounds that there is no leaking of the internal solution). Due to the high tendency of the asymmetric microelectrode to the low Er(III) concentration, the selectivity will be drastically improved [<xref rid="b8-sensors-07-03119" ref-type="bibr">8</xref>
].</p>
<p>Many experimental and theoretical investigations have been carried out to understand better the fundamental interaction between metal ions and neutral molecules and their relationship to molecular recognition. Computational models capable of reliably predicting ligand selectivity in a variety of cations have been shown to be valuable tools for the advancement of practical works [<xref rid="b54-sensors-07-03119" ref-type="bibr">54</xref>
-<xref rid="b58-sensors-07-03119" ref-type="bibr">58</xref>
]. In order to have a clear picture about the selectivity of ligand for various metal ions, in this work, we investigated its binding to Na<sup>+</sup>
, Ce<sup>3+</sup>
, Nd<sup>3+</sup>
, Pr<sup>3+</sup>
, Sm<sup>3+</sup>
, Gd<sup>3+</sup>
, Pb<sup>2+</sup>
, Cu<sup>2+</sup>
and Er<sup>3+</sup>
ions by using the Extended Hückel semi-empirical calculations. Because system contains atom (Er), which have not been assigned any basis function in ab-initio calculation, Extended Hückel semi-empirical calculations have been used [<xref rid="b60-sensors-07-03119" ref-type="bibr">60</xref>
,<xref rid="b61-sensors-07-03119" ref-type="bibr">61</xref>
]. The influence of the nature, size and charge of metal ions on the complexation reaction with the neutral ligand is explained on the basis of the calculation of gas–phase binding energies.</p>
<p>The binding energy of the uncomplexed ligand and its complexes with Er<sup>3+</sup>
, and other metal ions were carried out using Hyper Chem software (Version 6.01). The binding energy (ΔE) was calculated using <xref rid="FD2" ref-type="disp-formula">equation (1)</xref>
:
<disp-formula id="FD2"><label>(1)</label>
<mml:math id="mm2"><mml:mrow><mml:mi mathvariant="normal">Δ</mml:mi>
<mml:mtext>E</mml:mtext>
<mml:mo>=</mml:mo>
<mml:mi mathvariant="normal">Δ</mml:mi>
<mml:msub><mml:mtext>E</mml:mtext>
<mml:mtext>complex</mml:mtext>
</mml:msub>
<mml:mo>−</mml:mo>
<mml:mo stretchy="false">(</mml:mo>
<mml:mi mathvariant="normal">Δ</mml:mi>
<mml:msub><mml:mtext>E</mml:mtext>
<mml:mtext>ligand</mml:mtext>
</mml:msub>
<mml:mo>−</mml:mo>
<mml:mi mathvariant="normal">Δ</mml:mi>
<mml:msub><mml:mtext>E</mml:mtext>
<mml:mtext>cation</mml:mtext>
</mml:msub>
<mml:mo stretchy="false">)</mml:mo>
</mml:mrow>
</mml:math>
</disp-formula>
Where, ΔE<sub>complex</sub>
, ΔE<sub>ligand</sub>
and ΔE<sub>cation</sub>
are the total energies of the complex, uncomplexed ligand and metal ion, respectively. <xref rid="t1-sensors-07-03119" ref-type="table">Table 1</xref>
summarizes the theoretical data relating the stability of the ligand compelexes with Na<sup>+</sup>
, Ce<sup>3+</sup>
, Nd<sup>3+</sup>
, Pr<sup>3+</sup>
, Sm<sup>3+</sup>
, Gd<sup>3+</sup>
, Pb<sup>2+</sup>
, Cu<sup>2+</sup>
and Er<sup>3+</sup>
ions. Inspection of <xref rid="t1-sensors-07-03119" ref-type="table">Table 1</xref>
reveals that the cation binding energy with ligand shows a pronounced dependence on the nature of metal ions used.</p>
<p>Thus, based on the above ab-initio calculation results, ionophore could possibly be used as a suitable ionophore in preparation of an erbium ion-selective membrane microelectrode. The optimized structures of ionophore and its Er<sup>+3</sup>
complexes are shown in <xref rid="f3-sensors-07-03119" ref-type="fig">Fig. 3</xref>
.</p>
<p>At the next experiment, the HDB complexation with a number of transition metal ions including La(III), Yb(III), Gd(III), Er(III), Pr(III), Tb(III), Sm(III), Eu(III), Dy(III), Ho(III), Lu(III), Tm(III), Ca(II), Co(II), Pb(II), Cu(II) K(I), and Na(I) was investigated conductometrically in acetonitrile solutions (1.0 x 10<sup>-4</sup>
M of cation solution and 1.0 x 10<sup>-2</sup>
M of ligand) at 25 ± 0.1 °C [<xref rid="b51-sensors-07-03119" ref-type="bibr">51</xref>
]. 25 ml of each ion solution was titrated with 0.01 M of HDB solution in order to obtain a clue about the stability and selectivity of the resulting complexes.</p>
<p>The conductance of the solution was measured after each addition. Addition of the ligand was continued until the desired ligand-to-cation mole ratio was achieved. The 1:1 binding of the protonated amines with macrocyclic ligands can be expressed by the following equilibrium:
<disp-formula id="FD3"><label>(2)</label>
<mml:math id="mm3"><mml:mrow><mml:mtable><mml:mtr><mml:mtd><mml:mrow><mml:msup><mml:mrow><mml:msup><mml:mtext>M</mml:mtext>
<mml:mo>+</mml:mo>
</mml:msup>
<mml:mo>+</mml:mo>
<mml:mtext>L</mml:mtext>
<mml:mspace width="2.0em"></mml:mspace>
</mml:mrow>
<mml:mrow><mml:msub><mml:mtext>K</mml:mtext>
<mml:mtext>f</mml:mtext>
</mml:msub>
</mml:mrow>
</mml:msup>
<mml:mspace width="2.0em"></mml:mspace>
<mml:msup><mml:mrow><mml:mtext>ML</mml:mtext>
</mml:mrow>
<mml:mo>+</mml:mo>
</mml:msup>
</mml:mrow>
</mml:mtd>
<mml:mtd><mml:mo>⇄</mml:mo>
</mml:mtd>
</mml:mtr>
</mml:mtable>
</mml:mrow>
</mml:math>
</disp-formula>
</p>
<p>The corresponding equilibrium constant, K<sub>f,</sub>
is given by:
<disp-formula id="FD4"><label>(3)</label>
<mml:math id="mm4"><mml:mrow><mml:msub><mml:mtext>K</mml:mtext>
<mml:mtext>f</mml:mtext>
</mml:msub>
<mml:mrow><mml:mo>=</mml:mo>
</mml:mrow>
<mml:mfrac><mml:mrow><mml:mo stretchy="false">[</mml:mo>
<mml:msup><mml:mtext>ML</mml:mtext>
<mml:mo>+</mml:mo>
</mml:msup>
<mml:mo stretchy="false">]</mml:mo>
</mml:mrow>
<mml:mrow><mml:mo stretchy="false">[</mml:mo>
<mml:msup><mml:mtext>M</mml:mtext>
<mml:mo>+</mml:mo>
</mml:msup>
<mml:mo stretchy="false">]</mml:mo>
<mml:mo stretchy="false">[</mml:mo>
<mml:mtext>L</mml:mtext>
<mml:mo stretchy="false">]</mml:mo>
</mml:mrow>
</mml:mfrac>
<mml:mo>×</mml:mo>
<mml:mfrac><mml:mrow><mml:mtext>f</mml:mtext>
<mml:mo stretchy="false">(</mml:mo>
<mml:msup><mml:mtext>ML</mml:mtext>
<mml:mo>+</mml:mo>
</mml:msup>
<mml:mo stretchy="false">)</mml:mo>
</mml:mrow>
<mml:mrow><mml:mtext>f</mml:mtext>
<mml:mo stretchy="false">(</mml:mo>
<mml:msup><mml:mtext>M</mml:mtext>
<mml:mo>+</mml:mo>
</mml:msup>
<mml:mo stretchy="false">)</mml:mo>
<mml:mtext>f</mml:mtext>
<mml:mo stretchy="false">(</mml:mo>
<mml:mtext>L</mml:mtext>
<mml:mo stretchy="false">)</mml:mo>
</mml:mrow>
</mml:mfrac>
</mml:mrow>
</mml:math>
</disp-formula>
Where [ML<sup>+</sup>
], [M<sup>+</sup>
], [L] and f represent the equilibrium molar concentration of complexes, free cation, free ligand, and the activity coefficient of the species indicated, respectively.</p>
<p>Under the dilute condition we used, the activity coefficient of the unchanged ligand, f(L) can be reasonably assumed as unity [<xref rid="b62-sensors-07-03119" ref-type="bibr">62</xref>
,<xref rid="b63-sensors-07-03119" ref-type="bibr">63</xref>
]. The use of Deby-Huckel limiting law of 1:1 electrolytes [<xref rid="b64-sensors-07-03119" ref-type="bibr">64</xref>
] lead to the conclusion that f(M<sup>+</sup>
)∽f(ML<sup>+</sup>
), so the activity coefficient in <xref rid="FD4" ref-type="disp-formula">equation (3)</xref>
is canceled out. Thus the complex formation constant in term of the molar conductance can be expressed as [<xref rid="b65-sensors-07-03119" ref-type="bibr">65</xref>
,<xref rid="b66-sensors-07-03119" ref-type="bibr">66</xref>
]:
<disp-formula id="FD5"><label>(4)</label>
<mml:math id="mm5"><mml:mrow><mml:msub><mml:mtext>K</mml:mtext>
<mml:mtext>f</mml:mtext>
</mml:msub>
<mml:mo>=</mml:mo>
<mml:mfrac><mml:mrow><mml:mo stretchy="false">[</mml:mo>
<mml:msup><mml:mtext>ML</mml:mtext>
<mml:mo>+</mml:mo>
</mml:msup>
<mml:mo stretchy="false">]</mml:mo>
</mml:mrow>
<mml:mrow><mml:mo stretchy="false">[</mml:mo>
<mml:msup><mml:mtext>M</mml:mtext>
<mml:mo>+</mml:mo>
</mml:msup>
<mml:mo stretchy="false">]</mml:mo>
<mml:mo stretchy="false">[</mml:mo>
<mml:mtext>L</mml:mtext>
<mml:mo stretchy="false">]</mml:mo>
</mml:mrow>
</mml:mfrac>
<mml:mo>=</mml:mo>
<mml:mfrac><mml:mrow><mml:mo stretchy="false">(</mml:mo>
<mml:msub><mml:mi mathvariant="normal">Λ</mml:mi>
<mml:mrow><mml:mtext>M</mml:mtext>
<mml:mo>−</mml:mo>
</mml:mrow>
</mml:msub>
<mml:msub><mml:mi mathvariant="normal">Λ</mml:mi>
<mml:mtext>obs</mml:mtext>
</mml:msub>
<mml:mo stretchy="false">)</mml:mo>
</mml:mrow>
<mml:mrow><mml:mo stretchy="false">(</mml:mo>
<mml:msub><mml:mi mathvariant="normal">Λ</mml:mi>
<mml:mrow><mml:mtext>obs</mml:mtext>
<mml:mo>−</mml:mo>
</mml:mrow>
</mml:msub>
<mml:msub><mml:mi mathvariant="normal">Λ</mml:mi>
<mml:mtext>ML</mml:mtext>
</mml:msub>
<mml:mo stretchy="false">)</mml:mo>
<mml:mo stretchy="false">[</mml:mo>
<mml:mtext>L</mml:mtext>
<mml:mo stretchy="false">]</mml:mo>
</mml:mrow>
</mml:mfrac>
</mml:mrow>
</mml:math>
</disp-formula>
Where
<disp-formula id="FD6"><label>(5)</label>
<mml:math id="mm6"><mml:mrow><mml:mo stretchy="false">[</mml:mo>
<mml:mtext>L</mml:mtext>
<mml:mo stretchy="false">]</mml:mo>
<mml:mo>=</mml:mo>
<mml:msub><mml:mtext>C</mml:mtext>
<mml:mtext>L</mml:mtext>
</mml:msub>
<mml:mo>−</mml:mo>
<mml:mfrac><mml:mrow><mml:msub><mml:mtext>C</mml:mtext>
<mml:mtext>M</mml:mtext>
</mml:msub>
<mml:mo stretchy="false">(</mml:mo>
<mml:msub><mml:mi mathvariant="normal">Λ</mml:mi>
<mml:mrow><mml:mtext>M</mml:mtext>
<mml:mo>−</mml:mo>
</mml:mrow>
</mml:msub>
<mml:msub><mml:mi mathvariant="normal">Λ</mml:mi>
<mml:mtext>obs</mml:mtext>
</mml:msub>
<mml:mo stretchy="false">)</mml:mo>
</mml:mrow>
<mml:mrow><mml:mo stretchy="false">(</mml:mo>
<mml:msub><mml:mi mathvariant="normal">Λ</mml:mi>
<mml:mrow><mml:mtext>M</mml:mtext>
<mml:mo>−</mml:mo>
</mml:mrow>
</mml:msub>
<mml:msub><mml:mi mathvariant="normal">Λ</mml:mi>
<mml:mtext>ML</mml:mtext>
</mml:msub>
<mml:mo stretchy="false">)</mml:mo>
</mml:mrow>
</mml:mfrac>
</mml:mrow>
</mml:math>
</disp-formula>
Here, Λ<sub>M</sub>
is the molar conductance of the cation before addition of ligand, Λ<sub>ML</sub>
the molar conductance of the complex, Λ<sub>obs</sub>
the molar conductance of the solution during titration, C<sub>L</sub>
the analytical concentration of the HDB added, and C<sub>M</sub>
the analytical concentration of the cation. The complex formation constants, K<sub>f</sub>
, and the molar conductance of complex, Λ<sub>ML</sub>
, were obtained by computer fitting of <xref rid="FD5" ref-type="disp-formula">equations (4)</xref>
and <xref rid="FD6" ref-type="disp-formula">(5)</xref>
to the molar conductance-mole ratio data, using a nonlinear least-squares program KINFIT [<xref rid="b67-sensors-07-03119" ref-type="bibr">67</xref>
]. The logarithm of the formation constants (log K<sub>f</sub>
) of the resulting 1:1 complexes for mono and divalent cations were about two or less than 2., and for trivalent cations (other lanthanides except Er(III)) were in the range of 2.41-3.44. However, for Er(III) the logarithm of the formation constants was 4.74. Thus, HDB with the most stable complex with Er(III) ion is expected to act as a suitable ion-carrier for the fabrication of a Er(III) ion-selective membrane sensor.</p>
<p>In the next experiment, HDB was used in the construction of membrane microelectrodes for lanthanide ions (Er<sup>3+</sup>
, La<sup>3+</sup>
, Pr<sup>3+</sup>
, Sm<sup>3+</sup>
, Dy<sup>3+</sup>
, Ho<sup>3+</sup>
, Tm<sup>3+</sup>
, Nd<sup>3+</sup>
, Gd<sup>3+</sup>
, Ce<sup>3+</sup>
, Yb<sup>3+</sup>
, Eu<sup>3+</sup>
, and Lu<sup>3+</sup>
) and other representative and transition metal ions like Na<sup>+</sup>
, K<sup>+</sup>
, Ca<sup>2+</sup>
, Mg<sup>2+</sup>
, Pb<sup>2+</sup>
, Cu<sup>2+</sup>
and Co<sup>2+</sup>
ions. The curves of the best potential responses of the resulting membranes <italic>vs.</italic>
changing the concentration of the most important ions, that are shown in <xref rid="f4-sensors-07-03119" ref-type="fig">Fig. 4</xref>
, revealed that among the lanthanide, transition and representative ions tested, erbium(III) with the most Nernstian response, can be suitably determined with the PVC membrane microelectrode based on HDB. This selectivity for Er(III) was also shown by theoretical calculations. These kinds of behavior have been reported [<xref rid="b53-sensors-07-03119" ref-type="bibr">53</xref>
, <xref rid="b57-sensors-07-03119" ref-type="bibr">57</xref>
].</p>
<p>This behavior may be due to the result of the selective tendency of the ion carrier against erbium(III) (with relatively low charge density and hydration energy), in comparison to other metal ions, and the rapid exchange kinetics of the resulting HDB- erbium(III) complex.</p>
<p>Because the sensitivity and selectivity of any given membrane microelectrode is significantly related to the composition of the ion selective membrane, and the nature of the solvent mediators and additives used [<xref rid="b68-sensors-07-03119" ref-type="bibr">68</xref>
-<xref rid="b75-sensors-07-03119" ref-type="bibr">75</xref>
], we decided to study such effects on the behavior of the proposed microelectrode. The effect of the nature and amount of the plasticizer, the amount of PVC, and the additive on the potential response of the proposed erbium(III) microelectrode were investigated, and the results are given in <xref rid="t2-sensors-07-03119" ref-type="table">Table 2</xref>
. These data show that the three different plasticizers used, DBP, NB, and <italic>o</italic>
-NPOE have almost the same results if the optimum composition is used, however, because NB, and <italic>o</italic>
-NPOE are very polar, leading to the extraction of the polar interfering ions such as other rare earth ions which may have negative effects on the selectivity behavior of the microelectrode, DBP was chosen as the solvent mediator. This may seem to lead to the poorer extraction of erbium(III) ion, having relatively high charge density, by the average-polar solvent, but this seemed to be compensated by the selective complexation of HDB with the erbium(III) ions. This way, not only was the slope of the microelectrode response maintained, but its selectivity pattern was also improved.</p>
<p>In addition, from the data in <xref rid="t2-sensors-07-03119" ref-type="table">Table 2</xref>
, the optimum amount of the ion carrier is 5% (No. 8) which is select from a previously reported Er liquid membrane sensor [<xref rid="b47-sensors-07-03119" ref-type="bibr">47</xref>
], while the slope of the resulting emf <italic>vs.</italic>
log erbium(III) activity plot is about two-thirds of the expected Nernstian value (membranes No. 2). However, addition of 3% KTpClPB (membranes No. 5) will increase the sensitivity of the electrode response to a great extent. The fact that the presence of lipophilic anions in the composition of cationic-selective membrane microelectrodes, not only diminishes the ohmic resistance and enhances the potential behavior and selectivity, but also in poor extraction capacities, increases the sensitivity of the membrane electrodes, has long been known [<xref rid="b76-sensors-07-03119" ref-type="bibr">76</xref>
-<xref rid="b79-sensors-07-03119" ref-type="bibr">79</xref>
].</p>
<p><xref rid="t2-sensors-07-03119" ref-type="table">Table 2</xref>
shows the effect of the amount of PVC on the sensitivity of the fabrication of erbium(III) microelectrode. As it can be seen from <xref rid="t2-sensors-07-03119" ref-type="table">Table 2</xref>
, using 20% PVC as optimum amount in construction of the sensor presents a higher sensitivity (no. 1).</p>
<p>The potential response of the microelectrode was evaluated using the optimum composition of all ingredients, while lacking the ionophore, and it was found that under these conditions the response of the microelectrodes falls to very low values of 3.5 mV/decade, indicating that the sensitivity of the microelectrode is major due to the complexing behavior of HDB.</p>
<p>The mechanism of the response of the asymmetric membrane sensor is not well known up to now exactly. It is explained by an existence of a redox system Er<sup>3+</sup>
/Er in the surface of the gold as an internal reference electrode, in the presence of diffused ErCl<sub>3</sub>
from the outer layer of membrane to the inner layer (surface of the gold electrode).</p>
<p>The optimum equilibration time for the membrane electrode, after which it generates stable potentials when placed in contact with erbium(III) solutions, is found to be 48 h.</p>
<p>The critical response characteristics of the erbium(III) microelectrode were assessed according to IUPAC recommendations [<xref rid="b80-sensors-07-03119" ref-type="bibr">80</xref>
]. The potential response of the membrane at varying concentration of erbium(III) ions (<xref rid="f4-sensors-07-03119" ref-type="fig">Fig. 4</xref>
) indicates a rectilinear range from 1.0×10<sup>-3</sup>
-3.0×10<sup>-10</sup>
M. The slopes of the calibration curves were 17.5±0.3 mV/decade of erbium(III) activity. The limit of detection, as determined from the intersection of the two extrapolated segments of the calibration graph, was 2.0 ×10<sup>-10</sup>
M (∽30 ppt). The standard deviation of 8 replicate potential measurements for the proposed electrode is ±0.3. The potential drift within 5 minutes after each measurement is ±0.1 mV.</p>
<p>The influence of pH on the response of the membrane microelectrode to a 1.0 × 10<sup>-5</sup>
M erbium(III) solution, was studied over a pH range of 2.5 to 11.0, and the results show that in the pH range of 3.0 to 9.0, the potential does not change with changing pH that indicates the applicability of this microelectrode in the mentioned pH range. Beyond these limits, however, relatively drastic drifts in the potential <italic>vs.</italic>
pH behavior were observed. The observed drift at the higher pH values of this range could be due to the formation of insoluble Er(OH)<sub>3</sub>
ion in the solution. In acidic solutions having pH values of less than the minimum of this range, the ionophore used could be protonated to some extent, which results in improper functioning of the membrane electrode to the erbium(III) ion concentration.</p>
<p>The lifetime of the microelectrode, which is a measure of microelectrodes durability, was also studied in a 4 months period. During this period the microelectrode was used for 2 hours a day, 5 days a week. After each usage it was washed completely. After ten weeks only a relatively slight changes in the microelectrode's slope and detection limit from 17.5 ± 0.5 and 2.0×10<sup>-10</sup>
M to 15.5 ± 0.4 mV/decade and 1.0 × 10<sup>-9</sup>
M of erbium(III) activity.</p>
<p>One of the most important factors for any membrane microelectrode is its relative response for the primary ion over other ions present in solution, usually expressed in terms of potentiometric selectivity coefficients. In this work, matched potential method (MPM) and separated solution methods (SSM) [<xref rid="b81-sensors-07-03119" ref-type="bibr">81</xref>
,<xref rid="b82-sensors-07-03119" ref-type="bibr">82</xref>
] were used for determination of selectivity coefficients of the proposed erbium(III) microelectrode.</p>
<p>According to MPM method, the specified activity (concentration) of the primary ion (A, 1.0 × 10<sup>-8</sup>
M of Er(III) ion) is added to a reference solution (3.0 × 10<sup>-10</sup>
M) and, afterwards, the potential is measured. In a separation experiment, the interfering ions (B, 1.0 × 10<sup>-7</sup>
-1.0 × 10<sup>-5</sup>
M) are successively added to an identical reference solution until the measured potential matches that obtained before by adding the primary ions. The matched potential method selectivity coefficient, K<sub>MPM</sub>
, is then given by the resulting primary ion to the interfering ion activity (concentration) ratio. The selectivity coefficient,
<inline-formula><mml:math id="mm7"><mml:mrow><mml:msubsup><mml:mtext>K</mml:mtext>
<mml:mrow><mml:mtext>A</mml:mtext>
<mml:mo>,</mml:mo>
<mml:mtext>B</mml:mtext>
</mml:mrow>
<mml:mtext>Pot</mml:mtext>
</mml:msubsup>
</mml:mrow>
</mml:math>
</inline-formula>
, is determined as;
<disp-formula id="FD7"><label>(6)</label>
<mml:math id="mm8"><mml:mrow><mml:msubsup><mml:mtext>K</mml:mtext>
<mml:mrow><mml:mtext>A</mml:mtext>
<mml:mo>,</mml:mo>
<mml:mtext>B</mml:mtext>
</mml:mrow>
<mml:mtext>Pot</mml:mtext>
</mml:msubsup>
<mml:mo>=</mml:mo>
<mml:msub><mml:mtext>a</mml:mtext>
<mml:mtext>A</mml:mtext>
</mml:msub>
<mml:mo>/</mml:mo>
<mml:msub><mml:mtext>a</mml:mtext>
<mml:mtext>B</mml:mtext>
</mml:msub>
</mml:mrow>
</mml:math>
</disp-formula>
Where
<inline-formula><mml:math id="mm9"><mml:mrow><mml:mi mathvariant="normal">Δ</mml:mi>
<mml:mtext>A</mml:mtext>
<mml:mo>=</mml:mo>
<mml:msub><mml:mrow><mml:msup><mml:mtext>a</mml:mtext>
<mml:mo>′</mml:mo>
</mml:msup>
</mml:mrow>
<mml:mtext>A</mml:mtext>
</mml:msub>
<mml:mo>−</mml:mo>
<mml:msub><mml:mtext>a</mml:mtext>
<mml:mtext>A</mml:mtext>
</mml:msub>
</mml:mrow>
</mml:math>
</inline-formula>
, a<sub>A</sub>
is the initial primary ion activity and
<inline-formula><mml:math id="mm10"><mml:mrow><mml:msub><mml:mrow><mml:msup><mml:mtext>a</mml:mtext>
<mml:mo>′</mml:mo>
</mml:msup>
</mml:mrow>
<mml:mtext>A</mml:mtext>
</mml:msub>
</mml:mrow>
</mml:math>
</inline-formula>
is the activity of A in the presence of the interfering ion, a<sub>B</sub>
.</p>
<p>In separation solution method (SSM) the concentrations of a cell comprising an ion-selective electrode and a reference electrode (ISE cell) are adjusted with each of two separate solutions, one containing the ion A of the activity a<sub>A</sub>
(but no B), the other containing the ion B (but no A) of the activity as high as required to achieve the same measured cell voltage. From any pair of activities a<sub>A</sub>
and a<sub>B</sub>
giving the same cell voltage, the value of
<inline-formula><mml:math id="mm11"><mml:mrow><mml:msubsup><mml:mtext mathvariant="italic">K</mml:mtext>
<mml:mtext mathvariant="italic">AB</mml:mtext>
<mml:mtext mathvariant="italic">Pot</mml:mtext>
</mml:msubsup>
</mml:mrow>
</mml:math>
</inline-formula>
may be calculated from the equation:
<disp-formula id="FD8"><label>(7)</label>
<mml:math id="mm13"><mml:mrow><mml:msup><mml:mrow><mml:msubsup><mml:mi>K</mml:mi>
<mml:mrow><mml:mi>A</mml:mi>
<mml:mi>B</mml:mi>
</mml:mrow>
<mml:mtext mathvariant="italic">pot</mml:mtext>
</mml:msubsup>
<mml:mo>=</mml:mo>
<mml:msub><mml:mtext>a</mml:mtext>
<mml:mtext>A</mml:mtext>
</mml:msub>
<mml:mo>/</mml:mo>
<mml:mtext>a</mml:mtext>
</mml:mrow>
<mml:mrow><mml:msub><mml:mtext>Z</mml:mtext>
<mml:mtext>A</mml:mtext>
</mml:msub>
<mml:mo>/</mml:mo>
<mml:msub><mml:mtext>Z</mml:mtext>
<mml:mtext>B</mml:mtext>
</mml:msub>
</mml:mrow>
</mml:msup>
</mml:mrow>
</mml:math>
</disp-formula>
</p>
<p>The resulting selectivity coefficients values showed that the proposed erbium(III) microelectrode is highly selective with respect to the most of cations. In the case of lanthanide ions (holmium, cerium, neodymium, presidium, samarium, gadolinium, thulium, europium, dysprosium, lanthanum, ytterbium, lutetium) the selectivity coefficients are in the order of 1.0 × 10<sup>-3</sup>
or smaller in both methods, which seems to indicate that the erbium(III) ions can be determined in the presence of other lanthanide ions. The selectivity coefficients for other cations (Na<sup>+</sup>
, K<sup>+</sup>
, Mg<sup>2+</sup>
, Ca<sup>2+</sup>
, Cu<sup>2+</sup>
, and Pb<sup>2+</sup>
) are smaller than 2.0 × 10<sup>-5</sup>
, and they can not disturb the functioning of the erbium(III) membrane electrode. It means that the sensor can be used for the determination of the erbium ion when the concentration ratio of the interfering ion/Er is less than 10<sup>5</sup>
. In the other hand, at the ratio higher than this amount, interfering ions could interfere in the erbium concentration measurement.</p>
<p>The ionic radii, charge density and hydration energy of the lanthanide ions are not similar. Due to the radii of lanthanum ions (from Ce<sup>3+</sup>
to Lu<sup>3+</sup>
with the range of 1.02-0.80 A°, respectively) these elements have different properties such as charge densities and size and hydration energy (from Ce<sup>3+</sup>
to Lu<sup>3+</sup>
with the range of 3370-3760 kJ/mol) [<xref rid="b83-sensors-07-03119" ref-type="bibr">83</xref>
]. Thus, by using a suitable ionophore having the semi-cavity and relatively high flexibility, it is possible to construct a highly selective lanthanide ion sensor. Such remarkable selectivity of the Er(III) sensor over other metal ions reflects the high affinity of the HDB used as an ion carrier toward the Er(III) ions.</p>
<p>The dynamic response time of the microelectrode, being another very important parameter in the evaluation of a microelectrode, was studied by varying the concentration of a solution from 1.0 × 10<sup>-10</sup>
M to 1.0 × 10<sup>-3</sup>
M and recording the times needed for the microelectrode to reach ±1 mV of its equilibrium potential. The response time of the microelectrode was found to be about 10 seconds in the whole concentrations. This is highly likely due to the fast complexation-decomplexation exchange kinetics of Er(III) ion with the HDB at the test solution-membrane interface. To evaluate the electrode reversibility, a similar procedure in the opposite direction was adopted. The measurements were performed in the sequence of high-to-low sample concentrations. The results showed that the potentiometric response of the sensor is reversible, although the time needed to reach the equilibrium values was longer than that for the low-to-high sample concentration procedure.</p>
<p>The proposed erbium(III) membrane electrode was found to work well under laboratory conditions. It was applied as an indicator electrode for the titration of 20.0 ml of 1.0 ×10<sup>-6</sup>
M of erbium(III) solution with 1.0 × 10<sup>-3</sup>
M of standard EDTA, and vise versa. The resulting titration curve show that the amount of erbium(III) ions, can be determined with good accuracy (1.0 ± 0.1 ×10<sup>-6</sup>
M).</p>
</sec>
<sec sec-type="conclusions"><label>4.</label>
<title>Conclusion</title>
<p>The results obtained from the above mentioned study revealed that a potentiometric PVC-based membrane microelectrode based on HDB functions as an excellent erbium(III) selective membrane microelectrode and can be used for the determination of this ion in the presence of considerable concentrations of common interfering ions. Applicable pH range, lower detection limit, and potentiometric selectivity coefficients of the proposed microelectrode make it a superior device both compared to other methods used for the determinations of this ion.</p>
</sec>
</body>
<back><ack><p>The authors express their appreciation to the University of Tehran Research Council for financial support of this work.</p>
</ack>
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<element-citation publication-type="book"><person-group person-group-type="author"><name><surname>Greenwood</surname>
<given-names>N. N.</given-names>
</name>
<name><surname>Earnshaw</surname>
<given-names>A.</given-names>
</name>
</person-group>
<source>in Chemistry of the elements</source>
<publisher-name>Pergamon Press</publisher-name>
<year>1984</year>
<fpage>p. 1430</fpage>
</element-citation>
</ref>
</ref-list>
</back>
<floats-group><fig id="f1-sensors-07-03119" position="float"><label>Figure 1.</label>
<caption><p>The structure of ionophore HDB.</p>
</caption>
<graphic xlink:href="sensors-07-03119f1"></graphic>
</fig>
<fig id="f2-sensors-07-03119" position="float"><label>Figure 2.</label>
<caption><p>The surface of the microelectrode before (A) and after conditioning (B).</p>
</caption>
<graphic xlink:href="sensors-07-03119f2"></graphic>
</fig>
<fig id="f3-sensors-07-03119" position="float"><label>Figure 3.</label>
<caption><p>Optimal conformation of ionophore after complexation with Er<sup>3+</sup>
.</p>
</caption>
<graphic xlink:href="sensors-07-03119f3"></graphic>
</fig>
<fig id="f4-sensors-07-03119" position="float"><label>Figure 4.</label>
<caption><p>Potential response of lanthanide metal ion-selective microelectrode based on HDB with the composition of membrane no. 1.</p>
</caption>
<graphic xlink:href="sensors-07-03119f4"></graphic>
</fig>
<table-wrap id="t1-sensors-07-03119" position="float"><label>Table 1.</label>
<caption><p>Interaction energy between metal ions – ligand (HDB)</p>
</caption>
<table frame="hsides" rules="groups"><thead><tr><th align="center" valign="top" rowspan="1" colspan="1"><bold>Complexes</bold>
</th>
<th align="center" valign="top" rowspan="1" colspan="1"><bold>Stabilization Energy</bold>
<break></break>
<bold>(kJ/mole)</bold>
</th>
</tr>
</thead>
<tbody><tr><td align="center" valign="top" rowspan="1" colspan="1">Er<sup>3+</sup>
-ligand</td>
<td align="center" valign="top" rowspan="1" colspan="1">-29514.287</td>
</tr>
<tr><td align="center" valign="top" rowspan="1" colspan="1">Na<sup>+</sup>
-ligand</td>
<td align="center" valign="top" rowspan="1" colspan="1">-616.809</td>
</tr>
<tr><td align="center" valign="top" rowspan="1" colspan="1">Pb<sup>2+</sup>
-ligand</td>
<td align="center" valign="top" rowspan="1" colspan="1">-5900.406</td>
</tr>
<tr><td align="center" valign="top" rowspan="1" colspan="1">Cu<sup>2+</sup>
-ligand</td>
<td align="center" valign="top" rowspan="1" colspan="1">-24731.766</td>
</tr>
<tr><td align="center" valign="top" rowspan="1" colspan="1">Ce<sup>3+</sup>
-ligand</td>
<td align="center" valign="top" rowspan="1" colspan="1">-2799.706</td>
</tr>
<tr><td align="center" valign="top" rowspan="1" colspan="1">Nd<sup>3+</sup>
-ligand</td>
<td align="center" valign="top" rowspan="1" colspan="1">-7528.752</td>
</tr>
<tr><td align="center" valign="top" rowspan="1" colspan="1">Pr<sup>3+</sup>
-ligand</td>
<td align="center" valign="top" rowspan="1" colspan="1">-5083.861</td>
</tr>
<tr><td align="center" valign="top" rowspan="1" colspan="1">Sm<sup>3+</sup>
-ligand</td>
<td align="center" valign="top" rowspan="1" colspan="1">-12854.457</td>
</tr>
<tr><td align="center" valign="top" rowspan="1" colspan="1">Gd<sup>3+</sup>
-ligand</td>
<td align="center" valign="top" rowspan="1" colspan="1">-18643.196</td>
</tr>
</tbody>
</table>
</table-wrap>
<table-wrap id="t2-sensors-07-03119" position="float"><label>Table 2.</label>
<caption><p>Optimization of membrane ingredients</p>
</caption>
<table frame="hsides" rules="groups"><thead><tr><th align="center" valign="top" rowspan="3" colspan="1">Membrane No.</th>
<th colspan="4" align="center" valign="bottom" rowspan="1">Composition (%)</th>
<th align="center" valign="middle" rowspan="3" colspan="1">Slope<break></break>
(mV/decade)</th>
<th align="center" valign="bottom" rowspan="3" colspan="1">Linear<break></break>
range (M)</th>
<th align="center" valign="bottom" rowspan="3" colspan="1">Detection limit<break></break>
(M)</th>
</tr>
<tr><th valign="bottom" colspan="4" rowspan="1"><hr></hr>
</th>
</tr>
<tr><th align="center" valign="top" rowspan="1" colspan="1">PVC</th>
<th align="center" valign="top" rowspan="1" colspan="1">Plasticizer</th>
<th align="center" valign="top" rowspan="1" colspan="1">Ionophore</th>
<th align="center" valign="top" rowspan="1" colspan="1">KTpClPB</th>
</tr>
</thead>
<tbody><tr><td align="center" valign="top" rowspan="1" colspan="1">1</td>
<td align="center" valign="top" rowspan="1" colspan="1">20</td>
<td align="center" valign="top" rowspan="1" colspan="1">DBP, 72</td>
<td align="center" valign="top" rowspan="1" colspan="1">5</td>
<td align="center" valign="top" rowspan="1" colspan="1">3</td>
<td align="center" valign="top" rowspan="1" colspan="1">17.5 ± 0.4<xref rid="tfn1-sensors-07-03119" ref-type="table-fn">*</xref>
</td>
<td align="center" valign="top" rowspan="1" colspan="1">1.0×10<sup>-3</sup>
-3.0×10<sup>-10</sup>
</td>
<td align="center" valign="top" rowspan="1" colspan="1">2.0 × 10<sup>-10</sup>
</td>
</tr>
<tr><td align="center" valign="top" rowspan="1" colspan="1">2</td>
<td align="center" valign="top" rowspan="1" colspan="1">15</td>
<td align="center" valign="top" rowspan="1" colspan="1">DBP, 77</td>
<td align="center" valign="top" rowspan="1" colspan="1">5</td>
<td align="center" valign="top" rowspan="1" colspan="1">3</td>
<td align="center" valign="top" rowspan="1" colspan="1">12.5 ± 0.3</td>
<td align="center" valign="top" rowspan="1" colspan="1">7.3×10<sup>-4</sup>
-2.0×10<sup>-9</sup>
</td>
<td align="center" valign="top" rowspan="1" colspan="1">1.0 × 10<sup>-9</sup>
</td>
</tr>
<tr><td align="center" valign="top" rowspan="1" colspan="1">3</td>
<td align="center" valign="top" rowspan="1" colspan="1">20</td>
<td align="center" valign="top" rowspan="1" colspan="1">DBP, 72</td>
<td align="center" valign="top" rowspan="1" colspan="1">5</td>
<td align="center" valign="top" rowspan="1" colspan="1">2</td>
<td align="center" valign="top" rowspan="1" colspan="1">16.0 ± 0.4</td>
<td align="center" valign="top" rowspan="1" colspan="1">1.0×10<sup>-3</sup>
-5.0×10<sup>-10</sup>
</td>
<td align="center" valign="top" rowspan="1" colspan="1">3.5 × 10<sup>-10</sup>
</td>
</tr>
<tr><td align="center" valign="top" rowspan="1" colspan="1">4</td>
<td align="center" valign="top" rowspan="1" colspan="1">25</td>
<td align="center" valign="top" rowspan="1" colspan="1">DBP, 67</td>
<td align="center" valign="top" rowspan="1" colspan="1">5</td>
<td align="center" valign="top" rowspan="1" colspan="1">3</td>
<td align="center" valign="top" rowspan="1" colspan="1">15.1 ± 0.6</td>
<td align="center" valign="top" rowspan="1" colspan="1">9.0×10<sup>-4</sup>
-8.0×10<sup>-10</sup>
</td>
<td align="center" valign="top" rowspan="1" colspan="1">4.5 × 10<sup>-10</sup>
</td>
</tr>
<tr><td align="center" valign="top" rowspan="1" colspan="1">5</td>
<td align="center" valign="top" rowspan="1" colspan="1">30</td>
<td align="center" valign="top" rowspan="1" colspan="1">DBP, 62</td>
<td align="center" valign="top" rowspan="1" colspan="1">5</td>
<td align="center" valign="top" rowspan="1" colspan="1">3</td>
<td align="center" valign="top" rowspan="1" colspan="1">15.3 ± 0.2</td>
<td align="center" valign="top" rowspan="1" colspan="1">8.0×10<sup>-4</sup>
-7.0×10<sup>-10</sup>
</td>
<td align="center" valign="top" rowspan="1" colspan="1">5.0 × 10<sup>-10</sup>
</td>
</tr>
<tr><td align="center" valign="top" rowspan="1" colspan="1">6</td>
<td align="center" valign="top" rowspan="1" colspan="1">20</td>
<td align="center" valign="top" rowspan="1" colspan="1">NB, 72</td>
<td align="center" valign="top" rowspan="1" colspan="1">5</td>
<td align="center" valign="top" rowspan="1" colspan="1">3</td>
<td align="center" valign="top" rowspan="1" colspan="1">15.3 ± 0.3</td>
<td align="center" valign="top" rowspan="1" colspan="1">8.2×10<sup>-4</sup>
-7.0×10<sup>-10</sup>
</td>
<td align="center" valign="top" rowspan="1" colspan="1">5.0 × 10<sup>-10</sup>
</td>
</tr>
<tr><td align="center" valign="top" rowspan="1" colspan="1">7</td>
<td align="center" valign="top" rowspan="1" colspan="1">20</td>
<td align="center" valign="top" rowspan="1" colspan="1">NPOE, 72</td>
<td align="center" valign="top" rowspan="1" colspan="1">5</td>
<td align="center" valign="top" rowspan="1" colspan="1">3</td>
<td align="center" valign="top" rowspan="1" colspan="1">14.2 ± 0.6</td>
<td align="center" valign="top" rowspan="1" colspan="1">5.4×10<sup>-4</sup>
-3.0×10<sup>-10</sup>
</td>
<td align="center" valign="top" rowspan="1" colspan="1">1.0 × 10<sup>-5</sup>
</td>
</tr>
<tr><td align="center" valign="top" rowspan="1" colspan="1">8</td>
<td align="center" valign="top" rowspan="1" colspan="1">20</td>
<td align="center" valign="top" rowspan="1" colspan="1">DBP, 75</td>
<td align="center" valign="top" rowspan="1" colspan="1">5</td>
<td align="center" valign="top" rowspan="1" colspan="1">-</td>
<td align="center" valign="top" rowspan="1" colspan="1">11.6 ± 0.4</td>
<td align="center" valign="top" rowspan="1" colspan="1">6.0×10<sup>-4</sup>
-1.0×10<sup>-8</sup>
</td>
<td align="center" valign="top" rowspan="1" colspan="1">1.0 × 10<sup>-8</sup>
</td>
</tr>
<tr><td align="center" valign="top" rowspan="1" colspan="1">9</td>
<td align="center" valign="top" rowspan="1" colspan="1">20</td>
<td align="center" valign="top" rowspan="1" colspan="1">DBP, 77</td>
<td align="center" valign="top" rowspan="1" colspan="1">-</td>
<td align="center" valign="top" rowspan="1" colspan="1">3</td>
<td align="center" valign="top" rowspan="1" colspan="1">3.5 ± 0.2</td>
<td align="center" valign="top" rowspan="1" colspan="1">2.5×10<sup>-4</sup>
-4.0×10<sup>-6</sup>
</td>
<td align="center" valign="top" rowspan="1" colspan="1">2.2 × 10<sup>-6</sup>
</td>
</tr>
</tbody>
</table>
<table-wrap-foot><fn id="tfn1-sensors-07-03119"><label>*</label>
<p>The standard deviation of 8 replicate potential measurements</p>
</fn>
</table-wrap-foot>
</table-wrap>
</floats-group>
</pmc>
</record>
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