connect.2012.8.xml

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<front>
<journal-meta> 
<journal-id journal-id-type="pmc">connect</journal-id> 
<journal-id journal-id-type="publisher-id">connect</journal-id> 
<journal-title-group> 
<journal-title>QScience Connect</journal-title> 
</journal-title-group> 
<issn pub-type="ppub">2223-506X</issn> 
<publisher> 
<publisher-name>Bloomsbury Qatar Foundation Journals</publisher-name>
</publisher> 
</journal-meta> 
<article-meta> 
<article-id pub-id-type="publisher-id">8</article-id> 
<article-id pub-id-type="doi">10.5339/connect.2012.8</article-id> 
<article-categories> 
<subj-group subj-group-type="heading"> 
<subject>Research article</subject> 
</subj-group> 
</article-categories> 
<title-group> 
<article-title>Redox-responsive probes for selective chelation of bivalent cations</article-title> 
</title-group> 
<contrib-group>
<contrib contrib-type="author">
<string-name>          
<given-names>Noureddine</given-names>
<x> </x>                
<surname>Raouafi</surname>   </string-name> 
<xref ref-type="corresp" rid="c1"><sup>&#x204E;</sup></xref>               
<x>, </x></contrib>  
<contrib contrib-type="author">
<string-name>               
<given-names>Janet</given-names>
<x> </x>
<surname>Bahri</surname> </string-name>                
<x>, </x></contrib>  
<contrib contrib-type="author">
<string-name>                
<given-names>Rihab</given-names>
<x> </x>                 
<surname>Sahli</surname> </string-name>                
<x>, </x></contrib>
<contrib contrib-type="author">
<string-name>         
<given-names>Khaled</given-names>
<x> </x>                
<surname>Boujlel</surname> </string-name>               
<x> </x></contrib> 
<aff>University of Tunis El-Manar,
Faculty of Sciences of Tunis, Department of Chemistry, Tunis, Tunisia</aff></contrib-group>
<author-notes> 
<corresp id="c1"><sup>&#x204E;</sup>Email: noureddine.raouafi@gmail.com</corresp></author-notes> 
<pub-date pub-type="pub"> 
<day>30</day> 
<month>8</month>
<year>2012</year> </pub-date> 
<issue content-type="issue-sequence">2012</issue>
<elocation-id>8</elocation-id> 
<history> 
<date date-type="received">
<day>20</day>
<month>12</month>
<year>2011</year></date> 
<date date-type="accepted">
<day>24</day>
<month>7</month>
<year>2012</year></date></history>
<permissions> 
<copyright-statement>&#x00A9;&#x00A0;2012 Raouafi et&#x00A0;al., licensee Bloomsbury Qatar
Foundation Journals. This is an open access article distributed under the terms
of the Creative Commons Attribution License CC BY 3.0 which permits
unrestricted use, distribution and reproduction in any medium, provided the
original work is properly cited.</copyright-statement> 
<copyright-year>2012</copyright-year> 
<copyright-holder>Bloomsbury Qatar Foundation Journals</copyright-holder> </permissions>
<self-uri content-type="pdf" xlink:href="connect.2012.8.pdf" xmlns:xlink="http://www.w3.org/1999/xlink"/>  
<abstract>
<title>Abstract</title>
<p>N,N-disubstituted bis(furanyl-2-methyl)aminoanilines are new electrochemically-active probes for cations relying on the phenylenediamine moiety as an electroactive transducer and the difuranylamino group as an ionophore site. The electrochemical investigations, by means of cyclic and Osteryoung square wave voltammetries (CV and SWV, respectively), showed that these compounds are able to bind Mg<sup>2+</sup>, Ca<sup>2+</sup>, Ni<sup>2+</sup> and Zn<sup>2+</sup> cations with strong affinities. The addition of catalytic amounts of trifluoromethanesulfonic acid (TfOH) was found necessary to achieve rapid cation complexation. The electroactive redox features of the probes were drastically modified when the ionophore site was bonded to the cations. The anodic potential shifts of the oxidation peaks were between 905 and 1030 mV depending on the cations. The electrochemical investigations suggested the formation of a 1:2 stoichiometric complex: <!--\xdollar\rm -->            [M(L)
<sub>2</sub>]<sup>2+</sup><!--\xdollar-->, <!--\xdollar\rm -->            M=Mg<!--\xdollar-->, Ca, Ni and Zn. These probes were found to be selective of Ca<sup>2+</sup> and chelates, with strong preference for Ca<sup>2+</sup> even in presence of others cations (Ca<sup>2+</sup>&#x003E;&#x00A0;Mg<sup>2+</sup>, Ca<sup>2+</sup>&#x003E;&#x00A0;Ni<sup>2+</sup> and Ni<sup>2+</sup>&#x003E;&#x00A0;Zn<sup>2+</sup>). UV-visible spectrophotometric studies also showed blue shifts of the absorption bands comprising between 5 and 29 nm ligands when the metal ions were added to the solution, which confirmed the complexes formation.</p></abstract>
<kwd-group kwd-group-type="author">
<title>Keywords</title>
<kwd>Bivalent cations</kwd>
<x>, </x>
<kwd>tetraalkylated p-phenylenediamine (TAPD)</kwd>
<x>, </x>
<kwd>electroactive
probes</kwd>
<x>, </x>
<kwd>UV-visible spectrophotometry</kwd>
<x>, </x>
<kwd>cyclic and square wave voltammetries</kwd>
<x>, </x>
<kwd>selectivity</kwd></kwd-group> 
</article-meta>
</front> 
<body>
<sec>
<title>Introduction</title>
<p>Metal ions play pivotal roles in biological systems. Magnesium, calcium and zinc are
undoubtedly the most abundant cations in the human body. Other cations like nickel
are less abundant but also have great importance&#x00A0;<xref ref-type="bibr" rid="b1 b2 b3 b4">[1&#x2013;4]</xref>.
For instance, magnesium, calcium and zinc triad are the major actors in the central
nervous system&#x2019;s signal transmission reactions. Moreover, these cations act as enzyme
co-factors to trigger the enzymes action as they take part in cell exocytosis and in
immune system response&#x00A0;<xref ref-type="bibr" rid="b1 b2 b3 b4">[1&#x2013;4]</xref>. The amounts of these
ions necessary for functionality varies, e.g.,&#x00A0;it has been proven that small
amounts of nickel are necessary for living organisms and large amounts are
found to be toxic&#x00A0;<xref ref-type="bibr" rid="b5">[5]</xref>. Therefore, it is imperative to be able to
detect and evaluate the concentrations of such cations and eventually to
monitor their fluxes in biological medium&#x00A0;<xref ref-type="bibr" rid="b6 b7">[6,7]</xref>. A plethora
of chromogenic probes exist, which are used to determine concentration
and to monitor the flux of ionic species in solution or biological medium.
Reversible electrochemical probes are less known&#x00A0;<xref ref-type="bibr" rid="b8 b9
b10">[8&#x2013;10]</xref>. Many redox reversible groups such as ruthenium complex and
ferrocene&#x00A0;<xref ref-type="bibr" rid="b10 b11 b12">[10&#x2013;12]</xref>, and organic groups like
phenylenediamine and phenazine&#x00A0;<xref ref-type="bibr" rid="b13 b14">[13,14]</xref>, have been used
recently as transducers to build electrochemically responsive probes. The guest
detection or release can be monitored through the modification of the electrochemical
features of the redox center&#x00A0;<xref ref-type="bibr" rid="b15 b16 b17">[15&#x2013;17]</xref>. Pearson
et&#x00A0;al.&#x00A0;<xref ref-type="bibr" rid="b13 b18">[13,18]</xref> and Sibert et&#x00A0;al.&#x00A0;<xref ref-type="bibr" rid="b19 b20">[19,20]</xref>
popularized electroactive phenylenediamine-based probes containing various-sized
crown or thiocrown ethers to chelate metals ions.</p>
<p>Herein, we report the use of redox-responsive probes based on furanyl receptors for
magnesium, calcium, nickel and zinc sensing in acetonitrile. Electrochemical methods
have been used to investigate the chelation of the metal through the changes induced
in the electrochemical features of the probes. UV-visible spectrophotometry
was also used to detect the formation of complexes in acetonitrile solution.
Moreover, the selectivity of these probes toward cations was examined. All
compounds were prepared according to a previously published procedure
(<xref rid="fig1">Fig.&#x00A0;1</xref>)&#x00A0;<xref ref-type="bibr" rid="b9">[9]</xref>.
<fig  id="fig1">  
<label>Figure&#x00A0;1.</label>  
<caption> 
<title>&#x00A0;General route to tetraalkylated phenylenediamines 
<bold>1a-c</bold> preparation
<italic>via</italic> reductive alkylation reaction. </title> 
</caption>       
<graphic xlink:href="gr1.jpg" xmlns:xlink="http://www.w3.org/1999/xlink"/>      
</fig>
<!--\setFloat [h]{fig1}--></p>
</sec>
<sec>
<title>Experimental section</title>
<sec>
<title>Reagents</title>
<p>4-Dimethylaminoaniline, 4-piperidinylaniline, 4-morpholinoaniline, sodium
cyanoborohydride, acetic acid, perchlorate salts, tetraethylammonium
hexafluorophosphate, trifluoromethanesulfonic acid (TfOH) and methanol were
available from Sigma-Aldrich and were used without further purification.
Furfuryl-2-carboxaldehyde was freshly distilled before use. AnhydroScan<sup><!--\text{-->&#x00AE;<!--}--></sup>&#x00A0;acetonitrile
was purchased from LabScan and used as received.</p>
</sec>
<sec>
<title>Preparation of the probes</title>
<p>The reaction was performed under argon atmosphere at room temperature and
shielded from the light by an aluminium foil. To a solution of 2.0 mmol
of the primary aromatic amine dissolved in methanol (50 mL), 8.0 mmol
of furfural and 10.0 mmol of acetic acid were added and the mixture was
stirred for 12 h. Sodium cyanoborohydride (2.0 mmol) was added and stirring
was continued for a further 4 h. The mixture was neutralized with 50 mL
of saturated sodium bicarbonate solution and extracted twice with 20 mL
of dichloromethane. The organic phase was washed twice with 10 mL of
distilled water, dried over magnesium sulfate and evaporated to dryness.<!--\vadjust {\pagebreak }-->The oily brownish residue was chromatographed on silica gel and eluted
with 30:70 ethyl acetate&#x2013;cyclohexane binary system to afford the desired
products.</p>
</sec>
<sec>
<title>Instrumentation</title>
<p><!--\xdollar\rm --><sup>1</sup><!--\xdollar-->H,
<!--\xdollar\rm --><sup>13</sup><!--\xdollar-->C
NMR spectra were recorded on a Bruker Advance 300 MHz apparatus in deuterated
solvents. Chemical shift values are given in ppm relative to tetramethylsilane as
an internal reference. Infrared spectra were measured on a Perkin-Elmer
spectrophotometer as KBr pellets. The electrochemical experiments were conducted
at ambient temperatures and at potential sweep rates equal to 0.1&#x00A0;Vs<sup>-1</sup>; in 0.1 M
tetraethylammonium hexafluorophosphate acetonitrile solution. A three-electrode
glass cell was controlled by a Radiometer Analytical POL 150 with a MED 150
stand. The cell was fitted with a carbon glassy disk as a working electrode
(3&#x00A0;mm in diameter), with a platinum wire as a counter electrode and an
<mml:math altimg="e1.gif" display="inline">
<mml:mstyle mathvariant="normal">
<mml:mi>Ag</mml:mi></mml:mstyle>
<mml:mo>&#x002F;</mml:mo>
<mml:mstyle mathvariant="normal">
<mml:mi>AgCl</mml:mi></mml:mstyle></mml:math> (3M
KCl) electrode used as reference electrode. Data acquisition and treatment were
respectively performed with TraceMaster 5 Software for cyclic voltammetry (CV) and
square wave voltammetries (SWV) experiments. The working electrode was polished
at the beginning of each experiment.</p>
</sec>
<sec>
<title>Synthesis and characterization</title>
<p>The spectroscopic data for physical characterization of compounds 
<bold>1a-c</bold> are given in
the Supplementary Material File (SMF file).</p>
</sec>
</sec>
<sec>
<title>Results and Discussion</title>
<sec>
<title>UV-visible spectrophotometric study</title>
<p>The UV-visible spectrum of the free probe 
<bold>1a</bold> showed two major bands located at 267 and 326 nm
corresponding to 
<mml:math altimg="e2.gif" display="inline">
<mml:mi>&#x03C0;</mml:mi> 
<mml:mo>&#x2192;</mml:mo> 
<mml:msup>
<mml:mrow >
<mml:mi>&#x03C0;</mml:mi></mml:mrow>
<mml:mrow >
<mml:mo>&#x2217;</mml:mo></mml:mrow></mml:msup ></mml:math>
and 
<mml:math altimg="e3.gif" display="inline">
<mml:mi>n</mml:mi> 
<mml:mo>&#x2192;</mml:mo> 
<mml:msup>
<mml:mrow >
<mml:mi>&#x03C0;</mml:mi></mml:mrow>
<mml:mrow >
<mml:mo>&#x2217;</mml:mo></mml:mrow></mml:msup ></mml:math>
of the aromatic moieties (<xref rid="fig2">Fig.&#x00A0;2</xref>).
<fig  id="fig2">  
<label>Figure&#x00A0;2.</label>  
<caption> 
<title>&#x00A0;UV-visible absorption spectra before (&#x2014;) and after the addition of: a- 0.5
equivalent of Mg<sup>2+</sup><!--\unskip \break -->(<mml:math altimg="e4.gif" display="inline">
<mml:mi>&#x2022;</mml:mi>
<mml:mi>&#x2022;</mml:mi>
<mml:mi>&#x2022;</mml:mi></mml:math>),
0.5 equivalent of Ca<sup>2+</sup>(- - -), b- 0.5 equivalent of Zn<sup>2+</sup>(- - -), 0.5 equivalent of
Ni<sup>2+</sup>(<mml:math altimg="e5.gif" display="inline">
<mml:mi>&#x2022;</mml:mi>
<mml:mi>&#x2022;</mml:mi>
<mml:mi>&#x2022;</mml:mi></mml:math>)
to an acetonitrile solution of 
<bold>1a</bold> compound. </title> 
</caption>       
<graphic xlink:href="gr2.jpg" xmlns:xlink="http://www.w3.org/1999/xlink"/>      
</fig>
<!--\setFloat [h]{fig2}--></p>
<p>TfOH speeds up the chelation of the metal ions (<italic>vide infra</italic>), and the probes 
<bold>1a-c</bold>
contain two aromatic nitrogen atoms. The addition of catalytic amounts did not
influence the positions of the absorption bands or their intensities. As shown
in
<xref rid="fig2">Fig.&#x00A0;2</xref>(a), the addition of a 0.5 equivalent of Mg<sup>2+</sup> to a solution of free probe
<bold>1a</bold> induced simultaneously a 5 nm blue and an hyperchromic effect of the
band at 267 nm. The other weak band underwent a hypsochromic shift of
13 nm. No change was observed when 0.5 equivalent of Ca<sup>2+</sup> was added. In
addition, a 27 nm blue shift of the band at 326 nm and an increasing of the
absorption intensity of the band at 267 nm with a hypsochromic shift of about
6 nm were observed when Zn<sup>2+</sup> was added. Ni<sup>2+</sup> provoked similar effects, but
the band at 267 nm underwent a blue shift by 5 nm and saw its intensity
reduced.<!--\pagebreak --></p>
<p>The spectrophotometric behaviors of 
<bold>1b</bold> and 
<bold>1c</bold> in the presence of metal ions were
similar to that of 
<bold>1a</bold> (Figs. 1S-a and 1S-b see the SMF file). The absorption bands
shifts for compounds 
<bold>1a-c</bold> were summarized in
<xref rid="tbl1">Table&#x00A0;1</xref>.
<table-wrap  id="tbl1">  
<label>Table&#x00A0;1.</label>  
<caption> 
<title>&#x00A0;Shifts of the UV-visible spectrophotometric absorption
bands upon complexation of probes 
<bold>1a-c</bold> by metallic ions in acetonitrile. </title> 
</caption>
<table frame="hsides" rules="groups"> 
<colgroup>   
<col align="left"/>   
<col align="left"/>   
<col align="left"/>   
<col align="left"/>   
<col align="left"/>   
<col align="left"/>   
<col align="left"/></colgroup>
<thead> 
<tr valign="top"> 
<td align="left"> </td> 
<td colspan="2"  align="center"><!--\hspace *{-3pc}--><bold>1a</bold> (<mml:math altimg="e6.gif" display="inline">
<mml:msub>
<mml:mrow >
<mml:mi>&#x03BB;</mml:mi></mml:mrow>
<mml:mrow >
<mml:mstyle mathvariant="normal">
<mml:mi>max</mml:mi></mml:mstyle></mml:mrow></mml:msub >
<mml:mo>&#x002F;</mml:mo>
<mml:mstyle mathvariant="normal">
<mml:mi>nm</mml:mi></mml:mstyle></mml:math>) </td> 
<td colspan="2"  align="center"><!--\hspace *{-3pc}--><bold>1b</bold> (<mml:math altimg="e7.gif" display="inline">
<mml:msub>
<mml:mrow >
<mml:mi>&#x03BB;</mml:mi></mml:mrow>
<mml:mrow >
<mml:mstyle mathvariant="normal">
<mml:mi>max</mml:mi></mml:mstyle></mml:mrow></mml:msub >
<mml:mo>&#x002F;</mml:mo>
<mml:mstyle mathvariant="normal">
<mml:mi>nm</mml:mi></mml:mstyle></mml:math>) </td> 
<td colspan="2"  align="center">
<bold>1c</bold> (<mml:math altimg="e8.gif" display="inline">
<mml:msub>
<mml:mrow >
<mml:mi>&#x03BB;</mml:mi></mml:mrow>
<mml:mrow >
<mml:mstyle mathvariant="normal">
<mml:mi>max</mml:mi></mml:mstyle></mml:mrow></mml:msub >
<mml:mo>&#x002F;</mml:mo>
<mml:mstyle mathvariant="normal">
<mml:mi>nm</mml:mi></mml:mstyle></mml:math>)
 </td></tr></thead>
<tbody> 
<tr valign="top"> 
<td align="left">Without     </td> 
<td align="left">267 </td> 
<td align="left">326                                                                                                      </td> 
<td align="left">265 </td> 
<td align="left">319                                                                                                      </td> 
<td align="left">265 </td> 
<td align="left">319                                                                                                      </td>
</tr> 
<tr valign="top"> 
<td align="left">Mg<sup>2+</sup>            </td> 
<td align="left">262 </td> 
<td align="left">313                                                                                                      </td> 
<td align="left">262 </td> 
<td align="left">310                                                                                                      </td> 
<td align="left">265 </td> 
<td align="left">319                                                                                                      </td>
</tr> 
<tr valign="top"> 
<td align="left">Ca<sup>2+</sup>            </td> 
<td align="left">266 </td> 
<td align="left">326                                                                                                      </td> 
<td align="left">263 </td> 
<td align="left">310                                                                                                      </td> 
<td align="left">265 </td> 
<td align="left">318                                                                                                      </td>
</tr> 
<tr valign="top"> 
<td align="left">Ni<sup>2+</sup>             </td> 
<td align="left">262 </td> 
<td align="left">326                                                                                                      </td> 
<td align="left">261 </td> 
<td align="left">299                                                                                                      </td> 
<td align="left">265 </td> 
<td align="left">319                                                                                                      </td>
</tr> 
<tr valign="top"> 
<td align="left">Zn<sup>2+</sup>            </td> 
<td align="left">261 </td> 
<td align="left">299                                                                                                      </td> 
<td align="left">261 </td> 
<td align="left">299                                                                                                      </td> 
<td align="left">262 </td> 
<td align="left">295                                                                                                      </td>
</tr> 
</tbody> 
</table>
</table-wrap>
<!--\setTable [t]{tbl1}--></p>
</sec>
<sec>
<title>Electrochemistry of free probes</title>
<p>The cyclic voltammetry of compounds 
<bold>1a-c</bold> shows two mono-electronic
reversible oxidation peaks similar to those observable in tetramethylated
<italic>para</italic>-phenylenediamine (TMPD)&#x00A0;<xref ref-type="bibr" rid="b21">[21]</xref>, corresponding respectively to
the formation of a radical-cation and a dication. The electrochemical features of
<bold>1a-c</bold> did not change much comparatively to TMPD, although the probes
became less easy to oxidize likely due to an increase of inductive effects
(<xref rid="fig3">Fig.&#x00A0;3</xref>). SWV curves are given in (Fig. 2S).
<fig  id="fig3">  
<label>Figure&#x00A0;3.</label>  
<caption> 
<title>&#x00A0;Cyclic voltammograms free probes 
<bold>1a</bold> (&#x2014;), 
<bold>1b</bold> (- - -) and 
<bold>1c</bold>
(&#x2013;<!--{ --><mml:math altimg="e9.gif" display="inline">
<mml:mi>&#x2022;</mml:mi></mml:math><!--} -->&#x2013;)
(2.0 mM) in 0.1 M TEAPF
<sub>6</sub>&#x00A0;acetonitrile solution. Glassy carbon as
working electrode (diameter: 3&#x00A0;mm), Pt wire as counter-electrode and
Ag<!--\unskip --><mml:math altimg="e10.gif" display="inline">
<mml:mo>&#x002F;</mml:mo></mml:math>AgCl
(3M) as a reference electrode. </title> 
</caption>       
<graphic xlink:href="gr3.jpg" xmlns:xlink="http://www.w3.org/1999/xlink"/>      
</fig>
<!--\setFloat [h]{fig3}--></p>
<p>Moreover, the addition of catalytic quantities of TfOH did not alter the
electrochemical features of the redox system, but a 20 to 30 percent decrease in the
peaks&#x2019; currents was observed due to the protonation of the nitrogen compounds
(<xref rid="fig4">Fig.&#x00A0;4</xref>). Preliminary investigations showed that after several CV scans, a purple
coloration developed slowly, probably due to electro-generated protons occurring at
the electrode. These protons accelerated tremendously the cations&#x2019; chelation. The
same coloration was observed and was related solely to protons since the
same results were obtained by adding TfOH in presence and absence of the
cations. Furthermore, the acid did not influence the oxidation potentials of the
probes.</p>
<p>The CV for a 2.0 mM solution of 
<bold>1a</bold> in acetonitrile showed two anodic peaks during
the sweep potential scan associated with two cathodic peaks in the reverse scan
indicating that the two oxidation processes remained, as expected, reversible. Similar
conclusions were also observed for compounds 
<bold>1b</bold> and 
<bold>1c</bold>&#x00A0;<xref ref-type="bibr" rid="b22
b23">[22,23]</xref>, and these results are in agreement with our previous reported
works&#x00A0;<xref ref-type="bibr" rid="b8 b9 b29">[8,9,29]</xref>.
<fig  id="fig4">  
<label>Figure&#x00A0;4.</label><!--aboveskip=3pt,abovecapskip=6pt,belowskip=3pt--><caption> 
<title>&#x00A0;Cyclic (a) and square wave voltammograms (b) of 2.0 mM
of free probe 
<bold>1a</bold> (&#x2014;) and in 10 percent molar (0.2 mM) of TfOH (- - -)
in 0.1 M TEAPF
<sub>6</sub>&#x00A0;acetonitrile solution. Scan rate: 0.1&#x00A0;Vs<sup>-1</sup>; Glassy carbon as
working electrode (diameter: 3&#x00A0;mm), Pt wire as counter-electrode and
Ag<!--\unskip --><mml:math altimg="e11.gif" display="inline">
<mml:mo>&#x002F;</mml:mo></mml:math>AgCl
(3M) as a reference electrode. </title> 
</caption>       
<graphic xlink:href="gr4.jpg" xmlns:xlink="http://www.w3.org/1999/xlink"/>      
</fig>
<!--\setFloat [t]{fig4}--></p>
<p>The effect of the scan rate on the first oxidation peak current
(<mml:math altimg="e12.gif" display="inline">
<mml:msub>
<mml:mrow >
<mml:mi>i</mml:mi></mml:mrow>
<mml:mrow >
<mml:mi>p</mml:mi>
<mml:mi>a</mml:mi></mml:mrow></mml:msub >
<mml:mo>&#x002F;</mml:mo>
<mml:msup>
<mml:mrow >
<mml:mi>&#x03BD;</mml:mi></mml:mrow>
<mml:mrow >
<mml:mn>1</mml:mn>
<mml:mo>&#x002F;</mml:mo>
<mml:mn>2</mml:mn></mml:mrow></mml:msup ></mml:math>) was
studied in the range of 0.05&#x2013;1.0 Vs<sup>-1</sup> when the potential was poised approximately to
0.4&#x2013;0.6 V (Fig. 3S). The linearity of the straight line of the logarithm of the first
peak current as a function of logarithm of scan rate was close to 0.5&#x00A0;V (Fig. 4S)
indicating a diffusion-controlled electrochemical process&#x00A0;<xref ref-type="bibr" rid="b22
b23">[22,23]</xref>.</p>
<p>The electrochemical characteristics for the compounds 
<bold>1a-c</bold> are summarized
in
<xref rid="tbl2">Table&#x00A0;2</xref>.
<table-wrap  id="tbl2">  
<label>Table&#x00A0;2.</label> <!--aboveskip=3pt,belowskip=3pt--><caption> 
<title>&#x00A0;Thermodynamic data of probes 
<bold>1a-c</bold>
<xref rid="TF2.a"><sup>a</sup></xref>. </title> 
</caption>
<table frame="hsides" rules="groups"> 
<colgroup>   
<col align="left"/>   
<col align="left"/>   
<col align="left"/>   
<col align="left"/>   
<col align="left"/>   
<col align="left"/>   
<col align="left"/>   
<col align="left"/>   
<col align="left"/>   
<col align="left"/></colgroup>
<thead> 
<tr valign="top"> 
<td align="left"> </td> 
<td colspan="5"  align="center">1st oxidation wave/mV                                                                                                                                                                                                                                                                                                                      </td> 
<td colspan="4"  align="center">2nd oxidation wave/mV
 </td></tr> 
<tr valign="top"> 
<td align="left">Compd.            </td> 
<td align="left">
<mml:math altimg="e13.gif" display="inline">
<mml:msub>
<mml:mrow >
<mml:mi>E</mml:mi></mml:mrow>
<mml:mrow >
<mml:mi>p</mml:mi>
<mml:mi>a</mml:mi></mml:mrow></mml:msub ></mml:math> </td> 
<td align="left">
<mml:math altimg="e14.gif" display="inline">
<mml:msub>
<mml:mrow >
<mml:mi>E</mml:mi></mml:mrow>
<mml:mrow >
<mml:mi>p</mml:mi>
<mml:mi>c</mml:mi></mml:mrow></mml:msub ></mml:math> </td> 
<td align="left">
<mml:math altimg="e15.gif" display="inline">
<mml:msub>
<mml:mrow >
<mml:mi>&#x0394;</mml:mi>
<mml:mi>E</mml:mi></mml:mrow>
<mml:mrow >
<mml:mi>p</mml:mi></mml:mrow></mml:msub ></mml:math>
<xref rid="TF2.b"><sup>b</sup></xref> </td> 
<td align="left">
<mml:math altimg="e16.gif" display="inline">
<mml:msub>
<mml:mrow >
<mml:mi>E</mml:mi></mml:mrow>
<mml:mrow >
<mml:mn>1</mml:mn>
<mml:mo>&#x002F;</mml:mo>
<mml:mn>2</mml:mn></mml:mrow></mml:msub ></mml:math>
<xref rid="TF2.c"><sup>c</sup></xref> </td> 
<td align="left">
<mml:math altimg="e17.gif" display="inline">
<mml:msub>
<mml:mrow >
<mml:mi>i</mml:mi></mml:mrow>
<mml:mrow >
<mml:mi>p</mml:mi>
<mml:mi>a</mml:mi></mml:mrow></mml:msub >
<mml:mo>&#x002F;</mml:mo>
<mml:msub>
<mml:mrow >
<mml:mi>i</mml:mi></mml:mrow>
<mml:mrow >
<mml:mi>p</mml:mi>
<mml:mi>c</mml:mi></mml:mrow></mml:msub ></mml:math> </td> 
<td align="left">
<mml:math altimg="e18.gif" display="inline">
<mml:msub>
<mml:mrow >
<mml:mi>E</mml:mi></mml:mrow>
<mml:mrow >
<mml:mi>p</mml:mi>
<mml:mi>a</mml:mi></mml:mrow></mml:msub ></mml:math> </td> 
<td align="left">
<mml:math altimg="e19.gif" display="inline">
<mml:msub>
<mml:mrow >
<mml:mi>E</mml:mi></mml:mrow>
<mml:mrow >
<mml:mi>p</mml:mi>
<mml:mi>c</mml:mi></mml:mrow></mml:msub ></mml:math> </td> 
<td align="left">
<mml:math altimg="e20.gif" display="inline">
<mml:msub>
<mml:mrow >
<mml:mi>&#x0394;</mml:mi>
<mml:mi>E</mml:mi></mml:mrow>
<mml:mrow >
<mml:mi>p</mml:mi></mml:mrow></mml:msub ></mml:math>
<xref rid="TF2.b"><sup>b</sup></xref> </td> 
<td align="left">
<mml:math altimg="e21.gif" display="inline">
<mml:msub>
<mml:mrow >
<mml:mi>E</mml:mi></mml:mrow>
<mml:mrow >
<mml:mn>1</mml:mn>
<mml:mo>&#x002F;</mml:mo>
<mml:mn>2</mml:mn></mml:mrow></mml:msub ></mml:math>
<xref rid="TF2.c"><sup>c</sup></xref> </td>
</tr></thead>
<tbody> 
<tr valign="top"> 
<td align="left">
<bold>1a</bold>                </td> 
<td align="left">300                                                                                                    </td> 
<td align="left">190                                                                                                    </td> 
<td align="left">110                                                                                                    </td> 
<td align="left">250                                                                                                    </td> 
<td align="left">0,96                                                                                                   </td> 
<td align="left">910                                                                                                    </td> 
<td align="left">810                                                                                                    </td> 
<td align="left">100                                                                                                    </td> 
<td align="left">860                                                                                                    </td>
</tr> 
<tr valign="top"> 
<td align="left">
<bold>1b</bold>                </td> 
<td align="left">320                                                                                                    </td> 
<td align="left">230                                                                                                    </td> 
<td align="left">90                                                                                                     </td> 
<td align="left">280                                                                                                    </td> 
<td align="left">0,98                                                                                                   </td> 
<td align="left">880                                                                                                    </td> 
<td align="left">780                                                                                                    </td> 
<td align="left">100                                                                                                    </td> 
<td align="left">830                                                                                                    </td>
</tr> 
<tr valign="top"> 
<td align="left">
<bold>1c</bold>                </td> 
<td align="left">410                                                                                                    </td> 
<td align="left">320                                                                                                    </td> 
<td align="left">90                                                                                                     </td> 
<td align="left">370                                                                                                    </td> 
<td align="left">0,97                                                                                                   </td> 
<td align="left">920                                                                                                    </td> 
<td align="left">830                                                                                                    </td> 
<td align="left">90                                                                                                     </td> 
<td align="left">880                                                                                                    </td>
</tr> 
</tbody> 
</table>
<table-wrap-foot>
<fn id="TF2.a">
<label>a</label>
<p>0.1 M <!--\xdollar\rm -->          TEAPF
<sub>6</sub><!--\xdollar-->&#x00A0;in
acetonitrile, scan rate: 0.1 Vs<sup>-1</sup>; working electrode: glassy carbon
(diameter: 3&#x00A0;mm); counter-electrode: Pt wire and reference electrode:
<mml:math altimg="e22.gif" display="inline">
<mml:mstyle mathvariant="normal">
<mml:mi>Ag</mml:mi></mml:mstyle>
<mml:mo>&#x002F;</mml:mo>
<mml:mstyle mathvariant="normal">
<mml:mi>AgCl</mml:mi></mml:mstyle></mml:math>;</p></fn> 
<fn id="TF2.b">
<label>b</label>
<p><!--\xdollar\rm -->&#x0394;E
<sub>p</sub>=E
<sub>pa</sub>&#x2212;E
<sub>pc</sub><!--\xdollar-->;</p></fn> 
<fn id="TF2.c">
<label>c</label>
<p><!--\xdollar\rm -->E
<sub>1&#x002F;2</sub>=(E
<sub>pa</sub>+E
<sub>pc</sub>)&#x002F;2<!--\xdollar--></p></fn>
</table-wrap-foot></table-wrap>
<!--\setTable [t]{tbl2}--></p>
</sec>
<sec>
<title>Sensing of calcium and magnesium</title>
<p>CV and SWV studies in the presence of calcium or magnesium ions revealed that
compounds 
<bold>1a-c</bold> were able to bind these cations. As shown in
<xref rid="fig5">Fig.&#x00A0;5</xref>, the chelation of Ca<sup>2+</sup> or Mg<sup>2+</sup> by the compound 
<bold>1a</bold> provoked very important anodic shifts
in the CV curves when a 0.5 equivalent of ions is added. In fact, the two peaks of oxidation (<!--\xdollar\rm -->O
<sub>1</sub><!--\xdollar--> and <!--\xdollar\rm -->O
<sub>2</sub><!--\xdollar-->)
related to the free probe disappeared, and an irreversible peak (<!--\xdollar\rm -->O
<sub>3</sub><!--\xdollar-->) was
observed at much higher anodic potentials attributed to oxidation of the
complexes. On the reverse scan, two reduction peaks were observed: an <!--\xdollar\rm -->R
<sub>4</sub><!--\xdollar-->&#x00A0;peak occurred at the same
potential as an <!--\xdollar\rm -->          R
<sub>2</sub><!--\xdollar-->,
indicating that the dication generated from the oxidation of the complex
remained uncoordinated because of the electronic repulsion between
the positive charges on the phenylenediamine moiety and the cations.
Reduction into the neutral initial species took place at higher potentials (<!--\xdollar\rm -->R
<sub>5</sub><!--\xdollar-->)
compared to those without calcium or magnesium (<!--\xdollar\rm -->R
<sub>1</sub><!--\xdollar-->),
because the recomplexation reaction displaces the interfacial equilibrium, thus
facilitating the electron transfer&#x00A0;<xref ref-type="bibr" rid="b24 b25">[24,25]</xref>.
<fig  id="fig5">  
<label>Figure&#x00A0;5.</label><!--aboveskip=3pt,abovecapskip=6pt,belowskip=3pt--><caption> 
<title>&#x00A0;CV voltammograms of 2.0 mM solution of 
<bold>1a</bold> in acetonitrile 0.1M
TEAPF
<sub>6</sub>&#x00A0;solution in the absence <!--\unskip \break -->(&#x2014;) and in presence of 0.5 equivalent of Ca<sup>2+</sup>
(<mml:math altimg="e23.gif" display="inline">
<mml:mi>&#x2022;</mml:mi>
<mml:mi>&#x2022;</mml:mi>
<mml:mi>&#x2022;</mml:mi></mml:math>) of 0.5
equivalent of Mg<sup>2+</sup> (- - -), b- SWV voltammograms of 2 mM solution 
<bold>1a</bold> in acetonitrile
0.1M TEAPF
<sub>6</sub>&#x00A0;solution in the absence (&#x2014;) and in presence of 0.5 equivalents of Ca<sup>2+</sup>
(<mml:math altimg="e24.gif" display="inline">
<mml:mi>&#x2022;</mml:mi>
<mml:mi>&#x2022;</mml:mi>
<mml:mi>&#x2022;</mml:mi></mml:math>) of
0.5 equivalents of Mg<sup>2+</sup> (- - -). </title> 
</caption>       
<graphic xlink:href="gr5.jpg" xmlns:xlink="http://www.w3.org/1999/xlink"/>      
</fig>
<!--\setFloat [t]{fig5}--></p>
<p>CV as well as SWV showed that the third peak <!--\xdollar\rm -->O
<sub>3</sub><!--\xdollar--> is
probably due to the oxidation of the furan and the chelated phenylenediamine
rings&#x00A0;<xref ref-type="bibr" rid="b26 b27">[26,27]</xref>. The peak was more prominent in the case of
calcium compared to magnesium.</p>
<p>For CV, the potentials shifts reported in
<xref rid="tbl3">Table&#x00A0;3</xref> were between 780 and 990 mV. In SWV the increases of peaks potentials
were <!--\vadjust {\pagebreak }-->slightly less important and were between 710 and 930 mV. The substituent on
the para position to the ionophore site affected the magnitude of the potential shifts.
Beer et&#x00A0;al. established that the stability of the complexes was proportional to the
potential <!--\unskip \break -->shifts&#x00A0;<xref ref-type="bibr" rid="b28">[28]</xref>.
<table-wrap  id="tbl3">  
<label>Table&#x00A0;3.</label> <!--aboveskip=3pt,belowskip=3pt--><caption> 
<title>&#x00A0;CV and SWV potentials of first oxidation peak and third oxidation peak
(complex peaks) and their differences for compounds 
<bold>1a-c</bold> in the presence of 0.5 equivalent of
<!--\let \rm \relax \unboldmath --> Ca<!--\xdollar\rm -->           <!--\xxUB--><sup>2<!--\xxxUB-->+</sup><!--\xdollar-->,
Mg<!--\xdollar\rm -->          <!--\xxUB--><sup>2<!--\xxxUB-->+</sup><!--\xdollar-->, Ni<!--\xdollar\rm --><!--\xxUB--><sup>2<!--\xxxUB-->+</sup><!--\xdollar--> and Zn<!--\xdollar\rm --><!--\xxUB--><sup>2<!--\xxxUB-->+</sup><!--\xdollar--> in acetonitrile/0.1M
TEAPF<!--\xdollar\rm -->          <!--\xxUB--><sub>6</sub><!--\xdollar-->. </title> 
</caption>
<table frame="hsides" rules="groups"> 
<colgroup>   
<col align="left"/>   
<col align="left"/>   
<col align="left"/>   
<col align="left"/>   
<col align="left"/>   
<col align="left"/></colgroup>
<thead> 
<tr valign="top"> 
<td align="left">       </td> 
<td align="left">                                                                                                                                                          </td> 
<td align="left"><!--\xdollar\rm -->        Ca<sup>2+</sup><!--\xdollar--> </td> 
<td align="left"><!--\xdollar\rm -->        Mg<sup>2+</sup><!--\xdollar--> </td> 
<td align="left"><!--\xdollar\rm -->        Ni<sup>2+</sup><!--\xdollar--> </td> 
<td align="left"><!--\xdollar\rm -->        Zn<sup>2+</sup><!--\xdollar--> </td>
</tr></thead>
<tbody> 
<tr valign="top"> 
<td colspan="6"  align="center">CV Data
 </td></tr> 
<tr valign="top"> 
<td  rowspan="2"  align="left">
<bold>1a</bold> </td> 
<td align="left"><!--\xdollar\rm -->        E
<sub>pa</sub><!--\xdollar-->(<!--\xdollar\rm -->            O
<sub>1</sub><!--\xdollar-->)/mV   </td> 
<td align="left">300                                                                                                                </td> 
<td align="left">300                                                                                                                </td> 
<td align="left">300                                                                                                                </td> 
<td align="left">300                                                                                                                </td>
</tr> 
<tr valign="top"> 
<td align="left">             </td> 
<td align="left"><!--\xdollar\rm -->        E
<sub>pa</sub><!--\xdollar--> (<!--\xdollar\rm -->            O
<sub>3</sub><!--\xdollar-->)/mV  </td> 
<td align="left">1290                                                                                                              </td> 
<td align="left">1230                                                                                                              </td> 
<td align="left">1270                                                                                                              </td> 
<td align="left">1240                                                                                                              </td>
</tr> 
<tr valign="top"> 
<td align="left">             </td> 
<td align="left"><!--\xdollar\rm -->        &#x0394;E
<sub>p</sub><!--\xdollar-->/mV                                                                                                                         </td> 
<td align="left">990                                                                                                                </td> 
<td align="left">930 <!--\vspace *{5pt}-->                                                                                                               </td> 
<td align="left">970                                                                                                                </td> 
<td align="left">940                                                                                                                </td>
</tr> 
<tr valign="top"> 
<td  rowspan="2"  align="left">
<bold>1b</bold> </td> 
<td align="left"><!--\xdollar\rm -->        E
<sub>pa</sub><!--\xdollar-->(<!--\xdollar\rm -->            O
<sub>1</sub><!--\xdollar-->)/mV   </td> 
<td align="left">330                                                                                                                </td> 
<td align="left">330                                                                                                                </td> 
<td align="left">330                                                                                                                </td> 
<td align="left">330                                                                                                                </td>
</tr> 
<tr valign="top"> 
<td align="left">             </td> 
<td align="left"><!--\xdollar\rm -->        E
<sub>pa</sub><!--\xdollar--> (<!--\xdollar\rm -->            O
<sub>3</sub><!--\xdollar-->) /mV </td> 
<td align="left">1280                                                                                                              </td> 
<td align="left">1210                                                                                                              </td> 
<td align="left">1230                                                                                                              </td> 
<td align="left">1300                                                                                                              </td>
</tr> 
<tr valign="top"> 
<td align="left">             </td> 
<td align="left"><!--\xdollar\rm -->        &#x0394;E
<sub>p</sub><!--\xdollar-->/mV                                                                                                                         </td> 
<td align="left">950                                                                                                                </td> 
<td align="left">880 <!--\vspace *{5pt}-->                                                                                                               </td> 
<td align="left">900                                                                                                                </td> 
<td align="left">970                                                                                                                </td>
</tr> 
<tr valign="top"> 
<td  rowspan="2"  align="left">
<bold>1c</bold> </td> 
<td align="left"><!--\xdollar\rm -->        E
<sub>pa</sub><!--\xdollar-->(<!--\xdollar\rm -->            O
<sub>1</sub><!--\xdollar-->)/mV   </td> 
<td align="left">430                                                                                                                </td> 
<td align="left">430                                                                                                                </td> 
<td align="left">430                                                                                                                </td> 
<td align="left">430                                                                                                                </td>
</tr> 
<tr valign="top"> 
<td align="left">             </td> 
<td align="left"><!--\xdollar\rm -->        E
<sub>pa</sub><!--\xdollar--> (<!--\xdollar\rm -->            O
<sub>3</sub><!--\xdollar-->)/mV  </td> 
<td align="left">1210                                                                                                              </td> 
<td align="left">1220                                                                                                              </td> 
<td align="left">1260                                                                                                              </td> 
<td align="left">1230                                                                                                              </td>
</tr> 
<tr valign="top"> 
<td align="left">             </td> 
<td align="left"><!--\xdollar\rm -->        &#x0394;E
<sub>p</sub><!--\xdollar-->/mV                                                                                                                         </td> 
<td align="left">780                                                                                                                </td> 
<td align="left">790                                                                                                                </td> 
<td align="left">830                                                                                                                </td> 
<td align="left">800                                                                                                                </td>
</tr> 
<tr valign="top"> 
<td colspan="6"  align="center">SWV Data
 </td></tr> 
<tr valign="top"> 
<td  rowspan="2"  align="left">
<bold>1a</bold> </td> 
<td align="left"><!--\xdollar\rm -->        E
<sub>pa</sub><!--\xdollar-->(<!--\xdollar\rm -->            O
<sub>1</sub><!--\xdollar-->)/mV   </td> 
<td align="left">270                                                                                                                </td> 
<td align="left">270                                                                                                                </td> 
<td align="left">270                                                                                                                </td> 
<td align="left">270                                                                                                                </td>
</tr> 
<tr valign="top"> 
<td align="left">             </td> 
<td align="left"><!--\xdollar\rm -->        E
<sub>pa</sub><!--\xdollar--> (<!--\xdollar\rm -->            O
<sub>3</sub><!--\xdollar-->) /mV </td> 
<td align="left">1120                                                                                                              </td> 
<td align="left">1140                                                                                                              </td> 
<td align="left">1200                                                                                                              </td> 
<td align="left">1160                                                                                                              </td>
</tr> 
<tr valign="top"> 
<td align="left">             </td> 
<td align="left"><!--\xdollar\rm -->        &#x0394;E
<sub>p</sub><!--\xdollar-->/mV                                                                                                                         </td> 
<td align="left">850                                                                                                                </td> 
<td align="left">870 <!--\vspace *{5pt}-->                                                                                                               </td> 
<td align="left">930                                                                                                                </td> 
<td align="left">890                                                                                                                </td>
</tr> 
<tr valign="top"> 
<td  rowspan="2"  align="left">
<bold>1b</bold> </td> 
<td align="left"><!--\xdollar\rm -->        E
<sub>pa</sub><!--\xdollar-->(<!--\xdollar\rm -->            O
<sub>1</sub><!--\xdollar-->)/mV   </td> 
<td align="left">290                                                                                                                </td> 
<td align="left">290                                                                                                                </td> 
<td align="left">290                                                                                                                </td> 
<td align="left">290                                                                                                                </td>
</tr> 
<tr valign="top"> 
<td align="left">             </td> 
<td align="left"><!--\xdollar\rm -->        E
<sub>pa</sub><!--\xdollar--> (<!--\xdollar\rm -->            O
<sub>3</sub><!--\xdollar-->) /mV </td> 
<td align="left">1220                                                                                                              </td> 
<td align="left">1150                                                                                                              </td> 
<td align="left">1260                                                                                                              </td> 
<td align="left">1180                                                                                                              </td>
</tr> 
<tr valign="top"> 
<td align="left">             </td> 
<td align="left"><!--\xdollar\rm -->        &#x0394;E
<sub>p</sub><!--\xdollar-->/mV                                                                                                                         </td> 
<td align="left">930                                                                                                                </td> 
<td align="left">860 <!--\vspace *{5pt}-->                                                                                                               </td> 
<td align="left">890                                                                                                                </td> 
<td align="left">970                                                                                                                </td>
</tr> 
<tr valign="top"> 
<td  rowspan="2"  align="left">
<bold>1c</bold> </td> 
<td align="left"><!--\xdollar\rm -->        E
<sub>pa</sub><!--\xdollar-->(<!--\xdollar\rm -->            O
<sub>1</sub><!--\xdollar-->)/mV   </td> 
<td align="left">400                                                                                                                </td> 
<td align="left">400                                                                                                                </td> 
<td align="left">400                                                                                                                </td> 
<td align="left">400                                                                                                                </td>
</tr> 
<tr valign="top"> 
<td align="left">             </td> 
<td align="left"><!--\xdollar\rm -->        E
<sub>pa</sub><!--\xdollar--> (<!--\xdollar\rm -->            O
<sub>3</sub><!--\xdollar-->) /mV </td> 
<td align="left">1150                                                                                                              </td> 
<td align="left">1110                                                                                                              </td> 
<td align="left">1200                                                                                                              </td> 
<td align="left">1170                                                                                                              </td>
</tr> 
<tr valign="top"> 
<td align="left">             </td> 
<td align="left"><!--\xdollar\rm -->        &#x0394;E
<sub>p</sub><!--\xdollar-->/mV                                                                                                                         </td> 
<td align="left">750                                                                                                                </td> 
<td align="left">710                                                                                                                </td> 
<td align="left">800                                                                                                                </td> 
<td align="left">770                                                                                                                </td>
</tr> 
</tbody> 
</table>
<table-wrap-foot>
<fn id="TF3.1">
<p>* <!--\xdollar\rm -->           &#x0394;E
<sub>p</sub>=E
<sub>pa</sub>(O
<sub>3</sub>)&#x2212;E
<sub>pa</sub>(O
<sub>1</sub>).<!--\xdollar--></p></fn>
</table-wrap-foot></table-wrap>
<!--\setTable [t]{tbl3}--></p>
<sec>
<title>Sensing of nickel and zinc</title>
<p>Electrochemical studies of the probe 
<bold>1a</bold> in the presence of Ni<sup>2+</sup> and Zn<sup>2+</sup> gave similar
results to those observed with alkaline earth cations (Fig. 5S-a). The signals of
the uncoordinated probe disappeared due to the cations&#x2019; addition along
with the appearance of more anodic signals related to the oxidation of the
chelated forms.<!--\vadjust {\pagebreak }--> The potential peaks&#x2019; shifts were in the range of 800 to 970 mV
(<xref rid="tbl3">Table&#x00A0;3</xref>). For all compounds, the addition of TfOH accelerated the Zn<sup>2+</sup>
complexation; however, the reaction could occur without it. For Ni<sup>2+</sup>, a 10 percent
molar solution of TfOH was necessary to achieve the chelation. Effectively, if no acid
was added and in presence of 0.5 equivalent of Ni<sup>2+</sup>, the electrochemical features of the
free probe did change even if the Ni<sup>2+</sup>/ligand solution was kept under stirring
overnight. Compounds 
<bold>1b</bold> and 
<bold>1c</bold> behaved similarly in presence of nickel.
Probably, the complexation reaction needs a change in the conformation of
the probes induced by the protonation followed by metal-proton exchange
reaction.</p>
<p>In cases of Ni<sup>2+</sup> and Zn<sup>2+</sup>, no overlapping peaks were observed in the voltammograms due
to the oxidation of the furan ring as noticeable with the alkaline earth ions, a
phenomenon confirmed by single peaks observed in square wave voltammetry curves
(Fig. 5S-b).</p>
<p>For all cations, a 1:2 metal-to-ligand ratio was observed which lead
to the conclusion that the obtained complexes in solution were <!--\xdollar\rm -->[M(L)
<sub>2</sub>]<sup>2+</sup><!--\xdollar-->.</p>
</sec>
</sec>
<sec>
<title>Selectivity</title>
<p>As the studies showed, the probes can bind all the cations, and it was interesting to
investigate the selectivity between them. First, alkaline earth ions were compared,
then a comparison was made between Zn<sup>2+</sup> and Ni<sup>2+</sup> and finally alkaline earth were
compared to transition metal ions. 
<bold>1a</bold> did not allow for distinguishing between the
ions and 
<bold>1c</bold> was discarded because of the morpholine ring oxygen which could
compete with the ionophore site, so 
<bold>1b</bold> was the most fitted for this study. CV (Fig.
6S) and SWV (<xref rid="fig6">Fig.&#x00A0;6</xref>) voltammograms showed that 
<bold>1b</bold> in the presence of a mixture of Ca<sup>2+</sup> and
Mg<sup>2+</sup> gave curves which were close to those of Ca<sup>2+</sup> alone, although the intensity of the
peak current was less important. So it could be concluded that 
<bold>1b</bold> was more selective
to calcium over magnesium&#x00A0;<xref ref-type="bibr" rid="b29">[29]</xref>.
<fig  id="fig6">  
<label>Figure&#x00A0;6.</label><!--aboveskip=3pt,abovecapskip=6pt,belowskip=3pt--><caption> 
<title>&#x00A0;SWV voltammograms of (i) metal free 
<bold>1b</bold> (2.0mM) (), (ii) 
<bold>1b</bold> in the
presence of 0.5 equivalent of Mg<sup>2+</sup> (&#x2014;), (iii) 
<bold>1b</bold> in the presence of 0.5 equivalent of Ca<sup>2+</sup>
(&#x2013;<!--{ --><mml:math altimg="e25.gif" display="inline">
<mml:mi>&#x25B4;</mml:mi></mml:math><!--} -->&#x2013;)
and (iv) solution in the presence of 0.5 equivalent of Mg<sup>2+</sup> and of 0.5 equivalent of Ca<sup>2+</sup>
(- - -) in acetonitrile/0.1M TEAPF
<sub>6</sub>&#x00A0;solution. </title> 
</caption>       
<graphic xlink:href="gr6.jpg" xmlns:xlink="http://www.w3.org/1999/xlink"/>      
</fig>
<!--\setFloat [h]{fig6}--></p>
<p>In addition, analogous treatment showed that 
<bold>1b</bold> was more selective to Ni<sup>2+</sup> over Zn<sup>2+</sup>
(<xref rid="fig7">Fig.&#x00A0;7</xref>). In fact, even in the presence of a large amount of Zn<sup>2+</sup> (5 equivalent), the
addition of a stoichiometric quantity of Ni<sup>2+</sup> was enough to form the 1:2 complex, thus
zinc was ejected from the receptor ionophore site and nickel replaced it indicating
that the probe was very selective to nickel.
<fig  id="fig7">  
<label>Figure&#x00A0;7.</label>  
<caption> 
<title>&#x00A0;SWV voltammograms of (i) metal free 
<bold>1b</bold>
(2.0 mM) (&#x2014;), (ii) 
<bold>1b</bold> in the presence of 0.5 equivalent of Ni<sup>2+</sup>
<!--\unskip \break -->(&#x2013;<!--{ --><mml:math altimg="e26.gif" display="inline">
<mml:mi>&#x2022;</mml:mi></mml:math><!--} -->&#x2013;), (iii) 
<bold>1b</bold> in the presence
of 0.5 equivalent of Zn<sup>2+</sup> (&#x2013;<!--{ --><mml:math altimg="e27.gif" display="inline">
<mml:mi>&#x25B4;</mml:mi></mml:math><!--} -->&#x2013;)
and (iv) solution in the presence of 0.5 equivalent of Ni<sup>2+</sup> and of 5 equivalents of Zn<sup>2+</sup> (- -
-) in acetonitrile/0.1M TEAPF
<sub>6</sub>&#x00A0;solution. </title> 
</caption>       
<graphic xlink:href="gr7.jpg" xmlns:xlink="http://www.w3.org/1999/xlink"/>      
</fig>
<!--\setFloat [t]{fig7}--></p>
<p>As shown in the
<xref rid="fig8">Fig.&#x00A0;8</xref>, only one peak was observed after the addition of a equimolar mixture of
the Ca<sup>2+</sup> and Ni<sup>2+</sup>. The peak is almost superimposable with that of the oxidation of the
calcium complex, which lead to the conclusion that the 
<bold>1b</bold> compound was more
selective to calcium over nickel. These results indicated that 
<bold>1b</bold> was more selective to
Ca<sup>2+</sup>&#x003E;&#x00A0;Mg<sup>2+</sup>&#x003E;&#x00A0; Ni<sup>2+</sup>&#x003E;&#x00A0;Zn<sup>2+</sup>.<!--\pagebreak --><fig  id="fig8">  
<label>Figure&#x00A0;8.</label>  
<caption> 
<title>&#x00A0;SWV voltammograms of (i) metal free 
<bold>1b</bold> (2.0mM)
(&#x2013;<!--{ --><mml:math altimg="e28.gif" display="inline">
<mml:mi>&#x2022;</mml:mi></mml:math><!--} -->&#x2013;), (iii)
<bold>1b</bold> in the presence of 0.5 equivalent of Ca<sup>2+</sup> (&#x2014;), (ii) 
<bold>1b</bold> in the presence of 0.5 equivalent of
Ni<sup>2+</sup> (&#x2013;<!--{ --><mml:math altimg="e29.gif" display="inline">
<mml:mi>&#x25B4;</mml:mi></mml:math><!--} -->&#x2013;)
and (iv) solution in the presence of 0.5 equivalent of Ni<sup>2+</sup> and of 0.5 equivalent of Ca<sup>2+</sup> (-
- -) in acetonitrile/0.1M TEAPF
<sub>6</sub>&#x00A0;solution. </title> 
</caption>       
<graphic xlink:href="gr8.jpg" xmlns:xlink="http://www.w3.org/1999/xlink"/>      
</fig>
<!--\setFloat [t]{fig8}--></p>
</sec>
</sec>
<sec>
<title>Conclusion</title>
<p>In summary, UV-visible spectrophotometric, cyclic, square wave and differential pulse
voltammetry studies showed that a series of tetraalkylated pheneylenediamines
containing a bis(furan-2-methyl)amino group as the ionophore site were found to be
sensitive to the presence of calcium, magnesium, nickel and zinc ions in
solution. All the complexes were formed in 1:2 stoichiometries. Comparative
studies showed that 
<bold>1b</bold> was more selective to Ca<sup>2+</sup> over Mg<sup>2+</sup> and to Ni<sup>2+</sup> over
Zn<sup>2+</sup>. Further investigation revealed that 
<bold>1b</bold> was more selective to Ca<sup>2+</sup> over
Ni<sup>2+</sup>.</p>
</sec>
<sec>
<title>Acknowledgments</title>
<p>The authors gratefully acknowledge the Tunisian Ministry of Higher Education and
Scientific Research for financial help (Lab CH-02) and the DRGS-CNRS program for
research grant (10/R-1202) for RS.</p>
</sec>
<sec>
<title>Appendix. Supplementary materials</title>
<p>Supplementary materials associated with this article can be found, in the online
version 
<uri xlink:href="http://dx.doi.org/10.5339/connect.2012.8">doi:10.5339/connect.2012.8</uri>.</p></sec></body>
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