Selective colorimetric detection of Cu²⁺by lanthanide based hybrid complexes associated with a single crystal growth mediated transformation

Abstract

Two rare earth (Dy 3+ and Nd 3+ ) based complexes were synthesized using the ligands bipyridine, p-amino benzoic acid and nitro benzoic acid. The complexes were characterized by single crystal and powder XRD, IR spectroscopy and thermogravimetric analysis. Both complexes showed characteristics luminescent properties of the lanthanides and weak metamagnetic transitions at low temperature. Both the Dy and Nd compounds were found to detect Cu 2+ selectively in very low concentration of the transition metal salt in the mixture of other transition metals like Cu 2+ , Co 2+ , Ni 2+ , Zn 2+ , Cd 2+ , Ag + and Au 3+ . This selective copper uptake is associated with a fast distinct colour change as a result of which the detection of copper can be realized visually. The Nd compound was more selective and detected Cu 2+ selectively till 10 -5 M concentration of the Cu salt. The Cu 2+ encapsulation in the Nd complex was characterized through a rare single crystal growth mediated transformation involving an irreversible exchange of the metal ion of the complex. This single crystal transformation facilitates the easy understanding of the copper encapsulation mechanism and also provides information on the detector complex, which is rarely studied. compounds 1 and 2 can be used to identify the presence of various transition metals like Cu, Ni, Co etc. qualitatively and also may be quantitatively if a relation between the rate of TM diffusion with the TM salt concentration can be established.The next question that naturally arises is whether there is any selectivity for this metal absorption/ metal exchange. To check this, we prepared solutions of other metal salts, NiCl 2 , CoCl 2 , ZnSO 4 , CdCl 2 , AgNO 3 and AuCl 3 of strengths same as that of the Cu 2+ solution. The selectivity studies on 1/2 were done through microscopy and ICP-OES analysis on the crystals of 1/2 and the supernatant after exchange with all the metals cations (Cu 2+ , Co 2+ , Ni 2+ , Zn 2+ , Cd 2+ , Ag + , Au 3+ ) individually.. From the metal exchange studies it was found out that 1 was selective to the coinage metals (Cu 2+ , Ag 1+ and Au 3+ ) and selectively took only these from mixture of all the salts and also when done individually, as analyzed from ICP-OES (Table S5).2, on the other hand, was very much selective to Cu 2+ and did not take any other metal cations, neither in mixture of all the salts, nor individually ( Figure 10 and Table S5). To see the sensing property of the Cu 2+ detection, Cu 2+ exchange studies were also done using other anionic salts of Cu 2+ eg. Cu(NO 3 ) 2 , CuSO 4 and it was found that both the compounds could detect Cu 2+ irrespective of the type of anion/salt used ( Figure 11, Table S5).The microscopy studies on Co, Ni, exchange interestingly showed that these metals are not getting into the system and no color change of the crystal is happening (Figure S16, S17). The color that is appearing in Figures S16b and S16c is only the solution color and after the solution is taken out, the crystals looks almost similar with absolut...

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“…In addition, 10 μM Ag + had a minimal effect on the fluorescence intensity of Eu­(DPA) 3 @Lap-Tris. Few emission intensity changes were observed in the presence of these interfering metal ions, demonstrating that this assay method was highly selective for Cu 2+ and unaffected by the presence of other ionic species . The emission response in the presence of anions was examined under the same conditions, as anions present in solution can be another factor that affects such sensing systems (Figure E).…”

“…Few emission intensity changes were observed in the presence of these interfering metal ions, demonstrating that this assay method was highly selective for Cu 2+ and unaffected by the presence of other ionic species. 35 The emission response in the presence of anions was examined under the same conditions, as anions present in solution can be another factor that affects such sensing systems (Figure 3E). No substantial changes were observed from the anions tested, demonstrating that anion interference in solution is a minimal factor on this system.…”

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