Novel potentiometric and optical silver ion-selective sensors with subnanomolar detection limits

Abstract

Ten Ag+-selective ionophores have been characterized in terms of their potentiometric selectivities and complex formation constants in solvent polymeric membranes. The compounds with pi-coordination show much weaker interactions than those with thioether or thiocarbamate groups as the coordinating sites. Long-term studies with the best ionophores show that the lower detection limit of the best Ag+ sensors can be maintained in the subnanomolar range for at least 1 month. The best ionophores have also been characterized in fluorescent microspheres. The so far best lower detection limits of 3 x 10(-11) M (potentiometrically) and 2 x 10(-11) M Ag+ (optically) are found with bridged thiacalixarenes.

Figure 1

Potentiometric response behavior of the six Ag⁺-selective ionophores, I–VI, having π-electrons as coordinating sites.

Figure 2

Potentiometric response behavior of the four Ag⁺-selective ionophores, VII–X, having S as coordinating sites.

Figure 3

Calibration curves obtained with o-NPOE/PVC and DOS/PVC membranes based on ionophore IX. The lower detection limits are 3 × 10⁻¹¹ and 5 × 10⁻¹⁰M Ag⁺ (i.e., log a_Ag+ = −10.6 and −9.3), respectively.

Figure 4

Optical response of DOS/PVC microspheres based on ionophore IX (top) and X (bottom) expressed as the mole fraction of the protonated chromoionophore (ETH 5418, see Experimental). The sensing particles were immobilized on glass slides. The AgNO₃ solutions were applied in Mg(OAc)₂ buffer at pH 4.7 or MOPS buffer at pH 7.4. The theoretical response curves (solid lines) were calculated with the same exchange constant at both pH values.

Figure 5

Optical response behavior of DOS/PVC microspheres based on ionophore IX (top) and X (bottom) to different ions at pH 7.4. The theoretical curves (solid lines) were calculated with the exchange constants, $K_{\text{exch}}^{\text{Ag}}$, given in Table 5.