The solvent-dependent binding modes of a rhodamine-azacrown based fluorescent probe for Al3+ and Fe3+

• A new chemosensor based on rhodamine-azacrown derivative is reported.
• The sensor responds to Al3+ and Fe3+ with a simple 1:1 mode in aqueous solutions.
• The detection limit of Fe3+ is 1.2 × 10−8 M in aqueous solution.
• The sensor selectively responds to Al3+ with a 1:2 complex form in acetonitrile.
• The detection limit of Al3+ is 5.9 × 10−7 M in acetonitrile.

A rhodamine-azacrown derivative (2-(2-(1,4-dioxa-7,13-dithia-10-azacyclo- pentadecan-10-yl)ethyl)-3′,6′-bis(diethyl amino)spiro[isoindoline-1,9′-xanthen]-3-one) responded to Al3+ and Fe3+ in ethanol–water solutions (20/80, v/v, pH = 7.0) and selectively responded to Al3+ in acetonitrile. In addition, it bound to the cation with a simple 1:1 mode in aqueous solutions and 2:1 mode in acetonitrile. The association constants (logKa) of the sensor for Fe3+ and Al3+ were obtained to be 6.5 and 3.6 in ethanol–water, and the corresponding detection limits were calculated to be 1.2 and 8.6 × 10−8 M, respectively, while the association constant (logKa) for Al3+ was 3.9 with a detection limit of 5.9 × 10−7 M in acetonitrile under our experimental conditions. Nuclear magnetic resonance and other investigations indicated that in acetonitrile the existence of the second Al3+ shifted the binding location of the first Al3+, which led to the spirolactam ring-open and fluorescence formation. The mechanisms for the sensor to Al3+ and Fe3+ in different solvents are discussed in detail.

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