A highly selective carbazole-based D-π-D-A fluorescence turn-off sensor for Fe3+ detection in aqueous media

In this study, N-hexyl-6-(4-methoxyphenyl)carbazole-3-carbaldehyde with donor-it-donor-acceptor (D-it-D-A) structure was designated as a probe N and its fluorescence sensor properties were investigated against 30 different analytes. Probe N in DMSO gave a structureless emission band with a maximum at 470 nm and a high photoluminescence quantum yield (80 %). In going from non-polar to polar solvents, the emission maximum of probe N red-shifted by 72 nm and Stokes shifts observed between absorption and emission spectra was 71 nm (3689 cm-1). It was found that the emission maximum of probe N at 470 nm was more significantly quenched by Fe3+ than by Fe2+ among the analytes studied. Probe N showed high selectivity and high sensitivity with a 0.8 nM limit of detection (LOD) against Fe3+ in the presence of competing analytes. The LOD value is the lowest value reported so far for the detection of Fe3+. Job's plot indicated a 2:1 ratio between probe N and Fe3+. 1H NMR and FT-IR provided further evidence that probe N complexed Fe3+ via the non-bonding electrons on the oxygen atoms of the aldehyde and methoxy groups. In practical applications, probe N successfully detected Fe3+. These results revealed that probe N could be an effective turn-off fluorescence sensor in the determination of Fe3+ in liquid environments.