Cd-Based Nanoporous Metal–Organic Frameworks for Luminescent Sensing of Carcinoid Biomarkers and Al³⁺

Effective detection of biomarkers 5-hydroxytryptamine (5-HT) and 5-hydroxyindole-3-acetic acid (5-HIAA) in serum/urine components is important for the early diagnosis of carcinoids. Two Cd-metal–organic frameworks (MOFs), namely, {[Cd 3 (L) 2 (bpod) 2 (H 2 O) 2 ]·3DMF·4H 2 O} n ( LCU-129 ) and {[Cd 3 (L) 2 (btzyb) 2 (H 2 O) 2 ]·DMF·2H 2 O} n ( LCU-130 ), were synthesized from 3-(3,5-dicarboxylatobenzyloxy)benzoic acid (H 3 L) and two N-containing ligands (bpod (2,5-bis(4′-pridyl)-1,3,4-oxadiazole) and btzyb (1,4-bis(triazol-1-yl)benzene)). Two MOFs displayed 3D porous and 2D → 3D penetrating structures, respectively. Luminescence studies have shown that LCU-129 and LCU-130 exhibited significant luminescence enhancement/quenching in 5-HT/5-HIAA, respectively, without the interference of other neurotransmitters, glutamic acid (Glu) and aspartic acid (Asp). The limit of detection (LOD) values of LCU-129 toward 5-HT/5-HIAA were 0.06 and 0.19 μM and those of LCU-130 toward 5-HT/5-HIAA were 1.17 and 2.23 μM, respectively. Meanwhile, the rich nanoporous and oxadiazole rings within LCU-129 provided binding sites for metal ions, which can specifically recognize Al 3+ through a luminescence enhancement mechanism (with the LOD as low as 0.77 μM), reflecting the influence of the functional groups within frameworks. The recovery rate experiments showed that LCU-129 and LCU-130 could be used as fluorescent probes for the selective detection of biomarkers 5-HT/5-HIAA and Al 3+ simultaneously.