Obtaining high efficiency room temperature phosphorescence (RTP) by employing non-noble metals poses two challenges: (1) strengthening spin-orbit coupling of excitons to improve the rate of intersystem crossing (ISC) by using non-noble metals with small-atomic-number; (2) employing structural confinement to enhance radiation relaxation because harsh conditions, including carefully selected matrices, rigid solid-state crystalline structure and low temperature, are commonly needed. Here, layered double hydroxides (LDHs) with orderly non-noble metal arrangements were used as an inorganic matrix to activate RTP of carbon dots (CDs). The Zn orderly arranged on the LDH layer contributes to the enhancement in spin-orbit coupling of excitons and the decrease in the energy gap for the singlet-triplet state. The structural confinements of the LDH layer and nano-interlayer testify that the phosphorescence of CDs-LDHs originates from the suppressed radiationless relaxation processes. Using the high tunability of metal species and ratios on the LDH layer, this method can be widely applied to optimize ISC and phosphorescence properties.

Through the powerful hydrothermal method, five rare-earth (Re = Dy, Gd, Ho, Pr, and Sm) three-dimensional (3D) cluster-based metal–organic frameworks (MOFs) have been synthesized, namely, [Dy­(L)­(H2O)­(DMF)] n (1), {[Gd­(L)­(H2O)­(DMF)]·DMF} n (2), {[Ho­(L)­(H2O)­(DMF)]·0.5DMF} n (3), {[Pr­(L)­(H2O)­(DMF)]·0.5DMF} n (4), and {[Sm­(L)­(H2O)1.55(DMF)0.45]·DMF} n (5; H3L = terphenyl-3,4″,5-tricarboxylic acid), which have been determined by single crystal X-ray analyses and PXRD characterization. Structural analyses reveal that, in 1–5, these L3– ligands are linked by five different rare-earth centers, forming the iso-structural nanoporous frameworks. PXRD patterns of bulky samples 1–5 also are consistent with theoretical PXRD patterns confirming their purity. Solid state photoluminesce of free H3L and 1–5 at room temperature also has been investigated indicating strong ligand-based emissions. Besides these, fluorescent dye Rhodamine B (RhB) can be introduced into MOF1 forming the composite material RhB@MOF1 with a high quantum yield of 35%. It is noted that, through deliberately tuning the morphologies of nanoparticle MOF1 under different ultrasonic conditions, RhB@MOF1 can be utilized as the first ratiometric fluorescent sensor to effectively discriminate l- and d-lysine from other amino acid molecules with high K sv values and low LOD values. On the other hand, 2 was for the first time to be utilized as an excellent bifunctional MOFs-based sensing platform to detect insulin and Al3+ with a low detection limit in the human serum solution.

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