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Theoretically, the two aldehydes of terephthalaldehyde (TPA) are equivalent, so the single or double Schiff base from TPA and d -glucosamine (Glc) may be formed at the same time. However, it is preferred to produce separately a single Schiff base ( L 1 ) or double Schiff base ( L 2 ) for different synthesis systems of anhydrous methanol or water–methanol. We calculated the Δ r G of the formation of compounds L 1 and L 2 by density functional theory (DFT). In an anhydrous methanol system, the Δ r G values of L 1 and L 2 are both below zero and L 2 is lower, suggesting the spontaneous formation of the two Schiff bases. Though adjusting the molar ratio of Glc to TPA, L 1 and L 2 both were separately formed in anhydrous methanol. However, in the water–methanol system, L 2 was absent, which is most likely due to higher Δ r G (4.95 eV) and better water solubility. The results also exhibits that the positive charge of C in −CHO for TPA is smaller in a mixed solvent than that in methanol, which confirms that the nucleophilic reaction of the Schiff base is more difficult in a mixed solvent. Therefore, we could realize to control the synthesis of a pure single or double Schiff base from Glc and TPA by adjusting the molar ratio and solvent. The as-prepared two kinds of Schiff bases have strong optical properties, high bacteriostatic activity, and can be used as fluorescent probes for tumor cell imaging.

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Based on the mechanism of luminescence resonance energy transfer (LRET) and using a special single strand DNA as the recognition element, a portable paper-based sensor for the accurate detection of total heavy rare-earth ions (mainly Gd3+, Tb3+ and Dy3+) concentration was proposed. The RNA cleaving-DNAzyme should recognize rare-earth ions to cleave RNA on DNA duplexes linking UCNPs and AuNPs, causing UCNPs and AuNPs to approach each other, inducing LRET, which attenuated the green upconversion luminescence (UCL) triggered by the 980 nm laser. UCL was captured by a charge-coupled device (CCD) image sensor and processed with the red-green-blue (RGB) image to quantitatively analyze heavy rare-earth ions in the samples. In the range of 5–50 μmol·L-1, the sensor has good sensitivity, with the limit of detection of 1.26 μmol L−1.