Synthesis of water-soluble CdSe quantum dots by ligand exchange with p-sulfonatocalix(n)arene (n= 4, 6) as fluorescent probes for amino acids

A simple, rapid ligand exchange route to prepare highly fluorescent, stable and water-soluble CdSe quantum dots (QDs) is reported by using p-sulfonatocalix(n)arene (SFCA(n), n = 4 or 6) instead of original trioctylphosphine oxide (TOPO) ligands. The ligands were found to have a profound effect on the luminescence response of QDs to amino acids. The SFCA(4) coated CdSe QDs were sensitive to methionine. On the other hand, SFCA(6) coated CdSe QDs turned out to be sensitive to phenylalanine. To demonstrate the detection capability of these new probes, SFCA(n) (n = 4, 6) coated CdSe QDs were used to detect methionine and phenylalanine in physiological buffer solution. Under optimal conditions, the relative fluorescence intensities of the SFCA(n) coated QDs (n = 4, 6) increased linearly with increasing concentration of amino acids. Methionine and phenylalanine enhanced the luminescence of SFCA(n) coated QDs with a concentration dependence that was best described by a Langmuir-type equation. The possible mechanism is also discussed.