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Upconversion nanoparticles (UCNPs) are widely used as energy donors via a luminescence resonance energy transfer (LRET) technique in biosensing field. To enhance LRET efficiency and afford an improved sensitivity for Cu2+ detection, sandwich structured NaYF4@NaYF4:Er3+/Yb3+@NaYF4 UCNPs have been designed and prepared. For the core-internal shell-external shell structure, luminescence ions (Er3+) are located in the internal shell near the nanoparticle surface, which is close enough to energy acceptors outside. To study the sensing properties, the UCNPs are used as energy donors and rhodamine B hydrazide (RBH) as energy acceptors to construct the nanoprobes. It is found that the sandwich structured nanoparticles gains more intense luminescence than the naked NaYF4:Er3+/Yb3+ UCNPs, which is beneficial to the further improvement of signal sensitivity in biological detection. Moreover, the specificity for detecting Cu2+ is also checked and found that the designed probe is much more sensitive to Cu2+ than the other ions including K+, Na+, Li+, Ca2+, Ba2+, Mg2+, Zn2+ and Sr2+. The experiment on the stability of UCNPs-RBH probe and RBH shows the 980 nm laser produces less photodamage to RBH than that of visible light. Therefore, the UCNPs can be used to construct potential probes for realizing effective detection of Cu2+.

A bifunctional colorimetric and fluorescent chemosensor RbCS has been successfully designed and synthesized by simple condensation of 3,5-dichlorosalicylaldehyde and rhodamine B hydrazide. In aqueous media, chemosensor RbCS exhibits remarkably enhanced absorbance intensity and color change from colorless to pink for Cu 2+ , and shows significant “off–on” fluorescence accompanied with color changes from colorless to red upon binding to Hg 2+ . The absorbance and fluorescence signals of RbCS can be restored with addition of EDTA disodium into solutions of RbCS–Cu 2+ and RbCS–Hg 2+ , showing that the binding of chemosensor RbCS and Cu 2+ /Hg 2+ is chemically reversible.

A new rhodamine B derivative colorimetric and fluorescent sensor (1) was synthesized by condensation reaction of rhodamine B hydrazide and 2,4-dihydroxybenzaldehyde, which showed reversible and highly selective and sensitive recognition toward Cu2+ over other examined metal ions. Upon addition of Cu2+, sensor 1 exhibits remarkably enhanced absorbance intensity and color change from colorless to pink in DMSO and MeCN aqueous buffer solution or pure MeCN, and shows significant off–on fluorescence accompanied by color changes from colorless to orange in MeCN. The sensor 1 was also successfully applied to the determination of Cu2+ in water samples.