Design, Synthesis And Study Of Fluorescent Probes Based On Rhodamine Derivatives For Metal Ions

Detection of metal ions is very important in the field of environmental protection, biochemistry and analytic chemistry. Fluorescent probe has many advantages, such as high sensitivity, simple operation, low cost, real time monitoring of metal ions, not requiring sophisticated instrumentation and sample preparation and so on. Therefore, the fluorescent probe becomes a powerful means for detection of metal ions. Rhodamine are dyes used extensively as fluorescent labeling reagents and dye laser sources because of excellent spectroscopic properties, such as long absorption and emission wavelengths, high fluorescence quantum yield, large extinction coefficient, and high stability against light. Moreover, it is well known that many derivatives of rhodamine undergo equilibrium between the spirocyclic and ring-open forms, and the two forms always exhibit completely different fluorescent properties. The spirocyclic (closed-ring) form is basically colorless and non-fluorescent (turn-off), while the ring-open form gives rise to strong absorption and fluorescence emission. Therefore, many compounds based on rhodamine skeleton have been used for developing fluorescent probe.In this paper, we designed and synthesized three rhodamine derivatives L1-L3as fluorescent probes for metal ions. The fluorescent probe L1had high selectivity and sensitivity for Hg2+.The probe L2and L3were used for detecting Fe3+.We synthesized the fluorescent probe L1by combining the rhodamine B hydrazide and coumarin-3-isothiocyanate. In CH3OH/Tris-HCl (0.010mol-L"1, v: v=8:2, pH=7.00) buffer solution, the fluorescent probe L1exhibited a high selectivity and sensitivity for Hg2+. After adding Hg2+to the solution of L1, the probe L1showed a UV-vis absorption at about560nm, and the color of the solution changed from colorless to dark pink. A remarkable enhancement of fluorescent intensity at592nm for L1with Hg2+was detected. The stoichiometry of the probe L1and Hg2+was1:1, which had been proved by the Benesi-Hildebrand plots and the Job’s method. The binding constant was3.04×104L·mol-1. The limit of detection of L1towards Hg2+was7.67×10-6mo1·L-1.The fluorescent probe L2was synthesized by the condensation of rhodamine6G hydrazide and glyoxal. In the CH3CN/Tris-HCl (0.010mol·L-1, v:v=9:1, pH=7.00) buffer solution, the probe L2showed high selectivity and sensitivity to Fe3+. Upon the addition of Fe3+, the solution of L2exhibited a strong UV-vis absorption at530nm, and the color of the solution changed from colorless to orange. With Fe3+, the probe L2showed a remarkable enhancement of fluorescence intensity at561nm. According to the Benesi-Hildebrand equation, the Fe3+ion and L2showed a2:1stoichiometry, the binding constant between the probe L2and Fe3+was1.4×1010L2·mol-2. The [Fe3+]-2had a good linear relationship(R=0.9957) to the fluorescence intensity between3.33×10-6mol·L-1to2.0×10-4mol·L-1.The linear equation was Y=0.00281+X*0.00195. The limit of detection of L2to Fe3+was2.3×10-6mol·L-1We constructed a fluorescent ratiometric probe L3.The probe L3was designed on the basis of L2, which was synthesized by the combination of2-(quinolin-8-yloxy)acetohydrazide and L2. Continuous addition of Fe3+to the CH3OH/Tris-HCl (0.010mol·L-1,v:v=1:1, pH=7.00) buffer solution of L3, the fluorescent intensity of L3at433nm decreased gradually, meanwhile, the fluorescent intensity of L3at561nm increased gradually. Therefore, the probe L3was an excellent fluorescent ratiometric probe for Fe3+.