FLUORESCENCE ENHANCEMENT BY CHELATION OF Eu 3+ AND Tb 3+ IONS IN SOL GELS A. J. Silversmith a A. P. Magyar a, K.S. Brewer a, and D.M. Boye b a Physics Department, Hamilton College, Clinton, NY USA b Physics Department, Davidson College, Davidson, NC USA Chelation of rare earth (RE) ions has been used for many years as a way of enhancing the optical excitation of the ions in solution. The chelating molecules, which absorb strongly in the near uv, bind to the RE ion. Optical excitation of the chelate followed by efficient energy transfer to the RE results in visible fluorescence. In this work we incorporated the chelate-RE complex into sol-gels made with the organic precursor tetramethoxysilane (TMOS). Two chelating agents - 2,6-pyridine-dicarboxylic acid (PDC) and 3- pyridinepropionic acid (PPA) - and two different synthesis techniques are used. Optical properties of the dried gels (heated to 90˚C) and annealed SiO 2 doped glasses (heated to 900˚C) were studied to determine firstly, whether the chelate/RE complex remained intact after incorporation into the gel and secondly, whether the optical properties of the annealed glasses differed from those of glasses synthesized without chelation. In addition to studying energy transfer between the chelate molecule and the RE, we investigated whether incorporation of the chelate reduced fluorescence quenching due to residual OH - in the glass – a common problem in RE doped sol-gel glasses. Abstract Conclusions  PDC synthesis is effective at isolating the RE within the sol-gel. The synthesis results in enhanced excitation efficiency and reduced fluorescence quenching, resulting in intense red (Eu) or green (Tb) fluorescence under uv excitation.  In-situ synthesis with PPA does not result in fully chelated RE ions in gels. Experimental Setup Corresponding author: Dr. Ann Silversmith Physics Department, Hamilton College 198 College Hill Rd. Clinton, NY This work sponsored in part by the Research Corporation through a Cottrell College Science Award. RE 3+ Argon Laser PMT Dye Laser Monochromator Ammeter Oscilloscope Computer with DataLogger or Labview software aom Hg lamp Sample quality Discussion  Very bright green emission from Tb(PDC) dry gels under 254nm excitation  Tb(PDC) complex remains intact in sol-gel  Terbium behavior mirrors that of Europium, with the addition of the broad 4f 8  4f 7 5d 1 excitation line in the chelated gels.  All syntheses form optically clear gels  PDC gels crack and turn powdery after several weeks, but storage with a dessicant helps  PDC gels dissintegrate when annealing  PPA gels retain good optical clarity upon annealing  Other chelating agents, in particular a longer bidentate to replace PPA in the in-situ synthesis.  Adjustment of annealing conditions to improve quality of PDC annealed glasses.  Fabrication of thin films with chelated RE’s.  Synthesis with increased RE concentration. Further Investigation t [ms] ln (fluorescence) 5 D 0  7 F 2 Fluorescence Decays of chelated Eu 3+ dry gels Eu/Al “standard” gel Eu(PPA)3 gel Eu(PPA)6 gel Eu(PDC)3 gel Eu(PDC) crystal 5D05D0 Eu x10 3 cm -1 Energy 7F07F0 7F27F2 7F17F1 5D15D1 5D25D2 5D35D3 Fluorescence Decays of Annealed glasses Fluorescence (arb units) Wavelength (nm) Eu 2 O 3 powder Eu(PDC) glass Eu/Al glass Terbium results t [ms] 5 D 4  7 F 5 Fluorescence Decays Tb/Al gel Tb(PPA) gel Tb(PDC)3 gel Excitation spectra (monitoring 5 D 4  7 F 5 ) Wavelength [nm] Tb 3+ 7F67F6 7F07F0 7F57F5 5D45D4 5D35D x10 3 cm -1 Energ y 4f 8  4f 7 5d 1 Advantages Easy to vary recipe Versatile form Monoliths of good size and clarity Lower temperature processing Higher concentration of RE’s Disadvantages Fluorescence quenching due to: Lanthanide clustering OH - vibrations Pr,Nd,Er,Eu-doped sol-gel glasses: Sol-gel Glass versus Melt Glass Sol-Gel Recipes “Standard” RE 3+ -Doped Sol Gel Synthesis  dissolve europium nitrate and aluminum nitrate in water  acidify solution with concentrated HCl  add silica precursor, tetramethylorthosilicate (TMOS)  mix until homogeneous Crystalline Chelate RE 3+ -Doped Sol Gel Synthesis  suspend pyridinedicarboxylic acid (PDC) in water and heat to boiling  add rare earth nitrate salt as a solution in water  cool solution and adjust pH to 8 using 2 M sodium hydroxide  crystallize the rare earth chelate by slow evaporation  dissolve crystals in water  adjust pH to 4 using concentrated HCl  add TMOS and stir until homogeneous In Situ Chelate RE 3+ -Doped Sol Gel Synthesis  dissolve 3-pyridinepropionic acid (PPA) in water  add RE nitrate salt for RE 3+ :PPA 1:3 molar ratio  reduce volume of solution by half with gentle boiling (20 minutes)  cool solution and adjust pH to 4 using concentrated HCl  add TMOS and stir until homogeneous Processing of Sols  cast sols into polypropylene test tubes and cap tightly  heat until gelled at 40 ˚Cage gels 60 ˚C for 24 h, then remove test tube caps  heat at 60 ˚C for an additional 24 h  increase temperature to 90 ˚C and age gels for 48 h  anneal in air  Discussion  Strong Eu 3+ excitation band that correlates with the PDC absorption indicates that the Eu(PDC) association remains complete – after incorporation into the gel.  Long fluorescence lifetime of Eu(PDC) gel offers further evidence that the chelation is complete.  Eu(PDC) samples degrade and are partially opaque after annealing. The fluorescence spectrum has a peak at 611nm, which coincides with the strongest 5 D 0  7 F 2 line in Eu 2 O 3.  Fluorescence decay time in Eu(PPA) gels is longer than in Eu/Al gels, indicating partial association of the chelate and Eu 3+. The absence of the excitation band for wavelengths below 300nm implies little energy transfer from chelate to Eu 3+. The bi-dentate PPA is short and may not be able to bond at two sites.  The decay time from Eu(PPA)6 is longer than from Eu(PPA)3 and shorter than Eu(PDC). Further evidence chelation is incomplete in the PPA gels.  Incomplete chelation with PPA may be due to the physical size of the molecule - the PPA (bidentate) is a relatively short molecule and may not be long enough to grab on to the RE in two places.  The crystalline chelate synthesis ensures that the RE is completely associated; the in-situ technique is a “stir- and-hope” approach. Eu(PDC) Eu(PPA) Eu(PDC) gel fluorescence ex =254nm Eu(PDC) xtal Eu(PDC) gel excitation (monitoring 5 D 0  7 F 2 ) PDC absorption Fluorescence (arb units) wavelength(nm) Fluorescnece/absorbance (arb) Fluorescence (arb units) Eu/Al gel wavelength(nm) Fluorescnece/absorbance (arb) Eu(PPA) gel excitation (monitoring 5 D 0  7 F 2 ) 7F0 5DJ7F0 5DJ 7F0 5DJ7F0 5DJ Europium Results Eu(PPA) gel fluorescence ex =254nm x20  =0.18ms  =2.3ms  =2.1ms  =1.3ms  =0.83ms ln (fluorescence) Tb/Al “standard” gel Tb(PPA)3 gel TbPDC)3 gel Tb(PDC) crystal Fluorescence (arb units) Chelate absorption edge Fluorescence (arb units)