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FLUORESCENCE ENHANCEMENT BY CHELATION OF Eu3+ AND Tb3+ IONS IN SOL GELS

A. J. Silversmitha A. P. Magyara, K.S. Brewera, and D.M. Boyeb

aPhysics Department, Hamilton College, Clinton, NY 13323 USAbPhysics Department, Davidson College, Davidson, NC 28036 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 SiO2 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 College198 College Hill Rd. Clinton, NY 13323asilvers@hamilton.edu

This work sponsored in part by the Research Corporation through a Cottrell College Science Award.

RE3+

RE3+

Argon Laser

PMT

Dye Laser

Monochromator

Ammeter

Oscilloscope

Computer withDataLogger 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-gelTerbium behavior mirrors that of Europium, with the addition of the broad 4f8 4f75d1

excitation line in the chelated gels.

All syntheses form optically clear gelsPDC gels crack and turn powdery after several weeks, but storage with a dessicant helpsPDC gels dissintegrate when annealingPPA 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

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5D07F2 Fluorescence Decays of chelated Eu3+ dry gels

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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 clusteringOH- vibrations

Pr,Nd,Er,Eu-doped sol-gel glasses:

Sol-gel Glass versus Melt Glass

Sol-Gel Recipes

“Standard” RE3+-Doped Sol Gel Synthesisdissolve europium nitrate and aluminum nitrate in water

acidify solution with concentrated HCladd silica precursor, tetramethylorthosilicate (TMOS)mix until homogeneous

Crystalline Chelate RE3+-Doped Sol Gel Synthesissuspend pyridinedicarboxylic acid (PDC) in water and heat to boilingadd 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 wateradjust pH to 4 using concentrated HCladd TMOS and stir until homogeneous

In Situ Chelate RE3+-Doped Sol Gel Synthesisdissolve 3-pyridinepropionic acid (PPA) in water add RE nitrate salt for RE3+:PPA 1:3 molar ratioreduce volume of solution by half with gentle boiling (20 minutes) cool solution and adjust pH to 4 using concentrated HCladd TMOS and stir until homogeneousProcessing of Solscast sols into polypropylene test tubes and cap tightlyheat until gelled at 40 ˚Cage gels 60 ˚C for 24 h, then remove test tube capsheat at 60 ˚C for an additional 24 hincrease temperature to 90 ˚C and age gels for 48 hanneal in air

Discussion Strong Eu3+ 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 5D07F2 line in Eu2O3.Fluorescence decay time in Eu(PPA) gels is longer than in Eu/Al gels, indicating partial association of the chelate and Eu3+. The absence of the excitation band for wavelengths below 300nm implies little energy transfer from chelate to Eu3+. 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.

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Eu(PDC) gel fluorescence ex=254nm

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Eu(PDC) gel excitation (monitoring 5D07F2)

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Europium Results

Eu(PPA) gel fluorescence ex=254nm

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