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

1
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 USA bPhysics
Department, Davidson College, Davidson, NC 28036
USA
Abstract
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 90C) and
annealed SiO2 doped glasses (heated to 900C)
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.
Sample quality

• 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

Terbium results
5D0?7F2 Fluorescence Decays of chelated Eu3 dry
gels
Fluorescence Decays of Annealed glasses
t2.3ms
Eu2O3 powder Eu(PDC) glass Eu/Al glass
ln (fluorescence)
Fluorescence (arb units)
t0.18ms
t ms
Wavelength (nm)

• 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
  5D0?7F2 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.

5D4?7F5 Fluorescence Decays
Excitation spectra (monitoring 5D4?7F5 )
Fluorescence (arb units)
Chelate absorption edge
t2.1ms
ln (fluorescence)
4f8 ? 4f75d1
t1.3ms
t0.83ms
t ms
Wavelength nm
Experimental Setup

• 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 4f8 ? 4f75d1 excitation
  line in the chelated gels.

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.

Corresponding author Dr. Ann Silversmith
Physics Department, Hamilton College 198 College
Hill Rd. Clinton, NY 13323 asilvers_at_hamilton.edu
This work sponsored in part by the Research
Corporation through a Cottrell College Science
Award.
Further Investigation

• 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 REs.
• Synthesis with increased RE concentration.