RE3+
FLUORESCENCE ENHANCEMENT BY
3+
3+
CHELATION OF Eu AND Tb IONS IN SOL GELS
RE3+
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
Sol-gel Glass versus Melt Glass
Abstract
Advantages
• Easy
to vary recipe
•Versatile form
• Monoliths of good size and
clarity
• Lower temperature processing
•Higher concentration of RE’s
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 3pyridinepropionic 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.
Pr,Nd,Er,Eu-doped sol-gel glasses:
Sol-Gel Recipes
Europium Results
Eu(PDC)
5D
2
20
10
5
0
7F
2
7F
1
7F
0
585
595
Fluorescnece/absorbance (arb)
5D
0
15
x20
575
5D
1
Eu(PDC) xtal
605
615
625
560
Eu(PDC) gel excitation
(monitoring 5D07F2)
PDC absorption
7F
200
300
400
0
5 D J
500
Eu/Al gel
580
600
200
640
Eu(PPA) gel excitation
(monitoring 5D07F2)
7F
300
400
0
5 D J
500
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
600
Terbium results
wavelength(nm)
7F Fluorescence Decays

0
2
of chelated Eu3+ dry gels
Tb3+
x103cm-1
Fluorescence Decays of
Annealed glasses
25
0
5D
3
-3
t=0.18ms
-4
Eu2O3 powder
Eu(PDC) glass
15
Eu/Al glass
10
-5
7F
0.5
1
1.5
2
560
t [ms]
580
600
620
Tb(PPA)3 gel
TbPDC)3 gel
Tb(PDC) crystal
5
640
Wavelength (nm)
0
7F
5
7F
6
450
500
550
600
650
5D 7F Fluorescence
4
5
Excitation spectra
(monitoring 5D47F5 )
Fluorescence (arb units)
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 “stirand-hope” approach.
Tb/Al “standard” gel
0
Chelate
absorption edge
0
Fluorescence (arb units)
-2
20
5D
4
Energ
y
Eu/Al “standard” gel
Eu(PPA)3 gel
Eu(PPA)6 gel
Eu(PDC)3 gel
Eu(PDC) crystal
Fluorescence (arb units)
t=2.3ms
-1
ln (fluorescence)
620
In Situ Chelate RE3+-Doped Sol Gel Synthesis
dissolve 3-pyridinepropionic acid (PPA) in water
add RE nitrate salt for RE3+: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
Sample quality
wavelength(nm)
5D
600
“Standard” RE3+-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 RE3+-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
Decays
0
-0.5
Tb/Al gel
Tb(PPA) gel
ln (fluorescence)
5D
3
Fluorescence (arb units)
25
Eu(PPA) gel fluorescence
lex=254nm
Fluorescnece/absorbance (arb)
x103cm-1
Energy
Fluorescence (arb units)
Eu3+
Eu(PPA)
Eu(PDC) gel fluorescence
lex=254nm
Disadvantages
•Fluorescence quenching due to:
Lanthanide clustering
OH- vibrations
Tb(PDC)3 gel
4f8  4f75d1
-1
t=2.1ms
-1.5
-2
-2.5
t=1.3ms
t=0.83ms
-3
-3.5
230
280
330
380
430
480
530
0
1
2
3
4
5
t [ms]
Wavelength [nm]
Experimental Setup
Discussion
Argon Laser
Dye Laser
aom
Hg lamp
 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
Ammeter
Computer with
DataLogger or Labview
software
PMT
Monochromator
 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.
Oscilloscope
 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@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 RE’s.
 Synthesis with increased RE concentration.