Luminescent Colloidal Silica

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Luminescent Colloidal Silica

• Tom Schmedake
• Department of Chemistry
• University of North Carolina at Charlotte
• Charlotte, NC 28223

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Colloidal Silica
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Stöber type Sphere Preparation

Typical Recipe (250 nm spheres) Ethanol solvent
(200 mL scale) 2.0 M H2O 1.0 M NH3 0.17 M
TEOS 8 hour stirring _at_ 250C Properties Nonporo
us 80nm 1000nm range of particle sizes
Monodispersed (down to 2 RSD)
Stober, Fink, and Bohn. J. Colloid Interface
Sci., 1968, 26, 62. Bogush G.H., et. al. J. of
Non-Crystalline Solids 1988, 104, 95.
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Sol-gel chemistry
Hydrolysis
Condensation

• Initial particles aggregate until they reach
  colloidal stability
• Particles then grow spherically to minimize free
  surface energy
• Particles stop growing when they again reach
  colloidal stability

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Multi-layer growth and self-sharpening
1 Shell 650nm (20kx)
2 Shells 690nm (20kx)
Core 450nm (20kx)

• Self-sharpening growth leads to lower
  polydispersity
• Also allows fluorescence doping

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Making silica spheres luminescent

• Hydrophilic dyes can sometimes be incorporated
  during growth
• Covalent attachment via silylation chemistry
  then addition of more shells
• Dye incorporation usually prevents calcination

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Fluorescent core-shell silica particles
Ulrich Weisner, Chem. Soc. Rev., 2006, 35,
10281042
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Ulrich Weisner, Chem. Soc. Rev., 2006, 35,
10281042
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Hydrodynamic radius Dye - 1 nm Core - 2
nm Core-shell 15 nm
Ulrich Weisner, Chem. Soc. Rev., 2006, 35,
10281042
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Ulrich Weisner, Chem. Soc. Rev., 2006, 35,
10281042
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Ulrich Weisner, Chem. Soc. Rev., 2006, 35,
10281042
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Ulrich Weisner, Chem. Soc. Rev., 2006, 35,
10281042
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Sensing with colloidal silica PEBBLES

• PEBBLES R. Kopelman (Univ. Mich.)
• sol-gel, ormosil, and polymer spheres filled
  with analyte specific dye
• can be used as intracellular nano-biosensors

Xu, H. , et.al., Anal. Chem. 2001, 73, 4124.
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Mesoporous colloidal silica as sensor substrate

• Spheres doped with Ru(bipy)32 through ion
  exchange process
• Ru(bipy)32 emits 615 nm
• Emission is quenched by O2

Quenching is fast and reversible
A. M. Jakob, R. Hudgins, M. El-Kouedi, T. A.
Schmedake Submitted, 2006.
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Applications of Luminescent Colloidal Silica
1. Fluorescence detection of a single E-coli
3. Photonic Crystals / Colloidal Crystals / Opals
X. J. Zhao, et. al., Proc. of the Nat. Acad. of
Sci., 2004, 101, 15027.
2. Blood flow monitoring
V. L. Colvin, Adv. Mat., 2001, 13, 389.
See review L. Wang, K. M. Wang, S. Santra, X. J.
Zhao, L. R. Hilliard, J. E. Smith, J. R. Wu and
W. H. Tan, Anal. Chem., 2006, 78, 646.
Y. Chan, J. P. Zimmer, M. Stroh, J. S. Steckel,
R. K. Jain and M. G. Bawendi, Adv. Mat., 2004,
16, 2092.
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Immunoassays
L. Wang, et. al. , Anal. Chem., 2006, 78, 646.
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Immunoassays
L. Wang, et. al. , Anal. Chem., 2006, 78, 646.
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J.N. Anker and R. Kopelman. "Magnetically
modulated optical nanoprobes," Appl. Phys. Lett. 
82, 1102 (2003).
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Aminopropylsilica Sphere Preparation

Properties Same as SFB silica Aminopropyl groups
are spread throughout the silica network Positive
surface charge Modified SFB procedure Ethanol
solvent 3.0 M H2O 0.5 M NH3 0.17 M
TEOS Co-condensation of APTES Stir for 3 hours _at_
250C
Aminoproplytriethoxysilane (APTES)
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A. M. Jakob and T. A. Schmedake, Chem. Mater.,
2006, 18, 3173-3175.
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Tailored properties
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Luminescence
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Quantum Yield
Emission spectra of separate ethanol solutions
with the same absorbance Quantum yield was found
to be as high as 12 POPOP 2-2-(1,4-phenylene
)bis5-phenyloxazole
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Long-lifetime Photoluminescence

• Spheres calcined over 500oC exhibit long lifetime
  photoluminescence visible for more than 10
  seconds at room temperature.
• The long lifetime photoluminescence decay was
  modeled using multi-exponential decay function
  combining a simple decay term and a stretched
  exponential decay term.

Soriano, R. B., Schmedake T. A., et al., Appl.
Phys. Lett ., 2007, 91, 1-3
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The stretching parameter

• The ß parameter can result from dispersive
  transport of the photoexcited electrons and/or
  holes in the solid due to either
• Multiple trapping and detrapping carriers
• Hopping or tunnelling carriers

Soriano, R. B., Schmedake T. A., et al., Appl.
Phys. Lett ., 2007 91, 1-3
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Temperature dependence studies

• The ß parameter is temperature independent over
  the entire temperature range studied which is
  indicative of a hopping mechanism
• The simple exponential decay term appears to be
  temperature independent at least until 400K

Soriano, R. B., Schmedake T. A., et al., Appl.
Phys. Lett ., 2007, 91, 1-3
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Temperature dependence studies cont

• For single exponential decay, average lifetime
  (t) is
• t 1/k
• For stretched exponential decay, k is time
  dependent, average lifetime lttsgt is

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Temperature dependence studies cont

• The stretched exponential component is more
  susceptible to competing pathways

Soriano, R. B., Schmedake T. A., et al., Appl.
Phys. Lett ., 2007, 91, 1-3
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Potential Applications- Imaging/Bioassays

• Advantages of dye-doped silica particles over
  traditional fluorophores
• Magnified signal for increased sensitivity
• Easy conjugation to biomolecules
• Increased photostability
• Decreased sensitivity to environment
• Advantages of luminescent silica particles
  fabricated with APTES compared to other dye-doped
  particles
• stable at very high temperatures
• Long lifetimes allows gating of short lifetime
  interferences
• Low probable cytotoxicity

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Acknowledgements
Robert Hudgins Adam Jakob Essoyodou
Kpatcha Jasmine Gregory Ronald Sorianno
Funding UNC-Charlotte Junior Faculty Research
Grants Research Corporation Cottrell College
Award DARPA / ARL
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Radiative Processes in Dielectric Spheres
The rates of radiative processes can be
controlled by altering the electromagnetic vacuum
field to which the oscillating dipole is coupled
(e.g. via a dielectric boundary).

2 mm
H. Schniepp and V. Sandoghdar, Phys. Rev.Lett.
25, 257403-1 (2002).