1. Water-Soluble, Monolayer-Protected Quantum Dots
Joseph A. Giesen, Elizabeth M. Henry, April D. Dale, Adrienne C. Borchardt, and Deon T. Miles  Department of Chemistry
Acknowledgments
Collaborators: Dr. Ngee-Sing Chong (MTSU). Michael C. Leopold (U. Richmond). and Kevin W. Kittredge (Siena).
Faculty Research Grants, Academic Initiatives, Conduff Scientific Grants, and Croom Beatty Chemistry Research Internship
Project Overview
Monolayer-protected quantum dots (QDs) were synthesized in aqueous solutions. Several water-soluble thiols were used to protect the semiconducting core from surface oxidation and to improve the stability of the QDs. Thiolated crown ether molecules were place-exchanged onto the surface of QDs, and the resulting changes in spectral properties were observed. Knowledge of the quantum efficiency of the QDs is important prior to any potential use in sensing applications. A thorough study of the quantum yield of the QDs as a function of heating time is underway. The synthesis of PbSe versions of these water-soluble QDs have resulted in measured success. The first attempts to characterize these materials spectroscopically are presented here.
Water-Soluble Thiols
Prof. Miles
Joseph
April
QDs illuminated with Ambient Light
QDs illuminated with UV Light (λEXC = 365 nm)
References
Gaponik, N. et al. J. Phys. Chem. B 2002, 106,
Lin, S.-Y. et al. Anal. Chem. 2002, 74,
Grabolle, M. et al. Anal. Chem. 2009, 81,
Elizabeth
Adrienne
Heating Time
Wavelength (nm)
2.5
2.0
1.5
1.0
0.5
0 min
2 min.
4 min
16 min
6 min
8 min
20 min
In Situ Spectral Acquisition
Relative Quantum Yield Equation
Quantum Yield Determination3
Summary
The quantum yields of Fluorescein and Rhodamine 6G are used as the standards.
Fluorescence and ultraviolet-visible spectrophotometry are used to find absorbance and integrated intensity.
Greener Synthesis of QDs1
Reaction Mixture
Metal perchlorate: Cd(ClO4)2·H2O, Zn(ClO4)2·6H2O or Pb(ClO4)2·2H2O in Type 1 water.
One of eight thiolated ligands (shown above).
Typical ligand-to-metal ratio is 2.4:1.
Procedure
The pH is adjusted to ≥ 11 using 1 M NaOH while stirring.
Mixture was deaerated for ~30 minutes with N2.
NaHSe solution is added to reaction mixture.
Mixture was allowed to reflux over a period of time.
Aliquots were collected based on visible change in color.
Place-Exchange with Crown-Ether Thiols
Complexation of CE-modified QDs2
R= 15-crown-5 ether
Impact of Heating on Quantum Yield
= YES
= NO
Synthesis of Water-Soluble PbSe QDs
The synthesis of PbSe QDs has been a recent challenge in our laboratory.
Different thiol:metal ratios were used to synthesize a series of water-soluble, monolayer-protected PbSe QDs.
Small emission peak (in infrared region) is observed in fluorescence spectroscopy of successfully-prepared materials.
2.4:1 TIO-PbSe
λEXC = 250 nm
λMAX = 823 nm
ФX = Quantum yield of QDs
ФST = Quantum yield of standard
Grad = Slope of integrated
intensities from the plot
η = Refractive index
Effect of Added K+ to CE-modified QDs
Several CdSe QDs were place-exchanged with thiolated crown ether molecules.
Potential changes in the spectral properties of the crown ether-modified QDs (with and without the addition of metal ions) were monitored using fluorescence spectroscopy.
PLACE-EXCHANGE REACTION
Thiol:Metal Ratio
Successful Synthesis?
2.4:1 TIO
2.4:1 GLU
2.4:1 TGL
4.8:1 TIO
4.8:1 GLU
1.2:1 GLU
2.4:1 TGA
2.4:1 MPA