Energy Transfer of Fluorescent CdSe/ZnS Quantum
Dots and Gold Nanoparticles and Its Applications for
Mercuric (II) Ion Detection
By Ming Li and Nianqiang Wu
WVNano Initiative, Department of Mechanical and
Aerospace Engineering
at West Virginia University
Outline
 Background and Objectives
 Sensor Design
 Size Effect on Energy Transfer
 Mercuric (II) Detection
 Conclusions
Background
 Förster resonance energy transfer (FRET) is a process that occurs between
an excited molecular donor and a molecular acceptor;
Bio/environmental
monitoring
Therapeutics/diagno
stics
Energy
Transfer
Photovoltaic devices
Light emitters
The rate of FRET energy transfer strongly depends on the spectral overlap of the
emission spectrum of the donor with the absorption spectrum of the acceptor .
 Acceptors include conventional organic dyes such as
,
Fluorescein-5-isothiocyanate
(FITC),
,
Cy3
,
etc.
Cy5
 Ultrafine gold nanoparticles quench the fluorescence emission much
efficiently, following nanometal surface energy transfer (NSET).
 Large sized gold nanoparticles exhibit size-tunable localized surface
plasmon (LSPR), and the additional electromagnetic field should
affect energy transfer in a different manner.
Objectives
The overall objective is to develop an ultrasensitive strategy for trace
mercuric (II) detection based on the energy transfer between CdSe/ZnS
quantum dots and gold nanoparticles. Specific aims are as follows:
(1) Investigating size effect of gold nanoparticles on fluorescence
quenching of CdSe/ZnS quantum dots;
(2) Gaining mechanistic insights into the energy transfer mechanism;
(3) Developing the sensor for trace mercuric (II) ion detection.
Sensor Design
Energy transfer
QD
gold
spacer
Used DNA sequence:
1): 5’-HS-CAGTTTGGAC-3’
3’-GTCTTTCCTG-SH-5’
2): 5’-HS-CAGCAGGACTTTGGACCAAC-3'
3'-GTCGTCCTGTTTCCTGGTTG-SH-5’
3): 5’-HS-CAGCTGGACCGTAGTTTGGACCTACGTACG-3'
3'-GTCGACCTGGCATCTTTCCTGGATGCATGC-SH-5’
 CdSe/ZnS quantum dots as the energy donor and gold nanoaprticles as
the energy acceptor;
 Change the size of gold nanoparticles and the space between quantum
dots and gold nanoparticles by using different DNA lengths.
Size Effect on Energy Transfer
(a)
(b)
20 nm
(a) Absorption and fluorescence emission
spectra of the used quantum dots
(b) TEM image of the used quantum dots
The quantum dots exhibit a fluorescence emission band at 570 nm and
a particle size of 3.4 nm in diameter.
(a)
(b)
 3 nm gold nanoparticles
show no observable LSPR
absorption;
10 nm
(d)
(c)
 15 nm and 80 nm gold
nanoparticles have LSPR
peaks at 520 nm and 550 nm,
respectively;
 The spectra overlap
increases as the increasing
particle size.
50 nm
50 nm
(a) Extinction spectra of 3, 15 and 80 nm gold nanoparticles and fluorescence
emission spectra of DNA-functionalized quantum dots. (b) TEM images of 3, 15
and 80 nm sized gold nanoparticles, respectively.
Fluorescence quenching by different sized gold nanoparticles
(a)
(b)
3 nm
15 nm
80 nm
(c)
Quenching of fluorescence emission of the quantum dots by the gold nanoparticles with different
sizes. (a) fluorescence emission spectra, (b) emission intensity at 570 nm and (c) Stern-Volmer
plots as a function of mercuric (II) ion concentration.
(a)
(b)
(c)
Experimental data points and theoretical curves of the quenching efficiency versus the
separation distance.
• The quenching efficiency increases as the increasing particles size;
• 3 nm gold nanoparticles quench fluorescence emission following the NSET
mechanism with a 1/d4 distance dependence;
• 15 nm and 80 nm gold nanoparticles quench fluorescence emission following
the FRET mechanism with a 1/d6 distance dependence.
Mercuric (II) Detection
Excitation
Au
+
QD
Au
emission
Hg2+ (
)
• Increasing the DNA loading on
quantum dots;
• A quantum dot coupled to
several gold.
Energy transfer
Excitation
GTCTTTCCTG-S
QD
S-CAGTTTGGAC
no emission
Au
(a)
(b)
(c)
• Fluorescence emission intensity at 570 nm
decreases with the increasing mercuric (II)
concentration;
• The detection limit is down to 0.2 nM.
(a) Fluorescence emission spectra and (b) quenching efficiency of quantum dots as a
function of mercuric (II) concentration; (c) is the linear region of (b) at low concentration.
Fluorescence quenching efficiency in the presence of various metal ions. The
concentration of each metal ion is 100 nM (96 nM QDs, 104 nM Au NPs and 0.1 mM
ethylenediamine in 0.3 M PBS).
Conclusions
• The quenching efficiency of fluorescence emission of quantum dots
increases as the increasing particle size;
• The 3 nm gold nanoparticles quench fluorescence emission
following the NSET mechanism with a 1/d4 distance-dependence;
• The 15 nm and 80 nm gold nanoparticles quench fluorescence
emission following the FRET mechanism with a 1/d6 distancedependence;
• The increasing quenching efficiency with the increasing gold particle
size is attributed to the enhanced LSPR and the increasing spectral
overlap;
• The quantum dot/DNA/gold sensor exhibits an ultrasensitive
detection toward mercuric (II) and high selectivity over other
environmental ions.
Acknowledgments
This work was supported by an NSF grant (CBET-0754405). The resource
and facilities used were partially supported by an NSF RII grant (EPS
1003907) and a Research Challenge Grant from the State of West Virginia
(EPS08-01), the West Virginia University Research Corporation and the
West Virginia EPSCoR Office.