Site-Specific Synthesis and In Situ Immobilization of Fluorescent ...
Post on 10-Feb-2017
216 Views
Preview:
Transcript
Site-Specific Synthesis and In Situ Immobilization of Fluorescent Silver
Nanoclusters on DNA Nanoscaffolds by Use of the Tollens Reaction
Suchetan Pal, Reji Varghese,, Zhengtao Deng,
Zhao Zhao, Ashok Kumar, Hao Yan, Yan Liu
Department of Chemistry and Biochemistry and
The Biodesign Institute, Arizona State University, (USA)
Angew. Chem. Int. Ed. 2011, 50, 4176 –4179
Ammu Mathew7th May 2011
� Fluorescent silver nanoclusters (AgNCs)
� DNA-templated synthesis of AgNCs
� DNA nanostructures - templates for metallization
� DNA origami structures - superior nanoscale scaffolds
�Tollen’s reagent: commonly employed in carbohydrate chemistry to test for the
aldehyde functionality in reducing sugars
Introduction
Ag2O
aldehyde functionality in reducing sugars
�Site specificity and uniform distribution of the metal NCs along the DNA
templates remain a challenge
DNA-Origami-Directed Self-Assembly of Discrete Silver-Nanoparticle
Architectures
Angew. Chem. Int. Ed. 2010, 49, 2700 –2704
Angew. Chem. Int. Ed. 2011, 50, 2041 –2044
Encapsulation of Gold Nanoparticles in a DNA
Origami Cage
� A new DNA-based method for the synthesis of water-soluble fluorescent AgNCs with a narrow
size distribution by use of the well-known Tollens reaction.
� Site specific synthesis and in situ immobilization of AgNCs on a triangular DNA origami
scaffold.
�The addressability of DNA origami enables the site-specific synthesis and in situ incorporation
of fluorescent AgNCs on the predefined DNA scaffolds with nanometer-scale spatial resolution.
In this paper:
• Small number of sugar moieties were covalently incorporated into a DNA sequence at adjacent
positions.
• Synthesis of AgNCs by the specific stoichiometry of the Tollens reaction: that is, one aldehyde
sugar molecule can reduce two Ag+ ions to Ag02
• These Ag clusters could then act as nucleation sites for further Ag deposition under mild
reductive conditions.
• Tethering of the sugar functional groups to DNA offers stabilization of the AgNCs synthesized.
Strategy:
• DNA1, DNA2,and DNA3, each of which contains 15 nucleotides and one, two, or
three consecutive modified deoxyurinidine (dUm)units, respectively.
Synthesis of Ag NCs on sugar (galactose)-modified DNA strands
(DNA1, DNA2,and DNA3)
Procedure:
Tollens reagent added to the sugar modified DNA (DNA1, DNA2, or
DNA3) in TAE–Mg2+ (tris(hydroxymethyl) aminomethane [Tris]) buffer, and the
mixture was incubated overnight in the dark at room temperature.
each dUm unit carries a sugar unit.
A
B
DNA 1
Ex: 337 nm
Em: 412 nm
3.6 ns (12 %), 0.8 ns (37 %)
and 0.1 ns (51%)
DNA 2
Ex: 337 nm
Em: 411 nm
3.7 ns (12 %), 0.9 ns (42 %)
C
3.7 ns (12 %), 0.9 ns (42 %)
and 0.1 ns (47 %).
DNA 3
Ex: 337 nm
Em: 420 nm
3.8 ns (4 %), 0.9 ns (21 %),
0.3 ns (31 %) and 0.1 ns (44 %)
similar-sized emissive AgNCs with DNA strands carrying different no. of sugar units
a) TEM and b) STEM images of NCs synthesized
by using free DNA3. The inset is a high-
resolution TEM image of a nanocluster. Scale
bars: 10 nm (a and b) and 2 nm (inset of a).
The red shift of the DNA absorbance peak from
260 nm to 275 nm is due to complexation with Ag.
UV-Vis absorption spectra
DNA3 (black) and Tollens reagent (red) ; spectra taken in 30 minutes
interval (for 10 hours) after the addition of 200 fold Ag+.
•DNA 1 and DNA2 similar NCs
in TEM
•Excellent photostability
• No characteristic Ag plasmon
absorption band
• NCs were nearly
monodispersive (~ 2 nm).
Fluorescence emission spectra of DNA3 (5 µM) solutions in different buffer conditions and
Ag+/DNA ratio, after incubation for 12 hours.
1) Fluorescence intensity of the Ag-NC is proportional to Ag+ : sugar modified DNA
2) Effect of Buffer (TAE buffer, HEPES buffer and water)
3) Without the sugar modified DNA as the nucleation site, the fluorescent Ag-NC does not form.
� Formation of Ag NCs involves the initial reduction of Ag+ to Ag0n (n=2, 4, or 6) seed created by
the Tollens reaction by the sugar units , followed by further reduction of extra Ag+ ions by Tris in
the TAE buffer solution
� The maximum size of the cluster would depend on the Ag+/DNA molar ratio, irrespective of the
number of sugar units present on the DNA
Mechanism:
Kinetic evidence : by varying the Tris concentration
Fluorescence emission spectra of DNA3 (5 µM) solutions in different TAEMg2+ buffer concentrations at
fixed Ag+/DNA ratio = 200; after (a)12 hrs (b) 48 hrs, and (c) 96 hrs. Samples were excited at 340 nm.
Origami embellished with silver:
DNA strands with specific sequences and covalently attached sugar moieties were used
for the site-specific incorporation of the sugar units on a DNA origami scaffold. This
approach enabled the subsequent site-specific synthesis and in situ immobilization of
fluorescent Ag clusters at predefined positions on the DNA nanoscaffold by treatment
with the Tollens reagent
Schematic representation of the site-specific immobilization of fluorescent AgNCs on a
triangular DNA origami scaffold
Before After
AFM images of triangular
origami with DNA3 as the
probe Scale bar is 100 nm
and z-scale range is 10 nm.
Histograms showing the
height of the bright
feature on the origami
~ 2.1 nm height difference - apparent diameter of the AgNCs synthesized and deposited in situ
a) Free DNA3
Ex: 337 nm
Em: 420 nm
b) DNA origami
Ex: 340 nm
Em: 418 nm
Free DNA3 DNA origami
DNA2• Fluorescent Ag NCs grown in situ on the origami scaffold had
optical characteristics similar to those of the NCs obtained with
free DNA3.
• No formation of Ag NPs even after several days of incubation.
• No deposition of NCs was observed in other regions of the
triangular DNA origami scaffold, which demonstrates the
excellent site specificity of our DNA templated approach.
a) TEM image of the origami structure after treatment with the
Tollens reagent (the sample was negatively stained with uranyl
formate) and b) corresponding high-resolution TEM image of
the NCs immobilized on the modified arm. c) EDX spectrum of the AgNCs on the DNA origami structure.
Control experiment:
The sugar units act as the nucleation sites for Ag NC formation. The
unmodified DNA scaffolds do not play an active role in the reduction
of the Ag+ ions, but only act as a structural scaffold
A B
(A) AFM images of the triangular shaped DNA origami with one arm carrying ssDNA oligos,
which do not contain the suger modification. No site specific Ag nanocluster formation after
Tollens’ reaction was observed. (B) TEM images of the triangular shaped DNA origami with
one arm carrying ssDNA oligos, which do not contain the suger modification. No site specific
Ag nanocluster formation after Tollens’ reaction was observed. Scale bars are 100 nm.
� A new DNA-based method for the synthesis of water-soluble fluorescent AgNCs through the
use of the well-known Tollens reaction.
� DNA of specific sequences with sugar moieties covalently attached have been successfully
used for the site-specific incorporation of the sugar units on a triangular DNA origami scaffold to
enable the subsequent site-specific synthesis and in situ immobilization of AgNCs at predefined
positions on the DNA nanoscaffold.
Summary
positions on the DNA nanoscaffold.
� The resulting high-density array of emissive NCs may have potential for application in many
fields, such as the fabrication of semiconductor nanostructures.
� Our new approach is characterized by excellent site specific control of NC nucleation and
yields uniformly sized high-density arrays of AgNCs.
� Offers a unique platform for the future site-specific deposition of other metals, such as gold,
which may lead to advances in DNA-based nanoelectronics.
Thank you
top related