Grafting and coloring onto silver nanoparticles by photoinduced surface modification

Article abstract of DOI:10.1021/ja9047416

(Figure Presented) Surface modification of metal nanostructures can create multifunctional materials potentially very useful in many application fields and, consequently, has been the subject of intensive studies. This work reported the modification of silver nanoparticles (Ag NPs) by UV-induced interface reactions, a method controllable in both the color and the surface chemistry of the nanoparticles. Using poly(N-vinyl-2-pyrrolidone) (PVP) as the protecting polymer, Ag NPs were synthesized in ethanol and then mixed with F-bromoisobutyric acid (BIBA). When the mixture is exposed to UV irradiation, Ag NPs present themselves in a serial tone from pale blue to blue, dark blue, and finally purple with the progress of the interface reactions. It is shown that these color changes are directly related to the chemical components on the surface of Ag NPs, and hence the correlation of the colors with the chemical states of main components on the surface of Ag NPs has been made during the course of interface reactions.

Full text of DOI:10.1021/ja9047416

Published on Web 08/28/2009
Grafting and Coloring onto Silver Nanoparticles by Photoinduced Surface
Modification
Hucheng Zhang,* Liwei Zhang, and Jianji Wang
School of Chemistry & EnVironmental Sciences, Henan Normal UniVersity, Xinxiang, Henan 453007, China
Received June 10, 2009; E-mail: hzhang@henannu.edu.cn
Localized surface plasmon resonance (LSPR) is the general
1
optical phenomenon of noble metal nanoparticles. It results from
the collective oscillation of conduction electrons and is responsible
for the brilliant colors of metal colloidal solutions. The resonance
frequency of LSPR can be easily tuned by the size, shape,
2-4
composition, and microenvironment properties of the nanoparticles,
and hence noble metal nanoparticles are potentially very useful
5
materials in many application fields, such as optical devices, optical
6
7,8
energy conversion, catalysis, near field scanning optical mi-
9
10,11
croscopy, surface enhanced spectroscopies,
chemical and
Especially, the surface modification
of noble metal nanoparticles can be easily carried out with
1
2-14
biological sensors, etc.
1
5,16
alkanethiols, biomolecules, polymers, or other ligands.
Nano-
particles via surface modifications can create new functional
materials and meet the requirements in the applications described
above. Therefore, nanoparticles with versatile surface chemistry
have been the subject of intensive studies.
Figure 1. UV-vis spectra of samples at the different reaction stages. Inset
shows the photographs 1-5, respectively corresponding to samples 1-5.
Using R-bromoisobutyric acid (BIBA) as the surface modifier,
in this work, a new method was developed for the surface
modification of silver nanopartiles (Ag NPs). The modified Ag NPs
via this method possess several desirable characteristics. The surface
chemistry, and also the colors of Ag NPs, can be simply controlled
by UV irradiation. The purple Ag NPs have a core/shell nanostruc-
ture with a dense encapsulation layer on their surface. This
modification not only retains the dispersibility of the Ag NPs in
water but also produces nanoparticles with the surface of different
chemical components to effectively protect them from deterioration
in aqueous media and to be available for further surface modification.
Ag NPs were synthesized via ascorbic acid reduction of silver
nitrate using ethanol as solvent in the presence of poly(N-vinyl-2-
Figure 2. TEM images of the samples. Insets show the images of single
nanopartile magnified respectively from samples 1 and 5.
NPs in sample 5 possess a core/shell nanostructure and have a size
of 41 ( 9 nm with a dense organic shell of ∼8 nm in average
thickness. All these experimental results imply that chemical
reactions occur on the Ag NP surface.
17
pyrrolidone) (PVP). The ethanol colloidal solution of the Ag NPs
sample 1) appears as dark orange with the extinction maximum at
10 nm (Figure 1). As the ethanol solution of BIBA was injected
(
4
The optical properties of Ag NPs are one of the most interesting
aspects. This work shows that the interface reactions can alter the
frequency of conduction electron oscillation and, consequently,
change the color of Ag NPs. Presumably, the occurrence of strong
chemisorptions in sample 2 suppresses the free conduction electrons.
When sample 2 is exposed to UV irradiation, the interactions
between Br in BIBA and Ag NPs are gradually weakened, and
hence the collective oscillation of conduction electrons becomes
increasingly significant with the progression of interface reactions.
At the same time, Ag NPs are subjected to the varied circumstances
of surface chemistry and present themselves in a serial tone in the
range of visible spectrum. Obviously, the conversion of molecular
kind and composition on the surface of Ag NPs is definitively
responsible for the color evolution, and consequently it becomes
necessary to reveal the mechanisms of color variations by inves-
tigating the chemical reactions on the Ag surface and in the bulk
solutions.
into sample 1, unexpectedly, no deposition but a pale blue colloidal
solution (sample 2) was formed. Under UV irradiation, sample 2
exhibited consecutive color changes and presented blue after 10 h
(
7
sample 3), dark blue after 36 h (sample 4), and finally purple after
2 h (sample 5) with the extinction maximum at 531 nm. The red-
shift phenomenon can be also observed by controlling the morphol-
1
8
ogy of Ag NPs. Depending on the intended study, any color of
Ag NPs in the process can be obtained by removing the sample
from UV source. Thereafter, no color change was detected with
further radiation for 1 week.
Figure 2 shows the transmission electron microscope (TEM)
images of samples at three typical stages of the interface reactions.
The Ag NPs in sample 1 have an average size of 26 nm with a
standard deviation of 4 nm (26 ( 4 nm). Because of the strong
chemisorptions of Ag NPs toward BIBA, however, Ag NPs in
sample 2 form spindle-shaped agglomerates 226 ( 31 nm in length
and 64 ( 7 nm at maximum diameter. Each agglomerate consists
of ∼15-20 Ag NPs. Moreover, it is clearly observed that the Ag
The changes of chemical components with the reaction progress
in the solutions are revealed by gas chromatography-mass
spectrometry (GC-MS) (Scheme 1 and Figures S1-S8). PVP is
1
3206 9 J. AM. CHEM. SOC. 2009, 131, 13206–13207
10.1021/ja9047416 CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Chemical Changes of R-Bromoisobutyric Acid in the
FT-IR were carried out on the solid samples without volatilizable
molecules. For sample 2, the Ag 3d and Br 1s peaks in the X-ray
photoelectron spectra exhibit the strongest intensity (Figures S9
and S10), indicating that the chemisorptions of Ag NPs to BIBA
produce the nearly “naked” naoparticles. Both peaks are still very
significant in sample 4. However, they are almost undetected in
sample 5, because Br atoms fall away from the surface of Ag NPs
and then Ag NPs are densely capped by PVP. Correspondingly,
the X-ray photoelectron spectra in O 1s region show three chemical
states of oxygen and present the significant changes with the
progress of interface reactions (Figure 3b). The shift of O 1s peak
Solutions
proved to be a useful protecting polymer and has been applied
successfully to prepare noble metal nanoparticles in various
1
9
systems. However, the physical adsorption of Ag NPs is weak
toward the solvent-swollen PVP and results in the loose distribution
of PVP from 530.3 to 529.3 eV gives the indication of interactions
20,21
of oxygen with Ag NPs in sample 1.
It is noticed that the
1
5
of polymeric segments over nanoparticles. Hence, the strong
chemisorptions can still occur between BIBA and Ag NPs in sample
relative intensity of the peak at 529.3 eV decreases with the reaction
progress and finally disappears in sample 5. Instead, O 1s peak at
2
, and BIBA and R-ethoxyisobutyric acid are determined to be the
531.5 eV is detected in sample 5, which results from the occurrence
main components in the solution. This indicates that the interface
reactions hardly proceed after the chemisorptions, whereas they can
remarkably progress under UV irradiation and cause the change of
chemical components in the solutions. At the stage situated by
sample 4, the interface reactions have progressed to a great extent,
and 2-methyl-3,3-diethoxy-1-propylene is identified as the main
component in the solution. The color changes of Ag NPs from pale
blue to dark blue are directly related to the extent of interface
reactions and, hence, to the chemical components on the surface
of Ag NPs. For sample 5, the GC-MS analysis shows that the
main component in the solution is ethyl R-bromoisobutyrate (EBIB),
and its content is roughly the same as that of the originally added
BIBA within experimental error. In the control experiment without
Ag NPs, no change is detected in the ethanol solution of PVP and
BIBA except that BIBA turns readily into EBIB, confirming the
unique role played by Ag NPs in the coloration. From these results,
it follows that the coating onto Ag NPs does not result from BIBA
but rather from PVP; that is, the PVP segments attached to Ag
NPs are responsible for the color changes from dark blue to purple.
Presumably, when Br element leaves the Ag NPs, the activated
Ag atoms are left behind on the nanoparticle surface. The
interactions of ketone groups of PVP with the activated Ag atoms
are so strong that PVP segments are grafted on the Ag surface to
form the core/shell nanostructure. Figure 3a represents the main
components on the surface of Ag NPs in these samples, which can
be further confirmed by X-ray photoelectron spectroscopy (XPS)
and FT-infrared (FT-IR) spectroscopy.
of C-O as PVP segments are grafted to Ag NPs. In addition, the
FT-IR spectrum of sample 1 is similar to that of PVP but is very
different from that of sample 5 in the spectral range 1800-1000
-1
-1
cm (Figure S11). The peaks at 1175 and 1114 cm in the FT-
IR spectrum of sample 5 represent the presence of the stretching
22
vibration of C-O and affirm the PVP graft onto Ag NPs and
also the formation of the core/shell nanostructure during the
interface reactions.
Acknowledgment. The research was supported by the National
Natural Science Foundation of China (Grant Nos. 20873036,
2
0573034) and the Program for New Century Excellent Talents in
University of Henan Province.
Supporting Information Available: Full experimental details, GC-
MS study, characterization of XPS and FT-IR. This material is available
free of charge via the Internet at http://pubs.acs.org.
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JA9047416
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