Esterification of surface modified silver nanoparticles with n-butanol

Article abstract of DOI:10.1246/cl.2002.950

Surface modified nanometer-sized silver particles with mercaptosuccinic acid (MSA) are crystallized in the size range of few micron-meter. The esterification of carboxylic acid of MSA on the surface of particles is performed with n-butanol, enabling phase transfer from water to organic phase. The yield of esterification was found to be more than 90%.

Full text of DOI:10.1246/cl.2002.950

950
Chemistry Letters 2002
Esterification of Surface Modified Silver Nanoparticles with n-Butanol
Junya Tominaga, Seiichi Sato, Hiroshi Yao, and Keisaku Kimuraꢀ
Department of Material Science, Graduate School of Science, Himeji Institute of Technology, Kamigori, Hyogo 678-1297
(Received May 30, 2002; CL-020467)
Surface modified nanometer-sized silver particles with
mercaptosuccinic acid (MSA) are crystallized in the size range
of few micron-meter. The esterification of carboxylic acid of
MSA on the surface of particles is performed with n-butanol,
enabling phase transfer from water to organic phase. The yield of
esterification was found to be more than 90%.
The synthesis of nanometer-sized particles with well-defined
surfaces is a current interest from materials science. Following the
first modification on CdSe quantum dot,1 there are many reports
on the formation of self-assembled modification of gold
nanoparticles.2{4 Later, it was applied to silver5{8 and cobalt.9
In these studies, the solvents to suspend the nanoparticles are all
organic liquids owing to the limitation of modifiers. Recently we
have developed a synthetic route for water-soluble nano-
particles.10 Phase transfer of nanometer-sized particles from
water to organic solvent or vice versa is also a current interest,
because of wide technological applications. In the previous
report, we have demonstrated that gold nanoparticles are
transferred from water to hexane through the surface modification
with anion-cation interaction.11 As for the silver particles, it was
shown that Ag particles can be transferred through the adsorption
by oleic acid.12 Here we describe a phase transfer from water to
butanol using esterification reaction on the surface of silver
nanoparticles.
Figure 1. TEM photographs and the size histograms of Ag nanopar-
ticles a) before (Ag-MSA) and b) after (Ag-MSA-Bu) modification by
butyl alcohol. Scale bar is the same for both.
The basic preparation procedure is from Ref. 10. Briefly,
0.5 mmol of AgNO3 aqueous solution (5 w/v% preparation in
advance) was first mixed with 1.0 mmol of MSA in 100 mL
methanol to give a transparent solution. A freshly prepared 0.2 M
aqueous sodium borohydride solution 25 mL was then added
through a syringe under vigorous stirring. The color of the
solution changed dark-brown immediately and then resulted in a
flocculent dark-brown precipitate. After further stirring for 1.5 hr,
the sample precipitate was gathered by decantation and
centrifugation followed by washing with 20% methanol/water
solution. These processes were repeated twice and then washed
with ethanol. No dialysis was engaged. The mean diameter of the
particles, determined by a transmission electron microscope
(TEM), was 4.9 nm with FWHM of 2 nm (Figure 1a). Structure
assignment and size estimation were conducted using X-ray
diffraction (XRD). XRD patterns of Ag samples can be indexed to
that of fcc, indicating the formation of pure silver metallic
particles. The average particle size estimated from the Scherrer’s
equation, according to the half-width of (111) peak corrected with
that of bulk Ag, was estimated to be ca. 3.5 nm. This starting
material for the surface esterification was abbreviated with Ag-
MSA. In order to check the purity of the starting samples in
surface modification, crystallization of this particle was con-
ducted. The crystallization (3D super lattice formation) took
place 4–10 days in acidic conditions. Figure 2 shows numerous
ꢀm size crystallites with clear crystal habit (majority rectangular
Figure 2. An SEM photograph of 3D particle crystals with MSA modified
surface. Numerous rectangular-shaped crystallites are mixed with amor-
phous deposits from suspension.
fringe). Irrespective of the rather wide size distribution, we can
get crystallites of nanoparticles. No fractionation process was
needed for this crystallization showing that the starting particles
are uniformly modified.
Five mL n-butanol with 0.1 mL of conc. sulfuric acid was
added to 3 mg Ag-MSA powder described above and heated less
than 70 ꢁC for 20 min. The color of the suspension changed to
brown. Dialysis vz. ethanol was applied in order to remove
residual sulfuric acid until the conductivity of the effluent
indicated zero. This particle was named as Ag-MSA-Bu. The
optical absorption spectra in the visible region (Figure 3) were
taken in water (Ag-MSA) and in butanol (Ag-MSA-Bu). Slight
red-shift was observed for Ag-MSA-Bu. The size distribution is
shown in Figure 1b. The average size was found to be 4.1 nm. A
TEM study revealed that the silver particles smaller than 3 nm are
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
951
In order to examine the content of unmodified carboxylate
groups on the surface, we have conducted the energy dispersive
X-ray analysis (EDX). The result of area analysis
(100 ꢃ 100 ꢀm2) is listed in Table 1. Since the surface
carboxylate group is either ꢂCOONa or ꢂCOO(C4H9Þ, the most
important elements to be analyzed are Cand Na. The initial
atomic ratio of Cand Na, 4 : 2, changes to 12 : 0 if complete
esterification takes place. Hence C : Na ¼ 12 ꢂ 4x : x, in which x
is Na atomic content. We have found that the conversion rate of
esterification was 98%. That is to say, almost of all surface
carboxylate were converted to ester. To show directly this
estimate, we have suspended two samples, Ag-MSA powders and
Ag-MSA-Bu powders, into water/n-butanol mixed solution. The
photograph is shown in Figure 5. It goes without saying that the
surface character of Ag particles was quantitatively modified
through esterification.
Figure 3. Visible absorption spectra before (Ag-MSA) and after (Ag-
MSA-Bu) esterification.
Table 1. Energy dispersive X-ray analysis of Ag-MSA-Bu powders. Units
in atom%
very unstable under electron beam irradiation of normal intensity.
Therefore special care should be taken to use extensively dried
sample in vacuo and to keep the electron beam as weak as possible
during observation.
CO
Na
S
Sum
9.63
56.14
20.05
0.87
86.69
The Sum is less than 100 because of subtraction of background signals
from substrate.
The surface structure of Ag particles before and after
esterification was examined by IR spectra (Figure 4). The formal
surface species after esterification are also shown in the inset of
Figure 4. The existence of two strong peaks at 1579 cmꢂ1 and
1404 cmꢂ1 in Ag-MSA, which can be attributed to the
asymmetric and symmetric stretching vibration of carboxylate
ions, respectively, indicates that MSA exists in the form of
carboxylate anions before modification.10b From elemental
analysis, the counter cations were found to be sodium ion.10a
The absence of S–H vibration peak at 2548 cmꢂ1 of pure MSA
gives evidence that MSA combines with silver through sulfur
atoms in thiol group. After modification, the carbonyl peak at
1716 cmꢂ1 and ether peak at 1149 cmꢂ1 appeared.13 Small hump
at 2922 cmꢂ1 is from C–H stretching of butyl group. All these
findings support that surface carboxylate changed to ester. We
have also found that very weak peaks existed at the position of
carboxylate ions in this sample, indicating that not all carboxylic
acid were converted to ester. A strong O–H band at around
3400 cmꢂ1 in both samples revealed the presence of bound water
on the particle surface.
Figure 5. Photo of the dispersions; a) before and b) after esterification.
Note that the upper layer is n-butanol and the lower is water for both bottles.
This work is supported in part by Grants-in-aid for Scientific
Research on Basic Research (B: 13440212) from Ministry of
Education, Science, Sports and Culture, Japan.
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Figure 4. FT-IR spectra of Ag-MSA and Ag-MSA-Bu samples.