Phase Transfer of Au and Ag Nanoparticles
FULL PAPER
lowed by coupling with amphiphile
by using DCC/HOBt. The details of
the synthesis procedure and charac-
terization of the products are provid-
ed in the Supporting Information. For
CMC measurement,sodium salts of
the corresponding amphiphiles were
prepared by titration with sodium hy-
droxide in ethanol according to the
[
53]
literature report.
Preparation of amphiphile-capped
gold nanoparticles (amphiphile–
GNPs): In a typical reaction,an am-
phiphile Ole-Trp/Ste-Trp/Lau-Trp/
Ole-Tyr solution (1 mL,40 m in
m
methanol) was diluted with triple-dis-
tilled water (8.5 mL). The pH of the
medium was adjusted to alkaline by
adding NaOH for complete solubili-
zation of the amphiphile. An aqueous
4
solution of HAuCl (0.5 mL,10 m m)
Figure 10. UV/visible absorption spectra of the Ole-Trp–GNPs conjugate as a function of temperature in
A) water and B) toluene.
was then added dropwise to the
above solution with constant magnet-
ic stirring. The pH of the final reac-
tion mixture was adjusted to ꢀ11
with standard NaOH solution,and
the mixture was stirred magnetically for 3 days at ambient temperature.
We observed a color change from yellow to ruby red,indicating the for-
mation of colloidal GNPs through oxidation of the tryptophan/tyrosine
residue of the respective amphiphiles.
silver NPs to various nonpolar organic solvents without
causing any particle aggregation,simply by changing the pH
of the medium from alkaline to acidic. The UV/visible spec-
tral results showed no change of the SPR properties of these
NPs after transfer to organic solvents. The TEM study also
showed that the particle size and size distribution remain
unchanged after transfer to organic solvent. The presence of
an amino acid head group increases the thermal stability of
the GNPs in either phase. This method can also be applied
to the preparation of other MNPs by using a variety of
amino acid derivatized fatty acid molecules. In brief,our
method has several advantages over earlier reported meth-
ods,for example: 1) no external reducing agent is required;
Preparation of amphiphile-capped silver nanoparticles (amphiphile–
SNPs): In a typical reaction,an amphiphile Ole-Trp solution (1 mL,
40 mm in methanol) was dissolved in triple-distilled water (8.9 mL). The
pH of the medium was adjusted to alkaline for complete solubilization of
3
the amphiphile. An aqueous solution of AgNO (0.1 mL,10 m m) was
then added dropwise with constant magnetic stirring. The pH of the final
reaction mixture was adjusted to ꢀ11 with standard NaOH solution,and
the mixture was stirred for 3 days at room temperature. We observed a
color change from colorless to yellow,indicating the formation of colloi-
dal SNPs through oxidation of the tryptophan residue of the amphiphile.
Preparation of oleic acid capped gold nanoparticles (oleic acid–GNPs):
In a typical reaction,oleic acid solution (1 mL,40 m m in methanol) was
dissolved in triple-distilled water (8 mL). The pH of the medium was ad-
justed to alkaline (pH 11) for complete solubility of oleic acid. An aque-
2) 100% transfer occurs simply by changing the pH by using
any protonated acid; 3) the nature and position of the SPR
property of GNPs/SNPs remain unaltered during transfer;
and 4) the method is very simple to use and requires only
two steps.
ous solution of HAuCl (0.5 mL,10 m m) was added to the above solution
4
with constant stirring. Finally,NaBH
4
solution (0.5 mL,10 m m) was
added dropwise to the above mixture with constant magnetic stirring. A
change of color from yellow to ruby red indicated the formation of
oleate-stabilized GNPs and the solution was further stirred for 30 min.
Phase transfer of amphiphile–GNPs and amphiphile–SNPs: Typically,the
as-prepared colloidal amphiphile–GNPs suspension (2 mL) was mixed
with an equal volume of toluene to obtain two immiscible layers consist-
ing of the transparent organic phase on the top and the colored hydrosol
at the bottom. HCl (0.25n) solution was added to this biphasic mixture
under magnetic stirring to make the aqueous phase pH 2–3. The phase-
transfer process was completed within a few minutes. The complete
transfer of the GNPs from water to toluene can be visualized by the
color change of the aqueous phase from ruby red to colorless,and at the
same time the organic phase from colorless to ruby red. A similar proce-
dure was also followed for the transfer of conjugates to other nonpolar
organic solvents,such as benzene,xylene,cyclohexane,and hexane. The
amphiphile–SNPs conjugates were also transferred to toluene by using
the same procedures.
Experimental Section
Materials: Oleic acid,stearic acid,lauric acid, l-tryptophan (Trp), l-tyro-
sine (Tyr),dicyclohexylcarbodiimide (DCC),and 1-hydroxybenzotriazole
HOBt) were purchased from SRL India. Hydrogen tetrachloroaurate-
(
A
C
H
T
R
E
U
N
G
(III) trihydrate (HAuCl ·3H O) and silver nitrate (AgNO ) were pur-
4
2
3
chased from Sigma–Aldrich. All the aqueous solutions were made with
triple-distilled water. HPLC-grade organic solvents were used for other
purposes.
Synthesis of redox-active amphiphiles: Four new redox-active amphi-
philes (conjugates of a fatty acid and an amino acid),oleic-Trp-COOH
Ole-Trp),stearic-Trp-COOH (Ste-Trp),lauric-Trp-COOH (Lau-Trp),
and oleic-Tyr-COOH (Ole-Tyr),were synthesized by conventional solu-
tion-phase methods using a racemization-free fragmentation/condensa-
tion strategy. Scheme 1 depicts the chemical structures of these trypto-
phan- and tyrosine-based redox-active amphiphiles. The C terminus of an
amino acid was first protected by esterification with a methyl group,fol-
(
Characterization
[
59]
NMR experiments: All NMR studies of the redox-active amphiphiles in
CDCl (1–10 mm) were carried out with Bruker DPX 300 MHz spectrom-
3
eters. All NMR data are available in the Supporting Information.
Chem. Eur. J. 2007, 13,9850 – 9861
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA,Weinheim
9859
Si, Satyabrata
