COMMUNICATION
Straightforward green synthesis of ‘‘naked’’ aqueous silver nanoparticlesw
Salvatore Giuffrida,a Giorgio Ventimigliab and Salvatore Sortino*a
Received (in Cambridge, UK) 7th April 2009, Accepted 7th May 2009
First published as an Advance Article on the web 29th May 2009
DOI: 10.1039/b907075c
Water-soluble, exceptionally stable, ‘‘naked’’ silver nanoparticles
were obtained in a single step by simple decomposition of a
commercial silver complex at room temperature without the need
of external reducing agents and conventional stabilizing ligands.
temperature. Fig. 1 shows the absorption spectral changes
observed in the case of a 0.2 mM solution. It can be noted that
during the time course the main absorption band of Ag(acac)
at 292 nm bleaches and blue-shifts to 274 nm. Concurrently,
the growth of a new absorption at 420 nm with the formation
of two fairly clear isosbestic points at 255 and 355 nm, is
observed. These spectral changes are consistent with a
spontaneous thermal reaction involving the release of the acac
ligand, in its protonated form (Hacac), and formation of
AgNPs characterized by their typical plasmon absorption
band at ca. 420 nm. Interestingly, the release of the ligand is
essentially quantitativez and it can be easily removed from the
solution by drying under vacuum at 50 1C. Subsequent
resuspension of the AgNPs under air did not change either
the plasmon band intensity nor its profile, indicating preserva-
tion of the physicochemical properties after the ligand removal
(inset in Fig. 1).
In the fascinating realm of nanostructured materials, noble
metal nanoparticles (NPs) have been receiving a great deal of
attention.1 The wealth of their unique properties makes NPs
ideally suited for widespread applications in different fields
ranging from diagnostic imaging and drug delivery to opto-
electronics and catalysis.2 This scenario has stimulated a surge
of interest in the developing of synthetic protocols for NPs.
Syntheses have been performed in a variety of conditions by
reductive chemical methods, sonochemical and photochemical
techniques using a multiplicity of chemical and biochemical
stabilizing ligands to avoid aggregation.3
Recently, silver NPs (AgNPs) have attracted much attention
in biomedical applications by virtue of their surface plasmon
resonance and anti-microbial activity. In fact, AgNPs
have been used as optical indicators for molecules and
biomolecules,4 wound healing5 and cytoprotective agents
towards HIV-1 infected cells.6
In an excellent publication, Raveendran et al.7 outline the
three main aspects in the preparation of metal NPs that should
be evaluated from a green chemistry perspective, that are: the
appropriate choice of the solvent medium used for the syn-
thesis, the utilization of environmentally benign reducing
agents, and the choice of nontoxic materials for the stabiliza-
tion of the NPs. Besides, Mirkhalaf et al.8 also emphasize and
comment on the need for synthetic approaches that are
less reliant on the chemisorption of conventional stabilizing
ligands containing S, N or P. Surprisingly only few reports
have addressed these issues to date.9
A transmission electron microscopy (TEM) micrograph of a
representative sample is shown in Fig. 2. The AgNPs are quite
homogeneously distributed, exhibit a nearly spherical shape,
and are characterized by a mean diameter of 25.8 nm and
s = 10.2 nm. Electron diffraction shows that the NPs are
single crystals with fcc structure.
These AgNPs exhibited an outstanding stability. Indeed,
they remained well dispersed in water under air condition for
In this communication we report a facile synthesis of AgNPs
that fully satisfies all the above criteria. In particular we show
that indefinitely stable AgNPs can be obtained in aqueous
solution by using the commercial and cheap 2,4-pentandionate
Ag(I) complex, Ag(acac), as the only chemical reactant,
without the need of external reducing agents and conventional
stabilizing ligands.
The preparation of AgNPs is very straightforward and
was performed in one step simply dissolving a given amount
of Ag(acac) in an Ar-saturated water solution at room
Fig. 1 Absorption spectral changes observed at room temperature
for an Ar-saturated aqueous 0.2 mM solution of Ag(acac) in the time
interval 0–48 h in the dark. The arrows indicate the direction of the
spectral evolution with time. The inset shows the absorption spectrum
before (a) and after (b) drying (under vacuum at 50 1C) of the solution
at the end of the reaction and subsequent redispersion of the dried
sample in the same volume of water. The spectrum of the free Hacac
ligand (c) is also shown, for sake of comparison.
a Dipartimento di Scienze Chimiche, Universita´ di Catania, Viale
Andrea Doria 8, I-95125, Catania, Italy. E-mail: ssortino@unict.it
b CCI Group-Microfluidic Division-Molecular Diagnostic Business
Unit STMicroelectronics, I-95128, Catania, Italy
w Electronic supplementary information (ESI) available: Extinction
spectra of Ag(acac) and Hacac in aqueous solution and spectro-
photometric assay for H2O2. See DOI: 10.1039/b907075c
ꢀc
This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 4055–4057 | 4055