Skeleton pseudomorphs of nanostructured silver for the surface-enhanced Raman spectroscopy

Article abstract of DOI:10.1016/j.mencom.2019.07.012

Pseudomorphs (mesocages) of metallic silver were prepared via either a shape-preserving reduction of polyhedral Ag₂O precursors with H₂O₂ in aqueous ammonia or an ultrasonic spray pyrolysis of aqueous diamminesilver(i) hydroxide. Both the materials were revealed as suitable for the direct analysis at 0.1–1 μm concentrations of oil pollutants and pharmaceutical substances using surface-enhanced Raman spectroscopy due to their unique morphology providing either a larger surface area for sorption of analytes or additional ‘hot spots’ resulting in the enhancement factors of 10⁶–10⁸.

Full text of DOI:10.1016/j.mencom.2019.07.012

Mendeleev
Communications
Mendeleev Commun., 2019, 29, 395–397
Skeleton pseudomorphs of nanostructured silver
for the surface-enhanced Raman spectroscopy
Anna A. Semenova,a Sergey V. Savilov,b Alexander E. Baranchikov,c
Vladimir K. Ivanova,c and Eugene A. Goodilin*a,b,c
a Department of Materials Science, M. V. Lomonosov Moscow State University, 119991 Moscow,
Russian Federation. Fax: +7 495 939 0998; e-mail: goodilin@yandex.ru
b Department of Chemistry, M. V. Lomonosov Moscow State University, 119991 Moscow,
Russian Federation
c N. S. Kurnakov Institute of General and Inorganic Chemistry, Russian Academy Sciences,
119991 Moscow, Russian Federation
DOI: 10.1016/j.mencom.2019.07.012
Pseudomorphs (mesocages) of metallic silver were prepared
via either a shape-preserving reduction of polyhedral Ag2O
precursors with H2O2 in aqueous ammonia or an ultrasonic
spray pyrolysis of aqueous diamminesilver(i) hydroxide.
Both the materials were revealed as suitable for the direct
analysis at 0.1–1 mm concentrations of oil pollutants and
pharmaceutical substances using surface-enhanced Raman
spectroscopy due to their unique morphology providing either
a larger surface area for sorption of analytes or additional
‘hot spots’ resulting in the enhancement factors of 106–108.
Silver nanoparticles are highly demanded in practical applica-
tions as catalysts, conducting and biologically active materials,1–5
building units for nanoscale devices, in flexible electronics,6,7
and, especially, in modern devices utilizing highly sensitive
assays by the surface-enhanced Raman spectroscopy (SERS).8–13
Seeding, aggregation-based growth, kinetically controlled over-
growth, template-directed methods, selective etching, and ageing
have been developed for the preparation of Ag nanoparticles
(NPs) possessing controlled morphologies.1–4,9 These nano-
structures are efficient probes for SERS of biologically active
molecules,oncologicmarkers,livingcellsandtheircompartments
in the frame of development of new medical screening procedures
and personal medicine.1,9,14–19 Pseudomorphs are defined as the
result of shape-preserving processes of phase transformations. They
are further promising as SERS-active materials due to the expected
broader light absorption spectral range as compared to narrow
plasmonic bands of separatedAg NPs and their enhanced surface
area being available for target sensing of adsorbed substances.13,20
Here, we report on both the optimization of controlled synthesis
of silver pseudomorphic particles† (Figure 1)13,20,‡ to yield a
large fraction of porous octahedra with morphologically pre-
determined hot spots and the ensemble of randomized polyhedral
Ag NPs (Figure 2) produced by aerosol decomposition and self-
etching.§
An overall morphology of the polyhedra prepared according
to the PVP protocol demonstrates features of nucleation and
(a)
20
40
2q/deg
60
80
1
2
2 µm
(b) 1
3
2
1 µm
Figure 1 SEM images of metallic silver nanostructured particles obtained
by pseudomorphic reduction of octahedral Ag2O precursor with hydrogen
peroxide: (a) the general view of precipitate (the inset shows an XRD
pattern of cubic silver) and (b) the magnified view of porous polyhedral
particles showing (1) the smooth morphology, (2) octahedra overgrown with
Ag NPs, and (3) intergrown pseudomorphic silver.
†
At the first step, octahedral Ag2O was formed upon the treatment of
aqueous solution of AgNO3 (50 ml, 100 mm) with ammonia solution
(0.5 m) added dropwise in the presence of polyvinylpyrrolidone (PVP)
solution (0.1 mm) as a specific face-selective surfactant. The mixture
became transparent. Aqueous solution of NaOH (0.3 ml, 2 m) was added to
the colorless solution under vigorous stirring. The obtained Ag2O was several
times washed with distilled water and ethanol. At the second step, hydrogen
peroxide (10 ml, 10%) was added as a reducing agent to Ag2O under
stirring followed by the injection of etching ammonia solution (10 ml,
0.5 m). The synthesized silver particles were collected by centrifugation.
‡
Micrometer-sized single crystalline silver particles (coincided with the
geometrical size of nanostructured silver octahedra) have never been
used for SERS devices since such large particles are SERS ineffective
and possess a small surface area.3,10
© 2019 Mendeleev Communications. Published by ELSEVIER B.V.
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.
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Mendeleev Commun., 2019, 29, 395–397
(b)
(a)
part of initial silver complex and also quick solvent evaporation
followed by the formation of crystalline Ag2O intermediate. The
further pseudomorphic products of this transformation include
silver nanostructured particles (see Figure 2), water, ammonia, and
oxygen. Some molecular oxygen contacting the products was from
surrounding air. Polyhedral aggregates could result from initial
Ag2O formation within the mist droplets prior its decomposition
into silver and oxygen. Silver(i) oxide is capable of producing
cubic, edge- and corner-truncated cubic, rhombicuboctahedral,
edge- and corner-truncated octahedral, octahedral, and hexapod
structures.13 All of them could then transform into pure silver
forming pores due to the oxygen release. As soon as liquid water
and remaining traces of silver(i) complex reside together in
the condensation zone at decreased temperature (60–80°C), it
facilitates the achievement of both superficial decoration with
small Ag NPs and self-assembly of very complex, hierarchic
silver micro- and nanostructures (see Figure 2) resembling the
growth of skeleton forms of crystals under highly nonequilibrium
conditions with a lack of nutrient phase. Thus, the flat facet growth
was replaced with development of quickly growing rough grains
and dendrites. The growth of ‘porous bundles’ and nanoflowers
seems to be different steps of the same process of Ag2O thermo-
chemical transformation followed by kinetically controlled develop-
ment of the fastest growing grains co-oriented at once due to the
initial texture of bundles.
The silver cubes (see Figure 2) do not look as single crys-
talline species. Their size enlargement occurs due to the post-
reduction of silver in the presence of remaining Ag+ byproducts
and overgrowth of the initial particles. The cubes could be thus
considered as pseudocrystals formed in a topotactic manner
preserving the shape of precursor crystals of Ag2O. Their trans-
formation demands oxygen loss, and this develops nanoporosity.
Thermochemical etching and overgrowth are evident from the
morphology of cubes. This assumption explains quite well the
diversity in sizes and morphology of the cubes.
2
300 nm
1
Ag
Ag
(c)
C
20 nm
(d)
3
6
9
E/keV
1
2
1
2
1 µm
3 µm
Figure 2 Complex morphology of nanostructured particles formed after the
ultrasonic spray pyrolysis of aqueous diamminesilver(i) hydroxide: (a) SEM
image of typical pseudomorphic cubes superficially decorated with Ag NPs,
(b) TEM image, ED and local chemical analysis of the cubes, (c) SEM overall
view of Ag NPs growth in the condensation zone showing (1) pseudomorphic
cubes and (2) chaotically nucleated silver particles, and (d) SEM view of
(1) porous silver bundle overgrown with (2) textured silver crystallites.
growth of solid phases providing usually a broad particle size
distribution, however octahedral particles dominate certainly under
the selected experimental conditions. The octahedral particles are
the centres of silver regrowth and secondary nucleation upon the
redox process as seen from additional silver seeds deposited onto
facets of some octahedra. Each of the particles consists of silver
clusters (20–40 nm) forming pseudomorph octahedral shapes
with a large number of pores increasing partly the overall surface
area and, what is even more important, most probably providing
capillary channels for analytes to reach the ‘hot spots’ in the
pseudomorphs. The pseudomorphs consist evidently of pure
metallic silver without residual silver oxide and XRD texture (see
Figure 1) evidencing for their disordered internal polycrystalline
structure; the same is also confirmed indirectly by the presence
of multiple nanosized pores and quite rough surface of the
octahedra.¶
The unique nanostructures obtained by both the methods are
favourable for their implementation into SERS devices since
(a)
(b)
1000 imp.
1000 imp.
Oppositely, a thermal treatment of ultrasonic mist within a
hot zone resulted in the irreversible decomposition of the major
500
1500
2500
900 1100 1300 1500
Raman shift/cm–1
§
Diamminesilver(i) hydroxide was decomposed by aerosol spray pyrolysis.
Raman shift/cm–1
To prepare aqueous diamminesilver(i) hydroxide solutions for the aerosol
protocol, aqueous (0.1 m) NaOH (Aldrich) was added dropwise to fresh
aqueous silver nitrate solution (0.01 m) until the complete precipitation
of black-brownAg2O. This as-prepared oxide was thoroughly washed with
deionized water (high purity water, Milli-Q, Millipore) and dissolved in
aqueous ammonia solution (10%, twofold molar excess) to give solution
(0.03–0.3 m) of a silver(i) complex nebulized into a hot zone (550–950°C)
of a tubular furnace.
(c)
(d)
1000 imp.
1000 imp.
¶
The obtained materials were characterized by a transmission electron
microscopy (TEM) and electron diffraction (ED) using LEO912 AB
OMEGA (Carl Zeiss) and Jeol JEM 2100F (Jeol) instruments. The pristine
sample microstructure was studied using a Carl Zeiss NVision 40 field
emission scanning electron microscope at the accelerating voltage of
0.5–5 kV. The obtained substrates were examined using a Rigaku
D/MAX 2500 (Japan) instrument equipped with a rotating copper anode
(CuKa irradiation, 2q range of 5–80°, and step of 0.02°). Raman and
SERS experiments were performed using an InVia Raman microscope
(Renishaw, UK) equipped with an argon laser (20 mW, 514 nm) and
power neutral density filter (10%). All the spectra were collected using 20×
objective lens for the acquisition time of 10–120 s. A silicon wafer was
employed for the calibration.
1300 1400 1500 1600
Raman shift/cm–1
400
1200
2000
2800
1200
Raman shift/cm–1
Figure 3 SERS spectra of (a) an absorbed layer of evaporated CHT and
(b) aqueous meldonium, (c) reference spectrum of crystalline meldonium,
and (d) SERS spectrum of highly diluted (10–10 m) Rhodamine 123 dye.
Spectra (a),(b) were recorded using the prepared silver pseudomorphs (via
reduction of Ag2O octahedra) at 10% of nominal laser power and 1 mm
concentrations for the acquisition time of 10–60 s, and spectrum (d) was
recorded using nanostructured particles obtained after ultrasonic spray
pyrolysis.
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Mendeleev Commun., 2019, 29, 395–397
they reveal a pronounced effect of Raman scattering enhance-
for SERS measurements for the analysis of oil pollutants and
pharmaceutical substances.
ment. Figure 3 shows typical SERS spectra of the model dye
(Rhodamine 123), oil pollutant and pharmaceutical compound.††
In the case of Rhodamine 123, all major peaks were recorded
at the lowest detection threshold of about 0.1–0.05 nm. This
concentration is not detectable by a conventional Raman spectro-
scopy, but millimolar concentrations start to manifest the intense
fluorescence of rhodamine. This SERS effectiveness of the
produced materials confirms that they are composed of nano-
structured metallic silver as was also observed by TEM, EDX,
XRD and electron diffraction patterns (see Figure 2).
This work was supported in its instrumental part by the
M. V. Lomonosov Moscow State University Program of Develop-
ment. EAG acknowledges the support by the Russian Science
Foundation (grant no. 14-13-00871). AAS was supported by the
Grant of the President of the Russian Federation for the govern-
mental support of young scientists (grant no. MK-1146.2017.3).
References
The octahedral particles produced via the PVP protocol plays
the role of a highly hierarchic structure being ready for SERS
applications (see Figure 3) for model dye analytes, molecular
thiol products, crude oil desulfurization markers, and also for well-
known pharmaceutical compounds like meldonium [mildronate,
3-(2,2,2-trimethyldiazan-2-ium-1-yl)propanoate]. In particular, the
SERS spectrum of cyclohexanethiol (CHT) represents its adsorbed
layers anchored onto silver surface via the thiol groups. The S–H
stretching band usually appearing at 2570 cm–1 and also the CSH
bending mode at 800–900 cm–1 are missing in the SERS spectra
indicating that the molecules are dissociatively adsorbed on the
silver surface forming a link to the silver. Oppositely, the band at
721 cm–1 corresponds to the C–S stretching due to the interaction
with silver. Other vibration modes are typical for the various con-
formers of cyclohexane ring: ring deformation and stretching at
389 and 845 cm–1, and CH2 twisting, wagging, scissoring and
stretching at 1178, 1267, 1363, 1444, 2900 cm–1. Meldonium
species can be detected in water at the micromolar levels (see
Figure 3). Evidently, the most of bright vibrational modes are
present in both the original and enhanced Raman spectra with
varied intensities depending, as in the case of CHT, on the
conformations of absorbed molecules.
In conclusion, various production protocols of silver pseudo-
morphs from aqueous diamminesilver(i) hydroxide opened up
a new simple access to the nanostructured particles possessing
desired morphologies for practical applications. The growth
features seem to be related to the decomposition of diammine-
silver(i) hydroxide via Ag2O as the intermediate and its further
reduction into pure silver. Aqueous diamminesilver(i) hydroxide
as compared to other salts and complexes provides a large benefit
owing to simplicity, promptness and low temperature conditions
of the preparation process. The latter allows one, in particular,
to obtain nanoparticles with new types of morphologies suitable
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†† The enhancement factors (EF) were calculated as the ratio of corre-
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Received: 7th December 2018; Com. 18/5764
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