nanomaterials
Article
Spectroscopic and Microscopic Analyses of Fe3O4/Au
Nanoparticles Obtained by Laser Ablation in Water
Maurizio Muniz-Miranda 1,2, * , Francesco Muniz-Miranda 3 and Emilia Giorgetti 2
1
2
3
*
Department of Chemistry “Ugo Schiff”, University of Florence, Via Lastruccia 3, 50019 Sesto Fiorentino, Italy
Institute of Complex Systems (CNR), Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy;
emilia.giorgetti@fi.isc.cnr.it
École Nationale Supérieure de Chimie de Paris and PSL Research University, CNRS, Institute of Chemistry
for Life and Health Sciences (i-CLeHS), FRE 2027, 11, rue Pierre et Marie Curie, F-75005 Paris, France;
f.muniz-miranda@chimieparistech.psl.eu
Correspondence: maurizio.muniz@unifi.it
Received: 8 December 2019; Accepted: 8 January 2020; Published: 10 January 2020
Abstract: Magneto-plasmonic nanoparticles constituted of gold and iron oxide were obtained
in an aqueous environment by laser ablation of iron and gold targets in two successive steps.
Gold nanoparticles are embedded in a mucilaginous matrix of iron oxide, which was identified
as magnetite by both microscopic and spectroscopic analyses. The plasmonic properties of the
obtained colloids, as well as their adsorption capability, were tested by surface-enhanced Raman
scattering (SERS) spectroscopy using 2,2′ -bipyridine as a probe molecule. DFT calculations allowed for
obtaining information on the adsorption of the ligand molecules that strongly interact with positively
charged surface active sites of the gold nanoparticles, thus providing efficient SERS enhancement.
The presence of iron oxide gives the bimetallic colloid new possibilities of adsorption in addition
to those inherent to gold nanoparticles, especially regarding organic pollutants and heavy metals,
allowing to remove them from the aqueous environment by applying a magnetic field. Moreover,
these nanoparticles, thanks to their low toxicity, are potentially useful not only in the field of sensors,
but also for biomedical applications.
Keywords: laser ablation; gold; magnetite; SERS; 2,2′ -bipyridine
1. Introduction
Nanoparticles constituted of metals like silver, gold, or copper exhibit plasmonic properties and are
widely employed as biosensors, drug vectors, and SERS (surface-enhanced Raman scattering) [1,2] and
fluorescence markers, especially gold nanoparticles that are more biocompatible. Their applications
can be realized by adding additional functionalities like magnetic properties. Hence, in nanomedicine
they find diagnostic and/or therapeutic applications in magnetic resonance imaging or for generating
hyperthermia by applying locally intense magnetic fields [3–6]. In this regard, Fe3 O4 magnetic
nanoparticles, presenting good biocompatibility and low toxicity [7–9], are widely used in these
biomedical applications. Usually, different chemical procedures are employed to prepare these
nanocomposites [10–17] to be used for sensoristic and biomedical applications, but they involve
problems due to the presence of surfactants, stabilizers, residual reductants, and by-products,
which could interfere in both the adsorption and the detection of ligands. In this regard, laser-assisted
procedures have been recently employed to obtain metal nanoparticles with both plasmonic and
magnetic properties [18–20]. In the past, some of us adopted the laser ablation procedure of metal targets
in water to produce bimetallic colloidal nanoparticles [21,22]; in particular, two-step laser ablations
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of iron and silver [23] and of nickel and silver [24] were employed to obtain magneto-plasmonic
colloidal nanoparticles.
Here, we propose the fabrication of bifunctional Fe3 O4 /Au nanoparticles obtained by the
two-step laser ablation of iron and gold targets in water, along with microscopic and spectroscopic
characterization. To this end, high-resolution transmission electron microscopy (HRTEM) and selected
area electron diffraction (SAED) analyses have been performed, and visible absorption, XPS, Raman,
and SERS spectra have been obtained. To obtain information on the type of ligand/metal adsorption
provided by these nano-platforms, calculations based on density functional theory (DFT) have also
been carried out using 2,2′ -bipyridine as a molecular reporter.
The importance and novelty of the present investigation, in addition to producing “pure” colloidal
suspensions—that is, without the aid of chemical reagents and surfactants—are due to the fact
that gold nanoparticles are trapped in a ferromagnetic matrix, so they are preserved from colloidal
collapse. In addition, it is possible to remove all the bimetallic material, including the possible load of
adsorbed ligands, from the solvent and transport them thanks to the use of a magnetic field. In this
regard, the presence of iron oxide gives the bimetallic colloid new possibilities for the adsorption
of ligands, in addition to those inherent to gold nanoparticles, and also for removal of them from
the aqueous environment, especially with regard to organic pollutants [25] and heavy metals [26].
Finally, our bimetallic colloids exhibit plasmonic properties, in addition to magnetic ones, due to the
presence of gold nanoparticles, which allow application of the SERS technique for sensoristic purposes.
In practice, SERS spectroscopy provides huge intensification of the Raman signal of molecules adsorbed
on nanostructured gold or silver surfaces, usually up to 107 enhancement factors with respect to the
normal Raman response of non-adsorbed molecules.
2. Materials and Methods
2.1. Laser Ablation
Iron (Sigma-Aldrich, St. Louis, Missouri (USA), 99.99% purity) and gold (Goodfellow, Huntingdon
(UK), 99.95% purity) plates were used as targets for the laser ablation. Colloidal suspensions were
prepared by laser ablations of iron in deionized water (18.2 MX cm @ 25 C), and then of gold, by using
the fundamental wavelength (1064 nm) of a Q-switched Nd:YAG laser (Quanta System G90-10:
rep. rate 10 Hz, pulse width at FWHM of 10 ns). The laser pulse energy was set at 20 mJ/pulse,
corresponding to 200 mW average power, focusing the laser light into a laser spot of approximately 1
mm diameter and corresponding fluence of 2.5 J/cm2 . The target plate was fixed at the bottom of a glass
vessel filled with 6 mL of liquid (height above the target: 2 cm). The irradiation time of the metal targets
was about 20 min. To minimize effects due to crater formation in the metal targets, the glass vessel was
manually rotated and translated, stopping the ablation process every three minutes. The laser pulse
entered the vessel from above, thus impinging perpendicularly onto the target. These experimental
procedures were chosen in order to obtain a valid colloidal stability, following the indications of our
previous experiments [23].
2.2. UV–Visible Extinction Spectroscopy
UV–visible extinction spectra of the colloidal suspensions were obtained in the 200–800 nm region
by using a Cary 5 Varian spectrophotometer (OPL (optical path length) = 2 mm). The observed bands
were due to both absorption and scattering of the radiation.
2.3. Microscopic Techniques
TEM (transmission electron microscopy) and HRTEM (high-resolution TEM) images were obtained
after dipping Ni grids in the colloidal suspensions. Microscopic measurements, EDX (energy-dispersive
X-ray spectrometry) analysis, and SAED patterns were obtained using a Jeol 2010 instrument operating
at 200 kV and equipped with an EDS Link ISIS EDX micro-analytic system.
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2.4. Raman Spectroscopy
Raman spectra of the bimetallic nanoparticles deposited on aluminum plate were measured at
different points of the dried sample by using a Renishaw RM2000 micro-Raman instrument equipped
with a diode laser emitting at 785 nm. Sample irradiation was accomplished by using the 50×
microscope objective of a Leica Microscope DMLM. The backscattered Raman signal was fed into the
monochromator through 40 µm slits and detected by an air-cooled CCD (2.5 cm−1 per pixel) filtered by
a double holographic Notch filters system. Spectra were calibrated with respect to a silicon wafer at
520 cm−1 .
SERS spectra of 10−4 M 2,2′ -bipyridine (Sigma-Aldrich, St. Louis, Missouri (USA), 99% purity)
in bimetallic colloid were obtained after addition of 10−2 M NaCl (Sigma-Aldrich, St. Louis,
Missouri (USA), 99.999% purity) in order to increase the SERS enhancement without compromising
the colloidal stability. The 647.1 nm line of a Kripton ion laser and a Jobin-Yvon HG2S monochromator
equipped with a cooled RCA-C31034A photomultiplier were used. A defocused laser beam with
100 mW power was employed for impairing thermal effects. Power density measurements were made
using a power meter instrument (model 362; Scientech, Boulder, CO, USA) giving ∼5% accuracy in the
300–1000 nm spectral range.
2.5. X-ray Photoelectron Spectroscopy
XPS measurements were made using a non-monochromatic Mg Kα X-ray source (1253.6 eV) and a
VSW HAC 5000 hemispherical electron energy analyzer operating in the constant pass energy mode at
Epas = 44 eV. The bimetallic colloidal samples were prepared just before the analysis by depositing a few
drops of the colloidal suspensions on soda glass substrates and letting the solvent evaporate. In order
to increase the amount of deposited nanoparticles, this procedure was repeated several times. Then,
the glasses with bimetallic nanoparticles were introduced into the UHV system via a loadlock under
inert gas (N2 ) flux and kept in the introduction chamber overnight, allowing the removal of volatile
substances as confirmed by the achieved pressure value (2 × 10−9 mbar), just above the instrument
base pressure. The obtained spectra were referenced to the C 1s core peak at 284.8 eV assigned to the
adventitious carbon. The spectra were fitted using CasaXPS software version 2.3.15.
2.6. Density Functional Theory Calculations
All DFT calculations were carried out using the GAUSSIAN 09 package [27]. Optimized geometries
were obtained at the DFT level of theory, employing the widely adopted Becke 3-parameter hybrid
exchange functional (B3) combined with the Lee–Yang–Parr correlation functional (LYP) [28,29],
along with the Lanl2DZ basis set and pseudopotential [30–32]. All parameters were allowed to
relax and all calculations converged toward optimized geometries corresponding to energy minima,
as revealed by the lack of negative values in the frequency calculation. Dispersion interactions were
taken into account using Grimme’s D3 scheme along with Becke–Johnson damping [33]. A scaling
factor of 0.98 for the calculated harmonic wavenumbers was employed, as usually performed in
calculations at this level of theory [34–38]. The calculated Raman intensities were obtained by following
the indications of reference [24].
3. Results and Discussion
3.1. Microscopic Investigation
The colloid obtained by laser ablation of an iron target in water has a zeta potential value of +20.0
mV, which is lowered to +13.9 mV when a gold target is also ablated. This lowering is due to the
adsorption of negative ions deriving from the water environment on the (positive) surface of the gold
nanoparticles. However, the zeta potential is sufficient to provide stability to the bimetallic colloid,
with no precipitate visible a week after preparation. The zeta potential data are reported in Figure S1.
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The bimetallic colloid presents a red color and exhibits magnetic properties, as shown in Figure S2.
When approaching a magnet, the colloidal nanoparticles aggregate, until they appear as a dark red
precipitate visible to the naked eye.
Observing the TEM images (see Figure 1), the colloid consists of spheroidal particles with
dimensions ranging from a few nanometers to almost 20 nm in diameter. Based on the contrast,
two kind of nanoparticles, with weaker contrast (low contrast, LC) and stronger contrast (high contrast,
HC), can be distinguished. LC particles are mainly particles of a few nanometers, whereas the HC
particles have two size classes: particles of a few nanometers and particles with a diameter of 10–20 nm.
From the point of view of the metallic composition, the sample contains Fe and Au (in addition to
O). The large HC particles are substantially composed of Au. Large LC particles are composed of Fe.
The small particles, for which it is not possible to make EDX measurements on single individuals,
show both Au and Fe. The EDX analyses of typical HC and LC nanoparticles are reported in Figure S3.
Figure 1. Low-magnification TEM micrograph of the Fe3 O4 /Au colloid (right), showing Au particles
embedded in low-contrast matrix, along with SAED analysis (Mag: magnetite).
SAED on an enlarged field of the sample (Figure 1) shows interplanar distances consistent with
magnetite (Fe3 O4 ) and metallic gold. In particular, the ring around 2.36 Å is quite strong and must
be attributed to the 111 reflection of gold [39]. Analysis of the high-resolution microscopic images
(HRTEM) (Figure 2) shows interplanar distances typical of metallic Au for HC particles, both large and
small. Crystalline growth in HC particles is observed as icosahedrons. LC particles show interplanar
distances typical of magnetite [40], but also the presence of some small particles with amorphous
characteristics. In conclusion, the gold nanoparticles appear to be embedded in a mucilaginous
matrix consisting of magnetite in the form of nanoparticles of various sizes, with scarce tendency to
aggregation, which can be separated from the aqueous environment under the action of a magnetic field.
−
Figure 2. High-resolution TEM images of the Fe3 O4 /Au colloid, showing the interplanar distances in
gold (Au) and magnetite (Mag) particles.
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3.2. Raman Spectra
After centrifugation of a portion of the bimetallic colloid, the precipitate was examined using a
micro-Raman spectrometer and showed the typical Raman band of magnetite (Fe3 O4 ) at 665 cm−1 (see
Figure 3), in agreement with the literature [41,42], confirming the magnetic properties of the nanosystem.
Figure 3. Micro-Raman spectra of the Fe3 O4 /Au nanoparticles deposited on Al plate at different points
of the dry film. Excitation: 785 nm.
3.3. XPS Measurements
The XPS spectrum relative to the f7/2 –f5/2 gold spectral region (see Figure 4) can be fitted by two
components: the main one is located at 84.3 eV (f7/2 ), while the subordinate is located at a higher
energy value (85.4 eV). These components can be attributed to Au(0) and Au(I), respectively, as well as
occurring in the case of gold laser-ablated in deionized water [43].
Figure 4. XPS spectrum of the bimetallic nanoparticles in the gold f7/2 –f5/2 spectral region.
3.4. UV–Visible Extinction Spectra
Figure 5 shows the UV–visible absorption spectra of the colloidal samples obtained by laser
ablation of iron (Spectrum A) and then laser ablation of gold (Spectrum B). The band observed in
Spectrum B around 525 nm is attributable to the surface plasmon resonance of non-aggregated gold
nanoparticles. By adding 2,2′ -bipyridine (bpy), the plasmon band is shifted to 535 nm.
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′
Figure 5. UV–visible extinction spectra of the Fe3 O4 (A) and Fe3 O4 /Au (B) colloids. Spectrum C refers
to the bimetallic colloid in the presence of 2,2′ -bipyridine.
′
3.5. Surface-Enhanced Raman Scattering
The evidence of the surface plasmon band of nanosized gold particles (see Figure 5) suggests
the possibility of SERS activity of this bimetallic system. However, the molecular ligand needs to be
effectively adsorbed to provide a reliable Raman enhancement. Hence, in order to find confirmation
of our hypothesis, we checked the SERS response of the bimetallic colloid in the presence of 10−4 M
2,2′ -bipyridine (bpy). By activation with NaCl, we observed a satisfactory SERS spectrum of bpy in−
the bimetallic colloid (Figure 6), with results quite similar to those reported in the literature for the
′
adsorption of bpy on pure gold colloidal nanoparticles [44]; those frequencies are reported in Table 1
for comparison. This similarity indicates that our SERS spectrum is attributable to 2,2′ -bipyridine
bound to the gold nanoparticles present in the Fe3 O4 colloidal matrix, which does not impair the
′
ligand adsorption on gold. In Table 1 the IR and Raman frequencies of solid bpy [45] are also reported.
−
Figure 6. SERS spectrum of 2,2′′ -bipyridine in the bimetallic colloid. Excitation: 647.1 nm.
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Table 1. Observed and calculated frequencies (cm−1 ).
Symmetry
Species [45]
bpy
IR/Raman [45]
Bu
Ag
Bu
Ag
Ag
Ag
Bu
Bu
Bu
Bu
Ag
Ag
Ag
Bu
Ag
Bu
Ag
Bu
Ag
Bg
Bu
Au
Ag
Bu
Au
Bg
Bg
Bu
Bg
Au
Ag
Bg
Au
Bu
Ag
Bu
Bg
Au
Ag
Bg
Au
Ag
Bg
1575
1589
1550
1572
1482
1446
1450
1410
1265
1250
1309
1301
1236
1140
1146
1085
1094
1065
1044
——
1040
——
994
995
975
——
909
890
815
755
764
742
740
655
614
620
550
——
440
409
——
332
224
bpy/Au
SERS
1598
1567
1485
1358
1306
1283
1213
1179
1059
1016
935
890
764
748
651
356
bpy/Au+
Calc.
bpy/Au◦
Calc.
1603
1598
1590
1575
1491
1469
1445
1429
1324
1331
1306
1289
1294
1206
1190
1128
1114
1078
1062
1039
1038
1033
1007
993
993
991
923
825
908
786
760
746
747
658
653
632
555
447
441
422
405
353
226
1592
1594
1581
1570
1483
1460
1433
1419
1323
1298
1313
1286
1274
1184
1176
1113
1096
1077
1049
1032
1023
979
1022
985
983
976
913
827
913
774
767
754
756
656
636
617
561
479
415
415
380
327
241
bpy/Au
SERS [44]
1586
1562
1479
1301
1173
1057
1010
761
646
403
353
The addition of NaCl was necessary to obtain a satisfactory SERS spectrum of bpy. The presence
of chloride anions, which strongly adsorb on the surface of the gold nanoparticles, has double validity
because it can promote both the nanoparticle aggregation necessary for an efficient SERS response and
the formation of active sites capable of strongly binding ligand molecules, similar to what occurs with
silver nanoparticles activated by chloride anions [46–48]. In practice, in our bimetallic suspension it
was not possible to obtain a valid SERS spectrum of 2,2′ -bipyridine, even at 10−4 M concentration,
unless we added NaCl. To induce particle aggregation or concentration, magnetic attraction could be
employed, instead of adding chloride anions, in order to improve the SERS signal of the adsorbed
ligands. In the future, this method will be tested by also evaluating the occurrence of possible problems
in colloidal stability. In the present work, we used chloride activation to obtain an effective SERS
response in a stable aqueous suspension in order to evaluate the possible use of these nanoparticles in
biomedical applications.
However, one last problem remains to be solved: what kind of active site on the surface of the
gold nanoparticles is involved in the interaction with the molecule, given that the XPS spectrum also
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showed the presence of ionized gold such as Au(I)? DFT calculations on the molecule linked to a
neutral or a positively charged gold adatom can help in this purpose.
3.6. DFT Calculations
In Table 1 the experimental SERS frequencies of bpy are compared with those calculated for
bpy/gold model complexes, along with the IR and Raman frequencies of solid bpy [45], whose molecules
present a trans-planar structure. We observe that the prominent SERS bands (at 353, 651, 764, 1016,
1059, 1179, 1306, and 1485 cm−1 ) correspond to the bpy Raman bands of Ag symmetry species. For the
simulation of the SERS spectra of the adsorbed bpy, we used the functional B3LYP, along with the
Lanl2DZ basis set.
The choice to use this basis set was justified by the following considerations.
(a). This basis set has been widely employed in many literature articles to successfully reproduce
both the structural and vibrational properties of different molecules. Here we report only a few
very recent examples [49–55].
(b). Core electrons can be treated in an approximate way via effective core potentials (ECPs).
This treatment includes scalar relativistic effects, which are important for the proper description
of the geometric, electronic, and spectroscopic properties of heavy atoms. The LanL2DZ basis set
is the best known basis set for molecular systems containing these atoms and for the efficient
simulation of the Raman spectra of complexes with transition metals and the SERS spectra of
molecules adsorbed on silver or gold nanoparticles, as demonstrated by many recent papers (for
example, [38,50,52–55]).
We also tested the reliability of this basis set by examining the free 2,2′ -bipyridine molecule in its
typical trans conformation and comparing our DFT results with those reported in the literature [48] for
the same molecule, with the same functional but with a different basis, 6-31+G*. The Lanl2DZ basis
set used by us provided results generally comparable with those reported in the literature, as shown
in the Supplementary Materials, regarding both structural parameters (Table S1) and vibrational
frequencies (Table S2).
DFT calculations were performed for two gold complexes, where the bpy molecule in cis
conformation is linked by means of the nitrogen atoms to a neutral Au atom or to a gold cation, Au+ .
The complex bpy/Au+ better reproduces the observed SERS frequencies than the complex bpy/Au◦ .
In the first case, the average error between the calculated and observed frequencies is 7.75 cm−1 ; in the
second one, the average error is significantly larger at 13.27 cm−1 . In addition, the interaction of the
molecule with a neutral atom is quite weak, in comparison with the interaction with Au+ , as shown by
the bpy→gold electronic charge transfers and the N–gold bond distances reported in Table 2, with |e|
being the unsigned electron charge. The Mulliken partial charges are reported in Table S1. Hence,
it is possible to conclude that the ligand molecules, when they adsorb on gold, strongly interact with
positively charged active sites of the nanoparticle surface. Figure S4 shows the calculated normal
modes of the bpy/Au+ complex relative to the prominent SERS bands. All these correspond to in-plane
vibrations of the bpy molecule, in particular, the bands observed at 356, 651, 764, and 1016 cm−1
correspond to ring deformations, and those at 1306 and 1485 cm−1 to H bending modes.
To better quantify the charge transfer, we also employed a descriptor (called DCT , charge transfer
distance) [56] that was mainly proposed to describe electron–hole displacement in optical excitations
(Sn →S0 , n = 1, 2 . . . , with S being singlet electronic states). The DCT version adopted here is based on a
partial charge (namely Mulliken’s) approach, using the spreadsheet reported in the Supplementary
Materials of reference [56] and already employed with success for electronic transitions [37,57]. With the
DCT scheme, the difference between the electronic density of the ground state (S0 ) and the excited
state of interest (Sn ) gives rise to a charge separation that can be modeled in a dipolar fashion due to
a barycenter of reduced electronic charge (Q+ here) and a barycenter of increased electronic charge
(Q− here). The vector connecting the two points gives a straightforward depiction of the direction and
−
−
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→
−
magnitude of the overall charge movement and allows
for calculating the amount of charge transferred.
−
While this powerful yet easy approach was mainly developed to model different electronic states
of the same system, it can, in principle, also be adopted for ground states of systems with different
components (as long as the geometry of common moieties of the relaxed systems does not change
→
significantly); this is discussed in more detail in the Supplementary Materials.
Table 2. Calculated charge transfers and bond distances.
−
Model Complex
Bpy→Gold Charge Transfer
N–Gold
Bond Distance
bpy/Au◦
bpy/Au+
−0.232 |e|
−0.502 |e|
2.62 Å
2.23 Å
To the best of our knowledge, this is the first time the DCT index has been adopted to describe
charge rearrangements due to surface effects and not to light excitations, and it is reported in Figure 7.
−
Figure 7. Adsorption model of bpy on Au+ adatom. Points Q+ and Q− are the barycenters of the
depletion and the increment of electron density, respectively, with respect to an isolated bpy molecule
(cis conformation) and an isolated Au+ cation.
With this approach, the computed charge transfer distance is about ~1.95 Å and the amount of
charge moving is ~1.1 |e|, higher than that estimated from just the increase of electron charge on the
Au atom; this is due to the fact that the DCT takes into account the charge displacement over the
whole system.
Finally, it is appropriate to define the limits of the DFT calculation model used by us, based on
the chemical interaction between a bpy molecule and a single (positively charged) metal adatom.
This complex correctly reproduces the positions of the SERS bands, because it is able to predict how
the structure and, therefore, the force constants of the molecule change due to interaction with the
metal. However, our model fails to satisfactorily reproduce the observed SERS intensities, as shown in
the simulated SERS spectrum reported in Figure S5. Actually, in the case of 2,2′ -bipyridine, which is
linked to gold in a bidentate way by means of the lone pairs of the nitrogen atoms, our model cannot
simulate the effect that the gold nanoparticles have on the polarizability of the adsorbed molecule and,
therefore, on the intensities of the observed SERS spectrum.
4. Conclusions
Stable nanoparticles constituted of gold and iron oxide were obtained in an aqueous environment
by means of laser ablation of Fe and Au targets in two successive steps, avoiding the presence
of surfactants, stabilizers, residual reductants, and by-products which could interfere in both the
adsorption and the detection of ligands. By using this technique, a mere mixture of two different
metal colloids is not obtained, because gold nanoparticles are found to be embedded in the colloidal
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matrix of iron oxide. The latter was identified as magnetite by both microscopic and spectroscopic
analyses. The plasmonic properties of the obtained colloidal nanosystem, as well as its capability of
ligand adsorption, were tested by SERS spectroscopy using 2,2′ -bipyridine (bpy) as a probe molecule.
Thanks to the DFT calculations performed on model systems of gold/ligand complexes, it is possible to
argue that positively charged active sites of the gold nanoparticles are responsible for the adsorption
of ligand molecules when these approach the metal surface. In this way, strong interaction takes
place between molecule and metal, with consequent efficient SERS enhancement, involving the charge
transfer of one electron from the molecule to the metal.
Unlike the mixed Ag/Fe3 O4 and Ag/NiO colloids previously prepared by two-step laser
ablation [23,24], the present magneto-plasmonic nanoparticles are more biocompatible and are therefore
potentially useful not only in the field of sensors, but also for biomedical applications. Our bimetallic
colloidal suspensions are expected to have very low toxicity. Gold nanoparticles are known to be
biocompatible and chemically stable, making them ideally suitable for biological applications [58].
Also, magnetite nanoparticles can exhibit low toxicity [59–61], which is closely dependent on the
preparation method. In this respect, laser ablation in pure water represents the procedure of choice for
the best biocompatibility properties.
Finally, a possible interpretation of the connection between Fe3 O4 and Au nanoparticles can be
proposed. In our sample, colloidal gold is intimately linked to the ferromagnetic material constituted of
a mucilaginous matrix of small magnetite (Fe3 O4 ) nanoparticles. Hence, all the bimetallic material can
be completely separated by magnetic attraction from the aqueous environment wherein it is dispersed.
In the literature [26], ultrafine Fe3 O4 nanoparticles were employed to remove heavy metal ions from
contaminated waters, thanks to their excellent adsorption performance. In a similar way, the magnetite
nanoparticles obtained by laser ablation could act as adsorbents for the laser-ablated gold nanoparticles,
forming a mixed bimetallic colloidal suspension. In fact, we verified by XPS measurements and DFT
calculations that our gold particles have a positively charged surface. This could make them suitable
to be captured by the magnetite nanoparticles, similarly to what happens with heavy metal ions.
Supplementary Materials: The Supplementary Materials are available online at http://www.mdpi.com/2079-4991/
10/1/132/s1. Figure S1: Zeta potential data; Figure S2: Fe3 O4 /Au bimetallic colloidal sample, before and after
application of magnetic field; Figure S3: EDX analysis; Figure S4: Calculated normal modes of the bpy/Au+
complex relative to the prominent SERS bands. The hydrogen atoms are omitted; Figure S5: Simulated SERS
spectrum for the bpy/Au+ complex; Table S1: Mulliken partial charges in the bpy/gold complexes; Table S2:
Observed and calculated vibrational frequencies (cm−1 ) of bpy; Table S3: Mulliken partial charges.
Author Contributions: M.M.-M. and E.G. produced the bimetallic colloids; M.M.-M. recorded Raman and
absorption spectra; E.G. performed the microscopic analysis; F.M.-M. carried out quantum chemical calculations
and designed the DCT analysis; M.M.-M. coordinated and designed the research; M.M.-M. and F.M.-M. improved
the manuscript accordingly to the Reviewers’ comments. All authors have read and agree to the published version
of the manuscript.
Funding: This research received no external funding.
Acknowledgments: The authors acknowledge MDPI editorial’s invitation to contribute to the Nanomaterials
Special Issue dedicated to Laser Synthesis of Nanomaterials. They also wish to thank Stefano Caporali (UniFI) for
his support in the XPS measurements.
Conflicts of Interest: The authors declare no conflict of interest.
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