R. Taheri-Ledari, et al.
Ultrasonics-Sonochemistry61(2020)104824
used as considerable heterogeneous nanoscale catalysts in chemical and
biochemical reactions, due to their magnetic core and shells with
functionalization capability [9,10]. One of the most efficient methods
for surface functionalization of NPs is ultrasonication that provides a
proper driving force for chemical reactions and gives us more satisfying
results about physical properties of desired product [11–13]. This field
of study also investigates the effect of ultrasound (US) waves on uni-
formity of the new layers loaded onto the surface of NPs [14]. In ad-
dition, both morphology and particle size of the modified NPs are well
controlled via ultrasonication in comparison with other methods. Thus,
in this work, ultrasonication was used as an eco-friendly and convenient
procedure for the fabrication of the uniform NPs. The first and foremost
reason for using the US irradiation, instead of other methods such as
microwave (MW) and stirring under reflux condition is that the other
approaches do not give us the acceptable results. For instance, Mak-
tedar, et al. compared US irradiation with stirring method to prepare
graphene oxide, and found out that more desirable result is obtained
using US wave irradiation. In addition, they achieved desired products
in shorter times without using additive hazardous compounds [15]. As
another sample, Liu, et al. have compared ultrasonication with stirring
in production of polyaniline by using various analyses such as X-ray
diffraction, energy dispersive X-ray and transmission electron micro-
scopy [16]. The obtained results implied that the crystallinity of poly-
aniline produced via ultrasonication was considerably higher than that
produced via the stirring. Ultrasonication is cheaper, faster and more
effective method for preparation of heterogeneous NPs in liquid phase
in comparison with other methods. From physical aspect, ultrasonica-
tion creates an effective vibration that can form a monodispersed het-
erogeneous mixture and inhibit aggregation of the particles. From
chemical aspect, high-energy bubbles created by US waves (cavitation)
into a water bath provide the required energy for reactants to occur a
chemical reaction [17]. This phenomenon occurs when the fast vibra-
tion of the probe produces numerous bubbles in micro-scale.
In this study, firstly, we present a novel strategy for preparation of
DTT-functionalized silica coated Fe3O4 NPs (Fe3O4@Pd/CaCO3-DTT)
via ultrasonication, then its application is precisely investigated for S–S
bond reduction. A precise comparison between the different methods
has been made for preparation of the nanocomposite and this is de-
monstrated that ultrasonication is the most efficient method for this
purpose. In this regard, an ultrasonic bath with frequency 50 KHz and
power density 200 W L−1 has been suitably applied. Then, to show high
catalytic performance of Fe3O4@Pd/CaCO3-DTT nanocomposite, in-
itially a model reaction of S–S reduction by the prepared heterogeneous
nano-reductant was executed using diphenyl disulfide, and the optimal
conditions was indicated. Afterward, the catalytic performance Fe3O4@
Pd/CaCO3-DTT has been more investigated using different disulfide
derivatives such as bis(4-chlorophenyl) disulfide and bis(4-methyl-
phenyl) disulfide. Finally, the catalytic activity of the novel designed
reducing agent is monitored in a real antibody reduction on trastu-
zumab (Herceptin) antibody which is an IgG1 monoclonal antibody.
The characterization of the desired nanoscale product such as general
structure, size and morphology, surface functionalization ratio, mag-
netic property, biological features and crystal pattern were compre-
hensively investigated, as well. This novel nano-organocatalyst could be
a substantial alternative for homogeneous S–S bond reducing agents
through its significant properties. By using this product, dialysis process
is eliminated after reduction of antibodies and they would be easily
purified by using an external magnet. Here, to prepare the Fe3O4@Pd/
CaCO3-DTT NPs, a convenient strategy is presented in which palladium
deposited on calcium carbonate (Pd/CaCO3), known as Lindlar catalyst,
and US waves are employed. Also, through the precise comparisons,
this has been disclosed that there is a great synergistic catalytic effect
between the fabricated Fe3O4@Pd/CaCO3-DTT NPs and US waves.
2. Results and discussion
2.1. Preparation
To initiate our study, Fe3O4 NPs were synthesized according to the
literature [18]. Firstly, iron salts were dissolved in distilled water at
80 °C. Then, ammonia solution was added dropwise until pH = 12 was
achieved. Dark particles of Fe3O4 were collected by an external magnet
in the bottom of the flask and were washed for several times with
distilled water and ethanol. From here onwards, to obtain more uniform
particles and also to inhibit the agglomeration of the particles, the
preparation process was continued by US waves. In the next stage,
Fe3O4 NPs were coated by silicate network by using of tetraethyl or-
thosilicate (TEOS). A TEOS solution (20 wt%) were dropwise added to
the finely dispersed mixture of Fe3O4 NPs in polyethylene glycol (PEG-
300), distilled water and ethanol, during sonication [19–21]. Light
brown silica-coated iron oxide NPs (Fe3O4@SiO2) were magnetically
collected, washed and dried at 40 °C.
Lindlar catalyst is a heterogeneous catalyst that is formed from
palladium deposited on calcium carbonate then poisoned with various
forms of lead or sulphur, and used for hydrogenation of alkynes to al-
kenes [22]. As an advanced form of this reagent, palladium NPs were
prepared and immobilized on calcium carbonate, then was used for
coupling reaction through the presence of palladium NPs. This type of
Pd particles help us to have monodispersed Pd particles on a substrate
and use them for various aims without need to design difficult strate-
gies. The electronic interactions with oxygen atom is considered as one
the most important properties of Pd, as well [23,24]. In this work, Pd
NPs were simultaneously prepared and immobilized on calcium car-
bonate via US wave irradiation, because we needed an appropriate
resource to provide driving force for both production and immobiliza-
tion with acceptable uniformity onto the surface of Fe3O4@SiO2 NPs. In
this regard, different methods such as reflux condition, high tempera-
ture and pressure into an autoclave and MW irradiation were also ex-
perimented, and the obtained results were compared with US-assisted
method (Table 1). The obtained results exhibited that the core/shell
structure of Fe3O4@SiO2 NPs was damaged at high temperatures, and
MW was not an appropriate method due to its low performance.
Therefore, as the next stage of preparation process, we have used Pd/
CaCO3 to execute a convenient and high performance core-coating
method that was implemented by US waves. In this regard, Fe3O4@SiO2
core/shell NPs were well dispersed in dry tetrahydrofuran (THF) by
ultrasonication, then Pd/CaCO3 were added and the mixture was put
into an US bath. Time, temperature and frequency are three main fac-
tors in NPs coating reactions that were carefully monitored to obtain
the optimum conditions by investigating Fourier-transform infrared
Table 1
Comparison of the different methods for preparation of Fe3O4@SiO2@Pd/
CaCO3 NPs.
Entry Method
Temp. (oC) Conditions
Time (h) W (%)
1
2
3
4
Reflux
Autoclave
MW irradiation
64
150
–
THF
DMF
12
12
1
3.8
1.2
0.8
THF / F: 50 KHz,
PD: 200 W
Ultrasonication 0–5
2
* Optimum condition by considering particle size, uniformity, mono-
dispersity, morphology and surface functionalization ratio (W% of sulfur ele-
ment).
a
Weight percentage obtained from EDX and CHNS analyses (Tables S1 and
S2 in the SI section).
b
A conventional microwave oven at a power (P) of 300 W was used; Temp.:
temperature; MW-P: MW irradiation power; F: ultrasonication frequency (KHz),
PD: ultrasonication power density (W/L), US bath was used for all of the ul-
trasonication steps; All the reactions were carried out under N2 atmosphere.
2