Effect of different molecular coatings on the
heating properties of maghemite nanoparticles†
Marco Sanna Angotzi, ‡
ab
Valentina Mameli, ‡
ab
Shankar Khanal,
c
Miroslav Veverka,
c
Jana Vejpravova
*
c
and Carla Cannas
*
ab
In this work, the effect of different molecular coatings on the alternating magnetic field-induced heating
properties of 15 nm maghemite nanoparticles (NPs) in water dispersions was studied at different
frequencies (159–782 kHz) and field amplitudes (100–400 G). The original hydrophobic oleate coating
was replaced with dimercaptosuccinic acid (DMSA) or polyethylene glycol trimethoxysilane (PEGTMS),
while cetrimonium bromide (CTAB) or stearic acid-poloxamer 188 (SA-P188) was intercalated or
encapsulated, respectively, to transfer the dispersions into water. Surface modification, based on
intercalation processes, induced clustering phenomena with the formation of spherical-like assemblies
(CTAB and SA-P188), while ligand-exchange strategies kept the particles isolated. The clustering
phenomenon has detrimental effects on the heating performances compared with isolated systems, in
line with the reduction of Brown relaxation times. Furthermore, broader comprehension of the heating
phenomenon in this dynamic system is obtained by following the evolution of SPA and ILP with time and
temperature beyond the initial stage.
Introduction
Spinel ferrite nanoparticles (NPs), thanks to the excellent
control of magnetic properties through chemical manipulation,
represent ideal systems for many elds, such as environmental
applications
1–5
and biomedicine.
6–8
In particular, their ability to
release heat when subjected to an alternating magnetic eld
(i.e., magnetic heat generation) makes them appealing for
catalysis
9–12
and magnetic uid hyperthermia (MFH).
13–15
When
NPs are in the superparamagnetic (SPM) state, according to
linear-response theory (LRT),
16
heat is released through relaxa-
tion losses, which can be associated with vector magnetization
reversal inside the particle (N´ eel relaxation time, s
N
, eqn (1)),
and through physical rotation of the particle in a uid (Brown
relaxation time, s
B
, eqn (2)):
s
N
¼ s
0
e
KV
k
B
T
(1)
s
B
¼
3hV
H
k
B
T
(2)
where s
0
is the characteristic relaxation time (10
9
to 10
11
s),
17
K the anisotropy constant, V the inorganic volume of the
particle, k
B
the Boltzmann constant, T the temperature of the
system, h the viscosity of the medium, and V
H
the hydrody-
namic volume of the particle. Therefore, the effective relaxation
time (s) accounts for both N´ eel and Brown mechanisms and is
dened by:
1
s
¼
1
s
N
þ
1
s
B
(3)
which means that the faster relaxation time dominates the
other one. For instance, considering magnetite NPs (K ¼ 3 10
4
Jm
3
), the N´ eel relaxation is the dominant mechanism up to 15
nm, while beyond 20 nm, the Brown one prevails. Within the
validity of LRT, the specic power absorption (SPA, or specic
loss power SLP or specic absorption rate SAR) is related to the
loss power density by the mass density of the particles (SPA ¼ P/
r), where P is dened as:
P ¼
m
0
2
M
s
2
VH
0
2
3k
B
T s
ð2pf sÞ
2
1 þð2pf sÞ
2
(4)
where m
0
is the vacuum permeability, M
s
the saturation
magnetization, V the particle volume, H
0
the applied eld, k
B
the Boltzmann constant, T the temperature, and f the applied
frequency.
Other mechanisms responsible for the heat release are
hysteresis losses, typical of multi-domain or blocked single-
domain nanoparticles, which are associated with hysteretic
a
Department of Chemical and Geological Sciences, University of Cagliari, S.S. 554
Bivio per Sestu, Monserrato, 09042 CA, Italy. E-mail: ccannas@unica.it
b
Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali
(INSTM), Via Giuseppe Giusti 9, 50121 Firenze (FI), Italy
c
Department of Condensed Matter Physics, Charles University, Ke Karlovu 5, 12116
Prague 2, Czech Republic. E-mail: jana@mag.mff.cuni.cz
† Electronic supplementary information (ESI) available: TGA analyses; magnetic
measurements; MFH measurements; literature comparison. See DOI:
10.1039/d1na00478f
‡ These authors contributed equally to this work.
Cite this: Nanoscale Adv., 2022, 4, 408
Received 24th June 2021
Accepted 8th November 2021
DOI: 10.1039/d1na00478f
rsc.li/nanoscale-advances
408 | Nanoscale Adv., 2022, 4, 408–420 © 2022 The Author(s). Published by the Royal Society of Chemistry
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