nanomaterials

Review Magnetism of Nanoparticles: Effect of the Organic Coating

Maryam Abdolrahimi 1,2, Marianna Vasilakaki 3 , Sawssen Slimani 1,4,5, Nikolaos Ntallis 3, Gaspare Varvaro 1, Sara Laureti 1, Carlo Meneghini 2 , Kalliopi N. Trohidou 3, Dino Fiorani 1 and Davide Peddis 1,5,*

1 Istituto di Struttura della Materia—CNR, Monterotondo Scalo, 00015 Rome, Italy; [email protected] (M.A.); [email protected] (S.S.); [email protected] (G.V.); [email protected] (S.L.); dino.fi[email protected] (D.F.) 2 Dipartimento di Scienze, Università degli Studi Roma Tre, 00146 Rome, Italy; [email protected] 3 Institute of Nanoscience and Nanotechnology, NCSR “Demokritos”, 15310 Agia Paraskevi, Attiki, Greece; [email protected] (M.V.); [email protected] (N.N.); [email protected] (K.N.T.) 4 Laboratory of Applied Physics, Faculty of Sciences of Sfax, University of Sfax, B.P. 1171, Sfax 3000, Tunisia 5 Dipartimento di Chimica e Chimica Industriale, Università degli Studi di Genova, Via Dodecaneso 31, 16146 Genova, Italy * Correspondence: [email protected]; Tel.: +39-347-772-8174

Abstract: The design of novel multifunctional materials based on nanoparticles requires tuning of their magnetic properties, which are strongly dependent on the surface structure. The organic coating represents a unique tool to significantly modify the surface structure trough the bonds between the ligands of the organic molecule and the surface metal atoms. This work presents a critical overview of the effects of the organic coating on the magnetic properties of nanoparticles trough a selection



 of papers focused on different approaches to control the surface structure and the morphology of nanoparticles’ assemblies. Citation: Abdolrahimi, M.; Vasilakaki, M.; Slimani, S.; Ntallis, N.; Keywords: magnetic nanoparticles; molecular coating; magnetic properties Varvaro, G.; Laureti, S.; Meneghini, C.; Trohidou, K.N.; Fiorani, D.; Peddis, D. Magnetism of Nanoparticles: Effect of the Organic Coating. Nanomaterials 1. Introduction 2021, 11, 1787. https://doi.org/ 10.3390/nano11071787 Since the pioneering works of Louis Néel in 1944 [1], magnetic nanoparticles have been the subject of continuous interes [2–9], due to the possibilities of designing new Academic Editor: Oscar Iglesias magnetic systems with finely tuned properties, thanks to the enormous progress in the synthesis and diagnostic methods, and the increasing number of applications, e.g., Received: 21 May 2021 biomedicine [10–13], ferrofluid technology [14], catalysis [15,16], sensors [17–19], magnetic Accepted: 1 July 2021 energy storage [20,21], electronics [22] etc. Therefore, the understanding of the physics of Published: 9 July 2021 magnetic nanoparticles and the control of their properties still represent a very attractive and challenging topic for both fundamental research and technological applications. Publisher’s Note: MDPI stays neutral Entering the nanometer-scale regime (<100 nm), the magnetic properties of materials with regard to jurisdictional claims in change progressively with decreasing size until they show substantial differences with published maps and institutional affil- respect to the bulk state when the fraction of atoms lying on the surface becomes larger iations. than that in the particle core, making the role of the surface dominant. For this reason, in the last decades ever-increasing interest has been devoted to approaches aimed at controlling the contribution of the surface to optimize the magnetic properties for meeting the requirements for specific applications. Copyright: © 2021 by the authors. The surface structure is disordered, due to the breaking of lattice symmetry, with Licensee MDPI, Basel, Switzerland. random missing bonds and then incomplete atomic coordination sphere with respect to the This article is an open access article atoms in the core and in the bulk state. This induces significant changes in the magnetic distributed under the terms and properties of nanoparticles, which become more and more important with decreasing conditions of the Creative Commons particle size, i.e., increasing surface/volume ratio: Attribution (CC BY) license (https:// − creativecommons.org/licenses/by/ increase of magnetic anisotropy due to the surface contribution arising from the 4.0/). random spin pinning on the surface. The “surface anisotropy” was described by

Nanomaterials 2021, 11, 1787. https://doi.org/10.3390/nano11071787 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, 1787 2 of 17

Nanomaterials 2021, 11, 1787 − increase of magnetic anisotropy due to the surface contribution arising from2 of the 16 random spin pinning on the surface. The “surface anisotropy” was described by Néel [23], using a phenomenological model which assumes the lack of atomic bonds on the surface of a crystal at the origin of this form of anisotropy. The surface anisotropy Néel [23], using a phenomenological model which assumes the lack of atomic bonds on contribution was described as a pair interaction of nearest neighbors spins, the surface of a crystal at the origin of this form of anisotropy. The surface anisotropy depending on their distance and orientation. Later studies clarified the mechanism contribution was described as a pair interaction of nearest neighbors spins, depending of the surface anisotropy pointing out the very important role of spin-orbit coupling on their distance and orientation. Later studies clarified the mechanism of the surface anisotropyand ligand pointingfield [24,25]. out the very important role of spin-orbit coupling and ligand − fielddecrease [24,25 of]. saturation magnetization in ultrasmall particles because of the surface − decreasespin disorder of saturation that makes magnetization the surface in ultrasmall magnetically particles inactive because (dead of the surfacelayer). spinThis disorderrepresents that a strong makes limitation the surface for magnetically those applications inactive of (dead nanoparticles layer). This requiring represents high aparticle strong moments. limitation for those applications of nanoparticles requiring high particle moments.The surface [25,26] contribution can be controlled through the interaction with anotherThe surfacephase in [25 contact,26] contribution with it, cani.e., beexploi controlledting the through interface the interactioneffect, which with can another be of phasedifferent in contact type depending with it, i.e., on exploiting the nanoparticle the interface system effect, and the which nature can of be the of different two phases type in dependingcontact [27]. on As the an nanoparticle example, in system bimagnetic and the core/shell nature of particles, the two phasescombining in contact ferromagnetic [27]. As anand example, antiferromagnetic in bimagnetic phases, core/shell the exchange particles, combiningcoupling at ferromagnetic the interface andbetween antiferromag- the core neticand the phases, surface the shell exchange can give coupling rise to at the the exch interfaceange bias between phenomenon the core [28,29], and the which surface is shelloften canaccompanied give rise to by the an exchange increase biasof anisotropy, phenomenon due [to28 ,the29], so-called which is “exchange often accompanied anisotropy”. by anThe increase effect on of anisotropy,the anisotropy due and to the saturation so-called magnetization “exchange anisotropy”. can be tuned The controlling effect on the the anisotropycore diameter, and saturationthe thickness magnetization of the surface can and be tunedthe chemical controlling composition. the core diameter, The interface the thicknesscan play ofa thedifferent surface role and when the chemical the phase composition. in contact Theis non-magnetic, interface can playe.g., aan different organic rolemolecule. when theIt has phase been in proved contact that is non-magnetic, the surfactant e.g., plays an organica strong molecule. role on the It hasmagnetic been provedproperties that theof the surfactant NPs surface plays a reducing strong role the on disorder the magnetic by replacing properties the of the missing NPs surface bonds reducingthrough theproper disorder ligands by which replacing bind the to missingthe surface bonds restoring through in propersuch a way ligands the whichbulk crystal bind toenvironment the surface restoring(Figure 1). in such a way the bulk crystal environment (Figure1).

Figure 1. SchematicFigure diagram 1. Schematic showing diagram the effect showing of the theorganic effect coating of the organicon the surface coating spin on the structure. surface spin structure.

The nanoparticles can be functionalized by a post synthesis process allowing to finely tune the magnetic properties of the single particle. The main effects of the reduction of the surface disorder produced by the organic coating on the magnetic properties are: Nanomaterials 2021, 11, 1787 3 of 16

− increase of the particle saturation magnetization, due to the contribution of the surface, becoming magnetically active [30–33]. − decrease of the particle anisotropy, due to the decrease of the surface contribution [24,34]. A fine tuning of both saturation magnetization and anisotropy can be achieved through the control of the role of the factors that were found to significantly affect the above properties, such as the type of functional group of the ligands, their chain length, the nature and the strength of the bond (monodentate or bidentate), the packing density of the coating molecules, the electronic structure and surface disorder of the original particles, which depend on the synthesis route. It is worth pointing out that, due to the complexity and the number of the involved factors, in some cases contradictory results (e.g., decrease of the saturation magnetization) are observed, [35,36] in oxide particles with spinel structure. The organic coating can also produce other changes that have a direct impact on the magnetic properties of an assembly of nanoparticles, such as: − controlled change of the interparticle distance, functionalizing the particles with molecules of proper steric features, allowing in such a way a tuning of the strength of dipolar interparticle interactions, which contribute to the effective anisotropy [37,38]. − controlled change, by specific ligands, of morpho-structural features of the particle assembly, i.e., size, shape and the crystalline structure of NPs during the synthesis process, strongly affecting the resulting magnetic state and its thermal stability [39–41]. Besides the effects on the magnetic properties, magnetic coating can provide new selective surface functionalities exploitable for several applications and can control the physical and chemical stability of nanoparticles avoiding aggregation and favoring their biocompatibility. In this framework, the present paper is aimed at providing a critical overview, through a selection of publications on the different effects produced by the organic coating on the magnetic and morpho-structural properties of nanoparticle assemblies. The paper is structured in sections focused on the following effects: (a) Tuning of saturation magnetization and anisotropy (b) Control of interparticle interactions (c) Control of morpho-structural properties

2. Tuning of the Saturation Magnetization and Anisotropy As previously reported, organic molecules can modify the surface structure through the ligands which actually replace the missing bonds, reducing the spin disorder, leading to a change of surface anisotropy [24,34] and saturation magnetization (MS)[30–33,41]. The mechanism of bonding to the nanoparticle surface depends on the type and strength of bonds as well as the geometry and electronic structure of the surface. Metal atoms located at the surfaces containing available d-bands can interact with small molecules having accessible π* states through Blyholder-type interaction [42]. Actually, a new surface can be built whose atomic environment can be predetermined choosing the suitable organic molecule with proper ligands able to bind to surface atoms. Examples of different approaches to tune both saturation magnetization and anisotropy are reported below using different molecules with different functional groups of ligands. Advanced characterization techniques and theoretical models have been used to deeply investigate the interaction between molecules and surfaced atoms. Most evidence of the organic coating effects on the magnetic properties has been reported on oxide nanoparticles. A detailed structural and theoretical investigation of the coordination symmetry of ligands and calculation of atomic distances on the surface were performed by Salafranca et al. [33]. That paper provides evidence of the changes in the surface electronic structure produced the coating and highlights the key role of the nature and number of organic molecules in the fundamental magnetic properties of nanoparticles. The authors used aberration-corrected scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS), electron magnetic chiral dichroism (EMCD) measurements, Nanomaterials 2021, 11, 1787 4 of 16

combined with density functional theory (DFT) calculations to map the magnetization of oleic acid coated magnetite nanoparticles (from 6 to 15 nm) in real space with sub- nanometer spatial resolution. DFT calculations reproduced well the measured O K-edge EELS fine structure and provided the most stable atomic configuration. It was found that the surface bonding with the acid’s oxygen atoms results in O-Fe atomic configuration and distances close to the bulk values. Indeed, the oxygen of the carboxylic group completes the octahedral environment of the bonding Fe atoms, making their first coordination like bulk magnetite. For the Fe ions bonded to the organic acid oxygens, the Fe-O average distance is 2.05 Å close to 2.10 Å in the bulk. Consequently, their densities of states occupancies are like those of bulk Fe3O4. For the half of the surface Fe ions that do not bond to the acid’s oxygen atoms, the number of O nearest neighbors is 5, as in the bare surface, and the average Fe-O distance is 1.92 Å, only 0.04 Å smaller than that at the bare surface. The effect of the type and strength of bond between the organic molecule and the particle surface was also investigated by N. Pérez et al. [30] on 5 nm Fe3−xO4 nanoparticles, synthesized by high-temperature decomposition in organic phase, where oleic acid is covalently bonded to the nanoparticles, in low-temperature aqueous conditions, while polyvinyl alcohol (PVA) yielded just a protective coating without chemical bond. Magneti- zation and XMCD measurements (Figure2) showed a saturation magnetization close to bulk magnetite and an orbital moment effectively quenched in oleic acid-coated nanopar- ticles of high crystal quality. PVA coated nanoparticles showed a reduced value of the magnetization and threefold increase in the orbital moment. The total magnetic moment per formula unit, measured by XMCD, µFe, in the oleic acid sample is about 42% larger than that for the PVA sample [µFe (PVA) /µFe (oleic) = 0.706], in excellent agreement with the magnetization results [MS (PVA) /MS (oleic) = 0.71]. By means of high-resolution electron microscopy measurements, the authors correlated the different nanostructures, arising from different synthesis procedures, to the spin and orbital contribution to the magnetic moments. The functional group bound to the surface and the type and strength of the bond play a crucial role in tuning magnetic properties. The effect of the oleic acid (OA) on the magnetic anisotropy and saturation magnetization of ~5 nm spinel CoFe2O4 nanoparticles is compared to that of diethylene glycol (DEG) by Vasilakaki et al. [43] combining magne- tization and in field Mössbauer spectroscopy measurements with physical modeling by Density functional theory (DFT) and Monte Carlo simulations. DEG and OA interact with the surface of the particles through hydroxylic (R-OH) and carboxylic (R-COOH) groups, respectively. The difference between the two coatings is in the functional group through which the interaction with the surface occurs: R-OH for DEG and R-COOH for OA, and the type of bond between ligands and the metal atoms, electrostatic for DEG and covalent for OA. Starting from the cationic distribution, different for the two systems and determined by Mössbauer spectra, the relaxed structures of the nanoparticle systems, anisotropy energy (MAE) and the net magnetic moments were calculated by DFT. The DEG coated sample has a net magnetic moment larger (approximately 1.3 times) and an anisotropy energy smaller (approximately 1.5 times) than those of the OA coated sample. This is coherent with the measured hysteresis cycles at 5 K (Figure3), showing higher saturation magnetization and lower magnetic anisotropy for the DEG coated sample with respect to the OA one. This provides evidence that the DEG coating is more efficient in reducing the surface disorder. Monte Carlo (MC) simulations, assuming a random distribution of core shell particles and accounting for the interplay between intraparticle surface characteristics and interparticle interactions effect, reproduce very well all the experimental findings. Nanomaterials 2021, 11, 1787 5 of 16 Nanomaterials 2021, 11, 1787 5 of 17

FigureFigure 2. XMCDXMCD curves curves at at the the L L2,32,3 FeFe edges edges recorded recorded on on the the oleic oleic acid acid (dashed (dashed blue blue line) line) and and PVA PVA Nanomaterials 2021, 11, 1787 6 of 17 (solid(solid red line) samples. Upper inset:inset: detail ofof thethe high-energyhigh-energy XMCDXMCD curvecurve of of the the oleic oleic acid acid sample. sample.Bottom inset:Bottom detail inset: of detail the scaling of the ofscali theng XMCD of the curvesXMCD at curves the Fe at L 3theedge Fe measuredL3 edge measured on both samples.on both samples.The Figure The is Figure reprinted is reprinted with permission with permission from Prez´ from et al. Ṕ [rez30]. et al. [30].

The functional group bound to the surface and the type and strength of the bond play a crucial role in tuning magnetic properties. The effect of the oleic acid (OA) on the magnetic anisotropy and saturation magnetization of ∼5 nm spinel CoFe2O4 nanoparticles is compared to that of diethylene glycol (DEG) by Vasilakaki et al. [43] combining magnetization and in field Mössbauer spectroscopy measurements with physical modeling by Density functional theory (DFT) and Monte Carlo simulations. DEG and OA interact with the surface of the particles through hydroxylic (R-OH) and carboxylic (R- COOH) groups, respectively. The difference between the two coatings is in the functional group through which the interaction with the surface occurs: R-OH for DEG and R-COOH for OA, and the type of bond between ligands and the metal atoms, electrostatic for DEG and covalent for OA. Starting from the cationic distribution, different for the two systems and determined by Mössbauer spectra, the relaxed structures of the nanoparticle systems, anisotropy

energyFigure 3.(MAE)Experimental and the findings net magnetic (a) and Montemoments Carlo were simulation calculated (b) for by the DFT. hysteresis The DEG loops coated at 5 K sample has a net magnetic moment larger (approximately 1.3 times) and an anisotropy Figurefor an3. assemblyExperimental of interacting findings CoFe (a) 2andO4 nanoparticlesMonte Carlo coated simulation with DEG (b) (fullfor the circles) hysteresis and OA loops (empty at 5 K for anenergycircles) assembly surfactants.smaller of interacting (approximately The Figure CoFe is reprinted2 O1.54 nanoparticles times) with than permission thosecoated fromof with the Vasilakaki DEGOA coated(full et circles) al. sample. [43]. and ThisOA (emptyis circles)coherent surfactants. with the The measured Figure is hysteresis reprinted cycleswith permission at 5 K (Figure from 3), Vasilakaki showing higheret al. [43]. saturation magnetizationBeyond the and effect lower of functionalmagnetic anisotropy group, the for chemical the DEG nature coated of thesample whole with ligand respect repre- to thesentsBeyond OA an one. important the This effect provides factor of functional in evidence tuning the group,that magnetic the the DEG propertieschemical coating is ofnature more nanoparticles. efficientof the whole in The reducing correlation ligand the repre- sentssurfacebetween an important disorder. the chemical Montefactor nature Carloin tuning of (MC) the ligandthe simulations, magnet and theic magneticassumingpropertiesanisotropy a randomof nanoparticles. wasdistribution investigated The of corecorrela- by tionshellC. between R. Vestalparticles the et al. chemical [and24] comparing accounting nature the of effects forthe ligandthe produced interplay and by the a series betweenmagnetic of ligands intraparticleanisotropy (para-substituted wassurface investi- benzoic acid HOOC–C H –R; R=H, CH , Cl, NO , OH and substituted benzene Y–C H , gatedcharacteristics by C. R. Vestal and et interparticleal.6 [24]4 comparing interactions3 the effects effect,2 produced reproduce by a seriesvery wellof ligands all 6the (para-5 experimentalY=COOH, SH, findings. NH2, OH, SO3H) on the coercivity of ~4 nm MnFe2O4 nanoparticles. The substitutedpaper makes benzoic a very acid important HOOC–C contribution6H4–R; R to = the H, understanding CH3, Cl, NO of2, fundamentalOH and substituted properties ben- ∼ zeneof Y–C surface6H5, magnetism, Y = COOH, providing SH, NH2 evidence, OH, SO of3H) the on mechanisms the coercivity underlying of 4 nm the MnFe role of2O the4 nano- particles.key factors, The paper closely makes linked a to very surface important coordination contribution chemistry, to the responsible understanding for the effectsof funda- mental properties of surface magnetism, providing evidence of the mechanisms underly- ing the role of the key factors, closely linked to surface coordination chemistry, responsi- ble for the effects produced by the organic coating. Figure 4a shows the dependence of coercivity on the pKa of the ligands. The coercivity of bare particles is also reported as a reference. The strong decrease of coercivity (almost 50%) observed going from bare to coated particles, can be ascribed to the crystal field splitting energy (CFSE), generated from d orbitals splitting by the coordination ligands, the largest CFSE resulting in the strongest effect, associated to the reduction of spin orbit coupling on the magnetic cations. The acidity of the coordination ligand was found to be the major factor affecting the re- duction of the anisotropy. Larger is the pKa (i.e., weaker is the acid, being pKa the negative log of the dissociation constant Ka), higher is the electron donation capability of the ligand, stronger is the metal-ligand s bond and consequently the CFSE, and larger is the percent- age decrease of coercivity at 5 K. Effect of the coating on particles of different size (4, 12, and 25 nm), reported in Figure 4b, indicates a less evident reduction of coercivity in 12 nm particles and practically no effect in 25 nm particles. This is fully consistent with the re- duction of the role of the surface, due to the reduced volume fraction of atoms at the sur- face. Nanomaterials 2021, 11, 1787 6 of 16

produced by the organic coating. Figure4a shows the dependence of coercivity on the pKa of the ligands. The coercivity of bare particles is also reported as a reference. The strong decrease of coercivity (almost 50%) observed going from bare to coated particles, can be ascribed to the crystal field splitting energy (CFSE), generated from d orbitals splitting by the coordination ligands, the largest CFSE resulting in the strongest effect, associated to the reduction of spin orbit coupling on the magnetic cations. The acidity of the coordination ligand was found to be the major factor affecting the reduction of the anisotropy. Larger is the pKa (i.e., weaker is the acid, being pKa the negative log of the dissociation constant Ka), higher is the electron donation capability of the ligand, stronger is the metal-ligand s bond and consequently the CFSE, and larger is the percentage decrease of coercivity at 5 K. Nanomaterials 2021, 11, 1787 Effect of the coating on particles of different size (4, 12, and 25 nm), reported in Figure7 of4 b,17

indicates a less evident reduction of coercivity in 12 nm particles and practically no effect in 25 nm particles. This is fully consistent with the reduction of the role of the surface, due to the reduced volume fraction of atoms at the surface.

Figure 4. (a) variation of coercivity with pKa of MnFe2O4 coated with different ligands: p-hydroxybenzoic acid (PHBA), pFigure-toluic 4. acid (a) variation (PTA), benzoic of coercivity acid (BA), with pKap-chlorobenzoic of MnFe2O4 coated acid (PCBA), with differentp-nitrobenzoic ligands: acidp-hydroxybenzoic (PNBA)). Coercivity acid (PHBA), of bare p nanoparticles-toluic acid (PTA), is reported benzoic as acid a reference; (BA), p-chlorobenzoic (b) variation acid of coercivity (PCBA), p nanoparticles-nitrobenzoic ofacid different (PNBA)). sizes Coercivity coated with of bare different nano- ligands:particles pis-hydroxybenzoic reported as a reference; acid (PHBA), (b) variationp-toluic of acid coercivity (PTA), benzenesulfonicnanoparticles of di acidfferent (BSA) sizes aniline coated and with phenol. different ligands: p-hydroxybenzoic acid (PHBA), p-toluic acid (PTA), benzenesulfonic acid (BSA) aniline and phenol. The number of organic molecules attached to the surface and the type of bonding (mono-dentateThe number or bidentate)of organic withmolecules the surface attached atoms to significantlythe surface and affect the the type magnetic of bonding prop- (mono-dentateerties of nanoparticles. or bidentate) with the surface atoms significantly affect the magnetic prop- ertiesCosto of nanoparticles. et al. [44] investigated the magnetic properties of 3 nm maghemite nanoparticles (synthesizedCosto et byal. a[44] gas investigated phase method the and magnetic recrystallized properties in acid of 3 media)nm maghemite coated by nanoparti- phospho- clesnate (synthesized (phosphonoacetic by a gas acid phase or pamidronic method and acid) recrystallized and carboxylate-based in acid media) (carboxymethylcoated by phos- phonatedextran) (phosphonoacetic molecules. Magnetization acid or pamidronic measurements acid) at 5and K showedcarboxylate-based a decrease of(carboxyme- anisotropy thyl(i.e., dextran) bare particles: molecules. Hc = 2130Magnetization Oe; pamidronic measurements acid coated at particles:5 K showed Hc =a 1610decrease Oe) andof ani- an sotropyincrease (i.e., of saturation bare particles: magnetization Hc = 2130 Oe; (i.e., pamidronic bare particles: acid McoatedS = 35.2 particles: emu/g; H pamidronicc = 1610 Oe) andacid an coated increase particles: of saturation MS = 41.5 magnetization emu/g) after both(i.e., bare coating particles: processes, MS = due 35.2 to emu/g; the reduction pami- dronicof the surfaceacid coated disorder particles: and spinMS = canting.41.5 emu/g) Since after the both amount coating ofphosphate processes, groupsdue to the in there- ductionsample coatedof the surface by pamidronic disorder acid and is spin one thirdcanting. approximately Since the amount of that of of phosphate the sample groups coated inby the phosphonoacetic sample coated acid, by pamidronic it is suggested acid that is one the third first moleculesapproximately are attached of that toof the particlesample coatedsurface by by phosphonoacetic a double-monodentate acid, it bondis suggested (similar that to what the first reported molecules by Yee are et attached al. [45] whichto the particleshould facilitatesurface by the a double-monodentate coordination of ligands bond inducing (similar a to stronger what reported reduction by ofYee the etsurface al. [45] whichdisorder. should facilitate the coordination of ligands inducing a stronger reduction of the surfaceAn disorder. opposite effect on the saturation magnetization, i.e., a reduction, was reported in referencesAn opposite [35,36] effect where on the the particles saturation were magnetization, prepared by methodsi.e., a reduction, other than was that reported used byin referencesCosto et al. [35,36] [44]. Ngowhere et the al. [particles36] investigated were prepared the effect by ofmethods coating other on 3 than nm cobaltthat used ferrite by Costoparticles et byal. citric[44]. Ngo acid. et The al. particles, [36] investigated prepared the trough effect a micellar of coating solution, on 3 nm are characterizedcobalt ferrite particles by citric acid. The particles, prepared trough a micellar solution, are character- ized by cationic vacancies. A reduction of MS was observed, whereas the anisotropy is the same as for the uncoated particles. The same effect was observed by Yuan et al. [35] in carboxyl- and amine-functional- ized iron oxide nanoparticles, suspended in water, and in oleic acid functionalized sus- pended in hexane and heptane. A reduction of the magnetic phase was detected depend- ing on the coatings and different suspension media. The largest MS reduction was found for amine coated particles. Oleic acid coated samples showed a solvent-dependent reduc- tion. Besides the difference between the coating molecules, the above results provide evi- dence of the critical role of the preparation process and coating conditions, affecting the surface characteristics of both the original and coated particles, in determining the mag- netic properties of the functionalized particles. Nanomaterials 2021, 11, 1787 7 of 16

by cationic vacancies. A reduction of MS was observed, whereas the anisotropy is the same as for the uncoated particles. The same effect was observed by Yuan et al. [35] in carboxyl- and amine-functionalized iron oxide nanoparticles, suspended in water, and in oleic acid functionalized suspended in hexane and heptane. A reduction of the magnetic phase was detected depending on the coatings and different suspension media. The largest MS reduction was found for amine coated particles. Oleic acid coated samples showed a solvent-dependent reduction. Besides the difference between the coating molecules, the above results provide evidence of the critical role of the preparation process and coating conditions, affecting the surface characteristics of both the original and coated particles, in determining the magnetic properties of the functionalized particles. Besides functional groups bound to the nanoparticle surface and chemical nature of the ligands, the chain length of the molecules also plays a role in determining the magnetic properties of the coated particles. The effects of six organic ligands, characterized by different chain lengths and func- tional groups (caprylic acid-C8Ac, lauric acid-C12Ac, 1-octanethiol-C8T, 1-dodecanethiol- C12T, 1-octylamine-8Am and 1-dodecylamine -C12Am) on the MS value of 9 nm fcc-FePt nanoparticles were analyzed and compared with the bulk value by Y. Tanaka et al. [46]. They compared the MS values, with respect to the bulk one (75 emu/g), of particles coated by six organic ligands. FePt particles capped with oleic acid (OAc) were synthesized and then the Oac groups were converted to the above ones via ligand exchange. The difference in the MS values was interpreted as the difference in the electron donation ability (basicity) of the ligands to the Fe bands. Regardless of the functional group, the electron donation ability was found to be affected by the chain length of the ligand. Higher MS values were obtained capping the surface of the NPs with ligands with lower basicity (higher pKb) and lower surface coverage, i.e., reducing the number of Fe-ligand binding sites (Table1). For C8T and C12T samples, the M S values were found to be 7% and 14% higher, respectively, than that of as synthesized NPs capped with oleic acid, due to the thinning of the non-magnetic shell.

Table 1. Saturation magnetization and differential magnetization (with respect to the bulk value, 75 emu/g) of the coated NPs with the ligands.

M DM Ligand S S (emu/g) (emu/g) Oac 19.5 55.5 C8Ac 19.6 ± 0.4 55.4 ± 0.4 C12Ac 15.3 ± 2.8 59.7 ± 2.8 C8T 20.8 ± 0.1 54.2 ± 0.1 C12T 22.3 ± 0.1 52.7 ± 0.1 C8Am 21.0 ± 3.1 54.0 ± 3.1 C12Am 20.4 ± 2.0 54.6 ± 2.0

The surface coating of nanoparticles with molecules exhibiting themselves magnetic properties was found to be a very powerful tool for controlling the magnetic particles. An interesting examples is given by Prado et al. [47] who tuned the magnetic anisotropy of nanoparticles via the surface coordination of anisotropic molecular complexes. They II prepared 5 nm γ-Fe2O3 nanoparticles functionalized by [Co (TPMA)Cl2] and TMA (tris (2-pyridylmethyl) amine). A sample functionalized just with TMA was used as a reference. The presence of the {CoII(TPMA)}2+ complex at the surface of the nanoparticles and the formation of an oxo-bridge between Co(II) and Fe(III) ions were evidenced by XPS, AAS and XAS measurements. The temperature of the maximum in the ZFC curves was higher for 2 [CoII(TPMA)Cl ] sample (Tmax = 30 K) than for the reference one (Tmax = 11 K). Hysteresis II loops at 5 K showed a much larger coercivity for the [Co (TPMA)Cl2] (Hc ~840 Oe) with respect to the sample prepared with addition of Tetramethylammonium hydroxide Nanomaterials 2021, 11, 1787 8 of 16

57 (Hc ~60 Oe). In agreement with the above measurements, Fe Mössbauer spectra at 77 K indicated a slowdown of the relaxation phenomena of the magnetization, due to the attached Co(II) complexes which increased the magnetic anisotropy of the Fe(III) moments, strengthening thus the magnetization of each nanoparticle. Fe and Co specific (L2,3 edges) XMCD-detected magnetization curves at 5 K are superimposed demonstrating Nanomaterials 2021, 11, 1787 9 of 17 that the Co(II) was magnetically coupled to the Fe(III) ions of the maghemite nanoparticles (Figure5).

II II FigureFigure 5.5. XASXAS andand XMCD XMCD signals signals measured measured on on [Co [Co(TPMA)Cl(TPMA)Cl2] functionalized2] 9unctionalized sample sample at the at Fe the (a ,Fec) and(a,c) Coand (b Co,d)L (b2,3,d)edges L2,3 edges at 5 Kat and5 K and 6 T. Fe-specific6 T. Fe-specific and an Co-specificd Co-specific XMCD-detected XMCD-detected magnetization magnetiza- curvestion curves at 5 Kat ( e5). K The (e). FigureThe Figure is reprinted is reprinted with with permission permission from from Prado Prado et al. et [47 al.]. [47].

3.3. Control of Interparticle Interactions Besides thethe effectseffects described described above, above, the the coating coating layer layer and and its its thickness thickness allow allow reducing, reduc- throughing, through the controlthe control of the of the distance distance between between particle particle moments, moments, the the strength strength of of dipolardipolar interactionsinteractions [[36,37,48,49],36,37,48,49], whichwhich contributecontribute toto thethe effectiveeffective anisotropyanisotropy ofof thethe nanoparticlenanoparticle assembly.assembly. The The control control of of interparticle interparticle interactions, interactions, which which also also implies implies a tuning a tuning of the of thesat- saturationuration magnetization magnetization by by the the surface surface coating, coating, is is very very important important for for applications, applications, as theythey usuallyusually requirerequire concentratedconcentrated assembliesassemblies ofof magneticmagnetic nanoparticles.nanoparticles. Vasilakaki et al. in in their their paper paper [43], [43 ],discussed discussed in inthe the previous previous section, section, compared compared the theeffect effect on the on magnetic the magnetic properties properties of 5 nm of 5CoFe nm2 CoFeO4 nanoparticles2O4 nanoparticles produced produced by the coating by the coating with oleic acid (OA) and diethylene glycol (DEG). Lower anisotropy and larger with oleic acid (OA) and diethylene glycol (DEG). Lower anisotropy and larger MS (∼120 Memu/g)S (~120 were emu/g) found were for foundthe DEG for coated the DEG sample coated with sample respect with to respect the OA to coated the OA one coated (∼82 oneemu/g). (~82 For emu/g). an assembly For an of assembly single-domain of single-domain particles with particles uniaxial with anisotropy uniaxial and anisotropy magnet- andization magnetization reversal by reversalcoherent byrotation, coherent the rotation, type and the strength type and of interparticle strength of interparticle interactions interactions can be estimated using DM plot (∆M(H) = MDCD(H) − (1–2 × MIRM(H)) can be estimated using DM plot (ΔM(H) = MDCD(H) − (1–2 × MIRM(H)) through the Wohl- through the Wohlfarth model [50,51]. As shown by the DM plots, the dipolar interactions farth model [50,51]. As shown by the DM plots, the dipolar interactions were found to be were found to be stronger (2.5 times) for the DEG-coated sample. This is not due only to stronger (2.5 times) for the DEG-coated sample. This is not due only to the larger MS, but the larger M , but also to a shorter interparticle distance between DEG coated particles. In also to a shorterS interparticle distance between DEG coated particles. In the OA sample, the OA sample, for each particle a ~2 nm thickness of oleic acid produces a total distance for each particle a ∼2 nm thickness of oleic acid produces a total distance of ∼ 4 nm between of ~4 nm between surfaces. On the other hand, the side chain link of the shorter chain surfaces. On the other hand, the side chain link of the shorter chain DEG produces a single DEG produces a single layer of 0.5 nm with a total distance of 1 nm among particles’ layer of 0.5 nm with a total distance of 1 nm among particles’ surfaces. The above thick- surfaces. The above thicknesses of the OA and the DEG molecules were estimated by a nesses of the OA and the DEG molecules were estimated by a simple calculation starting simple calculation starting from density and surface area for 1 g monolayer. The calculated from density and surface area for 1 g monolayer. The calculated dipolar interactions en- dipolar interactions energies are ~31 K and ~12 K for DEG and OA sample, respectively. ergies are ∼31 K and ∼12 K for DEG and OA sample, respectively. The ratio between di- The ratio between dipolar energies and DM plot intensities is exactly the same, due to polar energies and DM plot intensities is exactly the same, due to the contribution of the the contribution of the coating to both the saturation magnetization and the interparticle distance.coating to The both stronger the saturation dipolar interactionsmagnetization in the and DEG thecoated interparticle sample distance. results also The in stronger a larger blockingdipolar interactions temperature in with the DEG respect coated to the sample OA coated results sample. also in a larger blocking tempera- ture Thewith effectrespect of to molecular the OA coated coating sample. on the interparticle interactions and anisotropy of cobaltThe ferrite effect nanoparticles of molecular wascoating also on investigated the interparticle by Peddis interactions et al. [37 and]. Nanoparticles, anisotropy of co- in powderbalt ferrite form, nanoparticles of 5–8 nm average was also size, investigated were synthesized by Peddis by et the al. high[37]. thermalNanoparticles, decomposition in pow- methodder form, of of metalorganic 5–8 nm average precursor size, inwere presence synthesized of oleic by acid the and high oleylamine. thermal decomposition As shown by X-raymethod diffraction of metalorganic and TEM precursor images, thein presence cobalt ferrite of oleic particles acid and are oleylamine. essentiallyspherical As shown and by X-ray diffraction and TEM images, the cobalt ferrite particles are essentially spherical and uniform, separated from each other by 2 nm surfactant on the surface. Thermal analysis results indicated that the weight percentage of oleic acid is around 30%, corresponding to a monolayer of the organic molecules adsorbed at the surface of the nanoparticles. To evaluate the effect of the coating removal on the interparticle interactions, the 5 nm sample (CoFe1 ∼5 nm; MS 70 emu/g; µ0Hc ∼1.3 T) was submitted to thermal treatments at 350 °C (CoFe1T350, ∼5 nm; MS 79 emu/g; µ0Hc ∼1.9 T) and 500 °C (CoFe1T500 ∼7.3 nm; MS 72 emu/g; µ0Hc ∼1.8 T). Figure 6 shows ΔM plots for the as prepared cobalt ferrite NPs and after the thermal treatments. Nanomaterials 2021, 11, 1787 9 of 16

uniform, separated from each other by 2 nm surfactant on the surface. Thermal analysis results indicated that the weight percentage of oleic acid is around 30%, corresponding to a monolayer of the organic molecules adsorbed at the surface of the nanoparticles. To evaluate the effect of the coating removal on the interparticle interactions, the 5 nm sample Nanomaterials 2021, 11, 1787 10 of 17 (CoFe1 ~5 nm; MS 70 emu/g; µ0Hc ~1.3 T) was submitted to thermal treatments ◦ ◦ at 350 C (CoFe1T350, ~5 nm; MS 79 emu/g; µ0Hc ~1.9 T) and 500 C (CoFe1T500 ~7.3 nm; MS 72 emu/g; µ0Hc ~1.8 T). Figure6 shows ∆M plots for the as prepared cobalt ferrite NPs and after the thermal treatments.

Figure 6. ∆M plot at 5 K for the CoFe1 (full circles), CoFe1T350 (empty squares), and CoFe1T500 (full Figure 6. ΔMtriangles) plot at 5 samples. K for the The CoFe1 figure (full is reproduced circles), CoFe1T350 with permission (empty fromsquares), ref. [and37]. CoFe1T500 (full triangles) samples. The figure is reproduced with permission from ref. [37]. For the three samples, the DM plot shows a negative peak, indicating the predomi- For thenance three of samples, dipolar interparticlethe DM plot interactions.shows a negative The increase peak, indicating of the DM the intensity predomi- after thermal nance of dipolartreatments interparticle at 350 ◦C interactions. can be ascribed The mainlyincrease to of the the decomposition DM intensity ofafter molecular thermal coating (i.e., treatments decreasingat 350 °C can of interparticle be ascribed interactions). mainly to the In decomposition fact, CoFe1 and CoFe1T350of molecular samples, coating show equal ◦ (i.e., decreasingvalue of of interparticle particle size, interactions). quite close value In fact, of CoFe1 MS. On and the CoFe1T350 other hand, samples, the treatment show at 500 C equal valueinduces of particle an increase size, quite of particle close value size and of M thenS. On the the further other increase hand, the of interparticle treatment at interactions 500 °C inducescan be an ascribed increase toof the particle increase size of and the then particle the momentfurther increase (i.e., µp of= MsV).interparticle The largest field interactionscorresponding can be ascribed to the to ∆theM increase peak in the of samplethe particle without moment surfactant (i.e., isµp a = clear MsV). indication The that the largest fieldelimination corresponding of the to molecular the ΔM coatingpeak in inducesthe sample a significant without increasesurfactant of magneticis a clear anisotropy, indication asthat confirmed the elimination by the increaseof the molecu of coercivelar coating field andinduces the effective a significant anisotropy increase constant. of magnetic anisotropy,The effect as confirmed of the oleate by ligandsthe incr onease the of interparticle coercive field interactions and the effective and surface ani- anisotropy sotropy constant.of three different capped nanoparticles, Fe3O4, CoFe2O4 and ZnFe2O4 with average size of The effect∼4.5 of nm, the was oleate investigated ligands on by Virumbrales-delthe interparticle Olmointeractions et al. [52 and]. The surface particles anisot- were prepared ropy of threeby thedifferent high temperaturecapped nanoparticles, decomposition Fe3O4 of, CoFe metal2O organic4 and ZnFe precursor2O4 with using average oleic acid and size of ∼ 4.5oleylamine nm, was investigated as a stabilizing by Virumbrales-del agent. The coordination Olmo et of al. carboxylate [52]. The particles groups ofwere the ligands to prepared bythe the surface high temperature of the particles decomposition was confirmed of metal by FTIR organic spectroscopy. precursor In using the CoFe oleic2O 4 sample, acid and oleylaminecarboxylate as groupsa stabilizing bridging agent. bidentate The coordination act as ligands of carboxylate and in the case groups of Fe of3O the4 the ligands ligands to theare surface chelating. of the However, particles for was ZnFe confirmed2O4 the bridging by FTIR bidentate spectroscopy. and chelating In the CoFe ligands2O4 coexist at sample, carboxylatethe surface groups of the bridging particles. bidentate The values act of as effective ligands magneticand in the volumes case of (calculatedFe3O4 the from the 2 ligands areexperimental chelating. However, blocking for temperature, ZnFe2O4 the Tb bridging, through bidentate the expression and chelatingTb = KVm ligands(1-H/H k) /25kB, coexist at thewhere surface H and of the H kparticles.are the applied The values and of anisotropy effective magnetic field, respectively), volumes (calculated Veff, for Fe3O4 and ZnFe O samples (71 and 82 nm3, respectively) are less than twice the experimental from the experimental2 4 blocking temperature, Tb, through the expression Tb = KVm (1- particle volumes, V , (61 and 54 nm3, respectively) indicating that in these samples H/Hk)2/25kB, where H and Hk areTEM the applied and anisotropy field, respectively), Veff, for 1–2 particles are interacting. However, for the CoFe O sample, V (331 nm3) is an order Fe3O4 and ZnFe2O4 samples (71 and 82 nm3, respectively) are 2less4 than twiceeff the experi- of magnitude higher than V (45 nm3), i.e., aggregates of 7–10 nanoparticles. These mental particle volumes, VTEM, (61 and 54TEM nm3, respectively) indicating that in these sam- ples 1–2 particles are interacting. However, for the CoFe2O4 sample, Veff (331 nm3) is an order of magnitude higher than VTEM (45 nm3), i.e., aggregates of 7–10 nanoparticles. These results suggest that for Fe3O4 and ZnFe2O4 nanoparticles the protecting organic layer min- imizes the interparticle interactions, while for CoFe2O4 NPs, despite presenting a similar particle–particle distance (∼ 2 nm), the molecular coating has less effect on the interparti- cle interaction. This can be ascribed to the interplay existing to interparticle interactions and magnetic anisotropy, a point that still needs some investigations [48,49]. Nanomaterials 2021, 11, 1787 10 of 16

results suggest that for Fe3O4 and ZnFe2O4 nanoparticles the protecting organic layer Nanomaterials 2021, 11, 1787 minimizes the interparticle interactions, while for CoFe2O4 NPs, despite presenting11 of 17 a similar particle–particle distance (∼2 nm), the molecular coating has less effect on the interparticle interaction. This can be ascribed to the interplay existing to interparticle interactions and magnetic anisotropy, a point that still needs some investigations [48,49]. The control of interparticle interactions, trough the surface coating, can be exploited The control of interparticle interactions, trough the surface coating, can be exploited in some applications improving the performance of nanoparticle based magnetic materi- in some applications improving the performance of nanoparticle based magnetic materials. als. An interesting example, related to biomedicine, is given by Blanco-Andujar et al. [38]. An interesting example, related to biomedicine, is given by Blanco-Andujar et al. [38]. They They investigated the effect of the interparticle interactions, controlled by coating iron investigated the effect of the interparticle interactions, controlled by coating iron oxide NPs oxideby using NPs different by using concentrations different concentrations of citric acid of (CA),citric onacid the (CA), intrinsic on the loss intrinsic parameter loss (ILP) pa- rameterfor magnetic (ILP) hyperthermia.for magnetic hyperthermia. AA set set of of citric citric acid-coated acid-coated iron iron oxide oxide NPs NPs was was prepared prepared by by a a coprecipitation coprecipitation method method inin a a microwave microwave reactor. reactor. Two Two NPs NPs assemblies, assemblies, prepared prepared by adding by adding 1 mmol 1 mmol (CA-ioA) (CA-ioA) and 2 −1 2and mmol 2 mmol (CA-ioH) (CA-ioH) CA in CA solution, in solution, showed showed the lower the lower (1.8 (1.8nHm nHmkg 2)kg and−1) higher and higher (4.1 2 −1 57 nHm(4.1 nHmkg )2 kgILP− 1values.) ILP values. Room temperature Room temperature Fe Mössbauer57Fe Mössbauer measurements measurements showed that showed the samplesthat the consist samples of consist a mixture of a of mixture magnetite of magnetite (25–35 wt%) (25–35 and wt%) maghemite and maghemite particles. particles.In order toIn investigate order to investigate the interparticle the interparticle interactions interactions,, field dependent field dependent measurements measurements of the rema- of the nentremanent magnetizations magnetizations (i.e., Δ (i.e.,M plot)∆M plot)have havebeen beenperformed performed at 5 K at (Figure 5 K (Figure 7). The7). The negative nega- ΔtiveM values∆M values indicate indicate that demagnetizing that demagnetizing dipole–dipole dipole–dipole interactions interactions are dominant. are dominant. The con- The centrationconcentration of citric of citric acid acid and and the thereaction reaction time time are are found found to toaffect affect the the strength strength of of interpar- interpar- ticleticle interactions. interactions. In In the the sample sample synthesized synthesized un underder lower lower concentrations concentrations of of citric citric acid acid and and longerlonger reaction reaction times times (CA-ioA) (CA-ioA) the the interparticl interparticlee core core interactions interactions are are stronger. stronger. The The results results provideprovide evidence evidence that that the the demagnetizing demagnetizing intera interactions,ctions, controlled controlled by by the the concentration concentration of of citriccitric acid, acid, reduce reduce the the heating heating properties. properties.

FigureFigure 7. 7. Δ∆MM plots plots and and (inset) (inset) Henkel Henkel plots plots of of representa representative’stive’s samples samples of of the the CA-io CA-io series series at at 5 5K. K. TheThe figure figure is is reproduced reproduced with with permission permission from from Blanco-Andujar Blanco-Andujar et et al. al. [38]. [38].

4.4. Control Control of of Morpho-Structural Morpho-Structural Properties Properties TheThe type type and and the the concentration concentration of of the the orga organicnic molecule molecule both both during during and and after after the the synthesissynthesis process process can can produce produce significant significant chan changesges in in the the morphology morphology of of the the nanoparticle nanoparticle assembly,assembly, allowing allowing to to control control the the particle particle size size and and shape, shape, their their distribution distribution and andtheir their ar- rangementsarrangements (i.e., (i.e., the thedegree degree and andtype typeof particles of particles aggregation) aggregation) strongly strongly affecting affecting the mag- the magnetic properties [39,40,53–55]. Many authors have investigated the effect of the oleic netic properties [39,40,53–55]. Many authors have investigated the effect of the oleic acid acid concentration on the particle size [40,54,55] and the morphology of nanoparticle concentration on the particle size [40,54,55] and the morphology of nanoparticle assem- assemblies. The effect of the oleic acid concentration on the size, shape, morphology and blies. The effect of the oleic acid concentration on the size, shape, morphology and ag- agglomeration type on the magnetic properties of CoFe2O4 particles was investigated by glomeration type on the magnetic properties of CoFe2O4 particles was investigated by Jo- Jovanovic et al. [55], who prepared the samples following the same procedure as in [54] vanovic et al. [55], who prepared the samples following the same procedure as in [54] using 0.1, 0.15, 0.2, 0.25, 0.5, and 2 M of surfactant. using 0.1, 0.15, 0.2, 0.25, 0.5, and 2 M of surfactant. TG and FT-IR analyses showed that oleic acid forms covalent bidentate bonds with metal ions on the particle surface and a complete monolayer at a critical concentration. Without oleic acid, agglomerated nanoplatelets with a crystallite size of about 19 nm were observed by XRD and TEM (Figure 8). A decrease of particles size (from 19 to 5 nm) and change of morphology (from nanoplatelets to sphere-like) was observed increasing the concentration of oleic acid up to 0.25 M, whereas a further increase of the oleic acid Nanomaterials 2021, 11, 1787 11 of 16

TG and FT-IR analyses showed that oleic acid forms covalent bidentate bonds with metal ions on the particle surface and a complete monolayer at a critical concentration. Without oleic acid, agglomerated nanoplatelets with a crystallite size of about 19 nm were Nanomaterials 2021, 11, 1787 12 of 17 observed by XRD and TEM (Figure8). A decrease of particles size (from 19 to 5 nm) and change of morphology (from nanoplatelets to sphere-like) was observed increasing the concentration of oleic acid up to 0.25 M, whereas a further increase of the oleic acid concentrationconcentration hadhad almostalmost nono effecteffect onon the the corresponding corresponding properties. properties. Thus,Thus, 0.250.25 MM oleic oleic acidacid cancan be identified identified as as a acritical critical concentr concentrationation across across which which the biggest the biggest changes changes in prop- in propertieserties occur. occur. Raman Raman spectroscopy spectroscopy indicated indicated that the that surface the surfaceof the NPs of theis under NPs isstrain under be- straincause becauseof the oleic of the acid oleic capping acid cappinglayer, while layer, the while oleic the acid oleic concentration acid concentration had no effect had no on effectthe composition on the composition and structure and structure of the NPs. of the Magnetization NPs. Magnetization measurements measurements at room at temper- room temperatureature provided provided evidence evidence that the that concentratio the concentrationn of OA enables of OA the enables control the of control the magnetic of the magneticbehavior behaviorof nanoparticles of nanoparticles producing producing a change afrom change ferrimagnetic from ferrimagnetic (below 0.2 (below M) to 0.2super- M) toparamagnetic superparamagnetic (above 0.25 (above M). 0.25 M). TheThe effectseffects ofof coatingcoating magnetitemagnetite nanoparticlesnanoparticles (Fe0)(Fe0) byby oleicoleic acidacid (FeOA)(FeOA) andand phos-phos- phatephate (FP1) (FP1) on on the the morpho-structural morpho-structural features features and and magnetic magnetic properties properties were were investigated investigated andand comparedcompared by Muthukumaran Muthukumaran and and Philip Philip [53]. [53 It]. was It was observed, observed, by IFTR, by IFTR, that the that phos- the phosphatephate capping capping leads leads to a nearly to a nearly cubic cubic shaped shaped particles particles because because of drastic of drastic binding binding of phos- of phosphatephate ions ionsover over magnetite magnetite crystallites crystallites at specific at specific planes planes as multilayer as multilayer islands. islands. On the On con- the contrary,trary, HRTEM HRTEM images images show show that that the the oleic oleic acid acid capping capping produces produces nearly nearly spherical spherical particles parti- clesbecause because of slow of slow binding binding of oleic of oleic acid acid and and the the homogenous homogenous growth growth of ofcrystal crystal planes planes in inall allcrystallographic crystallographic directions. directions. These These morphological morphological differences differences induce significantsignificant changeschanges inin the the magnetic magnetic properties, properties, strongly strongly affecting affecting the the magnetic magnetic stability stability of of the the nanoparticles. nanoparticles.

FigureFigure 8. 8.TEM TEM images images of of cobalt cobalt ferrite ferrite NPs NPs synthesized synthesized with with (a )( 0,a) (0,b )( 0.1,b) 0.1, (c) ( 0.15,c) 0.15, (d) ( 0.2,d) 0.2, (e) 0.25,(e) 0.25, (f) 0.5,(f) 0.5, (g) 1,(g and) 1, and (h) 2 (h M) of2 M oleic of oleic acid. acid. Scale Scale bar is bar 10 nm.is 10 Thenm. figure The figure is reproduced is reproduced with permission with permission from from Jovanovic et al. [55]. Jovanovic et al. [55].

NanomaterialsNanomaterials 20212021, 11, 11, 1787, 1787 13 12of of17 16

Figure 9 shows the probability distribution of particles, obtained from SAXS data, as Figure9 shows the probability distribution of particles, obtained from SAXS data, as a a functionfunction ofof theirtheir sizes. sizes. AlmostAlmost identical identical and and broad broad size size distribution distribution pattern pattern is is observed observed for forthe the Fe0 Fe0 and and FP1 FP1 samples, samples, but but both both differ differ widely widely in comparisonin comparison to FeOA, to FeOA, exhibiting exhibiting a very a verynarrow narrow size size distribution distribution pattern. pattern. This This indicates indicates that that the the interaction interaction of oleicof oleic acid acid with with the themagnetite magnetite NPs NPs was was very very slow slow and and hence hence the particlesthe particles grew grew sufficiently sufficiently through through ripening rip- of eningformed of formed nuclei and nuclei reached and reached a uniform a uniform size with size lower with defects lower before defects arresting before arresting the crystallite the crystallitegrowth. growth.

FigureFigure 9. 9. ProbabilityProbability distribution distribution as as a afunction function of of particle particle size, size, obtained obtained from from SAXS SAXS data, data, for for FP1, FP1, FeOAFeOA and and Fe0 Fe0 magnetite magnetite nanoparticles. nanoparticles. The The inset inset shows shows the the scattering scattering intensity intensity vs. vs. q. q. The The figure figure is is reproducedreproduced with with permission permission fr fromom Muthukumaran Muthukumaran et et al. al. [53]. [53].

OnOn the the other other hand, hand, the the strong strong interaction interaction of of phosphate phosphate ions ions with with the the NPs NPs appears appears to to arrestarrest the the crystallite crystallite growth growth before before attaining attaining a asufficient sufficient growth growth to to reach reach uniform uniform size size and and shape,shape, which which is isevident evident from from the the identical identical broad broad size size distribution distribution pattern pattern of FP1 of FP1 to that to that of Fe0.of Fe0. TheThe effect effect of ofthe the concentration concentration of triethylene of triethylene glycol glycol on the on size the of size particles of particles aggregates aggre- gates and the saturation magnetization of Mn Fe O spinel nanoparticles (average size and the saturation magnetization of Mn2Fe2O4 spinel2 2 nanoparticles4 (average size 7.4 nm) was7.4 nm)investigated was investigated by Aslibeiki by Aslibeikiet al. [31]. et The al. [ 31nanoparticles]. The nanoparticles were synthesized were synthesized by a sol- by a solvothermal route based on high temperature decomposition of metal nitrates in the vothermal route based on high temperature decomposition of metal nitrates in the pres- presence of different contents of triethylene glycol (Ml of TEG = 0 (T0), 5 (T5), 10 (T10), 20 ence of different contents of triethylene glycol (Ml of TEG = 0 (T0), 5 (T5), 10 (T10), 20 (T20), 30 (T30)). Field emission scanning electron microscopy (FE-SEM) showed that with (T20), 30 (T30)). Field emission scanning electron microscopy (FE-SEM) showed that with increasing TEG content, the particles agglomeration decreases moving to smaller clusters increasing TEG content, the particles agglomeration decreases moving to smaller clusters and enhances the nanoparticles’ polydispersion in the polymer. This leads to a decrease of and enhances the nanoparticles’ polydispersion in the polymer. This leads to a decrease the blocking temperature, due to the decrease of magnetic interactions between the particle of the blocking temperature, due to the decrease of magnetic interactions between the superspins, and to an increasing tendency to saturation, as shown by the field dependence particle superspins, and to an increasing tendency to saturation, as shown by the field of the magnetization at 300 K (in the superparamagnetic regime) and 5 K (in the blocked dependence of the magnetization at 300 K (in the superparamagnetic regime) and 5 K (in regime), and by the high-field differential susceptibility (χd), which was found to decrease the blocked regime), and by the high-field differential susceptibility (χd), which was found by increasing the TEG content, reflecting the tendency to the saturation and the decrease to decrease by increasing the TEG content, reflecting the tendency to the saturation and of magnetic disorder at the surface (Figure 10). This is due to O2- ions of the triethylene 2- theglycol decrease molecules of magnetic which disorder bind to Fe at andthe Mnsurface surface (Figure cations 10). reducingThis is due the to surface O ions disorder of the triethyleneby decreasing glycol the molecules oxygen vacancieswhich bind at to the Fe NPsand Mn surface. surface This cations leads reducing to an increase the surface of the disordermagnetization by decreasing up to a the value oxygen much vacancies higher than at the that NPs of the surface. bare particles. This leads to an increase of the magnetization up to a value much higher than that of the bare particles. Nanomaterials 2021, 11, 1787 14 of 17 NanomaterialsNanomaterials 20212021,, 1111,, 17871787 1413 of of 1617

80 6 80 6 70 70 5 5 60 60 4 )

-1 50

4 /kg) ) 3

-1 50 kg /kg) 2 3 m

40kg 3 2 m -6 40 3 -6 (10 (A m d 0 30 (10 (A m χ d 0 M 30 2 χ

M 2 20 20 1 10 1 10 0 0 0 0 5 10 15 20 25 30 0 0 5 10TEG (ml) 15 20 25 30 TEG (ml)

Figure 10. Saturation magnetization (extrapolated at zero field) and high field differential suscepti- Figure 10. Saturation magnetization (extrapolated at zero field) field) and high field field differential suscepti- bility (χd) of Mn2Fe2O4 NPs at 5 K with respect to different contents of triethylene glycol. bility (χd)) of of Mn Mn22FeFe22OO4 4NPsNPs at at 5 5K K with with respect respect to to different different contents contents of of triethylene triethylene glycol. glycol. The effect of albumin coating on the morphology and magnetic properties of an as- The effect of of albumin albumin coating coating on on the the morp morphologyhology and and magnetic magnetic properties properties of an of as- an sembly of ultra-small (∼ 2 nm) MnFe2O4 nanoparticles was investigated by Vasilakaki et assemblysembly of ofultra-small ultra-small (∼ ( ∼2 nm)2 nm) MnFe MnFe2O24O nanoparticles4 nanoparticles was was investigated investigated by byVasilakaki Vasilakaki et al. [39] by magnetization measurements and numerical simulations at an atomic and etal. al.[39] [39 by] by magnetization magnetization measurements measurements and and numerical numerical simulations simulations at at an atomic andand mesoscopic scale. The particles were entrapped in bovine serum albumin (BSA) by using mesoscopic scale. The particles were entrapped in bovine serum albuminalbumin (BSA)(BSA) byby usingusing a water-in-oil single microemulsion system. It was observed that the covering process in- a water-in-oil single single microemulsion microemulsion system. system. It Itwas was observed observed that that the the covering covering process process in- duces clustering of particles determining changes in their magnetic properties [56]. inducesduces clustering clustering of ofparticles particles determining determining changes changes in in their their magnetic magnetic properties properties [56]. [56 ]. By means of first-principles calculations, based on spin-polarized density functional By means of first-principlesfirst-principles calculations, based onon spin-polarizedspin-polarized densitydensity functionalfunctional theory, of the electronic structure, relaxed structures were determined for both the un- theory, ofof the the electronic electronic structure, structure, relaxed relaxed structures structures were were determined determined for both for theboth uncoated the un- coated and albumin coated particles. The clusters are surrounded by the long albumin andcoated albumin and albumin coatedparticles. coated particles. The clusters The areclusters surrounded are surrounded by the long by albumin the long molecules albumin molecules and then they do not touch each other, whereas the nanoparticles in the clusters andmolecules then they and then do not they touch do not each touch other, each whereas other, whereas the nanoparticles the nanoparticles in the clusters in the clusters are in are in physical contact (Figure 11). The coating with albumin produces a change in the physicalare in physical contact contact (Figure (Figure11). The 11). coating The withcoating albumin with albumin produces produces a change a in change the structure, in the structure, size and shape distribution of clusters of exchange coupled particles, giving rise sizestructure, and shape size and distribution shape distribution of clusters of cluste of exchangers of exchange coupled coupled particles, particles, giving giving riseto rise a to distributiona distribution of of blocking blocking temperatures. temperatures. The The calculations calculations showed showed that that thethe albuminalbumin coatingcoat- to a distribution of blocking temperatures. The calculations showed that the albumin coat- ingreduces reduces the the surface surface anisotropy anisotropy of of the the particles particles leading leading to ato lower a lower value value of theof the coercive coercive field, ing reduces the surface anisotropy of the particles leading to a lower value of the coercive field,as experimentallyas experimentally observed. observed. field, as experimentally observed. M.C.M.C. simulations, simulations, which which assumed assumed spherical particlesparticles with with core/shell core/shell randomly randomly located lo- M.C. simulations, which assumed spherical particles with core/shell randomly lo- catedat the at nodesthe nodes of a of cubic a cubic lattice, lattice, accounting accounting for bothfor both intra intra and interparticleand interparticle contributions contribu- to cated at the nodes of a cubic lattice, accounting for both intra and interparticle contribu- tionsthe to Hamiltonian, the Hamiltonian, are in are very in good very agreementgood agreement with the with results the results of experiments. of experiments. tions to the Hamiltonian, are in very good agreement with the results of experiments.

Figure 11. Modeling of the albumin coated clusters of ultra-small MnFe O nanoparticles with a core/surface morphology 2 4 (a); enlarged schematic representation of two selected dipolarly interacting pairs of nanoparticles, core (S1)/surface (S2,S3), Figure 11. Modeling of the albumin coated clusters of ultra-small MnFe2O4 nanoparticles with a core/surface morphology that belong to two neighboring clusters (b). In each pair, the exchange intraparticle (Jc1,Jc2,Jsrf) and the exchange Figure 11. Modeling of the albumin coated clusters of ultra-small MnFe2O4 nanoparticles with a core/surface morphology (a);interparticle enlarged schematic interactions representation (J ) between of two macrospins selected aredipolarly depicted. interacting The figure pairs is reproduced of nanoparticles, with permission core (S1)/surface from Vasilakaki (S2,S3), (a); enlarged schematic representationinter of two selected dipolarly interacting pairs of nanoparticles, core (S1)/surface (S2,S3), that belong to two neighboring clusters (b). In each pair, the exchange intraparticle (Jc1, Jc2, Jsrf) and the exchange interpar- etthat al. belong [39]. to two neighboring clusters (b). In each pair, the exchange intraparticle (Jc1, Jc2, Jsrf) and the exchange interpar- ticle interactions (Jinter) between macrospins are depicted. The figure is reproduced with permission from Vasilakaki et al. [39].ticle interactions (Jinter) between macrospins are depicted. The figure is reproduced with permission from Vasilakaki et al. [39]. Nanomaterials 2021, 11, 1787 14 of 16

5. Conclusions The selected papers provide evidence of the key role of the organic coating on the tuning of magnetic properties of nanoparticles. The exploitation of the effects induced by the molecular coating offers a great opportunity to design novel nanostructured magnetic materials (composite and hybrid nanomagnets) opening enormous prospects in the realiza- tion of advanced polyfunctional magnetoelectric devices. Moreover, the molecular coating of magnetic nanoparticles, combining inorganic magnetic cores (mainly oxides) and organic shells, plays a very important role in biomedicine (for diagnosis and therapy of tumor cells) not only controlling the anisotropy and increasing the saturation magnetization, making NPs more efficient since they are magnetically driven, but also enhancing their bio-cytocompatibility. In order to optimize the magnetic properties of nanoparticles for specific applications a deep knowledge of the mechanisms through which the molecule ligands bind to the surface cations is required to understand the role of the many different factors involved (the type of functional group of the ligands, their chain length, their electron donation capability, the nature and the strength of the bond, the thickness of the coating layer, the electronic structure and surface disorder of the original particles, which depend on the synthesis route). The investigations reported in the selected papers provide a contribution to the knowledge of such mechanisms and to a deeper understanding of the surface magnetism effects and their role in controlling the fundamental properties of magnetic nanoparticles. However, the effects of the complex interplay of such factors are not fully clear yet and further investigations are necessary requiring an interdisciplinary approach involving skills in different research fields, such as matter and surface physics, organic and inorganic chemistry and specifically coordination chemistry.

Author Contributions: Conceptualization, D.P. and D.F.; data curation, M.A., D.P. and D.F.; writing— Original draft preparation, M.A. and D.F; writing—Review and editing, M.A., D.P., G.V., S.L., S.S., C.M., D.F., M.V., N.N. and K.N.T.; visualization, D.P. and D.F.; supervision, D.P., D.F., C.M. and K.N.T.; project administration, D.P.; funding acquisition, D.P.; All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by European Union’s Horizon 2020 Research and Innovation Programme (grant agreement no. 731976 (MAGENTA) and by Fondazione Compagnia of San Paolo and University of Genova (Progetto Premialità 2020). Data Availability Statement: Not applicable. Acknowledgments: The authors thank A. Omelyanchik for editing and visualization. Conflicts of Interest: The authors declare no conflict of interest.

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