Standardisation of magnetic nanoparticles in liquid suspension

It has long been noted that batch-to-batch reproducibility in nanoparticle synthesis is a significant challenge, and a key obstacle to the development of marketable products. Variations in particle output are commonly the result of either the uncontrollable presence (or variation) of contaminants in raw materials, and/or the uncontrollable variation in production process parameters. Even with access to detailed procedural instructions, issues with reproducibility become amplified when trying to reproduce synthesis processes in different laboratories. Furthermore, the problem of reproducibility in synthesis increases with the batch size under production [11]. As a result, at present the standardisation of synthesis methods remains a significant challenge.

A further consideration is the large variety of synthesis routes which are available for producing MNPs. The precise details of individual synthesis techniques are frequently kept secret in order to protect the intellectual property of manufacturers, and prevent competitors from copying their products. Due to the reluctance of the MNP industry to share the precise details of individual manufacturing processes, it is unrealistic to attempt to create global standards for them. This, coupled with the complexities that would be involves in drawing up standardised techniques for every possible MNP synthesis route, mean that we do not regard the standardisation of synthesis methods as the best option.

In this review, we therefore choose to concentrate primarily on the standardisation of characterisation methods, rather than on synthesis procedures. It should be noted, however, that a small number of published standards do exist pertaining to vocabulary and processes relevant to MNP production [12–22], in addition, some standards of relevance are currently under preparation at the International Electrotechnical Commission [23]. The study of the exact parameters which dictate variations in nanoparticle synthesis, and the quest for reproducibility remain ongoing topics in front-line research [24]. The reproducible synthesis of identical MNPs in different labs remains a scientific goal. With greater understanding of the interplay between the important limiting factors, standards for MNP synthesis can be expected to develop in the future.

As the standardisation of MNPs synthesis requires significant R&D advances, post-synthesis characterisation promises a faster and in particular more reliable route towards achieving the harmonisation of MNP products. A vast variety of characterisation techniques exist, probing both magnetic and non-magnetic parameters. In many cases, multiple measurement techniques are capable of probing the same parameter through alternative means [25].

It is notable that while several standards have been published relating to the characterisation of non-magnetic properties of nanoparticles (see section 6.1 for details), to date, no standards for the magnetic characterisation of nanoparticles have been formulated. One stumbling block is that to develop standardised measurements, it is very helpful to be able to work with standard reference materials. This, as we have just discussed, remains a significant challenge for MNPs. At present, no approved reference materials exist for MNP characterisation measurements.

That said, some de facto 'quasi-standard' materials are being used for specific purposes. One example is the use of the commercially available MRI contrast agent Resovist^® in the development of magnetic particle imaging, a novel method for mapping MNP distributions [26]. This is a pragmatic approach, which, given the highly-fragmented nature of the MNP market sector, is an informative one. Indeed, it has led us to the conclusion that rather than attempting to establish only a few 'one-size-fits-all' standards, a more application-specific 'mix-and-match' approach should be adopted. The mix-and-match approach combines different, but complementary, items from a wider list in order to form a coordinated set which meets the necessary requirements. We discuss this below, both from a pedagogical perspective, and in the context of recent developments towards a MNP suspension standard that is being undertaken through the International Organization for Standardization (ISO).

These documents fulfil the role of introducing and defining terminology specific to a given topic within the standards structure. Definitions standards are those whose scope is limited to the definition of technical terms. Materials specifications typically contain definitions specific to the material in question, as well as additional information such as lists of appropriate characterisation measurement techniques, safe handling guidelines and labelling requirements.

While some vocabulary pertaining to nanoparticles has already been included within published standards, a number of key definitions, particularly relating to magnetic characteristics, have yet to be accepted universally. It is not, for example, possible to define a measurement technique to establish superparamagnetic behaviour in a MNP sample, until the exact meaning of the term is defined in the literature. Harmonisation of the terminology used to describe MNPs is therefore the first requisite step in the path towards standardisation.

The need to obtain a representative usable sample is universal and intrinsic to all techniques for the measurement of MNP suspensions. Sampling documents are reliant on the terminology contained within definitions standards, and are a requisite to feed into documents on best practise for conducting characterisation measurements. Other aspects of sample preparation and handling may also require the compilation of standardised codes of practise. Examples of this include the correct pipetting techniques for handling nanoparticle dispersions, or the correct methods for digestion of samples for measurement of iron concentration.

As previously described, a separate standard should be established for each characterisation technique. Measurement standards are reliant upon both definitions and sampling standards. They are in turn a prerequisite in order to formulate more specific standards for particular applications. While a number of characterisation techniques for nanoparticles have already been the subject of ISO standards, at present none exist for characterisation of the magnetic properties of MNPs. This is a key area which requires development.

Establishing industry consensus on the requirements for MNP labelling for use in specific applications can greatly aid end-users of the material. Appropriate storage, disposal and health warnings are also of significant importance, particularly given the unique and relatively unknown nature of nanomaterials. Labelling and storage documents are reliant on the previous definition of relevant terminology, and are a necessary reference in the compilation of a comprehensive application standard.

This class of document is the most advanced in terms of the standardisation infrastructure required, as it draws on the content of an established library of definitions, measurement and labelling standards. An application standard should define the important parameters of relevance for MNPs destined for a particular application, and give recommendations for, or define the characterisation techniques suitable to measure them. Information on the appropriate labelling of the characterised particles should also be included.

The proposed structure is intended to allow the development and cross referencing of many interlinked standards with the least possible confusion or amendment. Each application standard can draw upon the specific documents which are relevant to it. Additional measurement and application standards may be developed as and when the need arises.

To give a concrete example (see figure 2), one possible application standard that could be envisioned might be on the requirements for MNPs for use in magnetic hyperthermia therapy. Such an application standard would not stand on its own, but instead would be accompanied by both a definitions standard (defining the terms associated with single-core and multicore MNPs) and a material specification (defining the properties characteristic of MNP suspensions, and how they may be measured). There would also be a sampling standard on how to obtain appropriately representative aliquots from an MNP suspension, plus a series of measurement standards corresponding to each of the parameters needed to fully characterise the MNP suspension with respect to its intended application purpose: in this case, of magnetic heating [27]. The list of measurement standards provided in figure 2—of chemical analysis, hydrodynamic diameter, dynamic magnetic properties and intrinsic loss power—are examples of the sorts of standards that would be required here [28]. (Note this is a hypothetical list only: in practise, it will take a good deal of work to develop the standards before a definitive list may be determined.). Lastly there would be labelling standards (we list here two—concentration and stability of the suspension, and the magnetic heating capacity of the MNPs themselves—but there may be others too), and the application standard itself, which would bring the entire set together and in which the inter-relations between the constituent standards would be presented and explained.

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In sections 6–10 below, we will examine the current state-of-the-art in each of the classifications of standard described here. We shall then identify the gaps in the existing documentation, and future requirements for MNP standards content.

However, before that, it is important to recognise that nanomaterials standardisation work is currently ongoing in a number of organisations around the world, and in particular that there is a programme under way at ISO that is specifically directed towards nanotechnology standards. We review this groundwork within the next section. Details of draft standards relating to MNP suspensions which are currently under development at ISO can be found in section 9 of this review.

A vast variety of particle structures have been reported in the literature. However, all of the known variants have included some, or all, of the following components. As such, we consider that in order to aid the harmonisation between MNP manufacturers and users, the following concepts should be incorporated into the standardisation literature.

4.2.1.1. Magnetic cores.

The fundamental building blocks of magnetic nanoparticles are called cores [11]. They are individual nanoscale magnetic objects formed of a magnetic material. Each core may be a single crystallite of magnetic material (meaning that the crystalline lattice is coherent and unbroken throughout the object), or have a polycrystalline structure (meaning that the crystalline lattice within the object has two or more distinguishable orientations).

The simplest MNP is that which contains just one magnetic core – a 'single-core' magnetic nanoparticle. Figure 3 shows a schematic diagram of a single-core MNP, along with other constituent components (matrix, functional shell, hydrodynamic surface layer) that we discuss further below.

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The size and composition of each magnetic core, as well as their spatial distribution relative to other cores (see later with respect to 'multicore' MNPs), affects the magnetic properties of the overall particle system. When reduced below a critical size, it becomes energetically unfavourable for domain walls to form within a magnetic core. As a result, cores smaller than this limit will form single domains, with all of the spins aligned in a common direction.

Magnetic cores can be formed from a range of metals and metal oxides, often with the choice of material dictated by the choice of application. For example, while pure magnetic metals offer high magnetisation (magnetic moment per unit volume) values, they are generally not suitable for biomedical or environmental applications due their toxicity and sensitivity to oxidation. In general, particles formed from metal oxides exhibit a greater long term stability, although pure metal may be rendered stable if protected from oxidation by sufficient layers of oxide or other materials such as gold or carbon.

Magnetic iron oxides, such as magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃) are among the most commonly-used materials in MNP systems for biomedical application, due to their non-toxicity and stability from oxidation [51]. To date iron oxide particles are the only MNP products so far approved for administration to humans [52], these being: the MRI contrast agent Resovist^®, the iron replacement agent Feraheme®, the sentinel node detection agent Sienna+^®, and the magnetic thermoablation agent NanoTherm^™.

Other magnetic metals and oxides (including iron, nickel, cobalt and more complex oxides) have all been studied, with many exhibiting unique and useful properties. One of the key challenges to the introduction of products implementing these particles, however, is the need to conduct thorough studies of the environmental and health impact which such nanomaterials may incur. This is an ongoing topic in the development of MNP standardisation [53].

4.2.1.2. Non-magnetic matrix.

4.2.1.3. Functionalised shell and hydrodynamic layer.

4.2.1.4. Multicore particles.

Multicore magnetic nanoparticles—as illustrated in figure 4—are particles that contain more than one distinguishable magnetic core, where 'distinguishable' means that the cores are embedded within a non-magnetic matrix, and that there is a physical separation between neighbouring cores that is filled with the matrix material. If the cores of a muticore particle are sufficiently tightly bunched, then the term 'nanoflower' is used to describe them. This class of particles has been demonstrated to show great promise for a variety of applications including magnetic hyperthermia therapy [45, 60] and magnetic resonance imaging [61]. Note that if there were no physical separation between adjacent cores, then the assembly would behave as a polycrystalline single-core particle.

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In defining the structure of multicore MNPs, it is convenient to use the term 'core-cluster' to describe the dispersion of the cores within the matrix (see figure 4). This leads to other parametric descriptions that may be applied to multicore MNPs, including the number of cores per particle; the core-cluster diameter; and the density of the core-cluster. Changes in these parameters reflect significant changes in the packing arrangement, meaning that the spacing of the cores within the particle is variable, and can greatly affect its characteristics—as illustrated in figure 5.

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Multicore MNPs are often a natural product of one-pot synthesis routes in which the magnetic cores and the non-magnetic matrix material coexist at the time of formation. The presence of numerous cores may also be advantageous in functional terms, such as by increasing the overall magnetic content of given size particle, while still allowing the formation of stable colloids. Furthermore, magnetic dipole-dipole interactions between the cores in multicore particles have been implicated as being beneficial for applications including magnetic hyperthermia [62] and magnetic particle imaging [63].

Due to their internal structure, and the additional complexity of particles distributed in liquid suspensions, a number of important size quantities are necessary in order to properly describe MNPs.

4.2.2.1. Core, core-cluster, particle, functionalised shell and hydrodynamic sizes.

4.2.2.2. Size distributions.

The zeta (ζ) potential is a measure of the potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersed particle. The ζ-potential provides a key indicator of the stability of colloidal dispersions, which is critical to many applications.

It follows, then, that this value is an important metric for any nanoparticle solution, but particularly so for MNPs as small attractive forces between magnetic cores may exceed the repulsion, resulting in aggregation of the dispersion. In general, the larger the value of the ζ-potential, the greater the stability of the suspensions against aggregation. Further stability can be conferred to the particle dispersion by the addition of charged functionalised ligands to the particle surface through a process known as 'electrostatic stabilisation'. Finally, the choice of solvent used in the particle suspension will also greatly influence the ζ-potential value obtained as variation in the pH value can greatly affect the stability of the suspension.

Alternatively, MNPs may be stabilised sterically by the addition of a coating layer of large polymer molecules such as polysaccharides. Electrostatic and steric stabilisation methods may be combined to form electro-steric stabilisation. Steric stabilisation methods are less susceptible to changes in the in the ionic strength or pH, and thus steric or electro-steric stabilisation is preferable for some applications [71].

The chemical composition of nanoparticle products is of utmost importance, especially when validating them for use in medical, biological, or environmental technologies. Analysis of the chemical composition can also indicate the extent of impurities which are present within particles. For iron oxide MNPs, Mössbauer spectroscopy or x-ray diffraction can further aid in discerning which ferrite phases are present. Thus far magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃) are the only ferrites which have been approved for use in humans [72]. To characterise MNPs for some applications, a complete understanding of the particle composition including the crystal structure, extent of defects and the presence/extent of impurities may be necessary [73].

Magnetism in nanoparticle suspensions

In this section we introduce key magnetic concepts which are necessary considerations when characterising magnetic nanoparticles in liquid suspensions.

4.2.4.1. Superparamagnetism.

4.2.4.2. Relaxation mechanisms.

The orientation of MNPs in suspension can relax via two distinct mechanisms: Néel relaxation or Brownian relaxation. In Néel relaxation, the magnetic moment flips direction within the particle due to thermal agitation, with an average time between these reversal given by (τ_N) [77]. For a temperature T, a system of non-interacting (in zero field) single-core particles has a characteristic Néel relaxation time τ_N given by:

$$\begin{align} \displaystyle {{\tau}_{{\rm N}}}={{\tau}_{0}}\exp \left[\frac{K{{V}_{{\rm c}}}}{{{k}_{{\rm B}}}T} \right],\nonumber \end{align}$$

where τ₀ is a material dependent time constant (typically in the range of 10^–12–10^–9 s), KV_c is the total anisotropy energy of the particle core, and k_B is the Boltzmann constant. The Néel relaxation behaviour of a given type of particle is dictated by the size, shape and distribution of the cores (which may be interacting if the packing density in the core-cluster is high) within the matrix.

For liquid suspensions of MNPs, reorientation of a particle's magnetic moment may also be caused by a physical rotation of the whole particle within the suspension medium and unless they have been specifically immobilised, this provides an additional degree of freedom to the magnetisation alignment of nanoparticles. Magnetisation realignment through physical rotation of the particle is known as Brownian relaxation, which occurs with characteristic time τ_B which is given by:

$$\begin{align} \displaystyle {{\tau}_{{\rm B}}}=\frac{3{{V}_{{\rm H}}}\eta}{{{k}_{{\rm B}}}T},\nonumber \end{align}$$

where ${{V}_{{\rm H}}}$ is the hydrodynamic volume of the particle, and $\eta$ is the viscosity of the carrier liquid.

A liquid suspension of nanoparticles may be dominated by either Néel or Brownian relaxation depending upon a combination of factors including the particle size, structure (single or multicore), the liquid in which they are dispersed and the temperature of the system.

In a multicore particle, individual cores are unable to physically rotate due to the matrix in which they are anchored and whilst this prevents Brownian relaxation of the individual cores from occurring, the overall particle, however, may still rotate within the suspension. In this case, the relaxation behaviour of the particle is determined by a combination of Néel relaxation within the individual cores and the Brownian relaxation of the overall particle. As a result, a multicore particle may appear to exhibit either Néel or Brownian relaxation in the same manner as a single-core particle.

For both single- and multicore particles. The effective relaxation time, ${{\tau}_{{\rm eff}}}$,of the MNP suspension accounting for both Néel and Brownian contributions can be deduced using:

$$\begin{align} \displaystyle {{\tau}_{{\rm eff}}}=~{{\frac{{{\tau}_{{\rm N}}}{{\tau}_{{\rm B}}}}{{{\tau}_{{\rm N}}}+\tau}}_{{\rm B}}}.\nonumber \end{align}$$

Further discussion of the contributions from both Néel and Brownian relaxation processes is described in [78].

Relaxation behaviours are important in a variety of MNP applications, particularly in biomedicine. These include magnetic biosensor detection systems [79–82], inducing localized magnetic hyperthermia treatment to kill tumour cells [83], as well as providing or enhancing contrast in magnetic particle imaging [84] and magnetic resonance imaging [85]. The type of relaxation exhibited by a MNP suspension therefore has great implications for its applicable uses, and so this is a critical characterisation parameter.

4.2.4.3. Interactions between particles.

Interactions between MNPs within a system can have a strong influence the group behaviour [86], and thus are an important consideration during magnetic characterisation. The magnetic moment of individual MNPs can easily exceed 10 000 μ_B (Bohr magneton), and the resultant dipole interactions act over long distances to influence the magnetic properties of the system. For a set of randomly distributed particles with average magnetic moment µ and average separation d, an estimate of the dipole interaction energy of a particle using:

$$\begin{align} \displaystyle {{E}_{d}}\approx ~\frac{{{\mu}_{0}}}{4\pi}\frac{{{\mu}^{2}}}{{{d}^{3}}},\nonumber \end{align}$$

where µ₀ is the permeability of free space.

If the concentration of a suspension of superparamagnetic MNPs is sufficiently high then the magnetic dipole interaction can result in ordering of the particles at temperatures below a critical value T₀ [87], as given by:

$$\begin{align} \displaystyle {{T}_{0}}\approx \frac{{{E}_{d}}}{{{k}_{{\rm B}}}}.\nonumber \end{align}$$

Dipolar interactions can significantly affect the results obtained from static and dynamic magnetization as well as other characterization measurements. They are also a relevant factor when synthesising MNPs for optimal magnetic hyperthermia performance [88–91]. If particles are packed very closely within a core-cluster, then exchange interactions between the cores may start to influence the overall behaviour of the particle.

DC magnetometry describes a variety of measurements in which the magnetic moment of an MNP sample (either as an MNP suspension, an immobilised sample or as a powder) is measured as a function of either an applied static magnetic field H, temperature T or measurement time t. DC magnetometry represents one of the most common measurement techniques reported within the literature, and yet there no standardised methodologies suitable for the application of the technique to MNP characterisation. The unique properties of MNPs mean that the existing document standards, which relate to macroscopic magnetometry, are not transferable to this particular class of measurements [109] and so necessitates a need to develop additional standards that reflect the unique properties and requirements of MNPs.

Through this technique, one can measure the total magnetic moment of an MNP ensemble in saturation and, assuming the volume of the magnetic material is known, the saturation magnetization M_S can be deduced. In case of immobilised and/or thermally blocked MNPs, the magnetic remanence and the coercive field can also be measured. The models used to analyse the measurements vary in complexity and number of system parameters. In the case of an ensemble of non-interacting identical particles at T $\gg$ T_B (i.e. superparamagnetic particles) neglecting magnetic anisotropy effects, the ensemble magnetisation can be described by the Langevin function:

$$\begin{align} \displaystyle M\left(H,T \right)={{M}_{{\rm s}}}\left(\coth \left(\frac{{{\mu}_{{\rm p}}}{{\mu}_{0}}H}{{{k}_{{\rm B}}}T} \right)-\frac{{{k}_{{\rm B}}}T}{{{\mu}_{p}}{{\mu}_{0}}H} \right),\nonumber \end{align}$$

where µ_pµ₀H is the magnetic Zeeman energy and k_BT the thermal energy of an MNP with the magnetic moment µ_p.

In the case of T $\leqslant$ T_B (i.e. thermally blocked particles), magnetic anisotropy negates the use of the Langevin expression and instead there are a number of models in the literature that may be applied to describe the particles' behaviour, as discussed in [110–112]. Dipolar interactions can occur in MNP systems with complex core-cluster arrangements, in which case the modelling complexity often needs to be increased. In this case, Monte Carlo simulations have been demonstrated as a feasible method to model such systems [113, 114], or a numerical inversion process may be implemented [38]. The thermal response of the MNP sample can also be determined from DC magnetometry measurements. Such measurements may be conducted either in large (saturating or near saturating fields) (S/NS), or under small fields (zero field cooled/field cooled loops, ZFC/FC magnetisation). In S/NS measurements, the sample is initially cooled to cryogenic temperatures under a large applied field after which the sample magnetisation is recorded over the desired temperature range. The temperature dependence of the saturation magnetisation is typically fitted using Bloch-type law:

$$\begin{align} \displaystyle {{M}_{{\rm S}}}\left(T \right)=~{{M}_{{\rm S}}}(0)\left(1-B{{T}^{\alpha}} \right),\nonumber \end{align}$$

where B is the Bloch constant. In bulk materials, α has a value of 3/2, although deviations from this value have been reported for nanoparticulate samples [113] with suggested mechanisms including the low-temperature freezing of surface spins [115] and or the quantum confinement of spin-waves [116].

ZFC/FC magnetisation measurements are more commonly found in the literature and tend to form typical part of the characterisation of almost all MNP samples. To measure ZFC magnetisation curves, one begins by initially cooling a demagnetised sample in the absence of a magnetic field. Once the minimum desired temperature is reached (T  <  T_B, preferably  <  10 K), a small magnetic field (ca. 1–20 mT) is applied and the sample magnetisation is recorded as a function of increasing temperature until T  >  T_B (typically room temperature). Following this, the FC magnetisation curve—the cooling curve is then recorded as the temperature is decreased to T  <  T_B, under the same applied field. From such measurements it is possible to determine the blocking temperature and, provided that the core is size is well characterised, the magnetic anisotropy constant can be estimated [117].

The presence of dipolar interactions within an MNP sample—either within the core-cluster or between particles, will shift the blocking temperature peaks in the ZFC/FC magnetisation curve [86]. These are highly sensitive to sample preparation and may be minimised by using sufficiently dilute MNP suspensions, although such effects may still occur in systems where core-clusters are interacting amongst themselves. Further complications may also arise when studying very small particles (<10 nm). This is because for such ultrasmall particles, their anisotropy energy is comparable to the thermal energy of the system.

AC susceptibility (ACS) measurements probe the magnetisation of a sample under the application of a sinusoidal AC magnetic field. The field amplitude is selected to be small enough to probe only the linear dynamic magnetisation response of the MNP ensemble. The dynamic magnetic response is recorded as a function of the excitation frequency. It is described by two components, one that is in-phase with the excitation field (real part), and one which is out-of-phase (90 degree phase shift, the imaginary part). In relation to the applied field, the recorded signal is interpreted as a complex magnetic susceptibility (with the above mentioned real and imaginary parts). The frequencies typically employed are in the range of 1 Hz–10 MHz. The response of the sample at varying temperatures may also be measured.

From ACS spectra a number of parameters can be extracted, these include the median hydrodynamic size distribution of particles [118], the core size distribution (if additional parameters are known), whether Néel or Brownian relaxation dominates and the characteristic relaxation times of the system [78]. In some cases, an estimate of the specific absorption rate of particles (see section 6.2.3) may be made from the imaginary part of the AC susceptibility [122].

Temperature dependent AC susceptibility measurements (ACS versus temperature at different excitation frequencies) can provide information about the core volume distribution and allow the magnetic anisotropy constant to be obtained if the MNP system is non-interacting. Analysis is somewhat complicated if there is a distribution of particle shapes within the sample with differing anisotropies.

At present analysis of measurement data is currently conducted using the techniques based on those described in [119–121]. The development of AC susceptibility is a valuable tool for MNP analysis, with relevance to a wide variety of applications [80, 122]. A standardised approach to both measurement and analysis will provide great value to manufacturers.

A similar method to ACS is magnetorelaxometry (MRX), where the magnetic response versus time of an ensemble of MNP is observed after switching off an external pre-magnetizing field (typically in the mT range). Devices have been demonstrated using both SQUID magnetometry [123–125], as well as fluxgate magnetometers [126, 127] which have the advantage of not requiring cryogenic cooling, and other techniques [128]. Applications of MRX are rapidly developing at present. The major topics at present can be divided into those which aim at (1) quantification and localisation of MNPs administered to organisms, and (2) measurements that probe the interactions of magnetic nanoparticles with their environments. An excellent overview of these applications is given in [129].

Rotating magnetic field (RMF) is a characterisation method with similarity to ACS, although the technique is still under development. It measures the viscous drag forces between the particles in a liquid dispersion and the carrier fluid in which they are dispersed [130, 131]. A constantly rotating magnetic field is applied to the sample at a range of frequencies. The system measures the phase lag between the field vector and the rotating moment of the particle ensemble via monitoring the real and imaginary components of the complex magnetisation. RMF allows the Brownian time constant of particles in suspension to be measured and it can also be used to probe the colloidal stability and binding state of the particles [132–134]. It is also possible to find the magnetic moment of the particles, and if the saturation magnetisation is known, then the core size can also be determined.

When an alternating magnetic field is applied to MNP suspensions, it is possible to observe heating within MNP suspensions. There are number of mechanisms that underpin this effect, including the particle system used and the magnitude and frequency of the applied field. Magnetic hyperthermia therapy is based upon utilising the heat to cause damage to tumour cells that have been loaded with MNPs. A more complete discussion of the application can be found within section 8.1.2.1.

The effectiveness of a given particle type for magnetic hyperthermia therapy is characterised by the specific loss power (SLP), which is a measure of the heat which it dissipates. The SLP of an MNP suspension describes the heating power P generated per unit mass of magnetic material m within it [135], and can be expressed as:

$$\begin{align} \displaystyle {\rm SLP}=\frac{P}{m}.\nonumber \end{align}$$

While the heating power produced by a specific nanoparticle type is dictated by its physical and magnetic properties, the power produced also scales linearly with the frequency f and quadratically with the strength of the alternating magnetic field H applied (at low fields). The intrinsic loss power (ILP) of an MNP material attempts to remove these factors, and is given by:

$$\begin{align} \displaystyle {\rm ILP}=\frac{{\rm SLP}}{f{{H}^{2}}}.\nonumber \end{align}$$

A quantitative and comparable measurement of these values is deceptively difficult to achieve, as indicated by the vast scatter of results published in the literature. One major hurdle is that the majority of measurement setups are non-adiabatic, meaning that heat is continuously lost from the MNP suspension during measurement. A number of techniques to compensate for these losses are currently under investigation [136]. More rarely, attempts are made to conduct measurements under adiabatic conditions [45, 137, 138], which requires a significant investment of both time and resources.

At present there is no standard methodology for measuring the SLP of MNP suspensions and the current state-of-the-art allows only for the direct comparison of particle efficiencies using identical equipment. The realisation of a safe, efficient and reliable cancer therapy based on magnetic hyperthermia will be significantly furthered by the creation of a standardized methodology for characterizing the ILP of MNP systems [27, 135, 139–141].

The effectiveness of MNP suspensions as contrast agents is characterised using the NMR T1 and T2 parameters [142]. These describe the relaxation of hydrogen nuclei (i.e. protons) via the spin-lattice (longitudinal) relaxation and the spin-spin (transversal) relaxation, respectively, following the application a radio frequency pulse. These values are modified in the vicinity of MNPs due to magnetic field arising from the magnetic core [143, 144], although the spatial configuration of the core/core-cluster as well as and surface modifications of the particle is known to affect the relaxivity of protons [145, 146].

The relaxivity R_i of MNP suspensions is given by:

$$\begin{align} \displaystyle \frac{1}{{{T}_{i}}}={{R}_{i}}\cdot c\left({\rm Fe} \right)+\frac{1}{{{T}_{i(0)}}},\nonumber \end{align}$$

where T_i represents either the longitudinal (T1) or transversal (T2) relaxation time in at a concentration c(Fe) in (mmol l⁻¹) and the relaxation time T_i(0) of the pure solvent. A common testing method is to conduct T1 and T2 measurements for a series of concentrations of the same particle type, and compare the results with the above relationship [147].

MNP suspensions are regularly used to modify the contrast in magnetic resonance imaging (MRI), a non-invasive in vivo imaging technique with a broad range of diagnostic applications. For a longer discussion of this technique and the key literature of relevance, please see section 8.1.1.1. It is worth noting that the technique by which magnetic resonance relaxivity is assessed is not yet universal, and a truly harmonised system for calibration and measurement protocols is needed.

In magnetic particle spectroscopy (MPS), a MNP sample is exposed to a sinusoidally varying magnetic field at a specific excitation frequency (typically ca. 25 kHz). The excitation field has sufficient amplitude so as to exploit the nonlinearity of the magnetisation curve. The magnetic response is monitored using a single or gradiometric induction coil, while the measured signal is a convolution of the excitation frequency with higher odd harmonics that are dictated by the nonlinear magnetisation curve of the MNP sample. It follows, then, that a Fourier transformation of the signal separates the harmonics of the MNPs and the signal originating from the excitation field. The observed spectrum depends on a variety of characteristic parameters of the MNPs, including the core and particle size distributions, the magnetic anisotropy, temperature as well as the frequency of the excitation field.

As a specific quantitative method, MPS is suited to characterize the MNP content in a larger sample of biological material. Standardization of this method has not been performed so far.

MPS is closely related to the recent innovation of magnetic particle imaging (MPI) [84], with both techniques proportional to the non-linear magnetisation response of MNP suspensions [26]. Indeed, whilst MPS has great potential for the optimization of tracers for MPI, the results obtained by this technique are also influenced by properties of the suspension, such as the hydrodynamic size and the viscosity of the liquid in which they are suspended (if Brownian relaxation dominates) [148–150].

Magnetic separation is a technique by which the stability of an MNP suspension in the presence of varying magnetic field gradients can be probed. The application of a magnetic field gradient to the MNP suspension results in a magnetic force, causing the MNPs to move towards high field regions. The velocity of the MNP is determined by a combination of the magnetic properties of the MNP and the hydrodynamic properties of the whole suspension, described by the viscosity of the solvent, the hydrodynamic MNP diameter and the temperature, also taking stochastic Brownian motion of the particles into account [151, 152].

The MNP concentration is monitored by observing the intensity $I$ of light transmitted through the suspension. Concentration c follows then the Beer–Lambert-Bougers relationship:

$$\begin{align} \displaystyle I={{I}_{0}}{{{\rm e}}^{-\alpha Lc}},\nonumber \end{align}$$

where I₀ is the light intensity transmitted in the absence of any particles, L is the length of the optical light path, and α is the absorption coefficient of the particle suspension.

Provided that the transmission intensity is appropriately calibrated using suspensions of known concentration, the evolution of particle concentration under applied field gradients may be measured. A variety of field gradients can be applied in a typical system, with gradients up to 60 T m⁻¹ commonly used [153].

This technique provides a suitable method to test MNP suspensions which would otherwise be stable over the measurement time scales as well as assessing the stability against applied fields of a given particle system. Practically, magnetic separation utilizes typical sample volumes in the range of 40–100 µl, and can probe MNP suspensions with concentrations ranging from 0.05 mg_Fe ml⁻¹ up to 0.3 mg_Fe ml⁻¹. Separation times are in the range of hours for most field gradients used.

Magnetic separation is also critical to a number of applications where MNPs have functional groups added to the surface in order to bind to specific target molecules [6]. The particles and attached material are then removed from the fluid by the application of a field gradient. Examples of applications include the isolation and measurement of specific biomolecules, and the removal of pollutants from water (see section 8.1.3 for details).

Colloidal stability of a nanoparticle suspension is a key factor in determining its viability for a multitude of applications. As a result, the standardisation of this technique can be expected to have a strong positive impact on the industry. The implementation of round-robin measurements of known nanoparticle compositions may be used to compare the accuracy and reproducibility of measurements using different equipment.

Magnetic nanoparticles find uses in medical imaging as both contrast agents in MRI, and as tracers in MPI. Imaging technologies are not limited to magnetic nanoparticles; gold nanoparticles and nanorods are used in photoacoustic imaging, whilst radioactive tracers are used to generate contrast in positron emission tomography. However, MNPs offer high spatial resolution and so make a good contrast agent for use in MRI. The contrast arises from the very nature of the magnetism within the nanomaterial. In the case of MPI imaging, the imaging signal arises from the induced magnetisation measured directly from the MNP tracer within the imaging medium. The minimally invasive nature of both of these techniques, combined with the insights which they provide, make them powerful tools in current and future diagnostics. We discuss the two key applications, and refer to the key measurements outlined in sections 6.2.2, 6.2.4 and 6.2.5.

8.1.1.1. Magnetic resonance imaging (MRI).

8.1.1.2. Magnetic particle imaging.

Therapeutic applications of MNP suspensions span a broad market and includes magnetic hyperthermia therapy [194], targeted drug delivery [187, 188], gene delivery [189], and the stimulation/suppression of the human immune system [190–192]. Despite the differences in the physical principles of therapy, standardisation of technical terms as well as the synthesis and characterisation methods would provide a reliable understanding of the MNP properties [27, 193] which benefit them all.

8.1.2.1. Magnetic hyperthermia therapy.

8.1.2.2. Diagnostic applications/molecular sensing.

Magnetic nanoparticle suspensions form the cornerstone of a number of innovative techniques to aid in the desalination of seawater and the removal of heavy metal pollutants. With increasing global attention on sustainability and environmental protection this field can be expected to grow dramatically in the coming years. The requirements for MNPs for use in environmental application, as catalysts or in seals, lubricants and soaps are not, in general, as demanding as for other applications such as biomedical use. However, standards may help to improve and push the development of new products using the unique properties of MNPs, to make the application more reliable and to gain their acceptance in society. A summary of some of the most promising methods currently under development can be found below.

8.1.3.1. Forward osmosis and seawater desalination.

8.1.3.2. Heavy metal ion removal.

8.1.3.3. Catalysts for industrial reactions.

Magnetic nanoparticles in liquid suspension have been demonstrated for use in vacuum seals with products currently available from companies including Ferrotec and Roseal. The products are commonly described as ferrofluids or magnetic liquids. They are suitable for use in high-vacuum environments, in cases where it is necessary to facilitate rotation and movement of components [216–218].

The incorporation of MNP suspensions into lubricants allows precision targeting of the location of these materials [219, 220], which is of particular interest for the development of highly resilient and long-running precision mechanisms. In a similar way, the incorporation of MNP and MNP suspensions into soaps has the potential to aid a variety of applications including the clean-up of oil spills [221].