Cancer is a disease with a great impact on the world population. According to the most recent World Health Organization’s (WHO) report, among noncommunicable diseases, cancer was the leading cause of death in global population in 2019 (WHO, 2020). Cancer is commonly caused by accumulation of genetic mutations, which yield cells with high genomic instability and high capability of replication that finally leads to tissue invasion and tumor formation (Hanahan and Weinberg, 2011). Furthermore, the characteristics of cancer cells change and evolve with the growth of the solid tumor, generating a quite complex ecosystem, where the diagnosis and prescription of the treatment are quite challenging tasks.

Cancer treatment protocols such as surgery, radiotherapy, and chemotherapy are already consolidated. However, these are not always the most suitable ones for the patient clinical conditions since each cancer subtype has many characteristics that are specific to the organ of origin, thus increasing, even more, the complexity of diagnostics and treatment. Furthermore, such conventional therapies do not achieve high efficiency in all cases as they can promote unwanted selection of the most resistant and eventually aggressive subtypes. This can lead to an apparent improvement of patients’ clinical conditions, but that can degenerate rapidly due to recurrence with increased resistance to antitumor drugs (Cree and Charlton, 2017). Another negative feature of the current therapies is the frequent emergence of undesirable side effects, directly impacting the quality of life of the patients. In this context, nanotechnology shows special relevance, as it enables the optimization of the materials properties for the desired application. Hence, more effective and less harmful treatments of diseases are expected using nanomaterials since they make possible the combination of several functionalities on a single system in a synergic way (Svenson, 2013).

Nanotechnology aims at the development of materials with at least one of its dimensions in the nanometric scale (1 to ∼100 nm) that can present unique physico-chemical properties as a consequence of their nanometric dimensions and associated high surface areas (Boverhof et al., 2015). Nanomaterials properties can be modified and controlled according to their composition, shape, size, and surface chemistry, fundamental to make them suitable for applications in catalysis, energy conversion systems, sensors, medical diagnostics, and treatment, among others (Teja and Koh, 2009). Among them, possibly those for biomedical application are the most rewarding but also the most challenging ones, given the stringent requirements of purity, biocompatibility, stability, and effectiveness. Accordingly, they require the most exquisite functionalization processes to incorporate all the needed characteristics to make them suitable for real applications. For instance, the biological properties can be modulated by conjugating on their surface: therapeutic and bioactive molecules, vectorizing agents (antibodies, antigens, aptamers, and peptides), and molecular species for diagnostics such as fluorescent biomarkers, contrast agents, and radiotracers (Stark, 2011; McNamara and Tofail, 2017). Their combination in a single platform, i.e., on each single nanoparticle, can generate smart functional nanobiomaterials (NBMs) whose representative scheme is illustrated in Figure 1. NBMs focused on complex diseases such as cancer are emerging with the evolution especially of multi-conjugation methods providing a good control on the surface chemistry as well as on the colloidal stability.

Currently, increasing efforts are being focused on the development of NBMs aiming at the simultaneous diagnosis and treatment of cancer, an approach called theranostics (Jang et al., 2016; Dadfar et al., 2019). Promising materials have been achieved, but only a few were translated to clinical trials and approved by regulatory agencies. The heterogeneity and complexity of cancer as well as the lack of a more thorough understanding about the interactions of nanomaterials with biological systems are imposing difficulties (Singh et al., 2020). Hence, studies addressing issues such as nanomaterials fate in biological systems, the mechanisms of nanobio interaction, and further understanding of pathological, inflammatory, and other biological responses are fundamental. Moreover, to fully address these aspects, a multidisciplinary team encompassing different areas of knowledge such as chemistry, physics, biology, pharmacy, and medicine is necessary. The contribution of specialists with different expertise should lead to faster and more accurate analyses and solutions of the several issues. In this context, we expect to contribute to the progress of the field from a chemical perspective.

Therefore, this review is aimed to present the state of the art on NBMs based on titanium dioxide (TiO₂) and iron oxide (IO), discussing the current context and perspectives on nanomaterials for cancer diagnostics and/or treatment. Also, the correlation of surface chemistry with the mechanisms of cell-NBM interaction is addressed, as well as the effects induced by common phenomena such as aggregation and agglomeration, and corona effect, and how they can compromise the nanomaterials properties and change their biological fate. These will be illustrated and discussed based on the most recent literature, thus providing new perspectives and clues on how addressing the challenges for the development of new nanoagents for cancer diagnostics, therapy, and theranostics.

The control on nanomaterials’ surface chemistry is fundamental when designing NBMs for medical and biomedical applications since the molecular layer acts as the interface with the surrounding environment, thus playing a major role in the interactions and determining the biological fate of nanomaterials (Nel et al., 2009). Furthermore, the conjugation of different molecules on the surface enables the modulation of their physico-chemical properties and the incorporation of various functionalities, to adjust their biological activity.

Titanium dioxide is a semiconductor material with favorable physico-chemical properties for application in white paint pigments and sunscreens, due to its high chemical stability and interesting optical properties, especially the high refractive index and capacity to absorb UV light, and reflect/scatter visible light (Antoniou et al., 2008). On the other hand, charge separation can be induced by photoexcitation exploring its semiconductor properties, generating a high-energy hole in the valence band that has been extensively explored to promote photodegradation processes, for example, of contaminants in water. The concomitantly generated electron in the conduction band also can be used in reduction processes despite its relatively low energy.

TiO₂ nanoparticles are generally produced on a large scale by reaction of titanium tetrachloride and water, a high throughput, and efficient method, but more refined techniques are generally necessary for the preparation of NPs for biomedical applications. There are several methods available for the synthesis of TiO₂ NPs, such as chemical deposition methods (Seifried et al., 2000; Oh and Ishigaki, 2004), electrophoretic deposition methods (Nyongesa and Aduda, 2017), spray pyrolysis (Okuya et al., 2004), sonochemical and microwave-assisted methods (Zhu and Chen, 2014), hydro or solvothermal methods (Kim et al., 2003; Feng et al., 2005), sol-gel methods (Oskam et al., 2003), and template-assisted methods (Jiang et al., 2007; Yuan et al., 2013). The most used among all are the sol-gel, the hydro and solvothermal, and the template-assisted methods, given the simplicity allied to improve the control on the size and shape combined with colloidal stabilization and functionalization strategies.

The widespread application of TiO₂ NPs as a white pigment in cosmetic formulations, such as sunscreens, powders, and eyeshadows, is related to their high availability allied to low toxicity. However, it is important to notice that in those cases, the material is applied on the skin, a quite stringent barrier (Dréno et al., 2019). The situation changes significantly when intravenous or other internal applications are considered since it involves the administration of nanomaterials directly in the blood or tissues as discussed in the previous topics. In this context, the toxicity becomes very dependent on factors such as size, morphology, and surface chemistry of nanoparticles. Hence, toxicological studies are essential for the evaluation of the biocompatibility, biodistribution, clearance, and possible toxicity. Aula et al. have systematically described the in vitro and in vivo assays to assess the toxicological effects of NBMs (Aula et al., 2015). Ideally, both in vitro and in vivo experiments are necessary for the evaluation of the toxicological parameters in a complementary way. As mentioned in the previous topics, the in vitro and the in vivo conditions differ considerably due to the biological medium, biomolecules concentration, shear stress, dynamic flow, the corona effect, among other factors that affect the NPs stability and aggregation process, and ultimately the biological interactions and fate. Accordingly, biocompatible NBMs are achieved when they exert their functionality without showing significant adverse toxic effects to the organism.

In fact, the nature of the molecular species conjugated on NPs surface may be used to control the eventual toxic effects, thus adjusting it to the requirements of a given application. For instance, Thevenot et al. showed that the cytotoxic effects on cancer cells were dependent on the molecules conjugated on the NBMs as well as the cell line. Uncoated TiO₂ NPs reduced mice-derived LLC lung carcinoma viability considerably, but the mice-derived B16F1 skin melanoma viability was not affected at a concentration as high as 10 mg/ml. Interestingly, the cytotoxicity on LLC cells was decreased upon conjugation of polymers to the NPs surface (Thevenot et al., 2008). Uncoated TiO₂ NPs were shown to aggregate in 0.9% saline solution and accumulate mainly in the liver, spleen, and lungs upon i.v. administration in rats, but without toxic effects at a dose of 7.7–9.4 mg/kg (Elgrabli et al., 2015). Another critical aspect during the modification of the molecular outer layer is ensuring the colloidal stability of the suspensions since aggregation processes irreversibly compromise their interaction properties with biological systems (Albanese and Chan, 2011). Aggregation may even compromise the NPs functionality, for example, attenuating the photodynamic therapy efficiency (Kamps et al., 2013). In order to enhance the NP stability and to reduce the clearance by MPS and the accumulation in the liver, spleen, and kidney of mice, Li et al. utilized RBC-derived membranes to coat mesoporous TiO₂ NPs. This strategy enabled an NBM able to accumulate more effectively in MCF-7 tumor in vivo model (Li S et al., 2021).

The semiconductor properties of TiO₂ NPs conjugated with suitable molecules are being explored in therapeutic approaches such as photodynamic therapy, which is considered economical, more effective, and less invasive than conventional approaches, reducing the need of repeated and prolonged treatments. PDT is recommended primarily for oral, head, and neck cancer although the possibility of treating other subtypes is being currently explored (dos Santos et al., 2019). PDT combines the interaction between a photosensitizer (PS) and light, generally in the presence of molecular oxygen, which is excited to a very reactive singlet state, the active species (Kou et al., 2017). Most photosensitizers exhibit autofluorescence, which makes easier to track them when concentrated in specific parts of the body requiring treatment, but are activated only when combined with light of suitable wavelengths thus imparting good control on the PDT treatment (Usuda et al., 2007). In fact, despite the systemic application of PS, the production of reactive oxygen species (ROS), responsible for cells death, is circumvented only to the local where they were photoexcited, greatly reducing the toxicity to normal tissues (Wang et al., 2011). TiO₂ NPs, most frequently associated with photoactive organic dyes such as porphyrins and phthalocyanines, are being explored as theranostic agents against various cancer subtypes (Yurt et al., 2018; Shi et al., 2020).

Nanobiomaterials based on TiO₂ NPs have also been investigated for cancer therapy, but in a modest extent when compared with IO NPs. For instance, the combination of TiO₂ NPs with antitumoral agents has been proposed as an efficient drug delivery platform for chemotherapy (Chen et al., 2017). Different drug release mechanisms such as pH-triggered (Wang et al., 2019) and irradiation-triggered delivery (Li et al., 2009), among others, have been explored in that context. The release of drugs triggered by a stimulus grants a more effective and controlled delivery of bioactive molecules to tumor cells in contrast to healthy cells (Wang et al., 2016; Raja et al., 2020). This is a highly pursued condition since the amount of chemotherapeutic agent used in the treatment can be significantly decreased, while maintaining the desired pharmacological effects, thus minimizing the occurrence of adverse side effects.

The perspectives of such NBMs in cancer therapy are better illustrated by exploring the combination of TiO₂ NPs with Dox, a well-established antitumoral drug used against several cancer subtypes to block cellular replication and tumor growth. In addition, it is known to produce free radical species causing irreversible damage to biomolecules and consequent cell death (Tacar et al., 2013). Nevertheless, Dox has nonspecific action, and its usage leads to undesired side effects when devoid of a targeted delivery system. Hence, its combination with NPs was expected to reduce the cytotoxic effects to healthy tissues, while maintaining or enhancing the efficacy against cancer cells, as confirmed by Du et al. who presented exciting in vitro and in vivo results using MDA-MB-231-GFP-fLuc breast cancer cells as a model. Dox loaded onto PEG-coated TiO₂ NPs showed improved antitumoral efficiency with minimal side effects. Such results were attested by the absence of histological changes and minimal weight loss by the test animals suggesting no significant adverse systemic effects, thus indicating that TiO₂-based nanocarriers can increase the chemotherapeutic effects of doxorubicin in vivo (Du et al.,2015).

Several works reported the combination of doxorubicin with a PDT photosensitizer aiming to increase the cytotoxic effects toward cancer cells (Zeng et al., 2015; Tong, et al., 2017; Chen et al., 2017; Flak et al., 2017). Flak et al. (2017) presented two theranostic TiO₂-based nanoagents obtained by conjugation of zinc phthalocyanine and Dox. These NBMs were able to penetrate HeLa 3D-spheroid cells and impart enhanced cytotoxicity against fibroblast MSU-1.1 model cells. Furthermore, their potentiality for bioimaging by confocal fluorescence microscopy was demonstrated. A similar strategy was also explored in an elegant work by Zeng et al. (2015), who designed a Janus nanocomposite of rare-earth NaYF4:Yb/Tm and TiO₂, which act as an inorganic photosensitizer, loaded with Dox and folic acid (FA), for simultaneous PDT and drug delivery based on up-conversion luminescence (UCL). Rare-earth-based nanomaterials enable an alternative PDT approach due to the UCL property. Upon excitation with near-infrared light (NIR), the rare-earth nanoparticle emits UV radiation that is absorbed by the TiO₂ NPs producing high-energy electrons and holes capable of generating ROS. This is one of the few examples of nanomaterials using an inorganic PS to activate a semiconductor active component. The nanocomposite was tested with breast cancer cell lines, demonstrating low cytotoxicity and improved efficacy against multidrug-resistant MCF-7/ADR cells as compared to free Dox in in vitro assays. This result was confirmed in vivo by using nude mice model, which presented no alteration in the major organs 15 days post-intravenous administration of FA-NPs-Dox in comparison with the controls PBS, FA-NPs, and free Dox. A similar approach was proposed by Chen et al. and Tong et al., who proposed nanoparticles with rare-earth core and hollow mesoporous TiO₂ shell, conjugated with PEG and 3-aminopropyltriethoxysilane, and loaded with Dox, as pH-dependent DDS and simultaneous NIR-triggered PDT nanoagents (Chen et al., 2017; Tong, et al., 2017). According to the in vitro MTT assays, such a dual therapy approach enhanced the cytotoxicity and promoted a significant reduction in HeLa cells viability when compared to the drug delivery therapy alone. In short, nanoagents based on TiO₂ NPs combining chemotherapy with PDT, by concomitant local delivery of an antitumoral agent, such as Dox, with image diagnostic by fluorescence, are very promising NBMs to improve the treatment of cancer.

At this point, it is important to note that some Dox drug delivery nanosystems were co-conjugated with folic acid as a targeting agent (Tong, et al., 2017; Chen et al., 2017). Folic acid is a vitamin related to the DNA nucleic bases synthesis, essential for survival and growth of normal cells as well as of tumor cells in rapid proliferation. For this reason, folate receptors are overexpressed in the plasmatic membrane of tumor cells making them interesting targets (Fernández et al., 2018). Hence, folic acid can be explored to actively vectoring nanobiomaterials for diagnostic, drug delivery, and theranostic of diseases such as breast and ovarian carcinoma, among others. The incorporation of folic acid was shown to increase the selectivity inducing their concentration in MCF-7, SKOV3, and HeLa tumor cells in comparison with normal cells, thus demonstrating its effectiveness in targeting strategy. In another vectoring approach, Sette et al. had presented a novel antibody for conjugation with PEG-TiO₂ NPs, and targeting Kv 11.1 potassium channel, expressed on the plasma membranes of pancreatic ductal adenocarcinoma cells (Sette et al., 2013). Furthermore, Jana et al. presented an interesting approach for the dual co-conjugation of biomolecules on TiO₂ NPs by multi-functionalizing them with tris-(nitrilotriacetic acid) (Tris-NTA) and biotin anchoring sites for oligo-histidine and avidin-modified biomolecules, respectively (Jana et al., 2013). By using this approach, a vast class of targeting biomolecules such as antibodies can be conjugated to TiO₂ NPs generating NBMs aiming for specific diseases.

The works presented so far intended to illustrate the possibilities of application of TiO₂ NPs and how the results can be influenced by the composition and the size, as well as the molecules intentionally conjugated on the surface, or unintendedly adsorbed such as due to the formation of a corona. Of course, the variety of studies and approaches that can be found in the recent literature involving TiO₂ NBMs spams a much broader scope, and the examples presented below refer only to those published and consolidated in the last two decades. For example, it has been consistently demonstrated that TiO₂ NPs have no significant tendency to penetrate the gastrointestinal tract (Janer et al., 2014; Bachler et al., 2015). On the other hand, histopathological investigations for almost three months indicated that when i.v. administered, they tend to accumulate mainly in the liver, but also in the spleen, lungs, and kidneys in minor extent (Fabian et al., 2008; Geraets et al., 2014). A complementary work by Xie et al. showed that the main route of elimination of titanium dioxide-based nanomaterials is via renal excretion (Xie et al., 2011), a quite favorable route. Finally, the agglomeration of nanoparticles is dependent on several factors, such as pH and ionic strength, but also concentration since saturation can be reached leading to agglomeration. Nevertheless, this is a problem that can be solved by proper choice of the molecules conjugated on the NPs surface. In fact, there are bright perspectives for the development of efficient TiO₂-based nanoagents for diagnostic and treatment of cancer, as presented in Table 1, where examples are listed according to the type of nanoparticle, the stabilizing and/or targeting agents, the in vitro or in vivo models, and possible applications in oncological therapy.

Iron oxide can be found as several polymorphs, being hematite (α-Fe₂O₃) the thermodynamically most stable one, along with magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃). Magnetite and maghemite exhibit interesting magnetic as well as bonding properties for the preparation of functional nanomaterials, thanks to the possibility of easy conjugation of many molecular species on the surface, including biomolecules (Chouhan et al., 2021). More recently, the semiconductor properties of hematite have been more intensely explored for application in photo-driven processes, such as photodecomposition of contaminants and water splitting, as an alternative to TiO₂ (Mishra and Chun, 2015). Nevertheless, probably, nanomaterials based on magnetite and maghemite are among the most extensively studied ones for biomedical applications, especially given to their high biocompatibility and abundance allied to the intrinsic superparamagnetic and magnetic hyperthermia properties. The superparamagnetism is observed when the core is smaller than a specific critical size, below about 16 nm for maghemite, yielding superparamagnetic IO NPs (SPIONs). It is important to point out that superparamagnetic materials present no magnetic remanence and coercivity in the absence of an external magnetic field. Furthermore, the magnetic moments of these nanoparticles are thermally randomized resulting in the absence of any magnetic attraction between them, a very convenient feature for biomedical applications (Gyergyek et al., 2017). Magnetite and maghemite belong to the class of materials called ferrites and mixed ferrites that can be prepared by replacing the iron(II) ions in tetrahedral holes of inverse spinel structure by metal ions such as copper(II), nickel(II), cobalt(II), zinc(II), manganese(II), barium(II), and strontium(II) to get similar magnetic materials, whose nanoparticles also can be classified as SPIONs (Laurent et al., 2008; Melo et al., 2021). Accordingly, all such materials can be applied for the development of nanoagents for cancer diagnosis and therapy, but we should be aware that they can exhibit much larger toxicity than the ones solely based on iron.

There are several methods for the preparation of iron oxide-based nanomaterials but some of them are recurrent in the literature: hydrothermal reactions (Gyergyek et al., 2017), solgel synthesis (Lemine et al., 2014), microemulsion method (Zhang et al., 2010), and sonochemical reactions (Dolores et al., 2015). However, in the case of magnetic nanoparticles, the chemical co-precipitation from iron salts (Martínez-Mera et al., 2007; LaGrow et al., 2019) and the thermal decomposition methods (Guardia et al., 2010; Escoda-Torroella et al., 2021) are among the most popular given their simplicity. Briefly, in thermal decomposition, the IO NPs are prepared from organometallic precursors at high temperatures, in a nonaqueous solvent. This technique yields magnetic nanoparticles with narrow size distribution, high crystallinity, and controlled morphology (Wu et al., 2015; Roca et al., 2019). However, co-precipitation is the most conventional method, in which a mixture of ferrous and ferric ions is reacted with a strong base (Wu et al., 2015). In all cases, several contaminants can be present in the reaction mixture, and purification methods such as ultracentrifugation (Sjögren et al., 1997; Carney et al., 2011), size exclusion chromatography (Strable et al., 2001), magnetic filtration (Babes et al., 1999), or flow field gradient (Hassan et al., 2008) are necessary to remove them. Among the possible contaminants, there are salts, decomposition byproducts, and eventual free iron ions and nonmagnetic particles formed in the reaction. Those methods also can be used to select and narrow the size distribution, especially to exclude larger aggregated particles.

Toxicological studies, in vitro and in vivo, provide important insights about the toxicity and are fundamental to evaluate the biocompatibility of the NBMs. Prodan et al. demonstrated that IO NPs did not have cytotoxic effects on HeLa cells even at concentrations as high as 30 µg/ml, showing no morphological changes in histopathological analysis after intraperitoneal injection in concentration up to 1.4 mmol/kg in Brown Norway rats (Prodan et al., 2013). But IO NBMs showed different biological responses depending on the size and the surface chemistry. Feng et al. showed that smaller PEG-coated NPs exhibited much lower in vitro cellular uptake than polyethylenimine-coated ones, but slower clearance and higher accumulation in SKOV-3 ovarian tumor in vivo (Feng et al., 2018). Along in vitro and in vivo, ex vivo assays are also useful for studying the biocompatibility of the NBMs. Among them, the hemocompatibility can be verified by the hemolysis assay incubating them with animal or human blood samples. For instance, Janko et al utilized this assay to demonstrate that hemocompatibility can be achieved by performing a BSA corona on lauric acid-conjugated SPION (Janko et al., 2017). In vivo hematology and blood biochemistry tracking also may provide relevant information about the effects of IO NBMs in the organism. Gu et al. observed that oleic acid-coated IO NPs encapsulated in PEG-phospholipids micelles did accumulate in liver and spleen after i.v. injection (5 mg/kg) in Balb/c mice. IO NBMs are expected to be biodegraded without toxic effects due to natural iron metabolism, but they were not completely cleared even after a month despite a significant decrease in concentration. Interestingly, the hematological studies showed that blood cell levels were in the normal range, but with increased levels of enzymes, mainly those associated to liver cells that were suggested to be related with the metabolization of oleic acid in the liver (Gu et al., 2012). Sangnier et al. showed that the rate of intracellular biodegradation of IO NPs by hMSC spheroid model is dependent on NBMs characteristics (Sangnier et al., 2019) such as size, morphology, and surface chemistry demonstrating their relevance on the design of biocompatible theranostic nanoagents.

Magnetic resonance image is a powerful noninvasive and non-harmful diagnostic technique that is able to provide high spatial resolution images, by monitoring the relaxation of the nuclear spin usually of protons, whose precession is synchronized by a radio frequency pulse and the intensity decay monitored as a function of time to generate 3D images (Mukhtar, et al., 2020). The so-called “nuclear spin relaxation process” takes place by two mechanisms giving rise to longitudinal (T₁, spin-lattice) and transverse (T₂, spin-spin) relaxation times. Each compound has protons in different chemical environments, giving rise to characteristic Larmor frequencies and relaxation times, that are further influenced by the medium where they are, making it possible to generate the images. However, sometimes a contrast agent is required to improve the resolution and get more accurate information. Currently, paramagnetic gadolinium(III) chelates are generally used for such purposes in clinical practices. But it is well-known that T₁ and T₂ relaxation times are decreased by materials with magnetic properties, such as SPIONs, which can be used as contrast agents in MRI techniques (De León-Rodríguez et al., 2015). In the human body, the hydrogen atoms are present mainly in water, carbohydrates, and biomolecules. Hence, the presence or accumulation of SPIONs in certain sites/tissues (e.g. a tumor) can be used to create a much clear contrast in relation to the surrounding tissues (Mukhtar et al., 2020; Santhosh and Ulrih, 2013). There are some examples of IO-based MRI contrast nanoagents that were approved by drug regulatory agencies, such as the United States Food and Drug Administration (FDA) and the European Medicines Agency (EMA), which are listed in Table 2 (Wang, 2011; Anselmo and Mitragotri, 2015). Interestingly, the first FDA-approved IO-based NBM was prescribed for iron replacement therapies in patients with iron deficiency (Anselmo and Mitragotri, 2015). It consisted of an iron oxide core coated with polysaccharides such as dextran and sucrose. Later on, SPIONs with similar coating layers such as ferumoxide (Feridex I.V.®, Endorem®) were approved for clinical application as MRI contrast agents. However, some of these products have been discontinued for unknown reasons. Recently, the attention has been shifted to NBMs based on ultrasmall SPIONs (USPIONs), nanoparticles with hydrodynamic diameter <50 nm (Daldrup-Link, 2017). For instance, ferumoxytol (Feraheme®, Rienso®) is a USPION originally approved for iron replacement in patients with chronic kidney disease, but that has shown potential as MRI contrast agent in clinical angiography studies due to its longer blood half-life (Daldrup-Link, 2017).

Also, considering the magnetic properties of SPIONs, we will guess, why not pulling them onto a specific location using a magnet? The magnetic concentration is possible given to the high saturation magnetization that can be achieved with SPIONs and consequent forces induced by a gradient of magnetic field thus allowing to guide and induce their accumulation in the desired site. However, the magnetic field intensity is inversely proportional to the volume and the square of the distance to the magnet does not provide a high accuracy while demanding the knowledge of the exact location of the target (Santhosh and Ulrih, 2013; Uchiyama et al., 2015). Thus, the incorporation of molecular targeting properties to NBMs is a much more attractive strategy considering the advantages associated with the selective or specific interactions, as discussed below.

In addition to the intrinsic magnetic properties explored for the development of MRI contrast agents, the rich surface coordination chemistry enables the attachment of different types of chemical species, including molecules that provide preferential interactions with specific biological structures. This possibility is rather interesting since it can be used not only to induce their accumulation in desired sites, thus decreasing the required dose for an image in diagnosis, but also to bind bioactive molecules and deliver locally the medicines in a selective way. IO NPs can be designed as passive or active vectoring antitumoral drug carriers (Santhosh and Ulrih, 2013).

The magnetic properties of IO NPs also can enable a therapy mode called magnetic hyperthermia (MHT). In this approach, an alternating magnetic field (AMF) is applied to generate enough heat to increase the local temperature and promote cell death, and/or induce the activation of the immune system. This method has a great advantage for the treatment of solid tumors since it is possible to magnetically accumulate them in the target sites, and selectively induce cancer cell death without harming the surrounding healthy tissue (Périgo et al., 2015). In 2010, EMA approved the magnetic fluid named NanoTherm® (Table 2), the first example of a product based on IO NBMs for treatment by MHT by direct injection in the tumor, followed by sessions of AMF. Clinical studies have been carried out to evaluate the effect of MHT combined with radiotherapy for the treatment of patients with glioblastoma (Maier-Hauff et al., 2010; Grauer et al., 2018). More in-depth discussion about the treatment by magnetic hyperthermia was provided by Chang et al. in a review article (Chang et al., 2018).

Many are the possibilities for designing IO nanomaterials that can rely on a single mechanism of action, as reported by Li et al. and Vyas et al., who explored IO NPs as Dox nanocarriers for treatment, respectively, of breast and liver cancer cells (Vyas et al., 2015; Li and Zhang, 2020). Nevertheless, the current trend is the development of multifunctional NBMs combining different therapies in the quest for synergistic effects, thus enhancing the efficacy of the treatment, while incorporating simultaneous image diagnostics capability (Cai et al., 2017). For instance, several groups reported the use of IO NBMs for simultaneous treatment by Dox-targeted delivery and magnetic hyperthermia, coupled with image diagnostic by MRI, thus demonstrating the potentiality and good perspectives of such types of theranostic agents (Huang et al., 2017; Angelopoulou et al., 2019; Mansur et al., 2020). Furthermore, a common feature in these works is the use of FA as targeting agent to improve their selectivity towards cancer cell lines overexpressing folate receptors. A recent work showed that the conjugation of FA on SPIONs enhanced the cellular uptake of the corresponding NBM by HeLa cells, demonstrating the possibility of active targeting to selectively favor their accumulation in tumor tissues (Deda et al., 2017; Deda et al., 2021).

The design of nanosystems based on iron oxide NBMs for controlled drug release is another promising area that is being intensively pursued using different approaches, such as drug encapsulation in the polymeric coating layer (Khaledian et al., 2020; Tsai et al., 2020), or by drug co-encapsulation in micelles (Bai et al., 2018; Manigandan et al., 2018), hydrogels (Indulekha et al., 2017; Zou et al., 2020), liposomes (Jose et al., 2019; Szuplewska et al., 2019), lipid NPs (Świętek et al., 2020), polymeric NPs (Mosafer et al., 2017; Luque-Michel et al., 2021), and mesoporous silica nanostructures (Asghar et al., 2020). The stimulated release is a very interesting strategy since it allows a better control on the delivery, ensuring the medicine will be released in the desired site and the therapeutic dose will be achieved (Santhosh and Ulrih, 2013; Monnier et al., 2014). Another important issue that can be avoided is the burst effect, associated with a sudden increase of physiological drug concentration upon administration of NP-based formulations in living organisms. Hence, the cancer treatment efficacy will be remarkably improved by utilizing stimulus-response-based release nanosystems. For example, the release can be induced by local changes in pH (Bai et al., 2018) or temperature (Dorjsuren et al., 2020), or by irradiation of AMF (Fortes Brollo et al., 2020) or high-energy photons (Di Corato et al., 2015).

Another interesting strategy that has been explored in the treatment of cancer is photothermal therapy (PTT), following the current trend on new multifunctional nanosystems. In order to achieve a multimodal treatment, it is necessary to produce NBMs with synergic and/or complementary features, for example, conjugating antitumoral drugs and photosensitizers for PDT (Di Corato et al., 2015; Sun et al., 2019), or photothermal conversion agents (PTCA) such as organic dyes and gold NPs (Espinosa et al., 2015; Wang et al., 2018; Li and Zhang, 2020) for PTT. Briefly, PTT is a new cancer therapy modality that explores the photothermal property of NIR-light-absorbing materials, such as plasmonic or semiconducting carbon nanomaterials, to generate heat and induce tumor cells death, in a similar way to magnetic hyperthermia. However, until very recently, the use of IO NPs had been limited to MHT combined with drug delivery chemotherapy, such that the PTT property of magnetite NBMs had been barely investigated. But, this has changed after Chu et al. and Shen et al. showed that enough heat is generated upon NIR irradiation of Fe₃O₄ NPs to increase the local temperature and induce cancer cells death, thus demonstrating their potential as PTCAs (Chu et al., 2013; Shen et al., 2013). In addition, Espinosa et al. and Shen et al. reported that IO-based NBMs can have photothermal efficiencies comparable to gold nanomaterials (Shen et al., 2013; Espinosa et al., 2016). Nevertheless, the property was shown to be highly dependent on aggregation state and morphology of the iron oxide nanocrystals, where the effect can be enhanced by conjugation of NIR absorbing polymers such as polydopamine (Liao et al., 2012; Shen et al., 2013; Chen et al., 2014; Espinosa et al., 2016). Since these findings, the number of works reporting on IO magnetic NPs for PTT combined with other therapy modalities has increased (Huang et al., 2015; Guo et al., 2017; Oh et al., 2017; Yang et al., 2017; Fathy et al., 2021). In fact, magnetite NBMs are being intensively studied aiming for application as multimodal cancer theranostic agents.

The works presented to demonstrate the high potential of IO NBMs for real application in cancer theranostics. Iron oxides exhibit interesting optical and magnetic properties and have a versatile surface chemistry allowing easy conjugation of molecular species, as well as fine control of their size and morphology. Such possibilities are listed in Table 3 where some examples of NBMs and their related applications are presented, as well as the conjugated stabilizing and co-adjuvant agents employed, and the in vitro or in vivo model utilized in the biological assays.

Health problems caused by diseases and tumors are among the many problems faced by our society, where cancer probably is one of the most pernicious and complex, thus demanding innovative solutions. In all cases, early diagnostics and treatment probably are the most recommended protocol, especially if based on personalized materials that seek and concentrate on specific targets dispersed in the body. Such kind of technology can become a reality but depends on the development of innovative smart nanobiomaterials able to overcome the harsh conditions imposed by the biological environment, such as the clearance and immune system, which rapidly can filtrate them out from the vascular system. These processes make them unavailable imposing the need of much larger concentrations to achieve the therapeutic dose and proportionally larger risk of unwanted side effects. Accordingly, stealthing strategies such as the conjugation of zwitterionic species, extracellular vesicle, or cell membranes, or even the use of properly treated cells, such as emptied RBCs, as shuttles, are being proposed to improve the circulation time. However, targeting is a fundamental property that must be implemented in all, even in such more sophisticated NBMs, to make then smarter and safer, while fulfilling requirements of CS, size, and morphology, as well as of a functional load of diagnostic and/or therapeutic agents. Clearly, this is a description of multi-conjugated multi-functional nanomachines where iron oxide and titanium dioxide nanoparticles are two highly biocompatible materials that can be used as platform. They encompass high surface areas and structural stability, exhibit mild and flexible enough surface chemistry as well as useful magnetic, optical, and semiconducting properties that can be explored in diagnostic by resonance magnetic image, treatment by hyperthermia, and local generation of very reactive species, respectively. However, it is highly recommended to include in the development protocol, careful systematic studies in media mimicking as close as possible the biological fluids and biological environments of in vivo systems since tests carried out in water tend to provide misleading results. For instance, CS is a fundamental property that tends to be highly influenced by the medium in which the NBMs are dispersed, such that the agglomeration or aggregation can completely change the uptake, pharmacokinetics, biolocalization, and clearance. Accordingly, the formation of a corona should also be carefully evaluated, but it can be controlled by adjusting the surface chemistry or used to improve the colloidal stability and circulation time taking benefit of the stealthing properties it can impart. In short, this review summarized the current understanding and state of the art on the design of smart nanobiomaterials based on IO and TiO₂ nanoparticles as platform. In fact, innovative nanomaterials for application in personalized molecular diagnostics and therapy of diseases can be realized by multi-functionalization and co-conjugation of complimentary molecular species opening great new perspectives in the biomedical area.