## 1 Introduction  
Matter can reflect, transmit, or absorb light. The reflected or transmitted light is scattered in space, while the optical energy of absorbed light can be converted to heat. The heating, which occurs when light interacts with nanomaterials, can have different physical mechanisms, which depend on the nature of the nanomaterial [1]. In metals, such as silver, copper, or gold, heat occurs when light interacts with conduction electrons on the nanoparticles' surface. In semiconducting materials, such as iron oxides, optical radiation energy allows a temporary transition of electrons from the valence band to the conduction band, which results in heat generation when the electrons relax back to the valence band [1].  
Heat (or hyperthermia) is an attractive physical approach that can be used in cancer treatment [2; 3]. When the temperature in tissues increases, typically between 40 and 45 \({}^{\circ}\)C, the blood flow and the oxidation of tissues increase [4], collagen fibers slacken [5], and tumors become more sensitive to chemotherapeutics [6; 7] or radiation [8; 9].  
Currently, there are different clinical approaches which employ probes and needles to generate heat by microwaves, radiofrequencies, or ultrasound [10; 11]. While efficient, these approaches are invasive and may be detrimental to bystander tissue. On the other hand, promising preclinical approaches were reported in which nanoparticles were used to heat tumor tissues [5; 12; 13; 14].  
Among the different approaches, the nanoparticle light-to-heat conversion principle represents an excellent therapeutic alternative to current approaches and is particularly attractive for several reasons. First, nanoparticle suspensions can simply be injected into desired tissues, which does not require surgical intervention. Secondly, near-infrared light (650 to 1350 nm) may penetrate the tissue, meaning that light can be used to trigger heat generation in injected nanoparticles. Thirdly, the bystander tissue, between the light source and the target, can remain undamaged.  
Photothermal treatment mediated by plasmonic (gold) nanoparticles already reached clinical trials (NCT01679470, NCT01270139), and other materials [15; 16] are emerging for photothermal in preclinical research. Among the different materials, iron oxides appear to be excellent candidates not only because they effectively generate heat under different physical modalities [17], but also because they can be used as diagnostic tools and are magneto-responsive [18; 19] and could thus be magnetically guided to the site of interest [20]. Iron oxides are also safe [21] and biocompatible [22], and they were already used for magnetic hyperthermia in clinical settings [23; 24]. While several research groups reported the efficacy of photo-induced heating of iron oxides [19; 25; 26; 27], little is known about the effect of iron oxide's silica encapsulation and superparamagnetic iron oxide nanoparticles (SPIONs) clustering on the photothermal yield.  
In magnetic hyperthermia, intracellular confinement is now recognized as an important issue because nanoparticle aggregation within endosomes inhibits both physical mechanisms that account for heat generation: nanoparticle Brownian relaxation--the rotation of entire magnetic nanoparticles within their surroundings--and Neel relaxation, or the rotation of the magnetic moment within the magnetic cores [17]. However, when different types of iron oxide nanoparticles are internalized in endosomes, their photothermal yield can slightly increase, even in comparison to nanoparticle suspensions in water [17]. With the aim of evaluating the effect of SPIONs' encapsulation and clustering on the heating outcome, we present a study in which we compare the heating efficacy of non-encapsulated, individually encapsulated, or clustered and encapsulated SPIONs. The heating is compared in photothermal regime and in magnetic hyperthermia.