## 1 Introduction  
The iron (II) citrate complex represents an effective solution for resolving iron-deficiency anaemia in people with chronic kidney conditions. This complex is usually used to treat low levels of iron in the blood. Iron is an important metal for human life because it is a cofactor of many types of enzymes and cytochromes of the mitochondrial electron transport chain, and it is part of the haemoglobin and myoglobin structures, which act as oxygen transporters. The use of ferrous organic complexes, such as iron (II) citrate, represents an excellent option to improve health care, since they increase bioavailability and solubility, solving the iron deficiency problem [1; 2; 3; 4]. At the physiological pH of blood (7.4), high levels of Fe\({}^{2+}\)(aq) would theoretically result in the precipitation of iron (II) hydroxide. Complexation by citrate allows an almost 30-fold increase in the amount of Fe\({}^{2+}\) that can be dissolved in the blood. Iron-based systems are also proven to be effective for antitumor treatment. Daily uptake of iron through food is not sufficient to provide an adequate level of this metal in the blood. Some nonspecific symptoms linked to iron deficiency are impaired concentration, loss of energy, and fatigue. However, the most serious effects of iron deficiency, such as anaemia, are associated with loss of hair, angular stomatitis, and premature birth and bone resorption in pregnant women [5]. Iron deficiency can occur in surgical patients due to blood loss or pre-existing anaemia. The encapsulation of the iron (II) citrate complex in porous silica biomaterials allows a slow release of iron, thus resolving problems linked to the lowering of the iron levels in patients' blood [6]. Iron can also be toxic in excess; high concentrations of iron can promote pathogenic bacterial growth and modify their metabolic pattern [7]. Therefore, by modulating the iron concentration, it is possible to reduce bacterial growth. Silica biomaterials have high biocompatibility, avoiding negative outcomes as a result of their implantation in bodily tissues; in addition, it is known from the literature [8; 9; 10] and from medical practice [11] that silica-based materials are able to form a hydroxyapatite layer on their surface that improves the interaction between medical devices and tissue. Drug trapping in its structure is enabled by its porosity [12]. One of the techniques that allow the synthesis of a hybrid biomaterial is the sol-gel method. It is well known that the characteristics of the synthesis methodology have a great influence on the structure, morphology, and chemical composition, as well as on the mechanical, thermal, and optical properties of the resultant product. The formation of a sol followed by gelation, starting from inorganic materials as precursors, is the basis of the sol-gel technique. A three-dimensional network gel is formed via the condensation of hydrolys species in solution. Parameters such as temperature, pH, water content, and precursor concentrations affect the structure of the initial gel and the properties of the material [13]. The main advantage of using the sol-gel method for biomaterial synthesis is the possibility of encapsulating different organic compounds and other bioactive molecules that can be used as carriers for drug delivery [14; 15; 16]. Indeed, it has been reported that gels can release drugs (such as anti-inflammatory agents, antibiotics, and antitumor agents) in a controlled manner [17; 18; 19; 20]. Sol-gel preparation can be done with a wide variety of precursors, but silica-based carriers are preferred because they are a well-known system, inexpensive, and biodegradable [21]. Moreover, it has also been reported that silica/PEG hybrids allow us to obtain greater elasticity, to avoid particle agglomeration, and to create a network that entraps many kinds of drugs and compounds in only one step [22]. In this work, iron (II)-based biomaterials were synthesized via the sol-gel route.