## 1 Introduction  
The recent trends of using nanotechnology to resolve unmet medical needs have been recognized as a Key Enabling Technology in the European Union (EU) [1]. By developing advanced drug delivery systems (DDSs), therapeutic compounds of interest can be entrapped within nanoparticle matrices or conjugated to different nanosystems [2, 3], drug molecules can be delivered to designated molecular targets at much lower doses, and over longer time periods [2, 4, 3]. The development of nanoformulations has revolutionized the delivery and targeting of therapeutic compounds, providing an economic and innovative utilization of existing drugs [5, 6, 7, 8, 9]. Numerous nanocomposites have received considerable attention as promising DDSs. Mesoporous silica nanoparticles (MSNs), discovered in 1992 by the Mobile Oil Corporation, have emerged as one of the most advanced and versatile nano DDSs [10, 11, 12, 13]. Small sized MCM-41 MSNs are the most frequently used carriers for pharmaceutical applications, owing to their unique mesoporous structure, high surface area, large pore volume and tuneable pore diameter [10, 11, 12, 13]. In addition to this, MSNs also provide a solid framework with abundant surface chemical moieties, allowing surface functionalization with various ligand molecules, for the targeted and controlled release of encapsulated drug moieties to a particular site [10, 11, 14, 15, 16]. Even though MSNs has been widely used in drug delivery, considerate efforts towards novel compositions of MSN-based systems have been attempted through surface engineering with different targeting functionalities, to enhance the accuracy and selectivity of these systems as nanocarriers, beyond that of unfunctionalized MSNs. As a result, recently we presented an exciting conceptual study in which a novel functionalized MSN-based system was synthesized and characterized (Figure 1a) [16]. The fabricated MSNs are highly uniformed in spherical shapes and sizes (\(\approx\)150 nm), with well-ordered porous channels, without the interconnection between each individual channel (Figure 1b). This represented the first step of our journey into the development of a nano modality with the ability to transport anticancer agents selectively to breast tumours, to reduce off-target adverse effects. In that work, the pH-sensitive polymer Poly-L-Histidine (PLH) and the FDA-approved hormonal drug Tamoxifen (TAM) were appended to native MSNs. Our functional nanostructure is believed to lay a foundation for a new theoretical concept of triple-action DDSs, which includes: (i) passive targeting using nanosized MCM-41 as a DDS; (ii) active targeting through the selective binding of TAM to estrogen receptors (ERs)-overexpressing-breast tumours; (iii) controlled drug release with PLH molecules acting as a pH sensitive gatekeeper, preventing premature drug leakage [16]. A high drug loading capacity (the mass ratio of drugs to nanosystems) is an important parameter to determine whether a nanostructure can be employed as an ideal DDS [17]. In this study, we will load our previously synthesized functionalized nanostructures with Doxorubicin (DOX), to investigate loading ability of our system. DOX has been proven to be effective against both early and metastatic breast cancer, owing to its ability to interfere with DNA and RNA synthesis in solid tumours [18; 19; 20]. However, the potential clinical applications of DOX are hindered by its highly toxic profile and non-specific mechanisms of action which can cause fatal damage to healthy and sensitive tissues, leading to cardiac arrest in some cases [19; 20]. Therefore, successful loading of DOX onto MSN-based systems  
Figure 1: (**a**) The triple-action nanostructure DDS based on MSNs, functionalized with the pH sensitive PLH polymer and the hormonal drug Tamoxifen. Image adapted from [PERSON] et al. [16].