Cancer is one of the main causes of death globally, second only to cardiovascular diseases. Despite progress in cancer treatment, chemotherapy remains the primary treatment option [1,2]. Methotrexate (MTX) is an effective chemotherapeutic agent against diverse cancers, such as breast, epidermoid, brain, and lung cancers. However, the limitations of MTX, including toxicity, drug resistance, leukopenia, and nephrotoxicity, restrict its clinical application in tumor treatment [3]. Advanced drug delivery systems, specifically those that utilize drug nanocarriers such as liposomes, dendrimers, and inorganic and polymeric nanoparticles, have been designed to address the issue of severe side effects associated with cancer treatments [4]. Mesoporous silica nanoparticles (MSN) serve as ideal hosts for anticancer drugs owing to their unique characteristics, including biocompatibility, facile surface modification via silanol groups, stable mesoporous structure, high surface area, large pore volume, and tunable pore size [5]. However, rapid and less controllable drug release from MSN owing to their inherent mesoporous structure poses challenges. To overcome this issue and enhance drug release control, surface-modified MSN have been developed [6]. Employing various gatekeepers and stimuli-responsive strategies enables MSN to control cargo release at specific time intervals and targeted sites [7]. The pH variations between physiological and extracellular tumor tissues (pH 6.5–7.2) and the mild acidity in endosomes (pH 5.5–6.0) and lysosomes (pH 4.5–5.0) highlight pH as a reliable stimulus for developing responsive nanocarriers [8]. Surface modification of MSN using pH-sensitive polymers represents a unique approach in designing responsive nanocarriers for controlled drug release [9].

MSN can undergo polymer grafting by attaching previously formed polymer chains to the MSN surface (grafting to) or initiating new polymer chains from radical sites on the surface (grafting from) [10]. The "grafting from" method employs radical polymerization of vinylic monomers, ring-opening polymerization of cyclic monomers, and reversible addition-fragmentation chain transfer polymerization [11].

The unique molecular arrangement of vinyl groups, represented by CH2CH–CH2, allows their participation in radical polymerization [12,13]. Radical polymerization involves the initiation of polymer chain growth using radicals that are highly reactive with unpaired electrons. The presence of a double bond (=) in the vinyl groups facilitates radical formation, initiating polymerization reactions that result in long-chain polymer formation [14]. These specific chemical configurations make vinyl groups valuable for the synthesis of various polymeric materials [15].

Acrylic acid is a fundamentally unsaturated carboxylic acid with a vinyl group directly connected to its carboxylic acid end. Its structure comprises carboxyl groups that enable it to respond to pH changes via hydrogen bonding and dissociation. They can also transport positively charged substances via electrostatic interactions [16]. Allylamine, which has a high concentration of –NH2 groups, forms electrostatic interactions with negatively charged substances under weakly basic conditions [17]. Furthermore, allylamines can participate in typical amine reactions to connect with other functional groups, resulting in the formation of multifunctional polymeric allyl derivatives [18,19]. Therefore, acrylic acid and allylamine can function as unique adsorptive carriers for loading anticancer drugs such as MTX in the liquid phase. Moreover, both acrylic acid and allylamine, which act as pH-responsive polymers, initiate endosomal escape via the proton sponge hypothesis [16,20].

Research on the surface grafting of MSN using poly (acrylic acid-co-allylamine) (PAA) via radical polymerization for cancer therapy remains limited. PAA, which possesses reactive carboxyl and amine groups, offers extensive possibilities for modification [21]. The polymer's repeat unit, comprising pendant carboxyl/amine substituents, enables numerous chemical modifications compared to monolayer grafting of carboxyl and amine groups on MSN [22]. Additionally, the reactive PAA grafted from MSN can serve as "gatekeepers" for MSN pores [23]. Furthermore, pendant amine substituents have been used to covalently attach folic acid [24].

Thus, a successful pH-responsive controlled-release system was developed. This design aims to close the pores at high pH levels and reopen them at low pH levels. To this end, we grafted PAA polymeric chains onto the surface of MSN by free radical polymerization, and the mesoporous structure of the MSN remained unchanged. Additionally, folic acid was conjugated to MSN-grafted PAA to actively target specific MCF-7 cells (a breast cancer cell line) with folate receptors. Active targeting offers advantages by binding to overexpressed receptors in cancer cells, in contrast to passive targeting through enhanced permeability and retention effects [25]. Surface-conjugated target ligands may enhance the selectivity of drug delivery to specific tissues and cells [26]. In this study, we conducted a detailed investigation of MTX as an anticancer drug against MCF-7 cells. The novelty of this project was to use PAA as a safe gatekeeper as well as an efficient linker and to apply the final formulation for the delivery of MTX molecules.