SWCNTs—Single walled carbon nanotubes, MWCNTs—Multi-walled carbon nanotubes.

#### *3.5. Gold Nanoparticles (AuNPs)*

AuNPs have received current scientific interest among numerous nanocarriers developed for use in nanomedicines due to their unique uses in cancer therapy such as drug delivery, tumour sensing and photothermal agents [78]. For a variety of reasons, the use of AuNPs in cancer treatment and diagnosis is gaining a lot of interest. Furthermore, their inactivity toward biological systems has made them superior to conventional metal-

based drug delivery technologies [79]. The inorganic nanoparticles have non-sensitive physical-chemical properties and are meant to convert irradiation energy into harmful radicals for photodynamic or photothermal therapy for solid malignancies. Due to their unique features, inorganic nanoparticles serve an important role in a variety of domains, including drug processing, bioimaging and sensing. Inorganic nanocarriers such as gold nanoparticles perform an essential pharmacological role. When AuNPs are adjusted to a proper shape and size, they are likewise non-toxic and have low phototoxicity [80,81]. The schematic representation of multifunctional gold nanoparticles are shown in Figure 6. Molecules 2022, 27, x FOR PEER REVIEW 11 of 27

Figure 6. Schematic representation of multifunctional gold nanoparticles. **Figure 6.** Schematic representation of multifunctional gold nanoparticles.

3.6. Mesoporous Silica Nanoparticles (MSNs) Due to their extraordinary potential as nanocarriers for cancer therapy and imaging, mesoporous silica nanoparticles have received the attention of researchers [89–94]. MSNs have been studied and found to be promising carriers for biomedical imaging and drug delivery due to their good biocompatibility, high pore volume, uniform pore size distribution, large surface area and further chemical modification on the surface of MSNs to modulate the nanoparticle surface characteristics. Furthermore, pharmaceuticals can be placed onto the mesoporous, resulting in prolonged drug release [94,95]. Mesoporous sizes range from 2 to 50 nm. MCM-41 nanoparticles were the most extensively described MSNs for cancer therapy. This class of MSN is hexagonally structured homogeneous mesoporous that facilitates drugs to be loaded into micro-channels while also inhibiting the The optical properties, tuneability and surface plasmon resonance of gold nanoparticles drew researchers' attention nowadays. AuNPs can be modified easily by changing the appearance and applying a negative charge on the gold nanoparticles surface. This means that by combining various molecules such as ligands, medicine and genes can be easily functionalized. Furthermore, the non-toxicity and biocompatibility of gold nanocarriers make an excellent choice for utilizing as a drug carrier, for example, when methotrexate coupled with gold nanoparticles, which has been used to treat cancer, has shown to be more cytotoxic to a variety of tumour cell lines compared to free methotrexate. MTX was observed to rise at a faster rate and a higher concentration in tumour cells when conjugated with gold nanoparticles. When coupled with gold nanoparticles via an acid-labile connection, doxorubicin is a marker of enhanced toxicity to the MCF-7/ADR breast cancer cell line, which is multidrug-resistant [82–84].

pre-release of loaded drugs [2,96]. On surfaces of the amine groups of MSNs, polyethylene glycol was conjugated to create long-circulation MSNs [97]. The Schematic representation of multifunctional mesoporous silica nanoparticles are shown in Figure 7. For tumour cell targeting, several targeting ligands such as transferrin, mannose and folic acid (FA) have been coupled on surfaces of the MSNs. For example, the folate receptor (FR), which is typically overexpressed in many human tumour cells, has been widely employed in targeting the tumour cells and nanomaterial treatment. Researchers used an amide linkage to conjugate folate with polyethyleneimine and then this co-polymer coated with silica particles. When compared to non-targeted nanoparticles, FA-modified silica nanoparticles showed increased cytotoxicity in both human cervical and breast can-PEGylated gold nanoparticles can overcome the problem of RES uptake. Under physiological conditions, PEGylated-gold nanoparticles have better stability and solubility. The surface of gold nanoparticles could be modified to allow for targeted medication delivery via various ligands. For example, gold nanoparticles conjugated to fluorescent heparin might be utilised for cancer diagnostics and transferrin could be conjugated on the surface of gold nanocarriers for targeting [85]. To improve the effect of limited photodynamic therapy, Xin et al. created phthalocyanine chloride tetra sulphonic acid (AlPcS4) delivery systems using AuNPs. As AuNPs are not only easily accessible to AlPcS4, but also exhibit accelerated single oxygen production and directly cause cell death with photothermal effects, AlPcS4 has a significant anti-tumour action [86].

cer cells and tumour absorption [98–100]. MSNs are employed in nucleic acid-guided treatments and nucleic acid delivery because of their relatively large surface area, superior biocompatibility for functionalization and variable pore size used to encapsulate various cargos [101–104]. MSNs have recently been developed as nanocarriers for photodynamic therapy (PDT), photothermal therapy (PTT), or both. PTT and PDT, two important types of phototherapies, sparked a lot of interest in various cancer treatments [105]. The applications Apart from the synthetic approach of synthesising NPs, recently the herbal or biogenic approach has got much attention by the researchers and is been widely explored. In one such attempt, Xing et al. have studied innovative chemotherapeutic AuNPs to treat bladdercancer in a recent study and the AuNPs were prepared using *Citrus aurantifulia* seed extract. The outcomes of the clinical trial established that the AuNPs can be used as antioxidant, anticholinergics, anti-diabetic and anti-bladder cancer supplements in humans [87]. The biogenic nanoparticles are devoid of chemical neurotoxicity being of natural origin and

of MSNs are shown in Table 6.

hence are considered as the safest mode of augmenting cancer therapy with a reduced degree of toxicity. The applications of AuNPs in drug delivery for cancer therapy are shown in Table 5.

**Table 5.** Applications of gold nanoparticles (AuNPs) in drug delivery for cancer therapy. Reproduced with permission from reference [88].

