**1. Introduction**

Chemodynamic therapy (CDT) is an emerging cancer treatment, which depends on the Fenton or Fenton-like reactions to obtain highly toxic hydroxyl radicals (·OH) for killing cancer cells [1]. The Fe/hydrogen peroxide (H2O2) system is defined as the Fenton reagent, and the others (e.g., Co, Cd, Cu, Ag, Mn, Ni) are called Fenton-like reagents [2–6]. Compared with normal cells, the tumor microenvironment is characterized by overexpression of H2O2, glutathione (GSH) and weak acidity [7,8]. Up to now, most of the metal ions used to construct CDT nanotherapeutics include Mn, Fe and Cu. Fe ions in particular are the most commonly used for CDT [9]. However, the Fe-based Fenton reaction is only effective under strongly acidic conditions (pH 2–4) [10]. Therefore, the Fentonreaction of Fe will be limited under the neutral and weakly acidic microenvironment conditions encountered in tumors. Mn(II) used in Fenton-like reactions only remains stable when pH < 4. In contrast, the efficiency of Cu(I) is not influenced by the pH. Even under the best reaction conditions, the reaction rate of Cu(I) is 160 times than that of Fe(II) [11–13]. In addition, a large amount of GSH in tumor cells can react with reactive oxygen species and affect the concentration of H2O2 and ·OH. GSH can be depleted during the reduction of the Cu(II) to Cu(I), which is beneficial for CDT [14]. Therefore, Cu-based nanotherapeutic agents for CDT have attracted a lot of attention.

**Citation:** Hao, Y.-N.; Qu, C.-C.; Shu, Y.; Wang, J.-H.; Chen, W. Construction of Novel Nanocomposites (Cu-MOF/GOD@HA) for Chemodynamic Therapy. *Nanomaterials* **2021**, *11*, 1843. https://doi.org/10.3390/nano11071843

Academic Editor: Jose L. Arias

Received: 19 June 2021 Accepted: 14 July 2021 Published: 16 July 2021

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The relatively higher concentration of H2O2 in tumor cells than in normal cells is the basis for CDT due to Fenton/Fenton-like reactions with H2O2. However, the H2O2 concentration in tumor cells is still limited and this limitation actually influences the efficiency of CDT [15]. Different kinds of strategies have been proposed to overcome this drawback. The introduction of extraneous enzyme (glucose oxidase (GOD), superoxide dismutase and so on) into the cell interior may facilitate the production of H2O2, which ensures the continuous and effective treatment of tumor cells [16]. GOD can catalyze the reaction of glucose (Glu) to produce gluconic acid and H2O2 [16,17]. Therefore, combining GOD and a CDT agen<sup>t</sup> is a promising strategy. GOD consumes a large amount of Glu needed for physiological activities, which achieves a "cell starving" effect therapy during this process. This phenomenon also supplies H2O2 for CDT [18].

Metal-organic frameworks (MOFs) are organic-inorganic hybrid materials formed by the combination of inorganic metal ions or metal clusters and organic ligands. MOFs have been widely used in biomedical imaging and therapy [19], biosensing [20], catalysis and as drug carriers [21] due to their high specific surface area, porosity and diversified structures. MOFs can be easily modified with appropriate treatments during or after their synthesis [22]. Therefore, Cu-MOFs are suitable as modifiable CDT agents.

Hyaluronic acid (HA) is a natural acid mucopolysaccharide present in the synovial fluid and extracellular matrix [23]. HA has been widely used in tissue engineering, drug delivery and molecular imaging which all benefit from its biocompatibility and biodegradability [23–25]. More importantly, HA can specifically target CD44 overexpressed in various cancer cells and be decomposed by the intracellular hyaluronidase [26]. Therefore, HA is often used to bounding various drug-loaded nanoparticles as a targeting moiety for enhanced cancer therapy [27–30].

In this work, Cu-MOF are chosen as a cascade nanoreactor for CDT. As shown in Figure 1, GOD are first loaded onto Cu-MOF and Cu-MOF/GOD composites are thus obtained. HA acts as a shell on the Cu-MOF/GOD to avoid GOD leakage and as a targeting molecule to tumor site. The resulting Cu-MOF/GOD@HA nanocomposites are activated by GSH in tumors and catalyze H2O2 to produce ·OH for CDT for cancer treatment as illustrated in Figure 1.

**Figure 1.** Schematic of the Cu-MOF/GOD@HA preparation process ( **A**) and the Cu-containing nanoformulation mediated CDT (**B**).

## **2. Materials and Methods**

*2.1. The Preparation of Cu-MOF/GOD@HA Nanocomposites*

2.1.1. Cu-MOF/GOD

Cu-MOFs (HKUST-1) were prepared according to a previously reported approach [31]. Thus, 10 mg of Cu-MOF were dissolved in 1 mL of absolute ethanol. Next 10 mg of coupling agen<sup>t</sup> (1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride/Nhydroxysuccinimide) (EDC/NHS) were dissolved in 6 mL of DI water. The above Cu-MOF and EDC/NHS solutions were mixed with vigorous stirring for 30 min. Then 2 mg of GOD was dissolved in 3 mL of DI water and added dropwise into the above mixture under stirring for 4 h. Finally, the resultant Cu-MOF/GOD was washed with DI water three times.
