Tetraethyl orthosilicate (TEOS), dimethyl sulfoxide (DMSO), CDDP, Dopamine hydrochloride (DA), ammonium hydroxide solution (NH₃·H₂O, ∼28% NH₃ in water) and Anhydrous ethanol originated from Shanghai Maclin Biochemical Technology Co., LTD. Phosphate buffer solution (PBS) and Trimethylol aminomethane hydrochloric acid (Tris-HCl) were purchased from Beijing Coolaber Technology Co., LTD. Moreover, potassium permanganate (KMnO₄) and sodium carbonate (Na₂CO₃) were offered by Henan Yaoye Chemical Products Co., LTD. Amino Cy5.5 (NH₂-Cy5.5) was purchased from Solarbio Biotechnology Co., LTD. The Millipore system (Milli-Q, 18.2 MX cm) was used to purify deionized water (H₂O). All other chemicals were received in good time and were of reagent quality.

First, solid silica (sSiO₂) nanoparticles were synthesized. 120 ml anhydrous ethanol, 20 ml deionized water, and 10 ml ammonia were introduced into a 250 ml round bottomed flask in turn. After sealing, the sample was subjected to magnetic stirring for 15 min, and then 4.5 mL ethyl orthosilicate was slowly introduced into the mixed solution in the stirring. After sealing, the stirring continued for 8 h at ambient temperature to obtain SiO₂ NPs. The resulting SiO₂ NPs were washed 3 times with anhydrous ethanol and deionized water, respectively. Subsequently, the NPs were stored in deionized water for the next synthesis SiO₂@MnO₂.

600 mg KMnO₄ dispersed in 20 ml water was added dropwise to the stored SiO₂ NPs after ultrasonic treatment. Next, the mixture was ultrasonically treated for 0.5 h (37°C, 40 KHZ) and then stirred at ambient temperature overnight. The reaction solution was centrifuged at 11,000 rpm/min, the supernatant was discarded, and the obtained precipitates were washed with anhydrous ethanol and deionized water for three times, respectively. On that basis, SiO₂@MnO₂ nanospheres with core-shell structure were generated.

Lastly, core-shell SiO₂@MnO₂ nanoparticles were dissolved into Na₂CO₃ solution for 12 h at 60°C. Subsequently, the etched dispersion was centrifuged, the supernatant was discarded, and the obtained precipitates were washed with anhydrous ethanol and deionized water for three times in turn to obtain MnO₂ NPs.

10 mg MnO₂ was introduced to 10 mL DMSO containing 10 mg CDDP and stir in the dark for 12 h. Subsequently, the remaining CDDP was centrifuged out of the solution to generate MnO₂/CDDP NPs. The NPs were dispersed in 10 mL Tris-HCl buffer (pH 8.5, 10 × 10⁻³ M) containing 10 mg DA, and stirred in the dark for 6 h at ambient temperature to obtain MnO₂/CDDP NPs coated with PDA. The final product MnO₂/CDDP @PDA NPs was centrifuged and lyophilized for 48 h.

10 mg MnO₂/CDDP@PDA NPs and 1 mg NH₂-Cy5.5 were fully dispersed in 10 ml Tris-HCl buffer and reacted at ambient temperature for 6 h away from light. The mixture was then centrifuged for 10 min at 11,000 rpm/min and washed 3 times with deionized water to remove the remaining NH₂-Cy5.5 to obtain MnO₂/CDDP@PDA- Cy5.5 NPs. The finished product was freeze-dried for 48 h.

The identical volume of MnO₂ and MnO₂/CDDP@PDA- Cy5.5 NPs solutions with ultra-pure water and normal saline as the solvents were placed in Xilin bottles, respectively. After the samples stood for 24 h, photos were taken, such that the stability can be compared and analyzed. The morphology, structure and elemental composition of the nanoparticles were characterized through transmission electron microscopy (TEM, FEI Tecnai F20) and X-ray energy dispersion spectroscopy (EDS, Oxford X-Max 80 T). The nanoparticle size and zeta potential were characterized using the Marvin Nanoparticle Size Analyzer (Nano-ZS90). The valence states of the elements were detected in nanoparticles through X-ray photoelectron spectroscopy (XPS, The United States-Thermo SCIENTIFIC ESCALAB 250 Xi). A specific surface area and pore size were investigated using the Barrett–Joyner–Halenda (BJH) method and the Brunauer–Emmett–Teller (BET) method (Micromeritics Instrument Corp. ASAP2460). Ultraviolet-visible spectra of nanoparticles were examined using ultraviolet spectrophotometer (UV-1285, Shimadzu).

The content of CDDP was determined by UV-vis to evaluate the CDDP of MnO₂ loading. Using the method developed previously (Anirudhan et al., 2022), 5,400 μg CDDP powder was dissolved into 5 ml DMSO solution, and 1,080 μg/mL CDDP solution was prepared. The solutions of 540, 216, 108, 21.6 μg/mL CDDP were diluted with deionized water. The absorbance values of the CDDP solutions with different concentrations were examined under 301 nm ultraviolet light, and the standard curve was generated (Supplementary Figure S1). After CDDP and MnO₂ reacted fully, the resultant reaction mixture was centrifuged to separate the supernatant from the precipitate. The optical density of CDDP in the supernatant was measured using the ultraviolet visible spectrophotometer. In addition, in order to prove that the cavity in manganese dioxide improves the drug loading, the drug loading of SiO₂ before etching is also investigated. The drug loading rate (DL%) and encapsulation efficiency (EE%) of CDDP was examined indirectly by measuring the amount of CDDP in the supernatant, as expressed below: E E   % = Total   drug   addition − The   amount   of   drug   in   the   supernatant Total   drug   addition × 100 % DL  % = Total   drug   addition − The   amount   of   drug   in   the   supernatant mass   of   drugs   loaded   final   carriers × 100 %

With different solutions as the release medium, 2 mL of MnO₂/CDDP@PDA (1 mg/mL) solution was dissolved into the dialysis bag, and the dialysis bag was placed in the above beakers containing an appropriate number of media and placed on the magnetic stirrers (37°C, 100 rpm). At 1, 2, 4, 8, 12, and 24 h, 0.5 mL of the solution outside the dialysis bag was removed, and the corresponding release medium of equal volume was introduced. The Mn²⁺ content was analyzed by inductively coupled plasma atomic emission spectrometry (ICP-OES). The content of CDDP was determined using an ultraviolet spectrophotometer, and the CDDP release rate was calculated in accordance with the established standard curve and reference formula. C_n denotes the drug concentration in the release medium at the n time point, V represents the total volume of the release medium, C_k expresses the drug concentration in the release medium at the k time point, and V₀ is the sampling volume. The CDDP release percentage  % = Cn × V + ∑ 1 n − 1 C k × V 0 m × 100 %

The concentration of Mn²⁺ was determined as 90.788 mg/L by ICP-OES to determine the relaxation rate of MnO₂/CDDP@PDA-Cy5.5 and assess the MR imaging capabilities of nanoparticles. Next, MnO₂/CDDP@PDA-Cy5.5 was dispersed in different conditions. After a 4-h incubation, the MRI study was conducted with a 3.0 T magnetic resonance (MR) device. The T₁ value of MnO₂/CDDP@PDA-Cy5.5 under different conditions is determined as a function of concentration. The following acquisition parameters were used in T₁-weighted image acquisitions: field of view = 90 × 90 mm², matrix = 268 × 153, slice thickness = 2.0 mm, repetition time = 3,000 ms, echo time = 80 ms.

8305C human thyroid cancer cells originated from Wuhan Procell Life Science and Technology Co., Ltd., and cultured in MEM (including NEAA) supplemented with 10% fetal bovine serum (Corning), 10,000 U/mL penicillin and 10 mg/mL streptomycin (Corning) in a humidified environment of 37°C and 5% CO₂. Nthy-ori3-1 human thyroid normal cells were purchased from Zhaoqing Sea Star Biotechnology Co., LTD., and then cultured in RPMI-1640 supplemented with 10% fetal bovine serum (Corning), 10,000 U/mL penicillin, as well as 10 mg/mL streptomycin (Corning).

To examine the cell internalization behaviour of various formulations, 8305C cells were incubated with Cy5.5 and MnO₂/CDDP@PDA-Cy5.5 nanoparticles and then imaged by laser confocal scanning microscopy (CLSM) (Hao et al., 2016b). 8305C cells with a cell density of 2×10⁵/mL were incubated in an incubator for 24 h Cy5.5 and MnO₂/CDDP@PDA-Cy5.5 were added and incubated for another 8 h. Next, the cells were gradually washed three times with PBS (pH = 7.4). Hoechst was used to identify the nuclei, which were then incubated without light for 30 min. The cells were then given three PBS rinses before 1 mL of PBS buffer was added for CLSM imaging.

To assess the cytotoxicity of MnO₂@PDA-Cy5.5 NPs, 8305C and Nthy-ori3-1 cells were inoculated in 96-well plates at a density of 2×10⁵/well for 12 h. Next, MnO₂@PDA-Cy5.5 NPs dispersion in culture medium with different concentrations (Based on the concentration of MnO₂) was introduced to the wells. 10 mL of CCK-8 reagent was added to the relevant well after 24 h. After a light shake, the plates were put back under usual conditions for cell incubation for 0.5–1 h. The microplate reader found the OD value at 450 nm after 0.5 h. Cell viability was calculated using the following formula: C e l l   v i a b i l i t y % = O D s a m p l e − O D b l a n k O D c o n t r o l − O D b l a n k

In addition, nanoparticles carrying CDDP were used to evaluate the therapeutic effect. In this process, free CDDP, MnO₂/CDDP@PDA-Cy5.5 (CDDP concentration 0.625, 1.25, 2.5, 5, 10, 20 μg/mL) were incubated with 8305C cells for 24 h, and cell viability was detected by CCK-8 assay. The methods of determination and data processing were the same as those described in the cytotoxicity tests.

8305C cells with a density of 2×10⁵/mL were inoculated into a 6-well plate to assess the effect of MnO₂/CDDP@PDA-Cy5.5 on apoptosis of cancer cells. After 12 h incubation, MnO₂/CDDP@PDA-Cy5.5, MnO₂, free CDDP, and PBS (blank group) were introduced to the respective well and incubated for another 48 h. The tests conformed to the procedure described in the apoptosis kit. Apoptosis was characterized by flow cytometry.

Both BALB/c Nude female (15–18 g, 5–6 weeks) and male SD rats originated from Liaoning Changsheng Biotechnology Co., LTD. All animals were used in investigations following a protocol endorsed by the Qiqihar Medical College’s Experimental Animal Ethics Committee. All experimental nude mice were raised in the SPF environment of Medical College with consistent and suitable alternating light and shade, temperature (22°C ± 1°C) and humidity (50%–60%), during which they were fed and drinking water adequately. 8305C cells at logarithmic growth stage were digested with 0.25% trypsin. The cells were collected after centrifugation and resuspended with normal saline. The cell suspension was prepared at a concentration of 2×10⁵/mL and then subcutaneously injected into the right armpit of naked mice. Drug therapy and imaging studies were conducted on the tumor-bearing mice. Weight and tumor volume were examined every 1 day and detailed records were made.

The Caliper IVIS Spectrum bioluminescence imaging system was employed for fluorescence imaging of tumor-bearing mice. The anesthetized nude mice were placed in a small animal fluorescence imager, and the internal anesthetic gas release device was directed at the mouth and nose of the nude mice for continuous anesthesia. Fluorescence scanning was performed at 1 h, 2 h, 4 h and 12 h before and after MnO₂/CDDP@PDA-Cy5.5 injection.

After anesthesia, nude mice were placed on a 3.0 T Philips MRI bed for T₁ signal scanning. After scanning, MnO₂/CDDP@PDA-Cy5.5 NPs were injected into nude mice by tail vein at a dose of 5 mg/kg. After injecting NPs for 2 h, 4 h and 8 h, the nude mice were scanned for T₁ signal again, and the results were recorded and then analyzed. The following acquisition parameters were used in T₁-weighted image acquisitions: field of view = 90 × 90 mm², matrix = 268 × 153, slice thickness = 2.0 mm, repetition time = 3,000 ms, echo time = 80 ms.

Treatment is initiated when the tumor has grown to 80–100 mm³. Mice were randomly divided into four groups with three mice in the respective group. The four groups comprised the PBS group, the MnO₂ group, the CDDP group, and the MnO₂/CDDP@PDA-Cy5.5 group. The dose of MnO₂/CDDP@PDA-Cy5.5 was 3 mg/kg (based on the measurement of CDDP), and the dose of MnO₂ was 30 mg/kg. The drug was injected into the tail vein of nude mice, once every other day, for a total of 7 times. Before each administration, the weight and tumor volume of mice were examined with balance and vernier caliper, and the health status of mice was observed. The equation for calculating tumor size is V = 0.5×L×S², where V is the volume of the tumor, and L and S are the longest and shortest diameters of the tumor respectively.

After treatment, nude mice were killed by cervical dislocation, and tumors and major organs (heart, liver, spleen, lung, and kidney) of the respective group were extracted, fixed with 4% paraformaldehyde solution, embedded in paraffin, staining with hematoxylin and eosin (H&E), and imaging with digital microscope (HAMAMATSU S210 microscope). After that, UTP notch end labeling (TUNEL) was used to dye the tumor tissue in order to better determine the tumor’s level of apoptosis. Tumors and major organs (heart, liver, spleen, lung, and kidney) of the probe group were extracted and dissolved into aqua regia for 48h, and Pt²⁺ content was examined by ICP-AES for biological distribution study (Wang et al., 2019).

Biochemical analysis was conducted on 1.5 mL of blood from SD rats administrated with MnO₂/CDDP@PDA-Cy5.5NPs, and Saline treated rats were assigned into controls. To obtain serum, blood samples were centrifuged at 4,000 rpm for 10 min at 4 C. Subsequently, alanine aminotransferase (ALT), aspartate aminotransferase (AST), creatinine (Cr), and urea nitrogen (BUN) concentrations were measured in the serum with an automatic biochemical analysis (BS-430, Mindray).

Data are presented as mean ± standard deviation (SD). The Student's t-test was used for the statistical analysis. The indicators of significance were #p > 0.05, *p < 0.05, **p < 0.01, and ***p < 0.001.

Figure 1 depicts the synthesis of the MnO₂/CDDP@PDA-Cy5.5 as well as the mechanism of diagnosis and treatment. In brief, hollow mesoporous MnO₂ nanoparticles were first prepared by template etching and ultrasonic cavitation. The chemotherapeutic CDDP was then physically encapsulated in a hollow cavity of manganese dioxide nanoparticles to alleviate its inherent toxicity. To precisely control the release of CDDP, PDA shells containing many active groups were formed on MnO₂ NPs by self-polymerization of dopamine monomers under alkaline conditions to form MnO₂/CDDP@PDA NPs. In mildly acidic conditions, the PDA shells would depolymerize and release CDDP. Lastly, NH_2-Cy5.5 is attached to the surface of MnO₂/CDDP@PDA NPs to form MnO₂/CDDP@PDA-Cy5.5 NPs through the reaction of amino groups and active esters under weak base conditions.

As can be seen from the comparison before and after Figure 2 (A₁-A₂), manganese dioxide nanoparticles are unstable in ultra-pure water and normal saline, and obvious condensation occurs after 24 h of placement. As can be seen from the comparison before and after Figure 2(A₃-A₄), the stability of modified MnO₂ nanoparticles is significantly improved. In addition, the particle size variation of MnO₂/CDDP@PDA-Cy5.5 NPs in pure water and PBS solution was also studied. As shown in Supplementary Figure S2, the hydrated particle size of MnO₂/CDDP@PDA-Cy5.5 NPs in pure water and PBS has not changed significantly for seven consecutive days, indicating that it has good stability.

TEM was used to monitor the formation of hollow mesoporous MnO₂ NPs and core-shell MnO₂/CDDP@PDA-Cy5.5 NPs. All NPs had strong monodispersity and were generally spherical (Figures 2B, C). Figure 1B’s depiction of the nanospheres’ appearance as having a gray center and an atrous edge indicates that a hollow, mesoporous MnO₂ structure was successfully synthesized. Additionally, a unique two-layer structure (Figure 2C) indicated that the PDA shell had completely encapsulated the hollow mesoporous MnO₂ core. Due to the thicker hydration layer, the average particle size of MnO₂/CDDP@PDA-Cy5.5 NPs generated by TEM was nearly 90 nm, much smaller than that of DLS (Supplementary Figure S4). The TEM size conformed to the theoretical requirements of the EPR effect (Maeda et al., 2013; Wathoni et al., 2020). Furthermore, the element mapping of high-angle annular dark-field scanning (HAADF-STEM) (Figure 2(D-F)) further demonstrated the hollow structure of MnO₂ and exhibited the capability of distinguishing the manganese and oxygen elements covered in the nanoparticles. Supplementary Figure S3 further quantifies manganese and oxygen, hollow nanoparticles are characterized by higher element density in the edges and lower in the core.

The UV-vis absorption spectra of different preparations were tested, as presented Figure 3A, to verify the successful loading of CDDP into the manganese dioxide cavity and the successful coupling of Cy5.5 with MnO₂/CDDP@PDA. After CDDP was loaded into MnO₂, the strong absorption band of MnO₂/CDDP@PDA-Cy5.5 NPs and MnO₂/CDDP at 301 nm was evident, representing the characteristic peak of CDDP compared with pure MnO₂ NPs. MnO₂/CDDP@PDA-Cy5.5 NPs also displayed an absorption peak at 675 nm, consistent with the absorption wavelength of Cy5.5, suggesting that MnO₂/CDDP@PDA was successfully coupled with Cy5.5. Furthermore, EDS of MnO₂/CDDP@PDA-Cy5.5 NPs indicated the presence of platinum elements in Figure 3B, suggesting that CDDP was well loaded into the manganese dioxide cavity, consistent with the change of Zeta potential in the preparation process (Figure 3C).

XPS spectra provide further evidence for the successful formation of the PDA coating (Figure 3D) (Zeng et al., 2020). In other words, the spectrum of MnO₂ NPs coated with PDA revealed a brand-new N-peak corresponding to the amino group of PDA, suggesting that the preparation of core-shell MnO₂/CDDP@PDA NPs was successful. According to Figure 2E, the binding energies of the Mn 2p3/2 and 2p1/2 were centered at 642 eV and 653.9 eV, respectively, suggesting that Mn’s primary oxidation state was +4. Figure 2F shows the binding energy of C-OH and C=O centered at 531.65 eV and 533.14 eV, respectively. The nitrogen adsorption–desorption isotherm of MnO₂ was collected, and the corresponding specific surface area was calculated to be 64.04 m² g⁻¹ (Figure 4A). Beyond that, this adsorption–desorption isotherm belongs to the typical V-type isotherm, confirming its rich mesopores. The BJH model was employed to fit the corresponding pore size distribution curve. As indicated by the results, the pore size of MnO₂ before drug loading was 8.76 nm, and the pore size of MnO₂/CDDP decreased to 5 nm after drug loading. This variation indicates that cisplatin enters the hollow cavity through the hole (Figure 4B). According to the XRD statistics in Supplementary Figure S5, the distinctive MnO₂ diffraction peaks at 2θ = 22.4°, 37.1°, 42.6°, 56.4°, and 67.6°, which emerged at (120), (131), (300), (160), and (003), respectively, and corresponded to the MnO₂ crystal type (JCPDS 14–0644). The above results suggested that the obtained MnO₂ displayed a cubic crystal structure (Wang et al., 2023).

In order to increase the loading efficiency of CDDP, the loading efficiency was investigated at different feed ratios. As shown in Supplementary Figure S6, when the ratio of MnO₂ to CDDP was 1:1, the drug loading rate and encapsulation efficiency reached the maximum, which were 32.27% and 47.65%, respectively, which were 10.82% and 20.34% higher than those before SiO₂ etching. On that basis, the drug release performance of MnO₂/CDDP@PDA at different conditions was evaluated. As depicted in Figures 4C, D, Mn²⁺ and CDDP were rapidly released from MnO₂/CDDP@PDA under 2 mM GSH or acidic conditions (pH = 5.5), but released slowly in neutral PBS. Compared with the slow drug-release profiles of MnO₂/CDDP@PDA at pH 7.4, the release speeds of both CDDP and Mn²⁺ were found to be faster in weak redox environment. The above-mentioned significant pH and redox-responsive drug release behaviour led to the decreased CDDP release in a neutral environment, prolonged the blood circulation in the body, and facilitated the CDDP release of free pharmaceuticals in times, such that negative side effects on healthy tissues were reduced. Degradation of MnO₂/CDDP@PDA in acidic solution (pH 5.5) was also observed by TEM (Supplementary Figure S7). We found that the nanoparticles were distributed in sheet form after acid dissolution.

As depicted in Figures 4E, F; Supplementary Figure S8A, at pH 7.4, no significant difference was reported in MRI T₁-weighted imaging signal intensity of MnO₂/CDDP@PDA-Cy5.5 NPs different concentrations, and the relaxation rate reached 0.771 mM⁻¹S⁻¹. As revealed by the above-mentioned result, MnO₂/CDDP@PDA-Cy5.5 NPs did not exhibit any T₁-weighted imaging ability in neutral environment. Under 2 mM GSH or acidic conditions (pH = 5.5), the signal intensity of T₁-weighted MnO₂/CDDP@PDA-Cy5.5 NPs with different concentrations was notably enhanced, and the relaxation rate were 15.519 and 14.313 mM⁻¹S⁻¹. The possible reason for the above results may be the responsive degradation of MnO₂/CDDP@PDA-Cy5.5 NPs. Under acidic and redox conditions, MnO₂/CDDP@PDA-Cy5.5 NPs is reduced to Mn²⁺, which has MRI T₁-weighted imaging capability.

To evaluate whether the nanoparticles would still display redox responsive MR switchable properties after addition of biologically relevant concentrations of reducing agents, MnO₂/CDDP@PDA-Cy5.5 (Mn = 1 mM) NPs were treated with different concentrations of GSH at pH = 7.4 and imaged under 3.0 T MR. It can be clearly seen from Supplementary Figure S8B that at this pH, when GSH concentration is between 20 and 200 µM, imaging switching will occur.

The uptake of MnO₂/CDDP@PDA-Cy5.5 and its intracellular behaviour were observed through CLSM in depth. As depicted in Figure 5, the cell nuclei stained by DAPI are represented by blue, while Cy5.5 and MnO₂/CDDP@PDA-Cy5.5 are all represented by red. Notably, MnO₂/CDDP@PDA-Cy5.5 exhibited greater fluorescence intensity in the cytoplasm. Fluorescence pictures of 8305C cells treated with either MnO₂/CDDP@PDA-Cy5.5 or free Cy 5.5 were reconstructed in 2.5 dimensions by ZEN 2012 software by utilizing the “optical sectioning” capability of CLSM. Compared with free Cy5.5, MnO₂/CDDP@PDA-Cy5.5 entered the cell more, especially with stronger red and blue fluorescence, as evidenced by the presence of a fluorescence signal along the z-axis. In order to further study the uptake of MnO_2/ CDDP@PDA-Cy5.5 by 8305C cells, we carried out fluorescence quantification by flow cytometry. As shown in Figure 7(D-F), The mean fluorescence intensity of MnO₂/CDDP@PDA-Cy5.5 incubation group was significantly higher than that of Cy5.5 group. These results suggested that the uptake ability of 8305C cells to MnO₂/CDDP@PDA-Cy5.5 is better than Cy5.5.

As depicted in Figure 6A, after 8305C cells and Nthy-ori3-1 cells were administrated with different concentrations of MnO₂@PDA-Cy5.5 for 24 h, the survival rate of the cells reached nearly 70% even at the concentration of 100 μg/mL. As revealed by the above result, MnO₂@PDA-Cy5.5 exhibited high biocompatibility with 8305C cancer cells and Nthy-ori3-1 normal, and it can serve as a drug carrier for in-depth research.

Subsequently, 8305C cells were adopted to evaluate the therapeutic effect of MnO₂/CDDP@PDA-Cy5.5. As depicted in Figure 6B, with the increase of CDDP concentration, cell mortality in the MnO₂/CDDP@PDA-Cy5.5 group was notably higher than that in the free CDDP group. Furthermore, at the identical concentration of CDDP, the mortality rate in the MnO₂/CDDP@PDA-Cy5.5 group was significantly higher than that in the CDDP group, suggesting that MnO₂/CDDP@PDA-Cy5.5 has a better ability to inhibit the growth of tumor cells. This can be attributed to the higher amount of CDDP that the drug delivery system is able to internalize in the cell.

The results of flow cytometry are shown in Figure 6(C₁-C₄). The apoptosis rates of 8305C cells administrated with PBS, MnO₂, CDDP and MnO₂/CDDP@PDA-Cy5.5 were 5.28%, 6.11%, 14.98% and 19.04%, respectively. Compared with free CDDP, MnO₂/CDDP@PDA-Cy5.5 induced more apoptosis in 8305C cells. This can be attributed to the EPR effect of nanoparticles improved the targeting of nanoparticles and effectively enhanced the apoptosis of 8305C cells.

As depicted in Figures 7A, B, whole-body fluorescence imaging showed widespread Cy5.5 fluorescence in the mouse body 1 h after an intravenous injection of MnO₂/CDDP@PDA-Cy5.5. With the gradual accumulation and penetration of MnO₂/CDDP@PDA-Cy5.5, the fluorescence signal was detected only in the tumor area and reached its maximum intensity 4 h after injection, suggesting effective tumor accumulation by MnO₂/CDDP@PDA-Cy5.5 NPs through EPR effect. Notably, fluorescence at tumor sites was still evident 8 h after injection, arising from the prolonged blood circulation of NPs and their remarkable ability to target the tumor.

The in vivo imaging of MnO₂/CDDP@PDA-Cy5.5 is shown in Figure 8A. Following the injection of nanoparticles, the T₁ imaging signal in the tumor steadily grew over time, which was peaked at 4 h. This is also confirmed by comparing the intensity of T₁ signal corresponding to the tumor site and muscle Figures 8B, C. Accordingly, MnO₂/CDDP@PDA-Cy5.5 could be employed as an acid-responsive T₁ contrast agent.

As depicted in Figure 9A, tumor bearing nude mice were divided into groups for 14 days of treatment. Figure 9B showed that no significant inhibitory effect was observed on tumor volume in nude mice in the PBS and MnO₂ groups, but after 14 days of treatment, tumor growth rate in the free CDDP group was inhibited to a certain extent. In addition, tumor volume decreased in MnO₂/CDDP@PDA-Cy5.5 nude mice. Meanwhile, as depicted in Figure 9C, there was no significant difference in body weight among the groups. The mice were killed at the conclusion of the trial, and the tumors were removed, photographed, and then weighed. As depicted in Figure 9D, the MnO₂/CDDP@PDA-Cy5.5 group exerted the optimal therapeutic effect. Figure 9E illustrates the final tumor weight. The mean tumor weight of the PBS group, the MnO₂ group, the free CDDP group, and the MnO₂/CDDP@PDA-Cy5.5 group reached 0.367 g, 0.227 g, 0.132 g, and 0.100 g, respectively, consistent with the growth trend of tumor volume. As revealed by the above-mentioned results, MnO₂/CDDP@PDA-Cy5.5 can exert a notable therapeutic effect while outperforming free CDDP.

As depicted in Supplementary Figure S9, the kidneys of the free CDDP group had tubular collapse. The renal tubules in the MnO₂/CDDP@PDA-Cy5.5 group were organized in comparison to the free CDDP group, and no pathological alterations or necrosis were reported in the kidneys, suggesting that MnO₂/CDDP@PDA-Cy5.5 mitigated the renal toxicity of CDDP. There were no significant differences in all other major organs, including the heart, liver, spleen and lungs, in the MnO₂ or MnO₂/CDDP@PDA-Cy5.5 groups compared with the saline group.

Figure 10A presents the representative tumor H&E staining samples, suggesting that the MnO₂/CDDP@PDA-Cy5.5 group had tumor cell necrosis and shrinkage compared with the rest groups. Thus, the significantly improved therapeutic efficacy was shown for tumor-bearing mices administered MnO₂/CDDP@PDA-Cy5.5 in comparison to those administered with CDDP at the identical CDDP doses. Based on in situ TUNEL, the apoptosis levels of tumor tissues were investigated in depth, and little green coloration was observed in the tumors of nude mice administrated with PBS and MnO₂, suggesting that no cell apoptosis was identified (Figure 10A). In contrast, tumors of nude mice administrated with free CDDP displayed an increase in the amount of green cells, and the highest level of apoptosis was observed in tumors of nude mice administrated with MnO₂/CDDP@PDA-Cy5.5 NPs. Thus, compared with free CDDP, MnO₂/CDDP@PDA-Cy5.5 nanoparticles are more effective in inducing apoptosis.

As depicted in Figure 10B, compared with the control group, the indexes of liver function (ALT and AST) and renal function (CREA and BUN) in the experimental group are very similar to those in the control group. Moreover, referring to the indexes of liver and kidney in the literature, it was found that MnO₂/CDDP@PDA-Cy5.5 NPs did not induce obvious liver and kidney dysfunction (Rong et al., 2021). In a word, the above-mentioned results show that MnO₂/CDDP @ PDA-Cy5.5 NPS has good biocompatibility, and can provide passive drug delivery with low risk of off-target toxicity.

The biodistribution results indicated that much more Pt²⁺ existed in the tumor than in the other organs, suggesting that MnO₂/CDDP@PDA-Cy5.5 NPs exhibited significant tumor accumulation (Figure 10C), probably due to the EPR effect. Moreover, high amounts of Pt²⁺ were identified in the kidneys of the above-mentioned mice, suggesting that Pt²⁺ from MnO₂/CDDP@PDA-Cy5.5 NPs was quickly cleared by the kidneys.