Grain Boundaries Assisted Construction of Pt Colloidosomes for Tumor Catalytic Therapy Enhanced by Near-Infrared Photon-Excited Hot Electrons

doi:10.21203/rs.3.rs-2498108/v1

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Grain Boundaries Assisted Construction of Pt Colloidosomes for Tumor Catalytic Therapy Enhanced by Near-Infrared Photon-Excited Hot Electrons

https://doi.org/10.21203/rs.3.rs-2498108/v1

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Noble metal nanoparticles (NPs) are photo-responsive substrates that generate hot electrons under light excitation, and these hot electrons enhance the catalytic capability of the NPs, which is useful in many applications. For ultrasmall (≤ 5 nm) Pt NPs with excellently catalytic performance, it is challenging to utilize near-infrared (NIR) light-excited hot electrons to enhance the nanozyme activities of Pt NPs for tumor catalytic therapy because ultrasmall Pt NPs have no obvious absorption from the visible light band to the NIR band. Herein, we report on constructing Pt colloidosomes (Cs) with ultrasmall Pt NPs to elevate NIR light absorption to excite multitudes of hot electrons. The successful photon-to-electron conversion is based on the high density of electron states (d-band) at the Fermi energy in the Pt system, which leads to a high probability of generating NIR photon-excited hot electrons. NIR light absorption of Pt Cs is optimized by introducing grain boundaries (GBs) defects in Mn3O4 precursors by laser irradiation in liquids. Moreover, NIR photon-excited hot electrons largely promote the self-cascade catalytic production of reactive oxygen species (ROS) for enhanced tumor therapy. This study first validates the generation of hot electrons in the Pt nanostructure under NIR light irradiation by assembling ultrasmall Pt NPs, and then demonstrates the Pt Cs to be highly promising as efficient photo-responsive nanozymes with integrated photoactivity and chemical activity.

Physical sciences/Materials science/Biomaterials/Bioinspired materials

Biological sciences/Biotechnology/Biomimetics/Biomimetic synthesis

Highly energetic hot electrons can drive catalytic reactions to avoid the undesirable energy losses that occur in conventional catalysis activated by thermal energy. Thus, they are widely investigated for photochemical, photovoltaic, and photodetection applications¹^–⁵. For noble metal catalysts capable of harvesting light, hot electrons can be produced through the light-stimulated interband and/or intraband transition of d-band electrons⁶^–⁹. Excitation light for the generation of hot electrons in noble metals is mainly located in the visible band¹⁰^–¹³. The open problem is that the number of hot electrons reaching the metal surface is limited because visible light-excited hot electrons are highly energetic, so they have strong electron-phonon scattering and short lifetime¹⁴^–¹⁵. In contrast, near-infrared (NIR) photon-excited hot electrons possess lower energy, so, in principle, they should have a longer lifetime. However, the generation of hot electrons through NIR photon-excitation has mostly been observed in non-noble metal catalysts, which have broadly tunable plasmon frequency from visible to infrared¹⁶^–¹⁷.

Generation of hot electrons in noble metals has been strongly dependent on the d-band structure. Pt shows a wide d-band, and the top of the d-band can reach the Fermi energy^8,18^–²⁰. This leads to a very high density of electron states at the Fermi energy and ensures that electrons located in the part of the d-band lower than the Fermi energy (valence band) can be easily excited to the s-band (conduction band)²¹^–²². It can be inferred that Pt has the potential to supply sufficient electrons for the transition from valence band to conduction band under NIR photon-excitation, which is conducive to attaining a high yield of hot electrons with a long lifetime. Moreover, theoretical calculations have predicted that the generation rates of hot electrons in Pt nanocrystals peak when the wavelength of incident light is in the range of 700–1000 nm²¹. The aforementioned studies are valid only if the Pt NPs have significant NIR absorption capability. However, commonly studied catalytic Pt NPs are of ultrasmall (≤ 5 nm) size. Ultrasmall Pt NPs only show limited absorption from ultraviolet (UV) to the short wavelength of visible light, meaning they do not show NIR absorption²³^–²⁵.

One important application for ultrasmall Pt NPs is their ability to be utilized as nanozymes to promote the generation of reactive oxygen species (ROS) for tumor catalytic therapy²⁶. Pt nanozymes exhibit high stability against harsh chemical conditions in acidic tumor microenvironments (TME) and show multiple enzyme-like activities similar to peroxidase (POD), catalase (CAT), and oxidase (OXD) to inhibit tumors²⁷^–³¹. Theoretical calculations show that ultrasmall Pt NPs have powerful catalytic decomposition ability for H₂O₂ and O₂ molecules to produce ROS due to the very low adsorption energies and activation energy barriers for the two molecules on Pt surface³²^–³³. Nevertheless, in reality, the catalytic performance of Pt nanozymes has not met expectations because multitudes of d-band electrons are still limited in the valence band and unable to participate in the catalysis. A widely accepted solution is to utilize photo-excited hot electron transfer from some light-harvesting materials (Au, TiN, and GaN) to make up for the shortage of energetic electrons of Pt³⁴^–³⁷. But this strategy still does not fully release the contribution of the d-band electrons in the Pt system to its catalytic activity. Additionally, because the NIR band is the “optical window” for tumor catalytic therapy³⁸, the excitation band used to generate hot electrons is limited to the NIR band when Pt is taken as a nanozyme. Given that the d-band for ultrasmall Pt NPs is close to the Fermi energy, NIR photon-excitation (808 nm ≈ 1.5 eV) should be able to make d-band electrons jump across the Fermi energy as hot electrons to facilitate the catalytic performance of Pt nanozymes.

Enhancing the NIR light-harvesting ability of ultrasmall Pt NPs is the prerequisite for achieving the coexistence of their photoactivity and chemical activity. Recently, it was demonstrated that the light absorption of ultrasmall Pt NPs can be tuned by embedding them into SiO₂ substrate to adjust the light scattering on the interface instead of changing the metal particle size, and the formed composite exhibits broadened light absorption from the visible band to the NIR band²⁵. Noble metal colloidosomes have a highly scattered interface due to their collective structure comprising close-packed noble metal NPs^{25, 39}^–⁴⁰, this type of interface showcases great convenience for extending the optical absorption from the visible band to the NIR band without increasing the size of Pt NPs. Such colloidosomes have been well documented to exhibit improved catalytic efficiency by enriching a large number of photo-excited hot electrons⁴¹^–⁴³. For example⁴¹, Au-based colloidosomes have been proved for their validity in photo-enhanced catalysis for CO₂ to methane conversion and formic acid oxidation. Based on galvanic replacement reactions, Pt Cs can be formed on the substrate of Mn₃O₄ NPs, which are self-sacrificed to form a hollow structure consisting of Pt NPs. Pt Cs have already been utilized as the catalyst for oxygen reduction reaction and oxygen evolution reaction⁴⁴^–⁴⁵. Because the site for Pt deposition is decided by the divalent manganese (Mn²⁺) ions on the interface, Pt NPs tend to grow bigger in localized regions rather than exhibiting the uniform and closed-packed deposition as expected. Thus, the broadening of light absorption for Pt Cs in the NIR band is still limited.

To improve the uniform and closed-packed deposition of Pt NPs, the distribution of Mn²⁺ ions on the surface needs to be modulated. Laser-induced crystalline defects have been confirmed to promote atomic redistribution at the surface for optimizing the interfacial compressive strain, carrier transfer, and even phase separation^46–48. Remarkably, laser-induced GBs on the surface of a particle can change the atom distribution by breaking down the surrounding coordination^49–50. Furthermore, GBs can give rise to obvious compressive strain, which can boost the interfacial redox reactions⁵¹^–⁵³. By the technique of laser treatment in liquids, defects can be generated and modulated in metal oxide particles during laser-induced fragmentation or melting⁵⁴. Hence, it is rational to introduce GBs into Mn₃O₄ substrate for rearranging the location of Mn²⁺ ions so as to obtain uniform deposition of ultrasmall Pt NPs on the desired colloidosome structure.

In this study, we designed GBs-rich Mn₃O₄ NPs created by pulsed laser irradiation in deionized water and subjected them to a galvanic replacement reaction, ultimately obtaining close-packed Pt Cs with an ultrasmall particle size of 2.17 nm and an optimized interparticle distance of 0.29 nm. These Pt Cs exhibited remarkably broad absorption from the visible band to the NIR band, and they supplied hot electrons to enhance the self-cascade catalysis of ultrasmall Pt NPs like CAT and OXD mimics under NIR photon-excitation. Therefore, the Pt Cs can be used as a photo-responsive nanozyme for enhanced tumor catalytic therapy.

To obtain Pt Cs with uniform and closed-packed deposition of Pt NPs, we adopted laser irradiation to generate GBs-enriched Mn₃O₄ NPs for subsequent galvanic replacement reaction. Figure 1(a) schematically illustrates the formation processes from completely crystallized raw Mn₃O₄ (R-Mn₃O₄) to laser-treated Mn₃O₄ (L-Mn₃O₄) with enriched GBs and finally to the desired Pt Cs. Accordingly, the corresponding sample changed from yellow suspension to orange colloidal solution, and finally to black colloidal solution (Figure S1). For metal oxide NPs, absorbing laser with photon energy (hν) larger than the bandgap (E_g) can immediately promote strong photoionization, resulting in structure fragmentation into educts. These educts can accumulate and regrow as novel NPs with incomplete crystal structure⁵⁵. Herein, we found that the R-Mn₃O₄ NPs exhibited strong absorption of light from 200 to 450 nm, and the E_g of R-Mn₃O₄ NPs was calculated as 2.54 eV⁵⁶ (see Figure S2 for details), so a pulsed laser beam with a wavelength of 355 nm (photon energy of 3.49 eV) was selected as the irradiation source. The transmission electron microscopy (TEM) images in Figure S3 show the nice crystallinity for R-Mn₃O₄ NPs with a size of 50 nm. Figures 1(b) and 1(f) reveal that L-Mn₃O₄ NPs have a similar size and composition compared to R-Mn₃O₄ NPs, but GBs are generated on the particle surface after laser irradiation in Figures 1(c)–1(d). In our experiments, with laser induced photoionization, R-Mn₃O₄ NPs were broken into metastable educts in an aqueous environment (Figure S4), and then the educts underwent ripening and regrowth processes to transform into GBs-enriched L-Mn₃O₄ NPs⁵⁷. By analyzing the lattice structure for L-Mn₃O₄ NPs, we found that the {200} and {004} planes were the two major exposed planes, and GBs emerged between the two planes (see Figure S5 for details). Furthermore, the comparison between oxygen species for Mn₃O₄ before and after laser irradiation indicates the existence of abundant GBs (see Figure S6 for details).

To further study the impact of GBs in the deposition of Pt NPs, density function theory (DFT) calculations of absorption energy for PtCl₄²⁻ groups on different planes and the GBs were compared. According to the shape of tetragonal R-Mn₃O₄ NPs, the {200} and {004} planes were the two major exposed planes (Figure S7). In Figure 1(e), the adsorption energy values of PtCl₄²⁻ groups on the {200} and {004} planes on the Mn₃O₄ surface were recalculated to be −9.90 eV and −3.88 eV, respectively. This means that PtCl₄²⁻ groups tend to be adsorbed on the {200} planes. Moreover, the site for Pt deposition is decided by the Mn²⁺ ions on the surface of Mn₃O₄ NPs in the galvanic replacement reaction⁴⁴. The number of Mn²⁺ ions accounts for two-fifths on the {200} plane and one-half on the {004} plane. When PtCl₄²⁻ groups react with limited Mn²⁺ ions on the {200} plane, the formed Pt NPs easily deposit on some local regions near Mn²⁺ ions and have uncontrollable size. However, laser-induced GBs indicate that the miniaturization of grains can promote the random distribution of the {200} and {004} planes to support even deposition of ultrasmall Pt NPs⁴⁸. In addition, GBs are conducive to the galvanic replacement reaction, because they are beneficial for exposing more active sites⁵⁸^–⁶⁰. GBs also favorably alter metal-oxygen bonds and local electron density by inducing surface compressive strains, thereby indirectly tuning the electron configuration and the adsorption ability to surface adsorb reactants⁵¹^–⁵³. The calculated adsorption energy of PtCl₄²⁻ groups on GB was to be −4.51 eV. Therefore, L-Mn₃O₄ with abundant GBs can improve the galvanic replacement reaction with PtCl₄²⁻ groups, which is helpful for obtaining Pt Cs with high density and uniform Pt NPs loading.

As expected, Pt Cs possess a porous inner structure and an outer shell of closed-packed ultrasmall Pt NPs. Figure 1(f) shows that Pt Cs look like spherical NPs with an average size of 55 ± 6.6 nm. The energy-dispersive X-ray spectroscopy (EDX) mapping analysis in Figure 1(g) confirms the uniform distribution of Mn, Pt, and O, and the X-ray photoelectron spectroscopy (XPS) spectra in Figure S8 prove that the Pt and Mn in Pt Cs are Pt⁰and a mixture of Mn⁺² and Mn⁺³ species, indicating that the Pt and Mn exist as metal and oxide, respectively. Based on inductive coupled plasma emission spectrometer (ICP) data, the molar ratio Pt/Mn can reach as high as 2.23, but the value for R-Pt Cs synthesized from R-Mn₃O₄ under the same conditions only reaches 1.06 (Figure S9(a)). This indicates that the deposition rate of the galvanic replacement reaction on R-Mn₃O₄ NPs was much lower than that on L-Mn₃O₄ NPs. This phenomenon is also consistent with the Brunauer–Emmet–Teller (BET) method, which shows that Pt Cs have a larger porous inner structure (specific surface area is 101.7 m²·g^-1, see Figure 1(i)) compared to commonly etched R-Pt Cs (specific surface area is 65.15 m²·g^-1, see Figure S9(b)). The most part of Mn₃O₄ in Pt Cs was self-sacrificed during the galvanic replacement reaction, which is why the characteristic peaks of Mn₃O₄ almost disappeared in the X-ray diffraction (XRD) patterns (Figure 1(h)). As shown in Figure 1(j), the deposited Pt NPs on the Pt Cs surface had an average size of 2.17 ± 0.03 nm, and the measured interplanar spacing was about 0.225 nm, which corresponded to the (111) plane of cubic Pt NPs, consistent with the broadened Pt diffraction peaks (JCPDS No. 96-101-1104) in the XRD pattern of the Pt Cs. Additionally, the ultrasmall Pt NPs were closely packed with a tiny interparticle distance of 0.29 ± 0.04 nm. As shown in Figures S9(c)–S9(d), the size of deposited Pt NPs on R-Pt Cs and their interparticle distance were 2.63 ± 0.08 nm and 1.29 ± 0.1 nm, respectively. Both were larger than the corresponding values for Pt Cs. Ultrasmall Pt NPs (≤ 5 nm) are extremely helpful for subsequent catalytic reactions. More importantly, the generation of hot electrons in NPs is a quantum effect that is strongly size-dependent⁶¹^–⁶². In addition, the uniform and dense arrangement of Pt NPs is conducive to near-field coupling between adjacent Pt NPs, thereby expanding the broadening of light absorption for the Pt Cs^{25, 63}^–⁶⁵. The porous structure is also conducive to the continuous reflection of light in Pt Cs, thus greatly enhancing the light absorption efficiency of Pt Cs⁶⁶^–⁶⁸. All of the preceding analysis lays the foundation for the generation of photo-excited hot electrons.

The generation of photo-excited hot electrons in noble metal systems needs to meet two preconditions: first, the metal can absorb photons with a specific wavelength; second, the photon energy is high enough to excite the d-band electrons to cross the Fermi level^{6, 69}. As shown in Figure 2(a), Pt Cs indeed expanded the light absorption from the visible band to the NIR band for ultrasmall Pt NPs (Figure S10) as expected. Considering the NIR band, Pt Cs have a higher absorption capacity than R-Pt Cs (Figure S15(g)). The electronic density of the state distribution of Pt, determined by DFT calculations and presented in Figure 2(b), showed a wide d-band distribution (−6.4 eV to 0.41 eV, DOS > 0.5) and a high peak of d-band reaching the Fermi energy. This confirms a very high density of electron states at the Fermi energy, and ensures that the energy of NIR photons (808 nm = 1.53 eV) can easily excite the electrons located at the part of the d-band below the Fermi energy for interband (d-s) transition or intraband (d-d) transition. The long-lived hot electrons excited by NIR photons can participate in the catalytic reaction in two ways⁶: one is the hot electrons with higher kinetic energy and higher momentum than other electrons at the Fermi surface may transfer energy to certain chemical bonds of molecules to break these bonds, such as in the decomposition of H₂O₂ into O₂ and H₂O; the other is the hot electrons can inject directly into unoccupied orbits of target molecules and lower the potential of the molecules required by chemical reactions, such as in the conversion of O₂ to O₂^•−. In our experiments, Pt Cs have been proved as CAT and OXD mimics in catalyzing the decomposition of H₂O₂ and O₂, respectively (see Figures S11–S12 for details). Pt Cs also exhibit the self-cascade catalytic activities to produce O₂^•−- from H₂O₂ in a simulated TME⁷⁰^–⁷¹. Therefore, the changes in the enzyme-like activity for Pt Cs can be used to investigate the effect of possible hot electrons on the enzyme-like activity.

We prepared four groups of Pt Cs samples to investigate their enzyme-like activities corresponding to different Pt/Mn ratios (see Table 1 and Figures S13–S14 for details). When the molar ratio of Pt/Mn elements increased to 2.2, the relative mass activity of the Pt in Pt Cs reached a peak, and the impact of the residual Mn₃O₄ content could be eliminated due to tight surrounding of ultrasmall Pt NPs. The enzyme-like catalytic activities of Pt Cs and other controls with and without NIR light irradiation (808 nm laser as light source) were compared under room temperature. For CAT mimic, OXD mimic, and self-cascade catalytic reactions, the activity of Pt Cs under NIR light irradiation at room temperature increased by 109.3%, 53.1%, and 41.8%, compared to the same situations without NIR light irradiation. The corresponding activity enhancement values were 55.7%, 38.3%, and 18.6% for R-Pt Cs and 37.8%, 10.6%, and 11.9% for Pt NPs, respectively. These results indicated that the activity of Pt Cs significantly increased under NIR light irradiation, and the increased values were higher than those of R-Pt Cs and Pt NPs. In noble metals, the absorption of photons can lead to the generation of hot charges or the generation of heat by electron-phonon coupling⁷². To evaluate the heat conversion, we tested the photothermal effect of Pt Cs and R-Pt Cs under NIR light (see details in Figure S15). The photothermal conversion efficiencies of Pt Cs and R-Pt Cs were calculated to be 23.9% and 18.9%, respectively.

To further explore the effect of heat induced by NIR light on the enzyme-like activity, we simulated the photothermal effect through heating in a water bath⁷³. Figures 2(c)–2(e) demonstrate that the pure thermal effect limited the promotion of the activity of these samples. Taking CAT mimic as an example, the activity enhancement of Pt Cs affected by thermal effect was 76.7%, which was 32.6% lower than the contribution of NIR light. Moreover, the difference between the contribution of NIR light and thermal effect on the enzyme-like activity was 11.8% for R-Pt Cs and 3.5% for Pt NPs. These significant differences between the contribution of NIR light and thermal effect can be attributed to the hot electrons excited by NIR photons. For OXD mimic and self-cascade catalysis, hot electrons increased the catalytic activity of Pt Cs by 25.2% and 15.3%, respectively, that of R-Pt Cs by 13.9% and 5.6%, respectively, and that of Pt NPs by 3.4% and 2.5%, respectively. Compared with Pt Cs, the activity enhancement induced by hot electrons for R-Pt Cs was rather weak, while the increase in the activity of Pt NPs was almost negligible. Pt Cs can support a high efficiency for the generation of hot electrons, thereby facilitating the improvement of enzyme-like catalytic activity.

We also carried out the reverse verification experiments under an ice bath condition (Figures 2(f)–2(h)) to exclude the contribution of the heat converted from the NIR light. Compared with the ice bath groups in the dark, the ice bath groups irradiated by the 808 nm laser successfully increased the CAT mimic activity of Pt Cs by 73.3%, increased the OXD mimic activity by 38.8%, and increased the self-cascade catalytic activity by 30.3%. The enhancing trend of enzyme-like activity between different controls is consistent with the results above, which further verifies the contribution of hot electrons to enzyme-like activity. Visible light was also used as a light source to detect the enzyme-like activity of Pt Cs with a negligible photothermal effect (Figures S16(a)–S16(c)). The activity enhancement of Pt Cs under visible light further verifies the positive effect of hot electrons. More details associated with these experiments are presented in Figure S16.

To further confirm the conversion from NIR photons to hot electrons, Pt Cs, R-Pt Cs, Pt NPs, and Mn₃O₄ NPs were tested in view of the photocurrent in Figure 2(i). The photocurrent was induced by hot electrons immediately upon illumination and reached its maximum magnitude within several milliseconds. It usually takes longer (on the order of seconds) to reach the thermal diffusion equilibrium for the heat-induced temperature increase of the surroundings⁷⁴^–⁷⁵. Accordingly, a photoelectrochemical method with a millisecond time resolution would be able to disentangle the charge carrier effect from the photothermal effect. Figure S17 shows the I_photo-t curve of a Pt Cs electrode in a single current response process. The current reached the peak immediately upon 808 nm laser irradiation and then returned to the initial state immediately when the laser was turned off. The response and recovery times of the photocurrent were both at the millisecond level, which is consistent with the theoretical response time of the photoelectric effect caused by hot electrons. Figure 2(i) compares the photocurrent signals of Pt Cs with other controls. The photocurrent density of Pt Cs was 1.8 µA·cm^-1 under 808 nm laser irradiation, almost 3.6 times that of R-Pt Cs (0.5 µA·cm^-1). The photocurrent signals of Pt NPs and Mn₃O₄ NPs were so weak that they could be ignored. This further indicates that Pt Cs can generate a large number of hot electrons under NIR photon-excitation.

Furthermore, enhancement of the electric field (|E|/|E₀|) induced by NIR photon-excitation was calculated using the finite difference time-domain (FDTD) method, where |E| and |E₀| denote the photo-enhanced and initial incident electric fields, respectively. Hot electron injection will lead to significant enhancement of the electric field, so the stronger the electric field in Pt Cs, the greater the number of hot electrons generated^{51, 76}. Figures 2(j) and 2(k) show the illustration of the constructed model of Pt Cs and the calculated spatial distribution of the photo-induced electric field enhancement. The highest local electric field enhancement (|E_max|/|E₀|) of Pt Cs was 6.88, which was far higher than 0.914 of R-Pt Cs and 0.596 of pure Pt NPs (shown in Figure S18). The location of highest |E|/|E₀| was concentrated between the closed-packed Pt NPs on the surface of the Pt Cs, which was caused by the near-field coupling effect between adjacent ultrasmall Pt NPs^63–65. Therefore, NIR photon-excited hot electrons play an important role in promoting enzyme-like catalytic activity.

Figure 3 displays the investigated enzyme-like catalytic kinetics for Pt Cs with and without NIR photon-excitation. First, the CAT mimic activity of Pt Cs was modulated by a laser with power density in the range of 0–3 W·cm^-2. Figure S19 proves that an 808 nm laser and limited heat cannot lead to significant self-decomposition of H₂O₂ in systems. As shown in Figure 3(a), as the laser power density increased, so did the amount of O₂ produced by the catalytic decomposition of a specific amount of H₂O₂ in solution (100 µM) in 10 min. Compared with the system without laser irradiation, the concentration of generated O₂ under laser irradiation (2 W·cm^-2) increased to 203.0%, and even to 215.8% when the laser power density rose to 3 W·cm^-2. Figure 3(b) monitors the change in H₂O₂ concentration according to its characteristic absorption peak at 240 nm⁷⁷. The concentration of H₂O₂ decreased significantly with the increase in laser power density. Under 3 W·cm^-2 laser irradiation, the absorption peak of H₂O₂ almost disappeared within 10 min, indicating that H₂O₂ was completely decomposed. Comparatively, the concentration of H₂O₂ in the system without Pt Cs had no significant decreases under laser irradiation with different power densities. The impact from NIR light irradiation to the OXD substrate o-phenylenediamine (OPD) in solution can be neglected (Figure S20). Figure 3(e) shows that, in the range of 0–2 W·cm^-2, the characteristic absorbance of the oxidized OPD gradually increased with the increase in laser power density in 10 min. However, the absorption intensity of the oxidized OPD dropped when the 808 nm laser power density exceeded 2 W·cm^-2. This means that the OXD mimic activity of Pt Cs increased first and then decreased with the increase in laser power density, which may be induced by temperature-dependence of nanozymes. Additionally, the electron spin resonance spectroscopy (ESR) spectra in Figure 3(f) compare the O₂^•⁻ signals under different conditions⁷⁷. The peak intensity of O₂^•− under NIR light irradiation was about 4.1 times higher than in dark conditions, indicating that NIR photon-excitation can indeed promote the generation of O₂^•−. The steady-state catalytic kinetics for Pt Cs were analyzed by fitting the Michaelis–Menten curves and Lineweaver–Burk plots (Figures 3(c)–3(d), Figures 3(g)–3(h)). The maximum velocity (V_max) and Michaelis–Menten constant (K_m) for CAT mimic behaviors of Pt Cs under NIR light irradiation were calculated as 7.14 × 10^-4 M·s^-1 and 0.35 mM, respectively⁷⁸. For the control group without NIR light irradiation, the corresponding V_max and K_mvalues were 5.46 × 10^-4 M·s^-1 and 0.62 mM. In comparation, V_max increased by 30.8%, and K_m decreased by 43.5% under NIR light irradiation. Similarly, the V_max and K_m for OXD mimic behaviors before and after NIR light irradiation were 16.74 × 10^-8 M·s^-1, 0.056 mM and 21.77 × 10^-8 M·s^-1, 0.054 mM, respectively. The lower K_m indicates a stronger affinity to the nanozyme substrates, and the larger V_max indicates a faster speed in catalytic reactions. Thus, NIR light irradiation can obviously improve the affinity of H₂O₂ for Pt Cs and the chemical transformation of O₂ on the surface of Pt Cs.

The self-cascade catalytic activity of Pt Cs in simulated TME was tested. As shown in Figure 3(i), the production of ROS increased with the increase in laser power density and reached its peak at 2 W·cm^-2. Figure S21 compares the generated ROS levels in the OXD mimic catalysis test and the self-cascade catalysis test. The self-cascade catalytic activity of Pt Cs significantly promoted the generation of ROS in the reaction system. The ESR data in Figure 3(j) confirmed that the addition of H₂O₂ alone could not change the concentration of O₂^•− in solution, and neither the control group of Mn₃O₄ NPs nor Pt NPs could significantly promote the transformation from H₂O₂ to O₂^•− in solution. When Pt Cs emerged in the tested solution, the signal of O₂^•− was apparently enhanced in the ESR spectrum. NIR light irradiation further improved the peak intensity for O₂^•− by 2.9 times. Therefore, NIR light irradiation was confirmed to greatly enhance the self-cascade catalytic ability of Pt Cs to generate sufficient ROS to destroy tumor cells (Figure 3(k)).

Before the cell experiment, we used polyethylene glycol (PEG) to biomodify Pt Cs and verified the stability of PEG/Pt Cs in phosphate-buffered saline (PBS) buffer (Figure S22). Compared with normal cells, tumor cells produce excessive amounts of H₂O₂ (up to 0.5 nmol per 10⁴ cells per h)⁷⁹. H₂O₂ in TME can be converted into O₂ by CAT mimic and then catalyzed by OXD mimic to produce numerous highly toxic ROS, which can effectively kill tumor cells. In this experiment, DCFH-DA was used react with ROS to generate 2,7-dichlorofluorescein (DCF) with green fluorescence, which enabled the detection of ROS in cells. As shown in Figure 4(a), negligible green fluorescence was observed in the control, 808 nm laser, and Mn₃O₄ groups. The Pt group showed fluorescence, and the fluorescence of 4T1 cells treated with PEG/Pt Cs was more obvious. Further, compared with other experimental groups, the PEG/Pt Cs + laser group showed the most obvious green fluorescence, indicating that the cells co-incubated with PEG/Pt Cs can produce a large amount of ROS under NIR light irradiation. The flow cytometry in Figure 4(b) quantifies the ROS of the above experimental groups. The ROS production rate of the PEG/Pt Cs + laser group (84.8%) outclassed that of the PEG/Pt Cs group (40.2%), Pt group (26.8%), and other control groups.

The effects of PEG/Pt Cs on the cell viabilities of normal cells and tumor cells were compared. As shown in Figure 4(c), the increase in PEG/Pt Cs concentration can decrease the tumor cell viabilities. Regarding the PEG/Pt Cs + laser group, its half maximal inhibitory concentration (IC₅₀) was 83 µg·mL^-1, which was far lower than that of the experimental group without NIR light irradiation (180 µg·mL^-1). This further indicates that NIR light promotes the catalytic activity of Pt Cs and produces more ROS, leading to increased cytotoxicity. However, for normal cells, due to the low content of H₂O₂, the PEG/Pt Cs + laser group with different concentrations showed good safety and low toxicity in normal cells (293T and hl7702) (Figure 4(d)). Hence, the PEG/Pt Cs + laser group had good biosafety and low toxicity. To observe the living and dead cells, cells were co-stained with Calcian-AM and PI after treatment with different experimental groups. As shown in Figure 4(e), the PEG/Pt Cs + laser group exhibited more dead cells than the PEG/Pt group, pointing to the better antitumor activity of the PEG/Pt Cs + laser group.

As displayed in Figure 5(a), the therapeutic evaluations of PEG/Pt Cs were examined on 4T1 tumor inoculated mice. To design a more reasonable treatment scheme in vivo, we tracked the enrichment of Pt Cs at the tumor site through different imaging methods. Our previous study demonstrated that a payload of PtO_x on porous Mn₃O₄ shows enhanced longitudinal (T₁) contrast ability for magnetic resonance imaging (MRI)⁸⁰. Figure S23(a) shows that the longitudinal relaxivity (r₁) of PEG/Pt Cs is 3.5 mM^-1·s^-1, and the transverse relaxivity (r₂) is 7.4 mM^-1·s^-1, resulting in the r₂/r₁ value of 2.11, which is suitable for T₁ MRI⁸¹. Figure 5(b) shows T₁ MRI of the tumor site at different times after intravenous injection of PEG/Pt Cs in mice. There was no obvious signal at the tumor site before injection, whereas the tumor began to show enhanced contrast within 0.5 h after PEG/Pt Cs injection. T₁ contrast continued to increase until it reached the highest point at 6 h after injection. This may have been due to the passive accumulation of PEG/Pt Cs at the tumor site resulting from the enhanced permeability and retention effect. A quantitative analysis of the signal-to-noise ratio (SNR) in the tumor site was performed to evaluate the potential of PEG/Pt Cs as a contrast agent. Figure S23(b) shows that the SNR value increased from 52.7 (before injection) to 65.8 (6 h post injection) and then gradually decreased to 46.9 (24 h post injection). In addition, according to the MRI tracking data above, we used infrared thermography to observe the temperature change at the tumor site in mice after 6 h post injection. Figure 5(c) shows that under 808 nm laser irradiation, the temperature of the tumor site of mice injected with PEG/Pt Cs gradually increased with the increase in irradiation time, reaching 48.0℃ after 10 min of laser irradiation. In contrast, the tumor site of mice injected with PBS buffer was only 38.3℃ after 10 min of laser irradiation (Figure 5(d)). Moreover, infrared imaging of the dissected mice organs showed that the tumor signal was significantly higher than that of other organs (Figure S24). Our findings proved that PEG/Pt Cs accumulated at the tumor site 6 h after injection.

Due to differences in pH value and H₂O₂ content between tumor tissues and normal organs, cancer cells are more vulnerable to the impact of exogenous ROS compared with normal cells⁸². PEG/Pt Cs can generate a large number of ROS through efficient self-cascade catalysis under NIR photon-excitation, which has the potential to specifically inhibit tumor growth. As shown in Figure 5(e), during the 21-day treatment period, the average body weight of each group did not change significantly, indicating that the adverse reactions caused by PEG/Pt Cs injection during the anti-cancer treatment can be ignored. Compared with the other five groups, the PEG/Pt Cs + laser group had the best tumor treatment effect and significantly inhibited the tumor growth of mice (Figure 5(f)). The average tumor volume of the PEG/Pt Cs + laser group was 70.42 mm³ at 21 days after the first dose, much smaller than that of the PBS group (1200.95 mm³), PBS + laser group (1204.78 mm³), Mn₃O₄group (921.80 mm³), Pt group (437.79 mm³), and PEG/Pt Cs group (204.83 mm³) (Figure 5(g)). Furthermore, Figure 5(h) indicates that PEG/Pt Cs had outstanding therapeutic outcomes on tumors under NIR light. Hematoxylin and eosin (H&E), Ki67, and TUNEL staining assays of tumor tissues further proved that the tumor cells underwent severe apoptosis after 21 days of treatment (Figure 5(i)). Notably, the blood biochemical and haematological analysis showed that there were no comparable differences between the PEG/Pt Cs-treated group and the control group (Figure S25), demonstrating no significant infection and inflammation were arose during the whole evaluation period. Moreover, H&E analysis was performed on the main organs (Figure S26). As with the control group, the mice treated with the PEG/Pt Cs exhibited no obvious damage to the heart, liver, spleen, lung, or kidney, indicating that PEG/Pt Cs had no obvious side effects on the physiological function or health status of mice during the treatment. The above data show that the process of antitumor inhibition by PEG/Pt Cs had low toxicity to organisms and good biosafety.

In summary, Pt Cs with wide absorption from the visible band to the NIR band were successfully fabricated by galvanic replacement between PtCl₄²⁻ ions and laser-treated Mn₃O₄ NPs with enriched GBs. The GBs in Mn₃O₄ NPs can not only promote the even distribution of Mn²⁺ ions as the galvanic replacement sites, leading to the uniform deposition of ultrasmall Pt NPs, but also increase the rate of the galvanic replacement reaction due to the enhanced compressive strain. The immediate rising of photocurrent in Pt Cs under NIR light irradiation directly confirmed the generation of hot electrons. According to the results from enzyme-like activity analysis, photocurrent test, DFT calculation, and FDTD simulation, we established that NIR photon-excitation can greatly promote the dual-enzyme activity and self-cascade catalytic activity of Pt Cs. Additionally, in vivo experiments in mice further verified the superior antitumor ability of Pt Cs with the assistance of MRI guidance. These results highlight the achievements of constructing Pt Cs for generating hot electrons under NIR photon-excitation for photo-responsive enzyme-like activities.

Acknowledgement

This work was supported the Hefei National Laboratory for Physical Sciences at the Microscale (No. KF2020110), the Key Research and Development Plan of Anhui Province (No. 201904a05020049), Natural Science Foundation of Anhui Province (No. 2208085MH256), National Natural Science Foundation of China (No. 52071313), Hefei Institutes of Physical Science Director’s Fund, Chinese Academy of Sciences (YZJJKX202202).

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Grain Boundaries Assisted Construction of Pt Colloidosomes for Tumor Catalytic Therapy Enhanced by Near-Infrared Photon-Excited Hot Electrons

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