br of approximately nm and nm respectively indicating that f
of approximately 77 nm and 12 nm respectively, indicating that few-layer BP nanosheets were obtained . As evidenced by the high re-solution-TEM (HR-TEM) image (Fig. 1D) and selected area electron diﬀraction (SAED) pattern (inset of Fig. 1D), the BP structure was re-tained after exfoliation [18,27]. Besides, scanning transmission electron microscopy (STEM) showed excellent co-localization of abounding Pt with P element (Fig. 1E), visually confirming a strong coordination between DACHPt and BP. As shown in Fig. S3, the BP/DACHPt could be indexed into orthorhombic BP consistent with JCPDS No. 73-1358 .
In addition, the structural changes of bare BP and BP/DACHPt were characterized by Raman spectral analysis through determining detailed rotational and vibrational modes (Fig. 1F). The spectrum of BP/DACHPt exhibits three main Raman peaks, corresponding to A1g (out-of-plane phonon mode) at 359.0 cm−1, B2g and A2g (two in-plane modes) at 432.5 and 460.8 cm−1, respectively . Diﬀerent from bare BP, the A1g, B2g, and A2g peaks of BP/DACHPt are slightly red-shifted by about 2.8, 4.3, and 4.3 cm−1, respectively, because the coordination between DACHPt and BP inhibited the vibration of surface P atoms to reduce the corre-sponding Raman scattering energy.
To further evaluate the interaction between BP and DACHPt in BP/ DACHPt, high resolution-X-ray photoelectron spectroscopy (HR-XPS) was performed, using both bare BP and DACHPt as controls. Assuming Pt-P coordination was most probable, we focused on P and Pt core-level
Fig. 3. Photothermal and chemo therapy in vitro with BP/DACHPt. A) Fluorescence images of the calcein AM (green, live cells) and PI (red, dead cells) co-stained HeLa DETA NONOate after exposed to NIR irradiation, following incubation with bare BP and BP/DACHPt (scale bar = 100 μm). B) Cell viability of cells incubated with bare BP and BP/ DACHPt in the absence or presence of NIR irradiation. C) Cell viability of cells in-cubated with DACHPtCl2 and BP/DACHPt for 48 h. All the Bare BP and BP/DACHPt were predispersed in air-exposed water for 72 h.
regions. As shown in Fig. 1G, the 2p peak of bare BP is located at 129.7 eV, as the characteristic peak of BP crystal . Moreover, there are a small number of oxidized P species (PxOy) sub-bands at 134.7 eV, which can be attributed to the oxidation of bare BP surface . In contrast, the P 2p peak of BP/DACHPt is evidently attenuated, whereas the peak at 133.8 eV is enhanced. Since bare BP was only slightly oxi-dized, most P atoms on the surface of BP/DACHPt had coordinated with DACHPt, thereby forming P5+ species. In the meantime, the Pt 4f sig-nals of the three samples were measured. Bare BP has no Pt signal, but BP/DACHPt exhibits a distinct pair of Pt peaks (4f7/2 and 4f5/2) at 73.0 and 76.4 eV . The doublets of BP/DACHPt are shifted toward lower binding energies compared to those of DACHPt (73.4 and 76.8 eV), due to the electron donation from P to Pt(II) metal center. Furthermore, compared to the doublets of DACHPtCl2 without removal of Cl atoms (72.4 and 75.8 eV, Fig. S4), those of DACHPt move much higher, con-firming its electron deficiency. Whereas the doublets of BP/DACHPtCl2 (72.5 and 75.8 eV) move slightly higher compared to that of DACHPtCl2, which is consistent with previous literature  and at-tributed to the weak electron donation from the Pt(II) metal center in DACHPtCl2 to P. This opposite electron donation to BP/DACHPt fore-shadows only DACHPt but not DACHPtCl2 could coordinate and sta-bilize the lone pair electrons of P. All the results above confirmed the successful coordination between atom P and Pt in BP/DACHPt, also implying that DACHPt would inhibit the oxidation of BP.
3.2. Stability evaluation under ambient conditions
To study whether coordination with DACHPt improved the stability of BP, BP/DACHPt was dispersed in air-exposed water at 50 μg/mL for diﬀerent time periods, and the absorption was monitored at each time point. At the beginning, both the spectra of bare BP and BP/DACHPt exhibit broad absorption from UV to NIR regions . With extended time, the absorbance of bare BP in air-exposed water plummets (Fig. 2A). After 24 and 168 h of dispersion, the absorbance becomes only 53% and 42% respectively of the original one (Fig. 2C), suggesting that bare BP indeed rapidly and continuously degraded. In contrast, the absorbance of BP/DACHPt keeps stable (Fig. 2B), with 90% retained even after 168 h (Fig. 2C). Therefore, BP/DACHPt was much more stable than bare BP in air-exposed water.
Considering that BP must be exposed to physiological conditions for a long time before tumor photothermal therapy, its photothermal sta-bility was of great significance . The photothermal performances of BP/DACHPt (50 μg/mL) in air-exposed water at diﬀerent time points were also tested to confirm its stability. The bare BP and BP/DACHPt solutions were continuously irradiated by 808 nm NIR laser (power density: 1.0 W/cm2), and their temperature changes with prolonged time were monitored using IR thermal camera. When bare BP began to be dispersed, the temperature rose by 28.1 °C within 10 min, but by only 17.6 °C after 12 h (Fig. 2D). After 168 h, irradiation barely elevated the temperature, so the photothermal performance was quickly atte-nuated along with BP degradation. Contrarily, BP/DACHPt was evi-dently more photothermally stable, allowing the temperature to in-crease by 22.5 °C even after 168 h (Fig. 2E). In combination with the UV–Vis spectroscopic results, coordination with DACHPt considerably stabilized BP. Also, we studied the photothermal stabilities of BP/ DACHPtCl2, BP/CDDP and BP/Pt(NH3)2 which were prepared by mixing BP with DACHPtCl2, cisplatin (CDDP) and CDDP after removal of chlorine. As expected, BP/DACHPtCl2 and BP/CDDP had similar low photothermal stabilities to that of bare BP (Figs. 2F and S5). Having Cl atoms, DACHPtCl2 or CDDP cannot coordinate with BP to suppress its oxidation. After removal of Cl atoms, the active species of CDDP be-came able to coordinate with BP similarly to DACHPt did. Collectively, BP could be stabilized through coordination with the active species of platinum-based drugs.