Upconversion-bismuth Nanosystem as Theranostic Agent for NIR Laser-driven Chemo-photothermal Therapy

TU Gang，LING Dan-ping，LIU Jie，WANG Feng，WANG Hai-fang，SHI Li-yi*，SUN Li-ning*

（1. Research Center of Nano Science and Technology，College of Sciences，Shanghai University，Shanghai 200444，China；2. Department of Chemistry，College of Sciences，Shanghai University，Shanghai 200444，China；3. Institute of Nanochemistry and Nanobiology，Shanghai University，Shanghai 200444，China）* Corresponding Authors，E-mail：shiliyi@shu.edu.cn；lnsun@shu.edu.cn

Abstract：A hybrid nanosystem of upconversion-bismuth integration（denoted as UBDAs）is designed and synthesized for near infrared（NIR）light-driven chemo-photothermal therapy. The obtained UBDAs present excellent photothermal conversion capacity（～28.5%）and good biocompatibility. Meanwhile，under excitation of NIR，UBDAs can emit ultraviolet/visible light，which promotes the continuous rotation-flip movement of the photosensitizer azobenzene in the mesoporous，thereby achieving the controlled drug release and avoiding the side effects of traditional ultraviolet light excitation on biological tissues. Photothermal experiments show that UBDAs hybrid nanosystems have a good photothermal effect under 980 nm laser irradiation. In addition，based on Tm3+ and Bi element the UBDAs are expected to be used in upconversion luminescence and X-ray computed tomography（CT）imaging to achieve dual-mode imaging-mediated and single NIR-driven chemotherapy and photothermal therapy. Therefore，this work provides a new idea for the integration of diagnosis and synergistically enhanced antitumor therapy.

Key words：upconversion luminescence；imaging；photothermal therapy；drug release；hybrid nanosystems

1 Introduction

Because of its high efficiency, low invasion, and remote controllability, photothermal therapy is regarded as one of the most promising therapeutic strategies for antitumor therapy. However, long-term chemotherapy has caused serious toxic and side effects on biological specimens and living tissues,which has brought great difficulties to treatment. It is well known that synergistic treatment has achieved remarkable results in the treatment of various diseases. Therefore, it is necessary to combine other treatments with chemotherapy to achieve synergistic effect in order to overcome the shortcomings of chemotherapy[1].

Photothermal therapy（PTT）causes irreversible damage to tumor tissue and produces good therapeutic effects owing to its advantages, such as non-invasive, high selectivity, and deep penetration depth in biological therapy[2]. In photothermal therapy, nearinfrared（NIR）laser energy can be absorbed by the photothermal agent and converted into heat, causing the death of tumor cells[3]. Up to now, noble metal nanoparticles（Au, Pd, and Pt,etc.）[4-6], metal chalcogenide compounds, transition metal disulfides[7-9]and carbon nanomaterials,etc. have been demonstrated as promising photothermal agents for cancer treatment[10]. However, most of them have the disadvantages of high price, toxicity, and complicated preparation process. Therefore, developing photothermal reagents with low-cost, non-toxic, and environmentally friendly synthetic routes has become one of the hot research spots[11].

Bismuth（Bi）is a heavy metal element with a high atomic number（Z=83）and has a good X-ray attenuation coefficient（5.74 cm2·g-1, 100 keV）[12].More importantly, compared with other noblemetals（Au, Pd, and Pt,etc.）, Bi has the advantages of nontoxicity and low cost, and Bi is an inexpensive“green”metal as well[13]. Moreover, Bi shows a good photothermal conversion ability with a strong NIR absorption capacity. As well known, the long-wavelength NIR light displays better tissue-penetrating capability due to its high maximum permissible exposure and fine spatio-temporal resolution, also provides many possibilities for tumor therapy with improved effectiveness, especially in PTT. Therefore,the nanomaterials based on Bi that triggered by NIR light are expected to be potential photothermal therapy agents[14-15].

Among the reported candidate materials for optical imaging, rare-earth doped upconversion nanoparticles（UCNPs）have been widely used as imaging agents in tumor diagnosis due to their unique optical properties[16-25]. UCNPs can convert NIR light into ultraviolet light, visible light, or NIR light through anti-Stokes displacement. In addition, the hollow mesoporous-type nanocarriers can be used as light-responsive drug carriers, which have many advantages, such as low drug loading rate and good biocompatibility. The photomechanical azobenzene can create a continuous rotation-inversion movement under the upconverted UV/visible light that emitted by UCNPs, thereby achieving a photo-responsive controlled drug release to biological tissues[1,17].

Recent progress on the role of upconversion nanoparticles in cellular therapy suggests that its viability not only as a stimulator andin vivoimaging probe, but more importantly, as a real-time monitor of cellular treatments[16-17]. Here, we develop a dualmode（upconversion luminescence and CT）imaging-mediated and single NIR laser-driven theranostic agent for chemo-photothermal therapy. The final UBDAs exhibit good biocompatibility, and the single NIR laser responsive photothermal and drug-releasing properties were studied. Moreover, the live/dead cell staining assay was performed, demonstrating the effective tumor ablation through synergistic chemo-photothermal therapy.

2 Experiment

2.1 Materials

Rare-earth chloridesRECl3·6H2O（99.99%）（RE=Y, Yb, Tm）, 1-octadecane（90%）, and oleic acid（OA, 90%）, polyvinyl pyrrolidone（PVP）（99%）,hexadecyl trimethyl ammonium bromide（CTAB,99%）, tetraethyl orthosilicate（TEOS）, 3-amino-propyl trimethoxysilane（APTES, 98%）, and 4-phenylazobenzoyl chloride（AZO, 99.9%）were bought from Sigma-Aldrich Co.,Ltd. Methanol（CH3OH,99.5%）,Sodium borohydride（NaBH4, 98%）, Bismuth nitrate（Bi（NO3）3·5H2O, 99%）, dimethyl sulfoxide（C2H6OS,99.8%）, ammonium fluoride（NH4F, 98%）, and sodium hydroxide（NaOH, 96%）were obtained from Aladdin Company. Doxorubicin hydrochloride（98%）was obtained from Jingchun Biotech Co., Ltd. Cyclohexane（C6H12, 99.7%）, ethanol（C2H6O, 99.7%）were purchased from Sinopharm Co., Ltd. Ultrapure deionized water（Millipore system）was used for all experiments.

2.2 Synthesis of Bi-PVP Ultra-small Nanoparticles

Bi-PVP was synthesized according to the previous report with minor modification[13]. Bismuth nitrate（0.1 g）and PVP（0.3 g）were added to a solution containing 10 mL of glycerol and 5 mL of ethanol, heated to 60 ℃, and stirred for 1 h. Sodium borohydride（0.05 g）was quickly added and stirred for 1 min. Bi-PVP ultra-small nanoparticles were collected by centrifugation, washed two times with water and ethanol at 4 ℃, then dispersed in ultrapure water（10 mL）.

2.3 Synthesis of Mesoporous Silica-coated Upconversion Luminescent Nanomaterials（Denoted as UB@mSiO2）

The NaYF4∶20%Yb, 0.5%Tm（UCNPs）were obtained according to our previous method[25]. 2 mL of UCNPs, 0.1 g of CTAB and 20 mL of deionized water as a surfactant were injected into a small beaker, heated to 60 ℃and stirred for 1 h, then cooled to room temperature with stirring for 12 h. 40 mL of water, 6 mL of ethanol, Bi-PVP ultra-small nanoparticles,and 150 μL of 2 mol/L sodium hydroxide solution were injected into a 250 mL two-necked bottle connecting with a reflux condenser and continuously stirred at 70 ℃. 200 μL of TEOS was added to the beaker at a constant speed and stirred for 2 h under a reflux condenser[26-27]. The products were centrifugation, washed several times with ethanol, and then dispersed in 10 mL of ethanol. Subsequently, 1.2 g of ammonium nitrate in 190 mL of ethanol was dropped slowly and kept stirring for 12 h at 60 ℃.UB@mSiO2was collected by centrifugation, washed several times with ethanol, then dispersed in ethanol（10 mL）.

2.4 Synthesis of Amino-modified UB@mSiO2 Nanomaterial（sDenotedasUB@mSiO2-NH）2

200 μL of APTES and 15 mL of UB@mSiO2were injected into a 50 mL flask and stirred for 48 h at 25 ℃. Amino-modified nanomaterials UB@mSiO2-NH2were collected by centrifugation, washed several times with ethanol, and then dispersed in ultrapure water（10 mL）.

2.5 Synthesis of Hybrid Nanomaterials Loading with DOX（Named as UBDs）

5 mL of DOX（1 mg·mL-1doxorubicin hydrochloride aqueous solution）and 10 mL of UB@mSiO2-NH2were injected into a 50 mL flask and stirred for 48 h. The hybrid nanomaterials loaded with DOX were collected by centrifugation, washed several times with ethanol, denoted as UBDs, and then dispersed in dimethyl sulfoxide（DMSO,10 mL）.

2.6 Synthesis of UBDAs Hybrid Nanosystems Encapsulated with AZO（Named as UBDAs）

0.1 g of AZO and 10 mL of UBDs were injected into a 50 mL flask and stirred for 12 h, and the obtained samples were washed with DMSO and absolute ethanol, and then dispersed in 10 mL of ultrapure water,denoted as UBDAs hybrid nanosystems.

2.7 Synthesis of UDAs Hybrid Nanomaterials（Named as UDAs）

UDAs hybrid nanomaterials were synthesized according to the preparation of UBDAs, except that the Bi-PVP was not introduced in synthesis of UB@mSiO2.

2.8 Characterization

A JEM-2100F low-to-high resolution transmission electron microscope（TEM）was used to characterize different morphology at 120 kV. Fourier transform infrared（FTIR）spectra were measured with an Avatar 370 instrument in the spectral range from 4 000 cm-1to 500 cm-1. The upconversion luminescence spectra were acquired using a 980 nm laser with an Edinburgh FS5 fluorescence spectrometer with a 980 nm laser. UV-visible absorption spectra were carried out on a Shimadzu UV-2500PC ultraviolet-visible spectrometer. The zeta potentials were recorded by PCS analysis software on a Nano-ZS（Malvern Instruments Corporation）.

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2.9 Photothermal Performance

The photothermal effect of the UBDAs was preliminarily evaluated by exposing the corresponding aqueous solutions with various concentrations to a 980 nm laser irradiation（1.5 W·cm-2, 10 min）[28-29].The temperature was recorded using a thermocouple microprobe. To evaluate the photothermal stability,the temperature was measured every 10 s during the five cycles of 10 min laser irradiation and 10 min natural cooling for the aqueous solution of the nanoparticles（UBDAs: 400 μg·mL-1）. The photothermal conversion efficiency（η）was evaluated by recording the temperature variation in a cycle of alternating heating and cooling process[13,30].

2.10 Cell Experiments

HeLa（Human epithelial cervical cancer cell line）was obtained from the Cell Bank of Type Culture Collection of Chinese Academy of Sciences（Shanghai, China）. The cells were cultured in high glucose DMEM（4.5 g·L-1glucose）supplemented with 10%（v/v）fetal bovine serum（FBS, Sigma-Aldrich, USA） and 1% penicillin-streptomycin at 37 ℃and 5% CO2in a humidified incubator. HeLa cells were seeded into plates（1×104cells per well in 96-well plates for the live/dead staining, or, 15×104cells per well in 12-well plates for the cell survival）and incubated for 24 h. Subsequently, the cells were exposed to UBDAs for the following assays.

Typan blue staining was used to evaluate the cell survival of the cells treated with the UBDAs.HeLa cells were exposed to the culture medium（10% FBS）containing the UBDAs at different concentrations, and the cells cultured in culture medium without UBDAs as the control[31-32]. After 24 h,the medium was removed, and the typan blue solution（0.4 mg·mL-1）was added and cultured for 3 min, where dead cells were stained blue. The cell survival（%）is expressed as the percentage of the surviving cell number of treated groups in that of the control[33-34].

The live/dead staining of cells was conducted by using the kit（L-3224, Invitrogen, USA）following the instruction[35]. The dyes in the kit, Calcein AM and propidium iodide（PI）, can differentiate live cells（green,λex= 495 nm/λem= 515 nm）from dead cells（red,λex=535 nm/λem=635 nm）. The cells were cultured in the medium（10% fetal bovine serum）containing 400 μg·mL-1UBDAs for 4 h. The photothermal experimental group was irradiated with 980 nm laser（1.5 W·cm-2）for 10 min. The control group was not treated with laser. After that, the cells were washed with cold D-Hank’s solution. The cells were stained with the dyes for 30 min and then observed using a fluorescence microscope.

3 Results and Discussion

3.1 Synthesis and Characterization of UBDAs Hybrid Nanosystem

The design of the UBDAs hybrid nanosystem for synergistically enhanced chemo-photothermal therapy was illustrated in Fig. 1. First, rare earth doped up-conversion luminescence nanoparticles NaYF4∶Yb,Tm（UCNPs）and metal bismuth nanoparticles as the core are simultaneously coated in a mesoporous silica shell layer to form a mesoporous coated core-shell hybrid nanosystem UB@mSiO2with amino functionalization on the surface, and then loading DOX into mesoporous silica pores. Finally, the drug was encapsulated with AZO compound to obtain the final upconversion-bismuth hybrid nanosystems UBDAs. As shown in Fig. 2, the morphology and structure of the initial UCNPs to the final UBDAs nanosystem were characterized by transmission electron microscopy（TEM）. As displayed in Fig.2（a）and 2（b）, the UCNPs show good monodispersity with an average diameter of around 38 nm and Bi-PVP shows a very small size（below 10 nm）, respectively.

Fig.1 Schematic illustration of designing UBDAs hybrid nanosystem for potential application in synergistically enhanced chemophotothermal therapy of tumor

Fig.2 Transmission electron microscope（TEM）images of NaYF4∶Yb，Tm（a），Bi-PVP（b），UB@mSiO2（c），and UBDAs（d）.（e）Corresponding high-resolution TEM（HR-TEM）image of UB@mSiO2.（f）-（g）HAADF-STEM-EDS mapping image of UBDAs.

Fig. 2（c）displays that the SiO2mesoporous shell successfully coated on the surface of UCNPs and Bi-PVP, leading to the formation of UB@mSiO2with the average size of approximately 55 nm. And from the high-resolution transmission electron microscopy（HR-TEM）image of UB@mSiO2in Fig. 2（e）, it can be observed that the lattice fringe spacing of 0.295 nm is in agreement with the d-spacing of（110）lattice plane of hexagonal-system NaYF4（JCPDS 16-0334）, and 0.37 nm and 0.16 nm are attributed to（101）and（024）of Bi（JCPDS 85-1329）, respectively. It can be deduced that the UCNPs and Bi exist in the UB@mSiO2nanoparticles. From the TEM image（Fig. 2（d））and HR-TEM image of UBDAs（Fig. 2（f））, it can be observed that there is no obvious increase in particle size of UBDAs after loading with DOX and AZO. From Fig. 2（g）, the corresponding elements（Bi, Si, Tm, and N）in the hybrid nanosystem by elemental mapping image suggest that successful synthesis of the UBDAs assemblies by this facile method.

Fig.3（a）shows the zeta potentials of Bi-PVP,UCNPs@mSiO2, UB@mSiO2, UCNPs@mSiO2NH2,UBDs, and UBDAs nanosystem, respectively. In comparison with the zeta potentials of Bi-PVP and UCNPs@mSiO2being +38.2 mV and -20.6 mV, respectively, the UB@mSiO2displays -12.4 mV, indicating that the Bi-PVP was successfully encapsulated in mesoporous silica. After APTES modification, the zeta potential of UB@mSiO2-NH2is +45.1 mV,which shows that the —NH2group was successfully attached to the UB@mSiO2. The zeta potential of UBDs increases to +47.2 mV, suggesting the successful loading of DOX in UBDs[36]. Since AZO is negatively charged, leading to the zeta potential of UBDAs to be reduced to +23.2 mV, it suggests the successful installing of AZO on UBDs and the formation of UBDAs nanosystem.

Fig.3 （a）The zeta potentials of Bi-PVP，UCNPs@mSiO2，UB@mSiO2，UB@mSiO2-NH2，UBDs，and UBDAs dispersed in water（200 μg·mL-1）.（b）FTIR spectra of UCNPs，Bi-PVP，UB@mSiO2，UBDs，and UBDAs.（c）UV-visible absorption spectra of UB@mSiO2，UBDs，and UBDAs.（d）Upconversion luminescence spectra of UCNPs，UB@mSiO2，UBDs，and UBDAs dispersed in water（200 μg·mL-1）upon excitation of 980 nm.

Fig. 3（d）shows the upconversion emission spectra of UCNPs, UB@mSiO2, UBDs, and UBDAs under 980 nm excitation, respectively. The emission peaks located at 291 nm（1I6→3H6）, 345 nm（1I6→3F4）, 362 nm（1D2→3H6）, 450 nm（1D2→3F4）, 479 nm（1G4→3H6）, and 648 nm（1G4→3F4）belong to the characteristic emission of Tm3+ion. A significant decrease in the upconversion luminescence intensity can be observed in UB@mSiO2relative to that of UCNPs. It is worth noting that the red emission intensity at 648 nm of UBDs remains almost unchanged, while the emission intensities at 450 nm and 479 nm are reduced in comparison with those of UB@mSiO2. Because of overlap between the green emission of upconversion luminescence and the absorption spectrum of DOX（absorption band with maximum at around 480 nm）, the Fo¨rster resonance energy transfer（FRET）occurs between UB@mSiO2and DOX,resulting in green emission of UBDs which was partially diminished after loading of DOX. In addition, the emission peaks at 345 nm and 362 nm of UBDAs,compared with those of UBDs, decrease significantly after AZO is encapsulated. This is because the absorption spectrum of AZO overlaps with the upconversion luminescence spectrum of UBDs in the ultraviolet region（see Fig. 3（c）and 3（d））, leading to FRET between UBDs and AZO. Therefore, the results demonstrate the successful coating of AZO.

3.2 Photothermal Properties

The UBDAs hybrid nanosystem exhibits a broad UV-Vis-NIR absorbance, as shown in Fig. S3,which encourages us to study the potential photothermal property under NIR laser irradiation. The hybrid nanosystem was dispersed in water at different concentrations（50, 100, 200, 400 μg·mL-1）, and pure water was used as the control. As shown in Fig. 4（a）, under a 980 nm laser（1.5 W·cm-2, 10 min）irradiation the temperatures of UBDAs dispersion raise rapidly with the increase of concentration.The temperature of dispersion at concentration of 200 μg·mL-1and 400 μg·mL-1increase to 44 ℃and 52 ℃, respectively, after irradiation for 10 min.However, the temperature of pure water increases by only 3 ℃after 10 min irradiation. Besides the concentration-dependent photothermal effect, the UBDAs dispersion shows a power-dependent heating effect, and there is a large upward trend with the increase of power densities（0.50, 1.0, 1.5 W·cm-2）,as shown in Fig S4. The above results indicate that the UBDAs hybrid nanosystem can efficiently convert the NIR light into thermal energy[37]. In addition,the photothermal conversion efficiency（η）is determined to be 28.5% according to the data obtained from Fig. 4（c）and 4（d）by using the reported method[37-39]. The efficiency value is slightly higher in comparison with that of UCNPs@Bi@SiO2nanoparticles（28.4%）[40]. To test the photothermal stability,the temperature change of UBDAs dispersion was recorded as a function of time during the five on/off cycles of laser irradiation. As shown in Fig. 4（b）,the temperature increment of UBDAs almost maintains unchanged during the heating process after five cycles of irradiation, and the temperature of UBDAs after fifth irradiation is still around 97% of that of the first irradiation, indicating that the UBDAs has good photothermal stability. Furthermore,the infrared thermal images of different concentrations（50, 100, 200, 400 μg·mL-1）of UBDAs dispersion and pure water are displayed in Fig. 4（e）at the time points of 0, 2, 4, 6, 8, 10 min upon 980 nm laser irradiation（1.5 W·cm-2）. The UBDAs hybrid nanosystem exhibits notable time-dependent and concentration-dependent thermal effects. Since the death of cancer cells can be induced at temperature higher than 42 ℃, the UBDAs hybrid nanosystem is expected to be a potential candidate for PTT of tumor.

Fig.4 （a）Temperature curves of UBDAs suspensions at various concentrations（50，100，200，400 μg·mL-1）under 980 nm laser（1.5 W·cm-2）irradiation for 10 min.（b）Temperature curves of the UBDAs（400 μg·mL-1）over five cycles of laser on/off operation.（c）Photothermal circulation curves of the UBDAs suspensions.（d）Linear time data versus -lnθ obtained from the cooling period of panel Fig.4（c）.（e）Infrared thermal images of different concentrations（50，100，200，400 μg·mL-1）of UBDAs hybrid nanosystem and pure water at the time points of 0，2，4，6，8，10 min upon 980 nm laser irradiation（1.5 W·cm-2）.

3.3 Drug Release

To study the drug release of this nanosystem in response to NIR light, we first tested the absorbance spectra of DOX solution with different concentrations[41-42]. And the absorbance intensity（at 480 nm）as a function of DOX concentration was shown in Fig. 5（a）, from which the corresponding standard absorption curve can be simulated and established.Since the photomechanical AZO can create a transformation fromtrans-isomer intocis-isomer under UV light excitation[17]（Fig. S5）, DOX could be released from UBDAs under the upconverted UV light that emitted by UCNPs. Therefore, a photo-responsive controlled DOX release can be achieved from the continuous rotation-inversion movement of AZO depending on the 980 nm laser irradiation on the nanosystem.

Fig.5 （a）DOX standard curve by testing the absorbance（at 480 nm）of DOX solution with different concentrations（6.25，12.5，25.0，50.0，100 μg·mL-1）.（b）Stimuli-responsive DOX release of UBDAs，UDAs，and UBDs in PBS for 72 h after 980 nm laser（1.5 W·cm-2）irradiation for 10 min.

Subsequently, the DOX release behaviors of UBDAs, UDAs, and UBDs were further investigated under 980 nm laser irradiation（Fig. 5（b））. From the synthesis process, it is known that UDAs has no Bi being introduced and UBDs has no AZO for encapsulation. As displayed in Fig. 5（b）, it can be observed that the release rate of final UBDAs is faster than those of UDAs and UBDs, implying that the photothermal effect of Bi and thecis-transreversal of AZO structure could facilitate the DOX release, respectively. The release of DOX could be triggered by NIR lightviathetrans-isomer of AZO.The upconverted UV light emitted by the UCNPs creates a continuous rotation-inversion movement and the back and forth wagging motion of AZO molecules, which acts as a molecular impeller that propels the NIR-triggered release of DOX. Additionally, under 980 nm NIR laser stimuli, the accelerating release of DOX from UBDs（no loading of AZO）also occurs in the PBS solution. This is mainly attributed to the rapid raise of local temperature that induced by the photothermal effect under the laser irradiation, which can enhance the thermal vibration and weaken the interaction between DOX and nanosystem, resulting in the accelerated DOX release.

3.4 In Vitro Cytotoxic Effect Against HeLa Cells

Based on the excellent photothermal effect and controllable drug release of UBDAs nanosystem under 980 nm laser irradiation, the nanosystem is considered to be used in the toxicity study of HeLa cells and the effect of photothermal therapy and chemotherapy in killing HeLa cells[43]. Fig. 6 and Fig. S6 show the cell viabilities after the HeLa cells were coincubated with different concentrations of UBDAs and UB@mSiO2for 24 h, and HeLa cells incubated with PBS are used as a blank control group. It can be observed that even at the highest concentration of UBDAs（400 μg·mL-1）, the cell survival remains above 95%. This indicates that the nanosystem has low or no cytotoxicity to HeLa cells in the dosages range studied. Thus, the good biocompatibility and photothermal effect as well as the controllable drug release make the nanosystem potentially useful in synergistic photothermal therapy and chemotherapy for cancer cells in biomedical application.

Fig.6 HeLa cells survival after culturing in the medium containing UBDAs with different concentrations（0，25，50，100，200，400 μg·mL-1）for 24 h

Encouraged by the high biocompatibility, outstanding photothermal conversion capacity and controllable drug release of the UBDAs nanosystem, the antitumor performance of the nanosystem was evaluated by the live/dead cell staining assay. It is evident from Fig. 7 that UBDAs（400 μg·mL-1）do not affect the viability of the cells in the high concentration because all of the HeLa cells show bright green fluorescence. When HeLa cells were incubated with UBDAs as well as with NIR laser irradiation, nearly all cells were dead and showed red fluorescence,which effectively illustrates that the good antitumor performance of UBDAs upon 980 nm laser irradiation. The loaded DOX could be released from the UBDAs nanosystem by NIR-triggered release, in which the AZO can create a transformation fromtrans-isomer intocis-isomer under UV light that generated by the upconversion nanoparticles of the nanosystem. Simultaneously, local temperature raise rapidly that induced by the photothermal effect under the NIR laser irradiation, resulting in the accelerated DOX release. Therefore, the designed UBDAs can be used as the promising single NIR-light stimuli-responsive drug release and photothermal therapy.

Fig.7 Calcein AM/PI staining images of HeLa cells after incubation with UBDAs（400 μg·mL-1）for 24 h with or without NIR laser irradiation（1.5 W·cm-2，10 min）. Control：HeLa cells incubation with DMEM cell culture medium supplemented with 10%（v/v）fetal bovine serum.Scale bar is 100 μm.

4 Conclusions

In summary, a new upconversion-bismuth hybrid nanosystem UBDAs was successfully developed through a mild method,which can be expected to achieve for single NIR-light stimuli-responsive drug release and photothermal therapy. The upconverted UV light emitted by the UCNPs creates a continuous rotation-inversion movement that propels the NIRtriggered release of DOX. More importantly, the UBDAs exhibit extremely high biological safety and effective chemo-photothermal therapyin vitro. Therefore, this work opens an opportunity of exploring the new type of hybrid nanosystem for efficiently synergistic tumor therapy.

Supplementary Information and Response Letter are available for this paper at: http://cjl.lightpublishing.cn/thesisDetails#10.37188/CJL.20220142.