Light-induced tumor theranostics based on chemical-exfoliated borophene

Among 2D materials (Xenes) which are at the forefront of research activities, borophene, is an exciting new entry due to its uniquely varied optical, electronic, and chemical properties in many polymorphic forms with widely varying band gaps including the lightest 2D metallic phase. In this paper, we used a simple selective chemical etching to prepare borophene with a strong near IR light-induced photothermal effect. The photothermal efficiency is similar to plasmonic Au nanoparticles, with the added benefit of borophene being degradable due to electron deficiency of boron. We introduce this selective chemical etching process to obtain ultrathin and large borophene nanosheets (thickness of ~4 nm and lateral size up to ~600 nm) from the precursor of AlB2. We also report first-time observation of a selective Acid etching behavior showing HCl etching of Al to form a residual B lattice, while HF selectively etches B to yield an Al lattice. We demonstrate that through surface modification with polydopamine (PDA), a biocompatible smart delivery nanoplatform of B@PDA can respond to a tumor environment, exhibiting an enhanced cellular uptake efficiency. We demonstrate that borophene can be more suitable for safe photothermal theranostic of thick tumor using deep penetrating near IR light compared to gold nanoparticles which are not degradable, thus posing long-term toxicity concerns. With about 40 kinds of borides, we hope that our work will open door to more discoveries of this top-down selective etching approach for generating borophene structures with rich unexplored thermal, electronic, and optical properties for many other technological applications.


Cell viability assay
Cultured HCT-116, HeLa, MCF7 and A549 cells were placed in 96-well sterile plastic plates at an amount of 1.8×10 3 cells per well. Counting Kit-8 (CCK-8) was used to detect the cell viability. After 48 h of growth, the cells were treated in a dose-dependent of B @PDA. Then, the cells were added CCK-8 reagent (10 μL) and incubated for 0.5 hour at 37°C before assay. Absorbance of each well at 450 nm was measured through a microplate reader (Labsystem). The experiment was repeated in triplicate and the cell viability curve was plotted.

Endocytosis
The HCT-116 cells were cultured with Cy5-B@PDA for 30 min or 3 h, and then the cells were fixed with methyl alcohol for 15 min, permeabilized for 10 min with 0.1% Triton X-100, and blocked by 3 % bovine serum albumin (BSA) at room temperature for 2 h. Then, the coloring process was conducted with Mitochondrial Staining kit and Lysosome Staining Kit, respectively. Lastly, the cells were stained using DAPI for 10 min under dark condition at room temperature. For each step, cells were cleaned three times using PBS. With nail polish mounting, the cells were visualized and imaged on an Olympus FV1000 confocal microscope.

Detection of ROS
The HCT-116 cells were seeded on 12 well plates for one day. Then, the cells were incubated in the medium containing 25 ppm of B@PDA for 4h. After that, the cells were irradiated with an 808 nm laser (1 W cm -2 , 10 min). Then, the medium was

Fluorescence imaging
The IVIS Imaging equipment (PerkinElmer Inc., Waltham, MA, USA) with filter sets was employed for living imaging. The camera was set with the maximum gain, a luminescent exposure time of 10 s and a binning factor of 4. Scanning parameters are as below: field of view of 12.5 cm, excitation/emission wavelength of 500/600 nm, fluency rate of 2 mW cm -2 .

PA imaging
PA imaging was conducted by Vevo LAZR-2100 PAUS system (VisualSonics Inc., Toronto, Canada). A linear acoustic array transducer (24 mm width, 21 MHz) was configured to the LAZR system. Laser wavelength of 808 nm was used.

Statistical Analyses
All data was shown as the mean ± the standard deviation (SD). Statistical analysis was carried on with SPSS 10.0 (SPSS, Inc., Chicago, IL, USA). p<0.05 was regarded as statistically significant (n = 3). Analysis of variance was conducted for all groups.

Figure S6
Photothermal performance of B@PDA (200 ppm) and pure PDA under irradiation (808 nm, 1 W/cm 2 , 10 min). The preparation method of pure PDA is the same as that of B@PDA at the corresponding concentration. This data shows that the photothermal performance of B@PDA is significantly higher than that of pure PDA.

Figure S7
In vitro PA images of B@PDA and PDA as a function of concentration (0, 125, 250, 500, 1000, and 2000 µg mL −1 ). This data shows that the photoacoustic signal of B@PDA is significantly higher than that of pure PDA. Data was conducted by MOST imaging system (inVision 128; iThera Medical, Germany).

Figure S9
Relative viability of HCT-116 cell incubated with PDA for 4 h and after photothermal treatment (1 W cm -2 , 808 nm, 10 min). The preparation method of pure PDA is the same as that of B@PDA at the corresponding concentration. This data shows that the cell killing ability of B@PDA is significantly higher than that of pure PDA.   Cu2-xSe nanocrystals 22% 808 7 MoS2 nanosheets 23.8% 808 8 Graphene oxide (GO) 25% 532 9 Ti3C2 nanosheets 30.6% 808 10