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808 nm Light-triggered Thermometer-Heater Up-converting Platform based on Nd3+-sensitized Yolk-shell GdOF@SiO2 Hao Suo, Xiaoqi Zhao, Zhiyu Zhang, and Chongfeng Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12753 • Publication Date (Web): 27 Nov 2017 Downloaded from http://pubs.acs.org on November 28, 2017

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ACS Applied Materials & Interfaces

808 nm Light-triggered Thermometer-Heater Upconverting Platform based on Nd3+-sensitized Yolkshell GdOF@SiO2 Hao Suo, Xiaoqi Zhao, Zhiyu Zhang and Chongfeng Guo* National Key Laboratory of Photoelectric Technology and Functional Materials (Culture Base) in Shaanxi Province, National Photoelectric Technology and Functional Materials & Application of Science and Technology International Cooperation Base, Institute of Photonics & PhotonTechnology, Northwest University, Xi’an, 710069, China; --------------------------------------------------------------------------------------------------------------------KEYWORDS: Yolk-shell structure; Up-conversion; Thermometry; Light-to-heat conversion; Antibacterial activity. ABSTRACT: The realization of real-time and accurate temperature reading at subcutaneous level during the photo-thermal therapy (PTT) could maximally avoid the collateral damages induced by overheating effects, which remains a formidable challenge for biomedical applications. Herein, 808 nm light-driven yolk-shell GdOF: Nd3+/Yb3+/Er3+@SiO2 microcapsules were developed with thermal-sensing and heating bi-functions. Under 808 nm excitation, sensitive thermometry was implemented by monitoring thermo-responsive emissions from 2

H11/2/4S3/2 levels of Er3+, meanwhile the addition of Nd3+ with rich metastable intermediate

levels and the yolk-shell configuration with large specific surface area triggered efficient lightto-heat conversion via enhanced non-radiative channels. The potentiality of dual-functional samples for controlled photo-thermal subcutaneous treatment was validated through ex vivo . ACS Paragon Plus Environment

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experiments, and the antibacterial activity against E. coli was also elaborately evaluated. Results open a general avenue for designing and developing up-converting platforms with sensitive thermal-sensing and efficient heating bi-functions, which makes a significant step towards the achievement of real-time controlled PTT. 1. INTRODUCTION Photo-thermal therapy (PTT) is a newly-developed and promising therapeutic strategy for cancers owing to its advantages of high-specificity and minimal-invasiveness, in which external photon energy could be converted into massive heat by nano/micro-scaled photo-thermal therapeutic agents (PTAs) to induce thermal-ablation of cancer cells.1-3 Since the therapeutic effects of PTT strongly depend on the amount of heat produced by PTAs, insufficient heating or intracellular overheating would lead to ineffectual treatment to cancer cells or irreversible damage to surrounding normal cells.4-6 This problem could be overcome by real-time monitoring intracellular temperature via fluorescence intensity ratio (FIR) method during the process of PTT to achieve efficient cancer therapy with minimum collateral damages.7-9 Lanthanide-doped upconversion nano/microparticles (UCNPs/UCMPs) have been widely proposed as real-time subcutaneous FIR thermometers in biomedical fields due to their near-infrared (NIR) excitation light with deep tissue penetration and minimized background auto-luminescence, tunable size and high thermal resolution.10-13 Owing to large absorption cross-section at 980 nm of Yb3+ (~ 10-20 cm-1) and efficient energy transfer (ET) from Yb3+ to Er3+, Er3+/Yb3+ co-doped systems with intense green emission from thermally coupled levels (TCLs) 4S3/2/2H11/2 of Er3+ (∆E ≈ 800 cm-1) have been considered as one of the most promising candidates for luminescent thermometers.14-17 However, their further applications in biotechnology have been greatly hindered because the maximum absorption of water molecule also centers at around 980 nm in . ACS Paragon Plus Environment

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biological tissues (absorption coefficient ~ 0.48 cm-1). Continuous laser irradiation at 980 nm would inevitably trigger overheating effects and then damage normal cells and tissues.18 To break through above obstacles, Nd3+ ions with broad absorption cross-section at 808 nm (~ 10-19 cm-1) and high ET efficiency from Nd3+ to Yb3+ (~70%) were proposed as new NIR absorbers and sensitizers thanks to the weak absorption of water at 808 nm (absorption coefficient ~ 0.02 cm-1), which is beneficial to increasing the penetration depth and minimizing the laser-induced heating effect in biological tissues.19-22 Up-conversion (UC) process is always accompanied by the heat generation via phononassisted non-radiative (NR) channels, and excellent photo-thermal effects of Nd3+-sensitized UCNPs/UCMPs have been detected in previous works owing to high rates of NR process from abundant intermediate levels with small energy gaps in Nd3+ ion. These biocompatible UCNPs/UCMPs with effective NIR absorptions could also be applied in disinfection or tumor therapy as PTAs.23-28 For bettering therapeutic effects of PTAs, several attempts have been tried to enhancing the probability of NR process, such as increasing doping contents or developing unique nanostructures with high specific surface (e.g. So-called “yolk-shell” structures with a distinctive

core@void@shell

configuration).29-31

Above

mentioned

bi-functions

of

UCNPs/UCMPs make it possible to rationally design and develop all-in-one thermometer-heater platforms capable of remote thermometry and efficient heating under single beam 808 nm excitation, which establishes crucial building blocks toward the real-time controlled PTT.24-26, 3233

Considering the competitive relation between UC process and non-radiative heating, it is top-priority to maximally achieve excellent UC emissions and light-to-heat conversions in a single host. Lanthanide oxyfluorides (LnOF) exhibit higher UC efficiency than oxides yet better

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photo-thermal conversion ability than fluorides, which have been proposed as ideal hosts for allin-one thermometer-heater platforms.34-36 In order to enhance biocompatibility, a silica layer coated on the outer surface of original UCNPs/UCMPs was extensively used to facilitate their conjugations with various biomolecules for synergetic therapeutics.37 In present work, 808 nm light-driven yolk-shell GdOF: Nd3+/Yb3+/Er3+@SiO2 microcapsules (denoted as YS-GOF@Si) were constructed with a void space between the core and outer shell. As a contrast, Yb3+/Er3+ and Nd3+/Yb3+/Er3+ doped GdOF@SiO2 samples with core-shell microstructures were also synthesized and denoted as GOF@Si, and all samples prepared in this work were schematically depicted in Scheme 1. Sensitive thermometry and superior photo-thermal conversions were simultaneously achieved in yolk-shell GdOF: Nd3+/Yb3+/Er3+@SiO2 under the excitation of 808 nm light. The potentiality of bi-functional samples for controlled photo-thermal subcutaneous treatment and antibacterial activity against E. coli were also elaborately validated.

Scheme 1. Scaled models of the presented core-shell samples GOF: Yb3+/Er3+@Si and GOF:Nd3+/Yb3+/Er3+@Si as well as yolk-shell microcapsules YS-GOF:Nd3+/Yb3+/Er3+@Si. 2. EXPERIMENTAL SECTION Materials and chemicals. All raw materials were directly used without further purification, including high pure Er2O3, Yb2O3, Nd2O3, Gd2O3 (99.99%) and analytical grade reagents nitric . ACS Paragon Plus Environment

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acid (HNO3), sodium fluoride (NaF), urea (CO(NH2)2), glucose, ammonium hydroxide (NH4OH, 25 wt%), tetraethyl orthosilicate (TEOS), cetyltrimethylammonium bromide (CTAB) and phosphate buffered saline (PBS). Synthesis of the spindle-like precursor. First, 1 mmol Ln(NO)3 (Ln = Yb3+, Er3+, Nd3+ and Gd3+) solutions were formed by dissolving corresponding Ln2O3 in diluted HNO3 with heating and stirring, followed by adding 1 mmol NaF, 1.5 g urea and de-ionized (DI) water (100 mL, pH ~ 1.5) with stirring in a water bath at 85 ºC for 3 h. The resulting precipitates were separated by centrifugation after washing several times with DI water and ethanol. Then, the precursors were obtained after further dried at 70 ºC for 12 h and denoted as P. The precursor solutions were formed through ultrasonic re-dispersion of 0.1 g precursors in 10 mL DI water for 15 min for the following synthesis. Synthesis of core-shell and yolk-shell GdOF@SiO2 microcrystals. For the synthesis of coreshell precursors, 5 mL DI water and 60 mL ethanol were introducing into the prepared precursor solutions, and then 0.6 mL aqueous ammonia and 0.2 mL TEOS were slowly dropped in sequence under mildly stirring at room temperature (RT) for 6 h. The suspensions were collected by centrifugation, washed with deionized water and ethanol in turn and further dried at 70 ºC for 12 h to obtain the silica coated precursors (precursor@SiO2, named as P@Si). For the preparation of yolk-shell samples, 8 mL DI water and 12 mL ethanol were first adding into prepared precursor solutions with stirring for 15 min, which were sealed into a 50 mL Teflonlined autoclave and maintained at 190 ºC for 4 h. The resulting brown precipitates were centrifugally separated and washed several times with DI water and ethanol to form the carbon coated precursors (precursors@C, named as P@C). Then, P@C samples were re-dispersed in mixtures including 40 mL DI water, 30 mL ethanol, 0.6 mL aqueous ammonia, 0.15 g CTAB via


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ultrasonication for 15 min, followed by adding dropwise 0.2 mL TEOS with mildly stirring at RT for 6 h. The suspensions were collected by centrifugation, washed with deionized water and ethanol in turn and further dried at 70 °C for 12 h to get carbon and silica coated precursors (precursor@C@SiO2, named as P@C@Si). The final core-shell samples GdOF: RE@SiO2 (RE =

10%Yb3+/1%Er3+

and

1%Nd3+/10%Yb3+/1%Er3+)

and

yolk-shell

samples

GdOF:

1%Nd3+/10%Yb3+/1%Er3+@SiO2 were formed after heating P@Si and P@C@Si at 700 oC for 2 h (1 oC/min), which were respectively denoted as GOF@Si and YS-GOF@Si in the following parts. Characterization. The crystal phase and structure of samples were determined by X-ray diffraction (XRD) patterns and Fourier transform infrared (FTIR) spectra using a Rigaku-Dmax 3C powder X-ray diffractometer (Japan) with Cu-Kα radiation and a Bruker EQUINOX55 spectrometer. The microcosmic structures of samples were recorded on a field emission scanning electron microscope (FE-SEM, Hitachi SU-8010) and a transmission electron microscope (TEM, FEI TF-20). For TEM investigations, powders were suspended in ethanol solution, and a drop of this solution was placed on a carbon-coated copper grids. UC emission spectra were measured at different temperature on a FLS920 fluorescence spectrophotometer equipped with an Oxford OptistatDN2 as nitrogen cryogenics temperature controlling system and semiconductor lasers (980 and 808 nm) as excitation sources, and a digital pulse generator (DG, Rigol technologies, Beijing, China) coupled with excitation sources was employed to record the luminescent lifetimes. The duration of stay at the measured temperature is set as 20 min for thermal evolution UC emission spectra. The photo-thermal effects of samples were investigated using an InfReC R500 infrared thermal camera (Nippon electronic company, Japan).


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Thermometric and heating bi-functions of yolk-shell GdOF: Nd3+/Yb3+/Er3+@SiO2 in subtissues. For ex vivo experiments, 0.1 mL PBS solution containing yolk-shell GdOF: Nd3+/Yb3+/Er3+@SiO2 (1 mg/mL) was slowly injected into a piece of fresh chicken breast at around 1-1.5 mm injection depth. Under 808 nm excitation with different power density, the UC luminescent signals in sub-tissues were collected by the spectrophotometer while the temperature distributions in the surface were recorded by the infrared thermal camera. In vitro antibacterial activity of yolk-shell GdOF: Nd3+/Yb3+/Er3+@SiO2 microcrystal. Escherichia coli (ATCC25922, E.coli) were transferred to sterile nutrient broth from agar slant culture medium, and then sterile water was used to dilute the bacteria. Subsequently, 100 µL PBS buffer solution or PBS containing yolk-shell GdOF: Nd3+/Yb3+/Er3+@SiO2 (1 mg/mL) was mingled adequately with the diluted E. coli, respectively. The above two groups were exposed to 808 nm irradiation for 15 min (44 mW/mm2), and denoted as PBS+808 nm and PBS+sample+808 nm. The same two groups without any irradiation were set as control, and denoted as blank and PBS+sample. After that, 20 µL acquired solution was transferred to the agar slant culture medium based on spread plate method and further cultured at 37 oC for 24 h under air condition, followed by counting the numbers of bacteria colony. The repetitive times were set as three in present experiments to guarantee the reliability of the antibacterial results. 3. RESULTS AND DISCUSSION 3.1. Crystal phase and morphology. The precursors were firstly prepared via co-precipitation approach followed by coating SiO2 layer or carbon and SiO2 double layers in order on the surface of original precursors, the final core-shell and yolk-shell GdOF@SiO2 microcrystals with different doping ions were formed after necessary calcinations. In order to determine the phase and structure of precursors and annealed-samples, their XRD patterns were presented in Figure . ACS Paragon Plus Environment

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1a together with the standard profile of Gd2(CO3)3·H2O (JCPDS No. 37-0559), GdF3 (JCPDS No. 12-0788) and GdOF (JCPDS No. 50-0569). It is found that the most of diffraction peaks in precursor sample were identified to Gd2(CO3)3·H2O, and a small amount of peaks were originated from GdF3 (marked by *). After coating and heating process, well-crystallized rhombohedral GdOF crystal was obtained with a broad band centered at around 22o corresponding to the amorphous silica layer, indicating no significant influence on the phase purity of samples prepared by different synthetic procedures and rare earth ions doping. To further characterize the coating process and transformation from precursors to final samples, the FT-IR spectra of precursors, precursor@C, precursor@C@SiO2, core-shell and yolk-shell GdOF@SiO2 were displayed in Figure 1b, in which the broad absorption band at 3423 cm-1 was assigned to the stretching vibrations of hydroxyl groups. The absorption bands at 1409 and 1500 cm-1 were attributed to COO- group while the peaks centered at 688, 758, 840 and 1087 cm-1 in the precursors corresponds to the absorption of carbonate, which further revealed the main component of the precursors.35 After coating carbon layer on the surface of precursors, two new bands at 1317 and 1629 cm-1 from vibrations of -C-OH and -C=C- were observed, indicating the occurrence of carbonization process. With further coating with silica layer, the typical vibrations of SiO2 from Si-O-Si (1090 cm-1), Si-OH (964 cm-1) and Si-O (466 cm-1) were clearly detected, and two bands at 2925 and 2854 cm-1 were assigned to -CH3 and -CH2- from CTAB. After calcination treatment, the sharp decrease in -C=C- bands and appearance of new absorptions at 521 cm-1 in two annealed samples strongly confirmed the formation of GdOF.


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Figure 1. (a) XRD patterns of precursors, Yb3+/Er3+ doped GOF@Si, Nd3+/Yb3+/Er3+ doped GOF@Si and YS-GOF@Si; (b) FT-IR spectra of precursors, P@C, P@C@SiO2, YS-GOF@Si and GOF@Si microcrystals. The synthetic procedures of core-shell sample were divided into three stages: precursor, SiO2 layer coated precursor and calcined core-shell GdOF@SiO2, as schematically illustrated in Figure 2a. As representatives, the morphology of Nd3+/Yb3+/Er3+ tri-doped products at each step was characterized via SEM and TEM measurements, and the dopants barely affected the shape and size distribution of present samples. Precursor particles exhibited highly monodispersed and regular spindle-like shape with uniform size of about 360 nm in length and 150 nm in width, as shown in Figure 2b and e. Through a modified Stöber procedure, a smooth nonporous silica layer with a mean thickness of 25 nm was well coated on the surface of spindle-like particles without significant influence on their high uniformity and dispersibility (Figure 2c and f). After annealing treatment, resulting core-shell GdOF@SiO2 microcrystals still maintained the highly dispersed . ACS Paragon Plus Environment

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and original shape, as displayed in Figure 2d and g. A slight shrinkage was observed in the size of inner cores due to the decomposition of precursors and the crystallization of GdOF during the annealing process.

Figure 2. (a) Schematic diagram for the synthetic procedures of GOF@Si; typical SEM (up) and TEM (bottom) images of (b, e) precursors, (c, f) P@Si and (d, g) GOF@Si microcrystals. The synthetic routes for yolk-shell GdOF: Nd3+/Yb3+/Er3+@SiO2 can be summarized as precursor, carbon coated precursor, silica and carbon double layers coated precursor and final calcined yolk-shell product four steps, as schematically depicted in Figure 3a. Under hydrothermal environment, the core of spindle-like precursors was well coated with an extremely smooth spacer layer of carbon with an average thickness of 27 nm (Figure 3b and e). After further growing a 30 nm mesoporous silica shell on the surface of carbon layer, well-dispersed . ACS Paragon Plus Environment

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core@shell@shell structured microparticles were formed with a mean size of about 470 nm in length and 270 nm in width (Figure 3c and f). After annealing treatment, uniform yolk-shell GdOF@SiO2 microcapsules formed with the hollow interior cavity between the silica shell and GdOF core as results of the vanished carbon layer and the shrunken sizes of the core during the process of calcinations (Figure 3d and g). Clear lattice fringe with well-resolved 2D interplanar spacing (3.15 Å) in the inner core corresponding to the (012) crystal face of GdOF was clearly seen from the HR-TEM image (Figure 3h). The selected-area electron diffraction (SAED) pattern (Figure 3i) presented a common electron diffraction ring patterns indexed as the (012), (104), (018) and (1010) planes of GdOF crystal, which discloses the polycrystal nature of the resulting products. The obvious dark/bright contrast among the interior cavity, shell and core part in the high-angle annular dark-field scanning TEM (HAADF-STEM) image further confirmed the yolk-shell architectural prototype, as presented in Figure 3j. The elemental mapping analysis disclosed the elemental composition and distribution of yolk-shell particle (Figure 3k), in which Gd, Yb, F, O and Si were detected and well-distributed in a single microcapsule; whereas the doping contents of Er3+ and Nd3+ ions were too low (1%) to be measured. The cross-section compositional line profiles and the superimposed elements mapping further demonstrated the elemental localization within a single yolk-shell GdOF@SiO2 microcapsule.


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Figure 3. (a) Schematic diagram for the synthetic procedures of YS-GOF@Si; TEM images with different magnification of (b, e) P@C, (c, f) P@C@Si and (d, g) YS-GOF@Si; (h) HR-TEM image, (i) SAED pattern, (j) HAADF-STEM image and (k) elemental mapping together with cross-section compositional line profiles of YS-GOF@Si. 3.2. UC luminescence and photo-thermal conversion analysis. The UC emission spectra of asprepared 10%Yb3+/1%Er3+ and 1%Nd3+/10%Yb3+/1%Er3+ doped core-shell GdOF@SiO2 and 1%Nd3+/10%Yb3+/1%Er3+ tri-doped yolk-shell GdOF@SiO2 samples were recorded in visible region with 808 nm excitation to illuminate the effect of dopants and microcosmic architectures on UC properties. As displayed in Figure 4a, the characteristic emission bands of Er3+ ions peaked at 534 (2H11/2 → 4I15/2), 543 (4S3/2 → 4I15/2) and 670 nm (4F9/2 → 4I15/2) were clearly . ACS Paragon Plus Environment

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detected in all samples, but UC intensity and red to green (R/G) ratio were completely different. The weak UC intensity observed in Yb3+/Er3+co-doped core-shell samples under 808 nm excitation suggested the ineffective absorption ability of Yb3+ or Er3+ at 808 nm. The UC intensity was substantially enhanced as introducing Nd3+ into Er3+/Yb3+ co-doped samples owing to the larger absorption cross-section of Nd3+ and efficient ET process from Nd3+ to Yb3+.19 Such phenomenon indicates that Nd3+ ion could act as an efficient sensitizer to transfer absorbed photon energy through the intermediate bridging-sensitizer Yb3+ ion to the luminescent center Er3+ ion in tri-doped systems under the irradiation of 808 nm, as depicted in Figure 4c. Intriguingly, yolk-shell GdOF: Nd3+/Yb3+/Er3+@SiO2 microcapsules exhibited bigger R/G ratio and weaker emission intensity than that of tri-doped core-shell samples (inset of Figure 4a). The CIE chromaticity coordinates were calculated to be (0.31, 0.66) and (0.35, 0.63) for Nd3+/Yb3+/Er3+ doped core-shell and yolk-shell GdOF@SiO2, which were visualized by UC luminescent images of ethanol-dispersed samples under 808 nm excitation (Figure S1). This phenomenon could be well interpreted by the non-radiative (NR) process and surface quenching effect.13 Comparing with core-shell structure, the large internal void space endows yolk-shell configuration with higher specific surface area and more quenching sites, resulting in higher rate of NR process and weaker UC emission intensity.31 In the yolk-shell products, two enhanced NR processes (4S3/2 → 4F9/2 and 4I11/2 → 4I13/2) of Er3+ ion mainly trigger the increase of R/G ratio via facilitating the population of low-lying level 4F9/2 from green-emitting levels 4S3/2/2H11/2. To further justify above assumptions, the NIR emission spectra in range of 850-1150 nm and temporal curves in Nd3+/Yb3+/Er3+ tri-doped core-shell and yolk-shell GdOF@SiO2 microcrystals were measured with 808 nm excitation. As presented in Figure S2, the NIR spectra of two samples mainly consist of three bands peaked at 886, 1057 and 971 nm corresponding to


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the 4F3/2 → 4I9/2 and 4F3/2 → 4I11/2 of Nd3+ and 2F5/2 → 2F7/2 of Yb3+. It is clearly observed that the lifetimes of 4F9/2 → 4I15/2 (Er3+), 4S3/2 → 4I15/2 (Er3+), 4F3/2 → 4I9/2 (Nd3+), 4F3/2 → 4I11/2 (Nd3+) and 2

F5/2 → 2F7/2 (Yb3+) transitions in core-shell products were all longer than those in yolk-shell

samples, which further confirmed higher rates of NR processes in yolk-shell configuration with larger specific surface area. Since the radiative transition and non-radiative heating are two competitive processes, UC samples with higher NR possibility usually produce low quantum efficiency and more internal heat.13 In order to explore the potential applications of prepared samples as optical heaters, the photo-thermal effects of samples with different doping systems (Yb3+/Er3+ or Nd3+/Yb3+/Er3+), morphologies (core-shell or yolk-shell) and excitation wavelengths (808 or 980 nm) were evaluated and shown in Figure 4b. Monitored by infrared thermal camera, the temperature increased rapidly and then achieved the maximum as prolonging the exposure time to 195 s, and the power density of 808 and 980 nm lasers were both set as 39 mW/mm2. It is observed that the temperature increments (∆T) of core-shell samples GdOF: Yb3+/Er3+@SiO2 under 980 nm excitation (22.4 K) was higher than that of the same samples with 808 nm excitation (11.9 K), resulting from much broader absorption cross-section of Yb3+ at 980 nm than that at 808 nm. Obviously, the addition of Nd3+ ion in Er3+/Yb3+ co-doped systems triggered more heat under the excitation of 808 nm laser (27.5 K), indicating the crucial role of Nd3+ ion in the heating process. According to graphically depicted energy diagram (Figure 4c), Nd3+ ion not only plays the role of efficient sensitizer but also acts as an efficient heating unit to convert the absorbed energy to heat under the excitation of 808 nm via NR channels from its rich metastable intermediate levels with relatively small energy gaps.24-26 In comparison with core@shell architecture, better lightto-heat conversion ability was achieved in the yolk-shell tri-doped sample with higher specific


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surface area (34.1 K) due to its higher rate of NR process. The internal space between core and shell allows multiple reflections of NIR lights within the cavity and improves the energy absorption, which is also beneficial to light-to-heat conversion in yolk-shell samples.38

Figure 4. (a) UC emission spectra, integrated intensity and red to green ratio of Yb3+/Er3+, Nd3+/Yb3+/Er3+ doped GOF@Si and Nd3+/Yb3+/Er3+ doped YS-GOF@Si; (b) variation curves of laser-induced heating effects versus excitation time (λex = 808 nm); (c) simplified energy scheme of Nd3+/Yb3+/Er3+ tri-doped systems. 3.3. UC Thermal sensing performance. In an attempt to validate the potentiality of present samples as self-referenced thermometers, temperature-dependent UC spectra of core-shell GdOF: 10%Yb3+/1%Er3+@SiO2 (λex = 980 nm), 1%Nd3+/10%Yb3+/1%Er3+ tri-doped core-shell . ACS Paragon Plus Environment

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and yolk-shell GdOF@SiO2 microcrystals (λex = 808 nm) in green region were monitored in Figure 5a and Figure S3, respectively. It is worth noting that all spectra was normalized at 534 nm from 2H11/2 → 4I15/2 transition of Er3+ for better comparison and the excitation density (10 mW/mm2) is too low to trigger the laser-induced heating effect. As increasing the temperature from 260 to 490 K, the UC intensity of 4S3/2 → 4I15/2 (I543nm) monotonously declined in all samples without significant changes in band position. Followed by the Boltzmann-type distribution, the fluorescent intensity ratio (FIR) between quasi-thermal balanced 2H11/2 and 4S3/2 could be described as:15 FIR =

I 534 nm hω2 N 2 A2 ω2 g 2 A2 ∆E ∆E exp(− ) = B exp(− ) = = I 543nm hω1 N1 A1 ω1 g1 A1 k BT k BT

(1) where I, N, ω, g and A denote the integrated emission intensity (I534nm: 505-539 nm; I543nm: 540575 nm), number of ions, degeneracy and spontaneous emission rate of two transitions, respectively. The energy gap, Planck’s and Boltzmann’s constant are symbolized by ∆E, ω and kB, and B (ω2g2A2 /ω1g1A1) is the coefficient. The dynamical monitoring of FIR values on a monolog scale (LnFIR) in three samples were plotted against inverse absolute temperature (1/T) in Figure S4. According to the slopes (∆E/K) of optimal fitting straight lines, the energy separations between 2H11/2 and 4S3/2 levels were calculated to be approximately 756 ± 0.21, 748 ± 0.61 and 730 ± 0.67 cm-1 in GdOF: Yb3+/Er3+@SiO2 (λex = 980 nm), GdOF: Nd3+/Yb3+/Er3+@SiO2 and yolk-shell GdOF: Nd3+/Yb3+/Er3+@SiO2 microcrystals (λex = 808 nm) with approximately 0.5%, 1.6% and 3.9% of the relative error δ (|∆E - Em|/Em), respectively. In comparison with that of previous works based on Er3+ ions 39, the obtained relative errors were little enough to ensure the measurement accuracy. The corresponding values of coefficient B and regression parameters R were computed from the fitting curves of FIR versus T to be B1 = 19.773 . ACS Paragon Plus Environment

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± 0.123 and R1 = 0.999, B2 = 19.616 ± 0.222 and R2 = 0.999, B3 = 18.842 ± 0.349 and R3 = 0.999, respectively. For practical applications, the thermal-sensing capability of UC sensors could be quantitatively assessed by thermal sensitivity, namely relative sensitivity (Sr) and absolute sensitivity (Sa). Sr is the theoretical rate of the FIR changing with temperature while the variation of FIR along with temperature defines Sa, which could be mathematically expressed as:34 Sr =

1 dFIR ∆E = FIR dT k BT 2

(2) Sa =

dFIR ∆E =FIR dT k BT 2

(3) According to their expressions, the value of Sr is mainly determined by energy gap ∆E, yet the Sa is affect by two variables energy gap (∆E) and fluorescent intensity ratios (FIR). Figure 5b plotted the variation of Sr and Sa for three samples as a function of temperature, in which the close values of relative sensitivity (~1.6% K-1 at 260 K) were observed in three samples resulting from their similar energy gaps. Moreover, the absolute sensitivity of three samples gradually increased and achieved maximums at around 0.0098, 0.0097 and 0.0096 K-1 with continuously rising to 490 K. The temperature uncertainty (δT = δFIR/Sr) was determined to be around 0.67, 0.46 and 0.39 K for GdOF: Yb3+/Er3+@SiO2, GdOF: Nd3+/Yb3+/Er3+@SiO2 and yolk-shell GdOF: Nd3+/Yb3+/Er3+@SiO2, respectively. The obtained values of thermal sensitivity and temperature uncertainty are superior and comparable with other previous reported luminescent thermometers.23,

40

Above results revealed that the tri-doping of Nd3+ and yolk-shell

configuration hardly weaken the thermal sensitivity in present systems, which offers the . ACS Paragon Plus Environment

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possibility of 808 nm light-driven thermometry with relatively high sensitivity utilizing Nd3+/Yb3+/Er3+ tri-doped yolk-shell GdOF@SiO2 microcapsules as thermal sensors.

Figure 5. (a) Contour projection of thermal evolution UC spectrum of YS-GOF: Nd3+/Yb3+/Er3+@Si under 808 nm excitation; (b) temperature-dependent of absolute/relative sensitivity in GOF: Yb3+/Er3+@Si (λex = 980 nm), GOF: Nd3+/Yb3+/Er3+@Si (λex = 808 nm) and YS-GOF: Nd3+/Yb3+/Er3+@Si (λex = 808 nm) microcrystals. 3.4. Thermometry and heating in sub-tissues. The dual functionalities of simultaneous optical heating and thermal sensing of yolk-shell GdOF: Nd3+/Yb3+/Er3+@SiO2 microcapsules in subtissues were validated through designing ex vivo experiments. As schematically depicted in Figure 6a, 0.2 mL PBS solution containing yolk-shell microcapsules (1 mg/mL) was subcutaneously injected into a fresh chicken breast at a depth of 1-1.5 mm, and then 808 nm laser beam was tightly focused on the injected position with different excitation density. After irradiating for 10 min, the real-time temperatures in sub-tissues were calculated by spectral analysis of injected samples based on FIR technique, whereas the infrared thermal camera was employed to record the surface temperatures. Figure 6b plotted the steady state power densitydependent temperatures in the surface and sub-tissue of chicken breast under the excitation of . ACS Paragon Plus Environment

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808 nm laser. With adjusting the excitation density from 21 to 68 mW/mm2, monitored temperatures increased from 296 to 314 K in the surface and 297 to 336 K in the injected point. The difference in temperatures between surface and sub-tissue was attributed to the efficient heat transfer from tissue to air and heat diffusion through intra-tissues, which is accordance with previous researches.23-26,

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Results indicate that FIR technique provides an accurate thermal

sensing results in subcutaneously injected position unaffected by geometrical factors of the experiments, while two-dimensional thermal images of the surfaces of the irradiated position and its surroundings could be simultaneously monitored by the infrared thermometry. The combined use of two strategies could minimally avoid the collateral damages during the photo-thermal treatment. As a contrast, the temperature growth could be negligible in 808 nm irradiated chicken breast without samples injection (blank group), suggesting that 808 nm excitation barely generates heating effects in chicken breast within our experimental range. Therefore, the bicapabilities of optical heating and thermal sensing in subcutaneous tissues endow present 808 nm light-driven yolk-shell GdOF: Nd3+/Yb3+/Er3+@SiO2 microcapsules with great potential as all-inone heater-thermometer platforms towards real-time controlled PTT.


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Figure 6. (a) The scheme of the experimental setups for monitoring temperatures of fresh chicken breast; (b) power density-dependent temperatures in chicken breast with and without YS-GOF: Nd3+/Yb3+/Er3+@Si injection under 808 nm excitation. 3.5. Photo-thermal inactivation effects against bacteria. To further demonstrate 808 nm lightinduced photo-thermal effects of yolk-shell GdOF: Nd3+/Yb3+/Er3+@SiO2 samples, its antibacterial activity was evaluated using Escherichia coli (E. coli) bacteria as bacterium models based on surface plating method,41 as presented in Figure 7. In the typical procedure, E. coli were incubated with PBS solution or PBS containing yolk-shell GdOF: Nd3+/Yb3+/Er3+@SiO2 samples. Then, above two groups with 808 nm irradiating for 15 min (P = 44 mW/mm2) were denoted as PBS+808 nm and PBS+sample+808 nm, whereas the corresponding groups without any irradiation were denoted as blank and PBS+sample (Figure 7a). As depicted in Figure 7b, bacteria strains had a relatively high survival rate in PBS+808 nm (88.4%, p-value > 0.05) and PBS+sample groups (85.1%, p-value > 0.05) compared with blank group, revealing the harmlessness of 808 nm laser irradiation alone and the low toxicity of samples to bacteria strains under present experimental conditions. In sharp contrast, the bacteria viability in PBS solution containing yolk-shell GdOF: Nd3+/Yb3+/Er3+@SiO2 dramatically decreased to 53.1% (p-value < 0.001) upon exposure to 808 nm laser. As schematically proposed in Figure 7c, 808 nm lightinduced much heat generated by yolk-shell microcapsules would induce the denaturation of enzymes and then inhibit necessary intracellular reactions. The proteins and lipids on the cell membrane are also damaged at such temperature and some low-molecular-mass species (such as K+, PO42-, DNA and RNA) leak out from the distorted membrane, which seriously disturbs the normal function of the bacteria and leads to the death of bacteria (Figure S5).42 Above results implied that the present Nd3+-sensitized yolk-shell GdOF@SiO2 microcrystals could be worked . ACS Paragon Plus Environment

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as ideal antibacterial agents with excellent photo-thermal inactivation effect.

Figure 7. (a) Digital images and (b) relative viability of bacterial colonies formed by E. coli under different incubated conditions; (c) schematic illustration of photo-thermal inactivation effect of Nd3+/Yb3+/Er3+ doped YS-GOF@Si against E. coli under 808 nm excitation. How to simultaneously achieve sensitive thermometry and efficient non-radiative heating in a single host is a great challenge for all-in-one thermometer-heater platforms, and the reduced penetration depth of green emission of Er3+ in tissues also severely hampers the further progress of practical applications.22-26, 30, 43 In present systems, Nd3+ doping contents only reached up to 1% in yolk-shell microcrystals due to serious concentration quenching effect. Therefore, new strategies and systems (e.g. designing core-shell structure to allow higher doping level of Nd3+ or coating with semiconductor CuxS), utilizing Nd3+-triggered UCNPs as thermometers and heaters operating in biological windows, should be developed in near future with improved luminescent efficiency and photo-thermal effect. . ACS Paragon Plus Environment

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CONCLUSIONS In summary, up-converting yolk-shell microcapsules GdOF: Nd3+/Yb3+/Er3+@SiO2 with thermalsensing and optical heating bi-functions have been developed as thermometer-heater platforms. Sensitive luminescent thermometry (Sa(max) ~ 0.0096 K-1, Sr(max) ~1.6% K-1) and superior photothermal conversions were simultaneously achieved in yolk-shell samples irradiated by 808 nm laser. The yolk-shell configuration with large specific surface area and the addition of Nd3+ with rich metastable intermediate levels hardly affected the thermal sensitivity, but greatly boosted the light-to-heat conversion via enhanced non-radiative transitions. With increasing the power density from 21 to 68 mW/mm2, photo-thermal effects of yolk-shell samples triggered the rapid increase of subcutaneous maximum temperature from 297 to 336 K, which could be real-time monitored via FIR technique. Moreover, excellent antibacterial efficiency of yolk-shell samples against E. coli (viability ~ 53.1%) was realized under 808 nm excitation. Through integrating sensitive thermometry and efficient heat production into one up-converting platform, yolk-shell GdOF: Nd3+/Yb3+/Er3+@SiO2 microcapsules could be regarded as ideal candidates for real-time controlled PTT with high therapeutic accuracy. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. CIE diagram and UC luminescent photographs of samples; NIR emission spectra (λex = 808 nm) and temporal decay curves of samples; Temperature-dependent FIR value of samples; SEM images of blank group and E. coli incubated with yolk-shell samples under 808 excitation. . ACS Paragon Plus Environment

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (No. 51672215, 11274251), Research Fund for the Doctoral Program of Higher Education of China (RFDP) (No.20136101110017), Natural Science Foundation of Shaanxi Province (No.2014JM1004), Foundation of Shaanxi Province Educational Department (15JS101) and Northwest University Doctorate Dissertation of Excellence Funds (YYB17004)

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