NaYF4:Yb,Er,Nd@NaYF4:Nd Upconversion Nanocrystals Capped

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 4

4

NaYF:Yb,Er,Nd@NaYF:Nd Upconversion Nanocrystals Capped With Mn:TiO For 808-Nm NIR-Triggered Photocatalytic Applications 2

Zhiying Nie, Xiaoxia Ke, Danni Li, Yuling Zhao, Lanlan Zhu, Ru Qiao, and Xiao Li Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05234 • Publication Date (Web): 26 Aug 2019 Downloaded from pubs.acs.org on August 27, 2019

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The Journal of Physical Chemistry

NaYF4:Yb,Er,Nd@NaYF4:Nd Upconversion Nanocrystals Capped with Mn:TiO2 for 808-nm NIR-Triggered Photocatalytic Applications Zhiying Niea,†, Xiaoxia Kea,†, Danni Lia,†, Yuling Zhaoa, Lanlan Zhua, Ru Qiaoa,*, Xiao Li Zhangb,* a Key

Laboratory of the Ministry of Education for Advanced Catalysis Materials, College

of Chemistry and Life Sciences, Zhejiang Normal University, Jinhua 321004, China b

State Center for International Cooperation on Designer Low-Carbon & Environmental Materials, School of Materials Science and Engineering, Zhengzhou University, Zhengzhou 450001, China

*Corresponding Authors. E-mail address: [email protected], [email protected] † Z.

Nie, X. Ke and D. Li contributed equally to this work.

Abstract This paper described a rational design of core/shell heterostructure consisting of upconversion nanoparticles (UCNPs) as the core and Mn-doped TiO2 (Mn:TiO2) as the

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shell for promising near-infrared (NIR) photocatalytic dye degradation and antibacterial applications. The initial stage was the preparation of NaYF4:Yb,Er,Nd@NaYF4:Nd (Yb/Er/Nd@Nd) active-core/active-shell UCNPs via an epitaxial growth process which could solve the problem of intrinsic low luminescence efficiency. The UC luminescence of typically used NaYF4:18%Yb,0.5%Er,1%Nd@NaYF4:20%Nd could be enhanced by ~ 20 times after coating NaYF4:20%Nd active-shell around NaYF4:18%Yb,0.5%Er,1%Nd core particles, which was mainly attributed to the significant increase in the NIR absorption and efficient energy migration from shell to core. Subsequently, Mn:TiO2 nanocrystals were uniformly coated on Yb/Er/Nd@Nd to form Yb/Er/Nd@Nd@Mn:TiO2 core/shell NPs. Yb/Er/Nd@Nd acted as a NIR-to-visible transducer to sensitize Mn:TiO2 when illuminated with 808-nm NIR light. Importantly, Manganese as dopant increased the photocatalytic activity, extending the photoresponse of TiO2 to green light region and consequentially improving its harvesting efficiency of UC luminescence from Yb/Er/Nd@Nd NPs as well as playing the role of charge separators, which triggered the generation of sufficient reactive oxygen species to induce photocatalytic activity of the core/shell

system.

The

spread-plating

and

MIC

experiments

indicated

that

Yb/Er/Nd@Nd@Mn:TiO2 NPs possessed excellent photocatalytic antibacterial activities against Gram-negative and Gram-positive bacteria under 808-nm laser irradiation, for example, the MIC value of E. coli was only 12.5 μg·mL-1. Meanwhile, the Nd3+/Yb3+-based UC system with 808-nm excitation could significantly minimize the overheating effect on biological cells associated with the conventional 980-nm excitation.


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Preliminary study on biocompatibility of the photocatalyst showed no cytotoxic effect within a certain dose range as measured by MTT assay. A possible working mechanism of Yb/Er/Nd@Nd@Mn:TiO2 heterojunctions was proposed to understand the enhanced photochemical properties. 1. Introduction As one of the most widely studied UC luminescent materials, lanthanide (Ln3+)-doped NaREF4 (RE = rare earth) nanocrystals have the ability to convert low-energy NIR light to high-energy visible/UV light as well as NIR fluorescence in some cases1–4, via the anti-Stokes emission process. Due to their excellent photophysical properties, such as longer-lived luminescence, higher signal-to-noise ratio, and higher resistance to photobleaching, UCNPs are widely applied in fields of bioimaging and sensing5-8, photocatalysis9, three-dimensional displays10, and solar cells11,12. At present, the conventional UCNPs are simultaneously doped with Yb3+/Ln3+ couples, in which Yb3+ ions are employed as a sensitizer owing to their high UC efficiencies, and Ln3+ ions (Er3+, Tm3+, or Ho3+ in most cases)13 are used as an activator. However, the narrow band absorption nature of Yb3+-sensitized UC process requires laser excitation at ca. 980 nm corresponding to 2F7/2 → 2F5/2 transition which coincides with the strong absorption of water. This overlap of the absorption bands between Yb3+ ions and water molecules lowers NIR light absorption and upconversion efficiency in Yb3+-sensitized UCNPs, which greatly restricts their practical applications. Especially in widespread biological



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applications, the long irradiation at 980 nm can be strongly absorbed by water molecules in biological samples and trigger the overheating effect to induce cell death and tissue damage.14 The best way to minimize this overheating effect is to shift the excitation wavelength peak from 980 nm to 808 nm by introducing Nd3+ ions into Yb3+/Ln3+-coupled UC system because of the low absorption coefficient of water at 808 nm.15-17 In this Yb3+/Ln3+/Nd3+ tri-doped UC system, Nd3+ ions, having a maximum photon absorption around 808 nm, act as the NIR absorber and primary sensitizer, while Yb3+ ions play a role of bridging sensitizer to transfer energy from Nd3+ to the lanthanide activator (Nd3+ → Yb3+ → Ln3+ energy transfer process). Consequently, the overheating effect, especially for biological tissues, can be effectively suppressed by this treatment. However, the elevated doping level of Nd3+ ions can induce cross-relaxation energy transfer between Ln3+ activator and Nd3+ ions, which results in poor NIR absorption and decrease in UC intensity.18 In order to overcome this problem, constructing a core/shell architecture is thought to be an efficient strategy to improve the luminescence of UCNPs. Active-core/inert-shell

or

active-core/active-shell

LaPO4:Eu@LaPO419,

NaYF4:Yb,Er@NaYF420,

structures,

such

as

NaGdF4:Yb,Tm@NaGdF4:Eu21,

NaGdF4:Er@NaGdF4:Er22, NaGdF4:Yb,Er@NaGdF4:Yb23, etc. have been reported. The shell material with similar lattice constants around the luminescent core can protect the lanthanide ions in the core (especially those near the surface) from non-radiative decay caused by surface defects. Additionally, compared with the inert shell, the active shell


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can also deliver the excited energy from sensitizer ions in the shell to the core particles thereby increasing the intensity of upconversion. It is noteworthy that, if UCNPs can be coupled with the conventional semiconductor photocatalysts, the hybrid nanomaterials can utilize the upconverted UV and visible emissions from UCNPs upon NIR irradiation to sensitize semiconductor for photocatalytic applications, via a fluorescence resonance energy transfer (FRET) process.24-31 Serving as a semiconductor photocatalyst, TiO2 can efficiently harvest the UV light to further sensitize the ambient oxygen to produce reactive oxygen species (ROS). Coincidently, its bandgap of 3.2 eV (anatase TiO2) matches well with the energy of emission peaks (320 ~ 370 nm) of Yb/Tm-doped UCNPs, so a series of Yb/Tm-doped UCNPs@TiO2

composite

photocatalysts,

such

as

NaYF4:Yb,Tm@TiO232,33,

NaGdF4:Yb,Tm@TiO234, NaLuF4:Gd,Yb,Tm@TiO235, YF3:Yb,Tm@TiO236, have been prepared via sequential growth process. Generally, the NIR-to-visible light conversion of UCNPs upon NIR irradiation is much stronger than their NIR-to-UV light conversion because the fewer anti-stokes shift is happened in this process, however, the photoabsorbance of TiO2 in the visible region is still too low due to the large bandgap of over 3 eV, indicating the low energy transfer efficiency between UCNPs and the semiconductor.37,38 Constructing a UCNPs/visible light-induced semiconductor binary system is a feasible method to solve this problem effectively. Transition metal (such as Fe, Cu, Ni, Cr and V) doping in semiconductor production is a commonly used method to



extend the spectral response of wide bandgap semiconductor (for instance, TiO2) towards visible light.39-43 Our group once reported synthesis of Zn1-xNixO and Zn1-xMnxO by solvothermal method and co-precipitation method, respectively,9,44,45 the doped zinc oxides exhibited improved visible light-induced photocatalytic activities because the impurity doping not only narrowed the electronic energy band structure of ZnO but also inhibited the recombination of photogenerated carriers. Except for this impurity doping approach, several other significant efforts, such as coupling with narrow bandgap semiconductors and deposition of noble metals, have also been devoted for expanding the absorption of wide bandgap semiconductors for better use of solar energy and improving their photocatalytic activities.46,47 In order to improve synergistic utilization of UV, visible and NIR lights for photocatalysis, herein, we report our recent efforts on synthesis of novel TiO2-based NIR-responsive photocatalyst, which had a core/shell structure consisting of NaYF4:Yb,Er,Nd@NaYF4:Nd active-core/active-shell UCNPs as the core, and anatase Mn:TiO2 nanocrystals as the external shell (termed as Yb/Er/Nd@Nd@Mn:TiO2). Yb/Er/Nd@Nd with Er3+ as the activator is an improved visible light emitting UC phosphor with two dominant green emission peaks at 524 nm and 543 nm and a relatively weak red emission peak at 658 nm under 808 nm NIR irradiation. As expected, with enhanced UC emissions and stronger light absorption in visible light region, Yb/Er/Nd@Nd@Mn:TiO2 exhibited enhanced photocatalytic activity during either dye


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degradation or antibacterial test by using 808-nm infrared laser as irradiation source. More importantly, free radical trapping experiments and transient photocurrent responses were carried out to discuss the photocatalytic mechanism of Yb/Er/Nd@Nd@Mn:TiO2 NPs. Meanwhile, MTT (abbr. for 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide) assay was operated to evaluate their biocompatibility for future application in the biomedical field. 2. Experimental section All reagents were of analytical grade and used as received without further purification. 2.1 Synthesis of Yb/Er/Nd@Nd UCNPs Yb/Er/Nd@Nd active-core/active-shell structured NPs were synthesized according to the literature with a little modification.16,48 First, for preparation of high-quality NaYF4:Yb,Er,Nd NPs, 1 mmol of RE(CH3COO)3 (RE = 80.5% Y + 18% Yb + 0.5% Er + 1% Nd) was dissolved in a mixed solution containing 6 mL of oleic acid and 15 mL of 1-octadecene, which was heated at 160 oC for 1 h under stirring to obtain the lanthanide-oleate complexes and then cooled down to room temperature. After that, 10 mL of methanol containing NH4F (4 mmol) and NaOH (2.5 mmol) was added and the obtained solution was stirred at 50 oC for 30 min and then degassed at 100 oC for 15 min with the purpose of removing methanol. Subsequently, the solution was heated at 300 oC for 1 h under the protection of N2. After reaction, NaYF4:Yb,Er,Nd NPs were collected



by centrifugation, washed by ethanol and cyclohexane, and then dispersed in 10 mL of cyclohexane. Yb/Er/Nd@Nd active-core/active-shell NPs was prepared via an epitaxial growth protocol analogous to the synthetic process of NaYF4:Yb,Er,Nd, except that 1 mmol of RE(CH3COO)3 (RE = 80% Y + 20% Nd) and as-obtained NaYF4:Yb,Er,Nd NPs dispersed in 10 mL of cyclohexane were added to the mixture of oleic acid and 1-octadecene. The obtained Yb/Er/Nd@Nd UCNPs were re-dispersed in 10 mL of cyclohexane. 2.2 Synthesis of CTAB-stabilized Yb/Er/Nd@Nd UCNPs CTAB-stabilized Yb/Er/Nd@Nd UCNPs were synthesized by a reverse-micelle method49, which could render preliminary hydrophilicity to the as-synthesized oleate-capped UCNPs and facilitate the uniform deposition of Mn:TiO2 on the core surface. Typically, 0.05 g of cetyltrimethyl ammonium bromide (CTAB) was dissloved in 20 mL of water under vigorous stirring, which was followed by adding 2 mL of cyclohexane containing UCNPs into the above CTAB solution. The mixture was then put into a water-bath (set at 80 oC) to evaporate cyclohexane with stirring, resulting in a transparent water solution. The CTAB-stabilized UCNPs were then separated, washed and finally dispersed in 10 mL of ethanol. 2.3 Synthesis of Yb/Er/Nd@Nd@Mn:TiO2 core/shell NPs


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Yb/Er/Nd@Nd@Mn:TiO2 nanocomposites were synthesized via a sol-gel method.50 Typically, 10 mL of ethanol containing CTAB-stabilized Yb/Er/Nd@Nd UCNPs, 15 mL of ethanol, and 75 μL of ammonia (28 wt%) were mixed under ultrasonicaton for 15 min. After that, 60 μL of tetrabutyl titanate (TBOT) was introduced dropwise to the suspension solution in 5 min with stirring. A given amount of Mn(CH3COO)2 was then added to the above colloid solution. Subsequently, the solution was put into water-bath and stirred vigorously at 45 oC for 24 h. After that, the solid powder was collected and annealed at 400 oC for 3 h under a N2 atmosphere to achieve a crystalline anatase Mn-doped TiO2 shell. In this work, the proportion of Mn(CH3COO)2 was adjusted in order to obtain solids with different Mn atomic fraction from 0 to 5 mol%, the prepared samples

were

labled

as

Yb/Er/Nd@Nd@1%Mn:TiO2,

Yb/Er/Nd@Nd@TiO2,

Yb/Er/Nd@[email protected]%Mn:TiO2,

Yb/Er/Nd@Nd@3%Mn:TiO2,

and

Yb/Er/Nd@Nd@5%Mn:TiO2, where the number denoted the nominal Mn content (mol%). 2.4 Characterization XRD patterns were collected by using an X-ray diffractometer (X’Pert-MPD, Philips) with Cu Kα radiation. TEM images were obtained using a JEOL JEM-2100F TEM with an accelerating voltage of 200 kV. The chemical states of involved elements were studied on an ESCALAB 250Xi X-ray photoelectron spectrometer with an Al Kα (1486.6 eV) monochromatic source. UC fluorescence spectra were acquired on a Hitachi F-7000



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spectrometer equipped with a commercial 808-nm NIR diode laser. UV-Vis absorption spectra were recorded with a Shimazhu UV-2450 UV-Vis spectrophotometer. N2 adsorption-desorption isotherms were collected using a Micrometrics ASAP 2020 surface area and porosity analyzer at 77 K, after the sample had been degassed with vacuum and N2 flushing, at 120 oC for 4 h. Transient photocurrent measurements were performed on an electrochemical workstation (CHI-660D, Chenhua), wherein Na2SO4 solution (0.2 M, pH = 6.8) was used as the electrolyte solution. Pt wire and Ag/AgCl (saturated KCl) were used as the counter electrode and the reference electrode, respectively, and the resulting sample film on FTO served as the working electrode. 2.5 Evaluation of photocatalytic activities of Yb/Er/Nd@Nd@Mn:TiO2 Test

for

RhB

photodegradation.

Photocatalytic

activities

of

Yb/Er/Nd@Nd@Mn:TiO2 samples were first evaluated by Rhodamine B (RhB) degradation under irradiation of 808-nm NIR laser (set at 3 W). Typically, 30 mg of the photocatalyst was suspended in 15 mL of RhB solution (1 × 10−5 M) and stirred in the dark for 1 h to reach the adsorption–desorption equilibrium of the dye with the catalyst. The above solution was then exposed to the irradiation of 808-nm diode laser under stirring, and 1.5 mL of solution was taken out from the reaction mixture at each 30 min interval. After removing any suspended solid in sample aliquots by centrifugation, a UV-Vis spectrophotometer (Tu-1810, Puxi) was used to analyse the absorbance of RhB to follow its decomposition.


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In addition, ethylene diamine tetraacetic acid (EDTA), tert-butanol (t-BuOH), and 1,4-benzoquinone (BQ) were used as scavengers to investigate the crucial roles of h+, •OH and •O2−, respectively, in the process of RhB photodegradation. Test for photocatalytic antibacterial activities. We subsequently selected B. subtilis and E. coli as indicators to investigate NIR-induced photocatalytic antibacterial activities of the samples against Gram-positive and Gram-negative bacteria, respectively, by using a spread plate method.51 First, 10 mg of Yb/Er/Nd@Nd@Mn:TiO2 was mixed in 8 mL of nutrient broth containing 1.0 × 108 CFU mL-1 of bacteria. The suspension was then magnetically stirred in the dark for 10 min, which was followed by taking 200 µL of the mixed solution out and spreading it on a nutrient agar plate. After that, the remaining suspension was exposed to the irradiation of 808-nm NIR light with stirring. After each 10 min interval, a 200 µL aliquot was taken out and spread on an agar plate. In addition, 200 µL of suspended bacterial cells without the antibacterial agent was also spread over a nutrient agar plate to serve as a control. Finally, all the plates were incubated at 37 ± 0.5 oC

for 24 h and then the number of bacterial colonies was counted. Furthermore,

the

minimum

inhibitory

concentration

(MIC)

values

of

Yb/Er/Nd@Nd@Mn:TiO2 against B. subtilis and E. coli were determined by the micro-dilution method described by NCCLS with a little modification.52 In brief, Yb/Er/Nd@Nd@Mn:TiO2 was serially diluted two-fold ranging from 800 µg mL1 to 6.25 µg mL1 with 5 mL of freshly sterilized nutrient broth in a set of glass tubes, and



subsequently 100 µL of pre-cultured bacterial strain (1×108 CFU mL1) was added each tube by micropipette. All the tubes were then placed in a constant temperature shaking incubator under 808-nm NIR irradiation. After 18 h of incubation at 37 ± 0.5 oC, the MIC values were obtained by observing the turbidity of the bacterial growth. Bacterial growth rates in different Yb/Er/Nd@Nd@Mn:TiO2 solutions were also determined by measuring the optical density at 600 nm (OD600) using a UV-Vis spectrophotometer based on the turbidity of the cell suspension. 2.6 Cell viability assay Cytotoxicity of Yb/Er/Nd@Nd@Mn:TiO2 NPs in human umbilical vein endothelial cells (HUVEC) was determined by a standard MTT assay. The cells were seeded in 96-well plates (5000 cells per cell) and incubated for 12 h (5% CO2, 37 oC). After treatment with different concentrations of Yb/Er/Nd@Nd@Mn:TiO2 NPs, the cells were further cultured for another 24 h. Subsequently, 20.0 μL of MTT solution (5.0 mg/mL phosphate buffer solution) was added to each well, and incubated at 37 °C for 4 h. The supernatant was discarded, and then 150 μL of dimethyl sulfoxide (DMSO) was added to each sample with shaking for at least 15 min in order to dissolve the formazan. The corresponding spectra were recorded with a microplate reader at 595 nm. The cell viability rate (VR) was calculated using the following formula:

VR% 

Absorbance of test wells  100% Absorbance of control wells


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3. Results and discussion 3.1 Synthetic strategy of Yb/Er/Nd@Nd@Mn:TiO2 core/shell NPs Scheme 1 describes the synthetic process of Yb/Er/Nd@Nd@Mn:TiO2 core/shell NPs. To obtain Yb/Er/Nd@Nd UCNPs, NaYF4:Yb,Er,Nd nanocrystals were synthesized by a thermal decomposition method, which was followed by growth of an external shell (NaYF4:Nd) on their surface via an epitaxial growth method. This active-core/active-shell structure enables more efficient upconversion under 808-nm laser excitation. Then surfactant CTAB was used as a phase transfer agent to change the hydrophobic nature of the oleate-capped UCNPs. In this process, the hydrophobic tail of CTAB interacted with the hydrophobic oleate ligand on the surface of NaYF4:Nd, and its hydrophilic charged head group made the UCNPs water soluble, which facilitated the deposition of amorphous Mn:TiO2 via a sol-gel process. After annealing the product at 400 oC, a crystalline

Mn:TiO2

shell

(anatase

phase)

was

sequentially

obtained.

As

Yb/Er/Nd@Nd@Mn:TiO2 nanocomposites are constructed in a core/shell manner, Mn:TiO2 can sufficiently utilize UV and visible lights emitted by UC cores under illumination of 808-nm light, and sensitize ROS generation from the ambient oxygen. 3.2 Synthesis and Characterization of Yb/Er/Nd@Nd@Mn:TiO2 NPs Morphologies of uncoated NaYF4:Yb,Er,Nd, Yb/Er/Nd@Nd and the final Yb/Er/Nd@Nd@Mn:TiO2 NPs were observed by TEM. As shown in Figure 1a,



Yb/Er/Nd-tridoped core-only samples were in elliptical shape, and dispersed very well with a mean particle size of 24 nm (Figure 1b). XRD pattern of NaYF4:Yb,Er,Nd NPs was along with the standard card for hexagonal NaYF4 crystals (JCPDS no. 16-0334), and no impurity peaks were observed (Figure 1c). After epitaxial growth of a NaYF4:Nd shell on them, the obtained Yb/Er/Nd@Nd active-core/active-shell UCNPs still preserved the elliptical shape with an enlarged mean particle diameter of 48 nm (Figure 1d and e). By comparing XRD pattern of Yb/Er/Nd@Nd NPs (Figure 1f) with that of core-only NPs, it is found that all diﬀraction peaks of the samples are in good agreements with those in the JCPDS card. This fact implies that neither the introduction of RE3+ ions into the host nor the shell coating has effect on the crystal phase of the products. Moreover, Yb/Er/Nd@Nd NPs exhibit slightly narrower and sharper diffraction peaks and a little stronger peak intensity than NaYF4:Yb,Er,Nd only-core NPs, this improved crystallinity is caused by step-by-step high-temperature reaction for synthesis of the core/shell UCNPs. Figure 2 shows UC emission spectra of the core-only and core/shell UCNPs under excitation at 808 nm. Both samples exhibit the characteristic green UC emissions of Er3+ ions peaked at 524 nm and 543 nm, which are ascribed to the 2H11/2 → 4I15/2 and 4S3/2 → 4I

15/2

transitions via a two-photon process53. A relatively weak red emission at 658 nm

arises from the 4F9/2 → 4I15/2 transition of Er3+. Compared with NaYF4:Yb,Er,Nd core-only NPs, giant enhancement in UC luminescence was observed from


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Yb/Er/Nd@Nd NPs, which can be attributed to the assistance of active-core/active-shell design. First, the high concentration of Nd3+ in the shell greatly enhance the NIR absorption at 808 nm, which facilitates the energy transfer and photon upconversion of the Er3+ ions. Second, the shell not only protects the luminescent lanthanide ions in the core from nonradiative relaxation due to surface defects but also protects the ions from vibrational energies of surface quenching centers, such as ligands and solvent molecules.16,23 Using the classical sol-gel method followed by annealing treatment, CTAB-modified Yb/Er/Nd@Nd UCNPs were coated with a heterogeneous TiO2 shell, and the shell thickness could be increased gradually by increasing TBOT content (Figure S1). Figure S1c shows that the spacing of the lattice fringe in the outer layer is around 0.352 nm, matching well with the (101) plane of anatase TiO2. When TBOT content was increased from 45 μL to 75 μL, TiO2-coated UCNPs were likely to aggregate together due to cross-linking of the hydrolyzed TBOT molecules between the particles (Figure S1d). Photoluminescence (PL) spectra in Figure S2 show that, because both incident and emitted light are scattered by the extra coating layer12,25, the luminescence intensities of Yb/Er/Nd@Nd@TiO2 samples are weaker than that of bare Yb/Er/Nd@Nd UCNPs. In order to avoid aggregation phenomenon and reduce the relatively large loss of luminescence intensity, the usage of TBOT was determined to be 60 μL in the subsequent synthesis of Mn:TiO2 coated UCNPs.



After adding Mn(CH3COO)2 in the sol-gel process, while other terms remaining unchanged, Yb/Er/Nd@Nd@Mn:TiO2 core/shell nanocomposites were obtained (Figure 3a), which favors energy transfer between UCNPs and semiconductor shell. The relatively light and rough shell was around 3.6 nm thickness and did not change obviously before and after calcination (Figure 3b). EDX spectrum in Figure 3c shows coexistence of the main elements such as Na, Y, F, Ti, and O, and the doping elements such as Yb, Er, Nd, and Mn. Cu signal comes from the copper grid. Figure 4 shows XRD patterns of the as-prepared Yb/Er/Nd@Nd@Mn:TiO2 NPs with different Mn doping concentration in TiO2 shell (0 - 5 mol%). The characteristic diffraction peaks (Figure 4a) match well with two crystalline phases of β-NaYF4 (JCPDS No. 16-0334) and TiO2 (JCPDS No. 21-1272), which shows that the incorporation of Mn ions preserved the anatase structure of TiO2 and this doped semiconductor shell was successfully created on Yb/Er/Nd@Nd UCNPs. Additionally, it is worth noting from the enlarged view of TiO2 (101) peak that there is a slight shift of the diffraction peak toward lower angle values in Mn:TiO2 samples compared with pure TiO2, with the increase of Mn doping content from 0.5 mol% to 5 mol% (Figure 4b). This displacement of the (101) reflection can be associated with the perturbation in the anatase crystalline phase. Due to the small difference in Mn and Ti ionic radii (0.80 Å for Mn2+, 0.66 Å for Mn3+, 0.60 Å for Mn4+ and 0.68 Å for Ti4+), the interstitial incorporation of the dopant into the titania


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network can possibly take place, which produces a strain in TiO2 lattice and influences the displacement of the (101) reflection in XRD patterns.54 To get a further insight of the local chemical environments, XPS measurement was made on the selected sample, Yb/Er/Nd@Nd@3%Mn:TiO2. As expected, all elements in the selected sample could be found in the survey spectrum (Figure 5a). The high-resolution spectrum of Ti 2p (Figure 5b) exhibits two characteristic peaks located at 458.8 eV (Ti 2p3/2) and 464.5 eV (Ti 2p1/2) which match typical binding energy values of Ti4+ in anatase TiO2. The binding energy of O 1s around 529.5 eV is attributed to lattice oxygen of TiO2, while another small shoulder peak centered at 531.7 eV is due to the presence of OH surface groups and possible adsorbed water (Figure 5c). Figure 5d shows the characteristic Mn 2p spin-orbit doublet peaks. The broad and asymmetric peaks imply the coexistence of different oxidation states of Mn in TiO2 lattice55. Base on peak fitting results, the peaks located at about 640.12 eV, 641.32 eV and 642.87 eV are generally assigned to Mn2+, Mn3+ and Mn4+ species, respectively. Additionally, a signal at about 647.12 eV can be associated with a Mn2+ satellite band. Of course we can also see that the peaks corresponding to Mn 2p are very weak due to a very low concentration of Mn doping. The above-mentioned XPS measurement further confirms that Mn:TiO2 outer shell has been loaded on Yb/Er/Nd@Nd UCNPs. Based on FRET theory, the UC emission intensity of the luminescent donor can be decreased after combination with an energy acceptor (photosensitizer). Figure 6a



represents UC emission spectra of Yb/Er/Nd@Nd UCNPs, Yb/Er/Nd@Nd@TiO2, and Yb/Er/Nd@Nd@Mn:TiO2 with different Mn doping levels. As shown, the phenomena of fluorescence weakening can be observed after a semiconductor layer was coated. Because UV absorption of TiO2 did not match with green and red UC emissions from Yb/Er/Nd@Nd, a pure TiO2 layer coated on these UCNPs only caused a slight decrease of the two visible emissions via scattering both incident and emitted lights. In contrast, once a Mn:TiO2 shell was coated on the UCNPs, the green emission was significantly quenched, suggesting that the Mn:TiO2 layer can efficiently harvest the green emission from the UCNPs under NIR irradiation. Among these UC spectra, the fluorescent peaks for Yb/Er/Nd@Nd@3%Mn:TiO2 were most greatly quenched, illustrating this sample possesses the highest energy transfer efficiency. This FRET process in the core/shell system is mainly affected by the spectral overlap between the UC emissions of UCNPs and the absorption spectrum of Mn:TiO2. As revealed in Figure 6b, UV-Vis-NIR absorption spectra of all synthesized samples exhibit an intense peak in UV region from 200 - 380 nm due to the bandgap transition of TiO2, and ~ 20 nm red-shift for Yb/Er/Nd@Nd@Mn:TiO2 is also clearly observed. Meanwhile, a wide shoulder peak located in 400 - 600 nm for Yb/Er/Nd@Nd@Mn:TiO2 samples is observed, and Yb/Er/Nd@Nd@3%Mn:TiO2 shows the most obvious shoulder peak among these samples. This broadening of spectral response range is caused by the presence of impurity levels in the forbidden gap of TiO2 after Mn incorporation, which overlaps with the green emissions of Yb/Er/Nd@Nd UCNPs, thus the UC emissions can be quenched


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due to FRET process. Additionally, the peaks at 808 nm and 980 nm are the absorption of Nd3+ and Yb3+, respectively. Therefore, the Mn:TiO2 shell in this system can be activated by NIR light to produce photogenerated electrons and holes which can further react with surrounding oxygen and water to result in formation of a large amount of ROS to kill bacteria and photodegradate organic dyes. 3.3 Photocatalytic activities of Yb/Er/Nd@Nd@Mn:TiO2 NPs First, we used decoloration of RhB to evaluate the photocatalytic activities of Yb/Er/Nd@Nd@Mn:TiO2 NPs. In particular, the samples with different Mn-doping concentrations were investigated, and their photocatalytic performances are shown in Figure 7a. The direct photolysis of RhB dye without catalyst under 808-nm NIR irradiation can be negligible. Based on the kinetic curve of photodegradation of RhB in the presence of Yb/Er/Nd@Nd@TiO2, we can conclude that TiO2 shell could not absorb visible light emission from the UC cores. By contrast, all Yb/Er/Nd@Nd@Mn:TiO2 samples exhibited better activities than the pure TiO2-layered UCNPs, in which Yb/Er/Nd@Nd@3%Mn:TiO2 sample reached the highest photocatalytic activity, approximately

35%

of

RhB

was

degraded

in

the

presence

of

Yb/Er/Nd@Nd@3%Mn:TiO2 after irradiation for 4 h. Further increasing the dopant amount in TiO2 layer led to a decrease of activity, which indicates that a suitable doping concentration of Mn in the host lattice is crucial to the optimal photocatalytic properties. Such doping-concentration-dependent photocatalytic activities can be understood as



follows: (1) This NIR-driven activity results from the synergistic effect of UCNPs and Mn:TiO2 shell. When the doping level is less than or equal to 3 mol%, the increased Mn content can improve the spectral response range of Mn:TiO2 to overlap with green emissions of UCNPs and make full use of them. Meanwhile, Mn ions can act as a mediator of interfacial charge transfer to efficiently prevent some photogenerated e-h+ recombination. (2) When the doping level is above 3 mol%, Mn ions change into recombination centers for e and h+ because the distance between these trapping sites in host lattice decreases, which leads to the increase of recombination rate56. (3) This enhancement regularity of photocatalytic activity is also consistent with the change regularity of the surface characteristics of the photocatalysts with the increase of Mn doping amount. As shown in Figure S4, the specific surface area of the samples increased from 21 to 38 m2g with the increase of Mn doping amount from 0 to 5 mol%. In general, a photocatalyst with a high specific surface area and a large pore volume is indispensable for improving photocatalytic performance, which is mainly due to the combined effects of several factors, such as more surface active sites, ease of transportation of reactant molecules, and enhanced harvesting of irradiation light. As a result, a suitable Mn doping concentration in the host lattice is favorable for the best activity of the core/shell photocatalysts. The reaction kinetics of RhB photodegradation was analyzed using a Langmuir-Hinshelwood kinetic model. As shown in Figure 7b, the kinetics give a good fit to a first-order rate equation: -ln(C/C0) = kt, where C0 is the initial concentration of RhB, C is the concentration at time t, and k is the apparent first-order


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rate constant (min1). The rate constants (k) were given by the slopes of linear fit and estimated to be 3.0×105, 2.4×104, 6.7×104, 2.12×103 and 1.08×103 min1 for Yb/Er/Nd@Nd@TiO2, Yb/Er/Nd@[email protected]%Mn:TiO2, Yb/Er/Nd@Nd@1%Mn:TiO2, Yb/Er/Nd@Nd@3%Mn:TiO2 and Yb/Er/Nd@Nd@5%Mn:TiO2, respectively. The rate constant for Yb/Er/Nd@Nd@3%Mn:TiO2 is about 2 to 9 times as high as other Yb/Er/Nd@Nd@Mn:TiO2 samples. In this case, Yb/Er/Nd@Nd@3%Mn:TiO2 shows the best photocatalytic performance. In order to study its durability, the cycle run for photodegradation of RhB in the presence of Yb/Er/Nd@Nd@3%Mn:TiO2 sample was carried out in the same conditions, as presented in Figure S5. The observed results have no significant difference after 5 cycles, indicating the sample exhibits relatively good durability. The slight change of degradation ratio in the cycling test may be due to the loss of photocatalyst during the recycling by centrifugation and the effect of calcination to remove the residual RhB molecules on its crystallinity. This migration kinetics of photoexcited charge carriers were further investigated based on detailed measurements of transient photocurrent response under NIR irradiation. As shown in Figure 7c, Yb/Er/Nd@Nd@Mn:TiO2 samples exhibited much higher photocurrents than pure TiO2-coated UCNPs, and the photocurrent response ascended first and descended afterwards, in which Yb/Er/Nd@Nd@3%Mn:TiO2 possessed the highest value in comparison with other samples, indicating an obvious charge transfer in the semiconductor shell. Considering that the charge carriers can only be photogenerated



and transferred in Mn:TiO2 shell, we confirm that the 3 mol% Mn-doped TiO2 can most efficiently absorb the upconverted visible emission comes from Yb/Er/Nd@Nd UC cores, which is in agreement with the result shown in PL and UV-Vis-NIR absorption spectra of Yb/Er/Nd@Nd@Mn:TiO2. To compare the roles of different reactive species in this NIR-induced photodegradation process, we employed t-BuOH, BQ, and EDTA as scavengers for detection of •OH, •O2−, and h+, respectively (see Figure 7d). Compared with the scavenger-free system, the degradation ratio of RhB was slightly restrained with introduction of EDTA, indicating that h+ are not the key reactive species. We can also observe that the degradation ratio of the dye was partially inhibited to some extent when t-BuOH was added instead, however, it was obviously inhibited in the presence of BQ. The results illustrate that •O2 radicals are the dominant reactive species generated in this system, while a few •OH radicals are also generated as a minor reactive species. Based on its good NIR-driven photocatalytic activity, the application of Yb/Er/Nd@Nd@3%Mn:TiO2 as an antibacterial agent was also evaluated by using Gram-positive B. subtilis as model bacterium to investigate its photocatalytic antibacterial effect owing to its special energy transfer efficiency. The photocatalytic antibacterial activity of Yb/Er/Nd@Nd@3%Mn:TiO2 agent against B. subtilis is shown in Figure 8. We can see from the plate assay that, when there was no antibacterial agent existing in the initial cultivated suspension, colony forming units of the incubated B.


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subtilis did not decrease under 808-nm laser irradiation, even though the irradiation time was extended to 40 min, in comparison with those incubated under dark condition (Figure 8a-e). It indicates that Nd-responded 808-nm NIR irradiation can overcome the overheating effect of commonly used 980-nm NIR irradiation on bacterial cells (Figure S6) and does not inhibit the bacteria growth. As shown in Figure 8f, the survival number of B. subtilis was not affected in dark with Yb/Er/Nd@Nd@3%Mn:TiO2 agent, which shows that this agent itself does not inactivate bacteria during the testing experiment. By contrast, when Yb/Er/Nd@Nd@3%Mn:TiO2 agent was present in the bacterial suspension following irradiation, the photoinactivation of B. subtilis was obviously increased with increasing irradiation time, and the cells could be completely killed within 40

min

of

irradiation

(Figure

8g-j).

Compared

with

Yb/Er/Nd@Nd@TiO2,

Yb/Er/Nd@Nd@1%Mn:TiO2 and Yb/Er/Nd@Nd@5%Mn:TiO2 respectively (Figure S7), this Yb/Er/Nd@Nd@3%Mn:TiO2 sample significantly showed a higher antibacterial activity under 808-nm NIR irradiation, verifying the higher energy transfer efficiency between Yb/Er/Nd@Nd and 3%Mn:TiO2. Similar to the operation of detecting ROS in photodegradation of RhB, we employed t-BuOH, KBrO3 and EDTA as scavengers for •OH, •O2− and h+, respectively, to investigate the main ROS in photocatalytic antibacterial experiments based on the spread plate method. As shown in Figure 9a-d, without 808-nm laser irradiation, colonies of B. subtilis still spread all over the petri dish after 24 h of incubation whether in the presence



of EDTA, KBrO3 or t-BuOH, implying that these scavengers hardly inhibit the growth of bacteria. Compared with the control experiment without scavenger under NIR irradiation (Figure 9e), the antibacterial activity of Yb/Er/Nd@Nd@3%Mn:TiO2 agent did not decrease in the presence of EDTA although EDTA could trap the produced h+ (Figure 9f), which suggests that h+ is not the main reactive species in this antibacterial process. However, when the scavenger was replaced with KBrO3, the antibacterial activity of the agent greatly reduced and the residual colonies of B. subtilis increased obviously after 40 min of NIR illumination (Figure 9g). Besides, inhibition of the photocatalyst activity also occurred in the presence of t-BuOH (Figure 9h), we can observe residual colonies in the petri dish, but the colony number is less than that in the presence of KBrO3. The above results indicate that, during photocatalytic antibacterial process, both •O2− and •OH radicals have been considered as reactive species generated from 3%Mn:TiO2 layer, and the role of •O2− is much more significant than that of •OH. Interestingly, we can find that the reactive species at work are consistent with those in photodegradation of RhB, so we speculate that the photocatalytic mechanisms for RhB degradation and bacterial inactivation are the same. 3.4 Photocatalytic mechanism for NIR-driven Yb/Er/Nd@Nd@Mn:TiO2 Based on the above results, the proposed mechanism for NIR-responsive photocatalysis is illustrated in Scheme 2, including successive Nd3+→Yb3+→activator energy transfer, the activation of Mn:TiO2 through energy transfer and the generation of


Page 24 of 52

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reactive free radicals. Initially, under 808-nm laser irradiation, Nd3+ ions in the active-core/active-shell UCNPs are excited to their 4F5/2 state, which is followed with nonradiative relaxation to the 4F3/2 state. The energy can transfer to nearby Yb3+ ions crossing the shell and populate their 2F5/2 state. Subsequently, Yb3+ ions act as an effective bridge to relay the energy eventually to Er3+ ions. This energy transfer route initiates a typical UC process in the core, with Er3+ ions excited to high energy levels like 4F

9/2,

4S

3/2

and 2H11/2. As a result, the excited electrons of Er3+ relax to the ground state

with the emission of visible light. Because the effective Mn doping in TiO2 lattice can induce intermediate energy bands as stepping stones to narrow the anatase energy gap into the green light region, most of the visible light emission can be captured by Mn:TiO2 outer shell. Finally, the photogenerated electrons could easily transfer from the valence band of TiO2 to the localized Mn energy levels along with the d-d transitions and then to the conduction band of TiO2 partially, followed with migration of generated e and h+ to the particle surface to take part in surface reaction.57,58 Furthermore, we think that the coexistence of three different manganese oxidation states, acting as charge separators, also favors the improvement of the photocatalytic behavior of TiO2. We believe that Mn2+ and Mn3+ in the network, acting as hole traps, can attract the photogenerated holes to change their oxidation states to Mn3+ and Mn4+, respectively. This process can increase the amount of photogenerated electrons migrating to the surface of the solids. On the other hand, Mn4+ can trap the electrons to modify its oxidation state to Mn3+, however, the photogenerated holes can induce the recovery of Mn4+ species by oxidizing the Mn3+



Page 26 of 52

formed above. Of course, Mn3+ species also undergoes a similar valence transition process. Both processes promote the electrons to migrate towards the particle surface and inhibit the recombination of photogenerated e-h+ pairs. In order to elucidate the synergetic effect between the manganese oxidation states, an XPS analysis was carried out to study the surface stability of Mn-doped photocatalyst during the photocatalytic degradation. In the case of manganese (see Figure S8), the fitted XPS shows that the surface and structural stability was relatively good before and after photocatalytic reaction. Meanwhile, a slight increase of the Mn4+ content and a slight decrease of the Mn2+ and Mn3+ contents in Yb/Er/Nd@Nd@3%Mn:TiO2 could be observed. It is therefore assumed that some Mn2+ and Mn3+ were oxidized by the photogenerated holes. Finally the excited e− can reduce adsorbed O2 molecules to •O2 radicals which were proved to be the dominant photocatalytic oxidant in this work. Meanwhile, the photogenerated h+ can react with surrounding H2O (or OH) to form •OH radicals, these formed •OH radicals are also a strong oxidant leads to effective RhB degradation and bacterials death as minority of ROS. Thus, the possible photocatalysis performance of Yb/Er/Nd@Nd@Mn:TiO2 can be formulated as: Mn:TiO2 + hν (green emission from Yb/Er/Nd@Nd) → Mn:TiO2 (h+VB + eCB) eCB + O2 → •O2



h+VB + H2O → •OH + H+

(3)


(1)

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h+VB + OH → •OH

(4)

RhB + •O2 and •OH → degradation products B. subtilis + •O2 and •OH → inactivation

(5) (6)

Additionally, we used this spread plate method to evaluate the different antibacterial efficiencies

against

Gram-positive

and

Gram-negative

bacteria

using

Yb/Er/Nd@Nd@3%Mn:TiO2 agent in the presence of 808-nm NIR light. The obtained results (Figure S9) show that the survival number of Gram-negative E. coli was visibly less than that of B. subtilis within 20 min of light irradiation, and E. coli cells were almost completely inactivated if prolonging irradiation time to 30 min, so the inactivation rate of E. coli is greater than that of B. subtilis. Moreover, MIC values of E. coli and B. subtilis were tested to be 12.5 μg·mL1 and 50 μg·mL1, via a micro-dilution method, respectively (Figure 10). So the growth of Gram-negative bacteria can be restrained by Yb/Er/Nd@Nd@3%Mn:TiO2 more rapidly and effectively than that of Gram-positive bacteria under 808-nm NIR irradiation, which is contrary to the antibacterial efficiencies of previously reported Ni-TiO259 against different types of bacteria. The obtained experimental result can be ascribed to the difference in their cell wall structures. The thick cell wall in Gram-positive bacteria is composed of multiple layers of peptidoglycan and teichoic acids, which endows the cells with rigidity and extended cross-linking and makes them difficult to be penetrated by the oxidative •O2 and •OH. But these oxidative species can penetrate the cell wall of Gram-negative bacteria relatively easily because



Page 28 of 52

their cell wall is composed of covalently linked lipids and polysaccharides and lacks strength and rigidity. Consequently, NIR-driven photocatalytic inactivation of Gram-negative bacteria requires only a lower concentration of ROS than that of Gram-positive bacteria. With the intention of using Yb/Er/Nd@Nd@Mn:TiO2 in biological application, the influence of the prepared nanocomposites on cell viability was evaluated through MTT assay by using HUVEC as model cell and presented in Figure 11. The cell viability of HUVEC was not significantly changed after 24 h of incubation treatment with different concentrations of Yb/Er/Nd@Nd@3%Mn:TiO2 NPs and was still around 90% even the concentration of the agent was up to 300 μg·mL-1. Thus, Yb/Er/Nd@Nd@Mn:TiO2 NPs exhibit good biocompatibility and low cellular toxicity. As the core/shell NPs are a kind of good NIR-driven photocatalyst and biocompatible material, one may use them as an agent for purifying waste water and photodynamic therapy. 4. Conclusions In summary, we successfully prepared Yb/Er/Nd@Nd@Mn:TiO2 core/shell NPs and investigated their 808-nm NIR induced photocatalytic activity for dye degradation and antibacterial use. The core/shell structural design of the photocatalyst took advantage of the

synergistic

eﬀect

of

UCNPs

and

Mn:TiO2

NPs.

Yb/Er/Nd@Nd

active-core/active-shell NPs presented a significant intensive upconverted visible emission compared with Yb/Er/Nd core-only NPs. Meanwhile, this Nd3+/Yb3+-based UC


Page 29 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


system with 808-nm excitation could minimize the overheating effect on biological cells. Mn incorporation on Ti lattice sites narrowed the energy gap of TiO2 to permit its absorption well into the visible spectral region. And besides, the coexistence of Mn2+, Mn3+ and Mn4+ oxidation states, acting as charge separators, could inhibit the recombination of photogenerated e-h+ pairs. Together, these factors enabled Mn:TiO2 shell coated on UCNPs to be irradiated mainly by green light from UCNPs through FRET process, which triggered generation of oxidative •O2 and •OH to induce dye decomposition and bacterial death. Also, the MTT assay showed that this NIR-driven photocatalyst exhibited good biocompatibility and low cellular toxicity. The above-mentioned results could stimulate the further research for creation of NIR responsive photocatalysts and possibly stimulate their promising applications in fields of photocatalytic water treatment, water splitting, photodynamic therapy, etc. Notes The authors declare no competing financial interest. Supporting Information TEM images, PL spectra, XPS spectra, N2 adsorption-desorption isotherms, recycling tests for RhB photodegradation, and plate assay for samples. Acknowledgments



This work is financially supported by the Natural Science Foundation of Zhejiang Province (Grant No. LY18B010005, LY15B010003). The authors thank Dr. Dongmei Wang for her support on the measurements of BET surface area and pore size distribution.

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Schemes


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Scheme 1. Schematic illustration for preparation of Yb/Er/Nd@Nd@Mn:TiO2 core/shell photocatalysts.

Scheme

2.

Schematic

illustration

of

the

working

Yb/Er/Nd@Nd@Mn:TiO2 photocatalyst under 808-nm NIR irradiation. Figure Captions


mechanism

for


Figure 1. (a, b and c) TEM image, particle size distribution, and XRD pattern of NaYF4:Yb,Er,Nd core-only NPs, respectively. (d, e and f) TEM image, particle size distribution, and XRD pattern of Yb/Er/Nd@Nd active-core/active-shell UCNPs, respectively.


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Figure 2. UC emission spectra of NaYF4:Yb,Er,Nd core NPs and Yb/Er/Nd@Nd active-core/active-shell NPs under the excitation at 808 nm.

Figure 3. TEM images of (a) Yb/Er/Nd@Nd@amorphous-3%Mn:TiO2 and (b) Yb/Er/Nd@Nd@3%Mn:TiO2. (c) EDX spectrum of Yb/Er/Nd@Nd@3%Mn:TiO2. Signal of Cu is from the copper grid.



Figure 4. (a) XRD patterns of Yb/Er/Nd@Nd@Mn:TiO2 with different Mn doping concentrations (0 - 5 mol%), (b) high-resolution of (101) peak of Mn:TiO2.


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Figure 5. XPS spectra of the Yb/Er/Nd@Nd@3%Mn:TiO2: (a) survey spectrum, (b) Ti 2p, (c) O 1s, (d) Mn 2p.

Figure 6. (a) Fluorescence spectra and (b) UV-Vis-NIR diffuse reflection spectra of Yb/Er/Nd@Nd@Mn:TiO2 with different Mn doping concentrations (0 - 5 mol%).




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Figure 7. (a) Photocatalytic activity and (b) -ln(C/C0) vs. time plot for photodegradation of RhB in the presence of Yb/Er/Nd@Nd@Mn:TiO2 with different Mn-doping concentrations under 808-nm light irradiation. (c) Transient photocurrent density vs. time plot for Yb/Er/Nd@Nd@Mn:TiO2 samples with different Mn doping concentration (0 - 5 mol%) under 808-nm light irradiation. (d) Comparison of the sample activities in the presence of different scavengers for identifying ROS.



Figure 8. Photographs of colony forming units of B. subtilis (a - e) treated without antibacterial agent under dark condition, and exposed to 808-nm NIR-irradiation for 10, 20, 30, and 40 min, respectively, (f - j) treated with Yb/Er/Nd@Nd@3%Mn:TiO2 agent under dark condition, and exposed to 808-nm NIR-irradiation for 10, 20, 30, and 40 min, respectively.


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Figure 9. (a, e) Photographs of colony forming units of B. subtilis treated with Yb/Er/Nd@Nd@3%Mn:TiO2 under dark condition and 808-nm NIR irradiation for 40 min, respectively (Controls). (b, f) Detection of h+ in photocatalytic antibacterial tests of Yb/Er/Nd@Nd@3%Mn:TiO2 by adding 5×10-5 mol of EDTA under dark condition and 808-nm NIR irradiation for 40 min, respectively. (c, g) Detection of •O2− in photocatalytic antibacterial tests of Yb/Er/Nd@Nd@3%Mn:TiO2 by adding 5×10-5 mol of KBrO3 under dark condition and 808-nm NIR irradiation for 40 min, respectively. (d, h) Detection of •OH in photocatalytic antibacterial tests of Yb/Er/Nd@Nd@3%Mn:TiO2 with addition of 5×10-5 mol of t-BuOH under dark condition and 808-nm NIR irradiation for 40 min, respectively.



Figure 10. The turbidity measurements of (a) B. subtilis growth and (b) E. coli growth in the presence of Yb/Er/Nd@Nd@3%Mn:TiO2. Insets of (a) and (b): Photographs of MIC test results of Yb/Er/Nd@Nd@3%Mn:TiO2 agent against B. subtilis and E. coli, respectively, in which, from left to right, tubes are presented as 800, 400, 200, 100, 50, 25, 12.5, 6.25 μg mL-1 of the antibacterial agent, and the blank control without the antibacterial agent.


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Figure 11. Cell viability assays of HUVEC treated with different concentrations of Yb/Er/Nd@Nd@3%Mn:TiO2 NPs.



TOC Graphic


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NaYF4:Yb,Er,Nd@NaYF4:Nd Upconversion Nanocrystals Capped

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