Nitrogen and Fluorine Codoped, Colloidal TiO2 Nanoparticle: Tunable

Dec 19, 2017 - Centre for Advanced Materials, Indian Association for the Cultivation of Science, ... (F) codoped TiO2 nanoparticle with tunable doping...
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 1976−1986

Nitrogen and Fluorine Codoped, Colloidal TiO2 Nanoparticle: Tunable Doping, Large Red-Shifted Band Edge, Visible Light Induced Photocatalysis, and Cell Death Aritra Biswas, Atanu Chakraborty, and Nikhil R. Jana* Centre for Advanced Materials, Indian Association for the Cultivation of Science, Kolkata 700032, India

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S Supporting Information *

ABSTRACT: Visible light photocatalysis by TiO2 requires efficient doping of other elements with red-shifted band edge to the visible region. However, preparation of such TiO2 with tunable doping is challenging. Here we report a method of making nitrogen (N) and fluorine (F) codoped TiO2 nanoparticle with tunable doping between 1 and 7 at. %. The preparation of N, F codoped TiO2 nanoparticle involves reaction of colloidal TiO2 nanorods with an ammonium fluoride−urea mixture at 300 °C, and the extent of N/F doping is tuned by varying the amount of ammonium fluoride−urea and the reaction time. Resultant colloidal N, F codoped TiO2 nanoparticles show doping dependent shifting of the band edge from the UV to near-IR region, visible light induced generation of reactive oxygen species (ROS), and visible light photodegradation of bisphenol A. A colloidal form of doped TiO2 nanoparticle offers labeling of cells, visible light induced ROS generation inside a cell, and successive cell death. This work shows the potential advantage of anisotropic nanoparticle precursor for tunable doping and colloidal form of N, F codoped TiO2 nanoparticle as a visible light photocatalyst. KEYWORDS: nanoparticle, doping, band edge shifting, visible light photocatalysis, reactive oxygen species, bisphenol A, cytotoxicity

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INTRODUCTION Titanium dioxide (TiO2) is the most popularly used photocatalyst due to its chemical stability, low toxicity, and low cost.1−6 It occurs mainly in rutile, anatase, and brookite forms, among which the rutile form is thermodynamically more stable in the bulk phase, the anatase form is more stable in nanoscale structure,7 and the brookite form is relatively less studied.8 However, TiO2 nanoparticle has a wide band gap of 3.2 eV that restricts its use only under UV light (λ < 400 nm) and a small portion of the solar spectrum (2−3%) can be utilized for such catalysis.5 Thus, there is undergoing research for the development of visible light active TiO2 and other photocatalysts in order to utilize most of the solar spectrum.9,10 These approaches include doping with metal/nonmetal,6,11 introducing crystal defect,12 and making hybrid nanostructures with heterojunction structures.13 However, most of these approaches are not efficient enough for complete capturing of sunlight and for any practical application. The origin of such failure is the rapid electron−hole pair recombination that occurs just after light absorption, and thus various strategies are introduced for effective charge separation, hole trapping, and electron trapping.5 Although cation or anion doped TiO2 nanoparticles are successfully used for photocatalysis, anion doping has more substantial effect in extending the absorption spectra toward the visible region.5,6 In particular nitrogen (N) doped TiO2 nanoparticle with the extended visible light absorption has been widely used for photocatalysis.14−23 © 2017 American Chemical Society

However, N doping leads to oxygen vacancy sites in the TiO2 lattice and produces an unstable particle along with increased recombination kinetics of the electron−hole pair.6,21−23 In contrast codoping of N with fluorine (F) in the TiO2 nanoparticle can eliminate this defective crystal structure.24 This is because O2−/N3−/F− are isoelectric and comparable in size and two O2− ions are equivalent to one N3− ion and one F− ion.24 It is shown that codoping of N and F in the TiO2 nanoparticle can be achieved as high as 15 at. % and the band edge of TiO2 nanoparticle can be red-shifted as high as 550 nm.24 These types of N, F codoped TiO2 nanoparticles have been studied recently by various groups with the hope to improve the photocatalytic performance under visible light.24−32 Most of the syntheses use a solid phase approach that involves heating of a powder TiO2 sample with N/F precursors followed by annealing at >500 °C.29−32 In addition substrate TiO2 particles are >100 nm in size and nondispersible.25−32 Thus, resultant doped nanoparticles sinter with each other, being nondispersible or poorly dispersible, and doping is generally inefficient. Additionally the extent of N/F doping is difficult to control in most of these approaches. Considering the fact that doping can be enhanced by >4 times if Received: September 15, 2017 Accepted: December 19, 2017 Published: December 19, 2017 1976

DOI: 10.1021/acsami.7b14025 ACS Appl. Mater. Interfaces 2018, 10, 1976−1986

Research Article

ACS Applied Materials & Interfaces

fluorescence (F) images of cells were measured by an Olympus IX81 microscope using Image-Pro Plus version 7.0 software. Synthesis of N and F Codoped TiO2 Nanoparticle from TiO2 Nanorod. About 6.0 mL of oleic acid was loaded in a three-necked round-bottomed flask and purged with nitrogen for 15−20 min under 70−80 °C. Next, 0.2 mL of titanium isopropoxide was injected at the stirring condition, and temperature was increased in the range of 300 °C. Then 0.7 mL of oleylamine was injected under stirring conditions, and temperature was maintained for 2 h. Next, heating was stopped and solution was cooled to room temperature and mixed with 180− 360 mg of ammonium fluoride (4.86−9.72 mmol) and 180−360 mg of urea (3.0−6.0 mmol). Next, the temperature was raised to 300 °C under magnetic stirring condition in open air and maintained for 1−2 h, and then heating was stopped. As the temperature increased, bubbles were observed from solution that increased with time and then decreased within 15−30 min. In addition there was a gradual change of solution color from white to green within 15−30 min. Nanoparticles were precipitated from solution by adding acetone, and isolated precipitates were dissolved in chloroform. Next, a second round of purification of nanoparticles was performed by ethanol-based precipitation and chloroform/cyclohexane/toluene-based redispersion. Finally, purified solution was prepared in chloroform/cyclohexane/toluene and used as a stock solution. In order to characterize the TiO2 nanorods produced, a part of the solution was collected before adding ammonium fluoride−urea and precipitated from the solution by adding acetone and the isolated precipitate was dissolved in chloroform. Nanorods were further purified via ethanol-based precipitation and chloroform/cyclohexane/ toluene-based redispersion for two times. Finally, purified nanorods were dissolved in chloroform and used for characterization studies. Synthesis of Polyacrylate-Coated Doped TiO2 Nanoparticle. About 3 mg of doped TiO2 nanoparticle was dissolved in 2 mL of Igepal−cyclohexane reverse micelle, mixed with reverse micelle solution of different acrylate monomers to make a total volume of 10−12 mL and was taken in a 50 mL three-necked round-bottomed flask. Next, 100 μL of N,N,N,N-tetramethylethylenediamine base was added, and the whole solution was degassed under an argon atmosphere for 15 min. Next, 3 mg of ammonium persulfate, dissolved in 100 μL of water, was introduced, and reaction was continued for 30 min under stirring conditions. Next, ethanol was added to precipitate the particles and repeatedly washed with chloroform/ethanol. Finally, particles were dissolved in water and purified by dialysis against distilled water using dialysis membrane (MWCO ∼ 12,000−14 000 Da). Specifically, we have used poly(ethylene glycol) methacrylate, 3sulfopropyl methacrylate, and N-(3-aminopropyl) methacrylamide to introduce poly(ethylene glycol), anionic SO3−, and primary amine groups on the nanoparticle surface, respectively. Additionally, 5 mol % bis[2-(methyacryloyloxy)ethyl]phosphate was used as a cross-linker. In selected cases we have used fluorescein o-methacrylate as one of the monomers for preparing FITC functionalized nanoparticles. Photocatalytic Experiment. Colloidal doped nanoparticle was washed 4−5 times by precipitation and redispersion method described above and dried well, and then 3 mg of nanoparticle was dispersed in 50 mL of bisphenol A solution with a concentration of 100 mg/L. The suspension was stirred in the dark for 90 min to establish adsorption−desorption equilibrium and then irradiated with a visible light source, which was a 250 W Hg vapor lamp (>380 nm wavelength) with light intensity ∼ 5 mW/cm2. A certain volume of aliquot was withdrawn at different time intervals, nanoparticles were separated by high speed centrifuge, the aqueous solution was filtered with 0.2 μm centrifugal filter, and supernatant was passed through the HPLC column to verify the degradation of bisphenol A. A UV detector was used with the absorbance at 275 nm as the detection signal. In order to estimate the ROS generated by nanoparticles, 3 mg of as synthesized doped nanoparticle or polyacrylate coated doped nanoparticle was suspended in a 50 mL aqueous terephthalic acid solution with a concentration of 5 × 10−4 M, and the pH was adjusted to basic using NaOH solution. Next, the solution was stirred for 90

micrometer sized TiO2 substrate particles are replaced by 6− 10 nm particles,15 we presume that use of colloidal anisotropic nanoparticle as doping substrate may improve doping performance. Here we report a N and F codoping approach using colloidal TiO2 nanorods (2−3 nm diameter and 25−35 nm length) as substrate. We found that efficient doping can be achieved at 300 °C using a colloidal/dispersed nanorod substrate with the formation of N and F codoped TiO2 nanoparticles of 70−225 nm size. The presented doping approach and resultant N, F codoped TiO2 nanoparticles have five specific advantages over reported doping approaches/ doped materials. (see Supporting Information, Table S1, for details) First, the extent of N and F doping can be tuned between 1 and 7 at. % by simple variation of doping precursor and doping time. Second, N, F codoped nanoparticles are colloidal in nature and they can be further functionalized for specific application. Third, the band edge of colloidal doped nanoparticles is red-shifted by >600 nm, and they absorb visible to near-IR light. Fourth, colloidal doped nanoparticles show N/F doping dependent visible light photocatalytic activity. Fifth, colloidal doped nanoparticles can be used to produce reactive oxygen species (ROS) inside a cell under visible light irradiation and to induce cell death.

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EXPERIMENTAL SECTION

Materials. Ammonium fluoride was purchased from Loba Chemie Pvt. Ltd. Urea was purchased from Merck. Methylthiazolyl diphenyl tetrazolium bromide (MTT) was purchased from Himedia. Oleic acid, oleylamine, titanium(IV) isopropoxide, poly(ethylene glycol) methacrylate, 3-sulfopropyl methacrylate potassium salt, ammonium persulfate, bis[2-(methyacryloyloxy)ethyl]phosphate, Igepal CO-520, cyclohexane, N,N,N,N-tetramethylethylenediamine, N-(3aminopropyl)methacrylamide, dialysis tube (MWCO ∼ 12,000− 14,000 Da), Dulbecco’s modified Eagle medium (DMEM), 2′,7′dichlorofluorescin diacetate (DCFDA), propidium iodide (PI), and terephthalic acid were purchased from Sigma-Aldrich. Instrumentation. Transmission electron microscopy (TEM) images, energy dispersive X-ray spectra (EDS), and selected area electron diffraction (SAED) patterns were measured using an ultrahigh resolution field emission gun transmission electron microscope (UHRFEG-TEM), JEOL, JEM 2100 F field emission electron microscope. Wide-angle X-ray diffraction (XRD) was measured with a Bruker D8 Advance powder diffractometer using Cu Kα (λ = 1.5406 Å) as the incident radiation. X-ray photoelectron spectroscopy (XPS) measurement was performed using an Omicron (serial no. 0571) Xray photoelectron spectrometer. Raman spectra were recorded by J-Y Horiba confocal triple Raman spectrometer (Model T64000), fitted with gratings of 1800 grooves/mm, and a TE cooled Synapse CCD detector from J-Y Horiba. The samples were excited using a 532 nm Nd:YAG laser (Spectra Physics). The scattered signals were collected at 180° scattering angle to the excitations from an Olympus open stage microscope of 50× objective. The detector and the data acquisition were controlled by LabSpec 5 software of Horiba. HPLC (Waters-515) analysis was performed using a SunFire C18 column). UV−visible absorption spectra of samples were collected using a Shimadzu UV-2550 UV−visible spectrophotometer. Emission spectra were measured using a Synergy Mx multi-mode microplate reader. CHN analysis was performed with a Model No. 2400 SERIES II CHNS/O analyzer (PerkinElmer). Visible light intensity was measured using a visible light meter (Model HTC LX101). Fourier transform infrared (FTIR) spectra were measured on a PerkinElmer Spectrum 100 FTIR spectrometer using a KBr powder pressed pellet of solid samples. EPR spectra were recorded on a JEOL JES-FA 200 instrument. The hydrodynamic sizes and ζ potentials were measured via dynamic light scattering (DLS) using a NanoZS (Malvern) instrument. The differential interference contrast (DIC) and 1977

DOI: 10.1021/acsami.7b14025 ACS Appl. Mater. Interfaces 2018, 10, 1976−1986

Research Article

ACS Applied Materials & Interfaces Scheme 1. Synthetic Approach for N, F Codoped TiO2 Nanoparticlea

a In the first step TiO2 nanorod is prepared at 300 °C by hydrolysis of titanium isopropoxide in oleic acid medium. Next, colloidal TiO2 nanorod is reacted with ammonium fluoride and urea at 300 °C. The extent of N/F doping is varied by changing the amount of ammonium fluoride/urea and the reaction time.

Table 1. Synthetic Conditions and Property of N, F Codoped TiO2 Nanoparticle sample

doping conditions (NH4F, urea, time)a

TiO2 nanorod d-TiO2 (1)

180 mg, 180 mg, 120 min

d-TiO2 (2)

360 mg, 360 mg, 120 min

d-TiO2 (3)

360 mg, 360 mg, 60 min

sizeb 2−3 nm × 25−35 nm (30−100 nm) 70−100 nm (140−230 nm) 150−220 nm (180−350 nm) 175−225 nm (180−350 nm)

F, N content (at. %)c

band edge (absorption max)

visible light induced ROS generation, photocatalysis, cytotoxicity

0.0, 1.1 (1.1)

− (340 nm)

no, no, no

1.2, 1.4 (2.3)

800 nm (350 nm)

poor, poor, poor

6.6, 3.0 (3.1)

900 nm (450 nm)

good, good, good

3.5, 6.8 (6.5)

950 nm (500 nm)

good, good, good

a

All other conditions are mentioned in Experimental Section. bValues are derived from TEM. Values in parentheses are polyacrylate coated particles measured by DLS. cValues are derived from XPS. Values in parentheses are measured by elemental analysis. 500 μL of fresh DMEM medium was added. Then, cells were incubated with 50 μL of MTT (5 mg/mL) solution for 4 h, violet formazan was dissolved in a DMF−water solution of sodium dodecyl sulfate, and absorbance of the solution was measured at 570 nm in a microplate reader. The relative cell viability was measured by assuming 100% cell viability for control cells without nanoparticles. Control cytotoxicity experiments were performed under the same conditions, except that cells were not exposed under visible light.

min to reach homogeneity at room temperature, and then irradiated with a 250 W Hg vapor lamp with light intensity ∼5 mW/cm2. At different time intervals a part of the solution was collected, nanoparticles were separated by centrifuge, and fluorescence of the supernatant was measured by exciting at 315 nm. Investigation of Nanoparticle Induced Reactive Oxygen Species Generation inside HeLa Cells under Visible Light Exposure. HeLa cells were cultured in a 24-well plate under 37 °C with 5% CO2 using DMEM media with 10% heat activated fetal bovine serum (FBS) and 1% penicillin streptomycin. After 24 h cells were taken in fresh media and mixed with 50 μL of polyacrylate coated nanoparticle solution. After 60 min of incubation cells were washed with PBS buffer solution (pH = 7.4) to remove unbound nanoparticles. Then cells were taken in fresh media and irradiated with a 250 W Hg vapor lamp for 30 min with light intensity ∼ 5 mW/ cm2. For ROS detection, 5 μL of DCF solution (10 mM) was added to the media and incubated for 20−30 min under 37 °C. Next, cells were washed with PBS (pH = 7.4) and fresh DMEM media was added. After that, the cells were imaged under fluorescence microscope. In order to determine the membrane damage, visible light irradiated cells were treated with PI staining solution (10 μg/ mL), incubated in dark at 37 °C for 10 min, and washed cells were imaged under green excitation. MTT Assay. HeLa cells were seeded into 24-well tissue culture plates in the presence of 500 μL of DMEM medium supplemented with 10% fetal bovine serum and 1% penicillin/streptomycin at 37 °C and 5% CO2. After 24 h cells were taken in fresh media, and then cells were incubated with polyacrylate coated doped nanoparticles for 60 min. Next, cells were irradiated with visible light (250 W Hg vapor lamp with light intensity ∼ 5 mW/cm2) for 30 min followed by 24 h incubation under dark. Next, cells were washed with PBS buffer, and

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RESULTS AND DISCUSSION Transformation of TiO2 Nanorod to N, F Codoped TiO2 Nanoparticle. Synthesis approach of N and F codoped TiO2 nanoparticle involves reaction of colloidal TiO2 nanorod with a mixture of ammonium fluoride and urea under high temperature (Scheme 1). At first, TiO2 nanorod is prepared at 300 °C by hydrolysis of titanium isopropoxide in oleic acid medium under nitrogen atmosphere.33 Next, colloidal nanorods are mixed with solid ammonium fluoride and urea and heated in air at 300 °C under magnetic stirring condition. Under this condition, intermediates (HF and NH3) produced from ammonium fluoride−urea react with nanorod and produce N, F codoped TiO2 nanoparticle. The formation of doped TiO2 nanoparticle is observed by gradual change of solution color from white to green within 15−30 min. The extent of N/F doping is varied by changing the amount of ammonium fluoride/urea and the reaction time (Table 1). After reaction, doped nanoparticles are purified by conventional precipitation−redispersion method and dispersed in chloroform/cyclohexane/toluene. 1978

DOI: 10.1021/acsami.7b14025 ACS Appl. Mater. Interfaces 2018, 10, 1976−1986

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Transmission electron microscopic image of TiO2 nanorod and three different N, F codoped TiO2 nanoparticles at low and high resolutions. Selected area electron diffraction pattern is shown in the insets of the high resolution images. (b) (i) X-ray diffraction pattern of TiO2 nanorod and three different N, F codoped TiO2 nanoparticles with respect to the bulk anatase TiO2. (ii) Raman analysis of TiO2 nanorod and three different N, F codoped TiO2 nanoparticles. Signals at 144 cm−1 (Eg), 197 cm−1 (Eg), 399 cm−1 (B1g), 519 cm−1 (A1g, B1g), and 639 cm−1 (Eg) correspond to the anatase phase of undoped TiO2 nanorod. However, in doped nanoparticles a new band at 204 cm−1 appears (shown as red arrow) that corresponds to the first-order scatterings of nonstoichiometric titanium nitride (Ti−N). In addition the low frequency 144 cm−1 (Eg) band is slightly shifted to ∼155 cm−1 which is attributed to the nonstochiometric TiO2‑x phase.

ammonium fluoride−urea and changing the reaction time. (see Table 1 for details). Crystalline phases of TiO2 nanoparticle are characterized by wide-angle X-ray diffraction study (Figure 1b). Both the doped and undoped TiO2 nanoparticles are crystalline with anatase structure. The crystal planes of the synthesized nanoparticles are characteristics of tetragonal phase according to the standard data (JCPDS No. 84-1286). No additional reflections are observed for the doped TiO2 nanoparticles which indicate that either doping of N/F occurs inside the crystal lattice or is homogeneously distributed over the nanoparticle surface. However, some of the broad reflections of undoped TiO2 become sharp in doped TiO2, and this may be due to increased

Figure 1a and Supporting Information Figure S1 show the TEM images of TiO2 nanorod substrate and N, F codoped TiO2 nanoparticle, prepared under three different conditions. Table 1 summarizes their preparation condition, size, and extent of doping. While nanorods are 2−3 nm in diameter and 25−35 nm in length, doped nanoparticles are 70−225 nm in size. Depending on the increased extent of doping, they are termed as d-TiO2 (1), d-TiO2 (2), and d-TiO2 (3), where the doping of N and F is in the range of 1−7 at. %. The atomic percents of N and F are 1.4, 1.2 for d-TiO2 (1), 3.0, 6.6 for dTiO2 (2), and 6.8, 3.5 for d-TiO2 (3) (see below for details), and these variations are achieved by using different amounts of 1979

DOI: 10.1021/acsami.7b14025 ACS Appl. Mater. Interfaces 2018, 10, 1976−1986

Research Article

ACS Applied Materials & Interfaces

Figure 2. Evidence of N, F codoping via XPS and FTIR analysis of doped TiO2 nanoparticle. (a) Deconvoluted Ti 2p3/2 and Ti p1/2 signals that shift to higher binding energies after doping. While undoped TiO2 shows their signals at ∼458.6 and ∼464.2 eV, in d-TiO2 (1) they are shifted to ∼461.2 and ∼467.1 eV, respectively. These shiftings are relatively less for d-TiO2 (2) and almost insignificant for d-TiO2 (3). (b) Deconvoluted N 1s signal at ∼400.7 eV for d-TiO2 (1); at ∼402.0, ∼400.4, and ∼399.6 eV for d-TiO2 (2); and at ∼400.6, ∼399.3, and ∼398 eV for d-TiO2 (3). In undoped TiO2 it appears at ∼399.9 eV due to surface adsorbed oleylamine. The N 1s bands below ∼400.0 eV are attributed as interstitially doped or directly substituted N into the host lattice as they are present even after surface cleaning by argon ion sputtering. (c) Characteristic signals of F 1s in doped TiO2 nanoparticle, which is absent in undoped TiO2 nanorods. d-TiO2 (1) shows a signal at ∼688 eV for directly substituted fluorine atom in the TiO2 lattice, and d-TiO2 (3) shows an F 1s signal at ∼685 eV that can be assigned to the interstitially or surface doped fluorine atoms. However, d-TiO2 (2) shows signals at 688 and ∼685.8 eV due to both directly substituted fluorine and interstitial/surface doped fluorine. (d) FTIR spectra of doped TiO2 nanoparticles, showing Ti−F stretching vibration at 944−1030 cm−1.

The effect of F doping leads to the shifting of Ti 2p signals. Ti 2p3/2 and Ti p1/2 signals of undoped TiO2 nanorod appear at ∼458.6 and ∼464.2 eV, respectively. However, in d-TiO2 (1) they are shifted to higher binding energies of ∼461.2 and ∼467.1 eV, respectively. These shiftings are relatively less for d-TiO2 (2) (values are ∼459.2 and ∼464.9 eV, respectively), and shifting is almost insignificant for d-TiO2 (3) (values are ∼458.1 and ∼463.7 eV, respectively). Deconvoluted N 1s signal for the undoped TiO2 appears at ∼399.9 eV which can be assigned as surface adsorbed nitrogen atoms coming from oleylamine precursor. In contrast, d-TiO2 (1) shows an N 1s signal at ∼400.7 eV; d-TiO2 (2) shows signals at ∼402.0, ∼400.4, and ∼399.6 eV; and d-TiO2 (3) shows signals at ∼400.6, ∼399.3, and ∼398 eV. These bands may arise due to surface adsorbed, interstitial, or substitutional doping of the N atoms. Control experiments show that high temperature annealing at 400−500 °C or surface cleaning by argon ion sputtering before XPS cannot remove the N 1s signal below ∼400.0 eV, suggesting that N 1s signals below ∼400.0 eV are due to interstitially doped or directly substituted N into the host lattice (Supporting Information, Figure S3). The extents of N and F dopants are determined from XPS. In addition the nitrogen amount has been determined via

particle size. Fluorescence spectral study of doped TiO2 nanoparticles show no emission band around 600 nm, suggesting that oxygen vacancies are absent in the crystal structure (Supporting Information, Figure S2). Raman analysis shows the signature of anatase TiO2 phase and doping leads to the shifting of the low frequency 144 cm−1 (Eg) band to ∼155 cm−1 because of the nonstochiometric TiO2−x phase and appearance of a new band at 204 cm−1 corresponding to the first-order scatterings of nonstoichiometric titanium nitride (Ti−N)19 (Figure 1b). Core level X-ray photoelectron spectroscopy has been performed in order to determine the nature of N and F doping (Figure 2). In the case of undoped TiO2 there is no F 1s signal in XPS, but doped TiO2 shows characteristic signals of F 1s. In particular d-TiO2 (1) shows a signal at ∼688 eV for directly substituted F atom in the TiO2 lattice and d-TiO2 (3) shows an F 1s signal at ∼685 eV that can be assigned to the interstitially or surface doped F atoms. However, d-TiO2 (2) shows signals at 688 and ∼685.8 eV due to both directly substituted F and interstitial/surface doped F. F doping is further confirmed from FTIR analysis that shows a Ti−F stretching vibration at the 944−1030 cm−1 region. 1980

DOI: 10.1021/acsami.7b14025 ACS Appl. Mater. Interfaces 2018, 10, 1976−1986

Research Article

ACS Applied Materials & Interfaces

Figure 3. (a) Time dependent evaluation of UV−visible absorption spectra of doped TiO2 nanoparticle from colloidal TiO2 nanorods at their early stages. (b) Acetone solution of isolated doped TiO2 nanoparticle at different time points of reaction, showing that blue-green particles appear typically after 10−20 min of reaction and color becomes more intense as the time progress. (c) TEM image of starting TiO2 nanorods used with length distribution in the inset. (d) TEM image of TiO2 nanoparticles at different time points, showing that nanorods aggregate after 5 min (with length distribution in the inset), partially dissolve at 10−20 min (with size distribution in the inset), nucleate to cubic/tetragonal shaped particles at 20−30 min, and then grow into larger size with time (with size distribution in the inset).

shaped particles at 10−20 min time and cubic/tetragonal shaped particles of larger size as the time progresses. On the basis of these observations, we propose a tentative mechanism of formation of N, F codoped TiO2 nanoparticle (Supporting Information, Scheme S1). At the first step the capping ligands around TiO2 nanorods are partially removed and nanorods become unstable. Next, intermediate HF and NH3 (which are produced from ammonium fluoride and urea, respectively) start reacting with the nanorod and induce partial dissolution of nanorod (by reaction with HF) and nucleation of doped TiO2 nanoparticle which eventually lead to N, F codoping. Finally, well-developed nanocube/nanorhombohedra are produced depending upon the amount of doping precursor and reaction time. It is reported earlier that addition of hydrochloric acid to a titania suspension leads to rapid development of anatase particles.34 It is also known that impurity incorporation into TiO2 nanoparticle is easier under the conditions where nanoparticle acts as a unit (rather than atom) and the growth occurs via nanoparticle−nanoparticle attachment.35 Thus, acidic condition and high temperature offer structural non-equillibrium of the nanorod crystal phase, high reactivity of HF offers rapid F doping and induced N doping, and nucleation−growth of N, F codoped TiO2 occur from partially dissolved nanorods. We have also replaced ammonium fluoride−urea mixture by hydrazine/urea/5-fluorouracil and found that TiO2 nanorods remain intact and doping becomes insignificant, which further support the proposed mechanism (Supporting Information, Figure S9). Doping Dependent Large (>600 nm) Red Shifting of Band Edge. The most interesting aspect of N, F codoped TiO2 nanoparticle is their intense and stable blue-green color. While undoped solid TiO2 nanoparticle is white in color, the solid samples or colloidal solution of all N, F codoped TiO2 nanoparticles are blue-green in color. Figure 4 shows the solid samples and UV−visible absorption spectra of their dilute dispersions. All three doped TiO2 nanoparticles show strong

elemental analysis and correlated with XPS data. The atomic percentages of N and F are 1.4, 1.2 for d-TiO2 (1), 3.0, 6.6 for d-TiO2 (2), and 6.8, 3.5 for d-TiO2 (3). In order to further understand the origin of the unusual color, we have performed EPR study and observed a very weak signal of Ti3+ (g ∼ 1.98) and associated with paramagnetic oxygen (g ∼ 2.06) signals. (Supporting Information, Figure S4) This result indicates partially reduced TiO2 is present in a small percent in the doped nanoparticle. Formation Mechanism of N, F Codoped TiO2 Nanoparticle. We found that anisotropic nanorod structure, 300 °C temperature, and colloidal form are three important criteria for efficient N and F doping. Under the similar doping condition, if shorter nanorods or near-spherical nanoparticles are used, the doping becomes inefficient (Supporting Information, Figure S5). Similarly, longer nanowires are a less effective substrate for doping (Supporting Information, Figure S5). Moreover, if a thin film of TiO2 nanorods is mixed with a drop cast solution of urea−ammonium fluoride and the dried film is then heated at 300 °C, doping is less efficient (Supporting Information, Figure S6). However, if solid nanorods are mixed with solid urea−ammonium fluoride and then heated at 300 °C, doping can be achieved but the doping tunability is less effective via changing conditions (Supporting Information, Figure S7). If 600 nm. N, F codoped TiO2 nanoparticles are highly dispersible in organic solvents such as chloroform and toluene, and dispersion remains stable for weeks/months (Figure 4). However, doped TiO2 nanoparticles are insoluble in water or polar organic solvents such as ethanol and methanol. This is

Figure 5. (a) Visible light induced reactive oxygen species generation by N, F codoped TiO2 nanoparticle. Typically, the same concentrations of colloidal doped TiO2 nanoparticles and terephthalic acid are mixed together, irradiated by visible light under similar condition and then photogenerated ROS (ratio of emission at 420 nm after and before light irradiation, I/I0) is measured at different times of irradiation. Results show increased ROS generation with irradiation time and varied rate of ROS generation for different doped samples which are in the order of d-TiO2 (3) > d-TiO2 (2) > d-TiO2 (1) > undopped TiO2. (b) Visible light photodegradation of bisphenol A using N, F codoped TiO2 nanoparticle as photocatalyst showing that bisphenol A degradation kinetics by nanoparticles is in the order of d-TiO2 (3) > d-TiO2 (2) > d-TiO2 (1) > undopped TiO2. Colloidal doped TiO2 nanoparticle is mixed with bisphenol A and irradiated by visible light, and then degradation of bisphenol A (ratio of final to initial bisphenol A concentration, C/C0) is monitored via HPLC. 1982

DOI: 10.1021/acsami.7b14025 ACS Appl. Mater. Interfaces 2018, 10, 1976−1986

Research Article

ACS Applied Materials & Interfaces within 60−80 min. This result suggests that d-TiO2 (3) would be the most effective photocatalyst among the three nanoparticles. In the next experiment we have studied the visible light photodegradation of bisphenol A using N, F codoped TiO2 nanoparticle as photocatalyst. Bisphenol A is a well-known endocrine disrupting chemical , and its degradation by sunlight is an important scientific research aspect.39 Typically, colloidal doped TiO2 nanoparticle is mixed with bisphenol A and irradiated under visible light. Next, degradation of bisphenol A is monitored via HPLC (Figure 5b and Supporting Information, Figure S13). Results show that bisphenol A degradation kinetics follows trends similar to the ROS generation trend as shown in Figure 5a. The degradation rate of bisphenol A is highest by d-TiO2 (3), lowest by d-TiO2 (1), intermediate by d-TiO2 (2), and insignificant by undopped TiO2 nanoparticle. This result clearly suggests that the extent of N, F doping dictates the photocatalytic performance, and heavily doped TiO2 nanoparticle is preferred for better photocatalysis. Visible Light Induced Cell Death by N, F Codoped TiO2 Nanoparticle via ROS Generation. Next, we have investigated visible light induced cytotoxicity of doped TiO2 nanoparticle. We have selected d-TiO2 (2) and d-TiO2 (3) for this study as they produce significant amounts of ROS within 30 min, in comparison to d-TiO 2 (1). Doped TiO 2 nanoparticle is transformed into polyacrylate coated nanoparticle of 140−350 nm hydrodynamic size and with zwitterionic surface charge. The zwitterionic surface charge of the nanoparticle is introduced for efficient cell labeling without appreciable cytotoxicity.36,37 In order to study the cell labeling, these nanoparticles are further conjugated with FITC so that they can be seen under fluorescence microscope. When HeLa cells are incubated with colloidal solution of doped TiO2 nanoparticle, the cells get labeled by nanoparticles and distinct green emission is observed from HeLa cells (Figure 6 and Supporting Information, Figure S14). In the next experiment, cells are labeled with FITC nonconjugated, doped nanoparticle and then exposed under visible light for 30 min. Next, generation of ROS inside the cell is investigated via fluorescent imaging using dichlorofluorescein diacetate as probe.40 Results show that HeLa cells labeled with doped TiO2 nanoparticle and exposed with visible light show green emission due to the formation of ROS. In contrast, undoped TiO2 nanoparticle cannot produce ROS under visible light, and doped TiO2 nanoparticle cannot produce ROS if they are not exposed with visible light (Figure 7 and Supporting Information, Figure S15). Propidium ioide (PI) staining of nanoparticle labeled and visible light exposed cells show strong red emission, indicating the formation of membrane pores that induce permeation of PI to intercalate with the DNA of cell nucleus.41 Clearly such red emission is absent for control samples, suggesting that doping of TiO2 nanoparticle and visible light exposure are essential for membrane pore formation. ROS producing materials are widely used for antifungal42and antibacterial43−45 activity. It is known that ROS generation inside a cell leads to cell death via apoptosis and necrosis.46 So we have estimated the ROS mediated cytotoxicity using MTTbased cell viability assay. Typically, doped TiO2 nanoparticle labeled cells are exposed with visible light for 30 min and after 24 h of incubation in culture media, MTT assay has been performed. Results show that cell viability of doped TiO2

Figure 6. Labeling of HeLa cells by N, F codoped TiO2 nanoparticle. Typically, d-TiO2 (3) is transformed into polyacrylate coated nanoparticle of 180−350 nm hydrodynamic size with zwitterionic surface charge and conjugated with FITC. Next, cells are incubated with colloidal solution of nanoparticle for 3 h and washed cells are imaged under bright field (BF) or fluorescence (F) mode. Strong green emission from cells indicates that cells are labeled with nanoparticle.

nanoparticle labeled and visible light exposed cells greatly decreased and the effect becomes more prominent at higher nanoparticle concentration (Figure 8). In contrast, undoped TiO2 nanoparticle labeled cells exposed with visible light or doped TiO2 nanoparticle labeled cells without visible light exposure are less toxic as they do not produce ROS. N, F Codoped Colloidal TiO2 Nanoparticle: Advantage of TiO2 Nanorod-Based Doping Approach. The presented doping approach has three unique features and advantages as compared to reported N, F codoping approaches. First, the doping requires relatively less drastic conditions. While we have used 300 °C for doping, most of the earlier works used >300 °C. Second, nanometer size and dispersible TiO2 nanorods are used for doping in their colloidal form, while most of the earlier works use either micrometer sized particles or nondispersible particles. Third, the extent of N and F doping can be tuned between 1 and 7 at. %. There are only a few earlier reports that can tune N, F doping typically between 0.1 and 0.4 at. %.25 The reasons for this unique doping are the use of colloidal TiO2 nanorods as doping substrate. The small size, anisotropic shape, and 1983

DOI: 10.1021/acsami.7b14025 ACS Appl. Mater. Interfaces 2018, 10, 1976−1986

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Figure 7. (a) Evidence of visible light induced ROS generation inside cell by doped TiO2 nanoparticle. HeLa cells are labeled with N, F codoped TiO2 nanoparticle (FITC nonconjugated) and then exposed under visible light for 30 min. Next, cells are incubated with DCF followed by fluorescence imaging under bright field (BF) or fluorescence (F) mode. Strong green fluorescence indicates ROS generation inside cells. (b) ROS induced cell membrane rupture by doped TiO2 nanoparticle. Nanoparticle labeled HeLa cells are exposed to visible light for 30 min and then incubated with propidium iodide (PI) followed by fluorescence imaging under BF or F mode. Damaged membrane allows PI to enter a cell and intercalate with the DNA of the nucleus and are shown by red fluorescence. Strong red fluorescence from a cell nucleus indicates membrane rupture. (i) d-TiO2 (3) with visible light exposure for 30 min, (ii) d-TiO2 (3) without any visible light exposure, and (iii) undoped TiO2 nanorod exposed under visible light for 30 min.

nanorods are replaced by nanorod films or physical mixtures in the powder form, the doping becomes inefficient and nontunable. Resultant N, F codoped TiO2 nanoparticles have four specific advantages over reported N, F codoped TiO2 nanoparticles. First, colloidal doped nanoparticles have a redshifted band edge up to 900 nm and they can absorb visible to near-IR light. Considering the maximum red shift up to 550 nm for reported N, F codoped TiO2 nanoparticles, this is a significantly large shift. Second, doped nanoparticle is stable under long preservation and the color becomes stable for more than month, either before or after use as catalyst (Supporting Information, Figure S16). Third, N, F codoped TiO 2 nanoparticles show visible light photocatalysis depending on the extent of N/F doping. Clearly this type of doping dependent photocatalysis is not reported. Fourth, doped nanoparticles are colloidal in nature and they can be transformed into polymer coated and functional nanoparticles for specific applications. For example, they can be used for cell targeting and visible light induced cell death via ROS generation. Considering the unusually large red shift of band edge and strong absorption in visible region, further studies are necessary to understand the property of this material. There are earlier reports of N, F codoped green colored TiO2 or N doped green colored TiO2, and a small percent of Ti3+ is responsible for such color.28,47 HF doped blue TiO2 and NHF

Figure 8. Visible light induced toxicity of N, F codoped TiO2 nanoparticle labeled cells. Typically, nanoparticle labeled cells are exposed with visible light for 30 min, and after 24 h of incubation in culture media, MTT assay has been performed. Results show that cell viability greatly decreased due to visible light exposure, and the effect becomes more prominent at higher dose of nanoparticle. In contrast, undoped TiO2 nanorod exposed with visible light or doped TiO2 nanoparticle without visible light exposure is less toxic. The mean ± SD of three determinations are represented in bars.

dispersed form offer maximum exposure of surface atoms for reaction with dopant precursors at less drastic conditions. In addition dispersible substrate allows controlled doping by simple variation of doping precursor concentration and reaction time. Control experiments show that if colloidal 1984

DOI: 10.1021/acsami.7b14025 ACS Appl. Mater. Interfaces 2018, 10, 1976−1986

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ACS Applied Materials & Interfaces doped brown TiO2 are also reported.48 In our case, we are unable to detect Ti3+ species by XPS. This is either due to low concentration or they are deep inside lattice. However, we observed a very weak EPR signal associated with Ti3+ which may be one of the reasons for blue-green color and large red shift of band edge. In addition unique doping approach that involves dissolution of TiO2 nanorod followed by nucleation− growth of doped nanoparticle, may lead to efficient doping both in the lattice and at the surface. In particular, directly substituted F (Ti−F) is confirmed from XPS and FTIR and directly substituted N (Ti−N) is confirmed from XPS and Raman spectra. It is known that N, F codoping leads to the formation of isolated N 2p band at the top of the O 2p valence band and possibly high doping and nature of doping (substituted and interstitial/surface doping) further influence red shifting of the band edge.24 In addition good durability and color stability of the doped material indicate the high application potential of this material.

SR/NM/NS-1143/2016, and 02(0249)/15/EMR-II) A.B. and A.C. acknowledge CSIR, India for providing research fellowships.

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CONCLUSION Nitrogen and fluorine codoped TiO2 nanoparticle has been synthesized from colloidal TiO2 nanorods. The approach produces colloidal doped TiO2 nanoparticle and doping of nitrogen/fluorine can be tuned between 1 and 7 at. %. Doped TiO2 nanoparticles have a doping dependent heavily redshifted band edge (>600 nm), and their colloidal dispersion absorbs from visible to near-IR light. They produce reactive oxygen species under visible light exposure that can be used for photocatalytic degradation of toxic chemicals, cells, and pathogens. The colloidal form of doped TiO2 nanoparticle offers labeling of cells followed by visible light induced cell death via reactive oxygen species generation inside the cell. This work shows the potential advantage of nitrogen and fluorine codoping of TiO2 nanoparticle for efficient utilization of sunlight.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b14025. Summary of earlier N/F doped TiO2 nanoparticle with their properties, details of the characterization of N, F codoped TiO2 nanoparticle, control doping experimental data at different conditions, characterization of polyacrylated coated doped TiO2 nanoparticle, details of photocatalysis data, and cell labeling and ROS imaging data of doped nanoparticles (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Aritra Biswas: 0000-0002-8072-8274 Atanu Chakraborty: 0000-0003-2138-7913 Nikhil R. Jana: 0000-0002-4595-6917 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge DST and CSIR, Government of India for financial assistance (Grant Nos. DST/TM/WTI/2K16/02(G), 1985

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NOTE ADDED AFTER ASAP PUBLICATION Due to a production error, this paper was published on the Web on January 2, 2018, with errors in column 1 of Table 1. The corrected version was reposted on January 3, 2018.

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DOI: 10.1021/acsami.7b14025 ACS Appl. Mater. Interfaces 2018, 10, 1976−1986