Emission Wavelength Tuning in Rare Earth Fluoride Upconverting

Nov 27, 2012 - School of Chemistry, University of East Anglia, Norwich NR4 7TJ, United Kingdom. •S Supporting Information. ABSTRACT: The radiative ...
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Emission Wavelength Tuning in Rare Earth Fluoride Upconverting Nanoparticles Decorated with Dye-Coated Titanate Nanotubes Dmitry V. Bavykin,*,† Tanya L. Stuchinskaya,‡ Lefteris Danos,† and David A. Russell‡ †

Energy Technology Research Group, Faculty of Engineering and Environment, University of Southampton, Southampton SO17 1BJ, United Kingdom ‡ School of Chemistry, University of East Anglia, Norwich NR4 7TJ, United Kingdom S Supporting Information *

ABSTRACT: The radiative energy transfer from rare earth fluoride upconverting (UC) NaxLiyYF4:Yb3+,Er3+ nanoparticles to rhodamine dyes has been systematically studied in colloidal solutions at room temperature. The UC emission bands at 520 and 550 nm have been shifted to the longer-wavelength (ca. 600 nm) region suitable for biomedical applications. To decrease the optical length between the upconverting emitter and the fluorophore, the UC nanoparticles were decorated with titanate nanotubes coated with a dense layer of dye molecules providing possible resonance-energy transfer between them. The fabricated nanostructured composite shows efficient harvesting of UC emission within the proximity of the nanoparticles, allowing the local generation of light suitable for photodynamic therapy applications.

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INTRODUCTION The phenomenon of multiphoton upconversion of radiative energy was predicted in the late 1950s1 and experimentally observed2 mainly on rare earth materials, rendering them useful in applications such as infrared (IR) quantum counters and upconversion solid-state lasers.3 Further searches for new upconverting systems and improvements in the efficiency of existing systems have recently enabled their application in photovoltaic solar cells4,5 in which photons with an energy lower than the band gap of the solar cell semiconductor can be utilized via an upconversion process, increasing the overall energy efficiency of the solar cell. Recent advances in the synthesis methods for preparing nanostructured materials have resulted in the development of nanometer-sized solid crystal upconverters. Such UC nanoparticles can be stable in colloidal solutions, can permeate biological membranes, and are useful in various biomedical applications6 including imaging and biodetection assays both in vitro and in vivo.7 The main advantages of UC nanoparticles compared to traditional fluorophore biomarkers are lower background luminescence and the photoassisted degradation of biological specimens because of the lower energy of nearinfrared (NIR) excitation photons.8 In addition, the photoexcitation depth for NIR photons can reach 10 mm in soft tissues,9 which is significantly deeper than that for ultraviolet (UV) photons. Following the improvements in the quantum efficiency of UC nanoparticles that resulted in an increase in their upconverting emission intensity, the range of their bioapplications has extended into the area of photodynamic therapy (PDT). The method is based on the principle that the photons emitted from the UC nanoparticles are locally reabsorbed by immobilized photosensitizers, resulting in the generation of © 2012 American Chemical Society

cytotoxic reactive oxygen species that can lead to the destruction of diseased cells.10 The early results11,12 suggest the feasibility of this method for the effective treatment of cancer in deep tissues. An additional advantage of utilizing NIR light with a 980 nm excitation wavelength is that the interaction between the excitation beam and the tissues is less invasive compared to the shorter wavelength of light traditionally used for phototherapy.13 Indeed, light at 980 nm is absorbed only by water rather than biomolecules (e.g., absorption maximum for hemoglobin is at 550 nm). Despite the development of various sensitizers for PDT applications, the range of optical wavelengths that stimulates the generation of reactive oxygen species is limited. Therefore, the tuning of the UC emission wavelength is required for specific sensitizers. Such tuning is usually achieved by selecting a suitable lanthanide dopant,14,15 dopant proportion, 16 substitution of Na+ with Li+ or K+,17 or manufacturing core− shell nanoparticles.18 However, the choice of emission wavelength is limited by the spectrum of the doping lanthanides, which are also characterized by a narrow bandwidth restricting the range of available luminescence wavelengths. Other methods of wavelength shifting include a modification of the size of the UC nanoparticles,19 cutting off part of the UC emission spectrum by using dyes with a large extinction coefficient,20 coating the UC nanoparticles with cadmium selenide21 or cadmium telluride quantum dots with large Stokes shift,22 and downshifting the energy of the photons emitted from the UC nanoparticles via radiative or nonradiative Received: October 11, 2012 Revised: November 27, 2012 Published: November 27, 2012 17419

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nanotubes (0.5 g, pure or coated with rhodamine dye) were mixed under stirring with 25 cm3 of a 0.02 mol dm−3 CTAB solution for 3 h. This was followed by sequential washing with water and acetone using filtration, followed by drying at 120 °C for 45 min. The obtained CTAB-modified nanotubes (50 mg) were then suspended in 50 cm3 of CHCl3 and sonicated using a VWR ultrasonic bath (USC 300TH 80 W, 45 Hz) for 3 h, followed by the resting of the suspension for 2.5 h. Immediately following this, 40 cm3 of liquid from the top of the suspension that contained a stable suspension of TiNT was collected and used for the preparation of UC/TiNT composites. For the composite preparation, 10 cm3 of the stable suspension of TiNT in CHCl3 was mixed with 0.004 mg of UC nanoparticles, sonicated for 15 min, and left overnight. Sample Characterization. TEM images were obtained using a JEOL 3010 transmission electron microscope operating at 300 kV; the powder sample was dry deposited onto a copper grid covered with a perforated carbon film. SEM images were obtained using a JEOL 6500 FEG-SEM scanning electron microscope. The UV−vis absorbance and photoluminescence spectra of colloidal solutions were recorded using Neosys-2000 (Scinco) and LS-50B (PerkinElmer) spectrometers, respectively. For the excitation of luminescence from the UC nanoparticles, a 200 mW RLDH980-200-3 infrared laser diode (Roithner Lasertechnik, GMBH) was used. The digital photographs of upconversion emission were taken using a DMC-TZ10 digital camera (Panasonic) without the use of cutoff filters. Fluorescence decay curves were measured using the time-correlated single-photon-counting (TCSPC) technique29 with a FluoTime200 spectrometer (PicoQuant) equipped with a TimeHarp300 TCSPC board (PicoQuant) and a Hamamatsu photomultiplier (PMA-185). The excitation source was a 485 nm picosecond pulsed diode laser (PicoQuant, LDH485) driven by a PDL800-D driver (PicoQuant) operated at a variable pulse repetition rate (10−40 MHz). The emission from the solid powder samples pressed between glass plates was collected at right angles to the excitation laser beam. The emission arm was fitted with a long-pass filter (HQ510LP, Chroma) before the monochromator (Scientech 9030). The fwhm of the system’s instrument response function (IRF) was 200 ps. The fluorescence decay curves were analyzed using the FluoFit software (PicoQuant, version 4.2.1) based on multiexponential models.30

Förster resonance energy transfer (FRET) to various organic fluorescent dyes with a suitable emission band.23,24 The last method for emission wavelength tuning is versatile and is probably more suitable for PDT applications because it avoids the use of toxic quantum dots and achieves a large intensity of in-situ-generated light of the required wavelength. However, the effects of the chemical and electron structures, the luminescence quantum yield, and the method of immobilization of the dye on the efficiency of energy transfer, which may vary significantly, have not yet been systematically studied. For example, the adsorption of rhodamine B on the surface of amino-functionalized doped NaYF4 nanoparticles results in total quenching of the dye luminescence,20 whereas the adsorption of the same rhodamine B on the surface of the same UC nanoparticles coated with a poly(ethyleneglycol)based polymer results in the successful shifting of the emission band to the longer-wavelength region by a radiative or nonradiative energy-transfer mechanism.23 In this work, the possibility of tuning the emission wavelength of upconverting nanoparticles using rhodamine dyes has been explored for PDT applications. To reduce the optical path between the upconverter and the fluorophore, the UC nanoparticles were decorated with titanate nanotubes (TiNT) coated with the dye. Such nanostructured composites have allowed efficient harvesting of UC emission, utilizing resonance-energy transfer between close-packed molecules of rhodamine on the surface of nanotubes, followed by the local emission of downshifted light. The reported structures demonstrate versatility and are promising for PDT applications.

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EXPERIMENTAL PROCEDURE

Reagents. Yttrium(III) chloride hexahydrate (YCl3·6H2O), ytterbium(III) chloride (YbCl3), erbium(III) chloride hexahydrate (ErCl3·6H2O), sodium trifluoroacetate (Na-TFA), lithium trifluoroacetate (Li-TFA), trifluoroacetic acid (H-TFA), oleic acid (OA), octadecene (OD), rhodamine B (RdB), rhodamine 101 (Rd 101), cetyltrimethylammonium bromide (CTAB), sodium hydroxide (NaOH), potassium hydroxide (KOH), hydrochloric acid (HCl), titanium(IV) dioxide, ethanol pure grade, toluene, and chloroform (CHCl3) were all obtained from Aldrich and were used without further purification. Preparation of Upconverting Nanoparticles. The method of preparation was adapted from Veggel et al.’s method of solvothermal decomposition25 modified by the use of trifluoroacetate salts of rare earth metals as a precursor.26 In a round-bottom three-necked flask, 10 cm3 of H-TFA was mixed with 1.785 g of YCl3, 0.425 g of YbCl3, and 0.0415 g of ErCl3 and refluxed at 75 °C under a flow of argon for 12 h, followed by evaporation of the solvent. The resulting solids were mixed with 60 cm3 of an OA and OD mixture (1:1 v/v), followed by the addition of 0.542 g of Li-TFA and 0.0.1524 g of Na-TFA under vigorous stirring at 125 °C until the complete dissolution of solids within approximately 30 min. The molar ratio between Li+ and Na+ in the mixture was 4:1, which corresponds to the maximum upconversion efficiency.26 The temperature of the solution was then increased to 300 °C at a rate of 10 °C min−1 under stirring at 100 rpm in an argon atmosphere. After 90 min, the temperature was decreased to 25 °C. UC nanoparticles were precipitated by the addition of ethanol and separated by decanting. The as-synthesized nanoparticles were purified several times by repeated dissolution in toluene followed by precipitation in ethanol. Decoration of UC Nanoparticles with TiNT Composites. The titanate nanotubes (TiNT) were prepared by the alkaline hydrothermal treatment of TiO2 in a 10 mol dm−3 mixture of KOH−NaOH (1:25 mol/mol) at 106 °C for 4 days following the procedure in ref 27. Adsorption isotherms of rhodamine dyes on the surface of TiNT were measured using a method described elsewhere.28 The resulting

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RESULTS AND DISCUSSION Radiative Energy Transfer from UC Nanoparticles to Fluorophores in Solution. For the successful application of UC nanoparticles in photodynamic therapy (PDT), it is essential that the intensity of upconverted light emitted by the particles in vivo is relatively large in order to drive the photochemical reaction of cytotoxic reactive oxygen generation by photosensitizers. This can be achieved by both raising the intensity of photoexciting NIR light and increasing the quantum yield of upconversion. Raising the intensity of the incident NIR light also increases the apparent efficiency of upconversion as a result of the nonlinearity of the process.2 However, the extremely high intensity of the NIR beam can also damage the surrounding tissue. Among various rare earth materials, powdered NaYF4 doped with 20% Yb3+ and 2% Er3+ has already demonstrated the highest (23%) internal quantum efficiency of upconversion under excitation at 980 nm with 40 W cm−2 intensity.31 Further improvement in the efficiency of doped NaYF4 luminescence could be achieved by the partial (80%) substitution of Na+ to Li+ ions, as shown by Nann et al.26 Such substitution is also accompanied by a transition of the nanoparticle crystal structure from cubic to tetragonal, as well as a modification of the shape of the nanocrystallites from spheroidal to polyhedral.32 Figure 1 shows TEM images of rare earth fluoride UC nanoparticles of general composition NaxLiyYF4:Yb3+,Er3+. The particles are characterized by a single-crystal structure and a polyhedral (mostly octahedral) 17420

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Figure 1. Electron microscope (a) TEM, (b) HRTEM, and (c) SEM images of NaxLiyYF4:Yb3+,Er3+ UC nanoparticles.

shape (Figure 1c). The HRTEM image in Figure 1b shows characteristic fringes of UC nanoparticles indicating an interplanar distance of 0.48 nm, which corresponds to the (101) crystallographic planes of tetragonal LiYF4.32 The typical size of the particles is approximately 50 nm, which is optimal for PDT because larger particles could result in a slower penetration rate thorough the cellular walls and smaller particles could result in a decreased UC efficiency due to surface recombination.33 The substitution of Na+ with Li+ in UC nanoparticles also results in a modification of the shape of the emission spectrum of the particles, leading to the domination of the 4S3/2 → 4I15/2 series bands (at ca. 550 nm) relative to the 4F3/2 → 4I15/2 bands (at ca. 650 nm).26 Although the total intensity of upconverted light is increasing, the emission at 650 nm does not significantly increase. At the same time, most PDT photosensitizers (e.g., Chlorin e6 and Protoporphyrin IX34,35) were developed to operate at wavelengths longer than 600 nm to avoid reabsorption by the hemoglobin of red blood cells. Therefore, in order to utilize the emission effectively from UC nanoparticles the series of bands between 500 and 550 nm should be shifted to the longer range of wavelengths that matches the absorbance spectrum of the photosensitizer. Figure 2 shows photoluminescence spectra of NaxLiyYF4:Yb3+,Er3+ UC nanoparticles suspended in chloroform excited with a NIR laser at 980 nm. The spectra feature several narrow bands in the region of 510−560 nm (2H11/2 → 4 I15/2 and 4S3/2 → 4I15/2 transitions) and lower-intensity bands at 650−670 nm (4F3/2 → 4I15/2 transitions). The addition of either rhodamine fluorophore results in a decrease in luminescence at 510−560 nm, giving rise to the broad emission bands at ca. 570 nm and ca. 590 nm for RdB and Rd101, respectively. The position and intensity of peaks at 650−670 nm remain constant. In both cases, the increase in fluorophore concentration leads to the shifting of the dye luminescence peak to a longer wavelength because of reabsorption. The initial rise in the integral intensity of dye luminescence is followed by its decline above a certain concentration, which is larger for Rd101 because of its lower extinction coefficient (Figure 1S in the Supporting Information). Visually, the addition of rhodamine dyes results in the change in the apparent emission color from green to red as seen in Figure 3.

Figure 2. Photoluminescence spectra of UC nanoparticles in CHCl3 (0.4 wt %) after the addition of (a) RdB (the concentration along the arrow is 0, 8 × 10−6, 2 × 10−5, 3.6 × 10−5, 5.6 × 10−5, and 8 × 10−5 mol dm−3) and (b) Rd101 (the concentration along the arrow is 0, 2 × 10−6, 6 × 10−6, 10−5, 1.7 × 10−5, 3 × 10−5, 4.5 × 10−5, 6.5 × 10−5, and 10−4 mol dm−3). The excitation wavelength is 980 nm.

Figure 3. Photographic images of the upconverting luminescence in (a) a 0.4 wt % colloidal solution of NaxLiyYF4:Yb3+,Er3+ UC nanoparticles in CHCl3 and (b) after the addition of 3 × 10−4 mol dm−3 rhodamine B. The excitation wavelength is 980 nm. Images were obtained without the use of optical filters.

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In principle, there are two possible mechanisms of energy transfer between UC nanoparticles and fluorophore molecules in our system, namely, radiative (via emission/absorption acts) and resonance (FRET). Because covering the surface of UC nanoparticles with oleic acid results in poor adsorption of the charged dye, most of the fluorophore molecules should remain evenly distributed in the surrounding solution. In such a situation, the energy-transfer mechanism is probably dominated by radiative-type rather than resonance-type FRET, which requires efficient coupling of electronic states and usually occurs at much smaller distances between the donor and acceptor. The improvements in fluorophore adsorption on the surface of UC nanoparticles may stimulate resonance-energy transfer; however, it would occur only between subsurface donors (rare earth ions) and fluorophore molecules adsorbed on the surface of the nanoparticles. The emission from the core of the UC particles can be transferred only to the fluorophores via a radiative mechanism because the size of the particles is too large to provide efficient FRET. Although a decrease in particle size may further increase the FRET proportion, it would also result in a decrease in the efficiency of upconversion resulting from surface recombination.33 The distance between a donor (rare earth ions) and acceptor (fluorophore molecule), at which 90% of the total upconverted energy is transferred via a radiative mechanism, can be estimated using the Beer−Lambert formula as L = 1/(ε × C), where ε is the extinction coefficient of the fluorophore and C is its concentration. The maximal intensity of fluorophore luminescence was observed at concentrations of added RdB and Rd101 of CRdB = 2 × 10−5 mol dm−3 and CRd101 = 6.5 × 10−5 mol dm−3, respectively. Using these concentrations and the values of ε at the maximum fluorophore absorption (Figure S1 in the Supporting Information), the average length of upconverted light propagation, where it is absorbed by the fluorophore dye, can be estimated to be 4.2 and 5.6 cm for RdB and Rd101, respectively ( Supporting Information). Such a large distance between the donor and acceptor makes the system less suitable for PDT applications, in which it is desirable that the generation and utilization of the light by photosensitizer occurs locally in order to avoid the interaction of that light with cell structures resulting in losing the efficiency of generating reactive oxygen species. Radiative Energy Transfer from UC Nanoparticles to Fluorophore-Decorated Titanate Nanotubes. One of the possible methods to decrease the distance between donor and acceptor is to avoid homogeneous distributions of the fluorophore in the solution by localizing it near the UC nanocrystals. The surface area of UC nanoparticles itself is insufficient to accommodate enough dye molecules to absorb 90% of the emitted light. However, the decoration of UC nanoparticles with high-surface-area nanostructures that have a good affinity for the fluorophores can increase the local fluorophore concentration. The recently discovered titanate nanotubes (TiNT)36 are characterized by a tubular morphology, high surface area, and high capacity to surface adsorb cationic dyes from aqueous solutions.28 They are characterized by single-crystal structure, moderate photocatalytic activity under UV, and no activity under vis or NIR light.36 Figure 4 shows the isotherm of Rd101 adsorption on the surface of TiNT suspended in ethanol at 25 °C. The isotherm of adsorption follows Langmuir-type adsorption

Figure 4. Isotherm of rhodamine 101 (Rd101) adsorption on the surface of titanate nanotubes from ethanol solution at 25 °C.

aRd101 =

a satK adsC Rd101 1 + K adsC Rd101

(1)

where aRd101 is the uptake of rhodamine 101 by the surface of TiNT expressed as mol(Rd101)/mol(TiNT), asat is the maximal uptake corresponding to a saturated surface of nanotubes with dye molecules forming a monolayer, Kads is an adsorption constant, and CRd101 is the equilibrium concentration of Rd101 in solution. The values of asat and Kads equal to 3.1 × 10−3 mol(Rd101) mol(TiNT)−1 and 1.55 × 105 dm3 mol−1, respectively, can be obtained by fitting the isotherm to eq 1. Surface saturation with Rd101 molecules occurs when their concentration, CRd101, exceeds 5 × 10−5 mol dm−3 and the uptake of rhodamine approaches asat. Using this value, we can estimate the density of the surface layer of dye as 1.15 × 10−13 cm−2 (estimation in Supporting Information), which is similar to the surface density of the molecule rhodamine 6G, a smaller molecule of the same family, on the surface of quartz (2.7 × 10−13 cm−2).37 Despite the strong interaction of Rd101 molecules with TiNT, their adsorption on the surface of nanotubes does not result in luminescence quenching. Figure 5 shows photoluminescence spectra of Rh101 in ethanol after the addition of a colloidal solution of TiNT. Within the range of added concentrations of TiNT, the shape of the spectrum remains constant, whereas the intensity of photoluminescence decreases to only a small extent. The Stern−Volmer expression of luminescence intensity (I) as a function of quencher (colloidal TiNT) concentration is shown in the inset of Figure 5. The plot is characterized by a nonlinear dependence, which is probably due to (i) light scattering from colloidal TiNT at their high concentration and (ii) the nonlinear isotherm of adsorption of Rd101 on the surface of TiNT. The inability of titanate nanotubes to quench the photoexcited states of Rd101 together with their ability to adsorb Rd101 molecules from the solution can be utilized in lightharvesting structures. Indeed, the dense decoration of nanotubes with fluorophore dye provides an efficient way to increase the local concentration of acceptors of UC emission, which can be estimated to be 0.24 mol dm−3 in solid titanate nanotubes covered with a monolayer of Rd101 (Supporting Information). Such a high local concentration of dye can significantly decrease the radius of the sphere within which 90% of UC emission is absorbed by the Rd101 from several centimeters in 17422

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Figure 5. Photoluminescence spectra of 2.5 × 10−5 mol dm−3 Rh101 in ethanol after the addition of a colloidal solution of TiNT at concentrations of 0, 6 × 10−6, 3 × 10−5, 6 × 10−5, 2.4 × 10−4, and 1.2 × 10−3 mol dm−3. The excitation wavelength is 480 nm, and the temperature is 25 °C. The inset shows the Stern−Volmer transform of luminescence intensity, I, at 595 nm as a function of TiNT (quencher) concentration.

Figure 6. Normalized kinetic curves of the fluorescence decay of Rd101 deposited on the surface of titanate nanotubes with different loadings: (a) 0.06, (b) 0.5, and (c) 2.5 mmol(Rd101) mol(TiNT)−1. The excitation wavelength is 485 nm. Luminescence is observed at 600 nm.

Fitting kinetic curves with 2 exponential decay functions and the calculation of the average time constant allows us to estimate the characteristic luminescence lifetime of Rd101 on the surface of TiNT as 2.51, 1.92, and 1.67 ns with respect to the above loadings. The characteristic luminescence lifetime of Rd101 in ethanol solution was determined as 2.56 ± 0.01 ns, which is very similar to the lifetime of Rd101 on the surface of TiNT at the lowest loading. By assuming an even distribution of the dye on the surface of the nanotubes, the average distance between the Rd101 molecules on the surface of the nanotubes can be estimated to be 3.3, 7.3, and 21 nm with respect to the above values of aRd101 (Supporting Information). As prepared, the UC nanoparticles are coated with oleic acid, causing the surface to be hydrophobic, in contrast to the surface of neat TiNT, which is hydrophilic.36 As a result, it is difficult to find a suitable solvent in which both UC and TiNT can form stable suspensions. Our first attempt to suspend both materials in the mixture of toluene and ethanol resulted in the formation of a composite with low uniformity (Figure S3 in the Supporting Information) in which UC and TiNT formed agglomerates without efficient mixing. The surface of TiNT, however, can become hydrophobic thorough the adsorption of a cationic surfactant such as cetyltrimethylammonium bromide (CTAB). These modified nanotubes can be suspended in lesspolar solvents. It has also been found that the length of the nanotubes can affect the quality of mixing between TiNT and UC. It was found that shorter nanotubes can form better coatings around UC nanoparticles compared to longer nanotubes because of the steric difficulties associated with the long tubes. Because TiNT are relatively brittle, their length can be decreased using ultrasonic treatment, resulting in their breakage, thus leading to compact packing.41 Figure 7 shows a TEM image of UC nanoparticles decorated with titanate nanotubes. The composite was prepared in chloroform, and the surface of the nanotubes was preliminarily modified by CTAB. The molar ratio in the composite was selected to decorate one UC nanoparticle with several nanotubes. The surface of nanotubes in the UC/TiNT composites can be further coated with Rd101 by adsorption from the chloroform solution although the presence of CTAB on the surface can decrease the efficiency of FRET for Rd101. Figure 8 shows the photoluminescence spectra of colloidal UC/TiNT

homogeneous solution to 480 nm in the composite layer (Supporting Information). Additionally, the close packing of dye molecules onto the surface of the nanotubes decreases the average distance between them. If the deposited dyes follow the crystalline order of the titanate surface, their orientation on the surface may favor the effective coupling of their molecular orbitals, resulting in resonance-energy transfer (FRET) between dye molecules. Such a phenomenon is known to occur in dyes embedded in the channel of zeolites mimicking the antenna system of self-assembled chlorophyll molecules during photosynthesis.38 To examine the possibility of FRET between Rd101 molecules in the Rd101/TiNT composite, the dynamics of luminescence decay was studied as a function of dye loading on the nanotube surfaces. An increase in dye loading decreases the average distance between molecules. Because the FRET rate constant strongly depends on the distance between molecules,39 an increase in dye loading may stimulate resonanceenergy transfer, which would decrease the characteristic lifetime of Rd101 luminescence. Figure 6 shows the fluorescence decay curves from Rd101 deposited on the surface of TiNT at three different uptake values aRd101 (0.06, 0.5, and 2.5 mmol(Rd101) mol(TiNT)−1). The curves follow a multiexponential decay function. The increase in Rd101 surface concentration results in an increase in the rate of the luminescence decay. Such a decrease in the fluorescence lifetime can be associated with concentration quenching, Rd101 dimer formation, or resonance-energy transfer between Rd101 molecules. However, concentration quenching can be excluded because the deposition of fluorophore on the surface of the nanotubes does not result in luminescence quenching over a wide range of surface concentration of Rd101 (Figure 5). We also do not observe dimers of Rd101 molecules after their deposition on the surface of TiNT, which would result in the appearance of the absorbance band at lower energies.40 Figure 2S in the Supporting Information indicates that the absorbance spectrum of fluorophore remains unchanged under the addition of a colloidal suspension of TiNT. 17423

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CONCLUSIONS The wavelength of light emitted from upconverting rare earth nanocrystals of NaxLiyYF4:Yb3+,Er3+ can be effectively tuned (downshifted) using rhodamine-type fluorescent dyes in the solution via a radiative transfer-energy mechanism. The optical distance between UC nanoparticles and fluorophore molecules dissolved in solution can exceed several centimeters, making them less suitable for in vivo biomedical applications because of the absorption of UC emission by the tissue. However, the optical distance can be decreased by employing titanate nanotubes (TiNT) as light-harvesting systems that have recently demonstrated a good affinity for cationic dyes. It has been found that the adsorption of rhodamine 101 (Rd101) on the surface of TiNT does not result in the quenching of its luminescence or the formation of dimers. Additionally, an increase in the surface coverage with Rd101 molecules results in a decrease in their luminescence lifetime, probably because of the resonance energy transfer between dye molecules. The decoration of UC nanoparticles with Rd101-coated TiNT allows a significant decrease in the distance between the UC emitter and the fluorophore to a few hundred nanometers. For the effective penetration of such nanostructures through the cellular walls, however, their current size probably has to be decreased. This can be achieved by optimizing the nanotube length and the size of the UC nanoparticles. Although the methods of binding between UC and TiNT and the possibility of resonance transfer between deposited dyes need to be studied and improved systematically, the presented work demonstrates the feasibility of nanostructured UC/TiNT/ Rd101 composites for effective wavelength tuning and the local generation of light suitable for photodynamic therapy.

Figure 7. TEM image (left-hand side) of UC nanoparticles decorated with titanate nanotubes (TiNT) coated with Rd101 and a schematic drawing (right-hand side) of the upconversion of NIR (980 nm) light and the downshifting of UC emitted (550 nm) light using Rd101 fluorophores on the surface of TiNT.

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ASSOCIATED CONTENT

* Supporting Information S

Additional details of isotherm of adsorption measurements and various estimations. Absorbance and photoluminescence spectra of ethanol solutions of RdB and Rd101. SEM image of neat UC and the TiNT composite precipitated from toluene/ethanol. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 8. Photoluminescence spectra of UC nanoparticles decorated with TiNT in CHCl3 after the addition of Rd101 (the concentration along the arrow is 0, 8 × 10−6, 1.6 × 10−5, 3.2 × 10−5, 6 × 10−5, and 1.12 × 10−4 mol dm−3). The excitation wavelength is 980 nm. The peak at 610 nm is the signal from the second harmonic from scattered excitation light.

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composites after the addition of Rd101 under excitation with NIR light. In a similar manner (see Figure 2 for comparison), the adsorption of Rd101 on the surface of nanotubes results in an extinction of the emission bands at 500−550 nm and the appearance of Rd101 characteristic bands at 600 nm. However, in the case of the composite, the downshifted light is emitted locally, which can be further utilized in PDT using established photosensitizers. Although CTAB-modified TiNT was able to form a uniform mixture with hydrophobic oleic acid-coated UC nanoparticles, for in vivo applications of such composites it would be better to modify their surface using various ligands.10 This would allow us to avoid the use of CTAB, which competes with Rd101 during adsorption on the surface of TiNT. Despite the fact that nanostructured TiO2 or titanates are characterized by photocatalytic activity in the reaction of oxidation of organic molecules, the application of NIR or vis light should not result in the accelerated photodecomposition of Rd101 because the energy of the photons is much smaller than the band gap of both TiO2 and titanates.

AUTHOR INFORMATION

Corresponding Author

*Tel: + 44 2380598358. Fax: + 44 2380598754. E-mail: d. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the EPSRC, UK (grant EP/F044445/1: “A hydrothermal route to metal oxide nanotubes: synthesis and energy conversion applications”) and The Big C Cancer Charity (grant 10-20R).

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REFERENCES

(1) Bloembergen, N. Solid State Infrared Quantum Counters. Phys. Rev. Lett. 1959, 2, 84−85. (2) Auzel, F. Upconversion and Anti-Stockes Processes with f and d Ions in Solids. Chem. Rev. 2004, 104, 139−174. (3) Joubert, M. F. Photon Avalanche Upconversion in Rare Earth Laser Materials. Opt. Mater. 1999, 11, 181−203. 17424

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dx.doi.org/10.1021/la304043d | Langmuir 2012, 28, 17419−17425