Controlled Direct Growth of Polymer Shell on Upconversion

Article pubs.acs.org/Macromolecules

Controlled Direct Growth of Polymer Shell on Upconversion Nanoparticle Surface via Visible Light Regulated Polymerization Ali Bagheri,† Hamidreza Arandiyan,† Nik Nik M. Adnan,‡ Cyrille Boyer,*,‡ and May Lim*,† †

School of Chemical Engineering, The University of New South Wales, Sydney, NSW 2052, Australia Centre for Advanced Macromolecular Design (CAMD) and Australian Centre for NanoMedicine (ACN), School of Chemical Engineering, UNSW Australia, Sydney, NSW 2052, Australia

‡

S Supporting Information *

ABSTRACT: Lanthanide-doped upconversion nanoparticles (UCNPs) have unique photoluminescent properties that are useful in many biomedical applications. Modification of UCNPs with a polymer layer can confer additional functionality such as biocompatibility, stability in vivo, or drug delivery capability. It is also important that the modification process can be controlled precisely and without having adverse effects on the UCNPs luminescence properties. Herein, a polymer shell was grafted directly from the surface of UCNPs (grafting from) via visible light (λmax = 635 nm, 0.7 mW/cm2) regulated photoenergy/electron transfer−reversible addition fragmentation chain transfer polymerization (PETRAFT). The polymerization kinetics, grafting density, and thickness of the surface-tethered polymer chains can be tuned precisely by adjusting the monomer and RAFT agent ratio or the light exposure time. This approach also permits temporal control of the polymerization process. That is, the polymerization process can be initiated, halted, or terminated by switching the light source on and off. By limiting the non-radiative decay caused by surface defects, as well as from vibrational deactivation from solvents, the polymer shell enhanced the upconversion luminescence of the silica-coated UCNPs. This investigation paves the way for the development of UCNPs with controlled properties for various application requirements.

1. INTRODUCTION Lanthanide-doped upconversion nanoparticles (UCNPs) are a class of photoluminescent nanoparticles with the ability to combine low-energy photons to produce a single high-energy photon via an upconversion process.1,2 Particularly, ytterbiumsensitized UCNPs adsorb low-energy near-infrared (NIR; 980 nm) photons and emit high-energy radiation in the ultraviolet (UV), visible, and shorter wavelengths of NIR (800 nm).3,4 This unique property enables a wide range of biomedical applications for UCNPs, such as biodetection, luminescent labeling, bioimaging, sensing, photodynamic therapy, luminescent probe, and drug/gene delivery.5−7 These applications require the incorporation of UCNPs with other functional materials to enhance and broaden their use.8,9 Notably is the modification of the UCNPs surface with polymers to create water-soluble and biocompatible nanoplatforms and to confer biocompatibility, size, and chemical stability in biological systems and the capability for subsequent functionalization with drugs and biomolecules.10,11 This can be achieved using a ligand exchange process, where hydrophobic capping oleic acid (OA) ligands are exchanged with preformed hydrophilic polymers such as poly(ethylene glycol) phosphate,12 poly(ethylene glycol) diacid,13 poly(acrylic acid),14 and polyvinylpyrrolidone10 or via ligand oxidation, where the OA is oxidized with the Lemieux−von Rudloff reagent.15 Another © XXXX American Chemical Society

approach relies on the encapsulation of UCNPs using amphiphilic block copolymers, where the polymers are attached to the UCNPs via van der Waals interactions between the hydrophobic block on the polymer and hydrophobic alkyl chains on the surface of UCNPs.16 Haupt, Sum Bui, and coworkers proposed to create a polymer shell in situ around UCNPs via photopolymerization. In their approach, upconverted UV or blue emissions from the UCNPs were used to initiate a free radical polymerization of 2-hydroethyl methacrylate (HEMA) and a cross-linker, N,N′-ethylenebis(acrylamide), in the presence of photoinitiator (benzophenone/TEA or eosin/TEA). The addition of co-monomers with functional groups allows binding of the polymers to UCNP surface and the functionalization with biomolecules.7 Despite advancements using these approaches, several challenges remain. First, the aforementioned methods cannot produce polymers with molecular diversity and controllability on the UCNPs surface. Second, in the ligand exchange approach, incomplete displacement of the surface ligands by the polymers/ligands can lead to particle aggregation, an effect that is highly undesirable for subsequent in vivo applications.17 Received: July 1, 2017 Revised: August 3, 2017

A

DOI: 10.1021/acs.macromol.7b01405 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Scheme 1. Reaction Scheme of the Surface Modification of RAFT Agent-Functionalized UCNPs and the Subsequent Grafting of a POEGMA Layer Using PET-RAFT Polymerization

thickness in the case of surface polymerization) of the polymer chains can be controlled in a very precise manner. To the best of our knowledge, PET-RAFT has not been applied to the modification of inorganic materials, including nanoparticles, and specifically to growth of polymer chains from the surfaces of UCNPs. Herein, we describe a strategy to grow polymer chains directly from the surface of NaYF4:Yb/Tm UCNPs using the PET-RAFT approach. In this approach, a RAFT agent, 4cyano-4-[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid (CDTPA), was first grafted onto the silica-coated UCNPs to yield UCNP@CDTPA (Scheme 1). The effects of monomer to UCNP@CDTPA ratios, as well as the light exposure time, on the polymerization kinetics, monomer conversion, polydispersity, grafting density, and thickness of the surface-tethered polymer chains were investigated. Furthermore, by switching the visible light on and off, we showed the thickness of the polymer layer can easily be controlled. Consequently, the “living” character of our surface-initiated PET-RAFT approach was demosntrated. We further showed that, in contrast to the conventional surface modification approaches, the dense polymer shell bound onto the surface of the UCNPs protected the dopant ions in the core from non-radiative decay caused by surface defects as well as from vibrational deactivation from solvents in the colloidal dispersions, leading to enhancement of the upconverted emission intensity.

Furthermore, in both the ligand exchange and encapsulation methods, the density and thickness of polymer on the UCNPs surface cannot be easily controlled.18,19 These two parameters can affect the physiochemical and photoluminescence properties of the UCNPs.20 A facile and general polymerization technique that can grow tightly tethered polymer chains with high grafting is therefore highly desirable. One strategy would be to use surface-initiated controlled living radical polymerization, otherwise known as the “grafting from” approach, where a chain transfer agent or initiator is attached on the surface of nanoparticles and tethered polymer chains are grown directly from the solid surfaces.21,22 Using the “grafting from” approach, a high degree of control over the polymerization kinetics, grafting density, and thickness of polymer brushes on the particle surface can be achieved.23,24 Another important advancement is the use of visible light to intiate controlled living radical polymerization. Indeed, several light regulated polymerization techniques have emerged, including photo-ATRP, photo-NMP, and photo-RAFT.25−32 These light-regulated polymerization techniques have been utilized in a variety of contexts and were very recently employed to engineer live cell surfaces,33 protein−polymer conjugates,34 and magnetic nanoparticle surfaces35 using the “grafting from” approach. PET-RAFT, photoenergy/electron transfer−reversible addition fragmentation chain transfer polymerization, the well-established RAFT polymerization, is initiated by visible light in the presence of photocatalysts.36 PET-RAFT is considered one of the most versatile techniques due to its ability to rapidly polymerize a wide number of monomers under a broad range of wavelengths.37,38 More importantly, through the ability to turn the visible light source on and off, temporal regulation of the polymerization reaction can be achieved. Thus, growth and therefore the length (or

2. EXPERIMENTAL SECTION 2.1. Synthesis of NaYF4:Tm/Yb Core Upconversion Nanoparticles. In a typical procedure for the synthesis of β-NaYF4:Yb/Tm, 0.78 mmol of YCl3, 0.2 mmol of YbCl3, and 0.02 mmol of TmCl3 were added to a 100 mL flask and dissolved in 2 mL of DI water to form a clear solution after vigorous stirring. After 6 mL of OA and 15 mL of 1-octadecene were added, the solution was heated to 100 °C for 10 B


Article

Macromolecules min and then to 156 °C for 30 min and then cooled to room temperature. A solution of 4 mmol of NH4F (0.1482 g) and 2.5 mmol of NaOH (0.1 g) in 10 mL of methanol was added, and then the solution was kept at 50 °C for 30 min. After methanol was evaporated, the solution was heated to 300 °C under an argon atmosphere for 1.5 h and then cooled to room temperature. The nanocrystals were precipitated with 10 mL of ethanol, collected after centrifugation, and redispersed in cyclohexane.39 2.2. Synthesis of Positively Charged UCNP@SiO2-NH2 Nanoparticles. As-prepared UCNP−cyclohexane solution (1.5 mL) was added into the mixture of Igepal CO-520 (1 mL) and cyclohexane (20 mL). After stirring for 3 h, ammonium hydroxide (150 μL, 30%) was added, and the mixture was stirred for another 2 h. A syringe pump was used to control the adding rate, and the solution (1 mL) composed of tetraethyl orthosilicate (TEOS) (50 μL) and cyclohexane (0.8 mL) was introduced into the system within 1 h. The system was hermetically stirred for 24 h. To graft amino groups, 50 μL of 3aminopropyltriethoxysilane (APTES) was added into the system and stirred for another 5 h. Then methanol was added to precipitate the product before collecting nanoparticle by centrifugation. The asprepared nanoparticle was redispersed in ethanol under ultrasonic treatment, precipitated with excess hexane, and collected by centrifugation. This procedure was repeated for three times to remove excess Igepal CO-520. The as-obtained nanoparticles were finally dispersed in 20 mL of deionized water.40 2.3. Preparation of RAFT Agent NHS Ester and UCNP@ CDTPA. This strategy was inspired by the literature method for the surface modification of magnetic iron oxide nanoparticles.41 To a solution of CDTPA RAFT agent (50 mg, 0.125 mmol) in THF, DCC (0.136 mmol), NHS (0.136 mmol), and DMAP (5 mg) were added and allowed to stir for 20 h (Scheme 1). After that, the mixture was filtered using a 0.44 μm syringe filter. The filtered CDTPA-NHS ester was used in the next step synthesis without further purification. The as-prepared UCNP@SiO2-NH2 (20 mg) was dissolved in THF (10 mL), and then CDTPA-NHS ester (200 μL) and DIPEA (3 μL) were added. The reaction was allowed to continue for 24 h under dark conditions at room temperature. Afterward, the CDTPA-anchored UCNPs were collected and washed by ethanol, THF, and diethyl ether for three times. 2.4. Synthesis of POEGMA Chains from the Surface of Nanoparticles Using the “Grafting From” Approach, e.g., UCNP@POEGMA-F100. 150 mg (0.5 mmol, 100 equiv) of OEGMA and 10 mg of RAFT grafted UCNPs (UCNP@CDTPA) (0.005 mmol of total RAFT agent; ∼1:4 grafted RAFT:free RAFT), presonicated in 1.9 mL of DMSO and 17 μg of ZnTPP in 100 μL of DMSO, were added to a reaction vial. The glass vial was covered with aluminum foil and degassed with nitrogen for 20 min. The degassed mixture was then irradiated under red (λ = 610−655 nm, 0.7 mW/cm2) light at room temperature. After overnight reaction (∼14 h), the reaction mixture was washed, sonicated, and centrifuged three times in order to isolate the POEGMA-grafted nanoparticles, e.g., UCNP@POEGMA-F50, UCNP@POEGMA-F100, and UCNP@POEGMA-F150. Aliquots of the reaction mixture were withdrawn and analyzed by 1H NMR to measure monomer conversion. Free polymers collected from the supernatant were characterized by GPC to determine number-average molecular weight (Mn,GPC) and molecular weight distribution (Mw/ Mn). For the sake of quantifying/comparing upconverted emission intensities particles concentration and laser power density were kept constant. 2.5. ON/OFF Study of Synthesis of POEGA Chains from the Surface of UCNPs Using the “Grafting From” Approach. 75 mg (0.25 mmol, 100 equiv) of OEGMA and 5 mg of RAFT grafted nanoparticles UCNP@CDTPA (0.0025 mmol of total RAFT agent), presonicated in 0.9 mL of DMSO and 11 μg of ZnTPP in 100 μL of DMSO, were added to a reaction vial. The glass vial was covered with aluminum foil and degassed with nitrogen for 20 min. The degassed mixture was then irradiated under red (λ = 610−655 nm, 0.7 mW/ cm2) light at room temperature. Aliquots were withdrawn for subsequent characterizations.

2.6. Synthesis of Block Copolymer Functionalized UCNP@ PBzMA-b-POEGMA. 0.97 g (5.5 mmol, 500 equiv) of BzMA and 10 mg of RAFT grafted nanoparticles UCNP@CDTPA (0.011 mmol of total RAFT agent), presonicated in 2 mL of DMSO and 50 μg of ZnTPP in 100 μL of DMSO, were added to a reaction vial. The glass vial was covered with aluminum foil and degassed with nitrogen for 20 min. The degassed mixture was then irradiated under red (λ = 610− 655 nm) light at room temperature. After 20 h reaction, 0.3 mL of reaction aliquot was taken for analysis (conversion of ∼89%). Subsequently, a solution of 0.3 mL of DMSO containing 0.82 g of OEGMA monomer (250 equiv to RAFT) and 20 μg of ZnTPP was added to the reaction mixture and purged by N2 for 20 min. The degassed mixture was then irradiated under red (λ = 610−655 nm, 0.7 mW/cm2) light at room temperature for 8 h. Aliquots of the reaction mixture were withdrawn and analyzed. Free polymers collected from the supernatant were characterized by GPC to determine numberaverage molecular weight (Mn,GPC) and molecular weight distribution (Mw/Mn).

3. RESULTS AND DISCUSSION 3.1. Surface Modification of UCNPs with Silica and RAFT Agent. UCNPs were prepared via a thermal decomposition method in the presence of oleic acid (OA) as previously reported in the literature.42 TEM micrograph (Figure 1a) shows the synthesis of β-NaYF4 nanocrystals

Figure 1. TEM images of (a) NaYF4:Yb,Tm nanoparticles and (b) modified UCNP@SiO2-NH2 nanoparticles. (c) XPS results of UCNPOA, UCNP@SiO2-NH2, and UCNP@CDTPA.

doped with Yb3+ and Tm3+ with typical size of 40 nm (40 ± 5 nm, n = 100 particles). Subsequently, the UCNPs were coated with a silica shell (UCNP@SiO2) using a procedure described in the literature.39 The silica layer was then modified with APTES to yield UCNP@SiO2-NH2. Amine groups on the UCNP@SiO2 surface were confirmed by the presence of amino vibrational modes between 1610 and 1460 cm−1 in the FTIR spectra (Figure S1a).43,44 In addition, the broad peak at 3415 cm−1 could be attributed to the stretching and bending vibration bands of amine groups, which is in agreement with C


Article

Macromolecules previous reports.43,44 In addition, a shift in the zeta-potential values from negative (−19.9 mV for UCNP@SiO2) to positive values after functionalization verified the presence of amine groups on the surface (Figure S1b). The silica-coated UCNPs that were modified with amines were susceptible to aggregation due to the electrostatic interactions between the positively charged amines and the negatively charged surface silanol groups present on the particles surfaces, which is in agreement with previous reports.45,46 TEM micrographs of the modified nanoparticles show a thin and uniform coating of silica layer (thickness of approximately 3−5 nm) on the UCNPs (Figure 1b and Figure S2b,c). Dynamic light scattering (DLS) measurements were also carried out to further measure the size and the dispersity of the modified nanoparticles (Figure S1c). After coating the UCNPs with silica and further functionalization with APTES molecules, DLS showed an increase of the size of these nanoparticles in agreement with TEM data. Surface area and total pore volume of UCNP@SiO2 were also determined to be 138.7 m2 g−1 and 0.27 cm3 g−1, respectively, by Brunauer−Emmet−Teller (BET) analysis (Figure S4). UCNP@SiO2-NH2 was then modified via N,N’-dicyclohexylcarbodiimide (DCC) coupling between the carboxylic group of the RAFT agent (CDTPA) and the amine groups on the surface of particles (designated UCNP@CDTPA, Scheme 1). After grafting, we observed a slight change of color from white to pale yellow. More importantly, the particles were easily dispersed in different organic solvents (DMSO, THF, etc.). The substitution of the amine groups by the RAFT agent resulted in a decrease of zeta potential from +24.9 to −10.9 mV, which indicates that the CDTPA RAFT agent modification was successful (Figure S1b). The surface chemical composition of the nanoparticles at various stages of surface modification was also determined by X-ray photoelectron spectroscopy (XPS). XPS spectra of OA-capped UCNPs confirmed the presence of fluoride (F 1s), sodium (Na 1s), and yttrium (Y 3d5), which is consistent with the composition of NaYF4 matrix of UCNPs (Figure 1c). After modification with a silica layer and further functionalization with APTES, XPS analysis of UCNP@SiO2-NH2 revealed the presence of additional signals, including silica (Si 2p), oxygen (O 1s), carbon (C 1s), and nitrogen (N 1s). The presence of a N 1s peak centered at 399.31 and 400.81 eV indicated the presence of APTES molecules on the surface of nanoparticles, which is in agreement with values reported in the literature.47,48 After functionalization with RAFT agent, the XPS spectra of UCNP@CDTPA also verified the presence of F 1s, Si 2p, O 1s, N 1s, and C 1s peaks. In addition, successful conjugation of CDTPA onto the APTES layer was confirmed by the appearance of characteristic sulfur (S 2p) peaks, which can be fitted into two components with binding energies at about 163.55 eV (C−S) and 161.73 eV (CS) (Figure 1c).49 Interestingly, after surface modifications, the signals corresponding to the UCNPs became much weaker, indicating the formation of a silica layer. It is worthwhile to mention that the sampling depth of XPS was ∼4 nm; thus, the response of surface grafted UCNPs was expected to be strongly weakened compared with that of the bulk.48 Thermogravimetric analysis (TGA) was performed before and after RAFT functionalization to evaluate the RAFT density on the surface of these nanoparticles. As the values of weight loss (measured by TGA) of the modified particles is greatly dependent on various factors (such as thickness of the silica

layer, content of the covalently bounded APTES and CDTPA molecules, etc.),50 multiple measurements were taken to ensure provision of more precise values (Figure S5a,b). The amount of CDTPA RAFT agent grafted onto the surface of the UCNP@ SiO2-NH2 nanoparticles was then calculated based on the residual weight at 650 °C, as determined by TGA (see general calculations; eqs S1−S3), and used to calculate the amount of RAFT agent added in the system. This technique has been commonly used for similar systems in the literature.41,51 3.2. Effects of OEGMA Monomer and UCNP@CDTPA Ratio on the POEGMA Grafting Density and Thickness. POEGMA chains were successfully “grafted from” the surface of CDTPA-modified UCNPs via PET-RAFT polymerization in the presence of ZnTPP as an efficient photocatalyst36,52,53 under red LED light (λmax = 635 nm, 0.7 mW/cm2) irradiation (Scheme 1). DMSO was found to be the ideal solvent for dispersing the CDTPA-modified UCNPs. To improve the control of the polymerizations, we added additional RAFT agent in the polymerization solution. Indeed, it has been reported that surface-initiated polymerization without addition of excess of RAFT agent (or sacrificial RAFT agent) in solution results in high dispersity of grafted polymer and low efficiency of the chain transfer process, leading to an uncontrolled polymerization system.54 The presence of “sacrificial” RAFT agent allows an efficient exchange reaction between the graft and free polymer leading to good control of propagation of polymer chains, i.e., to yield low-dispersity polymers. Another advantage of using free RAFT agent is that it can increase the viscosity of the polymerization system due to the formation of free polymers produced by the sacrificial RAFT agent. This prevents interparticle coupling by reducing particle diffusion.23 The characteristics of chains anchored to the particle surfaces were assumed to be similar to the polymers in solution (specially at lower conversions), which was commonly used in the literature for “grafting from” systems using controlled polymerizations.23,55−60 Thus, sacrificial CDTPA was introduced in the reaction solution to control the polymerization. Free polymers were isolated and analyzed in order to estimate the molecular weights and polymer dispersities of the grafted polymer on the surface. Initial tests were performed using different molar ratios of [OEGMA] to [UCNP@CDTPA] + [sacrificial CDTPA], i.e., 50:1, 100:1, and 150:1, denoted as UCNP@POEGMAF50, -F100, and -F150, respectively (Table 1). Comparison between surface-grafted and free polymer in preliminary studies found no appreciable differences in the number-average molecular weight (Mn,GPC) and molecular weight distribution (Mw/Mn). Therefore, the free polymer was used to provide some information on the grafted polymer. Figure 2a shows the molecular weight distribution of free polymers. A good correlation was observed between the theoretical and experimental molecular weights (Table 1). More importantly, the polymer dispersities remained low (Mw/Mn < 1.21) despite the high monomer conversions (α > 80%). The TGA showed different amount of POEGMA was grafted onto the UCNPs as indicated by different weight loss (∼24, 32, and 38 wt % (±2%) corresponding to the polymer layer) (Figure S5b). The corresponding grafting densities of UCNP@POEGMA-F50, -F100, and -F150 calculated using the weight loss and the specific surface area of UCNP@SiO2 (from BET analysis) (Table 1, for general calculations see eqs S4 and S5).51 As determined from TGA analysis, an approximately constant value of grafting density was obtained with different target D


Article

Macromolecules Table 1. Results of Polymers Synthesized by the “Grafting From” Approach Using PET-RAFT Polymerization samples UCNP@ POEGMAF50 UCNP@ POEGMAF100 UCNP@ POEGMAF150

Mn,theo α (%)a (g/mol)b

Mn,GPC (g/mol)c

Mw/Mnc

grafting density (chains/nm2)d

86

13300

14500

1.16

0.094 ± 0.012

78

24100

24650

1.19

0.079 ± 0.012

76

34600

36800

1.21

0.072 ± 0.012

transfer between polymer chains. This is commonly reported in grafting from approach for ATRP and RAFT polymerization that a significant amount of ATRP or RAFT agents cannot initiate the polymerizations.51 TEM micrographs after polymerization show the presence of polymer shells of different thicknesses around the particles (Figure 2b−d). The thickness of the polymer shell increased from approximately 7 ± 1, 14 ± 3, and 20 ± 4 nm for UCNP@ POEGMA-50, -100, and -150, respectively. The EDS spectrum of the UCNP@POEGMA-F50 (Figure S6) shows very strong peaks of all the doped lanthanides (Yb, Y, Tm) and relatively strong peaks of F and Na, which indicates the existence of NaYF4:Yb/Tm in the nanohybrids. The high intensities of these peaks in the EDS spectrum suggest that the prepared polymer coated particles possess efficient upconversion fluorescence. The presence of peaks for other elements such as Si and C in the EDS spectrum confirms the formation of the silica layer and polymer chains tethered to the surface of nanoparticles. Element mapping (Figure 3a−i) of Na, F, Y, Yb, Tm, Si and C confirms the presence of silica layer and polymer chains around the UCNPs. The polymerization kinetics of POEGMA were determined (see Table S2). Figure 4a shows the evolution of the numberaverage molecular weight (Mn) and the polydispersity index (Mw/Mn) of the free polymer as a function of monomer conversion. The molecular weight increases linearly with monomer conversion together with low dispersity until 69% conversion (where polymerization was stopped, Figure S7), demonstrating good control/livingness of PET RAFT polymerization using this approach (Figure 4a and Table S2). Mn,GPC is in agreement with Mn,theo (see Table S2). The apparent propagation rate constants (kpapp) were also dependent on the concentration of the introduced ZnTPP photocatalyst (Figure 4b), which is consistent with previous reports in the literature.38 Higher concentration of the catalyst stimulated polymerization at a higher rate. Nonetheless, when the catalyst concentration exceeded 100 ppm (relative to the monomer concentration), the polymerization rate was no longer controlled. This can be attributed to detrimental self-quenching effect of the photoredox catalyst at high concentration.38 The ability to initiate, halt, or terminate the polymerization process by switching the light source on and off was investigated. Figure 5a shows temporal control of the polymer thickness grafted on the surface of UCNP@POEGMA-F100 by switching the light source off for 2 h at the time points of 3, 7, and 12 h. As can be seen from Figure 5b, the system remained dormant with no polymerization taking place in the absence of light. Switching the light back “ON” reactivated the polymerization process. These “activation” and “deactivation” processes enabled highly fine-tuned and facile control over the polymer thickness on the surface of the UCNP by simply switching the light source “ON” and “OFF” (Figure 5i−iii). The ability of visible light mediated surface-initiated RAFT polymerization to graft from diblock copolymers was also explored to demonstrate the versatility of PET-RAFT. A diblock copolymer was synthesized via chain extension of POEGMA layer grafted onto UCNPs with polybenzyl methacrylate (PBzMA; see the Experimental Section). Successful chain extension was confirmed by a complete shift of the molecular weight distribution before and after chain extension (Figure 6a). TEM micrographs confirm the growth of the polymer chains. A uniform 9 ± 2 nm thick PBzMA layer was observed before chain extension; after chain extension, a 18

a

OEGMA conversion determined by NMR (see Supporting Information). bTheoretical molecular weights determined by the following equation: Mn,theo = ([OEGMA]/([UCNP@CDTPA] + [free RAFT]) × αOEGMA × MWOEGMA + MWRAFT, with αOEGMA, MWOEGMA, and MWRAFT agent correspond to OEGMA conversion and molar mass of OEGMA and CDTPA, respectively. [UCNP@CDTPA]:[free CDTPA] = 1:4. cNumber-average molecular weight and dispersity (Mw/Mn) of free POEGMA corresponding UCNP@POEGMA-Fn after polymerization overnight determined by GPC using polystyrene d Grafting density calculated by grafting density = [(weight-loss/ Mn,GPC) × Na]/[mUCNPs × SUCNPs], where Mn,GPC, Na, mUCNPs, and SUCNPs correspond to number-average molecular weight, Avogadro number, mass of silica-coated UCNPs, and the specific surface area of silica-coated UCNPs, respectively. The reactions were performed at room temperature overnight under a 5 W red LED light (λ max = 635 nm) in DMSO using ZnTPP as catalyst with 50 ppm concentration relative to monomer.

Figure 2. (a) Molecular weight distribution of the corresponding free POEGMA homopolymers from UCNP@POEGMA-Fn and TEM images of (b) UCNP@POEGMA-F50, (c) UCNP@POEGMA-F100, and (d) UCNP@POEGMA-F150.

molecular weights (Table 1). A slight decrease of the grafting density with an increase of the molecular weight was attributed to an increase of the steric hindrance. These values are consistent with commonly observed grafting densities using the “grafting from” approach for other systems.51 Interestingly, we observe a significant difference between the grafting density of CDTPA units (1.4−2.3 molecule/nm2) and polymer chains (∼0.09 chain/nm2) on UCNPs, which is attributed to the steric hindrance of the polymer chains, limiting the reaction of E


Article

Macromolecules

Figure 3. (a) TEM and (b−i) element mapping images of the sample UCNP@POEGMA-F50.

Figure 4. Surface-initiated PET-RAFT polymerization of POEGMA at room temperature under red light irradiation. (a) Evolution of Mn and Mw/ Mn versus monomer conversion, using molar ratio of [OEGMA]:[UCNP@CDTPA]:[ZnTPP] = [50]:[1]:[3.25 × 10−3] in DMSO. (b) Plot of ln([M]0/[M]t) vs exposure time at two different ZnTPP concentrations (70 and 50 ppm relative to monomer concentration).

± 4 nm thick PBzMA-b-POEGMA layer can be seen (Figure 6b and i−ii). It should be noted that the presence of RAFT end group can be easily converted into a thiol, which provides the ability for further conjugation of biomolecules.61,62 3.3. Effect of Hydrophilic Polymer Shell on the Optical Properties of UCNPs. Both the as-synthesized (NaYF4:Yb,Tm) UCNPs and UCNP@SiO2-NH2 showed similar upconverted spectral character under the illumination of a 980 nm laser (Figure S1d) in accord with previous literature reports.63−65 Emissions at 475 and 650 nm can be ascribed to three-photon processes ( 1G4 to 3 H6 and 1G4 to 3 F4 , respectively), and emissions at 695 and 800 nm are twophoton pathways (3F2,3 to 3H6 and 3H4 to 3H6, respectively). This upconverted emission spectrum corresponds to what has been reported previously for Tm3+.66−69 However, the upconversion luminescence intensity of the UCNP@SiO2NH2 particles was lower than the OA-UCNPs. This decrease in fluorescence can be attributed to (1) light-scattering effect on both emission and excitation light by the presence of silica layer

and (2) the strong Si−O stretching, which affected the emission intensities through multiphoton relaxation.40,70−72 To overcome this problem, a thin layer of silica was generated on the particles surface to lower the loss of upconverting light emissions. In order to confirm the detected features were due to the upconversion process, a blank sample consisting of zirconium oxide was also illuminated under a 980 nm laser and showed no apparent upconverted emissions (Figure S3).73 The optical properties of the polymer-coated UCNPs were then studied in order to evaluate the effects of grafted hydrophilic POEGMA shell on the anti-Stokes emissions of NaYF4:Yb,Tm UCNPs. It should be noted that the ZnTPP which was used as a photocatalyst (to initiate the PET-RAFT polymerization under external visible light) has to be removed at the end of the polymerization. Indeed, the presence of photocatalyst residue would affect the physical and chemical properties of the polymer products and especially hinder biomedical applications.74 Thus, POEGMA-grafted nanoparticles were washed, sonicated, and centrifuged three times F


Article

Macromolecules

Figure 5. “ON−OFF” study of surface-initiated PET-RAFT polymerization of POEGMA. (a) Increase of polymer thickness vs time of light exposure (polymerization time) (HR TEM images corresponding to UCNP@ POEGMA-F100 after (i) 3 h, (ii) 5 h, and (iii) 8 h total red light exposure with 2 h interval/off without light exposure). (b) Plot of ln([M]0/[M]t) vs time of exposure. The reactions were performed at room temperature under 5 W red LED light (λmax= 635 nm, 0.7 mW/cm2) in DMSO using [POEGMA]:[UCNP@CDTPA]:[ZnTPP] = [100]:[1]:[6 × 10−3].

Figure 6. (a) Molecular weight distributions of corresponding free polymers of UCNP@PBzMA and UCNP@PBzMA-b-POEGMA synthesized by grafting from the approach using PET-RAFT in the presence of BzMA, followed by chain extension with OEGMA. (b) Grafted polymer thickness before and after chain extension; (i−ii) HR TEM images of the UCNP@PBzMA and UCNP@PBzMA-b-POEGMA.

in order to isolate purified particles. As expected, the corresponding upconversion luminescence spectrum of the designed POEGMA-coated UCNPs is similar to that of the CDTPA-modified nanoparticles (Figure 7a). These results strongly indicate that the characteristic upconversion property of the nanoparticles was unaffected by the polymer coating. More importantly, there was an enhancement of the upconversion luminescent intensity in the presence of the

POEGMA polymer shell, and the upconverted luminescent intensifies with increased polymer thickness (Figure 7b and Figure S8a,b). This increase in the upconverted luminescent intensity can be attributed to two main factors. First, as the polymer density increases, the degree of stability of the colloidal solution increases, leading to less aggregation and a reduction in precipitation/sedimentation of the nanoparticles, which consequently increase the number of nanoparticles in the path G


Article

Macromolecules

Figure 7. Room temperature visible-range upconversion spectrum of (a) UCNP@POEGMA-F50 and -F100 under the excitation of 980 nm NIR laser. For comparison, the upconversion emission spectrum of the corresponding nanocrystals before polymer coating (UCNP@CDTPA) is shown. (b) UCNP@POEGMA-F50 at different polymerization times and their corresponding monomer conversion (note: the emission spectra above 750 nm are over the range).

of the laser beam. Second, the dense layers of grafted polymer reduce the interaction between solvent and the surface of UCNPs. This has the effect of reducing quenching that is due to high-energy oscillators (e.g., surface impurities, ligands, and solvent molecules).75−78 The use of surface passivation to suppress quenching has been demonstrated previously.17,79 Here, the grafted polymer shielded the luminescent ions in the UCNPs core (especially those near the surface) from nonradiative decay caused by surface defects as well as from vibrational deactivation due to the solvents.16,80−84

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.L.). *E-mail: [email protected] (C.B.). ORCID

Ali Bagheri: 0000-0003-3484-5856 Cyrille Boyer: 0000-0002-4564-4702

4. CONCLUSION We demonstrated for the first time that highly efficient visible light mediated controlled/living PET-RAFT polymerization can be used to grow polymer chains directly on the surface of inorganic materials (“grafting from”), in this case, lanthanidedoped UCNPs. This polymerization process rendered these nanoparticles water dispersible. We also demonstrated that the grafting density and thickness of the polymer chains can be tuned precisely by adjusting the polymerization components and the light exposure time. The polymerization process, and therefore the thickness of the polymer chains, can also be initiated, halted, or terminated by switching the visible light on or off. The upconversion emission spectrum of the polymer coated particles was not affected by the grafting process, and the upconversion properties were conserved. More importantly, the polymer shell limits the accessibility of water on the UCNP surface, thus preventing a non-radiative dissipation. These findings would be useful for a variety of applications in the design of nanoplatforms that require accurate control on the hybrid of organic/inorganic materials. In addition, this study paves the way for the development of new generations of UCNPs, which can be decorated with diverse polymers that have tunable thicknesses to meet various application requirements. Investigations of monomer scope and extension of synthetic versatility of this system for the preparation of increasingly complex functional materials are currently underway.

■

TGA curve, NMR spectra of polymer, EDS analysis, and upconversion emission spectra (PDF)

Notes

The authors declare no competing financial interest.

■

REFERENCES

(1) Lu, Y.; Zhao, J.; Zhang, R.; Liu, Y.; Liu, D.; Goldys, E. M.; Yang, X.; Xi, P.; Sunna, A.; Lu, J.; Shi, Y.; Leif, R. C.; Huo, Y.; Shen, J.; Piper, J. A.; Robinson, J. P.; Jin, D. Tunable Lifetime Multiplexing Using Luminescent Nanocrystals. Nat. Photonics 2013, 8 (1), 32−36. (2) Wang, F.; Liu, X. Recent Advances in the Chemistry of Lanthanide-Doped Upconversion Nanocrystals. Chem. Soc. Rev. 2009, 38 (4), 976−989. (3) Shen, J.; Chen, G.; Vu, A. M.; Fan, W.; Bilsel, O. S.; Chang, C. C.; Han, G. Engineering the Upconversion Nanoparticle Excitation Wavelength: Cascade Sensitization of Tri-Doped Upconversion Colloidal Nanoparticles at 800 Nm. Adv. Opt. Mater. 2013, 1 (9), 644−650. (4) Xie, X.; Gao, N.; Deng, R.; Sun, Q.; Xu, Q.; Liu, X. Mechanistic Investigation of Photon Upconversion in Nd(3+)-Sensitized Core-Shell Nanoparticles. J. Am. Chem. Soc. 2013, 135 (34), 12608−12611. (5) Wu, X.; Zhang, Y.; Takle, K.; Bilsel, O.; Li, Z.; Lee, H.; Zhang, Z.; Li, D.; Fan, W.; Duan, C.; Chan, E. M.; Lois, C.; Xiang, Y.; Han, G. Dye-Sensitized Core/active Shell Upconversion Nanoparticles for Optogenetics and Bioimaging Applications. ACS Nano 2016, 10 (1), 1060−1066. (6) Bagheri, A.; Arandiyan, H.; Boyer, C.; Lim, M. Lanthanide-Doped Upconversion Nanoparticles: Emerging Intelligent Light-Activated Drug Delivery Systems. Adv. Sci. 2016, 3, 71500437 10.1002/ advs.201500437 10.1002/advs.20167003DOI (7) Beyazit, S.; Ambrosini, S.; Marchyk, N.; Palo, E.; Kale, V.; Soukka, T.; Tse Sum Bui, B.; Haupt, K. Versatile Synthetic Strategy for Coating Upconverting Nanoparticles with Polymer Shells through Localized Photopolymerization by Using the Particles as Internal Light Sources. Angew. Chem., Int. Ed. 2014, 53 (34), 8919−8923. (8) Li, X.; Zhang, F.; Zhao, D. Lab on Upconversion Nanoparticles: Optical Properties and Applications Engineering via Designed Nanostructure. Chem. Soc. Rev. 2015, 44 (6), 1346−1378. (9) Wang, C.; Cheng, L.; Liu, Z. Drug Delivery with Upconversion Nanoparticles for Multi-Functional Targeted Cancer Cell Imaging and Therapy. Biomaterials 2011, 32 (4), 1110−1120.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01405. Chemicals, characterizations, experimental setups, grafting density calculation, and additional figures including H


Article

Macromolecules (10) Johnson, N. J. J.; Sangeetha, N. M.; Boyer, J.-C.; van Veggel, F. C. J. M. Facile Ligand-Exchange with Polyvinylpyrrolidone and Subsequent Silica Coating of Hydrophobic Upconverting βNaYF4:Yb3+/Er3+ Nanoparticles. Nanoscale 2010, 2 (5), 771−777. (11) Bogdan, N.; Vetrone, F.; Roy, R.; Capobianco, J. A. Carbohydrate-Coated Lanthanide-Doped Upconverting Nanoparticles for Lectin Recognition. J. Mater. Chem. 2010, 20 (35), 7543−7550. (12) Boyer, J. C.; Manseau, M. P.; Murray, J. I.; Van Veggel, F. C. J. M. Surface Modification of Upconverting NaYF4 Nanoparticles with PEG-Phosphate Ligands for NIR (800 Nm) Biolabeling within the Biological Window. Langmuir 2010, 26 (2), 1157−1164. (13) Yi, G. S.; Chow, G. M. Synthesis of Hexagonal-Phase NaYF4:Yb,Er and NaYF 4:Yb,Tm Nanocrystals with Efficient upConversion Fluorescence. Adv. Funct. Mater. 2006, 16 (18), 2324− 2329. (14) Liu, Y.; Tu, D.; Zhu, H.; Li, R.; Luo, W.; Chen, X. A Strategy to Achieve Efficient Dual-Mode Luminescence of Eu3+in Lanthanides Doped Multifunctional NaGdF4 Nanocrystals. Adv. Mater. 2010, 22 (30), 3266−3271. (15) Chen, Z.; Chen, H.; Hu, H.; Yu, M.; Li, F.; et al. Versatile Synthesis Strategy for Carboxylic Acid-Functionalized Upconverting Nanophosphors as Biological Labels Versatile Synthesis Strategy for Carboxylic Biological Labels. J. Am. Chem. Soc. 2008, 130 (10), 3023− 3029. (16) Sedlmeier, A.; Gorris, H. H. Surface Modification and Characterization of Photon-Upconverting Nanoparticles for Bioanalytical Applications. Chem. Soc. Rev. 2015, 44 (6), 1526−1560. (17) Li, Z.; Guo, H.; Qian, H.; Hu, Y. Facile Microemulsion Route to Coat Carbonized Glucose on Upconversion Nanocrystals as High Luminescence and Biocompatible Cell-Imaging Probes. Nanotechnology 2010, 21 (31), 315105. (18) Viger, M. L.; Grossman, M.; Fomina, N.; Almutairi, A. Low Power Upconverted near-IR Light for Efficient Polymeric Nanoparticle Degradation and Cargo Release. Adv. Mater. 2013, 25 (27), 3733−3738. (19) Yan, B.; Boyer, J. C.; Branda, N. R.; Zhao, Y. Near-Infrared Light-Triggered Dissociation of Block Copolymer Micelles Using Upconverting Nanoparticles. J. Am. Chem. Soc. 2011, 133 (49), 19714−19717. (20) Liu, B.; Chen, Y.; Li, C.; He, F.; Hou, Z.; Huang, S.; Zhu, H.; Chen, X.; Lin, J. Poly(Acrylic Acid) Modification of Nd 3+ -Sensitized Upconversion Nanophosphors for Highly Efficient UCL Imaging and pH-Responsive Drug Delivery. Adv. Funct. Mater. 2015, 25 (29), 4717−4729. (21) Khabibullin, A.; Bhangaonkar, K.; Mahoney, C.; Lu, Z.; Schmitt, M.; Sekizkardes, A. K.; Bockstaller, M. R.; Matyjaszewski, K. Grafting PMMA Brushes from α-Alumina Nanoparticles via SI-ATRP. ACS Appl. Mater. Interfaces 2016, 8 (8), 5458−5465. (22) Zhao, Y.; Perrier, S. Reversible Addition-Fragmentation Chain Transfer Polymerization from Surfaces. Adv. Polym. Sci. 2015, 270, 77−106. (23) Ohno, K.; Ma, Y.; Huang, Y.; Mori, C.; Yahata, Y.; Tsujii, Y.; Maschmeyer, T.; Moraes, J.; Perrier, S. Surface-Initiated Reversible Addition-Fragmentation Chain Transfer (RAFT) Polymerization from Fine Particles Functionalized with Trithiocarbonates. Macromolecules 2011, 44 (22), 8944−8953. (24) Ciftci, M.; Batat, P.; Demirel, A. L.; Xu, G.; Buchmeiser, M.; Yagci, Y. Visible Light-Induced Grafting from Polyolefins. Macromolecules 2013, 46 (16), 6395−6401. (25) Chen, M.; Zhong, M.; Johnson, J. A. Light-Controlled Radical Polymerization: Mechanisms, Methods, and Applications. Chem. Rev. 2016, 116 (17), 10167−10211. (26) Leibfarth, F. A.; Mattson, K. M.; Fors, B. P.; Collins, H. A.; Hawker, C. J. External Regulation of Controlled Polymerizations. Angew. Chem., Int. Ed. 2013, 52 (1), 199−210. (27) Fors, B. P.; Hawker, C. J. Control of a Living Radical Polymerization of Methacrylates by Light. Angew. Chem., Int. Ed. 2012, 51 (35), 8850−8853.

(28) Pan, X.; Fang, C.; Fantin, M.; Malhotra, N.; So, W. Y.; Peteanu, L. A.; Isse, A. A.; Gennaro, A.; Liu, P.; Matyjaszewski, K. Mechanism of Photoinduced Metal-Free Atom Transfer Radical Polymerization: Experimental and Computational Studies. J. Am. Chem. Soc. 2016, 138 (7), 2411−2425. (29) McKenzie, T. G.; Fu, Q.; Wong, E. H. H.; Dunstan, D. E.; Qiao, G. G. Visible Light Mediated Controlled Radical Polymerization in the Absence of Exogenous Radical Sources or Catalysts. Macromolecules 2015, 48 (12), 3864−3872. (30) Chen, M.; Gu, Y.; Singh, A.; Zhong, M.; Jordan, A. M.; Biswas, S.; Korley, L. T. J.; Balazs, A. C.; Johnson, J. A. Living Additive Manufacturing: Transformation of Parent Gels into Diversely Functionalized Daughter Gels Made Possible by Visible Light Photoredox Catalysis. ACS Cent. Sci. 2017, 3 (2), 124−134. (31) Chen, M.; Deng, S.; Gu, Y.; Lin, J.; MacLeod, M. J.; Johnson, J. A. Logic-Controlled Radical Polymerization with Heat and Light: Multiple-Stimuli Switching of Polymer Chain Growth via a Recyclable, Thermally Responsive Gel Photoredox Catalyst. J. Am. Chem. Soc. 2017, 139 (6), 2257−2266. (32) Shen, L.; Lu, Q.; Zhu, A.; Lv, X.; An, Z. Photocontrolled RAFT Polymerization Mediated by a Supramolecular Catalyst. ACS Macro Lett. 2017, 6 (6), 625−631. (33) Niu, J.; Lunn, D.; Pusuluri, A.; Yoo, J.; O’Malley, M. A.; Mitragotri, S.; Soh, H. T.; Hawker, C. J. Engineering Live Cell Surface with Cytocompatible Controlled Living Radical Polymerization. Nat. Chem. 2017, 9 (6), 537−545. (34) Tucker, B. S.; Coughlin, M. L.; Figg, C. A.; Sumerlin, B. S. Grafting-From Proteins Using Metal-Free PET − RAFT Polymerizations under Mild Visible-Light Irradiation. ACS Macro Lett. 2017, 6, 452−457. (35) Yang, Y.; Liu, X.; Ye, G.; Zhu, S.; Wang, Z.; Huo, X.; Matyjaszewski, K.; Lu, Y.; Chen, J. Metal-Free Photoinduced Electron Transfer−Atom Transfer Radical Polymerization Integrated with Bioinspired Polydopamine Chemistry as a Green Strategy for Surface Engineering of Magnetic Nanoparticles. ACS Appl. Mater. Interfaces 2017, 9 (15), 13637−13646. (36) Bagheri, A.; Yeow, J.; Arandiyan, H.; Xu, J.; Boyer, C.; Lim, M. Polymerization of a Photocleavable Monomer Using Visible Light. Macromol. Rapid Commun. 2016, 37 (11), 905−910. (37) Xu, J.; Jung, K.; Atme, A.; Shanmugam, S.; Boyer, C. A Robust and Versatile Photoinduced Living Polymerization of Conjugated and Unconjugated Monomers and Its Oxygen Tolerance. J. Am. Chem. Soc. 2014, 136 (14), 5508−5519. (38) Shanmugam, S.; Xu, J.; Boyer, C. Exploiting Metalloporphyrins for Selective Living Radical Polymerization Tunable over Visible Wavelengths. J. Am. Chem. Soc. 2015, 137 (28), 9174−9185. (39) Gnanasammandhan, M. K.; Idris, N. M.; Bansal, A.; Huang, K.; Zhang, Y. Near-IR Photoactivation Using Mesoporous Silica-Coated NaYF4:Yb,Er/Tm Upconversion Nanoparticles. Nat. Protoc. 2016, 11 (4), 688−713. (40) Chien, Y. H.; Chou, Y. L.; Wang, S. W.; Hung, S. T.; Liau, M. C.; Chao, Y. J.; Su, C. H.; Yeh, C. S. Near-Infrared Light Photocontrolled Targeting, Bioimaging, and Chemotherapy with Caged Upconversion Nanoparticles in Vitro and in Vivo. ACS Nano 2013, 7 (10), 8516−8528. (41) Gonzato, C.; Courty, M.; Pasetto, P.; Haupt, K. Magnetic Molecularly Imprinted Polymer Nanocomposites via Surface-Initiated RAFT Polymerization. Adv. Funct. Mater. 2011, 21 (20), 3947−3953. (42) Qian, H. S.; Zhang, Y. Synthesis of Hexagonal-Phase Core-Shell NaYF4 Nanocrystals with Tunable Upconversion Fluorescence. Langmuir 2008, 24 (21), 12123−12125. (43) Xiao, Q.; Zheng, X.; Bu, W.; Ge, W.; Zhang, S.; Chen, F.; et al. A Core/Satellite Multifunctional Nanotheranostic for in Vivo Imaging and Tumor Eradication by Radiation/Photothermal Synergistic Therapy. J. Am. Chem. Soc. 2013, 135 (35), 13041−13048. (44) Li, W.; Wang, J.; Ren, J.; Qu, X. Near-Infrared Upconversion Controls Photocaged Cell Adhesion. J. Am. Chem. Soc. 2014, 136 (6), 2248−2251. I


Article

Macromolecules (45) Liu, F.; Zhao, Q.; You, H.; Wang, Z. Synthesis of Stable Carboxy-Terminated NaYF4: Yb3+, Er3+@SiO2 Nanoparticles with Ultrathin Shell for Biolabeling Applications. Nanoscale 2013, 5 (3), 1047−1053. (46) Bagwe, R. P.; Hilliard, L. R.; Tan, W. Surface Modification of Silica Nanoparticles to Reduce Aggregation and Non-Specific Binding. Langmuir 2006, 22 (9), 4357−4362. (47) Demirci, S.; Caykara, T. RAFT-Mediated Synthesis of Cationic Poly[(ar-Vinylbenzyl) Trimethylammonium Chloride] Brushes for Quantitative DNA Immobilization. Mater. Sci. Eng., C 2013, 33 (1), 111−120. (48) Lechevallier, S.; Hammer, P.; Caiut, J. M.; Mazeres, S.; Mauricot, R.; Verelst, M.; Dexpert, H.; Ribeiro, S. J.; Dexpert-Ghys, J. APTES-Modified RE2O3:Eu3+ Luminescent Beads: Structure and Properties. Langmuir 2012, 28 (8), 3962−3971. (49) Gurbuz, N.; Demirci, S.; Yavuz, S.; Caykara, T. Synthesis of Cationic N-[3-(Dimethylamino) Propyl] Methacrylamide Brushes on Silicon Wafer via Surface-Initiated RAFT Polymerization. J. Polym. Sci., Part A: Polym. Chem. 2011, 49 (2), 423−431. (50) Bottcher, H.; Hallensleben, M. L.; Nuss, S.; Wurm, H. ATRP Grafting from Silica Surface to Create First and Second Generation of Grafts. Polym. Bull. 2000, 44 (2), 223−229. (51) Basuki, J. S.; Esser, L.; Zetterlund, P. B.; Whittaker, M. R.; Boyer, C.; Davis, T. P. Grafting of P (OEGA) Onto Magnetic Nanoparticles Using Cu (0) Mediated Polymerization: Comparing Grafting “from” and “to” Approaches in the Search for the Optimal Material Design of Nanoparticle MRI Contrast Agents. Macromolecules 2013, 46 (15), 6038−6047. (52) Al Mousawi, A.; Poriel, C.; Dumur, F.; Toufaily, J.; Hamieh, T.; Fouassier, J. P.; Lalevée, J. Zinc Tetraphenylporphyrin as High Performance Visible Light Photoinitiator of Cationic Photosensitive Resins for LED Projector 3D Printing Applications. Macromolecules 2017, 50 (3), 746−753. (53) Yeow, J.; Xu, J.; Boyer, C. Polymerization-Induced SelfAssembly Using Visible Light Mediated Photoinduced Electron Transfer−Reversible Addition−Fragmentation Chain Transfer Polymerization. ACS Macro Lett. 2015, 4 (9), 984−990. (54) Zhao, Y.; Perrier, S. Synthesis of Poly(methyl Acrylate) Grafted onto Silica Particles by Z-Supported RAFT Polymerization. Macromol. Symp. 2007, 248 (1), 94−103. (55) Gao, C.; Muthukrishnan, S.; Li, W.; Yuan, J.; Xu, Y.; Müller, A. H. E. Linear and Hyperbranched Glycopolymer-Functionalized Carbon Nanotubes: Synthesis, Kinetics, and Characterization. Macromolecules 2007, 40 (6), 1803−1815. (56) Li, C.; Han, J.; Ryu, C. Y.; Benicewicz, B. C. A Versatile Method To Prepare RAFT Agent Anchored Substrates and the Preparation of PMMA Grafted Nanoparticles A Versatile Method To Prepare RAFT Agent Anchored Substrates and the Preparation of PMMA Grafted Nanoparticles. Macromolecules 2006, 39, 3175−3183. (57) Zhou, L.; He, B.; Huang, J.; Cheng, Z.; Xu, X.; Wei, C. Multihydroxy Dendritic Upconversion Nanoparticles with Enhanced Water Dispersibility and Surface Functionality for Bioimaging. ACS Appl. Mater. Interfaces 2014, 6 (10), 7719−7727. (58) Baum, M.; Brittain, W. J. Synthesis of Polymer Brushes on Silicate Substrates by Radical Addition Fragmentation Chain Transfer Technique. Macromolecules 2002, 35 (3), 610−615. (59) Zammarelli, N.; Luksin, M.; Raschke, H.; Hergenröder, R.; Weberskirch, R. grafting-from” Polymerization of PMMA from Stainless Steel Surfaces by a Raft-Mediated Polymerization Process. Langmuir 2013, 29 (41), 12834−12843. (60) Niu, J.; Lunn, D. J.; Pusuluri, A.; Yoo, J. I.; O’Malley, M. A.; Mitragotri, S.; Soh, H. T.; Hawker, C. J. Engineering Live Cell Surfaces with Functional Polymers via Cytocompatible Controlled Radical Polymerization. Nat. Chem. 2017, 9 (6), 537−545. (61) Boyer, C.; Bulmus, V.; Priyanto, P.; Teoh, W. Y.; Amal, R.; Davis, T. P. The Stabilization and Bio-Functionalization of Iron Oxide Nanoparticles Using Heterotelechelic Polymers. J. Mater. Chem. 2009, 19 (1), 111−123.

(62) Boyer, C.; Bulmus, V.; Davis, T. P. Efficient Usage of Thiocarbonates for Both the Production and the Biofmnctionalization of Poljnters. Macromol. Rapid Commun. 2009, 30 (7), 493−497. (63) Wang, G.; Qin, W.; Wang, L.; Wei, G.; Zhu, P.; Kim, R. Intense Ultraviolet Upconversion Luminescence from Hexagonal NaYF4:Yb3+/ Tm3+ Microcrystals. Opt. Opt. Express 2008, 16 (16), 11907−11914. (64) Chen, G.; Damasco, J.; Qiu, H.; Shao, W.; Ohulchanskyy, T. Y.; Valiev, R. R.; Wu, X.; Han, G.; Wang, Y.; Yang, C.; Ågren, H.; Prasad, P. N. Energy-Cascaded Upconversion in an Organic Dye-Sensitized Core/Shell Fluoride Nanocrystal. Nano Lett. 2015, 15 (11), 7400− 7407. (65) Xu, S.; Xu, S. H.; Zhu, Y. S.; Xu, W.; Zhou, P. W.; Zhou, C. Y.; Dong, B.; Song, H. W. A Novel Upconversion, Fluorescence Resonance Energy Transfer Biosensor (FRET) for Sensitive Detection of Lead Ions in Human Serum. Nanoscale 2014, 6 (21), 12573−12579. (66) Wei, W.; Zhang, Y.; Chen, R.; Goggi, J.; Ren, N.; Huang, L.; Bhakoo, K. K.; Sun, H.; Tan, T. T. Y. Cross Relaxation Induced Pure Red Upconversion in Activator-and Sensitizer-Rich Lanthanide Nanoparticles. Chem. Mater. 2014, 26 (18), 5183−5186. (67) Zhao, J.; Jin, D.; Schartner, E. P.; Lu, Y.; Liu, Y.; Zvyagin, A. V.; Zhang, L.; Dawes, J. M.; Xi, P.; Piper, J. A.; Goldys, E. M.; Monro, T. M. Single-Nanocrystal Sensitivity Achieved by Enhanced Upconversion Luminescence. Nat. Nanotechnol. 2013, 8 (10), 729−734. (68) Xu, X.; Clarke, C.; Ma, C.; Casillas, G.; Das, M.; Guan, M.; Liu, D.; Wang, L.; Tadich, A.; Du, Y.; Ton-That, C.; Jin, D. Depth-Profiling of Yb 3+ Sensitizer Ions in NaYF 4 Upconversion Nanoparticles. Nanoscale 2017, 9, 7719−7726. (69) Ma, C.; Xu, X.; Wang, F.; Zhou, Z.; Liu, D.; Zhao, J.; Guan, M.; Lang, C. I.; Jin, D. Optimal Sensitizer Concentration in Single Upconversion Nanocrystals. Nano Lett. 2017, 17, 2858−2864. (70) Li, Z.; Wang, L.; Wang, Z.; Liu, X.; Xiong, Y. Modification of NaYF4:Yb,Er@SiO2 Nanoparticles with Gold Nanocrystals for Tunable Green-to-Red Upconversion Emissions. J. Phys. Chem. C 2011, 115 (8), 3291−3296. (71) Mahalingam, V.; Vetrone, F.; Naccache, R.; Speghini, A.; Capobianco, J. A. Colloidal Tm3+/Yb3+-Doped LiYF4 Nanocrystals: Multiple Luminescence Spanning the UV to NIR Regions via LowEnergy Excitation. Adv. Mater. 2009, 21 (40), 4025−4028. (72) Li, Z.; Zhang, Y.; Shuter, B.; Idris, N. M. Hybrid Lanthanide Nanoparticles with Paramagnetic Shell Coated on Upconversion Fluorescent Nanocrystals. Langmuir 2009, 25 (20), 12015−12018. (73) Reisfeld, R.; Zelner, M.; Patra, A. Fluorescence Study of Zirconia Films Doped by Eu3+, Tb3+ and Sms and Their Comparison with Silica Films. J. Alloys Compd. 2000, 300-301, 147−151. (74) Matyjaszewski, K.; Tsarevsky, N. V. Macromolecular Engineering by Atom Transfer Radical Polymerization. J. Am. Chem. Soc. 2014, 136 (18), 6513−6533. (75) Chen, G.; Damasco, J.; Qiu, H.; Shao, W.; Ohulchanskyy, T. Y.; Valiev, R. R.; Wu, X.; Han, G.; Wang, Y.; Yang, C.; Ågren, H.; Prasad, P. N. Energy-Cascaded Upconversion in an Organic Dye-Sensitized Core/Shell Fluoride Nanocrystal. Nano Lett. 2015, 15 (11), 7400− 7407. (76) Zou, W.; Visser, C.; Maduro, J. A.; Pshenichnikov, M. S.; Hummelen, J. C. Broadband Dye-Sensitized Upconversion of nearInfrared Light. Nat. Photonics 2012, 6 (8), 560−564. (77) Wang, F.; Deng, R.; Wang, J.; Wang, Q.; Han, Y.; Zhu, H.; Chen, X.; Liu, X. Tuning Upconversion through Energy Migration in Core−shell Nanoparticles. Nat. Mater. 2011, 10 (12), 968−973. (78) Wang, F.; Wang, J.; Liu, X. Direct Evidence of a Surface Quenching Effect on Size-Dependent Luminescence of Upconversion Nanoparticles. Angew. Chem., Int. Ed. 2010, 49 (41), 7456−7460. (79) Lü, Q.; Guo, F.; Sun, L.; Li, A.; Zhao, L. Silica-/titania-Coated Y2O3Tm3+, Yb 3+ Nanoparticles with Improvement in Upconversion Luminescence Induced by Different Thickness Shells. J. Appl. Phys. 2008, 103 (12), 123533. (80) Wilhelm, S.; Kaiser, M.; Würth, C.; Heiland, J.; Carrillo-Carrion, C.; Muhr, V.; Wolfbeis, O. S.; Parak, W. J.; Resch-Genger, U.; Hirsch, T. Water Dispersible Upconverting Nanoparticles: Effects of Surface J


Article

Macromolecules Modification on Their Luminescence and Colloidal Stability. Nanoscale 2015, 7 (4), 1403−1410. (81) Li, L. Le.; Zhang, R.; Yin, L.; Zheng, K.; Qin, W.; Selvin, P. R.; Lu, Y. Biomimetic Surface Engineering of Lanthanide-Doped Upconversion Nanoparticles as Versatile Bioprobes. Angew. Chem., Int. Ed. 2012, 51 (25), 6121−6125. (82) Rojas-Gutierrez, P. A.; DeWolf, C.; Capobianco, J. A. Formation of a Supported Lipid Bilayer on Faceted LiYF 4 :Tm 3+ /Yb 3+ Upconversion Nanoparticles. Part. Part. Syst. Charact. 2016, 33, 859. (83) Ren, W.; Tian, G.; Jian, S.; Gu, Z.; Zhou, L.; Yan, L.; Jin, S.; Yin, W.; Zhao, Y. TWEEN Coated NaYF4:Yb,Er/NaYF4 Core/shell Upconversion Nanoparticles for Bioimaging and Drug Delivery. RSC Adv. 2012, 2 (18), 7037. (84) Yao, C.; Wang, P.; Zhou, L.; Wang, R.; Li, X.; Zhao, D.; Zhang, F. Highly Biocompatible Zwitterionic Phospholipids Coated Upconversion Nanoparticles for Efficient Bioimaging. Anal. Chem. 2014, 86 (19), 9749−9757.

K


Controlled Direct Growth of Polymer Shell on Upconversion

Recommend Documents