Luminescent and Magnetoresponsive Multifunctional Chalcogenide

The two-phase mixture was homogenized by ultrasonification for 1 min (10% ..... wt % CdS and 40–50 wt % magnetite), as estimated from thermogravimet...
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Luminescent and Magnetoresponsive Multifunctional Chalcogenide/Polymer Hybrid Nanoparticles Viktor Fischer,† Markus B. Bannwarth,† Gerhard Jakob,‡ Katharina Landfester,† and Rafael Muñoz-Espí*,† †

Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany Institute of Physics, University of Mainz, Staudingerweg 7, 55128 Mainz, Germany

‡

S Supporting Information *

ABSTRACT: Cadmium sulfide/magnetite/polymer multifunctional hybrid nanoparticles are prepared by crystallizing CdS in a controlled manner on the surface of phosphonate-functionalized polystyrene particles with a magnetic core. The supporting polymer magnetoresponsive nanoparticles are produced by a modified miniemulsion polymerization process: a first miniemulsion containing the core monomer (styrene) and a phosphonate-functionalized surfaceactive monomer is mixed with a second miniemulsion containing magnetite nanoparticles capped with oleic acid and the same surface-active monomer. The chalcogenide formation occurs in situ at the surface of the polymer particles by adding a precipitating agent (sodium sulfide) at a controlled rate. The phosphonate groups on the surface of the polymer particles have the ability to bind the cadmium ions and act as nucleating centers from which the controlled crystallization of CdS takes place. The resulting hybrid particles show a “raspberry-like” structure, with CdS nanocrystals surrounding the polymeric core. The superparamagnetic behavior of the initial iron oxide nanoparticles, without a recognizable blocking temperature, is retained in the final hybrids particles. The obtained hybrids show luminescence in the visible light with a maximum at 620 nm (2.00 eV).

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to the carrier particle2,30,31 or a layer-by-layer deposition technique.32−35 Although those methods are relatively well established, they may be tedious and time-consuming, due to the complex preparation of functional groups, coupling reactions, and subsequent purification steps. We have recently reported the synthesis of metal oxide/ polymer hybrid particles by a controlled in situ crystallization process at the surface of polymer particles prepared with phosphonate- and phosphate-functionalized surface-active monomers.36 Here, we demonstrate that it is possible to extend such approach to sulfides and, more importantly, we present the preparation of a “second generation” of multifunctional particles, which include a magnetoresponsive inorganic component (Fe3O4) inside and a light-responsive functionality (provided by CdS nanocrystallites) in the outer part. The polymer serves both as an incorporation matrix and as a carrier particle. We show how superparamagnetic magnetite can be embedded during polymerization by a modified emulsion polymerization procedure to create polymer nanocarrier particles, which provides an easy handling in terms of separation and concentration of colloidally stable nanoparticles.

INTRODUCTION Nature shows many examples in which different properties and functionalities are combined to create multifunctional hybrid systems with unique properties, often involving organic and inorganic components. A fascinating example of magnetic responsive hybrid structures is found in magnetotactic bacteria, apparently simple unicellular organisms with the ability to crystallize magnetite (Fe3O4) or greigite (Fe3S4) inside the cell, which allows them to orient along the magnetic field lines of the Earth. Such natural creations are a source of inspiration of many scientific minds, and intensive efforts have been dedicated to fabricate hybrid materials with different physical features, such as optical and magnetic responsivity for applications in imaging,1 cancer treatment,2 molecular imprinting,3 magnetolytic therapy,4 and catalysis.5 Hybrid particles containing inorganic components include organic−inorganic6 and inorganic−inorganic7 systems. Silica is widely applied in the generation of inorganic−inorganic hybrid systems as both shell8,9 and core,10−12 whereas surfactants and polymers are used as templates or supports,13−16 capping or structure-directing agents,17−20 and coating materials21,22 in organic−inorganic hybrid particles. Both inorganic and polymer particles can be utilized as carrier particles13−16,23 or to embed the inorganic components.24,25 The application of polymer particles as carriers allows an easy handling, and the polymer component can be eventually removed by dissolution26 or calcination.27−29 Usually, hybrid particles with functionalities inside and optically responsive materialssuch as CdS, PbS, or CdSe quantum dotsoutside are produced by an ex situ synthesis of functionalized quantum dots, followed by a coupling reaction © 2013 American Chemical Society

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EXPERIMENTAL SECTION Materials. Reagents were purchased from Sigma-Aldrich, unless otherwise stated. Ethanol (≥99.9%), octane (≥99.0%), potassium peroxydisulfate (KPS, puriss. p.a., ≥ 99.0%), ferric Received: January 9, 2013 Revised: February 20, 2013 Published: February 25, 2013 5999

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Cadmium acetate dihydrate was dissolved in ethanol (4 mL) and stirred for 2 h to ensure complete solution of the precursor. Subsequently, 100 μL of the latex dispersion (with or without magnetic core) was added, and the mixture was stirred for another 2 h. After the complexation time, 1 mL of an alcoholic Na2S·9H2O solution (0.1 M) was added with a syringe pump (dropping speed of 2 mL h−1). After crystallization, hybrid particles were purified either by three steps of centrifugation and redispersion in the corresponding solvent or by magnetic purification. The washed particles were centrifuged and dried under vacuum (48 h at 30 °C) for X-ray diffraction and magnetic measurements. Characterization Methods. Particle sizes of the prepared particles were determined by dynamic light scattering using a Nicomp particle sizer (model 380, PSS, Santa Barbara, CA) at a fixed angle of 90°. The solid content was determined by freezedrying a portion of the dispersion under vacuum for 24 h. The surface charge density was estimated by titration with a poly(diallyldimethylammonium chloride) solution (0.001 N, Mütek Analytik, M = 40 000−100 000 g mol−1) using a 702SM Titrino (Metrohm, Switzerland) automatic titrator (see Supporting Information for details). The zeta potential of the functionalized latexes was determined by electrophoretic mobility measurements in a Malvern ZetaSizer Nano-Z instrument. For the measurements, the dispersions were diluted 1:1000 with a KCl solution (0.001 M) to achieve optical transparency for the measurements and shear off all diffuse adsorbed ion layers. The measurements were conducted three times per sample to prove the repeatability. Scanning electron microscopy (SEM) was conducted in a field-emission microscope Leo Gemini 1530 operated at a voltage of 0.7 kV. Alternatively, a Hitachi SU8000 SEM microscope equipped with a Bruker AXS spectrometer was used for EDX analysis. Samples for SEM observation were prepared by drop-casting of diluted dispersions on silicon wafers. Transmission electron microscopy (TEM), scanning TEM (STEM), and energy-dispersive X-ray spectroscopy (EDX) were carried out in FEI Tecnai F20 microscope. X-ray diffraction (XRD) was conducted on a Philips PW 1820 diffractometer with monochromatic Cu Kα radiation (λ = 1.5418 Å, 40 kV, 30 mA, 5 s, Δθ = 0.02). Thermogravimetric analysis (TGA) was carried out with a thermobalance Mettler Toledo ThermoSTAR TGA/SDTA 851

chloride hexahydrate (Merck, 99%), ferrous chloride tetrahydrate (Merck, 99%), ammonium hydroxide (Riedel-de Haën, 26%), oleic acid (Riedel-de Haen, 58%), cadmium acetate dihydrate (puriss. p.a., ≥ 99.0%), sodium hydroxide (≥97.0%), and sodium sulfide nonahydrate (≥99.9%) were used without further purification. Styrene (≥99.0%) was passed over an aluminum oxide column before use to remove the stabilizer. The surfmer 11-(methacryloyloxy)undecylphosphonic acid was synthesized as described elsewhere.36 Synthesis of Magnetite Nanoparticles. The synthesis was performed following Urban et al.24 Briefly, FeCl3·6H2O (24.4 g, 90.1 mmol) and FeCl2·4H2O (12.0 g, 60.4 mmol) were dissolved in Milli-Q water (50 mL). Subsequently, concentrated ammonium hydroxide (40 mL) was added dropwise within 15 min under mechanical stirring, followed by the addition of oleic acid (6 g, 21.2 mmol) after precipitation of magnetite nanoparticles. The suspension was heated to 70 °C and stirred for 30 min. Afterward, the temperature was increased to 110 °C and the suspension was stirred for another hour. The product (black residue) was washed several times with Milli-Q water and dried for 16 h at 40 °C under vacuum. Synthesis of Control Phosphonate-Functionalized Polystyrene Nanoparticles. The latex particles were prepared as described elsewhere36 by miniemulsion polymerization from styrene and a small amount of 11-(methacryloyloxy)undecylphosphonic acid (abbreviated as RPO3H2 in the following), which serves at the same time as comonomer and surfactant. The disperse phase was prepared by mixing styrene (3 g) with hexadecane (125 mg) and 2,2′-azobis(2-methylbutyronitrile) (60 mg). The continuous phase was prepared by dissolving RPO3H2 (30 mg) and a defined amount of a 1 M NaOH aqueous solution (218 μL, equivalent to OH− groups) in water (18 mL). Both phases were mixed and stirred for 45 min. The emulsification was achieved by ultrasonication for 3 min (Branson Digital Sonifier 450-D; 1/2 in. tip, 90% intensity, pulse 10 s, pause 2 s) while cooling in an ice− water bath to avoid polymerization due to heating. The reaction occurred at 60 °C in a closed flask under constant stirring and was stopped after 12 h. The resulting dispersion was filtered and purified by centrifugation dialysis (Amicon Ultra centrifugal filter unit, 30 kDa, Millipore). Synthesis of Phosphonate-Functionalized Latex with Magnetic Core. Surface-functionalized polystyrene nanoparticles with a magnetic core were prepared from two miniemulsions. The first miniemulsion was obtained by mixing of water (24 g) containing a small amount of RPO3H2 (25 mg, 0.210 mmol) with a dispersion of oleic acid-capped magnetite particles (1 g of magnetite particles in 0.5 g of n-octane). Afterward, the blend was homogenized by ultrasound for 3 min. (50% intensity, pulse 10 s, pause 5 s). For the second miniemulsion, water (24 g) containing RPO3H2 (5 mg, 0.042 mmol) and sodium styrenesulfonate (50 mg) was mixed with a solution of styrene (1.2 g) and hexadecane (20 mg). The twophase mixture was homogenized by ultrasonification for 1 min (10% intensity, pulse 5 s, pause 5 s). Subsequently, both miniemulsions were combined and degassed by bubbling argon for 10 min. Afterward, KPS (20 mg) was added under mechanical stirring (250 rpm). For polymerization, the mixture was heated at 80 °C for 16 h. After complete polymerization, the dispersion was purified magnetically. Crystallization Experiments. All experiments were conducted at 25 °C in a closed flask using a ratio of 5 mmol of metal salt per gram of latex particles, which was found to show the best results in initial screening tests.

Figure 1. Preparation of phosphonate-functionalized polystyrene particles with magnetic iron oxide core by emulsion polymerization of two preformed miniemulsions. 6000

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Table 1. Characteristics of the Latex Dispersions Used in This Work surface charge density (nm−2)b control latex magnetic latex

no. of functional groups

solid content (wt %)

particle size (nm)

pH = 7.4

pH = 10.0

(nm−2)

(×104 per particle)

12.2 10.0a

172 ± 17 134 ± 44

0.21 ± 0.02 0.28 ± 0.09

0.70 ± 0.07 0.86 ± 0.29

0.35 0.43

2.78 3.12

a

The solid content was set to 10 wt % after magnetic purification. bThe error corresponds to the standard deviation of the calculated values from the particle charge detection with respect to the particle size distribution.

Figure 2. SEM and TEM images of phosphonate-functionalized particles (a,c) without and (b,d) with magnetoresponsive core (scale bar 450 nm in SEM images, 100 nm in TEM images). The photographs show the corresponding dispersions after synthesis.

Figure 3. (a) Magnetization measured at 300 K for the initial oleic acid-capped magnetite nanoparticles (dashed black line) and the polymer/magnetite hybrid particles (solid black line). The red line shows the magnetization of the hybrid material with respect to the iron oxide content, as determined by TGA. (b) Temperature-dependent measurements of the magnetization of hybrid nanoparticles at a magnetic field of 150 Oe under field cooled (FC) and zero field cooled (ZFC) conditions. Inset shows magnification of the temperature dependency as a line diagram.

under a nitrogen atmosphere (from 25 to 1000 °C with a heating rate of 10 °C min−1). Photoluminescence (PL) emission and excitation spectra were recorded in top-reading mode in a Tecan Infinite M100 plate reader by using a quartz plate (Hellma Analytics). Magnetic measurements were performed in a superconducting quantum interference device (SQUID, Quantum Design MPMS II). The weighted samples were filled in gelatin capsules and mounted in a low magnetic moment sample holder. The temperature sweeps were measured with a rate of 2 K min−1. The hysteresis curves were measured with the magnet in the superconducting state at each field value. The SQUID signals resembled closely the expected signal from point dipoles and were calibrated against a Pd reference sample.

polymerized under vigorous stirring at 80 °C, resulting in polystyrene particles that incorporate the magnetite and are functionalized at the surface with phosphonate groups. The iron oxide particles, prepared by coprecipitation with ammonia from an aqueous solution of Fe(II) and Fe(III), were capped with oleic acid according to an established procedure.24,25 The particles were of spherical shape and uniform in size, with a particle diameter of approximately 10 nm, and the crystal phase was confirmed as magnetite by X-ray diffraction (XRD) (see Supporting Information, Figure S1). Crystallite sizes of 10 nm, matching the observations by TEM, were calculated with the Scherrer equation37 from the broadening of the (311) XRD reflection. As control particles for the crystallization experiments described below, particles without magnetic core were also synthesized by miniemulsion polymerization of styrene in the presence of the same phosphonate surfmer. The main characteristics of the synthesized latex samples are presented in Table 1. The solid content of the control particle system was

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RESULTS AND DISCUSSION The polymeric particles used in this work were obtained by a modified miniemulsion copolymerization based on mixing two different precursor miniemulsions, as schematically depicted in Figure 1. The first dispersion was a miniemulsion containing styrene in the disperse phase and the second was a miniemulsion of oleic acid-capped magnetite particles in octane.25 Both miniemulsions were stabilized with 11-(methacryloyloxy)undecylphosphonic acid, a phosphonate-functionalized surfaceactive comonomer (a so-called surfmer) that acts simultaneously as surfactant and monomer. Both dispersions were mixed and 6001

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of magnetite was estimated to be as high as 67 wt % of the dry latex by thermogravimetric analysis (TGA). The surface charge density changes for both phosphonate-functionalized systems when increasing the pH from 7.5 to 10, which can be easily explained by the pH dependency of phosphonic acid. As the phosphonate groups of the surfmer have two protons, the number of functional groups per unit area can be estimated by dividing the number of surface charge density at pH 10 (i.e., under full deprotonation of the functional groups) by 2 (see Supporting Information for details). Approximately 3 × 10−4 groups per particle were estimated for both control and magnetic latexes. Figure 2 shows scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the latex particles without and with magnetite. The rough surface of the particles containing magnetite may be an indication that some oxide nanoparticles are also present on the surface. The incorporation of the oleic acid-capped magnetite particles in the polymer particle was proven by TEM. The magnetic properties of the initial magnetite nanoparticles and the produced magnetoresponsive latex were measured in a superconducting quantum interference device (SQUID) in a field range of −30 000 to 30 000 Oe. Measurements of the magnetization for the initial magnetite nanoparticles, presented in Figure 3 (dashed black line), showed a zerocrossing and no remanence, coercivity, or hysteresis. The nanoparticles can be thus considered as superparamagnetic with a saturation magnetization of 88 emu g−1, value obtained after recalculation taking into account the iron oxide content determined by thermogravimetric analysis. To prove that the quality of the magnetic iron oxide does not change during the formation of the

Figure 4. Mechanism of metal chalcogenide formation at the surface of phosphonate-functionalized latex particles with magnetic iron oxide core.

12.2 wt %. The latex loaded with magnetite had an initial solid content of 17.2 wt %, but it was fixed to 10.0 wt % after purification by magnetic separation. The magnetic latex has a smaller particle size with larger size distribution (134 ± 44 nm) than the control latex (172 ± 17 nm), which can be explained by the different preparation method. Whereas control particles were prepared with conventional miniemulsion polymerization, the magnetic latex was prepared by emulsion polymerization of two previously formed miniemulsions, which appears to increase the broadness of the size distribution. The content

Figure 5. SEM images of crystallized CdS at the surface of (a) the control particle system without magnetic core and (b) magnetoresponsive particles (scale bar 300 nm). The photographs show the corresponding hybrid-particle dispersion. (c) XRD pattern of a control system (CdS hybrid nanoparticles without magnetic core); vertical lines indicate the reference pattern of CdS (greenockite, JCPDS Card No. 41-1049). (d) EDX spectra of CdS hybrid particles with magnetoresponsive core (the Si peak is caused by the silicon wafer used for the measurement). 6002

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particle surface serve as nucleation centers. The addition rate was found to be crucial to control the supersaturation level and to drive effectively the crystallization to the particle surface while avoiding homogeneous nucleation. Scanning electron microscopy showed the formation of a “raspberry-like” morphology, with CdS nanocrystals on the surface of the phosphonate-functionalized particles, for both control and magnetic systems (Figure 5a,b). The in situ

polymer particles, the magnetization curve obtained for the hybrid particles (54.4 emu g−1, black line in Figure 3) was also recalculated for the iron oxide content. The recalculated magnetization curve shows an almost similar behavior and saturation magnetization (80.3 emu g−1) as the used magnetite particles. The slight difference in the saturation magnetization is within the range of error (considering weighing errors and errors of the SQUID magnetometer). Temperature-dependent measurements at a small and constant field of 150 Oe show a superparamagnetic behavior (Figure 3b). The zero-field measurement (ZFC) does not deviate from the field cooled measurement (FC), showing no observable blocking temperature. The inset of Figure 3b shows a high magnification of the ZFC and FC measurement at low temperatures, which indicates a very small path difference and proves the incorporation of real superparamagnets. To crystallize cadmium sulfide at the surface of the functionalized latex particles, an aliquot of the latex dispersion was first added to a cadmium precursor solution. The process of formation of CdS is illustrated in Figure 4. An accessible surface functionality is needed for a successful complexation of metal ions, which are trapped at the particle surface. After a sufficient complexation time (typically 2 h), the crystallization was induced by addition of the precipitating agent, Na2S, at a dropping rate of 2 mL h−1. The metal ions immobilized at the

Figure 7. Photoluminescence spectra of CdS hybrid particles (a) without and (b) with magnetic core. Blue curves show the photoluminescence emission spectra (λexc.= 400 nm), and red curves the photoluminescence excitation spectra (λem.= 620 nm). (c) Photographs showing the magnetic and the optical behavior of the colloidal suspensions of hybrid particles.

Figure 6. (a) Measurements of the zeta potential (ζ) for the prepared CdS/polymer hybrids. (b) Stabilization model of hybrid latex in dispersion. 6003

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encapsulation of magnetite in polymer particles, and on the other hand the in situ controlled crystallization of CdS on the surface of the same particles. Surface-active monomers containing phosphonate groups have been applied in the synthesis of magnetoresponsive particles for the incorporation of oleic acid-capped magnetite by a modified miniemulsion polymerization process. In the hybrid inorganic/polymer systems, the polymer serves simultaneous as incorporation matrix for the magnetite and as support for the cadmium sulfide. The prepared materials have an inorganic content of up to 80 wt % (30−40 wt % CdS and 40−50 wt % magnetite), as estimated from thermogravimetric analysis. As a consequence of the CdS nanocrystals present on the surface, the hybrid particles showed an intense photoluminescence in the orange visible range with a maximum at 620 nm (2.00 eV). In parallel, the encapsulated magnetite originates a superparamagnetic behavior without a recognizable blocking temperature and a high saturation magnetization. This work demonstrates how different inorganic systems can be used for the creation of hybrid particle systems that respond to multiple stimuli, such as light, a magnetic field, or both at the same time.

crystallization of cadmium sulfide was confirmed by X-ray diffraction. Figure 5c shows the XRD pattern of the control hybrid system, corresponding to hexagonal greenockite with an average crystallite sizeor, more precisely, a size of the coherently scattering domainsof ca. 13 nm, calculated from the full width at half-maximum of the (110) reflection by using the Scherrer equation. Analysis by energy-dispersive X-ray (EDX) spectroscopy confirmed the presence of Cd, Fe, and S on the magnetoresponsive hybrids (Figure 5d). Lattice spacing values corresponding to greenockite (CdS) and magnetite (Fe3O4) were identified by high-resolution TEM (Supporting Information, Figure S2). Scanning transmission electron microscopy (STEM) coupled with EDX and elemental mappings of iron and sulfur demonstrated that iron is homogeneously distributed along the particles, whereas cadmium and sulfur are mostly at the surface (Supporting Information, Figures S3−S5). These observations are consistent with the expected crystallization of CdS at the surface. The CdS content in the final hybrid materials was estimated to be between 30 and 40 wt % by TGA. The zeta potential of the particles increased drastically after the CdS crystallization (Figure 6a), which indicates the change in the chemical environment of the particle surface. The crystals formed at the particle surface have local Stern and diffuse layers and, thus, a local zeta potential (ζCdS+), as schematically depicted in Figure 6b. The overall measured zeta potential is a sum of the contributions of the free particle surface (ζP−) and the formed CdS crystals (ζ CdS +). Analogously to the observations that we have recently reported for metal oxides,36 we assume that sodium ions from the precipitating agent tend to strongly adsorb on the surface of the CdS nanoparticles, conferring a positive charge. According to this model, the zeta potential can be correlated with the coverage of the particles with the inorganic crystals, because the negative values of the functionalized polymer surface will be compensated by the contribution of the inorganic nanoparticles. Photoluminescence (PL) emission spectra of the control and the magnetic hybrid systems are depicted in Figure 7. Both hybrid particle systems show a broadband emission in the visible range with a maximum in the orange region at about 620 nm (2.00 eV, λexc= 400 nm), which can be attributed to a direct electron−hole recombination. A red shift of the normally observed green emission (518−530 nm) can be attributed to sulfur vacancies of the CdS crystal lattice.38,39 PL emission spectra show qualitatively the same behavior, but the PL excitation spectra (dotted curve) present some minor differences. The excitation edge is broader for the magnetoresponsive particles than for the control hybrids: for the latter, the excitation curve tends toward zero at 500 nm, whereas for the former it shows at least 20% of the maximum intensity at 500 nm. This observation may be related with a slightly different CdS crystal formation on magnetoresponsive polymer nanoparticles, because the crystal perfection and the presence of defects can strongly affect the photoluminescence features. The photograph in Figure 7c shows how the formed hybrid particles respond to different stimuli as UV light or a magnetic field. The hybrid nanoparticles are simultaneously magnetoresponsive and luminescent when irradiated with UV light.

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ASSOCIATED CONTENT

S Supporting Information *

Protocol for calculation of charged groups per square unit; TEM and XRD of magnetite nanoparticles; HR-TEM, STEM, EDX, and elemental mappings of hybrid particles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +49-6131-379-410. Fax: +49-6131-379-100. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank M. Steiert for XRD measurements, G. Glaßer for the help with SEM and EDX analysis, K. Kirchhoff and Dr. I. Lieberwirth for the assistance with the HR-TEM and STEM measurements and their interpretation, E. Muth for the polyelectrolyte titration experiments, and P. Kindervater for NMR measurements. M.B. gratefully acknowledges the financial support by the Graduate School of Excellence MAINZ.

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REFERENCES

(1) Song, E. Q.; Hu, J.; Wen, C. Y.; Tian, Z. Q.; Yu, X.; Zhang, Z. L.; Shi, Y. B.; Pang, D. W. Fluorescent-Magnetic-Biotargeting Multifunctional Nanobioprobes for Detecting and Isolating Multiple Types of Tumor Cells. ACS Nano 2011, 5, 761−770. (2) Cho, H. S.; Dong, Z. Y.; Pauletti, G. M.; Zhang, J. M.; Xu, H.; Gu, H. C.; Wang, L. M.; Ewing, R. C.; Huth, C.; Wang, F.; et al. Fluorescent, Superparamagnetic Nanospheres for Drug Storage, Targeting, and Imaging: A Multifunctional Nanocarrier System for Cancer Diagnosis and Treatment. ACS Nano 2010, 4, 5398−5404.

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CONCLUSIONS We have reported here a synthetic route for the preparation of multifunctional cadmium sulfide/magnetite/polymer nanoparticles with diameter