Thermoresponsive Nanoparticles of Self-Assembled Block

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Thermoresponsive nanoparticles of self-assembled

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block copolymers as potential carriers for drug

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delivery and diagnostics Antti Rahikkala†, Vladimir Aseyev‡, Heikki Tenhu‡, Esko I. Kauppinen,† and Janne Raula†,*

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†

Department of Applied Physics, Aalto University School of Science, FI-00079 Aalto, Finland

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‡

Laboratory of Polymer Chemistry, Department of Chemistry, University of Helsinki, P.O. Box

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55, FI-00014 Helsinki, Finland

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KEYWORDS: nanoparticle, self-assembly, block copolymer, drug release, temperature

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responsive.

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ABSTRACT:

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Thermally responsive hydrogel nanoparticles composed of self-assembled polystyrene-b-poly(N-

13

isopropyl

14

anilinonaphthalene-8-sulfonic acid have been prepared by aerosol flow reactor method. We

15

aimed exploring the relationship of intra particle morphologies, that were, PS spheres and

16

gyroids embedded in PNIPAm matrix as well PS-PNIPAm lamellar structure, to probe release in

17

aqueous solution below and above the cloud point temperature (CPT) of PNIPAm. The release

18

was detected by fluorescence emission given by the probe binding to bovine serum albumin.

19

Also, the colloidal behavior of hydrogel nanoparticles at varying temperatures were examined by

acrylamide)-b-polystyrene

block

copolymers

and

fluorescent

probe

1-

ACS Paragon Plus Environment

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scattering method. The probe release was faster below than above the CPT from all the

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morphologies of which gyroidal morphology showed the highest release. Colloidal behavior

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varied from single to moderately aggregated particles in order spheres-gyroids-lamellar.

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Hydrogel nanoparticles with tunable intra particle self-assembled morphologies can be utilized

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designing carrier systems for drug delivery and diagnostics.

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For Table of Contents only

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27 28

INTRODUCTION

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Approximately 80% of drugs are traditionally administrated in forms of tablets, capsules,

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dispersions, or fine aerosol particles. Downsizing these systems to nanoscopic scale provides

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outstanding opportunities —such as targeted delivery, controlled release, and increased

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bioavailability— for modern, advanced drug delivery systems to treat local (e.g. cancer) and

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systemic (e.g. diabetes) diseases.1-3 Small particle size has been shown to minimize side effects

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caused by the drug in cancerous tumors.4 Size reduction also enhances the solubility of drugs that

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are poorly soluble in their target organisms. Nanoparticles as vehicles can stabilize biomolecules,

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such as proteins, peptides, or DNA molecules from metabolic degradation, thus opening new

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possibilities for protein drug delivery and gene therapy.5,

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biocompatible, chemically and physically stable upon storage, provides control over

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Ideally, a drug delivery system is


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particle - and drug release, is capable of targeting, and functions exclusively in the site of action

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before being cleared from human body.

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Hydrogels are promising candidates for several applications, such as sensors,7 actuators,8

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filters,9

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temperature-responsive polymer that undergoes an abrupt coil-to-globule transition in water at

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the cloud point temperature (CPT) of 32 °C.11, 12 Hydrogels based on PNIPAm swell in water

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below the CPT and shrink upon heating —a feature suggested to be advantageous in controlled

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drug delivery systems.13 Previously, copolymeric micelles consisting of PNIPAm and fluorescent

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dyes have been studied as fluorescent dual probes to pH and temperature.14 An excellent review

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on stimuli-responsive polymers used in combination with fluorescent dyes for detection and

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sensing applications has been published by Liu et. al.15

and

drug

delivery

systems.10

Poly-N-isopropylacrylamide

(PNIPAm)

is

a

51 52

Aerosol techniques have been used in creating self-assembled nanoparticles.16-23 Using aerosol

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techniques to prepare solid, nano-sized particles from block copolymers enables efficiently

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encasing drug- and diagnostic molecules within the nanoparticles along with allowing self-

55

assembled structures inside the particles. Furthermore, different intra-particle morphologies may

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allow different drug release mechanisms upon controlled release.

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We have previously studied self-assembled aerosol nanoparticles prepared of polystyrene-

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block-poly(N-isopropylacrylamide)-block-polystyrene (PS-b-PNIPAm-b-PS) of three different

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morphological architectures: PS spheres in PNIPAm matrix, PS gyroids in PNIPAm matrix, and

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PS-PNIPAm lamellar structure.19 PNIPAm formed the outmost layer in all the nanoparticles. PS


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domains formed physical cross-links due to bridging PNIPAm blocks, which prevented the

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nanoparticles from disintegrating upon swelling in water at temperatures below the CPT. In A-B-

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A triblock copolymers, a copolymer may be looping or bridging depending on whether the end-

65

blocks reside in the same or different domains, respectively. The ratio of the bridging polymer

66

chains to the looping chains have been shown both experimentally and theoretically to be ~0.4.24-

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This work is a continuation of the aforementioned study. Here we incorporated water-soluble

70

fluorescent probe 1-anilinonaphthalene-8-sulfonic acid (1,8-ANS) into PS-b-PNIPAm-b-PS

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nanoparticles prepared in the aerosol flow reactor27 (AFR). We aimed to investigate how added

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components affect the copolymer assembly in the nanoparticles, and how different intra-particle

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structures affect the release of the component from the nanoparticles at varying temperatures.

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EXPERIMENTAL SECTION

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Materials. PS-b-PNIPAm-b-PS triblock copolymers were synthesized using controlled

77

reversible addition-fragmentation chain-transfer (RAFT)7 polymerization (see Figure 1 for

78

chemical structure and Table 1 for the composition. The fluorescent dye 1-anilinonaphthalene-8-

79

sulfonic acid, 1,8 ANS, (Sigma Aldrich, purity ≥ 98%), bovine serum albumin, BSA, (Sigma

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Aldric, (purity ≥ 98%) and the solvent dimethylformamide used in the aerosol process (Sigma

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Aldrich, purity ≥ 99.5%) were used as received. Water used in the release experiments was milli-

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Q water.

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Figure 1. The structure of PS-b-PNIPAm-b-PS with n denoting the number of repeating units of

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PS and m of PNIPAm, and 1,8-ANS is the fluorescent dye 1-anilinonaphthalene-8-sulfonic acid.

88 89

Table 1. The compositions of the polymers used in this study. The polymers are coded as

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PNx.yK with x denoting the weight fraction of PNIPAm and y the total number-averaged

91

molecular weight (Mn) of the polymer chain in kg mol-1. Abbreviations m and n give the amount

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of repeat units of PNIPAm and PS, respectively

Copolymer

w-% of PNIPAm

Mn (kg mol-1)

m

n

Mw/Mn

Morphology of the formed particles particles

PN77.118K

77

118.3

804

130

1.51

PS spheres

PN61.106

61

106.0

573

199

1.52

PS gyroid

PN43.65K

43

64.6

248

177

1.27

lamellar

93 94

Nanoparticle preparation. The AFR method to prepare nanoparticles in the aerosol phase

95

has been described in our earlier studies.21, 22, 27 Briefly, precursor solutions of PS-b-PNIPAm-b-

96

PS block copolymers (9.5 g L-1) and 1,8-ANS (0.95 g L-1) dissolved in DMF were atomized into

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the AFR using nitrogen jet with the flow rate of 2.5 L min-1. The atomization was carried out

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with a Collison-type air-jet atomizer operated in a recycling mode. In the AFR (inner diameter


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26 mm, length 900 mm) at 180 ± 2 °C the residence time of the aerosols was ~7.5 s. The aerosols

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were cooled and diluted downstream with excess nitrogen flowing at 30 L min-1 prior to a sample

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collection on aluminum foils by a Berner-type low pressure impactor.28

102 103

Particle morphology. Solid nanoparticles on a piece of aluminum paper taken from the

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BLPI collection stage 4 with D50 = 173 nm were immersed in a water droplet at 20 °C or 40 °C

105

for 1 minute or 4 hours. Subsequently, the samples were flash-freezed in liquid propane cooled

106

below -175 °C and transferred into a vacuum oven for freeze-drying for 10 hours. These samples

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and freshly prepared nanoparticles were then sputter-coated with ~2 nm thick layer of gold to

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enhance image contrast. Scanning electron microscopy (SEM) was performed using a Jeol JSM-

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7500FA operating at normal SEM mode using 2 kV high tension and 10 µA emission current.

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Transmission electron microscopy (TEM) was performed using a Jeol JEM-3200FSC cryo-

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transmission electron microscope operating at -188 °C. The micrographs were recorded with

112

Gatan Ultrascan 4000 camera in bright field mode using 300 kV acceleration voltage. The

113

samples were collected from aerosol phase onto holey carbon copper grids by the a point-to-

114

plane electrostatic precipitator (ESP) and imaged both with and without iodine staining.

115 116

Size determination in aqueous dispersions. The measurements were carried out by

117

means of a Malvern Instrument ZetaSizer Nano-ZS equipped with a 4 mW HeNe laser operating

118

at 633 nm in the temperature range from 15 to 66 °C and using square quartz cuvette. The

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hydrodynamic diameter, d, of the particles in aqueous media was measured at the scattering

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angle of 173°. Backscattering allows for suppressing the multiple scattering and avoiding sample

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filtration. Mean peak value of the intensity-weighted distributions of d estimated with multi-


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exponential fit to the collected intensity correlation functions was selected for further analysis.

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Dispersions were stored overnight in refrigerator at 4 °C. The samples were allowed to

124

equilibrate at each temperature for 10 min prior to measurement. Concentration of particles in

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the studied dispersions was the same of 1 w-%. The solvent was milli-Q water.

126 127

Release and binding of 1,8-ANS.

128 129

1,8-ANS fluorescence is weak in water, however its fluorescence increases significantly when

130

bound to nonpolar regions of Bovine serum albumin (BSA).29 The release of 1,8-ANS and its

131

binding to BSA was recorded using fluorescence spectrometry (QuantaMaster 40, Photon

132

Technology International, Edison, NJ, USA) with the detector bias voltage of -0.79 V. The

133

excitation wavelength was set at 372 nm and the emission was measured at 400-600 nm for

134

every 108 seconds for 120 times. The total time for one measurement was then 220 min. The

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spectrometer was equipped with a refrigerated circulator (ARCTIC A25, Thermo Scientific,

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Waltham, MA, USA) to adjust the temperature in the cuvette holder. The drug release was

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performed in a quartz cuvette, which was divided in two chambers; donor and acceptor, with a

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semi-permeable membrane (MWCO 12-14 kDa, ZelluTrans, Carl Roth GmbH, Karlsruhe,

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Germany) (see the set-up in Figure S2). The receiver chamber was filled with 3.8 mL of BSA

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(3 µmol L-1) in water and stirred with a magnet bar. The solid nanoparticle sample was placed on

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the membrane in the donor chamber followed by 0.2 mL of water. The release of 1,8-ANS was

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followed by its binding with BSA in the receiver compartment at 25 °C or 45 °C. Knowing that

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BSA has 5 binding sites for 1,8-ANS at pH 7,30 BSA in this experiment had sites for ~57 nmol of


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1,8-ANS. The nanoparticles of PN77.118K had 17 nmol, PN61.106K had 26 nmol, and

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PN43.65K had 23 nmol of 1,8-ANS.

146 147

Fluorescence signal was calibrated in respect to the aqueous 1,8-ANS solution within the

148

concentration range of 8.68 × 10-7 - 6.16 × 10-6 M and in presence of constant amount of BSA

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(3.00 × 10-6 M). The highest 1,8-ANS concentration in the calibration was close to the amount of

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1,8-ANS encapsulated within one sample of nanoparticles. The calibration was carried out at 25

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and 45 °C (see Figure S1) as the fluorescence yield of bound 1,8-ANS depends on temperature.31

152

The emissions were collected at 400 - 600 nm for ten times and the averages of the maximum

153

wavelengths at 479 nm were taken to the calibration.

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RESULTS AND DISCUSSION

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Copolymer assembly in nanoparticles. In our previous work

19

we observed that self-

157

assembled block copolymers formed lamellar, gyroidal and spherical inner architectures

158

depending on the weight ratio of PNIPAm and PS in the copolymers, see Figure 2 and Table 1.

159 160

Figure 2. TEM micrographs of aerosol polymer particles with spherical, gyroid-like and onion-

161

like morphologies from samples PN77.118K (A), PN61.106K (B), and PN55.91K (C),


8

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respectively. Samples were stained with iodine, which selectively stains the PNIPAM domains

163

appearing darker in the micrographs. Reproduced with permission from reference 19. Copyright

164

2012 American Chemical Society.

165

In this work, the internal self-assembled morphologies were not observed by TEM: 1,8-ANS

166

seemed to prevent the visualization of the contrast between phase-separated polymers (see Figure

167

3).

A

B

C

168 169

Figure 3. TEM micrographs of the 1,8-ANS containing (A) PN77.118K, (B) PN61.106K, and

170

(C) PN43.65K nanoparticles.

171

To reveal internal morphologies the nanoparticles were immersed in water droplet at 20 °C or

172

40 °C for 1 minute or 4 hours followed by quenching in liquid propane at -175 °C. After

173

quenching the samples were freeze-dryed in vacuum, where the ice sublimates away from the

174

nanoparticles. This treatment preserves the polymer network of the nanoparticles at the state they

175

were at the time of quenching. The appearance of the sphere-forming PN77.118K nanoparticles

176

was very porous below the CPT of PNIPAm similar to the previously observed (see Figure 4).

177

The particles spread on the aluminum sheets, which indicate their very loose internal structure.

178

Above the CPT at 40 °C, the nanoparticles had a compact, globular morphology due to the

179

collapse of PNIPAm segments.


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Figure 4. SEM micrographs of the PN77.118K nanoparticles flash-freezed from water at 20 °C

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for (A) 1 minute and (B) 4 hours and at 40 °C for (C) 1 minute and (D) 4 hours.

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The gyroid-forming PN61.106K nanoparticles formed a sponge-like morphology below and

184

above the CPT similar to the previously observed (see Figure 5). Above the CPT, the

185

nanoparticle structure seemed to be more globular than that below the CPT.


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188


189

The lamellar PN43.65K nanoparticles maintained their intact spherical form in water

190

regardless of applied temperature similar to the previously observed (see Figure 6). It appeared,

191

however, that below the CPT the particles had a wrinkled surface texture, which could be caused

192

by the swelling of PNIPAm.


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195


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Based on these findings, it can be concluded that PS-b-PNIPAm-b-PS copolymers assembled

197

in the presence of 1,8-ANS in the similar manner as in our previous work19 although the

198

structures could not be verified by TEM. The crosslinks between the particles seen in the images

199

are not present when the particles are dispersed in water but they are formed during sample

200

drying for SEM imaging.

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Particle size in water at elevating temperature. Based on the polymer chains and block

204

lengths one can expect that the particles formed by PN77.118K are less stable in water and the

205

most sensitive to a temperature change due to the high PNIPAm content. Without taking into

206

account the actual architecture of the particles formed by PN43.65K, one can expect that high PS

207

content makes the particles rigid and hydrophobic. The PN43.65K particles are expected to

208

aggregate even in cold water. Particles formed by PN61.106 are an intermediate case.

209 210

Thermal behaviour of the nanoparticles dispersed in water was studied using dynamic light

211

scattering as a function of increasing temperature. Figure 7 shows hydrodynamic diameters and

212

intensities of the scattered light. Figures S3-S5 show the corresponding size distributions. The

213

scattering intensities of the sphere-forming PN77.118K and gyroidal PN61.106K nanoparticles

214

increased abruptly at 32-34 ºC indicating a strong densification of the particles due to the

215

collapse of PNIPAm segments at its CPT. The PN77.118K particles showed lower intensities in

216

comparison to two other particle types below and above the CPT, which can be understood as

217

loose inner particle structure owing to the higher PNIPAM content. The lamellar PN43.65K

218

nanoparticles showed many-fold higher scattering intensity than those of the other nanoparticles

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below the CPT, which results from a compact, densified particle structure. The intensity

220

decreased slightly upon temperature raise but showed a moderate increase at 30-34 ºC owing to

221

the collapse of PNIPAm at its CPT. The hydrodynamic diameter of the PN77.118K particles

222

gradually decreased from 340 nm with increased temperature to level off to 190 nm at 33 ºC.

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Unfortunately, we cannot be sure that this size solely represents individual PN77.118K particles.

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Some degree of inter particle association is possible. The size of the PN61.106K nanoparticles

225

remained the same of ~200 nm at the whole temperature range. A sudden increase in the size at


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the vicinity of the CPT was caused by strong inter particle interactions and resulted in a bimodal

227

size distribution (see Fig. S4). This bimodality points to an intermediate stage where two

228

simultaneous particle populations occur: shrinking of individual particles and the formation of

229

aggregates, which further shrink upon heating. Between 32-40 ºC, all the particles are aggregated

230

and their size decreased to ~200 nm. The size of the PN43.65K particles slightly decreased along

231

temperature raise: below the CPT the size was above 600 nm, while below the CPT the size was

232

below 600 nm. This size indicated the formation of particle aggregates as it was compared to the

233

individual particles in the TEM images. PN43.65K particles are rigid and rather hydrophobic due

234

to high PS content. In hot water these “secondary” aggregates shrink to some extent. Knowing

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the actual lamellar structure of the PN43.65K particles one can expect water to be trapped within

236

the particle and therefore temperature response should be retarded which is fully supported by

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our experiments.


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Mean hydrodynamic diameter, nm

400

800

PN77.118K nanoparticles 350

700

300

600

250

500

200

400

150

300

100

200

50

100

0

Scattering intensity, cps

0 10

30

50

70

600

2500



2000

Due to bimodal distribution

400 1500 300 1000 200 500

100 0

0 10

30

50

70 2400


2300

600

2200 2100

500

2000 400 1900 300

1800

200

1700

100

1600

0



800

1500 10

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30 50 Temperature, °C

70

239

Figure 7. Mean hydrodynamic diameters (blue diamonds) and intensities of the scattered light

240

(red squares) obtained for the PS-block-PNIPAm-block-PS nanoparticle aqueous dispersions (1

241

w-%) upon increasing temperature. Circled data point for PN61.106K is an intermediate state of

242

particle interactions in the beginning of the micro phase separation and observed as a bimodal

243

size distribution (see figure S4 in the supporting information).

244 245


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Release and binding of 1,8-ANS.

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Pure 1,8-ANS. We studied the diffusion of free 1,8-ANS dissolved in water. This was done in

248

order to determine the controllability of the release of the fluorescent probe encapsulated in the

249

copolymer nanoparticles. The diffusion was measured from the donor chamber through the semi-

250

permeable membrane to the receiver chamber of the cuvette filled with 3.00 × 10-6 M BSA

251

solution. A 1.17 × 10-4 M solution of 1,8-ANS was used in the donor part. It was expected that as

252

soon as the probe enters to the receive compartment it binds to BSA and results in change in

253

fluorescence emission. At 25 °C the diffusion was constant during 3.6 h reaching release of

254

~39% whereas at 45 °C the diffusion was initially faster (8.5 % h-1 within the first 30 min) to that

255

at 25 °C (3.0 % h-1) until slowing at ~39%, ultimately reaching ~44% in 3.6 h, see Figure 8. The

256

difference in the diffusions is explained by temperature which affects the kinetics of the probe. In

257

order to compare the releases at different temperatures the releases from the nanoparticles at 45

258

°C were corrected by a factor given by the ratio of the intensity of emission at 25 °C divided by

259

the intensity of emission at 45 °C.


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Figure 8. The evolution of fluorescent emission intensities of the 1,8-ANS dye upon its binding

262

to BSA (3 µmol L-1) solution in the receiver compartment of the cuvette at 25 ºC and 45 ºC.

263

Temperature dependent diffusions of pure 1,8-ANS (i.e. without nanoparticles) through the

264

membrane are shown on left-up. Error bars from the two runs are shown for every tenth

265

measurement point. Empty nanoparticles refers to the nanoparticles without 1,8-ANS.

266 267

1,8-ANS from the nanoparticles. In all the sample cases, the release of 1,8-ANS from the

268

nanoparticles was faster and larger at 25 ºC than at 45 ºC. Table 2 collects the main results upon

269

the 3.6 h release time where the rate of initial release is characterized within the first 30 min of


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the experiment. As one can note, the nanoparticle samples PN77.118K and PN61.106K showed a

271

delayed release of ~10 min at 25 ºC. The initial release rates for these samples at 25 ºC were

272

calculated starting at the time the release began. At 45 °C, the release of 1,8-ANS in all the

273

samples started readily when inserted. The gyroidal PN61.106K nanoparticles showed the largest

274

release of 1,8-ANS (15.7 % at 25 ºC; 8.1% at 45 ºC) at 3.6 h below and above the CPT whereas

275

that of the sphere-forming PN77.118K nanoparticles was the lowest (10.6 % at 25 ºC; 4.3 % at

276

45 ºC). The release from the PN61.106K nanoparticles at 25 ºC was not only the largest but the

277

initial release rate of 1,8-ANS was clearly faster

278

nanoparticles.

(9.6 % h-1) than those of the other

279 280

Table 2. Initial release rates within the first 30 min and total release at 3.6 hours of 1,8-ANS

281

from different nanoparticles below and above the cloud point temperature of PNIPAm.

Copolymer

Temperature (°C)

Release at 3.6 h (%)

PN77.118K

25 45 25 45 25 45

10.6 ± 0.2 4.3 ± 0.2 15.7 ± 1.1 8.1 ± 2.6 13.1 ± 0.1 4.7 ± 1.1

PN61.106K PN43.65K

Initial release rate (% h-1) 5.8 2.6 9.6 4.0 4.2 2.6

282 283

Effect of nanoparticle structure to the 1,8-ANS release in water. Since we were not

284

able to clarify the phase-separated structures in the nanoparticles using TEM, we performed the

285

similar SEM analysis as was carried out in our previous study of PS-b-PNIPAm-b-PS

286

nanoparticles.19 In this study, the SEM images showed that the incorporation of the fluorescent


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1,8-ANS did not drastically change the self-assembly within the nanoparticles when compared to

288

our previous results without the probe. We hypothesize that 1,8-ANS molecules, which has three

289

phenyl rings, were located within the PS domains in the nanoparticles. This may explain

290

relatively low release percentages for 1,8-ANS from the nanoparticles.

291 292

Figure 9 schematically summarizes how the PS-b-PNIPAm-b-PS nanoparticle internal

293

structures in water direct the release of 1,8-ANS. The scheme which combines the DLS and the

294

fluorescence release studies shows roughly basic differences between the nanoparticles. It

295

appeared that PN77.108K nanoparticles dispersed as single particles in water and were

296

colloidally stable within studied time frame against aggregate formation below and above the

297

CPT of PNIPAm. It also showed the largest particle swelling, that is, water absorption by

298

PNIPAm below the CPT. The strongest aggregation even below the CPT was observed with the

299

lamellar PN43.65K nanoparticles. This could be expected based on the SEM images where the

300

nanoparticles appeared to be in a tightly packed form similar to freshly prepared nanoparticles.

301

These particles needed to aggregate for the colloidal stability, where this stability was provided

302

by PNIPAm chains at particle surfaces. The aggregates did not further aggregate but shrank

303

~10% upon heating. The gyroidal PN61.106K nanoparticles aggregated moderately at the

304

vicinity of the CPT forming colloidally stable aggregates. The highest and fastest 1,8-ANS

305

release can be understood and explained by the fact that PS domains form random cylindrical

306

tunnels throughout the PN61.106K particles of which some end at the particle surface. As it was

307

hypothesized above, the probe located mainly in the PS part. Below the CPT, 1,8-ANS diffuses

308

through the swollen PNIPAm matrix. Above the CPT, the gyroidal PS cylinders squeeze in as

309

the PNIPAm matrix collapses. This compression pushes the probe out from the PS tunnels to the


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Biomacromolecules

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310

outer water. The same type of squeezing out the probe does not apply to the sphere-forming

311

PN77.108K nanoparticles: the PS domains are embedded inside the PNIPAm matrix.

312 313

The lamellar PN43.65K nanoparticles with 150-200 nm diameter have ~4-5 layers of PS. The

314

outmost PS layer corresponds to ~50% of all the PS volume in a nanoparticle. However this PS

315

layer is also covered by a ~5 nm thick surface layer of PNIPAm.19 Considering that most of the

316

1,8-ANS reside inside PS domains, a major proportion of 1,8-ANS in the lamellar nanoparticles

317

have a close access to diffuse out from the outmost PS layer. The relatively constant release at

318

25 °C may be attributed to the sustained diffusion of 1,8-ANS from the outmost PS layer through

319

the swollen PNIPAm surface. The diffusion is much slower at 45 °C when the PNIPAm surface

320

has collapsed upon the PS layer. While the collapsing PNIPAm matrix directs pressure upon the

321

gyroidal tunnels from all sides at 45 °C, the outmost PS layer experiences only a modest pressure

322

from the thin PNIPAm surface layer. These structures do not allow an easy release of 1,8-ANS

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form the nanoparticles.

324 325


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Biomacromolecules

In water

Dry nanoparticles

TLCST Shrinking of individual particles at and above the LCST

PN77.108K

Aggregate formation at the vicinity of LCST followed by the shrinking of the aggregate above LCST

PN61.106K

Shrinking of aggregates at and above the LCST

PN43.65K 100 nm

326 327

Figure 9. Schematic illustration for the thermal behavior of the nanoparticles in water based on

328

the DLS measurements and the release of the fluorescent probe 1,8-ANS from the nanoparticles

329

below and above the LCST of PNIPAm.

330 331

CONCLUSIONS

332

We have demonstrated how thermal dependent swelling-deswelling behavior of self-assembled

333

hydrogel nanoparticles composed of PS-b-PNIPAm-b-PS block copolymers affect the shrinkage

334

and aggregate formation of the particles. At room temperature, the nanoparticles remained single

335

hydrogel particles dispersed in water when the content of PNIPAm was sufficient high: in this

336

study 77 w-%. However, the tendency toward aggregation increased as the content of PS

337

increased and the particle morphology became lamellar. Above the CPT of PNIPAm the particle

338

dispersions were stable over the studied time range. The influence of nanoparticle kinetics on the


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Biomacromolecules

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339

release of the fluorescent probe 1,8-ANS below and above the CPT was studied. It appeared that

340

the probe released better below the CPT than above it from all the nanoparticle hydrogels.

341

However, the release of the probe from the nanoparticles with gyroidal morphology showed the

342

highest and fastest release below and above the CPT of PNIPAm. This study allowed us to

343

understand the behavior of thermally responsive hydrogel nanoparticles in water dispersions and

344

its effect on the release of a molecule. This knowledge can be utilized in future studies in

345

designing controlled nanoparticulate drug delivery systems.

346

347 348

ASSOCIATED CONTENT

349

Supporting Information Available: The calibration of the 1,8-ANS fluorescent signal, a

350

schematic of the cuvette assembly used in the release experiments, and the intensity weighted

351

distributions of the hydrodynamic diameter. This material is available free of charge via the

352

Internet at http://pubs.acs.org.

353

354

AUTHOR INFORMATION

355

Corresponding Author

356

*email: [email protected]

357

Notes

358

The authors declare no competing financial interest.


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Biomacromolecules

ACKNOWLEDGMENT We thank the Academy of Finland (Proj. no. 140362) for financial support. This work made use of the Aalto University Nanomicroscopy Center (Aalto-NMC) premises.

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Thermoresponsive Nanoparticles of Self-Assembled Block

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