Anomalous Inverse Hysteresis of Phase Transition in Thermosensitive

Mar 26, 2018 - An anomalous inverse hysteresis behavior of the thermally induced LCST structural phase transition in dextran-graft-PNIPAM copolymer ...
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Article Cite This: J. Phys. Chem. C 2018, 122, 8003−8010

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Anomalous Inverse Hysteresis of Phase Transition in Thermosensitive Dextran-graf t-PNIPAM Copolymer/Au Nanoparticles Hybrid Nanosystem Oleg A. Yeshchenko,*,† Antonina P. Naumenko,† Nataliya V. Kutsevol,‡ Daria O. Maskova,† Iulia I. Harahuts,‡ Vasyl A. Chumachenko,‡ and Andrey I. Marinin§

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Physics Department and ‡Chemistry Department, Taras Shevchenko National University of Kyiv, 60 Volodymyrska St., 01601 Kyiv, Ukraine § Problem Research Laboratory, National University of Food Technology, 68 Volodymyrska St., 01601 Kyiv, Ukraine ABSTRACT: An anomalous inverse hysteresis behavior of the thermally induced LCST structural phase transition in dextran-graf t-PNIPAM copolymer macromolecules with Au nanoparticles has been observed by means of surface plasmon extinction and dynamic light scattering spectroscopy. Namely, the phase transition at heating occurs at temperature lower than one occurs at cooling. The inverse character of observed hysteresis has been rationalized to be caused by Au nanoparticles which play the key role in the change of the “sign” of the hysteresis. Also, Au NPs counteract the formation of PNIPAM aggregates in the solution. Moreover, it has been shown that the PNIPAM macromolecule shrinking at heating leads to an unexpected decrease of the surface plasmon peak area, its blue-shift, and narrowing. Such effects have been rationalized as the result of the sharp jump-like decrease of the refractive index of PNIPAM polymer occurring at the phase transition. It has been found that the observed LCST phase transition is reversible.

1. INTRODUCTION

PNIPAM-based polymers are a promising material in medical applications. The unique properties of poly(N-isopropylacrylamide) (PNIPAM)-based polymers and copolymers indicate them as an innovative drug carrier for drug delivery systems, diagnostic substance carriers, and also as biosensors. The main difficulty in the use of PNIPAM homopolymer in the preparation of delivery systems is its LCST, which is around 31−32 °C. This transition temperature is very close to the human skin; thus, this polymer can be applied for dermal drug delivery and also can be use in dermocosmetic applications as well as for photodynamic anticancer therapy. Hence, LCST for efficient application of the nanosystems based on PNIPAM should be shifted to around 37 °C. Currently, extensive research is going on with regard to the modification of the LCST of the thermosensitive polymers in drug delivery systems for specific targeting of the drugs. On the other hand, recent progress in the controlled syntheses of inorganic nanoparticles (NPs) such as metal NPs (Au NPs in particular) and semiconductor NPs (quantum dots) bring additional potential for their use as drugs such as in hyperthermia treatment or in imaging at the cellular level.7−12 However, these materials also need appropriate delivery vehicle in order for their controlled release and targeted delivery. In this respect, PNIPAM has been proposed as the stimuli-

The growing progress in nanotechnology and life sciences demonstrates an urgent need for novel advanced hybrid materials composed of biocompatible polymers.1−6 The past decade has seen the publication of hundreds of papers involving poly(N-isopropylacrylamide) (PNIPAM) as homopolymers as constituents of copolymers, gels, microgels, and surface layers. This interest is driven by the famous lower critical solution temperature (LCST) behavior, where heating an aqueous solution of PNIPAM above 32 °C induces phase separation. Such polymers, when introduced in the formulation in solution form, enable it to undergo a reversible, temperature-induced gel−sol transition upon heating or cooling of the solution. This reversible gel−sol transition is associated with the LCST (lower critical solution temperature) of the thermosensitive polymers. Below this temperature, the solution is homogeneous, the polymer chains are swollen, and the polymer exists in watersoluble form. At this stage, water and hydrophilic moieties of the polymer are bound to each other. This prevents interactions of the polymer chains and intrapolymer association. Above this temperature, a phase transition takes place. At this stage, the hydrogen bonds between the water molecules and the hydrophilic moieties are disrupted; water is expelled from the polymer chains which leads to their contraction, and subsequently they shrink. Hydrophobic interactions among the polymer chains persist and lead to the aggregation of the polymer. © 2018 American Chemical Society

Received: January 31, 2018 Revised: March 15, 2018 Published: March 26, 2018 8003

DOI: 10.1021/acs.jpcc.8b01111 J. Phys. Chem. C 2018, 122, 8003−8010

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The Journal of Physical Chemistry C responsive carrier of the NPs.13−15 These applications of PNIPAM/Au NPs hybrids need the knowledge of the peculiarities of the thermally induced phase transition in PNIPAM macromolecule at the nanoscale. The surface plasmon resonance (SPR) in the Au nanoparticles with its famous sensor property gives such possibility. The changes in the surrounding lead to change in the spectral characteristics of SPR. The monitoring of such changes give the information on the phase transition at the nanoscale. There have been some recent reports on the temperature-dependent study of Aufunctionalized PNIPAM using the SPR of Au NPs as the probe.16−24 These studies were devoted to the study of chemical attachment of the NPs on the polymer by appropriately modifying the backbone of the polymer. On the other hand, it is important to know the temperature-dependent interactions between functionalized Au NPs and PNIPAP polymer or its derivative. An understanding of the interaction between the functionalized NPs and the carrier polymer and its temperature sensitivity may play pivotal roles in futuristic drug delivery and chemical sensors.21,25−28 In present work we study the hybrid nanosystem containing the Au NPs with star-like branched PNIPAM with dextran core and grafted PNIPAM arms (D-g-PNIPAM). The aim of this work is to study the peculiarities of the thermally induced phase transition in D-g-PNIPAM at direct and reverse transition over the LCST point (Figure 1). To detect the phase transition in

room temperature for 24 h. The number of grafting sites per dextran backbone was predetermined by the molar ratio of acrylamide to cerium ions,31,32 and it was equal to 15. The sample was designated as D-g-PNIPAM. The reaction path is shown below: the calculated amount of dextran was dissolved in 100 mL of distilled water. This solution was stirred while removal of the dissolved oxygen was achieved by bubbling a gentle flux of argon for about 20 min. Then Ce(IV)/HNO3 initiator (0.125 N HNO3) was injected to obtain desirable grafts number. NIPAM monomer was added, and the polymerization proceeded at room temperature under an argon atmosphere for 24 h. The synthesized copolymers were precipitated into a mixture water−methanol, redissolved in water, and finally freeze-dried. 2.2. D-g-PNIPAM/AuNPs Nanosystem Synthesis. Reduction of Au ions and consequent formation of Au NPs were performed in solutions of the D-g-PNIPAM copolymer. 0.1 M HAuCl4 aqueous solution was added to 1 mL of polymer solution (C = 1 g/L) and stirred for 20 min at t = 25 C. Then, 0.1 M NaBH4 solution was added drop by drop at stirring. The obtained Au sols were stored in cold darkness. The nanosystems were prepared in polymer solution below the concentration of crossover (Guinier regime) for D-gPNIPAM. In the theory of diluted polymer solutions, the intrinsic viscosity is used as a criterion for the estimation of a solution concentration regime. The size of a macromolecule in the solution determines whether the solution is diluted or moderately concentrated, whereas the intrinsic viscosity is proportional to the macromolecular volume in the solution. The solution is diluted if its volume occupied by macromolecules is much smaller than the total solution volume. With the growth of the polymer concentration, the solution structure changes by transforming from isolate macromolecules to aggregates and forming a network of intermolecular links, when achieving a critical concentration of overlapping macromolecular coils, C*, the beginning of the so-called crossover region. In the case of flexible-chain polymers, the critical crossover concentration C* can be determined experimentally by the viscometric method:33,34 C* = 1/η, where η is the intrinsic viscosity. As it was reported in our previous paper35 on the synthesis and characterization of D-g-PNIPAM copolymers, the intrinsic viscosity for D70-g-PNIPAM sample was equal to 1.28 dL/g. Thus, C* = 1.25 g/dL for the sample studied in the present work. We have used the concentration of 0.1 g/dL, which is more than 10 times lower than the concentration of crossover C*. The molecular structure of D-g-PNIPAM polymers and the peculiarities of conformational transition for series of copolymers consisting of dextran core of various size and 15 PNIPAM grafts were reported in ref 35. The synthesized copolymers are star-like with low polydispersity. 2.3. Size-Exclusion Chromatography (SEC). Multidetection size exclusion chromatography (SEC) using a specific SEC line of the Institute Charles Sadron (ICS) at Strasbourg allowed determining the molecular weight distribution and average molecular weights of the branched PNIPAM sample. This SEC line involves a usual HPLC part (Shimadzu) and Malvern’s triple detection TDA 302 (Viscotek) incorporating a refractometer, a viscosimeter, and a two-angle (7° and 90°) light scattering apparatus. The fractionation was carried out through three columns PL gel Mixed B with a precolumn arranged in series. The eluent was N-methyl-2-pyrrolidone (NMP) of HPLC grade with 0.1 M LiBr. Measurements were

Figure 1. Schematics of reversible temperature-induced phase transition in D-g-PNIPAM/Au NPs system at heating and cooling.

D-g-PNIPAM polymer macromolecule, the temperature behavior of the extinction peak of SPR in Au NPs was monitored. The main obtained result is the observation of hysteresis in the temperature dependences of SPR peak area, wavelength, and width at the transition over the LCST point of the D-g-PNIPAM macromolecule. The decrease of light extinction, blue-shift, and narrowing of SPR peak were observed at the increase of temperature at direct transition over the LCST. The SPR blue-shift is quite surprising since usually the red-shift has been observed.19,23,27,29,30 The observed changes of SPR spectral characteristics were rationalized as the result of the local change of the refractive index of D-g-PNIPAM surrounding of Au NPs occurring at the transition over LCST.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Dextran-graft-poly(N-isopropylacrylamide) Copolymer. Dextrans with different molecular weights were purchased from Fluka with characteristics given by the manufacturer: Mw = 7 × 104. Cerium(IV) ammonium nitrate (CAN) from Aldrich was used as initiator. The dextran sample and the ceric salt were used without further purification. N-Isopropylacrylamide (NIPAM) obtained from Aldrich was twice recrystallized from hexane and dried under vacuum at 8004

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The Journal of Physical Chemistry C performed at T = 60 °C using a constant flow rate of 0.5 mL/ mn. PNIPAM solution of concentration 3.33 g/L was filtered on a 0.45 mm membrane prior to being injected. A volume of 100 mL of the solution was thus injected. The molecular parameters of D-g-PNIPAM copolymer we used as matrix for AuNPs fabrication were determined by SEC. Those are shown in Table 1. Table 1. Molecular Parameters of D-g-PNIPAM Determined by SEC sample

Mw × 10−6 (g/mol)

Mn × 10−6 (g/mol)

Mw/Mn

D-g-PNIPAM

1.03

0.674

1.52

It was reported31,32 that the distance between grafted chains determines its conformation. For individual D-g-PNIPAM sample, the conformation of grafted chains is worm-like. Decreasing of hydrodynamic diameter for D-g-PNIPAM macromolecule from 31 to 22 nm within the temperature interval 32.6−33.4 °C and sharp aggregation process at 33.7− 34.1 °C were observed in ref 35. Further heating to 36 °C does not cause additional aggregation. The reversibility of conformational transition for synthesized D-g-PNIPAM copolymers was proven. It was shown that the temperature of conformational transition was 2−4 °C higher than LCST point for linear PNIPAM of similar molecular weight and polydispersity.36 2.4. Transmission Electron Microscopy (TEM). For the sample preparation 400 mesh Cu grids with plain carbon film were rendered hydrophilic by a glow discharge treatment (Elmo, Cordouan Technologies, Bordeaux, France). A 5 μL drop was deposited and let adsorbed for 1 min; then the excess of solution was removed with a piece of filter paper. The observations of the GNPs were carried on two TEMs, Tecnai G2 or CM12 (FEI, Eindhoven, Netherlands), and the images were acquired with a ssCCD Eagle camera on the Tecnai and a Megaview SIS camera on the CM12. A typical TEM image of D-g-PNIPAM/AuNPs nanosystem obtained at temperature of 25 °C is presented in Figure 2. It is seen that AuNPs have size of d = 8 ± 3 nm and are spherical in shape. AuNPs were synthesized in dilute aqueous solution of D-g-PNIPAM (polymer concentration was below the concentration of crossover). Thus, the distance between the neighboring PNIPAM macromolecules is larger than the macromolecule size by several times. 2.5. Dynamic Light Scattering (DLS) and Zeta Potential Analysis. DLS measurements were carried out using a Zetasizer Nano ZS90 (Malvern Instruments Ltd., UK). The apparatus contains a 4 mW He−Ne laser with a wavelength 632.8 nm, and the scattered light is detected at an angle of 173° (backscattering). For accurate transition study, correlograms of 0.1 mg/mL aqueous D-PNIPAM were collected in the temperature range of 23−46 °C with step 0.1 °C. Each temperature point was held for 5 min before measurements to equilibrate the sample. At least 20 correlation curves for each temperature points were treated by the CONTIN algorithm.37 This analysis is reliable for complicate systems35 to get hydrodynamic diameter (DH) distributions. Zeta potential analysis is a technique for determining the surface charge of NPs in solution (colloids). NPs have a surface charge that attracts a thin layer of ions of opposite charge to the nanoparticle surface. This double layer of ions travels with the nanoparticle as it diffuses throughout the solution. The electric potential at the boundary of the double layer is known as the

Figure 2. TEM image of the D-g-PNIPAM/AuNPs hybrid nanosystem.

zeta potential of the particles. The zeta potential measurements were carried out using a Zetasizer Nano ZS90 (Malvern Instruments Ltd., UK). 2.6. Light Extinction Spectroscopy. The light extinction spectra were measured using a Cary 60 UV−vis spectrophotometer (Agilent Technologies, Inc.). The D-g-PNIPAM/ AuNPs aqueous solution was placed in a 1 cm × 1 cm × 1 cm cuvette. The cuvette was placed into an open furnace during the spectral measurements. The extinction spectra were measured in the temperature range of 23−46 °C in straight and reverse temperature directions. The transition from one temperature point to another was carried out for 10 min; thus, the heating and cooling of the sample were carried out quite slowly. Each temperature point was held for 5 min before measurement to equilibrate the sample. The spectra of optical density D = log(I0/I) were measured, where I0 is the intensity of light passed through the aqueous solution of D-g-PNIPAM polymer without Au NPs and I is the intensity passed through D-g-PNIPAM/AuNPs solution. Let us note that both spectra of the D-g-PNIPAM without Au NPs and the D-g-PNIPAM/AuNPs hybrid nanosystem were measured at the same temperature. Therefore, the spectra presented below are ones of gold NPs affected by the temperature-induced changes in their D-g-PNIPAM polymer surrounding.

3. RESULTS AND DISCUSSION The extinction spectra of D-g-PNIPAM/AuNPs dilute aqueous solution were measured at gradual heating of the sample from 23 up to 46 °C with subsequent its cooling down to 25 °C. The evolution of the extinction spectra of AuNPs was studied at the heating and cooling of the sample (Figures 3a and 3b, respectively). It is seen that the variation of the temperature leads to appreciable change of the spectrum. There is a clear SPR peak of Au NPs in the extinction spectra with maximum at about 528 nm. To quantitatively analyze the effects of 8005

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Figure 3. Behavior of extinction spectra Au NPs in D-g-PNIPAM/ AuNPs nanosystem at slow gradual heating (a) and cooling (b). The arrows show the spectral shift of the SPR peak.

temperature on AuNPs’ SPR spectral characteristics (area, wavelength, and width) in the studied nanosystem, the spectra were fitted by the basic Lorentzian peaks. The respective dependencies are shown in Figures 4a−c. The main features of the dependences of SPR spectral characteristics on temperature are the following. At the increase of temperature from 23 to 29 °C the SPR peak spectral characteristics do not change. Meanwhile, at temperature increase in the range of 29−42 °C an appreciable 25% decrease of area (decrease of extinction), blue-shift of 6.7 nm, and narrowing by 18% for SPR extinction peak occur. Such behavior of SPR in AuNPs indicates the fact of the structural phase transition in D-g-PNIPAM macromolecule, in which a molecule shrinks. The temperature range in which the phase transition occurs is ΔT = 13 °C. The reverse cooling of the sample leads to inverse behavior of the SPR peak spectral characteristics, i.e., to increase of the area, red-shift, and broadening. However, it is seen from Figures 4a−c that dependences obtained at heating and cooling do not coincide; i.e., the hysteresis are observed for temperature dependences of all SPR spectral characteristics. The hysteresis in LCST (or UCST) phase transition in polymer macromolecules was reported

Figure 4. Dependences of normalized area (a), spectral shift (b), and normalized width (c) of the extinction SPR peak of Au NPs in the D-gPNIPAM/AuNPs nanosystem on temperature at slow gradual heating and cooling. The values of the SPR peak area, shift, and width were taken in relation to the respective values for temperature of 23 °C. The arrows show the direction of the process.

earlier.38−43 The hysteresis effect is a result of the change of hydrophobic−hydrophilic balance of macromolecule which depends on the shape and size distribution of dispersed polymer molecules.42,43 Let us note that in all works known to us the hysteresis was determined by measuring the transmittance of the polymer solution which sharply decreased when the solution became turbid at the crossover the phase transition 8006

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The Journal of Physical Chemistry C point. In our work, we for the first time detected the hysteresis at LCST phase transition using the AuNPs’ plasmonic sensor. Let us note that the reverse changes at cooling occurs at temperatures higher by about 4−5 °C compared to changes at the heating. Taking into account that temperature range of phase transition is about 13 °C, the hysteresis loop is quite broad; i.e., the hysteresis character of the phase transition is clearly pronounced. Another interesting feature of observed phase transition is its unusual behavior; namely, the reverse changes of SPR spectral characteristics at the sample cooling occur at higher temperatures than ones which occur at the sample heating; i.e., an inverse hysteresis was observed. Let us note that in prevailing number of reports the direct hysteresis was observed; i.e., the phase transition at heating occurred at temperatures higher than at cooling (see e.g. refs 39−41). The inverse hysteresis, when the phase transition at heating occurred at temperatures lower than at cooling, is a quite rare phenomenon which was not observed for PNIPAM but was observed for poly(2-methacrylamidocaprolactam)-co-(N,Ndimethylacrylamide).38 Thus, our observation of the inverse hysteresis at LCST phase transition is the first one for PNIPAM to our knowledge. Thus, taking into account the fact of the direct hysteresis in “pure” (without AuNPs) PNIPAM macromolecules and the inverse hysteresis in the hybrid PNIPAM/AuNPs nanosystem, one can conclude that the presence of AuNPs plays the key role in the change of the “sign” of the hysteresis. The possible cause of such an effect can be the surface charge of AuNPs which would lead to the repulsion of the AuNPs. The repulsion would aid the volume expansion of the PNIPAM macromolecules causing the occurrence of the phase transition at higher temperature at the cooling. We measured the zeta potential of AuNPs synthesized in solution of D70-g-PNIPAM polymer. We obtained the value of −10 mV. Thus, Au NPs in the studied D70-g-PNIPAM/AuNPs nanosystem are negatively charged. Therefore, one can conclude that our assumption of the important role of repulsion of Au NPs in the inverse hysteresis of phase transition in D70-g-PNIPAM/AuNPs macromolecule seems to be quite reasonable. Note that the repulsion of AuNPs is only one of the possible physical mechanisms of the influence of Au NPs on the phase transition in studied system. The problem of such influence remains open and requires further study. Another point we would like to draw an attention to is the reversible character of the observed LCST phase transition that is seen from the reversible temperature dependences of the SPR peak spectral characteristics (Figures 4a−c). Note also that at next cycles of heating−cooling the temperature dependences of SPR peak spectral characteristics demonstrate the same inverse hysteresis behavior. The observation of the inverse hysteresis behavior of the phase transition in our D-g-PNIPAM/AuNPs nanosystem is confirmed by the results of DLS measurement (Figure 5a). It is seen that at heating the hydrodynamic diameter (size) of D-gPNIPAM/AuNPs macromolecule is 37 nm and is constant up to temperature of 33 °C; then it decreases sharply to 21 nm at 37 °C due to the macromolecule shrinking. At higher temperatures its size remains the same. The cooling from temperature of 42 to 34 °C leads to a sharp increase of the macromolecule size from 23 to 44 nm. Then the macromolecule size decreases slightly to 37 nm at 29 °C. Further cooling does not lead to change of the macromolecule size that is about 37 nm. The increase of the D-g-PNIPAM/AuNPs

Figure 5. Dependences of D-g-PNIPAM/AuNPs hydrodynamic diameter (a) and D-g-PNIPAM hydrodynamic diameter (b) on temperature at slow gradual heating and cooling obtained by DLS. The arrows show the direction of the process.

nanosystem size at the direct phase transition reflects the macromolecule shrinking and proves the fact of slight aggregation of the D-g-PNIPAM/AuNPs macromolecules at the phase transition. Thus, one can conclude that temperatureinduced effects discussed above and presented in Figures 3−5 are caused by the phase transition in separate (unaggregated) D-g-PNIPAM/AuNPs macromolecules. Let us note that similar to the temperature dependences of SPR peak wavelength, width, and area, the temperature dependence of D-g-PNIPAM/ AuNPs macromolecule size demonstrates reversible (reproducible) character (Figures 4a−c and 5a). Thus, one can conclude that the reproducibility of the inverse hysteresis character of temperature-induced phase transition in g-PNIPAM/AuNPs hybrid macromolecule is confirmed by both SPR extinction and DLS spectroscopy. To study the possible influence of Au NPs on the phase transition in D-g-PNIPAM/AuNPs macromolecules, we made the DLS measurements for D-g-PNIPAM without Au NPs. The results are presented in Figure 5b. It is seen that at heating from 21 to 32.5 °C the PNIPAM size is decreases slightly from 32 to 26 nm; then in the temperature range of 32.5−33.5 °C the sharp decrease of size from 26 to 14 nm occurs. Similar to the D-g-PNIPAM/AuNPs system, it is due to shrinking of the separate macromolecules. However, at temperatures higher than 33.5 °C the temperature behavior of the size of D-g8007

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The Journal of Physical Chemistry C PNIPAM without Au NPs differs drastically from one for D-gPNIPAM/AuNPs. Indeed, in the temperature range of 33.5− 34.5 °C the sharp increase of the PNIPAM size from 14 to 86 nm occurs that reflects the fact of the aggregation of PNIPAM macromolecules. At cooling, the temperature behavior of the PNIPAM size is fully reversible, but the temperature of destruction of aggregates (the size decreases) and the temperature of LCST phase transition in the separate macromolecule (the size increases) are lower than the respective temperatures at the heating. So, the usual direct hysteresis is observed for PNIPAM without Au NPs that is in accordance with reference data.39−41 Thus, it proves fully our above assumption of the role of the Au NPs in the change of the sign of the hysteresis at LCST phase transition in D-gPNIPAM/AuNPs macromolecules. Another conclusion that can be made is that gold NPs counteract the formation of aggregates. At last, it is important to note that the size of D-gPNIPAM/AuNPs at the temperatures lower than LCST point (37 nm) is somewhat larger than one of D-g-PNIPAM without Au NPs (32 nm) (Figure 5). All these facts confirm our above assumption of the repulsion of the Au NPs in D-g-PNIPAM/ AuNPs nanohybrid that differs drastically the character of the temperature-induced phase transformations in the PNIPAMbased system. Finally, let us consider the physical mechanisms of observed temperature behavior of Au NPs’ SPR caused by LCST transition in D-g-PNIPAM macromolecule. So, the increase of temperature at phase transition leads to decrease of SPR peak area, blue-shift, and narrowing (Figures 4a−c). First, it is wellknown that crossover the LCST point leads to increase of the light scattering by PNIPAM that leads to increase of the light extinction by PNIPAM solution (see e.g. refs 20 and 21). Let us remind, however, that as noted above in subsection 2.6, the change of the optical density of D-g-PNIPAM solution at phase transition was taken into account. Therefore, the change of extinction value that is seen in Figures 3 and 4a is associated exclusively with Au NPs. Second, the red-shift and broadening of SPR in PNIPAM/metal NPs hybrids at LCST phase transition are usually reported.19,23,27,29,30 These effects were rationalized as the result of the increase of the plasmonic coupling of metal NPs occurring due to their approach at the PNIPAM molecule shrinking. As it is noted above, our samples of D-g-PNIPAM/AuNPs solutions were quite dilute; namely, hybrid nanosystems were prepared in polymer solution below the concentration of crossover (Guiner regime). Therefore, since the Au NPs were located not very close to each other, the shrinking of PNIPAM molecule did not lead to considerable strengthening of the coupling between them. On the other hand, in many of cited works there is not considered an effect of the change of the refractive index of PNIPAM polymer at the LCST phase transition. Meanwhile, the increase of temperature during phase transition in PNIPAM polymer causes the sharp jump-like decrease of its refractive index.44 We considered an effect of sharp decrease of PNIPAM refractive index to rationalize the observed features of the Au NPs’ SPR peak spectral characteristics at the crossover LCST point. Respectively, we calculated the extinction spectra of Au NPs in dependence on the refractive index of surrounding medium nm using Mie theory in a quasi-static approximation45,46 (Figure 6). Performing the peak fitting of calculated spectra, we obtained the dependences of SPR peak area, spectral shift, and width on nm (Figures 7a−c). It is seen that decrease of nm would cause the decrease of SPR peak area, its

Figure 6. Calculated extinction spectra of isolated Au NPs in dependence on the refractive index of surrounding medium nm. The arrow shows the spectral shift of the SPR peak.

blue-shift, and narrowing, which is in full correspondence with effects observed experimentally. Another mechanism of the SPR peak narrowing can be the suppression of the fluctuations of the refractive index of medium that surrounds of the Au NPs occurring due to shrinking of PNIPAM molecules. Indeed, at the temperatures lower than LCST point, the Au NPs move freely with the PNIPAM chains to which NPs are attached. This leads to sufficient fluctuations of the refractive index of surrounding medium and, correspondingly, to broadening of SPR peak. At the temperatures higher than LCST point, the Au NPs become encapsulated into the dense tangle of shrunken PNIPAM molecule where the moving of Au NPs and polymer chains is constrained. Correspondingly, the fluctuations of nm are smaller, and the SPR peak narrows. Thus, the agreement of experimental and calculated results proves that an effect of the jump-like decrease of PNIPAM refractive index at LCST phase transition predominates the strengthening of plasmonic coupling of Au NPs occurring at PNIPAM molecule shrinking. Therefore, an origin of observed temperature effects on the spectral characteristics of Au NPs’ SPR in the D-g-PNIPAM/AuNPs hybrid nanosystem at the LCST phase transition is the jump-like decrease of the refractive index of PNIPAM polymer.

4. CONCLUSIONS In conclusion, the extinction spectra of Au nanoparticles in dilute aqueous solution of dextran-graf t-PNIPAM/AuNPs copolymer hybrid macromolecules have been measured at slow successive heating and cooling of the sample in the temperature range of 23−46 °C. Additionally, the dynamic light scattering measurements have been performed in the same temperature range. Both spectroscopic methods show the fact of existence of the thermally induced LCST structural phase transition in PNIPAM macromolecule which is accompanied by its shrinking. It has been shown that the macromolecule shrinking at heating leads to unexpected decrease of surface plasmon peak area, its blue-shift, and narrowing. Such effects have been rationalized as the result of the sharp jump-like decrease of the refractive index of PNIPAM polymer occurring at the phase transition. Moreover, an anomalous inverse hysteresis behavior of the observed LCST phase transition in D-g-PNIPAM macromolecules with Au nanoparticles has been observed by surface plasmon extinction and dynamic light scattering spectroscopy. Namely, the phase transition at heating 8008

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by Au NPs which play the key role in the change of the “sign” of the hysteresis. Another conclusion that can be made is that Au NPs counteract the formation of PNIPAM aggregates. At last, at the temperatures lower than LCST point the D-gPNIPAM/AuNPs macromolecules is larger than D-g-PNIPAM ones without Au NPs. The possible cause of such effects can be the negative surface charge of Au NPs which leads to the repulsion of the Au NPs and, correspondingly, to the volume expansion of the PNIPAM macromolecules. Finally, another important obtained result is the reversible character of the observed LCST phase transition that is confirmed both by the reversible temperature dependences of the SPR peak spectral characteristics and size of PNIPAM macromolecules.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] [email protected] (O.A.Y.). ORCID

Oleg A. Yeshchenko: 0000-0001-8927-5673 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NATO Science for Peace and Security (SPS) Programme (grant NUKR.SFPP 984617). REFERENCES

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Figure 7. Calculated dependences of normalized area (a), spectral shift (b), and normalized width (c) of extinction SPR peak of Au NPs in dependence on the refractive index of surrounding medium. The values of SPR peak area, shift, and width are taken in relation to the respective values for nm = 1.

occurs at temperature lower than one occurs at cooling. Meanwhile, the phase transition in PNIPAM macromolecules without Au NPs is common, i.e., the direct one. The inverse character of observed hysteresis has been assumed to be caused 8009

DOI: 10.1021/acs.jpcc.8b01111 J. Phys. Chem. C 2018, 122, 8003−8010

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