Fully Reversible Quantitative Phase Transfer of Gold Nanoparticles

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Fully Reversible, Quantitative Phase Transfer of Gold Nanoparticles using Bifunctional PNIPAM Ligands Tobias Honold, Dominik Skrybeck, Kristina G. Wagner, and Matthias Karg Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03874 • Publication Date (Web): 08 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016

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Fully Reversible, Quantitative Phase Transfer of Gold Nanoparticles using Bifunctional PNIPAM Ligands Tobias Honolda , Dominik Skrybecka, Kristina G. Wagnera,b and Matthias Kargb* a

Physical Chemistry I, University of Bayreuth, Universitaetsstr. 30, 95447 Bayreuth, Germany

b

Physical Chemistry I, Heinrich-Heine-University Duesseldorf, Universitaetsstr. 1, 40204 Duesseldorf, Germany

KEYWORDS: reversible phase transfer; ligand exchange; bifunctional polymer; stimuliresponsive; gold nanoparticle

Abstract: Ligand exchange with end-functionalized polymers is often applied to render nanoparticles with enhanced colloidal stability, to change the solubility in various environments and/or to introduce new functionalities. Here we show that exchange of citrate molecules with α-trithiocarbonate-ω-carboxyl terminated poly-(N-isopropylacrylamide) can successfully stabilize spherical gold particles of different diameters ranging from 15 to 53 nm. This is verified by transmission electron microscopy, dynamic light scattering and extinction spectroscopy. We show that the polymer-decorated nanoparticles respond to temperature and pH allowing access to control inter-particle interactions. In a range of pH slightly below the pKa of the terminal carboxyl groups, phase transfer of the particles from water to chloroform 1 ACS Paragon Plus Environment

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can be mediated by increasing the dispersion temperature above the lower critical solution temperature of poly-(N-isopropylacrylamide). Upon cooling fully reversible phase transfer to the water phase is observed. Extinction spectroscopy revealed phase transfer efficiencies close to 100% for every system under investigation.

INTRODUCTION Van der Waals forces between two bodies of the same material are always positive independent of the medium in between the two.1 Because of this, colloids tend to irreversibly aggregate and are considered to be thermodynamically metastable. Repulsive forces that typically counteract the attractive van der Waals forces in colloidally stable systems are electrostatic and steric interactions. The surface chemistry of colloids dictates the interparticle interactions, i.e. the presence of charges or polymer chains on the particle surface. Consequently, the modification of the colloid surface allows access to manipulate colloidal stability. Such a modification is often realized by ligand exchange where a new ligand replaces the initial ligands/molecules adsorbed on the colloid.2-4 Ligands are frequently used to enhance the colloidal stability5-7, to alter solubility in different environments8-10 and/or to introduce additional functionalities to the system.11-13 Ligand exchange can be conducted in a single phase employing mass exchange14 and/or a higher binding affinity15 of the new ligand to the colloids or in a two-phase system by phase transfer.16-18 Gold nanoparticles (AuNP) represent a convenient model system to study the ligand exchange process due to their optical properties related to the localized surface plasmon resonance (LSPR). This also allows the easy monitoring of the particle distribution in different solvents, e.g., whether the NPs are well dispersed in water or in an organic solvent. Furthermore, ligand exchange typically changes the refractive index environment around the NPs influencing the LSPR position and strength.19, 20 Particle destabilization can be monitored by far field UV-Vis spectroscopy, as this leads to a significant shift and broadening of the LSPR.21 Furthermore, AuNPs were in 2 ACS Paragon Plus Environment

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the focus of intense research in the last decades motivated by their unique optical, electric and catalytic properties making them interesting for different applications.22-26 Many of those applications require sufficient colloidal stability and compatibility with different dispersion media. A clear drawback of gold is its high Hamaker constant leading to strong attraction e.g. in water.27 This is the reason why AuNPs typically aggregate irreversibly and ligand exchange is challenging. In this work, we show that bifunctional α-trithiocarbonate-ω-carboxyl terminated, poly-(Nisopropylacrylamide) (PNIPAM) ligands successfully stabilize AuNPs with a broad range of sizes in water. Therefore, PNIPAM ligands of different molecular weights were synthesized by reversible addition-fragmentation chain-transfer polymerization (RAFT).28 Ligand exchange using citrate-stabilized AuNPs was achieved at neutral pH with the PNIPAM ligands offered in excess. As schematically illustrated in figure 1, ligand binding to the NP surface is achieved through the terminal trithiocarbonate group. The terminal carboxyl group renders the functionalized particles negatively charged at neutral pH. Hence, the PNIPAM encapsulated particles possess high colloidal stability and are well-dispersed in water as verified by dynamic light scattering (DLS) and extinction spectroscopy. We show that phase transfer of the polymer-decorated NPs from water to chloroform is possible at temperatures above the lower critical solution temperature (LCST) of PNIPAM. The success of the phase transfer strongly depends on the particle charge controlled by the dispersion pH. Only in a narrow pH regime, quantitative phase transfer occurs as manifested by extinction spectroscopy. Upon cooling below the LCST, the transfer is completely reversible. This behavior is fully reproducible for many heating/cooling cycles because the bifunctional PNIPAM ligands allow the reversible control of inter-particle interactions through pH and temperature.

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Figure 1. Illustration of the reversible phase transfer of bifunctional PNIPAM decorated AuNPs. The citrate ligands of as-prepared AuNPs are replaced with bifunctional PNIPAM by ligand exchange at pH 7. Lowering the pH to 2.7 reduces the charge density of the polymer decorated NPs. Fully reversible phase transfer between water and chloroform can be triggered by temperature. The inset shows the chemisorption of the trithiocarbonate functionality to the gold nanoparticle surface.

EXPERIMENTAL SECTION MATERIALS 2,2’-Azoisobutyronitrile (AIBN, 98 %, Aldrich) was recrystallized twice from methanol. Acetone (>99.8 %, Roth), 4,4´-Azobis(4-cyanovaleric acid) (ACVA, >98%, Aldrich), chloroform (>99 %, Fisher Scientific), dimethylformamide (>99.8 %, Aldrich), 2(dodecylthiocarbonothioylthio)-2-methylpropanoic acid (DDMAT, 98%, Aldrich), gold(III) chloride hydrate (HAuCl4, 99.999%, Aldrich), hydrochloric acid (0.1 M, Grüssing), N4 ACS Paragon Plus Environment

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isopropylacrylamide (NIPAM, >98%, TCI), and sodium citrate dihydrate (>99 %, Aldrich) were used as received. Phase transfer experiments were carried out in ultrapure water with a resistivity of 18 MΩ cm. SYNTHESIS

Au15 and Au19 particles. Citrate-stabilized AuNPs of 15 and 19 nm in diameter were synthesized by citrate reduction according to the Turkevich protocol.29 Two batches of AuNPs were prepared. Each time, 494.7 ml H2O and 5.25 ml of an aqueous solution of HAuCl4 (47.6 mM) were combined and heated until heavy boiling. Next, 8.33 ml of a hot (approximately 70°C) aqueous sodium citrate solution (100 mM) were added quickly under heavy stirring. After 15 min of additional boiling, the dispersion was cooled down rapidly in an ice bath. The second batch was prepared in the same way, however the particles were cooled down to room temperature slowly, without the use of an ice bath. Size analysis by TEM revealed mean diameters of 15 ± 2 nm (Au15) for the first batch and 19 ± 2 nm (Au19) for the second batch of particles, respectively.

Au22-Au53 particles. Citrate-stabilized AuNPs with diameters of 22 – 53 nm were synthesized by seed mediated growth according to the protocol by Bastús et al.30 Seed particles were prepared by heating 150 ml of an aqueous sodium citrate solution (2.2 mM) until heavy boiling. Next, 1 ml of an aqueous HAuCl4 solution (25 mM) was added quickly. The temperature was lowered to 90°C and the seeds were overgrown with gold by three repetitive additions of 1 ml HAuCl4 (25 mM), each in an interval of 30 min. After additional 30 min, 55 ml of the solution were removed and collected as the first generation of particles. Subsequent core growth was performed by addition of 53 ml H2O, 2 ml sodium citrate (60 mM) and HAuCl4 (25 mM) under the same conditions (3 x 1 ml, 30 min each). Seeded growth was continued until five generations of particles (with increasing core dimensions) were collected. Size analysis by TEM revealed average particle dimensions of 22 ± 3 (Au22), 29 ± 3 (Au29), 36 ± 3 (Au36), 44 ± 3 (Au44), and 53 ± 7 (Au53) nm in diameter. 5 ACS Paragon Plus Environment

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PNIPAM ligands. α-trithiocarbonate-ω-carboxyl terminated PNIPAM ligands were synthesised by RAFT polymerization according to the synthetic protocol of Ebeling et al.28 A detailed summary of the amounts of reactants is provided in table 1. In brief, NIPAM, DDMAT (RAFT agent), initiator and DMF were combined in a glass vial and degassed for 20 min with nitrogen at room temperature. The vial was immersed in a heated oil bath at 70°C. After 3 h (1 h for lowest molecular weight) of polymerization, the reaction was stopped by immersion of the vial in an ice bath and exposure to air. The reaction yield was determined by proton nuclear magnetic resonance spectroscopy (1H-NMR) and used for the calculation of the number-average molecular weight Mn. Details are found in the SI of the manuscript, equation (1). Next, the reaction products were precipitated three times in diethyl ether, collected by centrifugation and redissolved in acetone. Finally, the polymers were dried under vacuum. Size exclusion chromatography (SEC) traces and 1H-NMR spectra of the purified polymers are presented in the SI of the manuscript, figures S1 – S4. The different polymers are labelled PNIPAMy with y being the value of Mn. The LCSTs of PNIPAM9.5k, PNIPAM40k and PNIPAM82k were determined by temperature dependent UV-Vis spectroscopy as shown in figure S5. We found values of 31, 33 and 32°C for PNIPAM9.5k, PNIPAM40k and PNIPAM82k, respectively.

Table 1. RAFT polymerization conditions for the three different PNIPAM ligands. Sample

Initiator mol equiv. mol equiv. mol equiv. mol. equiv. Mn (NMR) NIPAM Initiator DDMAT DMF (kg/mol)

PNIPAM9.5k ACVA

1

3.6 x 10-4

5.0 x 10-3

3.0

9.5

PNIPAM40k

1

6.2 x 10-4

2.7 x 10-3

3.2

40.0

1

-4

3.2

82.1

PNIPAM82k

AIBN AIBN

6.1 x 10

1.48 x 10

-3

Particle functionalization via ligand exchange. At first, the as-synthesized AuNP dispersions were diluted by the same volume of H2O prior to ligand exchange. The exact concentrations of the nanoparticle dispersions are listed in table 2. Next, the particles were 6 ACS Paragon Plus Environment

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functionalized with PNIPAM ligands by dropwise addition of an aqueous PNIPAM solution (1 wt%) under heavy stirring. The polymers were added in 100 fold excess considering a grafting density of approx. 1 chain/nm2.7 As a typical example, 2 ml of a dispersion of Au15 particles were functionalized with 73.6 µl of a PNIPAM9.5k solution. Stirring was continued for 30 min. Afterwards excess polymer was removed by centrifugation at 5400 rcf until the supernatant was completely colourless. The same speed was used for systems under investigation. Depending on the particle size and ligand type the centrifugation time varied between 30 min and 12 hours. The precipitate was redispersed in H2O and the purification was repeated twice.

Table 2. Particle concentrations of AuNP dispersions and volume of ligand solution used for the functionalization of citrate-stabilized Au nanoparticles by ligand exchange. The concentrations of the aqueous PNIPAM dispersions were 1 wt% for all ligands.

Sample

cpart. V (PNIPAM9.5k) V (PNIPAM40k) V (PNIPAM82k) (particles/ml)a (µl) (µl) (µl)

Au15

3.3 x 1011

73.6

310.1

635

Au19

9.7 x 1010

35.7

150.2

308

Au22

5.8 x 1011

278.9

1174.5

2408

Au29

3.5 x 1011

363.3

1223.9

2509

Au36

2.7 x 1011

330.1

1389.8

2849

Au44

1.4 x 1011

267.2

1125.1

2306

Au53

9.3 x 1010

259.6

1093.2

2241

a

cpart, particle concentration of the Au dispersions calculated from the absorbance at λ = 400 nm. A detailed example for the calculation of cpart is provided in the SI, table S2.

Reversible Phase transfer. Phase transfer experiments were carried out in 2 ml snap-cap tubes. 500 µl of the respective polymer functionalized AuNP dispersion were combined with 7 ACS Paragon Plus Environment

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500 µl of an aqueous ligand solution (1 wt%) and incubated for 30 min at room temperature. Next, 500 µl CHCl3 was added and the pH of the water phase was lowered to approx. 2.7 – 2.9 by addition of 20 µl HCl solution (0.1 M). The tube was placed in a metal block thermostat (ThermoMix, MKR 13, HLC BioTech) at 60°C to promote phase transfer into the organic phase. Reversible phase transfer to the water phase was performed by lowering the temperature to 4°C. The reproducibility of the phase transfer process was investigated by switching between 4°C and 60°C for 8 times at least. Phase transfer experiments were performed under these conditions using all AuNP batches.

CHARACTERIZATION

DLS and zeta potential. Dynamic light scattering (DLS) and zeta potential (ζ-potential) experiments were carried out on a NanoZS Zetasizer (Malvern Instruments) using a He-Ne laser with λ = 633 nm at a detection angle of 173°. Intensity-time autocorrelation functions of dilute aqueous particle dispersions were recorded at 25°C. The hydrodynamic diameter were obtained by Cumulant analysis of the correlation functions provided by the instruments software. Every sample was equilibrated for 2 min prior to the measurement and three correlation functions were recorded with 60 sec acquisition time. ζ-potentials were determined from electrophoretic mobility measurements at 25°C using the particle dispersions as prepared for the DLS experiments. Electrophoretic mobilities were determined by electrophoresis and measurement of the particle velocity by Laser Doppler Velocimetry. This was performed three times for each sample. Electrophoretic mobilities were calculated into ζpotentials by the instruments software using the Einstein-Smoluchowski relation.

SEC. Size exclusion chromatography (SEC) was performed on a liquid chromatograph (Waters Associates) equipped with a UV- and refractive index (RI)-detector at 23°C. THF containing 0.25 wt. % tetrabutylammonium bromide was used as eluent at a flow rate of 0.5 ml/min. The column setup consisted of a guard column (PSS, 4 x 0.9 cm, styrene8 ACS Paragon Plus Environment

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divinylbenzene copolymer (SDV) gel, particle size 5µm, pore size 100 Å) and two separation columns (Varian, 30 x 0.8 cm, mixed C gel, particle size 5 µm). Polystyrene was used as a calibration standard.

TEM. Transmission electron microscopy (TEM) was carried out on a ZEISS CEM 902 TEM operating in bright field mode at 80 kV acceleration voltage. Samples were prepared by drop casting of dilute particle dispersions on carbon coated copper grids (200 mesh, Electron Microscopy Sciences). Particle diameters were obtained by image analysis using the ImageJ software.

UV-Vis extinction spectroscopy. Absorbance spectra were measured on a UV-Vis spectrophotometer (Specord 250, Analytik Jena) in a wavelength range of 325 to 1100 nm. AuNPs were measured in dilute aqueous dispersions before and after the ligand exchange using quartz cells (Hellma) with 1 cm light path. During the phase transfer experiments, spectra of H2O and CHCl3 phases were recorded in quartz cells (Hellma) with 1 mm light path.

RESULTS AND DISCUSSION

Ligand Exchange. We synthesized α-trithiocarbonate-ω-carboxyl terminated poly-Nisopropylacrylamide (PNIPAM) ligands with three different molecular weights by RAFT polymerization following a recently published synthetic protocol.28 These ligands combine the LCST behavior of PNIPAM with the pH-sensitive protonation/deprotonation property of the terminal carboxyl groups. Hence, the ligands respond to temperature and pH, rendering them bifunctional. We determined molecular weights of 9.5 (PNIPAM9.5k), 40 (PNIPAM40k) and 82 kg/mol (PNIPAM82k) by 1H-NMR. The molecular weight distributions were determined by SEC. The polydispersity indices (PDI) are in the range of 1.2 – 1.5, typical for RAFT polymerization. Further details can be found in the Supporting Information (SI), figures S1 – S4 and table S1. The terminal trithiocarbonate group is known to chemisorb to gold surfaces 9 ACS Paragon Plus Environment

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and can be employed as anchor group for the functionalization of AuNPs.28,

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31

In order to

study the ligand exchange with the three PNIPAM polymers, we synthesized AuNPs in aqueous phase by different citrate reduction protocols. Nanoparticles with diameters of 15 and 19 nm were obtained by a method adopted from the original protocol of Turkevich et al.29 Larger particles between 22 and 53 nm in diameter were obtained by seed mediated growth according to a recent protocol by Bastús et al.30 In the following, we will use the nomenclature Aux for the particles with x being the particle diameter in nanometer. All particles were of spherical shape with low polydispersity as analysed by TEM. Figure S6 in the SI shows representative bright-field TEM images and particle size distributions of all particles. The mean particle diameters d and the corresponding standard deviations are listed in table 3. The optical properties of the nanoparticles were analysed by UV-Vis extinction spectroscopy. All samples show single narrow peaks in the absorbance spectra, which correspond to the dipolar localized surface plasmon resonance (LSPR). The LSPR positions (see table 3) shift to higher wavelength with increasing particle size as expected.32 Ligand exchange was performed by mixing the aqueous AuNP dispersions with the polymer ligands dispersed in water. We used excess amounts of the PNIPAM ligands in order to avoid nanoparticle aggregation and to promote the ligand exchange. Further details can be found in the Experimental Section. Due to the higher binding affinity of the terminal trithiocarbonate group to gold, the initial citrate molecules can be replaced.28 This is expected to lead to a homogeneous encapsulation of the AuNPs with linear PNIPAM chains and terminal carboxyl groups as illustrated schematically in figure 1. Figure 2 shows selected bright-field TEM images of the Au15 and Au53 particles before (A, E) and after the ligand exchange with PNIPAM9.5k (B, F), PNIPAM40k (C, G) and PNIPM82k (D, H).


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Figure 2. TEM images of Au15 (top row) and Au53 particles (bottom row) before ligand exchange, stabilized with citrate (A and E), and after ligand exchange using PNIPAM9.5k (B and F), PNIPAM40k (C and G) and PNIPAM82k (D and H). The scale bar corresponds to 100 nm. TEM images of all particle/polymer combinations can be found in the SI, fig. S7 - 9. The as-prepared citrate-stabilized gold particles (fig. 2 A, E and fig. S6) form agglomerates on the TEM grids upon sample preparation by drop-casting from dilute aqueous dispersion. This points to a rather weak stabilization of the particles. It is important to note that the appearance of particle agglomerates is only a reason of the sample preparation. It is however possible to avoid particle aggregation by a more elaborate sample preparation as recently demonstrated by del Pino et al.2 In our work we used the same sample preparation for TEM for citrate stabilized particles and particles after ligand exchange to have directly comparable results. For this preparation the weak colloidal stability for citrate as stabilizer becomes evident by the aggregation on the TEM grids. In dispersion, the particles are well-dispersed without any evidence for agglomeration. This was confirmed by narrow LSPR peaks and the absence of any absorbance features in the high wavelength regime (> 700 nm) as measured by UV-Vis absorbance spectroscopy and through monomodal particle size distributions as measured by 11 ACS Paragon Plus Environment

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DLS. Once the particles were functionalized by PNIPAM, such agglomerates are no longer found. Instead, the particles are well separated and a low-contrast polymer shell homogeneously surrounding the particles is visible in the TEM images. We want to highlight that figure 2 is merely an overview of selected Aux-PNIPAMy systems demonstrating that the PNIPAM ligands can be grafted onto small particles, as well as substantially bigger ones. In total, 21 different combinations of AuNPs and PNIPAM ligands were investigated covering a size range of 15 to 53 nm of the particles and molecular weights of 9.5, 40 and 82 kg/mol for the polymer ligands. The polymer functionalization worked successfully for every combination tested in our study, irrespective of the size of the nanoparticle or the molecular weight of the polymer. The TEM images show the particles adsorbed on a TEM grid and exposed to the high vacuum in the TEM chamber. This results in a collapse of the polymer shell hampering a reliable analysis of the shell thickness. Furthermore, the significantly lower contrast of the PNIPAM ligands as compared to the AuNPs restricts any precise analysis of the shell morphology and dimensions. A much better technique to study the shell thickness of the Aux-PNIPAMy particles in bulk under equilibrium conditions is DLS. Investigation of the citrate-stabilized AuNP by DLS revealed single size populations without any indication for the presence of aggregates. Intensity weighted particle size distributions of all Aux-PNIPAMy particles are presented in the SI (figure S10). The hydrodynamic diameters dh, listed in table 3, are slightly larger as compared to the diameters obtained by TEM analysis. This stems from the solvation shell around the nanoparticles in solution. After ligand exchange with PNIPAM9.5k, a significant increase of the hydrodynamic diameters by approximately 20 nm is observed for all particles. This is a clear support for the success of the ligand exchange and the anchoring of the polymer ligands to the NP surface. For samples with the higher molecular weight ligands, PNIPAM40 and PNIPAM82, the increase of dh is in the order of 40 and 70 nm, respectively. As expected the ligand shell thickness increases with increasing


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molecular weight. The results from DLS underline that the ligand exchange was successful for all particle sizes independent of the molecular weight of the ligand.

Table 3. Summary of the results of the investigation of Aux-PNIPAMy particles by TEM, DLS and UV-Vis.

Citrate

PNIPAM9.5k

PNIPAM40k

PNIPAM82k

Sample

d [nm]a

dh [nm]b

λLSPR [nm]c

dh [nm]

λLSPR [nm]

dh [nm]

λLSPR [nm]

dh [nm]

λLSPR [nm]

Au15

15

18.4

519

44.2

525

60.0

525

82.5

525

Au19

19

23.6

521

48.1

526

72.6

526

96.1

526

Au22

22

32.3

523

54.5

527

76.4

527

99.5

528

Au29

29

34.4

526

58.7

531

75.3

531

91.5

531

Au36

36

39.9

533

62.6

537

81.9

537

107.1

538

Au44

44

41.8

534

68.6

539

89.2

539

122.9

539

Au53

53

54.7

537

75.5

542

99.4

542

135.8

542

a

d, AuNP diameters as obtained from TEM.

b

dh, Intensity weighted, z-average hydrodynamic diameter of Aux-PNIPAMy obtained by Cumulant analysis of intensity-time autocorrelation functions. Each measurement was performed at 25°C. The standard deviation of all measurements is below 5%. c

λLSPR, position of the LSPR.

Another convenient way to verify the ligand exchange procedure is UV-Vis extinction spectroscopy. The LSPR of the gold particles is very sensitive to changes in the refractive index environment. This is demonstrated for the ligand exchange of citrate-stabilized Au22 particles in aqueous dispersion in figure 3A, for example. After the PNIPAM functionalization with PNIPAM9.5k, PNIPAM40k and PNIPAM82k, a small redshift of the LSPR peak of approximately ∆λ = 5 nm is observable in all three cases. This redshift can be 13 ACS Paragon Plus Environment

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attributed to the increase of the surrounding refractive index due to the PNIPAM ligands that have a higher refractive index as compared to water. The shape and width of the LSPR peaks of all polymer-functionalized samples are almost identical to the citrate-stabilized particles. This confirms that the particles are well-stabilized after ligand exchange with the PNIPAM ligands. Aggregation – if present – would lead to significant broadening of the LSPR. Table 3 provides a detailed summary of the LSPR positions as measured for all polymerfunctionalized nanoparticles. Next, the effect of PNIPAM chain length on the LSPR shift ∆λ was studied for all Aux-PNIPAMy systems (figure 3 B). For all samples ∆λ is small (≈ 5 nm), independent of the size of the NPs and the molecular weight of the ligand. This indicates that all AuNPs were homogeneously encapsulated by the different PNIPAM ligands and we expect similar grafting densities.

Figure 3. Investigation of the PNIPAM functionalization of Aux particles by UV-Vis spectroscopy in aqueous dispersion. Absorbance spectra (A) of Au22 particles stabilized with citrate (black) and functionalized with PNIPAM9.5k (red), PNIPAM40k (green) and PNIPAM82k (blue). The spectra were normalized to the LSPR maximum and then shifted by vertical offsets for better comparison. Change of the LSPR position ∆λ as a function of the Au nanoparticle size for each PNIPAM ligand (B).


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The results from TEM, DLS and extinction spectroscopy confirm that the ligand exchange with α-trithiocarbonate-ω-carboxyl terminated PNIPAM was successful for a broad range of sizes of AuNPs. Furthermore, the ligand exchange could be performed with ligands of different molecular weights. Besides using spherical nanoparticles with citrate stabilization, we could successfully functionalize spherical citrate stabilized silver nanoparticles, as well as spherical gold nanoparticles and gold nanorods that were initially stabilized by cetyl trimethylammonium bromide (CTAB). This emphasizes the versatility of the PNIPAMy ligand as a coating material for silver and gold nanoparticles due to the high affinity of the trithiocarbonate group towards silver and gold surfaces. UV-Vis spectra before and after the ligand exchange are shown in the SI (figure S11).

Reversible phase transfer from water to the organic phase. Reversible phase transfer of NPs between two immiscible phases by means of an external stimulus requires a precise control of the colloidal stability and particle-solvent interactions. Aux-PNIPAMy particles obtained by ligand exchange with α-trithiocarbonate-ω-carboxyl PNIPAM possess both of these characteristics. The polymer-functionalized particles renders the particles stable in water, but they can even be dispersed in various organic solvents such as methanol, DMF and chloroform upon precipitation by centrifugation and redispersion in the corresponding solvent. The stability in these solvents is evidenced by absorbance spectra obtained from UVVis spectroscopy in figure S12. In aqueous solution and at neutral pH, the terminal carboxyl groups are deprotonated and hence the Au-PNIPAM particles possess negative surface charge piding electrostatic stabilization. Figure S13 A shows that the electrophoretic mobility µ of the Au22-PNIPAM9.5k particles at pH 7 is -2.4 10-8 m2/Vs. Lowering the pH below the pKa of the carboxyl groups, protonation leads to neutralization of the polymer shells. The magnitude of µ continuously decreases until a slightly negative value of -0.1 10-8 m2/Vs is reached at and below pH 3. In this regime, the particle stability depends strongly on the solubility of the 15 ACS Paragon Plus Environment

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PNIPAM ligands. Comparable trends are observed for Au22-PNIPAM40k and Au22PNIPAM82k particles in figure S13 B and C. Below the LCST of PNIPAM, polymer/water interactions are favoured and the PNIPAM chains are in the random coil formation stabilizing the gold particles through steric interactions, at least to some extent. In order to monitor the particle stability, we measured DLS below the ligand LCST (at 25°C) as a function of pH. Figure S13 D shows that dh of the Au22-PNIPAM9.5k particles does not change significantly in the pH range of 7.0 to 2.7. This indicates that the particles are stable in dispersion. Once a pH < 2.7 is reached, a strong increase of dh is observed. This increase is attributed to aggregation of the particles indicating poor colloidal stability. The good solvation of the PNIPAM ligands below the LCST does not sufficiently stabilize the particles against aggregation anymore. The same trend is observed for the other two systems shown in figure S13 E and F. Above the LCST of PNIPAM, polymer-polymer interactions are favoured and colloidally stable systems are only obtained at sufficiently high surface charges, i.e. at high pH. Under these poor solvent conditions for PNIPAM, aggregation of the particles starts already at significantly higher values of pH as compared to the scenario at temperatures below the LCST. Hence, pH as well as temperature control the colloidal stability of the particles allowing precise control of inter-particle interactions – from strongly repulsive to attractive. To investigate whether this behavior can be exploited for phase transfer from water to an (immiscible) organic phase, we performed experiments with chloroform as organic medium at various pH conditions of the aqueous phase. Due to the strong absorption of the gold particles in the visible, the success of the phase transfer can be conveniently judged by the optical appearance of the samples, i.e. whether the particles are in the upper water phase, in the lower chloroform phase or at the interface between chloroform and water. At high pH, above the pKa, the magnitude of µ of the particles is large, electrostatic repulsion is strong and the particles remain well-dispersed in the water phase at temperatures below the LCST. Instead, above the LCST, agglomeration can be observed. This is evidenced by a strong increase of the hydrodynamic dimensions of 16 ACS Paragon Plus Environment

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the particles and an increase in the turbidity of the water phase. Under these conditions, the appearing agglomerates were relatively stable and did not transfer to the chloroform phase but remained dispersed in the water phase. Shaking the two-phase system by hand or using a vortexer did not lead to a transfer of the particles to the organic phase. In order to find appropriate conditions for a successful phase transfer, we investigated the influence of pH, and thus surface charge, on the phase behavior at temperatures above the LCST. The results can be categorized in three different regimes. 1) In the pH regime 4 - 7, above the pKa of the carboxylic function of the polymer, phase transfer does not occur. The particles form agglomerates but are still dispersed in the aqueous phase. 2) In the pH regime 2.7 – 4, below the pKa of the carboxylic group, phase transfer to the chloroform phase becomes possible. The transfer is quantitative at a pH of 2.7. At this point, transfer to water is reversible if the temperature is lowered below the LCST of PNIPAM, e.g. to 4°C. In the region 2.7 < pH < 4 only partial transfer of the particles to chloroform is observed. 3) For pH < 2.7 rapid aggregation of the particles in the water phase is observed. It is still possible to transfer the particles to the organic phase, however a reversible transfer to water is not possible anymore. Based on these findings we chose a pH value of 2.7 for further phase transfer experiments of all particles. In this region, the electrostatic stabilization of the particles is very weak and the particles are mainly stabilized by the steric repulsion of the polymer chains. Phase transfer experiments were carried out with all AuNPs and with every PNIPAM ligand. Reversible phase transfer was successful with almost every AuNP and PNIPAM ligand combination. Only in the case of the smallest particles (Au15 and Au19) stabilized with the shortest ligand (PNIPAM9.5k), phase transfer was difficult. The particles only partially transferred to the organic phase and precipitation at the interface was observed. However, fully reversible transfer is possible for Au15 and Au19 decorated with the longer ligands (PNIPAM40k and PNIPAM82k). Figure 4 shows representative photographs of AuxPNIPAM40k samples at different steps of the phase transfer experiments. At pH 2.7 and 4°C 17 ACS Paragon Plus Environment

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the upper, aqueous phase is homogeneously coloured indicating that the particles are welldispersed. Upon heating to 60°C, all particles spontaneously accumulate at the H2O/CHCl3 interface within the course of 5 – 10 minutes. In this temperature and pH regime, the steric and electrostatic stabilization of the particles is very weak. It follows that attractive van der Waals forces cause a rapid agglomeration of the particles in the aqueous phase. These agglomerates sediment quickly to the water/chloroform interface where they remain. The photographs in figure 4 (top) reveal a sharp water/chloroform interface and particle accumulation extending into the water phase as visible by the coloration at the interface and the gradual decrease in color into the water phase. Since the temperature induced particle destabilization in the water phase happens very quickly and because the particle concentration in the aqueous phase by far exceeds the minimum particle number required to cover the liquid/liquid interface by a monolayer, we believe that jamming of the particles and particle agglomerates kinetically traps the NPs at and close to the interface. At this stage, the particles could be easily transferred to chloroform by means of slight agitation of the samples. We can conclude that the initial accumulation and trapping of the particles at the liquid/liquid interface is a kinetic effect. This is supported by the fact that the particles will cross the interface eventually after sufficiently long waiting times, reaching the thermodynamically stable state. The homogeneous coloration of the chloroform phases (figure 4, 60°C series) shows that the phase transfer was successful for all particles independent of the AuNP diameter. Once the temperature is lowered back to 4°C, accumulation of the particles at the water/chloroform interface is again observed. In difference to the previous scenario (heating to 60°C, particles initially dispersed in the water phase), this accumulation occurs during the course of 2-3 hours. Furthermore, agglomeration in the chloroform phase is not observed. Surprisingly the particles remain well-dispersed in the organic phase and nevertheless accumulate slowly at the liquid/liquid interface. Since the particles remain well-dispersed in chloroform at 4°C, the accumulation at the interface occurs as a result of Brownian motion 18 ACS Paragon Plus Environment

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and statistical collisions with the interface. This explains the long duration of the accumulation. Since now, the temperature is below the LCST, the solubility of the polymer ligands is significantly higher in the water phase. Hence, the Gibbs energy is reduced when the ligand shell is solvated by water. This results in a slow and spontaneous transfer of the particles to the water phase. This process however takes place in the course of two weeks. The process can be accelerated significantly by slightly shaking the sample resulting in an instantaneous and complete transfer of Aux-PNIPAMy to the water phase.

Figure 4. Reversible phase transfer of Aux-PNIPAM40k particles from H2O (upper phase) to CHCl3 (lower phase) and vice-versa. One image set contains photographs of Au22, Au29, Au36, Au44 and Au53 particles. At 4°C the particles are dispersed in the H2O phase. Increasing the temperature to 60°C results in particle accumulation at the interface. At this point phase transfer to the CHCl3 phase is possible by slight mechanical agitation. Lowering the temperature back to 4°C results in a fully reversible phase transfer to the water phase. The temperature dependent spontaneous transfer between water and chloroform clearly shows that the thermodynamically stable states are the homogeneous dispersion in either of the phases. The entropy and enthalpy gain by good solvation of the polymer shell overcome the energy gain of the interface due to particle adsorption. All phase transfer experiments were repeated multiple times and we found excellent reproducibility. This is demonstrated in figure S14 showing fully reversible phase transfer between water and chloroform upon 19 ACS Paragon Plus Environment

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heating/cooling of the system containing the Au15-PNIPAM82k particles, as an example. Moreover, phase transfer also works for other kinds of plasmonic particles. Citrate stabilized silver nanospheres, as well as CTAB stabilized gold nanospheres and nanorods could be transferred to chloroform under the same conditions as reported above by using PNIPAM40k as ligand. This is evidenced by UV-Vis spectra of the different particles in the chloroform phase shown in figure S15 in the SI. Hence, it is possible to use α-trithiocarbonate-ω-carboxyl terminated PNIPAM as a general approach for the phase transfer of plasmonic particles to chloroform. We note that it is difficult to transfer the particles to other organic solvents, besides chloroform. Phase transfer to a broad range of organic phases such as benzene, cyclohexane, ethyl acetate, heptane, hexane, toluene and trichloropropane was investigated. Surprisingly, only transfer to ethyl acetate was successful as can be seen by UV-Vis spectroscopy of the particles in figure S16 in the SI. In all the other cases the particles could not cross the liquid/liquid interface. The underlying mechanism appears to be rather complex and it is currently not clear why some solvents allow phase transfer and other do not. The strong absorbance of AuNPs in the visible allows for a quantitative analysis of the phase transfer process. Figure 5 shows absorbance spectra of the aqueous phase (solid lines) and the chloroform phase (dotted lines) at 4°C (A) and at 60°C (B) for the Au22, Au29, Au36, Au44 and Au53 particles stabilized by PNIPAM40k. In figure 5 A absorbance spectra of the water phase, recorded at 4°C, show the pronounced LSPR absorption for each type of particle. In contrast, the CHCl3 phase reveals very weak absorbance as clearly visible in the inset graph of figure 5 A. This underlines that the majority of the particles is well-dispersed in the water phase and that the chloroform phase is mostly free of particles. Based on these findings the difference of the absorbance of the water and chloroform phase at 400 nm (∆Abs400) was calculated for each type of particle as a measure for the particle distribution. The values of ∆Abs400 are listed in table 4. Comparing ∆Abs400 to the absorbance at 400 nm of the water phase before any phase transfer experiment allows the determination of the efficiency of the 20 ACS Paragon Plus Environment

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phase transfer η. The transfer has an efficiency η = 100% if the measured absorbance is identical to the initial value recorded in the water phase before the phase transfer experiment was performed. It is found that η is close to 100% for each type of particle. It follows that the phase transfer is quantitative for every AuNP under investigation. Spectra recorded at 60°C of the water and chloroform phase reveal no significant absorbance in the water phase anymore as can be observed in the inset graph of figure 5 B. In contrast, figure 5 B shows that the particles have been transferred almost completely to the chloroform phase as evidenced by the strong absorbance of the particles in the organic phase. This is further supported by the calculation of η being close to 100 % again. We note that the slight differences of ∆Abs400 between the 4°C and 60°C experiments might arise from a small fraction of particles which are still trapped at the interface or dispersed in the respective other phase.

Figure 5. Absorbance spectra of the water phase (solid lines) and the chloroform phase (dotted lines) measured for AuNPs with PNIPAM40k at 4°C (A) and at 60°C (B). Spectra are shown for the particles Au22 (black), Au29 (red), Au36 (green), Au44 (blue) and Au53 (pink). The insets show magnifications of the low absorbance region for the spectra of the chloroform phase (in A) and the water phase (in B).


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Table 4. Quantification of the phase transfer efficiency from absorbance spectroscopy. 4°C Sample

60°C

∆Abs400 (H2O – CHCl3)a

η4°C (%)b

∆Abs400 (CHCl3 – H2O)

η60°C (%)

Au22-PNIPAM40k

0.075

98

0.077

99

Au29-PNIPAM40k

0.101

97

0.119

99

Au36-PNIPAM40k

0.129

98

0.158

99

Au44-PNIPAM40k

0.134

99

0.155

98

Au53-PNIPAM40k

0.142

99

0.161

95

a

∆Abs400, Difference in absorbance at 400 nm from spectra for the H2O and CHCl3 phase at 4°C and 60°C. b

η, efficiency of the phase transfer calculated from the difference of ∆Abs400 relative to the initial Abs400 in water before the phase transfer experiment.

CONCLUSION In this work we have shown that α-trithiocarbonate-ω-carboxyl terminated PNIPAM polymers of different molecular weights, synthesized by RAFT polymerization, can be used for the functionalization of AuNPs within a broad range of sizes (15 – 53 nm). The functionalizations were performed by ligand exchange of the initial citrate molecules adsorbed onto the AuNP surface by PNIPAM. This ligand exchange yielded PNIPAMstabilized AuNPs with negative surface charge at neutral pH. At temperatures below the LCST of the PNIPAM ligands, stable dispersions were found for all combinations of NP and ligand. DLS confirmed the surface functionalization by a significant increase in hydrodynamic dimensions. Furthermore, the hydrodynamic particle size increased with increasing molecular weight of the PNIPAM ligands. Absorbance spectroscopy before and after ligand exchange revealed a redshift of the LSPR of the AuNPs, with a magnitude 22 ACS Paragon Plus Environment

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independent of particle size and ligand length. This LSPR shift is related to the increase in refractive index due to the PNIPAM encapsulation. Upon lowering the pH to values of 2.73.0, the surface charge of the Au-PNIPAM particles approached values close to zero due to protonation of the carboxyl groups of the PNIPAM ligands. In this range of pH, the LCST behavior of the PNIPAM ligands allowed quantitative phase transfer of the particles from aqueous phase to chloroform and vice-versa. At 60°C, where water is a poor solvent for PNIPAM, the particles were well-dispersed in the chloroform phase. Upon cooling to 4°C, the particles transferred back to the aqueous phase due to the increased solubility. Absorbance spectroscopy revealed that these phase transfers occurred with efficiencies in the order of 9599% for many heating/cooling cycles.

AUTHOR INFORMATION

Corresponding Author *Matthias Karg E-mail: [email protected] ACKNOWLEDGEMENTS The authors acknowledge financial support from the Deutsche Forschungsgemeinschaft (DFG) through the SFB 840 and the Emmy Noether programme. The authors thank Paul Reichstein (MC I, University of Bayreuth) for assistance with the SEC measurements.

Supporting Information Available: SEC traces of PNIPAM ligands, TEM images of citrate and polymer stabilized AuNP, polymer functionalization of Au nanorods, pH dependent ζ - potential and dh investigation of Au22-PNIPAMy particles and sequential phase


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transfer experiments of Au15-PNIPAM82k are available free of charge via the Internet at http://pubs.acs.org.

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For Table of Contents use only

Fully Reversible, Quantitative Phase Transfer of Gold Nanoparticles using Bifunctional PNIPAM Ligands Tobias Honolda , Dominik Skrybecka, Kristina G. Wagnera,b and Matthias Kargb* a

Physical Chemistry I, University of Bayreuth, Universitaetsstr. 30, 95447 Bayreuth, Germany

b

Physical Chemistry I, Heinrich-Heine-University Duesseldorf, Universitaetsstr. 1, 40204 Duesseldorf, Germany


Fully Reversible Quantitative Phase Transfer of Gold Nanoparticles

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