Salt-Responsive Polysulfabetaines from Acrylate and Acrylamide

Sep 22, 2015 - The polysulfabetaines derived from methacrylate (PSBMA) and methacrylamide (PSBMAm) also showed reversible salt-responsive and thermore...
1 downloads 8 Views 7MB Size
Download PDF
Article pubs.acs.org/Langmuir

Salt-Responsive Polysulfabetaines from Acrylate and Acrylamide Precursors: Robust Stabilization of Metal Nanoparticles in Hyposalinity and Hypersalinity Vivek Arjunan Vasantha,* Chen Junhui, Tay Boon Ying, and Anbanandam Parthiban* Institute of Chemical and Engineering Sciences (ICES), Agency for Science, Technology and Research (A*STAR), 1 Pesek Road, Jurong Island, Singapore 627833 S Supporting Information *

ABSTRACT: Metal nanoparticles (MNps) tend to be influenced by environmental factors such as pH, ionic strength, and temperature, thereby leading to aggregation. Forming stable aqueous dispersions could be one way of addressing the environmental toxicity of MNps. In contrast to the electrolyteinduced aggregation of MNps, novel zwitterionic sulfabetaine polymers reported here act as stabilizers of MNps even under high salinity. Polysulfabetaines exhibited unique solubility and swelling tendencies in brine and deionized water, respectively. The polysulfabetaines derived from methacrylate (PSBMA) and methacrylamide (PSBMAm) also showed reversible saltresponsive and thermoresponsive behaviors as confirmed by cloud-point titration, transmittance, and dynamic light scattering studies. The brine soluble nature was explored for its ability to be used as a capping agents to form metal nanoparticles using formic acid as a reducing agent. Thus, silver and noble metal (gold and palladium) nanoparticles were synthesized. The nanoparticles formed were characterized by UV−vis, XRD, TEM, EDX, and DLS studies. The size of the nanoparticles remained more or less the same even after 2 months of storage in 2 M sodium chloride solution under ambient conditions and also at elevated temperatures as confirmed by light-scattering measurements. The tunable, stimuli-responsive polysulfabetaine-capped stable MNp formed under low (hyposalinity) and hypersalinity could find potential applications in a variety of areas.

■

INTRODUCTION Nanotechnology, in spite of various health and environmental concerns, has emerged as one of the much sought after disciplines in the last few decades of the previous century. Nanomaterials and nanoparticles which are colloidal particles ranging in size from 10 to 100 nm in diameter are vital to nanotechnology-centered applications. Nanomaterials have been reported to be useful in a wide range of applications such as conductive inks, optics, and sensors as well as in a variety of biomedical applications such as drug delivery, imaging, therapeutic agents, and wastewater treatment.1,2 Because of its wide range of applications, the potential release of nanomaterials into the environment has raised concerns due to their toxic effects on the ecosystem.3−6 Therefore, developing an understanding of the effect of nanomaterials in a more complex, natural environment is more important than studying nanoparticle behavior under controlled conditions in the laboratory. For many applications, it is highly desirable to have nanoparticles with prolonged stability. Significant efforts have been devoted in recent years to the stabilization of nanomaterials by using small ligands, surfactants, and polymers. The stability of metal nanoparticles depended on the physiochemical properties and in particular the effectiveness of continued interaction between nanoparticle and stabilizer. Recent studies © XXXX American Chemical Society

have shown that the size, shape, and other physicochemical properties of nanoparticles can be influenced by various environmental parameters such as pH and ionic strength. Natural matters of organic or inorganic origin as well as other factors such as light and heat also affect the stability of nanoparticles.4 These external factors interfere with a variety of functions associated with nanomaterials such as mobility, dissolution, and aggregation.3 However, conditions prevailing in actual applications such as high ionic strength, high temperature, and high pressure could limit the application of nanoparticles. Factors such as high salt concentration could easily disrupt double layers which destabilize nanoparticles, thereby leading to the aggregation and precipitation of nanoparticles upon prolonged exposure.7 At elevated temperatures, the enhanced mobility of the nanoparticles will be accompanied by an increased number of collisions between nanoparticles, ultimately leading to precipitation. Notably, the unstable nature of nanoparticles upon prolonged exposure in a medium of high ionic strength has been indicated by changes in its surface plasmon resonance characteristics. Thus, environmental risks such as the reactivity and toxicity of nanoparticles Received: May 15, 2015 Revised: September 17, 2015

A

DOI: 10.1021/acs.langmuir.5b01768 Langmuir XXXX, XXX, XXX−XXX

Langmuir

■

are predominantly determined by their ability to form aggregates. Also, agglomerated nanoparticles will be adsorbed or form sediments irreversibly on surfaces, which in turn will reduce their mobility and transport.8 Therefore, it is fundamentally important to stabilize nanoparticles with environmentally benign dispersants or capping agents, whereby the stabilizing mechanism is through electrostatic interaction between nanoparticles and dispersing agents or by steric hindrance in order to prevent aggregation. A diverse range of polymeric materials have been reported as dispersants which help to stabilize various nanomaterials such as metal nanoparticles, carbon nanotubes, nanowires, graphene, and clay in either aqueous or organic media.9−16 Such dispersants play multiple roles such as controlling the particle size and shape, preventing aggregation, and in some cases also preventing the oxidation of metal nanoparticles. The use of a polymer matrix to stabilize nanoparticles helps to improve the compatibility, processability, thermal, and other physicochemical properties of nanodispersions, which are essential to developing novel applications. In the recent past, stimuliresponsive hydrophilic polymers such as polyacrylamide (PAm) and polyvinylpyrrolidone (PVP) have attracted much attention as stabilizers for metal nanoparticles.17−19 However, metal nanoparticles capped with PAm and PVP tend to destabilize under physiological (0.15 M NaCl solution) as well as moderately saline (0.9 M NaCl solution) conditions. The salinity of the medium decreased the thickness of the stabilizing shell.20,21 Additionally, PAm and PVP have a strong tendency to agglomerate in media of low to high ionic strength as well as in the presence of NaCl. These conditions induce the formation of globular structures due to the collapse of stretched polymer chains.22 Thus, it is desirable to have capping agents which stabilize nanoparticles under harsh environmental conditions. However, the stabilization of nanoparticles under such conditions has rarely been addressed in the literature.23−27 It is useful to note that stabilizers play a predominant role in determining the environmental footprint of nanoparticles. Zwitterionic polymers are an important and interesting class of materials which, depending on the chemical composition, display a diverse range of interactions such as H bonding, dipole interactions, electrostatic interactions, π−π interactions, and so forth. These interactions make zwitterionic polymers novel hybrid materials.28−34 Recently, we introduced novel zwitterionic polysulfobetaines35 and polysulfabetaines36 with distinct solubility characteristics along with thermal as well as salt-responsive behaviors. There is abundant literature available on zwitterionic sulfobetaines derived from polyacrylamide and acrylates.37−39 Although these polymers exhibit desirable solution behavior in deionized water, there is a vacuum for ionic polymers soluble in concentrated electrolyte solution such as brine. The recently reported polysulfabetaines36 address this gap. As reported in this article, the extension of the polysulfabetaine framework to polyacrylamides and polyacrylates not only helps to fulfill the aforementioned void but also vastly broadens the boundary and scope of zwitterionic polymers. These newly developed halophilic (salt-loving) zwitterionic polysulfabetaines act as excellent capping agents that stabilize monovalent and divalent metal nanoparticles in salt solutions of varying concentration. Furthermore, stabilized metal nanoparticles showed unique tunable, reversible, salt, and thermoresponsive behavior.

Article

EXPERIMENTAL SECTION

Materials. All reactions and polymerizations were performed with a Schlenk technique under an argon atmosphere. 1,3,2-Dioxathiolane 2,2-dioxide (1,2 ethylene sulfate), 1,3-propanediol cyclic sulfate (1,3 propylene sulfate), 2-(dimethylamino)ethyl methacrylate (DMAEMA), N-[3-(dimethylamino)propyl]methacrylamide (DMAPMAm), nitrobenzene, sodium bromide, 4,4′-azo bis(4-cyanovaleric acid) (ACVA), 1,1,1,3,3,3-hexafluoro-2-propanol (HFIPA), and formic acid were purchased from Sigma-Aldrich and used as received. Silver (AgNO3), gold (AuCl3·3H2O), and palladium (K2PdCl4) metal precursors were obtained from Sigma-Aldrich. Acetonitrile (ACN) was freshly dispensed from a Glass Contour Solvent Purification System. All other solvents used were analytical grade. Deionized water was used for swelling and other aqueous solution studies. Other chemicals used in this study were analytical-grade reagents obtained from commercial sources. Synthesis of Methacrylate and Methacrylamide Sulfabetaine Monomers. 2-((2-(Methacryloyloxy)ethyl)dimethylammonio)ethyl Sulfate (SBMA 1). In a 250 mL roundbottomed flask, 6.37 g (0.05 mol) of 1,2-ethylene sulfate was dissolved in dry acetonitrile (150 mL) followed by the addition of 2 drops of nitrobenzene. 2-(Dimethylamino)ethyl methacrylate (8.64 mL, 0.05 mol) was then added dropwise to the colorless solution. The reaction mixture was held at 50 °C for 24 h. The solvent was removed under vacuum to obtain a colorless solid. Yield = 13.7 g (94.9%). The crude solid was crystallized from 2:1:1 water/ethyl acetate/methanol. Melting point, 150.8 °C (DSC). 1H NMR (400 MHz, D2O, δ): 6.17 (s, 1H; CH), 5.78 (s, 1H; CH), 4.67 (t, 2H; −O−CH2), 4.52 (t, 2H; −O−O−CH2), 3.91−3.86 (m, 4H; CH2−N+−CH2), 3.30 (s, 6H; N(CH3)2), 1.95 (dd, 3H; CH3). LC-TOF (ESI) m/z: [M + H]+ calcd for C10H19NO6S, 281.09; found, 282.10. Elemental analysis calcd for C10H19NO6S: C 42.69, H 6.81, N 4.97, S 11.4, O 34.12; found: C 42.54, H 6.77, N 4.97, S 11.34, O 34.50. 3-((2-(Methacryloyloxy)ethyl)dimethylammonio)propyl Sulfate (SBMA 2). In a 250 mL round-bottomed flask, 4.04 g (0.03 mol) of 1,3-propanediol cyclic sulfate was dissolved in dry acetonitrile (150 mL), followed by the addition of 2 drops of nitrobenzene. 2(Dimethylamino)ethyl methacrylate (4.9 mL, 0.03 mol) was added dropwise to the colorless solution. The reaction mixture was held at 50 °C for 24 h. The reaction solution was cooled to room temperature and filtered via a Buchner funnel. The white powder was washed with the minimum amount of acetone and dried overnight in a vacuum desiccator. Yield = 7.15 g (82.7%). The crude compound was crystallized from 1:1 water/methanol. Melting point, 194.82 °C (DSC). 1H NMR (400 MHz, D2O, δ): 6.16 (s, 1H; CH), 5.78 (s, 1H; CH), 4.64 (t, 2H; −O−CH2), 4.17−4.14 (t, 2H; S−O−CH2), 3.83− 3.57 (m, 2H; −CH2−N+), 3.61−3.57 (m, 2H; -N+−CH2), 3.22 (s, 6H; N(CH3)2, 2.29−2.21 (m, 2H; CH2), 1.94 (dd, 3H; CH3). LC-TOF (ESI) m/z: [M + H]+ calcd for C11H21NO6S, 295.11; found, 296.11. Elemental analysis calcd for C11H21NO6S: C 44.73, H 7.17, N 4.74, S 10.85, O 32.5; found: C 44.73, H 7.10, N 4.74, S 10.90, O 32.81. 2-((3-Methacrylamidopropyl)dimethylammonio)ethyl Sulfate (SBMAm 1). In a 100 mL round-bottomed flask, 1.24 g (0.01 mol) of 1,2-ethylene sulfate was dissolved in dry acetonitrile (30 mL), followed by the addition of 2 drops of nitrobenzene. N-[3(Dimethylamino)propyl]methacrylamide (1.8 mL, 0.01 mol) was added dropwise to the colorless solution. The reaction mixture was held at 50 °C for 24 h. The solvent was then removed under reduced pressure. The colorless compound was filtered and washed with acetone and dried overnight in the vacuum desiccator to yield a white solid. Yield = 95.6%. The product is highly hygroscopic. 1H NMR (400 MHz, D2O, δ): 5.71 (s, 1H; CH), 5.47−5.46 (s, 1H; CH), 4.46 (t, 2H; −CH2−O), 3.75−3.72 (m, 2H; −N+−CH2), 3.46−3.42 (m, 2H; NH− CH2), 3.39−3.36 (m, 2H; −CH2−N+), 3.17 (s, 6H, −N+(CH3)2), 2.12−2.06 (m, 2H; CH2) 1.92 (dd, 3H; CH3). LC-TOF (ESI) m/z: [M + Na]+ calcd for C11H22N2O5S, 294.12; found: 317.11. Elemental analysis calcd for C11H22N2O5S: C 44.88, H 7.53, N 9.52, S 10.89, O 27.18; found: C 44.24, H 7.41, N 9.88, S 10.11, O 28.36. 3-((3-Methacrylamidopropyl)dimethylammonio)propyl Sulfate (SBMAm 2). In a 250 mL round-bottomed flask, 4.94 g (0.036 mol) B

DOI: 10.1021/acs.langmuir.5b01768 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir of 1,3-propanediol cyclic sulfate was dissolved in dry acetonitrile (150 mL), followed by the addition of 2 drops of nitrobenzene. (6.48 mL, 0.03 mol) N-[3-(Dimethylamino)propyl]methacrylamide was added dropwise to the colorless solution. The reaction mixture was held at 50 °C for 24 h. The reaction solution was cooled to room temperature and filtered via a Buchner funnel. The white powder was washed with the minimum amount of acetone and dried overnight in a vacuum desiccator. Yield = 9.80 g (88.8%). Melting point, 172.65 °C (DSC). 1 H NMR (400 MHz, D2O, δ): 5.72 (s, 1H; CH), 5.48 (s, 1H; CH), 4.16−4.13 (t, 2H; CH2−O), 3.48−3.44 (m, 2H; NH−CH2), 3.39− 3.34 (m, 4H; CH2−N+−CH2), 3.11 (s, 6H; N+(CH3)2), 2.20−2.16 (m, 2H; CH2), 2.07−2.03 (m, 2H; CH2), 1.93 (dd, 3H; CH3). LCTOF (ESI) m/z: [M + H]+ calcd for C12H24N2O5S, 308.14; found, 309.15. Elemental analysis calcd for C12H24N2O5S: C 46.74, H 7.84, N 9.08, S 10.4, O 25.94; found: C 46.64, H 7.75, N 9.11, S 10.30, O 26.11. General Procedure for the Preparation of Polysulfabetaine (PSBMA 1 and 2 and PSBMA 1 and 2). In a dry Schlenk flask (100 mL), SBMA (14 mmol) and initiator ACVA (0.5 mmol) were introduced, followed by 0.5 M NaBr solution (25 w/v%, 16 mL). The solution was purged with argon gas for 45 min. The Schlenk flask was then placed in a constant-temperature bath at 90 °C for 24 h. The resulting viscous polymer solution was cooled and diluted with 10 mL of 0.5 M NaBr solution. The solution was then dialyzed against deionized water for 3 days (MWCO = 3500) at room temperature. The resulting water-insoluble polymer was lyophilized and dried in vacuo at 50 °C for 48 h. Poly[2-((2-(methacryloyloxy)ethyl)dimethylammonio)ethyl Sulfate] (PSBMA 1). PSBMA 1 was synthesized from SBMA 1 (4.0 g, 14.2 mmol) and ACVA (0.16 g, 0.57 mmol) in 0.5 M NaBr solutions (16 mL) for 24 h at 90 °C. SLS analysis (HFIPA): Mw = 283 kDa; UV−vis (25 wt % NaCl), 216 nm. Poly[3-((2-(methacryloyloxy)ethyl)dimethylammonio)propyl Sulfate] (PSBMA 2). PSBMA 2 was synthesized from SBMA 2 (4.0 g, 13.5 mmol) and ACVA (0.15 g, 0.54 mmol) in 0.5 M NaBr solutions (16 mL) for 24 h at 90 °C. SLS analysis (HFIPA): Mw = 118 kDa; UV−vis (25 wt % NaCl), 216 nm. Poly[2-((3-methacrylamidopropyl)dimethylammonio)ethyl Sulfate] (PSBMAm 1). PSBMAm 1 was synthesized from SBMAm 1 (16.78 mmol) and ACVA (0.67 mmol) in 0.5 M NaBr solutions (20 mL) for 24 h at 90 °C. SLS analysis (HFIPA): Mw = 40 kDa; UV−vis (25 wt % NaCl), 223 nm. Poly[3-((3-methacrylamidopropyl)dimethylammonio)propyl Sulfate] (PSBMAm 2). PSBMAm 2 was synthesized from SBMAm 2 (5.0 g, 16.5 mmol) and ACVA (0.18 g, 0.66 mmol) in 0.5 M NaBr solutions (20 mL) for 24 h at 90 °C. SLS analysis (HFIPA): Mw = 113 kDa; UV−vis (25 wt % NaCl), 223 nm. General Procedure for the Synthesis of Metal Nanoparticles (Ag, Au, and Pd Nanoparticles). The metal nanoparticles were synthesized by a modified method of Wang et al. and Liu et al.40,41 In a clean, dry 25 mL round-bottom flask, freshly prepared 3 wt % PSBMA (2 mL) in formic acid was added under an argon atmosphere. The solution was stirred and purged with an argon atmosphere for 15 min. The metal precursors (4 mg) were added to the polymer solution all at once with stirring. The formic acid played a dual role as solvent and reducing agent. The color of the solution changed from colorless to dark within an hour for all nanoparticles. The solutions were stirred for 24 h at room temperature under an argon atmosphere. The resulting nanoparticle solution was washed with deionized water and ethanol to remove formic acid under centrifugation. The nanoparticles were dried under vacuum at 50 °C. The dried nanoparticles were dissolved in 2 M NaCl for UV−vis absorption and DLS studies and in HFIPA for TEM characterization. Characterization Methods. Instrumentation. 1H nuclear magnetic resonance (NMR) spectra were recorded at room temperature in a 400 MHz Bruker UltraShield AVANCE 400SB spectrometer. The zwitterionic polymers were soluble in 10% NaCl/D2O solution and HFIPA-D2. Residual solvent peaks were used as an internal standard. Infrared spectra were run as KBr pellets for solids using a Digilab Excalibur FT-IR spectrometer. High-resolution mass spectra were

recorded using electrospray ionization (ESI) techniques in positive and negative ion modes by Thermo Finnigan MAT 95 XP. Mass spectral data is reported as the mass-to-charge ratio (m/z). UV absorption spectra were measured on a Shimadzu UV−vis−UV 2550 spectrometer. The sample solutions were measured in a 1 cm path length cell and measured in the 200−600 nm region. The transmittance studies for critical salt concentration or cloud-point measurements were recorded at 600 nm. Static Light Scattering (SLS). SLS was performed by using a Zetasizer NanoZS Instrument (Malvern Instruments, U.K.) equipped with a He−Ne laser (633 nm) and with noninvasive backscattering (NIBS) detection at a scattering angle of 173° to measure molecular weight Mw and second virial coefficient A2. Prior to SLS, a sample of the polysulfabetaines was prepared in HFIPA in five to eight dilutions ranging from 0.05 to 10 mg/mL. The value of the refractive index (RI) was measured using an Abbe refractormeter 300. Seven polymer concentrations were measured for each refractive index, and a linear fit was used to obtain the refractive index increment, dn/dc value. It is worth noting that the measured refractive index is low due to HFIPA as a solvent (RI of HFIPA is 1.277 at 20 °C). The experimentally determined dn/dc values are summarized in Table 1.

Table 1. Free Radical Polymerizationa and Characterization of Zwitterionic Polysulfabetaines polymers

monomer/ initiator (mmol)

yield (%)

dn/dc

Mw, SLS (k Da)

A2 mL mol/g2

PSBMA 1 PSBMA 2 PSBMAm 1 PSBMAm 2

14.2/0.57 13.5/0.54 16.8/0.67 16.5/0.66

89 90 84 83

0.1774 0.1778 0.2061 0.1969

283 118 40 113

0.0050 0.0020 0.0013 0.0006

a

Polymerization was carried out in 0.5 M NaBr solution using ACVA (4,4′-azo bis(4-cyanovaleric acid) as the initiator; reaction time 24 h and temperature 90 °C. The apparent average molecular weight was determined by SLS using HFIPA as the solvent. The average scattering intensity from five to eight different concentrations for each polysulfabetaine was recorded. Samples were analyzed using the Malvern-supplied “molecular weight” operating procedure, with light being detected at an angle of 173° and a temperature of 20.0 °C in automatic mode. A special precaution was taken to avoid contamination with dust. Measurements were carried out using a cleaned glass cuvette. Toluene scattering was used as a reference. The resulting data lead to a Debye plot where the slope and the intercept allow the calculation of the molecular weight and second viral coefficient. It should be noted that since the polymer is highly charged, the obtained molecular weight is apparent (Table 1). Solubility. The solubility of PSBMA 1 and 2 and PSBMAm 1 and 2 in organic solvents at room temperature was determined by dispersing 1 wt % polymer in various solvents and holding the mixture at 70 °C for 1 h. Critical Salt Concentration or Cloud-Point Measurement. The cloud-point titration was carried out using 1 wt % PSBMA 1 and 2 and PSBMAm 1 and 2 in 10 wt % NaCl at 25 °C. The solution was titrated with deionized water and visually changed from clear to turbid at the end point. The CSC values were obtained by a visual determination of the cloud point (the clear solution became turbid) with an error value of ±0.2%. The CSC values are reported in Table 2. Phase-Behavior Measurements. The UCST of diluted PSBMA 1 and 2 and PSBMAm 1 and 2 solutions was determined with respect to temperature with varying NaCl concentration. The polymer solution was poured into a glass vial and placed directly into a temperaturecontrolled bath (Lauda Alpha RA 8) (temperature variation ±0.1 °C). The solutions, which were turbid at room temperature, became clear (transparent) upon heating. The transition point (UCST) was taken as the temperature at which the solution changed from cloudy to transparent. Aggregation Studies. Dynamic light scattering (DLS) was performed by using a Zetasizer NanoZS Instrument (Malvern C

DOI: 10.1021/acs.langmuir.5b01768 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir Table 2. Results of Swelling and CSC Studies of Polysulfabetaines physical properties

PSBMA 1

swelling ratio @ 24 h critical salt concentration (wt % NaCl)

titration DLS UV−vis

PSBMA 2

108.6 ± 4 7.35 (1.25 M) 7.96 (1.35 M) 8.22 (1.40 M)

91.9 4.96 5.77 5.05

± 0.2 (0.84 M) (0.98 M) (0.86 M)

PSBMAm 1

PSBMAm 2

140.1 ± 3.2 2.05 (0.35 M) 2.24 (0.38 M) 2.06 (0.35 M)

123.8 ± 2.4 3.48 (0.59 M) 4.08 (0.69 M) 3.41 (0.58 M)

Scheme 1. Synthesis of Methacrylate- and Methacrylamide-Based Zwitterionic Sulfabetaine Monomers and Polymers

Instruments, U.K.) equipped with a He−Ne laser (633 nm) and with noninvasive backscattering (NIBS) detection at a scattering angle of 173°. The autocorrelation function was converted in a volumeweighted particle size distribution from Malvern Instruments. Each measurement was repeated at least three times, and the average result was reported as the final z-average diameter (nm). The measurements were performed in the temperature range of 20−70 °C with a temperature interval of 5 °C and an equilibration time of 10 min. Polymer solutions with various concentrations were prepared in NaCl solution at 50 °C and filtered using a 0.45 μm disposable membrane filter to remove any dust in the solution. Hydration Studies. Triplicate samples of a known weight of PSBs were immersed in excess deionized water. The polymer kept in sealed containers was placed in a water bath at 23 °C and equilibrated for 48 h. The samples were then dried. The swelling ratio of the polymer gel was determined gravimetrically and was calculated from the ratio of the weight of the equilibrated gel to the dry weight. For each polymer, the average value was calculated. Hydrolytic Stability. The hydrolytic stability was determined in aqueous solution. The polymer was heated with deionized water (10 wt %) for 24 h at 95 °C. The reaction mixture was allowed to cool and then dialyzed with DI water using MWCO 3500, lyophilized, and dried in a vacuum oven at 50 °C for 24 h. The dried polymer samples were analyzed for elemental composition. Powder X-ray diffraction (XRD). XRD measurements were made on a Bruker D8 Advance X-ray powder diffractometer using Cu Kα radiation (λ = 0.15418 Å). The films were prepared by casting polymer solution on the Si substrate. Transmission Electron Microscopy (TEM). A Tecnai F20 S-twin transmission electron microscope at an acceleration voltage of 200 kV and complemented with an energy-dispersive X-ray spectrophotometer (EDX) was used for TEM analysis. TEM samples were prepared by drying aliquots (10 μL, 1 mg/mL) of nanoparticle solution in HFIPA onto a carbon-coated copper grid, followed by drying at room temperature.

■

reacting the corresponding vinyl monomer bearing 3° amino group with cyclic sulfates such as ethylene and propylene sulfates (Scheme S1). SBMA 1 and 2 were derived from acrylates, and SBMAm1 and 2 were derived from acrylamide. All of the monomers were thoroughly characterized and were found to be analytically pure (Table S1 and Figure S1). Conventional free radical polymerization was employed to prepare the sulfabetaine polymers. The polymerization was carried out in an electrolyte such as 0.5 M sodium bromide (NaBr) solution using 4,4′-azobis(4-cyanovaleric acid) (ACVA) as the initiator. The results of polymerization and the characteristics of polymers are summarized in Table 1. The polymers were characterized by nuclear magnetic resonance (1H NMR) spectroscopy, infrared spectroscopy (IR), and UV−vis spectroscopy (Figures S2−S4). Polysulfabetaines are more solvent-resistant than the corresponding polysulfobetaines. A similar trend was previously noticed by us in the case of polystyrene-based sulfabetaines.36 Polyacrylateand acrylamide-bearing sulfabetaines were soluble in aqueous electrolyte solutions, formic acid, and fluorinated alcohols (Table S2). Figure S2 shows the 1H NMR of PSBMA and PSBMAm. As expected, vinyl protons of SBMA and SBMAm appearing at 5.5 and 5.8 ppm completely disappeared upon polymerization. Two-step degradation was noticed in the thermogravimetric analysis (TGA) of polysulfabetaines (Figure S5). As reported before, polymers bearing a more thermally stable backbone and less stable pendant groups as well as block copolymers possessing segments of substantially different thermal stability tend to show two-step degradation curves in TGA.34,42−44 Polysulfobetaines also have been reported to show a two-step degradation curve in the TGA analysis.45 Among the polysulfabetaines, PSBMA 2 showed the highest thermal stability of 302 °C as determined by the loss of 5 wt % of polymer in TGA under a nitrogen atmosphere (Table S3). The thermal stability of polysulfabetaines was generally good, with polyacrylates being 19 °C more thermally stable than the polyacrylamide-based polysulfabetaines. The hydrolytic stability of the newly developed polysulfabetaines was determined under two different conditions, viz., in deionized water and 6 M HCl

RESULTS AND DISCUSSION

Synthesis and Characterization of Sulfabetaine Monomers and Polymers. The synthesis strategy employed for making hitherto unknown polyacrylate- and polyacrylamidederived sulfabetaines is shown in Scheme 1. Acrylate- and acrylamide-based sulfabetaine monomers were prepared by D

DOI: 10.1021/acs.langmuir.5b01768 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 1. Photographs of salt and thermoresponsive behavior of PSBMA 1 and 2 (vials 1 and 2) and PSBMAm 1 and 2 (vials 3 and 4) (a). Effect of salt concentration with respect to UV−vis transmittance (b). Size measurement (c) using 1 wt % PSBMA 1 and 2 and PSBMAm 1 and 2 in 10 wt % NaCl solution and subsequent titration with deionized water at 25 °C.

solution, both at 95 °C. As shown in Table S4, severe hydrolysis occurred under strongly acidic conditions, and the same was negligibly small in deionized water. Aqueous Behavior of Polysulfabetaines. Swelling Behavior. In addition to the favorable thermal stability, the unique brine solubility characteristics of polysulfabetaines also make these polymers potential candidates for applications in the oil field industry such as enhanced oil recovery. Methacrylate- and methacrylamide-based polysulfobetaines are soluble in water. However, polysulfabetaines studied here swell in water (Table 2), similar to that of polystyrene-based sulfabetaines.36 As expected, on the basis of the enhanced Hbonding ability of amide linkages, PSBMAm 1 and 2 showed a greater degree of swelling than did the corresponding acrylatebased zwitterions, PSBMA 1 and 2. The greater swelling of PSBMAm 1 and 2 is due to the fact that amide linkages possess both H-bond acceptor and donor atoms whereas ester linkages possess only acceptor atoms.46 Critical Salt Concentration (CSC). The strong ionic interactions prevailing in polysulfabetaines enabled these polymers to exhibit “salting in” behavior in electrolyte solutions. This also led to salt-responsive behavior as studied by critical salt concentration (CSC) measurements. CSC is the minimum salt concentration required to keep the polymer chains stretched in solution with minimized interaction between polymer chains through counterions. Thus, the polymer chains are highly solvated, with ionic interactions between ions present in the polymer and the respective counterions from the electrolyte. CSC was determined by dissolving polysulfabetaines in a known aqueous NaCl solution of higher concentration to form a transparent solution under ambient temperature (25 °C), followed by lowering the concentration through the addition of deionized water until

the appearance turned turbid. The point at which the transformation from a clear to turbid appearance occurred is reported as the CSC and is summarized in Table 2. Figure 1(a) shows the appearance of the polymer solution during the determination of the CSC. Between methacrylamide- and methacrylate-based polysulfabetaines, the latter required a higher salt concentration to form clear solutions. This trend is expected since methacrylamide-based polysulfabetaines swelled more than the methacrylate-based polysulfabetaines. Due to the fact that methacrylamide can be hydrated more, the amount of electrolyte required to separate the polymer chains further apart could be lower. In fact, a clear trend has been observed with polymers bearing more hydrophobic backbone requiring more salt to form a clear solution. Thus, among polysulfabetaines known and studied so far, the concentration of NaCl required to form a clear solution increases in the following order: methacrylamide < methacrylate < styrene. Halophiles (i.e., microorganisms that survive in high salt concentrations) exhibit characteristic folding and unfolding behavior by involving a charge-shielding effect through strategies such as the “salting-in effect”.47−49 On the basis of the amount of NaCl required to make clear polymer solutions, similar to halophiles,50 polysulfabetaines can also be divided into three categories. The first category of polysulfabetaines is those that dissolve in a hypersaline medium (3−5 M NaCl solution) where the salt concentration is higher than that of seawater. Low halophilic polysulfabetaines such as PSBMAm dissolve in 0.3−0.6 M NaCl solution (2−4 wt %) (hyposaline medium), and moderately halophilic polysulfabetaines are intermediate between the above two categories since these polymers dissolve in 0.8−2 M NaCl solution (4−10 wt %). Polystyrene-based sulfabetaines are examples of the first E

DOI: 10.1021/acs.langmuir.5b01768 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 2. UCST of PSBMA 1 and 2 and PSBMAm 1 and 2 in NaCl solution (a). Effect of temperature on size as studied by DLS (b). Transmittance at 600 nm versus heating (40 °C) and cooling (25 °C) of PSBMA 1 and PSBMAm 2 (c).

bimodal at lower temperature in the latter case. The bimodal distribution is an indication of the prevalence of association of polymer chains (i.e., interchain interaction) through counterions to a varying degree. As shown in Figures 1a and 2c, these transitions are reversible, thereby highlighting their physical nature. Such combined salt and thermoresponsive behaviors make these novel polysulfabetaines attractive smart materials. Synthesis and Characterization of PolysulfabetaineCapped Metal Nanoparticles. There is tremendous interest in forming stable nanomaterials in low (hypo-), high, and hypersaline media, specifically for biomedical and enhanced oil recovery applications. Stabilizing nanoparticles in the presence of electrolytes is quite challenging. The ability of polysulfabetaines to dissolve in aqueous solutions of varying salinity makes them interesting capping agents to stabilize metal nanoparticles. Metal nanoparticles (MNp) derived from silver (Ag) and noble metals such as gold (Au) and palladium (Pd) were prepared using PSBMA and PSBMAm as capping agents. Synthesis of Silver Nanoparticles. Ag nanoparticles have been widely studied and reported because of their broader spectrum of application and comparatively low cost.53−55 In general, Ag nanoparticles (AgNp) are prepared by a chemical reduction method using formic acid, which has been reported to be a green process.56−58 Thus, nanoparticles were prepared by dissolving polysulfabetaines in formic acid, followed by the addition of a metal precursor which underwent in situ reduction to form metal nanoparticles as shown in Figure 3.

category of extremophiles since these polymers dissolve in 2−4 M NaCl solution (10−25 wt %) and the methacrylate based polysulfabetaines, PSBMA, are examples of intermediate halophiles. Salt- and Temperature-Responsive Behavior. The transparent to translucent/turbid transition that occurs upon changing salt concentration is accompanied by conformational changes in the polymer chain. Thus, above CSC, stretched chains are formed, and below it the chains are collapsed.51 Figure 1 shows the salt-responsive nature of polysulfabetaines as studied by UV−vis spectroscopy (Figure 1b) and dynamic light scattering (DLS, Figure 1c). In addition to the CSC behavior, polysulfabetaines also exhibited upper critical solution temperature (UCST) phenomena (Figure 2a,b). Accordingly, the turbid solutions formed due to a lower concentration of NaCl solution could be transformed to clear, transparent polymer solutions upon heating. Thus, the salt-responsive nature can be tuned to thermoresponsive behavior. Therefore, transparent solutions of polymer can be made either at a higher concentration of NaCl solution (Figure 2a) or by a combination of higher temperature and low salt concentration (Figure 2a). The conformational changes which accompany these transitions from stretched chains to globular structures52 were further confirmed by DLS studies as shown in Figure 2b. The change in particle size from 13 nm at 40 °C to 1000 nm at 25 °C was noticed in DLS measurements, with distributions being unimodal at higher temperature in the former case and F

DOI: 10.1021/acs.langmuir.5b01768 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 3. Capping of silver nanoparticles using acrylamide-based polysulfabetaines (a). UV−vis absorption spectra of AgNp (λmax = 407 nm) in 2 M NaCl solution; (inset) AgNp-PSBMAs stabilized in 2 M NaCl (vial 1 no polymer; vials 2 and 3 AgNp-PSBMA 1 and 2; vials 4 and 5 AgNpPSBMAm 1 and 2) (b). FTIR spectra of PSBMAm 2 (bottom) and AgNp/PSBMAm 2 (top) (c).

Formic acid is a logical choice because of its ability to dissolve the polymer as well as to reduce the metal precursor. Thus, the capped metal salt upon in situ reduction formed the capped metal nanoparticle, which was conveniently separated by centrifugation followed by washing with water and ethanol. UV−vis and DLS studies were conducted on metal nanoparticles after dispersing the nanoparticles in 2 M NaCl solution. UV−vis and DLS Studies. Silver nanoparticles capped with PSBMA showed a unique yellow color. In the UV−vis spectrometric analysis an absorption maximum in the range of 405−407 nm caused by surface plasmon resonance (SPR) was observed with a full width at half-maximum (fwhm) of 58− 73 nm as shown in Figure 3b. It may be noted that the nanoparticles were stabilized in 2 M NaCl solution. Figure 3b (inset) also shows the color of Ag nanoparticles in different polysulfabetaines. The monodisperse nature of nanoparticles can be noticed by the symmetric nature of the peaks (Figure 3b) as well as the narrow range of fwhm values. Table 3 summarizes the maximum absorption (λmax) values as indicated by UV−vis spectroscopy and also compares the size of nanoparticles as determined by various other techniques such as transmission electron microscopy (TEM) and DLS. Infrared Spectroscopic Studies. A comparison of FT-IR spectra of nascent polysulfabetaine, PSBMAm 2, with that of

Table 3. Characteristics of Ag, Au, and Pd Nanoparticles Capped by Acrylate- and Acrylamide-Based Polysulfabetaines polymers PSBMA 1/AgNp PSBMA 2/AgNp PSBMAm 1/ AgNp PSBMAm 2/ AgNp PSBMAm 2/ AuNp PSBMAm 2/ PdNp

λmax (nm)

fwhm (nm)a

measured size (nm)b

TEM (nm)

407 407 407

70 60 58

50 40 14

30−40 40−60

405

73

44

10−30

540

48

17

20−30

280

36

21

8−15

The fwhm was determined by Gaussian fitting using the absorption of AgNP-polymer. bSize was measured by DLS. a

nanoparticle-capped polymer indicated a shift in the amide C O absorption band from 1653 to 1645 cm−1 (Figure 3c). This is likely due to the coordination of metal nanoparticles to the CO group.59 Though theoretically one could expect the interaction of metal ions with other counterions present in the polymer, the aforementioned shift in the CO absorption frequency is an indication that the predominant mode of association between polymer and metal nanoparticles is G

DOI: 10.1021/acs.langmuir.5b01768 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 4. TEM images of AgNp capped with PSBMA 1 (a), PSBMA 2 (b), and PSBMAm 2 (c). The insets are at a higher magnification of AgNp. DLS of polysulfabetaine-capped AgNps in 2 M NaCl solution (d).

through the coordinative linkage.60,61 No change in absorption corresponding to the sulfate anion was noticed with the asymmetric stretching of SO appearing at 1028 cm−1 in both native and capped polysulfabetaines. This may be due to the fact that the interaction between the sulfate anion and quaternary ammonium species remained unperturbed. The unchanged intensity of the absorption band corresponding to SO in both cases also confirms the above and indicates that the polymer remained intact and no untoward chemical reactions such as hydrolysis occurred during the preparation of nanoparticles (Table S4). Therefore, polysulfabetaines can be assumed to play a unique dual role, viz., stabilizing metal nanoparticles through coordination and at the same time protecting them further from the salinity of the medium through the screening of charges by the sulfabetaine-based zwitterionic moiety. Characterization of AgNP by XRD and TEM. X-ray diffraction analysis (XRD) (Figure S6) also confirmed the formation of Ag nanoparticles. In the XRD analysis, PSBMAm 2 showed an amorphous halo pattern. However, the formation of Ag nanoparticles was accompanied by the appearance of diffraction peaks at scattering angles of 38.3, 44.5, 64.8, and 77.8°. Ag nanoparticles appeared as nanosized spheres in TEM analysis (Figure 4a−c). TEM analysis also revealed that the nanoparticles were well dispersed in the polymer matrix with a mean diameter in the range of 10−60 nm. TEM analysis corroborated the observations made in UV−vis spectroscopic analysis and DLS measurements (Figure 4d). Energy-dispersive X-ray analysis (EDX, Figure S7) also showed a signal at 3 keV corresponding to the binding energy of Ag metal. Additionally, EDX indicated the presence of C, N, O, and S atoms, thereby confirming the presence of polysulfabetaine in the matrix. Stability of MNps in Saline Media. The stability of polysulfabetaine-capped Ag nanoparticles even in saline media is due to the salt-responsive nature of the sulfabetaine moiety. In particular, the ionic repulsions help to keep the capped nanoparticles apart from each other, thereby preventing aggregation. The role played by polysulfabetaine and its ionic interactions with the saline medium in stabilizing Ag nanoparticles is apparent from Figure 3b (inset). As shown in Figure 3b (inset), Ag nanoparticles aggregate and coagulate in saline in the absence of polysulfabetaine. In the absence of other ionic interactions as well as the protective capping agent surrounding the metal nanoparticle, chloride ions from the medium can combine with Ag metal to form AgCl, thereby destabilizing the nanoparticle. It is also useful to note that polysulfabetainecapped Ag nanoparticles showed very good stability in saline media of different concentrations (Figures 5a and S8). The previously described salt and thermoresponsive behavior of

Figure 5. Size of AgNp end-capped using PSBMAm 2 in NaCl solutions of various concentrations as determined by DLS. (Inset) Photographs of AgNp: vial 1 (DI water), vial 2 (0.06 M), vial 3 (0.125 M), vial 4 (0.25 M), vial 5 (0.5 M), vial 6 (1 M), vial 7 (2 M), vial 8 (4 M), and vial 9 (satd NaCl)] (a). Thermal reversibility of PSBMAm 2/ AgNp after storing under ambient conditions for 2 months in 0.5 M NaCl. (Inset) Photograph of nanoparticles in 0.5 M NaCl solution upon heating (bottom) and cooling (top) (b).

polysulfabetaine was also noticed in the nanoparticle dispersions. Thus, upon heating, the nanoparticle dispersion formed a transparent yellow solution (Figure 5b). Even after exposure to high salt concentration under ambient conditions for well over 2 months, no change in stability was noticed based on the UV−vis and DLS studies of metal nanoparticles (Table S5). H

DOI: 10.1021/acs.langmuir.5b01768 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 6. (a) UV−vis spectra of metal nanoparticels capped with polysulfabetaine (PSBMAm 2) in 2 M NaCl solution: (1) AgNp without polymer, resulting in coagulation; (2) AgNp (λmax = 407 nm); (3) AuNp (λmax = 540 nm); (4) PdNp (λmax = 280 nm). TEM images of AuNp (b) and PdNp (c) capped with PSBMAm 2. The inset is at a higher magnification of AuNp and PdNp. DLS of metal nanoparticles in 2 M NaCl (d).

Behavior of Metal Nanoparticles in Brine. Au and Pd Nps also showed salt and thermoresponsive behavior similar to that of Ag NPs. The thermoresponsive nature of Pd nanoparticles as studied by DLS at lower salt concentration (0.5 M) is shown in Figure S12. As explained earlier, at low temperature, the solution was turbid due to the aggregation of polymer chains. At elevated temperatures, the chains were stretched and well solvated through ionic interactions with the medium. Because of this the solution was transparent. The storage stability of metal nanoparticles capped with the novel polysulfabetaines are summarized in Table S5. As can be noticed from this table, the size of metal nanoparticles was very stable. No significant change in the size of the metal nanoparticles was noticed upon heating the aged sample at elevated temperatures of up to 100 °C as confirmed from DLS (Figures S13 and S14) studies. Heating the aged sample to 100 °C did not cause any appreciable change in the size of the nanoparticle; however, a change in PDI was noticed (Figure S14). Hence it can be concluded that polysulfabetaines are excellent capping agents for preparing stable metal nanoparticles in a saline medium of varying concentration.

The difference in size with electrolyte concentration (Figure 5a) is due to the varying degree of solvation of the polymer. Accordingly, in highly concentrated salt solutions, the polymers are well solvated with a complete screening of ions, leading to the formation of fully stretched chains. Hence, the particle size is the smallest in the saturated salt solution. Thus, the size of the nanoparticle is predominantly determined by the conformation of polymer in solution, which in turn is determined by the concentration of the salt solution. Synthesis of Gold and Palladium Nanoparticles. The ability of polysulfabetaines to cap Ag nanoparticles as well as the robust stability of nanoparticles encouraged us to extend this approach to other metal nanoparticles. Thus, polysulfabetaines were used as capping agents to form Au and Pd nanoparticles by employing formic acid as a solvent as well as a reducing agent. Analysis by UV−vis, DLS, and TEM showed the formation of metal nanoparticles as summarized in Table 3. Figure 6a shows the characteristics of metal nanoparticles formed using PSMAm 2 as a capping agent in 2 M NaCl solution. The SPR absorption maximum of 540 nm with a fwhm of 48 nm was noticed in the UV−vis spectroscopic analysis of Au nanoparticles (Figure 6a). In the case of Pd nanoparticles, SPR was observed at 280 nm with a fwhm of 36 nm. It is useful to note here that the absorption maximum of PSBMAm 2 was below 225 nm (Figure S4). This confirmed that the aforementioned absorptions were indeed due to the formation of metal nanoparticles. As in the case of Ag nanoparticles, a shift in the CO absorption frequency was noticed for Au and Pd nanoparticles as well as in the FT-IR analysis (Figure S9). This is a clear indication that the coordination of the metal−CO moiety is the dominant mode of capping by the zwitterionic polymer. Characterization of PSBMAm-Capped Au and Pd Nps. XRD analysis also indicated the diffraction pattern as expected for the presence of metals (Figure S10). PSBAm 2 showed a halo pattern characteristic of amorphous polymers. Diffraction peaks at scattering angles of 30 and 48° corresponding to reflections from 111 and 200 lattices of gold were observed in the case of Au nanoparticles. Additionally, EDX analysis also confirmed the presence of metal and polysulfabetaine (Figure S11). TEM images of metal nanoparticles capped by PSBMAm 2 are shown in Figure 6b,c. The image clearly shows discrete areas of high contrast, thereby suggesting the presence of nanoparticles. TEM also revealed that the particles were spherical in shape with sizes of 20−30 nm for Au and 8−15 nm for Pd nanoparticles. The size of the metal nanoparticles obtained from TEM is in good agreement with that obtained from DLS studies (Figure 6d).

■

CONCLUSIONS Novel halophilic sulfabetaines derived from poly(methacrylamide) and poly(methacrylate) showing salt-responsive and UCST behavior were synthesized and characterized. These polysulfabetaines were found to be very useful as capping agents for making highly stable metal nanoparticles in low (hypo-) to hypersaline media. Formic acid was used as a solvent as well as a reducing agent for making metal nanoparticles. Coordination through the CO functionality of ester and amide groups prevailing in polysulfabetaine was found to be the primary mechanism of interaction between the zwitterionic polymer and metal nanoparticle. The nanoparticles were stable under ambient conditions as well as upon heating for prolonged periods in the saline medium. The ability to form stable metal nanoparticles in low- and high-concentration saline media makes polysulfabetaine unique among zwitterionic polymers. We believe the metal nanoparticles reported here are suitable for biomedical and various other applications.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b01768. Detailed characterization of monomers and corresponding polymers by 1H NMR, FTIR, UV−vis, TGA, I

DOI: 10.1021/acs.langmuir.5b01768 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

■

(13) Ager, D.; Arjunan Vasantha, V.; Crombez, R.; Texter, J. Aqueous graphene dispersions−optical properties and stimuli-responsive phase transfer. ACS Nano 2014, 8 (11), 11191−11205. (14) Ning, J.; Li, G.; Haraguchi, K. Synthesis of highly stretchable, mechanically tough, zwitterionic sulfobetaine nanocomposite gels with controlled thermosensitivities. Macromolecules 2013, 46 (13), 5317− 5328. (15) Willcock, H.; Lu, A.; Hansell, C. F.; Chapman, E.; Collins, I. R.; O’Reilly, R. K. One-pot synthesis of responsive sulfobetaine nanoparticles by RAFT polymerisation: the effect of branching on the UCST cloud point. Polym. Chem. 2014, 5 (3), 1023−1030. (16) ShamsiJazeyi, H.; Miller, C. A.; Wong, M. S.; Tour, J. M.; Verduzco, R. Polymer-coated nanoparticles for enhanced oil recovery. J. Appl. Polym. Sci. 2014, 131 (15).10.1002/app.40576 (17) Motornov, M.; Roiter, Y.; Tokarev, I.; Minko, S. Stimuliresponsive nanoparticles, nanogels and capsules for integrated multifunctional intelligent systems. Prog. Polym. Sci. 2010, 35 (1−2), 174−211. (18) Doring, A.; Birnbaum, W.; Kuckling, D. Responsive hydrogels structurally and dimensionally optimized smart frameworks for applications in catalysis, micro-system technology and material science. Chem. Soc. Rev. 2013, 42 (17), 7391−7420. (19) Kowalczuk, A.; Trzcinska, R.; Trzebicka, B.; Müller, A. H. E.; Dworak, A.; Tsvetanov, C. B. Loading of polymer nanocarriers: Factors, mechanisms and applications. Prog. Polym. Sci. 2014, 39 (1), 43−86. (20) Li, X.; Lenhart, J. J.; Walker, H. W. Dissolution-accompanied aggregation kinetics of silver nanoparticles. Langmuir 2010, 26 (22), 16690−16698. (21) Sharma, V. K.; Siskova, K. M.; Zboril, R.; Gardea-Torresdey, J. L. Organic-coated silver nanoparticles in biological and environmental conditions: Fate, stability and toxicity. Adv. Colloid Interface Sci. 2014, 204, 15−34. (22) Huynh, K. A.; Chen, K. L. Aggregation kinetics of citrate and polyvinylpyrrolidone coated silver nanoparticles in monovalent and divalent electrolyte solutions. Environ. Sci. Technol. 2011, 45 (13), 5564−5571. (23) Estephan, Z. G.; Jaber, J. A.; Schlenoff, J. B. Zwitterion-stabilized silica nanoparticles: toward nonstick nano. Langmuir 2010, 26 (22), 16884−16889. (24) Lees, J. C.; Ellison, J.; Batchelor-McAuley, C.; Tschulik, K.; Damm, C.; Omanovic, D.; Compton, R. G. Nanoparticle impacts show high-ionic-strength citrate avoids aggregation of silver nanoparticles. ChemPhysChem 2013, 14 (17), 3895−3897. (25) Garcia, K. P.; Zarschler, K.; Barbaro, L.; Barreto, J. A.; O’Malley, W.; Spiccia, L.; Stephan, H.; Graham, B. Zwitterionic-coated ″stealth″ nanoparticles for biomedical applications: recent advances in countering biomolecular corona formation and uptake by the mononuclear phagocyte system. Small 2014, 10 (13), 2516−2529. (26) Foster, E. L.; Xue, Z.; Roach, C. M.; Larsen, E. S.; Bielawski, C. W.; Johnston, K. P. Iron oxide nanoparticles grafted with sulfonated and zwitterionic polymers: High stability and low adsorption in extreme aqueous environments. ACS Macro Lett. 2014, 3 (9), 867− 871. (27) Nap, R. J.; Park, S. H.; Szleifer, I. On the stability of nanoparticles coated with polyelectrolytes in high salinity solutions. J. Polym. Sci., Part B: Polym. Phys. 2014, 52 (24), 1689−1699. (28) Schulz, D. N.; Peiffer, D. G.; Agarwal, P. K.; Larabee, J.; Kaladas, J. J.; Soni, L.; Handwerker, B.; Garner, R. T. Phase-behavior and solution properties of sulfobetaine polymers. Polymer 1986, 27 (11), 1734−1742. (29) Lowe, A. B.; McCormick, C. L. Synthesis and solution properties of zwitterionic polymers. Chem. Rev. 2002, 102 (11), 4177−4189. (30) Kudaibergenov, S.; Jaeger, W.; Laschewsky, A. Polymeric Betaines: Synthesis, Characterization, and Application. In Supramolecular Polymers, Polymeric Betains, Oligomers; Advances in Polymer Science; Donnio, B., Guillon, D., Harada, A., Hashidzume, A., Jaeger, W., Janowski, B., Kudaibergenov, S., Laschewsky, A., Njuguna, J.,

solubility, thermal, hydrolytic stability. Nanoparticle characterization by XRD, EDX, stability in brine, FTIR, and DLS. (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was funded by the Agency for Science, Technology and Research (A*STAR), Singapore. We thank Dr. Zhu Yinghuai for providing metal precursors and Ms. Foo Ming Choo, Mr. Zhao Wenguang, Ms. Nur Zahirah Binte Zainal, and Ms. Noor Farhanah Binte Mohamed Barak for assistance with TGA and DLS studies.

■

REFERENCES

(1) Oliveira, O. N.; Iost, R. M.; Siqueira, J. R.; Crespilho, F. N.; Caseli, L. Nanomaterials for diagnosis: Challenges and applications in smart devices based on molecular recognition. ACS Appl. Mater. Interfaces 2014, 6 (17), 14745−14766. (2) Yin, Y. D.; Talapin, D. The chemistry of functional nanomaterials. Chem. Soc. Rev. 2013, 42 (7), 2484−2487. (3) Badawy, A. M. E.; Luxton, T. P.; Silva, R. G.; Scheckel, K. G.; Suidan, M. T.; Tolaymat, T. M. Impact of environmental conditions (ph, ionic Strength, and electrolyte Type) on the surface charge and aggregation of silver nanoparticles suspensions. Environ. Sci. Technol. 2010, 44 (4), 1260−1266. (4) Dwivedi, A. D.; Dubey, S. P.; Sillanpaa, M.; Kwon, Y. N.; Lee, C.; Varma, R. S. Fate of engineered nanoparticles: Implications in the environment. Coord. Chem. Rev. 2015, 287, 64−78. (5) Maurer-Jones, M. A.; Gunsolus, I. L.; Murphy, C. J.; Haynes, C. L. Toxicity of engineered nanoparticles in the environment. Anal. Chem. 2013, 85 (6), 3036−3049. (6) Gambinossi, F.; Mylon, S. E.; Ferri, J. K. Aggregation kinetics and colloidal stability of functionalized nanoparticles. Adv. Colloid Interface Sci. 2015, 222, 332. (7) Gambinossi, F.; Chanana, M.; Mylon, S. E.; Ferri, J. K. Stimulusresponsive Au@(meo2max-co-oegmay) nanoparticles stabilized by non-DLVO interactions: implications of ionic strength and copolymer (x:y) fraction on aggregation kinetics. Langmuir 2014, 30 (7), 1748− 1757. (8) Legg, B. A.; Zhu, M.; Comolli, L. R.; Gilbert, B.; Banfield, J. F. Impacts of ionic strength on three-dimensional nanoparticle aggregate structure and consequences for environmental transport and deposition. Environ. Sci. Technol. 2014, 48 (23), 13703−13710. (9) Song, Y.; Doomes, E. E.; Prindle, J.; Tittsworth, R.; Hormes, J.; Kumar, C. S. S. R. Investigations into sulfobetaine-stabilized cu nanoparticle formation: Toward development of a microfluidic synthesis. J. Phys. Chem. B 2005, 109 (19), 9330−9338. (10) Voorn, D. J.; Ming, W.; van Herk, A. M. Polymer-clay nanocomposite latex particles by inverse pickering emulsion polymerization stabilized with hydrophobic montmorillonite platelets. Macromolecules 2006, 39 (6), 2137−2143. (11) Vivek, A. V.; Pradipta, S. M.; Dhamodharan, R. Synthesis of silver nanoparticles using a novel graft copolymer and enhanced particle stability via a ″polymer brush effect″. Macromol. Rapid Commun. 2008, 29, 737−742. (12) Texter, J.; Vasantha, V. A.; Crombez, R.; Maniglia, R.; Slater, L.; Mourey, T. Triblock copolymer based on poly(propylene oxide) and poly(1-[11-acryloylundecyl]-3-methyl-imidazolium bromide). Macromol. Rapid Commun. 2012, 33, 69−74. J

DOI: 10.1021/acs.langmuir.5b01768 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir Pielichowski, J., Pielichowski, K., Takashima, Y., Eds.; Springer: Berlin 2006; Vol. 201, pp 157−224.10.1007/12_078 (31) Xuan, F.; Liu, J. Preparation, characterization and application of zwitterionic polymers and membranes: current developments and perspective. Polym. Int. 2009, 58 (12), 1350−1361. (32) Cao, Z.; Jiang, S. Super-hydrophilic zwitterionic poly(carboxybetaine) and amphiphilic non-ionic poly(ethylene glycol) for stealth nanoparticles. Nano Today 2012, 7 (5), 404−413. (33) Laschewsky, A. Structures and Synthesis of Zwitterionic Polymers. Polymers 2014, 6 (5), 1544−1601. (34) Vasantha, V. A.; Jana, S.; Lee, S. S. C.; Lim, C. S.; Teo, S. L. M.; Parthiban, A.; Vancso, J. G. Dual hydrophilic and salt responsive schizophrenic block copolymers - synthesis and study of self-assembly behavior. Polym. Chem. 2015, 6 (4), 599−606. (35) Vasantha, V. A.; Jana, S.; Parthiban, A.; Vancso, J. G. Water swelling, brine soluble imidazole based zwitterionic polymers– synthesis and study of reversible UCST behaviour and gel-sol transitions. Chem. Commun. 2014, 50, 46−8. (36) Vasantha, V. A.; Jana, S.; Parthiban, A.; Vancso, J. G. Halophilic polysulfabetaines - synthesis and study of gelation and thermoresponsive behavior. RSC Adv. 2014, 4 (43), 22596−22600. (37) McCormick, C. L.; Salazar, L. C. Water-soluble copolymers. 46. Hydrophilic sulfobetaine copolymers of acrylamide and 3-(2acrylamido-2-methylpropanedimethyl-ammonio)-1-propanesulphonate. Polymer 1992, 33 (21), 4617−4624. (38) Sin, M. C.; Chen, S. H.; Chang, Y. Hemocompatibility of zwitterionic interfaces and membranes. Polym. J. 2014, 46 (8), 436− 443. (39) Quintana, R.; Janczewski, D.; Vasantha, V. A.; Jana, S.; Lee, S. S. C.; Parra-Velandia, F. J.; Guo, S.; Parthiban, A.; Teo, S. L. M.; Vancso, G. J. Sulfobetaine-based polymer brushes in marine environment: Is there an effect of the polymerizable group on the antifouling performance? Colloids Surf., B 2014, 120, 118−124. (40) Wang, L.; Wang, H. J.; Nemoto, Y.; Yamauchi, Y. Rapid and efficient synthesis of platinum nanodendrites with high surface area by chemical reduction with formic acid. Chem. Mater. 2010, 22 (9), 2835−2841. (41) Liu, T.; Li, D. S.; Yang, D. R.; Jiang, M. H. Fabrication of flowerlike silver structures through anisotropic growth. Langmuir 2011, 27 (10), 6211−6217. (42) Likhitsup, A.; Parthiban, A.; Chai, C. L. L. Combining atomtransfer radical polymerization and ring-opening polymerization through bifunctional initiators derived from hydroxy benzyl alcoholPreparation and characterization of initiators, macroinitiators, and block copolymers. J. Polym. Sci., Part A: Polym. Chem. 2008, 46 (1), 102−116. (43) Parthiban, A.; Likhitsup, A.; Yu, H.; Chai, C. L. L. AB- and ABCtype di- and triblock copolymers of poly[styrene-block-(ε-caprolactone)] and poly[styrene-block-(ε-caprolactone)-block-lactide]: synthesis, characterization and thermal studies. Polym. Int. 2010, 59 (2), 145−154. (44) Parthiban, A.; Likhitsup, A.; Choo, F. M.; Chai, C. L. L. Triblock copolymers composed of soft and semi-crystalline segments-synthesis and characterization of poly[(n-butyl acrylate)-block-([varepsilon]caprolactone)-block-(L-lactide)]. Polym. Chem. 2010, 1 (3), 333−338. (45) Cardoso, J.; Rubio, L.; Albores-Velasco, M. Thermal degradation of poly(sulfobetaines). J. Appl. Polym. Sci. 1999, 73 (8), 1409−1414. (46) Kitano, H.; Imai, M.; Sudo, K.; Ide, M. Hydrogen-bonded network structure of water in aqueous solution of sulfobetaine polymers. J. Phys. Chem. B 2002, 106 (43), 11391−11396. (47) Niehaus, F.; Bertoldo, C.; Kahler, M.; Antranikian, G. Extremophiles as a source of novel enzymes for industrial application. Appl. Microbiol. Biotechnol. 1999, 51 (6), 711−729. (48) Oren, A. Bioenergetic aspects of halophilism. Microbiol. Mol. Biol. R. 1999, 63 (2), 334−348. (49) Madern, D.; Ebel, C.; Zaccai, G. Halophilic adaptation of enzymes. Extremophiles 2000, 4 (2), 91−98.

(50) Ollivier, B.; Caumette, P.; Garcia, J. L.; Mah, R. A. Anaerobicbacteria from hypersaline environments. Microbiol. Rev. 1994, 58 (1), 27−38. (51) Sukenik, S.; Boyarski, Y.; Harries, D. Effect of salt on the formation of salt-bridges in [small beta]-hairpin peptides. Chem. Commun. 2014, 50 (60), 8193−8196. (52) Seuring, J.; Agarwal, S. Polymers with upper critical solution temperature in aqueous solution: Unexpected properties from known building blocks. ACS Macro Lett. 2013, 2 (7), 597−600. (53) Kumar, A.; Vemula, P. K.; Ajayan, P. M.; John, G. Silvernanoparticle-embedded antimicrobial paints based on vegetable oil. Nat. Mater. 2008, 7 (3), 236−241. (54) Eckhardt, S.; Brunetto, P. S.; Gagnon, J.; Priebe, M.; Giese, B.; Fromm, K. M. Nanobio silver: Its interactions with peptides and bacteria, and its uses in medicine. Chem. Rev. 2013, 113 (7), 4708− 4754. (55) Hu, R.; Li, G.; Jiang, Y.; Zhang, Y.; Zou, J. J.; Wang, L.; Zhang, X. Silver−zwitterion organic−inorganic nanocomposite with antimicrobial and antiadhesive capabilities. Langmuir 2013, 29 (11), 3773−3779. (56) Wang, L.; Wang, H. J.; Nemoto, Y.; Yamauchi, Y. Rapid and efficient synthesis of platinum nanodendrites with high surface area by chemical reduction with formic acid. Chem. Mater. 2010, 22 (9), 2835−2841. (57) Zhao, H.; Yu, C.; You, H.; Yang, S.; Guo, Y.; Ding, B.; Song, X. A green chemical approach for preparation of PtxCuy nanoparticles with a concave surface in molten salt for methanol and formic acid oxidation reactions. J. Mater. Chem. 2012, 22 (11), 4780−4789. (58) Hebbalalu, D.; Lalley, J.; Nadagouda, M. N.; Varma, R. S. Greener techniques for the synthesis of silver nanoparticles using plant extracts, enzymes, bacteria, biodegradable polymers, and microwaves. ACS Sustainable Chem. Eng. 2013, 1 (7), 703−712. (59) Wang, H.; Qiao, X.; Chen, J.; Wang, X.; Ding, S. Mechanisms of PVP in the preparation of silver nanoparticles. Mater. Chem. Phys. 2005, 94 (2−3), 449−453. (60) Kirkland-York, S.; Zhang, Y.; Smith, A. E.; York, A. W.; Huang, F.; McCormick, C. L. Tailored design of Au nanoparticle-siRNA carriers utilizing reversible addition−fragmentation chain transfer polymers. Biomacromolecules 2010, 11 (4), 1052−1059. (61) Guo, Z.; Henry, L. L.; Palshin, V.; Podlaha, E. J. Synthesis of poly(methyl methacrylate) stabilized colloidal zero-valence metallic nanoparticles. J. Mater. Chem. 2006, 16 (18), 1772−1777.

K

DOI: 10.1021/acs.langmuir.5b01768 Langmuir XXXX, XXX, XXX−XXX