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pH Triggered Recovery and Reuse of Thiolated Poly(Acryl Acid) Functionalized Gold Nanoparticles with Applications in Colloidal Catalysis Siyam M. Ansar, Benjamin D. Fellows, Patrick Mispireta, O. Thompson Mefford, and Christopher L. Kitchens Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00870 • Publication Date (Web): 26 Jun 2017 Downloaded from http://pubs.acs.org on June 26, 2017

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pH Triggered Recovery and Reuse of Thiolated Poly(Acryl Acid) Functionalized Gold Nanoparticles with Applications in Colloidal Catalysis Siyam M. Ansar†, Benjamin Fellows+, Patrick Mispireta†, O. Thompson Mefford+, and Christopher L. Kitchens* # †

†

Department of Chemical and Biomolecular Engineering, Clemson University, Clemson, SC 29634, USA +

Department of Materials Science and Engineering, Clemson University, Clemson, SC 29634, USA #

Institute of Environmental Toxicology (CU-ENTOX), Clemson University, 509 Westinghouse Road, Pendleton, SC, USA

*

Corresponding author. Email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Thiolated poly(acrylic acid) (PAA-SH) functionalized gold nanoparticles were explored as a colloidal catalyst with potential application as a recoverable catalyst where the PAA provides pH-responsive dispersibility and phase transfer capability between aqueous and organic media. This system demonstrates complete nanoparticle recovery and redispersion over multiple reaction cycles without changes in nanoparticle morphology or reduction in conversion. The catalytic activity (rate constant) was reduced in subsequent reactions when recovery by aggregation was employed, despite unobservable changes in morphology or dispersability. When colloidal catalyst recovery employed a pH-induced phase transfer between two immiscible solvents, the catalytic activity of the recovered nanoparticles was unchanged over 4 cycles, maintaining the original rate constant and 100% conversion. The ability to recover and reuse of colloidal catalysts by aggregation/re-dispersion and phase transfer methods that occur at low and high pH, respectively could be used for different gold nanoparticle catalyzed reactions that occurs at different pH conditions.

KEYWORDS: Gold nanoparticles, thiolated PAA, catalytic activity, recovery, reuse


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INTRODUCTION Gold nanoparticles (AuNPs) have attracted increasing interest in catalysis due to their unique catalytic activities for particular reactions, which include alcohol oxidations,1 nitroarene reductions,2 hydrogenation of quinolone compounds3 and carbon-carbon cross coupling reactions.4 Supported AuNPs are commonly used in industries span from fine chemicals to pharmaceuticals synthesis due to the advantages of recovery/reuse of catalyst and ability to catalyze reactions in a continuous flow reactor.5-6 Supported AuNPs have been extensively explored, however colloidal AuNPs have greater catalytic activity and selectivity compared to supported AuNPs.5 Furthermore, colloidal AuNPs catalyze most reactions under mild conditions and have significant potential over supported catalysis in areas like chiral catalysis. The past 30+ years have seen an explosion in the number of colloidal AuNPs synthesized with different sizes, shapes, crystallinity, and atomic composition; all with unique properties that offer a plethora of opportunity for new catalysts. However, applications in catalysis are limited due to a lack of simple methods to recover these colloidal nanoparticles from reaction mixture and reuse. Designing reusable colloidal catalysts is a significant area of interest for colloidal catalysis. The design of recyclable nanoparticle-based catalysts has been a key area in green chemistry. Attempts at NP recovery have included membrane separation techniques and ultrafiltration,7-8 magnetic separation,9-10 and use of biphasic reaction conditions.11-12 In most cases, researchers have noted difficulty in recycling NPs for additional reactions which often suffers from complete loss of catalytic activity.1, 4, 13 Some success has been reported for recovery and reuse of colloidal AuNPs in water.14-16 Sakurai et al prepared thermosensitive AuNPs stabilized by vinyl ether star polymers and studied the catalytic activity and reusability for aerobic alcohol oxidation reaction.14 Their results indicated that the NPs effectively catalyzed the oxidation


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reaction and were easily separated from the reaction mixture by thermal-induced precipitation, allowing for repeated reuse. Zhang et al. prepared polyampholyte stabilized AuNPs, smaller than 4 nm in diameter, and demonstrated their pH-triggered reversible aggregation and re-dispersion ability.15 Their results demonstrated effective catalysis coupled with pH induced separation from the reaction mixture, including repeated recovery and reuse in aerobic alcohol oxidation reactions. However, polyampholyte stabilized AuNPs aggregate at very low pH (approximately pH 0) which is a harsh environment for most metal catalysts, reactor systems, and some reaction products. Furthermore, AuNPs interact weakly with aromatic groups in polyampholyte hence this polymer can be easily displaced by some educts during the reaction. In this work, we demonstrate a simple approach to preparing pH-responsive AuNPs and effective recovery and reuse as a catalyst. We have employed thiolated poly(acrylic acid) (PAASH) functionalized AuNPs (AuNP-SPAA) as a colloidal catalyst and demonstrated the application as a recoverable and reusable catalyst where the PAA provides pH-responsive dispersibility and phase transferability between aqueous and organic media. The activity of AuNP-SPAA catalyzed reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by sodium borohydride (NaBH4) is used as a model reaction. The as-prepared AuNP-SPAA are catalytically active towards the reduction of 4-NP to 4-AP and the recovery/reusability has been explored. Though colloidal AuNP aggregation and redispersion in water has been explored before, to our knowledge, the pH-triggered AuNP phase transfer between water and organic phase (without aggregation) and reuse in catalysis has been not reported.


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EXPERIMENTAL SECTION Chemicals and Instrumentation. All chemicals were purchased from Sigma-Aldrich except

1-ethyl-3-(3-dimethylaminopropyl)carbodiimide)

hydrochloride,

cysteamine

hydrochloride, and toluene, which were purchased from TCI, Chem-Impex, and Alfa Aesar, respectively. All chemicals were used without further purification. Water was deionized and filtered by a Milli-Q water system. UV-Vis spectra were taken using a Varian Cary 50 UV-VisNIR spectrophotometer. pH of the AuNP-SPAA solution was determined using a pH meter (sympHony SB90M5, VWR International). The particle size and morphological structure of AuNPs were examined with Transmission Electron Microscopy (TEM). The size distributions were obtained by image analysis with the ImageJ software package. TEM samples were prepared by drop casting ̴ 5 μL of as-synthesized citrate capped AuNPs dispersion onto a 300 mesh Cu grids covered with a formvar carbon film, followed by solvent evaporation. High-resolution TEM images were obtained using a Hitachi 9500 with an accelerating voltage of 300 kV. The purified AuNP-SPAA composites were diluted 5 times prior to Dynamic Light Scattering (DLS) measurements. The measurements were obtained using a Malvern instrument (Zeta sizer Nano series) at 25 °C. Hydrodynamic diameters and ζ potentials of AuNP-SPAA at different pH values were measured. The solutions were adjusted to the desired pH with either 0.1 M HCl or 0.1 M NaOH solutions. Synthesis of PAA-SH. The synthesis of the thiolated poly(acrylic acid) (PAA-SH)was done using carbodiimide amide coupling. One gram of poly(acrylic acid) (1,800 Mw, Sigma Aldrich), was dissolved in 50 mL of dimethyl sulfoxide (DMSO) (Sigma Aldrich, anhydrous). To this solution n-hydroxysuccinamide (NHS, 0.320 g, Sigma Aldrich, >98%) and 1-ethyl-3-(3-


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dimethylaminopropyl)carbodiimide) hydrochloride (EDC, 0.533 g, TCI, >98% ) were added in a 5:1 molar ratio. This solution was left to stir at room temperature for 30 minutes. After 30 minutes, cysteamine hydrochloride (0.316 g, Chem-Impex, 99.5%) in a 5:1 ratio (carboxyl to thiol) was then added to the reaction and left to stir for 4 hours. The PAA-SH was purified by adding toluene (Alfa Aesar, 99.5%) to small amounts of the PAA-SH containing solution. Centrifugation of this suspension yielded a highly viscous polymer layer which was separated from the supernatant and lyophilized. The PAA-SH was then suspended in ethanol for further use. PAA-SH Functionalized AuNP Synthesis. Citrate-stabilized AuNPs were first synthesized by the classic citrate reduction methods.17 In brief, 250 μL of 0.05 M HAuCl4 aqueous solutions were heated while gently stirring. When the solution start to boil, 2.0 mL of 0.05 M citrate in H2O was added and the resulting solution was stirred at 400 rpm for 15 min as the color of the solution changed from colorless to red. PAA-stabilized AuNPs were prepared by ligand exchange reaction between citrate-stabilized AuNPs and the PAA-SH. In brief, 10 mL of 3 mM PAA in dilute NaOH solution was added to 20 mL of as-synthesized AuNPs and incubated for 24 h before use it. Free PAA in the AuNP-SPAA was removed by centrifuagal precipitation and re-dispertion with H2O. Purification process was repeated 4 times. Thermogravimetric (TGA) Analysis. TGA experiment was conducted to quantify the amount of PAA grafted onto AuNPs. TGA measurements were performed on a TA instruments SDT Q600. 50 mL of purified AuNP-SPAA was concentrated to 60 µL by centrifugation at 14500 rpm for 1 h and 50 µL of concentrated AuNP-SPAA was deposited into a clean alumina TGA pan. The temperature was ramped to 100 °C at 10 °C/min and held for 10 min to remove


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all solvent. The temperature was then ramped to 600 °C at 10 °C/min under a N2 purge of 20 mL/min. 4-Nitrophenol Reduction Catalysis. The 4-NP reduction kinetics was performed in a 4 mL quartz cell using a Varian Cary 50 UV-Vis-NIR spectrophotometer. In brief, 0.25 mL of purified AuNP-SPAA (nominal concentration of 11 nM), 1.45 mL of H2O, and 1.00 mL of 0.2 mM 4-NP were mixed in a 4 mL quartz cell. Time-resolved UV-Vis spectra were taken immediately after addition of freshly prepared 0.30 mL of 0.1 M NaBH4 in water. The progress of the reaction was tracked by monitoring the change in intensity of 4-NP peak at 400 nm as function of time.

RESULTS AND DISCUSSION While poly (acrylic acid) is a widely used and robust polymer for many applications, it is not well suited as a stabilizing functionality for metal nanomaterials due to the lack of functional groups that bind strongly to the metal surface. In order to circumvent this issue and employ the pH-responsive functionality provided by the PAA polymer, we have modified a fraction of the carboxylic groups of the PAA with thiol functional groups (Figure 1a) that will covalently bind to gold nanoparticles. PAA-SH was synthesized using a common EDC-NHS coupling reaction between poly(acrylic acid) and cysteamine with molar ratios to yield 25 % modification of the carboxylic acid groups (1 in 4 carboxylic acid groups modified or 6 cysteamine groups per PAA polymer chain), where the degree of substitution was confirmed with the HNMR (Supporting Information, Figure S1). Citrate-stabilized AuNPs were synthesized by the classic citrate reduction methods.17 AuNP-SPAA was prepared by ligand exchange reaction between citratestabilized AuNPs and the PAA-SH dissolved in 0.01 M KOH solution. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy was used to analyze the PAA-


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SH binding, confirming the presence of PAA on the AuNP surface (Supporting Information, Figure S2). TEM was used to examine the AuNP size, size distribution after PAA-SH functionalization. As shown in Supporting Information, Figure S3, the average diameter of the AuNP-SPAA was 13.2 ± 0.9 nm and the inter-particle distance was increased due to the surface adsorbed PAA-SH layer, as determined by DLS. DLS indicated an increase in AuNP hydrodynamic diameter from 16.3 (PDI = 0.13) to 38.1 (PDI = 0.21) after PAA-SH functionalization. PDI stands for polydispersity index and such a small PDI values (< 0.4) suggest that the nanoparticles in solutions are relatively monodisperse. An increase in the magnitude of AuNP zeta-potential from -33.2 ± 1.5 mv for the citrate-stabilized AuNPs to -41.6 ± 2.9 mv after PAA-SH functionalization was also observed. DLS data and UV-Vis spectra (Supporting Information, Figure S3) further confirmed the colloidal stability of AuNP-SPAA in solution. AuNP-SPAA shows improved stability towards high pH (0.1M KOH), salt buffer and ionic strength compared to citrate capped AuNPs (Supporting Information, Figure S4). This improved stability is due to the negatively charged PAA polymer layer (in neutral and basic pH) that stabilizes the AuNP-SPAA by both steric and electrostatic repulsions. The quantity of PAA-SH adsorbed on AuNPs was determined by TGA (Supporting Information, Figure S5). The percent weight loss of PAA-SH adsorbed onto AuNPs is 7 % and the PAA-SH packing density on AuNPs was calculated to be 0.88 chain/nm2 (See SI) which is ~ 1.6 times smaller than Toth et al. reported thiolated PAA packing density on the 12 nm AuNPs (1.4 chain/nm2).18 Toth et al. synthesized thiolated PAA polymer by an alternative synthesis method resulting in a different molecular structure with different thiol functionality and a smaller number of thiol moities per PAA chain (3 sulfur atoms per chain). The difference in packing density is attributed to the difference in thiol arrangement on the PAA and conformation on the AuNP surface. The


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packing density and molecular weight of the polymer on the nanoparticle surface plays a key role in determining the surface properties of the nanoparticles especially colloidal phenomena and catalytic activity.19 In our recent work, we have demonstrated a direct correlation with decreased polymer chain packing density on AuNPs and increased catalytic activity for colloidal AuNPs.19

Figure 1: (a) Molecular structure of HS-PAA with a carboxyl to thiol ratio (x/y) of ~ 5, (b) Photographs visualizing the reversible AuNP-SPAA clustering and re-dispersion with changing pH. The left vial contains well-dispersed AuNPs at basic pH and the right vial contains aggregated and precipitated AuNPs at acidic pH, (c) Plot shows pH triggered reversibility of aggregation and re-dispersion monitored by the LSPR peak intensity at 526 nm, and (d) Hydrodynamic diameter and ζ-potential of the AuNP-SPAA as a function of pH.

AuNP-SPAA

showed

a

remarkable

pH-responsiveness,

undergoing

reversible

aggregation-precipitation and redispersion at acidic and basic pH visualized in Figure 1b and measured by UV-Vis absorbance spectroscopy in Figures 1c and S6. The UV-Vis spectra of AuNP-SPAA exhibited a characteristic localized surface plasmon resonance (LSPR) absorption at 526 nm (and intensity ca. 0.9) representative of the wine-red solution and confirming the stability of AuNP-SPAA in basic medium. The pH of the AuNP-SPAA solution was decreased


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by drop-wise titration with appropriate concentrations of HCl solution. Acidic conditions resulted in complete aggregation and precipitation of nanoparticles from the solution as indicated by disappearance of the LSPR peak demonstrated in Figure 1c and visualized as a colorless solution and black precipitate in Figure 1b. Adjustment of the pH back to basic conditions via NaOH titration resulted in complete redispersion of the precipitated AuNP-SPAA as evidenced by recovery of the LSPR peak maximum at 526 nm and absorbance value of ca.0.9, accounting for dilution (Figure 1c and Supporting Information, Figure S6). Recovery of the absorbance intensity and the lack of a red-shift in peak maxima indicates complete redispersion over multiple cycles without aggregation or morphology changes. TEM (Supporting Information, Figure S7) and DLS measurements confirm complete redispersion following multiple pHinduced precipitation cycles without changes in particle morphology. The precipitationredispersion process of AuNP-SPAA were repeated up to 4 cycles without losing the pHreversibility, however eventually salt formation will begin to influence colloidal stability. The pH dependent colloidal stability of the AuNP-SPAA was studied by ζ-potential, DLS hydrodynamic diameter, particle size measurements, and UV-Vis spectroscopy (Figures 1d and S8). As the pH drops below 4.5, the LSPR peak intensity decreases and peak maximum wavelength redshifts (Supporting Information, Figure S8) indicating aggregation and precipitation. DLS measurements show a dramatic increase in hydrodynamic diameter that results from aggregation and shows the onset of AuNP-SPAA instability commences at ca. pH 4.5. The measured ζ-.potential (Figure 1d) demonstrates the pH dependent surface charge where a highly negative charge is observed at pH 6 to 10 with a maximum magnitude ζ-potential of -42 mV indicating carboxylate group deprotonation. A dramatic decrease in the charge density occurred as the pH decreased from 6 to 2 where this decrease coincides with decreased


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electrostatic repulsion and colloidal stability as a result of carboxylate protonation.20-21 The onset of aggregation at ca. pH 4.5 (and ζ-potential of -20 mV) is consistent with the pKa of the carboxylate group (~4.5) in PAA.22-23 The significance of the pH responsive dispersion and reversibility is relevant for scenarios where nanoparticle recovery is required, such as colloidal catalysis.

Figure 2: Recovery and reuse of AuNP-SPAA in catalysis with pH triggered aggregation and redispersion. (A) Photographs depicting the progression of reaction, catalyst precipitation, redispersion and reuse for catalytic 4-nitrophenol reduction. (B) The progress of the reaction tracked by the change in 4-NP absorbance peak at 400 nm over the time, (C) Plot of -ln(Ct/C0) versus time spectra for the reduction of 4-NP over AuNP-SPAA catalyst at different catalytic cycles, and Table of the summary of catalytic reaction rate and conversion % as a function of catalytic reaction cycle.

Figure 2A demonstrates the facile and complete recovery of AuNP-SPAA from the reaction mixture, followed by complete redispersion for reuse in subsequent reactions. The pHtriggered precipitation/redispersion reversibility makes the AuNP-SPAA nanoparticle-ligand assembly a feasible colloidal catalyst that has the added capacity to be removed from the reaction


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phase and reused. The reaction kinetics were monitored in situ using time-resolved UV-Vis spectroscopy via changes in intensity of 4-NP peak at 400 nm (Supporting Information, Figure S9 and Figure 2B).24-28 The rate constant is obtained by fitting the data from Figure 2B to pseudo-first-order reaction kinetics with respect to 4-NP and the rate constant is indicative of catalytic activity. The linear relationship for -ln(C0/Ct) versus time (Figure 2C) confirms the pseudo-first-order kinetic assumption. Ct and C0 are the concentration of 4-NP at time t and t = 0. The reaction rate constants listed in Figure 2 are averaged from three independent measurements and reported for four consecutive catalytic cycles as depicted in Figure 2A. The rate constant of AuNP-SPAA is lower than that of the citrate-capped AuNPs which is 2.2 min-1 (Supporting Information, Figure S10). Despite lower catalytic activity, AuNP-SPAA can be recovered and reused in at least four consecutive catalytic cycles. Each catalytic cycle conducted the 4-NP reduction at a pH of 8.0, adjusted by KOH to ensure colloidal stability of the AuNP-SPAA catalyst. Following the reaction, the pH was adjusted to 4.0 with HCl, inducing nanoparticle agglomeration and precipitation. The 4-AP containing aqueous phase was decanted and the remaining black aggregated AuNP-SPAA precipitate was redispersed in fresh dilute KOH aqueous medium at pH = 8.0. The solution was then reused in three subsequent 4-NP reduction reactions. Despite achieving 100% conversion for each cycle, the reaction rate constants, listed in the Figure 2 table, showed a steady decrease with each reaction cycle. UV-Vis absorbance and TEM imaging of the AuNP-SPAA does not show any evidence of nanoparticle morphology change or irreversible aggregation that would result in reduced catalytic activity after 4 reaction cycles. High-resolution TEM imaging reveals that the AuNPs maintain well-defined crystalline phases and the lattice spacing for the planes are measured to be 0.23 nm before and the after each catalytic cycles (Supporting Information,


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Figure S11). This lattice spacing corresponds to the d spacing (0.236 nm) of the planes for facecentered cubic gold.29-30 UV-Vis absorbance peak intensity of AuNP-SPAA indicates no significant loss of AuNP during the recovery process for each reaction cycle (Supporting Information, Figure S9). Despite the inability to identify an obvious cause, the loss of catalytic activity as a result of the catalyst recovery and reuse is problematic for long-term catalyst use. Therefore, we have explored alternative mechanisms of recovering the AuNP-SPAA, avoiding aggregation. Specifically, inspiration was taken from the field of phase transfer catalysis, where AuNP recovery involved pH induced transfer of the AuNP-SPAA between two immiscible solvents. To date many strategies to modulate phase partitioning of nanoparticles have been developed and involve either manipulating the properties of the solvent or changing the wettability of the nanoparticle stabilizing ligands.31-32 Methods to manipulate the nanoparticle surface ligand structures to induce nanoparticle phase transfer include host-guest interactions,3334

electrostatic interactions,35-37 covalent modifications,38 and ligand exchanges39-44. In most

cases, the phase transfer is irreversible and preferred. For some applications, reversible and stimuli-responsive nanoparticle phase transfer is necessary. Reversible nanoparticle phase transfer between two immiscible solvents is far more challenging and complicated. Methods for nanoparticle transfer between solvents particularly between aqueous and organic phases, are very important for separation of metal nanoparticle catalyst from the reaction mixture and reuse it. In this work, it is evident that PAA-SH functionalized AuNPs can transfer between aqueous and chloroform layers with modulation of the aqueous phase pH to stimulate reversible phase transfer without aggregation.


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Figure 3: (a) Photographs demonstrating the pH-triggered reversible phase transfer of AuNPSPAA between water and chloroform. The left vial contains well-dispersed AuNP-SPAA in aqueous phase (top layer) at basic pH. The right vial contains AuNP-SPAA transferred into CHCl3 phase (bottom layer) with HCl addition and vigorous shaking. (b) Plot of LSPR peak intensity at 526 nm in aqueous phase showing pH-triggered reversible phase transfer of AuNPSPAA between water and chloroform (c) LSPR peak intensity at 526 nm in aqueous phase and percent transfer of AuNP-SPAA from aqueous to CHCl3 layer as a function of pH. The percent transfer was calculated by taking the absorbance of the AuNP-SPAA (in aqueous medium) at pH 12.5 as 0%. The red curve is a sigmoidal fit of the data and was used to identify the pH of transfer.

The AuNP-SPAA phase transfer between aqueous and organic phases was achieved as follows: 1.5 mL of AuNP-SPAA (pH = 12.5) was added to 1.5 mL of CHCl3 containing 0.1 wt% octadecylamine (Figure 3a, left vial). The pH was adjusted to 8.0 with dilute HCl and vigorously mixed for 2 minutes. The AuNP-SPAA completely transferred from the aqueous phase to the chloroform phase (Figure 3a, right vial). The transfer from aqueous to organic phase occurs when the pH of the aqueous phase decreased from above to below the pKa of amine headgroup of octadecylamine which is ca. 10.6.45 The role of the octadecylamine is to form an ion pair with


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the anionic carboxylate of the PAA and increase the hydrophobic character of the nanoparticleligand assembly.37

This is demonstrated by a control experiment where the absence of

octadecylamine results in AuNP-SPAA precipitation onto the glass vial surface and at the biphasic interface with pH change below 4.5 (Supporting Information Figure S12). Adjustment of the pH from 8.0 to 12.0 by KOH titration and vigorous mixing resulted in AuNPSPAA transfer from the chloroform back into the aqueous phase. The reversible phase transfer of AuNPs was again characterized by the UV-Vis absorption spectra of AuNP-SPAA in water phase over four cycles (Figure 3b, and Supporting Information Figure S13). The phase transfer was monitored by the LSPR peak intensity change of AuNP-SPAA in aqueous phase with pH titration from 12.5 to 8 (Figure 3c and Supporting Information Figure S14). The onset of phase transfer of AuNP-SPAA from aqueous to CHCl3 phase commences at pH 10.6 and complete transfer to CHCl3 phase at pH ≤ 9.5 (Figure 3c). The phase transfer of AuNPSPAA from the aqueous to organic phase is facilitated by the ion-pair formation between the negatively charged carboxylate group of PAA and protonated terminal amine group of octadecylamine at pH below ~10.6. The octadecylamine contains a long alkyl chain that imparts additional hydrophobic functionality to the AuNP-SPAA and facilitates phase transfer. Deprotonation of the octadecylamine above pH ~10 results in dissociation of the ion-pair and transfer of the negatively charged AuNP-SPAA back to the aqueous phase.


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Figure 4: Recovery and reuse of AuNP-SPAA catalyst via pH-induced phase transfer method. (A) Photograph representation of the recovery and reuse cycle of AuNP-SPAA after 4nitrophenol reduction reaction. The vials show, (a) Aqueous AuNP-SPAA and 4-NP mixture before reaction, (b) After complete reduction of 4-NP into 4-AP, (c) Addition of ODA in CHCl3, (d) H+ induced AuNP-SPAA phase transfer into CHCl3 layer, (e) Remove the supernatant containing 4-AP and decomposed NaBH4 without removing CHCl3 layer, and (f) Transfer of AuNP-SPAA from CHCl3 layer into water by adding the dilute NaOH. (B) The progress of the reaction tracked by the change in 4-NP absorbance peak at 400 nm over the time, (C) Plot of ln(Ct/C0) versus time and linear fit of the 4-NP reduction for four catalytic cycles and Table of catalytic reaction rate constants and conversion for each reaction cycle.

Direct phase transfer of nanoparticle is an extremely promising method for recovery and reuse of nanoparticle-based catalysts. With the AuNP-SPAA mediated reversible phase transfer without AuNP aggregation, it is feasible to use this strategy to recover and reuse the AuNPSPAA in catalytic conversion of 4-nitrophenol to 4-aminophenol reaction (Supporting Information Figure S15). Figure 4A depicts the use of the phase transfer method for complete


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recovery of AuNP-SPAA in the biphasic reaction mixture followed by reuse. The reaction rate constants from three independent measurements over four consecutive reaction cycles of AuNPSPAA are summarized in the Figure 4 Table. The subsequent catalytic data showed that AuNPSPAA could be reused at least four repeated times with negligible loss in catalytic activity and maintaining 100% conversion. This direct phase transfer method is better than the aggregation/redispersion method in terms of persevering the catalytic activity of AuNP-SPAA in subsequent catalytic cycles. Thus, by direct phase transfer method by switching the pH, the same batch of AuNP-SPAA can be recycled to catalyze multiple cycles of 4-nitrophenol reduction reaction, demonstrating the sustained catalytic activity of the AuNP-SPAA during the pHtriggered recovery and recycling process.

Conclusions

We have demonstrated for the first time pH triggered AuNP recovery using direct transfer of nanoparticles between two immiscible solvents without loss of catalytic activity. The synthesized PAASH nanoparticle ligand is a robust ligand that facilitates reversible aggregation/redispersion without changes in the nanoparticle morphology or colloidal properties. The versatile PAA ligand also facilitates reversible transfer of nanoparticles between aqueous and chloroform phases without degrading the nanoparticles. This pH-triggered reversible phase transfer was found to be particularly useful in the nanoparticle recovery following catalyzed reaction,


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enabling reuse in subsequent reactions without loss in activity. The AuNP-PAA catalyst was highly active and reusable in the catalytic reduction of 4-NP to 4-AP with 100% conversion and minimal reductions in the reaction rate with up to four catalyst recycles. Indeed, the design of recyclable nanoparticle-based catalysts has been a keen area of interest. The fundamental insight from this study allows the design of reusable nanoparticle catalysts with different surface functionality. Furthermore, this nanoparticle ligand platform is robust and has potential for extension to other nanoparticle surfaces for colloidal or catalytic applications.

ASSOCIATED CONTENT

Supporting Information: This material is available free of charge via the internet at http://pubs.acs.org. Synthesis of PAA-SH, FTIR and UV-Vis spectra of PAA-SH functionalized AuNPs, UV-Vis spectra of AuNP-SPAA as a function of pH, TGA curves of neat HS-SPAA and AuNP-SPAA, packing density of HS-SPAA on AuNP, AuNP-SPAA catalyzed 4-nitrophenol reduction reactions, and HR-TEM images of AuNP-SPAA before and after reaction cycles. Corresponding Author *Email: [email protected] ACKNOWLEDGMENT: This work was sponsored by the National Science Foundation grant No. CBET-1057633. We acknowledge the analytical lab manager, Kimberly Ivey at the materials science and engineering at Clemson University for assisting with the FTIR analysis.

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