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Research Article pubs.acs.org/journal/ascecg

Metal Nanoparticle Loaded Magnetic-Chitosan Microsphere: Water Dispersible and Easily Separable Hybrid Metal Nano-biomaterial for Catalytic Applications Thanusu Parandhaman,†,§ Nagaraju Pentela,‡,§ Baskaran Ramalingam,† Debasis Samanta,‡,§ and Sujoy K. Das*,†,§

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Biological Materials Laboratory, ‡Polymer Science Division, Council of Scientific and Industrial Research (CSIR)-Central Leather Research Institute (CLRI), Chennai, 600020, India § Academic of Scientific and Innovative Research (AcSIR), New Delhi, 110001, India S Supporting Information *

ABSTRACT: We report a green synthesis of magnetically separable hybrid metal nanobiomaterial as water dispersible and recyclable catalysts. The hybrid nanobiomaterial was prepared in a three-step process. The magnetic nanoparticles were initially synthesized by biomineralization process and coated with chitosan followed by binding and reduction of metal ions, which led to the formation of a magnetically separable hybrid nanobiomaterial (Fe3O4@Ch-MNPs, M = Au, Pd). The chemical composition, morphology, thermal stability, and magnetic behavior of the hybrid nanobiomaterial were characterized with zeta potential, Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), vibrating sample magnetometer (VSM), and field emission scanning electron microscopy (FESEM) analysis. The FESEM measurement demonstrated formation of highly dispersed Au and PdNPs on the surface of nanobiomaterial, while thermogravimetric and VSM analyses indicated high thermal stability and superparamagnetic behavior of the hybrid nanobiomaterial. The X-ray photoelectron spectroscopy studies revealed formation of pure metallic nanoparticles on the surface of the hybrid nanobiomaterial. The as-synthesized hybrid nanobiomaterial were tested for several model reactions such as photocatalytic reduction of dye, hydrogenation of p-nitrophenol, and Suzuki coupling reaction at ambient temperature and in aqueous solution. The catalytic efficiencies varied with the type of MNPs, and Fe3O4@Ch-PdNPs exhibited superior catalytic activities in all chemical reactions. In addition the hybrid nanobiomaterial demonstrated excellent recyclability and reusability without significant loss of catalytic activities. Furthermore, the leaching of metal ions was not detected during catalytic reaction confirming high stability and low environmental impact of the as-synthesized hybrid nanobiomaterial. We believe that our result will help to synthesize easily separable hybrid nanobiomaterial as a heterogeneous catalyst through a cost-effective and eco-benign synthetic route for the development of environmental sustainable nanotechnology. KEYWORDS: Metal nanoparticle, Magnetic properties, Supported catalyst, Photocatalytic reduction, Hydrogenation reaction, Suzuki coupling reaction, and Green chemistry

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etc. has received significant attention in recent years.8−10 However, the catalytic activity of the MNPs is largely hampered because of aggregation/agglomeration of the MNPs in the reaction medium.11 Additionally, the separation and recycling of the MNPs from the reaction medium remain a major challenge. Moreover, the accumulation of free MNPs and subsequent leaching of metal ions into the environment raise secondary pollution problems and cause harmful effects on living organisms.12,13 From a sustainable point of view, significant efforts have been devoted in recent years to design

INTRODUCTION Catalytic applications of noble metal nanoparticles (MNPs) have received significant interest because the nanoparticles possess large specific surface areas and large number of exposed metal atoms, which greatly enhance the catalytic and physicochemical properties of the MNPs.1,2 Among various MNPs, gold and palladium nanoparticles have attracted considerable attention in the reduction of nitro compounds, oxidation of alcohols, photocatalytic degradation of dyes, catalytic conversion, or decomposition of organic pollutants, electrochemical reduction of CO2, coupling reaction for C−C bond formation, and many more.3−7 Therefore, the utilization of AuNPs and PdNPs for environmental remediation and large scale production of agrochemicals, pharmaceuticals, polymers, © 2016 American Chemical Society

Received: August 5, 2016 Revised: November 29, 2016 Published: December 5, 2016 489

DOI: 10.1021/acssuschemeng.6b01862 ACS Sustainable Chem. Eng. 2017, 5, 489−501

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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic representation of synthesis of biogenic magnetic nanoparticles (A), encapsulation of magnetic nanoparticle with chitosan and formation of metal nanoparticles on the chitosan encapsulated magnetic nanoparticles (B) to form hybrid metal nanobiomaterial (Fe3O4@ChMNPs).

was decorated with Au and PdNPs through in situ reduction of metal ions by microwave irradiation to form magnetically separable hybrid nanobiomaterial (Fe3O4@Ch-MNPs, M = Au, Pd). The characterization, mechanism of formation, and catalytic efficiency of the synthesized Fe3O4@Ch-MNPs in photocatalytic reduction of methylene blue, hydrogenation of p-nitrophenol, and Suzuki coupling reaction have been demonstrated.

and synthesize solid-supported MNPs as a recyclable heterogeneous catalyst.14−16 However, recycling of the supported catalyst is still tedious as the separation process requires centrifugation or filtration. Iron oxide nanoparticles in this regard have aroused considerable interest as a supportive material to address the above issues because of their high saturation magnetization and low coercivity.17,18 Furthermore, the unique combination of magnetic and plasmonic properties may provide a novel hybrid nanobiomaterial for diverse catalytic applications. Despite several attempts being made to immobilize MNPs on iron oxide nanoparticles,19−22 development of an ecofriendly approach for the preparation of monodisperse and stable MNPs on the surface of magnetic support is a significant challenge. Since iron oxide nanoparticles are poor in functional groups, the ordinary impregnation often results in formation of polydisperse MNPs on the surface of magnetic nanoparticles.23 The chemical functionalization of the magnetic nanoparticle with appropriate ligands or molecules becomes an important prerequisite to improve the conjugation, loading efficiency, stability, and eventually the catalytic performance of the MNPs.24−26 Unfortunately, the functionalization of magnetic nanoparticles and subsequent conjugation of MNPs involve a complicated synthesis process, which requires harsh reaction conditions, toxic chemicals, and longer reaction time triggering environmental and safety issues.27 Very often the preparation process damages the magnetic structure, which reduces the magnetic response.28 Therefore, the current research focus has been converged on the design and facile synthesis of magnetically supported stable MNPs to address the present challenges. In this study, we report an eco-friendly synthesis process for preparation of Au and PdNPs on the surface of magnetic nanoparticles (Figure 1) as a reusable heterogeneous catalyst. Initially, biogenic magnetic nanoparticles were prepared using Shewanella algae at ambient conditions. In the next step the assynthesized magnetic nanoparticle was encapsulated by chitosan (Fe3O4@Ch) for immobilization of MNPs. Being a potent natural biomaterial with macro-chelating ligand and surface-rich hydroxylic groups along with tunable molecular structure, encapsulation of iron oxide nanoparticles with chitosan could provide uniform distribution of MNPs on the surface of the nanostructure.29−31 In the final step, Fe3O4@Ch

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MATERIALS AND METHODS

Materials. Chloroauric acid (purity≥ 99.99%), palladium chloride (purity≥ 99.9%), p-nitrophenol, iodoacetophenene, phenylboronic acid, and glutaraldehyde were purchased from Sigma-Aldrich, India. Chitosan (MW ≈ 300 kDa) with 90% degree of deacetylation was purchased from Panvo Organics, Chennai. Ferric chloride, light liquid paraffin, and other chemicals were purchased from Merck, India. The microbiological media were procured from Hi-media, India. Microorganism. Shewanella algae (MTCC−10455) was purchased from Institute of Microbial Technology, Chandigarh, India. The organism was cultured in nutrient agar (1% yeast extract, 1% peptone, 0.5% sodium chloride and 1.5% agar powder) slants and nutrient broth (1% yeast extract, 0.5% sodium chloride and 1% peptone) as necessary. The viability of the organism was maintained in nutrient agar slant through subculturing at 30 days intervals. Methods. Biosynthesis of Magnetic Nanoparticle. The biogenic synthesis of magnetic nanoparticle was carried out following incubation of the magnetite precursor, akaganeite (−FeOOH) with S. algae under anaerobic condition at 33 °C for 3 days. Lactate (10 mM) was served as the electron donor in this reaction. The color of the solution changed to black from the initial brown and attracted by the external magnet indicating formation of magnetic nanoparticles. On completion of the reaction, the biogenic magnetite nanoparticles were washed with double distilled water three times to remove the attached bacterial cells and dried in a hot air oven at 60 °C. A control experiment was carried out under similar conditions by incubating magnetite precursor akaganeite (−FeOOH) with lactate, however, without the addition of S. algae. The as-synthesized magnetic nanoparticles were calcined at 300 °C for 2 h and used for the remainder of the experiments. Encapsulation of Chitosan on Magnetic Nanoparticles. Chitosan encapsulated magnetic nanoparticles (Fe3O4@Ch) were prepared by water-in-oil emulsion method.32 Initially 0.1% (w/v) magnetic nanoparticle solution was added into 1% (w/v) chitosan solution (dynamic viscosity: 33.15 mPa·s at 30 °C) and sonicated for 30 min followed by incubation for 1 h at 30 °C under stirring. The chitosanmagnetic solution was added dropwise into light liquid paraffin containing Span-80 and stirred at 1800 rpm to form water-in-oil 490


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Figure 2. Experimental set up and biosynthesis of Fe3O4 nanoparticles (A) by S. algae; FESEM images of S. algae (B) and biosynthesized Fe3O4 nanoparticles (C); XPS survey spectra (D) of biosynthesized Fe3O4 nanoparticles. The inset picture of Figure D shows high resolution core level Fe 2p spectrum of Fe3O4 nanoparticles. emulsion. Subsequently, glutaraldehyde solution to a final concentration of 1% was added into the suspension and continuously stirred for another 5 h. The obtained material was then separated from the emulsion by centrifugation and washed with double distilled water and ethanol repeatedly to remove residual paraffin oil and Span-80. Finally, the collected material was dried by lyophilization to get Fe3O4@Ch powder. Synthesis of Gold and Palladium Nanoparticles Decorated Fe3O4 @Ch. The gold and palladium nanoparticles decorated Fe3O4@Ch were synthesized by microwave mediated in situ reduction of Au(III) and Pd(II) ions. In brief, 2 mL of aqueous solution of gold and palladium (2 mM) were taken separately in screw cap vials and 50 mg of as-prepared Fe3O4@Ch powder was added followed by incubation for 1 h at 30 °C under stirring. The Au and PdNPs were then synthesized by microwave reduction (100 W) of bound Au(III) and Pd(II) ions for a period of 60 and 240 min, respectively. The Au and PdNPs decorated Fe3O4@Ch were subsequently collected by centrifugation (20 000 g for 20 min) and washed with double distilled water several times to remove unreacted metal ions. Finally, the prepared materials were dried in a hot air oven at 50 °C. Hereafter the Au and PdNPs decorated Fe3O4@Ch were designated as Fe3O4@ChAuNPs and Fe3O4@Ch-PdNPs, respectively throughout the text. Characterization of Fe3O4, Fe3O4@Ch, Fe3O4@Ch-MNPs (M: Au and Pd). The morphology and chemical composition of all synthesized materials were characterized by several spectroscopic and microscopic techniques. The UV−vis spectra of dispersed solution of Fe3O4 nanoparticles, Fe3O4@Ch, Fe3O4@Ch-AuNPs, and Fe3O4@ChPdNPs were recorded on JASCO V650 UV−vis spectrophotometer. The crystal structure was recorded on Philips X-ray diffractometer at 0−80° with a Cu Kα source (λ = 1.54 Å), while thermal stability was measured by TGA (Philips) from 25 to 800 °C with a heating rate of 10 °C/min. The morphology of the samples was studied by optical (Olympus microscope) and field emission scanning electron microscopy (FESEM; JEOL JSM 6700F) equipped with energy dispersive X-ray analysis (EDAX). The Fourier transform infrared spectra were recorded on FTIR spectrometer (PerkinElmer) from 4000−500 cm−1 at a resolution of 1 cm−1. The chemical composition

and oxidation state of surface element was determined by XPS analysis on an Omicron Multiprobe (Omicron NanoTechnology, U.K.) spectrometer. The magnetic properties were examined by vibrating sample magnetometer (VSM, Lakeshore 7410) at room temperature with a high purity nickel sphere as a standard reference, whereas the magnetic hysteresis loop was obtained using an external magnetic field. Photocatalytic Reduction of Dye. The photocatalytic activity of the as-prepared Fe3O4@Ch-AuNPs and Fe3O4@Ch-PdNPs for the reduction of methylene blue (MB) solution was studied under various conditions as described below. Briefly, 2 mg of hybrid nanobiomaterial was added into 20 mL of MB (20 mg/L) solution taken in a 100 mL reagent bottle and incubated in the dark for 30 min with shaking to ensure the complete adsorption of MB. The dye solution was irradiated by sunlight, microwave (2.45 MHz, 100 W), visible (mercury vapor lamp, 400 W) and UV light (10 W, 285 nm) for different time intervals in the presence of 2.9 mM of NaBH4. The samples were then withdrawn at regular intervals, and the concentration of dye was measured spectrophotometrically by recording the absorbance at 665 nm. Catalytic Hydrogenation and Suzuki Coupling Reaction. The catalytic activity of the synthesized Fe3O4@Ch-AuNPs and Fe3O4@ Ch-PdNPs in the hydrogenation reaction of p-nitrophenol (p-NP) was evaluated using UV−vis spectrophotometer under stirring conditions. Typically, 100 μL of 3 × 10−3 M p-NP was added into 2.8 mL of aqueous solution containing 5 ± 0.1 mg of the Fe3O4@Ch-MNPs (M: Au, Pd) taken in a quartz cuvette. Freshly prepared 100 μL of 0.3 M sodium borohydride (NaBH4) was added into the reaction mixture, and the conversion of p-NP to p-amino phenol (p-AM) was monitored by measuring the absorbance value at 400 nm on a UV−vis spectrophotometer. The experiment was performed in triplicate and repeated at different temperatures (20−60 °C) under identical conditions. The control experiments were carried out under identical conditions as described above using Fe3O4@Ch and Fe3O4 nanoparticles. The Suzuki coupling reaction of p-idoacetophenone and phenylboronic acid was studied with Fe3O4@Ch-MNPs (M: Au, Pd) in the aqueous phase and oxic condition. In brief, a mixture of phenylboronic 491


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ACS Sustainable Chemistry & Engineering acid (182.9 mg, 1.5 mmol), 4-idoacetophenone (246 mg, 1 mmol), and double distilled water (3 mL) was taken in a round-bottom flask. Potassium carbonate (276.5 mg, 2 mmol) was added to this mixture under stirring. The reaction was initiated following the addition of 8 ± 0.5 mg of the hybrid nanobiomaterial and incubated under refluxing conditions for 5 h. At the end of the reaction, the product was extracted with dichloromethane (DCM) and subsequently analyzed by TLC and 1H NMR (400 MHz Bruker Avance-III spectrometer). The coupling reaction was also performed under different reaction conditions such as sunlight exposure, microwave irradiation, and in the laboratory at different temperatures (35−50 °C). The experiment with sunlight was conducted in a natural atmospheric environment in CLRI, Chennai, India, on sunny days between 11 AM to 4 PM in October 2015 with an average amount of solar radiation of 5.12 kWh/ m2, whereas the microwave irradiation was performed in a conventional microwave oven (2.45 MHz) at a power of 100 W for various time intervals. In all catalytic reactions the concentration of Au and Pd in the hybrid nanobiomaterial was 5.8 and 2.6 wt %, respectively, and catalytic reactions were repeated a minimum of five times. Recovery and Recyclability of Hybrid Fe3O4@Ch-MNPs (M: Au, Pd). The recovery and recyclability of the hybrid nanobiomaterial were executed to minimize the accumulation of the catalyst in the environment and reduce the production cost of the catalyst. After completion of the first cycle, Fe3O4@Ch-MNPs was separated using a strong neodymium magnet, washed with DCM and finally dried in a hot air oven. The dried catalysts were used in successive cycles of catalysis reaction adopting a similar procedure as described above. Leaching of Metal Ions from Hybrid Fe3O4@Ch-MNPs (M: Au, Pd). The leaching of metal ions from the synthesized hybrid nanobiomaterial were analyzed to understand the mechanism of catalysis reaction and stability of the Fe3O4@Ch-MNPs. The concentration of Au(III) and Pd(II) ions in the solution after catalysis reaction was measured using atomic absorption spectrometer (Analytic Jena, Nova 350) with appropriate standard solution.

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diffraction pattern at 2θ values of 30, 35.4, 43, 53.4, 56.9, and 62.5° corresponding to [220], [311], [400], [422], [511], and [440] planes of cubic Fe3O4 lattice, respectively. As per JCPDF data sheet (file no. 65-3107), these peaks indicated formation of a pure cubic spinel structure of Fe3O4.35 Further, the biogenic magnetic nanoparticles were calcined at 300 °C and the XRD pattern demonstrated that the calcination process did not affect the crystal structure of the magnetic nanoparticles, except the intensity of the peaks raised due to an increase in crystallinity. Moreover, removal of bound proteins from the nanoparticles surface might also be responsible for higher peak intensity. The size Fe3O4 nanoparticle was also calculated using the Scherrer equation (see Supporting Information), which demonstrated that the size of the Fe3O4 nanoparticles increased slightly from 25.6 to 29 nm after the calcination process. The remainder of the work was carried out with calcined magnetic nanoparticles. Synthesis and Characterization of Hybrid Fe3O4@ChMNPs (M: Au, Pd). The chitosan encapsulated magnetic nanoparticles (Fe3O4@Ch) were initially prepared by the water-in-oil emulsion method, followed by formation of metal nanoparticles on the surface of Fe3O4@Ch to prepare Fe3O4@ Ch-MNPs (Figure 1B). The encapsulation of Fe3O4 nanoparticles by chitosan was investigated by FTIR spectra. The IR spectrum (Figure 3) of chitosan showed characteristic

RESULTS AND DISCUSSION

Synthesis and Characterization of Biogenic Magnetic Nanoparticles. The extracellular biogenic magnetite nanoparticles were synthesized upon interaction of iron oxyhydroxides (FeOOH), as the magnetite precursor (akaganeite), with a dissimilatory metal-reducing bacterium, S. algae, under the anaerobic condition which is described graphically in Figure 1A.33 After 3 days of incubation the brown solution of akaganeite turned black, and the particles were attracted by an external magnetic bar (Figure 2A) indicating biosynthesis of magnetic nanoparticles by S. algae (Figure 2B). The formation and morphology of Fe3O4 nanoparticles were confirmed by a FESEM image (Figure 2C), which clearly depicted synthesis of ∼35 nm spherical magnetic nanoparticles. The extracellular synthesis of magnetite by S. algae did not require any harsh chemical or the addition of an exogenous electron carrier such as humic acids.33 The chemical composition of the biogenic magnetic nanoparticles was further investigated by XPS analysis. The survey spectrum showed core level Fe 2p peak along with C 1s, N 1s, and O 1s peaks as shown in Figure 2D. The high resolution Fe 2p peak had binding energies at 725.0 and 711.5 eV (inset Figure 2D) attributed to 2p1/2 and 2p3/2 of Fe, confirming the formation of Fe3O4 nanoparticles.34 The analysis of core level C 1s, N 1s, and O 1s peaks (Supporting Information Figure S1) revealed association of proteins with Fe3O4 nanoparticles. Therefore, the in situ conjugation of protein molecules with Fe3O4 resulted in formation of stable biogenic magnetic nanoparticles. The crystalline nature of the as-synthesized magnetic nanoparticles was investigated by XRD analysis (Supporting Information Figure S2), which showed a

Figure 3. FTIR spectra of chitosan, Fe3O4, Fe3O4@Ch, Fe3O4@ChAuNPs, and Fe3O4@Ch-PdNPs.

absorption bands at 3401 cm−1 due to −NH and/or −OH stretching vibrations and that at 2963 and 2852 cm −1 corresponded to −CH stretching vibration of copolymer of chitosan.36−38 The absorption bands at 1643, 1552, and 1265 cm−1 attributed to amide I (CO stretching vibration), amide II (−NH bending and C−N stretching) and amide III (N−H bending and the C−N stretching) band, respectively. The peak assigned to CH2OH of C-6 position of the sugar moiety of chitosan appeared at 1402 cm−1, while the band at 1092 cm−1 accounted for O−H bending.38,39 The peak at 1151 and 803 cm−1 corresponded to C−O−C antisymmetric stretching and glycoside linkage of saccharide structure, respectively.37 Similarly, the FTIR spectrum of calcined Fe3O4 nanoparticles demonstrated peaks respectively at 3401, 1631, 1422, 1395, 1115, 1058, and 596 cm−1 due to the asymmetric and symmetric stretching vibration of surface-bound residual protein molecules and Fe−O stretching vibration of Fe3O4,40 respectively. The IR spectrum of Fe3O4@Ch revealed characteristic bands of both chitosan (−CH, −CO, −CN, −NH and −OH stretching and deformation vibrations) and magnetite. However, in comparison to pure chitosan and magnetite clear shifting of the absorption spectra were recorded 492


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Figure 4. Color image (A) of Fe3O4@Ch, Fe3O4@Ch-AuNPs, and Fe3O4@Ch-PdNPs; UV−vis spectra (B) of chitosan, Fe3O4, Fe3O4@Ch, Fe3O4@ Ch-AuNPs, and Fe3O4@Ch-PdNPs; FESEM image of Fe3O4@Ch (C), Fe3O4@Ch-AuNPs (D), and Fe3O4@Ch-PdNPs (G); high resolution FESEM image of Fe3O4@Ch-AuNPs (E) and Fe3O4@Ch-PdNPs (H) depicting formation of Au and PdNP on the surface of material; EDXA spectrum of Fe3O4@Ch-AuNPs (F) and Fe3O4@Ch-PdNPs (I). The inset picture of Figure (D−G) showed high resolution FESEM image of single Fe3O4@Ch-AuNPs and Fe3O4@Ch-PdNPs.

in Fe3O4@Ch. The −NH, −OH, and −CH stretching vibrations of chitosan molecule were shifted to 3431, 2927, and 2860 cm−1, respectively, due to the binding of Fe3O4 nanoparticles with chitosan. Furthermore, amide II and III band shifted to 1564 and 1260 cm−1, respectively, whereas the absorption band for −OH bending appeared at 1065 cm−1 suggesting conjugation of Fe3O4 nanoparticles with chitosan through binding with amine and hydroxyl groups. The crosslinking of the amine group with glutaraldehyde in the formation of Fe3O4@Ch might also be responsible for the shifting of the amide band. In addition, the Fe−O stretching vibration shifted to 604 cm−1 and −COO− stretching vibration of surface proteins of magnetic nanoparticles disappeared. At the same time the absorption band at 1401 cm−1 was noticed due to CH2OH of chitosan in Fe3O4@Ch. The alterations of absorption bands therefore, supported the encapsulation of magnetic nanoparticles with chitosan forming Fe3O4@Ch. The FESEM image (Figure 4C) further revealed that the size of the as-prepared Fe3O4@Ch was ∼4 μm. The decoration of Au and PdNPs on the surface of Fe3O4@ Ch and subsequent formation of Fe3O4@Ch-AuNPs and Fe3O4@Ch-PdNPs was carried out through microwave-assisted reduction of surface-bound metal ions.41 In this synthesis process, the chitosan acted not only as linker between magnetite and MNPs, but also stabilized both the MNPs and magnetite preventing their aggregation. Furthermore, the formation of Fe3O4@Ch-AuNPs and Fe3O4@Ch-PdNPs did not require any reducing agent, organic solvent, or high temperature and thereby favors an eco-friendly green synthesis process. The formation of Au and PdNPs on the Fe3O4@Ch

surface was observed with subsequent color change of the material as shown in Figure 4A. The UV−vis spectra of the prepared Fe3O4@Ch-AuNPs and Fe3O4@Ch-PdNPs showed absorption peaks at 525 and 305 nm (Figure 4B),42,43 respectively, due to Surface Plasmon Resonance of Au and PdNPs; however, such a characteristic peak was absent in chitosan, magnetic nanoparticles, and Fe3O4@Ch. The metal ions-treated Fe3O4@Ch without exposure to microwave irradiation did not exhibit any SPR band of Au and PdNPs even after incubation for 24 h. This clearly demonstrated that reduction of Au(III) and Pd(II) ions to respective Au and PdNPs was assisted by microwave irradiation. The size and shape of the prepared Fe3O4@Ch-AuNPs and Fe3O4@Ch-PdNPs were determined by optical and FESEM images. The optical microscopic images of the as-prepared Fe3O4@Ch-MNPs are shown in Supporting Information Figure S3A,B, revealing homogeneous distribution of monodisperse and spherical Fe3O4@Ch-MNPs with size ranges from 2−7 μm. Further, the morphology and shape of Fe3O4@Ch-AuNPs and Fe3O4@Ch-PdNPs was investigated by FESEM attached with EDXA. The low resolution FESEM images demonstrated an average diameter of Fe3O4@Ch-AuNPs (Figure 4D) and Fe3O4@Ch-PdNPs (Figure 4G), 4.21 ± 0.51 and 4.08 ± 0.4 μm, respectively. The high resolution FESEM images (Figure 4D−G, inset) of individual material clearly indicated the spherical shape of the hybrid nanobiomaterial. The formations of metal nanoparticles on the surface of Fe3O4@Ch-MNPs are clearly visible with a further increase of the magnification of the images (Figure 4E,H). The elemental composition of the hybrid nanobiomaterial was confirmed by EDXA spectra. The 493


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ACS Sustainable Chemistry & Engineering area-profile analysis of Fe3O4@Ch-MNPs as shown in Figure 4F−I clearly depicted the appearance of Au and Pd peaks along with Fe, C, N, and O peaks. The metal peaks confirmed the formation of Au and PdNPs on the surface of Fe3O4@Ch, whereas Fe, C, N, and O signals originated from Fe3O4 and chitosan of Fe3O4@Ch-MNPs, respectively. The quantitative elemental analysis of the material counted 5.6 and 2.6% (w/w) of Au and Pd, respectively in Fe3O4@Ch-AuNPs and Fe3O4@ Ch-PdNPs. The FTIR spectra of Fe3O4@Ch-AuNPs and Fe3O4@ChPdNPs (Figure 3) were recorded in order to understand the formation and stabilization of the MNPs on the surface of the nanobiomaterial. The initial incubation of positively charged metal ions with Fe3O4@Ch caused rapid binding of metal ions (Au and Pd) on the electronegative (−32.6 mV ζ potential value) surface and void of Fe3O4@Ch through hydroxyl and amine groups of chitosan, resulting in a shift of the −NH/OH stretching band to 3124 cm−1. The intensities of amide II were reduced and appeared at 1560 cm−1, whereas the amide III band disappeared in Fe3O4@Ch-MNPs resulting from the binding of metal ions through the amine group of chitosan.36 The spectra of the hybrid materials also revealed the characteristic band of magnetite at 604 cm−1. The formation of Fe3O4@Ch-AuNPs and Fe3O4@Ch-PdNPs was further optimized by varying the microwave irradiation time, and the kinetics of the reduction of metal ions to MNPs was monitored spectrophotometrically. The broad SPR band of AuNPs appeared at 505 nm with minimum exposure of 30 min and a sharp peak appeared at 525 nm after exposure for 60 min indicating nucleation and growth of AuNPs on the Fe3O4@Ch (Supporting Information Figure S4A); however, further increase in exposure time beyond 85 min caused broadening of the SPR band attributed to the melting of AuNPs. In the same manner the process of Fe3O4@Ch-PdNPs preparation was optimized at 240 min for microwave irradiation of Pd(II) bound Fe3O4@Ch (Supporting Information Figure S4B). The result showed that microwave reduction of Au3+ ions required lesser time than that of Pd2+ ions. According to the hard soft acid base (HSAB) principle44 Au3+ is softer than Pd2+ as Au has higher atomic radius and is more polarizable than Pd. Therefore, it is easy to reduce the more labile Au3+ ions than the relatively hard Pd2+ ions. The oxidation state of Au and Pd in the synthesized hybrid nanobiomaterial was further investigated by XPS analysis. The survey spectra showed core level Au 4f and Pd 3d peaks in addition to Fe 2p, C 1s, N 1s, and O 1s peaks as shown in Figure 5A. The Au 4f spectrum (Figure 5B) in Fe3O4@ChAuNPs was composed of doublet peaks at a binding energy of 84.1 and 87.8 eV corresponding to 4f7/2 and 4f5/2 peaks of Au(0) respectively,43 whereas the related Pd 3d spectrum in Fe3O4@Ch-PdNPs (Figure 5C) was composed of doublet peaks corresponding to 3d5/2 and 3d3/2 peaks of Pd(0) and appeared at 335.2 and 340.4 eV, respectively.45 It is therefore, clear that all the bounded metal ions (Au(III) and Pd(II)) on Fe3O4@Ch were reduced to metallic nanoparticles. The signature of Fe 2p peak in the spectra appeared from magnetic nanoparticles, while C 1s, N 1s, and O 1s peaks were originated from chitosan of the Fe3O4@Ch-MNPs. On the basis of the obtained results a possible chemical structure of the Fe3O4@ Ch-MNPs is schematically presented in Figure 1B. Further confirmation of Fe3O4@Ch-AuNPs and Fe3O4@ChPdNPs was carried out by XRD analysis. The X-ray diffraction pattern of chitosan, magnetite, Fe3O4@Ch, Fe3O4@Ch-AuNPs,

Figure 5. XPS survey spectra of (A) Fe3O4@Ch-AuNPs and Fe3O4@ Ch-PdNPs, and core level high resolution Au 4f (B) and Pd 3d (C) spectra of Fe3O4@Ch-AuNPs and Fe3O4@Ch-PdNPs, respectively.

and Fe3O4@Ch-PdNPs are presented in Figure 6A. Chitosan had a broad peak at 21° attributed to the amorphous characteristic, whereas the calcined iron oxide nanoparticles showed a diffraction pattern at 2θ values of 30.2, 34.7, 43.5, 53.7, 57.2, and 62.8° corresponding to [220], [311], [400], [422], [511], and [440] planes of cubic Fe3O4 lattice, respectively. The XRD pattern of Fe3O4@Ch demonstrated strong diffraction peaks at 30.2, 35.5, 43.2, 53.6, 57.1, and 62.7° corresponding to [220], [311], [400], [422], [511], and [440] planes of cubic Fe3O4 lattice and a weak diffraction peak of amorphous chitosan at 21°. The intensity of the diffraction peak of chitosan was reduced in Fe3O4@Ch due to binding of Fe3O4 with chitosan through the −NH2/OH group causing disorder of the 3D crystal structure of chitosan.46 The final Fe3O4@Ch-AuNPs and Fe3O4@Ch-PdNPs exhibited additional peaks for face-centered cubic structure of Au and PdNPs along with strong diffraction pattern of Fe3O4 and a weak peak of chitosan. The diffraction peaks of [111], [200], [220] and [311] planes of AuNPs appeared at 39.3, 45.9, 62.1 and 76.8°, respectively (JCPDS no. 04-0784). On the other hand the diffraction peaks for PdNPs appeared at 39.3, 45.6, 64.7, and 79.6° assigning to the [111], [200], [220], and [331] planes, respectively, of face-centered cubic structure of metallic 494


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The thermal stability of the synthesized Fe3O4@Ch-AuNPs and Fe3O4@Ch-PdNPs was studied by TGA. The thermogram of chitosan, magnetite, Fe3O4@Ch, and Fe3O4@Ch-MNPs showed the decomposition temperature of the samples (Figure 6C). The TGA curve of the calcined Fe3O4 nanoparticle demonstrated that only 3.6% weight loss throughout the entire temperature range might be due to the removal of absorbed water and/or some residual organic molecules. On the other hand, pure chitosan showed three stages of decomposition. The initial weight loss below 100 °C was quite small (8%), which was associated with the evaporation of both adsorbed and bound water. The residual acetic acid was also removed in this stage.50,51 The second stage contributed about 41.9% of weight loss at the range of 180−270 °C due to the degradation and deacetylation of chitosan.52 The third stage, which started at 450 °C and ended at 800 °C demonstrated complete degradation of chitosan. The Fe3O4@Ch, Fe3O4@Ch-AuNPs, and Fe3O4@Ch-PdNPs demonstrated very similar thermogram patterns. Though the initial stage of water evaporation (7%) was same as that of chitosan, visible distinctions were observed in the second and third stages of weight loss. The Fe3O4@Ch, Fe3O4@Ch-AuNPs, and Fe3O4@Ch-PdNPs started decomposition at 180, 196, and 220 °C, respectively, and 32, 30, and ∼27% of weight loss were recorded at this stage. The final stage of decomposition started at 290 and 300 °C in both Fe3O4@Ch and Fe3O4@Ch-MNPs (Fe3O4@Ch-AuNPs and Fe3O4@ChPdNPs) and 48.3, 43.5, and 42.7% of weight loss were witnessed in Fe3O4@Ch, Fe3O4@Ch-AuNPs, and Fe3O4@ChPdNPs, respectively, which evidently demonstrated good thermal stability of the hybrid material. Compared to that of Fe3O4@Ch, the weight loss was less in Fe3O4@Ch-MNPs, suggesting incorporation of MNPs further stabilized the material. TGA data thus showed that the as-prepared Fe 3O 4@Ch-AuNPs and Fe 3 O4 @Ch-PdNPs had enough thermal stability and were very much suitable for catalytic reactions. Photocatalytic Reduction of Dye. Treatment of wastewater containing aromatic hydrocarbons especially organic dyes has been a major concern for environmental sustainability and remains a technical challenge. The catalytic performance of Fe3O4@Ch-MNPs in the reduction of methylene blue (MB) to leucomethylene blue (LMB) (Figure 7A) was therefore studied as a model dye pollutant. Figure 7D showed the disappearance of dye color following treatment with Fe3O4@Ch-AuNPs in the presence of NaBH4. The addition of Fe3O4@Ch-MNPs to the dye solution caused gradual decrease in absorption intensity of MB with time as measured by UV−vis spectra. The absorbance of MB at 665 nm became nearly zero within 9 min (Figure 7E) indicating complete reduction of MB to LMB by Fe3O4@ChAuNPs. On the other hand, Fe3O4@Ch-PdNPs took only 6 min (Figure 7F) to complete the reduction of MB. The control experiment performed with both Fe3O4 and Fe3O4@Ch did not exhibit reduction of absorption intensity of MB even after treatment for 5 h (Figure 7B,C). In the case of Fe3O4@Ch, the absorbance reduced slightly due to absorption of MB on the Fe3O4@Ch surface. The photocatalytic performance of Fe3O4@Ch-MNPs was further optimized at different reaction conditions (Table 1) such as microwave, UV, visible, and sunlight exposure. Very likely, the reaction was facilitated upon irradiation with UV light (Figure 7G) and the reduction process was accomplished just within 1 min. The rate of the reaction was calculated using pseudo-second order rate kinetics. The apparent rate constant (kapp) was calculated from the slop

Figure 6. XRD (A) of chitosan, Fe3O4, Fe3O4@Ch, Fe3O4@ChAuNPs, and Fe3O4@Ch-PdNPs; VSM analysis (B) of the synthesized Fe3O4@Ch-AuNPs and Fe3O4@Ch-PdNPs; and TGA (C) of chitosan, Fe3O4, Fe3O4@Ch, Fe3O4@Ch-AuNPs, and Fe3O4@Ch-PdNPs.

Pd crystal (JCPDS no. 46-1043). These results therefore, clearly demonstrated the fabrication of crystalline Au and PdNPs on the Fe3O4@Ch surface. The magnetic property of Fe3O4@Ch-AuNPs and Fe3O4@ Ch-PdNPs was analyzed by VSM measurement at room temperature, and Figure 6B displayed an “S”-shaped magnetic hysteresis loop of the samples demonstrating superparamagnetic behavior.47,48 The saturation magnetization value of Fe3O4@Ch-AuNPs was found at 23.1 emu/g, whereas the coercivity and retentivity values were 0.076 kOe and 2.5 emu/g, respectively. In the case of Fe3O4@Ch-PdNPs, saturation magnetization, coercivity, and retentivity values were observed, respectively, at 30.3 emu/g, 0.064 kOe, and 3.8 emu/g. Since the coercivity of Fe3O4@Ch-MNPs (Fe3O4@Ch-AuNPs and Fe3O4@Ch-PdNPs) and Fe3O4 nanoparticles (0.052 kOe) was almost the same and very close to zero, the superparamagnetic property of Fe3O4 remained the same in [email protected] The low saturation magnetization of Fe3O4@Ch-AuNPs and Fe3O4@Ch-PdNPs compared to the Fe3O4 nanoparticles (52.2 emu/g) was attributed to the encapsulation of Fe 3 O 4 nanoparticles by chitosan. However, the magnetism of Fe3O4@Ch-AuNPs and Fe3O4@Ch-PdNPs was sufficient enough for magnetic separation of the hybrid nanobiomaterial from aqueous solution using an external magnetic field leading to easy recycling and reuse. 495


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Figure 7. Catalytic activity of Fe3O4@Ch-MNPs in reduction of methylene blue to leucomethylene blue (A); color image (B) and corresponding UV−vis spectrum (C) of methylene blue following treatment with Fe3O4 NPs; color image (D) and time dependent UV−vis spectra (E) of methylene blue following treatment with Fe3O4@Ch-AuNPs; UV−vis spectra (F,G) of methylene blue following treatment with Fe3O4@Ch-PdNPs in laboratory condition and with UV irradiation; plot of ln A665 (absorbance at 665 nm) against the reaction time for MB reduction by Fe3O4@ChPdNPs (H), and estimated knor values (I) at different temperatures.

Table 1. Optimization of Catalytic Reduction of Methylene Blue to Leucomethylene Blue by Fe3O4@Ch-MNPsa: M = Au, PdNPs entry

time (min)

reaction condition

1 2 3 4 5 6 7 8 9 10

9 9 6 4 1 6 6 3 2 1

laboratory/H2O/Fe3O4@Ch-AuNPs microwave/H2O/Fe3O4@Ch-AuNPs visible light/H2O/Fe3O4@Ch-AuNPs sunlight/H2O/Fe3O4@Ch-AuNPs UV light/H2O/Fe3O4@Ch-PdNPs laboratory/H2O/Fe3O4@Ch-PdNPs microwave/H2O/Fe3O4@Ch-PdNPs visible light/H2O/Fe3O4@Ch-PdNPs sunlight/H2O/Fe3O4@Ch-PdNPs UV light/H2O/Fe3O4@Ch-PdNPs

temp (°C)

conversion (%)

± ± ± ± ± ± ± ± ± ±

99 99 100 100 100 99 100 100 100 100

32 52 33 35 32 32 52 33 35 32

1 1 1 1 4 3 2 2 2 2

Conditions: methylene blue (10 mL, 20 mg/L), NaBH4 (2.9 mM), Fe3O4@Ch-MNPs (2 ± 0.5 mg, containing 2.6% Pd in Fe3O4@Ch-PdNPs and 5.8% Au in Fe3O4@Ch-AuNPs), pH 7.2.

a

of linear regression of −ln A (absorbance at 665 nm) as a function of reaction time (Figure 7H). Finally the normalized rate constant (knor) was determined by dividing the kapp values

with concentration of metal present in the hybrid nanobiomaterial. It was observed from the estimated knor value (Figure 7I) that Fe3O4@Ch-PdNPs have higher photocatalytic 496


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Figure 8. UV−vis spectrum (A) of catalytic reduction of p-NP to p-AP by Fe3O4@Ch-MNPs; time-dependent absorption spectra of reduction p-NP by Fe3O4@Ch-AuNPs (B) and Fe3O4@Ch-PdNPs (C), respectively. Plot of ln A400 (absorbance at 400 nm) against the reaction time for p-NP reduction by Fe3O4@Ch-AuNPs (D) and Fe3O4@Ch-PdNPs (E), respectively; and normalized (F) rate constant (knor) estimated at different temperatures.

among the top 100 harmful organic pollutants according to the Environmental Protection Agency (EPA). Because of the large solubility and high stability, nitrophenols remain in the waterbody for a prolong time triggering detrimental effects on agricultural plants, animals, and human beings. The catalytic reduction of nitrophenol to aminophenol is considered as the most convenient approach to convert organic waste to a value added intermediate chemical as aminophenol is an important intermediate for synthesizing analgesic antipyretic drugs, agrochemicals, corrosion inhibitors, and dyes, etc.58,59 The catalytic hydrogenation performance of the hybrid nanobiomaterial was quantitatively evaluated in the reduction of pNP to p-AP in the presence of an excess NaBH4. Upon the addition of Fe3O4@Ch-MNPs to the reaction medium of p-NP and NaBH4, the absorption intensity at 400 nm decreased successively with time along with simultaneous appearance of a new peak at 300 nm corresponding to p-AP (Figure 8A).42,60 The reaction was completed within 3−14 min depending upon the MNPs on the hybrid nanobiomaterial and reaction temperature. The control experiments performed with the addition of Fe3O4@Ch and Fe3O4 nanoparticles showed that the bright yellow solution of p-NP remained almost the same. The maximum absorption peak did not exhibit any change in the case of Fe3O4 nanoparticles; however, when Fe3O4@Ch was used as catalyst, the maximum absorption peak was reduced slightly due to the adsorption of p-NP on the Fe3O4@

activity than Fe3O4@Ch-AuNPs in all reaction conditions. The catalytic degradation of MB has been reported under irradiation with UV and visible lights;53−55 however, only a limited study has been conducted on the reduction of MB. Zhu et al.56 reported that the reduction of MB using Fe3O4@Ch@silver core−shell nanostructures is completed within 10 min with a rate constant of 0.34/min, while Kim et al.57 demonstrated the complete reduction of MB by Pt decorated SiOx nanowire in 40 min. Interestingly in the present study, Fe3O4@Ch-AuNPs and Fe3O4@Ch-PdNPs took just 1 min under UV light to complete the reduction of MB to LMB with a kapp of 4.0/min and 5.0/ min (Supporting Information Table S1). The knor values were found to be 1.14 × 102 and 1.72 × 102 mmol−1·s−1, respectively, demonstrating superior catalytic activity of the synthesized Fe3O4@Ch-AuNPs and Fe3O4@Ch-PdNPs in the reduction of MB. Consistent with the obtained results, the photocatalytic activity of Fe3O4@Ch-MNPs is proposed to occur through the following steps; MB initially adsorbed on the surface of the hybrid nanobiomaterial and then reduced to LMB by Au or PdNPs through electron transfer process. The reduced dye desorbed from the Fe3O4@Ch-MNPs to create a free space for further reduction to continue. In contrast, Fe3O4 and Fe3O4@ Ch were unable to decolorized MB due to the absence of MNPs. Catalytic Hydrogenation and Suzuki Coupling Reaction of Fe3O4@Ch-MNPs. Nitrophenols have been listed 497


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ACS Sustainable Chemistry & Engineering Table 2. Optimization of the Suzuki Coupling Reaction Using Fe3O4@Ch-MNPsa: M = Au, PdNPs

entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

time (min) 540 540 540 300 540 300 180 60 1080 300 180 60 45 30 540

reaction condition

temp (°C)

refluxing/H2O/Fe3O4@Ch-PdNPs refluxing/H2O/Fe3O4@Ch-AuNPs sunlight/H2O/Fe3O4@Ch-PdNPs sunlight/H2O/Fe3O4@Ch-PdNPs microwave/H2O/Fe3O4@Ch-PdNPs microwave/H2O/Fe3O4@Ch-PdNPs microwave/H2O/Fe3O4@Ch-PdNPs microwave/H2O/Fe3O4@Ch-PdNPs laboratory condition/H2O/Fe3O4@Ch-PdNPs laboratory condition/H2O/Fe3O4@Ch-PdNPs laboratory condition/H2O/Fe3O4@Ch-PdNPs laboratory condition/H2O/Fe3O4@Ch-PdNPs laboratory condition/H2O/Fe3O4@Ch-PdNPs laboratory condition/H2O/Fe3O4@Ch-PdNPs microwave/H2O/Fe3O4@Ch-AuNPs

100 100 37 ± 37 ± 52 ± 52 ± 52 ± 52 ± 35 ± 50 ± 50 ± 50 ± 50 ± 50 ± 52 ±

4 3 1 1 1 1 2 2 2 2 2 2 1

conversion (%) 99 3 98 73 98 98 98 98 77 98 98 97 90 55 1

TON 50 769 1 276 50 256 37 435 50 256 50 256 50 256 50 256 39 487 50 256 50 256 49 743 46 153 28 205 425

TOF 5641 142 5584 7487 5584 10051 16752 50256 2193 10051 16752 49743 61538 56410 47

a

Conditions: Phenylboronic acid (183 mg, 1.5 mmol), p-iodoacetophenone (246.5 mg, 1 mmol), K2CO3 (276.5 mg, 2 mmol), Fe3O4@Ch-MNPs (8 ± 0.5 mg, containing 2.6% Pd in Fe3O4@Ch-PdNPs and 5.8% Au in Fe3O4@Ch-AuNPs), H2O, and under aerobic condition.

Ch surface. This clearly indicated that the reduction of p-NP to p-AP was catalyzed by Fe3O4@Ch-AuNPs and Fe3O4@ChPdNPs only. The time and temperature dependent absorption spectrum of p-NP reductions are presented in Figure 8B−C. It is reported that in most of the catalytic reaction a certain amount of time is required upon the addition of the catalyst, referred to as an induction time tind, to initiate the hydrogenation reaction.60−62 However, in the present system reaction started immediately after the addition of the Fe3O4@ Ch-MNPs. This suggested that perhaps p-NP adsorbed on the surface of Fe3O4@Ch-MNPs instantly favors the hydrogenation reaction. The catalysis reaction followed pseudo-first-order rate kinetics and the apparent rate constant (kapp) was determined from the slope of linear regression of ln A (absorbance at 400 nm) vs time (Figure 8D,E).62 Results showed that the average kapp value increased with temperature suggesting a temperaturedependent reduction process. It was also noted that Fe3O4@ Ch-PdNPs exhibited higher catalytic activity than the Fe3O4@ Ch-AuNPs over the entire temperature range. To further compare the catalytic efficiency of the Fe3O4@Ch-MNPs with that of other metal nanomaterials, as reported in the literature, normalized rate constant (knor) values were measured at different temperatures (Figure 8F) by dividing kapp values with the amounts of Au and Pd present in the hybrid nanobiomaterial. It was observed that the knor value of Fe3O4@ Ch-PdNPs is higher than that for Fe3O4@Ch-AuNPs; moreover, catalytic activities of both Fe3O4@Ch-AuNPs and Fe3O4@Ch-PdNPs are higher than those of the chemically synthesized metal nanomaterials (Supporting Information Table S2). The Suzuki coupling is another important reaction catalyzed by MNPs in C−C bond formation. Unfortunately, most of the reactions are carried out in highly flammable organic solvents and at high temperatures raising environmental concern.63−66 In this context, the coupling reaction in aqueous medium and oxic conditions using recyclable and reusable catalyst has significant importance for the development of sustainable chemistry and economy of concern. Therefore, the catalytic behavior of Fe3O4@Ch-MNPs in the C−C bond formation of phenylboronic acid and p-iodoacetophenone in aqueous

solution was explored under various conditions. We are delighted to observed a complete conversion of iodobenzene into biphenyl (Supporting Information Figure S5), when the reaction was carried out using Fe3O4@Ch-PdNPs under refluxing condition just for few hours (Table 2, entry 1). The coupling reaction was further optimized by changing the reaction conditions such as exposure to sunlight, microwave irradiation, and in normal laboratory. The results showed that under sunlight exposure 98% yield (TON = 50 256 and TOF = 5584) was achieved within 9 h (Table 2, entry 3), whereas under microwave irradiation (52 °C, 2.45 MHz frequency, and 100 W) 98% yield (TON = 50 256 and TOF = 50 256) was achieved just within 1 h of reaction (Table 2, entry 8) using Fe3O4@Ch-PdNPs. Similarly, when the same reaction was carried out in the normal laboratory condition 98% yield (TON = 50 256 and TOF = 16 752) was recorded after 3 h (Table 2, entry 11), and 90% yield (TON = 46 153 and TOF = 61 538) achieved just within 45 min (Table 2, entry 13). Although few reports are available in the literature for enhancement of the efficacy of the coupling reaction by artificial light irradiation using a palladium-based catalyst,67−69 to the best of our knowledge there is hardly any report, if any, demonstrating performance of the Suzuki coupling reaction under normal laboratory conditions in aqueous solution. On the other hand, Fe3O4@Ch-AuNPs demonstrated less activity and ∼3% yield (TON = 1276 and TOF = 142) (Table 2, entry 2) was observed under reflux conditions, and 1% yield (TON = 425 and TOF = 47) was obtained under microwave irradiation for 9 h (Table 2, entry 15). We also performed the coupling reaction with less reactive 4-chloroacetophenon under similar working conditions as in iodoacetophenone and observed that the catalyst did not exhibit any catalytic conversion.70,71 As a control experiment, Fe3O4@Ch-MNPs were replaced either with Fe3O4@Ch or Fe3O4 nanoparticles, giving rise to no product. Finally, the catalytic activity of Fe3O4@Ch-PdNPs was compared with the reported values, and it was observed that Fe3O4@Ch-PdNPs exhibited higher or similar TON and TOF compared to those chemically synthesized PdNPs (Supporting Information Table S3).42,72,73 Furthermore, most of the catalytic reactions of chemically synthesized PdNPs were 498


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ACS Sustainable Chemistry & Engineering performed in organic solvents, anoxic condition, and at high temperature; however, the catalytic reaction using Fe3O4@ChPdNPs occurred in aqueous solution at normal laboratory conditions. Moreover, there was no black precipitation of Pd after completion of the reaction, which supports the high stability and activity of the Fe3O4@Ch-PdNPs. The leaching of metal ions from the hybrid nanobiomaterial was also measured by AAS analysis and results demonstrated the absence of metal ions in the solution, which confirmed high electronic stabilization of Au and PdNPs on the hybrid nanobiomaterial. Immobilization of MNPs on solid support especially on magnetic nanoparticles often reduces its catalytic efficiency;67,74 however, such an adverse effect was not observed in the Fe3O4@Ch-MNPs. In the proposed hybrid nanobiomaterial the Fe3O4, chitosan, and MNPs acted independently; Fe3O4 favored easy magnetic separation of the catalyst, whereas MNPs catalyzed the reaction and chitosan provided stability to both Fe3O4 and MNPs. Overall the high stability of as-synthesized Fe3O4@Ch-MNPs not only provides catalytic efficiency, but the easy recovery and prevention of leaching of metal ions also protect the environment from the toxic effect of nanoparticles. Recovery and Recyclability of Fe3O4@Ch-MNPs. The recyclability and reuse of the catalysts in successive reactions are another important aspect for cost-effective application of the nanomaterials and for preventing their accumulation in waste. With regards to this critical and significant issue, Fe3O4@ Ch-MNPs was separated from the reaction medium using an external magnet, washed with dichloromethane, dried, and finally reused for the next cycle of the hydrogenation and coupling reaction. It was worth noting that during multiple cycles of hydrogenation reaction (Supporting Information Figure S6A), the conversion of p-NP was still as high as 95% in the fifth cycle of the reaction and did not exhibit significant loss of catalytic activity even after tenth repetitive cycles; the conversion efficiency remained above 87%. Similarly in the coupling reaction using Fe3O4@Ch-PdNPs, 96% conversion was achieved in the second cycle and dropped to 70% in fifth cycles of the reaction (Supporting Information Figure S6B). Therefore, the overall conversion was calculated to be 85%, which demonstrated good catalytic activity and good recycling potential of the Fe3O4@Ch-PdNPs. The high catalytic activity and recyclability of the synthesized hybrid nanobiomaterial is attributed to the synergistic effect of magnetic nanoparticles, chitosan, and metal nanoparticles. The many functional groups of chitosan favored the binding of aromatic compounds such as p-NP, phenylboronic acid, p-iodoacetophenone, and dye on Fe3O4@Ch-MNPs providing excellent contact of these aromatic compounds with the hybrid nanobiomaterial.75 Moreover, chitosan provides electronic stabilization of MNPs, thereby preventing aggregation and leaching of Au and PdNPs. The uniform distribution of MNPs (Au and Pd) on the surface favored the catalytic reactions, whereas the magnetic behavior facilitated their easy separation using an external magnetic field leading to recycling and reuse. The proposed Fe3O4@ChMNPs thus provide a promising opportunity for the preparation of highly efficient, stable, and recyclable metal nanocatalyst.

heterogeneous catalyst. In the process of hybrid nanobiomaterial preparation, the spherical magnetic nanoparticles (∼35 nm) were initially biosynthesized using S. algae and then functionalized with chitosan (Fe3O4@Ch, ∼4 μm) to decorate with Au and Pd NPs. Chitosan played a dual role in the decoration of stable Au and Pd NPs by in situ microwave reduction of metal ions and preventing leaching of the metal ions from the MNPs. Both Fe3O4@Ch-AuNPs and Fe3O4@ChPdNPs exhibited superparamagnetic properties with saturation magnetization values of 23.1 and 30.3 emu/g, respectively, and coercivity values of 0.076 and 0.064 kOe, respectively. More importantly, Fe3O4@Ch-MNPs exhibited very good thermal and electronic stability and excellent catalytic activity (>99% conversion) in photocatalytic reduction of dye, hydrogenation, and Suzuki coupling reactions in aqueous solution at ambient temperatures. The catalytic efficiency varied with the type of MNPs, and Fe3O4@Ch-PdNPs exhibited superior catalytic activity in all chemical reactions compared to other metal nanocatalysts. Furthermore, the prepared hybrid nanobiomaterial could be recovered easily with an external magnetic field and reused multiple times without significant loss in catalytic activity. We believe that our result will help in the synthesis of easily separable hybrid nanobiomaterials as heterogeneous catalysts through a cost-effective and eco-benign synthetic route to reduce the environmental burden of toxic chemicals and improve environmental sustainability.

CONCLUSION A novel eco-friendly synthetic route has been developed for the preparation of MNPs-decorated magnetically separable hybrid nanobiomaterials (Fe3O4@Ch-MNPs, M = Au, Pd) as

(1) Balanta, A.; Godard, C.; Claver, C. Pd Nanoparticles for C−C Coupling Reactions. Chem. Soc. Rev. 2011, 40, 4973−4985. (2) Mohanty, A.; Garg, N.; Jin, R. A Universal Approach to the Synthesis of Noble Metal Nanodendrites and Their Catalytic Properties. Angew. Chem., Int. Ed. 2010, 49, 4962−4966.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01862. Biosynthesis of magnetic nanoparticles; XPS analysis; comparison of rate constants (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Tel: +914424437133. Fax: +914424910846. ORCID

Sujoy K. Das: 0000-0002-2424-2851 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.K.D. acknowledges the Department of Science and Technology and Council of Scientific and Industrial Research, Government of India, for providing financial support under Fast Track Research Scheme (SR/FT/LS-80/2012) and STRAIT programme, respectively. T.P. gratefully acknowledges UGC for Rajiv Gandhi National Fellowship. We also acknowledge Central Sophisticated Instrumentation Facility (CSIL) of CLRI for TGA and zeta potential analysis.

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