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Preparation and characterization of poly(N-isoproylacrylamide-co-dimethylaminoethyl methacrylate) microgels and their composites of gold nanoparticles
Accepted Manuscript Title: Preparation and characterization of poly(N-isoproylacrylamide-co-dimethylaminoethyl methacrylate) microgels and their composites of gold nanoparticles Authors: Saif ur Rehman, Abdur Rahman Khan, Afzal Shah, Amin Badshah, Muhammad Siddiq PII: DOI: Reference:
S0927-7757(17)30194-2 http://dx.doi.org/doi:10.1016/j.colsurfa.2017.02.060 COLSUA 21415
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
19-10-2016 21-2-2017 22-2-2017
Please cite this article as: Saif ur Rehman, Abdur Rahman Khan, Afzal Shah, Amin Badshah, Muhammad Siddiq, Preparation and characterization of poly(N-isoproylacrylamide-co-dimethylaminoethyl methacrylate) microgels and their composites of gold nanoparticles, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2017.02.060 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation and characterization of poly(N-isoproylacrylamide-co-dimethylaminoethyl methacrylate) microgels and their composites of gold nanoparticles Saif ur Rehmana,b,*, Abdur Rahman Khana, Afzal Shahb, Amin Badshahb, Muhammad Siddiqb, *
a
Department of Chemistry, COMSATS Institute of Information Technology, Khyber Pakhtunkhwa, Abbottabad-22060, Pakistan. b
Department of Chemistry, Quaid-i-Azam University, Islamabad-45320, Pakistan.
Corresponding Author: 1. Muhammad Siddiq, Department of Chemistry, Quaid-i-Azam University, Islamabad-45320, Pakistan. Email: [email protected] 2. Saif ur Rehman. Department of Chemistry, COMSATS Institute of Information Technology, Khyber Pakhtunkhwa, Abbottabad-22060, Pakistan. Email; [email protected]
Graphical abstract
Highlights
Poly(NIPAM-co-DMAEMA) microgels are synthesized by emulsion polymerization. The synthesized microgels can be used for in situ synthesis of Au-NPs. Au-NPs are used as catalyst for the reduction of 4-NP. Tuning of optical properties of the synthesized Au-NPs microgel composites. Catalytic activities can be tuned using thermoresponsive behavior of the microgels.
Abstract This work reports the crosslinkage of N-Isopropylacrylamide (NIPAM) and N, Ndimethylaminoethyl methacrylate (DMAEMA) in water using N, N’- methylene-bis(acrylamide) (BIS) as crosslinker. The particle size, volume phase transition temperature (VPTT), swelling and deswelling behaviors of the obtained poly(NIPAM-co-DMAEMA) microgels were studied at different temperature and pH using dynamic light scattering (DLS) technique. Additionally, investigation of particle size and VPTT of the microgels was done by altering the content ratio of DMAEMA monomer. The prepared microgels were used as a nanoreactor for the synthesis of gold nanoparticles (Au-NPs) using sodium borohydride (NaBH4) as reducing agent. The in situ synthesized Au-NPs were characterized and visualized by UV-Visible Spectroscopy and TEM analysis, respectively. Optical properties of poly(NIPAM-co-DMAEMA)/Au-NPs composite
were studied by changing the concentration of gold(III) chloride trihydrate (HAuCl4.3H2O), sodium borohydride (NaBH4) and temperature of the system. The prepared microgel composite were used as a catalyst for the reduction of toxic 4-nitrophenol (4-NP) with apparent rate constant (kapp) of 0.336 min-1. Catalytic activities of the microgel composite were tuned by making use of thermoresponsive behavior of the microgels. Due to temperature and pH responsive nature of the synthesized microgels they have potential for application in drug delivery and sensory materials. Key words:
Microgels, nanoparticles, microgel composites, optical properties, catalysis
1. Introduction Metal nanoparticles (MNPs) possess unique physical and chemical properties as compared to their bulk which makes them promising materials in the fields of material science and engineering [1-5]. For this reason, researchers are focused on the synthesis of different MNPs like cobalt (Co), nickel (Ni), gold (Au) and silver (Ag) for use in different fields of science and technology [4, 6]. However, Au-NPs are considered more effective due to high conductivity, inertness, bright color and surface plasmon resonance (SPR) [7]. More importantly, the stability of Au-NPs makes them promising material in the field of optoelectronics and catalysis [8]. Nonetheless, MNPs’ formation of larger particles, as a result of easy agglomeration, lead to a decrease in their effectiveness [9]. Therefore, it is very important to control the size and shape of MNPs which are in strong relationship with their optical, electrical, medicinal, and catalytic properties [10-13]. The aggregation of MNPs can be prevented by using various stabilizing agents like polymers, biological macromolecules, latex particles, mesoporous inorganic materials, dendrimers, microgels or hydrogels, colloidal systems, and many others [4, 14-16]. However, microgels are not only useful in providing stability but also generate composite material that are useful in different fields of chemistry and engineering sciences [17]. Most importantly, microgels are biocompatible and environment friendly which have made them promising materials over conventional non-aqueous system [18, 19]. Microgels are utilized as nanoreactor in swollen state as they provide more free space between crosslinkage for nucleation and growth of NPs [20].
Antonietti et al. [21] was the first to employ microgels network for the synthesis of MNPs. He synthesized water soluble polystyrene based sulfonated microgels which were used as template for the synthesis of silver nanoparticles (Ag-NPs). Later on, Biffis et al. [22] stabilized palladium nanoparticles (Pd-NPs) by adopting a new method of developing a microgels system of sulfonic acid having the ability to load Pd2+ ions. Zhang and Kumacheva [23] also used microgels as template for the synthesis of MNPs of different size and shape by changing the density of crosslinker. This may have encouraging application in biolabeling, chemical and biological separation and catalysis [24, 25]. Most importantly the catalytic activities of MNPs remain stable for a long time by fabricating them inside microgels network. This is due to the fact that microgelsa network can act as scaffold to prevent MNPs from aggregation [4, 26]. Additionally, microgels can act as a good medium of mass transfer in catalysis by providing appropriate environment (controlled pH and temperature) to enhance the catalytic activities of MNPs [27]. Therefore, microgels loaded with MNPs are widely used as catalyst in aqueous media as the reactant can easily diffuse inside the microgels network to interact with catalyst (MNPs). Additionally, such microgel composites can easily recover for reuse by filtration and centrifugation [4, 28, 29]. Considering the above advantages, research has been focused on microgels for in situ synthesis of different MNPs to study their optical and catalytic properties [30]. Naeem et al. [31] used p(NIPAM-AA-AAm) microgels as a microreactor for the synthesis of Ag-NPs and tuned their optical properties by changing temperature, and content ratio of Acrylic acid (AA) in microgels but no prominent shift was observed in their optical properties. Similarly, Rehman et al. [5] also tuned the optical properties of Ag-NPs by changing the concentration of reducing agent, surfactant and silver salt (AgNO3) used in the preparation of Ag-NPs, but the observed trend in optical properties was not consistent. Li et al. [32] synthesized Au@copolymer nanocomposites and used them as template for in situ synthesis of Ag-NPs to form thermosensitive binary nanocomposites. The binary nanocomposites were efficiently used as heterogeneous catalyst in the reduction of 4-NP.Vadakkekara et al. [33] stabilized Au-NPs by gelatin for the reduction of nitrophenols with apparent rate constant (kapp) of 0.29 min-1 which is less than the calculated value in our work.
In this work, cationic poly(NIPAM-co-DMAEMA) microgels were prepared by simple free radical emulsion polymerization and the obtained microgels were used as microreactor for in situ synthesis of Au-NPs. Optical properties of the prepared Au-NPs were investigated in response to temperature, and also by changing the concentration of reducing agent (NaBH4) and gold (III) chloride trihydrate (HAuCl4.3H2O), respectively. The obtained result showed consistency in tuning the optical properties by changing the above mentioned parameters. In addition, the prepared microgel composites were used as catalyst for the reduction of 4-NP at different temperature. Initially the reduction rate of 4-NP was increased with increase in temperature but it decreased soon after VPTT of the microgels [32, 34]. Therefore, thermoresponsive behavior of the prepared microgels can be efficiently used to control the catalytic activities of different metal nanoparticles. More briefly, in this work we have not only tuned the optical properties of the synthesize poly(NIPAM-co-DMAEMA)/Au-NPs composite but also their catalytic activities. 2.
Materials and methods
2.1.
Materials N-Isopropylacrylamide (NIPAM, 97%) was recrystallized from a mixture of n-hexan and
benzene. similarly, N, N-Dimethylaminoethyl methacrylate (DMAEMA, 99%, Aldrich) was vacuum distilled over calcium hydride and kept in refrigerator at -20 °C for further use. Potassium persulfate (KPS, initiator) and N, N’- methylene-bis(acrylamide) (BIS, crosslinker) were recrystallized from methanol and ethanol, respectively, and stored at -20 °C. Other chemicals like sodium dodecyl Sulfate (SDS, 99%, Alfa), sodium borohydride (NaBH4, Alfa) and gold (III) chloride trihydrate (HAuCl4.3H2O, Alfa) were used as obtained. All the aqueous solutions were prepared in deionized (DI) water processed with Milli-Q SP reagent water system (resistivity of 18.4 MΩ cm). 2.2.
Synthesis of poly(NIPAM-co-DMAEMA) microgels Free radical emulsion polymerization was used for the synthesis of poly(NIPAM-co-
DMAEMA) microgels in DI water using BIS as crosslinker and KPS as initiator. Two different compositions of microgels were prepared with respect to DMAEMA monomer. For each composition, NIPAM (1g), DMAEMA (0.11 and 0.14 g, respectively), BIS (4 mol.%) and SDS
(0.025 g) were mixed in 95 mL DI water in a three necked round bottom flask with condenser. The mixture was heated to 70 °C under N2 purge with continuous stirring of 400 rpm. Polymerization reaction was initiated by adding 5 mL of freshly prepared KPS solution (0.05 M) into the reaction mixture after 1 h under the same experimental conditions. The reaction was further continued for 6 h to complete the synthesis of microgels. The prepared microgels were cooled down and passed through glass wool to remove the particulate matters. The obtained filtered microgels were dialyzed for 6 days against frequent change of DI water in a large beaker. Sodium acetate and phosphate/citrate buffer solutions were used to adjust pH of the microgels for further analysis. 2.3.
In situ synthesis of Au-NPs in poly(NIPAM-co-DMAEMA) microgels The synthesized poly(NIPAM-co-DMAEMA) microgels were used as template for in situ
synthesis of Au-NPs. Optical properties of the composite microgels were studied by changing the concentration of NaBH4 and HAuCl4.3H2O, respectively. For this purpose, 6.5 mM aqueous solution of HAuCl4.3H2O (0.75 mL, 1.00mL and 1.25mL, respectively) and 2 mL of poly(NIPAM-co-DMAEMA) microgels (1.25 wt.%) were added into 2.5 mL DI water and each sample was adjusted to pH~5.8 by using phosphate buffer (2.5 mL). The mixture was put on stirring in an ice bath to load Au-ions into the microgels network. Later on, the loaded Au-ions were reduced by 8.8 mM NaBH4 (3.5 mL, 5.0 mL, and 6.5 mL, respectively) to yield a ruby-red dispersion of Au-NPs which were then purified by dialysis for almost 12 h. Schematically the synthesis of microgels and Au-NPs is shown in Figure 1. The amount of loaded Au-ions by microgels was measured by Atomic Absorption Spectroscopy (AAS). For this purpose, the composite microgels were centrifuged and a specific amount was added into 100 mL solution of HCl (5.0 M) under stirring for 3 h to release the entrapped Au-NPs. A sample of 0.5 mL was taken from the medium and diluted tenfold to measure the concentration of Au-ions. 2.4.
Catalytic Reaction Catalytic activities of the synthesized microgels composite were conducted for reduction
of 4-NP in the presence of sodium borohydride (NaBH4) as reducing agent. For this purpose, 40 μL of 4-NP (0.01 M) and 160 μL of NaBH4 solutions (0.1 M) were added into 3 mL DI water in a quartz cuvette. It was followed by the addition of 40 μL of poly(NIPAM-co-DMAEMA)/Au-
NPs composites as a catalyst. Gradually, the color of solution started to change from yellow to transparent due to reduction of 4-NP. The progress in reduction of 4-NPwas recorded as reduction in absorbance at a regular interval of 2 min. 2.5.
Characterization Transition temperature and particle size of the synthesized microgels were observed by
dynamic LLS (ALV/DLS/SLS-5022F) at a wavelength of 632 nm from He-Ne laser. The light scattered from microgels was collected at an angle of 90° for duration of 10 min. All the plotted data of DLS were the average of three measurements for each sample. Similarly, the presence of Au-NPs in composite microgels was observed using High Resolution Transmission Electron Microscopy (HRTEM, JOEL 2010). For this purpose, a sample of the composite was put on a copper grid without staining. Moreover, optical properties and catalytic activities of poly(NIPAM-co-DMAEMA)/Au-NPs composite were conducted by UV-Visible spectrophotometer (Unico UV/vis 2802PCS) having thermally controlled cuvette. 3.
Results and discussion
3.1.
Effect of temperature, pH and DMAEMA concentration on poly(NIPAM-co-DMAEMA) microgels poly(NIPAM-co-DMAEMA) microgels are temperature and pH responsive due to the
presence of NIPAM and dimethylamino [-N(CH3)2], respectively. The digital camera image in Figure 2(a) shows thermoresponsive behavior of the microgels toward change in temperature. The microgels got turbid due to shrinkage with increase in temperature but showed reversible behavior by losing turbidity due to swelling with decrease in temperature. Moreover, the effect of change in temperature on particle size and VPTT of the microgels at different pH is also shown in Figure 2(b). The prepared microgels were swollen at low temperature due to hydrogen bonding between amide residues on microgels backbone and water molecules. Due to strong solvent-polymer interaction, water molecules were attracted into the microgels network and resulted in swelling of the microgels [35, 36].While at elevated temperature, hydrogen bonds were broken between microgels and water molecules, leading to entropically favored expulsion
of water from microgels network. Consequently polymer-polymer interactions became stronger than polymer-solvent interactions, resulting in shrinkage of the microgels. Similarly particle size of the prepared microgels was observed to decrease with increase in pH of the system. As shown in Figure 3(a), hydrodynamic diameter (Dh) of the microgels decreased from 194.56-120.35 nm with increase in pH form ~2.75-9.83. In acidic medium, amino group were protonated to create electrostatic repulsion inside the microgels network which resulted in attraction of water molecules inside microgels network to maintain electroneutrality and caused the microgels to swell [37]. However, in basic medium, electrostatic repulsion disappeared due to deprotonation of the amide groups leading to the expulsion of water molecules and caused the microgels to shrink [38]. Therefore, we can say that dimethylamino [-N(CH3)2] is the decisive factor to make the microgels responsive toward change in pH of the medium. The synthesized microgels are responsive toward temperature due to presence of NIPAM which is reported to have transition temperature at 32 °C in aqueous medium [39]. However, the transition temperature can be shifted by copolymerizing NIPAM with other reactive functional monomers to change hydrophobicity and hydrophilicity of the system [38, 40]. Incorporation of hydrophilic species increases and broaden VPTT of microgels while decreased with hydrophobic specie [41]. Similarly, incorporation of DMAEMA greatly affects thermoresponsive behavior of NIPAM based microgels due to its hydrophilic nature. Even a small amount of DMAEMA can affect particle size, swelling ratio and VPTT of the microgels as shown in the Figure 3(b). Without DMAEMA monomer, simply NIPAM based microgels are sterically stabilized while electrostatic stabilization generated by charged groups of initiator is not sufficient for their stability. On the other hand, copolymerization of NIPAM with DMAEMA monomer increased the crosslinking distribution and provided colloidal stability to nucleated microgels through electrostatic attraction. Therefore, copolymerization of DMAEMA with NIPAM resulted in decreasing the extent of swelling and broaden the VPTT of poly(NIPAM-co-DMAEMA) microgels. 3.2.
In situ synthesis of Au-NPs in poly(NIPAM-co-DMAEMA) microgels
Nanoparticles are high energetic with large surface area but highly unstable to cause aggregation. Therefore, poly(NIPAM-co-DMAEMA) microgels were used as microreactor for the synthesis and stability of Au-NPs. For this purpose, Au-ions were loaded onto the swollen microgels at low temperature. The loaded Au-ions were reduced to NPs by using NaBH4 as reducing agent. The reduction process was observed through naked eyes as the colour of reaction mixture was changed from light milky to ruby red as shown in Figure 4(a). The synthesized AuNPs are uniformly distributed inside the microgels network as shown by TEM image in Figure 4(b).The TEM image also showed spherical microgels which are densely loaded with Au-NPs from inside but the surface of the microgels do not show the presence of nanoparticles which may be due to dialysis of the microgel composite in DI water for quite long time. The synthesized nanoparticles were characterized by UV-Visible spectrophotometer as shown in Figure 4(c). Due to low refractive index and non-crystalline nature, pure microgels do not show any absorption peak [5]. However, a sharp absorption peak was observed at 525 nm with introduction of Au-NPs inside microgels network [42]. The adsorption capability of the prepared microgels for Au-ions was measured using atomic absorption spectroscopy (AAS). For this purpose a specific amount of the microgels composite was dissolved in a concentrated HCL (5.0 M) under constant stirring for 3 h. During this process, Au-NPs present inside microgels network were released in the form of Au-ions and their amount was found 93.75 mg/g as measured by AAS. Also, particle size of pure and Au-NPs loaded poly(NIPAM-co-DMAEMA) microgels composite was compared by using DLS technique. It was observed that Au-NPs not only decrease particle size but also VPTT of the microgels as shown in Figure 5(a). The microgels particle size was decrease from 168.23 to 132.64 nm after fabricating Au-NPs inside microgels network. The decrease in particle size can be attributed to electrostatic attraction between AuNPs and microgels network which leads to decrease in chain mobility of the microgels. As a result microgel composites lost their ability to expand freely at low temperature and observed smaller in size as compared to pure microgels of the same nature [5]. Moreover, the decrease in VPTT of microgels with Au-NPs is also due the joint effect of temperature and force of attraction between Au-NPs and microgels network. Additionally, as Figure 5(b) shows the effect of pH at room temperature on the prepared microgels composite, it was observed that pH of the medium has the same effect on the microgels composite as observed for microgels without Au-
NPs. As discussed earlier, at high pH amide groups of DMAEMA monomer were not ionized and hence less hydrophilic and exclude water from the microgels network. However, at pH below the pKa value of amide group; protonation of amide groups was observed to create positive charge inside microgels network [43]. The ionization of microgels generated electrostatic repulsion causing the microgels chains to move apart [44]. In order to maintain electroneutrality within microgels network, water molecules were attracted toward microgels and caged the positively charged amide groups. Both these phenomena led to increased sorption of water into the microgel and resulted in their swelling. 3.3.
Optical properties of poly(NIPAM-co-DMAEMA)/Au-NPs nanocomposites Optical properties of Au-NPs loaded poly(NIPAM-co-DMAEMA) microgels were
studied at different temperature using UV-Visible spectroscopy. A continuous red shift was observed in absorbance peak of Au-NPs with increase in temperature from 25-50 °C. At low temperature, the red shift in absorbance peak was not prominent but it was increased after VPTT due to abrupt shrinkage of microgels [Figure 6(a)]. The observed red shift in absorbance peak can be attributed to increase in size of Au-NPs with increase in temperature of the system [5]. The spectra appeared at higher temperature is the superposition of spectra of isolated Au-NPs that formed aggregates with increase in temperature. Due to the shrinkage of microgels, distance between neighboring Au-NPs became smaller and resulted in their aggregation to produce larger particles [45]. The effect of change in concentration of gold (III) chloride trihydrate was also studied on the optical properties of Au-NPs using UV-Visible spectroscopy as shown in Figure 6(b). At high Au-salt concentration, more nanoparticles were formed which enhanced their chances of aggregation to form larger particles and thus the absorbance peak was shifted to higher wavelength. Jiang et al. [46] also observed red shift in absorbance peaks of silver nanoparticles with increase in concentration of silver nitrate. Similarly, Hong Huang [47] also found the same phenomenon for Ag-NPs in the presence of polyvinyl pyrrolidone (PVP). Moreover, the absorbance peak was also found red shifted with increase in the volume of NaBH4 during the synthesis of Au-NPs as shown in Figure 6(c). The change in absorption peak toward longer wavelength can be attributed to the formation of larger NPs by using increased volume of the reducing agent [5]. Due to faster formation of nanoparticles with increase in NaBH4
concentration, chances of aggregation are increased and hence large size nanoparticles are synthesized. 3.4.
Catalytic properties in the reduction of 4-NP The catalytic reduction of 4-NP is extensively focused not only for decreasing the
toxicity of water but also to generate a useful compound in the form of 4-aminophenol (4-AP) which has been used in different industries like plastic, pharmacy, agriculture and biomedicine [48]. Thermodynamically, aqueous solution of NaBH4 seems to be feasible for reduction of organic compounds but the feasibility is suppressed by kinetic barrier due potential difference between donor and acceptor electrons [49]. Therefore, proper catalysts are used to overcome the energy barrier by facilitating electron transfer from reducing agent to nitro compounds. In this connection, MNPs are frequently used as catalyst on industrial scale for the reduction of organic nitrophenols and degradation of dyes [4, 50]. In order to gauge catalytic properties of the obtained Au-NPs microgel composites, reduction of 4-NP was investigated in the presence of NaBH4 aqueous solution. The catalytic reaction of Au-NPs was focused because such types of catalytic reactions are fast, easily characterized, effective and eco-friendly [4]. The reduction reaction of 4-NP can be seen through naked eye as the color of solution was changed from dark yellow to colorless as shown in Figure 7a. The absorption peak of pure 4-NP appeared at 317 nm and red shifted to 400 nm due to conversion of 4-NP to p-nitrophenolate ions (4-NP ions) after adding freshly prepared NaBH4 solution. The reduction of 4-NP started soon after the addition of microgels composite which was monitored as a decrease in absorbance peak of 4-NP as shown in Figure 7(a). Since the concentration of NaBH4 was higher than 4-NP, therefore, pseudo-firstorder kinetics was used to determine the apparent rate constant (kapp) of 4-NP reduction [32]. The concentration of 4-NP at time “t” was denoted as Ct while its initial concentration at time “0” was regarded as Co and the change in concentration of 4-NP (Ct/Co) was measured from the relative intensity of absorbance. From the linear relationship of ln(Ct/Co) versus time (t), it was concluded that reduction of 4-NP by microgels composite follow pseudo-first-order kinetics as shown in Figure 7(b). The calculated rate constants for the reduction of 4-NP was 0.336 min-1 showing that Au-NPs loaded poly(NIPAM-co-DMAEMA) microgels can act as good catalysts. Moreover, the catalytic activities of the microgels composite were studied at five different temperatures i.e. 25, 30, 35, 40 and 45 °C and the rate of reaction was recorded after 2 min of
reaction time at each temperature as shown in Figure 8. Initially the rate of reaction increased with increase in temperature but started to decrease very soon after VPTT (36 °C) of the microgels. The increase in apparent rate constant (kapp) before VPTT is due to thermo-activity of the catalytic reaction. However, decrease in kapp after VPTT is due to shrinkage of microgels which slow down the diffusion of 4-NP inside the microgels to interact with Au-NPs (catalyst). Hence, we can say that the increase in rate constant due to temperature is suppressed by shrinkage of microgels after VPTT.
4.
Conclusions In this study, multiresponsive poly(NIPAM-co-DMAEMA) microgels were synthesized by
free radical emulsion polymerization. Swelling and deswelling behavior of the microgels was investigated by dynamic light scattering (DLS) technique and observed to change with change in pH and temperature, respectively. The microgels were found swollen at low temperature and pH but started to shrink with increase in temperature and pH of the medium. The DLS results also showed the reduction in microgels particles size and broadening of VPTT with increase in content ratio of DMAEMA monomer. Moreover, the synthesized microgels were used as nanoreactor for in situ synthesis of Au-NPs. Optical properties of Au-NPs were found red shifted with increase in concentration of gold(III) chloride trihydrate (HAuCl4.3H2O), reducing agent (NaBH4) and temperature of the system, respectively. The synthesized poly(NIPAM-coDMAEMA)/Au-NPs composite can act as an excellent catalyst for the reduction of 4-NP and the catalytic activities of the composite microgels can be tuned using thermoresponsive behavior of the microgels.
Acknowledgements The Higher Education Commission (HEC) Pakistan is gratefully acknowledged for the financial support under the International Research Support Initiative Program (IRSIP). We are also thankful to the University of Science and Technology China (USTC) for facilitating us.
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Fig. 1: Schematic representation of poly(NIPAM-co-DMAEMA) microgels synthesis and in situ synthesis of Au-NPs.
Figure 2: (a) Digital camera image of poly(NIPAM-co-DMAEMA) microgels thermoresponsive behavior and (b) hydrodynamic diameter (Dh) of poly(NIPAM-co-DMAEMA) microgels as a function of temperature.
Fig. 3: (a) Hydrodynamic diameter (Dh) of poly(NIPAM-co-DMAEMA) microgels as a function of pH at room temperature and (b) the effect of DMAEMA concentration on Dh and VPTT of poly(NIPAM-co-DMAEMA) microgels.
Figure 4: (a) Digital camera image of Au-NPs dispersion, (b) TEM images of Au-NPs loaded poly(NIPAM-co-DMAEMA) microgels (TEM image bar = 0.2 µm), and (c) UV-Visible Spectra of poly(NIPAM-co-DMAEMA) microgel (i) before (ii) and after loading with Au-NPs. Figure 5: Hydrodynamic diameter (Dh) of poly(NIPAM-co-DMAEMA) microgels as a function of temperature (a) before and after loading with Au-NPs and (b) at different pH after loading with Au-NPs. Figure 6: Optical properties of Au-NPs loaded poly(NIPAM-co-DMAEMA) microgels with change in (a) temperature (b) volume (0.75, 1.00, and 1.25 mL) of gold (III) chloride trihydrate aqueous solution (6.5 mM) and (c) volume (3.50, 5.00,and 6.50 mL) of NaBH4 aqueous solution (8.8 mM). Fig. 7: (a) Time-dependent UV-Visible spectra of 4-NP and NaBH4 aqueous solution in the presence of poly(NIPAM-co-DMAEMA)/Au-NPs composites as a catalyst and (b) timedependent change in nitrophenol compounds (Ct/C0) in the presence of poly(NIPAM-coDMAEMA)/Au-NPs composites. Inset Figures (7b) is a linear relationship of ln(Ct/C0) as a function of time for 4-NP. Figure 8: Temperature-dependent UV-Visible spectra of 4-NP and NaBH4 aqueous solutions in the presence of poly(NIPAM-co-DMAEMA)/Au-NPs composites as a catalyst after 2 min of reaction time at each temperature.
Preparation and characterization of poly(N-isoproylacrylamide-co-dimethylaminoethyl methacrylate) microgels and their composites of gold nanoparticles
CH3
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S0927-7757(17)30194-2 http://dx.doi.org/doi:10.1016/j.colsurfa.2017.02.060 COLSUA 21415
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
19-10-2016 21-2-2017 22-2-2017
Please cite this article as: Saif ur Rehman, Abdur Rahman Khan, Afzal Shah, Amin Badshah, Muhammad Siddiq, Preparation and characterization of poly(N-isoproylacrylamide-co-dimethylaminoethyl methacrylate) microgels and their composites of gold nanoparticles, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2017.02.060 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation and characterization of poly(N-isoproylacrylamide-co-dimethylaminoethyl methacrylate) microgels and their composites of gold nanoparticles Saif ur Rehmana,b,*, Abdur Rahman Khana, Afzal Shahb, Amin Badshahb, Muhammad Siddiqb, *
a
Department of Chemistry, COMSATS Institute of Information Technology, Khyber Pakhtunkhwa, Abbottabad-22060, Pakistan. b
Department of Chemistry, Quaid-i-Azam University, Islamabad-45320, Pakistan.
Corresponding Author: 1. Muhammad Siddiq, Department of Chemistry, Quaid-i-Azam University, Islamabad-45320, Pakistan. Email: [email protected] 2. Saif ur Rehman. Department of Chemistry, COMSATS Institute of Information Technology, Khyber Pakhtunkhwa, Abbottabad-22060, Pakistan. Email; [email protected]
Graphical abstract
Highlights
Poly(NIPAM-co-DMAEMA) microgels are synthesized by emulsion polymerization. The synthesized microgels can be used for in situ synthesis of Au-NPs. Au-NPs are used as catalyst for the reduction of 4-NP. Tuning of optical properties of the synthesized Au-NPs microgel composites. Catalytic activities can be tuned using thermoresponsive behavior of the microgels.
Abstract This work reports the crosslinkage of N-Isopropylacrylamide (NIPAM) and N, Ndimethylaminoethyl methacrylate (DMAEMA) in water using N, N’- methylene-bis(acrylamide) (BIS) as crosslinker. The particle size, volume phase transition temperature (VPTT), swelling and deswelling behaviors of the obtained poly(NIPAM-co-DMAEMA) microgels were studied at different temperature and pH using dynamic light scattering (DLS) technique. Additionally, investigation of particle size and VPTT of the microgels was done by altering the content ratio of DMAEMA monomer. The prepared microgels were used as a nanoreactor for the synthesis of gold nanoparticles (Au-NPs) using sodium borohydride (NaBH4) as reducing agent. The in situ synthesized Au-NPs were characterized and visualized by UV-Visible Spectroscopy and TEM analysis, respectively. Optical properties of poly(NIPAM-co-DMAEMA)/Au-NPs composite
were studied by changing the concentration of gold(III) chloride trihydrate (HAuCl4.3H2O), sodium borohydride (NaBH4) and temperature of the system. The prepared microgel composite were used as a catalyst for the reduction of toxic 4-nitrophenol (4-NP) with apparent rate constant (kapp) of 0.336 min-1. Catalytic activities of the microgel composite were tuned by making use of thermoresponsive behavior of the microgels. Due to temperature and pH responsive nature of the synthesized microgels they have potential for application in drug delivery and sensory materials. Key words:
Microgels, nanoparticles, microgel composites, optical properties, catalysis
1. Introduction Metal nanoparticles (MNPs) possess unique physical and chemical properties as compared to their bulk which makes them promising materials in the fields of material science and engineering [1-5]. For this reason, researchers are focused on the synthesis of different MNPs like cobalt (Co), nickel (Ni), gold (Au) and silver (Ag) for use in different fields of science and technology [4, 6]. However, Au-NPs are considered more effective due to high conductivity, inertness, bright color and surface plasmon resonance (SPR) [7]. More importantly, the stability of Au-NPs makes them promising material in the field of optoelectronics and catalysis [8]. Nonetheless, MNPs’ formation of larger particles, as a result of easy agglomeration, lead to a decrease in their effectiveness [9]. Therefore, it is very important to control the size and shape of MNPs which are in strong relationship with their optical, electrical, medicinal, and catalytic properties [10-13]. The aggregation of MNPs can be prevented by using various stabilizing agents like polymers, biological macromolecules, latex particles, mesoporous inorganic materials, dendrimers, microgels or hydrogels, colloidal systems, and many others [4, 14-16]. However, microgels are not only useful in providing stability but also generate composite material that are useful in different fields of chemistry and engineering sciences [17]. Most importantly, microgels are biocompatible and environment friendly which have made them promising materials over conventional non-aqueous system [18, 19]. Microgels are utilized as nanoreactor in swollen state as they provide more free space between crosslinkage for nucleation and growth of NPs [20].
Antonietti et al. [21] was the first to employ microgels network for the synthesis of MNPs. He synthesized water soluble polystyrene based sulfonated microgels which were used as template for the synthesis of silver nanoparticles (Ag-NPs). Later on, Biffis et al. [22] stabilized palladium nanoparticles (Pd-NPs) by adopting a new method of developing a microgels system of sulfonic acid having the ability to load Pd2+ ions. Zhang and Kumacheva [23] also used microgels as template for the synthesis of MNPs of different size and shape by changing the density of crosslinker. This may have encouraging application in biolabeling, chemical and biological separation and catalysis [24, 25]. Most importantly the catalytic activities of MNPs remain stable for a long time by fabricating them inside microgels network. This is due to the fact that microgelsa network can act as scaffold to prevent MNPs from aggregation [4, 26]. Additionally, microgels can act as a good medium of mass transfer in catalysis by providing appropriate environment (controlled pH and temperature) to enhance the catalytic activities of MNPs [27]. Therefore, microgels loaded with MNPs are widely used as catalyst in aqueous media as the reactant can easily diffuse inside the microgels network to interact with catalyst (MNPs). Additionally, such microgel composites can easily recover for reuse by filtration and centrifugation [4, 28, 29]. Considering the above advantages, research has been focused on microgels for in situ synthesis of different MNPs to study their optical and catalytic properties [30]. Naeem et al. [31] used p(NIPAM-AA-AAm) microgels as a microreactor for the synthesis of Ag-NPs and tuned their optical properties by changing temperature, and content ratio of Acrylic acid (AA) in microgels but no prominent shift was observed in their optical properties. Similarly, Rehman et al. [5] also tuned the optical properties of Ag-NPs by changing the concentration of reducing agent, surfactant and silver salt (AgNO3) used in the preparation of Ag-NPs, but the observed trend in optical properties was not consistent. Li et al. [32] synthesized Au@copolymer nanocomposites and used them as template for in situ synthesis of Ag-NPs to form thermosensitive binary nanocomposites. The binary nanocomposites were efficiently used as heterogeneous catalyst in the reduction of 4-NP.Vadakkekara et al. [33] stabilized Au-NPs by gelatin for the reduction of nitrophenols with apparent rate constant (kapp) of 0.29 min-1 which is less than the calculated value in our work.
In this work, cationic poly(NIPAM-co-DMAEMA) microgels were prepared by simple free radical emulsion polymerization and the obtained microgels were used as microreactor for in situ synthesis of Au-NPs. Optical properties of the prepared Au-NPs were investigated in response to temperature, and also by changing the concentration of reducing agent (NaBH4) and gold (III) chloride trihydrate (HAuCl4.3H2O), respectively. The obtained result showed consistency in tuning the optical properties by changing the above mentioned parameters. In addition, the prepared microgel composites were used as catalyst for the reduction of 4-NP at different temperature. Initially the reduction rate of 4-NP was increased with increase in temperature but it decreased soon after VPTT of the microgels [32, 34]. Therefore, thermoresponsive behavior of the prepared microgels can be efficiently used to control the catalytic activities of different metal nanoparticles. More briefly, in this work we have not only tuned the optical properties of the synthesize poly(NIPAM-co-DMAEMA)/Au-NPs composite but also their catalytic activities. 2.
Materials and methods
2.1.
Materials N-Isopropylacrylamide (NIPAM, 97%) was recrystallized from a mixture of n-hexan and
benzene. similarly, N, N-Dimethylaminoethyl methacrylate (DMAEMA, 99%, Aldrich) was vacuum distilled over calcium hydride and kept in refrigerator at -20 °C for further use. Potassium persulfate (KPS, initiator) and N, N’- methylene-bis(acrylamide) (BIS, crosslinker) were recrystallized from methanol and ethanol, respectively, and stored at -20 °C. Other chemicals like sodium dodecyl Sulfate (SDS, 99%, Alfa), sodium borohydride (NaBH4, Alfa) and gold (III) chloride trihydrate (HAuCl4.3H2O, Alfa) were used as obtained. All the aqueous solutions were prepared in deionized (DI) water processed with Milli-Q SP reagent water system (resistivity of 18.4 MΩ cm). 2.2.
Synthesis of poly(NIPAM-co-DMAEMA) microgels Free radical emulsion polymerization was used for the synthesis of poly(NIPAM-co-
DMAEMA) microgels in DI water using BIS as crosslinker and KPS as initiator. Two different compositions of microgels were prepared with respect to DMAEMA monomer. For each composition, NIPAM (1g), DMAEMA (0.11 and 0.14 g, respectively), BIS (4 mol.%) and SDS
(0.025 g) were mixed in 95 mL DI water in a three necked round bottom flask with condenser. The mixture was heated to 70 °C under N2 purge with continuous stirring of 400 rpm. Polymerization reaction was initiated by adding 5 mL of freshly prepared KPS solution (0.05 M) into the reaction mixture after 1 h under the same experimental conditions. The reaction was further continued for 6 h to complete the synthesis of microgels. The prepared microgels were cooled down and passed through glass wool to remove the particulate matters. The obtained filtered microgels were dialyzed for 6 days against frequent change of DI water in a large beaker. Sodium acetate and phosphate/citrate buffer solutions were used to adjust pH of the microgels for further analysis. 2.3.
In situ synthesis of Au-NPs in poly(NIPAM-co-DMAEMA) microgels The synthesized poly(NIPAM-co-DMAEMA) microgels were used as template for in situ
synthesis of Au-NPs. Optical properties of the composite microgels were studied by changing the concentration of NaBH4 and HAuCl4.3H2O, respectively. For this purpose, 6.5 mM aqueous solution of HAuCl4.3H2O (0.75 mL, 1.00mL and 1.25mL, respectively) and 2 mL of poly(NIPAM-co-DMAEMA) microgels (1.25 wt.%) were added into 2.5 mL DI water and each sample was adjusted to pH~5.8 by using phosphate buffer (2.5 mL). The mixture was put on stirring in an ice bath to load Au-ions into the microgels network. Later on, the loaded Au-ions were reduced by 8.8 mM NaBH4 (3.5 mL, 5.0 mL, and 6.5 mL, respectively) to yield a ruby-red dispersion of Au-NPs which were then purified by dialysis for almost 12 h. Schematically the synthesis of microgels and Au-NPs is shown in Figure 1. The amount of loaded Au-ions by microgels was measured by Atomic Absorption Spectroscopy (AAS). For this purpose, the composite microgels were centrifuged and a specific amount was added into 100 mL solution of HCl (5.0 M) under stirring for 3 h to release the entrapped Au-NPs. A sample of 0.5 mL was taken from the medium and diluted tenfold to measure the concentration of Au-ions. 2.4.
Catalytic Reaction Catalytic activities of the synthesized microgels composite were conducted for reduction
of 4-NP in the presence of sodium borohydride (NaBH4) as reducing agent. For this purpose, 40 μL of 4-NP (0.01 M) and 160 μL of NaBH4 solutions (0.1 M) were added into 3 mL DI water in a quartz cuvette. It was followed by the addition of 40 μL of poly(NIPAM-co-DMAEMA)/Au-
NPs composites as a catalyst. Gradually, the color of solution started to change from yellow to transparent due to reduction of 4-NP. The progress in reduction of 4-NPwas recorded as reduction in absorbance at a regular interval of 2 min. 2.5.
Characterization Transition temperature and particle size of the synthesized microgels were observed by
dynamic LLS (ALV/DLS/SLS-5022F) at a wavelength of 632 nm from He-Ne laser. The light scattered from microgels was collected at an angle of 90° for duration of 10 min. All the plotted data of DLS were the average of three measurements for each sample. Similarly, the presence of Au-NPs in composite microgels was observed using High Resolution Transmission Electron Microscopy (HRTEM, JOEL 2010). For this purpose, a sample of the composite was put on a copper grid without staining. Moreover, optical properties and catalytic activities of poly(NIPAM-co-DMAEMA)/Au-NPs composite were conducted by UV-Visible spectrophotometer (Unico UV/vis 2802PCS) having thermally controlled cuvette. 3.
Results and discussion
3.1.
Effect of temperature, pH and DMAEMA concentration on poly(NIPAM-co-DMAEMA) microgels poly(NIPAM-co-DMAEMA) microgels are temperature and pH responsive due to the
presence of NIPAM and dimethylamino [-N(CH3)2], respectively. The digital camera image in Figure 2(a) shows thermoresponsive behavior of the microgels toward change in temperature. The microgels got turbid due to shrinkage with increase in temperature but showed reversible behavior by losing turbidity due to swelling with decrease in temperature. Moreover, the effect of change in temperature on particle size and VPTT of the microgels at different pH is also shown in Figure 2(b). The prepared microgels were swollen at low temperature due to hydrogen bonding between amide residues on microgels backbone and water molecules. Due to strong solvent-polymer interaction, water molecules were attracted into the microgels network and resulted in swelling of the microgels [35, 36].While at elevated temperature, hydrogen bonds were broken between microgels and water molecules, leading to entropically favored expulsion
of water from microgels network. Consequently polymer-polymer interactions became stronger than polymer-solvent interactions, resulting in shrinkage of the microgels. Similarly particle size of the prepared microgels was observed to decrease with increase in pH of the system. As shown in Figure 3(a), hydrodynamic diameter (Dh) of the microgels decreased from 194.56-120.35 nm with increase in pH form ~2.75-9.83. In acidic medium, amino group were protonated to create electrostatic repulsion inside the microgels network which resulted in attraction of water molecules inside microgels network to maintain electroneutrality and caused the microgels to swell [37]. However, in basic medium, electrostatic repulsion disappeared due to deprotonation of the amide groups leading to the expulsion of water molecules and caused the microgels to shrink [38]. Therefore, we can say that dimethylamino [-N(CH3)2] is the decisive factor to make the microgels responsive toward change in pH of the medium. The synthesized microgels are responsive toward temperature due to presence of NIPAM which is reported to have transition temperature at 32 °C in aqueous medium [39]. However, the transition temperature can be shifted by copolymerizing NIPAM with other reactive functional monomers to change hydrophobicity and hydrophilicity of the system [38, 40]. Incorporation of hydrophilic species increases and broaden VPTT of microgels while decreased with hydrophobic specie [41]. Similarly, incorporation of DMAEMA greatly affects thermoresponsive behavior of NIPAM based microgels due to its hydrophilic nature. Even a small amount of DMAEMA can affect particle size, swelling ratio and VPTT of the microgels as shown in the Figure 3(b). Without DMAEMA monomer, simply NIPAM based microgels are sterically stabilized while electrostatic stabilization generated by charged groups of initiator is not sufficient for their stability. On the other hand, copolymerization of NIPAM with DMAEMA monomer increased the crosslinking distribution and provided colloidal stability to nucleated microgels through electrostatic attraction. Therefore, copolymerization of DMAEMA with NIPAM resulted in decreasing the extent of swelling and broaden the VPTT of poly(NIPAM-co-DMAEMA) microgels. 3.2.
In situ synthesis of Au-NPs in poly(NIPAM-co-DMAEMA) microgels
Nanoparticles are high energetic with large surface area but highly unstable to cause aggregation. Therefore, poly(NIPAM-co-DMAEMA) microgels were used as microreactor for the synthesis and stability of Au-NPs. For this purpose, Au-ions were loaded onto the swollen microgels at low temperature. The loaded Au-ions were reduced to NPs by using NaBH4 as reducing agent. The reduction process was observed through naked eyes as the colour of reaction mixture was changed from light milky to ruby red as shown in Figure 4(a). The synthesized AuNPs are uniformly distributed inside the microgels network as shown by TEM image in Figure 4(b).The TEM image also showed spherical microgels which are densely loaded with Au-NPs from inside but the surface of the microgels do not show the presence of nanoparticles which may be due to dialysis of the microgel composite in DI water for quite long time. The synthesized nanoparticles were characterized by UV-Visible spectrophotometer as shown in Figure 4(c). Due to low refractive index and non-crystalline nature, pure microgels do not show any absorption peak [5]. However, a sharp absorption peak was observed at 525 nm with introduction of Au-NPs inside microgels network [42]. The adsorption capability of the prepared microgels for Au-ions was measured using atomic absorption spectroscopy (AAS). For this purpose a specific amount of the microgels composite was dissolved in a concentrated HCL (5.0 M) under constant stirring for 3 h. During this process, Au-NPs present inside microgels network were released in the form of Au-ions and their amount was found 93.75 mg/g as measured by AAS. Also, particle size of pure and Au-NPs loaded poly(NIPAM-co-DMAEMA) microgels composite was compared by using DLS technique. It was observed that Au-NPs not only decrease particle size but also VPTT of the microgels as shown in Figure 5(a). The microgels particle size was decrease from 168.23 to 132.64 nm after fabricating Au-NPs inside microgels network. The decrease in particle size can be attributed to electrostatic attraction between AuNPs and microgels network which leads to decrease in chain mobility of the microgels. As a result microgel composites lost their ability to expand freely at low temperature and observed smaller in size as compared to pure microgels of the same nature [5]. Moreover, the decrease in VPTT of microgels with Au-NPs is also due the joint effect of temperature and force of attraction between Au-NPs and microgels network. Additionally, as Figure 5(b) shows the effect of pH at room temperature on the prepared microgels composite, it was observed that pH of the medium has the same effect on the microgels composite as observed for microgels without Au-
NPs. As discussed earlier, at high pH amide groups of DMAEMA monomer were not ionized and hence less hydrophilic and exclude water from the microgels network. However, at pH below the pKa value of amide group; protonation of amide groups was observed to create positive charge inside microgels network [43]. The ionization of microgels generated electrostatic repulsion causing the microgels chains to move apart [44]. In order to maintain electroneutrality within microgels network, water molecules were attracted toward microgels and caged the positively charged amide groups. Both these phenomena led to increased sorption of water into the microgel and resulted in their swelling. 3.3.
Optical properties of poly(NIPAM-co-DMAEMA)/Au-NPs nanocomposites Optical properties of Au-NPs loaded poly(NIPAM-co-DMAEMA) microgels were
studied at different temperature using UV-Visible spectroscopy. A continuous red shift was observed in absorbance peak of Au-NPs with increase in temperature from 25-50 °C. At low temperature, the red shift in absorbance peak was not prominent but it was increased after VPTT due to abrupt shrinkage of microgels [Figure 6(a)]. The observed red shift in absorbance peak can be attributed to increase in size of Au-NPs with increase in temperature of the system [5]. The spectra appeared at higher temperature is the superposition of spectra of isolated Au-NPs that formed aggregates with increase in temperature. Due to the shrinkage of microgels, distance between neighboring Au-NPs became smaller and resulted in their aggregation to produce larger particles [45]. The effect of change in concentration of gold (III) chloride trihydrate was also studied on the optical properties of Au-NPs using UV-Visible spectroscopy as shown in Figure 6(b). At high Au-salt concentration, more nanoparticles were formed which enhanced their chances of aggregation to form larger particles and thus the absorbance peak was shifted to higher wavelength. Jiang et al. [46] also observed red shift in absorbance peaks of silver nanoparticles with increase in concentration of silver nitrate. Similarly, Hong Huang [47] also found the same phenomenon for Ag-NPs in the presence of polyvinyl pyrrolidone (PVP). Moreover, the absorbance peak was also found red shifted with increase in the volume of NaBH4 during the synthesis of Au-NPs as shown in Figure 6(c). The change in absorption peak toward longer wavelength can be attributed to the formation of larger NPs by using increased volume of the reducing agent [5]. Due to faster formation of nanoparticles with increase in NaBH4
concentration, chances of aggregation are increased and hence large size nanoparticles are synthesized. 3.4.
Catalytic properties in the reduction of 4-NP The catalytic reduction of 4-NP is extensively focused not only for decreasing the
toxicity of water but also to generate a useful compound in the form of 4-aminophenol (4-AP) which has been used in different industries like plastic, pharmacy, agriculture and biomedicine [48]. Thermodynamically, aqueous solution of NaBH4 seems to be feasible for reduction of organic compounds but the feasibility is suppressed by kinetic barrier due potential difference between donor and acceptor electrons [49]. Therefore, proper catalysts are used to overcome the energy barrier by facilitating electron transfer from reducing agent to nitro compounds. In this connection, MNPs are frequently used as catalyst on industrial scale for the reduction of organic nitrophenols and degradation of dyes [4, 50]. In order to gauge catalytic properties of the obtained Au-NPs microgel composites, reduction of 4-NP was investigated in the presence of NaBH4 aqueous solution. The catalytic reaction of Au-NPs was focused because such types of catalytic reactions are fast, easily characterized, effective and eco-friendly [4]. The reduction reaction of 4-NP can be seen through naked eye as the color of solution was changed from dark yellow to colorless as shown in Figure 7a. The absorption peak of pure 4-NP appeared at 317 nm and red shifted to 400 nm due to conversion of 4-NP to p-nitrophenolate ions (4-NP ions) after adding freshly prepared NaBH4 solution. The reduction of 4-NP started soon after the addition of microgels composite which was monitored as a decrease in absorbance peak of 4-NP as shown in Figure 7(a). Since the concentration of NaBH4 was higher than 4-NP, therefore, pseudo-firstorder kinetics was used to determine the apparent rate constant (kapp) of 4-NP reduction [32]. The concentration of 4-NP at time “t” was denoted as Ct while its initial concentration at time “0” was regarded as Co and the change in concentration of 4-NP (Ct/Co) was measured from the relative intensity of absorbance. From the linear relationship of ln(Ct/Co) versus time (t), it was concluded that reduction of 4-NP by microgels composite follow pseudo-first-order kinetics as shown in Figure 7(b). The calculated rate constants for the reduction of 4-NP was 0.336 min-1 showing that Au-NPs loaded poly(NIPAM-co-DMAEMA) microgels can act as good catalysts. Moreover, the catalytic activities of the microgels composite were studied at five different temperatures i.e. 25, 30, 35, 40 and 45 °C and the rate of reaction was recorded after 2 min of
reaction time at each temperature as shown in Figure 8. Initially the rate of reaction increased with increase in temperature but started to decrease very soon after VPTT (36 °C) of the microgels. The increase in apparent rate constant (kapp) before VPTT is due to thermo-activity of the catalytic reaction. However, decrease in kapp after VPTT is due to shrinkage of microgels which slow down the diffusion of 4-NP inside the microgels to interact with Au-NPs (catalyst). Hence, we can say that the increase in rate constant due to temperature is suppressed by shrinkage of microgels after VPTT.
4.
Conclusions In this study, multiresponsive poly(NIPAM-co-DMAEMA) microgels were synthesized by
free radical emulsion polymerization. Swelling and deswelling behavior of the microgels was investigated by dynamic light scattering (DLS) technique and observed to change with change in pH and temperature, respectively. The microgels were found swollen at low temperature and pH but started to shrink with increase in temperature and pH of the medium. The DLS results also showed the reduction in microgels particles size and broadening of VPTT with increase in content ratio of DMAEMA monomer. Moreover, the synthesized microgels were used as nanoreactor for in situ synthesis of Au-NPs. Optical properties of Au-NPs were found red shifted with increase in concentration of gold(III) chloride trihydrate (HAuCl4.3H2O), reducing agent (NaBH4) and temperature of the system, respectively. The synthesized poly(NIPAM-coDMAEMA)/Au-NPs composite can act as an excellent catalyst for the reduction of 4-NP and the catalytic activities of the composite microgels can be tuned using thermoresponsive behavior of the microgels.
Acknowledgements The Higher Education Commission (HEC) Pakistan is gratefully acknowledged for the financial support under the International Research Support Initiative Program (IRSIP). We are also thankful to the University of Science and Technology China (USTC) for facilitating us.
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Fig. 1: Schematic representation of poly(NIPAM-co-DMAEMA) microgels synthesis and in situ synthesis of Au-NPs.
Figure 2: (a) Digital camera image of poly(NIPAM-co-DMAEMA) microgels thermoresponsive behavior and (b) hydrodynamic diameter (Dh) of poly(NIPAM-co-DMAEMA) microgels as a function of temperature.
Fig. 3: (a) Hydrodynamic diameter (Dh) of poly(NIPAM-co-DMAEMA) microgels as a function of pH at room temperature and (b) the effect of DMAEMA concentration on Dh and VPTT of poly(NIPAM-co-DMAEMA) microgels.
Figure 4: (a) Digital camera image of Au-NPs dispersion, (b) TEM images of Au-NPs loaded poly(NIPAM-co-DMAEMA) microgels (TEM image bar = 0.2 µm), and (c) UV-Visible Spectra of poly(NIPAM-co-DMAEMA) microgel (i) before (ii) and after loading with Au-NPs. Figure 5: Hydrodynamic diameter (Dh) of poly(NIPAM-co-DMAEMA) microgels as a function of temperature (a) before and after loading with Au-NPs and (b) at different pH after loading with Au-NPs. Figure 6: Optical properties of Au-NPs loaded poly(NIPAM-co-DMAEMA) microgels with change in (a) temperature (b) volume (0.75, 1.00, and 1.25 mL) of gold (III) chloride trihydrate aqueous solution (6.5 mM) and (c) volume (3.50, 5.00,and 6.50 mL) of NaBH4 aqueous solution (8.8 mM). Fig. 7: (a) Time-dependent UV-Visible spectra of 4-NP and NaBH4 aqueous solution in the presence of poly(NIPAM-co-DMAEMA)/Au-NPs composites as a catalyst and (b) timedependent change in nitrophenol compounds (Ct/C0) in the presence of poly(NIPAM-coDMAEMA)/Au-NPs composites. Inset Figures (7b) is a linear relationship of ln(Ct/C0) as a function of time for 4-NP. Figure 8: Temperature-dependent UV-Visible spectra of 4-NP and NaBH4 aqueous solutions in the presence of poly(NIPAM-co-DMAEMA)/Au-NPs composites as a catalyst after 2 min of reaction time at each temperature.
Preparation and characterization of poly(N-isoproylacrylamide-co-dimethylaminoethyl methacrylate) microgels and their composites of gold nanoparticles
CH3
H2 C
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Microgel-Au-NPs = Au-NP
N-Isopropylacrylamide (NIPAM)
Fig. 1
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