Stimuli responsive microgel containing silver nanoparticles with tunable optical and catalytic properties

Chemicals

For the synthesis of microgel we used N-vinylcaprolactam (NVCL, 99%) and Itaconic acid (IA, 99%) as monomers, N,N-methylenebisacrylamide (MBA, 99%) as crosslinking agent, and ammonium persulfate (APS, 98%) as initiator. Sodium hydroxide (NaOH, 97%) was used as neutralizer. Silver nitrate (AgNO₃) was used as metal ion source and sodium borohydride (NaBH₄, 98%) was used as reducing agent. All the above chemicals were purchased from Sigma Aldrich. 4-Nitrophenol (4-NP, 99%) purchased from ACROS was used as model reductant. Distilled water (DW) water was used throughout the experiment.

Synthesis of P (NVCL-co-IA) copolymer microgel

P(NVCL-co-IA) copolymer microgel was synthesized by a surfactant free, free radical synthetic approach. Briefly, in a three-necked round bottom flask equipped with a condenser, nitrogen inlet and magnetic stirrer, 1.5 g of NVCL, 0.045 g of IA and 0.12 g of MBA (7 mol% w.r.t. numbers moles of monomers) were added in 95 ml of DW water. This mixture was neutralized by using NaOH. The temperature of reaction mixture was raised to 70°C and purged with N₂ for 20 min. A 0.015 g APS dissolved in 5 ml DW was added to start polymerization. After polymerization for 4 h, this mixture was cooled down to room temperature and dialyzed for 7 days with Millipore dialysis system (cellulose membrane having molecular weight cutoff of 14 000). For characterization, certain quantity of microgel was dried at 60 °C in an oven.

Synthesis of silver nanoparticles in P (NVCL-co-IA) microgel

For in situ synthesis of Ag NPs in P(NVCL-co-IA), 1 ml of prepared microgel dispersion was diluted with 25 ml of DW in a three-necked round bottom flask and purged with nitrogen for 15 min with continuous stirring. After that, 0.5 ml of 0.01 M AgNO₃ solution was added into the reaction medium and the reaction mixture was stirred for 45 min with continuous N₂ purging. Then, 5 ml of 0.05 M aqueous solution of NaBH₄ was added in the reaction mixture to reduce metal ions within microgel network. The colorless solution turned to yellowish immediately after the addition of aqueous solution of NaBH₄. The reaction was continued for 1 h and finally the resultant microgel fabricated with Ag NPs was dialyzed against DW for 2 h to remove any unreacted species. For characterization, certain quantity of microgel fabricated with Ag NPs was dried at 60 °C in an oven.

Catalytic study

To investigate the catalytic activity of synthesized P(NVCL-co-IA)-Ag, 10 ml of 0.1 mM 4-NP and 10 ml of 10 mM NaBH₄ were mixed together followed by adding specific amounts of P(NVCL-co-IA)-Ag. After specific interval of times, small amount of reaction mixture was withdrawn, and UV-Visible spectra were recorded until the yellow color of the mixture was faded away. The reaction mixture was continually stirred. Effect of temperature on reduction rate of 4-NP was investigated by carrying out the reduction reaction at 30 °C and 60 °C. The effect of amount of catalyst was also studied by performing the reaction with four different amounts of catalyst, which were 1.00, 0.50, 0.25 and 0.125 ml per 20 ml of reaction mixture.

P(NVCL-co-IA)-Ag microgel’s catalytic activity was also evaluated for the reduction of methylene blue (MB) and methyl orange (MO). Ten milliliter of 0.06 mM MB and 0.04 mM MO were treated separately with hundred folds excess of NaBH₄ and 0.2 ml of catalyst was added to proceed the reaction. Progress of reaction was investigated by using UV-Vis spectrometer.

Characterization techniques

FTIR spectroscopic analysis of the pure and hybrid microgels was performed in the powder form by using FTIR Spectrophotometer Model RZX (Perkin Elmer) to detect chemical structure. XRD analysis was carried out with (X-Ray diffractometer, model3040/60 X’Pert PRO in the 2θ range of 10–80°. Dynamic laser light scattering instrument (B1-200SM, Brookhaven Instruments Corporation, USA) was used to measure the size of the microgels. UV-Visible spectrophotometer (Shimadzu 1700) was used to carry out catalytic and optical studies of the hybrid microgels. Florescence study was performed with Perkin Elmer fluorescence spectrometer, model LS 55(120 V). TGA (Mettler Tolledo 851e) was used to observe the thermal stability. DSC analysis was done with differential scanning calorimeter Mettler Tolledo 823e. JEOL JEM 2100 F transmission electron microscope was used for microscopic studies of the hybrid microgels.

Stimuli sensitive behavior of P(NVCL-co-IA) and P(NVCL-co-IA)-Ag microgels

The presence of itaconic acid renders the pH sensitivity to the p(NVCL-co-IA) microgel. Therefore, the microgel particles exhibit volume phase transition in response to pH change in the surrounding medium. Keeping in view the pH responsiveness of microgel, the volume changes in microgel particles in response to pH changes in the surrounding medium of microgel was also studied by dynamic laser light scattering (LLS) at 20 °C and results are presented in Fig. 5a. An increase in volume of the microgel particles was observed with an increase in the pH of the medium. This increase is obvious to the deprotonation of acidic functional groups and developing of negative charge. The negatively charged deprotonated acidic functional groups repel each other and result in the extension of hydrogel network. As the polymer chains are pushed apart from each other the size of the particle is increased.

Fig. 5:

Change in hydrodynamic radius of microgel particles as a function of (a) pH and (b) temperature.

The presence of vinylcaprolactam makes the microgel sensitive to temperature of the medium and therefore we also studied the temperature responsive behavior of microgel particles. Fig. 5b shows the response of microgel to temperature at two different pH values; 4 and 10. At both pH values R_h of the microgel particles decreases significantly with increase in temperature. The value of R_h of the microgel particles decreased from 139.7 nm to 34.9 nm at pH 4 and from 261 nm to 49.2 nm at pH 10 upon increasing the temperature from 5 °C to 65 °C. This can be explained as the competition between the variable hydrophilic and hydrophobic interactions among the polymer chains of the microgel, upon varying temperature. At low temperature, higher interactions between hydrophilic groups of microgel and water molecules engenders more uptake of water molecules by the microgel spheres, lead to higher degree of swelling. Upon the gradual increase of temperature of the medium, thermal energy is absorbed by the polymer chains of microgel networks resulting in an increase in their average kinetic energy which in turn breaks the hydrophilic interactions. The breakdown of hydrophilic interaction is accompanied by the release of water molecules from microgel particles as well as collapse of polymer chains and results a decrease in size microgel particles. Transition in size was more significant at higher pH due to greater ionic strength of the medium. At high pH values, the polymer chains of microgel particles exist in more extended form due to electrostatic repulsion between negatively charged deprotonated groups and also larger amounts of water is entrapped in such extended or swollen polymeric networks. Upon collapse at high temperature, larger amount of water is released out of the particles and hence more significant volume phase transition is observed as a function of temperature at higher pH [17]. The surface plasmon resonance of silver nanoparticles is affected by the variation in inter-particle distance and refractive index of the medium [18]. When these nanoparticles are embedded in microgel networks then the inter-particle distance and refractive index of the surrounding medium can be varied by varying the temperature or pH of the medium. Along with a change in inter particle distance, a variation in temperature or pH of the medium changes the density, viscosity and ultimately refractive index of the microgel network. The change in refractive index then in turn changes the position of surface plasmon absorption band. This type of tuning of optical properties of silver nanoparticles was also studied in this work. The pH induced changes in absorption maxima of silver nanoparticles are show in Fig. 6a. A considerable movement of absorption maxima to higher wavelength in response to increase in pH is depicted Fig. 7a. The potential of the prepared microgel system to tune the optical properties is well exhibited by the variations in pH. Photoluminescence (PL) spectra of p(NVCL-co-IA)-Ag microgels were recorded at three different pH values to investigate the effect of pH on PL property of microgel at room temperature. PL spectra were taken in two sets: near ultraviolet wavelength range of 290–490 nm and the visible range of 420–700 nm, at 270 and 420 wavelengths of excitation, respectively as shown in Fig. 6b and c, respectively. Theoretical studies suggest that surface luminescence of metals can be attributed to the radiative incorporation of fermi electrons and d or sp band holes. Luminescence of the noble metals arouse due to d-band electrons excitation into higher energy states than fermi-level followed by energy loss due to hole-phonon and electron-phonon scattering and finally the fluorescence recombination of an electron from an occupied sp band with the hole occur. Fluorescence of nanoparticles originates due to the same reason [19]. The apparent emission peak at lower wavelength (around 300 and 420 nm for the excitation at 370 and 420 nm, respectively) appears to be due to Raman scattering of water [20]. A slight blue shift was observed in the λ_max of emission with decrease in pH of the microgel’s dispersion medium. Moreover the intensity of the peak is considerably enhanced at low pH. This pH sensitivity of the hybrid microgels originates due to two reasons; interaction of Ag NPs surfaces with the microgel changes with change in pH and the change in path of non-radiative energy loss. At low pH, microgels collapse due to the protonation of functional groups which actually decreases the charge density in polymeric network and hence around the surface of NPs [21]. A decrease in charge density also decreases the electric field around the surface of Ag NPs and results in increase in intensity and blue shift in maximum wavelength of emission. At high pH the opposite behavior was observed. These intelligent hybrid hydrogels having pH-sensitive fluorescence behavior and strong intensity at low pH may find applications in the fields of diagnostics, biomedicine and as fluorescence enhancers in electronic devices.

Fig. 6:

(a) The pH induced changes in UV-Visible spectra of p(NVCL-co-IA)-Ag microgels. The pH induced changes in PL spectra of p(NVCL-co-IA)-Ag microgels at excitation wavelength of (b) 260 nm and (c) 420 nm.

Fig. 7:

UV-Visible spectra of (a) 4-NP, (b) MB and (c) MO as a function of time in the presence of reducing agent and absence of catalyst.

Catalytic applications

A specific amount of the hybrid microgel was added as a catalyst in the reaction mixture containing either of 4-nitrophenol, methyl orange and methylene blue as a reactant which is reduced catalytically and sodium borohydride as a reducing agent. Owing to their UV-Vis active nature, the reduction of each of these reactants can be observed with UV-Vis spectrophotometer by measuring the absorbance of reaction mixture as a function of time. Among these reactants, 4-nitrophenol is reduced to 4-aminophenol, however, the activation energy required for this reaction is too high which cannot be achieved if reactants are allowed to interact in the absence of a suitable catalyst. Therefore, kinetically this reaction does not seem feasible but thermodynamics of this reaction suggests that it should take place because the product is more stable than the reactant. Like the reducing behavior of 4-nitrophenol, the reduction of methyl orange and methylene blue is also dead slow which may also be due to large kinetic barrier. The behavior of reduction of the chosen three reactants in the absence of catalyst is shown by UV-Visible spectra as a function of time in Fig. 7a–c which depicts that the reduction process was slow for methylene blue and almost static for the other two reactants. The kinetic hindrance against the reduction of these reactants can be overcome by the addition of a suitable catalyst in the reaction medium which provides a new route to these reactions with lower activation energy. Figure 8a–c represent the reduction of 4-NP, MO and MB, respectively, catalyzed by p(NVCL-co-IA)-Ag microgel. The decrease in the absorbance of 4-NP, MO and MB at 400, 462 and 664 nm, respectively, represents the consumption of reactant. The reduced product of 4-NP, the 4-AP, was also indicated by the appearance of new absorbance peak around 300 nm. The reducing agent was added in hundred folds excess as compared to the concentration of reactant and, therefore, the reactions were considered as pseudo first order. Taking the advantage of Beer–Lambert Law (direct proportionality of absorbance to concentration) reaction rates were calculated from the absorbance of reactants using mathematical expression of pseudo first order kinetics. The rate constants were calculated from the slope of ln(C_t/C_o) vs. time plot according to following expression

ln(A t / A o ) = ln(C t / C o ) = − k a p p ⋅ t

The plots of pseudo first order kinetics for the catalytic reduction of 4-NP, MB and MO are shown in Fig. 9a. The rate constant for pseudo first order kinetics is represented as apparent rate constant (k_app) and it was found 0.171, 0.0528 and 0.167 min⁻¹ for 4-NP, MB and MO, respectively at 25 °C.

Fig. 8:

UV-Visible spectra of (a) 4-NP, (b) MB and (c) MO as a function of time in the presence of reducing agent and of catalyst.

Fig. 9:

(a) Pseudo first order plots for the reduction of 4-NP, MB and MO. (b) Pseudo first plots for the reduction of 4-NP with different amounts of catalyst.

The reaction rate can also be tuned by variations in amount of catalyst as shown in Fig. 9b by increase in slopes of plots of pseudo first order kinetics for the reduction of 4-NP with increasing the dose of the catalyst in the reaction medium. By increase in catalyst dose, the catalytic centers are increased in the reaction medium which are responsible for the increase in effective collisions of reactants and hence an increase in reaction rate. The temperature of the reaction medium strongly influence the reaction rates so effect of temperature was also evaluated by carrying out the reduction of 4-NP at 15, 25, 40, and 60 °C. The reaction rate at each temperature was measured by pseudo first order kinetics. The plots of pseudo first order kinetics for the reduction of 4-NP at different temperatures are shown in Fig. 10a. An increase in apparent rate constant from 0.059 to 0.859 min⁻¹ was observed associated to an increase in temperature from 15 to 60 °C. At higher temperature, the average kinetic energy of reactants is enhanced while on the other hand microgel particles shrink. The shrinking of microgel particles decreases the diffusion distance between the reactants and catalyst surface while the higher kinetic energy of reactants helps to increase the diffusion rate. Both of these phenomena collectively increase the rate of reaction. So the rate of reaction can be tuned by the temperature of the reaction medium. Figure 10b and c show the Arrhenius and Eyring plots, respectively, for the calculation of activation energy (E_a), activation enthalpy change (ΔH^#) and activation entropy change (ΔS^#) and Gibbs free energy change (ΔG^#). Values of these parameters are given in Table 1. The slope of each of Arrhenius and Eyring plot was multiplied with general gas constant (R) to yield the value of E_a and ΔH^#, respectively. The value of ΔS^# was calculated from intercept of Eyring plot. The negative sign with the value of ΔS^# represents that the activated complex was formed by association mechanism and was in more ordered form in comparison to the reactants. The ΔG^# values were calculated by subtracting TΔS^# from ΔH^#. The positive sign with these values of ΔG^# indicates that reduction of 4-NP was non-spontaneous process and was initiated with catalyst [22].

Fig. 10:

(a) Pseudo first order plots for the reduction of 4-NP at different temperatures. (b) Arrhenius and (c) Eyring plot for the reduction of 4-NP.

The proposed mechanism of the catalytic reduction of 4-NP is illustrated in Fig. 11. The reactant species i.e. 4-NP and sodium borohydride first diffuse into hydrogel network and adsorbed on the surface of catalyst. The borohydride release its electron which is captured by the surface of catalyst. The electron is then transferred to 4-NP which is converted into the aminophenol through some intermediates as shown in Fig. 11. Once the aminophenol is formed, it is desorbed from the surface of catalyst and in this way the catalyst surface is regenerated which is then involved in further catalytic action. It is also assumed that the catalytic reduction of MO and MB takes place via same adsorption-desorption process at the surface of the silver nanoparticles.

Fig. 11:

Proposed mechanism for the catalytic reduction of 4-NP.