Zanru Guo*a,
Hongjian Gua,
Wei Maa,
Qiang Chena,
Zhanfeng He*b,
Jiali Zhanga,
Yongxin Liua,
Longzhen Zhenga and
Yujun Feng*c
aDepartment of Polymer Materials and Chemical Engineering, School of Materials Science and Engineering, East China Jiaotong University, Nanchang, Jiangxi 330013, P. R. China. E-mail: guozanru@ecjtu.edu.cn
bState Key Laboratory of Oil and Gas Reservoir Geology, Exploitation Southwest Petroleum University, Chengdu 610500, P. R. China. E-mail: he_zhanfeng2008@126.com
cPolymer Research Institute, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, P. R. China. E-mail: yjfeng@scu.edu.cn
First published on 26th October 2017
The design of controllable or “signal-triggered” metal nanoparticles is one of the emerging trends in nanotechnology and advanced materials. CO2-switchable polymer-hybrid silver nanoparticles (AgNPs) were prepared by a one-pot reaction reducing AgNO3 and trithioester terminated PDEAEMA with sodium borohydride (NaBH4). The hybrids showed a long-term stability, and their size and size distribution can be easily modulated by tuning the molar ratio of polymers to AgNO3. The hybrids not only exhibit hydrophobic–hydrophilic transitions in immiscible mixed solvents, but also undergo a switchable dispersion/aggregation states upon alternately treating with CO2 and N2. Moreover, this smart hybrid was preliminarily used as catalyst for the reduction of 4-nitrophenol. The catalytic activity of the hybrids can be switched and monotonously tuned by varying the flow rate of CO2 purged into the reaction system, which may open a new avenue for tailoring the catalytic activity of metal nanoparticles toward a given reaction.
Up to date, several kinds of stimuli-responsive polymers have been used to functionalize metal NPs to form “smart” catalysts. For example, temperature-responsive polymeric hydrogel,24 micelle,25,26 microgel,27 “yolk–shell” structure,28 and polymers29 were employed to support metal NPs, forming temperature switchable or tunable catalysts. Similarly, metal NPs were immobilized in pH-responsive polymeric hydrogel,30,31 microsphere,32 and micelle33 to get pH-responsive catalytic systems. Besides, light-controllable catalyst was obtained by the combination of a temperature-responsive polymer with metal nanoparticles which can convert light into heat through light irradiation.9 Though the use of temperature as a trigger for metal “smart” catalysts can switch reaction and modulate reaction rate, changing reaction temperature would result in some side reaction or affect reaction rate,9 which lead to a nonmonotone correlation with temperature. As for pH trigger based on acids and bases, it is hard to tune reaction rate as reaction usually happened in a fix pH range.23,31 Moreover, the use of acids and bases for tuning pH may contaminate or modify the final products.9 Therefore, to switch catalytic activity and modulate catalytic rate, it is desirable to develop a “green” and simple trigger for metal “smart” catalysts.
Very recently, we and others have employed CO2 and inert gas such as nitrogen to switch between the hydrophobic and hydrophilic property of polymers with amidine34,35 or amino groups,36–40 and then control the morphology of nanomaterials34,40 or its self-assemblies.36,39 As an abundant, economical, nontoxic, biocompatible, and renewable resource, CO2 trigger just bubble gas and leave no contamination during the stimulate process,34–39 which may satisfy the requirements of metal “smart” catalysts. Zhao and coworkers41 pioneered the fabrication of CO2-switchable gold nanoparticles (AuNPs) by functionalizing AuNPs with CO2-switchable polymers, and found the obtained AuNPs hybrids can be dispersed–re-dispersed in and separated from aqueous solution by CO2 and N2 bubbling. Besides, the hybrids exhibit high catalytic activity, easier separation and better reusability for 4-nitrophenol reduction. Yuan et al.42 embedded AuNPs into the shell of CO2-responsive magnetic hybrid nanospheres, and switch their catalytic activity through the access of swollen or collapsed of CO2 sensitive shell. Although these metal hybrid catalysts exhibit CO2-responsive properties, the studies were focused on gold metal, and its reusability and switchability. To the best of our knowledge, there have been few reports of other CO2-responsive metal catalyst and the gas-modulated catalytic activity. Compared with AuNPs and other metal nanoparticles (like Pt, Pd),43,44 silver nanoparticles (AgNPs) can be prepared more readily and inexpensively and also exhibits similar catalytic properties, which may have broad application prospects in catalysis.45 As such, the appeal for CO2-switchable AgNPs with gas-tunable catalytic activity for the general development of smart catalysts remains high.
In this report, we used an reversible addition–fragmentation transfer polymerization (RAFT) technique to synthesize a CO2-responsive polymer poly(2-(diethylamino)-ethyl methacrylate) (PDEAEMA), since RAFT polymerization has the advantage to prepare the thiol terminated polymer because the chain transfer agents always contain the dithioester or trithioester group that can be easily reduced to thiol terminated group.46–48 Resorting to reducing agent sodium borohydride (NaBH4), AgNO3 and trithioester end group of PDEAEMA were reduced to silver nanoparticles (AgNPs) and thiol moiety, respectively; then PDEAEMA adsorbed onto the surface of AgNPs via strong Ag–sulfur interaction, forming PDEAEMA–AgNPs (Ag–P) hybrids (Scheme 1). The hydrophilic–hydrophobic properties and dispersibility of the AgNPs were examined by bubbling CO2 or N2. The hybrids were applied as a catalyst in a model catalytic reduction of 4-nitrophenol, as it is one of the most refractory pollutants that can occur in industrial waste waters.42,48 Besides, the catalytic activity at different CO2 flow rate was preliminarily discerned, which may open a new avenue for tailoring the catalytic activity of metal nanoparticles toward a given reaction.
Infrared spectra were registered on a Nicolet MX-1E FTIR (USA) spectrophotometer in the scanning range of 4000–400 cm−1 using KBr pellet method.
The 1H NMR of the polymer was measured using a Nuclear Magnetic Resonance Spectrometer (AV CORP300, Bruker, Germany). The polymer was purified according to its solubility in CDCl3, and the molecular structure of the polymer was determined from the corresponding chemical shift and integral area in the 1H NMR.
The molecular weight and the molecular weight distribution of the polymers were determined using a gel permeation chromatography (GPC) system equipped with a Waters 515 pump and a 2410 detector. The column temperature was set at 25 °C and the THF was used as the mobile phase. Polystyrene (PSt) was used as the reference material.
Thermal gravimetric analysis (TGA, 299-F1, NETZSCH, Germany) was used to test the polymer modified silver nanoparticles in the polymer content. The sample was heated to a temperature of 800 °C at a rate of 10 °C min−1 in a nitrogen atmosphere (flow rate 20 mL min−1). The dialyzed dark brown concentrated mixture was purged with N2 for 1 h and centrifuged at 10000 rpm for 5 minutes, the upper layer was discarded, washed with deionized water and centrifuged. After three times, the mixture was suction filtered and the solid sample was vacuum dried at 50 °C for 24 hours prior to TGA and XRD characterizations.
The zeta potential values of silver colloid were measured with zetameter ZetaPALS (Brookhaven, USA). Each test was carried out for five times and the average values were taken as the final results.
X-ray diffraction (XRD) patterns of AgNPs hybrids was recorded using a RigakuDMAX2200 with Ni-filtered Cu Kα radiation over a scanning range of 30 to 80° at an X-ray power of 40 kV and 40 mA.
The conductivity of Ag–P1 dispersion was measured with a DDS-11A conductometer (Chengdu Fangzhou Instrument) at 25 °C, and the average values were calculated from three runs of a certain measurement.
Transmission electron microscope (Holland, Philips Company, Tecnai12) was used to observe the particle size distribution of silver nanoparticles. The size distributions of each sample were determined at least 1000 particles from photographs of the TEM images by image analysis software (Nano Measurer). As for negative staining TEM, the sample was stained with phosphotungstic acid for TEM observation.
To form CO2-switchable AgNPs, the silver nanoparticles were prepared by reducing silver nitrate (AgNO3) with NaBH4 in the presence of the PDEAEMA. Meanwhile, trithioester end group of PDEAEMA was reduced to thiol group with NaBH4,46–49 providing the necessary for chemical attaching PDEAEMA on the surface of AgNPs (Scheme 1). With such one-pot protocol, a series of AgNPs–PDEAEMA (Ag–P) dispersion were prepared by varying the molar ratios (1:6, 1:12 and 1:24) of PDEAEMA to AgNO3, as shown in Table 1. For characterization and storage, AgNPs hybrids were transferred from methanol to aqueous environment by bubbling CO2. And three dark brown dispersion were finally obtained (insert, Fig. 1), indicative of the formation of AgNPs.51
Samplea | [PDEAEMA]:[AgNO3]b | % PDEAEMA/% Agc | Average diameter of Ag–NPsd, nm |
---|---|---|---|
a Ag–P refers to PDEAEMA polymer-stabilized silver nanoparticles.b Molar ratios of PDEAEMA to AgNO3.c Weight percentage, determined by TGA.d Estimated from TEM images. | |||
Ag–P1 | 1:6 | 90/10 | 8.51 ± 2.8 |
Ag–P2 | 1:12 | 84/16 | 10.06 ± 3.7 |
Ag–P3 | 1:24 | 71/29 | 14.16 ± 6.6 |
To characterize the formation of AgNPs hybrids, UV-vis spectroscopy which is known for the sensitivity to the size, size distribution and morphology of metal nanoparticles51,52 was employed. As shown in Fig. 1, single absorption peak is found in the region of 320–600 nm, resulting from intense surface plasmon resonances (SPR) of the obtained AgNPs.53 It is noteworthy that the λmax gradually increases with increasing the silver contents of Ag-polymer dispersion. Usually, λmax of AgNPs are biased to shift to longer wavelengths with increasing nanoparticle size,52 suggesting that the AgNPs size increases slightly upon increasing the silver contents. This may arise from the increased collision frequency due to the formation of more Ag atoms.46,54 Furthermore, the full width at half-maximum (FWHM) could be calculated from the UV-vis spectra, since FWHM is useful to evaluate the polydispersity of AgNPs.46,56 The FWHM value of Ag–P1 is 105 nm, which are similar to or slightly smaller than those previously reported for AgNPs.46,52,55 Correlating with symmetric absorption peaks, this implies that the size of Ag–P1 is uniform.56 Compared with Ag–P1, the FWHM for Ag–P2 (118 nm), Ag–P3 (124 nm) become wider, suggesting that the polydispersity of the hybrids increased with increasing the silver ratio.
In order to get more direct information on the size, size distribution and morphology of AgNPs, TEM observations were performed. Fig. 2 shows TEM images and size distribution histograms of three AgNPs. One can find that the AgNPs hybrids display good dispersion and a spherical shape. With total 1000 particles counted by an image analysis software (Nano Measure) on number distribution, we found Ag–P1 have an average size of 8.51 ± 2.8 nm with a narrow distribution. With increasing the silver ratio, the size and size distribution become bigger. The sizes of the other two AgNPs are 10.06 ± 3.7 nm and 14.16 ± 6.6 nm, respectively. The TEM results are in good agreement with those of the UV-vis spectra analysis. Thus the size and size distribution of as obtained AgNPs can be easily modulated by varying the molar ratio of polymers to AgNO3. In addition, XRD was carried out to confirm the structure of AgNPs. As illustrated in Fig. S3,† their XRD pattern of AgNPs shows characteristic diffraction peaks for metallic silver [111], [200], [220] and [311] facets, indicative of the formation of pure Ag.46,55,57,58
As stated above, the AgNPs were clearly observed by TEM. However, the grafted PDEAEMA were not observed under TEM observation, which may attribute to the polymer with lower atomic mass.55 To confirm that PDEAEMA was grafted onto the surface of AgNPs, FT-IR spectroscopy was employed. As given in Fig. S4,† The IR spectra of the nanoparticles and the PDEAEMA are similar to one another, indicating that the polymer molecules have indeed grafted onto AgNPs. However, a remarkable difference in the peak intensity is found between the peaks in polymer and the hybrids. Those peaks correspond to the stretching mode of CS (1062 cm−1) and –CH2–S– (725 cm−1).49 After reaction, CS was disappeared because the trithioester group was reduced to thio group. The decreased intensity for –CH2–S– is believed to be that the thiol end group of the polymer on the nanoparticle form a relatively close packed thiol layer and molecular motion is constrained,56 which suggesting that polymer attached to AgNPs surface through a chemical bond between S ions and Ag atoms.
To determine the relative amount of PDEAEMA on AgNPs, thermal gravimetric analysis (TGA) measurement was carried out. From the TGA curve given in Fig. 3, the weight percentage of PDEAEMA in the Ag–P1, Ag–P2, Ag–P3 hybrids were ca. 90 wt%, 84 wt%, 71 wt%, respectively, from which we could calculate that one AgNPs was wrapped by roughly 2000 polymer chains (see ESI†). To visualize the polymer on nanoparticles, AgNPs samples treated with negative staining technique was used for TEM observation, which provides reverse-contrast negative electron optical images for the unstained component.55 One can find that black AgNPs dot surrounded by the brighter polymer part, showing typical cocoon-like morphology (inset, Fig. 2). The thickness of observed PDEAEMA layer is about 6–10 nm, indicating that the polymers were attached to AgNPs. Compared with Ag–P2 and Ag–P3, Ag–P1 has smaller size and narrower size distribution. Therefore, in the following experiments, we will mainly focus on Ag–P1.
In addition, the stability of AgNPs in aqueous environment is important for their application.55,56 To detect the stability of the PDEAEMA-protected AgNPs in water, we measured the absorption spectra of one of the AgNPs hybrids systems (Ag–P1) with the same concentration at different times. As shown from Fig. S5,† there is no obvious difference in the shape, position, and symmetry of the absorption peak during 12 months, indicative of the long-term stability of the hybrid.
Fig. 4 Hydrophobic–hydrophilic transition of Ag–P1 hybrids (0.08 mg mL−1, 25 °C) in DCM/water (1:1, v/v) monitored by the UV-vis spectrum: (a) treated with CO2 and (b) bubbling N2. |
During this process, UV-vis spectroscopy was used to investigate the dispersion state in the mixed solvent. As the hybrids was treated with CO2, the upper aqueous solution exhibited SPR at 408 nm (Fig. 4a), indicating that Ag–P1 hybrids were dispersed in the water phase. In contrast, the lower DCM solution shows no obvious signals in the range of 250–800 nm, suggesting no Ag–P1 presented in lower organic layer. When N2 was bubbled into the biphasic solution, the UV-vis spectra of the two phases was reversed, that is, the lower DCM phase exhibits strong SPR peak (Fig. 4b), while no signals appeared in the upper aqueous solution. These UV-vis spectra clearly show that Ag–P1 experiences a hydrophobic–hydrophilic transition. Based on these macroscopic results, it is noteworthy that Ag–P1 can switch between aqueous media and organic solvents, which is convenient for separation/collection.
To further reveal the dispersion state of the hybrids, UV-vis spectroscopy, was employed to monitor the variation of absorbance of hybrid suspension after bubbling and removing CO2, respectively. As shown in Fig. 5a, a strong SPR absorbance was observed after bubbling CO2. With bubbling N2, on the other hand, the absorbance gradually decreased, and concomitantly the SPR peak showed blue shift (from 408 nm to 425 nm), indicative of aggregation and precipitation.52,56 In addition, the FWHM values of the spectra increase with time of bubbling N2 (Fig. 5c), implying the formation of AgNPs aggregates with larger size and broad size distribution. In order to get more direct information on the aggregation state of AgNPs hybrids in water, TEM were performed. An obvious aggregation state of the AgNPs hybrids can be observed (Fig. S6†), which is in good agreement with the abovementioned results. When the dispersion was treated with CO2 again, the SPR peak and its FWHM reinstated (Fig. 5b and c), indicating that AgNPs hybrids were re-dispersed again.
To elucidate the dispersed/aggregated transition, electrical conductivity measurements were performed to monitor the change of conductivity for the suspension when cyclically bubbling CO2 and N2 (Fig. 6a). When CO2 was introduced into dispersion, the conductivity sharply rises from about ca. 5.8 to ∼130 μS cm−1, with a net enhancement of about 124 μS cm−1, which must be due to the tertiary amine groups in PDEAEMA reacting with CO2 in water to form charged ammonium bicarbonate.37–42 When CO2 was displaced by N2, the conductivity recovered to its original value, and this reversible change in conductivity could be repeated several times, which amply demonstrate that the response of the suspension to CO2 was fully reversible and reproducible.34
To further confirm that ionization happened on the cocoon of the hybrids, zeta potential of the silver colloidal solution has been measured. The zeta potential for the particle treated with CO2 reached +60.2 mV, as exhibited in Fig. 6b, supporting the formation of positive ammonium ions of the surface coated polymers. After removing CO2 by purging N2, the zeta potential decreases with increasing in the time of purging N2, and finally reduced to +0.66 mV (Fig. 6b), suggesting that positive ammonium ions of the polymer cocoon were mostly deprotonated due to the deportation of the CO2.
Scheme 2 Schematic illustration of the AgNPs hybrids response to the stimulus of CO2 (a), and switch and tune the catalytic activity for reduction of 4-nitrophenol (b). |
The reversible dispersion/aggregation states of AgNPs hybrids controlled by CO2 in water could also be understood based on the CO2-responsive behaviour of PDEAEMA. As shown in Scheme 2a, the tertiary amine groups of PDEAEMA coated on AgNPs were protonated when reacted with CO2 in water, leading an extended conformation of the polymer. Thus the interchain electrostatic repulsion and the steric hindrance among the AgNPs protected by the charged PDEAEMA should allow the long-term dispersing in water. After the removal of CO2, the electrostatic repulsion of PDEAEMA disappears owing to an opposite deprotonation effect,36,37 thus diminishing the polymer–polymer electrostatic repulsions and increasing the interaction among polymer chains,34 resulting in larger particles, and precipitate from aqueous solution.
Fig. S7a† shows the time-dependent UV spectra for the reduction of 4-nitrophenol in presence of CO2-treated AgNPs hybrids. One can find that the peak height at 400 nm exhibit a slight decreases within 40 min (Fig. S7a†), indicating a very low conversion of 4-nitrophenol. Based on the absorption at 400 nm values, linear correlation between ln(C/C0) (C is the concentration at a certain reaction time and C0 is the initial concentration of 4-nitrophenolate ions) versus reaction time was obtained (Fig. S7a†), indicating that such a catalytic reduction follows a pseudo-first-order law. The apparent reaction rate constant (kapp) was calculated from linear and is only 1.15 × 10−5 s−1. This result confronts earlier observations that CO2-bubbled AuNPs protected with PDEAEMA still exhibits a high catalytic activity for 4-nitrophenol.41,42 This may arise from that the density of PDEAEMA grafted on AgNPs hybrids, ca. 2000 polymer chains in one particle as aforedescribed, is higher than that of reported ones, though the previous papers did not provide this data.41,42 When CO2-treated AgNPs hybrids was introduce into the solution, the charged ammonium bicarbonates would be deprotonated because the basic NaBH4 reacted with carbonic acid formed by CO2 and water (i.e., NaBH4 + H+ + 3H2O = Na+ + 4H2↑ + H3BO3). Thus PDEAEMA cocoon became hydrophobic and wrapped AgNPs tightly, which inhibited access to the catalytic sites of AgNPs (Scheme 2b).
To open the access for 4-nitrophenol, we tried to bubble CO2 into the mixing solution. Firstly, we wonder whether 4-nitrophenol could be reduced by CO2. CO2 was bubbled into the mixing solution of 4-nitrophenol and NaBH4 (without AgNPs hybrids). After bubbling CO2 for 50 min, the peak height at 400 nm has no change (Fig. S8, ESI†), suggesting that 4-nitrophenol could not be reduced by CO2 in absent of AgNPs hybrids. In contrast, CO2 was purged into the mixture in presence of Ag–P1 hybrids. When the flow rate of CO2 is 10 mL min−1, it was found that the absorption at 400 nm decreased slowly within the first 18 min, and then dropped fast to nearly zero (Fig. S7b†). Meanwhile, the peak of 4-aminophenol at 300 nm was observed,42,55 implying that 4-nitrophenol was reduced to 4-aminophenol. It clearly shows an induction time before the conversion of reactants into products takes place (Fig. 7a and S7b†). After deducting the induction time (ti), the kapp is 1.57 × 10−3 s−1 and bigger than that of without bubbling CO2. This result suggests that access to the encapsulated AgNPs was opened, since polymer chains contacted with CO2 and extended in solution. When the flow rate increased, the induction time decreased and disappeared at 25 mL min−1 (Fig. 7a). And concomitantly the kapp increased with the flow rate, and reach to ca. 3.6 × 10−3 s−1 at 50 mL min−1 (Fig. S7h†), which is close to gold nanoparticle reaction rate constant.42 The kapp as function of CO2 flow rate was presented in Fig. 7b. Two linear regimes in the kapp curve are found: a monotonous linear increase in kapp is evidenced while CO2 flow within 30 mL min−1, after which it is slight change.
To elucidate this variation, pH value of the solution under bubbling CO2 was tested, since the degree of protonation increases with decreasing of pH.37 As exhibited in Fig. S9,† the pH decreased with bubbling CO2 and maintained at ca. 7 for catalysis. Note that if the pH was less than 6.7, the peak at 317 nm ascribed to 4-nitrophenol would appear, which would affect the catalytic reaction.60 During the reaction, additional NaBH4 should be added to compensate that of consumed by CO2 and keep pH stable. Besides, it clearly shows that the time for decreasing pH to ca. 7 increases with low flow rate, which may cause the induction time. As there was no obvious difference on pH for catalysis at different flow rate, the hybrid should show a similar protonation and has similar kapp. A possible explanation for increasing kapp is that the frequency for PDEAEMA cocoon contacting with CO2 increased under the high CO2 flow rate, which is favourable to opening the access to AgNPs. Such preliminary findings, particularly the linearity found in the flow rate below 30 mL min−1, imply that the catalytic activity of Ag–P1 hybrids can be switched and monotonously tuned by varying the flow rate of CO2 purged into the reaction system.
Footnote |
† Electronic supplementary information (ESI) available: 1H NMR and GPC of polymers, IR spectra, XRD data and TEM images of hybrids, the data of catalysis and pH variation during the catalytic process and some calculations. See DOI: 10.1039/c7ra09233d |
This journal is © The Royal Society of Chemistry 2017 |