Novel Smart Microreactors Equipped with Responsive Catalytic

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Novel Smart Microreactors Equipped with Responsive Catalytic Nanoparticles on Microchannels Lei Zhang, Zhuang Liu, Lu-Yue Liu, Xiao-Jie Ju, Wei Wang, Rui Xie, and Liang-Yin Chu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09939 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 9, 2017

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ACS Applied Materials & Interfaces

Novel Smart Microreactors Equipped with Responsive Catalytic Nanoparticles on Microchannels Lei Zhang,† Zhuang Liu,†,‡,* Lu-Yue Liu,† Xiao-Jie Ju,†,‡ Wei Wang,†,‡ Rui Xie,†,‡ Liang-Yin Chu†,‡,§,*

† School of Chemical Engineering, Sichuan University, Chengdu, Sichuan 610065, P. R. China. ‡ State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, Sichuan 610065, P. R. China. § Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing, Jiangsu 211816, P. R. China.

1 Environment ACS Paragon Plus


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ABSTRACT Nowadays efficient and reliable control of highly exothermic reactions to effectively prevent overheating or even explosion still remains a challenging task, although newly-developed microreactor technology has shown a promising way. Here we report a novel smart microreactor system equipped with responsive catalytic nanoparticles on microchannels for self-regulated control over highly exothermic reactions by responding to the reaction-generated heat. Based on shrinking/swelling behaviors of polymeric networks in the responsive catalytic nanoparticles, the smart microreactor could respond to the change of reaction temperature to tune the catalysis activity of catalytic particles in a thermo-feedback way. As a breakthrough result, highly exothermic reactions carried out in such a microreactor can be well controlled in a self-regulation way without any manual assistance, ensuring the safety of the reaction efficiently. Such smart responsive catalytic systems are highly potential and attractive as new generation of efficient tools that featured with self-regulation property for highly exothermic catalytic reactions.

KEYWORDS Microreactors; Responsive catalytic nanoparticles; Chemical self-regulation; Self-adjustable kinetics; Phase transitions


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INTRODUCTION In the past few years, continuous-flow microreactors with channel dimensions typically between tens and hundreds of micrometers1-3 have found widespread applications in hazardous reactions, nano-syntheses, pharmaceutical chemistry, etc.4-6 Such microreactors with high surface-tovolume ratio present exceptionally fast mass and heat transfer6-8 and provide sufficient surface area for on-wall immobilization of catalysts.8,9 Generally, for microreactors with catalysts, effective control of chemical reaction rate is crucial for the reactions3,10 with desired conversion,11,12 purity,13 selectivity14 and safety.7 Usually, the chemical reaction rate in microreactors can be regulated by adjusting certain parameters like concentration,15 pressure,16 temperature17 and flux.9 However, all these approaches are in passive ways relying on manual operations, i.e. the current microreactors cannot make an initiative adjustment for reactions responding to the change in environments. Besides, the on-wall fixations of catalysts in microreactors are usually achieved via chemical or physical methods.18-24 The current chemical methods enable effective and stable immobilization of catalysts on the inner surface of microchannels, but usually require several-step troublesome chemical modifications.18-22 The current physical methods for immobilizing catalysts on microchannels are almost based on adsorptions via Van der Waals' force or electrostatic interaction, which are easy processing but usually result in unstable immobilization of catalysts.23,24 So, fabrication of smart microreactors with self-regulation property that can adjust the chemical reaction rate in response to environmental stimuli by easily but stably immobilizing catalysts on microchannels still remains a challenge. Here we report a novel smart microreactor with responsive catalytic nanoparticles easily and stably immobilized on microchannels. The demonstrated smart microreactor can initiatively



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adjust the chemical reaction rate responding to the change in temperature. The immobilization of the responsive catalytic nanoparticles based on the adhesion and reduction properties of dopamine (DA), takes the advantages of both chemical stability and physical simplification. DA is a biomolecule containing catechol and amine functional groups, which can spontaneously polymerize into polydopamine (PDA) in weak alkaline conditions.25 The resultant PDA with chemical versatility enables strong adhesion to most of organic and inorganic surfaces, and can be used as a platform for further reactions.26,27 For example, certain catalytic metal nanoparticles, such as silver nanoparticles (Ag NPs), can be formed in situ by reduction and strongly located on the underlying surface of PDA layer, due to the reductive capability of catechol moieties of PDA.28 Poly(N-isopropylacrylamide) (PNIPAM) is a well-known thermo-responsive polymer, which can undergo reversible volume change near the volume phase transition temperature (VPTT, ~32 oC).29-31 Based on the thermo-responsive swelling/shrinking behaviors, PNIPAMbased hydrogels have been designed for various applications such as smart microvalves,32 actuators,33 and on/off switches for chemical reactions.34 In this work, the responsive catalytic nanoparticles are designed with PNIPAM nanogels (PNGs) as thermo-responsive carriers, loading Ag NPs catalysts formed in situ with assistance of a PDA surface layer. The PNGs-based responsive catalytic nanoparticles are simply fabricated by two steps at room temperature (25 oC) (Figure 1a-c). Firstly, PNGs (Figure 1a) are coated with PDA surface layer to form PNGs@PDA nanogels (Figure 1b) in DA solution (pH=8.5). After this process, the surface layer of PNGs@PDA nanogels is composed of the interpenetrated networks of PDA and PNIPAM. Then, the highly reductive catechol moieties of PDA (Figure 1b’) enable the seeded growth of Ag NPs (Figure 1c’) to form PNGs@PDA/Ag responsive catalytic nanoparticles (Figure 1c). The prepared PNGs@PDA/Ag nanoparticles are immobilized on the microchannel surface with


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assistance of the PDA adhesive layer to fabricate a smart microreactor (Figure 1d,e). The reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) with sodium borohydride (NaBH4) as reductant and the highly exothermic reaction of catalytic decomposition of hydrogen peroxide (H2O2) are selected as model reactions to investigate the temperature-responsive self-regulation performances of the responsive catalytic nanoparticles of PNGs@PDA/Ag and smart microreactors. The PNGs@PDA/Ag nanoparticles like “living” catalysts, is hopeful to adjust their activity in response to the change in temperature based on the swelling/shrinking behaviors of PNIPAM networks, which can affect the mass-transfer of the reactants reaching to the catalyst surface. As the schematic illustration of this catalyzing process shown (Figure 1f,g), when the temperature (T) is lower than the VPTT, the PNIPAM are in hydrophilic and swollen state, the reactants can easily diffuse into the networks of PNGs@PDA/Ag nanoparticles. As a result, the reactant molecule can easily contact with Ag NPs that enclosed by interpenetrated networks of PDA and PNIPAM, resulting in a high reaction rate (Figure 1f). On the contrary, when T>VPTT, the surface layer of PNGs@PDA/Ag nanoparticles becomes denser due to the shrunken and hydrophobic state of PNIPAM networks. Thus, although the high temperature enables increase in the reaction rate, the mass-transfer of reactant reaching to the catalyst surface is slowed down, causing a low reaction rate (Figure 1g).

EXPERIMENTAL SECTION Preparation of PNIPAM Nanogels (PNGs) PNGs were synthesized by precipitation polymerization.31 In a typical procedure, 2.26 g monomer

N-isopropylacrylamide

(NIPAM)

and

0.154

g

crosslinker

N,N´-

methylenebisacrylamide (BIS), for which the molar ratio of NIPAM to BIS was 20:1, were



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dissolved in 300 ml deionized (DI) water. The solution was then maintained at 70 oC using a thermo-stated water bath and bubbled with nitrogen for 20 min. Next, 0.08 g potassium persulfate (KPS), dissolved in 10 mL water, was added to the vessel to initiate polymerization. The reaction proceeded under stirring for 4 h at 70 oC. The prepared particles were washed by repeated centrifugations. To synthetize PNGs with small size, 1.13 g NIPAM, 0.0385 g BIS and 0.04 g KPS were used, and the reaction time was reduced to 15 min. Immediately after reaction, the round-bottom flask containing solution was placed in an ice bath to terminate the nucleation of PNGs. Then, the prepared PNGs were purified by dialysis.

Preparation of PDA-coated PNGs with Ag Nanoparticles (PNGs@PDA/Ag) PNGs@PDA were prepared by dispersing 10 mg PNGs in 2 mL of dopamine hydrochloride solution in Tris buffer (15 mM, pH=8.5). After stirring at room temperature (25 oC) for 2 h, PNGs@PDA nanoparticles were obtained by centrifugations for four times. The PNGs@PDA nanoparticles obtained with dopamine concentrations of 0.5, 1.0, and 2.0 mg/mL were referred as PNGs@PDA-1, PNGs@PDA-2 and PNGs@PDA-3, respectively. The growth of silver nanoparticles (Ag NPs) in the PDA surface layer of PNGs@PDA was performed by dispersing 10 mg PNGs@PDA-3 in 5 mL silver nitrate (AgNO3) solution with AgNO3 concentration of 20 or 15 mg/mL. After stirring at room temperature (25 oC) for 4 h, the resulted solution containing PNGs@PDA/Ag nanoparticles immobilized with Ag NPs was purified by centrifugations for four times. The resulted PNGs@PDA/Ag nanoparticles were prepared with different AgNO3 concentrations to obtain Ag NPs with different sizes. The size distribution of Ag NPs was determined by measuring the size of at least 100 metallic nanoparticles on PNGs@PDA/Ag nanoparticles using software Digital Micrograph from TEM micrographs.


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Preparation of Smart Microreactors with PNGs@PDA/Ag Nanoparticles on Microchannels The serpentine glass microchannels were fabricated by chemical wet etching according to a previously published work.35 Briefly, the patterns of microchannels were designed using software AutoCAD and then transferred onto the glass substrate with dry film photoresists as pattern transfer masks for wet etching. The microchannels with central width of 400 µm and depth of 65 µm were formed by etching in buffered oxide etchant (BOE) solution at 30 oC for 15 min. The BOE solution was made of ammonium fluoride (NH4F) solution (40 wt%), hydrogen fluoride (HF) solution (49 wt%) and hydrogen chloride (HCl) solution (37.5 wt %). The volume ratio of NH4F, HF, HCl solution was 7:1:1.6. Furthermore, the glass microchannel surface was coated with PDA layer before immobilizing PNGs@PDA/Ag nanoparticles. Briefly, the glass microchannels were coated with PDA by immersing in a 2 mg/mL dopamine solution (pH 8.5) for 2 h and then bonded to a microreactor using UV-curable glue. Subsequently, the smart microreactor was fabricated by introducing the solution containing PNGs@PDA/Ag (5 mg/mL) into the microchannels using a syringe pump (PHD 2000, Harvard Apparatus) at 100 µL/h for 2 h. Then, the microchannels were washed by pure water for 1 h with pumping speed of 100 µL/h. The capillary with inner diameter of 1000 µm was coated with PNGs@PDA/Ag nanoparticles using the same method. Because PDA enables strong adhesion to most of organic and inorganic surfaces,25-28 this PDA-assisted strategy can also be used to modify the microchannels of PDMS, stainless steel and other substrates with responsive catalytic nanoparticles. Comparing with the fabrication of PNIPAM/Ag composite nanoparticles by using NaBH4 as a reductant to immobilize Ag NPs,36,37 this polydopamine-assisted strategy is environment-friendly and takes the advantages of both chemical stability and physical simplification,25,27,28

Preparation of Control Microreactors with Blank Ag NPs Immobilized on Microchannels



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The same serpentine-shaped glass microchannel was used to fabricate the control microreactor. Briefly, the glass microchannel was firstly coated with PDA by simply immersing in DA solution (2 mg/mL, pH=8.5) with stirring for 12 h at room temperature (25 oC). Then, the PDA-coated microchannel was immersed in AgNO3 solution with concentration of 3 mg/mL under stirring for 5 h at room temperature (25 oC) to generate Ag NPs. Subsequently, the Ag NPs-coated microchannel was UV-bonded to the microreactor using UV-curable glue.

Similarly, the

capillary with inner diameter of 1000 µm was immobilized with blank Ag NPs on channel surface.

Catalytic Reduction of 4-Nitrophenol (4-NP) by PNGs@PDA/Ag 4-NP solution (30 µL, 5 mM) was mixed with aqueous solution of NaBH4 (3 mL, 25 mM). The NaBH4 solution was freshly prepared and used within 1 h at each time. PNGs@PDA/Ag solution (80 µL, 2.5 mg/mL) was then added to the mixture and shaken vigorously for mixing. UV spectra of the mixture were recorded with a UV-vis spectrometer every 1 min immediately after the addition of PNGs@PDA/Ag nanoparticles. The kinetic study of the reaction was performed by measuring the time-dependent absorption (A) at 400 nm at different temperatures ranging from 10 to 60 oC. At each temperature, the 4-NP reaction catalyzed by PNGs@PDA/Ag nanoparticles was parallelly carried out for three times to obtain the statistic errors. Because the ratio of the concentration Ct of the 4-NP at time t to its value C0 at t=0, defined as Ct/C0, could be given directly by the ratio of the absorbance At/A0.36,37 The ln(Ct/C0) values under the temperatures ranging from 10 to 60 oC were calculated and shown against time t. For the control experiments, a mixed nanoparticle solution (80 µL) containing PNGs (2.5 mg/mL) and unstuck Ag NPs (diameter of 30 nm, 0.5 mg/mL), referred as PNGs+Ag NPs, was added into the same 4NP reactant solution for catalytic reaction. The rate constant kapp values at different reaction


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temperatures were obtained from the slopes of ln(Ct/C0) to t lines. The reversibility and stability of the self-regulation performance of PNGs@PDA/Ag nanoparticles were studied by switching the temperature between 35 oC and 30 oC for 5 cycles. After each use for catalysis, the PNGs@PDA/Ag nanoparticles were rapidly separated from the solution by high-speed centrifugation and then used for the next cycle of catalysis. For a better separation of nanoparticles from the solution and avoiding the influence of particle loss on the catalysis reaction, the concentration of PNGs@PDA/Ag nanoparticles in reactant solution is increased to 1.25 mg/mL. Ten-cycle uses of PNGs@PDA/Ag nanoparticles were carried out with the same particle concentration of 1.25 mg/mL at 25 oC for 10 min. After each cycle, the nanoparticles were separated from solution by centrifugation and then reused in the next cycle. The conversion of 4-NP to 4-AP was calculated as 1-At/A0 after each use.

Catalytic Reduction of 4-NP in the Smart Microreactor A mixture of 4-NP (30 µL, 5mM) and NaBH4 (3 ml, 25 mM) was injected into the microchannel using a syringe pump (PHD 2000, Harvard Apparatus) with total flow rates of 500, 900 and 1800 µL/h, and the corresponding residence time in the microreactor is 6, 3.3 and 1.67 min, respectively. The reaction temperature in microreactor was controlled at 30 and 35 oC using a thermostatic stage system (TS62, Instec). The sample solutions used for UV-vis spectra were collected after flowing out of the microreactor for 30 min. At each temperature, the 4-NP reaction in the smart microreactor was carried out for three times to obtain the statistic errors. The kinetic study of the reaction was performed by measuring the time-dependent absorption of 4-NP at 400 nm via UV spectra. The reversible self-regulation performance of the smart microreactor was studied by continuously introducing reactant solution into the microreactor



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under the flow rate of 500 µL/h by switching the temperature between 35 oC and 30 oC for 5 cycles.

Catalytic Decomposition of H2O2 in Batch Reactor and Microchannel Reactor For the batch reaction, the reaction compartments were modified from a cell culture plate. 1 mL solution of commercial Ag NPs with a diameter of 30 nm was added into the left reaction compartment with a concentration of 0.14 mg/mL, and another 1 mL solution of prepared PNGs@PDA/Ag (25 mg/mL) was added into the right reaction compartment with the same Ag concentration of 0.14 mg/mL. The Ag content on PNGs@PDA/Ag nanocomposites is about 0.56 wt%, which is calculated from ICP-AES result. Then, 3 ml H2O2 (30 wt%) was added into each reaction compartment at the same time. For the microchannel reaction, two capillaries equipped with blank Ag NPs and PNGs@PDA/Ag nanoparticles respectively on the channel surfaces were fixed on a support with the same height. Then, H2O2 (30 wt%) solution was introduced into the two capillaries using a syringe pump (PHD 2000, Harvard Apparatus) with flow rates of 100 and 2000 µL/h.

Characterization of Components and Morphology The hydrodynamic radiuses of nanoparticles were measured by dynamic light scattering (DLS, Zetasizer Nano-ZEN3690, Malvern) at different temperatures with scattering angle of 90°. The UV spectra were recorded in the range 200-900 nm using a UV-vis spectrophotometer (UV1700, Shimadzu) equipped with a temperature-controlled cell (TCC-240A, Shimadzu). Transmission electron microscopy (TEM) was carried out on a Tecnai G2 F20 STWIN (FEI) transmission electron microscope operated at an acceleration voltage of 200 kV. All of the samples of PNGs, PNGs@PDA and PNGs@PDA/Ag are prepared with ultrasonic dispersion


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before TEM characterization. An X-ray Photoelectron Spectroscopy (XPS) (KRATOS, XSAM800) instrument using Al Kα (1486.6 eV) as radiation source was used to analyze the composition of the particles. The take-off angle of the photoelectron was set at 70° for XPS measurements. X-ray diffraction (XRD) crystal structure identification was carried out using an X’Pert Pro MPD (Philips) X-ray diffract meter with Cu Ke photoelectron. The samples for XRD were supported on glass substrates. Atomic force microscopy (AFM) was carried out in tapping mode using a Nanoscope V controller on a multimode microscope (Bruker). Inductively coupled plasma atomic emission spectrometry (ICP-AES) (Spectro Arcos, Germany) was utilized to assay the loss of Ag element from PNGs@PDA/Ag nanoparticles after each cycle of use for catalysis. The temperature changes in batch reaction compartments and microchannels during the catalytic decompositions of H2O2 were measured and recorded by an infrared camera (E40, FLIR system).

RESULTS AND DISCUSSION Fabrication and Characterization of PNGs@PDA/Ag Responsive Catalytic Nanoparticles The fabrication of PNGs@PDA/Ag responsive catalytic nanoparticles is simple and easy. The PNGs with PDA surface layer are prepared in 0.5, 1.0 and 2.0 mg/mL DA solutions, and are respectively referred as PNGs@PDA-1, PNGs@PDA-2 and PNGs@PDA-3 (Figure S1). The transmission electron microscopy (TEM) images of PNGs and PNGs@PDA show that PDA surface layers are successfully formed on the PNGs@PDA nanogels (Figure 2a1-a4). The thickness of PDA layer becomes larger with increasing the DA concentration (Figure 2a2-a4, and Figure S2). The X-ray photoelectron spectroscopy (XPS) spectra show a 2% rise of nitrogen in the surface layer of PNGs@PDA compared with that of PNGs (Figure S3), which also confirm



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the successful coating of PDA surface layer. PNGs@PDA-3 nanogels are selected for growing Ag NPs in situ in the PDA layer. The fabricated PNGs@PDA/Ag nanoparticles possess uniform spherical shape of Ag NPs with size of ~30 nm in the PDA layer, which are shown in the TEM images (Figure 2b1, b2) and the atomic force microscopy (AFM) image (Figure S4). The highresolution transmission electron microscopy (HRTEM) image (Figure 2b3) and the X-ray diffraction (XRD) analysis of PNGs@PDA/Ag (Figure 2b4) reveal the [111] lattice plane of Ag NPs crystalline. Moreover, the high-resolution XPS of Ag3d shows double peaks at 368.3 eV and 374.3 eV, which can also confirm the formation of Ag0 on PNGs@PDA/Ag (Figure S3). To obtain Ag NPs with different sizes in the PDA layer, variation of AgNO3 concentration can be utilized for preparation of the responsive catalytic nanoparticles. With higher AgNO3 concentration, smaller Ag NPs are prepared in the PDA layer (Figure S5a-c), because the increase in Ag+ concentration causes a growing number on the nucleation of Ag NPs that results in a decrease of particle size.38,39 The characteristic plasmon absorption of Ag NPs is confirmed at 435 nm by the UV-vis spectra of PNGs@PDA/Ag at 20 oC (Figure 2c and Figure S5d),40 which is not present in those of PNGs and PNGs@PDA-3. The synthesized PDA layer makes the diameter of nanogels larger, but does not influence the VPTT value (Figure 2d). Usually, introduction of non-responsive materials such as nanometal particles or polymers into PNIPAM gels affects the thermo-responsive performance. However, in this study, the PDA chains are interpenetrated with PNIPAM networks only in the thin surface payer of PNGs@PDA/Ag nanoparticles, and the Ag nanoparticles are only enclosed by the interpenetrated networks of PDA and PNIPAM in the thin surface layer. That is, the main body of the PNGs is not affected by the loading of Ag NPs via this PDA-assisted strategy. Therefore, after loading with Ag NPs, PNGs@PDA/Ag nanoparticles still exhibit excellent thermo-responsive volume phase transition


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at 32 oC (Figure 2d and Figure S5e), as well as satisfying reversible shrinking/swelling behavior from 20 oC to 50 oC (Figure S6). Meanwhile, both the hydrophilic groups in PDA polymer and the high surface area of Ag NPs in the PNGs@PDA/Ag nanogels can form more hydrogen bonds with water molecules and provide pathways in PNIPAM networks for water transport upon heating and cooling; thus, the equilibrium swelling ratio of the PNGs@PDA/Ag nanogels is a little larger than that of pure PNGs (Figure 2d).41,42 The UV-vis spectra of PNGs@PDA/Ag nanoparticles and blank Ag NPs with a diameter of 30 nm at different temperatures ranging from 20 oC to 50 oC are shown in Figure 3a and Figure S7, respectively. For the PNGs@PDA/Ag nanoparticles, with increasing the temperature across the VPTT, the absorbance at 400 nm increases significantly (Figure 3b), which indicates the effect of the thermo-responsive volume change on the status of PNGs@PDA/Ag. While the surface plasmon resonance (SPR) position of Ag NPs on PNGs@PDA/Ag decreases strongly due to the abrupt shrinkage of PNIPAM networks, which results in a blue shift from 435 nm to 421 nm for SPR positions of Ag NPs on PNGs@PDA/Ag upon the increase of temperature (Figure 3a). However, for the blank Ag NPs, no changes of both SPR positions and absorbance@400 nm are observed with increasing the temperature (Figure 3b and Figure S7). The results firmly confirm that the blue shift of Ag NPs on PNGs@PDA/Ag is caused by the shrinkage of PNIPAM networks. Generally, the optical properties of metal nanoparticles combining with PNIPAM polymers are very complicated.43 Both the red shift44-47 and the blue shift43,48,49 of their SPR positions during the collapse of PNIPAM networks have been reported. The SPR position of PNGs@PDA/Ag nanoparticles is depend on several factors, such as local refraction index around the metal and the interaction effects between the Ag NPs.50 The shrinkage of PNIPAM network possibly results in the increase of the local refractive index, which could cause a red shift of



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SPR.45-47 On the contrary, when the distance between the Ag NPs is decreased, the surface charge density of Ag NPs increases due to the decreased distance between the Ag NPs, which would promote a blue shift of the SPR because the increased frequency of the free-electron plasmon resonance of Ag NPs.37,51,52 Therefore, the shift of the SPR could be attributed to the competition effects of increased local refraction index and interaction between the Ag NPs. In our case, the interaction effects between the Ag NPs due to the shrinkage of the PNIPAM network are predominant, thus resulting in a blue shift.

Kinetics Study of the Catalytic Property of Responsive Catalytic Nanoparticles To investigate the catalytic properties of the responsive catalytic nanoparticles of PNGs@PDA/Ag, the reduction of 4-NP to 4-AP by NaBH4 is chosen as a model reaction in this work. The 4-NP solution is mixed with freshly prepared NaBH4 aqueous solution in the presence of catalytic nanoparticles. When catalyzed by PNGs@PDA/Ag at 30 oC, the color of 4-NP solution changes from originally bright yellow to transparent in 33 min (inserted picture in Figure 4a). The absorption peak of 4-NP at 400 nm decreases significantly, while that of 4-AP at 305 nm appears confirming the efficient conversion of 4-NP to 4-AP (Figure 4a). Furthermore, to perform the thermo-responsive catalysis properties of PNGs@PDA/Ag nanoparticels, the reduction of 4-NP to 4-AP using NaBH4 is catalyzed by PNGs@PDA/Ag at different temperatures ranging from 10 to 60 oC. The PNGs with unstuck Ag NPs (~30 nm) are utilized as control nanoparticles and referred as PNGs+Ag NPs. The reduction of 4-NP follows the first order kinetics since the concentration of NaBH4 greatly exceeds that of 4-NP.37,42,51-53 The fine linear relations between ln(Ct/C0) and time t of PNGs@PDA/Ag and PNGs+Ag NPs at different temperatures are shown in Figure 4b and Figure S8, respectively. The rate constant kapp values at different temperatures are obtained from the slopes of ln(Ct/C0)-t lines (Figure S9). Compared


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with the rate constant kapp data of the PNGs+Ag NPs, which are monotonically increased with temperature, those of PNGs@PDA/Ag obviously do not follow the classical Arrhenius-type with simple ascending tendency with increasing the temperature. The normal Arrhenius format is plotted for lnkapp versus T-1 (Figure 4c). Just as expected, the lnkapp data of the PNGs+Ag NPs present linear with T-1. The lnkapp variation tendency for PNGs@PDA/Ag nanoparticles includes three stages responding to the T-1 values (Figure 4c). First, when the temperature is lower than the VPTT (from 10 to 30 oC), the PNGs@PDA/Ag nanoparticles are in hydrophilic and swollen state. In this case, the mass-transfer of reactants is fast, so that the reactants could reach the Ag NPs easily, resulting in a simple increase of lnkapp with increasing temperature. Then, when the temperature is increased across the VPTT (from 30 to 40 oC), the PNGs@PDA/Ag nanoparticles turn into shrunken and hydrophobic state, resulting in a denser surface layer that prevents reactants from reaching the Ag NPs.37,44,53-55 As a result, the reaction rate dramatically decreases and reaches the minimum at 40 oC. Next, when the temperature increases further from 40 to 60 o

C, the PNGs@PDA/Ag nanoparticles do not shrink anymore and the density of surface layer

keeps unchanged. So, the increase of lnkapp with temperature becomes predominant and the reaction rate rises again. Interestingly, the slopes of Arrhenius curves for PNGs@PDA/Ag nanoparticles at temperatures regimes from 10 to 30 oC and from 40 to 60 oC are almost parallel, which indicates the same activation energies in the both temperature regimes. The thermoresponsive catalysis property of PNGs@PDA/Ag nanoparticles is reversible and reproducible by switching the temperature between 30 oC (below the VPTT) and 35 oC (above the VPTT) (Figure 4d). The difference between the rate constant kapp at 30 oC and 35 oC could be further enhanced by equipping smaller PNGs@PDA/Ag with higher Ag NPs coverage (Figure S10) due to the effective regulation of the transfer of reactant to active sites. As shown in Figure 4e, the



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PNGs@PDA/Ag nanoparticles still show high activity after ten-cycle uses at 25 oC. All the conversions are close to 98 % with reaction time of 10 min. The inductively coupled plasma atomic emission spectrometry (ICP-AES) data show that very little Ag element is detected in the centrifugal supernatant after each catalysis cycle (Figure S11a), which confirm the fine binding stability of Ag NPs on PNGs@PDA/Ag reduced by PDA. After ten cycles of catalysis, the PNGs@PDA/Ag nanoparticles still present an excellent thermo-responsive property with distinct SPR absorption of Ag NPs at 435 nm and the intact structure and morphology of PNGs@PDA/Ag (Figure S11b), which also confirm the high stability of the responsive catalytic nanoparticles.

Thermo-Responsive Catalytic Properties of the Smart Microreactor Recently, the chemo-mechano-chemical self-regulation for controlling the chemical reaction process has been reported,34 in which pH-responsive hydrogels of poly(acrylamide-co-acrylic acid) are incorporated into a microfluidic channel to realize the self-regulation for the chemical reaction responding to pH change. Here, our prepared responsive catalytic nanoparticles can be easily immobilized on microchannel surfaces to achieve smart microreactors for efficiently selfregulating the reaction rate in response to temperature change due to reaction heat. This is demonstrated by “gluing” PNGs@PDA/Ag nanoparticles on the serpentine glass microchannel of a microreactor (Figure 5a,b). For steady immobilization of PNGs@PDA/Ag nanoparticles, the glass microchannel has been coated with PDA layer before “gluing” the nanoparticles. AFM image shows that PNGs@PDA/Ag nanoparticles are uniformly immobilized on the channel surface through the adhesive PDA-coating (Figure 5c). The height profile from the AFM image shows the size of Ag NPs is ~30 nm (Figure 5d), which is in agreement with the result from TEM image. To test the adhesive strength of catalytic nanoparticles on the microchannels, a


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flow-shearing experiment has been carried out on the PNGs@PDA/Ag-immobilized surface of a capillary with an inner diameter of 400 µm. The PNGs@PDA/Ag-coated capillaries are rinsed by water with a flow rate of 5 mL/ h for 12 h, for which the boundary shear stress is about 0.2 N/m2. The SEM images of the inner surface of the PNGs@PDA/Ag-coated capillary after rinsing show that the PNGs@PDA/Ag are stably and uniformly immobilized on the microchannel surface (Figure S11c), which indicates that high adhesive strength is provided by the polydopamine. The chemical self-regulation property of the smart microreactor is investigated under continuous flow reactions at temperatures below the VPTT (30 oC) and above the VPTT (35 oC) respectively. A microreactor equipped only with blank Ag NPs on the microchannel is used as a control microreactor (Figure S12). In the control microreactor, reaction rate increases according to typical Arrhenius-type dependence with increasing temperature (Figure S13a). Therefore, according to the ln(Ct/C0) vs t lines at 30 and 35 oC (Figure 5e), the rate constant kapp in the control microreactor at 35 oC is higher than that at 30 oC (Figure 5f). While, the situation in the smart microreactor is significantly different (Figure S13b). The rate constant kapp in the smart microreactor at 30 oC is about twice higher than that at 35 oC (Figure 5f), i.e., the reaction rate at high temperature is much slower than that at low temperature. The slight difference of kapp between the catalytic nanoparticles in the microreactor and in the solution is resulted from the timely mass transfer rate in the microchannels that avoids the accumulation of reaction product. The reversibility and stability of the smart microreactor is further studied by continuously introducing reactant solution into the smart microreactor by switching the temperature between 35 oC and 30 oC for 5 cycles (Figure 5g). The smart microreactor shows a reversible selfregulation property responding to the temperature change. The results imply the excellent stability of the smart microreactor system. Such smart catalytic microreactors have highly



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potential application in the self-regulation of highly exothermic catalytic reactions to effectively prevent reactions from overheating. Namely, when the temperature of exothermic reaction increases above the VPTT, such smart catalytic systems can significantly slow down the reaction rate thus resulting in efficient reduce of reaction heat, which is crucial to highly explosive reactions. When the reaction temperature is decreased below the VPTT, the reaction rate increased again. As a result, the reaction rate of exothermic reactions can be self-regulatively controlled without any manual assistance, and thus the safety of reactions can be effectively ensured.

Self-regulation Performance of the Smart System for Highly Exothermic Reaction To demonstrate the self-regulation performances of smart microreactors equipped with PNGs@PDA/Ag nanoparticles, the catalytic decomposition of hydrogen peroxide (H2O2) is chosen as a model highly exothermic reaction and is carried out in both batch reactor (Figure 6a) and microchannel reactor with continous flow (Figure 6b). As shown in the scheme of setup of the batch reaction (Figure 6a), 3 ml H2O2 (30 wt%) is added into the two reaction compartments containing 1 mL solutions of blank Ag NPs (left) and PNGs@PDA/Ag (right) respectively, both with equaled silver concentration of 0.14 mg/mL. Catalyzed by blank Ag NPs, the decomposition of H2O2 is ultrafast. Concomitantly, vast amounts of oxygen is generated at once just after the adding of H2O2 into the compartment (Figure 6c, Movie S1). The temperature of the solution H2O2 decomposed by blank Ag NPs (left) is out of control in a short time (Figure 6d, Movie S1), which suddenly rises from 25.4 oC to 90.1 oC in 6.23 s with a fast heating-increased rate of 10.4 o

C/s (Figure 6g). Conceivably, the overheating and fast gas generation during the aggressive

decomposition of H2O2 with regular Ag NPs may cause a possible explosion hazard in a sealed batch reactor. Excitingly, the responsive PNGs@PDA/Ag nanoparticles show satisfactory self-


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regulation performance for the decomposition of H2O2 in batch reaction. The temperature of the solution in right compartment is well controlled to prevent the reaction from overheating. At the beginning, the temperature of solution with PNGs@PDA/Ag is lower than the VPTT of PNGs network, the PNGs@PDA/Ag nanoparticles are in hydrophilic and well dispersed state. As a result, the active sites on Ag surface are completely exposed to the reactant thus leading a fast generation of oxygen (Figure 6c) and rapid rising of temperature (Figure 6d and 6g). While the temperature is increased over the VPTT (Figure 6g), the PNGs@PDA/Ag nanoparticles are changed into hydrophobic and collapsed state, which causing the bulk aggregation of PNGs@PDA/Ag in solution (Figure 6c, Movie S1). So, the contact site between the catalysts and reactant is partly isolated. The reactant of H2O2 hardly gets access to the surface of Ag catalysts in PNGs@PDA/Ag nanoparticles. As a result, the reaction rate is obviously slowed down resulting in efficient reduction of reaction heat. The temperature of PNGs@PDA/Ag solution is increased from 25.2 oC to 59.2 oC within 8.63 s with a lower heating-increased rate of 3.9 oC/s, which is stably controlled between 40 oC and 45 oC during the long reaction time from 100 s to 1000 s (Figure 6g). The results demonstrate that the PNGs@PDA/Ag nanoparticles can control the reactivity by an internal parameter making a thermo-feedback regulation for the reactions, which can greatly improve the safety for the highly exothermic reactions. Although microreactors have been promoted for their excellent control of reaction heat, the heat in microreactors for the highly exothermic reaction may still be an issue. As a demonstration, the H2O2 is decomposed in microchannel (Figure 6b). Two capillaries with diameter of 1000 µm are respectively coated with blank Ag NPs and PNGs@PDA/Ag nanoparticles on internal surfaces of microchannels. With a lower flow rate of 100 µL/h, the temperature in the blank Ag NPscoated microchannel slightly increases from 25.0 oC to 31.7 oC within 12.47 s; while, the



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temperature in the PNGs@PDA/Ag-coated microchannel does not rise at all (Figure 6e and 6h, Movie S2). Although the volume change of PNGs@PDA/Ag is very significant around the VPTT (32 oC), the thermo-responsive volume change of PNGs@PDA/Ag is not a discontinuous change but a continuous one. That is, with increasing the temperature from 25 oC to 32 oC, the PNIPAM networks also shrink to some extent (Figures 2d and S5e). This thermo-induced shrinkage of the PNGs@PDA/Ag also causes the slowdown of the catalytic reaction. Thus, the thermo-feedback adjustment for the exothermic reaction can be still effective to some extent even through the temperature is lower than the VPTT. However, when the flow rate is increased to 2000 µL/h, the refreshing rate between the reactant and the catalysts is much increased due to the enhanced mass transfer in microchannels. It shows a sudden overheating in the blank Ag NPs-coated microchannel, where the temperature is rapidly increased from 25.7 oC to 55.4 oC within 6.53 s (Figure 6f and 6h, Movie S3). Thus, the overheating is still an inevitable issue for microchannel reactions especially when the flow rate is high and the microchannels are tightly packed in parallel integration. By employing the PNGs@PDA/Ag nanoparticles rather than the blank Ag NPs coated on the microchannel surface, the reaction in the microchannel presents a faster and sensitive self-regulation behavior. As shown in Figure 6f and 6h, the temperature in such microchannel is rapidly increased to 31.6 oC at the beginning time of 1.44 s with a heating rate of 4.51 oC/s. Then, the PNGs@PDA/Ag nanoparticles on the microchannel respond to the increased temperature and change into hydrophobic state. As a result, the reaction rate in the microchannel is obviously slowed down and the temperature in microchannel is well controlled, decreasing to 27.1 oC in a short responding time of 3.03 s with the assistance of enhanced heat and mass transfer in microchannels flow (Figure 6h). The results verify that the microchannel reactors coated with responsive catalytic nanoparticles of PNGs@PDA/Ag could efficiently


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respond to the change of temperature and make a thermo-feedback adjustment for the highly exothermic reaction independently without any manual assistance.

CONCLUSION In summary, a kind of responsive catalytic nanoparticles of PNGs@PDA/Ag and smart microreactors with PNGs@PDA/Ag nanoparticles equipped on the microchannels have been successfully developed based on thermo-responsive PNGs and versatile PDA coating. The demonstrated smart responsive catalytic systems exhibit efficient temperature-dependent regulation of the reaction rate depending on the thermo-responsive shrinking/swelling behaviors of PNIPAM networks inside the PNGs@PDA/Ag nanoparticles. As a breakthrough result, the metal-catalyst reactivity of the responsive catalytic systems can be self-regulatedly tuned down responding to the internally increased temperature during the heat-generated reaction process, thus reducing the reaction heat promptly. The proposed smart catalytic systems show excellent reaction stability. Compared with the passive controlling manner by adjusting the external conditions for avoiding run away of heat in reaction, the smart responsive systems proposed here can make a much faster and more reliable adjustment by directly changing the diffusion rate of reactant molecule to the surface of catalyst. By applying such smart systems in highly exothermic reactions, the overheating phenomenon of the reaction can be prevented in a selfregulation way in both batch reactor and continuous-flow of microreactor from the root, avoiding the accident effectively. Moreover, the VPTT of PNIPAM-based materials can be easily adjusted to a higher value by incorporating hydrophilic units (e.g., acrylamide (AAM)) or to a lower value by incorporating hydrophobic units (e.g., butyl methacrylate (BMA)), and the VPTT can be adjusted in a large range.56 The VPTT value can be linearly increased with increasing the molar



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ratio of hydrophilic monomer AAM in the PNIPAM-co-AAM polymer, while linearly decreased with increasing the molar ratio of hydrophobic monomer BMA in the PNIPAM-co-BMA polymer.56 Thus, the controllable temperature window of the smart responsive catalytic system could be adaptable for varied reactions. Functionalized with the thermo-responsive selfregulation of reaction rates, such smart responsive catalytic systems are highly promising to offer a unique way to perform catalytic reactions, particularly for ultrafast, highly exothermic reactions. Furthermore, feedback signals in the reaction environments, such as pH value, ionic and glucose concentration, can also be designed as reaction triggers in new generation of efficient smart catalytic systems by incorporating diverse functional polymers.

ASSOCIATED CONTENT Supporting Information The optical photographs of suspensions of PNGs, PNGs@PDA and PNGs@PDA/Ag; TEM images of PNGs and PNGs@PDA with different thicknesses of PDA layer; a large size AFM topography image of PNGs@PDA/Ag; the reversible shrinking/swelling behavior of PNGs@PDA/Ag nanoparticles; XPS spectra of PNGs, PNGs@PDA and PNGs@PDA/Ag; characterization of PNGs@PDA/Ag nanoparticles with size-controlled Ag NPs; UV-vis spectra of blank Ag NPs at temperatures ranging from 20 to 50 oC; influence of temperature on the reduction of 4-NP catalyzed by control nanoparticles of PNGs+Ag NPs; characterization of the as-prepared smaller PNGs@PDA/Ag nanoparticles with higher coverage of Ag NPs and the chemical self-regulation performance for 4-NP reduction; characterization of PNGs@PDA/Ag nanoparticles after cycle use; the apparent rate constant kapp of 4-NP reduction reaction catalyzed by PNGs@PDA/Ag at different temperatures; morphology and surface element characterization of the control microre-


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actor on-immobilized with Ag NPs; UV-vis spectra of 4-NP reduction in the smart microreactor and control microreactor operated at different temperatures. Movies showing chemical selfregulation for high exothermic reaction of decomposition of H2O2 in batch reactor, in microchannel with flow rate of 100 µL/h and in microchannel with flow rate of 2000 µL/h. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Address correspondence to [email protected] (Z.L.) or [email protected] (L.Y.C.).

Author Contributions L.-Y.C., Z.L. and L.Z. conceived and designed the study. L.Z. performed the experiments. All authors discussed the results and contributed to the data interpretation. L.-Y.C., Z.L., and L.Z. wrote the manuscript and all authors commented on the manuscript.

Funding Sources The National Natural Science Foundation of China (91434202, 21506127), the Program for Changjiang Scholars and Innovative Research Team in University (IRT15R48) and the State Key Laboratory of Polymer Materials Engineering (sklpme2014-1-01).

Notes The authors declare no competing financial interests.



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ACKNOWLEDGMENT The authors gratefully acknowledge support from the National Natural Science Foundation of China (91434202, 21506127), the Program for Changjiang Scholars and Innovative Research Team in University (IRT15R48) and the State Key Laboratory of Polymer Materials Engineering (sklpme2014-1-01). The authors are grateful to Prof. Guangsheng Luo at Tsinghua University for his valuable suggestions on the demonstration of catalytic decomposition of H2O2 as model exothermic reaction.

ABBREVIATIONS DA, dopamine; PDA, polydopamine; Ag NPs, silver nanoparticles; PNIPAM, poly(Nisopropylacrylamide); VPTT, volume phase transition tempearature; PNGs, PNIPAM nanogels; 4-NP, 4-nitrophenol; 4-AP, 4-aminophenol; NaBH4, sodium borohydride; SPR, surface plasmon resonance; TEM, transmission electron microscopy; HRTEM, high-resolution TEM; XPS, X-ray photoelectron spectroscopy; XRD, X-ray diffraction; AFM, atomic force microscope; ICP-AES, inductively coupled plasma atomic emission spectrometry.


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FIGURES

Figure 1. Schematic illustration of fabrication of proposed thermo-responsive catalytic PNGs@PDA/Ag nanoparticles and the smart microreactor equipped with PNGs@PDA/Ag nanoparticles on microchannel. a-c) Synthesis of PNGs (a), PNGs@PDA (b) and PNGs@PDA/Ag (c) nanoparticles. b’,c’) Chemical structure of the PDA layer (b’) and formation of Ag NPs (c’). The Ag NPs are enclosed by the interpenetrated networks of PDA and PNIPAM in the surface layer of the PNGs@PDA/Ag. d,e) Illustration of the smart microreactor (d) equipped with nanoparticles on the microchannel (e). f,g) The thermo-responsive catalytic mechanism of PNGs@PDA/Ag nanoparticles at temperatures TVPTT (g).


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Figure 2. Characterization of PNGs@PDA/Ag nanoparticles. a1-a4) TEM images of PNGs (a1) and PNGs@PDA-1 (a2), PNGs@PDA-2 (a3) and PNGs@PDA-3 (a4). b1) TEM image of PNGs@PDA/Ag. b2) Magnified view of Ag NPs marked in b1. b3) HRTEM image of Ag NPs marked in b2. b4) XRD patterns of PNGs@PDA-3 and PNGs@PDA/Ag. c) UV-vis spectra of 0.5 mg/mL solutions of PNGs, PNGs@PDA-3 and PNGs@PDA/Ag at 20 oC. d) Temperaturedependent hydrodynamic radius of PNGs, PNGs@PDA and PNGs@PDA/Ag in water. Scale bars are 100 nm in a1-a4 and b1.



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Figure 3. Influence of temperature on the optical property of PNGs@PDA/Ag nanoparticles. a) UV-vis spectra of PNGs@PDA/Ag solution in temperature ranging from 20 to 50 oC. b) Surface plasmon resonance (SPR) position and absorbance at 400 nm of PNGs@PDA/Ag (circle) and blank Ag NPs (triangle) with average diameter of 30 nm at different temperatures.


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Figure 4. Thermo-responsive catalytic properties of PNGs@PDA/Ag nanoparticles. a) UV-vis spectra and color change of 4-NP solution catalyzed by PNGs@PDA/Ag at 30 oC within 33 min. b) Influence of temperature on the reduction of 4-NP catalyzed with PNGs@PDA/Ag nanoparticles. c) Arrhenius plot of lnkapp(T-1) of PNGs@PDA/Ag and PNGs+Ag NPs at different temperatures. d) Reversible catalytic performance of PNGs@PDA/Ag nanoparticles by switching temperature from 30 oC (below the VPTT) to 35 oC (above the VPTT). e) The conversion of 4-NP in 10 min at 25 oC for ten cycles of recycling catalysts.



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Figure 5. Chemical self-regulation property of the smart microreactor verified by changing external temperature. a) Photograph of the as-prepared glass microreactor, in which the microchannels are filled with methylene blue solution for eye catching. b) Optical micrograph of the glass microchannel immobilized with PNGs@PDA/Ag nanoparticles. c) AFM topography image of dry PNGs@PDA/Ag nanoparticles immobilized on the microchannel. d) The height profile of the marked PNGs@PDA/Ag nanoparticles in (c), in which the peaks 1, 2 and 3 represent the positions of the Ag NPs marked in (c). e) The ln(Ct/C0) of 4-NP reactant versus the residence time in the control and smart microreactors at 30 and 35 oC. f) Influence of temperature on the kinetic constant kapp of 4-NP reduction in the control microreactor and smart microreactor. g) Cycle catalysis performance of the smart microreactor at switching temperatures of 30 oC (below the VPTT) and 35 oC (above the VPTT).


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Figure 6. Chemical self-regulation for highly exothermic reaction of decomposition of H2O2 by internal temperature change. a) Scheme of the setup of batch reactions. The left compartment containing blank Ag NPs solution while the right one containing the PNGs@PDA/Ag solution. b) Scheme of the setup of microchannel reactions. The upper microchannel is coated with blank Ag NPs on the inner surface, while the lower one is coated with PNGs@PDA/Ag nanoparticles on the inner surface. c,d) The optical photographs (c) and the photo-thermal images (d) of the batch reactions at different time intervals after adding H2O2 solution. e,f) The photo-thermal images of the microchannels at different time intervals after introducing H2O2 solution into the channels with flow rate of 100 µL/h (e) and 2000 µL/h (f). g,h) The instantaneous temperature changes in the batch reaction compartments (g) and in the microchannels with continuous flows (h) during the catalytic decomposition of H2O2.



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TOC FIGURE

ACS Paragon Plus Environment

38

Novel Smart Microreactors Equipped with Responsive Catalytic

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