A Positively Temperature-Responsive, Substrate-Selective Ag

J. Phys. Chem. B 2009, 113, 16501–16507

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A Positively Temperature-Responsive, Substrate-Selective Ag Nanoreactor Songjun Li† and Shaoqin Gong*,†,‡ Department of Mechanical Engineering, UniVersity of Wisconsin-Milwaukee, Milwaukee 53211, Wisconsin, Department of Materials, UniVersity of Wisconsin-Milwaukee, Milwaukee 53211, Wisconsin ReceiVed: August 5, 2009; ReVised Manuscript ReceiVed: October 12, 2009

An original Ag nanoreactor capable of positively temperature-responsive and substrate-selective catalysis was prepared in this study. This nanoreactor was made of Ag nanoparticles encapsulated in a 4-nitrophenol (NP)-imprinted polymer matrix that exhibited a temperature-sensitive interpolymer interaction between poly(acrylamide) (PAAm) and (2-acrylamide-2-methylpropanesulfonic acid) (PAMPS). At relatively low temperatures (such as 20 °C), this nanoreactor did not demonstrate significant NP-selective catalysis due to the interpolymer complexation between PAAm and PAMPS, which caused shrinking in the imprinted networks. Conversely, at relatively high temperatures (such as 40 °C), this nanoreactor provided significant NP-selective catalysis resulting from the dissociation of the interpolymer complexes between PAAm and PAMPS. Unlike traditional Ag nanoreactors, which lack positively temperature-responsive catalysis or substrate-selective ability, this unique nanoreactor employed both the imprinting of the substrate molecule (i.e., NP) and a temperaturesensitive PAAm/PAMPS network, thereby making positively temperature-responsive, substrate-selective catalysis feasible. 1. Introduction There has been growing interest in nanoreactors because of their importance in controlled catalysis.1-3 Unlike traditional catalytic nanoparticles, nanoreactors can provide tunable catalysis.4 Using nanoreactors, the reactivity of the ongoing reaction may be modulated as desired through a judicious selection of the specific nanoparticles and matrix materials.5,6 Impressive progress has been made recently in this area by adapting various multifunctional polymers as carriers for the catalytic metal nanoparticles.7-9 These multifunctional carriers, which primarily include core-shell-type microspheres and hydrogels, generally have one or more properties that can change the accessibility of the reactant to the encapsulated metal nanoparticles. The application of the molecular imprinting technique in nanoreactor carriers may provide additional functionality to the nanoreactors.10-12 Particularly, nanoreactor carriers made of molecularly imprinted polymer matrix are capable of specifically recognizing the imprint template (i.e., the substrate), thereby making substrate-selective catalysis feasible. On the forefront of nanoreactors are the intelligent nanoreactor systems that exhibit stimulus-responsive reactivity.13 One typical example is the core-shell-structured, poly(styrene)- PNIPAencapsulated Ag nanoreactors, which provide temperatureresponsive reactivity resulting from the thermal phase transition of PNIPA.14,15 The hydrophobicity of PNIPA increases with increasing temperatures; thus, at higher temperatures, the PNIPA network shrinks in water and reduces the accessibility of the reactants to the encapsulated Ag nanoparticles, leading to a lower catalysis; that is, the PNIPA-based Ag nanoreactors usually demonstrate negatively temperature-responsive catalysis. However, such negatively temperature-responsive catalysis generally does not lead to valuable applications because of the low reactivity and slow responsive kinetics at low temperatures.16,17 Moreover, it is very challenging to achieve substrate-selective catalysis using the * Corresponding author. Phone: 414-229-6614. E-mail: [email protected]. † Department of Mechanical Engineering. ‡ Department of Materials.

PNIPA-based Ag nanoreactors. Thus, an alternative approach is needed to prepare a new generation of highly intelligent, substrate-selective nanoreactors. This paper reports, for the first time, a positively temperatureresponsive, substrate-selective Ag nanoreactor (cf. Scheme 1). To prepare this nanoreactor, Ag nanoparticles were encapsulated in a unique molecularly imprinted polymer made of poly(acrylamide) (PAAm) and poly(2-acrylamide- 2-methylpropanesulfonic acid) (PAMPS). The interpolymer complexation and dissociation (i.e., the zipper-like interaction18,19) between PAMPS and PAAm in response to a temperature change can change the accessibility of the reactants to the encapsulated Ag nanoparticles (cf. Scheme 1). At relatively low temperatures, the interpolymer complexation between PAAm and PAMPS inhibited the reactants’ access to the encapsulated Ag nanoparticles. Conversely, at relatively high temperatures, the dissociation of the interpolymer complexes between PAAm and PAMPS improved the reactants’ access to the catalytic Ag species. In this way, this unique nanoreactor demonstrated the positively temperature-responsive, substrate-selective catalysis. In this study, 4-nitrophenol (NP) was selected as the imprint species to prepare the molecularly imprinted polymer matrix because it is a well-known, efficient substrate for Ag nanoreactors.14,15 To investigate the selectivity of the prepared nanoreactors, 2,6dimethyl-4-nitrophenol (DNP), a structural analogue of NP, was selected as the control. The objective of this study is to demonstrate that the positively temperature-responsive, substrateselective catalysis can be realized using this novel design. 2. Experimental Section 2.1. Materials. All chemicals used were commercially available products of analytic or regent grade and used as received. The 4-NP, DNP, and acetonitrile were purchased from SigmaAldrich (St. Louis, MO). AMPS, AAm, N,N′-methylenebisacrylamide (MBA), 2,2-azobisisobutyronitrile (AIBN), and dimethylsulfoxide were the products of Fischer Scientific (Milwaukee, WI). AgNO3 was purchased from Alfa Aesar (Ward Hill, MA).

10.1021/jp907527x  2009 American Chemical Society Published on Web 12/02/2009

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SCHEME 1: Proposed Mechanism for the Positively Temperature-Responsive, Substrate-Selective Ag Nanoreactor

SCHEME 2: Technical Outline for the Preparation of the AgNR-R Nanoreactor

2.2. Preparation of Nanoreactors. The positively temperature-responsive, substrate-selective Ag nanoreactor (viz., AgNRR) was prepared using molecular imprinting technology,20,21 as shown in Scheme 2. The template used was the complex of NP and Ag+ that was formed by adding AgNO3 (0.425 g; 2.5 mmol) into a NP-acetonitrile solution (1 mmol mL-1; 5 mL). The template, monomers (AMPS, AAm), cross-linker MBA, and initiator AIBN were dissolved into dimethylsulfoxide (20 mL) (cf. Table 1). After being dispersed and deoxygenated with sonication and nitrogen, the mixture system was irradiated by ultraviolet light (365 nm) until almost all monomers were polymerized. The resulting bulky polymer was roughly crushed, and the embedded Ag precursor was reduced with an excess

TABLE 1: Synthetic Composition of Nanoreactors nanoreactor template AMPS AAM MBA AIBN

AgNR-R +

Ag (2.5 mmol) and NP (5 mol) 3.72 g (18 mmol) 1.28 g (18 mmol) 23.12 g 0.5 g

AgNR +

Ag (2.5 mmol)

NR-R 0 mmol

0g 3.72 g (18 mmol) 2.56 g (36 mmol) 1.28 g (18 mmol) 23.12 g 23.12 g 0.5 g 0.5 g

amount of sodium borohydride. The resulting polymer (i.e., the nanoreactor precursor) was profusely washed with a mixture of methanol and acetic acid (9:1, v/v) to remove the template NP and minimal unreacted monomers. The prepared AgNR-R

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nanoreactor was dried in a vacuum vessel at room temperature and then ground into a size of 60-80 mesh. For comparison, two control nanoreactor systems, AgNR and NR-R, were also prepared under comparable conditions (cf. Table 1). The AgNR nanoreactor was also an NP-imprinted Ag nanoreactor, but its polymer matrix did not have PAMPS; thus, AgNR was not expected to show notable temperature-responsive catalysis due to the lack of temperature-responsive interpolymer interaction in this system. During the preparation of AgNR, the same procedures used for AgNR-R were used, except that AMPS was replaced with the same amount of AAm (in moles). NR-R had a nonimprinted, temperature-responsive PAMPS/ PAAm matrix and did not have catalytic Ag nanoparticles in the polymeric networks. 2.3. SEM, FTIR, and SPR Analysis. The morphology of the nanoreactors was studied using a scanning electron microscope (SEM, Hitachi S-450, Japan). The acceleration voltage used was 5 kV. The infrared spectra were recorded using a FTIR apparatus (Nicollet MX-1E, Madison, WI). The absorption bands of the surface plasma resonances (SPR) were recorded using a Lambda 25 UV spectrometer (Perkin-Elmer, Waltham, MA). 2.4. Temperature-Programmed Desorption. Temperatureprogrammed desorption (TPD) was carried out to evaluate the interaction between nanoreactors and analyte.22 Using a device composed of a gas chromatography (TCD) and a data processing system (Agilent 5890, Foster City, CA), the nanoreactors (0.1 g) were placed into an online U-shaped quartz tube (4 mm i.d.). After the nanoreactors were immersed into 10 µL of substrate (1.0 µmol mL-1 acetonitrile), the quartz tube was heated in a nitrogen flow (40 mL min-1; 0.22 MPa) at 10 °C min-1 from room temperature up to the temperature at which the absorbed substrate desorbed. The desorbing signal was recorded by the data processing system. 2.5. Swelling Test. The swelling experiments were performed to evaluate the accessibility of water to the polymer networks of the nanoreactors. In triplicate, dried samples of nanoreactors were immersed into deionized water. All nanoreactor systems were kept at specific temperatures for at least 48 h to reach swelling equilibrium. These wet samples were blotted with tissue to remove the water adhered on the surface, weighed (Wt), then dried in a vacuum vessel until a constant weight (Wd) was obtained. The swelling ratios were calculated using the following equation; the average of the triplicate runs was reported.

S)

Wt - Wd × 100% Wd

2.6. Dynamic Adsorbing-Desorbing Cyclic Voltammetry. The potential of reducing/oxidizing a binding molecule depends on the binding constant. A larger binding constant requires more energy to overcome the binding, thereby causing a larger redox potential. Thus, the dynamic adsorbing-desorbing cyclic voltammetry (DCV) can provide valuable information on the binding behavior between the nanoreactors and analyte.23 Using an electrochemical workstation equipped with a three-electrode configuration (Pt, working and auxiliary electrodes; Ag/Ag+, reference) (CHI-600, Austin, TX), the nanoreactors (10 mg) preadsorbed with ∼1 µmol of analyte were placed into an online cuvette equipped with a self-rotation unit (supporting electrolyte: 0.01 mmol mL-1 KNO3; 10 mL). The absorbed analyte in the adsorption-desorption equilibrium was consecutively scanned by the workstation up to 20 cycles until a stable DCV diagram was reached (scanning range, ∼1.2 to ∼-1.2 V; scanning rate, 0.25 V s-1).

Figure 1. FTIR spectra of the nanoreactors (a, AgNR-R precursor; b, AgNR-R; c, AgNR; d, NR-R).

2.7. Catalysis Test. In a batch format, the catalysis of the nanoreactors was studied at room temperature.24 NP (in triplicate) was added into the NaBH4 aqueous solution (0.2 µmol mL-1; 3 mL) with the initial concentration of 0.1 µmol mL-1. The solid content of the nanoreactors in this mixture system was 6 µg mL-1. The amount of NP being reduced was spectrophotometrically determined, and the average of the triplicate runs was reported. To study the catalytic specificity, the reduction of the NP analogue (i.e., DNP, as the control) was also performed under comparable conditions. 3. Results and Discussion 3.1. FTIR, SEM, and SPR Analysis of Molecular Imprinting. To study the imprinting behavior, Figure 1 presents the FTIR spectra of the nanoreactors. Three main absorption bands (3050-3500, 2800-3000, ∼1750 cm-1) and some fingerprints appeared in the spectra. These main absorption bands can be attributed to the stretching of O-H/N-H, C-H, and CdO, respectively.25 The fingerprints may arise from the vibration of the C-N and C-C bonds and the rotation of various groups.26 For comparison, we also included in Figure 1 the spectrum of the AgNR-R precursor (i.e., the AgNR-R nanoreactor system in which the imprinted NP had not yet been removed from the polymer matrix). The AgNR-R precursor demonstrated complicated absorption bands of NP at ∼33003500 cm-1. After washing, the spectrum of the resulting nanoreactor (i.e., AgNR-R) became comparable to that of NRR. Figure 2 presents the SEM images of these nanoreactors. The nonimprinted NR-R exhibited relatively smooth morphology, whereas the imprinted AgNR-R and AgNR appeared with speckles and cavities. The FTIR and SEM analysis indicate that imprinting of NP occurred during the preparation of these nanoreactors. Additional information regarding the imprinting behavior will be discussed in Section 3.2. Figure 3 presents the UV spectra of these nanoreactors. Both AgNR-R and AgNR

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Figure 4. TPD profiles of the nanoreactors (A, NP; B, DNP) ((1) AgNR-R, (2) AgNR, (3) NR-R).

Figure 2. SEM images of the nanoreactors (A, AgNR-R; B, AgNR; C, NR-R).

Figure 5. Swelling curves of the nanoreactors.

Figure 3. UV spectra of the nanoreactors (insert: digital photos of the nanoreactors; from left to right: NR-R, AgNR, and AgNR-R).

demonstrated the typical SPR adsorption bands of Ag nanoparticles at ∼400 nm.27,28 The NR-R did not show any SPR bands because there is no Ag species present in the polymeric network; thus, the SPR bands indicate that both AgNR-R and AgNR contained Ag nanoparticles, as expected. 3.2. Specific Interaction between the Nanoreactors and Analyte. Figure 4 presents the TPD profiles. The imprint species, NP, desorbed from AgNR-R, AgNR, and NR-R at 257, 263, and 234 °C, respectively (cf. Figure 4A). The NP-imprinted AgNR-R and AgNR interacted more strongly with NP as compared with the nonimprinted NR-R. For comparison, the TPD profiles of the NP analogue (DNP) are shown in Figure 4B. Although DNP is structurally similar to NP, all three

nanoreactors, including AgNR-R and AgNR, showed similar interactions with DNP. These results strongly indicate that the interactions offered by both AgNR-R and AgNR were highly NP-selective. Correlating with Section 3.1, the TPD profiles further indicate that the NP-imprinted nanoreactors had the expected NP-imprinted structure. Since molecular recognition by the imprinted polymers is a result of the specific interaction between the template and imprinted structures, the stronger interaction offered by the two NP-imprinted nanoreactors with NP (i.e., AgNR-R and AgNR) is expected. 3.3. Swelling Behavior. Figure 5 presents the swelling curves of the nanoreactors. The swelling ratios of these nanoreactors increased with increased temperature. The AgNR-R and NR-R showed a much stronger dependence on temperature compared with AgNR. A significant increase in the swelling behaviors of AgNR-R and NR-R appeared at around 30 °C. Both AgNR-R and NR-R exhibited low swelling ratios at relatively low temperatures; however, their swelling ratios were both significantly higher at higher temperatures. This, as previously explained, may be attributed to the interpolymer interaction between PAAm and PAMPS. The lower swelling ratio at relatively low temperatures may be due to the interpolymer complexation (cf. Scheme 1), which inhibited the access of water to the polymer networks of AgNR-R and NR-R. The increased

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Figure 6. DCV profiles of NP from the nanoreactors (a, NR-R at 20 °C; b, NR-R at 40 °C; c, AgNR-R at 20 °C; d, AgNR-R at 40 °C; e, AgNR at 20 °C; f, AgNR at 40 °C).

swelling ratios at relatively high temperatures can be attributed to the dissociation of the interpolymer complexes, which improved the access of water to the polymer networks. 3.4. Dynamic Binding Behavior. To further study the interpolymer interaction between PAAm and PAMPS, Figure 6 presents the DCV diagrams of the model reactant, NP. For comparison, two representative temperatures were studied, 20

and 40 °C, which are either lower or higher than the approximate transition temperature of AgNR-R and NR-R (∼30 °C), respectively. The model reactant NP that binds to AgNR-R at 20 °C exhibited a reduction peak at -884 mV; however, the reduction peak at 40 °C shifted to a much lower potential (-919 mV). Thus, AgNR-R showed a stronger interaction with the reactant at 40 °C than at 20 °C.

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Figure 7. Catalysis of the nanoreactors (a, 20 °C; b, 40 °C).

To further study this interaction, Figure 6 (cf. a, b, e, and f) shows the reduction potentials of NP from both controls. The reduction potential of NP that binds to AgNR-R at 20 °C was nearly comparable to that from NR-R (-884 vs -878 mV). The reduction potential of NP that binds to AgNR-R at 40 °C became as low as that from AgNR (-919 vs -923 mV). This result strongly indicates that AgNR-R provided a temperatureresponsive interaction with NP. Again, this observation can be attributed to the interpolymer interaction between PAAm and PAMPS. The complexation and dissociation between PAAm and PAMPS regulated the accessibility of the reactant to the imprinted networks, thereby making the temperature-responsive interaction feasible. 3.5. Catalysis and Molecular Recognition. Figure 7 presents the catalysis of the nanoreactors. The NR-R did not provide significant catalysis because Ag nanoparticles were not present within the polymer network. Both AgNR-R and AgNR demonstrated significant reactivity. Specifically, AgNR showed highly NP-selective reactivity at both 20 and 40 °C, whereas AgNR-R showed temperature-responsive selectivity with NP. For AgNR-R, the NP demonstrated a slightly lower activity than

Li and Gong

Figure 8. Relative specificity of the nanoreactors (a, 20 °C; b, 40 °C).

DNP at 20 °C (cf. Figure 7a); however, the activity of NP became much higher compared with that of DNP at 40 °C (cf. Figure 7b). To further study the substrate selectivity of the nanoreactors, Figure 8 presents the relative reactivity of NP vs DNP as a function of time. Both AgNR and AgNR-R demonstrated a relatively stable NP selectivity with time at both temperatures. In addition, AgNR showed a similar NP-specific selectivity at both temperatures, and AgNR-R provided a temperatureresponsive selectivity for NP. Specifically, AgNR-R demonstrated a significantly lower selectivity for NP at 20 °C than 40 °C. This, as previously explained, can be attributed to the interpolymer interaction within the imprinted network. The complexation and dissociation between PAAm and PAMPS regulated the accessibility of the reactants to the imprinted networks containing the catalytic Ag nanoparticles, thereby inducing the temperature-responsive substrate-selective catalysis. 4. Conclusions An original, positively temperature-responsive, substrateselective Ag nanoreactor was prepared and characterized. The

Ag Nanoreactor nanoreactor consisted of Ag nanoparticles embedded in a 4-nitrophenol (NP)-imprinted poly(2-acrylamide-2-methylpropanesulfonic acid) and poly(acrylamide) polymer matrix exhibiting temperature-responsive interpolymer interactions. At a relatively low temperature (such as 20 °C), this nanoreactor did not demonstrate significant NP-selective catalysis; however, at 40 °C, this nanoreactor exhibited very high NP-specific catalysis as compared with its analogue 2,6-dimethyl-4-nitrophenol. These results indicate that positively temperature-responsive, substrateselective nanoreactors can be fabricated using this novel design. References and Notes (1) Fischer, A.; Muller, J. O.; Antonietti, M.; Thomas, A. Synthesis of ternary metal nitride nanoparticles using mesoporous carbon nitride as reactive template. ACS Nano 2008, 2, 2489–2496. (2) Wei, G.; Zhang, W.; Wen, F.; Wang, Y.; Zhang, M. Suzuki reaction within the core-corona nanoreactor of poly(N-isopropylacrylamide)-grafted Pd nanoparticle in water. J. Phys. Chem. C 2008, 112, 10827–10832. (3) Graeser, M.; Pippel, E.; Greiner, A.; Wendorff, J. H. Polymer coreshell fibers with metal nanoparticles as nanoreactor for catalysis. Macromolecules 2007, 40, 6032–6039. (4) Wang, Y.; Wei, G.; Zhang, W.; Jiang, X.; Zheng, P.; Shi, L.; Dong, A. Responsive catalysis of thermoresponsive micelle-supported gold nanoparticles. J. Mol. Catal. A 2007, 266, 233–238. (5) Jiang, X.; Xiong, D.; An, Y.; Zheng, P.; Zhang, W.; Shi, L. Thermoresponsive hydrogel of poly(glycidyl methacrylate-co-N-isopropylacrylamide) as a nanoreactor of gold nanoparticles. J. Polym. Sci., Part A 2007, 45, 2812–2819. (6) Wang, Y.; Wei, G.; Wen, F.; Zhang, X.; Zhang, W.; Shi, L. Synthesis of gold nanoparticles stabilized with poly(N-isopropylacrylamide)co-poly(4-vinyl pyridine) colloid and their application in responsive catalysis. J. Mol. Catal. A 2008, 280, 1–6. (7) Li, D.; He, Q.; Cui, Y.; Li, J. Fabrication of pH-responsive nanocomposites of gold nanoparticles/poly(4-vinylpyridine). Chem. Mater. 2007, 19, 412–417. (8) Azzam, T.; Bronstein, L.; Eisenberg, A. Water-soluble surfaceanchored gold and palladium nanoparticles stabilized by exchange of low molecular weight ligands with biamphiphilic triblock copolymers. Langmuir 2008, 24, 6521–6529. (9) Kohut, A.; Voronov, A.; Samaryk, V.; Peukert, W. Amphiphilic invertible polyesters as reducing and stabilizing agents in the formation of metal nanoparticles. Macromol. Rapid Commun. 2007, 28, 1410–1414. (10) Riskin, M.; Tel-Vered, R.; Bourenko, T.; Granot, E.; Willner, I. Imprinting of molecular recognition sites through electropolymerization of functionalized Au nanoparticles: development of an electrochemical TNT sensor based on π-donor-acceptor interactions. J. Am. Chem. Soc. 2008, 130, 9726–9733. (11) Diltemiz, S. E.; Say, R.; Buyuktiryaki, S.; Hur, D.; Denizli, A.; Ersoz, A. Quantum dot nanocrystals having guanosine imprinted nanoshell for DNA recognition. Talanta 2008, 75, 890–896. (12) Tan, C. J.; Tong, Y. W. The effect of protein structural conformation on nanoparticle molecular imprinting of Ribonuclease A using miniemulsion polymerization. Langmuir 2007, 23, 2722–2730.

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