Green Synthesis of Hydroxylatopillar[5]arene ... - ACS Publications

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Green Synthesis of Hydroxylatopillar[5]arene-Modified Gold Nanoparticles and Their Self-Assembly, Sensing, and Catalysis Applications Genfu Zhao,† Xin Ran,† Xu Zhou,†,§ Xiaoping Tan,‡ Hong Lei,† Xiaoguang Xie,§ Long Yang,*,†,‡ and Guanben Du*,† †

Key Lab for Forest Resources Conservation and Utilization in the Southwest Mountains, Ministry of Education, Yunnan Province Key Lab of Wood Adhesives and Glued Products, School of Materials Science and Engineering, Southwest Forestry University, Kunming, 650224, China ‡ Key Lab of Inorganic Special Functional Materials, Chongqing Municipal Education Commission, School of Chemistry and Chemical Engineering, Yangtze Normal University, Chongqing, 408100, China § School of Chemical Science and Technology, Yunnan University, Kunming, 650091, China S Supporting Information *

ABSTRACT: A novel, green, one-pot synthesis of gold nanoparticles (AuNPs) was obtained by the redox reaction between AuCl4− and hydroxylatopillar[5]arene (HP5) in aqueous solution with the aid of OH− at room temperature without the need of a traditional harsh reducing agent such as NaBH4, N2H4, etc. Monodisperse AuNPs with a uniform diameter of ∼5.0 nm are fabricated via the proposed one-step colloidal synthesis route by using HP5 as both reducing agent and stabilizer, while AuNPs cannot be effectively protected by noncyclic monomers of HP5. The FTIR, 13C NMR, and XPS studies demonstrated that the hydroxy groups in HP5 reduce Au3+ into Au0, which leads to nucleation, growth, and formation of AuNPs, and the hydroxy groups themselves are oxidized to carboxyl groups. It is surprising that the HP5 functionalized AuNPs can self-assemble and form multiple well-defined architectures, including vesicles, like nanotubes, and one-/two-dimensional (1D/2D) nanostructures without the need of a guest mediator. The selfassembly mechanism was also studied. Moreover, the prepared HP5@AuNPs could be employed as not only scaffolds but energy acceptors for turn-on fluorescence sensing based on a competitive host−guest interaction. In addition, the AuNPs exhibited very excellent catalytic activity for the reduction of 4-nitrophenol (4-NP). We believe that the versatile HP5@AuNPs could be potentially used in the field of self-assembly, sensing, and catalysis. KEYWORDS: Pillar[5]arene, Gold nanoparticles, Self-assembly, Sensing, Catalysis

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and macrocycles (cyclodextrins,17,18 calixarenes,19−22 and cucurbiturils23). Integration of AuNPs and macrocyclic hosts significantly combines and enhances the merits and characteristics of the two component parts, including the optical, electronic, and catalytic performances of AuNPs and supramolecular recognition capability of the macrocyclic hosts, expanding their potential technological applications such as sensors, drug delivery, and catalysis.24,25 As a new developing family of macrocyclic hosts, pillar[n]arenes have received extensive attention due to their novel and rigid symmetric pillar-shaped architectures, hydrophobic electrondonating cavities, unique and excellent host−guest capabilities, and highly tunable functionalizable rims.26−29 On account of these

INTRODUCTION

Gold nanoparticles (AuNPs) play important roles in the fields of nanoscience and nanotechnology and have attracted continuous research interest due to their unique optical/electrical properties.1−5 Their physicochemical features and potential applications such as sensors,6,7 nanoelectronics,8 biomedicine,9 and catalysis10 have aroused strong repercussions. On account of their facile synthesis, high chemical stability, and easy surface functionalization, the AuNPs have become important building blocks for novel hybrid nanomaterials.3,4,11 In parallel, macrocyclic hosts (cyclodextrins, calixarenes, cucurbiturils, etc.) possess unique and sizetunable cavity structures and exhibit special properties.12−14 On the basis of host−guest interactions, they have been wellapplied for self-assembly, drug/gene delivery, separation, sensing, etc.4,15,16 Recently, researchers are pushing ahead to synthesize and assemble the hybrid nanomaterials of metal NPs © XXXX American Chemical Society

Received: November 17, 2017 Revised: January 22, 2018 Published: February 6, 2018 A

DOI: 10.1021/acssuschemeng.7b04292 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Schematic representation for the formation of HP5-modified AuNPs and their supramolecular self-assembly.

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unique properties, pillar[n]arenes are expected that can be used as building blocks for the assembly of supramolecular architectures30 and supramolecular sensing platforms.31 The synthesis, derivatization, host−guest chemistry, and supramolecular self-assembly of pillar[n]arenes have been widely explored,32−35 but the integration of pillar[n]arenes with AuNPs and the application of the resulting nanohybrids are rarely reported. Therefore, the conjugation of AuNPs and pillar[n]arenes not only provides a new kind of hybrid nanomaterial but is expected to bring new properties, functions, and applications.3,4,36 Li et al. reported that carboxylatopillar[5]arene modified AuNPs could be obtained by reduction of AuCl4̅ by NaBH4 in the presence of carboxylatopillar[5]arene.3 However, harsh reducing agent of NaBH4 was used, which is environmentally unfriendly. Recently, Xia’s group reported that cyclodextrin modified AuNPs can be assembled and form well-defined one- and two-dimensional (1D/2D) architectures.4 However, the formation of the assembly 1D/2D AuNPs needs additional guest mediators. In this study, a novel, green, one-pot synthesis of AuNPs was obtained by the redox reaction between AuCl4̅ and hydroxylatopillar[5]arene (HP5) in aqueous solution with the aid of OH− at room temperature without the need of a traditional harsh reducing agent, such as NaBH4, N2H4, etc. Monodisperse AuNPs with a uniform diameter of ∼5.0 nm are fabricated via the proposed one-step colloidal synthesis route by using HP5 as both reducing agent and stabilizer, while AuNPs cannot be effectively protected by noncyclic monomers of HP5. The FTIR, 13C NMR, and XPS studies demonstrated that the hydroxy groups in HP5 reduce Au3+ into Au0, which leads to nucleation, growth, and formation of AuNPs and the hydroxy groups themselves are oxidized to carboxyl groups. It is surprising that the HP5 functionalized AuNPs can self-assemble and form multiple well-defined architectures, including vesicles-like structures, 1D/2D nanostructures, and nanotubes without the need of a guest mediator. A schematic representation for the formation of HP5-modified AuNPs and their supramolecular self-assembly was provided in Figure 1. Moreover, the prepared HP5@AuNPs could be employed as not only scaffolds but energy acceptors for turnon fluorescence sensing based on a competitive host−guest interaction. In addition, the AuNPs exhibited very excellent catalytic activity for the reduction of 4-nitrophenol (4-NP).

EXPERIMENTAL SECTION

Chemicals and Materials. HAuCl4, paraquat (PQ), rhodamine B (RhB), and 4-NP were obtained from Shanghai Adamas Reagent Co., Ltd. (Shanghai, China). All the chemicals involved in these experiments were of analytical grade and used as received. Deionized water was used in all the experiments. The synthesis routes of hydroxylatopillar[5]arene (HP5), cationic pillar[5]arene (CP5), and noncyclic monomer of HP5 (compound 2) were provided in the Supporting Information. Instruments and Characterization. Transmission electron microscopy (TEM) images and high-resolution TEM (HRTEM) images were obtained on a FEI Tecnai F20 instrument at an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were carried out by using a Thermo ESCALAB 250 spectrometer with a twin anode Al Kα (1486.6 eV) X-ray source. Fourier transform infrared spectroscopy (FTIR) spectra were performed on a Vertex 80 V spectrometer. Ultraviolet−visible (UV−vis) spectra were recorded on a Shimadzu UV-2550 instrument. Fluorescence titration experiments were carried out using a Hitachi F-4500 fluorescence spectrophotometer (Tokyo, Japan) at room temperature. 1H NMR and 13C NMR spectra were recorded on a Bruker AV. DRX5 instrument operated at 400 MHz. Synthesis of HP5@AuNPs. In a typical synthesis, 200 μL of HAuCl4 (10.0 mM) was added to 2.0 mL of deionized water, followed by the addition of 200 μL of HP5 aqueous solution (50.0 mM, [HP5]/ [HAuCl4] = 5.0). Then, 100 μL of NaOH (0.5 M) was added into the above mixture. After stirring at room temperature for 30 min, the solution turned red and the HP5-modified AuNPs were thus obtained and termed [email protected]. The [email protected] ([HP5]/ [HAuCl4] = 0.5), [email protected] ([HP5]/[HAuCl4] = 1.0), and [email protected] ([HP5]/[HAuCl4] = 2.5) were also obtained by altering the concentration of HP5 from 50.0 mM to 25.0 mM, 10.0 mM, and 5.0 mM, respectively. Self-Assembly of HP5@AuNPs. [email protected] ([HP5]/ [HAuCl4] = 0.5), [email protected] ([HP5]/[HAuCl4] = 1.0), HP5@ AuNPs-2.5 ([HP5]/[HAuCl4] = 2.5), and [email protected] ([HP5]/ [HAuCl4] = 5.0) samples were aged for 3 days, 6 days, and 10 days at room temperature for self-assembly. All the samples were taken for measuring the UV−vis spectra and TEM. Fluorescence sensing of paraquat and catalytic reduction of 4-NP were provided in the Supporting Information.

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RESULTS AND DISCUSSION Synthesis and Characterization of HP5@AuNPs. We found that AuNPs with typical wine red3,4,36 could be facially fabricated using HP5 molecules as both stabilizer and reducing B


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HP5-modified AuNPs are stable and resistant to surrounding conditions as proved by the control experiments. The photographs of AuNPs formation procedure in the presence of HP5, CP5, and noncyclic monomer 2 under the same experimental conditions were provided in Figure 3. The color of HAuCl4

agent in mild conditions (Figure S7 in the Supporting Information). It should be noted that the formation of the HP5@AuNPs does not need of a traditional harsh reducing agent such as NaBH4, N2H4, etc. Figure 2A−D shows the TEM

Figure 2. TEM images of the synthesized AuNPs at the [HP5]/ [HAuCl4] molar ratio of 0.5 (A), 1.0 (B), 2.5 (C), and 5.0 (D), respectively. (E) HRTEM image of the AuNPs obtained at the [HP5]/ [HAuCl4] molar ratio of 2.5. The inset of part E shows that the lattice spacing of the AuNPs is 0.235 nm (111).

images of AuNPs synthesized at the [HP5]/[HAuCl4] ratio of 0.5, 1.0, 2.5, and 5.0, respectively. The four corresponding AuNPs are 5.2 ± 0.4, 5.3 ± 0.5, 5.3 ± 0.4, and 5.2 ± 0.3 nm in diameter, as demonstrated by the size distribution (Figure S8). The HRTEM image of the AuNPs obtained at the [HP5]/ [HAuCl4] molar ratio of 2.5 is illustrated in Figure 2E. The AuNPs possess distinct lattice fringes, indicating good crystallinity. The four AuNPs solutions possess a typical surface plasmon resonance (SPR) band centered at 520 nm (Figure S9), suggesting the formation of typical and stable [email protected],4,36 The obtained HP5@AuNPs products are comparable with that prepared by the conventional Turkevich method. Here, the AuNPs are fabricated via a one-step colloidal synthesis route by using HP5 as both reducing agent and stabilizer. Thus, compared with Turkevich method, the present synthesized AuNPs are covered with functional supermolecules, which may have versatile and potential applications in many fields such as selfassembly, sensing, catalysis, and so on. The HP5-modified AuNPs are very stable and resistant to the surrounding conditions. There are no obvious changes that are observed in the Au colloidal solutions after being stored for more than 6 months.

Figure 3. Photographs for the AuNPs formation procedure: (A) 2.0 mL of deionized water + 200 μL of HAuCl4 (10.0 mM); (B) A + 200 μL of HP5 aqueous solution (25.0 mM); (C) B + 100 μL of NaOH (0.5 M); (D) C stirred at room temperature for 30 min. (A′) 2.0 mL deionized water + 200 μL of HAuCl4 (10.0 mM); (B′) A′ + 200 μL of CP5 aqueous solution (25.0 mM); (C′) B′ + 100 μL of NaOH (0.5 M); (D′) C′ stirred at room temperature for 30 min. (A′′) 2.0 mL deionized water + 200 μL of HAuCl4 (10.0 mM); (B′′) A′′ + 200 μL of compound 2 aqueous solution (25.0 mM); (C′′) B′′ + 100 μL of NaOH (0.5 M); (D′′) C′′ stirred at room temperature for 30 min.

aqueous solution is typical pale yellow (A, A′, and A′′). Upon the addition of HP5, CP5, and compound 2 aqueous solution into the samples A, A′, and A′′, respectively, their color became orange red in seconds (B, B′, and B′′). The color change from yellow to orange red was caused by the displacement of Cl− in AuCl4̅ by Br− to form AuBr4−̅ , which was verified by a control experiment (Figure S10 in Supporting Information). Upon the addition of NaOH into the sample B, B′, and B′′, their orange red color disappeared in seconds (C, C′, and C′′). This color change was attributed to the fact that the addition of OH− resulted in the dissociation of Br− from AuBr4̅ and formed Au(OH)4̅ (Figure S10), suggesting C


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than those of AuBr4̅ and AuCl4−. In other words, the −OH groups on the HP5 molecule can be oxidized by Au(OH)4−; however, they cannot be oxidized by AuBr4̅ and AuCl4−. We think that is why the addition of OH− could induce the redox reaction between AuCl4̅ and the −OH groups on the HP5 molecule and finally produced the HP5@AuNPs colloidal solution. However, the color of sample C′ had almost no change (D′) due to the lack of the reducing groups on CP5 molecule.

that the stability of the Au(OH)4̅ complex ions is higher than that of AuBr4̅ as well as the stability of AuBr4̅ is higher than that of AuCl4−. After stirring at room temperature for 30 min, the color of sample C became the typical wine red, indicating the successful formation of HP5@AuNPs colloidal solution (D), which was caused by the redox reaction between Au(OH)4̅ and the hydroxyl (−OH) groups on the HP5 molecule. Thus, we concluded that the reduction potential of Au(OH)4̅ is higher

Figure 4. (A) FTIR spectra of HP5 and HP5@AuNPs. (B) Magnification of dotted box in part A. (C) 13C NMR spectra (100 MHz, D2O, rt) of HP5 and HP5@AuNPs. High-resolution XPS spectra of C 1s (D) and O 1s (E) of HP5@AuNPs. D


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Figure 5. Schematic illustration for synthesis of the HP5@AuNPs. The scheme only shows the possible synthesis process of the HP5@AuNPs, which does not consider the size proportion (between HP5 and the AuNPs) and the precise amounts of HP5 molecules on each AuNP.

Figure 6. TEM images of the HP5@AuNPs at the [HP5]/[HAuCl4] molar ratio of 0.5 (A, B, C), 1.0 (D, E, F), 2.5 (G, H, I), and 5.0 (J, K, L) for selfassembling 3d (A, D, G, J), 6d (B, E, H, K), and 10 d (C, F, I, L), respectively.

presence of C, N, and O elements in HP5 as well as the presence of C, N, O, and Au elements in HP5@AuNPs. The C 1s spectrum of HP5@AuNPs (Figure 4D) displays four peaks at 284.7, 285.1, 286.4, 287.6, and 288.8 eV, which are caused by the C−C, C−N, C−O, CO, and O−CO bonds, respectively,37−39 while the O 1s XPS spectrum (Figure 4E) exhibits three peaks at 531.4, 532.3, and 533.0 eV, which are attributed to *OC−O, O−C, and OC−O*, respectively.39 Compared with the XPS data for pure HP5 (Figure S12 in Supporting Information), a new group, namely, a carboxyl, is appeared in the HP5@AuNPs. On the basis of the decrease of hydroxy and the appearance of carboxyl groups, the chemical processes on the formation of the AuNPs can be simply understood as hydroxy groups in HP5 reduce Au3+ into Au0, which leads to nucleation, growth, and formation of AuNPs; meanwhile, the hydroxy groups themselves are oxidized to carboxyl groups (Figure 5). Furthermore, the formed carboxyl groups interact with the AuNPs surface by O−Au conjunction and prevent unlimited agglomeration of the AuNPs.4 Self-Assembly of the HP5@AuNPs. AuNP based selfassembly has attracted considerable attention because it is an ideal platform for studying the fundamental science of NP interactions at nanoscale; furthermore, it is a facile and effective system for various assay/bioassay applications.4 We found that the as-prepared HP5@AuNPs can self-assemble and form multiple well-defined architectures, including vesicles-like structures, nanotubes, and one-/two-dimensional (1D/2D) nanostructures. As shown in Figure 6A−C, the TEM images of the HP5@AuNPs at the [HP5]/[HAuCl4] molar ratio of 0.5 for self-assembling

In addition, the color of sample C′′ became black gray and some precipitations were observed, which implied that the noncyclic monomer-modified AuNPs aggregated and precipitated out from the solutions. Therefore, it can be concluded that the macrocyclic framework of HP5 plays an important role in the stabilization of AuNPs, while AuNPs cannot be effectively protected by noncyclic monomers of HP5 (compound 2 in Supporting Information). We further employed various characterization techniques to study the surface chemistry of the present synthesized HP5@ AuNPs. The FTIR spectra of HP5 and the HP5@AuNPs were first studied. As illustrated in Figure 4A, the HP5 displays several characteristic bands at approximately 3397, 3020, 2953, 1628, 1490, 1209, 1066, and 1406 cm−1, which are caused by the stretching vibrations of −OH, aromatic C−H, CH2, CC, aromatic skeleton, C−O−C, C−O, and bending vibrations of O−H, respectively. Compared with that of the pure HP5, the hydroxy band (3346 cm−1) of the HP5@AuNPs is obviously weaker than that of the original HP5, demonstrating the decrease in the amounts of hydroxy groups. Also, a new band appeared at 1660 cm−1 (Figure 4B), implying the presence of CO group in the HP5@AuNPs. Moreover, the 13C NMR spectra (100 MHz, D2O, rt) of HP5 and HP5@AuNPs were obtained as shown in Figure 4C. Compared with that of HP5, a new peak (168.29 ppm) appeared in the 13C NMR spectrum of HP5@AuNPs, which demonstrated the presence of CO group in the HP5@AuNPs. In addition, the XPS was used to investigate the structure information on HP5@AuNPs. The survey spectra of HP5 and HP5@AuNPs were illustrated in Figure S11, which confirm the E


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respectively, by calculating the height of the HP5 molecule (Figure S15), here the − OH groups on HP5 have been oxidized to −COOH groups, and the carboxyl groups is electronegative (−COO−) as the solution is alkaline. From the TEM results, the particle−particle distance (d) between each HP5@AuNPs ([HP5]/[HAuCl4] molar ratio is 2.5) in the 1D and 2D nanostructues is measured as shown in Figure S16, which indicated that the distance is approximately 2.2 ± 0.1 nm. The results indicated that the d is approximately equal to d2. Thus, it can be concluded that the predominant self-assembly way of the 1D and 2D AuNPs nanostructures should be through the second way. The typical TEM images for the self-assembly of the 1D and 2D nanostructures were illustrated in Figure S17. As for the HP5@ AuNPs ([HP5]/[HAuCl4] = 0.5), the amount of HP5 molecules are not enough for self-assembly. In the case of the HP5@AuNPs ([HP5]/[HAuCl4] = 1.0), vesicle-like structures were formed as the amount of HP5 increased. Then 1D and 2D nanostructures were further obtained at the [HP5]/[HAuCl4] molar ratio of 2.5. While typical nanotubes were observed in the HP5@AuNPs at the [HP5]/[HAuCl4] molar ratio of 5.0 because the amount of HP5 molecules are excess. As far as we know, it is the first time to report that dimension controllable nanoassembly is achieved without the need of any templates/scaffolds or guest mediators, using HP5@AuNPs as the sole building blocks. Furthermore, on the basis of rationally designing the structure of macrocyclic host molecules, it is possible to regulate the AuNP spacings, which is significant in the study of plamson coupling, surface enhanced Raman scattering, etc. Fluorescence Sensing of PQ. The HP5@AuNPs can act as acceptors for organic fluorophores, and the formed inclusion complex are promising for the employment of FRET based sensors.4,43 To demonstrate the potential applications, the interactions of the AuNPs and rhodamine B (RhB) molecules were first studied. As shown in Figure 7A,B, the fluorescence of

3d (A), 6d (B), and 10 d (C) were obtained, respectively. From the TEM images, it can be seen that the AuNPs are still monodispersed and no distinct self-assembly architectures are formed. Figure 6D−F show the TEM images of the HP5@AuNPs at the [HP5]/[HAuCl4] molar ratio of 1.0 for self-assembling 3d (D), 6d (E), and 10 d (F), respectively. It can be found that vesicleslike structures were formed for self-assembling 6d and 10d. In the case of the [HP5]/[HAuCl4] molar ratio of 2.5 (Figure 6G−I), 1D nanostructures were formed for self-assembling 3d and 2D nanostructures were formed for self-assembling 6d and 10d. At a [HP5]/[HAuCl4] molar ratio of 5.0 (Figure 6J−L), 2D nanostructures were formed for self-assembling 3d and nanotubes were formed for self-assembling 6d and 10d. Meanwhile, the SPR peak at 520 nm shows a gradual intensity decrease (Figure S13), further demonstrating that the self-assembly of the AuNPs occurred in solution.4 The self-assembly process of HP5 functionalized AuNPs needs a certain time to reach equilibrium. Thus, HP5@ AuNPs self-assembly will be more easy to reach equilibrium with the increase of self-assembly time. Also, with the increase of the [HP5]/[HAuCl4] molar ratio, there will be more active site for self-assembly. Thus, the assembly interactions will be more easy to reach equilibrium. To date, AuNP assemblies with 1D/2D topological nanostructures have been extensively studied and reported.40−42 However, good modulation of the assembly dimensionality without any templates/scaffolds has been a challenging issue. It has been reported that the self-assembly of 1D or 2D AuNPs can be obtained by the host−guest interaction of host− guest inclusion complexes.3,4 Interestingly, in the present system, the self-assembly of a 1D or 2D AuNPs structures can be facilely obtained without the need of a host−guest interaction. Here, there may be three possible ways for the self-assembly of the 1D and 2D AuNPs nanostructures (Figure S14 in Supporting Information). The particle−particle distances of the three possible ways were estimated to be 1.8 nm (d1), 2.2 nm (d2), and 3.2 nm (d3),

Figure 7. Fluorescent sensing of paraquat using the HP5@AuNPs. (A) Fluorescence spectra of RhB in the presence of different concentrations of the HP5@AuNPs. (B) Plots of relative fluorescence intensities (F/F0) of RhB versus the HP5@AuNP concentrations. (C) Fluorescence spectra of the HP5@AuNP−RhB inclusion complex after adding different concentrations of paraquat. (D) Plots of relative fluorescence intensities (F/F0) of the HP5@AuNP-RhB versus paraquat concentrations. F


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Figure 8. 1H NMR spectra (400 MHz, D2O, rt) of HP5@AuNPs (a), RhB (b), RhB + HP5@AuNPs (c), RhB + HP5@AuNPs + PQ (d), and PQ (e).

Figure 9. (A) Time-dependent UV−vis absorption spectra recorded during the catalytic reduction of 4-NP with the presence of the [email protected]. (B) Plots of ln(Ct/C0) as a function of the reaction time for the reduction of 4-NP catalyzed by [email protected] (red) and commercial Pd/C (black) catalysts.

HP5 host can bind paraquat with more affinity to release RhB. A schematic diagram was provided in Figure S18 in order to explain the competitive host−guest recognition process. Catalytic Reduction of 4-NP. Inspired by the highly monodispersed nature of AuNPs, we chose the reduction of 4-NP by NaBH4 as a model reaction to investigate the catalytic activity of the AuNPs. As shown in Figure S19A, the aqueous solution of 4-NP itself exhibits a strong absorption peak at 317 nm (black curve). Upon the addition of NaBH4 into the solution, the absorption peak at 317 nm disappears along with the appearance of a new peak at 400 nm (red curve) due to the formation of 4-nitrophenolate ion, at the same time the color of the solution changed from light yellow to bright yellow, which was consistent with the previous report.44 Upon the addition of [email protected] into the mixture of 4-NP and NaBH4, the absorption peak at 400 nm significantly decreases as the reaction proceeds. Meanwhile, a new peak appears at 300 nm (Figure 9A), revealing the reduction of 4-NP and formation of 4-AP, according to the previous report. Since the concentration of NaBH4 largely exceeds that of 4-NP, the reduction was considered as a pseudo-firstorder reaction with regard to 4-NP only. The absorbance was proportional to the concentration of 4-NP in this system and the value of ln(At/A0) reflects that of ln(Ct/C0), where Ct and C0 are the concentrations of 4-NP at time t and 0, respectively.

RhB is gradually quenched with an increase of the added HP5@ AuNPs. Because of matchable size/structure, the fluorophores can enter into the macrocyclic cavity by host−guest interactions. As a result, an effective energy transfer from the fluorophores to the AuNPs occurs, which results in fluorescence quenching. As paraquat molecules were introduced into the (HP5@AuNP-RhB) inclusion system, the quenched fluorescence is gradually recovered, as shown in Figure 7C. There is a good linear relationship between the relative fluorescence intensities (F/F0) and the concentrations of paraquat in the range 0.5−5.0 μM (Figure 7D), and the detection limit is 0.2 μM (S/N = 3). Such fluorescence recovery was caused by the replacement of RhB by the PQ molecules included in the HP5 host cavity. The competitive host− guest interaction was further studied by the 1H NMR spectra as shown in Figure 8. The 1H NMR spectra of HP5@AuNPs (a), RhB (b), RhB + HP5@AuNPs (c), RhB + HP5@AuNPs + PQ (d), and PQ (e) were obtained, respectively. We can see from the results that the aromatic protons of RhB underwent upfield shifting, which indicated that the HP5 can bind RhB significantly, although it was on the surface of AuNPs. After addition of paraquat, interestingly, the aromatic protons of RhB underwent downfield shifting (Figure 8d), which indicates the release of RhB. Moreover, comparison of Figure 8d with Figure 8e shows that the protons of paraquat shift upfield obviously, suggesting that G


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ACS Sustainable Chemistry & Engineering Therefore, the reaction rate constant k was calculated from the rate equation, ⎛C ⎞ ln⎜ t ⎟ = kt ⎝ C0 ⎠

might be an alternative strategy for manipulating the physicochemical properties of AuNPs.

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ASSOCIATED CONTENT

S Supporting Information *

(1)

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b04292. Synthesis and characterization of HP5, CP5 host molecules, and noncyclic monomer (Schemes S1−S3, Figures S1−S6); photographs of the synthesized HP5@ AuNPs (Figure S7); size distribution histograms of the synthesized HP5@AuNPs (Figure S8); UV−vis spectra of HP5@AuNPs (Figure S9); photographs for a control experiment (Figure S10); XPS survey spectra of HP5 and HP5@AuNPs (Figure S11); high-resolution XPS spectra of C 1s and O 1s for HP5 (Figure S12); UV−vis spectra of HP5@AuNPs at different self-assembly times (Figure S13); three possible self-assembly ways for the formation of the 1D and 2D AuNPs nanostructures (Figure S14); side view and top view of the optimized structures of the HP5 (−COO−) molecule (Figure S15); distribution histogram of the particle−particle distance between each HP5@ AuNPs (Figure S16); TEM images of the synthesized AuNPs at the [HP5]/[HAuCl4] molar ratio of 2.5 for selfassembling 0, 3, 6, and 10 days (Figure S17); schematic illustration of the competitive host−guest recognition (Figure S18); UV−vis spectra of 4-NP before and after adding NaBH4 solution, time-dependent UV−vis spectra of 4-NP solution in the presence of NaBH4 and different catalysts, plots of ln(Ct/C0) as a function of the reaction time for the reduction of 4-NP catalyzed by different catalysts (Figure S19); high-resolution XPS spectra of Au 4f for the HP5@AuNPs (Figure S20); and a proposed mechanism for the HP5@AuNPs catalytic reduction of 4-NP (Figure S21) (PDF)

Figure 9B shows the time-dependent UV−vis spectra of 4-NP catalyzed by the [email protected] and the rate constant k was calculated to be 0.380 min−1, according to the slope of the fitted line. The commercial Pd/C, [email protected], [email protected], and [email protected] were also studied for reduction of 4-NP (Figure S19B,C,E,G). The rate constant k values were calculated to be 0.053, 0.371, 0.374, and 0.332 min−1 for the commercial Pd/C, [email protected], [email protected], and [email protected] catalysts, respectively. As shown in Figure S19D,F,H, the rate constants on [email protected], HP5@ AuNPs-1.0, [email protected], and [email protected] were about 7.2, 7.0, 7.1, and 6.3-fold higher than that of the commercial Pd/C. Such excellent catalytic performance of the HP5@ AuNPs could be mainly attributed to synergistic effects of the HP5 molecules and AuNPs. According to Bader charge analysis,4,45 the Au atoms which are in contact with the anchored O are electropositive (+0.2−0.3 e), but the remaining ones are still electroneutral. The high-resolution XPS spectra of Au 4f in the HP5@AuNPs sample was obtained as shown in Figure S20. The Au 4f spectrum shows peaks at 84.0 and 87.6 eV, which are ascribed to the Au0. The peaks at 85.2 and 88.9 eV are caused by the Au+. Here, the positively charged surface of the AuNPs should possess high affinity toward the nitro anion, which greatly facilitated formation of the intermediates and the catalytic reactions.46 According to the above results, the proposed reaction processes using the HP5@AuNPs as the catalyst are shown in Figure S21 in the Supporting Information. Furthermore, the 4-NP molecules may interact with HP5 molecules by host−guest interactions, which may also facilitate the catalytic reactions. On the basis of these data, the resulting very high catalytic activities of the HP5@ AuNPs should be caused by the positively charged AuNPs surface and the unique integration of the HP5 molecules and AuNPs.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

CONCLUSIONS In summary, a novel, green, one-pot synthesis of AuNPs was obtained by the redox reaction between AuCl4̅ and HP5 in aqueous solution with the aid of OH− at room temperature without the need of a traditional harsh reducing agent such as NaBH4, ascorbic acid, etc. Monodisperse AuNPs with a uniform diameter of ∼6.0 nm are fabricated via the proposed one-step colloidal synthesis route by using HP5 as both reducing agent and stabilizer, while AuNPs cannot be effectively protected by noncyclic monomers of HP5. The FTIR, 13C NMR, and XPS studies demonstrated that the hydroxy groups in HP5 reduce Au3+ into Au0, which leads to nucleation, growth, and formation of AuNPs and the hydroxy groups themselves are oxidized to carboxyl groups. It is surprising that the HP5 functionalized AuNPs can selfassemble and form multiple well-defined architectures, including vesicles, nanotubes, and 1D/2D nanostructures without the need of a guest mediator. Moreover, the prepared HP5@AuNPs could be employed as not only scaffolds but energy acceptors for turnon fluorescence sensing based on a competitive host−guest interaction. In addition, the AuNPs exhibited very excellent catalytic activity for the reduction of 4-NP. This contribution, on one hand, demonstrates that the familiar AuNPs still have some “hidden” features; on the other hand, ligand structure design

ORCID

Xu Zhou: 0000-0002-6037-9281 Long Yang: 0000-0002-0286-1803 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Program for Leading Talents, Department of Science and Technology of Yunnan Province (Grant No. 2017HA013), the Basic Research Project of Science and Technology Commission of Chongqing (Grant Nos. cstc2017jcyjA0656 and cstc2017jcyjAX0031), and the Education Commission of Chongqing (Grant No. KJ1712298).

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