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Organic-Inorganic Hybrid Pillarene-Based Nanomaterial for Label-Free Sensing and Catalysis
Article
Organic-Inorganic Hybrid Pillarene-Based Nanomaterial for Label-Free Sensing and Catalysis Xin Wang, Zeng-Jie Liu, Eric H. Hill, ..., Paul S. Weiss, Jihong Yu, Ying-Wei Yang [email protected]
HIGHLIGHTS In situ synthesis of AuNPs coated with pillarenes via a reversed Turkevich method Ultrasensitive label-free detection of paraquat via the integration of two methods Efficient and robust catalysis of the hybrid material toward nitrophenol reduction
An organic-inorganic hybrid nanomaterial has been prepared by in situ coating gold nanoparticles with a nanoporous layer of carboxylatopillar[6]arenes through a reversed Turkevich method, and demonstrated not only an optimal performance in label-free sensing of herbicides but also an efficient and robust catalytic activity in organic synthesis.
Wang et al., Matter 1, 848–861 October 2, 2019 ª 2019 Elsevier Inc. https://doi.org/10.1016/j.matt.2019.03.005
Article
Organic-Inorganic Hybrid Pillarene-Based Nanomaterial for Label-Free Sensing and Catalysis Xin Wang,1 Zeng-Jie Liu,1 Eric H. Hill,2 Yuebing Zheng,2 Guanqi Guo,1 Yan Wang,1 Paul S. Weiss,3,4 Jihong Yu,1 and Ying-Wei Yang1,3,5,*
SUMMARY
Progress and Potential
Carboxylatopillar[6]arene sodium salt was used as an efficient reducing agent and stabilizer for the colloidal synthesis of gold nanoparticles coated with a nanoporous layer of synthetic macrocycles via the reversed Turkevich method. The carboxylatopillar[6]arene-functionalized nanoparticles were assembled in a controlled fashion utilizing the host-guest interactions between carboxylatopillar[6]arene and a cationic guest. Significantly, label-free detection of the widely used herbicide paraquat (methyl viologen) was achieved, whereby two different methods of spectral analysis were combined to reduce the detection limit and extend the concentration range of detection. Furthermore, the carboxylatopillar[6]arene-functionalized nanoparticles exhibited efficient catalysis of the reduction of nitrophenols. This facile, one-pot synthesis will be useful for future studies of organic-inorganic hybrid nanomaterials and applications in sensing and catalysis.
Organic-inorganic hybrid materials combining noble metal nanoparticles and novel molecular recognition elements hold great potentials in applications such as sensors, nanoelectronics, biomedicine, catalysis, and surface-enhanced Raman scattering (SERS) spectroscopy. Herein, we prepare an organicinorganic hybrid pillarene-based nanomaterial by coating monodisperse AuNPs with pillar [6]arenes with pillar-shaped rigid structures in situ through a reversed Turkevich method. Due to the achieved appropriate size of nanoparticles and the hostguest interactions between pillarene and guests, the nanomaterial not only possesses optimal performance in label-free detection of paraquat but also exhibit efficient and robust catalysis of nitrophenol reduction. The marriage of synthetic macrocycles and gold nanoparticles via different approaches offers fascinating insights into the fabrication of organic-inorganic hybrid nanomaterials, which will help to inspire and enable discoveries in advanced materials for ultimate applications.
INTRODUCTION Research on the synthesis and application of organic-inorganic hybrid nanomaterials has received increasing attention and has developed rapidly due to integration of tailored optical, magnetic, and electronic properties as well as enhanced stability, biocompatibility, and host-guest recognition characteristics compared with their individual counterparts.1–10 Among various nanoparticles, gold nanoparticles (AuNPs) have been extensively employed in different scientific areas including optical sensing,11–13 green catalysis,14,15 biomedicine,16–19 agriculture,20,21 nanoelectronics,22,23 and SERS sensing,24–28 due to their tunable size, shape, surface chemistry, and distancedependent optical properties.29–31 Accordingly, stabilizers such as surfactants play essential roles in the preparation of nanoparticles and control of their dispersion stability in solution and self-assembly.32–35 Supramolecular chemistry based on macrocycles such as crown ethers,36 cyclodextrins,37 cucurbiturils,38 calixarenes,39 pillararenes,40 and other synthetic macrocycles41–47 has drawn tremendous attention in terms of the design, synthesis, and functionalization of macrocyclic compounds and their versatile potential applications, particularly in hybrid nanomaterials utilizing their recognition properties.48–51 Pillar[n]arenes (pillarenes for short), a relatively new class of macrocyclic host first reported by Ogoshi et al. in 2008,52 continue to be of interest due to their symmetric pillar-shaped structures, adjustable cavity size, ease of chemical modification, and host-guest recognition capabilities.53 A series of water-soluble pillar[n]arenes have been investigated to stabilize noble metal nanoparticles, endowing them with water
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solubility and host-guest recognition.32,48,54–60 However, in most of these synthetic methods pillar[n]arenes serve as stabilizers under harsh reductive conditions. For instance, together with the Weiss group, we reported that water-soluble carboxylatopillar[5]arene (CP5) could serve as a stabilizer for the preparation of AuNPs with NaBH4 as a reductant (CP5-AuNPs-NaBH4).54 Huang and coworkers synthesized noble metal nanoparticles by utilizing reduction by NaBH4 and stabilization by pillar [6]arenes containing water-soluble functional groups such as imidazolium55 and carboxylate.56 They also synthesized amphiphilic pillar[5]arene bilayers-modified AuNPs by reduction of HAuCl4 with ascorbic acid in the presence of sufficient amine-pillar[5]arene.57 However, there are still limitations to these synthetic methods. (1) The controllable range of nanoparticle size is small in general (less than 5–6 nm) due to rapid nucleation induced by such strong reducing agents,61 resulting in difficulty separating the AuNPs from a colloidal suspension. (2) Compared with AuNPs of relatively larger size (7–30 nm), AuNPs with sizes <5 nm are not useful for detection methods using the ratio of the intensity of the surface plasmon resonance (SPR) peak, due to negligible changes in the SPR resulting from assembly induced by trace amounts of a guest compound. Hence, these small assemblies can stay in suspension for a period wherein they cannot register a change in the SPR signal. (3) The harsh reducing agents used in many of the synthetic methods, particularly NaBH4, are environmentally unfriendly.62 In 2014, Pastoriza-Santos and coworkers reported the synthesis of quasi-spherical AuNPs mediated by ammonium pillar[5]arene via a seeded-growth method, but unfortunately the preparation procedure is relatively complicated and the resulting particles showed poor homogeneity of morphology and size, greatly limiting ultrasensitive detection based on SPR.58 The reduction of Au(III) in HAuCl4 by the carboxylate moiety is a recent method for preparing AuNPs following the classic method based on citrate and its analogs, whereby the hydroxyl groups play important roles in reducing Au(III).63–68 CP5modified AuNPs (CP5-AuNPs-CT, 19.5 G 2.2 nm)64 have been prepared by the classic Turkevich method,69,70 whereby the macromolecule served as both reducing agent and stabilizer. However, it is difficult to prepare medium-sized AuNPs (6–10 nm) using the classic Turkevich method due to the influence of pH of the reaction mixture (3.2) on the nucleation and growth processes of AuNP formation. Reactive HAuCl4 species at acidic pH initially determine the reduction rate of HAuCl4 and particle-formation process, while neutral or weakly alkaline conditions of reaction mixtures favor effective stabilization.71 Therefore, taking into account that the pH of the HAuCl4 solution is 1.6 for the reversed Turkevich method (addition of HAuCl4 solution into the boiling reduction solution),71 which is lower than that for the classic Turkevich method (3.2), we set out to prepare medium-sized AuNPs (6–10 nm) from HAuCl4 in situ through the reversed Turkevich method with the aid of carboxylatopillarene. Herein, AuNPs synthesized using CP5 as both stabilizer and reducing agent (CP5-AuNPs-RT) were synthesized via a reversed Turkevich method (Scheme 1), whereby the resulting AuNPs were coated by a nanoporous layer of CP5 rings. The SPR peak of CP5-AuNPs-RT (520.5 nm) was in between that of CP5-coated AuNPs prepared via the NaBH4 reduction method (CP5-AuNPs-NaBH4, lmax = 490.0 nm) and CP5-coated AuNPs prepared via the classic Turkevich method (CP5-AuNPs-CT, lmax = 530.0 nm), suggesting a constitution of medium-sized CP5-AuNPs-RT, which was consistent with the histograms of size distribution determined by their transmission electron microscopy (TEM) images (dCP5-AuNPs-RT = 15.4 G 1.8 nm, dCP5-AuNPs-NaBH4 = 2.3 G 0.3 nm, dCP5-AuNPs-CT = 19.5 G 2.2 nm; Figure S5).
1State
Key Laboratory of Inorganic Synthesis and Preparative Chemistry, International Joint Research Laboratory of Nano-Micro Architecture Chemistry (NMAC), College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, P. R. China
2Department
of Mechanical Engineering, Materials Science & Engineering Program and Texas Materials Institute, The University of Texas at Austin, Austin, TX 78712, USA
3California
NanoSystems Institute and Department of Chemistry & Biochemistry, University of California, Los Angeles, CA 90095, USA
4Materials
Science & Engineering, University of California, Los Angeles, CA 90095, USA
5Lead
Contact
*Correspondence: [email protected] https://doi.org/10.1016/j.matt.2019.03.005
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Scheme 1. Schematic Comparisons of CP5-AuNP-NaBH4, CP5-AuNP-CT, CP5-AuNP-RT, and CP6-AuNP-RT
Since the strong complexation between methyl viologen (MV) and nanoporous macrocycles on the AuNP surfaces enhances detection sensitivity and accuracy, we prepared AuNPs along similar lines with CP6 (which binds to MV 104 times more strongly than CP5)72,73 through the same reversed Turkevich method (CP6-AuNPs-RT, abbreviated as CP6-AuNPs). Au(III) in HAuCl4 was reduced immediately by –COO on carboxylatopillarenes to form many Au(0) nuclei during mixing of HAuCl4 and carboxylatopillarenes, with a pH change from acidic to 7.0–8.0. Carboxylatopillarenes not only provide weakly alkaline conditions from the hydrolysis of carboxyl groups, but also act as efficient reducing agents and optimal stabilizers on the surfaces of the resulting AuNPs, in contrast with our previous report where carboxylatopillarenes only served as stabilizers.54 Significantly, we further demonstrated the successful application of CP6-AuNPs in label-free detection of MV (Scheme S3 and Figure S3), one of the world’s most widely used herbicides, and the integration of two methods of spectral analysis to improve the application of these hybrid nanoparticles for sensing. Specifically, one method is based on the ratio of integrated intensity of the two UV-visible (UV-vis) wavelength ranges of the aggregated state (A) and the dispersed state (D) of AuNPs, and the other is based on the ratio of the intensities of the SPR peaks of the aggregated state (AbsA) and dispersed state (AbsD). Their combination could not only achieve a lower detection limit but also an extended dynamic range in concentration for MV. Interestingly, CP6-AuNPs were used as a catalyst for the reduction of nitrophenols.
RESULTS AND DISCUSSION Water-soluble CP6 and its noncyclic monomer were synthesized according to a modified procedure from the literature (see the Supplemental Information for
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Figure 1. Characterization of CP6-AuNPs (A) UV-vis absorption spectrum and photograph of the solution (inset). (B) Transmission electron microscopy (TEM) image. Scale bar: 50 nm. (C) Histograms of size distribution (based on 300 particles) determined from TEM images. (D) Distribution of hydrodynamic diameters according to dynamic light-scattering measurements. (E) High-resolution TEM image. Scale bar: 2 nm. (F) Fourier transform infrared spectrum of CP6-AuNPs compared with CP6.
details; Schemes S1 and S4; Figures S1 and S4).74,75 The monodisperse AuNPs coated by a nanoporous layer of CP6 have been fabricated by a reversed Turkevich one-pot method in the absence of any additional reducing agents such as NaBH4. The procedure is as follows. The aqueous solution of CP6 was heated at reflux under vigorous stirring, followed by the addition of HAuCl4 aqueous solution (1.0 mL, 0.01 M). After 1.5 h of reaction, a wine-red colloidal solution of CP6-AuNPs was obtained, collected by centrifugation at 20,000 rpm for 20 min, and redispersed in deionized water for further studies. The CP6-AuNPs were characterized in detail by UV-vis spectroscopy, TEM, dynamic light scattering (DLS), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FT-IR) spectroscopy. As shown in the UV-vis spectrum (Figure 1A), the SPR of the CP6-AuNPs is at 516.0 nm. The average diameter of the CP6-AuNPs is 8.3 G 0.9 nm as determined by TEM (Figures 1B and 1C). The average hydrodynamic diameter is 13.9 nm (Figure 1D), where the increased size compared with TEM measurements can be ascribed to the hydration layer of CP6.62 The high-resolution TEM images show that CP6AuNPs have well-defined crystalline planes with distinct d spacings of 0.235 nm (Figure 1E) consistent with the (111) plane of face-centered cubic (fcc) Au. As shown in the O 1s XPS spectrum of CP6 (Figure S6A), the peaks at 531.4, 532.2, and 533.5 eV can be assigned to the -COOH, -COOH, and C-O-C, respectively. However, in the O 1s spectrum of CP6-AuNPs, besides the peak of C-O-C, there is only a single peak at 531.6 eV (Figure S6B) corresponding to O-C-O, which is ascribed to similar electronic environments of the oxygen atoms in the –COO anchored on the AuNP surface.76 Furthermore, to test for the presence of a nanoporous CP6 layer on the surface of the AuNPs, the material was characterized by FT-IR spectroscopy (Figure 1F), which shows similar characteristic peaks belonging to CP6 and CP6-AuNPs. In the FT-IR spectrum of CP6-AuNPs, typical peaks at
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1,6051 and 1,506 cm1 were observed, arising from the C=C stretching mode of the benzene ring in the backbone of CP6.54 The peaks at 1,650 cm1 and 1,407 cm1 can be ascribed to the -COO stretching mode. The peaks at 1,0531 and 1,202 cm1 result from the Ar-O-C group. In addition, the zeta-potential value of CP6-AuNPs is 74.0 mV (Figure S7), which endows them with remarkable stability in aqueous solution due to strong electrostatic repulsion. Significantly, AuNPs coated with CP6 rings (CP6-AuNPs) could remain stable in water for at least several months. As a control experiment, AuNPs coated with the noncyclic monomer of CP6 (M-AuNPs) were prepared using an identical experimental approach (Supplemental Information). The SPR peak of M-AuNPs was observed at 550.0 nm (Figure S8A). Compared with that of CP6-AuNPs, the red-shift and broadening of the SPR peak as well as the purple-colored solution of M-AuNPs revealed a larger AuNP size and poor homogeneity of M-AuNPs (Figure S8B), which is consistent with observations from TEM and DLS measurements (Figures S8C and S8D). In addition, M-AuNPs have poor stability in solution due to a low zeta-potential of 13.0 mV (Figure S8E). Based on the above results, the role played by CP6 as both the reducing agent and stabilizer in the preparation of AuNPs is supported by three factors: (1) –COO groups in CP6 reduce Au(III) into Au(0), resulting in the nucleation and growth of AuNPs by a one-pot hydrothermal method; (2) CP6 rings adsorb onto the AuNP surface via O-Au interactions to limit the growth and aggregation of AuNPs; and (3) the electrostatic interaction of –COO groups and the pillar-shaped rigid skeleton structure of CP6 stabilize the resulting system. Consequently, the CP6 monolayer on AuNP surface endows the AuNPs with potential advanced applications in detection and catalysis through their fascinating host-guest properties. Assembly of AuNPs via various approaches has aroused interest in the mechanisms of the self-assembly process and potential applications of nanoparticle assemblies. We surmised that the self-assembly of CP6-AuNPs could be controlled by host-guest interaction between the CP6 rings attached on the AuNP surface and certain cationic guest molecules (Figure 2A).57,74 Since the SPR peak of CP6-AuNPs is dependent on their assembly state, we anticipate that subtle changes in their UV-vis spectra could allow ultrasensitive and quantitative detection of trace cationic guests, which could induce the aggregation of CP6-AuNPs by reducing their negative zeta-potential.54 MV is known to form a stable 1:1 complex with CP6 in water with a high association constant of larger than 1 3 108 M1.72 Herein, ultrasensitive detection of trace amounts of MV by CP6-AuNPs is reported via the combination of two different methods of UV-vis spectroscopic analysis. Since the SPR peaks of monodisperse and aggregated AuNPs are located at approximately 516 and 600 nm, respectively, analysis of changes in the UV-vis spectra can be achieved by calculating the ratio of the integrated intensities of the ranges for the aggregated (A, ranging from 550 to 750 nm) and dispersed (D, ranging from 450 to 540 nm) AuNPs, respectively (Figures 2B and 2C).77 In this manner, a detection limit of 0.025 mM (0.011 ppm) was achieved (Figure 2D; Table S1), which is one order of magnitude lower than our previously reported system (0.2 mM).54 This increased sensitivity is attributed to two factors: (1) the complexation of CP6 and MV (1.02 G 0.10 3 108 M1)72 is stronger than that of CP5 and MV (8.2 G 1.7 3 104 M1),73 and (2) the SPR spectral change of CP6AuNPs is more obvious than that of CP5-AuNPs-NaBH4, owing to the increased
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Figure 2. Self-Assembly and Sensing Method (A) The self-assembly process of CP6-AuNPs controlled by host-guest interactions between the CP6 rings attached on the AuNP surface and methyl viologen (MV). (B) Plot of UV-vis spectrum showing the area integrals of the curve for the ranges of the aggregated (A, ranging from 550 to 750 nm) and dispersed (D, ranging from 450 to 540 nm) AuNPs. (C) Formula used to calculate the ratio A/D. (D) Sensing of MV (mM) by CP6-AuNPs via calculation of the DA/D. See also Table S1.
nanoparticle size of CP6-AuNPs (8.3 G 0.9 nm) in the present system compared with that of CP5-AuNPs-NaBH4 (2.3 G 0.3 nm) (Figures S5C and S5D). Note that detection based on the DA/D of the UV-vis spectrum only applies to detection of trace amounts of MV since significant changes in the UV-vis spectrum are induced by aggregation of CP6-AuNPs at higher concentrations of MV, e.g., broadening the peak at 516 nm and the appearance of a peak signifying the aggregated state between 550 and 700 nm. To address this limit, we use the ratio of the intensity of the SPR peaks at 600 and 516 nm (Abs600/Abs516) to achieve detection of MV over an extended concentration range (Table S2). A strong linear correlation between Abs600/ Abs516 and the concentration of MV is observed, and the linear range can be controlled by adjusting the concentration of CP6-AuNPs. The concentration of CP6-AuNPs has been optimized (3 3 108 M) to achieve quantification of MV over a concentration range of 2.0–15.0 mM (Figure 3), which is consistent with the detection range (from 0.025 to 2.0 mM) based on the DA/D discussed above. These results indicate that the CP6-AuNP system has great prospects in chemical detection applications. Upon adding deionized water instead of MV solution into AuNPs solution, no correlation between the DA/D and the volume of added water was observed, suggesting that the aggregation was purely the result of MV-induced self-assembly of the
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Figure 3. Sensing of Methyl Viologen with CP6AuNPs (A) UV-vis spectra of pure CP6-AuNPs and the mixture of CP6-AuNPs and MV at different concentrations. (B) Sensing of methyl viologen (MV) (mM) by CP6AuNPs via the ratio of the intensity of the SPR peaks at 600 nm and 516 nm (Abs600 /Abs 516 ). See also Table S2.
AuNPs with no effect from added water (Figure S9). As an additional control experiment, sensing of MV by M-AuNPs was also investigated (see Supplemental Information; Tables S3 and S4). However, M-AuNPs are burdened by larger particle size, wider size distributions (52.1 G 12.6 nm), and poor colloidal stability, and therefore exhibit poor performance in MV detection (Figure 4). In addition to their promising sensing capabilities, CP6-AuNPs exhibited good catalytic activity toward the reduction of nitrobenzene compounds into aminophenols, which are valuable synthetic intermediates of pesticides, medicines, dyestuffs, corrosion inhibitors, and more.78 To evaluate the catalytic activity of CP6-AuNPs, we carried out the reduction of nitrophenols with different stereochemistries, i.e., p-nitrophenol, m-nitrophenol, and o-nitrophenol, by NaBH4 as a reductant in aqueous solutions of CP6-AuNPs at 25 C.79 Monitoring the degradation over time by UV-vis spectroscopy showed a continuous decrease in the intensity of absorption peak at 400 nm attributed to nitrophenolate ions, with a concomitant increased absorbance at a new peak at 299 nm ascribed to aminophenol. Since this spectral change is directly associated with the catalytic reduction reaction, the linear correlation of ln(Ct/C0) with reaction time indicates that the reaction kinetics follow a pseudo-first-order rate law due to an excess of NaBH4 compared with nitrophenols (Figures 5 and S10).79 The order of the apparent rate constants (kapp) of the catalytic reductions for the three isomers of nitrophenols is m-nitrophenol > o-nitrophenol > p-nitrophenol (Figures 5A–5C and S10). More importantly, investigation of the recyclability of CP6-AuNPs catalysts indicated that there is negligible weakening of the catalytic activity after five cycles (Figures 5D and S11). In addition, the catalytic properties of the control material, M-AuNPs (without the nanoporous layer of pillarene rings on AuNPs), were also tested under the same con dC = kapp and the intrinsic rate ditions (Figure 6). The apparent rate constant kapp dt kapp ; where NAu is the dosage of Au ½mol , are summarized in constant k k = NAu Figure 7 and Table S5.80
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Figure 4. Sensing Study with Control Material (A) Plot of UV-vis spectrum showing the area integrals of the curve for the ranges of the aggregated (A, ranging from 600 to 800 nm) and dispersed (D, ranging from 470 to 580 nm) M-AuNPs. (B) Formula used to calculate the A/D ratio. (C) Sensing of methyl viologen (MV) (mM) by M-AuNPs via the calculation of the DA/D. (D) Sensing of MV (mM) by M-AuNPs via the calculation of the ratio of absorbance at 650–550 nm (Abs 650 /Abs 550 ). See also Tables S3 and S4.
As shown in Figure 7 and Table S5, the intrinsic rate constant (k) of the catalytic reduction of p-nitrophenol by CP6-AuNPs (or M-AuNPs) is much lower than that for m-nitrophenol or o-nitrophenol, because the para-substituted phenolic hydroxyl group weakens the electrophilicity of nitrogen atom more effectively than in meta- and/or ortho-substituted ones, which is unfavorable to the activated hydride consecutively attacking the reaction center in the nitro group.81 On the other hand, the catalytic activities for nitrophenol reduction of CP6-AuNPs are higher than that of M-AuNPs (Figure 7), which we attribute to the three following advantages. (1) The nanoporous layer of carboxylatopillarene rings adsorbed onto the AuNP surfaces endows them with stability in aqueous solution. (2) Size and the ratio of surface area to volume of the AuNPs are proportional to the increase in the rate constants.82 Fenger et al. compared the catalytic activities of CTAB-stabilized AuNPs with different sizes (from 3.5 to 56 nm) and revealed that the 13-nm AuNPs were optimal since they provided adequate interfacial area for adsorption of nitrophenol molecules compared with 3.5-nm AuNPs.83 The average particle size of AuNPs coated by the nanoporous layer of carboxylatopillarenes in the current system (CP6AuNP, 8.3 G 0.9 nm) is near the optimum size in the study by Fenger and coworkers. (3) The nanoporous layer of carboxylatopillarenes could potentially capture the nitrophenol based on the host-guest recognition capabilities, promoting the contact with the surface of AuNPs. Conclusions In summary, we first prepared monodisperse AuNPs coated by a nanoporous layer of carboxylatopillarenes in situ through a reversed Turkevich method, whereby carboxylatopillarenes acted as both reducing agent and stabilizer. Taking advantage of
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Figure 5. Catalytic Property of CP6-AuNP Material at 25 C (A) ln(C t/ C 0 ) versus time for the catalytic reduction of p-nitrophenol by CP6-AuNPs with NaBH 4 . (B) ln(C t/C 0 ) versus time for the catalytic reduction of m-nitrophenol by CP6-AuNPs with NaBH4 . (C) ln(C t/ C 0 ) versus time for the catalytic reduction of o-nitrophenol by CP6-AuNPs with NaBH 4 . (D) Stability of CP6-AuNPs catalyst over five cycles of nitrophenol reduction at 25 C. See also Figure S11.
the host-guest interactions between CP6 rings and cationic guest molecules, we determined the optimal performance of CP6-AuNPs in label-free detection of MV via the integration of two methods of UV-vis spectral analysis, achieving not only lower detection limits but also extended concentration intervals. Intriguingly, we found that CP6-AuNPs also possessed efficient catalytic activity for the reduction of nitrophenols. Moreover, the controlled assembly of CP6-AuNPs in the sizes presented in this work holds great promise for ultrasensitive SERS detection of herbicides and environmental pollutants in groundwater and effluent. The marriage of CP6 synthetic macrocycles and AuNPs provides a useful strategy for the fabrication of organic-inorganic hybrid nanomaterials, which could lead to promising candidates for applications in many different scientific areas, particularly sensing, catalysis, and biomedicine.
EXPERIMENTAL PROCEDURES Materials and Characterization Unless otherwise noted, the starting materials and reagents were purchased from commercial sources and used directly without further purification. Deionized water was used in all relevant experiments. 1H nuclear magnetic resonance (NMR) spectra were recorded on a Bruker-300 MHz NMR spectrometer at 25 C. Chemical shifts are noted relative to TMS. UV-vis spectroscopy was carried out in a standard quartz cuvette with 1-cm path length at 25 C using a Shimadzu UV-1800 spectrometer. TEM images were obtained on a Tecnai G2 S-Twin F20 instrument at an accelerating voltage of 200 kV. The zeta-potential values and hydrodynamic diameters were measured by DLS on an ALV/CGS-3 compact goniometer system. The XPS spectra were obtained with a PREVAC XPS/UPS System. FT-IR spectra were obtained on a Vertex 80v spectrometer.
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Figure 6. Catalytic Property of M-AuNP Material at 25 C (A and B) UV-vis absorption spectra at different time points (A) and the ln(C t /C 0 ) versus time (B) for the catalytic reduction of p-nitrophenol by M-AuNPs with NaBH4 . (C and D) UV-vis absorption spectra at different time points (C) and the ln(C t /C 0 ) versus time (D) for the catalytic reduction of m-nitrophenol by M-AuNPs with NaBH 4 . (E and F) UV-vis absorption spectra at different time points (E) and the ln(C t/C 0 ) versus time (F) for the catalytic reduction of o-nitrophenol by M-AuNPs with NaBH 4 .
Preparation of Carboxylatopillarene Sodium Salts Solution CP5 was synthesized according to our previously published procedure (Scheme S2 and Figure S2).54 The aqueous solution of CP5 sodium salts (0.02 M) was prepared as follows: 238.2 mg of CP5 was dissolved in 9.50 mL of deionized water, then NaOH solution (4.0 M, 500 mL) was added under sonication to generate a clear solution (molar ratio of CP5/NaOH = 1:10). The aqueous solution of CP6 sodium salts (0.01 M) was prepared as follows: 85.75 mg of CP6 was dissolved in 6.0 mL of deionized water, then NaOH solution (4.0 M, 180 mL) was added under sonication in a bath sonicator to generate a clear solution (molar ratio of CP6/NaOH = 1:12). Unless otherwise noted, this concentration of CP5/CP6 sodium salts was used in all experiments. Preparation of CP5-AuNPs by the Classic Turkevich Method (CP5-AuNPs-CT) The mixed solution of HAuCl4 aqueous solution (1.0 mL, 0.01 M) and deionized water (97.8 mL) was heated to reflux with vigorous stirring. CP5 aqueous solution
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Figure 7. Rates of Catalytic Reduction of Nitrophenols by CP6-AuNPs and M-AuNPs See also Table S5.
(1.2 mL, 0.02 M) was then added. The reaction mixture was refluxed for another 1.5 h and the CP5-AuNPs-CT colloidal solution was obtained. Preparation of CP5-AuNPs by the Reversed Turkevich Method (CP5-AuNPs-RT) Compared with the classic Turkevich method, this method simply reverses the order of addition of CP5 aqueous solution and HAuCl4 aqueous solution. Preparation of CP6-AuNPs by the Reversed Turkevich Method (CP6-AuNPs-RT) The mixture of the above CP6 aqueous solution (2.4 mL, 0.01 M) and deionized water (96.6 mL) was heated to slight boiling (99 C–100 C) under vigorous stirring, followed by the addition of HAuCl4 aqueous solution (1.0 mL, 0.01 M). The reaction mixture was then heated to maintain slight boiling for 1.5 h and the wine-red Au colloidal solution was obtained, collected by centrifugation at 20,000 rpm for 20 min, and redispersed in deionized water. Sensing of MV by CP6-AuNPs via the Calculation of the Ratio of Aggregated/ Dispersed The MV-induced aggregation of CP6-AuNPs was monitored by UV-vis spectroscopy during the addition of MV to CP6-AuNPs. For each sample, different concentrations of MV (Table S1) were added to a suspension of CP6-AuNPs (2.0 mL, [CP6-AuNPs] = 1 3 108 M, Abs516nm = 0.50), followed by mixing and standing for 20 min at room temperature (25 C). UV-vis absorption spectra of the mixtures were then collected in the range of 350–1,000 nm. The concentration of MV was quantified by calculating the DA/D from the integrated intensity of the aggregated (A, ranging from 550 to 750 nm) and dispersed (D, ranging from 450 to 540 nm) CP6-AuNPs. Sensing of MV by CP6-AuNPs via the Ratio of the Intensity of the SPR Peaks at 600 nm and 516 nm Different concentrations of MV (Table S2) were added to aqueous solutions of CP6modified AuNPs (1.0 mL, [CP6-AuNPs] = 3 3 108 M, Abs516nm = 1.40). After standing for 20 min at room temperature (25 C), the solution UV-vis absorption spectra were collected in the range of 450–1,000 nm. The concentration of MV was quantified by dividing the absorbance at 600 nm by that of 516 nm (Abs600/Abs516). Catalytic Reduction of Nitrophenol Isomers by CP6-AuNPs Nitrophenol (110 mL, 3.0 mM) and deionized water (2,974 mL) were added to a standard quartz cuvette with 1-cm path length. When freshly prepared NaBH4 (150 mL, 0.1 M) was added, the color of the mixture changed from light-yellow to yellow-green immediately as a consequence of the formation of nitrophenolate
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ion. CP6-AuNPs (66 mL, 4 3 108 M) was added and the UV-vis absorption spectra were recorded in the range of 200–700 nm at various time intervals (5 min, 2 min, and 1.5 min for p-, m-, and o-nitrophenol, respectively). The Stability of CP6-AuNPs for Repeated Catalytic Cycles Nitrophenol (110 mL, 3.0 mM) and fresh NaBH4 (25 mL, 0.01 M) were added into the above mixture. The timer was restarted at once and the UV-vis absorption spectrum was recorded in the same range at various time intervals. The procedure was repeated five times.
SUPPLEMENTAL INFORMATION Supplemental Information can be found online at https://doi.org/10.1016/j.matt. 2019.03.005.
ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (51673084, 21871108), Jilin Province-University Cooperative Construction Project – Special Funds for New Materials (SXGJSF2017-3), the 111 project (No. B17020), Jilin University Talents Cultivation Program, and the Fundamental Research Funds for the Central Universities for financial support.
AUTHOR CONTRIBUTIONS Conceptualization, X.W. and Y.-W.Y.; Methodology, X.W. and Y.-W.Y.; Investigation, X.W., Z.-J.L., G.G., and Y.W.; Writing – Original Draft, X.W., E.H.H., and Y.-W.Y.; Writing – Review & Editing, Y.Z., P.S.W., J.Y., and Y.-W.Y.; Funding Acquisition, Y.-W.Y.; Resources, Y.W., J.Y., and Y.-W.Y.; Supervision, Y.-W.Y.
DECLARATION OF INTERESTS The authors declare no competing interests. Received: January 20, 2019 Revised: February 28, 2019 Accepted: March 15, 2019 Published: August 07, 2019
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Organic-Inorganic Hybrid Pillarene-Based Nanomaterial for Label-Free Sensing and Catalysis Xin Wang, Zeng-Jie Liu, Eric H. Hill, ..., Paul S. Weiss, Jihong Yu, Ying-Wei Yang [email protected]
HIGHLIGHTS In situ synthesis of AuNPs coated with pillarenes via a reversed Turkevich method Ultrasensitive label-free detection of paraquat via the integration of two methods Efficient and robust catalysis of the hybrid material toward nitrophenol reduction
An organic-inorganic hybrid nanomaterial has been prepared by in situ coating gold nanoparticles with a nanoporous layer of carboxylatopillar[6]arenes through a reversed Turkevich method, and demonstrated not only an optimal performance in label-free sensing of herbicides but also an efficient and robust catalytic activity in organic synthesis.
Wang et al., Matter 1, 848–861 October 2, 2019 ª 2019 Elsevier Inc. https://doi.org/10.1016/j.matt.2019.03.005
Article
Organic-Inorganic Hybrid Pillarene-Based Nanomaterial for Label-Free Sensing and Catalysis Xin Wang,1 Zeng-Jie Liu,1 Eric H. Hill,2 Yuebing Zheng,2 Guanqi Guo,1 Yan Wang,1 Paul S. Weiss,3,4 Jihong Yu,1 and Ying-Wei Yang1,3,5,*
SUMMARY
Progress and Potential
Carboxylatopillar[6]arene sodium salt was used as an efficient reducing agent and stabilizer for the colloidal synthesis of gold nanoparticles coated with a nanoporous layer of synthetic macrocycles via the reversed Turkevich method. The carboxylatopillar[6]arene-functionalized nanoparticles were assembled in a controlled fashion utilizing the host-guest interactions between carboxylatopillar[6]arene and a cationic guest. Significantly, label-free detection of the widely used herbicide paraquat (methyl viologen) was achieved, whereby two different methods of spectral analysis were combined to reduce the detection limit and extend the concentration range of detection. Furthermore, the carboxylatopillar[6]arene-functionalized nanoparticles exhibited efficient catalysis of the reduction of nitrophenols. This facile, one-pot synthesis will be useful for future studies of organic-inorganic hybrid nanomaterials and applications in sensing and catalysis.
Organic-inorganic hybrid materials combining noble metal nanoparticles and novel molecular recognition elements hold great potentials in applications such as sensors, nanoelectronics, biomedicine, catalysis, and surface-enhanced Raman scattering (SERS) spectroscopy. Herein, we prepare an organicinorganic hybrid pillarene-based nanomaterial by coating monodisperse AuNPs with pillar [6]arenes with pillar-shaped rigid structures in situ through a reversed Turkevich method. Due to the achieved appropriate size of nanoparticles and the hostguest interactions between pillarene and guests, the nanomaterial not only possesses optimal performance in label-free detection of paraquat but also exhibit efficient and robust catalysis of nitrophenol reduction. The marriage of synthetic macrocycles and gold nanoparticles via different approaches offers fascinating insights into the fabrication of organic-inorganic hybrid nanomaterials, which will help to inspire and enable discoveries in advanced materials for ultimate applications.
INTRODUCTION Research on the synthesis and application of organic-inorganic hybrid nanomaterials has received increasing attention and has developed rapidly due to integration of tailored optical, magnetic, and electronic properties as well as enhanced stability, biocompatibility, and host-guest recognition characteristics compared with their individual counterparts.1–10 Among various nanoparticles, gold nanoparticles (AuNPs) have been extensively employed in different scientific areas including optical sensing,11–13 green catalysis,14,15 biomedicine,16–19 agriculture,20,21 nanoelectronics,22,23 and SERS sensing,24–28 due to their tunable size, shape, surface chemistry, and distancedependent optical properties.29–31 Accordingly, stabilizers such as surfactants play essential roles in the preparation of nanoparticles and control of their dispersion stability in solution and self-assembly.32–35 Supramolecular chemistry based on macrocycles such as crown ethers,36 cyclodextrins,37 cucurbiturils,38 calixarenes,39 pillararenes,40 and other synthetic macrocycles41–47 has drawn tremendous attention in terms of the design, synthesis, and functionalization of macrocyclic compounds and their versatile potential applications, particularly in hybrid nanomaterials utilizing their recognition properties.48–51 Pillar[n]arenes (pillarenes for short), a relatively new class of macrocyclic host first reported by Ogoshi et al. in 2008,52 continue to be of interest due to their symmetric pillar-shaped structures, adjustable cavity size, ease of chemical modification, and host-guest recognition capabilities.53 A series of water-soluble pillar[n]arenes have been investigated to stabilize noble metal nanoparticles, endowing them with water
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solubility and host-guest recognition.32,48,54–60 However, in most of these synthetic methods pillar[n]arenes serve as stabilizers under harsh reductive conditions. For instance, together with the Weiss group, we reported that water-soluble carboxylatopillar[5]arene (CP5) could serve as a stabilizer for the preparation of AuNPs with NaBH4 as a reductant (CP5-AuNPs-NaBH4).54 Huang and coworkers synthesized noble metal nanoparticles by utilizing reduction by NaBH4 and stabilization by pillar [6]arenes containing water-soluble functional groups such as imidazolium55 and carboxylate.56 They also synthesized amphiphilic pillar[5]arene bilayers-modified AuNPs by reduction of HAuCl4 with ascorbic acid in the presence of sufficient amine-pillar[5]arene.57 However, there are still limitations to these synthetic methods. (1) The controllable range of nanoparticle size is small in general (less than 5–6 nm) due to rapid nucleation induced by such strong reducing agents,61 resulting in difficulty separating the AuNPs from a colloidal suspension. (2) Compared with AuNPs of relatively larger size (7–30 nm), AuNPs with sizes <5 nm are not useful for detection methods using the ratio of the intensity of the surface plasmon resonance (SPR) peak, due to negligible changes in the SPR resulting from assembly induced by trace amounts of a guest compound. Hence, these small assemblies can stay in suspension for a period wherein they cannot register a change in the SPR signal. (3) The harsh reducing agents used in many of the synthetic methods, particularly NaBH4, are environmentally unfriendly.62 In 2014, Pastoriza-Santos and coworkers reported the synthesis of quasi-spherical AuNPs mediated by ammonium pillar[5]arene via a seeded-growth method, but unfortunately the preparation procedure is relatively complicated and the resulting particles showed poor homogeneity of morphology and size, greatly limiting ultrasensitive detection based on SPR.58 The reduction of Au(III) in HAuCl4 by the carboxylate moiety is a recent method for preparing AuNPs following the classic method based on citrate and its analogs, whereby the hydroxyl groups play important roles in reducing Au(III).63–68 CP5modified AuNPs (CP5-AuNPs-CT, 19.5 G 2.2 nm)64 have been prepared by the classic Turkevich method,69,70 whereby the macromolecule served as both reducing agent and stabilizer. However, it is difficult to prepare medium-sized AuNPs (6–10 nm) using the classic Turkevich method due to the influence of pH of the reaction mixture (3.2) on the nucleation and growth processes of AuNP formation. Reactive HAuCl4 species at acidic pH initially determine the reduction rate of HAuCl4 and particle-formation process, while neutral or weakly alkaline conditions of reaction mixtures favor effective stabilization.71 Therefore, taking into account that the pH of the HAuCl4 solution is 1.6 for the reversed Turkevich method (addition of HAuCl4 solution into the boiling reduction solution),71 which is lower than that for the classic Turkevich method (3.2), we set out to prepare medium-sized AuNPs (6–10 nm) from HAuCl4 in situ through the reversed Turkevich method with the aid of carboxylatopillarene. Herein, AuNPs synthesized using CP5 as both stabilizer and reducing agent (CP5-AuNPs-RT) were synthesized via a reversed Turkevich method (Scheme 1), whereby the resulting AuNPs were coated by a nanoporous layer of CP5 rings. The SPR peak of CP5-AuNPs-RT (520.5 nm) was in between that of CP5-coated AuNPs prepared via the NaBH4 reduction method (CP5-AuNPs-NaBH4, lmax = 490.0 nm) and CP5-coated AuNPs prepared via the classic Turkevich method (CP5-AuNPs-CT, lmax = 530.0 nm), suggesting a constitution of medium-sized CP5-AuNPs-RT, which was consistent with the histograms of size distribution determined by their transmission electron microscopy (TEM) images (dCP5-AuNPs-RT = 15.4 G 1.8 nm, dCP5-AuNPs-NaBH4 = 2.3 G 0.3 nm, dCP5-AuNPs-CT = 19.5 G 2.2 nm; Figure S5).
1State
Key Laboratory of Inorganic Synthesis and Preparative Chemistry, International Joint Research Laboratory of Nano-Micro Architecture Chemistry (NMAC), College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, P. R. China
2Department
of Mechanical Engineering, Materials Science & Engineering Program and Texas Materials Institute, The University of Texas at Austin, Austin, TX 78712, USA
3California
NanoSystems Institute and Department of Chemistry & Biochemistry, University of California, Los Angeles, CA 90095, USA
4Materials
Science & Engineering, University of California, Los Angeles, CA 90095, USA
5Lead
Contact
*Correspondence: [email protected] https://doi.org/10.1016/j.matt.2019.03.005
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Scheme 1. Schematic Comparisons of CP5-AuNP-NaBH4, CP5-AuNP-CT, CP5-AuNP-RT, and CP6-AuNP-RT
Since the strong complexation between methyl viologen (MV) and nanoporous macrocycles on the AuNP surfaces enhances detection sensitivity and accuracy, we prepared AuNPs along similar lines with CP6 (which binds to MV 104 times more strongly than CP5)72,73 through the same reversed Turkevich method (CP6-AuNPs-RT, abbreviated as CP6-AuNPs). Au(III) in HAuCl4 was reduced immediately by –COO on carboxylatopillarenes to form many Au(0) nuclei during mixing of HAuCl4 and carboxylatopillarenes, with a pH change from acidic to 7.0–8.0. Carboxylatopillarenes not only provide weakly alkaline conditions from the hydrolysis of carboxyl groups, but also act as efficient reducing agents and optimal stabilizers on the surfaces of the resulting AuNPs, in contrast with our previous report where carboxylatopillarenes only served as stabilizers.54 Significantly, we further demonstrated the successful application of CP6-AuNPs in label-free detection of MV (Scheme S3 and Figure S3), one of the world’s most widely used herbicides, and the integration of two methods of spectral analysis to improve the application of these hybrid nanoparticles for sensing. Specifically, one method is based on the ratio of integrated intensity of the two UV-visible (UV-vis) wavelength ranges of the aggregated state (A) and the dispersed state (D) of AuNPs, and the other is based on the ratio of the intensities of the SPR peaks of the aggregated state (AbsA) and dispersed state (AbsD). Their combination could not only achieve a lower detection limit but also an extended dynamic range in concentration for MV. Interestingly, CP6-AuNPs were used as a catalyst for the reduction of nitrophenols.
RESULTS AND DISCUSSION Water-soluble CP6 and its noncyclic monomer were synthesized according to a modified procedure from the literature (see the Supplemental Information for
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Figure 1. Characterization of CP6-AuNPs (A) UV-vis absorption spectrum and photograph of the solution (inset). (B) Transmission electron microscopy (TEM) image. Scale bar: 50 nm. (C) Histograms of size distribution (based on 300 particles) determined from TEM images. (D) Distribution of hydrodynamic diameters according to dynamic light-scattering measurements. (E) High-resolution TEM image. Scale bar: 2 nm. (F) Fourier transform infrared spectrum of CP6-AuNPs compared with CP6.
details; Schemes S1 and S4; Figures S1 and S4).74,75 The monodisperse AuNPs coated by a nanoporous layer of CP6 have been fabricated by a reversed Turkevich one-pot method in the absence of any additional reducing agents such as NaBH4. The procedure is as follows. The aqueous solution of CP6 was heated at reflux under vigorous stirring, followed by the addition of HAuCl4 aqueous solution (1.0 mL, 0.01 M). After 1.5 h of reaction, a wine-red colloidal solution of CP6-AuNPs was obtained, collected by centrifugation at 20,000 rpm for 20 min, and redispersed in deionized water for further studies. The CP6-AuNPs were characterized in detail by UV-vis spectroscopy, TEM, dynamic light scattering (DLS), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FT-IR) spectroscopy. As shown in the UV-vis spectrum (Figure 1A), the SPR of the CP6-AuNPs is at 516.0 nm. The average diameter of the CP6-AuNPs is 8.3 G 0.9 nm as determined by TEM (Figures 1B and 1C). The average hydrodynamic diameter is 13.9 nm (Figure 1D), where the increased size compared with TEM measurements can be ascribed to the hydration layer of CP6.62 The high-resolution TEM images show that CP6AuNPs have well-defined crystalline planes with distinct d spacings of 0.235 nm (Figure 1E) consistent with the (111) plane of face-centered cubic (fcc) Au. As shown in the O 1s XPS spectrum of CP6 (Figure S6A), the peaks at 531.4, 532.2, and 533.5 eV can be assigned to the -COOH, -COOH, and C-O-C, respectively. However, in the O 1s spectrum of CP6-AuNPs, besides the peak of C-O-C, there is only a single peak at 531.6 eV (Figure S6B) corresponding to O-C-O, which is ascribed to similar electronic environments of the oxygen atoms in the –COO anchored on the AuNP surface.76 Furthermore, to test for the presence of a nanoporous CP6 layer on the surface of the AuNPs, the material was characterized by FT-IR spectroscopy (Figure 1F), which shows similar characteristic peaks belonging to CP6 and CP6-AuNPs. In the FT-IR spectrum of CP6-AuNPs, typical peaks at
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1,6051 and 1,506 cm1 were observed, arising from the C=C stretching mode of the benzene ring in the backbone of CP6.54 The peaks at 1,650 cm1 and 1,407 cm1 can be ascribed to the -COO stretching mode. The peaks at 1,0531 and 1,202 cm1 result from the Ar-O-C group. In addition, the zeta-potential value of CP6-AuNPs is 74.0 mV (Figure S7), which endows them with remarkable stability in aqueous solution due to strong electrostatic repulsion. Significantly, AuNPs coated with CP6 rings (CP6-AuNPs) could remain stable in water for at least several months. As a control experiment, AuNPs coated with the noncyclic monomer of CP6 (M-AuNPs) were prepared using an identical experimental approach (Supplemental Information). The SPR peak of M-AuNPs was observed at 550.0 nm (Figure S8A). Compared with that of CP6-AuNPs, the red-shift and broadening of the SPR peak as well as the purple-colored solution of M-AuNPs revealed a larger AuNP size and poor homogeneity of M-AuNPs (Figure S8B), which is consistent with observations from TEM and DLS measurements (Figures S8C and S8D). In addition, M-AuNPs have poor stability in solution due to a low zeta-potential of 13.0 mV (Figure S8E). Based on the above results, the role played by CP6 as both the reducing agent and stabilizer in the preparation of AuNPs is supported by three factors: (1) –COO groups in CP6 reduce Au(III) into Au(0), resulting in the nucleation and growth of AuNPs by a one-pot hydrothermal method; (2) CP6 rings adsorb onto the AuNP surface via O-Au interactions to limit the growth and aggregation of AuNPs; and (3) the electrostatic interaction of –COO groups and the pillar-shaped rigid skeleton structure of CP6 stabilize the resulting system. Consequently, the CP6 monolayer on AuNP surface endows the AuNPs with potential advanced applications in detection and catalysis through their fascinating host-guest properties. Assembly of AuNPs via various approaches has aroused interest in the mechanisms of the self-assembly process and potential applications of nanoparticle assemblies. We surmised that the self-assembly of CP6-AuNPs could be controlled by host-guest interaction between the CP6 rings attached on the AuNP surface and certain cationic guest molecules (Figure 2A).57,74 Since the SPR peak of CP6-AuNPs is dependent on their assembly state, we anticipate that subtle changes in their UV-vis spectra could allow ultrasensitive and quantitative detection of trace cationic guests, which could induce the aggregation of CP6-AuNPs by reducing their negative zeta-potential.54 MV is known to form a stable 1:1 complex with CP6 in water with a high association constant of larger than 1 3 108 M1.72 Herein, ultrasensitive detection of trace amounts of MV by CP6-AuNPs is reported via the combination of two different methods of UV-vis spectroscopic analysis. Since the SPR peaks of monodisperse and aggregated AuNPs are located at approximately 516 and 600 nm, respectively, analysis of changes in the UV-vis spectra can be achieved by calculating the ratio of the integrated intensities of the ranges for the aggregated (A, ranging from 550 to 750 nm) and dispersed (D, ranging from 450 to 540 nm) AuNPs, respectively (Figures 2B and 2C).77 In this manner, a detection limit of 0.025 mM (0.011 ppm) was achieved (Figure 2D; Table S1), which is one order of magnitude lower than our previously reported system (0.2 mM).54 This increased sensitivity is attributed to two factors: (1) the complexation of CP6 and MV (1.02 G 0.10 3 108 M1)72 is stronger than that of CP5 and MV (8.2 G 1.7 3 104 M1),73 and (2) the SPR spectral change of CP6AuNPs is more obvious than that of CP5-AuNPs-NaBH4, owing to the increased
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Figure 2. Self-Assembly and Sensing Method (A) The self-assembly process of CP6-AuNPs controlled by host-guest interactions between the CP6 rings attached on the AuNP surface and methyl viologen (MV). (B) Plot of UV-vis spectrum showing the area integrals of the curve for the ranges of the aggregated (A, ranging from 550 to 750 nm) and dispersed (D, ranging from 450 to 540 nm) AuNPs. (C) Formula used to calculate the ratio A/D. (D) Sensing of MV (mM) by CP6-AuNPs via calculation of the DA/D. See also Table S1.
nanoparticle size of CP6-AuNPs (8.3 G 0.9 nm) in the present system compared with that of CP5-AuNPs-NaBH4 (2.3 G 0.3 nm) (Figures S5C and S5D). Note that detection based on the DA/D of the UV-vis spectrum only applies to detection of trace amounts of MV since significant changes in the UV-vis spectrum are induced by aggregation of CP6-AuNPs at higher concentrations of MV, e.g., broadening the peak at 516 nm and the appearance of a peak signifying the aggregated state between 550 and 700 nm. To address this limit, we use the ratio of the intensity of the SPR peaks at 600 and 516 nm (Abs600/Abs516) to achieve detection of MV over an extended concentration range (Table S2). A strong linear correlation between Abs600/ Abs516 and the concentration of MV is observed, and the linear range can be controlled by adjusting the concentration of CP6-AuNPs. The concentration of CP6-AuNPs has been optimized (3 3 108 M) to achieve quantification of MV over a concentration range of 2.0–15.0 mM (Figure 3), which is consistent with the detection range (from 0.025 to 2.0 mM) based on the DA/D discussed above. These results indicate that the CP6-AuNP system has great prospects in chemical detection applications. Upon adding deionized water instead of MV solution into AuNPs solution, no correlation between the DA/D and the volume of added water was observed, suggesting that the aggregation was purely the result of MV-induced self-assembly of the
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Figure 3. Sensing of Methyl Viologen with CP6AuNPs (A) UV-vis spectra of pure CP6-AuNPs and the mixture of CP6-AuNPs and MV at different concentrations. (B) Sensing of methyl viologen (MV) (mM) by CP6AuNPs via the ratio of the intensity of the SPR peaks at 600 nm and 516 nm (Abs600 /Abs 516 ). See also Table S2.
AuNPs with no effect from added water (Figure S9). As an additional control experiment, sensing of MV by M-AuNPs was also investigated (see Supplemental Information; Tables S3 and S4). However, M-AuNPs are burdened by larger particle size, wider size distributions (52.1 G 12.6 nm), and poor colloidal stability, and therefore exhibit poor performance in MV detection (Figure 4). In addition to their promising sensing capabilities, CP6-AuNPs exhibited good catalytic activity toward the reduction of nitrobenzene compounds into aminophenols, which are valuable synthetic intermediates of pesticides, medicines, dyestuffs, corrosion inhibitors, and more.78 To evaluate the catalytic activity of CP6-AuNPs, we carried out the reduction of nitrophenols with different stereochemistries, i.e., p-nitrophenol, m-nitrophenol, and o-nitrophenol, by NaBH4 as a reductant in aqueous solutions of CP6-AuNPs at 25 C.79 Monitoring the degradation over time by UV-vis spectroscopy showed a continuous decrease in the intensity of absorption peak at 400 nm attributed to nitrophenolate ions, with a concomitant increased absorbance at a new peak at 299 nm ascribed to aminophenol. Since this spectral change is directly associated with the catalytic reduction reaction, the linear correlation of ln(Ct/C0) with reaction time indicates that the reaction kinetics follow a pseudo-first-order rate law due to an excess of NaBH4 compared with nitrophenols (Figures 5 and S10).79 The order of the apparent rate constants (kapp) of the catalytic reductions for the three isomers of nitrophenols is m-nitrophenol > o-nitrophenol > p-nitrophenol (Figures 5A–5C and S10). More importantly, investigation of the recyclability of CP6-AuNPs catalysts indicated that there is negligible weakening of the catalytic activity after five cycles (Figures 5D and S11). In addition, the catalytic properties of the control material, M-AuNPs (without the nanoporous layer of pillarene rings on AuNPs), were also tested under the same con dC = kapp and the intrinsic rate ditions (Figure 6). The apparent rate constant kapp dt kapp ; where NAu is the dosage of Au ½mol , are summarized in constant k k = NAu Figure 7 and Table S5.80
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Figure 4. Sensing Study with Control Material (A) Plot of UV-vis spectrum showing the area integrals of the curve for the ranges of the aggregated (A, ranging from 600 to 800 nm) and dispersed (D, ranging from 470 to 580 nm) M-AuNPs. (B) Formula used to calculate the A/D ratio. (C) Sensing of methyl viologen (MV) (mM) by M-AuNPs via the calculation of the DA/D. (D) Sensing of MV (mM) by M-AuNPs via the calculation of the ratio of absorbance at 650–550 nm (Abs 650 /Abs 550 ). See also Tables S3 and S4.
As shown in Figure 7 and Table S5, the intrinsic rate constant (k) of the catalytic reduction of p-nitrophenol by CP6-AuNPs (or M-AuNPs) is much lower than that for m-nitrophenol or o-nitrophenol, because the para-substituted phenolic hydroxyl group weakens the electrophilicity of nitrogen atom more effectively than in meta- and/or ortho-substituted ones, which is unfavorable to the activated hydride consecutively attacking the reaction center in the nitro group.81 On the other hand, the catalytic activities for nitrophenol reduction of CP6-AuNPs are higher than that of M-AuNPs (Figure 7), which we attribute to the three following advantages. (1) The nanoporous layer of carboxylatopillarene rings adsorbed onto the AuNP surfaces endows them with stability in aqueous solution. (2) Size and the ratio of surface area to volume of the AuNPs are proportional to the increase in the rate constants.82 Fenger et al. compared the catalytic activities of CTAB-stabilized AuNPs with different sizes (from 3.5 to 56 nm) and revealed that the 13-nm AuNPs were optimal since they provided adequate interfacial area for adsorption of nitrophenol molecules compared with 3.5-nm AuNPs.83 The average particle size of AuNPs coated by the nanoporous layer of carboxylatopillarenes in the current system (CP6AuNP, 8.3 G 0.9 nm) is near the optimum size in the study by Fenger and coworkers. (3) The nanoporous layer of carboxylatopillarenes could potentially capture the nitrophenol based on the host-guest recognition capabilities, promoting the contact with the surface of AuNPs. Conclusions In summary, we first prepared monodisperse AuNPs coated by a nanoporous layer of carboxylatopillarenes in situ through a reversed Turkevich method, whereby carboxylatopillarenes acted as both reducing agent and stabilizer. Taking advantage of
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Figure 5. Catalytic Property of CP6-AuNP Material at 25 C (A) ln(C t/ C 0 ) versus time for the catalytic reduction of p-nitrophenol by CP6-AuNPs with NaBH 4 . (B) ln(C t/C 0 ) versus time for the catalytic reduction of m-nitrophenol by CP6-AuNPs with NaBH4 . (C) ln(C t/ C 0 ) versus time for the catalytic reduction of o-nitrophenol by CP6-AuNPs with NaBH 4 . (D) Stability of CP6-AuNPs catalyst over five cycles of nitrophenol reduction at 25 C. See also Figure S11.
the host-guest interactions between CP6 rings and cationic guest molecules, we determined the optimal performance of CP6-AuNPs in label-free detection of MV via the integration of two methods of UV-vis spectral analysis, achieving not only lower detection limits but also extended concentration intervals. Intriguingly, we found that CP6-AuNPs also possessed efficient catalytic activity for the reduction of nitrophenols. Moreover, the controlled assembly of CP6-AuNPs in the sizes presented in this work holds great promise for ultrasensitive SERS detection of herbicides and environmental pollutants in groundwater and effluent. The marriage of CP6 synthetic macrocycles and AuNPs provides a useful strategy for the fabrication of organic-inorganic hybrid nanomaterials, which could lead to promising candidates for applications in many different scientific areas, particularly sensing, catalysis, and biomedicine.
EXPERIMENTAL PROCEDURES Materials and Characterization Unless otherwise noted, the starting materials and reagents were purchased from commercial sources and used directly without further purification. Deionized water was used in all relevant experiments. 1H nuclear magnetic resonance (NMR) spectra were recorded on a Bruker-300 MHz NMR spectrometer at 25 C. Chemical shifts are noted relative to TMS. UV-vis spectroscopy was carried out in a standard quartz cuvette with 1-cm path length at 25 C using a Shimadzu UV-1800 spectrometer. TEM images were obtained on a Tecnai G2 S-Twin F20 instrument at an accelerating voltage of 200 kV. The zeta-potential values and hydrodynamic diameters were measured by DLS on an ALV/CGS-3 compact goniometer system. The XPS spectra were obtained with a PREVAC XPS/UPS System. FT-IR spectra were obtained on a Vertex 80v spectrometer.
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Figure 6. Catalytic Property of M-AuNP Material at 25 C (A and B) UV-vis absorption spectra at different time points (A) and the ln(C t /C 0 ) versus time (B) for the catalytic reduction of p-nitrophenol by M-AuNPs with NaBH4 . (C and D) UV-vis absorption spectra at different time points (C) and the ln(C t /C 0 ) versus time (D) for the catalytic reduction of m-nitrophenol by M-AuNPs with NaBH 4 . (E and F) UV-vis absorption spectra at different time points (E) and the ln(C t/C 0 ) versus time (F) for the catalytic reduction of o-nitrophenol by M-AuNPs with NaBH 4 .
Preparation of Carboxylatopillarene Sodium Salts Solution CP5 was synthesized according to our previously published procedure (Scheme S2 and Figure S2).54 The aqueous solution of CP5 sodium salts (0.02 M) was prepared as follows: 238.2 mg of CP5 was dissolved in 9.50 mL of deionized water, then NaOH solution (4.0 M, 500 mL) was added under sonication to generate a clear solution (molar ratio of CP5/NaOH = 1:10). The aqueous solution of CP6 sodium salts (0.01 M) was prepared as follows: 85.75 mg of CP6 was dissolved in 6.0 mL of deionized water, then NaOH solution (4.0 M, 180 mL) was added under sonication in a bath sonicator to generate a clear solution (molar ratio of CP6/NaOH = 1:12). Unless otherwise noted, this concentration of CP5/CP6 sodium salts was used in all experiments. Preparation of CP5-AuNPs by the Classic Turkevich Method (CP5-AuNPs-CT) The mixed solution of HAuCl4 aqueous solution (1.0 mL, 0.01 M) and deionized water (97.8 mL) was heated to reflux with vigorous stirring. CP5 aqueous solution
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Figure 7. Rates of Catalytic Reduction of Nitrophenols by CP6-AuNPs and M-AuNPs See also Table S5.
(1.2 mL, 0.02 M) was then added. The reaction mixture was refluxed for another 1.5 h and the CP5-AuNPs-CT colloidal solution was obtained. Preparation of CP5-AuNPs by the Reversed Turkevich Method (CP5-AuNPs-RT) Compared with the classic Turkevich method, this method simply reverses the order of addition of CP5 aqueous solution and HAuCl4 aqueous solution. Preparation of CP6-AuNPs by the Reversed Turkevich Method (CP6-AuNPs-RT) The mixture of the above CP6 aqueous solution (2.4 mL, 0.01 M) and deionized water (96.6 mL) was heated to slight boiling (99 C–100 C) under vigorous stirring, followed by the addition of HAuCl4 aqueous solution (1.0 mL, 0.01 M). The reaction mixture was then heated to maintain slight boiling for 1.5 h and the wine-red Au colloidal solution was obtained, collected by centrifugation at 20,000 rpm for 20 min, and redispersed in deionized water. Sensing of MV by CP6-AuNPs via the Calculation of the Ratio of Aggregated/ Dispersed The MV-induced aggregation of CP6-AuNPs was monitored by UV-vis spectroscopy during the addition of MV to CP6-AuNPs. For each sample, different concentrations of MV (Table S1) were added to a suspension of CP6-AuNPs (2.0 mL, [CP6-AuNPs] = 1 3 108 M, Abs516nm = 0.50), followed by mixing and standing for 20 min at room temperature (25 C). UV-vis absorption spectra of the mixtures were then collected in the range of 350–1,000 nm. The concentration of MV was quantified by calculating the DA/D from the integrated intensity of the aggregated (A, ranging from 550 to 750 nm) and dispersed (D, ranging from 450 to 540 nm) CP6-AuNPs. Sensing of MV by CP6-AuNPs via the Ratio of the Intensity of the SPR Peaks at 600 nm and 516 nm Different concentrations of MV (Table S2) were added to aqueous solutions of CP6modified AuNPs (1.0 mL, [CP6-AuNPs] = 3 3 108 M, Abs516nm = 1.40). After standing for 20 min at room temperature (25 C), the solution UV-vis absorption spectra were collected in the range of 450–1,000 nm. The concentration of MV was quantified by dividing the absorbance at 600 nm by that of 516 nm (Abs600/Abs516). Catalytic Reduction of Nitrophenol Isomers by CP6-AuNPs Nitrophenol (110 mL, 3.0 mM) and deionized water (2,974 mL) were added to a standard quartz cuvette with 1-cm path length. When freshly prepared NaBH4 (150 mL, 0.1 M) was added, the color of the mixture changed from light-yellow to yellow-green immediately as a consequence of the formation of nitrophenolate
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ion. CP6-AuNPs (66 mL, 4 3 108 M) was added and the UV-vis absorption spectra were recorded in the range of 200–700 nm at various time intervals (5 min, 2 min, and 1.5 min for p-, m-, and o-nitrophenol, respectively). The Stability of CP6-AuNPs for Repeated Catalytic Cycles Nitrophenol (110 mL, 3.0 mM) and fresh NaBH4 (25 mL, 0.01 M) were added into the above mixture. The timer was restarted at once and the UV-vis absorption spectrum was recorded in the same range at various time intervals. The procedure was repeated five times.
SUPPLEMENTAL INFORMATION Supplemental Information can be found online at https://doi.org/10.1016/j.matt. 2019.03.005.
ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (51673084, 21871108), Jilin Province-University Cooperative Construction Project – Special Funds for New Materials (SXGJSF2017-3), the 111 project (No. B17020), Jilin University Talents Cultivation Program, and the Fundamental Research Funds for the Central Universities for financial support.
AUTHOR CONTRIBUTIONS Conceptualization, X.W. and Y.-W.Y.; Methodology, X.W. and Y.-W.Y.; Investigation, X.W., Z.-J.L., G.G., and Y.W.; Writing – Original Draft, X.W., E.H.H., and Y.-W.Y.; Writing – Review & Editing, Y.Z., P.S.W., J.Y., and Y.-W.Y.; Funding Acquisition, Y.-W.Y.; Resources, Y.W., J.Y., and Y.-W.Y.; Supervision, Y.-W.Y.
DECLARATION OF INTERESTS The authors declare no competing interests. Received: January 20, 2019 Revised: February 28, 2019 Accepted: March 15, 2019 Published: August 07, 2019
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