Simultaneous determination of Acetaminophen and dopamine based on a water-soluble pillar[6]arene and ultrafine Pd nanoparticle-modified covalent organic framework nanocomposite

Abstract

While numerous sensing strategies have been applied in the determination of Acetaminophen (AP), dopamine (DA), and ascorbic acid (AA), the selectivity is always a critical challenge based on their similar structure and function. Accordingly, the development of a highly selective sensing method is not only necessary but also crucial. In this study, a novel electrochemical sensing platform for the simultaneous determination of AP and DA has been successfully constructed based on a multifunctional nanocomposite (WP6-Pd-COF) of water-soluble pillar[6]arene (WP6), ultrafine Pd nanoparticles, and triethylene glycol-modified covalent organic framework (COF). Pd nanoparticles with an average size of 4.2 nm are prepared by reducing K₂PdCl₄ under the stabilization of oxygen-rich COF, which shows superior catalytic activity in electrochemical detection. A supramolecular host–guest recognition system introduced between WP6 and analytes (AP, DA, and AA) can effectively recognize AP and DA, implying the simultaneous determination of AP and DA by this approach. The electrode, best operated at a working potential range from −0.2 to 0.8 V (vs. Hg/Hg₂Cl₂), works in the concentration ranges of 0.2–8 μM for DA and 0.1–7.5 μM for AP, and has a detection limit of 0.06 μM for DA and 0.03 μM for AP (S/N = 3). Therefore, this study presents potential application values in sensing, catalysis, and other fields.

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1. Introduction

Electrochemical simultaneous determination of analytes is a promising method and frequently applied in the detection of drugs and physiologically active molecules.^1,2 Nevertheless, this method is sometimes impeded by the selectivity and the stability of the electrochemical sensing platform. Electrochemical sensing of Acetaminophen (AP), dopamine (DA), and ascorbic acid (AA) as samples is always a harsh challenge because the redox potential of these compounds is particularly close.^3–6 The critical measures for enhancing the redox potential differences are to prepare various materials and modify the electrode. These strategies aim to vary the electrode kinetics of analytes in order to make the potential shift from the interfering species. Moreover, introducing carbon nanotubes to the electrochemical system can be an alternative approach.^7–10 Although abundant efforts have been made in order to improve these drawbacks, the selectivity is still a problem. Consequently, exploring an effective method to recognize these species with high selectivity is an urgent task. The supramolecular host–guest system based on an macrocyclic host and guest is an ideal option, which can solve the selectivity.

During the past decades, with the advancement of macrocyclic chemistry, there are several types of macrocyclic hosts (crown ether, cyclodextrin, calixarene, and cucurbituril) with different structures and chemical features,^11–14 and they bring a series of applications in supramolecular host–guest recognition, sensing, smart materials, drug delivery, and so forth. As a promising and new macrocyclic host, pillar[n]arenes (pillar[5]arene and pillar[6]arene), which were first reported by Ogoshi et al. in 2008, have shown numerous attractive characteristics such as easy synthesis and chemical modification, symmetric and rigid architecture, controlled cavity size and electron-rich cavity, and simultaneously display properties similar to those other traditional macrocyclic hosts.^15–21 Based on these qualities, pillar[n]arenes have achieved a good deal of advancements in disparate fields, particularly in the recognition and separation of isomers.^22–26 As a result, pillar[n]arenes play an important role in the supramolecular host–guest recognition system and have potential value in sensing. More importantly, the composites between pillar[n]arene and two-dimensional materials could be an ideal candidate for constructing an electrochemical sensing platform.²⁷

Covalent organic frameworks (COFs), as excellent two-dimensional materials, which were first pioneered by Yaghi et al. in 2005, exhibit unique traits of high crystallinity, well-organized porosity, and high surface area. In addition, they are applied in various fields including catalysis, gas and energy storage, enantio-separation, and sensing.^28–33 Meanwhile, they can serve as an outstanding support and skeleton for loading metal nanoparticles and single atoms.^34,35 As a result, the obtained heterogeneous catalyst with multiple recycling can be used in catalysis and energy conversion. With these reports in mind, the preparation of a hybrid material of COF-anchored metal nanoparticles, which can accelerate the sensitivity of the guest and improve the stability of the sensing system, is a prospective idea.

Our previous works have studied the composites of pillar[n]arenes and COF, which are made via π–π stacking and hydrogen bond interaction and demonstrate applications in electrochemical sensing of species.^36,37 DA and AP are a type of positive and electron-deficient molecules that can be easily captured by the negative and electron-rich cavity of water-soluble pillar[6]arene (WP6). Moreover, the size of Pd nanoparticles is easier to regulate as compared to that of Au nanoparticles. Thus, the hybrid material based on WP6, Pd, and covalent organic framework (COF) can be used to detect DA and AP type molecules. In this study, we report a multifunctional nanomaterial (WP6-Pd-COF) of WP6, ultrafine Pd nanoparticles, and triethylene glycol-modified COF. The WP6-Pd-COF nanomaterial has been successfully applied in electrochemical simultaneous detection towards AP and DA (Fig. 1). Triethylene glycol-modified COF shows numerous O atoms that offer numerous coordination sites between O and Pd atoms, thus obtaining ultrafine Pd nanoparticles with an average size of 4.2 nm. In addition, WP6 can recognize AP and DA via the supramolecular recognition interaction, while AA cannot be captured by WP6, which implies that AP and DA can be simultaneously detected using the electrochemical method. This advance might resolve the key issue of highly selective determination of these species with electrochemical sensing. We consider our research to be pioneering in the preparation of multifunctional nanocomposites in catalysis, sensing, and other fields.

Fig. 1 The preparation procedure of WP6-Pd-COF and its application in the simultaneous electrochemical sensing of DA and AP.

2. Materials and experiments

2.1. Materials

Benzene-1,3,5-tricarbaldehyde (denoted as B) is obtained from Jilin Chinese Academy of Sciences-Yanshen Technology Co. Ltd. K₂PdCl₄, 1,4-dioxane, mesitylene, sodium tetrahydroborate (NaBH₄), acetic acid (CH₃COOH), and tetrahydrofuran (THF) were purchased from Shanghai Titan Scientific Co. Ltd. All chemicals and solvents used in the syntheses are of reagent grade and can be used without further purification. The triethylene glycol monomer (denoted as A) and COF are synthesized according to the previously published study.³⁸ Water-soluble pillar[6]arene (WP6) is prepared according to a previously reported procedure.^39,40 The synthetic processes and NMR spectra of WP6 and compound A are shown in the ESI (Schemes S1, 2 and Fig. S1–6†). The phosphate buffer solution (PBS, 0.1 M) with pH = 7.0 value is prepared by mixing 0.1 M Na₂HPO₄ and NaH₂PO₄. The instruments used for characterizing the obtained materials are described in the ESI.†

2.2. Synthesis of triethylene glycol modified COF

Compound A (78.0 mg, 0.15 mmol) and B (16.2 mg, 0.1 mmol) were charged into a glass ampoule. Then, a mixed solvent of 0.3 mL 1,4-dioxane and 0.9 mL mesitylene was added into the ampoule, and the aqueous solution was sonicated for 10.0 min at room temperature in order to get homogenous dispersion. Further, a CH₃COOH (0.4 mL of 6.0 M) aqueous solution was added and the ampoule was immersed in an ultrasonic bath for another 10.0 min. Subsequently, the ampoule was flash-frozen in a liquid nitrogen bath and sealed under vacuum. Upon warming to 25 °C, the ampoule was placed in an oven at 120 °C and left undisturbed for 72 h, yielding a white solid. The white solid was isolated via centrifugation and washed with acetone and THF for three times, respectively. The solid was soaked in dry acetone for 12 h and dried at 80 °C under vacuum for 12 h to obtain pure COF as a white powder (76 mg, 86% yield). The crystallinity of COF was assessed via powder X-ray diffraction (PXRD).

2.3. Synthesis of COF anchoring Pd nanoparticles

COF anchoring Pd nanoparticles (Pd-COF) was prepared according to a similar procedure.⁴¹ The as-synthesized COF (10.0 mg) was dispersed into 10.0 mL methanol using an ultrasonic bath to get a well-dispersed suspension. Thereafter, an aqueous solution of K₂PdCl₄ (0.1 mL, 20.0 mM) was added into the suspension of COF. The mixed solution was stirred for 2 h under room temperature to deposit Pd metal precursors in the COF framework. Then, a fresh aqueous solution of NaBH₄ (0.5 M, 1 mL) was added to the above mixture and continuously stirred for 24 h. Finally, the nanocomposite of Pd-COF was obtained via centrifugation and washed with ultrapure water for three times and freeze-dried. The content of Pd in COF was evaluated via the inductively coupled plasma (ICP) analysis.

2.4. Preparation of the WP6-Pd-COF nanocomposite

By controlling the assembly of WP6 onto the surface of Pd-COF, we could obtain the WP6-Pd-COF nanocomposite (Fig. 1). As-prepared powder of Pd-COF (20.0 mg) was again dispersed into 10.0 mL ultrapure water using an ultrasonic bath. Then, an aqueous solution of WP6 (10.0 mL, 1.0 mg mL⁻¹) was added into the well-dispersed suspension and continuously stirred for 24 h. In the end, the mixture was centrifuged and washed with ultrapure water for three times to remove unmodified WP6.

2.5. Electrochemical simultaneous determination of AP and DA

All electrochemical measuring processes are illustrated in the ESI.†

3. Results and discussion

3.1. Characterization of WP6-Pd-COF

The COF contains –C=N– bonds, formed by the reaction between –CHO and –NH₂ groups, which can be confirmed by the Fourier transform infrared (FT-IR) analysis. As shown in Fig. 2a, after the comparison of FT-IR spectra of starting materials A and B, a vibration band at 1656 cm⁻¹ can be clearly found, which is ascribed to the –C=N– stretching. In addition, the characteristic peaks of –NH₂ and –CHO of compounds A and B disappear during the reaction, implying successful and complete conversion into imine bonds.^38,42,43 Simultaneously, the –C–O– stretching modes at 1220 and 1025 cm⁻¹ also appear in the FT-IR spectrum of COF, which further prove the conversion.^44,45 When the Pd nanoparticles are loaded onto the surface of COF, the FT-IR spectrum of COF exhibits no change, suggesting excellent stability of the COF framework. In the FT-IR spectra of WP6-Pd-COF and WP6, the typical peak of –O–C=O– at 1714 cm⁻¹ of WP6 can be also observed. This result shows that the WP6-Pd-COF nanocomposite has been successfully obtained. Excellent thermostability of WP6-Pd-COF is confirmed via the thermogravimetric analysis (TGA) technique (Fig. 2b).

Fig. 2 FT-IR spectra of A, B, COF, Pd-COF, WP6-Pd-COF, and WP6 (a); TGA curve of WP6-Pd-COF (b); PXRD patterns of COF and WP6-Pd-COF (c); N₂ sorption-desorption isotherms of the COF and WP6-Pd-COF (d); XPS survey spectrum of WP6-Pd-COF (e); high resolution XPS spectrum of Pd 3d (f).

PXRD was performed to study the crystallinity of COF and WP6-Pd-COF. Fig. 2c shows the experimental PXRD pattern of COF, and numerous peaks are observed at 3.67°, 7.2°, and 26.0°, which are assigned to the (100), (200), and (001) diffractions, respectively.³⁸ From the experimental PXRD pattern of WP6-Pd-COF, the characteristic peaks of Pd nanoparticles have not been found. This is because of the lower loaded amount and the small size of Pd particles, which do not show diffractions peaks in the PXRD and the result is in accordance with the reported work.⁴⁶ Moreover, the diffraction peaks of COF are present, implying that Pd nanoparticles do not affect the skeleton stability. The surface areas and porosities of COF and WP6-Pd-COF are evaluated using the N₂ adsorption–desorption test at 77 K (Fig. 2d). The Brunauer–Emmett–Teller (BET) surface areas of COF (Fig. S7†) and WP6-Pd-COF (Fig. S8†) are measured to be 16 m² g⁻¹ and 9.6 m² g⁻¹, respectively. The small surface area is caused by the fact that the inner channels of COF and WP6-Pd-COF are fully decorated by triethylene glycol segments, Pd nanoparticles, and WP6. This result further confirms the above experimental conclusions of the successful preparation of COF and WP6-Pd-COF.

The elements and chemical valence of WP6-Pd-COF are measured via X-ray photoelectron spectroscopy (XPS). For the XPS survey of WP6-Pd-COF, a clear Pd element can be observed (Fig. 2e), indicating that the Pd nanoparticles were successfully anchored. For the high resolution XPS spectrum of COF, the C 1s (Fig. S9†) and O 1s (Fig. S10†) spectra show C=C, C–O, C=N, and C=O bonds, which confirm the chemical composition of COF. Furthermore, the high resolution XPS spectrum of O 1s for WP6-Pd-COF demonstrates –O–C=O–, which is generated from the carboxylic acid group of WP6. The high resolution XPS spectrum of Pd 3d (Fig. 2f) indicates the presence of Pd(0) species. This appearance can be also observed in other metal nanoparticles that are in contact with air.⁴⁷ The energy dispersive X-ray (EDX) spectrum of WP6-Pd-COF shows the Pd content to be 1.67 wt% (Fig. S11†), which agrees with the XPS analysis.

3.2. Microstructure of WP6-Pd-COF

Transmission electron microscopy (TEM) was implemented to study the microstructure of the WP6-Pd-COF nanomaterial. As demonstrated in Fig. 3a, typical Pd nanoparticles are uniformly deposited onto the surface of COF. The high resolution TEM image of Pd nanoparticles (Fig. 3b) exhibits the interplanar spacing of 0.29 nm of the Pd sphere (Fig. 3c), which is ascribed to the face-centered cubic (fcc) structure. Fig. 3d displays the size distribution of Pd particles, suggesting that the average diameter of Pd particles is 3.812 ± 0.619 nm. This data is consistent with the PXRD pattern of WP6-Pd-COF. In addition, EDS elemental mappings (Fig. 3e–i) affirm that C, N, O, and Pd elements are uniformly distributed.

Fig. 3 TEM images of WP6-Pd-COF with different magnification times (a) and (b); high resolution TEM image of Pd nanoparticle (c); the size distribution histograms of the Pd nanoparticles (d); EDS mappings for C, N, O, and Pd elements of WP6-Pd-COF (e–i).

3.3. Electrochemical sensing of DA and AP

Due to the redox properties of DA and AP, an electrochemical technique can be applied to detect them. However, AA can cause interference during the detection of DA and AP. Therefore, we evaluated the interference ability of AA. Various modified electrodes were used to certify the detection ability towards DA and AP. As shown in Fig. 4a, two weaker peaks are found in the bare electrode (black curve). The peaks are enhanced by the Pd-COF-modified electrode (red curve), which is the result of the high catalytic activity of Pd nanoparticles. More importantly, the peak intensities are further increased by the WP6-Pd-COF electrode than that by bare and Pd-COF electrodes. The reason is the supramolecular host–guest interaction, which leads to a higher concentration of DA and AP on the surface of the electrode. Consequently, the WP6-Pd-COF electrode has excellent activity towards DA and AP and can detect them using the electrochemical method, simultaneously. More importantly, no signal can be detected at the WP6-Pd-COF electrode in the 0.1 M pH 7.0 PBS solution without DA and AP (Fig. S12†), implying that the observed peaks are caused by the reaction of DA and AP. The quantitative detection of DA and AP is carried out by the DPV method. As indicated in Fig. 4b, two oxidization clear peaks can be observed, and the response signal of the current intensity increases with the enhancement of DA and AP concentrations, indicating excellent linear relation between current intensity and concentration. Therefore, the calibration curves for the quantitative determination DA and AP are obtained and shown in Fig. 4c and d according to the data from Fig. 4b. The corresponding regression equation of I (μA) = 1.2 (μM) + 0.7 and correlation coefficient of 0.9957 for DA, the equation of I (μA) = 1.12 (μM) + 0.26 and correlation coefficient of 0.9912 for AP are analyzed with ideal results, respectively. The two detection limits of 0.06 μM for DA and 0.03 μM for AP (S/N = 3) are obtained. Moreover, the detection of DA in the presence of AP (6.0 μM) is implemented and still shows a good linear relationship (Fig. S13 and S14†). The detection of AP in the presence of DA (6.0 μM) also shows a good linear relationship (Fig. S15 and S16†), simultaneously. These results imply that the co-instantaneous detection of DA and AP is workable. Comparing the detection limit and the range in our strategy with reported works, our method promises a notable advancement, as demonstrated in Table S1.† We evaluate the selectivity of the functional electrochemical sensing strategy of the WP6-Pd-COF-modified electrode. By detecting numerous interferents containing 50.0 μM analogues (including AA, UA, cysteine, sucrose, and glucose) and common substances (including NaCl and KCl), almost no signal is found, suggesting high selectivity in our system (Fig. S17†). Furthermore, the practical analytical performance of the WP6-Pd-COF nanocomposite film-modified GCE has been evaluated for the detection of DA and PA via standard addition recovery methods in human urine samples. The DA and PA powders are dissolved in 0.1 M pH 7.0 PBS for the real sample analysis. Before the examination, collected human urine samples (pH 6.8) from the local school hospital of Yangtze Normal University were filtered several times using a filter paper. Filtered human urine samples were diluted in pH 7.0 PBS in the ratio of 1 : 20. As illustrated in Table S2,† the recoveries with 97.0–107% (97.6–102%) and the RSDs with 2.06–6.6% (2.9–5.06%) for DA (AP) are obtained by this method, implying that the highlighted accuracy and precision are satisfactory, and has potential applications in the field of sensing.

Fig. 4 CV curves of different modified electrodes with bare GCE, Pd-COF, and WP6-Pd-COF in solution of PBS (0.1 M pH 7.0) containing DA (2.0 μM) and AP (6.0 μM) (a); peak current intensity versus various concentrations of DA and AP (concentrations: 0.2, 0.8, 1.5, 2.0, 3.0, 4.5, 5.5, 6.0, 7.0, and 8.0 μM for DA; 0.1, 0.6, 1.2, 1.8, 2.5, 4.0, 5.0, 5.5, 6.0, and 7.5 μM for AP) at WP6-Pd-COF modified electrode (b); calibration plots for the determination of DA (c) and AP (d), respectively.

3.4. Host–guest interaction analysis

The outstanding selectivity and sensing performance are generated from the specific supramolecular recognition. Therefore, the host–guest interaction between WP6 and DA/AP is studied via¹H NMR. As shown in Fig. 5a and b, the protons H6, H7, and H8 in AP shift upfield when the same concentration of WP6 is added into the AP solution due to the shielding effect of the electron-rich cavity of WP6.⁴⁸ The protons H1, H2, H3, H4, and H5 in DA shift upfield distinctly after the addition of WP6 at the same time (Fig. 5c, d, and e ). More importantly, the protons of WP6 can shift. These results imply strong host–guest interaction between WP6 and DA/AP. However, almost no chemical shift can be observed in the host–guest spectrum of WP6 and AA (Fig. S18†), indicating that the AA cannot enter the cavity of WP6. This result explains why no AA signal is detected in the electrochemical method. In addition, the molecular size of DA and AP is calculated to be 0.43 nm by the Materials Studio simulation (Fig. S19†), which is smaller than the cavity of WP6 (0.67 nm) according to the reported study.⁴⁸ However, the size of xanthine (the width of 0.66 nm) is almost close to that of WP6, the length of xanthine is 0.72 nm (Fig. S19†), and thus we speculate that there is no recognition between WP6 and xanthine. Consequently, according to the above results, the DA/AP can be easily captured and detected by WP6 based on electrostatic interaction, matched molecular and cavity size, and hydrophobic effects.⁴⁸ These unique interactions are responsible for the electrochemical sensing platform of the WP6-Pd-COF-modified electrode to have high selectivity and sensitivity for DA/AP detection.

Fig. 5 ¹H NMR spectrum (500 MHz, D₂O, rt) of 10.0 mM AP (a), 10.0 mM AP + 10.0 mM WP6 (b), 10.0 mM WP6 (c), 10.0 mM DA + 10.0 mM WP6 (d), and 10.0 mM DA (e), respectively.

4. Conclusions

We have designed and constructed a functional nanocomposite based on water-soluble pillar[6]arene, ultrafine Pd nanoparticles, and covalent organic framework. The Pd nanoparticles can be obtained using a site reduction method by the multiple anchored sites in the triethylene glycol-modified COF. Then, the host molecule WP6 is loaded on the surface of COF by the hydrogen bond and π–π stacking. The obtained functional WP6-Pd-COF-modified electrode shows excellent electrochemical performance for the simultaneous determination of DA and AP. Therefore, our study offers potential applications in catalysis, sensing, and other fields.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Natural Science Foundation of Chongqing, China (Grant No. cstc2020jcyj-msxmX0341), the Education Commission of Chongqing, China (Grant No. KJQN202001410), the Science and Technology Program of Fuling, China (Grant No. FLKJ, 2019ABB2040), the authors thank the support of this work by the Program for Innovation Team Building at Institutions of Higher Education in Chongqing, China (Grant No. CXTDX201601039).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0an01717e

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