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Pyrene-functionalized halloysite nanotubes for simultaneously detecting and separating Hg(ii) in aqueous media: A comprehensive comparison on interparticle and intraparticle excimers

  • Haiyun Fan , Pengying Jia , Hailei Zhang EMAIL logo , Zhiyi Su , Xinwu Ba and Yonggang Wu EMAIL logo
Published/Copyright: May 26, 2022
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Nanotechnology Reviews
From the journal Nanotechnology Reviews Volume 11 Issue 1

Abstract

The development of a facile approach to simultaneously detect and separate Hg(ii) ions in an aqueous solution is a challenging topic in the chemosensing field. Herein, we focus on constructing the Hg(ii)-sensitive fluorescence “turn-on”-type composite materials by using halloysite nanotube (HNT) as substrate. Two types of HNTs-based chemosensors, HNTs-PHT and HNTs-BP, were developed in this study, which exhibit Hg(ii)-sensitive fluorescence “turn on” behavior by forming interparticle and intraparticle excimers, respectively. Fortunately, HNTs-PHT is able to effectively restrict the solvent relaxation of π–π* transition and make it a better detection tool in aqueous solution than HNTs-BP. The addition of Hg(ii) can trigger a dramatical increase at 469 nm in emission curves of HNTs-PHT, which cannot exhibit emission behaviors without the addition of Hg(ii). Furthermore, the formation of interparticle excimers makes Hg(ii) serve as a crosslinker to aggregate HNTs-PHT into precipitations. Following this way, Hg(ii) ions can be facilely removed from the water via a simple filtration or centrifugation approach. The as-prepared HNTs-PHT shows high specificity and precision in simultaneously detecting and separating Hg(ii) without the recourse to energy consumption, which will give a novel insight to deal with heavy metal pollution.

Keywords: halloysite nanotube; pyrene; clay; chemosensor; Hg(ii)

1 Introduction

The exceeded hazard of metal salts, especially the heavy metal ions in water, is an increasingly serious problem in the environmental protection field around the world in recent years [1,2]. The nonbiodegradable nature, bioaccumulation, and high toxicity give rise to severe health risks for humans [3,4]. Among the various heavy transition metals, mercury (Hg) is serving as the main role in heavy metal pollution, which has been reported to be usually involved in wastewater [5]. It should be noted that Hg is highly toxic that even 10–6 M can also result in irretrievable damages to health, even death [6,7]. A prime challenge for researchers is, therefore, to develop a facile, versatile, specific and sensitive system for simultaneous detection and separation of Hg(ii) ions in water.

Up to now, some analytical methods have been built for detecting Hg(ii) ions, including localized surface plasmon resonance (LSPR) [8], high-performance liquid chromatography (HPLC) [9] and electrochemistry technique [10,11]. The fluorescence-based method shows promising advantages including naked eye visualization capability, high selectivity and facile operation when used in the detection of metal ions [12,13]. It is worth noting that planar-conjugated molecules, the main source of fluorescent chromophores, often show poor water solubility and aggregate to form π–π accumulations in water [14]. So, some unexpected phenomena usually take place when dissolving or dispersing the planar-conjugated molecules in water, e.g. aggregation-caused quenching, formation of excimers, large fluctuation in fluorescence intensity, and poor reproducibility [15,16]. At present, the use of planar-conjugated molecules as detection tools is mainly limited to organic solvents or mixing solvents. Very few cases can be used in the pure water phase, which seriously reduces their practical application level. Functionalization of the planar-conjugated molecules with hydrophilic groups can improve the water solubility but may be not effective to solve the solvent relaxation problems of π–π* transition in water. The amphipathic molecules are easy to form micelles and also contribute to the unexpected influence on fluorescence behaviors [17,18]. These problems inspired us to explore novel detection systems with good feasibility in water.

There are some studies reported in the literature regarding Hg(ii) detection and removal based on physically immobilizing organic ligands onto nanomaterials aiming to meet the application in an aqueous solution [19,20]. However, the physical immobilization usually suffers from serious stability problems. Our hypothesis is to covalently functionalize the chemosensor on the surface of nanoparticles with good water dispersion and surface charge repulsion [21], which is quite beneficial to overcome the stability problems in physically immobilized nanocomposites.

In this way, the self-aggregation behavior and solvent relaxation of the π–π* transition can be effectively suppressed. Otherwise, the formation of interparticle excimers makes the Hg(ii) ions act as crosslinking points and drive the nanoparticles into precipitation, meeting the requirement of Hg(ii) separation. Halloysite nanotube (HNT) [22,23], a natural tubular nanomaterial, was selected as the substrate for the construction of the smart tool for simultaneous detection and separation of Hg(ii) ions. Benefiting from the hollow tubular structure, HNTs exhibit lower relative density and thereby better water dispersion than other commonly used nanoparticles such as silica [24,25]. The self-aggregation behavior, usually taking place in nanoparticles, is effectively suppressed based on the external charge distribution [26]. Other favorable merits including good biocompatibility [27,28], nontoxicity [29], non-degradation [30], hydrophilic [31] and low-cost [32] also make HNTs promising materials in the field of environmental protection [33,34,35,36], biomaterial [37,38,39,40], catalyst [41,42,43], battery [44], sensor [45,46], and coating [47]. Herein, the HNTs-based smart tool for the simultaneous detection and separation of Hg(ii) ions was facilely developed by anchoring the pyrene-containing chemosensor on the surface of HNTs, which exhibit an obvious “turn on” response on fluorescence after the addition of Hg(ii) in water. Moreover, the Hg(ii)-chelated aggregation would enlarge the particle sizes and result in macroscopic precipitations, which can serve as a facile separation approach without the recourse to energy consumption. As a result, a smart tool for simultaneously detecting and separating Hg(ii) in aqueous media was developed, which may pave a new path for managing metal ions not merely Hg(ii). Further studies will focus on exploiting more HNT-based detection systems with more simplified processes.

2 Experimental

2.1 Materials

HNTs were obtained from GuangZhou Shinshi Metallurgy and Chemical Co., Ltd., and purified before use [48]. Aminated HNTs (HNTs-NH2) and acyl chloride-bearing HNTs (HNTs-COCl) were prepared according to our previous work [49,50]. 4-Bromothiophene-2-carbaldehyde (A1), hydrazine hydrate, o-nitrophenol (B1), 1,3-dibromo-2-propanol (B2), 1-pyrenealdehyde (A3) and anhydrous SnCl2 in the analytical grade were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. 4-(Formylphenyl)boronic acid (A5) was purchased from Soochiral Chemical Science & Technology Co., Ltd. Oxalyl chloride and KH550® were obtained from Energy-Chemical.

2.2 Synthesis and preparation

2.2.1 Synthesis of A2

4-Bromothiophene-2-carbaldehyde (A1, 1.00 g, 5.24 mmol), MgSO4 (1.26 g, 10.48 mmol) and hydrazine hydrate (1.00 mL) were dissolved in ethanol (30 mL) and heated at 80°C under N2 atmosphere for 6 h. Then, the solution was cooled to room temperature and extracted three times with dichloromethane and saturated NaCl solution. After removal of the solvent in a vacuum, the residue was chromatographically purified on silica gel eluting with dichloromethane to afford A2 as a white solid (1.05 g, 98%). 1H NMR (400 MHz, CDCl3) δ (ppm): 7.80 (s, 1H), 7.11 (s, 1H), 6.94 (s, 1H), 5.55 (s, 2H). 13C NMR (100 MHz, CDCl3) δ (ppm): 181.84, 144.04, 137.97, 132.26, 111.51. MS (ESI): m/z: calcd for C5H5BrN2S: 205.07 [M]+; found: 206.86 [M + 2H]+.

2.2.2 Synthesis of A4

Compound A2 (0.70 g, 3.41 mmol), 1-pyrenealdehyde (A3, 0.86 g, 3.75 mmol) and MgSO4 (0.82 g, 6.82 mmol) were dissolved in absolute ethanol (30 mL) and stirred at 80°C under N2 atmosphere for 15 h. Then, the solution was cooled to room temperature and extracted for three times with dichloromethane and saturated NaCl solution. After removal of the solvent, the residue was chromatographically purified on silica gel eluting with petroleum ether/dichloromethane (3:1, v/v) to afford A4 as a faint yellow solid (0.98 g, 69%). 1H NMR (400 MHz, d 6-DMSO) δ (ppm): 9.67 (s, 1H), 9.14 (d, J = 6.4 Hz, 1H), 9.04 (s, 1H), 8.69 (d, J = 5.6 Hz, 1H), 8.42–8.35 (m, 4H), 8.32 (d, J = 6.0 Hz, 1H), 8.25 (d, J = 6.0 Hz, 1H), 8.15 (t, J = 10.4 Hz, 1H), 7.96 (s, 1H), 7.75 (s, 1H). 13C NMR (100 MHz, d 6-DMSO) δ (ppm): 161.96, 155.98, 155.60, 140.41, 140.01, 135.98, 135.82, 133.57, 131.28, 130.63, 130.42, 129.73, 129.66, 129.14, 128.91, 127.91, 127.34, 127.26, 126.97, 126.72, 126.62, 125.66, 124.60, 124.11, 123.82, 110.29. MS (ESI): m/z: calcd for C22H13BrN2S: 417.32 [M]+; found: 418.72 [M+H]+.

2.2.3 Synthesis of A6

Compound A4 (0.30 g, 0.72 mmol), (4-formylphenyl)boronic acid (A5, 0.13 g, 0.86 mmol), K2CO3 (0.6 g, 4.34 mmol) and Pd(PPh3)4 (8 mg, 0.007 mmol) in H2O (4 mL) and tetrahydrofuran (THF) (30 mL) were carefully degassed. The mixture was heated by microwave reactor at 65°C and stirred under N2 atmosphere for 2.5 h. After cooling to room temperature, saturated NaCl solution and dichloromethane (2:1, v/v) were added. The organic layer was separated and then stirred with anhydrous Na2SO4. After filtration and removal of the solvent in vacuum, the residue was chromatographically purified on silica gel eluting with petroleum ether/dichloromethane (2:1, v/v) to afford A6 as a faint yellow solid (302 mg, 95%). 1H NMR (400 MHz, d 6-DMSO) δ (ppm): 10.05 (s, 1H), 9.73 (s, 1H), 9.16 (d, J = 9.6 Hz, 1H), 9.13 (s, 1H), 8.74 (d, J = 8.4 Hz, 1H), 8.42 (m, 5H), 8.33 (s, 1H), 8.27 (d, J = 8.4 Hz, 2H), 8.17 (t, J = 15.6 Hz, 1H), 8.01 (s, 4H). MS (ESI): m/z: calcd for C29H18N2OS: 442.54 [M]+; found: 442.86 [M]+; FTIR: 3,447, 3,103, 3,039, 3,955, 2,921, 2,848, 2,760, 1,918, 1,875, 1,685, 1,600, 1,308, 1,294, 1,249, 1,231, 1,225, 1,211, 1,187, 1,171, 937, 901, 849, 830, 785, 716, 681, 660, 624, 612, 547, 535, 509, 499.

2.2.4 Synthesis of B3

O-Nitrophenol (B1, 3.00 g, 21.56 mmol), 1,3-dibromo-2-propanol (B2, 1.90 g, 8.72 mmol) and KOH (3.00 g, 17.86 mmol) were dissolved in dimethyl sulfoxide (15 mL) under N2 atmosphere. The solution was heated at 90°C for 4 h. After cooling to room temperature, the solution was extracted three times with dichloromethane and saturated NaCl solution. The organic layer was collected and then stirred with anhydrous Na2SO4. After filtration and removal of the solvent in a vacuum, the residue was chromatographically purified on silica gel eluting with dichloromethane to afford B3 as a faint yellow solid (85%). 1H NMR (400 MHz, CDCl3) δ (ppm): 7.88 (d, J = 8.0 Hz, 2H), 7.56 (t, J = 16.0 Hz, 2H), 7.16 (d, J = 8.0 Hz, 2H), 7.06 (t, J = 16.0 Hz, 2H), 4.43 (m, 2H), 4.34 (m, 2H), 3.04 (br, 1H). 13C NMR (100 MHz, CDCl3) δ (ppm): 151.98, 139.60, 134.73, 125.95, 121.11, 115.06, 69.56, 68.01. MS (ESI): m/z: calcd for C15H14N2O7: 334.28 [M]+; found: 335.00 [M+H]+.

2.2.5 Synthesis of B4

Compound B3 (1.50 g, 4.49 mmol) was dissolved in an ethanol/THF mixed solution (5:1, v/v) with the addition of an HCl solution containing anhydrous SnCl2. The system was carefully degassed and heated at 80°C for 8 h under a nitrogen atmosphere. After cooling to room temperature, saturated NaCl solution and dichloromethane (2:1, v/v) were added. The organic layer was separated and then stirred with anhydrous Na2SO4. After filtration and removal of the solvent, the residue was chromatographically purified on silica gel eluting with dichloromethane to afford B4 as a brown viscous liquid (1.11 g, 90%). 1H NMR (400 MHz, CDCl3) δ (ppm): 6.89–6.75 (m, 8H), 4.41 (m, 1H), 4.17 (m, 4H), 3.86 (s, 5H). 13C NMR (100 MHz, CDCl3) δ (ppm): 148.98, 135.76, 122.02, 118.99, 115.19, 111.35, 70.02, 68.89. MS (ESI): m/z: calcd for C15H18N2O3: 274.32 [M]+; found: 275.04 [M+H]+.

2.2.6 Synthesis of B5

Compound B4 (0.60 g, 2.19 mmol), 1-pyrenealdehyde (A3, 1.11 g, 4.81 mmol) and MgSO4 (3.87 g, 8.76 mmol) were dissolved in absolute ethanol (50 mL) and stirred at 80°C under N2 atmosphere for 15 h. Then, the solution was cooled to room temperature and extracted for three times with dichloromethane and saturated NaCl solution. After removal of the solvent, the residue was chromatographically purified on silica gel eluting with petroleum ether/dichloromethane (2:1, v/v) to afford B5 as a yellow solid (0.92 g, 60%). 1H NMR (400 MHz, d 6-DMSO) δ (ppm): 9.52 (s, 2H), 9.27 (d, J = 9.2 Hz, 2H), 8.64 (d, J = 8.0 Hz, 2H), 8.36 (d, J = 7.6 Hz, 2H), 8.28 (d, J = 7.6 Hz, 6H), 8.20–8.16 (m, 4H), 8.10 (t, J = 15.2 Hz, 2H), 7.28 (d, J = 9.2 Hz, 2H), 7.11–6.99 (m, 6H), 5.47 (d, J = 4.0 Hz, 1H), 4.35–4.26 (m, 5H). 13C NMR (100 MHz, d 6-DMSO) δ (ppm): 163.65, 155.25, 144.64, 134.36, 131.24, 130.24, 129.40, 129.33, 129.03, 127.86, 127.65, 127.03, 126.68, 126.35, 125.40, 124.51, 124.13, 123.77, 122.33, 121.62, 116.86, 116.24, 115.27, 72.90, 68.33. MS (ESI): m/z: calcd for C49H34N2O3: 698.82 [M]+; found: 698.49 [M]+; FTIR: 3,351, 3,044, 2,923, 2,868, 1,922, 1,801, 1,674, 1,610, 1,595, 1,579, 1,538, 1,506, 1,486, 1,448, 1,382, 1,241, 1,188, 1,111, 1,033, 898, 847, 819, 747, 712, 679, 607.

2.2.7 Preparation of HNTs-PHT

Compound A6 (80 mg, 0.18 mmol) and anhydrous MgSO4 (65 mg, 0.54 mmol) were dissolved into 20 mL of ethanol. Then, 200 mg of HNTs-NH2 was added to the solution. The mixture was refluxed for 8 h and then centrifuged to collect the residue. The residue was purified by washing with water, ethanol and dichloromethane. After drying in a vacuum, the pyrene-functionalized product HNTs-PHT was achieved as a yellow solid (160 mg).

2.2.8 Preparation of HNTs-BP

HNTs-COCl (270 mg), compound B5 (80 mg) and triethylamine (800 μL) in 5 mL of DMSO were vigorously stirred at 50°C under an N2 atmosphere. After cooling to ambient temperature, the suspension was centrifuged to collect the residue. The residue was then purified by washing with water and ethanol. After drying in a vacuum, the bispyrene-containing product HNTs-PB was achieved as a yellow solid (260 mg).

2.3 Characterizations

Transmission electron microscopy (TEM) observations and energy-dispersive spectrometer (EDS) mapping were performed on a field emission transmission electron microscope (Tecnai G2 F20 S-TWIN, FEI). Attenuated total refraction FTIR spectra were recorded on a Thermo Fisher Scientific NICOTET IS10 FTIR spectrophotometer in the region of 4,000–400 cm−1. NMR data were recorded on a WNMR spectrometer or a Bruker Advance III spectrometer (400 MHz). X-ray photoelectron spectroscopy (XPS) data were collected via a Thermo Scientific ESCALab 250Xi XPS equipped with 200 W monochromated Al Ka radiation. Thermogravimetric analysis (TGA) was performed on a Pyris1 TGA instrument under the nitrogen atmosphere. MS analyses were obtained by using a Bruker Daltonics Autoflex III. Turbidity test and fluorescence experiments were conducted on a Shimadzu UV-Vis spectrometer model UV-2550 and a Shimadzu RF-5301PC photometer, respectively.

3 Results and discussion

3.1 Synthesis and structural characterization

Two types of HNTs-based chemosensors were developed for simultaneously detecting and separating Hg(ii) ions (Scheme 1). As for Type I, HNTs-PHT was prepared by anchoring aldehyde-containing pyrene derivative (A6) on the surface of aminated HNTs (HNTs-NH2) via a typical Schiff base reaction, in which “P” represents “pyrene,” “H” represents “hydrazone” and “T” represents thiophene. The pyrene–hydrazone–thiophene moiety in obtained HNTs-PHT can serve as an ionophore to Hg(ii) ion and trigger a “turn on” fluorescence by the formation of interparticle excimer. As for the other type of HNTs-based chemosensor, HNTs-BP was synthesized by treating acyl chloride-bearing HNTs (HNTs-COCl) with bispyrene-based chemosensor (B5) via the reaction between –COCl and –OH groups, in which the “BP” is abbreviated for bispyrene. The nitrogen and oxygen atoms in the backbone of B5 moiety are able to chelate with Hg(ii) ions with the formation of intraparticle excimer. The reactants were kept in high concentrations in the above-mentioned Schiff base reaction and esterification reaction aiming to afford desirable grafting degrees. The as-obtained HNTs-PHT and HNTs-BP were carefully characterized by FTIR, TGA, solid-state NMR and XPS.

Scheme 1 Synthetic approaches to HNTs-PHT and HNTs-BP.
Scheme 1

Synthetic approaches to HNTs-PHT and HNTs-BP.

The FTIR spectrum of HNTs features three distinct peaks at 3,701, 3,625 and 1,030 cm–1 in Figure S18. The stretching vibration around 1,030 cm−1 should be attributed to the in-plane Si–O–Si. Owing to the curly and multilayer character of HNTs, the Al–OH groups are categorized into two different kinds. As a result, the band in the FTIR spectrum of HNTs divides into two peaks. The peak at 3,701 cm–1 should be attributed to the stretching vibrations of Al–OH on the lumen surface, meanwhile, the peak at 3,625 cm–1 can be ascribed to the Al–OH groups inside the multilayered structure. It should be noted that the peaks assigned to Al–OH groups and Si–O–Si groups can also be detected after the amination approach, Schiff base reaction and esterification reaction, suggesting that the aluminosilicate composites did not undergo serious damage in the modification processes.

13C solid-state NMR spectra of HNTs-PHT and HNTs-BP are shown in Figure 1, in which the peaks have been completed and interpreted. Original HNTs cannot display signals in the 13C solid-state NMR spectrum, which has been evidenced in our previous study [51]. So, the peaks in HNTs-PHT and HNTs-BP should be caused by the carbons in A6 and B5 moieties, respectively. As for HNTs-PHT, the carbons in –C═N– units give the signals in high-frequency regions, e.g. 159.90 and 148.46 ppm. The strong peaks located at 131.11, 128.27 and 122.38 ppm should be assigned to the carbons in pyrene groups and partly carbons in thiophene moiety. The peaks less than 40 ppm should be attributed to the carbons in saturated alkyl chains. The 13C solid-state NMR spectrum of HNTs-BP also displays the peaks at 159.42, 153.86, 14.54, 128.75, 124.47 and 108.88 ppm, which should be assigned to carbons in unsaturated groups, including benzene unit and pyrene groups. The weak peak at 64.01 ppm is caused by the carbons in saturated units near oxygen atoms. The NMR results match well with the 13C NMR data in Figure S11.

Figure 1 13C solid-state NMR spectra of HNTs-PHT (a) and HNTs-BP (b).
Figure 1

13C solid-state NMR spectra of HNTs-PHT (a) and HNTs-BP (b).

To further reveal the composition of HNTs-PHT and HNTs-BP, XPS analyses were conducted and are summarized in Figure 2. Figure 2a shows the XPS curves of HNTs, HNTs-PHT and HNTs-BP. The XPS spectrum of original HNTs shows the existence of aluminum (Al 2s and Al 2p) and silicon (Si 2s and Si 2p) in the range from 154 to 75 eV, matching well with the components in aluminosilicate clays. The strongest peak can be found at 532 eV, which should be assigned to O 1s signal. Regions from 290 to 280 eV relating to C 1s are expanded in Figure 2b. Original HNTs display a weak C 1s signal, which may be caused by the naturally curly process. HNTs-PHT and HNTs-BP show much stronger intensity for the C 1s peak at 284.8, which should be attributed to the anchoring of the chemosensors onto the HNTs. The presence of unsaturated groups contributes to the peaks around 288.5 eV.

Figure 2 XPS studies [(a) XPS curves of HNTs, HNTs-PHT and HNTs-BP; (b) C 1s regions of HNTs-PHT and HNTs-BP; (c) N 1s regions of HNTs-PHT and HNTs-BP; and (d) S 2p regions of HNTs-PHT].
Figure 2

XPS studies [(a) XPS curves of HNTs, HNTs-PHT and HNTs-BP; (b) C 1s regions of HNTs-PHT and HNTs-BP; (c) N 1s regions of HNTs-PHT and HNTs-BP; and (d) S 2p regions of HNTs-PHT].

Figure 2c shows the expanded regions from 411 to 394 eV relating to N 1s regions. The N 1s signal is undetected in original HNTs. Because of the presence of nitrogen in A6 and B5 moieties, both HNTs-PHT and HNTs-BP display N 1s peaks in the XPS curves. The difference in peak value and peak pattern can be clearly found between HNTs-PHT and HNTs-BP. As for HNTs-BP, nitrogen can be found in –NH– and –N═C– groups. The presence of –C═N–N═C– gives rise to a more complex peak pattern in HNTs-PHT. Moreover, the presence of S can be evidenced by the S 2p peak at 164.1 eV in Figure 2d. The XPS observations, as well as NMR findings, are in agreement with the expected structures of HNTs-PHT and HNTs-BP. Furthermore, the grafting degrees were calculated from the TGA curves depicted in Figure 3. The grafting degrees are 6.2, 9.2 and 9.8% for HNTs-NH2, HNTs-PHT and HNTs-BP, respectively.

Figure 3 TGA curves of HNTs, HNTs-NH2, HNTs-PHT and HNTs-BP.
Figure 3

TGA curves of HNTs, HNTs-NH2, HNTs-PHT and HNTs-BP.

3.2 Optical properties and the detection properties upon Hg(ii)

The fluorescent emission behavior of HNTs-BP was evaluated in water, which displayed an unexpected emission peak at 469 nm (Figure 4a) with the fluorescence quantum yield (Φ) at 0.06. Pyrene usually displays its emission peak at ca. 430 nm, while the emission peak of 469 nm should be attributed to the solvent effect rather than the monomer emission. Generally, pyrene is very sensitive to the polarity of the solvent [52,53]. In nonpolar solvents, e.g. THF and CH2Cl2, the fluorescence signal originates from pyrene only for monomer emission. In addition to the monomer emission, the formation of intense excimer emission can take place in polar solvents, due to the solvent relaxation of the π–π* transition. Hence, the titration of HNTs-BP with polarity was conducted by preparing the stock solution of HNTs-BP in only THF and, then, varying the water concentration from 0 to 90%. HNTs-BP does not show any excimer emission in only THF solvent (Figure S20). With the gradual addition of water, the fluorescence intensity increases significantly with obvious bathochromic shifts. The results show that the intramolecular rotation of the pyrene platform is restricted, which results in the formation of excimers without recourse to Hg(ii). After the addition of Hg(ii), the fluorescence intensity at 469 nm was increased by ca. 2-fold without any bathochromic or hypsochromic shift, in which the Φ increased to 0.09. In this case, Hg(ii) can chelate with N and O atoms in HNTs-BP and force the pyrene platform to form more excimers (Figure 4b). However, the strong background fluorescent emission and the inconspicuous difference in fluorescence intensity make HNTs-BP not a desirable detection tool for Hg(ii).

Figure 4 Study on luminescence properties. [(a) Fluorescence emission spectra of HNTs-BP with (iv) or without (iii) the addition of Hg(ii); (b) mechanism of the Hg(ii)-triggered intraparticle excimer in case of HNTs-BP; (c) fluorescence emission spectra of HNTs-PHT with (ii) or without (i) the addition of Hg(ii); and (d) mechanism of the Hg(ii)-triggered interparticle excimer in case of HNTs-PHT].
Figure 4

Study on luminescence properties. [(a) Fluorescence emission spectra of HNTs-BP with (iv) or without (iii) the addition of Hg(ii); (b) mechanism of the Hg(ii)-triggered intraparticle excimer in case of HNTs-BP; (c) fluorescence emission spectra of HNTs-PHT with (ii) or without (i) the addition of Hg(ii); and (d) mechanism of the Hg(ii)-triggered interparticle excimer in case of HNTs-PHT].

For the other case, when HNTs-PHT dispersed into water, non-fluorescent emission behavior can be observed (Φ < 0.001) (iii in Figure 4c). The solvent relaxation of the π–π* transition was effectively reduced. The N and S atoms in HNTs-PHT are able to chelate with Hg(ii) and force the pyrene moieties to afford interparticle excimers, which can cause a dramatical enhancement of fluorescence. Following this way, a remarkable fluorescence “turn on” behavior from nonfluorescent to strong fluorescent (iv in Figure 4c) can be found with the addition of Hg(ii) with an increased Φ value at 0.07. The remarkable fluorescence enhancement at 469 nm matches well with the proposed mechanism in Figure 4d. As a result, HNTs-PHT shows a much better detection performance in an aqueous solution than HNTs-BP.

To evaluate the detection accuracy and sensitivity of HNTs-PHT to Hg(ii), an aqueous solution with different quantities of Hg(ii) was added to HNTs-PHT aqueous suspension to reveal the relationship between fluorescence intensity and the concentration of Hg(ii). Plot of fluorescence intensity (I x ) of HNTs-BP aqueous suspension vs the concentration of Hg(ii) ([Hg(ii)]) was recorded. The fitting results indicate that the linear regression for I x and [Hg(ii)] exhibit a good linear correlation coefficient (R > 0.99). A good linearity relationship was achieved within the range from 1.0 × 10−5 to 1.0 × 10−4 M (Figure S21).

Some studies have shown that some other metal ions are also able to chelate with O, N or S atoms [54,55,56,57]. Therefore, the specificity of HNTs-PHT to Hg(ii) over commonly used ions, including Zn2+, Pb2+, Ni+, Na+, Mn2+, Mg2+, K+, Fe2+, Cu2+, Ba2+ and Al3+, was also investigated (the concentration is equal to 1 × 10−5 M, details are shown in Figure 5). Upon individual addition of a wide range of the abovementioned metal ions as their SO 4 2 − or Cl– salts, only Hg(ii) raised a conspicuous fluorescence enhancement at 469 nm with the I x value higher than 1,800 a.u. The fluorescence ratio “r” of the fluorescence intensity at 469 nm with and without the addition of ion was employed to evaluate the specificity upon Hg(ii). Except for Hg(ii), the r-value was found less than 5.0 for all cases. The addition of Hg(ii) raised a much higher r-value at 81.88. These results indicate that the as-obtained HNTs-PHT exhibits highly specific, precise and sensitive responses to Hg(ii) in an aqueous system.

Figure 5 Fluorescence spectra of HNTs-PHT in aqueous solution ([c] = 1.0 mg/mL) with the addition of Hg2+, Zn2+, Pb2+, Ni+, Na+, Mn2+, Mg2+, K+, Fe2+, Cu2+, Ba2+ and Al3+ (the concentration of each ion is equal to 1 × 10−5 M).
Figure 5

Fluorescence spectra of HNTs-PHT in aqueous solution ([c] = 1.0 mg/mL) with the addition of Hg2+, Zn2+, Pb2+, Ni+, Na+, Mn2+, Mg2+, K+, Fe2+, Cu2+, Ba2+ and Al3+ (the concentration of each ion is equal to 1 × 10−5 M).

3.3 Absorption studies

The absorption behaviors of HNTs-based chemosensors on Hg(ii) were confirmed by TGA curves. The weight (%) at 790°C for HNTs-PHT and HNTs-BP in TGA curves is 75.8 and 75.3%, respectively. After treating with excess Hg(ClO4)2, the Hg(ii)-chelated products, HNTs-PHT-Hg(ii) and HNTs-BP-Hg(ii), were also examined by TGA. The weight (%) at 790°C for HNTs-PHT-Hg(ii) and HNTs-BP-Hg(ii) in TGA curves increased to 78.2 and 77.5%, respectively, suggesting the absorption of Hg(ii). The absorption capacity of HNTs-PHT and HNTs-BP toward Hg(ii) is calculated as ca. 110 and 95 mg/g. HNTs-PHT shows a higher absorption capacity than that of HNTs-NP. Otherwise, the dynamic adsorption behaviors of HNTs-PHT and HNTs-BP displayed distinct differences. The interaction of HNTs-based chemosensors and Hg(ii) was investigated based on a turbidimetric assay method, which is widely used to evaluate the interactions of carbohydrates with proteins in the literature [58]. Plots of absorbance at 550 nm vs the time after the addition of Hg(ii) were recorded and are shown in Figure S22. A completely different behavior can be tracked between HNTs-PHT and HNTs-BP, which should be caused by the difference in the formation between interparticle and intraparticle excimers. The intraparticle excimer cannot give rise to the changes in particle sizes. So, the chelation of HNTs-BP with Hg(ii) cannot raise any significant changes in the absorbance values. The removal efficiency of HNTs-PHT toward Hg(ii) in an aqueous solution was evaluated by adding 5.0 mg of HNTs-PHT into 1.0 mL of Hg(ii) solution with different concentrations at 1 × 10−3, 1 × 10−4 and 1 × 10−5 M. The removal efficiency follows a concertation-dependent manner, which was calculated ad 72, 85 and 91%, respectively.

As for HNTs-PHT, the formation of interparticle excimers makes Hg(ii) ions serve as crosslinkers to aggregate the particles. As a result, a dramatic increase in absorbance values can be clearly found in HNTs-PHT after the formation of interparticle excimers, suggesting the formation of precipitations. Therefore, the separation of Hg(ii) was achieved by a simple precipitation approach without power consumption, when using HNTs-PHT as the adsorbent. TEM observations were conducted to further characterize the as-obtained precipitations. Figure 6b reveals that the HNTs-PHT is the monodisperse cylindrical-shaped nanotube with an open-ended lumen. After chelating with Hg(ii), the aggregates can be clearly observed in the TEM image (Figure 6c) of HNTs-PHT-Hg(ii). The corresponding elemental maps displaced in Figure 6d–g illustrate the distribution of the detected elements in EDS analysis, including Al (blue in Figure 6d), Hg (yellow in Figure 6e), O (purple in Figure 6f) and Si (green in Figure 6g). The mixture of distribution of the abovementioned elements is shown in Figure 6h. The signals from Al, O and Si mappings show their elemental distribution in the nanotube-based aggregation, which is consistence with the chemical composition of HNTs. The presence of Hg and the highly coincided distribution with the Al, O and Si indicate that the aggregates should be caused by Hg(ii) and thereby results in the macroscopic precipitations. Moreover, the Hg(ii) ions can be dissociated from HNTs-PHT-Hg(ii) with the addition of cysteamine, which endow HNTs-PHT a potential recyclable material for detecting and separating the Hg(ii) (Figure 6i).

Figure 6 Absorption studies. [(a) TGA curves of HNTs-PHT, HNTs-BP, HNTs-PHT-Hg(ii) and HNTs-BP-Hg(ii); (b) TEM image of HNTs-PHT; (c) TEM image of HNTs-PHT-Hg(ii); (d) EDS mapping of Al; (e) EDS mapping of Hg; (f) EDS mapping of O; (g) EDS mapping of Si; (h) mixture of the above-mentioned elements; and (i) aggregation and recycling mechanism for as-obtained HNTs-PHT-Hg(ii).
Figure 6

Absorption studies. [(a) TGA curves of HNTs-PHT, HNTs-BP, HNTs-PHT-Hg(ii) and HNTs-BP-Hg(ii); (b) TEM image of HNTs-PHT; (c) TEM image of HNTs-PHT-Hg(ii); (d) EDS mapping of Al; (e) EDS mapping of Hg; (f) EDS mapping of O; (g) EDS mapping of Si; (h) mixture of the above-mentioned elements; and (i) aggregation and recycling mechanism for as-obtained HNTs-PHT-Hg(ii).

4 Conclusion

We have developed two types of HNTs-based chemosensors, HNTs-PHT and HNTs-BP, for simultaneously detecting and separating Hg(ii) ions. The structural characterizations, including FTIR, solid-state NMR, XPS and TGA, demonstrated the successful modifications. The chemosensors have been covalently linked onto HNTs with good stability. Both HNTs-PHT and HNTs-BP are able to detect Hg(ii) ions via a “turn on” behavior on fluorescence, in which HNTs-PHT can effectively restrict the solvent relaxation of π–π* transition and make it a better detection tool in aqueous solution than HNTs-BP. The aqueous solution of HNTs-PHT displayed no emission behavior (Φ < 0.001), while a significant increase in emission behavior (Φ = 0.07) can be detected. HNTs-PHT shows highly specific, precise and sensitive responses to Hg(ii) in an aqueous system as compared to the literature. Moreover, HNTs-PHT exhibited a high absorption capacity upon Hg(ii) at ca. 110 mg/g, in which the Hg(ii) ions serve as crosslinking agents to aggregate HNTs-PHT into macroscopical precipitations. The established simultaneously detecting and separating approach upon Hg(ii) ions by HNTs-PHT gives a novel insight on dealing with a mercury pollution problem in water and widen the insight of environmental remediation.

NMR data, FTIR spectra, fluorescence spectra of HNTs-PB as a function of increasing water content, linear regression data and turbidity results are included in the Supporting information.

# These authors contributed equally to this work.

  1. Funding information: This work was financially supported by the National Natural Science Foundation of China (Grant No. 22102045) and the Key Project from the Natural Science Foundation of Hebei Province (No. B2020201072).

  2. Author contributions: The manuscript was written through contributions of all authors. Conceptualization: Y. Wu, X. Ba and H. Zhang; investigation and methodology: H. Fan, P. Jia and Z. Su; data curation: Y. Wu and H. Zhang; writing – original draft: H. Zhang; writing – review & editing: Y. Wu. All authors have accepted responsibility for the entire content of this manuscript and approved its submission.

  3. Conflict of interest: The authors state no conflict of interest.

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Received: 2022-03-02
Revised: 2022-04-23
Accepted: 2022-04-27
Published Online: 2022-05-26

© 2022 Haiyun Fan et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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  63. Promoted charge separation and specific surface area via interlacing of N-doped titanium dioxide nanotubes on carbon nitride nanosheets for photocatalytic degradation of Rhodamine B
  64. Elucidating the role of silicon dioxide and titanium dioxide nanoparticles in mitigating the disease of the eggplant caused by Phomopsis vexans, Ralstonia solanacearum, and root-knot nematode Meloidogyne incognita
  65. An implication of magnetic dipole in Carreau Yasuda liquid influenced by engine oil using ternary hybrid nanomaterial
  66. Robust synthesis of a composite phase of copper vanadium oxide with enhanced performance for durable aqueous Zn-ion batteries
  67. Tunning self-assembled phases of bovine serum albumin via hydrothermal process to synthesize novel functional hydrogel for skin protection against UVB
  68. A comparative experimental study on damping properties of epoxy nanocomposite beams reinforced with carbon nanotubes and graphene nanoplatelets
  69. Lightweight and hydrophobic Ni/GO/PVA composite aerogels for ultrahigh performance electromagnetic interference shielding
  70. Research on the auxetic behavior and mechanical properties of periodically rotating graphene nanostructures
  71. Repairing performances of novel cement mortar modified with graphene oxide and polyacrylate polymer
  72. Closed-loop recycling and fabrication of hydrophilic CNT films with high performance
  73. Design of thin-film configuration of SnO2–Ag2O composites for NO2 gas-sensing applications
  74. Study on stress distribution of SiC/Al composites based on microstructure models with microns and nanoparticles
  75. PVDF green nanofibers as potential carriers for improving self-healing and mechanical properties of carbon fiber/epoxy prepregs
  76. Osteogenesis capability of three-dimensionally printed poly(lactic acid)-halloysite nanotube scaffolds containing strontium ranelate
  77. Silver nanoparticles induce mitochondria-dependent apoptosis and late non-canonical autophagy in HT-29 colon cancer cells
  78. Preparation and bonding mechanisms of polymer/metal hybrid composite by nano molding technology
  79. Damage self-sensing and strain monitoring of glass-reinforced epoxy composite impregnated with graphene nanoplatelet and multiwalled carbon nanotubes
  80. Thermal analysis characterisation of solar-powered ship using Oldroyd hybrid nanofluids in parabolic trough solar collector: An optimal thermal application
  81. Pyrene-functionalized halloysite nanotubes for simultaneously detecting and separating Hg(ii) in aqueous media: A comprehensive comparison on interparticle and intraparticle excimers
  82. Fabrication of self-assembly CNT flexible film and its piezoresistive sensing behaviors
  83. Thermal valuation and entropy inspection of second-grade nanoscale fluid flow over a stretching surface by applying Koo–Kleinstreuer–Li relation
  84. Mechanical properties and microstructure of nano-SiO2 and basalt-fiber-reinforced recycled aggregate concrete
  85. Characterization and tribology performance of polyaniline-coated nanodiamond lubricant additives
  86. Combined impact of Marangoni convection and thermophoretic particle deposition on chemically reactive transport of nanofluid flow over a stretching surface
  87. Spark plasma extrusion of binder free hydroxyapatite powder
  88. An investigation on thermo-mechanical performance of graphene-oxide-reinforced shape memory polymer
  89. Effect of nanoadditives on the novel leather fiber/recycled poly(ethylene-vinyl-acetate) polymer composites for multifunctional applications: Fabrication, characterizations, and multiobjective optimization using central composite design
  90. Design selection for a hemispherical dimple core sandwich panel using hybrid multi-criteria decision-making methods
  91. Improving tensile strength and impact toughness of plasticized poly(lactic acid) biocomposites by incorporating nanofibrillated cellulose
  92. Green synthesis of spinel copper ferrite (CuFe2O4) nanoparticles and their toxicity
  93. The effect of TaC and NbC hybrid and mono-nanoparticles on AA2024 nanocomposites: Microstructure, strengthening, and artificial aging
  94. Excited-state geometry relaxation of pyrene-modified cellulose nanocrystals under UV-light excitation for detecting Fe3+
  95. Effect of CNTs and MEA on the creep of face-slab concrete at an early age
  96. Effect of deformation conditions on compression phase transformation of AZ31
  97. Application of MXene as a new generation of highly conductive coating materials for electromembrane-surrounded solid-phase microextraction
  98. A comparative study of the elasto-plastic properties for ceramic nanocomposites filled by graphene or graphene oxide nanoplates
  99. Encapsulation strategies for improving the biological behavior of CdS@ZIF-8 nanocomposites
  100. Biosynthesis of ZnO NPs from pumpkin seeds’ extract and elucidation of its anticancer potential against breast cancer
  101. Preliminary trials of the gold nanoparticles conjugated chrysin: An assessment of anti-oxidant, anti-microbial, and in vitro cytotoxic activities of a nanoformulated flavonoid
  102. Effect of micron-scale pores increased by nano-SiO2 sol modification on the strength of cement mortar
  103. Fractional simulations for thermal flow of hybrid nanofluid with aluminum oxide and titanium oxide nanoparticles with water and blood base fluids
  104. The effect of graphene nano-powder on the viscosity of water: An experimental study and artificial neural network modeling
  105. Development of a novel heat- and shear-resistant nano-silica gelling agent
  106. Characterization, biocompatibility and in vivo of nominal MnO2-containing wollastonite glass-ceramic
  107. Entropy production simulation of second-grade magnetic nanomaterials flowing across an expanding surface with viscidness dissipative flux
  108. Enhancement in structural, morphological, and optical properties of copper oxide for optoelectronic device applications
  109. Aptamer-functionalized chitosan-coated gold nanoparticle complex as a suitable targeted drug carrier for improved breast cancer treatment
  110. Performance and overall evaluation of nano-alumina-modified asphalt mixture
  111. Analysis of pure nanofluid (GO/engine oil) and hybrid nanofluid (GO–Fe3O4/engine oil): Novel thermal and magnetic features
  112. Synthesis of Ag@AgCl modified anatase/rutile/brookite mixed phase TiO2 and their photocatalytic property
  113. Mechanisms and influential variables on the abrasion resistance hydraulic concrete
  114. Synergistic reinforcement mechanism of basalt fiber/cellulose nanocrystals/polypropylene composites
  115. Achieving excellent oxidation resistance and mechanical properties of TiB2–B4C/carbon aerogel composites by quick-gelation and mechanical mixing
  116. Microwave-assisted sol–gel template-free synthesis and characterization of silica nanoparticles obtained from South African coal fly ash
  117. Pulsed laser-assisted synthesis of nano nickel(ii) oxide-anchored graphitic carbon nitride: Characterizations and their potential antibacterial/anti-biofilm applications
  118. Effects of nano-ZrSi2 on thermal stability of phenolic resin and thermal reusability of quartz–phenolic composites
  119. Benzaldehyde derivatives on tin electroplating as corrosion resistance for fabricating copper circuit
  120. Mechanical and heat transfer properties of 4D-printed shape memory graphene oxide/epoxy acrylate composites
  121. Coupling the vanadium-induced amorphous/crystalline NiFe2O4 with phosphide heterojunction toward active oxygen evolution reaction catalysts
  122. Graphene-oxide-reinforced cement composites mechanical and microstructural characteristics at elevated temperatures
  123. Gray correlation analysis of factors influencing compressive strength and durability of nano-SiO2 and PVA fiber reinforced geopolymer mortar
  124. Preparation of layered gradient Cu–Cr–Ti alloy with excellent mechanical properties, thermal stability, and electrical conductivity
  125. Recovery of Cr from chrome-containing leather wastes to develop aluminum-based composite material along with Al2O3 ceramic particles: An ingenious approach
  126. Mechanisms of the improved stiffness of flexible polymers under impact loading
  127. Anticancer potential of gold nanoparticles (AuNPs) using a battery of in vitro tests
  128. Review Articles
  129. Proposed approaches for coronaviruses elimination from wastewater: Membrane techniques and nanotechnology solutions
  130. Application of Pickering emulsion in oil drilling and production
  131. The contribution of microfluidics to the fight against tuberculosis
  132. Graphene-based biosensors for disease theranostics: Development, applications, and recent advancements
  133. Synthesis and encapsulation of iron oxide nanorods for application in magnetic hyperthermia and photothermal therapy
  134. Contemporary nano-architectured drugs and leads for ανβ3 integrin-based chemotherapy: Rationale and retrospect
  135. State-of-the-art review of fabrication, application, and mechanical properties of functionally graded porous nanocomposite materials
  136. Insights on magnetic spinel ferrites for targeted drug delivery and hyperthermia applications
  137. A review on heterogeneous oxidation of acetaminophen based on micro and nanoparticles catalyzed by different activators
  138. Early diagnosis of lung cancer using magnetic nanoparticles-integrated systems
  139. Advances in ZnO: Manipulation of defects for enhancing their technological potentials
  140. Efficacious nanomedicine track toward combating COVID-19
  141. A review of the design, processes, and properties of Mg-based composites
  142. Green synthesis of nanoparticles for varied applications: Green renewable resources and energy-efficient synthetic routes
  143. Two-dimensional nanomaterial-based polymer composites: Fundamentals and applications
  144. Recent progress and challenges in plasmonic nanomaterials
  145. Apoptotic cell-derived micro/nanosized extracellular vesicles in tissue regeneration
  146. Electronic noses based on metal oxide nanowires: A review
  147. Framework materials for supercapacitors
  148. An overview on the reproductive toxicity of graphene derivatives: Highlighting the importance
  149. Antibacterial nanomaterials: Upcoming hope to overcome antibiotic resistance crisis
  150. Research progress of carbon materials in the field of three-dimensional printing polymer nanocomposites
  151. A review of atomic layer deposition modelling and simulation methodologies: Density functional theory and molecular dynamics
  152. Recent advances in the preparation of PVDF-based piezoelectric materials
  153. Recent developments in tensile properties of friction welding of carbon fiber-reinforced composite: A review
  154. Comprehensive review of the properties of fly ash-based geopolymer with additive of nano-SiO2
  155. Perspectives in biopolymer/graphene-based composite application: Advances, challenges, and recommendations
  156. Graphene-based nanocomposite using new modeling molecular dynamic simulations for proposed neutralizing mechanism and real-time sensing of COVID-19
  157. Nanotechnology application on bamboo materials: A review
  158. Recent developments and future perspectives of biorenewable nanocomposites for advanced applications
  159. Nanostructured lipid carrier system: A compendium of their formulation development approaches, optimization strategies by quality by design, and recent applications in drug delivery
  160. 3D printing customized design of human bone tissue implant and its application
  161. Design, preparation, and functionalization of nanobiomaterials for enhanced efficacy in current and future biomedical applications
  162. A brief review of nanoparticles-doped PEDOT:PSS nanocomposite for OLED and OPV
  163. Nanotechnology interventions as a putative tool for the treatment of dental afflictions
  164. Recent advancements in metal–organic frameworks integrating quantum dots (QDs@MOF) and their potential applications
  165. A focused review of short electrospun nanofiber preparation techniques for composite reinforcement
  166. Microstructural characteristics and nano-modification of interfacial transition zone in concrete: A review
  167. Latest developments in the upconversion nanotechnology for the rapid detection of food safety: A review
  168. Strategic applications of nano-fertilizers for sustainable agriculture: Benefits and bottlenecks
  169. Molecular dynamics application of cocrystal energetic materials: A review
  170. Synthesis and application of nanometer hydroxyapatite in biomedicine
  171. Cutting-edge development in waste-recycled nanomaterials for energy storage and conversion applications
  172. Biological applications of ternary quantum dots: A review
  173. Nanotherapeutics for hydrogen sulfide-involved treatment: An emerging approach for cancer therapy
  174. Application of antibacterial nanoparticles in orthodontic materials
  175. Effect of natural-based biological hydrogels combined with growth factors on skin wound healing
  176. Nanozymes – A route to overcome microbial resistance: A viewpoint
  177. Recent developments and applications of smart nanoparticles in biomedicine
  178. Contemporary review on carbon nanotube (CNT) composites and their impact on multifarious applications
  179. Interfacial interactions and reinforcing mechanisms of cellulose and chitin nanomaterials and starch derivatives for cement and concrete strength and durability enhancement: A review
  180. Diamond-like carbon films for tribological modification of rubber
  181. Layered double hydroxides (LDHs) modified cement-based materials: A systematic review
  182. Recent research progress and advanced applications of silica/polymer nanocomposites
  183. Modeling of supramolecular biopolymers: Leading the in silico revolution of tissue engineering and nanomedicine
  184. Recent advances in perovskites-based optoelectronics
  185. Biogenic synthesis of palladium nanoparticles: New production methods and applications
  186. A comprehensive review of nanofluids with fractional derivatives: Modeling and application
  187. Electrospinning of marine polysaccharides: Processing and chemical aspects, challenges, and future prospects
  188. Electrohydrodynamic printing for demanding devices: A review of processing and applications
  189. Rapid Communications
  190. Structural material with designed thermal twist for a simple actuation
  191. Recent advances in photothermal materials for solar-driven crude oil adsorption
https://doi.org/10.1515/ntrev-2022-0119
Keywords for this article
halloysite nanotube; pyrene; clay; chemosensor; Hg(ii)
Creative Commons
BY 4.0

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  70. Research on the auxetic behavior and mechanical properties of periodically rotating graphene nanostructures
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  75. PVDF green nanofibers as potential carriers for improving self-healing and mechanical properties of carbon fiber/epoxy prepregs
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  80. Thermal analysis characterisation of solar-powered ship using Oldroyd hybrid nanofluids in parabolic trough solar collector: An optimal thermal application
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  82. Fabrication of self-assembly CNT flexible film and its piezoresistive sensing behaviors
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  84. Mechanical properties and microstructure of nano-SiO2 and basalt-fiber-reinforced recycled aggregate concrete
  85. Characterization and tribology performance of polyaniline-coated nanodiamond lubricant additives
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  89. Effect of nanoadditives on the novel leather fiber/recycled poly(ethylene-vinyl-acetate) polymer composites for multifunctional applications: Fabrication, characterizations, and multiobjective optimization using central composite design
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  98. A comparative study of the elasto-plastic properties for ceramic nanocomposites filled by graphene or graphene oxide nanoplates
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  101. Preliminary trials of the gold nanoparticles conjugated chrysin: An assessment of anti-oxidant, anti-microbial, and in vitro cytotoxic activities of a nanoformulated flavonoid
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  107. Entropy production simulation of second-grade magnetic nanomaterials flowing across an expanding surface with viscidness dissipative flux
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  109. Aptamer-functionalized chitosan-coated gold nanoparticle complex as a suitable targeted drug carrier for improved breast cancer treatment
  110. Performance and overall evaluation of nano-alumina-modified asphalt mixture
  111. Analysis of pure nanofluid (GO/engine oil) and hybrid nanofluid (GO–Fe3O4/engine oil): Novel thermal and magnetic features
  112. Synthesis of Ag@AgCl modified anatase/rutile/brookite mixed phase TiO2 and their photocatalytic property
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  115. Achieving excellent oxidation resistance and mechanical properties of TiB2–B4C/carbon aerogel composites by quick-gelation and mechanical mixing
  116. Microwave-assisted sol–gel template-free synthesis and characterization of silica nanoparticles obtained from South African coal fly ash
  117. Pulsed laser-assisted synthesis of nano nickel(ii) oxide-anchored graphitic carbon nitride: Characterizations and their potential antibacterial/anti-biofilm applications
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  119. Benzaldehyde derivatives on tin electroplating as corrosion resistance for fabricating copper circuit
  120. Mechanical and heat transfer properties of 4D-printed shape memory graphene oxide/epoxy acrylate composites
  121. Coupling the vanadium-induced amorphous/crystalline NiFe2O4 with phosphide heterojunction toward active oxygen evolution reaction catalysts
  122. Graphene-oxide-reinforced cement composites mechanical and microstructural characteristics at elevated temperatures
  123. Gray correlation analysis of factors influencing compressive strength and durability of nano-SiO2 and PVA fiber reinforced geopolymer mortar
  124. Preparation of layered gradient Cu–Cr–Ti alloy with excellent mechanical properties, thermal stability, and electrical conductivity
  125. Recovery of Cr from chrome-containing leather wastes to develop aluminum-based composite material along with Al2O3 ceramic particles: An ingenious approach
  126. Mechanisms of the improved stiffness of flexible polymers under impact loading
  127. Anticancer potential of gold nanoparticles (AuNPs) using a battery of in vitro tests
  128. Review Articles
  129. Proposed approaches for coronaviruses elimination from wastewater: Membrane techniques and nanotechnology solutions
  130. Application of Pickering emulsion in oil drilling and production
  131. The contribution of microfluidics to the fight against tuberculosis
  132. Graphene-based biosensors for disease theranostics: Development, applications, and recent advancements
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  135. State-of-the-art review of fabrication, application, and mechanical properties of functionally graded porous nanocomposite materials
  136. Insights on magnetic spinel ferrites for targeted drug delivery and hyperthermia applications
  137. A review on heterogeneous oxidation of acetaminophen based on micro and nanoparticles catalyzed by different activators
  138. Early diagnosis of lung cancer using magnetic nanoparticles-integrated systems
  139. Advances in ZnO: Manipulation of defects for enhancing their technological potentials
  140. Efficacious nanomedicine track toward combating COVID-19
  141. A review of the design, processes, and properties of Mg-based composites
  142. Green synthesis of nanoparticles for varied applications: Green renewable resources and energy-efficient synthetic routes
  143. Two-dimensional nanomaterial-based polymer composites: Fundamentals and applications
  144. Recent progress and challenges in plasmonic nanomaterials
  145. Apoptotic cell-derived micro/nanosized extracellular vesicles in tissue regeneration
  146. Electronic noses based on metal oxide nanowires: A review
  147. Framework materials for supercapacitors
  148. An overview on the reproductive toxicity of graphene derivatives: Highlighting the importance
  149. Antibacterial nanomaterials: Upcoming hope to overcome antibiotic resistance crisis
  150. Research progress of carbon materials in the field of three-dimensional printing polymer nanocomposites
  151. A review of atomic layer deposition modelling and simulation methodologies: Density functional theory and molecular dynamics
  152. Recent advances in the preparation of PVDF-based piezoelectric materials
  153. Recent developments in tensile properties of friction welding of carbon fiber-reinforced composite: A review
  154. Comprehensive review of the properties of fly ash-based geopolymer with additive of nano-SiO2
  155. Perspectives in biopolymer/graphene-based composite application: Advances, challenges, and recommendations
  156. Graphene-based nanocomposite using new modeling molecular dynamic simulations for proposed neutralizing mechanism and real-time sensing of COVID-19
  157. Nanotechnology application on bamboo materials: A review
  158. Recent developments and future perspectives of biorenewable nanocomposites for advanced applications
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  160. 3D printing customized design of human bone tissue implant and its application
  161. Design, preparation, and functionalization of nanobiomaterials for enhanced efficacy in current and future biomedical applications
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  163. Nanotechnology interventions as a putative tool for the treatment of dental afflictions
  164. Recent advancements in metal–organic frameworks integrating quantum dots (QDs@MOF) and their potential applications
  165. A focused review of short electrospun nanofiber preparation techniques for composite reinforcement
  166. Microstructural characteristics and nano-modification of interfacial transition zone in concrete: A review
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  168. Strategic applications of nano-fertilizers for sustainable agriculture: Benefits and bottlenecks
  169. Molecular dynamics application of cocrystal energetic materials: A review
  170. Synthesis and application of nanometer hydroxyapatite in biomedicine
  171. Cutting-edge development in waste-recycled nanomaterials for energy storage and conversion applications
  172. Biological applications of ternary quantum dots: A review
  173. Nanotherapeutics for hydrogen sulfide-involved treatment: An emerging approach for cancer therapy
  174. Application of antibacterial nanoparticles in orthodontic materials
  175. Effect of natural-based biological hydrogels combined with growth factors on skin wound healing
  176. Nanozymes – A route to overcome microbial resistance: A viewpoint
  177. Recent developments and applications of smart nanoparticles in biomedicine
  178. Contemporary review on carbon nanotube (CNT) composites and their impact on multifarious applications
  179. Interfacial interactions and reinforcing mechanisms of cellulose and chitin nanomaterials and starch derivatives for cement and concrete strength and durability enhancement: A review
  180. Diamond-like carbon films for tribological modification of rubber
  181. Layered double hydroxides (LDHs) modified cement-based materials: A systematic review
  182. Recent research progress and advanced applications of silica/polymer nanocomposites
  183. Modeling of supramolecular biopolymers: Leading the in silico revolution of tissue engineering and nanomedicine
  184. Recent advances in perovskites-based optoelectronics
  185. Biogenic synthesis of palladium nanoparticles: New production methods and applications
  186. A comprehensive review of nanofluids with fractional derivatives: Modeling and application
  187. Electrospinning of marine polysaccharides: Processing and chemical aspects, challenges, and future prospects
  188. Electrohydrodynamic printing for demanding devices: A review of processing and applications
  189. Rapid Communications
  190. Structural material with designed thermal twist for a simple actuation
  191. Recent advances in photothermal materials for solar-driven crude oil adsorption
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