Single-walled carbon nanohorn as new solid-phase extraction adsorbent for determination of 4-nitrophenol in water sample
Introduction
Solid-phase extraction is the most common technique for preconcentration of analytes, and plays a very important role in modern analytical science. It has the advantages of high recovery, short extraction time, high enrichment factor, low cost, and low consumption of organic solvents over liquid–liquid extraction. Various adsorbents for solid-phase extraction, such as silica-based materials, ion-pair and ion-exchange adsorbents, immunoaffinity extraction adsorbents, nanoparticle-based adsorbents, molecularly imprinted polymers, and various carbonaceous materials, have been used [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14].
Since the first use of carbonaceous adsorbents for solid-phase extraction in 1980s, various carbon materials have been adopted as solid-phase extraction adsorbents because of their specific properties and high stability [12]. Carbon nanotubes including single-walled carbon nanotube (SWCNT) and multi-walled carbon nanotube (MWCNT) are novel and interesting materials [15]. Because of their amazing adsorbent effects, carbon nanotubes have been reported as effective solid-phase extraction adsorbents for many analytes such as bisphenol A, 4-n-nonylphenol, and 4-tert-octylphenol [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. However, carbon nanotubes have several disadvantages. First, carbon nanotubes especially single-walled carbon nanotubes tend to form big bundles with narrow intertubular gaps, which hinder the permeation and extraction of analytes [25]. Second, it is difficult to remove impurities, which make purified carbon nanotubes expensive. Finally, metal catalyst residues in carbon nanotubes cannot be completely removed. These impurities can affect consequent study and result in controversial results [26], [27].
Recently, Iijima's group has reported an intriguing carbon nanomaterial named single-walled carbon nanohorn (SWCNH). SWCNH is synthesized by laser ablation of pure graphite without using metal catalyst with high production rate and high yield, making it potentially cheap [28]. It has been widely used for various applications, such as adsorption, drug delivery, fuel cells, super capacitors, and so on [25], [28], [29], [30], [31], [32], [33], [34], [35]. It has following distinguishing features. First, SWCNH is horn-shaped single-walled tubule with conical tip. The tubule has a typical diameter of about 2 nm with a length of 30–50 nm. Typically, SWCNH assembles and forms radial aggregates (80–100 nm) with the tips of individual tubules protruding out of the surface of the aggregate. The rough surface structure of the SWCNH aggregates results in weak Van der Waals interactions between aggregates and thus SWCNH has better dispersion in solvents than carbon nanotubes [34]. Second, in contrast to carbon nanotubes, SWCNH has enough large intertubular gaps for permeation of small molecules, ensuring both extremely large surface area and fast extraction. Third, the unique horn-shaped structure of SWCNH results in lots of edge plane graphite and/or defects, and thus the excellent extraction ability. Fourth, through tip opening and assembly of nanohorns, such SWCNH assemblies can provide various accessible pore structures, such as microporosity, mesoporosity, and macroporosity, allowing easy access of both the internal and interstitial spaces of SWCNH. Finally, SWCNH is very pure and essentially metal-free, which avoids cumbersome purification and makes it user-friendly. Based on its peculiar features metioned above, SWCNH is promising adsorbent for solid-phase extraction applications.
Phenol and substituted phenols have obtained considerable attention in wastewater and environmental analysis programs due to the human hazards they pose, even at μg/L levels. 4-Nitrophenol can cause significant damages to biodegradation and the human health including methemoglobinemia, the injuries to the liver and kidney. It can also damage the growth of the microbe, animals, and plants. Because of its high toxicity, 4-nitrophenol is included in the US Environmental Protection Agency List of Priority Pollutants [36]. Hence, determination of 4-nitrophenol has attracted much attention [37], [38], [39], [40], [41].
In this study, application of SWCNH as solid-phase extraction adsorbent is demonstrated for the first time using 4-nitrophenol as a model analyte. The solid-phase extraction of 4-nitrophenol was monitored by an electrochemical method. Fast solid-phase extraction of organic compounds to SWCNH was shown by effective extraction of 4-nitrophenol within 120 s without taking any special measures, such as applying a suitable potential at the electrodes during extraction or adding suitable dopants. Furthermore, highly sensitive and selective determination of 4-nitrophenol was achieved by the combination of solid-phase extraction to SWCNHs, medium exchange, and linear sweep voltammogram.
Section snippets
Reagents
Professor S. Iijima generously offered dahlia-like SWCNH that was prepared at room temperature by CO2 laser ablation. MWCNT was purchased from Shenzhen Nanotechnologies Port Co. Ltd. The CAS numbers of chemicals used were as follows: 4-Nitrophenol: 100-02-7; Phenol: 108-95-2; 2-Aminophenol: 95-55-6; 4-Aminophenol: 123-30-8; 2-Nitrophenol: 88-75-5; 3-Nitrophenol: 554-84-7; 2-Chlorophenol: 95-57-8; 4-Chlorophenol: 106-48-9. All chemicals used were of analytical grade purity and used as received.
Electrochemistry of 4-nitrophenol following solid-phase extraction
Fig. 1a shows the linear sweep voltammograms in 0.1 M pH 7.0 phosphate buffer solution following 60 s extraction from a 5 mL stirred 1.0 × 10−6 M 4-nitrophenol to SWCNH-modified GCE. A reduction peak appears at −0.71 V and the peak current is 15.72 μA. According to the currently accepted mechanism [40], [42], the reduction peak should be attributed to a four-electron transfer reduction of the nitro group (φ-NO2) to give hydroxylamine derivative.
The extraction of 4-nitrophenol to bare GCE and
Conclusions
This work shows that SWCNH is excellent solid-phase extraction adsorbent. It can extract 4-nitrophenol rapidly and efficiently. The combination of solid-phase extraction to SWCNH-modified GCE, medium exchange, and linear sweep voltammograms allows fast, sensitive, and selective determination of 4-nitrophenol. Compared with bare GCE and MWCNT-modified GCE, the sensitivity for the determination of 4-nitrophenol at SWCNH-modified GCE increases by 51.6 times and 5.55 times, respectively. Because of
Acknowledgements
The authors are very grateful to Professor S. Iijima (Solution Oriented Research for Science and Technology in Japan Science and Technology Agency) for generous offer of SWCNH. This work is kindly supported by the Ministry of Science and technology of the People's Republic of China (No.2006BAE03B08), the National Natural Science Foundation of China (No. No.20505016 & 20875086), the Department of Sciences & Technology of Jilin Province (20070108 and 20082104), and the Hundred Talents Program of
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