ReviewGraphene as a new sorbent in analytical chemistry
Introduction
The determination of trace organic species and metal ions in various samples is a subject of great interest. However, the direct determination of extremely low concentrations of analytes can be difficult because of matrix interferences or insufficient sensitivity of most analytical techniques. For this reason, preliminary separation and preconcentration of trace analytes from matrices are usually required. The most commonly employed techniques of sample treatment are solid-phase extraction (SPE) and liquid-liquid extraction (LLE). SPE, including its miniaturized mode, i.e. solid-phase microextraction (SPME), has become increasingly popular in trace analysis [1], as a result of the high recoveries and enrichment factors obtained, low cost and low consumption of organic solvents, and the possibility of combining with chromatography or spectroscopy techniques. However, high recovery and enrichment factor can be obtained only if a suitable solid sorbent is used. In recent years, nanomaterials have been used to develop new analytical procedures [2], [3], [4]. Due to their large surface area, chemical stability, durability, and corrosion resistance, nanomaterials have attracted greater interest than classical materials. In general, nanomaterials can be divided into three main groups according to their chemical nature: carbon-based, inorganic, and mixed. In carbon-based NPs, fullerenes and carbon nanotubes (CNTs) are the most popular in SPE of metal ions and organic compounds [5], [6]. In the past three years, we have seen intense interest grow in graphene (G).
Since the first experimental evidence of the electronic properties of G in 2004 [7], G has become the most intensively studied material. This results from its unique electrical, electrochemical, optical and mechanical properties. G is a new form of carbonaceous material, which possesses a single-layer or few-layer thickness of sp2-hybridized carbon atoms arranged in a honeycomb pattern. Many methods of synthesizing G have been proposed [8], but the most popular is the chemical method based on the oxidation of graphite to graphene oxide (GO) and subsequent chemical reduction of GO to G using a suitable reducing agent (e.g. hydrazine) (Fig. 1). The oxidation approach {e.g., Hummers method [9], [10]} is considered one of the most efficient methods of low-cost, large-scale production of G. Due to their huge surface area and spectacular physical and chemical properties, G and GO have attracted the greatest interest in many fields, including analytical chemistry. G and GO seem to be ideal sorbents in SPE or SPME because of their huge surface area {e.g., theoretical value for G is 2630 m2 g−1 [11]} and the hexagonal arrays of carbon atoms in G sheets that are ideal for strong interactions with other molecules. However, G is insoluble and hard to disperse in all solvents due to strong van der Waals interactions that can hamper sorption of organic compounds or metal ions. GO, in contrast, possesses large quantities of oxygen atoms on the surface of GO as epoxy, hydroxyl, and carboxyl groups [12], so GO is much more hydrophilic than G and can form stable colloidal suspensions. GO provides rich functional groups for the formation of hydrogen bonding or electrostatic interaction with organic compounds or metal ions. In general, G is considered a non-polar, hydrophobic sorbent with strong affinity for carbon-based ring structures, which can be applied in reversed-phase SPE. GO contains many more polar moieties, so it has a much more polar character than G and can therefore be applied as a sorbent in normal-phase SPE for preconcentration of organic compounds containing oxygen- and nitrogen-functional groups and metal species (Fig. 1).
Although CNTs and G have identical chemical composition, some differences in their adsorptive properties can be observed [13]. In contrast to CNTs, both surfaces of a planar sheet of G are accessible for adsorption of analytes. In the case of CNTs, the inner walls are not responsible for the adsorption, due to steric hindrance. G nanosheets are flexible and can be easily attached to a support, which is a strong advantage in preparing SPME fibers. However, certain features, such as softness and flexibility, can hamper classical SPE because of high pressure in the SPE column. G nanosheets can also escape from the SPE cartridge.
From a practical point of view, it is important that G can be synthesized by simple chemical methods in most chemical laboratories. In contrast to CNTs, G is usually prepared from graphite without using a metal catalyst, so it can be almost free from metallic impurities, which are practically impossible to remove in the case of CNTs.
G and GO have received much attention for their many potential applications in analytical chemistry and review papers were recently published [13], [14]. This article focuses on adsorptive properties of G and GO and their application in preconcentrating trace elements and organic compounds. We present several applications of G and GO for trace analysis of water, food, biological and environmental samples using chromatography and spectroscopy techniques. We also discuss some methods of modification or chemical functionalization, although comprehensive discussion on covalent and non-covalent functionalization of G and GO can be found elsewhere [15], [16].
Section snippets
Adsorption of organic compounds and metal species on G and GO
Organic compounds can be adsorbed on nanoparticles (NPs) or nanostructured materials via non-covalent interaction including electrostatic interaction, hydrogen bonds, π-π stacking, dispersion forces, dative bonds, and the hydrophobic effect [6]. In the case of G, the very large delocalized π-electron system plays the dominant role in formation of strong π–π stacking interaction with the aromatic rings of several organic compounds. Cation-π bonding can also be responsible for adsorption [17].
Solid-phase extraction
SPE is a well-known approach for preconcentration and analyte/matrix separation. It involves passing the liquid sample through a column containing an adsorbent that retains the analytes. In the next step, the adsorbed analytes are recovered upon elution with an appropriate solvent. High recovery can be obtained only if a sorbent appropriate to the particular application is chosen. In this area, G is a completely new sorbent. Recently published papers on application of G, GO and their
Solid-phase microextraction
SPME is one of the valuable techniques applied in preconcentration and separation of analytes from complex matrix samples. Because of being simple and solvent-free, SPME is becoming an alternative to conventional sample-extraction techniques. In SPME, the fiber or the wire coated with solid sorbent is exposed to the liquid sample (determination of both volatile and non-volatile analytes) or headspace above the sample (volatile analytes). After adsorption equilibrium is reached, the wire is
Magnetic solid-phase extraction
Separation methods based on magnetic materials have received considerable attention in recent years [70], [71]. The magnetic sorbent used in SPE is not packed into the cartridge, as in conventional SPE, and the preconcentration procedure is usually performed in the following stages:
- (1)
the magnetic sorbent is dispersed into aqueous sample solution and the analytes are adsorbed onto the magnetic sorbent under stirring;
- (2)
the sorbent with adsorbed analytes is recovered from the suspension using an
Conclusions and outlook
The past three years saw an intense interest growing in G and GO as new sorbents in analytical chemistry. Their chemical properties, very large delocalized π-electron system and huge surface area make G and GO favored in the analysis of groups of analytes with a wide range of polarity. It should be noted that the maximum adsorption capacities of G and GO are much higher than those of any of the currently reported sorbents, so G and GO nanosheets can be successfully applied in classical SPE of
Acknowledgements
This work was supported by the National Science Center (Poland) through the project No. DEC-2012/07/B/ST4/00568.
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