The Virtues of Graphene
In what should be our constant endeavor to improve sample preparation, we must look to new materials – and focus on bending them to our own needs.
I’m certainly not the first to say that sample preparation is extremely important in chemical analysis (and I doubt I will be the last). It eliminates unwanted matrix components, enriches or preconcentrates analytes, influences the quality of the obtained results and the total analysis time. Leading scientists in the field do give sample preparation the attention it deserves; the saying “if you fail to prepare, prepare to fail” is really not an overstatement (Fail to Prepare Prepare to Fail).
The marriage of materials and analytical chemistry is an important development in sample preparation. Today, there is a plethora of useful materials available to analytical scientists for sample processing. At the top of the list – or pretty close to the top – are carbon-based nanomaterials (including graphene in its various forms) for sorbent-based extraction procedures, such as classical solid-phase extraction (SPE), dispersive SPE, stir-bar sorptive extraction, and miniaturized solid-phase microextraction.
Graphene (a term recommended by the International Union of Pure and Applied Chemistry commission to replace the older term “graphite layers”) is a two-dimensional (actually one atom thick) carbon nanomaterial with a honeycomb pattern. Graphene’s exceptional properties – ultrahigh specific surface area (2630 m2g‑1), hydrophobicity, chemical versatility and tunability (especially in its oxygen-functionalized derivative, graphene oxide) and high chemical stability – make it a superior adsorbent candidate for many different sample preparation methods.
Other features that make graphene desirable as sorbent are its planar geometry of nanosheets and wrinkly surfaces that can interlock well with adsorbed targets, and its large delocalized π–π electron system, which can form a strong π–π stacking interaction with organic molecules.
Graphene oxide (also called graphitic oxide or graphitic acid) can be used for the extraction of metal ions and analytes exhibiting polar functionalities, such as hydroxyls, carbonyls, amines, heteroatoms (O, N, S, P), under normal-phase SPE conditions. In contrast, metal chelates and analytes exhibiting non-polar functionalities, such as aromatic, alkyl, alicyclic functional groups, can be extracted using graphene in reversed-phase SPE conditions.
Graphene is an ultra-light material, so it is typically hard to recover from suspension, even by high-speed centrifugation. But imbuing graphene with magnetic properties can solve this problem. Our group developed and used – for the first time – an iron oxide-oxyhydroxide/graphene composite material as a sorbent for dispersive SPE of polychlorinated biphenyls, polyaromatic hydrocarbons and phthalates. The material capitalizes on the adsorption features of both elements and the magnetic properties of iron oxide (1).
In the same context, we functionalized cotton fibers to produce a web of aminosilica-graphene cotton microfibrils for a novel and straightforward cotton-based extraction mode. The extraction of analytes on cotton fibers was followed by facile collection of cotton pieces, and the elution and subsequent injection into a gas chromatograph. To test the applicability of the functionalized cotton and to figure out the mechanism of extraction, several groups of pollutants were employed successfully, including polycyclic aromatic hydrocarbons, phthalates, musks, phenolic endocrine disrupters and haloacetic acids (2).
But it’s not all plain sailing. Although graphene is a highly efficient adsorbent, it usually shows little analyte specificity. To increase extraction selectivity, graphene (or graphene oxide) must be modified, which can be achieved through functionalization or hybridization with other groups, units, or materials that have specific affinity for target analytes. For example, aptamer-conjugated graphene oxide has been developed for the selective enrichment and ionization of cocaine and adenosine in human plasma (3).
It’s clear to me that we need more research focused on enhancing the selectivity and extraction efficiency of graphene-based sorbents. Another challenge will be the development of simple and environmentally friendly preparation methods to obtain high-quality graphene with homogeneous lateral size and shape. Nevertheless, in my view, we are only just starting to realize the full potential of graphene as sample preparation material.
- AA Karamani, et al., “Zero-valent iron/iron oxide-oxyhydroxide/graphene as a magnetic ssorbent for the enrichment of polychlorinated biphenyls, polyaromatic hydrocarbons and phthalates prior to gas chromatography–mass spectrometry”, J Chromatogr A, 1271, 1–9 (2013).
- I Montesinos, et al., “Graphene-coated cotton fibers as a sorbent for the extraction of multiclass pesticide residues from water and their determination by gas chromatography with mass spectrometry”, J Sep Sci, 38, 836–84 (2015).
- B Gulbakan, et al., “A dual platform for selective analyte enrichment and ionization in mass spectrometry using aptamer-conjugated graphene oxide”, J Am Chem Soc, 132 (49), 17408–17410 (2010).
After completing his PhD on flow injection analysis of glutamate, Constantine Stalikas joined the European Environmental Research Institute, where he became acquainted with liquid chromatography and flow injection analysis. In 1997, he moved to the Department of Analytical Chemistry at University of Cordoba, Spain, as a post-doctoral fellow. Shortly before joining the Institute for Hydrochemistry of Technical University of Munich in 2000, he was elected unanimously as lecturer of analytical chemistry at the University of Ioannina, Greece, where he is now full professor. He has published in the region of 85 scientific papers (h-index 30). Ηis main research interests are analytical applications of nanomaterials, especially in the field of sample preparation.
