Electrospun fibers as pre-concentration agent

In detection and analysis for specific analytes, sample preparation is important such that it is in a form suitable for analysis by the machine or to concentrate the analytes such that it reaches the detection limits of the machine. High surface area of nanofibers and the ease of functionalization for capturing specific analytes have made electrospun nanofibers a potential candidate for this application.

There are several considerations for the use of electrospun nanofibers as pre-concentration agent. Adsorption of analytes is often affected by the pH of the solution and in some cases, the competition from other contaminants in the water. Smaller diameter nanofibers are expected to be better in adsorption of analytes due to higher surface area [Ifegwu et al 2015]. Boyqci et al (2013) tested sol-gel based amine-functionalized solid phase microextraction (SPME) fibers fabricated from dip-coating and electrospinning of amino functionalized sol-gel solution on the fibers. Their experiment showed that fibers coated by electrospinning process performed much better than dip coated fibers. Fibers which are more hydrophilic or contain higher hydrophilic functional groups were also expected to adsorb analyte faster [Ifegwu et al 2015]. Apart from rapid adsorption, the fibers must also exhibit good desorption such that the analyte can be extracted for analysis [Ifegwu et al 2015]. Useful life of fibers in terms of the number of cycles of adsorption-desorption should also be considered if the material is meant for multiple usage.

It is vital that water source for human consumption is free of harmful contaminants. For some contaminants, even at very small quantities have the potential to cause long term harm due to accumulation within the human body. Functionalized electrospun membrane has been used for removal of heavy metal ions and other contaminants in waste water treatment. Similarly, it can also be used as a sorbent material to extract contaminants for analysis. Darko et al (2013) used electrospun polystyrene (PS) fibers loaded with pyrazole-1-carbodithioate and imidazole-1-carbodithioate for adsorption of divalent heavy metals, Cu(II), Ni(II) and Pb(II). The PS fiber containing diazole-incorporated sorbents was found to be effective in adsorption of the divalent heavy metals at low pH with a maximum saturation time of 42 minutes. Successive desorption of metal ions and regeneration of the sorbent showed good efficiency of between 99 and 95% up to the 4^th cycle. However, the efficiency drops below 95% after that which may be due to loss of ligands. To reduce the loss of ligands and increase the sensitivity of the sorbent material, surface functionalization through chemical bonding of the active agent to the surface of the nanofibers may be carried out. Moghimi (2011) used surface modified polyacrylonitrile nanofibers with ethanolamine (PAN-MEA) for solid phase extraction (SPE) of various metal ions. Their experiment found that PAN-MEA fibers has the greatest affinity for Cr(III) across a range of pH from 1 to 6. When tested with drinking tap water and double distilled water spiked with Cr(III), both results are similar therefore demonstrating that the presence of other inorganic and organic matters in tap water did not significantly affect the selective extraction and pre-concentration of Cr(III) by PAN-MEA fibers. The same material has also been demonstrated to be effective for pre-concentration of Mercury(II) in tap water [Moghimi and Abdouss 2012]. Another active agent, mSethanolamine (MMA) has also been successfully tested for for pre-concentration of Cu(II) using the same test method [Moghimi et al 2012]. Organic contaminants such as dimethylarsinic acid (DMA), monomethylarsonic acid (MMA) have also been extracted using electrospun fibers. Boyaci et al (2013) was able to demonstrate extraction of DMA, MMA and organometallic arsenic species using silica microfiber coated with electrospun polydimethylsiloxane (PDMS) end-functionalized with 3-(triethoxysilyl) propylamine (APTES). Ifegwu et al (2015) tested a range of polymers electrospun into nanofibrous membranes for their adsorption of urinary 1-hydroxypyrene. 1-hydroxypyrene is a biomarker of polycyclic aromatic hydrocarbons (PAH) which are the largest class of cancer-causing compounds. The polymers are selected based on anticipated hydrophobic interaction with 1-hydroxypyrene through hydrogen bonding, van der Waal interactions or pore-filling. In their tests of over 11 polymers, the sorption processes are highest within the first 30 min. Poly(vinyl benzyl chloride) (PVBC) showed the highest adsorption level at 93% but the desorption of hydroxypyrene is only 43% which may be due to chemical reaction between -OH group of the analyte and -CH2-Cl of the PVBC. The percentage recoveries were highest from nylon 6 and poly(styrene-co-divinylbenzene) (SDVB) at 72% and 70% respectively while polyethylene terephthalate (PET) has the lowest recovery at 34%. Thus nylon 6 and SDVB were the most sorbents for 1-hydroxypyrene in terms of their sorption capacity and percentage recoveries.

Another area where pre-concentration is useful is in solid phase extraction (SPE) of drugs in biological fluids. Kang et al (2007) used electrospun polystyrene nanofibers as sorbent material to extract trazodone from human plasma. The fibers are packed into a column and diluted blood samples with the protein removed are passed through the column. Methanol was then used to extract trazondone which has been sorbed on the fibers. Anaylsis using HPLC was able to demonstrate detection of trazodone from extracted samples. Ifegwu et al (2015) tested several electrospun polymers for the pre-concentration of urinary 1-hydroxyprene. Nylon-6 showed good results with more than 70% adsorption and desorption. Amphiphilic nature of Nylon-6 may account for its good recovery characteristic. Good adsorption of Nylon-6 may be due to its hydrophilic nature with possible bonding between the amide group of Nylon 6 and -OH group on the analyte. Hydrophobic interaction between the methylene groups of Nylon 6 and poly aromatic hyroxyprene may have also contributed.

Fig 1. Percentage recoveries of 1-hydoxypyrene at different concentrations [Ifegwu et al. Journal of Analytical Science and Technology (2015) 6:14].

High surface area of nanofibers offers several advantages as a pre-concentration agent. While specificity of adsorption and desorption is dependent on the material and the analyte pair, greater surface areas increases the rate of reactivity. In most experiments, it only takes 30 minutes for the extraction to reach equilibrium or for optimum extraction. A greater amount of analyte may also be adsorbed onto the fiber surface thereby increasing the concentration of analyte per unit mass of the fiber. This increases the productivity of the adsorption material and in some cases, improves the sensitivity of the analyte detection.