**3. Nanofiber based sorbents**

*Washing*: this serves to remove interferences retained on the sorbent leaving behind the analytes of interest. In some cases, it could be used to wash-off the analyte of interest while retaining the interferences. In mechanisms employing ion exchange, the pH of the wash solvent should be sufficient enough to disrupt the charged sites of interferences but not affect the

*Elution*: the elution solvent should be strong enough to disrupt all analyte-sorbent interactions in order to obtain the highest recoveries. However, there is a limit to the strength as harsh solvents would not only desorb analytes from the sorbent, but also strip strongly retained

Figure.4. shows schematic representations of the four approaches typically taken in practical SPE applications. Figure.4. (a) shows a three step SPE process in which; (1) the sorbent is conditioned after which the sample is loaded and finally (2) the analyte is eluted with the interferences being retained (clean up) or the analyte is simply eluted into a smaller sample

Figure.4. (b) shows a four step SPE process in which; (1) the sorbent is conditioned after which the sample is loaded, (2) the interferences eluted and finally (3) the analyte is eluted (clean up)

Figure.4. (c) shows a five step SPE process in which; (1) the sorbent is conditioned after which the sample is loaded, (2) the interferences eluted, (3) a fraction of analyte is selectively eluted

or the analyte is simply eluted into a smaller sample volume (preconcentration).

and finally (4) the second fraction of analytes is eluted.

**Figure 4.** Schematic representation of practical SPE operations

analyte.

6 Advances in Nanofibers

interferences.

volume (preconcentration).

Sorbent selection for SPE is normally guided by the ability of a material to retain analytes selectively as well as facilitate rapid and complete elution. The sorption process must be reversible [19, 20]. In addition to reversible sorption, SPE sorbents should possess a large specific surface area as well as exhibit stability in the sample matrix and elution solvents. Lastly, a sorbent material with a good surface contact with the sample solution would be most preferred.

Poole and co-workers [21] categorized SPE sorbents into three as; (i) general purpose, (ii) class specific and (iii) analyte specific. The most common retention mechanisms in SPE are based on van der Waals forces, π-π interactions, hydrogen bonding, dipole-dipole interactions and ion exchange interactions. As a result, sorbents can be classified on the basis of retention mechanisms as; (i) reversed phase (ii) normal phase and (iii) ion exchange. From the materials perspective, sorbents are classified into three as; (i) carbon based (ii) inorganic based and (iii) polymer based.

The SPE sorbent fabrication technique of choice could be viewed as one that produces a material that exhibits chemical and morphological properties that can be easily modified. Similarly, an optimal SPE sorbent material combines the following benefits: (i) small diameter, (ii) large specific surface area, (iii) simplified fabrication/synthesis, (iv) ability to be modified in order to incorporate all sorbent chemistries/functionalities, (v) ability to be modified in order to incorporate all sorbent morphologies (vi) a material that can be packed in the lower (less than 10) mg range without presenting a backpressure limitation or low analyte recoveries.

The advent of nanotechnology has been a major leap forward in the research area of sorbent based sample preparation techniques as it opened up possibilities for a new class of materials that could be used in SPE applications. The main benefit of nanostructured materials is their large specific surface area that facilitates the miniaturization of SPE allowing for the use of a reduced sorbent bed mass that achieves high extraction efficiency.

Although nanoparticles offer improved performance as sorbent material [22], they inherently exhibit some limitations. One of the main challenges is associated with their handling in packed SPE formats.

Given the fact that nanoparticles have shown excellent properties as sorbent material, it would seem prudent to focus developmental efforts on nanostructured material with the aim to address the challenges of nanoparticles. This brings about a need for an alternative fabrication approach that carries with it the benefits of nanoparticles at the same time addressing some (if not all) of their limitations. From our research group's perspective, the use of electrospun nanofibers as sorbent material is seen as a possible way of carrying along the benefits of nanoparticles, whilst addressing their limitations and increasing the possibilities for modifying sorbent morphology and functionality. It is upon this background that the section has been compiled to present and discuss relevant examples that demonstrate the potential of electro‐ spinning as an alternative fabrication technique.

## **3.1. Range of sorbent materials**

For it to be a good alternative, it should be able to fabricate a wide range of materials on the basis of type (inorganic, organic, inorganic/organic hybrid and biological), stability (mechan‐ ical and chemical) and morphology (pore structure and surface area).

*3.1.2. Silica based*

PVA resulting in amorphous silica fibers.

silanol groups for silylation [30].

adsorptive capacity of silica fibers in SPE applications.

to 600 m2

*3.1.3. Polymer based*

Over the years, silica microparticles have been used widely as a sorbent for solid phase extraction. Recently, the focus has shifted to the use of silica nanoparticles due to their large specific surface area and intrinsic surface reactivity [27]. However, the use of silica nanopar‐ ticles for packed sorbent SPE still remains a challenge chiefly because of high back pressure, which explains why to date the smallest microparticle that has been used for packed sorbent SPE has a diameter of 8 µm [28]. Due to the fact that the simplest electrospinning set-up allows the collection of nanofibers in the form of a porous nonwoven mesh, it follows that porous electrospun nanofiber based SPE sorbent formats could be easily fabricated. Therefore, it is anticipated that electrospun silica nanofibers would be used for packed sorbent SPE, thus

Electrospun Nanofiber Based Solid Phase Extraction

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9

overcoming the high back pressure limitation associated with silica nanoparticles.

The fabrication of inorganic nanofibers typically involves the electrospinning of a polymer/sol composite and subsequent calcination of the electrospun fibers. In 2002, Shao and co-workers were the first to report the fabrication of silica nanofibers [29]. The experimental approach involved first, the preparation of a silica sol from tetraethyl orthosilicate (TEOS), H3PO4, H2O followed by electrospinning of a PVA/silica sol. The PVA/ silica fibers were calcined to remove

In 2003, Choi and co-workers reported a simplified approach in which silica nanofibers were fabricated directly from a silica sol [30]. Their fabrication method involved the preparation of a silica sol from TEOS, distilled water, ethanol and HCL with subsequent electrospinning. An interesting aspect of the fabrication approach was the fact that unlike in the first report by Shao and co-workers [29], TEOS did not contain a polymer to help spinnability, thus there was no need for the calcination step. Spectroscopic characterizationof the silica nanofibers confirmed the extensive hydrolysis of the TEOS suggesting the availability of a substantial amount of

A way of improving the adsorption capacity of silica nanofibers is to increase the specific surface area as a function of the pore volume. Wei and co-workers fabricated porous silica nanofibers containing catalytic silver nanoparticles [31]. TEOS, poly [3- (trimethoxysily) propylmethacrylate] (PMCM) and AgNO3 were used as precursors for the production of silica/ polymer hybrid nanofibers. On heat treatment of the electrospun fibers, degradation of the PMCM polymer resulted in pores that led to the increase in specific surface area from 11 m2

surface area. Therefore it is expected that a similar approach could be used to improve the

Synthetic polymers are the most popular class of materials that have been electrospun for SPE applications. Some of these polymers include polystyrene or polystyrene copolymers,[32-41] Nylon 6,[42-44] and poly (ethylene terepthalate) [45]. Other reports have appeared in the Chinese database, unfortunately the polymeric material was not described in English [46-48]. It is expected that synthetic polymers will continue to be the most popular class of electrospun

/g. The porous fibers exhibited an improved catalytic activity due to the increased

/g

#### *3.1.1. Carbon based*

Commercial polyacrylonitrile (PAN) based carbon fibers account for nearly 90% of the total carbon fiber output worldwide due to the high carbon yield and easy carbonization process. Therefore, PAN is mostly chosen as the precursor polymer for the preparation of electrospun carbon nanofibers (CNFs) [23].

Porous carbon nanofibers fabricated by thermal activation of electrospun PAN based CNFs have improved sorptive properties due to their large specific surface area. In a report by Oh and co-workers, a study was carried out to determine the toluene adsorption capacity for PANbased steam activated carbon nanofibers (ACNFs) [24]. The CNFs were activated between 800°C and 1000°C in order to evaluate the effect of activation temperature on fiber porosity. A relatively large adsorption capacity (65 g toluene/100 g ACNFs) was achieved by ACNFs (activated at 1000°C) with a specific surface area of 1403 m2 g-1 as compared to (40 g toluene/100 g ACNFs) that was achieved for ACNFs (activated at 800°C) with a lower specific surface area of 853 m2 /g. The results obtained in the study clearly demonstrated the great potential of steam activation as a post electrospinning modification approach for the fabrica‐ tion of carbon nanofibers with a large sorptive capacity that could be used as sorbent material for SPE.

Shim and co-workers, compared the adsorption properties of electrospun steam activated carbon nanofibers and commercially available activated carbon fibers (CFs) [25]. Although there was a significant attenuation in the average fiber diameter from 20 µm (CFs) to 250 nm (ACNFs), the change in specific surface area was not significant (1015 m2 /g for CFs to 1193 m2 / g for ACNFs). Nevertheless, ACNFs exhibited a much larger adsorption capacity and faster adsorption/desorption kinetics due to their large number of shallow micropores and a more homogeneous surface, all these properties brought about by their nanoscale size. As demon‐ strated by the results obtained in the study, it was interesting to note that, besides the specific surface area, the pore structure and surface homogeneity also plays a significant role in improving sorptive capacity.

Bui and co-workers explored an alternative approach for fabricating porous carbon nanofibers in which PAN/pitch blends were electrospun with subsequent steam activation [26]. The specific surface area of the fabricated ACNFs increased from 723 m2 /g (activation temperature 700°C) to 1877 m2 /g (activation temperature 900°C). This was attributed to an increase in the mesopore fraction as the micropore fraction decreased. Given the fact that a specific surface area of 1877 m2 /g is among the highest ever reported for nanostructured materials, it is expected that carbon nanofibers fabricated in the study would exhibit an excellent sorptive capacity.

## *3.1.2. Silica based*

**3.1. Range of sorbent materials**

carbon nanofibers (CNFs) [23].

surface area of 853 m2

improving sorptive capacity.

700°C) to 1877 m2

area of 1877 m2

for SPE.

*3.1.1. Carbon based*

8 Advances in Nanofibers

For it to be a good alternative, it should be able to fabricate a wide range of materials on the basis of type (inorganic, organic, inorganic/organic hybrid and biological), stability (mechan‐

Commercial polyacrylonitrile (PAN) based carbon fibers account for nearly 90% of the total carbon fiber output worldwide due to the high carbon yield and easy carbonization process. Therefore, PAN is mostly chosen as the precursor polymer for the preparation of electrospun

Porous carbon nanofibers fabricated by thermal activation of electrospun PAN based CNFs have improved sorptive properties due to their large specific surface area. In a report by Oh and co-workers, a study was carried out to determine the toluene adsorption capacity for PANbased steam activated carbon nanofibers (ACNFs) [24]. The CNFs were activated between 800°C and 1000°C in order to evaluate the effect of activation temperature on fiber porosity. A relatively large adsorption capacity (65 g toluene/100 g ACNFs) was achieved by ACNFs

toluene/100 g ACNFs) that was achieved for ACNFs (activated at 800°C) with a lower specific

potential of steam activation as a post electrospinning modification approach for the fabrica‐ tion of carbon nanofibers with a large sorptive capacity that could be used as sorbent material

Shim and co-workers, compared the adsorption properties of electrospun steam activated carbon nanofibers and commercially available activated carbon fibers (CFs) [25]. Although there was a significant attenuation in the average fiber diameter from 20 µm (CFs) to 250 nm

g for ACNFs). Nevertheless, ACNFs exhibited a much larger adsorption capacity and faster adsorption/desorption kinetics due to their large number of shallow micropores and a more homogeneous surface, all these properties brought about by their nanoscale size. As demon‐ strated by the results obtained in the study, it was interesting to note that, besides the specific surface area, the pore structure and surface homogeneity also plays a significant role in

Bui and co-workers explored an alternative approach for fabricating porous carbon nanofibers in which PAN/pitch blends were electrospun with subsequent steam activation [26]. The

mesopore fraction as the micropore fraction decreased. Given the fact that a specific surface

that carbon nanofibers fabricated in the study would exhibit an excellent sorptive capacity.

/g (activation temperature 900°C). This was attributed to an increase in the

/g is among the highest ever reported for nanostructured materials, it is expected

/g. The results obtained in the study clearly demonstrated the great

g-1 as compared to (40 g

/g for CFs to 1193 m2

/g (activation temperature

/

ical and chemical) and morphology (pore structure and surface area).

(activated at 1000°C) with a specific surface area of 1403 m2

(ACNFs), the change in specific surface area was not significant (1015 m2

specific surface area of the fabricated ACNFs increased from 723 m2

Over the years, silica microparticles have been used widely as a sorbent for solid phase extraction. Recently, the focus has shifted to the use of silica nanoparticles due to their large specific surface area and intrinsic surface reactivity [27]. However, the use of silica nanopar‐ ticles for packed sorbent SPE still remains a challenge chiefly because of high back pressure, which explains why to date the smallest microparticle that has been used for packed sorbent SPE has a diameter of 8 µm [28]. Due to the fact that the simplest electrospinning set-up allows the collection of nanofibers in the form of a porous nonwoven mesh, it follows that porous electrospun nanofiber based SPE sorbent formats could be easily fabricated. Therefore, it is anticipated that electrospun silica nanofibers would be used for packed sorbent SPE, thus overcoming the high back pressure limitation associated with silica nanoparticles.

The fabrication of inorganic nanofibers typically involves the electrospinning of a polymer/sol composite and subsequent calcination of the electrospun fibers. In 2002, Shao and co-workers were the first to report the fabrication of silica nanofibers [29]. The experimental approach involved first, the preparation of a silica sol from tetraethyl orthosilicate (TEOS), H3PO4, H2O followed by electrospinning of a PVA/silica sol. The PVA/ silica fibers were calcined to remove PVA resulting in amorphous silica fibers.

In 2003, Choi and co-workers reported a simplified approach in which silica nanofibers were fabricated directly from a silica sol [30]. Their fabrication method involved the preparation of a silica sol from TEOS, distilled water, ethanol and HCL with subsequent electrospinning. An interesting aspect of the fabrication approach was the fact that unlike in the first report by Shao and co-workers [29], TEOS did not contain a polymer to help spinnability, thus there was no need for the calcination step. Spectroscopic characterizationof the silica nanofibers confirmed the extensive hydrolysis of the TEOS suggesting the availability of a substantial amount of silanol groups for silylation [30].

A way of improving the adsorption capacity of silica nanofibers is to increase the specific surface area as a function of the pore volume. Wei and co-workers fabricated porous silica nanofibers containing catalytic silver nanoparticles [31]. TEOS, poly [3- (trimethoxysily) propylmethacrylate] (PMCM) and AgNO3 were used as precursors for the production of silica/ polymer hybrid nanofibers. On heat treatment of the electrospun fibers, degradation of the PMCM polymer resulted in pores that led to the increase in specific surface area from 11 m2 /g to 600 m2 /g. The porous fibers exhibited an improved catalytic activity due to the increased surface area. Therefore it is expected that a similar approach could be used to improve the adsorptive capacity of silica fibers in SPE applications.

#### *3.1.3. Polymer based*

Synthetic polymers are the most popular class of materials that have been electrospun for SPE applications. Some of these polymers include polystyrene or polystyrene copolymers,[32-41] Nylon 6,[42-44] and poly (ethylene terepthalate) [45]. Other reports have appeared in the Chinese database, unfortunately the polymeric material was not described in English [46-48]. It is expected that synthetic polymers will continue to be the most popular class of electrospun materials for use as SPE sorbents. This is due to the fact that, of all the materials that have been electrospun, polymers show the greatest potential for tuning of the sorptive chemistries.

Sorptive capacity of polymer nanofibers could be improved by increasing the pore volume. There are several methods that have been reported for introducing porous structures in nanofibers which include electrospinning of polymer blends, [49] controlled humidity, [50] the salt induced process [51] and silica nanotemplating [52]. Silica nanotemplating is a relatively simple process compared to the other methods as they require handling of compli‐ cated interactions between the polymer matrix and pore generator. In addition, porosity and pore size of the resultant porous nanofibers may be controlled easily by adjusting the content and size of silica nanoparticles.

Shi and co-workers fabricated porous nylon 6 nanofibers using silica nanoparticles as the nanotemplate [52]. The experimental approach involved the fabrication of nylon 6/silica nanofiber composites, followed by removal of the silica nanoparticles through treatment with hydrofluoric acid. After the removal of the silica nanoparticles, the specific surface area and pore volume increased from 4.68 m2 /g to 8.31 m2 /g and 0.0133 cm3 /g to 0.0250 cm3 /g respec‐ tively. The increase in specific surface area demonstrated that the approach could be used to improve the sorptive capacity of nylon nanofibers for SPE applications.

#### **3.2. Control of selectivity**

Selectivity of sorbent material is a parameter of great concern in current SPE applications. Over the years, various sorbent modification strategies have been employed to impart selectivity based on basicity, acidity, polarity, size and more recently molecular recognition [53-55]. For it to be a good alternative, electrospinning should be able to allow the incorporation of an unlimited range of functionalities in order to target a broad spectrum of analytes.

hydrophobicity (polystyrene nanofibers) for non polar analytes, strong cation exchange properties (poly (styrene-co-*p*-sodium styrene sulphonate) nanofibers) for basic and neutral analytes and weak cation exchange properties (poly (styrene-co-methacrylic) nanofibers for strongly basic analytes. Due to the ease of spinnability of polystyrene copolymers and the demonstrated performance of the resultant fibers as SPE sorbents, a wide range of function‐ alities can be introduced on the polystyrene backbone by an experimental approach that involves copolymerization and electrospinning. It is expected that in the near future, SPE sorbents based on electrospun polystyrene copolymers will increase as there is a wide range

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**Scheme 1.** Schematic representation of synthesis and molecular structure of the porphyrin copolymers [56].

Of all SPE sorbent materials that have been reported to date, those fabricated via the molecular

Through electrospinning it is possible to incorporate the selectivity of MIPs either by encap‐ sulating MIP nanoparticles into electrospun nanofibers or by imprinting the electrospun fibers. Yoshimatsu and co-workers encapsulated molecularly imprinted nanoparticles into poly (ethylene terephthalate) (PET) nanofibers through electrospinning [45]. The composite nanofibers (Fig.5.) were used as a sorbent material for batch solid phase extraction of propa‐ nolol. As confirmed by radio ligand binding analysis, the specific binding sites in the composite nanofibers remained easily accessible and were chiral-selective. Furthermore, it was demon‐ strated that without the electrospun nanofiber based solid phase extraction step, the existence of propranolol residues in water could not be confirmed even with the sensitivity of HPLC–

imprinting technology have shown the best selectivity after immunosorbents [57].

of vinylic monomers that can be copolymerized with styrene.

*3.2.3. Molecular imprinting based*

MS/MS analysis [45].

#### *3.2.1. Polarity based*

Besides the conventional small functional groups like sulphonates and carboxylic acids, macromolecules can be incorporated into polymer nanofibers to improve their selectivity as SPE sorbents. Wan and co-workers reported the fabrication of porphyrinated nanofibers by copolymerization and electrospinning [56]. The fabrication approach involved solution copolymerization of acrylonitrile with vinyl porphyrins (see Scheme.1) and subsequent electrospinning of the resulting porphyrin copolymers.

#### *3.2.2. Ion exchange based*

At the beginning of 2008, Kang and co-workers reported a comparative study of the perform‐ ances of poly (styrene-co-methacrylic), poly (styrene-co-*p*-sodium styrene sulphonate) and polystyrene nanofibers for the extraction of steroidal compounds [35]. Of the three kinds of nanofibers, those of poly (styrene-co-*p*-sodium styrene) exhibited the highest extraction efficiencies, while those of polystyrene were the least efficient. The trend was attributed to the fact that the polar model analytes favored the polar sorbent. With respect to applications, the study provided a platform for different chemistries that may introduce selectivity based on

**Scheme 1.** Schematic representation of synthesis and molecular structure of the porphyrin copolymers [56].

hydrophobicity (polystyrene nanofibers) for non polar analytes, strong cation exchange properties (poly (styrene-co-*p*-sodium styrene sulphonate) nanofibers) for basic and neutral analytes and weak cation exchange properties (poly (styrene-co-methacrylic) nanofibers for strongly basic analytes. Due to the ease of spinnability of polystyrene copolymers and the demonstrated performance of the resultant fibers as SPE sorbents, a wide range of function‐ alities can be introduced on the polystyrene backbone by an experimental approach that involves copolymerization and electrospinning. It is expected that in the near future, SPE sorbents based on electrospun polystyrene copolymers will increase as there is a wide range of vinylic monomers that can be copolymerized with styrene.

#### *3.2.3. Molecular imprinting based*

materials for use as SPE sorbents. This is due to the fact that, of all the materials that have been electrospun, polymers show the greatest potential for tuning of the sorptive chemistries.

Sorptive capacity of polymer nanofibers could be improved by increasing the pore volume. There are several methods that have been reported for introducing porous structures in nanofibers which include electrospinning of polymer blends, [49] controlled humidity, [50] the salt induced process [51] and silica nanotemplating [52]. Silica nanotemplating is a relatively simple process compared to the other methods as they require handling of compli‐ cated interactions between the polymer matrix and pore generator. In addition, porosity and pore size of the resultant porous nanofibers may be controlled easily by adjusting the content

Shi and co-workers fabricated porous nylon 6 nanofibers using silica nanoparticles as the nanotemplate [52]. The experimental approach involved the fabrication of nylon 6/silica nanofiber composites, followed by removal of the silica nanoparticles through treatment with hydrofluoric acid. After the removal of the silica nanoparticles, the specific surface area and

tively. The increase in specific surface area demonstrated that the approach could be used to

Selectivity of sorbent material is a parameter of great concern in current SPE applications. Over the years, various sorbent modification strategies have been employed to impart selectivity based on basicity, acidity, polarity, size and more recently molecular recognition [53-55]. For it to be a good alternative, electrospinning should be able to allow the incorporation of an

Besides the conventional small functional groups like sulphonates and carboxylic acids, macromolecules can be incorporated into polymer nanofibers to improve their selectivity as SPE sorbents. Wan and co-workers reported the fabrication of porphyrinated nanofibers by copolymerization and electrospinning [56]. The fabrication approach involved solution copolymerization of acrylonitrile with vinyl porphyrins (see Scheme.1) and subsequent

At the beginning of 2008, Kang and co-workers reported a comparative study of the perform‐ ances of poly (styrene-co-methacrylic), poly (styrene-co-*p*-sodium styrene sulphonate) and polystyrene nanofibers for the extraction of steroidal compounds [35]. Of the three kinds of nanofibers, those of poly (styrene-co-*p*-sodium styrene) exhibited the highest extraction efficiencies, while those of polystyrene were the least efficient. The trend was attributed to the fact that the polar model analytes favored the polar sorbent. With respect to applications, the study provided a platform for different chemistries that may introduce selectivity based on

unlimited range of functionalities in order to target a broad spectrum of analytes.

/g and 0.0133 cm3

/g to 0.0250 cm3

/g respec‐

/g to 8.31 m2

improve the sorptive capacity of nylon nanofibers for SPE applications.

electrospinning of the resulting porphyrin copolymers.

and size of silica nanoparticles.

10 Advances in Nanofibers

pore volume increased from 4.68 m2

**3.2. Control of selectivity**

*3.2.1. Polarity based*

*3.2.2. Ion exchange based*

Of all SPE sorbent materials that have been reported to date, those fabricated via the molecular imprinting technology have shown the best selectivity after immunosorbents [57].

Through electrospinning it is possible to incorporate the selectivity of MIPs either by encap‐ sulating MIP nanoparticles into electrospun nanofibers or by imprinting the electrospun fibers.

Yoshimatsu and co-workers encapsulated molecularly imprinted nanoparticles into poly (ethylene terephthalate) (PET) nanofibers through electrospinning [45]. The composite nanofibers (Fig.5.) were used as a sorbent material for batch solid phase extraction of propa‐ nolol. As confirmed by radio ligand binding analysis, the specific binding sites in the composite nanofibers remained easily accessible and were chiral-selective. Furthermore, it was demon‐ strated that without the electrospun nanofiber based solid phase extraction step, the existence of propranolol residues in water could not be confirmed even with the sensitivity of HPLC– MS/MS analysis [45].

solvent treatments. Figure.6. shows a schematic representation of a possible binding site model

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What is the starting point in electrospun nanofiber based SPE method development? The

**i.** *Nature of the analyte*: What are the functional groups on the analyte? What is the logP

**ii.** *Nature of the electrospun fibers*: How does one introduce a functionality of the target

**iii.** *Nature of the SPE device*: What is the best way of handling the fibers (that is what sort

**iv.** *SPE operation*: What sample volume, analyte concentration, volume of SPE solvents

Of all these, the most important aspect is to come up with a feasible electrospun nanofiber based SPE device as that serves as a platform for a optimal SPE method development.

To date, fabricated SPE devices that employ electrospun nanofibers as a sorbent bed have been based on polystyrene or nylon polymers. This has resulted in a classification of electrospun nanofiber based SPE devices into two as: polystyrene type (polymer fibers of a relatively low mechanical strength); and, nylon type (polymer fibers of a relatively high

In 2007, Kang and co-workers were the first to report the use of electrospun polymer nanofibers for packed sorbent SPE [60]. They manually packed 1 mg of polystyrene nanofibers into a 200 µl micro pipette tip to form a micro column as shown in Fig.7. (a) and (b) as published in their 2009 article. Solvents were pushed through the electrospun nanofiber based SPE device manually by the pressure of air forced by a gas tight plastic syringe (2 mL) (Fig.7 (c)). The device demonstrated a leap forward regarding the use electrospun nanofibers for miniaturized SPE devices. Although the packing process involved the use of simple homemade tools, it seems the packing operation is not that simple as reproducibility relies very much on operator experience. Nevertheless, the study created a platform for further research as it clearly demonstrated that nanofibers allowed the miniaturization of SPE devices without compro‐

(that is what shape or format)? How much sorbent should be packed?

of the analyte? What is the pKa of the analyte? What is the solubility of the analyte?

analyte onto the surface of electrospun fibers? Which polymer (s) or spinnable precursor (s) material should be employed? Which electrospinning protocol should be adopted (electrospinning conditions and electrospinning set-up)? Is there a need to modify the morphology of the fibers, if there is, how could it be done? What

of SPE device (s) should be fabricated)? What is the best way of packing the nanofibers

for the reported 2,4-D imprinted nanofibers.

questions can be classified into four as;

and at what flow rate?

mechanical strength) [59].

**4.1. Polystyrene type SPE devices**

**4. Nanofiber based SPE method development**

diameter of the fibers should be fabricated?

**Figure 5.** SEM image of electrospun nanofiber composite membrane containing molecular imprinted nanoparticles [45].

**Figure 6.** Schematic representation of molecularly imprinted nanofibers with binding sites specific for 2,4-D template molecules [58].

Chronakis and co-workers reported a simplified approach that allowed the generation of template defined sites directly during electrospinning [58]. The electrospun nanofibers were prepared from a solution mixture of PET and polyallylamine in the presence of a template molecule, 2,4-dichlorophenoxyacetic acid (2,4-D). Polyallylamine was used to provide functional groups that interacted with the template during the electrospinning process, and PET was used as the supporting matrix to ensure easy fiber formation and to minimize the conformational change of the polymers when the nanofibers were subjected to different solvent treatments. Figure.6. shows a schematic representation of a possible binding site model for the reported 2,4-D imprinted nanofibers.
