thermal or chemical treatment to improve features
that include pore size, conductivity, and mechanical strength. Thermal posttreatment can be used
to control the diameter of nanofibers. Metal oxides
or salts in the nanofibers can be used to make the
fibers bactericidal, and graphitization of carbon
onto the nanofibers will enhance their specific
area surface. Chemical modification may add
properties similar to thermal treatment. Nanofibers generated by electrospinning can make use
of a wide range of inorganic, organic, and organometallic materials. Almost any soluble polymer
can be incorporated into nanofibers with the
electrospinning method with the constraint that
their molecular weight should be sufficiently
high. Examples include polyvinylpyrrolidone
(PVP), polyethylene oxide (PEO), polyvinyl acetate (PVAc), and polyvinyl alcohol (PVA), which
are water-soluble polymers, and non-watersoluble polymers such as polyimide (PI), polyacrylonitrile (PAN), polyvinylidene fluoride
(PVDF), polylactic acid (PLA), polymethacrylate
(PMMA), polystyrene (PS), polypyrrole (PPy),
and polyvinylchloride (PVC) [34].
Performance of Electrospun Nanofibers
Nanofiber filters have attracted a great deal of
interest since they have a high filtration efficiency
due to interception and a low pressure drop
because of the slip flow effect. Many parameters
impact filtration performance including the physical properties of the filter (such as nanofiber
diameter, surface area, pore size, packing density,
basis weight, and thickness) and the conditions of
the installation such as face velocity. Particulate
matter builds up on the filter in dendrimers which
can enhance particle filtration [25].
Nanofiber diameter is an important parameter
affecting filtration performance. Mao et al. [4]
evaluated the relationship between fiber diameter
and filtration efficiency and pressure drop, using
an yttria-stabilized zirconia (YSZ) nanofiber.
They found a decrease in the filtration efficiency
with an increase in nanofiber diameter. At the
same time, smaller fiber diameter decreased pressure drop, regardless of particle size (Fig. 4a, b). It
was recently reported that nanofiber diameters
less than 100 nm enhance the slip flow effect,
decreasing drag and pressure drop and subsequently increasing particle capture [37]. One
study used keratin-based nanofibers to demonstrate the effect [39]. Polymeric nanofiber felts
made of PU nanofibers [31], cellulose [40], and
PVP [41] have been produced by electrospinning
and also show improved filter performance due to
their reduced fiber diameter. In sum, electrospinning provides greatly improved filtration efficiency due in part to smaller fiber diameter that is
able to capture ultrafine particles; the smaller the
fiber, the greater the filtration efficiency.
The surface area and pore size of nanofibers are
affected by the fiber structure and the composition
of the blended solution. Liu et al. [42] prepared an
electrospun PAN/polyacrylic acid (PAA) composite nanofiber felt to study the effects of PAN/PAA
ratios on filtration performance. Blending PAA
with PAN allowed creation of a composite with
features that could be controlled. The tensile
strength of the nanofibers greatly improved,
going from 3.8 to 6.6 MPa as the PAA content
increased, creating robust mechanical strength in
pure PAN nanofibers. The smallest pore size in the
membranes resulted in a very high filtration efficiency, over 99.99%, with minimal pressure drop,
160 Pa. A porous bead-on-string polylactic acid
(PLA) membrane for capturing nanoparticles was
created by Wang et al. [43]. The large surface area,
coupled with microporous beads, showed
enhanced filtration efficiency. A smaller pore
size polyvinyl alcohol (PVA) nanofiber filter fabricated by Li et al. [44] has shown a high filtration
efficiency for nanoparticles. Wang et al. [45]
found that adding SiO 2 nanoparticles to multilayer PAN structures created bimodal distributions of fiber sizes that increased filtration
performance, with a removal efficiency of
99.99% at a pressure drop of Dp ¼ 116 Pa, for
nanoparticles relative to single-layer fibers. This
was said to be due to the roughness of the surface
fibers arising from the noncircular structure and
also from the greater specific surface area of the
multilayer fibers, as a result of adding SiO 2 . Two
membrane fibers, PA6 and PMIA, arranged in
sequence by Zhang et al. [46] showed high
Airborne Nanoparticles: Control and Detection
95
that include pore size, conductivity, and mechanical strength. Thermal posttreatment can be used
to control the diameter of nanofibers. Metal oxides
or salts in the nanofibers can be used to make the
fibers bactericidal, and graphitization of carbon
onto the nanofibers will enhance their specific
area surface. Chemical modification may add
properties similar to thermal treatment. Nanofibers generated by electrospinning can make use
of a wide range of inorganic, organic, and organometallic materials. Almost any soluble polymer
can be incorporated into nanofibers with the
electrospinning method with the constraint that
their molecular weight should be sufficiently
high. Examples include polyvinylpyrrolidone
(PVP), polyethylene oxide (PEO), polyvinyl acetate (PVAc), and polyvinyl alcohol (PVA), which
are water-soluble polymers, and non-watersoluble polymers such as polyimide (PI), polyacrylonitrile (PAN), polyvinylidene fluoride
(PVDF), polylactic acid (PLA), polymethacrylate
(PMMA), polystyrene (PS), polypyrrole (PPy),
and polyvinylchloride (PVC) [34].
Performance of Electrospun Nanofibers
Nanofiber filters have attracted a great deal of
interest since they have a high filtration efficiency
due to interception and a low pressure drop
because of the slip flow effect. Many parameters
impact filtration performance including the physical properties of the filter (such as nanofiber
diameter, surface area, pore size, packing density,
basis weight, and thickness) and the conditions of
the installation such as face velocity. Particulate
matter builds up on the filter in dendrimers which
can enhance particle filtration [25].
Nanofiber diameter is an important parameter
affecting filtration performance. Mao et al. [4]
evaluated the relationship between fiber diameter
and filtration efficiency and pressure drop, using
an yttria-stabilized zirconia (YSZ) nanofiber.
They found a decrease in the filtration efficiency
with an increase in nanofiber diameter. At the
same time, smaller fiber diameter decreased pressure drop, regardless of particle size (Fig. 4a, b). It
was recently reported that nanofiber diameters
less than 100 nm enhance the slip flow effect,
decreasing drag and pressure drop and subsequently increasing particle capture [37]. One
study used keratin-based nanofibers to demonstrate the effect [39]. Polymeric nanofiber felts
made of PU nanofibers [31], cellulose [40], and
PVP [41] have been produced by electrospinning
and also show improved filter performance due to
their reduced fiber diameter. In sum, electrospinning provides greatly improved filtration efficiency due in part to smaller fiber diameter that is
able to capture ultrafine particles; the smaller the
fiber, the greater the filtration efficiency.
The surface area and pore size of nanofibers are
affected by the fiber structure and the composition
of the blended solution. Liu et al. [42] prepared an
electrospun PAN/polyacrylic acid (PAA) composite nanofiber felt to study the effects of PAN/PAA
ratios on filtration performance. Blending PAA
with PAN allowed creation of a composite with
features that could be controlled. The tensile
strength of the nanofibers greatly improved,
going from 3.8 to 6.6 MPa as the PAA content
increased, creating robust mechanical strength in
pure PAN nanofibers. The smallest pore size in the
membranes resulted in a very high filtration efficiency, over 99.99%, with minimal pressure drop,
160 Pa. A porous bead-on-string polylactic acid
(PLA) membrane for capturing nanoparticles was
created by Wang et al. [43]. The large surface area,
coupled with microporous beads, showed
enhanced filtration efficiency. A smaller pore
size polyvinyl alcohol (PVA) nanofiber filter fabricated by Li et al. [44] has shown a high filtration
efficiency for nanoparticles. Wang et al. [45]
found that adding SiO 2 nanoparticles to multilayer PAN structures created bimodal distributions of fiber sizes that increased filtration
performance, with a removal efficiency of
99.99% at a pressure drop of Dp ¼ 116 Pa, for
nanoparticles relative to single-layer fibers. This
was said to be due to the roughness of the surface
fibers arising from the noncircular structure and
also from the greater specific surface area of the
multilayer fibers, as a result of adding SiO 2 . Two
membrane fibers, PA6 and PMIA, arranged in
sequence by Zhang et al. [46] showed high
Airborne Nanoparticles: Control and Detection
95