Fabrication of Silica Nanofluidic Tubing for Single Molecule Detection - Nanofluidics: Nanoscience and Nanotechnology - page 94

substrates with a void gap or patterned gold-electrode pairs on highly insulating substrates.
When a continuous fiber is deposited on the collector, the electrostatic interaction in as-
spun fibers initiates alignment. Another aligned nanofiber collection technique has been
developed by Zussman's group. 72 They demonstrated the effectiveness of using a tapered
and grounded wheel-like rotating bobbin as the collector to obtain parallel nanofibers.
Since the collector is not flat, the challenge when using this bobbin collector is to transfer
nanofibers onto a planar surface for possible integration with other devices and
applications. Additional rotating collectors have been proposed by Kameoka and co-
workers to define “scanning electrospinning” techniques. 73,74 A schematic of the process is
provided in Figure 5.3(a). The grounded collector is connected to an electric rotational
motor and rotates at high speed. A substrate is attached to the rotating collector where
nanofibers will be deposited. Nanofibers can be deposited in an aligned fashion due to the
rotation and relative motion between the electrospinning nozzle and the counter electrode.
The orientation of nanofibers is controlled by the linear velocity of rotating collector. In
this way, nanofibers can be aligned in a facile manner and collected on the flat substrate
for further treatment and applications. Figure 5.3(b) and (c) show parallel-aligned ZrO 2
nanofibers on silicon wafers.
The basic electrospinning process has also been modified in a variety of ways to
produce nanofibers with a range of secondary structures. For example, conjugated polymer
nanofibers, 75 core-sheath nanofibers, 76-78 hollow nanofibers, 79,80 porous nanofibers 81 have
all been produced by coaxial electrospinning from a two-fluid capillary spinneret.
Nanofibers conjugated with fluorescent probes, 82 metal oxide particles 83 and nanoparticles
of noble metals 84,85 have been reported.
For more detailed information about the electrospinning process, the reader is
directed to some excellent review articles published elsewhere. 58,59,61,86,87 Theoretical
analyses of the electrospinning process can also can be found. 88-91 However, due to
complex effects of diversified processing parameters, accurate modelling of
electrospinning is still challenging. In the following sections, we will focus on the
apparatus used for coaxial electrospinning of silica nanofluidic tubes.
5.2.2.2 Nano-Scale Silica Fibers and Hollow Tubing Structures
Silica is common substrate material in biological applications due to its
biocompatibility and excellent chemical and physical properties. Silica nanofibers can be
successfully produced using the basic electrospinning set-up shown in Figure 5.1. An
electrospinning solution can be prepared using the process developed by Kameoka and
coworkers. 68 This solution is a blend of spin-on glass (SOG) intermediate coating IC1-200
solution and polyvinylpyrrolidone (PVP) (MW~1,300,000). SOG is the precursor of silica
and PVP acts to introduce an appropriate viscosity to the precursor solution, which is
required to extract a continuous jet and establish a stable electrospinning process. In its
native form PVP is a white, hygroscopic powder which can be readily dissolved both in
deionised water and in a large number of organic solvents. In the experiments described by
Tsou, 92 the weight percentage of PVP in the whole solution is 4%. The mixture is stirred
until the PVP is completely dissolved, resulting in a viscous and transparent solution.
Subsequently the solution is dispensed into a glass syringe with a stainless steel needle as
the electrospinning nozzle. The solution is pumped from the needle tip at a flow rate
defined by an external syringe pump. The anode of a power supply is connected to the
needle while the cathode is connected with a grounded collector (counter electrode), which
is placed 5cm away from the needle tip. The applied voltage ranges from 3kV to 7kV. A
piece of aluminium foil is attached on the collector to harvest silica fibers. After