PAPER www.rsc.org/materials | Journal of Materials Chemistry
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Two dimensional anisotropic etching in tracked glass
Pradeep Ramiah Rajasekaran,a Justin Wolff,a Chuanhong Zhou,a Mary Kinsel,a Christina Trautmann,c
Samir Aouadib and Punit Kohli*a
Received 3rd July 2009, Accepted 19th August 2009
First published as an Advance Article on the web 24th September 2009
DOI: 10.1039/b913151e
We describe in this paper the creation of a two-dimensional pore gradient using hydrofluoric acid (HF)
chemical etching of tracked glasses (TGs). The first gradient was along the plane of TGs where the pore
diameters of the conical pores were modulated, and a second gradient was formed in pores along the
axis of each pore. The 2-D pore gradient in TGs was characterized with optical and electron
microscopies. We demonstrate that the pore gradient was formed only when the TGs had a thin layer of
polydimethylsiloxane (PDMS) on their surface and when the etching solution was not stirred. The 2-D
pore gradient was also found to be dependent upon the TG orientation with respect to the etching
solution. Following the reaction between HF and PDMS, the resulting insoluble precipitate was
deposited, due to gravity, at the bottom of the vertical TG’s etching surface, accruing at the mouths of
the pores. This precipitate deposition at pore mouths appeared to hinder the diffusion of HF to the pore
surface. This build up also caused the retention of the by-products inside the pores which further
suppressed the etching of the glass. The etching process was inhibited more at the bottom of the chips
than that at the top presumably due to the formation of a precipitate gradient on TG surface. Using
a 2-D pore gradient containing TGs, many different experiments can be performed simultaneously
which will improve the throughput rate and aid analysis in many potential applications in materials and
life sciences.
Introduction
Track etching1 is a well-known technique for making pores in
various materials such as polyesters,2 nylons,3 polycarbonate
membrane4,5 and glass.6,7 Tracking is usually performed by
irradiating substrates with heavy ions accelerated to �15% the
speed of light using a linear particle accelerator. Other irradia-
tion methods include nuclear reactors, radioactive sources and
scanning ion microbeams. These treatments allow the construc-
tion of damaged tracks in materials.1 The interactions between
accelerated heavy ions and substrates result in the damage of
chemical bonds in the material along the direction of ion
movement. Following chemical etching, the nanometre dimen-
sional tracks in materials can be transformed into pores of
various shapes and sizes.8 Due to the presence of damaged tracks
in materials, the chemical etching along the tracks is much faster
than the etching in the lateral direction. This results in pore
formation without significant reduction in the substrate’s thick-
ness. Depending upon the etching conditions and properties of
the material being etched, the pores can be conical,9 cylin-
drical,10,11 diamond,12 or cigar in shape.13 In general, the isotropic
etching of tracks in non-crystalline materials will result in
cylindrical pores whereas conical pores are formed using aniso-
tropic etching of damaged tracks.1 The etching of damaged
aThe Nanoscience Exploration Research and Development Group (NERDGroup), Departments of Chemistry and Biochemistry, Southern IllinoisUniversity, Carbondale, IL, 62901, USA. E-mail: [email protected] of Physics, Southern Illinois University, Carbondale, IL,62901, USAcGSI Helmholtzzentrum f€ur Schwerionenforschung GmbH, Planckstr,Darmstadt, 64291, Germany
8142 | J. Mater. Chem., 2009, 19, 8142–8149
tracks in crystalline materials can result in pore shapes other than
those with a circular cross-section.14 Commercially, the track
etch technology has been used in the fabrication of nano- and
micron-size filters15 and in the synthesis of nanomaterials using
porous membranes as templates.16–18
One-dimensional anisotropic etching for conical pore forma-
tion with relatively uniform diameters in polymeric materials
using track-etch technology is well-studied by the groups of
Siwy,19–26 Martin,9,12,19,20,26–29 and Azzaroni.30,31 We report here
the formation of a two-dimensional pore gradient in TG chips. In
this paper, the diameters of the conical pores were varied along
the y-axis in the x–y plane, and a second pore gradient was
formed along the length of the conical pores (z-axis) (Fig. 1).
Although there are reports of one-dimensional conical pore
formation in TGs, to the best of our knowledge, a 2-D pore
gradient in track-etch glass is not reported in the literature. We
also propose a mechanism that explains the 2-D pore gradient
Fig. 1 The schematic of a 2-D pore gradient formed in tracked etched
glass after HF chemical etching. The diameter of the conical pores is
gradually changed along the y-axis in the x–y plane and a second pore
gradient was formed along the length of conical pores (z-axis).
This journal is ª The Royal Society of Chemistry 2009
Fig. 3 The schematic of the etching apparatus showing various
components. TG was held between two polycarbonate blocks with
O-rings, and the whole apparatus is fastened with a clamp (not shown) to
hold TG in place. One side of TG was in contact with Ca(NO3)2 whereas
its second side was in contact with HF. Two Pt electrodes were connected
to a digital multimeter. The inset on the top right shows the whole
assembled device.
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formation in the TG. Through the use of 2-D gradient containing
chips many different experiments can be performed simulta-
neously using one chip. This will improve analysis efficiency and
throughput in many potential biological and materials sciences
applications.
Experimental
All the glass substrates used in our experiments were �70 mm
thick and were purchased from SCHOTT (catalogue # D263).
The glasses were tracked with an ion fluence of 104–105 ions cm�2
at GSI (Darmstadt, Germany).
Chemical etching of tracked glass (TG) chips
A 24.5% HF solution was used for etching the TG in all cases
unless otherwise stated. Fig. 2 shows the assembly used
for etching the TG in our experiments. The TG was adhered
to a microscope glass slide (MG, 25 mm by 25 mm, thickness
� 1 � 0.1 mm, drilled hole �5 mm in diameter) with a poly-
dimethylsiloxane (PDMS) film �0.2–0.5 mm thick, to provide
structural support for the delicate TG. We refer to this assembly
as TG-PDMS-MG in our discussion. PDMS was also used as
a cushion to absorb stresses when the assembly was positioned
within a two U-half-cell apparatus. The etching was performed
on the open circular area (diameter � 5 � 0.5) that was perme-
able to the HF solution. Fig. 3 shows the etching apparatus
which consists of the two U-half cells. TG-PDMS-MG was
placed in between two solutions, a 24.5% HF in 0.5 M NaCl
etching solution in one half cell and a saturated Ca(NO3)2
blocking solution in the other half U-cell. There were two
configurations of the U-cell apparatus utilized in this experiment.
The configuration illustrated in Fig. 3, we call the vertical
configuration. Some experiments were also performed in which
the chip along with the apparatus was rotated 90� from the
vertical configuration. This arrangement we refer to as the
horizontal configuration. In the horizontal configuration, HF
and Ca(NO3)2 were poured into the top and bottom cells, above
and below the TG respectively. NaCl served as an electrolyte to
increase the ionic conductivity of the HF solution. Two Pt wires,
(Alfa-Aesar) functioning as electrodes, were inserted in the cells,
one in each cell, situated on both sides of the TG. The measured
Fig. 2 The assembly used for the fabrication of 2-D pore gradient in the
tracked glass (TG) substrates. TGs were adhered to a thin PDMS layer
with microscope glass (MG). TG was exposed to HF via a hole drilled
through MG of (5 � 0.5) mm diameter.
This journal is ª The Royal Society of Chemistry 2009
increase in the ionic current or decrease in the ionic resistance was
used to assess the progress of the etching process. These
measurements were obtained using either a Fluke 8840A Multi-
meter or a Keithley 6487 picoammeter/voltage source. Etching
was stopped when a spike in ionic current or a decrease in ionic
resistance was observed. These observed spikes indicated the
point in time when a continuous ionic path was detected between
the two electrodes; i.e., it is the time when the TG’s width was fully
penetrated across the TG by HF along the tracks. Once etching
was complete, HF was neutralized by washing the TG chips in
copious amounts of Ca(NO3)2. The TG was then rinsed thor-
oughly with deionized water and ethanol. Under these experi-
mental conditions, the etching time for our chips was�700–900 s
corresponding to an etching rate of �75–100 nm s�1. The char-
acterization of pore formation after chemical etching was per-
formed with scanning electron microscopy (SEM) using a Hitachi
570 instrument and an optical microscope (Lieca DMIRB)
equipped with a QImage (Cooled Mono 12-bit) CCD camera.
Water contact angle measurements were performed on a goni-
ometer (CAM-Micro, Tantec Inc.). Our reported values of the
contact angle were the averages of at least three measurements.
Simulation and experiment
Although the chips were made of a glass with high optical
transparency, when utilizing an inverted optical microscope in
bright field mode, the pores appeared as dark circles with smaller
brighter circles at the center. The larger darker and smaller
brighter circles in the optical micrographs correspond to the
larger and smaller diameters of the pore. Using the 2-D finite
difference time domain (FDTD) method, we performed simula-
tions to gain a better understanding of the observed high-
contrast optical micrographs of the conical pores.32
The propagation of the light through the glass chip satisfies
Maxwell’s equations which are expressed by:
V � E ¼ �vB/v t (1)
V � H ¼ 3vE/v t (2)
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V � ¼ (iv/vx + jv/vy + kv/vz)x is curl operator; i, j and k are
orientation unit vectors; E and H are the electric and magnetic
fields of the light respectively; and B ¼ mH. m and 3 are
permeability and permittivity of glass with respect to the media
(air in our case). To make it easier to compute, we assume that
transverse magnetic (TM) polarized monochromatic light
(l ¼ 600 nm) is categorically incident on the larger open side of
the glass chip. Also in the simulation, we assumed a CCD
detector was fixed at the smaller pore side of the chip.
LDI-MS
A thin layer of the precipitate formed by the reaction between
TG, PDMS and HF, was deposited on a stainless steel target.
The target was inserted into the ion source of a Bruker Daltonics
Microflex LR time-of-flight mass spectrometer (Billerica, MA).
Laser desorption ionization (LDI) mass spectra were then
acquired by firing a pulsed nitrogen laser (337 nm, 20 Hz repe-
tition rate). The resultant positive ions were then recorded.
Results and discussion
The conical pores formation in the TG using anisotropic chem-
ical etching is well-established in literature.9,21,25 In the aniso-
tropic etching process, an enhanced etching rate was achieved on
one side of the TG (the etching side). On the contrary, a reduced
rate was obtained on the other side of the TG (the blocking side)
due to the neutralizing reaction between the etching and blocking
solutions (eqn (1)). In this paper, we report the preparation and
characterization of a 2-D anisotropic gradient in TG-PDMS-
MG. First, the gradient was established in the z-axis direction
where the pore diameter was uniformly modulated along the
thickness of the glass chip, and a second pore gradient was
created along the y-axis (Fig. 1). The smaller and larger diame-
ters of the conical pores can be controlled by means of altering
the etching conditions and by utilizing different tracked materials
with varying properties. We chose glass as our substrate because
it is a chemically, thermally and mechanically stable material.
Only a few chemicals such as HF and strong bases, for example,
Fig. 4 (A) shows the scanning electron micrograph (SEM) of a 2-D pore grad
area of TG exposed to HF (corresponding to the area of the drilled hole in th
bottom portions respectively of (A), zoomed-in to show the 2-D pore gradien
8144 | J. Mater. Chem., 2009, 19, 8142–8149
NaOH and KOH are known to chemically attack glass.33 In
particular, the reaction between glass (SiO2) and HF is extremely
fast due to attack by active species such as HF2� and HF on
glass34 (see eqn (2)).
The conical pore formation in the TG was easily inspected
with optical and electron microscopies. The gradient in the pore
diameters along the y-axis is shown in the SEM (Fig. 4) and
optical micrographs (Fig. 5). The top micrograph corresponds
to a top etched portion of the glass with a dl � (43 � 1) mm
whereas the bottom micrograph has a dl � (15 � 1) mm. Fig. 5B
shows the theoretical predictions of q of the pores formed in our
studies. The cone angle (q) that we calculated turned out to be
17.81�. We estimated q based on simple trigonometric calcula-
tions. For these calculations, it is assumed that (1) the HF
concentration and the etching rate are constant, (2) the larger
diameter dimension was assumed to be 45 mm; (3) the thickness
of the glass chip was 70 mm. These simple calculations provide
some guidance about cone angles of pores. q is highly dependent
upon the etching conditions (such as concentration and
temperature of the etching solution, humidity, etching time, and
the presence of impurities in the etching solution) and material
properties of the etching surface. We also note that q can be
accurately estimated only for break-through pores (i.e. both
smaller and larger pores are open). This is because the height of
the conical pores that are closed (at smaller openings) is hard to
determine using electron or optical micrographs under our
experimental conditions.
Fig. 6A and 6B show the optical micrograph and its surface
intensity plot respectively for a typical conical pore. The larger
pore appears as a dark circle whereas the smaller pore as
a smaller bright circle. At first, we were surprised to see a large
color contrast between the smaller and larger pores in the
optical micrographs because the whole chip is composed of
transparent glass, and it should be fully transparent to visible
light.
To gain more understanding regarding the optical micro-
graphs of the conical pores, we performed simulations by
solving Maxwell’s electromagnetic equations (Fig. 7). The
brighter and darker circles in the optical micrographs
ient containing tracked-etched glass. The black dotted line represents the
e MG). Figures (B), (C) and (D) are SEM images of the top, middle, and
t in TG.
This journal is ª The Royal Society of Chemistry 2009
Fig. 5 (A) The optical micrographs of conical pores fabricated using
2-D pore gradient technique. The Figure on the left shows the schematic
of an area of TG exposed to HF. The images on the right side are
a collection of optical micrographs from 5 areas of TG corresponding to
an area shown in the schematic in the left figure. The 50 micron scale bar
is common to all the micrographs. (B) Theoretical calculations for cone
angle with assumptions that the larger pore diameter is 45 mm, the small
side a couple 100 nm (just after ‘‘breakthrough’’ of the pores), and the
thickness of the TG did not change significantly after etching. The slant
height and the cone angles can be calculated using simple trigonometry
formulas. The cone angle (q), in general, is highly dependent on the ratio
of larger to smaller pore diameters.
Fig. 6 (A) shows is an optical micrograph of a conical pore open at both
ends using an inverted optical microscope in bright field mode. (B) shows
a 3-D optical surface intensity plot of the pore in (A).
Fig. 7 Simulation of light propagation through a hollow conical pore
using a 2-dimensional finite difference time domain (FDTD) method.
(A) shows the interference of the reflected light (J0K0 interface) from the
inside of the cone with incident light. (C) depicts the refracted light
(at J0K0 interface) and reflected light (KK0). (G) Incident light penetrating
through the pore (corresponds to the bright area in Fig. 6A). (D) Inter-
ference of light from area B (JJ0) and C (J0K0 and K0K interfaces).
(F) light from C is refracted by the KK0 plane, it corresponds to dark area
in the Fig. 6A.
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correspond to the smaller and larger pore diameters respectively
in the TG. Our simulations provided us detailed information on
this interesting phenomenon. Most of the incident light that
impinged on the pore was reflected or refracted by the inner wall
of the conical pore. The reflective light then interfered with the
incident light, which is shown in region A. The refractive light
was redirected into the glass (region C) with the refractive angle
qt satisfying Snell’s law: nisin(qi) ¼ ntsin(qt). ni and ni are
refractive indices of the incident and transmission medium
respectively, and qi is the incident angle. We also observed that
some of the incident light directly penetrated into the smaller
pore opening. In region G, a distinct diffraction pattern was
observed which corresponds to the smaller bright circles in our
experiments. In our simulation, we also see that some interfer-
ence patterns developed near the edges of the glass (i.e. near
points J and J0) due to the scattering caused by their sharp edges
and to the given boundary conditions used in the simulation.
This phenomenon was not seen in the optical micrographs
because experimentally there were no latent boundary condi-
tions such as sharp edges and planes, except at the chip corners.
In region D, the light transmitted normally, interfered with the
refractive light from region C, and this resulted in brighter
interference strips. Region D sparks special interest because its
simulated intensity is 2–3 times larger than the intensity in
region F and is consistent with our experimental results
regarding intensity (red circle at the edge of larger in Fig. 6B). F
is the dark circle region where the direction of the penetrating
light from region C was refracted away due to a ‘‘prism-like’’
effect of the conical pore. This ‘‘prism-like’’ effect led to
a decrease in the number of photons that reached the detector.
Ultimately, the high contrast between the dark and light areas
in the optical micrographs is a result of the interference due to
J. Mater. Chem., 2009, 19, 8142–8149 | 8145
Fig. 8 Comparison of pore diameter (dl)-versus-distance (D) from
bottom of TG after HF etching under different experimental conditions.
A significant 2-D pore gradient was observed only for TG-PDMS-MG
without stirring HF in vertical configuration. Each pore diameter is an
average of three measurements.
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the conical shape of the pore. It presents an easier, faster and
less expensive way for characterizing the conical pores in glass.
In our experiments, a reproducible 2-D anisotropic gradient
was only observed when the following two conditions were met:
(a) the presence of a thin PDMS film in between the TG and MG;
and (b) an unstirred HF solution during etching when the TG
was in vertical orientation. Fig. 8 shows four graphs comparing
pore diameter-versus-distance (D) starting from the bottom of
the TG etched area under four different experimental conditions:
(1) the etching of TG-PDMS-MG without stirring in vertical
configuration (blue squares); (2) the etching of TG-PDMS-MG
while stirring in the vertical configuration (red triangles); (3) the
etching of TG-PDMS-MG without stirring in the horizontal
configuration (green triangles); and (4) the etching of TG-3M
super 77-MG without stirring in the vertical configuration
(orange triangles).
The effect of stirring on the track-etched pore formation and
on the etching rate is previously described.35–37 In the present
study, the etching rate of the tracked substrate was found to
increase with stirring which is attributable to the removal of the
HF-PDMS reaction’s precipitate from the etching surface at
the HF–glass interface due to the solution’s circulation within the
U-half cell chamber. The pore gradient along the y-axis for
TG-PDMS-MG in (1) was�7.7 mm per 1 mm of TG. In contrast,
Fig. 9 Proposed mechanism of 2-D
8146 | J. Mater. Chem., 2009, 19, 8142–8149
we did not observe any pore gradient along the y-axis of the chip
for experiment (2) in which the etching solution was stirred.
These experiments suggest that a 2-D pore gradient in
TG-PDMS-MG in vertical configuration only occurs when the
etching solution was not stirred.
Similarly, the results of experiments (1) and (3) suggest that the
vertical configuration is necessary for the modulation of the
diameters of conical pores in the y-axis of the TG. Finally, to
evaluate the effect of adhesive chemistry on the etching rate of
the damaged tracks in glass substrates, we fabricated a TG glass
assembly with 3M Super 77 multi-purpose adhesive in place of
PDMS for sticking the TG to the MG (experiments (1) and (4)).
We chose this adhesive because its chemistry is very different
from PDMS. Since it does not contain siloxane polymer, it is
more resistant to HF attack than PDMS. HF is a weak acid, and
it reacts vigorously with Si containing materials due to the strong
affinity between Si and F38 (eqn (2)). The HF etching of the 3M
Super 77 containing TG did not show a uniform 2-D pore
gradient in the substrates. The pores were uniform throughout
the etching area (dl ¼ (13 � 1) mm) except at the adhesive–HF
interface where the pore diameter was somewhat smaller
(dl ¼ (8 � 1) mm). The dimensions of all the pores were smaller
because the etching was performed for only �250 s (about 1/3rd
of the total etching time). The reduced pore diameter at the
adhesive–HF interface was probably due to the hydrophobic
nature of the adhesive where the HF concentration was lowered.
More etching experiments were conducted in which the TG was
coated with a thin coating of wax. In these experiments, no
conical pore gradient was observed in the TG along the y-axis.
These experiments clearly indicate that the hydrophobicity of the
TG surface played a minor role in the formation of the 2-D pore
gradient.
We propose a mechanism to explain the formation of the 2-D
pore gradient in the PDMS containing TG after it was etched
with HF (Fig. 9). We propose that the TG surface exposed to HF
contained a thin film of PDMS. In addition, PDMS was also
present along the circular edge and in-between the TG and MG.
We argue that following the reaction between PDMS and HF
some of PDMS was etched away in the HF solution, and the
hydrophobic by-products of this reaction precipitated on TG.
Due to gravity, the precipitate was deposited at the mouth
openings of the pores and was built up against the HF exposed
TG. The precipitate blocked the diffusion of HF into the pores
and inhibited the PDMS-HF by-products from exiting out of the
pores. We further propose that a precipitate gradient formed
pore formation in tracked glass.
This journal is ª The Royal Society of Chemistry 2009
Table 1 Structural assignments of ion signals observed for the LDI ofthe PDMS-HF reaction by-products.
m/z Chemical structure
101.6
106.4
171.6
330.7
766.6
826.6
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along the y-axis in the opposite direction of the pore gradient.
Since the pore formation is a surface chemical reaction, it
appears that there is a increased restriction of HF to the pore
sites at the lower portion of the HF exposed TG than to the sites
present in the upper portion. The pore blockage and the degree of
blockage of HF resulted in the incrementally decreased chemical
etching rate in the lower portion of the exposed etching area of
the TG, and this, we suggest, contributed to the creation of the
pore gradient along the y-axis.
To further test our hypothesis that the pore gradient was
formed due to the presence of PDMS film on glass, we treated
TG-PDMS-MG first with O2 plasma39 (for 10 min, 100 W power,
and flow rate of O2 40 sccm using a FA2000 Reactive Ion Etcher
from BSETEQ Inc.) followed by five H2SO4 washings (18 M
H2SO4 and 10 s each). Since both of these treatments are known
to chemically attack polysiloxane,40 we expected a decrease or
elimination of a pore gradient along the y-axis as a result of these
treatments. Indeed, we observed a relatively uniform pore
distribution (dl � (45 � 10) mm and ds � (5 � 1) mm) over the
whole etching area of O2-plasma/H2SO4 treated TG. These
observations reinforce our hypothesis that the PDMS coating
contributed to the formation of 2-D pore formation in TGs.
Additional evidence regarding the contribution of PDMS to
the 2-D pore gradient formation came from the experiments
involving different PDMS film thicknesses in the TG-PDMS-
MG assemblies. Thicker PDMS adhesive films were expected to
generate a larger 2-D pore gradient in the TG than for thinner
PDMS adhesive films because of the increased amount of
precipitate formation with thicker PDMS films. We fabricated
two different TG-PDMS-MG assemblies that contained PDMS
thicknesses of (0.5 � 0.1) mm and (0.2 � 0.1) mm. As expected,
a larger pore gradient formed when utilizing thicker PDMS
layered samples as compared to a thinner PDMS layer on the TG
(Fig. 10).
Finally, we also tested the chemical composition of the
precipitate formed from the reaction between TG-PDMS-MG
and HF. For this, we synthesized the precipitate by reacting an
assembly made up of TG, PDMS and microscopic cover slide in
Fig. 10 Comparison of 2-D pore gradients formed in TG with PDMS of 2 different thicknesses. A larger pore gradient in y-axis was seen for TGs with
thicker PDMS (0.5 � 0.05 mm) than for a TG with thinner PDMS film (0.2 � 0.05 mm). Each pore diameter was an average of three measurements.
This journal is ª The Royal Society of Chemistry 2009 J. Mater. Chem., 2009, 19, 8142–8149 | 8147
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a mass ratio of 1 : 0.7 : 0.3 with a 24.5% HF solution overnight.
The major product of the reaction between glass and HF is
H2SiF6 which is water soluble. Thus, it did not affect the etching
of the TG. Table 1 shows the composition of the precipitate
formed by the reaction between the assembly and HF using mass
spectrometric analysis (also see Fig. 11). The major component
of the precipitate was composed of silicon, methyl and fluorine
containing oligomers. The mass spectrometry and water solu-
bility analysis of the precipitate suggested that the precipitate was
hydrophobic in nature. Fig. 11 shows a representative LDI mass
spectrum in the mass-to-charge (m/z) range of 100 to 2000
formed by the summation of 1000 laser shots. The ion formation
efficiency in LDI rapidly decreases as analyte molecular masses
increase above 2000 amu. This behavior was also observed in this
experiment, where no ion signals were observed at m/z > 2000.
Much larger molecular mass oligomers may be present in the
precipitate but would not be desorbed and ionized using this
technique. Table 1 summarizes the major ions observed in the
low and high mass-to-charge regions, 100 < m/z < 600 and
600 < m/z < 1500, respectively. The ion signals appearing at
m/z 101.6, 106.4, 171.5 and 330.7 were assigned to small sodiated
(or potassiated) oligomers composed of silicon, methyl,
hydrogen, oxygen and fluorine. A short series of ion signals was
observed between m/z 766.5 and 1035.5, and was assigned to
sodium- and potassium-containing linear poly(dimethyl, meth-
ylhydrogen)siloxane (n ¼ 5 to 7). The most abundant ion signal
appearing at m/z 766.0 belonged to the sodiated linear series
member with n ¼ 5. A longer series of ion signals was observed
between m/z 692.6 and 1244.9. These ion signals were assigned to
sodium and potassium cyclic poly(dimethyl, methylhy-
drogen)siloxane (n¼ 5 to 9), the most abundant ion signal in this
series at m/z 826.5 belonging to the sodiated cyclic series member
with n ¼ 6.
The water-insoluble precipitate from the TG-PDMS-MG and
HF reaction was also filtered out and was deposited over an
�2 cm2 area of a TG. The etching of this precipitate-covered TG
showed a 2-D pore formation only in the area that was not
Fig. 11 A representative LDI mass spectrum with mass-to-charge (m/z) ran
observed between m/z � 766.6 and 1035.6 with an average repeat unit of m/z
8148 | J. Mater. Chem., 2009, 19, 8142–8149
exposed to the precipitate. The etching on the area covered with
precipitate was retarded as evidenced by the comparison of the
pore diameters in the two regions of the TG. Furthermore,
a uniform 2-D gradient was not observed in this precipitate
devoid area. These results conclusively indicated that a PDMS
coating is needed for the 2-D pore gradient formation in
damaged tracked glass. We also note that glass used in the
experiments also contained small quantities of metal oxides like
BaO, CaO, ZnO etc. These metal-oxides form sparingly soluble
metal fluorides upon reaction with HF. We cannot rule out the
minor contributions these metal fluorides might have made to the
formation of the 2-D pore gradient in the TG.
Now the question is, how siloxane polymer film is formed on
the TG surface? Sylgard 184 (Dow Corning) is a two-part silicone
elastomer, and it contains a number of low-molecular weight
oligomers and high vapor pressure monomers. We believe that
some of siloxane monomers and oligomers formed a thin film
during TG-PDMS-MG fabrication, especially during curing
time when the temperature was �90–100 �C. At these tempera-
tures, it is possible that some monomers and oligomers diffused
across the TG surface resulting in the formation of a thin film of
PDMS. Our water contact angle on the TG was (65� 15)0. These
measurements suggested a hydrophobic TG which is consistent
with our argument for the presence of a PDMS film on the TG.
Finally, 2-D pore gradient substrates can be used in many
potential applications where a conical pore gradient is needed.
This paper provides a fast and simple fabrication method for
making cones of different dimensions in the tracked glass,
a capability which was previously not available to the scientific
community. The conical pores have been used in a wide range of
applications including bioseparation, biosensing, nanofluidic
diodes and rectifiers, and artificial ion channels and ion
pumps.22–29,41,42 In all of these experiments, either a single pore or
pores of uniform diameters were used. In principle, with the glass
chips fabricated in the present study, many different experiments
can be performed simultaneously using one single chip. Thus,
these chips are expected to increase the throughput rate of
ging between 100 and 2000. The inset shows a short series of ion signals
� 134.0 �0.2.
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analysis and may also help in reducing the cost of analysis in
biological and materials sciences.
In conclusion, we demonstrated the fabrication of glass
substrates with pore gradients in two directions. The 2-D pore
gradient was characterized with optical and electron micros-
copies. Our experiments indicated that a 2-dimensional pore
gradient was formed due to the blocking of the pore mouths in
the tracked-glass substrates by a precipitate formed by a reaction
between PDMS and HF. We proposed that a precipitate gradient
was formed in the TG which led to a pore gradient direction
along the y-axis.
Acknowledgements
We acknowledge financial support from the National Institute of
Health and the National Science Foundation. We also
acknowledge the Southern Illinois University Carbondale Mass
Spectrometry Facility. Instrumentation in the Mass Spectrom-
etry Facility was obtained with support of the National Science
Foundation Grant No. 0405819 and the State of Illinois. We
thank Rashid Zakeri (Indiana University) for his assistance in
chemical etching of glass and Drs. John Bozzola and Steve
Schmitt and Ms. Hillary Gates (IMAGE center at SIUC) for
electron microscopy analysis.
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