However, the three relationships indicate that the effects of the three geometrical dimensions on the sensitivity of a microwire sensor vary with the conducting polymer materials and the
Trang 1Effects of dimensions on the sensitivity of a conducting polymer
microwire sensor
a
Institute for Micromanufacturing, Louisiana Tech University, 911 Hergot Avenue, Ruston, LA 71272, USA
a r t i c l e i n f o
Article history:
Received 26 August 2008
Received in revised form
20 November 2008
Accepted 24 November 2008
Keywords:
Conducting polymers
Microwire sensors
Surface-to-volume ratio
Sensitivity
Intermediate-layer lithography
a b s t r a c t
It is commonly considered that the sensitivity of a microsensor increases with its increasing surface-to-volume ratio However, it is not exactly clear how the surface-to-surface-to-volume ratio affects the sensitivity of a conducting polymer microsensor The change in any of the three geometrical dimensions (i.e., length, width and thickness) of a microsensor changes the surface-to-volume ratio In designing a microsensor
of desired sensitivity, it is important to know the effect of each dimension on the sensitivity for properly defining the sizes and shapes of the microsensor As such, in this work, we have investigated the effects
of each individual dimension on the sensitivity of a conducting polymer microwire sensor Polypyrrole (PPy) and Poly (3,4-dimethlydioxythiophene) poly(styrenesulfonate) (PEDOT–PSS) microwire sensors of different dimensions were fabricated using an intermediate-layer lithography (ILL) method They were further employed to detect methanol and acetone vapors at concentrations in the range of 0.6–7.1 parts per thousand (ppt) The corresponding three relationships between the three geometrical dimensions and the sensitivities were found using a statistical program, SAS From the point view of surface-to-volume ratio, the thickness should affect the sensitivity much more than the other two dimensions However, the three relationships indicate that the effects of the three geometrical dimensions on the sensitivity of a microwire sensor vary with the conducting polymer materials and the targets to detect
In other words, which dimension has more effects on sensitivity is case-dependent Results presented in this work can be potentially used to aid in the design of conducting polymer microwire sensors of high sensitivity
&2008 Elsevier Ltd All rights reserved
1 Introduction
Conducting polymers have received much attention since their
discovery in 1977 Applications of conducting polymer
micro-systems span from electronic devices to biological and chemical
sensors Conducting polymers offer some unique advantages like
low weight, easy tailoring of properties and a wide spectrum of
color, mass, work function and conductivity when exposed
is through conductivity measurements of an exposed polymer
change of a conducting polymer film upon exposure to a
particular chemical analyte Most conducting polymers respond
to the exposure of an analyte with a unique change in
conductivity This response is reversible with original behavior
recovered as soon as the exposure is stopped The adsorption/
desorption-related conductivity changes normally occur at room temperature These so-called ‘‘chemiresistors’’ are easier to
and Poly (3,4-dimethlydioxythiophene) poly(styrenesulfonate)
Compared to film sensors, microsensors generally have exhibited higher sensitivity in detecting analytes of low concen-trations It is normally considered that the higher sensitivity is induced by the higher surface-to-volume ratio of the micropat-terns However, it is not exactly clear how the surface-to-volume ratio affects the sensitivity of a conducting polymer microsensor
A recently developed intermediate-layer lithography (ILL) enables
us to properly fabricate conducting polymer microsensors There-fore, in this work, PPy and PEDOT–PSS microwires of different dimensions have been fabricated using the ILL method and subsequently applied to detect methanol and acetone vapors of concentrations in the range of 0.6–7.1 parts per thousand (ppt)
Microelectronics Journal
0026-2692/$ - see front matter & 2008 Elsevier Ltd All rights reserved.
doi: 10.1016/j.mejo.2008.11.064
Corresponding author Current address: Department of Mechanical and
Aerospace Engineering, University of Texas at Arlington, 500 W First Street,
Arlington, TX 76019, USA Tel.: +1817 272 7366; fax: +1817 272 5010.
E-mail address: chengluo@uta.edu (C Luo).
Trang 2The microwire response was also compared with the response of a
square film (1 cm 1 cm) The corresponding relationships
between the three geometrical dimensions and the sensitivities
were found using a statistical program, SAS, to find the effects of
each individual dimension
The outline of this work is as follows Section 2 discusses the
detection principle of the conducting polymer film and microwire
sensors In Section 3, the fabrication of the conducting polymer
microwires of various dimensions is detailed with experimental
results The experimental setup for detecting methanol and
acetone vapors is presented in Section 4 Section 5 compares the
sensing results of conducting polymer film and microwire sensors
In Section 6, the effects of each individual dimension of a sensor
on the sensitivity are addressed This work is finally summarized
and concluded in Section 7
2 Sensing principles
In this section, we first show that the sensitivity of a
conducting polymer microsensor actually depends on the
sensitivity of a unit block, and then discuss ways to increase the
sensitivity of the unit block As what has been done by many
(RexposureRBase)/RBase, where RBase and Rexposure represent the
resistances of a sensor before and after exposure to a target,
respectively The SI indicates how large the sensor response is
to a particular concentration, and is used as a measure of the
sensitivity of the corresponding sensor
Geometrically, the sensing area of a film sensor can be
modeled to be made up of multiple microwires of unit width,
connected in parallel between the opposite edges at the
electro-des These microwires may be further divided into blocks of unit
may be regarded as individual ‘‘chemiresistor’’ elements with the
of analytes with unique changes in conductivity These
‘‘chemir-esistor’’ blocks may be treated to be electrically connected in a
serial fashion between the opposite electrodes Let the resistance
i,jrBase i;j )/rBase
into ‘‘n’’ identical blocks, the total base resistance of a single
connected in parallel, the overall base resistance would be
ðDR=RÞTotal¼ f½ðn=mÞ r0
i;j g
It is observed from Eq (1) that the SI of a film sensor equals that
of a single unit block, which does not depend on how many unit
blocks this film sensor has
In deriving Eq (1) for a film sensor, it is assumed that the unit blocks have the same sensitivity This assumption holds when the top surface of the film is much larger than the side surfaces During the detection, a film sensor has five surfaces exposed to a target: the top and four side surfaces The bottom surface interfaces with the substrate, and is not exposed to a target Since the top surface is much larger than the four exposed side surfaces, most of the unit blocks only get exposed to a target through their top surfaces of unit area That is, most blocks get the same exposure to a target Furthermore, these unit blocks have the same geometry Therefore, Eq (1) is reasonably true In the case of, for example, microwire sensors, the sizes of the top surface may be comparable with those of the side surfaces Unit blocks located at the edges of the sensors get more exposure to a target than those in the central area of the sensor The unit bocks may have different sensitivities Accordingly, the assumption in deriving Eq (1) may not hold In this case, the surface-to-volume ratio should be considered to address the average sensitivities of the unit blocks This ratio means that how much surface of a block, which has a unit volume, is exposed to a target In principle, more exposed surface implies that the block should be more affected, having higher sensitivity
Consider a rectangular pattern, which has a length a, a width b,
(a b+2 a t+2 b t) The volume of the film is (a b t) Therefore, the surface-to-volume ratio is (1/t+2/a+2/b) It can be seen that this ratio increases with decrease in length, width and thickness For a microsensor fabricated out of thin films, which normally have thicknesses ranging from tens of nanometers to
and much larger than the thickness As such, 1/t is much larger than 2/a and 2/b In other words, the changes of a and b do not
thickness is the most important dimension among the three in affecting surface-to-volume ratio In this work, we explored the effects of the surface-to-volume ratio on the sensitivity of a microsensor We further examined the effects of each individual dimension on the sensitivity of a microsensor We particularly
Base
r i, j
Contacts
Sensing area
Unit block
m
2 3 1
j
Microwire
Fig 1 Schematic view of the relationship between a film sensor and individual unit blocks.
a
b
t
Fig 2 The dimensions of a micropattern.
Trang 3studied microwire sensors, whose lengths were much larger than
the widths A microsensor may also have a rectangular shape, i.e.,
the length is about the same as the width If the length is large, the
corresponding surface-to-volume ratio is larger than that of a
microwire sensor When the length is small, it is not easy to make
a contact to the sensor As such, microwire sensors became the
focus of this work
3 Fabrication of PPy and PEDOT–PSS microwires
Conducting polymer micropatterns were generated using the
conducting polymer coatings and a layer of a non-conducting
polymer polymethyl methacrylate (PMMA) are heated up to the
printing temperature, which is above the glass transition
desired patterns and the substrate are brought into physical
contact by applied pressure, followed by subsequent cooling
(Fig 3b), and (iii) they are separated when their temperatures
pattern transfer from the mold to the conducting polymer
difference is that the substrate in the hot-embossing process has
only the layer of the material to be printed, while the substrate in
the ILL approach involves an additional intermediate layer
of a non-conducting polymer As a result of this difference,
the conducting polymer patterns would be electrically isolated
over the insulating intermediate layer, and patterns would be
imprinted on the conducting polymer layer even if there were
height differences existing between the features of the mold
[20–22]
PMMA was chosen as the intermediate-layer material, because
it is a good hot-embossing material The PMMA has small thermal
105 1C PPy (Sigma Aldrich Co.) and PEDOT–PSS (Baytron Co.) were used as received (5 wt% PPy in water and 1–1.4 wt% PEDOT–PPS in water) from the manufacturers Their thin layers were generated
by spin-coating the corresponding solutions on the PMMA sheet Before coating the conducting polymers over the PMMA, all polymer solutions were kept in an ultrasonic bath for 1 h to remove any aggregate formation in solution from prolonged
300 W watts for 45 s) to make it hydrophilic such that the water soluble conducting polymer solutions could be spin-coated over
it The key fabrication parameters in ILL are imprinting tempera-ture, imprinting force and imprinting time The imprinting
on these conducting polymers The mold was slowly inserted into the substrate to avoid the dynamic effects in the embossed polymer The silicon molds were fabricated using conventional ultraviolet lithography and deep reactive ion etch The embossing temperature and pressure were 150 1C and 50 MPa, respectively
Fig 4 shows a representative set of generated PPy microwires which have been used for sensing Every sensor comprised six PPy
or PEDOT–PSS microwires which were connected in parallel
Ag epoxy was placed at the two ends of these microwires as contact pads for electrical connection The dimensions of micro-wires were changed to vary the surface-to-volume ratios of the microwires One type of film and five types of microwire sensors were fabricated using the ILL method for either conducting
these sensors
Si mold
Conducting
polymer layer
Intermediate polymer
layer
PMMA substrate
Convex mold structure
Concave mold structure
Fig 3 The three-step procedures to fabricate polymeric patterns using the proposed ILL method: (a) heating of the substrate, (b) insertion of the mold into the two polymer layers, and (c) separation of the mold and the substrate.
Overall embossed area
PMMA substrate
PPy
Fig 4 (a) Perspective and (b) close-up (optical) views of PPy sensors generated on a PMMA sheet Each PPy microwire has a width of 50mm and a length of 2000mm.
Trang 44 Experimental setup for detection
chamber All the tested sensors were placed at the same location
inside the chamber, and the two contact wires for each sensor
were taken out and connected to a Keithley probe station for
I–V measurements The humidity and temperature of the chamber
were maintained at the room level and kept constant After the
chamber was closed, the sensor current was measured at 10 V to
determine the base resistance PPy and PEDOT–PSS microwires
were exposed to methanol and acetone vapors, respectively, since
they were sensitive to these two vapors, respectively Methanol of
a known volume was introduced into the chamber in a liquid form
(as a droplet) using a micro-liter syringe The same applied to
acetone The methanol droplet evaporated in 5–10 s After the
methanol droplets had evaporated completely, the current of a
sensor was measured at 10 V continuously for 180 s Similarly,
when acetone was introduced in the experimental chamber as a
droplet, it evaporated in 2–4 s After the methanol droplet had
evaporated completely, the sensor current was monitored con-tinuously for 120 s The observation time was reduced from 180 s for methanol to 120 s for acetone, since according to preliminary tests the PEDOT–PSS sensors responded to acetone exposure within 120 s After a test, the chamber was purged by nitrogen and vented For the next round of testing, the chamber was closed and the above procedure was repeated for detecting vapors of different concentrations
The masses of methanol and acetone were calculated from their known volumes (i.e., the evaporated volumes) and their densities at room temperature The mass of air was calculated from the known volume (i.e., the volume of the chamber) and the density of air at room temperature The concentration of the methanol was calculated from the ratio between the mass of methanol and that of air inside the test chamber The same applied to acetone The concentrations of methanol vapor ranged from 1.3 to 6.4 ppt This range of methanol concentrations was
about 1.5–5.0 ppt The detection of methanol of lower
concentration of this work was varied from 0.6 to 5.8 ppt, which
(that is, 5% of acetone vapor pressure at 21 1C) and below the
5 Sensing results 5.1 Exposure of PPy sensors to methanol vapor When the PPy film and microwire sensors were exposed to methanol vapor, response currents at 10 V varied with time in a
surface-to-volume ratios, the peak currents were reached be-tween 60 and 120 s after the methanol droplet had evaporated,
contact resistance and intrinsic resistance of PPy wire As
resis-tance could be neglected, and the intrinsic resisresis-tance of the PPy wire dominated the detected resistance The same applied to the
PSS–Ag contact was also ohmic The measured contact resistance
Table 1
Dimensions of the PPy sensors used in the tests.
(mm)
Length (mm)
Thickness of PPy layer (mm)
Surface-to-volume ratio (mm 1 )
Microwire Type III 100 2000 0.19 5.176
Table 2
Dimensions of the PEDOT–PSS sensors using in the tests.
PEDOT–PSS Width
(mm)
Length (mm)
Thickness of PEDOT–PSS layer (mm)
Surface-to-volume ratio (mm 1 )
Microwire Type III 100 2000 0.30 3.354
N2 inlet
N2 outlet
Test chamber
µL syringe
Keithley probe station
N2 outlet
N2 inlet
Microwire sensor
Fig 5 Experimental setup to determine the sensitivity of PPy and PEDOT–PSS sensors in detecting methanol and acetone, respectively.
Trang 5was 5.00 103O Hence, the effect of contact resistance was also
neglected in considering the PEDOT/PSS sensors For the PPy
film sensor, the response current reached a peak between 120
response current was due to the fact that the methanol molecules
were not stationary on the sensor causing the peak in the current
After the methanol droplet evaporated, it diffused inside the
chamber and reached the sensors dynamically The response
current first increased and then decreased The reason for this
increase in response current may be attributed to the fact that
methanol is a polar molecule which helps in interchain electron
transfer in PPy Also, the small size of the methanol molecules
helped it to diffuse into the polymer chain more effectively, thus
aiding conduction The whole behavior was similar to the PPy
maximum, the methanol vapor diffused out of PPy, since the
methanol concentration in the PPy microwires was higher than
that in the environment This caused the decrease in the current
The time to reach the peak current was defined as the response
time Accordingly, microwire sensors have a shorter response time
than film sensors The same transient phenomenon of the
initial exposure of PPy films to methonal The current had a rapid
increase of more than two orders of magnitudes during the first
The current settled down to a steady value below the maximum
made the concentration of the methanol around the sensor was higher than that of our case, which did not provide continuous supply of the methanol Therefore, the steady current obtained in
the steady value was just a little higher than the original value Therefore, the peak current was used in this work to calculate
much larger SI compared with the case of adopting the steady
Except for Type V microwires, the sensitivity increased in the
lowest methanol concentration of 1.3 ppt, the sensitivity of PPy
as compared to PPy microwire Type IV with sensitivity of 36.44%
highest acetone concentration of 6.4 ppt, the PPy film sensitivity was 10.4% and Type IV microwire was 55.6% These results indicate that in general the sensitivities of these sensors increase with the increasing surface-to-volume ratios
PPy film and microwires of Types I, II and III had the same
and microwire sensors had an approximately linear relationship
methanol concentration of 1.3 ppt, the sensitivities of PPy film sensor were 1.6% and Type III microwire sensor was 8.2% At the
Time (s)
5.30 5.23 5.17 5.10 5.03 4.97
Time (s)
1.68 1.64 1.60 1.56 1.52 1.48
PPy microwire sensors
PPy film sensor
-7A)
-7A)
Fig 6 Representative current responses of PPy (a) microwire and (b) film sensors during the 180-s exposure to methanol at a concentration of 3.8 ppt.
Trang 6highest methanol concentration of 6.4 ppt, the sensitivities of the
film and Type III microwire sensors were 10.4% and 17.5%,
respectively
The PPy thicknesses were varied for Types III, IV and
V microwire sensors with their lengths and widths kept constant
This was done to study the effects of the PPy thicknesses on the
sensitivity responses of the microwires The thicknesses of the
V microwires, respectively When the PPy thicknesses were varied,
there were large variations in the surface-to-volume ratios
Type IV microwires had the highest surface-to-volume ratio and
the highest sensitivities at all the methanol concentration levels
(Fig 7) The sensitivity of Type IV microwires at the lowest
methanol concentration was 36.4% and at the highest
concentra-tion was 55.6% These results imply that for the PPy microwires
their thicknesses may have larger effects on sensitivity than the
length and width
It is also worth pointing out that, although the width of Type I
wires was three times as large as that of Type II wires (they have
the same length and width), their surface-to-volume ratios only
differed by 0.013 Similarly, the 2.5-times difference in the widths
between Types II and III led to only 0.001 difference in their
surface-to-volume ratios These two comparisons support the
point raised in Section 2 That is, the changes of the length and
width do not cause much change in the surface-to-volume ratio of
these three types of microwires still had several percents of
difference in their sensitivities of detecting methanol
It is noted that PPy sensors that other researchers used have
demonstrated different sensitivities For example, as indicated in
respectively For this concentration, the SI’s of our five types of
sensors ranged from about 7–48% The PPy films used to generate
our sensors were spin-coated on substrates Naturally, the
sensitivity of a sensor should be affected by the sensing material
to make the film may also affect the sensitivity of a sensor
Manufacturing approaches affected the surface morphologies of
generated films and subsequently the sensitivities of these films
spin-coated films have relatively flat surfaces According to Eq (1), the
SI of the inkjet-printed PPy films may approximately equal that of
the islands if each island is considered as a unit block of the film
Compared to a large film of flat surfaces, these small islands of the
same thickness as the film have a higher surface-to-volume ratio
Therefore, their SI (and consequently the SI of the inkjet-printed film) should be higher than that of a spin-coated film On the other hand, microstructures can be further generated in spin-coated films, functioning as sensing components and yielding higher sensitivities This is implied by the different sensor responses of our five types of sensors Thus, essentially, it should
be feature sizes and shapes that affected the sensitivity of a sensor
in addition to the sensing materials
5.2 Exposure of PEDOT–PSS sensors to acetone vapor
Fig 8shows the wave-like variation of the response current in detecting acetone using PEDOT–PSS sensors The response current first decreased and then increased back to a steady value a little lower than its original value At the initial stage, exposure of PEDOT–PSS to acetone reduced the conductivity of the PEDOT–PSS
polar molecule, it dispersed inside the PPy matrix by hydrogen bonding This mechanism disrupted the ordered structure and hence reduced the conductivity of PPy A similar mechanism may
be playing a role in reducing the conductivity of the PEDOT–PSS microwires in our case Alternatively, acetone molecules diffused inside PEDOT–PSS, expanding the matrix, hindering the flow of charge carriers and thereby reducing conductivity of the micro-wires As the response current reached a minimum, the acetone vapor diffused out of the PEDOT–PSS due to the fact that the acetone concentration in the PEDOT–PSS was higher than that in the environment This caused the increase in the current The sensing response of the PEDOT–PSS to acetone was different from that in the case when PPy sensors were used to detect methanol The sensor current decreased to a minimum after about 90 s of exposure The response times of film and microwire sensors were about the same The same transient phenomenon was also found,
to detect methanol and ethanol However, due to the same reason addressed in Section 5.1, their steady currents were much different from the original currents, while in our case the steady value was just a little lower than the original value Thus, the
determining the corresponding SI, since this gave a much larger SI compared with the case of adopting the steady current to
Except for Type V, the sensitivity of these sensors increased in
lowest acetone concentration of 0.64 ppt, the sensitivity of the
as compared to Type IV microwires with sensitivity of 2.27% for a
acetone concentration of 5.8 ppt, the PEDOT–PSS film sensitivity was 0.5% and Type IV microwires was 20.6% These results indicate that in general the sensitivities of these sensors increase with the increasing surface-to-volume ratios On the other hand,
as what we observed from the case of PPy detection, the large changes in widths and lengths among Types I, II and III microwires made only small changes in their surface-to-volume ratios
microwires also had several percents of difference in their sensitivities of detecting acetone
The thickness of the PEDOT–PSS layer for the film and Type I, II
PEDOT–PSS microwires were more closely placed in the sensitiv-ity scale than the PPy microwires, while the overall trend was
increased from 0.05% for film sensor to 3% for Type III microwires,
60
55
50
45
40
35
30
25
20
15
10
5
0
Methanol concentration (ppth)
Type III Type II Type IV
Type I;
Type V Film
Fig 7 Sensitivity responses of the PPy sensors at various concentrations of
methanol exposure.
Trang 7at a concentration of 0.68 ppt and from 0.5% for film sensor to
20.7% for Type III microwires at a concentration of 5.8 ppt
The thickness of the PEDOT–PSS layer was varied with the
length and width kept constant, similar to that in the PPy
microwires The thicknesses of the PEDOT–PSS layers were
The sensitivity of Type IV microwires varied from 2.2% at 0.6 ppt
microwires were more than Type V and less than Type IV
microwires This trend is aligned with the increasing
surface-to-volume ratios in order from Type V to III to IV
The sensitivities of these five types of sensors ranged from
0.05% to 20.7% when they were exposed to 0.6–5.8 ppt of acetone
which detected 12.7 ppt of acetone using 2.5-mm-wide composite
films The composite films consisted of PEDOT–PSS/insulating
polymers or carbon black/insulating polymers The difference in
the sensitivities implies that both feature sizes and sensing
materials affected the sensitivities
to detect acetone, whose concentrations ranged from 104 to
416 ppt The corresponding sensitivities varied from 3% to 9% The
nanowires were synthesized using anodic aluminum oxide
membranes These results imply that our sensors also generally
have higher sensitivities than the nanowire sensors It has been
sensors do not show higher sensitivities They considered this was due to the impact of substrate roughness during film formation of the film sensors In other words, different manufacturing approaches generate different features, making sensors have different sensitivities, as discussed in Section 5.1 In this work,
-3 A) 0.60 0.58 0.56
0.52
0.48 0.50
0.66 0.64 0.62
Time (s) 0.54
PEDOT-PSS microwire sensors
-3 A)
3.477 3.480
3.504 3.501 3.498
Time (s)
3.495
PEDOT-PSS film sensors
3.492 3.489 3.486 3.483
Fig 8 Representative current responses of PEDOT–PSS (a) microwire and (b) film sensors during the 120-s exposure to acetone at a concentration of 5.8 ppt.
25 20 15 10 5
0 0.5 1.5 2.5 3.5 4.5 5.5
Acetone concentration (ppth)
Type IV Type III Type V Type II Type I Film
Fig 9 Sensitivity responses of the PEDOT–PSS sensors at various concentrations
of acetone.
Trang 8the same manufacturing approach (as well as the same sensing
material) has been used to generate the five types of sensors
Therefore, the manufacturing effect (as well as the sensing
material) is not a concern here in comparing the sensitivities of
these five types of sensors
6 Statistical analysis of sensing data
SAS has been run to fit the data points for further analyzing the
sensing results and examining the effects of each individual
dimension We intended to find the relationship of the SI with the
three geometrical dimensions and the vapor concentration It was
noticed that surface-to-volume ratio is a linear combination
of the inverse of the three geometrical dimensions, and that the
sensitivity should increase as this ratio increases Therefore, we
assumed that the SI was related with the inverse of these three
SI had an approximately linear relationship with the vapor
con-centration Therefore, the SI was assumed to be directly related to
to be the vapor concentration y stood for the SI Then, based on a
obtained in detecting methanol using PPy sensors, we got
dataset obtained in detecting acetone using PEDOT–PSS sensors,
the following equation was found:
individual dimension on the sensitivity from the above two
equations, let’s consider an example In designing a conducting
polymer microwire sensor, the initially chosen dimensions could
we considered how the changes in these three dimensions affect
the sensitivity Let alternative length, width and thickness be
1000m, 100n, and 0.1 l, respectively, where m, n and l were three
positive constants and their values determine the final values of
the three dimensions Substituting the inverse of these three
Eq (4) indicates that, for the detection of methanol using PPy
microwire sensors, the change in thickness had more effects than
the change in length on the sensitivity, while the latter had more
effects than the change in width For example, y increased by
13.25, 1.35, and 45.5, respectively, when we respectively set m, n
of the PEDOT–PSS microwire sensors in detecting acetone were
ordered from the highest to the lowest as: the change in length,
the change in thickness, and the change in width For example,
y increased by 8.65, 0.55, and 6, respectively, when we
respectively set m, n and l to be 0.5 As could be seen from these
two relationships, the changes in the dimensions had more effects
on the sensitivities of PPy microwire sensors than those on the
sensitivities of PEDOT–PSS microwire sensors Also, the degree of
influence of each individual dimension might vary with different
conducting polymer microwire sensors
As discussed in Section 2, the thickness affected surface-to-volume ratio much more than the length and the width Also, in principle the sensitivity increased with the increasing surface-to-volume ratio However, the two relationships given in Eq (4) indicate that the length had the same order of effects as the thickness on the sensitivity Therefore, in addition to the thickness, it is also important to reduce the length of a microwire for increasing the sensitivity These two relationships also imply that the length had more effects than the width To see this clearly, we compared the effects of the length with those of the width via the detection of acetone using PPy sensors PPy film and Types I, II and IIII microwire sensors were chosen to detect acetone vapors, whose concentrations were 1.3, 3.2, 4.5, 5.8, and
described in Section 4 were used The overall trend of the sensor responses was similar to what has been found in the previous two
3.6% for film sensor to 13.0% for Type III microwires at a concentration of 1.3 ppt and from 10.1% for film sensor to 29.0% for Type III microwires at a concentration of 7.1 ppt For a particular concentration, the sensitivity increased in the order: FilmoType IoType IIoType III Since these sensors had the same
length and width The corresponding fitting result was
was used to obtain Eq (4) from Eqs (2) and (3), by Eq (5) we had
This equation indicates that, for the detection of acetone using PPy microwire sensors, the change in length had the same order of effects as the change in width For example, y increased by 8.15 and 4.6, respectively when we, respectively, set m and n to be 0.5 The exact mechanism that caused the different effects of dimensions on the sensitivities is not clear We speculate that it is related to the internal structures and orientations of PPy and PEDOT–PSS For example, if the internal structure of a polymer is orientated upward, then its width should have more effect than the thickness, while the thickness of another polymer should be more important than the width in detection when its internal structure is pointed horizontally We leave this to future investigation
7 Summary and conclusions
In this work, microwires of PPy and PEDOT–PSS were fabricated using the ILL technique The microwires had different dimensions for achieving different surface-to-volume ratios For
0 5 10 15 20 25 30 35
1.3
Acetone cencentration (ppth)
Type III Type II Type I Film
Fig 10 Sensitivity responses of the PPy sensors at various concentrations of acetone.
Trang 9For PEDOT–PSS, the surface-to-volume ratio varied from 0.890 to
methanol vapor whose concentrations ranged from 1.3 to 6.4 ppt
Methanol exposure increased the response current of the PPy
sensors The PEDOT–PSS film and microwire sensors were
exposed to acetone vapor whose concentrations ranged from 0.6
to 5.8 ppt The response current of the sensors was reduced upon
exposure to acetone vapor In general, the sensitivities of the
sensors were found to increase with increasing surface-to-volume
ratios at various concentrations of the methanol and acetone
vapors The sensitivity data obtained from experiments were
analyzed with the aid of a statistical program, SAS From the point
view of surface-to-volume ratio, the thickness should affect the
sensitivity much more than the other two dimensions However,
the three relationships obtained from three sets of experiments,
respectively, indicate that the effects of the three geometrical
dimensions on the sensitivity of a microwire sensor vary with the
conducting polymer materials and the targets to detect In other
words, which dimension has more effects on sensitivity is
case-dependent Results presented in this work can be potentially used
to aid in the design of conducting polymer microwire sensors of
high sensitivity
Acknowledgements
This work was supported in part through NSF–DMI-0508454,
NSF/LEQSF(2006)-Pfund-53 and NSF-ECS-0529104 Grants
References
[1] L Rupprecht, Conductive Polymers and Plastics in Industrial Applications,
Plastic Design Library, 1999.
[2] B Adhikari, S Majumdar, Polymers in sensor applications, Progress in
Polymer Science 29 (2004) 699–766.
[3] S.T McGovern, G.M Spinks, G.G Wallace, Micro-humidity sensors based on a
processable polyaniline blend, Sensors and Actuators B 107 (2005) 657–665.
[4] J.W Gardner, P.N Bartlett, A brief history of electronic noses, Sensors and
Actuators B 18–19 (1994) 210–211.
[5] M.F Mabrook, C Pearson, M.C Petty, Inkjet-printed polymer films for the
detection of organic vapors, IEEE Sensors Journal 6 (2006) 1435–1444.
[6] S.V Patel, M.W Jenkins, R.C Hughes, W.G Yelton, A.J Ricco, Differentiation of
chemical components in a binary solvent vapor mixture using carbon/
polymer composite-based chemiresistor, Analytical Chemistry 72 (2000)
1532–1542.
[7] G.A Sotzing, S.M Briglin, R.H Grubbs, N.S Lewis, Preparation and properties
of vapor detector arrays formed from
Poly(3,4-ethylenedioxy)thiophene-Poly(styrenesulphonate)/Insulating polymer composites, Analytical
Chemis-try 72 (2000) 3181–3190.
[8] M.F Marbrook, C Pearson, M.C Petty, Inkjet printed PPy thin films for vapor sensing, Sensors and Actuators B 115 (2006) 547–551.
[9] W.A Daoud, J.H Xin, Y.S Szeto, Polyethylenedioxythiophene coatings for humidity, temperature and strain sensing polyamide fibers, Sensors and Actuators B 109 (2005) 329–333.
[10] J.H Cho, J.B Yu, J.S Kim, S.O Sohn, D.D Lee, J.S Huh, Sensing behaviors of polypyrrole sensor under humidity condition, Sensors and Actuators B 108 (2005) 389–392.
[11] C.P de Melo, B.B Neto, E.G de Lima, L.F B de Lira, J.E.C de Souza, Use of conducting Polypyrrole blends as gas sensors, Sensors and Actuators B 109 (2005) 348–354.
[12] H.K Jun, Y.S Huh, B.S Lee, S.T Lee, J.O Lim, D.D Lee, J.S Huh, Electrical properties of polypyrrole gas sensors fabricated under various pre-treatment conditions, Sensors and Actuators B 96 (2003) 576–581.
[13] L Jiang, H.K Jun, Y.S Huh, J.O Lim, D.D Lee, J.S Hoh, Sensing characteristics
of polypyrrole-poly(vinyl alcohol) methanol sensors prepared by in situ vapor state polymerization, Sensors and Actuators B 105 (2005) 132–137 [14] Q Ameer, S Adeloju, Polypyrrole-based electronic noses for environmental and industrial analysis, Sensors and Actuators B 106 (2005) 541–552 [15] T Yamauchi, K Kojima, K Oshima, M Shimomura, S Miyauchi, Glucose-sensing characteristics of conducting polymer bound with glucose oxidase, Synthetic Metals 102 (1999) 1320.
[16] L Ruangchuay, A Sirivat, J Schwank, Electrical conductivity response of polypyrrole to acetone vapor: effect of dopant anions and interaction mechanisms, Synthetic Metals 140 (2004) 15–21.
[17] L.M Torres-Rodriguez, M Billon, A Roget, G Bidan, A Polypyrrole-biotin based biosensor: elaboration and characterization, Synthetic Metals 102 (1999) 1328–1329.
[18] K Suri, S Annapoorni, A.K Sarkar, R.P Tandon, Gas and humidity sensors based on iron oxide-polypyrrole nanocomposites, Sensors and Actuators B 81 (2002) 277–282.
[19] Y Dan, Y Cao, T.E Mallouk, A.T Johnson, S Evoy, Dielectrophoretically assembled polymer nanowires for gas sensing, Sensors and Actuators B 125 (2007) 55–59.
[20] C Luo, R Poddar, X Liu, Innovative approach for replicating micropatterns in
a conducting polymer, Journal of Vacuum Science and Technology B 24 (2006) L19–L22.
[21] A Chakraborty, X Liu, C Luo, An intermediate-layer lithography method for generating multiple microstructures made of different conducting polymers, Microsystem Technologies 13 (2007) 1175–1184.
[22] X Liu, A Chakraborty, C Luo, Generation of all-polymeric diodes and capacitors using an innovative intermediate-layer lithography, in: J.P Martingale (Ed.), Progress in Solid State Electronics Research, Nova Science Publishers, Inc., 2007, pp 127–139.
[23] M Hecklele, W Bacher, K.D Mu¨ller, Hot embossing-the molding technique for plastic microstructures, Microsystem Technologies 4 (1998) 122–124 [24] S.Y Chou, P.R Krauss, P.J Renstorm, Nanoimprint Lithography, Journal of Vacuum Science and Technology B 14 (1996) 4129–4133.
[25] Z Liu, Y Su, K Varahramyan, Inkjet-printed silver conductors using silver nitrate ink and their electrical contacts with conducting polymers, Thin Solid Films 478 (2005) 275–279.
[26] L Ruangchuay, A Sirivat, J Schwank, Electrical conductivity response of polypyrrole to acetone vapor: effect of dopant anions and interaction mechanisms, Synthetic Metals 140 (2004) 15–21.
[27] R.L Plackett, The discovery of the method of least squares, Biometrika 59 (1972) 239–251.
[28] E.F Fama, J.D MacBeth, Risk, return and equilibrium: empirical tests, Journal
of Political Economy 71 (1973) 607–636.