In this paper, we describe the use of a non-contact, radio-fre-quency detector to externally interrogate the conductance of polyaniline films prepared by inkjet and screen printing polyan
Trang 1Non-contact, radio frequency detection of ammonia
with a printed polyaniline sensor
N.B Clarka,*, L.J Maherb
a
CSIRO Materials Science and Engineering, Private Bag 10, Clayton South 3169, Victoria, Australia
b Detection Systems Pty Ltd., P.O Box 397, Bayswater 3153, Victoria, Australia
a r t i c l e i n f o
Article history:
Received 9 February 2009
Received in revised form 17 March 2009
Accepted 29 March 2009
Available online xxxx
Keywords:
Sensor
Remote interrogation
Radio frequency
Polyaniline
Inkjet printing
Screen printing
a b s t r a c t
A novel system for the detection of ammonia was developed by monitoring the conductance of inkjet printed or screen printed polyaniline films with a radio frequency detector The system has the advantage
of non-contact detection of ammonia within sealed packages Since the sensor is a passive printed film that is externally interrogated, it does not require an internal power source or associated circuitry, and therefore may be a low-cost device suitable for smart packaging applications When printed on a suitable substrate, the sensor can be cycled several times using heat or a volatile acid to regenerate the polyaniline surface
Crown Copyright Ó 2009 Published by Elsevier Ltd All rights reserved
1 Introduction
A range of devices are available for the detection of ammonia
but most are costly and complex precision instruments
Accord-ingly, there has been continuing interest in simpler, lower-cost
designs that could be deployed more widely, such as those based
on thin films of polyaniline However, even these simple
polyani-line ammonia sensors generally require internal electrical power
and associated circuitry to operate as designed
Polyaniline (PAn) is a polymer that changes conductivity with
change in pH as a result of changes in the degree of protonation
of the polymer backbone, making it useful as a sensor for volatile
bases such as ammonia Unfortunately, polyaniline in the
conduct-ing state is difficult to process because it cannot be dissolved in
common solvents or melted below the decomposition
tempera-ture A common approach to overcoming this difficulty is to
prepare polyaniline in the form of dispersions suited to
conven-tional application methods
One method of preparing aqueous dispersions with high
stabil-ity is based on polyaniline nanofibres, as extensively reported by
Kaner’s group[1–17] Information and insights have accumulated
over several years but broadly, aqueous dispersions of polyaniline
can be formed by manipulating the reaction conditions to favour
homogenous nucleation, either by interfacial polymerisation, or more recently, by rapid mixing during the polymerisation reaction induction period, then allowing the reaction to proceed to comple-tion undisturbed Both methods result in the formacomple-tion of polyan-iline nanofibres with diameters in the 30–50 nm range, depending upon the acid used in the reaction to protonate the polymer and make it hydrophilic The polyaniline nanofibres are small enough
to be stabilised by the positive charge they carry in aqueous disper-sion Such stable aqueous dispersions can be used in inkjet ink, but concentrations are often lower than desirable for printing, as several ink layers may be required before useful conductivity is obtained
An alternative approach is the preparation of dispersions with a bulky hydrocarbon component to increase solubility and plasticity; for example, using dodecylbenzenesulfonic acid (DBSA) as a dop-ant as reported by Killard’s group[18–23] The DBSA acts as both
an anionic dopant and as a surfactant that stabilises the polyaniline dispersion against agglomeration Because of the improved stabil-ity of the polyaniline dispersion, PAn loading concentrations can be higher, which is better for printing
In this paper, we describe the use of a non-contact, radio-fre-quency detector to externally interrogate the conductance of polyaniline films prepared by inkjet and screen printing polyani-line-nanofibre and polyaniline-DBSA dispersions Since the sensor
is externally interrogated, it does not require an internal power source or associated circuitry, allowing the detection of ammonia inside sealed packages
1381-5148/$ - see front matter Crown Copyright Ó 2009 Published by Elsevier Ltd All rights reserved.
doi:10.1016/j.reactfunctpolym.2009.03.011
* Corresponding author Tel.: +61 395452259; fax: +61 395452448.
E-mail address: noel.clark@csiro.au (N.B Clark).
Contents lists available atScienceDirect Reactive & Functional Polymers
j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / r e a c t
Trang 22 Experimental
2.1 PAn dispersion preparation
DBSA (90%, technical grade, Fluka, Germany), ammonium
per-oxydisulfate (Analar, BDH, UK), Teric BL8 surfactant (C12
ethoxy-lated fatty acid alcohol, Huntsman, Australia) and sodium
dodecylsulfate (Analar, BDH, UK) were used as received
Hydro-chloric acid (37%, AR-grade, Merck, Germany) was diluted to
1 M with deionised water Aniline (AR-grade, Univar, Australia)
was distilled under vacuum with vigorous stirring to prevent
bumping
A PAn-nanofibre dispersion was prepared using the methods
described by Huang and Kaner[11] The purified aniline (3.2 mmol
or 0.3 g) was mixed with 10 mL of 1.0 M HCl acid solution
Ammo-nium peroxydisulfate (0.8 mmol or 0.18 g) was mixed into another
10 mL aliquot of the acid solution The aniline-acid solution was
added to the oxidant and the two solutions rapidly mixed for
30 s and then allowed to react undisturbed overnight The
follow-ing day, the polyaniline was washed with water and centrifuged
After three washings, the supernatant liquor had a pH of 3.2 and
was strongly green in colour, indicating the presence of polyaniline
particles too small to settle Before use in printing trials, any
remaining particles larger than 1lm were removed by passing
the dispersion through a 55-mm glassfibre filter (Whatman GFA,
UK) with vacuum assistance Teric BL8 was added to adjust surface
tension to a level within the operating envelope of the printer
PAn-DBSA dispersions were prepared, based on the method
reported by Ngamna et al.[21], but with several minor variations
First, the amount of DBSA used was half of that found by Ngamna
et al to be optimum for maximum PAn concentration (i.e 0.095 M
DBSA, rather than 0.19 M; because of the other modifications
em-ployed, the stability of the dispersion at this DBSA addition level
was acceptable) Second, the reaction was conducted with mixing
only during the reaction induction period and none thereafter, as
Kaner’s group had shown that continuous mixing during PAn
syn-thesis may increase particle agglomeration Third, ultrasonication
was used to disperse the PAn formed prior to filtration, to break
up any large particles and maximise dispersion concentration
and stability The procedure used was as follows:
DBSA (1.24 g) was added to 20 mL H2O, 0.60 g An, 20 mL 1 M
HCl and 0.73 g (NH4)2S2O8, and mixed for 30 s, and then reacted
undisturbed for 2.5 h The dispersion formed was dialysed against
600 mL of 0.05 M sodium dodecylsulfate through a cellulose
mem-brane (Sigma–Aldrich D9402) for 48 h with two solution changes,
then ultrasonicated with a 750 W Sonics Vibracell 1-cm diameter
titanium probe (treatment cycle was 1 s on, 2 s off, 70% of
maxi-mum amplitude, 10 min treatment time, equivalent to 3.3 min
to-tal ultrasonication time) and passed through a 55-mm glassfibre
filter (GFA, Whatman, UK) with vacuum assistance
The viscosity of the dispersions was measured with a Brookfield
DV II+ viscometer and surface tension was measured with the
rod-pull method[24,25] The contact angle of water drops applied to
each of the substrates was measured with a KSV Instruments Ltd
Cam 200 The UV–Vis spectra of the dispersions were collected
with a Perkin–Elmer Lambda 35 spectrophotometer, after dilution
to comparable absorbance PAn film thickness was measured on
PVC substrates by taking microtome cross sections and imaging
them with a Philips XL 30 Field Emission Scanning Electron
Microscope
The two PAn dispersions were cast directly on selected
sub-strates, or injected into the cartridge of a Dimatix Materials Printer
(model 2811 FUJIFILM Dimatix, USA) for inkjet printing trials An
aliquot of the PAn–DBSA dispersion was also evaporated to a paste
for application with a screen printing kit (Riso PG-5, Japan)
2.2 Substrates For the application trials, a range of substrates were employed
of commercial and general laboratory origin, the majority being polymer label stock that may be candidates for future commercial sensor deployment Based on Raman spectra (data not given for commercial reasons) the label stock samples were composed of polyethylene with TiO2 filler, with an ink receptor layer applied
to the surface Substrate pH was measured with a Hanna Instru-ments model 211 pH meter, on solutions prepared by chopping 0.5 g of each substrate into 2 mm strips and soaking in 10 g of deionised water for 72 h
2.3 Radio frequency conductance measurements
A PCIS-3000 10-95 6536 radio frequency detector (Detection Systems, Melbourne, Australia)[26]was used to monitor the con-ductance of sensor strips coated with the polyaniline dispersions PCIS equipment is typically used to detect missing and damaged items inside sealed packs as they are conveyed on high speed pro-duction lines This electromagnetic inspection technique is based
on a low voltage, low frequency AC electric field Sealed packages are conveyed through the PCIS transducer by means of a motorised conveyor and a series of ‘slices’ are recorded and assembled into an overall picture of the internal contents of the package A computer analyses this overall picture for any defects In traditional PCIS applications it is desirable to focus on the contents and ignore the RF signal generated by the packaging However, for the exper-iment described here, it was desirable to detect the signal from a sensor embedded in, or on, the packaging Further, the sensor con-ductivity was modulated by the local environment (i.e the concen-tration of ammonia inside the box) To our knowledge this is the first time such a device has been used as a component of a RF sen-sor system
2.4 Lumped circuit approximation
An equivalent circuit of the sensor while centred in the scan-ning head can be described as follows (refer toFig 1) The trans-mitted signal reaches the receiving electrode by two parallel paths – (1) directly capacitively coupled, and (2) coupled via the sensor Here, Cbypassis the direct coupling from the transmitting
Fig 1 Simple lumped circuit approximation of sensor in the PCIS transducer.
Trang 3to receiving electrode across the scanning head (substantial
rela-tive to the sensor path) C1is the coupling from the transmitting
electrode to the sensor strip Zsensoris the impedance of the PAn
sensor strip The sensor strip is mostly resistive in its conducting
state C2is the coupling from the top of the sensor strip to the
receiving electrode This model is useful for understanding the
ba-sics of the scanning head Some limitations of this model include:
Impedances involved are very high, circuit elements are
distrib-uted and stray capacitances are present
Packaging materials are neglected The impedances of materials
generally thought of as insulating (paper, cardboard, plastic, or
the conveyor belt) can be significant in value, but in practice
their effect can generally be mitigated Their response is
gener-ally constant, localised, or at a lower frequency than that of the
sensor or product
For the polyaniline sensor tests, a hinged polypropylene box of
dimensions 195 mm 55 mm 45 mm was employed, with the
sensor strip taped to the inside, vertically near the centre of the
longitudinal axis of the box (Fig 2) The composition of the plastic
was not important as the only requirement was for a constant
cross section Three drops of the test liquid, either ammonia
solu-tion or acetic acid, were applied by pipette to the inside of the box
along the front edge When closed, the box would quickly fill with
vapour from the applied liquid, potentially inducing a change in
the degree of protonation of the PAn chains on the sensor surface
Passing the box over the transmitter on the conveyor belt
al-lows the conductance of the PAn strip to be monitored, which in
turn is an indicator of the atmosphere within the box The
trans-mitter generates a low sinusoidal voltage, typically 8–20 V rms,
in the low frequency range The conveyor was 3-mm thick and
was driven by a variable speed drive, but with the same speed used
for all experiments The receiving assembly on the PCIS equipment
actually has two ‘lanes’ or inspection zones Because only one zone
was required for these tests the two zones were electrically
con-nected together, effectively forming one double sized inspection
area The amplifier converted the received signal to a low
imped-ance suitable for subsequent signal processing The amplified
sig-nal was converted to a DC voltage of relatively low frequency
These waveforms are the ‘RF signal vs time’ plots presented in this
paper Positive RF signals resulted if a conductive strip or sensor
was placed in the scanning head The strip or sensor acts as a
‘par-tial reduced impedance path’, compared with the air/bypass path The computer converted and stored the waveforms to disk Each waveform consisted of 500 samples over a duration of 2 s (250 Hz sampling rate) The range of the ‘RF signal’ is 0–4095, with
a baseline (nothing in the scanning head) around 2270 The stored waveforms were analysed using a spreadsheet Of particular inter-est was the height of the peak caused by the PAn sensor in the mid-dle of the package The time series of the height of this peak over time was also of key interest
3 Results and discussion 3.1 Substrate properties Important properties of the substrates used to support polyan-iline sensor films were the contact angle of applied water drops and pH (Table 1) Contact angle predicts wettability, which in turn may affect the uniformity of the applied polyaniline film, whereas substrate pH may affect the performance of the polyaniline film as
an ammonia sensor Accordingly, the best performance would be expected from substrates that were both hydrophilic (contact an-gles less than 90°) and acidic (pH of <7) Only the glass and filter paper met both these criteria
3.2 PAn dispersion properties The properties of the prepared PAn dispersions are listed in Ta-ble 2 The PAn-nanofibre dispersion had a total solids concentra-tion less than 1%, and not surprisingly, had properties not dissimilar to water This meant that surface tension in particular was higher than desirable for the Dimatix printer and two drops
of Teric BL8 surfactant (C12 ethoxylated fatty acid alcohol, Hunts-man) was added to reduce surface tension to 37 dyn cm 1prior to printing The PAn-DBSA dispersion had a solids concentration al-most two and a half times higher than that of the nanofibre disper-sion, and surface tension was within the range of the Dimatix printer The UV–Vis spectra were consistent with those expected for PAn-nanofibre [10] and PAn-DBSA dispersions, respectively
[21] 3.3 PAn-nanofibre application Test prints were made with the PAn-nanofibre dispersion on filter paper Because the PAn concentration was low, print intensity
Fig 2 Test arrangement using PCTS-3000 radio frequency detector.
Table 1 Substrates tested.
Label stock B KW Doggett, Australia 101 7.6
Label stock D Avery Dennison, Australia 74 8.1
Table 2 Dispersion properties relative to dimatix printer requirements.
Liquid Total solids (%) Viscosity (mPa S) Surface tension (dyn/cm)
Trang 4remained poor even after several layers were applied in succession
(examples are therefore not reproduced here) When the
PAn-nanofibre dispersion was cast (rather than printed) on glass, a thin,
uniform film was formed that was soft and easily damaged by
handling
3.4 PAn-nanofibre radio frequency response
The PAn-coated glass slide was installed in the test box and
passed beneath the radio frequency head several times, with
ammonia or acetic acid solutions added successively to change
the box atmosphere as described above The radio frequency
wave-form was collected at each pass Output traces representing acidic
and alkaline atmospheres are presented in Fig 3 Under acidic
conditions, a large peak was evident in the output from the radio
frequency detector, corresponding to the position of the
PAn-nanofibre sensor strip in the box axial dimension Since the
position of the peak relative to the horizontal axis of the chart
reflected the physical position of the PAn strip inside the plastic box, moving the sensor strip within the box would result in a cor-responding lateral movement of the peak in the output trace The PAn peak amplitude varied according to the atmosphere within the box Peak maxima were obtained either initially, after the sensor strip was first installed with the PAn-nanofibre in the emeraldine salt form, or after exposure to acetic acid following pre-vious exposure to ammonia Peak minima (i.e baseline levels) were reached after exposure of the sensor strip to ammonia, caused by de-protonation of the amine groups in the PAn polymer
to the emeraldine base form Overall, the results demonstrated that the PAn-nanofibre sensor worked as reported by Kaner’s group[2], although the low concentration of the PAn-nanofibre dispersion meant that satisfactory prints would require many passes of the print head
Since the detector was sensitive enough to register disturbances
in the radio frequency field caused by transitions from air to plastic
as the box was conveyed through the detector, two peaks were
Box axis (mm)
2250 2300 2350 2400 2450 2500 2550 2600 2650
Acid Alkali
Fig 3 Radio frequency signal obtained from PAn-nanofibre dispersion cast on glass, with successive exposure to ammonia and acetic acid.
2360 2380 2400 2420 2440 2460 2480
Surface resistivity (KOhms/sq)
Fig 4 Relationship between radio frequency signal and surface resistivity measured with a 4-point probe.
Trang 5evident in the device output corresponding to the two ends of the
box, and three smaller bumps on the baseline (at 60 mm, 75 and
90 mm in the axial dimension) associated with shallow decorative
mouldings incorporated by the manufacturer in the box design
3.5 PAn-DBSA application
The image intensity obtained by inkjet printing the PAn-DBSA
dispersion on filter paper was satisfactory However, sharpness
suffered as a result of lateral migration of the dispersion along
the fibres and coverage was not completely uniform, with mottle
and print head scan lines both evident To determine the
relation-ship between RF signal and electrical resistance, filter paper was
inkjet printed with varying layers of the PAn-DBSA The surface
resistivities of the prints were measured with a 4-point probe
and related to the corresponding RF signal (Fig 4) It was evident
that the relationship was linear over much of the surface resistivity
range, although at low resistivity the RF Signal appeared to plateau
To achieve a fair comparison, the PAn-DBSA was cast on glass for the initial tests with the radio frequency detector In this case, the dispersion shrank dramatically on drying, leaving only a small area of the glass covered by the polymer, which appeared to be a film much thicker than that achieved using the PAn-nanofibre dis-persion Nevertheless, successive exposure to ammonia and acetic acid produced a series of peaks in the radio frequency detector out-put similar to those obtained with the PAn-nanofibre dispersion (Fig 5)
By abstracting the signal maxima of the sensor, it was possible
to plot signal strength against time (Fig 6) The sensor could be cycled by successive exposure to ammonia and acetic acid but on the third such cycle the baseline showed evidence of a rise in con-ductance, probably from accumulation of ammonium acetate Repeating this process for PAn-DBSA cast on each of the sub-strates listed in Table 1, several observations were made First, the results were primarily influenced by pH, with those substrates with alkaline surfaces (label stocks A, B, D and E) giving poor
Box axis (mm)
2250 2300 2350 2400 2450 2500 2550 2600 2650
Initial signal After ammonia After acetic acid
Fig 5 Radio frequency signal obtained from PAn-DBSA dispersion cast on glass, with successive exposure to ammonia and acetic acid.
2250 2300 2350 2400 2450 2500 2550 2600 2650
Time (min)
Fig 6 Radio frequency maxima obtained from PAn-DBSA dispersion cast on glass, with successive exposure to ammonia (squares) and acetic acid (circles).
Trang 6results because of partial de-protonation of the PAn-DBSA film
be-fore exposure to ammonia Second, shrinkage of the PAn-DBSA film
on drying was a significant issue for smooth substrates like glass
and PVC, and also for the non-absorbent label stocks Overall, the
best results were obtained using the filter paper substrate, which
was low in pH and had a micro-texture that prevented PAn film
shrinkage on drying
Another method of cycling the sensor may be by simply heating the printed substrate in air, driving off the volatile ammonia as re-cently detailed by Crowley et al.[23] This approach was adopted for PAn-DBSA inkjet printed on filter paper (Fig 7) Although a sta-ble baseline was obtained because ammonium acetate could not accumulate, the sensor did not recover quickly on heating The DBSA dispersion, while more concentrated than the PAn-nanofibre dispersion, was still quite low in viscosity, as required for inkjet printing A consequence of this low viscosity was deep penetration of the dispersion in the z-direction, almost to the underside of the paper sheet Such deep penetration could slow the evaporation of ammonia, as the diffusion path through the coated paper matrix would be tortuous To test this concept, the dispersion was thickened by evaporation, forming a paste that could be applied by screen printing The thickness of a screen-printed film on PVC was measured from SEM images of cross
2250 2300 2350 2400 2450 2500 2550 2600 2650
Box axis (mm)
Fig 7 Radio frequency maxima obtained from PAn-DBSA dispersion, inkjet printed on filter paper, with exposure to ammonia and recovery by heating in air on a hotplate at
70 °C Ammonia additions (square points) result in steep declines in the RF signal, followed by gradual recovery with heating.
Table 3
Film thickness of PAn dispersions applied to PVC.
2250 2300 2350 2400 2450 2500 2550 2600 2650
Time (min)
Fig 8 Radio frequency maxima obtained from PAn-DBSA dispersion, screen printed on filter paper, with repeated application of ammonia (square points) and recovery by heating in air on a hotplate at 70 °C.
Trang 7sections and compared with inkjet printed DBSA and
PAn-nanofibre (Table 3) Screen printing the PAn-DBSA dispersion onto
filter paper gave a rough and thick film compared to other methods
of application However, it gave an ammonia sensor that could be
rapidly and repeatedly cycled by heating in air on a hotplate at
70 °C (Fig 8)
4 Conclusions
Ammonia sensors were prepared by printing polyaniline
disper-sions on various substrates and using a radio frequency detector to
monitor conductance Dispersions of polyaniline doped with
dode-cylbenzenesulfonic acid could be prepared at higher concentration
than those of polyaniline nanofibres and consequently the former
were more suitable for inkjet printing on a range of substrates
However, these substrates varied in their suitability as sensor
sup-ports because of variations in both pH and contact angle, with the
best substrate found to be a filter paper with a low pH and a
micro-texture that obviated PAn-DBSA film shrinkage on drying The
ink-jet printed PAn-DBSA dispersion worked as an ammonia sensor but
cycling time on exposure to volatile acid or heat was slow,
proba-bly because of deep penetration of the low viscosity PAn-DBSA
ink-jet formulation through the sheet A screen-print paste prepared by
evaporating the PAn-DBSA inkjet formulation to a higher viscosity
was more successful, rapidly and repeatedly cycling simply by
heating in air Together, the PAn-DBSA and the radio frequency
detector form a non-contact sensor for the detection of ammonia
within sealed packages, and is therefore a low-cost device that
might be suitable for smart packaging applications
Acknowledgements
The authors are indebted to Dr Orawan Winther-Jensen (nee
Ngamna), of Monash University, for advice on the preparation of
PAn-DBSA dispersions The SEM images were collected by Mark
Greaves at CSIRO Materials Science and Engineering Contact angle
measurements were made with the assistance of Dr Christian
Kugge of CSME, using equipment located at CSIRO Molecular and
Health Technologies The authors are indebted to Dr Ken Wong
of Scion, New Zealand, Dr Nafty Vanderhoek and Dr Warwick
Rav-erty at CSIRO Materials Science and Engineering for valuable discussions and suggestions The work formed part of a New Zea-land Government FRST project entitled Functional Packaging Sys-tems for Food Exports and was financially supported by the Cooperative Research Centre for Functional Communication Sur-faces (CRC SmartPrint) and CSIRO Materials Science and Engineering
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