EDP SCIENCES JOURNAL - AUTHOR QUERY FORM
Journal Code: EPJAP Article Number: ap140289
Article Title: Pulsed laser deposition of polytetrafluoroethylene-gold composite layers
AUTHOR QUERIES – TO BE ANSWERED BY THE CORRESPONDING AUTHOR Dear Author,
During the preparation of your manuscript for typesetting, the queries listed below have arisen. Please answer these queries by marking the required corrections at the appropriate point in the text.
Electronic file usage
Sometimes we are unable to process the electronic file of your article and/or artwork. If this is the case, we have proceeded by:
Scanning (parts of) your
article Rekeying (parts of) your
article Queries and/or remarks
Location Query/remark Response
No queries.
CE: Nitheya
Thank you for your assistance.
DOI: 10.1051/epjap/2014140289
P HYSICAL J OURNAL A
PPLIEDP
HYSICS Regular ArticlePulsed laser deposition of polytetrafluoroethylene-gold composite layers
Gabriella Kecskem´eti1,a, Tomi Smausz2,1, Zs´ofia Berta1, B´ela Hopp1, and G´abor Szab´o1,2
1 Department of Optics and Quantum Electronics, University of Szeged, 6720 Szeged, D´om t´er 9, Hungary
2 MTA-SZTE Research Group on Photoacoustic Spectroscopy, University of Szeged, 6720 Szeged, D´om t´er 9, Hungary
Received: 11 July 2014 / Received in final form: 11 September 2014 / Accepted: 24 September 2014 Published online: (Inserted later) – cEDP Sciences 2014
Abstract. PTFE-metal composites are promising candidates for use as sensor materials. In present study PTFE-Au composite layers were deposited by alternated ablation of pressed Teflon pellets and gold plates with focused beam of an ArF excimer laser at 6 J/cm2 fluence, while keeping the substrate at 150 ◦C temperature. The morphology and chemical composition of the ∼3–4 µm average thickness layers was studied by electron microscopy and energy dispersive X-ray spectroscopy. The layers were mainly formed of PTFE gains and clusters which are covered by a conductive Au film. For testing the applicability of such layers as sensing electrodes, composite layers were prepared on one of the two neighbouring electrode of a printed circuit board. Cholesterol and glucose solutions were prepared using 0.1M NaOH solvent containing 10% Triton X-100 surfactant. The electrodes were immersed in the solutions and voltage between the electrodes was measured while a constant current was drawn through the sample. The influence of the analyte concentration on the power spectral density of the voltage fluctuation was studied.
1 Introduction
1
Due to the good mechanical, thermal and chemical sta-
2
bility polytetrafluoroethylene (PTFE) is a promising can-
3
didate for sensor preparation where its role can be either
4
the immobilization of the component responsible for the
5
sensing or even the participation in the sensing mecha-
6
nisms when detecting humidity [1], SO2[2,3], O2, CO2[4]
7
or other gases [5,6]. The pulsed laser deposition (PLD) of
8
PTFE thin layers is a thoroughly studied research field,
9
the method allows the deposition of stoichiometric thin
10
films with morphology ranging from compact to sponge-
11
like structure [7–9]. The electrical and wetting properties
12
can be tuned by addition of metals. Recent studies showed
13
that PTFE/silver composite structures deposited by PLD
14
using PTFE/Ag targets have a rough morphology with
15
increased specific surface attributed to the deposition of
16
PTFE grains and show improved conductive and wetting
17
properties due to the Ag content [10].
18
In the last few years several attempts have been made
19
for fabrication of non-enzymatic sensors for the detection
20
clinically important analytes, as glucose [11,12], choles-
21
terol [13,14] or urea [15]. These researches are motivated
22
by the fact that, although the amperometric and potentio-
23
metric detectors based on incorporation of enzymes into
24
the active electrodes [16–19] show good selectivity, the
25
enzyme immobilization process is the most difficult step
26
of the production process. The non-enzymatic sensors are
27
a e-mail:kega@physx.u-szeged.hu
based on conductive electrodes with high specific surface 28
and charges involved in electrocatalytic process are 29
detected by amperometric measurement methods. While 30
classical detection techniques are based on the measure- 31
ment of the time-averaged value of the sensor signal, in 32
some cases the existence of “fingerprints” of the analytes 33
[20–22] in the low amplitude time-varying components of 34
the signal were also demonstrated. This detection method 35
is called fluctuation-enhanced sensing (FES). In a recent 36
work the PTFE/Ag composite layer covered electrodes 37
were immersed in cholesterol solution and the voltage fluc- 38
tuation was measured while driving a constant current 39
through the electrodes. It was found that power spectral 40
density of the “noise” depended on the cholesterol con- 41
centration; however a quick aging of the electrodes due to 42
the silver oxidation was observed [23]. 43
In this work we present our results on the pulsed laser 44
deposition of PTFE/gold composite layers onto electrodes 45
of printed circuit boards and their behavior in fluctua- 46
tion enhanced sensing measurements is monitored when 47
immersed in solutions of cholesterol and glucose and their 48
mixture. 49
2 Experimental
502.1 Thin film deposition 51
Composite layers formed of PTFE and gold were prepared 52
by pulsed laser deposition onto one electrode of a printed 53
The European Physical Journal Applied Physics
Fig. 1.Measurement procedure for the fluctuation-enhanced sensing.
(a) (b) (c)
Fig. 2.Electron microscopic image of the rough composite layer covered electrode (a) and elemental distribution on the same area for F (b) and Au (c).
circuit sample board containing a pair of 2×2 mm2 gold
54
plated electrodes with 1 mm separation distance as pre-
55
sented in detail earlier in reference [23]. The disk-shaped
56
target was composed of two halves of disks: one of bulk
57
Au and one of pressed PTFE powder. The experimental
58
conditions were chosen based on the results of our ear-
59
lier studies [9,10] as to assure degradation-free transfer of
60
PTFE and appropriate mixing of Teflon and metal in
61
order to obtain a rough surface conductive composite
62
layer. The continuously rotated target was ablated with
63
5000 pulses of an ArF excimer laser (λ = 193 nm,
64
FWHM = 20 ns) focused onto a 0.8 mm2 area while
65
the applied fluence was 6 J/cm2. During the deposition
66
the sample board facing the target at 4 cm distance was
67
kept at 150 ◦C temperature. The morphology and the
68
elemental distribution of the prepared layers were studied
69
with a Hitachi S4700 scanning electron microscope (SEM)
70
equipped with a R¨ontec QuanTax energy-dispersive X-ray
71
spectrometer (EDX).
72
2.2 PTFE/Au composite layers as sensor electrodes
73
A 0.1M NaOH solvent containing 10% Triton X-100 sur-
74
factant was used to prepare solutions of 2 and 5 mM
75
cholesterol, 5 and 15 mM glucose and their mixtures.
76
The fluctuation based sensing measurements were per-
77
formed as follows: the sample board was immersed ver-
78
tically into the solutions until the two electrodes became
79
fully covered (Fig. 1). A constant current of 5 μA was 80
drawn through the circuit and the U(t) voltage between 81
the two electrodes was measured with 38 nV resolution 82
at a sampling rate of 4000 Hz for a period of 15 s. 83
TheS(f) power spectral density of the voltage fluctuation 84
was obtained by fast Fourier transform (FFT) in LabView 85
software environment. The signal was divided in 30 pcs. 86
of 0.5 s segments and their FFT spectra were averaged. 87
Reference measurements on untreated electrodes were also 88
carried out. 89
3 Results
903.1 Thin film characterization 91
As the electron microscopic image in Figure2a shows, the 92
layers have a rough surface, since the PFFE (-[C2F4]n-) 93
is mainly transferred in form of grains and larger clus- 94
ters, only a minor part can be originated from repolymer- 95
ization from larger polymer chain fragments. Elemental 96
microanalysis was realized by EDX and the results proved 97
that the gold is more uniformly distributed over the 98
deposited area. More detailed previous studies showed 99
that the darker areas on the image showing the elemental 100
map of the gold (Fig.2c) can be attributed to the shield- 101
ing effect of the PTFE grains, since their size is larger 102
than the ∼1 μm detection depth of the EDX. As the 103 alternate ablation of the two components results in the 104
0 200 400 600 800 1000
(3)
(2) (5)
(4)
(1) solvent (2)2mM cholesterol (3)5mM cholesterol (4)5mM glucose (5)15mM glucose
(a)
(1)
(4) (3) (2)
(b) (1) solvent
(2)2mMol cholesterol (3)15mMol glucose (4)2mMol cholesterol
+ 15mMol glucose
1E-7 (1)
1E-8 1E-6 1E-5 1E-4 1E-3 0.01
S (f)
1E-7 1E-8 1E-6 1E-5 1E-4 1E-3 0.01
S (f)
FREQUENCY (Hz)
0 200 400 600 800 1000
FREQUENCY (Hz)
Fig. 3. Spectra of the voltage fluctuation for different solution types when using composite layer covered electrode (a) and comparison of the spectra recorded with a two component solution to the corresponding single component spectra (b).
covering/mixing of the Teflon structures with the metal,
105
the layers became conductive; moreover the wettability is
106
also increased as compared to pure PTFE. These proper-
107
ties of the layers assure an increased contact area when
108
the electrode is immersed in the solution, as compared to
109
the original, uncovered electrodes.
110
3.2 Fluctuation-enhanced sensing
111
For voltage fluctuation measurements each solution was
112
tested with a new sample board.S(f) power spectral den-
113
sity function was obtained in 0–2 kHz frequency range
114
with 2 Hz resolution according to the sampling rate and
115
the length of theU(t) signal used for FFT. In most cases
116
the harmonics of the 50 Hz grid frequency appeared in
117
the spectra, which were cut off. In case of untreated
118
electrodes the analyte and its concentration did not show
119
observable influence on the obtainedS(f) spectra. In con-
120
trast to this, there was a noticeable difference between the
121
spectra obtained in presence of cholesterol and glucose
122
solutions with different concentrations (Fig. 3a). In case
123
of the conducting pure solvent the noise spectrum origi-
124
nates from its characteristic resistance fluctuation, which
125
is a general property of conductive elements in electronics.
126
In case of cholesterol and glucose solution the charge
127
transfer related to their electrocatalytic reaction at the
128
surface of the rough electrode also contributes to the
129
detected noise. This indicates that due to their rough
130
surface such composite layers may serve as active elec-
131
trodes in non-enzymatic electrocatalytic sensors. In case
132
of real measurements the interference between the differ-
133
ent analytes being present in monitored solution has to
134
be taken into account. The Figure 3b shows an exam-
135
ple spectrum for a solution containing both cholesterol
136
and glucose as compared to appropriate single component
137
solutions. Although we expected the spectra of the
138
mixtures to be situated somewhere between those cor-
139
responding to the single component solutions, there is
140
no straightforward relationship between the correspond-
141
ing spectra.
142
A quantitative comparison of the spectra was real-
143
ized by principal component analysis (PCA). Since value
-10 -8 -6 -4 -2 0 2 4 6 8 10 12 14
-0.4 -0.2 0.0 0.2 0.4
Solvent 5mM chol.+
15mM gluc.
2mM chol.+
5mM gluc.
Glucose
PC-1
PC-2
Cholesterol
Fig. 4.Result of PCA analysis of the voltage fluctuation spec- tra recorded for different solutions represented in PC-1 - PC-2 plane. The arrows indicate the tendency for increasing concen- trations of single component solutions.
ranges of the S(f) functions cover several orders of 144
magnitudes, their logarithm was calculated and then fit- 145
ted with a third order exponential decay function. Thus 146
smooth curves following the tendencies of the spectral 147
functions were obtained and their 2–100 Hz frequency 148
range was submitted to PCA analysis. Figure4shows the 149
plotted values of the first two components of the resulting 150
scores matrix (PC-1 vs. PC-2). The data points indicate 151
that even if there is a tendency when varying the concen- 152
trations of cholesterol and glucose solutions, one cannot 153
find a connection between the position of the data points 154
for the mixtures and the concentration of the components. 155
PCA can be used also for evaluation of infrared absorp- 156
tion spectra of multicomponent samples. Supposing no 157
interaction between the components, the total absor- 158
bance is the summarized absorbance of the individual con- 159
stituents. In case of a three component sample, performing 160
a PCA analysis on IR spectra recorded for different mix- 161
tures and plotting the results in plane a ternary graph- 162
like triangular point distribution can be obtained with
The European Physical Journal Applied Physics
0 500 1000 1500 2000
1E-7 1E-6 1E-5 1E-4 1E-3 0.01
0 min 10 min
FREQUENCY (Hz)
S (f)
Fig. 5.Averaged spectra recorded at the beginning and at the end of 10 min aging test.
points corresponding to the three pure samples on the
163
corners of the triangle, as demonstrated in by Bacci et al.
164
(calcareous samples [24]) and Norgaart et al. (sucrose
165
and its components [25]). Obviously, in case of four con-
166
stituents the first three PC components of the score
167
matrix have to be plotted in a 3D coordinate system, etc.
168
In our case the solution can be considered as a three com-
169
ponent sample formed of solvent, glucose and cholesterol.
170
When either the cholesterol or glucose was dissolved, the
171
linear shift in PC-1 - PC-2 plane of the data points as
172
the function of concentration was visible in our case, too.
173
The behavior of the cholesterol-glucose mixtures was
174
different from the above mentioned case of IR spectral
175
analysis, which can be partly attributed to the lack of
176
component-specific peaks in the recorded noise spectra
177
and partly to the fact that the presence of one solved com-
178
ponent influences the interaction of the electric charges
179
with the other solved component resulting in a non-
180
additive aspect of the noise spectra. This suggests that
181
in presented experimental parameters the fluctuation
182
enhanced sensing accompanied with principal component
183
analysis is not a practicable way for multicomponent
184
analysis in liquid phase. A possible solution could be the
185
monitoring of the noise when altering the driving current
186
(and consequently the constant component of the mea-
187
sured voltage), since the electrocatalytic process of glucose
188
and cholesterol is voltage dependent.
189
In an earlier study a quick degradation of the silver
190
based composite electrodes (probably caused by oxida-
191
tion) was observed during similar experiments and only
192
the spectra recorded in the first 15 s could be used for eval-
193
uation. Therefore the stability of the PTFE/gold compos-
194
ite layers and temporal behavior of the recorded spectra
195
were also tested. There was no observable discoloration
196
of the electrodes even after 10 min continuously running
197
experiment. Figure 5 shows an example on the noise
198
spectral stability recorded in case of 5 mM cholesterol
199
solution.
200
4 Summary
201Pulsed laser deposition method was used to prepare 202
conducting PTFE-gold composite layers for sensor elec- 203
trode purposes. The increased specific surface of the layers 204
increased the sensitivity of the electrodes as compared to 205
the original smooth gold plating, when measuring voltage 206
fluctuations in presence of cholesterol and glucose 207
solutions. While the earlier studied PTFE-silver compos- 208
ite electrodes showed a fast aging due to the oxidation of 209
the silver, the use of gold as conducting element resulted 210
in significant increase in stability. Although there is obvi- 211
ous influence of the concentration on the recorded spectra, 212
in case of two component solution the separation of the 213
components’ effect is not straightforward. Similar difficul- 214
ties were encountered in multicomponent gas sensing with 215
FES method. Further studies on optimization of measur- 216
ing parameters (electric current value, sampling rate, etc.) 217
and data processing methods are needed for enhancing the 218
cross-selectivity of the method. 219
This research was supported by the European Union and the 220
State of Hungary, co-financed by the European Social Fund 221
in the framework of T ´AMOP 4.2.4. A/2-11-1-2012-0001 222
“National Excellence Program” and the “Biological and 223
environmental responses initiated by new functional materials” 224
Grant no. T ´AMOP-4.2.2.A-11/1/KONV-2012-0047. 225
References
2261. M.V. Fuke, A. Vijayan, M. Kulkarni, R. Hawaldar, R.C. 227
Aiyer, Talanta76, 1035 (2008) 228
2. I. Tarsiche, D. Ciurchea, Sens. Actuat. B72, 94 (2001) 229
3. M. Nebel, S. Neugebauer, H. Kiesele, W. Schuhmann, 230
Electrochim. Acta55, 7923 (2010) 231
4. T.G. Anjos, C.E.W. Hahn, Sens. Actuat. B 135, 224 232
(2008) 233
5. Z. Qin, P.-N. Wang, Y. Wang, Sens. Actuat. B 107, 805 234
(2005) 235
6. M. Wienecke, M.-C. Bunescu, M. Pietrzak, K. Deistung, 236
P. Fedtke, Synth. Met.138, 165 (2003) 237
7. S.T. Li, E. Arenholz, J. Heitz, D. Bauerle, Appl. Surf. Sci. 238
125, 17 (1998) 239
8. G.B. Blanchet, S.I. Shah, Appl. Phys. Lett. 62, 1026 240
(1993) 241
9. T. Smausz, B. Hopp, N. Kresz, J. Phys. D 35, 1859 242
(2002) 243
10. G. Kecskem´eti, B. Hopp, T. Smausz, Z. T´oth, G. Szab´o, 244
Appl. Surf. Sci.258, 7982 (2012) 245
11. X. Wang, Y. Zhang, C.E. Banks, Q. Chen, X. Ji, Colloids 246
Surf. B78(2), 363 (2010) 247
12. S. Hui, J. Zhang, X. Chen, H. Xu, D. Ma, Y. Liu, B. Tao, 248
Sens. Actuat. B 155, 592 (2011) 249
13. Y.-J. Lee, J.-Y. Park, Biosens. Bioelectron. 26, 1353 250
(2010) 251
14. Y. Li, H. Bai, Q. Liu, J. Bao, M. Han, Z. Dai, Biosens. 252
Bioelectron.25, 2356 (2010) 253
15. S. Mondal, M.V. Sangaranarayanan, Sens. Actuat. B177,
254
478 (2013)
255
16. R. Nenkova, D. Ivanova, J. Vladimirova, T. Godjevargova,
256
Sens. Actuat. B148, 59 (2010)
257
17. P.-C. Nien, P.-Y. Chen, K.-C. Ho, Sensors9, 1794 (2009)
258
18. S. Saha, S.K. Arya, S.P. Singh, K. Sreenivas, B.D.
259
Malhotra, V. Gupta, Biosens. Bioelectron.24, 2040 (2009)
260
19. M. Guo, J. Chen, J. Li, L. Nie, S. Yao, Electroanalysis16,
261
1992 (2004)
262
20. G. Schmera, C. Kwan, P.M. Ajayan, R. Vajtai, L.B. Kish,
263
IEEE Sens. J.8, 714 (2008)
264
21. L.B. Kish, J. Smulko, P. Heszler, C.-G. Granqvist, 265
Nanotechnology Perceptions3, 43 (2007) 266
22. J.R. Biard, L.B. Kish, Fluct. Noise. Lett.5, 153 (2005) 267
23. T. Smausz, G. Kecskem´eti, T. Csizmadia, F. Benedek, 268
B. Hopp, Appl. Surf. Sci.278, 117 (2013) 269
24. M. Bacci, R. Chiari, S. Porcinai, B. Radicati, Chemometr. 270
Intell. Lab.39(1), 115 (1997) 271
25. L. Nørgaard, R. Bro, S. Balling Engelsen, “Principal 272
Component Analysis and Near Infrared Spectroscopy”, 273
a FOSS white paper, http://www.foss.de/industry- 274
solution/chemical-analysis/papers 275
