Author's Accepted Manuscript
A new room temperature gas sensor based on pigment-sensitized TiO2 thin film for amines determination Li Yanxiao, Zou Xiao-bo, Huang Xiao-wei, Shi Ji-yong, Zhao Jie-wen, Mel Holmes, Limin Hao
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S0956-5663(14)00373-X http://dx.doi.org/10.1016/j.bios.2014.05.040 BIOS6802
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Biosensors and Bioelectronics
Cite this article as: Li Yanxiao, Zou Xiao-bo, Huang Xiao-wei, Shi Ji-yong, Zhao Jie-wen, Mel Holmes, Limin Hao, A new room temperature gas sensor based on pigment-sensitized TiO2 thin film for amines determination, Biosensors and Bioelectronics, http://dx.doi.org/10.1016/j.bios.2014.05.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A new room temperature gas sensor based on pigment-sensitized TiO2 thin
1
film for amines determination
2
Li Yanxiaoa
3 4
Mel Holmesb a
5 6 7 8 9
Zou Xiao-boa * Huang Xiao-weia
Shi Ji-yonga
Zhao Jie-wena ,
Limin Haoc
School of Food and Biological Engineering, Jiangsu university, 301 Xuefu Rd.,
212013 Zhenjiang, Jiangsu, China b
School of Food Science and Nutrition, University of Leeds, Leeds LS2 9JT,
United Kingdom c
The Research center of China Hemp Materials, Beijing, China
10 11
*Corresponding author. Prof. Zou Xiao-bo
12
Tel: +86 511 88780085; Fax: +86 511 88780201
13
Email address: [email protected]
14
Abstract᧶
15
A new room temperature gas sensor was fabricated with pigment-sensitized TiO2
16
thin film as the sensing layer. Four natural pigments were extracted from spinach
17
(Spinacia oleracea), red radish (Raphanus sativus L), winter jiasmine (Jasminum
18
nudiflorum), and black rice (Oryza sativa L. indica) by ethanol. Natural
19
pigment-sensitized TiO2 sensor was prepared by immersing porous TiO2 films in an
20
ethanol solution containing a natural pigment for 24 h. The hybrid organic-inorganic
21
formed films here were firstly exposed to atmospheres containing methylamine
22
vapours with concentrations over the range 2–10 ppm at room temperature. The films
23
sensitized by the pigments from black-rice showed an excellent gas-sensitivity to 1
24
methylamine among the four natural pigments sensitized films thanking to the
25
anthocyanins. The relative change resistance, S , of the films increased almost linearly
26
with increasing concentrations of methylamine (r=0.931). At last, the black rice
27
pigment sensitized TiO2 thin film was used to determine the biogenic amines
28
generated by pork during storage. The developed films had good sensitivity to
29
analogous gases such as putrscine, and cadaverine that will increase during storage.
30
Keywords: Titanium dioxide film; nature pigment; sensitize; amine; anthocyanin
31
1. Introduction
32
Amines are generally biologically active and even toxic compounds (Raible et al.
33
2005) as such the need to determine amine concentrations has increased during the
34
last few years. Often amines have characteristic odors and high doses of biogenic
35
amines can promote uncontrolled reactions in the human body and can, therefore, lead
36
to health problems, such as cancer, strokes or other diseases (Suzzi and Gardini 2003;
37
Tang et al. 2011). In the past decade, many kinds of amine sensors have been
38
developed based on different sensing mechanisms. These include electrical (Liao et al.
39
2010; Pacquit et al. 2006; Xia et al. 2011; Zamani et al. 2009), mass (Lu et al. 2009;
40
Raible et al. 2005; Wang et al. 2002), or optical-based (Alimelli et al. 2007; Kang and
41
Meyerhoff 2006; Liu and Lu 2007; Moradian et al. 2000) methods.
42
Titanium dioxide (TiO2) is a semiconductor material that has received a lot of
43
attention recently as a thin film sensor for gas detection (Bisquert et al. 2008;
44
Karunagaran et al. 2007; Mor et al. 2006). It has been established that the sensing
45
properties of the films are improved by decreasing the grain size of the TiO2 so as to
46
increase the surface to volume ratio of the films (Xiao-wei et al. 2013; Zhu et al. 2
47
2007). Films based on TiO2 have been used to detect amines (Bernacka-Wojcik et al.
48
2010; Boscornea et al. 2001; Moradian et al. 2000; Persad et al. 2008; Raible et al.
49
2005; Schweizer-Berberich et al. 1994; Suska et al. 2009). The TiO2 sensing films
50
described so far operate at high temperatures (250-450ഒ) (Karunagaran et al. 2007,
51
Bernacka-Wojcik et al. 2010) or use rear metals films. Maintenance of sensor at high
52
temperature increases power consumption, reduces sensor life and complicates the
53
design of the sensor due to need for integration of heater and temperature sensors with
54
gas sensing film (Sen et al. 2004). Some rear metals films, such as tellurium (Sen et al.
55
2004) films, could be use as amines gas sensors operable at room temperature. The
56
use of rare metal is expensive and difficult for industry produce. Thus, there is a need
57
for ammonia sensors which are operable at room temperature, and easy producible.
58
Nanocrystalline TiO2 films sensitized with pigments were first used in solar cells.
59
By light excitation, the pigment absorbs photons and injects electrons into the
60
semiconductor’s conduction band. Similarly, under an amines gas condition, the
61
pigment absorbs electrons from the amines and injects electrons into the
62
semiconductor’s conduction band. Therefore, the conductivity of the TiO2 films
63
correlate with the concentration of the amines. Natural pigments of the flavonoid class,
64
found in leaves and fruits and responsible for the colours of various vegetal tissues
65
have been studied for application as sensitizers of solar cells (Bruder et al. 2009;
66
Cheng et al. 2012; Huang et al. 2014; Lee et al. 2011; Reijnders 2010). Due to their
67
cost efficiency, non-toxicity and complete biodegradation, natural pigments have been
68
a popular subject of research. The use of natural pigments to sensitize nanocrystalline
69
TiO2 film offers promising prospects for the advance of this technology in amines 3
70
detection.
71
China is the world's biggest market for pork in terms of production as well as
72
consumption(Huang et al. 2014). Clearly, a major concern for consumers is the quality
73
and safety of the product. Studies show that biogenic amines such as cadaverine,
74
tryptamine, putrescine, and tyramine, are significantly related to traditional quality
75
indices (e.g. total aerobic bacterial counts, pH, and TVBN) (Hernández-Jover et al.
76
1996; Suzzi and Gardini 2003) for pork. The need to determine pork-borne amines
77
has increased during the last few years.
78
In this study, we report on the preparation of novel gas sensors based on thin
79
TiO2 films sensitized with natural pigments. The hybrid organic-inorganic films
80
formed here firstly were used to detect methylamine reversibly at room temperature.
81
Then the films were used to monitoring the freshness of pork during storage.
82
2. Experimental
83
2.1. Film preparation
84
The preparation of films consists of four steps as shown in Fig. 1.
85
(1) Rinsing of the microscope glass slides (step 1)
86
The microscope glass slides were boiled in 200 ml H2SO4 and H2O2 solution
87
with the volume ratio 1:3 for 30 min in order to eliminate the ions from the surface of
88
the matrix which are deleterious to the sensor activity. The substrates were then
89
consecutively rinsed in acetone and ethanol in an ultrasonic bath for 15 min. Two
90
counter gold comb electrodes were prepared on the glass by sputtering as shown in
91
Fig. 1. Finally the microscope glass slides were cleaned by de-ionized water and dried
92
by a N2 gas gun. 4
Fig. 1
93 94
(2) Preparation and characterization of TiO2 thin films (step 2)
95
Thin titanium oxide films were deposited onto the glass substrates with a
96
sputtered-Au counter electrode using a home built DC magnetron system. 99.999%
97
pure titanium of 100 mm diameter and 6 mm thickness has been used as the sputtering
98
target. Prior to the introduction of the sputtering gas, the vacuum of the chamber was
99
evacuated to lower than 8×104 Pa. Sputtering pressure was kept at 5.0 Pa. The flow
100
rates of Ar (99.999%) and O2 (99.999%) were kept at constant values of 44.9 and 10
101
sccm (standard-state cubic centimeter per minute), respectively. The discharges
102
were generated at a constant power of 300 W. Initially the Ti target was pre-sputtered
103
in an argon atmosphere of 2.0 Pa in order to remove the surface oxide layer. When the
104
discharge colour changes from pink to blue, it indicates that the oxide layer has been
105
removed from the target surface. Next, oxygen and argon were introduced into the
106
vacuum chamber and the sputtering process commenced. The temperature of the
107
substrate was maintained at 200°C and the deposition time was 10 h. The thickness of
108
the films was monitored by a stylus profiler (Alpha-step 500). The adherence strength
109
of the TiO2 thin film on the microscope glass slide was monitored by a coating
110
adherence tester (CSM micro-Combi Tester) using a technique published in (Dutta et
111
al. 2011).
112
(3) Preparation of natural pigments (step 3)
113
The pigments from spinach (Spinacia oleracea), red radish (Raphanus sativus
114
L), winter jiasmine (Jasminum nudiflorum), and black rice (Oryza sativa L. indica)
115
were extracted with ethanol by the following steps: spinach (Spinacia oleracea), red 5
116
radish (Raphanus sativus L), winter jiasmine (Jasminum nudiflorum), and black rice
117
(Oryza sativa L. indica) were washed with water and vacuum dried at 60 °C. After
118
crushing into fine powder using a mortar and pestle, these materials were immersed in
119
absolute ethanol at room temperature in the dark for one week. Then the solids were
120
filtrated out, and the filtrates were concentrated at 40 °C for use.
121
(4) Preparation of pigments-sensitized TiO2 thin films (step 4)
122
The TiO2 film was solidified and sintered by heating the glass sheet at 450 °C in
123
air for 30 min, and then cooled to around 80 °C. The glass solidified TiO2 was
124
immersed in a natural pigment alcohol solution for 24 h. The other impurities were
125
washed up with anhydrous ethanol and dried in moisture-free air. Finally, a
126
pigment-sensitized TiO2 gas sensor was prepared.
127
2.2 Characterization solution and films
128
The adsorption spectrum and band gap energies of the samples were determined
129
by using a UV-2450 UV–VIS spectrophotometer (Shimadzu Corporation, Japan)
130
equipped with diffuse reflectance accessory (DRA). This technique allows the study
131
of the reflectance spectra of the samples in the solid form. The crystalline structure of
132
the films was measured by X-ray diffraction (XRD) equipment (Model D/max 2550V,
133
Rigaku Co. Tokyo, Japan), using Cu K ( = 1.5406 Å) radiation. The broadening of
134
XRD peak at 2 = 25.4° (d1 0 1) for anatase TiO2, was used to calculate the crystallite
135
size according to the well-known Scherrer equation. The surface morphology of the
136
fabricated films was examined using a scanning electron microscope (SEM: S-3000N,
137
Hitachi, Japan) on gold-coated specimens.
138 6
139
2.3 Measurement of the sensor in response to methylamine
140
The electrical current of the film was measured by an Agilent digital electrometer
141
(34401A) (Fig. 1). The response of the sensor is defined as the relative change of the
142
electrical current of the films in ambient air and in the analytes:
143
S = abs (Ig Io)/Io × 100% (1)
144
where Ig is the electrical current of the sensor in methylamine with air
145
background and Io is the electrical current in air.
146
The devices were mounted in a specially constructed glass chamber where a
147
carrier gas (air) containing known concentrations of the methylamine vapours was
148
passed over the films. The concentration of the methylamine vapours could be varied
149
rapidly for response time measurements. All the measurements were made at room
150
temperature and under normal atmospheric pressure. As shown in Fig. 2, the carrier
151
gases with known concentrations of the methylamine vapours were produced in two
152
stages. In the first stage, air gas at a flow rate of 100 ml/min was passed over a
153
permeation vial containing a known quantity of methylamine and held at a constant
154
temperature of 25°C. The concentration of the methylamine vapour (C1) in the stream
155
of air with a flow rate of F1 was calculated using a technique published in (Choi and
156
Hawkins 1997; Mabrook and Hawkins 2001; Namiesik 1984): PU F1
157
C1
158
where P is the permeation rate and the reciprocal vapour density of the
159
methylamine . The permeation rate was calculated by determining the liquid volume
160
lost from the vial over a known time period. The concentration of the vapours in the
(2)
7
161
air stream during this stage was determined to be 10 ppm. In the second stage, the air
162
flowing from the vial was mixed with a second air stream and a range of
163
concentrations between 0.1 and 10 ppm obtained. The resulting concentration was
164
calculated from (Namiesik 1984): C1 F1 F1 F2
(3)
165
C
166
where C is the concentration of the measured gas and F2 the flow rate of the
167
dilution air stream. Fig. 2
168 169
2.4 Pork measurement
170
2.4.1 Pork meat.
171
A piece of meat (pork shoulder) of ca. 1 kg was purchased from a commercial
172
plant (Danyang Meat Processing Corp., Zhenjiang, China), and placed in a sealed ice
173
container. Aseptic techniques such as the use of disposable gloves, bactericide built-in
174
cutting board and flame sterilized scalpel were used to avoid sample contamination.
175
31 tissue samples of 20mm×40 mm×40 mm (length × width × thickness) were
176
prepared, and placed in zip lock freezer bags. Samples were kept at 5 °C for 7 days.
177
Chemical determinations and sensor measurements were performed on the samples
178
and were taken at each day of storage. During the sensor measurements, the freezer
179
bags were opened in the thermostated permeation chamber (Fig. 2) for 30 minutes to
180
strengthen the aromatic concentration.
181 182
2.4.2 Biogenic amine determination. Biogenic amines [spermine (SM), putrescine (PU), cadaverine (CA), and
8
183
spermidine (SD)] were determined by a liquid chromatographic method as described
184
by Hernández-Jover et al. (Hernández-Jover et al. 1996). The method involves the
185
separation of ion pairs formed between biogenic amines and octanesulfonic acid, a
186
postcolumn derivatization with o-phthalaldehyde (OPT), and spectrofluorometric
187
detection. All reagents were analytical grade except HPLC reagents that were LC
188
grade. Biogenic amine standards were purchased from Sigma Chemical (Shanghai,
189
China).
190
3. Results and discussion
191
3.1 Characterization of natural pigment
192
Fig. 3
193
Fig. 3A shows the UV–VIS absorption spectra of spinach spinach (Spinacia
194
oleracea), red radish (Raphanus sativus L), winter jiasmine (Jasminum nudiflorum),
195
and black rice (Oryza sativa L. indica) extracts in alcohol. From Fig. 3A, it can be
196
seen that there is the same absorbance peak at approximately 525 nm for the extracts
197
of black rice, and red radish, which is ascribed to the same pigment cores
198
(anthocyanins) contained in the two extracts. Anthocyanins are the core compositions
199
of some natural pigments and is often found in fruits, owers and the leaves of plants,
200
because anthocyanins shows the colour in the range of visible light from red to blue.
201
The molecular structure of anthocyanins is shown in Fig. 3 C. Spinach (Spinacia
202
oleracea) and Jiasmine (Jasminum nudiflorum) show green and yellow colour
203
pigments, their extracts also exhibiting similar colours. From Fig.3 A, it can be seen
204
that the characteristic absorption peak of chlorophyll with wavelength approximately
205
660 nm and a characteristic absorption peak of carotenoid with wavelength at 455 nm 9
206
appear in the extract of spinach (Spinacia oleracea) and jiasmine (Jasminum
207
nudiflorum).
208
3.2 Characterization of TiO2 thin film
209
The thickness of the films was ca. 0.5 m monitored by an -step surface profiler.
210
The adherence strength of the TiO2 thin film on the microscope glass slide as
211
deposited was 9.5 N monitored by a coating adherence tester. The microstructure and
212
the morphology of films were examined using a SEM as shown in Fig. S1 (a) and (b).
213
Fig. S1a clearly show that the surface is composed of faceted polygonal particles or
214
grains (‘crystallites’) of 50–150 nm in size. A cross-sectional cut of the sample, Fig.
215
S1b, shows that the film has a dense columnar nano-structure. This agrees with the
216
film being polycrystalline (Kluth et al. 2003). There are gaps or fissures between the
217
individual grains. The top surface has a very rough or protruding appearance. The
218
crystalline microstructure of the thin films was characterized using XRD. Fig. S1c
219
shows the XRD spectra of TiO2 thin films annealed at different temperatures (as
220
deposited, 450°C) in air for 5 h. The TiO2 thin films still remained in a complete
221
anatase phase. From Fig. S1c, we can see increased crystallinity of the TiO2 thin film
222
annealed at 450°C. This occurs because the (1 0 1) plane of anatase films are the most
223
exposed face in the nanocrystal structure (Zheng et al. 2001).
224
Fig. 3 B illustrates the absorption spectra of black rice extract in ethanol solution
225
and on TiO2 film. The peak wavelength absorption of black rice extract on TiO2 film
226
(around 590nm) is greatly red-shift, as compared with the extract in ethanol solution
227
(around 525 nm). Anthocyanins (structural formula given in Fig. 3C, D) are strongly
228
adsorbed on TiO2 as a result of complexation with TiIV ions on the surface. Surface 10
229
complexation can readily occur via elimination of a proton (Fig. 3D). The shift to the
230
infrared region at the absorption maxima of the pigment sensitized film confirmed the
231
reaction between the TiO2 and the anthocyanins, since the interaction caused a
232
reduction of the electron density in the chromophore group, thus reducing its polarity
233
(bathochromic effect). These observations are in agreement with values reported in
234
natural pigment sensitized solar cells (Calogero and Marco 2008; Gómez-Ortíz et al.
235
2010; Hao et al. 2006; Zhou et al. 2011).
236
The alkyl group rather than carboxyl group or hydroxyl group on the pigment
237
molecule cannot form a chemical bond with a TiO2 porous film. In addition, strong
238
steric hindrance of long chain alkane of chlorophyll and carotenoid also prevent the
239
pigment molecules from arraying on TiO2 films efficiently. Due to these two factors,
240
the extracts of spinach and jiasmine are poorly absorbed onto the TiO2 film, and the
241
sensitizing effect to the TiO2 film is low, which is in accordance with our
242
experimental results.
243 244
3.3 Alternative natural pigment sensitized TiO2 thin film responses to methylamine
245
The response characteristics of sensors to volatile organic amines were first
246
tested by applying methylamine vapour. Ambient conditions were simulated by
247
adjusting the relative humidity (R.H.) to 40% at room temperature. The response of
248
TiO2 thin films sensitized with pigment extracts from spinach (Spinacia oleracea),
249
winter jiasmine (Jasminum nudiflorum) , red radish (Raphanus sativus L), and black
250
rice (Oryza sativa L. indica)
251
response of dyes free TiO2 film was nearly zero. This behaviour can be explained with
were shown in Table 1. As control experiments, the
11
252
the analogy to that of the mechanism of gas adsorption and desorption on TiO2 film
253
as described by Karunagaran et al (2007). High response values (S%) are obtained
254
from the sensor sensitized by the natural pigment extracts of black rice and red radish
255
which contain higher anthocyanins. Black rice extracts, which has the highest
256
anthocyanins content, generated the highest response values among the four organic
257
materials (Table 1). The lower response values for the spinach and winter jiasmine
258
which ascribes to weak bonds between their chlorophyll, -carotene molecule and
259
TiO2 film is inconsistent with the above discussions. Although natural pigment
260
chlorophyll play a key role in the photosynthesis ability in the plant body, their
261
functional capabilities do not translate into a good sensitizing compound because of
262
the lack of available bonds between the pigments molecules and TiO2 film through
263
which electrons can transport from excited pigment molecules to the TiO2 film.
264
Obviously, as a sensitizing compound, the interaction and bond between sensitizing
265
compound (pigment) and TiO2 film is very important in enhancing the gas sensitivity
266
of the thin film.
267
3.4 Black rice pigment sensitized TiO2 thin film response to methylamine
268
Fig. 4
269
Fig 4 A. shows the response of the pigment sensitized TiO2 film based sensor to
270
different concentrations of methylamine in the background of ambient air with R.H.
271
ฏ40% at room temperature. The resistance (R0) of the sensor after manufacture in air
272
is 5.58 M Ohm. The sensors indicated responses to methylamine within a
273
concentration range from 2 to 10 ppm in steps of 2 ppm. After each increase in
274
concentration of the vapours, the glass sample chamber was flushed with air. The 12
275
resistance of the TiO2 film decreased with exposure to methylamine (Fig. 4A). This
276
analysis was carried out three times: the sensor exhibited a good stability, due to the
277
fact that the curves referred to the different cycles overlapped between them. The
278
calibration curves for the films were obtained by plotting the relative variation, S, of
279
the sensor resistance against the concentration of the vapours in the carrier gas. Fig. 4
280
B shows the changes in S for a film exposed to different concentrations of
281
methylamine. The relative resistance of the sensor increased almost linearly with
282
increasing concentrations of methylamine (r = 0.931) over the concentration range
283
2–10 ppm. This linearity change of the films also indicates that the gold layers make
284
good Ohmic contacts with the films. Even if metal–semiconductor Schottky-type
285
junctions are formed then they are of no consequence in the sensors as they are well
286
away from where the sensing of the vapours occurs on the surface of the films.
287
The response time, defined as the time taken for the sensor's resistance to reach
288
the 90% of the steady-state resistance, is around 200-240 s. Likewise, the recovery
289
time represents the time required by the sensitivity factor to return to 10% below its
290
equilibrium value in air following the zeroing of the test gas methylamine and it was
291
found to be around 260-290 s. The response and recovery times were not as high as
292
those reported in the literature (Joshi et al. 2011)and (Capone et al. 2000), indicating
293
that molecules of amines are weakly attached to the sensing materials, or previously
294
adsorbed methylamine molecules. These results indicate that anthocyanins are
295
responsible for the sensing action. However, the sensors appear to have some form of
296
drift. This is mainly caused by the same response and recovery times set in the cycles.
297
In the dynamic flow changes, there is little methylamine remain in the chamber or on 13
298
the film when the air flux is restored after the gas test. This caused the incomplete
299
recovery of the sensor.
300
The sensing mechanism for the metal oxides working in the humidified air at
301
room temperatures (usually 0–30 °C) is often proposed to be related to the electrolytic
302
dissociations of the gas species in the adsorbed water molecules covered on the
303
surface of the metal oxides (Helwig et al. 2009; Ostrick et al. 1999). The formation of
304
a thin water film supports the diffusion of methylamine into the interfacial region by
305
dissolving the analyte. Furthermore, the ionized species in the water molecule would
306
ensure a change in the pH value on the oxide surface and thus change the
307
electrons/holes densities at the conduction or valence bands of the oxide
308
semiconductor, consequently, this would result in the sensor signals observed (Ostrick
309
et al. 1999). However, as pointed out by Helwig et al. (Helwig et al. 2009), this
310
“dissociative gas sensing mechanism” requires that the analyte gases should have
311
good water solubility and be easily ionized in water. Moreover, the sensor signals
312
induced by such type of sensing mechanism are usually small. The interactions of the
313
amines with the electrode material could be different in the presence of natural
314
pigments and water and should be considered:
315
Anthocyanin + RNH2(g) l Anthocyanin- + RNH3+
(4)
316
Anthocyanin + H2O(g) l Anthocyanin- + H3O+
(5)
317
RNH2(g) + H3O+ l RNH3+ + H2O(g)
(6)
318
where Anthocyanin- is anion of the Anthocyanin. Relatively low humidity is
319
necessary for sensing amines by the anthocyanin functionalized TiO2 film because the
320
anthocyanin is soluble in water. 14
321
An influence of the humidity on the clean air resistance of our samples is
322
illustrated in Fig. 4 C. The resistance decreased significantly if the R.H. increased
323
from 20% to 90%. The low humidity is necessary to ionized anthocyanin molecules
324
adsorbed on the surface of TiO2. Firstly, at room temperature, amines solves very well
325
in water under formation of RNH3+ and OH- ions. A similar reaction of methylamine
326
may occur on the sample surface with the adsorbed water. Secondly, at low humidity,
327
amines could be ionized by the anthocyanins on the TiO2 film and thereby decrease in
328
the proton concentration at the surface. Consequently, more positively charged holes
329
at the surface of the TiO2 have to be extracted to neutralize the adsorbed anthocyanins
330
and water film. However the presence of water promotes desorption of the pigment
331
from TiO2 film which is undesirable. High humidity in the flow cell (above 50%)
332
could lower the efficiency of the composite sensor.
333
Temperature tests of the bioamines gas sensors were performed with mini-heater.
334
The temperature was fixed at some selected values within the interval from 10 to
335
50°C. The dependencies were obtained from the measurements of the resistance
336
response to methylamine gas (R.H. 40%, 10 ppm of methylamine). The resistance
337
response to methylamine was slightly decreased on temperature from 10 to 50°C,as
338
shown in Fig. 4 D. The solubility of methylamine in water decreases as well as the
339
amount of water itself on the surface. Therefore, the amount of methylamine which is
340
present at the film and which is able to react with adsorption sites is reduced and the
341
sensitivity decreases (Ostrick et al. 2000). Long-term aging tests of the bioamines gas
342
sensors have been performed. The aging tests were performed in the dark and in air.
343
After the sensor element was stored in a container at room temperature over for 4 15
344
weeks, the response to 2 ppm methylamine was statistically indistinguishable from
345
that prior to the storage period.
346
3.5 Black rice pigment sensitized TiO2 thin film monitoring pork quality
347
Fig 5.
348
Fig. 5A shows that the amine content changes during 7 days storage of pork meat
349
at 4-6 °C. Spermidine remained constant, spermine slightly decreased, and the
350
formation of putrescine and cadaverine occurs. In particular, a continuous increase in
351
putrescine and cadaverine was observed with increasing storage days. These
352
observations are in agreement with values reported by other authors for fresh pork
353
meat (Hernández-Jover et al. 1996).
354
pigment sensitized TiO2 thin film to pork meat during storage. The response values
355
show an increase after 3 days. A rapid increase in the total amount of putrescine and
356
cadaverine was accompanied by parallel increases the response of gas sensors during
357
the storage days. A linear regression model (S%=0.289T+0.12) was build between the
358
response values of sensor and the sum content of cadaverine and putrescine during the
359
storage days as shown in Fig 5C. The correlation coefficients is 0.942 (p
A new room temperature gas sensor based on pigment-sensitized TiO2 thin film for amines determination Li Yanxiao, Zou Xiao-bo, Huang Xiao-wei, Shi Ji-yong, Zhao Jie-wen, Mel Holmes, Limin Hao
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PII: DOI: Reference:
S0956-5663(14)00373-X http://dx.doi.org/10.1016/j.bios.2014.05.040 BIOS6802
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Biosensors and Bioelectronics
Cite this article as: Li Yanxiao, Zou Xiao-bo, Huang Xiao-wei, Shi Ji-yong, Zhao Jie-wen, Mel Holmes, Limin Hao, A new room temperature gas sensor based on pigment-sensitized TiO2 thin film for amines determination, Biosensors and Bioelectronics, http://dx.doi.org/10.1016/j.bios.2014.05.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A new room temperature gas sensor based on pigment-sensitized TiO2 thin
1
film for amines determination
2
Li Yanxiaoa
3 4
Mel Holmesb a
5 6 7 8 9
Zou Xiao-boa * Huang Xiao-weia
Shi Ji-yonga
Zhao Jie-wena ,
Limin Haoc
School of Food and Biological Engineering, Jiangsu university, 301 Xuefu Rd.,
212013 Zhenjiang, Jiangsu, China b
School of Food Science and Nutrition, University of Leeds, Leeds LS2 9JT,
United Kingdom c
The Research center of China Hemp Materials, Beijing, China
10 11
*Corresponding author. Prof. Zou Xiao-bo
12
Tel: +86 511 88780085; Fax: +86 511 88780201
13
Email address: [email protected]
14
Abstract᧶
15
A new room temperature gas sensor was fabricated with pigment-sensitized TiO2
16
thin film as the sensing layer. Four natural pigments were extracted from spinach
17
(Spinacia oleracea), red radish (Raphanus sativus L), winter jiasmine (Jasminum
18
nudiflorum), and black rice (Oryza sativa L. indica) by ethanol. Natural
19
pigment-sensitized TiO2 sensor was prepared by immersing porous TiO2 films in an
20
ethanol solution containing a natural pigment for 24 h. The hybrid organic-inorganic
21
formed films here were firstly exposed to atmospheres containing methylamine
22
vapours with concentrations over the range 2–10 ppm at room temperature. The films
23
sensitized by the pigments from black-rice showed an excellent gas-sensitivity to 1
24
methylamine among the four natural pigments sensitized films thanking to the
25
anthocyanins. The relative change resistance, S , of the films increased almost linearly
26
with increasing concentrations of methylamine (r=0.931). At last, the black rice
27
pigment sensitized TiO2 thin film was used to determine the biogenic amines
28
generated by pork during storage. The developed films had good sensitivity to
29
analogous gases such as putrscine, and cadaverine that will increase during storage.
30
Keywords: Titanium dioxide film; nature pigment; sensitize; amine; anthocyanin
31
1. Introduction
32
Amines are generally biologically active and even toxic compounds (Raible et al.
33
2005) as such the need to determine amine concentrations has increased during the
34
last few years. Often amines have characteristic odors and high doses of biogenic
35
amines can promote uncontrolled reactions in the human body and can, therefore, lead
36
to health problems, such as cancer, strokes or other diseases (Suzzi and Gardini 2003;
37
Tang et al. 2011). In the past decade, many kinds of amine sensors have been
38
developed based on different sensing mechanisms. These include electrical (Liao et al.
39
2010; Pacquit et al. 2006; Xia et al. 2011; Zamani et al. 2009), mass (Lu et al. 2009;
40
Raible et al. 2005; Wang et al. 2002), or optical-based (Alimelli et al. 2007; Kang and
41
Meyerhoff 2006; Liu and Lu 2007; Moradian et al. 2000) methods.
42
Titanium dioxide (TiO2) is a semiconductor material that has received a lot of
43
attention recently as a thin film sensor for gas detection (Bisquert et al. 2008;
44
Karunagaran et al. 2007; Mor et al. 2006). It has been established that the sensing
45
properties of the films are improved by decreasing the grain size of the TiO2 so as to
46
increase the surface to volume ratio of the films (Xiao-wei et al. 2013; Zhu et al. 2
47
2007). Films based on TiO2 have been used to detect amines (Bernacka-Wojcik et al.
48
2010; Boscornea et al. 2001; Moradian et al. 2000; Persad et al. 2008; Raible et al.
49
2005; Schweizer-Berberich et al. 1994; Suska et al. 2009). The TiO2 sensing films
50
described so far operate at high temperatures (250-450ഒ) (Karunagaran et al. 2007,
51
Bernacka-Wojcik et al. 2010) or use rear metals films. Maintenance of sensor at high
52
temperature increases power consumption, reduces sensor life and complicates the
53
design of the sensor due to need for integration of heater and temperature sensors with
54
gas sensing film (Sen et al. 2004). Some rear metals films, such as tellurium (Sen et al.
55
2004) films, could be use as amines gas sensors operable at room temperature. The
56
use of rare metal is expensive and difficult for industry produce. Thus, there is a need
57
for ammonia sensors which are operable at room temperature, and easy producible.
58
Nanocrystalline TiO2 films sensitized with pigments were first used in solar cells.
59
By light excitation, the pigment absorbs photons and injects electrons into the
60
semiconductor’s conduction band. Similarly, under an amines gas condition, the
61
pigment absorbs electrons from the amines and injects electrons into the
62
semiconductor’s conduction band. Therefore, the conductivity of the TiO2 films
63
correlate with the concentration of the amines. Natural pigments of the flavonoid class,
64
found in leaves and fruits and responsible for the colours of various vegetal tissues
65
have been studied for application as sensitizers of solar cells (Bruder et al. 2009;
66
Cheng et al. 2012; Huang et al. 2014; Lee et al. 2011; Reijnders 2010). Due to their
67
cost efficiency, non-toxicity and complete biodegradation, natural pigments have been
68
a popular subject of research. The use of natural pigments to sensitize nanocrystalline
69
TiO2 film offers promising prospects for the advance of this technology in amines 3
70
detection.
71
China is the world's biggest market for pork in terms of production as well as
72
consumption(Huang et al. 2014). Clearly, a major concern for consumers is the quality
73
and safety of the product. Studies show that biogenic amines such as cadaverine,
74
tryptamine, putrescine, and tyramine, are significantly related to traditional quality
75
indices (e.g. total aerobic bacterial counts, pH, and TVBN) (Hernández-Jover et al.
76
1996; Suzzi and Gardini 2003) for pork. The need to determine pork-borne amines
77
has increased during the last few years.
78
In this study, we report on the preparation of novel gas sensors based on thin
79
TiO2 films sensitized with natural pigments. The hybrid organic-inorganic films
80
formed here firstly were used to detect methylamine reversibly at room temperature.
81
Then the films were used to monitoring the freshness of pork during storage.
82
2. Experimental
83
2.1. Film preparation
84
The preparation of films consists of four steps as shown in Fig. 1.
85
(1) Rinsing of the microscope glass slides (step 1)
86
The microscope glass slides were boiled in 200 ml H2SO4 and H2O2 solution
87
with the volume ratio 1:3 for 30 min in order to eliminate the ions from the surface of
88
the matrix which are deleterious to the sensor activity. The substrates were then
89
consecutively rinsed in acetone and ethanol in an ultrasonic bath for 15 min. Two
90
counter gold comb electrodes were prepared on the glass by sputtering as shown in
91
Fig. 1. Finally the microscope glass slides were cleaned by de-ionized water and dried
92
by a N2 gas gun. 4
Fig. 1
93 94
(2) Preparation and characterization of TiO2 thin films (step 2)
95
Thin titanium oxide films were deposited onto the glass substrates with a
96
sputtered-Au counter electrode using a home built DC magnetron system. 99.999%
97
pure titanium of 100 mm diameter and 6 mm thickness has been used as the sputtering
98
target. Prior to the introduction of the sputtering gas, the vacuum of the chamber was
99
evacuated to lower than 8×104 Pa. Sputtering pressure was kept at 5.0 Pa. The flow
100
rates of Ar (99.999%) and O2 (99.999%) were kept at constant values of 44.9 and 10
101
sccm (standard-state cubic centimeter per minute), respectively. The discharges
102
were generated at a constant power of 300 W. Initially the Ti target was pre-sputtered
103
in an argon atmosphere of 2.0 Pa in order to remove the surface oxide layer. When the
104
discharge colour changes from pink to blue, it indicates that the oxide layer has been
105
removed from the target surface. Next, oxygen and argon were introduced into the
106
vacuum chamber and the sputtering process commenced. The temperature of the
107
substrate was maintained at 200°C and the deposition time was 10 h. The thickness of
108
the films was monitored by a stylus profiler (Alpha-step 500). The adherence strength
109
of the TiO2 thin film on the microscope glass slide was monitored by a coating
110
adherence tester (CSM micro-Combi Tester) using a technique published in (Dutta et
111
al. 2011).
112
(3) Preparation of natural pigments (step 3)
113
The pigments from spinach (Spinacia oleracea), red radish (Raphanus sativus
114
L), winter jiasmine (Jasminum nudiflorum), and black rice (Oryza sativa L. indica)
115
were extracted with ethanol by the following steps: spinach (Spinacia oleracea), red 5
116
radish (Raphanus sativus L), winter jiasmine (Jasminum nudiflorum), and black rice
117
(Oryza sativa L. indica) were washed with water and vacuum dried at 60 °C. After
118
crushing into fine powder using a mortar and pestle, these materials were immersed in
119
absolute ethanol at room temperature in the dark for one week. Then the solids were
120
filtrated out, and the filtrates were concentrated at 40 °C for use.
121
(4) Preparation of pigments-sensitized TiO2 thin films (step 4)
122
The TiO2 film was solidified and sintered by heating the glass sheet at 450 °C in
123
air for 30 min, and then cooled to around 80 °C. The glass solidified TiO2 was
124
immersed in a natural pigment alcohol solution for 24 h. The other impurities were
125
washed up with anhydrous ethanol and dried in moisture-free air. Finally, a
126
pigment-sensitized TiO2 gas sensor was prepared.
127
2.2 Characterization solution and films
128
The adsorption spectrum and band gap energies of the samples were determined
129
by using a UV-2450 UV–VIS spectrophotometer (Shimadzu Corporation, Japan)
130
equipped with diffuse reflectance accessory (DRA). This technique allows the study
131
of the reflectance spectra of the samples in the solid form. The crystalline structure of
132
the films was measured by X-ray diffraction (XRD) equipment (Model D/max 2550V,
133
Rigaku Co. Tokyo, Japan), using Cu K ( = 1.5406 Å) radiation. The broadening of
134
XRD peak at 2 = 25.4° (d1 0 1) for anatase TiO2, was used to calculate the crystallite
135
size according to the well-known Scherrer equation. The surface morphology of the
136
fabricated films was examined using a scanning electron microscope (SEM: S-3000N,
137
Hitachi, Japan) on gold-coated specimens.
138 6
139
2.3 Measurement of the sensor in response to methylamine
140
The electrical current of the film was measured by an Agilent digital electrometer
141
(34401A) (Fig. 1). The response of the sensor is defined as the relative change of the
142
electrical current of the films in ambient air and in the analytes:
143
S = abs (Ig Io)/Io × 100% (1)
144
where Ig is the electrical current of the sensor in methylamine with air
145
background and Io is the electrical current in air.
146
The devices were mounted in a specially constructed glass chamber where a
147
carrier gas (air) containing known concentrations of the methylamine vapours was
148
passed over the films. The concentration of the methylamine vapours could be varied
149
rapidly for response time measurements. All the measurements were made at room
150
temperature and under normal atmospheric pressure. As shown in Fig. 2, the carrier
151
gases with known concentrations of the methylamine vapours were produced in two
152
stages. In the first stage, air gas at a flow rate of 100 ml/min was passed over a
153
permeation vial containing a known quantity of methylamine and held at a constant
154
temperature of 25°C. The concentration of the methylamine vapour (C1) in the stream
155
of air with a flow rate of F1 was calculated using a technique published in (Choi and
156
Hawkins 1997; Mabrook and Hawkins 2001; Namiesik 1984): PU F1
157
C1
158
where P is the permeation rate and the reciprocal vapour density of the
159
methylamine . The permeation rate was calculated by determining the liquid volume
160
lost from the vial over a known time period. The concentration of the vapours in the
(2)
7
161
air stream during this stage was determined to be 10 ppm. In the second stage, the air
162
flowing from the vial was mixed with a second air stream and a range of
163
concentrations between 0.1 and 10 ppm obtained. The resulting concentration was
164
calculated from (Namiesik 1984): C1 F1 F1 F2
(3)
165
C
166
where C is the concentration of the measured gas and F2 the flow rate of the
167
dilution air stream. Fig. 2
168 169
2.4 Pork measurement
170
2.4.1 Pork meat.
171
A piece of meat (pork shoulder) of ca. 1 kg was purchased from a commercial
172
plant (Danyang Meat Processing Corp., Zhenjiang, China), and placed in a sealed ice
173
container. Aseptic techniques such as the use of disposable gloves, bactericide built-in
174
cutting board and flame sterilized scalpel were used to avoid sample contamination.
175
31 tissue samples of 20mm×40 mm×40 mm (length × width × thickness) were
176
prepared, and placed in zip lock freezer bags. Samples were kept at 5 °C for 7 days.
177
Chemical determinations and sensor measurements were performed on the samples
178
and were taken at each day of storage. During the sensor measurements, the freezer
179
bags were opened in the thermostated permeation chamber (Fig. 2) for 30 minutes to
180
strengthen the aromatic concentration.
181 182
2.4.2 Biogenic amine determination. Biogenic amines [spermine (SM), putrescine (PU), cadaverine (CA), and
8
183
spermidine (SD)] were determined by a liquid chromatographic method as described
184
by Hernández-Jover et al. (Hernández-Jover et al. 1996). The method involves the
185
separation of ion pairs formed between biogenic amines and octanesulfonic acid, a
186
postcolumn derivatization with o-phthalaldehyde (OPT), and spectrofluorometric
187
detection. All reagents were analytical grade except HPLC reagents that were LC
188
grade. Biogenic amine standards were purchased from Sigma Chemical (Shanghai,
189
China).
190
3. Results and discussion
191
3.1 Characterization of natural pigment
192
Fig. 3
193
Fig. 3A shows the UV–VIS absorption spectra of spinach spinach (Spinacia
194
oleracea), red radish (Raphanus sativus L), winter jiasmine (Jasminum nudiflorum),
195
and black rice (Oryza sativa L. indica) extracts in alcohol. From Fig. 3A, it can be
196
seen that there is the same absorbance peak at approximately 525 nm for the extracts
197
of black rice, and red radish, which is ascribed to the same pigment cores
198
(anthocyanins) contained in the two extracts. Anthocyanins are the core compositions
199
of some natural pigments and is often found in fruits, owers and the leaves of plants,
200
because anthocyanins shows the colour in the range of visible light from red to blue.
201
The molecular structure of anthocyanins is shown in Fig. 3 C. Spinach (Spinacia
202
oleracea) and Jiasmine (Jasminum nudiflorum) show green and yellow colour
203
pigments, their extracts also exhibiting similar colours. From Fig.3 A, it can be seen
204
that the characteristic absorption peak of chlorophyll with wavelength approximately
205
660 nm and a characteristic absorption peak of carotenoid with wavelength at 455 nm 9
206
appear in the extract of spinach (Spinacia oleracea) and jiasmine (Jasminum
207
nudiflorum).
208
3.2 Characterization of TiO2 thin film
209
The thickness of the films was ca. 0.5 m monitored by an -step surface profiler.
210
The adherence strength of the TiO2 thin film on the microscope glass slide as
211
deposited was 9.5 N monitored by a coating adherence tester. The microstructure and
212
the morphology of films were examined using a SEM as shown in Fig. S1 (a) and (b).
213
Fig. S1a clearly show that the surface is composed of faceted polygonal particles or
214
grains (‘crystallites’) of 50–150 nm in size. A cross-sectional cut of the sample, Fig.
215
S1b, shows that the film has a dense columnar nano-structure. This agrees with the
216
film being polycrystalline (Kluth et al. 2003). There are gaps or fissures between the
217
individual grains. The top surface has a very rough or protruding appearance. The
218
crystalline microstructure of the thin films was characterized using XRD. Fig. S1c
219
shows the XRD spectra of TiO2 thin films annealed at different temperatures (as
220
deposited, 450°C) in air for 5 h. The TiO2 thin films still remained in a complete
221
anatase phase. From Fig. S1c, we can see increased crystallinity of the TiO2 thin film
222
annealed at 450°C. This occurs because the (1 0 1) plane of anatase films are the most
223
exposed face in the nanocrystal structure (Zheng et al. 2001).
224
Fig. 3 B illustrates the absorption spectra of black rice extract in ethanol solution
225
and on TiO2 film. The peak wavelength absorption of black rice extract on TiO2 film
226
(around 590nm) is greatly red-shift, as compared with the extract in ethanol solution
227
(around 525 nm). Anthocyanins (structural formula given in Fig. 3C, D) are strongly
228
adsorbed on TiO2 as a result of complexation with TiIV ions on the surface. Surface 10
229
complexation can readily occur via elimination of a proton (Fig. 3D). The shift to the
230
infrared region at the absorption maxima of the pigment sensitized film confirmed the
231
reaction between the TiO2 and the anthocyanins, since the interaction caused a
232
reduction of the electron density in the chromophore group, thus reducing its polarity
233
(bathochromic effect). These observations are in agreement with values reported in
234
natural pigment sensitized solar cells (Calogero and Marco 2008; Gómez-Ortíz et al.
235
2010; Hao et al. 2006; Zhou et al. 2011).
236
The alkyl group rather than carboxyl group or hydroxyl group on the pigment
237
molecule cannot form a chemical bond with a TiO2 porous film. In addition, strong
238
steric hindrance of long chain alkane of chlorophyll and carotenoid also prevent the
239
pigment molecules from arraying on TiO2 films efficiently. Due to these two factors,
240
the extracts of spinach and jiasmine are poorly absorbed onto the TiO2 film, and the
241
sensitizing effect to the TiO2 film is low, which is in accordance with our
242
experimental results.
243 244
3.3 Alternative natural pigment sensitized TiO2 thin film responses to methylamine
245
The response characteristics of sensors to volatile organic amines were first
246
tested by applying methylamine vapour. Ambient conditions were simulated by
247
adjusting the relative humidity (R.H.) to 40% at room temperature. The response of
248
TiO2 thin films sensitized with pigment extracts from spinach (Spinacia oleracea),
249
winter jiasmine (Jasminum nudiflorum) , red radish (Raphanus sativus L), and black
250
rice (Oryza sativa L. indica)
251
response of dyes free TiO2 film was nearly zero. This behaviour can be explained with
were shown in Table 1. As control experiments, the
11
252
the analogy to that of the mechanism of gas adsorption and desorption on TiO2 film
253
as described by Karunagaran et al (2007). High response values (S%) are obtained
254
from the sensor sensitized by the natural pigment extracts of black rice and red radish
255
which contain higher anthocyanins. Black rice extracts, which has the highest
256
anthocyanins content, generated the highest response values among the four organic
257
materials (Table 1). The lower response values for the spinach and winter jiasmine
258
which ascribes to weak bonds between their chlorophyll, -carotene molecule and
259
TiO2 film is inconsistent with the above discussions. Although natural pigment
260
chlorophyll play a key role in the photosynthesis ability in the plant body, their
261
functional capabilities do not translate into a good sensitizing compound because of
262
the lack of available bonds between the pigments molecules and TiO2 film through
263
which electrons can transport from excited pigment molecules to the TiO2 film.
264
Obviously, as a sensitizing compound, the interaction and bond between sensitizing
265
compound (pigment) and TiO2 film is very important in enhancing the gas sensitivity
266
of the thin film.
267
3.4 Black rice pigment sensitized TiO2 thin film response to methylamine
268
Fig. 4
269
Fig 4 A. shows the response of the pigment sensitized TiO2 film based sensor to
270
different concentrations of methylamine in the background of ambient air with R.H.
271
ฏ40% at room temperature. The resistance (R0) of the sensor after manufacture in air
272
is 5.58 M Ohm. The sensors indicated responses to methylamine within a
273
concentration range from 2 to 10 ppm in steps of 2 ppm. After each increase in
274
concentration of the vapours, the glass sample chamber was flushed with air. The 12
275
resistance of the TiO2 film decreased with exposure to methylamine (Fig. 4A). This
276
analysis was carried out three times: the sensor exhibited a good stability, due to the
277
fact that the curves referred to the different cycles overlapped between them. The
278
calibration curves for the films were obtained by plotting the relative variation, S, of
279
the sensor resistance against the concentration of the vapours in the carrier gas. Fig. 4
280
B shows the changes in S for a film exposed to different concentrations of
281
methylamine. The relative resistance of the sensor increased almost linearly with
282
increasing concentrations of methylamine (r = 0.931) over the concentration range
283
2–10 ppm. This linearity change of the films also indicates that the gold layers make
284
good Ohmic contacts with the films. Even if metal–semiconductor Schottky-type
285
junctions are formed then they are of no consequence in the sensors as they are well
286
away from where the sensing of the vapours occurs on the surface of the films.
287
The response time, defined as the time taken for the sensor's resistance to reach
288
the 90% of the steady-state resistance, is around 200-240 s. Likewise, the recovery
289
time represents the time required by the sensitivity factor to return to 10% below its
290
equilibrium value in air following the zeroing of the test gas methylamine and it was
291
found to be around 260-290 s. The response and recovery times were not as high as
292
those reported in the literature (Joshi et al. 2011)and (Capone et al. 2000), indicating
293
that molecules of amines are weakly attached to the sensing materials, or previously
294
adsorbed methylamine molecules. These results indicate that anthocyanins are
295
responsible for the sensing action. However, the sensors appear to have some form of
296
drift. This is mainly caused by the same response and recovery times set in the cycles.
297
In the dynamic flow changes, there is little methylamine remain in the chamber or on 13
298
the film when the air flux is restored after the gas test. This caused the incomplete
299
recovery of the sensor.
300
The sensing mechanism for the metal oxides working in the humidified air at
301
room temperatures (usually 0–30 °C) is often proposed to be related to the electrolytic
302
dissociations of the gas species in the adsorbed water molecules covered on the
303
surface of the metal oxides (Helwig et al. 2009; Ostrick et al. 1999). The formation of
304
a thin water film supports the diffusion of methylamine into the interfacial region by
305
dissolving the analyte. Furthermore, the ionized species in the water molecule would
306
ensure a change in the pH value on the oxide surface and thus change the
307
electrons/holes densities at the conduction or valence bands of the oxide
308
semiconductor, consequently, this would result in the sensor signals observed (Ostrick
309
et al. 1999). However, as pointed out by Helwig et al. (Helwig et al. 2009), this
310
“dissociative gas sensing mechanism” requires that the analyte gases should have
311
good water solubility and be easily ionized in water. Moreover, the sensor signals
312
induced by such type of sensing mechanism are usually small. The interactions of the
313
amines with the electrode material could be different in the presence of natural
314
pigments and water and should be considered:
315
Anthocyanin + RNH2(g) l Anthocyanin- + RNH3+
(4)
316
Anthocyanin + H2O(g) l Anthocyanin- + H3O+
(5)
317
RNH2(g) + H3O+ l RNH3+ + H2O(g)
(6)
318
where Anthocyanin- is anion of the Anthocyanin. Relatively low humidity is
319
necessary for sensing amines by the anthocyanin functionalized TiO2 film because the
320
anthocyanin is soluble in water. 14
321
An influence of the humidity on the clean air resistance of our samples is
322
illustrated in Fig. 4 C. The resistance decreased significantly if the R.H. increased
323
from 20% to 90%. The low humidity is necessary to ionized anthocyanin molecules
324
adsorbed on the surface of TiO2. Firstly, at room temperature, amines solves very well
325
in water under formation of RNH3+ and OH- ions. A similar reaction of methylamine
326
may occur on the sample surface with the adsorbed water. Secondly, at low humidity,
327
amines could be ionized by the anthocyanins on the TiO2 film and thereby decrease in
328
the proton concentration at the surface. Consequently, more positively charged holes
329
at the surface of the TiO2 have to be extracted to neutralize the adsorbed anthocyanins
330
and water film. However the presence of water promotes desorption of the pigment
331
from TiO2 film which is undesirable. High humidity in the flow cell (above 50%)
332
could lower the efficiency of the composite sensor.
333
Temperature tests of the bioamines gas sensors were performed with mini-heater.
334
The temperature was fixed at some selected values within the interval from 10 to
335
50°C. The dependencies were obtained from the measurements of the resistance
336
response to methylamine gas (R.H. 40%, 10 ppm of methylamine). The resistance
337
response to methylamine was slightly decreased on temperature from 10 to 50°C,as
338
shown in Fig. 4 D. The solubility of methylamine in water decreases as well as the
339
amount of water itself on the surface. Therefore, the amount of methylamine which is
340
present at the film and which is able to react with adsorption sites is reduced and the
341
sensitivity decreases (Ostrick et al. 2000). Long-term aging tests of the bioamines gas
342
sensors have been performed. The aging tests were performed in the dark and in air.
343
After the sensor element was stored in a container at room temperature over for 4 15
344
weeks, the response to 2 ppm methylamine was statistically indistinguishable from
345
that prior to the storage period.
346
3.5 Black rice pigment sensitized TiO2 thin film monitoring pork quality
347
Fig 5.
348
Fig. 5A shows that the amine content changes during 7 days storage of pork meat
349
at 4-6 °C. Spermidine remained constant, spermine slightly decreased, and the
350
formation of putrescine and cadaverine occurs. In particular, a continuous increase in
351
putrescine and cadaverine was observed with increasing storage days. These
352
observations are in agreement with values reported by other authors for fresh pork
353
meat (Hernández-Jover et al. 1996).
354
pigment sensitized TiO2 thin film to pork meat during storage. The response values
355
show an increase after 3 days. A rapid increase in the total amount of putrescine and
356
cadaverine was accompanied by parallel increases the response of gas sensors during
357
the storage days. A linear regression model (S%=0.289T+0.12) was build between the
358
response values of sensor and the sum content of cadaverine and putrescine during the
359
storage days as shown in Fig 5C. The correlation coefficients is 0.942 (p
