A new room temperature gas sensor based on pigment-sensitized TiO2 thin film for amines determination

A new room temperature gas sensor based on pigment-sensitized TiO2 thin film for amines determination

Author's Accepted Manuscript A new room temperature gas sensor based on pigment-sensitized TiO2 thin film for amines determination Li Yanxiao, Zou Xi...

606KB Sizes 4 Downloads 116 Views

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

www.elsevier.com/locate/bios

PII: DOI: Reference:

S0956-5663(14)00373-X http://dx.doi.org/10.1016/j.bios.2014.05.040 BIOS6802

To appear in:

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<0.001). This

360

good agreement can be explained because the thin film response to the development

361

of volatile components that result from the bacterial degradation of pork meat. Since

362

the sensorial spoilage of pork meat is mostly due to the amines, it seems appropriate

363

to use amines as an indicator of pork meat spoilage. The results demonstrated that the

364

sensor developed in this study was suitable for the monitoring of amines in pork and

365

enabled the evaluation of the freshness and quality of pork meat.

Fig. 5B shows the response of black rice

366 16

367

4. Conclusions

368

A new room temperature gas sensor was fabricated with pigment-sensitized TiO2

369

thin film as the sensing layer. A thin TiO2 film was deposited by a DC reactive

370

magnetron sputtering technique onto a well-cleaned glass substrate equipped with

371

interdigitated comb shaped electrodes. Natural pigments were extracted from spinach

372

(Spinacia oleracea), red radish (Raphanus sativus L), winter jiasmine (Jasminum

373

nudiflorum), and black rice (Oryza sativa L. indica) using alcohol. The natural

374

pigment sensitized TiO2 thin film was used to detect methylamine gas. It was found

375

that black rice pigment possesses the best sensitized effect among the four kinds of

376

natural pigments. This is due to the better interaction between the carbonyl and

377

hydroxyl groups of anthocyanins and the TiO2 film. The enhancement of sensitivity

378

with increasing humidity suggests that the adsorption of water promotes the

379

interaction of methylamine at the thin film. Furthermore, the fabricated sensor was

380

used to detect the amines generated in pork meat during its gradual abiotic

381

decomposition with the storage days. The sensor response values were highly

382

positively correlated with the content of cadaverine and putrescine. An intended

383

application for this sensor film is in a handheld instrument for the monitoring of food

384

and environment. The sensor must be used in dark, TiO2 in air and under illumination

385

act as photocatalyst. Therefore illumination would cause de degradation of the

386

adsorbed pigment and consequently a reduction of sensitivity and degradation of the

387

sensor is expected.

388 389 17

390

Acknowledgement

391

The authors acknowledge the financial support of Chinese 863 Program, (Grant

392

2011AA008007), Jiangsu appointed professor program, New Century Excellent

393

Talents in University (NCET-11-0986), Jiangsu province funds for distinguished

394

young scientists (BK2013010). We also wish to thank many of our colleagues for

395

many stimulating discussions in this field.

396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428

References Alimelli, A., Pennazza, G., Santonico, M., Paolesse, R., Filippini, D., D’Amico, A., Lundström, I., Di Natale, C., 2007. Anal. Chim. Acta 582(2), 320-328. Bernacka-Wojcik, I., Senadeera, R., Wojcik, P.J., Silva, L.B., Doria, G., Baptista, P., Aguas, H., Fortunato, E., Martins, R., 2010. Biosens. Bioelectron. 25(5), 1229-1234. Bisquert, J., Fabregat-Santiago, F., Mora-Ser, I., Garcia-Belmonte, G., Barea, E.M., Palomares, E., 2008. Inorg. Chim. Acta 361(3), 684-698. Boscornea, C., Tomas, S., Hinescu, L.G., Tarabasanu-Mihaila, C., 2001. J. Mater. Process. Technol. 119(1–3), 344-347. Bruder, I., Karlsson, M., Eickemeyer, F., Hwang, J., Erk, P., Hagfeldt, A., Weis, J., Pschirer, N., 2009. Sol. Energy Mater. Sol. Cells 93(10), 1896-1899. Calogero, G., Marco, G.D., 2008. Sol. Energy Mater. Sol. Cells 92(11), 1341-1346. Capone, S., Siciliano, P., Quaranta, F., Rella, R., Epifani, M., Vasanelli, L., 2000. Sens. Actuators, B 69(3), 230-235. Cheng, X., Sun, S., Liang, M., Shi, Y., Sun, Z., Xue, S., 2012. Dyes Pigments 92(3), 1292-1299. Choi, M.F., Hawkins, P., 1997. Sens. Actuators B 38/39, 390–394. Dutta, R.S., Majumdar, S., Laik, A., Singh, K., Kulkarni, U.D., Sharma, I.G., Dey, G.K., 2011. Surf. Coat. Technol 205(19), 4720-4725. Gómez-Ortíz, N.M., Vázquez-Maldonado, I.A., Pérez-Espadas, A.R., Mena-Rejón, G.J., Azamar-Barrios, J.A., Oskam, G., 2010. Sol. Energy Mater. Sol. Cells 94(1), 40-44. Hao, S., Wu, J., Huang, Y., Lin, J., 2006. Sol. Energy 80(2), 209-214. Helwig, A., Müller, G., Sberveglieri, G., Eickhoff, M., 2009. J. Sens 2009, 1-7. Hernández-Jover, T., Izquierdo-Pulido, M., Veciana-Nogués, M.T., Vidal-Carou, M.C., 1996. J. Agric. Food. Chem. 44(10), 3097-3101. Huang, X.-w., Zou, X.-b., Shi, J.-y., Guo, Y., Zhao, J.-w., Zhang, J., Hao, L., 2014. Food Chem. 145, 549-554. Joshi, A., Gangal, S.A., Gupta, S.K., 2011. Sens. Actuators, B 156(2), 938-942. Kang, Y., Meyerhoff, M.E., 2006. Anal. Chim. Acta 565(1), 1-9. Karunagaran, B., Uthirakumar, P., Chung, S.J., Velumani, S., Suh, E.K., 2007. Mater. Charact. 58(8–9), 680-684. Kluth, O., Schöpe, G., Hüpkes, J., Agashe, C., Müller, J., Rech, B., 2003. Thin Solid Films 442(1–2), 80-85. Lee, D.H., Lee, M.J., Song, H.M., Song, B.J., Seo, K.D., Pastore, M., Anselmi, C., Fantacci, S., De 18

429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465

Angelis, F., Nazeeruddin, M.K., Gräetzel, M., Kim, H.K., 2011. Dyes Pigments 91(2), 192-198. Liao, F., Yin, S., Toney, M.F., Subramanian, V., 2010. Sens. Actuators, B 150(1), 254-263. Liu, C.-H.J., Lu, W.-C., 2007. J. Chin. Inst. Chem. Eng, 38(5–6), 483-488. Lu, H.-H., Rao, Y.K., Wu, T.-Z., Tzeng, Y.-M., 2009. Sens. Actuators, B 137(2), 741-746. Mabrook, M., Hawkins, P., 2001. Sens. Actuators, B 75(3), 197-202. Mor, G.K., Varghese, O.K., Paulose, M., Shankar, K., Grimes, C.A., 2006. Sol. Energy Mater. Sol. Cells 90(14), 2011-2075. Moradian, A., Mohr, G.J., Linnhoff, M., Fehlmann, M., Spichiger-Keller, U.E., 2000. Sens. Actuators, B 62(2), 154-161. Namiesik, J., 1984. J. Chromatogr. 300, 79–108. Ostrick, B., Muhlsteff, J., Fleischer, M., Meixner, H., Doll, T., Kohl, C.-D., 1999. Sens. Actuators, B 57, 115–119. Ostrick, B., Pohle, R., Fleischer, M., Meixner, H., 2000. Sens. Actuators, B 68(1–3), 234-239. Pacquit, A., Lau, K.T., McLaughlin, H., Frisby, J., Quilty, B., Diamond, D., 2006. Talanta 69(2), 515-520. Persad, A., Chow, K.-F., Wang, W., Wang, E., Okafor, A., Jespersen, N., Mann, J., Bocarsly, A., 2008. Sens. Actuators, B 129(1), 359-363. Raible, I., Burghard, M., Schlecht, U., Yasuda, A., Vossmeyer, T., 2005. Sens. Actuators, B 106(2), 730-735. Reijnders, L., 2010. J. Cleaner Production 18(4), 307-312. Schweizer-Berberich, P.M., Vaihinger, S., Göpel, W., 1994. Sens. Actuators, B 18(1–3), 282-290. Sen, S., Muthe, K.P., Joshi, N., Gadkari, S.C., Gupta, S.K., Jagannath, Roy, M., Deshpande, S.K., Yakhmi, J.V., 2004. Sens. Actuators, B 98(2–3), 154-159. Suska, A., Ibáñez, A.B., Lundström, I., Berghard, A., 2009. Biosens.Bioelectron 25(4), 715-720. Suzzi, G., Gardini, F., 2003. Int. J. Food Microbiol. 88(1), 41-54. Tang, T., Qian, K., Shi, T., Wang, F., Li, J., Cao, Y., Hu, Q., 2011. Food Control 22(8), 1203-1208. Wang, C., He, X.-W., Chen, L.-X., 2002. Talanta 57(6), 1181-1188. Xia, H., Liu, T., Gao, L., Yan, L., Wu, J., 2011. Appl. Surf. Sci. 258(1), 254-259. Xiao-wei, H., Xiao-bo, Z., Ji-yong, S., Jie-wen, Z., Yanxiao, L., Limin, H., Jianchun, Z., 2013. Anal. Chim. Acta. 787(17), 233-238 Zamani, C., Casals, O., Andreu, T., Morante, J.R., Romano-Rodriguez, A., 2009. Sens. Actuators, B 140(2), 557-562. Zheng, S.K., Wang, T.M., Xiang, G., Wang, C., 2001. Vacuum 62(4), 361-366. Zhou, H., Wu, L., Gao, Y., Ma, T., 2011. J. Photochem. Photobiol., A 219(2–3), 188-194. Zhu, X., Nanny, M.A., Butler, E.C., 2007. J. Photochem. Photobiol., A 185(2–3), 289-294.

Tables captions:

466

Table 1. Response (S%) of the TiO2 thin films sensitized by natural pigments to 10

467

ppm methylamine vapour at 40% R.H., measured after 150 s exposure.

468 469 19

470

Figures captions:

471

Fig. 1 The diagram for assembling the pigment-sensitized TiO2 thin film gas sensor

472

Fig. 2 Schematic diagram of the flow rig for the generation of analytes vapour

473

standards in air.

474

Fig. 3 Absorbance spectra, molecular structure for natural pigments. (A)Absorbance

475

spectra for the extracts of black rice, red radish, winter jiasmine and spinach in

476

alcohol. (B)Absorbance spectra of black rice in ethanol solution on a TiO2 film. (C)

477

molecular structure of anthocyanin. (D) anthocyanin binding with the surface of the

478

TiO2 film, (E) The reaction mechanism of amines (R-NH2) and anthocyanin functionalized

479

TiO2 film

480

Fig. 4 Typical response signals of methylamine in air for black rice pigment sensitized

481

TiO2 film based sensors (A) Variation of conductance with the flow of 2-10 ppm

482

methylamine (response) and air (recovery) at room temperature (22 ± 1 °C) with

483

R.H.40%, n=3, (B) The response (S%) vs. concentration, (C) on relative humidity in

484

air at the ambient temperature (22 ± 1 °C). (D) S10 and R0 on relative temperature in

485

the ambient humidity air (R.H. 40%).

486

Fig. 5 Pork borne amines (spermidine, spermine, putrescine, and cadaverine) changes

487

(A) and its response with black rice pigment sensitized TiO2 thin film sensor (B)

488

during storage at 5 °C, the scattering plot (C) between sensor response (S%) and sum

489

of putrescine and cadaverine (T)

490

20

Table 1 Response (S%) of the TiO2 thin films sensitized by natural dyes to 10

491 492

ppm methylamine vapour at 40% rh, measured after 150 s exposure (n=5). Natural pigment

Responses (S%)

Black rice (Oryza sativa L. indica)

11.3 ± 0.15

Red radish (Raphanus sativus L)

5.6 ± 0.13

Spinach (Spinacia oleracea)

1.12 ± 0.08

Winter jiasmine (Jasminum nudiflorum),

1.33 ± 0.07

None

0.1 ± 0.05

493 494

Highlights

495 496 497

z

Four natural pigments sensitized TiO2 films were fabricated as the sensing layer

498

z

Anthocyanins in the natural dyes are very important in enhancing the gas sensitivity of the thin film

499 500

z

methylamine

501 502 503

Black rice pigment sensitized TiO2 film showed an excellent gas-sensitivity to

z

Pigment-sensitized TiO2 film was used to determine the biogenic amines generated by pork during storage

504 505

21

Figure 1

Figure 2

Figure-3

Figure-4

Figure 5