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Quartz crystal microbalance sensor for detection of aliphatic amines vapours
Sensors and Actuators B 147 (2010) 481–487
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Quartz crystal microbalance sensor for detection of aliphatic amines vapours Mohamad M. Ayad ∗ , Nagy L. Torad Department of Chemistry, Faculty of Science, University of Tanta, Tanta, Egypt
a r t i c l e
i n f o
Article history: Received 12 November 2009 Received in revised form 16 March 2010 Accepted 19 March 2010 Available online 27 March 2010 Keywords: Quartz crystal microbalance Polyaniline/emeraldine base Sensor Amines vapours
a b s t r a c t A new sensitive and on-line detection of primary aliphatic amines vapours will be established in this work. Based on polyaniline/emeraldine base (PANI/EB) film and quartz crystal microbalance (QCM), sensor for amines vapours detection was fabricated. The frequency shifts (f) of the QCM were increased due to the vapour adsorption into the PANI/EB film. f were found to be linearly correlated with the concentrations of amines vapours in mg L−1 . The sensor shows a good reproducibility and reversibility. The diffusion, the diffusion coefficient (D) and the adsorption kinetics of some aliphatic amines vapour into the PANI/EB film were determined and discussed. The D is dependent on the molecular radius of various amines molecules and the thickness of the polymer film thickness. The selectivity of the sensor is based on the differences in the values of the D and the adsorption kinetics of various amine vapours. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Sensing low concentrations of chemical vapours and volatile organic compounds (VOCs) are an area of great interest in many various applications. Amines vapours are of particular interest since both aliphatic and aromatic amines can induce toxicological responses at low concentrations [1,2]. Biogenic amines are formed by the activity of bacterial amino acid decarboxylase during the degradation processes of proteins, which have a carcinogenic effect on human body and can be used to indicate bacterial contamination. Aliphatic amines can be found in atmosphere from many wastewater effluents from industry, agriculture (waste incineration) [3], cattle feedlot operations [4], and car exhaust [5]. Trimethylamine, dimethylamine and ammonia are given off during the deterioration of fish after death [6]. In addition, trimethylamine is a good target for the detection of biogenic amines to evaluate the quality of meat food products [7,8]. Most of alkylamines are toxic and can irritate the skin, mucous membrane and respiratory tract, through all routes of exposure from inhalation ingestion and direct contact [9,10]. According to the American Conference of Government Industrial Hygienists (ACGIH), the threshold limit values are in the range between 5 and 10 mg L−1 for various alkylamines [10], this makes detection of these hazardous compounds is an ongoing challenging area. Hence sensitive and rapid detection of amines is an important task in environmental and industrial monitoring as well as in food quality control [11]. Several different techniques have been
∗ Corresponding author. Tel.: +20 40 3404398; fax: +20 40 3350804. E-mail address: [email protected] (M.M. Ayad). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.03.064
developed for amine sensing. Conventional real-time monitoring of gases has commonly been performed employing electrochemical sensors [12]. These are usually based on the oxidation of amines on various anode materials or on chemically modified electrodes [13]. Other sensing methods including, gas sensitive resistors [14], gas and liquid chromatography [10,15]. All these methods have involved complicated steps and pretreatments such as derivatization [15]. In addition, most of primary and secondary amines exhibit poor chromatographic performance making quantitative analysis difficult. Amongst types of chemical sensors there has been an increasing attention paid to the application of functionalized polymers coated the surface of quartz crystal microbalance (QCM) sensors. QCM is monitoring the mass change induced by the analyte adsorbed into the polymer films which is first introduced in 1964 [16]. The mass change (m ) can be monitored by measuring the oscillating frequency (f) of a quartz crystal and using the well-known Sauerbrey equation [17]:
f = −
2f02 √ Q Q
m
(1)
where f0 (Hz) is the natural frequency of the quartz crystal, Q is the quartz density (2.649 g cm−3 ) and Q is the shear modulus (2.947 × 1011 dyne/cm2 ). The performance characteristics of the QCM sensor from selectivity, response time and reversibility will depend on the chemical nature and physical properties of the polymeric coating. A number of polymeric coatings have been successfully employed in amines sensor applications as they exhibit changes in mass when they interact with certain chemicals. Mirmohseni and Oladegaragoze [18] have used polyvinylpyrrolidone (PVP)
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M.M. Ayad, N.L. Torad / Sensors and Actuators B 147 (2010) 481–487
letter films were dried in an oven at 50–60 ◦ C for at least 30 min until the QCM attains a constant frequency. The thickness of the film, L (cm) can be determined if the density of the PANI/EB polymer, = 1.244 g/cm3 , is known from the relation: m = L
(2)
2.4. Procedures
Scheme 1. Protonated (doped) polyaniline/emeraldine salt (ES) is deprotonated (dedoped) by treatment with an alkali to polyaniline/emeraldine base (EB).
coated QCM for determination of ammonia, methylamine, ethylamine, n-propylamine, iso-propylamine and n-butylamine. Li et al. [19,20] have constructed a sensor using water soluble PANI and PANI–TiO2 nano-composite for detection of trimethylamine and triethylamine. This sensor suffers from relatively lower detection sensitivity and reversibility as well as time consuming. Recently, we have exploit PANI coating on the electrode of the QCM as pH [21], phosphoric acid [22], chlorinated hydrocarbons and alcohols vapour [23–25] sensors. The sensors show a good reproducibility and reversibility. The frequency shifts of the QCM were linearly correlated with the concentration of the analytes. In the same context, we intended to use thin PANI in the state of emeraldine base (PANI/EB) film coated the QCM electrode as a sensor for detection of methylamine, dimethylamine, trimethylamine and triethylamine vapours in air. A simple, rapid, sensitive and reproducible sensor was developed. The diffusion and the adsorption kinetics of the adsorbed amine vapours into the polymer film coating on QCM were evaluated and discussed. 2. Experimental 2.1. Reagents and materials Aniline (ADWIC, Egypt) was distilled twice under atmospheric pressure using zinc dust. Ammonium peroxydisulphate (APS) (MP Biomedicals, LLC), sulphuric acid (ADWIC, Egypt), methylamine 40% solution (LOBA Chemie, India), dimethylamine 40% solution (Oxford, India), trimethylamine 25% solution (PROLABO, France), and triethylamine 99% solution (Merck, Germany) were used as received. 2.2. Instrumentation A 5 MHz AT-cut quartz crystal with gold electrodes in both sides is used. The QCM apparatus used for the frequency measurements was previously described [26]. The resonance frequency of the crystal was determined by GW frequency counter, Model GFC-8055G. 2.3. Coating the electrode of QCM with PANI film A 0.08 M solution of aniline was prepared in 50 mL of 0.1 M H2 SO4 and a solution of 0.1 M APS was prepared in 50 mL of 0.1 M H2 SO4 . The APS solution was added to the aniline solution. The APS/aniline molar ratio after mixing was 1.25. The solutions of the reactants were added to the polyethylene cell. As the polymerization proceeds, the in situ PANI/emeraldine salt (PANI/ES) films were deposited onto the electrode of QCM. The PANI/ES film formed at the end of the polymerization was rinsed with 0.1 M H2 SO4 . The dedoping process was made by exposure of the PANI/ES film to 0.1 M ammonia to give PANI/EB film as shown in Scheme 1. The
All measurements were carried out in a polyethylene cell with an internal volume of 140 mL. Hamilton microliter syringe (Hamilton Bonaduz AG, Switzerland) was used for analyte injections. The concentration of injected alcohol in the cell was calculated in mg L−1 using its density, purity percent and volume. The PANI/EB film coated the gold electrode of the QCM was exposed to hot dry air after the adsorption of the analyte to desorb the analyte and recover the electrode. The backshift of the crystal frequency to its initial value was taken as an indication of full desorption. All measurements were carried out at room temperature (∼24 ± 1 ◦ C). 3. Results and discussion 3.1. QCM measurements for determination of amines vapours The PANI/EB film coating on the electrode of QCM was exposed to different concentrations of amines vapours. The frequency changes due to the adsorption of amine vapours onto the film as a function of time were recorded. Fig. 1(a) shows the exposure of PANI/EB film of thickness 274 nm to different concentrations of methylamine vapours. The frequency of the quartz crystal decreased due to the adsorption of the vapours onto the polymer film according to Eq. (1). The time dependence of frequency shift (f), which corresponds to methylamine adsorption, was constructed. From the figure, f increases linearly till the steady state with increasing the concentration of the tested methylamine vapour. This is expected since more methylamine molecules provided in the test atmosphere, more molecules would be adsorbed onto the PANI/EB film coating on the QCM. After the equilibrium has been attained, the responses of the sensor can be recovered by drying the electrode using hot dry air.The response obtained from the sensor should be linear against the concentration of analyte. Thus, a calibration curve was plotted of f versus different concentrations of methylamine vapours ranged from 2 to 25 mg L−1 and is shown in Fig. 2(a). A linear relationship was obtained of correlation coefficient and slope equal 0.997 and 35.318 Hz/mg L−1 , respectively. It is expected that the polymer thickness would affect the response time and sensitivity of the sensor, thus further investigations were carried out. The frequency responses of the quartz crystal electrode, having different film thicknesses (96 and 274 nm), were recorded upon exposure to a different concentration of methylamine vapours. The plots of f with the vapour concentration for the different PANI/EB films of different thicknesses were linear as shown in Fig. 3. The correlation coefficients and the slopes of these linear relations were calculated and are given in Table 1. It is clear that as the film thickness increases, the magnitudes of the response (f) and the slopes increase due to the increasing of the active sites of the polymer available for the interaction with methylamine molecules. The PANI/EB film coating on QCM was also exposed to different concentrations of dimethylamine, trimethylamine and triethylamine vapours. The time dependence of f was obtained for different concentrations of vapours as shown in Fig. 1(b)–(d). The same profiles were obtained like that of methylamine. It is clear
M.M. Ayad, N.L. Torad / Sensors and Actuators B 147 (2010) 481–487
483
Fig. 1. Frequency change of QCM coated with PANI/EB film when exposed to the vapours of different concentrations of: (a) methylamine (film thickness 274 nm), (b) dimethylamine (film thickness 96 nm), (c) trimethylamine (film thickness 274 nm), (d) triethylamine (film thickness 274 nm).
from the graphs that as the concentration of the vapour increases, the magnitude of the response also increases. Calibration curves were constructed by plotting the f against the concentration of the vapours as shown in Fig. 2(b)–(d) and a linear relationship were obtained. The sensitivities (the slopes of the calibration graphs) and correlation coefficients (R) of the PANI/EB film coated quartz crystal electrode towards the tested amines vapours was calculated and given n in Table 1. Lower limit of detection (LLD) is the lowest concentration of analyte that can be detected. The values of LLD of
PANI/EB film coated quartz crystal electrode obtained for injected amines are presented in Table 1. 3.2. Reversibility of the sensor The PANI/EB film coated the electrode of QCM of thickness 96 nm was exposed to 2 mg L−1 of methylamine and the frequency of the QCM was recorded after reaching the equilibrium. The regeneration of the electrode was carried out by drying the electrode using hot air
Table 1 Analytical characteristic parameters for the determination of some aliphatic amines vapours using PANI/EB coated the QCM. Compound
Film thickness (nm)
Sensitivity (Hz/mg L−1 )
Ra
LLD (mg L−1 )
S.D.b (n = 5)
Methylamine
96 158 274
6.714 19.59 35.318
0.999 0.999 0.997
1.061 1.195 2.294
2.375 7.804 27.001
Dimethylamine Trimethylamine Triethylamine
96 274 274
7.187 38.072 6.4
0.999 0.999 0.998
1.384 0.827 3.202
3.315 10.491 6. 831
n: Number of times for each injected amine. a Correlation coefficient. b Standard deviation.
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Fig. 2. Calibration curves for the determination of aliphatic amines vapours in air using PANI/EB film coated QCM: (a) methylamine, (b) dimethylamine, (c) trimethylamine and (d) triethylamine.
until fully desorption was achieved. The experiment was repeated for five times to insure the complete reversibility of the sensor as shown in Fig. 4. From the figure it is clear that, the sensor shows a good reproducibility and reversibility. 3.3. The vapour diffusion and the adsorption kinetics study The diffusion and diffusion coefficient (D) of the various amines vapours of 5 mg L−1 concentration onto the PANI/EB film of thickness 184 nm coated QCM electrode was studied, and are shown in Fig. 5(a). The experimental data was analysed using Fick’s second equation, which has been reviewed by Crank [27]: ft =4 f∞
D t 1/2 L
(3)
where ft is the frequency change due to the adsorption of the vapour into the EB film at any time t and f∞ is the frequency change in the equilibrium state at the end of the adsorption process, these two parameters can be given as follows: ft = fEB − ft
and f∞ = fEB − f∞
Fig. 3. The effect of film thickness on the detection of different concentrations of methylamine vapours.
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485
Table 2 k (×104 s−1 ), and D (×1013 cm2 /s) of the tested aliphatic amines using PANI/EB coated the QCM. Compound
Film thickness (nm)
Concentration (mg L−1 )
D (×1013 cm2 /s)
k (×104 s−1 )
184 274
5 2 7 12 17 25
0.55 0.49 0.57 0.66 0.85 1.02
15.5 15.60 17.80 18.60 19.00 19.3
184 184 184
5 5 5
0.43 0.35 0.27
16.00 ± 0.10 18.2 ± 0.20 25.70 ± 0.15
Methylamine
Dimethylamine Trimethylamine Triethylamine
± ± ± ± ± ±
0.078 0.30 0.18 0.31 0.28 0.29
equation:
4r 3 3
r=
Fig. 4. Reproducibility and reversibility of QCM electrode coated with PANI/EB film of thickness 96 nm when exposed to 2 mg L−1 methylamine: (a) methylamine injection and (b) methylamine desorption using hot dry air.
where ft is the frequency during the exposure process at time t. f∞ is the frequency at the equilibrium state and fEB is the frequency of the film. The plot between ft /f∞ and of t1/2 /L for trimethylamine vapours is shown in Fig. 5(b). From the figure it is obvious that the process obeys Fickian kinetics [27]. The calculated values of D for the tested amines are shown in Table 2. The diffusion coefficients for a series of similar compounds in a particular system should be related by the molecular volume. The Stokes–Einstein equation predicts that the diffusion coefficient is inversely proportional to the molecular radius of the diffusant. Assuming that the molecules are spheres, the molecular radii can be estimated from the molecular volume according to the following
= Vm =
3 4N
·
MW
MW pN
(4)
1/3 (5)
where r is the molecular radius, Vm is the molecular volume MW is the molecular weight, is the density, and N is Avogadro, s number. The values of D for the tested amines were used in the construction of a plot of D versus the estimated molecular radius of each tested injected amine (Fig. 6(a)). From the figure, it was observed that the obtained results using thin PANI/EB films coated QCM adhere to establish the theory. On the other hand, the study of diffusion as a function of film thickness is an important study. Fig. 6(b) shows the relation between D and film thickness. It is obvious that, as the film thickness increases, the diffusion of the vapour increases. This is may be due to the increase of the active sites of the PANI/EB film which leads to more molecules have more chances to interact with the polymer chains. The amine vapour uptakes into polymer film may be considered as a pseudo-first order mass transfer between the vapour phase and the film adsorption sites. The rates of uptake of vapour can then be compared [28] in terms of the pseudo-first order rate constant (k) following the equation: ft = 1 − e−kt f∞
(6)
where ft and f∞ refer to the adsorbate weight uptake at time t and at equilibrium, respectively are previously defined. A graph of ln(1 − ft /f∞ ) against t should be a straight line of gradient k.
Fig. 5. (a) Frequency change of QCM coated with PANI/EB film of thickness 184 nm when exposed to 5 mg L−1 vapours of methylamine, dimethylamine, trimethylamine and triethylamine. (b) ft /f∞ as a function of t1/2 /L when exposed to 5 mg L−1 of trimethylamine vapours.
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Fig. 6. (a) Plot of the diffusion coefficient, D versus the estimated molecular radius for the four injected amines. (b) Diffusion coefficient, D, as a function of film thickness for 2 mg L−1 concentration of methylamine vapour.
Fig. 5(a) shows the response time of the PANI/EB film of thickness 184 nm towards a concentration of 5 mg L−1 of different amines vapour. The plot of ln(1 − ft /f∞ ) against t is shown in Fig. 7. It is apparent that the vapour uptakes for different vapours into the film follow the linear driving force mass transfer kinetic model [28]. The k values were determined from the gradient of the kinetic plot and are listed in Table 2. They are in the order: methylamine < dimethylamine < trimethylamine < triethylamine. It can be concluded that the kinetic selectivity for the film towards various analytes and the D values can be used as a basis for the sensor selectivity. It is interesting to see the effect of amines vapour concentration increment on the values of D and k. Therefore, the PANI/EB film coating on the electrode of QCM was exposed to different concentrations of methylamine vapours and the frequency changes due to the vapours adsorption onto the film of thickness 274 were recorded. D and k values were calculated as previously described using Eqs. (3) and (6) and are presented in Table 2. The obtained data show observed increases of D and k values as the concentration increases. This is justifies the present results and conclusion.
Fig. 7. Variation of ln(1 − ft /f∞ ) against time for the adsorption of 5 mg L−1 methylamine, dimethylamine, trimethylamine, and triethylamine.
4. Conclusion A sensor of polyaniline in the base form (PANI/EB) coated the QCM electrode was constructed for determination of primary aliphatic amines vapours in air. The sensor shows a rapid and sensitive detection for amines vapours and the sensitivity increases by increasing the concentration of the analyte species. The sensor exhibits satisfactory linear correlation also a good reproducibility and reversibility of the sensor was obtained. The diffusion and the adsorption kinetics of amine vapours were studied. The diffusion of amines vapours into PANI/EB films was found to establish the Stokes–Einstein equation, in which the diffusion coefficient is inversely proportional to the molecular radius of the diffusant. Also the diffusion was found to be increased with increasing the film thickness. The differences in the calculated values of the diffusion coefficient (D) and the adsorption kinetics can be used for the selective detection between these pollutants.
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M.M. Ayad, N.L. Torad / Sensors and Actuators B 147 (2010) 481–487 [12] W.N. Opdycke, S.J. Parks, M.E. Meyerhoff, Polymer-membrane pH electrodes as internal elements for potentiometric gas-sensing systems, Anal. Chim. Acta 155 (1983) 11–20. [13] M.D. Koppang, M. Witek, J. Blau, G.M. Swain, Electrochemical oxidation of polyamines at diamond thin-film electrodes, Anal. Chem. 71 (1999) 1188–1195. [14] B.P.J.D. Costello, P. Evans, N.M. Ratcliffe, Preparation of polypyrrole composites and the effect of volatile amines on their electrical-properties, Analyst 121 (1996) 793–797. [15] H. Kataoka, Derivatization reactions for the determination of amines by gas-chromatography and their application in environmental analysis, J. Chromatogr. A 733 (1996) 19–34. [16] W.H. King Jr., Piezoelectric sorption detector, Anal. Chem. 36 (1964) 1735–1739. [17] G. Sauerbrey, The use of quartz oscillators for weighing thin layers and for microweighing, Z. Phys. 155 (1959) 206–222. [18] A. Mirmohseni, A. Oladegaragoze, Construction of a sensor for determination of ammonia and aliphatic amines using polyvinylpyrrolidone coated quartz crystal microbalance, Sens. Actuators B: Chem. 89 (2003) 164–172. [19] G. Li, J. Zheng, X. Ma, Y. Sun, J. Fu, G. Wu, Development of QCM trimethylamine sensor based on water soluble polyaniline, Sensors 7 (2007) 2378–2388. [20] J. Zheng, G. Li, X. Ma, Y. Wang, G. Wu, Y. Cheng, Polyaniline-TiO2 nanocomposite-based trimethylamine QCM sensor and its thermal behavior studies, Sens. Actuators B: Chem. 133 (2008) 374–380. [21] M.M. Ayad, N.A. Salahuddin, A.K. Abou-Seif, M.O. Al-Ghayesh, pH sensor based on polyaniline and aniline–anthranilic acid copolymer films using quartz crystal microbalance and electronic absorption spectroscopy, Polym. Adv. Technol. 19 (8) (2008) 1142–1148. [22] M.M. Ayad, N.A. Salahuddin, M.O. Alghaysh, R.M. Issa, Phosphoric acid and pH sensors based on polyaniline films, Curr. Appl. Phys. 10 (2010) 235–240. [23] M.M. Ayad, G. El-Hefnawey, N.L. Torad, Chlorinated hydrocarbon vapors sensor using polyaniline coated quartz crystal microbalance sensor, Sens. Actuators B: Chem. 134 (2008) 887–894. [24] M.M. Ayad, G. El-Hefnawey, N.L. Torad, A sensor of alcohol vapours based on thin polyaniline base film and quartz crystal microbalance, J. Hazard. Mater. 168 (2009) 85–88.
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Biographies Mohamad M. Ayad is a Professor of Physical Chemistry at Department of Chemistry, Faculty of Science, University of Tanta, Egypt. He received his PhD degree in Physical Chemistry at the University of Tanta and University of Aston, Birmingham, UK under the auspices of channel system (1986). He was a postdoctoral fellow in the Department of Chemistry, University of Aston, Birmingham, UK (1988), Fulbright fellowship in IBM Almaden Research Center, San Jose, California, USA (1990), Royal society fellowship in Institute of Bioscience and Technology, Cranfield University, UK (2005) and JSPS invited Professor in Toyohashi University of Technology, Department of Materials Science, Toyohashi, Japan (2008). His research interests can in different broad areas: material science (application of conducting polymers, thin films, polymer interfaces and nano-composites) and sensors as well as charge transfer complexes. Currently, his group is pursuing research into chemically modifying conducting polymers and their use in the design of new sensors. Nagy L. Torad received his undergraduate education at Faculty of Science, University of Tanta, Egypt and is currently pursuing postgraduate studies in the Department of Chemistry, Faculty of Science, University of Tanta. His current research interests involve the characterization of polymer films as sensitive layers and their incorporation and application in various sensor architectures.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Quartz crystal microbalance sensor for detection of aliphatic amines vapours Mohamad M. Ayad ∗ , Nagy L. Torad Department of Chemistry, Faculty of Science, University of Tanta, Tanta, Egypt
a r t i c l e
i n f o
Article history: Received 12 November 2009 Received in revised form 16 March 2010 Accepted 19 March 2010 Available online 27 March 2010 Keywords: Quartz crystal microbalance Polyaniline/emeraldine base Sensor Amines vapours
a b s t r a c t A new sensitive and on-line detection of primary aliphatic amines vapours will be established in this work. Based on polyaniline/emeraldine base (PANI/EB) film and quartz crystal microbalance (QCM), sensor for amines vapours detection was fabricated. The frequency shifts (f) of the QCM were increased due to the vapour adsorption into the PANI/EB film. f were found to be linearly correlated with the concentrations of amines vapours in mg L−1 . The sensor shows a good reproducibility and reversibility. The diffusion, the diffusion coefficient (D) and the adsorption kinetics of some aliphatic amines vapour into the PANI/EB film were determined and discussed. The D is dependent on the molecular radius of various amines molecules and the thickness of the polymer film thickness. The selectivity of the sensor is based on the differences in the values of the D and the adsorption kinetics of various amine vapours. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Sensing low concentrations of chemical vapours and volatile organic compounds (VOCs) are an area of great interest in many various applications. Amines vapours are of particular interest since both aliphatic and aromatic amines can induce toxicological responses at low concentrations [1,2]. Biogenic amines are formed by the activity of bacterial amino acid decarboxylase during the degradation processes of proteins, which have a carcinogenic effect on human body and can be used to indicate bacterial contamination. Aliphatic amines can be found in atmosphere from many wastewater effluents from industry, agriculture (waste incineration) [3], cattle feedlot operations [4], and car exhaust [5]. Trimethylamine, dimethylamine and ammonia are given off during the deterioration of fish after death [6]. In addition, trimethylamine is a good target for the detection of biogenic amines to evaluate the quality of meat food products [7,8]. Most of alkylamines are toxic and can irritate the skin, mucous membrane and respiratory tract, through all routes of exposure from inhalation ingestion and direct contact [9,10]. According to the American Conference of Government Industrial Hygienists (ACGIH), the threshold limit values are in the range between 5 and 10 mg L−1 for various alkylamines [10], this makes detection of these hazardous compounds is an ongoing challenging area. Hence sensitive and rapid detection of amines is an important task in environmental and industrial monitoring as well as in food quality control [11]. Several different techniques have been
∗ Corresponding author. Tel.: +20 40 3404398; fax: +20 40 3350804. E-mail address: [email protected] (M.M. Ayad). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.03.064
developed for amine sensing. Conventional real-time monitoring of gases has commonly been performed employing electrochemical sensors [12]. These are usually based on the oxidation of amines on various anode materials or on chemically modified electrodes [13]. Other sensing methods including, gas sensitive resistors [14], gas and liquid chromatography [10,15]. All these methods have involved complicated steps and pretreatments such as derivatization [15]. In addition, most of primary and secondary amines exhibit poor chromatographic performance making quantitative analysis difficult. Amongst types of chemical sensors there has been an increasing attention paid to the application of functionalized polymers coated the surface of quartz crystal microbalance (QCM) sensors. QCM is monitoring the mass change induced by the analyte adsorbed into the polymer films which is first introduced in 1964 [16]. The mass change (m ) can be monitored by measuring the oscillating frequency (f) of a quartz crystal and using the well-known Sauerbrey equation [17]:
f = −
2f02 √ Q Q
m
(1)
where f0 (Hz) is the natural frequency of the quartz crystal, Q is the quartz density (2.649 g cm−3 ) and Q is the shear modulus (2.947 × 1011 dyne/cm2 ). The performance characteristics of the QCM sensor from selectivity, response time and reversibility will depend on the chemical nature and physical properties of the polymeric coating. A number of polymeric coatings have been successfully employed in amines sensor applications as they exhibit changes in mass when they interact with certain chemicals. Mirmohseni and Oladegaragoze [18] have used polyvinylpyrrolidone (PVP)
482
M.M. Ayad, N.L. Torad / Sensors and Actuators B 147 (2010) 481–487
letter films were dried in an oven at 50–60 ◦ C for at least 30 min until the QCM attains a constant frequency. The thickness of the film, L (cm) can be determined if the density of the PANI/EB polymer, = 1.244 g/cm3 , is known from the relation: m = L
(2)
2.4. Procedures
Scheme 1. Protonated (doped) polyaniline/emeraldine salt (ES) is deprotonated (dedoped) by treatment with an alkali to polyaniline/emeraldine base (EB).
coated QCM for determination of ammonia, methylamine, ethylamine, n-propylamine, iso-propylamine and n-butylamine. Li et al. [19,20] have constructed a sensor using water soluble PANI and PANI–TiO2 nano-composite for detection of trimethylamine and triethylamine. This sensor suffers from relatively lower detection sensitivity and reversibility as well as time consuming. Recently, we have exploit PANI coating on the electrode of the QCM as pH [21], phosphoric acid [22], chlorinated hydrocarbons and alcohols vapour [23–25] sensors. The sensors show a good reproducibility and reversibility. The frequency shifts of the QCM were linearly correlated with the concentration of the analytes. In the same context, we intended to use thin PANI in the state of emeraldine base (PANI/EB) film coated the QCM electrode as a sensor for detection of methylamine, dimethylamine, trimethylamine and triethylamine vapours in air. A simple, rapid, sensitive and reproducible sensor was developed. The diffusion and the adsorption kinetics of the adsorbed amine vapours into the polymer film coating on QCM were evaluated and discussed. 2. Experimental 2.1. Reagents and materials Aniline (ADWIC, Egypt) was distilled twice under atmospheric pressure using zinc dust. Ammonium peroxydisulphate (APS) (MP Biomedicals, LLC), sulphuric acid (ADWIC, Egypt), methylamine 40% solution (LOBA Chemie, India), dimethylamine 40% solution (Oxford, India), trimethylamine 25% solution (PROLABO, France), and triethylamine 99% solution (Merck, Germany) were used as received. 2.2. Instrumentation A 5 MHz AT-cut quartz crystal with gold electrodes in both sides is used. The QCM apparatus used for the frequency measurements was previously described [26]. The resonance frequency of the crystal was determined by GW frequency counter, Model GFC-8055G. 2.3. Coating the electrode of QCM with PANI film A 0.08 M solution of aniline was prepared in 50 mL of 0.1 M H2 SO4 and a solution of 0.1 M APS was prepared in 50 mL of 0.1 M H2 SO4 . The APS solution was added to the aniline solution. The APS/aniline molar ratio after mixing was 1.25. The solutions of the reactants were added to the polyethylene cell. As the polymerization proceeds, the in situ PANI/emeraldine salt (PANI/ES) films were deposited onto the electrode of QCM. The PANI/ES film formed at the end of the polymerization was rinsed with 0.1 M H2 SO4 . The dedoping process was made by exposure of the PANI/ES film to 0.1 M ammonia to give PANI/EB film as shown in Scheme 1. The
All measurements were carried out in a polyethylene cell with an internal volume of 140 mL. Hamilton microliter syringe (Hamilton Bonaduz AG, Switzerland) was used for analyte injections. The concentration of injected alcohol in the cell was calculated in mg L−1 using its density, purity percent and volume. The PANI/EB film coated the gold electrode of the QCM was exposed to hot dry air after the adsorption of the analyte to desorb the analyte and recover the electrode. The backshift of the crystal frequency to its initial value was taken as an indication of full desorption. All measurements were carried out at room temperature (∼24 ± 1 ◦ C). 3. Results and discussion 3.1. QCM measurements for determination of amines vapours The PANI/EB film coating on the electrode of QCM was exposed to different concentrations of amines vapours. The frequency changes due to the adsorption of amine vapours onto the film as a function of time were recorded. Fig. 1(a) shows the exposure of PANI/EB film of thickness 274 nm to different concentrations of methylamine vapours. The frequency of the quartz crystal decreased due to the adsorption of the vapours onto the polymer film according to Eq. (1). The time dependence of frequency shift (f), which corresponds to methylamine adsorption, was constructed. From the figure, f increases linearly till the steady state with increasing the concentration of the tested methylamine vapour. This is expected since more methylamine molecules provided in the test atmosphere, more molecules would be adsorbed onto the PANI/EB film coating on the QCM. After the equilibrium has been attained, the responses of the sensor can be recovered by drying the electrode using hot dry air.The response obtained from the sensor should be linear against the concentration of analyte. Thus, a calibration curve was plotted of f versus different concentrations of methylamine vapours ranged from 2 to 25 mg L−1 and is shown in Fig. 2(a). A linear relationship was obtained of correlation coefficient and slope equal 0.997 and 35.318 Hz/mg L−1 , respectively. It is expected that the polymer thickness would affect the response time and sensitivity of the sensor, thus further investigations were carried out. The frequency responses of the quartz crystal electrode, having different film thicknesses (96 and 274 nm), were recorded upon exposure to a different concentration of methylamine vapours. The plots of f with the vapour concentration for the different PANI/EB films of different thicknesses were linear as shown in Fig. 3. The correlation coefficients and the slopes of these linear relations were calculated and are given in Table 1. It is clear that as the film thickness increases, the magnitudes of the response (f) and the slopes increase due to the increasing of the active sites of the polymer available for the interaction with methylamine molecules. The PANI/EB film coating on QCM was also exposed to different concentrations of dimethylamine, trimethylamine and triethylamine vapours. The time dependence of f was obtained for different concentrations of vapours as shown in Fig. 1(b)–(d). The same profiles were obtained like that of methylamine. It is clear
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Fig. 1. Frequency change of QCM coated with PANI/EB film when exposed to the vapours of different concentrations of: (a) methylamine (film thickness 274 nm), (b) dimethylamine (film thickness 96 nm), (c) trimethylamine (film thickness 274 nm), (d) triethylamine (film thickness 274 nm).
from the graphs that as the concentration of the vapour increases, the magnitude of the response also increases. Calibration curves were constructed by plotting the f against the concentration of the vapours as shown in Fig. 2(b)–(d) and a linear relationship were obtained. The sensitivities (the slopes of the calibration graphs) and correlation coefficients (R) of the PANI/EB film coated quartz crystal electrode towards the tested amines vapours was calculated and given n in Table 1. Lower limit of detection (LLD) is the lowest concentration of analyte that can be detected. The values of LLD of
PANI/EB film coated quartz crystal electrode obtained for injected amines are presented in Table 1. 3.2. Reversibility of the sensor The PANI/EB film coated the electrode of QCM of thickness 96 nm was exposed to 2 mg L−1 of methylamine and the frequency of the QCM was recorded after reaching the equilibrium. The regeneration of the electrode was carried out by drying the electrode using hot air
Table 1 Analytical characteristic parameters for the determination of some aliphatic amines vapours using PANI/EB coated the QCM. Compound
Film thickness (nm)
Sensitivity (Hz/mg L−1 )
Ra
LLD (mg L−1 )
S.D.b (n = 5)
Methylamine
96 158 274
6.714 19.59 35.318
0.999 0.999 0.997
1.061 1.195 2.294
2.375 7.804 27.001
Dimethylamine Trimethylamine Triethylamine
96 274 274
7.187 38.072 6.4
0.999 0.999 0.998
1.384 0.827 3.202
3.315 10.491 6. 831
n: Number of times for each injected amine. a Correlation coefficient. b Standard deviation.
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Fig. 2. Calibration curves for the determination of aliphatic amines vapours in air using PANI/EB film coated QCM: (a) methylamine, (b) dimethylamine, (c) trimethylamine and (d) triethylamine.
until fully desorption was achieved. The experiment was repeated for five times to insure the complete reversibility of the sensor as shown in Fig. 4. From the figure it is clear that, the sensor shows a good reproducibility and reversibility. 3.3. The vapour diffusion and the adsorption kinetics study The diffusion and diffusion coefficient (D) of the various amines vapours of 5 mg L−1 concentration onto the PANI/EB film of thickness 184 nm coated QCM electrode was studied, and are shown in Fig. 5(a). The experimental data was analysed using Fick’s second equation, which has been reviewed by Crank [27]: ft =4 f∞
D t 1/2 L
(3)
where ft is the frequency change due to the adsorption of the vapour into the EB film at any time t and f∞ is the frequency change in the equilibrium state at the end of the adsorption process, these two parameters can be given as follows: ft = fEB − ft
and f∞ = fEB − f∞
Fig. 3. The effect of film thickness on the detection of different concentrations of methylamine vapours.
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Table 2 k (×104 s−1 ), and D (×1013 cm2 /s) of the tested aliphatic amines using PANI/EB coated the QCM. Compound
Film thickness (nm)
Concentration (mg L−1 )
D (×1013 cm2 /s)
k (×104 s−1 )
184 274
5 2 7 12 17 25
0.55 0.49 0.57 0.66 0.85 1.02
15.5 15.60 17.80 18.60 19.00 19.3
184 184 184
5 5 5
0.43 0.35 0.27
16.00 ± 0.10 18.2 ± 0.20 25.70 ± 0.15
Methylamine
Dimethylamine Trimethylamine Triethylamine
± ± ± ± ± ±
0.078 0.30 0.18 0.31 0.28 0.29
equation:
4r 3 3
r=
Fig. 4. Reproducibility and reversibility of QCM electrode coated with PANI/EB film of thickness 96 nm when exposed to 2 mg L−1 methylamine: (a) methylamine injection and (b) methylamine desorption using hot dry air.
where ft is the frequency during the exposure process at time t. f∞ is the frequency at the equilibrium state and fEB is the frequency of the film. The plot between ft /f∞ and of t1/2 /L for trimethylamine vapours is shown in Fig. 5(b). From the figure it is obvious that the process obeys Fickian kinetics [27]. The calculated values of D for the tested amines are shown in Table 2. The diffusion coefficients for a series of similar compounds in a particular system should be related by the molecular volume. The Stokes–Einstein equation predicts that the diffusion coefficient is inversely proportional to the molecular radius of the diffusant. Assuming that the molecules are spheres, the molecular radii can be estimated from the molecular volume according to the following
= Vm =
3 4N
·
MW
MW pN
(4)
1/3 (5)
where r is the molecular radius, Vm is the molecular volume MW is the molecular weight, is the density, and N is Avogadro, s number. The values of D for the tested amines were used in the construction of a plot of D versus the estimated molecular radius of each tested injected amine (Fig. 6(a)). From the figure, it was observed that the obtained results using thin PANI/EB films coated QCM adhere to establish the theory. On the other hand, the study of diffusion as a function of film thickness is an important study. Fig. 6(b) shows the relation between D and film thickness. It is obvious that, as the film thickness increases, the diffusion of the vapour increases. This is may be due to the increase of the active sites of the PANI/EB film which leads to more molecules have more chances to interact with the polymer chains. The amine vapour uptakes into polymer film may be considered as a pseudo-first order mass transfer between the vapour phase and the film adsorption sites. The rates of uptake of vapour can then be compared [28] in terms of the pseudo-first order rate constant (k) following the equation: ft = 1 − e−kt f∞
(6)
where ft and f∞ refer to the adsorbate weight uptake at time t and at equilibrium, respectively are previously defined. A graph of ln(1 − ft /f∞ ) against t should be a straight line of gradient k.
Fig. 5. (a) Frequency change of QCM coated with PANI/EB film of thickness 184 nm when exposed to 5 mg L−1 vapours of methylamine, dimethylamine, trimethylamine and triethylamine. (b) ft /f∞ as a function of t1/2 /L when exposed to 5 mg L−1 of trimethylamine vapours.
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Fig. 6. (a) Plot of the diffusion coefficient, D versus the estimated molecular radius for the four injected amines. (b) Diffusion coefficient, D, as a function of film thickness for 2 mg L−1 concentration of methylamine vapour.
Fig. 5(a) shows the response time of the PANI/EB film of thickness 184 nm towards a concentration of 5 mg L−1 of different amines vapour. The plot of ln(1 − ft /f∞ ) against t is shown in Fig. 7. It is apparent that the vapour uptakes for different vapours into the film follow the linear driving force mass transfer kinetic model [28]. The k values were determined from the gradient of the kinetic plot and are listed in Table 2. They are in the order: methylamine < dimethylamine < trimethylamine < triethylamine. It can be concluded that the kinetic selectivity for the film towards various analytes and the D values can be used as a basis for the sensor selectivity. It is interesting to see the effect of amines vapour concentration increment on the values of D and k. Therefore, the PANI/EB film coating on the electrode of QCM was exposed to different concentrations of methylamine vapours and the frequency changes due to the vapours adsorption onto the film of thickness 274 were recorded. D and k values were calculated as previously described using Eqs. (3) and (6) and are presented in Table 2. The obtained data show observed increases of D and k values as the concentration increases. This is justifies the present results and conclusion.
Fig. 7. Variation of ln(1 − ft /f∞ ) against time for the adsorption of 5 mg L−1 methylamine, dimethylamine, trimethylamine, and triethylamine.
4. Conclusion A sensor of polyaniline in the base form (PANI/EB) coated the QCM electrode was constructed for determination of primary aliphatic amines vapours in air. The sensor shows a rapid and sensitive detection for amines vapours and the sensitivity increases by increasing the concentration of the analyte species. The sensor exhibits satisfactory linear correlation also a good reproducibility and reversibility of the sensor was obtained. The diffusion and the adsorption kinetics of amine vapours were studied. The diffusion of amines vapours into PANI/EB films was found to establish the Stokes–Einstein equation, in which the diffusion coefficient is inversely proportional to the molecular radius of the diffusant. Also the diffusion was found to be increased with increasing the film thickness. The differences in the calculated values of the diffusion coefficient (D) and the adsorption kinetics can be used for the selective detection between these pollutants.
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Biographies Mohamad M. Ayad is a Professor of Physical Chemistry at Department of Chemistry, Faculty of Science, University of Tanta, Egypt. He received his PhD degree in Physical Chemistry at the University of Tanta and University of Aston, Birmingham, UK under the auspices of channel system (1986). He was a postdoctoral fellow in the Department of Chemistry, University of Aston, Birmingham, UK (1988), Fulbright fellowship in IBM Almaden Research Center, San Jose, California, USA (1990), Royal society fellowship in Institute of Bioscience and Technology, Cranfield University, UK (2005) and JSPS invited Professor in Toyohashi University of Technology, Department of Materials Science, Toyohashi, Japan (2008). His research interests can in different broad areas: material science (application of conducting polymers, thin films, polymer interfaces and nano-composites) and sensors as well as charge transfer complexes. Currently, his group is pursuing research into chemically modifying conducting polymers and their use in the design of new sensors. Nagy L. Torad received his undergraduate education at Faculty of Science, University of Tanta, Egypt and is currently pursuing postgraduate studies in the Department of Chemistry, Faculty of Science, University of Tanta. His current research interests involve the characterization of polymer films as sensitive layers and their incorporation and application in various sensor architectures.