Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University – Suceava 

  Year IX, No3 - 2010 
 

 40 

 
SELECTIVITY OF CALIX [6]ARENE-COATED PIEZOELECTRIC 

 QUARTZ CRYSTAL SENSOR FOR THE DETECTION  
OF SOME ORGANIC AMINE IN GAS FLOW 

 
Iosif GERGEN1, Despina Maria BORDEAN1, Monica HARMANESCU3,  

Florina RADU1, Marilena MICLAU2 
 

 1. USAMVB-Faculty of Food Technology, Timisoara, Calea Ardului 119, RO 300645, igergen@yahoo.com 
2. INCEMC (Institutul National de  C-D pentru Electrochimie si Materiale Condensate), Timisoara  

3. USAMVB-Agriculture Faculty, Timisoara 
 
 

Abstract: In this study, we developed calixarene-coated PQC sensors for the determination of 
organic amines in gas faze and study the selectivity of this coated sensor for normal and iso- form of 
propyl and butyl amines. p-tert-Butylcalix[6]arene compound was applied as piezoelectric quartz 
crystal (PQC) sensory coatings for the selective detection of volatile organic in gas phase. The AT-cut 
quartz crystals used in this study (Stanford Research Systems) having a diameter of 25.4 mm and a 
thickness of 0.33 mm. An oscillator circuit providing an alternating voltage will produce a 
fundamental frequency of 5 MHz for this particular crystal diameter and thickness. About 10.0 l of 
analyte was injected into the mixing chamber with an approximately volume of 250 ml. The measuring 
chamber with coated PQC has a volume of approximately 5 mL. The concentration of amine sample in 
mixing chamber and respectively in gas flow was around 40 ppm. Al experiments were conducted at 
30oC. The complexation based on the host–guest interaction between the calixarene coating and the 
analyte makes the coated PQC sensors sensitive to organic amines and results in quasi-irreversible 
frequency changes. A PQC sensor coated with a p-tert-Butylcalix[6]arene exhibits different selectivity 
for amines in gas phase, in the order: iso-propylamine< iso-butylamine<<n-butylamine. The relative 
selectivity of the analytes is explained by the cavity inclusion of analyte molecule, complete or partial, 
or by steric hindrance interaction. The application of PQC sensors to gas faze has provided new 
analytical possibilities for the food control, environmental science, biotechnology and medicine.  

 
Keywords: food quality, volatile amines, calixarene, sensitive membranes 

 
 
 
 

Introduction 
 
The detection of food quality and freshness 
is of increasing interest both for the 
consumer and food industry. Amines, 
especially volatile amines, are the principal 
compounds 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, respectively food quality 
and freshness. Gas chromatography 
methods are the best for control and 

monitoring the quality and freshness of 
food. But these methods usually require 
long time and pre-treatment processes 
which are not suitable to perform an in situ 
analysis and field application [1]. 
Developments in quartz crystal 
microbalance (QCM) sensor technology 
has progressed in the area of gas phase 
analysis since the first report in 1964, 
where King [2] used a QCM as a gas 
chromatograph absorption detector. Since 
then, a series of reports of other detection 
schemes for different gas phase analytes 
have appeared in the literature. The change 



 
Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University – Suceava 

  Year IX, No3 - 2010 
 

 41 

of resonance frequency (F in Hz) related 
to the mass (m in g) deposited on the 
crystal surface of area (A in cm2) and the 
fundamental frequency (F0 in MHz) of the 
PQC can be calculated from Sauerbrey’s 
equation [3]: 

F=−2.26×10−6×F0 2 ×(m/A)    (1) 
The theoretical detection limit of an 
oscillating quartz crystal is reported to be 
as low as 10-12 g (if F0 = 10 MHz) for 
coating materials according to Sauerbrey’s 
equation. Piezoelectric quartz crystal 
(PQCs) with appropriate coating materials 
can interact effectively with specific 
analytes. To improve the sensitivity and 
selectivity of PQC sensors, lately 
macrocyclics, e.g., crown ethers, 
cryptands, cavitants, cyclodextrins and 
calixarenes, have been used as coatings for 
gas detection. Simple dispersion forces and 
dipole–dipole interactions lead to sensor 
signals with poor selectivity but good 
reversibility. Hydrogen bonding 
(biochemistry) and steric interactions 
(key– lock principle) between the sensor 
coating and the analyte result in an 
increased signal intensity but it is quasi-
irreversible. The application of PQC 
sensors to gas faze has provided new 
analytical possibilities for the food control, 
environmental science, biotechnology and 
medicine [4].  
In this study, we developed calixarene-
coated PQC sensors for the determination 
of organic amines in gas faze and studied 
the selectivity of this coated sensor for 
normal and iso- form of propyl and butyl 
amines. 

 
 Experimental  
 
 Quartz Crystal Microbalance  
 
The AT-cut quartz crystals used in this 
study were purchased from Stanford 
Research Systems (Sunnyvale, California) 
[5] having a diameter of 25.4 mm and a 

thickness of 0.33 mm. They were patterned 
with two concentric gold-on-chrome 
electrodes having a wrap around geometry 
that allows both ground and radio 
frequency Q connections to be made to one 
side.  
Application of a voltage to the two 
electrodes produces a strain in the surface 
of the QCM along the cut of the crystal. 
An oscillator circuit providing an 
alternating voltage will produce a 
fundamental frequency of 5 MHz for this 
particular crystal diameter and thickness. 
The QCM200 System is a stand-alone 
instrument with a built-in frequency 
counter and resistance meter. It includes 
controller, crystal oscillator electronics, 
crystal holder, and quartz crystals. Series 
resonance frequency and resistance are 
measured and displayed directly on the 
front panel, and there is an analogue output 
proportional to the relative frequency to 
interface directly with potentiostats.  
In addition, the QCM200 has an RS-232 
interface and comes with both Windows 
and Mac software providing real-time 
display, analysis and storage of your QCM 
data. Figure 1 depicts the experimental set-
up of the PQC sensor detection system 
with the assembled computer interface. 
The frequency of the vibrating crystal was 
measured with a QCM 200 (Stanford 
Research Systems) type frequency counter 
connected to a microcomputer system. 
About 10.0 l of analyte was injected into 
the mixing chamber with an approximately 
volume of 250 ml. The measuring chamber 
with coated PQC has a volume of 
approximately 5 mL.  
The concentration of amine sample in 
mixing chamber and respectively in gas 
flow was around 40 ppm. All experiments 
were conducted at 30oC. 

 



 
Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University – Suceava 

  Year IX, No3 - 2010 
 

 42 

computer

QCM 200

M
sample injection

mixture chamber

flow meter

waste colection

N2 flow

N2
carrier
gas

coated quartz 
crystal 
chamber

 
Figure 1. Experimental set-up for the calixarene-

coated PQC sensor and amines. 
Coatings  
The coating solutions were prepared by 
dissolving 20 mg of the calixarene in 5 mL 
of chloroform. The concentrations of the 
resulting solutions were about 4 mg/ mL. 
Superior sides of the crystals were coated 
with the calixarene solution via the 
dropping method with a micro-syringe. An 
aliquot of 3.0 L of calixarene was 
dropped on to superior side of the quartz 
crystals. A thin membrane was obtained 
after solvent evaporation and dried with a 
hot air blower for approximately 30 s.  
Reagents 
The analytes iso-propylamine (98%), n-
butylamine (99%), iso-butylamine 
(98.5%), and pure solvents were all of 
analytical-reagent or chemically pure grade 
(Sigma Aldrich). The boiling point and 
molecular mass is presented in Table 1. 
Pure N2 was used as carrier gas. 

Table 1.  
Boiling point and molecular mass of amines 

Nr. Compound Bp (0C) MW 
1 Iso-propylamine 33 59.11 
2 Iso-butylamine 68-69 73.14 
3 N-butylamine 78 73.14 

Sensing material, calixarenes (CA) are 
cyclic oligomers containing large-scaled 
molecular cavities, dimensions of which 
depend on the number of aryl fragments 
(usually 4, 6, 8) in macro-cycle (Figure 2). 
Various functional groups can be bound to 
upper and lower rim. It is known that 
calixarenes can form “host–guest” 
complexes. Calixarene molecule is as a 

“host”, while metal ions and different 
organic compounds are as “guests”. 
Varying the number of aryl fragments and 
functional groups one can easily manage 
with sensitivity and selectivity of 
calixarene films [6]. Especially the cavity 
of calix[6]arene (2.1-2.8 Å cavity 
diameter) is sufficiently large to 
accommodate organic amine guests, 
whereas that of calix-[4]arene is too small 
[7]. It is probable that the close 
correspondence between the relative 
magnitudes of the organic amine (ionic 
diameter of ammonium ion = 2.96 Å) and 
the calixarene cavity diameters is an 
important factor in determining the 
complexation interactions between the 
calixarene and the amine guests [8]. 

 
Figure 2. General structures of the p-R-

calix[n] arene [7]; R=tert-butyl, Ri= OH and n=3 
for p-tert-butylcalix[6]arene. 

 
Results and Discussion 
The response curves for calixarene coated 
PQC sensor detection systems for 40 ppm 
of 3 organic amines at 30 °C in the 
measuring chamber are shown in Figure 3 
and absolute area values of measurement 
in Table 2. 

 
Figure 3. The response characteristics of PQC 

sensors coated with calyx[6]arene 
to 40 ppm of 3 organic amines. 

 



 
Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University – Suceava 

  Year IX, No3 - 2010 
 

 43 

Table 2.  
Absolute response for three amines 

detected in N2 flow with the PQC sensor 

The response characteristics of PQC 
sensors coated with p-tert-
butylcalyx[6]arene to 40 ppm of organic 
amines decrease in the order                                                                        
n-butylamine>>iso-butylamine>iso-
propylamine. This is attributed to the extra 
lypophilic stabilized interaction between 
the host (calixarene) and guest (amines) 
molecules. Organic amines are 
incorporated in the cavity of calixarene 
(CA) to form endo-CA complex and the 
complex involves hydrogen bonding with 
inclusion of the non-polar moiety of the 
organic amines in the CA cavity. On the 
basis of the data in Figure 3, it can be seen 
that n-butylamine have remarkable higher 
responses and iso-butylamine and iso-
propylamine have relative lower responses. 
This is due to the bulkiness of their non-
polar moieties that cannot be incorporated 
into the cavity of CA, while n butylamine, 
can be incorporated into the cavity of CA 
to form hydrogen bonding [4]. In similar 
experiment but with acetate derivatized 
calixarene (Ri = acetate), Wang et al [4] 
obtain similar results for linear long chain 
amines (n-butylamine, n-hexylamine, n-
octylamine and dodecylamine). It is 
concluded that a long alkyl group is 
another important factor controlling the 
response of organic amines. 

 
Conclusion 

 
In this study, we investigated p-tert-
butylcalix[6]arene-coated PQC sensors for 
the detection of organic amines in gas 

flow. The relative sensitivity of amines is 
based on host–guest complexation between 
the organic amine and the calixarene. 
Stronger responses were observed for n-
butylamine molecules which are easy 
included in the calyx[6]arene cavity. The 
response of the amines with a bulky group 
(iso-forms) is weak, since the bulky group 
hinders the amine from being included in 
the cavity. 

 
Acknowledgement 
The authors are grateful to the CNCSIS 
Romania for financial support (GRANT 
PNCD II Prog.  4 –  Nr. 102/01.10.2007, 
New α-quartz structure compounds- 
performat piezoelectric sensor for food 
quality and safety, SENZALI). 
 
References 
1. GUANG L., JUNBAO Z., XINGFA M., YU S., 
JUN F., GANG W.; Development of QCM 
Trimethylamine Sensor Based on Water Soluble 
Polyaniline, Sensors, 2007, 2378-2388. 
2. KING, W.H. Piezoelectric Sorption Detector, 
Anal. Chem.,1964, 36, 1735-39 
3. SAUERBREY, G. Verwendung von 
Schwingquarzen zur Wagung diinner Schichten und 
zur Mikrowiigung, Z. Phys., 1959, 155,206. 
4. CHUN W., XI-WEN H., CHEN L.X. : A 
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Analyst, 1999, 124, 1787–1790. 

Amines 
(Number 

of 
determina

tion) 

Iso- 
propyl 

amine (4) 

Iso-
butyl 
amine 

(4) 

N-Butyl 
amine (2) 

 Area, sqm 
Average 
values 

118 222 1090