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Phthalocyanines as sensitive coatings for QCM sensors: Comparison of gas and liquid sensing properties
Sensors and Actuators B 155 (2011) 298–303
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Phthalocyanines as sensitive coatings for QCM sensors: Comparison of gas and liquid sensing properties Mika Harbeck a,∗ , Dilek D. Erbahar a , Ilke Gürol a , Emel Musluo˘glu a , Vefa Ahsen a,b , Zafer Ziya Öztürk a,c a b c
TÜBITAK Marmara Research Center, Materials Institute, P.O. Box 21, 41470 Gebze, Kocaeli, Turkey Gebze Institute of Technology, Department of Chemistry, P.O. Box 141, 41400 Gebze, Kocaeli, Turkey Gebze Institute of Technology, Department of Physics, P.O. Box 141, 41400 Gebze, Kocaeli, Turkey
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Article history: Received 7 September 2010 Received in revised form 15 December 2010 Accepted 20 December 2010 Available online 28 December 2010 Keywords: Chemical sensor Quartz crystal microbalance Liquid sensing Phthalocyanine Organic pollutant Response modeling
a b s t r a c t The quartz crystal microbalance (QCM) was used to investigate the liquid sensing properties of a set of phthalocyanines (Pcs) which were systematically varied by attaching the substituent 2,2,3,3tetrafluoropropyloxy to different positions and by introducing a central metal ion (i.e. Ni2+ , Zn2+ , and Cu2+ ). The responses to low concentrations of organic compounds such as hydrocarbons and chlorocarbons dissolved in water were recorded. The materials were very sensitive to the tested compounds with detection limits in the lower parts-per-million range and they exhibited a good sensing performance as the sensors have been working fully reversibly and reliably over long periods of time. Besides, the influence of substitution pattern and choice of central metal ion on the liquid sensing properties of Pcs were studied for the first time. The results show that the responses differ notably from each other depending on the modifications made to the Pc. Finally, it is demonstrated that the gas and liquid sensing responses of the materials are highly correlated and can be linked to each other with the help of a basic physical model. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Sensor systems based on acoustic transducers, namely the quartz crystal microbalance (QCM) or surface acoustic wave sensors, are an ongoing topic of intensive research and development for many gas and liquid sensing applications such as environmental monitoring, security and safety, or health analysis [1–3]. Ample work is ongoing in the development of new or improved sensing materials. In the gas phase the QCM transducer is modified with a variety of sensing materials such as polymers or organic macromolecules, which are carefully chosen to suit best the intended application. In the field of liquid sensing the use of the QCM as biosensor platform dominates. Reports of QCM chemical sensors operating in liquids are sparse. Recently, phthalocyanines (Pcs) were introduced as new sensitive materials for the detection of different classes of organic pollutants in water [4,5]. Other current work describes the use of a choline monolayer [6]. Most common are investigations into molecularly imprinted polymers (MIPs) [7–12]. However, these sensitive materials are intended for
∗ Corresponding author. Tel.: +90 262 677 3123; fax: +90 262 641 2309. E-mail addresses: [email protected] (M. Harbeck), [email protected] (D.D. Erbahar), [email protected] (I. Gürol), [email protected] (E. Musluo˘glu), [email protected] (V. Ahsen), [email protected] (Z.Z. Öztürk). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.12.038
very specific targets and not for the measurement of a broad range of chemical compounds as each MIP is specially produced to match a single type of molecule. Besides, the approach is considered less suitable for small molecules without specific interaction pathways. The smallest molecules are still rather complex in their chemical structure [12]. However, small and simple compounds are the main target analytes in many applications such as hydrocarbon or chlorocarbon pollution monitoring. Thus, the same approach as in gas sensing, i.e. using an array of semi-selective sensors and data evaluation methods, is more appropriate for multi-analyte detection and classification of such chemicals of different but quite unspecific nature [13]. More research effort is considered crucial to overcome the lack of available liquid chemical sensors and experimental data in this field. As a large background experience and knowledge in the area of gas sensing are at hand it would be quite advantageous if one could benefit from this in the development of sensing materials for QCM liquid sensors. Experimental or computational results and theoretical models obtained in one area could then be applied also to the other. This work addresses both the development of new sensitive materials for liquid sensing of organic compounds and the linkage to the gas sensing field. A set of Pcs with fluoroalkyloxy substituents attached in different configurations and three different central metal ions were characterized for their QCM sensing performance towards common organic compounds. For the first time the influ-
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The phthalocyanines were synthesized according to reported standard procedures using the corresponding phthalonitrile as starting material. The experimental details of the synthesis and the results of the chemical analysis and spectroscopic characterization are described in [15,16] and references therein.
2.2. Sensor preparation and test procedures
Fig. 1. Chemical structure of the phthalocyanine molecule with an optional central metal ion M and the peripheral substituents R and R and non-peripheral substituent R and a list of compounds characterized with the QCM sensors in this work.
ence of substitution type and central metal on the liquid sensing properties of Pcs were studied. The particular type of substituent and central metal ion were chosen due to good results obtained previously with QCM gas sensors using the same materials [14,15]. The Pcs were found highly sensitive with detection limits in the low parts-per-million range in liquids, as well. Furthermore, the liquid phase sensor responses are compared with gas phase data of the same materials. A basic model allows predicting the liquid sensing results using gas phase data reaching a good approximation of the experimental results.
2. Experimental 2.1. Sensitive materials In total seven different Pc derivatives were characterized for their liquid sensing performance. The Pc cores were modified by adding the substituent 2,2,3,3-tetrafluoropropyloxy (–O–CH2 CF2 CHF2 ) and by introducing a central metal ion (nickel, zinc or copper) replacing the two inner hydrogen atoms. The codes used here to identify the individual compounds specify metal type (1: Ni, 2: Zn, and 3: Cu) as well as substitution pattern (denoted by the letters a–d). The substituents are introduced forming octa (compounds denoted with a) and tetra substituted (b–d) Pcs. Tetra compounds are available with substituents in peripheral, p (b and c) and non-peripheral, np (d) positions. Compounds denoted with b contain an additional chlorine atom. The full list of materials and their identification codes are given in Fig. 1.
The employed transducers, coating procedure, test system and measurement procedures of the liquid sensing experiments have been already described in detail previously in [4] and shall be summarized here only in short. Polished AT-cut QCMs of 4.95 MHz fundamental frequency were used for the liquid sensing experiments, but the more stable 3rd overtone was preferred as sensor signal. Coating of the transducers with the sensitive material was achieved using the air-brush technique. The spraying solution consisted of 1 mg Pc in 1 ml of a suitable organic solvent. Sensors were coated with an amount of sensitive material equivalent to 1 kHz frequency shift (fundamental) on one side. Seven common organic solvents were used as test analytes representing different chemical classes: dichloromethane (DCM), chloroform (TCM), tetrachloroethylene (TCE), chlorobenzene (CB), toluene (TLN), o-xylene (oXLN), and p-xylene (pXLN). The analytes were high purity chemicals and used as obtained from the supplier. 10 or 100 l of an analyte was added to 500 ml ultrapure water via a precision pipette to prepare the respective stock solutions. Test samples were prepared by diluting the stock solution to the desired concentration level. Typical concentration levels were in the range of 0.1–30 ppm (mg/l) with the exception of DCM and TCM. In these two cases the highest concentrations were 300 ppm. Caution was taken to avoid loss of analyte into the gas phase during preparation of the samples. Besides, stock solutions and test samples were prepared freshly before the experiments. QCM sensor signals were recorded with a QCM Z-500 measurement system based on impedance analysis (KSV Instruments Ltd., Finland). All measurements were made at a constant temperature of 20 ◦ C.
3. Results and discussion 3.1. Liquid sensing properties of phthalocyanines A typical sensor response of a QCM sensor coated with a Pc during intermitted exposure to water samples containing different concentrations of o-xylene in the low parts-per-million range is depicted in Fig. 2(a). Besides zeroing the baseline by referencing to the initial sensor value when no analyte was present no correction was made to the sensor data. The negative frequency shifts are the consequence of the analyte exposure. When the sensor is purged with clean water the sensor resonance frequency returns to a value approximating the initial baseline. This example depicts the response of a sensor with 3a to test samples with o-xylene in concentrations from 1 to 20 ppm. The other analytes and sensors give similar results, but differ in the amplitude of the response as well as response and recovery times. The response curve illustrates the excellent response characteristics of the Pc coated QCM sensors in liquid environments. The baseline stability of the sensor signal is very good over time. The equilibrium sensor signal is reached quickly and recovery is fast after exposure to the analyte. 90% of the full sensor signal is typically reached in a period below 90 s and 90% recovery of the baseline after analyte free water is introduced to the sensor chamber is reached in less than 1 min. The sensors could be used for repeated experiments over a period of several weeks without noticeable chance in the sensing performance.
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Fig. 2. (a) Transient signal of a QCM sensor (3rd harmonics) with 3a during exposure to samples containing o-xylene in concentrations between 1 and 20 ppm and (b) resulting sensor calibration curve.
The sensor responses increase with increasing concentration of the analyte and reach nearly 400 Hz for the highest tested o-xylene concentration. The response data can be represented by a linear calibration curve passing through the origin and the frequency shift per unit concentration of analyte (sensitivity) is as high as 20 Hz/ppm (see Fig. 2(b)). The water-sensitive material partition coefficient Kl (dimensionless) is a good descriptor for the sensing performance of a material, as it describes the degree of enrichment of the analyte in the sensing material. High enrichment is usually associated with high sensor responses. Kl is given as the ratio of cs and caq , the concentrations of the analyte in the sensing material and aqueous phase, respectively: Kl =
cs . caq
(1)
From the sensor sensitivity S as the slope of the sensor calibration curve S=
f caq
(2)
the partition coefficient can be calculated using Sauerbrey’s equation for the relation of mass to frequency changes as Kl =
S · s × 106 , fs
(3)
where S is the sensitivity of the QCM sensor for the analyte (in Hz/mg/l), fs the frequency shift of the sensor due to the coating with the sensitive material, and s the density of the sorbent layer (in g/cm3 ). The factor 106 is a result of the conversion of units from SI base units into commonly used units for density (g/cm3 ) and concentration (mg/l). Using Eq. (3) the partition coefficients were calculated for all sensor–analyte combinations from the experimental QCM data. The partition coefficients are presented in the bar diagrams in Fig. 3. The values span a wide range from about 30 for DCM up to 9200 for pXLN. Repetitions of the experiments with newly prepared sensors yielded a high reproducibility of sensors and measurements,
Fig. 3. Experimental partition coefficients of all compounds as obtained in the liquid sensing experiments at 20 ◦ C: comparison of differently substituted Pcs with (top) nickel, and (middle) zinc central metal ion as well as (bottom) results of all octa compounds. Insets provide the results for DCM and TCM on a magnified scale.
as deviations in sensitivity were less than 4%. The materials presented here are more sensitive than other Pcs studied previously under similar conditions [4]. The estimated limits of detection for a sensor with 3a are in a range of 5 ppm for dichloromethane and 0.05 ppm for o-xylene. 3.2. Discussion of liquid sensing results and comparison to gas phase data On the basis of the data presented in Fig. 3 the influences of substitution pattern and central metal ion on the sensing properties of the Pcs, i.e. the partition coefficient and sensitivity, can be summarized as follows: • The amount of absorbed analyte and thus the sensor sensitivity increases for all materials in the order of dichloromethane, chloroform, toluene, chlorobenzene, tetrachloroethylene, p-xylene, and o-xylene. For example, the partition coefficients of 1a for the analytes dichloromethane, chlorobenzene, and o-xylene are 30, 3090, and 5270, respectively (Fig. 3 (top)). • Octa substitution is most effective in increasing analyte uptake: 1a and 2a are more sensitive than their tetra counterparts 1b–d (see Fig. 3 (top)) and 2b (see Fig. 3 (middle)), respectively, with the exception of the analytes DCM and TCM. • The presence of the additional chlorine in R position in the tetra compounds decreases sensitivity (1b vs. 1c and 1d, Fig. 3 (top)). • Tetra substitution in peripheral position is slightly more favorable than in non-peripheral position to increase sensitivity (1c vs. 1d, Fig. 3 (top)). • However, next to the influence of position and number of substituents there is also a strong central metal ion influence, e.g. see data of the octa compounds in Fig. 3 (bottom). A decreasing
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0.8
Height
0.6 Fig. 5. Illustration of the path independency of phase equilibria.
0.4
3.3. Correlation between liquid and gas sensing 0.2
1c
1d
1b
1a
2a
2b
3a
0.0
Fig. 4. Result of a hierarchical cluster analysis of the normalized and centered sensor data.
sensitivity for all analytes can be observed in the order of Pcs with a Cu2+ , Zn2+ , and Ni2+ central metal ions.
Hierarchical cluster analysis (HCA) was used to analyze the high dimensional response data. It allows exploring similarities or dissimilarities among the individual materials according to their responses as sensing materials. The goal is to identify possible variations in the sensing characteristics caused by the changes of central metal ion and substitution pattern. HCA was performed using normalized and centered sensor responses. Euclidian distance was used as a dissimilarity measure and cluster linkage was based on the averaging method. The HCA result presented in Fig. 4 clearly shows a high variation in the response behavior of the materials caused by both a modification of the central metal ion and the substitution pattern. Regarding the octa compounds, the CuPc 3a is much different from the quite similar NiPc 1a and ZnPc 2a. The central metal ions in NiPcs and ZnPcs are in contrast to the one in CuPcs only weak in terms of interaction capabilities with analyte molecules. Furthermore, it was found that the tetra compounds 1b–d and 2b differ from the respective octa Pcs, but show little variation among each other for a given central metal ion type (see 1b–d). A direct comparison with the gas sensing results obtained previously in dry air (see [14]) for the same list of analytes as tested here reveals a generally good agreement between the two methods. The Pcs were found to be highly effective VOC absorbers both from the liquid and the gas phase. Furthermore, the relative responses to the analytes are very similar, e.g. the octa compounds 1a and 2a showed the highest gas phase responses to o-xylene, and then to chlorobenzene, toluene, and chloroform vapors. This finding is in agreement with the results here. For 1a also the same influence of the substitution pattern on sensor sensitivity was found: generally sensitivity is decreasing in the order of compounds with octa, peripheral tetra, and non-peripheral tetra substitution. However, a difference in the results was observed for the tetra compound with an additional chlorine atom (1b). In the gas sensing experiments 1b was found to be more sensitive than the other tetra compounds. This finding is not in agreement with the liquid sensing results. Besides, 1a is more sensitive than 2a in the gas phase. These observed deviations are attributed to the humidity contained in the sensitive material in the liquid sensing experiment. Apparently, the absorbed water has different influences on the sensitivities of the Pcs with a Zn and Ni central metal ions.
The desired quantitative link between gas and liquid sensing results is made using a basic physical model. The partition coefficient between two phases is describing a state which is independent of any intermediate steps required to reach this state. This is illustrated in Fig. 5 for the aqueous-sensitive material phase system. Instead of one direct phase equilibrium one could also imagine a path via two equilibria with an intermediate gas phase (e.g. air). The partitioning between liquid phase and sensitive material, described by Kl , would be unaffected. Accordingly, Kl can be rewritten as Kl = Kg · KH
(4)
or log Kl = log Kg + log KH ,
(5)
where Kg is the gas phase-sensitive material partition coefficient and KH Henry’s law constant for the gas phase–water system (KH = cgas /caq ). Using Eq. (5), Kl can be estimated using experimental gas sensor data (log Kg ) and tabulated material data (log KH ). Henry’s constants for a variety of compounds collected from various sources can be found e.g. in [17], however, all values used here were taken from one source only [18]. The gas phase-sensitive material partition coefficients, Kg , of the Pcs were calculated using the gas phase sensitivity data listed in Table 2 found in [14]. In Fig. 6 the various partition coefficients for TCM, TLN, CB, and oXLN of the octa compounds 2a and 1a are shown. A comparison of log Kl and log Kg shows that gas phase and liquid phase partition coefficients are of the same magnitude, but Kl is generally smaller than Kg . Using the experimental gas phase values and tabulated values Kl was predicted using Eq. (5) and compared to the experimental data obtained in this work. Fig. 7 shows the correlation between the calculated and experimental data in truepredicted plots for 1a,b and 2a,b. The model gives a good estimate of the actual experimental values, but generally the values of Kl are underestimated, i.e. the experimental liquid sensing values are higher than the predicted ones. Even though the absolute values are not predicted fully the correlation between gas and liquid phase data is high. The correlation coefficients R are as high as 0.99. The degree of overestimation varies by material as seen in the variations in the best fit slopes. An explanation for the deviation of the model in absolute values may be found in the fact that the gas sensing data was acquired under dry air conditions. In contrast to this the sensitive material has absorbed the maximum possible amount of water in the case of liquid sensing as it is exposed to a pure water phase. Absorbed water molecules are known to change the sorption properties of a sensitive materials to some extend. However, as water uptake by the Pc as used here is quite modest, the model gives already good results. The agreement is better than in the case of some polymers [19] where the deviation was found to be much larger. To conclude, the approach made here is suitable to estimate the performance of a Pc in liquid sensing experiments when gas phase data is already available or vice versa, but could possibly be improved using gas phase data obtained at high humidity levels.
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tive towards common organic compounds dissolved in aqueous samples. Due to the numerous positive aspects such as extended stability and short response and recovery times they are considered highly suitable for the use as components of a QCM sensor array. From the materials under test the best results in terms of sensitivity were obtained with octa substituted compounds having Cu2+ , Zn2+ , or Ni2+ central metal ions. A wide correspondence to gas phase sensor results was found in a direct comparison of the two techniques. Similar to gas sensing there is an influence of substitution pattern and central metal ion on the sensing properties. Furthermore, using a simple physical model a correlation was established with gas sensing data with good agreement. This is an important step to apply the experience and knowledge obtained in this field to the area of liquid sensing, as in the latter only little experience is available so far. The field of liquid chemical sensing is expected to expand in the future and this link will contribute to accelerate and ease the progress. Acknowledgement This study was supported by the DPT (Devlet Planlama Tes¸kilatı), the Turkish State Planning Organization. References
Fig. 6. Partition coefficients of selected analytes for the octa compounds (a) 2a and (b) 1a. Kl (exp) is the experimentally obtained water-Pc partition coefficient (this work), Kg is the gas phase-sensitive material partition coefficient (experimental data from [14]) and KH the Henry’s law constant for the gas phase–water system (tabulated in [17,18]).
Fig. 7. True-predicted plots showing Kl (exp) vs. Kl , the water-Pc partition coefficients obtained experimentally and calculated using Eq. (5), respectively: (a–b) 1a and 1b, (c–d) 2a and 2b.
4. Conclusions Phthalocyanines with fluoroalkyl oxy substituents varied systematically in substitution type and choice of central metal ion were extensively characterized as sensitive materials for QCM liquid sensing applications. They were found to be highly sensi-
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ether, and thia ether functional groups, the synthesis of their alkaline and transition metal complexes and the investigation of their liquid crystal, semiconductor, and gas sensing properties.
Biographies
Vefa Ahsen BSc, MSc, PhD is the dean of science faculty at the Gebze Institute of Technology (GYTE), Turkey. His research interest lies in the synthesis of stable ligands, e.g. phthalocyanines, vic-dioximes with such functional group as crown ether, aza ether, and thia ether, the synthesis of their alkaline and transition metal complexes and the investigation of their liquid crystal, semiconductor, gas sensor, photophysical and photochemical properties by enlightening the structure of these macrocyclic compounds.
Mika Harbeck obtained his PhD from the University of Tübingen, Germany, in 2005 and now works as a senior researcher at the TÜBITAK Marmara Research Center in Gebze, Turkey. His research activities lie in the field of chemical gas and liquid sensors for safety and environmental applications using acoustic transducers as well as the investigation of sensing properties of organic materials, especially phthalocyanines. Dilek D. Erbahar is a researcher at the TÜBITAK Marmara Research Center. Currently she is pursuing a PhD in liquid sensing with QCM sensors. Ilke Gürol received her PhD degree from the Department of Chemistry of Gebze Institute Technology in Turkey. Her research interests are the synthesis of stable ligands, e.g. phthalocyanines, Schiff base’s vic-dioximes with crown ether, aza
Emel Musluo˘ glu BSc, MSc, PhD is an associate professor at the TÜBITAK Marmara Research Center and Deputy Director of the Materials Institute. Her research interests are the synthesis of phthalocyanines, vic-dioximes with functional groups such as crown ether, aza ether, and thia ether, synthesis of their transition metal complexes and investigation of their chemical gas sensor, semiconductor and phase transfer catalysis properties.
Zafer Ziya Öztürk is professor of solid-state physics at the Gebze Institute of Technology (GYTE), Turkey. He received his PhD from the Technical University of Darmstadt, Germany, in 1982. He was a postdoctoral fellow at the Institute for Physical Chemistry, University of Tübingen, Germany, and has held several research, teaching and scientist positions including Dicle University, Diyarbakir, TÜBITAK Marmara Research Center, Gebze, and Marmara University, Istanbul, Turkey. His research interests involve solid-state device sensors, molecular electronics as well as chemical and biochemical sensors.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Phthalocyanines as sensitive coatings for QCM sensors: Comparison of gas and liquid sensing properties Mika Harbeck a,∗ , Dilek D. Erbahar a , Ilke Gürol a , Emel Musluo˘glu a , Vefa Ahsen a,b , Zafer Ziya Öztürk a,c a b c
TÜBITAK Marmara Research Center, Materials Institute, P.O. Box 21, 41470 Gebze, Kocaeli, Turkey Gebze Institute of Technology, Department of Chemistry, P.O. Box 141, 41400 Gebze, Kocaeli, Turkey Gebze Institute of Technology, Department of Physics, P.O. Box 141, 41400 Gebze, Kocaeli, Turkey
a r t i c l e
i n f o
Article history: Received 7 September 2010 Received in revised form 15 December 2010 Accepted 20 December 2010 Available online 28 December 2010 Keywords: Chemical sensor Quartz crystal microbalance Liquid sensing Phthalocyanine Organic pollutant Response modeling
a b s t r a c t The quartz crystal microbalance (QCM) was used to investigate the liquid sensing properties of a set of phthalocyanines (Pcs) which were systematically varied by attaching the substituent 2,2,3,3tetrafluoropropyloxy to different positions and by introducing a central metal ion (i.e. Ni2+ , Zn2+ , and Cu2+ ). The responses to low concentrations of organic compounds such as hydrocarbons and chlorocarbons dissolved in water were recorded. The materials were very sensitive to the tested compounds with detection limits in the lower parts-per-million range and they exhibited a good sensing performance as the sensors have been working fully reversibly and reliably over long periods of time. Besides, the influence of substitution pattern and choice of central metal ion on the liquid sensing properties of Pcs were studied for the first time. The results show that the responses differ notably from each other depending on the modifications made to the Pc. Finally, it is demonstrated that the gas and liquid sensing responses of the materials are highly correlated and can be linked to each other with the help of a basic physical model. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Sensor systems based on acoustic transducers, namely the quartz crystal microbalance (QCM) or surface acoustic wave sensors, are an ongoing topic of intensive research and development for many gas and liquid sensing applications such as environmental monitoring, security and safety, or health analysis [1–3]. Ample work is ongoing in the development of new or improved sensing materials. In the gas phase the QCM transducer is modified with a variety of sensing materials such as polymers or organic macromolecules, which are carefully chosen to suit best the intended application. In the field of liquid sensing the use of the QCM as biosensor platform dominates. Reports of QCM chemical sensors operating in liquids are sparse. Recently, phthalocyanines (Pcs) were introduced as new sensitive materials for the detection of different classes of organic pollutants in water [4,5]. Other current work describes the use of a choline monolayer [6]. Most common are investigations into molecularly imprinted polymers (MIPs) [7–12]. However, these sensitive materials are intended for
∗ Corresponding author. Tel.: +90 262 677 3123; fax: +90 262 641 2309. E-mail addresses: [email protected] (M. Harbeck), [email protected] (D.D. Erbahar), [email protected] (I. Gürol), [email protected] (E. Musluo˘glu), [email protected] (V. Ahsen), [email protected] (Z.Z. Öztürk). 0925-4005/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2010.12.038
very specific targets and not for the measurement of a broad range of chemical compounds as each MIP is specially produced to match a single type of molecule. Besides, the approach is considered less suitable for small molecules without specific interaction pathways. The smallest molecules are still rather complex in their chemical structure [12]. However, small and simple compounds are the main target analytes in many applications such as hydrocarbon or chlorocarbon pollution monitoring. Thus, the same approach as in gas sensing, i.e. using an array of semi-selective sensors and data evaluation methods, is more appropriate for multi-analyte detection and classification of such chemicals of different but quite unspecific nature [13]. More research effort is considered crucial to overcome the lack of available liquid chemical sensors and experimental data in this field. As a large background experience and knowledge in the area of gas sensing are at hand it would be quite advantageous if one could benefit from this in the development of sensing materials for QCM liquid sensors. Experimental or computational results and theoretical models obtained in one area could then be applied also to the other. This work addresses both the development of new sensitive materials for liquid sensing of organic compounds and the linkage to the gas sensing field. A set of Pcs with fluoroalkyloxy substituents attached in different configurations and three different central metal ions were characterized for their QCM sensing performance towards common organic compounds. For the first time the influ-
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The phthalocyanines were synthesized according to reported standard procedures using the corresponding phthalonitrile as starting material. The experimental details of the synthesis and the results of the chemical analysis and spectroscopic characterization are described in [15,16] and references therein.
2.2. Sensor preparation and test procedures
Fig. 1. Chemical structure of the phthalocyanine molecule with an optional central metal ion M and the peripheral substituents R and R and non-peripheral substituent R and a list of compounds characterized with the QCM sensors in this work.
ence of substitution type and central metal on the liquid sensing properties of Pcs were studied. The particular type of substituent and central metal ion were chosen due to good results obtained previously with QCM gas sensors using the same materials [14,15]. The Pcs were found highly sensitive with detection limits in the low parts-per-million range in liquids, as well. Furthermore, the liquid phase sensor responses are compared with gas phase data of the same materials. A basic model allows predicting the liquid sensing results using gas phase data reaching a good approximation of the experimental results.
2. Experimental 2.1. Sensitive materials In total seven different Pc derivatives were characterized for their liquid sensing performance. The Pc cores were modified by adding the substituent 2,2,3,3-tetrafluoropropyloxy (–O–CH2 CF2 CHF2 ) and by introducing a central metal ion (nickel, zinc or copper) replacing the two inner hydrogen atoms. The codes used here to identify the individual compounds specify metal type (1: Ni, 2: Zn, and 3: Cu) as well as substitution pattern (denoted by the letters a–d). The substituents are introduced forming octa (compounds denoted with a) and tetra substituted (b–d) Pcs. Tetra compounds are available with substituents in peripheral, p (b and c) and non-peripheral, np (d) positions. Compounds denoted with b contain an additional chlorine atom. The full list of materials and their identification codes are given in Fig. 1.
The employed transducers, coating procedure, test system and measurement procedures of the liquid sensing experiments have been already described in detail previously in [4] and shall be summarized here only in short. Polished AT-cut QCMs of 4.95 MHz fundamental frequency were used for the liquid sensing experiments, but the more stable 3rd overtone was preferred as sensor signal. Coating of the transducers with the sensitive material was achieved using the air-brush technique. The spraying solution consisted of 1 mg Pc in 1 ml of a suitable organic solvent. Sensors were coated with an amount of sensitive material equivalent to 1 kHz frequency shift (fundamental) on one side. Seven common organic solvents were used as test analytes representing different chemical classes: dichloromethane (DCM), chloroform (TCM), tetrachloroethylene (TCE), chlorobenzene (CB), toluene (TLN), o-xylene (oXLN), and p-xylene (pXLN). The analytes were high purity chemicals and used as obtained from the supplier. 10 or 100 l of an analyte was added to 500 ml ultrapure water via a precision pipette to prepare the respective stock solutions. Test samples were prepared by diluting the stock solution to the desired concentration level. Typical concentration levels were in the range of 0.1–30 ppm (mg/l) with the exception of DCM and TCM. In these two cases the highest concentrations were 300 ppm. Caution was taken to avoid loss of analyte into the gas phase during preparation of the samples. Besides, stock solutions and test samples were prepared freshly before the experiments. QCM sensor signals were recorded with a QCM Z-500 measurement system based on impedance analysis (KSV Instruments Ltd., Finland). All measurements were made at a constant temperature of 20 ◦ C.
3. Results and discussion 3.1. Liquid sensing properties of phthalocyanines A typical sensor response of a QCM sensor coated with a Pc during intermitted exposure to water samples containing different concentrations of o-xylene in the low parts-per-million range is depicted in Fig. 2(a). Besides zeroing the baseline by referencing to the initial sensor value when no analyte was present no correction was made to the sensor data. The negative frequency shifts are the consequence of the analyte exposure. When the sensor is purged with clean water the sensor resonance frequency returns to a value approximating the initial baseline. This example depicts the response of a sensor with 3a to test samples with o-xylene in concentrations from 1 to 20 ppm. The other analytes and sensors give similar results, but differ in the amplitude of the response as well as response and recovery times. The response curve illustrates the excellent response characteristics of the Pc coated QCM sensors in liquid environments. The baseline stability of the sensor signal is very good over time. The equilibrium sensor signal is reached quickly and recovery is fast after exposure to the analyte. 90% of the full sensor signal is typically reached in a period below 90 s and 90% recovery of the baseline after analyte free water is introduced to the sensor chamber is reached in less than 1 min. The sensors could be used for repeated experiments over a period of several weeks without noticeable chance in the sensing performance.
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Fig. 2. (a) Transient signal of a QCM sensor (3rd harmonics) with 3a during exposure to samples containing o-xylene in concentrations between 1 and 20 ppm and (b) resulting sensor calibration curve.
The sensor responses increase with increasing concentration of the analyte and reach nearly 400 Hz for the highest tested o-xylene concentration. The response data can be represented by a linear calibration curve passing through the origin and the frequency shift per unit concentration of analyte (sensitivity) is as high as 20 Hz/ppm (see Fig. 2(b)). The water-sensitive material partition coefficient Kl (dimensionless) is a good descriptor for the sensing performance of a material, as it describes the degree of enrichment of the analyte in the sensing material. High enrichment is usually associated with high sensor responses. Kl is given as the ratio of cs and caq , the concentrations of the analyte in the sensing material and aqueous phase, respectively: Kl =
cs . caq
(1)
From the sensor sensitivity S as the slope of the sensor calibration curve S=
f caq
(2)
the partition coefficient can be calculated using Sauerbrey’s equation for the relation of mass to frequency changes as Kl =
S · s × 106 , fs
(3)
where S is the sensitivity of the QCM sensor for the analyte (in Hz/mg/l), fs the frequency shift of the sensor due to the coating with the sensitive material, and s the density of the sorbent layer (in g/cm3 ). The factor 106 is a result of the conversion of units from SI base units into commonly used units for density (g/cm3 ) and concentration (mg/l). Using Eq. (3) the partition coefficients were calculated for all sensor–analyte combinations from the experimental QCM data. The partition coefficients are presented in the bar diagrams in Fig. 3. The values span a wide range from about 30 for DCM up to 9200 for pXLN. Repetitions of the experiments with newly prepared sensors yielded a high reproducibility of sensors and measurements,
Fig. 3. Experimental partition coefficients of all compounds as obtained in the liquid sensing experiments at 20 ◦ C: comparison of differently substituted Pcs with (top) nickel, and (middle) zinc central metal ion as well as (bottom) results of all octa compounds. Insets provide the results for DCM and TCM on a magnified scale.
as deviations in sensitivity were less than 4%. The materials presented here are more sensitive than other Pcs studied previously under similar conditions [4]. The estimated limits of detection for a sensor with 3a are in a range of 5 ppm for dichloromethane and 0.05 ppm for o-xylene. 3.2. Discussion of liquid sensing results and comparison to gas phase data On the basis of the data presented in Fig. 3 the influences of substitution pattern and central metal ion on the sensing properties of the Pcs, i.e. the partition coefficient and sensitivity, can be summarized as follows: • The amount of absorbed analyte and thus the sensor sensitivity increases for all materials in the order of dichloromethane, chloroform, toluene, chlorobenzene, tetrachloroethylene, p-xylene, and o-xylene. For example, the partition coefficients of 1a for the analytes dichloromethane, chlorobenzene, and o-xylene are 30, 3090, and 5270, respectively (Fig. 3 (top)). • Octa substitution is most effective in increasing analyte uptake: 1a and 2a are more sensitive than their tetra counterparts 1b–d (see Fig. 3 (top)) and 2b (see Fig. 3 (middle)), respectively, with the exception of the analytes DCM and TCM. • The presence of the additional chlorine in R position in the tetra compounds decreases sensitivity (1b vs. 1c and 1d, Fig. 3 (top)). • Tetra substitution in peripheral position is slightly more favorable than in non-peripheral position to increase sensitivity (1c vs. 1d, Fig. 3 (top)). • However, next to the influence of position and number of substituents there is also a strong central metal ion influence, e.g. see data of the octa compounds in Fig. 3 (bottom). A decreasing
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0.8
Height
0.6 Fig. 5. Illustration of the path independency of phase equilibria.
0.4
3.3. Correlation between liquid and gas sensing 0.2
1c
1d
1b
1a
2a
2b
3a
0.0
Fig. 4. Result of a hierarchical cluster analysis of the normalized and centered sensor data.
sensitivity for all analytes can be observed in the order of Pcs with a Cu2+ , Zn2+ , and Ni2+ central metal ions.
Hierarchical cluster analysis (HCA) was used to analyze the high dimensional response data. It allows exploring similarities or dissimilarities among the individual materials according to their responses as sensing materials. The goal is to identify possible variations in the sensing characteristics caused by the changes of central metal ion and substitution pattern. HCA was performed using normalized and centered sensor responses. Euclidian distance was used as a dissimilarity measure and cluster linkage was based on the averaging method. The HCA result presented in Fig. 4 clearly shows a high variation in the response behavior of the materials caused by both a modification of the central metal ion and the substitution pattern. Regarding the octa compounds, the CuPc 3a is much different from the quite similar NiPc 1a and ZnPc 2a. The central metal ions in NiPcs and ZnPcs are in contrast to the one in CuPcs only weak in terms of interaction capabilities with analyte molecules. Furthermore, it was found that the tetra compounds 1b–d and 2b differ from the respective octa Pcs, but show little variation among each other for a given central metal ion type (see 1b–d). A direct comparison with the gas sensing results obtained previously in dry air (see [14]) for the same list of analytes as tested here reveals a generally good agreement between the two methods. The Pcs were found to be highly effective VOC absorbers both from the liquid and the gas phase. Furthermore, the relative responses to the analytes are very similar, e.g. the octa compounds 1a and 2a showed the highest gas phase responses to o-xylene, and then to chlorobenzene, toluene, and chloroform vapors. This finding is in agreement with the results here. For 1a also the same influence of the substitution pattern on sensor sensitivity was found: generally sensitivity is decreasing in the order of compounds with octa, peripheral tetra, and non-peripheral tetra substitution. However, a difference in the results was observed for the tetra compound with an additional chlorine atom (1b). In the gas sensing experiments 1b was found to be more sensitive than the other tetra compounds. This finding is not in agreement with the liquid sensing results. Besides, 1a is more sensitive than 2a in the gas phase. These observed deviations are attributed to the humidity contained in the sensitive material in the liquid sensing experiment. Apparently, the absorbed water has different influences on the sensitivities of the Pcs with a Zn and Ni central metal ions.
The desired quantitative link between gas and liquid sensing results is made using a basic physical model. The partition coefficient between two phases is describing a state which is independent of any intermediate steps required to reach this state. This is illustrated in Fig. 5 for the aqueous-sensitive material phase system. Instead of one direct phase equilibrium one could also imagine a path via two equilibria with an intermediate gas phase (e.g. air). The partitioning between liquid phase and sensitive material, described by Kl , would be unaffected. Accordingly, Kl can be rewritten as Kl = Kg · KH
(4)
or log Kl = log Kg + log KH ,
(5)
where Kg is the gas phase-sensitive material partition coefficient and KH Henry’s law constant for the gas phase–water system (KH = cgas /caq ). Using Eq. (5), Kl can be estimated using experimental gas sensor data (log Kg ) and tabulated material data (log KH ). Henry’s constants for a variety of compounds collected from various sources can be found e.g. in [17], however, all values used here were taken from one source only [18]. The gas phase-sensitive material partition coefficients, Kg , of the Pcs were calculated using the gas phase sensitivity data listed in Table 2 found in [14]. In Fig. 6 the various partition coefficients for TCM, TLN, CB, and oXLN of the octa compounds 2a and 1a are shown. A comparison of log Kl and log Kg shows that gas phase and liquid phase partition coefficients are of the same magnitude, but Kl is generally smaller than Kg . Using the experimental gas phase values and tabulated values Kl was predicted using Eq. (5) and compared to the experimental data obtained in this work. Fig. 7 shows the correlation between the calculated and experimental data in truepredicted plots for 1a,b and 2a,b. The model gives a good estimate of the actual experimental values, but generally the values of Kl are underestimated, i.e. the experimental liquid sensing values are higher than the predicted ones. Even though the absolute values are not predicted fully the correlation between gas and liquid phase data is high. The correlation coefficients R are as high as 0.99. The degree of overestimation varies by material as seen in the variations in the best fit slopes. An explanation for the deviation of the model in absolute values may be found in the fact that the gas sensing data was acquired under dry air conditions. In contrast to this the sensitive material has absorbed the maximum possible amount of water in the case of liquid sensing as it is exposed to a pure water phase. Absorbed water molecules are known to change the sorption properties of a sensitive materials to some extend. However, as water uptake by the Pc as used here is quite modest, the model gives already good results. The agreement is better than in the case of some polymers [19] where the deviation was found to be much larger. To conclude, the approach made here is suitable to estimate the performance of a Pc in liquid sensing experiments when gas phase data is already available or vice versa, but could possibly be improved using gas phase data obtained at high humidity levels.
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tive towards common organic compounds dissolved in aqueous samples. Due to the numerous positive aspects such as extended stability and short response and recovery times they are considered highly suitable for the use as components of a QCM sensor array. From the materials under test the best results in terms of sensitivity were obtained with octa substituted compounds having Cu2+ , Zn2+ , or Ni2+ central metal ions. A wide correspondence to gas phase sensor results was found in a direct comparison of the two techniques. Similar to gas sensing there is an influence of substitution pattern and central metal ion on the sensing properties. Furthermore, using a simple physical model a correlation was established with gas sensing data with good agreement. This is an important step to apply the experience and knowledge obtained in this field to the area of liquid sensing, as in the latter only little experience is available so far. The field of liquid chemical sensing is expected to expand in the future and this link will contribute to accelerate and ease the progress. Acknowledgement This study was supported by the DPT (Devlet Planlama Tes¸kilatı), the Turkish State Planning Organization. References
Fig. 6. Partition coefficients of selected analytes for the octa compounds (a) 2a and (b) 1a. Kl (exp) is the experimentally obtained water-Pc partition coefficient (this work), Kg is the gas phase-sensitive material partition coefficient (experimental data from [14]) and KH the Henry’s law constant for the gas phase–water system (tabulated in [17,18]).
Fig. 7. True-predicted plots showing Kl (exp) vs. Kl , the water-Pc partition coefficients obtained experimentally and calculated using Eq. (5), respectively: (a–b) 1a and 1b, (c–d) 2a and 2b.
4. Conclusions Phthalocyanines with fluoroalkyl oxy substituents varied systematically in substitution type and choice of central metal ion were extensively characterized as sensitive materials for QCM liquid sensing applications. They were found to be highly sensi-
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ether, and thia ether functional groups, the synthesis of their alkaline and transition metal complexes and the investigation of their liquid crystal, semiconductor, and gas sensing properties.
Biographies
Vefa Ahsen BSc, MSc, PhD is the dean of science faculty at the Gebze Institute of Technology (GYTE), Turkey. His research interest lies in the synthesis of stable ligands, e.g. phthalocyanines, vic-dioximes with such functional group as crown ether, aza ether, and thia ether, the synthesis of their alkaline and transition metal complexes and the investigation of their liquid crystal, semiconductor, gas sensor, photophysical and photochemical properties by enlightening the structure of these macrocyclic compounds.
Mika Harbeck obtained his PhD from the University of Tübingen, Germany, in 2005 and now works as a senior researcher at the TÜBITAK Marmara Research Center in Gebze, Turkey. His research activities lie in the field of chemical gas and liquid sensors for safety and environmental applications using acoustic transducers as well as the investigation of sensing properties of organic materials, especially phthalocyanines. Dilek D. Erbahar is a researcher at the TÜBITAK Marmara Research Center. Currently she is pursuing a PhD in liquid sensing with QCM sensors. Ilke Gürol received her PhD degree from the Department of Chemistry of Gebze Institute Technology in Turkey. Her research interests are the synthesis of stable ligands, e.g. phthalocyanines, Schiff base’s vic-dioximes with crown ether, aza
Emel Musluo˘ glu BSc, MSc, PhD is an associate professor at the TÜBITAK Marmara Research Center and Deputy Director of the Materials Institute. Her research interests are the synthesis of phthalocyanines, vic-dioximes with functional groups such as crown ether, aza ether, and thia ether, synthesis of their transition metal complexes and investigation of their chemical gas sensor, semiconductor and phase transfer catalysis properties.
Zafer Ziya Öztürk is professor of solid-state physics at the Gebze Institute of Technology (GYTE), Turkey. He received his PhD from the Technical University of Darmstadt, Germany, in 1982. He was a postdoctoral fellow at the Institute for Physical Chemistry, University of Tübingen, Germany, and has held several research, teaching and scientist positions including Dicle University, Diyarbakir, TÜBITAK Marmara Research Center, Gebze, and Marmara University, Istanbul, Turkey. His research interests involve solid-state device sensors, molecular electronics as well as chemical and biochemical sensors.