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Self-assembled monolayers of mercaptoporphyrins as sensing material for quartz crystal microbalance chemical sensors
Sensors and Actuators B 47 (1998) 70 – 76
Self-assembled monolayers of mercaptoporphyrins as sensing material for quartz crystal microbalance chemical sensors Roberto Paolesse a,*, Corrado Di Natale b, Antonella Macagnano b, Fabrizio Davide b, Tristano Boschi a, Arnaldo D’Amico b a
Dipartimento di Scienze e Tecnologie Chimiche, Uni6ersita` di Roma ‘Tor Vergata’, Via della Ricerca Scientifica, 00133 Rome, Italy b Dipartimento di Ingegneria Elettronica Uni6ersita` di Roma ‘Tor Vergata’, 00173 Roma, Italy
Abstract Thiol-functionalized metalloporphyrin have been deposited as self-assembled monolayers onto the gold pad of quartz crystal microbalances (QMBs). The sensitivities of the resulting sensors have been measured with respect to model volatile organic compounds (VOCs), interesting for future practical applications. The sensitivities of these functionalized QMBs depend on the number of the sulfide groups present at the peripheral positions of the porphyrins, but they are higher if compared with the corresponding casting coated sensors. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Porphyrins; Self-assembled monolayers; Quartz crystal microbalance; Gas sensors
1. Introduction The development of chemical sensors for compounds detection in the gaseous state has been object of several investigations [1]. This great interest can be explicate by the numerous and fruitful applications of these devices, ranging from the environmental control to food analysis. From a schematic point of view, a chemical sensor is constituted by both a sensing material, interacting with the species present in the environment and a transducer which converts these chemical informations in some measurable data [2]. While a large number of different transducers has been developed, due to the continuous technological progress, a critical point for the development of a chemical sensor is the choice of the sensing material. A flurry of different classes of molecules have been proposed during these studies, able to bind volatile organic compounds (VOCs) by means of different types of interactions, such as electrostatic, coordination, hydrogen bonding, etc.. The first attempt was to design molecules with high selectivity, i.e. able to bind a single compound of a complex mixture. The overwhelming complexity of the chemical composition of the analyzed * Corresponding author. Tel.: + 39 6 72594386; fax: + 39 6 72594328; e-mail: [email protected] 0925-4005/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0925-4005(98)00052-5
environments lead to the development of a different approach, where the analysis is carried out with an array of different sensors, each of them having different properties of selectivity towards the chemical species present in the mixture. In this way all the sensors give an information related to the whole environment and the subsequent data analysis interprets these informations related to the chemical environments. Due to the analogies with the mammalian olfact, this approach of chemical sensing has been called electronic nose procedure. In the last few years the authors have been interested in the development of a sensor array composed by quartz crystal microbalances (QMB) coated with metal complexes of porphyrins for the analysis of VOCs [3–6]. The choice of metalloporphyrins as sensing material for these sensors was particularly significant: in fact they are able to bind in a reversible way a large number of chemical compounds, so mimicking their biological activity, for example in the hemoglobin [7], furthermore they are stable compounds and can be stored and used without strong precautions. Porphyrins are probably the most versatile ligands and almost all metals present in the Periodic Table have been coordinated to this macrocycle: this feature allows the preparation of a wide range of different sensors with the same porphyrin, by changing the metal coordinated.
R. Paolesse et al. / Sensors and Actuators B 47 (1998) 70–76
This aspect can be expanded further by operating substitutions on the molecular skeleton of the macrocycle, peripheral groups can influence the electronic distribution on the p-aromatic system of the porphyrin, so influencing the coordination and the related sensing properties. The developed synthetic chemistry of the porphyrin [8] allows the preparation of a wide range of different macrocycles and consequently different sensors. In the previous studies on the metalloporphyrin coated QMBs the authors discovered that the metal coordinated drives the selectivity of the corresponding QMB, whereas the different porphyrins influence the overall sensibility of the related sensor [3]. This prototype of electronic nose has been successfully tested both with test reagents and then in the analysis of the aroma of spoiling foods, beside these encouraging results, anyway, some aspects remain to be improved. First of all, the deposition of a thin film of metalloporphyrins onto the QMBs has been carried out by evaporation of a solution of the complexes in an organic solvent, this approach has the advantage to be extremely simple, but, on the other hand, it does not allow the control of the formation of the resulting film onto the quartz surface. The consequence of this problem is that the reproducibility of the film is not allowed and the authors had significant variations in the sensitivity performances of QMBs coated with the same metalloporphyrins. Furthermore, the adhesion of the film onto the quartz surface is not perfect and some amount of the metalloporphyrin can be mechanically lost during the time. As the stability of these compounds allows their use for a long time period (up to 1 year in the laboratories), this aspect can lead to a slow decrease in the performance of these sensors. Different systems have been developed for the deposition of thin films of organic substrated onto inorganic surfaces and the great challenge is to create surfaces with controlled structures [9]. One of the most fascinating approach has been the chemiadsorption of organic compounds onto inorganic substrates, the strong interactions between the organic molecule and the inorganic substrate causes the spontaneous formation of a monolayer film. The resulting self assembled monolayer is chemically bonded to the surface, so having improved chemical and mechanical stability. One of the most promising examples of this approach, reported in the literature, has been the formation of monolayers of organic thiols onto gold surfaces [9]. In this paper the authors report the deposition of self-assembled monolayers of metalloporphyrins functionalized with thiol groups onto the gold pad of QMBs and the performances of the resulting sensors toward the detection of model VOCs.
71
2. Experimental An array consisting of eight QMBs has been used, six sensors have been coated with self-assembling monolayers of different porphyrins and the two remaining QMBs have been coated by casting with metallopolphyrins, in order to compare the relative performances. Thiol functionalized meso-tetraphenylporphyrins (TPP(SH)x), reported in Fig. 1, have been prepared by a stepwise approach, starting from the corresponding para(2-bromoethoxy)-TPP, prepared from the condensation of para-(2-bromoethoxy)-benzaldehyde with pyrrole and benzaldehyde, following literature methods [10]. The bromo-substituted porphyrins have been reacted with sodium thioacetate to give the corresponding thioester, which, after acidic hydrolysis, afforded the related TPP(SH)x in good yields. Cobalt and Manganese metal complexes of these thiol-functionalized porphyrins have been prepared following literature methods [11], using carefully deoxygenated solvents. All these compounds have been fully characterized by spectroscopic and elemental analysis. The deposition of the self-assembled monolayers have been carried out by immersion of the AT-cut quartz crystals with gold electrodes (oscillatory frequency 20 MHz) in a 10 − 3 M dichloromethane solution of the thiol-functionalized metalloporphyrin for 1 day. After this time, the quartz crystals have been
Fig. 1. Structural formulae of mercaptoporphyrins. CoTPP(SH) (R2 =R3 =R4 =H; R1 =OCH2CH2SH; M =Co). CoTPP(SH)2 (R2 =R4 =H; R1 =R3 =OCH2CH2SH; M = Co). CoTPP(SH)Br (R2 =R4 =H; R1 =OCH2CH2SH; R3 =OCH2CH2Br; M= Co). MnTPP(SH) (R2 =R3 =R4 =H; R1 =OCH2CH2SH; M = MnCl). MnTPP(SH)4 (R1 =R2 =R3 =R4 =OCH2CH2SH; M =MnCl).
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R. Paolesse et al. / Sensors and Actuators B 47 (1998) 70–76
Fig. 2. Response curves of the QMBs towards triethylamine.
sonicated in dichloromethane for 30 min., then washed several times with ethanol and then dichloromethane. The measurement set-up consisted of a test chamber with a volume of 50 ml, housing eight QMB sensors. All measurements were carried out at room temperature, test reagents were injected into the test chamber by a gas syringe and a flux of nitrogen was used to evacuate the chamber after the measurements. These chemicals were reagent grade (Aldrich) and were used without further purifications.
Measurements of changes in frequency of QMBs were carried out through a multiplexer stage and a counter connected to a personal computer for data elaboration.
3. Results and discussion The first step of this research has been the preparation of the thiol-functionalized TPPs: these compounds
R. Paolesse et al. / Sensors and Actuators B 47 (1998) 70–76
73
Fig. 3. Response curves of the QMBs towards ethanol.
have been synthesized by a stepwise approach, using a 2-bromoethoxy group as precursor of the thiol. The number of the appendages present at the peripheral positions of the TPP can be varied by changing the relative ratio of the 2-bromoethoxybenzaldehyde and unsubstituted benzaldehyde during the synthesis of the intermediate bromo-substituted TPPs. The number of the sulfide groups present at the periphery of the TPP is important, because it can influence the orientation of the macrocycle onto the gold surface. In the case of cobalt, anyway, it was impossible to obtain the corre-
sponding CoTPP(SH)4 complexes, because the formation of disulfide bridges, catalyzed by the cobalt ion, induced the polymerization of the starting porphyrin affording an insoluble material [12]. The deposition of the monolayer films of metalloporphyrins onto the gold pad of QMBs has been carried out by immersion of the sensor into a dichloromethane solution of the metalloporphyrin for 24 h. Prolonged deposition times did not give significant changes in the performances of the resulting sensors, so indicating a good coverage of the gold surface.
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R. Paolesse et al. / Sensors and Actuators B 47 (1998) 70–76
Fig. 4. Orientation of the selfassembled monolayers of mercaptoporphyrins depending on the number of the sulfide groups at the peripheral positions.
To measure the performances of these self-assembled monolayer coated QMBs an eight sensor array has been used, two QMBs coated by casting with CoTPP and MnTPPC1, have been present in this array, in order to compare the performances of these new sensors with those previously used. Test reagents have been used for these measurements, they have been chosen looking at the possible applications of these sensors and can be considered as model compounds of the chemical species present, for example, in the aroma of spoiling foods (amine, aldehydes, etc.) or in some working environment (aromatic solvents). Some example of the sensitivities obtained with the self-assembled QMBs to respect the reference sensors are reported in Figs. 2 and 3. The response obtained is generally comparable, if not higher, in the case of the Co and Mn complexes of mono thiol-functionalized TPP, so showing that the monolayer deposited ensures a good sensitivity to the resulting sensor. It is worth noting that the gold pad, where the monolayer is deposited, is in the middle of the quartz crystal, the region more important to determine the sensitivity of the QMB. While the presence of a second thiol group did not significantly influence the sensitivity of the corresponding QMB, as in the case of the CoTPP(SH)2, a significant increase is observed in the case of MnTPP(SH)4, which showed a higher response. This feature can be explicated by the different orientation that these metalloporphyrins can assume onto gold, depending on the number of the sulfide group at the peripheral positions. When one SH group is present, the probable orientation of the porphyrin macrocycle is perpendicular to the gold surface (Fig. 4), this orientation favored also by the possible p – p interactions between the aromatic systems of the macrocycles [13]. In the case of MnTPP(SH)4, the more plausible orientation is parallel to the surface (Fig. 4), in the literature, in fact, this orientation has been reported for a similar CoTPP(SH)4 complex [14]. This orientation allows a greater binding property for the resulting monolayer, because it rules out all the p–p interactions between the macrocycles, that compete with the axial coordination. This hypothesis was confi-
rmed by the behavior showed in the case of the ethylendiamine (Fig. 5), in this case the sensitivity of the MnTPP(SH)4 functionalized QMB was lower than the other functionalized sensors. This result can be explicated by the bidentate nature of the ethylendiamine, while in the perpendicular oriented monolayers the metalloporphyrins can cooperate in the binding of this specie; this is impossible for the MnTPP(SH)4 so explicating the unusual lower sensitivity. It is relevant to note that these functionalized QMBs allowed the detection of species, such as benzene and other hydrocarbons, that were impossible with the casting coated sensors. In this case, in fact, the absence of donor atoms in the molecules rules out their coordination to the metal and weaker interactions must be present, because of the higher sensitivity of these selfassembled monolayer QMBs, these weak interactions allowed the detection of these compounds, that are important for environmental monitoring. The selectivities of these sensors are depending on the nature of the metal coordinated to the thiol-functionalized porphyrin, as previously reported for the casting coated QMBs, these selectivities can be predicted by the Hard–Soft Acid–Base principle, Mn-porphyrinates showed a higher sensitivity to hard ligands, such as alcohols, whereas Cobalt complexes had a relative higher response to molecules with soft donor atoms, like dimethylsulfide. The advantage of these self-assembled monolayers is also the reproducibility of the depositions, a set of four QMBs have been functionalized with CoTPP(SH), showing similar performances. It is also appropriate to point out that the deposition can be controlled further by changing the deposition times, so varying the coverage of the surface.
4. Conclusions The deposition of monolayers of thiol-functionalized metalloporphyrins is a useful approach to develop QMB sensors with improved properties. The performances of these sensors have been measured with respect to model volatile compounds of interest for practical applications.
R. Paolesse et al. / Sensors and Actuators B 47 (1998) 70–76
75
Fig. 5. Response curves of the QMBs towards ethylendiamine.
The results obtained showed the suitability of these sensors to be used for electronic nose applications.
Acknowledgements This work was supported by Murst and CNR. The valuable technical assistance of Mr. M. Zarlenga is gratefully acknowledged.
References [1] M.J. Madou, S.R. Morrison, Chemical sensing with Solid State Devices, Academic Press, New York, 1989, p. 1. [2] J. Janata, Principle of Chemical Sensors, Plenum Press, New York, 1989, p. 1. [3] J.A.J. Brunink, C.Di Natale, F. Bungaro, F.A.M. Davide, A. D’Amico, R. Paolesse, T. Boschi, M. Faccio, G. Ferri, The application of metalloporphyrins as coating material for QMBbased chemical sensors, Anal. Chim. Acta 325 (1996) 53.
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[4] C. Di Natale, J.A.J. Brunink, F. Bungaro, F. Davide, A. D’Amico, R. Paolesse, T. Boschi, M. Faccio, G. Ferri, Recognition of fish storage time by a metallo-porphyrin coated QMB sensor array, Meas. Sci. Technol. 7 (1996) 1. [5] C. Di Natale, A. Macagnano, F. Davide, A. D’Amico, R. Paolesse, T. Boschi, M. Faccio, G. Ferri, An electronic nose for food analysis, Sens. Actuators B 44 (1997) 521. [6] C. Di Natale, A. Macagnano, G. Repole, A. D’Amico, R. Paolesse, T. Boschi, The exploitation of metalloporphyrins as chemically interactive material in chemical sensors, Mater. Sci. Eng. (in press). [7] D. Dolphin (Ed.), The Porphyrins vol. VII, Academic Press, New York, 1978, p. 445. [8] D. Dolphin (Ed.), The Porphyrins, vol. I, Academic Press, New York, 1978, p. 289. [9] C.D. Bain, G.M. Whitesides, Modeling organic surfaces with self-assembled monolayers, Angew. Chem. Int. Ed. Engl. 101 (1989) 522.
.
[10] J.B. Kim, A.A. Adler, F.R. Longo, Synthesis of Poprhyrin from Monopyrroles, in: D. Dolphin (Ed.), The Porphyrins vol. I, Academic Press, New York, 1978, p. 85. [11] J.W. Buchler, Synthesis and Properties of Metalloporphyrins, in: D. Dolphin (Ed.), The Porphyrins vol. I, Academic Press, New York, 1978, p. 389. [12] L. Wen, M. Li, J.B. Schlenoff, Polyporphyrin thin films from the interfacial polymerization of mercaptoporphyrins, J. Am. Chem. Soc. 119 (1997) 7726. [13] J. Zak, H. Yuan, M. Ho, L.K. Woo, M.D. Porter, Thiol-derivatized metalloporphyrins: monomolecular films for the electrocatalytic reduction of dioxygen at gold electrodes, Langmuir 9 (1993) 2722. [14] T.A. Postlethwaite, J.E. Hutchison, K.W. Hathcock, R.W. Murray, Optical, electrochemical and electrocatalytic properties of self-assembled thiol derivatized porphyrins on transparent gold films, Langmuir 11 (1995) 4109.
Self-assembled monolayers of mercaptoporphyrins as sensing material for quartz crystal microbalance chemical sensors Roberto Paolesse a,*, Corrado Di Natale b, Antonella Macagnano b, Fabrizio Davide b, Tristano Boschi a, Arnaldo D’Amico b a
Dipartimento di Scienze e Tecnologie Chimiche, Uni6ersita` di Roma ‘Tor Vergata’, Via della Ricerca Scientifica, 00133 Rome, Italy b Dipartimento di Ingegneria Elettronica Uni6ersita` di Roma ‘Tor Vergata’, 00173 Roma, Italy
Abstract Thiol-functionalized metalloporphyrin have been deposited as self-assembled monolayers onto the gold pad of quartz crystal microbalances (QMBs). The sensitivities of the resulting sensors have been measured with respect to model volatile organic compounds (VOCs), interesting for future practical applications. The sensitivities of these functionalized QMBs depend on the number of the sulfide groups present at the peripheral positions of the porphyrins, but they are higher if compared with the corresponding casting coated sensors. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Porphyrins; Self-assembled monolayers; Quartz crystal microbalance; Gas sensors
1. Introduction The development of chemical sensors for compounds detection in the gaseous state has been object of several investigations [1]. This great interest can be explicate by the numerous and fruitful applications of these devices, ranging from the environmental control to food analysis. From a schematic point of view, a chemical sensor is constituted by both a sensing material, interacting with the species present in the environment and a transducer which converts these chemical informations in some measurable data [2]. While a large number of different transducers has been developed, due to the continuous technological progress, a critical point for the development of a chemical sensor is the choice of the sensing material. A flurry of different classes of molecules have been proposed during these studies, able to bind volatile organic compounds (VOCs) by means of different types of interactions, such as electrostatic, coordination, hydrogen bonding, etc.. The first attempt was to design molecules with high selectivity, i.e. able to bind a single compound of a complex mixture. The overwhelming complexity of the chemical composition of the analyzed * Corresponding author. Tel.: + 39 6 72594386; fax: + 39 6 72594328; e-mail: [email protected] 0925-4005/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0925-4005(98)00052-5
environments lead to the development of a different approach, where the analysis is carried out with an array of different sensors, each of them having different properties of selectivity towards the chemical species present in the mixture. In this way all the sensors give an information related to the whole environment and the subsequent data analysis interprets these informations related to the chemical environments. Due to the analogies with the mammalian olfact, this approach of chemical sensing has been called electronic nose procedure. In the last few years the authors have been interested in the development of a sensor array composed by quartz crystal microbalances (QMB) coated with metal complexes of porphyrins for the analysis of VOCs [3–6]. The choice of metalloporphyrins as sensing material for these sensors was particularly significant: in fact they are able to bind in a reversible way a large number of chemical compounds, so mimicking their biological activity, for example in the hemoglobin [7], furthermore they are stable compounds and can be stored and used without strong precautions. Porphyrins are probably the most versatile ligands and almost all metals present in the Periodic Table have been coordinated to this macrocycle: this feature allows the preparation of a wide range of different sensors with the same porphyrin, by changing the metal coordinated.
R. Paolesse et al. / Sensors and Actuators B 47 (1998) 70–76
This aspect can be expanded further by operating substitutions on the molecular skeleton of the macrocycle, peripheral groups can influence the electronic distribution on the p-aromatic system of the porphyrin, so influencing the coordination and the related sensing properties. The developed synthetic chemistry of the porphyrin [8] allows the preparation of a wide range of different macrocycles and consequently different sensors. In the previous studies on the metalloporphyrin coated QMBs the authors discovered that the metal coordinated drives the selectivity of the corresponding QMB, whereas the different porphyrins influence the overall sensibility of the related sensor [3]. This prototype of electronic nose has been successfully tested both with test reagents and then in the analysis of the aroma of spoiling foods, beside these encouraging results, anyway, some aspects remain to be improved. First of all, the deposition of a thin film of metalloporphyrins onto the QMBs has been carried out by evaporation of a solution of the complexes in an organic solvent, this approach has the advantage to be extremely simple, but, on the other hand, it does not allow the control of the formation of the resulting film onto the quartz surface. The consequence of this problem is that the reproducibility of the film is not allowed and the authors had significant variations in the sensitivity performances of QMBs coated with the same metalloporphyrins. Furthermore, the adhesion of the film onto the quartz surface is not perfect and some amount of the metalloporphyrin can be mechanically lost during the time. As the stability of these compounds allows their use for a long time period (up to 1 year in the laboratories), this aspect can lead to a slow decrease in the performance of these sensors. Different systems have been developed for the deposition of thin films of organic substrated onto inorganic surfaces and the great challenge is to create surfaces with controlled structures [9]. One of the most fascinating approach has been the chemiadsorption of organic compounds onto inorganic substrates, the strong interactions between the organic molecule and the inorganic substrate causes the spontaneous formation of a monolayer film. The resulting self assembled monolayer is chemically bonded to the surface, so having improved chemical and mechanical stability. One of the most promising examples of this approach, reported in the literature, has been the formation of monolayers of organic thiols onto gold surfaces [9]. In this paper the authors report the deposition of self-assembled monolayers of metalloporphyrins functionalized with thiol groups onto the gold pad of QMBs and the performances of the resulting sensors toward the detection of model VOCs.
71
2. Experimental An array consisting of eight QMBs has been used, six sensors have been coated with self-assembling monolayers of different porphyrins and the two remaining QMBs have been coated by casting with metallopolphyrins, in order to compare the relative performances. Thiol functionalized meso-tetraphenylporphyrins (TPP(SH)x), reported in Fig. 1, have been prepared by a stepwise approach, starting from the corresponding para(2-bromoethoxy)-TPP, prepared from the condensation of para-(2-bromoethoxy)-benzaldehyde with pyrrole and benzaldehyde, following literature methods [10]. The bromo-substituted porphyrins have been reacted with sodium thioacetate to give the corresponding thioester, which, after acidic hydrolysis, afforded the related TPP(SH)x in good yields. Cobalt and Manganese metal complexes of these thiol-functionalized porphyrins have been prepared following literature methods [11], using carefully deoxygenated solvents. All these compounds have been fully characterized by spectroscopic and elemental analysis. The deposition of the self-assembled monolayers have been carried out by immersion of the AT-cut quartz crystals with gold electrodes (oscillatory frequency 20 MHz) in a 10 − 3 M dichloromethane solution of the thiol-functionalized metalloporphyrin for 1 day. After this time, the quartz crystals have been
Fig. 1. Structural formulae of mercaptoporphyrins. CoTPP(SH) (R2 =R3 =R4 =H; R1 =OCH2CH2SH; M =Co). CoTPP(SH)2 (R2 =R4 =H; R1 =R3 =OCH2CH2SH; M = Co). CoTPP(SH)Br (R2 =R4 =H; R1 =OCH2CH2SH; R3 =OCH2CH2Br; M= Co). MnTPP(SH) (R2 =R3 =R4 =H; R1 =OCH2CH2SH; M = MnCl). MnTPP(SH)4 (R1 =R2 =R3 =R4 =OCH2CH2SH; M =MnCl).
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R. Paolesse et al. / Sensors and Actuators B 47 (1998) 70–76
Fig. 2. Response curves of the QMBs towards triethylamine.
sonicated in dichloromethane for 30 min., then washed several times with ethanol and then dichloromethane. The measurement set-up consisted of a test chamber with a volume of 50 ml, housing eight QMB sensors. All measurements were carried out at room temperature, test reagents were injected into the test chamber by a gas syringe and a flux of nitrogen was used to evacuate the chamber after the measurements. These chemicals were reagent grade (Aldrich) and were used without further purifications.
Measurements of changes in frequency of QMBs were carried out through a multiplexer stage and a counter connected to a personal computer for data elaboration.
3. Results and discussion The first step of this research has been the preparation of the thiol-functionalized TPPs: these compounds
R. Paolesse et al. / Sensors and Actuators B 47 (1998) 70–76
73
Fig. 3. Response curves of the QMBs towards ethanol.
have been synthesized by a stepwise approach, using a 2-bromoethoxy group as precursor of the thiol. The number of the appendages present at the peripheral positions of the TPP can be varied by changing the relative ratio of the 2-bromoethoxybenzaldehyde and unsubstituted benzaldehyde during the synthesis of the intermediate bromo-substituted TPPs. The number of the sulfide groups present at the periphery of the TPP is important, because it can influence the orientation of the macrocycle onto the gold surface. In the case of cobalt, anyway, it was impossible to obtain the corre-
sponding CoTPP(SH)4 complexes, because the formation of disulfide bridges, catalyzed by the cobalt ion, induced the polymerization of the starting porphyrin affording an insoluble material [12]. The deposition of the monolayer films of metalloporphyrins onto the gold pad of QMBs has been carried out by immersion of the sensor into a dichloromethane solution of the metalloporphyrin for 24 h. Prolonged deposition times did not give significant changes in the performances of the resulting sensors, so indicating a good coverage of the gold surface.
74
R. Paolesse et al. / Sensors and Actuators B 47 (1998) 70–76
Fig. 4. Orientation of the selfassembled monolayers of mercaptoporphyrins depending on the number of the sulfide groups at the peripheral positions.
To measure the performances of these self-assembled monolayer coated QMBs an eight sensor array has been used, two QMBs coated by casting with CoTPP and MnTPPC1, have been present in this array, in order to compare the performances of these new sensors with those previously used. Test reagents have been used for these measurements, they have been chosen looking at the possible applications of these sensors and can be considered as model compounds of the chemical species present, for example, in the aroma of spoiling foods (amine, aldehydes, etc.) or in some working environment (aromatic solvents). Some example of the sensitivities obtained with the self-assembled QMBs to respect the reference sensors are reported in Figs. 2 and 3. The response obtained is generally comparable, if not higher, in the case of the Co and Mn complexes of mono thiol-functionalized TPP, so showing that the monolayer deposited ensures a good sensitivity to the resulting sensor. It is worth noting that the gold pad, where the monolayer is deposited, is in the middle of the quartz crystal, the region more important to determine the sensitivity of the QMB. While the presence of a second thiol group did not significantly influence the sensitivity of the corresponding QMB, as in the case of the CoTPP(SH)2, a significant increase is observed in the case of MnTPP(SH)4, which showed a higher response. This feature can be explicated by the different orientation that these metalloporphyrins can assume onto gold, depending on the number of the sulfide group at the peripheral positions. When one SH group is present, the probable orientation of the porphyrin macrocycle is perpendicular to the gold surface (Fig. 4), this orientation favored also by the possible p – p interactions between the aromatic systems of the macrocycles [13]. In the case of MnTPP(SH)4, the more plausible orientation is parallel to the surface (Fig. 4), in the literature, in fact, this orientation has been reported for a similar CoTPP(SH)4 complex [14]. This orientation allows a greater binding property for the resulting monolayer, because it rules out all the p–p interactions between the macrocycles, that compete with the axial coordination. This hypothesis was confi-
rmed by the behavior showed in the case of the ethylendiamine (Fig. 5), in this case the sensitivity of the MnTPP(SH)4 functionalized QMB was lower than the other functionalized sensors. This result can be explicated by the bidentate nature of the ethylendiamine, while in the perpendicular oriented monolayers the metalloporphyrins can cooperate in the binding of this specie; this is impossible for the MnTPP(SH)4 so explicating the unusual lower sensitivity. It is relevant to note that these functionalized QMBs allowed the detection of species, such as benzene and other hydrocarbons, that were impossible with the casting coated sensors. In this case, in fact, the absence of donor atoms in the molecules rules out their coordination to the metal and weaker interactions must be present, because of the higher sensitivity of these selfassembled monolayer QMBs, these weak interactions allowed the detection of these compounds, that are important for environmental monitoring. The selectivities of these sensors are depending on the nature of the metal coordinated to the thiol-functionalized porphyrin, as previously reported for the casting coated QMBs, these selectivities can be predicted by the Hard–Soft Acid–Base principle, Mn-porphyrinates showed a higher sensitivity to hard ligands, such as alcohols, whereas Cobalt complexes had a relative higher response to molecules with soft donor atoms, like dimethylsulfide. The advantage of these self-assembled monolayers is also the reproducibility of the depositions, a set of four QMBs have been functionalized with CoTPP(SH), showing similar performances. It is also appropriate to point out that the deposition can be controlled further by changing the deposition times, so varying the coverage of the surface.
4. Conclusions The deposition of monolayers of thiol-functionalized metalloporphyrins is a useful approach to develop QMB sensors with improved properties. The performances of these sensors have been measured with respect to model volatile compounds of interest for practical applications.
R. Paolesse et al. / Sensors and Actuators B 47 (1998) 70–76
75
Fig. 5. Response curves of the QMBs towards ethylendiamine.
The results obtained showed the suitability of these sensors to be used for electronic nose applications.
Acknowledgements This work was supported by Murst and CNR. The valuable technical assistance of Mr. M. Zarlenga is gratefully acknowledged.
References [1] M.J. Madou, S.R. Morrison, Chemical sensing with Solid State Devices, Academic Press, New York, 1989, p. 1. [2] J. Janata, Principle of Chemical Sensors, Plenum Press, New York, 1989, p. 1. [3] J.A.J. Brunink, C.Di Natale, F. Bungaro, F.A.M. Davide, A. D’Amico, R. Paolesse, T. Boschi, M. Faccio, G. Ferri, The application of metalloporphyrins as coating material for QMBbased chemical sensors, Anal. Chim. Acta 325 (1996) 53.
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