Langmuir−Blodgett Films of a Manganese Corrole Derivative

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Langmuir-Blodgett Films of a Manganese Corrole Derivative Roberto Paolesse,*,† Corrado Di Natale,‡ Antonella Macagnano,‡ Francesco Sagone,† Manuela A. Scarselli,§ Piero Chiaradia,§ Vladimir I. Troitsky,| Tatiana S. Berzina,| and Arnaldo D’Amico‡ Dipartimento di Scienze e Tecnologie Chimiche, Dipartimento di Ingegneria Elettronica, and Dipartimento di Fisica and Istituto Nazionale di Fisica della Materia, Universita` di Roma “Tor Vergata”, Via della Ricerca Scientifica, 00173 Roma, Italy, and Technobiochip s.r.l. 57030 Marciana, (LI), Italy Received November 24, 1997. In Final Form: November 23, 1998 The first example of deposition of Langmuir-Blodgett (LB) films of a manganese complex of corrole is described. Characterization of the morphological and molecular structure of these LB films has been carried out by means of different microscopic and spectroscopic methods. Depositions onto different substrates afford very uniform films of determined thickness; all the experiments carried out suggest that manganese corrolates in LB films are normally oriented to the substrate, with a face-to-face packing of the macrocycles, due to π-π interactions. The interactions of these LB films with different volatile organic compounds (VOCs) have been investigated using the quartz microbalance (QMB) technique.

Introduction Molecular materials represent one of the most promising tools for the development of new technologies with important applications in different fields, such as for example sensors and nanoelectronics.1 One of the great advantages of these organic materials lies in the possibility of tailoring the desired compounds, so tuning physicochemical properties for specific applications. Further, it is possible to build up highly ordered thin films of these materials through their supramolecular interactions, and this control at a molecular level is a necessary requirement for their future applications in different devices.1 Among organic compounds, porphyrins represent one of the most fascinating species, because of their rich and almost unique properties, which are of interest for different disciplines.2 In fact they are widely present in Nature, where they play functions necessary for life,3 and they have been intensively studied to obtain information on biological systems. Their ability in natural systems to catalyze reactions, that are normally difficult if not impossible to carry out in laboratories, has attracted interest for their application as catalysts,4 trying to mimick their biological activity for synthetic applications. It is also relevant to note that porphyrins represent only the most important example, but a large number of * To whom correspondence should be addressed. Phone: 39.6.72594386. Fax: 39.6.72594328. E-mail: roberto.paolesse@ uniroma2.it. † Dipartimento di Scienze e Tecnologie Chimiche, Universita ` di Roma “Tor Vergata”. ‡ Dipartimento di Ingegneria Elettronica, Universita ` di Roma “Tor Vergata”. § Dipartimento di Fisica and Istituto Nazionale di Fisica della Materia, Universita` di Roma “Tor Vergata”. | Technobiochip s.r.l. 57030 Marciana. (1) Go¨pel, W.; Ziegler, Ch. In Nanostructure Based on Molecular Materials; Go¨pel, W., Ziegler, Ch., Eds.; VCH: Weinheim, 1992; p 1. (2) Porphyrins and Metalloporphyrins; Smith, K. M., Ed.; Elsevier: Amsterdam; 1975. (3) The Porphyrins; Dolphin, D., Ed.; Academic Press: New York; 1978. (4) Metalloporphyrins in Catalytic Oxidations; Sheldon, R. A., Ed.; Marcel Dekker: New York, 1994.

Figure 1. Molecular structure of corrole.

tetrapyrrolic macrocycles are known, each of them with its own physicochemical properties. Nature for example uses simple modification of the molecular skeleton of porphyrins to optimize the biological activities of these compounds: for example iron porphyrins are used in the hemoproteins for oxygen transport whereas chlorins or bacteriochlorins, where one or two peripheral double bonds are saturated, are present in photosynthetic processes.3 A flurry of different polypyrrolic macrocycles, the so-called “porphyrinoids”, have been synthesized in the laboratories, to further expand and study this close structure/properties relationship, for both theoretical and applicative interest.5 Among these modified porphyrins, corrole (Figure 1) is one the first examples reported;6 in fact, it was synthesized almost 40 years ago by Johnson during his attempt to prepare Vitamin B12. After a long period of silence, this macrocycle has recently received new attention,7 because it has demonstrated interesting and peculiar characteristics as a ligand, for example stabilizing higher oxidation (5) (a) Sessler, J. L.; Cyr, M.; Furuta, H.; Kra´l, V.; Mody, T.; Morishima, T.; Shionoya, M.; Weghorn, S. Pure Appl. Chem. 1993, 65, 393. (b) Vogel, E. J. Heterocycl. Chem. 1996, 33, 1461. (6) Johnson, A. W.; Kay, I. T. J. Chem. Soc. (C) 1965, 1620. (7) (a) Licoccia, S.; Paolesse, R. In Metal Complexes with Tetrapyrrole Ligands III; Buchler, J. W., Ed.; Springer-Verlag: Berlin and Heidelberg, Germany, 1995. (b) Will, S.; Lex, J.; Vogel, E.; Schmickler, H.; Gisselbrecht, J.-P.; Haubtmann, C.; Bernard, M.; Gross, M. Angew. Chem., Int. Ed. Engl. 1977, 36, 357.

10.1021/la9712909 CCC: $18.00 © 1999 American Chemical Society Published on Web 01/27/1999

LB Films of a Manganese Corrole Derivative

states for coordinated metals than the corresponding porphyrins. These interesting properties make corrole derivatives promising compounds for their modeling as organic materials; in particular, in the past few years we have been interested in the exploitation of porphyrin derivatives as sensing materials for chemical sensors to be used in electronic nose systems.8 For these applications, sensors should exhibit sensitivity toward a large number of different chemical species, but it is also important that the broad-band selectivity properties should not overlap. During these studies we reported that the selectivity/ sensitivity properties of the resulting sensors can be finely tuned by operating variations both on the coordinated metal and on the substitution pattern of the macrocycle.9 The use of corrole, a macrocycle related to porphyrin but with its own peculiar characteristics, can allow the development of a novel important family of sensing materials with peculiar selectivity/sensitivity properties, so greatly expanding the number of different sensors that can be built up for electronic nose applications. One of the crucial points of these studies is to develop methods to obtain a controlled deposition of corroles as ordered films onto inorganic substrates. The LangmuirBlodgett (LB) technique10 is one of the most successful approaches to obtain ordered thin films by supramolecular interactions of organic materials, and it has been widely applied for sensor applications. For example LB films of porphyrins have been reported, and short descriptions of these studies can be found in books by Roberts10 and Ulman.11 Both natural modified and synthesized porphyrins can be deposited by the LB technique.12,13 Tredgold and co-workers14 have demonstrated the possibility of gas sensing by LB films of complexes of mesoporphyrin IX and tetraarylporphyrin on the basis of conductance measurements. Sun and co-workers15 fabricated a field effect transistor with the LB film of a symmetrically substituted cobalt porphyrin. It was shown that such a sensor is very sensitive with good selectivity to the presence of NO2 gas. In the subsequent work of this group,16 different side substituted porphyrins with Co, Fe, Cu, and Ni central atoms have been used in a modified device to improve sensing characteristics. To our knowledge, LB films of corrole derivatives have never been reported in the literature. In this work, we present the first example of deposition and characterization of LB films of a manganese corrole complex. The (8) (a) Di Natale, C.; Brunink, J.; Bungaro, F.; Davide, F.; D’Amico, A.; Paolesse, R.; Boschi, T.; Faccio, M.; Ferri, G. Proceedings of the 2nd East Asia Conference on Chemical Sensors; International Academic Publishing: Beijing (China), 1995. (b) Di Natale, C.; Repole, G.; Macagnano, A.; Saggio, G.; Davide, F.; D’Amico, A.; Paolesse, R.; Boschi, T.; Faccio, M.; Ferri, G. In Sensors and Microsystems; Di Natale, C., D’Amico, A., Eds.; World Scientific: Singapore, 1996. (c) Di Natale, C.; Macagnano, A.; Davide, F.; D’Amico, A.; Paolesse, R.; Boschi, T.; Faccio, M.; Ferri, G. Sens. Actuators, B 1997, 44, 521. (9) Brunink, J.; Di Natale, C.; Bungaro, F.; Davide, F.; D’Amico, A.; Paolesse, R.; Boschi, T.; Faccio, M.; Ferri, G. Anal. Chim. Acta 1996, 325, 53. (10) Roberts, G. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (11) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-assembly; Academic Press: Boston, 1991. (12) Jones, R.; Tredgold, R. H.; Hodge, P. Thin Solid Films 1983, 99, 25. (13) Ruaudel-Teixier, A.; Barraud, A.; Relbeoch B.; Roulliay, M. Thin Solid Films 1983, 99, 33. (14) Tredgold, R. H.; Young, M. C. J.; Hodge, P.; Hoorfar, A. Proc. IEEE, Part 1 1985, 132 151. (15) Sun, L.; Gu, C.; Wen, K.; Chao, X.; Li, T.; Hu, G.; Sun, J. Thin Solid Films 1992, 210/211, 486. (16) Gu, C.; Sun, L.; Zhang, T.; Li, T. Thin Solid Films 1996, 284/ 285, 863.

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sensitivity of these films toward volatile organic compounds (VOCs) has been investigated by the nanogravimetric technique, using LB coated quartz microbalances (QMBs). Experimental Section (7,13-Dimethyl-2,3,8,12,17,18-hexamethylcorrolato)manganese(III) [Mn(EMC)] has been prepared as previously reported;17 the general conditions have been reported elsewhere.18 The Mn(EMC) was dissolved in a mixture of hexane and chloroform (1:1 v/v, 0.25 mg mL-1), and this solution was used to spread the compound at the air-water interface. The experiments on film deposition were performed in a clean room of Class 100. Pure water with a resistivity higher than 18 MΩ cm (after preparation) was used as a subphase. The surface pressurearea isotherms were recorded under different conditions, and the films were fabricated by using KSV System 5000. The speed of monolayer compression was equal to 0.015 nm2 molecule-1 min-1. Film deposition was fulfilled at a speed of 5 mm min-1. The surface pressure during the deposition of the samples for electrical, QMB, and AFM measurements was maintained at 17.0 mN m-1, while, during working out the process itself and studying the behavior of monolayers at the air-water interface, the deposition was carried out at surface pressures of 15.0, 17.0, and 30.0 mN m-1. The films composed of 40 monolayers were deposited onto QMBs for sensor measurements, and the films of various thicknesses were deposited onto silicon and silicon nitride surfaces for optical microscopy, interference, and AFM studies. Silicon substrates were treated by dimethyldichlorosilane to make them hydrophobic. The morphology of the films was observed with an Axioscop Carl Zeiss optical microscope (magnification up to 1000). When the thickness of the film on the silicon (or silicon nitride) substrate exceeds 20-30 nm, variations of interference color in the defective places are strong and 3-5 nm thick nonuniformity can be detected (with a lateral resolution of about 1-2 µm). If the film is absorbing in the visible region, as in the case of Mn(EMC), the color variations become even stronger. The total thickness of the films was measured with Carl Zeiss Mirau interference equipment (source of light: λ ) 545.4 nm) after forming a sharp step in the layer, covering the sample with a 10-20 nm thick collodion film to make the surface physically homogeneous, and evaporating an aluminum reflecting mirror. Infrared spectra were measured with a Perkin-Elmer FT-IR spectrometer System 2000. The films composed of 80 monolayers were deposited for this purpose onto silicon hydrophobic substrates at 15 and 30 mN m-1. The AFM apparatus has been described in detail elsewhere.19 It consists of a unit made of two separable cylindrical supports: the lower one contains the sample holder mounted on top of the piezoelectric scanning unit (movements 6 × 6 × 3 µm3), both inserted in a motor-controlled x-y-z stage (8 × 8 × 1 mm3). A laser deflection circuit is mounted on the top cylinder. The microscope is suspended through a vibration isolation system inside a stainless steel chamber. After the sample loading, the chamber is closed in order to work in an isolated and controlled environment. An optical microscope (180× enlargement) placed on top of the cylinder allows us to monitor from the outside the sample surface and to select, together with the x-y stage, a suitable area to image. Constant-force images have been obtained in air with the microscope working in the repulsive mode with a force of 2-4 nN from zero cantilever deflection. Gold-coated Si3N4 microlevers (from Park Scientific Instruments) with a spring constant of 0.023 N/m were used. All the images shown were untreated apart from rigid plane subtraction. A color map has been superimposed that varies from black for the deeper zones to white for the highest. The QMB technique8,9 has been utilized to study the adsorption of VOCs onto LB films of Mn(EMC). These films have been deposited on both the surfaces of quartz disks having a (17) Boschi, T.; Licoccia, S.; Paolesse, R.; Tagliatesta, P.; Tehran, M. A.; Pelizzi, G.; Vitali, F. J. Chem. Soc., Dalton Trans. 1990, 463. (18) Paolesse, R.; Licoccia, S.; Spagnoli, M.; Boschi, T.; Khoury, R. G.; Smith, K. M. J. Org. Chem. 1997, 62, 5133. (19) Cricenti, A.; Generosi, R. Rev. Sci. Instrum. 1995, 66, 2843.

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fundamental frequency of 5 MHz; QMBs have been housed in a test chamber having a volume of 250 mL. To measure the variation of fundamental oscillating frequency, due to adsorption of VOCs, the QMBs were used as the nonlinear component of an oscillator circuit. To maximize the electric dynamic range of the quartz oscillations, the Pierce circuital solution has been adopted. Frequency measurement has been accomplished utilizing the frequency counter of a Tektronix 2252 digital oscilloscope; a PC supervised all the measurements, also collecting the data.

Results and Discussion The necessary requirement for an organic compound to be deposited by the LB technique is to have an amphiphilic character. Natural porphyrins, for example, can be deposited by the LB technique, whereas synthetic porphyrins, like meso-tetraphenylporphyrin (TPP), cannot be used directly and a peripheral functionalization is necessary to obtain satisfying depositions. Among corrole derivatives, we decided to study the deposition of the manganese complex of this macrocycle; we have recently reported the synthesis and characterization of Mn(EMC),16,20 and some characteristics of this compound seem interesting for LB film formation. This compound, in fact, represents the first example of a “bare” Mn(III) tetrapyrrolic macrocycle complex: such coordination behavior is highly unusual for Mn(III) derivatives,21 and this feature allows, in the crystal structure of the complex, a molecular packing characterized by a strong overlap of the π systems, with the formation of infinite stacks of molecules in an ordered way, with the direct pyrrole-pyrrole link pointing to the same side of the columnar stacks. Furthermore, these supramolecular interactions seemed particularly promising for future sensing applications of this compound. Study of Langmuir Monolayers and Deposition of LB Films. The Mn(EMC) molecule does not contain long-chain substituents, and it was discovered unexpectedly that these molecules reveal high surface activity. To study the properties of Langmuir monolayers, first, surface pressure-area isotherms were recorded under different conditions. Namely, we tried to detect the variations of surface pressure-area isotherm shape as well as the shifts of isotherms under the change of pH and the temperature of water. To account for the chemical structure of the molecule being unusual for the LB technique, surface pressure-area isotherms were recorded also at different doses of the spread solutions. From these experiments we expected to obtain more reliable data on the determination of area per molecule because, due to probable gradients of surface pressure arising during the monolayer compression, an increase of surface pressure can result in the formation of aggregates with the thickness exceeding one monolayer. This effect should depend on the total area occupied by the monolayer. The surface pressure-area isotherm (Figure 2) reveals a bend in the region between 10 and 20 mN m-1. This isotherm was recorded at 22 °C using the dose of solution of 150 L spread at the surface of pure water. (A custom Langmuir trough with the width 120 mm was used.) One of the goals of our study was to understand the nature of this bend. It can be caused by a phase transition (due to structure and/or physical state changes), by monolayer collapse, or by a more complicated process which results in a phase transition caused by the morphological rearrangement of the layer at the water surface (with the

increase of the average thickness of the latter) under compression. The values of compressibilities and areas per molecule at the two different linear parts of the surface pressurearea isotherm shown in Figure 2 may indicate that a simple phase transition from liquid condensed to solid states is observed in the above-mentioned region. However, in our case the bend is shifted downward with the increase of temperature (Figure 3) while for ordinary phase transitions the opposite shift should take place.22,23 (It should be noted that the surface pressure-area isotherms in Figure 3 were recorded for the dose of 100 L of spread solution. As a result, the measured areas per molecule differ from those reported in Figure 2. This fact will be discussed below.) The hypothesis of simple collapse has failed as well. The morphology of the film deposited at optimal surface pressure (17 mN m-1) is demonstrated in Figure 4. Transfer ratios, when dipping substrate both up and down, were equal to 1 with the accuracy of (0.1. The film appears to be quite uniform and practically defectless. Thus, perfect monolayer by monolayer transfer during film growth can be expected in this case. On the other hand, the morphology of the film deposited at 30 mN m-1 (Figure 5) is not so perfect. One can see some inclusion of collapsed material and thickness nonuniformities. The interference color of the film deposited at 30 mN m-1 differs from that of the film deposited at 17 mN m-1. The thickness of the latter is less. However, considerable variations of area per molecule in the region of bend observed in Figures 2 and 3 obviously cannot be explained by collapse. When the film was deposited at 15 mN m-1, transfer ratios were less than 1 and the areas of the decreased thickness are

(20) Licoccia, S.; Morgante, E.; Paolesse, R.; Polizio, F.; Senge, M. O.; Tondello, E.; Boschi, T. Inorg. Chem. 1997, 36, 1564. (21) Kemmit, R. D. W. In Comprehensive Inorganic Chemistry; Trotman-Dickenson, A. F., Ed.; Pergamon Press: Oxford, U.K., 1973.

(22) Kellner, M. J.; Muller-Landau, F.; Cadenhead, D. A. J. Colloid Interface Sci. 1978, 82, 597. (23) Baret, J. F.; Hasmonay, H.; Firpo, J. L.; Dupin, J. J.; Dupeyrat, M. Chem. Phys. Lipids 1982, 30, 177.

Figure 2. Surface pressure-area isotherm of the Mn(EMC) monolayer spread at the surface of pure water at 22 °C. The dose of spread solution was 150 L.

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Figure 5. Microphotograph of 40 monolayers of Mn(EMC) deposited onto hydrophobic silicon substrate at 30 mN m-1.

Figure 3. Surface pressure-area isotherms of the Mn(EMC) monolayer spread at the surface of pure water at 32, 22, and 10 °C. The dose of the spread solution was 100 L.

Figure 6. Interference pattern at the step film/substrate formed in 40 monolayers of Mn(EMC) deposited onto a hydrophobic Si3N4 surface. The total thickness (H) is equal to 56.6 nm.

Figure 4. Microphotograph of 40 monolayers of Mn(EMC) deposited onto a hydrophobic silicon substrate. The area of the film boundary.

easily observed. (The film morphology is not shown.) In the perfectly deposited places, the interference color of the film is the same as that in the case of deposition at 17 mN m-1. Thus, we can expect that the average thicknesses of layers transferred from the air-water interface onto the solid substrate are approximately the same in both these cases. The total thicknesses of the films were measured by the interference technique, and the average thicknesses of the monolayers were calculated. They appeared to be equal to 1.40 ( 0.06, 1.70 ( 0.1, and 1.36 ( 0.1 nm for the films deposited at 17, 30, and 15 mN m-1, respectively. In the last case we tried to choose perfectly deposited places for measurements. An example of the interference pattern is shown in Figure 6. The value of the monolayer thickness of about 1.4 nm is well consistent with the supposition of almost normal arrangement of the planes of macrocycles in the deposited films and in appropriate monolayers at the air-water interface. Thus one can see that the average

thickness of the layer at the surface of water compressed over the bend obviously exceeds the maximum possible thickness of one monolayer. Finally, we should note that practically no changes of surface pressure-area isotherms were observed under pH variations in the range from 4 to 8, as expected regarding the nature of the molecule. On the basis of these data, the following model can be proposed to describe the variations of the monolayer state under compression. First, a liquid condensed monolayer is formed with either inclined or almost vertical arrangement of rather closely packed molecules. Under compression, the process of partial forcing out of the molecules from the monolayer is initiated, since the diameter of the macrocycle is only about 1.5 nm. This corresponds to the initial part of the bend. Thus, the areas with higher thicknesses are formed. However, considerable deterioration of the layer uniformity does not arise, especially if forced out molecules lie flat over the monolayer. Under such conditions the layer becomes more rigid because of the additional interaction between the molecules and, finally, a phase transition from liquid to solid state takes place (end of the bend). At this moment the forcing out of the molecules from the monolayer becomes more difficult and simple compression of the quasi-solid layer proceeds. Just in such a model, shifting down of the bend under the increase of temperature should take place, since temperature facilitates pushing out of the molecules from the monolayer. Comparison of the isotherms recorded under identical conditions, but using different doses (150 and 100 L) of

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Figure 7. Dependence of surface pressure on time at a fixed area of compressed monolayer.

the solution for monolayer spreading (curves in Figures 2 and 3 recorded at 22 °C), confirms this model. The first linear region gives the smaller extrapolated area per molecule for the higher dose, and this result is quite reproducible. Indeed, when the compressing barriers are far from the Wilhelmy plate but the molecules already begin to contact, due to the appearance of a gradient of surface pressure, the above-suggested process of pushing out of the molecules from the monolayer will start near the barrier while near the Wilhelmy plate the surface pressure is not yet high enough to initiate it. This just means that the recorded effective area per molecule at a given measured surface pressure will be less than that in the case of a smaller dose at the same surface pressure. Finally, however, equal amounts of molecules will be pushed out in both cases. When the further compression in the solid state is fulfilled, this process does not continue and the recorded areas per molecule are approximately the same. To understand in more detail what is the physical state of the monolayer at low surface pressure values, we tried to evaluate the equilibrium spreading pressure (ESP). It is well-known that the problem of ESP determination is very difficult because of slow and indefinite, as a rule, kinetics of monolayer spreading from the bulk solid phase. It appeared that just such problems arise when one deals with Mn(EMC). Thus, we were able only to evaluate the approximate region where the ESP is located. These measurements were carried out as follows. In one small section of definite area, we started from a large amount of a collapsed material, so that surface pressure decreased in time beginning from a high value. In the other similar section, we started from the powder distributed over the surface of water. In this section, surface pressure increased in time (from zero). To facilitate the monolayers to come to equilibrium with the bulk materials, the compounds at the surface were subjected to the action of an ultrasonic wave coming from piezoelements located under the water. In 12 h the surface pressure in the first section reached 16.4 mN m-1 while in the second one it was 9.8 mN m-1 (drift of balance readings was taken into account). Thus, surely the ESP is located between 10 and 16 mN m-1 and our supposition that the right part of the surface pressurearea isotherm corresponds to the liquid condensed phase becomes more substantiated. Such a conclusion was confirmed also by recording the dependence of surface pressure on time at a fixed area of the monolayer (Figure 7) and the dependence of the barrier coordinate on time at a fixed surface pressure (Figure 8). In the first case, relaxation typical for solid monolayers is observed at high surface pressure over the bend. At the same time, practically no change of the surface pressure

Figure 8. Dependence of surface pressure on time at a fixed area of compressed monolayer.

Figure 9. AFM image (8 nm × 8 nm) of a 100 monolayer LB film of Mn(EMC) deposited onto silicon.

value is observed below 9 mN m-1, which is quite typical for liquid monolayers. On the other hand, the curves in Figure 8 show that under the conditions of deposition (feedback is on) the monolayers are always unstable. Even at low surface pressure the slow process of pushing out of the molecules from the compressed monolayer takes place. The microscopic structure of LB films of Mn(EMC) was further investigated by using atomic force microscopy (AFM). The first studies of LB films were performed with scanning tunneling microscopy (STM);24,25 unfortunately, this technique requires the sample to be conductive, and this means that the layers deposited must be thin enough to allow the electrons to tunnel through the conductive substrate. The introduction of AFM26 allowed us to overcome this limitation because it is not restricted to conductive samples. Figure 9 (8 nm × 8 nm) shows the LB films (100 monolayers) of Mn(EMC) deposited onto silicon substrates. The presence of a well-ordered structure is clearly evident; different areas of the sample surface have been imaged to verify the absence of defects and to confirm the homogeneity of the deposition. (24) Binnig, G.; Rohrer, H.; Gerber, C.; Weibel, E. Appl. Phys. Lett. 1982, 40, 178. (25) Guckenberger, R.; Kosslinger, C.; Gatz, R.; Breu, H.; Levai, N.; Baumeister, W. Ultramicroscopy 1988, 25, 11. (26) Binnig, G.; Quate, C. F. Phys. Rev. Lett. 1986, 56, 930.

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results in a variation of the oscillating mass which induces, to a first approximation, the variation of the vibrating frequency:

∆f ) kq∆m where kq is the mass sensitivity constant. When exposed to only one species, the amount of mass can be expressed as

∆m ) µ‚∆n where µ is the molecular mass of the considered species and ∆n is the number of adsorbed molecules. The interaction with the analyte can be described in terms of the sorption isotherm. The analytical form of the isotherm can be given assuming a combined Langmuir and Henry-type interaction, as proposed in ref 29. In this way the relation between the QMB signal and the analyte concentration is given by

∆f ) kH‚c + kL ‚ Figure 10. Visible spectra of Mn(EMC) in a chloroform solution (solid line) and of a 40 monolayer LB film of Mn(EMC) (dashed line).

The thickness of the film was obtained by measuring the step height from a boundary zone to the sample surface and is about 150 nm, which is in good agreement with the value obtained with interference measurements and with the theoretical height estimated for a normal orientation of the molecules to the surface. The preference of Mn(EMC) for the “edge-on” orientation is probably due to the π-π interactions between macrocycles that are allowed in this “face-to-face” arrangement. This hypothesis is confirmed by the visible spectrum performed on the LB film deposited onto the quartz surface (Figure 10); the absorption spectrum shows significant line broadening and red shift of the absorbances: all these effects can be attributed to interactions between the π systems of the macrocycles.27 We tried also to detect the preferential orientation of the Mn(EMC) molecules in the deposited films by the measurement of transmission infrared spectra using a polarized beam. The angle of sample inclination was gradually changed from 0 to 60°. However, for both s- and p-polarized light no significant changes in the spectra were detected, while the amplitudes of some absorption bands were different for the samples deposited at 15 and 30 mN m-1. We analyzed the region of stretching vibrations of CH3 and CH2 groups (3000-2800 cm-1) and the region 1800-1300 cm-1, in which various vibrations attributed to the macrocycle can be observed. Such a situation can be explained by a symmetrical structure of molecules having numerous identical bonds directed in different ways and arbitrary azimuthal orientations of the macrocycle planes with respect to the normal to the layer plane. QMB Measurements. The adsorption interactions of these LB films with VOCs have been investigated using QMBs: the principle and application of QMB measurement is based on the variation (∆f) of the fundamental oscillating frequency of the quartz as a consequence of the adsorption of molecules from the gas phase. As shown by Sauerbrey,28 the adsorption, or desorption, of molecules (27) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525. (28) Sauerbrey, G. Z. Phys. 1959, 155, 206.

k‚c 1 + k‚c

where c is the analyte concentration, kH and kL are constants, and k denotes the ratio of the kinetic constants of the adsorption and desorption processes of the analyte at or from the recognition site. A useful parameter to evaluate these adsorption phenomena is the partition coefficient: it is defined as the ratio between the concentration of the analyte in the quartz coating (cb) and its concentration in the gas phase (ci). This parameter has been used to evaluate the sensitivity and the selectivity of chemical sensors.29,30 The concentration in the LB films can be expressed as the number of adsorbed molecules divided by the film volume, so that the partition coefficient is given as

K)

cb n 1 ‚ ) ci A‚δ ci

where δ is the thickness of the LB film and A is the coating area. The number of adsorbed molecules can then be calculated by the expression of ∆f, and the final expression of the partition coefficient of the j-th species in the LB film is given as

Kj )

1 k k + kL ‚ A‚δ‚kq‚µ H 1 + k‚c

[

]

In our case A ) 615 mm2 and δ ) 600 Å. The mass constant of the quartz was calibrated and its value was found to be kq)154.4 Hz/µg. It is worth to mention that the detection limit was in the nanometric range (1 Hz ) 6 ng) a common figure for this kind of sensors. QMB responses have been measured exposing LB-filmcoated quartz at different concentrations of various VOCs, representative of different compounds, such as hydrocarbons, aldehydes, alcohols, and amines. For each analyte a sorption isotherm was then fitted by a nonlinear least squares procedure determining for each analyte the set of parameters kH, kL, and k describing its interaction with the corrole LB film. Figure 11 shows the experimental data and the fitted isotherm in the case of benzene; the nonlinear isotherm results from a specific π-π interaction between the (29) Bo¨denhofer, K.; Hierleman, A.; Juza, M.; Schurig, V.; Go¨pel, W. Anal. Chem. 1997, 69, 4017. (30) Grate, J. W.; Abraham, M. H. Sens. Actuators, B, 1991, 3, 85.

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Figure 12. Partition coefficients of the different VOCs. Figure 11. Experimental data and fitted isotherm for benzene. Sensor response is expressed through the vibrating frequency variation of the quartz coated by the LB corrole film. The two components of the isotherm are also shown; the Langmuir part shows a saturation response of about 38 Hz. Table 1. Minimum Detectable Concentration and Maximum and Minimum Values of the Partition Coefficient for the Considered Analytes VOC

Cmin (ppm)

Kmax

Kmin

hexane valeraldeyhde n-hexanol benzene triethylamine dipropylamine ethylendiamine

7.2330 64.881 15.834 5.0856 12.833 8.2576 6.7627

16.3 28.9 998.2 4065.1 6243.6 5936.4 5972.9

10.9 25.5 37.9 78.6 780.9 767.5 741.5

aromatic system of the benzene and the aromatic system of the corrole. This interaction takes place at a low concentration of benzene while, at higher concentration, all the corrole specific sites are saturated and only the nonspecific absorptions occur and the shape of the isotherm becomes linear (Henry-type behavior). In the same figure both the components of the isotherm are plotted. The increase of sensitivity at low concentrations is not observed for all the analytes; indeed while the Henrytype isotherm is nonspecific in principle, being a simple dilution from the gas phase to the solid phase of the analyte, the Langmuir-type isotherm depends on the affinity of the analyte with the corrole. To recognize the effect of the interactions analyte-Mn(EMC), it useful to compare the sensor performances evaluated at low and high concentrations. For each analyte the minimum detectable concentration was determined imposing, on the fitted isotherm, a conventional value of 1 Hz as the lowest reliable frequency variation measurement. The partition coefficient was then calculated at the limit of detection and at very high concentrations in order to give values at the two opposite regimes: the specific adsorption (Langmuir behavior) and

the nonspecific sorption (Henry behavior). All these values are listed in Table 1. It is interesting to note that at high concentrations the partition coefficient is very small; this is expected because of the small thickness of the LB film, which does not allow an efficient sorption at high concentration. On the other hand, the high values of the partition coefficients calculated, for each analyte, at the estimated lowest measurable concentration have to be mentioned. An estimation of the molecular recognition efficiency of the corrole compound can be obtained considering the partition coefficient amplification between the two regimes occurring at high concentration (nonspecific sorption) and at low concentrations (specific interaction). In Figure 12 the ratio between these two values is plotted for each considered analyte. It has to be noted that the high gain is obtained for benzene, whose mechanism of interaction with the corrole LB film has been outlined previously. It also has to be noted that no gain is shown for hexane and valeraldehyde, which are expected to have no specific interactions with the corrole ring, and so no Langmuirlike contribution at low concentration can be expected. On the other hand, coordination to the manganese ion of the corrole complex must be considered for the three considered amines and hexanol, giving rise to specific Langmuir-like interactions, increasing the sensitivity at low concentrations. In conclusion, the deposition of LB films of Mn(EMC) is a useful approach to develop QMBs with improved and peculiar sensing properties. The obtained results showed for these QMBs a wide selectivity; this feature is particularly promising for their exploitation in sensor arrays, because a wide selectivity is one of the necessary requirements for sensors to be used for electronic nose applications.8c Acknowledgment. This work was supported by Murst and CNR. The valuable technical assistance of Mr. M. Zarlenga is gratefully acknowledged. LA9712909