Langmuir-Blodgett Films of a Manganese CorroleDerivative

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, andDipartimento di Fisica and Istituto Nazionale di Fisica della Materia,

Universita di Roma “Tor Vergata”, Via della Ricerca Scientifica, 00173 Roma, Italy, andTechnobiochip 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 isdescribed. Characterization of the morphological and molecular structure of these LB films has beencarried out by means of different microscopic and spectroscopic methods. Depositions onto different substratesafford very uniform films of determined thickness; all the experiments carried out suggest that manganesecorrolates 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 promisingtools for the development of new technologies withimportant applications in different fields, such as forexample sensors and nanoelectronics.1 One of the greatadvantages of these organic materials lies in the possibilityof tailoring the desired compounds, so tuning physico-chemical properties for specific applications. Further, itis possible to build up highly ordered thin films of thesematerials through their supramolecular interactions, andthis control at a molecular level is a necessary requirementfor their future applications in different devices.1

Among organic compounds, porphyrins represent oneof the most fascinating species, because of their rich andalmost unique properties, which are of interest for differentdisciplines.2 In fact they are widely present in Nature,where they play functions necessary for life,3 and theyhave been intensively studied to obtain information onbiological systems. Their ability in natural systems tocatalyze reactions, that are normally difficult if notimpossible to carry out in laboratories, has attractedinterest for their application as catalysts,4 trying to mimicktheir biological activity for synthetic applications.

It is also relevant to note that porphyrins representonly the most important example, but a large number of

tetrapyrrolic macrocycles are known, each of them withits own physicochemical properties. Nature for exampleuses simple modification of the molecular skeleton ofporphyrins to optimize the biological activities of thesecompounds: for example iron porphyrins are used in thehemoproteins for oxygen transport whereas chlorins orbacteriochlorins, where one or two peripheral double bondsare saturated, are present in photosynthetic processes.3A flurry of different polypyrrolic macrocycles, the so-called“porphyrinoids”, have been synthesized in the laboratories,to further expand and study this close structure/propertiesrelationship, for both theoretical and applicative interest.5

Among these modified porphyrins, corrole (Figure 1) isone the first examples reported;6 in fact, it was synthesizedalmost 40 years ago by Johnson during his attempt toprepare Vitamin B12. After a long period of silence, thismacrocycle has recently received new attention,7 becauseit has demonstrated interesting and peculiar character-istics as a ligand, for example stabilizing higher oxidation

* 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 diRoma “Tor Vergata”.

‡ Dipartimento di Ingegneria Elettronica, Universita di Roma“Tor Vergata”.

§ Dipartimento di Fisica and Istituto Nazionale di Fisica dellaMateria, Universita di Roma “Tor Vergata”.

| Technobiochip s.r.l. 57030 Marciana.(1) Gopel, W.; Ziegler, Ch. In Nanostructure Based on Molecular

Materials; Gopel, 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.

(5) (a) Sessler, J. L.; Cyr, M.; Furuta, H.; Kral, 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.

Figure 1. Molecular structure of corrole.

1268 Langmuir 1999, 15, 1268-1274

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

states for coordinated metals than the correspondingporphyrins.

These interesting properties make corrole derivativespromising compounds for their modeling as organicmaterials; in particular, in the past few years we havebeen interested in the exploitation of porphyrin derivativesas sensing materials for chemical sensors to be used inelectronic nose systems.8 For these applications, sensorsshould exhibit sensitivity toward a large number ofdifferent chemical species, but it is also important thatthe broad-band selectivity properties should not overlap.During these studies we reported that the selectivity/sensitivity properties of the resulting sensors can be finelytuned by operating variations both on the coordinatedmetal and on the substitution pattern of the macrocycle.9The use of corrole, a macrocycle related to porphyrin butwith its own peculiar characteristics, can allow thedevelopment of a novel important family of sensingmaterials with peculiar selectivity/sensitivity properties,so greatly expanding the number of different sensors thatcan be built up for electronic nose applications.

One of the crucial points of these studies is to developmethods to obtain a controlled deposition of corroles asordered films onto inorganic substrates. The Langmuir-Blodgett (LB) technique10 is one of the most successfulapproaches to obtain ordered thin films by supramolecularinteractions of organic materials, and it has been widelyapplied for sensor applications. For example LB films ofporphyrins have been reported, and short descriptions ofthese studies can be found in books by Roberts10 andUlman.11 Both natural modified and synthesized por-phyrins can be deposited by the LB technique.12,13 Tredgoldand co-workers14 have demonstrated the possibility of gassensing by LB films of complexes of mesoporphyrin IXand tetraarylporphyrin on the basis of conductancemeasurements. Sun and co-workers15 fabricated a fieldeffect transistor with the LB film of a symmetricallysubstituted cobalt porphyrin. It was shown that such asensor is very sensitive with good selectivity to thepresence of NO2 gas. In the subsequent work of thisgroup,16 different side substituted porphyrins with Co,Fe, Cu, and Ni central atoms have been used in a modifieddevice to improve sensing characteristics.

To our knowledge, LB films of corrole derivatives havenever been reported in the literature. In this work, wepresent the first example of deposition and characteriza-tion of LB films of a manganese corrole complex. The

sensitivity of these films toward volatile organic com-pounds (VOCs) has been investigated by the nanogravi-metric technique, using LB coated quartz microbalances(QMBs).

Experimental Section(7,13-Dimethyl-2,3,8,12,17,18-hexamethylcorrolato)manga-

nese(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 andchloroform (1:1 v/v, 0.25 mg mL-1), and this solution was usedto spread the compound at the air-water interface. The experi-ments on film deposition were performed in a clean room of Class100. Pure water with a resistivity higher than 18 MΩ cm (afterpreparation) was used as a subphase. The surface pressure-area isotherms were recorded under different conditions, andthe films were fabricated by using KSV System 5000. The speedof 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 forelectrical, QMB, and AFM measurements was maintained at17.0 mN m-1, while, during working out the process itself andstudying 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 weredeposited onto QMBs for sensor measurements, and the films ofvarious thicknesses were deposited onto silicon and silicon nitridesurfaces for optical microscopy, interference, and AFM studies.Siliconsubstrateswere treatedbydimethyldichlorosilane tomakethem hydrophobic.

The morphology of the films was observed with an AxioscopCarl Zeiss optical microscope (magnification up to 1000). Whenthe thickness of the film on the silicon (or silicon nitride) substrateexceeds 20-30 nm, variations of interference color in the defectiveplaces are strong and 3-5 nm thick nonuniformity can be detected(with a lateral resolution of about 1-2 µm). If the film is absorbingin the visible region, as in the case of Mn(EMC), the colorvariations become even stronger. The total thickness of the filmswas measured with Carl Zeiss Mirau interference equipment(source of light: λ ) 545.4 nm) after forming a sharp step in thelayer, covering the sample with a 10-20 nm thick collodion filmto make the surface physically homogeneous, and evaporatingan aluminum reflecting mirror.

Infrared spectra were measured with a Perkin-Elmer FT-IRspectrometer System 2000. The films composed of 80 monolayerswere deposited for this purpose onto silicon hydrophobic sub-strates 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 thepiezoelectric scanning unit (movements 6 × 6 × 3 µm3), bothinserted in a motor-controlled x-y-z stage (8 × 8 × 1 mm3). Alaser deflection circuit is mounted on the top cylinder. Themicroscope is suspended through a vibration isolation systeminside a stainless steel chamber. After the sample loading, thechamber is closed in order to work in an isolated and controlledenvironment. An optical microscope (180× enlargement) placedon top of the cylinder allows us to monitor from the outside thesample surface and to select, together with the x-y stage, asuitable area to image.

Constant-force images have been obtained in air with themicroscope working in the repulsive mode with a force of 2-4nN from zero cantilever deflection. Gold-coated Si3N4 microlevers(from Park Scientific Instruments) with a spring constant of 0.023N/m were used. All the images shown were untreated apart fromrigid plane subtraction. A color map has been superimposed thatvaries from black for the deeper zones to white for the highest.

The QMB technique8,9 has been utilized to study the adsorptionof VOCs onto LB films of Mn(EMC). These films have beendeposited on both the surfaces of quartz disks having a

(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 2ndEast Asia Conference on Chemical Sensors; International AcademicPublishing: 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-BlodgettFilms;PlenumPress: NewYork,1990.

(11) Ulman, A. An Introduction to Ultrathin Organic Films: FromLangmuir-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. ThinSolid 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. ThinSolid Films 1992, 210/211, 486.

(16) Gu, C.; Sun, L.; Zhang, T.; Li, T. Thin Solid Films 1996, 284/285, 863.

(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.

LB Films of a Manganese Corrole Derivative Langmuir, Vol. 15, No. 4, 1999 1269

fundamental frequency of 5 MHz; QMBs have been housed in atest chamber having a volume of 250 mL. To measure thevariation of fundamental oscillating frequency, due to adsorptionof VOCs, the QMBs were used as the nonlinear component of anoscillator circuit. To maximize the electric dynamic range of thequartz oscillations, the Pierce circuital solution has been adopted.Frequency measurement has been accomplished utilizing thefrequency counter of a Tektronix 2252 digital oscilloscope; a PCsupervised all the measurements, also collecting the data.

Results and Discussion

The necessary requirement for an organic compound tobe deposited by the LB technique is to have an amphiphiliccharacter. Natural porphyrins, for example, can bedeposited by the LB technique, whereas synthetic por-phyrins, like meso-tetraphenylporphyrin (TPP), cannotbe used directly and a peripheral functionalization isnecessary to obtain satisfying depositions.

Among corrole derivatives, we decided to study thedeposition of the manganese complex of this macrocycle;we have recently reported the synthesis and character-ization of Mn(EMC),16,20 and some characteristics of thiscompound seem interesting for LB film formation. Thiscompound, in fact, represents the first example of a “bare”Mn(III) tetrapyrrolic macrocycle complex: such coordina-tion behavior is highly unusual for Mn(III) derivatives,21

and this feature allows, in the crystal structure of thecomplex, a molecular packing characterized by a strongoverlap of the π systems, with the formation of infinitestacks of molecules in an ordered way, with the directpyrrole-pyrrole link pointing to the same side of thecolumnar stacks. Furthermore, these supramolecularinteractions seemed particularly promising for futuresensing applications of this compound.

Study of Langmuir Monolayers and Depositionof LB Films. The Mn(EMC) molecule does not containlong-chain substituents, and it was discovered unexpect-edly that these molecules reveal high surface activity. Tostudy the properties of Langmuir monolayers, first, surfacepressure-area isotherms were recorded under differentconditions. Namely, we tried to detect the variations ofsurface pressure-area isotherm shape as well as the shiftsof isotherms under the change of pH and the temperatureof water. To account for the chemical structure of themolecule being unusual for the LB technique, surfacepressure-area isotherms were recorded also at differentdoses of the spread solutions. From these experiments weexpected to obtain more reliable data on the determinationof area per molecule because, due to probable gradientsof surface pressure arising during the monolayer com-pression, an increase of surface pressure can result in theformation of aggregates with the thickness exceeding onemonolayer. This effect should depend on the total areaoccupied by the monolayer.

The surface pressure-area isotherm (Figure 2) revealsa bend in the region between 10 and 20 mN m-1. Thisisotherm was recorded at 22 °C using the dose of solutionof 150 L spread at the surface of pure water. (A customLangmuir trough with the width 120 mm was used.) Oneof the goals of our study was to understand the nature ofthis bend. It can be caused by a phase transition (due tostructure and/or physical state changes), by monolayercollapse, or by a more complicated process which resultsin a phase transition caused by the morphological re-arrangement of the layer at the water surface (with the

increase of the average thickness of the latter) undercompression.

The values of compressibilities and areas per moleculeat the two different linear parts of the surface pressure-area isothermshowninFigure2may indicate thatasimplephase transition from liquid condensed to solid states isobserved in the above-mentioned region. However, in ourcase the bend is shifted downward with the increase oftemperature (Figure 3) while for ordinary phase transi-tions the opposite shift should take place.22,23 (It shouldbe noted that the surface pressure-area isotherms inFigure 3 were recorded for the dose of 100 L of spreadsolution. As a result, the measured areas per moleculediffer from those reported in Figure 2. This fact will bediscussed below.)

The hypothesis of simple collapse has failed as well.The morphology of the film deposited at optimal surfacepressure (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 appearsto be quite uniform and practically defectless. Thus, perfectmonolayer by monolayer transfer during film growth canbe expected in this case. On the other hand, the morphologyof the film deposited at 30 mN m-1 (Figure 5) is not soperfect. One can see some inclusion of collapsed materialand thickness nonuniformities. The interference color ofthe film deposited at 30 mN m-1 differs from that of thefilm deposited at 17 mN m-1. The thickness of the latteris less. However, considerable variations of area permolecule in the region of bend observed in Figures 2 and3 obviously cannot be explained by collapse. When thefilm was deposited at 15 mN m-1, transfer ratios wereless 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. ColloidInterface 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. Thedose of spread solution was 150 L.

1270 Langmuir, Vol. 15, No. 4, 1999 Paolesse et al.

easily observed. (The film morphology is not shown.) Inthe perfectly deposited places, the interference color ofthe film is the same as that in the case of deposition at17 mN m-1. Thus, we can expect that the averagethicknesses of layers transferred from the air-waterinterface onto the solid substrate are approximately thesame in both these cases.

The total thicknesses of the films were measured by theinterference technique, and the average thicknesses ofthe monolayers were calculated. They appeared to be equalto 1.40 ( 0.06, 1.70 ( 0.1, and 1.36 ( 0.1 nm for the filmsdeposited at 17, 30, and 15 mN m-1, respectively. In thelast case we tried to choose perfectly deposited places formeasurements. An example of the interference pattern isshown in Figure 6. The value of the monolayer thicknessof about 1.4 nm is well consistent with the supposition ofalmost normal arrangement of the planes of macrocyclesin the deposited films and in appropriate monolayers atthe air-water interface. Thus one can see that the average

thickness of the layer at the surface of water compressedover the bend obviously exceeds the maximum possiblethickness of one monolayer.

Finally, we should note that practically no changes ofsurface pressure-area isotherms were observed underpH variations in the range from 4 to 8, as expectedregarding the nature of the molecule.

On the basis of these data, the following model can beproposed to describe the variations of the monolayer stateunder compression. First, a liquid condensed monolayeris formed with either inclined or almost vertical arrange-ment of rather closely packed molecules. Under compres-sion, the process of partial forcing out of the moleculesfrom the monolayer is initiated, since the diameter of themacrocycle is only about 1.5 nm. This corresponds to theinitial part of the bend. Thus, the areas with higherthicknesses are formed. However, considerable deteriora-tion of the layer uniformity does not arise, especially ifforced out molecules lie flat over the monolayer. Undersuch conditions the layer becomes more rigid because ofthe additional interaction between the molecules and,finally, a phase transition from liquid to solid state takesplace (end of the bend). At this moment the forcing out ofthe molecules from the monolayer becomes more difficultand simple compression of the quasi-solid layer proceeds.Just in such a model, shifting down of the bend under theincrease of temperature should take place, since temper-ature facilitates pushing out of the molecules from themonolayer.

Comparison of the isotherms recorded under identicalconditions, but using different doses (150 and 100 L) of

Figure 3. Surface pressure-area isotherms of the Mn(EMC)monolayer spread at the surface of pure water at 32, 22, and10 °C. The dose of the spread solution was 100 L.

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

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

Figure 6. Interference pattern at the step film/substrateformed in 40 monolayers of Mn(EMC) deposited onto ahydrophobic Si3N4 surface. The total thickness (H) is equal to56.6 nm.

LB Films of a Manganese Corrole Derivative Langmuir, Vol. 15, No. 4, 1999 1271

the solution for monolayer spreading (curves in Figures2 and 3 recorded at 22 °C), confirms this model. The firstlinear region gives the smaller extrapolated area permolecule for the higher dose, and this result is quitereproducible. Indeed, when the compressing barriers arefar from the Wilhelmy plate but the molecules alreadybegin to contact, due to the appearance of a gradient ofsurface pressure, the above-suggested process of pushingout of the molecules from the monolayer will start nearthe barrier while near the Wilhelmy plate the surfacepressure is not yet high enough to initiate it. This justmeans that the recorded effective area per molecule at agiven measured surface pressure will be less than that inthe case of a smaller dose at the same surface pressure.Finally, however, equal amounts of molecules will bepushed out in both cases. When the further compressionin the solid state is fulfilled, this process does not continueand the recorded areas per molecule are approximatelythe same.

To understand in more detail what is the physical stateof the monolayer at low surface pressure values, we triedto evaluate the equilibrium spreading pressure (ESP). Itis well-known that the problem of ESP determination isvery 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 dealswith Mn(EMC). Thus, we were able only to evaluate theapproximate region where the ESP is located. Thesemeasurements were carried out as follows. In one smallsection of definite area, we started from a large amountof a collapsed material, so that surface pressure decreasedin time beginning from a high value. In the other similarsection, we started from the powder distributed over thesurface of water. In this section, surface pressure increasedin time (from zero). To facilitate the monolayers to cometo equilibrium with the bulk materials, the compounds atthe surface were subjected to the action of an ultrasonicwave coming from piezoelements located under the water.In 12 h the surface pressure in the first section reached16.4 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 andour supposition that the right part of the surface pressure-area isotherm corresponds to the liquid condensed phasebecomes more substantiated.

Such a conclusion was confirmed also by recording thedependence of surface pressure on time at a fixed area ofthe monolayer (Figure 7) and the dependence of the barriercoordinate on time at a fixed surface pressure (Figure 8).In the first case, relaxation typical for solid monolayersis observed at high surface pressure over the bend. At thesame time, practically no change of the surface pressure

value is observed below 9 mN m-1, which is quite typicalfor liquid monolayers. On the other hand, the curves inFigure 8 show that under the conditions of deposition(feedback is on) the monolayers are always unstable. Evenat low surface pressure the slow process of pushing outof the molecules from the compressed monolayer takesplace.

The microscopic structure of LB films of Mn(EMC) wasfurther investigated by using atomic force microscopy(AFM). The first studies of LB films were performed withscanning tunneling microscopy (STM);24,25 unfortunately,this technique requires the sample to be conductive, andthis means that the layers deposited must be thin enoughto allow the electrons to tunnel through the conductivesubstrate. The introduction of AFM26 allowed us toovercome this limitation because it is not restricted toconductive samples.

Figure 9 (8 nm × 8 nm) shows the LB films (100monolayers) 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 imagedto verify the absence of defects and to confirm thehomogeneity 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.

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

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

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

1272 Langmuir, Vol. 15, No. 4, 1999 Paolesse et al.

The thickness of the film was obtained by measuringthe step height from a boundary zone to the sample surfaceand is about 150 nm, which is in good agreement with thevalue obtained with interference measurements and withthe theoretical height estimated for a normal orientationof the molecules to the surface.

The preference of Mn(EMC) for the “edge-on” orientationis probably due to the π-π interactions between macro-cycles that are allowed in this “face-to-face” arrangement.This hypothesis is confirmed by the visible spectrumperformed on the LB film deposited onto the quartz surface(Figure 10); the absorption spectrum shows significantline broadening and red shift of the absorbances: all theseeffects can be attributed to interactions between the πsystems of the macrocycles.27

We tried also to detect the preferential orientation ofthe Mn(EMC) molecules in the deposited films by themeasurement of transmission infrared spectra using apolarized beam. The angle of sample inclination wasgradually changed from 0 to 60°. However, for both s- andp-polarized light no significant changes in the spectra weredetected, while the amplitudes of some absorption bandswere different for the samples deposited at 15 and 30 mNm-1. We analyzed the region of stretching vibrations ofCH3 and CH2 groups (3000-2800 cm-1) and the region1800-1300 cm-1, in which various vibrations attributedto the macrocycle can be observed. Such a situation canbe explained by a symmetrical structure of moleculeshaving numerous identical bonds directed in differentways and arbitrary azimuthal orientations of the macro-cycle planes with respect to the normal to the layer plane.

QMB Measurements. The adsorption interactions ofthese LB films with VOCs have been investigated usingQMBs: the principle and application of QMB measure-ment is based on the variation (∆f) of the fundamentaloscillating frequency of the quartz as a consequence ofthe adsorption of molecules from the gas phase. As shownby Sauerbrey,28 the adsorption, or desorption, of molecules

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

where kq is the mass sensitivity constant. When exposedto only one species, the amount of mass can be expressedas

where µ is the molecular mass of the considered speciesand ∆n is the number of adsorbed molecules.

The interaction with the analyte can be described interms of the sorption isotherm. The analytical form of theisotherm can be given assuming a combined Langmuirand Henry-type interaction, as proposed in ref 29. In thisway the relation between the QMB signal and the analyteconcentration is given by

where c is the analyte concentration, kH and kL areconstants, and k denotes the ratio of the kinetic constantsof the adsorption and desorption processes of the analyteat or from the recognition site.

A useful parameter to evaluate these adsorption phe-nomena is the partition coefficient: it is defined as theratio between the concentration of the analyte in the quartzcoating (cb) and its concentration in the gas phase (ci).This parameter has been used to evaluate the sensitivityand the selectivity of chemical sensors.29,30

The concentration in the LB films can be expressed asthe number of adsorbed molecules divided by the filmvolume, so that the partition coefficient is given as

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

In our case A ) 615 mm2 and δ ) 600 Å. The massconstant of the quartz was calibrated and its value wasfound to be kq)154.4 Hz/µg. It is worth to mention thatthe 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-film-coated quartz at different concentrations of various VOCs,representative of different compounds, such as hydro-carbons, aldehydes, alcohols, and amines. For each analytea sorption isotherm was then fitted by a nonlinear leastsquares procedure determining for each analyte the setof parameters kH, kL, and k describing its interaction withthe corrole LB film.

Figure 11 shows the experimental data and the fittedisotherm in the case of benzene; the nonlinear isothermresults from a specific π-π interaction between the

(27) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112,5525.

(28) Sauerbrey, G. Z. Phys. 1959, 155, 206.

(29) Bodenhofer, K.; Hierleman, A.; Juza, M.; Schurig, V.; Gopel, W.Anal. Chem. 1997, 69, 4017.

(30) Grate, J. W.; Abraham, M. H. Sens. Actuators, B, 1991, 3, 85.

Figure 10. Visible spectra of Mn(EMC) in a chloroform solution(solid line) and of a 40 monolayer LB film of Mn(EMC) (dashedline).

∆f ) kq∆m

∆m ) µ‚∆n

∆f ) kH‚c + kL ‚ k‚c1 + k‚c

K )cb

ci) n

A‚δ‚ 1

ci

Kj ) 1A‚δ‚kq‚µ[kH + kL ‚ k

1 + k‚c]

LB Films of a Manganese Corrole Derivative Langmuir, Vol. 15, No. 4, 1999 1273

aromatic system of the benzene and the aromatic systemof the corrole. This interaction takes place at a lowconcentration of benzene while, at higher concentration,all the corrole specific sites are saturated and only thenonspecificabsorptionsoccurandtheshapeof the isothermbecomes linear (Henry-type behavior). In the same figureboth the components of the isotherm are plotted.

The increase of sensitivity at low concentrations is notobserved for all the analytes; indeed while the Henry-type isotherm is nonspecific in principle, being a simpledilution from the gas phase to the solid phase of theanalyte, the Langmuir-type isotherm depends on theaffinity of the analyte with the corrole. To recognize theeffect of the interactions analyte-Mn(EMC), it useful tocompare the sensor performances evaluated at low andhigh concentrations.

For each analyte the minimum detectable concentrationwas determined imposing, on the fitted isotherm, aconventional value of 1 Hz as the lowest reliable frequencyvariation measurement. The partition coefficient was thencalculated at the limit of detection and at very highconcentrations in order to give values at the two oppositeregimes: the specific adsorption (Langmuir behavior) and

the nonspecific sorption (Henry behavior). All these valuesare listed in Table 1.

It is interesting to note that at high concentrations thepartition coefficient is very small; this is expected becauseof the small thickness of the LB film, which does not allowan efficient sorption at high concentration. On the otherhand, the high values of the partition coefficients calcu-lated, for each analyte, at the estimated lowest measurableconcentration have to be mentioned.

An estimation of the molecular recognition efficiency ofthe corrole compound can be obtained considering thepartition coefficient amplification between the two regimesoccurring at high concentration (nonspecific sorption) andat low concentrations (specific interaction). In Figure 12the ratio between these two values is plotted for eachconsidered analyte. It has to be noted that the high gainis obtained for benzene, whose mechanism of interactionwith the corrole LB film has been outlined previously. Italso has to be noted that no gain is shown for hexane andvaleraldehyde, which are expected to have no specificinteractions with the corrole ring, and so no Langmuir-like contribution at low concentration can be expected.On the other hand, coordination to the manganese ion ofthe corrole complex must be considered for the threeconsidered amines and hexanol, giving rise to specificLangmuir-like interactions, increasing the sensitivity atlow concentrations.

In conclusion, the deposition of LB films of Mn(EMC)is a useful approach to develop QMBs with improved andpeculiar sensing properties. The obtained results showedfor these QMBs a wide selectivity; this feature is par-ticularly promising for their exploitation in sensor arrays,because a wide selectivity is one of the necessary require-ments for sensors to be used for electronic nose applica-tions.8c

Acknowledgment. This work was supported by Murstand CNR. The valuable technical assistance of Mr. M.Zarlenga is gratefully acknowledged.

LA9712909

Figure 11. Experimental data and fitted isotherm for benzene.Sensor response is expressed through the vibrating frequencyvariation of the quartz coated by the LB corrole film. The twocomponents of the isotherm are also shown; the Langmuir partshows a saturation response of about 38 Hz.

Table 1. Minimum Detectable Concentration andMaximum and Minimum Values of the Partition

Coefficient for the Considered Analytes

VOC Cmin (ppm) Kmax Kmin

hexane 7.2330 16.3 10.9valeraldeyhde 64.881 28.9 25.5n-hexanol 15.834 998.2 37.9benzene 5.0856 4065.1 78.6triethylamine 12.833 6243.6 780.9dipropylamine 8.2576 5936.4 767.5ethylendiamine 6.7627 5972.9 741.5

Figure 12. Partition coefficients of the different VOCs.

1274 Langmuir, Vol. 15, No. 4, 1999 Paolesse et al.