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Biosensors and Bioelectronics xxx (2004) xxx–xxx

Sorption of amines by the Langmuir–Blodgett films of soluble cobaltphthalocyanines: evidence for the supramolecular mechanisms

3

4

L. Valkovaa,b,∗, N. Borovkovc, O. Koifmanc, A. Kutepovc, T. Berzinad,M. Fontanad, R. Rellae, L. Valli f

5

6

a Ivanovo State University, Lab. of liq. cryst.,189-36, Gen. Khlebnikov St., 153048 Ivanovo, Russia7b INFM, Ancona, Italy8

c Institute of Solutions Chemistry of the Russian Academy of Sciences, Division of Chemistry and Material Science,1, Akademicheskaya St., 153045 Ivanovo, Russia

9

10d University of Parma, Department of Physics and INFM, Viale delle Scienze, 43100 Parma, Italy11

e Institute of Microelectronics and Microsystems IMM – CNR, Lecce, Italy12f Universita di Lecce, Lecce, Italy13

Received 12 November 2003; received in revised form 11 June 2004; accepted 14 June 2004

14

A15

zylamineb andC the sorbatem omplexes.B cular factora

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BIOS 1449 1–8

bstract

By means of microgravimetry, UV–Vis spectroscopy and optic microscopy, sorption of pyridine, primary aliphatic amines and beny the Langmuir–Blodgett (LB) films of tetra-4-tert-butyl- and tetra-(3-nitro-5-tert-butyl)-substituted cobalt phthalocyanines (CoPc’oPc*, respectively) was studied over a broad concentration range. In general, sorption occurs as stepwise intercalation ofolecules into the supramolecular 3D structure of the phthalocyanine assembly followed by formation of the donor–acceptor coth intercalation depth and stoichiometry of the complexes are determined by the molecular structure of amines. The supramolellows discrimination between amines in air but not in aqueous solutions because of concurrent intercalation of water.2004 Elsevier B.V. All rights reserved.

eywords:Cobalt phthalocyanines; Amines; Langmuir–Blodgett films; Nanoaggregates; Sorption

. Introduction

A major problem concerned with biomimetic sensor ar-ays (E-noses and E-tongues) is poor selectivity of sensi-ive layers (Di Natale et al., 2000). To enhance selectivity,hin film sorbents on the basis of chelate compounds, es-ecially porphyrines (Di Natale et al., 1996; Brunink et al.,996; Angnes et al., 1996; Leray et al., 1996; Delmarre andied-Charreton, 2000) and azaporphyrines (Schierbaum etl., 1995; Gupta and Misra, 1997; Fitzek et al., 2002) wereroposed. Because of extended aromatic systems and intense

ight absorbance, these compounds are applicable for chem-

∗ Corresponding author. Tel.: +7 0932 326210; fax: +7 0932 326600.E-mail address:valkova@ivanovo.ac.ru (L. Valkova).

ical sensors with different transducers (gravimetric, optresistive, fluorescent etc.).

An advantageous feature of porphyrin films as sensoterials is thought to be the opportunity to orient them towdifferent classes of analytes by changing the molecular sture (Di Natale et al., 2000). In particular, differently substtuted metal tetraphenylporphins with Lewis acid prope(MTpp, M=Zn, Co, Mn, etc.) were used to fabricate thin filthat sorbed selectively such Lewis bases as amines, alcand thio-compounds (Di Natale et al., 1996; Brunink et a1996; Angnes et al., 1996). Polymer films doped with ZnTpand CoTpp were studied to detect optically pyridine (piperidine and aliphatic amines (Leray et al., 1996; Delmarand Bied-Charreton, 2000). However, sorbent–sorbate intactions of the coordination type are so strong that the opsensor effect was reversible only under heating.

956-5663/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.bios.2004.06.047

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2 L. Valkova et al. / Biosensors and Bioelectronics xxx (2004) xxx–xxx

The first trial (Schierbaum et al., 1995) to use the films27

of Lewis acid phthalocyanines to detect organic bases in air28

showed that both alcohols and hydrocarbons were sorbed29

similarly. On the other hand, substantial increase in conduc-30

tivity of the film of manganese phthalocyanine due to sorp-31

tion of trimethylamine (Gupta and Misra, 1997) indicates that32

formation of the donor–acceptor complex in the solid state is33

quite probable, and may be utilized to develop selective sen-34

sitive layers. Recently, sorption of normal aliphatic amines35

by the films of alkyl-substituted phthalocyanines was stud-36

ied gravimetrically (Fitzek et al., 2002). Retention of these37

amines by the zinc phthalocyanine film was found to be 20–3038

times higher than that by the free-base phthalocyanine one.39

The authors (Fitzek et al., 2002) came to the conclusion that40

even at low equilibrium concentrations the amine molecules41

intercalate into the phthalocyanine stacks in such a way that42

the Zn atoms become coordinated. Fractions of the coordi-43

nated Zn atoms were estimated as 0.7–1.0 depending on the44

length of alkyl substituents.45

The works (Di Natale et al., 2000; Di Natale et al., 1996;46

Brunink et al., 1996; Angnes et al., 1996; Leray et al., 1996;47

Delmarre and Bied-Charreton, 2000; Schierbaum et al., 1995;48

Gupta and Misra, 1997; Fitzek et al., 2002) show that there are49

at least two mechanisms of interaction between solid Lewis50

acid sorbent and gaseous Lewis base sorbate, namely van der51

Waals and coordination ones. Relative contributions of the52

m cular53

s well54

a s of55

s first,56

s cid57

a g58

s orp-59

t60

,61

262

v y of63

t bsti-64

t ups65

c erties66

o lms67

o ing68

g69

a ding70

d71

B e72

C73

d ak to74

b udied75

i 6;76

B nd77

t n the78

o ined79

o ects.80

I po-81

s82

gravimetric one (Di Natale et al., 1996), whereas selectivity 83

of the interaction of ZnTpp with Py and piperidine is close84

to experimental error in spite of great difference in stability85

of the corresponding donor–acceptor complexes (Karmanova 86

et al., 1983). 87

Thus, neither van der Waals (Di Natale et al., 2000; Di 88

Natale et al., 1996; Brunink et al., 1996) nor coordination 89

(Angnes et al., 1996; Leray et al., 1996) mechanisms of the 90

sorbent–sorbate interaction seem to allow highly selective91

sorption of Lewis base analytes by the films of porphyrines92

and phthalocyanines. Therefore, this work aims at revealing93

of other mechanisms of the sorbent–sorbate interaction that94

may be utilized to enhance selectivity of thin film sorbents.95

2. Materials and methods 96

CoPc’ and CoPc* were synthesized according to the pro-97

cedure used earlier for copper derivatives (Stynes et al., 98

1973). Their basicities were determined by titration with sul-99

furic acid in the acetic acid medium. Amines (Fluka) were100

distilled before using. 101

The Langmuir–Blodgett (LB) films were fabricated on a102

MDT Langmuir trough (Zelenograd, Russia). Solutions (5103

× 10−4 mol/l) in benzene were spread on tridistilled water104

and compressed to surface pressure of 15 mN/m. Then the105

l ping106

a 107

ized108

g al-109

a th a110

f des111

p ents112

i om113

t apor114

f - 115

p Hz116

a in wa-117

t with118

s er 119

Q 120

s in121

a etic122

s per-123

f the124

c tion125

l pro-126

d by127

a f- 128

f as129

s ll at130

2 astic131

c ther132

w three133

s ing134

w 135

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TE

echanisms into retention are dependent on the moletructure and physical state of chelate compounds ass experimental conditions. The most reliable symptomorption by the coordination mechanism seem to be:ignificant differences in retention by the films of Lewis and neutral chelates (Fitzek et al., 2002); second, the stronorbent–sorbate interaction manifesting itself in poor desion (Leray et al., 1996; Fitzek et al., 2002).

Considering the data on the MTpp films (Di Natale et al.000; Di Natale et al., 1996; Brunink et al., 1996) from thisiewpoint, it should be noted that increase in Lewis acidithe MTpp molecules by changing the metal atom and suution of hydrogen atoms with electron withdrawing groauses the disproportionately low effect on sensor propf the films. In particular, retentions of amines by the fif MTpp with electron donating and electron withdrawroups differ from one another only by 5–10% (Brunink etl., 1996), whereas stability constants of the correspononor–acceptor complexes differ by ca. 50 times (Vogel andeckman, 1976). Full desorption of diethylamine from thoTpp film at room temperature (Brunink et al., 1996) in-icates that the sorbent–sorbate interaction is too wee considered as coordination. Thus, sensor effects st

n the works (Di Natale et al., 2000; Di Natale et al., 199runink et al., 1996) have the van der Waals nature, a

herefore are not specific with respect to Lewis bases. Other hand, sensor properties of porphyrin films determnly by coordination seem to be of vague applied prosp

n particular, optic response of the ZnTpp–polymer comition towards amines (Leray et al., 1996) is lower than the

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BIOS 1449 1–8

ayers were transferred onto solid supports by 20-fold dipccording to the Schafer technique.

Retention of sorbates by the LB films was characterravimetrically using two AT-cut quartz-crystal microbnces (QCM), each 12 mm in diameter. The QCM wi

undamental frequency of 4 MHz and silver-plated electroassivated with hydrogen sulfide were used for measurem

n air. The films deposited on the QCM were dried at roemperature and aged by multiple expositions to Py vollowed by heating at 100◦C until stabilization was comleted. The QCM with a fundamental frequency of 10 Mnd gold-plated electrodes was used for measurements

er. The shifts of resonance frequency after the coatingorbent films (�fS) were 635 Hz for CoPc’ on the formCM and 4120 Hz for CoPc* on the latter QCM.Sorption from air was studied under static condition

365 ml glass cell equipped with a water jacket, magntirrer and pipeline for purging. All measurements wereormed at 23◦C. Sorbate quantities were injected intoell with 10- and 500-mkl Hamilton syringes. Concentraeveling after injection takes not more than 3 min. Reucibility of gravimetric data in parallel runs was limitedccuracy of injection and reached±5%. Desorption was e

ected by purging the cell with air. Sorption from water wtudied under dynamic conditions in a 10 mkl Plexiglas ce0◦C. One side of the quartz crystal was sealed with a plasing maintaining it in an air environment, while the oas exposed to amine solutions. The runs consisted ofequential injections of amine solutions followed by purgith distilled water.

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L. Valkova et al. / Biosensors and Bioelectronics xxx (2004) xxx–xxx 3

UV–Vis spectra of the films were recorded on Specord136

M400 (Germany). Micrographs were taken on a Zeiss Ax-137

iotech light microscope (Germany).138

3. Results and discussion139

3.1. Substances and phenomena under investigation140

Amines as detection targets may be classified as follows:141

first, aliphatic amines (end products of protein foodstuff de-142

composition); second, Py derivatives (Vitamin PP, its anti-143

vitamins and nicotine) and third, aliphatic amines bearing144

aromatic cycles (drugs and hormones of the phenethylamine145

and histamine groups). Because of such diversity of molecu-146

lar structures, it seems reasonable to begin with structurally147

similar amines having the same reaction center but differently148

structured hydrocarbon fragments. Therefore, the present149

work deals with a series of primary amines and Py (Table 1).150

When choosing primary amines, the length and geometry of151

the hydrocarbon fragments as well as volatility closeness (cf.152

b.p. for PA and TBA) are taken into account. Py is taken153

because it coordinates as easily as primary aliphatic ones in-154

spite of its ternary nature (Karmanova et al., 1983; Stynes et155

al., 1973; Sibrina et al., 1997). Benzene whose sorption by156

the azaporphyrine LB films was studied earlier by scattering157

t ce158

i159

n160

f nted161

s162

i ating163

b Sec-164

TP

S

nntnBPB

Table 2Properties of cobalt phthalocyanines

Abbr. FW pK (20◦C, CH3COOH)

CoPc’ 795.9 ca. 2CoPc* 975.9 ca.−3

ond, two coordination mechanisms, namely bimolecular and165

trimolecular ones, are supposed to allow discrimination be-166

tween amines with different basicity (Table 1, pK). Bimolec- 167

ular coordination yields the axial donor–acceptor complexes168

whose stability in solution correlates with the amine basicity169

(Karmanova et al., 1983; Stynes et al., 1973). Trimolecular 170

(biomimetic (Valkova et al., 2004)) coordination occurs when 171

the amine molecule interacts with such a chelate that has one172

of two axial sites occupied with anotherπ-donating orπ- 173

accepting ligand (Stynes et al., 1973). Unlike porphyrin-like 174

chelates, azaporphyrines are able to exert thetrans-effect on 175

an adjacent chelate molecule that coordinates a Lewis base176

(Cariati et al., 1975). As a result, unique opportunity arises177

to coordinate weak bases more efficiently than strong ones178

(Cariati and Morazzoni, 1976). The study of the methylene 179

matrix doped with CoPc’ (Sibrina et al., 1997) showed that 180

the trimolecular coordination mechanism favors sorption of181

aliphatic amines. In particular, at 40◦C when sorption of 182

amines by the doped methylene matrix is fully reversible,183

retention of piperidine is by four times higher than that of Py.184

Because in the trimolecular coordination both azaporphyrine185

molecules act as electron acceptors (one asn-acceptor and 186

another asπ-acceptor), perceptible enhancement of coordi-187

nation efficiency is expected due to increase inπ-deficiency 188

of the azaporphyrine nucleus. Therefore, CoPc* whose ba-189

sicity is ca. five orders lower than that of CoPc’ (Table 2, pK) 190

i 191

3 192

rlier193

i - 194

r . 195

F ond-i 5 (4)a

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echnique (Valkova et al., 2003) is selected as a referennert sorbate.

Phthalocyanines CoPc’ and CoPc* (Table 2) are choserom supramolecular considerations. First, the fragmetructure of the phthalocyanine films (Valkova et al., 2003)s supposed to function as a separator device discriminetween amines with different hydrocarbon fragments.

able 1roperties of sorbates

orbate Abbr. FW b.p.(◦C) pK (25◦C, H2O)

-Propylamine PA 59.1 48 ca. 10.5-Butylamine BA 73.1 78 ca. 10.5ert-Butylamine TBA 73.1 46 ca. 10.5-Hexylamine HA 101.2 132 ca. 10.5enzylamine BzA 107.2 184 9.4yridine Py 79.1 115 5.3enzene B 78.1 80 –

P

BIOS 1449 1–8

s studied in the present work.

.2. Sorption of benzene and pyridine by the CoPc’ film

In the case of benzene, normal kinetic curves found ean the work (Schierbaum et al., 1995) are obtained at equilibium concentrations of [B]<300 ml/m3 (Fig. 1, curves 1–3)

ig. 1. Frequency changes of the QCM coated with the CoPc’ film respng to vapor of benzene at concentrations of 54 (1), 81 (2), 270 (3), 40nd 540 (5) ml/m3.

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4 L. Valkova et al. / Biosensors and Bioelectronics xxx (2004) xxx–xxx

Fig. 2. Frequency changes of the QCM coated with the CoPc’ film respond-ing to vapor of pyridine at concentrations of 135 (1), 270 (2), 405 (3), 540(4), 675 (5), 810 (6), 1080 (7) and 1350 (8) ml/m3.

At higher [B], the curves are anomalous (Fig. 1, curves 4 and196

5); retention at the first stage exceeds sorption capacity in-197

dicating that relaxation phenomena occur in the film heavily198

doped with benzene. Relaxation runs very slow, therefore,199

no plateau on the anomalous curve arises (curve 4). Thus,200

equilibrium conditions in the sorbent–sorbate system are not201

generally reachable even in the simplest case of benzene sorp-202

tion. It is noteworthy that though retention at [B]>500 ml/m3203

is anomalously high (curve 5), high rate and full reversibility204

of benzene desorption hold.205

In the case of Py, normal kinetic curves are obtained206

only at [Py]<20 ml/m3 (not shown). At 20<[Py]<450 ml/m3,207

the curves are pseudo-normal (Fig. 2, curves 1–3); they208

have tilted non-linear portions corresponding to the diffu-209

sion stage that transform to plateaus within ca. 40 min. At210

450<[Py]<650 ml/m3 (curve 4), relaxation anomaly vanish-211

ing in ca. 60 min is observed. At [Py]>650 ml/m3, oscillation212

anomaly arises (curves 5–8). Oscillations of retention being213

reproducible within experimental error are dependent on the214

film prehistory at the first stage of sorption (Fig. 3). It is note-215

worthy that analogous anomaly was found earlier for sorption216

F ond-i eo nera-t 0(r

Fig. 4. Isotherms of vapor sorption by the CoPc’ film for benzene (1) andpyridine (2). In the case of anomalous sorption curves, retention values mea-sured after 60-min exposition were used. Solid lines show fittings with thefirst (1) and third (2) order polynomials.

of pinene by the LB films of fatty acids (Shiratori et al., 2000). 217

At very high [Py], retention becomes nearly constant (Fig. 2, 218

curves 7 and 8). Except for the lowest concentration range,219

desorption of Py is not complete. 220

Fig. 4shows sorption isotherms for benzene and Py. The221

former isotherm is linear at [B]<300 ml/m3; the latter one is 222

sigmoid indicating several mechanisms of sorption. To re-223

veal the mechanisms, such points on kinetic curves should224

be considered in which a sorbent–sorbate system deviates225

from the equilibrium state minimally. Such points areT and 226

R (Fig. 3) corresponding to total and residual retentions.227

From the�fT, �fR and�fS values, two plots are constructed228

(Fig. 5). The (Py/CoPc’)R versus [Py] plot is linear up to [Py] 229

ca. 600 ml/m3 indicating the first order of coordination with 230

respect to Py. At larger [Py], i.e. when oscillations of reten-231

tion occur, molar residual retention is close to unity. These232

data show that the supramolecular structure of the film does233

not interfere with intercalation of the Py molecules onto the234

molecular level followed by coordination by the cobalt atoms.235

Because residual retention of Py is controlled only by coordi-236

nation, it may be used to construct the (�fR/�fT) versus [Py] 237

plot and determine dependence of coordination mechanism238

F oPc’fi lr t (1)a

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ig. 3. Frequency changes of the QCM coated with the CoPc’ film respng to vapor of pyridine at concentration of 675 ml/m3. The curves werbtained in the first cycle (1), in the second cycle after 10-min rege

ion at 100◦C (2) and in the third cycle after 30-min regeneration at 10◦C3). Italic letters show the points corresponding to total (T) and residual (R)etention.

BIOS 1449 1–8

ig. 5. Mechanistic characteristics of the pyridine sorption by the Clm: (1) molar residual retention, (�fR/�fS)(FWS/FWA); (2) residual/totaetention ratio. Solid lines show results of the data fittings with the firsnd fourth (2) order polynomials.

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L. Valkova et al. / Biosensors and Bioelectronics xxx (2004) xxx–xxx 5

Fig. 6. Frequency changes of the QCM coated with the CoPc’ film respond-ing to vapors oftert-butylamine at concentrations of 54 (1), 135 (2), 270 (3),540 (4) ml/m3 and propylamine at concentrations of 54 (5), 135 (6), 270 (7)ml/m3.

efficiency on [Py] (Fig. 5, curve 2). At low [Py], coordina-239

tion contributes into retention poorly. At [Py]>200 ml/m3, its240

contribution is nearly constant and estimated as 0.20± 0.05.241

3.3. Sorption of aliphatic amines and benzylamine by242

the CoPc’ film243

Among the primary amines studied, the simplest pseudo-244

normal curves are found for TBA (Fig. 6, curves 1–4).245

Anomalous curves are observed in the case of normal246

amines (Fig. 6, curve 7 andFig. 7, curve 1). Anomalous247

behavior shows the complex nature and low rates of the248

diffusion–relaxation phenomena in the doped film. It is note-249

worthy that sorption of PA is by far more efficient than that250

of TBA in spite of close volatilities.251

Molar residual retention of TBA increases linearly from252

0.3 to 0.4 within 50<[TBA]<300 ml/m3. Further increase in253

[TBA] favors irreversible sorption; at [TBA] = 550 ml/m3,254

molar residual retention reaches 0.85. PA behaves similarly;255

within the [PA] range studied, molar residual retention in-256

creases linearly and reaches ca. 4 at [PA] = 270 ml/m3 (Fig. 6,257

�fR=195 Hz). Retention of HA is the most efficient; the sorp-258

F ond-if n areg

Fig. 8. UV–Vis spectra of the CoPc’ (a) and CoPc* (b) films in air (1) andsaturated vapors of benzene (2), triethylamine (3), diethylamine (4),tert-butylamine (5), pyridine (6) and hexylamine (7).

tion curve is anomalous and molar residual retention reaches259

2.5 even at [HA] = 54 ml/m3 (Fig. 7, �fR=215 Hz). 260

Because the strong sorbent–sorbate interactions are non-261

stoichiometric in the case of aliphatic amines, composition of262

the donor–acceptor complexes may not be found from gravi-263

metric data. Therefore, effect of amine vapors on UV–Vis264

spectra of the LB films is studied. Profiles of the spectra, i.e.265

resolution and relative intensity of the Q band at ca. 680 nm,266

change discretely in sorbate vapors, and therefore may be267

used to classify sorbates by intensity of their disaggregating268

effect. 269

Fig. 8a shows that Py and HA exert the strongest effect270

(curves 6 and 7), the corresponding profiles being close to one271

in solution. Here, it should be noted that appearance of the Q272

band in UV–Vis spectra of the doped azaporphyrine films in-273

dicates low probability of the intermolecular HOMO–LUMO274

transition, but should not be considered as an evidence of the275

molecular form existence (Valkova et al., 2002a). The aggre- 276

gated state of CoPc’ in the film saturated with Py is proved277

by oscillations of retention (Fig. 3) caused by structural re- 278

arrangements. Benzene and triethylamine exert the moderate279

disaggregating effect that manifests itself in the higher in-280

tensity of the Q band than one of the band at ca. 620 nm281

corresponding to the intermolecular HOMO–LUMO transi-282

tion (Fig. 8a, curves 2 and 3). Diethylamine and TBA show283

the weakest effect (curves 2 and 3). 284

en-285

s re of286

s re-287

g Sec-288

o e) is289

a ence290

o in-291

d n of292

a but293

a ment294

i e sys-295

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ig. 7. Frequency changes of the QCM coated with the CoPc’ film respng to vapor of hexylamine (1) at concentration of 54 ml/m3. The curvesor propylamine (2) and pyridine (3) obtained at the same concentratioiven for comparison.

BIOS 1449 1–8

The UV–Vis spectra allow finding relation between intity of the disaggregating effect and the molecular structuorbates. First, full disintegration of the initial CoPc’ aggates is caused only by Py and primary normal amines.nd, high molecular symmetry (benzene and triethylaminstronger intercalation factor than polarity and the presf primary amino group if there is some sort of steric hrances (diethylamine and TBA). Third, residual retentioliphatic amines is determined not only by coordinationlso interactions of the van der Waals nature. This state

s supported by data obtained on other sorbent–sorbat

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6 L. Valkova et al. / Biosensors and Bioelectronics xxx (2004) xxx–xxx

Fig. 9. Frequency changes of the QCM coated with the CoPc’ film respond-ing to vapor of benzylamine at concentrations of 27 (1), 135 (2) and 405 (3)ml/m3. The curve for propylamine (4) obtained at 135 ml/m3 is given forcomparison.

tems. In particular, we observed residual retention of ca. 0.3 in296

the CuPc’–triethylamine system even at low equilibrium con-297

centrations, though no coordination of such hindered amine298

by the copper atom is possible. Moreover, residual retention299

was found even in the purely hydrophobic systems (Shiratori300

et al., 2000). Thus, a jamming of aliphatic molecules in the301

nanostructured sorbents is evidenced.302

Sorption of BzA occurs in a different way. Kinetic curves303

being pseudo-normal at comparatively low [BzA] (Fig. 9,304

curves 1 and 2) show weakly pronounced oscillations of re-305

tention at [BzA] = 405 ml/m3 (curve 3). The following points306

are of importance. First, sorption of BzA is less efficient than307

that of PA inspite of great difference in volatility (Table 1).308

Second, the equilibrium state after desorption is established309

quite rapidly. Third, at [BzA]>150 ml/m3, residual retention310

is independent of [BzA] being close to 0.5 (Fig. 9, �fR =311

45 Hz). To desorb BzA, long-duration heating at 150◦C of312

the doped film is needed.313

These data indicate that interaction between CoPc’ and314

BzA at the molecular level occurs stoichiometrically by the315

trimolecular coordination mechanism. Contribution of coor-316

dination into retention determined from the�fR and�fT =317

460 Hz values is ca. 0.1. Intercalation of BzA is not accom-318

panied by the jamming in spite of its low volatility. Thus, the319

BzA and Py molecules intercalate by the common mecha-320

nism.321

3322

w323

t324

a a-325

t om-326

e rent327

i h. To328

c ount.329

F ains330

m trum331

Fig. 10. Frequency changes of the QCM coated with the CoPc* film re-sponding to aqueous solutions oftert-butylamine (a) andn-butylamine (b)in the first cycle at concentrations of 0.15 (1) and 1.5 (2) mol/l. The in-sert shows response to 1.5 mol/l solution ofn-butylamine in three sequentialcycles.

with the resolved Q band arises in Py vapor (curve 6). The332

strongest spectral effect is caused by HA; the corresponding333

spectrum (curve 7) is fully devoid of the Q band indicat-334

ing formation of a new supramolecular form of CoPc* with335

strongly collectivizedπ-density. Therefore, the CoPc*–TBA 336

and CoPc*–BA systems are studied gravimetrically and mi-337

croscopically. 338

In the former case (Fig. 10a), kinetic curves and retention339

in three sequential sorption–desorption cycles are indepen-340

dent of an ordinal cycle number (not shown). The equilib-341

rium state is reached within 10–15 min. Full desorption takes342

ca. 5 min. Sorption of BA (Fig. 10b) occurs in a different 343

way. In the first cycle, the kinetic curves have kinks after344

7.8- and 6.4-min expositions at [BA] = 0.15 and 1.5 mol/l,345

respectively. In the following cycles, retention sequentially346

increases, the kinks do not appear, intense diffusion phenom-347

ena arise and the datum line shifts sequentially to higher fre-348

quencies (Fig. 10b, insert). Sorption selectivity is poor in the349

first cycle, but becomes noticeable in the third one. 350

The visual observation shows that in the pristine film351

(Fig. 11a), the CoPc* molecules are assembled in the nanoag-352

gregates sized of 50–400 nm and united in grapes (Fig. 11, 353

insert). A minor portion of CoPc* is distributed more or less354

evenly over the whole surface. Treatment of the film with355

TBA solution entails growth of some nanoaggregates, but356

not changes the film morphology (Fig. 11b). Treatment of 357

t es358

t ge-359

n 360

The361

C fil-362

t mak-363

i CM364

f dra-365

t urs366

i e BA367

m sinte-368

g man-369

i ur-370

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.4. Sorption of aliphatic amines by the CoPc* film fromater

Opportunity of E-nose/E-tongue integration (Di Natale el., 2000) evokes interest in sorption of amines from w

er. Because it might be interfered with hydration phenna, the sorbent–sorbate systems with principally diffe

nteraction mechanisms should be chosen to begin withoose the systems, UV–Vis spectra are taken into accig. 8b shows that the profile of the CoPc* spectra remostly unchanged in vapors (curves 1–5). The spec

BIOS 1449 1–8

he film with BA solution not only destroys but also liquefihe film (Fig. 11c); stable microscopic droplets of homoeous solution arise instead of the nanoaggregates.

The data obtained may be rationalized as follows:oPc* film surface is hydrated. The TBA molecules in

rate into the grapes and reside on the nanoaggregatesng its surface hydrophilic. Therefore, the changes in Qrequency are determined primarily by a growth of the hyion layer rather than sorption of TBA. Sorption of BA occn the same way during the 6–7 min lag phase. Then th

olecules intercalate into the nanoaggregates and dirate them. Corresponding changes in the film structure

fest itself in the reproducible kinks on the kinetic curves. F

CTE

D P

RO

OF

BIOS 1449 1–8

L. Valkova et al. / Biosensors and Bioelectronics xxx (2004) xxx–xxx 7

Fig. 11. Micrographs of the CoPc* film as deposited (a) and after treatment with aqueous solutions oftert-butylamine (b) andn-butylamine (c) at concentrationsof 1.5 mol/l followed by drying at room temperature (×1000). The insert shows a grape of the coarse nanoaggregates (×10000).

ther interaction occurs by the coordination mechanism and re-371

sults in formation of the triple liquid-phase CoPc*–BA–water372

system. High stability of the system may be explained by for-373

mation of ionized species, such as (HA·CoPc*)+·(CoPc*)−.374

Liquefaction of the film is accompanied by a loss of the sur-375

face hydration layer as the datum line shift to higher frequen-376

cies indicates.377

3.5. Mechanistic insight into sorption phenomena378

General scheme of amine sorption by the films of sol-379

uble phthalocyanines may be sketched as follows. Initially380

the films are constructed from the multi-stacked nanoaggre-381

gates that weakly interact with one another (Fig. 12, 1). As382

a result, the films are characterized by the high free volume383

that is filled with the sorbate molecules first of all (Fig. 12,384

2a). Such sorption is described by the normal kinetic curves385

and therefore may be called “simple intercalation”. It was386

observed in the work (Schierbaum et al., 1995) for the sys-387

tems with non-polar sorbates at low equilibrium concentra-388

tions.389

The nanoaggregates possess degree of translational390

freedom. Therefore, at sufficiently high equilibrium con-391

centrations, the free volume suffers changes. In the case392

of non-aliphatic sorbates, translations result in formation of393

s ech-394

a395

P sorp-396

t tra-397

c398

e399

s in-400

c alo-401

c ay that402

t en-403

t ter-404

c405

In the case of primary normal aliphatic amines, interca-406

lation occurs onto the deepest levels. Localization of the407

amine molecules in the inter-stack space of the nanoaggre-408

gates (Fitzek et al., 2002) may be called “internal intercala- 409

tion” (Fig. 12, 3). The final stage is coordination by one or410

another mechanism (Fig. 12, 4a,b). 411

Fig. 12. Schematic representation of sorption phenomena in solid filmsof soluble cobalt phthalocyanines. (1) Initial supramolecular structure. (2)Micro-level: (a) simple intercalation; (b) condensed intercalation and (c)jammed intercalation. (3) Nano-level: inter-stack intercalation. (4) Molecu-lar level: (a) bimolecular coordination; (b) trimolecular coordination.

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Etructural voids filled with the sorbate molecules. Such mnism may be called “condensed intercalation” (Fig. 12, 2b).robably this mechanism determines anomalously high

ion capacity of the phthalocyanine films with respect to tehloroethane (Schierbaum et al., 1995) and benzene (Valkovat al., 2002b).

In the case of aliphatic sorbates, especially polar onelined to cluster formation, mutual orientation of the phthyanine nanoaggregates seems to change in such a whe intercalation species become well stabilized by theropy factor. Such mechanism may be called “jammed inalation” (Fig. 12, 2c).

DO

F

8 L. Valkova et al. / Biosensors and Bioelectronics xxx (2004) xxx–xxx

4. Conclusions412

General mechanistic scheme describing sorption of413

amines by the nanostructured films of soluble cobalt phthalo-414

cyanines is proposed. Amines with differently structured or-415

ganic fragments may be readily discriminated gravimetrically416

in air, but not in water because of strong hydration of the ph-417

thalocyanine films.418

Acknowledgements419

The work is supported by Russian Foundation for Basic420

Research (Grant No. 03-02-16643a) and Russian Academy421

of Sciences (Contracts 03-18 and P7/03).422

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