Adsorption mechanism and acidic behavior of naphthazarin on Ag nanoparticles studied by Raman spectroscopy

Vibrational Spectroscopy 30 (2002) 203–212

Adsorption mechanism and acidic behavior of naphthazarin on Ag nanoparticles studied by Raman spectroscopy G. Fabriciovaa, J.V. Garcı´a-Ramosa, P. Miskovskyb,c, S. Sanchez-Cortesa,* a

Instituto de Estructura de la Materia, CSIC, Serrano 121, 28006 Madrid, Spain Department of Biophysics, P.J. Safarik University of Kosice, Jessenna´ 5, 041 54 Kosice, Slovak Republic c International Laser Center, Ilkovicova 3, Bratislava, Slovak Republic

b

Received 19 December 2001; received in revised form 20 March 2002; accepted 2 April 2002

Abstract Surface-enhanced Raman scattering (SERS) spectroscopy is applied in this work to the study of the adsorption of naphthazarin (NZ) on Ag nanoparticles. Spectra recorded at different excitation wavelengths and pHs revealed that this molecule is adsorbed on the metal through several mechanisms. Two main types of adsorbed molecules can be identified that correspond to neutral and ionized NZ, which may be physisorbed or chemisorbed on the metal. The existence of these different forms can be due to different binding sites on the surface or to the formation of a multilayer architecture on the metal surface giving rise to different adsorbate states. Although the amount of the ionized molecule attached on the surface is higher at neutral pH, the neutral molecule may also exist even at very high pH. The amount of neutral NZ increases with the time and also as the NZ concentration is raised or as the dimethylsulfoxide (DMSO) concentration existing in the medium is increased. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Naphthazarin; Adsorption; Chemisorption; Charge transfer; Resonance Raman; SERS

1. Introduction Naphthazarin (NZ) (Fig. 1) is a diphenol derivative of naphthoquinone (5,8-dihydroxy-1,4-naphthoquinone). The interest of this molecule is based on the fact that a NZ structure is found in a variety of drugs with antitumoral application, such as hypericin [1], tricrozarin B [2] and 2,3,6-trichloronaphthazarin [3]. NZ is the basic unit of several tetracyclic antibiotics [4] and also is used by scientists as an analog of the active site of drugs used for treating cancer by photodynamic therapy [5]. Moreover, many NZ derivatives *

Corresponding author. Tel.: þ34-915-61-6800; fax: þ34-915-64-5557. E-mail address: [email protected] (S. Sanchez-Cortes).

show also antimicrobial activity [2,6]. Some ones are toxins or pigments produced by various fungi [7,8]. NZ is able to chelate ionic metals with the formation of charge transfer complexes with a large change in color and this fact allows the use of NZ in spectrophotometric determination of a variety of ionic metals [9–11]. Furthermore, another application of NZ is the induction of apoptosis in cells [12]. The structure of NZ has been the subject of numerous spectroscopic [13–16], X-ray crystallographic [17,18] and nuclear magnetic resonance spectroscopic [19,20] investigations. In addition, ab initio theoretical studies have been driven to elucidate the geometry of NZ with special emphasis in the role of hydrogen bonds in the general structure of the molecule [21,22]. NZ is a planar molecule and its molecular symmetry

0924-2031/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 2 0 3 1 ( 0 2 ) 0 0 0 2 0 - 6

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In the present paper, we report a study of the adsorption of NZ on Ag nanoparticles at different pH, excitation wavelengths, and adsorbate and dimethylsulfoxide (DMSO) concentrations. Finally, the evolution in time of the SERS spectrum was also studied.

Fig. 1. Different structures proposed for NZ: (a) 5,8-dihydroxy1,4-naphthoquinone, with C2v symmetry; (b) 4,8-dihydroxy-1,5naphthoquinone, with C2h symmetry; (c) conjugated structure with D2h symmetry.

depends on the positions of the two mobile protons in hydrogen bonds, which occur between the adjacent hydroxyl and carbonyl oxygen. For this molecule are possible three symmetry groups: D2h, C2h (with keto groups in 1,4-positions) and C2v (with keto groups in 1,5-positions) (Fig. 1). Crystal-structure analyses of several derivatives of NZ and anthraquinone and ab initio HF-SCF study of NZ established that the 1,4diketo structure is the most important one at room temperature [23–27]. There is a great controversy as to whether NZ has a D2h [18] or a C2v symmetry in solid state [16]. In fact NZ can crystallize under three polymorphic forms, designated as A, B and C. In the first two (A and B), only intramolecular hydrogen bonds are established between C=O and adjacent OH group, whereas in the C form, intermolecular hydrogen bonds are also established. In solution the predominant NZ structure seems to be the 1,4-diketo one as deduced by dipole moment determination [28], and NMR [29] and IR spectroscopy [30]. NZ is highly fluorescent and practically insoluble in water. This properties seriously limit the application of Raman spectroscopy to carry out a structural study of this molecule. However, near infrared FT-Raman and surface-enhanced Raman scattering (SERS) spectroscopy can be employed in the characterization of this molecule in aqueous media. The FT-Raman spectroscopy can be applied, since the near infrared light lies far from the absorption region of NZ. SERS technique consists in the employ of rough metal surfaces to enhance the Raman emission [31]. This technique can be successfully applied in the study of very insoluble compounds in water, since very low concentrations are required, with the additional advantage of the fluorescence quenching occurring on the metal surface [32].

2. Experimental 2.1. Materials NZ was purchased from Aldrich (95%). Stock solutions of NZ were prepared in DMSO. All the reagents employed were of analytical grade and purchased from Sigma. The aqueous solutions were prepared by using triply distilled water. Silver colloids were prepared by reduction of silver nitrate with citrate following the method of Lee and Meisel [33]. The colloid was activated before adding NZ. This activation consisted in a partial aggregation of the colloidal particles and to accomplish this, 12 ml of 0.5 M potassium nitrate solution was added to 0.5 ml of the original colloid. Then 0.5 ml of DMSO solution of NZ of different concentrations was added to the preaggregated colloid to obtained the desired final concentration. Nitric acid and sodium hydroxide were employed to vary the pH before the addition of the NZ solution to the colloid in pH-dependent experiments. The solid sample for the FT-Raman was prepared by putting the compounds powdered in a metal holder. Samples for the UV–VIS spectroscopy in water were prepared by adding an aliquot of the DMSO solution of NZ up to a concentration of 105 M. Quartz cells of 1 cm were used. 2.2. Instrumentation The SERS spectra with excitation in the visible region were recorded in a U-1000 Jobin–Yvon spectrophotometer by using 514.5, 488 and 457.9 nm radiation lines of a Spectra Physics model 165 argon ion laser and by using the 647.1 nm radiation line of a Spectra Physics model 165 krypton ion laser. Resolution was set at 4 cm1 and a 908 geometry was used to record the data. The output laser power was 180 mW. All the spectra were recorded at 1 cm1 step intervals with an integration time of 1 s. The SERS spectrum obtained at 782 nm was measured in a Renishaw 1000

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microspectrophotometer with an objective 50. The output laser power was 30 mW. FT-Raman and SERS spectra were obtained by using a RFS 100/S Brucker spectrophotometer. The 1064 nm line, provided by a Nd:YAG laser, was used as excitation line. The resolution was set to 4 cm1 and a 1808 geometry was employed. The output laser power was 150 mW in the case of SERS measurements and 50 mW in the solid samples.UV–VIS absorption spectra were recorded with a Cintra 5 spectrometer. 2.3. Calculation The geometry optimization and RHF calculations were carried out by using the GAUSSIAN 94 package of programs [34] and employing the RHF/6-31G basis set.

3. Results and discussion 3.1. Absorption spectra Fig. 2 shows the absorption spectra of NZ in water solution and 0.1% DMSO. This solution also was employed for obtaining SERS measurements. In aqueous medium, NZ can exist in the 3.8–12.8 pH interval under three different forms (NZH2, NZH and NZ2), showing absorption maxima at about 488, 514 (doublet), 575 and 614 nm. The pH dependence of the absorbance indicates a gradual dissociation of hydroxyl groups in the NZH2 form in going from acidic to alkaline pH. The absorption bands at 488 and 514 nm can be attributed to the neutral form of NZ (NZH2). This bands disappear completely at about pH 6.8. Another maximum of the absorption can be observed at 575 nm when the pH is raised to 6.8, which may correspond to the monoionized form (NZH). The latter band shifts to 614 nm on increasing the pH of the solution due to the appearance of the dianionic form (NZ2). The pK values corresponding to the NZH2 dissociation were deduced from the absorption spectra shown in Fig. 2 by using the graphical method described by Kuban and Havel [35]. The pK values deduced for NZ were pK1 ¼ 6:0 and pK2 ¼ 10:4. These values are significantly lower than those deduced for NZ by Idriss and Saleh in 50% (v/v)

Fig. 2. UV–VIS absorption spectra of NZ (105 M) at the following pH: (a) 12.8; (b) 12.1; (c) 11.0; (d) 10.1; (e) 8.8; (f) 7.8; (g) 6.8; (h) 5.7; (i) 5.5; (j) 5.3; (k) 5; (l) 4.6; (m) 4.4; (n) 3.8.

water/ethanol solution [36]. The significant difference of pK values is attributed to the different solvents where NZ is solubilized. 3.2. Raman of solid NZ and normal mode calculation The Raman spectrum of NZ in solid state, exciting at 1064 nm (Fig. 3a), is very similar to previously reported FT-Raman spectrum of NZ C structure [4] which forms not only intramolecular hydrogen bonding but also an intermolecular one. The spectrum of NZ derived from the ab initio calculations is shown in Fig. 3b. The main wavenumbers of these spectra, as well as the assignments derived from calculation are displayed in Table 1. In general, there is a concordance regarding the Raman intensities as well as the position of the peaks between the experimental and calculated

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Fig. 3. Raman spectrum of solid NZ (a) and theoretical Raman intensities deduced from the ab initio calculations (b).

spectra, except for some bands corresponding to the keto and phenol groups. The O–H and C=O stretching modes are expected at 3550 and 1740 cm1 (Fig. 3b), respectively. In the Raman spectrum the O–H stretching was not detected and the C=O stretching mode is markedly downward shifted. This result can be explained by taking into account that theoretical data are obtained from an isolated molecule, whereas NZ in solid state undergoes inter and intramolecular vibrations involving both keto and phenol groups. 3.3. SERS spectra at different pH Fig. 4. shows the SERS spectra of NZ at different pH exciting at 1064 nm. In general, a great similarity is observed between the SERS spectrum obtained at acidic conditions and the Raman spectrum of solid (Fig. 4c and d) and also between the SERS spectra at alkaline and neutral pH (Fig. 4a and b). At pH 3, the

Fig. 4. SERS of NZ (105 M) at pH 11.5 (a); 7.0 (b) and 3 (c) and FT-Raman spectrum of NZ in powder (d). Excitation at 1064 nm.

SERS spectrum is dominated by a very intense band at 1408 cm1, which can be observed also in the Raman spectrum of the solid, along with other bands appearing at 1632, 1555, 1269, 1088 cm1 which are slightly shifted in comparison to the Raman spectrum of solid (Fig. 4d). The similarity existing between the spectrum profile of NZ in solid state and the SERS spectrum recorded at acidic conditions suggests that at this pH the molecule exists on the Ag nanoparticles as the neutral form (NZH2), which is the one existing in solid state. Even so, there are differences concerning keto and hydroxyl bands which suggest a different arrangement of the molecule on the surface. For instance, the bands appearing in the 1300– 1100 cm1, related to C–O stretching motions, are very weak in the SERS at acidic pH, while the bands falling in the 650–450 cm1 region, associated to C–OH and C=O groups (Table 1), are also changed in the SERS respect to the Raman of solid.

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Table 1 Main experimental and calculated Raman wavenumbersa of NZ spectra and assignments derived from the calculation Raman (solid)

SERS pH 3

pH 7

pH 11

3078m 3067m 1652s

1644s

1642s

1593vw

1593vw

1502w

1508w

1632m 1596m 1560w

1555w

1448vw 1409vs

1444sh 1408s 1386w

Calculated wavenumbers

Assignmentsb

3550 3069vs 3068vs 1740vs 1661w 1628s 1599s 1470vw

n(OH) n(CH) n(CH) n(C=O) n(C=O) þ n(CC) n(CC) þ d(asymmetric def.) n(CC) þ d(asymmetric def.) n(CC) þ d(COH)

1470vw 1387vs 1382w

n(C–O) þ n(CC) þ d(COH) n(C–O) þ n(CC) n(CC) þ d(CH) þ d(trigonal def.) n(CC) þ d(CH) n(CC) þ d(COH) n(C–O) n(C–O) þ n(CC) d(COH) n(CC) þ d(CH) n(CC) þ d(CH) d(trigonal def.) n(CC) t(COH) n(CC) g(C–O) þ g(C=O) t(asymmetric def.) n(CC) þ d(asymmetric def.) d(asymmetric def.) n(CC) þ d(asymmetric def.) g(C–O) þ g(CH) þ t(puckering) þ t(asymmetric def.)

1356sh 1306vw 1264w 1230m 1201vw 1135w 1096vw 947w 623m

1332vw

1331vw

1240vs

1240s

1111w

1111vw

1269w

1088w 953w 625vw 596w 533w

467w 456w 422vw 409vw 352vw a b

489m

553vw 492w

460w 430w 347w

1323w 1314vw 1254m 1211vw 1175m 1120m 1077vw 920w 624vw 592w 545vw 467vw 457w 430vw 386vw 361vw

vw: very weak; w: weak; m: medium; ms: moderately strong; s, strong; vs: very strong. n: stretching; d: in-plane bending; g: out of plane bending; t: torsion.

SERS spectra obtained at pH 11.5 and 7 are very similar (Fig. 4a and b), thus indicating that these spectra may correspond to the monoionized form (NZH). This result indicates that the amount of NZ2 forms existing on the surface at the above pH is negligible because no significant changes were observed between neutral and alkaline pH. The only difference between the SERS at neutral and alkaline pH is the band at 553 cm1 which is stronger at high pH and which may be a consequence of the NZ deprotonation at alkaline pH, since it is attributed to gðCOÞ þ gðC¼OÞ motions (Table 1). At neutral pH, NZ tends to be adsorbed as the monoionized form,

which is the more abundant form existing in aqueous solution, as deduced from the results obtained by UV– VIS absorption spectra. In going from acidic to neutral or alkaline pH remarkable changes are observed in the SERS spectra. The band appearing in the SERS spectrum of NZ in acidic conditions at 1555 cm1 (Fig. 4c) disappears at both neutral (Fig. 4b) and basic pH (Fig. 4a), a strong band at appears at 1240 cm1 and the bands at 1408 and 1269 cm1 are markedly weakened. In addition, the bands appearing at 1632 and 1088 cm1 are upward shifted. All these changes are related to the molecule deprotonation since they involve bands attributed to C–OH bending or C–O

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stretching motions, which are sensitive to the internal H-bonds established with the vicinal C=O group, as indicated by the theoretical calculation (Table 1). The NZ deprotonation effects are also noted in the low wavenumber region where the bands appearing in the 600–300 cm1 region, in the SERS at low pH, disappear or shift in the spectra at pH neutral and alkaline. In fact, in the last spectra, only a prominent band at about 490 cm1 is observed in this region. The disappearance of a band at 533 cm1 at higher pH is also related to the deprotonation of NZ, since this band is assigned to out-of-plane C=O and C–O bending (Table 1) which are also sensitive to NZ deprotonation. The above changes allowed us to assign the Raman spectra corresponding to the neutral (NZH2) and monoionized (NZH) forms of NZ. The effect of pH on the SERS of NZ was also studied when exciting at 514.5 nm (Fig. 5). As can be

Fig. 5. SERS of NZ (105 M) at pH 11.5 (a); 7.0 (b) and 3 (c). Excitation at 514.5 nm.

observed, the profiles of the SERS spectra at both basic and neutral pH are again similar (Fig. 5a and b) and exhibit a different profile when compared to the SERS at acidic pH (Fig. 5c). In general, the changes observed in the SERS spectra in going from high to low pH obtained at 514.5 and 1064 nm are very similar. However, the bands corresponding to NZH2 are still seen at neutral and high pH, in contrast to what it is observed at 1064 nm. This result can be explained by assuming that there are different NZ forms adsorbed on the metal, whose photoactivity may dramatically change at different excitation wavelength, as we will see below. The existence of neutral NZ at so high pH indicates that the pK of the molecule is dramatically increased upon adsorption on the metal surface. 3.4. SERS spectra excited with different excitation wavelengths SERS spectra of NZ obtained at different excitation lines are shown in Fig. 6. The most intense bands are seen in the 1750–1200 cm1 region for all excitation wavelengths. By considering the 680 cm1 band of DMSO as an internal standard, we deduced a maximum resonant enhancement at 488 and 514.5 nm (Fig. 6). Among these spectra we can identify three different SERS profiles: (a) at 782 and 1064 nm; (b) at 647.1 nm; and (c) at 457.9, 488 and 514.5 nm. When compared to the SERS obtained at different pH, we observed similarities that can suggest the presence of two different NZ molecules: (a) NZH molecules, with characteristic bands at 1650–1644 and 1243– 1240 cm1; and (b) NZH2 with characteristic bands at 1560, 1405 and 1098 cm1 which are progressively enhanced when the excitation wavelength shifts towards the blue. However, the SERS spectrum of NZ at 647.1 nm is different to the others: two prominent band are seen at 1261 and 1229 cm1 along with a weak one at 1405 cm1, which can be related to the spectrum of the neutral molecule (see Raman of the solid in Fig. 3). This difference in relative intensities can be connected to a resonant effect on changing the excitation wavelength. In the SERS spectra obtained by exciting at high wavelengths (782 and 1064 nm) all molecular forms should be manifested with an intensity proportional to their concentration, because no resonance effect is expected. These spectra are clearly dominated by

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Fig. 7. SERS of NZ at 104 M (a) and 105 M (b) maintaining the DMSO concentration constant at 0.1% and at different DMSO concentrations: 0.1% (b); 1% (c) and 10% (d) at a NZ constant concentration (105 M). Excitation at 514.5 nm. Fig. 6. SERS of NZ (105 M) at different excitation wavelengths (pH ¼ 7:0).

NZH molecules, thus indicating the existence of a lower concentration of NZH2 molecules attached to the colloid surface. However, the intensity of NZH2 SERS bands is enhanced when the excitation wavelength is decreased. This effect can be attributed to the resonance enhancement that undergo the neutral molecules. In fact, the UV–VIS spectrum reveals an increase of absorbance at 514.5 nm at acidic pH (Fig. 2), corresponding to these molecules. Thus, the few neutral molecules attached to the surface can be put in evidence at lower wavelengths by means of a resonant mechanism involving a surface-enhanced resonance Raman scattering (SERRS) effect.

3.5. SERS spectra at different concentrations of NZ and of DMSO SERS spectra of NZ exciting at 514.5 nm at several concentrations are shown in Fig. 7a and b. NZ spectrum undergoes remarkable changes when varying the adsorbate concentration. At a low concentration, the SERS spectrum is dominated by characteristic bands of both NZH2 and NZH forms appearing at 1649, 1405, 1262 and 1240 cm1 (Fig. 7b). When increasing the NZ concentration the intensity of NZH bands at 1650, 1262 and 1127 cm1 markedly decrease. All these changes are quite similar to those occurring in going from neutral to acidic pH (Fig. 5) and those observed with the time (Fig. 8) where an increase of

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Fig. 8. Time evolution of NZ SERS: (a) SERS just after being prepared; (b) 1.5 h after the preparation. Excitation at 514.5 nm.pH ¼ 7:0.

(1650, 1240 and 1127 cm1) decreases and the spectrum is clearly dominated by bands corresponding to NZH2. In addition, the band at 1262 cm1 shifts to 1293 cm1, becoming a prominent band at high DMSO concentration. For two structurally related molecules of NZ, hypericin and emodin, a parallel effect was observed on increasing the concentration of both the adsorbate and DMSO. For these molecules, the similarity found between both effects was explained by taking into account that they may form microcrystals in aqueous media [37]. A similar situation seems to occur in the case of NZ. This molecule may form microcrystals which can be solubilized in DMSO, giving rise to an increase of free molecules and thus of the adsorbate effective concentration. In addition, the adsorption of DMSO may induce a desorption of NZ from the surface thus leading to a subsequent increase of the free NZ concentration on the surface. This is supported by differences observed between the SERS spectra at high NZ concentration and at high DMSO concentration: inversion of doublets appearing at 1293/1240 and 1599/1578 cm1, and an increase of the shoulder at 1195 cm1 (Fig. 7b–d). This changes can be a consequence of the presence of DMSO in the surroundings of the surface, which may also alter the interaction of NZ with the metal. 3.6. Time-dependence of SERS spectra

the NZH2 was also deduced. This fact can be related to the formation of multilayers of the adsorbate on the metal, once the binding sites to which NZ is adsorbed under the phenolate form are completely occupied. These multilayers may be integrated by neutral molecules whose relative amount increase in relation to the monoionized one. This result suggests that monoionized NZH molecules may be chemisorbed on the surface, while the neutral NZH2 molecules seem to be only weakly attached to the metal, i.e. physisorbed on the surface, or attached to the chemisorbed NZH species through H-bonds. The effect of DMSO concentration on SERS spectra of NZ was also examined (Fig. 7b–d). The increase of the DMSO concentration caused a similar effect than that observed when increasing the NZ concentration (Fig. 7a and b). As in the case of high NZ concentration, on increasing the DMSO concentration the intensity of SERS bands corresponding to NZH species

The SERS profile of NZ undergoes a remarkable change in time (Fig. 8). This change is more evident at 514.5 nm, since at this wavelength the spectra corresponding to all the adsorbed forms are seen. The evolution in time occurs in the same direction than changes observed when decreasing the pH (Fig. 5): the bands at 1650, 1260 and 1240 cm1 decrease and that at 1404 cm1 is progressively enhanced. In addition, in the low wavenumber region, an intensity increase of the band appearing at 360 cm1 (Fig. 8b) is observed. This result can be again explained on the basis of the existence of different NZ adsorbed molecules, whose relative concentrations changes with the time towards a more stable situation. In particular, the spectral changes described above indicate that an increase of NZH2 molecules is observed with the time. This is attributed to a progressive protonation of NZH on the surface due to: (a) an increase of Hþ concentration on the surface, induced by a partition phenomenon and

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a diffusion of protons to the surface; (b) a possible passivation of active binding sites on the Ag surface with a subsequent desorption of the strongly attached NZ molecules; or (c) a diffusion of NZ molecules to the surface leading to a concentration increase of neutral NZ, which may be organized in multilayers. This concentration increase could be also induced by a progressive dissolution of NZ microcrystals in the presence of DMSO, as mentioned above.

4. Conclusions SERS spectroscopy is applied to the study of the adsorption of NZ on Ag nanoparticles. Two main adsorbed molecules can be identified corresponding to the non-ionized NZ (NZH2) and the monoanion (NZH). These two molecular forms correspond to physisorbed and chemisorbed molecules. The existence of these different forms can be related to different binding site or to the formation of a multilayer architecture on the metal surface. Although the amount of the ionized molecule is greater, the neutral molecule may also exist at very high pH and can be put in evidence by shifting the excitation wavelength toward shorter values, i.e. the different molecular species adsorbed on the metal have a different response to the light. This effect may be used to select the spectral profile of each species by changing the excitation wavelength. The relative amount of the molecular species adsorbed on the Ag surface can be modified by changing the experimental conditions: pH, NZ concentration and DMSO concentration. At acidic pH the SERS spectrum is dominated by NZH2, but at alkaline pH the characteristic bands of this molecular form is still observed. The spectrum of neutral NZ is also increased at high concentrations of NZ and DMSO. This indicates that the neutral form is adsorbed in multilayers on the metal surface.

Acknowledgements This work has been supported by Direccio´ n General de Investigacio´ n (Ministerio de Ciencia y Tecnologı´a) project number BFM2001-2265 and by the Slovak grant agency VEGA, Grant no. 1/627/99 (P.M.). G.F.

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