Complexation of metal ions with Langmuir–Blodgett films of novel calixarene azo-derivative

Thin Solid Films 327–329 (1998) 686–689

Complexation of metal ions with Langmuir–Blodgett films of novel calixarene azo-derivative A.K. Hassan a ,*, A.V. Nabok a, A.K. Ray a, F. Davis b, C.J.M. Stirling b a

Physical Electronics and Optic Fibre Research Laboratories, School of Engineering, Sheffield Hallam University, City Campus, Pond Street, Sheffield S1 1WB, UK b Centre for Molecular Materials and Department of Chemistry, University of Sheffield, Brook Hill, Sheffield S3 7HF, UK

Abstract The complexation properties of Langmuir–Blodgett (LB) films of an amphiphilic calix(4)resorcinarene substituted by four arylazo groups with some heavy metal ions were studied using UV-vis spectral measurements. It was shown that soaking samples in metal salt solutions leads to irreversible decrease of absorbance at 460 nm. Efficiency of complexation is found to depend on both metal cations and anions. Effects of film thickness, solution concentration and time of exposure have also been studied. It was also shown that the addition of a small amount of aromatic compounds such as aniline, leads to a further decrease in the absorbance of the films. The presence of metal salts in the film was directly confirmed by XPS. The mechanism of ion complexation is discussed.  1998 Elsevier Science S.A. All rights reserved Keywords: Ion complexation; Calixarenes; Azo-dyes; Langmuir–Blodgett films

1. Introduction Calixarenes [1] are well known macrocyclic compounds which are noted for their size-related selectivity in binding cations [2,3] as well as organic guest molecules [4]. Thin films can be constructed from calix(n)arenes and the similar calix(4)resorcinarenes for sensing applications using various deposition techniques. In particular, calix(4)arene incorporated into polymer membranes has been used in ChemFETs for detection of heavy metal ions [5]. Thermal evaporation has been shown to yield calix(4,6 and 8)arene films with unchanged molecular structure [6]. These films were chemically stable allowing their use as sensing membranes in liquid media. Films of higher ordered structures and controlled thickness have been obtained using the Langmuir–Blodgett (LB) technique for calix(4)resorcinarenes [7] and calix(8)rene [8], and have also shown similar suitability for use as sensing elements in liquid as well as in gaseous media. LB films of an amphiphilic calix(4)resorcinarene substi-

* Corresponding author. Tel.: +44 114 2533512; fax: +44 1142533306; e-mail: [email protected]

0040-6090/98/$19.00  1998 Elsevier Science S.A. All rights reserved PII S0040-6090 (98 )0 0741-X

tuted by four arylazo groups, have been previously characterised [9] and were used in the present work to study their complexation with some heavy metal ions. The presence of azo-chromophores in this compound allows detection of changes of optical absorption in the visible range resulting from the interaction of metal cations with the p-electrons of the azo-chromophores [10,11]. Possible hydrogen bonding interaction with calixarene hydroxy groups together with ion binding to the azo-groups seem to offer a very promising dual effect for development of optical ion sensing elements.

2. Experimental details The amphiphilic calix(4)resorcinarene (Fig. 1), substituted by four arylazo groups, referred to as Azo1 throughout the text, was used in the present study. The synthesis of this material and characterisation of their LB films have been described in a previous publication [9]. LB films of Azo1 have been produced using a NIMA 622 trough containing deionised water as subphase with a nominal pH of 5.5 at a temperature of 22°C. The compound was spread from 0.5 mg/ml solution in chloroform, containing 10% ethanol, onto the Milli-Q water subphase. Glass microscope slides and

A.K. Hassan et al. / Thin Solid Films (1998) 686–689

silicon wafers, rendered hydrophobic by sonication in appropriate solvents and subsequent treatment with dimethyldichlorosilane, were used as substrates. Azo1 LB films were deposited onto them at a constant surface pressure of 25 mN/m and dipping speed of 10 mm/min for both down- and up-strokes. UV-vis absorption spectra were measured using a Hitachi U2000 spectrophotometer in the wavelength range 350–700 nm. Spectra of the LB films were measured before and after being soaked for 1 h in aqueous solutions of the following heavy metal salts: CuSO4, NiCl2, AgNO3, Cr2(SO4)3 and Hg(CH3COO)2. The pH value of all solutions used was in the range 5.0–5.5. The choice of certain salts is based on their water solubility. The work is mainly concerned with the qualitative analysis of the effect of metal ions. Effect of counterions has also been considered. More detailed analysis of the effects of concentration and exposure time has been performed in relation to AgNO3 solutions only. To confirm ion complexation, X-ray photoelectron spectroscopy (XPS) measurements have been made for an Azo1 LB film (four layers) deposited on a silicon wafer. For these measurements samples were soaked for 1 h in aqueous 10 mM CuSO4 solution and then washed for 1 h in several changes of pure water to remove loosely bound ions. XPS spectra were acquired on a VG Clam 2 spectrometer, using Mg Ka X-rays (1253.6 eV). The X-ray source was operated at 100 W, with the pass energy of the analyser set to 100 eV. The take-off angle was 30° relative to the sample surface. This gives a sampling depth of ca. 1.8 nm [12], which is about the same as the thickness of a monolayer of Azo1 in LB film [9].

3. Results and discussion

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saturation above 100 mM (Fig. 4). The dependencies presented in Figs. 3 and 4 demonstrate general trends in Azo1 film behaviour. Ion binding to Azo1 LB films is a permanent effect since rinsing of samples in deionised water for long periods (from a few hours to 2 days) causes no restoration of the initial spectra. It also confirms that water cannot wash out the Azo1 molecules from the film during treatment in solutions of metal salts. Similarly, prolonged soaking of the fresh samples in pure water did not result in any changes in their respective spectra. Varying the number of LB layers (two, four, eight, 16 and 20) does not show any significant change of absorbance after soaking into AgNO3 solution. It indicates that this type of reaction is merely a surface mechanism, i.e. ion binding is mostly taking place at the upper layers of the Azo1 LB film. It should be noticed that 100 mM AgNO3 solution containing 6 × 1019 cm−3 of Ag+ ions seems to be an unlimited source for diffusion into LB film which have a surface concentration of 1.5 × 1013 cm−2 of Azo1 molecules, as calculated from the value of area per molecule (1.6 nm2) [9]. The long time response and limitation of ion binding to the surface can be attributed to the extremely high hydrophobicity of calix(4)resorcinarene films [14]. The effect of Ag+ ion binding in LB films is smaller than that observed in a non-aqueous solution of Azo1 in a chloroform/ethanol mixture (1:10) containing 10 mM AgNO3 (Fig. 2, curves 4 and 5). In this case alkyl chains cannot limit ion penetration, and the reaction occurs immediately after addition of the salt, yielding DA/A of about 12% even in 10 mM AgNO3 solution. Addition of very small amounts (about 0.1% in volume of aqueous ion solutions) of aromatic compounds, such as aniline, caused further decreases in the absorption intensity of the Azo1 LB films (Fig. 2, curve 3). It is known that aro-

3.1. UV-vis absorption spectra Typical absorption spectra of Azo1 LB films (20 layers) are shown in Fig. 2. The main absorption band around 455 nm corresponds to the p → p* transition of azo-chromophore in trans-conformation [13]. It can be seen that the position of the absorption maximum is slightly shifted to the high wavelength side (red shift) as compared with the peak position in the solution spectrum at 430 nm, which usually corresponds to isolated molecules. This shift can simply be explained by aggregation of Azo1 molecules in the LB film. Soaking of the LB films of Azo1 in aqueous solution of AgNO3 leads to a decrease of the absorption maximum. Effects of exposure time and solution concentration on the absorption intensity have been studied in more details. The relative change in the intensity of absorption (DA/A) is found to increase monotonically with time and reaches saturation after 2 h of soaking in 10 mM AgNO3 solution (Fig. 3). Increase in the concentration of the AgNO3 solution is also found to cause an increase of DA/A followed by

Fig. 1. Chemical structure of calix(4)resorcinarene Azo1 molecule.

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matic molecules can penetrate into the calixarene LB film matrix and cause its swelling [7]. Additionally, interaction between benzene derivatives and the alkyl chains can lead to a decrease of film hydrophobicity. This consequently will allow water to penetrate into Azo1 LB film and make further ion binding possible. The effect of other heavy metal ions on the absorption maximum of Azo1 LB films has been studied. The resultant relative change in the intensity of absorption (DA/A) after soaking different samples of the same thickness (four LB layers) in 10 mM aqueous solutions for 1 h is shown in the bar chart (Fig. 5). It can be seen that the greatest response (about 10%) was found on binding of CuSO4, and efficiency of complexation is found to decrease in the sequence CuSO4, NiCl2, AgNO3, Cr2(SO4)3, Hg(CH3COO)2. This sequence does not correlate with the radius of the metal ions. The effect of counterions on the absorption spectra of Azo1 LB films has also been studied using different copper salts. It was found that DA/A value increases in sequence of 9, 14, 24 and 42% after soaking for 1 h in 10 mM solutions of CuSO4, Cu(NO3)2, CuCl2 and Cu(CH3COO)2 salts, respectively. It shows that both cations and anions can contribute to the complexation process. Possibly this is an effect of hydrogen bonding of the calixarene to the various anions. The presence of ortho-hydroxyl groups has been shown to favour complex formation between metal ions and simple azobenzene dyes [11], and may occur in the similar resorcinarene. Possibly the copper ions are incorporated between adjacent azo-linkages, allowing complexation to both azo and hydroxyl functions. The pKa of the hydroxyl groups of a similar azo-substituted resorcinarene has been measured at 10, therefore at the pHs used in these experiments, depro-

Fig. 2. UV-vis absorption spectra of 20 LB layers of Azo1 ((1) initial spectrum, (2) after 1 h soaking in 100 mM AgNO3 solution, (3) after the same treatment in mixture containing 0.1% aniline) and spectra of solution of Azo1 ((4) in chloroform/ethanol (1:10) mixture and (5) in the same mixture containing 10 mM AgNO3).

Fig. 3. Time dependence of the percentage absorbance changes of Azo1 LB films at 455 nm due to exposure to 10 mM AgNO3 aqueous solution.

tonation of these groups and salt formation is exceedingly unlikely [15]. Complexation appears to lead to a loss of intensity but no great shift in the absorption peak position. It is not known why this occurs but is found to happen for both LB films and solutions of Azo1 compound. The mechanism of complexation is not, therefore, fully controlled by ion inclusion in the calixarene cavity and the interaction with the azo-group seems to be the main mechanism of complexation [11]. The role of anions can be related to their abilities to penetrate through the matrix of Azo1 LB film.

Fig. 4. Dependence of the percentage absorbance changes of Azo1 LB films at 455 nm on AgNO3 concentration in water. Soaking time was 1 h for each sample.

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Fig. 5. A diagram showing the percentage changes of absorbance of Azo1 LB films at 455 nm due to 1 h exposure to 10 mM aqueous solutions of different heavy metal salts.

3.2. X-ray photoelectron spectroscopy The XPS spectrum of Azo1 LB film (four layers) after exposure to aqueous CuSO4 solution showed characteristic peaks corresponding to C, Cu, N, O, S and Si (substrate). The peak for nitrogen showed a 2:1 doublet at 402 and 408 eV, corresponding to azo- and nitro-N atoms. Integration of peak areas gave, after normalisation for the cross-section factor, ratios of 1:1 for Cu and S indicating that the stoichiometry of Cu2+ and SO2− 4 is maintained in the film. The Cu/N ratio was 1:3.1 indicating that each Azo1 molecule bound approximately four CuSO4 units, or one CuSO4 moiety per each arylazo group. It confirms that the interaction of metal ions with azo-groups in Azo1 LB films is dominant over the salt formation. The Cu/O ratio was 1:10 and Cu/C 1:56, which are higher than expected stoichiometric ratios 1:8 and 1:24, respectively. This apparent discrepancy for the Cu/C ratio can be explained by the fact that XPS intensities are dependent on the depth of the species detected in the film. Elements at the surface of films show much greater signals than those buried within films. In the case of Azo1 LB films the uppermost 1–2 nm of the film consists entirely of the hydrophobic hydrocarbon sidechains, thereby enhancing the signal for carbon. The same effect could be occurring for oxygen or perhaps the ‘excess oxygen’ could be due to water bound within the film.

4. Conclusions An irreversible decrease of absorption intensity at 455 nm

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in LB films of a calix(4)resorcinarene substituted with four azo-chromophores was found after soaking of samples in aqueous solutions of metal salts. Response is slow taking 2 h to saturate and efficiency of ion complexation depends on both metal cations and counter ions. Penetration of aqueous solutions into the films is restricted by its high hydrophobicity, and ion binding is taking place in the uppermost LB layers. Addition of small quantities of aromatic compounds to the solutions increases ion binding presumably because of decreased film hydrophobicity. The presence of Cu2 + and SO24 − in the LB films after soaking was directly confirmed by XPS. It was shown that the mechanism of complexation is based on the interaction between metal cations and azo-groups. The properties of the anions may also affect the penetration of the salts through the film matrix. Further work is currently underway in order to investigate the mechanism of complexation and salt penetration in Azo1 films.

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