Francesca
Scarpelli
a,
Alessandra
Crispini
*a,
Iolinda
Aiello
*a,
Nicolas
Godbert
a,
Fabio
Marchetti
*b,
Sonila
Xhafa
b,
Giovanni
De Filpo
c,
Mariafrancesca
Baratta
c,
Riccardo
Berardi
d,
Pasquale
Alfano
d and
Eugenia
Giorno
a
aMAT-InLAB, Dipartimento di Chimica e Tecnologie Chimiche, Università della Calabria, 87036 Arcavacata di Rende, CS, Italy. E-mail: a.crispini@unical.it; iolinda.aiello@unical.it
bSchool of Science and Technology, Chemistry Section, University of Camerino, Via S. Agostino 1, 62032 Camerino, MC, Italy. E-mail: fabio.marchetti@unicam.it
cNOPTEA, Dipartimento di Chimica e Tecnologie Chimiche, Università della Calabria, 87036 Arcavacata di Rende, CS, Italy
dTiFQLab – Centro di sperimentazione ricerca e analisi applicate alle tecnologie alimentari e dell'acqua potabile – Department DIMES, Università della Calabria, 87036 Arcavacata di Rende, CS, Italy
First published on 6th September 2024
The effects exerted by new bioactive acylpyrazolonate Ag(I) derivatives of the general formula [Ag(QPy,CF3)(R-Im)] containing different substituents on the imidazole (R-Im) ancillary ligands and the natural plasticizer castor oil when both are added to the ethylcellulose (EC) biopolymer in the preparation of thin films as potential active food packaging materials are presented. The Ag(I) complexes [Ag(QPy,CF3)(Bn-Im)] and [Ag(QPy,CF3)(Bu-Im)], having benzyl and butyl substituents, whose single crystal molecular structures are reported, have proved to be highly compatible for efficient incorporation between the EC polymer and the hydrophobic plasticizer chains, giving rise, even at low concentrations, to homogeneous, robust and elastic films. The concomitant presence of these Ag(I) complexes and castor oil in the polymer EC matrix gives rise to thin films with improved antibacterial activity against Escherichia coli (E. coli) as a model of Gram-negative bacterial strains when compared to the non-plasticized ones, with very low Ag(I) migration in the three food simulants used (distilled water, ethanol 10% v/v and acetic acid 3% v/v) under two assay conditions (70 °C for 2 h and 40 °C for 10 days).
On the other hand, the ability of EC to incorporate reinforcing agents such as natural plasticizers represents an additional method not only to obtain EC films for active packaging, but also to improve their mechanical properties, which are partly limited by the semi-crystalline nature of EC.24,25 Additives such as soybean and sunflower oil can reduce the intermolecular interactions between EC polymer chains, enhancing the free space between polymers and the mobility of chains.26,27 In this regard, among all vegetable oils, castor oil shows good performance at low temperatures, high kinematic viscosity and excellent lubrication properties.28–30 The high viscosity of this vegetable oil is rather unusual and is the result of the hydrogen bonding network formed by the hydroxyl group in the predominant ricinoleic fatty acid.28 It is noteworthy that thin films of EC plasticized with castor oil and containing anthocyanins were recently developed for intelligent packaging to monitor the freshness of pork meat.31
In addition, in our research studies on eco-friendly bioactive thin films for food packaging, we recently reported on the preparation and antibacterial properties of EC films embedded with antimicrobial Ag(I) 4-acyl-5-pyrazolonate (Qpy) complexes bearing imidazole (Im) ancillary ligands.32 As an extension of our previous work, herein we present the concomitant effects exerted by new Ag(I) derivatives of the general formula [Ag(QPy,CF3)(R-Im)] containing different substituents on the Im ancillary ligands and the natural plasticizer castor oil when both are added to the EC biopolymer in the preparation of bioactive thin films (Fig. 1).
All the complexes were synthesised according to the procedure reported in the literature,33 and the single crystal X-ray structural characterization of complexes 1 and 2 is reported for the first time.
The identification of the best performing [Ag(Qpy,CF3)(R-Im)] complexes with respect to the different substituents held by the Im ancillary ligands, their suitable concentration and the right amount of the plasticizer to be used as dopants in the preparation of bioactive EC films has been achieved through both physical-chemical and mechanical characterization. All the new films are tested for their antimicrobial activity against Escherichia coli (E. coli) as a model of Gram-negative bacterial strains and the results are compared to those obtained from the EC films specifically prepared incorporating only the Ag(I) complexes. Moreover, Ag(I) migration tests towards specific food simulants, according to the EU Food Contact Regulations for Plastics 10/2011 on the migration of chemicals from plastic, are reported.
Infrared (IR) spectra were recorded on a PerkinElmer Spectrum One FT-IR spectrometer. Fourier transform infrared (FT-IR) analysis was carried out on the film samples in the mid-infrared area (4000–400 cm−1) with a PerkinElmer Spectrum 100 FT-IR spectrometer. KBr pellets were prepared only to acquire the spectra of the Ag(I) complexes. Differential Scanning Calorimetry (DSC) analysis was performed using a TA DSC Q2000 instrument with a refrigerated cooling unit (RCS 90). Indium metal standard was used for temperature calibration. The samples were accurately weighed (5–6 mg) and crimped in non-hermetic aluminium pans. The samples were heated at a heating rate of 5 °C min−1, under a dry nitrogen atmosphere (flow rate: 50 ml min−1), using an empty non-hermetic aluminium pan as a reference.
Powder X-ray diffraction (PXRD) measurements were carried out on a Bruker D2-Phaser equipped with Cu Kα radiation (λ = 1.5418 Å) and a Lynxeye detector, at 30 kV and 10 mA, with a step size of 0.01° (2θ). The obtained diffractograms were analysed using DIFFRAC.EVA diffraction software.
Tensile strength measurements were recorded on a Sauter TVO-S tensile tester equipped with a Sauter FH-1k digital dynamometer and AFH FAST software (Sauter GmbH, Balingen, Germany) at a strain rate of 5 mm min−1. EC-based films were cut into rectangular strips of dimensions 5 cm × 1 cm (length × width).
All the ECnx-P4 films were prepared by adding 0.15 mL of castor oil (29% w/w, P) into 45 mL of a dichloromethane solution containing 0.500 g of EC, followed by the dissolution of different amounts of [Ag(Qpy,CF3)(R-Im)], 1–4 (2.5%, a, 0.05%, b, and 0.02%, c, weight ratios between the complex and EC). The solutions were stirred for 2 h and then poured into a Petri glass dish (9 cm diameter). After the slow solvent evaporation for 24 h at room temperature, the films were desorbed with 5 mL of distilled water from the glass surface.
The same procedure was used to prepare the reference films without and with castor oil: EC0, EC-AgNO3a–c, ECnx, and EC-AgNO3a–c-P4.
The Ag(I) migration tests of all the Ag-based films were performed under two different contact conditions and using three different simulants: distilled water (simulant A), ethanol 10% v/v (simulant B) and acetic acid 3% v/v (simulant C). The films, previously cut into squares of dimensions 5 × 5 cm, were immersed in 50 mL of simulant in a conical flask, so that both faces of the sample were in contact with the simulant. The measurements were performed to correlate Ag(I) migration as a function of contact liquid type, temperature, relative exposure time, and amount of Ag(I) complexes present in the film. All conical flasks, covered with aluminium foil, were kept in a controlled atmosphere under two assay conditions: 40 °C for 10 days and 70 °C for 2 h, according to the EU legislation on the migration of chemicals from plastic materials.44 Currently, the Food and Drug Administration (FDA/CFSAN) allows the application of silver nitrate (AgNO3) in bottled water and the European Food Safety Authority (EFSA) has declared a specific migration limit of equal to or lower than 0.05 mg kg−1 of food.44,45 After the immersion period, the samples were removed, and the simulant was extracted to analyse the Ag(I) released in the simulants by using ICP-OES. Calibration curves for the investigated element (silver) were obtained by using aqueous standard solutions (3% nitric acid) prepared by appropriate dilution of stock standards. For each sample, three experiments were carried out to evaluate the reproducibility of the release.
Complexes 1 and 2 have been crystallized through slow evaporation of a dichloromethane solution and the single crystal X-ray structural characterization is herein reported. As shown in Fig. 2, the HQpy,CF3 ligand is found in a mono-anionic N2-chelating mode, with the Ag(I) coordination sphere completed by the imidazole ligand (1-benzylimidazole, Bn-Im, and 1-butylimidazole, Bu-Im) bound through its nitrogen atom.
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Fig. 2 Molecular structure of (a) [Ag(Qpy,CF3)(Bn-Im)], 1, and (b) [Ag(Qpy,CF3)(Bu-Im)], 2, with the atomic labelling scheme. |
The bond distances and angles around the Ag(I) ion are comparable to those found in the crystal structure of an analogous Ag(I) complex of the HQpy,CF3 ligand and similar coordinated imidazole ligands (Table 1).33 Similarly, a distinct asymmetry between the Ag–N bond distances within the N,N-chelated ring is found, being more pronounced in complex 2 compared to the corresponding values found in the reference compounds. While both complexes show a similar “bite” angle, the trigonal-planar geometry around the metal ion is found to be much more distorted in complex 2 than in 1. The rotation of the Bn-Im ligand with respect to the N,N-chelated ring in 1 is quite pronounced, with a twist angle around the Ag–N(4) bond of 53.9(3)° and a dihedral angle between its mean plane and the N,N-chelated ring of 48.7(1)°. Moreover, the overall distortion from planarity in 1 is indicated by the torsional angle around the N(2)–C(7) bond within the N,N-chelated ring of 9.1(3)°. The phenyl ring of the imidazole benzyl substituent is nearly perpendicular to the imidazole ring, with a dihedral angle between the two mean planes of 91.3(1)°. The 3D packing of 1 is characterized by weak intermolecular interactions involving the H atoms of the aromatic rings establishing C–H–O and C–H-π contacts with the O atom of the –CO group of the ligand and the aromatic ring of the imidazole benzyl substituent (Fig. 3a). Moreover, π–π interactions are established between the pyridine ligands of the coordinated acylpyrazolonate ligands. Differently, molecules of complex 2 exhibit a greater tendency towards flatness. In this case, the “twist angle” around the Ag–N(4) bond and the dihedral angle between imidazole and the N,N-chelated ring mean planes have values equal to 7.3(2) and 13.8(1), respectively. The butyl chain bound to the N atom of the imidazole ligand adopts an all-trans-configuration in the average molecular plane. In this case, the overall molecular geometry induces the formation of dimers of molecules held together via argentophilic interactions, with a minimum Ag–Ag distance of 3.273(1) Å approximately along the b direction (Fig. 3b). Within the dimers, weak C–H–F contacts are established with the H atoms of the hydrocarbon chain.
1 | 2 | |
---|---|---|
Ag–N(1) | 2.261(2) | 2.195(2) |
Ag–N(3) | 2.362(2) | 2.414(2) |
Ag–N(4) | 2.141(2) | 2.127(3) |
N(1)–Ag–N(3) | 70.8(1) | 71.0(1) |
N(1)–Ag–N(4) | 143.6(1) | 165.3(1) |
N(3)–Ag–N(4) | 145.1(1) | 120.9(1) |
N(1)–N(2)–C(7)–N(3) | 9.1(3) | 1.0(4) |
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Fig. 3 Crystal packing view of (a) complex 1 (down the a direction) showing π–π–π C–H-π, C–H–O and C–H–F interactions, and (b) (c) complex 2 along the b direction and in the ac plane. |
In the ac plane, where the average molecular plane lies, one of the imidazole H atoms at the ortho-position with respect to the substituted nitrogen atom, is involved in hydrogen bonding (a direction) with one of the O atoms of the coordinated HQpy,CF3 ligand. Moreover, C–H–O and C–H–F contacts are found along the c direction (Fig. 3c). The experimental PXRD patterns of complexes 1 and 2 are in agreement with the simulated ones derived from the single X-ray crystal structure (Fig. S1 in the ESI†).
Sample | Castor oil amount (% w/w) | Young's modulus (MPa) | Elongation at break (%) | Stress (MPa) |
---|---|---|---|---|
EC0 | 0 | 914 ± 42 | 1.4 ± 0.1 | 12.2 ± 0.9 |
EC-P2 | 134 | 117 ± 7 | 16.3 ± 0.2 | 7.7 ± 1.2 |
EC-P3 | 56 | 158 ± 9 | 15.7 ± 0.2 | 8.0 ± 1.1 |
EC-P4 | 29 | 180 ± 11 | 18.6 ± 0.3 | 8.9 ± 0.8 |
As previously reported, the DSC trace of the pure EC polymer presents on heating a first slow endothermal process at low temperature (from 35 °C to 60 °C) characterised by a broad low intensity peak, corresponding to the evaporation of solvent molecules entrapped into the drop-casted film membrane. This process is then followed by a slight change of the slope at ca. 135 °C, corresponding to the glass transition temperature (Tg) of the EC polymer, and ultimately by an endothermal process corresponding to the melting of the polymer occurring at ca. 183 °C.32 When castor oil is introduced into the EC0 polymer, even at a low concentration of the plasticizer (29% w/w), the registered DSC trace is characterised by the loss of slope change during heating, thus preventing the exact determination of the glass transition temperature in agreement with an increase of the plasticity of the system. This is accompanied by a broadening of the melting point temperature, which spreads on a wider range of temperatures up to 200 °C. Another piece of evidence of this increase of “fluidity” of the system resides in the absence of the exothermic peak of the crystallization process observed upon cooling. This occurs because castor oil increases the mobility of the polymer chains, thereby enhancing their ability to remain in an amorphous state rather than crystallizing.47 While for pure EC0, this process occurs upon cooling at ca. 178 °C, when castor oil is used, this process is not observed, proving that the presence of the plasticizer impedes the crystallization process (Fig. 5).
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Fig. 5 DSC traces of EC1a-P4 (blue line), EC-P4 (red line) and EC0 (black line) (from ref. 32) obtained at a heating rate of 5 °C min−1. |
To investigate the structural changes of the polymer EC backbone after castor oil incorporation, powder X-ray diffraction (PXRD) analysis was carried out on the plasticized and non-plasticized films. As already reported, the PXRD pattern of EC films casted from dichloromethane solution is characterized by two reflection peaks attributable to the interlayer distance of the ordered polymer structures and to the interchain distance within the same polymer layer, respectively.32 However, this diffraction profile can also be associated with the lyotropic liquid crystalline nature of EC films, specifically their cholesteric phase. The comparison between EC0 and EC-P4 diffractograms (Fig. 6, black and red lines, respectively) highlights that the two referenced peaks are found at different angle values within the two samples. The first reflection, associated with the interlayer distance and centred at 2θ = 8.2° (d = 10.8 Å) in the EC0 pattern, shifts to a lower angle value, corresponding to a larger d value (2θ = 7.2°; d = 12.2 Å), in the diffraction profile of EC-P4.32
This finding, already reported for other plasticized polymers, clearly demonstrates the effectiveness of the plasticizer in intercalating between the layers of the polymer chains, reducing the intermolecular interactions existing between them.48–50 Moreover, in the EC0-P pattern, the second peak, arising from the interchain distance within the same polymer layer, shifts to slightly higher angles (2θ = 20.2°), corresponding to smaller d-spacings (d = 4.4 Å), compared to the EC0 pattern (2θ = 19.2°; d = 4.6 Å). This may also be ascribed to the interposition of the plasticizer between the polymer layers, which forces the chains within each layer to pack more closely.
The introduction of Ag(I) complexes into plasticized films can alter the internal polymer structure by the establishment of new chemical interactions between the EC polymer chains, plasticizer and Ag(I) derivatives. Therefore, the mechanical properties of the plasticized films doped with Ag(I) were properly investigated in order to find which Ag(I) complex, and in what percentage, can ensure a mechanical behaviour of the derived film similar to or even better than that of EC-P4. Since EC0 is poorly elastic and highly stiff, the mechanical characterization of the non-plasticized EC films containing the Ag(I) complexes was not performed. Furthermore, since AgNO3 is the standard reference material in antibacterial tests, EC-P4 films containing AgNO3 in different weight amounts were also prepared and their mechanical properties were evaluated on par with the other samples. All experimental results are reported in Fig. 7 and Table S2.†
What immediately stands out is that Ag(I) doping enhances film robustness, with Young's modulus values higher than that of EC-P4 (180 MPa) and quite similar to that of the pristine EC0 film (914 MPa) (Fig. 7). Nonetheless, the elastic properties are clearly reduced, going back to very poor values, especially when complexes 3 and 4 are used as dopant agents (Fig. 7). In contrast, interesting results are obtained with complex 2, where the film at 0.05% w/w concentration exhibits a Young's modulus of 418 MPa, two times higher than that of EC-P4, and a percentage of elongation of 21%, representing an additional 13% increase over the corresponding value in EC-P4. This improvement in both directions, robustness and elasticity, proves that complex 2 manages to insert itself into the hydrocarbon chains of both castor oil and the EC polymer via compatible intermolecular interactions with the substituent on the imidazole ring, without altering in a substantial way the structure of the neat EC-P4 film.
When the Ag(I) complexes are blended together with castor oil into the polymer EC matrix, the only observable change is the decrease of the melting point temperature of the polymer by a few degrees, while the DSC thermogram is still characterised by a relative broadening of the melting point endothermic peak that spreads from 177 °C up to ca. 200 °C. Fig. 5 reports the DSC trace of the EC1a-P4 film as an example, together with the traces of the EC-P4 blend and pure EC0 for a direct comparison. A similar behaviour is observed for all complexes (Fig. S2 in the ESI†).
The PXRD patterns of the plasticized films containing the Ag(I) complexes, compared to the corresponding non-plasticized samples and to the reference matrices EC0 and EC0-P, are illustrated in Fig. 8. The effectiveness of the plasticizer persists even in the presence of Ag(I) complexes within the polymer matrix, since the PXRD patterns of all the Ag(I)-containing plasticized films (Fig. 8, pink lines) exhibit reflections associated with the interlayer and interchain distances at the same positions as in the EC0-P4 film (Fig. 8, red lines), thus shifted with respect to the corresponding non-plasticized films (Fig. 8, blue lines). Previously, it has been demonstrated that complex 1 displays a strong affinity toward the EC polymer matrix.32 In fact, upon the incorporation of complex 1 into the EC polymer matrix, a local hexagonal sub-organization of the polymer backbone, which coexists with the chiral nematic order (N*), is formed. This is evidenced by the emergence of three weak reflections in the typical diffraction profile of EC0, as seen in the PXRD pattern of EC1a (Fig. 8a, blue line, reflections are marked with stars). These diffraction peaks correspond to interplanar spacings of d = 11.26 Å, 6.6 Å, and 2.4 Å, and can be indexed to a 2D hexagonal lattice (H). The same applies when complex 1 is incorporated into EC0 in the presence of castor oil (Fig. 8a, pink line), with weak reflections, consistent with a 2D hexagonal lattice, occurring at the same d-values as in the corresponding non-plasticized film. A similar phenomenon is observed when complex 2 is incorporated into the polymer, as the diffraction peaks, associated with the hexagonal sub-organization of the EC matrix, also appear in the PXRD patterns of EC2a and EC2a-P4 (Fig. 8b, blue and pink lines, respectively; reflections are marked with stars). The explanation for this observation lies in the similar structures of the two complexes, which allow for efficient incorporation between the polymer chains, thus modifying the organization of the polymer matrix. Moreover, the presence of castor oil, hydrophobic in nature, does not change the miscibility of both complexes 1 and 2 with the ethylcellulose polymer; in contrast, it forms a perfectly homogeneous blend.
A different situation is observed when complexes 3 and 4 are incorporated into the polymer matrix, both in the presence and absence of the plasticizer. Indeed, in the diffraction profiles of EC3a, EC3a-P4, EC4a and EC4a-P4 (Fig. 8c and d, blue and pink lines), the distinctive reflections of these Ag(I) compounds (Fig. S3c and S3d in the ESI†) arise from the typical diffraction profile of EC. This finding clearly indicates that the crystallization of complexes 3 and 4 occurs during the film forming process in both plasticized and non-plasticized cases, leading to the formation of inhomogeneous samples. Finally, the PXRD patterns of the films containing AgNO3, ECAgNO3a and ECAgNO3a-P4, are perfectly superimposable with the corresponding PXRD patterns of the reference films, EC0 and EC-P4, respectively (Fig. S4 in the ESI†). The absence of both characteristic AgNO3 reflections and new diffraction peaks, as previously observed in plasticizer-free EC films, indicates excellent incorporation of the salt into the polymer matrix, effectively acting as a solvent-like dispersive medium for AgNO3.
The FT-IR spectra of the Ag(I)-incorporating films, along with those of the corresponding Ag(I) complexes, are illustrated in Fig. 10, and compared to those of EC0 and EC0-P (Fig. 9). First of all, the addition of castor oil to the EC polymer is evident due to the presence of the 1743 cm−1 band in the spectrum of EC0-P (Fig. 9, red line; the band is marked with a triangle), which is characteristic of the CO vibration of the ester groups of the plasticizer.
With the addition of the Ag(I) complexes, the ester band remains unchanged, indicating the absence of significant interactions between the plasticizer and the Ag(I) compounds (Fig. 10). In addition, the FT-IR spectra of the Ag(I)-containing films, both plasticized and non-plasticized (Fig. 10, pink and blue lines, respectively), show no significant differences compared to the undoped films. However, some differences are observed between the spectra of the Ag(I)-containing films and those of the corresponding Ag(I) complexes. Specifically, the band at ca. 1665 cm−1, arising from the CO stretching vibrations of the Ag(I) complex carbonyl groups, shifts to a higher wavenumber in the spectra of the corresponding films (Fig. 10). As previously reported, this finding suggests the existence of weak van der Waals interactions between the polymer and the Ag(I) additives.32,51 However, this effect is negligible in the film containing complex 3, while it is enhanced in the FT-IR spectra of the films incorporating the other Ag(I)-compounds. This indicates that the presence of benzyl, butyl, and phenyl substituents on the imidazole ring of complexes 1, 2 and 4, respectively, plays a significant role in the interaction with the matrix.
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Fig. 11 Antibacterial activity, expressed as the relative killing percentage (%) of ECnb,c-P4 and ECnb,c films and relative controls. |
The embedding of silver derivatives introduces antibacterial activity even at low concentrations in all cases. However, only in the EC1b,c-P4, EC2b,c-P4 and EC3c-P4 films, the concomitant presence of the plasticizer and the Ag(I) additives in the EC polymer makes the derived films more active against E. coli with respect to the non-plasticized ones. The best relative killing percentage is found in the case of EC1b-P4 and EC2b-P4 films, containing complexes [Ag(Qpy,CF3)(Bn-Im)], 1, and [Ag(Qpy,CF3)(Bu-Im)], 2, in a concentration of 0.05% w/w, whose activities are comparable with that found for ECAgNO3b-P4. These results are particularly encouraging since in the Ag(I) complexes under study, the concomitant presence of the coordinated 4-acyl-5-pyrazolone and the imidazole ligands, both biologically active organic molecules,54 synergistically imparts a significant bactericide behavior at least against E. coli even in the absence of water solubility.
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Fig. 12 Ag(I) migration values in milligrams per volume of EC1b,c-P4, EC1b,c, EC2b,c-P4 and EC2b,c films in simulant A at (a) and (b) 2 h and 70 °C and (c) and (d) 10 days at 40 °C. |
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Fig. 13 Ag(I) migration values in milligrams per volume of EC1b,c-P4, EC1b,c, EC2b,c-P4 and EC2b,c films in simulant B at (a) and (b) 2 h and 70 °C and (c) and (d) 10 days at 40 °C. |
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Fig. 14 Ag(I) migration values in milligrams per volume of EC1b,c-P4, EC1b,c, EC2b,c-P4 and EC2b,c films in simulant C at (a) and (b) 2 h and 70 °C and (c) and (d) 10 days at 40 °C. |
As shown in Fig. 12–17, the addition of complexes 1 and 2 and castor oil into the EC polymer matrix produces a distinct effect of Ag(I) release with respect to complexes 3 and 4, despite the simulant and the assay conditions. Indeed, in the case of EC1b,c-P4 and EC2b,c-P4, the Ag(I) migration values are always below the limit set by the EU legislation of 0.05 mg kg−1 or the instrumental limit of quantification (LOQ) of 0.004 mg L−1, and equal to or lower than the corresponding values found for the non-plasticized EC1b,c and EC2b,c (Fig. 12, 13 and 14) films.43 Another important common aspect is that in EC1b,c-P4 and EC2b,c-P4, the Ag(I) migration is not concentration dependent, with the only exception being EC2b-P4 in simulant C at the shortest exposure time and at the highest temperature (Fig. 14b).
In distilled water (simulant A), the effect of the presence of the plasticizer is much more evident in the presence of complex 2, where the difference in Ag(I) migration is very significant with respect to the non-plasticized film at the 0.05% weight ratio between the complex and EC under both working conditions (Fig. 12b and d). Differently, in simulants B and C, the greatest difference between the plasticized and non-plasticized EC films is clearly dependent on the working conditions (Fig. 13 and 14). Indeed, in simulant B, the reduction of the swelling effect exerted by ethanol on the EC polymeric matrix55 in the plasticized films is favoured by a temperature of 70 °C and an exposure time of 2 h in the case of EC1b-P4, while for EC2b-P4, a lower temperature and a longer exposure time are necessary (Fig. 13a and d). Even if both complexes 1 and 2 show structures compatible with efficient incorporation between the EC polymer chains and the hydrophobic plasticizer, thereby forming perfectly homogeneous blends, some differences in Ag(I) migration in the various simulants and under the different working conditions can only be revealed at a higher content of complexes within the films. As reported in Tables S3 and S4,† the Ag(I) migration from EC2a-P4 films is generally lower than that observed in the case of the EC1a-P4 ones, at 70 °C and two hours of exposure and in the three simulants, while the opposite trend is recorded when lowering the temperature and increasing the time of exposure. The presence of the butyl substituent on the Bu-Im ligand in complex 2 yields a predominance of hydrophobic interactions with the aliphatic chains of the environment (both castor oil and EC polymer) compared to what can be hypothesized in the case of complex 1. Indeed, in 1, the presence of the phenyl ring in the coordinated Bn-Im ligand introduces more interactions such as aromatic ones, as already highlighted by the analysis of the intermolecular interactions in the solid crystalline state. Therefore, as an attempt of explanation, at high temperature, the increased motion induced on the rotationally free phenyl ring of the Bn-Im coordinated ligand in complex 1 causes the weakening of the aromatic interactions established within the polymeric matrix, with the consequent increase of silver migration. This effect is particularly evident in simulants B and C. While in simulant B, a concomitant swelling effect of the polymer EC matrix in the presence of ethanol amplifies the effect of Ag(I) release, in simulant C, degradation effects in an acidic medium may not be irrelevant (Table S4 in the ESI†). On the other hand, at a lower temperature and a longer exposure time, an opposite trend in silver migration between EC1a-P4 and EC2a-P4 is observed (Table S5 in the ESI†).
The cooperative hydrophobic and aromatic intermolecular interactions between complex 1 and the plasticized EC polymer matrix help keep the complex molecules tightly embedded within the system for a longer time, with a consequent reduction of silver migration with respect to EC2a-P4 films.
In all cases, the Ag(I) migration from EC1b,c-P4 and EC2b,c-P4 films is always found to be lower than that recorded for the reference EC-AgNO3b,c-P4 films (Fig. 15–17). Indeed, when AgNO3 is embedded with castor oil within the EC matrix, the Ag(I) release from the resulting films is concentration dependent, higher than that from the non-plasticized ones and, in the case of 0.05% w/w concentration, it is higher than the limit set by the EU legislation (0.05 mg kg−1). As proved by the PXRD pattern of the AgNO3a-P4 film (Fig. S3 in the ESI†), even in the presence of the plasticizer, the Ag(I) salt is deeply and homogeneously dispersed within the matrix, similar to the case of the non-plasticized film.32 Even if Ag(I) ions do not cause any inner or surface structural variation of the polymeric backbone, the presence of castor oil, inducing a kind of inner “fluidity”, favors a greater mobility of the free ions, increasing their release when compared to the non-plasticized films.
The same trends in terms of Ag(I) release are observed in the case of EC3b,c-P4 and EC4b,c-P4 films when compared to the non-plasticized ones (Fig. 15–17). In these cases, both complexes give rise to inhomogeneous films, crystallizing on their surface (see PXRD characterization), regardless of the presence or absence of the plasticizer. However, the major Ag(I) release from EC3b,c-P4 and EC4b,c-P4 films when compared to the non-plasticized ones could be explained by the significant incompatibility between both complexes and the hydrophobic plasticizer.
Generally, the highest values of Ag(I) migration are detected in all simulants A, B and C under both working conditions, at a complex concentration of 0.05% w/w, overcoming the limit set by the EU legislation of 0.05 mg kg−1. The Ag(I) release decreases on decreasing the amount of complexes 3 and 4 added to the EC matrix. No significant difference is observed in the Ag(I) migration values between the EC3-P4 and EC4-P4 films in all the complex concentrations, in all simulants and under the working conditions used in Fig. 15–17 and Tables S6 and S7 in the ESI.†
Crystallographic data for 1 and 2 have been deposited at the CCDC under 2375073 and 2375074.†
F. S. is grateful to the project PON “Ricerca e Innovazione” 2014–2020, Asse IV “Istruzione e ricerca per il recupero” and Azione IV.6 “Contratti di ricerca su tematiche Green” (CUP: H25F21001230004; identification code: 1062_R8_GREEN).
The University of Camerino is gratefully acknowledged for having financed part of S. X.'s doctoral scholarship. This work was also financially supported by MUR – PNRR (Decreto Direttoriale n. 703 del 20-4-2022), within the framework of the project “VITALITY – Innovation, digitalisation and sustainability for the diffused economy in Central Italy” (cod. ECS_00000041), Spoke 9 – Nanostructured materials and devices.
Footnote |
† Electronic supplementary information (ESI) available. CCDC 2375073 and 2375074. For crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4dt02201g |
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