From fluorescent layered double hydroxides to fluorescent polymer-based nanocomposites
Serena Coiaia*, Stefano Javaronea, Elisa Passagliaa, Francesca Cicognaa, Werner Oberhauserb, Massimo Onora, Andrea Puccic, Pierpaolo Mineic
aIstituto di Chimica dei Composti Organo Metallici (ICCOM), Consiglio Nazionale delle Ricerche, SS Pisa, Via G. Moruzzi 1, 56124 Pisa, Italy
bIstituto di Chimica dei Composti Organo Metallici (ICCOM), Consiglio Nazionale delle Ricerche, Via Madonna del Piano 10, 50019 Sesto Fiorentino, FI, Italy
cDipartimento di Chimica e Chimica Industriale, Università degli Studi di Pisa, Via G. Moruzzi 13, 56124 Pisa, Italy
Corresponding author: Serena Coiai, e-mail [email protected]
Highlights
Fluorescein intercalated LDH samples by anion exchange and calcination-rehydration. Transfer of fluorescent features from hybrid particles to PLA and LDPE based
nanocomposites.
Investigation of humidity sensing properties of fluorescent polymer-based nanocomposites. Abstract
Fluorescent polymer nanocomposites were prepared by dispersion of hybrid layered nanoparticles into poly(lactic acid) (PLA) and linear low density polyethylene (LDPE). Layered double hydroxide (LDH) was used as a host matrix for fluorescein dye. Co-intercalation of a surfactant was carried out to space apart the fluorophore molecules thus preventing the fluorescence quenching. The co-intercalated LDHs were found to be fluorescent even in the solid state and after dispersion in PLA and LDPE their fluorescent features were transferred to both the polymer matrices with some differences depending on morphology and compatibility between the polymer phase and hybrid particles. Finally, the responsiveness of polymer composites to wet and dry conditions for possible use in water vapor sensors was preliminary investigated.
Surfactant Fluorescein Co-intercalatedLDH O O O O O Polymer dispersion Fluorescent nanocomposite
Keywords: Fluorescein co-intercalated layered double hydroxides, fluorescent polymer-based nanocomposites, optical properties, responsiveness to humidity
1. Introduction
In recent years, much effort has been devoted to extend applications of commodity plastics through mixing and blending with different additives. In this context, polymer nanocomposites prepared by dispersion of functional nanostructured substrates are of great interest due to the possibility to combine improvements in mechanical properties, heat resistance, flammability, gas permeability, etc. with functional properties, which are transferred from the filler to the polymer composites [1,2].
The hybrid inorganic-organic assembly is a suitable approach for obtaining multifunctional materials that can be used also as fillers for polymers [3]. In particular, the intercalation of guest species into layered inorganic solids is a method for producing ordered inorganic-organic assemblies with unique micro-structure controlled by host-guest and guest-guest interactions [4]. Functional hybrid materials obtained by intercalation of specific active species between inorganic layers can be in principle dispersed into a polymeric matrix thus obtaining multifunctional polymer nanocomposites, but up to now only a few examples are reported in the literature [2].
Both cationic clays (silicate-layered materials) and anionic clays, such as layered double hydroxides (LDHs), are two-dimensional (2D) host materials suitable to be modified by intercalation of organic and inorganic ions. In particular, LDHs, also known as anionic clays, are made of positively charged and pillared hydroxide lamellae balanced by hydrated anions intercalated between the layers. LDHs
are excellent host matrices for storage and delivery of bioactive molecules, drugs and photofunctional molecules [5-7]. The intercalation protects molecules from oxidation during storage, reduces the effects of migration, shows release behavior suitable for their use in controlled delivery systems and, in the case of chromophores, inhibits the aggregation of dye molecules and reduces the fluorescence quenching. Numerous examples are reported about the intercalation of organic dyes in LDHs (i.e. azo-dyes, fluorescein, pyrene/perylene derivatives, etc.) [8-12] for applications as dye lasers, solid-state self-emission devices and optical sensors [13]. Notably, to optimize the optical properties of the guest molecules, molecular dispersion within the LDHs galleries is needed, because only in this way the intermolecular interactions inducing fluorescence dissipation can be reduced [13-17].
The xanthene dye fluorescein (Flu), 2-(3,6-dihydroxyxanthyl)-benzoic acid, and its sodium salt are commonly used as fluorescent indicators, pH probes of intercellular fluids, fluorescent probes and fluorescent sensors for biogenic matter. Indeed, the various protolytic forms of Flu (cation, neutral, anion and dianion with pK1= 2.1, pK2= 4.3, and pK3= 6.4, Figure 1) give different UV-Vis absorption and emission spectra. In particular, the intensity of emission at 510-520 nm increases with the pH and the dianion has the most intense fluorescence and quantum yield [18].
Cation Neutral species Monoanion Dianion O O C O O H O O O C O O H O O O CO O O O HO OH CO OH O O CO OH HO HO O O O C O O O C O O HO
Figure 1. Possible protolytic forms of Flu
Nevertheless the aggregation of Flu in the solid state leads to its partial or whole fluorescence quenching which greatly restricts its application in solid state dye devices. Accordingly, the incorporation into a matrix such as LDH has been seen as a great advantage to confine Flu in a stable environment and to distribute homogenously the Flu molecules. Notably, thanks to host-guest interactions (electrostatic attraction and hydrogen bonding) Flu aggregation is prevented, thus reducing fluorescence quenching. In particular, Flu has been successfully immobilized via anion exchange and co-precipitation methods in the interlayer region and/or on the surface of LDHs. It has been observed that if Flu is directly loaded, the anions uncontrollably fill in the gallery or attach at the surface of the LDH nanoparticles with very high local concentration. In these conditions, due to close spacing between molecules, the resultant hybrid has very low or even no fluorescence [19]. Costantino et al. [10] carried out a few experiments modulating the concentration of the dye thus preparing hybrid materials in which Flu was only adsorbed onto the surface or even intercalated between the inorganic layers but in a low packing density. In this way, the hybrids resulted fluorescent and the authors evidenced a different optical behavior as a function of the dye intercalation or adsorption extent that led to different dye-matrix interactions and consequently to different emission spectra. Excellent results were also achieved by co-intercalating Flu anions and alkyl sulfonates with different alkyl chain lengths [19-22]. It was found that the co-intercalation of alkylsulfonates is an effective strategy for preventing the aggregation of the dye by changing the interlayer microenviroment. Moreover, surfactant molecules reduce the fluorescence quenching by inhibiting non-radiative processes and influencing the orientation order and aggregation characteristics of the dye molecules. Indeed, photoluminescence properties of the host-guest system can be finely controlled by varying the fluorophore/alkylsulfonate molar ratio. These authors also tested the application of these co-intercalated LDHs as optical sensors for dopamine and pH and very recently Sasai and Morita [23] investigated their use as humidity sensors.
Despite numerous studies about preparation and properties of functional inorganic/organic assemblies, their use as fillers for polymers is scarcely reported. Guo et al. [8] described the dispersion of pigment-intercalated LDH into polypropylene and evidenced that the thermo- and photo-stability of the composite containing the intercalated LDH was higher than that of the sample containing the free pigment dispersed into the polymer. Marangoni et al. [24] prepared
organo-LDHs intercalated with different blue dye molecules by the co-precipitation method and then dispersed the hybrids into polystyrene. It was found that dyes were disposed face to face in the LDH gap and that the layers preserved them from dissolution and migration into the polymer. However, by analyzing the morphology it was established that the fillers were immiscible with the polymer and polystyrene chains are not able to diffuse into the LDH galleries. An example of functional nanocomposite obtained by adding a non-linear optical (NLO) dye-modified montmorillonite into a functionalized polypropylene was also reported [4] showing that some effects can be successfully transferred to polymer matrix.
Herein, Flu intercalated LDH samples are prepared by anion exchange and calcination-rehydration methods. The hybrids are characterized from a structural point of view and their optical properties are investigated and compared with respect to composition and method of preparation. Interestingly, the most performing hybrid systems are, for the first time, used for the preparation of functional polymer-based nanocomposites by dispersing them in two different polymer matrices: poly(lactic acid) (PLA) and linear low density polyethylene (LDPE). Indeed, the hybrid assembly can transfer its functional properties to the polymer and at the same time the layers of the 2D host material can be separated until exfoliation giving rise to a disordered material. The structural and morphological properties as well as the optical features of the resulting composites are discussed. Moreover, preliminary experiments are also reported to investigate the responsiveness of these functional polymer nanocomposites to wet and dry conditions and accordingly to design possible optical humidity sensors.
2. Experimental
2.1 Materials
A magnesium aluminum hydroxy carbonate (LDH-CO3) Pural MG63HT with molecular formula [Mg0.66Al0.34(OH)2](CO3)0.17·0.62 H2O as previously determined [25], was kindly supplied by Sasol Germany GmbH. Fluorescein sodium salt (Sigma-Aldrich), sodium dodecyl sulfate (Sigma-Aldrich, ACS reagent, ≥99.0%) methanol (Sigma-Aldrich), toluene (Sigma-Aldrich, ACS reagent), and chloroform (Carlo Erba, RPE grade and Sigma-Aldrich, HPLC grade, ≥99.8%, ethanol stabilized) were used as received. Analytical grade chemicals including NaOH and NaNO3 were used. Deionized and free-CO2 water was also used. PLA IngeoTM Biopolymer 2003D 96% L-lactide produced by
NatureWorks®, USA, MFI (2.16 kg/190 °C) 4-8 g/10 min and LDPE Riblene FL34 by Polimeri Europa, Italy, MFI (2.16 kg/190 °C) 2.1 g/10 min were used as polymer matrices. PLA was dried in a vacuum oven for 18 h at 110 °C before use.
2.2 Preparation of fluorescein intercalated LDH and fluorescein/dodecyl sulfate co-intercalated LDH by anion exchange
LDH-CO3 was first converted into the nitrate form (LDH-NO3) according to the titration procedure reported by Muksing et al. [26]. The LDH-CO3 was dispersed in a 1 M NaNO3 solution (mass/volume = 2 g/100 ml) and the suspension was titrated with a 1 M HNO3 solution until pH=5. The white solid was recovered by filtration, washed several times with deionized and CO2-free water, and finally dried at 60 °C in a vacuum oven. The calculated anion-exchange capacity (AEC) of the LDH-NO3 having the formula [Mg0.66Al0.34(OH)2](LDH-NO3)0.34·0.56H2O (determined according to the analytic procedure described in Ref. [26]) is 3.76 mequiv/g, calculated as follows: AEC = x/Mw·103 (mequiv/g), where Mw and x are the molecular weight and the layer charge per octahedral unit, respectively.
Fluorescein (Flu) was individually intercalated (LDH-Flu) and also co-intercalated with dodecyl sulfate (DS) (LDH-(DS/Flu)1) between the LDH layers by anion exchange. In the first case, an amount of Flu sodium salt (1.410 mmol) corresponding to 1.5 times the AEC of LDH-NO3was dissolved in 50 ml of CO2-free deionized water. The pH of the solution was maintained at 7 by using 0.1 M NaOH solution. Once Flu was completely solubilized, the LDH-NO3 (0.5 g) was added to the solution under nitrogen atmosphere, and then kept in the dark and under stirring for 48 h. The solid was recovered by filtration, washed extensively with CO2-free deionized water, and then dried under vacuum at 60 °C to constant weight. In the case of the co-intercalated product (LDH-(DS/Flu)1), LDH-NO3 (0.5 g) was added under nitrogen to a water/ethanol (1:1 v/v) solution (150 ml) containing Flu (0.037 mmol) and DS (2.745 mmol). The molar amount of the two anions corresponds to 1.5 times the AEC of the LDH-NO3precursor; Flu/(DS+Flu) molar ratio (1.34 x 10-2) was selected according to Refs. [19,20]. The pH of the solution was maintained at 7 by using 0.1 M NaOH solution. The suspension was kept in the dark and under stirring in an inert atmosphere for 48 h. The product was recovered by filtration, washed extensively with CO2-free deionized water, and then dried under vacuum at 60°C to constant weight.
LDH-CO3 was first thermally treated in a muffle furnace at 450 °C to obtain its calcined form (calcined-LDH). The weight loss due to the calcination is about 40 wt%. The first product (LDH-(DS/Flu)2) was obtained by dispersing 0.6 g of calcined-LDH in an aqueous solution (100 ml) containing Flu (0.084 mmol) and DS (6.138 mmol). The pH of the solution was maintained at 9 by using 0.1 M NaOH solution. The Flu/(DS+Flu) molar ratio (1.34 x 10-2) was analogous to that used for the preparation of LDH-(DS/Flu)1. The molar amount of the two anions corresponds to 1.5 times the AEC of the LDH-CO3 (AEC = 4.22 meq/g). The suspension was maintained under stirring for 24 h at 60 °C for 72 h to ensure the regeneration of the LDH structure. The recovered LDH-(DS/Flu)2 was washed extensively with CO2-free deionized water and then dried under vacuum at 60 °C to constant weight. LDH-(DS/Flu)3 was prepared by setting Flu/(DS+Flu) molar ratio as 1.34 x 10-3. The molar amount of the two anions corresponds to 1.5 times the AEC of the LDH-CO3. Accordingly 0.6 g of LDH-calcined were dispersed in an aqueous solution (100 ml) containing Flu (0.0085 mmol) and DS (6.313 mmol). The pH of the solution was maintained at 9 by using 0.1 M NaOH solution. LDH-(DS/Flu)3 was washed extensively with CO2-free deionized water, and then dried under vacuum at 60 °C to constant weight.
2.4 Preparation of LDH/polymer samples
LDH/PLA samples, containing 5 wt.% of modified-LDH with respect to the polymer matrix, were prepared by solution mixing. In a typical experiment, modified-LDH (0.1 g) was suspended in a chloroform/methanol 70/30 (v/v) solution (5 ml), stirred and bath-sonicated for 20 min. The modified-LDH suspension was then added dropwise to the PLA chloroform solution (2 g of PLA per 30 ml of CHCl3) and kept under stirring for 18 h at room temperature. Finally, the solvent was removed by evaporation under vacuum and composites were dried under vacuum at 60 °C for 24 h until constant weight. LDPE/modified-LDH composites, containing 5 wt% of modified-LDH with respect to the polymer matrix, were prepared by solution mixing. In a typical experiment, modified-LDH (0.1 g) was suspended in toluene (5 ml), stirred and sonicated for 20 min. The modified-LDH suspension was then added dropwise into a solution of LDPE (2 g of LDPE per 30 ml of toluene) at 80 °C and kept under stirring for 18 h. Finally, the solvent was removed by evaporation under vacuum and composites were dried under vacuum at 60 °C for 24 h until constant weight. Thin films of polymer nanocomposites (about 30 m) were prepared by compression moulding with a Carver press, model 3912, working at 180°C and 9-10 bar.
2.5 Characterization
Wide-angle X-ray diffraction (WAXD) analysis was performed at room temperature with a X’Pert PRO (PANalytical) powder diffractometer in the 1.5 - 30° 2 range applying a scan speed of 0.01601°/min, using a Cu Kα radiation (1.5406 Å). The basal spacing of LDHs, d003, was computed by applying the Bragg’s law. The LDH-NO3, LDH-CO3 and modified-LDHs were characterized as powders, whereas PLA and LDPE-based nanocomposites as films prepared by compression moulding. In the case of PLA-based nanocomposites, polymeric films were annealed at 80°C for 18 h before being analysed.
Infrared spectra were recorded with a Fourier Transform Spectrometer PerkinElmer Spectrum 100 over the wavenumber range of 450 - 4000 cm-1. The spectra of LDHs as well as those of the organic anions were obtained by mixing the samples with potassium bromide (KBr 99.4% spectroscopic grade purchased from Sigma-Aldrich).
Thermogravimetric analysis (TGA) was performed using an Exstar TG/DTA Seiko 7200 instrument. Samples (5-10 mg) were placed in alumina sample pans and runs were carried out at the standard rate of 10 °C min-1 from 30 to 800 °C under air flow (200 ml min-1).
Emission spectra (λexc = 480 nm) of modified-LDHs (in the form of solid powders) and of polymer based nanocomposites (in the form of films) were recorded at room temperature on a Horiba Jobin-Yvon Fluorolog®-3 spectrofluorometer equipped with a 450 W xenon arc lamp, double-grating excitation and single-double-grating emission monochromators and the solid-sample holder. Fluorescence was collected with the front-face mode at 30°.
Emission spectra (λexc = 480 nm) of films (30 m) of polymer based nanocomposites exposed to different dry and wet conditions were acquired under isotropic excitation with a Perkin Elmer luminescence spectrometer LS55 controlled by FL Winlab software and equipped with the front-surface accessory.
A Dionex DX-500 ion chromatograph equipped with a GP40 gradient pump and a Rheodyne 7125 injector (25 μl sample loop) was employed for the determination of nitrate content of LDH samples after equilibrating the fillers with 1 M Na2CO3 solution. The chromatographic separation was carried out in isocratic mode (flow rate: 1.0 ml/min; eluent: 2.1 mmol Na2CO3 and 0.3 mmol NaHCO3) on a Dionex IonPac AG12 column (4 × 50 mm, 9 μm particle size). Ion suppression was achieved using a Dionex ASRS 300 (4 mm) self-regenerating suppressor in recycle mode. Detection of nitrate was obtained with an AD-20 variable wavelength UV-Vis detector at 215 nm. A Dionex
PeakNet chromatography data system (version 4.5) was used for data acquisition and processing. For the calibration of the instrumental response, standard solutions of nitrate were prepared in water and used for the construction of the calibration plot (1, 2.5, 5, 7.5, and 10 mg/Kg NO3). The nitrate was extracted from the sample by equilibrating 25 mg of LDH with 5 ml of 1 M Na2CO3 solution. The solution was recovered by centrifugation. Before injection, the extracts were filtered through a 0.45 mm Agilent Captiva premium syringe filter.
The Flu content was also determined by HPLC analysis after having equilibrated 25 mg of LDH with 1 M Na2CO3 solution. An HPLC system 1260, Agilent Technologies equipped with a vacuum membrane degasser, an auto sampler and diode array detector with 60 mm optical length was used for determining the Flu content in LDH-Flu and co-intercalated LDHs. Separations were carried out using a reversed-phase HPLC column ZORBAX Eclipse Plus C18 (4.6 mm × 100.0 mm, 3 μm, Agilent Technologies. Column temperature was set at 40 °C and injection volume was 10 μL. The determination of Flu in standards and samples solutions was performed using an isocratic elution in 40% acetonitrile–60% water (0.05% phosphoric acid). Elution was performed at a solvent flow rate of 1 ml min-1. The detection of Flu was performed in the absorbance mode at 225 and 497 nm; UV-Vis spectra were recorded in the range of 220-650 nm.
The ICP-OES analysis was performed using a Varian 720 ES ICP-OES with SPS-3 Autosampler from Varian Inc (USA). 100 mg of each modified-LDH was sampled and placed in a Teflon container with 8 ml of HNO3 at 69% and 2 ml of H2O2 at 30%. The liquid was then transferred in 50 ml flask filled up with deionized water. The solutions were analyzed for Mg and Al using a 1:20 dilution. The instrument calibration was performed using two standards obtained by diluting 2.5 ml of 50 ppm solutions of the elements in a total volume of 50 ml. The emission wavelengths used for building the calibration curves and for the analysis of the samples are: Al 396.153 nm and Mg 279.077 nm.
2.6 Determination of LDH composition
The quantity of NO3- and Flu anions was estimated by ion and reverse-phase HPLC analysis of solutions obtained after equilibrating a given amount of LDH with 1 M Na2CO3 solution (Table 1). The amount of DS into the co-intercalated products and of water in all LDHs was calculated by TGA; whereas the atomic Mg/Al atomic ratio was determined by ICP-OES (Table 1).
Sample Flu [mg/kg]a) DS [wt.%]b NO3 -[mg/kg]a Water [wt.%]b Mg/Al [atomic ratio] LDH-NO3 - - 240,000 2.2 1.9 LDH-Flu 100,000 - 50,000 10.9 1.7
LDH-(DS/Flu)1 15 35.6 Not foundc 8.3 1.8
LDH-(DS/Flu)2 40 31.0 - 7.5 2.3
LDH-(DS/Flu)3 3 29.3 - 8.4 2.2
a) The quantity of Flu and NO3 anions is given as mg of Flu or NO3- per 1 kg of LDH
b) The quantity of DS anions and the amount of water are given as percentage by weight of LDH
c) No NO3 anions were found by analyzing this sample. Likely, a nitrate-carbonate exchange partially
occurred during the preparation or purification of this modified LDH.
On the basis on the values reported in Table 1 the chemical formula of LDH-NO3, LDH-Flu and co-intercalated LDHs was stated (see Table S1, Supporting Information).
2.7 Preliminary test of humidity sensitivity of polymer nanocomposites
Emission spectra (λexc = 480 nm) of polymer nanocomposites were registered after having exposed a thin film (30 m thickness) of each sample to different humidity conditions. In particular, a first spectrum of the film dried in a vacuum oven at 110°C was collected; later the dried film was exposed to a relative humidity of 99% at 25 °C for 6 hours and the spectrum was registered once more. The relative humidity was created inside an apparatus where humid Ar gas was produced by passing dry Ar through a pure water medium. The dried film was put on a 47 mm diameter PFA filter inside this apparatus and exposed at a constant flow rate of the humid gas of 50 cm3/min, monitored by a mass flow controller.
The relative emission variation (I/I0), which was calculated as (I-I0)/I0, where I is the peak maximum of the fluorescence spectrum of the wet film and I0 the peak maximum of the fluorescence spectrum of the dried film, was determined.
3. Results and discussion
3.1 Fluorescein modified LDHs
Anion exchange and calcination-rehydration methods were used for preparing modified-LDH samples containing fluorescein anions (Flu) between the layers. Flu anions were intercalated alone
or co-intercalated with an alkylsulfate (i.e. dodecylsulfate, DS) as an effective strategy for preventing the aggregation of the dye reducing intermolecular quenching and improving fluorescence efficiency [20,21]. A low Flu/(Flu+DS) molar ratio, ranging between 10-2 and 10-3, was selected since this condition enabled to optimize the optical properties of similar hybrids [19]. The anion exchange method was used to prepare LDH-Flu, and LDH-(DS/Flu)1 containing solely Flu or a mixture of the two, respectively; whereas co-intercalated products, (DS/Flu)2 and LDH-(DS/Flu)3, were prepared by calcination-rehydration. The Flu/(Flu+DS) molar ratio selected for the preparation of LDH-(DS/Flu)2 was the same used for preparing LDH-(DS/Flu)1, while LDH-(DS/Flu)3 was obtained by using a lower Flu/(Flu+DS) value in the feed (see Experimental).
3.1.1 Structure and composition of fluorescein modified LDHs
In order to verify the effective intercalation of the organic anions, XRD patterns and FT-IR spectra of all modified LDHs were collected and compared with those of LDH-NO3 and LDH-CO3 precursors (Figure 2). The XRD profiles of LDH-NO3 and LDH-CO3 evidence both (003) and (006) reflections corresponding to basal spacing (d003) of 8.9 Å and 7.6 Å, respectively. LDH-Flu, obtained by anion exchange, shows symmetric and equally spaced basal reflections between 1.5 and 30° (2 with the (003) reflection corresponding to a basal spacing of 15.1 Å. This result reveals a significant increase of the interlayer distance with respect to LDH-NO3 precursor due to intercalation of Flu anions. The basal spacing value corresponds to a geometric arrangement of Flu anions within the galleries where the dyes are aligned to the layers in such a way to interact through the anionic groups with their surface and to build up interactions between each other as in fluorescein sodium salt [10].
5 10 15 20 25 30 006 003 (f) (d) (e) (c) (b) 006 003 012 009 006 003 006 In te ns ity 2(°) 003 (a)
Figure 2. XRD patterns of (a) CO3, (b) NO3, (c) Flu, (d) (DS/Flu)1, (e) LDH-(DS/Flu)2, (f) LDH-(DS/Flu)3
The XRD patterns of the co-intercalated LDH samples (Figure 2) are all very similar to that displayed by LDH intercalated only with dodecylsulfate and previously prepared by anion exchange [26]. Only one series of (00l) reflections can be observed even if two anions were used. Notably, the basal spacing (about 24 Å), which is consistent with a monolayer arrangement of the sulfate anions [27], does not change significantly for the three co-intercalated samples (i.e. (DS/Flu)1, LDH-(DS/Flu)2, and LDH-(DS/Flu)3) independently of the preparation method used and of the amount of intercalated Flu and DS species (Table 1).
The FTIR spectra of modified and purified LDHs confirmed the presence of the organo-anions. In Figures 3 and 4, FTIR spectra of CO3 and NO3 are compared with those of Flu, LDH-(DS/Flu) samples and organic salts (i.e. Na2Flu and NaDS) used for modifications.
3500 3000 1600 1400 1200 1000 800 600 1581 14651391 1334 1214 1362 1465 1580 1214 11721112 1332 1112 1172 1385 Na2Flu LDH-Flu LDH-NO3 LDH-CO3 Wavenumber (cm-1) T ra sm itt an ce ( a. u. ) 1370
Figure 3. FT-IR spectra in the regions 3800-2500 and 1700-500 cm-1 of LDH-CO3, LDH-NO3, LDH-Flu, and Na2Flu
The spectrum of LDH-Flu shows bands at 1580, 1465, 1214, 1172, and 1112 cm-1 due to Flu anions and corresponding to asymmetric COO- stretching, xanthene ring skeletal C-C stretching conjugated with symmetric COO-, C-O-C stretching of xanthene ring, C-OH phenolic stretching, and aromatic C-H in plane bending, respectively [28]. A band at 1362 cm-1 is also well visible with a shoulder at about 1332 cm-1. For comparison with the FTIR spectra of LDH-CO3 and LDH-NO3 it seems plausible that this band is due to carbonate anions rather than nitrate anions originally present in the LDH precursor. Likely, a nitrate-carbonate exchange partially occurred during the preparation or purification of the modified filler.
Figure 4. FT-IR spectra in the regions 3800-2500 and 1700-500 cm-1 of (DS/Flu)1, LDH-(DS/Flu)2, LDH-(DS/Flu)3, and NaDS
Spectra of co-intercalated LDHs show the C-H stretching modes of DS in the high-frequency region with the C-H stretching vibration bands at 2957, 2920, and 2850 cm-1 and the C-H bending mode at 1467 cm-1, while in the low frequency region spectra display the sulphate S=O asymmetric and symmetric stretching modes at 1221 and 1083 cm-1. No evidence of vibrational bands due to Flu was observed in the spectra of co-intercalated LDHs. The typical Flu bands are merged with DS absorptions because the quantity of the xanthene dye intercalated between the layers is quite low, as demonstrated by HPLC analysis (Table 1).
TGA analysis of the modified-LDHs performed under air flow was consistent with the degradation of the hydroxide layers and with that of the intercalated organic anions. In the case of LDH-Flu three main degradation steps were observed (Figure 5). The first step between 30 to 130 °C can be attributed to the loss of co-intercalated water; the second step between 150 and 200 °C is likely due to the initial dehydroxylation of the layer as suggested by Yang et al. [29] even if it may fit with the thermal desorption of Flu anions up taken on the surface of the microcrystals [10]. The main step of thermal decomposition of intercalated Flu occurred over 400 °C (see TGA curve of Na2Flu in Figure S1, Supporting Information) and is overlapped by the dehydroxylation of the layers. In the
case of the co-intercalated samples (Figure 6), no separate weight loss step for decomposition of Flu anions could be discerned. Overall the TGA curves are comparable with that of a dodecyl sulfate intercalated LDH [26]. Four main steps can be observed. The first step up to 150 °C corresponds to the loss of co-intercalated water. The decomposition of DS occurs between 150 and 300 °C, whereas dehydroxylation of the layers, Flu decomposition and eventual decomposition of nitrate and carbonate anions take place successively.
Figure 5. TGA and DTG curves of LDH-NO3 and LDH-Flu
Figure 6. TGA and DTG curves of LDH-(DS/Flu)1, LDH-(DS/Flu)2, LDH-(DS/Flu)3
Differences in chemical composition can affect the optical properties of the hybrids as well as their dispersion capability and extent of interfacial interactions with a polymer matrix. The three co-intercalated LDHs have different composition in terms of quantity of DS and Flu (Table 1) and they also have a different chemical formula (Table S1, Supporting Information). The amount of Flu in the co-intercalated LDHs is related to the feed content and it is smaller when a lower Flu/(Flu+DS)
molar ratio was used (LDH-(DS/Flu)3 vs. LDH-(DS/Flu)1 and LDH-(DS/Flu)2). In addition, for equal feed conditions (LDH-(DS/Flu)1 vs. LDH-(DS/Flu)2), the anion exchange method makes possible to intercalate more DS species than the calcination-rehydration one, whereas the amount of Flu is lower. These differences may be attributed to the kinetics of the anion exchange and calcination-rehydration processes and to the competition between DS and Flu anchoring and intercalation between the layers. Indeed, while the calcination-rehydration method involves two main steps, i.e. rehydration and reconstruction of the LDH structure, the anion exchange depends on the exchange between nitrate and organic anions. Therefore the two methods may give rise to a different distribution of the Flu molecules between the surface and lamellae of LDH. As a consequence, a higher local dye concentration may be encountered.
3.1.2 Photoluminescence properties of fluorescein modified LDHs
The fluorescence emission spectra of LDH-Flu and co-intercalated LDHs were recorded. A different emission was observed for the three co-intercalated samples (Table 2).
Table 2. Optical properties of LDH-Flu and LDH-(DS/Flu) samples Sample name Emission max (λem, nm)a
LDH-(DS/Flu)1 538
LDH-(DS/Flu)2 550
LDH-(DS/Flu)3 531
aWavelength corresponding to peak fluorescence intensity when irradiated at λex=480 nm
Emission spectra of LDH-(DS/Flu) samples are shown in Figure 7, where intensities have been
normalized to facilitate the comparison. The emission spectrum of LDH-Flu is not shown because
no fluorescence emission was observed. In this case, indeed, Flu anions were intercalated between lamellae in higher quantity with respect to the co-intercalated LDHs (Table 1) and experienced J- or H-type dye aggregation, which adversely affected fluorescence emission [19].
500 550 600 650 700 750 0.0 0.2 0.4 0.6 0.8 1.0 LDH-(DS/Flu)2 LDH-(DS/Flu)1 LDH-(DS/Flu)3 N o rm al iz e d E m is si o n (a .u .) Wavelength (nm)
Figure 7. Fluorescence emission spectra (λex=480 nm) spectra of LDH-(DS/Flu)1, LDH-(DS/Flu)2 and LDH-(DS/Flu)3
Upon addition of DS and decrease of Flu concentration (LDH-(DS/Flu)3<LDH-(DS/Flu)1<LDH-(DS/Flu)2) the fluorescence emerged, likely because the surfactant partially prevented the Flu aggregation inside the LDH gallery region. The lambda of emission of all the co-intercalated LDHs is red shifted with respect to that of the spectrum of Flu in solution at different pH (Figure S2, Supporting Information) and this behavior is attributed to the remaining dye aggregates that persist in spite of the presence of DS [19]. In the case of LDH-(DS/Flu)2 containing the higher Flu concentration, the lambda of emission was the most red shifted, possibly attributed to the greater number of aggregates. On the contrary, the lowest emission wavelength was reached for LDH-(DS/Flu)3 containing the lowest quantity of Flu (Table 1). Finally, by using the same Flu/(DS+Flu) molar ratio in the feed, the hybrid obtained by the anion exchange method is that showing the lowest lambda of emission. This feature can be likely correlated with an optimized separation of the dye molecules obtained by this method with respect to calcination-rehydration, even if this last method also led to a lower Flu concentration in the hybrid. These results show that in case of co-intercalated LDHs the aggregation of Flu decreases resulting in more isolated Flu molecules, surrounded by surfactant molecules.
3.2 Polymer nanocomposites based on fluorescein modified LDHs
Modified LDHs (LDH-Flu and co-intercalated LDHs) containing 5 wt% of Flu were solvent-dispersed in two different polymer matrices, i.e. PLA and LDPE. These polymer matrices were selected considering their different interaction capability with the fillers. The morphology of the composites was investigated by XRD analysis and it was tentatively correlated with the optical properties, as follows.
3.2.1 Comparison between the morphology of PLA and LDPE nanocomposites with fluorescein modified LDHs
The XRD pattern of PLA/LDH-Flu did not show (003) and (006) basal reflections of neat hybrid (Figure 8a). On the contrary, in the XRD patterns of PLA/LDH-(DS/Flu)1 and PLA/LDH-(DS/Flu)2 diffraction peaks due to co-intercalated LDHs are present (Figure 8b). Indeed, in both the cases, there is a weak signal at low 2 degrees likely due to the (003) reflection of the dispersed filler, which is slightly shifted to smaller angles compared to that of the corresponding neat hybrids. This result indicates an increase in the interlamellar spacing, thus suggesting an intercalated morphology. Conversely, in the case of the PLA/LDH-(DS/Flu)3 sample, the basal reflections of the filler are not visible and likely a more disordered or exfoliated morphology was obtained.
Regarding the LDPE-based composites, the XRD patterns (Figure 9) show, in addition to the reflections of LDPE crystals, the typical (003) and (006) reflections of LDHs, which are only slightly shifted towards the smaller angles. This result suggests that the particles remained as in the original packed assembly with some intercalation. Probably, the poor compatibility between the matrix and the modified LDHs did not favor the dispersion of the layers.
4 6 8 10 12 14 16 18 20 (006) LDH-Flu PLA/LDH-Flu In te ns ity ( a. u. ) 2 (°) PLA (003)
(a)
4 6 8 10 12 14 16 18 20 (006) PLA/LDH-(DS/Flu)3 LDH-(DS/Flu)1 PLA/LDH-(DS/Flu)1 PLA/LDH-(DS/Flu)2 In te ns ity ( a. u. ) 2 (°) PLA (003)(b)
Figure 8. X-ray diffraction patterns of (a) PLA, Flu and LDH-Flu; (b) PLA, PLA/LDH-(DS/Flu)1, PLA/LDH-(DS/Flu)2, PLA/LDH-(DS/Flu)3, and LDH-(DS/Flu)1.
4 6 8 10 12 14 16 18 20 22 24 26 (006) LDPE/LDH-Flu LDH-Flu In te ns ity ( a. u. ) 2 (°) LDPE (003)
(a)
4 6 8 10 12 14 16 18 20 22 24 26 (006) (003) LDH-(DS/Flu)1 LDPE/LDH-(DS/Flu)3 LDPE/LDH-(DS/Flu)2 LDPE/LDH-(DS/Flu)1 LDPE In te na si ty ( a. u. ) 2 (°)(b)
Figure 9. X-ray diffraction patterns of (a) LDPE, Flu and LDH-Flu; (b) LDPE, LDPE/LDH-(DS/Flu)1, LDPE/LDH-(DS/Flu)2, LDPE/LDH-(DS/Flu)3, and LDH-(DS/Flu)1.
3.2.2 Photoluminescence properties of PLA and LDPE based nanocomposites with fluorescein modified LDHs
Thin films of PLA and LDPE composites were then characterized by fluorescence spectroscopy. As a result, we observed a partial reactivation of the fluorescence emission (Figure 10), due to the dispersion of the non-fluorescent LDH-Flu in the two polymer matrices. Indeed, even if the fluorescence intensity of PLA/LDH-Flu and LDPE/LDH-Flu samples was very low, emission can be appreciated upon excitation at 480 nm. The dispersion of LDH-Flu in the two polymer matrices
allowed to re-establish the fluorescence, most likely owing to the intercalation of polymer chains between the layers as well as the delamination of the host-guest systems, as supposed by XRD analysis, partially destroyed dye aggregates.
500 525 550 575 600 625 650 675 700 725 750 0 2000 4000 6000 8000 10000 12000 14000 16000 LDH-Flu PLA/LDH-Flu LDPE/LDH-Flu E m is si o n In te ns ity ( a. u .) Wavelength (nm)
Figure 10. Emission spectra (λex.= 480 nm) of LDH-Flu, PLA/LDH-Flu, and LDPE/LDH-Flu
In the case of the co-intercalated hybrids, the dispersion in the two polymer matrices evidenced a blue shifted emission (20-25 nm) with respect to the corresponding co-intercalated LDHs (Figure 11), and the effect was more evident when PLA was used as matrix compared to LDPE. This noteworthy result indicates a further destruction of dye aggregates due to the dispersion and distribution of the hybrid into polymer matrices. Notably, in case of PLA, XRD patterns show either a shift of the characteristic (003) diffraction peak to a lower 2 value or its absence (Figure 8b), which is in accordance with an intercalated or exfoliated morphology of the filler. Accordingly, it can be supposed that the dispersion of the filler in the polymers caused a further change in the microenvironment of Flu molecules by slipping the layers and separating the guest dyes.
500 550 600 650 700 750 0.0 0.2 0.4 0.6 0.8 1.0 PLA/LDH-(DS/Flu)1 LDPE/LDH-(DS/Flu)1 LDH-(DS/Flu)1 N o rm al iz e d E m is is o n In te n si ty Wavelength (nm) (a) 500 550 600 650 700 750 0.0 0.2 0.4 0.6 0.8 1.0 LDH-(DS/Flu)2 LDPE/LDH-(DS/Flu)2 PLA/LDH-(DS/Flu)2 N o rm a liz e d E m is si o n In te n si ty Wavelength (nm) (b) 500 550 600 650 700 750 0.0 0.2 0.4 0.6 0.8 1.0 LDH-(DS/Flu)3 LDPE/LDH-(DS/Flu)3 PLA/LDH-(DS/Flu)3 N o rm al iz e d E m is si o n In te n si ty Wavelength (nm) (c)
Figure 11. Emission (λex = 480 nm) spectra of LDPE and PLA based nanocomposites with: (a) LDH-(DS/Flu)1, (b) LDH-(DS/Flu)2, and (c) LDH-(DS/Flu)3
3.2.3 Preliminary investigation about relative humidity of polymer composites
It has been recently reported that LDH co-intercalated with both Flu and alkyl surfactant possesses luminous sensing properties versus relative humidity making this hybrid suitable for optical humidity sensors [23]. If this property is preserved in the composites, remarkable benefits are gained for sensors application due to thermo-mechanical features of polymer-based materials compared to hybrid powders. Thermoplastic materials can be indeed processed and can assume specifically designed shapes, thus improving device uses. Accordingly, as a preliminary investigation, fluorescence emission spectra of PLA and LDPE-based composites were collected after having exposed thin film specimens to wet and dry conditions. To perform this experiment emission spectra of vacuum dried films and films exposed to relative humidity of 99% were registered (Figure 12 and Figure 13). Notably, it was observed an enhanced fluorescence emission by increasing humidity for both PLA and LDPE based composites. This result can be explained considering the equilibrium between the dianionic and monoanionic Flu forms and the different intensity of emission of these two species (Figure S2, Supporting Information). As previously suggested for a similar co-intercalated LDH [23], the presence of water between the layers can shift the monoanion-dianion equilibrium by changing the relative concentrations. Notably, the water in contact with the metal hydroxide surfaces induces a basic interlayer spacing that favors the dianionic Flu species; meanwhile, the monoanionic Flu is favored under dry conditions owing to the reduction in polarity of the LDH interlayer spacing caused by the desorption of hydrated water molecules.
Figure 12 and Figure 13 show that the relative emission intensity (under wet and dry conditions) is higher for the PLA-based composites than for the LDPE-based ones. Notably, for both the matrices the largest values are obtained by using LDH-(DS/Flu)2 probably due to the higher concentration of Flu in this sample (Table 1) that maximizes the responsiveness of the dye-based hybrid substrate. However, the relative emission variation (I/I0) as a function of the type of LDH indicates that the largest response occurs for LDPE-based films featuring a more efficient interaction of water molecules with the Flu species in LDPE-based composites than in PLA-based samples.
Figure 12. On the left: emission spectra (λex = 480 nm) of PLA-based nanocomposites with 5 wt% LDH-(DS/Flu)1, LDH-(DS/Flu)2 and LDH-(DS/Flu)3, respectively. Spectra were recorded on films of 30 m thickness exposed to dry and wet conditions. On the right: relative intensity variation of a peak maximum (I/I0) of PLA-based nanocomposites as a function of the different type of LDH.
Figure 13. On the left: emission spectra (λex = 480 nm) of LDPE-based nanocomposites with 5 wt% LDH-(DS/Flu)1, LDH-(DS/Flu)2 and (c) LDH-(DS/Flu)3, respectively. Spectra were recorded on films of 30 m thickness exposed to dry and wet conditions. On the right: relative intensity variation of a peak maximum (I/I0) of LDPE-based nanocomposites as a function of the different type of LDH.
Such changes in emission could be attributed to the different polymer matrix and morphology of the two sets of composites. In the case of LDPE-based samples, the XRD analysis has revealed the formation of intercalated nanocomposites in which the LDH phase has maintained a stacked and parallel lamellar structure, while increasing the basal interlayer distance. In this condition, the
500 520 540 560 580 600 620 0 100 200 300 400 500 600 PLA/LDH-(DS/Flu)1 dried PLA/LDH-(DS/Flu)1 wet PLA/LDH-(DS/Flu)2 dried PLA/LDH-(DS/Flu)2 wet PLA/LDH-(DS/Flu)3 dried PLA/LDH-(DS/Flu)3 wet F lu or es ce nc e In te ns ity ( a. u. ) Wavelength (nm)
PLA/LDH-(DS/Flu)1 PLA/LDH-(DS/Flu)2 PLA/LDH-(DS/Flu)3
0,0 0,5 1,0 1,5 2,0 2,5 3,0 I/I 0 500 520 540 560 580 600 620 0 100 200 300 400 500 600 LDPE/LDH(DS/Flu)1 dried LDPE/LDH(DS/Flu)1 wet LDPE/LDH(DS/Flu)2 dried LDPE/LDH(DS/Flu)2 wet LDPE/LDH(DS/Flu)3 dried LDPE/LDH(DS/Flu)3 wet E m is si on In te ns ity ( a. u. ) Wavelength (nm)
LDPE/LDH-(DS/Flu)1 LDPE/LDH-(DS/Flu)2 LDPE/LDH-(DS/Flu)3 0,0 0,5 1,0 1,5 2,0 2,5 3,0 I/I 0
fluorescence observed is due to Flu molecules confined between the hydrophilic inorganic lamellae, which are likely more susceptible to local variation in pH in presence of humidity. On the contrary, in the case of PLA-based samples the morphology is more disordered, possibly exfoliated, as evidenced by XRD, owing to more effective interfacial interactions. Accordingly, Flu molecules are in this case more affected by the surrounding PLA chains. The different water vapor permeability of PLA and LDPE [30-32] may be also responsible of the differences observed. Finally in the case of PLA it is also possible that the exposition to humidity caused a partial hydrolysis of ester bond of PLA chains thus increasing the number of carboxylic acid chain ends contributing to maintain the monoanionic form of Flu even in presence of humidity.
On the whole, this first screening proves that the humidity sensing property of the hybrids can be successfully transferred to thermoplastic polymer matrices. Moreover, it was also found that the response is partially reversible. Indeed, for the sample LDPE/LDH-(DS/Flu)2 which showed the highest variation of fluorescence emission, from dry to wet conditions, it was found that after a second cycle of drying and exposure to humidity there is a similar change in emission as observed during the first cycle (Figure S3, Supporting Information).
4. Conclusions
Co-intercalated fluorescent LDH nanoparticles have been successfully prepared by both anion exchange and calcination-rehydration methods. The co-intercalation of Flu molecules and alkyl sulfate anions is indeed an effective strategy for preventing the aggregation of the dye reducing intermolecular quenching and obtaining hybrid LDH particles that are fluorescent even in the solid state. Moreover, it has been found that the method of preparation, as well as the concentration of reagents, affects the chemical composition and arrangement of Flu species between lamellae. This last results in different optical properties of the hybrids as well as in different dispersion capability of particles into PLA or LDPE.
Fluorescent polymer nanocomposites have been obtained thus transferring the fluorescent features of LDHs to the polymer matrices with some differences depending on morphology and compatibility between the polymer phase and hybrid particles. The exposition of dried films to relative humidity of 99% showed an increase of their emission due to the shift of the monoanion-dianion Flu equilibrium. Even if the effect was generally more marked for LDPE nanocomposites than for PLA based samples, results suggest the possible use of both materials as humidity sensors.
Acknowledgements
This work was partially supported by the National Project ‘New Polymer Systems with Electric and Optical Functionalities via Nano and Micro Adhesive Dispersion to Produce Materials and Devices for Smart Applications’ POLOPTEL 2011–2014, La Fondazione CARIPISA conv. 167/09. S. C. acknowledges Marco Carlo Mascherpa and Roberto Spiniello (ICCOM-CNR SS Pisa) for ICP-OES and TGA analyses. Dr. Ilaria Domenichelli (ICCOM-CNR SS Pisa) is also acknowledged for her contribution to the experimental work.
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