In this study we propose a suitable method for the solar-activated controlled release of volatile compounds from polymeric microcapsules bonded with photocatalytic nanoparticles. These reservoirs can find applications, for example, in the controlled release of insecticides, repellents, or fragrances, amongst other substances. The surfaces of the microcapsules have been functionalized with TiO2 nanoparticles. Upon ultraviolet irradiation, redox mechanisms are initiated on the semiconductor surface resulting in the dissociation of the polymer chains of the capsule wall and, finally, volatilization of the encapsulated compounds. The quantification of the output release has been performed by gas chromatography analysis coupled with mass spectroscopy.
Microencapsulation and release has been an area of growing interest in the last years due to the many application perspectives, like in pharmaceutical products for drug controlled release, such as of insulin and other proteins, or cosmetics [
Scheme of the polymerization reaction.
The immobilization of titanium dioxide nanoparticles in microcapsules can offer a wide range of practical applications and combines the main advantages of photocatalytic reactions with the possibility of controlled release by solar activation. These photoactive nanomaterials can be deposited onto several types of surfaces, including tents, curtains, and windows, amongst other surfaces [
The main objective of this work was to produce polymeric microreservoir systems for the controlled release of volatile compounds (e.g., insecticides, deodorants, fragrances, etc.) upon solar activation. The TiO2 nanoparticles were synthesized by a modified sol-gel method and its microstructure was characterized by X-ray diffraction for different calcination temperatures. The controlled release of the encapsulated dodecane was studied in the presence of TiO2 catalysts with and without ultraviolet (UV) irradiation. In order to extend the semiconductor absorbance into the visible light region, aiming to reduce the semiconductor band gap, the TiO2 nanoparticles were doped with nitrogen anions [
Titanium (IV) isopropoxide (>99%, Sigma-Aldrich), propyl alcohol (>98%, Sigma-Aldrich), triethylamine (>99%, Sigma-Aldrich), hydrochloric acid (37%, Sigma), poly(vinyl alcohol) (>98%, Sigma-Aldrich), ethylenediamine (>99.5%, Sigma-Aldrich), dodecane (>99%, Sigma-Aldrich), and sebacoyl chloride (>95%, Sigma-Aldrich) were used as received. Commercial TiO2 P25 powder was purchased from Degussa and used as reference photocatalytic material.
The N-doped TiO2 nanoparticles were synthesized by a modified sol-gel method [
After the sol-gel synthesis, the N-doped TiO2 nanoparticles were collected by repeated centrifugation and washed with propyl alcohol. Amorphous powders were first annealed at 80°C in a conventional electric oven for 8 h and later calcined at 300°C, 500°C, 700°C, and 800°C during 2 h to produce nanometric powders. The resulting samples were named considering the calcination temperature as NP300, NP500, NP700, and NP800, respectively. The sample without any thermal treatment was identified as NPRT.
The photocatalytic oxidation of methylene blue (MB) in the presence of the N-doped TiO2 nanoparticles under UV irradiation was investigated in order to evaluate the photocatalytic activity at different calcination temperatures [
In a typical experiment, 2 mg of photocatalyst was suspended in aqueous methylene blue solution (10−5 M) in a quartz cell (40 mm × 40 mm × 10 mm) at pH 7.2 [
The absorbance of the MB was monitored at intervals of 5 min using a spectrophotometer (ScanSpec UV-Vis, ScanSci) in the range of 300–900 nm. The rate of photodegradation of MB was analyzed by monitoring the intensity variation of the main absorption peak at 665 nm.
The kinetics of photocatalytic degradation of MB is a pseudo-first-order reaction and can be expressed according to the equation [
Photoluminescence technique (PL) was used for the detection of hydroxyl radicals (•OH) produced during the photocatalysis reaction. Coumarin was chosen as molecular probe, which readily reacted with •OH radicals to produce a highly fluorescent subproduct, 7-hydroxycoumarin (7HC), which shows a strong PL signal at 456 nm [
The microcapsules were prepared by using an adapted method based on interfacial polycondensation in an ultrasonic bath at a frequency of 30 kHz [
The gas chromatography coupled with mass spectrometry (GC-MS) analysis of dodecane output release from within the microcapsules was performed using a Varian 4000 Performance apparatus, equipped with a CP8944 VF-5 column and an ion trap mass spectrometer as detector. The carrier gas was helium, at a flow rate of 1 mL·min−1. Column temperature was initially 40°C, and then gradually increased to 270°C at 8°C·min−1. For GC-MS detection an electron impact ionization system was used with ionization energy of 70 eV.
The crystalline structure of the photocatalysts was characterized by X-ray diffraction analysis (XRD, Bruker D8 Discover diffractometer) using Cu K
Figure
XRD patterns of the N-doped TiO2 powders calcined at (a) room temperature (b) 300°C (c) 500°C, (d) 700°C, and (e) 800°C. A: anatase; R: rutile.
Comparative studies of the adsorption of methylene blue on the synthetized nanocatalyst powders were performed. A solution of methylene blue (10−5 M) was stirred with the different TiO2 nanoparticles (0.15 g·L−1) in the dark to ensure complete surface adsorption of dye. The change in methylene blue concentration was investigated by monitoring the maximum of absorbance of this dye at 665 nm, as plotted in Figure
UV-vis absorption variation at 665 nm measured in the dark of MB (10−5 M, pH 7.2) solution as a function of time for nano-TiO2 powders annealed at different temperatures. This experiment has also been performed for the P25 reference powder.
These results indicate that the TiO2 nanoparticles strongly adsorb the methylene blue molecules at the surface. This can be explained by the pH effect on methylene blue/TiO2 nanoparticle suspensions. The pH of the solution influences significantly the characteristics of the TiO2 surface charge. Point of zero charge (PZC) is the value of pH at which the surface charge is zero. The knowledge of PZC of titania (PZC = 6.8) helps to predict the type and nature of the charge transfer that occurs preferentially. Since the photocatalytic assays were performed at pH higher than the reference PZC of titania, the surface of TiO2 nanoparticles has a negative charge and there is an electrostatic adsorption between negative-charged surfaces of TiO2 and methylene blue cations that carry a positive charge (
The photocatalytic reaction is very sensitive to the catalyst surface. The MB reacts with electrons and superoxide anions generated on the modified TiO2 particles under UV irradiation. The photocatalytic degradation reactions of MB on UV were investigated using the prepared N-doped TiO2 powders at different calcination temperature. In order to compare their UV light activities, the experiments were also carried out using the TiO2 powders without thermal treatment and the reference TiO2 P25 in the same way as using the N-doped TiO2. The evolution of MB concentration with time for each sample at 665 nm is presented in Figure
Photodegradation of MB (10−5 M, pH 7.2) solution as a function of UV-A irradiation time for TiO2 powders annealed at different temperatures and reference P25.
The photocatalytic degradation is a pseudo-first-order reaction and its kinetics,
Rate constant (
Samples |
|
Degradation (%) of MB solution after 120 min. |
---|---|---|
NPRT | — | 2 |
NP300 | 1.6 × 10−3 | 16 |
NP500 | 6.0 × 10−3 | 40 |
NP700 | 2.4 × 10−2 | 95 |
NP800 | 1.5 × 10−2 | 85 |
TiO2 P25 | 1.7 × 10−2 | 88 |
Under ultraviolet irradiation the valence-band electrons of TiO2 are excited to conduction band, leading to the formation of photogenerated electrons (e−) and holes (h+) pairs. In aqueous medium •OH radicals are generated by the reaction between photogenerated holes and H2O. Related reactions may be summarized as follows:
The detection by coumarin fluorescence is based on the fact that its hydroxylation generates various subproducts, in which only one is strongly fluorescent, 7-hydroxycoumarin, as follows:
After the correction of the coumarin PL signal, which is almost negligible, the 7-hydroxycoumarin fluorescence signal can be used to determine the quantity of •OH generated in reaction. Figure
(a) Photoluminescence spectra of coumarin solution (10−3 mol L−1, pH 3.3) after UV-A irradiation and (b) the time dependence of the photoluminescence intensity of 7-hydroxycoumarin at 456 nm, in the presence of the NP700 powder.
In order to investigate the effect of temperature annealing on the formation rate of radicals, PL experiments were performed in same conditions of NP700 for all N-doped TiO2 prepared by sol-gel method. After 2 h of irradiation PL spectra were recorded and compared (Figure
Photoluminescence spectra of coumarin solution (10−3 mol L−1, pH 3.3) after 120 min of UV-A irradiation in the presence of N-TiO2 powders annealed at different temperatures.
It is clear that the NP700 sample yields a higher PL intensity, implying a high formation rate of 7HC and •OH. It is interesting to note that the PL intensity decreased when NP800 was used. This can be explained by the fact that NP800 contains a two-phase mixture of anatase and rutile, which can be related to the increase of the recombination in photogenerated electrons and holes and subsequently block of •OH production on the TiO2 surface. The •OH formation rate on rutile phase is much lower than that on anatase phase. The observed PL signal was lower for calcined powders at temperatures below 700°C, meaning that a prominent anatase phase is directly correlated with enhanced •OH production.
The FTIR spectrum of the synthesized polyamide microcapsules functionalized with TiO2 is presented in Figure
IR spectra of microcapsules with TiO2 prepared by interfacial polymerization.
Figure
The spectral region between 3500 and 3060 cm−1 corresponds to the –NH stretching vibration of the primary and secondary amines, which could be related to the excess of amine used. The large band at 3310 cm−1 is ascribed to the –C=O characteristic band. The peaks from 2970 cm−1 to 2830 cm−1 are attributed to the TiO2 showing that the nanoparticles were successfully adsorbed on the surface.
Figure
Scanning electron microscopy micrographs of dodecane-loaded polyamide microcapsules (a) with N-doped TiO2 nanoparticles adsorbed onto its surface and (b) microcapsule degradation upon UV-A irradiation.
The GC-MS analysis of the UV-A-irradiated samples containing dodecane-loaded microcapsules adsorbed with TiO2 nanoparticles on the surface revealed the presence of dodecane as a principal compound from the output yield. In order to compare the results with a commercial sample of TiO2 P25, GC-MS experiments were performed only for the sample NP700, since it showed the best results in the photocatalysis and photoluminescence assays discussed before (Figures
Chromatograms obtained by GC-MS for UV-A-irradiated dodecane-loaded polyamide microcapsules in the presence and absence of N-doped TiO2 nanoparticles (NP700).
GC-MS chromatograms obtained for dodecane-loaded polyamide microcapsules adsorbed with N-doped TiO2 nanoparticles (NP700), with and without UV-A irradiation.
From the analysis of Figures
Figure
Concentration (ppm) of the output release of dodecane (Dod) from the polyamide microcapsules, for different samples and experimental conditions.
A simple method was developed for the synthesis of a stable nanocrystalline N-doped TiO2 photocatalysts by a modified sol-gel method, which were adsorbed on dodecane-loaded polyamide-based microcapsules. The effect of annealing temperature on the photocatalytic activity of the nanoparticles under ultraviolet irradiation was investigated. It was found that photocatalytic activity of these nanoparticles is strongly dependent on the calcination temperature. The nanoparticles calcined at 700°C revealed the highest photocatalytic activity upon degrading methylene blue under UV-A irradiation. Furthermore, the formation of •OH radicals on the photocatalyst surface under UV-A irradiation was also monitored by fluorescence experiments. In the same way of MB degradation, the N-doped TiO2 powders calcined at 700°C form a higher number of hydroxyl radicals on its surface than powders annealed at lower and higher temperatures (800°C). This result can be related to the more stable and prominent anatase phase in this sample, in detriment to rutile formation at a higher temperature.
This work has shown that polyamide based microcapsules with TiO2 adsorbed on the surface can be successfully used to trigger the release of a model organic volatile compounds by UV-A irradiation. By means of GC-MS experiments it was possible to identify and quantify the amount of volatile compound released.
The proof of concept presented in this work can be extended to afford the preparation of microcapsules loaded with organic compounds that can be released in a controlled manner through UV-A irradiation.
This work was supported by FEDER through the COMPETE Program and by the Portuguese Foundation for Science and Technology (FCT) in the framework of the Strategic Project PEST-C/FIS/UI607/2011 and PTDC/CTM-NAN/119979/2010 Project.