Freezing-Induced Loading of TiO2 into Porous Vaterite Microparticles: Preparation of CaCO3/TiO2 Composites as Templates To Assemble UV-Responsive Microcapsules for Wastewater Treatment.

The photocatalytic degradation of organic molecules is one of the effective ways for water purification. At this point, photocatalytic microreactor systems seem to be promising to enhance the versatility of the photoassisted degradation approach. Herein, we propose photoresponsive microcapsules prepared via layer-by-layer assembly of polyelectrolytes on the novel CaCO3/TiO2 composite template cores. The preparation of CaCO3/TiO2 composite particles is challenging because of the poor compatibility of TiO2 and CaCO3 in an aqueous medium. To prepare stable CaCO3/TiO2 composites, TiO2 nanoparticles were loaded into mesoporous CaCO3 microparticles with a freezing-induced loading technique. The inclusion of TiO2 nanoparticles into CaCO3 templates was evaluated with scanning electron microscopy and elemental analysis with respect to their type, concentration, and number of loading iterations. Upon polyelectrolyte shell assembly, the CaCO3 matrix was dissolved, resulting in microreactor capsules loaded with TiO2 nanoparticles. The photoresponsive properties of the resulted capsules were tested by photoinduced degradation of the low-molecule dye rhodamine B in aqueous solution and fluorescently labeled polymer molecules absorbed on the capsule surface under UV light. The exposure of the capsules to UV light resulted in a pronounced degradation of rhodamine B in capsule microvolume and fluorescent molecules on the capsule surface. Finally, the versatility of preparation of multifunctional photocatalytic and magnetically responsive capsules was demonstrated by iterative freezing-induced loading of TiO2 and magnetite Fe3O4 nanoparticles into CaCO3 templates.

Introduction

The development of novel effective systems to degrade water pollutants and bacterial contamination is an essential task of modern chemistry. Photoassisted catalytic degradation of organic compounds in the presence of titanium dioxide appears to be a promising approach to wastewater treatment, disinfection, antifouling, and elimination of various pollutants. TiO2 nanoparticles possess unique photocatalytic properties[1] for the decomposition of organic and inorganic waste,[2] deactivation of viruses,[3] bacteria,[4] microfungi and microalgae, and destruction of cancer cells.[5] On the other hand, the employment of bare TiO2 particles remains unfavorable with respect to practical applications.[6] One way to enhance the safety and selectivity of photoinduced decomposition with TiO2 particles is to encapsulate them into photoactive microcontainers.[7] From this point, the hybrid[8] microcapsules were prepared via layer-by-layer (LbL) self-assembly[9] on the template cores[10] appear the most promising as they suggest several ways to improve photocatalytic efficiency. The first one is the composite containers locally increase the concentration of the TiO2 particles and the pollutant in the given microvolume.[11] The pollutant may absorb on the container surface or diffuse into the cavity. Second, the controllable stepwise assembly allows to adjust the structure, chemical, and functional properties of the capsules, opening a broad avenue for their modification. For instance, the capsules may be modified with self-assembled layers of organic molecules with various molecular recognition sites to improve the selectivity to the desired pollutant.[12−14] Additionally, the noble metal particles acting as an electron sink can be enclosed to improve the efficiency of photocatalysis because of the reduction of charge carrier recombination[15,16] and by shifting the spectral maximum of absorption to the visible band, utilizing more energy to trigger the photocatalytic reaction from the visible light.[17] Finally, the composite microcontainers can be more easily removed from the purified water compared to the bare TiO2 particles. The inclusion of magnetic particles into containers allows for magnetic separation[18,19] to remove the containers upon purification, with the possibility of further recovery and regeneration of the catalyst.

From the variety of methods to combine the functional nanoparticles with self-assembled microcapsules, the one resulting in the highest particle containment is preferable as the efficiency of the photocatalytic reaction is directly related to the amount of the catalyst.[20] Regarding the microcapsules’ assembly, the freezing-induced loading (FIL) method was shown as an efficient approach for the loading of inorganic nanoparticles and organic macromolecules into the pores of template microparticles.[21] This approach is based on the successive freezing and thawing of a mixture of nanoparticle and template core suspension. Additionally, FIL may be employed for the preparation of complex composite templates by iterative loading of various functional particles or molecules, resulting in novel and enhanced properties of assembled capsules.[22] From this point, photocatalytic and magnetically responsive microcapsules might be of interest because of ease of preparation and removal after the water treatment is carried out.

In this paper, the FIL method was employed for the first time to prepare the stable CaCO3/TiO2 composite microparticles with high TiO2 loading as template cores for the preparation of multilayered self-assembled microcapsules for photocatalytic degradation of organic pollutants. The photocatalytic activity of these capsules was tested in two ways, mimicking possible options for capsules to be involved in degradation activity. In particular, these are the decomposition of low-molecular dye under UV light in an aqueous medium in inner capsule microvolume and decomposition of preabsorbed dye under UV light onto the capsule surface. Additionally, we have demonstrated the possibility of preparation of multifunctional microcapsules possessing photocatalytic and magnetic properties by iterative FIL loading of TiO2 and Fe3O4 nanoparticles into CaCO3 cores.

Results and Discussion

Preparation of CaCO3/TiO2 Composite Microparticles

A comparison of the measured anatase and P25 anatase + rutile particle properties is shown in Table S1 (Supporting Information). On the one hand, the specific surface of the particles is almost the same and, therefore, should not affect their comparative photocatalytic activity. On the other hand, according to dynamic light scattering (DLS) measurements, anatase particles tend to form larger aggregates in the aqueous medium compared to P25 anatase + rutile particles (Figure a), which can be related to their lower ζ-potential (5.5 and 17.8 mV, respectively, under pH 5, Figure b).

Figure 1

TiO2 nanoparticle size distribution measured by DLS (a), ζ-potential (b), and time dependence of the sedimentation stability of anatase and P25 anatase + rutile nanoparticle suspension (c).

Figure c demonstrates the time dependence of the sedimentation stability of nanoparticles in the aqueous medium estimated with respect to particle suspension transmittance. The inset shows the scheme of the experiment. The transmittance of the anatase particle solution climbs from 50% to almost 100% in 10 min, indicating that most of the particles passed the laser line and sedimented on the cuvette bottom. In contrast, the transmittance of P25 anatase + rutile particle suspension remains at the same level (about 3–5%) for 20 min and even further (the measurement was carried out for 3 h, the data are not shown). Accomplished with the DLS and ζ-potential data, this indicates that P25 anatase + rutile nanoparticles are more stable in the aqueous solution.

Mesoporous calcium carbonate microparticles with vaterite structure are widely used as templates for assembling of multifunctional carriers because of their high porosity (with an average pore size of 20–70 nm)[23] and solubility under mild pH conditions (pH 7).[24] Moreover, the porous nature and developed surface of CaCO3 microparticles are preferable in some applications, for example, sensing.[25]

Currently, the most common methods for the preparation of vaterite particles loaded with active agents and inorganic nanoparticles are the coprecipitation method (encapsulation during particle preparation)[26,27] and adsorption from solution.[28] Additionally, the synthesis of inorganic nanoparticles in CaCO3 pores in situ can be employed,[29] yet it is more complicated and less common. However, as the aqueous dispersion of TiO2 nanoparticles has a pH of 4.5–5,[30] and the formation of vaterite particles occurs in alkaline medium, it is challenging to obtain CaCO3/TiO2 composites using these methods. Figure a,b shows the scanning electron microscopy (SEM) images of the resulted CaCO3/TiO2 particles prepared with the coprecipitation technique. The addition of TiO2 suspension with acidic pH during the CaCO3 nucleation leads to the formation of cubic calcite microparticles that are not suitable for further capsule preparation.

Figure 2

SEM images of CaCO3 microparticles loaded with TiO2 nanoparticles by coprecipitation of anatase (a) and P25 anatase + rutile (b) and by adsorption of anatase (c) and P25 anatase + rutile (d). The scale bars correspond to 5 μm.

The attempt to adsorb the TiO2 nanoparticles on vaterite templates (the SEM image of the initial vaterite template is shown in Figure S3, Supporting Information) from aqueous solution resulted in a clear separation of micro- and nanoparticles (Figure c,d). This can be explained by the fact that both CaCO3 and TiO2 particles in an aqueous medium under pH 7 are charged slightly negatively[30,31] and, therefore, their adsorption is hardly possible because of electrostatic repulsion.

In turn, FIL implies the pushing off of TiO2 and CaCO3 particles in aqueous solution by the moving of the water crystallization front during controllable freezing. The particles are concentrated ahead of the crystallization front, reducing the water freezing point. Finally, upon nano- and microparticles being concentrated in a given microvolume, the nanoparticles are embedded into the CaCO3 pores, substituting the residual water under the growing pressure of the ice front.

Figure a,b demonstrates the SEM images of the CaCO3/TiO2 particles prepared by FIL under various conditions as described in Table in the Experimental Section. The images show that the visual appearance of the FIL-loaded CaCO3 templates varies depending on the type and concentration of TiO2 nanoparticles. This is the most prominent for the CaCO3/TiO2 composites with P25 anatase + rutile. The surface becomes more developed and rougher with an increasing amount of loaded TiO2 (Figure b) unlike the composites with anatase (Figure a).

Figure 3

SEM images of CaCO3/TiO2 composites loaded with (a) anatase (A1–A4) and (b) P25 anatase + rutile (B1 – B4) nanoparticles. The scale bars correspond to 2 μm on the main images and to 500 nm on the insets. (c) Relative distribution of Ti in CaCO3/TiO2 composites measured by EDX (SEM). (d) EDXS of the B4 composite (yellow color corresponds to the Ca K-line and green to the Ti K-line) measured by STEM. The scale bar corresponds to 500 nm

Table 1

List of CaCO3/TiO2 Composite Samples with Corresponding Preparation Conditions

CaCO3/TiO2 composites	TiO2 nanoparticle type	concentration of TiO2 in H2O, mg/mL	number of loading cycles
A1	anatase	0.5	1
A2		0.5	2
A3		1	1
A4		1	2
B1	P25 anatase + rutile	0.5	1
B2		0.5	2
B3		1	1
B4		1	2

The amount of Ti in the composite particles was estimated with energy-dispersive X-ray spectroscopy (EDX) analysis. Although EDX is a semiquantitative method and cannot provide direct information on the loading efficiency of TiO2 nanoparticles, it is still useful for comparative sample analysis with respect to the percentage of Ti atoms relative to the total number of atoms in the region of interest. According to EDX data (SEM) (Figure c), the amount of Ti in the samples loaded with P25 anatase + rutile nanoparticles (B1–B4) is higher than that of the samples loaded with anatase nanoparticles (A1–A4). This can be attributed to the lower loading of anatase into CaCO3 particles as the anatase is less stable in an aqueous medium. Noticeably, the amount of Ti in A2 and A3 samples is almost the same (within a margin of error) as in B2 and B3 samples. Therefore, it can be concluded that the loading efficiency of the TiO2 nanoparticles with FIL is defined by the total amount of loaded particles and can be adjusted by the concentration and/or the number of loading cycles (within the particle concentration range used in this work).

Figure d shows the HR-STEM cross-sectional image of the CaCO3/TiO2 B4 composite microparticle merged with an energy-dispersive X-ray spectroscopy system (EDXS) map of the volume and near-surface area. The HR-STEM confirms a highly porous structure of CaCO3 particles with the pore density decreasing from the center to the particle surface. The EDXS map reveals that the distribution of TiO2 nanoparticles over the CaCO3 microparticle surface is rather nonuniform. The nanoparticles are mainly located in the surface layer of 100 nm; however, in some areas, the depth of particle penetration reaches up to 400 nm (highlighted with a red dotted line), which is confirmed by the elemental analysis.

Preparation of UV-Responsive Microcontainers and Decomposition of the Low-Molecular Dye

The polyelectrolyte shells were assembled on the prepared composite CaCO3/TiO2 particles by the LbL method. The shells were assembled of poly(allylamine hydrochloride) (PAH) and poly(4-styrenesulfonate)sodium salt (PSS) polyelectrolytes because of the good stability and well-established properties of the resulted polyelectrolyte complex. As the shell is assembled and the CaCO3 matrix is removed, the TiO2 nanoparticles anchored onto the inner layer of the polyelectrolyte shell. The polyelectrolyte shell is permeable for low-molecular compounds, allowing for their photocatalytic degradation in the container microvolume directly on contact with preconcentrated TiO2 particles.

Figure a,b demonstrates the SEM images of the capsules assembled on the CaCO3/TiO2 composite cores after the dissolution of the CaCO3 matrix. Compared to the control PAH/PSS capsules (Figure c), the surface of TiO2-loaded capsules becomes rougher and TiO2 particle aggregates can be seen inside the capsule shells. This correlates with the data of EDX analysis, indicating the growth of Ti amount in the capsules with the surface roughness and the loading of TiO2 in the initial composite microparticles (Figure d). However, the Ti amount in the resulted capsules is lower than in the corresponding CaCO3/TiO2 composites, which may be related to the desorption of some number of TiO2 nanoparticles during the shell assembly. Nevertheless, the overall tendency in the distribution of Ti atoms depending on the loading and type of TiO2 nanoparticles remains the same.

Figure 4

SEM images of PAH/PSS capsules assembled on CaCO3/TiO2 composites loaded with (a) anatase (A1–A4) and (b) P25 anatase + rutile (B1–B4) nanoparticles; (c) SEM image of a control PAH/PSS capsule assembled on a CaCO3 particle. The scale bar is 1 μm. (d) Relative distribution of Ti in capsules assembled on CaCO3/TiO2 composites measured by EDX (SEM).

To evaluate the photocatalytic activity in the first set of experiments, microcapsules loaded with TiO2 nanoparticles were incubated in the Rho B solution under UV light. Prior to this, the appropriate UV lamp wavelength was selected. It is known that TiO2 is an n-type semiconductor.[32] The measured values of the band gap for anatase and P25 anatase + rutile particles were 2.85 and 3.22 eV, respectively (the absorption spectra and Tauc plots are given in Figure S4 in the Supporting Information). The measured band gap for P25 anatase + rutile is in a good agreement with the reference one of the anatase phase, which is 3.2 eV.[33] On the other hand, the measured band gap for anatase particles is different, which might be due to the fast sedimentation of the particles affecting the data acquisition. Nevertheless, the irradiation with a 367 nm UV lamp (corresponding to 3.38 eV photon energy) should result in the effective generation of electron–hole pairs in both types of nanoparticles.

Figure a,b,e,f shows the dependence of relative change of the concentration of Rho B dye incubated with photoactive microcontainers under UV light and the percentage of dye decomposition rate. The first point of every plot corresponds to the absorbance of an as-prepared mixture of capsules and Rho B, whereas the zero points were measured after 30 min of incubation without UV irradiation to reach an adsorption equilibrium in the mixture.[34] The degradation rate curves (Figure b,f) demonstrate the decomposition of the dye incubated with TiO2-loaded capsules (the standard plot for Rho B aqueous solution is given in Figure S6 in the Supporting Information). For both particle types, the degradation efficiency depends on the number of loaded particles. Noticeably, the capsules demonstrate almost the same photocatalytic activity, except those with minimal loading. Comparing the photocatalytic efficiency of capsules loaded with anatase and P25 anatase + rutile nanoparticles, the latter demonstrate more than 50% degradation of Rho B after 3 h of incubation under UV light, whereas incubation of the dye solution with anatase-loaded capsules resulted in only 30% degradation in the same time. This can be associated with better loading of P25 anatase + rutile particles. Additionally, some studies hypothesized that anatase with rutile inclusions appears to be more preferable for photocatalytic applications on comparing to pure anatase.[35−37] Although the rutile phase is considered as low-active itself, the junction between anatase and rutile phases promotes the separation of charge carriers because of electron migration from rutile to anatase and hole migration to the surface. The rutile phase plays the role of a charge separator and provides additional sites for oxidation, which results in enhanced photocatalytic activity of P25 anatase + rutile particles.

Figure 5

Relative change of the concentration (a) and percentage degradation of Rho B (b) in aqueous solution incubated with photoactive capsules loaded with anatase (A1–A4 samples) nanoparticles under UV light; ln(C/C0) plots (c) and corresponding reaction rate constants (d) for anatase-loaded (A1–A4 samples) capsules. The relative change of the concentration (e) and percentage degradation of Rho B (f) in aqueous solution incubated with photoactive capsules loaded with P25 anatase + rutile nanoparticles (B1–B4 samples) under UV light; ln(C/C0) plots (g) and corresponding reaction rate constants (h) for P25 anatase + rutile-loaded (B1–B4 samples) capsules.

The kinetics of dye decomposition in aqueous solution is generally described by the Langmuir–Hinshelwood equation, which also takes into account the adsorption of the degraded agent onto the catalyst’s surface.[38] At low dye concentrations, the equation can be written as the first-order kinetics reactionwhere C0 is an initial dye concentration, C is a measured dye concentration, k is a pseudo-first-order rate constant, and t is degradation time.[39]Figure c,g shows the semi-log plots of C0 versus time for the capsules loaded with two types of TiO2 nanoparticles plotted based on the degradation rate measurements. The experimental data are well approximated with a linear fitting curve, which implies the pseudo-first-order of the degradation kinetics. The reaction constants can be figured from these plots as slopes of the fitting lines. Figure d,h shows the comparison in reaction constants of the capsules loaded with various numbers and types of TiO2 nanoparticles. This indicates that the capsules loaded with P25 anatase + rutile particles have higher photocatalytic performance compared with capsules loaded with anatase particles. It is also noticeable that the capsules loaded with the same number of TiO2 particles (A2/A3 and B2/B3 samples) have the same photocatalytic performance. Therefore, it can be concluded that the resulted photocatalytic efficiency of the photoactive containers depends on the type and the final number of the TiO2 particles loaded by FIL, which gives certain adjustability in the container preparation.

Decomposition of the Fluorescent Label on the Surface of UV-Responsive Microcontainers

The prepared photoactive containers are effective in the decomposition of the small-molecule dye that is able to penetrate into the capsule microvolume. However, some water treatment applications require decomposition of large molecules or microorganisms that will contact mainly the capsule surface. In this regard, it is of interest to figure out the degradation efficiency of the capsules with respect to the substances attached to the external layer that are unable to go inside the capsule volume. For this purpose, TRITC–PAH was absorbed instead of PAH during the assembly of the finishing capsule bilayer. The TRITC label was selected as it is known as a fairly photostable fluorophore under UV light.[40]

Figure shows the fluorescent confocal laser scanning microscopy (CLSM) images of the microcapsules assembled on the regular CaCO3 particles and on the CaCO3/TiO2 composites. The difference in size and shape of the resulted fluorescent microcontainers assembled on various templates is clearly seen. The capsules assembled on the control CaCO3 cores (Figure a) demonstrate an almost round shape with prominent fluorescent properties. In turn, the capsules assembled on the composite CaCO3/TiO2 cores are less spherical because of the roughness of the initial templates. The difference in size and shape of the capsules assembled on the initial CaCO3 cores and CaCO3/TiO2 composite cores can be related to the various surface roughnesses of the templates, which were confirmed by SEM analysis. In turn, the capsules loaded with anatase and P25 anatase + rutile nanoparticles have a difference in size and shape as well, which can be associated with various loading efficiencies of the nanoparticles. The P25 particles demonstrate better loading and, therefore, the resulted composite cores have a more developed surface, which can be seen also by nonuniform adsorption of fluorescently labeled molecules and inhomogeneous capsule shell thickness. Additionally, the initial CaCO3 particles are not monodisperse intrinsically, which results in capsule size variation. Moreover, the fluorescent images captured with the same settings demonstrate a decrease in the fluorescent signal of the capsules that may be associated with the initiation of fluorophore photodegradation.

Figure 6

CLSM images of the capsules assembled on various templates: (a) control CaCO3, (b) anatase-loaded CaCO3/TiO2 composite (A4 sample), (c) P25 anatase + rutile-loaded CaCO3/TiO2 composite (B4 sample). The scale bar corresponds to 2 μm.

The collected data on the dye photodegradation after 3 h under UV light are shown in Figure . Photodegradation curves for all fluorescent capsules loaded with TiO2 nanoparticles lie below the control curve with statistical confidence. This indicates that TiO2 nanoparticles significantly affect the photodegradation process. Noticeably, the absolute values of the relative change of the concentration of the degradable fluorescent agent Δc/c for all capsules loaded with P25 anatase + rutile nanoparticles (Figure b) are greater than that for capsules loaded with anatase (Figure a). The Δc/c values are in the range of 10–30% for A1–A4 samples, and in the range of 25–50% for B1–B4 samples for 1 to 3 h of exposure with UV light. This is comparable with the degradation rates of small-molecule dye in the capsule volume for the highest loading of TiO2 particles. However, in the case of degradation on the surface, the dependence of degradation on the particle loading is more pronounced.

Figure 7

Relative change of the concentration of the degradable fluorescent agent (Δc/c, %) for the capsules loaded with (a) anatase (A1–A4 samples) and (b) P25 anatase + rutile (B1–B4 samples) nanoparticles under UV light.

Preparation of UV-Responsive Magnetic Microcontainers

Finally, the FIL method was employed for iterative loading of CaCO3 templates with TiO2 and Fe3O4 nanoparticles for the preparation of multifunctional capsules possessing photocatalytic activity and magnetic responsiveness. To do this, P25 anatase + rutile nanoparticles were used as previous experiments demonstrated their better stability in an aqueous medium, better loading ability, and, what is more important, better photocatalytic activity. Figure a shows the SEM image of CaCO3/TiO2/Fe3O4 composite particle loaded by 1 FIL cycle with P25 anatase + rutile nanoparticles (1 mg/mL) and by 1 FIL cycle with Fe3O4 nanoparticles. The morphology and surface roughness of these composites look similar to those of B4 core samples loaded by 2 FIL cycles with P25 anatase + rutile particles (see Figure b). The EDX analysis (Figure c) shows a relatively high amount of Ti (about 5 at. %) and Fe (about 4 at. %) atoms in the composite cores. Analogous to TiO2 loaded capsules, the content of Ti and Fe in the resulted capsules (Figure b) is considerably lower (about 1%) in comparison with initial composite particles. On the other hand, the content of Ti atoms is similar to that of the B3 capsule sample prepared with the same TiO2 loading and, considering only the Ti amount, these capsules should have the same photocatalytic properties.

Figure 8

(a) SEM image of the CaCO3/TiO2/Fe3O4 composite particle (scale bar is 2 μm); (b) SEM image of the capsule assembled on CaCO3/TiO2/Fe3O4 composite particle (scale bar is 1 μm); (c) relative distribution of Ti and Fe in CaCO3/TiO2/Fe3O4 composite particles and corresponding capsules measured by EDX (SEM); (d) mobility of the capsules loaded with TiO2 and Fe3O4 nanoparticles in the magnetic field measured as time dependence of the capsule suspension transparency.

Figure d shows the mobility of the TiO2/Fe3O4-loaded capsules in the nonuniform magnetic field of a permanent magnet. The capsule mobility was measured analogous to the experiment on TiO2 particle sedimentation (see Figure ) except that the gravity field was substituted with a magnetic field applied alongside the cuvette. The capsule suspension became almost transparent in about 100 s after the magnetic field was applied because of the capsule attachment to the cuvette wall near the magnet. This clearly indicates the response of TiO2/Fe3O4 capsules to the applied magnetic field.

Finally, the photocatalytic properties of TiO2/Fe3O4-loaded capsules were figured out by incubation of the capsules in the Rho B solution under UV light. Figure a,b shows the comparison of the relative change of the concentration and percentage degradation curves for TiO2/Fe3O4-loaded capsules and B3 capsule samples loaded with the same type and amount of TiO2 nanoparticles. The acquired data reveal the concentration decrease of Rho B incubated with TiO2/Fe3O4-loaded capsules is half as much of those of Rho B incubated with the B3 sample after 3 h under UV light. The ln(C/C0) plots (Figure c) indicate pseudo-first-order kinetics of dye degradation for TiO2/Fe3O4-loaded capsules as well, although the figured out reaction constants (Figure d) show relatively low photocatalytic performance in comparison with TiO2-loaded capsules. This may be related to several points. First, Fe3O4 nanoparticles may absorb part of the light and, thus, reduce the photocatalytic efficiency. Although Fe3O4 is a semiconductor as well, it was shown that it does not make a significant impact itself on photocatalytic reaction under UV or visible light when combined with UV/vis-responsive materials.[41] Second, Fe3O4 nanoparticles may be prone to the so-called photodissolution phenomenon, which results in growth of recombination of photogenerated electrons and holes in TiO2.[42] This can be avoided by covering Fe3O4 particles with the SiO2 shell.[43,44] Furthermore, it should be noted that the photocatalytic activity of the TiO2/Fe3O4-loaded capsules can be adjusted by additional loading of TiO2 nanoparticles.

Figure 9

Relative change of the concentration (a) and percentage degradation of Rho B (b) in aqueous solution incubated with photoactive capsules loaded with TiO2 (B3) and TiO2/Fe3O4 (B3 + Fe3O4) nanoparticles under UV light; ln(C/C0) plots (c) and corresponding reaction rate constants (d) for TiO2 and TiO2/Fe3O4-loaded capsules.

Conclusions

For the first time, TiO2 nanoparticles were loaded into mesoporous CaCO3 microparticles with the FIL technique. Moreover, the FIL method was shown to be only effective for the preparation of CaCO3/TiO2 composite particles in mild aqueous conditions and potentially can be extended to any type of nanoparticles incompatible with CaCO3 in an aqueous medium. The P25 anatase + rutile nanoparticles were found to be the most suitable for CaCO3/TiO2 composite preparation. The loading efficiency of P25 particles was found to be higher than that of anatase that may be associated with less sedimentation stability of anatase particles in aqueous solution. The EDXS analysis demonstrated that the nanoparticles are mostly located in the surface layer with an average penetration depth of about 100 nm.

The CaCO3/TiO2 composite particles may be employed as template cores for the preparation of photoresponsive polyelectrolyte microcapsules. The inclusion of TiO2 nanoparticles effectively promotes the photodegradation of rhodamine B fluorescent dye into the capsule microvolume and TRITC fluorescent label adsorbed onto the surface bilayer of the capsule shell. Noticeably, the relative change of the concentration of the degradable fluorescent agent is higher for the capsules loaded with P25 anatase + rutile particles rather than anatase-loaded capsules because of the higher photocatalytic activity of anatase with rutile inclusions. Additionally, multifunctional photocatalytic and magnetically responsive microcapsules were prepared by iterative loading of TiO2 and Fe3O4 nanoparticles to the CaCO3 template. The capsules exhibit photocatalytic properties and can be easily collected by the magnetic field for further recovery and regeneration of the catalyst.

The reported data demonstrate the potential of TiO2-loaded capsules prepared with the FIL method for photoinduced degradation of organic molecules, bacteria, and other water pollutants either diffusing into the capsule volume or attaching to their surface. Additionally, FIL allows for multiple loading of various functional materials and, thus, the application field may be extended with further capsule modification and preparation of multifunctional containers, which was demonstrated by the addition of magnetic nanoparticles. We believe, these capsules are promising for water treatment applications and antifouling materials.

Experimental Section

Materials

Calcium chloride dihydrate, anhydrous sodium carbonate, sodium chloride, iron(II) chloride tetrahydrate, iron(III) chloride hexahydrate, sodium hydroxide, citric acid, hydrochloric acid (HCl), PAH (Mw = 70 kDa), PSS (Mw = 70 kDa), polyallylamine hydrochloride labeled with tetramethylrhodamine isothiocyanate (TRITC–PAH, Mw = 70 kDa), rhodamine B (Rho B), anatase TiO2 polymorph with the mean particle size of 25 nm, and Aeroxide P25 anatase + rutile nanoparticles (composition of anatase and rutile TiO2 polymorph with the mean particles size of 21 nm) were purchased from Sigma-Aldrich. All chemicals were used as received without further purification. Deionized (DI) water (specific resistivity higher than 18.2 MΩ·cm) from a Milli-Q plus 185 (Millipore) water purification system was used to make all solutions.

Synthesis of CaCO3 Microparticles

Spherical porous CaCO3 microparticles were synthesized as described by Volodkin et al.[45] In particular, 0.615 mL of 1 M CaCl2 and 0.615 mL of 1 M Na2CO3 solutions were injected into 2.5 mL of DI water under vigorous stirring. After 1 min, the stirring was stopped and the resulted suspension of particles was separated by centrifugation and washed two times with DI water. The resulted microparticles had a spherical shape with an average size of 2.8 ± 0.4 μm.

Synthesis of Magnetite Nanoparticles

Magnetite Fe3O4 nanoparticles (MNPs) were prepared by coprecipitation of Fe2+ and Fe3+ salts in the alkaline medium as described by Massart.[46] A homemade automated reactor setup was employed for nanoparticle synthesis.[47] This setup provides precise remote control over delivered components in the reaction chamber and preserves the inert atmosphere during synthesis. As-prepared nanoparticles were stabilized with 0.1 M citric acid and dialyzed in DI water for 4 days at room temperature. The concentration of the resulted colloid was found to be 0.98 mg/mL as measured by the colorimetric titration which is based on the reaction of iron(III) ions with SCN– and ferric rhodanidecomplexes formation. The average size of the nanoparticles measured by the DLS method was 11 ± 3 nm.

Preparation of CaCO3/TiO2 and CaCO3/TiO2/MNPs Composite Microparticles with FIL

To prepare CaCO3/TiO2 composites, 2 mL of an aqueous suspension of TiO2 nanoparticles with different concentrations (0.5 and 1 mg/mL) was added to 40 mg of CaCO3 cores. The mixture was placed in a freezer (−20 °C) in a continuously rotating flask for 2 h to ensure the water is completely frozen. After freezing, the samples were thawed at room temperature and rinsed with DI water. Then, the samples were dried (60 °C) or were sent to a repeated freezing/thawing procedure (a loading cycle). The loading was carried out for one and two cycles for each type of TiO2 nanoparticles. Two groups of composite CaCO3/TiO2 cores loaded with each type of TiO2 particles were prepared for this study as listed in Table , where A1–A4 are the composites loaded with anatase nanoparticles, whereas B1–B4 the ones loaded with P25 anatase + rutile nanoparticles.

To prepare CaCO3/TiO2/Fe3O4 composites, B3 samples were additionally loaded with MNPs by FIL as well. To do this, 2 mL of Fe3O4 nanoparticle suspension was added to CaCO3/TiO2 composite particles and the mixture was again placed in a freezer for 2 h, thawed, rinsed with DI water, and dried (60 °C).

CaCO3/TiO2 composites were also prepared by coprecipitation[27] and adsorption from solution methods[48] for comparison with FIL. For coprecipitation, 0.615 mL of 1 M CaCl2 and 0.615 mL of 1 M Na2CO3 solutions were injected into 2.5 mL of titanium dioxide aqueous suspension with the concentration of 1 mg/mL under vigorous stirring. After 1 min, the stirring was stopped and the resulted suspension of particles was separated by centrifugation and washed two times with DI water. To obtain CaCO3/TiO2 composites by the adsorption method, 40 mg of CaCO3 microparticles were suspended in 2 mL of titanium dioxide aqueous suspension with a concentration of 1 mg/mL. After 40 min of incubation, the particles were washed three times with DI water. After washing all types of particles were dried.

Synthesis of Polyelectrolyte Composite Microcapsules

Synthesis of polyelectrolyte composite microcapsules was carried out according to Sukhorukov et al.[49] Alternating adsorption of polyelectrolytes (PAH and PSS, 1 mg/mL in 0.15 M NaCl) onto the CaCO3/TiO2 microparticles was carried out for 15 min with each layer followed by three DI rinsing/centrifugation steps (1 min, 1500 rcf). After assembly of four PAH/PSS bilayers, the CaCO3 was dissolved with 0.1 M HCl, which resulted in the formation of composite PAH/PSS capsules loaded with TiO2 nanoparticles. The HCl solution can be safely used to remove CaCO3 because of the absence of the specific sorption of Cl– ions onto the surface of TiO2 particles.[30] In the same manner, the shell was assembled onto CaCO3/TiO2/Fe3O4 cores. The control samples were assembled on regular CaCO3 cores with the same shell structure. To study the photodegradation on the capsule surface, the PAH in the finishing bilayer was substituted for the TRITC–PAH deposited from 1 mg/mL in a 0.15 M NaCl solution.

Hydrodynamic Diameter of Nanoparticles

The hydrodynamic diameter of nanoparticles was measured by DLS with a Zetasizer Nano ZS analyzer (Malvern Instruments Co, United Kingdom).

Stability of Aqueous Suspensions

The stability of aqueous suspensions was estimated with respect to a sedimentation ratio measured with the homemade setup analogous to the one described in ref (21). In particular, a quartz cuvette was filled with 3 mL of aqueous nanoparticle suspension with a concentration of 1 mg/mL. The measured pH value was 6.2 for anatase and 6.5 for P25 anatase + rutile particle suspensions. The cuvette was placed between the laser diode and photodetector in a way that the laser light passed through the cuvette parallel to the air/water interface. The device measured the time dependence of the suspension transparency increasing along with the particle sedimentation. Prior to measurement, the nanoparticle suspensions were redispersed with a horn ultrasound (Bandelin, Germany) for 1 min. The ultrasound source operates at 20 kHz with a power density of 1 W/cm2.

Surface Morphology (SEM) and Elemental Analysis (EDX)

Surface morphology (SEM) and elemental analysis (EDX) were performed with a MIRA II LMU scanning electron microscope (Tescan, Brno, Czech Republic) equipped with the INCA Energy 350 energy-dispersive microanalysis system.

(TEM) and (STEM)

Transmission (TEM) and scanning/transmission (STEM) electron microscopy imaging was performed with a Titan 80-300 TEM/STEM (FEI, USA) electron microscope equipped with a Schottky field emission gun, spherical aberration corrector (Cs probe corrector) and EDXS (EDAX, USA).

Absorption Spectra

The absorption spectra of TiO2 suspensions were measured with a Shimadzu UV-1800 spectrophotometer (Shimadzu, Japan) with an acquisition step of 1 nm. The measured UV–vis absorption spectra were further processed to estimate the band gap of anatase and P25 anatase + rutile particles by Tauc plot of (hνα)2 versus (hν) and extrapolation of linear plot region to the energy axis.[50] α is the absorption coefficient that is calculated from the measured absorbance A as α = 2.303A/d, where d is the path length of the cuvette (1 cm).[51]

Confocal Laser Scanning Microscopy

CLSM images were obtained with a Leica TCS SP8 X (Leica, Germany) microscope equipped with immersion objective ×100 (numerical aperture of 1.44). A 552-nm laser was used to excite the TRITC–PAH fluorescence.

Photocatalytic Properties of TiO2 Loaded Microcapsules

The photocatalytic properties of TiO2-loaded microcapsules were evaluated under UV light of Camelion LH9-U 9W lamp (λ = 367 nm, the spectrum is given in Figure S1 in the Supporting Information). To do this, the capsules were mixed with Rho B solution (0.01 mg/mL) and dispensed in a 96-well plate that was constantly shacked by a mini-shaker with a speed of 260 rpm. The lamp was set 5 cm above the irradiated surface, whereas the irradiated area of the single well was 38.5 mm2. The absorption spectra of Rho B and the capsule mixture were measured using the Synergy H1 Multi-Mode Reader UV–vis spectrometer (BioTek Instruments, Inc., USA). The data were collected before UV exposure and after every 30 min of irradiation, whereas the total irradiation time was 3 h. The absorption spectra were measured from 480 to 620 nm range with a 1 nm step. The same experiment was carried out with aqueous suspension of TRITC-labeled capsules.

Afterward, the obtained data were processed with the standard data processing software and were presented as dependences of the relative absorbance change (ΔA/A) on the irradiation time. The time dependence of ΔA/A was derived as the ratio of the absorbance intensity change measured before and after irradiation to the initial absorbance intensitywhere A is absorbance measured before UV exposure, whereas Aafter is that measured after UV exposure, Δc/c is a relative change of the degradable fluorescent agent concentration. The values were figured out as an absolute peak magnitude at 560 nm. According to the Beer–Lambert law, the absorbance is directly related to the concentration (except extremely low or high analyte concentrations), and, therefore, the measured time dependence of ΔA/A can be associated with the relative change of the concentration Δc/c of the degradable fluorescent agent.

Additionally, the percentage of degradation of Rho B was figured out. To do this, the Rho B standard plot was measured first. Afterward, the corresponding decrease in Rho B concentration depending on the UV irradiation time was plotted.