Phthalocyanine-Grafted Titania Nanoparticles for Photodegradation of Ibuprofen

Abstract

The natural environment is constantly under threat from man-made pollution. More and more pharmaceuticals are recognized as emerging pollutants due to their growing concentration in the environment. One such chemical is ibuprofen which has been detected in processed sewage. The ineffectiveness of water methods treatment currently used raises the need for new remediation techniques, one of such is photodegradation of pollutants. In the present study, zinc(II) and copper(II) phthalocyanines were grafted onto pure anatase TiO₂ nanoparticles (5 and 15 nm) to form photocatalysts for photodecomposition of ibuprofen in water. The nanoparticles were subjected to physicochemical characterization, including: thermogravimetric analysis, X-ray powder diffraction, X-ray photoelectron spectroscopy, Brunauer–Emmett–Teller surface area analysis and particle size measurements. In addition, they were assessed by means of electron spin resonance spectroscopy to evaluate the free radical generation. The materials were also tested for their photocatalytic activity under either UV (365 nm) or visible light (665 nm) irradiation. After 6 h of irradiation, almost complete removal of ibuprofen under UV light was observed, as assessed by liquid chromatography coupled to mass spectrometry. The reaction kinetics calculations revealed that the copper(II) phthalocyanine-containing nanoparticles were acting at a faster rate than those with zinc(II) derivative. The solutions after the photoremediation experiments were subjected to Microtox^® acute toxicity analysis.

1. Introduction

The natural environment all around the globe is constantly deteriorating due to human activity. The wholesale loss of animal species is associated with environmental damage as a result of decreasing natural habitats, an increase in air and seas temperatures, as well as change in ecosystems caused by alarmingly increasing concentrations of chemicals [1,2]. With the constant development of our civilization and growing industrialization, we are facing new problems within our environment. They concern an improper utilization of chemical contaminants, including industrial wastes, pharmaceuticals and cosmetics [3]. Many substances end up in the natural environment due to thoughtless disposal of wastes or abuse of drugs. Now labeled as emerging contaminants, they often pose a threat to the environment due to their inherent toxicity to plants and animals. When deposited in the ground, these wastes can travel and end up in water reservoirs spreading toxicity and endangering human health. Furthermore, this is an issue of grave importance given that technology employed in contemporary water treatment facilities cannot effectively remove all of the substances belonging to this broad group [4].

As previously stated, drugs and drug-derived molecules constitute a group of compounds that are considered an emerging threat to the environment [5]. Traces of many pharmaceutically and biologically active compounds are often reported in the environment as the result of their manufacturing, use and improper disposal (Figure 1) [6,7]. Some pharmaceuticals undergo degradation in wastewaters and therefore do not pose any human health or environmental threat but some compounds can remain unchanged for a significant time [8]. Pharmaceuticals and their metabolites escape into wastewater mostly through the domestic, hospital or veterinary use, the pharmaceutical industry or animal farming [8,9,10,11,12,13]. Since many sewage treatment plants do not possess tools to effectively remove these compounds, they remain in the effluents and, eventually, their traces can be found in drinking water [14]. One of such compounds is a non-steroid anti-inflammatory drug–ibuprofen, dispensed over the counter in the pharmacy, which is not removed entirely from the waste-water in water-treatment plants [4]. Considerable adverse effects on human health resulting from exposure to very low levels of pharmaceuticals in drinking water are very unlikely, according to the World Health Organization (WHO) report from 2011 [15]. However, the report highlighted a few issues, which remain unsolved until this day, one of which being knowledge gaps associated with long-term, low-level exposure to pharmaceuticals. Little is also known about the combined effects of their mixtures, especially for sensitive subpopulations. It is noteworthy that global spending on medicines has been growing in recent years and is predicted to exceed $1.5 trillion by 2023 [16]. This is one of the main reasons why new technologies of remediation are being pursued.

Although the WHO report provided recommendations on managing such concerns [15], technological solutions for wastewater, which would guarantee complete removal of all residues, are not yet in place [13]. Conventional wastewater treatments are often not effective enough, for example due to the low biodegradability of many pharmaceutical compounds. Therefore, attention more recently has been turned to alternative approaches, which include bioremediation, advanced oxidation processes (AOPs), ultrasonic treatment or adsorption techniques [5,17]. Among other AOPs, which employ the intermediacy of highly reactive species in order to decay the pollutant [7], photoremediation seems to be a promising method, especially when titanium(IV) oxide (titanium dioxide, TiO₂) is used as the photocatalyst [18]. Various studies have already assessed this way of decomposing ibuprofen (IBU) in water solutions [4,19,20,21]. There is also evidence that TiO₂ samples impregnated with photosensitizers, for example, porphyrins and phthalocyanine derivatives, enhance the degradation efficiency of organic pollutants [22].

Photoremediation is a relatively cheap and efficient method for removal of chemical contaminants. The process uses photocatalytic degradation of chemicals by exposing the photosensitizer to UV-Vis light. The photosensitizer absorbs energy from the light, which can then be transferred to the molecules present in the reaction environment. The released energy can interact with oxygen producing reactive oxygen species such as superoxides, singlet oxygen or hydroxyl radicals [23]. Metal oxides, such as zinc(II) oxide and titanium(IV) oxide, belong to chemical compounds that have so far been most commonly applied as photosensitizers in various photoremediation studies [24]. TiO₂ has been used for the degradation of pharmaceuticals belonging to various groups, including antibiotics, analgesics, anticonvulsants, β-blockers, lipid regulators, non-steroid anti-inflammatory drugs, psychiatric drugs [25]. The results of broad studies are promising, with near 100% removal efficiency of pollutants in many cases being achieved. However, it is worth noting that in most experiments, the UV light, which is not an optimal source of light in water solutions, was applied [26]. This is due to the TiO₂ band gap that only allows absorption of light below 387 nm. Ideally, for water remediation purposes one would require a material that is active under exposure to visible light, which is mainly dictated by its cost-effectiveness. Functionalization of TiO₂ surface can significantly improve its physicochemical properties. One way of such functionalization is grafting the surface with macrocyclic porphyrinoid compounds like porphyrazines and phthalocyanines (Pc). These compounds reveal a broad excitation band that includes a visible light region. Additionally, the periphery of porphyrinoids can be modified, which even further enhances the desired characteristics [27]. By depositing porphyrinoid compounds on TiO₂ surface it was possible to obtain hybrid materials that can be excited by the visible light [28].

In this work, we present the results of our studies where nanometric size TiO₂ particles were functionalized with copper(II) and zinc(II) phthalocyanines (CuPc and ZnPc, respectively). The obtained materials were characterized in terms of their physicochemical properties, using particle size distribution, X-ray powder diffraction (XRPD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), Brunauer–Emmett–Teller (BET) surface area analysis and electron spin resonance (ESR) spectroscopy, as well as prospective utility for water remediation. Therefore, the TiO₂-Pc composites were assessed for their photocatalytic activity towards photodegradation of ibuprofen, which is a common pharmaceutical water pollutant. The decomposition was monitored by liquid chromatography coupled to mass spectrometry (LC-MS/MS) and the acute toxicity of the resulting solutions was evaluated using Microtox^®.

2. Results and Discussion

2.1. Preparation and Characterization of the Nanoparticles

The prepared materials were based on commercially available anatase phase titanium(IV) oxide nanoparticles. The choice of this crystalline phase was based firstly on the highest photoactivity of the anatase and secondly on the best stability of anatase nanoparticles in such a small diameter [28,29,30]. The nanoparticles (NPs) were prepared using the Pc : TiO₂ ratio of 1:100, based on the optimal activity observed for similar materials [31]. The researched phthalocyanines were chosen according to their high photoactivity—ZnPc [32,33] and earlier reported effectiveness in photoremediation—CuPc [22,31,34]. Prepared nanomaterials, based on 5 nm and 15 nm TiO₂ grafted with zinc(II) or copper(II) phthalocyanine, are summarized in

Table 1. Hybrid TiO₂-Pc materials applied in the study.

TiO2 Nanoparticle Size	Pc, Pc : TiO2 Ratio (m/m)	Symbol of the Material
5 nm	CuPc, 1 : 100	1%CuPc@TiO2(5 nm)
5 nm	ZnPc, 1 : 100	1%ZnPc@TiO2(5 nm)
15 nm	CuPc, 1 : 100	1%CuPc@TiO2(15 nm)
15 nm	ZnPc, 1 : 100	1%ZnPc@TiO2(15 nm)

.

All the prepared materials, along with neat TiO₂ NPs, were first subjected to physicochemical characterization to confirm later their potential usefulness for photoremediation. The applied techniques included: TGA, XRPD, particle size and BET surface area measurements, as well as ESR.

2.2. Thermoanalysis

The determination of the phase composition of Pc-grafted TiO₂ photocatalysts was based on thermogravimetry results by analyzing the loss of mass (%), as well as the rate of mass loss during heating. In the case of unmodified commercial TiO₂ (anatase), two distinct stages were observed. The first (up to 120 °C) was associated with water desorption (2.1%) [35]. In the second stage (temperature range of 120 °C-500 °C), which was accompanied by approx. 3.71% weight loss, there was a release of chemically bound water molecules observed. Water molecules in this step were formed as a result of the condensation of the surface hydroxyl groups. The obtained results are consistent with the literature data. Many authors emphasize that the temperature range of 120-500 °C is suitable for separating physically adsorbed and chemically bound water molecules [35]. It is assumed that as a result of further heating to 900 °C, the TiO₂ surface becomes free of OH surface groups.

In the thermograms of the two analyzed phthalocyanines–ZnPc and CuPc–a similar effect can be observed–heating to 120 °C resulted in approx. 2.6% loss of adsorbed water. However, further heating to 670 °C, resulted in rapid decompositions of the Pcs, which were accompanied by about 62% weight loss. After exceeding this temperature, reductions in the decomposition rates (to 900 °C) were observed.

Titanium(IV) oxide composites with ZnPc and CuPc deposited by surface impregnation were also subjected to TGA. Samples containing commercial TiO₂ with a particle size of 5 nm and 15 nm were analyzed. Quantitative evaluation of the obtained thermograms showed that heating in the temperature range of 20-120 °C resulted in approx. 3% weight loss caused by the desorption of adsorbed water. However, as a result of further heating, approx. 5% weight loss was observed, which could be associated with the condensation of surface hydroxyl groups (120-500 °C) accompanied by water release and the decomposition of phthalocyanine (300-500 °C). It was found that the smaller 5 nm particles expressed a higher loss of mass during the analysis than their 15 nm counterparts, regardless of their functionalization. This fact could be attributed to the higher surface area of 5 nm particles and would further translate to the number of hydroxyl groups present at the surface that could be thermally condensed with water release. However, no regularities were found when comparing bare titania nanoparticles to ZnPc or CuPc grafted materials, regardless of their sizes.

Exemplary thermograms of TiO₂ nanocomposites with Pcs are presented in Figure 2.

2.3. X-ray Powder Diffraction Analysis

All the prepared materials were subjected to XRPD experiments. As presented in Figure 3, there is no difference between the diffraction patterns of neat TiO₂ nanoparticles and NPs grafted with either ZnPc or CuPc. It is consistent with the literature reports since it is assumed that in such composites, macrocyclic compounds occur as amorphous substances dispersed on TiO₂ surface [32,35,36]. The reflections observed in the diffractograms of 1%Cu/1%ZnPc@TiO₂ correspond to the anatase phase, suggesting that the Pc deposition method used does not change the polymorph of titanium dioxide. In diffraction patterns of 5 nm TiO₂ materials a decrease in peak intensity and broadening of peaks in comparison to 15 nm TiO₂ materials is observed. This finding can be related to the smaller size of crystallites [37]. Calculation of the crystallite sizes using Scherrer equation showed no differences between the initial bare titania nanoparticles and the final materials prepared from the respective size particles.

2.4. XPS

All the prepared materials were subjected to X-ray photoelectron spectroscopy experiments to detect the binding energy on the bare TiO₂ and TiO₂-phthalocyanine catalysts. The obtained spectra are collected in Figure 4. In the spectra of all the materials, signals from Ti, O and C can be observed, as labeled in Figure 4a. The drop in intensity of the peaks is visible in the 15 nm Pc@TiO₂ materials (see Figure 4d–f) in comparison to 5 nm Pc@TiO₂ (see Figure 4a–c), which is related to their lower surface area. Most notably, due to lack of covalent bonding, there were no noticeable shifts found when comparing the spectra of bare TiO₂ and Pc@TiO₂. The probable mode of binding of the phthalocyanines to titanium dioxide nanoparticles is via the oxygen atoms of TiO₂, either through hydrogen bonding or coordination of the Pc by the oxygen lone electron pairs [38]. Therefore, in the spectra, the presence of O 1s signals at 530.75 eV and 530.91 eV for a bare TiO₂ (5 nm) and both Pc-grafted TiO₂ (5 nm), respectively, were noted. The presence of the low-intensity C 1s carbon signal for bare TiO₂ samples was attributed to the adsorption of airborne CO₂ on their surface [39]. The significant increase of C 1s intensity is observed in the spectra of Pc-grafted TiO₂ materials. The lack of nitrogen, as well as Zn and Cu signals in the spectra, is caused by the very low content of these elements in the studied 1%Pc@TiO₂ materials [40].

2.5. Particle Size

The size of the particles was acquired using nanoparticle tracking analysis (NTA). The results are summarized in

Table 2. Particle size measured for TiO₂ based materials.

Material	Particle Size	Polydispersity Index a
TiO2 (5 nm)	145.7 ± 68.4 nm	0.22
1%CuPc@TiO2 (5 nm)	103.5 ± 87.8 nm	0.72
1%ZnPc@TiO2 (5 nm)	158.7 ± 74.4 nm	0.22
TiO2 (15 nm)	201.6 ± 139.8 nm	0.48
1%CuPc@TiO2 (15 nm)	144.5 ± 129.1 nm	0.80
1%ZnPc@TiO2 (15 nm)	236.2 ± 128.1 nm	0.29

^a–calculated according to the formula PDI = (SD/mean diameter)² [42].

. The obtained particle sizes reveal that the materials were prone to agglomeration, which is strongly marked. The example size distribution of the NPs is shown in Figure 5. As a result of the agglomeration, none of the particles can be regarded as monodisperse. The polydispersity index of all the materials exceeds 0.2 [41].

2.6. Electron Spin Resonance Spectroscopy

To check the effect of exposure on free radical generation in samples, the materials were also studied by ESR method. The exemplary ESR spectra are shown in Figure 6.

For both types of samples, containing either zinc(II) or copper(II) phthalocyanine, a single line is visible. Typical spectroscopic parameters were determined: for zinc(II) phthalocyanine (1%ZnPc@TiO₂) g=2.0032, ΔH=0.75 mT and for copper(II) phthalocyanine (1%CuPc@TiO₂) g=2.0506, ΔH=4.67 mT. After the exposure to 665 nm light irradiation, the signal intensity changed, which means that the free radicals were generated. It seems that these changes depend on the time the material was irradiated: after 15 min the signal rapidly decreases and after another 15 min it increases but is still lower than non-irradiated sample (Figure 6a). Such changes were not observed for pure ZnPc, for which the signal intensity was stable (Figure 6b). In the case of differently sized TiO₂ nanoparticles it was observed that the non-irradiated sample with the particle size of 5 nm gave a very weak, trace signal. In contrast, the measurements of the sample of the larger particle size resulted in a complex signal, which decreased significantly after exposure. The observed changes in signal intensity are probably associated with the surface defects that are likely to promote the rapid recombination of generated free radicals. A thorough understanding of these changes and mechanisms that take place requires further study.

2.7. BET Surface Area Analysis

BET surface area analyses were performed for all the studied materials to assess whether the modification of titania with the phthalocyanines revealed any effect on the materials’ surface. The results are summarized in

Table 3. Brunauer–Emmett–Teller (BET) surface area measured for the materials prepared.

Material	BET Surface Area [m2/g]
TiO2 (5 nm)	264.8 ± 1.2
1%CuPc@TiO2 (5 nm)	249.7 ± 1.5
1%ZnPc@TiO2 (5 nm)	244.3 ± 1.5
TiO2 (15 nm)	104.6 ± 0.3
1%CuPc@TiO2 (15 nm)	96.8 ± 0.3
1%ZnPc@TiO2 (15 nm)	100.4 ± 0.3

.

The difference found in the surface areas between 5 nm and 15 nm materials is evident. The 5 nm nanoparticles exhibit a surface area of around 250 m²/g, while the 15 nm ones present values close to 100 m²/g. By comparing the data, the methodology used for the modification of both TiO₂ materials with 1%CuPc and 1%ZnPc caused small decreases in the materials’ surfaces, resulting from two factors. Firstly, the presence of phthalocyanines in titania nanoparticles pores. Secondly, the fact that the combination of nanoparticles with macrocycles can lead to an agglomeration of the received materials.

2.8. Photochemical Studies

2.8.1. Photodegradation of Ibuprofen

Ibuprofen is one of the most commonly used drugs in the world, for many years included in the WHO Model List of Essential Medicines [43]. Similar to other nonsteroidal anti-inflammatory drugs, it is used to treat pain, inflammation and to reduce fever both in adult and pediatric patients [44]. As an over-the-counter (OTC) drug with many indications, it is often taken without medical supervision [45]. Recent advances in analytical technology facilitate the detection of pharmaceutical compounds. The presence of ibuprofen in both wastewater influents and effluents has already been reported by numerous researchers [6,46,47,48]. The removal of ibuprofen or its metabolites in the course of the drinking water treatment process is not always efficient [49]. This raises concerns over potential risks to human health and aquatic ecosystems from exposure to ibuprofen traces in drinking water. The above considerations demonstrate that a modern, efficient technology ought to be applied for the removal of pharmaceuticals from water.

The photocatalytic systems based on Pc-TiO₂ or TiO₂ nanoparticle suspension as photocatalysts were used for the assessment of ibuprofen removal from water via photodegradation process. To assess the photodegradation efficiency of both nanomaterials, IBU removal using 1%CuPc@TiO₂ and 1%ZnPc@TiO₂ with TiO₂ alone was compared. It has been previously observed that using pure TiO₂ it is possible to almost completely remove IBU in 2–3 h [50,51,52,53]. Furthermore, an approach was previously undertaken, where commercially available P25 TiO₂ nanoparticles (~21 nm) were used in combination with UV light to successfully degrade IBU [54]. What is more, the authors managed to identify a series of resulting IBU decomposition products. However, the authors did not use TiO₂ based visible-light photocatalysts, only a neat P25 TiO₂ and UV irradiation. It is worth noting, that the use of a sensitizing moiety was presented by Patterson et al., who used natural plant extracts to broaden the absorption spectrum of P25 TiO₂ [55]. The composites were used to form thin films and natural sunlight was tested as the light source, which allowed to double the efficacy of the photodegradation of IBU with neat P25 TiO₂. In the herein presented study the solution of IBU in water was irradiated with the photocatalyst and accompanied by careful monitoring of the IBU concentration. The results are summarized in Figure 7 and Figure 8.

In contrast to the literature findings [53], after 2 h of our experiment, the levels of IBU were around 60% when referred to the IBU level at t₀. In the case of UV-light driven photocatalytic degradation with 1%CuPc@TiO₂ and pure TiO₂, almost complete removal of IBU was observed after six hours of our experiment. The use of the 665 nm red light did not cause any change in the IBU content in the tested experiments. Although one would expect such outcome in the case of neat TiO₂ nanoparticles, ZnPc and CuPc grafted materials also did not yield any effect in these conditions despite the fact that both these phthalocyanines absorb the light at this wavelength. This is in contrast to the earlier literature reports, where ZnPc deposited on P25 TiO₂ in an analogical manner was found photocatalytically active and enabled photodegradation of erythromycin [32]. Nevertheless, there are certain differences between the herein presented and the literature procedures. In our study the calcination step, following the Pc deposition on TiO₂ nanoparticles, was omitted. It is believed that this step irrevocably leads to the decomposition of the phthalocyanine, as proven by the TGA measurements. Presumably, the material obtained before [32] was composed of P25 TiO₂ doped with C, N or Zn on its surface with some residual phthalocyanine. Herein, in the presented study, probably due to the non-stabilized nature of the tested nanoparticles, only a portion of grafted macrocycles was present on the surface and could come in close proximity of IBU for the photocatalytic reaction to occur.

Although the factors mentioned above may have had the greatest impact on the result of the experiment, other differences will also be briefly discussed below. Factors such as the size and load of TiO₂ particles, the initial IBU concentration, temperature were found to influence the photodegradation of IBU in aqueous solutions and have been explored in several studies [51]. Throughout our experiment the temperature did not exceed 35 °C, which seems to be consistent with other photocatalytic degradation research studies. As mentioned before, prior studies have also indicated the influence of the initial drug concentration on the process. The observed efficiency was usually increasing with the decrease of the initial concentration of IBU [51,56]. Additionally, the effect of the catalyst load has been reported in the literature. Several reports have shown that the optimal load of the suspended TiO₂ is around 1000 mg/L, however, these values typically varied between 50-3000 mg/L [50,51,54,56,57]. Consistent with the literature, the initial IBU concentration in our study was 10 mg/L, whereas the TiO₂ load was 100 mg/0.1 L. Hence, these parameters were expected to reveal similar degradation rates at similar sampling times. Another major factor, which could not be omitted, is the average particle size. In our experiment nano-grade TiO₂ was used. Nano-scale particles have a higher surface-to-volume ratio than microparticles and thanks to the larger surface available for photocatalytic reactions, they are expected to be more efficient in the photodegradation studies. However, in the present study, the NPs tended to form agglomerates. Therefore, the access of IBU to the surface of the photocatalyst might have been limited, as it was suggested in other studies [51,58].

In the case of the UV irradiation experiments, another interesting fact was observed. Namely, the neat TiO₂ was photocatalytically active to a similar extent as CuPc grafted titania, regardless of the nanoparticle size and the surface area presented by the nanomaterial. On the opposite, deposition of ZnPc on titanium(IV) oxide resulted in the decrease of IBU degradation rate in the case of both, 5 nm and 15 nm particles. This effect could be explained by the absorption of light by ZnPc deposited on the surface of TiO₂ with the formation of Pc-derived reactive oxygen species, in particular very short-living singlet oxygen molecules. ZnPc is known to produce singlet oxygen, which would probably be dissipated before reaching IBU molecules [59]. In the case of CuPc, because of lower singlet oxygen generation quantum yields, the energy absorbed by the phthalocyanine had to be transferred to TiO₂. In addition, the same results obtained for photocatalysts, regardless of the 5 or 15 nm particle size used, can be associated with the agglomeration of the non-stabilized nanoparticles [60]. As can be observed in Table 2., the measured mean sizes of the materials are much higher than those obtained for neat TiO₂ nanoparticles and the wide-spread of the results is revealed by the high values of standard deviations. All these facts suggest that stabilization of the nanoparticles is necessary for the potential photocatalytic material.

2.8.2. Calculation of Photodegradation Constants for Various Conditions Applied

The kinetic parameters (degradation rate constants k of IBU decay) were calculated for experiments with UV light irradiation and for the following series of photocatalysts: 1%ZnPc@TiO₂(5 nm), 1%ZnPc@TiO₂(15 nm), 1%CuPc@TiO₂(5 nm) 1%CuPc@TiO₂(15 nm), TiO₂(5 nm) and TiO₂(15 nm).

The results are depicted in

Table 4. The regression and kinetic parameters of IBU degradation rate constants (k, s⁻¹) under UV irradiation with 1%ZnPc@TiO₂ (5 nm), 1%ZnPc@TiO₂ (15 nm), 1%CuPc@TiO₂ (5 nm) 1%CuPc@TiO₂ (15 nm), TiO₂ (5 nm) and TiO₂ (15 nm) as a photocatalysts.

Samples	Regression Parameters a		Kinetic Parameters [k (s−1)] b
1%ZnPc@TiO2(5 nm)	a	−0.1214	k	2.024 × 10−3	s−1
	Δα	0.0303	Δκ	5.051 × 10−4	s−1
	b = y(0)	1 504 745.2
	Δβ	1.098
	sa	0.0117
	sb	0.0364
	sy	0.0611
	r	−0.982
1%ZnPc@TiO2(15 nm)	a	−0.1548	k	2.580 × 10−3	s−1
	Δα	0.0136	Δκ	2.279 × 10−4	s−1
	b = y(0)	912 073.1
	Δβ	1.043
	sa	0.00531
	sb	0.0164
	sy	0.02757
	r	−0.998
1%CuPc@TiO2(5 nm)	a	−0.3832	k	6.387 × 10−3	s−1
	Δα	0.0601	Δκ	1.002 × 10−3	s−1
	b = y(0)	1 160 993.9
	Δβ	1.204
	sa	0.0233
	sb	0.0722
	sy	0.121
	r	−0.993
1%CuPc@TiO2(15 nm)	a	−0.3649	k	6.083 × 10−3	s−1
	Δα	0.0548	Δκ	9.149 × 10−4	s−1
	b =y(0)	1 242 276.5
	Δβ	1.184
	sa	0.0213
	sb	0.0659
	sy	0.11
	r	−0.993
TiO2(5 nm)	a	−0.391	k	6.517 × 10−3	s−1
	Δα	0.0292	Δκ	4.876 × 10−4	s−1
	b = y(0)	1 158 957.5
	Δβ	1.0945
	sa	0.011381
	sb	0.0351
	sy	0.0589
	r	−0.998
TiO2(15 nm)	a	−0.359	k	5.988 × 10−3	s−1
	Δα	0.0252	Δκ	4.202 × 10−4	s−1
	b = y(0)	1 111 784.1
	Δβ	1.08
	sa	0.0098
	sb	0.0302
	sy	0.0508
	r	−0.999

^a The parameters: a, sa, b, sb and r were calculated by the least square regression and stand for slope, the standard deviation of the slope, intercept, the standard deviation of intercept and correlation coefficient, respectively. ^b The kinetic parameters k [s⁻¹] is the photodegradation reaction of IBU constant rate.

. and in Figure 9 and Figure 10. The studied reaction was defined as the first-order kinetics and the degradation rate constant was calculated using the formula (1):

ln P_t = ln P₀–k · t,

(1)

where: P_t surface area of the sample in time t [h] in the isothermal test, P₀ is the surface area of the sample in time 0 and k [s⁻¹] is the reaction rate constant. According to the theory of first-order kinetics, the semi-logarithmic plot P_t = f(t) is linear and its slope corresponds to the magnitude of the degradation rate constant k (Figure 9 and Figure 10). Thus, the least-squares method was used to calculate the regression parameters: y = ax + b, a ± Δa, b ± Δb, standard errors S_a, S_b and the correlation coefficient r. The ±Δa, ±Δb were estimated for f = n − 2 degrees of freedom and α = 0.05. Table 4. summarizes the results for each experiment.

2.9. Acute Toxicity Assessment

The solutions of ibuprofen were subjected to ecotoxicity testing using the Microtox^® test. The test is based on Aliivibrio fischeri bacterial cell suspension. The changes in bioluminescence after a sample is added can be directly correlated with the metabolism of the bacteria. So that the luminescence decreases linearly with the sample toxicity increase [61,62]. As can be seen in Figure 11a, the toxicity of the ibuprofen solutions photoremediated with UV light slightly increases in comparison with IBU solutions before the irradiation. Since ibuprofen is oxidized to other products which might express higher toxicity, these results can be associated with degradation of IBU to its photoproducts [63].

As can be seen in Figure 11b, the solutions photocatalyzed with red light by neat TiO₂ nanoparticles exhibit unchanged toxicity to A. fischeri bacteria. In case of phthalocyanine-grafted titania nanoparticles, despite the UV or red light irradiation, the toxicities of both solutions are comparable. A slight ca. 10% toxicity increase of the ibuprofen irradiated solutions after 6 h of the UV light irradiation was noted. In addition, this effect was weaker for Pc@TiO₂(15 nm) than for Pc@TiO₂(5 nm). It indicates that thephotocatalytic activity of anatase is potentiated with the use of proper modification of titania nanoparticles.

3. Materials and Methods

3.1. Materials and Instruments

All reaction mixtures were stirred using Radleys Heat-On™ heating system (Saffron Walden, United Kingdom). Solvents and all reagents were obtained from commercial suppliers (Merck, Darmstadt, Germany; Fluorochem, Hadfield, United Kingdom; Chempur, Piekary Śląskie, Poland; Avantor Performance Materials Poland S.A., Gliwice, Poland) and used without further purification. TiO₂ anatase nanoparticles were purchased from US Research Nanomaterials Inc., Houston, TX, USA.

Mass spectrometry-grade methanol and ammonium acetate were purchased from Sigma–Aldrich (Saint Louis, MO, USA). Mass spectrometry-grade water was prepared by reverse osmosis in a Demiwa system from Watek (Ledec nad Sazavou, Czech Republic), followed by double distillation from a quartz apparatus.

3.2. Preparation of Nanoparticles

TiO₂-Pc nanoparticles were prepared as a modification of chemical deposition method [64]. As an example, for the preparation of 1%ZnPc@TiO₂ material, zinc(II) phthalocyanine (10 mg, 0.017 mmol) was dissolved in analytical grade dichloromethane in a round-bottom flask. Titanium dioxide (1000 mg) of appropriate particle size was added and the suspension was sonicated for 0.5 h and then stirred for 1 hour. After that, the solvent was removed using a rotary evaporator. The resulting blue powder was dried in air for 10 h to remove any traces of the solvent.

For the means of comparison, pure TiO₂ was treated according to the same procedure but without the addition of the macrocycle.

3.3. Nanoparticle Characterization

3.3.1. Thermogravimetric Analysis

Thermogravimetric analysis was performed at the Wielkopolska Center for Advanced Technologies in Poznań, Poland, with the use of a thermogravimetric analyzer (TGA 4000, Perkin Elmer, Kraków, Poland). The analyzed materials were: TiO₂ (5 nm), TiO₂ (15 nm), neat ZnPc, neat CuPc and Pc@TiO₂ composites. Samples of around 5 mg were heated from room temperature to 900 °C at the speed of 10 °C/min. The experiments were conducted in a nitrogen atmosphere. Temperature measurements were accurate up to 10⁻⁵ °C, while the masses with the accuracy of 10⁻⁵ mg.

3.3.2. X-ray Powder Diffraction

X-ray diffraction patterns of the samples were recorded at the Poznan University of Medical Sciences Core Facility on a Bruker AXS D2 Phaser diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) with a Cu Kα anode (λ = 1.54060 Å) operating at 30 kV and 10 mA. The diffraction patterns were collected at ambient temperature over an angular measurement range of 5° to 60° 2θ with a step size of 0.02° and a counting rate of 2 s/step with the sample spinning.

3.3.3. X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy (XPS) experiments were performed under 10⁻⁹ mbar vacuum using SPECS Multimethod system with XPS, SPECS Surface Nano Analysis GmbH (Berlin, Germany). Al anode was used as the source of non-monochromatic X-rays. CasaXPS software was used to analyze the data.

3.3.4. Particle Size

The nanoparticle size distribution was analyzed using Malvern Panalytical (Malvern, United Kingdom) NanoSight LM10 instrument (sCMOS camera, 405 nm laser) using NTA (Nanoparticle Tracking Analysis) 3.2 Dev Build 3.2.16 software. Before the measurements, the NP dispersions were diluted with water to achieve the operating range of nanoparticle concentration. The temperature of the sample chamber was set and maintained at 25.0 ± 0.1 °C and the syringe pump infusion rate was set to 200. For each sample, three movies of 30 seconds in length were recorded.

3.3.5. Electron Spin Resonance Spectroscopy

All samples were studied using ESR spectroscopy. The measurements were carried out on an X-band Bruker EMX-10 spectrometer (Bruker, Billerica, MA, USA) with a magnetic field second modulation frequency of 100 kHz. The ESR spectra were recorded at room temperature (293 K) in two magnetic field ranges: 60 mT or 4 mT. Each sample was measured three times: before exposure, after exposure for 15 min and after subsequent exposure for another 15 min. Typical spectroscopic parameters: g-factor value, peak-to-peak line width (ΔH) and ESR signal intensity (corresponding to the concentration of free radicals in the sample) were determined for ESR spectra.

3.3.6. BET Surface Area Analysis

The Brunauer-Emmet-Teller surface area analysis was performed at the Wielkopolska Center for Advanced Technologies in Poznań, Poland, with the use of ASAP™2420 system (Micromeritics®, Norcross, GA, USA).

3.4. Photochemical Studies

3.4.1. Set-Up of the Photocatalytic Experiment

The source of light in the irradiation experiments was a set of three laser diodes with λ_max = 665 nm for the red light irradiation or λ_max = 365 nm for UV irradiation. Light intensity was set to 20 mW/cm² and monitored with a RD 0.2/2 radiometer (Optel, Wrocław, Poland). Distilled water was used in all experiments.

The photodegradation experiment was carried out in a reactor, which consisted of three beakers placed on a magnetic stirrer in the middle and three UV-Vis lamps with peak emission wavelength of either 365 nm or 665 nm, positioned on a circular line (Figure 12). A layer of aluminum foil was placed behind and above the photoreactor to prevent scattering of light or heat energy and to protect the operator. To each of the beakers containing 100 mL of IBU solution at concentration 10 mg/L, 100 mg of TiO₂-based material was added, sonicated for about 1 minute and stirred vigorously in the dark for 30 min. After that, LED lamps (either UV or Vis) were turned on and the mixtures were irradiated for 6 h while being constantly stirred. Samples (~2 mL) were taken at time-points: -0.5 h, 0 h, 0.5 h, 1.0 h, 2.0 h, 4.0 h, 6.0 h after the irradiation was started. The temperature of the solution during the photocatalytic reaction did not exceed 35 °C.

Collected samples were centrifuged (10000 rpm, 30 min) and the supernatant was filtered through 0.2 µm poly(tetrafluoroethylene) (PTFE) syringe filters. Before the liquid chromatography coupled to mass spectrometry (LC-MS/MS) analysis the samples were diluted 20 times in a methanol/water 1:1 (v/v) mixture.

3.4.2. LC-MS/MS Analysis of the Samples

A chromatographic system UltiMate 3000 RSLC from Dionex (Sunnyvale, CA, USA) coupled with the API 4000 QTRAP triple quadrupole mass spectrometer from AB Sciex (Foster City, CA, USA) was used and 5 µL samples were injected into a Synergi Fusion-RP column (50 mm × 2.0 mm I.D.; 2,5 µm) from Phenomenex (Torrance, CA, USA) maintained at 35 °C. The mobile phase employed in the analysis consisted of 5 × 10⁻³ mol L⁻¹ ammonium acetate in water and methanol at a flow rate of 0.3 mL min⁻¹. Gradient elution was performed by linearly increasing the percentage of organic modifier from 70 to 100% in 2.5 min and then holding it at 100% to 3 min. A pre-run time of 3 min was used before the next injection. The effluent from the LC column was introduced into the electrospray ionization source (Turbo Ion Spray) operated in negative ion mode. The following settings were used for the ion source and mass spectrometer—curtain gas 10 psi, nebulizer gas 40 psi, auxiliary gas 40 psi, temperature 400 °C, ion spray voltage −3500 V, declustering potential −35 V and collision gas set to medium. The dwell time for mass transition detected in the reaction monitoring mode was set to 100 ms. The quantitative transition was from 205 to 161 m/z at collision energy set to −10 eV.

The linearity of the method was tested in a range of 2∓1000 [µg L⁻¹]. The instrumental limit of detection (LOD) and the instrumental limit of quantitation (LOQ) were calculated on the basis of signal to noise (S/N) ratio. The S/N = 3 was used for calculation of LOD and the S/N = 10 for calculation of LOQ. Precision and accuracy were not tested because sample preparation included only a dilution step. Therefore, only the injection precision of the instrument applies in this procedure, which was always below 1%. For ibuprofen LOD = 0.5 [µg L⁻¹], LOQ = 1.7 [µg L⁻¹].

3.5. Microtox^® Acute Toxicity Assessment

The samples of the remediated solutions were tested using Microtox^® acute toxicity test–81.9% Screening Test which was performed using Microtox^® M500 equipment, according to the protocols distributed by the producer (ModernWater plc, London, United Kingdom) [61,65]. Cell viability was calculated according to bioluminescence emitted by the Aliivibrio fischeri bacteria as measured with Microtox^® M500 with Modern Water MicrotoxOmni 4.2 software.

4. Conclusions

To sum up, materials based on pure anatase titania nanoparticles and phthalocyanines were prepared by a chemical deposition method, as well as carefully analyzed by TGA, XRPD, XPS, particle size measurements and ESR. The analytical techniques revealed that the applied TiO₂ modification method was suitable because of having no effect on anatase crystallographic phase. As exhibited in ESR measurements, the materials were able to generate free radicals and their formation changed upon red-light exposure. As the materials were found to agglomerate, proper stabilization of the nanoparticles will be considered in our further studies.

The experiments with ibuprofen aqueous solutions indicated that the best results can be obtained after UV light irradiation. CuPc doped TiO₂ was found to be as effective as neat TiO₂ when exposed to 365 nm light, whilst the ZnPc modified titania was shown to photocatalyze the degradation of ibuprofen at a slower rate. The experiments performed using the modified titania and red light at 665 nm demonstrated the lack of effectiveness of the obtained materials, as well as neat TiO₂, in these conditions. The acute toxicity assessment showed only a slight increase in toxicity of the ibuprofen solutions after the photodegradation experiment in the presence of modified TiO₂ nanoparticles. The results show that the use of such approach does not impede the excellent photocatalytic activity of anatase and it might be potentiated with the use of proper modification of titania nanoparticles in the form of robust molecules preventing agglomeration or the use of surfactants.