halloysite nanotube composites

Journal Pre-proof Effects of molecular aggregation on photostability of protoporphyrin-IX/halloysite nanotube composites José R. Tozoni, Alexandre Marletta, Raigna A. Silva, Erick Piovesan, Kaluana P. de Oliveira, Noélio O. Dantas, Anielle C.A. Silva, Tito J. Bonagamba, PatríciaTargon Campana, Francisco M. Braz Fernandes, Maria Raposo PII:

S0254-0584(19)31414-2

DOI:

https://doi.org/10.1016/j.matchemphys.2019.122604

Reference:

MAC 122604

To appear in:

Materials Chemistry and Physics

Received Date: 4 October 2019 Revised Date:

10 December 2019

Accepted Date: 30 December 2019

Please cite this article as: José.R. Tozoni, A. Marletta, R.A. Silva, E. Piovesan, K.P. de Oliveira, Noé.O. Dantas, A.C.A. Silva, T.J. Bonagamba, PatríTargon. Campana, F.M. Braz Fernandes, M. Raposo, Effects of molecular aggregation on photostability of protoporphyrin-IX/halloysite nanotube composites, Materials Chemistry and Physics (2020), doi: https://doi.org/10.1016/j.matchemphys.2019.122604. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Intensity (a.u.)

Figure abstract 24 PL x Irradiation exposure time (min) 21 (PpIX/HNTs-1) 18 15 12 9 6 3 0 550 600 650 700

0 1 3 5 7 15 30 45 60 75 90 105 120

750

Wavelength (nm) 0.42

Absorbance

0.35

PpIX/HNTs-1 PpIX/HNTs-2 PpIX/HNTs-3 HNTs PpIX0 (solution)

0.28 0.21 0.14 0.07 0.00 200

300

400

500

600

700

800

Wavelength (nm)

1

Effects of molecular aggregation on photostability of protoporphyrin-IX/halloysite nanotube composites José R. Tozoni1, Alexandre Marletta1, Raigna A. Silva1, Erick Piovesan1, Kaluana P. de Oliveira1, Noélio O. Dantas.2, Anielle C. A. Silva2, Tito J. Bonagamba3, PatríciaTargon Campana4, Francisco M. Braz Fernandes5 and Maria Raposo6 1- Instituto de Física, Universidade Federal de Uberlândia, Uberlândia, Minas Gerais, Brasil 2- Universidade Federal de Alagoas, Maceió, Alagoas, Brasil 3- Instituto de Física de São Carlos, Universidade de São Paulo, CP 369, 13560-970, São Carlos, São Paulo, Brazil 4- Escola de Artes, Ciências e Humanidades, Universidade de São Paulo, São Paulo, São Paulo, Brasil 5- CENIMAT, Departamento de Materiais, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal 6- CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia, Universidade Nova de Lisboa, 2829-516 Caparica, Portugal

Abstract This article presents a successfully development of hybrid functional nanomaterials composed of Protoporphyrin-IX/Halloysite nanotubes (PpIX/HNTs), in which the PpIX molecules were adsorbed on the surface of the HNTs at different degree of aggregation. This article also presents a possible explanation for the PpIX adsorption by the HNTs surface and, moreover, the results show that the PpIX molecules adsorption on the HNTs surfaces, and its degree of aggregation, can be controlled by changing the ratio between PpIX and HNTs concentrations. Interestingly, from photodegradation experiments, it is observed that, after the photodegradadion of isolated monomers, the PL spectra and the photodegradation curves show that there is a ratio between the amount of residual monomers and dimers in which the PpIX/HNTs photodegradation decreases significantly, leading to a more photostable system. Therefore, these PpIX/HNTs systems can lead to the development of more efficient porphyrin-based devices and processes, and help to understand the processes of aggregation and formation of porphyrin supramolecular structures. Keywords: hybrid functional nanomaterials, Protoporphyrin-IX/Halloysite nanotubes, aggregation, photostability and photodegradation.

1

1. Introduction The highly conjugated macrocyclic arrangement of porphyrins, composed of four pyrrole rings connected by unsaturated methylene bridges [1-4], simultaneously with the ability of porphyrins to form supramolecular structures [4-8], provides porphyrins systems with electronic, structural, biologic, and functional unique properties [2]. The porphyrins unique properties leads to several possible applications in fields such as: artificial photosynthesis [9], photomedicine [10,11], gas sensors [12], photovoltaic devices [8,13], hybrid materials (metallic-organic) to be used in spintronics [14] and molecular electronics [15], among others. Porphyrins biological, photophysical and photochemical processes are affected by several factors and, among them, the intermolecular interactions is a very important one [3,16-19]. Porphyrin molecules dispersed in a solution or into a solid matrix, can interact with each other in the ground state, forming monomers, dimers, H and J aggregates, highly ordered molecular arrangements, and others [3,16-19]. In both soluble and solid state, the chemical structure of the porphyrin, solvent and porphyrins concentrations, and the pH of the solution have a great influence on the porphyrins aggregation states [3,16-26]. Generally, the aggregation of porphyrin molecules changes the characteristics of electronic states, modifying their electronic energy levels and spectral line shape [3,1726]. Aggregation also influences the energy and charge transfer processes of the porphyrins-based systems, decreasing the lifetime of the singlet and triplet excited states, the emission quantum yield and the generation of Reactive Oxygen Species (ROS) [3,17-26]. On the other hand, in the field of supramolecular chemistry, the aggregation tendency of the porphyrin is a desired and a very important characteristic [5-8,16]. Particularly, amphiphilic porphyrins like the Protoporphyrin-IX (PpIX) [16,20-22,24-26] that has two propionic acid side chains in the lower pyrrole rings, are of great interest since they allow the fabrication of supramolecular structures with highly ordered architectures, which are stabilized by the π-π stacking of porphyrin rings and also by the non-covalent interactions between their hydrophilic substituent [16,22,24-26]. Several studies have been carried out to: develop systems in which porphyrins are at different states of aggregation [7,8,16-26]; identify the major factors that influence in the formation of these aggregates [16]; understand the aggregation effects on the formation of supramolecular structures and on the functional properties of porphyrins-based systems [3-8,16-26]. In this sense, the formation of hybrid inorganic2

organic host-guest systems involving porphyrins or conjugated polymers has been presented as a new window of research [4,27-30] in the areas such as: photoactivity, luminescence, photocatalysis, optical anisotropy, nonlinear optics and drug delivery systems etc. [4,27-30]. Moreover, in these host-guest systems, organic molecules deaggregation could be obtained through adsorption of organic molecules on clay nanoparticles surfaces or through the intercalation of organic molecules in the interlamellar space of the clays sheets [4, 27-30]. Recently, there have been reports on the development of a porphyrin/clay ‘Artificial Light-Harvesting System’, in which the cationic porphyrin molecules are adsorbed on the anionic surface of the clays without aggregation [28]. According to the authors, the non-aggregation of porphyrin molecules is rationalized by size matching the distances between the charged sites in the porphyrin molecule and those between anionic sites on the clay surface. The authors call this effect Size-Matching Rule [28]. In the field of host-guest hybrid inorganic-organic systems, one of the materials that has been widely used as host are the Halloysite nanotubes (HNTs) [31-34]. The HNTs present nanosized dimensions, high mechanical strength, thermal stability, biocompatibility, and abundance, leading to a large number of possible applications [31-34]. Based on the abovementioned settings and on the difficulty to find hybrid systems formed by porphyrins and clays that favor the porphyrins de-aggregation while maintaining their functional, photophysical and photochemical properties [4,28-30], in this article we developed and studied functional photoluminescent hybrid nanocomposites composed of PpIX/HNTs. The present research results show that the simple casting process (PpIX-HNTs-Solvent) is able to produce PpIX/HNTs nanocomposites that still preserve the photophysical and photochemical properties of PpIX, besides promoting the PpIX de-aggregation.

2. Experimental section Materials. Acetonitrile, dimethylsulfoxide, PpIX (562.66 g/mol) and HNTs (294.19 g/mol) were purchased from Sigma-Aldrich. The HNTs molecular formula is Al2Si2O5(OH)4-2 H2O. According to information obtained from the manufacturer HNTs have tubes of 30-70 nm / 1-3 µm diameter/length; typical specific surface area 64 m2/g; pore volume of 1.26-1.34 mL/g; refractive index 1.54; and true specific gravity 2.53 g/cm3. All reagents and materials were used without further purifications. All solutions were prepared using ultrapure water. 3

Analysis. Photophysical characterization of the samples was performed using the following optical techniques: UV-Vis absorbance, steady-state photoluminescence excitation (PLE) and emission spectra (PL). PpIX/HNTs powder and HNTs nanocomposites samples were characterized by UV-Vis absorbance spectra using an integrating sphere coupled to the Shimadzu UV-3600 spectrophotometer. Both the PLE and emission spectra PL were recorded on a Hitachi U-2001 spectrofluorimeter. The photochemical properties of the PpIX/HNTs were also studied using the steady-state photoluminescence techniques as a function of the irradiation time on ambient atmosphere (photodegradation), using an Ocean Optics USB2000 spectrophotometer and a Xenon (450 W) lamp coupled to an IHR320 monochromator (Jobin Yvon). Structural and microstructural characterization techniques such as wide-angle X-ray scattering (WAXD) and scanning electron microscopy (SEM) were also performed using a X-ray diffractometer XRD6000 (Shimadzu) and a scanning electron microscope VEGA3(Tescan), respectively.

2.1.Samples preparation methods. Due to its low solubility in water [20,21,24-26], the PpIX (5.0 mg) was first dissolved in acetonitrile (6.0 mL) and dimethylsulfoxide (3.0 mL) solution. After that, 41.0 mL of ultrapure water was added to the mixture. The concentration of the final PpIX/solvent solution was ~1.77×10-4mol/L, this solution was called PpIX0. Different concentrations of the PpIX0 solutions were mixed with HNTs nanoparticles according to Table 1. To favor mixture homogeneity and adsorption of PpIX on HNTs surfaces, each mixture was mechanically stirred and sonicated twice for 5.0 min. The HNTs have a good dispersion in aqueous solution on mechanical stirred, due the HNTs negatively charged hydrophilic external surfaces. Then samples were left to rest for 30.0 min for the decantation of the PpIX/HNTs nanocomposites. After the decantation period, solution excess was removed and samples were left in a drying oven at 60 °C for two days, to promote evaporation of the residual solvent. For the experiments involving UVVis absorbance, steady-state photoluminescence and scanning electron microscopy (SEM), solid state samples were prepared by pressing (p=1.0kN) PpIX/HNTs (200mg), resulting in ~10.0x0.5 mm (diameter-thickness) plates.

4

Table 1: PpIX/HNTs samples. PpIX / HNTs Sample

PpIX0

HNTs

Ultrapure

Solution

(mL)

(mg)

water(mL)

Concentrations mg / mg

PpIX0/HNTs/H2O -1

1.60

1,000.00

30.00

0.16 / 1,000.00

PpIX0/HNTs/H2O -2

3.40

1,000.00

30.00

0.34 / 1,000.00

PpIX0/HNTs/H2O -3

6.00

1,000.00

30.00

0.60 / 1,000.00

3. Results and discussions Figure 1(a) shows the normalized absorption spectra of the PpIX/DMSO/ACN (1.55 x10-5 mol/L) and PpIX0/H2O (2.96 x10-5 mol/L) solutions. Figures 1(b), 1(c) and 1(d) show the absorption spectra of the PpIX0/HNTs/H2O-1,-2 and -3 solutions used for the preparation of the PpIX/HNTs-1, -2, and -3 samples, before the addition of the HNTs and after the decantation of the PpIX/HNTs nanocomposites (supernatant solution). Before the HNTs addition, the absorption spectra for all samples are very similar and present an extremely broadened shift in the Soret band, with maxima at ~466 nm. Compared to the absorption spectra of the monomers in the PpIX/DMSO/ACN solution, Figure 1 (a), the broadened shift indicates PpIX aggregation due to the presence of water in the solution [24,25]. According to Fuhrhop et al.[24], when the protoporphyrin IX or an amphiphilic derivative is dissolved in water, a very stable dimer formation is predicted, in which the porphyrin units are rotated by 180o to obtain a symmetric distribution of the hydrophilic head groups. Similar PpIX absorption spectra have been previously observed by Fuhrhop et al. [24] for PpIX dissolved in water with 20% of DMSO and by Inamuraet al. [25] for PpIX at various pH’s. The observed shifts were associated to PpIX dimerization and aggregation, since PpIX is an amphoteric electrolyte and the medium is moderately acidic [24-25]. In the present case, after the decantation of PpIX/HNTs nanocomposites, the pH of all solutions was ~5. The presence of water in the solution affects the absorption spectra at the Q band (475-650nm) in the same way.

5

0.8

0.4

a)

PpIX/DMSO/ACN PpIX0/H2O

Soret Band

Normalized Intensity

1.2

Q Band (x 16)

IV III

0.0

II

I

400 450 500 550 600 650 700 750

Wavelength(nm)

0.45

Absorbance

b)

PpIX0/HNTs/H2O -1

0.30

0.15

0.00

Before (HNTs) After (HNTs)

400 450 500 550 600 650 700 750

Wavelength(nm) 0.45

Absorbance

c)

PpIX0/HNTs/H2O -2

0.30 Before (HNTs) After (HNTs)

0.15

0.00

400 450 500 550 600 650 700 750

Wavelength(nm)

0.30

0.15

0.00

d)

PpIX0/HNTs/H2O -3 Normalized intensity

Absorbance

0.45

1.0 0.8 0.6 0.4 0.2 0.0

400 450 500 550 600 650 700 750

Before (HNTs) After (HNTs)

400 450 500 550 600 650 700 750

Wavelength(nm)

Figure 1- (a) Absorption spectra of the PpIX/DMSO/ACN and PpIX0/H2O solutions, (b), (c) and (d) Absorption spectra of the PpIX0/HNTs/H2O-1, -2, and -3 solutions used for the preparation of the PpIX/HNTs-1, -2, and -3 samples, before the addition of HNTs and after the decantation of PpIX/HNTs nanocomposites (supernatant solution). 6

Moreover, the absorbance spectrum of the supernatant solution of the PpIX0/HNTs/H2O -3 shows that, after de mixed process and the decantation period, the addition of HNTs did not change the PpIX0/H2O solution wavelength position of the absorbance peaks. Due the adsorption of the PpIX molecules on the HNTs surfaces the absorbance intensity of the supernatant decrease, but the wavelength position of the absorbance peaks stay unchanged, see the normalized spectra inserted in the Figure1(d). The absorbance decrease suggests that PpIX molecules are adsorbed on HNTs surfaces, figure 1(b), 1(c) and 1(d). As shown from the absorbance spectra of solutions after the HNTs addition; see Figure 1(b), 1(c) and 1(d); in the case of the PpIX0/HNTs/H2O-1solution, all PpIX molecules were adsorbed on the HNTs surfaces, while the PpIX0/HNTs/H2O-2, and -3 solutions, respectively, presented a residual amount of PpIX molecules in the supernatant solution. Considering that the absorption is proportional to the number of absorber centers, i.e., the amount of PpIX contained in the solution, the absorbance values suggest that, under the experimental mechanical stirring and sonication conditions used, the PpIX adsorption limit was ~ 0.300 mg for every 1,000.00 mg of HNTs. Using this result the ratio between the quantity of molecules of PpIX and the surface area of the HNTs (nm2) was ~1/200. The WAXD spectra of the HNTs and PpIX/HNTs-1,-2, and -3 samples were carried out to verify the possible intercalation occurrence of the PpIX molecules into the interlamellar space of the HNTs, see Figure 2. All samples show a broad maximum at the region around 2θ =12 degrees, revealing that the HNTs are in an various hydration states with water in the interlayer spaces [31,32,34]. Comparing the WAXD spectra of the HNTs with the spectra of the PpIX/HNTs-1, -2, and -3 samples, no significant changes are observed, which suggests that the PpIX intercalation in the interlayer spaces between the clays sheets did not occur (see region around 2θ =12 degrees in figure 2) and there was no exfoliation of HNTs. It is also observed that the maximum at the region around 2θ =12 degrees of the PpIX/HNTs 1, 2, and 3 samples present a small shift due to the hydration caused by the sample preparation process. The maximum around 2θ =26 degrees, showing the presence of quartz [34].

7

Intensity (a. u.)

3

* 2

*

1

*

HNTs PpIX/HNTs-1 PpIX/HNTs-2 PpIX/HNTs-3

* 0

10

20

30 40 2θ (degree)

50

60

Figure 2 - WAXD spectra of HNTs and PpIX/HNTs 1, 2, and 3 samples, the maximum around 2θ =26 degrees shows the presence of quartz[34]. These results suggest that the PpIX molecules are being adsorbed only on HNTs surfaces. These results also demonstrate that the HNTs preserve the HNTs crystalline structures after the samples preparation processes, which is not significantly different from pristine HNTs. Figure 3 presents SEM images of PpIX/HNTs-3 (3(a) and 3(b)) and PpIX/HNTs-1 (3(c) and 3(b)) pressed samples. In the references 31, 32 and 34 the authors refer to the HNT as a nanoscroll and discuss about the three most important HNTs characteristics that is: (i) the hollow tubular nanostructure, (ii) the small interaction between the nanotubes and (iii) the nanoscopic dimensions, a qualitative analysis of the SEM images reveals the characteristics (ii) and (iii),see Figure 3(b). The HNTs small interaction leads to a small area of contact between the adjacent tubes, and uniform dispersion of HNTs in the solutions on mechanical stirred [34] and in the PpIX/HNTs solid state nanocomposites.

8

a)

b)

c)

d)

Figure 3- SEM images of PpIX/HNTs-3 ((a) and (b)) and PpIX/HNTs-1 ((c) and (b)) pressed samples. The UV-Vis absorption, PLE, and PL spectra of the PpIX/HNTs-1, -2, and -3 samples were carried out to verify if, after the PpIX molecules have been adsorbed on HNTs surfaces, the PpIX photophysical properties are preserved. Figure 4 show the UV-Vis absorption spectra of pristine HNTs (solid state, nanopowder), PpIX/HNTs-1, 2, and -3 samples (solid state, nanopowder), and PpIX0/H2O (solution 2.96 x10-5 mol/L). The UV-Vis absorption spectra show that the amount of PpIX adsorbed on the HNTs surfaces increases with the rise of the PpIX0 concentration. Moreover, the Soret band absorption spectra of the samples PpIX/HNTs-1, -2, and -3 (solid-state) do not present the same line shape; see the normalized spectra inserted in figure-4. This indicates, as already expected, that the aggregation states of the PpIX molecules adsorbed on the surfaces of HNTs differ in function of the PpIX0 concentration. 9

PpIX/HNTs-1 PpIX/HNTs-2 PpIX/HNTs-3 HNTs PpIX0/H2O

0.63 0.56

Absorbance

0.49 0.42

Normalized Intensity

0.70 1.0 0.8 0.6 0.4 0.2 0.0 300

0.35

400

500

600

700

Wavelength(nm)

IV III

0.28 0.21

II

0.14

I

0.07 0.00 200

300

400

500

600

700

800

Wavelength (nm) Figure 4- Absorption spectra of the PpIX/HNTs-1, -2 and -3 samples, HNTs and the PpIX0/H2O solution. On the other hand, when the Soret band absorption spectrum of the PpIX0/H2O solution is compared with the absorption spectra of the PpIX/HNTs-1, -2, and -3 solidstate samples, a large blue shift (approximately ~ 70 nm) accompanied by a wellresolved and narrowing line-shape are observed, see the normalized absorbance spectra inserted in the Figure-4. Despite the effects of the medium acidity, the spectral shift and narrowing line suggest that the aggregations states of the PpIX molecules adsorbed on the HNTs surface are lowest than when they are in solution. Moreover, when the absorption spectrum at the Q-band region of the PpIX0/H2O solution is compared with the ones of the PpIX/HNTs-1, -2, and -3 solid-state samples, is observed changes in the intensities of the Q-band maxima. The bands intensities exchange from the ethio-type (IV>III>II>I) (solution) to rhodo-type (III >IV>II>I) (solid-state). Literature associates this alteration to an electronegative substituent at the pyrrole β sites conjugated with the ring

[35].

Bennet

et

al.

observed

similar

absorption

spectra

in

anionic

tetraphenylporphyrins intercalated into layered double hydroxides [36]. Additionally, Fuhrhop et al. suggested the possibility of half-neutralization of the PpIX carboxylic acid side chains, which could produce intra- and intermolecular COOH--̶ OOC hydrogen bonds [24]. Therefore, a possible explanation for the PpIX adsorption on the HNTs surface is: the half-neutralization change the electronegativity of the carboxylic acid side chains 10

attached at the pyrrole β sites of PpIX, which are attracted by the hydroxyl groups present on the surface of the HNTs. The stabilization occurs via cooperative hydrogen bonds between the half-neutralized carboxylic acid tail of the PpIX (monomers or dimers) and the hydroxyl groups located on the HNTs external surface defects or edges of the tubes [31-34], as show in the Figure 5. The external surface of the HNTs is predominantly covered by siloxane (Si-O-Si) groups with a few silanol (Si-OH) and by Aluminol (Al-OH) groups exposed at the edges of the tube [31-34]. Thus, the possible explanation for the PpIX molecules de-aggregation is the presence of a few number of hydroxyl groups on the surface of the HNTs [~1/(200 nm2)], available to make cooperative hydrogen bonds between the half-neutralized carboxylic acid tails of the PpIX (monomers and dimers), similar to the ‘Size-Matching Rule effect’ mechanism [28].

Figure 5- Scheme of proposed cooperative hydrogen bonds between half-neutralized carboxylic acid tail of the PpIX (monomer or dimer) and the hydroxyl group located on the HNTs external surface and the PpIX chemical structure. 11

Figure 6 (a) shows the normalized PLE spectra of the PpIX/HNTs-1, -2, and -3 samples, detection wavelength at 604 nm. Figure 6(b) shows the PLE (at the Soret band region) and the PL spectra of the PpIX/HNTs-1, -2, and -3 samples, with detection and excitation wavelength at 604 and 415 nm, respectively. It should be also here referred that to analyze the PLE spectra, one should consider: the fluorescence emission of the PpIX is only due to the monomers [26], the initial absorption of a photon by the singlet ground state produces short-lived first or second excited singlet states, the second electronic excited singlet state (Soret band~400 nm) relax to the first electronic excited singlet state (Q-band~475-650nm) via vibrational processes and then first electronic excited singlet state can lose energy by fluorescence, or by internal conversion or intersystem crossing [3]. The comparison between the PLE spectra shown in Figure 6(a) with the absorption spectra, shown in Figure 4, demonstrates this behavior. All PLE spectra show a maximum around 562 nm linked to the highest emission efficiency of the Qband electronic transitions. It is interesting to note that, at the Soret band region, the PLE spectra have two main maxima, one around 413 nm, due to the monomers, and another around 389 nm, due to, preferably, to the dimers [21,22]. Since, as well established in the literature, the emission of the PpIX is only owing to the monomers [26] and the PLE spectra are only due to the PpIX molecules that emitted, it can be concluded that there is a very efficient channel of excitation energy transfer between the dimmers (donor) and the monomers (acceptor) [3,18,28]. On the other hand, when comparing the intensities of the UV-Vis absorption and PLE spectra of the PpIX/HNTs1,-2, and -3 solid-state samples at the region around 413 nm (absorption of the monomers), it is verified that there has been an inversion of the intensities as a function of the PpIX0 concentrations used. As expected, the lower is the PpIX0 concentration (de-aggregation), the greater is the emission efficiency. Figure 6- (b) (normalized PLE spectra) shows that the ratio between the number of dimers and monomers is strongly affected by the concentration of the PpIX0 solutions used, and that the increase in PpIX0 solution concentration leads to an increase in a quantity of dimers.

12

Normalized Intensity

1.2

a)

1.0

PLE (λdet= 604 nm) PpIX/HNTs- 1 PpIX/HNTs- 2 PpIX/HNTs- 3

0.8 0.6 0.4 0.2 0.0 350

400

450

500

550

600

Wavelength(nm)

Normalized Intensity

1.2 1.0 0.8 0.6

b) PLE (λdet= 604 nm) and PL (λexc= 415 nm) PpIX/HNTs-1 PpIX/HNTs-2 PpIX/HNTs-3

0.4 0.2 0.0 350 400 450 500 550 600 650 700 750

Wavelength(nm) Figure 6- (a) Normalized PLE spectra of the PpIX/HNTs-1, 2, and 3 samples, (b) normalized PLE (at the Soret band region) and PL spectra of the PpIX/HNTs-1, -2, and -3 samples. The effects of the presence or/and excitation energy transfer of the dimers to the monomers on the PL spectra (fig. 6(b)) of the PpIX/HNTs samples can also be observed by the small red shift (~3nm) of the maximum around 604 nm. The increase in the number of dimers around the monomers implies in an increase in the red shift, Figure 6(b). This shows that the PL spectra of the PpIX/HNTs systems were composed of the emission of monomers with different amounts of dimers around them.

13

In order to discern about the aggregation effect on the ROS generation efficiency of the PpIX/HNTs system, photodegradation measurements were carried out. The light excitation wavelength used was 415 nm, the power of the excitation light was 790 µW and the irradiated area was 0.3 cm2, for all samples. Total irradiation time was 120 min and the PL spectra were acquired in periods of 1 min. Figure 7 show some of the PL spectra of the PpIX/HNTs-1, -2, and -3 acquired during irradiation. From the analysis of PL spectra intensities (at exposure time equal to zero) of the PpIX/HNTs-1, -2, and -3 samples (Figure 7(a), 7(b) and 7(c)), it is noticed that the increase in PL intensities emission follows the PpIX0 concentration decrease. It suggests that the fluorescence self-quenching increases as PpIX molecules concentration increases in the PpIX/HNTs system. It is been stated that the fluorescence self-quenching can be attributed to both: electronic transfer (from one excited porphyrin to another porphyrin’s ground state) and Förster type resonance energy transfer, being both processes highly dependent on the aggregation degree [2, 28, 30, 37]. Due to the increase in nonradioactive processes, the formation of aggregates decreases the lifetime of the singlet and triplet states of the photosensitizer. It makes the transfer of electrons (Type I oxidative mechanism) or energy (Type II oxidative mechanism) from the photosensitizer excited triplet state to the oxygen in the ground state more difficult, which decreases the efficiency of the photosensitizer to generate ROS [3,17-26,37,38]. These ROS can, in turn, interact with the

photosensitize,

leading

to

its

photodegradation

[3,

17-26,37,38].

The

photodegradation effects of PpIX molecules can be seen at PpIX/HNTs PL spectra (Figures 7(a), 7(b) and 7(c)). For the PpIX/HNTs-1 system (Figure 7(a)), a PL spectrum intensity decrease is observed as irradiation time increases, pointing to the degradation of PpIX molecules. Besides the decrease in PL intensity, a red shift is clearly observed as long as irradiation time increases. From the first irradiation time (t=0) up to 7 minutes, the PL maximum emission shifts about 4nm, denoting the photodegradation of isolated PpIX monomers. Additionally, a shoulder around 630 nm, mainly associated to the chlorin-type photoproducts formation, is also clearly observed at PpIX/HNTs PL spectra, showing that a dimer photodegradation is occurring, which is in accordance with refs (37,38). As photodegradation is due to a photo-oxidation process, that depends on the ROS creation rate[3, 37-38], our results demonstrate that, even in the PpIX/HNTs system, the PpIX molecules present photochemical effects. Although these effects are present in all systems showed here (PpIX/HNTs-2, Figure 7 (b); and PpIX/HNTs-3, Figure 7(c)), our results show that photodegradation, thus the ROS 14

formation, became less evident when the PpIX molecules concentration and the aggregates increase. Figure 7(d) show the photodegradation curves (integrated PL spectra versus irradiated time) adjusted using mono-exponential decays. Following the photodegradation behavior already observed for the PL intensities, the time decay is shorter for PpIX/HNTs-1 (low PpIX concentration) in comparison to the PpIX/HNTs-2 and PpIX/HNTs-3 (higher PpIX concentration). It is noted that the emission and photodegradation efficiencies for the PpIX molecules depend strongly on the chemical environment around the monomers, i.e., more isolated the monomer, more efficient is its emission and photodegradation. Thus, as the PpIX concentration increases, the efficiency of the photosensitizer to generate ROS decreases. The photodegradation curves also showed that, after the degradation of isolated monomers, there is a ratio between the amount of residual monomers and dimmers at which PpIX/HNTs photodegradation decreases significantly, leaving the system more photostable. It also suggests that there is a very efficient channel of excitation energy and/or electrons transfer between the dimers and the monomers. As stated earlier in this article, the PL spectra of the PpIX/HNTs samples are composed of the emission of monomers with different amounts of dimers around them. This finding is corroborated by the red shift and photostability of the PpIX/HNTs PL spectra after a period of photodegradation.

15

21

a)

Intensity (a.u.)

18

PL x Irradiation exposure time (PpIX/HNTs-1)

0 (min) 1 (min) 3 (min) 5 (min) 7 (min) 15 (min) 30 (min) 45 (min) 60 (min) 75 (min) 90 (min) 105 (min) 120 (min)

15 12 9 6 3 0 550

21

600

b)

Intensity (a.u.)

18

650 700 Wavelength (nm)

PL x Irradiation exposure time (PpIX/HNTs-2) 0 (min) 1 (min) 3 (min) 5 (min) 7 (min) 15 (min) 30 (min) 45 (min) 60 (min) 75 (min) 90 (min) 105 (min) 120 (min)

15 12 9 6 3 0 550

21

600

c)

Intensity (a.u.)

18

650 700 Wavelength (nm)

750

PL x Irradiation exposure time (PpIX/HNTs-3) 0 (min) 1 (min) 3 (min) 5 (min) 7 (min) 15 (min) 30 (min) 45 (min) 60 (min) 75 (min) 90 (min) 105 (min) 120 (min)

15 12 9 6 3 0 550

600

d)

35 Integrated PL (a. u.)

750

650 700 Wavelength (nm)

750

PpIX/HNTs-1 Exponential 1 decay (t1= 7.80 min)

30 25

PpIX/HNTs-2 Exponential 1 decay (t1= 17.97 min)

20 15 10

PpIX/HNTs-3 Exponential 1 decay (t1= 24.77 min)

5 0

0

20

40 60 80 Time (min)

100

120

16

Figure 7- PL spectra as a function of irradiation time (a) PpIX/HNTs-1, (b) PpIX/HNTs-2 and (c) PpIX/HNTs-3, (d) photodegradation curves (integrated PL spectra versus irradiated time) of PpIX/HNTs samples 1, 2, and 3. The excitation wavelength used was 415 nm. Total irradiation time was 120 min and the PL spectra were acquired in periods of 1 min.

4. Conclusions In this article we have successfully developed hybrid functional materials composed of PpIX/HNTs in which the PpIX molecules, due to the concentrations used and their interaction with the surface of the HNTs, were adsorbed on the surface of the HNTs at different degree of aggregation. This article presents an explanation for the PpIX adsorption by the HNTs surface and shows that the PpIX molecules adsorption on the HNTs surfaces, and its degree of aggregation, can be controlled by changing the ratio between PpIX and HNTs concentrations. The degree of PpIX aggregation exerts a strong influence, both on the PpIX light emission efficiency and on the efficiency of reactive oxygen species generation. The analysis of the PLE, PL spectra intensities and the photodegradation curves as a function of the PpIX concentration allowed us to conclude that the photoluminescence spectra of the PpIX/HNTs samples are composed by the emission of monomers with different amounts of dimers around them. Moreover, the photodegradation curves showed that, under the experimental conditions used, there is a ratio between the amount of monomers and dimers at which PpIX/HNTs photodegradation decreases significantly, leaving the system more photostable. It also suggests that there is a very efficient channel of excitation energy and/or electrons transfer between the dimers and the monomers. Finally, the development of systems in which the porphyrin molecules aggregation/de-aggregation can be controlled, retaining its photophysical and photochemical properties, can lead to the development of more efficient porphyrin based devices and processes, and help to understand the processes of aggregation and formation of porphyrin supramolecular structures.

Author information Corresponding Author: [email protected] Acknowledgment 17

This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001, Bilateral Project (2013/14262-7) FCT-MEC (Brazil/Portugal), CNPq and FAPEMIG.The authors also acknowledge the financial support from FEDER, through Programa Operacional Factores de Competitividade − COMPETE and Fundação para a Ciência e a Tecnologia − FCT, by the project PTDC/FIS-NAN/0909/2014 and for the Portuguese research Grant UID/FIS/00068/2013.

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Hybrid functional nanomaterials composed of Protoporphyrin-IX/Halloysite nanotubes Cooperative hydrogen bonds between carboxylic acid tail and hydroxyl group Protoporphyrin-IX/Halloysite nanotubes photostability Protoporphyrin-IX degree of aggregation, monomers and dimmers De-aggregation of Protoporphyrin-IX, emission efficiency and energy transfer

Declaration of interests xThe authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: