Enhanced Oxygen Singlet Production by Hybrid System of Porphyrin

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Enhanced Oxygen Singlet Production by Hybrid System of Porphyrin and Enriched (6,5) Single-Walled Carbon Nanotubes for Photodynamic Therapy Gustavo A. M. Sáfar, Rafael Nunes Gontijo, Cristiano Fantini, Dayse Carvalho da Silva Martins, Ynara Marina Idemori, Maurício V. B. Pinheiro, and Klaus Krambrock J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5111289 • Publication Date (Web): 29 Jan 2015 Downloaded from http://pubs.acs.org on February 13, 2015

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The Journal of Physical Chemistry

Enhanced Oxygen Singlet Production by Hybrid System of Porphyrin and Enriched (6,5) Single-Walled Carbon Nanotubes for Photodynamic Therapy Gustavo A. M. Sáfar1,*, Rafael N. Gontijo1, Cristiano Fantini1, Dayse C. S. Martins2, Ynara M. Idemori2, Maurício V. B. Pinheiro1, Klaus Krambrock1. 1

Departamento de Física, Universidade Federal de Minas Gerais, Belo Horizonte-MG, 31270-901,

Brazil. 2

Departamento de Química, Universidade Federal de Minas Gerais, Belo Horizonte-MG, 31270-901,

Brazil.

Keywords: Photodynamic therapy, oxygen singlet, porphyrin, single-walled carbon nanotube

ABSTRACT We demonstrate that hybrid systems of porphyrins and chirality enriched (6,5) single-walled carbon nanotubes (E-SWCNTs) are better candidates for photodynamic therapy (PDT) than theirs components alone. Surprisingly, the E-SWCNTs act as optical absorption enhancers to the porphyrins increasing the oxygen singlet production when illuminated by a light source with energy higher than the E-SWCNT gap plus the equivalent in energy of an E-SWCNT phonon. The phenomenon is explained based on energy transfer from the E-SWCNT to the porphyrin which finally transfers it to the oxygen molecule. The large optical absorption cross section of ESWCNT and the resonance of the porphyrin to the oxygen singlet-triplet transition are the responsible for the synergistic effect.

INTRODUCTION Photodynamic therapy (PDT) is a technique currently used to treat malignant tumors1,

2

and

microbial parasites.3 It is based on three key components: a photosensitizer, a light source and tissue oxygen. The photosensitizer is non-toxic in the dark and becomes highly toxic under appropriate illumination in the presence of oxygen producing reactive oxygen species (ROS).

ACS Paragon Plus Environment


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The reactive oxygen species are either free radicals (called type I process) or singlet oxygen (called type II process) due to electron transfer or energy transfer, respectively. Porphyrins are well-known photosensitizers for PDT because of different molecular properties: (i) high absorption coefficients in the red spectral region which is favorable for deep penetration of light in tissues, (ii) low cytotoxic effects in dark, (iii) high quantum yields for singlet oxygen production due to energy transfer from the excited triplet state to the oxygen molecule and (iv) long lived excited triplet state (~µs).4 One possible way to enhance oxygen singlet production under illumination is to find a resonant compound able to capture more energy than the molecule alone and transfer it efficiently to the porphyrins. Recent works have shown that quantum dots (QD) could be used in the photodynamic therapy of cancer as resonant energy donors for porphyrin-type photosensitizers.5-8 In one of these, fluorescence lifetime imaging revealed Förster resonance energy transfer (FRET) from quantum dots to chlorin e6 (Ce6) within live cells.5 The system harvests light in a broad energy range and transfer the excitation from the dot through the porphyrin to oxygen, generating singlet oxygen.8 The Förster coupling between the excitons residing on the quantum dot and the porphyrin was calculated and ascribed as the main responsible mechanism of energy transfer.8 The investigation on the capacity of such complex to generate 1O2 showed that the QD-Ce6 complex irradiated by visible light is able to produce 1O2 more efficiently than QDs or Ce6 taken separately.7 Here, we demonstrate that hybrid systems of porphyrins and chirality enriched (6,5) singlewalled carbon nanotubes (E-SWCNT) transfer directly energy to oxygen to promote a tripletsinglet transition. This is the inverse of an effect already reported in the literature.9 Moreover, we show that the hybrid system of E-SWCNT and meso-tetrakis(4-pyridyl)porphyrin tosylate salt H2TM4PyP(OTs)4 can synergistically generate oxygen singlet. We used H2TM4PyP(OTs)4 and its

myrystil

analogous

meso-tetrakis(N-myristyl-4-pyridinium)porphyrin

tosylate

salt

(H2TMy4PyP(OTs)4 because H2TM4PyP4+ has been identified as a good candidate for photodynamic therapy.10 We compare the efficiency of the process with commercial verteporfin. We show that the efficiency of H2TM4PyP(OTs)4 is comparable to verteporfin, but E-SWCNT/ H2TM4PyP(OTs)4 is much more efficient related to the porphyrin concentration.


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EXPERIMENTAL SECTION 1. Porphyrin Synthesis. The

preparation

of

meso-tetrakis(N-myristyl-4-pyridinium)porphyrin

tosylate

salt

(H2TMy4PyP(OTs)4) is described elsewhere.11 H2TMy4PyPBr4 was prepared via alkylation of the parent meso-tetrakis(4-pyridyl)porphyrin according to an adaptation of the method of Okuno et al.12 Briefly, to a refluxing solution of H2TM4PyP (104.38 mg, 0.17 mmol) in N,Ndimethylformamide (20 mL) were added tetradecyl bromide (4.00 mL, 13.44 mol). The reaction was monitored by TLC (silica) and UV-visible absorption spectroscopy. This solution was kept under reflux for 7 hours and the H2TMy4PyPBr4 was precipitated by the addition of 80 mL diethyl ether. The product was thoroughly washed with 125 mL diethyl ether and dried in vacuum at room temperature. The compound was then dissolved in a CH2Cl2:CH3OH (9:1) mixture, the solution was filtered, and dried in vacuum at room temperature. The product was purified by column chromatography on neutral alumina using CH2Cl2:CH3OH (9:1) mixture as eluent. The samples in the bromide form were converted to the tosylate form by percolation through an anion-exchange resin Dowex 2X8 (OTs– form) with a CH2Cl2:CH3OH (9:1) mixture. 2. Single-Walled Carbon Nanotube Dispersion and Separation through Hydrogel Chromatography. A single-walled carbon nanotube (SWCNT) dispersion was made using 100 mg of (6,5) enriched Comocat (SweNT® SG65i) put in a 70 mM solution of sodium dodecyl sulfate (SDS, Sigma Aldrich). The resulting mixture was then tip sonicated at 40W for a total of 3 hours with intervals of 30 minutes (VCX-500, Sonics & Materials). After sonication, the solution was centrifuged at 10000 rpm for 1 hour followed by an ultracentrifugation at 40000 rpm for another hour to remove impurities.13 The resulting dispersion was then used for the hydrogel chromatography separation process. This step consists of making a column containing a bit (approximately 2 mL) of the hydrogel Sephacryl® S200 (Sigma Aldrich) in a plastic syringe (10 mL; 8 cm in length and 1.5 cm in inner diameter) and passing the dispersion through the column. As the affinity of the metallic SWCNT with the gel is negligible, its interaction with gel beads is very weak, thus they flow through the column without being stuck to the beads. The semiconducting SWCNT (E-



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SWCNT), on the other hand, have a chiral dependent interaction with the gel, being attached to the gel beads.13, 14 For the separation process, 1 mL of the SWCNT dispersion was added to the column. The dispersion ran through the column, leaving some residual nanotubes along the way. The column was cleaned of the non-interacting SWCNT with a 70 mM solution of SDS. The column is now rich with certain types of semiconducting SWCNT, specially (6,5) SWCNT, and these can be obtained by simply washing the column with a 175 mM solution of SDS.13

3. Sample Preparation and Measurements. Oxygen singlet 1O2 spin trap TEMP (2,2,6,6-tetrametylpiperidinol, CAS 2403-88-5, ALDRICH, 98 %) was used without purification. The stock solution was prepared in dark consisting of 1 mL of 1M TEMP in distilled and de-ionized water. Stable nitroxide radical TEMPOL (4-hydroxyTEMPO, 2,2,6,6 tetramethylpiperidine-1-oxyl, CAS 2226-96-2, ALDRICH, 97 %) was used without further purification. 1M TEMPOL stock solution was prepared in distilled and deionized water and used in EPR analysis for calibration of absolute spin concentrations. H2TM4PyP(OTs)4 (POR) and H2TMy4PyP(OTs)4 (MYR) solution were prepared in distilled and de-ionized water with concentration of 5.5x10-5mol/L. Verteporfin (VER) (CAS 129497-78-5, ALDRICH, ≥ 94 %), a well known porphyrin photosensitizer for PDT, was dissolved in DMSO (because of its poor solubility in H2O), diluted in distilled water and used as a reference sample. Approximately 1 mg of verteporfin was dissolved in 12.5 mL DMSO and after mixed with same volume in water. Illumination of different samples in the visible spectral range was done by a white LED lamp filtered either by a yellow or red filter at 570 nm or 630 nm, respectively. For the infrared illuminations an array consisting of five LEDs irradiating at 940 nm were used. The measured integrated radiance for the different illuminations under experimental conditions at 570 nm, 630 nm and 940 nm were 45, 28 and 10 mW sr-1 m-2 calibrated by Coherent Lasermate/D power meter. Optical absorption spectra of different photosensitizer aqueous solutions were measured using a Shimadzu UV-VIS-NIR (Shimadzu UV-3600) spectrometer with sample solutions in quadratic quartz cuvettes with side length of 10 mm. Electron paramagnetic resonance (EPR)


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measurements were carried out on Miniscope MS400 spectrometer (Magnettech, Germany) using the following parameters: microwave power 10 mW, modulation field 2G, 100 kHz, center field 335 mT, 3 scans with a sweep time of 60 s each. EPR measurements were carried out with 50 µL of previously prepared aqueous solutions in glass capillaries, either in the dark, or after different illumination times. In order to determine the oxygen singlet production as a function of light energy and illumination time EPR spin trapping method was used with oxygen singlet specific spin trap TEMP in mixed solutions with the different photosensitizers. Due to the oxygen singlet production the diamagnetic spin trap TEMP transforms into the stable nitroxide radical TEMPOL which is paramagnetic. A typical experimental run included: (i) a blank solution containing 50 µL TEMP (1M) and 50 µL H2O; (ii) 50 µL TEMP (1M) and 25 µL photosensitizer (POR, MYR, VER or E-SWCNT) plus 25 µL H2O and (iii) 50 µL TEMP (1M) and mixed 50 µL photosensitizers in 1:1 ratio (POR + E-SWCNT; MYR + E-SWCNT). Another set was done for calibration with verteporfin, for 570 nm filter, were 25 µL H2O was replaced by 25 µL DMSO/H2O (1:1). RESULTS Figure 1 shows the optical absorption spectra of aqueous solutions of (a) MYR, (b) POR, (c) VER and (d) aqueous suspension of E-SWCNTs. MYR solution present two absorption bands at 530 nm and 570 nm, whereas POR solution four bands centered at 521 nm, 556 nm, 587 nm and 641 nm. On the other hand commercial photosensitizer VER shows three bands centers centered at 575, 630 and 695 nm. All three porphyrin solutions show also weak absorption bands in between 900 and 1000 nm spectral range. The optical absorption spectrum of E-SWCNTS aqueous suspension presents the expected optical bands centered at 572 and 992 nm associated with E22 and E11 optical transitions of the (6,5) nanotube.15 The other low intensive features present in the spectra are related to optical transitions from other semiconducting nanotube chiralities present, in small amount, in the sample. The absorption spectra of the mixtures are nearly simple linear combinations of the spectra presented in fig. 1 (see Supporting Information, for both before and after illumination at 570 nm, figure S5), which was already expected.16



570

0.2 (a)

630

940

MYR

0.0

Absorbance

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0.2

(b) POR

0.0 VER

0.2 (c) 0.0

E-SWCNT

0.8 (d) 0.0 400

600

800

1000

Wavelength (nm)

Figure 1. Optical absorption spectra of aqueous solutions of different porphyrins (5.5 x 10-5mol/L) (a) MYR, (b) POR, (c) VER, and (d) aqueous suspension of E-SWCNT.

The EPR spectra for all samples, as a function of time of illumination at the three different wavelengths (see Supporting Information, figures S1 - S3) are typical for the stable nitroxide radical TEMPOL. The nitroxide spectrum consists of a triplet due to hyperfine interaction of the radical electron with nuclear spin of nitrogen. From EPR spectral simulations using the spin Hamiltonian H= β + the g factor and hyperfine interaction a of the nitroxide radical TEMPOL are obtained (nuclear spin I = 1, 100% natural abundance) as g = 2.0084(2) and a = 1.69(2) mT.17 At the beginning of illuminations the nitroxide triplet EPR spectra of the spin adducts show intensity ratios of 1:1:1, which transform in slightly asymmetric triplet EPR spectra for longer illumination times. The lateral two EPR lines broaden, due to different relaxation times, in relation to the middle line resulting in a decrease in intensity. The total integrated area of each individual line is, however, the same. The spectra of EPR (see Supporting Information) indicate that some nitroxide radicals exist as impurities in the solution (dark spectra) which increase strongly in intensity by red light (630 nm) or yellow (570 nm) illumination.


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VER POR MYR POR+E-SWNT MYR+E-SWNT E-SWNT TEMP

570 nm

1.2

Concentration (mM/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.9

0.6

0.3

0.0 0

30

60

90

Time (min)

Figure 2. Absolute concentrations of formed spin adduct TEMPOL after illumination of different aqueous solutions containing spin trap TEMP and different porphyrins, E-SWCNT or mixtures of both as a function of yellow (570 nm) light illuminations. The concentration of porphyrins is the same in all samples containing porphyrins.




2


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630 nm

1

0 0

30

60

90

120

150

180

Time (min)

Figure 3. Absolute concentrations of formed spin adduct TEMPOL after illumination of different aqueous solutions containing spin trap TEMP and different porphyrins, E-SWCNT or mixtures of both as a function of red light (630 nm) light illuminations. The concentration of porphyrins is the same in all samples containing porphyrins.

Figure 2, 3 and 4 present the absolute concentrations of stable nitroxide radical for the different porphyrin solutions, the E-SWCNT suspensions and the mixed solution containing porphyrins and E-SWCNT as a function of illumination times for the visible spectral range at 570 nm (fig. 2), 630 nm (fig. 3) and for the infrared spectral range at 940 nm (figure 4). The absolute concentrations of the spin adducts were obtained by double integration of the EPR spectra and calibrated with an aqueous solution of (0.01 M) of stable free radical TEMPOL.


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940 nm


0.4


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.3

0.2

0.1

0.0 0

30

60

90

120

Time (min)

Figure 4. Relative concentration of formed spin adduct TEMPOL after illumination of different aqueous solutions containing spin trap TEMP and different porphyrins, E-SWCNT or mixtures of both as a function infrared light illumination (940 nm). The concentration of porphyrins is the same in all samples containing porphyrins.

The formation of a stable spin adduct TEMPOL from spin trap TEMP indicates that the dominant process is a type II process (energy transfer) due to formation of oxygen singlet. For all three porphyrin solutions yellow and red light illuminations show higher efficiencies for the production of oxygen singlet and therefore the formation of stable spin adducts when compared with infrared illumination. This is consistent with the higher absorption intensity of the porphyrins in the yellow to red spectral region (see figure 1) when compared with the infrared spectral region. However, although the absolute concentration of spin adducts produced by infrared light is slightly lower for the infrared, we should take also into account that the radiance of the infrared light is lower by a factor of 4 and 3 when compared with the yellow and red light illuminations, respectively. The decrease of the spin adduct concentrations for longer illumination times in the visible spectral range observed in the case of POR and MYR solutions (see figures 2 and 3) is yet



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unclear but it could be explained by attack of the spin adducts by high concentration of oxygen singlet. It is interesting to note that this does not occur for the commercial porphyrin VER solution. However, in this case verteporfin was not easily solved in pure water and a mixture of DMSO and water was used to prepare the solutions. Figures 2 and 3 demonstrate clearly that illuminating (6,5)-enriched semiconducting singlewalled nanotubes E-SWCNT suspensions by yellow and red light leads also to the formation of singlet oxygen and therefore to nitroxide spin adduct formation with comparable efficiencies to porphyrin solutions. The rate constant for spin adduct formation is somewhat slower compared to the porphyrin solutions, however, no attack of the formed spin adducts is observed for longer illumination times. It should be noted also that although the absorption intensity of E-SWCNT suspensions is much higher in the infrared spectral region oxygen singlet is not produced at 940 nm. The results for the mixed solutions containing E-SWCNT suspensions and porphyrin aqueous solutions are very interesting. The nitroxide adduct formation is enhanced and the possible attack of formed spin adducts, i.e. the absolute spin adduct concentration formed by quantities of oxygen singlet, is higher when compared to the sum of the pure solutions. Also, the mixed solution show less effect on the degradation of nitroxide spin adducts for longer illumination times. On the other hand, illumination of the mixed solutions in the infrared spectral range leads to smaller absolute nitroxide concentrations when comparing with the sum of the pure E-SWCNT or porphyrin solutions. In order to compare the efficiency in oxygen singlet generation of the porphyrins in the same solution, due to verteporfin insolubility in water, we had measured all in the same DMSO/H2O (1:8) solution (see Supporting Information). The results show that the little amount of DMSO does not change significantly the overall behavior.

DISCUSSIONS


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Earlier we mentioned that porphyrins are well-known photosensitizers for PDT because of different molecular properties: (i) high absorption coefficients in the red spectral region which is favorable for deep penetration of light in tissues, (ii) low cytotoxic effects in dark, (iii) high quantum yields for singlet oxygen production due to energy transfer from the excited triplet state to the oxygen molecule and (iv) long lived excited triplet state (~µs).4 All three porphyrins studied here are excellent photosensitizers for oxygen singlet production. Much less is known about the photophysical properties of E-SWCNT regarding PDT. From our data presented in figures 2 and 3 the first observation is that E-SWCNT transfer directly energy to oxygen triplet-singlet transition, 3O2→1O2 at least in the visible spectral range 570 nm and 630 nm with similar efficiencies compared with the porphyrins in the former case. This is explained with the high optical absorption band at 566 nm which is due to the E22 transition of (6,5) SWCNTs.15 This is somewhat surprising and on the contrary of an effect already reported in the literature where a quenching of the oxygen singlet formation by SWCNTs was observed during excitation with a monochromatized xenon lamp light source in the range of 500 to 950 nm.9 However, in that work, an unsorted SWCNT sample was used and consequently quenching of the energy transfer from oxygen singlet to semiconductor SWCNT was probably facilitated by the presence of metallic SWCNT that, among other non-radiative processes, compete with the absorption of light, preventing exciton formation. The mechanism for the EPR signal generation of the TEMP spin adduct in the hybrid system may be described by the following expressions:

+

+ + ℎυ ⇆ + + → + +

→

Where OS is the oxygen singlet, OT is the oxygen triplet, STRAP is the spin trap, P is the porphyrin (either POR or MYR), hν is the photon, SAD is the spin adduct and Sdeg is the spin adduct degraded by oxygen singlet. Using the law of mass action, we have:



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= − ℎ# + !

(1)

= − + ℎ# − − ! = − + ! = !

(2)

(3)

(4)

If we take all the equations at once we get a system of coupled differential equations of first order. However, we can verify the solution for the early stages of the reaction. If [P] is constant and we name k5 as k4/[P]and we group all the terms with [OS ] in (2), then we have: = (− − − * ) + ℎ# !

(5)

At the early stages of time, we have [OS] very small and [SAD] small then: ≅ ℎ# !

(6)

From equation (3) we have with the same argument: ≅ !

(7)

For the same photon energy, we have kept [OT], [STRAP] and [hν] constant. If we integrate equation (6) and then equation (7), we have: [OS](t) = [OT][P][STRAP][hν] k1+ [OS](0)

(8)

and then [SAD](t) = k1k3[OT][P][STRAP][hν]t2 + k3 [OS](0)t + [SAD](0)


(9).

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If we have k1 and k3 with small values then we get a linear relation for [SAD](t). Indeed, we verify that the slope seems to be linear at the early stages of the reaction, which corroborates the approximation for d[OS]/dt and d[SAD]/dt. [SAD](t) ≅ k3[OS](0)t+ [SAD](0)

(10)

For the case of the hybrid system we just change [P] for [P + E-SWCNT] and replace k1 and k3 by k’1 and k’3 in all equations from (1) to (10) and then we have [SAD’](t) ≅ k’3[OS](0)t + [SAD](0)

(11).

All we need is to know if k3 is higher or lower than k’3, if it is higher or lower for the porphyrin either adsorbed or not to an E-SWCNT. The slope for the hybrid system is steeper compared to the porphyrin alone at least for the photon energy in the red spectral range (630 nm). Therefore, the experimental results on mixed solution of porphyrins and E-SWCNTs indicate enhanced oxygen singlet formation compared with pure porphyrin solutions for the early stage of illumination at least for excitation in the visible spectral range for which there is good superposition of absorption bands between porphyrins and E-SWCNTs. We suggest that the mechanism through which the enhanced oxygen singlet is produced lies upon an energy transfer between the E-SWCNT and the porphyrin molecule (figure 5). The energy could be transferred either by the photoinduced charge transfer (PCT) Förster effect

5-8, 35-37

18-34

or through

, or even through a photonic emission-absorption mechanism. Whenever the

incident photon energy is higher than the electronic gap of the semiconducting E-SWCNT, an exciton would be created and the energy of its recombination, minus the energy of phonon scattering, would be transferred to the porphyrin via all the aforementioned mechanisms. The energy of the gap (minus some phonon scattering energy) being higher than the oxygen tripletsinglet conversion energy, the energy transfer would be very efficient, because of the multipathway process, even if some relaxation mechanism takes place in between the energy passing through the porphyrin (figure 5a). Then the porphyrin could transfer the energy to the oxygen triplet to promote it to the singlet state. The porphyrin alone is able to absorb the same energy. However, at the gap energy, the density of states of the E-SWCNT is much higher than the density of states of the porphyrin, because it coincides with the Van Hove singularities of the E-



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SWCNT

38, 39

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, which means an optical absorption cross section much higher for the nanotubes

when compared to the porphyrins. The combination of POR and SWCNT (a sample of SWCNT with 60 % wt. of (6,5) chirality) exhibits energy and charge transfer, a phenomenon already reported in the literature.16 Other porphyrin/SWCNT hybrid systems have shown the same kind of interaction. For instance, porphyrins and carbon nanotubes show charge transfer, along with energy transfer, either photoinduced or in the dark.11,

40-42

Depending on the porphyrin molecule, and the nanotube

chirality, the charge transfer is mild 40 or strong.42 The influence of the charge transfer is seen on the magnetic, optical or conduction properties of the nanotube or the interacting molecule. One particular example is the nanometrological porphyrin.11 This porphyrin shows an extra luminescence line in the visible (green) whenever it is in close contact with a conducting nanoparticle11. The Hamiltonian of the molecule is changed because of the charge transfer along with a curvature change induced by the nanoparticle (a nanotube, for instance)11. A similar porphyrin can dope nanotubes strong enough to induce a Breit-Wigner-Fano signal in the nanotube Raman spectrum, and completely modify the magnetic behavior of the nanotube43. All these phenomena occur via charge transfer through π − π interactions between the nanotube and the porphyrin and the electron carries the energy during the process. In the infrared spectral range at 940 nm, the production of oxygen singlet by the hybrid system is not as efficient as at 630 nm (figure 5 b). The reason could be that phonon scattering takes away energy, preventing exciton formation. Conveniently, the difference in energy of the incident photon (940 nm) and the energy necessary for singlet-triplet transition (1270 nm) is around 2600~3000 cm-1, which corresponds to 2-phonon Raman bands of E-SWCNT. A less probable possibility could be a combination of 2 phonon scattering by the porphyrin, in case of a photonic emission-absorption process10 or the generation of phonons during a charge transfer process (see for instance ref 44). Certainly, one major phonon scattering process is the Raman scattering of ESWCNT because Raman cross section of carbon nanotubes is huge near the resonance.45 Particularly, this could explain the virtual quenching of the signal (fig. 4), on the E-SWCNT curve or on the hybrid system curves with both POR and MYR.


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Although some reports state that singlet oxygen singlet is not efficiently converted by a wavelength longer than 850 nm, oxygen singlet can be produced even without a chromophore at 1064 nm.46,47

Figure 5. Mechanism of energy transfer from the E-SWCNT to porphyrins, and then to oxygen, in place of photoluminescence: a) hν is enough to create electron-hole pair and excitons even with Raman scattering, which enables several pathways through FRET and other processes (like PCT) to porphyrin singlet states; b) hν is just enough to promote exciton formation while there is energy loss through phonons (Raman) which makes the density of resonant states between SWCNT and porphyrin lower. After being transferred to the porphyrin, the energy passes through an intersystem crossing (ISC) and is transferred to the oxygen.

Finally, one must consider that there is some influence in the energy levels of both components of the hybrid system by the simple proximity of the porphyrins to the SWCNT in terms of charge and dielectric constant, as already reported in the literature.16,40,41 The influence of the mutual orientation of the porphyrin and the SWCNT in this case could also be important.48,49 However, further studies are necessary.



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CONCLUSIONS Hybrid systems of porphyrins and (6,5)-SWCNT can be a promising candidate for photodynamic therapy. Indeed, (6,5)-SWCNT alone are able to produce oxygen singlet, most probably via direct energy transfer from E-SWCNT excitons to the dissolved O2, an inverse phenomenon of one reported in the literature.9 However, the hybrid system is much more efficient than the ESWCNT alone and, in some cases, in the range of the wavelength used in medical therapy, more efficient even than the isolated porphyrins investigated in this study. Either by photoinduced charge transfer,18-34 by Förster effect or even by a photonic emission-absorption mechanism, energy transfer is achieved, through the nanotube to the porphyrins and again to the dissolved oxygen. AUTHOR INFORMATION Corresponding author *E-mail: [email protected]. Fax: 3409-5600. Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Supporting Information Available: We present EPR spectra of all samples, a comparison between EPR signal in H2O and DMSO/H2O and optical absorbance spectra of the samples before and after illumination for 570 nm. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGEMENTS The authors thank FAPEMIG, CNPq, CAPES for financial support.


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TOC graphic



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Porphyrins and nanotubes combine to form a hybrid system that generates oxygen singlet synergistically, a phenomenon detected by electron paramagnetic resonance using the spin trapping method. 450x349mm (96 x 96 DPI)


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Enhanced Oxygen Singlet Production by Hybrid System of Porphyrin

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