ISSN 1061933X, Colloid Journal, 2014, Vol. 76, No. 5, pp. 539–545. © Pleiades Publishing, Ltd., 2014. Original Russian Text © O.V. Dement’eva, M.M. Vinogradova, E.A. Luk’yanets, L.I. Solov’eva, V.A. Ogarev, V.M. Rudoy, 2014, published in Kolloidnyi Zhurnal, 2014, Vol. 76, No. 5, pp. 587–593.
Zinc PhthalocyanineBased WaterSoluble Thiolated Photosensitizer and Its Conjugates with Gold Nanoparticles: Synthesis and Spectral Properties O. V. Dement’evaa, M. M. Vinogradovaa, b, E. A. Luk’yanetsc, L. I. Solov’evac, V. A. Ogareva, and V. M. Rudoya a
Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow, 119071 Russia email:
[email protected] b Mendeleev University of Chemical Technology, Higher Chemical College, Russian Academy of Sciences, Miusskaya pl. 9, Moscow, 125047 Russia c State Scientific Center, Research Institute of Organic Intermediates and Dyes, ul. Bol’shaya Sadovaya 1/4, Moscow, 123995 Russia Received February 10, 2014
Abstract—A new photosensitizer, monoconjugate of zinc octa4,5carboxyphthalocyanine with Lcys teine, which is rather readily soluble in water, has been synthesized and characterized. The chemisorption of its molecules on the surface of gold nanoparticles with different sizes has been studied. A comparative analysis of the spectral characteristics of the synthesized conjugates of gold nanoparticles with the photosensitizer has shown that the photosensitizer grafted to gold nanoparticles exhibits rather intense fluorescence. Therefore, such conjugates can be used for photodynamic therapy of tumors. DOI: 10.1134/S1061933X14050068
INTRODUCTION Nanoparticles (NPs) of noble metals (primarily gold) are used to solve a number of applied problems. In particular, they may be used as catalysts, active ele ments of bio and chemosensors, remedies for diag nostics and/or treatment of different diseases, and carriers for targeted drug delivery [1–7]. It follows from analysis of published data [2, 5–7] that one of the most promising fields is the application of Au nanoparticles as carriers for drugs. This is prima rily explained by the relative biological inertness (low toxicity) of gold NPs and the feasibility of controlling their distribution in an organism by grafting biologi cally active molecules to the surface of particles or varying their sizes or shapes. This makes it possible to selectively accumulate NPs carrying different func tional compounds in a desired organ or tissue through the mechanisms of active [1, 2, 5, 7] or passive [8, 9] targeted delivery. This is of fundamental significance for a number of drugs, in particular, photosensitizers (PSs), i.e., compounds that induce the formation of singlet oxygen and peroxide radicals under the action of light and are used for photodynamic therapy of tumors and other diseases [10, 11]. Many of these compounds have poor solubility in water and low selectivity coefficients, which makes their practical application difficult. The immobilization of PS mole cules on the surface of gold NPs may not only facilitate
the solution of this problem, but also, under certain conditions, improve the properties of a drug (i.e., increase its fluorescence [12] or the quantum yield of singlet oxygen [12, 13]). In the opinion of authors of [13], one of the reasons for this effect is the enhance ment of the electromagnetic field near NPs, which exhibit localized surface plasmon resonance (LSPR). PS molecules can be immobilized on the surface of NPs by means of physical adsorption [12, 14–17] or solubilization in a layer of a polymer chemisorbed on particles [17–19]. However, it should be taken into account that a substantial part of a drug must be rather rapidly desorbed form the particle surface upon intra venous injection of the nanostructures thus obtained. An approach based on the covalent bonding (con jugation) of PS molecules to gold NPs via S–Au bonds [18, 20–25] seems more promising. Note that all the cited works dealt with hydrophobic (i.e., actually waterinsoluble) compounds. Therefore, in most cases, additional efforts were necessary to provide the obtained conjugates with solubility in water, which was necessary for the use of these structures in experiments in vivo. In particular, to solve this problem, the authors of [23, 24] grafted a watersoluble polymer, αmer captoωcarboxy poly(ethylene glycol), which played the role of a phase transfer agent, together with octaalkylsubsituted zinc phthalocyanine to a gold NP surface.
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HOOC
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Fig. 1. Zinc mono[N(1carboxy methyl1mercaptome thyl)methyl]imide octa4,5carboxyphthalocyanine.
The goal of this work was to synthesize a watersol uble photosensitizer, zinc mono[N(1carboxy methyl1mercaptomethyl)methyl]imide octa4,5 carboxyphthalocyanine (MOCPZn), and its conju gates with gold NPs of different sizes and study their spectral characteristics. EXPERIMENTAL Materials and Methods The following compounds were used in the experi ments: chloroauric acid (HAuCl4); sodium citrate; sodium borohydride; aqueous solutions of sodium hydroxide (1 M), ammonium hydroxide (28 wt %), and tetrakis(hydroxymethyl)phosphonium chloride (80 wt %) (all ACS Reagent grade, Aldrich, Ger many); poly(ethylene glycol) (PEG) with molecular mass Mw = 2000 and Mw/Mn = 1.02 and having one terminal thiol group (Polymer Source, Canada); and dimethyl sulfoxide (DMSO, reagent grade, OOO KomponentReaktiv, Russia). All solutions were prepared in distilled water addi tionally deionized using an Arium 611 system (Sarto rius, Germany), with water having a specific conduc tivity of no higher than 0.056 µS/cm. The pH values of prepared colloidal solutions were measured with an I500 ion meter (NPKF Akvilon, Russia). NPs were centrifuged in a 320R centrifuge (Het tich, Germany). Amicon Ultra4 centrifugal filters were additionally used for 2 and 5nm particles. Par ticles were resuspended with the help of a Sapphire ultrasonic bath or an IKAT10 Basic ultrasonic dis perser (IKAWerke, Germany). NP sizes were measured with an LEO912 AB Omega highresolution transmission electron micro scope (HRTEM) (Karl Zeiss, Germany). In some cases, a Zetasizer Nano ZS dynamic lightscattering
spectrometer (DLS) (Malvern, United Kingdom) was used. Absorption spectra of colloidal solutions were mea sured in a wavelength range of 300–1100 nm with an Evolution 300 twobeam scanning spectrophotometer (Thermo Scientific, United States) using cells with light paths of 2 and 10 mm; the reference beam was passed through cells filled with distilled water or DMSO. Fluorescence spectra for solutions of MOCPZn with different concentrations and dispersions of its conjugates with gold NPs were recorded with a Cary Eclipse spectrofluorimeter (Varian, United States) using a xenon lamp as an exciting radiation source. The quantum yield of fluorescence (Φ) was measured for the studied dispersions relative to a reference solu tion of unsubstituted zinc phthalocyanine in DMSO (Φ = 0.20) [26]. In a number of cases, an RF 5301 PC spectrofluorimeter (Shimadzu, Japan) was also used. Synthesis of Zinc Octa4,5CarboxyPhthalocyanine– LCysteine Conjugates Zinc mono[N(1carboxy methyl1mercaptome thyl)methyl]imide octa4,5carboxyphthalocyanine (I). A mixture of zinc octa4,5carboxyphthalocyanine tetraanhydride [27] (0.42 g), Nmethylpyrrolidone (42 mL), and cysteine (0.06 g) was stirred for 8 h at 150–155°C under nitrogen. Phthalocyanine complex was precipitated with a petroleum ether–benzene mixture (4 : 1, vol/vol). The suspension was filtered, and the precipitate was washed on the filter with ben zene and hot water; then, it was purified by extracting impurities with hot chloroform, additional washing with acetone, and boiling in distilled water for 6 h for complete hydrolysis of anhydride groups; after that, the suspension was filtered. The precipitate was dried at 100–110°C to obtain compound I (Fig. 1) with a yield of higher than 90%. IR spectrum (KBr): ν 1437 (–S–CH2); 1707, 1769 (C=O, imide); 2581 (–S–H, traces); 2942 (CH2); 3231, 3253, 3398 (OH) cm–1. Electronicabsorption spectrum (phosphate buffer, pH 8): λmax 686, 657, 354 nm (relative intensities of 0.91 : 0.78 : 1). Elementalanalysis data: Found (%): C, 50.98, 51.03; H, 2.06, 2.10; N, 12.63, 12.52; S, 3.19, 3.27. Calculated for C43H19N12O16SZn (%): C, 50.87; H, 1.89; N, 12.42; S, 3.16. The addition of a dilute aqueous sodium hydroxide solution to a suspension of I in distilled water to pH 8.5 under stirring followed by filtration of a resulting solu tion from mechanical impurities and evaporation of the filtrate until dry with a rotary evaporator in vac uum (30 mmHg) gave rise to corresponding sodium salt (II) with a quantitative yield. COLLOID JOURNAL
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Electronicabsorption spectrum (water): λmax 682, 655, 355 nm (relative intensities of 0.88 : 0.84 : 1); (phosphate buffer, pH 8): 686, 655, 354 (relative intensities of 0.87 : 0.79 : 1). Zinc tri[N(1carboxy methyl1mercaptome thyl)methyl]imide octa4,5carboxyphthalocyanine (III). Compound III was obtained with a yield of 94% in the same way from the tetranhydride (0.35 g) and cysteine (0.5 g) in Nmethylpyrrolidone for 15 h. Electronicabsorption spectrum (DMF): λmax 685 (sh), 658, 362 nm (relative intensities of 1.06 : 1.41 : 1); (aqueous 0.05% sodium hydroxide solution): 658, 358 (relative intensities of 1.21 : 1). Elementalanalysis data: For C43H25N11O16S3Zn anal. calcd. (%) C, 49.64; H, 2.13; N, 13.00; S, 8.11. Found (%) C, 49.21, H, 2.32, N, 13.21, S, 7.88, 49.24; 2.38; 13.43; 7.71
Synthesis of MOCPZn Conjugates with Gold Nanoparticles of Different Sizes Three gold hydrosols with different particle sizes obtained by classical procedures with the use of sodium citrate [28], sodium borohydride [29], or tet rakis(hydroxymethyl)phosphonium chloride [30] as reductants were used in the experiments. After the synthesis was completed, gold NPs were precipitated by centrifugation for 1 h at 14000 rpm (for NPs with a diameter of 15 nm) or 5000 rpm (for NPs with diame ters of 5 and 2 nm); then, they were resuspended in phosphate buffer with pH 7.2. For more compete dep osition of 5 and 2nm NPs, Amicon Ultra4 centrif ugal filters were used. When synthesizing gold NPs conjugated with MOCPZn, an MOCPZn solution (2 mg/mL) in phos phate buffer was added to a preset volume of an NP dispersion in the same solvent. The photosensitizer was added in a fivefold excess relative to the calculated amount necessary for the monolayer coverage of NPs (an area of MOCPZn molecule projection onto a plane equal to 2 nm2 was used in the calculations). A mixture thus obtained was allowed to stand for 7 days, excess MOCPZn was removed by repeated centrifuga tion/resuspension of conjugate particles; then, they were dispersed in water, phosphate buffer, or DMSO. Mixed conjugates of Au nanoparticles with PEG and MOCPZn molecules were obtained in the same way. Both components were introduced in a threefold excess relative to the amount necessary for monolayer coverage of gold NPs. A mixture thus obtained was allowed to stand for at least 7 days; then, excess MOCPZn and PEG were removed in accordance with the abovedescribed scheme. The amount of MOCPZn chemisorbed on gold NPs was determined spectrophotometrically from variations in the light absorption by a supernatant at a COLLOID JOURNAL
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wavelength of 685 nm relative to the absorption by an initial PS solution with the help of a calibration curve. In addition, fluorescence spectra were measured for dispersions of conjugates after the removal of excess MOCPZn. RESULTS AND DISCUSSION Using the abovedescribed procedure, monocon jugate I of zinc octa4,5carboxyphthalocyanine with cysteine was obtained by heating an equimolar mix ture of Lcysteine and zinc octa4,5carboxyphtha locyanine tetraanhydride [27] in Nmethylpyrroli done under an inert gas. Compound I was soluble in polar aprotic organic solvents, phosphate buffer, and dilute aqueous solutions of alkalis. In the presence of excess cysteine, triconjugate III was synthesized. Sodium salt II of monoconjugate I was obtained by neutralizing a dilute aqueous solution to pH 8.5. The electronicabsorption spectra measured for solutions of the abovelisted compounds suggested that they occurred in an aggregated state. At the next stage of the study, MOCPZn conjugates with Au particles of different sizes were synthesized. For this purpose, we primarily used NPs of a citrate Au hydrosol obtained through the Turkevich procedure using sodium citrate as a reductant of metal ions and a stabilizer of particles being formed [28]. A micrograph of these particles is shown in Fig. 2. According to the HRTEM data, their average diameter was 15 ± 2 nm. The sol thus obtained had a ruby color and exhibited an intense symmetric LSPR peak with a maximum at a wavelength of 519 nm (Fig. 3, curve 1); the number concentration of the sol was 2 × 1012 particle/mL. Grafting of MOCPZn molecules to a gold NP sur face caused a very small (from 519 to 521 nm) batho chromic shift of the LSPR maximum, this shift being, obviously, due to a change in the dielectric permittivity of a medium near the particle surface, and the appear ance of a weakly pronounced shoulder in a range of 650–750 nm, i.e., in a region of the maximum absorp tion of the photosensitizer (Fig. 3, curve 2). The com parative analysis of the absorption spectra of MOCPZn measured in its initial solution and in the supernatant resulting from the centrifugation of the synthesized conjugate NPs showed that nearly 1000 MOCPZn molecules were grafted to one gold NP; hence, the NP surface area occupied by one MOCPZn molecule was about 0.7 nm2. This area is significantly smaller than the value aforementioned for the area of MOCPZn molecule projection (2 nm2), thereby indicating a nonplanar orientation of photo sensitizer molecules upon the formation of the conju gate. As a whole, this result agreed with the data of [24], in which a value of 155 molecules of octaalkyl subsituted zinc phthalocyanine per one Au NP with a diameter of ≈ 4.3 nm was obtained; moreover, work [24] concerned a mixed conjugate in which molecules
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Fig. 2. Panel (a): micrograph of citrate gold hydrosol nanoparticles and panel (b): histogram of particlesize dis tribution.
of two compounds, PEG and hydrophobic PS, were chemisorbed on the surface of gold NPs. As follows from the data presented in Fig. 4, the aqueous dispersion of our conjugate is characterized by a rather intense fluorescence with a maximum in the vicinity of 700 nm. Even large amounts of DMSO (in which the solubility of MOCPZn is much higher than that in water) did not substantially affect the shape and/or position of the fluorescence peak. All of the aforementioned suggested that dye molecules grafted to particle surface were mainly in the mono meric form. The amount of chemisorbed MOCPZn estimated from the fluorescence spectra appeared to be some what smaller that that mentioned above—nearly 460 MOCPZn molecules per one gold NP. This dis crepancy may be due to the effect of dyefluorescence quenching near plasmonic NPs (e.g., via the mecha nism of energy transfer [22]). This effect is known to be observed at relatively small distances (smaller than
8–10 nm) between a particle surface and a fluoro phore group [31–33]. Obviously, it is this situation that that we are dealing with. It should be emphasized that, taking into account the feasibility of using Au/PS conjugates in both diag nostics and therapy of tumors, the selection of the necessary distance between a PS molecule and a plas monic NP is a rather complex problem. Indeed, the feasibility of enhancing the fluorescence of PS mole cules by their conjugation with metal NPs is of greatest 1
interest for the purposes of diagnostics. At the same time, the quantum yield of singlet oxygen is of sub stantially greater importance for photodynamic ther apy. According to [13], this yield markedly increases, when a PS molecule is located in immediate proximity (at a distance of ~1 nm) to a metal particle surface. Au/PS conjugates are, as a rule, introduced into an organism in the form of a hydrosol. Therewith, it is of principle importance to ensure their long circulation in the bloodstream. Most commonly, this problem is solved by an additional modification of a particle sur face with PEG [8]. We synthesized mixed conjugate of citrate sol NPs with molecules of MOCPZn and PEG having one terminal thiol group. Experimental results testified that, as was to be expected, the amount of the dye chemisorbed on the surface of Au nanoparticles of this conjugate was reduced (by nearly two times) to compare with Au/MOCPZn conjugate. The aforementioned data suggest that gold nano particles can, indeed, play the role of carriers for chemisorbed MOCPZn molecules. At the same time, it is more reasonable to use NPs with sizes that are as small as possible, because the amount of a drug being delivered increases with the specific surface area (per 1 Remember that the distance between a dye molecule and an NP
must be from 8–10 to 20–25 nm for this enhancement [33]. COLLOID JOURNAL
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Fig. 4. Fluorescence spectrum of aqueous dispersion of MOCPZn conjugate with 15nm NPs of citrate gold sol.
Fig. 5. Normalized absorption spectra of phosphatebuffer dispersions of gold NPs (1) 5 and (2) 2 nm in diameter conjugated with MOCPZn.
unit mass) of particles. Therefore, our next step was the synthesis of MOCPZn conjugates with Au parti cles having average diameters of 5 and 2 nm. NPs with an average diameter of 5 nm were synthe sized by reducing Au(III) ions with sodium borohy dride in the presence of sodium citrate, which, in this case, played the role of a stabilizer of metal particles that were being formed [28]. The hydrosol thus obtained had a ruby color and exhibited an intense symmetric LSPR peak with a maximum at a wave length of 514 nm. The particlenumber concentration in the sol was approximately 1013 mL–1. Nanoparticles with an average diameter of 2 nm were obtained by a procedure [30] using tet rakis(hydroxymethyl)phosphonium chloride as a reductant. The resulting sol had deepbrown color and a particlenumber concentration of about 1015 mL–1. LSPR was not observed for this sol because of the overly small sizes and defectiveness of its particles [30]. MOCPZn conjugates with NPs of both sols (here after, they are denoted as Au(5) and Au(2), respec tively) were synthesized in accordance with the above described scheme (see the experimental section). After washing was completed, Au(2)/MOCPZn and Au(5)/MOCPZn conjugates were resuspended in phosphate buffer or DMSO. The absorption spectra of Au(2)/MOCPZn and Au(5)/MOCPZn are shown in Fig. 5. It can be seen that, in the case of the Au(5)/MOCPZn conjugate (curve 1), a bathochromic shift takes place for the LSPR maximum to compare with the initial sol (from 514 to 520 nm). The value of this shift is signifi cantly larger than that for the MOCPZn conjugate with 15nm gold particles of the citrate sol (see above). From our point of view, this may result from two rea sons. First, it cannot be excluded that, as the size of
gold NPs decreases, the surface grafting density of PS molecules increases, and, as a consequence, the local dielectric permittivity of the medium near the NP sur face changes more greatly. Second, under these condi tions (even at a constant grafting density), as the size of gold NPs decreases, the fraction of their conduction electrons involved in the formation of S–Au chemical bonds increases; the decrease in the concentration of free electrons that participate in the plasmon forma tion may also lead to the bathochromic shift of the LSPR band (a similar size effect took place upon ozone adsorption on gold NPs in their hydrosols [34]). Moreover, the spectrum of the dispersion of this conjugate comprises a pronounced absorption band in a range of 600–700 nm (Fig. 5, curve 1), which is, obviously, attributed to the light absorption by grafted MOCPZn molecules. A similar band with a markedly higher relative intensity is registered in the spectrum of a dispersion of Au(2)/MOCPZn conjugate nanoparti cles (Fig. 5, curve 2). Unfortunately, the analysis of corresponding supernatants did not yield the number of MOCPZn molecules grafted under our conditions to one Au(5) or Au(2) NP. The issue is that washing of the conjugates was accompanied by partial aggrega tion of dye molecules in pores of centrifugal filters. As follows from the data presented in Fig. 5, the resuspension of Au(2)/MOCPZn and Au(5)/ MOCPZn conjugates in phosphate buffer is accompa nied by the aggregation of chemisorbed MOCPZn molecules. This is evident from the hypsochromic shift and the change in the shape of the main absorption band of MOCPZn, which are especially pronounced for Au(2)/MOCPZn (curve 2). A similar situation remains for the corresponding Au/MOCPZn/PEG mixed conjugates (their spectra are not presented). It should be emphasized that MOCPZn fluorescence is
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REFERENCES
Fluorescence intensity, rel. units 1.0 0.8 0.6 0.4 0.2 0 600
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Fig. 6. Normalized fluorescence spectrum of Au(5)/MOCPZn/PEG mixed conjugate dispersion in DMSO.
not observed for these dispersions. From our point of view, this may be due to not only the aggregation of dye molecules, but also the effect of dye fluorescence quenching near the surface of plasmonic NPs (at least for 5nm particles). At the same time, the resuspension of Au(2)/MOCPZn and Au(5)/MOCPZn conjugates in DMSO is not accompanied by the aggregation of chemisorbed MOCPZn molecules. As a consequence, these dispersions fluoresce rather intensely. The fluo rescence spectrum of Au(5)/MOCPZn/PEG mixed conjugate is exemplified in Fig. 6. Note that, accord ing to our estimation, the quantum yield of fluores cence (Φ = 0.11) of MOCPZn chemisorbed on Au(5) NPs is even somewhat higher than that measured for its molecular solution with the same concentration (Φ = 0.07). The reasons for this effect remain to be studied. CONCLUSIONS Mono and triconjugates of zinc octa4,5car boxyphthalocyanine with Lcysteine have been syn thesized and characterized, the conjugates being highly soluble in phosphate buffer. Conjugate molecules have been grafted onto gold nanoparticles of different sizes. The optical properties of Au NP/photosensitizer dispersions in different sol vents (phosphate buffer and DMSO) have been stud ied. The number of phthalocyanine molecules grafted to the surface of 15nm gold nanoparticles in a citrate sol has been determined. It has been shown that the photosensitizer chemisorbed on gold nanoparticles exhibits rather intense fluorescence.
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