Preparation and Characterization of Magnesium Phthalocyanines Nanoparticles: Effect of Surfactant on the Shape and Size of Nanoparticles.
Summary: Magnesium phthalocyanine nanoparticles (MgPc) were developed in aqueous phase and the effect of anionic, cationic and non-ionic surfactant on the preparation of nanoparticles was evaluated. The sonication time, concentration of dye and the dispersing agents showed significant effects on the shape and size of the nanoparticles. The absorption spectra of nano MgPc dyes were investigated to confirm the formation of nanoparticles. The current study is expected to open a new gateway toward various applications for water insoluble dyes, especially towards biological activity of the nanomaterials.
Keywords: Nanoparticles, surfactant, sonication, dyes, biological activity
Phthalocyanines (PCs) are most commonly used as electro-optic devices, photoconducting agents, photovoltaic cell elements, non-linear optics as well as in electro-catalysis. PC derivatives exhibit high absorption coefficients (Iu (greater than)105 M-1cm-1) in the visible region of the spectrum, mainly in the phototherapeutic window (600-800 nm) and long lifetime triplet excited states for the efficient production of singlet molecular oxygen . Based on these properties, PCs were recently used in medicine for field detection of tumors and cure by photodynamic therapy (PDT) . Additionally, PCs are promising photosensitizers for the treatment of microbial infections involving photodynamic process (PDI). This requires the administration of a photosensitizer, which is accumulated preferentially in the microbial cells. Subsequent irradiation with visible light, in the presence of oxygen, results in cell damage that inactivates the microorganisms .
Thus, PCs were proposed for PDI of microorganisms in an attempt to overcome the problem of microbial strain resistance [4-6]. A long-standing problem with MgPc is their low solubility. Thus, it was important to create a facile route to nanoparticles in aqueous solution.
In this study, a new, efficient method for the preparation of magnesium phthalocyanine nanoparticles is described. The influence of various parameters such as concentration of magnesium phthalocyanine, concentration of surfactant, time of ultrasonic irradiation has been discussed. After preparation, the nanoparticles were characterized by Fourier Transform Infrared (FTIR), UV-visible Spectroscopy, Transmission Electron Microscopy (TEM) and Scanning Electron Microcopy (SEM). Scheme-1 shows a schematic representation of the preparation of magnesium phthalocyanine nanoparticles.
Results and discussion
Effect of MgPc Dye Concentration
Metal phthalocyanines have typical electronic absorption spectra containing a distinct band in the visible spectrum called the Q band (Fig. 1);a broader band appears in the blue region near 340 nm and is also termed the B or Soret band. Both Q and B bands of phthalocyanines are attributed to Ia" I transitions. The position of the Q band in the spectrum depends on the central metal atom, peripheral and non-peripheral substituents, axial ligands, the solvent in which the phthalocyanine is dissolved, aggregation, and the extent of I njugation. The Q band can be either blue-shifted (a hypsochromic shift) or red-shifted (a bathochromic shift) caused by change in the structure of the phthalocyanine. Variation in the shape of the Q band occurs when the symmetry of the complex is altered. The formation of phthalocyanine aggregates is a disadvantage since aggregation has an adverse effect on the photophysical behavior of phthalocyanines .
Addition a surface acting agent (surfactant) leads to dispersion of the aggregates and also dispersion of MPc in water (i.e to form nanoparticles). Factors affecting the formation of MgPc nanoparticles were monitored by UV- visible spectroscopy. Ten milliliters of an aqueous suspension of 5mg TritonX-100, or CTABor SDBS were irradiated for 1hr with different amounts of MgPc. Fig. 2 (a) and (b), show the UVa"Visible spectra of MgPc nanoparticles prepared by using TritonX-100 as dispersing agent.
The spectra showed a new Q band at I= 824 nm (nearr IR region) due to the formation of MgPc nanoparticles. We also observed that the absorbance increased with increasing concentration of MgPc to reach a 0.4g\L concentration led to a decrease in absorbance maximum at 0.4 g\L. Addition of MgPc above the In the case of CTAB [Fig. 3 (a) and (b)], we observed a shift in the Q band at I= 675 nm due to formation of MgPc nanoparticles but no new Q band was observed. We again observed that absorbance increased with increasing concentration of MgPc to a maximum absorbance at 0.5g\L. With SDBS, [Fig. 4(a) and (b)], we observed a new Q band at I= 824 nm due to the formation of MgPc nanoparticles and the absorbance increased with increased MgPc concentration to reach amaximum at 0.3g\L. Addition of MgPc more beyond 0.3g/L decreased absorbance.
Visibly, the intense blue colour, characteristic of these magnesium phthalocyanine solutions, was found to be much lighter in the nanoparticles samples.
Effect of Type and Concentration of Surfactant
Surfactant molecules have the ability to prevent aggregation in aqueous solution and were used as stabilizers for the preparation of organic nanoparticles. The surfactants are usually organic compounds that are amphiphilic in nature, which enables them to surround nanoparticles, thus making it difficult for them to self-associate. Three types of surfactants were used in the experiments, namely CTAB, SDBS, and Triton as cationic, anionic and neutral surfactants respectively. Aqueous suspensions of MgPc (4mg in 10ml H2O) withTritonX-100, 5 mg MgPc with CTAB and 3 mg MgPc with SDBS were sonicated for 1h with and without surfactants. As shown in Fig. 7-9 the intensity of absorbance increased with concentration of the surfactant. Thus, surfactants prevent aggregation and increase the efficiency of nanoparticle generation.
The surfactant molecules tightly surrounding the MgPc nanoparticles stabilize the excited state of the molecules and lower the HOMO energy (Fig. 5) causing a bathochromic shift of the Q band compared with MgPc in methanol. It is significant that the new Q band for MgPc developed in water. Also the Q band of the MgPc nanoparticles showed a bathochromic shift in both SDBS and Triton when compared with MgPc in CTAB (Table-1) due to interaction with the micelles of the former two surfactants . With TritonX-100, [Fig. 7 (a) and (b)], we observed a new Q band at I=824 nm (near IR region) due to the formation of MgPc nanoparticles that reached maximum intensity at 0.77 mM. Exceeding the critical micelle concentration (approximately 0.77 mM concentration) caused a decrease in absorbance intensity with CTAB, [Fig. 8(a) and (b)], we observed the Q band at I=675 nm indicating formation of MgPc nanoparticles but the new Q band at 820 nm was not observed.
This can be explained in terms of electrostatic repulsion of positive charges between the metal Pc and the surfactants destabilizing the LUMO of the MgPc nanoparticles and hence giving rise to a hypsochromic shift. The absorbance of the nanoparticles solution increases with increasing CTAB concentration and the highest absorbance value was observed with CTAB at 1.92 mM. Beyond this point the absorbance intensity gradually decreased. With SDBS as surfactant, we again observed a new Q band at I=824 nm due to formation of MgPc nanoparticles which can be explained by van der Waals and charge attraction that provides different stabilization of the MgPc LUMO. The absorbance increased as the concentration of the surfactant increased up to 2.28 mM.
Table-1: UV-visible absorption values of MgPc bulk, solution and nano dispersed particles.
(Q) band###(Q) band (Q) band (B) band
-###675###645 (sh) 395###MgPc CTAB
In general, the results reveal enhancement of the absorption of MPc dyes as the concentration of surfactants increases until the CMC concentration is reached. The result can be rationalized on the basis of MPc micellization in micelle aggregate and the second case is the further enhancement of absorption values above CMC which was until certain point of surfactant concentration. The surfactant molecules connect with the superficial molecules of the particles by hydrophobic force on one side and by other side it connects with water molecules by hydrophilic force. At high concentration of the surfactants, all the initial particles of metal phthalocyanine will be surrounded by surfactant molecules. Fig. 10 illustrates the effect of surfactant on the colo rof the MgPc dispersion.
Effect of Sonication Time
The ultrasonic irradiation time has profound effects on the particle size of the MgPc nanoparticles. The absorbance increase is due to an increase in the number of nanoparticles and a decrease in size resulting in a larger surface area. Fig. 11-13 shows absorbance dependence vs irradiation time for an optimum sample of MgPc nanoparticles. Before irradiation, the associated microcrystals of MgPc precipitate in the cell leaving the solution colorless.
After irradiation, a transparent blue colloidal solution was obtained with different degrees of color depending on the surfactants used. High irradiation time gave higher dispersion and hence higher ntensity of the absorbance band. The new bands in the region of 800-900 nm are not characteristic of metal phthalocyanines. Fig.. 11a-13a shows the spectra of magnesium phthalocyanines nanoparticles using different surfactants. The formation of a new absorbance bands in the region of 800-900 nm represent a breakthrough in the field of photodynamic therapy.
An electron microscope (SEM andTEM) was used to acquire micrographs of nanoparticles under optimum conditions. For scanning electron microscopy, the magnesium phthalocyanine nanoparticles were centrifuged, the solid mass collected and subjected to measurement by SEM and TEM. The types of surfactants effects and the reactant concentration determine the size and morphology of the MgPc nanoparticles. Fig. 14 shows SEM pictures of MgPc nanoparticles prepared using different types of surfactants. Fig. 14 (A-C) shows SEM images of MgPc-TritonX-100, MgPc- CTAB and MgPc-SDBS nanoparticles. The MgPc- Triton nanoparticles have a smoother surface appear to be in the formand appear to be in the form of nanospheres.
The MgPc-CTAB and MgPc-SDBS nanoparticles appear as rods with much smaller size compared to Triton nanoparticles. Fig. 15 (AandBandC) shows a TEM image which reveals that the sample is composed of a large quantity of nanoparticles with uniform shape and approximately the same size. The average size of the nanoparticles (quantum dots) range from 5-10 nm.
For IR spectra, the magnesium phthalocyanine nanoparticles were centrifuged and washed with de-ionized water three times. Metal phthalocyanine complexes possess similar infrared (IR) spectra. The shift in characteristic MPc bands was about 50 cm-1 as one central metal was replaced by another. The significant bands are the C-H stretching vibration around 3030 cm-1, C-C ring skeletal stretching vibrations around 1600 and 1475 cm-1, and the C-H out of plane bending vibrations around 750-790 cm-1. All these bands are due to the aromatic ring of the MPc. Metal sensitive bands appear at about 1490 and 1410 cm-1. show the IR spectra of the magnesium phthalocynine nanoparticles obtained with different surfactants.
All chemicals were of analytical reagent- grade. Magnesium phthalocyanines (MgPc) were purchased from A Bcross organics. Methanol, ethanol, cationic surfactant, cetyltrimethyl ammonium bromide (CTAB), non-ionic surfactant, Triton X- 100 and anionic surfactant, sodium p- dodecylbenzene sulfonate (SDBS) were purchased from Sigmaa"Aldrich and used without further purification. De-ionized water was used throughout the experiments.
Preparation of MgPc Nanoparticles Using Triton X- 100 Surfactant. Some tests were carried out in order to optimize experimental conditions such as magnesium phthalocyanine concentrations, concentration of surfactants and sonication time. In a typical procedure for non-ionic surfactant, the following three procedures were carried out.
Effect of MgPc Concentration
A glass rod-milled mixture of different amounts of MgPc (1,2,3,4,5 mg) and Triton X-100 (5mg) were prepared by dropwise addition of 10 ml de-ionized water. Each mixture was sonicated at 400 C for 1 h, and then left to settle at room temperature for one day prior to measuring the absorption spectra of the dispersion.
Effect of Surfactant Concentration
The above procedure was carried out by using different amounts of Triton X-100 (0, 1,2,4,5,7,15 mg) and MgPc (4 mg).
Effect of Sonication Time
A milled mixture of MgPc (4mg), Triton X- 100 (4mg) and 10 ml de-ionized water was prepared as above. The mixtures were sonicated for different time intervals (0 - 60 min) at 400 C and again left to settle at room temperature for one day.
Preparation of MgPc Nanoparticles Using CTAB and SDBS Surfactants. The same method was applied as in the case of Triton-X except the temperature used was 30 oC. Milled mixtures of MgPc (5mg), CTAB (7mg) and 10 ml de-ionized water or MgPc (3mg) and SDBS (9mg) and 10 ml de-ionized water were prepared as above and sonicated for different time intervals (0 - 60 min) at 300 C. The mixtures were left to settle at room temperature for one day.
Effect of MgPc Concentration
The influence of MgPc was studied by changing its concentration which are given in Table 2.
Table-2: Conditions of preparation; 5 mg of CTAB or SDBS at 300 C for 1h by using different MgPc concentrations.
Effect of Surfactant Concentration
The effect of surfactant was investigated by using different amounts of surfactant given in Table 3.
Table-3: Preparative conditions, 5mg MgPc, in case of CTAB or 3mg MgPc in case of SDBS at 300 C for 1h by using different surfactant concentrations.
Effect of Sonication Time
Milled mixtures of MgPc (5mg), CTAB (7mg) and 10 ml de-ionized water or MgPc (3mg) and SDBS (9mg) and 10 ml de-ionized water were prepared as above and sonicated for different time intervals (0 - 60 min) at 300 C. The mixtures were left to settle at room temperature for one day.
Characterization of Magnesium Phthalocyanine Nanoparticles A UV-visible spectrum was recorded on a Varian Cary 50 with 1 cm (path length) quartz cuvettes. In the case of MgPc however, a plastic cell was used to monitor the formation of magnesium phthalocyanines nanoparticles under different experimental conditions. The FT-IR spectra were recorded on a Perkin-Elmer 100 series (Beaconsfield, Bucks, and UK). A centrifuge from Sigma-Aldrich with speed 3900 rpm (St. Louis, Mo, USA) was used to separate the nanoparticles. A Barnstead\lab-line "Aqua Wave 9372" ultrasonic bath was used.
Transmission electron microscopy (TEM, Model JEM-1011; JEOL, Tokyo Japan) was used to assess the formation of nanoparticles. TEM samples were prepared by direct deposition on a carbon-coated copper grid. A scanning electron microscope (SEM, Model JSM 6380LA; JEOL, Tokyo Japan) was used to monitor the formation of nanoparticles. De-ionized water (12 Ma,,|cm) from a Millipore Milli-Q water purification system was used for all aqueous solutions.
Magnesium phthalocyanine nanoparticles were synthesized using ultrasonic irradiation in the presence of surfactant. The absorption spectra of the surfactant encapsulated nanoparticles exhibited a large bathochromic shift compared to that of the bulk particles. This property of MgPc nanoparticles may prove useful for information storage or display applications. It is expected that this work will afford new approaches to applications in aqueous medium of water-insoluble dyes particularly applications relevant to biological activity such as photodynamic therapy.
This work was funded by King Abdulaziz City for Science and Technology (National Program for Advanced and Strategic Technologies) Saudi Arabia under grant No. (8-ENE198-3).
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Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, Jeddah 21589, P.O. Box 80203, Saudi Arabia. Chemistry Department, Faculty of Science, King Abdulaziz University, P. O. Box 80203, Jeddah 21589, Saudi Arabia. aasiri firstname.lastname@example.org
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|Author:||Asziri, Abdullah Mohamed; Otaibi, Maha Moteb al-; Shishtawy, Reda Muhammad el-; Alamry, Khalid Ahmad|
|Publication:||Journal of the Chemical Society of Pakistan|
|Date:||Oct 31, 2013|
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