Characterization of liposomes
Figure 2 shows the fluorescence image of calcein loaded liposomes at room temperature (a) and AFM (Atomic Force Microscope) image of the liposomal formulation (b). The fluorescence image shows round hollow nanosize particles containing fluorescence agent as green spots; and AFM image indicate uniform spherical particles. The particle size of liposomes was estimated to be around 100 nm by AFM.
Figure 3 shows particle size and zeta potential of prepared liposomes obtained by nanozetasizer. Liposomes showed a partially negative zeta potential of -4.66 ± 0.7 mV (mean ± SD, n = 3). In term of size, liposomes showed a particle size of 83.87 ± 8.49 nm (mean ± SD, n = 3) in good agreement with AFM results. Their PDI was 0.163, which represent a good uniformity, in good agreement with imaging results.
Calcein calibration curve
Calibration curve was drawn up from 10
-9 to 10
-7 mM calcein using 11 concentrations at pH 7.5 (
Figure 4). Calcein concentration vs. intensity showed a good linear relationship (R
2 = 0.995) over the studied range (
Figure 4).
Release of calcein from liposomes
Figure 5 shows calcein release from liposomes in several conditions including lysis with methanol, placing in darkness and exposing to the indoor daylight, sunlight, sunlight exposure using vials covered with an opaque film (to study the thermal effect of sunlight), white LED light, red LED light, green LED light and blue LED light. In this study, liposome lysis by methanol which have the maximum release, was considered as 100% release and other liposomal formulations were evaluated in comparison to this value.
According to the results shown in
Figure 5, liposomes, placed in darkness (at room temperature) and opaque vials exposed to sunlight showed no significant release over time, while exposing to the indoor daylight and direct sunlight led to up to 10 and 34% release, respectively. White LED with approximately 50% release, showed the highest effect (
Figure 5). Light insensitive liposomal formulation (without DC
8,9PC) also did not show any calcein release. Statistical analysis showed that the difference between sunlight and indoor daylight (
P-value = 0.000) and white LED and sunlight (
P-value = 0.021) are significant. These results show that the DC
8,9PC liposomes release their cargo only in the presence of light and that the effect of light is not a thermal effect. These results indicate that liposomes should be kept in darkness to prevent any possible release due to light from other sources such as daylight. Therefore, such systems require special packaging. However, heating by sunlight did not result in release of the cargo and no protection is required in this regard.
DC
8,9PC is a lipidic monomer which will be polymerized when exposed to the light as demonstrated by Yavlovich,
et al. (
54), using DC
8,9PC-containing liposomes exposed to UV laser radiation for up to 45 min. Application of LED in our system provides some advantages. In comparison to laser, LED is much safer and more user friendly with lower cost (
55,
56). Besides this, the LED used in the present investigation is in the wavelength range of visible light that is safer than UV.
It worth to note that Kenaan
et al. (
53) studied DC
8,9PC reactions with UV irradiation and showed that exposing DC
8,9PC liposomes to UV radiation could result in oxidative reactions and formation of products such as ketones, aldehydes and alcohols, in a time-dependent manner and if they are exposed to UV for more than 8 min. Accordingly, and in spite of the fact that LED is expected to be safer than UV, the radiation time in the present investigation was chosen to be 5 min to reduce such a risk. However, further studies are required to assess reactions of these lipids with LED lights.
Our results show that light-triggered calcein release from liposomes depends on light type and intensity. Exposure to sunlight showed higher release than indoor daylight (10% vs 34%). Intensity of sunlight is about 2000 times of that of indoor daylight. Considering our results (
Figure 5) and light properties (
Table 1), we can conclude that the sensitivity of liposomes to light more depends on the type (wavelength) of the light than to intensity. This is in good agreement with Leung and Romanowski who suggested that wavelength is the most affecting factor on cargo release from light-sensitive liposomes (
57). Each wavelength has a specific effect on a specific compound and, therefore, specific release percentage. Therefore, we cannot expect full release for all wavelengths, even by increasing the intensity. This will explain the small difference in calcein release between sunlight and indoor-daylight, in spite of huge difference between their intensities.
White light contains all the wavelengths in the visible area. There are some reports about different effects of various wavelengths on biological materials. P. Moore,
et al. investigated the effect of wavelength on proliferation of cultured murine cells and concluded that both wavelength and cell type influence the cell proliferation response to low intensity laser irradiation (
58). It has also been shown that different doses and wavelengths of low-level laser therapy (LLLT) show different effects on cytochrome-c oxidase activity in intact skeletal muscle of rats (
59). Wang
et al. also reported that photobiomodulation of human adipose-derived stem cells under different wavelengths operates via different mechanisms of action (
60). However, there is no systematic study on the effects of wavelength of light on the release of cargos from light-sensitive liposomes and to the best of our knowledge, reported experiments in the literatures usually deal with only one wavelength (
57,
61,
62).
To determine which part of white LED shows more effect, we studied several wavelengths (red, green and blue) in the present investigation. These wavelengths are used for clinical and cosmetic applications. For instances, red range of wavelengths is used in wrinkles, scars, and persistent wounds treatments, green range of wavelengths is used for skin calming, anti-aging and increasing the collagen in the skin and blue range of wavelengths is used for treatment of acne vulgaris, combination therapy and seasonal affective disorders (
63).
Our findings indicate that different wavelengths show different effects on calcein release from liposomes. The highest release was observed for blue LED with about 30% calcein release, followed by 24% release by red LED (
Figure 5). Green LED showed lowest amount of calcein release (less than 10%) (
Figure 5). Statistical analysis show that the differences between all light LEDs (blue, green and red) or these lights with white LED, are significant (
P-value < 0.003). Their effects on release are different and red LED with the lowest intensity was more effective than green light, by about 2.5 times. These data are in good agreement with Leung and Romanowski who have suggested that wavelength is the most affecting factor on cargo release from light-sensitive liposomes (
57).
.
| LED light | Wavelength (nm)a | Intensities (lx, in 5 cm distance) |
|---|
| Blue | 452 | 53200 |
| Green | 511 | 40700 |
| Red | 630 | 17400 |
| White (blue+green+red) | 452 + 511 + 630 | 83600 |
| Sunlight | | 99300 |
| Indoor daylightb | | 44 |