different solubility parameters. Micellar structures are favored in the case of
hydrophobic substrates solubilized within the hydrophobic core of the micelle
[253, 254]. Hydrophilic substrates are typically encapsulated by solubilization
within the water-filled cavity of liposomes or vesicles. Several block copolymers
have been used, including PB-b-PEO [255, 256]. The hydrophobic encapsulation
into the vesicle shell of various polymers has been reported recently [257–263], but
most of these works address the interface by simply utilizing the amphiphilic
substrate itself [264–266]. However, it has not yet been possible to verify the full
hydrophobic nature of the encapsulation, especially in the case of colloidal
substrates.
Considering potential pharmaceutical applications, encapsulation of different
substrates at the same time is demanded but is also a scientific challenge [267]. The
polymersome system presented here offers the possibility for encapsulation of both
hydrophilic and hydrophobic substances at the very same time in the very same
polymersome. Potential loading systems are typically limited by the lack of suitable
characterization methods. We utilized fluorescence correlation spectroscopy (FCS)
in combination with cryogenic TEM imaging and DLS to characterize hydrophobic
loading.
In order to understand the influence of different molecular parameters on
structure formation and stability, we report on the directed encapsulation of two
hydrophobic model substrates inside the polymersome shell. The system both
complies with requirements like water insolubility and has sufficient fluorescence
intensity for monitoring. Fluorescent CdSe/CdS/ZnS core–shell quantum dots
(QDs), which carry hydrophobic surface ligands, serve as a model substrate of
the nanosize regime (core size approximately 6 nm). Nile Red, a lipophilic fluorescent dye, represents the molecular size regime [268–270].
Polymersome dispersions were produced following two different routes, the
cosolvent method and the rehydration method:
• In the cosolvent method, small polymersomes with a narrow size distribution
were obtained starting with a copolymer solution in THF and dropwise addition
of water. Controlled by a syringe pump, the dropping velocity of water addition
was set to 9.9 mL/h. At approximately 30 wt% THF, the addition of water was
stopped and the THF evaporated over 2–3 days. Standard final polymer concentration was around 1 g/L in water. The samples were filtered through 0.45-μm
filters before proceeding. Nile Red loading of those polymersomes was achieved
by the addition of dye to the starting copolymer/THF solution with different
dye/copolymer weight ratios. Further treatment was done the same way.
• Employing the rehydration method, huge polymersomes with broad size
distribution were obtained starting with a copolymer solution in THF or chloroform and creating a film in a Teflon vessel. After film drying at 50
C under
vacuum, water was added. Film rehydration was supported by intense ultrasonic
use and an elevated temperature of 50
C. The dispersions were filtered through
5 μm filters before proceeding. For DLS and FCS, additional filtration through
0.45-μm filters was used to remove dust and larger structures. QD loading of
Structure Formation of Polymeric Building Blocks: Complex Polymer Architectures
189
hydrophobic substrates solubilized within the hydrophobic core of the micelle
[253, 254]. Hydrophilic substrates are typically encapsulated by solubilization
within the water-filled cavity of liposomes or vesicles. Several block copolymers
have been used, including PB-b-PEO [255, 256]. The hydrophobic encapsulation
into the vesicle shell of various polymers has been reported recently [257–263], but
most of these works address the interface by simply utilizing the amphiphilic
substrate itself [264–266]. However, it has not yet been possible to verify the full
hydrophobic nature of the encapsulation, especially in the case of colloidal
substrates.
Considering potential pharmaceutical applications, encapsulation of different
substrates at the same time is demanded but is also a scientific challenge [267]. The
polymersome system presented here offers the possibility for encapsulation of both
hydrophilic and hydrophobic substances at the very same time in the very same
polymersome. Potential loading systems are typically limited by the lack of suitable
characterization methods. We utilized fluorescence correlation spectroscopy (FCS)
in combination with cryogenic TEM imaging and DLS to characterize hydrophobic
loading.
In order to understand the influence of different molecular parameters on
structure formation and stability, we report on the directed encapsulation of two
hydrophobic model substrates inside the polymersome shell. The system both
complies with requirements like water insolubility and has sufficient fluorescence
intensity for monitoring. Fluorescent CdSe/CdS/ZnS core–shell quantum dots
(QDs), which carry hydrophobic surface ligands, serve as a model substrate of
the nanosize regime (core size approximately 6 nm). Nile Red, a lipophilic fluorescent dye, represents the molecular size regime [268–270].
Polymersome dispersions were produced following two different routes, the
cosolvent method and the rehydration method:
• In the cosolvent method, small polymersomes with a narrow size distribution
were obtained starting with a copolymer solution in THF and dropwise addition
of water. Controlled by a syringe pump, the dropping velocity of water addition
was set to 9.9 mL/h. At approximately 30 wt% THF, the addition of water was
stopped and the THF evaporated over 2–3 days. Standard final polymer concentration was around 1 g/L in water. The samples were filtered through 0.45-μm
filters before proceeding. Nile Red loading of those polymersomes was achieved
by the addition of dye to the starting copolymer/THF solution with different
dye/copolymer weight ratios. Further treatment was done the same way.
• Employing the rehydration method, huge polymersomes with broad size
distribution were obtained starting with a copolymer solution in THF or chloroform and creating a film in a Teflon vessel. After film drying at 50
C under
vacuum, water was added. Film rehydration was supported by intense ultrasonic
use and an elevated temperature of 50
C. The dispersions were filtered through
5 μm filters before proceeding. For DLS and FCS, additional filtration through
0.45-μm filters was used to remove dust and larger structures. QD loading of
Structure Formation of Polymeric Building Blocks: Complex Polymer Architectures
189