The synthesis of nonspherical (and other) nanoparticle morphologies has traditionally been accomplished using a multistep self-assembly procedure starting with the synthesis and purification of well-defined amphiphilic diblock (or multiblock) copolymers. One of the most common self-assembly techniques was popularized by Eisenberg in the 1990s and involves the dissolution of the amphiphilic block copolymer in a common solvent for both polymer blocks followed by the slow addition of a solvent selective for one of the blocks1-3. As the selective solvent (typically water) is added, the block copolymer undergoes self-assembly to form polymeric nanoparticles. The final morphology (or mixtures of morphologies) of the nanoparticles are determined by a large number of factors such as the relative lengths of each polymer block, rate of water addition and the nature of the common solvent. However, this approach generally only allows for the production of nanoparticles at relatively low solids content (less than 1 wt%) and so limits its practical scalability4. In addition, the reproducible formation of "intermediate" phases such as worm-like micelles can be difficult owing to the narrow range of parameters required to stabilize this nonspherical morphology5.
The polymerization-induced self-assembly (PISA) approach partially addresses the drawbacks of the Eisenberg approach by utilizing the polymerization process itself to drive self-assembly in situ allowing for nanoparticle synthesis at much higher solids content (typically 10-30 wt%)6-8. In a typical PISA approach, a living polymerization process is used to chain extend a solvent soluble macroinitiator (or macro-CTA) with a monomer that is initially soluble in the reaction medium but forms an insoluble polymer. The PISA approach has been used to synthesize worm-like micelles by systematically testing a number of experimental parameters and using detailed phase diagrams as a synthetic "roadmap"5,9.
Despite their challenging synthesis, there is great interest in worm-like nanoparticles due to their interesting properties relative to their spherical counterparts. For example, we have demonstrated that drug loaded short and long worm-like micelles synthesized using a PISA approach have significantly higher in vitro cytotoxicity compared to spherical micelles or vesicles10. Others have shown a correlation between nanoparticle aspect ratio and blood circulation time in in vivo models11. Others have shown that the synthesis of worm-like nanoparticles using an appropriate PISA methodology yields a macroscopic gel due to the nanoscale entanglement of the nanoparticle filaments. These gels have demonstrated potential as sterilizable gels owing to their thermoreversible sol-gel behavior12.
This protocol describes a method allowing for the in situ monitoring of the formation of worm-like micelles by simply observing the solution viscosity during the polymerization. Previous studies of similar worm-like micellar gels have demonstrated that above a critical temperature, these nanoparticles undergo a reversible worm-sphere transition and so form free-flowing dispersions at elevated temperatures. To date, these systems have utilized a thermally sensitive azo compound to initiate the controlled polymerization13,14 and so gelation may not be readily observed in these systems during the thermal polymerization. From these studies, it was hypothesized that synthesizing PISA derived nanoparticles at lower temperatures may allow for observation of this gelation behavior in situ.
Recently we reported the use of a facile room temperature photopolymerization technique to mediate the PISA process to yield nanoparticles of different morphologies15. Here, a visualized protocol is presented for the reproducible synthesis of worm-like micelles by observing the solution viscosity behavior during the polymerization. The dispersion polymerization proceeds readily using commercially available light-emitting diodes (LEDs) (λ = 460 nm, 0.7 mW/cm2).
The overall goal of this protocol is to demonstrate the Facile Synthesis of Worm-like Micelles using Visible Light Mediated Polymerization. This method involves the preparation of polymerate nanoparticles, which can be used for a range of applications such as nano medicine The main advantage of this procedure is the production of worm-like nanoparticles using two-step procedure. The implication of this technique extends towards the field of drug delivery because in a number of studies, drug loaded worm-like nanoparticles have shown to give increased cellular uptake.
To begin this procedure, add the appropriate reagents as indicated in the text protocol, a magnetic stir bar and 50 milliliters of acetone nitrile to 100 milliliter round bottom flask. Seal the flask with an appropriately sized rubber septum and steel wire. Then, cool the flask to less than four degrees Celsius in an ice water bath.
Following this, de-oxygenate the flask for thirty minutes by bubbling nitrogen directly into the reaction mixture through a 21 gauge needle with a second 21 gauge needle acting as a vent. Place the flask in an oil bath at 70 degrees Celsius for five and a half hours. Once the reaction is compl