Calcium Phosphate Nanoparticle Synthesis with DNA Encapsulation
Much of the protocol is adapted from Thomas Morgan's report on calcium phosphate encapsulation. Another good resource is Encapsulation of Plasmid DNA by Xia Cao in Ximing Xu's lab in China.
Double Microemulsion Preparation
A microemulsion is a mixture of oil, water, and surfactant. In our case, we are using reverse microemulsion win which nano-scale water droplets are dispersed in an oil phase. The surfactant allows for the formation of the emulsion of other otherwise immiscible phases of oil and water. The microemulsion provides us a controlled nanoscale environment for synthesis of the microparticles within the nano-scale droplets of water.
Aqueous solution preparation
- Vacuum aspirate for two hours distilled water (and ethanol) to create CO2 free dionized water solution. This is to prevent the CO2 from affecting the reaction or from microbubbles from forming with freezing solutions. The gas free dionized water can be stored for up to one weeks and should be used for all aqueous solutions.
- Make solution of 1 x 10-1 M CaCl2. Use probe sonicator for at least 10 minutes.
- Make solution of 6 x 10-2 disodium phosphate and 8.2 x 10. 8.2 x 10-3 disodium silicate. Stir for at least 30 minutes. The silicate serves as a nucleation agent for the calcium phosphate .
- Also, add the DNA origami to the phosphate solution. The DNA and phosphate solution should have little interaction until the calcium is introduced.
- Prepare 1 x 10-2 M disodium hydrogen citrate dihydrate solution. This is for coating the CP nanoparticle to allow disperal.
- The molar ratio of water to surfactant should be 4 to 1. If we use 3 ml of Igepal then we will want an aqueous volume of 490 ul.
- Create 10 mL of 29% (v/v) nonionic surfactant poly(oxyethylene)-nonylphenyl ether (Igepal CO-520) in cyclohexane.
- Watch out! Igepal CO-520 is cancer-suspecting agent. Use gloves.
- Add 444 μL of distilled water.
- Add 46 μL of 1 x 10-2 M CaCl2 (creates 9.4 mM calcium), and allow to equilibrate for one hour.
- Create 10 mL of 29% (volume/volume) of Igepal CO-520 in cycloehxane.
- Add 444 μL of distilled water (minus however much DNA origami solution is added).
- Mix 46 μL of the phosphate/silicate solution, (creates 5.6 mM phosphate and 0.77 mM sodium silicate)
- Allow to equilibrate for one hour.
Mix A and B
- Use syringe pump to slowly add microemulsion A to B for ~2-10 minutes under constant stirring. The ratio of calcium to phosphate is 10:6, which is the same as the hydroxyapatite. For 2 minute times, use plastic disposable pipettes.
- Quench the precipitation by adding 16 μL aliquot of the 1 x 10-2 sodium citrate solution and allow to react for 10-15 minutes.
- The sodium citrate will serve to disperse the nanoparticles and thus halt the mineralization of the calcium phosphate . It will also present carboxyl functional groups on the surface of the nanoparticle, for later amide conjugation if we so desire. 4 x 10-2 M aminopropyltriethoxysilane(APTES) can also be used as a dispersant for a longer period of time, if we want to have amine-functionalized nanoparticles.
- 'Dissolve the emulsions with 35 mL of ethanol (pH 7, adjust with 1 M KOH).' This will dissolve the micelles.
- Use Aeorsol OT or TritonX-100 from Sigma as surfactants. Try stirring the microemulsions at 4°C for 12 hrs if not optically clear, and add B to *A at 4 ml/hr for 12 h in 4°C cold room. See Mozumdar's protocol 
- We may also want to dope with metasilicate. See another of Adair's protocols.
- See also Polyacrylic acid-PEG to skip the citrate quenching.
- See BSA-Rhodamine flourophore used by Sokolova. Be sure to use slicing by confocal to ensure that flourophores are within the cell and not accumulating upon the surface.
- We need to build/predict a phase diagram for the surfactant vs. hexane vs. aq(Ca/p)
High Performance Liquid Chromatography
We will use HPLC with van der Waals interaction to seperate the nanoparticles from the leftover reagents, DNA, and cyclohexane. The nanoparticles will then be eluted into a high-concentration well-dispersed solution. Much is adapted from Wang et al.
- Load the stationary phase with 20 μm silica microspheres. Use the APS-treated negative-charged microspheres, which will bind the carboxyl-terminated nanoparticles.
- The terminal end of the colum should be connected to a spectrometer with the wavelength set an absorbance that is suited to whichever marker or dye is used.
- Load the unwashed nanoparticle suspension into the column, followed by ethanol.
- Elute the column, with the flow rate fixed at 0.5-2mL/min.
- The silica stationary particles will bind the nanoparticles due to van der Waals attraction when the mobile phase is nonionic. The ethanol will thus wash away the surfactants/cyclohexane.
- Continue washing until the free DNA content is below limit of UV detection in the fractions. Usually only one pass of the eluent is enough.
- Once washing is complete, elute the nanoparticles using 70:30 ethanol-water solution (with dionized degassed water of 5 x 10-4 M NaCl pH 7).
- The water provides enough ionic attraction to overwhelm the van der Waals interaction of silica and nanoparticles. The nanoparticles will become mobile and will exit the column into a concentration solution.
- Collect the fractions, and use the fraction with the first major peak in absorbance observed.
Note: In the event HPLC is not availabe, then try following Mozumdar's protocol for microemulsions: (Her 2008 protocol differs from 2003)
- Optional 2008:Evaporate off the cyclohexane, and dissolve the cake in 10 ml of ehtanol.
- Centrifuge at 8000 rpm for 30 minutes
- Then wash pelleted nanoparticles with ethanol three times.
- Redisperse in 10 ml of distilled water or PBS by sonication (22 kHz) for 2 hrs.
- Dialyze the nanoparticles for ten hours.
Or try without microemulsion:
- The nanoparticles can now be conjugate with a variety of compounds to provide it with a secondary functionalization. See secondary functionalization protocol.
- The nanoparticles were also taken under images, see microscopy protocol.
- Morgan TT, Muddana HS, Altinoglu EI, Rouse SM, Tabaković A, Tabouillot T, Russin TJ, Shanmugavelandy SS, Butler PJ, Eklund PC, Yun JK, Kester M, and Adair JH. . pmid:19367837.
- Adair, JH et al. Heterogeneous Deposition of Calcium Phosphates at the Silicon(Hydrous) Oxide-Water Interface. Urolithias 2. New York: Plenum Press; (1995) p. 181-187.
- Leeuwenburgh SC, Ana ID, and Jansen JA. . pmid:19751849.
- Singh R, Saxena A, and Subho Mozumdar. Calcium Phosphate-DNA Nanocomposites: Morphological Studies and Their Bile Duct Infusion for Liver-Directed Gene Therapy. Int. J. Appl. Ceramics Technology 5; (2008) p. 1-10.
- Barth BM, Sharma R, Altinoğlu EI, Morgan TT, Shanmugavelandy SS, Kaiser JM, McGovern C, Matters GL, Smith JP, Kester M, and Adair JH. . pmid:20180585.
- Sokolova V, Kovtun A, Heumann R, and Epple M. . pmid:17031704.
- Wang J, White WB, and Adair JH. . pmid:16526702.
- Paul M Gruenbacher 21:18, 18 April 2013 (EDT):