Biorelevant Latexes and Microgels for the Interaction with Nucleic Acids

Amphoteric core-shell microgels: contraphilic two-compartment colloidal particles

pH-responsive amphoteric core-shell microgel particles were synthesized by emulsion copolymerization of the appropriate functional monomers with ethylene glycol dimethacrylate as the cross-linker. 2-(Diethylamino)ethyl methacrylate (DEA) was used as the ionizable basic monomer, and tert-butyl methacrylate served as the hydrophobic monomer precursor, which gave the methacrylic acid (MAA) moieties following acid hydrolysis of the ester groups. The core of the polyampholyte microgels comprised a cross-linked poly(2-(diethylamino)ethyl methacrylate) (PDEA) or poly(methacrylic acid) (PMAA) network surrounded by a cross-linked PMAA or PDEA shell, respectively. A polyampholyte random copolymer microgel with the DEA and MAA units randomly distributed within the gel phase was also prepared. Scanning electron microscopy studies showed spherical particles of a narrow size distribution, and transmission electron microscopy verified the core-shell topology of the particles. Potentiometric titration curves revealed two plateau regions for the polyampholyte core-shell microgels attributed to the independent ionization process of the core and the shell of the particles, in contrast to the random copolymer microgel particles that exhibited a single plateau region as a result of the simultaneous protonation/deprotonation process of the basic and acidic moieties of the microgels. The core and the shell of the particles were found to swell independently upon ionization of the DEA or MAA moieties at low or high pH, respectively, whereas collapsed latex particles were obtained in the intermediate pH range when both the core and the shell of the particles were neutral, in agreement with the potentiometric titration data. These core-shell microgels comprise novel two-compartment nanostructures that exhibit contraphilic properties in the core and the shell of the particles in response to a single external stimulus.

Chemiluminescent bead-based hybridization assay for the detection of genomic DNA from E. coli in purified plasmid samples

A bead-based hybridization assay was developed for detection of traces of E. coli genomic DNA (gDNA) present in purified plasmid DNA (pDNA) samples. Standards of gDNA and pDNA samples were sheared by sonication and adsorbed onto aminopropyl controlled pore glass (CPG) particles (130 microm). A preliminary study was conducted to optimize the amount of DNA adsorbed on the particles. Results indicated that maximum attachment efficiency was obtained by adsorbing DNA for 2 h in 0.2 x SSC, pH 5.7. The DNA-bound particles were hybridized overnight with a 181-bp digoxigenin-labeled probe, specific for gDNA. Following a chemiluminescent detection protocol, signal intensities of the standards were plotted as a function of initial gDNA concentration. The calculated detection limit (LOD) was 1.4 pM of gDNA. The assay was able to detect gDNA in pure plasmid preparations at the 1% level even in the presence of 1,000-fold excess of noncomplementary target. Hybridization results were compared with a quantitative real-time PCR assay. Both methods afforded similar accurate results at the 95% confidence level.

Enzymatic manipulations of DNA oligonucleotides on microgel: towards development of DNA-microgel bioassays

We demonstrate that DNA oligonucleotides covalently coupled to colloidal microgel can be manipulated by T4 DNA ligase for DNA ligation and by Phi29 DNA polymerase for rolling circle amplification (RCA). We also show that the long single-stranded RCA product can generate intensive fluorescence upon hybridization with complementary fluorescent DNA probe. We believe DNA-microgel conjugates can be explored for the development of DNA based bioassays and biosensors.

Metal nanocrystals incorporated within pH-responsive microgel particles

Cross-linked sterically stabilized latexes of approximately 250 nm diameter were synthesized by emulsion polymerization of 2-(diethylamino)ethyl methacrylate using a bifunctional oligo(propylene oxide)-based diacrylate cross-linker and a poly(ethylene oxide)-based macromonomer as the stabilizer at pH 9. These particles exhibit reversible swelling properties in water by adjusting the solution pH. At low pH, they exist as swollen microgels as a result of protonation of the tertiary amine units. Deswelling occurs above pH 7 [the effective pK(a) of poly(2-(diethylamino)ethyl methacrylate)], leading to the formation of the original compact latex particles. The swollen microgels can be used as nanoreactors: efficient impregnation with Pt nanoparticles can be achieved by incorporating precursor platinum compounds, followed by metal reduction. Dynamic light scattering was used to compare two methods of Pt nanoparticle impregnation with respect to the size and stability of the final Pt-loaded microgel particles. In the first method, the H2PtCl6 precursor was added to hydrophobic latex particles at high pH, followed by metal reduction. In the second method, H2PtCl6 was added to hydrophilic swollen microgel particles at low pH, and then this metal salt was reduced in situ using NaBH4 and the pH was raised by the addition of base. Both the Pt salt-loaded (metalated) microgels and the final Pt nanoparticle-loaded microgels had well-defined structures that were independent of the synthesis route. Polymer-metal interactions were investigated by UV-visible absorption spectroscopy, which confirmed that the Pt salt was completely reduced to zero-valent Pt. Transmission electron microscopy and X-ray diffraction studies verified the formation of nanometer-sized Pt nanoparticles within these microgels, which can be used as recoverable colloidal catalyst supports for various organic reactions.

Magnetic particles for the separation and purification of nucleic acids

Nucleic acid separation is an increasingly important tool for molecular biology. Before modern technologies could be used, nucleic acid separation had been a time- and work-consuming process based on several extraction and centrifugation steps, often limited by small yields and low purities of the separation products, and not suited for automation and up-scaling. During the last few years, specifically functionalised magnetic particles were developed. Together with an appropriate buffer system, they allow for the quick and efficient purification directly after their extraction from crude cell extracts. Centrifugation steps were avoided. In addition, the new approach provided for an easy automation of the entire process and the isolation of nucleic acids from larger sample volumes. This review describes traditional methods and methods based on magnetic particles for nucleic acid purification. The synthesis of a variety of magnetic particles is presented in more detail. Various suppliers of magnetic particles for nucleic acid separation as well as suppliers offering particle-based kits for a variety of different sample materials are listed. Furthermore, commercially available manual magnetic separators and automated systems for magnetic particle handling and liquid handling are mentioned.

Adsorption of oligonucleotides on PMMA/PNIPAM core-shell latexes: polarity of the PNIPAM shell probed by fluorescence

The adsorption of a rhodamine X labeled oligonucleotide composed of 25-mers of thymine (dT(25)-ROX) onto the thermosensitive shell of PMMA/PNIPAM core-shell latex particles was studied at 22 and 40 degrees C, below and above the T(VPT) (volume phase transition temperature) of the PNIPAM shell, respectively. The experimental binding isotherms were well fitted with the cooperative Hill model. The Hill coefficient is lower than 1 at both temperatures showing that the adsorption is anticooperative. The polarity of the shell was probed by both the lifetimes and solvatochromic shifts of the zwitterionic form of rhodamine X. For temperatures below the shell T(VPT) has a polarity similar to that of water, while for temperatures above the transition the polarity is equivalent to that of a water/dioxane mixture with 30% (v/v) water.