Jukka Niskanen

How to manipulate the upper critical solution temperature (UCST)

In this mini-review, we discuss multi-stimuli-responsive polymers, which exhibit upper critical solution temperature (UCST) behavior mainly in aqueous solutions. Firstly, we discuss both the lower and upper critical solution temperature behavior of thermoresponsive polymers, to understand the differences between the two. This will be followed by examples of polymers that undergo a UCST phase transition. Secondly, we show how the solution properties of multi stimuli-responsive polymers can be influenced by several factors in addition to temperature, such as counter ions, electricity, light, or pH. Common to all stimuli are their capabilities to induce changes in the conformations and interactions of the polymers. With UCST polymers, the thermoresponsiveness is predominantly dependent upon the presence of strong supramolecular interactions between the polymer side groups. These are known to be affected by the molecular weight of the polymer, the solution concentration, and the presence of salts. With all these different ways to affect the cloud point of the polymers, we have systems that are readily tunable to many applications.

Temperature and pH responsive hybrid nanoclay grafted with PDMAEMA

Recent advances in grafting from techniques have made it possible to produce hybrid materials from montmorillonite clay by covalently grafting polymers to the surface of montmorillonite particles. In this article the temperature and pH responsiveness of a hybrid material consisting of poly(dimethylaminoethyl methacrylate) (PDMAEMA) grafted clay is investigated by differential scanning calorimetry, pressure perturbation calorimetry and fluorescence spectroscopy. The phase transition is affected not only by the pH of the aqueous medium, but also by the electrostatic interactions between the positively charged polymer and negatively charged clay particles.

Boron nitride nanotubes as vehicles for intracellular delivery of fluorescent drugs and probes

AIM To evaluate the response of cells to boron nitride nanotubes (BNNTs) carrying fluorescent probes or drugs in their inner channel by assessment of the cellular localization of the fluorescent cargo, evaluation of the in vitro release and biological activity of a drug (curcumin) loaded in BNNTs. METHODS Cells treated with curcumin-loaded BNNTs and stimulated with lipopolysaccharide were assessed for nitric oxide release and stimulation of IL-6 and TNF-α. The cellular trafficking of two cell-permeant dyes and a non-cell-permeant dye loaded within BNNTs was imaged. RESULTS BNNTs loaded with up to 13 wt% fluorophores were internalized by cells and controlled release of curcumin triggered cellular pathways associated with the known anti-inflammatory effects of the drug. CONCLUSION The overall findings indicate that BNNTs can function as nanocarriers of biologically relevant probes/drugs allowing one to examine/control their local intracellular localization and biochemical effects, leading the way to applications as intracellular nanosensors.

Interfacial and fluorescence studies on stereoblock poly(N-isopropylacryl amide)s.

Aqueous solution and water-air interfacial properties of associative thermally responsive A-B-A stereoblock poly(N-isopropylacryl amide), PNIPAM, polymers were studied and compared to atactic PNIPAM. The A-B-A polymers consist of atactic PNIPAM as a hydrophilic block (either A or B) and a water-insoluble block of isotactic PNIPAM. The surface tensions of aqueous PNIPAM solutions were measured as a function of both temperature and concentration. The isotactic blocks did not have an effect on the surface activity of the solutions. Rheological measurements on the water-air interface showed that the aggregated PNIPAMs containing isotactic blocks increased the elasticity of the surface significantly as compared to the atactic reference upon heating. Two fluorescence probes, pyrene and (4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (4HP), added to the aqueous polymer solutions were concluded to reside in surroundings with lower polarity and increased microviscosity in cases when the polymers contained isotactic blocks, as compared to ordinary atactic polymers.

Interparticle distance in monolayers controlled by soft spacers

This paper addresses the use of graft polymer layers as spacers to control interparticle distance in 2-dimensional monolayers. Gold nanoparticles grafted with thermosensitive PNIPAM-based polymers with a large range of molecular mass and different degrees of hydrophobicity have been studied. The hydrophobicity of the polymer is adjusted by incorporation of a comonomer n-propylamine. The resulting copolymer, PNIPAM-co-NPAM, exhibits lower collapse transition temperature and increased cooperativity in the collapse process with n ≈ 150 compared to n ≈ 100 for PNIPAM, n being the number of monomers per collapse domain. Langmuir isotherms of these polymers under moderate compression follow closely a π ≈ c3 behavior with corresponding critical exponent ν = 3/4 as predicted for 2-dimensional polymer conformation in good solvent. Nanoparticles grafted with these polymers form stable Langmuir monolayers where the graft polymer chains adopt a 2-D stretched conformation that tethers the nanoparticles to the interface. The nanoparticle cores are thus isolated by the polymer shells resulting in nanoparticle areas that increase with polymer chain length. Correspondingly, the interparticle distance is found to vary with chain length as Dp ≈ N0.8. For the Au-PNIPAM-NPAM, a moderate increase in temperature to near-θ conditions decreases the nanoparticle area by about 30% through lateral collapse of the polymer layer. This thermally induced molecular collapse in a 2-D monolayer is an unusual and novel observation that may be attributed to cooperative effects of the collapse transition of the new copolymer PNIPAM-co-NPAM.