Pick up, move and release of nanoparticles utilizing co-non-solvency of PNIPAM brushes.

A critical complication in handling nanoparticles is the formation of large aggregates when particles are dried e.g. when they need to be transferred from one liquid to another. The particles in these aggregates need to disperse into the destined liquid medium, which has been proven difficult due to the relatively large interfacial interaction forces between nanoparticles. We present a simple method to capture, move and release nanoparticles without the formation of large aggregates. To do so, we employ the co-non-solvency effect of poly(N-isopropylacrylamide) (PNIPAM) brushes in water-ethanol mixtures. In pure water or ethanol, the densely end-anchored macromolecules in the PNIPAM brush stretch and absorb the solvent. We show that under these conditions, the adherence between the PNIPAM brush and a silicon oxide, gold, polystyrene or poly(methyl methacrylate) colloid attached to an atomic force microscopy cantilever is low. In contrast, when the PNIPAM brushes are in a collapsed state in a 30-70 vol% ethanol-water mixture, the adhesion between the brush and the different counter surfaces is high. For potential application, we demonstrate that this difference in adhesion can be utilized to pick up, move and release 900 silicon oxide nanoparticles of diameter 80 nm using only 10 × 10 µm2 PNIPAM brush.

Mimicking natural cell environments: design, fabrication and application of bio-chemical gradients on polymeric biomaterial substrates.

Gradients of biomolecules on synthetic, solid substrates can efficiently mimic the natural, graded variation of properties of the extracellular matrix (ECM). Such gradients represent accessible study platforms for the understanding of cellular activities, and they also provide functional supports for tissue engineering (TE). This review describes the most relevant methods to produce 2-dimensional (2D) as well as 3-dimensional (3D) gradient supports for cell manipulations, and also addresses the response of cells from different origins when seeded on these constructs. The fabrication strategies summarized encompass the combination of polymer and surface chemistries, micro- and nano-engineering construction strategies and biotechnological approaches. This multidisciplinary scheme has enabled the design and realization of diverse, synthetic supports as cellular environments, spanning from the first gradient self-assembled monolayer (SAM) to multilayers, and hybrid constructs mimicking the complexity of natural tissue environments. The standing challenge is bringing these advances in the fabrication of supports to a dynamic functioning in space and time, towards the successful imitation of the most complex bio-chemical system ever studied: our body.

Optical imaging beyond the diffraction limit by SNEM: effects of AFM tip modifications with thiol monolayers on imaging quality.

Tip-enhanced nanoscale optical imaging techniques such as apertureless scanning near-field optical microscopy (a-SNOM) and scanning near-field ellipsometric microscopy (SNEM) applications can suffer from a steady degradation in performance due to adhesion of atmospheric contaminants to the metal coated tip. Here, we demonstrate that a self-assembled monolayer (SAM) of ethanethiol (EtSH) is an effective means of protecting gold-coated atomic force microscopy (AFM) probe tips from accumulation of surface contaminants during prolonged exposure to ambient air. The period over which they yield consistent and reproducible results for scanning near-field ellipsometric microscopy (SNEM) imaging is thus extended. SNEM optical images of a microphase separated polystyrene-block-poly (methylmethacrylate) (PS-b-PMMA) diblock copolymer film, which were captured with bare and SAM-protected gold-coated AFM probes, both immediately after coating and following five days of storage in ambient air, were compared. During this period the intensity of the optical signals from the untreated gold tip fell by 66%, while those from the SAM protected tip fell by 14%. Additionally, gold coated AFM probe tips were modified with various lengths of alkanethiols to measure the change in intensity variation in the optical images with SAM layer thickness. The experimental results were compared to point dipole model calculations. While a SAM of 1-dodecanethiol (DoSH) was found to strongly suppress field enhancement we find that it can be locally removed from the tip apex by deforming the molecules under load, restoring SNEM image contrast.

Direct surface structuring of organometallic resists using nanoimprint lithography.

The availability of suitable resist materials is essential for nanoimprint lithography (NIL). In this work, the application of poly(ferrocenylmethylphenylsilane) (PFMPS) as a new type of imprint resist is reported. As PFMPS contains iron and silicon in the main chain, it possesses a very high resistance to reactive ion etching. Polymer patterns formed after imprinting were transferred into silicon substrates owing to the high etch resistivity of PFMPS. The parameters for imprinting, such as polymer molar mass and initial film thickness, were investigated. A decrease in the initial film thickness facilitated the residual layer removal, as well as the pattern transfer. Only upon complete removal of the residual layer with argon plasma did pattern transfer result in aspect ratios up to 4:1 and less surface roughness.

Temperature-modulated quenching of quantum dots covalently coupled to chain ends of poly(N-isopropyl acrylamide) brushes on gold.

A thermo-responsive polymer/quantum dot platform based on poly(N-isopropyl acrylamide) (PNIPAM) brushes 'grafted from' a gold substrate and quantum dots (QDs) covalently attached to the PNIPAM layer is presented. The PNIPAM brushes are grafted from the gold surface using an iniferter-initiated controlled radical polymerization. The PNIPAM chain ends are functionalized with amine groups for coupling to water-dispersible COOH-functionalized QDs. Upon increasing the temperature above the lower critical solution temperature (LCST) of PNIPAM the QD luminescence is quenched. The luminescence was observed to recover upon decreasing the temperature below the LCTS. The data obtained are consistent with temperature-modulated thickness changes of the PNIPAM layer and quenching of the QDs by the gold surface via nonradiative energy transfer.