Self-assembly of colloidal cube superstructures with critical Casimir attractions.

The structure of self-assembled materials is determined by the shape and interactions of the building blocks. Here, we investigate the self-assembly of colloidal 'superballs', i.e. cubes with rounded corners, by temperature-tunable critical Casimir forces to obtain insight into the coupling of a cubic shape and short range attractions. The critical Casimir force is a completely reversible and controllable attraction that arises in a near-critical solvent mixture. Using confocal microscopy and particle tracking, we follow the self-assembly dynamics and structural transition in a quasi-2D system. At low attraction, we observe the formation of small clusters with square symmetry. When the attraction is increased, a transition to a rhombic Λ1-lattice is observed. We explain our findings by the change in contact area at faces and corners of the building blocks combined with the increase in attraction strength and range of the critical Casimir force. Our results show that the coupling between the rounded cubic shape and short-range attraction plays a crucial role for the superstructures that form and provide new insights for the active assembly control of micro and nanocubes.

3D test sample for the calibration and quality control of stimulated emission depletion (STED) and confocal microscopes.

Multiple samples are required to monitor and optimize the quality and reliability of quantitative measurements of stimulated emission depletion (STED) and confocal microscopes. Here, we present a single sample to calibrate these microscopes, align their laser beams and measure their point spread function (PSF) in 3D. The sample is composed of a refractive index matched colloidal crystal of silica beads with fluorescent and gold cores. The microscopes can be calibrated in three dimensions using the periodicity of the crystal; the alignment of the laser beams can be checked using the reflection of the gold cores; and the PSF can be measured at multiple positions and depths using the fluorescent cores. It is demonstrated how this sample can be used to visualize and improve the quality of STED and confocal microscopy images. The sample is adjustable to meet the requirements of different NA objectives and microscopy techniques and additionally can be used to evaluate refractive index mismatches as a function of depth quantitatively.

Seeded-Growth of Silica Rods from Silica-Coated Particles.

Seeded growth of silica rods from colloidal particles has emerged as a facile method to develop novel complex particle structures with hybrid compositions and asymmetrical shapes. However, this seeded-growth technique has been so far limited to colloidal particles of only a few materials. Here, we first develop a general synthesis for the seeded-growth of silica rods from silica particles. We then demonstrate the growth of silica rods from silica-coated particles with three different cores which highlight the generality of this synthesis: fluorescently labeled organo-silica (fluorescein), metallic (Ag), and organic (PS latex). We also demonstrate the assembly of these particles into supraparticles. This general synthesis method can be extended to the growth of silica rods from any colloidal particle which can be coated with silica.

Luminescence thermometry for in situ temperature measurements in microfluidic devices.

Temperature control for lab-on-a-chip devices has resulted in the broad applicability of microfluidics to, e.g., polymerase chain reaction (PCR), temperature gradient focusing for electrophoresis, and colloidal particle synthesis. However, currently temperature sensors on microfluidic chips either probe temperatures outside the channel (resistance temperature detector, RTD) or are limited in both the temperature range and sensitivity in the case of organic dyes. In this work, we introduce ratiometric bandshape luminescence thermometry in which thermally coupled levels of Er3+ in NaYF4 nanoparticles are used as a promising method for in situ temperature mapping in microfluidic systems. The results, obtained with three types of microfluidic devices, demonstrate that temperature can be monitored inside a microfluidic channel accurately (0.34 °C) up to at least 120 °C with a spot size of ca. 1 mm using simple fiber optics. Higher spatial resolution can be realized by combining luminescence thermometry with confocal microscopy, resulting in a spot size of ca. 9 µm. Further improvement is anticipated to enhance the spatial resolution and allow for 3D temperature profiling.

Phase diagram of binary colloidal rod-sphere mixtures from a 3D real-space analysis of sedimentation-diffusion equilibria.

Self-assembly of binary particle systems offers many new opportunities for materials science. Here, we studied sedimentation equilibria of silica rods and spheres, using quantitative 3D confocal microscopy. We determined not only pressure, density and order parameter profiles, but also the experimental phase diagram exhibiting a stable binary smectic liquid-crystalline phase (Sm2). Using computer simulations we confirmed that the Sm2-phase can be stabilized by entropy alone, which opens up the possibility of combining new materials properties at a wide array of length scales.