Label-free tracking of whole-cell response on RGD functionalized surfaces to varied flow velocities generated by fluidic rotation

Fluidic flow plays important roles in colloid and interface sciences. Measuring adsorption, aggregation processes and living cell behavior under a fluidic environment with varied flow velocities in a parallel and high-throughput manner remains to be a challenging task. Here a method is introduced to monitor cell response to well-defined flow with varied velocities over an array of label-free resonant waveguide grating (RWG) based optical biosensors. The arrangement consists of a circular well with an array of biosensors at the bottom surface. By rotating the liquid over the biosensor array using a magnetic stirrer bar, flow velocities from zero to a predefined maximum can be easily established over different locations within the biosensor array as characterized in detail by numerical simulations. Cell adhesion and detachment measurements on an Arg-Gly-Asp (RGD) peptide functionalized surface were performed to demonstrate i) measurements at a wide range of simultaneous flow velocities over the same interface; ii) the possibility of parallel measurements at the same flow conditions in one run; and iii) the simple tuning of the employed range of flow velocities. Our setup made it possible to analyze the magnitude and rate of cell detachment at various flow velocities in parallel and determine the critical velocity and force where cells start to detach from the RGD motif displaying biomimetic surface. Furthermore, cellular response to simultaneous mechanical (flow) and chemical stimulation was also investigated using trypsin as a model. This study opens a new possibility to investigate interface phenomena under predefined and conveniently varied flow conditions.

Robust Contact Angle Determination for Needle-in-Drop Type Measurements

One of the main approaches for contact angle determination using sessile drops with a missing apex (e.g., because of the presence of the needle tip) is the polynomial drop-profile fitting method. The major disadvantage of this fitting procedure is that the derived contact angle is highly sensitive to the polynomial order and the number of pixels involved in the actual fit. In the present work, an easily implementable method is introduced to effectively tackle these drawbacks. Instead of fitting the drop-profile itself, the polynomial fitting is applied to the difference between the drop profile and the circumcircle, independently for both sides of the drop. The derivative value of this difference at the contact point is used to correct the slope obtained analytically from the circumcircle. It is shown that this approach allows the robust determination of the contact angle with high (≤0.6°) accuracy in a straightforward manner, and the results are not affected by the actual contact angle, drop volume, or the resolution of the captured image. Validation of this new approach is also given in the contact angle range of 20°-150° by comparing the results to the values calculated by the Young-Laplace fit.

Detecting patchy nanoparticle assembly at the single-particle level

Patchy colloidal particles offer a unique platform to explore and investigate spatially inhomogeneous colloidal interactions. In the present work we provide direct, in situ experimental observation at the single particle level on the colloidal interaction driven formation of a heterodimer, consisting of a patchy nanorod and a sphere. The gold nanorods employed during the experiments feature positively charged tip regions, while the side of the rods is covered by surface grafted PEG. The driving force of the assembly is the electric double layer interaction between the nanorod and the nanosphere possessing opposite surface charge. The great advantage of using gold heterodimers is that their optical scattering spectrum characteristics are known to be very sensitive to the actual structure of the assembly due to plasmon hybridization. This was exploited to determine the structure of the heterodimer in situ upon formation in aqueous medium. The results indicate, that despite the attractive patches being located at the nanorod tips, for the given particle pairs the spheres readily bind to the side region of the nanorods in the aqueous medium during the assembly. Finite element simulations of the electric double layer interaction reveal that this is the energetically favorable configuration for the given heterodimer. This work demonstrates the potential of in situ experiments performed at single particle level for the characterization of self-assembling structures, which can contribute to the development of rationally designed nanoscale building blocks.