Artificial Cell Membranes Interfaced with Optical Tweezers: A Versatile Microfluidics Platform for Nanomanipulation and Mechanical Characterization

Aurora Dols-Perez, Victor Marin, Guillermo J. Amador, Roland Kieffer, Daniel Tam, Marie-Eve Aubin-Tam

Cell lipid membranes are the site of vital biological processes, such as motility, trafficking, and sensing, many of which involve mechanical forces. Elucidating the interplay between such bioprocesses and mechanical forces requires the use of tools that apply and measure piconewton-level forces, e.g., optical tweezers. Here, we introduce the combination of optical tweezers with free-standing lipid bilayers, which are fully accessible on both sides of the membrane. In the vicinity of the lipid bilayer, optical trapping would normally be impossible due to optical distortions caused by pockets of the solvent trapped within the membrane. We solve this by drastically reducing the size of these pockets via tuning of the solvent and flow cell material. In the resulting flow cells, lipid nanotubes are straightforwardly pushed or pulled and reach lengths above half a millimeter. Moreover, the controlled pushing of a lipid nanotube with an optically trapped bead provides an accurate and direct measurement of important mechanical properties. In particular, we measure the membrane tension of a free-standing membrane composed of a mixture of dioleoylphosphatidylcholine (DOPC) and dipalmitoylphosphatidylcholine (DPPC) to be 4.6 × 10–6 N/m. We demonstrate the potential of the platform for biophysical studies by inserting the cell-penetrating trans-activator of transcription (TAT) peptide in the lipid membrane. The interactions between the TAT peptide and the membrane are found to decrease the value of the membrane tension to 2.1 × 10–6 N/m. This method is also fully compatible with electrophysiological measurements and presents new possibilities for the study of membrane mechanics and the creation of artificial lipid tube networks of great importance in intra- and intercellular communication.

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Interface-Dependent Radiative Lifetimes of Yb3+, Er3+ Co-doped Single NaYF4 Upconversion Nanowires

Xuezhe Zhou, Xiaojing Xia, Bennett E. Smith, Matthew B. Lim, Alexander B. Bard, Anupum Pant, Peter J. Pauzauskie

The development of upconversion nanomaterials for many photonic applications requires a detailed understanding of their radiative lifetimes that in turn depend critically on local environmental conditions. In this work, hexagonal (β-phase) sodium-yttrium-fluoride (NaYF4) nanowires (NWs) were synthesized and substitutionally co-doped with a luminescent solid solution of trivalent erbium and ytterbium ions. A single-beam laser trapping instrument was used in tandem with a piezo-controlled, variable-temperature stage to precisely vary the nanowire’s distance from the substrate. The spontaneous photoluminescence lifetime of the 4S3/2 → 4I15/2 transition from Er3+ ions was observed to change by >60% depending on the ions’ separation distance from a planar (water/glass) dielectric interface. The 4S3/2 state lifetime is observed to increase by a factor of 1.62 ± 0.01 as the distance from the quartz coverslip increases from ∼0 nm to ∼40 μm. Less significant changes in the luminescence lifetime (≤10%) were observed over a temperature range between 25 and 50 °C. The distance dependence of the lifetime is interpreted quantitatively in the context of classical electromagnetic coupling between Er3+ ions within the nanowire and the adjacent dielectric interface. We also demonstrate potential applications of the NaYF4 NWs for both controlling and probing temperatures at nanometer scales by integrating them within a poly(dimethylsiloxane) composite matrix.

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High Trap Stiffness Microcylinders for Nanophotonic Trapping

Ryan P. Badman, Fan Ye, Wagma Caravan, Michelle D. Wang

Nanophotonic waveguides have enabled on-chip optical trap arrays for high-throughput manipulation and measurements. However, the realization of the full potential of these devices requires trapping enhancement for applications that need large trapping force. Here, we demonstrate a solution via fabrication of high refractive index cylindrical trapping particles. Using two different fabrication processes, a cleaving method and a novel lift-off method, we produced cylindrical silicon nitride (Si3N4) particles and characterized their trapping properties using the recently developed nanophotonic standing-wave array trap (nSWAT) platform. Relative to conventionally used polystyrene microspheres, the fabricated Si3N4 microcylinders attain an approximately 3- to 6-fold trap stiffness enhancement. Furthermore, both fabrication processes permit tunable microcylinder geometry, and the lift-off method also results in ultrasmooth surface termination of the ends of the microcylinders. These combined features make the Si3N4 microcylinders uniquely suited for a broad range of high-throughput, high-force, nanophotonic waveguide-based optical trapping applications.

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Contact electrification of individual dielectric microparticles measured by optical tweezers in air

Haesung Park and Thomas W. LeBrun

We measure charging of single dielectric microparticles after interaction with a glass substrate using optical tweezers to control the particle, measure its charge with a sensitivity of a few electrons and precisely contact the particle with the substrate. Polystyrene (PS) microparticles adhered to the substrate can be selected based on size, shape, or optical properties and repeatedly loaded into the optical trap using a piezoelectric (PZT) transducer. Separation from the substrate leads to charge transfer through contact electrification. The charge on the trapped microparticles is measured from the response of the particle motion to a step excitation of a uniform electric field. The particle is then placed onto a target location of the substrate in a controlled manner. Thus, the triboelectric charging profile of the selected PS microparticle can be measured and controlled through repeated cycles of trap loading followed by charge measurement. Reversible optical trap loading and manipulation of the selected particle leads to new capabilities to study and control successive and small changes in surface interactions.

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Cell-Scaffold Adhesion Dynamics Measured in First Seconds Predicts Cell Growth on Days Scale – Optical Tweezers Study

Rok Podlipec and Janez Štrancar

Understanding the cell–biomaterial interface from the very first contact is of crucial importance for their successful implementation and function in damaged tissues. However, the lack of bio- and mechano-analytical methods to investigate and probe the initial processes on the interface, especially in 3D, raises the need for applying new experimental techniques. In our study, optical tweezers combined with confocal fluorescence microscopy were optimized to investigate the initial cell–scaffold contact and to investigate its correlation with the material-dependent cell growth. By the optical tweezers-induced cell manipulation accompanied by force detection up to 100 pN and position detection by fluorescence microscopy, accurate adhesion dynamics and strength analysis was implemented, where several attachment sites were formed on the interface in the first few seconds. More importantly, we have shown that dynamics of cell adhesion on scaffold surfaces correlates with cell growth on the days scale, which indicates that the first seconds of the contact could markedly direct further cell response. Such a contact dynamics analysis on 3D scaffold surfaces, applied for the first time, can thus serve to predict scaffold biocompatibility.

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Ceragenin Mediated Selectivity of Antimicrobial Silver Nanoparticles

Mark A Hoppens , Christopher B Sylvester , Ammar T Qureshi , Thomas Scherr , Desiree R Czapski , Randolph S Duran , Paul B Savage , and Daniel J. Hayes

The understanding that common broad spectrum antimicrobials disrupt natural microbial flora important in acquiring nutrients and preventing infection has resulted in a paradigm shift favoring more selective antimicrobials. This report explores silver nanoparticles conjugated with ceragenin, or cationic antimicrobials (CSA-SNPs), as a potential gram-positive selective antimicrobial. Herein, CSA-SNPs are characterized using TEM, DLS, zeta potential, and HPLC-ESI-TOF-MS. The antimicrobial properties are determined through MIC/MBC and time-kill studies. Spatial selectivity of the conjugate nanoparticle was evaluated using confocal imaging, MATLAB statistical analysis, and video monitored interactions between bacteria and CSA-SNPs via laser trapping techniques. Cytotoxicity is also determined by live/dead staining and flow cytometry. Average particle size as determined through TEM analysis and hydrodynamic diameter determined via DLS are 63.5 +/- 38.8 nm and 102.23 +/- 2.3 nm respectively. The zeta potential of the SNP before and after CSA attachment is -18.23 mV and -8.34 mV. MIC/MBC data suggests CSA-SNPs are eight times more effective against Staphylococcus aureus than SNPs alone. Furthermore, MATLAB analysis of confocal imaging found that 70% of CSA-SNPs are within 2 µm of S. aureus whereas this percentage falls to below 40% with respect to Escherichia coli. These results are bolstered further by laser trapping experiments demonstrating selective adherence of CSA-SNPs conjugates with bacterial strains. Cytotoxicity studies of CSA-SNPs against 3T3 fibroblasts indicate 50% cell viability at 50 ppm.

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