All posts by Linda Hirst

Congratulations Joe Lopes – CREST fellow spring 2017

Congratulations go to Joe, who was selected as one of just four UC Merced graduate students to recieve funding for the Spring semester as a CREST fellow.

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The CREST center (Center for cellular and biomolecular machines) is funded by the National Science Foundation and you can read more about their work and opportunities here.

Joe is working on kinesin – based  microtubule transport and active matter.

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Microtubules gliding on kinesin motors

 

 

 

 

Studying microtubule spools

Understanding the role of transport velocity in biomotor-powered microtubule spool assembly

Amanda J. Tan, Dail E. Chapman, Linda S. Hirst and Jing Xu

In this new paper we examined the sensitivity of microtubule spools to transport velocity.
Perhaps surprisingly, we determined that the steady-state
number and size of spools remained constant over a seven-fold
range of velocities. Our data on the kinetics of spool assembly
further suggest that the main mechanisms underlying spool growth
vary during assembly.
Read the paper in RSC Advances here

Amanda Tan – winner 2016 faculty mentor fellowship

Congratulations to physics graduate student Amanda Tan for winning a UC Merced “faculty mentor” fellowship.  This prestigious fellowship is awarded to prepare future faculty and provides a year’s funding plus a travel stipend.

Amanda’s research project focuses on active biological materials, in particular microtubules and molecular motors. She is collaborating with the Xu lab at UC Merced and will have her first paper with the group out soon.

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The award assists recipients in acquiring and developing advanced research skills under faculty mentorship and is aimed at increasing the number of students who complete their Ph.D. degree and successfully acquire a faculty appointment.

 

 

Using molecular motors to extract lipid tubules

A Simple Experimental Model to Investigate Force Range for Membrane Nanotube Formation

Chai Lor1, Joseph D. Lopes2, Michelle K. Mattson-Hoss3, Jing Xu2* and Linda S. Hirst2*
  • 1Biological Engineering and Small-scale Technologies Graduate Program, School of Engineering, University of California Merced, Merced, CA, USA
  • 2Physics Department, School of Natural Science, University of California Merced, Merced, CA, USA
  • 3Developmental and Cell Biology, School of Biological Sciences, University of California Irvine, Irvine, CA, USA

fmats-03-00006-g001The presence of membrane tubules in living cells is essential to many biological processes. In cells, one mechanism to form nanosized lipid tubules is via molecular motor induced bilayer extraction. In this paper, we describe a simple experimental model to investigate the forces required for lipid tube formation using kinesin motors anchored to 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) vesicles. Previous related studies have used molecular motors actively pulling on the membrane to extract a nanotube. Here, we invert the system geometry; molecular motors are used as static anchors linking DOPC vesicles to a two-dimensional microtubule network and an external flow is introduced to generate nanotubes facilitated by the drag force. We found that a drag force of ≈7 pN was sufficient for tubule extraction for vesicles ranging from 1 to 2 μm in radius. By our method, we found that the force generated by a single molecular motor was sufficient for membrane tubule extraction from a spherical lipid vesicle.

Congratulations Dr Lor!

Dr Chai Lor successfully defended his PhD thesis in the BEST (Bioengineering and small scale technologies) program on October 26th. Chai was a bioengineering undergraduate at UC Merced and a member of the first graduating class.

“Phase Behavior and Nanotube Formation in Lipid Membranes”

Biological cells are protected by a complex barrier called the lipid membrane. The lipid membrane is a soft material structure consisting of many lipid molecules held together by hydrophobic forces in an aqueous solution. Two simple experimental models were employed to investigate the role of specific lipid molecules in biological membranes. The first model investigated the phase behavior of the binary lipid mixture, 1-dipalmitoyl-2-docosahexaenoyl-sn-glycero-3-phosphoethanolamine (DHA-PE) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), using small-angle x-ray scattering (SAXS) and wide-angle x-ray scattering (WAXS). Our results shows that DHA-PE induces phase separation into a DHA rich liquid crystalline (Lα) phase and a DHA poor gel (Lβ’) phase at overall DHA-PE concentrations as low as 0.1mol%. In addition, we find that the structure of the Lβ’ phase, from which the DHA-PE molecules are largely excluded, is modified in the phase-separated state at low DHA-PE concentrations, with a decrease in bilayer thickness of 1.34nm for 0.1mol% at room temperature compared to pure DPPC bilayers. The second model investigated the formation of lipid nanotubes using an anchor system consisting of lipids, kinesin molecular motors, and microtubules in a flow cell. Lipid tubulation was conducted on two different membranes, 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and DPPC. Lipid nanotubes were pulled from anchored giant unilamellar vesicles (GUVs) by drag force generated from the flow inside the channel. The results showed that DPPC membranes cannot generate lipid nanotubes while DOPC can, which was expected. We find that a drag force of approximately ≈7.9 pN is sufficient for tubule extraction and that it only requires 1-2 kinesin motor proteins for anchoring the GUV.

“Low concentrations of docosahexanoic acid significantly modify membrane structure and phase behavior”
C. Lor and L.S. Hirst, MEMBRANES, 5(4), 857-874 Link (2015)

 

Soft particle trapping in a dual-beam optical trap

Stability and instability for low refractive-index-contrast particle trapping in a dual-beam optical trap

Alison Huff, Charles N. Melton, Linda S. Hirst, and Jay E. Sharping

Biomedical optics express, 6 (10), 3812-3819 (2015)

A dual-beam optical trap is used to trap and manipulate dielectric particles. When the refractive index of these particles is comparable to that of the surrounding medium, equilibrium trapping locations within the system shift from stable to unstable depending on fiber separation and particle size. This is due to to the relationship between gradient and scattering forces. We experimentally and computationally study the transitions between stable and unstable trapping of poly(methyl methacrylate) beads for a range of parameters relevant to experimental setups involving giant unilamellar vesicles. We present stability maps for various fiber separations and particle sizes, and find that careful attention to particle size and configuration is necessary to obtain reproducible quantitative results for soft matter stretching experiments.

Read the full paper here

Bifurcation plots showing the axial stability locations vs. particle radius for three different traps with fiber separations of (a) 45 μm, (b) 92.9 μm, and (c) 129.1 μm. The red dots represent the simulated stable trapping location, and the symbols represent experimentally observed stably trapped particles. The trap center is located at z=0. Trapping locations are shown schematically within (c). The error bars were determined empirically through measurements of the inner and outer edges of both the fiber and beads.
Bifurcation plots showing the axial stability locations vs. particle radius for three different traps with fiber separations of (a) 45 μm, (b) 92.9 μm, and (c) 129.1 μm. The red dots represent the simulated stable trapping location, and the symbols represent experimentally observed stably trapped particles. The trap center is located at z=0. Trapping locations are shown schematically within (c). The error bars were determined empirically through measurements of the inner and outer edges of both the fiber and beads.

Kyle Kabasaras, CAMP summer student, presents research at UC Merced Symposium

This summer Kyle Kabasaras presented his original research project at the UC Merced undergrad research symposium.

Investigating quantum dot assembly in a cholesteric liquid crystal

An ongoing goal in condensed matter physics is directly controlling the self-assembly of quantum dots (QDs) into specific structures while maintaining their original electronic and optical properties. One method of controlling the self-assembly of QDs is to disperse them within a liquid crystal (LC) medium and apply a variety of thermal stimulations. Recently, our lab developed a method of creating spherical, vesicle-shaped QDs within a nematic LCVesicle formation depends on the QD concentration in the LC as well as the LC’s intermolecular dispersion forces and thermal properties. In this project, we investigate the dispersion of CdSe/ZnS (core/shell) QDs in a cholesteric LC (CLC) medium and predict the QD aggregations to cluster near the LC defects. By varying parameters such as QD concentration and temperature, we exploit the CLC’s sensitive optical and thermal propertiesTo observe these effects, we apply spectrophotometry, polarized optical microscopy, and fluorescence microscopy. These techniques highlight the aggregation of QDs within the host CLC and identify how LC phase transitions determine where QDaggregates form. This work illustrates the possibility of new LC-based QD devices, and we will continue by exploring the lasing potential of our sample.

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CAMP is a National Science Foundation funded project to promote undergraduate research participation.

 

 

 

Makiko Quint wins faculty mentor fellowship

Congratulations to graduate student Makiko Quint! Makiko was awarded the UC Merced faculty mentor fellowship for 2015/2016. Her research project, in collaboration with the Ghosh group at UC Merced focuses on liquid crystal nanoparticle composites.

Read her recent paper in optical express here

The award assists recipients in acquiring and developing advanced research skills under faculty mentorship and is aimed at increasing the number of students who complete their Ph.D. degree and successfully acquire a faculty appointment.

The fellowship provides one year’s funding for Makiko’s graduate work – congratulations!

 

A plasmon induced liquid crystal device

“All optical switching of nematic liquid crystal films driven by localized surface plasmons” M.T. Quint, S. Delgado, Z.S. Nuno, L.S. Hirst and S. Ghosh, OPTICS EXPRESS, 23,5, 6888 (2015) Link

We have demonstrated an all-optical technique for reversible in-plane and out-of-plane switching of nematic liquid crystal molecules in few micron thick films. Our method leverages the highly localized electric fields (“hot spots”) and plasmonic heating that are generated in the near-field region of densely packed gold nanoparticle layers optically excited on-resonance with the localized surface plasmon absorption. Using polarized microscopy and transmission measurements, we observe this switching from homeotropic to planar over a temperature range starting at room temperature to just below the isotropic transition, and at on-resonance excitation intensity less than 0.03 W/cm2. In addition, we controllably vary the in-plane directionality of the liquid crystal molecules in the planar state by altering the linear polarization of the incident excitation. Using discrete dipole simulations and control measurements, we establish spectral selectivity in this new and interesting perspective for photonic application using low light power.

Quantum dot micro-shells

Self-assembled nanoparticle micro-shells templated by liquid crystal sorting”A. R. Rodarte, B.H. Cao, H. Panesar, R.J. Pandolfi, M. Quint, L. Edwards, S. Ghosh, J.E. Hein and L.S. Hirst, Soft Matter, 10.1039/C4SM02326A (2015) Link

A current goal in nanotechnology focuses on the assembly of different nanoparticle types into 3D organized structures. In this paper we report the use of a liquid crystal host phase in a new process for the generation of micron-scale vesicle-like nanoparticle shells stabilized by ligand–ligand interactions. The constructs formed consist of a robust, thin spherical layer, composed of closely packed quantum dots (QDs) and stabilized by local crystallization of the mesogenic ligands. Ligand structure can be tuned to vary QD packing within the shell and made UV cross-linkable to allow for intact shell extraction into toluene. The assembly method we describe could be extended to other nanoparticle types (metallic, magnetic etc.), where hollow shell formation is controlled by thermally sorting mesogen-functionalized nanoparticles in a liquid crystalline host material at the isotropic to nematic transition. This process represents a versatile method for making non-planar 3D nano-assemblies.