Selected from recent publications

1: Cell mechanics and dynamics

References: 

1: Zheng Liu#, Yang Liu#, Yuan Chang, Hamid Reza Seyf, Asegun Henry, Alexa L Mattheyses, Kevin Yehl, Yun Zhang, Zhuangqun Huang, Khalid Salaita. Nature methods, 13, 143-156 (2016) (co-first author)

2: Hanquan Su#, Zheng Liu#, Yang Liu, Victor Pui-Yan Ma, Aaron Blanchard, Jing Zhao, Kornelia Galior, R. Brian Dyer, and Khalid Salaita,

Nano Letters, ASAP (2018) (co-first author)

Mechanical force has a critical role in many cellular functions, including cell division, gene expression, differentiation and motility. Despite its fundamental importance to cell biology, significant gaps remain in our understanding of the coupling between chemical and mechanical signals. As a first step to understanding mechanotransduction circuits operating within cells, a number of techniques have been developed to investigate the response of individual cells to spatially confined physical perturbations. The current state-of-the-art tools for force manip­ulation of living cells, such as the atomic force microscope and optical and magnetic tweezers have been hampered by their low experimental throughput. Another general approach involves using magnetic actuation of nanoparticles and micropillars are lack of spatial and tempo resolution in mechanical stimulation and molecular specificity.

 To address these challenges, we developed a new class of optomechanical actuator (OMA) nanoparticles that allow one to optically manipulate receptor tension (4 pN <F< 50 pN) with optical spatial resolution and 5 msec temporal resolution using low intensity NIR illumination. OMA nanoparticles were comprised of a plasmonic gold nanorod core coated with a thermo-responsive polymer shell (poly(N-isopropylmethacrylamide, pNIPMAm) . With this design, the gold rod functions as a photothermal transducer, converting NIR excitation to localized heat that drives a drastic and transient collapse of the polymer shell. OMA nanoparticles rapidly shrink upon photo-illumination, and exclusively apply forces to cell receptors that are bound to their cognate ligands on the nanoparticle surface. The amplitude, duration, repetition and loading rate of the mechanical input are controlled by the NIR illumination profile. We demonstrate optomechanical actuation by controlling integrin-based focal adhesion formation, cell protrusion and migration, and t cell receptor activation.

Nanostructured composite materials containing electron donating and accepting components, such as dye- sensitized nanocrystalline oxides, conjugated polymer/C60 blends, and quantum dot (QD) sensitized oxides or QD solids, are widely used in novel solar cells. The dynamics of interfacial charge separation and recombination across the electron donor/acceptor interface plays essential roles in determining the energy conversion efficiencies of these devices. Ensemble-averaged time-resolved spectroscopic studies showed that these dynamics are often highly heterogeneous and intimately dependent on the nanoscale and molecular structures of the interfaces. In recent years, in an effort to provide insight into the structural/chemical origins of the complex charge separation properties, interfacial charge transfer (CT) dynamics at the single donor−acceptor level have also been examined by single molecule fluorescence spectroscopy. This technique can provide the sensitivity, spectral resolution, and time resolution needed to reveal the static and dynamic heterogeneities that are hidden in the ensemble-averaged measurements. However, it remains challenging to correlate the observed heterogeneity of interfacial CT dynamics to the material and interfacial structures based on fluorescence measurement alone. Therefore, further progresses in material design and improvement would require new methods that can provide spectral and dynamical information offered by fluorescence spectroscopy as well as simultaneous spatial/morphological resolution on the nanometer or smaller length scales.

We have developed a new method for controlling and probing the spatial dependence of photo-induced charge transfer dynamics in nanoscale electron donor−acceptor materials that are widely used in solar cells. The spatial control is achieved by functionalizing either the donor or acceptor on the AFM tip, whereas the charge transfer dynamics is probed by fluorescence spectroscopy in an integrated AFM/confocal fluorescence microscopy setup. We show that single QD-modified AFM tips can be prepared by bifunctional molecular linkers and can be used for AFM and fluorescence imaging studies. We demonstrate that using single CdSe/CdS core/shell QD functionalized AFM tips, the spatial dependence of photoinduced electron transfer dynamics from the single QD to TiO2 nanoparticles can be controlled and probed with a sub optical-diffraction-limited spatial resolution and high tem-poral resolution. Our findings demonstrate a new general method for high-resolution imaging of the spatial dependence of ultrafast charge transfer dynamics in nanocomposite materials.

References:

1: Zheng Liu et al. Nano letters 13 (11), 5563-5569 (2013)

2: Zheng Liu et al,  The Journal of Physical Chemistry Letters 4 (14), 2284-2291(2013)

3: Zheng Liu et al, Nature communications 2, 305 (2011)