Gang-Yu Liu

Confounded by global energy needs, much research has been devoted to convert solar energy to various usable forms, such as chemical energy in the form of hydrogen via water splitting. Most photoelectrodes, such as hematite, utilize UV and visible radiation, whereas ∼40% infrared (IR) energy remains unconverted. This work represents our initial attempt to utilize IR radiation, that is, adding rare-earth materials to existing photoelectrodes. A simple substrate composed of hematite film and rare-earth nanocrystals (RENs) was prepared and characterized. Spectroscopy evidence indicates that the RENs in the composite absorb IR radiation (980 nm) and emit at 550 and 670 nm. The emitted photons are absorbed by surrounding hematite films, leading to improvement of water splitting efficiency as measured by photocurrent enhancement. This initial work demonstrates the feasibility and concept of using RENs for utilizing more solar radiation, thus improving the efficiency of existing solar materials and devices.

Integrin clustering, typically nanometers in dimension, is the first and an important step in integrin-mediated cellular signaling processes such as focal adhesion. Engineered nanostructures mimicking extracellular matrices (ECM) provide a new approach for investigation and regulation of this initial step and of downstream cascades of focal adhesion. This work reveals that fibronectin (Fn) nanostructures, even at a small height of 3.2 nm ± 0.5 nm, exhibit high efficacy in guiding cellular orientation and polarization. More interestingly, the Fn nanostructures also impact intracellular structures such as preferential filopodia attachment, and commensurate alignment of intracellular actin stress fibers. The impact can be rationalized by the strong and specific interaction between integrin and Fn, leading to integrin clusters and then focal adhesion assemblies following the underlying nanostructure of Fn. This guided assembly further mediates the downstream behavior, such as actin stress fiber alignment and overall cellular morphology. Our observations collectively demonstrate that engineered nanostructures of Fn provide an alternative and high efficacy biochemical cue for regulation of cellular signaling processes.

Extracellular matrices (ECM) triggered cellular signaling processes often begin with the clustering of the cellular receptors such as integrin and FcεRI. The sizes of these initial protein complexes or clusters are tens to 100 nm in dimension; therefore, engineered nanostructures could provide effective mimics of ECM for investigation and control of the initial and downstream specific signaling processes. This current topic discusses recent advances in nanotechnology in the context of design and production of matching chemical functionality and geometry for control of specific cellular signaling processes. Two investigations are reported to demonstrate this concept: (a) how the presentation of antigen at the nanometer scale would influence the aggregation of FcεRI, which would impact the formation of activation complexes, leading to the rearrangement of actin in cytoskeleton and degranulation or activation of mast cells; (b) how the engineered nanostructure could guide the initial integrin clustering, which would impact the formation of focal adhesion and downstream cell signaling cascades, leading to polarization, migration, and morphological changes. Complementary to engineered ECMs using synthetic ligands or peptides, or topographic control at the micrometer scale, nanostructures of designed geometry and chemical functionality provide new and effective biochemical cues for regulation of cellular signaling processes and downstream behaviors.

Dendrimers and telodendrimer micelles represent two new classes of vehicles for drug delivery that have attracted much attention recently. Their structural characterization at the molecular and submolecular level remains a challenge due to the difficulties in reaching high resolution when imaging small particles in their native media. This investigation offers a new approach towards this challenge, using scanning tunneling microscopy (STM) and atomic force microscopy (AFM). By using new sample preparation protocols, this work demonstrates that (a) intramolecular features such as drug molecules and dendrimer termini can be resolved; and (b) telodendrimer micelles can be immobilized on the surface without compromising structural integrity, and as such, high resolution AFM imaging may be performed to attain 3D information. This high-resolution structural information should enhance our knowledge of the nanocarrier structure and nanocarrier-drug interaction and, therefore, facilitate design and optimization of the efficiency in drug delivery.

Dendrimers have shown great potential in drug delivery because of their enhancement of drug solubility In aqueous media, leading to an increase in in vivo circulation and efficacy to targets. The structure of drug-dendrimer complexes however, is not well-known owing to the difficulties associated with visualizing individual drug molecules attached to dendrimers. Scanning tunneling microscopy (STM) enables visualization of dendrimer intramolecular structures using our approach of metal ion tagging. This work extends the approach to reveal the hierarchical structure of indomethacin-loaded poly(amidoamine) hydroxyl-terminated dendrimers. STM imaging provides structural information such as their height, lateral dimensions, and volume. High-resolution STM images enable the identification, and count of, individual indomethacin molecules bound to the anterior of dendrimers. Removal of drug molecules by the STM tip allows the calculation of individual drug-dendrimer binding energy, which is consistent with 1-3 hydrogen bonds. These investigations provide new Insight into the hierarchical structure and nature of indomethacin-dendrimer interactions and deepen our understanding of the stability and pharmacokinetic behavior of dendrimer-based drug delivery vehicles.

By using a highly sensitive technique of atomic force microscopy-based single-cell compression, the rigidity of cultured N2a and HT22 neuronal cells was measured as a function of amyloid-β42 (Aβ42) protein treatment. Aβ42 oligomers led to significant cellular stiffening; for example, 90–360% higher force was required to reach 80% deformation for N2a cells. Disaggregated or fibrillar forms of Aβ42 showed much less change. These observations were explained by a combination of two factors: (i) incorporation of oligomer into cellular membrane, which resulted in an increase in the Young's modulus of the membrane from 0.9 ± 0.4 to 1.85 ± 0.75 MPa for N2a cells and from 1.73 ± 0.90 to 5.5 ± 1.4 MPa for HT22 cells, and (ii) an increase in intracellular osmotic pressure (e.g., from 7 to 40 Pa for N2a cells) through unregulated ion influx. These findings and measurements provide a deeper, more characteristic, and quantitative insight into interactions between cells and Aβ42 oligomers, which have been considered the prime suspect for initiating neuronal dysfunction in Alzheimer's disease.

This article addresses the question of what information and new insights atomic force microscopy (AFM) provides that are of importance and relevance to cellular biophysical chemistry research. Three enabling aspects of AFM are discussed: (a) visualization of membrane structural features with nanometer resolution, such as microvilli, ridges, porosomes, lamellapodia, and filopodia; (b) revealing structural evolution associated with cellular signaling pathways by time-dependent and high-resolution imaging of the cellular membrane in correlation with intracellular components from simultaneous optical microscopy; and (c) qualitative and quantitative measurements of single cell mechanics by acquisition of force-deformation profiles and extraction of Young’s moduli for the membrane as well as cytoskeleton. A future prospective of AFM is also presented.

Cell tracing dyes are very frequently utilized in cellular biology research because they provide highly sensitive fluorescent tags that do not compromise cellular functions such as growth and proliferation. In many investigations concerning cellular adhesion and mechanics, fluorescent dyes have been employed with the assumption of little impact on the results. Using the single cell compression technique developed by our team, the single cell mechanics of MDA-MB-468 and MLC-SV40 cells were investigated as a function of dye uptake. Cell tracing dyes increase living cell stiffness 3−6 times and cell-to-probe adhesion up to 7 times. These results suggest a more significant effect than toxins, such as thrombin. A simple analytical model was derived to enable the extraction of the Young’s moduli of the cell membrane and cytoskeleton from the force−deformation profiles measured for individual cells. The increase in Young’s modulus of the membrane is 3−7 times, which is more significant than that of the cytoskeleton (1.1−3.4 times). We propose that changes in cell mechanics upon the addition of fluorescent tracing dye are primarily due to the incorporation of amphiphilic dye molecules into the cellular plasma membrane, which increases the lateral interaction among phospholipid chains and thus enhances their rigidity and adhesion.