In recent years, nanostructures created using optical vortices have attracted much attention. However, the details of the nanostructure formation process have not yet been clarified. In this study, focusing on nanostructures formed by Laguerre-Gaussian beam irradiation, we investigated the assembly dynamics of nanoparticles (NPs) as a model to understand the formation process of chiral nanostructures. Analyzing the fluorescence intensity and areas at the laser focal spot, we evaluated the assembled structure of NPs. Furthermore, particle tracking analysis for NPs attracted to the focal spot from the outside was performed. As a result, NPs assembled in the x-y plane and stacked vertically, where NPs outside the laser focal spot were attracted to the toroidal potential well along the orbit and were eventually trapped.
Optical vortices possess an orbital angular momentum (OAM). This is characterized by a topological charge (l ) associated with their helical wavefronts. Although difficult to observe directly, the OAM of optical vortices can be clarified by transferring it to materials. The OAM of optical vortices causes a physical "twist" in materials such as metal, silicon, photo-responsive polymer, and even liquid-phase resin. In recent studies, the fabrication of twisted metal nanoneedles melted by nanosecond optical vortex irradiation [1] and the helical polymer fiber formed via photopolymerization reaction by CW optical vortex irradiation [2] have been reported. By using femtosecond laser, photopolymerization occurs only at the focal spot due to two-photon absorption of ultraviolet (UV)-curable resin [3]. Femtosecond optical vortex irradiation resulted in the formation of microstructure that precisely reflect the distribution of electric field intensity [4]. However, the interactions between OAM of optical vortices and materials remain unclear. In this study, we utilize two-photon polymerization reaction of UV-curable resin by femtosecond optical vortex laser irradiation to evaluate the transfer mechanism of the OAM to materials.
Decades of research in the field of tissue engineering have allowed important findings to control cellular behaviors in the lab by designing artificial scaffolds. However, it is still challenging to engineer tissues (i.e. cell collectives with specific functions) that have intrinsic functions equivalent to those in our body. One key element in building such advanced functional tissues is the understanding of structure-cellular function correlations. Specifically, helical structures are seen in many of the tissues in our body, such as the helical structure of skeletal muscle fibers. Yet, no research has investigated the effects of helical structure on cell or tissue level functions due to the lack of technologies to design such helical scaffolds. Herein, we utilized a novel class of helical light field, referred to as an optical vortex, to realize the fabrication of helical scaffolds. By implementing the optical vortex in the photopolymerization of a biocompatible poly(ethylene glycol diacrylate) (PEGDA) scaffold, we expected that the orbital angular momentum of the optical vortex would transfer the helical structure on the fabricated PEGDA gels. Adopting the photo-initiated radical polymerization chemistry, we successfully created PEGDA gels using the optical vortex via single photon and two photon absorption. Although further characterizations are necessary, the helical PEGDA gels fabricated in this study will potentially provide a novel means to investigate how the helical structures affect cellular and tissue-level functions.
In biological membranes, lipids and proteins interact with each other to regulate their functions with complex structure. Manipulation techniques of molecular dynamics are desired to elucidate the regulation mechanism mediated by the interaction of membrane molecules. Optical trapping has been applied to study the biological molecular dynamics since it allows manipulation of biomolecules labeled with single μm-sized particle at the laser focal spot in solution. Due to the complex structure of biological membrane and weak optical trapping forces, it is difficult to investigate the effects of optical trapping on molecules in the biological membrane. In this study, a simple biological membrane model, the substrate-supported lipid bilayer (SLB), was used instead of the complex biological membrane. We investigated the diffusion properties of SLB in an optical trap to clarify the optical trapping dynamics of cell surface molecules. To evaluate the diffusion of lipid molecules, a fluorescent molecule, Texas Red conjugated lipid molecule (TR-PE), was mixed in SLB. The lateral diffusion of TR-PE in an optical trap was estimated by fluorescence correlation spectroscopy (FCS). The diffusion of TR-PE in SLB was slowed down with increasing laser power, suggesting that optical forces act slightly on the molecules in the lipid bilayer. Optical trapping has the potential to assemble molecules in biological membranes due to the difference in diffusion rate of molecules between inside and outside of the focal spot.
Neurons form complex networks and communicate through synaptic connections. The molecular dynamics of cell surface molecules at synaptic terminals are essential for elucidating synaptic transmission and plasticity in biological neural networks. To achieve artificial control of synaptic transmission in neural networks at the single-synapse level, we propose and demonstrate the application of optical trapping for laser-induced perturbation to cellular molecules on neurons. In this study, we investigated the effects of optical forces on the dynamics of cell molecules in an optical trap on neurons. The diffusion properties of the cell surface molecules under optical trapping were evaluated using fluorescence analysis with single-particle tracking and fluorescence correlation spectroscopy. Molecular diffusion at the cell surface of neurons was compared to that of lipid molecules in artificial bilayers. Moreover, the molecular dynamics in an optical trap without fluorescent labeling under live cell conditions was evaluated using Raman spectroscopy.
Neuronal stimulation is essential to understand information processing in brain systems. Spatiotemporal patterns of neuronal activity can be modified by external stimuli. Recent studies have shown that neurons can be stimulated by short-pulse laser processing of the cell membrane. An optical vortex with a helical wavefront possesses an orbital angular momentum (OAM) enables the inward twisting of ablated materials, thereby processing further precisely cells beyond a conventional Gaussian beam. We herein study the mechanisms of neuronal stimulation with a focused nanosecond optical vortex. The focused nanosecond optical vortex on the cell membrane of rat hippocampal neurons induces extracellular Ca2+ influx and neuronal activity elicitation. Morphological changes of the neuronal cell membrane due to nanosecond optical vortex irradiation is also evaluated with fluorescence recovery after photobleaching. After the deposition of a single pulse of nanosecond optical vortex on the cell membrane of neurons, the fluorescence intensity of membrane probe at the focal region significantly decreases, however, it recovers within 5 seconds. Such dynamics suggests that the transient disruption occurs at the cell membrane based on laser ablation and recovers due to lateral diffusion of membrane molecules. The diffusion coefficients of membrane molecules after optical vortex irradiation are larger than those of Gaussian beam irradiation, and the disrupted membrane areas are smaller than the expected ones as the optical vortex focal region. These differences are attributed to the fact that the disruption of cell membrane owing to laser ablation and subsequent membrane diffusion are assisted by OAM transfer effects.
Neurons in the brain communicate by releasing and receiving neurotransmitters at synapse. Synaptic vesicles (SVs) that encapsulate neurotransmitters play an important role for neuronal communication. We demonstrate that optical trapping of synaptic vesicles in cultured rat hippocampal neurons regulates the neuronal network activity. The neuronal electrical activity was evaluated by extracellular potential measurement using microelectrodes arrays (MEAs). When a near-infrared trapping laser was focused on synaptic vesicles labeled with FM1-43 dye, fluorescence caused by two-photon absorption was observed at the focal spot. The fluorescence intensity gradually increased during the laser irradiation time at the laser power of 500 mW, indicating that optical trapping forces cause the assembly of SVs at the focal spot. In the extracellular potential measurement of neuronal electrical activity, spike number of spontaneous neuronal activity increased under optical trapping of SVs. The synchronicity of neuronal network activity by cross-correlation analysis increased after the laser irradiation under higher laser power conditions. These results suggest that neuronal electrical activity can be manipulated by optical trapping of synaptic vesicles.
For the purpose of precise manipulation of single nanoparticles by optical trapping, we demonstrated optical trapping of nanoparticles enhanced depending on the wavelength of excitation laser. The optical trapping dynamics of quantum dot (QD) nanoparticles at the focal spot was evaluated by fluorescence correlation spectroscopy (FCS). The simultaneous irradiation with excitation and near-infrared lasers increased the average transit time of QDs at the focal spot, which depended on the laser power and the wavelength of the excitation laser. This suggests that the particle motion of QD nanoparticles is constrained at the laser focus due to enhancement of optical trapping based on the resonant optical response.
We demonstrate surface plasmon resonance (SPR) based optical trapping of quantum-dot (QD) nanoparticles suspended
in water with a bull’s eye-type plasmonic chip. The particle dynamics of QD suspensions at the laser focus was evaluated
by fluorescence correlation spectroscopy. The average transit time of QD suspensions on the plasmonic chip increased than
that on the coverslip, suggesting that single QD was more constrained at the focal spot due to optical trapping enhanced
with SPR.
We numerically investigate the convection of surrounding fluid in optical trapping of micro- and nanoparticles. The
effects of the laser irradiation on the fluid simulation are twofold. First, we take into account the temperature increase of
the fluid due the photothermal effect of the solvent, that is, the fluid flow is described by the Navier-Stokes equations
under the Boussinesq approximation. Second, we assume that the suspended particles drag the fluid when they are
transported by the optical force. This dragging effect is considered in the fluid simulation by adding to the Navier-Stokes
equation an external forcing term, which is modelled by considering the counterbalance between the optical scattering
force and the Stokes drag. It is shown that the latter effect is dominant under the usual experimental setup in optical
trapping of particles with the diameter larger than 0.5 μm. Furthermore, the particle size dependence on the convective
flow speed is investigated. The numerical results are supported by optical trapping experiment qualitatively.
AMPA-type glutamate receptor (AMPAR) is one of neurotransmitter receptors at excitatory synapses in neuronal cell. For realizing the artificial control of synaptic transmission, we have applied optical trapping of quantum-dot (QD) conjugated AMPARs on neuronal cells. Here, we demonstrate simultaneous measurement combined with optical trapping and patch-clamp recordings to evaluate the neuronal electrical activity. The relationship between optical trapping dynamics of QD-AMPARs located on neuronal cells and the neuronal electrical activity was discussed.
AMPA-type glutamate receptor (AMPAR) is one of the major neurotransmitter receptors at excitatory synapses. The initial assembling states of AMPARs at cell surface are essential for synaptic transmission, which is related with learning and memory in living neural systems. To realize artificial control of synaptic transmission, we demonstrate to modulate the initial assembling states of quantum-dot conjugated AMPARs (QD-AMPARs) with optical trapping. The optical trapping dynamics of QD-AMPARs on living neurons was evaluated with fluorescence imaging and fluorescence correlation spectroscopy (FCS). The transit time at laser focus of QD-AMPARs on neurons estimated from FCS analysis increased with the culturing days and addition of neurotransmitter, which suggests that QD-AMPARs are confined at the focal spot due to optical trapping.
Molecular dynamics of glutamate receptor, which is major neurotransmitter receptor at excitatory synapse located on neuron, is essential for synaptic plasticity in the complex neuronal networks. Here we studied molecular dynamics in an optical trap of AMPA-type glutamate receptor (AMPAR) labeled with quantum-dot (QD) on living neuronal cells with fluorescence imaging and fluorescence correlation spectroscopy (FCS). When a 1064-nm laser beam for optical trapping was focused on QD-AMPARs located on neuronal cells, the fluorescence intensity of QD-AMPARs gradually increased at the focal spot. Using single-particle tracking of QD-AMPARs on neurons, the average diffusion coefficient decreased in an optical trap. Moreover, the decay time obtained from FCS analysis increased with the laser power and the initial assembling state of AMPARs depended on culturing day, suggesting that the motion of QD-AMPAR was constrained in an optical trap.
Molecular dynamics at synaptic terminals in neuronal cells is essential for synaptic plasticity and subsequent modulation
of cellular functions in a neuronal network. For realizing artificial control of living neuronal network, we demonstrate
laser-induced perturbation into molecular dynamics in the neuronal cells. The optical trapping of cellular molecules such
as synaptic vesicles or neural cell adhesion molecules labeled with quantum dots was evaluated by fluorescence imaging
and fluorescence correlation spectroscopy. The trapping and assembling dynamics was revealed that the molecular
motion was constrained at the focal spot of a focused laser beam due to optical trapping force. Our method has a
potential to manipulate synaptic transmission at single synapse level.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.