Jung-Dae Kim

Single-cell optoporation and transfection using femtosecond laser and optical tweezers

In this paper, we demonstrate a new single-cell optoporation and transfection technique using a femtosecond Gaussian laser beam and optical tweezers. Tightly focused near-infrared (NIR) femtosecond laser pulse was employed to transiently perforate the cellular membrane at a single point in MCF-7 cancer cells. A distinct technique was developed by trapping the microparticle using optical tweezers to focus the femtosecond laser precisely on the cell membrane to puncture it. Subsequently, an external gene was introduced in the cell by trapping and inserting the same plasmid-coated microparticle into the optoporated cell using optical tweezers. Various experimental parameters such as femtosecond laser exposure power, exposure time, puncture hole size, exact focusing of the femtosecond laser on the cell membrane, and cell healing time were closely analyzed to create the optimal conditions for cell viability. Following the insertion of plasmid-coated microparticles in the cell, the targeted cells exhibited green fluorescent protein (GFP) under the fluorescent microscope, hence confirming successful transfection into the cell. This new optoporation and transfection technique maximizes the level of selectivity and control over the targeted cell, and this may be a breakthrough method through which to induce controllable genetic changes in the cell.

Anomalous magnetoresistance at low temperatures (T⩽10K) in a single crystal of GdB4

We successfully synthesized pure single crystals of GdB4, which reveals RRR=550, defined as RRR≡ρ(300K)∕ρ(2K). The temperature derivative of resistivity dρ(T)∕dT shows a transitionlike change of electronic property at Ta≈10K, in addition to the known antiferromagnetic transition at TN=42K. We also found that the ρ(T) shows an anomalous increase under the applied magnetic fields H below Ta, which leads to a positive magnetoresistance ratio MR≡[ρ(H,T)−ρ(0,T)]∕ρ(0,T)=58800% at T=2K and H=7T. Furthermore, the electronic feature near Ta manifests itself by a broad bump near T≈10K in a specific heat measurement, which indicates that the transition at Ta is a bulk property. We found that the positive and large magnetoresistance can be described by an empirical function, based on Kohler’s rule for the electron dynamics of a pure simple metal.

Trapping of a single DNA molecule using nanoplasmonic structures for biosensor applications

Conventional optical trapping using a tightly focused beam is not suitable for trapping particles that are smaller than the diffraction limit because of the increasing need of the incident laser power that could produce permanent thermal damages. One of the current solutions to this problem is to intensify the local field enhancement by using nanoplasmonic structures without increasing the laser power. Nanoplasmonic tweezers have been used for various small molecules but there is no known report of trapping a single DNA molecule. In this paper, we present the trapping of a single DNA molecule using a nanohole created on a gold substrate. Furthermore, we show that the DNA of different lengths can be differentiated through the measurement of scattering signals leading to possible new DNA sensor applications.

Traceable assembly of microparts using optical tweezers

Assembly of components with a size in the order of tens of micrometers or less is difficult because the gravitational forces become smaller than weak forces such as capillary, electrostatic and van der Waals forces. As such, the picked-up components commonly adhere to the manipulator, making the release operation troublesome, and the repeatable supply of components cannot be guaranteed because the magazining and bunkering scheme available in conventional scale assembly cannot be extended to these small objects. Moreover, there are also no effective ways known to deliver the finalized assembly externally. In this paper, we present the manipulation and assembly of microparts using optical tweezers, which by nature do not have stiction problems. Techniques allowing bunkering and finalizing the assembly for exporting are also presented. Finally, we demonstrate an exemplary microassembly formed by assembling two microparts: a movable microring and a microrod fixed on a glass substrate. We believe this traceable microassembly to be an important step forward for micro- and nano-manufacturing.

Magnetic anisotropy and magnon gap state of SmB4 single crystal

This paper investigates the magnetic and electronic properties of a single-crystal SmB4. Two magnetic phase transition temperatures at TN=25 K and Tm≈7 K were revealed by the temperature-dependent magnetization for an applied magnetic field perpendicular to the c-axis. Magnetic transition at TN=25 K is caused by the antiferromagnetic order with Sm3+ moments that lie on the ab-plain. However, the antiferromagnetic transition temperature was not observed in a temperature-dependent magnetization for an applied magnetic field along the c-axis. The second transition occurring at Tm≈7 K was observed for directions both parallel and perpendicular to the c-axis and was suppressed by applying magnetic fields. Interestingly, the second transition exhibited local maximum behavior, implying the second order phase transition. Furthermore, temperature-dependant resistivity showed a magnon gap feature below the second magnetic transition temperature T

Anisotropic magnetic phase diagrams of HoB4 single crystal

We have investigated the magnetic and electronic properties of a single-crystal HoB4. Antiferromagneticlike transitions were revealed by the temperature-dependent magnetization exhibiting two transition temperatures at TN1=7.1 K and TN2=5.7 K for an applied magnetic field along the c-axis, and showed only one transition at T=5.7 K for an applied magnetic field perpendicular to the c-axis. The isothermal magnetization with an applied magnetic field along the c-axis exhibited three distinct metamagnetic phase transitions at Hc1=2 T, Hc2=3.5 T, and Hc3=3.9 T at T=2 K. On the other hand, the isothermal magnetization with an applied magnetic field perpendicular to the c-axis showed two metamagneticlike transitions. A rapid change in the temperature and the field-dependent resistivity was found to be intimately correlated with magnetic transitions. Based on the isothermal magnetization and field-dependent resistivity for both magnetic field directions (H∥c and H⊥c), anisotropic phase diagrams of HoB4 were const...

Weak ferromagnetism in single crystalline YbB6−δ

We present a study of magnetization and transport properties of single crystals of YbB6 and YbB6±δ (δ=0.3 as a nominal value). The YbB6−δ crystal revealed a weak ferromagnetism, manifested by hysteresis loop in M(H) at room temperature with a saturation magnetic moment of 1.5×10−4μB∕f.u. and the ferromagnetic transition temperature higher than T=300K, whereas YbB6 and YbB6+δ crystals revealed diamagnetism and paramagnetism, respectively. It was found that the bulk magnetic property of YbB6±δ crystals is intrinsically diamagnetic and that the magnetism, ferromagnetism, or paramagnetism is a surface effect. Based on x-ray photoelectron spectroscopy and electron probe microanalyzer measurements, we believed that the extraneous magnetic impurities, such as Fe and Ni, are not the origin of the ferromagnetism or paramagnetism. While we observed from electrical resistivity and Hall resistivity that the off stoichiometry in YbB6±δ causes carrier doping effect, it is not clear at present what surface state creates...

A measurement of the maximal forces in plasmonic tweezers.

Plasmonic tweezers that are designed to trap nanoscale objects create many new possibilities for single-molecule targeted studies. Numerous novel designs of plasmonic nanostructures are proposed in order to attain stronger forces and weaker laser intensity. Most experiments have consisted only of immobilization observations--that is, particles stick when the laser is turned on and fall away when the laser is turned off. Studies of the exertable forces were only theoretical. A few studies have experimentally measured trap stiffness. However, as far as we know, no studies have addressed maximal forces. In this paper, we present a new experimental design in which the motion of the trapped particle can be monitored in either parallel or orthogonal directions to the plasmonic structures symmetric axis. We measured maximal trapping force through such monitoring. Although stiffness would be useful for force-calibration or immobilization purposes, for which most plasmonic tweezers are used, we believe that the maximal endurable force is significant and thus, this paper presents this aspect.

Stiffness measurement of a biomaterial by optical manipulation of microparticle

Since the discovery of the trapping nature of laser beam, optical tweezers have been extensively employed to measure various parameters at micro/nano level. Optical tweezers show exceptional sensitivity to weak forces making it one of the most sensitive force measurement devices. In this work, we present a technique to measure the stiffness of a biomaterial at different points. For this purpose, a microparticle stuck at the bottom of the dish is illuminated by the trapping laser and respective QPD signal as a function of the distance between the focus of the laser and the center of the microparticle is monitored. After this, microparticle is trapped and manipulated towards the target biomaterial and when it touches the cell membrane, QPD signal shows variation. By comparing two different QPD signals and measuring the trap stiffness, a technique is described to measure the stiffness of the biomaterial at a particular point. We believe that this parameter can be used as a tool to identify and classify different biomaterials.

Construction and actuation of a microscopic gear assembly formed using optical tweezers

The assembly of micrometer-sized parts is an important manufacturing process; any development in it could potentially change the current manufacturing practices for micrometer-scale devices. Due to the lack of reliable microassembly techniques, these devices are often manufactured using silicon, which includes etching and depositions with little use of assembly processes. The result is the requirement of specialized manufacturing conditions with hazardous byproducts and limited applications where only simple mechanisms are allowed. Optical tweezers are non-contact type manipulators that are very suitable for assembling microparts and solve one of the most difficult problems for microassembly, which is the sticking of the physical manipulator to the micropart. Although contact type manipulators can be surface modified to be non-sticky, this involves extra preprocessing—optical tweezers do not require such additional efforts. The weakness of using optical tweezers is that the permanent assembly of parts is not possible as only very small forces can be applied. We introduce an advanced microassembly environment with the combined use of optical tweezers and a motorized microtip, where the former is used to position two parts and the latter is used to introduce deformation in the parts so that they form a strongly fitted assembly.