Optical tweezers help in nanoparticle manipulation and trapping in a non-invasive manner, hence are widely used in medical and biological fields. In an article recently published in the journal Nanomaterials, the authors presented an integrated multifunctional two-dimensional (2D) plasmonic optical tweezer with an array of graphene disks and a substrate circuit.
Study: Integrated Multifunctional Graphene Discs 2D Plasmonic Optical Tweezers for Manipulating Nanoparticles.Image Credit: Rost9/Shutterstock.com
The substrate circuits allowed the application of bias voltage to configure each graphene disk’s Fermi energy.
Optical Tweezers to Trap Nanoparticles
The optical tweezers technology is widely applied in the medical and biological fields to manipulate and trap the nanoparticles in a non-invasive manner. Traditional laser optical tweezers are powerful tools to trap proteins, cells, viruses, metal nanowires, and dielectric particles. Particle trapping is achieved by balancing the scattering force and the gradient force formed through the lens laser beams induced strong electric field gradient. The focused beam’s focal size should be higher or equal to the incident light wavelength, which limits the focused laser beam’s application in nanoparticle trapping.
The conventional beam tweezers should incident a high-power laser light for greater trapping stiffness, which causes thermal or optical damage to biological nanoparticles. To this end, artificial micro-nano antenna containing plasmonic optical tweezers are an effective solution to eliminating the influence due to the photothermal effect. Plasmonic resonance is formed on the metal surface when the incident light frequency equilibrates with the system’s overall resonant frequency.
Graphene is a 2D material with a single-layered carbon structure. The unique photoelectric and optical properties of graphene along with their zero bandgaps are receiving considerable attention from researchers. The electrical conductivity of graphene can be adjusted through chemical doping or by an electric gate. Moreover, graphene can be transformed into nanodisks, nanoribbons, and nanoholes.
2D Plasmonic Optical Tweezers
In the present work, the authors theoretically proposed multifunctional integrated graphene disks with 2D plasmonic optical tweezers to manipulate the nanoparticles. The optical tweezers can trap and transport nanoparticles throughout the 2D plane without a preset path and by applying a bias voltage.
The present work involves the finite element method, which is a numerical method that allowed the authors to simulate the optical force distribution on nanoparticles and their trapping potential. Furthermore, they carried out the simulations of photothermal fluid and estimated the magnitude of nanoparticle forces, Stokes’s drag force, Brownian motion force, and thermophoretic force. The authors demonstrated the system potential in applying the Langevin equation for nanoparticle trapping, transport, and fusion and also they plotted the nanoparticle’s motion trajectory.
The probe was fixed at 50 nanometers above the graphene disk’s center and the near-field enhancement efficiency was determined. The electric field intensity at the probe was 24 times higher than the incident light. Fermi energy level and specific wavelength were required to enhance the graphene’s localized surface plasmon resonance (LSPR) intensity.
The authors investigated the resonance wavelength for Fermi energies 0.1 and 0.6 electron volts, and observed that under incident light, when the Fermi energy level was 0.1 electron volts, the graphene disk’s plasmonic resonance was enhanced by two times, and when the Fermi energy level was 0.6 electron volts, they observed 24 times enhancement.
The graphene disk’s strongest resonance wavelength was 8.28 nanometers. In an incident light of 8.2 micrometers wavelength, the intensity enhancement of plasmonic resonance was intense at the graphene disk’s center. However, when the wavelength was more than 8.4 micrometers, the intensity enhancement of plasmonic resonance was dispersed at the graphene disk’s center. Moreover, the incident light of 8.26 micrometers can trap the nanoparticles at the center.
Next, the authors introduced a spherical nanoparticle in the simulation. They configured the Fermi energy of the graphene disc as 0.6 electron volts and the optical forces on the nanoparticles were investigated at different locations with the intensity of incident light of 1 milliwatt per square micrometer.
The trapping potential and optical forces on the nanoparticles employing the time-averaged tensor were calculated. The results showed the optical force components as the nanoparticle’s function centered along the x-axis, with a y-center at 0 and a z-center at 50 nanometers. The results revealed that the optical force located at the graphene disk’s center along the x-axis traps the nanoparticles.
In summary, the team prepared multifunctional graphene discs with 2D plasmonic optical tweezers to manipulate the nanoparticles. They demonstrated that when the graphene disk’s Fermi energy and incident light wavelengths were 0.6 electron volts and 8.26 micrometers, respectively, the LSPR formed with compact electric field enhancement.
The theoretical calculations to obtain trapping potential in the near-field enhancement and the optical forces on the nanoparticles were successful.
The nanoparticle’s manipulation by modulating the LSPR position in the x-y plane was also demonstrated.
The present study showed the positive correlation of nanoparticle optical forces with relative refractive index and radius of the nanoparticles.
Yang, H., Mei, Z., Li, Z., Liu, H., Deng, H., Xiao, G., Li, J., Luo, Y., & Yuan, L. Integrated Multifunctional Graphene Discs 2D Plasmonic Optical Tweezers for Manipulating Nanoparticles. Nanomaterials 2022, 12(10), 1769. https://www.mdpi.com/2079-4991/12/10/1769