# Plasmonic Tweezers towards Biomolecular and Biomedical Applications
## 1 Introduction  
In 1970, [PERSON] published two letters in Physical Review Letters to demonstrate the radiation pressure from laser beams exerted on particles [1] and atoms [2]. In 1986, he developed the seminal single beam gradient force optical tweezers (OTs) [3], and he successfully trapped viruses, bacteria and single cells with this technique in 1987 [4; 5]. Inspired by his pioneering work, researchers have utilized OTs to study mechanical properties of biological particles and systems [6; 7; 8], e.g., torsional compliance of flagella in tethered _Escherichia coli_ and motile _Streptococcus_[9], tensions of DNA molecules [10; 11], motions of motor protein myosin-I and kinesin [12; 13], discrete step motion of RNA polymerase in transcription processes [14], and topological organization of a T4 lysozyme protein [15]. OTs have also been applied in vivo, e.g., manipulating red blood cells in subdermal capillaries of a living mouse for a potential blockage clearance [16] and manipulating the ear bone of a laver zebrafish [17]. Due to the profound importance of OTs in biological and medical applications, [PERSON] won one half of the Nobel Physics Prize in 2018. OTs can apply pico-Newton scale forces to bioparticles in the range of hundreds of nanometers to a few microns. However, to trap Rayleigh bioparticles well below the diffraction limit (<100 nm), directly using conventional OTs is difficult. First, since the size of the particle is much smaller than the focused laser beam, the amount of light can be used for trapping is limited by the volume of the nanoparticle. Second, the difference between the refractive index of the bioparticle and the aqueous environment is extremely small, which reduces the polarizability of the particle and consequently the optical force. Third, when the size of the particle is reduced, the viscous drag is decreased, which is not favorable. It is extremely important to have stable, non-invasive and flexible techniques to trap and manipulate nanoscale bio-samples in the intracellular environments [18], so alternative photonic methodologies that could generate nanoscale near fields have been developed [19]. Using evanescent fields, micron and sub-micron particles have been trapped and transported for sorting purposes [20; 21; 22; 23]. Due to the field gradient of the evanescent field only pulling the particle to the surface of the waveguide but pushing the particle along the light propagation direction, 3D nanostructures, e.g., photonic crystals (PCs) cavities [24] and silicon nano-antennas [25], have been designed to tailor the near fields for 3D light confinements. Besides nanophotonic techniques based on dielectric materials, plasmonic tweezers (PTs) based on surface plasmon polaritons (SPPs) generated at the interface of a dielectric medium and a conductive material is another major branch of nanoparticles manipulating methodologies. Optical force generated by surface plasmon resonance (SPR) was first proposed by [PERSON] and his colleagues in 2001 [26]. In 2004, [PERSON] et al. demonstrated trapping 200 nm fluorescent latex particles using the intense near field gradients generated around holes in gold thin films [27]. In 2008, PTs based on pairs of gold nanodots was demonstrated by [PERSON] et al. [28]. With the development of the fabrication technologies, simulation methodologies and materials science, PTs have been developed quickly in the last 10 years, which leads to improved trapping performance [29; 30] and applications in biological research [31]. In this short review, PTs engineered toward biomolecular and biomedical applications are discussed. In Section 2, optical trapping using self-induced back-action effect and chiral optical forces of the enhanced near chiral fields are discussed. In Section 3, PTs based on a few different nano-aperture designs are covered. In Section 4, an outlook containing future trends in PTs and challenges needs to be conquered to achieve meaningful applications in biomolecular and biomedical areas is provided.