# A Review on Optoelectrokinetics-Based Manipulation and Fabrication of Micro/Nanomaterials
## 3 OEK-Based Manipulation of Micro/Nanomaterials
### Manipulation of Nano-Scaled Particles  
OEK has also been used to dynamically manipulate nano-scaled entities, including the separation of nanowires [64; 65; 66; 67], the patterning of two-dimensional nanomaterials [68], and manipulation of nanoparticles [69; 70; 71; 72; 73; 74]. OEK allows the use of ODEP forces to effectively separate individual nanowires with different conductance levels, which has facilitated the development of nanodevices. Dynamic separation of semiconducting and metallic nanowires based on their difference in translational velocity was reported [64]. When the external voltage was higher than the \"separation voltage,\" i.e., the threshold one for moving the silicon nanowire, the silver nanowire could be separated from the silicon nanowire. Due to the high polarization of metallic nanowires in a non-uniform electric field, the silver nanowire moved at a much higher velocity than the silicon nanowire. When the OEK chip was dynamically illuminated by an optically-projected laser line at a scanning velocity greater than 2 \(\upmu\)m/s and an AC bias potential of 8 V\({}_{\mathrm{PP}}\), both of these two types of nanowires were trapped by the laser line initially. The silicon nanowire, however, could not follow the movement of the laser line and became \"uncontrollable.\" Hence, these two types of nanowires were successfully separated. In addition, it was experimentally validated that real-time and large-scale assembly of silver nanowires is possible using an array of image-defined traps, suggesting the potential for massively parallel assembly at the nanoscale. Furthermore, the manipulation and assembly of nanoparticles have been studied both theoretically and experimentally. First, a rapid and automatic assembly of 100 nm diameter gold nanoparticle (AuNP)-based microstructures was experimentally investigated [72]. Figure 9 indicates the experimental process in which AuNP-based microstructures were rapidly assembled. It was observed that each geometry of the four AuNP-based microstructures could reflect the optical pattern. However, using the triangle pattern as the virtual electrodes attracted most of the AuNPs into the locations of angular bisectors and formed a circle structure with three lines pointing toward the center of the triangle. The square pattern pushed the AuNPs to the diagonals of the pattern and most of them were moved into the central area. Figure 8: A series of OEK-based robotic micromanipulations. (**A**) A fully enclosed microrobot; (**B**) the load mode of a partially enclosed microrobot; (**C**) the partially enclosed microrobot loaded one bead of interest; (**D**) the fully enclosed microrobot transported the bead; (**E**) the partially enclosed microrobot delivered the bead; (**F**) the bead was unloaded; (**G**) schematic illustration of the “load,” “transport,” and “deliver” micromanipulations (reproduced from Ref. [63]). Fabrication of various nanomaterial-based microelectrodes in an OEK chip was achieved using two different methods [73]. A composite solution consisting of conductive polyaniline (PANI) nanoparticles and multi-walled carbon nanotubes (MWCNTs) was used to examine the possibility of creating microelectrodes by OEK forces. Microelectrodes with various geometrical sizes could be fabricated within 1-3 min. Compared to the resistance change of the MWCNTs bundles, that of the microelectrodes could be neglected; the ethanol concentration was successfully reflected by the resistance change of the sensitive elements. Rapid assembly of carbon nanoparticles (CNPs) with a diameter of 50 nm into electrical elements was realized [74]. A series of experiments were conducted to rapidly assemble CNPs into various electrical elements within 45 s. The results demonstrated that the microstructures had resistance properties. Specifically, their resistance value could be controlled by the width and length of the microstructures: inversely increasing with the width (Figure 10a-c) and linearly increasing with the length (Figure 10d-f).