*1.2.2 Nanorobots*

*Solid State Physics - Metastable, Spintronics Materials and Mechanics of Deformable...*

tion. The formation of hydrogen molecules requires that protons (H+

sunlight absorption produces the electron hole pair. The water oxidation by holes

**1.2 General and basic concepts about nanotechnology, nano-/microrobots** 

Spintronic concept raised in the late 1980s refers to the use of spins to information transmission and computational operations [32, 33]. Spintronics is an emergent technology grounded in the information transmission by electronic charge and electron spin [34–39]. Spintronic represents a paradigm break in the field of information to combine charge and magnetism in processing and storage. The beginning of spintronics is marked by the discovery of giant magnetoresistance (GMR) effect, in 1988, which resulted in the award of Nobel Prize in Physics in 2007 to Fert and Grunberg [40, 41]. Firstly, spintronic was associated with inorganic oxides, metals, and semiconductors because of the dependence of spin-orbit coupling (SOC). However, organic molecules have wanted properties such as biocompatibility, flexibility, abundance, the possibility of synthesis, low cost [32, 42], and rapidly gained interest in the spintronic studies. The potential applications for spintronics, particularly for electronic devices, are spin filters, spin diodes, spin transistors, spin field-effect transistors, and spin qubits in semiconductor nanostructures [42]. Spintronic has some emerging and promising subfields that are current-induced torque (CIT), spin Hall effect (SHE), spin caloritronics, silicon spintronics, spintronic aspects of graphene and topological insulators (TIs), and chiral-induced spin selectivity effect [32, 34]. The electron spins are degenerate in energy, but the level of degeneracy is broken inside the helix because the electron velocity generates an effective magnetic field that couples with the chiral potential. In a model of DNA double helix, the spin-down electrons aligned preferentially parallel to their velocity in a right-handed helix, while the same occurred with spin-up electrons in the left-handed helix. In an experimental approach, self-assembled monolayers (SAMs) of 3′ thiolated single- and double-strand DNAs (ssDNA and dsDNA, respectively) were attached on a clean 200 nm-thick polycrystalline gold film that was evaporated on glass slides. Photoelectrons were ejected from the gold film by clockwise and counterclockwise circularly polarized light and transmitted through ssDNA and dsDNA monolayers. A more intense transport of electrons ejected with

**(motors), and spintronics**

*1.2.1 Spintronic*

) produces hydroxyl free radicals as intermediates of molecular oxygen evolu-

from the combination of hydroxyl radicals as molecular oxygen, accept the electrons promoted to the conduction band. However, hydrogen gas production competes with the combination of hydroxyl radicals as hydrogen peroxide that is favored by spin-antiparallel photogenerated holes. In the absence of a spin filter, the combination of spin-antiparallel hydroxyl radicals produces singlet molecular oxygen and requires an overpotential of 1 eV, since molecular oxygen is a triplet species in the fundamental state. Chiral molecules act as a spin filter in the electron transfer favoring the production of spin-parallel hydroxyl free radicals and consequently oxygen evolution simultaneously with H2 production [28]. In the literature, the association of porphyrins and/or hemeproteins with nanostructures, especially for photodynamic therapy purposes, is reported [14, 29]. The reason for this association refers to an enormous quantity of studies and recent findings involving nanostructure properties and manipulation, particularly the potential for drug delivery systems [30]. Nanostructured materials have at least one dimension between 1 and 100 nm. They usually have different (electronic, mechanic, magnetic, optical, etc.) properties from the bulk material, which results in multiple potential applications [31].

), resulting

**146**

(h+

Nowadays, a new field of study involving nanotechnology is gaining importance: micro-/nanorobotics. Micro-/nanorobots (MNRs) have autonomous motion provided by micro-/nanomotors (MNMs) that are micro-/nanometerscale devices powered with the ability to convert chemical, optical, acoustic, magnetic, and electrical energies into mechanical energy [47]. MNRs can be functionalized to perform complex tasks in a microcosm that constitutes the so-called micro-/nanorobots (MNRs) [48]. MNRs have an extensive range of potential applications such as remediation, nanofabrication, repair of materials, engineering, computing, environment monitoring, and especially in theranostics. Drug delivery systems, cell transport, and DNA and RNA insertions are some of the most numerous studies [49, 50]. The size of MNRs allows their application in minimally invasive diagnosis and treatments [51]. There is a basic classification for nanorobots. They can be biological, artificial, or biohybrid [52]. Also, they are classified according to the type of propulsion: self-propelled or external field-propelled ones. The self-propelled nanorobots convert energy from the environment to kinetic energy for independent movement, and it can be done by self-electrophoresis, self-thermophoresis, self-diffusiophoresis, and tiny bubbles [52]. Among the energy sources that self-propelled MNRs can use, light is highly attractive [47]. Light-powered MNMs can obtain energy from an external source and surrounding chemicals to get efficient propulsion through a photocatalytic process and constitute the photocatalytic micro-/nanomotors (PMNMs). Selfpropelling PMNMs can be controlled in various ways such as chemical concentration or light intensity [47, 48, 53]. Furthermore, these PMNMs can be operated at low levels of optical and chemical energy input, which are highly desired scenarios. An important aspect is that the photocatalytic reactions of PMNMs can generate the superoxide radicals (O2 −• ) that give these devices great potential for environmental remediation, especially in the degradation of organic pollutants. The Janus model can be used to explain the basic principles that respond to the self-propulsion of the photocatalytic MNMs (**Figure 3**).

The external field-propelled MNRs depend on an external force such as electric and magnetic field, light impulses, sonic waves, etc. [52]. The fabrication of MNRs can be direct, indirect, or by self-assembly [51]. The techniques used for the MNR fabrication are the same for the regular nanoparticles: top-down (lithography and scanning probe microscopy) and bottom-up (deposition, a solution with reducing agents). The materials used for MNR fabrication could be super magnetic substances, organic and inorganic compounds, and biological substances [51, 54].

*Solid State Physics - Metastable, Spintronics Materials and Mechanics of Deformable...*

#### **Figure 3.**

*Self-propelling mechanisms of Janus micromotors. The photocatalytic mechanisms are represented by the three first representations. The formation of a concentration gradient of superoxide ions and bubbles results from the oxidation of hydrogen peroxide in solution. In the right image, irradiation with infrared light on the nanostructured gold layer creates a thermal gradient due to the plasmonic effect. Warming promotes agitation of water molecules that generate one-way movement.*
