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Soft Materials Driven by Photothermal Effect and Their Applications

Hari K. Bisoyi 1 Augustine M. Urbas 2 and Quan Li 1

1?Kent State University, Liquid Crystal Institute and Chemical Physics Interdisciplinary Program, 1425 Lefton Esplanade, Kent, OH 44242, USA

2?Wright-Patterson Air Force Base, Materials and Manufacturing Directorate, Air Force Research Laboratory, Dayton, OH 45433, USA

1.1 Introduction

We are surrounded by soft materials. Most organs of the human body are made of soft materials. One defining characteristic of soft materials is that they can be readily deformed by external stimuli and stress. Even the thermal energy available at room temperature is sufficient to cause significant deformation in soft materials [1-13]. de Gennes in his 1991 Nobel lecture emphasized "flexibility" as a defining characteristic of soft materials. Colloids, polymers, liquid crystals (s), gels, and biological materials are categorized as soft materials. Being pervasive in nature, soft materials have been a source of inspiration for scientists and engineers for the design and fabrication of stimuli-responsive smart materials and systems for scientific investigations to understand the basic principles and technological application in devices and health care. Accordingly, a wide variety of engineered soft materials have been developed, which play a crucial role in modern technology. Soft nanotechnology, soft robotics, and soft lithography are examples that reflect the far-reaching implications of soft materials in biology and engineering [14-22]. Soft materials constitute one of the most stimulating interdisciplinary frontiers of modern science and are built on diverse experimental and theoretical foundations. Although soft materials in neat state have found a wide range of applications, fabrication of composites of soft materials by integrating other functional materials has recently become a common practice to realize advanced and tunable materials with enhanced properties for high-tech applications. Incorporation of photothermal agents into the matrices of soft materials is one such emerging approach.

Photothermal agents absorb light and convert it into heat, which is referred to as the photothermal effect. Over the past decade, many different types of photothermal agents have been reported, including inorganic nanomaterials (e.g. noble metal nanoparticles and carbon-based materials) (Figure 1.1) and organic compounds or materials (e.g. indocyanine green and polyaniline) [23-54]. Different classes of photothermal agents are associated with their own advantages and disadvantages. Near infrared ()-absorbing photothermal agents have earned a special position owing to their suitability in biological applications since biological tissues are largely transparent to NIR light. They have been applied in photothermal therapy and drug delivery remotely actuated by NIR light. Local heat produced by these agents raises the temperature and causes cell death in cancer treatment. Photothermal agents have also been used to trigger and accelerate drug release in biomedical applications. A useful photothermal agent is required to exhibit strong absorption of light and efficient transduction of light into heat. Gold nanoparticles (s), gold nanorods (s), carbon nanotubes (s), graphene, and iron oxide nanoparticles have been employed as photothermal agents owing to their high photothermal conversion efficiency [55-67]. Similarly, conjugated polymers and dye molecules have also been utilized as photothermal agents. These organic compounds convert absorbed light through a non-radiative relaxation process. Plasmonic particles such as gold nanoparticles produce heat subsequent to light absorption through a distinct mechanism. Light absorbed by GNPs excites the electrons in the plasmonic band. These excited electrons relax through electron-phonon interaction by colliding with the gold lattice. This collision produces heat, which is transferred to the surrounding medium through phonon-phonon coupling, resulting in increasing the surrounding temperature (Figure 1.1e).

Figure 1.1 Commonly employed photothermal agents for driving soft materials by light irradiation. (a) Gold nanoparticle, (b) gold nanorod, (c) carbon nanotube, and (d) graphene. (e) Schematic describing the principle of photothermal light to heat conversion by plasmonic nanostructures.

Source: Qin and Bischof [23]. Copyright 2014. Reproduced with permission from The Royal Society of Chemistry.

Recently, remote driving of soft materials by combining with photothermal agents is an emerging endeavor that reaps the benefits of both classes of promising materials. Accordingly, both inorganic and organic photothermal agents have been incorporated into the matrices of soft materials. The functional composite materials have been driven by light where the photothermal agent absorbs the light and converts it into heat, thereby increasing the temperature. The remotely triggered local temperature increase causes various physical and morphological changes including phase transitions. The occurrence of such changes in the soft matter matrix has been exploited for different applications (Figure 1.2). In this chapter, we present the developments on soft materials driven by photothermal effect of various photothermal agents. The combination of well-known soft materials such as LCs, polymers, and gels with inorganic nanoparticles, carbon nanomaterials, and organic dyes is discussed.

Figure 1.2 Different demonstrated applications of the hybrid systems that have been fabricated by combining soft materials and photothermal agents.

1.2 Liquid Crystals Driven by Photothermal Effect

Liquid crystals (LCs) represent a state of matter that appears between the crystalline solid state and isotropic liquid state [68-75]. This thermodynamically stable state of matter has been recognized as the fourth state of matter after solid, liquid, and gas. In this state of matter, the constituent elements, i.e. molecules, macromolecules, and molecular aggregates, simultaneously exhibit order and mobility, which renders it very intriguing and fascinating. The presence of order makes this state anisotropic whereas molecular mobility facilitates stimuli responsiveness. LCs are commonly known for their extensive applications in liquid crystal display () devices. However, they have been applied in numerous non-display applications [76-86]. In addition to their technological applications, LCs are interesting for fundamental studies to understand self-assembly and self-organization of matter in multiple dimensions and over different length scales [87-97]. LCs serve as model supramolecular systems. They have been regarded as an important class of soft materials owing to their large response to small stimuli [98-108]. Moreover, they play very critical roles in living systems. LCs offer a significant contribution in nanoscience and nanotechnology. LCs have been classified into two broad classes: thermotropic and lyotropic. In thermotropic materials, the occurrence of LC phases is dependent on temperature. In lyotropic systems, the LC phase formation is primarily determined by the concentration of solutes in appropriate solvents although temperature also plays a role in the appearance and stability of LC phases.

Thermotropic and lyotropic LC phases undergo phase transition in response to the effect of temperature change. The temperature of the systems can be changed by directly heating or cooling in contact with heat sources. However, temperature variation by remote control and noncontact methods is appealing in many instances where application of direct heat is either detrimental or impractical. In this context, the use of light energy to vary the temperature of thermotropic and lyotropic LCs is very promising. In order to achieve this goal, it is often necessary to have a component in the system that can absorb the light energy and efficiently convert it into heat, thereby increasing the temperature of the medium. This role of converting light energy into heat has been played by certain metal nanoparticles, carbon nanomaterials, and organic compounds that exhibit photothermal effect. Thus, LC materials can be effectively driven by light using such photothermal agents in their matrices. The temperature variation by photothermal effect can either cause phase transitions or modification of physical properties of LC phases. Therefore, a variety of photothermal agents including inorganic nanoparticles and organic compounds have been employed for driving LC materials for fundamental studies and different applications.

Sun et al. have prepared gold-nanocrystal-doped LC elastomer microparticles and studied their light-driven shape morphing [109]. Soft lithography and polymerization techniques were used to fabricate microcylinders from the LC elastomer nanocomposite. The processing conditions were such that weakly undulating director orientation along the long axis of the microcylinders was obtained. Upon irradiation with infrared () laser, the dispersed GNPs efficiently produce heat and transfer it to...