SMART NANOMATERIALS *
CHAPTER 3 CHAPTER 3
3.3 MECHANISMS OF RESPONSE
The responsiveness of a nanoparticle is a result of a sequence of several events shown in Fig. 3.2:
reception of an external signal (either physical or chemical), change of material properties due to signal (pH, thermo, light) or chemical reaction of the material properties (due to interaction with solvents or enzymes), and transduction of the changes into a macro/microscopically significant event (i.e., aggre- gation or desegregation of nanoparticles, release of cargo) (Motomov, Roiter, Tokarev, & Minko, 2010).
Most “smart” materials contain polymer branches/brushes which allow for response to different stimuli. This section addresses some of these external stimuli.
Table 3.1 Mechanical Properties of Various Engineering Fibers
Fiber Material Specific Density E (TPa) Strength (GPa) Strain at
Break (%)
CNTs 1.3–2 1 10–60 10
HS steel 7.8 0.2 4.1 <10
Carbon fiber PAN 1.7–2 0.2–0.6 1.7–5 0.3–2.4
Carbon fiber pitch 2–2.2 0.4–0.96 2.2–3.3 0.27–0.6
E/S glass 2.5 0/07/0.08 2.4/4.5 4.8
Kevlar 49 1.4 0.13 3.6–4.1 2.8
Table 3.2 Electronic Properties of Various Engineering Substances
Material Thermal Conductivity (W/mK) Electrical Conductivity
CNTs >3000 106–107
Copper 400 6 × 107
Carbon fiber pitch 1000 2–8.5 × 106
Carbon fiber PAN 8–105 6.5–14 × 106
51 3.3 MEchANISMS Of RESpONSE
3.3.1 pH-RESPONSIVE NANOMATERIALS
Most pH-responsive polymers are composed of weak polyelectrolytes with carboxylic, phosphoric, or amino functional groups (basic and acidic). By changing the solution’s pH, chemical equilibrium is shifted changing the ionization degree (charging neutral particles) of the polymer chains (Ruhe et al., 2004). As a response to this stimulus, the particle expands or collapses its polymer brushes, which is very useful for drug delivery systems in biomedicine (pH throughout the human body differs:
7.35–7.45 for blood and 4.5–5 in lysosomes).
Most pH-responsive materials are core–shell particles produced by bottom–up approaches: chemi- cal synthesis and grafting techniques. The core is usually made up of metals, metal oxides, polymers, etc. Fig. 3.3 is an example of the response of these polymer chains to pH increase.
It is an example of a PVP-based copolymer, in this case poly(2-vinylpyridine). Ionization changes of polyelectrolytes result in coil-to-globule transition. At low pH, the chains retain coil formations which retain mostly the elevation of single chains. As the pH is increased, the polymer chains agglom- erate due to intermolecular forces. This agglomeration of polymer brushes is also apparent in hybrid gold–PVP nanoparticles, where these brushes swell and shrink with pH stimulus.
3.3.2 LIGHT-RESPONSIVE NANOMATERIALS
In the case of light-responsive polymers, photoactive groups (azobenzene, spirobenzopyran, or cinna- monyl) are linked to the chains. These light responses are reversible structural changes and the stimulus is usually UV–visible light. Changes in size and shape, or formation of ionic or zwitter-ionic species are outcomes of receiving irradiation (Li & Keller, 2009). Another application of such technology is combining these conjugated polymers with CNTs which provide a good photoactive layer on the sur- face of the tubes.
FIGURE 3.2
potential stimuli and responses of synthetic polymers.
From Schmaljohann, D. (2006). Thermo- and pH-responsive polymers in drug delivery. Elsevier, 1655–1670.
52 CHAPTER 3 SMART NANOMATERIALS
An example from Motomov et al. is using amphiphilic-responsive nanoparticles to stabilize foam.
By shining light onto the particles, the shell-forming polymer undergoes a chemical reaction by absorp- tion of the light and forms new functional groups. These groups become ionized and change the amphi- philic particles to hydrophobic particles that destabilize the foam.
3.3.3 TEMPERATURE-RESPONSIVE NANOMATERIALS
The response of change in temperature for many nanoparticles varies according to application.
Increasing the temperature of a solution can affect the UV–visible absorption spectra of a core–shell nanoparticle, induce changes in wettability of a surface for synthetic and bio-inspired stimulus-respon- sive systems, and drug release capabilities, similar to “coiled-to-globular” transition in pH (Yusa et al., 2007) (Chen, Ferris, Zhang, Ducker, & Zauscher, 2010).
The coiled to globular effect can be seen in Fig. 3.4, where by increasing the temperature, Yusa et al.
(2007) decrease the hydrodynamic radius (Rh) of the poly(N-isopropylacrylamide) (PNIPAM)-based molecule.
FIGURE 3.3
changes of the polyelectrolyte ionization result in a pronounced coil-to-globule transition of chains, AfM- visualized conformations of adsorbed poly(2-vinylpyridine). (A) ph 3.89; (B) ph 4.04; (c) ph 4.24.
From Roiter, Y., & Minko, S. (2005). Single molecule experiments at the solid-liquid interface: in situ conformation of absorbed flexible polyelectrolyte chains. Journal of the American Chemical Society, 15688–15689.
53 3.3 MEchANISMS Of RESpONSE
It is imperative to understand that different polymer chains will act differently under the same stimuli. In some cases, the particle Rh increases with increasing temperature due to balance between segment–segment interactions and segment–solvent intermolecular interactions (Motomov et al., 2010). Examples of polymer–solvent pairs are: PNIPAM, polylactic acid, polylactide (homo- and copolymers), proteins, and polysaccharides in aqueous solutions (Motomov et al., 2010).
3.3.4 MAGNETIC FIELD RESPONSIVE NANOMATERIALS
Magnetic fields (MFs) are used as stimuli for drug release systems. One of the most popular magnetic nanoparticles used in ferrogels is iron oxide (Fe3O4). The response here is due to agglomeration and expansion of these magnetic particles within a carrier gel. The mechanism of these responsive materials is summarized in Fig. 3.5.
3.3.5 BIOLOGICAL AND CHEMICAL RESPONSIVE NANOMATERIALS
The response to chemical and biochemical signals is mainly due to interaction between functional groups and chemical reactions within molecules and polymer brushes. This selective interaction relies on colligation of responsive polymers with biological molecules, such as DNA, enzymes, antibodies, and other proteins, and is known as selective molecular recognition phenomena.
The main application of using enzymes as a signaling biochemical is polymers with immobilized enzymes, where a substrate that diffuses from the surrounding aqueous medium to the polymer can be bio-catalytically converted into products which interact with the responsive polymer and cause chemical changes in the polymer and materials with fragments that are substrates for enzymes.
Enzyme is used as an “external stimulus” that cleaves the chemical bonds in the material (Motomov et al., 2010).
FIGURE 3.4
The hydrodynamic radium (Rh) change of block copolymer pNIpAM-b-pMOEGMA/gold nanocomposites.
From Yusa, S., Yamago, S., Sugahara, M., Morikawa, S., Yamamoto, T., & Morishima, Y. (2007). Macromolecules. 5907.
54 CHAPTER 3 SMART NANOMATERIALS
An example of using enzymes as external stimulus is the experiment done by Jin et al. (Fig. 3.6), where a copolymer made of phenyboronic acid and sugar-based side chains was loaded with insulin.
Once free glucose was added, it competed to bind with the boronic acid within the polymer, breaking the cross-links and inducing swelling of the nanoparticle, therefore releasing its payload of insulin.