**4. Structural characterization**

230 Biomaterials – Physics and Chemistry

πγ *kTN* <sup>⎡</sup> <sup>⎤</sup> <sup>=</sup> <sup>⎢</sup> <sup>⎥</sup> <sup>⎢</sup> <sup>⎥</sup> <sup>⎣</sup> <sup>⎦</sup>

π

Biomolecules often have sensitive bio-active sites that can change under external stimuli such as temperature, light, electrical signals, PH and chemical/biochemical reactions of their environs. Such switchable biomolecules are of tremendous usefulness in diverse areas including biological, medical and bio-electronic technology. Most research groups in this field are interested in investigating new class of switchable biological systems albeit the field is still at its infancy stage. Chu et al. (2010) reported electro-switchable oligopetides as a function of surface potential. Oligolysine peptides exhibit protonated amino side chain at PH-7 providing the basis of switching ''ON'' and ''OFF'' of the biological activity on the surface upon application of negative potential. Switching initiated by PH changes has been observed in other biomolecules and biopolymers (Zimmermann et al., 2006). Biomolecular motors of actomyosin experience rapid and reversible on-off switching by thermal activation (Mihajlovi et al., 2004). The most optimistic approach of integrating photo switchable biomolecules into opto-electronic devices is provided by highly photo sensitive bacteriorhodopsin. This molecule has shown remarkable photo sensitive switching of its electrical properties that mimic conventional Gate transistors (Roy et al., 2010; Pandey, 2006; Qun et al., 2004). Bottom-up approach toward building optical nano-electronic devices is also feasible with the discoveries of switchable photoconductivity in even the smallest

8 *VRH*

*W*

structures such as quantum dots of cross-linked ligands (Lilly et al., 2011).

Electrical switching in biomolecules has wider applications in electronic industry. However, a lucid understanding of microscopic switching mechanism in these biological systems is still an outstanding challenge. In most investigations probing electrical switching in organic molecules, an external electrical signal is applied to the sample sandwiched between metal electrodes. Many materials have been reported to show hysteretic impedance switching where a system in its high impedance state (OFF) is switched by a threshold voltage into a low impedance state (ON) and remains in the ON state even with the reversal of applied voltage. This phenomenon is also known as resistive switching. Switching mechanism depend on whether the contribution comes from thermal, electronic, or ionic effects (Waser et al., 2007). Resistive switching is generally dependent on number of mobile charges, their mobility and the average electric field. In the case where the current is highly localized within a small sample area, filamental conduction take place (Scott et al., 2007). This simply involves formation of metallic bridge connecting the two electrodes. Filamental conduction accounts for negative differential resistance (NDR) model which is the basis of many molecular switching processes (Ren et al., 2010). Although this model was originally applied to inorganic materials, it can also explain resistive switching in organic samples (Tseng et al., 2006). The model assumes a trap- controlled channel where tunnelling take place in between chains of metallic islands. Similarly a decrease in electron transport channels and weak coupling between electrodes and the contact molecule causes NDR switching behaviour. Electric field induced switching mechanism is common in literature (Waser et al., 2007). In

*R*

**3. Electrical switching mechanism in bioploymers** 

9

3 ( )

3 4 *EF* 1/4

(9)

*R N* <sup>=</sup> (10)

( )

*EF*
