Gold nanoparticle devices

Direct observation of molecular diffusion is the most powerful approach to evaluate the bilayer fluidity and molecular diffusivity. Recent advances in optics and CCD devices enable us to detect and track the diffusive motion of a single molecule with an optical microscope. Usually, a fluorescent dye, gold nanoparticle, or fluorescent microsphere is used to label the target molecule in order to visualize it in the microscope [31-33]. By tracking the diffusive motion of the labeled-molecule in an artificial lipid bilayer, random Brownian motion was clearly observed (Figure 13.3) [31]. As already mentioned, the artificial lipid bilayer can be treated as a two-dimensional fluid. Thus, an analysis for a two-dimensional random walk can be applied. Each trajectory observed on the microscope is then numerically analyzed by a simple relationship between the displacement, r, and time interval, T,... 

Historically, OTEs were used to investigate complex redox reactions including mediated reactions of enzymes (-> mediators). Recently OTEs have been extensively applied to study spectroelectrochemical properties of thin films of electrochromic materials (- elec-trochromic devices, -> electrochromism), and various chromic deposits including bioelectrochemical materials (e.g. - enzymes) or network films of gold -> nanoparticles. They are often used in -> photoelectrochemistry as electrode materials. 

Abraham, Wlosnewski, Buck, and Jacob [95] invented a new type of photoswitchable rotaxane based on the above principles where the diaryl-methoxy-cycloheptatriene unit is replaced by 9-aryl-9-methoxy-acridanes (Fig. 9b4) that undergo photoheterolysis (313 nm light) with formation of acridinium ions (Fig. 9b5). Duo, Jacob, and Abraham [96] demonstrated that such devices can be deposited and can operate on gold nanoparticles. 

LSPs are detected as resonance peaks in the absorption or scattering spectra or as dips in the transmission spectra of the metallic nanoparticles. Nanoparticles of very conductive metals like gold, silver, and copper are ideal materials for excitation of localized surface plasmons due to an extremely high ratio of the modulus of the real (Sr) to the imaginary parts (8i) of its dielectric constant. Silver and copper nanoparticles are prone to oxidation and therefore often require coatings of protective over layers. Gold nanoparticles are chemically stable and are employed for the development of devices based on plasmon resonances of nanoparticles. 

Fiber optic chemical and biological sensors, based on SPR in continuous metallic films deposited on optical fibers, have been previously reported. " Sensors based on optical fibers have advantages like high response times, ability to be remotely interrogated, and low electromagnetic interference. In our study, fiber optic devices were developed based on excitation of LSPs in gold nanoparticles, deposited on the surface and tip of different optical fiber stractures. As refractive index of the medium around the sensors was varied, or biological molecules attached to the nanoparticles, a shift in the plasmon resonance related dip in the transmission spectram was observed. 

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