Nanorods aspect ratio

Gao JX, Bender CM, Mmphy CJ (2003) Dependence of the gold nanorod aspect ratio on the nature of the directing smfactant in aqueous solution. Langmuir 19 9065-9070... 

To obtain higher nanorod aspect ratios and lengths in the presence of Ag+ ions, a co-surfactant is required benzyldimethylhexadecylammonium chloride for nanorods with aspect ratios up to 10 [164] or Pluronic F-127 for aspect ratios up to 20 [165]. The average length of the nanorods can be increased continuously by adding new amounts of growth solution. However, for longer nanorods or nano wires (i.e., aspect ratios above 80) the synthesis should be performed in the absence of Ag+ ions. 

In order to assemble gold nanorods (aspect ratio 2.4) by the click reaction, one batch of nanorods capped with 4-azidobutane-l-thiol, A, and another with hex-5-yn-l-thiol, B, were reacted in the acetonitrile-water mixture under standard conditions [12], The occurrence of the click reaction was established by infrared spectroscopy [13], The product of the reaction was investigated by electronic absorption spectroscopy and transmission electron microscopy. Fig. 1 compares the electronic absorption spectra of the gold nanorods before and after the click reaction between the A and B type nanorods. Isolated gold nanorods show transverse and longitudinal plasmon bands around 520 nm and 630 nm, respectively (see Fig. la). 

Figure 11.11 E-field enhancement contours external to differently shaped silver nanoparticles for the in-plane dipole SPR from DDA calculations, for (A) a nanoprism (edge length = 60 nm, thickness = 12 nm.) and (B) a nanorod (aspect ratio = 2.8 1, effective radius = IS nm). The arrows indicate the regions of maximum field intensity. Reprinted with permission from reference [5]. (2004) American Institute of Physics.

Our seed-mediated growth approach has been successful for the synthesis of gold nanorods, with diameters of ca. 20 nm and aspect ratios from 2 to 20, in a controllable fashion (Figure 9.3). For silver, we have been able to make both short nanorods (aspect ratio 4) and long nanowires (aspect ratios 50-350) in the case of nanowires the level of control we have is more limited. 

Fig. 9.13. Transmission electron micrographs of gold nanorods, aspect ratio 18, before (a) and 24 h after (b) cyanide treatment. Reprinted with permission from [62], Copyright (2002) American Chemical Society.

We also examined the dissolution reaction of gold nanospheres, spheroids, and nanorods (aspect ratios 1, 2-5, and 18 respectively) with persulfate, which is thermodynamically a favorable reaction [62] ... 

Our results, based on UV-vis spectroscopy and TEM [62], indicate that spheroids (aspect ratio 2-5 with 12-30 nm short axis) are more reactive than spheres (20-30 nm) and nanorods (aspect ratio 18 with 16 nm short axis). In the presence of persulfate, spheroids convert to spheres (Figure 9.14), but similar size spheres and rods do not react at all, on the timescale of days. We presume that during the shape transition from spheroid to sphere, a fraction of gold atoms at the spheroid edges oxidize thus the particle diameter shrinks ( 5-10%) after persulfate treatment. 

Figure 16.6 Tunability of the plasmon resonance maximum in gold nanostructures. Variation of the surface plasmon extinction maximum with (a) nanospheie diameter D (b) nanoshell total radius R2 with fixes R1/R2 = 0.857 (c) nanoshell core/shell ratio R1/R2 at fixed R2 = 70 nm (d) nanorod effective radius = (3V/4II) / at fixed aspect ratio R = A/B = 3.9 (e) nanorod aspect ratio R at fixed r ff = 11.43 nm. (Reproduced with permission firam P. K. Jain et al., 2006. J. Phys. Chem. B 110 7238-7248. Copyright 2006 American Chemical Society.)...

The influence of nanorod aspect ratio on the SERS surface enhancement factor (SEF) was systematically investigated by Murphy and was shown to be a significant parameter for the case of isolated nanorods 23). Therefore, the SEF dependence on nanorod length for the Ag nanorod array substrates was probed. Figure 2 highhghts the results of this study, while details can be found elsewhere (28,33). It is concluded that optimum SERS substrates are those with nanorod lengths of 900 mn which provide a SEF of 5 x lO for BPE (33). 

PbSe nanorods were synthesized by the reaction of Pb-oleate with tris(diethylamino)-phosphine selenide. The amount of water present in the reaction, either added intentionally or present as a contaminate, had a dramatic effect on both the PbSe nanorod morphology and yield. The precursors were carefully dried and water was intentionally added which had an effect on the nanorod aspect ratio which was controlled from 1.1 to 10 and the yield from 1 to 14% by varying the water concentration from 0 to 204 mM as shown in Fig. 13. 

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