Fine-tuning the shape of gold nanorods

Gold nanorods prepared by a seed-mediated growth approach use similar to 4-nm gold nanospheres as the seeds and subsequent reduction of metal salt with a weak reducing agent (ascorbic acid) in the presence of a directing surfactant to produce nanorods. If insufficient ascorbic acid is added in the growth step, then metal salt remains. Additional input of ascorbic acid preferentially deposits more metal at the ends of the nanorods, to yield "dogbone"- like structures. Surprisingly, heat treatment of the unpurified gold nanorods (prepared with an insufficient amount of ascorbic acid) yielded fatter gold nanorods; the oxidation product of ascorbic acid appears to act as a reductant at higher temperature. These modified shapes of the gold nanorods directly influence their optical properties.