A golden time for nanotechnology
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troduction Gold nanoparticles (AuNPs) have become an enduring example of how bulk properties of a material are altered at the nanoscale. It is now well known that ∼4–200-nm diameter particles of gold, suspended in solvents, appear red, blue, green, and brown, depending on their crystal size, shape, and state of aggregation. The strong visible and near-infrared optical resonances of AuNPs, now called plasmons, can be pictured as coherent oscillations of conduction-band electrons that move around the surface of the particle upon resonant illumination. Innovations in shape control have allowed for tuning of the plasmonic properties of AuNPs for applications that require precise optical response. Gold nanorods (AuNRs), in particular, are extremely popular for their tunable plasmonic resonances from ∼520 nm to ∼2000 nm. Many preparations based on wet chemical synthesis have been developed to increase the yield and vary the size and aspect ratio of these particles.1–6 In addition to AuNP synthesis, much of the innovations in nanoparticle applications have come from developments in surface chemistry modifications. This has ranged from soft organic coatings such as polyelectrolytes7 and poly(ethylene glycol) (PEG)8 to hard coatings, including silica9 and metal– organic frameworks (MOFs).10 This has widened the reach of gold in nanotechnology by allowing precise control of the
particle interface with other nano- or biomaterials. Study of the nano–bio interface is of special importance due to increasing interest in applications such as photothermal therapy,11 photoacoustic imaging,12 and biochemical sensing.13 Here, we review developments in the synthesis of AuNPs and AuNRs, focusing on recent innovations in the control of shape, size, and plasmonic properties. Methods to alter the surface chemistry through ligand exchange or shell growth are also summarized. Finally, recent applications of AuNPs are discussed, with an emphasis on those with biological and environmental impact, including advances in photothermal therapy, effects on soil chemistry, and AuNP effects on bacterial gene regulation.
Making gold on the nanoscale Metallic nanoparticle synthesis has a rich history, first presented (albeit unknowingly) in chemical protocols dating back to the eighth century, where metal salts added to molten glass resulted in brilliant colors.14 Centuries later, Faraday performed the first systematic study of AuNP synthesis, deducing that the colors are the result of finely divided metallic gold particles with dimensions smaller than the wavelength of light.15 More recently, modern methods of AuNP syntheses have been developed that focus on the chemical reduction of HAuCl4 in
Huei-Huei Chang, Department of Chemistry, University of Illinois at Urbana-Champaign, USA; [email protected] Matthew T. Gole, Department of Chemistry, University of Illinois at Urbana-Champaign, USA; [email protected] Catherine J. Murphy, Department of Chemistry, University of Illinois at Urbana-Champaign, USA; [email protected] *These authors contribu
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