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Investigation of Gold Nanoparticles

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    Nanoparticles have diameters of 1-100 nm, so a 1 nm particle has a diameter of 1 x 10-9 m.  Nanoparticle research is currently a very active field of investigation due to a large variety of applications spanning from medicine (1), optics and electronics (2) and chemical and biological detection and measurement (3).  Nanoparticles are special because they display characteristics that are different from those of the same material in bulk form.  In fact, one can fine tune physical and optical properties of nanoparticles by controlling their size and shape. 

    You are familiar with the shiny, metallic appearance of bulk gold and silver.  However, reducing the dimensions of the particle drastically changes the appearance as a result of the way it interacts with light. Gold nanoparticles with diameters of 25 nm absorb green light and appear red in color. Silver nanoparticles absorb violet light and are yellow. The ability of nanoparticles to absorb or scatter light is not new knowledge. Artisans as far back as the times of Mesopotamia used nanoparticles to generate a glittering effect on ceramic pots and artists in medieval times used them for stained glass. Because nanoparticles are stable, the red and yellow color in these windows remains today.  Nanostructures like those that produce bright, shimmering colors on butterfly wings are composed of multiple layers with air gaps in between that refract, diffract and reflect light generating luminous colors (4).  Nanoparticle shape also affects its optical properties.  For example, spherical gold nanoparticles absorb in the 500 nm spectral region while irregularly shaped nanorods and nanostars absorb in the near-infrared (5).

    A large area of application of nanoparticles is in the development of optical sensors for detection and measurement of a wide array of analytes (3).  Spherical gold nanoparticles change color from red to blue depending whether they are dispersed or aggregated.  Thus, any ion, small molecule or even protein that can trigger gold nanoparticles to aggregate or disperse can be detected.


    Figure 1.  Example of colorimetric sensing of metal ions using gold nanoparticles functionalized with chelating agents.

    For example, sensors have been developed that rely on gold nanoparticles coated with chelating agents.  In the absence of the target ion, the nanoparticles are in their dispersed state and appear red.  In the presence of the specific ion, the interaction between the chelating agent and the ion bring the nanoparticles together, shifting their color to blue.  Similar approaches using different functional groups on the nanoparticles have extended this detection approach to small organic molecules, oligonucleotides and proteins (3).

    The color change of the gold and silver nanoparticles illustrates an important concept about nanoscale science.  Chemical and physical properties such as color, conductivity, and reactivity do not depend on the identity of the substance but on the size of the particle.

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