Gold nanoparticles are of interest for biomedical applications; their varied usage includes cellular imaging, molecular diagnosis and targeted therapy. Their unique optoelectrical properties have been extensively researched and utilized in high-tech applications ranging from organic photovoltaics, sensory probes, drug delivery in biology and medical applications and in electronic conductors and catalysis.
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From bulk to the nano
The optical, magnetic, structural and electronic properties of materials change as their size reduces from the bulk to the nanometer. The optical and electronic properties of nanoparticles such as gold can be tuned by controlling the particle size, shape, surface chemistry and aggregation state.
When a metal particle is excited by a specific wavelength of light, conduction electrons near the surface undergo collective oscillation. The amplitude of this oscillation reaches a maximum at a specific frequency, known as surface plasmon resonance (SPR). SPR induces a strong absorption of the incident light, which is measured using UV-visible absorption spectrometry.
The total light extinction – the energy loss of an electromagnetic wave after passing through matter -results from absorption and scattering. Absorption is the result of a photon’s energy being dissipated, whilst scattering occurs when a photon’s energy causes electrons to vibrate in the matter and re-emit photons either at the same frequency as the scattered light (Rayleigh scattering) or at in shift frequency (Raman scattering).
Gold nanoparticles (GNPs) absorb and scatter light with incredible efficiency; they are easy to make and modify and exhibit strongly enhanced and tunable optical characteristics. Unlike the bulk gold, we are used to seeing, Au NPs exhibit different optical properties. Unaggregated nanoparticles such as colloidal gold - a suspension of nanoparticles of gold in water - is intense red in color. Aggregated gold nanoparticles are blue/purple in solution, which can progress to a clear solution with a black precipitate.
The optical properties of GNPs alter when they aggregate; the conduction electrons near the particle’s surface become delocalized, sharing amongst neighboring particles. When this occurs, SPR shifts to lower energies causing absorption and scattering peak to shift to longer wavelengths.
Spherical gold nanoparticles’ optical properties are highly dependent on the diameter of the particle; smaller nanospheres primarily exhibit peaks near 520 nm, while larger spheres have larger peaks and exhibit increased scattering. They also depend on the refractive index near the particle surface. As the refractive index increases, the nanoparticle extinction spectrum shifts to longer wavelengths – known as red-shifting.
Gold nanoparticles make ideal probes for biological and cell imaging applications. Their nano-size makes them comparable to cellular components and proteins, allowing them to bypass natural barriers easily. The fact that they can be tailor-made in various sizes to exhibit differing properties is another great advantage.
SPR provides excellent fluorescence and photostability which is required for imaging applications. They are often used as labels for microscopy in biomedical applications, and as bioimaging tags in dark field microscopy techniques, where scattering from individual nanoparticles with diameters larger than 40-50 nm can be visualized.
Scanning electron microscopy (SEM) has been used to show that GNPs at 10 nm can accumulate in various cell organelles – the cytoplasm and nucleus for example – causing damage to the nuclear membrane. This has been confirmed by atomic force microscopy (AFM).
The two techniques have also shown that GNPs of 25 nm diameter can enter the cytoplasm and surround the nuclear membrane; nanoparticles of 50 nm are too large to enter the nucleus.
Through-focus scanning optical microscopy has become an important tool for determining the size of nanoparticles; it employs 4D optical information collected at different focus positions. Low partial coherence illumination combined with analysis of content from this process enables size determination with nanometer-scale sensitivity.
References and further reading
Gold Nanoparticles: Optical Properties
Gold nanoparticles: Optical properties and implementations in cancer diagnosis and photothermal therapy
Gold Nanoparticles: Properties and Applications
Microscopic Studies of Various Sizes of Gold Nanoparticles and Their Cellular Localizations
Nanoparticle size determination using optical microscopes