Nanoparticles have become materials of great interest for their unusual optical properties. Part of what makes them so remarkable is that even nanoparticles with the same chemical composition as a bulk material can have very different properties.
Image Credit: CI Photos/Shutterstock.com
These small-scale particles often behave very differently to their bulk counterparts because nanoparticles have a very large surface area to volume ratio and can exhibit what is known as ‘quantum confinement effects’.
Confinement effects mean nanoparticles behave more like isolate atoms in terms of their electronic structure than bulk material. This can result in an enhancement of the optical properties of the nanoparticle as the electronic states resemble the discreet energy levels of an atom rather than there being a sufficiently high density of states that they resemble the continuous broad features found in materials.
Silver nanoparticles have become popular in nanomedical as anti-fungal and anti-cancer agents. Their small size and good solubility make them relatively easy to process, so they can be incorporated into more complex materials such as bandages or biomedical devices where they can help destroy pathogens and ensure wounds remain sterile.
As well as their anti-microbial properties, silver nanoparticles also have remarkable optical properties as they can both scatter and absorb light highly efficiently.
Size Dependence
The optical properties of nanoparticles are often size-dependent. Small nanoparticles have their absorption maxima shifted towards shorter wavelengths, while larger particles approaching 100 nm in size have a much broader absorption across much of the visible spectrum extending to 800 nm.
One convenient aspect of the size dependence of the optical properties of nanoparticles is that – assuming all the nanoparticle solutions are at the same concentration - UV-vis absorption spectroscopy can be used to profile the particle sizes.
Being able to use a relatively straightforward characterization technique for particle sizing of nanoparticles is advantageous as the small size of many particle species can make it challenging to use direct imaging methods such as microscopy.
Standard microscopy techniques are limited in their spatial resolution by the diffraction limit of the incident light. This is wavelength dependent, and in the optical regime can limit the spatial resolution of visible light microscopy techniques to the nanometer range.
Characterization
Particle size is not the only variable that affects the optical properties and absorption of silver nanoparticles. Aggregation of the nanoparticles, which is a concentration-dependent effect, can also lead to spectral changes.
When silver nanoparticles aggregate, their local ‘surface’ becomes shared with the other particles they are aggregated with. This essentially provides a much larger area that the electrons can be delocalized across, which shifts the plasmon resonances to much longer wavelengths.
Not all nanoparticle aggregates are stable and aggregation can be a reversible process. The shape of the overall optical absorption spectrum is also slightly different for nanoparticles with larger particle sizes versus aggregates.
On aggregation, typically, a second broader redshifted feature is formed as well as the main absorption feature at a higher wavelength.
Due to this change in the optical properties and the time scales associated with the aggregation of nanoparticles, which are often on the order of minutes, UV-vis spectroscopy can be used to determine the degree of aggregation and also the rate at which it occurs. This is particularly useful to check whether or not nanoparticles have become unstable in solution.
Environmental Effects
Silver nanoparticles can be dispersed in a number of solvents, but their electronic structure and properties are also highly sensitive to their local environment. In the same way, their absorption properties depend on the particle size and the presence of any neighboring particles to aggregate with. Their absorption spectrum is also sensitive to the refractive index of the materials around them.
When the environment around the nanoparticle has a larger refractive index, it results in the redshifting of the nanoparticle optical absorption spectrum. The main absorption peak can shift by nearly 100 nm as a result.
Tuneability
The tuneabiltiy and control over these optical properties make silver nanoparticles so attractive for applications such as sensing.
Recent work has made use of this sensitivity to use the strong optical signal for silver nanoparticles as a way of probing their own electrochemistry using optical microscopy.
Understanding the electrochemistry of such particles is important for knowing how silver nanoparticles will behave as part of devices such as DNA sensors, where electron transfer processes between the target and a reference molecule are used to identify different DNA sequences.
Silver nanoparticles are a good choice for molecular sensors due to their high brightness due to the surface plasmon resonances and environmental sensitivity, meaning there should be a clear color change on binding a target chemical with a wavelength shift that is characteristic of the species being bound.
Continue reading: Could Nanoparticles Create Sustainable Lighting?
References and Further Reading
Neal Blackman III, G. N. B., & Genov, D. A. (2018). Bounds on quantum confinement effects in metal nanoparticles. Physical Review B, 115440, 115440. Available at: https://doi.org/10.1103/PhysRevB.97.115440
X. Zhang, Z. Liu, W. Shen and S. Gurunathan, (2016) Silver Nanoparticles: Synthesis, Characterization, Properties, Applications, and Therapeutic Approaches. Int. J. Mol. Sci., 17, 1534. Available at: https://doi.org/10.3390/ijms17091534
Rasmagin, S. I., & Apresyan, L. A. (2020). Analysis of the Optical Properties of Silver Nanoparticles. Spectroscopy of Condensed Matter, 128(3), 339–342. Available at: https://doi.org/10.1134/S0030400X20030169
Noël, J., & Lemineur, J. (2021). Optical microscopy to study single nanoparticles electrochemistry: From reaction to motion. Current Opinion in Electrochemistry, 25, 100647. Available at: https://www.sciencedirect.com/science/article/abs/pii/S2451910320301885
Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.