ChemTalk

Dr. George Schatz: The Mystery of Colloidal Gold Nanoparticles

Colloidal Gold Nanoparticles

Nanoparticles In Perspective

[Alaina Rumrill] Although nanoparticles have been utilized throughout human history, the term “nanoscience” was not formally used by scientists like Dr. George Schatz until the 1990s [1]. Nanoscience encompasses the study, manipulation, and engineering of matter, particles, and structures on the nanometer scale (1 millionth of a meter) [2]. To grasp the size of a nanoparticle, consider that one human hair is about 60,000 to 100,000 nanometers wide. DNA, by comparison, measures about 2.5 nanometers in diameter, while a person standing at 6 feet 6 inches would span two billion nanometers [1].  

When a material is incredibly tiny, or nanometer-sized, it exhibits unique interactions with light. Plasmonic nanoparticles absorb light significantly in the visible and near-infrared spectrums, resulting in a pronounced excitation of the electric field [3].  For thousands of years, plasmonic nanoparticles have been used in stained glass windows. However, the use of nanoparticles was made completely by accident [1]. To achieve the red and yellow colors in stained glass, colloidal gold and colloidal silver are used. Colloidal gold, or the suspension of nanoparticles of gold in a fluid, is a fascinating example of plasmonic nanoparticles. In reaction with light, the tiny gold particles appear as a red liquid. Colloidal silver exhibits the same phenomenon, as tiny suspended silver particles appear yellow with light [1]. To explore this reaction, scientists looked at fine particle properties, concerned mostly with light scattering properties.

Colloidal Gold Nanoparticles

Michael Faraday, among the first scientists to write extensively about colloidal gold in the 1850s, found that when water is evaporated from colloidal gold, it would return to its original chunks of gold [1]. After this discovery, he concluded that he should eventually be able to achieve the red color from colloidal gold by chopping the chunks into smaller pieces, however he was unsuccessful. He discovered basic laws of electricity magnetism which could be used to understand the colorful properties of colloidal gold. These laws were later termed Maxwell’s equations for light scattering [1]. Early 1950s physicists David Bohm and David Pines concluded that metal particles are a bunch of moving electrons, including free electrons and positive charges of ions left behind. With this, the realization was made that the optical properties of metal particles are similar to that of plasmon: an oscillation of electrons in a material when they are excited by electromagnetic waves [1]. 

Figure 1. Surface plasmon resonance effect.

In plasmon and metal particles, the motion of electrons resembles a pendulum swinging back and forth. This is similar to that of a surface plasmon formed when light strikes metal, initiating a collective movement of electrons across the surface [Figure 1]. These electrons, in turn, absorb and scatter light, producing a prism of distinct colors based on wavelength. This describes colloidal gold’s light-scattering properties. Prominently today, gold nanoparticles and plasmon resonance are used in sensing, imaging, biomedical diagnostics, and Raman spectroscopy [1].

Raman Spectroscopy

A significant advancement in the study of nanoparticles is Raman spectroscopy. Raman spectroscopy is a technique used to analyze the vibrational and rotational modes of molecules. This provides valuable information about the chemical composition, molecular structure, and bonding properties of substances [1]. In his extensive research on nanoparticles, Dr. George Schatz has greatly contributed to the understanding of Raman signals [1]. More specifically, Raman scattering, or an inelastic scattering of light, shows when photons interact with the vibrational modes of molecules. This results in a shift in the energy of scattered photons [1].

Raman spectroscopy is especially useful for studying nanoparticles. Other forms of spectroscopy are not as effective in analyzing such small particles. In the 1970s, scientists began to explore the idea of analyzing a monolayer of molecules on a surface. In 1993, several papers claimed that they observed Raman spectra of an individual molecule that was absorbed onto a combination of colloidal particles [1]. Breakthrough was eventually made by Richard Van Dyne in 2005, as he performed Raman measurements on a single molecule and successfully received signals from a single particle [1]. Newer understanding includes advancements by Dr. George Schatz, who has successfully developed a newer theoretical framework and computational methods to understand the Raman scattering process [1]. Particularly, he has been impactful in his work on Surface-Enhanced Raman Spectroscopy (SERS). SERS is a technique that enhances Raman signals through the interaction of molecules with specifically designed nanostructured surfaces. His efforts have contributed to clarifying the mechanisms underlying the enhancement effect and optimizing conditions for practical applications. Without the progress made in Raman spectroscopy specifically by scientists today like Dr. George Schatz, our understanding of plasmonic nanoparticles would not be as comprehensive as it is today.

Dr. Schatz’s Findings

Dr. George Schatz’s interest in nanoparticles stemmed from his upbringing on a farm. He began to wonder about the chemicals used in agriculture, bridging the connection to chemistry [1]. Dr. Schatz earned his Ph.D. in chemistry at the California Institute of Technology and was a postdoc at the Massachusetts Institute of Technology. After completing his studies, Dr. Schatz has contributed to his research at Northwestern University for the past 47 years [1].

Dr. Schatz’s research encompasses collaborations and expanding upon the findings and theories of earlier scientists on nanoparticles [1]. His research on nanoparticles involves the interaction of light with nanostructured materials, particularly metallic nanoparticles, to amplify Raman signals. Dr. Schatz has made tremendous contributions to the field of Raman spectroscopy such as improving sensing platforms and imaging methods. He emphasizes that no one believed in Raman spectroscopy in the 1970s, and the way it worked was certainly misunderstood. Now, Dr. Schatz’s work includes utilizing molecular dynamics simulations to study the dynamics of molecules under Raman excitation. Dr. Schatz has also improved the field by developing computational methods and models and studying the electromagnetic enhancement mechanism in surface-enhanced Raman scattering [1].  

Learn More

If you’d like to hear more about Dr. George Schatz’s journey and his findings in the field of nanoscience, visit us on Spotify, Apple Podcasts, and many other streaming services to listen to our ChemTalk Podcast with Dr. George Schatz, Professor of Chemistry and Chemical and Biological Engineering at Northwestern University.

Find the ChemTalk podcast here.

Works Cited

[1] Schatz, George. Personal Interview. Bella Liguori and Yeongson Seo. 7 July 2023.

[2] “What Is Nanoscience & Nanotechnology?” EMM Nano. Accessed March 15, 2024. https://www.emm-nano.org/what-is-nanoscience-nanotechnology/. 

[3] “Plasmonic Nanoparticles.” Plasmonic Nanoparticles – an overview | ScienceDirect Topics. Accessed March 15, 2024. https://www.sciencedirect.com/topics/medicine-and-dentistry/plasmonic-nanoparticles#:~:text=Plasmonic%20nanoparticles%20have%20a%20significant,result%2C%20a%20higher%20field%20inte