Silicate glass is the most well-known and widely used type of glass, for example, it is found in most households, in drinking glasses or windowpanes. The integration of gold nanoparticles (NPs) in silicate glass has been used in art and decoration for centuries. These NPs impact the way the silicate glass interacts with light through the now well-known phenomenon called localized surface plasmon resonance (LSPR). This unique light modulation behaviour has opened up applications from coloured glass to special optical components.

The ability to uniquely modulate light in silicate glasses by gold NPs has sparked the scientific community to utilise these NPs in other glass types to generate new optical functionalities. Of the many glass types investigated, tellurite glass has been of particular interest since it exhibits a unique combination of properties. Tellurite glass is somewhat easy to fabricate, is durable, and exhibits low phonon energy, wide transmission window, and high solubility for luminescent rare earth ions, allowing these ions to emit bright light over a wide spectral range from the visible to the infrared. These are important features for fibre optics, laser systems, and sensing technologies. To achieve the desired light modulation behaviour, the size, shape, distribution, and quantity of the gold NPs must be controlled carefully. However, the technique commonly used for precisely forming gold NPs in silicate glass, the so-called striking technique, has proven insufficient to achieve precise control of gold NPs in tellurite glass.

The researchers from the University of Adelaide have had a long-term and twisted relationship with gold NPs in tellurite glass. Initially, more than 10 years ago, they developed a technique to incorporate diamond nanocrystals into tellurite glass fibres, so that the unique properties of diamond nanocrystals could be harnessed in a flexible and inert platform ¹. The addition of diamond nanocrystals to a tellurite glass melt, contained in a gold crucible, was found to chemically reduce the gold ions dissolved into the glass melt from the gold crucible and turning them into gold NPs – giving the glass a beautiful dichroic colour of blue in transmission and orange in reflection, similar to the famous ~2000 years old Lycurgus cup in the British museum, which appears red in transmission and green in reflection. However, the colourful behaviour of the gold NPs was undesired for the development of diamond-doped tellurite glass fibres as it deteriorated the optical performance of the fibres ². Therefore, a large amount of work was dedicated over the subsequent ~5 years to minimize and eventually even completely prevent the gold NP formation ³, which allowed the demonstration of magnetic field sensing with a diamond-doped tellurite glass fibre ⁴.

In a parallel project, the addition of upconversion nanocrystals to a tellurite glass made by melting in a gold crucible also led to the formation of a small amount of gold NPs ⁵. It was hypothesized that the organic residues on the surface of the nanocrystals could have caused the chemical reduction of a few gold ions to gold NPs. Another outcome of this project was that the doping method of adding nanocrystals to a glass melt does not allow homogeneous dispersion of the nanocrystals in the volume of the glass. These results sparked undertaking dedicated research to solve the mystery of gold NP formation in tellurite glass and to identify a method for better nanocrystal dispersion in glass.

For this new research avenue, the so-called powder doping method ⁷ was tested using ruby nanocrystals prepared via ball milling ⁶ and thus without surface groups. To the great surprise of the research group in Adelaide, the glass made by reheating powder of colourless tellurite glass (melted in a gold crucible) blended with ruby nanocrystals showed the same characteristic dichroic colour appearance as the glasses made by adding diamond nanocrystals to the melt. As the ruby nanocrystals did not exhibit any chemically reducing ability (unlike diamond nanocrystals), the formation of the gold NPs could have only been caused by the fabrication process and the glass itself. This was confirmed by repeating the reheating of powder of tellurite glass (containing dissolved gold ions) but without added ruby nanocrystals – this undoped tellurite glass showed an almost identical homogeneous dichroic coloured glass. By contrast, if the tellurite glass was reheated in a bulk form, the resulting glass remained colourless.

This fascinating and serendipitous discovery motivated the Adelaide group to collaborate with the glass researchers in Germany to pivot the project towards investigating the use of the powder reheating technique as well as the dissolution of gold ions from the gold crucible for controlled formation of gold NPs to enable manipulation of the LSPR properties of gold NPs in tellurite glass. Building upon the knowledge of the conventional striking technique about control of reheating temperature and duration, they were able to produce a range of tellurite glasses with tunable LSPR properties (Figure 1). In addition, the team also demonstrated that the Mie theory is a viable tool to correlate the gold NP quantity, size and size distribution in tellurite glass to its LSPR properties (peak position, intensity and bandwidth, as well as the corresponding colour effect). This advance in technology of creating gold NPs is expected to provide guidance for designing and manipulating the plasmonic properties in tellurite glass for exciting photonics research and applications in the future.

Of course, there are many more future avenues to look at. For example, the team is keen to find out why the powdered form of tellurite glass provides the reducing power to transform gold ions into NPs but not in the bulk form. Another key step towards practical application is how to eliminate the gas bubbles trapped in the powder-reheated glasses so that the desired LSPR properties can be fully exploited.

There is another learning beyond the scientific content – please don’t easily throw away “unwanted” findings because you’ll never know where they might lead you to.

More information of this work entitled “Controlled formation of gold nanoparticles with tunable plasmonic properties in tellurite glass” can be found in the Light: Science & Applications journal (DOI: https://doi.org/10.1038/s41377-023-01324-x).

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