Testing time: Nanorepair UV being applied to limestone samples from Cadiz (Courtesy: Javier Becerra)
From the Egyptian pyramids to The Pentagon in Washington DC, limestone has long been one of the most popular materials for ornate buildings and monuments. But this rock’s aesthetic qualities and workability come at a price – it can be vulnerable to degradation by natural weathering, air pollution and other processes.
New research by scientists in Spain and Greece might provide a new way of preserving limestone architecture for future generations. The group has created a nano-composite doped with quantum dots, which can be applied to reinforce limestone structures while providing a way of monitoring the changes.
When consolidating historic architecture, you want durability and chemical compatibility, without changing its aesthetic features. Traditionally, limestones have been treated using limewater, a common name for a dilute aqueous solution of calcium hydroxide. Limewater can create a durable seal, but it does not tend to penetrate deeply into the rock. Also, calcium hydroxide has a limited solubility in water.
An alternative developed the past decade are “nanolimes” – calcium hydroxide nanoparticles dispersed in water or organic solvents. The tiny size of nanoparticles enables them to penetrate deeper into stone, while the increased specific surface area can enhance the carbonation process. Indeed, it is the calcium carbonate created in this process that cements the original limestone structure, not the nanoparticles themselves.
Nanolimes are still a relatively young technology so there is uncertainty over how they will perform over long time periods. In some cases, nanoparticles have been known to migrate back to the surface as the solvent evaporates. So, when applying nanolimes in the real world it is important to monitor their performance over time.
Nanolimes with a twist of quantum dots
To help with monitoring, a new type of nanolime has been created by a team led by Javier Becerra Luna from the Universidad Pablo de Olavide in Spain. Dubbed “Nanorepair UV”, the material takes advantage of the optical properties of quantum dots. These are tiny pieces of semiconductor just a few nanometres across that are sometimes described as “artificial atoms” because they absorb and emit light at specific wavelengths.
Becerra’s nanolime comprises calcium hydroxide nanoparticles doped with zinc oxide quantum dots, which act as markers. When illuminated with UV light, the quantum dots emit fluorescent light, highlighting the newly formed limestone against the original structure. In practice, conservation workers would remove a small sample from an inconspicuous part of a monument to establish how deeply the consolidant has penetrated and how it is performing.
“The major technical challenge was creating a nanocomposite and not a mixture of two different nanoparticles. It is a key point that the quantum dots are posed on the calcium hydroxide, so we can measure the real penetration depth of the treatment,” Becerra told Physics World.
