LACRYS is a Proof of Concept project aimed to develop a new restoration technique based on light-controlled dissolution and precipitation of calcium carbonate. The main idea behind the project is that light controlled precipitation and dissolution of CaCO3 can be achieved using photobase (PBG ) and photoacid generators (PAG), respectively. These molecules can induce a pH increase (PBG), or decrease (PAG) after light absorption, undergoing a photochemical reaction. Therefore, the light-controlled pH variation can change the solubility of calcium carbonate, which is highly dependent on pH. These processes can be obtained in not invasive conditions, using aqueous solutions and a not harmful light wavelength.
Calcium carbonate dissolution was achieved by using diphenyliodonium hexafluorophosphate (DIP) as a PAG, which generates a Bronsted acid under UV light irradiation by means of a radical mechanism. Photodissolution of calcium carbonate was first performed in solution, to understand the reaction mechanism and then, in order to be closer to the final application in artworks restoration, it was performed on calcium carbonate powder deposited on a glass surface. In these conditions, we obtain complete CaCO3 dissolution in 15 minutes of irradiation. Then, the system can be efficiently washed with MilliQ water and no traces of DIP solution or CaCO3 spot remains on the glass slide. However, CaCO3 dissolution was still too slow for a real application. Instead of exciting DIP at shorter wavelengths, which would have been unsafe for the operator, we fasten the process using a sensitizer, Thioxanthene-9-one, that has an absorbance in the range of 360-400 nm and that thus increases the performances and extend the application to higher wavelengths. Indeed, CaCO3 dissolution occurs in 5 minutes and is highly localized both at 365 nm and at 395 nm.
PBGs have been used to induce a pH increase by light, in order to precipitate CaCO3. Also, the process is conducted in water-based solutions, avoiding the use of organic solvents and it can be controlled locally, by using a not harmful irradiation light (365 nm). In particular, two PBGs were exploited for this purpose: Sodium tetraphenylborate and ketoprofen. The former promotes CaCO3 formation of a pH jump and the latter the pH increase is coupled with CO2 production. Sensitization of Sodium Tetraphenylborate by Thioxanthene-9-one considerably fasten the process and the photoinduced precipitation can be achieved with just 5 minutes of irradiation by a 365 nm source. Furthermore, photoinduced precipitation of CaCO3 by ketoprofen was also studied for application in lithography and patterning. After many problems encountered with the coprecipitation of the by-products of the photobase, we have performed advances and confirmed that it is possible to literally write CaCO3 structures with light. This simple and clean new technique could have an important impact in artworks restoration as well as in the design of advanced nanomaterials. The results from the ERC - PoC could be the basis for further exploitation and eventual licensing by stakeholders in the field. In order to asses protection a draft document has been submitted to the corresponding units of CSIC and UNIBO, and is presently under evaluation of patentability requirements. It is expected to file a European Patent application in the Q2 of 2021.
