How a pigment can enhance near infrared imaging

Materials World magazine
24 Apr 2020

An ancient pigment, Egyptian Blue, brings novel capabilities to near infrared imaging. Idha Valeur reports.  

A nanomaterial made from exfoliated layers of Egyptian Blue (EB) pigment at a miniature width, can improve imaging with near infrared (NIR) spectroscopy and microscopy, according to researchers from the University of Göttingen, Germany. 

‘We have analysed the photophysical properties of this ancient yet innovative nanomaterial, highlighting how it could efficiently be used as a new working tool for future works in the biophotonics field,’ outlines Gabriele Selvaggio, PhD student at the university.

‘By performing toxicity tests on cells, we have also shown that this material is non-toxic, which is a fundamental requirement for its employment in the biomedical field.’ 

According to the team, it is already known that the pigment is visible at NIR range, however, it has not been previously shown that the pigment’s fluorescence at NIR is intense and stable even in nanostructured (NS) form. ‘This means that EB-NS could work very well as a labelling agent in multiple scenarios, e.g. particle tracking in embryos of fruit flies, in plants, etc.,’ Selvaggio explains. ‘One could also envision image-guided surgery, or labelling of cancer tissue,…[and]…fingermark detection on banknotes.’ 

A new exfoliation process has been developed to separate the layered silicate to make it as small as possible to yield the 2D sheets. Selvaggio says the method involves a combination of milling and ultrasounds, which is then followed by centrifugation or syringe-filtration. 

He explains that previous exfoliation methods take a long time to complete and result in large nanosheets that are not suitable for this application. 

‘For these reasons, we employed a mixed approach of planetary ball milling in water and tip sonication i.e. ultrasounds in water or propanol. To sum it up, a first milling step is performed, the obtained supernatant – which contains the smallest crystals yielded from the milling – is then dried and introduced into a specific volume of liquid, water or propanol, which is then tip sonicated for six hours, typically. Finally, to remove unexfoliated material and leave only the smallest nanosheets in the sample, either liquid cascade centrifugation was performed, or syringe-filtration was carried out. In the latter case, this means that filters of specific pore diameter were used to “sieve” the material and leave larger stuff out,’ he says. 

Labelling agents in the form of fluorophores are often used in microscopy and spectroscopy, most commonly emitting in the visible range of the spectrum, but they may be affected by autofluorescence and phototoxicity problems. Selvaggio states that these labels’ fluorescence intensities decrease in time, known as ‘bleaching’, during longer imaging experiments. ‘On the other hand, EB-NS emits in the NIR and is not affected by bleaching, therefore optimal for particle tracking in living systems.’

Currently, the team is working on further developing the exfoliation of the pigment into the nanostructured form and applying the approach to other similar pigments. ‘A further challenge that we have started to address is also the “functionalisation” of this material, i.e. the creation of chemical bonds between EB-NS and molecules able to increase its water solubility as well as its affinity to analytes of interest e.g. cell receptors,’ Selvaggio adds.