Improving photodynamic therapy by turning visible light into infrared energy

Materials World magazine
,
27 Feb 2019

Visible light can be converted into infrared energy through a chemical process. Idha Valeur finds out how it works.

A chemical process by which visible light is turned into infrared energy makes it possible for innocuous radiation to infiltrate living tissue. This development could improve both reach and the level of effectiveness of photodynamic therapy for diseases like cancer, say researchers at Columbia University, USA.

The researchers carried out a series of chemical transformations, which normally need high-energy visible light, but they found they could use non-invasive infrared light instead.

The new technique combines photoredox catalysis – a field of organic chemistry using visible light to perform chemical transformations – with triplet fusion upconversion. During this process, two infrared photons get merged into one visible light photon.

Unlike other technologies where only visible light is caught, wasting the remaining solar spectrum, triplet fusion upconversion gathers the low-energy infrared light and transforms it into light that is absorbable by solar panels. Although visible light can be reflected without a problem, infrared wavelengths are longer so they can pass through thicker materials.

‘With this technology, we were able to fine-tune infrared light to the necessary, longer wavelengths that allowed us to non-invasively pass through a wide range of barriers, such as paper, plastic moulds, blood and tissue,’ said Columbia University Associate Professor of Chemistry Luis M Campos.

Columbia University PhD student and visiting scholar at the Rowland Institute at Harvard University, USA, Andrew Pun, told Materials World, ‘The major use of this new method is for photodynamic therapy (PDT). We can make drugs that can kill basically any infection, the problem is not killing the human at the same time. If you could inject a drug in its inactive form, prodrug, then turn that drug active with localised light radiation – you could treat diseases like cancer locally without harming the rest of the body.’

Pun explained how most of the chemistry enabled by photoredox catalysis uses blue or UV light, which is unable to penetrate the skin to a satisfactory level. ‘Blue and UV light don’t penetrate the skin well, so they can’t be used for photodynamic therapy. With our technique, we can use infrared light to activate photoredox reactions, opening the door for using them in photodynamic therapy,’ he said.

Deeper tissues

Several attempts have been made to enable visible light to pass through skin and blood without causing damage to healthy tissue and organs. The current method used, photodynamic therapy, makes use of a photosensitiser drug, which is triggered by light and subsequently produces very reactive oxygen which then kills or hinders the cancer cells’ growth. However, this therapy is restricted to treating localised or surface cancers.

‘This new technology could bring PDT into areas of the body that were previously inaccessible. Rather than poisoning the entire body with a drug that causes the death of malignant cells and healthy cells, a non-toxic drug combined with infrared light could selectively target the tumour site and irradiate cancer cells,’ said Columbia University Professor of Chemistry and co-author of the study Tomislav Rovis.

Not only can this new process treat cancer, but it could also be used in cases of traumatic brain injuries, hearing loss, and damaged nerves or spinal cords.

Beyond medical conditions, the technology could find applications in remote management of chemical storage solar power production and data storage, drug development and food safety methods.

‘Our biggest challenge will be to incorporate this technique into biological systems,’ said Pun. ‘Currently, these reactions are done in organic solvents, but it would be great to show they can be done in aqueous systems, paving the way for their use in photodynamic therapy.’