Measuring temperature changes caused by light

Researchers from the Daegu Gyeongbuk Institute of Science and Technology (DGIST) and Korea Institute of Science and Technology (KIST) have jointly developed a transparent temperature sensor capable of precisely and quickly measuring temperature changes caused by light. Described in the journal Materials Horizons, the technology is expected to contribute to the advancement of various applied bio devices that rely on sensitive temperature changes.

The photothermal effect using plasmonic nanomaterials has been widely proposed in various bio-application fields, such as brain nerve stimulation, drug delivery, cancer treatment and ultrahigh-speed PCR, due to its unique heating properties using light. However, measuring temperature changes by photothermal phenomena still relies on an indirect and slow measurement method using a thermal imaging camera, meaning it is not suitable for local temperature measurement at the level of a single cell, which changes rapidly at the level of several milliseconds to tens of micrometres. Due to the absence of precise information on temperature changes, photothermal effect technology has raised concerns about the understanding of biological changes and stable clinical application resulting from precise temperature changes.

Accordingly, the joint research team developed a temperature sensor technology that can measure even rapid temperature changes in less than a few milliseconds by using the thermoelectric effect, in which a voltage signal is generated by rapid charge transfer triggered by a difference in temperature. The team established a direct photothermal phenomenon measurement technology with reduced interference by light utilising an organic thermoelectric layer of transparent ‘PEDOT:PSS’, a conductive polymer suitable for storing charges.

The 50 nm-thin PEDOT:PSS thermoelectric sensor secures high transparency at 97% on average in the visible light zone and can be directly applied to the area of photothermal phenomena, minimising light interference for various photothermal bioengineering and medical applications. In addition, since a low-temperature solution process could be used for the polymer thermoelectric material used, it was prepared using an inkjet printing process, which is simpler to manufacture than a general semiconductor process, with a high degree of design freedom — giving it an advantage in the printing process.

The transparent thermoelectric temperature sensor technology can be used to understand the mechanism of the optical neural interface for controlling brain activity using light, which has recently been known broadly through optogenetics. It is a key technology in that it can be utilised to analyse the principles in treating cancer cells with local high heat. In addition, it is expected that it can be applied to next-generation semiconductor technologies, such as wearable devices, transparent display devices and analysis of local deterioration of power semiconductors, based on the principle of powerless operation.

“It is significant in that we proposed a technology that directly and precisely measures the photothermal effect, the biggest advantage of which is rapid generation of local heat,” said DGIST Professor Kang Hong-gi. “We look forward to the possibility of in-depth bioengineering analysis and biomedical application by combining it with various bio-electronic chips through micro-semiconductor processes in the future.”

Image caption: Calcium imaging of neuronal networks within the PEDOT:PSS region. Credit: Materials Horizons (2022).