A study published in Dalton Transactions by the Changchun Institute of Optics, Fine Mechanics and Physics and the Royal Society of Chemistry, with collaborators from Chongqing University of Posts and Telecommunications and, reports the creation of a new material that emits high-purity red light when excited by a specific type of near-infrared light. This advancement holds potential for improved temperature sensing deep inside biological tissues and for creating more secure anti-counterfeiting features.

Conventional upconversion materials, which convert low-energy light into higher-energy visible light, often produce multiple colors simultaneously. This mixture of colors can limit their effectiveness in applications like precise bio-imaging, where a single, pure color is highly desirable. Furthermore, many are excited by light within the first biological window, which does not penetrate tissue as deeply as light in the second window.

To address these limitations, the research team focused on a material called CaSc₂O₄ doped with erbium (Er³⁺) ions, which can be excited by 1532 nm light falling within the advantageous second near-infrared biological window. However, Er³⁺ alone typically emits a mix of green and red light. The researchers introduced holmium (Ho³⁺) ions as a co-dopant to manipulate the internal energy pathways.

They synthesized the phosphor powder using a high-temperature solid-state reaction method. Spectroscopic analysis confirmed that the Ho³⁺ ions effectively modified the energy transfer processes within the material. This manipulation suppressed the green emission and significantly enhanced the red light, resulting in an approximately 19-fold increase in the red-to-green emission ratio and yielding near-pure red luminescence.

The team then explored the material's application in optical thermometry. They leveraged the fine splitting of the red emission lines, whose intensity ratio changes predictably with temperature. This property allowed them to create a highly sensitive thermometer, achieving a maximum relative sensitivity of 0.48% per Kelvin. In a demonstration, the red emission signal could be detected through 6 mm of biological tissue, while a separate near-infrared signal from the same material reached 8 mm, with the temperature readings remaining reliable at these depths.

Additionally, the material displayed different shades of red emission when excited by 980 nm versus 1532 nm light. The researchers used this property to create printed patterns that are invisible under daylight but reveal distinct red hues under different infrared lights, showcasing a potential application in optical anti-counterfeiting with enhanced concealment.

This work demonstrates a method to achieve high-color-purity red emission from a deep-penetrating NIR-II excitable material. It provides a promising tool for non-invasive deep-tissue temperature monitoring and for developing advanced security features.