Optical Temperature Sensing With Infrared Excited Upconversion Nanoparticles.

Area of Science:

• Nanotechnology
• Biophysics
• Optical Imaging

Background:

• Upconversion nanoparticles (UCNPs) are utilized for sensitive, high-resolution local temperature measurements.
• Conventional continuous wave (CW) infrared excitation for UCNPs can cause localized thermal heating, limiting applications.
• The impact of pulsed excitation and higher intensities on UCNPs for molecular-scale bioimaging remains unclear.

Purpose of the Study:

• To investigate the feasibility of 800 and 980 nm excited UCNPs for thermal sensing under pulsed excitation.
• To assess local heating effects associated with pulsed UCNP excitation at intensities relevant for single-particle imaging.
• To evaluate the biocompatibility of pulsed infrared excitation for UCNP bioimaging applications.

Main Methods:

• UCNPs were excited using pulsed lasers at 800 and 980 nm.
• Temperature sensing was performed ratiometrically, analyzing the intensity ratio of specific emission peaks (I525/I545).
• Optical measurements quantified temperature dependence (ln(I525/I545) vs. 1/T).
• Local heating was assessed in air and water.
• DNA tightropes were exposed to pulsed infrared excitation to evaluate sample viability.

Main Results:

• UCNPs demonstrated ratiometric temperature sensing with sensitivities of 1 × 10⁻⁴ K⁻¹ under both 800 and 980 nm pulsed excitation.
• No significant local heating was observed in air or water at pulsed laser intensities suitable for single-particle imaging.
• DNA tightropes showed no appreciable change in viability when exposed to pulsed 800 or 980 nm infrared excitation.
• A clear ln(I525/I545) vs. 1/T dependence was observed for both excitation wavelengths.

Conclusions:

• Pulsed 800 and 980 nm excitation is feasible for UCNP-based thermal sensing without inducing significant local heating.
• UCNPs are suitable reporters for molecular-scale studies, including protein-DNA interactions, under pulsed infrared excitation.
• This approach overcomes limitations of CW excitation, enabling advanced bioimaging applications at the single-molecule level.