Using 915 nm Laser Excited Tm³⁺/Er³⁺/Ho³⁺-Doped NaYbF4 Upconversion Nanoparticles for in Vitro and Deeper in Vivo Bioimaging without Overheating Irradiation

Abstract

Successful further development of superhigh-constrast upconversion (UC) bioimaging requires addressing the existing paradox: 980 nm laser light is used to excite upconversion nanoparticles (UCNPs), while 980 nm light has strong optical absorption of water and biological specimens. The overheating caused by 980 nm excitation laser light in UC bioimaging is computationally and experimentally investigated for the first time. A new promising excitation approach for better near-infrared to near-infrared (NIR-to-NIR) UC photoluminescence in vitro or in vivo imaging is proposed employing a cost-effective 915 nm laser. This novel laser excitation method provides drastically less heating of the biological specimen and larger imaging depth in the animals or tissues due to quite low water absorption. Experimentally obtained thermal-graphic maps of the mouse in response to the laser heating are investigated to demonstrate the less heating advantage of the 915 nm laser. Our tissue phantom experiments and simulations verified that the 915 nm laser is superior to the 980 nm laser for deep tissue imaging. A novel and facile strategy for surface functionalization is utilized to render UCNPs hydrophilic, stable, and cell targeting. These as-prepared UCNPs were characterized by TEM, emission spectroscopy, XRD, FTIR, and zeta potential. Specifically targeting UCNPs excited with a 915 nm laser have shown very high contrast UC bioimaging. Highly stable DSPE-mPEG-5000-encapsulated UCNPs were injected into mice to perform in vivo imaging. Imaging and spectroscopy analysis of UC photoluminescence demonstrated that a 915 nm laser can serve as a new promising excitation light for UC animal imaging.

Excitation energy migration dynamics in upconversion nanomaterials

“…One of them uses CW laser excitation at 915 nm, instead of 980 nm, to reduce the radiation heating to a certain extent at the expense of upconversion efficiency. 9 Another approach is to introduce Nd 3+ ions as an additional NIR absorber and sensitizer in conven- (Fig. 3).…”

808 nm driven Nd3+-sensitized upconversion nanostructures for photodynamic therapy and simultaneous fluorescence imaging

“…As a consequence, overheating effects limit the power of the excitation light. 14 Various efforts have been made to improve the performance of upconversion nanomaterials in this regard. Recently, Nd 3+ -sensitized UCNPs with 800 nm laser excitation properties were reported.…”

Recent advances in the optimization and functionalization of upconversion nanomaterials for in vivo bioapplications

“…For in vivo bioapplications in particular, a high excitation power (4500 mW cm À2 ) leads to a greater heating effect and increases the cost of the instruments. 23 To solve this problem, recent efforts have attempted to improve the upconversion efficiency of UCNPs by reducing the required excitation power density and improving the performance of UCNPs in bioapplications.…”

“…However, new efforts have been recently undertaken to design new structures to solve this problem. For example, Zhan and coworkers 23 reported using a 915 nm laser as an excitation source to achieve in vivo bioimaging with lower overheating effects. More recently, Nd 3 þ ions have been introduced as antenna to absorb the excitation light and further transfer energy to Yb 3 þ ions to sensitize activators to generate UCL at excitations of 808 nm.…”