KEYWORDS: Quantum dots, Multiplexing, Atmospheric particles, Short wave infrared radiation, Quantum efficiency, Infrared radiation, Infrared imaging, In vivo imaging, Signal to noise ratio, Shortwaves
Fluorescence bioimaging utilizing the near infrared-II (NIR-II; 1000 – 1700 nm) window, also named shortwave infrared (SWIR) imaging, exhibits superior penetration depth and signal-to-noise ratio compared to NIR-I imaging. We synthesized and characterized multiple lead sulfide/cadmium sulfide (PbS/CdS) core/shell quantum dots (QDs) with distinct, monodisperse size distributions, narrow emission, high quantum yield, and photostability for use in multiplexed NIR-II imaging. Following micelle encapsulation, the QDs are water-soluble, stably dispersed, and functionalized for easy click chemistry conjugation with targeting moieties. In vivo mice imaging with IV injected QDs showed very strong and distinctive signals for multiplexed imaging.
We utilized these NIR QDs to demonstrate the utility of an image processing algorithm capable of determining the relative concentration ratios of two distinct emitters at various tissue depths. This algorithm is based on the dual probe imaging method, which we modified for use in wide-field imaging systems. The algorithm was validated using skin-mimicking tissue phantoms that were used to vary the effective imaging depth from 0-1.2 mm. The algorithm correctly determined concentration ratios of a two-color QD system for variations in concentration ≥ 10%, independently of depth. These contrast agents and the imaging approach are being adapted for use for in vivo targeted tumor imaging.ng adapted for use for in vivo targeted tumor imaging.
For the first time, we assess the biodistribution and toxicity of unshelled CIS and partially zinc-alloyed CISZ QDs in a murine model at 1-day, 7-day, and 1-month timepoints. We show that bare CIS QDs breakdown quickly, with >75% of the initial dose being cleared by 1-month. Surprisingly, we also demonstrate a significant toxic response to these QDs as measured by organ weight, blood chemistry, and histology. Specifically, low doses of CIS particles (15mg/kg) induce severe hepatotoxicity and splenotoxicity. Similarly, CISZ demonstrated significant, but lower, toxicity compared to bare CIS. Overall, our data suggests that reconsideration of CIS as a translatable QD system is required: degradation-based toxicity is an important aspect of biocompatibility that needs to be assessed in “non-toxic” QDs, if QDs are to ever be clinically successful. Finally, we suggest a non-toxic biodegradable alternative.
Cadmium-free near infrared (NIR) emitting quantum dots (QDs) have significant potential for multiplexed tissue-depth imaging applications in the first optical tissue window (i.e., 650 – 900 nm). Indium phosphide (InP) chemistry provides one of the more promising cadmium-free options for biomedical imaging, but the full tunability of this material has not yet been achieved. Specifically, InP QD emission has been tuned from 480 – 730 nm in previous literature reports, but examples of samples emitting from 730 nm to the InP bulk bandgap limit of 925 nm are lacking. We hypothesize that by generating inverted structures comprising ZnSe/InP/ZnS in a core/shell/shell heterostructure, optical emission from the InP shell can be tuned by changing the InP shell thickness, including pushing deeper into the NIR than current InP QDs. Colloidal synthesis methods including hot injection precipitation of the ZnSe core and a modified successive ion layer adsorption and reaction (SILAR) method for stepwise shell deposition were used to promote growth of core/shell/shell materials with varying thicknesses of the InP shell. By controlling the number of injections of indium and phosphorous precursor material, the emission peak was tuned from 515 nm to 845 nm (2.41 – 1.47 eV) with consistent full width half maximum (FWHM) values of the emission peak ~0.32 eV. To confer water solubility, the nanoparticles were encapsulated in PEGylated phospholipid micelles, and multiplexing of NIR-emitting InP QDs was demonstrated using an IVIS imaging system. These materials show potential for multiplexed imaging of targeted QD contrast agents in the first optical tissue window.
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