Next-generation short-wave infrared fluorescent probes

The short-wavelength infrared (SWIR) spectral window can lead to significant improvements in contrast, depth and resolution for in vivo imaging. The LBRC is one of the international leaders in the development of SWIR quantum dots (QDs) and their application in intravital imaging. We aim to establish facile synthesis of SWIR QDs that outperform existing organic SWIR fluorophores to enable molecularly targeted SWIR imaging. We also seek to enable high-speed and high-resolution optical imaging in the SWIR by synthesizing core-shell-shell QDs (CSS QDs) with narrow emission lines and high quantum yields spanning the entire sensitivity range of modern SWIR cameras. In addition, we plan to conjugate SWIR CSS QDs to multiple targeting mediators to enable genetically-targeted imaging of microscopic metastases in cancer models and to elucidate immune responses of leukocyte subclasses using in vivo flow cytometry. SWIR CSS QDs will enhance the capabilities of the wide-field SWIR multiphoton imaging setup in TRD1.2. Furthermore, the synthesis of new, non-toxic SWIR nanoparticles will ease the translation of SWIR imaging applications into clinical settings.

Developing next-generation of SWIR fluorescent probes

Figure 1: (a) Schematic band alignment in InAsCdSeCdS CSS QDs. (b) Growth of two shells onto an InAs QD. While the CdSe shell leads to a strong redshift in the PL peak, the QY initially increases and then decreases during shell growth, indicative of the formation of a quasi type-II structure. The position of the PL peak stays approximately constant during growth of the outer CdS shell, while the QY shows steady improvements, indicative of the formation of a type-I structure.

To address the challenges of biomedical imaging in vivo, we have developed bright, fluorescent probes, mainly core-shell-shell quantum dots (CSS QDs) with narrow emission lines and high quantum yields spanning the entire sensitivity range of modern InGaAs-based SWIR cameras. The ideal imaging probe needs to exhibit high brightness for fast imaging speeds and narrow emission profiles that can be tuned over the entire SWIR camera sensitivity range for multiplexed and wavelength-selective imaging. Furthermore, probes should exhibit high stability of the quantum yield and wavelength of the emission peak, both under laser irradiance as well as during long-term storage. Indium arsenide (InAs) QDs are among the most promising SWIR probes to address these challenges as they exhibit size-tunable emission, broad absorption spectra, and show higher quantum yields than rare earth nanocrystals (NCs), silver chalcogenide NCs, or organic SWIR dyes. In contrast to other SWIR QDs, such as PbS or Ag2S, InAs QDs can exhibit higher quantum yields and probe stability after transfer from the organic phase to aqueous media. A synthetic challenge of translating InAS QDs has been the ability to synthesize large scale, SWIR-emissive, InAs NCs with narrow emission linewidths remains challenging and conventional syntheses require additional post-synthetic purification, which drastically reduces reaction yields (Fig. 1).

We have made significant progress in improving our understanding of the underlying mechanism of InAs QD growth and in turn have developed a new synthetic route that overcomes the challenges of previous methods. With high relative QYs at the red edge of the sensitivity range of SWIR cameras, our probes are more than six times brighter than previously published InAs-based QD systems and further maintain high QYs in aqueous media after phase transfer 1]. Ultimately, we demonstrate how our SWIR fluorophores exhibit a 2-3 order of magnitude higher QY and demonstrate their use in non-invasive through-skull fluorescence imaging in mice (Fig. 2). The improved material properties position our InAs CSS QDs as an alternative material of choice for SWIR fluorescence imaging.

Figure 2: Fluorescence imaging of mouse brain vasculature through intact skin and skull using a mixture of SWIR CSS QDs. The four images were collected using four bandpass filters (50 nm spectral width) centered at 950, 1,100, 1,300 and 1,600 nm with integration times of 150, 50, 100 and 5,000 ms, respectively. The images show that longer imaging wavelengths enhance the spatial resolution of fine vascular brain structures by significantly improving the signal to background (S/B) ratio in the region of interest, which stresses the need for SWIR emitters with high QYs and narrow emission at the red edge of SWIR detectors.

References

  1. "Continuous injection synthesis of indium arsenide quantum dots emissive in the short-wavelength infrared," Nature Communications, 7:12749, Nov 2016 [ Pubmed ]