Extending the near infrared emission range of indium phosphide quantum dots for multiplexed 'In Vivo' imaging

This report of the reddest emitting indium phosphide quantum dots (InP QDs) to date demonstrates tunable, near infrared (NIR) photoluminescence and fluorescence multiplexing in the first optical tissue window with a material that avoids toxic constituents. This synthesis overcomes the InP synthesis “growth bottleneck” and extends the emission peak of InP QDs deeper into the first optical tissue window using an inverted QD heterostructure. The ZnSe/InP/ZnS core/shell/shell structure is designed to produce emission from excitons with heavy holes confined in InP shells wrapped around larger-bandgap ZnSe cores and protected by a second shell of ZnS. The InP QDs exhibit InP shell thickness-dependent tunable emission with peaks ranging from 515 – 845 nm. The high absorptivity of InP leads to effective absorbance and photoexcitation of the QDs with UV, visible, and NIR wavelengths in particles with diameters of eight nanometers or less. These nanoparticles extend the range of tunable direct-bandgap emission from InP-based nanostructures, effectively overcoming a synthetic barrier that has prevented InP-based QDs from reaching their full potential as NIR imaging agents. Multiplexed lymph node imaging in a mouse model shows the potential of the NIR-emitting InP particles for in vivo imaging.First author draf

Bandgap Engineering of Indium Phosphide-Based Core/Shell Heterostructures Through Shell Composition and Thickness

The large bulk bandgap (1.35 eV) and Bohr radius (~10 nm) of InP semiconductor nanocrystals provides bandgap tunability over a wide spectral range, providing superior color tuning compared to that of CdSe quantum dots. In this paper, the dependence of the bandgap, photoluminescence emission, and exciton radiative lifetime of core/shell quantum dot heterostructures has been investigated using colloidal InP core nanocrystals with multiple diameters (1.5, 2.5, and 3.7 nm). The shell thickness and composition dependence of the bandgap for type-I and type-II heterostructures was observed by coating the InP core with ZnS, ZnSe, CdS, or CdSe through one to ten iterations of a successive ion layer adsorption and reaction (SILAR)-based shell deposition. The empirical results are compared to bandgap energy predictions made with effective mass modeling. Photoluminescence emission colors have been successfully tuned throughout the visible and into the near infrared (NIR) wavelength ranges for type-I and type-II heterostructures, respectively. Based on sizing data from transmission electron microscopy (TEM), it is observed that at the same particle diameter, average radiative lifetimes can differ as much as 20-fold across different shell compositions due to the relative positions of valence and conduction bands. In this direct comparison of InP/ZnS, InP/ZnSe, InP/CdS, and InP/CdSe core/shell heterostructures, we clearly delineate the impact of core size, shell composition, and shell thickness on the resulting optical properties. Specifically, Zn-based shells yield type-I structures that are color tuned through core size, while the Cd-based shells yield type-II particles that emit in the NIR regardless of the starting core size if several layers of CdS(e) have been successfully deposited. Particles with thicker CdS(e) shells exhibit longer photoluminescence lifetimes, while little shell-thickness dependence is observed for the Zn-based shells. Taken together, these InP-based heterostructures demonstrate the extent to which we are able to precisely tailor the material properties of core/shell particles using core/shell dimensions and composition as variables

Synthesis and characterization of indium phosphide-based quantum dot heterostructures

Colloidal semiconductor nanocrystal quantum dots (QDs) have been extensively studied for applications in optoelectronic devices, biosensing, and imaging. Recent interest has turned to heavy metal-free compositions such as indium phosphide as an alternative to cadmium- and lead-based materials. Photoluminescence emission from InP QDs is size-tunable over a wide spectral range, providing superior color tuning compared to traditional CdSe QD but their optical properties and chemical synthesis is less well established. This study examines how InP-based heterostructures can be engineered to enhance their utility as heavy metal-free fluorophores emitting throughout the visible and near infrared (NIR) wavelength ranges by addressing three fundamental materials design and synthesis issues. First, the bandgap engineering of InP-based QDs is achieved by varying the core size, shell composition, and shell thickness of a core/shell heterostructures, generating emitters spanning 500 – 1100 nm. Second, the brightness mismatch between small blue/green emitters and large red-emitting QDs is addressed by tuning the absorption cross-section and extinction coefficient by synthesizing a series of QDs with a combination of core sizes, shell thicknesses, and shell compositions, resulting in a rainbow of brightness-matched InP emitters. Finally, the synthesis of inverted InP heterostructures, producing the reddest-emitting InP QDs ever reported by generating photoluminescence from a quantum confined InP shell, was significantly improved. The non-toxic nature of InP in conjunction with its unique optical properties render it an excellent candidate for use in in vitro and in vivo clinical or commercial settings

Correlating ZnSe Quantum Dot Absorption with Particle Size and Concentration

The focus on heavy metal-free semiconductor nanocrystals has increased interest in ZnSe semiconductor quantum dots (QDs) over the past decade. Reliable and consistent incorporation of ZnSe cores into core/shell heterostructures or devices requires empirical fit equations correlating the lowest energy electron transition (1S peak) to their size and molar extinction coefficients (ε). While these equations are known and heavily used for CdSe, CdTe, CdS, PbS, etc., they are not well established for ZnSe and are non-existent for ZnSe QDs with diameters < 3.5 nm. In this study, a series of ZnSe QDs with diameters ranging from 2 to 6 nm were characterized with small angle X-ray scattering (SAXS), transmission electron microscopy (TEM), UV-Vis spectroscopy, and microwave plasma atomic emission spectroscopy (MP-AES). SAXS-based size analysis enabled practical inclusion of small particles in the evaluation, and elemental analysis with MP-AES elucidates a non-stoichiometric Zn:Se ratio consistent with zinc-terminated spherical ZnSe QDs. Using these combined results, empirical fit equations correlating QD size with its lowest energy electron transition (i.e., 1S peak position), Zn:Se ratio, and molar extinction coefficients for 1S peak, 1S integral, and high energy wavelengths are reported. Finally, the equations are used to track the evolution of a ZnSe core reaction. These results will enable the consistent and reliable use of ZnSe core particles in complex heterostructures and devices