This review summarizes recent progress in the design and applications of cadmium-free quantum dots (Cd-free QDs), with an emphasis on their role in biophotonics and nanomedicine. We first present the features of Cd-free QDs and describe the physics and emergent optical properties of various types of Cd-free QDs whose applications are discussed in subsequent sections. Selected specific QD systems are introduced, followed by the preparation of these Cd-free QDs in a form useful for biological applications, including recent advances in achieving high photoluminescence quantum yield (PL QY) and tunability of emission color. Next, we summarize biophotonic applications of Cd-free QDs in optical imaging, photoacoustic imaging, sensing, optical tracking, and photothermal therapy. Research advances in the use of Cd-free QDs for nanomedicine applications are discussed, including drug/gene delivery, protein/peptide delivery, image-guided surgery, diagnostics, and medical devices. The review then considers the pharma...

New Generation Cadmium-Free Quantum Dots for Biophotonics and Nanomedicine

Any design of nanoparticle vectors for cancer therapy or imaging must take into account the interaction of the nanoparticles with the tumor microenvironment. Size, charge, and shape have been shown to dominate this interaction.[1, 2] In vivo probing of solid tumors with particles of different sizes simultaneously has thus far been challenging due to the limitations of available nano-sized probes.[3–5] Fluorescent dextrans and other macromolecule probes have been used in studies with intravital microscopy, but heterogeneities across samples has prevented their use for the simultaneous imaging of a size series of probes within the same tumour.[5] MRI contrast agents are another attractive set of probes due to the minimally-invasive nature of the technology,[6, 7] but the lower spatial resolution of MRI limits the imaging of heterogeneity within tumors, and the technique does not allow for simultaneous imaging and tracking of a size series of probes within the same tumor.

A Nanoparticle Size Series for In Vivo Fluorescence Imaging

The homogeneous (single-cluster) and inhomogeneous contributions to the low temperature electronic absorption spectrum of 35-50 ? diameter CdSe clusters are separated using transient photophysical hole burning. The clusters have the cubic bulk crystal structure, but their electronic states are strongly quantum confined. The inhomogeneous broadening of these features arises because the spectrum depends upon cluster size and shape, and the samples contain similar, but not identical, clusters. The homogeneous spectrum, which consists of a peak 140 cm -1 (17 meV) wide, with a phonon sideband and continuum absorption to higher energy, is compared to a simple molecular orbital model. Electron-vibration coupling, which is enhanced in small clusters, contributes to the substantial broadening of the homogeneous spectrum. The inhomogeneous width of the lowest allowed optical transition was found to be 940 cm -1 , or seven times the homogeneous width, in the most monodisperse sample.

Electronic states of semiconductor clusters : Homogeneous and inhomogeneous broadening of the optical spectrum

This study focuses on the synthesis of Cu2S particles as the core wrapped-up with thin layer of TiO2-shell for improved surface and interface control of photo-excitons for H2 production. Chemical states of metal Cu+, Ti4+ ions were confirmed by X-ray Photo-electron and Electron Paramagnetic Resonance (g-factor of 2.03) Spectroscopy. Transmission Electron Microscopic images revealed the hexagonal shape of Cu2S wrapped-up with TiO2 having the shell thickness varying from 12.0 to 16.7 nm. Time resolved transient photoluminescence decay spectra confirmed that three folds prolonged life-time of excitons generated on Cu2S@TiO2 core-shell than pristine Cu2S. The batch photo-ractor used to evaluate the performance of photocatalysts in an aqueous solution containing Na2S/Na2SO4 promoted oxidation reaction for H+ generation. The control over shell thickness directly influenced the optical and surface-interface properties, resulting in the enhanced H2 production of 41.6 mmol h−1 g−1 of catalyst for an optimized catalyst with UV–vis light to H2 production efficiency of 10.3%. The same catalyst also exhibited excellent stability for un-interrupted H2 production upto 30 h, along with good results on recyclability and reusable efficiency. Scale-up operation performed to recover H2 from sulphide wastewater in tank photo-reactor triggered H2 gas 16.1 mmol h−1 g−1 of catalyst.

Photocatalytic recovery of H2 from H2S containing wastewater: Surface and interface control of photo-excitons in Cu2S@TiO2 core-shell nanostructures

Colloidal nanocrystals are nanometer-sized, solution-grown inorganic particles stabilized by a layer of surfactants attached to their surface. The inorganic cores exhibit useful properties controlled by composition as well as size and shape, while the surfactant coating ensures that these structures are easy to fabricate and process. It is this combination of features that makes colloidal nanocrystals attractive and promising building blocks for advanced materials and devices. But their full potential can only be exploited if we achieve exquisite control over their composition, size, shape, crystal structure and surface properties. Here we review what is known about nanocrystal growth and outline strategies for controlling it.

Colloidal nanocrystal synthesis and the organic-inorganic interface

Core/intermediate/shell (C/I/S) structures with Type-I emission are well-known and are gaining immense importance due to their superior luminescence properties. Here, we report a unique C/I/S structure composed of CdSe/CdS/ZnSe that exhibits both Type-I and Type-II phenomena. The structures have been well characterized using a combination of optical and structural techniques. The photoluminescence (PL) and photoluminescence excitation (PLE) data indicate the formation of a combined Type-I and Type-II structure in one material, results supported by simple theoretical calculations. Single particle fluorescence reveals colocalization of both the emissions. The X-ray diffraction (XRD) and transmission electron microscopy (TEM) results confirm the structure of these particles. The time-resolved fluorescence studies show the possibility of tuning the lifetime of these materials by changing the Type-I/Type-II thickness ratios. It is possible to form these two separate excitons in the same system separated by a CdS intermediate layer that acts both as a barrier and an active member of the Type-II system allowing the generation and recombination of two excitons, in violation of Kasha's rule.

Simultaneous Type-I/Type-II Emission from CdSe/CdS/ZnSe Nano-Heterostructures

In the recent past, visible-light-mediated photoredox catalysis has made a huge impact on the development of new synthetic methods under very mild and ecologically benign conditions. Although semiconductor nanocrystals or quantum dots (QDs) possess suitable optoelectronic and redox properties for photoredox catalytic applications, surprisingly, their use for the activation of challenging chemical bonds in the synthesis of organic molecules is little explored. We report here the application of ZnSe/CdS core/shell QDs for the synthetically important photoredox catalytic activation of carbon–halogen bonds in dehalogenation and C–H arylation reactions using (hetero)aryl halides as bench-stable inexpensive bulk starting materials, under very mild reaction conditions. The outstanding catalytic activity of ZnSe/CdS core/shell QDs is a direct consequence of the high specific surface area and homogeneity of QDs in solution and their high photostability toward oxidation.

Quantum Dots in Visible-Light Photoredox Catalysis: Reductive Dehalogenations and C–H Arylation Reactions Using Aryl Bromides

The decrease in photoluminescence (PL) of four different sized CdSe colloidal quantum dots (donors) has been investigated in the presence of three different Cobalt(III) complexes (acceptors). The steady-state and time-resolved PL (TRPL) spectroscopy have been used to investigate the mechanism of quenching. The complex concentration driven change in lifetimes of QDs and stronger PL quenching than predicted solely by TRPL data indicate that the quenching is neither purely static nor purely dynamic in nature. Further, the absence of any ground state complex absorption feature suggests that the static quenching contribution is due to the close proximity of the QDs fluorophores and deactivating sites of complexes. The dynamic quenching processes like diffusion mediated collisional quenching, Dexter energy transfer, and hole transfer have been methodically ruled out, leaving Forster resonant energy transfer (FRET) and the electron transfer (eT) between the QDs and complexes as the possible mechanisms. The Marcu...

Probing the Mechanism of Fluorescence Quenching of QDs by Co(III)-Complexes: Size of QD and Nature of the Complex Both Dictate Energy and Electron Transfer Processes

Fluorescence quenching of CdSe–ZnS quantum dots (core–shell QDs) is shown to be affected in the presence of cobalt(III) complexes with pyridyl anchors. Steady-state and time-resolved fluorescence spectra indicate that the quenching has primarily a static component. The decrease in photoluminescence intensity can be best explained by charge transfer from the QDs to the Co(III) complexes whereas the energy transfer pathways have been methodically ruled out. The fact that quenching is a result of electron transfer is also supported by electrochemical data showing the positions of the energy levels.

Anuushka Pal

Papers

Simultaneous Type-I/Type-II Emission from CdSe/CdS/ZnSe Nano-Heterostructures

Quantum Dots in Visible-Light Photoredox Catalysis: Reductive Dehalogenations and C–H Arylation Reactions Using Aryl Bromides

Probing the Mechanism of Fluorescence Quenching of QDs by Co(III)-Complexes: Size of QD and Nature of the Complex Both Dictate Energy and Electron Transfer Processes

Electron transfer from CdSe-ZnS core-shell quantum dots to cobalt(III) complexes.

Conjugated‐Polymer/Quantum‐Confined Nanomaterials‐Based Hybrids for Optoelectronic Applications