Anodic near-infrared electrochemiluminescence from Cu-doped CdTe quantum dots for tetracycline detection

A sensitive anodic near-infrared electrochemiluminescence (ECL) immunosensor for the detection of tetracycline, based on Cu-doped CdTe quantum dots, was fabricated for the first time in this work. We have synthesized Cu-doped CdTe quantum dots by co-precipitation. The emission spectrum of the Cu-doped CdTe quantum dots could reach the near-infrared region at 730 nm in a short reflux time. More importantly, the ECL intensity of the CdTe quantum dots was enhanced by 2 fold after Cu element doping, which was attributed to the Cu d-orbital mixed with the conduction band and valence band of the host CdTe quantum dots. Inspired by the strong anodic ECL intensity of Cu-doped CdTe quantum dots, the anodic near infrared ECL sensor was constructed to detect tetracycline by competitive immunoassay. The detection range of the developed biosensor was 0.01-10 ng mL-1 and the detection limit was 0.0030 ng mL-1. In addition, the biosensor showed outstanding selectivity, long-term stability and high reproducibility, which has great potential in the field of analysis and detection.

Recent research on the luminous mechanism, synthetic strategies, and applications of CuInS2 quantum dots

We discuss the synthesis and luminescence mechanisms of CuInS₂ QDs, the strategies to improve their luminous performance and their potential application in light-emitting devices, solar energy conversion, and the biomedical field.

Cu-doped quantum dots: a new class of near-infrared emitting fluorophores for bioanalysis and bioimaging

Transition metal ion-doped quantum dots (QDs) exhibit unique optical and photophysical properties that offer significant advantages over undoped QDs, such as larger Stokes shift to avoid self-absorption/energy transfer, longer excited-state lifetimes, wider spectral window, and improved chemical and thermal stability. Among the doped QDs emitters, Cu is widely introduced into the doped QDs as novel, efficient, stable, and tunable optical materials that span a wide spectrum from blue to near-infrared (NIR) light. Their unique physical and chemical characteristics enable the use of Cu-doped QDs as NIR labels for bioanalysis and bioimaging. In this review, we discuss doping mechanisms and optical properties of Cu-doped QDs that are capable of NIR emission. Applications of Cu-doped QDs in in vitro biosensing and in in vivo bioimaging are highlighted. Moreover, a prospect of the future of Cu-doped QDs for bioanalysis and bioimaging are also summarized.

Synthesis of far-red- and near-infrared-emitting Cu-doped InP/ZnS (core/shell) quantum dots with controlled doping steps and their surface functionalization for bioconjugation

In this study, we designed and synthesized far-red- and near-infrared-emitting Cu-doped InP-based quantum dots (QDs), and we also demonstrated their highly specific and sensitive biological imaging ability. Cu-doped InP/ZnS (core/shell) QDs were prepared using the hot colloidal synthesis method in the organic phase. The ZnS shell passivates the surface and improves the photoluminescence (PL) intensity. However, the InP : Cu/ZnS (core : dopants/shell) QDs, which were obtained after the Cu dopant was incorporated into bare InP QDs, followed by ZnS shell coating, had relatively low PL intensities (maximum PL quantum yield (QY) was only ∼16%) presumably due to the formation of defect sites in the InP-core QDs caused by dopant migration from the InP core to the ZnS shell. We prepared high-quality InP/ZnS : Cu/ZnS (core/shell : dopant/outer-shell) QDs, where thin ZnS shell layers were grown on bare InP QDs prior to Cu ion doping to prevent dopant migration and obtained PL QYs as high as 40%. The native hydrophobic ligands of the as-synthesized Cu-doped QDs were replaced with hydrophilic ligands including dihydrolipoic acid and a zwitterionic ligand, which rendered the QDs water-soluble. These QDs exhibited remarkable colloidal stabilities over a wide pH range, with hydrodynamic diameters less than 10 nm. Modified QD surfaces can also be used in conjugation with other functional moieties to apply highly specific and sensitive imaging probes with very low background levels. As a proof-of-concept study, we successfully demonstrated the selective imaging of streptavidin beads with biotin-conjugated QDs. These decorated Cu-doped InP/ZnS (core/shell) QDs are promising biological-probe candidates for imaging and assaying with reduced concerns regarding toxicity.

Preparing CdS QDs in sodium alginate gel: realizing water solubility and stimuli responsiveness of QDs in an integrative way

Quantum dots (QDs) are of great interest due to their excellent fluorescence properties and thus, they have been widely studied. Compared with the typical organometallic synthetic routes and hydrothermal methods usually carried out under high temperatures, methods employing colloidal templates can be used for preparing QDs in mild conditions and have gained increasing attention. In this prospect, a hydrogel is an ideal colloidal template for the preparation of QDs in an aqueous medium, while the related study for in situ preparation of QDs in a gel and the consequent functionalization of QDs are in demand. In this paper, we proposed a two-step method to prepare CdS QDs in a sodium alginate (SA) gel, which showed effective constraint in the uniform size distribution of QDs. Without the introduction of additional ligands, the prepared CdS-SA QDs exhibited responsiveness to pH and detection of Fe3+, thus providing a simplified way for the functionalization of QDs. CdS-SA QDs showed good biocompatibility and stability in a certain concentration, which indicated the prospective applications of CdS-SA QDs in the fields of biological labeling and environmental sensing.

Impurity Location-Dependent Relaxation Dynamics of Cu:CdS Quantum Dots

Various types of 2% Cu-incorporated CdS (Cu:CdS) quantum dots (QDs) with very similar sizes have been prepared via a water soluble colloidal method. The locations of Cu impurities in CdS host nanocrystals have been controlled by adopting three different synthetic ways of doping, exchange, and adsorption to understand the impurity location-dependent relaxation dynamics of charge carriers. The oxidation state of incorporated Cu impurities has been found to be +1 and the band-gap energy of Cu:CdS QDs decreases as Cu2S forms at the surfaces of CdS QDs. Broad and red-shifted emission with a large Stokes shift has been observed for Cu:CdS QDs as newly produced Cu-related defects become luminescent centers. The energetically favored hole trapping of thiol molecules, as well as the local environment, inhibits the radiative recombination processes of Cu:CdS QDs, thus resulting in low photoluminescence. Upon excitation, an electron is promoted to the conduction band, leaving a hole on the valence band. The hole is transferred to the Cu+ d-state, changing Cu+ into Cu2+, which then participates in radiative recombination with an electron. Electrons in the conduction band are ensnared into shallow-trap sites within 52 ns. The electrons can be further captured on the time scale of 260 ns into deep-trap sites, where electrons recombine with holes in 820 ns. Our in-depth analysis of carrier relaxation has shown that the possibilities of both nonradiative recombination and energy transfer to Cu impurities become high when Cu ions are located at the surface of CdS QDs.

"Quantized" Doping of Individual Colloidal Nanocrystals Using Size-Focused Metal Quantum Clusters

The insertion of intentional impurities, commonly referred to as doping, into colloidal semiconductor quantum dots (QDs) is a powerful paradigm for tailoring their electronic, optical, and magnetic behaviors beyond what is obtained with size-control and heterostructuring motifs. Advancements in colloidal chemistry have led to nearly atomic precision of the doping level in both lightly and heavily doped QDs. The doping strategies currently available, however, operate at the ensemble level, resulting in a Poisson distribution of impurities across the QD population. To date, the synthesis of monodisperse ensembles of QDs individually doped with an identical number of impurity atoms is still an open challenge, and its achievement would enable the realization of advanced QD devices, such as optically/electrically controlled magnetic memories and intragap state transistors and solar cells, that rely on the precise tuning of the impurity states (i.e., number of unpaired spins, energy and width of impurity levels) within the QD host. The only approach reported to date relies on QD seeding with organometallic precursors that are intrinsically unstable and strongly affected by chemical or environmental degradation, which prevents the concept from reaching its full potential and makes the method unsuitable for aqueous synthesis routes. Here, we overcome these issues by demonstrating a doping strategy that bridges two traditionally orthogonal nanostructured material systems, namely, QDs and metal quantum clusters composed of a "magic number" of atoms held together by stable metal-to-metal bonds. Specifically, we use clusters composed of four copper atoms (Cu₄) capped with d-penicillamine to seed the growth of CdS QDs in water at room temperature. The elemental analysis, performed by electrospray ionization mass spectrometry, X-ray fluorescence, and inductively coupled plasma mass spectrometry, side by side with optical spectroscopy and transmission electron microscopy measurements, indicates that each Cu:CdS QD in the ensemble incorporates four Cu atoms originating from one Cu₄ cluster, which acts as a "quantized" source of dopant impurities.

Dually enriched Cu:CdS@ZnS QDs with both polyvinylpyrrolidone twisting and SiO2 loading for improved cell imaging

Through harvesting of the increased Stokes shift of CdS QDs via Cu-doping, the concentration-quenching or aggregation-quenching of CdS QDs was largely alleviated. A dually-enriched strategy with both polyvinylpyrrolidone (PVP) twisting and SiO2 loading was developed for generating a highly luminescent doped-dots (d-dots) assembly for improved cell imaging.

The facile one-step aqueous synthesis of near-infrared emitting Cu+ doped CdS quantum dots as fluorescence bioimaging probes with high quantum yield and low cytotoxicity

The water-soluble Cu⁺:CdS QDs with NIR emission and high PLQY were prepared in a N₂ atmosphere and employed as bioimaging probes for 3T3 cells.

Facile synthesis of functional gadolinium-doped CdTe quantum dots for tumor-targeted fluorescence and magnetic resonance dual-modality imaging

Magnetic quantum dots (MQDs) are an important class of agents for fluorescence (FL)/magnetic resonance (MR) dual-modal imaging due to their excellent optical and magnetic properties. However, functional MQDs prepared by a simple room-temperature route as FL/MR dual-modal imaging probes are lacking. Herein, we report the fabrication of Gd-doped CdTe quantum dots (Gd:CdTe QDs) as an agent for FL/MR dual-modality imaging. The as-designed QDs with an ultrasmall particle size are synthesized by a facile one-pot aqueous synthesis approach at room temperature. They emit strong fluorescence at 640 nm with a quantum yield of 37% in water, and they have a high longitudinal relaxation rate (r₁) value of 3.27 mM^-1 s^-1. With the further conjugation of folic acid, the Gd:CdTe QDs can successfully label live HepG2 cells for targeted cellular imaging and present no evidence of cellular toxicity up to the concentration of 0.5 mg mL^-1. They have been employed as a suitable contrast agent successfully for tumor-targeted FL/MR dual-modal imaging in a mouse model.

The effects of composition and surface chemistry on the toxicity of quantum dots

Recently, researchers have paid much attention to the toxicity of QDs because of their rapidly increasing application in biomedicine. Recent investigations have indicated that QDs have influences on biological systems at the cellular, subcellular, and protein levels during the processes of imaging and therapeutic applications. The toxicity of QDs is related to their composition, surface functionality, size, shape, and charge, etc. among which composition and surface modification are two important elements. This feature article mainly concentrates on the latest developments in reducing QD toxicity by manipulating their composition and surface modification. Besides the cadmium-based QDs, the assessment of toxicity in vitro and in vivo for other QDs such as carbon dots, graphene QDs, silicon QDs, Ag₂Se QDs, CuInS₂@ZnS, InP QDs, hybrid QDs of carbon and CdSe@ZnS, etc., is generalized. Each kind of QD has its own advantages. Cadmium-based QDs have broad UV excitation, narrow emission and bright photoluminescence (PL), while cadmium-free ones present low toxicity. In fact, a lot of investigations show that the toxicity of QDs is dose dependent, whatever the composition. Consequently, surface modification becomes very important to reduce toxicity and simultaneously impart biocompatibility, stability, and specificity to QDs. Therefore, the functionalization of QDs with inorganic shells (e.g., CdSe@ZnS, CdSe@SiO₂), polymers, bio- or natural macromolecules is summarized. Future research work should concentrate on preparing novel QDs with appropriate surface modification and investigating the long-term influence of QDs on absorption, distribution, metabolism, and elimination in vivo, especially for cadmium-free QDs such as carbon-based QDs, Ag₂Se QDs, CuInS₂@ZnS QDs and InP QDs, etc.