Electrostatically Driven Resonance Energy Transfer in an All-Quantum Dot Based Donor-Acceptor System

Single- and two-photon-induced Förster resonance energy transfer in InP-mCherry bioconjugates

Indium phosphide (InP) quantum dots (QDs) have recently garnered considerable interest in the design of bioprobes due to their non-toxic nature and excellent optical properties. Several attempts for the conjunction of InP QDs with various entities such as organic dyes and dye-labeled proteins have been reported, while that with fluorescent proteins remains largely uncharted. This study reports the development of a Förster resonance energy transfer pair comprising glutathione-capped InP/GaP/ZnS QDs [InP(G)] and the fluorescent protein mCherry. Glutathione on InP(G) undergoes effective bioconjugation with mCherry consisting of a hexahistidine tag, and the nonradiative energy transfer is investigated using steady-state and time-resolved measurements. Selective one-photon excitation of InP(G) in the presence of mCherry shows a decay of the emission of the QDs and a concomitant growth of acceptor emission. Time-resolved investigations prove the nonradiative transfer of energy between InP(G) and mCherry. Furthermore, the scope of two-photon-induced energy transfer between InP(G) and mCherry is investigated by exciting the donor in the optical transparency range. The two-photon absorption is confirmed by the quadratic relationship between the emission intensity and the excitation power. In general, near-infrared excitation provides a path for effective light penetration into the tissues and reduces the photodamage of the sample. The two-photon-induced energy transfer in such assemblies could set the stage for a wide range of biological and optoelectronic applications in the foreseeable future.

Nanoparticle Self-Assembly: From Design Principles to Complex Matter to Functional Materials

The creation of matter with varying degrees of complexities and desired functions is one of the ultimate targets of self-assembly. The ability to regulate the complex interactions between the individual components is essential in achieving this target. In this direction, the initial success of controlling the pathways and final thermodynamic states of a self-assembly process is promising. Despite the progress made in the field, there has been a growing interest in pushing the limits of self-assembly processes. The main inception of this interest is that the intended self-assembled state, with varying complexities, may not be "at equilibrium (or at global minimum)", rendering free energy minimization unsuitable to form the desired product. Thus, we believe that a thorough understanding of the design principles as well as the ability to predict the outcome of a self-assembly process is essential to form a collection of the next generation of complex matter. The present review highlights the potent role of finely tuned interparticle interactions in nanomaterials to achieve the preferred self-assembled structures with the desired properties. We believe that bringing the design and prediction to nanoparticle self-assembly processes will have a similar effect as retrosynthesis had on the logic of chemical synthesis. Along with the guiding principles, the review gives a summary of the different types of products created from nanoparticle assemblies and the functional properties emerging from them. Finally, we highlight the reasonable expectations from the field and the challenges lying ahead in the creation of complex and evolvable matter.

Blue-emitting InP quantum dots participate in an efficient resonance energy transfer process in water

Development of stable blue-emitting materials has always been a challenging task because of the necessity of high crystal quality and good optical properties. We have developed a highly efficient blue-emitter, based on environmentally friendly indium phosphide/zinc sulphide quantum dots (InP/ZnS QDs) in water, by controlling the growth kinetics of the core as well as the shell. A rational combination of less-reactive metal-halides, phosphorus, and sulphur precursors is the key for achieving the uniform growth of the InP core and ZnS shell. The InP/ZnS QDs showed long-term stable photoluminescence (PL) in the pure-blue region (∼462 nm), with an absolute PL quantum yield of ∼50% and a colour purity of ∼80% in water. Cytotoxicity studies revealed that the cells can withstand up to ∼2 micromolar concentration of pure-blue emitting InP/ZnS QDs (∼120 μg mL^-1). Multicolour imaging studies show that the PL of InP/ZnS QDs was well-retained inside the cells as well, without interfering with the fluorescence signal of commercially available biomarkers. Moreover, the ability of InP based pure-blue emitters to participate in an efficient Förster resonance energy transfer (FRET) process is demonstrated. Installing a favorable electrostatic interaction turned out to be crucial in achieving an efficient FRET process (E ∼75%) from blue-emitting InP/ZnS QDs to rhodamine B dye (Rh B) in water. The quenching dynamics fits well with the Perrin formalism and the distance-dependent quenching (DDQ) model, which confirms an electrostatically driven multi-layer assembly of Rh B acceptor molecules around the InP/ZnS QD donor. Furthermore, the process of FRET was successfully translated into the solid state, proving their suitability for device-level studies as well. In short, our study expands the spectrum of aqueous QDs based on InP towards the blue region for future biological and light harvesting studies.

All-quantum dot based Förster resonant energy transfer: key parameters for high-efficiency biosensing

While colloidal quantum dots (QDs) are commonly used as fluorescent donors within biosensors based on Förster resonant energy transfer (FRET), they are hesitantly employed as acceptors. On the sole basis of Förster theory and the well-known behaviour of organic dyes, it is often argued that the QD absorption band over the UV-visible range is too wide. Discarding these preconceptions inherited from classical fluorophores, we experimentally examine the FRET process occurring between donor and acceptor CdTe QDs and provide a mathematical description of it. We evidence that the specific features of QDs unexpectedly lead to the enhancement of acceptors' emission (up to +400%), and are thus suitable for the design of highly efficient all-QD based FRET sensors. Our model enables us to identify the critical parameters maximizing the contrast between positive and negative biosensing readouts: the concentrations of donors and acceptors, their spectral overlap, the densities of their excitonic states, their dissipative coupling with the medium and the statistics of QD-QD chemical pairing emerge as subtle and determinant parameters. We relate them quantitatively to the measured QD-QD FRET efficiency and discuss how they must be optimized for biosensing applications.

Impact of coherent core/shell architecture on fast response in InP-based quantum dot photodiodes

Solution-processed, cadmium-free quantum dot (QD) photodiodes are compatible with printable optoelectronics and are regarded as a potential candidate for wavelength-selective optical sensing. However, a slow response time resulting from low carrier mobility and a poor dissociation of charge carriers in the optically active layer has hampered the development of the QD photodiodes with nontoxic device constituents. Herein, we report the first InP-based photodiode with a multilayer device architecture, working in photovoltaic mode in photodiode circuits. The photodiode showed the fastest response speed with rising and falling times of τ _r = 4 ms and τ _f = 9 ms at a voltage bias of 0 V at room temperature in ambient air among the Cd-free photodiodes. The single-digit millisecond photo responses were realized by efficient transportation of the photogenerated carriers in the optically active layer resulting from coherent InP/ZnS core/shell QD structure, fast separation of electron and hole pairs at the interface between QD and Al-doped ZnO layers, and optimized conditions for uniform deposition of each thin film. The results suggested the versatility of coherent core/shell QDs as a photosensitive layer, whose structures allow various semiconductor combinations without lattice mismatch considerations, towards fast response, high on/off ratios, and spectrally tunable optical sensing.