One of the fastest moving and most exciting interfaces of nanotechnology is the use of quantum dots (QDs) in biology. The unique optical properties of QDs make them appealing as in vivo and in vitro fluorophores in a variety of biological investigations, in which traditional fluorescent labels based on organic molecules fall short of providing long-term stability and simultaneous detection of multiple signals. The ability to make QDs water soluble and target them to specific biomolecules has led to promising applications in cellular labelling, deep-tissue imaging, assay labelling and as efficient fluorescence resonance energy transfer donors. Despite recent progress, much work still needs to be done to achieve reproducible and robust surface functionalization and develop flexible bioconjugation techniques. In this review, we look at current methods for preparing QD bioconjugates as well as presenting an overview of applications. The potential of QDs in biology has just begun to be realized and new avenues will arise as our ability to manipulate these materials improves.

Quantum dot bioconjugates for imaging, labelling and sensing

Microporous metal–organic frameworks (MOFs) that display permanent porosity show great promise for a myriad of purposes. The potential applications of MOFs can be developed further and extended by encapsulating various functional species (for example, nanoparticles) within the frameworks. However, despite increasing numbers of reports of nanoparticle/MOF composites, simultaneously to control the size, composition, dispersed nature, spatial distribution and confinement of the incorporated nanoparticles within MOF matrices remains a significant challenge. Here, we report a controlled encapsulation strategy that enables surfactant-capped nanostructured objects of various sizes, shapes and compositions to be enshrouded by a zeolitic imidazolate framework (ZIF-8). The incorporated nanoparticles are well dispersed and fully confined within the ZIF-8 crystals. This strategy also allows the controlled incorporation of multiple nanoparticles within each ZIF-8 crystallite. The as-prepared nanoparticle/ZIF-8 composites exhibit active (catalytic, magnetic and optical) properties that derive from the nanoparticles as well as molecular sieving and orientation effects that originate from the framework material.

/pdf/imparting-functionality-to-a-metal-organic-framework-2ch180mgdf.pdf

Imparting functionality to a metal–organic framework material by controlled nanoparticle encapsulation

Inorganic colloidal nanoparticles are very small, nanoscale objects with inorganic cores that are dispersed in a solvent. Depending on the material they consist of, nanoparticles can possess a number of different properties such as high electron density and strong optical absorption (e.g. metal particles, in particular Au), photoluminescence in the form of fluorescence (semiconductor quantum dots, e.g. CdSe or CdTe) or phosphorescence (doped oxide materials, e.g. Y(2)O(3)), or magnetic moment (e.g. iron oxide or cobalt nanoparticles). Prerequisite for every possible application is the proper surface functionalization of such nanoparticles, which determines their interaction with the environment. These interactions ultimately affect the colloidal stability of the particles, and may yield to a controlled assembly or to the delivery of nanoparticles to a target, e.g. by appropriate functional molecules on the particle surface. This work aims to review different strategies of surface modification and functionalization of inorganic colloidal nanoparticles with a special focus on the material systems gold and semiconductor nanoparticles, such as CdSe/ZnS. However, the discussed strategies are often of general nature and apply in the same way to nanoparticles of other materials.

https://faculty.washington.edu/mzhang/mse599/Surface-modification.full.pdf

Surface modification, functionalization and bioconjugation of colloidal inorganic nanoparticles

Doping--the intentional introduction of impurities into a material--is fundamental to controlling the properties of bulk semiconductors. This has stimulated similar efforts to dope semiconductor nanocrystals. Despite some successes, many of these efforts have failed, for reasons that remain unclear. For example, Mn can be incorporated into nanocrystals of CdS and ZnSe (refs 7-9), but not into CdSe (ref. 12)--despite comparable bulk solubilities of near 50 per cent. These difficulties, which have hindered development of new nanocrystalline materials, are often attributed to 'self-purification', an allegedly intrinsic mechanism whereby impurities are expelled. Here we show instead that the underlying mechanism that controls doping is the initial adsorption of impurities on the nanocrystal surface during growth. We find that adsorption--and therefore doping efficiency--is determined by three main factors: surface morphology, nanocrystal shape, and surfactants in the growth solution. Calculated Mn adsorption energies and equilibrium shapes for several nanocrystals lead to specific doping predictions. These are confirmed by measuring how the Mn concentration in ZnSe varies with nanocrystal size and shape. Finally, we use our predictions to incorporate Mn into previously undopable CdSe nanocrystals. This success establishes that earlier difficulties with doping are not intrinsic, and suggests that a variety of doped nanocrystals--for applications from solar cells to spintronics--can be anticipated.

Doping semiconductor nanocrystals

Temperature (T) is probably the most fundamental parameter in all kinds of science. Respective sensors are widely used in daily life. Besides conventional thermometers, optical sensors are considered to be attractive alternatives for sensing and on-line monitoring of T. This Review article focuses on all kinds of luminescent probes and sensors for measurement of T, and summarizes the recent progress in their design and application formats. The introduction covers the importance of optical probes for T, the origin of their T-dependent spectra, and the various detection modes. This is followed by a survey on (a) molecular probes, (b) nanomaterials, and (c) bulk materials for sensing T. This section will be completed by a discussion of (d) polymeric matrices for immobilizing T-sensitive probes and (e) an overview of the various application formats of T-sensors. The review ends with a discussion on the prospects, challenges, and new directions in the design of optical T-sensitive probes and sensors.

Luminescent probes and sensors for temperature.

Highly luminescent CdSe and CdTe quantum dots (QDs) are prepared in a hot solvent of capping molecules (TOP/TOPO/HDA for CdSe and TOP/DDA for CdTe). The influence of exchange of the capping molecules with different types of thiol molecules (amino ethanethiol, (3-mercaptopropyl)trimethoxysilane, hexanethiol, 2-propenethiol, and 4-mercaptophenol) is investigated for both CdSe and CdTe QDs. A remarkable difference is observed:  capping exchange with thiol molecules results in an increased luminescence efficiency for CdTe QDs but induces quenching of the excitonic emission of CdSe QDs. The striking difference between the two types of II-VI QDs is explained by the difference in the energy of the valence band top. The lower energetic position of the valence band for CdSe results in hole trapping of the photogenerated hole on the thiol molecule, thus quenching the luminescence. For CdTe the valence band is situated at higher energies with respect to the redox level of most thiols, thus inhibiting hole trapping a...

/pdf/influence-of-thiol-capping-on-the-exciton-luminescence-and-4ubd346ybp.pdf

Influence of thiol capping on the exciton luminescence and decay kinetics of CdTe and CdSe quantum dots

Colloidal CdTe quantum dots prepared in TOP/DDA (trioctylphosphine/dodecylamine) are transferred into water by the use of amino−ethanethiol•HCl (AET) or mercaptopropionic acid (MPA). This results in an increase in the photoluminescence quantum efficiency and a longer exciton lifetime. For the first time, water-soluble semiconductor nanocrystals presenting simultaneously high band-edge photoluminescence quantum efficiencies (as high as 60% at room temperature), monoexponential exciton decays, and no observable defect-related emission are obtained.

/pdf/highly-luminescent-water-soluble-cdte-quantum-dots-3sems2cyhk.pdf

Highly Luminescent Water-Soluble CdTe Quantum Dots

A comprehensive investigation is presented on the factors governing the photoluminescence (PL) quantum yields (QYs) and size dispersion of colloidal CdSe nanocrystals. The temporal evolution of the ensemble PL properties (absorption and luminescence spectra, QYs and lifetimes) during growth at different temperatures (170−310 °C) and different Cd:Se ratios was followed for several hours (2−6 h). The QY values increase during the growth to a maximum and, after a variable time interval (from minutes to hours, depending on the growth temperature), decrease gradually. Low QYs are due to poor passivation, surface disorder, and surface degradation, which arise at different stages of the growth. High QYs can be achieved and maintained only under an ideal combination of growth temperature, solvent composition, and Cd:Se ratio, which leads to an optimum surface. The overgrowth of a fresh surface layer restores high QYs to CdSe nanocrystals with decreased efficiencies because of surface degradation. The insight gain...

/pdf/single-step-synthesis-to-control-the-photoluminescence-547bl3s2ky.pdf

Single-step synthesis to control the photoluminescence quantum yield and size dispersion of CdSe nanocrystals

The influence of the synthesis conditions on the properties of nanocrystalline ZnS:Mn2+ is discussed. Different Mn2+ precursors and different ratios of the precursor concentrations [S2-]/[Zn2+] were used. The type of Mn2+ precursor does not have an effect on the luminescence properties in the synthesis method described. On going from an excess of [Zn2+] to an excess of [S2-] during the synthesis, the particle diameter increases from 3.7 to 5.1 nm, which is reflected by a change in the luminescence properties. Photoluminescence measurements also showed the absence of the ZnS defect luminescence around 450 nm when an excess [S2-] is used during the synthesis. This effect is explained by the filling of sulfur vacancies. The ZnS luminescence is quenched with an activation energy of 62 meV, which is assigned to the detrapping of a bound hole from such a vacancy.

/pdf/synthesis-and-photoluminescence-of-nanocrystalline-zns-mn-2-1weo2agfso.pdf

Synthesis and Photoluminescence of Nanocrystalline ZnS:Mn^(2+)

Nanocrystalline ZnS:Pb2+ is synthesised ia a precipitation method. The influence of the size of the nanocrystals
 and the sulfide concentration used in the synthesis on the luminescence properties is investigated.
 Nanocrystalline ZnS:Pb2+ shows a white emission under UV excitation with a rather high quantum efficiency (∽5%).
 At least two luminescence centres are involved. One centre is identified as a Pb2+ ion located on a regular
 Zn2+ site and gives a red emission under 480 nm excitation. The luminescence properties of this emission
 are characteristic of 3P0
→
1S0 (A-band) or charge transfer (D-band) transitions on Pb2+ ions. The other centres
 are not as well defined and give a broad green emission band under 380 nm excitation and also show
 luminescence properties typically observed for Pb2+. The green emission probably originates from a charge-transfer
 like D-band emission of Pb2+ in ZnS close to a defect (e.g. an S2− vacancy or an O2− ion on an S2− site).
 A relation between the temperature quenching of the emissions and the band gap is observed and indicates
 that photoionisation occurs. The higher quenching temperature for the Pb2+ luminescence in smaller particles can be
 explained by widening
 of the band gap
 as a result of quantum size effects in the ZnS host.

/pdf/luminescence-of-nanocrystalline-znse-mn2-2p79ql2bof.pdf

Sander Frederik Wuister

Papers

Influence of thiol capping on the exciton luminescence and decay kinetics of CdTe and CdSe quantum dots

Highly Luminescent Water-Soluble CdTe Quantum Dots

Single-step synthesis to control the photoluminescence quantum yield and size dispersion of CdSe nanocrystals

Synthesis and Photoluminescence of Nanocrystalline ZnS:Mn^(2+)

Luminescence of nanocrystalline ZnSe:Mn2+