Semiconductor Quantum Dots as Components of Photoactive Supramolecular Architectures

Luminescent quantum dots (QDs) are colloidal semiconductor nanocrystals consisting of an inorganic core covered by a molecular layer of organic surfactants. Although QDs have been known for more than thirty years, they are still attracting the interest of researchers because of their unique size-tunable optical and electrical properties arising from quantum confinement. Moreover, the controlled decoration of the QD surface with suitable molecular species enables the rational design of inorganic-organic multicomponent architectures that can show a vast array of functionalities. This minireview highlights the recent progress in the use of surface-modified QDs - in particular, those based on cadmium chalcogenides - as supramolecular platforms for light-related applications such as optical sensing, triplet photosensitization, photocatalysis and phototherapy.

Highly Responsive Ultraviolet Sensor Based on ZnS Quantum Dot Solid with Enhanced Photocurrent

Detection of visible blind UV radiation is not only interesting but also of technologically important. Herein, we demonstrate the efficient detection of UV radiation by using cluster like ZnS quantum dot solid nanostructures prepared by simple reflux condensation technique. The short-chain ligand 3-mercaptopropionic acid (MPA) involved in the synthesis lead to the cluster like formation of ZnS quantum dots into solids upon prolonged synthesis conditions. The ZnS QD solid formation resulted in the strong delocalization of electronic wave function between the neighboring quantum dots. It increases the photocurrent value, which can be further confirmed by the decrease in the average lifetime values from 64 to 4.6 ns upon ZnS cluster like QD solid formation from ZnS QDs. The ZnS quantum dot solid based UV sensor shows good photocurrent response and a maximum responsivity of 0.31 (A/W) at a wavelength of 390 nm, is not only competitive when compared with previous reports but also better than ZnS and metal oxide-based photodetectors. The device exhibits a high current value under low-intensity UV light source and an on/off ratio of IUV/Idark = 413 at zero biasing voltage with a fast response. Further, photocurrent device has been constructed using ZnS quantum dot solid nanostructures with graphene hybrids as an active layer to improve the enhancement of photoresponsivity.

Electronic Structure with Dipole Moment and Rovibrational Calculation of Cadmium Chalcogenide Molecules CdX (X = Se, Te)

Ab initio calculations of 51 electronic states in the representation 2s+1Λ(±) of CdX (X = Se, Te) molecules have been carried out by using the complete active space self-consistent field and multireference configuration interaction (single and double excitations with the Davidson correction). The potential energy along with the static and transition dipole moment curves for the investigated electronic states of the CdX molecules has been mapped. Consequently, the spectroscopic constants R e, ωe, B e, and T e have been computed for the bound states. The spectroscopic dissociation energy D e, the zero-point energy, and the ionicity are also calculated for the bound electronic states X3Π, (1)1Σ+, (1)1Π, and (1)3Σ+. Rovibrational calculation is performed for the X3Π, (1)1Σ+, (1)1Π, and (1)3Σ+ states of CdSe together with the X3Π, (1)1Σ+, and (1)1Π states of a CdTe molecule. The Einstein coefficients of spontaneous and induced emissions, A 21 and B 21, are computed for the transition between the electronic states (1)3Σ+ and X3Π. In the present work, the values are well-consistent with those available in the literature.

Sensing with photoluminescent semiconductor quantum dots

Fluorescent sensors benefit from high signal-to-noise and multiple measurement modalities, enabling a multitude of applications and flexibility of design. Semiconductor nanocrystal quantum dots (QDs) are excellent fluorophores for sensors because of their extraordinary optical properties. They have high thermal and photochemical stability compared to organic dyes or fluorescent proteins and are extremely bright due to their large molar cross-sections. In contrast to organic dyes, QD emission profiles are symmetric, with relatively narrow bandwidths. In addition, the size tunability of their emission color, which is a result of quantum confinement, make QDs exceptional emitters with high color purity from the ultra-violet to near infrared wavelength range. The role of QDs in sensors ranges from simple fluorescent tags, as used in immunoassays, to intrinsic sensors that utilize the inherent photophysical response of QDs to fluctuations in temperature, electric field, or ion concentration. In more complex configurations, QDs and biomolecular recognition moieties like antibodies are combined with a third component to modulate the optical signal via energy transfer. QDs can act as donors, acceptors, or both in energy transfer-based sensors using Förster resonance energy transfer (FRET), nanometal surface energy transfer (NSET), or charge or electron transfer. The changes in both spectral response and photoluminescent lifetimes have been successfully harnessed to produce sensitive sensors and multiplexed devices. While technical challenges related to biofunctionalization and the high cost of laboratory-grade fluorimeters have thus far prevented broad implementation of QD-based sensing in clinical or commercial settings, improvements in bioconjugation methods and detection schemes, including using simple consumer devices like cell phone cameras, are lowering the barrier to broad use of more sensitive QD-based devices.

Highly photoluminescent and temperature-sensitive P, N, B-co-doped carbon quantum dots and their highly sensitive recognition for curcumin

Temperature-sensitive P, N, B-co-doped carbon quantum dots (PNBCDs) synthesized using one-pot method exhibit many excellent features, such as strong fluorescence, good stability and sensitive detection for curcumin.

Development of a portable device for Ag+ sensing using CdTe QDs as fluorescence probe via an electron transfer process

Ag⁺ as one of the most commonly seen toxic heavy metal ions is involved in numerous vital biological processes, which would cause fatal damages and environmental contamination when Ag⁺ is excessive. In the present work, CdTe quantum dots (QDs) with green, orange, and red emission capped by mercaptoacetic acid (TGA) were synthesized at one time by controlling the synthesis time and utilized for Ag⁺ detection. Both fluorescence spectral red-shift and intensity decrease could be used for Ag⁺ discrimination. Fluorescence lifetime, Zeta potential, and XRD, etc. were carried out to analyze the detection mechanism. Results displayed that surface passivation and electron transfer due to binding effects of Ag⁺ to Te atom on traps of QDs could be relied on to explain the sensing mechanism. Additionally, in accordance with PCA analysis, Ag⁺ could be also be successfully differentiated from Hg²⁺ and the other metal ions. Importantly, a home-made portable device based on a 32 bit embed Micro Control Unit (MCU) system was first proposed for Ag⁺ detection. The power supply system adopt the mini-sized lithium cell instead of the power supply system, which ensure its practical applicability. The relative position of light source and detector is set at 90° to minimize the interference. According to the detection results, the linear detection range using the device was from 5 nM to 200 nM (with a larger coefficient of determination, R²), and the detection limit was calculated to be about 5 nM, which indicated that this proposed method and device could fulfil the practical application requirements.

A double fluorescent nanoprobe based on phosphorus/nitrogen co-doped carbon dots for detecting dichromate ions and dopamine

An “on–off–on” fluorescent phosphorus/nitrogen co-doped carbon dot (PNCD) probe was explored for the determination of Cr(vi) and dopamine resulting from the inner filter effect (IFE). The blue-emitting carbon dots with high quantum yields of 25.47% as well as a narrow size distribution were synthesized by a rapid, convenient route using H₃PO₄ and ethylenediamine as the precursors without any surface passivation. A wide linear region in the range of 7–70 μM with a detection limit of 0.71 μM was achieved for Cr(vi). Moreover, the proper reductants can weaken the inner filter effect to recover the PNCD fluorescence by converting Cr(vi) into Cr(iii). Therefore, the PNCDs/Cr(vi) hybrid could also be used as an “off–on” fluorescent probe for detecting dopamine (DA) with a detection limit of 0.49 μM. Consequently, the PNCDs could serve as a powerful fluorescent bi-sensor for detection of both Cr(vi) and DA in practical applications.

An “on–off–on” fluorescent phosphorus/nitrogen co-doped carbon dot (PNCD) probe was explored for the determination of Cr(vi) and dopamine resulting from the inner filter effect (IFE).

A study on the synthesis, longitudinal optical phonon–plasmon coupling and electronic structure of Al doped ZnS nanorods

First principles density functional theory (DFT) calculations were employed to study the structural and electronic properties of pure and Al doped ZnS nanorods.

Biosynthesis of zinc sulfide quantum dots using waste off-gas from a metal bioremediation process

Waste H₂S biogas from a mine-water remediation bioprocess is used to make zinc sulfide quantum dots which are identical to ZnS QDs made by chemical methods.

Local structural studies on Co doped ZnS nanowires by synchrotron X-ray atomic pair distribution function and micro-Raman shift

The atomic structures of nanowires were studied by X-ray atomic pair distribution function analysis and total synchrotron X-ray scattering data. A PDF method was used to describe a wurtzite and zinc-blended mixed phase model.

A study on the synthesis, pair distribution function and diverse properties of cobalt doped ZnS nanowires

The fundamental atomic structure of pure and Co doped ZnS nanowires has been studied using pair distribution function (PDF) analysis. It was confirmed that samples have hexagonal (wurtzite) structure. The interatomic distance was calculated using PDF analysis. It was observed that the energy band gap decreases as Co content increases.

Determination of tryptophan and tyrosine by chemiluminescence based on a luminol–N-bromosuccinimide–ZnS quantum dots system

ZnS QDs as a catalyst can catalyze luminol–NBS system CL, based on Trp and Tyr can inhibit this system CL intensity, we were designed a rapid and sensitive sensor for determination of Trp and Tyr.

Shell thickness modulation in ultrasmall CdSe/CdS(x)Se(1-x)/CdS core/shell quantum dots via 1-thioglycerol

In this study, we report on the synthesis of CdSe/CdS core-shell ultrasmall quantum dots (CS-USQDs) using an aqueous-based wet chemistry protocol. The proposed chemical route uses increasing concentration of 1-thioglycerol to grow the CdS shell on top of the as-precipitated CdSe core in a controllable way. We found that lower concentration of 1-thioglycerol (3 mmol) added into the reaction medium limits the growth of the CdSe core, and higher and increasing concentration (5-11 mmol) of 1-thioglycerol promotes the growth of CdS shell on top of the CdSe core in a very controllable way, with an increase from 0.50 to 1.25 nm in shell thickness. The growth of CS-USQDs of CdSe/CdS was confirmed by using different experimental techniques, such as optical absorption (OA) spectroscopy, fluorescence spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and Raman spectroscopy. Data collected from OA were used to obtain the average values of the CdSe core diameter, whereas Raman data were used to assess the average values of the CdSe core diameter and CdS shell thicknesses.

Fluorescence enhancement detection of uric acid based on water-soluble 3-mercaptopropionic acid-capped core/shell ZnS:Cu/ZnS

A simple core/shell ZnS:Cu/ZnS QD based fluorescence enhancement system is constructed for detecting uric acid. The method with high sensitivity, selectivity and lower toxicity features was applied for determination of UA in real samples.

Graphitic carbon nitride solid nanofilms for selective and recyclable sensing of Cu2+ and Ag+ in water and serum

Graphitic carbon nitride hybrid nanofilms are reported as sensitive, rapid and recyclable sensor for Cu²⁺ and Ag⁺, and the sensing in serum demonstrates their potential in medical applications.

Cu(ii)4L4coordination-driven molecular container: a reusable visual colorimetric sensor for Ag(i) ions

A Cu₄L₄square-like molecular container which can be a reusable visual sensor for Ag⁺is reported. The present results can be a useful stepwise approach for the construction of heterometallic supramolecular complexes with potential applications.

Mn2+ ion influenced optical and photocatalytic behaviour of Mn–ZnS quantum dots prepared by a microwave assisted technique

(a) Photon absorption and exciton formation, (b) interstitial sulfur emission, (c) interstitial zinc emission, (d) blue emission, (e) electron trapping by Mn ions' d state, (f) orange light emission and (g) orange emission quenching by electrons trapped by the neighbouring Mn²⁺ ions.

Behaviour of zinc complexes and zinc sulphide nanoparticles revealed by using screen printed electrodes and spectrometry

In this study, we focused on microfluidic electrochemical analysis of zinc complexes (Zn(phen)(his)Cl₂, Zn(his)Cl₂) and ZnS quantum dots (QDs) using printed electrodes. This method was chosen due to the simple (easy to use) instrumentation and variable setting of flows. Reduction signals of zinc under the strictly defined and controlled conditions (pH, temperature, flow rate, accumulation time and applied potential) were studied. We showed that the increasing concentration of the complexes (Zn(phen)(his)Cl₂, Zn(his)Cl₂) led to a decrease in the electrochemical signal and a significant shift of the potential to more positive values. The most likely explanation of this result is that zinc is strongly bound in the complex and its distribution on the electrode is very limited. Changing the pH from 3.5 to 5.5 resulted in a significant intensification of the Zn(II) reduction signal. The complexes were also characterized by UV/VIS spectrophotometry, chromatography, and ESI-QTOF mass spectrometry.

Highly Ag+ selective tripodal gold(I) acetylide-based "off-on" luminescence chemosensors based on 3(ππ*) emission switching

Newly prepared gold(I) acetylide-based luminescent cation sensors 1c and 1d, which bear a novel three-armed flexible conformation, together with control molecule 2c, were synthesized in four steps. 1c and 1d exhibit the π → π* absorption of the acetylide ligands in fluid solutions and produce the (3)(ππ*) emission of the acetylide ligands in the solid state and degassed THF solution. The (3)(ππ*) emission of 1c in DMSO can be turned on upon addition of Ag(+). The binding ratio between 1c and Ag(+) as well as the binding constant log K were determined as 1:1 and 4.35 ± 0.12 respectively by UV-vis titration experiments. The formation of [1c·Ag](+) adduct, which leads to the appearance of new up-field peaks, was confirmed by (1)H NMR spectroscopic titrations. The control (1)H NMR titration experiments for the acetylide-free analogue 1a and single-armed analogue 2c indicate the acetylide groups and tripodal structures are responsible for the binding of Ag(+). The control experiment for tripodal gold(I) acetylide analogue 1d suggests the change of PPh3 with P(2-Py)Ph2 induces similar binding affinity toward Ag(+) but less selectivity toward Ag(+). Free 1c also exhibits the anion binding affinity toward F(-), but the Ag(+) adduct [1c·Ag](+) shows less affinity toward F(-).