The design of quantum dots and their conjugates as luminescent probes for analyte sensing

The design and applications of quantum dots (QDs) as fluorescent probes for analyte sensing is presented. Cadmium based thiol-capped QDs were employed as probe for the detection of analytes. Comparative studies between core CdTe and core-shell CdTe@ZnS QDs showed that the overall sensitivity and selectivity of the sensor was dependent on the nature of the capping agent and the QDs employed, hence making CdTe@ZnS QDs a more superior sensor than the core. To explore the luminescent sensing of QDs based on the fluorescence “turn ON” mode, L-glutathione-capped CdTe QDs was conjugated to 4-amino-2,2,6,6-tetramethylpiperidine-N-oxide (4AT) to form a QDs-4AT conjugate system. The QDs-4AT nanoprobe was highly selective and sensitive to the detection of bromide ion with a very low limit of detection. Subsequently, metallo-phthalocyanines (MPcs) were employed as host molecules on the surface of QDs based on the covalent linking of the QDs to the MPc. Elucidation of the reaction mechanism showed that the fluorescence “turn ON” effect of the QDs-MPc probe in the presence of the analyte was due to axial ligation of the analytes to the Pc ring. Studies showed that the type of substituent attached to the MPc ring influenced the overall sensitivity of the probe. Additionally, a comparative investigation using newly synthesized phthalocyanine and triaza-benzcorrole complexes was conducted when these complexes were conjugated to CdSe@ZnS QDs for analyte sensing. Results showed that the triaza-benzcorrole complex can be employed as a host-molecule sensor but displayed a lower sensitivity for analyte sensing in comparison to the phthalocyanine complex

Ultralow detection of 3,4-methylenedioxymethylamphetamine using an immunofluorescence nanoconjugate of heavy metal-free alloyed quantum dots and NiCeFe₃O₄ magnetic nanoparticles

The recreational use of 3,4-methyl​enedioxy​methylamphetamine (MDMA) remains consistently high on a global level. This reinforces the importance of early, rapid and accurate detection of MDMA. This work reports on the novel development of a rapid, ultrasensitive and selective immunofluorescence nanobiosensor probe for MDMA. To develop the nanoprobe, heavy metal-free AuZnFeSeS alloyed quantum dots (QDs) were synthesized and coated with carboxy (COOH)-silica and subsequently conjugated to an anti-MDMA antibody (Ab). Secondly, spinel NiCeFe3O4 magnetic nanoparticles (MNPs) were synthesized, surface functionalized with L-cysteine and conjugated to the same anti-MDMA Ab. An immunocomplex was established where both AuZnFeSeS QDs-Ab and NiCeFe3O4-Ab each captured the target MDMA drug. The bound QDs fluorescently reported the surface biomolecular interaction between the nanomaterials and Ab while the bound NiCeFe3O4 MNPs functioned as an adsorbent and as a signal amplifier. The Ab binding on AuZnFeSeS QDs surface switched off the fluorescence of the QDs but upon interaction of AuZnFeSeS QDs-Ab with NiCeFe3O4-Ab, the fluorescence of the bound QDs was switched on. Experimental analysis revealed the inefficiency of AuZnFeSeS QDs-Ab (without NiCeFe3O4-Ab) to detect MDMA ultrasensitively and selectively; hence, the use of NiCeFe3O4-Ab was justified in the probe system. Under optimum reaction conditions, MDMA was quantitatively detected using the AuZnFeSeS QDs-Ab-NiCeFe3O4-Ab nanocomplex in the concentration range of 0.05–50,000 nM and a detection limit of 0.02 nM (0.0046 ng/mL) was obtained. The developed AuZnFeSeS QDs-Ab-NiCeFe3O4-Ab nanoprobe was successfully used to detect MDMA in urine with satisfactory recoveries

Plasmonic Oleylamine-Capped Gold and Silver Nanoparticle-Assisted Synthesis of Luminescent Alloyed CdZnSeS Quantum Dots

We report on a novel strategy to tune the structural and optical properties of luminescent alloyed quantum dot (QD) nanocrystals using plasmonic gold (Au) and silver (Ag) nanoparticles (NPs). Alloyed CdZnSeS QDs were synthesized via the organometallic synthetic route with different fabrication strategies that involve alternative utilization of blends of organic surfactants, ligands, capping agents, and plasmonic oleylamine (OLA)-functionalized AuNPs and AgNPs. Ligand exchange with thiol l-cysteine (l-cyst) was used to prepare the hydrophilic nanocrystals. Analysis of the structural properties using powder X-ray diffraction revealed that under the same experimental condition, the plasmonic NPs altered the diffractive crystal structure of the alloyed QDs. Depending on the fabrication strategy, the crystal nature of OLA-AuNP-assisted CdZnSeS QDs was a pure hexagonal wurtzite domain and a cubic zinc-blende domain, whereas the diffraction pattern of OLA-AgNP-assisted CdZnSeS QDs was dominantly a cubic zinc-blende domain. Insights into the growth morphology of the QDs revealed a steady transformation from a heterogeneous growth pattern to a homogenous growth pattern that was strongly influenced by the plasmonic NPs. Tuning the optical properties of the alloyed QDs via plasmonic optical engineering showed that the photoluminescence (PL) quantum yield (QY) of the AuNP-assisted l-cyst-CdZnSeS QDs was tuned from 10 to 31%, whereas the PL QY of the AgNP-assisted l-cyst-CdZnSeS QDs was tuned from 15 to 90%. The low PL QY was associated with the surface defect state, while the remarkably high PL QY exhibited by the AgNP-assisted l-cyst-CdZnSeS QDs lends strong affirmation that the fabrication strategy employed in this work provides a unique opportunity to create single ensemble, multifunctional, highly fluorescent alloyed QDs for tailored biological applications

Development of a Thiol-capped Core/Shell Quantum Dot Sensor for Acetaminophen

Acetaminophen (AC) is a frequently used pharmaceutical which has been detected in water systems and is of concern due to its potential environmental impacts. In this study, three quantum dot (QD)-ligand systems, namely L-cysteine (L-cys)-, N-acetyl- L-cysteine (NAC)- and glutathione (GSH)-capped CdSe/ZnS quantum dots, were synthesized and tested for the fluorescence detection of acetaminophen. Among the synthesized aqueous core/shell quantum dots, L-cys-CdSe/ZnS QDs were found to be optimal with high sensitivity for the fluorescence detection of acetaminophen. The L-cys-CdSe/ZnS QDs were of a zinc blende crystal structure and displayed excellent fluorescence intensity and photostability and provided a photoluminescence quantum yield of 77 % . The fluorescence of L-cys-CdSe/ZnS QDs was enhanced by the introduction of AC enabling the development of a fast and simple method for the detection of AC. Under optimal conditions, F-F0 was linearly proportional to the concentration of AC from 3.0–100 nmol L–1 with limits of detection and quantification of 1.6 and 5.3 nmol L–1, respectively. Some related pharmaceutical compounds including epinephrine hydrochloride (EP), L-ascorbic acid (AA), uric acid (UA), dopamine hydrochloride (DA) and 4-aminophenol (4-AP) did not interfere with the sensing ofAC. The probe was also successfully applied in the determination of AC in tap and river water matrices.The University of Pretoria, the Water Research Commission (Grant K5/2438/1 and K5/2752) as well as the Photonics Initiative of South Africa (Grant PISA-15-DIR-06).http://www.journals.co.za/sajchemam2019Chemistr

Cadmium-free silica-encapsulated molecularly imprinted AuZnCeSeS quantum dots nanocomposite as an ultrasensitive fluorescence nanosensor for methamphetamine detection

One of the major challenges facing forensic drug analysis is the difficulty in detecting ultralow concentration of illicit drugs in biological matrices without the need for an extraction or a pre-treatment step. This work report on the development of a novel AuZnCeSeS quantum dots (QDs)-molecular imprinted polymer (MIP) nanocomposite fluorescent probe for methamphetamine (METH) recognition. Silica-coated AuZnCeSeS QDs were synthesized and characterized using spectrophotometric, spectroscopic and electron microscopy techniques. Via a free radical polymerization reaction, a thin layer of MIP shell with METH as the template was coated around the QDs surface leading to the formation of a QDs-MIP nanocomposite probe. The MIP coating passivated the QDs surface leading to radiative fluorescence enhancement of the bound QDs. Under optimum reaction conditions, METH was selectively and quantitatively detected via a fluorescence quenching reaction process. The unique selectivity of the nanoprobe for METH recognition showed clearly that METH was able to precisely re-bind to the MIP surface with size and shape reorganization. While the MIP shell functioned to provide the required selectivity, the AuZnCeSeS QDs functioned to fluorescently report the surface binding interaction. The use of a AuZnCeSeS QDs-non-imprinted polymer as probe to detect METH resulted in poor sensitivity and selectivity; hence, demonstrating the suitability of the AuZnCeSeS QDs-MIP nanoprobe to accurately detect METH. METH was detected within a wide concentration range from 0.05 to 50,000 nM with a detection limit of ∼0.02 nM (0.0036 ng/mL). The developed AuZnCeSeS QDs-MIP nanoprobe was efficiently used to detect METH in untreated urine sample with recovery efficiency from ∼100 to 110%

A localized surface plasmon resonance-amplified immunofluorescence biosensor for ultrasensitive and rapid detection of nonstructural protein 1 of Zika virus

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Blue-emitting SiO₂-coated Si-doped ZnSeS quantum dots conjugated aptamer-molecular beacon as an electrochemical and metal-enhanced fluorescence biosensor for SARS-CoV-2 spike protein

The outbreak of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) which was first reported in early January 2020, continues to devastate the worlds public health system. Herein, we report on the development of a novel metal-enhanced fluorescence (MEF) and electrochemical biosensor for SARS-CoV-2 spike (S) protein. To develop the MEF biosensor, SiO2-coated Si-doped ZnSeS quantum dots (QDs) were newly synthesized and conjugated to an aptamer-molecular beacon (Apta-MB) probe. Thereafter, cationic AuNPs, used as a localised surface plasmon resonance (LSPR) signal amplifier, were self-assembled on the QDs-Apta-MB conjugate to form a QDs-Apta-MB-AuNP probe. To develop the electrochemical biosensor, the QDs-Apta-MB assay was carried out on a carbon nanofiber-modified screen-printed carbon electrode. Cyclic voltammetry (CV), differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS) were used to characterize the electrode surface whilst spectrophotometric, spectroscopic, fluorescence polarization and electron microscopic techniques were used to characterize the materials. Under optimal experimental conditions, the QDs binding to the Apta-MB, quenched the QDs’ fluorescence and with SARS-CoV-2 S protein binding to the Apta-MB, LSPR signal from cationic AuNPs of different sizes and shapes were used to tune the fluorescence signal to obtain enhanced sensitivity. On the other hand, using [Fe(CN)6]/K3−/4- buffered with NaAc-KAc-TrizmaAc-KSCN-Borax as the electrolyte solution, anodic peaks of the QDs from the CV and DPV plots were unravelled. Electrochemical detection of SARS-CoV-2 S protein was accomplished by a systematic increase in the QDs anodic peak current generated from the DPV plots. The limits of detection obtained for the SARS-CoV-2 S protein were 8.9 fg/mL for the QDs-Apta-MB-AuNP MEF probe and ∼0.5 pg/mL for the QDs-Apta-MB electrochemical probe. Detection of SARS-CoV-2 S protein in saliva was demonstrated using the QDs-Apta-MB-AuNP MEF probe

Passivating effect of ternary alloyed AgZnSe shell layer on the structural and luminescent properties of CdS quantum dots

The surface passivation of luminescent CdS quantum dots (QDs) via epitaxial overgrowth of new alloyed ternary AgZnSe shell layer is reported here. Two synthetic fabrication strategies were used to tune the optical properties of CdS/AgZnSe core/alloyed shell QDs across the visible region. Transmission electron microscopy, powder X-ray diffraction, Raman, UV/vis and fluorescence spectrophotometric techniques were used to characterize the nanocrystals. Analysis of the internal structure of the QDs revealed that homogeneity of the particle reduced as the size increased, thus indicating that the QDs growth transitioned from an interfacial epitaxial homogenous state to a heterogeneous state. The crystal structure of the QDs revealed a distinct zinc-blende diffraction pattern for CdS while CdS/AgZnSe core/alloyed shell QDs kinetically favoured a phase change process from the zinc-blende phase to a wurtzite phase. Analysis of the photophysical properties revealed varying degrees of interfacial defect state suppression in CdS/AgZnSe QDs which was dependent on the QDs size. Specifically, the fluorescence quantum yield (QY) of CdS/AgZnSe QDs was at most ~5-fold higher than the CdS core and varied from 35% to 73%. We found that band gap modulation via the synthetic fabrication strategy employed, influenced the optical properties of the core/alloyed shell QDs. The CdS/AgZnSe QDs produced in this work hold great promise in light-emitting optoelectronic applications.The Water Research Council (WRC) project K5/2752, South Africa and the University of Pretoria.http://www.elsevier.com/locate/mssphj2020Chemistr