An organic-inorganic polyacrylamide-based surface imprinted quantum dots for the impedimetric and voltammetric detection of diazepam in saliva with smartphone readout

Diazepam (DZP) is a muscle-relaxing, anxiety-relieving sedative drug; nonetheless, it is also an addictive drug that may be abused. This work reports on the development of a novel electrochemical nanosensor for diazepam using SiO2-encapsulated-3-mercaptopropionic acid-capped AuZnCeSeS quantum dots (QDs) overcoated with a molecularly imprinted polymer (MIP) on screen-printed carbon electrodes (SPCEs). Electrochemical, spectroscopic and electron microscopic characterization of the nanomaterial and modified electrode surface was carried out and is reported herein. Specifically, electrochemical characterization of the QDs/SPCE using cyclic voltammetry (CV) revealed that the QDs exhibit a higher electrode surface area whilst electrochemical impedance spectroscopy (EIS) characterization demonstrated a lower charge transfer resistance (Rct). To fabricate the electrochemical nanosensor, firstly, alloyed AuZnCeSeS QDs were synthesized in the organic phase and thereafter capped with 3-mercaptopropionic acid (MPA) via a ligand exchange reaction. The MPA-AuZnCeSeS QDs were encapsulated in a SiO2 layer to form a SiO2-MPA AuZnCeSeS QDs system. The QDs were drop-casted onto SPCEs to form a SiO2-MPA AuZnCeSeS QDs/SPCE transducer interface. Organic based acrylamide, used as a functional monomer, was electropolymerized via CV on the QDs/SPCE in the presence of the diazepam template with ethylene glycol dimethacrylate as a crosslinker and 2,2'-azobis(2-methylpropionitrile) as an initiator. Under optimum experimental conditions, DZP was detected using EIS and square wave voltammetry (SWV). Using a portable potentiostat and a hand-held smartphone-based potentiostat, DZP was quantitatively detected in saliva using the MIP@QDs/SPCE with a limit of detection (LOD) of 2.3 μM and 2.7 μM, respectively. The LOD for DZP from SWV analysis was 1.0 μM.

Water/oil nanoemulsion-based synthesis of BixSn6-2xSy (0.33 ≤ × ≤ 2.95) semiconductor QDs for efficient photocatalytic degradation of MB dye

The development of efficient photocatalysts for the photodegradation of organic dyes in wastewater is highly worthwhile. Herein, the nanoemulsion tactic was utilized to synthesize Bi_xSn_6-2xS_y (0.33 ≤ x ≤ 2.95) photocatalysts with morphological structures that changed from nanowhiskers to quantum dots (QDs). The optical properties of these materials were examined by UV-visible absorbance spectroscopy and photoluminescence, while Mott-Schottky analysis was utilized to study their electronic properties. Bi_xSn_6-2xS_y materials exhibit appreciable absorption in the UV-visible light range with a direct band gap that increases from 1.23 to 1.46 eV. Both crystal structure and composition greatly affect the photocatalytic activity of Bi_xSn_6-2xS_y semiconductors. Among the various synthesized photocatalysts, BiSn₄S_4.5 can efficiently photodegrade methylene blue dye (MB) in the shortest time under UV-visible light. The photocatalytic activity is positively affected by the change of crystal structure from orthorhombic to cubic symmetry. Based on the Mott-Schottky plots, the flat band potential (E_fb) and the semiconductor behavior of the fabricated Bi_xSn_6-2xS_y nanomaterials were determined. The obtained E_fb values for SnS, Bi_0.33Sn_5.34S_5.8, BiSn₄S_5.5, and Bi_2.14Sn_1.71S_4.7 are -0.18 V, -0.42 V, -0.53 V, and -0.51 V (vs. Ag/AgCl), respectively. The E_fb value is clearly shifted towards more negative potential values with increasing the Bi molar ratio (x). However, Bi_2.95Sn_0.1S_4.5 semiconductor was found to be of n-type character, having a positive E_fb value of +0.66 V (vs. Ag/AgCl). Photocurrent and EIS responses confirm the high stability and photocatalytic activity of BiSn₄S_5.5, which also achieves the lowest charge transfer resistance. The modified electronic properties of the Bi_xSn_6-2xS_y semiconductors significantly improve their photocatalytic activity, rendering them to be promising absorbers for sunlight harvesting applications.

Tuning of (E)-(4-fluorophenyl)-1,1-diamino-2,3-diazabuta-1,3-diene nanostructures for the selective detection of imidacloprid

The present work demonstrates a facile route for synthesizing the organic nanoparticles (ONPs) and the blue fluorescent Quantum Dots (QDs) based on an organic molecule named (E)-(4-fluorophenyl)-1,1-diamino-2,3-diazabuta-1,3-diene. The synthesis process possesses advantages viz green synthesis, non-toxic degraded products, and amount of organic compound. Initially, the ONPs were prepared using the nanoprecipitation method and were screened for their recognition potential against various pesticides, however, no selectivity has been observed. This motivated us to tune the ONPs into QDs. The QDs were prepared using the hydrothermal method and a color change was observed in the QDs solution under daylight and under a UV lamp. The emission wavelength was observed at 400 nm (λexcitation = 278 nm). The synthesized QDs exhibited selective sensing potential towards imidacloprid via a quenching mechanism. A normalised decrement in the luminescence intensity of QDs was observed on raising the concentration of imidacloprid and a good linear response was noticed over a concentration varies from 1 μM to 100 μM with a regression coefficient of 0.99. The detection limit was estimated to be 4.53 nM and quantification limit was calculated to be and 13.72 nM.

Optical Dynamics of Copper-Doped Cadmium Sulfide (CdS) and Zinc Sulfide (ZnS) Quantum-Dots Core/Shell Nanocrystals

Recently, quantum-dot-based core/shell structures have gained significance due to their optical, optoelectronic, and magnetic attributes. Controlling the fluorescence lifetime of QDs shells is imperative for various applications, including light-emitting diodes and single-photon sources. In this work, novel Cu-doped CdS/ZnS shell structures were developed to enhance the photoluminescence properties. The objective was to materialize the Cu-doped CdS/ZnS shells by the adaptation of a two-stage high-temperature doping technique. The developed nanostructures were examined with relevant characterization techniques such as transmission electron microscopy (TEM) and ultraviolet–visible (UV–vis) emission/absorption spectroscopy. Studying fluorescence, we witnessed a sharp emission peak at a wavelength of 440 nm and another emission peak at a wavelength of 620 nm, related to the fabricated Cu-doped CdS/ZnS core/shell QDs. Our experimental results revealed that Cu-doped ZnS shells adopted the crystal structure of CdS due to its larger bandgap. Consequently, this minimized lattice mismatch and offered better passivation to any surface defects, resulting in increased photoluminescence. Our developed core/shells are highly appropriate for the development of efficient light-emitting diodes.

Metal Nanoparticle and Quantum Dot Tags for Signal Amplification in Electrochemical Immunosensors for Biomarker Detection

With the increasing importance of healthcare and clinical diagnosis, as well as the growing demand for highly sensitive analytical instruments, immunosensors have received considerable attention. In this review, electrochemical immunosensor signal amplification strategies using metal nanoparticles (MNPs) and quantum dots (Qdots) as tags are overviewed, focusing on recent developments in the ultrasensitive detection of biomarkers. MNPs and Qdots can be used separately or in combination with other nanostructures, while performing the function of nanocarriers, electroactive labels, or catalysts. Thus, different functions of MNPs and Qdots as well as recent advances in electrochemical signal amplification are discussed. Additionally, the methods most often used for antibody immobilization on nanoparticles, immunoassay formats, and electrochemical methods for indirect biomarker detection are overviewed.

An electrochemical tyrosinamide aptasensor using a glassy carbon electrode modified by N-acetyl-l-cysteine-capped Ag-In-S QDs

This paper reports an aptamer-based green approach for the electrochemical evaluation of tyrosinamide (Tyr-NH₂). In this regard, at the first step, an aqueous synthetic strategy for preparing N-acetyl-l-cysteine (NAC)-capped Ag-In-S (AIS) quantum dots (QDs) with bright yellow/orange emission was developed. The conjugation of AIS QDs to NAC-biomolecules provides opportunities for using them as luminescent contrast agents for living cell tracking and labeling or sensing studies. In the next step, the design stage of the aptasensor, the glassy carbon electrode (GCE) was modified with the AIS QDs and then the Tyr-NH₂ special aptamer, which has an amine group at its end, interacts with silver and indium ions at the surface of the AIS QDs and through the formation of covalent bonding of AgN and InN, attaches to the GCE surface modified with the AIS QDs. In this approach, for the first time, NAC-capped AIS QDs have been used to modify the electrode surface in the aptamer-based electrochemical sensor. The response changes of the [Fe(CN)₆]^4-/3- as redox probe, during the modification of GCE surface, the fabrication and assessment of proposed aptasensing, using the cyclic voltammetry, differential pulse voltammetry and electrochemical impedance spectroscopy were recorded. The designed aptasensor for the Tyr-NH₂ evaluation showed good linearity from 0.01 to 2.81 nM and 2.81-10.81 nM, and low detection limit of 3.34 pM. The obtained results of the stability, reproducibility and selectivity investigations implying that the reported aptasensor as the first aptamer-based electrochemical assay for Tyr-NH₂, can be reliable for the determination of Tyr-NH₂ in serum samples.

Optical Sensors Based on II-VI Quantum Dots

Fundamentals of quantum dots (QDs) sensing phenomena show the predominance of these fluorophores over standard organic dyes, mainly because of their unique optical properties such as sharp and tunable emission spectra, high emission quantum yield and broad absorption. Moreover, they also indicate no photo bleaching and can be also grown as no blinking emitters. Due to these properties, QDs may be used e.g., for multiplex testing of the analyte by simultaneously detecting multiple or very weak signals. Physico-chemical mechanisms used for analyte detection, like analyte stimulated QDs aggregation, nonradiative Förster resonance energy transfer (FRET) exhibit a number of QDs, which can be applied in sensors. Quantum dots-based sensors find use in the detection of ions, organic compounds (e.g., proteins, sugars, volatile substances) as well as bacteria and viruses.

Specific bioanalytical optical and photoelectrochemical assays for detection of methanol in alcoholic beverages

Methanol is a poison which is frequently discovered in alcoholic beverages. Innovative methods to detect methanol in alcoholic beverages are being constantly developed. We report for the first time a new strategy for the detection of methanol using fluorescence spectroscopy and photoelectrochemical (PEC) analysis. The analytical system is based on the oxidation of cysteine (CSH) with hydrogen peroxide (H₂O₂) enzymatically generated by alcohol oxidase (AOx). H₂O₂ oxidizes capping agent CSH, modulating the growth of CSH-stabilized cadmium sulphide quantum dots (CdS QDs). Disposable screen-printed carbon electrodes (SPCEs) modified with a conductive osmium polymer (Os-PVP) complex were employed to quantify resulting CdS QDs. This polymer facilitates the "wiring" of in situ enzymatically generated CdS QDs, which photocatalyze oxidation of 1-thioglycerol (TG), generating photocurrent as the readout signal. Likewise, we proved that our systems did not suffer from interference by ethanol. The PEC assays showed better sensitivity than conventional methods, covering a wide range of potential applications for methanol quantification.

3D Printed Graphene Based Energy Storage Devices

3D printing technology provides a unique platform for rapid prototyping of numerous applications due to its ability to produce low cost 3D printed platforms. Herein, a graphene-based polylactic acid filament (graphene/PLA) has been 3D printed to fabricate a range of 3D disc electrode (3DE) configurations using a conventional RepRap fused deposition moulding (FDM) 3D printer, which requires no further modification/ex-situ curing step. To provide proof-of-concept, these 3D printed electrode architectures are characterised both electrochemically and physicochemically and are advantageously applied as freestanding anodes within Li-ion batteries and as solid-state supercapacitors. These freestanding anodes neglect the requirement for a current collector, thus offering a simplistic and cheaper alternative to traditional Li-ion based setups. Additionally, the ability of these devices’ to electrochemically produce hydrogen via the hydrogen evolution reaction (HER) as an alternative to currently utilised platinum based electrodes (with in electrolysers) is also performed. The 3DE demonstrates an unexpectedly high catalytic activity towards the HER (−0.46 V vs. SCE) upon the 1000th cycle, such potential is the closest observed to the desired value of platinum at (−0.25 V vs. SCE). We subsequently suggest that 3D printing of graphene-based conductive filaments allows for the simple fabrication of energy storage devices with bespoke and conceptual designs to be realised.

Microbead QD-ELISA: Microbead ELISA Using Biocatalytic Formation of Quantum Dots for Ultra High Sensitive Optical and Electrochemical Detection

Electrochemical detection strategies employing semiconductor quantum dots (QDs) open up new opportunities for highly sensitive detection of biological targets. We designed a new assay based on microbead linked enzymatic generation of CdS QDs (Microbead QD-ELISA) and employed it in optical and electrochemical affinity assays for the cancer biomarker superoxide dismutase 2 (SOD2). Biotinylated antibodies against SOD2 were immobilized on the surface of polyvinyl chloride microbeads bearing streptavidin. In order to prevent any non-specific adsorption the microbeads were further blocked with bovine serum albumin. The analyte, SOD2 was captured on microbeads and labeled with alkaline phosphatase-conjugated antibody linked with mouse antibody against SOD2. Hydrolysis of para-nitrophenylphosphate by immobilized alkaline phosphatase triggered the rapid formation of phosphate-stabilized CdS QDs on the surface of microbeads. The resulting semiconductor nanoparticles were detected by fluorescence spectroscopy, microscopy, and square-wave voltammetry (SWV). The electrochemical assay based on the detection with square-wave voltammograms of Cd²⁺ ions originating from immobilized CdS QDs showed linearity up to 45 ng mL^-1, and the limit of SOD2 detection equal to 0.44 ng mL^-1 (1.96 × 10^-11 M). This detection limit is lower by 2 orders of magnitude in comparison with that of other previously published assays for superoxide dismutase. The electrochemical assay was validated with HepG2 (Human hepatocellular carcinoma) cell lysate containing SOD2.

Determining the core, corona, and total size of CdSeS/ZnS quantum dots by SEC/QELS and TEM

The size (hydrodynamic or Stokes radius, R H) of non-functionalized CdSeS/ZnS (core/shell) quantum dots (QDs) was characterized by size-exclusion chromatography with on-line quasi-elastic light scattering (SEC/QELS). Accurate determination of the size of QDs is important, because many of the optical properties of these materials are size dependent. A clear advantage of SEC/QELS over many batch techniques (e.g., QELS without separation) is the capability of the hyphenated technique to characterize the entire size range of a disperse sample, rather than merely providing a statistical average of the sizes present. Here, the SEC/QELS-determined R H values of CdSeS/ZnS QDs are compared to those determined by a traditional SEC experiment employing a calibration curve based on polystyrene standards, providing for the first reported study on SEC/QELS of non-functionalized QDs while also demonstrating the shortcomings of the widely-employed calibration curve approach. Furthermore, combining the R H of the QDs obtained by SEC/QELS with core size measurements derived from transmission electron microscopy allowed further calculation of the size of the QDs' coronas. The latter result was found to be in close agreement to the previously measured dimension of the main corona constituent, as well as with the calculated size of this constituent.

Nanobiosensors in diagnostics

Medical diagnosis has been greatly improved thanks to the development of new techniques capable of performing very sensitive detection and quantifying certain parameters. These parameters can be correlated with the presence of specific molecules and their quantity. Unfortunately, these techniques are demanding, expensive, and often complicated. On the other side, progress in other fields of science and technology has contributed to the rapid growth of nanotechnology. Although being an emerging discipline, nanotechnology has raised huge interest and expectations. Most of the enthusiasm comes from new possibilities and properties of nanomaterials. Biosensors (simple, robust, sensitive, cost-effective) combined with nanomaterials, also called nanobiosensors, are serving as bridge between advanced detection/diagnostics and daily/routine tests. Here we review some of the latest applications of nanobiosensors in diagnostics field.

Size-exclusion chromatography of metal nanoparticles and quantum dots

This review presents an overview of size-exclusion chromatographic separation and characterization of noble metal nanoparticles (NPs) and quantum dots (QDs) over the past 25 years. The properties of NPs and QDs that originate from quantum and surface effects are size dependent; to investigate these properties, a separation technique such as size-exclusion chromatography (SEC) is often needed to obtain narrow distribution NP populations that are also separated from the unreacted starting materials. Information on the size distributions and optical properties of NPs have been obtained by coupling SEC to detection methods such as ultraviolet-visible and/or fluorescence spectroscopy. Problems associated with the sorption of NPs and QDs onto various SEC stationary phases, employing both aqueous and organic eluents, are also discussed here.

Electrochemical Immunoassay of Escherichia coli O157:H7 Using Ag@SiO2 Nanoparticles as Labels

Silica coated silver (Ag@SiO2) nanoparticles were prepared and characterized by transmission electron microscope (TEM) and UV-vis absorption, and the nanoparticles were used as labels in sandwich-type immunosensor of Escherichia coli O157:H7 (E. coli O157:H7). The labels involved in immunoreaction were dissolved by mixed acid of hydrofluoric acid and nitric acid, and the released Ag(+) ions were electrochemical stripping analyzed (via differential pulse voltammetry, DPV) at poly(acrylic acid)/poly(diallyldimethylammonium chloride)/carbon nanotubes (PAA/PDCNT) modified glass carbon electrode (GCE), which obviously enhanced the signal of Ag(+) stripping. Then, the number of E. coli O157:H7 could be indirectly reflected by the signal intensity of labeled Ag(+). And the results showed that the DPV signals were proportional to the logarithm of the E. coli O157:H7 concentration in the range from 20 cfu/mL to 8.0 × 10(3) cfu/mL with the detection limit of 13 cfu/mL.

In situ anodic growth of rod-like TiO2 coating on a Ti wire as a selective solid-phase microextraction fiber

A novel rod-like TiO₂ based SPME coating was directly fabricated by in situ anodization of Ti wire (Fig. 1). It has larger surface area and longer service time for sensitive determination of ultraviolet filters in environmental water samples.