Functional ZnS:Mn(II) quantum dot modified with L-cysteine and 6-mercaptonicotinic acid as a fluorometric probe for copper(II)

Patulin-imprinted origami 3D-ePAD based on graphene screen-printed electrode modified with Mn-ZnS quantum dot coated with a molecularly imprinted polymer

We developed a novel, compact, three-dimensional electrochemical paper-based analytical device (3D-ePAD) for patulin (PT) determination. The selective and sensitive PT-imprinted Origami 3D-ePAD was constructed based on a graphene screen-printed electrode modified with manganese-zinc sulfide quantum dots coated with patulin imprinted polymer (Mn-ZnS QDs@PT-MIP/GSPE). The Mn-ZnS QDs@PT-MIP was synthesized using 2-oxindole as the template, methacrylic acid (MAA) as a monomer, N,N'-(1,2-dihydroxyethylene) bis (acrylamide) (DHEBA) as cross-linker and 2,2'-azobis (2-methylpropionitrile) (AIBN) as initiator, respectively. The Origami 3D-ePAD was designed with hydrophobic barrier layers formed on filter paper to provide three-dimensional circular reservoirs and assembled electrodes. The synthesized Mn-ZnS QDs@PT-MIP was quickly loaded on the electrode surface by mixing with graphene ink and then screen-printing on the paper. The PT-imprinted sensor provides the greatest enhancement in redox response and electrocatalytic activity, which we attributed to synergetic effects. This arose from an excellent electrocatalytic activity and good electrical conductivity of Mn-ZnS QDs@PT-MIP, which improved electron transfer between PT and the electrode surface. Under the optimized DPV conditions, a well-defined PT oxidation peak appears at +0.15 V (vs Ag/AgCl) using 0.1 M of phosphate buffer (pH 6.5) containing 5 mM K₃Fe(CN)₆ as the supporting electrolyte. Our developed PT imprinted Origami 3D-ePAD revealed excellent linear dynamic ranges of 0.001-25 μM, with a detection limit of 0.2 nM. Detection performance indicated that our Origami 3D-ePAD possesses outstanding detection performance from fruits and CRM in terms of high accuracy (%Error for inter-day is 1.11%) and precision (%RSD less than 4.1%). Therefore, the proposed method is well-suited as an alternative platform for ready-to-use sensors in food safety. The imprinted Origami 3D-ePAD is an excellent disposable device with a simple, cost-effective, and fast analysis, and it is ready to use for determining patulin in actual samples.

A ratiometric luminescence sensing platform based on lanthanide-based silica nanoparticles for selective and sensitive detection of Fe3+ and Cu2+ ions

Detection of Fe(III) and Cu(II) in water is highly desirable because their abnormal levels can cause serious harm to human health and environmental safety. In this work, a ratiometric luminescence sensing platform based on lanthanide-based silica nanoparticles was constructed for the detection of Fe³⁺ and Cu²⁺ ions. The terbium-silica nanoparticles (named SiO₂@Tb) with dual-emission signals were successfully prepared by grafting Tb³⁺ ions onto trimellitic anhydride (TMA) functionalized silica nanospheres. It can serve as a ratiometric fluorescent probe for the detection of Fe³⁺ and Cu²⁺ ions in water with the green emission of Tb³⁺ ions as a response signal and the blue emission of silica nanospheres as the reference signal. Significantly, an easy-to-differentiate color change for visual detection was also realized. SiO₂@Tb shows high sensitivity even in very low concentration regions towards the sensing of Fe³⁺ and Cu²⁺ with low detection limits of 0.75 μM and 0.91 μM, respectively. Moreover, the mechanism for the luminescence quenching of SiO₂@Tb was systematically investigated, and was attributed to the synergetic effect of the absorption competition quenching (ACQ) mechanism and cation exchange. This study demonstrates that SiO₂@Tb can be employed as a promising fluorescent probe for the detection of Fe³⁺ and Cu²⁺ ions, and the combination of lanthanide ions with silica nanoparticles is an effective strategy to construct a ratiometric fluorescent sensing platform for the determination of analytes in environmental detection.

Colorimetric detection of heavy metal ions with various chromogenic materials: Strategies and applications

Detection of heavy metal ions has drawn significant attention in environmental and food area due to their threats to the human health and ecosystem. Colorimetry is one of the most frequently-used methods for the detection of heavy metal ions owing to its simplicity, easy operation and rapid on-site detection. The development of chromogenic materials and their sensing mechanisms are the key research direction in the area of colorimetric method. Since each chromogenic material has their unique optical and chemical properties, they have totally different colorimetric sensing mechanisms. This review focuses on the chromogenic materials and their sensing strategies for the colorimetric detection of heavy metal ions. We divide the chromogenic materials into three types, including organic materials, inorganic materials, and other materials. As for each type of chromogenic material, we discuss their detailed sensing strategies, sensing performance, and real sample applications. Moreover, current challenges and perspectives related to the colorimetry of heavy metal ions are also discussed in this review. The aim of this review is to help readers to better understand the principles of colorimetric methods for heavy metal ions and push the development of rapid detection of heavy metal ions.

Metal-Organic Frameworks Encapsulating Carbon Dots Enable Fast Speciation of Mono- and Divalent Copper

Copper is an essential element to play significant roles in human health associated to the strong redox properties of Cu(I) and Cu(II). The concurrent monitoring of copper species in biological matrixes is highly desired. Herein, a dual-channel fluorescence nanoprobe was designed for the speciation of mono- and divalent copper by conjugating carbon dots (CDs) with Eu-based metal-organic frameworks (Eu-MOFs). The obtained Eu-MOFs@CD nanoprobe exhibits fluorescence at λ_ex/λ_em = 380/454 nm from CDs and λ_ex/λ_em = 275/615 nm from Eu-MOFs. Bathocuproine disulfonate (BCS) specifically chelates Cu⁺ to produce a BCS-Cu⁺ adduct with absorption at 480 nm, which quenches the fluorescence of CDs at 454 nm due to the inner filter effect. On the other hand, Cu²⁺ quenches the fluorescence of Eu-MOFs due to the replacement of Eu³⁺ by Cu²⁺. Thus, Eu-MOFs@CDs enable extremely fast detection of Cu⁺ and Cu²⁺ within 1 min. Furthermore, the nanoprobe is demonstrated by monitoring the variation of Cu⁺ and Cu²⁺ in the degradation process of copper nanoparticles and Cu-based MOFs.

Multifunctional dendritic molecular probe for selective detection of Cu2+ ions using potentiometric and fluorometric techniques

We have designed and synthesized a multifunctional dendritic molecular probe that selectively detects Cu2+ ions via potentiometric and fluorometric techniques with low detection limits (3.5 μM in potentiometry, 15 nM in fluorometry). The selective and reversible binding of the molecule with the Cu2+ ion was used to make a solid-state microsensor (diameter of 25 μm) by incorporating the molecular probe into the carbon-based membrane as an ionophore for Cu(II). The Cu(II) microelectrode has a broad linear range of 10 μM to 1 mM with a near Nernstian slope of 30 mV/log [aCu2+] and detection limit of 3.5 μM. The Cu(II) microsensor has a fast response time (1.5 s), and it has a broad working pH range from 3.5 to 6.0. The incorporation of the hydrophobic dendritic moiety makes the ionophore less prone to leaching in an aqueous matrix for potentiometric measurement. The cinnamaldehyde component of the molecule helps detection of Cu2+ ions fluorometrically, as indicated by a change in fluorescence upon selective and reversible binding of the molecular probe to the Cu2+ ions. The strategic design of the molecular probe allows us to detect Cu2+ ions in drinking water by using this novel dendritic fluoroionophore and solid-state Cu2+ - ion-selective microelectrode.

Anchoring zinc-doped carbon dots on a paper-based chip for highly sensitive fluorescence detection of copper ions

In this work, zinc-doped carbon dots (Zn-CDs) were anchored on a three-dimensional wheel type paper-based microfluidic chip, and were decorated with 6-mercaptonicotinic acid (MNA) and L-cysteine (L-Cys) for highly sensitive and rapid fluorescence detection of Cu²⁺. Zn-CDs were first anchored on paper through the amide bonds between the carboxyl groups of the Zn-CDs and the amino groups of the paper. Afterwards, Zn-CDs were decorated with MNA and L-Cys, effectively preventing the Zn-CDs from aggregation. The nitrogen atom on the pyridine ring and the carboxylic acid groups in MNA and L-Cys coordinated with Cu²⁺ to form a nonfluorescent ground-state complex, causing the fluorescence quenching of the Zn-CDs. The three-dimensional rotary design could simplify the operation process and achieve simultaneous analysis of multiple samples with different concentrations. Under optimal conditions, the fluorescent sensor exhibits linear response for the determination of Cu²⁺ in the range from 0.1 to 60 μg L^-1 with the detection limit (LOD) of 0.018 μg L^-1. The proposed strategy provides a novel way for the highly sensitive detection of Cu²⁺ in a complex water environment.

Excellent fluorescence detection of Cu2+in water system using N-acetyl-L-cysteines modified CdS quantum dots as fluorescence probe

View of the negative influence of metal ions on natural environment and human health, fast and quantitative detection of metals ions in water systems is significant. Ultra-small grain size CdS quantum dots (QDs) modified with N-acetyl-L-cysteines (NALC) (NALC-CdS QDs) are successfully prepared via a facile hydrothermal route. Based on the changes of fluorescence intensity of NALC-CdS QDs solution after adding metal ions, the fluorescence probe made from the NALC-CdS QDs is developed to detect metal ions in water systems. Among various metal ions, the fluorescence of NALC-CdS QDs effectively quenched by the addition of Cu²⁺, the probe shows high sensitivity and selectivity for detecting Cu²⁺in other interferential metal ions coexisted system. Importantly, the fluorescence intensity of NALC-CdS QDs changes upon the concentration of Cu²⁺, the probe displays an excellent linear relationship between the fluorescence quenching rate and the concentration of Cu²⁺in ranging from 1 to 25μM. Besides, the detected limitation of the probe towards Cu²⁺as low as 0.48μM. The measurement of Cu²⁺in real water sample is also carried out using the probe. The results indicate that NALC-CdS QDs fluorescence probe may be a promising candidate for quantitative Cu²⁺detection in practical application.

pH-controlled preferential binding of cucurbit[7]uril-coated iron-oxide nanoparticles to 6-mercaptonicotinic acid for fluorescent detection of cadmium ions in the solid state

A host-guest complex of 6-mercaptonicotinic acid (MNA) and cucurbit[7]uril (CB7) was prepared and conjugated to γ-Fe₃O₄ nanoparticles (NPs) to detect toxic cadmium ions in water as a solid-state sensor. The formation of an inclusion host-guest complex with CB7 was confirmed by UV-vis absorption and proton NMR spectroscopy. CB7 preferentially binds the protonated MNA form compared to the neutral form, demonstrated by a binding constant for the protonated form that is four orders of magnitude higher than that of the neutral form. An increase in the pK_a of MNA by 1.2 units was demonstrated after the addition of CB7, which further supports preferential binding between MNA and CB7. The NMR results confirm binding to cadmium via the carboxylic acid moiety. Stationary and time-resolved fluorescence results, in solution and in the solid state, indicate that cadmium and CB7 cause a blue shift in the MNA emission bands and extend its excited-state lifetime due to dissociation of the MNA dimer. In the solid state, switching the emission signals between Cd²⁺-MNA/CB7NPs (ON state) and MNAH⁺/CB7NPs (OFF state) was achieved by controlling the pH. An efficient, regenerable, and stable sensor device was fabricated for sensitive and selective detection of Cd²⁺ in contaminated water samples. Graphical abstract Regeneration of MNA/CB7 nanoparticles for the detection of cadmium ions in the solid state by a visible blue emission signal upon suppression of photoinduced electron transfer (PET).

Silica-anchored cadmium sulfide nanocrystals for the optical detection of copper(II)

A fluorometric assay was developed for the determination of copper(II) ion based on its quenching effect on the green fluorescent probe of SiO₂-anchored CdS nanocrystals (SiO₂/CdS NCs). The fluorescent probe was prepared by a surface ion-directing strategy for homogeneous precipitation of CdS NCs onto the carboxyl-capped SiO₂ core surfaces. In comparison to CdS NCs, the SiO₂/CdS NCs has strong fluorescence emission and good photostability. Moreover, SiO₂/CdS NCs show higher fluorescence selectivity for copper(II) ions than for other common metal ions because copper(II) ions have a strong fluorescence quenching effect on SiO₂/CdS NCs. At excitation/emission wavelengths of 300/516 nm, the resulting fluorescent probe shows wide linear ranges from 0.01 to 2 μM with a detection limit of 6.3 nM for copper(II) ions. The method has been applied to the determination of trace copper(II) ions in tea infusions with satisfactory results. Graphical abstract.

Ethylenediamine-assisted growth of multi-dimensional ZnS nanostructures and study of its charge transfer mechanism on supercapacitor electrode and photocatalytic performance

In recent years, much interest has been raised by materials with multi-purpose characteristics as the performance of electrochemical energy devices such as supercapacitors and photocatalytic activities depend strongly on the properties of materials. This study delineates various parameters like morphology, energy band gap, charge transfer resistance, different defect states, diffusion coefficient and functional groups adsorbed on the surface of material to assess the performance of supercapacitor electrodes and photocatalytic degradation efficiency of synthesised multi-dimensional ZnS nanostructures. Ethylenediamine (EN)-mediated multi-dimensional ZnS nanostructures were grown by the solvothermal route. One-dimensional (1D), 2D and 3D morphologies were obtained by varying the ratio of de-ionised water and EN, taken as 1:3, 1:2 and 1:1, respectively. The EN molecules effectively capped most of the surfaces of the ZnS nanoparticles formed, preventing agglomeration of nanoparticles due to the decrement in surface energy. The oriented attachment of these clusters resulted in the formation of 1D, 2D and 3D morphologies. The plausible chemistry in the formation of 1D, 2D and 3D nanostructures has been elaborated. Charge transfer properties of prepared electrodes have been examined using the electrochemical impedance spectroscopy (EIS) technique because better charge transfer causes diminishing electron/hole recombination and hence better photodegradation efficiency. Among the synthesised materials, the 2D nanostructure degraded the eosin Y dye to maximum 90.71% efficiency with rate constant 34 × 10^-3 min^-1. 2D nanostructures possess better charge transfer and hence better photodegradation efficiency. Various studies using methods of UV-vis, Fourier-transform infrared, Brunauer-Emmett-Teller, x-ray photoelectron spectroscopy and photoluminescence spectra are in good agreement with the obtained photodegradation results. After analysing cyclic voltammetry curves and EIS, a higher diffusion coefficient is obtained for 1D nanostructure material, hence a higher specific capacitance and higher energy density of 159.12 F g^-1 and 22.75 KWh kg^-1 are found in this case. Only 9% loss of specific capacitance is found after 1000 cycles, showing a relatively high cycling stability in 3D nanostructures. The excellent supercapacitive property can be attributed to the porous structure and high specific surface area. Thus, the synthesised multi-dimensional ZnS nanostructures are proved to be a potential candidate for both photocatalytic and supercapacitor electrode performance.

CdSe quantum dots capped with a deep eutectic solvent as a fluorescent probe for copper(II) determination in various drinks

The present study shows that copper(II) ions can be determined with a new fluorescent probe that is based on the use of CdSe quantum dots capped with deep eutectic solvent (DES-CdSe QDs). The capped QDs were prepared in aqueous phase by a one-step procedure under ambient atmosphere using selenium dioxide as a stable precursor for selenium, and ascorbic acid as non-toxic reducing agent. The deep eutectic solvent is composed of choline chloride and thioglycolic acid and acts as stabilizing and functionalizing agent. The fluorescent probe undergoes an increase in the fluorescence intensity (with excitation/emission wavelengths at 380/560 nm) in the presence of Cu(II). Other ions display no significant effect on fluorescence. The effects of sample pH value, concentration of buffer, and volume of QDs solution were optimized by response surface methodology using a Box-Behnken statistical design. Under the optimal conditions, the response of the probe is linear in the 10-600 nM Cu(II) concentration range, with a 5.3 nM limit of detection. This is lower than the allowable maximum Cu(II) concentration in drinking water. The relative standard deviation of the method for five replicate measurements of Cu(II) at a 100 nM concentration level is 2.0%. The probe was successfully applied to the determination of Cu(II) in various drinks. Graphical abstractSchematic representation of a fluorometric method for the determination of Cu(II) at nanomolar concentration levels. The fluorescent system consists of deep eutectic solvent-capped cadmium selenide quantum dots (DES-CdSe QDs). Fluorescence is strongly enhanced by copper(II).

Fluorometric sensing of oxygen using manganese(II)-doped zinc sulfide nanocrystals

Manganese(II)-doped zinc sulfide nanocrystals (Mn:ZnS NCs) with dual-emission fluorescence (peaks at 445 nm and 590 nm under 330 nm excitation), good water stability and low toxicity were synthesized by hot injection. The fluorescence intensity of both emission bands of the nanocrystals can change rapidly by the content of gaseous and dissolved oxygen. The process is fully reversible. Compared with the maximum intensity of Mn:ZnS sensing film in 100% nitrogen, the emission of the blue emission decreases by 72% in the presence of 100% oxygen, and the yellow emission by 32%. Response is linear in the presence of 3% to 12% of oxygen percentage in gas. For water-dissolved oxygen, the linear response occurs between 0.54 and 11.4 mg·L^-1. Graphical abstractMn-doped ZnS NCs with dual-emission fluorescence were synthesized by hot-injection method. The reversible and rapid sensing characteristics of Mn-doped ZnS NCs to oxygen were studied, and the possible sensing mechanism was investigated.

Nanoparticle- and microparticle-based luminescence imaging of chemical species and temperature in aquatic systems: a review

Most aquatic systems rely on a multitude of biogeochemical processes that are coupled with each other in a complex and dynamic manner. To understand such processes, minimally invasive analytical tools are required that allow continuous, real-time measurements of individual reactions in these complex systems. Optical chemical sensors can be used in the form of fiber-optic sensors, planar sensors, or as micro- and nanoparticles (MPs and NPs). All have their specific merits, but only the latter allow for visualization and quantification of chemical gradients over 3D structures. This review (with 147 references) summarizes recent developments mainly in the field of optical NP sensors relevant for chemical imaging in aquatic science. The review encompasses methods for signal read-out and imaging, preparation of NPs and MPs, and an overview of relevant MP/NP-based sensors. Additionally, examples of MP/NP-based sensors in aquatic systems such as corals, plant tissue, biofilms, sediments and water-sediment interfaces, marine snow and in 3D bioprinting are given. We also address current challenges and future perspectives of NP-based sensing in aquatic systems in a concluding section. Graphical abstract ᅟ.

Fabrication of cefotaxime sodium-functionalized gold nanoclusters for the detection of copper ions in Chinese herbal medicines

Antibiotic-based gold nanoclusters (AuNCs) are a good sensing platform for specific recognition; however, the related studies are few. Herein, a simple and facile strategy was proposed for the fabrication of bright blue fluorescent AuNCs through the degradation product (DCTX) of cefotaxime sodium that induced the reduction of HAuCl₄. Various analytical techniques were applied to characterize the prepared AuNCs@DCTX. AuNCs@DCTX exhibited a strong emission peak centered at 420 nm and a quantum yield of 11.8%. Furthermore, an aggregation-induced fluorescence quenching mode endowed the AuNCs@DCTX probe with good specificity and sensitivity for Cu²⁺ detection. The proposed probe had a linear range of 0.01–40 μM, a precision with a relative standard deviation of 1.2% (n = 8), and a detection limit of 8 nM (signal/noise = 3). Interestingly, the probe could be reused through switching the “off” and “on” states by the addition of Cu²⁺ and EDTA. The practicality of the sensing platform was investigated for the determination of Cu²⁺ in four Chinese herbal medicines (CHMs), and the results were in accordance with those obtained by the FAAS method. This study has provided an alternate way for the fabrication of thiolate-protected AuNCs for sensing applications.

Antibiotic-based gold nanoclusters (AuNCs) are a good sensing platform for specific recognition; however, the related studies are few.