Determination of arsenic based on quenching of CdS quantum dots fluorescence using the gas-diffusion flow injection method

Arsenic field test kits based on solid-phase fluorescence filter effect induced by silver nanoparticle formation

Millions of people worldwide are affected by naturally occurring arsenic in groundwater. The development of a low-cost, highly sensitive, portable assay for rapid field detection of arsenic in water is important to identify areas for safe wells and to help prioritize testing. Herein, a novel paper-based fluorescence assay was developed for the on-site analysis of arsenic, which was constructed by the solid-phase fluorescence filter effect (SPFFE) of AsH3-induced the generation of silver nanoparticles (AgNPs) toward carbon dots. The proposed SPFFE-based assay achieves a low arsenic detection limit of 0.36 μg/L due to the efficient reduction of Ag+ by AsH3 and the high molar extinction coefficient of AgNPs. In conjunction with a smartphone and an integrated sample processing and sensing platform, field-sensitive detection of arsenic could be achieved. The accuracy of the portable assay was validated by successfully analyzing surface and groundwater samples, with no significant difference from the results obtained through mass spectrometry. Compared to other methods for arsenic analysis, this developed system offers excellent sensitivity, portability, and low cost. It holds promising potential for on-site analysis of arsenic in groundwater to identify safe well locations and quickly obtain output from the global map of groundwater arsenic.

A highly selective self-powered sensor based on the upconversion nanoparticles/CdS nanospheres for chlorpyrifos detection

Light sources are crucial for photoelectrochemical (PEC) self-powered sensing, where visible light is widely used. However, due to its high energy, it has some downsides as an irradiation source for overall system, so it is urgent to achieve effective near-infrared (NIR) light absorption because it makes up a significant portion of the solar spectrum. Herein, up-conversion nanoparticles (UCNPs) that could increase the energy of low-energy radiation were combined with semiconductor CdS as the photoactive material (UCNPs/CdS), which broadens the response range of solar spectrum. The NIR light-excited self-powered sensor could be produced via oxidizing H2O at photoanode and lowering dissolved oxygen at cathode under the NIR light without external voltage. Meanwhile, molecularly imprinted polymer (MIP) was added to photoanode as a recognition element to increase the sensor's selectivity. The open-circuit voltage of the self-powered sensor grew linearly as chlorpyrifos concentration climbed from 0.01 to 100 ng mL-1, showing good selectivity as well as reproducibility. This work provides valuable basis for the preparation of efficient and practical PEC sensor with NIR light response.

Nanomaterials-enriched sensors for detection of chiral pharmaceuticals

Advancements in nanoscience and nanotechnology have opened new pathways to fabricate novel nanostructures with interesting properties that would be used for different applications. In this respect, nanostructures comprising chirality are one of the most rapidly developing research fields encompassing chemistry, physics and biology. Chirality, also known as mirror asymmetry, describes the geometrical property of an object that is not superimposable on its mirror image. This characteristic plays a crucial role because these identical forms of chiral species in pharmaceuticals or food additives may exhibit different effects on living organisms. Therefore, chiral analysis is an important field of modern chemical analysis in health-related industries that are reliant on the production of enantiomeric compounds involving pharmaceuticals. This review covers the recent advances dealing with the synthesis, design and advantageous analytical performance of nanomaterials-enriched sensors used for chiral pharmaceuticals. We conclude this review with the challenges existing in this research field and our perspectives on some potential strategies with cutting-edge approaches for the rational design of sensors for chiral pharmaceuticals. We expect this comprehensive review will inspire future studies in nanomaterials-enriched chiral sensors.

Photoelectrochemical detection of microRNAs based on target-triggered self-assembly of energy band position-matched CdS QDs and C3N4 nanosheets

An ultrasensitive photochemical biosensor based on the target miRNA-triggered catalytic hairpin assembly (CHA) reaction between Au nanoparticles (AuNPs)/C₃N₄ nanosheets and CdS quantum dots (QDs) was developed for the determination of miRNAs. Firstly, AuNPs/C₃N₄ nanosheets were immobilized onto a working glassy carbon electrode. Then, the hairpin probe 1 (H1) was loaded through Au-S bonding. Afterward, the unbound sites were blocked with 6-mercaptohexanol to avoid nonspecific adsorption. In the presence of the target miRNA, the CHA reaction between the H1 and hairpin probe 2-CdS QDs (H2-CdS QDs) could be triggered. As a result, the AuNPs/C₃N₄ nanosheet and CdS QDs were linked by the double helix structure H1-H2. Unlike the other CHA reactions, H2 used in this work is longer than H1 so that the AuNPs/C₃N₄ nanosheets could touch the CdS QDs. Given the matched energy band positions between the C₃N₄ nanosheet and CdS QDs, a strong photocurrent could be obtained after the CHA reaction was triggered by the target miRNA. In addition, p-type C₃N₄ nanosheets and n-type CdS QDs presented reduction photocurrents and oxidation photocurrents, respectively. Therefore, the photocurrents were vectors in this design that can eliminate the interference of nonspecific adsorption and avoid the generation of false-positive signals. Under the optimal conditions, the limit of detection was 92 aM. The constructed photoelectrochemical biosensor showed good reproducibility and selectivity in the analysis of serum samples, which indicates its great prospects in disease diagnostics and bioanalysis.

Development of a microfluidic membraneless vaporization flow system for trace analysis of arsenic

A new design of a membraneless vaporization (MBL-VP) unit coupled with a specific flow system is presented for the determination of arsenic at trace levels using a hydride generation process. The MBL-VP unit contains two concentric conical reservoirs, with the outer cone selected as the donor reservoir. The volume of the outer donor reservoir is thereby greater than the acceptor volume, necessary for holding sufficient sample and reagents for the generation of arsine gas by reaction between As(iii) and sodium borohydride under acidic conditions. The arsine gas diffuses into the narrow headspace and is absorbed by an aliquot of 150 μL of mercuric chloride acceptor solution. The resulting reaction produces hydronium ions which is monitored by the absorbance change at 530 nm of the methyl orange indicator added in the acceptor solution. To decrease the detection limit, the aspiration and removal of the donor plug, comprising the sample, borohydride and acid, into and out of the donor cone are repeated several times, while the acceptor solution is kept unchanged. As a result, analysis of arsenic was achieved in the range of 10 to 100 μg L-1 with a detection limit of 8 μg L-1. Application to surface water was investigated. Percent recoveries of spiked surface water samples were in the range of 94-110%. For comparison of total arsenic (As(iii) and As(v)), the results obtained from the developed method are not statistically different from the ICP-OES method.

An electrochemiluminescence sensor for 17β-estradiol detection based on resonance energy transfer in α-FeOOH@CdS/Ag NCs

An electrochemiluminescence (ECL) resonance energy transfer system is constructed for 17β-estradiol (E2) detection using α-FeOOH@CdS nanospheres as the ECL-active substrates and Ag NCs as an efficient quencher. CdS QDs loaded onto three-dimensional (3D) urchin-like α-FeOOH nanospheres (α-FeOOH@CdS nanospheres) exhibited excellent ECL responses, which is attributed to dual-amplification of α-FeOOH frameworks. The 3D hierarchical structure of the α-FeOOH nanospheres provided abundant sites for loading ECL-active species, thus significantly improving the ECL performance of substrates; While Fe³⁺ presented on surface of α-FeOOH nanospheres could be reduced to Fe²⁺ in negative potentials, after which might activate persulfate in a Fenton-like process, resulting in more sulfate free radicals for more effective ECL responses via electron transfer reactions. Additionally, Ag nanoclusters (Ag NCs) stabilized by single stranded oligonucleotide were introduced as quenching probes for CdS QDs owing to the well-matched donor-acceptor spectrum for efficient energy transfer, which makes them appropriate for detection of E2. The proposed strategy displayed a desirable dynamic range from 0.01 to 10 pg mL^-1 with a limit of detection of 0.003 pg mL^-1. The proposed strategy based on the ECL-RET strategy offered an ideal way for E2 detection, and also revealed an alternative platform for detection of other small molecules.

Portable smartphone-integrated paper sensors for fluorescence detection of As(III) in groundwater

Arsenic contamination in groundwater is a supreme environmental problem, and levels of this toxic metalloid must be strictly monitored by a portable, sensitive and selective analytical device. Herein, a new system of smartphone-integrated paper sensors with Cu nanoclusters was established for the effective detection of As(III) in groundwater. For the integration system, the fluorescence emissive peak of Cu nanoclusters at 600 nm decreased gradually with increasing As(III) addition. Meanwhile, the fluorescence colour also changed from orange to colourless, and the detection limit was determined as 2.93 nM (0.22 ppb) in a wide detection range. The interfering ions also cannot influence the detection selectivity of As(III). Furthermore, the portable paper sensors based on Cu nanoclusters were fabricated for visual detection of As(III) in groundwater. The quantitative determination of As(III) in natural groundwater has also been accomplished with the aid of a common smartphone. Our work has provided a portable and on-site detection technique toward As(III) in groundwater with high sensitivity and selectivity.

Selective and sensitive fluorescent nanoprobe based on AgInS2-ZnS quantum dots for the rapid detection of Cr (III) ions in the midst of interfering ions

We herein report a novel eco-friendly method for the fluorescent sensing of Cr (III) ions using green synthesized glutathione (GSH) capped water soluble AgInS₂-ZnS (AIS-ZnS) quantum dots (QDs). The as-synthesized AIS-ZnS QDs were speherical in shape with average diameter of ∼2.9 nm and exhibited bright yellow emission. The fluorimetric analyses showed that, compared to Cr (VI) ions and other 20 metal ions across the periodic table, AIS-ZnS QDs selectively detected Cr (III) ions via fluorescent quenching. In addition, AIS-ZnS QDs fluorescent nanoprobes exhibited selective detection of Cr (III) ions in the mixture of interfering divalent metal ions such as Cu (II), Pb (II), Hg (II), Ni (II). The mechanism of Cr (III) sensing investigated using HRTEM and FTIR revealed that the binding of Cr (III) ions with the GSH capping group resulted in the aggregation of QDs followed by fluorescence quenching. The limit of detection of Cr (III) ions was calculated to be 0.51 nM. The present method uses cadmium free QDs and paves a greener way for selective determination of Cr (III) ions in the midst of other ions in aqueous solutions.

A simple paper-based approach for arsenic determination in water using hydride generation coupled with mercaptosuccinic-acid capped CdTe quantum dots

This research aims to develop a simple paper-based device for arsenic detection in water samples where a hydride generation technique coupled with mercaptosuccinic acid-capped CdTe quantum dots (MSA-CdTe QDs) as a detection probe was applied to the detection system. MSA-CdTe QDs were coated on a paper strip, inserted into the cover cap of a reaction bottle, to react with the developed arsine gas. Fluorescent emission of the QDs was quenched upon the presence of arsenic in solutions, whereby only a small amount of the MSA-CdTe QDs was required. The excitation and emission wavelengths for fluorescent detection were 278.5 nm and 548.5 nm, respectively. The proposed system provided a limit of detection of 0.016 mg L-1 and a limit of quantitation of 0.053 mg L-1, and a detection range of 0.05-30.00 mg L-1. In addition, the tolerance level of the detection approach to interference by other vapor-generated species was successfully improved by placing another paper strip coated with a solution of saturated lead acetate in front of the detection paper strip. This developed approach offered a simple and fast, yet accurate and selective detection of arsenic contaminated in water samples. In addition, the mechanism of fluorescent quenching was also proposed.

Development of ultrasensitive and As(iii)-selective upconverting (NaYF4:Yb3+,Er3+) platform

Solid-phase, LRET-based NaYF₄:Yb³⁺,Er³⁺ platform for the ultrasensitive (1 nM) detection of arsenic.

A Colorimetric Probe Based on Functionalized Gold Nanorods for Sensitive and Selective Detection of As(III) Ions

A colorimetric probe for determination of As(III) ions in aqueous solutions on basis of localized surface plasmon resonance (LSPR) was synthesized. The dithiothreitol molecules with two end thiols covalently combined with Au Nanorods (AuNRs) with an aspect ratio of 2.9 by Au-S bond to form dithiothreitol coated Au Nanorods (DTT-AuNRs), acting as colorimetric probe for the determination of As(III) ions. With the adding of As(III) ions, the AuNRs will be aggregated and leading the longitudinal SPR absorption band of DTT-AuNRs decrease due to the As(III) ions can bind with three DTT molecules through an As-S linkage. The potential factors affect the response of DTT-AuNRs to As(III) ions including the concentration of DTT, pH values of DTT-AuNRs, reaction time and NaCl concentration were optimized. Under optimum assay conditions, the DTT-AuNRs colorimetric probe has high sensitivity towards As(III) ions with low detection limit of 38 nM by rules of 3σ/k and excellent linear range of 0.13–10.01 μM. The developed colorimetric probe shows high selectivity for As(III) ions sensing and has applied to determine of As(III) in environmental water samples with quantitative spike-recoveries range from 95.2% to 100.4% with low relative standard deviation of less than 4.4% (n = 3).

Ultrasensitive point-of-care testing of arsenic based on a catalytic reaction of unmodified gold nanoparticles

Ultrasensitive arsenic detection based on inhibition of a catalytic reaction between Rhodamine B and sodium borohydride.

A "Turn-On" thiol functionalized fluorescent carbon quantum dot based chemosensory system for arsenite detection

Carbon quantum dots (CQDs) have emerged out as promising fluorescent probes for hazardous heavy metals detection in recent past. In this study, water soluble CQDs were synthesized by facile microwave pyrolysis of citric acid & cysteamine, and functionalized with ditheritheritol to impart thiol functionalities at surface for selective detection of toxic arsenite in water. Microscopic analysis reveals that the synthesized CQDs are of uniform size (diameter ∼5nm) and confirmed to have surface SH groups by FT-IR. The functionalized probe is then demonstrated for arsenite detection in water by "Turn-On" read out mechanism, which reduces the possibility of false positive signals associated with "turn off' probes reported earlier. The blue luminescent functionalized CQDs exhibit increase in fluorescence intensity on arsenite addition in 5-100ppb wide detection range. The probe can be used for sensitive detection of arsenite in environmental water to a theoretical detection limit (3s) of 0.086ppb (R²=0.9547) with good reproducibility at 2.6% relative standard deviation. The presented reliable, sensitive, rapid fCQDs probe demonstrated to exhibit high selectivity towards arsenite and exemplified for real water samples as well. The analytical performance of the presented probe is comparable to existing organic & semiconductor based optical probes.

Circular dichroism sensor based on cadmium sulfide quantum dots for chiral identification and detection of penicillamine

A new chemical sensor based on the measuring of circular dichroism signal (CD) was fabricated from cysteamine capped cadmium sulfide quantum dots (Cys-CdS QDs). The chiral-thiol molecules, d-penicillamine (DPA) and l-penicillamine (LPA), were used to evaluate potentials of this sensor. Basically, DPA and LPA provide very low CD signals. However, the CD signals of DPA and LPA can be enhanced in the presence of Cys-CdS QDs. The CD spectra of DPA and LPA exhibited a mirror image profile. Parameters affecting the determination of DPA and LPA were thoroughly investigated in details. Under the optimized condition, the CD signals of DPA and LPA displayed a linear relationship with the concentrations of both enantiomers, ranging from 1 to 35 μM. Detection limits of this sensor were 0.49 and 0.74 μM for DPA and LPA, respectively. To demonstrate a potential application of this sensor, the proposed sensor was used to determine DPA and LPA in real urine samples. It was confirmed that the proposed detection technique was reliable and could be utilized in a broad range of applications.

A step towards mobile arsenic measurement for surface waters

Surface modified quantum dots (QDs) are studied using a bio-inspired cysteine rich ligand (glutathione, GSH) and their quenching response and selectivity to arsenic examined. As predicted from As(3+) binding with highly crosslinked phytochelatin-(PCn)-like molecules, better arsenic selectivity is obtained for a thicker more 3-dimensional GSH surface layer, with exposed sulfhydryl groups. A detection limit of at least 10 μM can be achieved using CdSe/ZnS core-shell QDs capped with this GSH structure. The system is also demonstrated using a mobile phone camera to record the measurement, producing a detection limit of 5 μM. However, copper remains the main interferent of concern. Water-soluble CdTe QDs show little sensitivity to As(3+) even with a GSH surface, but they remain sensitive to Cu(2+), allowing a copper baseline to be established from the CdTe measurement. Despite anticipating that spectrally non overlapping fluorescence would be required from the two types of QDs to achieve this, a method is demonstrated using RGB channels from a mobile phone and processing the raw data for CdTe QDs, with an emission wavelength of 600 nm, and CdSe/ZnS QDs, with emission maximum of 630 nm. It is shown that As(3+) measurement remains feasible at the WHO guideline value of 10 μg L(-1) up to a copper concentration of around 0.3 μM Cu(2+), which corresponds to the highest recorded level in a selection of large rivers world-wide.

An improved method for ratiometric fluorescence detection of pH and Cd2+ using fluorescein isothiocyanate-quantum dots conjugates

In this study, thioglycolic acid capped-CdTe quantum dots (QDs) were modified by polyethylenimine (PEI), and then combined with fluorescein isothiocyanate (FITC) to fabricate FITC-CdTe conjugates. The self-assembly of FITC, CdTe and PEI was ascribed to electrostatic interactions in aqueous solution. The resulting conjugates were developed toward two routes. In route one, ratiometric photoluminescence (PL) intensity of conjugates (IFITC/IQDs) was almost linear toward pH from 5.3 to 8.7, and a ratiometric PL sensor of pH was favorable obtained. In route two, firstly added S(2-) induced remarkable quenching of QDs PL peak (at the "OFF" state), which was restored due to following addition of Cd(2+) (at the "ON" state). In the conjugates, successive introduction of S(2-) and Cd(2+) hardly influenced on FITC PL peaks. According to this PL "OFF-ON" mode, a ratiometric PL method for the detection of Cd(2+) was achieved. Experimental results confirmed that the IFITC/IQDs exhibited near linear proportion toward Cd(2+) concentration in the range from 0.1 to 15μM, and the limit of detection was 12nM. Interferential experiments adequately testified that the proposed sensors of pH and Cd(2+) were practicable in real samples and complex systems. In comparison with conventional analytical techniques, the ratiometric PL method was simple, rapid, economic and highly selective.

Speciation of arsenic (III) and arsenic (V) based on quenching of CdS quantum dots fluorescence using hybrid sequential injection-stopped flow injection gas-diffusion system

A hybrid sequential injection-stopped flow injection system was developed for the speciation of arsenic based on the quenching of mercaptoacetic acid capped cadmium sulfide quantum dots (CdS-MAA QDs) fluorescence intensity. The analytical procedure involves the generation of arsine from As(III) by sodium borohydride in acetate buffer medium pH 6.0. The generated arsine (donor stream) diffuses across the PTFE membrane of the gas-diffusion unit into an acceptor stream and then interacts with CdS-MAA QDs. Total arsenic was determined after pre-reduction of As(V) to As(III) with 1% (m/v) mercaptoacetic acid. Concentration of As(V) in the sample solutions can be deduced from the difference of total arsenic and As(III). Optimization of the experimental conditions and instrumental parameters were investigated. Under optimal conditions, limits of detection were 20 μg L(-1) for As(III) and 40 μg L(-1) for As(V). Recoveries in the range 84-103% were obtained from sediment sample.

Application of quantum dots as analytical tools in automated chemical analysis: a review

Colloidal semiconductor nanocrystals or quantum dots (QDs) are one of the most relevant developments in the fast-growing world of nanotechnology. Initially proposed as luminescent biological labels, they are finding new important fields of application in analytical chemistry, where their photoluminescent properties have been exploited in environmental monitoring, pharmaceutical and clinical analysis and food quality control. Despite the enormous variety of applications that have been developed, the automation of QDs-based analytical methodologies by resorting to automation tools such as continuous flow analysis and related techniques, which would allow to take advantage of particular features of the nanocrystals such as the versatile surface chemistry and ligand binding ability, the aptitude to generate reactive species, the possibility of encapsulation in different materials while retaining native luminescence providing the means for the implementation of renewable chemosensors or even the utilisation of more drastic and even stability impairing reaction conditions, is hitherto very limited. In this review, we provide insights into the analytical potential of quantum dots focusing on prospects of their utilisation in automated flow-based and flow-related approaches and the future outlook of QDs applications in chemical analysis.