Rapid and ultrasensitive colorimetric detection of mercury(II) by chemically initiated aggregation of gold nanoparticles

Colorimetric response of lysine-caped gold/silver alloy nanocomposites for mercury(II) ion detection

A simple and rapid colorimetric assay for determination of mercury(II) ion (Hg²⁺) in aqueous solutions was developed based on aggregation of gold/silver alloy nanocomposites (Au/Ag NCs). Au/Ag NCs were aggregated after the addition of Hg²⁺ and the positively charged amino acid, lysine. The different aggregation degrees of Au/Ag NCs is indicated by variations in the absorption spectra and accompanied by a color change from orange yellow to yellowish green. Under the optimal conditions, the shift of absorbance ratio (A₆₅₀/A₄₄₇) was proportional to Hg²⁺ concentrations in the range of 0.01-10.0 μM. The limit of detection is 4.8 nM. The proposed colorimetric assay was extremely specific for Hg²⁺ and other environmentally relevant metal ions did not interfere with the determination.

Architecture of multi-channel and easy-to-make sensors for selective and sensitive Hg2+ ion recognition through Hg‒C and Hg‒N bonds of naphthoquinone-aniline/pyrene union

The aim of this work is, for the first time, to develop new inexpensive, easy-to-make and multi-channel receptors, naphthoquinone-aniline/pyrene union ((Nq-An) and (Nq-Pyr)) and their Hg²⁺ complexes [Hg-(Nq-An)₂] and [Hg-(Nq-Pyr)₂] to supply an efficient solution to critical deficiencies to be encountered for Hg²⁺ recognition. This study is based on colorimetric, fluorometric, and voltammetric methods for determination of Hg²⁺ ions through Hg-C and Hg-N binding mode of the naphthoquinone-aniline/pyrene union in aqueous media. The binding mode of the receptors with Hg²⁺ cation was confirmed by usual characterization techniques for the synthesized Hg²⁺-complexes [Hg-(Nq-An)₂] / [Hg-(Nq-Pyr)₂] and voltammetric, ¹H NMR titration experiments as well as Job's method, indicating a 2:1 complex between the receptors and Hg²⁺ cation. The receptors showed a considerable color switching from orange to pink along with a red-shift of absorption wavelength, and fluorescence enhancement via the Chelation Enhanced Fluorescence effect (CHEF), and distinctive changes on the voltammogram of the electroactive naphthoquinone unit with Hg²⁺ cation. The experiments indicate that the sensors are highly selective and sensitive toward Hg²⁺ among the studied metal ions in aqueous media compared with other reported Hg²⁺ sensors.

Limitations for colorimetric aggregation assay of metal ions and ways of their overcoming

The development of analytical methods for the determination of metal ions in water is one of the priority tasks for efficient environmental monitoring. The use of modified gold nanoparticles and the colorimetric detection of their aggregation initiated by ions binding with specific receptors on the nanoparticle surface has high potential for simple testing. However, the limits of this approach and the parameters determining the assay sensitivity are not clear, and the possibilities of different assay formats are estimated only empirically. We have proposed a mathematical description of the aggregation processes in the assay and have estimated the detection limits of an aptamer-based assay of Pb2+ ions theoretically and experimentally. In the studied assay, gold nanoparticles modified with G,T-enriched aptamer were used, and their aggregation caused by the interaction with Pb2+ ions was controlled via a color change. The experimentally determined limit of Pb2+ detection was 700 ppb, which was in good agreement with theoretical calculations. An examination of the model showed that the limiting parameter of the assay is the binding constant of the aptamer-Pb2+ ion interaction. To overcome this limitation without searching for alternate receptors, two methods have been proposed, namely additional aggregation-causing components or centrifugation. These approaches lowered the detection limit to 150 ppb and even to 0.4 ppb. The second value accords with regulatory demands for the permissible levels of water source contamination, and the corresponding approach has significant competitive potential due to its rapidity, simple implementation, and the visual assessment of the assay results.

A gold nanoparticle-based colorimetric mercury(II) biosensor using a DNA probe with phosphorothioate RNA modification and exonuclease III-assisted signal amplification

Herein, we report a rapid and sensitive colorimetric detection of Hg²⁺ by designing a specific DNA probe with phosphorothioate RNA modification (PS-probe) for Hg²⁺ recognition and utilizing DNA-modified gold nanoparticles (DNA-AuNPs) as the transducer. The distance between two DNA-AuNPs is controlled by a linker DNA, providing the linker DNA-regulated aggregation or dispersion status of AuNPs in solution. Exonuclease III (Exo III) can trigger the recycled digestion of linker DNA strands, inhibiting the reformation of aggregated nanoparticles and hence leading to a color shift from purple to red. However, the Hg²⁺-induced cleavage of the PS-probe can efficiently prevent the digestion of linker DNA strands by Exo III and hence reassemble the modified AuNPs to form aggregates in purple color. Thus, a positive correlation between the linker DNA strands left and the addition of Hg²⁺ provides a quantitative basis for Hg²⁺ sensing. A linear range of A₅₂₀/A₇₀₀ versus Hg²⁺ concentration is achieved in the range 2-100 nM associated with a detection limit as low as 1.30 ± 0.04 nM. Moreover, the biosensor exhibits excellent selectivity for Hg²⁺. The strong selectivity behavior was confirmed by recoveries ranging from 96 to 114% in real water samples. Graphical abstractSchematic representation of sensing mechanism of Hg²⁺ using a DNA probe with phosphorothioate RNA modification (PS-probe) and Exo III-assisted signal amplification.

Progress in rapid optical assays for heavy metal ions based on the use of nanoparticles and receptor molecules

This review (with 230 refs.) covers recent progress in rapid optical assays for heavy metals (primarily lead and mercury as the most relevant) based on the use of nanoparticles and receptor molecules. An introduction surveys the importance, regulatory demands (such as maximum permissible concentrations) and potential and limitations of various existing methods. This is followed by a general discussion on the use of nanoparticles in optical assays of heavy metals (including properties, basic mechanisms of signal generation). The next sections cover methods for the functionalization of nanoparticles with (a) sulfur-containing compounds (used for modification of nanoparticles or added to the reaction medium), (b) nitrogen-containing compounds (such as amino acids, polypeptides, and heterocyclic molecules), and (c) oxygen-containing species (such as hydroxy and carbonyl compounds). This is continued by a specific description of specific assays based on the use of aptamers as receptors, on the use of deoxyribozymes as synthetic reaction catalysts, of G-quadruplex aptamers, of aptamers in logic gate-type of assays of linear (unstructured) aptamers ("hairpins"), and on the use of aptamers in lateral flow assays. A next section covers assays based on the employment of antibodies as receptors (used in the immunoassay development). The properties of various nanoparticles and their applicability in optical assays are also discussed in some detail. Final sections discuss the selectivity of assays, potential interferences by other cations, methods for their elimination, and also matrix effects and approaches for sample pretreatment. A concluding section discusses current challenges and future trends. Analysis based on enzyme inhibition assay is not treated here but enzyme-like action of some receptor molecules such as DNAzymes is discussed. Graphical abstract Schematic presentation of main principles of application of various nanoparticles with receptor molecules (S-, N-, O-containing, heterocyclic compounds, proteins, antibody, aptamers) for heavy metals ions detection. The included methods cover optical assays with description of mechanisms of interactions and signal generation.

Colorimetric and visual mercury(II) assay based on target-induced cyclic enzymatic amplification, thymine-Hg(II)-thymine interaction, and aggregation of gold nanoparticles

A colorimetric biosensor and visual test is described for the determination of mercury(II). It relies on the specific thymine-Hg(II)-thymine (T-Hg²⁺-T) interaction which induces a cyclic amplification process (caused by the enzyme exonuclease III) and the aggregation of gold nanoparticles. These results in a color change from red to violet. Under optimized conditions, this colorimetric assay (best performed at 524 nm) has a detection limit as low as 0.9 nM with a detection range over 4 orders of magnitude (from 1 nM to 10 μM). Graphical abstract Schematic of a colorimetric method for determination of mercury ions (Hg²⁺) based on the thymine-Hg²⁺-thymine interaction-triggered cyclic enzymatic amplification and aggregation of gold nanoparticles with the aid of exonuclease III (Exo III).

Dual detection of bioaccumulated Hg2+ based on luminescent bacteria and aggregation-induced emission

We develop a dual detection strategy for bioaccumulated Hg²⁺ based on turn-off of the bioluminescence of P. phosphoreum bacteria by disrupting the quorum sensing system and turn-on of the photoluminescence of an aggregation-induced emission (AIE) probe by forming aggregates with Hg²⁺ inside the bacteria.

New portable smartphone-based PDMS microfluidic kit for the simultaneous colorimetric detection of arsenic and mercury

A smartphone-based microfluidic platform was developed for point-of-care (POC) detection using surface plasmon resonance (SPR) of gold nanoparticles (GNPs). The simultaneous colorimetric detection of trace arsenic and mercury ions (As3+ and Hg2+) was performed using a new image processing application (app). To achieve this goal, a microfluidic kit was fabricated using a polydimethylsiloxane (PDMS) substrate with the configuration of two separated sensing regions for the quantitative measurement of the color changes in GNPs to blue/gray. To fabricate the microfluidic kit, a Plexiglas mold was cut using a laser based on the model obtained from AutoCAD and Comsol outputs. The colorimetric signals originated from the formation of nanoparticle aggregates through the interaction of GNPs with dithiothreitol - 10,12-pentacosadiynoic acid (DTT-PCDA) and lysine (Lys) in the presence of As3+ and Hg2+ ions. This assembly exhibited the advantages of simplicity, low cost, and high portability along with a low volume of reagents and multiplex detection. Heavy Metals Detector (HMD), as a new app for the RGB reader, was programmed for an Android smartphone to quantify colorimetric analyses. Compared with traditional image processing, this app provided significant improvements in sensitivity, time of analysis, and simplicity because the color intensity is measured through a new normalization equation by converting RGB to an Integer system. As a simple, real-time, and portable analytical kit, the fabricated sensor could detect low concentrations of As3+ (710 to 1278 μg L-1) and Hg2+ (10.77 to 53.86 μg L-1) ions in water samples at ambient conditions.

Mentha-Stabilized Silver Nanoparticles for High-Performance Colorimetric Detection of Al(III) in Aqueous Systems

The present paper reports a facile and selective colorimetric method for the detection of potential environmental and health hazardous metal ions using green synthesized silver nanoparticles (AgNPs). Here the organic functional groups present in the plant extract (Mentha arvensis) are used as reductants and stabilizers in the synthesis of AgNPs. They also provide a suitable binding site to the (Al(III)) analyte in the detection mechanism. The leaf extract of Mentha arvensis was used to synthesize AgNPs at room-temperature and at 80 °C. The AgNPs synthesized at 80 °C exhibit excellent selective colorimetric detection of Al(III). The as-synthesized AgNPs have been characterized, and the synthesis, stabilization of NPs and detection mechanism has also been illustrated by using UV-vis, XPS, FTIR, TEM, EDX, SEM, AAS, and TGA analytical tools and techniques. The selectivity of detection probe was supported by the reaction between probe and metal ions followed first-order kinetics having the highest value of the regression coefficient (R² = 0.99) for Al(III) and the analysis of thermodynamic parameters. The prepared sensor showed a lower limit of detection (LOD) of 1 nM (S/N = 3.2) in real water samples. The proposed method can be successfully utilized for the detection of Al(III) from both drinking and real water samples at the nanomolar level.

Functionalization of silver nanoparticles with mPEGylated luteolin for selective visual detection of Hg2+ in water sample

A novel colorimetric sensor for selective detection of Hg²⁺ based on mPEGylated luteolin functionalized silver nanoparticles was prepared.

A dual-mode colorimetric and fluorometric “light on” sensor for thiocyanate based on fluorescent carbon dots and unmodified gold nanoparticles

A novel, highly sensitive and selective dual-readout (colorimetric and fluorometric) sensor based on fluorescent carbon dots (CDs) and unmodified gold nanoparticles (AuNPs) for the detection of thiocyanate (SCN⁻) was proposed.