Label-free colorimetric sensor for mercury(II) and DNA on the basis of mercury(II) switched-on the oxidase-mimicking activity of silver nanoclusters

In this paper, a novel colorimetric biosensor for Hg²⁺ and DNA molecules is presented based on Hg²⁺ stimulated oxidase-like activity of bovine serum albumin protected silver clusters (BSA-Ag NCs). Under mild conditions, Hg²⁺ activated BSA-Ag NCs to show high catalytic activity toward the oxidation of 3,3′,5, 5′-tetramethylbenzidine (TMB) using ambient dissolved oxygen as an oxidant. The oxidase-like activity of BSA-Ag NCs was “switched-on” selectively in the presence of Hg²⁺, which permitted a novel and facile colorimetric sensor for Hg²⁺. As low as 25 nmol L⁻¹ Hg²⁺ could be detected with a linear range from 80 nmol L⁻¹ to 50 mmol L⁻¹. In addition, the sensing strategy was also employed to detect DNA molecules. Hg²⁺ is known to bind very strongly and specifically with two DNA thymine bases (T) to form thymine–Hg²⁺–thymine (T–Hg²⁺–T) base pairs. The hairpin-structure was disrupted and Hg²⁺ ions were released after hybridization with the DNA target. By coupling the Hg²⁺ switched-on the oxidase-mimicking activity of BSA-Ag NCs, we developed a novel label-free strategy for facile and fast colorimetric detection of DNA molecules. More important, target DNA can be detected as low as 10 nmol L⁻¹ with a linear range from 30 to 225 nmol L⁻¹. Compared with other methods, this method presents several advantages such as the independence of hydrogen peroxide, high sensitivity and good selectivity, avoiding any modification or immobilization of DNA, which holds a great potential of metal NCs for clinical application in biosensing and biotechnology.     

Catalytic gold nanoparticles for fluorescent detection of mercury(II) and lead(II) ions

In this study, we developed a fluorescence assay for the highly sensitive and selective detection of Hg²⁺ and Pb²⁺ ions using a gold nanoparticle (Au NP)-based probe. The Hg–Au and Pb–Au alloys that formed on the Au NP surfaces allowed the Au NPs to exhibit peroxidase-mimicking catalytic activity in the H₂O₂-mediated oxidation of Amplex UltraRed (AUR). The fluorescence of the AUR oxidation product increased upon increasing the concentration of either Hg²⁺ or Pb²⁺ ions. By controlling the pH values of 5 mM tris–acetate buffers at 7.0 and 9.0, this H₂O₂–AUR–Au NP probe detected Hg²⁺ and Pb²⁺ ions, respectively, both with limits of detection (signal-to-noise ratio: 3) of 4.0 nM. The fluorescence intensity of the AUR oxidation product was proportional to the concentrations of Hg²⁺ and Pb²⁺ ions over ranges 0.05–1 μM (R² = 0.993) and 0.05–5 μM (R² = 0.996), respectively. The H₂O₂–AUR–Au NP probe was highly selective for Hg²⁺ (>100-fold) and Pb²⁺ (>300-fold) ions in the presence of other tested metal ions. We validated the practicality of this simple, selective, and sensitive H₂O₂–AUR–Au NP probe through determination of the concentrations of Hg²⁺ and Pb²⁺ ions in a lake water sample and of Pb²⁺ ions in a blood sample. To the best of our knowledge, this system is the first example of Au NPs being used as enzyme-mimics for the fluorescence detection of Hg²⁺ and Pb²⁺ ions.     

Ultrasensitive and rapid screening of mercury(II) ions by dual labeling colorimetric method in aqueous samples and applications in mercury-poisoned animal tissues

Rapid and ultrasensitive detection of trace heavy metal mercury(II) ions (Hg²⁺) are of significant importance due to the induced serious risks for environment and human health. This presented article reports the gold nanoparticle-based dual labeling colorimetric method (Dual-COLO) for ultrasensitive and rapid detection of Hg²⁺ using the specific thymine–Hg²⁺–thymine (T–Hg²⁺–T) as recognition system and the dual labeling strategy for signal amplification. Both qualitative and quantitative detections of Hg²⁺ are achieved successfully in aqueous samples. More importantly, the achieved detection limit of 0.005 ng mL⁻¹ (0.025 nM) without any instruments is very competitive to other rapid detection methods even ICP-MS based methods. This Dual-COLO method is also applied directly for real water sample monitoring and, more importantly, applied in analysis of mercury poisoned animal tissues and body fluidic samples, indicating a potentially powerful and promising tool for environmental monitoring and food safety control.     

Multiplex sensor for detection of different metal ions based on on–off of fluorescent gold nanoclusters

In this study, a multiplex fluorescence sensor for successive detection of Fe³⁺, Cu²⁺ and Hg²⁺ ions based on “on–off” of fluorescence of a single type of gold nanoclusters (Au NCs) is described. Any of the Fe³⁺, Cu²⁺ and Hg²⁺ ions can cause quenching fluorescence of Au NCs, which established a sensitive sensor for detection of these ions respectively. With the introduction of ethylene diamine tetraacetic acid (EDTA) to the system of Au NCs and metal ions, a restoration of fluorescence may be found with the exception of Hg²⁺. A highly selective detection of Hg²⁺ ion is, thus, achieved by masking Fe³⁺ and Cu²⁺. On the other hand, the masking of Fe³⁺ and Cu²⁺ leads to the enhancement of fluorescence of Au NCs, which in turn provides an approach for successive determination of Fe³⁺ and Cu²⁺ based on “on–off” of fluorescence of Au NCs. Moreover, this assay was applied to the successful detection of Fe³⁺, Cu²⁺ and Hg²⁺ in fish, a good linear relationship was found between these metal ions and the degree of quenched fluorescent intensity. The dynamic ranges of Hg²⁺, Fe³⁺ and Cu²⁺ were 1.96 × 10⁻¹⁰–1.01 × 10⁻⁹, 1.28 × 10⁻⁷–1.27 × 10⁻⁶ and 1.2 × 10⁻⁷–1.2 × 10⁻⁶ M with high sensitivity (the limit of detection of Fe³⁺ 2.0 × 10⁻⁸ M, Cu²⁺ 1.9 × 10⁻⁸ M and Hg²⁺ 2 × 10⁻¹⁰ M). These results indicate that the assay is suitable for sensitive detection of these metal ions even under the coexistence, which can not only determine all three kinds of metal ions successively but also of detecting any or several kinds of metal ions.     

A multi-channel sensor based on 8-hydroxyquinoline ferrocenoate for probing Hg(II) ion

A new sensing molecule 8-hydroxyquinoline ferrocenoate (Fc-Q) which combines ferrocene and 8-hydroxyquinoline moieties was synthesized and applied as a multi-channel sensor for the detection of Hg²⁺ ion. Fc-Q can coordinate with Hg²⁺ to give colorimetric, fluorescent and electrochemical responses. Upon complexation with Hg²⁺ ion, the characteristic absorption peak is red-shifted (Δλ = 45 nm), the fluorescent intensity is quenched at 303 nm, and the oxidation peak is cathodic shifted (ΔE_1/2 = −149 mV). Quantitatively analyzed Hg²⁺ ions at the range of ppb level could be achieved by electrochemical response. For the practical application of sensing Hg²⁺ in real world water, Fc-Q modified screen-printed carbon electrodes were obtained for facile, sensitive, and on-site analysis of Hg²⁺.     

A ratiometric electrochemical biosensor for sensitive detection of Hg based on thymine–Hg–thymine structure

In this paper, a simple, selective and reusable electrochemical biosensor for the sensitive detection of mercury ions (Hg²⁺) has been developed based on thymine (T)-rich stem–loop (hairpin) DNA probe and a dual-signaling electrochemical ratiometric strategy. The assay strategy includes both “signal-on” and “signal-off” elements. The thiolated methylene blue (MB)-modified T-rich hairpin DNA capture probe (MB-P) firstly self-assembled on the gold electrode surface via Au–S bond. In the presence of Hg²⁺, the ferrocene (Fc)-labeled T-rich DNA probe (Fc-P) hybridized with MB-P via the Hg²⁺-mediated coordination of T–Hg²⁺–T base pairs. As a result, the hairpin MB-P was opened, the MB tags were away from the gold electrode surface and the Fc tags closed to the gold electrode surface. These conformation changes led to the decrease of the oxidation peak current of MB (I_MB), accompanied with the increase of that of Fc (I_Fc). The logarithmic value of I_Fc/I_MB is linear with the logarithm of Hg²⁺ concentration in the range from 0.5 nM to 5000 nM, and the detection limit of 0.08 nM is much lower than 10 nM (the US Environmental Protection Agency (EPA) limit of Hg²⁺ in drinking water). What is more, the developed DNA-based electrochemical biosensor could be regenerated by adding cysteine and Mg²⁺. This strategy provides a simple and rapid approach for the detection of Hg²⁺, and has promising application in the detection of Hg²⁺ in real environmental samples.     

A highly selective nanogold-aptamer catalytic resonance scattering spectral assay for trace Hg using HAuCl4-ascorbic acid as indicator reaction

Single strand DNA (ssDNA) was used to modify nanogold to obtain a nanogold-aptamer resonance scattering (RS) probe (NGssDNA) for Hg²⁺, based on the formation of stable thymine-Hg²⁺-thymine (T-Hg²⁺-T) mismatches and aggregation of the released nanogold particles. After removing the aggregated particles by filtrate membrane, the excess NGssDNA in the filtration solution exhibit catalytic effect on the gold particle reaction between HAuCl₄ and ascorbic acid (AA) that appear as RS peak at 596 nm. When Hg²⁺ concentration increased, the RS intensity at 596 nm decreased. The decreased intensity is linear to Hg²⁺ concentration in the range of 0.00008-0.888 ng/mL Hg²⁺, with detection limit of 0.000034 ng/mL. The nanogold-aptamer catalytic RS assay was applied to determination of Hg²⁺ in water with satisfactory results.     

Synthesis of cysteamine-coated CdTe quantum dots and its application in mercury (II) detection

High-quality cysteamine-coated CdTe quantum dots (CA-CdTe QDs) were successfully synthesized in aqueous phase by a facile one-pot method. Through hydroxylamine hydrochloride-promoted kinetic growth strategy, water-soluble CA-CdTe QDs could be obtained conveniently in a conical flask by a stepwise addition of raw materials. The photoluminescence quantum yield (PL QY) of the obtained QDs reached 9.2% at the emission peak of 520 nm. The optical property and the morphology of the QDs were characterized by UV–vis absorption spectra, photoluminescence spectra (PL) and transmission electron microscopy (TEM) respectively. Furthermore, the fluorescence of the resultant QDs was quenched by copper (II) (Cu²⁺) and mercury (II) (Hg²⁺) meanwhile. It is worthy of note that to separately detect Hg²⁺, cyanide ion could be used to eliminate the interference of Cu²⁺. Under the optimal conditions, the response was linearly proportional to the logarithm of Hg²⁺ concentration over the range of 0.08–3.33 μM with a limit of detection (LOD) of 0.07 μM.     

Fluorescence detection of mercury(II) and lead(II) ions using aptamer/reporter conjugates

We have developed a fluorescence technique for the detection of Hg²⁺ and Pb²⁺ ions using polythymine (T₃₃)/benzothiazolium-4-quinolinium dimer derivative (TOTO-3) and polyguanine (G₃₃)/terbium ions (Tb³⁺) conjugates, respectively. Hg²⁺ ions induce T₃₃ to form folded structures, leading to increased fluorescence of the T₃₃/TOTO-3 conjugates. Because Pb²⁺ ions compete with Tb³⁺ ions to form complexes with G₃₃, the extent of formation of the G₃₃-Tb³⁺ complexes decreases upon increasing the Pb²⁺ concentration, leading to decreased fluorescence at 545 nm when excited at 290 nm. To minimize interference from Hg²⁺ ions during the detection of Pb²⁺ ions, we conducted two-step fluorescence measurements; prior to addition of the G₃₃/Tb³⁺ probe, we recorded the fluorescence of a mixture of the T₃₃/TOTO-3 conjugates and Hg²⁺ ions. The fluorescence signal obtained was linear with respect to the Hg²⁺ concentration over the range 25.0-500 nM (R² = 0.99); for Pb²⁺ ions, it was linear over the range 3.0-50 nM (R² = 0.98). The limits of detection (at a signal-to-noise ratio of 3) for Hg²⁺ and Pb²⁺ ions were 10.0 and 1.0 nM, respectively. Relative to other techniques for the detection of Hg²⁺ and Pb²⁺ ions in soil and water samples, our present approach is simpler, faster, and more cost-effective.     

A label-free and signal-on supersandwich fluorescent platform for Hg sensing

A label-free supersandwich fluorescent assay was demonstrated for the first time by taking Hg²⁺ as a detection candidate. The principle of the proposed supersandwich fluorescent platform is based on the formation of supersandwich structure by T-Hg²⁺-T coordination and the fluorescence enhancement of the intercalated Genefinder (GF) in double strand DNA (dsDNA). Such supersandwich fluorescent DNA sensor exhibits a linear range of 10–300 nM for the detection of Hg²⁺, with a detection limit of 2.5 nM on the basis of the 3σ/slope (σ represents the standard deviation of the blank samples), which is well below the permit of the U.S. Environmental Protection Agency (<10 nM). The detection can be fulfilled in less than 10 min. The proposed mix-and-detect fluorescent platform exhibits excellent sensitivity, selectivity, and convenient manipulation. The assay was successfully used to detect Hg²⁺ in the lake water samples, which suggested its potential in practical samples.     

A fluorescent chemosensor for Hg based on naphthalimide derivative by fluorescence enhancement in aqueous solution

Naphthalimide derivative (compound 1) containing hydrophilic hexanoic acid group was synthesized and used to recognize Hg²⁺ in aqueous solution. The fluorescence enhancement of 1 is attributed to the formation of a complex between 1 and Hg²⁺ by 1:1 complex ratio (K = 2.08 × 10⁵), which has been utilized as the basis of fabrication of the Hg²⁺-sensitive fluorescent chemosensor. The comparison of this method with some other fluorescence methods for the determination of Hg²⁺ indicated that the method can be applied in aqueous solution rather than organic solution. The analytical performance characteristics of the proposed Hg²⁺-sensitive chemosensor were investigated. The chemosensor can be applied to the quantification of Hg²⁺ with a linear range covering from 2.57 × 10⁻⁷ to 9.27 × 10⁻⁵ M and a detection limit of 4.93 × 10⁻⁸ M. The experiment results show that the response behavior of 1 toward Hg²⁺ is pH independent in medium condition (pH 4.0–8.0). Most importantly, the fluorescence changes of the chemosensor are remarkably specific for Hg²⁺ in the presence of other metal ions, which meet the selective requirements for practical application. Moreover, the response of the chemosensor toward Hg²⁺ is fast (response time less than 1 min). In addition, the chemosensor has been used for determination of Hg²⁺ in hair samples with satisfactory results, which further demonstrates its value of practical applications.     

A highly selective fluorescent probe for Hg based on a rhodamine-coumarin conjugate

A fluorescent probe 1 for Hg²⁺ based on a rhodamine-coumarin conjugate was designed and synthesized. Probe 1 exhibits high sensitivity and selectivity for sensing Hg²⁺, and about a 24-fold increase in fluorescence emission intensity is observed upon binding excess Hg²⁺ in 50% water/ethanol buffered at pH 7.24. The fluorescence response to Hg²⁺ is attributed to the 1:1 complex formation between probe 1 and Hg²⁺, which has been utilized as the basis for the selective detection of Hg²⁺. Besides, probe 1 was also found to show a reversible dual chromo- and fluorogenic response toward Hg²⁺ likely due to the chelation-induced ring opening of rhodamine spirolactam. The analytical performance characteristics of the proposed Hg²⁺-sensitive probe were investigated. The linear response range covers a concentration range of Hg²⁺ from 8.0 × 10⁻⁸ to 1.0 × 10⁻⁵ mol L⁻¹ and the detection limit is 4.0 × 10⁻⁸ mol L⁻¹. The determination of Hg²⁺ in both tap and river water samples displays satisfactory results.     

Development of silver/gold nanocages onto indium tin oxide glass as a reagentless plasmonic mercury sensor

We demonstrate the utilization of silver/gold nanocages (Ag/Au NCs) deposited onto transparent indium tin oxide (ITO) film glass as the basis of a reagentless, simple and inexpensive mercury probe. The localized surface plasmon resonance (LSPR) peak wavelength was located at ∼800 nm. By utilizing the redox reaction between Hg²⁺ ions and Ag atoms that existed in Ag/Au NCs, the LSPR peak of Ag/Au NCs was blue-shifted. Thus, we develop an optical sensing probe for the detection of Hg²⁺ ions. The LSPR peak changes were lineally proportional to the concentration of Hg²⁺ ions over the range from 10 ppb to 0.5 ppm. The detection limit was ∼5 ppb. This plasmonic probe shows good selectivity and high sensitivity. The proposed optical probe is successfully applied to the sensing of Hg²⁺ in real samples.     

Target-induced formation of gold amalgamation on DNA-based sensing platform for electrochemical monitoring of mercury ion coupling with cycling signal amplification strategy

Heavy metal ion pollution poses severe risks in human health and environmental pollutant, because of the likelihood of bioaccumulation and toxicity. Driven by the requirement to monitor trace-level mercury ion (Hg²⁺), herein we construct a new DNA-based sensor for sensitive electrochemical monitoring of Hg²⁺ by coupling target-induced formation of gold amalgamation on DNA-based sensing platform with gold amalgamation-catalyzed cycling signal amplification strategy. The sensor was simply prepared by covalent conjugation of aminated poly-T₍₂₅₎ oligonucleotide onto the glassy carbon electrode by typical carbodiimide coupling. Upon introduction of target analyte, Hg²⁺ ion was intercalated into the DNA polyion complex membrane based on T–Hg²⁺–T coordination chemistry. The chelated Hg²⁺ ion could induce the formation of gold amalgamation, which could catalyze the p-nitrophenol with the aid of NaBH₄ and Ru(NH₃)₆³⁺ for cycling signal amplification. Experimental results indicated that the electronic signal of our system increased with the increasing Hg²⁺ level in the sample, and has a detection limit of 0.02 nM with a dynamic range of up to 1000 nM Hg²⁺. The strategy afforded exquisite selectivity for Hg²⁺ against other environmentally related metal ions. In addition, the methodology was evaluated for the analysis of Hg²⁺ in spiked tap-water samples, and the recovery was 87.9–113.8%.     

Ultrasensitive mercury(II) ion detection by europium(III)-doped cadmium sulfide composite nanoparticles

With the biomolecule glutathione (GSH) as a capping ligand, Eu³⁺-doped cadmium sulfide composite nanoparticles were successfully synthesized through a straightforward one-pot process. An efficient fluorescence energy transfer system with CdS nanoparticles as energy donor and Eu³⁺ ions as energy accepter was developed. As a result of specific interaction, the fluorescence intensity of Eu³⁺-doped CdS nanoparticles is obviously reduced in the presence of Hg²⁺. Moreover, the long fluorescent lifetime and large Stoke's shift of europium complex permit sensitive fluorescence detection. Under the optimal conditions, the fluorescence intensity of Eu³⁺ at 614 nm decreased linearly with the concentration of Hg²⁺ ranging from 10 nmol L⁻¹ to 1500 nmol L⁻¹, the limit of detection for Hg²⁺ was 0.25 nmol L⁻¹. In addition to high stability and reproducibility, the composite nanoparticles show a unique selectivity towards Hg²⁺ ion with respect to common coexisting cations. Moreover, the developed method was applied to the detection of trace Hg²⁺ in aqueous solutions. The probable mechanism of reaction between Eu³⁺-doped CdS composite nanoparticles and Hg²⁺ was also discussed.     

A cationic porphyrin-based self-assembled film for mercury ion detection

A cationic 5,15-(p-(9,9-bis(6-trimethylammoniumhexyl)fluorenylethynyl)phenyl)porphyrin tetrabromide was synthesized and the self-assembled films were used for Hg²⁺ detection in aqueous media. The detection response is based on fluorescence quenching of the porphyrin molecule upon coordination with Hg²⁺. The detection shows high selectivity for Hg²⁺ over Cu²⁺, Zn²⁺, Pb²⁺, Cd²⁺, Mn²⁺, Ni²⁺, Co²⁺ and Ca²⁺. A linear response toward Hg²⁺ in a concentration range of 1 × 10⁻¹⁰-1 × 10⁻⁶ M was observed for the film with a detection limit of 0.1 nM. The cationic porphyrin film shows higher stability and significant improvement in detection sensitivity, as compared to other porphyrin-based sensors. The amphiphilic cationic nature of the porphyrin synthesized also allows for the direct deposition of a porphyrin layer on a bare glass surface through self-assembly.     

Colorimetric signaling of Hg ions by a nitrobenzoxadiazole-appended cyclen-triester

A novel Hg²⁺-selective colorimetric sensor based on a cyclen–nitrobenzoxadiazole (NBD) conjugate was investigated. A cyclen derivative with three ester ligands was used as the binding site and the NBD moiety acted as the reporting chromogenic subunit. Interaction of 1 with Hg²⁺ ions resulted in a pronounced color change from pink to yellow, and fluorescence signaling was also possible. Selective colorimetric signaling of Hg²⁺ ions by NBD-functionalized cyclen with a detection limit of 1.5 × 10⁻⁶ M in aqueous environments was successfully achieved.     

Highly sensitive and selective detection of biothiols using graphene oxide-based “molecular beacon”-like fluorescent probe

A fluorometric method for quantity analysis of biothiols was developed using a graphene oxide (GO)-based “molecular beacon”-like probe, which consisted of FITC labeled thymine (T)-rich single-stranded DNA (ssDNA), GO and Hg²⁺ ions. The labeled ssDNA containing T–T mismatches would self-hybridize to duplex in the presence of Hg²⁺, which can avoid its adsorption on GO and the fluorescence of this GO-based probe was recovered. The fluorescence of the probe quenched after the addition of biothiols such as glutathione (GSH) and cysteine (Cys) owing to thiol groups can selectively competitive ligation of Hg²⁺ ions with T–T mismatches. In the present work, the GO-based probe was used for the determination of GSH and Cys. Under the optimal conditions, a linear correlation was established between fluorescence intensity ratio I₀/I and the concentration of GSH in the range of 2.0 × 10⁻⁹–5.0 × 10⁻⁷ mol L⁻¹ with a detection limit of 1.0 × 10⁻⁹ mol L⁻¹. The linear range for Cys is from 5.0 × 10⁻⁹ to 4.5 × 10⁻⁷ mol L⁻¹ with a detection limit of 2.0 × 10⁻⁹ mol L⁻¹. The proposed method was applied to the determination of GSH in human serum and cell extract samples with satisfactory results.     

A highly selective and sensitive fluorescein-based chemodosimeter for Hg2+ ions in aqueous media

A new fluorescein-based chemodosimeter (II) for Hg²⁺ ion was designed and synthesized, and it displayed excellent selective and sensitive toward Hg²⁺ ion over other commonly metal ions in aqueous media. II was a colorless, non-fluorescent compound. Upon addition of Hg²⁺ to the solution of II, the thiosemicarbazide moiety of II would undergo an irreversible desulfurization reaction to form its corresponding oxadiazole (IV), a colorful and fluorescent product. During this process, the spirocyclic ring of II was opened, causing instantaneous development of visible color and strong fluorescence emission in the range of 500-600 nm. Based on the above mechanism, a fluorogenic Hg²⁺-selective chemodosimeter was developed. The fluorescence increase is linearly with Hg²⁺ concentration up to 1.0 μmol L⁻¹ with a detection limit of 8.5 × 10⁻¹⁰ mol L⁻¹ (3σ). Compared with the rhodamine-type chemodosimeter, II is more stable in aqueous media and exhibits higher sensitivity toward Hg²⁺. The findings suggest that II will serve as a practical chemodosimeter for rapid detection of Hg²⁺ concentrations in realistic media.     

p-Dimethylaminobenzaldehyde thiosemicarbazone: a simple novel selective and sensitive fluorescent sensor for mercury(II) in aqueous solution

A novel and simple fluorophore, p-dimethylaminobenzaldehyde thiosemicarbazone (DMABTS), was prepared in order to find available fluorescent chemosensor for mercuric ion in aquesous solution. DMABTS emitted fluorescence at 448 nm in aqueous solution and its fluorescence intensity was completely quenched upon interaction with Hg²⁺ ions, which should be attributed to the 1:1 complex formation between DMABTS and Hg²⁺. The binding constant of the complex was determined as 7.48 × 10⁶ mol l⁻¹. The linear range of quantitative detection of 0 to 5.77 × 10⁻⁶ mol l⁻¹ and the detection limit of 7.7 × 10⁻⁷ mol l⁻¹ for Hg²⁺ in the 6.3 × 10⁻⁶ mol l⁻¹ DMABTS aqueous solution were obtained from a calibration curve. The coexistence of several transition metal ions and anions did interfere the fluorometric titration of Hg²⁺ ion by less than 4% in the emission change.