Evaluating the treatment effectiveness of copper-based algaecides on toxic algae Microcystis aeruginosa using single cell-inductively coupled plasma-mass spectrometry

Assessing the disparity: comparative toxicity of Copper in zebrafish larvae exposes alarming consequences of permissible concentrations in Brazil

Copper (Cu) is a naturally occurring metal with essential micronutrient properties. However, this metal might also pose increased adverse environmental and health risks due to industrial and agricultural activities. In Brazil, the maximum allowable concentration of Cu in drinking water is 2 mg/L. Despite this standard, the impact of such concentrations on aquatic organisms remains unexplored. This study aimed to evaluate the toxicity of CuSO4 using larval zebrafish at environmentally relevant concentrations. Zebrafish (Danio rerio) larvae at 72 hr post-fertilization (hpf) were exposed to nominal CuSO4 concentrations ranging from 0.16 to 48 mg/L to determine the median lethal concentration (LC50), established at 8.4 mg/L. Subsequently, non-lethal concentrations of 0.16, 0.32, or 1.6 mg/L were selected for assessing CuSO4 -induced toxicity. Morphological parameters, including body length, yolk sac area, and swim bladder area, were adversely affected by CuSO4 exposure, particularly at 1.6 mg/L (3.31 mm ±0.1, 0.192 mm2 ±0.01, and 0.01 mm2 ±0.05, respectively). In contrast, the control group exhibited values of 3.62 mm ±0.09, 0.136 mm2 ±0.013, and 0.3 mm2 ±0.06, respectively. Behavioral assays demonstrated impairments in escape response and swimming capacity, accompanied by increased levels of reactive oxygen species (ROS) and lipid peroxidation. In addition, decreased levels of non-protein thiols and reduced cellular viability were noted. Data demonstrated that exposure to CuSO4 at similar concentrations as those permitted in Brazil for Cu adversely altered morphological, biochemical, and behavioral endpoints in zebrafish larvae. This study suggests that the permissible Cu concentrations in Brazil need to be reevaluated, given the potential enhanced adverse health risks of exposure to environmental metal contamination.

Study of Legionella pneumophila treatment with copper in drinking water by single cell-ICP-MS

Legionella pneumophila is a persistent opportunistic pathogen that poses a significant threat to domestic water systems. Previous studies suggest that copper (Cu) is an effective antimicrobial in water systems. A rapid and sensitive quantification method is desired to optimize the conditions of L. pneumophila treatment by Cu and to better understand the interaction mechanisms between Cu and cells. In this study, we developed a highly sensitive single cell (SC)-ICP-MS method to monitor L. pneumophila cell concentration and track their uptake of Cu. The SC-ICP-MS method showed excellent sensitivity (with a cell concentration detection limit of 1000 cells/mL), accuracy (good agreement with conventional hemocytometry method), and precision (relative standard deviation < 5%) in drinking water matrix. The cupric ions (Cu2+) treatment results indicated that the total L. pneumophila cell concentration, Cu mass per cell, colony-forming unit counting, and Cu concentration in supernatant all exhibited a dose-dependent trend, with 800-1200 µg/L reaching high disinfection rates in drinking water. The investigation of percentages of viable and culturable, viable but nonculturable (VBNC), and lysed cells suggested there always were VBNC present at any Cu concentration. Experimental results of different Cu2+ treatment times further suggested that L. pneumophila cells developed an antimicrobial resistant mechanism with the prolonged Cu exposure. This is the first quantification study on the interactions of Cu and L. pneumophila in drinking water using SC-ICP-MS.

Synergistic influence of iodine and hydrogen peroxide towards the degradation of harmful algal bloom of Microcystis aeruginosa

Cyanobacterial blooming due to the influence of temperature and increased nutrients in ponds/lakes aided by the runoff from agricultural lands, is a serious environmental issue. The presence of cyanotoxins in water may poison the health of aquatic organisms, animals, and humans. In this study, we focus on chemical assisted degradation of Microcystis aeruginosa- an alga that is of special relevance owing to its consistent blooming, especially in tropical regions. The study aims to ascertain the individual iodine (I) and hydrogen peroxide (H2O2) and their combination (hereinafter referred to as IH) effects on the degradation of Microcystis aeruginosa. As expected, the collected pond water revealed the presence of metal ions viz., Ni, Zn, Pb, Cu and Mn, which enriched the blooming of M. aeruginosa. Interestingly, a complete rupture of the cells - pigment loss, biochemical degradation and oxidative damage-was observed by the IH solution after exposure for ∼9 h under ambient conditions. In comparison to control (original water without chemicals), the addition IH completely eliminated the pigments phycocyanin (99.5%) and allophycocyanin (98%), and degraded ∼81% and 91% of carbohydrates and proteins, respectively due to the synergistic action of I and H. Superior degradation of algae through a simple and eco-friendly approach presented in this study could be explored more effectively towards its large-scale applicability.

Expanding the boundaries of atomic spectroscopy at the single-cell level: critical review of SP-ICP-MS, LIBS and LA-ICP-MS advances for the elemental analysis of tissues and single cells

Metals have a fundamental role in microbiology, and accurate methods are needed for their identification and quantification. The inability to assess cellular heterogeneity is considered an impediment to the successful treatment of different diseases. Unlike bulk approaches, single-cell analysis allows elemental heterogeneity across genetically identical populations to be related to specific biological events and to the effectiveness of drugs. Single particle-inductively coupled plasma-mass spectrometry (SP-ICP-MS) can analyse single cells in suspension and measure this heterogeneity. Here we explore advances in instrumental design, compare mass analysers and discuss key parameters requiring optimisation. This review has identified that the effect of pre-treatment of cell suspensions and cell fixation approaches require further study and novel validation methods are needed as using bulk measurements is unsatisfactory. SP-ICP-MS has the advantage that a large number of cells can be analysed; however, it does not provide spatial information. Techniques based on laser ablation (LA) enable elemental mapping at the single-cell level, such as laser-induced breakdown spectroscopy (LIBS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The sensitivity of commercial LIBS instruments restricts its use for sub-tissue applications; however, the capacity to analyse endogenous bulk components paired with developments in nano-LIBS technology shows great potential for cellular research. LA-ICP-MS offers high sensitivity for the direct analysis of single cells, but standardisation requires further development. The hyphenation of these trace elemental analysis techniques and their coupling with multi-omic technologies for single-cell analysis have enormous potential in answering fundamental biological questions.

Single-cell ICP-MS for studying the association of inorganic nanoparticles with cell lines derived from aquaculture species

The current research deals with the use of single-cell inductively coupled plasma-mass spectrometry (scICP-MS) for the assessment of titanium dioxide nanoparticle (TiO₂ NP) and silver nanoparticle (Ag NP) associations in cell lines derived from aquaculture species (sea bass, sea bream, and clams). The optimization studies have considered the avoidance of high dissolved background, multi-cell peak coincidence, and possible spectral interferences. Optimum operating conditions were found when using a dwell time of 50 μs for silver and 100 μs for titanium. The assessment of associated TiO₂ NPs by scICP-MS required the use of ammonia as a reaction gas (flow rate at 0.75 mL min^-1) for interference-free titanium determinations (measurements at an m/z ratio of 131 from the ⁴⁸Ti(NH)(NH₃)₄ adduct). The influence of other parameters such as the number of washing cycles and the cell concentration on accurate determinations by scICP-MS was also fully investigated. Cell exposure trials were performed using PVP-Ag NPs (15 and 100 nm, nominal diameter) and citrate-TiO₂ NPs (5, 25, and 45 nm, nominal diameter) at nominal concentrations of 10 and 50 μg mL^-1 for citrate-TiO₂ NPs and 5.0 and 50 μg mL^-1 for PVP-Ag NPs. Results have shown that citrate-TiO₂ NPs interact with the outer cell membranes, being quite low in the number of citrate-TiO₂ NPs that enters the cells (the high degree of aggregation is the main factor which leads to the aggregates being in the extracellular medium). In contrast, PVP-Ag NPs have been found to enter the cells.

Evaluation of Cd2+ stress on Synechocystis sp. PCC6803 based on single-cell elemental accumulation and algal toxicological response

With the development of single cell analysis techniques, the concept of precision toxicology has been proposed in recent years. Due to the heterogeneity of cells, we need to perform toxicological assessments on individual cells. Microalgae, one kind of important primary producers, play as a major pathway by which heavy metals enter the food chain and thus accumulate/transfer to higher trophic levels. Herein, the biosorption of Cd (Ex-Cd) and bioaccumulation of Cd (In-Cd) for Synechocystis sp. PCC 6803 were investigated by online 3D droplet microfluidic device combined with inductively coupled plasma mass spectrometry detection. Meanwhile, the algal toxicological responses of the algae cell to Cd²⁺ exposure under different concentration (50, 100, and 150 μg L ^- 1) and time (15 min, 24, 48 and 96 h) were studied. Combining single-cell analysis with toxicological indicators, the toxicity mechanism of Cd²⁺to algal was discussed. The single cell analysis results revealed heterogeneity in cellular uptake of Cd²⁺. The proportion of Cd-containing cells and Cd content in single algal cells all reached the maximum at 24 h. The uptake of Cd²⁺ occurred within 15 min under all tested exposure concentrations and a large part of Cd²⁺ were adsorbed on the algal cells surface. The Pearson correlation analysis showed that cell density, chlorophyll a and carotenoids were significantly negatively correlated with Cd accumulation, whereas ROS level and SOD activity were significantly positively correlated with Cd accumulation. It suggested that Cd²⁺accumulated intracellular would show toxic effects on the algal cells and oxidative stress is the main mechanism of Cd toxicity to algal cells. This work promotes our understanding of the toxicological responses of microalgae under Cd stress at single cells level.

Effect of three commercial algaecides on cyanobacteria and microcystin-LR: implications for drinking water treatment using activated carbon

Toxic cyanobacterial blooms in aquatic ecosystems are associated to both public health and environmental concerns worldwide. Depending on the treatment technologies used, the removal capacity of cyanotoxins by drinking water treatment plants (DWTPs) is not sufficient to reach safe levels in drinking water. Likewise, controlling these blooms with algaecide may impair the efficiency of DWTPs due to the possible lysis of cyanobacterial cells and consequent release of cyanotoxins. We investigated the effects of three commercial algaecides (cationic polymer, copper sulfate, and hydrogen peroxide) on the growth parameters of the cyanobacterium Microcystis aeruginosa and the release of microcystin-LR (MC-LR). The potential interference of each algaecide on the MC-LR removal by adsorption on activated carbon (AC) was also tested through adsorption isotherms and kinetics experiments. Most algaecides significantly decreased the cell density and biovolume of M. aeruginosa, as well as increased the release of MC-LR. Interestingly, the presence of the algaecides in binary mixtures with MC-LR affected the adsorption of the cyanotoxin. Relevant adsorption parameters (e.g., maximum adsorption capacity, adsorption intensity, and affinity between MC-LR and AC) were altered when the algaecides were present, especially in the case of the cationic polymer. Also, the algaecides influenced the kinetics (e.g., by shifting the initial adsorption and the desorption constant), which may directly affect the design and operation of DWTPs. Our study indicated that algaecides can significantly impact the fate and the removal of MC-LR in DWTPs when the adsorption process is employed, with important implications for the management and performance of such facilities.

Single-cell ICP-MS to address the role of trace elements at a cellular level

The heterogeneity properties shown by cells or unicellular organisms have led to the development of analytical methods at the single-cell level. In this sense, considering the importance of trace elements in these biological systems, the inductively coupled plasma mass spectrometer (ICP-MS) configured for analyzing single cell has presented a high potential to assess the evaluation of elements in cells. Moreover, advances in instrumentation, such as coupling laser ablation to the tandem configuration (ICP-MS/MS), or alternative mass analyzers (ICP-SFMS and ICP-TOFMS), brought significant benefits, including sensitivity improvement, high-resolution imaging, and the cell fingerprint. From this perspective, the single-cell ICP-MS has been widely reported in studies involving many fields, from oncology to environmental research. Hence, it has contributed to finding important results, such as elucidating nanoparticle toxicity at the cellular level and vaccine development. Therefore, in this review, the theory of single-cell ICP-MS analysis is explored, and the applications in this field are pointed out. In addition, the instrumentation advances for single-cell ICP-MS are addressed.

Monitoring and control methods of harmful algal blooms in Chinese freshwater system: a review

Harmful algal blooms (HABs) are a worldwide problem with substantial adverse effects on the aquatic environment as well as human health, which have prompted researchers to study measures to stem and control them. Meanwhile, it is key to research and develop monitoring methods to establish early warning HABs. However, both the current monitoring methods and control methods have some shortcomings, making the field application limited. Thus, we need to improve current approaches for monitoring and controlling HABs efficiently. Based on the freshwater system features in China, we review various monitoring and control methods of HABs, summarize and discuss the problems with these methods, and propose the future development direction of monitoring and control HABs. Finally, we envision that it can combine physical, chemical, and biological methods to inhibit HAB expansion in the future, complementing each other with advantages. Further, we promise to establish a long-term strategy of controlling HABs with various algicidal bacteria co-cultivate for field applications in China. Efforts in studying algicidal bacteria must be increased to better control HABs and mitigate the risks of aquatic ecosystems and human health in China.

Detection of Histidine-Tagged Protein in Escherichia coli by Single-Cell Inductively Coupled Plasma-Mass Spectrometry

We have developed a rapid and precise quantification method for a histidine (His)-tagged recombinant protein produced in Escherichia coli (E. coli) by single-cell inductively coupled plasma-mass spectrometry (SC-ICP-MS). Plasmid vector containing enhanced green fluorescent protein (EGFP) or red fluorescent protein (mCherry) gene fused with His-tag was transformed into E. coli. The transformed E. coli was exposed to nickel (Ni) chloride or cobalt (Co) chloride for labeling His-tag with the Ni or Co ion. Then, E. coli was analyzed by SC-ICP-MS to determine the amount of EGFP or mCherry protein on the basis of the signal of Ni or Co bound to His-tag. By comparing Ni and Co contents in E. coli expressing His-tagged mCherry with those in nontagged mCherry, the specific binding of Co to His-tag was more clearly detected than that of Ni. The Co contents were increased until 6 h after the protein induction, and this observation was coincident with the increases in fluorescence intensity of EGFP or mCherry measured by a flow cytometer. However, the Co contents were decreased for EGFP and kept at a constant level for mCherry from 6 to 24 h despite the continuous increase in the fluorescence intensity through incubation. The fluorescent proteins were mainly recovered in the insoluble fraction 24 h after the induction. This can be explained by the fact that the overexpressed fluorescent proteins with His-tag are transferred into inclusion bodies, which hampers the binding of the fluorescent proteins to the Co ion. SC-ICP-MS can be a useful technique to precisely quantify soluble recombinant proteins in E. coli without the extraction and purification process.

Efficient photocatalytic inactivation of Microcystis aeruginosa by a novel Z-scheme heterojunction tubular photocatalyst under visible light irradiation

The design of a photocatalyst for efficient algal inactivation under visible light is essential for the application of photocatalysis to the control of harmful algal blooms. In this study, a novel Z-scheme heterojunction tubular photocatalyst, Ag₂O@PG, was synthesized by chemically depositing silver oxide compounded with P-doped hollow tubular graphitic carbon nitride for the photocatalytic inactivation of Microcystis aeruginosa (M. aeruginosa). The photocatalytic algal inactivation experiments showed that the photocatalytic activity of Ag₂O@PG was influenced by the ratio of the composition of the obtained materials. The optimal algal inactivation efficiency was observed when using Ag₂O@PG-0.4 at a dosage of 0.2 g/L. It was able to achieve a 99.1 % M. aeruginosa inactivation at an initial concentration of 4.5 × 10⁶ cells/mL following 5 h' visible light irradiation. During the process, the cell membrane permeability and cell morphology changed. Furthermore, under the constant attack of superoxide radicals and holes caused by Ag₂O@PG, the superoxide dismutase, glutathione and malondialdehyde of algae cells increased during the experiments to alleviate oxidative damage. Eventually, the antioxidant system of algae cells was destroyed. To further validate the potential application of Ag₂O@PG-0.4 in real algal bloom environment, an experiment in real water samples was carried out. Overall, the Ag₂O@PG-0.4 as an efficient photocatalyst has a promising potential for emergency treatment measures to alleviate algal blooms.

Living in a transient world: ICP-MS reinvented via time-resolved analysis for monitoring single events

After 40 years of development, inductively coupled plasma-mass spectrometry (ICP-MS) can hardly be considered as a novel technique anymore. ICP-MS has become the reference when it comes to multi-element bulk analysis at (ultra)trace levels, as well as to isotope ratio determination for metal(loid)s. However, over the last decade, this technique has managed to uncover an entirely new application field, providing information in a variety of contexts related to the individual analysis of single entities (e.g., nanoparticles, cells, or micro/nanoplastics), thus addressing new societal challenges. And this profound expansion of its application range becomes even more remarkable when considering that it has been made possible in an a priori simple way: by providing faster data acquisition and developing the corresponding theoretical substrate to relate the time-resolved signals thus obtained with the elemental composition of the target entities. This review presents the underlying concepts behind single event-ICP-MS, which are needed to fully understand its potential, highlighting key areas of application (e.g., single particle-ICP-MS or single cell-ICP-MS) as well as of future development (e.g., micro/nanoplastics).

Quantification of silver nanoparticle interactions with yeast Saccharomyces cerevisiae studied using single-cell ICP-MS

Silver nanoparticles (AgNPs) have been used in many fields due to their anticancer, antimicrobial, and antiviral potential. Single-cell ICP-MS (SC-ICP-MS) is an emerging technology that allows for the rapid characterization and quantification of a metal analyte across a cell population in a single analysis. In this study, a new rapid and sensitive SC-ICP-MS method was developed to quantitatively study the interactions of AgNPs with yeast Saccharomyces cerevisiae. The method can quantify the cell concentration, silver concentration per cell, and profile the nanoparticle distribution in a yeast cell population. AgNP dosing time, concentration, and AgNP size were quantitatively evaluated for their effects on AgNP-yeast cell interactions. The results showed that the initial uptake of AgNPs was rapid and primarily driven by the mass of Ag per cell. The optimal dosing particle concentrations for highest uptake were approximately 1820, 1000, and 300 AgNPs/cell for 10, 20, and 40 nm AgNPs, respectively. Furthermore, this study also validated a washing method for the application to a microorganism for the first time and was used to quantitatively determine the amount of cell surface-adsorbed AgNPs and intracellular AgNPs. These results indicated that the mass (Ag in ag/cell) ratios of intracelluar vs cell surface-adsorbed AgNPs were similar for different AgNP sizes. This high throughput and ultrasensitive SC-ICP-MS method is expected to have many potential applications, such as optimization of methods for green synthesis of AgNPs, nanotoxicity studies, and drug delivery. This is the first quantification study on the interactions of AgNPs and S. cerevisiae using SC-ICP-MS.

Recent advances in copper analyses by inorganic mass spectrometry

Copper (Cu) participates in the biological redox reaction in the body, and its deficiency is fatal to the body. At the same time, Cu is extremely toxic when it exists in excess. Thus, the body has to tightly and spatiotemporally regulate the concentration of Cu within a physiological range by several groups of Cu-regulating proteins. However, entire mechanisms underlying the maintenance of Cu homeostasis in body and cells have not fully understood. It is necessary to analyze Cu itself in a body and in a cell to reveal the Cu homeostasis. In this review, recent advances in the analytical techniques to understand the Cu metabolism such as speciation, imaging and single-cell analysis of Cu were highlighted.

A new perspective of copper-iron effects on bloom-forming algae in a highly impacted environment

Relatively little work has been done on the role of micronutrients in influencing development and progression of harmful algal blooms, yet micronutrients are ineluctably required for growth. Relatively small changes in micronutrient status have wide-ranging consequences. Here, we report results from mesocosm experiments with Microcystis and Desmodesmus spp., in mono- and mixed-cultures, to probe questions of how copper, iron, and copper-iron amendments affect growth, short-term assemblage progression, and production of siderophore, chalkophore, and microcystin in lake water from a large, hypereutrophic lake (Taihu, China). Our approach offers an entirely new perspective to understanding micronutrient dynamics in aqueous environments, as this is the first work to systematically screen for chalkophores and siderophores separately, as a function of copper/iron amendment, and using community trajectory analysis. Singular findings are summarized as follows: 1) unlike lab-based studies, in our work we observe neither dramatic copper-modulation of iron demand, nor evidence of an iron-protective effect from copper toxicity. 2) The interplay between chalkophore/siderophore production supports a concept model wherein Microcystis spp. varies behavior to individually and uniquely manage copper/iron requirements in a phased manner. In being able to specifically screen for chalkophores, we observe a previously unreported link between chalkophore and microcystin production that may relate to iron-limitation. 3) Regarding harmful algal bloom (HAB) persistance, the lake water itself influences mesocosm changes; differentiated effects for iron regarding growth indicators and/or reduction of Fe-limitation stress were found at an HAB-free field station (Xukou Bay), likely a consequence of low bioavailability of iron in this station as compared to HAB-impacted stations (half the initial dissolved iron concentration, persisting throughout experiments). The low dissolved iron accompanies more intense chalkophore/siderophore community trajectories.

Characteristics of the copper-induced viable-but-non-culturable state in bacteria

The antimicrobial applications of copper (Cu) are exploited in several industries, such as agriculture and healthcare settings. While Cu is capable of efficiently killing microorganisms, sub-lethal doses can induce a viable-but-non-culturable (VBNC) state in bacteria of many distinct clades. VBNC cells cannot be detected by standard culture-based detection methods, and can become a threat to plants and animals as they often retain virulent traits upon resuscitation. Here we discuss the putative mechanisms of the Cu-induced VBNC state. Common observations in Cu-induced VBNC cells include a cellular response to reactive oxygen species, the exhaustion of energy reserves, and a reconfiguration of the proteome. While showing partial overlap with other VBNC state-inducing stressors, these changes seem to be part of an adaptive response to Cu toxicity. Furthermore, we argue that Cu resistance mechanisms such as P-type ATPases and multicopper oxidases may ward off entry into the VBNC state to some extent. The spread of these mechanisms across multi-species populations could increase population-level resistance to Cu antimicrobials. As Cu resistance mechanisms are often co-selected with antibiotic resistance mechanisms, this threat is exacerbated.

Inductively coupled plasma mass spectrometry based platforms for studies involving nanoparticle effects in biological samples

The widespread application of nanoparticles (NPs) in recent times has caused concern because of their effects in biological systems. Although NPs can be produced naturally, industrially synthesized NPs affect the metabolism of a given organism because of their high reactivity. The biotransformation of NPs involves different processes, including aggregation/agglomeration, and reactions with biomolecules that will be reflected in their toxicity. Several analytical techniques, including inductively coupled plasma mass spectrometry (ICP-MS), have been used for characterizing and quantifying NPs in biological samples. In fact, in addition to providing information regarding the morphology and concentration of NPs, ICP-MS-based platforms, such as liquid chromatography/ICP-MS, single-particle ICP-MS, field-flow fractionation (asymmetrical flow field-flow fractionation)-ICP-MS, and laser ablation-ICP-MS, yield elemental information about molecules. Furthermore, such information together with speciation analysis enlarges our understanding of the interaction between NPs and biological organisms. This study reports the contribution of ICP-MS-based platforms as a tool for evaluating NPs in distinct biological samples by providing an additional understanding of the behavior of NPs and their toxicity in these organisms.

Sequential colorimetric sensing of cupric and mercuric ions by regulating the etching process of triangular gold nanoplates

A triangular gold nanoplate (AuNPL)-based colorimetric assay is presented for ultrasensitive determination of cupric ions (Cu²⁺⁾ and mercuric ions (Hg²⁺) in sequence. AuNPLs were found to be etched efficiently when producing triiodide ions (I₃^-) by a redox reaction between Cu²⁺ and iodide ions (I^-), leading to a change of the shape of AuNPLs from triangular to sphere along with a color change from blue to pink. In the presence of Hg²⁺ the etching of AuNPLs was suppressed due to the consumption of I^- by the formation of HgI₂. With an increase of the concentration of the Hg²⁺ a transformation from sphere to triangular in the shape of AuNPLs occurred with a color change from pink to blue. The evolution of AuNPLs from etching to anti-etching state by sequential addition of Cu²⁺ and Hg²⁺ was accompanied with color variations and band shifts of localized surface plasmon resonance (LSPR), allowing for visual and spectroscopic determination of Cu²⁺ and Hg²⁺ successively within 15 min. In the range 0.01-1.5 μM for Cu²⁺ and 0.02-3.0 μM for Hg²⁺, the linear relationship between the band shift values and the target ions concentration was found good (R² > 0.996). The limit of detections (3S/k) was 19 nM for Cu²⁺ and 9 nM for Hg²⁺, respectively. The lowest visual estimation concentration was 80 nM for both Cu²⁺ and Hg²⁺ through the distinguishable color changes. This system exhibited desirable selectivity for Cu²⁺ and Hg²⁺ over other common ions tested. The method has been successfully applied to sequential determination of Cu²⁺ and Hg²⁺ in real water and food samples. Graphical abstract Scheme 1 Schematic illustration for sequential detection of Cu²⁺ and Hg²⁺ based on etching of AuNPLs.