A Biosensor Platform for Metal Detection Based on Enhanced Green Fluorescent Protein

Surface display provides an efficient expression system for production of recombinant proteins and bacterial whole cell biosensor in E. coli

A novel bacterial display vector based on Escherichia coli has been engineered for recombinant protein production and purification. Accordingly, a construct harboring the enhanced green fluorescent protein (EGFP) and the ice nucleation protein (INP) was designed to produce EGFP via the surface display in E. coli cells. The fusion EGFP-expressed cells were then investigated using fluorescence measurement, SDS- and native-PAGE before and after TEV protease digestion. The displayed EGFP was obtained with a recovery of 57.7 % as a single band on SDS-PAGE. Next, the efficiency of the cell surface display for mutant EGFP (EGFP S202H/Q204H) was examined in sensing copper ions. Under optimal conditions, a satisfactorily linear range for copper ions concentrations up to 10 nM with a detection limit of 0.073 nM was obtained for cell-displayed mutant EGFP (mEGFP). In the presence of bacterial cell lysates and purified mEGFP, response to copper was linear in the 2-10 nM and 0.1-2 μM concentration range, respectively, with a 1.3 nM and 0.14 μM limit of detection. The sensitivity of bacterial cell lysates and surface-displayed mEGFP in the detection of copper ions is higher than the purified mEGFP.

Rapid Detection of Cd2+ Ions in the Aqueous Medium Using a Highly Sensitive and Selective Turn-On Fluorescent Chemosensor

Herein, a novel optical chemosensor, (CM1 = 2, 6-di((E)-benzylidene)-4-methylcyclohexan-1-one), was designed/synthesized and characterized by ¹H-NMR and FT-IR spectroscopy. The experimental observations indicated that CM1 is an efficient and selective chemosensor towards Cd²⁺, even in the presence of other metal ions, such as Mn²⁺, Cu²⁺, Co^2+, Ce³⁺, K⁺, Hg²⁺,^, and Zn²⁺ in the aqueous medium. The newly synthesized chemosensor, CM1, showed a significant change in the fluorescence emission spectrum upon coordination with Cd²⁺. The formation of the Cd²⁺ complex with CM1 was confirmed from the fluorometric response. The 1:2 combination of Cd²⁺ with CM1 was found optimum for the desired optical properties, which was confirmed through fluorescent titration, Job's plot, and DFT calculation. Moreover, CM1 showed high sensitivity towards Cd²⁺ with a very low detection limit (19.25 nM). Additionally, the CM1 was recovered and recycled by the addition of EDTA solution that combines with Cd²⁺ ion and, hence, frees up the chemosensor.

Oral Metal-Organic Gel Protected Whole-Cell Biosensor for in Situ Monitoring Nitrosamines in the Gastrointestinal Tract

Nitrite, a type of food additive, has been proved convertible to genotoxic nitrosamines in the gastrointestinal tract by intestinal flora. There is no appropriate method for in situ detection of nitrosamines. Herein, plasmid-introduced Saccharomyces cerevisiae, which can respond to nitrosamine-induced DNA damage and activate pMAG1-based DNA damage repair (DDR), was designed as whole-cell biosensors (WCBs) for monitoring the in situ generated nitrosamines by a reporter gene expressing enhanced green fluorescent protein (EGFP). In order to protect the validity of WCBs (pMAG1 yeast) from the gastric acid environment, a type of metal-organic gel (MOG), coordinated by Fe³⁺ and 2,2'-thiodiacetic acid (TDA), was prepared to embed the WCBs. The MOG(Fe-TDA) is gastric acid resistant and can deliver the pMAG1 yeast to the gut without compromising the performance of pMAG1 yeast to detect in situ generated nitrosamines. The genotoxicity of nitrosamines converted from nitrite was successfully detected in the gastrointestinal tract of mice.

Detection of Cd2+ in Aqueous Solution by the Fluorescent Probe of CdSe/CdS QDs Based on OFF–ON Mode

The detection of heavy metals in aqueous solutions has always attracted much attention from all over the world. A fluorescent probe of CdSe/CdS core-shell quantum dots (QDs) was designed to detect trace Cd²⁺ in aqueous solutions using the OFF–ON mode rapidly and efficiently, likely based on adsorption and desorption reactions between ethylenediaminetetraacetic acid disodium salt (EDTA) and CdSe/CdS QDs. In the OFF mode, the optical shielding function of EDTA results in fluorescence quenching owing to the strong adsorption ability of EDTA with Cd²⁺ on the sites of CdSe/CdS QDs surface. In the ON mode, the introduction of Cd²⁺ promotes the desorption of EDTA from the EDTA-CdSe/CdS QDs and restores the fluorescence intensity. There were two linear response ranges which were 0.1–20 µmol/L and 20–90 µmol/L for the EDTA-CdSe/CdS system to detect Cd²⁺. The detection limit was 6 nmol/L, and the standard deviation was below 4% for the detection of Cd²⁺ concentration in tap water.

Metal-Induced Fluorescence Quenching of Photoconvertible Fluorescent Protein DendFP

Sensitive and accurate detection of specific metal ions is important for sensor development and can advance analytical science and support environmental and human medical examinations. Fluorescent proteins (FPs) can be quenched by specific metal ions and spectroscopically show a unique fluorescence-quenching sensitivity, suggesting their potential application as FP-based metal biosensors. Since the characteristics of the fluorescence quenching are difficult to predict, spectroscopic analysis of new FPs is important for the development of FP-based biosensors. Here we reported the spectroscopic and structural analysis of metal-induced fluorescence quenching of the photoconvertible fluorescent protein DendFP. The spectroscopic analysis showed that Fe²⁺, Fe³⁺, and Cu²⁺ significantly reduced the fluorescence emission of DendFP. The metal titration experiments showed that the dissociation constants (K_d) of Fe²⁺, Fe³⁺, and Cu²⁺ for DendFP were 24.59, 41.66, and 137.18 μM, respectively. The tetrameric interface of DendFP, which the metal ions cannot bind to, was analyzed. Structural comparison of the metal-binding sites of DendFP with those of iq-mEmerald and Dronpa suggested that quenchable DendFP has a unique metal-binding site on the β-barrel that does not utilize the histidine pair for metal binding.

Recent advances in bacterial biosensing and bioremediation of cadmium pollution: a mini-review

Cadmium (Cd) pollution has become a global environmental issue because Cd gets easily accumulated and translocated in the food chain, threatening human health. Considering the detrimental effects and non-biodegradability of environmental Cd, this is an urgent issue that needs to be addressed through the development of robust, cost-effective, and eco-friendly green routes for monitoring and remediating toxic levels of Cd. This article attempts to review various bacterial approaches toward biosensing and bioremediation of Cd in the environment. This review focuses on the recent development of bacterial cell-based biosensors for the detection of bioavailable Cd and the bioremediation of toxic Cd by natural or genetically-engineered bacteria. The present limitations and future perspectives of these available bacterial approaches are outlined. New trends for integrating synthetic biology and metabolic engineering into the design of bacterial biosensors and bioadsorbers are additionally highlighted.

Xenobiotics-Division and Methods of Detection: A Review

Xenobiotics are compounds of synthetic origin, usually used for domestic, agricultural, and industrial purposes; in the environment, they are present in micropollutant concentrations and high concentrations (using ng/L to µg/L units). Xenobiotics can be categorized according to different criteria, including their nature, uses, physical state, and pathophysiological effects. Their impacts on humans and the environment are non-negligible. Prolonged exposure to even low concentrations may have toxic, mutagenic, or teratogenic effects. Wastewater treatment plants that are ineffective at minimizing the release of xenobiotic compounds are one of the main sources of xenobiotics in the environment (e.g., xenobiotic compounds reach the environment, affecting both humans and animals). In order to minimize the negative impacts, various laws and regulations have been adopted in the EU and across the globe, with an emphasis on xenobiotics removal from the environment, in a way that is economically, environmentally, and socially acceptable, and will not result in their accumulation, or creation of compounds that are more harmful. Detection methods allow detecting even small concentrations of xenobiotics in samples, but the problem is the diversity and mix of compounds present in the environment, in which it is not known what their effects are). In this review, the division of xenobiotics and their detection methods will be presented.

Detection of Bioavailable Cadmium by Double-Color Fluorescence Based on a Dual-Sensing Bioreporter System

Cadmium (Cd) is carcinogenic to humans and can accumulate in the liver, kidneys, and bones. There is widespread presence of cadmium in the environment as a consequence of anthropogenic activities. It is important to detect cadmium in the environment to prevent further exposure to humans. Previous whole-cell biosensor designs were focused on single-sensing constructs but have had difficulty in distinguishing cadmium from other metal ions such as lead (Pb) and mercury (Hg). We developed a dual-sensing bacterial bioreporter system to detect bioavailable cadmium by employing CadC and CadR as separate metal sensory elements and eGFP and mCherry as fluorescent reporters in one genetic construct. The capability of this dual-sensing biosensor was proved to simultaneously detect bioavailable cadmium and its toxic effects using two sets of sensing systems while still maintaining similar specificity and sensitivity of respective signal-sensing biosensors. The productions of double-color fluorescence were directly proportional to the exposure concentration of cadmium, thereby serving as an effective quantitative biosensor to detect bioavailable cadmium. This novel dual-sensing biosensor was then validated to respond to Cd(II) spiked in environmental water samples. This is the first report of the development of a novel dual-sensing, whole-cell biosensor for simultaneous detection of bioavailable cadmium. The application of two biosensing modules provides versatile biosensing signals and improved performance that can make a significant impact on monitoring high concentration of bioavailable Cd(II) in environmental water to reduce human exposure to the harmful effects of cadmium.

In-situ monitoring of xenobiotics using genetically engineered whole-cell-based microbial biosensors: recent advances and outlook

Industrialisation, directly or indirectly, exposes humans to various xenobiotics. The increased magnitude of chemical pesticides and toxic heavy metals in the environment, as well as their intrusion into the food chain, seriously threatens human health. Therefore, the surveillance of xenobiotics is crucial for social safety and security. Online investigation by traditional methods is not sufficient for the detection and identification of such compounds because of the high costs and their complexity. Advancement in the field of genetic engineering provides a potential opportunity to use genetically modified microorganisms. In this regard, whole-cell-based microbial biosensors (WCBMB) represent an essential tool that couples genetically engineered organisms with an operator/promoter derived from a heavy metal-resistant operon combined with a regulatory protein in the gene circuit. The plasmid controls the expression of the reporter gene, such as gfp, luc, lux and lacZ, to an inducible gene promoter and has been widely applied to assay toxicity and bioavailability. This review summarises the recent trends in the development and application of microbial biosensors and the use of mobile genes for biomedical and environmental safety concerns.

Sensitive and Specific Cadmium Biosensor Developed by Reconfiguring Metal Transport and Leveraging Natural Gene Repositories

Whole-cell biosensors are useful for monitoring heavy metal toxicity in public health and ecosystems, but their development has been hindered by intrinsic trade-offs between sensitivity and specificity. Here, we demonstrated an effective engineering solution by building a sensitive, specific, and high-response biosensor for carcinogenic cadmium ions. We genetically programmed the metal transport system of Escherichia coli to enrich intracellular cadmium ions and deprive interfering metal species. We then selected 16 cadmium-sensing transcription factors from the GenBank database and tested their reactivity to 14 metal ions in the engineered E. coli using the expression of the green fluorescent protein as the readout. The resulting cadmium biosensor was highly specific and showed a detection limit of 3 nM, a linear increase in fluorescent intensities from 0 to 200 nM, and a maximal 777-fold signal change. Using this whole-cell biosensor, a smartphone, and low-tech equipment, we developed a simple assay capable of measuring cadmium ions at the same concentration range in irrigation water and human urine. This method is user-friendly and cost-effective, making it affordable to screen large amounts of samples for cadmium toxicity in agriculture and medicine. Moreover, our work highlights natural gene repositories as a treasure chest for bioengineering.

Emergent Biosensing Technologies Based on Fluorescence Spectroscopy and Surface Plasmon Resonance

The purpose of this work is to provide an exhaustive overview of the emerging biosensor technologies for the detection of analytes of interest for food, environment, security, and health. Over the years, biosensors have acquired increasing importance in a wide range of applications due to synergistic studies of various scientific disciplines, determining their great commercial potential and revealing how nanotechnology and biotechnology can be strictly connected. In the present scenario, biosensors have increased their detection limit and sensitivity unthinkable until a few years ago. The most widely used biosensors are optical-based devices such as surface plasmon resonance (SPR)-based biosensors and fluorescence-based biosensors. Here, we will review them by highlighting how the progress in their design and development could impact our daily life.

Spectroscopic Analysis of Fe Ion-Induced Fluorescence Quenching of the Green Fluorescent Protein ZsGreen

The fluorescence of fluorescent proteins (FPs) is quenched when they are exposed to certain transition metals, which makes them promising receptor materials for metal biosensors. In this study, we report the spectroscopic analysis of metal-induced fluorescence quenching of the fluorescent protein ZsGreen from Zoanthus sp. The fluorescence of ZsGreen was reduced to 2%, 1%, and 20% of its original intensity by Fe²⁺, Fe³⁺, and Cu²⁺, respectively. Metal titration experiments indicated that the dissociation constants of Fe²⁺, Fe³⁺, and Cu²⁺ for ZsGreen were 11.5, 16.3, and 68.2 μM, respectively. The maximum binding capacities of ZsGreen for Fe²⁺, Fe³⁺, and Cu²⁺ were 103.3, 102.2, and 82.9, respectively. Reversibility experiments indicated that the fluorescence of ZsGreen, quenched by Fe²⁺ and Fe³⁺, could be recovered, but only to about 15% of its original intensity, even at a 50-fold molar excess of EDTA. In contrast, the fluorescence quenched by Cu²⁺ could be recovered up to 89.47% of its original intensity at a Cu²⁺: EDTA ratio of 1:5. The homology model of ZsGreen revealed that the protein does not share any metal-binding sites with previously reported FPs, suggesting that ZsGreen contains unprecedented binding sites for fluorescence quenching metal ions.

Construction of a mApple-D6A3-mediated biosensor for detection of heavy metal ions

Pollution of heavy metals in agricultural environments is a growing problem to the health of the world’s human population. Green, low-cost, and efficient detection methods can help control such pollution. In this study, a protein biosensor, mApple-D6A3, was built from rice-derived Cd²⁺-binding protein D6A3 fused with the red fluorescent protein mApple at the N-terminus to detect the contents of heavy metals. Fluorescence intensity of mApple fused with D6A3 indicated the biosensor’s sensitivity to metal ions and its intensity was more stable under alkaline conditions. mApple-D6A3 was most sensitive to Cu²⁺, then Ni²⁺, then Cd²⁺. Isothermal titration calorimetry experiments demonstrated that mApple-D6A3 successfully bound to each of these three metal ions, and its ability to bind the ions was, from strongest to weakest, Cu²⁺  > Cd²⁺  > Ni²⁺. There were strong linear relationships between the fluorescence intensity of mApple-D6A3 and concentrations of Cd²⁺ (0–100 μM), Cu²⁺ (0–60 μM) and Ni²⁺ (0–120 μM), and their respective R² values were 0.994, 0.973 and 0.973. When mApple-D6A3 was applied to detect concentrations of heavy metal ions in water (0–0.1 mM) or culture medium (0–1 mM), its accuracy for detection attained more than 80%. This study demonstrates the potential of this biosensor as a tool for detection of heavy metal ions.

ZnAl nano layered double hydroxides for dual functional CRISPR/Cas9 delivery and enhanced green fluorescence protein biosensor

Evaluation of the effect of different parameters for designing a non-viral vector in gene delivery systems has great importance. In this manner, 2D crystals, precisely layered double hydroxides, have attracted the attention of scientists due to their significant adjustability and low-toxicity and low-cost preparation procedure. In this work, the relationship between different physicochemical properties of LDH, including pH, size, zeta potential, and synthesis procedure, was investigated and optimized for CRISPR/Cas9 delivery and reverse fluorescence response to the EGFP. In this manner, ZnAl LDH and ZnAl HMTA LDH were synthesized and characterized and applied in the HEK-293 cell line to deliver CRISPR/Cas9. The results were optimized by different characterizations as well as Gel Electrophoresis and showed acceptable binding ability with the DNA that could be considered as a promising and also new gold-standard for the delivery of CRISPR/Cas9. Also, the relationship of the presence of tertiary amines (in this case, hexamethylenetetramine (HMTA) as the templates) in the structure of the ZnAl LDH, as well as the gene delivery application, was evaluated. The results showed more than 79% of relative cell viability in most of the weight ratios of LDH to CRISPR/Cas9; fully quenching the fluorescence intensity of the EGFP/LDH in the presence of 15 µg mL-1 of the protoporphyrins along with the detection limit of below 2.1 µg mL-1, the transfection efficiency of around 33% of the GFP positive cell for ZnAl LDH and more than 38% for the ZnAl LDH in the presence of its tertiary amine template.

Spectroscopic and Structural Analysis of Cu2+-Induced Fluorescence Quenching of ZsYellow

Fluorescent proteins exhibit fluorescence quenching by specific transition metals, suggesting their potential as fluorescent protein-based metal biosensors. Each fluorescent protein exhibits unique spectroscopic properties and mechanisms for fluorescence quenching by metals. Therefore, the metal-induced fluorescence quenching analysis of various new fluorescent proteins would be important step towards the development of such fluorescent protein-based metal biosensors. Here, we first report the spectroscopic and structural analysis of the yellow fluorescent protein ZsYellow, following its metal-induced quenching. Spectroscopic analysis showed that ZsYellow exhibited a high degree of fluorescence quenching by Cu²⁺. During Cu²⁺-induced ZsYellow quenching, fluorescence emission was recovered by adding EDTA. The crystal structure of ZsYellow soaked in Cu²⁺ solution was determined at a 2.6 Å resolution. The electron density map did not indicate the presence of Cu²⁺ around the chromophore or the β-barrel surface, which resulted in fluorescence quenching without Cu²⁺ binding to specific site in ZsYellow. Based on these results, we propose the fluorescence quenching to occur in a distance-dependent manner between the metal and the fluorescent protein, when these components get to a closer vicinity at higher metal concentrations. Our results provide useful insights for future development of fluorescent protein-based metal biosensors.

Preparation of "Ion-Imprinting" Difunctional Magnetic Fluorescent Nanohybrid and Its Application to Detect Cadmium Ions

In this work, we have fabricated a novel difunctional magnetic fluorescent nanohybrid (DMFN) for the determination of cadmium ions (Cd²⁺) in water samples, where the “off-on” model and “ion-imprinting” technique were incorporated simultaneously. The DMFN were composed of CdTe/CdS core-shell quantum dots (QD) covalently linked onto the surface of polystyrene magnetic microspheres (PMM) and characterized using ultraviolet-visible spectroscopy (UV-Vis), fluorescence spectroscopy, and transmission electron microscopy (TEM). Based on the favorable magnetic and fluorescent properties of the DMFN, the chemical etching of ethylene diamine tetraacetic acid (EDTA) at the surface produced specific Cd²⁺ recognition sites and quenched the red fluorescence of outer CdTe/CdS QD. Under optimal determination conditions, such as EDTA concentration, pH, and interfering ions, the working curve of determining Cd²⁺ was obtained; the equation was obtained Y = 34,759X + 254,894 (R = 0.9863) with a line range 0.05–8 μM, and the detection limit was 0.01 μM. Results showed that synthesized magnetic fluorescent microspheres had high sensitivity, selectivity, and reusability in detection. Moreover, they have significant potential value in fields such as biomedicine, analytical chemistry, ion detection, and fluorescence labeling.

Specific heavy metal/metalloid sensors: current state and perspectives

Heavy metal(loid)s play pivotal roles in regulating physiological and developmental aspects in living organisms depending on their concentration. For example, a trace amount of heavy metal(loid)s is essential for living organisms, but heavy metal(loid)s in high concentrations negatively affect their physiology and development. Because of rapid industrial developments, heavy metal(loid)s have been accumulating in environmental systems, thereby becoming a threat to human health and the earth's ecosystem. Thus, the development of tools to quantify and monitor heavy metal(loid)s in environmental systems has become essential. Typically, risk has been determined through instrument-based analysis, regardless of the shortcomings regarding expense and duration. Nowadays, the need for alternative tools, besides instrumental analysis, to detect heavy metals has prompted the development of new techniques, and many different methods have been reported from various research areas, including new techniques based on electrochemistry and biological systems. Nonetheless, it seems that the gap between laboratory and fieldwork is still greater than it should be when it comes to applying these systems. In this mini-review, we discuss the current status of heavy metals/metalloid detection techniques, with an emphasis on biosensors. Moreover, we discuss the advantages and disadvantages as well as the mechanisms behind newly developed sensors and make suggestions to improve applicability and to develop new objective targeting sensors. Although many different types of metal(loid) sensors are available, we focused on metal sensors based on biological systems. Additionally, we suggest potent approaches to developing new biosensor systems based on current metal sensor mechanisms.

Heavy metal(loid) biosensor based on split-enhanced green fluorescent protein: development and characterization

Heavy metal(loid)s such as Cd and Hg adversely affect human health and are therefore strictly regulated and monitored; however, their quantitation in the environment is usually performed by expensive and time-consuming instrumental analysis techniques, which necessitates the search for more practical alternatives. Herein, we prepare enhanced green fluorescent protein (eGFP)-based biomolecules for metal(loid) sensing by insertion of metal-binding loops (MBLs) into a loop region of eGFP to render this protein inactive and show that the binding of metal ions to MBLs induces a conformational change and restores the original activity. Specifically, eGFP with an MBL sequenced as CTTCGCG regains fluorescence upon exposure to Cd and Hg, which allows the above metals to be quantified in the concentration range of 0-5 μM. For practical applicability verification, the developed sensing platform is used to quantify Cd in artificially amended soil and water samples. Although the obtained results imply that sensor performance needs to be significantly improved, the presented design concept is believed to be of high value to researchers in the field of heavy metal sensing and facilitate the development of new biosensors.