Spectrafluorimetric method for the rapid screening of toxic heavy metals in water samples

A fast and inexpensive sensitive screening test for recognising potential wastewater contamination by the presence of highly toxic heavy metals is described. The test is based on the reaction of the toxic heavy metals Hg(II), Cd(II), Pb(II) and Ag(I) with 6-mercaptopurine (6-MP) to produce highly fluorescent complexes.Optimum experimental conditions include a buffer of pH 7.2 (0.1 M citric acid/0.2 M Na₂HPO₄), a chelating reagent concentration of 6×10⁻⁴ M and the addition of 10⁻⁴ M of o-phenanthroline. The fluorescence emitted by the complexes was measured at 380 and 540 nm for excitation and emission wavelengths, respectively.Detection limits of 4, 3, 6 and 3 μg l⁻¹ were achieved for Hg, Cd, Pb and Ag. Relative standard deviation (R.S.D.) were between ±2 and ±6% of the fluorescence signals for five identical samples. Potential interference effects from other heavy metals (Zn, Mn, Co, Fe, Ni and Cu), which could affect the response of the proposed screening test was investigated. Results showed that none of these metals give rise to noticeable fluorescence signals under the above described experimental conditions.Finally, the capability of the proposed heavy metal screening test for the analysis of contaminated water samples is discussed.     

Simple optical fibre biosensor based on immobilised enzyme for monitoring of trace heavy metal ions

A simple optical fibre biosensor based on immobilised enzyme for monitoring of trace heavy metal ions has been developed. The biosensor recognition system was designed based on the inhibition of urease activity, where the urease is immobilised on ultrabind membrane. The studies of inhibition by the heavy metal ions Hg(II), Ag(I), Cu(II), Ni(II), Zn(II), Co(II) and Pb(II) were performed using a fibre-optic biosensor configuration, where the pH change resulting from the bio-catalytic hydrolysis of urea was monitored at the wavelength 615 nm spectroscopically, using commercial pH indicator strip before and after the exposure to the heavy metal ions. The immobilised urease was regenerated by l-cysteine. The linear response range between 1×10^-9–1×10^-5 M and the limit of detection 1×10^-9 M (0.2 g/L) for Hg(II) ions was achieved by employing the flow method. The optimisation of experimental parameters, including flow method, is also discussed.     

Cockle shell-derived nanoparticles for optical urea biosensor development based on reflectance transduction

An optical biosensor for urea based on urease enzyme immobilised on functionalised calcium carbonate nanoparticles (CaCO₃-NPs) was successfully developed in this study. CaCO₃-NPs were synthesised from discarded cockle shells via a simple and eco-friendly approach, followed by surface functionalisation with succinimide ester groups. The fabricated biosensor is comprised of two layers. The first (bottom layer) contained functionalised NPs covalently immobilised to urease, and the second (uppermost layer) was alginate hydrogel physically immobilised to the pH indicator phenolphthalein. The biosensor provided a colorimetric indication of increasing urea concentrations by changing from colourless to pink. Quantitative urea analysis was performed by measuring the reflectance intensity of the colour change at a wavelength of 633.16 nm. The determination of urea concentration using this biosensor yielded a linear response range of 30–1000 mM (R² = 0.9901) with a detection limit of 17.74 mM at pH 7.5. The relative standard deviation of reproducibility was 1.14%, with no signs of interference by major cations, such as K⁺, Na⁺, NH?⁺, and Mg²⁺. The fabricated biosensor showed no significant difference with the standard method for the determination of urea in urine samples.     

Interactions between heavy metals and photosynthetic materials studied by optical techniques

In this work studies on rapid inhibitory interactions between heavy metals and photosynthetic materials at different organization levels were carried out by optical assay techniques, investigating the possibility of applications in the heavy metal detection field. Spinach chloroplasts, thylakoids and Photosystem II proteins were employed as biotools in combination with colorimetric assays based on dichlorophenol indophenole (DCIP) photoreduction and on fluorescence emission techniques. It was found that copper and mercury demonstrated a strong and rapid photosynthetic activity inhibition, that varied from proteins to membranes, while other metals like nickel, cobalt and manganese produced only slight inhibition effects on all tested photosynthetic materials. By emission measurements, only copper was found to rapidly influence the photosynthetic material signals. These findings give interesting information about the rapid effects of heavy metals on isolated photosynthetic samples, and are in addition to the literature data concerning the effects of growth in heavy metal enriched media.     

Free and sol-gel immobilized alkaline phosphatase-based biosensor for the determination of pesticides and inorganic compounds

Alkaline-phosphatase (ALP) catalyses the hydrolysis of 1-naphthyl phosphate to fluorescent 1-naphthol (λ_ex=346 nm, λ_em=463 nm). This enzymatic reaction was investigated in presence of inhibitors: organochlorine (tetradifon), carbamate (metham-sodium) and organophosphorus pesticides (fenitrothion), heavy metal (Ag⁺) and CN⁻. The fluorescent signal, which is inversely dependent on the inhibitor concentration, is related to the amount of the inhibitor. Detection limits between 4.1 μM for tetradifon and 91.2 μM for metham-sodium were found. The relative standard deviation (R.S.D.) was between 2.6 and 6.2%.Sol-gel matrices derived from tetramethyl orthosilicate were doped with ALP using microencapsulation. The response of the biosensor based ALP sol-gel encapsulated to 1-naphthyl phosphate was reproducible (R.S.D.=6.6%). Inhibition plots obtained for test pesticides (metham-sodium and tetradifon) display linear calibration in the ranges 194-774 μM and 3.5-28 μM, detection limits of 4.9 and 292.3 μM and R.S.D. of 3.9 and 7.3% for metham-sodium and tetradifon, respectively. The results show that the system is able to detect class compounds such as pesticides and inorganic compounds.     

A blood-assisted optical biosensor for automatic glucose determination

A new approach for glucose determination in blood based on the spectroscopic properties of blood hemoglobin (Hb) is presented. The biosensor consists of a glucose oxidase (GOx) entrapped polyacrylamide (PAA) film placed in a flow cell. Blood is simply diluted with bidistilled water (150:1, v:v) and injected into the carrier solution. When reaching the PAA film, the blood glucose reacts with the GOx and the resulting H₂O₂ reacts with the blood Hb. This produces an absorbance change in this compound. The GOx-PAA film can be used at least 100 times. Lateral reactions of H₂O₂ with other blood constituents are easily blocked (by azide addition). The linear response range can be fitted between 20 and 1200 mg dL⁻¹ glucose (R.S.D. 4%, 77 mg dL⁻¹). In addition to the use of untreated blood, two important analytical aspects of the method are: (1) the analyte concentration can be obtained by an absolute calibration method; and (2) the signal is not dependent on the oxygen concentration.A mathematical model relating the Hb absorbance variation during the reaction with the glucose concentration has been developed to provide theoretical support and to predict its application to other compounds after changing the GOx by another enzyme. The method has been applied to direct glucose determination in 10 blood samples, and a correlation coefficient higher than 0.98 was obtained after comparing the results with those determined by an automatic analyzer. As well as sharing some of the advantages of disposable amperometric biosensors, the most significant feature of this approach is its reversibility.     

Optical biosensor for the determination of BOD in seawater

An automatic sensing system was developed using an optical BOD sensing film. The sensing film consists of an organically modified silicate (ORMOSIL) film embedded with an oxygen-sensitive Ru complex. A multi-microorganisms immobilization method was developed for the BOD sensing film preparation. Three different kinds of microorganisms, Bacillus licheniformis, Dietzia maris and Marinobacter marinus from seawater, were immobilized on a polyvinyl alcohol ORMOSILs. After preconditioning, the BOD biosensor could steadily perform well up to 10 months. The linear fluctuant coefficients (R²) in the range of 0.3-40 mg L⁻¹ was 0.985 when a glucose/glutamate BOD standard was applied. The reproducible response for the BOD sensing film could be obtained within ±2.3% of the mean value in a series of 10 samples in 5.0 mg L⁻¹ BOD standard GGA solution. The effects of temperature, pH and sodium chloride concentration on the two microbial films were studied as well. The BOD sensing system was tested and applied for the BOD determination of seawater.     

Liquid crystal-based sensors for the detection of heavy metals using surface-immobilized urease

In this study, a new method for the detection of heavy metals in aqueous phase was developed using liquid crystals (LCs). When UV-treated nematic LC, 4-cyano-4'-pentyl biphenyl (5CB) that was confined in the urease-modified gold grid was immersed in a urea solution, an optical response from bright to dark was observed under a polarized microscope, indicating that a planar-to-homeotropic orientational transition of the LC occurred at the aqueous/LC interface. Since urease hydrolyzes urea to produce ammonia, which would be ionized into ammonium and hydroxide ions, the main product of the photochemically degraded 5CB, 4-cyano-4'-biphenylcarboxylic acid (CBA), was deprotonated and self-assembled at the interface, inducing the orientational transition in the LC. Due to the high sensitivity and rapid response of this system, detection of heavy metal ions was further exploited. The divalent copper ion, which could effectively inhibit the activity of urease, was used as a model heavy metal ion. The optical appearance of the LC did not change when urea was in contact with the copper nitrate hydrate-blocked urease. After the copper-inhibited urease was reactivated by EDTA, a bright-to-dark shift in the optical signal was regenerated, indicating an orientational transition of the LC. This type of LC-based sensor shows high spatial resolution due to its optical characteristics and therefore could potentially be used to accurately monitor the presence of enzyme inhibitors such as heavy metal ions in real-time.     

Three-step extraction procedure for determination of heavy metals availability to vegetables

A three-step sequential extraction procedure was used to determine the concentration of heavy metal speciation forms in soil. The procedure allows one to identify the pool of heavy metals that can be potentially mobilised under changes in soil pH value or redox potential. It has been shown that similar portions of heavy metals are present in reducible, oxidisable and residual fractions. It was found that soil chemical properties significantly affected the distribution of heavy metals among different fractions and their uptake by vegetables. Cadmium was a dominant element which occurred in the exchangeable fraction—the most bioavailable and potentially toxic.     

Feasibility of the use of disposable optical tongue based on neural networks for heavy metal identification and determination

This study presents the development and characterization of a disposable optical tongue for the simultaneous identification and determination of the heavy metals Zn(II), Cu(II) and Ni(II). The immobilization of two chromogenic reagents, 1-(2-pyridylazo)-2-naphthol and Zincon, and their arrangement forms an array of membranes that work by complexation through a co-extraction equilibrium, producing distinct changes in color in the presence of heavy metals. The color is measured from the image of the tongue acquired by a scanner working in transmission mode using the H parameter (hue) of the HSV color space, which affords robust and precise measurements. The use of artificial neural networks (ANNs) in a two-stage approach based on color parameters, the H feature of the array, makes it possible to identify and determine the analytes. In the first stage, the metals present above a threshold of 10⁻⁷ M are identified with 96% success, regardless of the number of metals present, using the H feature of the two membranes. The second stage reuses the H features in combination with the results of the classification procedure to estimate the concentration of each analyte in the solution with acceptable error. Statistical tests were applied to validate the model over real data, showing a high correlation between the reference and predicted heavy metal ion concentration.     

Toxicity assessment of heavy metals and organic compounds using CellSense biosensor with E.coli

A new strategy using an arnperometric biosensor with Escherichia coli （E. coli） that provides a rapid toxicity determination of chemical compounds is described. The CellSense biosensor system comprises a biological component immobilized in intimate contact with a transducer which converts the biochemical signal into a quantifiable electrical signal. Toxicity assessment of heavy metals using E.coli biosensors could be finished within 30 min and the 50% effective concentrations （ECso） values of four heavy metals were determined. The results shows that inhibitory effects of four heavy metals to E.coli can be ranked in a decreasing order of Hg^2＋ 〉 Cu^2＋ 〉 Zn^2＋ 〉 Ni^2＋, which accords to the results of conventional bacterial counting method. The toxicity test of organic compounds by using CellSense biosensor was also demonstrated. The CellSense biosensor with E. coli shows a good, reproducible behavior and can be used for reproducible measurements.     

溶胶凝胶法制备固定化酶柱并应用于化学发光型的葡萄糖传感器

溶胶凝胶过程以期纯度高,均匀性强,处理温度低,反应条件易于控制等优点成为生物传感器中一种颇具前途的固化方法。利用硅酸乙酯的溶胶凝胶化过程对葡萄糖氧化酶进行固化,并制备了不同载体下的ＧＯＤ酶柱。实验结果表明ＧＯＤ可在ＳｉＯ２的溶胶凝胶体中保持较高的活性。     

ICP-MS determination of heavy metals in submerged cultures of wood-rotting fungi

Accumulation of five heavy metal ions by five species of wood-rotting basidiomycetes during a 9-day cultivation was studied. Contents of Cd, Cu, Pb, and Zn were measured using ICP-MS; the amount of mercury was determined directly in solid samples using the Advanced Mercury Analyser. A standard operation procedure for the sample preparation and determination of metal content was developed and validated. Presence of Cd, Cu, Hg, and Pb decreased the accumulation of zinc by the fungi. The basidiomycete Pycnoporus cinnabarinus exhibited the highest metal binding capacity of all fungi tested.     

Glucose concentration determination based on silica sol-gel encapsulated glucose oxidase optical biosensor arrays

Optical biosensor arrays for rapidly determining the glucose concentrations in a large number of beverage and blood samples were developed by immobilizing glucose oxidase (GOD) on oxygen sensor layer. Glucose oxidase was first encapsulated in silica based gels through sol-gel approach and then immobilized on 96-well microarrays integrated with oxygen sensing film at the bottom. The oxygen sensing film was made of an organically modified silica film (ORMOSIL) doped with tris(4,7-diphenyl-1,10-phenanthroline) ruthenium dichloride (Ru(dpp)₃Cl₂). The oxidation reaction of glucose by glucose oxidase could be monitored through fluorescence intensity enhancement due to the oxygen consumption in the reaction. The luminescence changing rate evaluated by the dynamic transient method (DTM) was correlated with the glucose concentration with the wide linear range from 0.1 to 5.0 mM (Y = 13.28X − 0.128, R = 0.9968) and low detection limit (0.06 mM). The effects of pH and coexisting ions were systemically studied. The results showed that the optical biosensor arrays worked under a wide range of pH value, and normal interfering species such as Na⁺, K⁺, Cl⁻, PO₄³⁻, and ascorbic acid did not cause apparent interference on the measurement. The activity of glucose oxidase was mostly retained even after 2-month storage, indicating their long-term stability.     

Simultaneous determination of traces of heavy metals by solid-phase spectrophotometry

A coupling sensitive solid phase spectrophotometric (SPS) procedure for determination of traces of heavy metals (Me-SPS) and multicomponent analysis by multiple linear regressions (MA), a simple methodology for simultaneous determination of metals in mixtures was inaugurated. The Me-SPS procedure is based on sorption of heavy metals on PAN-resin and direct absorbance measurements of colour product Me-PAN sorbed on a solid carrier in a 1-mm cell. This methodology (Me-SPS-MA) was checked by simultaneous determination of metals in synthetic mixtures with different compositions and contents of metals important in pharmaceutical practice: Zn, Pb, Cd, Cu, Co, and Ni. Good agreement between experimental and theoretical amounts of heavy metals is obtained from the recovery test (78.3–110.0%). The proposed method enables determination of particular metal ion at the ng mL⁻¹ level and it was successfully applied to the determination impurities from heavy metal traces in pharmaceutical substances (Cu in ascorbic acid, Pb in glucose, and Zn in insulin). The proposed procedure could be possible contribution to the development of pharmacopoeial methodology for a heavy metals test.     

Signal enhancement of electrochemical DNA biosensors for the detection of trace heavy metals

Heavy-metal pollution has attracted intensive attention from the public because of the severe threats of heavy metals to the ecosystem and human health. Ultralow concentration of heavy metals in aquatic environment leads to the urgent needs of sensitive approaches for heavy-metal detection. Electrochemical DNA biosensors present outstanding superiority in convenience, selectivity, and sensitivity compared with conventional methods. To achieve the ultralow detection limit, efforts have been made to implement signal enhancement strategies to develop electrochemical DNA biosensors with enhanced sensing performance. This review focuses on the recent progress in signal enhancement strategies applied to electrochemical DNA biosensors for heavy-metal-ion detection including nicking enzyme–assisted amplification, the utilization of core–shell nanoparticles, and nanocomposites modification.     

A new optical platform for biosensing based on fluorescence anisotropy

A novel fluorescence-based optical platform for the interrogation of an optical biochip was designed and developed. The optical biochip was made of poly(methyl methacrylate) (PMMA) formed by two pieces of PMMA appropriately shaped in order to obtain four microchannels that are 500-μm wide and 400-μm high. The lower part includes the microchannels and the inlet and outlet for the fluidics, while the sensing biolayer was immobilized on the upper part. The optical signal comprised the fluorescence emitted by the biolayer, which was anisotropically coupled to the PMMA cover and suitably guided by the PMMA chip. The potentiality of the optical chip as a biosensor was investigated by means of a direct IgG/anti-IgG interaction carried out inside the flow channels. The mouse-IgG was covalently immobilized on the internal wall of the PMMA cover, and the Cy5-labelled anti-mouse IgG was used for the specific interaction. Several chemical treatments of the PMMA surface were investigated, poly(L-lactic acid), Eudragit L100 and NaOH, in order to obtain the most effective distribution of carboxylic groups useful for the covalent immobilisation of the mouse-IgG. The treatment with Eudragit L100 was found to be the most successful. Limits of detection and quantification of 0.05 μg mL⁻¹ and 0.2 μg mL⁻¹, respectively, were obtained with the configuration described.     

Layer double hydroxides (LDHs)- based electrochemical and optical sensing assessments for quantification and identification of heavy metals in water and environment samples: A review of status and prospects

One of the most severe environmental problems is heavy metal contamination, putting the world's sustainability at risk. Much effort has been put into developing sensors that can be taken anywhere to detect the environmental effects of heavy metals. Sensitivity, selectivity, multiplexed detection ability, and mobility enhance significantly when nanoparticles and nanostructures are incorporated into sensors. LDHs (layered double hydroxides) have gotten much attention in analytical chemistry in recent years because of their benefits, including their large specific surface area, ease of synthesis, low cost, and high catalytic efficiency and biocompatibility. LDHs are often manufactured as nanomaterial composites or created with specialized three-dimensional structures depending on the application. However, in these settings, LDHs (as color indicators, extracting sorbents, and electrochemical sensing) are usually restricted. Upcoming signs of progress and development possibilities of LDHs in analytical chemistry are reviewed in this paper to assist overcome these problems. Furthermore, the approaches used in the design of LDHs, including structural aspects, are defined and assessed in preparation for future analytical applications. The latest advances in optical and electrochemical sensors to detect heavy metals are described in this review. The sorts and characteristics of LDHs will be explored first. We will then go into microelectrode (or nanoelectrode) arrays, nanoparticle-modified electrodes, and microfluidic optical and electrochemical sensing assays in detail. This paper also discusses design strategies for LDH-based nanostructured sensors and the advantages of using nanomaterials and nanostructures.     

Sol-gel derived metal dispersed ceramic-graphite composite electrode for amperometric determination of dopamine

A new silver dispersed composite electrode made of sol-gel derived ceramic-graphite has been demonstrated for the determination of dopamine. Silver has been immobilized within the porous and rigid silicate network in the composite by the specific interaction with (3-mercaptopropyl)trimethoxysilane (MPS). The modified composite electrode coated with Nafion exhibited good catalytic activity for the oxidation of dopamine at a reduced potential of 0.35 V with good sensitivity and selectivity. The sensor showed a linear response to dopamine in the concentration range from 6.6×10⁻⁶ to 1.2×10⁻³ M with a correlation coefficient of 0.9987. The electrode surface can be easily renewed by a simple mechanical polishing and has the advantages of good reproducibility, rapid response and remarkable stability.     

Assessment of heavy metals leachability from metallo-organic sorbent-iron humate-with the aid of sequential extraction test

The sequential extraction test, known as a BCR procedure, was used to assess a leachability of heavy metals (Zn, Cd, Pb, Cu) from the metallo-organic sorbent—iron humate—loaded with these metals. The sequential test allowed to discriminate between various fractions of heavy metals, namely the acid-extractable fraction, the fraction bound to Fe oxides, and the fraction bound to organic matter. It was proven that the heavy metals are bound mainly to Fe oxides and organic matter, and thus they may be relatively hardly liberated into the environment. The BCR sequential extraction test exhibited a very good repeatability, when it was applied to the loaded sorbent—relative standard deviations were typically lower than 10%.