Immobilization of metallothionein as a sensitive biosensor chip for the detection of metal ions by surface plasmon resonance

On the Strong Binding Affinity of Gold-Graphene Heterostructures with Heavy Metal Ions in Water: A Theoretical and Experimental Investigation

Minimum energy configurations in 2D material-based heterostructures can enable interactions with external chemical species that are not observable for their monolithic counterparts. Density functional theory (DFT) calculations reveal that the binding energy of divalent toxic metal ions of Cd, Pb, and Hg on graphene-gold heterointerfaces is negative, in contrast to the positive value associated with free-standing graphene. The theoretical predictions are confirmed experimentally by Surface Plasmon Resonance (SPR) spectroscopy, where a strong binding affinity is measured for all the heavy metal ions in water. The results indicate the formation of a film of heavy metal ions on the graphene-gold (Gr/Au) heterointerfaces, where the adsorption of the ions follows a Langmuir isotherm model. The highest thermodynamic affinity constant K = 3.1 × 107 L mol-1 is observed for Hg2+@Gr/Au heterostructures, compared to 1.1 × 107 L mol-1 and 8.5 × 106 L mol-1 for Pb2+@Gr/Au and Cd2+@Gr/Au, respectively. In the case of Hg2+ ions, it was observed a sensitivity of about 0.01°/ppb and a detection limit of 0.7 ppb (∼3 nmol L-1). The combined X-ray photoelectron spectroscopy (XPS) and SPR analysis suggests a permanent interaction of all of the HMIs with the Gr/Au heterointerfaces. The correlation between the theoretical and experimental results indicates that the electron transfer from the graphene-gold heterostructures to the heavy metal ions is the key for correct interpretation of the enhanced sensitivity of the SPR sensors in water.

Surface plasmon resonance and microscale thermophoresis approaches for determining the affinity of perforin for calcium ions

Perforin is a pore-forming protein that plays a crucial role in the immune system by clearing virus-infected or tumor cells. It is released from cytotoxic granules of immune cells and forms pores in targeted lipid membranes to deliver apoptosis-inducing granzymes. It is a very cytotoxic protein and is therefore adapted not to act in producing cells. Its activity is regulated by the requirement for calcium ions for optimal activity. However, the exact affinity of perforin for calcium ions has not yet been determined. We conducted a molecular dynamics simulation in the absence or presence of calcium ions that showed that binding of at least three calcium ions is required for stable perforin binding to the lipid membrane. Biophysical studies using surface plasmon resonance and microscale thermophoresis were then performed to estimate the binding affinities of native human and recombinant mouse perforin for calcium ions. Both approaches showed that mouse perforin has a several fold higher affinity for calcium ions than that of human perforin. This was attributed to a particular residue, tryptophan at position 488 in mouse perforin, which is replaced by arginine in human perforin. This represents an additional mechanism to control the activity of human perforin.

Engineered Whole-Cell-Based Biosensors: Sensing Environmental Heavy Metal Pollutants in Water-a Review

The frequent exposure and accumulation of heavy metals in organisms cause serious health issues affecting a range of organs such as the brain, liver, and reproductive organs in adults, infants, and children. Several parts of the world have high levels of heavy metals affecting millions of people, costing millions of dollars for improving the potability of water and medical treatment of the affected. Hence, water quality assessment is required to monitor the degree of heavy metal contamination in potable water. In nature, organisms respond to various environmental pollutants such as heavy metals, allowing their survival in a diverse environmental niche. With the advent of recombinant DNA technology, it is now possible to manipulate these natural bioreporters into controlled systems which either turn on or off gene expression or activity of enzymes in the presence of specific heavy metals (compound-specific biosensors) otherwise termed as whole-cell biosensors (WCBs). WCBs provide an upper hand compared to other immunosensors, enzyme-based sensors, and DNA-based sensors since microbes can be relatively easily manipulated, scaled up with relative ease, and can detect only the bioavailable heavy metals. In this review, we summarize the current knowledge of the various mechanisms of toxicity elicited by various heavy metals, thence emphasizing the need to develop heavy metal sensing platforms. Following this, the biosensor-based platforms including WCBs for detecting heavy metals developed thus far have been briefly elaborated upon, emphasizing the challenges and solutions associated with WCBs.

X-ray Photoelectron Spectroscopy Analysis of Chitosan-Graphene Oxide-Based Composite Thin Films for Potential Optical Sensing Applications

In this study, X-ray photoelectron spectroscopy (XPS) was used to study chitosan-graphene oxide (chitosan-GO) incorporated with 4-(2-pyridylazo)resorcinol (PAR) and cadmium sulfide quantum dot (CdS QD) composite thin films for the potential optical sensing of cobalt ions (Co2+). From the XPS results, it was confirmed that carbon, oxygen, and nitrogen elements existed on the PAR-chitosan-GO thin film, while for CdS QD-chitosan-GO, the existence of carbon, oxygen, cadmium, nitrogen, and sulfur were confirmed. Further deconvolution of each element using the Gaussian-Lorentzian curve fitting program revealed the sub-peak component of each element and hence the corresponding functional group was identified. Next, investigation using surface plasmon resonance (SPR) optical sensor proved that both chitosan-GO-based thin films were able to detect Co2+ as low as 0.01 ppm for both composite thin films, while the PAR had the higher binding affinity. The interaction of the Co2+ with the thin films was characterized again using XPS to confirm the functional group involved during the reaction. The XPS results proved that primary amino in the PAR-chitosan-GO thin film contributed more important role for the reaction with Co2+, as in agreement with the SPR results.

Grating-coupled interferometry reveals binding kinetics and affinities of Ni ions to genetically engineered protein layers

Reliable measurement of the binding kinetics of low molecular weight analytes to their targets is still a challenging task. Often, the introduction of labels is simply impossible in such measurements, and the application of label-free methods is the only reliable choice. By measuring the binding kinetics of Ni(II) ions to genetically modified flagellin layers, we demonstrate that: (1) Grating-Coupled Interferometry (GCI) is well suited to resolve the binding of ions, even at very low protein immobilization levels; (2) it supplies high quality kinetic data from which the number and strength of available binding sites can be determined, and (3) the rate constants of the binding events can also be obtained with high accuracy. Experiments were performed using a flagellin variant incorporating the C-terminal domain of the nickel-responsive transcription factor NikR. GCI results were compared to affinity data from titration calorimetry. We found that besides the low-affinity binding sites characterized by a micromolar dissociation constant (Kd), tetrameric FliC-NikRC molecules possess high-affinity binding sites with Kd values in the nanomolar range. GCI enabled us to obtain real-time kinetic data for the specific binding of an analyte with molar mass as low as 59 Da, even at signals lower than 1 pg/mm2.

Investigating the Properties of Cetyltrimethylammonium Bromide/Hydroxylated Graphene Quantum Dots Thin Film for Potential Optical Detection of Heavy Metal Ions

The modification of graphene quantum dots (GQDs) may drastically enhance their properties, therefore resulting in various related applications. This paper reported the preparation of novel cetyltrimethylammonium bromide/hydroxylated graphene quantum dots (CTAB/HGQDs) thin film using the spin coating technique. The properties of the thin film were then investigated and studied. The functional groups existing in CTAB/HGQDs thin film were confirmed by the Fourier transform infrared (FTIR) spectroscopy, while the atomic force microscope (AFM) displayed a homogenous surface of the thin film with an increase in surface roughness upon modification. Optical characterizations using UV-Vis absorption spectroscopy revealed a high absorption with an optical band gap of 4.162 eV. Additionally, the photoluminescence (PL) spectra illustrated the maximum emission peak of CTAB/HGQDs thin film at a wavelength of 444 nm. The sensing properties of the as-prepared CTAB/HGQDs thin film were studied using a surface plasmon resonance technique towards the detection of several heavy metal ions (HMIs) (Zn2+, Ni2+, and Fe3+). This technique generated significant results and showed that CTAB/HGQDs thin film has great potential for HMIs detection.

Cry Protein Crystal-Immobilized Metallothioneins for Bioremediation of Heavy Metals from Water

Cry proteins have been the subject of intense research due to their ability to form crystals naturally in Bacillus thuringiensis (Bt). In this research we developed a new strategy that allows for the removal of cadmium and chromium from wastewater by using one Cry protein, Cry3Aa, as a framework to immobilize tandem repeats of the cyanobacterial metallothionein SmtA from Synechococcus elongatus (strain PCC 7942). SmtA is a low molecular weight cysteine-rich protein known to bind heavy metals. A series of Cry3Aa-SmtA constructs were produced by the fusion of one, three, or six tandem repeats of SmtA to Cry3Aa. Overexpression of these constructs in Bt resulted in the production of pure Cry3Aa-SmtA fusion crystals that exhibited similar size, crystallinity, and morphology to that of native Cry3Aa protein crystals. All three Cry3Aa-SmtA constructs exhibited efficient binding to cadmium and chromium, with the binding capacity correlated with increasing SmtA copy number. These results suggest the potential use of Cry3Aa-SmtA crystals as a novel biodegradable and cost-effective approach to the removal of toxic heavy metals from the environment.

Label-free optical spectroscopy for characterizing binding properties of highly sensitive nanocrystalline cellulose-graphene oxide based nanocomposite towards nickel ion

Surface plasmon resonance (SPR) is a label-free optical spectroscopy that is widely used for biomolecular interaction analysis. In this work, SPR was used to characterize the binding properties of highly sensitive nanocrystalline cellulose-graphene oxide based nanocomposite (CTA-NCC/GO) towards nickel ion. The formation of CTA-NCC/GO nanocomposite has been confirmed by FT-IR. The SPR analysis result shows that the CTA-NCC/GO has high binding affinity towards Ni²⁺ from 0.01 until 0.1 ppm with binding affinity constant of 1.620 × 10³ M^-1. The sensitivity for the CTA-NCC/GO calculated was 1.509° ppm^-1. The full width at half maximum (FWHM), data accuracy (DA), and signal-to-noise ratio (SNR) have also been determined using the obtained SPR curve. For the FWHM, the value was 2.25° at 0.01 until 0.08 ppm and decreases to 2.12° at 0.1 until 10 ppm. The DA for the SPR curves is the highest at 0.01 until 0.08 ppm and lowest at 0.1 until 10 ppm. The SNR curves mirrors the curves of SPR angle shift where the SNR increases with the Ni²⁺ concentrations. For the selectivity test, the CTA-NCC/GO has the abilities to differentiate Ni²⁺ in the mixture of metal ions.

Exploration of surface plasmon resonance for sensing copper ion based on nanocrystalline cellulose-modified thin film

In this research, surface plasmon resonance (SPR) spectroscopy was used for sensing copper ion by combining the SPR with nanocrystalline cellulose modified by hexadecyltrimethylammonium bromide and graphene oxide composite (CTA-NCC/GO) thin film. The binding of Cu²⁺ on CTA-NCC/GO thin film was monitored by using SPR spectroscopy. By using the obtained SPR curve, detection range, binding affinity, sensitivity, full width at half maximum (FWHM), data accuracy (DA), and signal-to-noise ratio (SNR) have been calculated. The results showed that the sensor detection range was 0.01 until 0.5 ppm, and that it reached a saturation value. Moreover, the resonance angle shift followed the Langmuir isotherm model with a binding affinity constant of 4.075 × 10³ M^-1. A high sensitivity of 3.271° ppm^-1 also was obtained for low Cu²⁺ concentration ranged from 0.01 to 0.1 ppm. For the FWHM, the lowest value calculated was at 0.08 and 0.1 ppm, which is 3.35°. The DA of the SPR signal consecutively highest at 0.08 and 0.1 ppm. Besides that, the SNR of the SPR signal increases with the Cu²⁺ concentrations. The CTA-NCC/GO thin film morphological properties were also studied by using atomic force microscopy. The rms roughness values, which were obtained before and after in contact with Cu^2+, were 3.51 nm and 2.46 nm, respectively.

pH Controlled Impedimetric Sensing of Copper(II) Ion Using Oxytocin as Recognition Element

We report the modulation of the specific metal gation properties of a peptide and demonstrate a highly selective sensor for copper(II) ion. The neuropeptide oxytocin (OT) is reported for its high affinity towards Zn2+ and Cu2+ at physiological pH. The binding of the metal ions to OT is tuned by altering the pH of the medium. OT was self-assembled on glassy carbon electrode using surface chemistry, and electrochemical impedance spectroscopy (EIS) was used to probe the binding of Cu2+. Our results clearly indicate that at pH 10.0, the binding of Cu2+ to OT is increased compared to that at pH 7.0, while the binding to Zn2+ becomes almost negligible. This proves that the selectivity of OT towards each of the ions can be regulated simply by controlling the pH of the medium and hence allows the preparation of a sensing device with selectivity to Cu2+.

Oxytocin-Monolayer-Based Impedimetric Biosensor for Zinc and Copper Ions

Zinc and copper are essential metal ions for numerous biological processes. Their levels are tightly maintained in all body organs. Impairment of the Zn²⁺ to Cu²⁺ ratio in serum was found to correlate with many disease states, including immunological and inflammatory disorders. Oxytocin (OT) is a neuropeptide, and its activity is modulated by zinc and copper ion binding. Harnessing the intrinsic properties of OT is one of the attractive ways to develop valuable metal ion sensors. Here, we report for the first time an OT-based metal ion sensor prepared by immobilizing the neuropeptide onto a glassy carbon electrode. The developed impedimetric biosensor was ultrasensitive to Zn²⁺ and Cu²⁺ ions at physiological pH and not to other biologically relevant ions. Interestingly, the electrochemical impedance signal of two hemicircle systems was recorded after the attachment of OT to the surface. These two semicircles suggest two capacitive regions that result from two different domains in the OT monolayer. Moreover, the change in the charge-transfer resistance of either Zn²⁺ or Cu²⁺ was not similar in response to binding. This suggests that the metal-dependent conformational changes of OT can be translated to distinct impedimetric data. Selective masking of Zn²⁺ and Cu²⁺ was used to allow for the simultaneous determination of zinc to copper ions ratio by the OT sensor. The OT sensor was able to distinguish between healthy control and multiple sclerosis patients diluted sera samples by determining the Zn/Cu ratio similar to the state-of-the-art techniques. The OT sensor presented herein is likely to have numerous applications in biomedical research and pave the way to other types of neuropeptide-derived sensors.

Binding Affinity of a Highly Sensitive Au/Ag/Au/Chitosan-Graphene Oxide Sensor Based on Direct Detection of Pb2+ and Hg2+ Ions

The study of binding affinity is essential in surface plasmon resonance (SPR) sensing because it allows researchers to quantify the affinity between the analyte and immobilised ligands of an SPR sensor. In this study, we demonstrate the derivation of the binding affinity constant, K, for Pb²⁺ and Hg²⁺ ions according to their SPR response using a gold/silver/gold/chitosan-graphene oxide (Au/Ag/Au/CS-GO) sensor for the concentration range of 0.1-5 ppm. The higher affinity of Pb²⁺ to binding with the CS-GO sensor explains the outstanding sensitivity of 2.05 °ppm^-1 against 1.66 °ppm^-1 of Hg²⁺. The maximum signal-to-noise ratio (SNR) upon detection of Pb²⁺ is 1.53, and exceeds the suggested logical criterion of an SNR. The Au/Ag/Au/CS-GO SPR sensor also exhibits excellent repeatability in Pb²⁺ due to the strong bond between its functional groups and this cation. The adsorption data of Pb²⁺ and Hg²⁺ on the CS-GO sensor fits well with the Langmuir isotherm model where the affinity constant, K, of Pb²⁺ and Hg²⁺ ions is computed. The affinity of Pb²⁺ ions to the Au/Ag/Au/CS-GO sensor is significantly higher than that of Hg²⁺ based on the value of K, 7 × 10⁵ M^-1 and 4 × 10⁵ M^-1, respectively. The higher shift in SPR angles due to Pb²⁺ and Hg²⁺ compared to Cr³⁺, Cu²⁺ and Zn²⁺ ions also reveals the greater affinity of the CS-GO SPR sensor to them, thus supporting the rationale for obtaining K for these two heavy metals. This study provides a better understanding on the sensing performance of such sensors in detecting heavy metal ions.

A Simple Metallothionein-Based Biosensor for Enhanced Detection of Arsenic and Mercury

Metallothioneins (MTs) are a family of cysteine-rich proteins whose biological roles include the regulation of essential metal ions and protection against the harmful effects of toxic metals. Due to its high affinity for many toxic, soft metals, recombinant human MT isoform 1a was incorporated into an electrochemical-based biosensor for the detection of As³⁺ and Hg²⁺. A simple design was chosen to maximize its potential in environmental monitoring and MT was physically adsorbed onto paper discs placed on screen-printed carbon electrodes (SPCEs). This system was tested with concentrations of arsenic and mercury typical of contaminated water sources ranging from 5 to 1000 ppb. The analytical performance of the MT-adsorbed paper discs on SPCEs demonstrated a greater than three-fold signal enhancement and a lower detection limit compared to blank SPCEs, 13 ppb for As³⁺ and 45 ppb for Hg²⁺. While not being as low as some of the recommended drinking water limits, the sensitivity of the simple MT-biosensor would be potentially useful in monitoring of areas of concern with a known contamination problem. This paper describes the ability of the metal binding protein metallothionein to enhance the effectiveness of a simple, low-cost electrochemical sensor.

A metal-responsive interdigitated bilayer for selective quantification of mercury(ii) traces by surface plasmon resonance

An innovative design of reusable SPR chips allowing the quantitative and selective determination of mercury(ii) at the ppt level is reported.

Optical nano antennas: state of the art, scope and challenges as a biosensor along with human exposure to nano-toxicology

The concept of optical antennas in physical optics is still evolving. Like the antennas used in the radio frequency (RF) regime, the aspiration of optical antennas is to localize the free propagating radiation energy, and vice versa. For this purpose, optical antennas utilize the distinctive properties of metal nanostructures, which are strong plasmonic coupling elements at the optical regime. The concept of optical antennas is being advanced technologically and they are projected to be substitute devices for detection in the millimeter, infrared, and visible regimes. At present, their potential benefits in light detection, which include polarization dependency, tunability, and quick response times have been successfully demonstrated. Optical antennas also can be seen as directionally responsive elements for point detectors. This review provides an overview of the historical background of the topic, along with the basic concepts and parameters of optical antennas. One of the major parts of this review covers the use of optical antennas in biosensing, presenting biosensing applications with a broad description using different types of data. We have also mentioned the basic challenges in the path of the universal use of optical biosensors, where we have also discussed some legal matters.

Highly sensitive and selective chip-based fluorescent sensor for mercuric ion: development and comparison of turn-on and turn-off systems

Miniaturization is currently an important trend in environmental and food monitoring because it holds great promise for on-site monitoring and detection. We report here two ready-to-use chip-based fluorescent sensors, compatible with microarray technology for reagentless, one-step, fast, highly sensitive and selective detection of the mercuric ion (Hg(2+)) in the turn-on and turn-off operation modes. Both operation modes are based on the highly selective T-Hg(2+)-T coordination between two neighboring polythymine (T) strands at a high probe density and its induced displacement of the complementary polyadenine strand labeled with either fluorophore or quencher, which enables the turn-off and turn-on detection of Hg(2+), respectively. The turn-off sensor is slightly more sensitive than the turn-on sensor, and their detection limits are 3.6 and 8.6 nM, respectively, which are both lower than the U.S. Environmental Protection Agency limit of [Hg(2+)] for drinkable water (10 nM, 2 ppb). Compared to the turn-off sensor with the dynamic Hg(2+) detection range from 3.6 nM to 10 μM (R(2) = 0.99), the turn-on sensor has a broader dynamic Hg(2+) detection range, from 8.6 nM to 100 μM (R(2) = 0.996). Both sensors exhibited superior selectivity over other reported sensors using thymine-rich probes for Hg(2+) detection over other common metal ions. In addition, the practical application of the chip-based sensors was demonstrated by detecting spiked Hg(2+) in drinking water and fresh milk. The sensor has great potential for on-site practical applications due to its operational convenience, simplicity, speed, and portability.

Optical biosensors for food quality and safety assurance-a review

Food quality and safety is a scientific discipline describing handling, preparation and storage of food in ways that prevent food borne illness. Food serves as a growth medium for microorganisms that can be pathogenic or cause food spoilage. Therefore, it is imperative to have stringent laws and standards for the preparation, packaging and transportation of food. The conventional methods for detection of food contamination based on culturing, colony counting, chromatography and immunoassay are tedious and time consuming while biosensors have overcome some of these disadvantages. There is growing interest in biosensors due to high specificity, convenience and quick response. Optical biosensors show greater potential for the detection of pathogens, pesticide and drug residues, hygiene monitoring, heavy metals and other toxic substances in the food to check whether it is safe for consumption or not. This review focuses on optical biosensors, the recent developments in the associated instrumentation with emphasis on fiber optic and surface plasmon resonance (SPR) based biosensors for detecting a range of analytes in food samples, the major advantages and challenges associated with optical biosensors. It also briefly covers the different methods employed for the immobilization of bio-molecules used in developing biosensors.

Development of surface plasmon resonance-based sensor for detection of silver nanoparticles in food and the environment

Silver nanoparticles are recognized as effective antimicrobial agents and have been implemented in various consumer products including washing machines, refrigerators, clothing, medical devices, and food packaging. Alongside the silver nanoparticles benefits, their novel properties have raised concerns about possible adverse effects on biological systems. To protect consumer's health and the environment, efficient monitoring of silver nanoparticles needs to be established. Here, we present the development of human metallothionein (MT) based surface plasmon resonance (SPR) sensor for rapid detection of nanosilver. Incorporation of human metallothionein 1A to the sensor surface enables screening for potentially biologically active silver nanoparticles at parts per billion sensitivity. Other protein ligands were also tested for binding capacity of the nanosilver and were found to be inferior to the metallothionein. The biosensor has been characterized in terms of selectivity and sensitivity towards different types of silver nanoparticles and applied in measurements of real-life samples-such as fresh vegetables and river water. Our findings suggest that human MT1-based SPR sensor has the potential to be utilized as a routine screening method for silver nanoparticles, that can provide rapid and automated analysis dedicated to environmental and food safety monitoring.

Surface plasmon resonance for rapid screening of uranyl affine proteins

A sensitive immunoassay based on SPR analysis was developed to measure uranyl cation (UO(2)(2+)) affinity for any protein in a free state under physiological conditions. The technique involves immobilization of a specific monoclonal antibody (mAb) raised against UO(2)(2+) and 1,10-phenanthroline-2,9-dicarboxylic acid (DCP) used as a probe of UO(2)(2+) captured by the mAb. Calibration curves were established for accurate determination of UO(2)(2+) concentrations with a detection limit of 7 nM. The remaining free UO(2)(2+) could be accurately quantified from the different protein-metal equilibrium and a dose-response curve established for K(D) determination. This generic method was applied not only to proteins such as transferrin and albumin but also to small phosphonated ligands. Its robustness allows the fast UO(2)(2+) K(D) determination of any kind of macromolecules and small ligands using very few amount of compounds, thus opening new prospects in the field of uranium toxicity.

Recent developments and applications of hybrid surface plasmon resonance interfaces in optical sensing

Nanostructured noble metals exhibit an intense optical near field due to surface plasmon resonance, therefore promising widespread applications and being of interest to a broad spectrum of scientists, ranging from physicists, chemists, and materials scientists to biologists. A wealth of research is available discussing the synthesis, characterization, and application of noble metal nanoparticles in optical sensing. However, with respect to the sensitivity of the frequency and width of these surface plasmon resonance modes to the particle's shape, size, and environment, in nearly every case, success strongly depends on the availability of highly stable, adhesive, and sensitive nanoparticles. This undoubtedly presents a challenging task to nanofabrication. The past decade has witnessed fascinating advances in this field, in particular, the construction of oxide-based hybrid plasmonic interfaces to overcome the problem addressed above by (1) coating the metallic nanostructures with thin overlayers to form sandwiched structures or (2) embedding metallic nanostructures in a dielectric matrix to obtain metal/dielectric matrix nanocomposite films. In this critical review, we focus on recent work related to this field, beginning with a presentation of hybrid films with enhanced structural and optical stability, readily and selectively designed using chemical and physical techniques. We then illustrate their interesting optical properties and demonstrate exciting evidence for the postulated application in surface plasmon sensing fields. Finally, we survey the work remaining to be done for that potential to be realized.

Protein-based electrochemical biosensor for detection of silver(I) ions

Silver(I) ions are extremely toxic to aquatic animals. Hence, monitoring of these ions in the environment is needed. The aim of the present study was to suggest a simple biosensor for silver(I) ions detection. The suggested biosensor is based on the modification of a hanging mercury drop electrode (HMDE) by the heavy metal binding protein metallothionein (MT) for silver(I) ions detection. Metallothionein accumulated for 120 s onto the HMDE surface. After rinsing the electrode, the biosensor (MT modified HMDE) was prepared prior to detection of silver(I) ions. The biosensor was immersed in a solution containing silver(I) ions. These ions were bound to the MT structure. Furthermore, the electrode was rinsed and transferred to a pure supporting electrolyte solution, in which no interference was present. Under these experimental conditions, other signals relating to heavy metals naturally occurring in MT were not detected. This phenomenon confirms the strong affinity of silver(I) ions for MT. The suggested biosensor responded well to higher silver(I) ion concentrations. The relative standard deviation for measurements of concentrations higher than 50 microM was approximately 2% (n = 8). In the case of concentrations lower than 10 microM, the relative standard deviation increased to 10% (n = 8). The detection limit (3 signal/noise) for silver(I) ions was estimated as 500 nM.

Studies of metal ion binding by apo-metallothioneins attached onto preformed self-assembled monolayers using a highly sensitive surface plasmon resonance spectrometer

The use of a flow-injection surface plasmon resonance (FI-SPR) spectrometer equipped with a bicell detector or a position-sensitive device for determining coordination of heavy metal ions (Cd(2+) and Hg(2+)) by surface-confined apo-metallothionein (apo-MT) molecules is described. To facilitate the formation of a compact MT adsorbate layer with a uniform surface orientation, MT molecules were attached onto a preformed alkanethiol self-assembled monolayer. The method resorts to the generation of apo-MT at the surface by treating the MT-covered sensor chip with glycine-HCl and the measurement of the apo-MT conformation changes upon metal ion incorporation. Domain-specific metal ion binding processes by the apo-MT molecules were observed. Competitive replacement of one metal ion by another can be monitored in real time by FI-SPR. The tandem use of an immobilization scheme for forming a sub-monolayer of MT molecules at the sensor surface and the highly sensitive FI-SPR instrument affords a low concentration detection level. The detection level for Cd(2+) (0.1 μM or 15 ppb) compares favorably with similar studies and the methodology complements to other well-established sensitive analytical techniques. The extent of metal incorporation by apo-MT molecules was also determined.

Immobilization of bovine serum albumin as a sensitive biosensor for the detection of trace lead ion in solution by piezoelectric quartz crystal impedance

A biosensor based on bovine serum albumin (BSA) for the detection of lead (Pb(2+)) ion was developed and characterized. BSA was immobilized onto a colloidal Au-modified piezoelectric quartz crystal (PQC) as a biosensor for the detection of Pb(2+) ion by piezoelectric quartz crystal impedance (PQCI). Calibration curves for the quantification of Pb(2+) ion showed excellent linearity throughout the concentration range from 1.0 x 10(-7) to 3.0 x 10(-9)mol/L. The interaction between the Pb(2+) ions and the sensor chip is influenced significantly by the pH of the reaction buffer, and the optimal pH for the experiment was 5.4. Under the optimal conditions, the detection limit of 1.0 x 10(-9)mol/L for Pb(2+) was obtained. Kinetic parameters of the Pb(2+)-BSA interactions were also determined by using this chip. The sensor chip could be regenerated for use by dipping in the ethylenediaminetetraacetic acid (EDTA) solution for approximately 2h, and the chip was used to detect Pb(2+) ion for eight times without obvious signal attenuation.

Cr(VI) quantification using an amperometric enzyme-based sensor: Interference and physical and chemical factors controlling the biosensor response in ground waters

The development of an amperometric enzyme-based sensor for chromate (CrO(4)(2-)) quantification in ground waters was investigated. Crucial physical and chemical factors characterising ground waters were tested for their influence or interference on chromate quantification: pH (7.6-8.5), temperature (9-25 degrees C), ionic strength (0-0.2M), oxygen, metals, bicarbonate and sulphate. The biosensor's response was dependent on temperature and pH as sensitivity increased with temperature and was higher at pH 7.6 than at pH 8.5. Sensitivity decreased with ionic strength until 0.1M, and was stable for higher values. Dissolved oxygen did not allow chromate quantification when it was present, but O(2) could be eliminated by adding Na(2)SO(3) or bubbling nitrogen gas into the solution. Bicarbonate did not interfere with chromate quantification by the biosensor. Sulphate was detected with a detection threshold 80 times higher than that of chromate and a lower sensitivity. Several metals (V(V), W(VI), Mn(VII), Mo(VI)) similar to chromate due to their oxidative properties and structure (oxyanions) were tested as possible interfering compounds. The sensitivity of the biosensor for these metals was low and the detection level was 30 times higher than that of chromate. These metal concentrations are usually weaker than chromate concentration in polluted ground waters so that dilution of the sample should allow chromate quantification by the biosensor. This study shows that the cytochrome c(3)-based sensor can detect compounds other than chromate but with a lower sensitivity. Although non-specific for the detection of chromate, it can however be adapted and used for the quantification of chromate in ground waters containing low sulphate concentration.