Optimization of a portable microanalytical system to reduce electrode fouling from proteins associated with biomonitoring of lead (Pb) in saliva

There is a need to develop reliable portable analytical systems for on-site and real-time biomonitoring of lead (Pb) from both occupational and environmental exposures. Saliva is an appealing matrix since it is easily obtainable, and therefore a potential substitute for blood due to existing reasonably good correlation between Pb levels in blood and saliva. The microanalytical system is based on flow-injection/stripping voltammetry with a wall-jet (flow-onto) microelectrochemical cell. Samples that contain as little as 1% saliva can cause electrode fouling, resulting in significantly reduced responsiveness and irreproducible quantitations. In addition, incomplete Pb release from salivary protein can also yield a lower Pb response than expected. This paper evaluates the extent of in vitro Pb-protein binding and the optimal pretreatment for releasing Pb from the saliva samples. Even in 50% by volume of rat saliva, the electrode fouling was not observed, due to the appropriate sample pretreatment and the constant flow of the sample and acidic carrier that prevented passivation by the protein. The system offered a linear response over a low Pb range of 1-10 ppb, low detection limit of 1 ppb, excellent reproducibility, and reliability. It also yielded the same Pb concentrations in unknown samples as did the ICP-MS. These encouraging results suggest that the microanalytical system represents an important analytical advancement for real-time non-invasive biomonitoring of Pb.     

Simultaneous detection of cadmium, copper, and lead using a carbon paste electrode modified with carbamoylphosphonic acid self-assembled monolayer on mesoporous silica (SAMMS)

A new sensor was developed for simultaneous detection of cadmium (Cd²⁺), copper (Cu²⁺), and lead (Pb²⁺), based on the voltammetric response at a carbon paste electrode modified with carbamoylphosphonic acid (acetamide phosphonic acid) self-assembled monolayer (SAM) on mesoporous silica (Ac-Phos SAMMS). The adsorptive stripping voltammetry (AdSV) technique involves preconcentration of the metal ions onto Ac-Phos SAMMS under an open circuit, then electrolysis of the preconcentrated species, followed by a square wave potential sweep towards positive values. Factors affecting the preconcentration process were investigated. The voltammetric responses increased linearly with the preconcentration time from 1 to 30 min or with metal ion concentrations ranging from 10 to 200 ppb. The responses also evolved in the same fashion as adsorption isotherm in the pH range of 2-6. The metal detection limits were 10 ppb after 2 min preconcentration and improved to 0.5 ppb after 20 min preconcentration.     

Detection of Cd, Pb, and Cu in non-pretreated natural waters and urine with thiol functionalized mesoporous silica and Nafion composite electrodes

Electrochemical sensors have great potential for environmental monitoring of toxic metal ions in waters due to their portability, field-deployability and excellent detection limits. However, electrochemical sensors employing mercury-free approaches typically suffer from binding competition for metal ions and fouling by organic substances and surfactants in natural waters, making sample pretreatments such as wet ashing necessary. In this work, we have developed mercury-free sensors by coating a composite of thiol self-assembled monolayers on mesoporous supports (SH-SAMMS™) and Nafion on glassy-carbon electrodes. With the combined benefit of SH-SAMMS™ as an outstanding metal preconcentrator and Nafion as an antifouling binder, the sensors could detect 0.5 ppb of Pb and 2.5 ppb of Cd in river water, Hanford groundwater, and seawater with a minimal amount of preconcentration time (few minutes) and without any sample pretreatment. The sensor could also detect 2.5 ppb of Cd, Pb, and Cu simultaneously. The electrodes have long service times and excellent single and inter-electrode reproducibility (5% R.S.D. after 8 consecutive measurements). Unlike SAMMS™-carbon paste electrodes, the SAMMS™-Nafion electrodes were not fouled in samples containing albumin and successfully detected Cd in human urine. Other potentially confounding factors affecting metal detection at SAMMS™-Nafion electrodes were studied, including pH effect, transport resistance of metal ions, and detection interference. With the ability to reliably detect low metal concentration ranges without sample pretreatment and fouling, SAMMS™-Nafion composite sensors have the potential to become the next-generation metal analyzers for environmental and bio-monitoring of toxic metals.     

Automated portable analyzer for lead(II) based on sequential flow injection and nanostructured electrochemical sensors

A fully automated portable analyzer for toxic metal ion detection based on a combination of a nanostructured electrochemical sensor and a sequential flow injection system has been developed in this work. The sensor was fabricated from a carbon paste electrode modified with acetamide phosphonic acid self-assembled monolayer on mesoporous silica (Ac-Phos SAMMS) which was embedded in a very small wall-jet (flow-onto) electrochemical cell. The electrode is solid-state and mercury-free. Samples and reagents were injected into the system and flowed through the electrochemical cell by a user programmable sequential flow technique which required minimal volume of samples and reagents and allowed the automation of the analyzer operation. The portable analyzer was evaluated for lead (Pb) detection due to the excellent binding affinity between Pb and the functional groups of Ac-Phos SAMMS as well as the great concern for Pb toxicity. Linear calibration curve was obtained in a low concentration range (1-25 ppb of Pb(II)). The reproducibility was excellent; the percent relative standard deviation was 2.5 for seven consecutive measurements of 10 ppb of Pb(II) solution. Excess concentrations of Ca, Ni, Co, Zn, and Mn ions in the solutions did not interfere with detection of Pb, due to the specificity and the large number of the functional groups on the electrode surface. The electrode was reliable for at least 90 measurements over 5 days. This work is an important milestone in the development of the next-generation metal ion analyzers that are portable, fully automated, and remotely controllable.     

Voltammetric detection of lead(II) and mercury(II) using a carbon paste electrode modified with thiol self-assembled monolayer on mesoporous silica (SAMMS)

The anodic stripping voltammetry at a carbon paste electrode modified with thiol terminated self-assembled monolayer on mesoporous silica (SH-SAMMS) provides a new sensor for simultaneous detection of lead (Pb2+) and mercury (Hg2+) in aqueous solutions. The overall analysis involved a two-step procedure: an accumulation step at open circuit, followed by medium exchange to a pure electrolyte solution for the stripping analysis. Factors affecting the performance of the SH-SAMMS modified electrodes were investigated, including electrode activation and regeneration, electrode composition, preconcentration time, electrolysis time, and composition of electrolysis and stripping media. The most sensitive and reliable electrode contained 20% SH-SAMMS and 80% carbon paste. The optimal operating conditions were a sequence with a 2 min preconcentration period, then a 60 s electrolysis period of the preconcentrated species in 0.2 M nitric acid, followed by square wave anodic stripping voltammetry from -1.0 V to 0.6 V in 0.2 M nitric acid. The areas of the peak responses were linear with respect to metal ion concentrations in the ranges of 10-1500 ppb Pb2+ and 20-1600 ppb Hg2+. The detection limits for Pb2+ and Hg2+ were 0.5 ppb Pb2+ and 3 ppb Hg2+ after a 20 min preconcentration period.     

New functional materials for heavy metal sorption: "supramolecular" attachment of thiols to mesoporous silica substrates

A new class of sorbent material, which exhibits exceptional metal capture from contaminated natural water, features aromatic thiol ligands reversibly bound to functionalized mesoporous silica through non-covalent interactions and have the potential of being regenerable.     

Voltammetric analysis of europium at screen-printed electrodes modified with salicylamide self-assembled on mesoporous silica

Mercury-free sensors for europium (Eu(3+)) assay based on the chemical modification of screen-printed carbon electrodes (SPCEs) with self-assembled salicylamide on mesoporous silica (Sal-SAMMS) have been developed. The preconcentration of Eu(3+) at SAMMS-based sensors utilizes the binding affinity of the salicylamide and Eu(3+), accomplished at open circuit potential without electrolyte and solution de-gassing. Optimal Eu detection was obtained after 3-5 min preconcentration in Eu solution (pH 2-6), electrolysis at -0.9 V for 60 s in a new medium (0.1-0.2 M NH(4)Cl, pH 3.5), followed by a square-wave voltammetric detection of Eu in the same electrolyte. Attributed to the strong covalent bonding of the functional groups on mesoporous silica and silane cross-linking, the SAMMS-modified SPCEs with a built-in 3-electrode system can be re-used for tens of measurements with minimal degradation, enabling the establishment of the calibration curve and lowering the costs. A linear calibration curve was found in the range of 75 to at least 500 ppb Eu(3+) after 5 min preconcentration. The experimental detection limit was 10 ppb after 10 min preconcentration, which can be improved with increased preconcentration time. Reproducibility (% RSD) of 100 ppb Eu(2+) was 10% for a single sensor and 10% for 5 sensors, which can be improved through the precision of sensor manufacturing, in which SAMMS modification can be made in-situ.     

Microanalyzer for biomonitoring lead (Pb) in blood and urine

Biomonitoring of lead (Pb) in blood and urine enables quantitative evaluation of human occupational and environmental exposures to Pb. State-of-the-art ICP–MS instruments can only analyze metals in laboratories, resulting in lengthy turnaround times, and they are expensive. In response to the growing need for a metal analyzer capable of on-site, real-time monitoring of trace toxic metals in individuals, we developed a portable microanalyzer based on flow-injection/stripping voltammetry (ASV), and validated the system using rat blood and urine spiked with known amounts of Pb. Fouling of electrodes by proteins often prevents the effective use of electrochemical sensors in biological matrices. Minimization of such fouling was accomplished with suitable sample pretreatment and by establishing turbulent flow of blood and urine containing Pb onto the electrode inside the microanalyzer, which resulted in no apparent electrode fouling even when the samples contained 50% urine or 10% blood by volume. No matrix effect was observed for the voltammetric Pb signals, even when the samples contained 10% blood or 10% urine. The microanalyzer offered linear concentration ranges relevant to Pb exposure levels in humans (0–20 ppb in 10% blood samples, 0–50 ppb in 50% urine samples). The device showed excellent sensitivity and reproducibility; Pb detection limits were 0.44 ppb and 0.46 ppb, and % R.S.D. was 4.9 and 2.4 in 50% urine and 10% blood samples, respectively. It gave similar Pb concentrations in blood and urine to those measured by ICP–MS. It offered high throughput (3 min per sample) and economical use of samples (60 μL per measurement) as well as low reagent consumption (1 μg of Hg per measurement), thus minimizing environmental concerns associated with mercury use. Since it is miniaturized, the microanalyzer is portable and field-deployable. Thus, it shows much promise as the next-generation analyzer for the biomonitoring of toxic metals.     

Direct detection of Pb in urine and Cd, Pb, Cu, and Ag in natural waters using electrochemical sensors immobilized with DMSA functionalized magnetic nanoparticles

Urine is universally recognized as one of the best non-invasive matrices for biomonitoring exposure to a broad range of xenobiotics, including toxic metals. Detection of metal ions in urine has been problematic due to the protein competition and electrode fouling. For direct, simple, and field-deployable monitoring of urinary Pb, electrochemical sensors employing superparamagnetic iron oxide (Fe3O4) nanoparticles with a surface functionalization of dimercaptosuccinic acid (DMSA) has been developed. The metal detection involves rapid collection of dispersed metal-bound nanoparticles from a sample solution at a magnetic or electromagnetic electrode, followed by the stripping voltammetry of the metal in acidic medium. The sensors were evaluated as a function of solution pH, the binding affinity of Pb to DMSA-Fe3O4, the ratio of nanoparticles per sample volume, preconcentration time, and Pb concentrations. The effect of binding competitions between the DMSA-Fe3O4 and urine constituents for Pb on the sensor responses was studied. After 90 s of preconcentration in samples containing 25 vol.% of rat urine and 0.1 g L(-1) of DMSA-Fe3O4, the sensor could detect background level of Pb (0.5 ppb) and yielded linear responses from 0 to 50 ppb of Pb, excellent reproducibility (%RSD of 5.3 for seven measurements of 30 ppb Pb), and Pb concentrations comparable to those measured by ICP-MS. The sensor could also simultaneously detect background levels (<1 ppb) of Cd, Pb, Cu, and Ag in river and seawater.