Solid phase extraction of trace elements

Here, we report a catalytic beacon sensor for uranyl (UO22+) based on an in vitro-selected UO22+-specific DNAzyme. The sensor consists of a DNA enzyme strand with a 3′ quencher and a DNA substrate with a ribonucleotide adenosine (rA) in the middle and a fluorophore and a quencher at the 5′ and 3′ ends, respectively. The presence of UO22+ causes catalytic cleavage of the DNA substrate strand at the rA position and release of the fluorophore and thus dramatic increase of fluorescence intensity. The sensor has a detection limit of 11 parts per trillion (45 pM), a dynamic range up to 400 nM, and selectivity of >1-million-fold over other metal ions. The most interfering metal ion, Th(IV), interacts with the fluorescein fluorophore, causing slightly enhanced fluorescence intensity, with an apparent dissociation constant of ≈230 μM. This sensor rivals the most sensitive analytical instruments for uranium detection, and its application in detecting uranium in contaminated soil samples is also demonstrated. This work shows that simple, cost-effective, and portable metal sensors can be obtained with similar sensitivity and selectivity as much more expensive and sophisticated analytical instruments. Such a sensor will play an important role in environmental remediation of radionuclides such as uranium.

A catalytic beacon sensor for uranium with parts-per-trillion sensitivity and millionfold selectivity

Colorimetric uranium sensors based on uranyl (UO2(2+)) specific DNAzyme and gold nanoparticles (AuNP) have been developed and demonstrated using both labeled and label-free methods. In the labeled method, a uranyl-specific DNAzyme was attached to AuNP, forming purple aggregates. The presence of uranyl induced disassembly of the DNAzyme functionalized AuNP aggregates, resulting in red individual AuNPs. Once assembled, such a "turn-on" sensor is highly stable, works in a single step at room temperature, and has a detection limit of 50 nM after 30 min of reaction time. The label-free method, on the other hand, utilizes the different adsorption properties of single-stranded and double-stranded DNA on AuNPs, which affects the stability of AuNPs in the presence of NaCl. The presence of uranyl resulted in cleavage of substrate by DNAzyme, releasing a single stranded DNA that can be adsorbed on AuNPs and protect them from aggregation. Taking advantage of this phenomenon, a "turn-off" sensor was developed, which is easy to control through reaction quenching and has 1 nM detection limit after 6 min of reaction at room temperature. Both sensors have excellent selectivity over other metal ions and have detection limits below the maximum contamination level of 130 nM for UO2(2+) in drinking water defined by the U.S. Environmental Protection Agency (EPA). This study represents the first direct systematic comparison of these two types of sensor methods using the same DNAzyme and AuNPs, making it possible to reveal advantages, disadvantages, versatility, limitations, and potential applications of each method. The results obtained not only allow practical sensing application for uranyl but also serve as a guide for choosing different methods for designing colorimetric sensors for other targets.

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Highly Sensitive and Selective Colorimetric Sensors for Uranyl (UO22+): Development and Comparison of Labeled and Label-Free DNAzyme-Gold Nanoparticle Systems

The recovery of uranium (U) from seawater has been investigated for over six decades in efforts to secure uranium sources for future energy production. The majority of the research activities have focused on inorganic materials, chelating polymers, and nanomaterials. Previous studies of uranium adsorption from aqueous solutions, mainly seawater, are reviewed here with a focus on various adsorbent materials, adsorption parameters, adsorption characterization, and marine studies. Continuous progress has been made over several decades, with adsorbent loadings approaching 3.2 mg U/g adsorbent in equilibrium with seawater. Further research is needed to improve first, the viability including improved capacity, selectivity, and kinetics, and second, the sorbent regeneration for multicycle use. An overview of the status of the uranium adsorption technology is provided and future research needs to make this technology commercially competitive are discussed.

Recovery of Uranium from Seawater: A Review of Current Status and Future Research Needs

Square-wave voltammetry (SWV) is one of the four major voltammetric techniques provided by modern computer-controlled electroanalytical instruments, such as Autolab and μAutolab (both EcoChemie, Utrecht), BAS 100 A (Bioanalytical Systems), and PAR Model 384 B (Princeton Applied Research) [1]. The other three important techniques are single scan and cyclic staircase, pulse, and differential pulse voltammetry (see Chap. II.2). All four are either directly applied or after a preconcentration to record the stripping process. The application of SWV boomed in the last decade, first because of the widespread use of the instruments mentioned above, second because of a well-developed theory, and finally, and most importantly, because of its high sensitivity to surface-confined electrode reactions. Adsorptive stripping SWV is the best electroanalytical method for the determination of electroactive organic molecules that are adsorbed on the electrode surface [2].

Square-wave voltammetry

Spatial distribution of trace metals in the Krka River, Croatia: an example of the self-purification.

Stripping voltammetric determination of trace levels of uranium by synergic adsorption

An evaluation of the quality status of the pristine karst, tufa depositing aquatic environment of the Plitvice Lakes National Park based on the analysis of heavy (ecotoxic) metals was examined for the first time. Analyses of trace metals in water, sediment and fish (Salmo trutta, Oncorhynchus mykiss, Squalius cephalus) samples were conducted either by stripping voltammetry (Zn, Cd, Pb and Cu) or cold vapour atomic absorption spectrometry (Hg). The concentration of dissolved trace metals in water was very low revealing a pristine aquatic environment (averages were, in ng/L: 258 (Zn), 10.9 (Cd), 11.7 (Pb), 115 (Cu) and 1.22 (Hg)). Slightly enhanced concentrations of Cd (up to 50 ng/L) and Zn (up to 900 ng/L) were found in two main water springs and are considered as of natural origin. Observed downstream decrease in concentration of Cd, Zn and Cu in both water and sediments is a consequence of the self-purification process governed by the formation and settling of authigenic calcite. Anthropogenic pressure was spotted only in the Kozjak Lake: Hg concentrations in sediments were found to be up to four times higher than the baseline value, while at two locations, Pb concentrations exceeded even a probable effect concentration. The increase of Hg and Pb was not reflected on their levels in the fish tissues; however, significant correlations were found between Cd level in fish tissues (liver and muscle) and in the water/sediment compartments, while only partial correlations were estimated for Zn and Cu. A high discrepancy between values of potentially bioavailable metal fraction estimated by different modelling programs/models raised the question about the usefulness of these data as a parameter in understanding/relating the metal uptake and their levels in aquatic organism. The aquatic environment of the Plitvice Lakes National Park is characterized, in general, as a clean ecosystem.

Heavy metal contents in water, sediment and fish in a karst aquatic ecosystem of the Plitvice Lakes National Park (Croatia)

Distributions of Hg, Cd, Pb, Cu and Zn in seawater and sediment from Mljet National Park, Adriatic Sea are presented for the first time. Natural and anthropogenic factors play an important role in determining resultant trace metals' concentrations in the region. We place particular emphasis on the saline “lakes” of Malo Jezero and Veliko Jezero, which have restricted flows of seawater. In Malo Jezero lake, fresh karstic spring water generated by flooding, and weathering of dolomites are the main sources of naturally elevated Cd, Pb and Zn concentrations (20.7 ± 1.6, 289 ± 19, 1260 ± 0.08 ng L−1, respectively); anthropogenic input is negligible. In Veliko Jezero lake enhanced Cu and Zn contents originate from anthropogenic input (tourism and agriculture). Distributions of the Pb and Zn in the water columns of both lakes are influenced by natural aragonite precipitation and sedimentation. Exceptionally high total Hg concentrations of 24.2 and 33.7 ng L−1 in the water column of Malo Jezero, sampled during periods of high rainfall associated with strong eastern winds, suggest an airborne input. Total Hg concentrations in waters of both lakes are elevated because of inefficient mixing. Two different metal distribution patterns exist in the sediment columns. First, Hg, Pb, Cu and Zn show elevated concentrations in recent sediments due to anthropogenic input. Second, Cd content increases with depth due to reprecipitation via a downward redox boundary shift. Described natural processes, as well as anthropogenic influence, enhance levels of trace metals. Careful study followed by suitable interpretation based on geochemical data were necessary to distinguish natural from anthropogenic sources.

Marina Mlakar

Papers

Spatial distribution of trace metals in the Krka River, Croatia: an example of the self-purification.

Stripping voltammetric determination of trace levels of uranium by synergic adsorption

Heavy metal contents in water, sediment and fish in a karst aquatic ecosystem of the Plitvice Lakes National Park (Croatia)

Natural and anthropogenic sources of Hg, Cd, Pb, Cu and Zn in seawater and sediment of Mljet National Park, Croatia

Revision of iron(III)-citrate speciation in aqueous solution. Voltammetric and spectrophotometric studies.