Determination of Technetium and Its Speciation by Surface-Enhanced Raman Spectroscopy

Speciation Analysis of Metals and Metalloids by Surface Enhanced Raman Spectroscopy

The presence of metalloids and heavy metals in the environment is of critical concern due to their toxicological impacts. However, not all metallic species have the same risk level. Specifically, the physical, chemical, and isotopic speciation of the metal(loids) dictate their metabolism, toxicity, and environmental fate. As such, speciation analysis is critical for environmental monitoring and risk assessment. In the past two decades, surface-enhanced Raman spectroscopy (SERS) has seen significant developments regarding trace metal(loid) sensing due to its ultrahigh sensitivity, readiness for in situ real-time applications, and cost-effectiveness. However, the speciation of metal(loid)s has not been accounted for in the design and application of SERS sensors. In this Perspective, we examine the potential of SERS for metal(loid) speciation analysis and highlight the advantages, progress, opportunities, and challenges of this application.

3D Ionic Olefin-Linked Conjugated Microporous Polymers for Selective Detection and Removal of TcO4-/ReO4- from Wastewater

Technetium (⁹⁹Tc) is a highly toxic radioactive nuclear wastewater contaminant. Real-time detection of ⁹⁹Tc is very difficult due to its difficult-to-complex nature. Herein, a novel three-dimensional ionic olefin-linked conjugated microporous polymer (TFPM-EP-Br) is constructed using tetrakis(4-aldehyde phenyl)methane (TFPM) as the central monomer. The unique cationic cavity and highly hydrophobic framework enable TFPM-EP-Br to act as a fluorescent sensor for TcO₄^-. The fluorophores of TFPM-EP-Br can be quenched due to electron transfer from TFPM-EP-Br to TcO₄^- and the formation of strongly nonfluorescent complexes. Meanwhile, the regular pore channels are beneficial for the fast mass transfer of TcO₄^-, resulting in an ultrafast response time (less than 2 s) with an ultralow detection limit (33.3 nM). In addition, the ultrahigh specific surface area enables TFPM-EP-Br to combine the ability to synergistically detect and remove radioactive ⁹⁹Tc. From this perspective, the novel conjugated microporous polymer has made a breakthrough in the detection and extraction of radioactive contaminants.

Exploring the Reduction Mechanism of 99Tc(VII) in NaClO4: A Spectro-Electrochemical Approach

Technetium (Tc) is an environmentally relevant radioactive contaminant whose migration is limited when Tc(VII) is reduced to Tc(IV). However, its reaction mechanisms are not well understood yet. We have combined electrochemistry, spectroscopy, and microscopy (cyclic voltammetry, rotating disk electrode, X-ray photoelectron spectroscopy, and Raman and scanning electron microscopy) to study Tc(VII) reduction in non-complexing media: 0.5 mM KTcO₄ in 2 M NaClO₄ in the pH from 2.0 to 10.0. At pH 2.0, Tc(VII) first gains 2.3 ± 0.3 electrons, following Tc(V) rapidly receives 1.3 ± 0.3 electrons yielding Tc(IV). At pH 4.0-10.0, Tc(IV) is directly obtained by transfer of 3.2 ± 0.3 electrons. The reduction of Tc(VII) produced always a black solid identified as Tc(IV) by Raman and XPS. Our results narrow a significant gap in the fundamental knowledge of Tc aqueous chemistry and are important to understand Tc speciation. They provide basic steps on the way from non-complexing to complex media.

Immobilization of perrhenate using synthetic pyrite particles: Effectiveness and remobilization potential

Radioactive pertechnetate (TcO4^-) has been detected in nuclear waste affected soil and groundwater, posing significant effect on human health and the environment. Yet, cost-effective remediation of Tc-contaminated soil and groundwater remains challenging. To address this critical technology need, we prepared a class of pyrite (FeS₂) particles for effective immobilization of pertechnetate. Using perrhenate (ReO₄^-) as a non-radioactive surrogate of TcO₄^-, we tested the immobilization effectiveness of the material through batch kinetic experiments, and evaluated the remobilization potential of immobilized Re under anoxic (sealed from air) and oxic (exposed to air) conditions and in the presence of humic acid (HA), EDTA, nitrate, and a Chinese loess soil. The results showed that more acidic pH gave faster Re(VII) removal due to more abundant electron sources (Fe²⁺ and S₂^2-). X-ray diffraction (XRD) and/or X-ray photoelectron spectroscopy (XPS) analyses confirmed formation of ReO₂/ReS₂ as the major reduction products. The immobilized Re remained highly stable when aged for 360 days under anoxic conditions at different influence factors. Yet, the immobilized Re was vulnerable to oxygen oxidation, and about 78% of Re was remobilized after 40 days of exposure to air regardless of the initial pH (3.5-9.0) due to excessive pyrite oxidation and the associated pH drop (~2). HA at 120 mg/L inhibited Re remobilization under oxic conditions, which lowered the Re remobilization by ~21% after 40 days of oxic aging. The presence of EDTA facilitated dissolution of Fe but inhibited the dissolution of Re under oxic conditions. Nitrate showed negligible effect on Re remobilization. The presence of a Chinese loess soil effectively inhibited Re remobilization under both oxic and anoxic conditions, lowering the leachable Re by ~32% under oxic conditions. The findings may guide engineered application of pyrite particles as a long-lasting reducing material for immobilization pertechnetate or similar redox-active contaminants in soil and water.

In Situ Spectroscopic Analysis and Quantification of [Tc(CO)3]+ in Hanford Tank Waste

The quantitative conversion of nonpertechnetate [Tc(CO)₃]⁺ species in nuclear waste storage tank 241-AN-102 at the Hanford Site is demonstrated. A waste sample containing the [Tc(CO)₃]⁺ species is added to a developer solution that rapidly converts the nonemissive species into a luminescent complex, which is detected spectroscopically. This method was first demonstrated using a [Tc(CO)₃]⁺ sample of nonwaste containing matrix to determine a detection limit (LOD), resulting in a [Tc(CO)₃]⁺ LOD of 2.20 × 10^-7 M, very near the LOD of the independently synthesized standard (2.10 × 10^-7 M). The method was then used to detect [Tc(CO)₃]⁺ in a simulated waste using the standard addition method, resulting in a [Tc(CO)₃]⁺ concentration of 1.89 × 10^-5 M (within 27.7% of the concentration determined by β liquid scintillation counting). Three samples from 241-AN-102 were tested by the standard addition method: (1) a 5 M Na adjusted fraction, (2) a fraction depleted of ¹³⁷Cs, and (3) an acid-stripped eluate. The concentrations of [Tc(CO)₃]⁺ in these fractions were determined to be 9.90 × 10^-6 M (1), 0 M (2), and 2.46 × 10^-6 M (3), respectively. The concentration of [Tc(CO)₃]⁺ in the as-received AN-102 tank waste supernatant was determined to be 1.84 × 10^-5 M.

In Situ Quantification of [Re(CO)3]+ by Fluorescence Spectroscopy in Simulated Hanford Tank Waste

A pretreatment protocol is presented that allows for the quantitative conversion and subsequent in situ spectroscopic analysis of [Re(CO)₃]⁺ species in simulated Hanford tank waste. In this test case, the nonradioactive metal rhenium is substituted for technetium (Tc-99), a weak beta emitter, to demonstrate proof of concept for a method to measure a nonpertechnetate form of technetium in Hanford tank waste. The protocol encompasses adding a simulated waste sample containing the nonemissive [Re(CO)₃]⁺ species to a developer solution that enables the rapid, quantitative conversion of the nonemissive species to a luminescent species which can then be detected spectroscopically. The [Re(CO)₃]⁺ species concentration in an alkaline, simulated Hanford tank waste supernatant can be quantified by the standard addition method. In a test case, the [Re(CO)₃]⁺ species was measured to be at a concentration of 38.9 μM, which was a difference of 2.01% from the actual concentration of 39.7 μM.

Review of SERS Substrates for Chemical Sensing

The SERS effect was initially discovered in the 1970s. Early research focused on understanding the phenomenon and increasing enhancement to achieve single molecule detection. From the mid-1980s to early 1990s, research started to move away from obtaining a fundamental understanding of the phenomenon to the exploration of analytical applications. At the same time, significant developments occurred in the field of photonics that led to the advent of inexpensive, robust, compact, field-deployable Raman systems. The 1990s also saw rapid development in nanoscience. This convergence of technologies (photonics and nanoscience) has led to accelerated development of SERS substrates to detect a wide range of chemical and biological analytes. It would be a monumental task to discuss all the different kinds of SERS substrates that have been explored. Likewise, it would be impossible to discuss the use of SERS for both chemical and biological detection. Instead, a review of the most common metallic (Ag, Cu, and Au) SERS substrates for chemical detection only is discussed, as well as SERS substrates that are commercially available. Other issues with SERS for chemical detection have been selectivity, reversibility, and reusability of the substrates. How these issues have been addressed is also discussed in this review.

Intracellular surface-enhanced Raman scattering (SERS) with thermally stable gold nanoflowers grown from Pt and Pd seeds

SERS provides great sensitivity at low concentrations of analytes. SERS combined with near infrared (NIR)-resonant gold nanomaterials are important candidates for theranostic agents due to their combined extinction properties and sensing abilities stemming from the deep penetration of laser light in the NIR region. Here, highly branched gold nanoflowers (GNFs) grown from Pd and Pt seeds are prepared and their SERS properties are studied. The growth was performed at 80 °C without stirring, and this high temperature growth method is assumed to provide great shape stability of sharp tips in GNFs. We found that seed size must be large enough (>30 nm in diameter) to induce the growth of those SERS-active and thermally stable GNFs. We also found that the addition of silver nitrate (AgNO3) is important to induce sharp tip growth and shape stability. Incubation with Hela cells indicates that GNFs are taken up and reside in the cytoplasm. SERS was observed in those cells incubated with 1,10-phenanthroline (Phen)-loaded GNFs.

Complexation of rhenium with ethylenediaminetetraacetic acid: studies of technetium analogue using Raman spectroscopy

The complexation of rhenium, to the polyamino-polycarboxylate EDTA was investigated using Raman spectroscopy. Complexation occurred when ReVIIO4 - was placed in the presence of a Sn(II) reducing agent and EDTA. Rhenium complexation was confirmed by the Raman and UV/Vis spectra of the solution samples. Binding modes were assigned using the Raman spectra of a solid-phase sample. Three complexes are proposed, a six-coordinate dimer H4[Re2(μO)2(EDTA)2], a seven-coordinate monomer H2[ReO(EDTA)(OH2)2] and a mono-oxorhenium [ReO(EDTA)] 2- . The solubility of Re(IV)-EDTA complexes is greater than that of ReO4. If this behaviour is analogous to Tc, it is proposed that complexation to EDTA by Tc(IV) may increase the solubility and enhance the environmental transport of 99Tc from geological repositories.

Functionalized gold nanoparticle supported sensory mechanisms applied in detection of chemical and biological threat agents: a review

There is a great necessity for development of novel sensory concepts supportive of smart sensing capabilities in defense and homeland security applications for detection of chemical and biological threat agents. A smart sensor is a detection device that can exhibit important features such as speed, sensitivity, selectivity, portability, and more importantly, simplicity in identifying a target analyte. Emerging nanomaterial based sensors, particularly those developed by utilizing functionalized gold nanoparticles (GNPs) as a sensing component potentially offer many desirable features needed for threat agent detection. The sensitiveness of physical properties expressed by GNPs, e.g. color, surface plasmon resonance, electrical conductivity and binding affinity are significantly enhanced when they are subjected to functionalization with an appropriate metal, organic or biomolecular functional groups. This sensitive nature of functionalized GNPs can be potentially exploited in the design of threat agent detection devices with smart sensing capabilities. In the presence of a target analyte (i.e., a chemical or biological threat agent) a change proportional to concentration of the analyte is observed, which can be measured either by colorimetric, fluorimetric, electrochemical or spectroscopic means. This article provides a review of how functionally modified gold colloids are applied in the detection of a broad range of threat agents, including radioactive substances, explosive compounds, chemical warfare agents, biotoxins, and biothreat pathogens through any of the four sensory means mentioned previously.

Determination of technetium-99 in environmental samples: a review

Due to the lack of a stable technetium isotope, and the high mobility and long half-life, (99)Tc is considered to be one of the most important radionuclides in safety assessment of environmental radioactivity as well as nuclear waste management. (99)Tc is also an important tracer for oceanographic research due to the high technetium solubility in seawater as TcO(4)(-). A number of analytical methods, using chemical separation combined with radiometric and mass spectrometric measurement techniques, have been developed over the past decades for determination of (99)Tc in different environmental samples. This article summarizes and compares recently reported chemical separation procedures and measurement methods for determination of (99)Tc. Due to the extremely low concentration of (99)Tc in environmental samples, the sample preparation, pre-concentration, chemical separation and purification for removal of the interferences for detection of (99)Tc are the most important issues governing the accurate determination of (99)Tc. These aspects are discussed in detail in this article. Meanwhile, the different measurement techniques for (99)Tc are also compared with respect to advantages and drawbacks. Novel automated analytical methods for rapid determination of (99)Tc using solid extraction or ion exchange chromatography for separation of (99)Tc, employing flow injection or sequential injection approaches are also discussed.

Dissolution of technetium(IV) oxide by natural and synthetic organic ligands under both reducing and oxidizing conditions

Technetium-99 (Tc) in nuclear waste is a significant environmental concern due to its long half-life and high mobility in the subsurface. Reductive precipitation of technetium(IV) oxides [TcO(2)(s)] is an effective means of immobilizing Tc, thereby impeding its migration in groundwater. However, technetium(IV) oxides are subject to dissolution by oxidants and/or complexing agents. In this study we ascertain the effects of a synthetic organic ligand, ethylenediaminetetraacetate (EDTA), and two natural humic isolates on the dissolution and solubility of technetium(IV) oxides. Pure synthetic technetium(IV) oxide (0.23 mM) was used in batch experiments to determine dissolution kinetics at pH ∼6 under both reducing and oxidizing conditions. All organic ligands were found to enhance the dissolution of technetium(IV) oxides, increasing their solubility from ∼10(-8) M (without ligands) to 4 × 10(-7) M under strictly anoxic conditions. Reduced Tc(IV) was also found to reoxidize rapidly under oxic conditions, with an observed oxidative dissolution rate approximately an order of magnitude higher than that of ligand-promoted dissolution under reducing conditions. Significantly, oxidative dissolution was inhibited by EDTA but enhanced by humic acid compared to experiments without any complexing agents. The redox functional properties of humics, capable of facilitating intramolecular electron transfer, may account for this increased oxidation rate under oxic conditions. Our results highlight the importance of complex interactions for the stability and mobility of Tc and thus for the long-term fate of Tc in contaminated environments.

Interactions of Tc(IV) with humic substances

To understand the key processes affecting 99Tc mobility in the subsurface and help with the remediation of contaminated sites, the binding constants of several humic substances (humic and fulvic acids) with Tc(IV) were determined, using a solvent extraction technique. The novelty of this paper lies in the determination of the binding constants of the complexes formed with the individual species TcO(OH)+ and TcO(OH)2(0). Binding constants were found to be 6.8 and between 3.9 and 4.3, for logβ1,-1,1 and logβ1,-2,1, respectively; these values were little modified by a change of ionic strength, in most cases, between 0.1 and 1.0 M, nor were they by the nature and origin of the humic substances. Modeling calculations based on these show TcO(OH)-HA to be the predominant complex in a system containing 20 ppm HA and in the 4-6 pH range, whereas TcO(OH)2(0) and TcO(OH)2-HA are the major species, in the pH 6-8 range.

A review on the fabrication of substrates for surface enhanced Raman spectroscopy and their applications in analytical chemistry

This work reviews different types of substrates used for surface-enhanced Raman scattering (SERS) that have been developed in the last 10 years. The different techniques of self-assembly to immobilize metallic nanoparticles on solid support are covered. An overview of SERS platforms developed using nanolithography methods, including electron-beam (e-beam) lithography and focused ion beam (FIB) milling are also included, together with several examples of template-based methodologies to generate metallic nano-patterns. The potential of SERS to impact several aspects of analytical chemistry is demonstrated by selected examples of applications in electrochemistry, biosensing, environmental analysis, and remote sensing. This review shows that highly enhancing SERS substrates with a high degree of reliability and reproducibility can now be fabricated at relative low cost, indicating that SERS may finally realize its full potential as a very sensitive tool for routine analytical applications.

Perchlorate detection at nanomolar concentrations by surface-enhanced Raman scattering

Perchlorate (ClO4-) has emerged as a widespread environmental contaminant and has been detected in various food products and even in human breast milk and urine. This research developed a sensing technique based on surface-enhanced Raman scattering (SERS) for rapid screening and monitoring of this contaminant in groundwater and surface water. The technique was found to be capable of detecting ClO4- at concentrations as low as 10(-9) M (or approximately 0.1 microg/L) by using 2-dimethylaminoethanethiol (DMAE) modified gold nanoparticles as a SERS substrate. Quantitative analysis of ClO4- was validated with good reproducibility by using both simulated and contaminated groundwater samples. When coupled with a portable Raman spectrometer, this technique has the potential to be used as an in situ, rapid screening tool for perchlorate in the environment.