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  • 1.
    Xu, Kequan
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.
    Electrochemical detection of trace metals: from traditional techniques to new ultrathin membrane electrodes2021Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Accurate detection of trace metals in environmental waters is an analytical challenge that is still open for the time being. The current state of the field reveals the predominance of the so-called hanging mercury drop electrode (HMDE) for multi-metal detection by means of anodic stripping voltammetry (ASV) readout. Being aware of the high toxicity of mercury and the high risk of a serious environmental footprint when water measurements are performed with the HMDE, in the past years, the electrochemistry field has rapidly moved towards the provision of tangible alternatives. Yet, none of the proposed methodologies has reached appropriate maturation and/or analytical features to substitute the use of the HMDE in the detection of trace metal ions in water.

    The investigations presented in this thesis are framed within the direction of new analytical strategies for the detection of trace metals in water, with special focus on the silver ion (Ag+). Voltammetric ion-selective electrodes (ISEs) with a working mechanism conceived on the basis of interconnected charge-transfer (CT) and ion-transfer (IT) processes are selected for such purpose due to their unique characteristics towards decentralized measurements.

    The first chapter of the thesis aims to provide a general background about electrochemistry measurements of ions, providing special attention to all-solid-state voltammetric ISEs based on ultrathin membranes that provide the CT–IT mechanism. Fundaments about ASV and the use of the HMDE for trace metal detection are also revised. Of particular interest is the case of Ag+ determination, which is not fully addressed with the HMDE. Accordingly, the state-of-the-art of electrochemical analysis of trace Ag+ has been established (Paper I).

    The second chapter shows the experimental details and the third chapter presents and discusses all the results obtained in this thesis.

    The first section is about a new analytical strategy for nanomolar detection of Ag+ in waters by coupling a silver-selective electrode (AgSE) based on a CT–IT mechanism with IT stripping voltammetry readout (Paper II). Specifically, the IT occurs via providing the CT process in electrodes that are modified with a redox-active conductive polymer and an ultrathin silver-selective membrane placed on top. Thus, the CT–IT tandem in voltammetric ISEs is unprecedently demonstrated for the detection of Ag+ in different water samples.

    The second section is based on the improvement of the limit of detection of the developed AgSE to detect sub-nanomolar concentration of Ag+ even in the presence of high interference levels, e.g., sodium ion (Paper III). Through the reduction of the total ion-exchange capacity of the ultrathin membrane, it is possible to increase the effectiveness towards the intake of Ag+ versus sodium ions (Na+) when IT stripping voltammetry is applied. The resulting ISE displayed a limit of detection of 0.05 nM, with a linear range of response up to 10 nM and is successfully applied for the analysis of Ag+ in several water samples, including seawater.

    The third section presents the investigation of the HMDE for multi-metal detection at trace levels in soil waters while establishing the fundaments, features and controversies of the technique (Paper IV). The entire replacement of the HMDE will only occur when multi-metal trace detection is provided by a sole electrode or an electrode array able to provide similar analytical characteristics, which are collected in this thesis, while avoiding the use of mercury or any other pollutant in the electrode manufacturing.

    The fourth section inquiries the possibility of using voltammetric ISE based on interconnected CT–IT processes for other trace metals, in particular lead and copper ions (Pb2+ and Cu2+) (Paper V). Despite more work being necessary in that direction, preliminary insights have revealed the potential of the CT–IT technique developed in this thesis towards multi-metal detection either with the incorporation in the membrane of multiple ionophores with different selectivity profiles for each metal or with a multi-sensor array. Accordingly, the research work presented in this thesis has a strong potential towards future investigations in this direction.

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  • 2.
    Xu, Kequan
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.
    The Exploration of Ultrathin Ion Selective Membranes for TraceDetection of Pb2+ and Cu2+Manuscript (preprint) (Other academic)
  • 3.
    Xu, Kequan
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry.
    Cuartero, Maria
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.
    Crespo, Gaston A.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.
    Lowering the limit of detection of ion-selective membranes backside contacted with a film of poly(3-octylthiophene)2019In: Sensors and actuators. B, Chemical, ISSN 0925-4005, E-ISSN 1873-3077, Vol. 297, article id UNSP 126781Article in journal (Refereed)
    Abstract [en]

    Nanometer-sized membranes (thickness of ca 200 nm) backside contacted with a film of poly(3-octylthiophene) (POT) are here interrogated by an electrochemical protocol based on the accumulation and stripping of the target ion aiming at lowering its limit of detection (i.e., in the sub-micromolar range). Thus, using a membrane based on silver ionophore IV (Sigma-Aldrich), which is one of the ionophores regularly used in ion-selective membranes presenting a large binding constant (log beta(Ag-)(ionophore )approximate to 12), it is possible to detect 5 nM concentration of silver with the established methodology. Importantly, this is a 1000-fold lower concentration of silver compared with the case in which the same membrane is subjected to traditional cyclic voltammetry. Essentially, the control of the oxidation state of the POT film by applying a constant potential during a certain period of time (i.e., E-app = 0 V for 720 s) in the presence of silver ions in the sample solution (from 5 to 100 nM) allows for an enrichment of the selective membrane in silver ions. As a result, a subsequent anodic linear sweep potential generates a voltammetric peak for silver transfer across the membrane that comprises a well-defined wave for such very low concentrations of silver in high sodium ion concentration background solution (10 mM NaNO3). Detection of nanomolar levels of silver in different types of natural and environmental waters is herein demonstrated and the results are validated using inductively coupled plasma mass spectrometry.

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  • 4.
    Xu, Kequan
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.
    Cuartero, Maria
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.
    Crespo, Gaston A.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.
    Subnanomolar detection of ions using thin voltammetric membranes with reduced Exchange capacity2020In: Sensors and actuators. B, Chemical, ISSN 0925-4005, E-ISSN 1873-3077, Vol. 321, article id 128453Article in journal (Refereed)
    Abstract [en]

    Herein, we report on a new strategy to improve the limit of detection of ionophore-based thin membranes interrogated under accumulation/stripping electrochemical protocol. Accordingly, we demonstrate subnanomolar detection of silver ion (Ag+) in water samples by re-formulating the membrane content with a reduced amount of the cation exchanger sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (Na+TFPB–), i.e. 10 mmol kg–1 compared to 40 mmol kg–1 commonly used in previous thin cation-selective membranes. Thoughtfully, by decreasing the amount of NaTFPB in the membrane phase, a diminution of its total ion-exchange capacity is to be seen. Essentially, a lower exchange capacity causes that the saturation of the membrane occurs at a lower concentration of Ag+, allowing us to reach a lower limit of detection. This effect is indeed promoted by achieving the total replacement of the Na+ present in the membrane by Ag+ entering from the solution (even at the subnanomolar level) at shorter accumulation times in the readout protocol. For the silver-selective electrode, we found a linear range of response with the peak current from 0.05–10 nM Ag+ concentration. The developed silver-selective electrode is successfully applied to the determination of Ag+ at the (sub)nanomolar level in different water samples (i.e., tap water, seawater and freshwater samples), with the results validated using inductively coupled plasma mass spectrometry as well as recovery studies. In addition, the electrode is suitable for dynamic studies involving the interaction of Ag+ with compounds forming natural organic matter in aquatic resources such as humic acid.

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  • 5.
    Xu, Kequan
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.
    Liu, Yujie
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry.
    Crespo, Gaston A.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.
    Cuartero, Maria
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.
    Ultrathin ion-selective membranes for trace detection of lead, copper and silver ions2022In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 427, article id 140870Article in journal (Refereed)
    Abstract [en]

    Voltammetric ion-selective electrodes (ISEs) based on poly(3-octylthiophene) (POT) in connection with ultra-thin membranes formulated with different selective receptors (i.e., ionophores) are proposed for detection of lead, copper and silver ions (Pb2+, Cu2+ and Ag+). The working mechanism of the POT-membrane electrode is based on interconnected charge transfer processes on both sides of the membrane, with the overall process depending on the electron transfer in the POT lattice ultimately linked to the ion transfer at the membrane–sample interface. This latter is demonstrated to be controlled by (i) the membrane composition and (ii) the accumulation/stripping electrochemical protocol, allowing the detection of traces of Ag+, Pb2+ and Cu2+. In the case of the Pb2+-selective electrode, the voltammogram displays several peaks that are hypothesized to correspond to different ion–ionophore stoichiometries. Following the signal related to the principal stoichiometry (1:1), a Pb2+ concentration as low as 0.1 nM is measurable. In contrast, the Cu2+- and Ag+-selective electrodes show only one peak for the corresponding ion analyte, which can be also detected at nanomolar concentrations. The results obtained with the three electrodes support their further usage for multi-ion detection in water samples through either a multi-ionophore-based electrode or multiple-electrode device. In any case, the membrane composition, in terms of the ionophore/exchanger molar ratio, is key to achieving a successful analytical application. Upcoming efforts may be directed at the replacement of traditional trace metal ion detection with the hanging mercury drop electrode to develop a more sustainable electrochemical approach without diminishing the analytical performance.

  • 6.
    Xu, Kequan
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.
    Pérez Ràfols, Clara
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry.
    Cuartero, Maria
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.
    Crespo, Gaston A.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.
    Electrochemical detection of trace silver2021In: Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 374, article id 137929Article in journal (Refereed)
    Abstract [en]

    Increasing utilization of silver and silver nanoparticles (AgNPs) in daily processes and products has led to a significant growth in scientific interest in methods for monitoring silver. In particular, the amount of silver ions (Ag +) released to the environment is known to have a detrimental effect on aquatic ecology, and thus some control actions have been implemented in recent years; for example, the manufacturing industry is now required to control and certify the quantity of AgNPs present in products. Electrochemical sensors are well suited to the task of silver monitoring due to several beneficial properties, including low costs, fast measurements, and facile adaptation to miniaturized, portable instrumentation. The predominant method for electrochemical silver determination involves potentiometric ion selective electrodes (ISEs) and voltammetric measurements. Reviewing the literature of the last ten years reveals significant improvements in the analytical performance of electrochemical sensors, mainly related to the development of new protocols, selective receptors, and electrode materials. Remarkably, ISEs with limits of detection (LOD) in the nanomolar range have been reported, employing careful control of ion fluxes across the membrane interfaces. What's more, sub-nanomolar LODs are attainable by stripping voltammetry using either ligand-based deposition strategies or thin layer membranes coupled to conducting polymers. Selectivity has also been optimized through the membrane composition of ISEs, with special focus on Ag+ ionophores. Furthermore, novel voltammetric methods allow for discrimination between Ag+ and AgNPs. However, there is still a dearth of studies applying such electrochemical sensors to on-site water analysis, and hence, further research is needed in order to translate these laboratory scale achievements to real-world contexts. Overall, this review describes the state-of-the-art in electrochemical silver detection, and provides a comprehensive description of those aspects contributing to the further development and improvement of analytical performance.

  • 7.
    Xu, Kequan
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.
    Pérez-Ràfols,, Clara
    Marchoud, Amine
    Cuartero, María
    Crespo, Gaston A.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Chemistry, Applied Physical Chemistry.
    Anodic Stripping Voltammetry with the Hanging Mercury Drop Electrode for Trace Metal Detection in Soil Samples2021In: chemosensors, ISSN 2227-9040, Vol. 9, p. 107-Article in journal (Refereed)
1 - 7 of 7
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