Silicon-based electrochemical microdevices for silicate detection in seawater (original) (raw)
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First Deployment and Validation of in Situ Silicate Electrochemical Sensor in Seawater
Frontiers in Marine Science
An electrochemical sensor is proposed to measure silicate concentration, in situ, in the ocean without any addition of liquid reagent. From the analytical principle to the laboratory prototype toward the first in situ, immersible sensor, the evolution of the mechanical design is presented and discussed. The developed in situ electronics were compared to the commercial potentiostat and gave promising results to detect low silicate signals with a limit of quantification of 1 µmol L −1 .The flow rate of the pump appeared to be a crucial parameter in order to transfer the silicomolybdic complex formed from the "complexation cell" to the "detection cell" without dilution as well as to fill and rinse the whole circuit. The study of temperature effect revealed no influence on the electrochemical signal between ∼7 • and ∼21 • C. Finally the sensor was successfully deployed for the very first time on a mooring off Coquimbo, Chile and also integrated onto a PROVOR profiling float in the Mediterranean Sea off Villefranche-sur-Mer, France. The data collected and/or sent through satellite were in good agreement with the 2 reference samples and previously published values illustrating the great potential of this electrochemical sensor. A 7 days silicate time series from the mooring deployment off Chile is also presented.
Reagentless and calibrationless silicate measurement in oceanic waters
Talanta, 2012
Determination of silicate concentration in seawater without addition of liquid reagents was the key prerequisite for developing an autonomous in situ electrochemical silicate sensor (Lacombe et al., 2007) [11]. The present challenge is to address the issue of calibrationless determination. To achieve such an objective, we chose chronoamperometry performed successively on planar microelectrode (ME) and ultramicroelectrode (UME) among the various possibilities. This analytical method allows estimating simultaneously the diffusion coefficient and the concentration of the studied species. Results obtained with ferrocyanide are in excellent agreement with values of the imposed concentration and diffusion coefficient found in the literature. For the silicate reagentless method, successive chronoamperometric measurements have been performed using a pair of gold disk electrodes for both UME and ME. Our calibrationless method was tested with different concentrations of silicate in artificial seawater from 55 to 140 Â 10 À 6 mol L À 1. The average value obtained for the diffusion coefficient of the silicomolybdic complex is 2.2 7 0.4 Â 10 À 6 cm 2 s À 1 , consistent with diffusion coefficient values of molecules in liquid media. Good results were observed when comparing known concentration of silicate with experimentally derived ones. Further work is underway to explore silicate determination within the lower range of oceanic silicate concentration, down to 0.1 10 À 6 mol L À 1 .
Talanta, 2008
From the study of molybdenum oxidation in aqueous solutions we developed a semi-autonomous method to detect silicate in aqueous samples. Molybdenum oxidation was used to form molybdate in acidic media. The silicomolybdic complex formed with silicate is detectable by amperometry or cyclic voltammetry. The new electrochemical method is in good agreement with the method conventionally used for environmental water silicate analysis. In the second stage, a completely reagentless method was developed using molybdate and proton produced during molybdenum oxidation. Reproducibility tests show a precision of 2.6% for a concentration of 100 μmol L −1. This new method will be very suitable for the development of new autonomous silicate sensors easy to handle and without reagents. In this paper we present the analytical and chemical aspects necessary for a complete documentation of the method before the development of a new reagentless sensor.
Analytical and Bioanalytical Chemistry, 2006
Voltammetric procedures for trace metals analysis in polluted natural waters using homemade bare golddisk microelectrodes of 25-and 125-μm diameters have been determined. In filtered seawater samples, square wave anodic stripping voltammetry (SWASV) with a frequency of 25 Hz is applied for analysis, whereas in unfiltered contaminated river samples, differential pulse anodic stripping voltammetry (DPASV) gave more reliable results. The peak potentials of the determined trace metals are shifted to more positive values compared to mercury drop or mercury-coated electrodes, with Zn always displaying 2 peaks, and Pb and Cd inversing their positions. For a deposition step of 120 s at −1.1 V, without stirring, the 25-μm gold-disk microelectrode has a linear response for Cd, Cu, Mn, Pb and Zn from 0.2 μg L −1 (1 μg L −1 for Mn) to 20 μg L −1 (30 μg L −1 for Zn, Pb and 80 μg L −1 for Mn). Under the same analytical conditions, the 125-μm golddisk microelectrode shows linear behaviour for Cd, Cu, Pb and Zn from 1 μg L −1 (5 μg L −1 for Cd) to 100 μg L −1 (200 μg L −1 for Pb). The sensitivity of the 25-μm electrode varied for different analytes from 0.23 (±0.5%, Mn) to 4.83 (±0.9%, Pb) nA L μmol −1 , and sensitivity of the 125-μm electrode varied from 1.48 (±0.7%, Zn) to 58.53 (±1.1%, Pb nA L μmol −1 . These microelectrodes have been validated for natural sample analysis by use in an on-site system to monitor Cu, Pb and Zn labile concentrations in the Deûle River (France), polluted by industrial activities. First results obtained on sediment core issued from the same location have shown the ability of this type of microelectrode for in situ measurements of Pb and Mn concentrations in anoxic sediments.
Limnology and Oceanography: Methods, 2008
This article reports on the field trials of a membrane-free amperometric microelectrode dissolved oxygen sensor, which were performed during oceanographic cruise D279 of RRS Discovery. The sensor was used to obtain full depth oxygen profiles while mounted on a wire-operated CTD (conductivity, temperature, depth) instrument. A stable performance was achieved by carefully designed electrochemical cleaning conditions of the sensing platinum microdisk cathode. The flow issues inherent to moving probes were resolved by a novel stop-flow cell fitted with a pumping system for sample exchange and flow control. The details of the sensor operation, calibration, and construction, including the flow control system, are described. The sensor response is validated by calibration and by an analytical approach that yields oxygen data directly from the current readings. The accuracy of the microelectrode response is critically assessed using Winkler titrations on bottle samples taken during the relevant sensor deployments. The results lead to the conclusion that due to high accuracy, fast response time, and lack of membrane-related problems the device is particularly suitable for moving probes and high spatial resolution water column oxygen profiling.
Towards Chip-Based Salinity Measurements for Small Submersibles and Biologgers
International Journal of Oceanography, 2013
Water's salinity plays an important role in the environment. It can be determined by measuring conductivity, temperature, and depth (CTD). The corresponding sensor systems are commonly large and cumbersome. Here, a 7.5 × 3.5 mm chip, containing microstructured CTD sensor elements, has been developed. On this, 1.5 mm 2 gold finger electrodes are used to measure the impedance, and thereby the conductivity of water, in the MHz frequency range. Operation at these frequencies resulted in higher sensitivities than those at sub-MHz frequencies. Up to 14 kΩ per parts per thousand salt concentration was obtained repeatedly for freshwater concentrations. This was three orders of magnitude higher than that obtained for concentrations in and above the brackish range. A platinum electrode is used to determine a set ambient temperature with an accuracy of 0.005 ∘ C. Membranes with Nichrome strain gauges responded to a pressure change of 1 bar with a change in resistance of up to 0.21 Ω. A linear fit to data over 7 bars gave a sensitivity of 0.1185 Ω/bar with an R 2 of 0.9964. This indicates that the described device can be used in size-limited applications, like miniaturized submersibles, or as a bio-logger on marine animals.
Remote Stripping Analysis of Lead and Copper by a Mercury-Coated Platinum Microelectrode
Electroanalysis, 2004
The performance of a remote stripping sensor based on mercury microelectrodes (MM-RS) for the in situ detection of trace metals in aquatic systems, was investigated. The submersible device employed here consists of a single mercurycoated platinum disk microelectrode assembled in a two-electrode cell configuration, and connected remotely by a 30 m long shielded cable. First, the MM-RS device is characterized in Ru(NH 3) 3 6 and Pb 2 synthetic aqueous solutions by applying cyclic voltammetry and anodic stripping voltammetry (ASV), respectively. The results obtained show that the small electrode dimensions and the related low currents involved, the long remote connection cable or the use of a two-electrode system do not cause noise effects or uncompensated resistance problems in the measurements. Using square-wave voltammetry in the stripping step, linear calibration graphs for Pb 2 ions over the concentration range 1 Â 10 À9 À 5 Â 10 À7 M were obtained, and a detection limit, DL, of 0.15 nM was found. The relative standard deviation (RSD), at 5 Â 10 À8 M Pb 2 level, was within 5%. The effect of humic acid and of sodium dodecylsulfate surfactants on the stripping responses was also investigated. The performance of the submersible MM-RS system was tested for the in situ monitoring of the labile fraction of lead and copper on a site of the Lagoon of Venice. In situ Pb 2 and Cu 2 concentrations were monitored for about 8 hours, by leaving the sensor immersed in the lagoon waters (2 m depth) and recording the response every hour. Under these field conditions, reliable in situ data for the labile fraction of these metal ions with a satisfactory precision, the RSD being within 7 and 9 % for lead and copper, respectively, were obtained.
In Situ Detection of Macronutrients and Chloride in Seawater by Submersible Electrochemical Sensors
Analytical chemistry, 2018
A new submersible probe for the in situ detection of nitrate, nitrite, and chloride in seawater is presented. Inline coupling of a desalination unit, an acidification unit, and a sensing flow cell containing all-solid-state membrane electrodes allows for the potentiometric detection of nitrate and nitrite after removal of the key interfering ions in seawater, chloride and hydroxide. Thus, the electrodes exhibited attractive analytical performances for the potentiometric detection of nitrate and nitrite in desalinated and acidified seawater: fast response time ( t < 12 s), excellent stability (long-term drifts of <0.5 mV h), good reproducibility (calibration parameter deviation of <3%), and satisfactory accuracy (uncertainties <8%Diff compared to reference technique). The desalination cell, which can be repetitively used for about 30 times, may additionally be used as an exhaustive, and therefore calibration-free, electrochemical sensor for chloride and indirect salinity ...