Low-cost Electrochemical Sensors Research Articles

The use of electrochemical sensors for monitoring urban air quality in low-cost, high-density networks

Measurements at appropriate spatial and temporal scales are essential for understanding and monitoring spatially heterogeneous environments with complex and highly variable emission sources, such as in urban areas. However, the costs and complexity of conventional air quality measurement methods means that measurement networks are generally extremely sparse. In this paper we show that miniature, low-cost electrochemical gas sensors, traditionally used for sensing at parts-per-million (ppm) mixing ratios can, when suitably configured and operated, be used for parts-per-billion (ppb) level studies for gases relevant to urban air quality. Sensor nodes, in this case consisting of multiple individual electrochemical sensors, can be low-cost and highly portable, thus allowing the deployment of scalable high-density air quality sensor networks at fine spatial and temporal scales, and in both static and mobile configurations.In this paper we provide evidence for the performance of electrochemical sensors at the parts-per-billion level, and then outline results obtained from deployments of networks of sensor nodes in both an autonomous, high-density, static network in the wider Cambridge (UK) area, and as mobile networks for quantification of personal exposure. Examples are presented of measurements obtained with both highly portable devices held by pedestrians and cyclists, and static devices attached to street furniture. The widely varying mixing ratios reported by this study confirm that the urban environment cannot be fully characterised using sparse, static networks, and that measurement networks with higher resolution (both spatially and temporally) are required to quantify air quality at the scales which are present in the urban environment. We conclude that the instruments described here, and the low-cost/high-density measurement philosophy which underpins it, have the potential to provide a far more complete assessment of the high-granularity air quality structure generally observed in the urban environment, and could ultimately be used for quantification of human exposure as well as for monitoring and legislative purposes.

Facile and versatile approaches to enhancing electrochemical performance of screen printed electrodes

Screen printed carbon electrodes provide attractive opportunity for the development of miniaturized low cost electrochemical sensors. However, the electrodes display very low level of electrochemical performance due to the nonelectroactive components in the ink formulation and relatively low graphitic carbon content imposed by the constraints of screen-printing. In this work, selective etching action of a judiciously selected organic solvent was envisaged to improve the electrochemical characteristics of the electrodes. The procedure involves soaking the electrode strips in N,N-dimethylformamide (DMF) for just minutes of etching followed by drying. Five minutes DMF treated electrode, for instance, showed a 100-fold increase in the heterogeneous charge transfer kinetics compared to the untreated electrodes. The effective area of the solvent treated electrode was 57-fold of its apparent geometric area. The solvent treated electrodes also showed a dramatically decreased charge transfer resistance from ca. 18,000Ω to 180Ω. The results of surface examination using scanning probe microscopy and electrochemical impedance spectroscopy revealed enhancement in the porosity of the solvent treated electrodes. The procedure is versatile and applicable to other screen-printed electrodes such as gold, silver or silver/silver chloride to improve the performances characteristics including stability. The ability to activate several electrodes simultaneously and multiple re-usability of the solvent makes this approach cost effective and very promising to facilitate mass production of miniaturized and disposable electrochemical sensors.

Hydrogen emissions from Erebus volcano, Antarctica

The continuous measurement of molecular hydrogen (H2) emissions from passively degassing volcanoes has recently been made possible using a new generation of low-cost electrochemical sensors. We have used such sensors to measure H2, along with SO2, H2O and CO2, in the gas and aerosol plume emitted from the phonolite lava lake at Erebus volcano, Antarctica. The measurements were made at the crater rim between December 2010 and January 2011. Combined with measurements of the long-term SO2 emission rate for Erebus, they indicate a characteristic H2 flux of 0.03 kg s–1 (2.8 Mg day–1). The observed H2 content in the plume is consistent with previous estimates of redox conditions in the lava lake inferred from mineral compositions and the observed CO2/CO ratio in the gas plume (∼0.9 log units below the quartz–fayalite–magnetite buffer). These measurements suggest that H2 does not combust at the surface of the lake, and that H2 is kinetically inert in the gas/aerosol plume, retaining the signature of the high-temperature chemical equilibrium reached in the lava lake. We also observe a cyclical variation in the H2/SO2 ratio with a period of ∼10 min. These cycles correspond to oscillatory patterns of surface motion of the lava lake that have been interpreted as signs of a pulsatory magma supply at the top of the magmatic conduit.

Structural characterisation of 1,10-phenanthroline–montmorillonite intercalation compounds and their application as low-cost electrochemical sensors for Pb(II) detection at the sub-nanomolar level

A low-cost electrochemical sensor for lead detection was designed and developed in the present study, using an organo-clay obtained by the intercalation of 1,10-phenanthroline (OP) within montmorillonite (MMT). Intercalates of 1,10-phenanthroline in montmorillonite were prepared from a 1/1 water–ethanol aqueous solutions at pHs 3, 5 and 8. Their structural characterisation was done by a variety of spectroscopic and analytical techniques: XRD, FTIR, 13C and 23Na NMR, ICP and thermal analysis. Under acidic conditions, OP was protonated and a cation exchange took place with the interlayer sodium ions, leading to a two-dimensional arrangement of protonated OP molecules laying parallel to the clay mineral interlayer surfaces. Under basic conditions, neutral OP was intercalated in the presence of hydrated Na + ions forming two OP layers plausibly H-bonded to the water molecules coordinating the sodium ions. This latter OP-MMT intercalate was used as Pb(II) sensor via a carbon paste electrode (CPE) by adsorptive stripping voltammetry because it should lead to the exchange of hydrated sodium ions by Pb(II). The amount of accumulated Pb(II) increased with an increase of the accumulation time and remained constant after saturation. The optimal pH of the accumulation medium was 6 and the best electrolysis time and potential were 60 s and − 0.8 V respectively. The results obtained in the case of Pb(II) led to a limit of detection in the sub-nanomolar range (4 × 10 − 10 M), two orders of magnitude lower than in the case of Cu(II) (4.0 × 10 − 8 M). There were no interferences with Cu(II) when it was at concentrations 0.1 × [Pb(II)]. For equimolar concentrations, the Pb(II) signal was reduced by 50%. The interferences were minimal in the case of Ag(I). In contrast, the Pb(II) signal was reduced by 70% in the presence of Hg(II) at concentrations 0.1 × [Pb(II)], showing a strong Hg(II) interference.