The paper describes the use of a microfluidic system to synthesize carbon dots and their use as optical pH sensors.

A flow injection system based on potentiometric detection and designed for on-line monitoring of ammonia in freshwater streams is described. A specially constructed all-solid state tubular flow-through ammonium electrode was used in conjunction with a gas-diffusion chamber to enhance its selectivity. Construction of the electrode involved the in situ formation and direct casting of the sensing nonactin-poly(vinyl chloride) membrane on to a mouldable conducting filled polymer (epoxy resin loaded with graphite), which acted as an internal solid contact, without an inner solution. The electrode response under dynamic conditions both with and without the gas-diffusion chamber was evaluated. After optimisation of the flow system parameters, over 30 samples with a wide range of ammonia concentrations (2 × 10^–5–10^–2 M) could be processed per hour with a detection limit of ca. 10^–6 M. The performance of the monitoring system under on-line conditions in a water treatment plant is also described.

A new liquid membrane, mercury(II) ion-selective electrode (ISE) based on 1,3-diphenylthiourea as a neutral carrier was developed. In solubility tests of 1,3-diphenylthiourea in six different plastizicers, tris(2-ethylhexyl) phosphate (TEHP) showed the best performance and was selected for the membrane. Several ISEs were conditioned and tested for the selection of seven metal ions (Hg²⁺, Cu²⁺, Zn²⁺, Pb²⁺, Ca²⁺, Mg²⁺ and Ag⁺). The best Nernstian response was obtained for Hg²⁺ with an experimental slope of 58.6 ± 0.8 mV per decade in the concentration range 6.0 × 10⁻⁶–5.0 × 10⁻⁴ mol l⁻¹; the presence of the complex Hg(OH)⁺ ion explains the slope of the response curve. The ISE showed stable detection reproducibility during 6 weeks, with a practical detection limit of 10⁻⁶ mol dm⁻³ and a high selectivity for Cd²⁺, Pb²⁺, Co²⁺, Cu²⁺, Sr²⁺, Zn²⁺, Ca²⁺, Mg²⁺, Mn²⁺, K⁺, Na⁺ and NH₄ ⁺ ions. Only one interference for Ag⁺ was found. A second linear response curve for Hg²⁺ was obtained at pH 4 with a slope of 30.8 mV per decade in the concentration range 2.0 × 10⁻⁶–2.1 × 10⁻⁴ mol l⁻¹.