N-CDs were prepared with a high fluorescence quantum yield of 35.08%. N-CDs have been employed to detect metronidazole in commercial metronidazole tablets. N-CDs exhibited quick response time, high selectivity and sensitivity, and low cytotoxicity.

We use the natural zeolite clinoptilolite as the sensitive element in a plasticised PVC membrane. Separated, a sample pool and a reference pool in water-gated SnO₂ thin-film transistor (SnO₂ WGTFT) with such a membrane leads to membrane potential, and thus transistor threshold shift in response to the common drinking water pollutants Pb²⁺ or Cu²⁺ in the sample pool. Threshold shift with ion concentration, c, follows a Langmuir–Freundlich (LF) characteristic. As the LF characteristic shows the steepest slope in the limit c → 0, this opens a window to limits-of-detection (LoDs) far below the ‘action levels’ of the ‘lead-and-copper rule’ for drinking water: Pb²⁺: LoD 0.9 nM vs 72 nM action level, Cu²⁺: LoD 14 nM vs 20.5 μM action level. LoDs are far lower than for membranes using organic macrocycles as their sensitive elements. Threshold shifts at the lead and copper action levels are more significant than shifts in response to variations in the concentration of non-toxic co-cations, and we discuss in detail how to moderate interference. The selective response to lead and copper qualifies clinoptilolite-sensitised WGTFTs as a low footprint sensor technology for monitoring the lead-and-copper rule, and to confirm the effectiveness of attempts to extract lead and copper from water.

Cesium (Cs⁺) cations are rare in nature, but the β⁻ active radioisotope ¹³⁷Cs can be released from nuclear accidents and find its way into the water supply, where it is harmful to humans and animals who drink it. We here report a water-gated thin film transistor (WGTFT) that allows the detection of Cs⁺ in drinking water at very low concentrations. The transistor channel is formed from spray-pyrolyzed tin dioxide, SnO₂, which gives WGTFTs with near-zero initial threshold. When the WGTFT is sensitized with a plasticized PVC membrane containing the Cs⁺-selective zeolite “mordenite,” it displays a threshold shift when exposed to drinking water samples carrying traces of Cs⁺. The response characteristic is given by the Langmuir adsorption isotherm instead of the Nikolsky-Eisenman law commonly found for ion-sensitive WGTFTs sensitized with organic ionophores. We find a complex stability constant K = (3.9 ± 0.4) × 10⁹ l/mol and a limit of detection (LoD) of 33 pM. Our LoD is far lower than the Cs⁺ potability limit of 7.5 nM, which cannot be met by organic-sensitized membranes where the LoD is typically in the order of 100 nM or more.

It is essential to remove rhodamine 6G and acid orange 10 dyes from contaminated water because they can induce cancer and irritate the lungs, skin, mucous, membranes, and eyes. Hence, in the current work, the Pechini sol–gel method was used for the facile synthesis of ZrO₂/CdMn₂O₄/CdO as novel nanocomposites at 600 and 800 °C. The synthesized nanocomposites were used as novel adsorbents for the efficient removal of rhodamine 6G and acid orange 10 dyes from aqueous media. The nanocomposites, which were synthesized at 600 and 800 °C, were abbreviated as EK600 and EK800, respectively. The synthesized nanocomposites were characterized by EDS, XRD, N₂ adsorption/desorption analyzer, and FE-SEM. The patterns of XRD showed that the average crystal size of the EK600 and EK800 nanocomposites is 68.25 and 85.32 nm, respectively. Additionally, the images of FE-SEM showed that the surface of the EK600 nanocomposite consists of spherical, polyhedral, and rod shapes with an average grain size of 99.36 nm. Additionally, the surface of the EK800 nanocomposite consists of polyhedral and spherical shapes with an average grain size of 143.23 nm. In addition, the BET surface area of the EK600 and EK800 nanocomposites is 46.33 and 38.49 m²/g, respectively. The optimal conditions to achieve the highest removal of rhodamine 6G and acid orange 10 dyes are pH = 8, contact time = 24 min, and temperature = 298 kelvin. The greatest removal capacity of the EK600 and EK800 adsorbents towards rhodamine 6G dye is 311.53 and 250.63 mg/g, respectively. Additionally, the greatest removal capacity of the EK600 and EK800 adsorbents towards acid orange 10 dye is 335.57 and 270.27 mg/g, respectively. The removal of rhodamine 6G and acid orange 10 dyes using the EK600 and EK800 adsorbents is spontaneous, exothermic, follows the Langmuir adsorption isotherm, and fits well with the pseudo-first-order kinetic model.

We introduce fluoride-selective anion exchange resin sorbents as sensitisers into membranes for water-gated field effect transistors (WGTFTs). Sorbents were prepared via metal (La or Al)-loading of a commercial macroporous aminophosphonic acid resin, Puromet^TM MTS9501, and were filled into a plasticised poly(vinyl chloride) (PVC) phase transfer membrane. We found a potentiometric response (membrane potential leading to WGTFT threshold shift) to fluoride following a Langmuir–Freundlich (LF) adsorption isotherm with saturated membrane potential up to ~480 mV, extremely low characteristic concentration c_1/2 = 1/K, and picomolar limit of detection (LoD), even though ion exchange did not build up charge on the resin. La-loading gave a superior response compared to Al-loading. Membrane potential characteristics were distinctly different from charge accumulating sensitisers (e.g., organic macrocycles) but similar to the Cs⁺ (cation) selective ion-exchanging zeolite mineral ‘mordenite’. We propose a mechanism for the observed threshold shift and investigate interference from co-solutes. Strong interference from carbonate was brought under control by ‘diluting’ metal loading in the resin. This work sets a template for future studies using an entirely new ‘family’ of sensitisers in applications where very low limit of detection is essential such as for ions of arsenic, mercury, copper, palladium, and gold.

Hydrogen sulphide (H₂S) is a toxic gas soluble in water, H₂S_aq, as a weak acid. Since H₂S_aq usually originates from the decomposition of faecal matter, its presence also indicates sewage dumping and possible parallel waterborne pathogens associated with sewage. We here present a low footprint (‘frugal’) H₂S_aq sensor as an accessible resource for water quality monitoring. As a sensing mechanism, we find the chemical affinity of thiols to gold (Au) translates to H₂S_aq. When an Au electrode is used as a control gate (CG) or floating gate (FG) electrode in the electric double layer (EDL) pool of an extended gate field effect transistor (EGFET) sensor, EGFET transfer characteristics shift along the CG voltage axis in response to H₂S_aq. We rationalise this by the interface potential from the adsorption of polar H₂S molecules to the electrode. The sign of the shift changes between Au CG and Au FG, and cancels when both electrodes are Au. The sensor is selective for H₂S_aq over the components of urine, nor does urine suppress the sensor’s ability to detect H₂S_aq. Electrodes can be recovered for repeated use by washing in 1M HCl. Quantitatively, CG voltage shift is fitted by a Langmuir-Freundlich (LF) model, supporting dipole adsorption over an ionic (Nernstian) response mechanism. We find a limit-of-detection of 14.9 nM, 100 times below potability.