Free chlorine detection based on EC’ mechanism at an electroactive polymelamine-modified electrode

Waste DVD polycarbonate substrate for screen-printed carbon electrode modified with PVP-stabilized AuNPs for continuous free chlorine detection

An electrochemical free chlorine sensor was developed by modifying a lab-made screen-printed carbon electrode (SPCE) with gold nanoparticles synthesized with polyvinylpyrrolidone (AuNPs-PVP). The electrode was made by screen printing carbon ink on a waste digital versatile disc (SPC-wDVD). PVP was used to stabilize AuNPs. Scanning electron microscopy showed that AuNPs aggregated without the stabilizer. The electrochemical behavior of the SPC-wDVD was evaluated by comparison with commercial SPCEs from two companies. Electrochemical characterization involved cyclic voltammetry and electrochemical impedance spectroscopy. The detection of free chlorine in water samples was continuous, facilitated by a flow-injection system. In the best condition, the developed sensor exhibited linearity from 0.25 to 3.0 and 3.0 to 500 mg L-1. The limit of detection was 0.1 mg L-1. The stability of the sensor enabled the detection of free chlorine at least 475 times with an RSD of 3.2 %. The AuNPs-PVP/SPC-wDVD was able to detect free chlorine in drinking water, tap water and swimming pool water. The agreement between the results obtained with the proposed method and the standard spectrophotometric method confirmed the precision of the developed sensor.

CuO-ZIF-8 modified electrode surface as a new electrochemical sensing platform for detection of free chlorine in aqueous solution

This work has applied metal-organic frameworks (MOFs) with high adsorbability and catalytic activity to develop electrochemical sensors to determine free chlorine (free-Cl) concentrations in aqueous media. A zeolitic imidazolate frameworks, Zn(Hmim)2 (ZIF-8) has been synthesized and incorporated with CuO nanosheets to decorate a glassy carbon electrode (GCE) and provide a new sensor for free-Cl determination. The as-prepared ZIF-8 and CuO-ZIF-8 composites have been characterized by FESEM, EDX, XRD, and FT-IR analyses. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) utilized to characterize the CuO-ZIF-8/GC modified electrode electrochemically, demonstrated the ability of the sensor to measure free-Cl concentration. Using differential pulse voltammetry (DPV) and under the optimal conditions, the prepared CuO-ZIF-8/GC modified electrode showed a linear response in the 0.25-60 ppm range with a 12 ppb detection limit (LOD) for free-Cl concentration. Finally, the fabricated sensor was applied to analyze free-Cl from actual swimming pool water samples with promising 97.5 to 103.0% recoveries.

Impact of gate electrode on free chlorine sensing performance in solution-gated graphene field-effect transistors

Free chlorine is widely used to disinfect water used for drinking and food processing. This requires highly sensitive, simple, and capable continuous-measurement sensors to enable the concentration of free chlorine in water to be monitored and controlled. Free chlorine sensors based on solution-gated graphene field-effect transistors (GFETs) are a suitable platform for highly sensitive and continuous measurements. However, their sensing mechanisms require further elucidation to improve their performance. In this study, we focused on the gate electrode and investigated its influence on the sensing performance. Using the free chlorine sensor based on the solution-gate GFET, we showed that the Dirac point voltage in the transfer curve changed significantly as the free chlorine concentration changed, and the electric double-layer capacitance of the gate electrode decreased. Furthermore, we demonstrated that a solution-gated GFET using graphene or boron-doped diamond as the gate electrode could be used to detect changes in the free chlorine concentration in the concentration range of tap water. The sensing performance in the low concentration range benefits from the wide potential window of carbon-based electrodes, which do not have electrochemically active sites. Using these carbon-based materials as gate electrodes, GFETs have the potential to be used as durable sensors that are resistant to surface fouling and oxidation.

Cobalt-based metal-organic framework-functionalized graphene oxide modified electrode as a new electrochemical sensing platform for detection of free chlorine in aqueous solution

This work reports the profit of using a MOF compound for developing a sensitive electrochemical sensor to free chlorine detection in an aqueous solution. Co-MOF and FGO composites were synthesized and combined with the carbon paste (CP) to prepare an efficient electrochemical sensor with high sensing ability. The fabricated Co-MOF and FGO composites were characterized by SEM, EDX, FT-IR, and XRD techniques. Meanwhile, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were utilized to assess the electrochemical performance of the Co-MOF-FGO/CP modified electrode. Under the optimized condition, the amperometric detection showed that the reduction current of free chlorine increased linearly with a coefficient determination of 0.995 during its wide concentration range of 0.1-700 ppm. Also the detection limit (LOD) (S/N = 3) was 0.01 ppm. The selectivity of the sensor was tested with possible interferences, and satisfactory results were obtained. The proposed sensor was successfully used to determine the free chlorine in tap water and swimming pool water real samples. The results suggested that this proposed sensor could pave the way for developing the electrochemical sensor of free chlorine in aqueous media with MOFs.

Flow-based green ceramics microdevice with smartphone image colorimetric detection for free chlorine determination in drinking water

The residual free chlorine concentration is an important parameter to evaluate the potability of water and the efficiency of disinfection in the water treatment system. As a restricted range of residual free chlorine concentration at all points of the distribution network is needed to ensure efficiency and to avoid deleterious effects, fast and in situ quantification of this specie is important. This work deals with the development and validation of two procedures based on DPD (N,N-diethyl-p-phenylenediamine) and OT (ortho-tolidine, 3,3-dimethylbenzidine) for the determination of residual free chlorine in water by exploiting a flow-based microdevice built with Low Temperature Co-Fired Ceramic (LTCC) technology. The analytical signal was monitored by a smartphone camera through RGB values obtained by a free application (Color Grab®). Under optimized conditions, linear ranges within 0.6-2.5 mg/L and 0.1-2.3 mg/L were obtained for DPD and OT methods, with limits of detection and quantification of 0.023 and 0.077 mg/L (DPD) and 0.026 and 0.089 mg/L (OT). Precision expressed as RSD (2.0 mg/L free chlorine, n = 10), was 1.3 % and 0.7 %, respectively. Both procedures were successfully applied to the analysis of samples from a water treatment plant. The flow-based microdevice coupled to digital-image colorimetry is an innovative, sustainable, and cost-effective analytical tool for in-field chemical analysis.

Poly(caffeic acid) Redox Couple Decorated on Electrochemically Reduced Graphene Oxide for Electrocatalytic Sensing Free Chlorine in Drinking Water

Regular water quality measurements are essential to the public water supply. Moreover, selective free chlorine (disinfectant) level monitoring without an interfering agent is necessary. The present work aimed to fabricate poly(caffeic acid) (p-CFA) coated on an electrochemically reduced graphene oxide (ERGO) surface for the selective detection of free chlorine. Electron microscopy and various spectroscopic techniques confirmed the p-CFA@ERGO/glassy carbon (GC) electrode. The p-CFA@ERGO/GC coated probe surface coverage was calculated to be 4.75 × 10^-11 mol cm^-2. The p-CFA@ERGO/GC showed superior catechol/o-quinone oxidation/reduction peaks for electrocatalytic free chlorine determination. The performance of the developed sensor electrode was outstanding, with an extensive range of free chlorine detection (20 μM to 20 mM), high sensitivity (0.0361 µA µM^-1), and low detection limit (0.03 µM). The p-CFA@ERGO/GC capability of the realist water samples, such as the tested commercial and tap water, yielded a good range of recovery (from 98.5% to 99.9%). These values align with the standard N,N'-diethyl-p-phenylenediamine reagent method results.

Electrogenerated chemiluminescence of luminol at a boron-doped diamond electrode for the detection of hypochlorite

Electrogenerated chemiluminescence (ECL) of luminol at a boron-doped diamond electrode has been used for hypochlorite determination. The presence of H₂O₂ induces the generation of the ECL signals of luminol. In contrast, the presence of hypochlorite oxidizes luminol directly to decrease the ECL signals of luminol. Accordingly, a decrease of the ECL signals of luminol in the presence of H₂O₂ was used as the signal response for hypochlorite detection. A linear decrease of ECL signals with the NaClO concentration in the range from 0 to 20 μM was observed with a sensitivity of 18.56 a.u. μM^-1 cm^-2. An estimated detection limit of 0.88 μM was achieved, which is around one order lower than the detection limit obtained using the normal electrochemical method with the same electrode. The system also provides a good selectivity towards Cu²⁺ and Na⁺. A reproducibility of 3.40%RSD was noted for 15 repetitive measurements. The analytical performance was found to be favourable in comparison to those of other typical electrochemical and electrochemiluminescence methods, indicating that it is applicable for real sample detection.

Detection of free chlorine in water using graphene-like carbon based chemiresistive sensors

Free chlorine is the most commonly used water disinfectant. Measuring its concentration during and after water treatment is crucial to ensure its effectiveness. However, many of the existing methods do not allow for continuous on-line monitoring. Here we demonstrate a solid state chemiresistive sensor using graphene-like carbon (GLC) that overcomes that issue. GLC films that were either bare or non-covalently functionalized with the redox-active phenyl-capped aniline tetramer (PCAT) were successfully employed to quantify aqueous free chlorine, although functionalized devices showed better performance. The response of the sensors to increasing concentrations of free chlorine followed a Langmuir adsorption isotherm in the two tested ranges: 0.01–0.2 ppm and 0.2–1.4 ppm. The limit of detection was estimated to be 1 ppb, permitting the detection of breaches in chlorine filters. The devices respond to decreasing levels of free chlorine without the need for a reset, allowing for the continuous monitoring of fluctuations in the concentration. The maximum sensor response and saturation concentration were found to depend on the thickness of the GLC film. Hence, the sensitivity and dynamic range of the sensors can be tailored to different applications by adjusting the thickness of the films. Tap water samples from a residential area were tested using these sensors, which showed good agreement with standard colorimetric measurement methods. The devices did not suffer from interferences in the presence of ions commonly found in drinking water. Overall, these sensors are a cost-effective option for the continuous automated monitoring of free chlorine in drinking water.

Chemiresistive sensors based on graphene-like carbon films are very stable and sensitive. They can be used for continuous online monitoring of free chlorine.

Biodegradable asparagine–graphene oxide free chlorine sensors fabricated using solution-based processing

A free chlorine-sensing biodegradable ink was made by functionalizing asparagine onto graphene oxide then deposited on an electrode. The sensor showed a sensitivity of 0.30 μA ppm−1, selectivity amid interfering ions, and low temperature dependence.

A reusable, reagent-less free chlorine sensor using gold thin film electrode

Free chlorine is widely used as a disinfectant in the water industry. Accurate monitoring of the residual free chlorine concentration in water cycles is critical to maintain public health safety. Here, we report on a thin gold film-based reusable and reagent-less free chlorine sensor. A gold thin film of 300 nm thickness was deposited on a polyimide tape, which was placed on a glass substrate and a simple Styrofoam adhesive tape was used to cover the film and expose 0.36 cm2 circular area as the sensing surface. The sensor showed a high sensitivity of 0.327 μA ppm-1, with a linear range of 0 to 6 ppm, and an accuracy of <0.1 ppm with high selectivity in the presence of commonly interfering ions. The sensor response time was 50 s with a negligible hysteresis of 0.06 ppm. The sensor showed very little change in output current in the pH range between 5.2 to 8.4, and temperature range of 20 to 30 °C. Therefore, the sensor operation is reagent-less, does not need frequent calibration, and showed consistent sensing performance with real water samples. The simple fabrication, ease-of-use and reliable sensing performance of the proposed sensor shows feasibility for mass-production and application in remote and resource-limited areas.

Potentiometric Sensor Based on Carbon Paste Electrode for Monitoring Total Residual Chlorine in Electrolytically-Treated Ballast Water

A new potentiometric sensor based on modified carbon paste electrode (CPE) was prepared for the sensitive and selective detection of total residual chlorine (TRC) in simulated electrolytically-treated ballast water (BW). The modified CPE was prepared using ferrocene (Fc) as the sensing species and paraffin oil as the binder. It is revealed that the addition of Fc can significantly shorten the response time and improve the reproducibility, selectivity, and stability of the sensor. The open circuit potential of the Fc-CPE is in linear proportion to the logarithm of TRC within the TRC concentration range from 1 mg∙dm-3 to 15 mg∙dm-3. In addition, the Fc-CPE sensor exhibits good selectivity to TRC over a wide concentration range of the possible co-exiting interference ions in seawater. The Fc-CPE electrode can be used as a convenient and reliable sensor for the continuous monitoring of TRC during the electrolytic treatment of BW.

Electrochemical determination of free chlorine on pseudo-graphite electrode

Pseudo-graphite from the University of Idaho Thermolyzed Asphalt Reaction also known as GUITAR is a new form of carbon. It shares morphological features with graphites, including basal and edge planes. Unlike graphites and other sp²-hybridized carbons, GUITAR has fast heterogeneous electron transfer across its basal planes and resistance to corrosion similar to boron-doped diamond electrodes. In this contribution GUITAR electrodes were examined as sensors for aqueous free chlorine (HOCl and OCl^-) at pH 7.0 with cyclic voltammetric (CV) and chronoamperometric (CA) methods. Using CV at 50 mV s^-1 GUITAR has a limit of detection of 1.0 μmol L^-1, linear range of 0-5,000 μmol L^-1, sensitivity of 215.8 μA L mmol^-1 cm^-2 and a signal stability of 4 days in constant exposure to 1 mmol L^-1 free chlorine in pH 7.0, 0.1 mol L^-1 phosphate buffer system. After 7 days of exposure GUITAR electrodes lost 37% of the former sensitivity, which was recovered by an in-situ regeneration procedure. The common aqueous ions, Ca²⁺, Na⁺, NO₃^-, SO₄^2-, Cl^-, CO₃^2- and dissolved oxygen did not affect the response of the GUITAR-based sensor. The combination of limit of detection, linear range, sensitivity, sensor lifetime and its relative lack of interferences indicate that GUITAR is one of the best performers in free chlorine sensors.

Simple aminophenol-based electrochemical probes for non-enzymatic, dual amperometric detection of NADH and hydrogen peroxide

Non enzymatic detection of NADH and H₂O₂ is of practical significance for both environmental and biological prospective. However, there is no simple, straight forward electrochemical sensor available for sensing of them in real samples. Addressing this challenge, we report a simple stimuli responsive aminophenol, pre-anodized screen printed carbon electrode (SPCE*/AP) based electrochemical probes for dual detection of NADH and H₂O₂. Aminophenol prepared and adsorbed on the electrode from aminophenylboronic acid via boronic acid deprotection with H₂O₂. The SPCE*/AP fabricated with this process was characterized by cyclic voltammetry (CV), scanning electron microscope (SEM), Raman spectroscopy, UV-visible spectroscopy, and X-ray photoelectron spectroscopy (XPS). Amperometric detection results showed that SPCE*/AP electrodes exhibited linearity from 50 µM to 500 µM and from 200 µM to 2 mM with a detection limit (S/N = 3) of 4.2 µM and 28.9 µM for NADH and H₂O₂, respectively. Excellent reproducibility and selectivity for NADH and H₂O₂ were observed for this electrochemical platform. In addition, the matrix effect was investigated further using the same technique to analyze NADH and H₂O₂ in human urine samples, human serum samples, cell culture medium (containing 10% fetal bovine serum, FBS), and environmental water samples (tap water and rain water). Also, the present sensor demonstrated promising outcomes with living cells (normal cells and cancer cells).

Highly sensitive solution-gated graphene transistor based sensor for continuous and real-time detection of free chlorine

The concentration of free chlorine used for sterilizing drinking water, recreational water, and food processing water is critical for monitoring potential environmental and human health risks, and should be strictly controlled. Here, we report a highly efficient solution-gated graphene transistor (SGGT) device, for the detection of free chlorine in a real-time and convenient manner with excellent selectivity and high sensitivity. The detection mechanism of the SGGT with Au gate electrode is attributed to two combined effects: the reduction of the free chlorine on Au gate electrode; and the direct oxidization of graphene by the free chlorine in solution. The SGGT device shows a linear response range of free chlorine from 1 μM to 100 μM, with detection limit as low as 100 nM, far beyond the sensitivity required for practical applications. Finally, we also demonstrate the performance of the SGGT for determination of free chlorine in local tap water samples. The results presented herein have important implications in the development of portable and disposable devices based on SGGT sensing platform for the simple, real-time, and selective determination of free chlorine.

Improved Cl2 sensing characteristics of reduced graphene oxide when decorated with copper phthalocyanine nanoflowers

A novel gas sensing platform involving a hybrid of reduced graphene oxide (rGO) sheets with unsubstituted copper phthalocyanine (CuPc) nanoflowers has been explored as a room temperature ppb level chemiresistive chlorine (Cl₂) sensor with a detection limit as low as 1.97 ppb.

Visual and optical detection of hypochlorite in water samples based on etching of gold/silver alloy nanoparticles

In this work, we have described the cost-effective, simple, selective and sensitive approach for the detection of hypochlorite (ClO⁻) using gold/silver alloy nanoparticles (Au/Ag alloy NPs) as a colorimetric probe.

Microfabricated electrochemical pH and free chlorine sensors for water quality monitoring: recent advances and research challenges

Recent advances of micro-electrochemical ph and free chlorine sensors are reviewed and their technological challenges and perspectives are provided.