Fluorometric and colorimetric determination of hypochlorite using carbon nanodots doped with boron and nitrogen

Why hypochlorite in water cannot be detected by ion chromatography?

Free chlorine (FC), primarily existing as hypochlorite anion in alkaline water (pKa 7.5), could theoretically be analyzed by an ion chromatography (IC) system. However, the feasibility of direct FC detection by IC remains controversial in previous researches. To address this issue, this study systematically examined the fate of FC under varying IC eluent pH, anion exchange column (AEC) types, detectors, and suppression conditions. The results ultimately disprove it and show that the primary obstacle to direct FC analysis with IC system is its transformation: 31 %-41 % became Cl- and 59 %-69 % became immobile organic chloramines in AEC, due to its reaction with quaternary ammonium compounds (QACs) in AECs. Moreover, IC analysis of Cl- might be interfered by coexisting FC, although FC concentrations are typically much lower than Cl-. Eluent suppressor can also consume FC even when residual FC after AEC is limited. In contrast, variations in water pH do not affect FC stability, and a UV detector effectively distinguishes FC from other anions. This study hence for the first time elucidates several FC consumption mechanisms in IC system, which represent key barriers hindering FC analysis by IC, and accordingly provide some guidance on future efforts to develop novel AEC to overcome these barriers.

Monocationic versus dicationic-based monomethine cyanine dyes for ultrasensitive colorimetric detection of hypochlorite ion in water

Detecting residual chlorine as a hypochlorite ion (ClO-) in drinking water is crucial for ensuring disinfection effectiveness and safety. In the present study, we report two novel Quinolium Benzothiazole-Based Cyanine (3ethylbenzothiazol-2(3 H)-ylidene)methyl)-1-(4-iodobutyl)quinolin-1-ium tetrafluoroborate (IBTQ) and 1-(3-(4-(dimethylamino)pyridin-1-ium-1-yl)propyl)-4-((3-methylbenzothiazol-2(3 H)-ylidene)methyl)quinolin-1-ium diiodide (DMP-BTQ) hypochlorite (ClO-) sensors using UV- visible, colorimetric, and quartz crystal microbalance (QCM) techniques. The two sensors generate distinct absorption spectra, frequency shifts, and color changes that are visible to the naked eye. They exhibit high sensitivity and selectivity towards ClO-. The sensors have limits of detection (LOD) values in the range of 13.92 ppm and 0.127 ppm for IBTQ and DMP-BTQ, respectively, based on absorption performance with no interference of potential ions in drinking water. The method yields good recovery results, ranging from 97.4 to 103.0%, for ClO- detection in the studied water samples. In addition, the LOD for the QCM technique is 0.06 ppm for IBTQ and 0.045 ppm for DMP-BTQ with low quantification. The sensors can be loaded on paper strips for naked-eye detection of ClO- in domestic tap water and water treatment facilities. The sensors also provide low-cost, low cytotoxicity, high sensitivity, selectivity, and reusability of ClO- in water. The sensing mechanism was rationalized in terms of radical cation generation upon ClO- oxidizing action. The ease of cyanine oxidation was substantiated by quantum chemical studies including density functional theory (DFT) calculations, natural bond orbital (NBO) analysis, molecular electrostatic potential (MESP), and time-dependent density functional theory to support the experimental results.

Carbon dot-based fluorescent sensors for pharmaceutical detection: Current innovations, challenges, and future prospects

Environmental contamination by pharmaceuticals has become a matter of concern as they are released in sewage systems at trace levels, thus impacting biological systems. Increasing concerns about the low-level occurrence of pharmaceuticals in the environment demands sensitive and selective monitoring. Owing to their high sensitivity and specificity carbon dots (CDs) have emerged as suitable fluorescent sensors. This review discusses the current scenario of the status of pharmaceuticals in the environment, limitations associated with traditional techniques employed for their detection, and benefits offered by CDs like easy surface modification and tunable optical properties for sensing applications. Several representative means by which CDs interact with other molecules such as inner filter effect (IFE), dynamic quenching (DQ), static quenching (SQ), Förster resonance energy transfer (FRET), among others, are also discussed along with co-referencing fluorophores to design sensors. Based on developments described herein, CDs-based sensors can be expected to sense pharmaceuticals ranging from nanogram to picogram, target real-time industrial and spiked sample analysis, etc., which provides direction for future research.

Rapid detection and quantification of milk adulterants using a nanoclusters-based fluorescent optical tongue

A paper-based sensor array consisting of eight nanoclusters (NCs) combined with multivariate analysis was used as a rapid method for the determination of animal sources of milk; goat, camel, sheep and cow. It was also used to detect and quantify three adulterants including sodium hypochlorite, hydrogen peroxide and formaldehyde in milk. The changes in fluorescence intensity of the NCs were quantified using a smartphone when the sensor array was immersed in the milk samples. The device generated a specific colorimetric signature for milk samples from different animals and for different adulterants. This allowed simultaneous identification of animal and adulterant sources with 100% accuracy. The device was found to be capable of accurately measuring the level of contaminants with a detection limit as low as 0.01% using partial least squares regression. In conclusion, a paper-based optical tongue device has been developed for the detection of adulterants in milk with point-of-need capability.

Colorimetric and fluorometric dual-mode determination of hypochlorite based on redox-mediated quenching

We have successfully created a dual-modal probe, labeled as of iron(ii)-ortho-phenanthroline/N, S@g-CDs, which combines both fluorometric and colorimetric techniques for the accurate and sensitive detection of hypochlorite (ClO-). The mechanism behind this probe involves the fluorescence quenching interaction between nitrogen and sulfur co-doped green emissive carbon dots (N, S@g-CDs) and the iron(ii)-ortho-phenanthroline chelate, utilizing both the inner filter effect and redox processes. As the concentration of ClO- increases, the iron(ii) undergo oxidation to iron(iii) as follows: Fe(ii) + 2HClO = Fe(iii) + Cl2O + H2O, leading to the restoration of N, S@g-CDs fluorescence. Simultaneously, the color of the system transitions gradually from red to colorless, enabling colorimetric measurements. In the fluorometric method with an excitation wavelength of 370 nm, the ClO- concentration exhibits a wide linear correlation with fluorescence intensity ranging from 0.07 to 220 μM. The detection limit achieved in this method is 0.02 μM (S/N = 3). In contrast, the colorimetric method exhibits a linear range of 1.12 to 200 μM, with a detection limit of 0.335 μM (S/N = 3). The proposed selective absorbance for this method is 510 nm. The probe has been effectively utilized for the detection of ClO- in various samples, including water and milk samples. This successful application showcases its potential for determining ClO- in complex matrices, highlighting its broad range of practical uses.

Boron-enriched rice-like homologous carbon nanoclusters with a 51.5% photoluminescent quantum yield for highly sensitive determination of endogenous hydroxyl radicals in living cells

Exploring the ultrahigh quantum efficiency of a carbon-based probe via a green and simple technique, and utilisation of its sensing ability for highly bioactive molecule detection is still highly challenging. Herein, we prepared a novel boron-enriched rice-like homologous carbon nanoclusters (BRCNs) with an ultrahigh quantum efficiency of ∼51.5% by introduction of a conjugated structure attached to the CN bond and an electron-withdrawing boron active centre. Unexpectedly, the BRCNs obtained showed a stable dispersion of rice-like carbon nanograins, composed of small carbon dot assembled nanoclusters with an average diameter size of ∼30 nm, and containing boron units of ∼24.68 at%. What's exciting is that the BRCNs obtained exhibited an "on-off-on" three-state emission with the addition of an hydroxyl radical (OH˙) and its antioxidants. Thus, two distinctive fluorescent responses for OH˙ and antioxidants based on the BRCN probe had been developed, and the mechanism has been determined using TEM, XPS, FT-IR, FL, UV-vis spectrophotometry, UPS and fluorescent lifetimes. The OH˙, generated from the Fenton's reagent, preferentially attack the electron-deficient vacancy p orbit of the boron atom in the surface of the BRCNs, which results in the boron atom being easily substituted/attacked by OH˙, and leading to spontaneous aggregation induced quenching (AIQ) due to the existence of a strong intermolecular hydrogen bond between denatured BRCNs. Furthermore, the proposed method was also successfully applied to monitor endogenous OH˙ generation in HeLa cells by confocal imaging, which could be used for elucidating OH˙-induced oxidative damage to biological tissues and proteins.

3-Aminophenylboronic acid-functionalized molybdenum disulfide quantum dots for fluorescent determination of hypochlorite

A simple method is reported for hypochlorite determination based on fluorescence 3-aminophenylboronic acid-functionalized molybdenum disulfide quantum dots (B-MoS₂ QDs). B-MoS₂ QDs with strong fluorescence at 380 nm have been successfully synthesized by the amidation reaction between APBA and hydrothermal MoS₂ QDs. Hypochlorite sensing was proposed utilizing the fluorescent quenching effect of 3,3',5,5'-tetramethylbenzidine dihydrochloride (TMB) on B-MoS₂ QDs and the fast redox reaction between hypochlorite and TMB. The fluorescent quenching effect of TMB to B-MoS₂ QDs was proved to be caused by static dynamic quenching and inner filter effect. A good linear relationship was obtained in the hypochlorite concentration range from 1 to 20 μM, and the limit of detection (LOD) was 36.8 nM. The proposed fluorescent detection assay was simple and fast, taking only 5 min at room temperature. Satisfactory results were obtained in the standard spike recovery tests on tap water and milk samples, which indicate high potential in constructing fluorescent bio-detection assays.

Nitrogen-doped carbon quantum dots as fluorescent nanosensor for selective determination and cellular imaging of ClO. SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2022;271:120941. [PMID: 35114635 DOI: 10.1016/j.saa.2022.120941]

The carbon nanomaterial based fluorescent probes have been widely applied in biological imaging. In the current research, we propose an interesting strategy for selective sensing of hypochlorite (ClO^-) by a water-soluble and highly fluorescent nanosensor based on the N-doped carbon quantum dots (CDs) which was fabricated by a facile and environmental friendly hydrothermal approach from polyvinyl pyrrolidone, L-arginine and tryptophan. The structural characteristics of the probe were measured by multitudinous methods which proved the nanometer spherical structure of the probe and the successfully N-doping. Fluorescent investigation demonstrated that the probe is not only highly stable under interferences of pH, ionic strength, and irradiation, but also significantly selective toward ClO^- amongst a variety of attractive bioactive species through the fluorescent quenching process which was correlative with the concentration of ClO^- and linearly in the range of 0.1-50 μmol·L^-1 with the sensitivity of 0.03 μmol·L^-1. The probe can also be further illustrated in a prospective application for determination of ClO^- in environmental water through both solution response and filer paper sensing. Moreover, the positive biocompatibility and ignorable cytotoxicity made the probe a promising effective agent for detection and visualizing ClO^- in living cells which can facilitate the understanding the oxidative stress from the overexpressing ClO^-.

A simple and green method for direct determination of hydrogen peroxide and hypochlorite in household disinfectants based on personal glucose meter

A convenient and green method was developed using a personal glucose meter (PGM) for direct determination of hydrogen peroxide and hypochlorite in household disinfectants based on the acetylthiocholine iodide (ATCI)-mediated reaction. In this method, acetylcholinesterase catalyzes the hydrolysis of ATCI to generate thiocholine iodide, which can trigger the reduction of K3[Fe(CN)6] to K4[Fe(CN)6] and generate a PGM detectable signal. When the hydrogen peroxide or hypochlorite is pre-incubated with the ATCI, the yield of thiocholine will be decreased because the iodine molecules can initiate the oxidation of thiocholine to disulphides, leading to a lower PGM readout. Thus, the hydrogen peroxide or hypochlorite can be directly quantified by PGM as simple as detecting glucose in blood. After being systematically investigated, the optimum conditions of the PGM method are as follows: 5.0 min of enzymatic reaction time, 6.0 mM final concentration of ATCI solution, and 10.0 min of reaction time between ATCI/H2O2 and thiocholine. Moreover, the recoveries of hydrogen peroxide in real sample spiked with three different concentrations (final concentrations of 5.0, 10.0, and 20.0 mM) are in the range of 96.3-108.4%. The recoveries of hypochlorite in real sample spiked with three different concentrations (final concentrations of 1.0, 5.0, and 10.0 mM) are in the range of 99.4-117.0%. These results indicate that the developed method can be employed for the detection of hydrogen peroxide and hypochlorite in household disinfectants.

Green detection of trace cyanuric acid and free chlorine together via ion chromatography

Chlorinated cyanurates (CCAs) are a type of disinfectants currently used worldwide for fight of Coronavirus. However, CCAs upon dosed into water can release not only free chlorine (FC), a strong disinfectant, but also cyanurate (CYA), a persistent compound potentially harmful to human and environment. Therefore, detecting CYA and FC in water are very important not only for ensuring sufficient disinfection but also for monitoring the impacts of FC and CYA on receiving watershed. However, conventional analytical methods for them are mostly based on colorimetric methods, which have high method detection limits (MDLs) and rely on chemical reactions that are likely sensitive to coexisting chemicals. To overcome these issues, we herein proposed a facile and reaction-free method to detect CYA and FC together in just one run by ion chromatography (IC) equipped with both conductivity and ultraviolet absorbance detectors. The method features obvious advantages over colorimetric methods in being lower MDLs (3.6 μg/L for CYA and 9.0 μg/L for FC), environmental-friendly (i.e., no organic solvent involved), and more resistant to alkaline solution. With this method, trace levels of CYA (i.e., 34-44 μg/L), which were nondetectable by conventional method, were found in two river water samples, implying that the local environment was already polluted by CCAs during the pandemic period. Overall, this study demonstrates a robust tool that may assist better understanding and monitoring the fate and transport of trace CCA derivatives in water.

One-pot synthesis of efficient multifunctional nitrogen-doped carbon dots with efficient yellow fluorescence emission for detection of hypochlorite and thiosulfate

CD-based ratiometric fluorescence probes are of great significance for visual detection, but accomplishing this goal is still a particularly challenging task.

High quantum yield nitrogen and boron co-doped carbon dots for sensing Ag+, biological imaging and fluorescent inks

Herein, bright blue-green fluorescent nitrogen and boron co-doped carbon dots (N, B-CDs) with a quantum yield (QY) up to 33.04% were synthesized viahydrothermal treatment from ammonium citrate tribasic and 3-aminophenylboronic acid. The synthesized N, B-CDs showed outstanding water solubility. According to the principle of the static quenching effect (SQE), the synthesized N, B-CDs were utilized as an efficient sensor for sensing Ag⁺. The linear range and limit of detection (LOD) of the sensor for Ag⁺ are 0.99-26.04 μM and 9.03 nM (3σ/m). The proposed method was successfully adopted to detect Ag⁺ in environmental water, which is of great significance to environmental detection. Furthermore, due to the excellent fluorescence performance, the N, B-CDs were found to be an effective tool for biological imaging and as a fluorescent ink, which widens the horizons for the multifunctional applications of N, B-CDs.

A facile synthesis of long-wavelength emission nitrogen-doped carbon dots for intracellular pH variation and hypochlorite sensing

Intracellular pH and hypochlorite (ClO^-) concentration play an important role in life activities, so there is an urgent need to develop a valid strategy to monitor pH and ClO^- in biological systems with high sensitivity and specificity. In this study, we report long-wavelength emission nitrogen-doped carbon dots (N-CDs) and their potential applications in intracellular pH variation, ClO^- sensing and cell imaging. The N-CDs were prepared via a facile one-pot hydrothermal method of neutral red (NR) and glutamine (Gln). N-CDs exhibited a pH-sensitive response in the range of 4.0-9.0 and a good linear relationship in the range of 5.6-7.4, which indicated that N-CDs are an ideal agent for monitoring pH fluctuations in living cells. In addition, ClO^- was capable of reducing the photoluminescence of N-CDs based on static quenching. The linear range is 1.5-112.5 μM and 112.5-187.5 μM, and the LOD is 0.27 μM. Besides, the as-fabricated N-CDs have been smoothly achieved to monitor pH and ClO^- in PC-12 living cells due to their great biocompatibility and lower cytotoxicity, demonstrating their promising applications in the biomedical field. Compared with other CD-based methods, the as-proposed N-CDs have a longer fluorescence emission, which makes them potentially valuable in biological systems. The results pave a way towards the construction of long-wavelength carbon-based nanomaterials for fluorescence sensing and cell imaging.

Photoluminescent sea urchin-shaped carbon-nanobranched polymers as nanoprobes for the selective and sensitive assay of hypochlorite

This work reports donor-acceptor type sea urchin-like carbon nanobranched polymers (SUCNPs). As a novel carbon-based nanomaterial, SUCNPs were effectively synthesized for the first time through a facile and economical solvothermal approach employing uric acid and l-cysteine as nitrogen/sulfur sources. The nitrogen-rich structure of the heterocylic aromatic polymer led to a blue fluorescence at the excitation/emission maxima of 350/436 nm with robust photostability. SUNCPs showed highly selective ability towards hypochlorite (ClO-) against other relevant interfering substances. Upon exposure to a growing concentration of ClO-, SUCNPs fluorescence presented a gradual rise with a remarkable blue shift by virtue of the inhibition of photoinduced charge transfer (PCT) process. A linear relationship was established between the fluorescence intensity ratio (I 401 nm/I 436 nm) and the ClO- concentration in the range of 0.1-200 μM. The detection limit was as low as 30 nM (3σ/k). The "turn-on" type nanoprobe was further used in real samples and paper-based analytical chips efficiently, implying its application in a sophisticated and convenient platform.

N, P-co-doped carbon dots as a dual-mode colorimetric/ratiometric fluorescent sensor for formaldehyde and cell imaging via an aminal reaction-induced aggregation process

Novel colorimetric and ratiometric fluorometric dual-mode N, P-co-doped carbon nanodots, BPEI-CDs, for highly sensitive and selective detection of formaldehyde (FA) were successfully prepared from N-(phosphonomethyl)iminodiacetic acid (PMIDA) and branched polyethyleneimine (BPEI). The treatment of FA caused a remarkable linear enhancement of ratiometric fluorescence (F_501 nm/F_408 nm) in a wide range of 0-40 μM with a detection limit (LOD) of 0.47 μM (3σ/k), along with distinct color changes from colorless to light yellow. Mechanistic study shows that this electron-rich system, formed by the cooperative roles of N and P, promoted the FA-induced Schiff bases formation reaction, which contributed to the CD aggregation-induced emission (AIE) "turn-on" response and enhancement of π-conjugation-induced bathochromic behaviors. Furthermore, N, P-co-doped BPEI-CDs were successfully applied to the determination of FA in bean sprout samples. Using the standard addition method, the recoveries ranged from 96.9 to 101.8%, and the relative standard deviation (RSD) was in the range 2.23 to 3.21%. The application for intracellular FA sensing further verified that this novel nanoprobe may offer a new venue for the design of simple, low-cost, and sensitive biosensors. Graphical abstract.

Design of long-wavelength emission carbon dots for hypochlorous detection and cellular imaging

A facile strategy for the preparation of nitrogen and phosphorus co-doped carbon dots (N, P-CDs) with long-wavelength emission is attractively proposed in one-pot hydrothermal strategy. The resulting N, P-CDs hold exceptional optical features and display excitation wavelength-independent properties with the emission wavelength at 590 nm, which enable it with the satisfactory relative quantum yield (QY) of 15.6% in long-wavelength region. In addition, the proposed N, P-CDs demonstrates specific selectivity towards ClO^- over other competitive reactive oxygen species and exhibits rapid fluorescence response time to ClO^-. Moreover, the N, P-CDs exhibits low-cytotoxicity and excellent cell membrane permeability for recognizing ClO^- in SMMC-7721 cells, which demonstrates their enormous potential in biological system.

Highly sensitive fluorescent carbon dots probe with ratiometric emission for the determination of ClO−

A ratiometric fluorescent N,S co-doped carbon dots (N,S-CD) probe for ClO⁻ has been facilely obtained via a one-step hydrothermal method.

Luminescent metal organic frameworks with recognition sites for detection of hypochlorite through energy transfer

A luminescent metal organic framework (LMOF) of type UiO-66-NH₂ was chosen for specific and sensitive detection of trace levels of hypochlorite. Hypochlorite causes the quenching of the blue fluorescence of nano-UiO-66-NH₂ (with excitation/emission maxima at 325/430 nm), and this finding forms the basis for a fluorometric assay for hypochlorite. The method overcomes disadvantages of conventional redox-probes which are interfered by oxidants with oxidation capability stronger than that of hypochlorite. Compared with other fluorescent probes for sensing hypochlorite, UiO-66-NH₂ has a comparable detection limit of 0.3 μmol L^-1 and a broad linearity relationship in the range of 1-8 μmol L^-1. The probe was successfully applied to the detection of hypochlorite in complex water samples and living Hela cells. Graphical abstract Schematic representation of hypochlorite induced quenching of the blue fluorescence of nano-UiO-66-NH₂ (with excitation/emission maxima at 325/430 nm) through energy transfer. It overcomes disadvantages of conventional redox-probes which are interfered by oxidants with oxidation capability stronger than that of hypochlorite.