Facile synthesis of carbon nanodots with surface state-modulated fluorescence for highly sensitive and real-time detection of water in organic solvents

A highly water-soluble hydrogen-bond-induced emission carbon dots for ratiometric fluorescent detection of water content in organic solvents

Water in organic solvents is a prevalent impurity that significantly influences chemical reactions and industrial processes. Carbon dots (CDs) gained attention as promising tools for chemosensing due to their advantageous characteristics, including easy synthesis, cost-effectiveness, and excellent adjustability and stability. However, limited solubility in water and turn off fluorescence response mode hinder the practical utilization of CDs for water sensing. To tackle such dilemma, a highly water-soluble CDs with distinctive hydrogen-bond-induced emission (HBIE) was rationally designed through introducing sulfone group into the widely employed p-phenylenediamine precursor. The inclusion of sulfone group imparts the CDs with notable water solubility, as well as distinctive ratiometric fluorescent response towards water content, exhibiting high sensitivity and selectivity. Upon exposure to water, the emission color of CDs undergoes a real-time transition from blue to yellow-green, which can be readily discerned by naked eyes. The CDs have been successfully applied in detecting water in commercially available alcohol. This study presents a straightforward yet efficacious approach for rationally design of CDs with unique HBIE characteristics and ratiometric fluorescent response, offering great potential for practical chemosensing applications.

A dynamic Eu(III)-macrocycle served as the turn-on fluorescent probe for distinguishing H2O from D2O

H2O and D2O are an important pair of analogues, and their high-efficient detections are closely related to fields of chemical industry, food processing, semiconductor, environmental monitoring, etc. Because of their extremely similar physical and chemical properties, H2O and D2O can be mutually soluble in any ratios, and it is generally thought that the discrimination of H2O and D2O is an enormous challenge. Herein, upon the fact that vibrational frequency of O-H is greater than O-D, we design a dynamic Eu(III)-macrocycle Eu-2a with two emitters which exhibits the imine bond breakage of macrocycle emitter H2L2a in H2O or D2O, resulting in the turn-on fluorescence of Eu(III) emitter. For their differential fluorescence sensing signals of Eu-2a on three emission bands (433, 500 and 615 nm), the statistical analysis method is employed to produce fully separated fingerprints and thus high-throughput discrimination of 13 common solvents, especially the H2O and D2O. Fluorescent titration experiments by instrumental or smartphone-based analysis method also prove the successful determination of proportional H2O/D2O mixtures together with the good sensitivity and wide linear response range. Moreover, this H2O-triggered fluorescent complex Eu-2a used as the fluorescence ink also shows its potential in information encryption application. This article must be a valuable reference for the areas of lanthanide-based luminescent material, multianalyte detection and information encryption.

Yellow Carbon Dots for Fluorescent Water Sensing, Relative Humidity Sensing, and Anticounterfeiting Applications

Most fluorescent probes based on carbon dots (CDs) fluorescence color or intensity change are still used for detection in solution, but in practical fluorescence detection applications, detection in the solid state is necessary. Therefore, a CDs-based fluorescence sensing device is designed in this paper, which can be used for water detection in liquid and solid states. Using oPD as a single precursor, yellow fluorescent CDs (y-CDs) were prepared by hydrothermal method, which can be used in the field of water detection and anti-counterfeiting by using its solvent-sensitive properties. First, y-CDs can be used to visually and intelligently detect the water content in ethanol. Secondly, it can be used to detect the Relative Humidity (RH) of the environment by combining it with cellulose to form a fluorescent film. Finally, y-CDs can also be used as a fluorescent material for fluorescence anti-counterfeiting.

Fluorescence-Based Portable Assays for Detection of Biological and Chemical Analytes

Fluorescence-based detection techniques are part of an ever-expanding field and are widely used in biomedical and environmental research as a biosensing tool. These techniques have high sensitivity, selectivity, and a short response time, making them a valuable tool for developing bio-chemical assays. The endpoint of these assays is defined by changes in fluorescence signal, in terms of its intensity, lifetime, and/or shift in spectrum, which is monitored using readout devices such as microscopes, fluorometers, and cytometers. However, these devices are often bulky, expensive, and require supervision to operate, which makes them inaccessible in resource-limited settings. To address these issues, significant effort has been directed towards integrating fluorescence-based assays into miniature platforms based on papers, hydrogels, and microfluidic devices, and to couple these assays with portable readout devices like smartphones and wearable optical sensors, thereby enabling point-of-care detection of bio-chemical analytes. This review highlights some of the recently developed portable fluorescence-based assays by discussing the design of fluorescent sensor molecules, their sensing strategy, and the fabrication of point-of-care devices.

Novel fluorescence sensor for the selective recognition of tetracycline based on molecularly imprinted polymer-capped N-doped carbon dots

A novel fluorescent probe based on molecularly imprinted polymers (MIPs) coupled with N-doped carbon dots (CDs) was prepared and used for specific recognition and sensitive determination of tetracycline (TC). N-doped CDs were synthesized using citric acid as a carbon source and ethylenediamine as a nitrogen source by a microwave assisted pyrolysis method. The determination conditions such as the solvents, material amount, pH value, and temperature were optimized. The CDs-MIPs have the best quenching on TC in water. The proposed method used for TC determination in milk powder samples had a detection limit of 0.054 μg mL-1 and a wide range of 0.5-30 μg mL-1. Meanwhile, satisfactory recoveries were obtained ranging from 95 to 108%. Oxytetracycline, chlorotetracycline and most of the coexisting substances showed no obvious interference indicating that the CDs-MIP probe exhibited high selectivity due to the presence of imprinted sites. Charge transfer from CDs-MIPs to TC may be through the mechanism of fluorescence quenching. This work gives a feasible strategy for the synthesis of N-doped carbon dot based molecularly imprinted polymers used as a fluorescent sensor in the food analysis field.

Reversible fluorescent test strip with red fluorescent carbon dots for monitoring water in organic solvents: Visual detection via a smartphone

Herein, a novel type of red-emitting carbon dots called TN-CDs was created via a one-step hydrothermal approach using neutral red and tartaric acid as raw materials. The fluorescence of TN-CDs was gradually quenched as the amount of water increased, and the color of the solution changed from yellow to pink mauve (or purple to pink). The reaction could be completed within only 5 s in various organic solvents such as N,N-Dimethylformamide (DMF), methanol (MeOH), acetonitrile (ACN), and ethanol (EtOH) with linear detection ranges of 1.2%-35.0%, 0.5%-20.0%, 0.25%-5.0% and 0%-16.0%, respectively. In addition, we prepared a reusable test strip and then combined it with TN-CDs to detect water content in DMF, as well as integrated it with smartphone software, a UV lamp, and a dark chamber for real-time, on-site, visual quantitative detection of the water content.

A handy imaging sensor array based on the phase transformation from CsPbBr3 to CsPb2Br5: highly sensitive and rapid detection of water content in ethanol

This work proposes a handy imaging sensor array based on the phase transformation from CsPbBr3 to CsPb2Br5 for highly sensitive and rapid detection of the water content in ethanol.

N, Cl-doped carbon dots for fluorescence and colorimetric dual-mode detection of water in tetrahydrofuran and development of a paper-based sensor

N, Cl-doped carbon dots (N, Cl-CDs) were prepared by hydrothermal method from rhodamine B (RhB) and ethylenediamine (EDA). The resulting N, Cl-CDs exhibited fascinating solvent dependence and strict excitation independence. As the polarity of the solvent increased (from tetrahydrofuran (THF) to water), the emission spectrum of N, Cl-CDs was redshifted and the fluorescence efficiency decreased, which were attributed to hydrogen bond-induced aggregation. Taking advantage of these attributes, the N, Cl-CDs were used as suitable probes for fluorescence and colorimetric dual-mode detection of water in THF. The linear relationship was 0.5-100% water with the detection limit down to 0.093%. Moreover, the sensing platform was converted into a paper-based sensor for handy, real-time, and visible humidity sensing. N, Cl-CDs/PVA films were fabricated and realized continuously tunable solid-state fluorescence, further expanding their practical application.

An ethanethioate functionalized polythiophene as an optical probe for sensitive and fast detection of water content in organic solvents

A new polythiophene-based optical probe, namely PTS, was designed and prepared for detection and quantification of the water present in organic solvents. PTS exhibited sensitive and fast absorption and fluorescence signaling response to the changes of water content in tetrahydrofuran (THF), N,N-dimethylformamide (DMF) and N,N-dimethylacetamide (DMAc) due to the water-induced interpolymer-stacking aggregation as demonstrated by dynamic light scattering (DLS) analysis. The fluorescence intensity of PTS at 550 nm linearly reduced as a function of the water content in detection ranges of 0-30% (v/v) in THF, 0-10% in DMF and 0-10% in DMAc with the limit of detection (LOD) for water being 0.034% (v/v) in THF, 0.013% (v/v) in DMF, and 0.014% (v/v) in DMAc, respectively. Additionally, PTS-incorporated test paper was fabricated to successfully achieve naked-eye detection of water in DMF and DMAc. PTS was further applied to estimate the water content in real samples, convincingly demonstrating that our method was comparable with the standard Karl Fischer titration.

Ratiometric dual-emission of Rhodamine-B grafted carbon dots for full-range solvent components detection

Quick and visual detection of component contents, such as water, in a mixed solvent is important for many practical applications, and a full range detection is especially preferred. In this work, a carbon dots based ratiometric fluorescent sensor was synthesized by grafting fluorescent group (Rhodamine B, RhB) on carbon dots, and the dual emission peaks exhibited a linear ratiometric response with the change of polarity and hydrogen bond of Solvent Hansen solubility parameters. This responsive behavior is attributed to surface state photoluminescence mechanisms, and has been used for the quantitative detection of water content in ethanol with an excellent linear relationship (R² = 0.996), a low detection limit (0.2%), and a full detection range (0-100%). Furthermore, a paper-based ratiometric fluorescence sensing strip is also demonstrated, which exhibits good storage stability and sensitivity. This study suggests that RhB grafted carbon dots could be feasibly and effectively used as ratiometric fluorescent sensors for solvent content detection.

Concentration-dependent photoluminescence carbon dots for visual recognition and detection of three tetracyclines

Concentration-dependent photoluminescence carbon dots (CDs) have been successfully synthesized through the one-step hydrothermal treatment of o-phthalic acid and ethylenediamine. The CDs possessed higher fluorescence quantum yield, up to 39.22%, exhibiting distinguished optical property, water solubility, and stability. The CDs that emit strong blue-green fluorescence can visually identify and determine tetracycline (TC), oxytetracycline (OTC), and chlortetracycline (CTC). TC quenched the fluorescence of CDs at 500 nm owing to the inner filter effect; OTC behaved similarly, but the emission wavelength of CDs was red-shifted to 515 nm. Inversely, once CTC was introduced to CDs solution, the fluorescence increased and the emission peak was blue-shifted to 450 nm. Bandgap transition and electrostatic interaction were proposed to be the mechanisms for the detection of OTC and CTC by CDs. Wide linear relationships were established for TC, OTC, and CTC with the limits of detection to be 50 nM, 36 nM, and 373 nM, respectively. Furthermore, the nanoscale probe constructed by this system has been applied to detect tetracyclines (TCs) in complex samples with satisfying recoveries (93.2-114%) and was designed as a portable test strip sensor for visually on-site TCs of honey sample screening. Accordingly, the preparation process of the nano fluorescent probe is simple and environmentally friendly, and the probe has a specific recognition ability for tetracyclines. The synthesized CDs in this work provide a new orientation for fast, effective, and visual real-time detection of tetracycline in actual samples.

Application of reduced graphene oxide-based actuators for real-time chemical sensing of liquid and vapour phase contaminants

The possibility of employing responsive materials for the in situ detection of chemical contaminants in the liquid phase is demonstrated here.

Surface chemistry tuning the selectivity of carbon nanodots towards Hg2+ recognition

Fluorescence quenching of carbon nanodots by metal ions has been extensively applied for the determination of oligonucleotides, proteins, small molecules and metal ions. However, the problem of poor selectivity originating from the coordination of surface oxygen-containing groups to many kinds of metal ions has limited the prosperity of carbon nanodots in detection field. Herein, the specific recognition of carbon nanodots to Hg²⁺ is controlled by rational regulation of the surface structure of carbon nanodots. Passivation of the surface carboxyl and hydroxyl groups plays a decisive role in inhibiting the binding of metal ions with carbon nanodots. Upon the attachment of Hg²⁺ specific recognition unit, carbon nanodots exhibited a high selectivity to Hg²⁺. This work facilitates to rationally design the surface structure of carbon nanodots to obtain the desirable selective recognition ability.

Monitoring of reaction kinetics and determination of trace water in hydrophobic organic solvents by a smartphone-based ratiometric fluorescence device

A smartphone-based ratiometric fluorescence device was designed to monitor the reaction kinetic process under vigorous mixing conditions, demonstrated by the hydrolysis of Cs₄PbBr₆ nanocrystals (NCs). In the presence of trace water, part of Cs₄PbBr₆ NCs (non-fluorescent) was converted to CsPbBr₃ NCs (strong fluorescent). Using anthracene as the reference fluorophore, the brightness ratio of the green (from CsPbBr₃ NCs) to blue (from anthracene) components in the fluorescence image which was recorded in situ by the smartphone camera was measured as the signal for kinetic analysis. It was shown that the water-triggered conversion reaction from Cs₄PbBr₆ NCs to CsPbBr₃ NCs follows the pseudo-second-order kinetic model in the early rapid hydrolysis stage (up to 4 min). With increasing water content, the hydrolysis of Cs₄PbBr₆ NCs is promoted to yield more CsPbBr₃ NCs, which was used to determine trace water in n-hexane, dichloromethane, and toluene with detection limits of 0.031, 0.043, and 0.057 μL mL^-1, respectively. The device offers the advantages of portability and low cost for rapid field determination of trace water in hydrophobic organic solvents. Graphical abstract.

Lignin-derived red-emitting carbon dots for colorimetric and sensitive fluorometric detection of water in organic solvents

Water contained in organic solvents or products in chemical industries, as contaminants, poses an adverse risk in chemical reaction, life or environmental safety. However, conventional fluorescent water sensing suffers from drawbacks, including limited organic solvents, narrow linear range, lack of visual detection, single detection strategy, and others. Herein, a novel type of red-emitting carbon dots (RCDs) has been created via one-step solvothermal synthesis based on biomass (e.g., lignin) as the carbon source and p-phenylenediamine (PPD) as the nitrogen source. Colorimetric and fluorometric detection of water in organic solvents has been demonstrated. The RCDs showed excitation-independent photoluminescence (PL) in different solvents and solvatochromic behavior, red in water, orange in ethanol, yellow in N,N-dimethyl formamide (DMF), and green in acetone. Remarkably, detection of water content in six organic solvents, including polar solvents (ethanol, acetone, dimethyl sulfoxide (DMSO), tetrahydrofuran (THF), and DMF) and apolar solvent (ether), was performed. With increasing water content in solvents, emission colors changed from green to red, or yellow to red, offering qualitative sensing of water. Furthermore, a broad linear detection range (10-90%), low limits of detection (LOD) (e.g., 0.36% for ethanol and 0.082% for acetone), and good generality for various organic solvent systems were realized. Particularly, dual sensing strategies, including PL quenching and shift with water in various solvents, were achieved simultaneously, showing great potential for the development of advanced optical sensors with high performance.

Effect of Solvent-Derived Highly Luminescent Multicolor Carbon Dots for White-Light-Emitting Diodes and Water Detection

Recently, the multicolor fluorescent carbon dots (CDs) have drawn much attention due to their various applications. Herein, we report multicolor emissive CDs by solvent-controlled and solvent-responded approaches. The blue to red color emissive CDs are obtained by the solvothermal method by varying the solvent during the reaction. The red color emissive CDs (R-CDs) with good quantum yield is obtained in a water medium. The detailed characterization revealed that the solvent controls the particle size, band gap, and nitrogen doping concentration. Specifically, in the protic solvent, the high N content and presence of imine nitrogen are the reason for red emission. However, in an aprotic solvent, the least N doping and a lack of C-O groups are responsible for a blueshift. Interestingly, it was observed that the R-CDs provide a full range of visible color by dispersing in different immiscible solvents. The fluorescence emission in immiscible solvents is redshifted by enhancing the polarity. Moreover, the developed CDs detected the low water concentrations (≤0.2%, v/v) visually and fluorometrically in various organic solvents. Simultaneously, we have employed synthesized CDs in white-light-emitting diodes and fluorescent ink.

Covalent organic frameworks as a sensing platform for water in organic solvent over a broad concentration range

A highly crystalline covalent organic frameworks (COFs) formed by condensation reaction between 1, 3, 5-tris (4-aminophenyl) benzene and 4, 4'-biphenyldicarboxaldehyde is utilized as a sensing platform for water in organic solvent over a broad concentration range. The resulting COFs exhibits brilliant fluorescence in various organic solvents such as methanol, DMF, acetonitrile and ethanol, moreover its fluorescence intensity has a significant and rapid response to the content of water in organic solvent over a broad concentration range. The broadest sensing range is achieved over 7%-70% (v/v) for water in DMF, and the lowest limit of detection is 0.042% (v/v) for water in methanol among the investigated organic solvents. The superior properties of the sensing platform expand the application ranges of COFs and endow the resulting COFs with a great prospect in practical applications for highly efficient detecting water in organic solvent.

Core-shell upconversion nanoparticles of type NaGdF4:Yb,Er@NaGdF4:Nd,Yb and sensitized with a NIR dye are a viable probe for luminescence determination of the fraction of water in organic solvents

Lanthanide-doped core-shell upconversion nanoparticles (UCNPs) of type NaGdF₄:Yb,Er@NaGdF₄:Yb,Nd were prepared by the co-precipitation method. The luminescence intensity was further enhanced by adding the sensitizer dye IR-808. If water is added to organic solvents [such as N,N-dimethylformamide (DMF), dimethyl sulfoxide, methanol, acetone, acetonitrile, and ethanol] containing the probe, its luminescence intensity peaking at 545 nm is reduced. The decrease is linearly related to the percentage of water in the respective organic solvent. Water fractions ranging from 0.05% to 10% (volume %) can be sensitively detected, and the detection limit is 0.018% of water in DMF. The detection scheme is mainly attributed to the fact that the transfer of energy from the near-infrared light (NIR) dye to the UCNPs is strongly reduced in the presence of traces of water. Graphical abstract The near infrared dye (IR-808) transfer efficiency to NaGdF₄:Yb, Er@NaGdF₄:Yb, Nd upconversion nanoparticles in water is far less than that in organic phase. Several methods for determination of trace water in organic solvents were developed by using this effect.

Ratiometric and Turn-On Luminescence Detection of Water in Organic Solvents Using a Responsive Europium-Organic Framework

The development of simple, rapid-response sensors for water detection in organic solvents is highly desirable in the chemical industry. Here we demonstrate a unique luminescence water sensor based on a dual-emitting europium-organic framework (Eu-MOF), which is assembled from a purposely selected 2-aminoterephthalic acid ligand with responsive fluorescence inherent in its intramolecular charge transfer (ICT) process. This ICT process can be rapidly switched-on in the presence of water owing to its ability to boost and stabilize the ICT state. In contrast, the Eu³⁺ emission within the framework is insensitive to water and can serve as a reference, thus enabling highly sensitive water detection in a turn-on and ratiometric way. In addition, the significant ratiometric luminescence response induced by water makes Eu-MOF undergo a distinct change of emitting color from red to blue, which is favorable for visual analysis with the naked eye. Sensitive determination of water content (0.05-10% v/v) in various organic solvents is achieved in multiple readouts including ratiometric emission intensity, emission color, or the Commission Internationale de l'Eclairage (CIE) chromaticity coordinate. The present Eu-MOF sensor featuring high sensitivity and reusability, self-calibration, simple fabrication and operation, and capability for real-time and in situ detection is expected to have practical applications in water analysis for industrial processes.

Impact of Hydrogen Bonding on the Fluorescence of N-Amidinated Fluoroquinolones

The fluorescence properties of AIE-active N-amidinated fluoroquinolones, efficiently obtained by a perfluoroaryl azide-aldehyde-amine reaction, have been studied. The fluorophores were discovered to elicit a highly sensitive fluorescence quenching response towards guest molecules with hydrogen-bond-donating ability. This effect was evaluated in a range of protic/aprotic solvents with different H-bonding capabilities, and also in aqueous media. The influence of acid/base was furthermore addressed. The hydrogen-bonding interactions were studied by IR, NMR, UV/Vis and time-resolved fluorescence decay, revealing their roles in quenching of the fluorescence emission. Due to the pronounced quenching property of water, the N-amidinated fluoroquinolones could be utilized as fluorescent probes for quantifying trace amount of water in organic solvents.

Nanomaterials for luminescent detection of water and humidity

Significant advances of typical nanomaterials in the luminescent detection of water and humidity are presented.