The construction of dual-emissive ratiometric fluorescent probes based on fluorescent nanoparticles for the detection of metal ions and small molecules

With the rapid development of fluorescent nanoparticles (FNPs), such as CDs, QDs, and MOFs, the construction of FNP-based probes has played a key role in improving chemical sensors. Ratiometric fluorescent probes exhibit distinct advantages, such as resistance to environmental interference and achieving visualization. Thus, FNP-based dual-emission ratiometric fluorescent probes (DRFPs) have rapidly developed in the field of metal ion and small molecule detection in the past few years. In this review, firstly we introduce the fluorescence sensing mechanisms; then, we focus on the strategies for the fabrication of DRFPs, including hybrid FNPs, single FNPs with intrinsic dual emission and target-induced new emission, and DRFPs based on auxiliary nanoparticles. In the section on hybrid FNPs, methods to assemble two types of FNPs, such as chemical bonding, electrostatic interaction, core satellite or core-shell structures, coordination, and encapsulation, are introduced. In the section on single FNPs with intrinsic dual emission, methods for the design of dual-emission CDs, QDs, and MOFs are discussed. Regarding target-induced new emission, sensitization, coordination, hydrogen bonding, and chemical reaction induced new emissions are discussed. Furthermore, in the section on DRFPs based on auxiliary nanoparticles, auxiliary nanomaterials with the inner filter effect and enzyme mimicking activity are discussed. Finally, the existing challenges and an outlook on the future of DRFP are presented. We sincerely hope that this review will contribute to the quick understanding and exploration of DRFPs by researchers.

Metal-free auxiliary pyrophosphate detection based on near-infrared carbon dots

The excessive use of pyrophosphate (PPi) anions as additives poses a serious threat to human health and the environment. Considering the current status of PPi probes, the development of metal-free auxiliary PPi probes has important applications. In this study, a novel near-infrared nitrogen and sulfur co-doped carbon dots (N,S-CDs) were prepared. The average particle size of N,S-CDs was 2.25 ± 0.32 nm with average height was 3.05 nm. The probe N,S-CDs showed a special response to PPi, and a good linear relationship was obtained with PPi concentrations ranging from 0 to 1 μM, with the limit of detection being 0.22 nM. Tap water and milk were used for practical inspection, and ideal experimental results were acquired. In addition, the probe N,S-CDs also showed good results in biological systems, such as cell and zebrafish experiments.

Semiconductor/Carbon Quantum Dot-based Hue Recognition Strategy for Point of Need Testing: A Review

The requirement to establish novel methods for visual detection is attracting attention in many application fields of analytical chemistry, such as, healthcare, environment, agriculture, and food. The research around subjects like "point-of-need", "hue recognition", "paper-based sensor", "fluorescent sensor", etc. has been always aimed at the opportunity to manufacture convenient and fast-response devices to be used by non-specialists. It is possible to achieve economic rationality and technical simplicity for optical sensing toward target analytes through introduction of fluorescent semiconductor/carbon quantum dot (QD) and paper-based substrates. In this Review, the mechanisms of anthropic visual recognition and fluorescent visual assays, characteristics of semiconductor/carbon QDs and ratiometric fluorescence test paper, and strategies of semiconductor/carbon QD-based hue recognition are described. We cover latest progress in the development and application of point-of-need sensors for visual detection, which is based on a semiconductor/carbon quantum dot-based hue recognition strategy generated by ratiometric fluorescence technology.

Facile synthesis of multifunctional carbon dots with 54.4% orange emission for label-free detection of morin and endogenous/exogenous hypochlorite

Carbon dots with long-wavelength emission (orange to red), high quantum yield (QY) and good biocompatibility are of great significance for biomedical applications, but achieving this is still a highly challenging task. In this work, multifunctional carbon dots with 54.4% orange emission (O-CDs) were prepared through one-pot solvothermal treatment of nileblueasulphate and citric acid as precursor for label-free recognition of morin and endogenous/exogenous hypochlorite (ClO^-) and bioimaging in cellular and zebrafish. Morin can quench the luminescence of O-CDs by static quenching (SQ). The linear range is 5-125 μM and LOD is 0.84 μM. ClO^- reduce the photoluminescence intensity of O-CDs via SQ. The linear range is 2.5-90 μM and LOD was 0.46 μM. In addition, The obtained O-CDs have successfully realized the monitoring of morin and endogenous/ exogenous ClO^- in living cells and zebrafish owing to its superior biocompatibility, exceptional photostability and lower toxicity. This work opens up a novel opportunity for the development of long-wavelength emission multifunctional nanomaterial with high quantum yield based on CDs for biosensing, biolabeling and biomedical optical imaging.

Recent Advances and Applications in Paper-Based Devices for Point-of-Care Testing

Point-of-care testing (POCT), as a portable and user-friendly technology, can obtain accurate test results immediately at the sampling point. Nowadays, microfluidic paper-based analysis devices (μPads) have attracted the eye of the public and accelerated the development of POCT. A variety of detection methods are combined with μPads to realize precise, rapid and sensitive POCT. This article mainly introduced the development of electrochemistry and optical detection methods on μPads for POCT and their applications on disease analysis, environmental monitoring and food control in the past 5 years. Finally, the challenges and future development prospects of μPads for POCT were discussed.

Integration detection of mercury(ii) and GSH with a fluorescent "on-off-on" switch sensor based on nitrogen, sulfur co-doped carbon dots

Using aurine and citric acid as precursors, we have synthesized stable blue-fluorescent nitrogen and sulfur co-doped carbon dots (NS-CDs), with a high quantum yield of up to 68.94% via a thermal lysis method. The fluorescent NS-CDs were employed as a sensitive sensor for the integration detection of Hg2+ and glutathione (GSH). This was attributed to Hg2+ effectively quenching the fluorescence of the NS-CDs by static quenching, and then GSH was able to recover the fluorescence owing to the stronger binding between Hg2+ and the sulfhydryl of GSH. Based on the "on-off-on" tactic, the detection limits of Hg2+ ions and GSH were 50 nM and 67 nM respectively. The fluorescence sensor was successfully applied to detect Hg2+ ions and GSH in actual samples (tap water and fetal bovine serum). Furthermore, we have proved that the sensor had good reversibility. Overall, our NS-CDs can serve as effective sensors for environmental and biological analysis in the future.

Iron ion sensing and in vitro and in vivo imaging based on bright blue-fluorescent carbon dots

Herein, we propose an eco-friendly synthesis of carbon dots (CDs) and ingeniously design a rapid and label-free "turn-off" sensing platform for ultrasensitive recognition of Fe³⁺ in vitro and in vivo. CDs with extraordinary advantages involving exceptional stability, ultra-low toxicity as well as admirable biocompatibility were simply prepared via one-step hydrothermal strategy of Caulis polygoni multiflora. Result indicated that as-acquired CDs not only exhibit excitation dependency, but also have a high quantum yield of (QY) up to 42%. Miraculously, the fluorescence of CDs can be extinguished sharply by Fe³⁺ because of static quenching effect with linear range of 0-400 µM, yielding a detection limit of 0.025 μM. Benefiting from these characteristics, CDs have been extended for multicolourful imaging and tracking Fe³⁺ fluctuations in living cells. Bioimaging of zebrafish larvae exposed to CDs confirmed that it is smoothly circulated to other tissues and organs owing to their small size. Eventually, as-prepared CDs have been implemented for the real-time detection of Fe³⁺ in nude mice.

Green-emissive water-dispersible silicon quantum dots for the fluorescent and colorimetric dual mode sensing of curcumin

Curcumin, an active ingredient in Curcuma longa, which possesses good biological and pharmacological activities, is effective in treating many diseases. Developing simple and sensitive methods for the detection of curcumin is of great significance. In this study, novel water-dispersible silicon quantum dots (SiQDs), which can sensitively respond to curcumin through fluorescent and colorimetric dual modes were synthesized via a one-step hydrothermal treatment of N-[3-(trimethoxysilyl) propyl]-ethylenediamine (DAMO) and p-phenylenediamine. The fluorescence of SiQDs could be remarkably quenched by curcumin via the inner filter effect (IFE) and static quenching effect (SQE). A good linear relationship was obtained in the range of 0.25-75 μM with a detection limit of 91 nM. More interestingly, curcumin could also be visually detected using SiQDs via an obvious color change of the solution from pale yellow to orange-red, which allows the establishment of a sensitive colorimetric method for curcumin detection in the linear range of 0.05-57.5 μM with a detection limit of 32 nM. The proposed method was successfully applied to detect curcumin in health care products and spices. Notably, to realize rapid and convenient visual detection of curcumin, a paper sensor was also fabricated.

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.

A simple fluorescence assay for trypsin through a protamine-induced carbon quantum dot-quenching aggregation platform

The development of a simple detection strategy for trypsin (Try) is urgent, and is ascribed to the diagnostic value of Try in several diseases. Herein, a facile but effective fluorescence strategy for Try was developed based on the protamine (Pro)-induced aggregation of carbon quantum dots (CQDs). The fluorescence of negatively charged CQDs was quenched with Pro due to the assembly of CQDs and Pro (CQDs/Pro) through electrostatic interaction. However, the highly positively charged Pro, which is rich in basic arginine residues, was preferred to be hydrolyzed by Try. Try can induce the deaggregation of CQDs/Pro, thereby enabling the release of CQDs to restore the fluorescence intensity. Thus, the use of CQDs/Pro as a testing platform will be employed as a “turn-on” method for Try. In addition, the fluorescence-resuming response was proportional to Try, ranging from 25 ng mL⁻¹ to 500 ng mL⁻¹ with a limit of detection (LOD) of 8.08 ng mL⁻¹. This “turn-on” fluorescence assay for Try was label-free, convenient, and relatively free of interference from coexisting substances. Actual applications for Try monitoring and trypsin inhibitor screening also illustrated the considerable prospect of CQDs in the clinical field, combined with the superiority of the simple mixing operation.

In this work, a simple melting method was developed for carbon quantum dot fabrication to integrate with protamine as an effective signal-on fluorescence strategy for trypsin detection.