A novel method for determination of peroxide value and acid value of extra-virgin olive oil based on fluorescence internal filtering effect correction

Peroxide value (PV) and acid value (AV) are widely used indicators for evaluating oxidation degree of olive oils. Fluorescence spectroscopy has been extensively studied on the detection of oil oxidation, however, the detection accuracy is limited due to internal filtering effect (IFE). Due to the primary and secondary IFE, at least two wavelengths of absorption information are required. Least squares support vector regression (LSSVR) models for PV and AV were established based on two absorption coefficients (μa) at 375 nm and emission wavelength and one fluorescence intensity at corresponding wavelength. The regression results proved that the model based on 375 and 475 nm could reach the best performance, with the highest correlation coefficient for prediction (rp) of 0.889 and 0.960 for PV and AV respectively. Finally, the explicit formulations for PV and AV were determined by nonlinear least squares fitting, and the rp could reach above 0.94 for two indicators.

Highly water-soluble and biocompatible hyaluronic acid functionalized upconversion nanoparticles as ratiometric nanoprobes for label-free detection of nitrofuran and doxorubicin

Antibiotic detection is crucial and challenging because the widespread consumption of antibiotics has shown extensive harmful effects on food, environment and human health. Here, we propose highly water-soluble and biocompatible hyaluronic acid (HYA) functionalized upconversion nanoparticles (UCNPs) for ratiometric detection of multiple antibiotics. The ultraviolet upconversion luminescence (UCL) from UCNPs was significantly quenched by nitrofurazone (NFZ)/nitrofurantoin (NFT), and blue UCL was quenched by doxorubicin (DOX), while red UCL remained unchanged for internal reference. The UCNPs-HYA nanoprobes exhibit excellently sensitive and selective NFZ, NFT and DOX detection in linear range of 2.5-100 μM, 2.5-80 μM, and 2.5-200 μM with the LOD at 0.28 μM (55 μg/kg), 0.20 μM (48 μg/kg) and 0.17 μM (97 μg/kg), respectively. The nanoprobes achieved detecting real samples of NFZ in lake water, liquid milk and chicken meat with satisfactory results, and UCL bioimaging of DOX in HeLa cells. The UCNPs-HYA ratiometric nanoprobes are promising for food samples detection and potential biosensing in the cellular environment.

Carbon nanospheres dual spectral-overlapped fluorescence quenching lateral flow immunoassay for rapid diagnosis of toxoplasmosis in humans

Toxoplasmosis is a common zoonotic disease caused by a protozoan parasite Toxoplasma gondii (Tox), approximately infecting one-third of human populations worldwide. This study developed the carbon nanospheres (CNPs) based dual spectral-overlapped fluorescence quenching lateral flow immunoassay (CNPs-FQLFIA) for detection of Tox antibodies (ToxAbs). The CNPs have been effectively coupled with Tox antigen (ToxAg), which can completely overlap the excitation and emission spectra of europium nanospheres (EuNPs) and CdSe/ZnS quantum dots (QDs) in testing strips of CNPs-QDs-FQLFIA or CNPs-EuNPs-FQLFIA. The sensitivity of CNPs-EuNPs-FQLFIA or CNPs-QDs-FQLFIA was 4 or 8 IU/mL under natural light readout, or both 4 IU/mL ToxAbs under ultraviolet (UV) light readout by the naked eyes, respectively. The limit of detection (LOD) of two types of CNPs-FQLFIA was both 1 IU/mL ToxAbs under UV light by a dry fluorescence analyzer, but no cross-reaction was found with other antibodies. The intra-assay coefficient variation (CV) of both CNPs-EuNPs-FQLFIA and CNPs-QDs-FQLFIA was less than 8%, while the inter-assay CV was less than 14%, respectively. The correlation coefficient (R2) of CNPs-EuNPs-FQLFIA or CNPs-QDs-FQLFIA to measure the different concentrations of ToxAbs spiked serum samples was 0.99712 and 0.99896, respectively. The CNPs-FQLFIA presented a characteristics of 94.3% sensitivity, 100% specificity and 98% accuracy for detection of ToxAbs in clinical serum samples. In conclusion, CNPs-FQLFIA with EuNPs or QDs fluorescence reporter was an easy, rapid, sensitive, precise and quantitative assay for detecting Tox antibodies in human blood samples.

Highly efficient dual-mode detection of AFB1 based on the inner filter effect: Donor-acceptor selection and application

BACKGROUND

The utilization of inner filter effect (IFE) brings more opportunities for construction of fluorescence immunoassays but remains a great challenge, especially how to select best donor in the face of extensive fluorescent nanomaterials. Aflatoxin B1 possesses high toxicity among mycotoxins and is frequently found in agricultural products that may significantly threaten to human health. Therefore, with the help of signal transduction mechanism of IFE to develop a convenient and sensitive approach for AFB1 detection is of great significance in ensuring food safety.

RESULTS

Herein, the classical alkaline phosphatase (ALP) catalyzes hydrolysis of p-nitrophenylphosphate to produce p-nitrophenol (PNP) was employed as a model reaction, which intends to explore tunable multicolor fluorescence of gold nanoclusters (AuNCs) for matching PNP to maximize IFE efficiency. The luminescent green-emitting AuNCs were selected as an optimal donor in terms of excellent spectral overlap, high photoluminescence, and adequate system adaptability, thus achieving a 22-fold increase in sensitivity improvement compared to colorimetric method for ALP detection. The fluorescence quenching mechanism between PNP and AuNCs was validated as IFE by studying ultraviolet absorption, zeta potentials and fluorescence lifetime. In light of this, we integrated a highly specific antibody-antigen recognition system, efficient enzymatic reaction and excellent optical characteristics of AuNCs to develop dual-mode immunoassay for AFB1 monitoring. The sensitivity of fluorometric immunoassay was lower to 0.06 ng/mL, which obtained a 3.5-fold improvement compared to "gold standard" ELISA. Their practicability and applicability were confirmed in the tap water, corn, wheat and peanuts samples.

SIGNIFICANCE

This work provides an easy-to-understand screening procedure to select optimal donor-acceptor pairs in IFE analysis. Furthermore, we expect that integration of IFE-based signal conversion strategy into mature immunoassay not only extends the signal types, simplifies signal amplification steps, and reduces the false-positive/false-negative rates, but also provides a simple, convenient, and versatile strategy for monitoring of trace other contaminants.

A metal-organic framework and quantum dot-based ratiometric fluorescent probe for the detection of formaldehyde in food

A green and sensitive ratio fluorescence strategy was proposed for the detection of formaldehyde (FA) in food based on a kind of metal-organic frameworks (MOFs), MIL-53(Fe)-NO2, and nitrogen-doped Ti3C2 MXene quantum dots (N-Ti3C2 MQDs) with a blue fluorescence at 450 nm. As a type of MOFs with oxidase-like activity, MIL-53(Fe)-NO2 can catalyze o-phenylenediamine (OPD) into yellow fluorescent product 2,3-diaminophenazine (DAP) with a fluorescent emission at 560 nm. DAP has the ability to suppress the blue light of N-Ti3C2 MQDs due to inner filter effect (IFE). Nevertheless, Schiff base reaction can occur between FA and OPD, inhibiting DAP production. This results in a weakening of the IFE which reverses the original fluorescence color and intensity of DAP and N-Ti3C2 MQDs. So, the ratio of fluorescence intensity detected at respective 450 nm and 560 nm was designed as the readout signal to detect FA in food. The linear range of FA detection was 1-200 µM, with a limit of detection of 0.49 µM. The method developed was successfully used to detect FA in food with satisfactory results. It indicates that MIL-53(Fe)-NO2, OPD, and N-Ti3C2 MQDs (MON) system constructed by integrating the mimics enzyme, enzyme substrate, and fluorescent quantum dots has potential application for FA detection in practical samples.

An off-on fluorescence method for acid phosphatase assay based on the inner filter effect of MnO2 nanosheets on vitamin B2

Fluorescence analysis has attracted much attention due to its rapidity and sensitivity. The present work describes a novel fluorescence detection method for acid phosphatase (ACP) on the basis of inner-filter effect (IFE), where MnO2 nanosheets (MnO2 NSs) and vitamin B2 (VB2) are served as absorbers and fluorophores, respectively. In the absence of ACP, the absorption band of MnO2 NSs overlaps well with the excitation band of VB2, resulting in effective IFE and inhibition of VB2 fluorescence. In the presence of ACP, 2-phospho-L-ascorbic acid trisodium salt (AAP) is hydrolyzed to generate ascorbic acid (AA), which efficiently trigger the reduction of MnO2 NSs into Mn2+ ions, causing the weakening of the MnO2 NSs absorption band and the recovery of VB2 fluorescence. Further investigation indicates that the fluorescence recovery degree of VB2 increases with the increase of ACP concentration. Under selected experimental conditions, the proposed method can achieve sensitive detection of ACP in the ranges of 0.5-4.0 mU/mL and 4.0-15 mU/mL along with a limit of detection (LOD) as low as 0.14 mU/mL. Finally, this method was successfully applied for the detection of ACP in human serum samples with satisfactory recoveries in the range of 95.0 %-108 %.

Facile and green synthesis of chlorophyll-derived multi-color fluorescent carbonized polymer dots and their use for sensitive detection of hemin

Due to the very important role in physiological process, a simple and sensitive hemin detection method is necessarily required. Biomass-based carbonized polymer dots (CPDs) have been widely studied especially as fluorescence probe owing to the advantages of low toxicity and the variety of fluorescence color, yet there are still challenges in developing their multi-color emission property from the same raw materials. In this work, red, white and blue emissive CPDs derived from chlorophyll have been synthesized via hydrothermal method. Then white-emitted CPDs (white-CPDs) with the Commission International d'Eclairage (CIE) coordinates at (0.34, 0.32) were used to develop a fluorescence quenched sensing system for hemin determination. There is a good linear relationship between (F0-F)/F0 and concentration of hemin in the range of 0.1-0.95 μM with a detection limit of 0.043 μM, and the quenching mechanism was considered to be caused by inner filter effect (IFE). Moreover, it has been successfully used for hemin detection in serum and also for visual determination, which indicating great potential in applications of disease diagnoses and trace identification.

A ratiometric fluorescence platform based on WS2 QDs/CoOOH nanosheet system for α-glucosidase activity detection

In this study, we have constructed a ratiometric fluorescence sensor for sensitive sensing of α-glucosidase activity based on WS2 QDs/ CoOOH nanosheet system. In this system, as an oxidase-imimicking nanomaterial, CoOOH nanosheet could convert o-phenylenediamine into 2,3-diaminophenazine (DAP), which had a high fluorescence emission at 575 nm. The DAP subsequently could quench the fluorescence of WS2 QDs via the inner filter effect (IFE). L-Ascorbic acid-2-O-α-D-glucopyranose could be hydrolyzed by α-glucosidase to yield ascorbic acid. CoOOH nanosheet can be converted to Co2+ ions by ascorbic acid, leading to the fluorescence decrease of DAP and the fluorescence recovery of WS2 QDs. Therefore, a novel ratio fluorescence sensing strategy was established for α-glucosidase detection based on WS2 QDs/CoOOH nanosheet system. This WS2 QDs/CoOOH nanosheet system has a low detection limit of 0.009 U/mL for α-Glu assay. The proposed strategy succeeded in detecting α-Glu in human serum samples.

Facile synthesis of nitrogen-doped carbon dots for ultrasensitive detection of anticancer drug gefitinib based on IFE

Gefitinib, a highly significant antitumor drug, is now commonly employed in clinical settings as a first-line treatment for patients with advanced or metastatic non-small cell lung cancer, colon cancer, and breast cancer. Herein, a convenient, rapid, and accurate fluorescence method based on nitrogen-doped carbon dots (NCDs) was designed for ultrasensitive detection of gefitinib. The NCDs were easily synthesized through one-pot hydrothermal process using p-phenylenediamine and D-glutamic acid as the precursors. The sensing strategy relied on the fluorescence of NCDs at 345 nm, which was selectively reduced by gefitinib based on the inner filter effect (IFE). With a broad linear range of 0.025-30 μg/mL and a low limit of detection of 5.5 ng/mL, the probe was successfully applied to the detection of gefitinib in human serum samples, demonstrating strong practicality, affordability, and high accuracy. The proposed sensor is simple in design, fast in detection and cost-effective, and exhibits promising application in drug real-time analysis.

Schiff base fluorescent sensor with aggregation induced emission characteristics for the sensitive and specific Fe3+ detection

An aggregation induced emission based compound ((E)-4-((2-hydroxy-5-methoxybenzylidene)amino)benzoic acid) was synthesized through facile Schiff base condensation and characterized by various spectral techniques. The as-prepared compound represented a typical aggregation induced emission behavior in aqueous solution and exploited as a turn-off fluorescent sensor for Fe3+ detection in THF-H2O system (3:7, v/v) with high sensitivity and selectivity. The mechanism of the fluorescence quenching was intensively studied, which was attributed to both dynamic quenching and inner filter effect. The fluorescence probe displayed a highly broad dynamic response range (0.5-500 μM) for selective detection of Fe3+ with a limit of detection of 0.079 μM. The proposed method was successfully employed for detection and quantification of Fe3+ in human urine samples and proved to have potential for practical applications in biological field.

Förster-free Effect Fluorescence Quenching of 5,10,15,20-tetraphenylporphyrin in Solution

The absorption of exciting light by an inner filter may be the cause of large errors in estimating the efficiency of fluorescence quenching. We performed fluorescence measurements of 5,10,15,20-tetraphenylporphyrin toluene solution in a wide concentration range. We have shown a Förster-free fluorescence quenching in a right-angle geometry experiment, caused by second order inner filter. We proposed to perform measurements in a front-surface geometry to investigate the nature of quenching not related to the inner filters. We have shown that concentration effects in tetraphenylporphyrin solutions in toluene are not accompanied by a decrease in the fluorescence intensity in a front-surface geometry at concentrations of 10- 3-10- 7 mol/l. It was possible to separate the phenomena of the internal filters from the processes occurring in a liquid medium. Our results are of great importance due to the widespread fundamental research of porphyrin-based dyes properties.

Inner filter effect-based upconversion nanosensor for rapid detection of thiram pesticides using upconversion nanoparticles and dithizone-cadmium complexes

Herein, we propose a method for detecting thiram based on the fluorescence inner filter effect using upconversion nanoparticles and dithizone-cadmium complexes (DZ-Cd2+). The ultraviolet absorption of DZ-Cd2+ was in the range of 480-600 nm under alkaline conditions, resulting in fluorescence quenching of the nanoparticles at 540 nm. Thiram had a stronger coordination effect with Cd2+ than dithizone; thus, more thiram-cadmium complex (T-Cd2+) formed when thiram was added, leading to fluorescence recovery at 540 nm. The standard thiram curve was found to have a detection limit of 6.75 ng/mL in the linear range of 0.01-1000 µg/mL. In addition, high-performance liquid chromatography results for detecting thiram in apple samples revealed good application performance. The results demonstrate that the developed method has great potential to detect thiram residues in food.

A dual-emitting fluoroprobe fabricated by aloe leaf-based N-doped carbon quantum dots and copper nanoclusters for nitenpyram detection in waters by virtue of inner filter effect and static quenching principles

Fluorescence sensing technique has been used in environmental analysis due to its simplicity, low cost, and visualization. Although the fruit pulp-based biomass carbon quantum dots (CQDs) have excellent luminescent properties, aloe leaves possess the superiority of being easily accessible in all seasons compared to fruit pulp. Thus, we fabricated Aloe carazo leaf-based nitrogen doping-CQDs (N-CQDs) using a facile hydrothermal approach, which emitted bright blue fluorescence with a quantum yield of 21.4 %. By comparison, the glutathione-encapsulated copper nanoclusters (GSH-CuNCs) displayed strong red fluorescence. A blue/red dual emission based on the N-CQDs/CuNCs mixture was established for nitenpyram detection. At the 350-nm excitation, the N-CQD/CuNCs system produced dual-wavelength emitting peaks at 440 and 660 nm, respectively. Moreover, when nitenpyram was introduced into the system, the fluorescence intensities (FIs) of N-CQDs significantly decreased, whereas the FIs of GSH-CuNCs varied slightly; simultaneously, the solution color changed from bright blue to dark red. Both the spectral overlapping between nitenpyram's UV-Vis absorption and N-CQDs' excitation and almost unchanged fluorescence lifetimes indicated the occurrence of inner-filtering effect (IFE) in the dual-emitting fluoroprobe. In addition, the Stern-Volmer constant (Ksv = 6.92 × 103 M-1), temperature effect, as well as UV-Vis absorption of N-CQD/CuNCs before and after the addition of nitenpyram corroborated the static-quenching behavior. Consequently, the fluorescence-quenching of N-CQDs by nitenpyram was attributable to the joint IFE and static-quenching principles. A good linearity existed between the F660/F440 values and nitenpyram concentrations (0.5-200 μM) with a method detection limit of 0.15 μM. The dual-emitting fluoroprobe provided the satisfactory recoveries (95.0%-107.0 %) for nitenpyram detection in real-world waters, which were comparable with the results of traditional liquid chromatography coupled to tandem mass spectrometry method. Owing to its simple operations, low-cost, and adaptability for on-site outdoor monitoring, the newly developed dual-emitting fluoroprobe possesses great potential applications in routine monitoring of nitenpyram under field conditions.

Carbon quantum dot with co-doped nitrogen and phosphorus for tazettine ratiometric fluorescent sensing

An overdose of tazettine (TZ) has toxic side effects, which makes it extremely important to rapidly and subtly determine the TZ. In this study, double-emitting, nitrogen-phosphorus co-doped carbon quantum dots (N,P-CQDs) were prepared using cis-butenedioic acid, phosphoric acid, and p-phenylenediamine. An effective inner filter effect (IFE) can occur between TZ and N,P-CQDs, resulting in their fluorescence quench. Therefore, a ratio fluorescent probe was constructed for detecting the TZ was constructed. After optimizing the experimental conditions, the quenching efficiency showed a strong linear connection with the TZ concentration in the range of 0.01-30 µmol/L, with the detection limit of 0.002 µmol/L. This method could be satisfactorily applied to detect TZ in mouse plasma samples.

Ratiometric fluorescence sensor based on europium-organic frameworks for selective and quantitative detection of cerium ions

BACKGROUND

Due to the unavoidable use of cerium in daily life, the accumulation of cerium in the environment increases health risks for humans. Therefore, it is crucial to develop a chemical sensing technology for the rapid, sensitive, and selective detection of cerium ions.

RESULTS

In this research work, a novel two-dimensional chain structure of a europium-based metal organic framework (Eu-MOF) [Eu2(tcpa)(Htcpa)2] was synthesized by using 3,4,5,6-tetrachloro-1,2-benzenedicarboxylic acid (H2TCPA) as the ligand and europium nitrate as the metal source. The results of powder X-ray diffraction and thermogravimetric analysis show that the synthesized Eu-MOF has excellent chemical and thermal stability. When the Eu-MOF suspension was excited by ultraviolet light at 292 nm, four fluorescence emissions were observed at 420, 595, 620 and 705 nm. It was particularly interesting that when cerium ions (Ce3+/Ce4+) were added to the Eu-MOF suspension, the fluorescence intensity at 420 nm was enhanced, while the fluorescence at 620 nm was quenched. On this basis, a ratiometric fluorescent sensor for detecting cerium ions was constructed, which has a good linear relationship in the range of 0.05-15 μM and a detection limit of 16 nM. The plausible mechanism of the change in the fluorescence characteristics of Eu-MOF caused by cerium ions was discussed in detail. Through the study of fluorescence lifetime and ultraviolet absorption, it was proven that the mechanism of Ce3+-quenching Eu-MOF fluorescence is the inner filter effect. Photoinduced electron transfer and internal filtering effects lead to fluorescence quenching at 620 nm, while redox reactions lead to fluorescence enhancement of the ligand at 420 nm.

SIGNIFICANCE

The proposed ratiometric fluorescence sensor was successfully employed for the detection of cerium ions in real water samples, confirming that it can be used as an alternative method for the detection of Ce3+ and Ce4+ in environmental samples.

On-site quantitation of xanthine in fish and serum using a smartphone-based spectrophotometer integrated with a dual-readout nanosensing assay

Rapid and quantitative biochemical analysis at points-of-need is imperative for food safety inspection. This work reports on: 1) a stand-alone smartphone-based "two-in-one" spectrophotometer (the SAFS) installed with a self-developed application (the SAFS-App) which can precisely collect both absorption spectra and fluorescence spectra in a reproducible manner within 5 s; and 2) a straightforward protocol for xanthine detection using fluorescent carbon nanodots and silver nanoparticles. The assay performed with the SAFS demonstrates high specificity towards xanthine, and a linear range of 1-60 μM with LODs of 0.38 and 0.58 μM for colorimetric and fluorometric readouts, respectively. The reliability and robustness of the SAFS are validated by on-site quantitation of xanthine in fish and serum samples, with comparable accuracy to HPLC method. More importantly, the SAFS presents itself as an appealing device which is accessible to everyone through the Internet of Things and can be tailored for diverse point-of-care testing applications.

A single recognition unit-based virtual sensor Array: Applying 3D fluorescence spectroscopy to inner filter effect-based sensing

A convenient, fast, low-cost detection and discrimination method is demanded for environmental monitoring but still it remains more technological challenges. Herein, we demonstrate that the inner filter effect (IFE), in combination with three-dimensional fluorescence spectroscopy, can offer a virtual sensor array (VSA) as apropersolution. And with the aid of pattern recognition techniques, it is feasible to recognize compounds with structural similarities economically and effectively. In this study, with the help of visual clustering plots of principal component analysis (PCA), a prediction model based on hierarchical strategy was made using support vector machine (SVM) method for the qualitative profiling of aromatic pollutants. The VSA was constructed by a single metal-organic framework (MOF) recognition unit (MOF-74 (Zn)) with the excitation wavelength as external regulatory factors. Pattern characteristics of four aromatics with very similar structures (phenylamine, chlorobenzene, nitrobenzene, and phenol), both single analyte and binary mixtures, were acquired. The primary constituents of multi-dimensional spectral signals were subsequently extracted and fed into a vector machine to construct a prediction model through 10-fold cross-validation optimization, resulting in a classification accuracy of 100% for single analytes and 96% for mixtures. Quantitative research has shown that, except for chlorobenzene, all three other analytes can be predicted in concentration within an acceptable error range, and the mixture can be predicted proportionally. Moreover, the VSA can be used to distinguish these pollutants in tap and river water also. We propose for the first time a new tack for the construction of VSA in a general manner, namely using three-dimensional full range fluorescence scanning for IFE based sensing to get multiple times of information resulting from different weak interaction between analyte and sensor for decision-making.

A water-stabilized Tb-MOF can be used as a sensitive and selective fluorescence sensor for the detection of oxytetracycline hydrochloride

Oxytetracycline hydrochloride (OTC) is a commonly used over-the-counter antibiotic, valued for its potent antibacterial properties. However, the inappropriate and excessive use of OTC can result in the accumulation of the drug in both the environment and human body, causing significant harm to ecosystems and human health. Therefore, the development of a fast and sensitive method for detecting OTC is of great significance. Lanthanide metal-organic frameworks (LnMOFs) can effectively excite lanthanide metals to emit long-lifetime, narrow and stable fluorescence based on the antenna effect, but their application in fluorescence sensing is rarely reported. In this work, a strongly fluorescent material Tb-MOF was synthesized by a facile solvothermal method using 1,2,4,5-phenylenetetracarboxylic acid (H4btec) and 1,10-phenanthroline (1,10-phen) as organic ligands and lanthanide metal Tb as the luminescent center. Tb-MOF is a stable material in water and shows excellent linearity with OTC in the concentration range of 0 ∼ 70 μM, with low detection limit (0.12 μM) and luminescence color transition from bright green to dark green during the detection process. X-ray diffraction, UV-vis absorption and fluorescence lifetime analyses revealed that the fluorescence quenching of Tb-MOF by OTC is caused by the inner filter effect in static quenching. Test strips for OTC detection were successfully prepared using Tb-MOF. These strips are not only low cost and easy to prepare but can also be used as portable sensing devices that can be easily distinguished by the naked eye during OTC testing. This study not only presents a fluorescent probe for the detection of OTC in water but also offers a practical method for converting fluorescent luminescent materials into functional devices for OTC detection.

A ratiometric fluorescence sensor based on the inner filtration effect of gold nanoparticles on quantum dots for monitoring dopamine

In this study, a smart phone assisted ratiometric fluorescence sensor was designed for detecting dopamine (DA). The ratiometric fluorescence sensor was prepared by simple physical mixing green quantum dots (GQDS) and red quantum dots (RQDS). DA could induce gold nanoparticles (AuNPs) aggregate via hydrogen-bonding interactions, and further changed the absorption spectrum of gold nanoparticles to overlap with a certain emission spectrum of ratiometric fluorescence sensor. AuNPs had inner filtration effect (IFE) on the ratiometric fluorescence sensor. Due to the IFE, the dispersive AuNPs could quench GQDS, whereas the clustered AuNPs could quench RQDS. With the addition of DA, the color of ratiometric fluorescence changed from orange red to green. To simplify the detection process, a smartphone was employed to detecting DA in human urine by measuring RGB value of fluorescence color changes with a detection limit of 86 nM. This proposed method has the advantages of low cost, easy prevalence and simple operation, thus provides a great promise for rapid detection of biomarker in biological samples.

Exploring fluorescence of metal nanoparticles for effective utility in drug sensing: A Promising ''on-off'' fluorescence probe for analysis of cephalosporins is fabricated

A promising fluorescent nano sensor was fabricated exploiting the unique optical and physicochemical properties of silver nanoparticles (AgNPs). AgNPs were prepared following a chemical reduction technique to get a highly water-soluble nano sensor, stable for at least 1 month without the need of organic stabilizers. Full characterization of AgNPs was done using different spectroscopic and microscopic techniques. They exhibit excellent water solubility, physicochemical and optical properties, enabling them to be successfully applied in chemical sensing of drugs. The prepared AgNPs could be conceived as a fluorescent probe for the fluorimetric determination of two commonly administered cephalosporins ceftriaxone (CTX) and cefepime (CFP) based on the quenching behavior of the fluorescence omitting the need for pre-derivatization or chromogenic reagents. The fluorescence intensity of AgNPs at 485 nm after excitation at 242 nm was quantitively quenched upon increasingly adding the studied drugs over the concentration ranges of 1-10 µg/mL and 0.9-9 µg/mL with detection limits of 0.178 µg/mL and 0.145 µg/mL for CTX and CFP, respectively. The quenching mechanisms were investigated and illustrated. The influence of different experimental parameters was studied and optimized. The suggested sensor provides an innovative, sensitive, and eco-friendly approach for the assay of the drugs in their pharmaceutical vials and quality control laboratories with excellent % recoveries of 99.88 ± 1.15, 99.95 ± 1.15 for CTX and CFP, respectively. The method was validated in accordance with ICH Q2 R1 recommendations. The greenness evaluation was performed through both Eco-Scale and GAPI revealing the green criteria of the developed method.

Receptor-free phenothiazine derivative as fluorescent probe for picric acid: Investigation of the inner filter effect channel

In this study, a highly fluorescent and receptor-free phenothiazine derivative (PDAB) was developed to detect picric acid. A combination of steady-state and time-resolved fluorescence studies was conducted to examine the excited state behavior of PDAB with picric acid in solution. The PDAB probe displayed a significant degree of selectivity and was highly sensitive to picric acid, with an extremely low detection limit of 9.82 nM. Time-resolved fluorescence quenching studies exhibit direct evidence of an inner filter effect-based sensing mechanism. Using the Parker equation, a thorough analysis was done to correct the inner filter effect on the sensing of picric acid. Overall, these studies provide critical information on the sensing mechanism for picric acid detection.

Sensitive determination of naftazone using carbon quantum dots nanoprobe by fluorimetry and smartphone-based techniques

A simple, fast, and direct mix-and-read spectrofluorimetric method has been developed for the sensitive determination of naftazone (NFZ) utilizing graphene quantum dots (GQDs) as a greener and highly luminescent nanosensor. NFZ effectively quenches the strong fluorescence of GQDs at λex/λem of 350/440 nm via the inner filter effect mechanism. The nanosensor exhibits excellent linearity for NFZ over the concentration range of 0.46 to 186 μM with a limit of detection of 0.04 μM. The proposed method was validated for the successful determination of NFZ in tablets and on manufacturing equipment surfaces with good % recoveries of 98.4-101.6 and 96.3 - 102.2%, respectively. Furthermore, an integrated smartphone-based reader has been implemented and successfully applied for the determination of NFZ. The smartphone-based reader consists of a 365 nm UV torch as an excitation source, a smartphone for recording images, and smartphone-powered image analysis software for signal interpretation, together with a paper-based analytical device (PAD) utilizing filter paper as a substrate and correction fluid as a barrier for creation of detection zones. This smart platform showed excellent sensitivity with a limit of detection down to 0.12 nmol/zone, and it could be used for in-site determination of NFZ, especially for the limited resources laboratories.

Paper-assisted ratiometric fluorescent sensors for on-site sensing of sulfide based on the target-induced inner filter effect

Dissolved sulfide tends to species transformation and loss upon leaving the matrix, thus the development of a practical on-site determination of sulfide is crucial for environmental monitoring and human health. In this work, a novel paper-based ratiometric fluorescence sensor was developed for the field analysis of sulfide, which system was constructed by the inner filter effect (IFE) of CdS quantum dots (QDs) toward carbon dots (C-dots). Instead of an aqueous phase system, the conversion of sulfide to its hydride would induce the in-situ formation of CdS QDs on the paper, which acted as an energy acceptor to quench the emission of C-dots, leading to a variation of ratiometric fluorescence from blue to yellow with the increasing concentration of sulfide. Moreover, we proposed a smartphone-based fluorescence capture device integrated with a programmed Python program, accomplishing both color recognition and accurate detection of sulfide. Under the optimal condition, this ratiometric fluorescence sensor allowed for the on-site analysis of sulfide with a limit of detection of 0.05 μM. The accuracy of the sensor was validated via the successful field analysis of environmental water samples with satisfactory recoveries. Compared to other fluorescence methods used for sulfide analysis, this developed system retains the advantages of label-free, low-cost, ease of operation, and miniaturization, showing great potential for the measurement of sulfide on-site, as well as environmental monitoring.

Highly sensitive detection of microRNA-21 by nitrogen-doped carbon dots-based ratio fluorescent probe via nuclease-assisted rolling circle amplification strategy

Highly sensitive and selective detection of microRNA-21 (miRNA-21) in biological samples is critical for the disease diagnosis and cancer treatment. In this study, a nitrogen-doped carbon dots (N-CDs)-based ratio fluorescence sensing strategy was constructed for miRNA-21 detection with high sensitivity and excellent specificity. Bright-blue N-CDs (λ_ex/λ_em = 378 nm/460 nm) were synthesized by facile one-step microwave-assisted pyrolysis method by using uric acid as the single precursor, and the absolute fluorescence quantum yield and fluorescence lifetime of N-CDs were 35.8% and 5.54 ns separately. The padlock probe hybridized with miRNA-21 firstly and then was cyclized by T4 RNA ligase 2 to form a circular template. At the present of dNTPs and phi29 DNA polymerase, the oligonucleotide sequence in miRNA-21 was prolonged to hybridize with the surplus oligonucleotide sequences in circular template, generating long and reduplicated oligonucleotide sequences containing abundant guanine nucleotides. Separate G-quadruplex sequences were generated after the addition of Nt.BbvCI nicking endonuclease, and then hemin bound with G-quadruplex sequence to construct the G-quadruplex DNAzyme. Such G-quadruplex DNAzyme catalyzed the redox reaction of o-phenylenediamine (OPD) with H₂O₂, finally producing the yellowish-brown 2,3-diaminophenazine (DAP) (λ_em = 562 nm). Due to the inner filter effect between N-CDs and DAP, the ratio fluorescence signal of DAP with N-CDs was utilized for sensitive detection of miRNA-21 with detection limit of 0.87 pM. Such approach has practical feasibility and excellent specificity for miRNA-21 analysis during highly homological miRNA family in HeLa cell lysates and human serum samples.

Spectroscopic and Molecular Docking Studies of the Interaction of Non-steroidal Anti-inflammatory Drugs with a Carrier Protein: an Interesting Case of Inner Filter Effect and Intensity Enhancement in Protein Fluorescence

Interaction of diclofenac and indomethacin with lysozyme was studied using several spectroscopic and molecular docking methods. Difference UV-visible spectra showed that the absorption profile of lysozyme changed when both diclofenac and indomethacin were mixed with the former. The sequential addition of both drugs to the lysozyme solution caused the decrease of the intrinsic fluorescence of the latter, however, when the data were corrected for inner filter effect, an enhancement in the fluorescence of lysozyme was detected. Accordingly, the fluorescence enhancement data were analyzed using Benesi-Hildebrand equation. Both, diclofenac and indomethacin showed good interaction with lysozyme, although, the association constants of indomethacin were nearly two-fold higher as compared to that of diclofenac. The binding was slightly more spontaneous in case of indomethacin and the major forces involved in the binding of both drugs with lysozyme were hydrogen bonding and hydrophobic interactions. Secondary structural analysis revealed that both drugs partially unfolded lysozyme. Results obtained through molecular docking were also in good agreement with the experimental outcomes. Both, diclofenac and indomethacin, are bounded at the same site inside lysozyme which is located in the big hydrophobic cavity of the protein.

In-situ synthesis of ionic liquid-based-carbon quantum dots as fluorescence probe for hemoglobin detection

Carbon quantum dots (CQDs) have emerged as a potential fluorescent probe in bio/analytical chemistry in the present decade. The optical characteristics of CQDs may be tuned by their functional groups, which can also be used to selectively produce stable bonds with target molecules. Along with them, ionic liquids (ILs) are now demonstrating their important relevance in the field of pharmaceuticals for the creation of potent therapeutics. In the article, we have discussed the use of high fluorescent ILs-decorated-CQDs (CQDs-IM@OTf) as a straightforward and quick-acting fluorescence probe for sensitive and precise hemoglobin (Hb) determination with minimum detectability of 6.7 nM. The proposed mechanism behind this involves static mode of quenching which leads to the formation of a ground state complex [CQDs-IM@OTf-Hb complex] between the Hb protein and the drug. Despite the fact that Hb can quench the fluorescence of CQDs due to the inner filter effect (IFE) of the protein, which effects both the excitation and emission spectra of the CQDs, the addition of H₂O₂ improved the sensitivity of Hb detection. The present assay predicated on Hb interaction with H₂O₂, which produces reactive oxygen species such as hydroxyl (OH^.) and superoxide (O₂^.-) radicals under heme degradation and/or iron release from Hb. The subsequent reaction of hydroxyl radicals with CQDs, which acts as a strong oxidising agent, causes a high fluorescence quenching. The designed fluorescence probe was used to measure Hb in the concentration range of 3-90 nM with a precise detection limit of 0.33 nM. The quantification of hemoglobin (Hb) in diluted human blood samples is done using this observation.

Synergistic effect-triggered fluorescence quenching enables rapid and sensitive detection of alkaline phosphatase

The development of biosensors mediated by synergistic quenching effect is of great significance for rapid and accurate clinical diagnosis. Hence, we prepared a cyan-emitting fluorescent Si dots for alkaline phosphatase (ALP) detection through the synergistic quenching effect of inner filter effect (IFE) and photo-induced electron transfer (PET). Si dots were prepared by microwave-assisted method, which displayed high quantum yield (28.7%), as well as good physiochemical properties, such as photo-stability, pH stability, and chemical stability. As the hydrolysate of 4-nitrophenyl phosphate disodium salt hexahydrate catalyzed by ALP, both IFE and PET of 4-nitrophenyl to Si dots were used for the turn-off mode detection of ALP. The linear relationships were established between the change of fluorescence intensity and ALP concentration in the range of 0.05 U L^-1 to 5.0 U L-1, and 5.0 U L^-1 to 80.0 U L^-1, respectively. The detection limit was 0.01 U L^-1. The synergistic quenching effect caused the turn-off mode detection to be more sensitive, and it can also be used for the accurate detection of ALP in human serum, thereby showing great anti-interference ability in complex environments.

Fluorescent Carbon Dots Derived From Soy Sauce for Picric Acid Detection and Cell Imaging

Picric acid (PA) is a powerful nitro-aromatic explosive that harms the environment and human health. Developing non-toxic and low-cost sensors for the rapid detection of PA is essential. An environment-friendly fluorescent probe for PA detection is designed based on carbon dots (CDs) directly separated from edible soy sauce by silica gel column chromatography. Neither organic reagents nor heating process was needed to prepare CDs. The obtained CDs exhibit bright blue fluorescence, good water solubility, and photostability. The fluorescent probe for PA was developed according to the CD's fluorescence can be significantly quenched via the inner filter effect between CDs and PA. The linear range was 0.2-24 µM with a limit of detection of 70 nM. This proposed method was successfully employed to detect PA in the real water samples with satisfactory recoveries between 98.0-104.0%. Moreover, the CDs were suitable for fluorescence imaging of HeLa cells owing to their low toxicity and good biocompatibility.

Selective determination of ellagic acid in aqueous solution using blue-green emissive copper nanoclusters

Development of beneficial sensors to analyze ellagic acid concentrations is of great importance for food safety and human health. Herein, a facile and fast fluorescent probe was carried out for the excellently selective and sensitive measurement of ellagic acid in real samples through histidine protected copper nanoclusters (histidine@Cu NCs) as a nanosensor. This as-developed histidine@Cu NCs were performed through UV-vis absorption spectroscopy, fluorescence spectroscopy, transmission electron microscopy, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy and fluorescence lifetime analysis. The TEM image revealed that this nanomaterial had spherical features with the average diameter of 2.5 ± 0.05 nm. The blue-green fluorescence of this Cu NCs was found under the UV light. Meanwhile, the maximum excitation and emission wavelength were located at 387 nm and 488 nm. After addition of ellagic acid, the fluorescence of histidine@Cu NCs was slowly weakened with excellent linear range of 0.5-300 μM and detection limit of 0.077 μM. The fluorescence weakening mechanism of this nanosensor were attributed to the inner filter effect (IFE) and static quenching. Finally, this as-established analysis platform was successfully employed to measure ellagic acid in real samples.

Ratiometric fluorescence assay for sulfide ions with fluorescent MOF-based nanozyme

A novel ratiometric fluorescence strategy for sulfide ions (S^2-) analysis has been developed using metal-organic framework (MOF)-based nanozyme. NH₂-Cu-MOF displays blue fluorescence (λem = 435 nm) originating from 2-amino-1,4-benzenedicarboxylic acid ligand. Besides, it possesses oxidase-like activity due to Cu²⁺ node, which can trigger chromogenic reaction. o-Phenylenediamine (OPD), as a common enzyme substrate, can be oxidized by NH₂-Cu-MOF to form luminescent products (oxOPD) (λem = 570 nm). Inner filter effect occurs between oxOPD and MOF. Upon exposure to S^2-, oxidase-like activity of MOF is depressed significantly because of the generation of CuS. On one hand, the amount of free Cu²⁺ decreases, affecting the yielding of oxOPD. On the other hand, CuNPs with larger size are obtained during the oxidation-reduction reaction between Cu²⁺ and OPD, which show weaker autocatalytic ability for OPD oxidation. These result in the decrease and increase of intensities at 570 and 435 nm, respectively. This method exhibits sensitive and selective responses towards S^2- with LOD of 0.1 μM. Furthermore, such ratiometric strategy has been applied to detect S^2- in food samples.

Rapid and sensitive detection of methyl parathion in rice based on carbon quantum dots nano-fluorescence probe and inner filter effect

A highly sensitive fluorescent sensing system of novel carbon quantum dots nano-fluorescent probe based on corn stalks was established for the determination of methyl parathion by alkaline catalytic hydrolysis and inner filter effect mechanism. The carbon quantum dots nano-fluorescent probe was prepared from corn stalks using an optimized one-step hydrothermal method. The detection mechanism of methyl parathion was revealed. The reaction conditions were optimized. The linear range, sensitivity and selectivity of the method were evaluated. Under the optimal conditions, the carbon quantum dots nano-fluorescent probe exhibited high selectivity and sensitivity to methyl parathion, achieving a linear range of 0.005-14 µg/mL. The fluorescence sensing platform was applied to the detection of methyl parathion in rice samples, and the results showed that the recoveries range from 91.64 to 104.28 %, and the relative standard deviations were less than 4.17 %. The detection limit for methyl parathion in rice samples was 1.22 µg/kg, and the limit of quantitation (LOQ) was 4.07 µg/kg, which was very satisfactory.

A test strip constructed by molecular imprinting for ratiometric fluorescence with ultra-low limit of detection for selective monitoring of Sudan I in chili powder

An up-conversion molecularly imprinted ratiometric fluorescent probe with a monodisperse nuclear-satellite structure and its test strip are designed which can avoid fluorescent background interference to detect Sudan I in chili powder highly selective and sensitive. The detection mechanism is based on the selective recognition of Sudan I by imprinted cavities on the surface of ratiometric fluorescent probe and the inner filter effect between Sudan I molecules and the emission of up-conversion materials (NaYF₄:Yb,Tm). Under optimized experimental conditions, the response of fluorescent ratio signals (F₄₇₅/F₆₄₅) of this test strip show a good linear relationship in the range 0.02-50 μM Sudan I. The limits of detection and quantitation are as low as 6 nM and 20 nM, respectively. Sudan I is selectively detected in the presence of fivefold higher concentrations of interfering substances (imprinting factor up to 4.4). Detection of Sudan I in chili powder samples show ultra-low LOD (44.7 ng/g), satisfactory recoveries (94.99-105.5%) and low relative standard deviation (≤ 2.0%). This research offers a reliable strategy and promising scheme for highly selective and sensitive detection of illegal additives in complex food matrix via an up-conversion molecularly imprinted ratiometric fluorescent test strip.

Sulfur quantum dots as a fluorescent sensor for N-acetyl-beta-D-glucosaminidase detection

N-acetyl-beta-D-glucosaminidase (NAG) is an important biomarker for early clinical diagnosis of renal disease, suggesting the necessity to develop a fast and sensitive method for its detection. In this paper, we developed a fluorescent sensor based on polyethylene glycol (400) (PEG-400)-modified and H₂O₂-assisted etched sulfur quantum dots (SQDs). According to the fluorescence inner filter effect (IFE), the fluorescence of SQDs can be quenched by the p-nitrophenol (PNP) generated by NAG-catalyzed hydrolysis of p-Nitrophenyl-N-acetyl-β-D-glucosaminide (PNP-NAG). We successfully used the SQDs as a nano-fluorescent probe to detect the NAG activity from 0.4 to 7.5 U·L^-1, with a detection limit of 0.1 U·L^-1. Furthermore, the method is highly selective and was successfully used in the detection of NAG activity in bovine serum samples, suggesting its great application prospect in clinical detection.

Dual-mode immunochromatographic assay based on dendritic gold nanoparticles with superior fluorescence quenching for ultrasensitive detection of E. coli O157:H7

We presented a colorimetric/fluorescent dual-mode immunochromatographic assay (ICA) for the sensitive detection of Escherichia coli O157:H7. The use of polydopamine (PDA)-modified gold nanoparticles (AuNPs) with broadband absorption allowed for excellent colorimetry signals for the ICA detection. Moreover, the absorption spectrum of PDA-AuNPs significantly overlaps with the excitation and emission spectra of ZnCdSe/ZnS quantum dots (QDs), resulting in effective quenching of the QDs fluorescence due to the inner filter effect. The fluorescence intensity changes induced by PDA-AuNPs were utilized for the sensitive detection of E. coli O157:H7, achieving a detection limit of 9.06 × 10¹ CFU/mL, which was 46-fold lower than that of traditional AuNPs-based immunoassay. The proposed immunosensor exhibited the recovery rate between 80.12% and 114.69% in detecting actual samples, indicating its reliability and satisfactory accuracy. This study provides insights into dual-mode signal outputs and the ICA development for food safety applications.

Selective detection of nitenpyram by silica-supported carbon quantum dots

In this study, a fluorescent sensor of nitrogen-doped carbon quantum dots (N-CQDs) and silica gel hybrid was developed for the quantitative detection of nitenpyram, a toxic neonicotinoid existing in groundwater and/or surface water.The prepared N-CQDs@SiO₂ sensor exhibited remarkable sensing selectivity and sensitivity towards nitenpyram among the four pesticides and six metal ions. A prominent fluorescence quenching of N-CQDs@SiO₂ at 445 nm was observed in the presence of nitenpyram with a linear response range of 0-300.0 mg L^-1 and an estimated limit of detection of 1.53 mg L^-1. The main cause for selective sensing is that nitenpyram absorbs the excitation light of N-CQDs@SiO₂, leading to fluorescence quenching of the sensor through the inner filter effect.

Terbium-triggered aggregation-induced emission of bimetallic nanoclusters for anticancer drugs sensing via the inner filter effect

The development of accurate and sensitive detection methods of anticancer drugs is of significant importance because they play vital roles in biological systems. In recent years, bimetallic nanoclusters (BMNCs) incorporating the advantages of two metals have gained more and more attention, and can be widely applied in sensing applications. In this work, for the first time, we designed a sensing platform based on terbium ion (Tb³⁺) triggered aggregation-induced emission (AIE) of BMNCs. Tb³⁺ hybrid glutathione (GS) protected Ag/Cu nanoclusters (Tb³⁺@GS-AgCuNCs) were facilely fabricated according to the complexation reaction between Tb³⁺ and the carboxyl group of GS. Due to the inner filter effect (IFE), the fluorescence of Tb³⁺@GS-AgCuNCs decreased significantly in the presence of anticancer drugs with 6-thioguanine and methotrexate as representatives. In addition, the sensing platform was applied to monitor 6-thioguanine and methotrexate in real serum samples, indicating that it has great potential in anticancer drugs related applications.

Carbon dots doped with nitrogen as an ultrasensitive fluorescent probe for thrombin activity monitoring and inhibitor screening

A simple and sensitive fluorometric assay based on nitrogen-doped carbon dots (N-CDs) was developed for the determination of thrombin (TB) activity in human serum samples and living cells. The novel N-CDs were prepared by a facile one-pot hydrothermal method using 1,2-ethylenediamine and levodopa as precursors. Such N-CDs exhibited green fluorescence with excitation/emission peaks at 390/520 nm and a high fluorescence quantum yield of approximately 39.2%. H-D-Phenylalanyl-L-pipecolyl-Larginine-p-nitroaniline-dihydrochloride (S-2238) was hydrolyzed by TB to produce p-nitroaniline which was capable of quenching the fluorescence of N-CDs due to an inner filter effect. This assay was used to detect TB activity with a low detection limit of 11.3 fM. The proposed sensing method was then expanded to the TB inhibitor screening and exhibited excellent applicability. As a typical TB inhibitor, argatroban was determined in a concentration as low as 1.43 nM. The method has also been successfully employed for the determination of TB activity in living HeLa cells. This work showed significant potential for TB activity assay in clinical and biomedicine applications.

Internal reflectance cell fluorescence measurement combined with multi-way analysis to detect fluorescence signatures of undiluted honeys and a fusion of fluorescence and NIR to enhance predictability

Honey is a complex food matrix that contains diverse polyphenolic compounds. Some phenolics exhibit fluorescence signatures which can be used to evaluate honey quality, and authenticity and to determine botanical origin. Mānuka honey contains two unique fluorescence markers: Leptosperin (MM1) and Lepteridine^TM (MM2) that are derived from Leptospermum scoparium nectar. Fluorescence measurement of supersaturated solutions such as undiluted honeys can be challenged by complex inner filter effects. The current study shows the ability of internal reflectance cell fluorescence measurement and multi-way analysis to detect fluorophores in undiluted honeys. This study scanned honeys from different geographic districts generating excitation emission matrices (250-400/300-600 nm), and by near infrared (NIR) hyperspectral camera (547-1701 nm). PARAFAC and tri-PLS could track two fluorescence markers: MM1 (R² = 0.82 & RMSEP = 138.65) and MM2 (R² = 0.82 & RMSEP = 2.75) from undiluted honey fluorescence data with > 80 % accuracy. Classification of mono-floral, multi-floral and non-mānuka honeys achieved 90 % overall accuracy. Fusion of fluorescence data at ƛ_ex 270 & 330 nm and NIR hyperspectral data combined with multi-block PLS analysis enhances predictability of fluorescence markers further. The study revealed the potential of internal reflectance cell fluorescence measurement combined with chemometrics and data fusion for rapid evaluation of honey quality and botanical origin.

Copper nanoclusters stabilized by D-penicillamine for ultrasensitive and visual detection of oxytetracycline

Copper nanoclusters (DPA@CuNCs) with red fluorescence were successfully synthesized by a one-step method based on D-penicillamine (DPA), which acted not only as a reducing agent but also as a stabilizer. The products were characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, particle-size analysis, ultraviolet-visible spectrophotometry, and fluorescence spectrometry. When the excitation wavelength was 280 nm, DPA@CuNCs emitted bright red fluorescence at 640 nm with a fluorescence quantum yield of 5.8 %. Due to the inner filter effect, oxytetracycline (OTC) effectively quenched the fluorescence of DPA@CuNCs, and then DPA@CuNCs were applied to the trace detection of OTC. The method showed a good linear range for OTC from 5 to 60 μmol/L, with a detection limit of 0.026 μmol/L and a correlation coefficient R² of 0.9983. Moreover, a paper-based sensor for the visual detection of OTC has been developed, which can conveniently and rapidly distinguish the concentration ranges of OTC through the color changes of the test papers.

A water-soluble naphthalimide fluorescent probe for Cr2O72- and Fe3+ based on inner filter effect

A probe 3 (2-ethoxy-N-(2-(2-(2-hydroxyethoxy)ethyl)-1,3-dioxo-2,3-dihydro-1H-benzo[de] isoquinolin-6-yl)benzamide) that could selectively respond to Cr₂O₇^2- and Fe³⁺ was reported in this paper. The selectivity, pH titration, concentration titration, detection limit, time dependence, quenching constant and recognition mechanism of probe 3 for Cr₂O₇^2- and Fe³⁺ were studied in CH₃CN/HEPES buffer solution. The results showed that Cr₂O₇^2- and Fe³⁺ could rapidly quench the fluorescence of probe 3 through the inner filter effect (IFE). The quenching kept constant after 30 s, and the quenching constants were 7.99 × 10³ L^.mol^-1 and 4.13 × 10³ L^.mol^-1, respectively. The detection limits of probe 3 for Cr₂O₇^2- and Fe³⁺ were 1.15 μmol^.L^-1 and 1.95 μmol^.L^-1, respectively, which were lower than the maximum allowable concentrations in drinking water stipulated by EPA. The determination results of Cr₂O₇^2- and Fe³⁺ in water samples indicated that probe 3 could be used as a potential detection tool in practical applications.

Quantification of organic fluorophores in absorbing media by solid-phase fluorescence excitation-emission matrix (SPF-EEM) spectroscopy of modeled mixtures containing bovine serum albumin (BSA) and colorants

Solid-phase fluorescence excitation-emission matrix (SPF-EEM) spectroscopy is beneficial for investigating the characteristics of natural organic matter (NOM) in the solid phase without extraction procedures. However, inner filter effect (IFE) due to the presence of dark components in samples can make it difficult to quantify the fluorophore concentration. To establish a new method to determine unknown concentrations of a fluorescent material in a sample containing various absorbing materials by SPF spectroscopy, modeled mixtures containing bovine serum albumin (BSA) and colorants at different ratios were examined. Fluorescence intensities of BSA against various concentrations afforded different saturation curves for different colorants in the mixtures, suggesting that it is difficult to use the SPF intensity for quantifying the concentration of fluorescent samples in which IFE has occurred, because one cannot obtain a single calibration curve that does not depend on the absorbing medium that it is mixed in. However, products of the fluorescence intensity and Kubelka-Munk (KM) function at the excitation wavelength were proportional to the first order of BSA weight concentrations, regardless of the colorant type. By using this trend as a calibration curve, it may be possible to quantify the amount of BSA from its SPF-EEM spectrum. In this study, the KM function was obtained using an ultraviolet-visible (UV-Vis) spectrometer with an integrating sphere. To reduce the labor and equipment cost of UV-Vis spectroscopy, a substrate of the KM function also was obtained from the Rayleigh scattering in an SPF-EEM spectrum, which could be used as a parameter for calibration curves that quantify the BSA concentration. Although further studies are required, this study proposed that the product of the SPF intensity and KM function at the excitation wavelength can be partially used for an empirical formula to quantify a variety of fluorescent materials mechanically mixed with various absorbing materials.

Utilizing inner filter effect in resonance Rayleigh scattering technique: a case study with silver nanocubes as RRS probe and several analytes as absorbers

It was demonstrated that the mechanism of the inner filter effect (IFE) can emerge well in the resonance Rayleigh scattering (RRS) technique and be utilized as a new analytical method in the design of innovative IFE-based sensors. To prove this process, silver nanocubes (Ag NCs) with tunable extinction spectra were selected as RRS probes, and three analytes, doxorubicin (DOX), sunitinib (SUN), and Alizarin Red S (ARS), were considered as the typical absorbers. In addition, in the presence of SUN as a typical analyte, the quenching of the RRS signal of Ag NCs, with λ_max of 419 nm, was linear in the range 0.01 to 2.5 µM of SUN. The limit of detection (LOD) was 0.0025 µM. The introduced method was then used to develop a dual-signal assay for the ratiometric determination of Al³⁺ ions. The suggested dual-signal assay was based on the color changes of ARS caused by Al³⁺ and the IFE between ARS and Ag NCs. The obtained results showed that the two characteristics of response sensitivity and linear dynamic range are very satisfactory for sensing Al³⁺ ions. The findings of this study demonstrate that the newly developed IFE mechanism can be employed as an attractive and highly efficient analytical technique for measuring different analytes.

A highly selective fluorescent sensor for chlortetracycline based on histidine-templated copper nanoclusters

In this study, histidine-protected copper nanoclusters (Cu NCs@His) were established by using a one-pot method, which histidine and ascorbic acid were applied as the template and reducing agent, respectively. The as-developed Cu NCs@His endued green emission wavelength at 494 nm with the excitation of 378 nm. The Cu NCs@His exhibited green fluorescence under UV light (365 nm). Using Cu NCs@His as a pattern nanosensor, the fluorescent "turn off" mechanism was fabricated for the determination of chlortetracycline in the light of the linear decrease of fluorescence intensities around 494 nm. The chlortetracycline conducted as a quencher, leading to reveal an excellent linear relationship between ln(F₀/F) of Cu NCs@His and chlortetracycline concentrations with the range of 0.5-200 μM, and the detection limit was 0.876 μM. The fluorescence quenching of Cu NCs@His revealed excellent selectivity for chlortetracycline over other potential interfering substances in the human body. This strategy was exhibited to be a convenient sensing platform for the detection of chlortetracycline in real medical samples, which could unfold a brand new and direct system for the sensing of chlortetracycline in real samples.

One-step synthesis of blue emission copper nanoclusters for the detection of furaltadone and temperature

Polyvinyl pyrrolidone (PVP), playing roles as a templating agent, can be applied to prepare blue-emitting copper nanoclusters (Cu NCs@PVP) on the basis of a rapid chemical reduction synthesis method. The Cu NCs@PVP displayed a blue emission wavelength at 430 nm and the corresponding quantum yield (QY) could reach 10.4%. Subsequently, the as-synthesized Cu NCs@PVP were used for the trace analysis of furaltadone based on the inner filter effect (IFE) between Cu NCs@PVP and furaltadone, which caused the fluorescence to be effectively quenched. Additionally, this proposed determination platform based on the Cu NCs@PVP for furaltadone sensing possessed an excellent linear range from 0.5 to 100 μM with a lower detection limit of 0.045 μM (S/N = 3). Meanwhile, the Cu NCs@PVP also could be applied for the sensing of temperature. Furthermore, the practicability of the sensing platform has been successfully verified by measuring furaltadone in real samples, affirming its potential to increase fields for the determination of furaltadone.

MoS2 quantum dots as fluorescent probe for methotrexate detection

Herein, we developed a unique fluorescence biosensor for methotrexate assay based on MoS₂ quantum dots, which were synthesized in one step using sodium molybdate and cysteine as raw materials. The fluorescence of MoS₂ QDs could be quenched when encountered with methotrexate, which was attributed to the inner filter effect (IFE). Furthermore, this present IFE-based method showed the linearity between the MoS₂ QDs fluorescence intensity and the methotrexate concentration in the range of 0.05-1 μM with the LOD of 42 nM. The practical applicability of this strategy was successfully demonstrated by detecting methotrexate in real samples. Results indicated that the proposed method could be a promising sensing platform for methotrexate analysis.

Paper-based device for the selective determination of doxycycline antibiotic based on the turn-on fluorescence of bovine serum albumin-coated copper nanoclusters

An enhanced ratiometric fluorescence sensor was built for on-site visual detection of doxycycline (DOX) through the interaction with bovine serum albumin on the surface of red emissive copper nanoclusters. Upon the addition of weakly fluorescent DOX, the red fluorescence from copper nanoclusters gradually decreased through the inner-filter effect (IFE), while a green fluorescence appears and significantly increases, forming an interesting fluorescent isosbestic point, which was assigned to DOX due to sensitization effect of bovine serum albumin. On the basis of this ratiometric fluorescence, the system possessed good limit of detection (LOD) of 45 nM and excellent selectivity for DOX over other tetracyclines. Based on these findings, a paper-based sensor has been fabricated for distinct visual detection of trace DOX and combined with smartphone color recognizer for quantitative detection of DOX (LOD = 83 nM). This method shows broad application prospects in environmental monitoring and food safety.

Fluorescent carbon dots as selective nano probe for determination of diacerein in presence of co-formulated drugs

In this work, a new and simple carbon dots (CDs) based fluorescent probe was introduced for selective determination of diacerein (DIA) in presence of two co-formulated drugs. This highly fluorescent sensor was constructed using chitosan as a carbon and nitrogen source by single step carbonization. The constructed probe is based on the inner filter effect (IFE), in which DIA serves as a strong absorber, influencing the excitation of the fluorescer (CDs). This overlap leads to quenching of CDs fluorescence upon increasing DIA concentration within the range of 2.5-17.5 µg/mL with mean % recovery reached to 99.7 ± 0.7. The performance of the constructed sensor had been validated according to the ICH guidelines and the results revealed that it is precise and accurate. Moreover, it has many advantages such as simplicity, saving time and good selectivity for the determination of DIA as a minor component in presence of co-formulated drugs in its tablet dosage form.

A solid-phase capture probe based on upconvertion nanoparticles and inner filter effect for the determination of ampicillin in food

Ampicillin (AMP) is commonly used to treat diseases caused by bacterial infections as a veterinary drug. However, the abuse of AMP can lead to residues in food and ultimately cause harm to humans. Thus, it is significant to construct a reliable system for AMP detection. Here, we developed an inner filter effect system based on a solid-phase capture probe and the catalysis of platinum nanoparticles (PtNPs) for AMP determination in food. In the presence of AMP, PDMS captured AMP then combined with aptamer-functionalized PtNPs, which catalyzed the oxidation of 3,3',5,5'-tetramethylbenzidine, resulting in upconversion fluorescence quenching. The results showed the fluorescence intensity of upconversion nanoparticles was related to AMP concentration (0.5-100 ng·mL^-1) with an LOD of 0.32 ng·mL^-1, which made quantification of AMP possible. The method also achieved a satisfactory recovery rate (96.89-112.92%) and can be used for AMP detection in food samples with selectivity and sensitivity.

Spectrally tunable humic acid-based carbon dots: a simple platform for metronidazole and ornidazole sensing in multiple real samples

Humic acid-based carbon dots (HACDs) have excellent properties and are widely used in environmental detection, bioimaging, and optoelectronic materials. Herein, we investigated the structure-activity relationship between the morphology and optical properties of HACDs, and reported on a novel strategy for metronidazole (MNZ) and ornidazole (ONZ) sensing in multiple real samples. It was found that the average particle size decreased from 3.28 to 2.44 nm, optimal emission wavelength was blue-shifted from 500 to 440 nm, and the quantum yield (QY) improved from 5 to 23% with the temperature increasing from 110 to 400 °C. Under the oxidation of hydrogen peroxide (H₂O₂) and potassium permanganate (KMnO₄), the UV-vis spectra of HACD aqueous solution showed time-dependent behavior, and the fluorescence emission of HACDs achieved spectrally tunable multi-color luminescence in the temporal dimension. The surface of HACDs contained a large number of hydroxyl (-OH) and carboxyl (-COOH) fluorophores, resulting in excellent pH sensing. Meanwhile, the synthesized HACDs revealed sensitive response to MNZ and ONZ with the limit of detection (LOD) of 60 nM and 50 nM in aqueous solutions, which had also been successfully applied in various actual samples such as lake water, honey, eggs, and milk with satisfactory results because of the inner filter effect (IFE). Our research is advantageous to enhance the potential applications of HACDs in advanced analytical systems.

Cascade amplification strategy combined with analyte-triggered fluorescence switching of dual-quenching system for highly sensitive detection of isoniazide

A sensitive fluorescence sensing platform consisting of manganese dioxide nanosheets (MnO₂) and gold nanoparticles (AuNPs) as dual nanoquenchers has been constructed to detect isoniazid combined with analyte-triggered cascade reactions. The fluorescence of 2,3-diaminophenazine (DAP) is quenched simultaneously by MnO₂ and AuNPs via inner filter effect. MnO₂ is decomposed by isoniazid to generate Mn²⁺, which makes AuNPs aggregated. The quenching abilities of both the decomposed MnO₂ and aggregated AuNPs are inhibited, causing remarkable fluorescence recovery. The usage of dual nanoquenchers enhances the quenching efficiency and reduces the fluorescence background. Moreover, the isoniazid-triggered cascade reaction further amplifies the readout signal. Thus, this strategy exhibits higher sensitivity towards the detection of isoniazid. Compared with MnO₂-based fluorescence assay, this strategy possesses lower limit of detection. This strategy has been successfully used to detect isoniazid in pharmaceutical preparations, which is of great significance for drug analysis.