An investigation of the Excitation Wavelength-Dependent Dynamic Changes in the Mechanism of Detection of Picric Acid using Pyrene-Based Donor-Acceptor Systems.

Picric acid (PA) is an important industrial feedstock and hence the release of industrial effluents without proper remediation results in its buildup in soil and water bodies. The adverse effects of PA accumulation in living beings necessitate the development of efficient methods for its detection and quantification. Herein, we describe pyrene-based fluorescent sensors for PA, where pyrene is appended with electron-withdrawing groups, malononitrile, and 2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene) malononitrile (DCDHF). These molecules displayed the typical emission of pyrene monomers, as well as a broad red-shifted emission resulting from an intramolecular charge transfer (ICT) in the excited state. Both the emissions displayed a turn-off response to PA with high selectivity and sensitivity and the lowest limit of detection was estimated as 27 nM. To prove the feasibility of on-site detection, test paper strips were prepared, which could detect PA up to 4.58 picograms. Using a combination of experimental and theoretical studies the mechanism of the detection was identified as primary/secondary inner filter effect, oxidative photoinduced electron transfer, or a combination of both depending on the excitation wavelength. Interestingly, the contribution of each of these mechanisms to the total quenching process varied with a change in the excitation wavelength.

High Luminescent Carbon Dots Derived from Fermented Beverages for Sensitive Sensing of Tartrazine in Foods.

Highly luminescent carbon dots (CDs) derived from fermented beverages-kvass (K-CDs) were synthesized through a one-step hydrothermal method with ethylenediamine (EDA) as a surface passivation reagent. Purified K-CDs with a fluorescent quantum yield of 35.1% were obtained after a dialysis process. The K-CDs were characterized by TEM, FT-IR, XPS, fluorescence and UV-vis spectroscopy. The results indicated that K-CDs possess typical excitation wavelength-dependent blue fluorescence emission, and the strongest excitation and emission wavelengths are 350 nm and 440 nm, respectively. The great spectral overlap between the emission peak (440 nm) of K-CDs and the absorption peak (430 nm) of tartrazine (TAR) leads to an effective fluorescence quenching phenomenon by TAR through inner filter effect (IFE) and the calculated (lg(I0/I)) showed a linear response to TAR concentration in the range of 0.1-70 µM. The detection limit of the developed method is 23 nM for TAR, and the relative standard deviation (RSD) is 3.9% (c = 10 µM, n = 7). The fluorescent sensor for TAR based on K-CDs through the IFE mechanism possesses the characteristics of rapid, sensitive, and high selectivity. It has been successfully applied to detect of trace TAR in foods.

Fluorescent Nitrogen-doped Carbon Dots-based Turn-off Sensor for Bilirubin.

Bilirubin (BR), a heme protein produced from breakdown of haemoglobin is present in aged red blood cells; whose abnormal concentration is associated with diseases like hyperbilirubinemia, coronary disease, iron deficiency, and so on. Herein, we have synthesized a selective, sensitive, and low-cost sensing platform using fluorescent nitrogen doped carbon dots (NCDs), prepared from precursors; citric acid and urea via a simple microwave-assisted method. The emission at 444 nm on excitation with 360 nm was well quenched in presence of BR suggesting a direct turn-off detection for BR. Characterization of developed probe was done by UV-Visible absorption studies, photoluminescence studies, SEM, TEM, ATR-FTIR, XPS, and DLS analysis. BR was detected with a Limit of Detection (LoD) and Limit of Quantification (LoQ) of 0.32 µM and 1.08 µM respectively. NCDs exhibited excellent selectivity and sensitivity towards BR in the presence of co-existing biomolecules and ions. Practical feasibility was checked by paper-strip-based sensing of BR and spiked real human samples were used for conducting real sample analysis.

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.

Terbium Phenanthroline Complex as a Luminescent Probe for the Detection of Anthrax Biomarker: Dipicolinic acid.

Dipicolinic acid (DPA) is a prominent biomarker for Anthrax disease. Bacillus anthracis bacterial endospores is composed of DPA as the significant component, which on over inhalation can cause severe health issues. Such contagious and life-threatening pathogens can be employed as bioweapons or biothreat agents for spreading bioterrorism which is a major risk for national security and public health concerns. Hence, effective detection or a surveillance system is essential for preventing the growth of bioterrorism events. Herein, we have developed a Terbium - 1,10 Phenanthroline (Tb-Phen) based lanthanide luminescence complex with bright green fluorescence. On addition of DPA, the green fluorescence is turn-off at a linear range from 0.6 to 4.762 mM. In this effect, 5D4- 7F5 transition caused by 1,10-phenanthroline to Tb3+ at 544 nm is restricted due to energy transfer quenching and Inner Filter Effect (IFE). The developed probe shows good sensitivity towards the detection of DPA with other coexisting biomolecules and ions with a low Limit of Detection (LOD) of 5.029 µM. The practical feasibility was evaluated in paper strip assay and extended in real samples such as human serum and tap water with satisfactory recovery percentage. Thereby, probe finds promising application in sensing of anthrax spore biomarker (DPA) and biothreat agents.

Fluorescent Carbon Nitride Nanoparticles for Picric Acid Sensing.

Detection of nitroaromatic explosives is essential in the area of environmental safety. Fluorescent carbon nitride nanoparticles is a promising material for this purpose. Herein, we have prepared fluorescent carbon nitride nanoparticles (CNNPs) by one step thermal treatment of formamide. These fluorescent CNNPs is sensitive towards picric acid (PA) than other analytes both in aqueous medium and on test paper which is witnessed by fluorescence quenching based on inner filter effect (IFE). The PA detection with the fluorescent CNNPs is observed in the concentration ranges, 0 µM to 60 µM with linear range of 10 nM to 25 µM. The minimum detection limit in aqueous medium and solid phase are determined to be 26.20 nM and 10 µM respectively. Finally, the fluorescent CNNPs is applied for detection of PA in real water samples. The recoveries are in the ranges from 99.54 to 116.35% with relative standard deviation less than 3.85%. This proposed fluorescent method can act as suitable analytical technique to monitored PA concentration in water samples.

Microwave synthesized novel biomass carbon dots applied in the fluorescent detection of crystal violet.

To establish a new method for detecting crystal violet (CV), a harmful dye, herein, a genre of novel biomass carbon dots (CDs) was synthesized via a microwave method and employed as a fluorescent probe, in which water spinach and polyethylene glycol (PEG) performed as raw materials. Based on the inner filter effect (IFE) between the luminescent CDs and CV, the blue emission of this probe at 430 nm could be quenched by CV. Hence, a new strategy was proposed to selectively determine CV in aquaculture ambient. Moreover, under the optimal experiment conditions, this method showed a good linearity between the concentration of CV (c) and fluorescence quenching rate (ΔF/F0) in the concentration range of 4-200 µmol/L with the corresponding correlation coefficient (r) and the detection limit of 0.997 and 710 nmol/L, respectively. With advantages of environmental protectivity, sensitivity, affordability, and user-friendliness, the facilely fabricated CDs could be successfully applied in detecting CV in aquaculture samples, providing a technical foundation for monitoring the pollution of CV and ensuring the quality and safety of aquatic products.

P-doped carbon dot nano-probe for inner filter effect-based determination of sarecycline in pharmaceutical dosage form and human plasma.

Based on novel phosphorus-doped carbon dots (PCDs), a simple, quick, and accurate fluorescence probe for sarecycline (SAR) determination has been created. The PCDs were prepared in just five minutes using green, straightforward one-step microwave pyrolysis. To create the PCD probe, sodium phosphate monobasic was utilized as a phosphorus dopant and citric acid as a carbon supply. The proposed synthesis method was energy efficient and yielded CDs with a narrow particle size distribution. Based on inner-filter effect mechanism, the generated PCDs were used as nano-probe for SAR determination. The fluorescence quenching intensity showed a strong linear relationship with SAR concentration in the 3-90-µM range with a detection limit of 0.88 µM. Because there is no surface alteration of the CDs or creation of a covalent bond between SAR and PCDs, the developed approach is quick, easy, inexpensive, and requires less time. The new probe's enhanced sensitivity, broad linear range, and acceptable selectivity made it suitable for SAR measurement in pharmaceutical formulations and spiked human plasma. Most importantly, the Green Analytical Procedure Index (GAPI) and Analytical GREEnness (AGREE) assessments showed that the suggested method was environmentally friendly.

Sustainable synthesis of fluorescent polymer carbon dots@PVA for sensitive chlortetracycline detection.

Antibiotic residues persist in the environment and represent serious health hazards; thus, it is important to develop sensitive and effective detection techniques. This paper presents a bio-inspired way to make water-soluble fluorescent polymer carbon dots (PCDs@PVA) by heating biomass precursors and polyvinyl alcohol (PVA) together. For example, the synthesized PCDs@PVA are very stable with enhanced emission intensity. This property was observed in a wide range of environmental conditions, including those with changing temperatures, pH levels, UV light, and ionic strength. PCDs@PVA detected the antibiotic chlortetracycline (CTCs) with great selectivity against structurally related compounds and a low detection limit of 20 nM, demonstrating outstanding sensitivity and specificity. We confirmed the sensor's practical application through real sample analysis, yielding recovery rates of 98%-99% in samples of milk, honey, and river water. The synthesized PCDs@PVA fluorescence sensor was successfully used for CTCs detection in real samples.

Asparagus officinalis Herb-Derived Carbon Quantum Dots: Luminescent Probe for Medical Diagnostics.

The utilization of natural materials for the synthesis of highly fluorescent carbon quantum dots (CQDs) presents a sustainable approach to overcome the challenges associated with traditional chemical precursors. Here, we report the synthesis of novel S,N-self-doped CQDs (S,N@CQDs) derived from asparagus officinalis herb. These S,N@CQDs exhibit 16.7 % fluorescence quantum yield, demonstrating their potential in medical diagnostics. We demonstrate the efficacy of S,N@CQDs as luminescent probes for the detection of anti-pathogenic medications metronidazole (MTZ) and nitazoxanide (NTZ) over concentration ranges of 0.0-180.0 µM (with a limit of detection (LOD) of 0.064 µM) and 0.25-40.0 µM (LOD of 0.05 µM), respectively. The probes were successfully applied to determine MTZ and NTZ in medicinal samples, real samples, and spiked human plasma, with excellent recovery rates ranging from 99.82 % to 103.03 %. Additionally, S,N@CQDs demonstrate exceptional efficacy as diagnostic luminescent probes for hemoglobin (Hb) detection over a concentration range of 0-900 nM, with a minimal detectability of 9.24 nM, comparable to commercially available medical laboratory diagnostic tests. The eco-friendly synthesis and precise detection limits of S,N@CQDs meet necessary analytical requirements and hold promise for advancing diagnostic capabilities in clinical settings. This research signifies a significant step towards sustainable and efficient fluorescence-based medical diagnostics.

3D printing-based lateral flow visual assay utilizing the dual-excitation property of nitrogen-doped carbon dots for the ratiometric determination and chemometric discrimination of flavonoids.

A ratiometric fluorescence sensor has been established based on dual-excitation carbon dots (D-CDs) for the detection of flavonoids (morin is chosen as the typical detecting model for flavonoids). D-CDs were prepared using microwave radiation with o-phenylenediamine and melamine and exhibit controllable dual-excitation behavior through the regulation of their concentration. Remarkably, the short-wavelength excitation of D-CDs can be quenched by morin owing to the inner filter effect, while the long-wavelength excitation remains insensitive, serving as the reference signal. This contributes to the successful design of an excitation-based ratiometric sensor. Based on the distinct and differentiated variation of excitation intensity, morin can be determined from 0.156 to 110 µM with a low detection limit of 0.156 µM. In addition, an intelligent and visually lateral flow sensing device is developed for the determination  of morin content in real samples with satisfying recoveries, which indicates the potential application for human health monitoring.

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.

Novel immunosensor based on electrochemiluminescence inner filter effect and static quenching between fibrillary Ag-MOGs and SiO2@PANI@AuNPs for enabling the sensitive detection of neuron-specific enolase.

Metal-organic gels (MOGs) are unique supramolecular gels that are convenient to synthesize. In this work, a cathodic electrochemiluminescence (ECL) system based on Ag-MOGs as a luminophore and K2S2O8 as a co-reactor was developed. The ECL spectrum of the Ag-MOGs overlapped significantly with the strong UV-Vis spectrum of the SiO2@PANI@AuNPs, which effectively quenched the ECL luminescence of the Ag-MOGs. Relying on the inner filter effect between Ag-MOGs and SiO2@PANI@AuNPs, a novel ECL-IFE immunosensor was developed for the detection of neuron-specific enolase (NSE). Under optimal conditions, the ECL signal of the immunosensor displayed excellent linearity over the NSE concentration range of 10 fg/mL-100 ng/mL. The limit of detection (LOD) was 2.6 fg/mL (S/N = 3) with a correlation coefficient R2 of 0.9975. The ECL immunosensor also exhibited excellent stability and reproducibility for the detection of NSE. The results reported provide a feasible concept for the development analytical methods for the detection of other clinically relevant biomarkers.

Three in one: coordination-induced emission for inherent fluorescent Al-MOF synthesis combined with inner filter effect@aggregation-induced emission mechanisms for designing color tonality and ratiometric sensing platforms.

Three phenomena, namely coordination-induced emission (CIE), aggregation-induced emission (AIE), and inner filter effect (IFE), were incorporated into the design of a ratiometric and color tonality-based biosensor. Blue fluorescent Al-based metal-organic frameworks (FMIL-96) were prepared from non-emissive ligand and aluminum ions via CIE. Interestingly, the addition of tetracycline (TC) led to ratiometric detection and color tonality, as the blue emission at 380 nm was quenched (when excited at 350 nm) due to IFE, while the green-yellowish emission at 525 nm was enhanced due to AIE. Based on that, an ultra-sensitive visual-based color tonality mode with smartphone assistance was developed for detection of TC. The sensor exhibited a linear relationship within a broad range of 2.0 to 85.0 µM TC with a detection limit of 68.0 nM. TC in milk samples was quantified with high accuracy and precision. This integration of smartphone and visual fluorescence in solution is accurate, reliable, cost-effective, and time-saving, providing an alternative strategy for the semi-quantitative determination of TC on-site.

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.

A Design-assisted Spectrofluorometric Method Utilizing a One-pot Fluorescent Probe for the Quantitation of some Calcium Channel Blockers.

Based on their reaction with highly fluorescent carbon quantum dots (CQDts), a precise and reliable spectrofluorometric approach was developed for the determination of three calcium channel blockers. The studied drugs are: lercanidipine, nimodipine and nifedipine. (CQDts) were produced using a one-step hydrothermal method with ascorbic acid as the carbon source. The produced CQDts were capped by alcohol to create yellow emitters displaying a high fluorescence emission at 524 nm when excited at 325 nm. The fluorescence intensity of CQDts was noticeably quenched by each of the three calcium channel blockers. The relation between their concentrations and fluorescence quenching is linear over the concentration range of 0.5-20 µg/mL for each of the three drugs. A full factorial design was used to optimize the effect of variable factors. Therefore, under optimum experimental design conditions, the detection limits for lercanidipine, nimodipine, and nifedipine were 0.11 ± 1.09, 0.10 ± 0.25 and 0.12 ± 0.71 µg/mL, respectively. The LOQ was 0.33, 0.30, and 0.37 µg/mL respectively. The quenching of fluorescent CQDts occurred through the inner filter effect (IFE) for nimodipine, while it was mixed with dynamic quenching for lercanidipine and nifedipine. The proposed method was effectively used to determine the cited drugs in their pharmaceutical products and had an acceptable level of precision. The selectivity of the CQDts system towards the studied drugs was examined indicating no interference from interfering species.

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.

Upconversion luminescence nanosensor for detection of Fe3+ and phosphate ion based on the inner-filter effect.

In this work, an upconversion luminescence (UCL) nanosensor for fast detection of ferric ion (Fe3+) and phosphate ion (Pi) is developed based on the inner-filter effect (IFE) between NaYF4:Yb/Er upconversion nanoparticles (UCNPs) and Fe3+-hypocrellin B (HB) complex. Fe3+-HB complex has strong absorption band (450-650 nm), which overlaps with the green emission peak of UCNPs at 545 nm. By adding Fe3+ and Pi, the UCNPs-HB system produces the red-shift change of absorption spectrum, which leads to the "on-off-on" process of IFE. So, with the specific recognition ability of HB for Fe3+ and the competitive complexation of Pi for Fe3+, the proposed nanosensor utilizes the UCL change to achieve the detection of the targets. For the detections of Fe3+, the linear range is 10-600 µM with a limit of detection (LOD) of 2.62 µM, and for Pi, the linear range is 5-100 µM with a LOD of 1.25 µM. The results for selectivity, precision, and recovery test are also satisfactory. Furthermore, the real sample detection shows that the proposed nanaosensor has a great potential in environmental and biological systems. An upconversion luminescence (UCL) nanosensor based on the inner-filter effect (IFE) between upconversion nanoparticles (UCNPs) and Fe3+-hypocrellin B (HB) complex for the detection of Fe3+ and phosphate ion has been proposed, which is promising to be a convenient and sensitive assay for monitoring Fe3+ and phosphate ion in different environments and biological systems.

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.

Construction of an "ON-OFF" fluoroprobe using ionic liquids-modified orange peel-based carbon quantum dots for selective/sensitive permanganate assay in waters and the underlying quenching mechanisms.

Herein, we fabricated blue-fluorescence carbon quantum dots modified by ionic liquids (ILs-CQDs) with a quantum yield of 18.13% by employing orange peel as a carbon source and [BMIM][H2PO4] as a dopant. The fluorescence intensities (FIs) of ILs-CQDs were significantly quenched upon the addition of MnO4- with excellent selectivity and sensitivity in waters, and this phenomenon provided a feasibility for constructing a sensitive "ON-OFF" fluoroprobe. The prominent overlapping between the maximum excitation/emission of ILs-CQDs and the UV-Vis absorption of MnO4- implied an inner filter effect (IFE). The higher Kq value demonstrated that the fluorescence-quenching phenomenon was a static-quenching process (SQE). Coordination between MnO4- and oxygen/amino-rich groups in ILs-CQDs resulted in the alteration of zeta potential in the fluorescence system. Consequently, the interactions between MnO4- and ILs-CQDs belong to a joint mechanism of IFE and SQE. When plotting the FIs of ILs-CQDs vs. the concentrations of MnO4-, a satisfactorily linear correlation was obtained across the range of 0.3-100 µM with a detectable limit of 0.09 µM. This fluoroprobe was successfully applied to detect MnO4- in environmental waters with satisfactory recoveries of 98.05-103.75% and relative standard deviations (RSDs) of 1.57-2.68%. Also, it gave more excellent performance metrics as compared to the Chinese standard indirect iodometry method and other previous approaches for MnO4- assay. Overall, these findings offer a new avenue to engineer/develop a highly efficient fluoroprobe based on the combination of ILs and biomass-derived CQDs for the rapid/sensitive detection of metal ions in environmental waters.