An optical sensing system with ratiometric and turn-off dual-mode of CDs@MnO2 nanosheets for the determination of H2O2 and glucose based on a combination of first-order scattering, fluorescence, and second-order scattering

Physicochemically modulated fluorescence-scattering ratiometric sensor for selective and visual detection of levodopa

In this study, a facile fluorescence-scattering ratiometric sensor was designed for visual and selective detection of levodopa (LD) via a clever physicochemical modulation scheme. The alkalized products of LD can rapidly react with polyethyleneimine (PEI) to exhibit an intense blue fluorescence and decrease the second-order scattering (SOS) signal of PEI. As the concentration of LD increased, the fluorescence intensity at 420 nm increased and the SOS intensity at 675 nm decreased synchronously. Thus the fluorescence-scattering ratiometric sensor was constructed by virtue of the two simultaneously changed signals. Furthermore, red light-emitting Au nanoclusters (AuNCs) were added into the above mixture solution to enlarge the SOS signal and provide a stable red background fluorescence. The intensity ratio of fluorescence to SOS (F/(S/Sblank)) is linear dependent on CLD in the wide range of 50.0---30000.0 nM, and LD as low as 50.0 nM can be identified with the naked eye via change of fluorescence color. The developed ratiometric sensor is smart, simple and efficient, and has been applied to the convenient assay of LD in real samples. The proposed physicochemical modulation strategy provides a new and facile path for selectively and visually identifying the target from its analogues.

A dually emissive MPA-CdTe QDs@N, S-GQD nanosensor for sensitive and selective detection of 4-nitrophenol using two turn-off signals

4-Nitrophenol (4-NP) is an extremely poisonous and carcinogenic phenol that poses serious health issues to humans. Therefore, it becomes highly demanded and urgent to determine 4-NP in water samples. In this study, we developed a facile and effective dually-emissive nanosensor containing simply mixed CdTe quantum dots (CdTe QDs) and N, S modified graphene quantum dots (N, S-GQDs) for 4-NP. The synthesized CdTe QDs and N, S-GQDs exhibited excitation-independent emission located at 540 nm and 420 nm, respectively. The nanosensor displayed two turn-off fluorescent signals when exposed to 4-NP. The degree of quenching varied depending on the excitation wavelength range used, which can be explained by the quenching phenomenon based on the inner filter effect (IFE). Moreover, analysis of the recorded excitation-emission matrix (EEM) data using the parallel factor analysis (PARAFAC) technique revealed a negative emission spectrum corresponding to non-emissive 4-NP. On the other hand, the species with no peak in fluorescence data had a negative spectrum as the PARAFAC emission loading. Under the optimized conditions, the CdTe QDs@GQD nanosensor achieved fast and highly sensitive detection of 4-NP within the concentration range of 0.0-30.0 μM, with a detection limit of 0.52 μΜ.

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.

A wide-range ratiometric sensor mediating fluorescence and scattering based on carbon dots/metal-organic framework composites for the detection of bisulfite/sulfite in sugar

Bisulfite (HSO₃^-) and sulfite (SO₃^2-) are commonly employed in food preservatives and are also significant environmental pollutants. Thus, developing an effective method for detecting HSO₃^-/SO₃^2- is crucial for food safety and environment monitoring. In this work, based on carbon dots (CDs) and zeolitic imidazolate framework-90 (ZIF-90), a composite probe (named CDs@ZIF-90) is constructed. The fluorescence signal and the second-order scattering signal of CDs@ZIF-90 are employed to ratiometricly detect HSO₃^-/SO₃^2-. This proposed strategy exhibits a broad linear range for HSO₃^-/SO₃^2- determination (10 µM to 8.5 mM) with a limit of detection of 2.74 μM. This strategy is successfully applied for evaluating HSO₃^-/SO₃^2- in sugar with satisfactory recoveries. Therefore, this work has uniquely combined the fluorescence and second-order scattering signals to establish a novel sensing system with a wide linear range, which is applicable for ratiometric sensing of HSO₃^-/SO₃^2- in actual samples.