Supramolecularly Assembled Ratiometric Fluorescent Sensory Nanosystem for "Traffic Light"-Type Lead Ion or pH Sensing

Synthesis Strategies, Optical Mechanisms, and Applications of Dual-Emissive Carbon Dots

Tuning the optical properties of carbon dots (CDs) and figuring out the mechanisms underneath the emissive phenomena have been one of the most cutting-edge topics in the development of carbon-based nanomaterials. Dual-emissive CDs possess the intrinsic dual-emission character upon single-wavelength excitation, which significantly benefits their multi-purpose applications. Explosive exploitations of dual-emissive CDs have been reported during the past five years. Nevertheless, there is a lack of a systematic summary of the rising star nanomaterial. In this review, we summarize the synthesis strategies and optical mechanisms of the dual-emissive CDs. The applications in the areas of biosensing, bioimaging, as well as photoelectronic devices are also outlined. The last section presents the main challenges and perspectives in further promoting the development of dual-emissive CDs. By covering the most vital publications, we anticipate that the review is of referential significance for researchers in the synthesis, characterization, and application of dual-emissive CDs.

A ratiometric electrochemical sensor for detecting lead in fish based on the synergy of semi-complementary aptamer pairs and Ag nanowires@zeolitic imidazolate framework-8

This work describes the synergistic application of semi-complementary aptamer pairs and signals on-off ratio strategies on glassy carbon electrodes (GCE) for detecting lead ions (Pb²⁺) in fish. Gold nanoparticles (AuPNs) as the electrode substrate can provide added binding sites for the aptamers and improve the conductivity of the electrodes. Pb²⁺ aptamers containing ferrocene (Fc) molecules act as molecular recognizers in the sensing system. In the presence of target ions, Fc signals are affected by conformational changes of the aptamer. The "Ag nanowires@zeolitic imidazolate framework-8 with methylene blue (AgNWs@ZIF-8/MB)" can be semi-complementary to the Pb²⁺ aptamer after binding to single-stranded DNA (S₁). However, S₁/AgNWs@ZIF-8/MB self-assembled with Pb²⁺ aptamer (Apt) by hybridization incubation was quickly replaced by Pb²⁺ competitively, resulting in the loss of methylene blue (MB) signaling molecules. Hence, the internal reference signal (MB) and conformation change signal (Fc) comprise the ratio sensing system well. Morphology, spectroscopy, and electrochemistry methods have validated the modification and sensing behaviors. The used Apt has made considerable progress in analytical performance. In interference studies and stability checks, the ratio measurement signal I_Fc/I_MB is a more reliable signal than the single signal readout. Following a log-linear relationship, this sensor provides a wide linear range. Furthermore, the proposed sensor can be used to determine Pb²⁺ in fish samples, and the results agree with those obtained using ICP-MS and recovery tests.

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

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

Cellulose-Based Fluorescent Material for Extreme pH Sensing and Smart Printing Applications

Environment-responsive fluorescence materials are being widely investigated for instrument-free determination of various environmental factors. However, developing an eco-friendly cellulose-based fluorescent pH sensor for sensing extreme acidity and alkalinity is still challenging. Herein, a highly fluorescent and multifunctional material is developed from biopolymer-based cellulose acetate. A biopolymer-based structure containing responsive functional groups such as -C═O and -NH is constructed by chemically bonding 5-amino-2,3-dihydrophthalazine-1,4-dione (luminol) onto cellulose acetate using 4,4'-diphenylmethane diisocyanate (MDI) as a cross-linking agent. The prepared material (Lum-MDI-CA) is characterized by UV-vis, Fourier transform infrared, ¹H NMR, ¹³C NMR spectroscopies, and fluorescence techniques. The material exhibits excellent aqua blue fluorescence and demonstrates extreme pH sensing applications. Interesting results are further revealed after adding a pH-unresponsive dye such as MTPP as the reference to develop the ratiometric method. The ratiometric system clearly differentiates the extreme acidic pH 1 from pH 2 and extreme alkaline pH 12, 13, and 14 by visual and fluorescence color change response under a narrow pH range. In addition, the material is fabricated into transparent flexible fluorescent films which demonstrate an outstanding UV shielding, security printing, and haze properties for smart food packaging and printing applications.

Ag2ZnSnS4-ZnS core-shell colloidal quantum dots: a near-infrared luminescent material based on environmentally friendly elements

Low cost, multinary colloidal quantum dots (QDs) based on environmentally friendly elements, with bright, narrow-width, tunable near-infrared (NIR) luminescence are promising alternatives to Cd and Pb chalcogenide QDs for in vivo bio-imaging, LED and sensing applications. Herein, we demonstrate Pb/Cd free solution-processed colloidal luminescent Ag2ZnSnS4-ZnS (AZTS-ZnS) core-shell QDs with precise control over the ZnS shell thickness and thereby its optical properties. Unlike indium based multinary (I-III-VI group) core-shell QDs these nanocrystals show a narrow photoluminescence (PL) full width at half maximum (fwhm) of 105-110 meV in the first NIR window. By monitoring the starting AZTS core size, we achieve tunable emission over a small NIR window in these QDs with the best PL quantum yield (PLQY) of 17.4%.

Nanomaterial-based fluorescent sensors for the detection of lead ions

Lead (Pb) poisoning has been a scourge to the human to pose sighnificant health risks (e.g., organ disorders, carcinogenicity, and genotoxicity) as observed from many different parts of the world, especially in developing countries. The demand for accurate sensors for its detection, especially in environmental media (soil, water, food, etc.) has hence been growing steadily over the years. The potential utility of fluorescent nanosensors as an important analytical tool is recognized due to their astonishing characteristics (e.g., high sensitivity/selectivity, enhanced detection performance, low cost, portability, and rapid on-site detection ability). This review is organized to offer insight into the recent developments in fluorescent nanosensing technology for the detection of lead ions (Pb²⁺). To this end, different types of nanomaterials explored for such applications have been classified and evaluated with respect to performance, especially in terms of sensitivity. This review will help researchers gain a better knowledge on the status and importance of optical nanosensors so as to remediate the contamination of lead and associated problems. The technical challenges and prospects in the development of nanosensing systems for Pb²⁺ are also discussed.

A rapid and simple ratiometric fluorescent sensor for patulin detection based on a stabilized DNA duplex probe containing less amount of aptamer-involved base pairs

In this study, a sensor is described for determination of patulin by using ratiometric fluorescence measurement and strand displacement strategy. In the presence of patulin, the ratiometric fluorescence response decreases, owing to disassembly of DNA duplex structure and target-mediated release of TAMRA-labeled complementary DNA sequence2 (cDNA2). While, in the absence of target, the fluorescence resonance energy transfer (FRET) phenomenon happens between FAM and TAMRA under excitation at 490 nm, resulting in the enhancement of ratiometric signal. The use of ratiometric fluorescence signal with different signal indicators avoids the problem of environmental interference and improves the sensitivity of the aptasensor. Also, the DNA duplex structure contains minimum aptamer-involved base pair sequence, resulting in further improvement of the aptasensor sensitivity. This sensing platform provided a wide linear range from 15 ng/L to 35 μg/L and a detection limit of 6 ng/L for patulin. The aptasensor was used to determine patulin in spiked apple juice samples and showed satisfactory results.

A simple paper-based aptasensor for ultrasensitive detection of lead (II) ion

In this study, a simple paper-based aptasensor has been developed for the ultrasensitive detection of lead (Pb²⁺) ion within about 10 min. The aptasensor has been successfully designed by taking advantages of the Förster Resonance Energy Transfer (FRET) process and the super fluorescence quenching property of graphene oxide (GO) sheet. The sensing mechanism of the aptasensor is based on the conformational switch of the Pb²⁺-specific aptamer from a random coil to a G-quadruplex structure. An injection of Pb²⁺ on the paper-based platform induces the release of the specific aptamer from the GO surface that recovers the fluorescence emission. Under the optimal experimental conditions, there is a good linear relationship between the fluorescence recovery and the Pb²⁺concentration in the ranges of 5-70 pM and 0.07-20 nM. Moreover, the aptasensing array exhibits a high sensitivity to Pb²⁺ with an ultra-low detection limit of 0.5 pM. The developed aptasensor has been successfully applied to determine Pb²⁺ in tap water, lake water, milk, and human blood serum. The paper-based aptasensor can be efficiently utilized to detect other metal ions and biological molecules by substituting target specific aptamer.

Biomimetic Sensing System for Tracing Pb2+ Distribution in Living Cells Based on the Metal-Peptide Supramolecular Assembly

Metal-peptide interactions provide plentiful resource and design principles for developing functional biomaterials and smart sensors. Pb²⁺, as a borderline metal ion, has versatile coordination modes. The interference from competing metal ions and endogenous chelating species greatly challenges Pb²⁺ analysis, especially in complicated living biosystems. Herein, a biomimetic peptide-based fluorescent sensor GSSH-2TPE was developed, starting from the structure of a naturally occurring peptide glutathione. Lewis acid-base theory was employed to guide the molecular design and tune the affinity and selectivity of the targeting performance. The integration of peptide recognition and aggregation-induced emission effect provides desirable sensing features, including specific turn-on response to Pb²⁺ over 18 different metal ions, rapid binding, and signal output, as well as high sensitivity with a detection limit of 1.5 nM. Mechanism investigation demonstrated the balance between the chelating groups, and the molecular configuration of the sensor contributes to the high selectivity toward Pb²⁺ complexation. The ion-induced supramolecular assembly lights up the bright fluorescence. The ability to image Pb²⁺ in living cells was exhibited with minimal interference from endogenous biothiols, no background fluorescence, and good biocompatibility. With good cell permeability, GSSH-2TPE can monitor changes in Pb²⁺ levels and biodistribution and thus predict possible damage pathways. Such metal-peptide interaction-based sensing systems offer tailorable platforms for designing bioanalytical tools and show great potential for studying the cell biology of metal ions in living biosystems.