Organic fluorophores-based molecular probes with dual-fluorescence ratiometric responses to in-vitro/in-vivo pH for biosensing, bioimaging and biotherapeutics applications

In recent years, organic fluorophores-based molecular probes with dual-fluorescence ratiometric responses to in-vitro/in-vivo pH (DFR-MPs-pH) have been attracting much interest in fundamental application research fields. More and more scientific publications have reported the exploration of various DFR-MPs-pH systems that have unique dual-fluorescence ratiometry as the signal output, in-built and signal self-calibration functions to improve precise detection of targets. DFR-MPs-pH systems possess high-performance applications in biosensing, bioimaging and biomedicine fields. This review has comprehensively summarized recent advances of DFR-MPs-pH for the first time. First of all, the compositions and types of DFR-MPs-pH are introduced by summarizing different organic fluorophores-based molecule systems. Then, construction strategies are analyzed based on specific components, structures, properties and functions of DFR-MPs-pH. Afterward, biosensing and bioimaging applications are discussed in detail, primarily referring to pH sensing and imaging detection at the levels of living cells and small animals. Finally, biomedicine applications are fully summarized, majorly involving bio-toxicity evaluation, bio-distribution, biomedical diagnosis and therapeutics. Meanwhile, the current status, challenges and perspectives are rationally commented after detailed discussions of representative and state-of-the-art studies. Overall, this present review is comprehensive, in-time and in-depth, and can facilitate the following further exploration of new and versatile DFR-MPs-pH systems toward rational design, facile preparation, superior properties, adjustable functions and highly efficient applications in promising fields.

Histidine-capped copper nanoclusters for in situ amplified fluorescence monitoring of doxycycline through inner filter effect

There is a significant need to accurately measure doxycycline concentrations in view of the adverse effects of an overdose on human health. A fluorescence (FL) detection method was adopted and copper nanoclusters (CuNCs) were synthesized using chemical reduction technology. Based on FL quenching with doxycycline, the prepared CuNCs were used to explore a fluorescent nanoprobe for doxycycline detection. In an optimal sensing environment, this FL nanosensor was sensitive and selective in doxycycline sensing and displayed a linear relationship in the range 0.5-200 μM with a detection limit of 0.092 μΜ. A characterization test demonstrated that CuNCs offered active functional groups for identifying doxycycline using electrostatic interaction and hydrogen bonds. Static quenching and the inner filter effect (IFE) resulted in weakness in the FL of His@CuNCs with doxycycline with great efficiency. This suggested nanosensor was revealed to be a functional model for simple and rapid detection of doxycycline in real samples with very pleasing accuracy.

Synthesis of Greenish-Yellow Fluorescent Copper Nanocluster for the Selective and Sensitive Detection of Fipronil Pesticide in Vegetables and Grain Samples

In this paper, a new synthetic route is introduced for the synthesis of high-luminescent greenish-yellow fluorescent copper nanoclusters (PVP@A. senna-Cu NCs) using Avaram senna (A. senna) and polyvinylpyrrolidone (PVP) as templates. A. senna plant extract mainly contains variety of phytochemicals including glycosides, sugars, saponins, phenols, and terpenoids that show good pharmacological activities such as anti-inflammatory, antioxidant, and antidiabetic. PVP is a stable and biocompatible polymer that is used as a stabilizing agent for the synthesis of PVP@A. senna-Cu NCs. The size, surface functionality, and element composition of the fabricated Cu NCs were confirmed by various analytical techniques. The as-prepared greenish-yellow fluorescent Cu NCs exhibit significant selectivity towards fipronil, thereby favoring to assay fipronil pesticide with good linearity in the range of 3.0-30 μM with a detection limit of 65.19 nM. More importantly, PVP@A. senna-Cu NCs are successfully applied to assay fipronil in vegetable and grain samples.

A ratiometric fluorescence sensor based on gold silver nanoclusters and tungsten disulfide quantum dots with simple fabrication for the detection of copper ions in river water

Copper plays a key role in the human body; meanwhile, excess Cu²⁺ ions can result in various diseases. Nanoclusters (NCs) are often used to measure Cu²⁺ ions, but there are two difficulties. On the one hand, a single probe of NCs is easily affected by environmental factors. On the other hand, it is difficult to mask the interference of Pb²⁺ ions and Cd²⁺ ions in the process of detecting Cu²⁺ ions. As a new type of quantum dots (QDs), tungsten disulfide quantum dots (WS₂-QDs) have some advantages of simple synthesis and stable luminescence properties. Stable WS₂-QDs with blue fluorescence are used as a reference probe, while gold silver nanoclusters (AuAgNCs) with red fluorescence are used as a response probe. A ratiometric fluorescent sensor was constructed by mixing the two styles of fluorescent probes, which is abbreviated as NCs/QDs. This nano-sensor can be used to detect the concentration of Cu²⁺ ions, in which the fluorescence of QDs does not change significantly, while the fluorescence of NCs can be quenched by Cu²⁺ ions. The concentration of Cu²⁺ ions can be determined as low as 0.12 μM with a linear range from 0.3 to 3 μM. The common interference caused by Pb²⁺ and Cd²⁺ ions can be eliminated by the phosphate buffer solution (PBS). This sensor was used to detect the concentration of Cu²⁺ in river water with satisfactory results.

Highly Sensitive Detection of Chymotrypsin Based on Metal Organic Frameworks with Peptides Sensors

In this study, peptides and composite nanomaterials based on copper nanoclusters (CuNCs) were used to detect chymotrypsin. The peptide was a chymotrypsin-specific cleavage peptide. The amino end of the peptide was covalently bound to CuNCs. The sulfhydryl group at the other end of the peptide can covalently combine with the composite nanomaterials. The fluorescence was quenched by fluorescence resonance energy transfer. The specific site of the peptide was cleaved by chymotrypsin. Therefore, the CuNCs were far away from the surface of the composite nanomaterials, and the intensity of fluorescence was restored. The limit of detection (LOD) using Porous Coordination Network (PCN)@graphene oxide (GO) @ gold nanoparticle (AuNP) sensor was lower than that of using PCN@AuNPs. The LOD based on PCN@GO@AuNPs was reduced from 9.57 pg mL^-1 to 3.91 pg mL^-1. This method was also used in a real sample. Therefore, it is a promising method in the biomedical field.

Ratiometric upconversion luminescence nanoprobes from construction to sensing, imaging, and phototherapeutics

In terms of the combined advantages of upconversion luminescence (UCL) properties and dual-signal ratiometric outputs toward specific targets, the ratiometric UCL nanoprobes exhibit significant applications. This review summarizes and discusses the recent advances in ratiometric UCL nanoprobes, mainly including the construction of nanoprobe systems for sensing, imaging, and phototherapeutics. First, the construction strategies are introduced, involving different types of nanoprobe systems, construction methods, and ratiometric dual-signal modes. Then, the sensing applications are summarized, involving types of targets, sensing mechanisms, sensing targets, and naked-eye visual detection of UCL colors. Afterward, the phototherapeutic applications are discussed, including bio-toxicity, bio-distribution, biosensing, and bioimaging at the level of living cells and small animals, and biomedicine therapy. Particularly, each section is commented on by discussing the state-of-the-art relevant studies on ratiometric UCL nanoprobe systems. Moreover, the current status, challenges, and perspectives in the forthcoming studies are discussed. This review facilitates the exploration of functionally luminescent nanoprobes for excellent sensing, imaging, biomedicine, and multiple applications in significant fields.

In situ reaction-based ratiometric fluorescent assay for alkaline phosphatase activity and bioimaging

Alkaline phosphatase (ALP) is an important biomarker, it is of great significance to develop a sensitive and efficient analytical method for ALP. In this study, an in situ reaction based ratiometric fluorescence assay for ALP was proposed. l-ascorbic acid-2-phosphate (AA2P) was used as a substrate for ALP, and Cu²⁺/o-phenylenediamine (OPD) were involved in this system. Cu²⁺ can oxidize OPD to 2,3-diaminophenazine (OPDox) with an emission centered at 566 nm. The presence of ALP can catalyze the hydrolysis of AA2P to ascorbic acid (AA), which will inhibit the production of OPDox and reduce the corresponding fluorescence intensity, and AA will react with OPD to generate 3-(dihydroxyethyl)furan[3,4-b]quinoxalin-1-one (DFQ) with an emission peak at 447 nm. The fluorescence ratio of F447/F566 has a linear relationship with ALP activity. The proposed method is highly sensitive, finely selective, cost efficiency and easy to operate, it exhibits good linearity in the range of 0.5-22 and 22-40 mU·mL^-1, with a detection limit as low as 0.06 mU·mL^-1. The excellent applicability of this strategy in human serum samples and MCF-7 cells imaging suggests that this method has promising prospects for biomedical research.

Ratiometric detection of doxycycline in pharmaceutical based on dual ligands-enhanced copper nanoclusters

A water-soluble, stable, simple and dual ligands (bovine serum albumin and L-histidine)-enhanced copper nanoclusters (BSA-CuNCs@L-His) was synthesized by one-step wet chemical method. Interestingly, the introduction of L-His ligand could improve evidently the quantum yields (QYs, 3.47%) and stability of BSA-CuNCs due to forming the stronger interaction of L-His and Cu and producing bigger diameter CuNCs by coordination-induced aggregation. Thus, a new ratiometric fluorescent probe (RF-probe) was successfully exploited for sensitively and selectively mensurating doxycycline (DOX) because DOX could simultaneously regulate the fluorescence (FL) intensities of BSA-CuNCs@L-His at 410 and 520 nm. The FL quenching of BSA-CuNCs@L-His at 410 nm by DOX was mainly originated from the static quenching process, while DOX could bind to Trp-212 in BSA from the skeleton of BSA-CuNCs@L-His by electrostatic interaction causing the appearance of new emission peak at 520 nm. The content of DOX was monitored by the RF-probe with a linear range of 0.05-14.0 μM and a LOD (limit of detection) and LOQ (limit of quantification) of 6.4 and 21.3 nM (at 3σ/slope and 10σ/slope). Moreover, compared to the standard HPLC method, the proposed RF-probe was extended to the detection of DOX in doxycycline hydrochloride (DOXH) tablets, DOXH injections and DOXH capsules with satisfactory results.

Synthesis of bio-templated clickable quantum dots and dual-emitting organic/inorganic complex for ratiometric fluorescence visual assay of blood glucose

With the prevalence of diabetes, rapid and simple blood glucose monitoring becomes more and more important. Here, we reported the synthesis of the bio-templated N3-CdZnTeS quantum dots (QDs), which was...

Copper nanoclusters: designed synthesis, structural diversity, and multiplatform applications

Atomically precise metal nanoclusters (MNCs) have gained tremendous research interest in recent years due to their extraordinary properties. The molecular-like properties that originate from the quantized electronic states provide novel opportunities for the construction of unique nanomaterials possessing rich molecular-like absorption, luminescence, and magnetic properties. The field of monolayer-protected metal nanoclusters, especially copper, with well-defined molecular structures and compositions, is relatively new, about two to three decades old. Nevertheless, the massive progress in the field illustrates the importance of such nanoobjects as promising materials for various applications. In this respect, nanocluster-based catalysts have become very popular, showing high efficiencies and activities for the catalytic conversion of chemical compounds. Biomedical applications of clusters are an active research field aimed at finding better fluorescent contrast agents, therapeutic pharmaceuticals for the treatment and prevention of diseases, the early diagnosis of cancers and other potent diseases, especially at early stages. A huge library of structures and the compositions of copper nanoclusters (CuNCs) with atomic precisions have already been discovered during last few decades; however, there are many concerns to be addressed and questions to be answered. Hopefully, in future, with the combined efforts of material scientists, inorganic chemists, and computational scientists, a thorough understanding of the unique molecular-like properties of metal nanoclusters will be achieved. This, on the other hand, will allow the interdisciplinary researchers to design novel catalysts, biosensors, or therapeutic agents using highly structured, atomically precise, and stable CuNCs. Thus, we hope this review will guide the reader through the field of CuNCs, while discussing the main achievements and improvements, along with challenges and drawbacks that one needs to face and overcome.

Visual bio-detection and versatile bio-imaging of zinc-ion-coordinated black phosphorus quantum dots with improved stability and bright fluorescence

Zero-dimensional black phosphorus quantum dots (BPQDs) have unique structural characteristics and excellent properties for promising applications over other BP structures. With the decrease of BP stacked layers, BP becomes much unstable and is easy to be oxidized and degraded in air and water. To prevent BP oxidation and degradation is crucial during the preparation process of highly stable BPQDs with strong fluorescence (FL). Herein, we explored a zinc-ion-coordinated strategy to achieve the emerging Zn@BPQDs with improved colloidal and FL stabilities. Zn ions can be adsorbed on BP surface via cation-π interactions, which passivates lone pair electrons of phosphorus and makes BP much stable upon exposure to air and water. Zn@BPQDs were prepared through sonication-assisted liquid-phase exfoliation of bulk BP crystals in the presence of Zn ions and solvothermal reaction of exfoliated few-layer Zn@BP nanosheets. Experimental results confirm the preparation of Zn@BPQDs with improved stability and high FL. Zn@BPQDs were used for both FL spectral detection and naked-eye visual FL detection of glutathione in practical samples. As emerging FL reagents, biocompatible Zn@BPQDs were further used for efficient in-vitro cell imaging and in-vivo imaging in natural plants and living aquatic animals.

Biocompatible thiolate protected copper nanoclusters for an efficient imaging of lung cancer cells

We report, the one-pot synthesis of water-soluble and biocompatible 3-mercaptopropylsulfonate (MPS) protected novel copper nanoclusters (CuNCs). Interestingly, the TEM image of MPS protected CuNCs exhibits an ultrasmall nanoclusters of particle size <2-nm, similar to its Au and Ag analogue. The hydrophilic and biocompability property of thiolate protected CuNCs. i.e., MPS stabilized CuNCs and its luminescent nature gave rise to maximum quantum yield of 1.5%. Further, as achieved CuNCs was investigated for haemocompatibility, cell viability and fluorescent microscopic analysis with A549 lung cancer cell line. Haemolytic study was examined using human RBCs in the concentration range of 4 to 22 μg/mL for which 7.5% of haemolysis was obtained for an optimum concentration of 22 μg/mL of CuNCs. The cell viability analysis was carried out by MTT assay using A549 lung cancer cells for the minimum (10 μg/mL) and maximum (45 μg/mL) concentration of CuNCs which reports 93.1% and 38.2% cell viability respectively. The inverted light microscopic images from the control and CuNCs treated (20 μg/mL) cells exhibited an excellent biocompatibility with a normal morphology. Upon increasing the concentration of CuNCs upto 45 μg/mL, the cell viability trends to decrease and the cell morphology also denature gradually. Further, the bio-imaging application of CuNCs was analyzed with A549 lung cancer cells. The efficient imaging with CuNCs treated (20 μg/mL) A549 cells resulted in a green colour emission using FITC filter (460- 490 nm). Thereby the obtained results confirm the applicability of CuNCs for the biomedical and cancer diagnosis applications.

Assembly of Black Phosphorus Nanosheets and MOF to Form Functional Hybrid Thin-Film for Precise Protein Capture, Dual-Signal and Intrinsic Self-Calibration Sensing of Specific Cancer-Derived Exosomes

As the emerging and noninvasive biomarkers, exosomes play an important role in cancer screening, cancer-related immune response, and the physiological process. The sensitive, specific, and efficient detection of cancer cell-derived exosomes is of significance for early cancer diagnosis of patients. This work developed a novel dual-signal and intrinsic self-calibration aptasensor of exosomes based on a functional hybrid thin-film platform. This platform was constructed via facile assembly of black phosphorus nanosheets (BPNSs) and ferrocene (Fc)-doped metal-organic frameworks (ZIF-67) on indium tin oxide (ITO) slice, followed by combining methylene blue (MB)-labeled single- strand DNA aptamer on ITO slice. The resultant aptamer-BPNSs/Fc/ZIF-67/ITO platform had dual redox-signal responses of MB (labeled on aptamer) and Fc (doped into ZIF-67). In the presence of specific cancer cell-derived exosomes, the redox current of MB regularly reduced and that of Fc (as reference) hardly changed. An intrinsic self-calibration aptasensor was achieved and enabled sensitive detection of exosomes, showing a limit of detection down to 100 particles mL^-1. The aptasensor with a capability of precise protein capture can efficiently determine specific cancer cell-derived exosomes in practical human serum and plasma samples from healthy individuals and breast cancer patients. In light of excellent performances, this aptasensor can be expanded to multiple biomarkers from cell line exosomes and is beneficial for exploring advanced techniques for high-performance detection of exosomes derived from different types of cancer cells. This work promotes the development of current techniques for early cancer screening and clinical diagnosis.

A new dual-recognition strategy for hybrid ratiometric and ratiometric sensing perfluorooctane sulfonic acid based on high fluorescent carbon dots with ethidium bromide

In this work, a novel strategy for perfluorooctane sulfonic acid (PFOS) detection was established by using a ratiometric nanosensor with the combination of fluorescence and second-order scattering (SOS). The practical ratiometric nanosensor was synthesized simply by mixing fluorescent dye ethidium bromide (EB) and nitrogen doped carbon dots (N-CDs) which was synthesized by a one-step hydrothermal method with Victoria Blue B, possessing three emission peaks at 472 nm, 560 nm and 600 nm under a single wavelength excitation of 280 nm, respectively. The EB served as the reference signal label, and the N-CDs, having response to the analytes, acted as the response signal label. To achieve ratiometric detection, the fluorescence emission of the N-CDs was turned off and the SOS emission was turned on with the addition of the target PFOS. To achieve colorimetric detection, with the help of EB, the fluorescence of the system changed from green to orange. Under the optimal conditions, the difference of F₄₇₂/I₅₆₈ of the nanosensor had good linearity against the concentrations of PFOS within a linear range of 0-2.0 μM. The limit of detection was as low as 27.8 nM, which was low enough for the detection of PFOS in water samples. The proposed method has been successfully applied to the detection of PFOS with RSD <1.67%. The results show that the as-prepared N-CDs/EB ratiometric nanosensor has potential application for the detection of PFOS in environmental monitoring.

The synthesis of switch-off fluorescent water-stable copper nanocluster Hg2+ sensors via a simple one-pot approach by an in situ metal reduction strategy in the presence of a thiolated polymer ligand template

The fabrication of stable fluorescent copper nanoclusters (CuNCs) in aqueous media is still challenging, despite the low price and potential biomedical applications. Herein, we report a facile and efficient strategy for assembling CuNCs using multifunctional thiolated copolymers with pH and thermoresponsive features. The new nanohybrids are formed via a simple one-pot approach through the reduction of a copper salt with hydrazine in the presence of a multithiolated polymer, which provides a template during nanocluster assembly and further efficient protection against oxidation and aggregation. Furthermore, the thermo- and pH-responsive properties of the pristine copolymers endow the nanohybrids with these stimuli-responsive features. The thiol content and the macromolecular size of the polymer ligands exert strong influences on the final photophysical properties of these new hybrid luminescent nanoclusters. The existence of stable bright greenish-yellow emission in water over long periods of time, the high photostability under UV irradiation and the strong oxidation resistance toward hydrogen peroxide of the hybrid CuNCs suggest that they have great promise for nanomedicine, bioassay and nanosensor use. Furthermore, the polymeric CuNCs obtained have been successfully tested as optical switch-off sensors for the sensitive and highly selective detection of Hg2+ in the presence of other metal ions in liquid and solid states. Finally, we demonstrate the practical application of the new hybrid to Hg2+ detection in human urine.

A ratiometric electrochemical sensor for lead ions based on bismuth film coated porous silicon nanoparticles

A ratiometric electrochemical sensor for the detection of lead ions was developed based on porous silicon nanoparticles with in situ plated bismuth to improve the accuracy and reliability.

Ultrasensitive biosensor for microRNA-155 using synergistically catalytic nanoprobe coupled with improved cascade strand displacement reaction

MicroRNAs, essential for gene expression and physiological regulation, are considered to be reliable biomarkers for the early diagnosis and treatment of cancers. Herein, a sensitive biosensor that uses a synergistically catalytic nanoprobe and improved toehold strand displacement reaction (TSDR) has been fabricated, and successfully applied to microRNA-155 (miR-155) detection. A nanoscale copper-based metal organic framework assembled by Pt nanoparticles and horseradish peroxidase (Cu-NMOF@PtNPs/HRP) served as a co-catalytic nanoprobe and was coupled with improved TSDR to achieve multiple amplifications. In the absence of miR-155, the tetrahedral DNA nanostructures (TDNs) immobilized on the gold electrode were independent of the TSDR system because of the binding of the shielding region of the locked probe (LP) with the template probe (TP). Instead, the target would initiate the TSDR system, leading to the conformational change of TDNs and hybridization of the nanoprobe. Cu-NMOF@PtNPs/HRP exhibited extraordinary catalytic property towards the hydroquinone-hydrogen peroxide system, demonstrating that the nanoprobe exerted a concerted effect on the electrochemical performance of the biosensor. Under optimal conditions, the cathodic current exhibited a logarithmic relation over 0.50-1.0 × 10⁵fM miR-155, with a detection limit of 0.13 fM, indicating that the constructed biosensor has considerable potential in the field of clinical disease diagnostics for miR-155.

Preparation and applications of electrochemical chemosensors based on carbon-nanomaterial-modified molecularly imprinted polymers

The past few decades have witnessed a rapid development in electrochemical chemosensors (ECCSs). The integration of carbon nanomaterials (CNMs) and molecularly imprinted polymers (MIPs) has endowed ECCSs with high selectivity and sensitivity toward target detection. Due to the integrated merits of MIPs and CNMs, CNM-modified MIPs as ECCSs have been widely reported and have excellent detection applications. This review systematically summarized the general categories, preparation strategies, and applications of ECCSs based on CNM-modified MIPs. The categories include CNM-modified MIPs often hybridized with various materials and CNM-encapsulated or CNM-combined imprinting silica and polymers on working electrodes or other substrates. The preparation strategies include the polymerization of MIPs on CNM-modified substrates, co-polymerization of MIPs and CNMs on substrates, drop-casting of MIPs on CNM-modified substrates, self-assembly of CNMs/MIP complexes on substrates, and so forth. We discussed the in situ polymerization, electro-polymerization, and engineering structures of CNM-modified MIPs. With regard to potential applications, we elaborated the detection mechanisms, signal transducer modes, target types, and electrochemical sensing of targets in real samples. In addition, this review discussed the present status, challenges, and prospects of CNM-modified MIP-based ECCSs. This comprehensive review is desirable for scientists from broad research fields and can promote the further development of MIP-based functional materials, CNM-based hybrid materials, advanced composites, and hybrid materials.

Integrating Target-Responsive Hydrogels with Smartphone for On-Site ppb-Level Quantitation of Organophosphate Pesticides

Precise on-site profiling of organophosphate pesticides (OPs) is of significant importance for monitoring pollution and estimating poisoning. Herein, we designed a simple and convenient portable kit based on Ag⁺-responsive hydrogels for accurate detection of OPs. The newly developed hydrogels employed o-phenylenediamine (OPD) and silicon quantum dots (SiQDs) as indicator, which possessed ratiometric response. In this sensor, OPs as inhibitor of acetylcholinesterase prevented the generation of thiocholine, which blocked the formation of metal-polymer with Ag⁺, further triggered the oxidation of OPD to yield yellow 2,3-diaminophenazine (DAP) with fluorescence emission at 557 nm. The fluorescence intensity of SiQDs (444 nm) was quenched by DAP through inner filter effect (IFE) process, emerging a typical ratiometric response. Interestingly, the ratiometric signal of kit, which was recorded by smartphone's camera, can be transduced by ImageJ software into the hue parameter that was linearly proportional to the concentration of OPs. The simplicity of portable kit combined with smartphone operation, which possessed high sensitivity (detection limit <10 ng mL^-1) and rapid sample-to-answer detection time (45 min) in agricultural sample, indicating that the methodology offered a new sight for portable monitoring of food safety and human health.

Ketjen black/ferrocene dual-doped MOFs and aptamer-coupling gold nanoparticles used as a novel ratiometric electrochemical aptasensor for vanillin detection

In this work, a facile ratiometric electrochemical aptasensor was developed towards sensitive and selective detection of vanillin, based on Ketjen black/ferrocene dual-doped zeolite-like MOFs (Fc-KB/ZIF-8) and electrodeposited gold nanoparticles (AuNPs) coupling with DNA aptamer. Fc-KB/ZIF-8 composites were prepared via one-pot solvothermal reaction and drop-coated on glassy carbon electrode (GCE) surface to form Fc-KB/ZIF-8@GCE. AuNPs were in-situ electro-deposited on the modified GCE. 5'-SH terminated aptamer of vanillin was combined with AuNPs via Au-S coupling to form aptamer-AuNPs/Fc-KB/ZIF-8@GCE as a new sensing platform. Under optimal conditions, electrochemical (square wave voltammetry) curves of this sensing platform were measured in electrolyte solutions containing vanillin. With increase of vanillin concentration (C_van), vanillin had an increased peak current intensity (I_van, as response signal). Fc doped into ZIF-8 had slight changes in its peak current intensity (I_Fc, as reference signal). There is a well plotting linear relationship between I_van/I_Fc and the logarithm of C_van ranging from 10 nM to 0.2 mM, with a low limit of detection of 3 nM. The aptamer-AuNPs/Fc-KB/ZIF-8@GCE was applied as a ratiometric electrochemical aptasensor of vanillin. This aptasensor had sensitive and selective electrochemical signal responses on vanillin, over potential interferents. This aptasensor enabled vanillin detection in real food samples, showing high detection performance. Experimental results testified that this aptasensor had high reliability and practicability for vanillin determination in real samples.

Ratiometric Sensing for Alkaline Phosphatase Based on Two Independent Signals from in Situ Formed Nanohybrids of Semiconducting Polymer Nanoparticles and MnO2 Nanosheets

Ratiometric sensing systems transduced through independent analyte-sensitive response signals, which are simultaneously obtained from a single material, are highly desired to improve sensing reliability and sensitivity. In this study, a dual-model ratiometric sensing system with fluorescence and second-order light scattering (SOS) as transducing signals has been designed for the ratiometric detection of alkaline phosphatase (ALP). Semiconducting polymer nanoparticles (SPNs) made of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1',3}-thiadiazole)] are prepared and used as reducing and stabilizing agents to prepare MnO₂ nanosheets in situ through the reduction of KMnO₄. The formed SPNs@MnO₂ nanohybrids exhibit independent fluorescence and SOS response to ALP by using l-ascorbic acid 2-phosphate trisodium salt as the enzyme substrate. Benefiting from the simultaneous availability of fluorescence and SOS signals under the same excitation, a ratiometric probe has been constructed successfully for ALP sensing. Under optimal conditions, the SPNs@MnO₂ nanohybrids for ALP detection show a good linear detection range from 0.1 to 9.0 U L^-1 with a detection limit of 0.034 U L^-1. Additionally, a visual and portable sensing device for ALP detection is also constructed based on the fluorescent performances of the SPNs@MnO₂ nanohybrids. We believe the proposed method with the in situ preparation of SPN-based hybrid probes via the reducing ability of SPNs will pave a new way for the construction of multifunctional sensing materials in chemo-/biosensing applications.

Copper nanoclusters: an efficient fluorescence sensing platform for quinoline yellow

Fluorescence quenching behavior of artificial food colorant quinoline yellow (QY), on interaction with l-cysteine stabilized copper nanoclusters (l-Cys-CuNCs) is investigated in this work. For this purpose, l-cysteine stabilized CuNCs were synthesized and characterized using various analytical techniques. Results demonstrated that the synthesized probe (size ~2 nm) had very promising optical features such as bright blue fluorescence, significant quantum yield and excellent photostability. l-Cys-CuNCs can function as a fluorescence sensor by selectively sensing QY among other yellow colorants, giving a detection limit as low as 0.11 μM. The developed sensor exhibited a linear concentration range from 5.50 to 0.20 μM. The developed fluorescence assay was successfully applied for testing commercial samples, thereby making this sensing strategy significant for quality control of food stuffs.

Ion imprinted dual reference ratiometric fluorescence probe for respective and simultaneous detection of Fe3+ and Cu2+

Dual detection of two kinds of metal ions was achieved by a dual reference ratiometric fluorescence probe.

A carbon-dot-based dual-emission probe for ultrasensitive visual detection of copper ions

In this work, we successfully developed an ultrasensitive dual-emission fluorescent sensor (CRB) for instant visual determination of Cu²⁺ by modifying a Cu-sensitive rhodamine B derivative (RB) onto photostable carboxyl modified carbon dots (CDs).