N,S-GQDs mixed with CdTe quantum dots for ratiometric fluorescence visual detection and quantitative analysis of malachite green in fish

Fabrication of SERS composite substrates using Ag nanotriangles-modified SiO2 photonic crystal and the application of malachite green detection

A novel surface-enhanced Raman scattering (SERS) composite substrates on the basis of Ag triangular nanoplates(Ag TNPs)-modified SiO2 photonic crystals (PC) is fabricated and applied to the SERS detection of malachite green (MG). It consists of uniformly arranged Ag TNP@SiO2, a new PC. Notably, Ag TNP are uniformly aligned on the SiO2 surface, forming a three-dimensional high-density hotspot nanostructure. With the tip "hot spots" of Ag TNPs, Bragg diffraction of SiO2 and coupling enhancement between Ag TNPs and SiO2, the SERS enhancement of this composite substrates was multiplied. The effect on the SERS of Ag TNP@SiO2 composite substrate was systematically optimized by tuning Ag TNP size, size of SiO2 microspheres, coverage of Ag TNPs on SiO2 and fabrication method of Ag TNPs and PC. Moreover, the uniform of SERS composite substrates and Raman signal was dramatically increased by the method of vertical deposition. Eventually, the SERS composite substrates were employed in MG detection. Its broad detection range of 1 pM-1 μM and low limit of detection (LOD) of 0.49 pM indicated acceptable sensitivity and repeatability. This work illustrates the promising applicability in food safety analysis based on SERS composite substrates composed by Ag TNP@SiO2 with numerous SERS enhancements and excellent stability.

A highly selective dual-signal response ratiometric fluorescence sensing strategy for malachite green in fish based on carbon dots/copper nanoclusters nanocomposite

Malachite green (MG), a widely used antiparasitic agent, poses health risks to human due to its genotoxic and carcinogenic properties. Herein, a stable dual-emission fluoroprobe of carbon dots/copper nanoclusters is prepared for highly selective detection of MG based on the inner filter effect. This probe exhibits characteristic emission bands at 435 and 625 nm when excited at 376 nm. After adding MG, the both emission signals were significantly quenched, and the ratio of fluorescence intensity (F435/F625) was linearly related to the concentration of MG in the range of 0.05-40 μmol L-1 with a limit of detection of 18.2 nmol L-1. Meanwhile, the two signals exhibit linear relationships with the concentration of MG, respectively, and the corresponding detection results were consistent. The fluoroprobe was successfully used for the detection of MG in fish samples with the recoveries ranging from 96.0% to 103.8% and a relative standard deviation of <3.3%.

Reliable detection of malachite green by self-assembled SERS substrates based on gold-silicon heterogeneous nano pineapple structures

Morphology regulation of heterodimer nanoparticles and the use of their asymmetric features for further practical applications are crucial because of the rich optical properties and various combinations of heterodimers. This work used silicon to asymmetrically wrap half of a gold sphere and grew gold branches on the bare gold surface to form heterogeneous nano pineapples (NPPs) which can effectively improve Surface-enhanced Raman scattering (SERS) properties through chemical enhancement and lightning-rod effect respectively. The asymmetric structures of NPPs enabled them to self-assemble into the monolayer membrane with consistent branch orientation. The prepared substrate had high homogeneity and better SERS ability than disorganized substrates, and achieved reliable detection of malachite green (MG) in clams with a detection limit of 7.8 × 10-11 M. This work provided a guide to further revise the morphology of heterodimers and a new idea for the use of asymmetric dimers for practically photochemical and biomedical sensing.

Less interference fluorescence analytical strategy: Bridging substance-triggered ratiometric sensor with convenient preparation and application

High interference and narrow application range are key of bottleneck of recent fluorescence analysis methods, which limit their wide application in the sensing field. Therefore, to overcome these disadvantages, a ratiometric fluorescence sensing system utilizing berberine (BER) and silver nanoclusters protected by dihydrolipoic acid (DHLA-AgNCs) was constructed for the first time in this work, to achieve determination of BER and daunorubicin (Dau). BER aqueous solution (non-planar conformation) has no fluorescence emission. When it was mixed with DHLA-AgNCs, the conformation of BER became planar, producing fluorescence emission at 515 nm besides the fluorescence emission peak of DHLA-AgNCs at 653 nm. With the increase of BER concentration added in system, the fluorescence intensity of BER (planar conformation) at 515 nm increased obviously and the fluorescence intensity of DHLA-AgNCs decreased slightly. Therefore, the dual emission fluorescence sensing system was constructed based on a fluorescence substance and non fluorescence substance, to achieve determination of BER. Meanwhile, based on the bridging effect of BER and fluorescence resonance energy transfer effect from Dau, the altering of two peaks intensity was utilized to achieve determination of Dau. Thus, this dual emission sensing system can not only be used for fluorescence analysis of BER and its analogues, but also based on the bridging effect of BER, allowing the determination of Dau and its analogues that could not be directly measured with silver nanoclusters, expanding the application range of traditional dual emission detection systems. Meanwhile, this system has strong anti-interference ability and low toxicity to the human body and less pollution to the sample and environment. This provides a new direction and universal research strategy for the construction of new fluorescence sensing systems in the future for the analysis of target substances that cannot be directly detected with conventional fluorescence analysis methods.

An ultrasensitive smartphone-assisted bicolor-ratiometric fluorescence sensing platform based on a "noise purifier" for point-of-care testing of pathogenic bacteria in food

Non-specific binding in fluorescence resonance energy transfer (FRET) remains a challenge in foodborne pathogen detection, resulting in interference of high background signals. Herein, we innovatively reported a dual-mode FRET sensor based on a "noise purifier" for the ultrasensitive quantification of Escherichia coli O157:H7 in food. An efficient FRET system was constructed with polymyxin B-modified nitrogen-sulfur co-doped graphene quantum dots (N, S-GQDs@PMB) as donors and aptamer-modified yellow carbon dots (Y-CDs@Apt) as acceptors. Magnetic multi-walled carbon nanotubes (Fe@MWCNTs) were employed as a "noise purifier" to reduce the interference of the fluorescence background. Under the background purification mode, the sensitivity of the dual-mode signals of the FRET sensor has increased by an order of magnitude. Additionally, smartphone-assisted colorimetric analysis enabled point-of-care detection of E. coli O157:H7 in real samples. The developed sensing platform based on a "noise purifier" provides a promising method for ultrasensitive on-site testing of trace pathogenic bacteria in various foodstuffs.

Smartphone-based dual inverse signal MOFs fluorescence sensing for intelligent on-site visual detection of malachite green

The development of intelligent, sensitive, and visual methods for the rapid detection of veterinary drug residues is essential to ensure food quality and safety. Here, a smartphone-based dual inverse signal MOFs fluorescence sensing system was proposed for intelligent in-site visual detection of malachite green (MG). A UiO-66-NH2@RhB-dual-emission fluorescent probe was successfully synthesized in one step using a simple one-pot method. The inner filter effect (IFE) quenches the red fluorescence, while hydrogen bonding interaction enhances the blue fluorescence, enabling highly sensitive, accurate, and visual detection of MG dual inverse signals through fluorescence analysis. The probe showed great linearity over a wide range of 0.1-100 μmol/L, with a limit of detection (LOD) of 20 nmol/L. By integrating smartphone photography and RGB (red, green, and blue) analysis, accurate quantitative analysis of MG in water and actual fish samples can be achieved within 5 min. This developed platform holds great promise for the on-site detection of MG in practical applications, with the advantages of simplicity, cost-effectiveness, and rapidity. Consequently, it may open up a new pathway for on-site evaluation of food safety and environmental health.

A ratiometric fluorescent sensor based on S-doped BCNO quantum dots and Au nanoclusters combined with 3D-printing portable device for the detection of malachite green

Sulfur-doped BCNO quantum dots (S-BCNO QDs) emitting green fluorescence were prepared by elemental doping method. The ratiometric fluorescence probe with dual emissions was simply established by mixed S-BCNO QDs with gold nanoclusters (GSH-Au NCs). Because the emission spectrum of Au NCs (donor) at 615 nm overlapped well with the ultraviolet absorption of malachite green (MG), fluorescence resonance energy transfer (FRET) can be achieved. When the concentration of MG increased, the fluorescence intensity (F495) of S-BCNO QDs decreased slowly, while the fluorescence intensity (F615) of Au NCs decreased sharply. The fluorescence intensity ratio of F615/F495 decreased with the increase of MG. By plotting the F615/F495 values against MG concentration, a sensitive and rapid detection of MG was possible with a wide detection range (0.1-50 µM) and a low detection limit of 10 nM. Due to the accompanying fluorescence color change from pink to blue-green, it can be used for visual detection. A three dimensional-printing device utilizing digital image colorimetry to capture color changes through the built-in camera, enables quantitative detection of MG with a good linearity between the values of red/green ratio and MG concentrations at the range 1-50 µM. This sensing platform had a range of advantages, including high cost-effectiveness, portability, ease of operation, and high sensitivity. Furthermore, the sensing platform was successfully applied to the detection of MG in real water sample and fish samples, thereby verifying the reliability and effectiveness of this sensing platform in water quality monitoring and food safety.

Facile fluorescence detection of malachite green in fish using molecularly imprinted polymers doped CdTe quantum dots based system

Malachite green (MG) possesses high toxicity, therefore, the detection of MG in fish tissues is of vital importance. A novel core-shell MIPs doped CdTe quantum dots coated silica nanoparticles (CdTe-MIP/SiO2 NPs) were synthesized via a simple one-pot strategy. The materials were characterized carefully. The resulting CdTe-MIP/SiO2 NPs were coated on the thin layer chromatography plate, and coupled with miniaturized fluorimeter for fluorescence detection of MG in fish samples. The resulting CdTe-MIP/SiO2 NPs based system possessed good linearity (0.01 ∼ 20 μmol/L), high recoveries (98.36 %∼101.45 %) and low detection limit (3.7 nmol/L) for MG. Furthermore, CdTe-MIP/SiO2 NPs based system were employed to measure fish samples spiked with MG, meanwhile, HPLC was utilized to evaluate the accuracy and reliability. And the paired t-test was conducted to evaluate differences between fluorescence method and HPLC, P > 0.05 means no significant difference was observed, the results demonstrated that both fluorescence method and HPLC are suitable for MG analysis.

A new and highly efficient CuMOF-based nanoenzyme and its application to the aptamer SERS/FL/RRS/Abs quadruple-mode analysis of ultratrace malachite green

Malachite green (MG) is highly toxic, persistent, and carcinogenic, and its widespread use is a danger to the ecosystem and a threat to public health and food safety, making it necessary to develop new sensitive multimode molecular spectroscopy methods. In this work, a new copper-based nanomaterial (CuNM) was prepared by a high-temperature roasting using a copper metal-organic framework (CuMOF) as precursor. The as-prepared CuNM was characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy, transmission electron microscopy (TEM), and BET surface area analysis. CuNM was found to catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by H2O2 to produce the oxidation product TMBOX; however, subsequently, the MG aptamer (Apt) could be adsorbed on the CuNM surface by intermolecular interaction, which would inhibit the catalytic performance. After the addition of MG to be tested, the CuNM previously adsorbed by the Apt was transformed into its free state, thus restoring its catalytic activity. This new nanocatalytic indicator reaction could be monitored by surface-enhanced Raman scattering (SERS)/resonance Rayleigh scattering (RRS)/fluorescence (FL)/absorption (Abs) quadruple-mode methods. The SERS determination range was 0.004-0.4 nmol L-1 MG, with a limit of detection of 0.0032 nM. In this way, a rapid, stable, and sensitive method for the determination of MG residues in the environment was established.

Cobalt oxyhydroxide nanosheet-modulated ratiometric fluorescence platform for the selective detection of malachite green in fish

A ratiometric fluorescence platform was developed based on the cobalt oxyhydroxide (CoOOH) nanosheet-modulated fluorescence response of blue emissive copper nanoclusters (Cu NCs) and yellow emissive o-phenylenediamine (OPD). CoOOH nanosheets showed dual function of strong absorption and oxidation ability, which can effectively quench the blue fluorescence of Cu NCs, with an excitation and emission peak maximum at 390 and 450 nm, respectively , and transfer the OPD into yellow fluorescence products, with an excitation and emission peak maximum at 390 and 560 nm, respectively. Upon introducing butyrylcholinesterase (BChE) and its substrates, CoOOH nanosheets were decomposed into Co2+, and malachite green (MG) showed strong inhibition ability to this  process. This resulted in the obvious difference on the ratio of blue and yellow fluorescence recorded on the system in the presence and absence of MG, which was utilized for the quantitative detection of MG, with a limit of detection of 0.140 μM and a coefficient of variation of 3.5%. The fluorescence ratiometric assay showed excellent detection performances in practical sample analysis.

The construction of dual-emissive ratiometric fluorescent probes based on fluorescent nanoparticles for the detection of metal ions and small molecules

With the rapid development of fluorescent nanoparticles (FNPs), such as CDs, QDs, and MOFs, the construction of FNP-based probes has played a key role in improving chemical sensors. Ratiometric fluorescent probes exhibit distinct advantages, such as resistance to environmental interference and achieving visualization. Thus, FNP-based dual-emission ratiometric fluorescent probes (DRFPs) have rapidly developed in the field of metal ion and small molecule detection in the past few years. In this review, firstly we introduce the fluorescence sensing mechanisms; then, we focus on the strategies for the fabrication of DRFPs, including hybrid FNPs, single FNPs with intrinsic dual emission and target-induced new emission, and DRFPs based on auxiliary nanoparticles. In the section on hybrid FNPs, methods to assemble two types of FNPs, such as chemical bonding, electrostatic interaction, core satellite or core-shell structures, coordination, and encapsulation, are introduced. In the section on single FNPs with intrinsic dual emission, methods for the design of dual-emission CDs, QDs, and MOFs are discussed. Regarding target-induced new emission, sensitization, coordination, hydrogen bonding, and chemical reaction induced new emissions are discussed. Furthermore, in the section on DRFPs based on auxiliary nanoparticles, auxiliary nanomaterials with the inner filter effect and enzyme mimicking activity are discussed. Finally, the existing challenges and an outlook on the future of DRFP are presented. We sincerely hope that this review will contribute to the quick understanding and exploration of DRFPs by researchers.

A recyclable Eu3+-functionalized dual-emissive metal-organic framework for portable, rapid detection and efficient removal of malachite green

A europium-functionalized, dual-emissive, metal-organic framework-based fluorescence sensor (EuUCNDA) was constructed via post-synthetic modification of an UiO-66-type precursor through coordination interactions. EuUCNDA exhibited extremely high selectivity and sensitivity for malachite green (MG) with a low detection limit of 13.01 nM, a wide linear concentration range (0.05-50 μM), excellent anti-interference properties, a rapid response (<1 min), and the possibility of recycling. The good sensing performance of EuUCNDA enables the practical detection of MG in fish pond water and grass carp with good recoveries. Moreover, EuUCNDA can be reused for sensing MG and over 90% of fluorescence intensity can be restored after 7 cycles. Furthermore, EuUCNDA-embedded paper-based sensors combined with smartphone imaging afford portable and visual monitoring of MG in real samples. Notably, besides good sensing performance, EuUCNDA could efficiently remove MG from water. Hence, this work provides a recyclable and sensitive fluorescence sensor for portable, visual, rapid detection and efficient removal of MG.

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.

The ratiometric fluorescent sensor based on the mixture of CdTe quantum dots and graphene quantum dots for quantitative analysis of silver in drinks

Silver nanoparticles can be used in antibacterial packaging or disinfection. Research has shown that sugary fluid induces the leaching of silver nanoparticles into water, which may be harmful to humans. Single wavelength fluorescence analysis has been used for quantitative analysis of silver nanoparticles but suffers from low specificity and poor anti-interference ability. In this paper, a ratiometric fluorescence sensor system (GCS) was used for the detection of Ag+, which realized both visual detection and quantitative analysis of silver in drinks. The color changes of GCS with different concentrations of Ag+ are distinguishable and easy to analyze. There is also a good linear relationship between the concentrations of Ag+ and varieties of F424 nm/F570 nm, and the lowest detection limit reached 0.2266 nmol/L. This GCS shows good selectivity and recovery and could be used for the detection of Ag+ in drink samples.

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.

One-pot preparation of Mn3O4/GSH/CdTe quantum dots complex for T1-weighted MRI/fluorescence detection of H3PO4

Magnetic resonance and fluorescence dual mode imaging with both advantages of high sensitivity and high spatial resolution, which could realize real-time, in situ, and dynamic imaging, is very suitable for detecting small molecules. And the synthesis of imaging materials with good imaging performance has become the core content. In this paper, based on biomimetic mineralization technology, GSH has been used as a material to design a one-pot method for the preparation of Mn₃O₄/GSH/CdTe quantum dots composites (MGQ in short). Furthermore, MGQ with good T₁MR and fluorescence response to the variety concentration of H₃PO₄, has been used for quantitative analysis of H₃PO₄ in serum. And the lowest limit detection could reach 0.1769 nmol/L for fluorescent detection, 0.02207 mol/L for MRI detection. MGQ would be applied as a sensor in diagnostic and clinical applications.

Ratiometric Fluorescence Immunoassay Based on Carbon Quantum Dots for Sensitive Detection of Malachite Green in Fish

Malachite green (MG) is a synthetic poisonous organic compound that has been banned in many countries as a veterinary drug for aquaculture. An efficient, fast and sensitive method is urgently needed for monitoring the illegal use of malachite green (MG) in aquaculture. In this study, a novel ratiometric fluorescence immunoassay was established. Nitrogen-doped carbon quantum dots were used as ratiometric fluorescent probes with a fluorescence peak at 450 nm. Horseradish peroxidase was employed to convert o-phenylenediamine to 2,3-diaminophenazine, with a new fluorescence peak at 580 nm and a strong absorption at 420 nm. The inner filter effect between N-CQD fluorescence and DAP absorption was identified. It allows for the ratiometric detection of MG using a fluorescent immunoassay. The results demonstrated a linear ratiometric fluorescence response for MG between 0.1 and 12.8 ng·mL^-1. The limit of detection of this method was verified to be 0.097 μg·kg^-1 with recoveries ranging from 81.88 to 108%, and the relative standard deviations were below 3%. Furthermore, this method exhibited acceptable consistency with the LC-MS/MS results when applied for MG screening in real samples. These results demonstrated a promising application of this novel ratiometric fluorescence immunoassay for MG screening with the merits of rapid detection, simple sample preparation, and stable signal readout. It can be an alternative to other traditional methods if there are difficulties in the availability of expensive instruments, and achieve comparable results or even more sensitivity than other reported methods.

Facile fabrication of Ag@C@C8 nanoparticles as a SERS substrate and their environmental applications

Carbon-coated silver (Ag@C) nanoparticles were prepared by a one-step hydrothermal synthesis method, the surface was modified with chlorodimethyloctylsilane (C8) to generate C8 functionalized Ag@C@C8 nanoparticles with long-term stability and high sensitivity.