Ratiometric fluorescence probe of MIPs@CdTe QDs for trace malachite green detection in fish

Current trends of functional monomers and cross linkers used to produce molecularly imprinted polymers for food analysis

Molecularly imprinted polymers (MIPs) as artificial synthetic receptors are in high demand for food analysis due to their inherent molecular recognition abilities. It is common practice to employ functional monomers with basic or acidic groups that can interact with analyte molecules via hydrogen bonds, covalent bonds, and other interactions (π-π, dipole-ion, hydrophobic, and Van der Waals). Therefore, selecting the appropriate functional monomer and cross-linker is crucial for determining how precisely they interact with the template and developing the polymeric network's three-dimensional structure. This study summarizes the advancements made in MIP's functional monomers and cross-linkers for food analysis from 2018 to 2023. The subsequent computational design of MIP has been thoroughly explained. The discussion has concluded with a look at the difficulties and prospects for MIP in food analysis.

A tailored ratiometric fluorescent sensor based on CdTe and MgF2 quantum dots for trace N-ethylpentylone detection

The development of low-cost and highly sensitive ratiometric fluorescence sensor, CdTe@MIPs/MgF2, for N-Ethylpentylone (NEP) detection in wastewater samples is described. In this system, CdTe@MIPs (λex = 370, λem = 570) are employed as the receptor and response unit for NEP, with MgF2 (λex = 370, λem = 470) as the reference signal to enhance stability. Under optimal conditions, the sensor shows fluorescent quenching response at 570 nm to NEP in linear range of 2-200 nM, with LOD of 0.6 nM. The sensor also demonstrates significant selectivity for NEP over other analogues and interferents, making it ideal for practical applications in wastewater analysis. This approach is potentially more cost-effective and sensitive than conventional mass spectrometry in detecting abused substances in sewage. Additionally, the MgF2 fluorescent nano-material was first-ever developed and investigated, which may be significant in future research.

Smartphone-assisted colorimetric dual-mode sensing system based on europium-doped metal-organic frameworks for rapid on-site visual detection of Fe3+ and doxycycline

Exploring a rapid, sensitive, low-cost, in-situ intelligent monitoring multi-target fluorescence detection platform is important for food safety and environmental monitoring. A dual-mode ratiometric fluorescence sensing system integrated with a smartphone based on a luminescent metal-organic framework (NH2-MIL-53) and CdTe/Eu was developed for visual, in-situ analysis of Fe3+ and doxycycline (DOX) in this paper. Interestingly, with increasing Fe3+ concentration, the fluorescence sensing system exhibits dual-emission with CdTe QDs at 540 nM as the response signal and NH2-MIL-53 at 438 nm as the reference signal, resulting in a significant color shift of fluorescence color from blue-green to blue, with a linear range of 5--1550 nM and a detection limit of 1.08 nM. In the presence of DOX, the blue fluorescence of NH2-MIL-53 and the green fluorescence of CdTe QDs were quenched respectively by the internal filtering effect and the photoelectron transfer effect. While DOX enhances the red fluorescence of Eu3+ by the antenna effect, forming a triple-emission fluorescence sensor. The visual color of this fluorescent sensor shifted from blue green to grey to pink-white to pink to fuchsia to red as the DOX concentration increased with a detection limit of 0.11 nM. Furthermore, the developed intelligent sensing platform achieved real-time in-situ detection of Fe3+ and DOX with detection limit of 1.47 nM and 6.43 nM, respectively. The platform was applied to detection actual samples with satisfactory results, which proved a promising application for real-time on-site food safety monitoring and human health monitoring.

A smartphone-assisted down/up-conversion dual-mode ratiometric fluorescence sensor for visual detection of mercury ions and l-penicillamine

Establishment of a rapid, sensitive, visual, accurate and low-cost fluorescence detection system to detect multiple targets was of great significance in food safety evaluation, ecological environment monitoring and human health monitoring. In this work, a smartphone-assisted down/up-conversion dual-mode ratiometric fluorescence sensor was proposed based on metal-organic framework (NH₂-MIL-101(Fe)) and CdTe quantum dots (CdTe QDs) for visual detection of mercury ions (Hg²⁺) and L-penicillamine (L-PA), in which NH₂-MIL-101(Fe) was used as the reference signal and CdTe QDs was used as the response signal. The down-conversion fluorescence system at excitation wavelength of 300 nm (ex: 330 nm) was used to detect Hg²⁺ and L-PA, in which the detection limit of Hg²⁺ was 0.053 nM with the fluorescence color changed from green to blue, and the detection limit of L-PA was 1.10 nM with the fluorescence color changed from blue to green. Meanwhile, the up-conversion fluorescence system at excitation wavelength of 700 nm (ex: 700 nm) was used to detect Hg²⁺ and L-PA. The detection limits of Hg²⁺ and L-PA were 0.11 nM and 2.93 nM, respectively. The detection of Hg²⁺ and L-PA were also carried out based on the color extraction RGB values identified by the smartphone with a detection limit of 0.091 nM for Hg²⁺ and 8.97 nM for L-PA. In addition, the concentrations of Hg²⁺ and L-PA were evaluated by three-dimensional dynamic analysis in complex environments. The smartphone-assisted down/up-conversion dual-mode ratiometric fluorescence sensor system provides a new strategy for detection Hg²⁺ and L-PA in food safety evaluation, environmental monitoring and human health monitoring.

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.

Genetic Algorithm-Assisted Design of Sandwiched One-Dimensional Photonic Crystals for Efficient Fluorescence Enhancement of 3.18-μm-Thick Layer of the Fluorescent Solution

One-dimensional photonic crystal structures have been widely used to enhance fluorescence. However, its fluorescence enhancement is low-fold because of a weak excitation field region. In this paper, we used a genetic algorithm to assist in the design of two photonic crystals based on Al₂O₃ and TiO₂ materials. One of them has a defect consisting of SiO₂. The Fabry-Perot cavity (FP cavity) formed by the sandwiched photonic crystal achieves up to 14-fold enhancement of the excitation electric field. We modulate the electric field radiation distribution of the fluorescent material by using photonic forbidden bands. For a 3.18 μm thick layer of the fluorescent solution, the structure achieves up to 60-fold fluorescence enhancement. We also discussed that the reason for the different enhancement abilities in different places is the phase change caused by the optical path difference. This design is expected to have applications in display, imaging, etc.

Review on molecular imprinting technology and its application in pre-treatment and detection of marine organic pollutants

Molecular imprinting technology (MIT) has been considered as an attractive method to produce artificial receptors with the memory of size, shape and functional groups of the templates and has become an emerging technique with the potential in various fields due to recognitive specificity, high efficient selectivity and mechanical stability, which can effectively remove background interference and is suitable for the pre-treatment and analysis of trace level substances in complex matrix samples. Nearly 100 papers about the application of MIT in the detection of marine pollutants were found through Science Citation Index Expanded (SCIE). On this basis, combined with the application of MIT in other fields, the pre-treatment process of marine environmental samples was summarized and the potential of four types of different molecularly imprinted materials in the pre-treatment and detection of marine organic pollutants (including antibiotics, triazines, organic dyes, hormones and shellfish toxins) samples was evaluated, which provides the innovative configurations and progressive applications for the analysis of marine samples, and also highlights future trends and perspectives in the emerging research field.

Detection of tartrazine in fake saffron containing products by a sensitive optical nanosensor

A fluorescent assay for the selective analysis of tartrazine was developed. Tartrazine is a health-threatening food additive commonly used as fake saffron. An optical nanosensor was fabricated based on molecular imprinting technique in which carbon dots (CDs) as fluorophores and tartrazine as a template molecule were embedded in molecularly imprinted polymer (MIP) matrix. The synthesized CDs embedded in MIP (CDs-MIP) was characterized by various methods. The fluorescence intensity of (CDs-MIP) was selectively quenched in the presence of tartrazine in comparison with other similar food color additives. The correlation between the quenching of CD-MIP and the concentration of tartrazine was used as an optical sensing for rapid detection of tartrazine in the range of 3.3-20.0 nM (1.8-10.7 μg L^-1) with detection limit of 1.3 nM (0.70 μg L^-1). Eventually, the designed nanosensor was successfully applied for tartrazine detection in foodstuffs such as fake saffron, saffron tea and saffron ice cream samples.

Biological Sample-Compatible Ratiometric Fluorescent Molecularly Imprinted Polymer Microspheres by RAFT Coupling Chemistry

Ratiometric fluorescent molecularly imprinted polymer (MIP) sensors hold great promise in many bioanalytical areas because of their high sensitivity and selectivity as well as excellent self-referencing and visual detection capability. However, their synthetic strategies are rather limited and the development of such optosensing MIPs that can directly and selectively quantify small organic analytes in complex biological samples remains a formidable challenge owing to the complexity of sample matrices. Herein, a versatile and modular strategy to obtaining well-defined ratiometric fluorescent MIP microspheres capable of directly and selectively detecting an organic herbicide [2,4-dichlorophenoxyacetic acid (2,4-D)] in undiluted pure milks is described. First, it involves the synthesis of uniform "living" polymer particles via RAFT precipitation polymerization, their successive well-controlled grafting of a polymer shell labeled with red CdTe QDs (being inert to 2,4-D) and an MIP shell labeled with green 4-nitrobenzo[c][1,2,5]oxadiazole (NBD) units (showing fluorescence "light-up" upon binding 2,4-D) via surface-initiated RAFT polymerization, and final grafting of hydrophilic poly(N-isopropylacrylamide) brushes via an efficient coupling reaction (i.e., RAFT coupling chemistry). The resulting hydrophilic dual fluorescent MIP particles showed excellent photostability and reusability. They exhibited obvious analyte binding-induced "turn-on"-type ratiometric fluorescence (and color) change and high 2,4-D optosensing selectivity and sensitivity in pure bovine milk (with a detection limit of 0.13 μM). Moreover, they were directly applied to 2,4-D determination in undiluted pure goat milk with good recoveries (96.0-103.2%) and high accuracy (RSD = 1.5-5.5%), even in the presence of several analogues of 2,4-D. The general applicability of our strategy was also demonstrated. This study paves the way for efficiently developing various advanced MIP optosensors (of easily tunable structures and desired properties) highly promising in many bioanalytical applications.

Direct extraction and detection of malachite green from marine sediments by magnetic nano-sized imprinted polymer coupled with spectrophotometric analysis

This research describes the application of magnetic molecularly imprinted nano-sized polymers (MMIPs) for the selective extraction and fast detection of malachite green (MG) from marine sediment samples followed by UV-Vis spectrophotometry. The novel material was prepared by surface imprinting using methacrylic acid as the functional monomer for fixing the template molecules. The polymers obtained at each step were thoroughly studied by transmission electron microscopy, FTIR spectroscopy and thermogravimetric analysis. Simultaneously, the adsorption performances of the resulting nanoparticles were analysed in detail and an excellent affinity with the MG was revealed. Further, the main parameters of magnetic molecular imprinted solid-phase extraction (MMIP-SPE) were screened via multivariate optimization methods. The magnetic nanoparticles were used as special adsorbents to directly extract MG from crude marine sediment extracts. The developed method exhibits satisfactory recoveries from the spiked samples, ranging from 80.40 to 92.96% with an RSD of less than 5.18% (n = 3).

Upper Excited State Photophysics of Malachite Green in Solution and Films

Relaxation pathways of upper excited electronic states of malachite green (MG) in ethanol and in films are studied by steady-state and time-resolved spectroscopic techniques. In contrast to ethanol, where MG emits weak short-lived spectrally well separated S₂ and S₁ fluorescence with the lifetimes ∼0.3 and ∼0.9 ps, MG films show a much stronger broadband fluorescence within 430-700 nm, revealing multiexponential kinetics with the characteristic decay times τ₁ ≈ 1 ps, τ₂ ≈ 10 ps, τ₃ ≈ 0.05-0.8 ns, and τ₄ ≈ 2-3 ns. By the analysis of spectroscopic responses of MG in ethanol and in films as well as by theoretical modeling, we demonstrate that significant increase of fluorescence lifetimes and substantial enhancement of fluorescence intensity in MG films are stipulated by the decrease of efficiency of the S₂ → S₁ and S₁ → S₀ internal conversion, which in turn is caused by hindrance of rotation of MG's phenyl rings controlling the S₂/S₁ and S₁/S₀ conical intersections. These findings indicate that MG films may become promising non-Kasha materials (with reasonable S₂ emission) with numerous photophysical and photochemical applications.