TP-FRET-Based Fluorescent Sensor for Ratiometric Detection of Formaldehyde in Real Food Samples, Living Cells, Tissues, and Zebrafish

Advancements in small molecule fluorescent probes for the detection of formaldehyde in environmental and food samples: A comprehensive review

Formaldehyde (FA), a hazardous substance with carcinogenicity and mutagenicity, necessitates sensitive and accurate detection methods for protecting public health and the environment. While numerous reviews have explored FA fluorescent probes, the current literature predominantly emphasizes biological systems, leaving a gap in addressing FA's roles in environmental monitoring and food safety. This review discusses recognition mechanisms for FA detection, including 2-aza-Cope rearrangement, methylenehydrazine reaction, formimine formation, and other mechanisms. Furthermore, this review underscores the practical applications of these probes in real-world contexts, namely their incorporation into test strips, hydrogels, and membranes for environmental monitoring and food safety. Moreover, this review highlights future directions for developing intelligent detection systems that combine fluorescent probes with data processing algorithms and artificial intelligence technologies. By synthesizing the current knowledge in this area, this review aims to stimulate future research and advancements in FA detection technology, ultimately contributing to improved environmental management and public health protection.

A new bioluminescent probe for detecting formaldehyde in real food samples

Formaldehyde (FA) is an important type of reactive carbonyl species (RCS). The development of a rapid and efficient method to quantify FA in food and biological samples is necessary. Herein, we have developed the first bioluminescent probe FA-Fluc with homoallyl amine to cage firefly D-luciferin (Fluc), which could react almost completely with FA via aza-Cope rearrangement in 4 h to afford a turn-on Bioluminescence (BL). The detection limit of FA-Fluc was 0.047 ppm; and could detect FA in different seafood, agricultural products, and cooking processes. FA-Fluc has a good selectivity for FA, which shows that it does not react with other RCS. In addition, the FA content detected in complex food substrates is almost the same as that detected by acetylacetone and high-performance liquid chromatography (HPLC) methods. We also found that FA-Fluc was a powerful tool for real-time and dynamic imaging of FA in live cells and animals.

A highly sensitive Golgi-targeted fluorescent probe for the simultaneous detection of malondialdehyde and formaldehyde in living systems and foods

Malondialdehyde (MDA) and formaldehyde (FA) are highly active carbonyl substances widely present in both biological and abiotic systems. The detection of MDA and FA is of great significance for disease diagnosis and food safety monitoring. However, due to the similarity in structural properties between MDA and FA, very few probes for synergistically detecting MDA and FA were reported. In addition, functional abnormalities in the Golgi apparatus are closely related to MDA and FA, but currently there are no fluorescent probes that can detect MDA and FA in the Golgi apparatus. Therefore, we constructed a simple Golgi-targetable fluorescent probe GHA based on hydrazine moiety as the recognition site to produce a pyrazole structure after reaction with MDA and to generate a CN double bond after reaction with FA, allowing MDA and FA to be distinguished due to different emission wavelengths during the recognition process. The probe GHA has good specificity and sensitivity. Under the excitation of 350 nm, the blue fluorescence was significantly enhanced at 424 nm when the probe reacted with MDA, and the detection limit was 71 nM. At the same time, under the same excitation of 350 nm, the reaction with FA showed a significant enhancement of green fluorescence at 520 nm, with a detection limit of 12 nM for FA. And the simultaneous and high-resolution imaging of MDA and FA in the Golgi apparatus of cells was achieved. In addition, the applications of the probe GHA in food demonstrated it can provide a powerful method for food safety monitoring. In summary, this study offers a promising tool for the synergistic identification and determination of MDA and FA in the biosystem and food, facilitating the revelation of their detailed functions in Golgi apparatus and the monitoring of food safety.

Progress in the Study of Optical Probes for the Detection of Formaldehyde

Formaldehyde, one of the simplest reactive carbonyl substances, is involved in many physiological and pathological processes in living organisms. There is a large amount of data showing that abnormal elevation of formaldehyde is associated with a variety of diseases in the body, such as neurodegenerative diseases, Alzheimer's disease, cardiovascular diseases and cancer, and is also a representative carcinogen, so monitoring formaldehyde is of great importance for disease diagnosis and treatment. In this review, In this paper, we summarize and classify the last ten years of probes for the detection of formaldehyde according to different reaction mechanisms and discuss the structures and applications of the probes. Finally, we briefly describe the challenges and possible solutions in this field. We believe that more new probes provide powerful tools to study the function of formaldehyde in living systems.

Excellent ratiometric two-photon fluorescent probes for hydrogen sulfide detection based on the fluorescence resonance energy transfer mechanism

Fluorescence resonance energy transfer (FRET) is an important mechanism to design ratiometric fluorescent probes that are able to detect analytes quantitatively according to the ratio of two well-resolved emission signals. Two-photon (TP) fluorescent probes can realize the detection in living cells and tissues with deeper penetration depth, higher resolution, and lower photodamage in contrast to one-photon fluorescent probes. However, to date, fabricating TP-FRET ratiometric fluorescent probes possessing large two-photon absorption (TPA), high fluorescence quantum yield and perfect FRET efficiency is still challenging. Consequently, to develop excellent TP-FRET ratiometric probes and explore the relationship between their molecular structures and TP fluorescence properties, in this paper, we designed a series of H2S-detecting TP fluorescent probes employing the FRET mechanism based on an experimental probe BCD. Thereafter, we comprehensively evaluated the TP sensing performance of these probes by means of time-dependent density functional theory and quadratic response theory. Furthermore, we determined energy transfer efficiency and fluorescence quantum yield. Significantly, through regulating benzene-fused positions, we successfully improved fluorescence quantum yield and TPA cross-section simultaneously. Large spectral overlap between energy donor emission and acceptor absorption was achieved and near perfect energy transfer efficiency was acquired for all the studied probes. We revealed that these probes exhibit two well-resolved TPA bands, which are contributed by FRET donors and acceptors, respectively. Especially, both the wavelengths and the cross-sections of the two TPA bands agree well with those of energy donors and acceptors, which is the unique TPA spectral profile of FRET probes and has never been previously reported. Moreover, we proposed an excellent TP-FRET probe BCD3 and its product molecule BCD3-H2S, which exhibit large Stokes (141 nm and 88 nm) and emission shifts (5931 cm-1), as well as greatly increased TP action cross-sections (24-fold and 60-fold) in the near-infrared region with respect to BCD and BCD-H2S. Our detailed study can give an insight into the efficient design of novel TP-FRET fluorescent probes.

Metal-organic frameworks (MOFs) based luminescent and electrochemical sensors for food contaminant detection

With the increasing population, food toxicity has become a prevalent concern due to the growing contaminants of food products. Therefore, the need for new materials for toxicant detection and food quality monitoring will always be in demand. Metal-organic frameworks (MOFs) based on luminescence and electrochemical sensors with tunable porosity and active surface area are promising materials for food contaminants monitoring. This review summarizes and studies the most recent progress on MOF sensors for detecting food contaminants such as pesticides, antibiotics, toxins, biomolecules, and ionic species. First, with the introduction of MOFs, food contaminants and materials for toxicants detection are discussed. Then the insights into the MOFs as emerging materials for sensing applications with luminescent and electrochemical properties, signal changes, and sensing mechanisms are discussed. Next, recent advances in luminescent and electrochemical MOFs food sensors and their sensitivity, selectivity, and capacities for common food toxicants are summarized. Further, the challenges and outlooks are discussed for providing a new pathway for MOF food contaminant detection tools. Overall, a timely source of information on advanced MOF materials provides materials for next-generation food sensors.

A novel fluorescent probe for the detection of formaldehyde in real food samples, animal serum samples and gaseous formaldehyde

Formaldehyde (FA) is widely used as an adhesion promoter and dyeing aid in industrial production. Ingestion of a certain amount of formaldehyde may cause corrosive burns in the mouth, throat, and digestive tract. Therefore, it is very necessary to use simple and effective detection methods to ensure human health and food safety. Herein, a novel fluorescent probe NFD based on naphthalimide for the detection of formaldehyde in food was designed and synthesized. The probe had a remarkable fluorescence response to formaldehyde at 554 nm. And it exhibited fascinating advantages of good selectivity, high sensitivity, and low detection limit. In addition, the solid sensor prepared by loading the probe on the filter paper was successfully realized the visual detection of liquid and gaseous formaldehyde. More importantly, the probe possessed excellent stability in the detection of formaldehyde in real food samples and animal serum samples.

A new quinoline based probe with large Stokes shift and high sensitivity for formaldehyde and its bioimaging applications

As a common reactive metabolite in living organisms, abnormal levels of formaldehyde may cause diseases such as cancer and Alzheimer's disease. Therefore, it is important to develop a sensitive and efficient method to understand the role of formaldehyde in physiology and pathology. Herein, a new fluorescent probe 4-phenyl-2-(trifluoromethyl) quinolin-7-hydrazino (QH-FA) was prepared for the detection of formaldehyde in near-total aqueous media with hydrazine as the reaction site and quinoline derivatives as the fluorophore. After reacting with formaldehyde, the hydrazine group formed methylenehydrazine and the fluorescence was significantly enhanced (223-fold) with large Stokes shift of 140 nm. Furthermore, the response of QH-FA to formaldehyde could be finished with in only 10 min with good selectivity, and can distinguish formaldehyde from other aldehydes. More remarkably, the estimated limit of detection of QH-FA is 8.1 nM, which is superior to those of previously reported formaldehyde fluorescent probes. At the end, we detected formaldehyde in cells and zebrafish using QH-FA in a near-total aqueous system and obtained fluorescence images by confocal microscopy.

Ratiometric fluorescence sensing of formaldehyde in food samples based on bifunctional MOF

A new fluorescence strategy was described for ratiometric sensing of formaldehyde (FA) with bifunctional MOF, which acted as a fluorescence reporter as well as biomimetic peroxidase. With the assistance of H₂O₂, NH₂-MIL-101 (Fe) catalyzes the oxidation of non-luminescent substrate o-phenylenediamine (OPD) to produce fluorescent product (oxOPD) with the maximum emission at 570 nm. Besides, intrinsic fluorescence of MOF (λem = 445 nm) was quenched by oxOPD through inner filter effect (IFE). However, FA and OPD reacted to generate Schiff bases, which competitively consumed OPD inhibiting the generation of oxOPD. Under the excitation wavelength of 375 nm, a ratiometric strategy was designed to detect FA with the fluorescence intensity ratio at 445 nm and 570 nm (F₄₄₅/F₅₇₀) as readout signal. This strategy exhibited a wide linear range (0.1-50 μM) and low detection limit of 0.03 μM. This method was confirmed for FA detection in food samples. In addition to establishing a new method to detect FA, this work will open new applications of MOF in food safety.

A highly sensitive fluorescent nanoprobe for the amplified detection of formaldehyde

Non-conjugated polymer nanoparticles (PNPs) have been widely reported for analytical applications; however, the development of an effective fluorescence signal-amplification scheme based on PNPs remains challenging. In this study, polyethyleneimine-based polymer nanoparticles (PEI-PNPs) were synthesized for interrogating the fluorescence signal-amplification analytical application of the PNPs. The PEI-PNPs with an aggregated PEI polymer structure were able to confine a large density of sub-fluorophores on an individual nanoparticle, enabling the realization of a signal-amplification effect. Herein, formaldehyde (FA) was utilized for enhancing the fluorescence intensity of the PEI-PNPs as a model to confirm our proof-of-concept strategy. Our results showed that a more than 9-fold signaling-enhancing ability for the sensing of FA was observed using the PEI-PNPs prepared with a higher PEI concentration. The possible mechanism for the FA amplified sensing was studied. In particular, the FA-recognition units were sub-fluorophores of PEI-PNPs, which were simultaneously formed with the preparation of the PEI-PNPs avoiding the leakage effect of dyes. We believe that the water-soluble and biocompatible PEI-PNPs are promising candidates for the detection of endogenous FA in living systems.

A new amine moiety-based near-infrared fluorescence probe for detection of formaldehyde in real food samples and mice

A new amine moiety-based near-infrared fluorescent probe (Probe-NH₂) is developed for detection of formaldehyde in food samples and mice. Probe-NH₂ is constructed and synthesized from the IR-780 via two-step reactions as a hemicyanine skeleton bearing an amino moiety. The response mechanism is based on Schiff base reaction that formaldehyde reacts with amine group to form the corresponding imines. Probe-NH₂ for detection of formaldehyde exhibits excellent analytical performance, including near-infrared fluorescence emission at 708 nm, high selectivity and sensitivity, also provides a response time as low as 30 min with a detection limit of 1.87 μmolL^-1. Notably, we constructed a simple, rapid and visual formaldehyde detection platform based on paper chips in the near-infrared region for the first time. The accurate detection of formaldehyde in real food samples is of great significance, Probe-NH₂ was detected in dried beancurd sticks, endive sprout, frozen shrimp and squid, with good recoveries of 99.60%-112.72%, indicating the reliability of Probe-NH₂ for spiked determination of formaldehyde in contaminated foods. More importantly, Probe-NH₂ has been successfully applied to the detection of endogenous formaldehyde in mice.

Reaction-based fluorescent and chemiluminescent probes for formaldehyde detection and imaging

Formaldehyde (FA), a reactive carbonyl species, is classified as Group 1 carcinogen by International Agency for Research on Cancer (IARC) in 2004. In addition, clinical studies have implicated that elevated levels of FA have been associated with different kinds of diseases, such as neurodegenerative diseases, diabetes, and chronic liver and heart disorders. However, in addition to the direct inhalation of FA in the environment, most organisms can also produce FA endogenously by demethylases and oxidases during the metabolism of amino acids and xenobiotics. Since FA plays an important role in physiological and pathological processes, developing reliable and efficient methods to monitor FA levels in biological samples is crucial. Reaction-based fluorescent/chemiluminescent probes have provided robust methods for FA detection and real-time visualization in living organisms. In this highlight, we will summarize the major developments in the structure design and applications of FA probes in recent years. Three main strategies for designing FA probes have been discussed and grouped by different reaction mechanisms. In addition, some miscellaneous reaction mechanisms have also been discussed. We also highlight novel applications of these probes in biological systems, which offer powerful tools to discover the diverse functions of FA in physiology and pathology processes.

PVA-SM microstructure enhanced ratiometric fluorescence probe for formaldehyde detection in solution and gas

Formaldehyde (FA) is one of the most common pollutants, which has tremendous harm to humans and environment. In this work, 4-amino-3-pentene-2-one (Fluoral-p) and SiO₂ coated quantum dot (QD@SiO₂) were combined to implement a new ratiometric fluorescence probe QD@SiO₂-Fluoral-p for FA detection. In addition, by utilization of polyvinyl alcohol (PVA) and SiO₂ microsphere (SM), a kind of PVA-SM microstructure was assembled with QD@SiO₂-Fluoral-p to composite a signal enhanced sensing film. The QD@SiO₂-Fluoral-p exhibited good response to 0-400 mg/L FA solution and an enhancement around 15 folds was realized after introducing PVA-SM. In both situations, the probe showed linear relationship to FA concentration (C_FA), with detection limits of 14 and 0.5 mg/L, respectively. Also, the sensing film showed a good linear response to FA gas in the range of 0 to 2 ppm, with a detection limit 0.03 ppm. As a result, the PVA-SM enhanced ratiometric fluorescence probe features high sensitivity, low detection limit, good selectivity, as well as portable, which can serve as a useful tool for investigating FA in solution and gas at room temperature.

Systematic investigation of the aza-Cope reaction for fluorescence imaging of formaldehyde in vitro and in vivo

Increasing evidence has highlighted the endogenous production of formaldehyde (FA) in a variety of fundamental biological processes and its involvement in many disease conditions ranging from cancer to neurodegeneration. To examine the physiological and pathological relevance and functions of FA, fluorescent probes for FA imaging in live biological samples are of great significance. Herein we report a systematic investigation of 2-aza-Cope reactions between homoallylamines and FA for identification of a highly efficient 2-aza-Cope reaction moiety and development of fluorescent probes for imaging FA in living systems. By screening a set of N-substituted homoallylamines and comparing them to previously reported homoallylamine structures for reaction with FA, we found that N-p-methoxybenzyl homoallylamine exhibited an optimal 2-aza-Cope reactivity to FA. Theoretical calculations were then performed to demonstrate that the N-substituent on homoallylamine greatly affects the condensation with FA, which is more likely the rate-determining step. Moreover, the newly identified optimal N-p-methoxybenzyl homoallylamine moiety with a self-immolative β-elimination linker was generally utilized to construct a series of fluorescent probes with varying excitation/emission wavelengths for sensitive and selective detection of FA in aqueous solutions and live cells. Among these probes, the near-infrared probe FFP706 has been well demonstrated to enable direct fluorescence visualization of steady-state endogenous FA in live mouse brain tissues and elevated FA levels in a mouse model of breast cancer. This study provides the optimal aza-Cope reaction moiety for FA probe development and new chemical tools for fluorescence imaging and biological investigation of FA in living systems.

ICT based water-soluble fluorescent probe for discriminating mono and dicarbonyl species and analysis in foods

A unique and highly water-soluble ICT-based fluorescent probe is developed for efficient detection and discrimination of reactive monocarbonyl formaldehyde (FA) from dicarbonyl methylglyoxal (MGO)/glyoxal (GO) by modulating the ICT process, which was confirmed by photophysical and TD-DFT analysis. The probe is applied in cellular imaging and quantifying FA in preserved food and MGO in manuka honey.

A ratiometric fluorescent sensing of proanthocyanidins by MnO2 nanosheets simultaneously tuning the photoluminescence of Au/AgNCs and thiamine

By simultaneously regulating the photoluminescence of alloy Au/Ag nanoclusters (NCs) and thiamine (VB1) through MnO₂ nanosheets (MnO₂ NS), a novel ratiometric fluorescent probe (RF-probe) was established for sensitively and selectively monitoring proanthocyanidins (PAs). The introduction of Ag (I) ions could enhance significantly the quantum yields (QYs, 11.1%) of AuNCs based on the synthetic method of UVI (UV irradiation) combined with MWH (microwave heating). MnO₂ NS could quench the fluorescence (FL) of Au/AgNCs mainly coming from Förster resonance energy transfer (FRET), while it could act as a nanozyme catalyst for directly catalyzing the oxidation of VB1 to produce highly fluorescent oxVB1. In the presence of PAs, MnO₂ was reduced to Mn²⁺, which caused that its quenching capacity and oxidase-like activity were vanished, thus the FL of oxVB1 and Au/AgNCs was reduced and recovered. The concentration of PAs could be monitored by the RF-probe with a linear range of 0.27-22.4 μmol L^-1 and corresponding limit of detection (LOD) and limit of quantification (LOQ) were calculated to be 75.9 and 250.5 nmol L^-1. Furthermore, the RF-probe was successfully used for the determination of PAs in mineral water, PAs additive and PAs capsule with satisfactory results compared to the standard HPLC method.

Construct a lysosome-targeting and highly selective fluorescent probe for imaging of hydrogen sulfide in living cells and inflamed tissues

Since the fluctuation of cellular hydrogen sulfide (H₂S) is a very important third endogenously generated gaseous signaling molecule and plays a key role in the development of numerous human disorders, the real-time fluorescence detection of H₂S in living systems has attracted plenty of interest during past decade. Although a lot of H₂S fluorescent probes have been reported, the relationship between the physiology and pathology of H₂S in organelles remains unclear, especially for inflammatory tissue. In this work, by adopting a weakly basic morpholine group as the lysosome-targeting site, a naphthalimide derivative as the signal reporter group and a 4-dinitrobenzene-ether (DNB) as fluorescence signal quencher and H₂S-selective recognition moiety, we reported a new lysosome-targeting TP fluorescent probe LyNP-H₂S for H₂S detection and imaging in living cells and inflamed tissues. The probe LyNP-H₂S exhibits very low fluorescence signal in the absence of H₂S, and displays a significant 262-fold fluorescence intensity enhancement in the presence of H₂S at 540 nm. Moreover, LyNP-H₂S has the capability of quantitative detection of H₂S at concentrations ranging from 0 to 12.0 μM (limit of detection = 9.8 nM), rapid response, as well as high sensitivity and selectivity toward H₂S. Impressively, the results of living cell and inflamed tissues imaging test demonstrate that LyNP-H₂S has the potentiality of being an ideal probe for real-time H₂S detection in biosystems.

Recent advances in selective formaldehyde detection in biological and environmental samples by fluorometric and colorimetric chemodosimeters

Formaldehyde, a highly reactive carbonyl species, has been widely used in day-to-day life owing to its numerous applications in essential commodities, etc.; the extrusion of formaldehyde from these sources basically leads to increased formaldehyde levels in the environment. Additionally, formaldehyde is endogenously produced in the human body via several biological processes. Considering the adverse effects of formaldehyde, it is highly important to develop an efficient and reliable method for monitoring formaldehyde in environmental and biological samples. Several chemodosimeters (reaction-based sensing probes) have been designed and synthesized to selectively detect the presence of formaldehyde utilizing the photophysical properties of molecules. In this review, we have comprehensively discussed the recent advances in the design principles and sensing mechanisms of developed probes and their biological/environmental applications in selective formaldehyde detection and imaging endogenous formaldehyde in cells. We have summarized the literature based on three different categories: (i) the Schiff base reaction, (ii) the 2-aza-Cope sigmatropic rearrangement reaction and (iii) miscellaneous approaches. In all cases, reactions are accompanied by changes in color and/or emission that can be detected by the naked eye.

Measuring calibration factors by imaging a dish of cells expressing different tandem constructs plasmids

Three-cube Förster resonance energy transfer (FRET) method is the most extensively applied approach for live-cell FRET quantification. Reliable measurements of calibration factors are crucial for quantitative FRET measurement. We here proposed a modified TA-G method (termed as mTA-G) to simultaneously obtain the FRET-sensitized quenching transition factor (G) and extinction coefficients ratio (γ) between donor and acceptor. mTA-G method includes four steps: (1) predetermining the ratio ranges of the sensitized emission of acceptor (F_C ) to the donor excitation and donor channel image (I_DD [(DA])) for all FRET plasmids; (2) culturing the cells which express every FRET plasmid in one dish respectively; (3) distinguishing and marking the cells expressing different FRET plasmids by detecting their F_C /I_DD (DA) values; (4) linearly fitting F_C /I_AA (DA) (acceptor excitation and acceptor channel image) to I_DD (DA)/I_AA (DA) for different kinds of cells. We implemented mTA-G method by imaging tandem constructs cells with different FRET efficiency cultured in one dish on different days, and obtained consistent G and γ values. mTA-G method not only circumvents switchover of different culture dishes but also keep the constant imaging conditions, exhibiting excellent robustness, and thus will expands the biological applications of quantitative FRET analysis in living cells.

A mitochondria-targeting ratiometric fluorescent probe for the detection of sulfur dioxide in living cells

A mitochondria-targeting ratiometric fluorescent probe was developed for the detection of sulfur dioxide in living cells.