1 Introduction
Formaldehyde is a kind of chemical active substance which widely exists in nature. It has been used in the industry for polymer synthesis (adhesives, resins and leather retanning agents, etc.), and also as raw materials for the production or processing of plastic products, wood furniture, decorative materials, leather and fur and flocculating materials. Or can be directly used for biological antisepsis, antibacterial daily necessities, fabric surface treatment and the like
[1⇓⇓⇓⇓⇓~7]. In addition, formaldehyde is also a signal molecule of human physiological metabolism. Formaldehyde is continuously produced in biological metabolic processes, such as the cleavage of serine or the demethylation of DNA, RNA, and histones. Under the mediation of tetrahydrofolate (THF) and the catalysis of alcohol dehydrogenase 5 (ADH5) and aldehyde dehydrogenase 2 (ALDH2), it is converted into formate, which is absorbed or eliminated by human body, thus realizing the transfer of 1 C source between human genetic material and protein skeleton. At the same time, in order to ensure the stability of the internal environment, the concentration of formaldehyde should be maintained in a reasonable range: 50-100 μmol/L in blood, 300-300 μmol/L in brain, and 200-400 μmol/I in cells, but the concentration of formaldehyde in cancer sites will rise to 800 μmol/L
[8⇓⇓⇓~12]. Medical research shows that excessive formaldehyde intake from the external environment mainly accumulates in the respiratory system such as the nasal cavity. For endogenous formaldehyde, it is present in all cellular tissues, especially in the nervous system, where it plays a role in regulating memory and cognition. Abnormal levels of formaldehyde can cause obvious harm to the human body, not only potentially inducing cancer, leukemia and Alzheimer's disease, but also a genetic toxic agent, causing human bone marrow failure (BMF) and gene mutations (fancd2 and Aldh2)
[13,14]. Therefore, formaldehyde has been listed as a carcinogen (2004) and one of the causes of leukemia (2012) by the International Agency for Cancer (IARC). However, the pathogenicity of external formaldehyde is still controversial in the international medical community, and the results need to be further verified
[15,16].
At present, the traditional methods for detecting formaldehyde mainly include Radiometric method, High-performance liquid chromatography (HPLC) and Preconcentration-chemical ionization mass spectrometry (PCIMS).PCI-MS), Gas chromatography-mass spectrometry (GC-MS) and Colorimetric method, but these methods not only have the problems of expensive instruments, complex operation and time-consuming, but also cause irreparable damage to the sample, and the detection sensitivity and selectivity need to be improved
[17][18][19][20][21].
According to the principles and strategies of targeting or diversity design, chemosensing integrates chemistry, biology, physics and other disciplines to form a system aiming at molecular recognition, revealing physiological processes and solving biomedical problems
[22⇓⇓~25]. As one of the important branches of chemical sensors, reactive organic small molecule fluorescent probes have attracted more and more attention because of their obvious advantages, such as simple operation, fast recognition rate, high sensitivity and selectivity, and non-destructive to biological samples. By designing probe structures, proposing effective organic synthesis routes, and simulating the delocalization and energy level changes of electrons in the conjugate-push-pull framework by means of calculation, or by means of molecular self-assembly, the new synthesis reactions are more reasonable and simplified
[26⇓~28]. In the optical response experiment, if the probe has the advantages of high sensitivity and selectivity to the recognition reaction of the analytical substrate, the concentration of the substrate can be quantified. At the same time, under multi-photon or near-infrared fluorescence imaging, organisms can show higher spatial and temporal resolution, and the real-time imaging effect of cells and tissues is better than that of conventional single-photon imaging
[29⇓~31]. In addition, the fluorescent probe with remarkable effect, simple structure and convenient use can also be used for detecting harmful substances in actual samples (commodities), which lays the foundation for constructing a novel fluorescent detection method
[32⇓~34].
In the past five years, the recognition and application of organic small molecule fluorescent probes for formaldehyde have developed rapidly. It has formed a momentum of starting from biomedicine and expanding to real life, so it has a wider practical significance. In this paper, the research progress of fluorescent probes for formaldehyde was reviewed, including the reaction types, the fluorescent imaging of formaldehyde in organisms and the application of formaldehyde detection in real samples (commodities).The development of fluorescent probes for formaldehyde was summarized and prospected.
2 Reaction type of formaldehyde fluorescent probe
The formaldehyde fluorescent probe and the target formaldehyde mainly undergo condensation reaction between the amino group of the recognition group on the probe and the carbonyl group on the formaldehyde, undergo structural rearrangement, dehydration and release of fluorescence signals in the fluorophore skeleton; Or no rearrangement occurs, and the signal of the fluorophore is restored by weakening the PET. Based on this reaction principle, the probes can be classified into 2-aza-Cope rearrangement type, methylene hydrazine type, methylimido type and other types according to the change of the probe structure after the reaction with formaldehyde. The intrinsic photophysical mechanisms of fluorescence changes of the probes include Photo-induced electron transfer (PET), Internal charge transfer (ICT), Aggregation-induced emission (AIE), Excited-state intramolecular proton transfer (ESIPT) and fluorescence resonance energy transfer (FRET
[35⇓~37]. In the following, the representative achievements are selected from the above three main types, and the basic structural design and response mechanism are briefly described, which are summarized in Table 1.
2.1 2-aza-Cope rearrangement
Under the heating condition (such as 37 ℃), the homoallyl amino group reacts with formaldehyde to generate an unstable imine intermediate through a reversible condensation reaction, and the intermediate promotes the allyl amino group [3,3] -sigma bond to migrate, rearrange and hydrolyze through the charge effect, thereby releasing a fluorophore signal. As for the 2-aza-Cope rearranged formaldehyde fluorescent probe, the structure of rhodamine as the fluorophore parent was reported in the early stage. In 2015, Chan et al. designed and developed the first silica-rhodamine-based formaldehyde fluorescent probe 1 (Figure 1)
[38].
The quenching group (4-nitrobenzyl) contained in the probe 1 well reduces the background fluorescence of the probe through the donor excitation-photoinduced electron transfer (d-PET) effect, and does not produce the interference of autofluorescence before the reaction. After the reaction with formaldehyde, it undergoes condensation, rearrangement and hydrolysis, accompanied by the departure of 4-nitrobenzyl, which blocks the PET process and restores the fluorescence. The results of optical properties showed that probe 1 had high sensitivity and good selectivity to formaldehyde: the fluorescence of probe 1 was enhanced by about 7 and 33. 5 times to 0. 25 and 5 mmol/L formaldehyde, respectively, and it was not interfered by other substances and conditions (aldehydes, active sulfur, active oxygen and pH (7. 0-10. 4)).
In addition, Chang et al. reported probe 2 at the same time (fig. 2)
[39]. In contrast, probe 2 not only maintains high sensitivity and good selectivity to formaldehyde, but also has simpler structure (no quenching group) and better water solubility. After that, Chang et al continued to improve the structure of the probe and developed a series of 2-aza-Cope rearranged formaldehyde fluorescent probes with excitation wavelengths from ultraviolet to visible light
[40,41]. For example, probes 3 and 4 mainly introduce methyl and dimethyl groups into the 3 and 4 positions of the recognition group respectively, and connect the fluorophore skeleton with the oxygen-containing alkyl group. As a result, the reaction with formaldehyde, in addition to undergoing a 2-aza-Cope rearrangement, also underwent a spontaneous β-elimination to produce phenol, which reacted with the fluorophore to produce stronger fluorescence. The improvement of this series of probe recognition groups improves the reaction activity and luminescence efficiency of fluorophores, and provides guidance for subsequent researchers in structural design.
Subsequently, the probe structure design tends to be simple and delicate, mainly ratiometric probes, with strong practicability, and the introduction of reference internal standards can maximize the avoidance of external environmental interference, ensuring good sensitivity and stability. Among them, naphthalene and its derivatives (quinoline and coumarin) are important fluorophores
[42⇓⇓⇓⇓⇓⇓⇓~50]. In 2016, Lin et al. Developed a ratiometric formaldehyde fluorescent probe 5 (fig. 3)
[42]. The construction of the probe 5 is completed by one-step synthesis with 6-hydroxynaphthalene as a fluorophore. In the process of forming the internal charge transfer (ICT), the fluorophore is excited and the wavelength is red-shifted to generate a ratio signal: the fluorescence intensity decreases at the λ
em of 359 nm, while the fluorescence intensity increases at the λ
em of 451 nm. The fluorescence intensity of probe 5 (10 μmol/L) was 53.2 times higher than that of the I
451/I
359 when it reacted with 300 equivalents of formaldehyde, and it was stable in the pH range of 4 ~ 10.
Qian et al. reported a ratiometric fluorescent probe 6 (fig. 4) for imaging formaldehyde in plants, whose fluorophore parent was selected as N, N-dimethylquinoline structure with excellent photophysical and chemical properties
[43]. Compared with probe 5, probe 6-a underwent the same 2-aza-Cope rearrangement reaction and ICT process, but the ratio signal of probe 6-a was located at a longer emission wavelength I
570/I
495, with a red shift of about 120 nm. As a result, the interference of the autofluorescence background in the biological tissue can be avoided in addition to the interference of the external environment. Therefore, probe 6 has a wider pH range (pH 3-11) and a lower detection limit (0.5 μmol/L). These advantages provide even more favorable factors for the expansion of biological imaging in plant tissues.
Chang et al. Also explored a series of aminocoumarin-based ratio probes 7 a-c (fig. 5) with the aid of a 2-aza-Cope rearrangement reaction platform
[44]. By designing the difference between the X group on the fluorophore and the R group on the recognition group, the probe with better optical properties and faster response rate is expected to be optimized. The probe 7A is taken as a control structure, that is, the R group on the recognition group is a H atom. It was demonstrated that probe 7A (10 μmol/L) had only a weak ratio enhancement (1.6-fold) to formaldehyde (100 μmol/L) and a low reaction rate (0.017 L·mol
-1·s
-1). However, when the H atom was replaced by Me (methyl), probe 7B increased the ratio enhancement by 3.2 times and the reaction rate by 7 times under the same conditions. This is due to the Thorpe-Ingold effect, which accelerates the transition of the cyclization transition state in the 2-aza-Cope rearrangement reaction, thus improving the kinetic and thermodynamic efficiency of the reaction. On this basis, the hydroxypropyl side chain of X group was replaced by a hydrophilic long chain containing amide group and ether bond (7c). After hydrophilic modification, probe 7c not only enhanced homogeneous staining and subcellular localization, but also had a higher ratio enhancement (6 times) to formaldehyde, which further improved the sensitivity of formaldehyde detection in biological tissues.
At the same time, other research groups have also designed and synthesized a variety of probes with different fluorophore structures, most of which simplify the synthesis and ensure the stability of photophysical and chemical properties, providing more options for the dual detection of biological cell tissues and actual samples (commodities)
[51⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~67].
2.2 Methylenehydrazine type
At room temperature, the amino group of hydrazine reacts with formaldehyde to form methylene hydrazine containing Schiff base structure, which weakens the fluorescence process and restores the fluorescence. For methylene hydrazine type formaldehyde fluorescent probes, the structure of quinolyl fluorophore as the parent was first reported. In 2015, Yu et al. designed and synthesized 8-hydroxyquinoline derivative-based probe 8 based on PET mechanism (Fig. 6)
[68]. Before the reaction, the fluorescence emission of the group was quenched due to the delocalization of the lone pair electrons on the hydrazine group, resulting in a strong PET effect, resulting in only weak fluorescence. However, after the condensation reaction with formaldehyde, the PET pathway was inhibited and the fluorescence was turned on. The detection limit of the probe is 0. 9 μmol/L, but the selectivity for formaldehyde needs to be improved.
Since then, 1,8-naphthalimide has been widely used to design the fluorophore skeleton structure of formaldehyde fluorescent probes due to its excellent photostability and high quantum yield
[69⇓⇓⇓⇓⇓⇓~76]. Among them, the more classical one is the two-photon probe 9 (fig. 7) designed and synthesized based on the PET mechanism with the 4-hydrazine group as the recognition group. This fluorescent probe 9b was first reported by Lin et al
[69]. Probes 9 exhibited excellent photophysical properties, including a 900-fold increase in fluorescence intensity, a high reaction rate (0.39 min
-1), and a low detection limit (0.71 μmol/L). In addition, it has good selectivity and pH stability (weak acid to neutral). By adjusting the structure, probes 9c ~ e were prepared, which are more suitable for the localization and imaging of cancer cells and organelles
[71⇓~73]. On this basis, Yuan et al. synthesized a fast response formaldehyde fluorescent probe 9 f by combining rhodamine-based fluorophores, and its response time to formaldehyde was shortened from about 30 min to 5 min, with a lower detection limit (0. 21 μmol/L)
[75]. The ratiometric probe 9 G developed by Zhou et al., through the non-conjugated connection of 1,8-naphthalimide fluorophore (energy acceptor) and two-photon fluorophore (energy donor), increases the internal energy conversion based on FRET mechanism, shortens the response time of formaldehyde to 65 s again, and the detection limit can reach (5. 8 ± 0.2) nmol/L
[76].
In addition, there are methylene hydrazine probes with different structures
[77⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~90]. In the development trend, the photophysical mechanism of formaldehyde response is more abundant and mature, in addition to PET and FRET, ICT and ESIPT have improved the selectivity, sensitivity and response speed of formaldehyde respectively, providing a broader application prospect.
2.3 Azomethine and other types
Similarly, at room temperature, the free amino group on the probe reacts with formaldehyde to form a Schiff base, which inhibits the electron transfer process on the amino group, forms a cyclic or terminal chain structure of azomethine group, and releases a fluorophore signal. As for the formimine formaldehyde fluorescent probe, it has been developed for a long time and its structure is diversified. In the early stage, it was still reported in the form of rhodamine as the fluorophore parent
[91⇓~93].
In 2011, Han et al. Designed and synthesized a formaldehyde fluorescent probe 10 based on a rhodamine B derivative (fig. 8)
[91]. During the reaction, the amino group reacted with aldehydes to form a Schiff base intermediate, which promoted the intramolecular deoxolactam ring opening to form a rhodamine product containing imidazolidine, which developed color and emitted fluorescence within 1 H, thus realizing the selective detection of various alkyl aldehydes and aromatic aldehydes. Since then, two kinds of ultra-fast fluorescent probes for formaldehyde detection have been developed, which can quickly respond to the formation of cyclic imidazole products within 5 min and 10 s, respectively, and the selectivity for single formaldehyde has been improved
[92,93].
Since then, the BODIPY dye has been used as the fluorophore of the methylimido-type formaldehyde fluorescent probe
[94⇓~96]. Among them, Lv et al. Designed and synthesized a rarely reported fluorescent "off" type fluorescent probe 11 (fig. 9)
[94]. Probe 11 reacts with formaldehyde and undergoes a reversible aldehyde-ammonia condensation reaction to form azomethine, and the fluorescence intensity decreases. The fluorescence quenching is due to the isomerization and rotation of the formed C = N bond, which leads to the "turn-off" of the probe fluorescence. By increasing the viscosity of the solution, isomerization and rotation are inhibited and fluorescence is "switched on". The reversible detection property of the probe 11 to formaldehyde realizes a recycling value of at least three times. The probe showed good linearity (R
2=0.9991) and low detection limit (50 nmol/L) for formaldehyde in the range of 0 ~ 500 μmol/L.
In other reported methylimido-type reactions, Yin et al. introduced a colorimetric probe 12 (fig. 10) for formaldehyde detection under weakly acidic conditions, which uses a cyanine derivative as a fluorophore and ethylenediamine as a formaldehyde recognition group
[97⇓⇓⇓⇓⇓⇓⇓~105][97]. Under weak acidic conditions (pH 3 ~ 6), the color of probe 12 changed from blue to green when it contacted with formaldehyde. However, under neutral or alkaline conditions, the solution is always blue. In addition, probe 12 has a short response time (30 s) to formaldehyde and good selectivity.
In addition to the three main types mentioned above, there are other types of formaldehyde fluorescent probes
[106⇓⇓~109]. In this paper, we focus on a kind of probe which can simultaneously detect sulfur dioxide and formaldehyde under reversible conditions to study the dynamic changes and interaction between sulfur dioxide and formaldehyde
[106⇓~108]. One example is the first reported probe 13 (fig. 11), which has carbazole as the fluorophore and a benzopyridine unit as the recognition site for sulfur dioxide. The reaction mechanism is as follows: sulfur dioxide is added to the benzopyridine unit through Michael addition reaction, and after formaldehyde is added, due to the reversibility of the Michael addition reaction and the fact that the ethylamine part in carbazole promotes the shedding of sulfite and is captured by formaldehyde, the reverse equilibrium is promoted. With the increase of sulfur dioxide concentration (0 ~ 50 μmol/L), the fluorescence intensity decreased at the λ
em=630 nm of the emission wavelength and increased at the λ
em=370 nm, and the solution changed from purplish red to colorless under visible light. After that, with the increase of formaldehyde concentration (0-200 μmol/L), the fluorescence intensity of the two sites returned to the original position, and the solution changed from colorless to purplish red under visible light. This process allows one cycle in a very short time of 65 s and can be repeated 9 times.
3 Fluorescence imaging applications in vivo
In view of the real-time imaging of biological tissues by formaldehyde fluorescence probe, it can play a visualization function with high spatial and temporal resolution, so it can also be used to study the concentration changes and physiological metabolism of formaldehyde in vivo. The application in organism is mainly to study the change of formaldehyde concentration in cells, tissues and living bodies. In the following examples, several common animal and plant models are selected to introduce different types of biological imaging, which are summarized in Table 1.
3.1 Detection of intracellular formaldehyde
For intracellular fluorescence imaging, the detection of both exogenous and endogenous formaldehyde is included. The probe 1 reported by Chan et al. (Fig. 12) was used to image human embryonic kidney cells (HEK293). A control group and three experimental groups were designed, and the blue fluorescence gradually increased, achieving the purpose of detecting exogenous formaldehyde
[38]. The cells were incubated with probe 1 (2 μmol/L) at 37 ℃ for 8 min, and then treated with 0, 1, 2. 5 and 5 mmol/L formaldehyde for 3 H. With the increase of formaldehyde concentration, the fluorescence of HEK293 cells increased gradually. Different from normal cells, Chang et al also applied probe 2 (under Figure 12) to human breast cancer cells (MCF7) with relatively high formaldehyde content (700-800 μmol/L), designed a control group and two experimental groups, and the red fluorescence was attenuated, achieving the purpose of detecting endogenous formaldehyde
[39]. During the experiment, the blank cancer cells and the experimental cancer cells (treated with different concentrations of formaldehyde inhibitors TCP and GSK) were incubated with probe 2 (10 μmol/L) at 37 ℃ for 60 min.
After fluorescence confocal imaging, it was found that the blank cancer cells maintained strong fluorescence, and the intensity was higher than that of the cancer cells with formaldehyde inhibitor, which proved that probe 2 could detect endogenous formaldehyde in disease models. This imaging study provides a reference for the detection of formaldehyde in different normal cells and cancer cells. However, due to the lack of sufficient experimental groups, it is only qualitative analysis in general, and does not quantitatively detect the concentration of formaldehyde in unknown cells to provide a basis for medical research.
3.2 Detection of Formaldehyde in Tissue
In addition, the formaldehyde fluorescence probe has also been applied to the fluorescence imaging of biological tissue sections (liver, kidney, plant root tips, etc.). For example, Lin et al. First used fluorescent probe 9b for two-photon (880 nm) imaging of tissue (liver)
[69]. One control group and two experimental groups were designed for liver tissue sections from 0 to 60.m u.m depth, resulting in a change in green fluorescence (fig. 13). There was no significant change in the first group of blank samples after imaging. In the second experimental group, after adding formaldehyde inhibitor NaHSO
3 for 30 min and then incubating with probe 9b (10 μmol/L) for 1 H, there was no obvious fluorescence after imaging, indicating that the formaldehyde content decreased after inhibitor treatment. In the third experimental group, the cells were directly incubated with probe 9 B (10 μmol/L) for 1 H, and produced obvious green fluorescence after imaging. The results show that the probe 9b can detect the physiological concentration of formaldehyde in living tissues by two-photon imaging. Under two-photon imaging, the lower energy ensures the integrity of cells and the permeability of photons in tissues, and the formaldehyde fluorescence imaging in tissues is realized for the first time. Moreover, the discovery and use of a new formaldehyde inhibitor, NaHSO
3, will facilitate the future study of formaldehyde bioimaging.
In addition, Qian et al. used the ratiometric probe 6 developed by Qian et al. to image the root tip tissue of Arabidopsis thaliana for the first time, and the fluorescence changed from green to red
[43]. As shown in fig. 14, in the first control group, the root tip tissues were treated with Probe 6 (50 μmol/L) for 45 min; In the second experimental group, the cells were incubated with formaldehyde (3 mmol/L) for 2 H. After imaging, it was found that probe 6 could penetrate well into the root tip tissue and produce uniform and intense initial green fluorescence. However, the fluorescence intensity of the red channel in the control group was weak, indicating that the physiological formaldehyde content in the root tip tissue was low. However, after the addition of external formaldehyde, strong red fluorescence was produced, and the ratio signal xinhao red </sub>/I<sub> green </sub > was significantly enhanced, indicating that probe 6 could monitor the change of formaldehyde content in Arabidopsis root tip tissue. The use of ratio probe makes it possible to accurately detect the concentration of formaldehyde in animal and plant tissues without external environmental interference.
3.3 Detection of Formaldehyde in Vivo
For in vivo imaging, You et al. carried out an experiment with the reported ratiometric probe14 for in vivo zebrafish, designed a control group and three experimental groups, and the fluorescence changed from blue to green
[58]. As shown in Figure 15, in the first control group, after 3 H of incubation with probe 14 (5 μmol/L), a weak fluorescence signal was collected from the green channel, indicating that endogenous formaldehyde was detected in zebrafish; In the second experimental group, formaldehyde was removed by NaHSO
3, and then probe 14 was added in the same way, no green fluorescence was produced, which verified the conclusion of the first experimental group. On this basis, the second experimental group was treated with different high concentrations of formaldehyde (1 and 2 mmol/L) as the third and fourth experimental groups. After 3 hours, it was found that the blue fluorescence disappeared and the green fluorescence increased with the increase of exogenous formaldehyde concentration, indicating that the high concentration of formaldehyde consumed the NaHSO
3 completely and was recognized by the probe again. On the whole, the in vivo detection of endogenous and exogenous formaldehyde in zebrafish is realized. However, it still only highlights the influence of the change of exogenous formaldehyde concentration on the fluorescence intensity of the probe, and fails to achieve the goal of detecting the actual endogenous formaldehyde concentration of cells with high sensitivity.
Finally, Chang et al. used the first reported near-infrared fluorescence probe 15 (fig. 16) for deep tissue imaging in anesthetized living mice, designed a control group and three experimental groups, and the fluorescence changed from blue to red
[41]. In the first control group, the mice were injected intraperitoneally with water; In the second experimental group, formaldehyde (2.5 mg/kg) was injected; In the third experimental group, the same amount of formaldehyde and formaldehyde scavenger NaHSO
3 were injected. In the fourth experimental group, only the formaldehyde scavenger NaHSO
3 was injected; Probe 15 (100 μmol/L) was injected into each group of samples. Compared with the blank sample, the fluorescence of the mice injected with formaldehyde and NaHSO
3 or only NaHSO
3 was not significantly different, and the fluorescence of the mice injected with formaldehyde alone was about 3-fold enhanced. The results showed that the probe 15 could successfully detect the changes of formaldehyde in vivo. Although the use of near-infrared probes maximizes the avoidance of damage to living organs and better photon penetration depth, it mainly adopts the research methods of cells and tissues similar to those introduced before (adding formaldehyde externally), lacking in-depth research on physiological metabolic processes and the interior of diseased bodies.
4 Determination of Formaldehyde in Actual Samples (Commodities)
Because the organic small molecule fluorescent probe can realize rapid, high-sensitivity and high-selectivity detection of formaldehyde in a complex solution system, the organic small molecule fluorescent probe can be used for trace detection of formaldehyde residues in actual samples (commodities). It mainly includes the detection of food and formaldehyde gas, and the comprehensive summary is shown in Table 1.
4.1 Determination of Formaldehyde in Food
Formaldehyde is used as a preservative (or illegally) to extend the shelf life of food, or to change the color and odor of food to attract consumers, resulting in formaldehyde residues that pose a serious threat to human health. Li et al. Reported two probes 8 and 16 (fig. 17) for the detection of formaldehyde in commercially available dry goods
[68,98]. Five grams of samples (shredded squid, fish fillets, dried bean curd stick and laver) were cut up and soaked in water for 12 hours, and then centrifuged to obtain the supernatant. Under the preliminary detection of the two probes, the formaldehyde contents in the two batches of samples were 30/53.28 mg/kg (shredded squid), 11/14.80 mg/kg (fish fillet), 8.3/10.75 mg/kg, (dried bean curd stick) and 83/171.2 mg/kgrespectively (laver), but no comparison and evaluation were made with reference to the accurate results. Only from the comparison of the detection results of the two probes, the selectivity of probe 8 (methylene hydrazine type) is better than that of probe 16 (methylimino type).
In the process of high temperature production, the Maillard reaction between amino acids and reducing sugars will produce formaldehyde, which will cause product pollution. Zeng et al. developed probe 17 (fig. 17) for the detection of formaldehyde in taffy and compared the results with standard high performance liquid chromatography (HPLC) analysis
[60]. Four kinds of toffee in supermarket were taken, 2 G samples were dissolved in 8 mL deionized water, Zn(AcO)
2(1 mL,0.1 mol/L) and K
4Fe(CN)
6(1 mL,0.1 mol/L) were added, and the supernatant was centrifuged. The formaldehyde contents in the four samples were 5.2, 2.1, 5.4 and 2.4 mg/kg, respectively, after preliminary detection using probe 17, and the HPLC analysis results were 2.5, 2.1, 2.7 and 2.1 mg/kg. The results showed that the formaldehyde content of No.2 and No.4 samples were basically consistent under the two detection methods. The detection results of No.1 and No.3 samples were also in the same order of magnitude, which verified the reliability of probe 17 for detecting formaldehyde content in taffy.
In addition, there are tests for fungi and meat products. For example, Lentinus edodes can produce endogenous formaldehyde through a continuous catalytic process, and Lin et al. Developed Probe18 to detect formaldehyde in Lentinus edodes and compared it with the standard phenol reagent (MBTH) method (Fig. 18)
[92]. Samples were taken from the local market and subjected to simple ultrasonic extraction and centrifugation to obtain a test water sample containing formaldehyde. Through the preliminary detection of probe 18 (Figure 18d), the formaldehyde content in the water sample was (2.1 ± 0.1) μmol/L, which was equivalent to the content of (23.6 ± 1.2) mg/kg formaldehyde in dried mushrooms. In order to verify the reliability of the test results, the MBTH method was used to detect the formaldehyde content in the extracted water sample (Fig. 18a ~ C), and the formaldehyde content was (21.9 ± 0.4) mg/kg, which was basically consistent with the fluorescence method, thus verifying the reliability of the method.
4.2 Detection of Formaldehyde in Air
Formaldehyde is also one of the major indoor pollutants. To date, probes can be loaded onto filter paper strips, fabric fibers, and silica gel plates as chromogenic and fluorescent indicators of formaldehyde. The change of color or fluorescence caused by the interaction with formaldehyde gas provides a way to qualitatively analyze formaldehyde and track the actual indoor formaldehyde gas in a wide concentration range.
Lin et al. wetted (DMF solution) the filter paper strips loaded with probe 18 (fig. 19) and suspended in weighing bottles containing formaldehyde solution (37 wt%) and distilled water, respectively
[92]. After 10 s, the filter strip suspended above the formaldehyde quickly turned red, and obvious fluorescence changes could also be observed under the ultraviolet lamp (Fig. 19 a, B). Subsequently, the same treated filter strips were placed in a ventilated room and in a drawer of a plywood table, respectively. After 5 min, the filter strips placed in the plywood drawer were found to turn red and produce more intense fluorescence under ultraviolet light (Fig. 19c, d). Li et al. Also loaded AIE probes on textiles and cotton
[57]. Under the same conditions, the fluorescence response intensity of the probe loaded on cotton to formaldehyde was higher than that on textile.
Unlike the above fiber-based loading matrix, Yin et al. loaded the AIE probe 19 (Figure 20) on the high-performance thin-layer chromatography silica gel plate (HPTLC), and achieved the purpose of high sensitivity, selectivity, portable and rapid detection of formaldehyde gas by virtue of the advantage that the AIE probe can produce fluorescence in the solid state
[56]. The experimental process is as follows: the analyte solution (formaldehyde) is added into the gas dispersion device, and the gas volatilizes and slowly enters the test chamber with HPTLC under the action of the pump. When the formaldehyde instrument shows that the formaldehyde content in the chamber is stable, after 60 minutes of reaction, the fluorescence change can be observed by naked eyes. The detection limit of formaldehyde is 0.036 mg/m
3, which is better than the World Health Organization (WHO) requirement (0.1 mg/m
3), and is not affected by temperature (20 ~ 30 ℃) and humidity (25% ~ 40%). However, only the standard sample of formaldehyde gas was detected, and the content of formaldehyde in the actual air was not detected after indoor sampling.
5 Conclusion and prospect
In recent years, there are three types of formaldehyde fluorescent probes, including 2-aza-Cope rearrangement, methylene hydrazine and azomethine. The first type is the most reported and is most widely used in the detection of organisms and formaldehyde gas. The reason is that the recognition group (homoallylamino) on the 2-aza-Cope rearrangement fluorescent probe has great steric hindrance and strong specificity in the reaction with formaldehyde, so the selectivity of this kind of probe is the best. The disadvantages are that the reaction rate is slow, the reaction time is long (usually more than 2 H) and the heating condition is needed. Therefore, how to improve the reaction kinetics and thermodynamics is an important goal for the future development of such probes. Although the second type of probe is not as selective as the first type (better than the third type), its preparation process is relatively simple, the cost is low, and the photophysical properties of the fluorophore skeleton are excellent (for example, 1,8-naphthalimide, the fluorescence enhancement factor can be as high as 900 times), so in addition to biological aspects, the detection of formaldehyde in food has steadily increased in the past two years. The third type of probe is the earliest, which has a rapid response to formaldehyde, and has many applications in the detection of exogenous formaldehyde and actual samples (commodities), but how to improve its selectivity is an urgent problem to be solved.
From the perspective of biological application of formaldehyde fluorescent probes, the detection and imaging of endogenous and exogenous formaldehyde in basic biological models such as cells, tissues and living bodies have been achieved, as well as the verification of a small number of physiological metabolic phenomena (such as folate degradation and oxidative damage), but there is a lack of in-depth research on formaldehyde quantification and pathology
[41,58]. This requires the development of near-infrared or two-photon formaldehyde fluorescence probes with higher sensitivity and spatial and temporal resolution, as well as the combination of near-infrared two-region and photoacoustic imaging.It expands the in-depth study of the correlation between formaldehyde and various cases (there are reports of probe studies on brain and Alzheimer's disease), and then assists other important medical means to achieve real-time monitoring of the dynamic process of pathology in different periods
[9,110].
For the detection of actual samples (commodities), the application of fluorescent probes has been increasing in recent years, mainly for food and air. For the detection of food, the probe is added after simple solution extraction. The detection in the air is mainly the volatile gas of formaldehyde solution, which is adsorbed on the filter paper, fabric fiber or silica gel plate loaded with the probe. However, there are some deficiencies in these two aspects of detection, such as too simple sample preparation, far from the actual model, lack of comparison with the standard method, and so on, which can not fully verify the actual operability of the fluorescence probe method and the reliability of the detection results. In the future, in addition to overcoming these shortcomings, improvements should be made in optimizing the detection device, in-situ and non-destructive detection of solid samples, and real-time detection of actual air samples.
Formaldehyde, as a widely used chemical, has been widely used in many fields, such as leather textile, wood processing, decorative materials, cosmetics and pharmaceuticals. These fields have long relied on high performance liquid chromatography (HPLC), gas chromatography-mass spectrometry (GC-MS) and spectrophotometry to detect formaldehyde, but these methods generally have some problems, such as expensive instruments, complex relying on international detection standards, time-consuming, uneven detection sensitivity and selectivity. At present, the fluorescent probe method for detecting formaldehyde has been newly popularized and applied in the fields of fur products and plywood
[85,102]. In a word, from the wide use of formaldehyde and the lack of detection methods, there is still much room for expansion in many fields to promote the demand of national application and the establishment of international standards.
National Key Research and Development Program of China in the 13th Five-Year Plan(2017YFB0308500)
National Natural Science Foundation of China(21907080)
National Natural Science Foundation of China(21476134)
Natural Science Foundation of Shaanxi Province(2020JM-069)
Natural Science Basic Research Program of Shaanxi Province(2019JQ-456)
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