The Application of Hydrogen Sulfide Fluorescent Probe in Food Preservation, Detection and Evaluation

This work primarily reviewed the response mechanism of fluorescent probes for H2S detection in foodstuffs in recent years, as well as the methodologies employed for detecting foodstuffs. Firstly, the significance of studying H2S gas as an important signaling molecule is introduced. Subsequently, a review of the response mechanism of the scientific community on how to detect H2S in foodstuffs samples by fluorescent probe technology is carried out. Secondly, the methods commonly used for detecting foodstuffs samples are discussed, including the test strip method and the spiking recovery methods. Nevertheless, despite the significant advancements in this field, there remain some research gaps. Finally, the article identifies the remaining issues that require further attention in current research and proposes avenues for future investigation. More importantly, this work identifies the current limitations of research in this field and proposes future applications of fluorescent probes for H2S in assessing food freshness and determining food spoilage. Therefore, this review will provide robust technical support for the protection of consumer health and the advancement of the sustainable development of the food industry and also put forward some new ideas and suggestions for future research.


Introduction
For decades, hydrogen sulfide (H 2 S) has been considered a toxic gas with a rotten egg smell.However, recent studies have revealed the key role of H 2 S as an important gas signaling molecule and redox balance molecule in living organisms.Produced naturally from L-cysteine, cystathionine γ-lyase (CSE) and cystathionine β-synthase (CBS) [1], H 2 S acts as a gaseous signaling molecule and plays important roles in several physiological systems.In the nervous system, H 2 S is involved in neurotransmission processes, regulating NMDA receptors and TRP channels through S-mercapturization modifications [2], and influencing intracellular calcium ion concentrations and neurotransmitter transmission [3].In the cardiovascular system, H 2 S has a significant antihypertensive effect, mainly through inhibition of the renin-angiotensin system, inhibition of vascular remodeling, and direct diastole of the vascular smooth muscle [4].In addition, H 2 S is involved in cell proliferation [5], apoptosis regulation [6], and antioxidant [7] and anti-inflammatory [8] physiological processes.However, abnormal levels of H 2 S in the body can also cause several health problems, such as Down syndrome, Alzheimer's disease, cirrhosis of the liver, and atherosclerosis [9].
In addition, H 2 S is inextricably linked to most high-protein foods [10].As one of the major volatiles produced by spoiled foods, H 2 S concentration is commonly used as an indicator of the freshness of high-protein foods.For example, yeasts in wine produce H 2 S gas, which can negatively affect the sensory quality of fermented foods and beverages [4].Excessive concentrations of H 2 S can cause quality degradation of alcoholic beverages [11,12], resulting in economic losses or serious food safety problems.Therefore, the detection of H 2 S levels in real food samples is of great importance, and the development of a faster and more convenient method for detecting H 2 S in real food samples has become particularly important.
Fluorescence imaging has become a fundamental tool for detecting gas molecules, renowned for its non-invasive, real-time imaging capabilities, high sensitivity, and high spatial and temporal resolution.Concurrently, its exemplary biocompatibility, high selectivity, non-destructive analytical attributes, and straightforward operation have established it as a principal analytical methodology for biomolecular cellular observation and food monitoring [13][14][15][16][17].As a result of ongoing research, a variety of fluorescent probes for the detection of H 2 S have been developed and synthesized.These include probes based on the azide/nitro reduction reaction [18], nucleophilic reaction [19,20], substitution reaction [21], strong S-Cu bonding ability [22], binding to unsaturated double bonds [23], and metal sulfide precipitation reaction [24].
It is noteworthy that MOF, as a novel material, can also be employed in the fabrication of biochemical sensors with H 2 S detection probes.Metal-organic frameworks (MOFs) are a novel class of porous crystalline hybrid materials comprising metal ions and organic linkers.They exhibit high porosity and biodegradability, which are advantageous for various applications [25].The combination of MOFs and a fluorescent probe has the potential to significantly enhance its sensitivity.Additionally, MOFs are frequently employed in the development of biosensors.The fundamental principle underlying this biosensor design methodology can be elucidated as follows: The initial step involves reducing the substance under examination to generate an electrochemical signal.Secondly, the substance under examination reacts with the substance to be tested, resulting in a change to its molecular structure.This, in turn, gives rise to a luminescent phenomenon.Thirdly, certain substances themselves emit fluorescence.When they react with the substance under examination, they produce a fluorescence enhancement phenomenon.The specific substances employed for detection are contingent upon the analyte in question [26].MOF structures are particularly well-suited for detecting hydrogen sulfide in food products.However, the number of fluorescent probes for detecting H 2 S in food remains relatively limited.
This review examines the fluorescent probes designed for detecting H 2 S in real food samples (Scheme 1).These probes employ a variety of chemical reactions, including the 2,4-dinitrophenyl (DNP) thiolysis reaction, the double bond addition-based 7-nitrobenzo-2oxa-1,3-diazole (NBD) thiolysis reaction, and the azide-based reaction.Furthermore, two prevalent techniques employed in the analysis of food samples are elucidated, and the document additionally contemplates the prospective utilization of H 2 S fluorescent probes in the evaluation of food safety, particularly in the identification of food preservation and deterioration characteristics.In conclusion, this review offers researchers a comprehensive overview of the latest developments in the design and application of fluorescent probes for food testing.Additionally, it provides food safety experts with a valuable repository of fundamental research and innovative concepts.The implementation of enhanced detection techniques for H 2 S in food samples, coupled with the expedient identification of any contamination, can markedly enhance the overall safety of the food supply chain.

Fluorescent Probes for the Detection of H2S in Foods
The synthesis of a variety of fluorescent probes has enabled the development of methods for detecting hydrogen sulfide (H2S) in real food samples.Moreover, the recognition group of H2S determines whether the fluorescent probe can be responded to and influences the response time.The recent fluorescent probes for detecting H2S in food can be classified into the following five categories.

Thiolysis Reactions Based on 2,4-dinitrophenyl (DNP)
DNP can trap H2S, so the reaction of this group with H2S can be well applied in the design of fluorescent probes.Under the nucleophilic effect of H2S, the DNP ether is thiolated by the H2S-based group, and the sulfation process is fully triggered, which is accompanied by a change in fluorescence intensity, and thus the fluorescent probe can respond (Figure 1a).At present, DNP-based fluorescent probes have been widely used for detecting H2S in food.However, different fluorophores and backbones can give different properties to the probes.The fluorescent probe 7 designed by Xie et al. (Figure 2) has triphenylamine and 2,4-dinitrobenzenesulfonyl groups incorporated into the styrylpyridinium scaffold, and the pyridinium salt in the scaffold can increase the solubility of the probe and its emission wavelength, which makes the probe mitochondrially targeted.In addition, the probe has a color change that appears in the "naked eye" state (Figure 1b), Scheme 1. Formation and action of H 2 S and its application to fluorescent probes.

Fluorescent Probes for the Detection of H 2 S in Foods
The synthesis of a variety of fluorescent probes has enabled the development of methods for detecting hydrogen sulfide (H 2 S) in real food samples.Moreover, the recognition group of H 2 S determines whether the fluorescent probe can be responded to and influences the response time.The recent fluorescent probes for detecting H 2 S in food can be classified into the following five categories.

Thiolysis Reactions Based on 2,4-dinitrophenyl (DNP)
DNP can trap H 2 S, so the reaction of this group with H 2 S can be well applied in the design of fluorescent probes.Under the nucleophilic effect of H 2 S, the DNP ether is thiolated by the H 2 S-based group, and the sulfation process is fully triggered, which is accompanied by a change in fluorescence intensity, and thus the fluorescent probe can respond (Figure 1a).At present, DNP-based fluorescent probes have been widely used for detecting H 2 S in food.However, different fluorophores and backbones can give different properties to the probes.The fluorescent probe 7 designed by Xie et al. (Figure 2) has triphenylamine and 2,4-dinitrobenzenesulfonyl groups incorporated into the styrylpyridinium scaffold, and the pyridinium salt in the scaffold can increase the solubility of the probe and its emission wavelength, which makes the probe mitochondrially targeted.In addition, the probe has a color change that appears in the "naked eye" state (Figure 1b), revealing the possibility of visualizing H 2 S content in food samples [27].Probe 1 utilizes the excellent properties of high stability and large Stokes shift of 4-diethylaminosalicylaldehyde [28], so it was chosen as the fluorophore of probe 1.It is worth mentioning that Zhong et al. also detected H 2 S in the gas phase and prepared nanofiber membranes doped with probe 1 by electrostatic spinning and exposed them to saturated H 2 S vapor.The results were obtained under daylight, as shown in Figure 1c, indicating that the fiber membrane was able to detect H 2 S in the gas phase, and the high porosity and large specific surface area of the membrane composed of electrostatically spun filaments could improve the sensitivity of the probe to some extent [29].However, it remains to be seen whether this structured fiber membrane can be used for trace analysis in food.Surprisingly, test strips impregnated with probe 12 could be used for visual determination of H 2 S gas due to food spoilage [30].Thus, this may support the design and establishment of a food spoilage assessment system in the future.
Compared with other H 2 S-specific fluorescent probes, probe 2 has a simple structure, is easy to synthesize, and achieves the detection of H 2 S in wine using a spiked recovery assay with recoveries of 90-103%.It has great potential for application in wine quality control and freshness assessment [31].The structure may hold some promise for future rapid synthesis of probes for use in the detection field, but its application is not yet broad enough and there is still room for development.Compared to probe 2, probe 3 uses benzothiazole to provide fluorescent signals.Meanwhile, this probe has been used for in situ imaging of rice in addition to real-time imaging of cells and zebrafish.A positive correlation between drought stress treatment and H 2 S levels was observed.The fluorescence intensity of the red channel increased with increasing drought stress treatment time (Figure 1d) [32].However, probe 4 utilizes the nopinone-based cyanopyridine amine structure with stable optical properties [33], but adds the same benzothiazole moiety as probe 3 in the neighboring position of the hydroxyl group, which results in the ESIPT phenomenon, and the fluorescence ability can be greatly enhanced.In addition, probe 4 can be used for detecting H 2 S in a variety of food samples [34].It is expected to provide some ideas for food quality analysis.Probe 6 was constructed using N,N-diethyl-4-vinylaniline with a 1-methylpyridinium iodide moiety to form the SPy-OH fluorophore, and it was constructed to form a donor-π-acceptor (D-π-A) structure by combining N,N-diethyl-4-vinylaniline with a 1methylpyridinium iodide moiety, which was chosen as the mitochondrial targeting moiety [38,39].Also, in this case, the reaction of the DNS ester group was initiated by HS-via a nucleophilic reaction, releasing the Sy-OH fluorophore.Surprisingly, the probe was able to respond to H2S at an ultrafast rate of 4 s, which is a great advantage in detecting H2S content in food samples to infer the freshness of such foods [4].Probe 14, which has a similar response mechanism to probe 6, releases a red fluorophore upon complete Probe 6 was constructed using N,N-diethyl-4-vinylaniline with a 1-methylpyridinium iodide moiety to form the SPy-OH fluorophore, and it was constructed to form a donor-πacceptor (D-π-A) structure by combining N,N-diethyl-4-vinylaniline with a 1-methylpyridinium iodide moiety, which was chosen as the mitochondrial targeting moiety [38,39].Also, in this case, the reaction of the DNS ester group was initiated by HS-via a nucleophilic reaction, releasing the Sy-OH fluorophore.Surprisingly, the probe was able to respond to H 2 S at an ultrafast rate of 4 s, which is a great advantage in detecting H 2 S content in food samples to infer the freshness of such foods [4].Probe 14, which has a similar response mechanism to probe 6, releases a red fluorophore upon complete cleavage of its dinitrophenyl group moiety, resulting in fluorescence enhancement.Water quality can be detected and visualized in food samples (Figure 1f) [37].
Similar to probe 6, probe 15 was constructed using a combination of 2-benzothiazole acetonitrile and 7-hydroxytetrahydroquinoxaline-6-carboxaldehyde in the D-π-A system, which was designed to extend the emission wavelength of the probe well [40].DNP triggers the fluorescence burst of the probe via a photo-induced electron transfer (d-PET) process excited by the electron donor.Upon reaction with H 2 S, the hydrogen sulfide induces CeO bond cleavage of the probe.Subsequently, the resulting phenol salts attack the a,bunsaturated nitrile moiety by intramolecular cyclization, resulting in a fluorophore.Since the fluorophore is highly conjugated, it restricts the rotation of the C=C double bond, causing the fluorescent probe to produce intense red fluorescence upon reaction with H 2 S [41].A tandem reaction (thiolation-cyclization) has been proposed in the design, whereby more diverse NIR fluorescent probes can be designed by substituting the protecting group [42].This point can give ideas to many researchers.A two-step tandem reaction initiated by HSwas used to construct probe 15.
In addition, probe 8 is based on a natural flavonol and has been used to detect H 2 S in environmental water samples, beer, wine, milk, acid sprouts, and eggs [43].However, probe 10 is based on (E)-2-(benzo[d]thiazol-2-yl)-3-(5-(4-hydroxyphenyl)thiophen-2-yl)acrylonitrile as the fluorophore, and due to the bursting effect of the DNP fragment on the fluorescence, probe 10 hardly fluoresces.However, in the presence of H 2 S, the recognition group is dissociated and the fluorophore (BAOH) is released, allowing the probe to respond and fluoresce yellow [44].
In general, researchers select stronger electron-absorbing groups for probe design.As an analog of DNP, the introduction of DNBS tends to enable the visualization of H 2 S under lower background signal conditions.Its strong electron-withdrawing ability accelerates the H 2 S-mediated thiolation reaction, further improving the response time.Therefore, in addition to DNP, DBNS is often used as a recognition group for H 2 S. Xu et al. selected phenothiazine derivatives as the fluorophore portion of probe 9 to make the probe with low cytotoxicity, easy modification, and good stability for more efficient, sensitive, and safe detection of H 2 S in food samples [45].Xiao et al. designed fluorescent probe 5 by combining naphthalimide fluorophore; it was selected as the fluorophore of the probe due to the high chemical stability it imparts to the probe, its easy modification, and, most importantly, its low cost.The probe also achieved "naked eye" observation of the reaction state in real food samples (Figure 1e) [36], which will be useful for the future mass production of the probe and its application in practice.
The design concept of Probe 11 is somewhat distinct from that of other probes.The 2,4-dinitrobenzenesulfonate (DNBS) group serves two distinct roles in the probe: it acts as the corresponding site and the electron acceptor.The fluorescence is extinguished by the photoelectron transfer effect (PET).The probe is essentially non-fluorescent due to the the PET effect, which is eliminated with the addition of H 2 S as a result of the reaction between the sulfonate bond and H 2 S. Once more, red fluorescence was observed as a result of the restoration of the intramolecular charge transfer effect (ICT) [46].Probe 13 is a novel corrole-based fluorescent probe that exhibits sensitivity and can be employed to detect water quality.When the 2,4-dinitrobenzenesulfonate ester bond of probe 13 reacted with H 2 S and was cleaved, the foregoing PET effect was terminated, resulting in a bright red fluorescence signal [47].
Both DNP and DNBS are commonly utilized H 2 S recognition molecules for fluorescent probes.As illustrated in Table 1, most probes with recognition molecules DNP and DBNS exhibit a rapid response capability, enabling them to respond swiftly.Given that food is susceptible to environmental influences during storage, an inappropriate storage environment can result in accelerated spoilage.This characteristic of the probe, coupled with its rapid response time, enables the detection of H 2 S in food, thus determining its freshness.This type of fluorescent probe can, therefore, play a pivotal role in the assessment of food preservation.

Based on Double Bond Addition
The addition of H 2 S to a C=C double bond results in a Michael addition reaction, which effectively disrupts the original compound's conjugated structure (Scheme 2a).This process ultimately leads to the emission of fluorescence.With this strategy, Shen et al. synthesized probe 16 (Figure 3).The probe changed color from blue to yellow when observed with the naked eye (Figure 4a), and a change in fluorescence from no fluorescence to orange fluorescence was observed under a fluorescence microscope (Figure 4b).The probe exhibits excellent photostability, maintaining a stable fluorescence intensity for at least 48 h [49].In contrast, probe 17 designed by Xu et al. (Figure 3) employs a nucleophilic addition of H 2 S in the olefinic portion, which results in the disruption of π-conjugation and the blocking of intramolecular charge transfer (ICT).The switching transition of the ICT process gives rise to a change in fluorescence.The probe exhibits a color change from reddish brown to white when observed visually, and under fluorescence microscopy, the color shifts gradually from orange-red to blue (Figure 4c).Furthermore, the probe demonstrated excellent optical response characteristics in the detection of trace H 2 S in living cells (Figure 4e) and in zebrafish (Figure 4f) [10].It is noteworthy that probe 18 designed by Magesh et al. in 2023 (Figure 3) is capable of simultaneous detection of CN-and HS-due to the electrondeficient nature of benzo-hemicyanine and the presence of electron-rich sulfur atoms in 4-(methylthio)benzaldehyde.This facilitates a donor between the polar C=C double bonds, thereby generating donor-π-acceptor interactions.CN-and HS-are capable of undergoing selective nucleophilic addition reactions at their respective reaction sites, which results in alterations in fluorescence [50].The aforementioned fluorescent probes are suitable for use in the detection of food samples.In contrast to other fluorescent probes that have been synthesized based on the C=C double bond mechanism, probe 19 exhibits an "ON-OFF" fluorescence response.The ICT effect is disrupted by the Michael addition of H2S to the C=C double bond, which results in the loss of fluorescence [51].The mechanism of probe 20 is essentially analogous to that of probe 19, with the fluorescence response also occurring in an "ON-OFF" manner.The probe is employed to analyze water and beer samples [62].In contrast to other fluorescent probes that have been synthesized based on the C=C double bond mechanism, probe 19 exhibits an "ON-OFF" fluorescence response.The ICT effect is disrupted by the Michael addition of H 2 S to the C=C double bond, which results in the loss of fluorescence [51].The mechanism of probe 20 is essentially analogous to that of probe 19, with the fluorescence response also occurring in an "ON-OFF" manner.The probe is employed to analyze water and beer samples [62].

Thiolation Reactions Based on 7-Nitrobenzo-2-oxa-1,3-diazole (NBD)
The thiolysis of NBD ethers or NBD amines is triggered by H2S via nucleophilic aromatic substitutions, resulting in the release of fluorophores (Scheme 3).The fluorescent probe 21 (Figure 3), designed by Luo et al., exhibits a double PET effect.When the morpholine moiety acts as an electron donor, the electrons on it are able to be transferred to the naphthylimide core, thereby triggering the α-PET effect.In this process, the electrons flow from the electron-rich donor (morpholine) to the electron-deficient acceptor (naphthylimide).Concurrently, this electron flow indirectly generates further interactions between the naphthimide core and another electron-deficient fragment (NBD), thereby enhancing the overall photoinduced electron transfer phenomenon.It is noteworthy that, in addition to this, the d-PET effect coexists in the system.The combination of these two dual effects allows probe 21 to produce minimal autofluorescence under acidic conditions, thereby facilitating optimal optical imaging.Furthermore, the probe exhibits a visible

Thiolation Reactions Based on 7-Nitrobenzo-2-oxa-1,3-diazole (NBD)
The thiolysis of NBD ethers or NBD amines is triggered by H 2 S via nucleophilic aromatic substitutions, resulting in the release of fluorophores (Scheme 3).The fluorescent probe 21 (Figure 3), designed by Luo et al., exhibits a double PET effect.When the morpholine moiety acts as an electron donor, the electrons on it are able to be transferred to the naphthylimide core, thereby triggering the α-PET effect.In this process, the electrons flow from the electron-rich donor (morpholine) to the electron-deficient acceptor (naphthylimide).Concurrently, this electron flow indirectly generates further interactions between the naphthimide core and another electron-deficient fragment (NBD), thereby enhancing the overall photoinduced electron transfer phenomenon.It is noteworthy that, in addition to this, the d-PET effect coexists in the system.The combination of these two dual effects allows probe 21 to produce minimal autofluorescence under acidic conditions, thereby facilitating optimal optical imaging.Furthermore, the probe exhibits a visible color change from orange to light yellow and a shift in fluorescence emission from no fluorescence to green fluorescence.The probe is capable of detecting changes in the endogenous and lysosomal levels of H 2 S in tumor cells in living systems.Moreover, the probe was successfully employed for the detection of H 2 S in environmental wastewater and food samples, including beer (Figure 4h).However, probe 24 (Figure 3) exhibits a slightly different mechanism than probe 21.Additionally, probe 24 is an open fluorescent probe.Probe 21 employs the coumarin 2212-dicyanoisophorone conjugate as a fluorophore, with NBD serving as the specific recognition site for H 2 S. When the fluorophore is attached to NBD through ether bonding, the electron-donating ability of the hydroxyl group is constrained, thereby inhibiting the ICT effect and resulting in a rapid increase in fluorescence.Upon adding H 2 S, the NBD undergoes partial cleavage, releasing the fluorophore.Consequently, the probe's fluorescence was restored following the completion of the reaction [55].
Molecules 2024, 29, x FOR PEER REVIEW 13 of 25 color change from orange to light yellow and a shift in fluorescence emission from no fluorescence to green fluorescence.The probe is capable of detecting changes in the endogenous and lysosomal levels of H2S in tumor cells in living systems.Moreover, the probe was successfully employed for the detection of H2S in environmental wastewater and food samples, including beer (Figure 4h).However, probe 24 (Figure 3) exhibits a slightly different mechanism than probe 21.Additionally, probe 24 is an open fluorescent probe.Probe 21 employs the coumarin 2212-dicyanoisophorone conjugate as a fluorophore, with NBD serving as the specific recognition site for H2S.When the fluorophore is attached to NBD through ether bonding, the electron-donating ability of the hydroxyl group is constrained, thereby inhibiting the ICT effect and resulting in a rapid increase in fluorescence.Upon adding H2S, the NBD undergoes partial cleavage, releasing the fluorophore.Consequently, the probe's fluorescence was restored following the completion of the reaction [55].

Reactions Based on Azide Groups
The azide group has been demonstrated to possess excellent reducing capabilities, which can effectively convert the electron-withdrawing azide group into an electron-donating amino group (Scheme 4a).This process serves to restore the fluorescence that was previously extinguished by the azide group.Based on this, probe 31 (Figure 3) was designed and applied to the detection of H2S in red wine samples.As illustrated in Figure 4g, the test strips display enhanced fluorescence intensity (dark blue) in response to elevated H2S concentrations within the fluorescence field.This suggests that the probe exhibits excellent capability for detecting authentic food samples, with a detection limit as low as 1 µM for red wine samples [62].

Reactions Based on Azide Groups
The azide group has been demonstrated to possess excellent reducing capabilities, which can effectively convert the electron-withdrawing azide group into an electrondonating amino group (Scheme 4a).This process serves to restore the fluorescence that was previously extinguished by the azide group.Based on this, probe 31 (Figure 3) was designed and applied to the detection of H 2 S in red wine samples.As illustrated in Figure 4g, the test strips display enhanced fluorescence intensity (dark blue) in response to elevated H 2 S concentrations within the fluorescence field.This suggests that the probe exhibits excellent capability for detecting authentic food samples, with a detection limit as low as 1 µM for red wine samples [62].
Molecules 2024, 29, x FOR PEER REVIEW 13 of 25 color change from orange to light yellow and a shift in fluorescence emission from no fluorescence to green fluorescence.The probe is capable of detecting changes in the endogenous and lysosomal levels of H2S in tumor cells in living systems.Moreover, the probe was successfully employed for the detection of H2S in environmental wastewater and food samples, including beer (Figure 4h).However, probe 24 (Figure 3) exhibits a slightly different mechanism than probe 21.Additionally, probe 24 is an open fluorescent probe.Probe 21 employs the coumarin 2212-dicyanoisophorone conjugate as a fluorophore, with NBD serving as the specific recognition site for H2S.When the fluorophore is attached to NBD through ether bonding, the electron-donating ability of the hydroxyl group is constrained, thereby inhibiting the ICT effect and resulting in a rapid increase in fluorescence.Upon adding H2S, the NBD undergoes partial cleavage, releasing the fluorophore.Consequently, the probe's fluorescence was restored following the completion of the reaction [55].

Reactions Based on Azide Groups
The azide group has been demonstrated to possess excellent reducing capabilities, which can effectively convert the electron-withdrawing azide group into an electron-donating amino group (Scheme 4a).This process serves to restore the fluorescence that was previously extinguished by the azide group.Based on this, probe 31 (Figure 3) was designed and applied to the detection of H2S in red wine samples.As illustrated in Figure 4g, the test strips display enhanced fluorescence intensity (dark blue) in response to elevated H2S concentrations within the fluorescence field.This suggests that the probe exhibits excellent capability for detecting authentic food samples, with a detection limit as low as 1 µM for red wine samples [62].

Others
It has been demonstrated that molecules exhibiting AIE properties are capable of maintaining high light stability and robust fluorescence emission in the polymerized state [65].Probe 25 designed by Wang et al. (Figure 3) is an AIE ratiometric probe, which is capable of accurate analysis in food safety testing based on the acquisition of the ratio of the intensities of the two emission peaks of a fluorescent probe as well as its unique response products.In a 70% ethanol aqueous solution, the addition of hydrogen sulfide (H 2 S) results in the formation of new hydrogen bonds between the N and H atoms of the N-H-S moiety.The formation of new hydrogen bonds can further enhance the rigidity of the probe 25.Furthermore, the formation of hydrogen bonds requires the participation of water molecules.The formation of hydrogen bonds facilitates the transition of probe AIE from a green to a red aggregated state.In essence, the addition of H 2 S gas induces a transformation from the aggregated green state of rod AIE to a globular red state (Figure 5a,b).This probe is also suitable for trace analysis [56].
In contrast to probe 25, probe 26 (Figure 3) employed a thiophenecarboxylic ester as the response unit for H 2 S, while naphthofluorescein was utilized as the fluorescent signaling group.The low fluorescence intensity of probe 26 was due to the protection of the probe's hydroxyl group by the thiophenecarboxylic ester.The release of the phenolic hydroxyl group upon treatment with an excess of H 2 S restores the structure of the fluorescent signaling moiety and enhances the fluorescence intensity [57].Although it is capable of detecting the concentration of H 2 S in red wine samples, the fluorescence intensity can only be significantly enhanced after treatment with excess H 2 S. As a result, it is challenging to achieve rapid detection in practical applications, and its range of applications is limited.Probe 22 (Figure 3), which employs a similar mechanism to probe 26 (Scheme 4b), is also based on the formation of 7-hydroxy-4-methylcoumarin through the disruption of the thiophenate ether group in the probe, which is induced by H 2 S.This process results in an enhanced fluorescence intensity, as illustrated in Figure 3.This method is suitable for the determination of H 2 S levels in not only red wine but also beer [53].
Furthermore, based on the principle that H 2 S can trigger the release of protective groups (PG), Hu et al. introduced a quinoline-phenol vinyl conjugate to design a fluorescent probe.The release of PG by H 2 S results in the formation of unstable amphiphiles (QL-Oi), which subsequently undergo self-incineration and ultimately give rise to a nonfluorescent neutral quinoline-phenol vinyl conjugate.The two daylight color changes with high contrast (Figure 5d) facilitate a clear determination of food freshness and offer new possibilities for solid-state paper test experiments with H 2 S in real food samples (Figure 5e) and food freshness assessment [54].
In the presence of H 2 S, the oxygen-carbonitrile bond in DDAO-CN is rapidly hydrolyzed, releasing the fluorophore 1,3-dichloro-7-hydroxy-9,9-dimethylacridin-2(9H)-one (DDAO), which results in a change of the solution from yellow to blue.This resulted in an on-type fluorescent response, exhibiting intense red fluorescence under UV light irradiation [58].However, probe 29 (Figure 3) is composed of a novel electron-available phenyl thiochlorocarbonate, which inhibits fluorescence by preventing charge transfer within the molecule.In the presence of H 2 S, the electron donor is removed, thereby restoring the intermolecular charge transfer (ICT) effect and resulting in fluorescence.Meanwhile, during the detection of the food sample, the color of the test strip undergoes a gradual transition from green to red (Figure 5g) [60].The introduction of H 2 S to probe solutions 27 and 29 produces a gradual alteration in solution color or test strip color, respectively, resulting in a markedly contrasting and readily discernible hue that differs significantly from the original.This enables the tracing of H 2 S produced during the deterioration of foodstuffs.
In particular, probe 28 (Figure 3) is a dual-channel fluorescent probe that exhibits selective recognition of H 2 S. The fluorophore of the mercapto compound and the reactive site of 2-thiophene carbonyl enable the specific recognition of H 2 S. The removal of thiophenecarboxylic acid by the addition of H 2 S results in the release of the yellow dye T-B and enhances the ICT effect [51], which is responsible for the observed fluorescence changes.
It is noteworthy that the reaction must be conducted in an aqueous solution both before and after, which has a deleterious impact on the expeditious detection of H 2 S gas produced during the spoilage of food samples.In the presence of H2S, the oxygen-carbonitrile bond in DDAO-CN is rapidly hydrolyzed, releasing the fluorophore 1,3-dichloro-7-hydroxy-9,9-dimethylacridin-2(9H)-one (DDAO), which results in a change of the solution from yellow to blue.This resulted in an on-type fluorescent response, exhibiting intense red fluorescence under UV light irradiation [58].However, probe 29 (Figure 3) is composed of a novel electron-available phenyl thiochlorocarbonate, which inhibits fluorescence by preventing charge transfer within the molecule.In the presence of H2S, the electron donor is removed, thereby restoring the intermolecular charge transfer (ICT) effect and resulting in fluorescence.Meanwhile, during the detection of the food sample, the color of the test strip undergoes a gradual tran- In addition to the aforementioned design concepts, probes may also be constructed through the principles of copper ion bursting and substitution (Scheme 2b).For example, probe 30 (Figure 3) is based on this principle.The fluorescent molecule IR820, a derivative of indocyanine green (ICG), is employed as the fluorophore, and the probe's fluorescence is triggered following the chelation with copper ions.Following the reaction with H 2 S, the fluorescence intensity is restored.The probe demonstrated an effective and discernible fluorescence response in detecting environmental water and food samples (Figure 5f).Probe 32 (Figure 3) is more complex in structure but has the capacity for CN -and HS - detection, is versatile, and the detection in fresh beef samples may be visualized under fluorescent conditions (Figure 5c).This could prove a valuable tool in detecting food safety issues [63].
and enhances the ICT effect [51], which is responsible for the observed fluorescence changes.It is noteworthy that the reaction must be conducted in an aqueous solution both before and after, which has a deleterious impact on the expeditious detection of H2S gas produced during the spoilage of food samples.
In addition to the aforementioned design concepts, probes may also be constructed through the principles of copper ion bursting and substitution (Scheme 2b).For example, probe 30 (Figure 3) is based on this principle.The fluorescent molecule IR820, a derivative of indocyanine green (ICG), is employed as the fluorophore, and the probe's fluorescence is triggered following the chelation with copper ions.Following the reaction with H2S, the fluorescence intensity is restored.The probe demonstrated an effective and discernible fluorescence response in detecting environmental water and food samples (Figure 5f).Probe 32 (Figure 3) is more complex in structure but has the capacity for CN -and HS - detection, is versatile, and the detection in fresh beef samples may be visualized under fluorescent conditions (Figure 5c).This could prove a valuable tool in detecting food safety issues [63].

Methods for H 2 S Detection in Foods
At a glance, there are three principal methods for the detection of H 2 S in real food samples using fluorescent probes.At present, the primary objective is the utilization of test strips for the visualization of H 2 S content through coloration, a technique that is predominantly employed for the detection of foodstuffs such as beef, chicken, pork, eggs, and shrimp.The second method is the spiked recovery test, which is typically employed for the detection of red wine, beer, and other similar beverages.

Colorimetric Method
The most prevalent colorimetric method employed in the present study is the test strip method.The rationale behind the design of the test strip is to employ a straightforward physical deposition technique to load a fluorescent probe onto a test strip [66], thereby enabling the strip to respond to the H 2 S [67] that is generated during the deterioration of the foodstuff (Figure 6a,b).The method primarily employs the use of filter paper sheets, which are soaked in the probe solution and subsequently air-dried to create filter paper sheets that are loaded with the probe, thereby producing test strips.The freshness of the food can be determined by certain color changes [61].Cai et al., in 2024, designed a turn-on fluorescent probe 10 with a large Stokes shift, a short response time, and a low detection limit.The probe was tested on real food samples (shrimp, pork, and chicken) and showed a gradual deepening of the yellow color over time (0 h, 12 h, and 24 h) (Figure 6e).
Furthermore, as illustrated in Figure 6c, they devised a smartphone-based analytical approach.As the concentration of H 2 S increased, the intensity of the yellow fluorescence also increased.The H 2 S content of the product was calculated from a linear relationship graph after the G/B value was determined by a color recognition program on the smartphone under UV light conditions.This value was then converted to a specific RGB value [44].This application provides a portable and prospective strategy with significant potential for quantitative analysis of H 2 S in food preservation.Furthermore, food testing applications employ the probe 19 solutions to specifically respond to H 2 S.This includes the "ON-OFF" type fluorescent probe designed by Shang et al.As the time is prolonged, the more severe the food spoilage, the lighter the color of the probe solution in the vial.Fluorescence intensity is observed to be weaker when examined under fluorescent conditions (Figure 6d) [50].Conversely, the test strip method may offer greater convenience in terms of detection, and the test strips are more portable.Furthermore, it is capable of visualizing the freshness of food samples and assisting in evaluating the efficacy of preservation methods.
Nevertheless, current research has yet to identify test strips that can achieve a "naked eye" response in a short period and be applied on a large scale to detect H 2 S in real samples in daily life.It is necessary to conduct further research to determine whether the color recognition program on a smartphone can be detached from the fluorescent field and accurately recognized in daylight.It is also important to note that the test strips or probe solutions must be placed near the food product to obtain an accurate response.Additionally, further investigation is required to ascertain whether this procedure affects the safety of the food product.Therefore, further experimental verification is required.

Spiked Recovery Test
The spiked recovery test is typically employed to identify the presence of red wine, beer, and water quality samples.These samples possess distinctive colors that can intensify the background hue of the test paper, potentially obscuring the results.Accordingly, the spiked recovery test is typically employed to detect the presence of hydrogen sulfide.Following the measurement of the hydrogen sulfide content, a comparison is made

Spiked Recovery Test
The spiked recovery test is typically employed to identify the presence of red wine, beer, and water quality samples.These samples possess distinctive colors that can intensify the background hue of the test paper, potentially obscuring the results.Accordingly, the spiked recovery test is typically employed to detect the presence of hydrogen sulfide.Following the measurement of the hydrogen sulfide content, a comparison is made between the measured value and the standard to ascertain whether the concentration of hydrogen sulfide in the food sample exceeds the specified limit.If the standard limit is exceeded, the food in question is rendered unfit for human consumption or commercial use.In 2017, Wang et al. designed probe 22 for the detection of hydrogen sulfide in red wine and beer.The results demonstrated that the recoveries for the detection of H 2 S in wine ranged from 91.2 to 110.0%, and were validated by methylene blue spectrophotometry, effectively illustrating the feasibility of the method [53].In 2018, Probe 2 was also designed to detect hydrogen sulfide in the same sample.The recovery for red wine was superior to that observed in 2017, with values ranging from 90.0% to 103.0% [31].Given the extensive existing data on the efficacy of fluorescent probes for the precise detection of hydrogen sulfide in authentic food samples (including water, red wine, and beer), no validation experiments for the methylene blue spectrophotometric method have been identified as having been conducted in recent years.In 2023, Xie et al. designed probe 20 to detect hydrogen sulfide in beer, with recoveries ranging from 95.6% to 103.7%.It was compatible with the determination of the H 2 S recovery concentrations, which were matched [51].This suggests that the technology of fluorescent probes is becoming increasingly sophisticated, with recoveries approaching the established standard values.Xu et al. employed three water samples (tap water, Black Lake Spring, and Daming Lake) to detect hydrogen sulfide, yielding detection recoveries that ranged from 91 to 112%.Notwithstanding slight discrepancies in the recoveries.However, they made a noteworthy observation in the paper regarding the application of probe detection.According to the GB11607-1989 [68], the concentration of H 2 S in water should be less than 0.2 mg/L.The detection limit of this probe in water quality is 5.73 µM, which demonstrates that the probe NBT can detect the hydrogen sulfide content in water quality [69].

Conclusions and Outlook
In recent years, an increasing number of studies have revealed that H 2 S is not only a key gaseous signaling molecule in living organisms but also a key molecule that is indispensable in the process of maintaining redox homeostasis.The intricate functions and significance of this gas have prompted a reexamination of its role in biological processes, offering novel insights into its involvement in physiological regulation and pathological states.Furthermore, H 2 S is inextricably linked to most high-protein foods.In the contemporary era, the gradual development of the Internet and food science and technology has led to a heightened focus on food safety and quality issues among the general public.Consumers have a heightened preference for food that is fresh.In order to maintain optimal freshness and maximize shelf life, effective measures to prevent food spoilage are essential and have become a significant concern in the food industry.In the contemporary food industry, the presence of H 2 S is inextricably linked to a diverse array of high-protein foods.This substance, renowned for its distinctive volatile properties, serves as a crucial indicator of the freshness of these foods.Fluorescent sensors are proving an effective solution to this problem.The use of high-performance fluorescent sensors offers a rapid and low-biologicaldestructive method of monitoring, preventing, and reducing food spoilage caused by H 2 S. If the threshold is exceeded, prompt action can be taken to avert potential food safety concerns, thereby guaranteeing that consumers have access to safe and wholesome food.
Currently, most fluorescent probes designed for use with real samples are oriented towards applications such as the thiolation of DNP, Michael's addition of C=C double bonds, and thiolation of NBD, among others.The incorporation of a fluorophore renders the probes highly efficacious (Figure 7).Several high-performance probes with low detection limits and low response times have been developed, including probes 13, 25, and 27, among others.Some probes are fully water-soluble, such as probe 32, but their structures are complex.Simultaneously, probes such as probe 2, which are simple and rapid to synthesize, are worthy of greater respect.However, they lack sufficient rapidity and versatility in their applications.In contrast, probe 4 has a broad range of applications and a multitude of variants.It can thus be concluded that, despite the design and synthesis of numerous fluorescent probes with excellent performance for detecting H 2 S, these probes still exhibit some limitations.In recent years, fluorescent probes have been developed that impede the ESIPT process by masking the phenolic hydroxyl groups with detection groups, thereby facilitating the detection of H 2 S [70].This concept serves as a crucial reference point for the development of novel fluorescent probes.The advancement of novel fluorescent probes for the detection of H 2 S in foodstuffs must address the following criteria (Figure 8): (1) It remains unclear whether it is possible to achieve complete water solubility without the involvement of organic reagents, which is still the case with only a few probes.(2) The question thus arises as to whether it is possible to achieve complete water solubility and to detect gaseous H 2 S. (3) The objective is to ascertain whether it is feasible to observe a change in color for trace amounts of H 2 S.This would entail enhancing the sensitivity and color change of the probes for small concentrations.(4) The objective is to ascertain whether a color change can be observed for trace amounts of H 2 S. In other words, the sensitivity of the probe must be enhanced for small concentrations, and it must be determined whether the color change is affected by the concentration of H 2 S. Also, it is essential to pursue continuous improvements in the loading capacity of the probe on the test strip.
The extensive use of excellent NIR fluorophores such as BODIPY and coumarin, as well as signal molecules such as azide and 2,4-nitrophenyl ether, helps improve the water solubility and sensitivity of the probe.As fluorescent probes continue to be researched and improved, it is anticipated that they will not only remain at the scientific research level but also be utilized in real-life applications, playing an increasingly pivotal role in food safety supervision and food preservation assessment.

3 of 24 25 Scheme 1 .
Scheme 1. Formation and action of H2S and its application to fluorescent probes.
29,  x FOR PEER REVIEW 6 of 25 fluorescent photographs of probe 14 test paper under natural light and 365 nm UV irradiation with different concentrations of H2S[37].

Figure 2 .
Figure 2. Various hydrogen sulfide fluorescent probes based on the thiolysis reaction of 2, 4-dinitrophenyl (DNP).The numbers on the diagram are used in place of the name of the compound.

Figure 2 .
Figure 2. Various hydrogen sulfide fluorescent probes based on the thiolysis reaction of 2, 4dinitrophenyl (DNP).The numbers on the diagram are used in place of the name of the compound.
Molecules 2024, 29, x FOR PEER REVIEW 11 of 25 sites, which results in alterations in fluorescence [50].The aforementioned fluorescent probes are suitable for use in the detection of food samples.

Scheme 2 .
Scheme 2. Approximate response mechanism of (a) double bond addition and (b) precipitation from mental binding [64].

Scheme 2 .
Scheme 2. Approximate response mechanism of (a) double bond addition and (b) precipitation from mental binding [64].

Figure 3 .
Figure 3. Various fluorescent probes for the detection of hydrogen sulfide in food samples.The numbers on the diagram are used in place of the name of the compound.

Figure 3 .
Figure 3. Various fluorescent probes for the detection of hydrogen sulfide in food samples.The numbers on the diagram are used in place of the name of the compound.

Figure 5 .
Figure 5. (a) Images of probe 25-based test paper for NaHS detection in sunlight (up) and under 365 nm UV light (down) [56].(b) Image of probe 25-based test paper for the detection of H 2 S gas under 365 nm light [56].(c) The color of beef test paper changes over time in long-wave light [63].(d) Color change of the detection strip loaded with probe 23 for different concentrations of H 2 S in ambient light [54].(e) Detection strips of probe 23 for the detection of H 2 S produced during the processing of pork (left) and eggs (right).Each group was stored at −4 • C (left) and 25 • C (right), respectively [54].(f) Probe 30 is based on the copper ion burst and substitution mechanism and its near-infrared fluorescence detection of H 2 S [61].(g) The test paper of probe 29 monitors the phenomenon of H 2 S production and its color change during spoilage of pork (left) and shrimp (right) [60].

Figure 6 .
Figure 6.(a) Preparation of test strips loaded with H 2 S fluorescent probes.(b) Colorimetric detection of H 2 S fluorescent probes.(c) H 2 S in actual samples was quantitatively detected by smartphone [50].(d) Colorimetric and fluorescence colorimetric response of the probe 19 to H 2 S produced during the spoilage of three food products.(I) Pork sample.(II) Garlic sample.(III) Chicken sample.(1) XDS (10 µM) solution was preserved in the dark at room temperature for 7 days.(2-9) Colorimetric and fluorescent colorimetric images of XDS (10 µM) solutions when foodstuff samples were preserved in the dark at room temperature for 0 day, 1 day, 2 days, 3 days, 4 days, 5 days, 6 days and 7 days, respectively, under natural light and UV light (365 nm).(IV)(1-3) XDS (10 µM) solution and pork sample were coexisted in refrigerator at 5 • C for 1 day, 3 days and 7 days, respectively; (4-6) XDS (10 µM) solution and chicken sample were coexisted in refrigerator at 5 • C for 1 day, 3 days and 7 days, respectively; (7-9) XDS (10 µM) solution and garlic sample were coexisted in refrigerator at 5 • C for 1 day, 3 days and 7 days, respectively [50].(e) Colorimetric and fluorescence photographs of test paper and food samples from Probe 10 under natural light and UV (365 nm) illumination [44].

Figure 8 .
Figure 8.The future outlook of the fluorescent probe of H2S.Author Contributions: All authors have read and agreed to the published version of the manuscript.

Figure 8 .
Figure 8.The future outlook of the fluorescent probe of H 2 S. Funding: This work was supported by Hunan Provincial Natural Science Foundation of China (No. 2023JJ30988).Institutional Review Board Statement: Not applicable.Informed Consent Statement: Not applicable.

Table 1 .
The main information of the probes detecting H 2 S in food samples.